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Emissions and Sinks: 1990-2008
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How to Obtain Copies
You can electronically download this document on the U.S. EPA's homepage at . To request free copies of this report, call the National Service
Center for Environmental Publications (NSCEP) at (800) 490-9198, or visit the website above and click on "order
online" after selecting an edition.
All data tables of this document are available for the full time series 1990 through 2008, inclusive, at the internet
site mentioned above.
For Further Information
Contact Mr. Leif Hockstad, Environmental Protection Agency, (202) 343-9432, hockstad.leif@epa.gov.
Or Mr. Brian Cook, Environmental Protection Agency, (202) 343-9135, cook.brianb@epa.gov.
For more information regarding climate change and greenhouse gas emissions, see the EPA web site at .
Released for printing: April 15, 2010
Energy Consumption—Stationary and Mobile Combustion Sources
The photos on the front and back cover of this report depict the energy consuming sources responsible for the
most greenhouse gas emissions, as calculated by the Inventory of U.S. Greenhouse Gas Emissions and Sinks:
1990-2008: stationary and mobile sources combusting fossil fuels. In 2008, these sources made up approximately
81 percent of gross U.S. greenhouse gas emissions. Despite their large contribution, both sources experienced
a drop in emissions from 2007 to 2008. Carbon dioxide, methane, and nitrous oxide emissions from fossil fuel
combustion at stationary sources decreased by 2.0 percent from 2007-2008 as a result of decreases in energy
consumption. In 2008, electricity demand decreased in response to higher energy prices, and a cooler summer. In
addition, higher gasoline prices led to a 5.9 percent decrease in overall emissions from the transportation sector,
or mobile sources from 2007 to 2008.
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INVENTORY OF U.S. GREENHOUSE GAS
EMISSIONS AND SINKS:
199O-2OO8
April 15, 2010
U.S. Environmental Protection Agency
1200 Pennsylvania Avenue, N.W.
Washington, DC 20460
U.S.A.
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Acknowledgments
The Environmental Protection Agency would like to acknowledge the many individual and organizational
contributors to this document, without whose efforts this report would not be complete. Although the complete
list of researchers, government employees, and consultants who have provided technical and editorial support is
too long to list here, EPA's Office of Atmospheric Programs would like to thank some key contributors and reviewers whose
work has significantly improved this year's report.
Work on emissions from fuel combustion was led by Leif Hockstad, Brian Cook, and Barbora Master. Work on industrial
process emissions was led by Mausami Desai. Work on methane emissions from the energy sector was directed by Lisa
Hanle and Kitty Sibold. Calculations for the waste sector were led by Rachel Schmeltz. Tom Wirth directed work on the
Agriculture, and together with Kimberly Todd and Jennifer Jenkins, directed work on the Land Use, Land-Use Change,
and Forestry chapters. Work on emissions of HFCs, PFCs, and SF6 was directed by Deborah Ottinger and Dave Godwin.
Venu Ghanta directed the work on mobile combustion and transportation.
Within the EPA, other Offices also contributed data, analysis, and technical review for this report. The Office of
Transportation and Air Quality and the Office of Air Quality Planning and Standards provided analysis and review for several
of the source categories addressed in this report. The Office of Solid Waste and the Office of Research and Development
also contributed analysis and research.
The Energy Information Administration and the Department of Energy contributed invaluable data and analysis on
numerous energy-related topics. The U.S. Forest Service prepared the forest carbon inventory, and the Department of
Agriculture's Agricultural Research Service and the Natural Resource Ecology Laboratory at Colorado State University
contributed leading research on nitrous oxide and carbon fluxes from soils.
Other government agencies have contributed data as well, including the U.S. Geological Survey, the Federal Highway
Administration, the Department of Transportation, the Bureau of Transportation Statistics, the Department of Commerce,
the National Agricultural Statistics Service, the Federal Aviation Administration, and the Department of Defense.
We would also like to thank Marian Martin Van Pelt, Randy Freed, and their staff at ICF International's Energy,
Environment, and Transportation Practice, including Don Robinson, Diana Pape, Susan Asam, Michael Grant, Robert
Lanza, Chris Steuer, Toby Krasney, Lauren Pederson, Joseph Herr, Kamala Jayaraman, Jeremy Scharfenberg, Mollie Averyt,
Ashley Labrie, Hemant Mallya, Sandy Seastream, Douglas Sechler, Ashaya Basnyat, Kristen Schell, Victoria Thompson,
Mark Flugge, Tristan Kessler, Katrin Moffroid, Veronica Kennedy, Aaron Beaudette, Anna Chavis, Larry O'Rourke, Rubab
Bhangu, Deborah Harris, Emily Rowan, Erin Gray, Roshni Rathi, Lauren Smith, Nikhil Nadkarni, Caroline Cochran, and
Neha Mukhi for synthesizing this report and preparing many of the individual analyses. Eastern Research Group, RTI
International, Raven Ridge Resources, and Arcadis also provided significant analytical support.
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The United States Environmental Protection Agency (EPA) prepares the official U.S. Inventory of Greenhouse Gas
Emissions and Sinks to comply with existing commitments under the United Nations Framework Convention
on Climate Change (UNFCCC). Under decision 3/CP.5 of the UNFCCC Conference of the Parties, national
inventories for UNFCCC Annex I parties should be provided to the UNFCCC Secretariat each year by April 15.
In an effort to engage the public and researchers across the country, the EPA has instituted an annual public review
and comment process for this document. The availability of the draft document is announced via Federal Register Notice
and is posted on the EPA web site. Copies are also mailed upon request. The public comment period is generally limited
to 30 days; however, comments received after the closure of the public comment period are accepted and considered for
the next edition of this annual report.
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Table of Contents
Acknowledgments i
Preface ii
Table of Contents ill
List of Tables, Figures, and Boxes vi
Tables vi
Figures xv
Boxes xvii
Executive Summary ES-1
ES.l. Background Information ES-2
ES.2. Recent Trends in U.S. Greenhouse Gas Emissions and Sinks ES-3
ES.3. Overview of Sector Emissions and Trends ES-12
ES.4. Other Information ES-15
1. Introduction 1-1
1.1. Background Information 1-2
1.2. Institutional Arrangements 1-7
1.3. Inventory Process 1-8
1.4. Methodology and Data Sources 1-10
1.5. Key Categories 1-11
1.6. Quality Assurance and Quality Control (QA/QC) 1-13
1.7. Uncertainty Analysis of Emission Estimates 1-15
1.8. Completeness 1-16
1.9. Organization of Report 1-16
2. Trends in Greenhouse Gas Emissions 2-1
2.1. Recent Trends in U.S. Greenhouse Gas Emissions and Sinks 2-1
2.2. Emissions by Economic Sector 2-17
2.3. Indirect Greenhouse Gas Emissions (CO, NOX, NMVOCs, and SO2) 2-28
3. Energy 3-1
3.1. Fossil Fuel Combustion (IPCC Source Category 1A) 3-4
3.2. Carbon Emitted from Non-Energy Uses of Fossil Fuels (IPCC Source Category 1A) 3-31
3.3. Incineration of Waste (IPCC Source Category lAla) 3-36
3.4. Coal Mining (IPCC Source Category IBla) 3-39
3.5. Abandoned Underground Coal Mines (IPCC Source Category IBla) 3-42
3.6. Natural Gas Systems (IPCC Source Category lB2b) 3-45
3.7. Petroleum Systems (IPCC Source Category lB2a) 3-50
III
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3.8. Energy Sources of Indirect Greenhouse Gas Emissions 3-54
3.9. International Bunker Fuels (IPCC Source Category 1: Memo Items) 3-56
3.10. Wood Biomass and Ethanol Consumption (IPCC Source Category 1A) 3-60
4. Industrial Processes 4-1
4.1. Cement Production (IPCC Source Category 2A1) 4-5
4.2. Lime Production (IPCC Source Category 2A2) 4-7
4.3. Limestone and Dolomite Use (IPCC Source Category 2A3) 4-10
4.4. Soda Ash Production and Consumption (IPCC Source Category 2A4) 4-13
4.5. Ammonia Production (IPCC Source Category 2B1) and Urea Consumption 4-15
4.6. Nitric Acid Production (IPCC Source Category 2B2) 4-19
4.7. Adipic Acid Production (IPCC Source Category 2B3) 4-20
4.8. Silicon Carbide Production (IPCC Source Category 2B4) and Consumption 4-23
4.9. Petrochemical Production (IPCC Source Category 2B5) 4-25
4.10. Titanium Dioxide Production (IPCC Source Category 2B5) 4-26
4.11. Carbon Dioxide Consumption (IPCC Source Category 2B5) 4-30
4.12. Phosphoric Acid Production (IPCC Source Category 2B5) 4-32
4.13. Iron and Steel Production (IPCC Source Category 2C1) and Metallurgical Coke Production 4-35
4.14. Ferroalloy Production (IPCC Source Category 2C2) 4-44
4.15. Aluminum Production (IPCC Source Category 2C3) 4-46
4.16. Magnesium Production and Processing (IPCC Source Category 2C4) 4-50
4.17. Zinc Production (IPCC Source Category 2C5) 4-53
4.18. Lead Production (IPCC Source Category 2C5) 4-56
4.19. HCFC-22 Production (IPCC Source Category 2E1) 4-58
4.20. Substitution of Ozone Depleting Substances (IPCC Source Category 2F) 4-60
4.21. Semiconductor Manufacture (IPCC Source Category 2F6) 4-64
4.22. Electrical Transmission and Distribution (IPCC Source Category 2F7) 4-69
4.23. Industrial Sources of Indirect Greenhouse Gases 4-74
5. Solvent and Other Product Use 5-1
5.1. Nitrous Oxide from Product Uses (IPCC Source Category 3D) 5-1
5.2. Indirect Greenhouse Gas Emissions from Solvent Use 5-4
6. Agriculture 6-1
6.1. Enteric Fermentation (IPCC Source Category 4A) 6-2
6.2. Manure Management (IPCC Source Category 4B) 6-7
6.3. Rice Cultivation (IPCC Source Category 4C) 6-12
6.4. Agricultural Soil Management (IPCC Source Category 4D) 6-18
6.5. Field Burning of Agricultural Residues (IPCC Source Category 4F) 6-32
7. Land Use, Land-Use Change, and Forestry 7-1
7.1. Representation of the U.S. Land Base 7-4
7.2. Forest Land Remaining Forest Land 7-13
7.3. Land Converted to Forest Land (IPCC Source Category 5A2) 7-27
iv
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7.4. Cropland Remaining Cropland (IPCC Source Category 5B1) 7-27
7.5. Land Converted to Cropland (IPCC Source Category 5B2) 7-38
7.6. Grassland Remaining Grassland (IPCC Source Category 5C1) 7-42
7.7. Land Converted to Grassland (IPCC Source Category 5C2) 7-47
7.8. Wetlands Remaining Wetlands 7-51
7.9. Settlements Remaining Settlements 7-55
7.10. Land Converted to Settlements (Source Category 5E2) 7-61
7.11. Other (IPCC Source Category 5G) 7-61
8. Waste 8-1
8.1. Landfills (IPCC Source Category 6A1) 8-2
8.2. Wastewater Treatment (IPCC Source Category 6B) 8-6
8.3. Composting (IPCC Source Category 6D) 8-17
8.4. Waste Sources of Indirect Greenhouse Gases 8-19
9. Other 9-1
10. Recalculations and Improvements 10-1
11. References 11-1
List of Annexes (Annexes available on CD version only)
ANNEX 1. Key Category Analysis
ANNEX 2 Methodology and Data for Estimating C02 Emissions from Fossil Fuel Combustion
2.1. Methodology for Estimating Emissions of CO2 from Fossil Fuel Combustion
2.2. Methodology for Estimating the Carbon Content of Fossil Fuels
2.3. Methodology for Estimating Carbon Emitted from Non-Energy Uses of Fossil Fuels
ANNEX 3. Methodological Descriptions for Additional Source or Sink Categories
3.1. Methodology for Estimating Emissions of CIL,, N2O, and Indirect Greenhouse Gases from Stationary Combustion
3.2. Methodology for Estimating Emissions of CIL,, N2O, and Indirect Greenhouse Gases from Mobile Combustion
and Methodology for and Supplemental Information on Transportation-Related GHG Emissions
3.3. Methodology for Estimating CIL, Emissions from Coal Mining
3.4. Methodology for Estimating CH4 and CO2 Emissions from Natural Gas Systems
3.5. Methodology for Estimating CIL, and CO2 Emissions from Petroleum Systems
3.6. Methodology for Estimating CO2, N2O and CFLj Emissions from the Incineration of Waste
3.7. Methodology for Estimating Emissions from International Bunker Fuels used by the U.S. Military
3.8. Methodology for Estimating HFC and PFC Emissions from Substitution of Ozone Depleting Substances
3.9. Methodology for Estimating CIL, Emissions from Enteric Fermentation
3.10. Methodology for Estimating CH4 and N2O Emissions from Manure Management
3.11. Methodology for Estimating N2O Emissions from Agricultural Soil Management
3.12. Methodology for Estimating Net Carbon Stock Changes in Forest Lands Remaining Forest Lands
3.13. Methodology for Estimating Net Changes in Carbon Stocks in Mineral and Organic Soils on Cropland
and Grassland
3.14. Methodology for Estimating CIL, Emissions from Landfills
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ANNEX 4. IPCC Reference Approach for Estimating C02 Emissions from Fossil Fuel Combustion
ANNEX 5. Assessment of the Sources and Sinks of Greenhouse Gas Emissions Excluded
ANNEX 6. Additional Information
6.1. Global Warming Potential Values
6.2. Ozone Depleting Substance Emissions
6.3. Sulfur Dioxide Emissions
6.4. Complete List of Source Categories
6.5. Constants, Units, and Conversions
6.6. Abbreviations
6.7. Chemical Formulas
ANNEX 7. Uncertainty
7.1. Overview
7.2. Methodology and Results
7.3. Planned Improvements
7.4. Additional Information on Uncertainty Analyses by Source
List of Tables, Figures, and Boxes
Tables
Table ES-1: Global Warming Potentials (100-Year Time Horizon) Used in this Report ES-3
Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq. or million
metric tons CO2 Eq.) ES-5
Table ES-3: CO2 Emissions from Fossil Fuel Combustion by Fuel Consuming End-Use Sector
(Tg C02 Eq.) ES-9
Table ES-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector
(Tg CO2 Eq.) ES-12
Table ES- 5: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) ES-14
Table ES-6. Emissions from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) ES-15
Table ES-7: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq.) ES-16
Table ES-8: U.S Greenhouse Gas Emissions by Economic Sector with Electricity-Related
Emissions Distributed (Tg CO2 Eq.) ES-17
Table ES-9: Recent Trends in Various U.S. Data (Index 1990 = 100) ES-18
Table ES-10: Emissions of NOX, CO, NMVOCs, and SO2 (Gg) ES-19
Table 1-1: Global Atmospheric Concentration, Rate of Concentration Change, and Atmospheric
Lifetime (years) of Selected Greenhouse Gases 1-3
Table 1-2: Global Warming Potentials and Atmospheric Lifetimes (Years) Used in this Report 1-7
Table 1-3: Comparison of 100-Year GWPs 1-8
Table 1-4: Key Categories for the United States (1990-2008) 1-12
Table 1-5. Estimated Overall Inventory Quantitative Uncertainty (Tg CO2 Eq. and Percent) 1-16
Table 1-6: IPCC Sector Descriptions 1-17
Table 1-7: List of Annexes 1-18
Table 2-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.) 2-4
Table 2-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg) 2-6
vi
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Table 2-3: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector
(Tg CO2 Eq.) 2-8
Table 2-4: Emissions from Energy (Tg CO2 Eq.) 2-10
Table 2-5: CO2 Emissions from Fossil Fuel Combustion by Fuel Consuming End-Use Sector
(Tg CO2 Eq.) 2-11
Table 2-6: Emissions from Industrial Processes (Tg CO2 Eq.) 2-13
Table 2-7: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq.) 2-14
Table 2-8: Emissions from Agriculture (Tg CO2 Eq.) 2-15
Table 2-9: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) 2-16
Table 2-10: Emissions from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) 2-16
Table 2-11: Emissions from Waste (Tg CO2 Eq.) 2-17
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq. and
Percent of Total in 2008) 2-18
Table 2-13: Electricity Generation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 2-21
Table 2-14: U.S. Greenhouse Gas Emissions by Economic Sector and Gas with Electricity-Related
Emissions Distributed (Tg CO2 Eq.) and Percent of Total in 2008 2-22
Table 2-15: Transportation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 2-24
Table 2-16: Recent Trends in Various U.S. Data (Index 1990 = 100) 2-27
Table 2-17: Emissions of NOX, CO, NMVOCs, and SO2 (Gg) 2-29
Table 3-1: CO2, CH4, and N2O Emissions from Energy (Tg CO2 Eq.) 3-2
Table 3-2: CO2, CH4, and N2O Emissions from Energy (Gg) 3-3
Table 3-3: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion (Tg CO2 Eq.) 3-4
Table 3-4: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion (Gg) 3-4
Table 3-5: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq.) 3-5
Table 3-6: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and
Sectors (Tg CO2 Eq. and Percent) 3-6
Table 3-7: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion by Sector (Tg CO2 Eq.) 3-8
Table 3-8: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion by End-Use Sector
(Tg CO2 Eq.) 3-9
Table 3-9: CO2 Emissions from Stationary Combustion (Tg CO2 Eq.) 3-10
Table 3-10: CH4 Emissions from Stationary Combustion (Tg CO2 Eq.) 3-11
Table 3-11: N2O Emissions from Stationary Combustion (Tg CO2 Eq.) 3-12
Table 3-12: CO2 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector
(Tg C02 Eq.) 3-15
Table 3-13: CH4 Emissions from Mobile Combustion (Tg CO2 Eq.) 3-17
Table 3-14: N2O Emissions from Mobile Combustion (Tg CO2 Eq.) 3-18
Table 3-15: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg CO2 Eq./QBtu) 3-21
Table 3-16: Carbon Intensity from All Energy Consumption by Sector (Tg CO2 Eq./QBtu) 3-22
Table 3-17: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Energy-related
Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq. and Percent) 3-25
Table 3-18: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from
Energy-Related Stationary Combustion, Including Biomass (Tg CO2 Eq. and Percent) 3-27
Table 3-19. Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from On-Road
Sources (Tg CO2 Eq. and Percent) 3-29
Table 3-20: CO2 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg CO2 Eq.) 3-31
VII
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Table 3-21: Adjusted Consumption of Fossil Fuels for Non-Energy Uses (TBtu) 3-22
Table 3-22: 2008 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions 3-33
Table 3-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Non-Energy Uses of
Fossil Fuels (Tg CO2 Eq. and Percent) 3-34
Table 3-24: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of Non-Energy Uses of
Fossil Fuels (Percent) 3-34
Table 3-25: CO2 and N2O Emissions from the Incineration of Waste (Tg CO2 Eq.) 3-37
Table 3-26: CO2 and N2O Emissions from the Incineration of Waste (Gg) 3-37
Table 3-27: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted 3-38
Table 3-28: Tier 2 Quantitative Uncertainty Estimates for CO2 and N2O from the Incineration of
Waste (Tg CO2 Eq. and Percent) 3-38
Table 3-29: CH4 Emissions from Coal Mining (Tg CO2 Eq.) 3-40
Table 3-30: CH4 Emissions from Coal Mining (Gg) 3-40
Table 3-31: Coal Production (Thousand Metric Tons) 3-41
Table 3-32: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Coal Mining
(Tg CO2 Eq. and Percent) 3-42
Table 3-33: CH4 Emissions from Abandoned Underground Coal Mines (Tg CO2 Eq.) 3-43
Table 3-34: CH4 Emissions from Abandoned Underground Coal Mines (Gg) 3-43
Table 3-35: Number of gassy abandoned mines occurring in U.S. basins grouped by class
according to post-abandonment state 3-45
Table 3-36: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Abandoned
Underground Coal Mines (Tg CO2 Eq. and Percent) 3-46
Table 3-37. CH4 Emissions from Natural Gas Systems (Tg CO2 Eq.) 3-47
Table 3-38. CH4 Emissions from Natural Gas Systems (Gg) 3-47
Table 3-39. Non-combustion CO2 Emissions from Natural Gas Systems (Tg CO2 Eq.) 3-47
Table 3-40. Non-combustion CO2 Emissions from Natural Gas Systems (Gg) 3-47
Table 3-41: Tier 2 Quantitative Uncertainty Estimates for CH4 and Non-energy CO2 Emissions from
Natural Gas Systems (Tg CO2 Eq. and Percent) 3-49
Table 3-42: CH4 Emissions from Petroleum Systems (Tg CO2 Eq.) 3-51
Table 3-43: CH4 Emissions from Petroleum Systems (Gg) 3-51
Table 3-44: CO2 Emissions from Petroleum Systems (Tg CO2 Eq.) 3-51
Table 3-45: CO2 Emissions from Petroleum Systems (Gg) 3-51
Table 3-46: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petroleum Systems
(Tg CO2 Eq. and Percent) 3-53
Table 3-47: Potential Emissions from CO2 Capture and Transport (Tg CO2 Eq.) 3-55
Table 3-48: Potential Emissions from CO2 Capture and Transport (Gg) 3-55
Table 3-49: NOX, CO, and NMVOC Emissions from Energy-Related Activities (Gg) 3-56
Table 3-50: CO2, CH4, and N2O Emissions from International Bunker Fuels (Tg CO2 Eq.) 3-58
Table 3-51: CO2, CH4 and N2O Emissions from International Bunker Fuels (Gg) 3-58
Table 3-52: Aviation Jet Fuel Consumption for International Transport (Million Gallons) 3-59
Table 3-53: Marine Fuel Consumption for International Transport (Million Gallons) 3-59
Table 3-54: CO2 Emissions from Wood Consumption by End-Use Sector (Tg CO2 Eq.) 3-61
Table 3-55: CO2 Emissions from Wood Consumption by End-Use Sector (Gg) 3-61
Table 3-56: CO2 Emissions from Ethanol Consumption (Tg CO2 Eq.) 3-62
VIM
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Table 3-57: CO2 Emissions from Ethanol Consumption (Gg) 3-62
Table 3-58: Woody Biomass Consumption by Sector (Trillion Btu) 3-62
Table 3-59: Ethanol Consumption by Sector (Trillion Btu) 3-62
Table 4-1: Emissions from Industrial Processes (Tg CO2 Eq.) 4-3
Table 4-2: Emissions from Industrial Processes (Gg) 4-4
Table 4-3: CO2 Emissions from Cement Production (Tg CO2 Eq. and Gg) 4-5
Table 4-4: Clinker Production (Gg) 4-6
Table 4-5: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Cement Production
(Tg CO2 Eq. and Percent) 4-6
Table 4-6: CO2 Emissions from Lime Production (Tg CO2 Eq. and Gg) 4-7
Table 4-7: Potential, Recovered, and Net CO2 Emissions from Lime Production (Gg) 4-7
Table 4-8: High-Calcium- and Dolomitic-Quicklime, High-Calcium- and Dolomitic-Hydrated, and
Dead-Burned-Dolomite Lime Production (Gg) 4-8
Table 4-9: Adjusted Lime Productiona (Gg) 4-9
Table 4-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lime Production
(Tg CO2 Eq. and Percent) 4-10
Table 4-11: CO2 Emissions from Limestone & Dolomite Use (Tg CO2 Eq.) 4-10
Table 4-12: CO2 Emissions from Limestone & Dolomite Use (Gg) 4-11
Table 4-13: Limestone and Dolomite Consumption (Thousand Metric Tons) 4-11
Table 4-14: Dolomitic Magnesium Metal Production Capacity (Metric Tons) 4-12
Table 4-15: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Limestone and
Dolomite Use (Tg CO2 Eq. and Percent) 4-13
Table 4-16: CO2 Emissions from Soda Ash Production and Consumption (Tg CO2 Eq.) 4-14
Table 4-17: CO2 Emissions from Soda Ash Production and Consumption (Gg) 4-14
Table 4-18: Soda Ash Production and Consumption (Gg) 4-14
Table 4-19: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Soda Ash
Production and Consumption (Tg CO2 Eq. and Percent) 4-15
Table 4-20: CO2 Emissions from Ammonia Production and Urea Consumption (Tg CO2 Eq.) 4-16
Table 4-21: CO2 Emissions from Ammonia Production and Urea Consumption (Gg) 4-16
Table 4-22: Ammonia Production, Urea Production, Urea Net Imports, and Urea Exports (Gg) 4-17
Table 4-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ammonia
Production and Urea Consumption (Tg CO2 Eq. and Percent) 4-18
Table 4-24: N2O Emissions from Nitric Acid Production (Tg CO2 Eq. and Gg) 4-19
Table 4-25: Nitric Acid Production (Gg) 4-20
Table 4-26: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Nitric Acid
Production (Tg CO2 Eq. and Percent) 4-20
Table 4-27: N2O Emissions from Adipic Acid Production (Tg CO2 Eq. and Gg) 4-21
Table 4-28: Adipic Acid Production (Gg) 4-22
Table 4-29: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Adipic Acid
Production (Tg CO2 Eq. and Percent) 4-23
Table 4-30: CO2 and CH4 Emissions from Silicon Carbide Production and Consumption
(Tg C02Eq.) 4-24
Table 4-31: CO2 and CH4 Emissions from Silicon Carbide Production and Consumption (Gg) 4-24
Table 4-32: Production and Consumption of Silicon Carbide (Metric Tons) 4-24
IX
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Table 4-33: Tier 2 Quantitative Uncertainty Estimates for CH4 and CO2 Emissions from
Silicon Carbide Production and Consumption (Tg CO2 Eq. and Percent) 4-24
Table 4-34: CO2 and CH4 Emissions from Petrochemical Production (Tg CO2 Eq.) 4-25
Table 4-35: CO2 and CH4 Emissions from Petrochemical Production (Gg) 4-25
Table 4-36: Production of Selected Petrochemicals (Thousand Metric Tons) 4-26
Table 4-37: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock
(Secondary Feedstock) Consumption (Thousand Metric Tons) 4-27
Table 4-38: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petrochemical
Production and CO2 Emissions fromCarbon Black Production (Tg CO2 Eq. and Percent) 4-27
Table 4-39: CO2 Emissions from Titanium Dioxide (Tg CO2 Eq. and Gg) 4-28
Table 4-40: Titanium Dioxide Production (Gg) 4-29
Table 4-41: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Titanium Dioxide Production (Tg CO2 Eq. and Percent) 4-29
Table 4-42: CO2 Emissions from CO2 Consumption (Tg CO2 Eq. and Gg) 4-30
Table 4-43: CO2 Production (Gg CO2) and the Percent Used for
Non-EOR Applications for Jackson Dome and Bravo Dome 4-31
Table 4-44: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
CO2 Consumption (Tg CO2 Eq. and Percent) 4-32
Table 4-45: CO2 Emissions from Phosphoric Acid Production (Tg CO2 Eq. and Gg) 4-32
Table 4-46: Phosphate Rock Domestic Production, Exports, and Imports (Gg) 4-33
Table 4-47: Chemical Composition of Phosphate Rock (percent by weight) 4-34
Table 4-48: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Phosphoric Acid Production (Tg CO2 Eq. and Percent) 4-35
Table 4-49: CO2 and CH4 Emissions from Metallurgical Coke Production (Tg CO2 Eq.) 4-37
Table 4-50: CO2 and CH4 Emissions from Metallurgical Coke Production (Gg) 4-37
Table 4-51: CO2 Emissions from Iron and Steel Production (Tg CO2 Eq.) 4-37
Table 4-52: CO2 Emissions from Iron and Steel Production (Gg) 4-37
Table 4-53: CH4 Emissions from Iron and Steel Production (Tg CO2 Eq.) 4-38
Table 4-54: CH4 Emissions from Iron and Steel Production (Gg) 4-38
Table 4-55: Material Carbon Contents for Metallurgical Coke Production 4-38
Table 4-56: Production and Consumption Data for the Calculation of CO2 and
CH4 Emissions from Metallurgical Coke Production (Thousand Metric Tons) 4-39
Table 4-57: Production and Consumption Data for the Calculation of CO2 Emissions from
Metallurgical Coke Production (million ft3) 4-39
Table 4-58: CO2 Emission Factors for Sinter Production and Direct Reduced Iron Production 4-40
Table 4-59: Material Carbon Contents for Iron and Steel Production 4-40
Table 4-60: CH4 Emission Factors for Sinter and Pig Iron Production 4-40
Table 4-61: Production and Consumption Data for the Calculation of CO2 and
CH4 Emissions from Iron and Steel Production (Thousand Metric Tons) 4-41
Table 4-62: Production and Consumption Data for the Calculation of CO2 Emissions from
Iron and Steel Production (million ft3 unless otherwise specified) 4-42
Table 4-63: Tier 2 Quantitative Uncertainty Estimates for CO2 and CH4 Emissions from
Iron and Steel Production and Metallurgical Coke Production (Tg. CO2 Eq. and Percent) 4-43
Table 4-64: CO2 and CH4 Emissions from Ferroalloy Production (Tg CO2 Eq.) 4-44
Table 4-65: CO2 and CH4 Emissions from Ferroalloy Production (Gg) 4-44
Table 4-66: Production of Ferroalloys (Metric Tons) 4-45
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Table 4-67: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ferroalloy
Production (Tg CO2 Eq. and Percent) 4-46
Table 4-68: CO2 Emissions from Aluminum Production (Tg CO2 Eq. and Gg) 4-47
Table 4-69: PEC Emissions from Aluminum Production (Tg CO2 Eq.) 4-47
Table 4-70: PEC Emissions from Aluminum Production (Gg) 4-47
Table 4-71: Production of Primary Aluminum (Gg) 4-49
Table 4-72: Tier 2 Quantitative Uncertainty Estimates for CO2 and PEC Emissions from Aluminum
Production (Tg CO2 Eq. and Percent) 4-50
Table 4-73: SF6 Emissions from Magnesium Production and Processing (Tg CO2 Eq. and Gg) 4-51
Table 4-74: SF6 Emission Factors (kg SF6 per metric ton of magnesium) 4-51
Table 4-75: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Magnesium
Production and Processing (Tg CO2 Eq. and Percent) 4-53
Table 4-76: CO2 Emissions from Zinc Production (Tg CO2 Eq. and Gg) 4-54
Table 4-77: Zinc Production (Metric Tons) 4-55
Table 4-78: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Zinc Production
(Tg CO2 Eq. and Percent) 4-56
Table 4-79: CO2 Emissions from Lead Production (Tg CO2 Eq. and Gg) 4-57
Table 4-80: Lead Production (Metric Tons) 4-57
Table 4-81: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lead Production
(Tg CO2 Eq. and Percent) 4-58
Table 4-82: HFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and Gg) 4-58
Table 4-83: HCFC-22 Production (Gg) 4-59
Table 4-84: Quantitative Uncertainty Estimates for HFC-23 Emissions from HCFC-22 Production
(Tg CO2 Eq. and Percent) 4-60
Table 4-85: Emissions of HFCs and PFCs from ODS Substitutes (Tg CO2 Eq.) 4-61
Table 4-86: Emissions of HFCs and PFCs from ODS Substitution (Mg) 4-61
Table 4-87: Emissions of HFCs and PFCs from ODS Substitutes (Tg CO2 Eq.) by Sector 4-61
Table 4-88: Tier 2 Quantitative Uncertainty Estimates for HFC and PEC Emissions from ODS
Substitutes (Tg CO2 Eq. and Percent) 4-63
Table 4-89: PEC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg CO2 Eq.) 4-65
Table 4-90: PEC, HFC, and SF6 Emissions from Semiconductor Manufacture (Mg) 4-65
Table 4-91: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from
Semiconductor Manufacture (Tg CO2 Eq. and Percent) 4-68
Table 4-92: SF6 Emissions from Electric Power Systems and Electrical Equipment Manufacturers
(Tg CO2Eq.) 4-69
Table 4-93: SF6 Emissions from Electric Power Systems and Electrical Equipment
Manufacturers (Gg) 4-70
Table 4-94: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Electrical
Transmission and Distribution (Tg CO2 Eq. and percent) 4-72
Table 4-95: 2008 Potential and Actual Emissions of HFCs, PFCs, and SF6 from Selected Sources
(Tg C02Eq.) 4-73
Table 4-96: NOX, CO, and NMVOC Emissions from Industrial Processes (Gg) 4-74
Table 5-1: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq. and Gg) 5-1
Table 5-2: N2O Production (Gg) 5-2
Table 5-3: N2O Emissions from N2O Product Usage (Tg CO2 Eq. and Gg) 5-2
XI
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Table 5-4: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from N2O Product Usage
(Tg CO2 Eq. and Percent) 5-3
Table 5-5: Emissions of NOX, CO, and NMVOC from Solvent Use (Gg) 5-4
Table 6-1: Emissions from Agriculture (Tg CO2 Eq.) 6-1
Table 6-2: Emissions from Agriculture (Gg) 6-2
Table 6-3: CH4 Emissions from Enteric Fermentation (Tg CO2 Eq.) 6-3
Table 6-4: CH4 Emissions from Enteric Fermentation (Gg) 6-3
Table 6-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Enteric Fermentation
(Tg CO2 Eq. and Percent) 6-5
Table 6-6: CH4 and N2O Emissions from Manure Management (Tg CO2 Eq.) 6-8
Table 6-7: CH4 and N2O Emissions from Manure Management (Gg) 6-8
Table 6-8: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O (Direct and Indirect)
Emissions from Manure Management (Tg CO2 Eq. and Percent) 6-11
Table 6-9: CH4 Emissions from Rice Cultivation (Tg CO2 Eq.) 6-14
Table 6-10: CH4 Emissions from Rice Cultivation (Gg) 6-14
Table 6-11: Rice Areas Harvested (Hectares) 6-15
Table 6-12: Ratooned Area as Percent of Primary Growth Area 6-15
Table 6-13: Non-USDA Data Sources for Rice Harvest Information 6-16
Table 6-14: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Rice Cultivation
(Tg CO2 Eq. and Percent) 6-17
Table 6-15: N2O Emissions from Agricultural Soils (Tg CO2 Eq.) 6-20
Table 6-16: N2O Emissions from Agricultural Soils (Gg) 6-20
Table 6-17: Direct N2O Emissions from Agricultural Soils by Land Use Type and N Input Type
(Tg CO2 Eq.) 6-20
Table 6-18: Indirect N2O Emissions from all Land-Use Types (Tg CO2 Eq.) 6-21
Table 6-19: Quantitative Uncertainty Estimates of N2O Emissions from Agricultural Soil
Management in 2008 (Tg CO2 Eq. and Percent) 6-30
Table 6-20: CH4 and N2O Emissions from Field Burning of Agricultural Residues (Tg CO2 Eq.) 6-32
Table 6-21: CH4, N2O, CO, and NOX Emissions from Field Burning of Agricultural Residues (Gg).... 6-33
Table 6-22: Agricultural Crop Production (Gg of Product) 6-35
Table 6-23: Percent of Rice Area Burned by State 6-35
Table 6-24: Key Assumptions for Estimating Emissions from Field Burning of Agricultural
Residues 6-35
Table 6-25: Greenhouse Gas Emission Ratios and Conversion Factors 6-36
Table 6-26: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Field
Burning of Agricultural Residues (Tg CO2 Eq. and Percent) 6-36
Table 7-1: Net CO2 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and Forestry
(Tg C02Eq.) 7-2
Table 7-2: Net CO2 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and Forestry
(Tg C) 7-2
Table 7-3: Emissions from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) 7-3
Table 7-4: Emissions from Land Use, Land-Use Change, and Forestry (Gg) 7-3
Table 7-5: Size of Land Use and Land-Use Change Categories on Managed Land Area by Land
Use and Land Use Change Categories (Thousands of Hectares) 7-5
Table 7-6: Net Annual Changes in C Stocks (Tg CO2/yr) in Forest and Harvested Wood Pools 7-15
XII
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Table 7-7: Net Annual Changes in C Stocks (Tg C/yr) in Forest and Harvested Wood Pools 7-16
Table 7-8: Forest area (1000 ha) and C Stocks (Tg C) in Forest and Harvested Wood Pools 7-16
Table 7-9: Estimates of CO2 (Tg/yr) Emissions for the Lower 48 States and Alaska 7-18
Table 7-10: Tier 2 Quantitative Uncertainty Estimates for Net CO2 Flux from Forest Land
Remaining Forest Land: Changes in Forest C Stocks (Tg CO2 Eq. and Percent) 7-21
Table 7-11: Estimated Non-CO2 Emissions from Forest Fires (Tg CO2 Eq.) for U.S. Forests 7-23
Table 7-12: Estimated Non-CO2 Emissions from Forest Fires (Gg Gas) for U.S. Forests 7-23
Table 7-13: Estimated Carbon Released from Forest Fires for U.S. Forests 7-24
Table 7-14: Tier 2 Quantitative Uncertainty Estimates of Non-CO2 Emissions from Forest Fires in
Forest Land Remaining Forest Land (Tg CO2 Eq. and Percent) 7-24
Table 7-15: Direct N2O Fluxes from Soils in Forest Land Remaining Forest Land (Tg CO2 Eq. and
Gg N2O) 7-25
Table 7-16: Quantitative Uncertainty Estimates of N2O Fluxes from Soils in Forest Land Remaining
Forest Land (Tg CO2 Eq. and Percent) 7-26
Table 7-17: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining Cropland
(Tg CO2 Eq.) 7-28
Table 7-18: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining Cropland (Tg C) 7-28
Table 7-19: Tier 2 Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within
Cropland Remaining Cropland (Tg CO2 Eq. and Percent) 7-33
Table 7-20: Emissions from Liming of Agricultural Soils (Tg CO2 Eq.) 7-34
Table 7-21: Emissions from Liming of Agricultural Soils (Tg C) 7-34
Table 7-22: Applied Minerals (Million Metric Tons) 7-35
Table 7-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Liming of
Agricultural Soils (Tg CO2 Eq. and Percent) 7-36
Table 7-24: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg CO2 Eq.).... 7-37
Table 7-25: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C) 7-37
Table 7-26: Applied Urea (Million Metric Tons) 7-37
Table 7-27: Quantitative Uncertainty Estimates for CO2 Emissions from Urea Fertilization
(Tg CO2 Eq. and Percent) 7-38
Table 7-28: Net CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg CO2 Eq.)... 7-39
Table 7-29: Net CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg C) 7-39
Table 7-30: Tier 2 Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within
Land Converted to Cropland (Tg CO2 Eq. and Percent) 7-42
Table 7-31: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland
(Tg CO2 Eq.) 7-43
Table 7-32: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg C) 7-43
Table 7-33: Tier 2 Quantitative Uncertainty Estimates for C Stock Changes occurring within
Grassland Remaining Grassland (Tg CO2 Eq. and Percent) 7-46
Table 7-34: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland
(Tg C02Eq.) 7-48
Table 7-35: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C) 7-48
Table 7-36: Tier 2 Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within
Land Converted to Grassland (Tg CO2 Eq. and Percent) 7-50
Table 7-37: Emissions from Peatlands Remaining Peatlands (Tg CO2 Eq.) 7-52
Table 7-38: Emissions from Peatlands Remaining Peatlands (Gg) 7-52
Table 7-39: Peat Production of Lower 48 States (in thousands of Metric Tons) 7-53
XIII
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Table 7-40: Peat Production of Alaska (in Thousands of Cubic Meters) 7-53
Table 7-41: Tier-2 Quantitative Uncertainty Estimates for CO2 Emissions from Peatlands
Remaining Peatlands 7-54
Table 7-42: Net C Flux from Urban Trees (Tg CO2 Eq. and Tg C) 7-55
Table 7-43: C Stocks (Metric Tons C), Annual C Sequestration (Metric Tons C/yr), Tree Cover
(Percent), and Annual C Sequestration per Area of Tree Cover (kg C/m2-yr) for 14 U.S. Cities .... 7-57
Table 7-44: Tier 2 Quantitative Uncertainty Estimates for Net C Flux from Changes in C Stocks in
Urban Trees (Tg CO2 Eq. and Percent) 7-58
Table 7-45: N2O Fluxes from Soils in Settlements Remaining Settlements
(Tg CO2 Eq. and Gg N2O) 7-59
Table 7-46: Quantitative Uncertainty Estimates of N2O Emissions from Soils in Settlements
Remaining Settlements (Tg CO2 Eq. and Percent) 7-60
Table 7-47: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg CO2 Eq.) 7-61
Table 7-48: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C) 7-61
Table 7-49: Moisture Content (%), C Storage Factor, Proportion of Initial C Sequestered (%),
Initial C Content (%), and Half-Life (years) for Landfilled Yard Trimmings and Food Scraps
in Landfills 7-63
Table 7-50: C Stocks in Yard Trimmings and Food Scraps in Landfills (Tg C) 7-64
Table 7-51: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Yard Trimmings and
Food Scraps in Landfills (Tg CO2 Eq. and Percent) 7-64
Table 8-1: Emissions from Waste (Tg CO2 Eq.) 8-1
Table 8-2: Emissions from Waste (Gg) 8-2
Table 8-3: CH4 Emissions from Landfills (Tg CO2 Eq.) 8-3
Table 8-4: CH4 Emissions from Landfills (Gg) 8-3
Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills
(Tg CO2 Eq. and Percent) 8-5
Table 8-6: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment
(Tg C02 Eq.) 8-7
Table 8-7: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Gg) 8-7
Table 8-8: U.S. Population (Millions) and Domestic Wastewater BOD5 Produced (Gg) 8-9
Table 8-9: Industrial Wastewater CH4 Emissions by Sector for 2008 8-9
Table 8-10: U.S. Pulp and Paper; Meat and Poultry; Vegetables, Fruits, and Juices Production; and
Fuels Production (Tg) 8-10
Table 8-11: Variables Used to Calculate Percent Wastewater Treated Anaerobically by Industry (%).... 8-11
Table 8-12: Wastewater Flow (m3/ton) and BOD Production (g/L) for U.S. Vegetables, Fruits, and
Juices Production 8-12
Table 8-13: U.S. Population (Millions), Available Protein (kg/person-year), and Protein Consumed
(kg/person-year) 8-15
Table 8-14: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Wastewater
Treatment (Tg CO2 Eq. and Percent) 8-16
Table 8-15: CH4 and N2O Emissions from Composting (Tg CO2 Eq.) 8-18
Table 8-16: CH4 and N2O Emissions from Composting (Gg) 8-18
Table 8-17: U.S. Waste Composted (Gg) 8-18
Table 8-18 : Tier 1 Quantitative Uncertainty Estimates for Emissions from Composting
(Tg CO2 Eq. and Percent) 8-18
Table 8-19: Emissions of NOX, CO, and NMVOCs from Waste (Gg) 8-19
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg CO2 Eq.) 10-3
XIV
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Table 10-2: Revisions to Net Flux of CO2 to the Atmosphere from Land Use,
Land-Use Change, and Forestry (Tg CO2 Eq.) 10-5
Figures
Figure ES-1: U.S. Greenhouse Gas Emissions by Gas ES-4
Figure ES-2: Annual Percent Change in U.S. Greenhouse Gas Emissions ES-4
Figure ES-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 1990 ES-4
Figure ES-4: 2008 Greenhouse Gas Emissions by Gas (percents based on Tg CO2 Eq.) ES-7
Figure ES-5: 2008 Sources of CO2 Emissions ES-7
Figure ES-6: 2008 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type ES-8
Figure ES-7: 2008 End-Use Sector Emissions of CO2, CH4, and N2O from Fossil Fuel
Combustion ES-8
Figure ES-8: 2008 Sources of CH4 Emissions ES-10
Figure ES-9: 2008 Sources of N2O Emissions ES-11
Figure ES-10: 2008 Sources of HFCs, PFCs, and SF6 Emissions ES-12
Figure ES-11: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector ES-12
Figure ES-12: 2008 U.S. Energy Consumption by Energy Source ES-13
Figure ES-13: Emissions Allocated to Economic Sectors ES-16
Figure ES-14: Emissions with Electricity Distributed to Economic Sectors ES-17
Figure ES-15: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic
Product ES-18
Figure ES-16: 2008 Key Categories ES-20
Figure 1-1: U.S. QA/QC Plan Summary 1-14
Figure 2-1: U.S. Greenhouse Gas Emissions by Gas 2-1
Figure 2-2: Annual Percent Change in U.S. Greenhouse Gas Emissions 2-2
Figure 2-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 1990 2-2
Figure 2-4: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector 2-8
Figure 2-5: 2008 Energy Chapter Greenhouse Gas Emission Sources 2-8
Figure 2-6: 2008 U.S. Fossil Carbon Flows (Tg CO2 Eq.) 2-9
Figure 2-7: 2008 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 2-9
Figure 2-8: 2008 End-Use Sector Emissions from Fossil Fuel Combustion 2-11
Figure 2-9: 2008 Industrial Processes Chapter Greenhouse Gas Emission Sources 2-12
Figure 2-10: 2008 Agriculture Chapter Greenhouse Gas Emission Sources 2-14
Figure 2-11: 2008 Waste Chapter Greenhouse Gas Emission Sources 2-17
Figure 2-12: Emissions Allocated to Economic Sectors 2-20
Figure 2-13: Emissions with Electricity Distributed to Economic Sectors 2-21
Figure 2-14: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product... 2-27
Figure 3-1: 2008 Energy Chapter Greenhouse Gas Sources 3-1
Figure 3-2: 2008 U.S. Fossil Carbon Flows (Tg CO2 Eq.) 3-2
Figure 3-3: 2008 U.S. Energy Consumption by Energy Source 3-6
Figure 3-4: U.S. Energy Consumption (Quadrillion Btu) 3-6
Figure 3-5: 2008 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 3-6
Figure 3-6: Annual Deviations from Normal Heating Degree Days for
the United States (1950-2008) 3-7
XV
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Figure 3-7: Annual Deviations from Normal Cooling Degree Days for the United States
(1950-2008) 3-7
Figure 3-8: Aggregate Nuclear and Hydroelectric Power Plant Capacity Factors in the United States
(1974-2008) 3-7
Figure 3-9: Electricity Generation Retail Sales by End-Use Sector 3-11
Figure 3-10: Industrial Production Indices (Index 2002=100) 3-13
Figure 3-11: Sales-Weighted Fuel Economy of New Passenger Cars and Light-Duty Trucks,
1990-2008 3-16
Figure 3-12: Sales of New Passenger Cars and Light-Duty Trucks, 1990-2008 3-16
Figure 3-13: Mobile Source CH4 and N2O Emissions 3-18
Figure 3-14: U.S. Energy Consumption and Energy-Related CO2 Emissions Per Capita and Per
Dollar GDP 3-22
Figure 4-1: 2008 Industrial Processes Chapter Greenhouse Gas Sources 4-1
Figure 6-1: 2008 Agriculture Chapter Greenhouse Gas Emission Sources 6-1
Figure 6-2: Sources and Pathways of N that Result in N2O Emissions from Agricultural Soil
Management 6-19
Figure 6-3: Major Crops, Average Annual Direct N2O Emissions Estimated Using the DAYCENT
Model, 1990-2008 (Tg CO2 Eq./year) 6-22
Figure 6-4: Grasslands, Average Annual Direct N2O Emissions Estimated Using the DAYCENT
Model, 1990-2008 (Tg CO2 Eq./year) 6-22
Figure 6-5: Major Crops, Average Annual N Losses Leading to Indirect N2O Emissions Estimated
Using the DAYCENT Model, 1990-2008 (Gg N/year) 6-23
Figure 6-6: Grasslands, Average Annual N Losses Leading to Indirect N2O Emissions Estimated
Using the DAYCENT Model, 1990-2008 (Gg N/year) 6-23
Figure 6-7: Comparison of Measured Emissions at Field Sites with Modeled Emissions Using the
DAYCENT Simulation Model 6-30
Figure 7-1. Percent of Total Land Area in the General Land-Use Categories for 2008 7-6
Figure 7-2: Forest Sector Carbon Pools and Flows 7-14
Figure 7-3: Estimates of Net Annual Changes in C Stocks for Major C Pools 7-17
Figure 7-4: Average C Density in the Forest Tree Pool in the Conterminous United States, 2008 7-17
Figure 7-5: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within
States, 2008, Cropland Remaining Cropland 7-29
Figure 7-6: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within
States, 2008, Cropland Remaining Cropland 7-29
Figure 7-7: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within
States, 2008, Land Converted to Cropland 7-40
Figure 7-8: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within
States, 2008, Land Converted to Cropland 7-40
Figure 7-9: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within
States, 2008, Grassland Remaining Grassland 7-44
Figure 7-10: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within
States, 2008, Grassland Remaining Grassland 7-44
Figure 7-11: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within
States, 2008, Land Converted to Grassland 7-49
Figure 7-12: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within
States, 2008, Land Converted to Grassland 7-49
Figure 8-1: 2008 Waste Chapter Greenhouse Gas Sources 8-1
XVI
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Boxes
Box ES- 1: Recalculations of Inventory Estimates ES-2
Box ES-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data ES-18
Box 1-1: The IPCC Fourth Assessment Report and Global Warming Potentials 1-8
Box 1-2: IPCC Reference Approach 1-11
Box 2-1: Methodology for Aggregating Emissions by Economic Sector 2-26
Box 2-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data 2-27
Box 2-3: Sources and Effects of Sulfur Dioxide 2-28
Box 3-1: Weather and Non-Fossil Energy Effects on CO2 from Fossil Fuel Combustion Trends 3-7
Box 3-2: Carbon Intensity of U.S. Energy Consumption 3-21
Box 3-3. Carbon Dioxide Transport, Injection, and Geological Storage 3-55
Box 4-1: Potential Emission Estimates of HFCs, PFCs, and SF6 4-73
Box 6-1. Tier 1 vs. Tier 3 Approach for Estimating N2O Emissions 6-24
Box 6-2: Comparison of Tier 2 U.S. Inventory Approach and IPCC (2006) Default Approach 6-34
Box 7-1: CO2 Emissions from Forest Fires 7-18
Box 7-2: Tier 3 Approach for Soil C Stocks Compared to Tier 1 or 2 Approaches 7-31
Box 8-1: Biogenic Emissions and Sinks of Carbon 8-6
XVII
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Executive Summary
An emissions inventory that identifies and quantifies a country's primary anthropogenic1 sources and sinks of
greenhouse gases is essential for addressing climate change. This inventory adheres to both (1) a comprehensive
and detailed set of methodologies for estimating sources and sinks of anthropogenic greenhouse gases, and (2)
a common and consistent mechanism that enables Parties to the United Nations Framework Convention on Climate Change
(UNFCCC) to compare the relative contribution of different emission sources and greenhouse gases to climate change.
In 1992, the United States signed and ratified the UNFCCC. As stated in Article 2 of the UNFCCC, "The ultimate
objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in
accordance with the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in the atmosphere
at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved
within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is
not threatened and to enable economic development to proceed in a sustainable manner."2
Parties to the Convention, by ratifying, "shall develop, periodically update, publish and make available...national
inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse gases not controlled by the
Montreal Protocol, using comparable methodologies..."3 The United States views the Inventory report as an opportunity
to fulfill these commitments.
This chapter summarizes the latest information on U.S. anthropogenic greenhouse gas emission trends from 1990 through
2008. To ensure that the U.S. emissions inventory is comparable to those of other UNFCCC Parties, the estimates presented
here were calculated using methodologies consistent with those recommended in the Revised 1996IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA1997), the IPCC Good Practice Guidance and Uncertainty
Management in National Greenhouse Gas Inventories (IPCC 2000), and the IPCC Good Practice Guidance for Land
Use, Land-Use Change, and Forestry (IPCC 2003). Additionally, the U.S. emissions inventory has begun to incorporate
new methodologies and data from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006). The
structure of this report is consistent with the UNFCCC guidelines for inventory reporting.4 For most source categories, the
Intergovernmental Panel on Climate Change (IPCC) methodologies were expanded, resulting in a more comprehensive
and detailed estimate of emissions.
1 The term "anthropogenic," in this context, refers to greenhouse gas emissions and removals that are a direct result of human activities or are the result
of natural processes that have been affected by human activities (IPCC/UNEP/OECD/IEA 1997).
2 Article 2 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate Change. See .
3 Article 4(1 )(a) of the United Nations Framework Convention on Climate Change (also identified in Article 12). Subsequent decisions by the Conference
of the Parties elaborated the role of Annex I Parties in preparing national inventories. See .
4 See .
Executive Summary ES-1
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Box ES-1: Recalculations of Inventory Estimates
Each year, emission and sink estimates are recalculated and revised for all years in the Inventory of U.S. Greenhouse Gas Emissions and
Sinks as attempts are made to improve both the analyses themselves, through the use of better methods or data, and the overall usefulness
of the report. In this effort, the United States follows the IPCC Good Practice Guidance (IPCC 2000), which states, regarding recalculations
of the time series, "It is good practice to recalculate historic emissions when methods are changed or refined, when new source categories
are included in the national inventory, or when errors in the estimates are identified and corrected." In general, recalculations are made to
the U.S. greenhouse gas emission estimates either to incorporate new methodologies or, most commonly, to update recent historical data.
In each Inventory report, the results of all methodology changes and historical data updates are presented in the "Recalculations and
Improvements" chapter; detailed descriptions of each recalculation are contained within each source's description contained in the report, if
applicable. In general, when methodological changes have been implemented, the entire time series (in the case of the most recent inventory
report, 1990 through 2007) has been recalculated to reflect the change, per IPCC Good Practice Guidance. Changes in historical data are
generally the result of changes in statistical data supplied by other agencies. References for the data are provided for additional information.
ES.1. Background Information
Naturally occurring greenhouse gases include water
vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O), and ozone (O3). Several classes of halogenated
substances that contain fluorine, chlorine, or bromine are also
greenhouse gases, but they are, for the most part, solely a
product of industrial activities. Chlorofluorocarbons (CFCs)
andhydrochlorofluorocarbons (HCFCs) are halocarbons that
contain chlorine, while halocarbons that contain bromine
are referred to as bromofluorocarbons (i.e., halons). As
stratospheric ozone depleting substances, CFCs, HCFCs,
and halons are covered under the Montreal Protocol on
Substances that Deplete the Ozone Layer. The UNFCCC
defers to this earlier international treaty. Consequently,
Parties to the UNFCCC are not required to include these
gases in their national greenhouse gas emission inventories.5
Some other fluorine-containing halogenated substances—
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and
sulfur hexafluoride (SF6)—do not deplete stratospheric ozone
but are potent greenhouse gases. These latter substances are
addressed by the UNFCCC and accounted for in national
greenhouse gas emission inventories.
There are also several gases that do not have a direct
global warming effect but indirectly affect terrestrial and/or
solar radiation absorption by influencing the formation or
destruction of greenhouse gases, including tropospheric and
stratospheric ozone. These gases include carbon monoxide
(CO), oxides of nitrogen (NOX), andnon-CH4 volatile organic
compounds (NMVOCs). Aerosols, which are extremely small
particles or liquid droplets, such as those produced by sulfur
dioxide (SO2) or elemental carbon emissions, can also affect
the absorptive characteristics of the atmosphere.
Although the direct greenhouse gases CO2, CH4, and
N2O occur naturally in the atmosphere, human activities
have changed their atmospheric concentrations. From
the pre-industrial era (i.e., ending about 1750) to 2005,
concentrations of these greenhouse gases have increased
globally by 36, 148, and 18 percent, respectively (IPCC
2007).
Beginning in the 1950s, the use of CFCs and other
stratospheric ozone depleting substances (ODS) increased
by nearly 10 percent per year until the mid-1980s, when
international concern about ozone depletion led to the
entry into force of the Montreal Protocol. Since then, the
production of ODS is being phased out. In recent years, use
of ODS substitutes such as HFCs and PFCs has grown as
they begin to be phased in as replacements for CFCs and
HCFCs. Accordingly, atmospheric concentrations of these
substitutes have been growing (IPCC 2007).
Global Warming Potentials
Gases in the atmosphere can contribute to the greenhouse
effect both directly and indirectly. Direct effects occur when
the gas itself absorbs radiation. Indirect radiative forcing
occurs when chemical transformations of the substance
produce other greenhouse gases, when a gas influences
the atmospheric lifetimes of other gases, and/or when a
gas affects atmospheric processes that alter the radiative
5 Emissions estimates of CFCs, HCFCs, halons and other ozone-depleting
substances are included in the annexes of this report for informational
purposes.
ES-2 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
balance of the earth (e.g., affect cloud formation or albedo).6
The IPCC developed the Global Warming Potential (GWP)
concept to compare the ability of each greenhouse gas to trap
heat in the atmosphere relative to another gas.
The GWP of a greenhouse gas is denned as the ratio of
the time-integrated radiative forcing from the instantaneous
release of 1 kilogram (kg) of a trace substance relative to
that of 1 kg of a reference gas (IPCC 2001). Direct radiative
effects occur when the gas itself is a greenhouse gas. The
reference gas used is CO2, and therefore GWP-weighted
emissions are measured in teragrams (or million metric tons)
of CO2 equivalent (Tg CO2 Eq.).7'8 All gases in this Executive
Summary are presented in units of Tg CO2 Eq.
The UNFCCC reporting guidelines for national
inventories were updated in 2006,9 but continue to require
the use of GWPs from the IPCC Second Assessment Report
(SAR) (IPCC 1996). This requirement ensures that current
estimates of aggregate greenhouse gas emissions for 1990
to 2008 are consistent with estimates developed prior to the
publication of the IPCC Third Assessment Report (TAR)
and the IPCC Fourth Assessment Report (AR4). Therefore,
to comply with international reporting standards under the
UNFCCC, official emission estimates are reported by the
United States using SAR GWP values. All estimates are
provided throughout this report in both CO2 equivalents and
unweighted units. A comparison of emission values using the
SAR GWPs versus the TAR and AR4 GWPs can be found
in Chapter 1 and, in more detail, in Annex 6.1 of this report.
The GWP values used in this report are listed in Table ES-1.
Global warming potentials are not provided for CO,
NOX, NMVOCs, SO2, and aerosols because there is no
agreed-upon method to estimate the contribution of gases
that are short-lived in the atmosphere, spatially variable, or
have only indirect effects on radiative forcing (IPCC 1996).
Table ES-1: Global Warming Potentials (100-Year Time
Horizon) Used in the Inventory Report
6 Albedo is a measure of the Earth's reflectivity, and is defined as the fraction
of the total solar radiation incident on a body that is reflected by it.
7 Carbon comprises 12/44ths of carbon dioxide by weight.
8 One teragram is equal to 1012 grams or one million metric tons.
9 See .
Gas
C02
CH4*
N20
HFC-23
HFC-32
HFC-125
HFC-134a
HFC-143a
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
CF4
C2F6
C^F-io
C6Fi4
SF6
GWP
1
21
310
11,700
650
2,800
1,300
3,800
140
2,900
6,300
1,300
6,500
9,200
7,000
7,400
23,900
Source: IPCC (1996).
* The CH4 GWP includes the direct effects and those indirect effects due
to the production of tropospheric ozone and stratospheric water vapor.
The indirect effect due to the production of C02 is not included.
ES.2. Recent Trends in U.S.
Greenhouse Gas Emissions
and Sinks
In 2008, total U.S. greenhouse gas emissions were
6,956.8 Tg CO2 Eq. Overall, total U.S. emissions have risen
by approximately 14 percent from 1990 to 2008. Emissions
declined from 2007 to 2008, decreasing by 2.9 percent
(211.3 Tg CO2 Eq.). This decrease is primarily a result of a
decrease in demand for transportation fuels associated with
the record high costs of these fuels that occurred in 2008.
Additionally, electricity demand declined in 2008 in part due
to a significant increase in the cost of fuels used to generate
electricity. In 2008, temperatures were cooler in the United
States than in 2007, both in the summer and the winter.
This lead to an increase in heating related energy demand
in the winter; however, much of this increase was offset by a
decrease in cooling-related electricity demand in the summer.
Executive Summary ES-3
-------
Figure ES-1 through Figure ES-3 illustrate the overall
trends in total U.S. emissions by gas, annual changes, and
absolute change since 1990. Table ES-2 provides a detailed
summary of U.S. greenhouse gas emissions and sinks for
1990 through 2008.
Figure ES-4 illustrates the relative contribution of the
direct greenhouse gases to total U.S. emissions in 2008.
The primary greenhouse gas emitted by human activities
in the United States was CO2, representing approximately
85.1 percent of total greenhouse gas emissions. The largest
source of CO2, and of overall greenhouse gas emissions,
was fossil fuel combustion. CH4 emissions, which have
declined by 5.5 percent since 1990, resulted primarily from
enteric fermentation associated with domestic livestock,
decomposition of wastes in landfills, and natural gas systems.
Agricultural soil management and mobile source fuel
combustion were the major sources of N2O emissions. Ozone
depleting substance substitute emissions and emissions of
HFC-23 during the production of HCFC-22 were the primary
contributors to aggregate HFC emissions. PFC emissions
resulted as a by-product of primary aluminum production
and from semiconductor manufacturing, while electrical
transmission and distribution systems accounted for most
SF6 emissions.
Overall, from 1990 to 2008 total emissions of CO2
increased by 820.4 Tg CO2 Eq. (16.1 percent), while CH4 and
N2O emissions decreased by 45.8 Tg CO2 Eq. (7.5 percent)
and 4.1 Tg CO2 Eq. (1.3 percent), respectively. During the
same period, aggregate weighted emissions of HFCs, PFCs,
and SF6 rose by 59.4 Tg CO2 Eq. (65.9 percent). From 1990
to 2008, HFCs increased by 90.0 Tg CO2 Eq. (243.7 percent),
PFCs decreased by 14.1 Tg CO2 Eq. (67.8 percent), and
SF6 decreased by 16.5 Tg CO2 Eq. (50.5 percent). Despite
being emitted in smaller quantities relative to the other
principal greenhouse gases, emissions of HFCs, PFCs,
and SF6 are significant because many of these gases have
extremely high global warming potentials and, in the cases
of PFCs and SF6, long atmospheric lifetimes. Conversely,
U.S. greenhouse gas emissions were partly offset by carbon
sequestration in forests, trees in urban areas, agricultural
soils, and landfilled yard trimmings and food scraps, which,
in aggregate, offset 13.5 percent of total emissions in 2008.
The following sections describe each gas' contribution to
total U.S. greenhouse gas emissions in more detail.
Figure ES-1
U.S. Greenhouse Gas Emissions by Gas
8,000 -
7,000 -
6,000 -
Ł 5,000 -
o
|> 4,000 -
3,000 -
2,000 -
1,000-
HFCs, PFCs, & SF,
Nitrous Oxide
Methane
I Carbon Dioxide
Figure ES-2
Annual Percent Change in U.S. Greenhouse Gas Emissions
3%-
2%-
1%-
0%
-1% -
-2% -
-3% -
1?%1,%160/
Illiliiil •-•• I
i
ii
-2.9%
CM co ^ in to r- oo
Figure ES-3
Cumulative Change in Annual U.S. Greenhouse Gas
Emissions Relative to 1990
1,100
1,000
900
800
7°°
"1 600
500
400
300
200
100
-100
918
969
1,006
1,041
840
. -38
ES-4 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg C02 Eq. or million metric tons C02 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Iron and Steel Production &
Metallurgical Coke Production
Cement Production
Natural Gas Systems
Lime Production
Incineration of Waste
Ammonia Production and Urea
Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and
Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands
Petroleum Systems
Zinc Production
Lead Production
Silicon Carbide Production and
Consumption
Land Use, Land-Use Change, and
Forestry (Sink)*
Biomass - Woodb
International Bunker Fuelsb
Biomass - Ethanolb
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Wastewater Treatment
Forest Land Remaining Forest
Land
Rice Cultivation
Stationary Combustion
Abandoned Underground Coal
Mines
Mobile Combustion
1990
5,100.8
4,735.7
1,820.8
1,485.8
845.4 1
339.1
216.7
27.9
119.6
102.6
33.3
37.3
11.5
8.0 1
16.8
7.1
5.1
6.8
4.1 1
3.3
1.2
1.4
2.2
1.5
1.0
0.6
0.9
0.3
0.4 1
(909.4)
215.2
111.8
4.2
613.4
132.4
149.3
129.5
84.1
29.3
33.9
23.5
3.2
7.1 1
7.4 1
6.0 I
4.7
1995 2000 2005
5,427.3 5,977.2 6,108.4
5,029.5 5,593.4 5,753.3
1,947.9 2,296.9 1 2,402.1
1,608.0 1,809.5 1
862.6 852.2
353.3 371.2
223.2 227.7
34.5 35.9
142.9 146.1
95.7
36.8
42.2
13.3
11.5
17.8
7.0
6.7
5.7
4.3
4.1
1.5
1.4
2.0
1.5
1.0
0.5
1.0
0.3
0.3
(842.9)
88.1
41.2
29.4
14.1
11.3
16.4
7.5
5.1
6.1
4.2
4.5
1.8
1.4
1.9
1.4
1.2
0.5
1.1
0.3
0.2
1,895.3
825.6
358.4
221.3
50.6
136.5
67.7
45.9
29.5
14.4
12.6
12.8
7.9
6.8
4.1
4.2
4.2
1.8
1.3
1.4
1.4
1.1
0.5
0.5
0.3
0.2
(664.2) 1 (950.4)
229.1 21 8.1 • 206.9
99.8 98.5
7.7 9.2
613.2 586.0
143.7 136.8
144.1 120.7
132.6 130.7
67.1 60.4
33.9 38.6
32.0 30.2
24.8 25.2
4.3 14.3
8.2 7.4
4.3 3.4
110.5
22.6
553.2
136.7
125.6
103.6
56.9
42.2
28.2
24.3
9.8
6.8
6.6
5.6
2.5
2006
6,017.2
5,652.8
2,346.4
1,876.7
850.7
322.1
206.0
50.9
141.4
70.5
46.6
29.5
15.1
12.7
12.3
7.9
8.0
3.8
4.2
3.8
1.8
1.7
1.5
1.2
0.9
0.5
0.5
0.3
0.2
(959.2)
207.9
129.1
30.5
568.2
139.0
127.1
103.1
58.3
42.3
28.2
24.5
21.6
5.9
6.2
5.5
2.3
2007
6,120.2
5,757.0
2,412.8
1,893.7
842.2
341.7
217.4
49.1
135.3
72.8
45.2
30.8
14.6
13.3
14.0
8.3
7.7
4.3
4.1
3.9
1.9
1.9
1.6
1.2
1.0
0.5
0.4
0.3
0.2
(955.4)
207.4
127.1
38.3
569.2
141.2
126.5
99.5
58.1
45.9
28.8
24.4
20.0
6.2
6.5
5.7
2.2
2008
5,921.2
5,572.8
2,363.5
1,785.3
819.3
342.7
219.5
42.5
134.2
69.0
41.1
30.0
14.3
13.1
11.8
7.6
6.6
4.5
4.1
3.4
1.8
1.8
1.6
1.2
0.9
0.5
0.4
0.3
0.2
(940.3)
198.4
135.2
53.3
567.6
140.8
126.3
96.4
67.6
45.0
29.1
24.3
11.9
7.2
6.7
5.9
2.0
Executive Summary ES-5
-------
Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg C02 Eq. or million metric tons C02 Eq.)
(continued)
Gas/Source
1990
1995
2000
2005
2006
2007
2008
Composting
Field Burning of Agricultural
Residues
Petrochemical Production
Iron and Steel Production &
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and
Consumption
Incineration of Waste
International Bunker Fuelsb
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Forest Land Remaining Forest
Land
Wastewater Treatment
N20 from Product Uses
Adipic Acid Production
Composting
Settlements Remaining
Settlements
Field Burning of Agricultural
Residues
Incineration of Waste
Wetlands Remaining Wetlands
International Bunker Fuels'3
MFCs
Substitution of Ozone Depleting
Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and
Distribution
Magnesium Production and
Processing
Semiconductor Manufacture
0.3 I
0.8
0.9
1.0
+ 1
1
0.2 •
322.3
203.5
43.9
18.9
14.4
12.8
15.8
.._
20.8
18.5
2.2
32.6
26.6
5.4l
0.5
0.7 1.3
0.7 0.9
1.1 1 1.2
1.ol 0.9
+ l +
1
+ 1 +
0.1 0.1
342.5 345.5
205.9
54.0
21.0
15.5
13.3
3.7
4.0
4.6
17.6
0.8
1.2
0.4
0.5
+
0.9
62.2
29.0
33.0
0.3
15.6
11.8
3.8
27.9
21.4
5.6
210.1
53.2
20.7
16.7
14.5
12.1
4.5
4.9
5.5
1.4
1.1
0.5
0.4
+
0.9
103.2
74.3
28.6
0.3
13.5
8.6
4.9
19.1
15.0
3.0
1.6
0.9
1.1
0.7
+
+
+
0.1
328.3
215.8
36.9
17.6
16.6
14.7
8.4
4.7
4.4
5.0
1.7
1.5
0.5
0.4
+
1.0
119.3
103.2
15.8
0.2
6.2
3.0
3.2
17.8
14.0
2.9
0.9 1.1 1.0
1.6
0.9
1.0
0.7
+
+
+
0.2
329.5
211.2
33.6
17.2
17.3
14.5
18.0
4.8
4.4
4.3
1.8
1.5
0.5
0.4
+
1.2
121.8
107.7
13.8
0.3
6.0
2.5
3.5
17.0
13.2
2.9
1.0
1.7
1.0
1.0
0.7
+
+
+
0.2
327.7
211.0
30.3
20.5
17.3
14.6
16.7
4.9
4.4
3.7
1.8
1.6
0.5
0.4
+
1.2
127.4
110.1
17.0
0.3
7.5
3.8
3.6
16.1
12.7
2.6
0.8
1.7
1.0
0.9
0.6
+
+
+
0.2
318.2
215.9
26.1
19.0
17.1
14.2
10.1
4.9
4.4
2.0
1.8
1.6
0.5
0.4
+
1.2
126.9
113.0
13.6
0.3
6.7
2.7
4.0
16.1
13.1
2.0
1.1
Total
6,126.8
6,488.8
7,044.5
7,133.2 7,059.9 7,168.1
6,956.8
5,217.3
5,646.0
6,380.2
6,182.8 6,100.7 6,212.7
Net Emissions (Sources and Sinks)
+ Does not exceed 0.05 Tg C02 Eq.
a Parentheses indicate negative values or sequestration. The net C02 flux total includes both emissions and sequestration, and constitutes a sink in the
United States. Sinks are only included in net emissions total.
b Emissions from International Bunker Fuels and Biomass Combustion are not included in totals.
c Small amounts of RFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
6,016.4
ES-6 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Figure ES-4
Figure ES-5
2008 Greenhouse Gas Emissions by Gas
(percents based on Tg C02 Eq.)
MFCs, PFCs,
Carbon Dioxide Emissions
The global carbon cycle is made up of large carbon
flows and reservoirs. Billions of tons of carbon in the form
of CO2 are absorbed by oceans and living biomass (i.e.,
sinks) and are emitted to the atmosphere annually through
natural processes (i.e., sources). When in equilibrium, carbon
fluxes among these various reservoirs are roughly balanced.
Since the Industrial Revolution (i.e., about 1750), global
atmospheric concentrations of CO2 have risen about 36
percent (IPCC 2007), principally due to the combustion of
fossil fuels. Within the United States, fossil fuel combustion
accounted for 94.1 percent of CO2 emissions in 2008.
Globally, approximately 30,377 Tg of CO2 were added to the
atmosphere through the combustion of fossil fuels in 2008,
of which the United States accounted for about 19 percent.10
Changes in land use and forestry practices can also emit
CO2 (e.g., through conversion of forest land to agricultural
or urban use) or can act as a sink for CO2 (e.g., through
net additions to forest biomass). In addition to fossil-fuel
combustion, several other sources emit significant quantities
of CO2. These sources include, but are not limited to non-
energy use of fuels, iron and steel production and cement
production (Figure ES-5).
As the largest source of U.S. greenhouse gas emissions,
CO2 from fossil fuel combustion has accounted for
approximately 79 percent of GWP-weighted emissions
since 1990, growing slowly from 77 percent of total GWP-
weighted emissions in 1990 to 80 percent in 2008. Emissions
10 Global CO2 emissions from fossil fuel combustion were taken from
Energy Information Administration International Energy Statistics 2009
< http://tonto.eia.doe.gov/cfapps/ipdbproject/IEDIndex3.cfm> EIA (2009).
2008 Sources of C02 Emissions
Fossil Fuel Combustion
Non-Energy Use of Fuels
Iron and Steel Production
& Metallurgical Coke Production
Cement Production
Natural Gas Systems
Lime Production
Incineration of Waste
Ammonia Production and
Urea Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands I
Petroleum Systems I <0.5
Zinc Production
5,573
C02 as a Portion
of all Emissions
Lead Production
Silicon Carbide Production and
Consumption
<0.5
<0.5
<0.5
75 100
TgCO,Eq.
125 150
of CO2 from fossil fuel combustion increased at an average
annual rate of 1 percent from 1990 to 2008. The fundamental
factors influencing this trend include: (1) a generally growing
domestic economy over the last 19 years, and (2) significant
overall growth in emissions from electricity generation
and transportation activities. Between 1990 and 2008, CO2
emissions from fossil fuel combustion increased from 4,735.7
Tg CO2 Eq. to 5,572.8 Tg CO2 Eq.—an 18 percent total
increase over the nineteen-year period. From 2007 to 2008,
these emissions decreased by 184.2 Tg CO2 Eq. (3.2 percent).
Historically, changes in emissions from fossil fuel
combustion have been the dominant factor affecting U.S.
emission trends. Changes in CO2 emissions from fossil fuel
combustion are influenced by many long-term and short-term
factors, including population and economic growth, energy
price fluctuations, technological changes, and seasonal
temperatures. On an annual basis, the overall consumption
of fossil fuels in the United States generally fluctuates in
response to changes in general economic conditions, energy
prices, weather, and the availability of non-fossil alternatives.
For example, in a year with increased consumption of
goods and services, low fuel prices, severe summer and
Executive Summary ES-7
-------
winter weather conditions, nuclear plant closures, and lower
precipitation feeding hydroelectric dams, there would likely
be proportionally greater fossil fuel consumption than a
year with poor economic performance, high fuel prices,
mild temperatures, and increased output from nuclear and
hydroelectric plants.
The five major fuel consuming sectors contributing to
CO2 emissions from fossil fuel combustion are electricity
generation, transportation, industrial, residential, and
commercial. Carbon dioxide emissions are produced by the
electricity generation sector as they consume fossil fuel to
provide electricity to one of the other four sectors, or "end-
use" sectors. For the discussion below, electricity generation
emissions have been distributed to each end-use sector on
the basis of each sector's share of aggregate electricity
consumption. This method of distributing emissions assumes
that each end-use sector consumes electricity that is generated
from the national average mix of fuels according to their
carbon intensity. Emissions from electricity generation are
also addressed separately after the end-use sectors have
been discussed.
Note that emissions from U.S. territories are calculated
separately due to a lack of specific consumption data for the
individual end-use sectors.
Figure ES- 6, Figure ES- 7, and Table ES-3 summarize
CO2 emissions from fossil fuel combustion by end-use sector.
Transportation End-Use Sector. Transportation activities
(excluding international bunker fuels) accounted for 32
percent of CO2 emissions from fossil fuel combustion in
2008.n Virtually all of the energy consumed in this end-use
sector came from petroleum products. Nearly 53 percent
of the emissions resulted from gasoline consumption for
personal vehicle use. The remaining emissions came from
other transportation activities, including the combustion of
diesel fuel in heavy-duty vehicles and jet fuel in aircraft.
Industrial End-Use Sector. Industrial CO2 emissions,
resulting both directly from the combustion of fossil fuels
and indirectly from the generation of electricity that is
consumed by industry, accounted for 27 percent of CO2
from fossil fuel combustion in 2008. Approximately 54
percent of these emissions resulted from direct fossil fuel
11 If emissions from international bunker fuels are included, the
transportation end-use sector accounted for 35 percent of U.S. emissions
from fossil fuel combustion in 2008.
combustion to produce steam and/or heat for industrial
processes. The remaining emissions resulted from consuming
electricity for motors, electric furnaces, ovens, lighting, and
other applications.
Residential and Commercial End-Use Sectors. The
residential and commercial end-use sectors accounted for
21 and 19 percent, respectively, of CO2 emissions from
fossil fuel combustion in 2008. Both sectors relied heavily
on electricity for meeting energy demands, with 71 and
79 percent, respectively, of their emissions attributable to
electricity consumption for lighting, heating, cooling, and
Figure ES-6
2008 C02 Emissions from Fossil Fuel Combustion by
Sector and Fuel Type
2,500 -|
2,000 -
1,500 -
1,000 -
500 -
0 -1
Petroleum
2,363
• Coal ^^m
Natural Gas
Relative Contribution -j ygg
by Fuel Type
• II
-••_•
Note: Electricity generation also includes emissions of less than 0.5 Tg C02 Eq. from
geothermal-based electricity generation.
Figure ES-7
2008 End-Use Sector Emissions of C02, CH4, and
N20 from Fossil Fuel Combustion
2,000 -,
^1,500 -
cT
u
i?
1,000 -
500 -
0 -
I From Direct Fossil
Fuel Combustion
I From Electricity
Consumption
1,050
1,818
1,518
1,193
43
U.S. Commercial Residential Industrial Transportation
Territories
ES-8 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table ES-3: C02 Emissions from Fossil Fuel Combustion by Fuel Consuming End-Use Sector (Tg C02 Eq.)
End-Use Sector
Transportation
Combustion
Electricity
Industrial
Combustion
Electricity
Residential
Combustion
Electricity
Commercial
Combustion
Electricity
U.S. Territories3
Total
Electricity Generation
1990
1,488.8
1,485.8
3.0
1,532.2
845.4
686.8
932.2
339.1
593.0
754.6
216.7
538.0
27.9
4,735.7
1,820.8
1995
1,611.0
1, 608.0 1
3.1
1,578.8 1
862.6
716.2
995.1
353.3
641.8
810.0
586.8
34.5
5,029.5
1,947.9
2000
1,813.0
1,809.5
3.4
1,642.0
852.2
789.8
1,133.6
371.2
762.4
968.9
227.7
741.3 •
35.9
5,593.4
2,296.9 |
2005
1,900.1
1,895.3
4.7
1,562.5
825.6
737.0
1,215.1
358.4
856.7
1,025.0
221.3
803.7
50.6
5,753.3
2,402.1
2006
1,881.2
1,876.7
4.5
1,562.8
850.7
712.0
1,152.9
322.1
830.8
1,005.0
206.0
799.0
50.9
5,652.8
2,346.4
2007
1,898.8
1,893.7
5.0
1,572.2
842.2
730.0
1,197.9
341.7
856.1
1,039.1
217.4
821.7
49.1
5,757.0
2,412.8
2008
1,789.9
1,785.3
4.7
1,510.9
819.3
691.6
1,184.5
342.7
841.8
1,044.9
219.5
825.4
42.5
5,572.8
2,363.5
Note: Totals may not sum due to independent rounding. Combustion-related emissions from electricity generation are allocated based on aggregate
national electricity consumption by each end-use sector.
a Fuel consumption by U.S. territories (i.e., American Samoa, Guam, Puerto Rico, U.S. Virgin Islands, Wake Island, and other U.S. Pacific Islands) is
included in this report.
operating appliances. The remaining emissions were due to
the consumption of natural gas and petroleum for heating
and cooking.
Electricity Generation. The United States relies on
electricity to meet a significant portion of its energy demands,
especially for lighting, electric motors, heating, and air
conditioning. Electricity generators consumed 37 percent of
U.S. energy from fossil fuels and emitted 42 percent of the
CO2 from fossil fuel combustion in 2008. The type of fuel
combusted by electricity generators has a significant effect
on their emissions. For example, some electricity is generated
with low CO2 emitting energy technologies, particularly non-
fossil options such as nuclear, hydroelectric, or geothermal
energy. However, electricity generators rely on coal for over
half of their total energy requirements and accounted for 95
percent of all coal consumed for energy in the United States
in 2008. Consequently, changes in electricity demand have
a significant impact on coal consumption and associated
CO2 emissions.
Other significant CO2 trends included the following:
• Carbon dioxide emissions from non-energy use of fossil
fuels have increased 14.6 Tg CO2 Eq. (12.2 percent) from
1990 through 2008. Emissions from non-energy uses
of fossil fuels were 134.2 Tg CO2 Eq. in 2008, which
constituted 2.3 percent of total national CO2 emissions,
approximately the same proportion as in 1990.
Carbon dioxide emissions from iron and steel production
and metallurgical coke production decreased from
2007 to 2008 (3.8 Tg CO2 Eq.), continuing a trend of
decreasing emissions from 1990 through 2008 of 33
percent. This decline is due to the restructuring of the
industry, technological improvements, and increased
scrap utilization.
In 2008, CO2 emissions from cement production
decreased by 4.1 Tg CO2 Eq. (9.0 percent) from 2007.
After decreasing in 1991 by two percent from 1990
levels, cement production emissions grew every year
through 2006; emissions decreased in the last two years.
Overall, from 1990 to 2008, emissions from cement
production increased by 24 percent, an increase of 7.9
Tg CO2 Eq.
Net CO2 flux from Land Use, Land-Use Change, and
Forestry increased by 30.9 Tg CO2 Eq. (3 percent) from
1990 through 2008. This increase was primarily due
to an increase in the rate of net carbon accumulation
in forest carbon stocks, particularly in aboveground
and belowground tree biomass, and harvested wood
pools. Annual carbon accumulation in landfilled yard
trimmings and food scraps slowed over this period, while
the rate of carbon accumulation in urban trees increased.
Executive Summary ES-9
-------
Methane Emissions
According to the IPCC, CH4 is more than 20 times as
effective as CO2 at trapping heat in the atmosphere. Over the
last two hundred and fifty years, the concentration of CH4
in the atmosphere increased by 148 percent (IPCC 2007).
Anthropogenic sources of CH4 include landfills, natural gas
and petroleum systems, agricultural activities, coal mining,
wastewater treatment, stationary and mobile combustion, and
certain industrial processes (see Figure ES- 8).
Some significant trends in U.S. emissions of CH4 include
the following:
• Enteric Fermentation is the largest anthropogenic source
of CH4 emissions in the United States. In 2008, enteric
fermentation CH4 emissions were 140.8 Tg CO2 Eq.
(25 percent of total CH4 emissions), which represents
an increase of 8.5 Tg CO2 Eq. (6.4 percent) since 1990.
• Landfills are the second largest anthropogenic source
of CH4 emissions in the United States, accounting for
22 percent of total CH4 emissions (126.3 Tg CO2 Eq.)
in 2008. From 1990 to 2008, net CH4 emissions from
landfills decreased by 23.0 Tg CO2 Eq. (15 percent),
with small increases occurring in some interim years.
This downward trend in overall emissions is the result
of increases in the amount of landfill gas collected and
combusted,12 which has more than offset the additional
CH4 emissions resulting from an increase in the amount
of municipal solid waste landfilled.
• Methane emissions from natural gas systems were 96.4
Tg CO2 Eq. in 2008; emissions have declined by 33.1 Tg
CO2 Eq. (26 percent) since 1990. This decline is due to
improvements in technology and management practices,
as well as some replacement of old equipment.
• In 2008, CH4 emissions from coal mining were 67.6
Tg CO2 Eq., a 9.6 Tg CO2 Eq. (16 percent) increase
over 2007 emission levels. The overall decline of
16.4 Tg CO2 Eq. (20 percent) from 1990 results
from the mining of less gassy coal from underground
mines and the increased use of CH4 collected from
degasification systems.
• Methane emissions from manure management increased
by 54 percent since 1990, from 29.3 Tg CO2 Eq. in
Figure ES-8
2008 Sources of CH4 Emissions
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems ^^|
Wastewater Treatment ^H
Forest Land Remaining Forest Land ^
Rice Cultivation |
Stationary Combustion |
Abandoned Underground Coal Mines |
Mobile Combustion
Composting
Field Burning of Agricultural Residues
Petrochemical Production
Iron and Steel Production
& Metallurgical Coke Production I
Ferroalloy Production | <0.5
Silicon Carbide Production and Consumption | <0.5
Incineration of Waste I <0.5
20 4G
60 80 100 120 140
Tg CO, Eq.
12 The CO2 produced from combusted landfill CH4 at landfills is not counted
in national inventories as it is considered part of the natural C cycle of
decomposition.
1990 to 45.0 Tg CO2 Eq. in 2008. The majority of this
increase was from swine and dairy cow manure, since
the general trend in manure management is one of
increasing use of liquid systems, which tends to produce
greater CH4 emissions. The increase in liquid systems is
the combined result of a shift to larger facilities, and to
facilities in the West and Southwest, all of which tend
to use liquid systems. Also, new regulations limiting
the application of manure nutrients have shifted manure
management practices at smaller dairies from daily
spread to manure managed and stored on site.
Nitrous Oxide Emissions
Nitrous oxide is produced by biological processes that
occur in soil and water and by a variety of anthropogenic
activities in the agricultural, energy-related, industrial, and
waste management fields. While total N2O emissions are
much lower than CO2 emissions, N2O is approximately
300 times more powerful than CO2 at trapping heat in
the atmosphere. Since 1750, the global atmospheric
concentration of N2O has risen by approximately 18 percent
(IPCC 2007). The main anthropogenic activities producing
N2O in the United States are agricultural soil management,
fuel combustion in motor vehicles, nitric acid production,
ES-10 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
stationary fuel combustion, manure management, and adipic
acid production (see Figure ES-9).
Some significant trends in U.S. emissions of N2O include
the following:
• Agricultural soils accounted for approximately 68
percent of N2O emissions in the United States in 2008.
Estimated emissions from this source in 2008 were 215.9
Tg CO2 Eq. Annual N2O emissions from agricultural
soils fluctuated between 1990 and 2008, although
overall emissions were 6.1 percent higher in 2008
than in 1990. N2O emissions from this source have not
shown any significant long-term trend, as they are highly
sensitive to the amount of N applied to soils (which has
not changed significantly over the time-period), and to
weather patterns and crop type.
• In 2008, N2O emissions from mobile combustion were
26.1 Tg CO2 Eq. (approximately 8 percent of U.S. N2O
emissions). From 1990 to 2008, N2O emissions from
mobile combustion decreased by 40 percent. However,
from 1990 to 1998 emissions increased by 26 percent,
due to control technologies that reduced NOX emissions
while increasing N2O emissions. Since 1998, newer
control technologies have led to a steady decline in N2O
from this source.
Figure ES-9
2008 Sources of N?0 Emissions
216
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Forest Land Remaining Forest Land
Wastewater Treatment ^^|
N20 from Product Uses ^H
Adipic Acid Production |
Composting |
Settlements Remaining Settlements I
Field Burning of Agricultural Residues
Incineration of Waste | <0.5
Wetlands Remaining Wetlands | <0.5
N20 as a Portion
of all Emissions
10
20 30
Tg C02 Eq.
40
• Nitrous oxide emissions from adipic acid production
were 2.0 Tg CO2 Eq. in 2008, and have decreased
significantly since 1996 from the widespread installation
of pollution control measures. Emissions from adipic
acid production have decreased by 87 percent since
1990, and emissions from adipic acid production have
remained consistently lower than pre-1996 levels since
1998.
HFC, PFC, and SF6 Emissions
HFCs and PFCs are families of synthetic chemicals that
are used as alternatives to ODS, which are being phased out
under the Montreal Protocol and Clean Air Act Amendments
of 1990. HFCs and PFCs do not deplete the stratospheric
ozone layer, and are therefore acceptable alternatives under
the Montreal Protocol.
These compounds, however, along with SF6, are
potent greenhouse gases. In addition to having high
global warming potentials, SF6 and PFCs have extremely
long atmospheric lifetimes, resulting in their essentially
irreversible accumulation in the atmosphere once emitted.
Sulfur hexafluoride is the most potent greenhouse gas the
IPCC has evaluated.
Other emissive sources of these gases include HCFC-22
production, electrical transmission and distribution systems,
semiconductor manufacturing, aluminum production, and
magnesium production and processing (see Figure ES-10).
Some significant trends in U.S. HFC, PFC, and SF6
emissions include the following:
• Emissions resulting from the substitution of ODS (e.g.,
CFCs) have been consistently increasing, from small
amounts in 1990 to 113.OTgCO2Eq. in 2008. Emissions
from ODS substitutes are both the largest and the fastest
growing source of HFC, PFC, and SF6 emissions. These
emissions have been increasing as phase-outs required
under the Montreal Protocol come into effect, especially
after 1994, when full market penetration was made
for the first generation of new technologies featuring
ODS substitutes.
• HFC emissions from the production of HCFC-22
decreased by 63 percent (22.8 Tg CO2 Eq.) from 1990
through 2008, due to a steady decline in the emission
rate of HFC-23 (i.e., the amount of HFC-23 emitted
per kilogram of HCFC-22 manufactured) and the use
Executive Summary ES-11
-------
Figure ES-10
Figure ES-11
2008 Sources of MFCs, PFCs, and SF6 Emissions
Substitution of Ozone
Depleting Substances
HCFC-22 Production
Electrical Transmission
and Distribution
Aluminum Production
Magnesium Production |
and Processing
10
20 30
Tg C02 Eq.
40
50
of thermal oxidation at some plants to reduce HFC-23
emissions.
Sulfur hexafluoride emissions from electric power
transmission and distribution systems decreased by 51
percent (13.6 Tg CO2 Eq.) from 1990 to 2008, primarily
because of higher purchase prices for SF6 and efforts by
industry to reduce emissions.
PFC emissions from aluminum production decreased by
85 percent (15.8 Tg CO2 Eq.) from 1990 to 2008, due
to both industry emission reduction efforts and lower
domestic aluminum production.
U.S. Greenhouse Gas Emissions and Sinks
by Chapter/IPCC Sector
7,500 -
7,000 -
6,500 -
6,000 -
5,500 -
5,000 -
. 4,500-
S 4,000 -
o1 3,500-
Ł 3,000-
"- 2,500 -
2,000 -
1,500-
1,000-
500-
0
(500) -
(1,000) -I
Waste
Industrial Processes / LULUCF (sources)
Agric
Land Use, Land-Use Change and Forestry (sinks^
CD r^ CQ
Note: Relatively smaller amounts of GWP-weighted emissions are also emitted from the
Solvent and Other Product Use Sector.
ES.3. Overview of Sector Emissions
and Trends
In accordance with the Revised 1996IPCC Guidelines
for National Greenhouse Gas Inventories (IPCC/UNEP/
OECD/IEA 1997), and the 2003 UNFCCC Guidelines on
Reporting and Review (UNFCCC 2003), Figure ES-11 and
Table ES-4 aggregate emissions and sinks by these chapters.
Emissions of all gases can be summed from each source
category from IPCC guidance. Over the nineteen-year period
of 1990 to 2008, total emissions in the Energy, Industrial
Processes, and Agriculture sectors climbed by 775.0 Tg CO2
Table ES-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg C02 Eq.)
Chapter/IPCC Sector
Energy
Industrial Processes
Solvent and Other Product Use
Agriculture
Land Use, Land-Use Change, and
Forestry (Emissions)
Waste
Total Emissions
Net C02 Flux from Land Use, Land-Use
Change, and Forestry (Sinks)3
Net Emissions (Sources and Sinks)
1990
5,224.1
318.3
4.4l
387.8
15.0
177.2
6,126.8
(909.4)
5,217.3
1995
5,545.8
339.1
4.6 1
407.7 1
17.2
174.5
6,488.8
(842.9)
5,646.0
2000
6,087.5
351.9
4.9 1
410.9
36.3
153.0
7,044.5
(664.2)
6,380.2
2005
6,187.9
334.7
4.4
419.7
28.6
158.0
7,133.2
(950.4)
6,182.8
2006
6,089.1
339.7
4.4
417.2
49.8
159.7
7,059.9
(959.2)
6,100.7
2007
6,182.9
350.9
4.4
423.0
47.6
159.3
7,168.1
(955.4)
6,212.7
2008
5,999.0
334.5
4.4
427.5
32.2
159.1
6,956.8
(940.3)
6,016.4
"The net C02 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included in net emissions total.
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values or sequestration.
ES-12 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Eq. (15 percent), 16.2 Tg CO2 Eq. (5 percent), and 39.7 Tg
CO2 Eq. (10 percent), respectively. Emissions decreased in
the Waste and Solvent and Other Product Use sectors by
18.1 Tg CO2 Eq. (10 percent) and less than 0.1 Tg CO2 Eq.
(0.4 percent), respectively. Over the same period, estimates
of net C sequestration in the Land Use, Land-Use Change,
and Forestry sector (magnitude of emissions plus CO2 flux
from all LULUCF source categories) increased by 13.7 Tg
CO2Eq. (1.5 percent).
Energy
The Energy chapter contains emissions of all greenhouse
gases resulting from stationary and mobile energy activities
including fuel combustion and fugitive fuel emissions.
Energy-related activities, primarily fossil fuel combustion,
accounted for the vast majority of U.S. CO2 emissions for
the period of 1990 through 2008. In 2008, approximately
84 percent of the energy consumed in the United States (on
a Btu basis) was produced through the combustion of fossil
fuels. The remaining 16 percent came from other energy
sources such as hydropower, biomass, nuclear, wind, and
solar energy (see Figure ES-12). Energy-related activities are
also responsible for CH4 and N2O emissions (37 percent and
13 percent of total U.S. emissions of each gas, respectively).
Overall, emission sources in the Energy chapter account for a
combined 86 percent of total U.S. greenhouse gas emissions
in 2008.
Figure ES-12
2008 U.S. Energy Consumption by Energy Source
Renewable
Energy
7.4%
Nuclear Electric
Power
Industrial Processes
The Industrial Processes chapter contains byproduct
or fugitive emissions of greenhouse gases from industrial
processes not directly related to energy activities such as
fossil fuel combustion. For example, industrial processes
can chemically transform raw materials, which often release
waste gases such as CO2, CH4, and N2O. These processes
include iron and steel production and metallurgical coke
production, cement production, ammonia production and
urea consumption, lime production, limestone and dolomite
use (e.g., flux stone, flue gas desulfurization, and glass
manufacturing), soda ash production and consumption,
titanium dioxide production, phosphoric acid production,
ferroalloy production, CO2 consumption, silicon carbide
production and consumption, aluminum production,
petrochemical production, nitric acid production, adipic
acid production, lead production, and zinc production.
Additionally, emissions from industrial processes release
HFCs, PFCs, and SF6. Overall, emission sources in the
Industrial Process chapter account for 5 percent of U.S.
greenhouse gas emissions in 2008.
Solvent and Other Product Use
The Solvent and Other Product Use chapter contains
greenhouse gas emissions that are produced as a by-product
of various solvent and other product uses. In the United States,
emissions from N2O from product uses, the only source of
greenhouse gas emissions from this sector, accounted for
about 0.1 percent of total U.S. anthropogenic greenhouse gas
emissions on a carbon equivalent basis in 2008.
Agriculture
The Agriculture chapter contains anthropogenic
emissions from agricultural activities (except fuel combustion,
which is addressed in the Energy chapter, and agricultural
CO2 fluxes, which are addressed in the Land Use, Land-
Use Change, and Forestry Chapter). Agricultural activities
contribute directly to emissions of greenhouse gases through
a variety of processes, including the following source
categories: enteric fermentation in domestic livestock,
livestock manure management, rice cultivation, agricultural
soil management, and field burning of agricultural residues.
Methane and N2O were the primary greenhouse gases
emitted by agricultural activities. Methane emissions from
Executive Summary ES-13
-------
enteric fermentation and manure management represented
25 percent and 8 percent of total CH4 emissions from
anthropogenic activities, respectively, in 2008. Agricultural
soil management activities such as fertilizer application
and other cropping practices were the largest source of U.S.
N2O emissions in 2008, accounting for 68 percent. In 2008,
emission sources accounted for in the Agriculture chapter
were responsible for 6.1 percent of total U.S. greenhouse
gas emissions.
Land Use, Land-Use Change, and Forestry
The Land Use, Land-Use Change, and Forestry chapter
contains emissions of CH4 and N2O, and emissions and
removals of CO2 from forest management, other land-use
activities, and land-use change. Forest management practices,
tree planting in urban areas, the management of agricultural
soils, and the landfilling of yard trimmings and food scraps
have resulted in a net uptake (sequestration) of C in the United
States. Forests (including vegetation, soils, and harvested
wood) accounted for 84 percent of total 2008 net CO2 flux,
urban trees accounted for 10 percent, mineral and organic soil
carbon stock changes accounted for 5 percent, and landfilled
yard trimmings and food scraps accounted for 1 percent of the
total net flux in 2008. The net forest sequestration is a result
of net forest growth and increasing forest area, as well as a net
accumulation of carbon stocks in harvested wood pools. The
net sequestration in urban forests is a result of net tree growth
in these areas. In agricultural soils, mineral and organic soils
sequester approximately 5.9 times as much C as is emitted
from these soils through liming and urea fertilization. The
mineral soil C sequestration is largely due to the conversion
of cropland to permanent pastures and hay production, a
reduction in summer fallow areas in semi-arid areas, an
increase in the adoption of conservation tillage practices,
and an increase in the amounts of organic fertilizers (i.e.,
manure and sewage sludge) applied to agriculture lands. The
landnlled yard trimmings and food scraps net sequestration is
due to the long-term accumulation of yard trimming carbon
and food scraps in landfills.
Land use, land-use change, and forestry activities in
2008 resulted in a net C sequestration of 940.3 Tg CO2 Eq.
(Table ES- 5). This represents an offset of 16 percent of
total U.S. CO2 emissions, or 14 percent of total greenhouse
gas emissions in 2008. Between 1990 and 2008, total land
use, land-use change, and forestry net C flux resulted in a
3.4 percent increase in CO2 sequestration, primarily due
to an increase in the rate of net C accumulation in forest C
stocks, particularly in aboveground and belowground tree
biomass, and harvested wood pools. Annual C accumulation
in landfilled yard trimmings and food scraps slowed over this
period, while the rate of annual C accumulation increased
in urban trees.
Table ES- 5: Net C02 Flux from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Sink Category
Forest Land Remaining Forest Land
Cropland Remaining Cropland
Land Converted to Cropland
Grassland Remaining Grassland
Land Converted to Grassland
Settlements Remaining Settlements
Other (Landfilled Yard Trimmings and
Food Scraps)
Total
1990
(729.8)
(29.4)
2.2
(52.0)
(19.8)
(57.1)
(23.5)
(909.4)
1995
(692.6)
(22.9)
2.9 1
(26.7)
(22.3)
(67.3)
(13.9)
(842.9)
2005
(467.7)
(30.2)
(2.4)
(52.6)
(27.3)
(77.5)
(11.3)
(664.2) |
2005
(806.6)
(18.3)
5.9
(9.0)
(24.6)
(87.8)
(10.1)
(950.4)
2006
(812.5)
(19.1)
5.9
(8.9)
(24.5)
(89.8)
(10.3)
(959.2)
2007
(806.9)
(19.7)
5.9
(8.8)
(24.3)
(91.9)
(9.8)
(955.4)
2008
(791.9)
(18.1)
5.9
(8.7)
(24.2)
(93.9)
(9.5)
(940.3)
Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
ES-14 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table ES-6. Emissions from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Source Category
C02
Cropland Remaining Cropland: Liming of Agricultural
Soils
Urea Fertilization
Wetlands Remaining Wetlands: Peatlands Remaining
Peatlands
CH4
Forest Land Remaining Forest Land: Forest Fires
N20
Forest Land Remaining Forest Land: Forest Fires
Forest Land Remaining Forest Land: Forest Soils
Settlements Remaining Settlements: Settlement Soils
Wetlands Remaining Wetlands: Peatlands Remaining
Peatlands
Total
1990
8.1
4.7
2.4
I
3.2
3.2
3.7
2.6
0.1
1.0
15.0 |
1995
8.1
4.4
2.7 1
1.0
4.3
4.3
4.9
3.5
0.2
,2!
17.2
2000
8.8
1.2l
14.3
14.3
13.2
11.7
0.4
1.1 1
36.3
2005
8.9
4.3
3.5
1.1
9.8
9.8
9.8
8.0
0.4
1.5
28.6
2006
8.8
4.2
3.7
0.9
21.6
21.6
19.5
17.6
0.4
1.5
49.8
2007
9.3
4.5
3.8
1.0
20.0
20.0
18.3
16.3
0.4
1.6
47.6
2008
8.6
3.8
3.8
0.9
11.9
11.9
11.7
9.7
0.4
1.6
32.2
+ Less than 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Emissions from Land Use, Land-Use Change, and
Forestry are shown in Table ES-6. The application of
crushed limestone and dolomite to managed land (i.e.,
liming of agricultural soils) and urea fertilization resulted
in CO2 emissions of 7.6 Tg CO2 Eq. in 2008, an increase
of 8 percent relative to 1990. The application of synthetic
fertilizers to forest and settlement soils in 2008 resulted in
direct N2O emissions of 1.9 Tg CO2 Eq. Direct N2O emissions
from fertilizer application to forest soils have increased by
422 percent since 1990, but still account for a relatively
small portion of overall emissions. Additionally, direct N2O
emissions from fertilizer application to settlement soils
increased of 62 percent since 1990. Non-CO2 emissions from
forest fires in 2008 resulted in CH4 emissions of 11.9 Tg CO2
Eq., and in N2O emissions of 9.7 Tg CO2 Eq. CO2 and N2O
emissions from peatlands totaled 0.9 Tg CO2 Eq. and less
than 0.01 Tg CO2 Eq. in 2008, respectively.
Waste
The Waste chapter contains emissions from waste
management activities (except incineration of waste, which
is addressed in the Energy chapter). Landfills were the
largest source of anthropogenic greenhouse gas emissions
in the Waste chapter, accounting for just over 79 percent of
this chapter's emissions, and 22 percent of total U.S. CH4
emissions.13 Additionally, wastewater treatment accounts
for 18 percent of Waste emissions, 4 percent of U.S. CH4
emissions, and 2 percent of U.S. N2O emissions. Emissions of
CH4 and N2O from composting are also accounted for in this
chapter; generating emissions of 1.7 Tg CO2 Eq. and 1.8 Tg
CO2 Eq., respectively. Overall, emission sources accounted
for in the Waste chapter generated 2.3 percent of total U.S.
greenhouse gas emissions in 2008.
ES.4. Other Information
Emissions by Economic Sector
Throughout the Inventory of U.S. Greenhouse Gas
Emissions and Sinks report, emission estimates are grouped
into six sectors (i.e., chapters) defined by the IPCC: Energy;
Industrial Processes; Solvent Use; Agriculture; Land Use,
Land-Use Change, and Forestry; and Waste. While it is
important to use this characterization for consistency with
UNFCCC reporting guidelines, it is also useful to allocate
emissions into more commonly used sectoral categories.
This section reports emissions by the following economic
sectors: Residential, Commercial, Industry, Transportation,
Electricity Generation, Agriculture, and U.S. Territories.
Table ES-7 summarizes emissions from each of these
sectors, and Figure ES-13 shows the trend in emissions by
sector from 1990 to 2008.
13 Landfills also store carbon, due to incomplete degradation of organic
materials such as wood products and yard trimmings, as described in the
Land-Use, Land-Use Change, and Forestry chapter of this report.
Executive Summary ES-15
-------
Table ES-7: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg C02 Eq.)
Implied Sectors
Electric Power Industry
Transportation
Industry
Agriculture
Commercial
Residential
U.S. Territories
Total Emissions
Land Use, Land-Use Change, and
Forestry (Sinks)
Net Emissions (Sources and Sinks)
1990
1,867.2
1, 545.0 1
1,506.6
433.2
395.1
345.9
33.7
6,126.8
(909.4)
5,217.3
1995
1,993.7
1,695.21
1,531. 3 1
460.8
399.6
367.6
40.7
6,488.8
(842.9)
5,646.0
2000
2,336.8
1, 932.3 1
1,469.1
485.3
387.3
386.7
46.9
7,044.5
(664.2)
6,380.2 |
2005
2,443.5
2,016.1
1,350.9
494.1
399.0
370.7
58.9
7,133.2
(950.4)
6,182.8
2006
2,387.5
1,993.0
1,380.2
515.1
389.2
334.9
60.0
7,059.9
(959.2)
6,100.7
2007
2,454.0
2,003.5
1,374.2
518.0
404.4
356.2
57.8
7,168.1
(955.4)
6,212.7
2008
2,404.2
1,886.1
1,342.4
504.1
410.9
359.3
49.9
6,956.8
(940.3)
6,016.4
Note: Totals may not sum due to independent rounding. Emissions include C02, CH4, N20, MFCs, PFCs, and SF6.
See Table 2-12 of this report for more detailed data.
Using this categorization, emissions from electricity
generation accounted for the largest portion (35 percent)
of U.S. greenhouse gas emissions in 2008. Transportation
activities, in aggregate, accounted for the second largest
portion (27 percent), while emissions from industry
accounted for the third largest portion (19 percent) of U.S.
greenhouse gas emissions in 2008. In contrast to electricity
generation and transportation, emissions from industry have
in general declined over the past decade. The long-term
decline in these emissions has been due to structural changes
in the U.S. economy (i.e., shifts from a manufacturing-
based to a service-based economy), fuel switching, and
energy efficiency improvements. The remaining 19 percent
of U.S. greenhouse gas emissions were contributed by,
in order of importance, the agriculture, commercial, and
residential sectors, plus emissions from U.S. territories.
Activities related to agriculture accounted for 7 percent of
U.S. emissions; unlike other economic sectors, agricultural
sector emissions were dominated by N2O emissions from
agricultural soil management and CH4 emissions from enteric
fermentation. The commercial sector accounted for 6 percent
of emissions while the residential sector accounted for 5
percent of emissions and U.S. territories accounted for 1
percent of emissions; emissions from these sectors primarily
consisted of CO2 emissions from fossil fuel combustion.
Carbon dioxide was also emitted and sequestered by a
variety of activities related to forest management practices,
Figure ES-13
Emissions Allocated to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
0-
Electric Power Industry
Transportation
s
Industry
griculture
Commercial
Residential
Note: Does not include U.S. Territories.
tree planting in urban areas, the management of agricultural
soils, and landfilling of yard trimmings.
Electricity is ultimately consumed in the economic
sectors described above. Table ES-8 presents greenhouse
gas emissions from economic sectors with emissions related
to electricity generation distributed into end-use categories
(i.e., emissions from electricity generation are allocated to
the economic sectors in which the electricity is consumed).
To distribute electricity emissions among end-use sectors,
emissions from the source categories assigned to electricity
ES-16 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table ES-8: U.S Greenhouse Gas Emissions by Economic Sector with Electricity-Related Emissions Distributed (Tg
C02 Eq.)
Implied Sectors
Industry
Transportation
Commercial
Residential
Agriculture
U.S. Territories
Total Emissions
Land Use, Land-Use Change, and
Forestry (Sinks)
Net Emissions (Sources and Sinks)
1990
2,179.8
1,548.2l
946.8
954.0
464.2
33.7
6,126.8
(909.4)
5,217.3
1995
2,228.0
1,698.3
1,000.21
1, 024.5 1
497.1
40.7
6,488.8
(842.9)
5,646.0
2000
2,239.2
1,935.8
1,141.5 1
1,162.4!
518.7
46.9
7,044.5
(664.2)
6,380.2
2005
2,071.1
2,020.9
1,216.5
1,242.2
523.5
58.9
7,133.2
(950.4)
6,182.8
2006
2,077.3
1,997.6
1,202.2
1,180.3
542.5
60.0
7,059.9
(959.2)
6,100.7
2007
2,084.2
2,008.6
1,240.1
1,226.9
550.5
57.8
7,168.1
(955.4)
6,212.7
2008
2,018.4
1,890.8
1,250.6
1,215.6
531.6
49.9
6,956.8
(940.3)
6,016.4
See Table 2-14 of this report for more detailed data.
Figure ES-14
Emissions with Electricity Distributed
to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
0-
Industry
Transportation
Residential
Commercial
Agriculture
Note: Does not include U.S. Territories.
generation were allocated to the residential, commercial,
industry, transportation, and agriculture economic sectors
according to retail sales of electricity.14 These source
categories include CO2 from fossil fuel combustion and the
use of limestone and dolomite for flue gas desulfurization,
CO2 and N2O from incineration of waste, CH4 and N2O from
stationary sources, and SF6 from electrical transmission and
distribution systems.
14 Emissions were not distributed to U.S. territories, since the electricity
generation sector only includes emissions related to the generation of
electricity in the 50 states and the District of Columbia.
When emissions from electricity are distributed among
these sectors, industry accounts for the largest share of U.S.
greenhouse gas emissions (29 percent) in 2008. Emissions
from the residential and commercial sectors also increase
substantially when emissions from electricity are included,
due to their relatively large share of electricity consumption
(e.g., lighting, appliances, etc.). Transportation activities
remain the second largest contributor to total U.S. emissions
(27 percent) despite the considerable decline in emissions
from this sector during the past year. In all sectors except
agriculture, CO2 accounts for more than 80 percent of
greenhouse gas emissions, primarily from the combustion of
fossil fuels. Figure ES-14 shows the trend in these emissions
by sector from 1990 to 2008.
Indirect Greenhouse Gases (CO, NOX,
NMVOCs, and S02)
The reporting requirements of the UNFCCC15 request
that information be provided on indirect greenhouse gases,
which include CO, NOX, NMVOCs, and SO2. These gases do
not have a direct global warming effect, but indirectly affect
terrestrial radiation absorption by influencing the formation
and destruction of tropospheric and stratospheric ozone, or,
in the case of SO2, by affecting the absorptive characteristics
of the atmosphere. Additionally, some of these gases may
react with other chemical compounds in the atmosphere to
form compounds that are greenhouse gases.
' See .
Executive Summary ES-17
-------
Box ES-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data
Total emissions can be compared to other economic and social indices to highlight changes over time. These comparisons include: (1)
emissions per unit of aggregate energy consumption, because energy-related activities are the largest sources of emissions; (2) emissions
per unit of fossil fuel consumption, because almost all energy-related emissions involve the combustion of fossil fuels; (3) emissions per
unit of electricity consumption, because the electric power industry—utilities and nonutilities combined—was the largest source of U.S.
greenhouse gas emissions in 2008; (4) emissions per unit of total gross domestic product as a measure of national economic activity; and
(5) emissions per capita.
Table ES-9 provides data on various statistics related to U.S. greenhouse gas emissions normalized to 1990 as a baseline year.
Greenhouse gas emissions in the United States have grown at an average annual rate of 0.7 percent since 1990. This rate is slightly slower
than that for total energy, approximately the same as for fossil fuel consumption, and much slower than that for either electricity consumption
or overall gross domestic product. Total U.S. greenhouse gas emissions have also grown slightly slower than national population since 1990
(see Figure ES-15).
Table ES-9: Recent Trends in Various U.S. Data (Index 1990 = 100)
Variable
1990
1995
2000
2005 2006 2007 2008
Growth
Rate"
GDPb
Electricity Consumption0
Fossil Fuel Consumption0
Energy Consumption0
Population"1
Greenhouse Gas Emissions6
100
100
100
100
100
100
113
112
107
107
107
106
140
127
117
116
113
115
157
134
119
119
118
116
162
135
117
118
119
115
165
138
119
120
120
117
166
136
115
118
121
114
2.9%
1.8%
0.8%
0.9%
1.1%
0.7%
1 Average annual growth rate.
1 Gross Domestic Product in chained 2000 dollars (BEA 2009).
: Energy content-weighted values (EIA 2009).
J U.S. Census Bureau (2009).
'- GWP-weighted values.
Figure ES-15
U.S. Greenhouse Gas Emissions Per Capita and
Per Dollar of Gross Domestic Product
Real GDP
Population
Emissions
per capita
Emissions
per $GDP
Source: BEA (2009), U.S. Census Bureau (2009), and emission estimates in this report.
Note: Does not include U.S. Territories.
ES-18 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table ES-10: Emissions of NOx, CO, NMVOCs, and S02 (Gg)
Gas/Activity
1990
1995
2000
2005
2006
2007
2008
NOX 21,728
Mobile Fossil Fuel Combustion 10,862
Stationary Fossil Fuel Combustion 10,023
Industrial Processes 591
Oil and Gas Activities 139
Incineration of Waste 82
Agricultural Burning 30
Solvent Use 1
Waste 0
CO 130,536
Mobile Fossil Fuel Combustion 119,360
Stationary Fossil Fuel Combustion 5,000
Industrial Processes 4,125
Incineration of Waste 978
Agricultural Burning 766
Oil and Gas Activities 302
Waste 1
Solvent Use 5
NMVOCs 20,930
Mobile Fossil Fuel Combustion 10,932
Solvent Use 5,216
Industrial Processes 2,422
Stationary Fossil Fuel Combustion 912
Oil and Gas Activities 554
Incineration of Waste 222
Waste 673
Agricultural Burning NA
S02 20,935
Stationary Fossil Fuel Combustion 18,407
Industrial Processes 1,307
Mobile Fossil Fuel Combustion 793
Oil and Gas Activities 390
Incineration of Waste 38
Waste 0
Solvent Use 0
Agricultural Burning NA
21,227
10,536
9,862
6071
1001
30
109,114
97,630
5,383
3,959
1,073
745
316
19,520
8,745
5,609
2,642
9731
582
237
731
NA
16,891
14,724
1,117
672
335
42
IVJ I L. H
335
<
NA
19,145
10,199
8,053
626
111
114
37
3
2
92,872
83,559
4,340
2,126
1,670
888
146
8
45
15,227
7,229
4,384
1,773
1,077
388
257
119
NA
14,830
12,849
1,031
632
287
29
1
1
NA
15,933
9,012
5,858
569
321
129
40
3
2
71,555
62,692
4,649
1,555
1,403
930
318
7
2
13,761
6,330
3,851
1,997
716
510
241
114
NA
13,466
11,541
831
889
181
24
1
0
NA
15,071
8,488
5,545
553
319
121
40
4
2
67,909
58,972
4,695
1,597
1,412
905
319
7
2
13,594
6,037
3,846
1,933
918
510
238
113
NA
12,388
10,612
818
750
182
24
1
0
NA
14,410
7,965
5,432
537
318
114
38
4
2
64,348
55,253
4,744
1,640
1,421
960
320
7
2
13,423
5,742
3,839
1,869
1,120
509
234
111
NA
11,799
10,172
807
611
184
24
1
0
NA
13,578
7,441
5,148
520
318
106
40
4
2
60,739
51,533
4,792
1,682
1,430
970
322
7
2
13,254
5,447
3,834
1,804
1,321
509
230
109
NA
10,368
8,891
795
472
187
23
1
0
NA
Source: EPA (2009), disaggregated based on EPA (2003) except for estimates from field burning of agricultural residues.
NA (Not Available).
Note: Totals may not sum due to independent rounding.
Since 1970, the United States has published estimates
of annual emissions of CO, NOX, NMVOCs, and SO2 (EPA
2008),16 which are regulated under the Clean Air Act. Table
16 NOX and CO emission estimates from field burning of agricultural residues
were estimated separately, and therefore not taken from EPA (2008).
ES-10 shows that fuel combustion accounts for the majority
of emissions of these indirect greenhouse gases. Industrial
processes—such as the manufacture of chemical and allied
products, metals processing, and industrial uses of solvents—
are also significant sources of CO, NOX, and NMVOCs.
Executive Summary ES-19
-------
Figure ES-16
2008 Key Categories
C02 Emissions from Stationary Combustion - Coal
C02 Emissions from Mobile Combustion: Road & Other
C02 Emissions from Stationary Combustion - Gas
C02 Emissions from Stationary Combustion - Oil
Direct N20 Emissions from Agricultural Soil Management
C02 Emissions from Mobile Combustion: Aviation
CH4 Emissions from Enteric Fermentation
C02 Emissions from Non-Energy Use of Fuels
CH4 Emissions from Landfills
Emissions from Substitutes for Ozone Depleting Substances
Fugitive CH4 Emissions from Natural Gas Systems
C02 Emissions from Iron and Steel Production & Metallurgical Coke Production
Fugitive CH4 Emissions from Coal Mining
Indirect N20 Emissions from Applied Nitrogen
CH4 Emissions from Manure Management
C02 Emissions from Cement Production
Fugitive CH4 Emissions from Petroleum Systems
N20 Emissions from Manure Management
Non-CO, Emissions from Stationary Combustion
Key Categories as a
Portion of all Emissions
0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
Tg C02 Eq.
Note: For a complete discussion of the key category analysis, see Annex 1 of this report. Darker bars indicate a Tier 1 level assessment key category. Lighter bars indicate a Tier 2 level assessment key category.
Key Categories
The IPCC's Good Practice Guidance (IPCC 2000)
defines a key category as a "[source or sink category] that
is prioritized within the national inventory system because
its estimate has a significant influence on a country's total
inventory of direct greenhouse gases in terms of the absolute
level of emissions, the trend in emissions, or both."17 By
definition, key categories are sources or sinks that have the
greatest contribution to the absolute overall level of national
emissions in any of the years covered by the time series. In
addition, when an entire time series of emission estimates
is prepared, a thorough investigation of key categories
must also account for the influence of trends of individual
source and sink categories. Finally, a qualitative evaluation
of key categories should be performed, in order to capture
any key categories that were not identified in either of the
quantitative analyses.
17 See Chapter 7 "Methodological Choice and Recalculation" in IPCC
(2000).
Figure ES-16 presents 2008 emission estimates for
the key categories as defined by a level analysis (i.e., the
contribution of each source or sink category to the total
inventory level). The UNFCCC reporting guidelines request
that key category analyses be reported at an appropriate
level of disaggregation, which may lead to source and sink
category names which differ from those used elsewhere in
this report. For more information regarding key categories,
see section 1.5 and Annex 1 of this report.
Quality Assurance and Quality Control
(QA/QC)
The United States seeks to continually improve the
quality, transparency, and credibility of the Inventory of
U.S. Greenhouse Gas Emissions and Sinks. To assist in these
efforts, the United States implemented a systematic approach
to QA/QC. While QA/QC has always been an integral part
of the U.S. national system for inventory development, the
procedures followed for the current inventory have been
formalized in accordance with the QA/QC plan and the
UNFCCC reporting guidelines.
ES-20 Executive Summary of the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Uncertainty Analysis of Emission Estimates
While the current U.S. emissions inventory provides a
solid foundation for the development of a more detailed and
comprehensive national inventory, there are uncertainties
associated with the emission estimates. Some of the current
estimates, such as those for CO2 emissions from energy-
related activities and cement processing, are considered
to have low uncertainties. For some other categories
of emissions, however, a lack of data or an incomplete
understanding of how emissions are generated increases
the uncertainty associated with the estimates presented.
Acquiring a better understanding of the uncertainty
associated with inventory estimates is an important step in
helping to prioritize future work and improve the overall
quality of this report. Recognizing the benefit of conducting
an uncertainty analysis, the UNFCCC reporting guidelines
follow the recommendations of the IPCC Good Practice
Guidance (IPCC 2000) and require that countries provide
single estimates of uncertainty for source and sink categories.
Currently, a qualitative discussion of uncertainty is
presented for all source and sink categories. Within the
discussion of each emission source, specific factors affecting
the uncertainty surrounding the estimates are discussed. Most
sources also contain a quantitative uncertainty assessment,
in accordance with UNFCCC reporting guidelines.
Executive Summary ES-21
-------
1. Introduction
This report presents estimates by the United States government of U.S. anthropogenic greenhouse gas emissions
and sinks for the years 1990 through 2008. A summary of these estimates is provided in Table 2.1 and Table 2.2
by gas and source category in the Trends in Greenhouse Gas Emissions chapter. The emission estimates in these
tables are presented on both a full molecular mass basis and on a global warming potential (GWP) weighted basis in order
to show the relative contribution of each gas to global average radiative forcing.1 This report also discusses the methods
and data used to calculate these emission estimates.
In 1992, the United States signed and ratified the United Nations Framework Convention on Climate Change (UNFCCC).
As stated in Article 2 of the UNFCCC, "The ultimate objective of this Convention and any related legal instruments that the
Conference of the Parties may adopt is to achieve, in accordance with the relevant provisions of the Convention, stabilization
of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with
the climate system. Such a level should be achieved within a time-frame sufficient to allow ecosystems to adapt naturally
to climate change, to ensure that food production is not threatened and to enable economic development to proceed in a
sustainable manner."2-3
Parties to the Convention, by ratifying, "shall develop, periodically update, publish and make available...national
inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse gases not controlled by the
Montreal Protocol, using comparable methodologies..."4 The United States views this report as an opportunity to fulfill
these commitments under the UNFCCC.
In 1988, preceding the creation of the UNFCCC, the World Meteorological Organization (WMO) and the United
Nations Environment Programme (UNEP) jointly established the Intergovernmental Panel on Climate Change (IPCC). The
role of the IPCC is to assess on a comprehensive, objective, open, and transparent basis the scientific, technical, and socio-
economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential
impacts and options for adaptation and mitigation (IPCC 2003). Under Working Group 1 of the IPCC, nearly 140 scientists
and national experts from more than thirty countries collaborated in the creation of the Revised 1996 IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997) to ensure that the emission inventories submitted
to the UNFCCC are consistent and comparable between nations. The IPCC accepted the Revised 1996 IPCC Guidelines
at its Twelfth Session (Mexico City, September 11-13, 1996). This report presents information in accordance with these
guidelines. In addition, this Inventory is in accordance with the IPCC Good Practice Guidance and Uncertainty Management
1 See the section below entitled Global Warming Potentials for an explanation of GWP values.
2 The term "anthropogenic," in this context, refers to greenhouse gas emissions and removals that are a direct result of human activities or are the result
of natural processes that have been affected by human activities (IPCC/UNEP/OECD/IEA 1997).
3 Article 2 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate Change. See .
Introduction 1-1
-------
in National Greenhouse Gas Inventories and the Good
Practice Guidance for Land Use, Land-Use Change, and
Forestry, which further expanded upon the methodologies
in the Revised 1996 IPCC Guidelines. The IPCC has also
accepted the 2006 Guidelines for National Greenhouse
Gas Inventories (IPCC 2006) at its Twenty-Fifth Session
(Mauritius, April 2006). The 2006 IPCC Guidelines build
on the previous bodies of work and include new sources
and gases ".. .as well as updates to the previously published
methods whenever scientific and technical knowledge have
improved since the previous guidelines were issued." Many
of the methodological improvements presented in the 2006
Guidelines have been adopted in this Inventory.
Overall, this Inventory of anthropogenic greenhouse gas
emissions provides a common and consistent mechanism
through which Parties to the UNFCCC can estimate
emissions and compare the relative contribution of individual
sources, gases, and nations to climate change. The structure of
this report is consistent with the current UNFCCC Guidelines
on Annual Inventories (UNFCCC 2006).
1.1. Background Information
Greenhouse Gases
Although the earth's atmosphere consists mainly of
oxygen and nitrogen, neither plays a significant role in
enhancing the greenhouse effect because both are essentially
transparent to terrestrial radiation. The greenhouse effect
is primarily a function of the concentration of water
vapor, carbon dioxide (CO2), and other trace gases in the
atmosphere that absorb the terrestrial radiation leaving the
surface of the earth (IPCC 2001). Changes in the atmospheric
concentrations of these greenhouse gases can alter the balance
of energy transfers between the atmosphere, space, land, and
the oceans.5 A gauge of these changes is called radiative
forcing, which is a measure of the influence a factor has in
altering the balance of incoming and outgoing energy in the
earth-atmosphere system (IPCC 2001). Holding everything
else constant, increases in greenhouse gas concentrations in
the atmosphere will produce positive radiative forcing (i.e.,
a net increase in the absorption of energy by the earth).
Climate change can be driven by changes in
the atmospheric concentrations of a number of
radiatively active gases and aerosols. We have
clear evidence that human activities have affected
concentrations, distributions and life cycles of these
gases (IPCC 1996).
Naturally occurring greenhouse gases include water
vapor, CO2, methane (CH4), nitrous oxide (N2O), and
ozone (O3). Several classes of halogenated substances that
contain fluorine, chlorine, or bromine are also greenhouse
gases, but they are, for the most part, solely a product
of industrial activities. Chlorofluorocarbons (CFCs) and
hydrochlorofluorocarbons (HCFCs) are halocarbons that
contain chlorine, while halocarbons that contain bromine
are referred to as bromofluorocarbons (i.e., halons). As
stratospheric ozone depleting substances, CFCs, HCFCs,
and halons are covered under the Montreal Protocol on
Substances that Deplete the Ozone Layer. The UNFCCC
defers to this earlier international treaty. Consequently,
Parties to the UNFCCC are not required to include these gases
in national greenhouse gas inventories.6 Some other fluorine-
containing halogenated substances—hydrofluorocarbons
(HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride
(SF6)—do not deplete stratospheric ozone but are potent
greenhouse gases. These latter substances are addressed by
the UNFCCC and accounted for in national greenhouse gas
inventories.
There are also several gases that, although they do not
have a commonly agreed upon direct radiative forcing effect,
do influence the global radiation budget. These tropospheric
gases include carbon monoxide (CO), nitrogen dioxide
(NO2), sulfur dioxide (SO2), and tropospheric (ground
level) ozone O3. Tropospheric ozone is formed by two
precursor pollutants, volatile organic compounds (VOCs)
and nitrogen oxides (NOX) in the presence of ultraviolet
light (sunlight). Aerosols are extremely small particles or
liquid droplets that are often composed of sulfur compounds,
carbonaceous combustion products, crustal materials
and other human-induced pollutants. They can affect the
absorptive characteristics of the atmosphere. Comparatively,
however, the level of scientific understanding of aerosols is
still very low (IPCC 2001).
5 For more on the science of climate change, see NRC (2001).
6 Emission estimates of CFCs, HCFCs, halons and other ozone-depleting
substances are included in this document for informational purposes.
1-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 1-1: Global Atmospheric Concentration, Rate of Concentration Change, and Atmospheric Lifetime (years) of
Selected Greenhouse Gases
Atmospheric Variable
Pre-industrial atmospheric concentration
Atmospheric concentration3
Rate of concentration change
Atmospheric lifetimed
C02
278 ppm
385 ppm
1.4 ppm/yr
50-2006
CH4
0.715 ppm
1.741-1. 865 ppmb
0.005 ppm/yra
12f
N20
0.270 ppm
0.321 -0.322 ppmb
0.26%/yr
114f
SF6
Oppt
5.6 ppt
Linear0
3,200
CF4
40 ppt
74 ppt
Linear0
>50,000
Source: Pre-industrial atmospheric concentrations and rate of concentration changes for all gases are from IPCC (2007). The current atmospheric
concentration for C02 is from NOAA/ESRL (2009).
a The growth rate for atmospheric CH4 has been decreasing from 14 ppb/yr in 1984 to almost 0 ppb/yr in 2001,2004, and 2005 (IPCC 2007).
b The range is the annual arithmetic averages from a mid-latitude Northern-Hemisphere site and a mid-latitude Southern-Hemisphere site for October
2006 through September 2007 (CDIAC 2009).
c IPCC (2007) identifies the rate of concentration change for SF6 and CF4 as linear.
"Source: IPCC (1996).
e No single lifetime can be defined for C02 because of the different rates of uptake by different removal processes.
f This lifetime has been defined as an "adjustment time" that takes into account the indirect effect of the gas on its own residence time.
Note: ppt (parts per thousand), ppm (parts per million), ppb (parts per billion).
Carbon dioxide, CH4, and N2O are continuously emitted
to and removed from the atmosphere by natural processes
on earth. Anthropogenic activities, however, can cause
additional quantities of these and other greenhouse gases
to be emitted or sequestered, thereby changing their global
average atmospheric concentrations. Natural activities such
as respiration by plants or animals and seasonal cycles of
plant growth and decay are examples of processes that only
cycle carbon or nitrogen between the atmosphere and organic
biomass. Such processes, except when directly or indirectly
perturbed out of equilibrium by anthropogenic activities,
generally do not alter average atmospheric greenhouse gas
concentrations over decadal timeframes. Climatic changes
resulting from anthropogenic activities, however, could
have positive or negative feedback effects on these natural
systems. Atmospheric concentrations of these gases, along
with their rates of growth and atmospheric lifetimes, are
presented in Table 1-1.
A brief description of each greenhouse gas, its sources,
and its role in the atmosphere is given below. The following
section then explains the concept of GWPs, which are
assigned to individual gases as a measure of their relative
average global radiative forcing effect.
Water Vapor (H20). Overall, the most abundant and
dominant greenhouse gas in the atmosphere is water vapor.
Water vapor is neither long-lived nor well mixed in the
atmosphere, varying spatially from 0 to 2 percent (IPCC
1996). In addition, atmospheric water can exist in several
physical states including gaseous, liquid, and solid. Human
activities are not believed to affect directly the average
global concentration of water vapor, but, the radiative
forcing produced by the increased concentrations of other
greenhouse gases may indirectly affect the hydrologic cycle.
While a warmer atmosphere has an increased water holding
capacity, increased concentrations of water vapor affect the
formation of clouds, which can both absorb and reflect solar
and terrestrial radiation. Aircraft contrails, which consist of
water vapor and other aircraft emittants, are similar to clouds
in their radiative forcing effects (IPCC 1999).
Carbon Dioxide. In nature, carbon is cycled between
various atmospheric, oceanic, land biotic, marine biotic,
and mineral reservoirs. The largest fluxes occur between the
atmosphere and terrestrial biota, and between the atmosphere
and surface water of the oceans. In the atmosphere,
carbon predominantly exists in its oxidized form as CO2.
Atmospheric CO2 is part of this global carbon cycle, and
therefore its fate is a complex function of geochemical and
biological processes.Carbon dioxide concentrations in the
atmosphere increased from approximately 280 parts per
million by volume (ppmv) in pre-industrial times to 385
ppmv in 2008, a 37.5 percent increase (IPCC 2007 and
NOAA/ESRL 2009).7'8 The IPCC definitively states that "the
present atmospheric CO2 increase is caused by anthropogenic
7 The pre-industrial period is considered as the time preceding the year
1750 (IPCC 2001).
8 Carbon dioxide concentrations during the last 1,000 years of the pre-
industrial period (i.e., 750-1750), a time of relative climate stability.
fluctuated by about 10 ppmv around 280 ppmv (IPCC 2001).
Introduction 1-3
-------
emissions of CO2" (IPCC 2001). The predominant source
of anthropogenic CO2 emissions is the combustion of fossil
fuels. Forest clearing, other biomass burning, and some non-
energy production processes (e.g., cement production) also
emit notable quantities of CO2. In its fourth assessment, the
IPCC stated that "most of the observed increase in global
average temperatures since the mid^O* century is very likely
due to the observed increased in anthropogenic greenhouse
gas concentrations," of which CO2 is the most important
(IPCC 2007).
Methane. Methane is primarily produced through
anaerobic decomposition of organic matter in biological
systems. Agricultural processes such as wetland rice
cultivation, enteric fermentation in animals, and the
decomposition of animal wastes emit CH4, as does the
decomposition of municipal solid wastes. CH4 is also
emitted during the production and distribution of natural
gas and petroleum, and is released as a by-product of coal
mining and incomplete fossil fuel combustion. Atmospheric
concentrations of CH4 have increased by about 143 percent
since 1750, from a pre-industrial value of about 722 ppb
to 1,741-1,865 ppb in 2007,9 although the rate of increase
has been declining. The IPCC has estimated that slightly
more than half of the current CH4 flux to the atmosphere is
anthropogenic, from human activities such as agriculture,
fossil fuel use, and waste disposal (IPCC 2007).
Methane is removed from the atmosphere through a
reaction with the hydroxyl radical (OH) and is ultimately
converted to CO2. Minor removal processes also include
reaction with chlorine in the marine boundary layer, a soil
sink, and stratospheric reactions. Increasing emissions of CH4
reduce the concentration of OH, a feedback that may increase
the atmospheric lifetime of CH4 (IPCC 2001).
Nitrous Oxide. Anthropogenic sources of N2O emissions
include agricultural soils, especially production of nitrogen-
fixing crops and forages, the use of synthetic and manure
fertilizers, and manure deposition by livestock; fossil fuel
combustion, especially from mobile combustion; adipic
(nylon) and nitric acid production; wastewater treatment and
waste incineration; and biomass burning. The atmospheric
concentration of N2O has increased by 18 percent since 1750,
from a pre-industrial value of about 270 ppb to 321-322 ppb
in 2007,10 a concentration that has not been exceeded during
the last thousand years. Nitrous oxide is primarily removed
from the atmosphere by the photolytic action of sunlight in
the stratosphere (IPCC 2007).
Ozone. Ozone is present in both the upper stratosphere,11
where it shields the Earth from harmful levels of ultraviolet
radiation, and at lower concentrations in the troposphere,12
where it is the main component of anthropogenic
photochemical "smog." During the last two decades,
emissions of anthropogenic chlorine and bromine-containing
halocarbons, such as CFCs, have depleted stratospheric ozone
concentrations. This loss of ozone in the stratosphere has
resulted in negative radiative forcing, representing an indirect
effect of anthropogenic emissions of chlorine and bromine
compounds (IPCC 1996). The depletion of stratospheric
ozone and its radiative forcing was expected to reach a
maximum in about 2000 before starting to recover. As of
the date of publication of IPCC's Fourth Assessment Report,
"whether or not recently observed changes in ozone trends
are already indicative of recovery of the global ozone layer
is not yet clear" (IPCC 2007).
The past increase in tropospheric ozone, which is also
a greenhouse gas, is estimated to provide the third largest
increase in direct radiative forcing since the pre-industrial
era, behind CO2 and CH4. Tropospheric ozone is produced
from complex chemical reactions of volatile organic
compounds mixing with NOX in the presence of sunlight.
The tropospheric concentrations of ozone and these other
pollutants are short-lived and, therefore, spatially variable
(IPCC 2001).
Halocarbons, Perfluorocarbons, and Sulfur Hexafluoride.
Halocarbons are, for the most part, man-made chemicals
that have both direct and indirect radiative forcing effects.
Halocarbons that contain chlorine (CFCs, HCFCs, methyl
9 The range is the annual arithmetic averages from a mid-latitude Northern-
Hemisphere site and a mid-latitude Southern-Hemisphere site for October
2006 through September 2007 (CDIAC 2009).
10 The range is the annual arithmetic averages from a mid-latitude Northern-
Hemisphere site and a mid-latitude Southern-Hemisphere site for October
2006 through September 2007 (CDIAC 2009).
11 The stratosphere is the layer from the troposphere up to roughly 50
kilometers. In the lower regions the temperature is nearly constant but in the
upper layer the temperature increases rapidly because of sunlight absorption
by the ozone layer. The ozone layer is the part of the stratosphere from 19
kilometers up to 48 kilometers where the concentration of ozone reaches
up to 10 parts per million.
12 The troposphere is the layer from the ground up to 11 kilometers near
the poles and up to 16 kilometers in equatorial regions (i.e., the lowest
layer of the atmosphere, where people live). It contains roughly 80 percent
of the mass of all gases in the atmosphere and is the site for most weather
processes, including most of the water vapor and clouds.
1-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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chloroform, and carbon tetrachloride) and bromine (halons,
methyl bromide, and hydrobromofluorocarbons [HBFCs])
result in stratospheric ozone depletion and are therefore
controlled under the Montreal Protocol on Substances that
Deplete the Ozone Layer. Although CFCs and HCFCs
include potent global warming gases, their net radiative
forcing effect on the atmosphere is reduced because they
cause stratospheric ozone depletion, which itself is an
important greenhouse gas in addition to shielding the
earth from harmful levels of ultraviolet radiation. Under
the Montreal Protocol, the United States phased out the
production and importation of halons by 1994 and of CFCs by
1996. Under the Copenhagen Amendments to the Protocol, a
cap was placed on the production and importation of HCFCs
by non-Article 5 countries13 beginning in 1996, and then
followed by a complete phase-out by the year 2030. While
ozone depleting gases covered under the Montreal Protocol
and its Amendments are not covered by the UNFCCC; they
are reported in this Inventory under Annex 6.2 of this report
for informational purposes.
HFCs, PFCs, and SF6 are not ozone depleting substances,
and therefore are not covered under the Montreal Protocol.
They are, however, powerful greenhouse gases. HFCs
are primarily used as replacements for ozone depleting
substances but also emitted as a by-product of the HCFC-
22 manufacturing process. Currently, they have a small
aggregate radiative forcing impact, but it is anticipated that
their contribution to overall radiative forcing will increase
(IPCC 2001). PFCs and SF6 are predominantly emitted from
various industrial processes including aluminum smelting,
semiconductor manufacturing, electric power transmission
and distribution, and magnesium casting. Currently, the
radiative forcing impact of PFCs and SF6 is also small,
but they have a significant growth rate, extremely long
atmospheric lifetimes, and are strong absorbers of infrared
radiation, and therefore have the potential to influence climate
far into the future (IPCC 2001).
Carbon Monoxide. Carbon monoxide has an indirect
radiative forcing effect by elevating concentrations of CH4
13 Article 5 of the Montreal Protocol covers several groups of
countries, especially developing countries, with low consumption
rates of ozone depleting substances. Developing countries with per
capita consumption of less than 0.3 kg of certain ozone depleting
substances (weighted by their ozone depleting potential) receive
financial assistance and a grace period of ten additional years in
the phase-out of ozone depleting substances.
and tropospheric ozone through chemical reactions with
other atmospheric constituents (e.g., the hydroxyl radical,
OH) that would otherwise assist in destroying CH4 and
tropospheric ozone. Carbon monoxide is created when
carbon-containing fuels are burned incompletely. Through
natural processes in the atmosphere, it is eventually oxidized
to CO2. Carbon monoxide concentrations are both short-lived
in the atmosphere and spatially variable.
Nitrogen Oxides. The primary climate change effects of
nitrogen oxides (i.e., NO and NO2) are indirect and result
from their role in promoting the formation of ozone in the
troposphere and, to a lesser degree, lower stratosphere,
where it has positive radiative forcing effects.14 Additionally,
NOX emissions from aircraft are also likely to decrease CH4
concentrations, thus having a negative radiative forcing
effect (IPCC 1999). Nitrogen oxides are created from
lightning, soil microbial activity, biomass burning (both
natural and anthropogenic fires), fuel combustion, and,
in the stratosphere, from the photo-degradation of N2O.
Concentrations of NOX are both relatively short-lived in the
atmosphere and spatially variable.
Nonmethane Volatile Organic Compounds (NMVOCs).
Non-CHt volatile organic compounds include substances such
as propane, butane, and ethane. These compounds participate,
along with NOX, in the formation of tropospheric ozone
and other photochemical oxidants. NMVOCs are emitted
primarily from transportation and industrial processes, as
well as biomass burning and non-industrial consumption
of organic solvents. Concentrations of NMVOCs tend to be
both short-lived in the atmosphere and spatially variable.
Aerosols. Aerosols are extremely small particles or liquid
droplets found in the atmosphere. They can be produced by
natural events such as dust storms and volcanic activity, or by
anthropogenic processes such as fuel combustion and biomass
burning. Aerosols affect radiative forcing differently than
greenhouse gases, and their radiative effects occur through
direct and indirect mechanisms: directly by scattering and
absorbing solar radiation; and indirectly by increasing droplet
counts that modify the formation, precipitation efficiency, and
radiative properties of clouds. Aerosols are removed from
the atmosphere relatively rapidly by precipitation. Because
aerosols generally have short atmospheric lifetimes, and
14 NOX emissions injected higher in the stratosphere, primarily from fuel
combustion emissions from high altitude supersonic aircraft, can lead to
stratospheric ozone depletion.
Introduction 1-5
-------
have concentrations and compositions that vary regionally,
spatially, and temporally, their contributions to radiative
forcing are difficult to quantify (IPCC 2001).
The indirect radiative forcing from aerosols is typically
divided into two effects. The first effect involves decreased
droplet size and increased droplet concentration resulting
from an increase in airborne aerosols. The second effect
involves an increase in the water content and lifetime
of clouds due to the effect of reduced droplet size on
precipitation efficiency (IPCC 2001). Recent research has
placed a greater focus on the second indirect radiative forcing
effect of aerosols.
Various categories of aerosols exist, including
naturally produced aerosols such as soil dust, sea salt,
biogenic aerosols, sulfates, and volcanic aerosols, and
anthropogenically manufactured aerosols such as industrial
dust and carbonaceous aerosols15 (e.g., black carbon, organic
carbon) from transportation, coal combustion, cement
manufacturing, waste incineration, and biomass burning.
The net effect of aerosols on radiative forcing is believed
to be negative (i.e., net cooling effect on the climate),
although because they remain in the atmosphere for only days
to weeks, their concentrations respond rapidly to changes in
emissions.16 Locally, the negative radiative forcing effects
of aerosols can offset the positive forcing of greenhouse
gases (IPCC 1996). "However, the aerosol effects do not
cancel the global-scale effects of the much longer-lived
greenhouse gases, and significant climate changes can still
result" (IPCC 1996).
The IPCC's Third Assessment Report notes that "the
indirect radiative effect of aerosols is now understood to also
encompass effects on ice and mixed-phase clouds, but the
magnitude of any such indirect effect is not known, although
it is likely to be positive" (IPCC 2001). Additionally, current
research suggests that another constituent of aerosols, black
carbon, has a positive radiative forcing, and that its presence
"in the atmosphere above highly reflective surfaces such as
snow and ice, or clouds, may cause a significant positive
radiative forcing" (IPCC 2007). The primary anthropogenic
emission sources of black carbon include diesel exhaust and
open biomass burning.
Global Warming Potentials
A global warming potential is a quantified measure of
the globally averaged relative radiative forcing impacts of
a particular greenhouse gas (see Table 1-2). It is defined as
the ratio of the time-integrated radiative forcing from the
instantaneous release of 1 kilogram (kg) of a trace substance
relative to that of 1 kg of a reference gas (IPCC 2001).
Direct radiative effects occur when the gas itself absorbs
radiation. Indirect radiative forcing occurs when chemical
transformations involving the original gas produce a gas or
gases that are greenhouse gases, or when a gas influences
other radiatively important processes such as the atmospheric
lifetimes of other gases. The reference gas used is CO2,
and therefore GWP weighted emissions are measured in
teragrams of CO2 equivalent (Tg CO2 Eq.).17 The relationship
between gigagrams (Gg) of a gas and Tg CO2 Eq. can be
expressed as follows:
Tg
Tg CO2 Eq. = (Gg of gas) x (GWP) x
1,000 Gg
15 Carbonaceous aerosols are aerosols that are comprised mainly of organic
substances and forms of black carbon (or soot) (IPCC 2001).
16 Volcanic activity can inject significant quantities of aerosol-producing
sulfur dioxide and other sulfur compounds into the stratosphere, which can
result in a longer negative forcing effect (i.e., a few years) (IPCC 1996).
where,
Tg CO2 Eq. = Teragrams of CO2 equivalent
Gg = Gigagrams (equivalent to a thousand
metric tons)
GWP = Global warming potential
Tg = Teragrams
GWP values allow for a comparison of the impacts of
emissions and reductions of different gases. According to the
IPCC, GWPs typically have an uncertainty of ±35 percent.
The parties to the UNFCCC have also agreed to use GWPs
based upon a 100-year time horizon although other time
horizon values are available.
Greenhouse gas emissions and removals should
be presented on a gas-by-gas basis in units of
mass... In addition, consistent with decision 2/
CP.3, Parties should report aggregate emissions
and removals of greenhouse gases, expressed in
C02 equivalent terms at summary inventory level,
17 Carbon comprises 12/44ths of carbon dioxide by weight.
1-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 1-2: Global Warming Potentials and Atmospheric
Lifetimes (Years) Used in this Report
Gas
C02
CH4b
N20
HFC-23
HFC-32
HFC-125
HFC-134a
HFC-143a
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
CF4
C2F6
C^F-io
C6Fi4
SF6
Atmospheric
Lifetime
50-200
12±3
120
264
5.6
32.6
14.6
48.3
1.5
36.5
209
17.1
50,000
10,000
2,600
3,200
3,200
GWPa
1
21
310
11,700
650
2,800
1,300
3,800
140
2,900
6,300
1,300
6,500
9,200
7,000
7,400
23,900
Source: (IPCC1996).
a 100-year time horizon.
b The GWP of CH4 includes the direct effects and those indirect
effects due to the production of tropospheric ozone and stratospheric
water vapor. The indirect effect due to the production of C02 is not
included.
using GWP values provided by the IPCC in its
Second Assessment Report... based on the effects
of greenhouse gases over a 100-year time horizon.n
Greenhouse gases with relatively long atmospheric
lifetimes (e.g., CO2, CH4, N2O, HFCs, PFCs, and SF6)
tend to be evenly distributed throughout the atmosphere,
and consequently global average concentrations can be
determined. The short-lived gases such as water vapor, carbon
monoxide, tropospheric ozone, ozone precursors (e.g., NOX
andNMVOCs), and tropospheric aerosols (e.g., SO2 products
and carbonaceous particles), however, vary regionally, and
consequently it is difficult to quantify their global radiative
forcing impacts. No GWP values are attributed to these
18 Framework Convention on Climate Change; ; 1 November 2002; Report of the Conference of the
Parties at its eighth session; held at New Delhi from 23 October to 1
November 2002; Addendum; Part One: Action taken by the Conference
of the Parties at its eighth session; Decision -/CP.8; Communications
from Parties included in Annex I to the Convention: Guidelines for the
Preparation of National Communications by Parties Included in Annex I to
the Convention, Part 1: UNFCCC reporting guidelines on annual inventories:
p. 7 (UNFCCC 2003).
gases that are short-lived and spatially inhomogeneous in
the atmosphere.
1.2. Institutional Arrangements
The U.S. Environmental Protection Agency (EPA), in
cooperation with other U.S. government agencies, prepares
the Inventory of U.S. Greenhouse Gas Emissions and Sinks.
A wide range of agencies and individuals are involved in
supplying data to, reviewing, or preparing portions of the
U.S. Inventory—including federal and state government
authorities, research and academic institutions, industry
associations, and private consultants.
Within EPA, the Office of Atmospheric Programs (OAP)
is the lead office responsible for the emission calculations
provided in the Inventory, as well as the completion of the
National Inventory Report and the Common Reporting
Format tables. The Office of Transportation and Air Quality
(OTAQ) is also involved in calculating emissions for the
Inventory. While the U.S. Department of State officially
submits the annual Inventory to the UNFCCC, EPA's
OAP serves as the focal point for technical questions and
comments on the U.S. Inventory. The staff of OAP and
OTAQ coordinates the annual methodological choice, activity
data collection, and emission calculations at the individual
source category level. Within OAP, an inventory coordinator
compiles the entire Inventory into the proper reporting
format for submission to the UNFCCC, and is responsible
for the collection and consistency of cross-cutting issues in
the Inventory.
Several other government agencies contribute to the
collection and analysis of the underlying activity data
used in the Inventory calculations. Formal relationships
exist between EPA and other U.S. agencies that provide
official data for use in the Inventory. The U.S. Department
of Energy's Energy Information Administration provides
national fuel consumption data and the U.S. Department of
Defense provides military fuel consumption and bunker fuels.
Informal relationships also exist with other U.S. agencies to
provide activity data for use in EPA's emission calculations.
These include: the U.S. Department of Agriculture, the U.S.
Geological Survey, the Federal Highway Administration, the
Department of Transportation, the Bureau of Transportation
Statistics, the Department of Commerce, the National
Agricultural Statistics Service, and the Federal Aviation
Introduction 1-7
-------
Box 1-1: The IPCC Fourth Assessment Report and Global Warming Potentials
In 2007, the IPCC published its Fourth Assessment Report (AR4), which provided an updated and more comprehensive scientific
assessment of climate change. Within this report, the GWPs of several gases were revised relative to the SAR and the IPCC's Third
Assessment Report (TAR) (IPCC 2001). Thus the GWPs used in this report have been updated twice by the IPCC; although the SAR GWPs
are used throughout this report, it is interesting to review the changes to the GWPs and the impact such improved understanding has on the
total GWP-weighted emissions of the United States. Since the SAR and TAR, the IPCC has applied an improved calculation of C02 radiative
forcing and an improved C02 response function. The GWPs are drawn from IPCC/TEAP (2005) and the TAR, with updates for those cases
where new laboratory or radiative transfer results have been published. Additionally, the atmospheric lifetimes of some gases have been
recalculated. In addition, the values for radiative forcing and lifetimes have been recalculated for a variety of halocarbons, which were not
presented in the SAR. Table 1-3 presents the new GWPs, relative to those presented in the SAR.
Table 1-3: Comparison of 100-Year GWPs
Gas
C02
CH4*
N20
HFC-23
HFC-32
HFC-125
HFC-134a
HFC-143a
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
CF4
C2F6
C^F-io
CeF-i4
SF6
SAR
1
21
310
11,700
650
2,800
1,300
3,800
140
2,900
6,300
1,300
6,500
9,200
7,000
7,400
23,900
TAR
1
23
296
12,000
550
3,400
1,300
4,300
120
3,500
9,400
1,500
5,700
11,900
8,600
9,000
22,200
AR4
1
25
298
14,800
675
3,500
1,430
4,470
124
3,220
9,810
1,640
7,390
12,200
8,860
9,300
22,800
Change from
TAR
NC
2
(14)
300
(100)
600
NC
500
(20)
600
3,100
200
(800)
2,700
1,600
1,600
(1,700)
SAR
AR4
0
4
(12)
3,100
25
700
130
670
(16)
320
3,510
340
890
3,000
1,860
1,900
(1,100)
Source: (IPCC 2007, IPCC 2001).
NC (No Change).
Note: Parentheses indicate negative values.
* The GWP of CH4 includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor. The
indirect effect due to the production of C02 is not included.
To comply with international reporting standards under the UNFCCC, official emission estimates are reported by the United States using
SAR GWP values. The UNFCCC reporting guidelines for national inventories19 were updated in 2002 but continue to require the use of GWPs
from the SAR so that current estimates of aggregate greenhouse gas emissions for 1990 through 2008 are consistent and comparable
with estimates developed prior to the publication of the TAR and AR4. For informational purposes, emission estimates that use the updated
GWPs are presented in detail in Annex 6.1 of this report. All estimates provided throughout this report are also presented in unweighted units.
'See
Administration. Academic and research centers also provide
activity data and calculations to EPA, as well as individual
companies participating in voluntary outreach efforts with
EPA. Finally, the U.S. Department of State officially submits
the Inventory to the UNFCCC each April.
1.3. Inventory Process
EPA has a decentralized approach to preparing the
annual U.S. Inventory, which consists of a National Inventory
Report (NIR) and Common Reporting Format (CRF)
tables. The Inventory coordinator at EPA is responsible for
compiling all emission estimates, and ensuring consistency
1-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
and quality throughout the NIR and CRF tables. Emission
calculations for individual sources are the responsibility
of individual source leads, who are most familiar with
each source category and the unique characteristics of its
emissions profile. The individual source leads determine the
most appropriate methodology and collect the best activity
data to use in the emission calculations, based upon their
expertise in the source category, as well as coordinating
with researchers and contractors familiar with the sources.
A multi-stage process for collecting information from the
individual source leads and producing the Inventory is
undertaken annually to compile all information and data.
Methodology Development, Data
Collection, and Emissions and
Sink Estimation
Source leads at EPA collect input data and, as necessary,
evaluate or develop the estimation methodology for the
individual source categories. For most source categories,
the methodology for the previous year is applied to the
new "current" year of the Inventory, and inventory analysts
collect any new data or update data that have changed from
the previous year. If estimates for a new source category are
being developed for the first time, or if the methodology is
changing for an existing source category (e.g., the United
States is implementing a higher tiered approach for that
source category), then the source category lead will develop
a new methodology, gather the most appropriate activity
data and emission factors (or in some cases direct emission
measurements) for the entire time series, and conduct a
special source-specific peer review process involving relevant
experts from industry, government, and universities.
Once the methodology is in place and the data are
collected, the individual source leads calculate emissions and
sink estimates. The source leads then update or create the
relevant text and accompanying annexes for the Inventory.
Source leads are also responsible for completing the relevant
sectoral background tables of the Common Reporting
Format, conducting quality assurance and quality control
(QA/QC) checks, and uncertainty analyses.
Summary Spreadsheet Compilation and
Data Storage
The inventory coordinator at EPA collects the source
categories' descriptive text and Annexes, and also aggregates
the emission estimates into a summary spreadsheet that
links the individual source category spreadsheets together.
This summary sheet contains all of the essential data in
one central location, in formats commonly used in the
Inventory document. In addition to the data from each source
category, national trend and related data are also gathered
in the summary sheet for use in the Executive Summary,
Introduction, and Recent Trends sections of the inventory
report. Electronic copies of each year's summary spreadsheet,
which contains all the emission and sink estimates for the
United States, are kept on a central server at EPA under the
jurisdiction of the inventory coordinator.
National Inventory Report Preparation
The NIR is compiled from the sections developed
by each individual source lead. In addition, the inventory
coordinator prepares a brief overview of each chapter that
summarizes the emissions from all sources discussed in the
chapters. The inventory coordinator then carries out a key
category analysis for the Inventory, consistent with the IPCC
Good Practice Guidance, IPCC Good Practice Guidance for
Land Use, Land Use Change and Forestry, and in accordance
with the reporting requirements of the UNFCCC. Also at
this time, the Introduction, Executive Summary, and Recent
Trends sections are drafted, to reflect the trends for the most
recent year of the current Inventory. The analysis of trends
necessitates gathering supplemental data, including weather
and temperature conditions, economic activity and gross
domestic product, population, atmospheric conditions, and
the annual consumption of electricity, energy, and fossil
fuels. Changes in these data are used to explain the trends
observed in greenhouse gas emissions in the United States.
Furthermore, specific factors that affect individual sectors
are researched and discussed. Many of the factors that affect
emissions are included in the inventory document as separate
analyses or side discussions in boxes within the text. Text
boxes are also created to examine the data aggregated in
different ways than in the remainder of the document, such
as a focus on transportation activities or emissions from
Introduction 1-9
-------
electricity generation. The document is prepared to match
the specification of the UNFCCC reporting guidelines for
National Inventory Reports.
Common Reporting Format
Table Compilation
The CRF tables are compiled from individual tables
completed by each individual source lead, which contain
source emissions and activity data. The inventory coordinator
integrates the source data into the UNFCCC's "CRF
Reporter" for the United States, assuring consistency across
all sectoral tables. The summary reports for emissions,
methods, and emission factors used, the overview tables
for completeness and quality of estimates, the recalculation
tables, the notation key completion tables, and the emission
trends tables are then completed by the inventory coordinator.
Internal automated quality checks on the CRF Reporter, as
well as reviews by the source leads, are completed for the
entire time series of CRF tables before submission.
QA/QC and Uncertainty
QA/QC and uncertainty analyses are supervised by the
QA/QC and Uncertainty coordinators, who have general
oversight over the implementation of the QA/QC plan and
the overall uncertainty analysis for the Inventory (see sections
on QA/QC and Uncertainty, below). These coordinators work
closely with the source leads to ensure that a consistent QA/
QC plan and uncertainty analysis is implemented across all
inventory sources. The inventory QA/QC plan, detailed in
a following section, is consistent with the quality assurance
procedures outlined by EPA and IPCC.
Expert and Public Review Periods
During the Expert Review period, a first draft of the
document is sent to a select list of technical experts outside
of EPA. The purpose of the Expert Review is to encourage
feedback on the methodological and data sources used in
the current Inventory, especially for sources which have
experienced any changes since the previous Inventory.
Once comments are received and addressed, a second
draft of the document is released for public review by
publishing a notice in the U.S. Federal Register and posting
the document on the EPA Web site. The Public Review
period allows for a 30 day comment period and is open to
the entire U.S. public.
Final Submittal to UNFCCC and
Document Printing
After the final revisions to incorporate any comments
from the Expert Review and Public Review periods,
EPA prepares the final National Inventory Report and
the accompanying Common Reporting Format Reporter
database. The U.S. Department of State sends the official
submission of the U.S. Inventory to the UNFCCC. The
document is then formatted for printing, posted online,
printed by the U.S. Government Printing Office, and made
available for the public.
1.4. Methodology and Data Sources
Emissions of greenhouse gases from various source and
sink categories have been estimated using methodologies
that are consistent with the Revised 1996IPCC Guidelines
for National Greenhouse Gas Inventories (IPCC/UNEP/
OECD/IEA 1997). In addition, the United States references
the additional guidance provided in the IPCC Good Practice
Guidance and Uncertainty Management in National
Greenhouse Gas Inventories (IPCC 2000), the IPCC Good
Practice Guidance for Land Use, Land-Use Change, and
Forestry (IPCC 2003), and the 2006 IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC 2006). To the
extent possible, the present report relies on published activity
and emission factor data. Depending on the emission source
category, activity data can include fuel consumption or
deliveries, vehicle-miles traveled, raw material processed,
etc. Emission factors are factors that relate quantities of
emissions to an activity.
The IPCC methodologies provided in the Revised
1996 IPCC Guidelines represent baseline methodologies
for a variety of source categories, and many of these
methodologies continue to be improved and refined as new
research and data become available. This report uses the
IPCC methodologies when applicable, and supplements them
with other available methodologies and data where possible.
Choices made regarding the methodologies and data sources
used are provided in conjunction with the discussion of each
source category in the main body of the report. Complete
documentation is provided in the annexes on the detailed
methodologies and data sources utilized in the calculation
of each source category.
1-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Box 1-2: IPCC Reference Approach
The UNFCCC reporting guidelines require countries to
complete a "top-down" reference approach for estimating C02
emissions from fossil fuel combustion in addition to their "bottom-
up" sectoral methodology. This estimation method uses alternative
methodologies and different data sources than those contained
in that section of the Energy chapter. The reference approach
estimates fossil fuel consumption by adjusting national aggregate
fuel production data for imports, exports, and stock changes rather
than relying on end-user consumption surveys (see Annex 4 of
this report). The reference approach assumes that once carbon-
based fuels are brought into a national economy, they are either
saved in some way (e.g., stored in products, kept in fuel stocks,
or left unoxidized in ash) or combusted, and therefore the carbon
in them is oxidized and released into the atmosphere. Accounting
for actual consumption of fuels at the sectoral or sub-national
level is not required.
1.5. Key Categories
The IPCC's Good Practice Guidance (IPCC 2000)
defines a key category as a "[source or sink category] that
is prioritized within the national inventory system because
its estimate has a significant influence on a country's total
inventory of direct greenhouse gases in terms of the absolute
level of emissions, the trend in emissions, or both."20 By
definition, key categories include those sources that have
the greatest contribution to the absolute level of national
emissions. In addition, when an entire time series of
emission estimates is prepared, a thorough investigation of
key categories must also account for the influence of trends
and uncertainties of individual source and sink categories.
This analysis culls out source and sink categories that
diverge from the overall trend in national emissions. Finally,
a qualitative evaluation of key categories is performed to
capture any categories that were not identified in any of the
quantitative analyses.
A Tier 1 approach, as defined in the IPCC's Good
Practice Guidance (IPCC 2000), was implemented to
identify the key categories for the United States. This analysis
was performed twice; one analysis included sources and
sinks from the Land Use, Land-Use Change, and Forestry
(LULUCF) sector, the other analysis did not include the
LULUCF categories. Following the Tier 1 approach, a
Tier 2 approach, as defined in the IPCC's Good Practice
Guidance (IPCC 2000), was then implemented to identify
any additional key categories not already identified in the
Tier 1 assessment. This analysis, which includes each
source category's uncertainty assessments (or proxies) in its
calculations, was also performed twice to include or exclude
LULUCF sources.
In addition to conducting Tier 1 and 2 level and trend
assessments, a qualitative assessment of the source categories,
as described in the IPCC's Good Practice Guidance (IPCC
2000), was conducted to capture any key categories that were
not identified by either quantitative method. One additional
key category, international bunker fuels, was identified
using this qualitative assessment. International bunker fuels
are fuels consumed for aviation or marine international
transport activities, and emissions from these fuels are
reported separately from totals in accordance with IPCC
guidelines. If these emissions were included in the totals,
bunker fuels would qualify as a key category according to
the Tier 1 approach. The amount of uncertainty associated
with estimation of emissions from international bunker fuels
also supports the qualification of this source category as
key, because it would qualify bunker fuels as a key category
according to the Tier 2 approach.
Table 1-4 presents the key categories for the United
States (including and excluding LULUCF categories) using
emissions and uncertainty data in this report, and ranked
according to their sector and global warming potential-
weighted emissions in 2008. The table also indicates the
criteria used in identifying these categories (i.e., level, trend,
Tier 1, Tier 2, and/or qualitative assessments). Annex 1 of
this report provides additional information regarding the key
categories in the United States and the methodologies used
to identify them.
20 See Chapter 7 "Methodological Choice and Recalculation" in IPCC
(2000).
Introduction 1-11
-------
Table 1-4: Key Categories for the United States (1990-2008)
IPCC Source Categories
Energy
C02 Emissions from Stationary Combustion - Coal
C02 Emissions from Mobile Combustion:
Road & Other
C02 Emissions from Stationary Combustion - Gas
C02 Emissions from Stationary Combustion - Oil
C02 Emissions from Mobile Combustion: Aviation
C02 Emissions from Non-Energy Use of Fuels
C02 Emissions from Mobile Combustion: Marine
C02 Emissions from Natural Gas Systems
Fugitive CH4 Emissions from Natural Gas Systems
Fugitive CH4 Emissions from Coal Mining
Fugitive CH4 Emissions from Petroleum Systems
N20 Emissions from Mobile Combustion:
Road & Other
Non-C02 Emissions from Stationary Combustion
International Bunker Fuelsb
Industrial Processes
C02 Emissions from Iron and Steel Production &
Metallurgical Coke Production
C02 Emissions from Cement Production
C02 Emissions from Ammonia Production and
Urea Consumption
N20 Emissions from Adipic Acid Production
Emissions from Substitutes for Ozone
Depleting Substances
HFC-23 Emissions from HCFC-22 Production
SF6 Emissions from Electrical Transmission
and Distribution
PFC Emissions from Aluminum Production
Agriculture
CH4 Emissions from Enteric Fermentation
CH4 Emissions from Manure Management
Direct N20 Emissions from Agricultural
Soil Management
Indirect N20 Emissions from Applied Nitrogen
N20 Emissions from Manure Management
Waste
CH4 Emissions from Landfills
Land Use, Land Use Change, and Forestry
C02 from Changes in Forest Carbon Stocks
C02 Emissions from Urban Trees
Gas
C02
C02
C02
C02
C02
C02
C02
C02
CH4
CH4
CH4
N20
N20
Several
C02
C02
C02
N20
Several
MFCs
SF6
PFCs
CH4
CH4
N20
N20
N20
CH4
C02
C02
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(93.9)
1-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 1-4: Key Categories for the United States (1990-2008) (continued)
IPCC Source Categories
C02 Emissions from Cropland Remaining Cropland
C02 Emissions from Landfilled Yard Trimmings
and Food Scraps
C02 Emissions from Grassland Remaining Grassland
CH4 Emissions from Forest Fires
N20 Emissions from Forest Fires
Subtotal Without LULUCF
Total Emissions Without LULUCF
Percent of Total Without LULUCF
Subtotal With LULUCF
Total Emissions With LULUCF
Percent of Total With LULUCF
Gas
C02
C02
C02
CH4
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Qualitative criteria.
bEmissions from this source not included in totals.
Note: Parentheses indicate negative values (or sequestration).
1.6. Quality Assurance and Quality
Control (QA/QC)
As part of efforts to achieve its stated goals for inventory
quality, transparency, and credibility, the United States has
developed a quality assurance and quality control plan
designed to check, document and improve the quality of
its inventory over time. QA/QC activities on the Inventory
are undertaken within the framework of the U.S. QA/QC
plan, Quality Assurance/Quality Control and Uncertainty
Management Plan for the U.S. Greenhouse Gas Inventory:
Procedures Manual for QA/QC and Uncertainty Analysis.
Key attributes of the QA/QC plan are summarized in
Figure 1-1. These attributes include:
• specific detailed procedures and forms that serve to
standardize the process of documenting and archiving
information, as well as to guide the implementation
of QA/QC and the analysis of the uncertainty of the
inventory estimates;
• expert review as well as QC—for both the inventory
estimates and the Inventory (which is the primary
vehicle for disseminating the results of the inventory
development process). In addition, the plan provides
for public review of the Inventory;
both Tier 1 (general) and Tier 2 (source-specific) quality
controls and checks, as recommended by IPCC Good
Practice Guidance;
consideration of secondary data quality and source-
specific quality checks (Tier 2 QC) in parallel and
coordination with the uncertainty assessment; the
development of protocols and templates provides for
more structured communication and integration with the
suppliers of secondary information;
record-keeping provisions to track which procedures
have been followed, and the results of the QA/QC
and uncertainty analysis, and contains feedback
mechanisms for corrective action based on the results
of the investigations, thereby providing for continual
data quality improvement and guided research efforts;
implementation of QA/QC procedures throughout the
whole inventory development process—from initial
data collection, through preparation of the emission
estimates, to publication of the Inventory;
a schedule for multi-year implementation; and
promotion of coordination and interaction within the
EPA, across Federal agencies and departments, state
government programs, and research institutions and
consulting firms involved in supplying data or preparing
Introduction 1-13
-------
Figure 1-1
U.S. QA/QC Plan Summary
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• Obtain data in electronic
format (if possible)
• Review spreadsheet
construction
• Avoid hardwiring
• Use data validation
• Protect cells
• Develop automatic
checkers for:
• Outliers, negative
values, or missing data
• Variable types match
values
• Time series
consistency
• Contact reports for
non-electronic
communications
• Provide cell references for
primary data elements
• Obtain copies of all data
sources
• List and location of
any working/external
spreadsheets
• Document assumptions
• Clearly label parameters,
units, and conversions
factors
• Review spreadsheet
integrity
• Equations
• Units
• Input and output
• Develop automated
checkers for:
• Input ranges
• Calculations
• Emission aggregation
• Common starting versions
for each Inventory year
• Utilize unalterable
summary tab for each
source spreadsheet
NT for linking to a master
• Maintain tracking tab for
status of gathering efforts
n IT
• Check input data for • Check citations in • Reproduce calculations
transcription errors
• Inspect automatic
checkers
• Identify spreadsheet
modifications that could
provide additional QA/QC
checks
spreadsheet and text for
accuracy and style
• Check reference docket for
new citations
• Review documentation
for any data/methodology
changes
• Review time series for
consistency
• Review changes in
data/consistency with
IPCC methodology
summary spreadsheet
• Follow strict version
control procedures
• Document QA/QC
procedures
Data Gathering Data Documentation Calculating Emissions Cross-Cutting Coordination
estimates for the inventory. The QA/QC plan itself is
intended to be revised and reflect new information that
becomes available as the program develops, methods
are improved, or additional supporting documents
become necessary.
In addition, based on the national QA/QC plan for
the Inventory, source-specific QA/QC plans have been
developed for a number of sources. These plans follow the
procedures outlined in the national QA/QC plan, tailoring
the procedures to the specific text and spreadsheets of the
individual sources. For each greenhouse gas emissions source
or sink included in this Inventory, a minimum of a Tier 1Q A/
QC analysis has been undertaken. Where QA/QC activities
for a particular source go beyond the minimum Tier 1 level,
further explanation is provided within the respective source
category text.
The quality control activities described in the U.S. QA/
QC plan occur throughout the inventory process; QA/QC
is not separate from, but is an integral part of, preparing
the Inventory. Quality control—in the form of both good
practices (such as documentation procedures) and checks on
whether good practices and procedures are being followed—
is applied at every stage of inventory development and
document preparation. In addition, quality assurance occurs
at two stages—an expert review and a public review. While
both phases can significantly contribute to inventory quality,
the public review phase is also essential for promoting the
openness of the inventory development process and the
transparency of the inventory data and methods.
The QA/QC plan guides the process of ensuring
inventory quality by describing data and methodology
checks, developing processes governing peer review and
public comments, and developing guidance on conducting
1-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
an analysis of the uncertainty surrounding the emission
estimates. The QA/QC procedures also include feedback
loops and provide for corrective actions that are designed to
improve the inventory estimates over time.
1.7. Uncertainty Analysis of
Emission Estimates
Uncertainty estimates are an essential element of a
complete and transparent emissions inventory. Uncertainty
information is not intended to dispute the validity of the
inventory estimates, but to help prioritize efforts to improve
the accuracy of future inventories and guide future decisions
on methodological choice. While the U.S. Inventory calculates
its emission estimates with the highest possible accuracy,
uncertainties are associated to a varying degree with the
development of emission estimates for any inventory. Some
of the current estimates, such as those for CO2 emissions from
energy-related activities, are considered to have minimal
uncertainty associated with them. For some other categories
of emissions, however, a lack of data or an incomplete
understanding of how emissions are generated increases the
uncertainty surrounding the estimates presented. Despite
these uncertainties, the UNFCCC reporting guidelines follow
the recommendation in the 1996 IPCC Guidelines (IPCC/
UNEP/OECD/IEA1997) and require that countries provide
single point estimates for each gas and emission or removal
source category. Within the discussion of each emission
source, specific factors affecting the uncertainty associated
with the estimates are discussed.
Additional research in the following areas could help
reduce uncertainty in the U.S. Inventory:
• Incorporating excluded emission sources. Quantitative
estimates for some of the sources and sinks of greenhouse
gas emissions are not available at this time. In particular,
emissions from some land-use activities and industrial
processes are not included in the Inventory either
because data are incomplete or because methodologies
do not exist for estimating emissions from these source
categories. See Annex 5 of this report for a discussion
of the sources of greenhouse gas emissions and sinks
excluded from this report.
• Improving the accuracy of emission factors. Further
research is needed in some cases to improve the accuracy
of emission factors used to calculate emissions from a
variety of sources. For example, the accuracy of current
emission factors applied to CH4 and N2O emissions from
stationary and mobile combustion is highly uncertain.
• Collecting detailed activity data. Although methodologies
exist for estimating emissions for some sources,
problems arise in obtaining activity data at a level
of detail in which aggregate emission factors can be
applied. For example, the ability to estimate emissions
of SF6 from electrical transmission and distribution is
limited due to a lack of activity data regarding national
SF6 consumption or average equipment leak rates.
The overall uncertainty estimate for the U.S. greenhouse
gas emissions Inventory was developed using the IPCC
Tier 2 uncertainty estimation methodology. Estimates of
quantitative uncertainty for the overall greenhouse gas
emissions inventory are shown below, in Table 1-5.
The IPCC provides good practice guidance on two
approaches—Tier 1 and Tier 2—to estimating uncertainty
for individual source categories. Tier 2 uncertainty analysis,
employing the Monte Carlo Stochastic Simulation technique,
was applied wherever data and resources permitted; further
explanation is provided within the respective source category
text and in Annex 7. Consistent with the IPCC Good Practice
Guidance (IPCC 2000), over a multi-year timeframe, the
United States expects to continue to improve the uncertainty
estimates presented in this report.
Emissions calculated for the U.S. Inventory reflect
current best estimates; in some cases, however, estimates
are based on approximate methodologies, assumptions, and
incomplete data. As new information becomes available in
the future, the United States will continue to improve and
revise its emission estimates. See Annex 7 of this report for
further details on the U.S. process for estimating uncertainty
associated with the emission estimates and for a more detailed
discussion of the limitations of the current analysis and
plans for improvement. Annex 7 also includes details on the
uncertainty analysis performed for selected source categories.
Introduction 1-15
-------
Table 1-5: Estimated Overall Inventory Quantitative Uncertainty (Tg C02 Eq. and Percent)
2008 Emission Estimate3 Uncertainty Range Relative to Emission Estimate11 Mean0 Standard Deviation0
Gas (TgC02Eq.) (TgC02Eq.) (%) (Tg C02 Eq.)
C02
CH4e
N20e
RFC, HFC & SF6e
Total
Net Emissions
(Sources and Sinks)
5,920.8
567.1
314.3
146.7
6,949.0
6,008.6
Lower Boundd
5,828.0
503.8
280.3
144.0
6,887.2
5,898.9
Upper Boundd
6,234.1
662.9
460.3
162.2
7,360.4
6,448.4
Lower Bound
-2%
-11%
-11%
-2%
-1%
-2%
Upper Bound
+5%
+ 17%
+46%
+ 11%
+6%
+7%
6,027.2
576.4
360.8
153.1
7,117.5
6,174.1
104.0
39.6
46.1
4.6
120.9
142.1
"Emission estimates reported in this table correspond to emissions from only those source categories for which quantitative uncertainty was performed
this year. Thus, the totals reported in this table exclude approximately 7.8 Tg C02 Eq. of emissions for which quantitative uncertainty was not assessed.
Hence, these emission estimates do not match the final total U.S. greenhouse gas emission estimates presented in this Inventory.
b The lower and upper bounds for emission estimates correspond to a 95 percent confidence interval, with the lower bound corresponding to 2.5th
percentile and the 95th percentile corresponding to 97.5th percentile.
c Mean value indicates the arithmetic average of the simulated emission estimates; standard deviation indicates the extent of deviation of the simulated
values from the mean.
d The lower and upper bound emission estimates for the sub-source categories do not sum to total emissions because the low and high estimates for total
emissions were calculated separately through simulations.
e The overall uncertainty estimates did not take into account the uncertainty in the GWP values for CH4, N20 and high GWP gases used in the inventory
emission calculations for 2008.
1.8. Completeness
This report, along with its accompanying CRF reporter,
serves as a thorough assessment of the anthropogenic sources
and sinks of greenhouse gas emissions for the United States
for the time series 1990 through 2008. Although this report
is intended to be comprehensive, certain sources have been
identified yet excluded from the estimates presented for
various reasons. Generally speaking, sources not accounted
for in this inventory are excluded due to data limitations or
a lack of thorough understanding of the emission process.
The United States is continually working to improve upon the
understanding of such sources and seeking to find the data
required to estimate related emissions. As such improvements
are implemented, new emission sources are quantified and
included in the Inventory. For a complete list of sources
excluded, see Annex 5 of this report.
1.9. Organization of Report
In accordance with the Revised 1996IPCC Guidelines
for National Greenhouse Gas Inventories (IPCC/UNEP/
OECD/IEA 1997), and the 2003 UNFCCC Guidelines on
Reporting and Review (UNFCCC 2003), this Inventory of
U.S. Greenhouse Gas Emissions and Sinks is segregated
into six sector-specific chapters, listed below in Table 1-6. In
addition, chapters on Trends in Greenhouse Gas Emissions
and Other information to be considered as part of the U.S.
Inventory submission are included.
Within each chapter, emissions are identified by the
anthropogenic activity that is the source or sink of the
greenhouse gas emissions being estimated (e.g., coal mining).
Overall, the following organizational structure is consistently
applied throughout this report:
1-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Chapter/IPCC Sector: Overview of emission trends for
each IPCC denned sector
Source category: Description of source pathway and
emission trends.
Methodology: Description of analytical methods
employed to produce emission estimates and
identification of data references, primarily for
activity data and emission factors.
Uncertainty: A discussion and quantification of the
uncertainty in emission estimates and a discussion
of time-series consistency.
QA/QC Verification: A discussion on steps taken to
QA/QC and verify the emission estimates, where
beyond the overall U.S. QA/QC plan, and any key
findings.
Recalculations: A discussion of any data or
methodological changes that necessitate a
recalculation of previous years' emission estimates,
and the impact of the recalculation on the emission
estimates, if applicable.
Planned Improvements: A discussion on any source-
specific planned improvements, if applicable.
Special attention is given to CO2 from fossil fuel
combustion relative to other sources because of its share of
emissions and its dominant influence on emission trends.
For example, each energy consuming end-use sector (i.e.,
residential, commercial, industrial, and transportation),
as well as the electricity generation sector, is described
individually. Additional information for certain source
categories and other topics is also provided in several
Annexes listed in Table 1-7.
Table 1-6: IPCC Sector Descriptions
Chapter/IPCC Sector Activities Included
Energy
Industrial Processes
Solvent and Other Product Use
Agriculture
Land Use, Land-Use Change, and
Forestry
Waste
Emissions of all greenhouse gases resulting from stationary and mobile energy activities
including fuel combustion and fugitive fuel emissions.
By-product or fugitive emissions of greenhouse gases from industrial processes not directly
related to energy activities such as fossil fuel combustion.
Emissions, of primarily NMVOCs, resulting from the use of solvents and N20 from product uses.
Anthropogenic emissions from agricultural activities except fuel combustion, which is
addressed under Energy.
Emissions and removals of C02, CH4, and N20 from forest management, other land-use
activities, and land-use change.
Emissions from waste management activities.
Source: IPCC/UNEP/OECD/IEA (1997)
Introduction 1-17
-------
Table 1-7: List of Annexes
ANNEX 1 Key Category Analysis
ANNEX 2 Methodology and Data for Estimating C02 Emissions from Fossil Fuel Combustion
2.1. Methodology for Estimating Emissions of C02 from Fossil Fuel Combustion
2.2. Methodology for Estimating the Carbon Content of Fossil Fuels
2.3. Methodology for Estimating Carbon Emitted from Non-Energy Uses of Fossil Fuels
ANNEX 3 Methodological Descriptions for Additional Source or Sink Categories
3.1. Methodology for Estimating Emissions of CH4, N20, and Indirect Greenhouse Gases from Stationary Combustion
3.2. Methodology for Estimating Emissions of CH4, N20, and Indirect Greenhouse Gases from Mobile Combustion and
Methodology for and Supplemental Information on Transportation-Related Greenhouse Gas Emissions
3.3. Methodology for Estimating CH4 Emissions from Coal Mining
3.4. Methodology for Estimating CH4 Emissions from Natural Gas Systems
3.5. Methodology for Estimating CH4 and C02 Emissions from Petroleum Systems
3.6. Methodology for Estimating C02 and N20 Emissions from Incineration of Waste
3.7. Methodology for Estimating Emissions from International Bunker Fuels used by the U.S. Military
3.8. Methodology for Estimating HFC and RFC Emissions from Substitution of Ozone Depleting Substances
3.9. Methodology for Estimating CH4 Emissions from Enteric Fermentation
3.10. Methodology for Estimating CH4 and N20 Emissions from Manure Management
3.11. Methodology for Estimating N20 Emissions from Agricultural Soil Management
3.12. Methodology for Estimating Net Carbon Stock Changes in Forest Lands Remaining Forest Lands
3.13. Methodology for Estimating Net Changes in Carbon Stocks in Mineral and Organic Soils on Croplands and Grasslands
3.14. Methodology for Estimating CH4 Emissions from Landfills
ANNEX 4 IPCC Reference Approach for Estimating C02 Emissions from Fossil Fuel Combustion
ANNEX 5 Assessment of the Sources and Sinks of Greenhouse Gas Emissions Excluded
ANNEX 6 Additional Information
6.1. Global Warming Potential Values
6.2. Ozone Depleting Substance Emissions
6.3. Sulfur Dioxide Emissions
6.4. Complete List of Source Categories
6.5. Constants, Units, and Conversions
6.6. Abbreviations
6.7. Chemical Formulas
ANNEX 7 Uncertainty
7.1. Overview
7.2. Methodology and Results
7.3. Planned Improvements
7.4. Additional Information on Uncertainty Analyses by Source
1-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
2. Trends in Greenhouse Gas
Emissions
2.1. Recent Trends in U.S. Greenhouse Gas Emissions and Sinks
In 2008, total U.S. greenhouse gas emissions were 6,956.8 teragrams of carbon dioxide equivalents (Tg CO2 Eq.);
net emissions were 6,016.4 Tg CO2 Eq. reflecting the influence of sinks (net CO2 flux from Land Use, Land Use
Change, and Forestry).1 Overall, total U.S. emissions have risen by almost 14 percent from 1990 to 2008. Emissions
decreased from 2007 to 2008 by 2.9 percent (211.3 Tg CO2 Eq.). The following factors were primary contributors to this
decrease: (1) a decrease in electricity demand and a resulting decrease in energy consumption, (2) higher energy prices
leading to a decrease in energy consumption, and (3) cooler summer conditions in 2008 compared to 2007 reducing energy
demand and offsetting the increased energy demand for heating in the colder winter. In addition, the high price of gasoline
combined with the economic downturn led to a significant decline in petroleum consumption by the transportation sector in
2008. Figure 2-1 through Figure 2-3 illustrate the overall trend in total U.S. emissions by gas, annual changes, and absolute
changes since 1990.
Figure 2-1
U.S. Greenhouse Gas Emissions by Gas
8,000 -
7,000 -
6,000 -
S 5,000 -
o
p 4,000 -
3,000 -
2,000 -
1,000-
o-
HFCs, PFCs, & SF,
Nitrous Oxide
Methane
I Carbon Dioxide
S3 ° S i-
Ł 5 S S
As the largest source of U.S. greenhouse gas emissions,
carbon dioxide (CO2) from fossil fuel combustion has
accounted for approximately 79 percent of global warming
potential (GWP) weighted emissions since 1990, growing
slowly from 77 percent of total GWP-weighted emissions
in 1990 to 80 percent in 2008. Emissions from this source
category grew by 17.7 percent (837.1 Tg CO2 Eq.) from
1990 to 2008 and were responsible for most of the increase
in national emissions during this period. From 2007 to 2008,
these emissions decreased by 3.2 percent (184.2 Tg CO2
Eq.). Historically, changes in emissions from fossil fuel
combustion have been the dominant factor affecting U.S.
emission trends.
Changes in CO2 emissions from fossil fuel combustion
are influenced by many long-term and short-term factors,
including population and economic growth, energy
price fluctuations, technological changes, and seasonal
1 Estimates are presented in units of teragrams of carbon dioxide equivalent (Tg CO2 Eq.), which weight each gas by its global warming potential, or
GWP, value. (See section on global warming potentials, Executive Summary.)
Trends in Greenhouse Gas Emissions 2-1
-------
Figure 2-2
Annual Percent Change in
U.S. Greenhouse Gas Emissions
1%-
-1% -
T
-1.0%
i— CNJ oo ** LO to r— oo
Figure 2-3
Cumulative Change in Annual U.S. Greenhouse Gas
Emissions Relative to 1990
1,100
1,000
900
800
s 70°
ca 600
p 500
400
300
200
100
0
-100J
1,006
1041
840
temperatures. On an annual basis, the overall consumption
of fossil fuels in the United States generally fluctuates in
response to changes in general economic conditions, energy
prices, weather, and the availability of non-fossil alternatives.
For example, in a year with increased consumption of
goods and services, low fuel prices, severe summer and
winter weather conditions, nuclear plant closures, and lower
precipitation feeding hydroelectric dams, there would likely
be proportionally greater fossil fuel consumption than in
a year with poor economic performance, high fuel prices,
mild temperatures, and increased output from nuclear and
hydroelectric plants.
In the longer-term, energy consumption patterns
respond to changes that affect the scale of consumption (e.g.,
population, number of cars, and size of houses), the efficiency
with which energy is used in equipment (e.g., cars, power
plants, steel mills, and light bulbs) and consumer behavior
(e.g., walking, bicycling, or telecommuting to work instead
of driving).
Energy-related CO2 emissions also depend on the type
of fuel or energy consumed and its carbon (C) intensity.
Producing a unit of heat or electricity using natural gas
instead of coal, for example, can reduce the CO2 emissions
because of the lower C content of natural gas.
A brief discussion of the year-to-year variability in fuel
combustion emissions is provided below, beginning with
2004.
Emissions from fuel combustion increased from 2004 to
2005 at a rate slightly lower than the average annual growth
rate since 1990. A number of factors played a role in this
slight increase. This small increase is primarily a result of the
restraint on fuel consumption, primarily in the transportation
sector, caused by rising fuel prices. Although electricity
prices increased slightly, there was a significant increase in
electricity consumption in the residential and commercial
sectors due to warmer summer weather conditions. This
led to an increase in emissions in these sectors with the
increased use of air-conditioners. As the amount of fuels used
to generate electricity increased among all end-use sectors,
electricity emissions increased as well. Despite a slight
decrease in industrial energy-related emissions, industrial
production and manufacturing output actually increased. The
price of natural gas escalated dramatically, causing a decrease
in consumption of natural gas in the industrial sector. Use
of renewable fuels decreased slightly due to decreased use
of biofuels and decreased electricity output by hydroelectric
power plants.
From 2005 to 2006, emissions from fuel combustion
decreased for the first time since 2000 to 2001. This decrease
occurred across all sectors, with the exception of the industrial
sector, due to a number of factors. The decrease in emissions
from electricity generation was a result of a smaller share of
electricity generated by coal and a greater share generated by
natural gas. Coal and natural gas consumption for electricity
generation decreased by 1.3 percent and increased by 6.0
percent in 2006, respectively, and nuclear power increased by
less than 1 percent. The transportation decrease was primarily
a result of the restraint on fuel consumption caused by rising
fuel prices, which directly resulted in a decrease of petroleum
consumption within this sector of about 1.3 percent in 2006.
2-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
The decrease in emissions from the residential sector was
primarily a result of decreased electricity consumption due
to increases in the price of electricity, and warmer winter
weather conditions. A moderate increase in industrial
sector emissions was a result of growth in industrial output
and growth in the U.S. economy. Renewable fuels used to
generate electricity increased in 2006, with the greatest
growth occurring in generation from wind.
After experiencing a decrease from 2005 to 2006,
emissions from fuel combustion grew from 2006 to 2007 at
a rate somewhat higher than the average growth rate since
1990. There were a number of factors contributing to this
increase. Unfavorable weather conditions in both the winter
and summer resulted in an increase in consumption of heating
fuels, as well as an increase in the demand for electricity.
This demand for electricity was met with an increase in coal
consumption of 1.7 percent, and with an increase in natural
gas consumption of 9.9 percent. This increase in fossil fuel
consumption, combined with a 14.7 percent decrease in
hydropower generation from 2006 to 2007, resulted in an
increase in emissions in 2007. The increase in emissions from
the residential and commercial sectors is a result of increased
electricity consumption due to warmer summer conditions
and cooler winter conditions compared to 2006. In addition
to these unfavorable weather conditions, electricity prices
remained relatively stable compared to 2006, and natural
gas prices decreased slightly. Emissions from the industrial
sector increased slightly compared to 2006 as a result of a 1.5
percent increase in industrial production and the increase in
fossil fuels used for electricity generation. Despite an overall
decrease in electricity generation from renewable energy in
2007 driven by decreases in hydropower generation, wind
and solar generation increased significantly.
Emissions from fossil fuel combustion decreased from
2007 to 2008. Several factors contributed to this decrease
in emissions. An increase in energy prices coupled with
the economic downturn led to a decrease in energy demand
and a resulting decrease in emissions from 2007 to 2008.
In 2008, the price of coal, natural gas, and petroleum used
to generate electricity, as well as the price of fuels used for
transportation, increased significantly. As a result of this
price increase, coal, natural gas, and petroleum consumption
used for electricity generation decreased by 1.3 percent, 2.6
percent, and 29.5 percent, respectively. The increase in the
cost of fuels to generate electricity translated into an increase
in the price of electricity, leading to a decrease in electricity
consumption across all sectors except the commercial sector.
The increase in transportation fuel prices led to a decrease in
vehicle miles traveled (VMT) and a decrease of 5.7 percent
in transportation fossil fuel combustion emissions from 2007
to 2008. Cooler weather conditions in the summer led to a
decrease in cooling degree days by 8.7 percent and a decrease
in electricity demand compared to 2007, whereas cooler
winter conditions led to a 5.6 percent increase in heating
degree days compared to 2007 and a resulting increase in
demand for heating fuels. The increased emissions from
winter heating energy demand was offset by a decrease in
emissions from summer cooling related electricity demand.
Lastly, renewable energy2 consumption for electricity
generation increased by 7.1 percent from 2007 to 2008,
driven by a significant increase in solar and wind energy
consumption (of 12 percent and 51 percent, respectively).
This increase in renewable energy generation contributed to
a decrease in the carbon intensity of electricity generation.
Overall, from 1990 to 2008, total emissions of CO2
increased by 820.4 Tg CO2 Eq. (16.1 percent), while CH4
and N2O emissions decreased by 45.8 Tg CO2 Eq. (7.5
percent) and 4.1 Tg CO2Eq. (about 1.3 percent) respectively.
During the same period, aggregate weighted emissions
of HFCs, PFCs, and SF6 rose by 59.4 Tg CO2 Eq. (65.9
percent). Despite being emitted in smaller quantities relative
to the other principal greenhouse gases, emissions of HFCs,
PFCs, and SF6 are significant because many of them have
extremely high GWPs and, in the cases of PFCs and SF6,
long atmospheric lifetimes. Conversely, U.S. greenhouse gas
emissions were partly offset by C sequestration in managed
forests, trees in urban areas, agricultural soils, and landfilled
yard trimmings, which was estimated to be 13.5 percent of
total emissions in 2008.
Table 2-1 summarizes emissions and sinks from all U.S.
anthropogenic sources in weighted units of Tg CO2 Eq., while
unweighted gas emissions and sinks in gigagrams (Gg) are
provided in Table 2-2.
2 Renewable energy includes the following energy sources: hydroelectric
power, geothermal energy, biofuels, solar energy, and wind energy.
Trends in Greenhouse Gas Emissions 2-3
-------
Table 2-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg C02 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Iron and Steel Production &
Metallurgical Coke Production
Cement Production
Natural Gas Systems
Lime Production
Incineration of Waste
Ammonia Production and Urea
Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and
Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands
Petroleum Systems
Zinc Production
Lead Production
Silicon Carbide Production and
Consumption
Land Use, Land-Use Change,
and Forestry (Sink)*
Biomass - Woodb
International Bunker Fuelsb
Biomass - Ethanolb
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Wastewater Treatment
Forest Land Remaining Forest
Land
Rice Cultivation
Stationary Combustion
Abandoned Underground Coal
Mines
Mobile Combustion
1990
5,100.8
4,735.7
1,820.8 1
1,485.8
845.4
339.1
216.7
27.9 1
119.6
102.6
33.3
37.3
11.5
8.0
16.8
7.1
4.1
3.3
,2
1.4 1
2.2
1.5
1.0 1
0.6
0.9
0.3
0.4 I
(909.4)
215.2
111.8
42l
613.4
132.4
149.3
129.5
84.1 1
29.3 1
33.9 1
23.5
3.2l
7.1
7.4l
6.0
4.7
1995 2000 2005
5,427.3 5,977.2 6,108.4
5,029.5 5,593.4 5,753.3
1,947.9 2,296.9 1 2,402.1
1,608.0 1,809.5 1,895.3
862.6 852.2
353.3 371.2
223.2 227.7
34.5 35.9
142.9
95.7
36.8
42.2
13.3
11.5
17.8
7.0
6.7
5.7
4.3
4.1
1.5
1.4
2.0
1.5
1.0
0.5
1.0
0.3
0.3
(842.9)
146.1
88.1
41.2
29.4
14.1
11.3
16.4
7.5
5.1
6.1
4.2
4.5
1.8
1.4
1.9
1.4
1.2
0.5
1.1
0.3
0.2
825.6
358.4
221.3
50.6
136.5
67.7
45.9
29.5
14.4
12.6
12.8
7.9
6.8
4.1
4.2
4.2
1.8
1.3
1.4
1.4
1.1
0.5
0.5
0.3
0.2
(664.2) 1 (950.4)
229.1 21 8.1 • 206.9
99.8 98.5
7.7l 9.2
613.2 586.0
143.7 136.8
144.1 120.7
132.6 130.7
67.1 60.4
33.9 38.6
32.0 30.2
24.8 1 25.2
4.3 14.3
7.6 1 7.5
7.1 1 6.6
8.2 7.4
4.3
3.4
110.5
22.6
553.2
136.7
125.6
103.6
56.9
42.2
28.2
24.3
9.8
6.8
6.6
5.6
2.5
2006
6,017.2
5,652.8
2,346.4
1,876.7
850.7
322.1
206.0
50.9
141.4
70.5
46.6
29.5
15.1
12.7
12.3
7.9
8.0
3.8
4.2
3.8
1.8
1.7
1.5
1.2
0.9
0.5
0.5
0.3
0.2
(959.2)
207.9
129.1
30.5
568.2
139.0
127.1
103.1
58.3
42.3
28.2
24.5
21.6
5.9
6.2
5.5
2.3
2007
6,120.2
5,757.0
2,412.8
1,893.7
842.2
341.7
217.4
49.1
135.3
72.8
45.2
30.8
14.6
13.3
14.0
8.3
7.7
4.3
4.1
3.9
1.9
1.9
1.6
1.2
1.0
0.5
0.4
0.3
0.2
(955.4)
207.4
127.1
38.3
569.2
141.2
126.5
99.5
58.1
45.9
28.8
24.4
20.0
6.2
6.5
5.7
2.2
2008
5,921.2
5,572.8
2,363.5
1,785.3
819.3
342.7
219.5
42.5
134.2
69.0
41.1
30.0
14.3
13.1
11.8
7.6
6.6
4.5
4.1
3.4
1.8
1.8
1.6
1.2
0.9
0.5
0.4
0.3
0.2
(940.3)
198.4
135.2
53.3
567.6
140.8
126.3
96.4
67.6
45.0
29.1
24.3
11.9
7.2
6.7
5.9
2.0
2-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg C02 Eq.) (continued)
Gas/Source
Composting
Field Burning of Agricultural
Residues
Petrochemical Production
Iron and Steel Production &
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and
Consumption
Incineration of Waste
International Bunker Fuelsb
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Forest Land Remaining Forest
Land
Wastewater Treatment
N20 from Product Uses
Adipic Acid Production
Composting
Settlements Remaining
Settlements
Field Burning of Agricultural
Residues
Incineration of Waste
Wetlands Remaining Wetlands
International Bunker Fuelsb
MFCs
Substitution of Ozone Depleting
Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and
Distribution
Magnesium Production and
Processing
Semiconductor Manufacture
Total
Net Emissions (Sources and Sinks)
1990
0.3 1
0.8
0.9
1.0
+ 1
+ 1
+ 1
0.2
322.3
203.5
43.9 1
18.9
14.4
12.8
,7
3.7
4.4
15.8
0.4l
0.4l
0.5
+ 1
1.1 1
36.9
_
20.8
18.5
2.2l
32.6
26.6
5.4
0.5
6,126.8
5,217.3
1995 2000 2005
0.7
0.7
1.1
1.0
+
+
+
0.1
342.5
205.9
54.0
21.0
15.5
13.3
3.7
4.0
4.6
17.6
0.8
1.2
0.4
0.5
+
1.0
1.3
0.9
1.2
0.9
+
+
+
0.1
345.5
210.1
53.2
20.7
16.7
14.5
12.1
4.5
4.9
5.5
1.4
1.1
0.5
0.4
+
0.9
62.2 103.2
74.3
28.6
0.3
13.5
8.6
4.9
19.1
15.0
1.6
0.9
1.1
0.7
+
+
+
0.1
328.3
215.8
36.9
17.6
16.6
14.7
8.4
4.7
4.4
5.0
1.7
1.5
0.5
0.4
+
1.0
119.3
103.2
15.8
0.2
6.2
3.0
3.2
17.8
14.0
3.0 2.9
0.9 1.1 1.0
6,488.8 7,044.5 7,133.2
5,646.0 6,380.2 6,182.8
2006
1.6
0.9
1.0
0.7
+
+
+
0.2
329.5
211.2
33.6
17.2
17.3
14.5
18.0
4.8
4.4
4.3
1.8
1.5
0.5
0.4
+
1.2
121.8
107.7
13.8
0.3
6.0
2.5
3.5
17.0
13.2
2.9
1.0
7,059.9
6,100.7
2007
1.7
1.0
1.0
0.7
+
+
+
0.2
327.7
211.0
30.3
20.5
17.3
14.6
16.7
4.9
4.4
3.7
1.8
1.6
0.5
0.4
+
1.2
127 '.4
110.1
17.0
0.3
7.5
3.8
3.6
16.1
12.7
2.6
0.8
7,168.1
6,212.7
2008
1.7
1.0
0.9
0.6
+
+
+
0.2
318.2
215.9
26.1
19.0
17.1
14.2
10.1
4.9
4.4
2.0
1.8
1.6
0.5
0.4
+
1.2
126.9
113.0
13.6
0.3
6.7
2.7
4.0
16.1
13.1
2.0
1.1
6,956.8
6,016.4
+ Does not exceed 0.05 Tg C02 Eq.
a The net C02 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included in net emissions
total. Parentheses indicate negative values or sequestration.
b Emissions from International Bunker Fuels and Wood Biomass and Ethanol Combustion are not included in totals.
c Small amounts of RFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
Note: One teragram (Tg) equals one million metric tons.
Trends in Greenhouse Gas Emissions 2-5
-------
Table 2-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Iron and Steel Production & Metallurgical
Coke Production
Cement Production
Natural Gas Systems
Lime Production
Incineration of Waste
Ammonia Production and Urea Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands
Petroleum Systems
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
Land Use, Land-Use Change, and
Forestry (Sink)3
Wood Biomass and Ethanol Consumption11
International Bunker Fuels'1
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Wastewater Treatment
Forest Land Remaining Forest Land
Rice Cultivation
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
Composting
Field Burning of Agricultural Residues
1990
5,100,840
4,735,701
1,820,818
1,485,773
845,429
339,142
216,656
27,882
119,602
102,564
33,278
37,317
11,533
8,049
16,831
7,084
5,127
6,831
4,141
3,311
1,195
1,416
2,152
1,529
1,033
555
929
285
375
(909,422)
219,341
111,828
29,209
6,303
7,111
6,169
4,003
1,395
1,613
1,120
152
339
353
288
223
15
36
1995
5,427,306
5,029,498
1,947,925
1,607,972
862,575
353,308
223,204
34,513
142,922
95,748
36,847
42,249
13,325
11,461
17,796
7,049
6,683
5,659
4,304
4,101
1,526
1,422
2,036
1,513
1,018
528
993
298
329
(842,852)
236,775
99,817
29,202
6,844
6,860
6,313
3,193
1,612
1,524
1,183
203
363
340
392
204
35
35
2000
5,977,201
5,593,381
2,296,894
1,809,537
852,212
371,212
227,674
35,853
146,141
8,110
41,190
29,394
14,088
11,270
16,402
7,541
5,056
6,086
4,181
4,479
1,752
1,421
1,893
1,382
1,227
534
1,115
311
248
(664,247)
227,276
98,482
27,903
6,513
5,747
6,223
2,877
1,837
1,439
1,199
681
357
315
350
160
60
42
2005
6,108,424
5,753,342
2,402,142
1,895,323
825,590
358,399
221,261
50,626
136,539
67,731
45,910
29,472
14,379
12,616
12,849
7,854
6,768
4,142
4,228
4,181
1,755
1,321
1,392
1,386
1,079
490
506
266
219
(950,396)
229,419
110,505
26,341
6,509
5,980
4,935
2,710
2,011
1,344
1,158
467
326
312
266
119
75
44
2006
6,017,221
5,652,845
2,346,406
1,876,678
850,742
322,053
206,024
50,941
141,382
70,539
46,562
29,526
15,100
12,684
12,300
7,875
8,035
3,801
4,162
3,837
1,836
1,709
1,505
1,167
879
488
513
270
207
(959, 158)
238,323
129,104
27,058
6,619
6,050
4,907
2,776
2,015
1,344
1,166
1,027
282
294
264
112
75
43
2007
6,120,220
5,756,999
2,412,825
1,893,749
842,242
341,737
217,355
49,090
135,306
72,802
45,229
30,816
14,595
13,289
13,968
8,319
7,702
4,251
4,140
3,931
1,930
1,867
1,552
1,166
1,012
474
411
267
196
(955,410)
245,706
127,054
27,105
6,723
6,023
4,738
2,765
2,183
1,372
1,162
953
295
309
269
105
79
46
2008
5,921,204
5,572,760
2,363,497
1,785,252
819,297
342,719
219,467
42,528
134,200
69,010
41,147
29,973
14,330
13,128
11,755
7,638
6,617
4,477
4,111
3,449
1,809
1,780
1,599
1,187
941
453
402
264
175
(940,349)
251,763
135,226
27,030
6,707
6,016
4,591
3,221
2,144
1,384
1,158
568
343
319
281
97
80
46
2-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg) (continued)
Gas/Source
1990
1995
2000
2005
2006
2007
2008
Petrochemical Production
Iron and Steel Production & Metallurgical
Coke Production
Ferroalloy Production
Silicon Carbide Production and Consumption
Incineration of Waste
International Bunker Fuelsb
41
59
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Forest Land Remaining Forest Land
Wastewater Treatment
N20from Product Uses
Adipic Acid Production
Composting
Settlements Remaining Settlements
Field Burning of Agricultural Residues
Incineration of Waste
Wetlands Remaining Wetlands
International Bunker Fuelsb
MFCs
Substitution of Ozone Depleting Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture _
+ Does not exceed 0.5 Gg.
M Mixture of multiple gases.
"The net C02 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included
emissions total. Parentheses indicate negative values or sequestration.
b Emissions from International Bunker Fuels and Wood Biomass and Ethanol Consumption are not included in totals.
c Small amounts of RFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
51
34
+
+
+
7
1,059
696
119
57
54
47
27
15
14
16
6
5
2
1
3
M
M
1
M
M
M
1
1
48
35
8
1,063
681
108
56
56
47
58
15
14
14
6
5
2
1
4
M
M
1
M
M
M
1
1
48
33
7
1,057
681
98
66
56
47
54
16
14
12
6
5
2
1
4
M
M
1
M
M
M
1
1
in net
43
31
8
1,026
696
84
61
55
46
33
16
14
7
6
5
2
1
4
M
M
1
M
M
M
1
1
Emissions of all gases can be summed from each source
category from Intergovernmental Panel on Climate Change
(IPCC) guidance. Over the nineteen-year period of 1990 to
2008, total emissions in the Energy, Industrial Processes,
and Agriculture sectors grew by 775.0 Tg CO2 Eq. (15
percent), 16.2 Tg CO2 Eq. (5 percent), and 39.7 Tg CO2 Eq.
(10 percent), respectively. Emissions decreased in the Waste
and Solvent and Other Product Use sectors by 18.1 Tg CO2
Eq. (10 percent) and less than 0.1 Tg CO2 Eq. (less than 0.4
percent), respectively. Over the same period, estimates of
net C sequestration in the Land Use, Land-Use Change, and
Forestry sector increased by 30.9 Tg CO2 Eq. (3.4 percent).
Trends in Greenhouse Gas Emissions 2-7
-------
Table 2-3: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg C02 Eq.)
Chapter/IPCC Sector
Energy
Industrial Processes
Solvent and Other Product Use
Agriculture
Land Use, Land-Use Change, and
Forestry (Emissions)
Waste
Total Emissions
Net C02 Flux from Land Use, Land-Use
Change, and Forestry (Sinks)3
Net Emissions (Sources and Sinks)
1990
5,224.1
318.3
387.8
15. ol
177.2
6,126.8
(909.4)
5,217.3
1995
5,545.8
339.1
4.6 1
407.7
17.2
174.5
6,488.8
(842.9)
5,646.0
2000
6,087.5
351.9
4.9 1
410.9
36.3
153.0
7,044.5
(664.2)
6,380.2
2005
6,187.9
334.7
4.4
419.7
28.6
158.0
7,133.2
(950.4)
6,182.8
2006
6,089.1
339.7
4.4
417.2
49.8
159.7
7,059.9
(959.2)
6,100.7
2007
6,182.9
350.9
4.4
423.0
47.6
159.3
7,168.1
(955.4)
6,212.7
2008
5,999.0
334.5
4.4
427.5
32.2
159.1
6,956.8
(940.3)
6,016.4
a The net C02 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included in net
emissions total.
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values or sequestration.
Figure 2-4
U.S. Greenhouse Gas Emissions and Sinks
by Chapter/IPCC Sector
Industrial Processes
Waste
7,500 -
7,000 -
6,500 -
6,000 -
5,500 -
5,000
^ 4,500
"J 4,000 -
o 3,500-
Ł 3,000-
'- 2,500 -
2,000 -
1,500-
1,000-
500-
0
(500) -
(1,000)-I
Agrii
Land Use, Land-Use Change and Forestry (sinks)
iiiiiiiiiiisiiiiiii
Note: Relatively smaller amounts of GWP-weighted emissions are also emitted from the
Solvent and Other Product Use sector
Energy
Energy-related activities, primarily fossil fuel
combustion, accounted for the vast majority of U.S. CO2
emissions for the period of 1990 through 2008. In 2008,
approximately 84 percent of the energy consumed in the
United States (on a Btu basis) was produced through the
combustion of fossil fuels. The remaining 16 percent came
from other energy sources such as hydropower, biomass,
nuclear, wind, and solar energy (see Figure 2-5 and Figure
2-6). A discussion of specific trends related to CO2 as well as
other greenhouse gas emissions from energy consumption is
presented in the Energy chapter. Energy-related activities are
also responsible for CH4 and N2O emissions (37 percent and
13 percent of total U.S. emissions of each gas, respectively).
Figure 2-5
2008 Energy Chapter Greenhouse Gas Emission Sources
5,573
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Coal Mining
Petroleum Systems
Mobile Combustion
Stationary Combustion
Incineration of Waste
Abandoned Underground •
Energy as a Portion
of all Emissions
Coal Mines
25
50 75 100
Tg C02 Eq.
125 150
Table 2-4 presents greenhouse gas emissions from the Energy
chapter, by source and gas.
Carbon dioxide emissions from fossil fuel combustion
are presented in Table 2-5 based on the underlying U.S.
energy consumer data collected by EIA. Estimates of CO2
emissions from fossil fuel combustion are calculated from
these EIA "end-use sectors" based on total consumption
and appropriate fuel properties (any additional analysis
and refinement of the EIA data is further explained in the
Energy chapter of this report). El As fuel consumption data
for the electric power sector comprises electricity-only and
combined-heat-and-power (CHP) plants within the NAICS
22 category whose primary business is to sell electricity,
or electricity and heat, to the public (nonutility power
2-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Figure 2-6
2008 U.S. Fossil Carbon Flows (Tg C02 Eq.)
NEU Emissions
5
Natural Gas Emissions
1,235
NEU Emissions 51
Non-Energy Use
Carbon Sequestered
210
Fossil Fuel Non-Energy
Non-Energy Consumption Use U.S.
Use Imports U.S. Territories
47 Territories o
43
Balancing Item
3
Note: Totals may not sum due to independent rounding.
The "Balancing Item" above accounts for the statistical imbalances
and unknowns in the reported data sets combined here.
NEU = Non-Energy Use
NG = Natural Gas
producers can be included in this sector as long as they
meet they electric power sector definition). EIA statistics
for the industrial sector include fossil fuel consumption
that occurs in the fields of manufacturing, agriculture,
mining, and construction. EIA's fuel consumption data
for the transportation sector consists of all vehicles whose
primary purpose is transporting people and/or goods from
one physical location to another. EIA's fuel consumption
data for the industrial sector consists of all facilities and
equipment used for producing, processing, or assembling
goods (EIA includes generators that produce electricity
and/or useful thermal output primarily to support on-site
industrial activities in this sector). EIA's fuel consumption
data for the residential sector consists of living quarters for
private households. EIA's fuel consumption data for the
commercial sector consists of service-providing facilities
and equipment from private and public organizations and
businesses (EIA includes generators that produce electricity
and/or useful thermal output primarily to support the
activities at commercial establishments in this sector). Table
2-5, Figure 2-7, and Figure 2-8 summarize CO2 emissions
from fossil fuel combustion by end-use sector.
The main driver of emissions in the energy sector is
CO2 from fossil fuel combustion. The transportation end-
use sector accounted for 1,789.9 Tg CO2 Eq. in 2008, or
approximately 32 percent of total CO2 emissions from fossil
Figure 2-7
2008 C02 Emissions from Fossil Fuel
Combustion by Sector and Fuel Type
Relative Contribution
by Fuel Type
2,500 -i
2,000 -
1,500 -
1,000 -
500 -
0 -1
Note: Electricity generation also includes emissions of less than 0.5 Tg C02 Eq. from
geothermal-based electricity generation.
2,363
Trends in Greenhouse Gas Emissions 2-9
-------
Table 2-4: Emissions from Energy (Tg C02 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Natural Gas Systems
Incineration of Waste
Petroleum Systems
Wood Biomass and Ethanol Consumption3
International Bunker Fuels3
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
Incineration of Waste
International Bunker Fuels3
N20
Mobile Combustion
Stationary Combustion
Incineration of Waste
International Bunker Fuels3
Total
+ Does not exceed 0.05 Tg C02 Eq.
a These values are presented for informational purposes
categories.
Note: Totals may not sum due to independent rounding.
1990
4,901.2
4,735.7
1,820.8
1,485.8
845.5
339.1
216.7
27.9
119.6
37.3
8.0 1
0.6 1
279.3
111.8
265.6
129.5
84.1
33.9
74|
6.0 1
0.2
57.2
43.9
12.8
0.5 1
1.1
5,224.1
1995
5,226.7
5,029.5
1,947.9
1,608.0
862.6
353.3
223.2
34.5
142.9
42.2
11.5
0.5 1
236.8
99.8
251. 3 1
132.6
67.1
32.0
7.1 1
8.2
4.3l
1
67.8
54.0
13.3
0.5 1
0.9
5,545.8
only and are not included in totals
2000
5,780.7
5,593.4
2,296.9
1,809.5
852.2
371.21
227.7 1
35.9
146.1
29.4
11.3
0.5 1
227.3
u.im
68.1
53.2
14.5
0.4
0.9
6,087.5
2005
5,932.5
5,753.3
2,402.1
1,895.3
825.6
358.4
221.3
50.6
136.5
29.5
12.6
0.5
229.4
110.5
203.4
103.6
56.9
28.2
6.6
5.6
2.5
0.1
52.0
36.9
14.7
0.4
1.0
6,187.9
2006
5,836.9
5,652.8
2,346.4
1,876.7
850.7
322.1
206.0
50.9
141.4
29.5
12.7
0.5
238.3
129.1
203.6
103.1
58.3
28.2
6.2
5.5
2.3
0.2
48.5
33.6
14.5
0.4
1.2
6,089.1
2007
5,936.9
5,757.0
2,412.8
1,893.7
842.2
341.7
217.4
49.1
135.3
30.8
13.3
0.5
245.7
127.1
200.7
99.5
58.1
28.8
6.5
5.7
2.2
0.2
45.3
30.3
14.6
0.4
1.2
6,182.9
2008
5,750.5
5,572.8
2,363.5
1,785.3
819.3
342.7
219.5
42.5
134.2
30.0
13.1
0.5
251.8
135.2
207.8
96.4
67.6
29.1
6.7
5.9
2.0
0.2
40.8
26.1
14.2
0.4
1.2
5,999.0
or are already accounted for in other source
fuel combustion, the largest share of any end-use sector.3 The
industrial end-use sector accounted for 27 percent of CO2
emissions from fossil fuel combustion. The residential and
commercial end-use sectors accounted for an average 21
and 19 percent, respectively, of CO2 emissions from fossil
fuel combustion. Both end-use sectors were heavily reliant
on electricity for meeting energy needs, with electricity
consumption for lighting, heating, air conditioning, and
operating appliances contributing to about 71 and 79 percent
of emissions from the residential and commercial end-use
sectors, respectively. Significant trends in emissions from
energy source categories over the nineteen-year period from
1990 through 2008 included the following:
• Total CO2 emissions from fossil fuel combustion
increased from 4,735.7 Tg CO2 Eq. to 5,572.8 Tg CO2
Eq.—an 18 percent total increase over the nineteen-year
period. From 2007 to 2008, these emissions decreased
by 184.2 Tg CO2 Eq. (3.2 percent).
• Carbon dioxide emissions from non-energy use of fossil
fuels have increased 14.6 Tg CO2 Eq. (12.2 percent) from
1990 through 2008. Emissions from non-energy uses
of fossil fuels were 134.2 Tg CO2 Eq. in 2008, which
constituted 2.3 percent of total national CO2 emissions.
3 Note that electricity generation is the largest emitter of CO2 when
electricity is not distributed among end-use sectors.
2-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-5: C02 Emissions from Fossil Fuel Combustion by Fuel Consuming End-Use Sector (Tg C02 Eq.)
End-Use Sector
Transportation
Combustion
Electricity
Industrial
Combustion
Electricity
Residential
Combustion
Electricity
Commercial
Combustion
Electricity
U.S. Territories
Total
Electricity Generation
1990
1,488.8
1,485.8
3.0 1
1,532.21
845.4 1
686.8
932.2 1
339.1
593.0
754.6
216.7
538.0
27.9
4,735.7
1,820.8
1995
1,611.0
1,608.0
1
1,578.8
862.6
716.2
995.1
353.3
641.8 1
810.0
223.2
586.8
34.5
5,029.5
1,947.9
2000
1,813.0
1,809.5
3.4
1,642.0
852.2
789.8
1,133.6
371.2
762.4
968.9
227.7
741.3
35.9 1
5,593.4
2,296.9 |
2005
1,900.1
1,895.3
4.7
1,562.5
825.6
737.0
1,215.1
358.4
856.7
1,025.0
221.3
803.7
50.6
5,753.3
2,402.1
2006
1,881.2
1,876.7
4.5
1,562.8
850.7
712.0
1,152.9
322.1
830.8
1,005.0
206.0
799.0
50.9
5,652.8
2,346.4
2007
1,898.8
1,893.7
5.0
1,572.2
842.2
730.0
1,197.9
341.7
856.1
1,039.1
217.4
821.7
49.1
5,757.0
2,412.8
2008
1,789.9
1,785.3
4.7
1,510.9
819.3
691.6
1,184.5
342.7
841.8
1,044.9
219.5
825.4
42.5
5,572.8
2,363.5
Note: Totals may not sum due to independent rounding. Combustion-related emissions from electricity generation are allocated based on aggregate
national electricity consumption by each end-use sector.
Figure 2-8
2008 End-Use Sector Emissions
from Fossil Fuel Combustion
2,000 -,
1,000 -
500 -
0 -
I From Direct Fossil
Fuel Combustion
From Electricity
Consumption
1,050
1,818
1,518
1,193
43
U.S. Commercial Residential Industrial Transportation
Territories
Methane emissions from natural gas systems were 96.4
Tg CO2 Eq. in 2008; emissions have declined by 33.1 Tg
CO2 Eq. (25.6 percent) since 1990. This decline has been
due to improvements in technology and management
practices, as well as replacement of old equipment.
Methane emissions from coal mining were 67.6 Tg CO2
Eq. In 2008, a decline in emissions of 16.4 Tg CO2 Eq.
(19.5 percent) from 1990, is aresult of the mining of less
gassy coal from underground mines and the increased
use of CH4 collected from degasincation systems.
• In 2008, N2O emissions from mobile combustion were
26.1 Tg CO2 Eq. (approximately 8.2 percent of U.S.
N2O emissions). From 1990 to 2008, N2O emissions
from mobile combustion decreased by 40.5 percent.
However, from 1990 to 1998 emissions increased by 26
percent, due to control technologies that reduced NOX
emissions while increasing N2O emissions. Since 1998,
newer control technologies have led to a steady decline
in N2O from this source.
• Carbon dioxide emissions from incineration of waste
(13.1 Tg CO2Eq. in 2008) increased by 5.1 Tg CO2Eq.
(63 percent) from 1990 through 2008, as the volume of
plastics and other fossil carbon-containing materials in
municipal solid waste grew.
Industrial Processes
Greenhouse gas emissions are produced as the
byproducts of many non-energy-related industrial activities.
For example, industrial processes can chemically transform
raw materials, which often release waste gases such as
CO2, CH4, and N2O. These processes include iron and
steel production and metallurgical coke production, cement
production, ammonia production and urea consumption, lime
production, limestone and dolomite use (e.g., flux stone,
flue gas desulfurization, and glass manufacturing), soda ash
production and consumption, titanium dioxide production,
Trends in Greenhouse Gas Emissions 2-11
-------
Figure 2-9
2008 Industrial Processes Chapter
Greenhouse Gas Emission Sources
Substitution of Ozone Depleting Substances
Iron and Steel Production &
Metallurgical Coke Production
Cement Production
Nitric Acid Production |
Lime Production |
HCFC-22 Production |
Electrical Transmission and Distribution |
Ammonia Production and Urea Consumption |
Aluminum Production |
Limestone and Dolomite Use |
Semiconductor Manufacture |
Petrochemical Production |
Soda Ash Production and Consumption |
Adipic Acid Production
Magnesium Production and Processing
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production <0-5
Lead Production <0.5
Silicon Carbide Production and Consumption <0.5
Industrial Processes
as a Portion of
all Emissions
25
50
75
100
125
Tg CO, Eq.
phosphoric acid production, ferroalloy production, CO2
consumption, silicon carbide production and consumption,
aluminum production, petrochemical production, nitric
acid production, adipic acid production, lead production,
and zinc production (see Figure 2-9). Industrial processes
also release HFCs, PFCs and SF6. In addition to their use
as ODS substitutes, HFCs, PFCs, SF6, and other fluorinated
compounds are employed and emitted by a number of other
industrial sources in the United States. These industries
include aluminum production, HCFC-22 production,
semiconductor manufacture, electric power transmission
and distribution, and magnesium metal production and
processing. Table 2-6 presents greenhouse gas emissions
from industrial processes by source category.
Overall, emissions from industrial processes increased
by 5.1 percent from 1990 to 2008 despite decreases in
emissions from several industrial processes, such as iron
and steel production and metallurgical coke production,
aluminum production, HCFC-22 production, and electrical
transmission and distribution. The increase in overall
emissions was driven by a rise in the emissions originating
from cement manufacture and, primarily, the emissions
from the use of substitutes for ozone depleting substances.
Significant trends in emissions from industrial processes
source categories over the nineteen-year period from 1990
through 2008 included the following:
• HFC emissions from ODS substitutes have been
increasing from small amounts in 1990 to 113.0 Tg
CO2 Eq. in 2008. This increase results from efforts to
phase out CFCs and other ODSs in the United States.
In the short term, this trend is expected to continue, and
will likely accelerate over the next decade as HCFCs—
which are interim substitutes in many applications—are
phased out under the provisions of the Copenhagen
Amendments to the Montreal Protocol.
• Carbon dioxide and CH4 emissions from iron and steel
production and metallurgical coke production decreased
by 5.2 percent to 69.7 Tg CO2 Eq. in 2008, and have
declined overall by 33.9 Tg CO2 Eq. (32.7 percent)
from 1990 through 2008, due to restructuring of the
industry, technological improvements, and increased
scrap utilization.
• PFC emissions from aluminum production decreased
by about 85 percent (15.8 Tg CO2 Eq.) from 1990 to
2008, due to both industry emission reduction efforts
and lower domestic aluminum production.
• Nitrous oxide emissions from adipic acid production
were 2.0 Tg CO2 Eq. in 2008, and have decreased
significantly in recent years from the widespread
installation of pollution control measures. These
emissions from adipic acid production have decreased
nearly 87 percent since 1990, and except for slight
increases in 2002 and 2003 these declined by 62 percent
from 1998 to 2008.
• Carbon dioxide emissions from ammonia production
and urea application (11.8 Tg CO2 Eq. in 2008) have
decreased by 5.1 Tg CO2 Eq. (30 percent) since 1990,
due to a decrease in domestic ammonia production. This
decrease in ammonia production can be attributed to
market fluctuations and high natural gas prices.
2-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-6: Emissions from Industrial Processes (Tg C02 Eq.)
Gas/Source
C02
Iron and Steel Production & Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Cement Production
Lime Production
Ammonia Production & Urea Consumption
Limestone and Dolomite Use
Aluminum Production
Soda Ash Manufacture and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
CH4
Petrochemical Production
Iron and Steel Production & Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and Consumption
N20
Nitric Acid Production
Adipic Acid Production
MFCs
Substitution of Ozone Depleting Substances3
HCFC-22 Production
Semiconductor Manufacturing MFCs
PFCs
Semiconductor Manufacturing PFCs
Aluminum Production
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconducter Manufacturing SF6
Total
1990 1995 2000 2005
191.5 192.6 187.7 167.0
102.6
97.?
5.5
33.3
11.5
16.8
5.1
6.8
4.1
3.3
1.2
1.4
2.2
1.5
0.9
0.3
95.7 88.1 67.7
90.7 83.7 63.9
5.0 4.4M 3.8
36.8 1 41.2 45.9
13.3
17.8
6.7
5.7
4.3
4.1
1.5
1.4
2.0
1.5
1.0
0.3
0.4 0.3
i.g| 2.1
0.9 1.1
1.0| 1.0
torn 1.0
+ l
+ l
+ l
34.7 38.6
18.9 21.0
15.8 17.6
36.9 62.2
0.3 29.0
14.1
16.4
5.1
6.1
4.2
4.5
1.8
1.4
1.9
1.4
1.1
0.3
0.2
2.2
1.2
0.9
0.9
+
+
+
26.3
20.7
5.5
103.2
74.3
14.4
12.8
6.8
4.1
4.2
4.2
1.8
1.3
1.4
1.4
0.5
0.3
0.2
1.8
1.1
0.7
0.7
+
+
+
22.6
17.6
5.0
119.3
103.2
36.4 33.0 28.6 15.8
0.2 0.3 1 0.3 1 0.2
20.8 15.6 13.5 6.2
2.2l 3.8 1 4.9 1 3.2
18.5 11.8 8. G| 3.0
32.6 27.9 19.1 17.8
26.6 21.4 15.0 14.0
5.4 5.6 1 3.0 2.9
0.5 0.9 1.1 1.0
318.3 339.1 351.9 334.7
2006
171.5
70.5
66.9
3.7
46.6
15.1
12.3
8.0
3.8
4.2
3.8
1.8
1.7
1.5
1.2
0.5
0.3
0.2
1.7
1.0
0.7
0.7
+
+
+
21.5
17.2
4.3
121.8
107.7
13.8
0.3
6.0
3.5
2.5
17.0
13.2
2.9
1.0
339.7
2007
174.0
72.8
69.0
3.8
45.2
14.6
14.0
7.7
4.3
4.1
3.9
1.9
1.9
1.6
1.2
0.4
0.3
0.2
1.7
1.0
0.7
0.7
+
+
+
24.2
20.5
3.7
127.4
110.1
17.0
0.3
7.5
3.6
3.8
16.1
12.7
2.6
0.8
350.9
2008
162.1
69.0
63.7
5.3
41.1
14.3
11.8
6.6
4.5
4.1
3.4
1.8
1.8
1.6
1.2
0.4
0.3
0.2
1.6
0.9
0.6
0.6
+
+
+
21.1
19.0
2.0
126.9
113.0
13.6
0.3
6.7
4.0
2.7
16.1
13.1
2.0
1.1
334.5
+ Does not exceed 0.05 Tg C02 Eq.
a Small amounts of RFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
Trends in Greenhouse Gas Emissions 2-13
-------
Solvent and Other Product Use
Greenhouse gas emissions are produced as a byproduct
of various solvent and other product uses. In the United
States, N2O Emissions from Product Uses, the only source of
greenhouse gas emissions from this sector, accounted for 4.4
Tg CO2 Eq., or less than 0.1 percent of total U.S. emissions
in 2008 (see Table 2-7).
In 2008, N2O emissions from product uses constituted
slightly more than 1 percent of U.S. N2O emissions. From
1990 to 2008, emissions from this source category decreased
by less than 0.5 percent, though slight increases occurred in
intermediate years.
Agriculture
Agricultural activities contribute directly to emissions of
greenhouse gases through a variety of processes, including
the following source categories: enteric fermentation in
domestic livestock, livestock manure management, rice
cultivation, agricultural soil management, and field burning
of agricultural residues.
In 2008, agricultural activities were responsible for
emissions of 427.5 Tg CO2 Eq., or 6.1 percent of total U.S.
greenhouse gas emissions. Methane and N2O were the
primary greenhouse gases emitted by agricultural activities.
CH4 emissions from enteric fermentation and manure
management represented about 25 percent and 8 percent
of total CH4 emissions from anthropogenic activities,
respectively, in 2008. Agricultural soil management
activities, such as fertilizer application and other cropping
practices, were the largest source of U.S. N2O emissions in
2008, accounting for almost 68 percent.
Some significant trends in U.S. emissions from
Agriculture include the following:
• Agricultural soils produced approximately 68 percent of
N2O emissions in the United States in 2008. Estimated
emissions from this source in 2008 were 215.9 Tg
CO2 Eq. Annual N2O emissions from agricultural soils
Figure 2-10
2008 Agriculture Chapter Greenhouse Gas
Emission Sources
Agricultural Soil Management
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of
Agricultural Residues
Agriculture
as a Portion of
all Emissions
50 100
Tg C02 Eq.
150
fluctuated between 1990 and 2008, although overall
emissions were 6.1 percent higher in 2008 than in 1990.
Methane emissions from this source have not shown any
significant long-term trend, as their estimation is highly
sensitive to the amount of N applied to soils, which has
not changed significantly over the time-period, and to
weather patterns and crop type.
Enteric fermentation was the largest source of CH4
emissions in 2008, at 140.8 Tg CO2 Eq. Although
emissions from enteric fermentation have increased by
6.4 percent between 1990 and 2008, emissions increased
about 9 percent between 1990 and 1995 and decreased
about 6 percent from 1995 to 2004, mainly due to
decreasing populations of both beef and dairy cattle
and improved feed quality for feedlot cattle. Emissions
increased by 5 percent from 2004 through 2007, as both
dairy and beef populations increased and the literature
for dairy cow diets indicated a trend toward a decrease
in feed digestibility. Emissions decreased again in 2008
as beef cattle populations decreased again. During
this timeframe, populations of sheep have decreased
48 percent since 1990 while horse populations have
Table 2-7: N20 Emissions from Solvent and Other Product Use (Tg C02 Eq.)
Gas/Source
N20
N20 from Product Uses
Total
1990
4.4
4.4
4.4 |
1995
4.6
4.6 1
4.6
2000
4.9
4.9 1
4.9
2005
4.4
4.4
4.4
2006
4.4
4.4
4.4
2007
4.4
4.4
4.4
2008
4.4
4.4
4.4
2-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-8: Emissions from Agriculture (Tg C02 Eq.)
Gas/Source
CH4
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of Agricultural Residues
N20
Agricultural Soil Management
Manure Management
Field Burning of Agricultural Residues
Total
1990
169.6
132.4
29.3
1
0.8 1
218.3
203.5
14.4
0.4
387.8 1
1995
185.9
143.7
33.9
1
07 1
221.8
205.9
15.5
0.4
407.7
2000
183.7
136.8
38.6
1
0.9 1
227.2
210.1
16.7
0.5
410.9
2005
186.7
136.7
42.2
6.8
0.9
233.0
215.8
16.6
0.5
419.7
2006
188.1
139.0
42.3
5.9
0.9
229.1
211.2
17.3
0.5
417.2
2007
194.2
141.2
45.9
6.2
1.0
228.8
211.0
17.3
0.5
423.0
2008
194.0
140.8
45.0
7.2
1.0
233.5
215.9
17.1
0.5
427.5
Note: Totals may not sum due to independent rounding.
increased by almost 87 percent, mostly over the last
seven years. Goat and swine populations have increased
1 percent and 25 percent, respectively, during this
timeframe.
• Overall, emissions from manure management increased
42 percent between 1990 and 2008. This encompassed
an increase of 54 percent for CH4, from 29.3 Tg CO2
Eq. in 1990 to 45.0 Tg CO2 Eq. in 2008; and an increase
of almost 19 percent for N2O, from 14.4 Tg CO2 Eq. in
1990 to 17.1 Tg CO2 Eq. in 2008. The majority of this
increase was from swine and dairy cow manure, since the
general trend in manure management is one of increasing
use of liquid systems, which tends to produce greater
CH4 emissions.
Land Use, Land-Use Change, and Forestry
When humans alter the terrestrial biosphere through land
use, changes in land use, and land management practices,
they also alter the background carbon fluxes between
biomass, soils, and the atmosphere. Forest management
practices, tree planting in urban areas, the management of
agricultural soils, and the landfilling of yard trimmings and
food scraps have resulted in an uptake (sequestration) of
carbon in the United States, which offset about 14 percent
of total U.S. greenhouse gas emissions in 2008. Forests
(including vegetation, soils, and harvested wood) accounted
for approximately 84 percent of total 2008 net CO2 flux,
urban trees accounted for 10 percent, mineral and organic soil
carbon stock changes accounted for 5 percent, and landfilled
yard trimmings and food scraps accounted for 1 percent of
the total net flux in 2008. The net forest sequestration is a
result of net forest growth, increasing forest area, and a net
accumulation of carbon stocks in harvested wood pools.
The net sequestration in urban forests is a result of net tree
growth and increased urban forest size. In agricultural soils,
mineral and organic soils sequester approximately 5.9 times
as much C as is emitted from these soils through liming and
urea fertilization. The mineral soil C sequestration is largely
due to the conversion of cropland to hay production fields,
the limited use of bare-summer fallow areas in semi-arid
areas, and an increase in the adoption of conservation tillage
practices. The landfilled yard trimmings and food scraps net
sequestration is due to the long-term accumulation of yard
trimming carbon and food scraps in landfills.
Land use, land-use change, and forestry activities in
2008 resulted in a net C sequestration of 940.3 Tg CO2
Eq. (256.5 Tg C). (Table 2-9). This represents an offset
of approximately 16 percent of total U.S. CO2 emissions,
or 14 percent of total greenhouse gas emissions in 2008.
Between 1990 and 2008, total land use, land-use change,
and forestry net C flux resulted in a 3.4 percent increase in
CO2 sequestration.
Land use, land-use change, and forestry source
categories also resulted in emissions of CO2, CH4, and N2O
that are not included in the net CO2 flux estimates presented
in Table 2-10. The application of crushed limestone and
dolomite to managed land (i.e., soil liming) and urea
fertilization resulted in CO2 emissions of 7.6 Tg CO2 Eq. in
2008, an increase of about 8 percent relative to 1990. Lands
undergoing peat extraction resulted in CO2 emissions of 0.9
Tg CO2 Eq. (941 Gg), and N2O emissions of less than 0.01
Tg CO2 Eq. N2O emissions from the application of synthetic
Trends in Greenhouse Gas Emissions 2-15
-------
Table 2-9: Net C02 Flux from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Sink Category
Forest Land Remaining Forest Land
Cropland Remaining Cropland
Land Converted to Cropland
Grassland Remaining Grassland
Land Converted to Grassland
Settlements Remaining Settlements
Other (Landfilled Yard Trimmings and
Food Scraps)
Total
1990
(729.8)
(29.4)
2.2|
(52.0)
(19.8)
(57.1)1
(23.5)
(909.4)
1995
(692.6)
(22.9)
2.9 1
(26.7)
(22.3)
(67.3)
(13.9)
(842.9)
2005
(467.7)
(30.2)
(2.4)
(52.6)
(27.3)
(77.5)
(11.3)
(664.2)
2005
(806.6)
(18.3)
5.9
(9.0)
(24.6)
(87.8)
(10.1)
(950.4)
2006
(812.5)
(19.1)
5.9
(8.9)
(24.5)
(89.8)
(10.3)
(959.2)
2007
(806.9)
(19.7)
5.9
(8.8)
(24.3)
(91.9)
(9.8)
(955.4)
2008
(791.9)
(18.1)
5.9
(8.7)
(24.2)
(93.9)
(9.5)
(940.3)
Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
Table 2-10: Emissions from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Source Category
1990
1995
2000
2005 2006 2007 2008
C02
Cropland Remaining Cropland: Liming of Agricultural
Soils
Cropland Remaining Cropland: Urea Fertilization
Wetlands Remaining Wetlands: Peatlands Remaining
Peatlands
CH4
Forest Land Remaining Forest Land: Forest Fires
N20
Forest Land Remaining Forest Land: Forest Fires
Forest Land Remaining Forest Land: Forest Soils
Settlements Remaining Settlements: Settlement Soils
Wetlands Remaining Wetlands: Peatlands Remaining
Peatlands
8.1
47 I
2.4 I
i
2.6
0.1
8.1
4.4
27
1.0
4.3
4.3
4.9
3.5
0.2
1.2
8.8
4.3
3.2
1.2
14.3
13.2
11.7
8.9
4.3
3.5
1.1
9.8
9.8
9.8
8.0
0.4
1.5
8.8
4.2
3.7
0.9
21.6
21.6
19.5
17.6
0.4
1.5
9.3
4.5
3.8
1.0
20.0
20.0
18.3
16.3
0.4
1.6
8.6
3.8
3.8
0.9
11.9
11.9
11.7
9.7
0.4
1.6
Total
15.0
17.2
36.3
28.6 49.8 47.6
32.2
+ Less than 0.05 Tg C02zEq.
Note: Totals may not sum due to independent rounding.
fertilizers to forest soils have increased from 1990 to 0.4 Tg
CO2 Eq. in 2008. Settlement soils in 2008 resulted in direct
N2O emissions of 1.6 Tg CO2 Eq., a 61 percent increase
relative to 1990. Non-CO2 emissions from forest fires in
2008 resulted in CH4 emissions of 11.9 Tg CO2 Eq., and in
N2O emissions of 9.7 Tg CO2 Eq. (Table 2-10).
Other significant trends from 1990 to 2008 in land use,
land-use change, and forestry emissions include:
• Net C sequestration by forest land has increased by
almost 9 percent. This is primarily due to increased forest
management and the effects of previous reforestation.
The increase in intensive forest management resulted in
higher growth rates and higher biomass density. The tree
planting and conservation efforts of the 1970s and 1980s
continue to have a significant impact on sequestration
rates. Finally, the forested area in the United States
increased over the past 19 years, although only at an
average rate of 0.23 percent per year.
Net sequestration of C by urban trees has increased by
65 percent over the period from 1990 to 2008. This is
primarily due to an increase in urbanized land area in
the United States.
Annual C sequestration in landfilled yard trimmings and
food scraps has decreased by 59 percent since 1990.
This is due in part to a decrease in the amount of yard
trimmings and food scraps generated. In addition, the
proportion of yard trimmings and food scraps landfilled
has decreased, as there has been a significant rise in
the number of municipal composting facilities in the
United States.
2-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Waste
Waste management and treatment activities are sources
of greenhouse gas emissions (see Figure 2-11). In 2008,
landfills were the second largest source of anthropogenic
CH4 emissions, accounting for 22 percent of total U.S. CH4
emissions.4 Additionally, wastewater treatment accounts
for 4 percent of U.S. CH4 emissions, and 2 percent of N2O
emissions. Emissions of CH4 and N2O from composting grew
from 1990 to 2008, and resulted in emissions of 3.5 Tg CO2
Eq. in 2008. A summary of greenhouse gas emissions from
the Waste chapter is presented in Table 2-11.
Overall, in 2008, waste activities generated emissions
of 159.1 Tg CO2Eq., or 2.3 percent of total U.S. greenhouse
gas emissions.
Some significant trends in U.S. emissions from Waste
include the following:
• From 1990 to 2008, net CH4 emissions from landfills
decreased by 23.0 Tg CO2 Eq. (15 percent), with small
increases occurring in interim years. This downward
trend in overall emissions is the result of increases in
the amount of landfill gas collected and combusted,5
which has more than offset the additional CH4 emissions
resulting from an increase in the amount of municipal
solid waste landfilled.
• From 1990 to 2008, CH4 and N2O emissions from
wastewater treatment increased by 0.8 Tg CO2 Eq. (3.5
percent) and 1.3 Tg CO2 Eq. (34 percent), respectively.
Figure 2-11
2008 Waste Chapter Greenhouse Gas Emission Sources
Landfills
Wastewater
Treatment
Composting
Waste as a Portion
of all Emissions
0 20
40
60 80
Tg C02 Eq.
100
120
140
• Methane and N2O emissions from composting each
increased by less than 0.1 Tg CO2 Eq. (1 percent) from
2007 to 2008. Emissions from composting have been
continually increasing since 1990, from 0.7 Tg CO2 Eq.
to 3.5 Tg CO2 Eq. in 2008, an over four-fold increase
over the time series.
2.2. Emissions by Economic Sector
Throughout this report, emission estimates are grouped
into six sectors (i.e., chapters) defined by the IPCC and
detailed above: Energy; Industrial Processes; Solvent and
Other Product Use; Agriculture; Land Use, Land-Use
Change, and Forestry; and Waste. While it is important to
use this characterization for consistency with UNFCCC
Table 2-11: Emissions from Waste (Tg C02 Eq.)
Gas/Source
CH4
Landfills
Wastewater Treatment
Composting
N20
Wastewater Treatment
Composting
Total
Note: Totals may not sum due to independent rounding.
1990
•
173.2
149.3
23.5
0.3 1
4.0 1
177.2
1995
169.6
144.1
24.8
0.7 1
4.8 1
4.0 1
0.8
174.5
2000
147.1
120.7
25.2
1
5.8
45|
1.4
153.0
2005
151.5
125.6
24.3
1.6
6.5
4.7
1.7
158.0
2006
153.1
127.1
24.5
1.6
6.6
4.8
1.8
159.7
2007
152.5
126.5
24.4
1.7
6.7
4.9
1.8
159.3
2008
152.3
126.3
24.3
1.7
6.8
4.9
1.8
159.1
4 Landfills also store carbon, due to incomplete degradation of organic
materials such as wood products and yard trimmings, as described in the
Land Use, Land-Use Change, and Forestry chapter.
5 The CO2 produced from combusted landfill CH4 at landfills is not counted
in national inventories as it is considered part of the natural C cycle of
decomposition.
Trends in Greenhouse Gas Emissions 2-17
-------
reporting guidelines, it is also useful to allocate emissions
into more commonly used sectoral categories. This section
reports emissions by the following U.S. economic sectors:
residential, commercial, industry, transportation, electricity
generation, and agriculture, as well as U.S. territories.
Using this categorization, emissions from electricity
generation accounted for the largest portion (35 percent)
of U.S. greenhouse gas emissions in 2008. Transportation
activities, in aggregate, accounted for the second largest
portion (27 percent). Emissions from industry accounted
for about 19 percent of U.S. greenhouse gas emissions in
2008. In contrast to electricity generation and transportation,
emissions from industry have in general declined over the
past decade. The long-term decline in these emissions
has been due to structural changes in the U.S. economy
(i.e., shifts from a manufacturing-based to a service-based
economy), fuel switching, and efficiency improvements. The
remaining 19 percent of U.S. greenhouse gas emissions were
contributed by the residential, agriculture, and commercial
sectors, plus emissions from U.S. territories. The residential
sector accounted for 5 percent, and primarily consisted
of CO2 emissions from fossil fuel combustion. Activities
related to agriculture accounted for roughly 7 percent of
U.S. emissions; unlike other economic sectors, agricultural
sector emissions were dominated by N2O emissions from
agricultural soil management and CUt emissions from enteric
fermentation, rather than CO2 from fossil fuel combustion.
The commercial sector accounted for roughly 6 percent
of emissions, while U.S. territories accounted for about 1
percent.
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors
(Tg C02 Eq. and Percent of Total in 2008)
Sector/Source
Electric Power Industry
C02from Fossil Fuel Combustion
Incineration of Waste
Electrical Transmission and
Distribution
Stationary Combustion
Limestone and Dolomite Use
Transportation
C02from Fossil Fuel Combustion
Substitution of Ozone Depleting
Substances
Mobile Combustion
Non-Energy Use of Fuels
Industry
C02from Fossil Fuel Combustion
Natural Gas Systems
Non-Energy Use of Fuels
Iron and Steel & Metallurgical
Coke Production
Coal Mining
Cement Production
Petroleum Systems
Nitric Acid Production
Lime Production
HCFC-22 Production
Ammonia Production and Urea
Consumption
Aluminum Production
Substitution of Ozone Depleting
Substances
1990
1,867.2
1,820.8
8.5!
26.6
8.6 1
2.6
1,545.0
1,485.8
+ 1
47.4
11.8
1,506.6
814.4
166.9
102.0
103.5
84.1
33.3
34.4
18.9
11.5
36.4
16.81
25.4
+
1995
1,993.7
1,947.9
11.9
21.4
I"
_
1,695.2
1,608.0
19.0
56.9
11.3
1,531.3
826.0
174.8
125.6
96.7
67.1
36.8
32.5
21.0
13.3
33.0
17.8
17.5
1.2
2000
2,336.8
2,296.9
n|
15.0
10.6
2.5 1
1,932.3
1,809.5
55.7
55.1
12.1
1,469.1
813.4
160.1
123.1
89.0
60.4
41.2
30.7
20.7
14.1
28.6
16.4
14.7
3.1
2005
2,443.5
2,402.1
13.0
14.0
11.0
3.4
2,016.1
1,895.3
72.9
37.6
10.2
1,350.9
778.8
133.1
118.3
68.4
56.9
45.9
28.7
17.6
14.4
15.8
12.8
7.1
5.2
2006
2,387.5
2,346.4
13.1
13.2
10.8
4.0
1,993.0
1,876.7
72.2
34.1
9.9
1,380.2
801.7
132.6
122.6
71.3
58.3
46.6
28.7
17.2
15.1
13.8
12.3
6.3
5.7
2007
2,454.0
2,412.8
13.7
12.7
11.0
3.9
2,003.5
1,893.7
68.8
30.7
10.2
1,374.2
793.8
130.3
116.6
73.5
58.1
45.2
29.3
20.5
14.6
17.0
14.0
8.1
6.1
2008
2,404.2
2,363.5
13.5
13.1
10.8
3.3
1,886.1
1,785.3
64.9
26.4
9.5
1,342.4
773.9
126.4
117.5
69.7
67.6
41.1
29.5
19.0
14.3
13.6
11.8
7.2
6.6
Percent3
34.6%
34.0%
0.2%
0.2%
0.2%
0.0%
27.1%
25.7%
0.9%
0.4%
0.1%
19.3%
11.1%
1.8%
1.7%
1.0%
1.0%
0.6%
0.4%
0.3%
0.2%
0.2%
0.2%
0.1%
0.1%
2-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (continued)
(Tg C02 Eq. and Percent of Total in 2008)
Sector/Source
Abandoned Underground Coal
Mines
Semiconductor Manufacture
N20 from Product Uses
Petrochemical Production
Soda Ash Production and
Consumption
Stationary Combustion
Limestone and Dolomite Use
Adipic Acid Production
Magnesium Production and
Processing
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Mobile Combustion
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and
Consumption
Agriculture
N20 from Agricultural Soil
Management
Enteric Fermentation
Manure Management
C02from Fossil Fuel Combustion
CH4 and N20 from Forest Fires
Rice Cultivation
Liming of Agricultural Soils
Urea Fertilization
Field Burning of Agricultural
Residues
C02 and N20 from Managed
Peatlands
Mobile Combustion
N20 from Forest Soils
Stationary Combustion
Commercial
C02from Fossil Fuel Combustion
Landfills
Substitution of Ozone Depleting
Substances
Wastewater Treatment
Human Sewage
Composting
Stationary Combustion
Residential
C02from Fossil Fuel Combustion
Substitution of Ozone Depleting
Substances
1990 1995 2000 2005
6.0 8.2
2.9 1 4.9
4.4l 4.6
4.2l 5.2
4.1 1 4.3
4.7 4.9
2.6 3.3
15.8 17.6
5.4l 5.6
1.2l 1.5
1.4l 1.4
2.2l 2.0
0.9 1 1.0
1.5l 1.5
0.9 1.0
0.3 0.3
0.4 0.3
433.2 460.8 1
7.4
6.2
4.9
5.7
4.2
4.8
2.5
5.5
3.0
1.8
1.4
1.9
1.1
1.4
1.1
0.3
0.3
485.3
5.6
4.4
4.4
5.3
4.2
4.4
3.4
5.0
2.9
1.8
1.3
1.4
1.3
1.4
0.5
0.3
0.2
494.1
203.5 205.9 210.1 215.8
132.4 143.7
43.7 49.3
31.0 36.6
5.8 7.7
7.1 7.6
4.7 4.4
2.4 2.7
1.2 1.1
1.0 1.0
0.3 0.4
0.1 0.2
+ +
395.1 399.6
216.7 223.2
136.8 136.7
55.2 58.9
38.7 46.8
26.0 17.8
7.5 1 6.8
4.3 1 4.3
3.2l 3.5
1 1.5
12 1 1.1
0.4l 0.5
0.4 0.4
+ 1 +
387.3 399.0
227.7 1 221.3
149.3 144.1 120.7 125.6
+ 1 0.7 1 5.5l 18.5
23.5 24.8 25.2 24.3
3.7 1 4.0 1 4.5l 4.7
0.7 1 1.5l 2.6 1 3.3
1.2 1.3 1.2 1.2
345.9 367.6 386.7 370.7
339.1 353.3 371.2 358.4
0.3 8.1 10.1 6.5
2006
5.5
4.7
4.4
4.8
4.2
4.6
4.0
4.3
2.9
1.8
1.7
1.5
1.3
1.2
0.5
0.3
0.2
515.1
211.2
139.0
59.6
49.0
39.2
5.9
4.2
3.7
1.4
0.9
0.5
0.4
+
389.2
206.0
127.1
22.4
24.5
4.8
3.3
1.1
334.9
322.1
7.5
2007
5.7
4.7
4.4
4.9
4.1
4.5
3.9
3.7
2.6
1.9
1.9
1.6
1.3
1.2
0.4
0.3
0.2
518.0
211.0
141.2
63.2
48.4
36.3
6.2
4.5
3.8
1.5
1.0
0.5
0.4
+
404.4
217.4
126.5
26.6
24.4
4.9
3.5
1.2
356.2
341.7
8.6
2008
5.9
5.4
4.4
4.4
4.1
4.1
3.3
2.0
2.0
1.8
1.8
1.6
1.3
1.2
0.4
0.3
0.2
504.1
215.9
140.8
62.1
45.4
21.7
7.2
3.8
3.8
1.5
0.9
0.5
0.4
+
410.9
219.5
126.3
31.1
24.3
4.9
3.5
1.2
359.3
342.7
10.3
Percent3
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
0.0%
7.2%
3.1%
2.0%
0.9%
0.7%
0.3%
0.1%
0.1%
0.1%
0.0%
0.0%
0.0%
0.0%
0.0%
5.9%
3.2%
1.8%
0.4%
0.3%
0.1%
0.1%
0.0%
5.2%
4.9%
0.1%
Trends in Greenhouse Gas Emissions 2-19
-------
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors
(Tg C02 Eq. and Percent of Total in 2008) (continued)
Sector/Source
1990
1995
Stationary Combustion
Settlement Soil Fertilization
U.S. Territories
C02from Fossil Fuel Combustion
Non-Energy Use of Fuels
Stationary Combustion
Total Emissions
Sinks
C02 Flux from Forests
Urban Trees
C02 Flux from Agricultural Soil
Carbon Stocks
Landfilled Yard Trimmings and
Food Scraps
Net Emissions
(Sources and Sinks)
5.5
1.0
33.7
27.9
5.7
0.1
6,126.8
(909.4)
(729.8)
(57.1)
(99.1)
(23.5)
5,217.3
5.0
1.2
40.7
34.5
6.0
0.1
6,488.8
(842.9)
(692.6)
(67.3)
(69.0)
(13.9)
5,646.0
4.3
„
46.9
35.9
10.9
0.1
7,044.5
(664.2)
(467.7)
(77.5)
(107.7)
(11.3)
6,380.2
4.3
1.5
58.9
50.6
8.1
0.2
7,133.2
(950.4)
(806.6)
(87.8)
(45.9)
(10.1)
6,182.8
3.9
1.5
60.0
50.9
8.9
0.2
7,059.9
(959.2)
(812.5)
(89.8)
(46.5)
(10.3)
6,100.7
4.2
1.6
57.8
49.1
8.5
0.2
7,168.1
(955.4)
(806.9)
(91.9)
(46.9)
(9.8)
6,212.7
4.6
1.6
49.9
42.5
7.2
0.2
6,956.8
(940.3)
(791.9)
(93.9)
(45.0)
(9.5)
6,016.4
0
.1%
0.0%
0.7%
0
0
0
.6%
.1%
.0%
100.0%
-13.5%
-11
-1
-0
-0
.4%
.3%
.6%
.1%
86.5%
+ Does not exceed 0.05 Tg C02 Eq. or 0.05 percent.
a Percent of total emissions for year 2008.
Note: Includes all emissions of C02, CH4, N20, MFCs, PFCs,
independent rounding.
and SF6. Parentheses indicate negative values or sequestration. Totals may not sum due to
Carbon dioxide was also emitted and sequestered by a
variety of activities related to forest management practices,
tree planting in urban areas, the management of agricultural
soils, and landfllling of yard trimmings.
Table 2-12 presents a detailed breakdown of emissions
from each of these economic sectors by source category, as
they are denned in this report. Figure 2-12 shows the trend
in emissions by sector from 1990 to 2008.
Figure 2-12
Emissions Allocated to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
0-
Electric Powerlndustry
Transportation
\
Industry
.griculture
Commercial
Residential
Note: Does not include U.S. Territories.
Emissions with Electricity Distributed to
Economic Sectors
It can also be useful to view greenhouse gas emissions
from economic sectors with emissions related to electricity
generation distributed into end-use categories (i.e., emissions
from electricity generation are allocated to the economic
sectors in which the electricity is consumed). The generation,
transmission, and distribution of electricity, which is the
largest economic sector in the United States, accounted for
35 percent of total U.S. greenhouse gas emissions in 2008.
Emissions increased by 30 percent since 1990, as electricity
demand grew and fossil fuels remained the dominant
energy source for generation. Electricity generation-related
emissions decreased from 2007 to 2008 by 2 percent,
primarily due to decreased CO2 emissions from fossil fuel
combustion. The electricity generation sector in the United
States is composed of traditional electric utilities as well as
other entities, such as power marketers and non-utility power
producers. The majority of electricity generated by these
entities was through the combustion of coal in boilers to
produce high-pressure steam that is passed through a turbine.
Table 2-13 provides a detailed summary of emissions from
electricity generation-related activities.
2-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-13: Electricity Generation-Related Greenhouse Gas Emissions (Tg C02 Eq.)
Gas/Fuel Type or Source
C02 1
C02from Fossil Fuel Combustion 1
Coal 1
Natural Gas
Petroleum
Geothermal
Incineration of Waste
Limestone and Dolomite Use
CH4
Stationary Combustion3
Incineration of Waste
N20
Stationary Combustion3
Incineration of Waste
SF6
Electrical Transmission and Distribution
Total 1
+ Does not exceed 0.05 Tg C02 Eq. or 0.05 percent.
"Includes only stationary combustion emissions related to
Note: Totals may not sum due to independent rounding.
1990
,831.4
,820.8
,547.6 1
175.3
97.5
0.4l
8.0 1
2.6 1
0.6 1
0.6 1
8.5
8.1 1
0.5 1
26.6
26.6
,867.2
1995
1,962.7
1,947.9
1,660.7
228.1
58.7
Ojl
11.5
3.3l
0.6 1
0.6 1
9.0
8.6 1
0.5 1
21.4
21.4
1,993.7
2000
2,310.7
2,296.9
1,927.4
280.8
88.4
0.4l
11.3
25
0.7 1
0.7
+
10.4
10.0
0.4
15.0
15.0
2,336.8
2005
2,418.1
2,402.1
1,983.8
318.8
99.2
0.4
12.6
3.4
0.7
0.7
+
10.7
10.3
0.4
14.0
14.0
2,443.5
2006
2,363.1
2,346.4
1,953.7
338.0
54.4
0.4
12.7
4.0
0.7
0.7
+
10.5
10.1
0.4
13.2
13.2
2,387.5
2007
2,430.0
2,412.8
1,987.3
371.3
53.9
0.4
13.3
3.9
0.7
0.7
+
10.6
10.2
0.4
12.7
12.7
2,454.0
2008
2,379.9
2,363.5
1,962.6
361.6
38.9
0.4
13.1
3.3
0.7
0.7
+
10.5
10.1
0.4
13.1
13.1
2,404.2
the generation of electricity.
To distribute electricity emissions among economic
end-use sectors, emissions from the source categories
assigned to the electricity generation sector were allocated
to the residential, commercial, industry, transportation,
and agriculture economic sectors according to retail sales
of electricity (EIA 2009a and Dufneld 2006). These three
source categories include CO2 from Fossil Fuel Combustion,
CH4 and N2O from Stationary Combustion, and SF6 from
Electrical Transmission and Distribution Systems.6
When emissions from electricity are distributed among
these sectors, industry accounts for the largest share of U.S.
greenhouse gas emissions (29 percent), followed closely by
emissions from transportation activities, which account for
27 percent of total emissions. Emissions from the residential
and commercial sectors also increase substantially when
emissions from electricity are included, due to their relatively
large share of electricity consumption. In all sectors except
agriculture, CO2 accounts for more than 80 percent of
greenhouse gas emissions, primarily from the combustion
of fossil fuels.
Table 2-14 presents a detailed breakdown of emissions
from each of these economic sectors, with emissions from
electricity generation distributed to them. Figure 2-13 shows
the trend in these emissions by sector from 1990 to 2008.
Figure 2-13
Emissions with Electricity Distributed to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
Industry
Transportation
Residential
Commercial
Agriculture
Note: Does not include U.S. Territories.
6 Emissions were not distributed to U.S. territories, since the electricity
generation sector only includes emissions related to the generation of
electricity in the 50 states and the District of Columbia.
Trends in Greenhouse Gas Emissions 2-21
-------
Table 2-14: U.S. Greenhouse Gas Emissions by Economic Sector and Gas with Electricity-Related
Emissions Distributed (Tg C02 Eq.) and Percent of Total in 2008
Sector/Gas
Industry
Direct Emissions
C02
CH4
N20
MFCs, PFCs, and SF6
Electricity-Related
C02
CH4
N20
SF6
Transportation
Direct Emissions
C02
CH4
N20
HFCsb
Electricity-Related
C02
CH4
N20
SF6
Commercial
Direct Emissions
C02
CH4
N20
MFCs
Electricity-Related
C02
CH4
N20
SF6
Residential
Direct Emissions
C02
CH4
N20
MFCs
Electricity-Related
C02
CH4
N20
SF6
Agriculture
Direct Emissions
C02
CH4
N20
1990
2,179.8
1,506.6
1,130.3
294.8
32.7
48.8 1
673.3 1
660.41
0.2l
3.1
9.6 1
1,548.2
1,545.0
1,497.6
4.5l
42.95
+
3.1 1
3.1 1
1
1
+
946.8
395.1
216.71
174.0
4.4l
551.71
541.1
0.2 1
0.3 1
608.1
596.5
0.2l
2.8 1
8.7!
464.2
433.2
39.2!
172. g|
221.2
1995 2000
2,228.0 2,239.2
1,531.3 1, 469.1 1
1,160.5 1,136.3
286.5 262.5
35.4 32.8
48.9 37.5
696.71 770.1
685.9 761.5
0.2l 0.2
3.2l 3.4|
7.5 5.ol
1,698.3 1,935.8
1,695.2 1,932.3
1,619.3 1,821.e!
4.o! 3.1 1
52.87 51.95
19.0 55.7
3.1 1 3.s!
3.1 1 3.5l
+ l +l
+ l +l
+ 1 +1
1,000.2 1.141.5
399.6
223.2
170.5
5.2
0.7
600.6
591.3
0.2
2.7
6.4
1,024.5
367.6
353.3
4.0
2.2
8.1
656.9
646.7
0.2
3.0
7.1
497.1
460.8
44.7
190.3
387.3
227.7
148.0
6.2!
5.5l
754.1
745.7
0.2l
3.3l
4.8
1,162.4
386.7
371.2
1
10.1
775.7
767.0
0.2
34l
5.0
51 8.?!
485.3
47.6
198.2
225.8 239.6
2005
2,071.1
1,350.9
1,062.2
227.7
29.0
31.9
720.2
712.8
0.2
3.2
4.1
2,020.9
2,016.1
1,905.5
2.2
35.46
72.9
4.8
4.8
+
+
+
1,216.5
399.0
221.3
152.4
6.8
18.5
817.6
809.1
0.2
3.6
4.7
1,242.2
370.7
358.4
3.4
2.4
6.5
871.5
862.5
0.3
3.8
5.0
523.5
494.1
55.7
196.7
241.7
2006
2,077.3
1,380.2
1,090.6
228.5
28.8
32.3
697.1
690.0
0.2
3.1
3.9
1,997.6
1,993.0
1,886.6
2.0
32.12
72.2
4.6
4.6
+
+
+
1,202.2
389.2
206.0
154.0
6.9
22.4
813.0
804.7
0.2
3.6
4.5
1,180.3
334.9
322.1
3.1
2.3
7.5
845.4
836.8
0.3
3.7
4.7
542.5
515.1
57.8
209.9
247.4
2007
2,084.2
1,374.2
1,083.0
226.6
31.7
32.9
710.0
703.0
0.2
3.1
3.7
2,008.6
2,003.5
1,904.0
1.9
28.78
68.8
5.1
5.1
+
+
+
1,240.1
404.4
217.4
153.4
7.0
26.6
835.8
827.6
0.2
3.6
4.3
1,226.9
356.2
341.7
3.4
2.5
8.6
870.7
862.2
0.3
3.8
4.5
550.5
518.0
57.8
214.3
245.9
2008
2,018.4
1,342.4
1,049.5
232.5
29.3
31.1
676.0
669.2
0.2
2.9
3.7
1,890.8
1,886.1
1,794.8
1.7
24.65
64.9
4.8
4.7
+
+
+
1,250.6
410.9
219.5
153.2
7.1
31.1
839.6
831.2
0.2
3.7
4.6
1,215.6
359.3
342.7
3.7
2.5
10.3
856.3
847.7
0.3
3.7
4.7
531.6
504.1
54.0
206.1
244.0
Percent3
29.0%
19.3%
15.1%
3.3%
0.4%
0.4%
9.7%
9.6%
0.0%
0.0%
0.1%
27.2%
27.1%
25.8%
0.0%
0.4%
0.9%
0.1%
0.1%
0.0%
0.0%
0.0%
18.0%
5.9%
3.2%
2.2%
0.1%
0.4%
12.1%
11.9%
0.0%
0.1%
0.1%
17.5%
5.2%
4.9%
0.1%
0.0%
0.1%
12.3%
12.2%
0.0%
0.1%
0.1%
7.6%
7.2%
0.8%
3.0%
3.5%
2-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-14: U.S. Greenhouse Gas Emissions by Economic Sector and Gas with Electricity-Related
Emissions Distributed (Tg C02 Eq.) and Percent of Total in 2008 (continued)
Sector/Gas
Electricity-Related
C02
CH4
N20
SF6
U.S. Territories
1990
31.01
30.41
+ 1
0.11
0.4l
33.7
1995
2000
2005
2006
2007
2008 Percent3
36.4
35.8
0.2
0.4
40.7
33.3
33.0
0.2
46.9
29.4
29.1
+
0.1
0.2
58.9
27.4
27.1
+
0.1
0.2
60.0
32.5
32.2
+
0.1
0.2
57.8
27.5
27.2
+
0.1
0.1
49.9
0.4%
0.4%
0.0%
0.0%
0.0%
0.7%
Total
6,126.8
6,488.8
7,044.5
7,133.2 7,059.9 7,168.1 6,956.8 100.0%
+ Does not exceed 0.05 Tg C02 Eq. or 0.05 percent.
a Percent of total emissions for year 2008.
b Includes primarily HFC-134a.
Note: Emissions from electricity generation are allocated based on aggregate electricity consumption in each end-use sector.
Totals may not sum due to independent rounding.
Industry
The industrial end-use sector includes CO2 emissions
from fossil fuel combustion from all manufacturing facilities,
in aggregate. This sector also includes emissions that are
produced as a byproduct of the non-energy-related industrial
process activities. The variety of activities producing these
non-energy-related emissions, to name a few includes
fugitive CH4 emissions from coal mining, by-product CO2
emissions from cement manufacture, and HFC, PFC, and
SF6 by-product emissions from semiconductor manufacture.
Overall, direct industry sector emissions have declined since
1990, while electricity-related emissions have risen. In
theory, emissions from the industrial end-use sector should
be highly correlated with economic growth and industrial
output, but heating of industrial buildings and agricultural
energy consumption are also affected by weather conditions.
In addition, structural changes within the U.S. economy
that lead to shifts in industrial output away from energy-
intensive manufacturing products to less energy-intensive
products (e.g., from steel to computer equipment) also have
a significant effect on industrial emissions.
Transportation
When electricity-related emissions are distributed
to economic end-use sectors, transportation activities
accounted for 27 percent of U.S. greenhouse gas emissions
in 2008. The largest sources of transportation GHGs in 2008
were passenger cars (33 percent), light duty trucks, which
include sport utility vehicles, pickup trucks, and minivans
(29 percent), freight trucks (21 percent) and commercial
aircraft (7 percent). These figures include direct emissions
from fossil fuel combustion, as well as HFC emissions from
mobile air conditioners and refrigerated transport allocated to
these vehicle types. Table 2-15 provides a detailed summary
of greenhouse gas emissions from transportation-related
activities with electricity-related emissions included in the
totals.
From 1990 to 2008, transportation emissions rose by 22
percent due, in large part, to increased demand for travel and
the stagnation of fuel efficiency across the U. S. vehicle fleet.
The number of vehicle miles traveled by light-duty motor
vehicles (passenger cars and light-duty trucks) increased
37 percent from 1990 to 2008, as a result of a confluence
of factors including population growth, economic growth,
urban sprawl, and low fuel prices over much of this period.
A similar set of social and economic trends has led to a
significant increase in air travel and freight transportation by
both air and road modes during the time series.
Although average fuel economy over this period
increased slightly due primarily to the retirement of older
vehicles, average fuel economy among new vehicles sold
annually gradually declined from 1990 to 2004. The decline
in new vehicle fuel economy between 1990 and 2004 reflected
the increasing market share of light duty trucks, which grew
from about one-fifth of new vehicle sales in the 1970s to
slightly over half of the market by 2004. Increasing fuel
prices have since decreased the momentum of light duty
truck sales, and average new vehicle fuel economy has
improved since 2005 as the market share of passenger cars
increased. VMT growth among all passenger vehicles has
also been impacted, remaining stagnant from 2004 to 2007,
compared to an average annual growth rate of 2.5 percent
Trends in Greenhouse Gas Emissions 2-23
-------
Table 2-15: Transportation-Related Greenhouse Gas Emissions (Tg C02 Eq.)
Vehicle Type/Gas
Passenger Cars
C02
CH4
N20
MFCs
Light-Duty Trucks
C02
CH4
N20
MFCs
Medium- and Heavy-Duty Trucks
C02
CH4
N20
MFCs
Buses
C02
CH4
N20
MFCs
Motorcycles
C02
CH4
N20
Commercial Aircraft3
C02
CH4
N20
Other Aircraft
C02
CH4
N20
Ships and Boats'
C02
CH4
N20
MFCs
Rail
C02
CH4
N20
MFCs
1990H
657.3
629.2
2.6 1
0.4
8.4l
1
+ •
1.8 1
136.8
135.41
0.1 1
I
44.4
43.9 1
0.1
0.4l
45.1
44.5
0.6 1
+ l
39.0
38.5
0,1
0.3 •
+
1995 2000 2005
645.9
606.7
2.1
26.9
10.1
436.5
406.4
1.4
22.1
6.5
277.7
274.8
0.2
1.0
1.7
9.2
9.2
+
+
+
1.8
1.8
+
+
143.1
141.6
0.1
1.4
32.3
32.0
0.1
0.3
58.6
57.7
+
0.8
+
43.7
42.7
0.1
0.3
0.5
695.2
644.1
1.6
25.2
24.3
512.0
466.9
1.1
22.4
21.7
354.5
345.8
0.1
1.2
7.4
11.2
11.1
709.3
662.0
1.1
17.8
28.4
551.0
505.6
0.7
13.7
31.0
408.3
396.0
0.1
1.1
11.1
12.0
11.8
+ 1 +
+ 1 +
0.1
1.9
1.8
+
+
170.9
169.2
0.1
1.6
33.5
33.1
0.1
0.3
61.3
60.2
+
0.9
0.1
48.0
45.6
0.1
0.3
2.0
0.2
1.7
1.6
+
+
162.8
161.2
0.1
1.5
35.1
34.7
0.1
0.3
45.2
44.5
+
0.6
+
53.0
50.3
0.1
0.4
2.2
2006
682.6
638.8
1.0
15.7
27.1
563.6
519.2
0.7
12.6
31.2
418.6
406.0
0.1
1.1
11.4
12.3
12.0
+
+
0.3
1.9
1.9
+
+
138.5
137.1
0.1
1.3
34.3
34.0
0.1
0.3
48.4
47.7
+
0.7
+
55.1
52.4
0.1
0.4
2.2
2007
671.6
632.4
0.9
13.8
24.6
569.9
528.0
0.6
11.2
30.1
425.2
412.5
0.1
1.1
11.5
12.5
12.1
+
+
0.3
2.1
2.1
+
+
139.5
138.1
0.1
1.3
33.7
33.4
0.1
0.3
55.2
54.4
+
0.8
+
54.3
51.6
0.1
0.4
2.2
2008
632.1
597.5
0.8
11.7
22.1
552.4
513.7
0.6
9.5
28.6
401.2
388.6
0.1
1.0
11.6
12.1
11.7
+
+
0.4
2.2
2.1
+
+
123.4
122.2
0.1
1.2
33.7
33.3
0.1
0.3
38.7
38.1
+
0.5
+
50.6
47.9
0.1
0.4
2.3
Other Emissions from Electricity
Generationd
Pipelines6
C02
Lubricants
C02
0,
36.0
36.0
11.8
11.8
0.1
38.2
38.2
11.3
11.3
0.1
32.3
32.3
10.2
10.2
0.1
32.3
32.3
9.9
9.9
0.1
34.3
34.3
10.2
10.2
0.1
34.9
34.9
9.5
9.5
2-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-15: Transportation-Related Greenhouse Gas Emissions (Tg C02 Eq.) (continued)
Vehicle Type/Gas
Total Transportation
International Bunker Fuels'
1990
1,548.2
113.0
1995
1,698.3
100.9
2000
1,935.8
99.5
2005
2,020.9
111.7
2006
1,997.6
130.5
2007
2,008.6
128.4
2008
1,890.8
136.6
+ Does not exceed 0.05 Tg C02 Eq.
a Consists of emissions from jet fuel consumed by domestic operations of commercial aircraft (no bunkers).
b Consists of emissions from jet fuel and aviation gasoline consumption by general aviation and military aircraft.
c Fluctuations in emission estimates are associated with fluctuations in reported fuel consumption, and may reflect data collection problems.
d Other emissions from electricity generation are a result of waste incineration (as the majority of municipal solid waste is combusted in "trash-to-steam"
electricity generation plants), electrical transmission and distribution, and a portion of limestone and dolomite use (from pollution control equipment
installed in electricity generation plants).
eC02 estimates reflect natural gas used to power pipelines, but not electricity. While the operation of pipelines produces CH4 and N20, these emissions
are not directly attributed to pipelines in the US Inventory.
'Emissions from International Bunker Fuels include emissions from both civilian and military activities; these emissions are not included in the
transportation totals.
Note: Totals may not sum due to independent rounding. Passenger cars and light-duty trucks include vehicles typically used for personal travel and less
than 8500 Ibs; medium- and heavy-duty trucks include vehicles 8501 Ibs and above. HFC emissions primarily reflect HFC-134a.
over the period 1990 to 2004. The recession supplemented
the effect of increasing fuel prices in 2008 and VMT declined
by 2.1 percent, the first decrease in annual passenger vehicle
VMT since 1990.
Almost all of the energy consumed for transportation
was supplied by petroleum-based products, with more than
half being related to gasoline consumption in automobiles
and other highway vehicles. Other fuel uses, especially diesel
fuel for freight trucks and jet fuel for aircraft, accounted for
the remainder. The primary driver of transportation-related
emissions was CO2 from fossil fuel combustion, which
increased by 20 percent from 1990 to 2008. This rise in CO2
emissions, combined with an increase in HFCs from virtually
no emissions in 1990 to 64.9 Tg CO2 Eq. in 2008, led to an
increase in overall emissions from transportation activities
of 22 percent.
Commercial
The commercial sector is heavily reliant on electricity
for meeting energy needs, with electricity consumption for
lighting, heating, air conditioning, and operating appliances.
The remaining emissions were largely due to the direct
consumption of natural gas and petroleum products, primarily
for heating and cooking needs. Energy-related emissions from
the residential and commercial sectors have generally been
increasing since 1990, and are often correlated with short-
term fluctuations in energy consumption caused by weather
conditions, rather than prevailing economic conditions.
Landfills and wastewater treatment are included in this
sector, with landfill emissions decreasing since 1990, while
wastewater treatment emissions have increased slightly.
Residential
The residential sector is heavily reliant on electricity
for meeting energy needs, with electricity consumption for
lighting, heating, air conditioning, and operating appliances.
The remaining emissions were largely due to the direct
consumption of natural gas and petroleum products, primarily
for heating and cooking needs. Emissions from the residential
sectors have generally been increasing since 1990, and
are often correlated with short-term fluctuations in energy
consumption caused by weather conditions, rather than
prevailing economic conditions. In the long-term, this sector
is also affected by population growth, regional migration
trends, and changes in housing and building attributes (e.g.,
size and insulation).
Agriculture
The agricultural sector includes a variety of processes,
including enteric fermentation in domestic livestock,
livestock manure management, and agricultural soil
management. In 2008, enteric fermentation was the largest
source of CH4 emissions in the United States, and agricultural
soil management was the largest source of N2O emissions in
the United States. This sector also includes small amounts
of CO2 emissions from fossil fuel combustion by motorized
farm equipment like tractors.
Trends in Greenhouse Gas Emissions 2-25
-------
Box 2-1: Methodology for Aggregating Emissions by Economic Sector
In presenting the Economic Sectors in the annual Inventory of U.S. Greenhouse Gas Emissions and Sinks, EPA expands upon the
standard IPCC sectors common for UNFCCC reporting. EPA believes that discussing greenhouse gas emissions relevant to U.S.-specific
sectors improves communication of the report's findings.
Electricity Generation: Carbon dioxide emissions from the combustion of fossil fuels included in the EIA electric-utility fuel-consuming
sector are apportioned to this economic sector. Stationary combustion emissions of CH4 and N20 are also based on the EIA electric-utility
sector. Additional sources include C02 and N20 from waste incineration, as the majority of municipal solid waste is combusted in "trash-to-
steam" electricity generation plants. The Electricity Generation economic sector also includes SF6 from electrical transmission and distribution,
and a portion of C02 from limestone and dolomite use (from pollution control equipment installed in electricity generation plants).
Transportation: Carbon dioxide emissions from the combustion of fossil fuels included in the EIA transportation fuel-consuming sector
are apportioned to this economic sector (additional analyses and refinement of the EIA data is further explained in the Energy chapter of this
report). Additional emissions are apportioned from CH4 and N20 from mobile combustion, based on the EIA transportation sector. Substitutes
for ozone depleting substances are apportioned to this economic sector based on their specific end-uses within the source category, along
with emissions from transportation refrigeration/air-conditioning systems. Finally, C02 emissions from non-energy uses of fossil fuels identified
as lubricants for transportation vehicles are included in the Transportation economic sector.
Industry: Carbon dioxide emissions from the combustion of fossil fuels included in the EIA industrial fuel-consuming sector, minus the
agricultural use of fuel explained below, are apportioned to this economic sector. Stationary and mobile combustion emissions of CH4 and
N20 are also based on the EIA industrial sector, minus emissions apportioned to the Agriculture economic sector described below. Substitutes
for ozone depleting substances are apportioned based on their specific end-uses within the source category, with most emissions falling
within the Industry economic sector (emissions from the other economic sectors are subtracted to avoid double-counting). Additionally, all
process-related emissions from sources with methods considered within the IPCC Industrial Process guidance have been apportioned to
this economic sector. This includes the process-related emissions (i.e., emissions resulting from the processes used to make materials, and
not from burning fuels to provide power or heat) from such activities as cement production, iron and steel production and metallurgical coke
production, and ammonia production. Additionally, fugitive emissions from energy production sources, such as natural gas systems, coal
mining, and petroleum systems are included in the Industry economic sector. A portion of C02 emissions from limestone and dolomite use
(from pollution control equipment installed in large industrial facilities) are also included in the Industry economic sector. Finally, all remaining
C02 emissions from non-energy uses of fossil fuels are assumed to be industrial in nature (besides the lubricants for transportation vehicles
specified above), and are attributed to the Industry economic sector.
Agriculture: As agricultural equipment is included in ElA's industrial fuel-consuming sector surveys, additional data is used to separate
out the fuel used by agricultural equipment, to allow for accurate reporting in the Agriculture economic sector from all sources of emissions,
such as motorized farming equipment. Energy consumption estimates are obtained from Department of Agriculture survey data, in combination
with separate EIA fuel sales reports. This supplementary data is used to apportion C02 emissions from fossil fuel combustion and CH4 and
N20 emissions from stationary and mobile combustion (this data is subtracted from the Industry economic sector to avoid double-counting).
The other emission sources included in this economic sector are non-combustion sources of emissions that are included in the Agriculture
and Land Use, Land-Use Change and Forestry chapters: N20 emissions from agricultural soils, CH4 from enteric fermentation (i.e., exhalation
from the digestive tracts of domesticated animals), CH4 and N20 from manure management, CH4 from rice cultivation, C02 emissions from
liming of agricultural soils and urea application, and CH4 and N20 from forest fires. Nitrous oxide emissions from the application of fertilizers
to tree plantations (termed "forest land" by the IPCC) are also included in the Agriculture economic sector.
Residential: This economic sector includes the C02 emissions from the combustion of fossil fuels reported for the EIA residential sector.
Stationary combustion emissions of CH4 and N20 are also based on the EIA residential fuel-consuming sector. Substitutes for ozone depleting
substances are apportioned based on their specific end-uses within the source category, with emissions from residential air-conditioning
systems distributed to this economic sector. Nitrous oxide emissions from the application of fertilizers to developed land (termed "settlements"
by the IPCC) are also included in the Residential economic sector.
Commercial: This economic sector includes the C02 emissions from the combustion of fossil fuels reported in the EIA commercial
fuel-consuming sector data. Stationary combustion emissions of CH4 and N20 are also based on the EIA commercial sector. Substitutes for
ozone depleting substances are apportioned based on their specific end-uses within the source category, with emissions from commercial
refrigeration/air-conditioning systems distributed to this economic sector. Public works sources including direct CH4from landfills and CH4
and N20 from wastewater treatment and composting are included in this economic sector.
2-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Electricity Generation
The process of generating electricity, for consumption in
the above sectors, is the single largest source of greenhouse
gas emissions in the United States, representing 35 percent
of total U.S. emissions. Electricity generation also accounted
for the largest share of CO2 emissions from fossil fuel
combustion, approximately 42 percent in 2008. Electricity
was consumed primarily in the residential, commercial,
and industrial end-use sectors for lighting, heating, electric
motors, appliances, electronics, and air conditioning.
Box 2-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data
Total emissions can be compared to other economic and social indices to highlight changes over time. These comparisons include: (1)
emissions per unit of aggregate energy consumption, because energy-related activities are the largest sources of emissions; (2) emissions
per unit of fossil fuel consumption, because almost all energy-related emissions involve the combustion of fossil fuels; (3) emissions per
unit of electricity consumption, because the electric power industry—utilities and nonutilities combined—was the largest source of U.S.
greenhouse gas emissions in 2008; (4) emissions per unit of total gross domestic product as a measure of national economic activity; or
(5) emissions per capita.
Table 2-16 provides data on various statistics related to U.S. greenhouse gas emissions normalized to 1990 as a baseline year. Greenhouse
gas emissions in the United States have grown at an average annual rate of 0.9 percent since 1990. This rate is slightly slower than that for
total energy or fossil fuel consumption and much slower than that for either electricity consumption or overall gross domestic product. Total
U.S. greenhouse gas emissions have also grown slightly slower than national population since 1990 (see Figure 2-14).
Table 2-16: Recent Trends in Various U.S. Data (Index 1990 = 100)
Variable
GDPb
Electricity Consumption0
Fossil Fuel Consumption0
Energy Consumption0
Population"1
Greenhouse Gas Emissions6
1990
100
100
100
100
100
100
1995
113
112
107
107
107
106
2000
140
127
117
116
113
115
2005
157
134
119
119
118
116
2006
162
135
117
118
119
115
2007
165
138
119
120
120
117
2008
166
136
115
118
121
114
Growth
Rate"
2.9%
1.8%
0.8%
0.9%
1.1%
0.7%
3 Average annual growth rate.
b Gross Domestic Product in chained 2000 dollars (BEA 2008).
c Energy content-weighted values (EIA 2008a).
d U.S. Census Bureau (2008).
e GWP-weighted values.
Figure 2-14
U.S. Greenhouse Gas Emissions Per Capita and
Per Dollar of Gross Domestic Product
170
160
150
g 140
- 130
g 120
!L 110
| 100
I 90
80
70
60
Real GDP
Population
Emissions
per capita
Emissions
per $GDP
NNNNNNNNN
Source: BEA (2008), U.S. Census Bureau (2008), and emission estimates in the Inventory report.
Note: Does not include U.S. Territories.
Trends in Greenhouse Gas Emissions 2-27
-------
2.3. Indirect Greenhouse Gas
Emissions (CO, NOX, NMVOCs, and S02)
The reporting requirements of the UNFCCC7 request
that information be provided on indirect greenhouse gases,
which include CO, NOX, NMVOCs, and SO2. These gases
do not have a direct global warming effect, but indirectly
affect terrestrial radiation absorption by influencing the
formation and destruction of tropospheric and stratospheric
ozone, or, in the case of SO2, by affecting the absorptive
characteristics of the atmosphere. Additionally, some of
these gases may react with other chemical compounds in the
atmosphere to form compounds that are greenhouse gases.
Carbon monoxide is produced when carbon-containing
fuels are combusted incompletely. Nitrogen oxides (i.e.,
NO and NO2) are created by lightning, fires, fossil fuel
combustion, and in the stratosphere from N2O. Non-CH4
volatile organic compounds—which include hundreds of
organic compounds that participate in atmospheric chemical
reactions (i.e., propane, butane, xylene, toluene, ethane, and
many others)—are emitted primarily from transportation,
industrial processes, and non-industrial consumption of
organic solvents. In the United States, SO2 is primarily
emitted from coal combustion for electric power generation
and the metals industry. Sulfur-containing compounds
emitted into the atmosphere tend to exert a negative radiative
forcing (i.e., cooling) and therefore are discussed separately.
One important indirect climate change effect of
NMVOCs and NOX is their role as precursors for tropospheric
ozone formation. They can also alter the atmospheric lifetimes
of other greenhouse gases. Another example of indirect
greenhouse gas formation into greenhouse gases is CO's
interaction with the hydroxyl radical—the major atmospheric
sink for CH4 emissions—to form CO2. Therefore, increased
atmospheric concentrations of CO limit the number of
hydroxyl molecules (OH) available to destroy CH4.
Since 1970, the United States has published estimates
of annual emissions of CO, NOX, NMVOCs, and SO2 (EPA
2009),8 which are regulated under the Clean Air Act. Table
2-17 shows that fuel combustion accounts for the majority
of emissions of these indirect greenhouse gases. Industrial
processes—such as the manufacture of chemical and allied
products, metals processing, and industrial uses of solvents—
are also significant sources of CO, NOX, and NMVOCs.
Box 2-3: Sources and Effects of Sulfur Dioxide
Sulfur dioxide (S02) emitted into the atmosphere through natural and anthropogenic processes affects the earth's radiative budget
through its photochemical transformation into sulfate aerosols that can (1) scatter radiation from the sun back to space, thereby reducing
the radiation reaching the earth's surface; (2) affect cloud formation; and (3) affect atmospheric chemical composition (e.g., by providing
surfaces for heterogeneous chemical reactions). The indirect effect of sulfur-derived aerosols on radiative forcing can be considered in
two parts. The first indirect effect is the aerosols' tendency to decrease water droplet size and increase water droplet concentration in the
atmosphere. The second indirect effect is the tendency of the reduction in cloud droplet size to affect precipitation by increasing cloud lifetime
and thickness. Although still highly uncertain, the radiative forcing estimates from both the first and the second indirect effect are believed
to be negative, as is the combined radiative forcing of the two (IPCC 2001). However, because S02 is short-lived and unevenly distributed
in the atmosphere, its radiative forcing impacts are highly uncertain.
Sulfur dioxide is also a major contributor to the formation of regional haze, which can cause significant increases in acute and chronic
respiratory diseases. Once S02 is emitted, it is chemically transformed in the atmosphere and returns to the earth as the primary source of
acid rain. Because of these harmful effects, the United States has regulated S02 emissions in the Clean Air Act.
Electricity generation is the largest anthropogenic source of S02 emissions in the United States, accounting for 87 percent in
2007. Coal combustion contributes nearly all of those emissions (approximately 92 percent). Sulfur dioxide emissions have decreased
in recent years, primarily as a result of electric power generators switching from high-sulfur to low-sulfur coal and installing flue gas
desulfurization equipment.
7 See .
8 NOX and CO emission estimates from field burning of agricultural residues
were estimated separately, and therefore not taken from EPA (2009).
2-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 2-17: Emissions of NOx, CO, NMVOCs, and S02 (Gg)
Gas/Activity
1990
1995
2000
2005
2006
2007
2008
NOX
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Oil and Gas Activities
Incineration of Waste
Agricultural Burning
Solvent Use
Waste
CO
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Incineration of Waste
Agricultural Burning
Oil and Gas Activities
Waste
Solvent Use
NMVOCs
Mobile Fossil Fuel Combustion
Solvent Use
Industrial Processes
Stationary Fossil Fuel Combustion
Oil and Gas Activities
Incineration of Waste
Waste
Agricultural Burning
S02
Stationary Fossil Fuel Combustion
Industrial Processes
Mobile Fossil Fuel Combustion
Oil and Gas Activities
Incineration of Waste
Waste
Solvent Use
Agricultural Burning
21,728
10,862
10,023
591
139
82
30
i
130,536
119,360
5,000
4,125
978
766
302
20,930
10,932
5,216
2,422
912
554
222
673
NA
20,935
18,407
1,307
793
390
38
°
0
NA
21,227
10,536
9,862
607
100
88
30
3
1
109,114
97,630
5,383
3,959
1,073
745
316
2
5
19,520
8,745
5,609
2,642
973
582
237
731
NA
16,891
14,724
1,117
672
335
42
1
1
NA
19,145
10,199
8,053
626
111
114
37
3
2
92,872
83,559
4,340
2,126
1,670
888
146
15,227
7,229
4,384
1,773
1,077
388
2571
ng|
NA|
14,830
12,849
1,031
632
287
29
1
1
NA
15,933
9,012
5,858
569
321
129
40
3
2
71,555
62,692
4,649
1,555
1,403
930
318
7
2
13,761
6,330
3,851
1,997
716
510
241
114
NA
13,466
11,541
831
889
181
24
1
0
NA
15,071
8,488
5,545
553
319
121
40
4
2
67,909
58,972
4,695
1,597
1,412
905
319
7
2
13,594
6,037
3,846
1,933
918
510
238
113
NA
12,388
10,612
818
750
182
24
1
0
NA
14,410
7,965
5,432
537
318
114
38
4
2
64,348
55,253
4,744
1,640
1,421
960
320
7
2
13,423
5,742
3,839
1,869
1,120
509
234
111
NA
11,799
10,172
807
611
184
24
1
0
NA
13,578
7,441
5,148
520
318
106
40
4
2
60,739
51,533
4,792
1,682
1,430
970
322
7
2
13,254
5,447
3,834
1,804
1,321
509
230
109
NA
10,368
8,891
795
472
187
23
1
0
NA
NA (Not Available).
Note: Totals may not sum due to independent rounding.
Source: EPA (2009), disaggregated based on EPA (2003) except for estimates from field burning of agricultural residues.
Trends in Greenhouse Gas Emissions 2-29
-------
3. Energy
Energy-related activities were the primary sources of U.S. anthropogenic greenhouse gas emissions, accounting
for 86.2 percent of total emissions on a carbon dioxide (CO2) equivalent basis in 2008. This included 97, 37,
and 13 percent of the nation's CO2, methane (CH4), and nitrous oxide (N2O) emissions, respectively. Energy-
related CO2 emissions alone constituted 82.7 percent of national emissions from all sources on a CO2 equivalent basis,
while the non-CO2 emissions from energy-related activities represented a much smaller portion of total national emissions
(3.6 percent collectively).
Emissions from fossil fuel combustion comprise the vast majority of energy-related emissions, with CO2 being the
primary gas emitted (see Figure 3-1). Globally, approximately 30,377 Tg of CO2 were added to the atmosphere through
the combustion of fossil fuels in 2008, of which the United States accounted for about 19 percent.1 Due to their relative
importance, fossil fuel combustion-related CO2 emissions are considered separately, and in more detail than other energy-
related emissions (see Figure 3-2). Fossil fuel combustion also emits CH4 and N2O, as well as indirect greenhouse gases
such as nitrogen oxides (NOX), carbon monoxide (CO), andnon-CH4 volatile organic compounds (NMVOCs). Mobile fossil
fuel combustion was the second largest source of N2O emissions in the United States, and overall energy-related activities
were collectively the largest source of the indirect greenhouse gas emissions mentioned above.
Energy-related activities other than fuel combustion,
such as the production, transmission, storage, and distribution
of fossil fuels, also emit greenhouse gases. These emissions
consist primarily of fugitive CH4 from natural gas systems,
petroleum systems, and coal mining. Smaller quantities of
CO2, CO, NMVOCs, and NOX are also emitted.
The combustion of biomass and biomass-based fuels
also emits greenhouse gases. CO2 emissions from these
activities, however, are not included in national emissions
totals because biomass fuels are of biogenic origin.2 It is
assumed that the carbon (C) released during the consumption
of biomass is recycled as U.S. forests and crops regenerate,
causing no net addition of CO2 to the atmosphere. The net
impacts of land-use and forestry activities on the C cycle
are accounted for separately within the Land Use, Land-Use
Change, and Forestry chapter. Emissions of other greenhouse
Figure 3-1
2008 Energy Chapter Greenhouse Gas Emission Sources
5,573
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Coal Mining
Petroleum Systems
Mobile Combustion
Stationary Combustion |
Incineration of Waste |
Abandoned Underground •
Energy as a Portion
of all Emissions
Coal Mines
25
50 75 100
Tg C02 Eq.
125 150
1 Global CO2 emissions from fossil fuel combustion were taken from Energy Information Administration International Energy Statistics 2009 < http://
tonto.eia.doe.gov/cfapps/ipdbproject/IEDIndex3.cfm> EIA (2009).
2 The 2006IPCC Guidelines, states that CO2 from the combustion or decay of short-lived biogenic material removed from where it is grown, is reported
as zero in the energy, Industrial Processes Product Use (IPPU) and waste sectors as classified in section 1.1.
Energy 3-1
-------
Figure 3-2
2008 U.S. Fossil Carbon Flows (Tg C02 Eq.)
Fossil Fuel
Energy Exports
457
Natural Gas Emissions
1,235
NEU Emissions 51
Non-Energy Use
Carbon Sequestered
210
Fossil Fuel Non-Energy
Non-Enerqy Consumption Use U.S.
Use Imports U.S.
47 Territories
43
alancing Item
3
Territories
Note: Totals may not sum due to independent rounding.
The "Balancing Item" above accounts for the statistical imbalances
and unknowns in the reported data sets combined here.
NEU = Non-Energy Use
NG = Natural Gas
gases from the combustion of biomass and biomass-based
fuels are included in national totals under stationary and
mobile combustion.
Table 3-1 summarizes emissions from the Energy sector
in units of teragrams of CO2 equivalents (Tg CO2 Eq.), while
unweighted gas emissions in gigagrams (Gg) are provided in
Table 3-2. Overall, emissions due to energy-related activities
were 5,999.0 Tg CO2 Eq. in 2008, an increase of 15 percent
since 1990.
Table 3-1: C02, CH4, and N20 Emissions from Energy (Tg C02 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Natural Gas Systems
Incineration of Waste
Petroleum Systems
Wood Biomass and Ethanol Consumption3
International Bunker Fuels3
1990
4,901.2
4,735.71
1,820.8
1,485.8
845.41
339.1
216.71
27.9 1
119.6
37.3
8.0!
219.3
111.8
1995
5,226.7
5,029.5
1,947.9
1,608.0
862.6
353.3
223.2
34.5
142.9
42.2
11.5
0.5 1
236.8
99.8
2000
5,780.7
5,593.4
2,296.9
1,809.5
852.2
371.21
227. 7 '!
35.9
146.1
29.4
11.3
0.5 1
227.3
98.5
2005
5,932.5
5,753.3
2,402.1
1,895.3
825.6
358.4
221.3
50.6
136.5
29.5
12.6
0.5
229.4
110.5
2006
5,836.9
5,652.8
2,346.4
1,876.7
850.7
322.1
206.0
50.9
141.4
29.5
12.7
0.5
238.3
129.1
2007
5,936.9
5,757.0
2,412.8
1,893.7
842.2
341.7
217.4
49.1
135.3
30.8
13.3
0.5
245.7
727.7
2008
5,750.5
5,572.8
2,363.5
1,785.3
819.3
342.7
219.5
42.5
134.2
30.0
13.1
0.5
257.8
735.2
3-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-1: C02, CH4, and N20 Emissions from Energy (Tg C02 Eq.) (continued)
Gas/Source
1990
1995
2000
2005
2006
2007
2008
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
Incineration of Waste
International Bunker Fuels3
N20
Mobile Combustion
Stationary Combustion
Incineration of Waste
International Bunker Fuels3
Total
203.6
103.1
58.3
28.2
6.2
5.5
2.3
+
0.2
48.5
33.6
14.5
0.4
1.2
200.7
99.5
58.1
28.8
6.5
5.7
2.2
+
0.2
45.3
30.3
14.6
0.4
1.2
207.8
96.4
67.6
29.1
6.7
5.9
2.0
+
0.2
40.8
26.1
14.2
0.4
1.2
5,545.8 6,087.5
6,187.9 6,089.1 6,182.9 5,999.0
+ Does not exceed 0.05 Tg C02 Eq.
a These values are presented for informational purposes only and are not included or are already accounted for in totals.
Note: Totals may not sum due to independent rounding.
Table 3-2: C02, CH4, and N20 Emissions from Energy (Gg)
Gas/Source
C02
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Incineration of Waste
Petroleum Systems
Wood Biomass and
Ethanol Consumption3
International Bunker Fuels3
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
Incineration of Waste
International Bunker Fuels3
N20
Mobile Combustion
Stationary Combustion
Incineration of Waste
International Bunker Fuels3
1990
4,901,225
4,735,701
119,602
37,317
8,049
555
219,341
111,828
12,649
6,169
4,003
1,613
353
288
223
+
8
185
142
41
2
3
1995
5,226,659
5,029,498
142,922
42,249
11,461
528
236,7751
99,817
11,966
6,313
3,193
1,524
340 1
1 392
Im 204
+ 1
•
219
1741
43 1
1
3
2000
5,780,718
5,593,381 1
146,141 1
29,394
11,270
5341
227,276
98,482
11,364
j
2005
5,932,458
5,753,342
136,539
29,472
12,616
490
229,419
110,505
9,685
4,935
2,710
1,344
312
266
119
+
7
168
119
47
1
3
2006
5,836,924
5,652,845
141,382
29,526
12,684
488
238,323
129, 104
9,697
4,907
2,776
1,344
294
264
112
+
8
156
108
47
1
4
2007
5,936,884
5,756,999
135,306
30,816
13,289
474
245,706
127,054
9,559
4,738
2,765
1,372
309
269
105
+
7
146
98
47
1
4
2008
5,750,514
5,572,760
134,200
29,973
13,128
453
251,763
135,226
9,893
4,591
3,221
1,384
319
281
97
+
8
132
84
46
1
4
+ Does not exceed 0.05 Tg C02 Eq.
a These values are presented for informational purposes only and are not included or are already accounted for in totals.
Note: Totals may not sum due to independent rounding.
Energy 3-3
-------
3.1. Fossil Fuel Combustion (IPCC
Source Category 1 A)
Emissions from the combustion of fossil fuels for energy
include the gases CO2, CH4, and N2O. Given that CO2 is
the primary gas emitted from fossil fuel combustion and
represents the largest share of U.S. total emissions, CO2
emissions from fossil fuel combustion are discussed at the
beginning of this section. Following that is a discussion of
emissions of all three gases from fossil fuel combustion
presented by sectoral breakdowns. Methodologies for
estimating CO2 from fossil fuel combustion also differ from
the estimation of CH4 and N2O emissions from stationary
combustion and mobile combustion. Thus, three separate
descriptions of methodologies, uncertainties, recalculations,
and planned improvements are provided at the end of this
section. Total CO2, CH4, and N2O emissions from fossil fuel
combustion are presented in Table 3-3 and Table 3-4.
CO2 from Fossil Fuel Combustion
Carbon dioxide is the primary gas emitted from fossil
fuel combustion and represents the largest share of U.S. total
greenhouse gas emissions. Carbon dioxide emissions from
fossil fuel combustion are presented in Table 3-5. In 2008,
CO2 emissions from fossil fuel combustion decreased by
3.2 percent relative to the previous year. This decrease is
primarily a result of a decrease in electricity demand and a
decrease in energy consumption, coupled with a significant
increase in the cost of fuels used to generate electricity
and an increased cost of electricity. In 2008, temperatures
were cooler in the United States than in 2007, both in the
summer and the winter. This led to an increase in heating
related energy demand in the winter; however, much of this
increase was offset by a decrease in cooling related electricity
demand in the summer. In addition, the high price of gasoline
combined with the economic downturn led to a significant
decline in petroleum consumption by the transportation
sector in 2008. In 2008, CO2 emissions from fossil fuel
combustion were 5,572.8 Tg CO2 Eq., or almost 18 percent
above emissions in 1990 (see Table 3-5).3
Trends in CO2 emissions from fossil fuel combustion are
influenced by many long-term and short-term factors. On
a year-to-year basis, the overall demand for fossil fuels in
the United States and other countries generally fluctuates in
response to changes in general economic conditions, energy
prices, weather, and the availability of non-fossil alternatives.
For example, in a year with increased consumption of
goods and services, low fuel prices, severe summer and
winter weather conditions, nuclear plant closures, and lower
Table 3-3: C02, CH4, and N20 Emissions from Fossil Fuel Combustion (Tg C02 Eq.)
Gas
C02
CH4
N20
Total
1990
4,735.7
12.1
56.8
4,804.6
1995
5,029.5
11.4
67.4
5,108.3
2000
5,593.4
10.0
67.7
5,671.1 |
2005
5,753.3
9.1
51.7
5,814.1
2006
5,652.8
8.5
48.1
5,709.5
2007
5,757.0
8.7
44.9
5,810.6
2008
5,572.8
8.7
40.4
5,621.9
Note: Totals may not sum due to independent rounding.
Table 3-4: C02, CH4, and N20 Emissions from Fossil Fuel Combustion (Gg)
Gas
C02
CH4
N20
1990
4,735,701
576 1
183
1995
5,029,498
544
217
2000
5,593,381
4751
218
2005
5,753,342
431
167
2006
5,652,845
406
155
2007
5,756,999
414
145
2008
5,572,760
416
130
Note: Totals may not sum due to independent rounding.
3 An additional discussion of fossil fuel emission trends is presented
in the Trends in U.S. Greenhouse Gas Emissions chapter.
3-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-5: C02 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg C02 Eq.)
Fuel/Sector
Coal
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Natural Gas
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Petroleum
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Geothermal3
Total
1990
1,718.4
3.0
12.0
155.2
NE|
1,547.
0.
1,000.
238.
6
6
7
0
142.1
409.2
36.0
175.3
NO|
2,016.
98.
62.
281.
1,449.
97.
27.
3
2
6
0
7
5
2
0.40
4,735.
7
1995
1,819.0
11.2
144.4
NE
1,660.7
0.9
1,156.6
262.7
164.2
463.3
38.4
228.1
NO
2,053.5
88.9
47.8
254.9
1,569.6
58.7
33.6
0.34
5,029.5
2000
2,065.5
1.1
8.8
127.3
NE
1,927.4
0.9
1,217.4
270.7
172.5
457.1
35.6
280.8
0.7
2,310.2
99.4
46.4
267.8
1,773.9
88.4
34.2
0.36
5,593.4
2005
2,112.9
0.8
9.3
115.3
NE
1,983.8
3.7
1,159.9
262.4
163.0
381.2
33.1
318.8
1.3
2,480.2
95.2
48.9
329.1
1,862.2
99.2
45.7
0.38
5,753.3
2006
2,077.2
0.6
6.2
112.6
NE
1,953.7
4.1
1,141.4
237.3
153.8
377.8
33.1
338.0
1.4
2,433.8
84.1
45.9
360.3
1,843.5
54.4
45.5
0.37
5,652.8
2007
2,105.7
0.7
6.7
107.0
NE
1,987.3
4.1
1,215.0
256.5
164.1
386.3
35.3
371.3
1.4
2,435.9
84.5
46.6
348.9
1,858.4
53.9
43.6
0.38
5,757.0
2008
2,076.6
0.7
6.4
102.9
NE
1,962.6
4.1
1,227.0
264.7
169.7
393.6
35.8
361.6
1.6
2,268.8
77.3
43.4
322.9
1,749.4
38.9
36.9
0.38
5,572.8
NE (Not Estimated)
NO (Not Occurring)
"Although not technically a fossil fuel, geothermal energy-related C02 emissions are included for reporting purposes.
Note: Totals may not sum due to independent rounding.
precipitation feeding hydroelectric dams, there would likely
be proportionally greater fossil fuel consumption than a
year with poor economic performance, high fuel prices,
mild temperatures, and increased output from nuclear and
hydroelectric plants.
Longer-term changes in energy consumption patterns,
however, tend to be more a function of aggregate societal
trends that affect the scale of consumption (e.g., population,
number of cars, size of houses, and number of houses), the
efficiency with which energy is used in equipment (e.g.,
cars, power plants, steel mills, and light bulbs), and social
planning and consumer behavior (e.g., walking, bicycling,
or telecommuting to work instead of driving).
Carbon dioxide emissions also depend on the source of
energy and its carbon (C) intensity. The amount of C in fuels
varies significantly by fuel type. For example, coal contains the
highest amount of C per unit of useful energy. Petroleum has
roughly 75 percent of the C per unit of energy as coal, and
natural gas has only about 55 percent.4 Producing a unit of
heat or electricity using natural gas instead of coal can reduce
the CO2 emissions associated with energy consumption,
and using nuclear or renewable energy sources (e.g., wind)
can essentially eliminate emissions (see Box 3-2). Table
3-6 shows annual changes in emissions during the last five
years for coal, petroleum, and natural gas in selected sectors.
In the United States, 84 percent of the energy consumed
in 2008 was produced through the combustion of fossil fuels
such as coal, natural gas, and petroleum (see Figure 3-3 and
Figure 3-4). The remaining portion was supplied by nuclear
electric power (9 percent) and by a variety of renewable
energy sources (7 percent), primarily hydroelectric power
and biofuels (ElA 2009a). Specifically, petroleum supplied
the largest share of domestic energy demands, accounting
for an average of 42 percent of total fossil fuel based energy
4 Based on national aggregate carbon content of all coal, natural
gas, and petroleum fuels combusted in the United States.
Energy 3-5
-------
Table 3-6: Annual Change in C02 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors
(Tg C02 Eq. and Percent)
Sector
FuelType 2004 to 2005 2005 to 2006 2006 to 2007 2007 to 2008 Total 2008
Electricity Generation Coal 40.8 2.1% -30.1 -1.5% 33.6 1.7% -24.7 -1.2% 1,962.6
Electricity Generation Natural Gas 22.1 7.5% 19.2 6.0% 33.3 9.9% -9.6 -2.6% 361.6
Electricity Generation Petroleum 2.2 2.3% -44.8 -45.2% -0.5 -0.9% -15.0 -27.9% 38.9
Transportation3 Petroleum 27.8 1.5% -18.7 -1.0% 14.9 0.8% -109.0 -5.9% 1,749.4
Residentia
Natural Gas -2.0 -0.7% -25.1 -9.6% 19.2 8.1% 8.2 3.2% 264.7
Commercial Natural Gas -6.9 -4.0% -9.2 -5.7% 10.2 6.6% 5.7 3.5% 169.7
Industrial
Industrial
Coal -3.0 -2.5% -2.6 -2.3% -5.6 -5.0% -4.1 -3.9% 102.9
Natural Gas -33.0 -8.0% -3.4 -0.9% 8.5 2.3% 7.2 1.9% 393.6
All Sectors" All Fuels" 44.5 0.8% -100.5 -1.7% 104.2 1.8% -184.2 -3.2% 5,572.8
a Excludes emissions from International Bunker Fuels.
b Includes fuels and sectors not shown in table.
Figure 3-3
Figure 3-5
2008 U.S. Energy Consumption by Energy Source H
Renewable
Energy
7.4%
Nuclear Electric
Power
O CO/ .^^^^^^ ^^^^.
O.il /o
Coal
22.6%
^fl Natural Gas
^M 24.0%
•
2,000 -
1 ,500 -
S
u
" 1 ,000 -
500 -
o -
2008 C02 Emissions from Fossil Fuel
Combustion by Sector and Fuel Type
• Petroleum 2,363
• Coal
• Natural Gas
Relative Contribution i 735
by Fuel Type
• 1
910
—••••r
3-| 1 1 1 1 :f|
o |
Note: Electricity generation also includes emissions of less than 0.5 Tg C02 Eq. from
geothermal-based electricity generation.
Figure 3-4
•••••••••••••ll|pl||ll||||ll||H consumption in 2008. Natural gas and coal followed in
100-
•=
| 80-
_O
| 60-
O
f 40-
Ł
20 —
o-
Total Energy
— — -" ^
^- ' Fossil Fuels
Renewable & Nuclear
aaaaaaaaaa§§§§§§§§§
Note: Expressed as gross calorific values.
uiuci ui iiiipui uui^c, (UAAJiuiuiig lui appiuAimaiciy ju cuiu
28 percent of total consumption, respectively. Petroleum
was consumed primarily in the transportation end-use sector,
the vast majority of coal was used in electricity generation,
and natural gas was broadly consumed in all end-use sectors
except transportation (see Figure 3-5) (El A 2009a).
Fossil fuels are generally combusted for the purpose
of producing energy for useful heat and work. During the
*
i s~i . -t • ,1 j? i
COniDUStlOn piuucss, uic i^. siuicu 111 uic lucis is UAIUIZ.CU tuiu
emitted as CO2 and smaller amounts of other gases, including
3-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Box 3-1: Weather and Non-Fossil Energy Effects on C02 from Fossil Fuel Combustion Trends
In 2008, weather conditions were cooler in the winter and much cooler in the summer, as heating degree days increased by 5.6 percent
and cooling degree days decreased by 8.7 percent, compared to 2007. Although winter conditions were cooler in 2008 compared to 2007,
the winter was slightly warmer than normal, with heating degree days in the United States only 1 percent below normal (see Figure 3-6).
Cooler winter conditions compared to 2007 led to an increase in demand for heating fuels. Although summer conditions were slightly cooler
in 2008 compared to 2007, summer temperatures were slightly warmer than usual, with cooling degree days 3 percent above normal (see
Figure 3-7) (EIA 2009f).5 Despite being warmer than normal in 2008, cooling degree days decreased by 8.7% from 2007, which offset the
increase in heating related energy demand.
Figure 3-6
Annual Deviations from Normal Heating Degree Days for the United States (1950-2008)
|| 10-
-15 -
Normal (4,524 Heating Degree Days)
T"'
O)O>O>O>O)O>O>O>O>O>O>O>
Note: Climatological normal data are highlighted. Statistical confidence interval for
•-III .. • •!
99% Confidence
O)O>O>O>O>O>O>O>
normal" climatology period of 1971 through 1990.
•'
O) O) O)
J
in
a> a> o
imp
1
CM «± (O CO
0000
Figure 3-7
Annual Deviations from Normal Cooling Degree Days for the United States (1950-2008)
_ 20-
g =
il o.
1-1 -10-
c &
— o^
-20-
l
• III 1
Normal (1 ,242 Cooling Degree Days)
J
O CM ^- (O CO O CM «± (O
0)0)0) 0) 0) 0) 0) 0) 0)
Note: Climatological normal data are highlighted. Statistics
99% Confidence
• • _ -.
|P "• !•[ 1 "1 " •
OOOCM«±(OOOOCM«±(OOO
0>0>0)0>0>0>0>0>0>0>0>
confidence interval for "normal" climatology period of 1971 through 1990.
.1 . 1,
|- M
0 CM -± (0 CO
0) 0) 0) 0) 0)
, 1.1
0 tM -f
=> ° °
III.
(O CO
=> °
Figure 3-8
Aggregate Nuclear and Hydroelectric Power Plant
Capacity Factors in the United States (1974-2008)
100-
80-
60-
j
[
f 40-
!
[
20-
0-
Wind
Although no new U.S. nuclear power plants have been constructed
in recent years, the utilization (i.e., capacity factors)6 of existing plants
in 2008 remained high at just over 90 percent. Electricity output by
hydroelectric power plants increased in 2008 by approximately 0.2
percent. Electricity generated by nuclear plants in 2008 provided just
over 3 times as much of the energy consumed in the United States
as hydroelectric plants (EIA 2009a). Nuclear, hydroelectric, and wind
power capacity factors since 1974 are shown in Figure 3-8.
5 Degree days are relative measurements of outdoor air temperature.
Heating degree days are deviations of the mean daily temperature below 65°
F, while cooling degree days are deviations of the mean daily temperature
above 65° F. Heating degree days have a considerably greater affect
on energy demand and related emissions than do cooling degree days.
Excludes Alaska and Hawaii. Normals are based on data from 1971 through
2000. The variation in these normals during this time period was ±10
percent and ±14 percent for heating and cooling degree days, respectively
(99 percent confidence interval).
6 The capacity factor equals generation divided by net summer capacity.
Summer capacity is defined as "The maximum output that generating
equipment can supply to system load, as demonstrated by a multi-hour
test, at the time of summer peak demand (period of June 1 through
September 30)." Data for both the generation and net summer capacity
arefromEIA(2009a).
Energy 3-7
-------
CH4, CO, and NMVOCs.7 These other C containing non-
002 gases are emitted as a by-product of incomplete fuel
combustion, but are, for the most part, eventually oxidized
to CO2 in the atmosphere. Therefore, it is assumed that all
of the C in fossil fuels used to produce energy is eventually
converted to atmospheric CO2.
Fossil Fuel Combustion Emissions
by Sector
In addition to the CO2 emitted from fossil fuel
combustion, CH4 and N2O are emitted from stationary and
mobile combustion as well. Table 3-7 provides an overview of
the CO2, CUt, and N2O emissions from fossil fuel combustion
by sector.
Other than CO2, gases emitted from stationary
combustion include the greenhouse gases CH4 and N2O and
the indirect greenhouse gases NOX, CO, and NMVOCs.8
Methane and N2O emissions from stationary combustion
sources depend upon fuel characteristics, size and vintage,
along with combustion technology, pollution control
equipment, ambient environmental conditions, and operation
and maintenance practices. Nitrous oxide emissions from
stationary combustion are closely related to air-fuel mixes
and combustion temperatures, as well as the characteristics
of any pollution control equipment that is employed.
Methane emissions from stationary combustion are primarily
a function of the CH4 content of the fuel and combustion
efficiency.
Table 3-7: C02, CH4, and N20 Emissions from Fossil Fuel Combustion by Sector (Tg C02 Eq.)
End-Use Sector/Gas
Electricity Generation
C02
CH4
N20
Transportation
C02
CH4
N20
Industrial
C02
CH4
N20
Residential
C02
CH4
N20
Commercial
C02
CH4
N20
U.S. Territories3
Total
1990 •
1,829.5
1, 820.8 1
0.6
1
1,534.4
1,485.8
4.7 1
43.9 1
850.2
845.4
1
344.6
339.1 1
4.4l
1.ll
21 7.9 1
216.71
0.9
0.4l
28.0
4,804.6 1
1995
1,957.1
1,947.9
0.6 1
8.6
1,666.3
1,608.0
4.3|
54.0
867.5
862.6
_
353.3
4.0 1
1
224.5
223.2
0.9 1
0.4
34.7
5,108.3
2000 2005
2,307.5 2,413.2
2,296.9 2,402.1
0.7 1 0.7
10.0
1,866.1
1,809.5
3.4
53.2
857.0
852.2
1.6
3.2
375.5
371.2
3.4
10.3
1,934.7
1,895.3
2.5
36.9
830.1
825.6
1.5
3.0
362.7
358.4
3.4
0.9 0.9
228.9 222.5
227.7 221.3
0.9 1 0.9
0.3 0.3
36.0 50.8
5,671.1 5,814.1
2006
2,357.2
2,346.4
0.7
10.1
1,912.6
1,876.7
2.3
33.6
855.4
850.7
1.5
3.2
325.9
322.1
3.1
0.8
207.2
206.0
0.8
0.3
51.1
5,709.5
2007
2,423.8
2,412.8
0.7
10.3
1,926.2
1,893.7
2.2
30.3
846.7
842.2
1.5
3.0
346.0
341.7
3.4
0.9
218.6
217.4
0.9
0.3
49.3
5,810.6
2008
2,374.3
2,363.5
0.7
10.1
1,813.4
1,785.3
2.0
26.1
823.4
819.3
1.3
2.8
347.4
342.7
3.7
0.9
220.7
219.5
0.9
0.3
42.7
5,621.9
a U.S. Territories are not apportioned by sector, and emissions are total greenhouse gas emissions from all fuel combustion sources.
Note: Totals may not sum due to independent rounding. Emissions from fossil fuel combustion by electricity generation are allocated based on aggregate
national electricity consumption by each end-use sector.
7 See the sections entitled Stationary Combustion and Mobile
Combustion in this chapter for information on non-CO2 gas
emissions from fossil fuel combustion.
8 Sulfur dioxide (SO2) emissions from stationary combustion are
addressed in Annex 6.3.
3-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-8: C02, CH4, and N20 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg C02 Eq.)
End-Use Sector/Gas
Transportation
C02
CH4
N20
Industrial
C02
CH4
N20
Residential
C02
CH4
N20
Commercial
C02
CH4
N20
U.S. Territories3
Total
1990H
1,537.5
1, 488.8 1
41
44.0
1,540.2
1,532.2
isl
6.3l
940.5
932.21
4.6 1
.„...,
754.6 1
1.0!
2.8 1
28.0
4,804.6 |
1995
1,669.3
1,611. ol
4.3l
54.0
1, 587.1 1
1,578.8
isl
6.5!
1,003.1
995.1
4.2l
3.8 1
814.0
810.0
1
2.9
34.7
5,108.3
2000
1,869.5
1,813.ol
3i
53.2
1,650.5
1,642.0
i.s|
.
3.6 1
4.2l
973.6
968.9
5,671.1 |
2005
1,939.5
1,900.1
2.5
37.0
1,570.4
1,562.5
1.7
6.2
1,223.4
1,215.1
3.7
4.6
1,029.9
1,025.0
1.1
3.8
50.8
5,814.1
2006
1,917.2
1,881.2
2.4
33.6
1,570.7
1,562.8
1.7
6.2
1,160.6
1,152.9
3.3
4.4
1,009.8
1,005.0
1.1
3.7
51.1
5,709.5
2007
1,931.3
1,898.8
2.2
30.3
1,580.0
1,572.2
1.7
6.2
1,206.0
1,197.9
3.6
4.5
1,044.0
1,039.1
1.1
3.8
49.3
5,810.6
2008
1,818.1
1,789.9
2.0
26.2
1,518.2
1,510.9
1.5
5.7
1,193.0
1,184.5
4.0
4.5
1,049.9
1,044.9
1.1
3.8
42.7
5,621.9
a U.S. Territories are not apportioned by sector, and emissions are total greenhouse gas emissions from all fuel combustion sources.
Note: Totals may not sum due to independent rounding. Emissions from fossil fuel combustion by electricity generation are allocated based on aggregate
national electricity consumption by each end-use sector.
Mobile combustion produces greenhouse gases other
than CO2, including CH4, N2O, and indirect greenhouse
gases including NOX, CO, and NMVOCs. As with stationary
combustion, N2O and NOX emissions from mobile
combustion are closely related to fuel characteristics, air-fuel
mixes, combustion temperatures, and the use of pollution
control equipment. N2O from mobile sources, in particular,
can be formed by the catalytic processes used to control
NOX, CO, and hydrocarbon emissions. Carbon monoxide
emissions from mobile combustion are significantly
affected by combustion efficiency and the presence of post-
combustion emission controls. Carbon monoxide emissions
are highest when air-fuel mixtures have less oxygen than
required for complete combustion. These emissions occur
especially in idle, low speed, and cold start conditions.
Methane and NMVOC emissions from motor vehicles are
a function of the CH4 content of the motor fuel, the amount
of hydrocarbons passing uncombusted through the engine,
and any post-combustion control of hydrocarbon emissions
(such as catalytic converters).
An alternative method of presenting combustion
emissions is to allocate emissions associated with electricity
generation to the sectors in which it is used. Four end-use
sectors were defined: industrial, transportation, residential,
and commercial. In the table below, electricity generation
emissions have been distributed to each end-use sector based
upon the sector's share of national electricity consumption,
with the exception of CH4 and N2O from transportation.9
Emissions from U.S. territories are also calculated
separately due to a lack of end-use-specific consumption
data. This method of distributing emissions assumes that
564 combustion sources focus on the alternative method as
presented in Table 3-8.
9 Separate calculations were performed for transportation-related
CH4 and N2O. The methodology used to calculate these emissions
are discussed in the mobile combustion section.
Energy 3-9
-------
emissions from stationary combustion are closely related to
air-fuel mixes and combustion temperatures, as well as the
The direct combustion of fuels by stationary sources characteristics of any pollution control equipment that is
in the electricity generation, industrial, commercial, and employed. Methane emissions from stationary combustion
residential sectors represent the greatest share of U.S. are primarily a function of the CH4 content of the fuel and
greenhouse gas emissions. Table 3-9 presents CO2 emissions combustion efficiency. Please refer to Table 3-7 for the
from fossil fuel combustion by stationary sources. The CO2 corresponding presentation of all direct emission sources of
emitted is closely linked to the type of fuel being combusted fuel combustion.
in each sector (see Methodology section for CO2 from
fossil fuel combustion). Other than CO2, gases emitted Electricity Generation
from stationary combustion include the greenhouse gases The Process of generating electricity is the single
CH4 and N20. Table 3-10 and Table 3-11 present CH4 and larSest source of C°2 emissions in the United States,
N20 emissions from the combustion of fuels in stationary representing 40 percent of total CO2 emissions from all
sources. Methane and N2O emissions from stationary C°2 emissions sources across the United States. Methane
combustion sources depend upon fuel characteristics, size wd N2° accounted for a small portion of emissions from
and vintage, along with combustion technology, pollution electricity generation, representing less than 0.1 percent
control equipment, ambient environmental conditions, md °-4 Percent' respectively.10 Electricity generation also
and operation and maintenance practices. Nitrous oxide accounted for the largest share of CO2 emissions from
Table 3-9: C02 Emissions from Stationary Combustion (Tg C02 Eq.)
Sector/Fuel Type
Electricity Generation
Coal
Natural Gas
Fuel Oil
Geothermal
Industrial
Coal
Natural Gas
Fuel Oil
Commercial
Coal
Natural Gas
Fuel Oil
Residential
Coal
Natural Gas
Fuel Oil
U.S. Territories3
Coal
Natural Gas
Fuel Oil
Total
1990
1,820.8
1, 547.6 1
175.sl
97.51
0.4l
845.4
155.21
409.2
281.0
216.7 1
12.ol
142.1 1
62.6 1
339.1
3.ol
238.0
98.2
27.9
0.6 1
NO!
27.2
3,249.9 |
1995
1,947.9
1,660.7
228.1
58.7
0.3 1
862.6
144.4
463.3
254.9
223.2
11.2
164.2
47.8
353.3
"
262.7
88.9
34.5
0.9 1
NO
33.6
3,421.5
2000
2,296.9
1,927.4
280.8
88.4
0.4l
852.2
127.3
457.1
267.8
227.7
8.8 1
172.5
46.4
371.2
1.1 1
270.7
99.4
35.9
0.9
34.2
3,783.8
2005
2,402.1
1,983.8
318.8
99.2
0.4
825.6
115.3
381.2
329.1
221.3
9.3
163.0
48.9
358.4
0.8
262.4
95.2
50.6
3.7
1.3
45.7
3,858.0
2006
2,346.4
1,953.7
338.0
54.4
0.4
850.7
112.6
377.8
360.3
206.0
6.2
153.8
45.9
322.1
0.6
237.3
84.1
50.9
4.1
1.4
45.5
3,776.2
2007
2,412.8
1,987.3
371.3
53.9
0.4
842.2
107.0
386.3
348.9
217.4
6.7
164.1
46.6
341.7
0.7
256.5
84.5
49.1
4.1
1.4
43.6
3,863.3
2008
2,363.5
1,962.6
361.6
38.9
0.4
819.3
102.9
393.6
322.9
219.5
6.4
169.7
43.4
342.7
0.7
264.7
77.3
42.5
4.1
1.6
36.9
3,787.5
3 U.S. Territories are not apportioned by sector, and emissions are from all fuel combustion sources (stationary and mobile) are presented in this table.
10 Since emissions estimates for U.S. territories cannot be
disaggregated by gas in Table 3-7and Table 3-8, the percentages
for CH4 and N2O exclude U.S. territory estimates.
3-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-10: CH4 Emissions from Stationary Combustion (Tg C02 Eq.)
Sector/Fuel Type
1990
1995
2000
2005
2006
2007
2008
Electricity Generation
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial
Coal
Fuel Oil
Natural Gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
0.6 0.6 0.7
0.3 1 0.4
0.1 1 +
0.1 1 0.1
0.1 1 0.1
1.5l 1.6
0.3 1 0.3
0.2! 0.1
0.2l 0.2
0.9 1 1.0
0.9
+
0.2
0.3
0.4
4.4
0.2
0.3
0.4
3.5
+
+
+
+
0.9
+
0.1
0.3
0.4
4.0
0.1
0.3
0.5
3.1
+
+
+
+
0.4
0.1
0.1
0.1
1.6
0.3
0.1
0.2
1.0
0.9
+
0.1
0.3
0.4
3.4
0.1
0.3
0.5
2.5
0.1
+
+
+
0.7
0.4
0.1
0.1
0.1
1.5
0.3
0.2
0.1
0.9
0.9
+
0.1
0.3
0.4
3.4
0.1
0.3
0.5
2.6
0.1
+
0.1
+
+ + + +
0.7
0.4
+
0.1
0.1
1.5
0.3
0.2
0.1
0.9
0.8
+
0.1
0.3
0.4
3.1
+
0.3
0.4
2.3
0.1
+
0.1
+
+
0.7
0.4
+
0.1
0.1
1.5
0.2
0.2
0.1
0.9
0.9
+
0.1
0.3
0.4
3.4
+
0.3
0.5
2.6
0.1
+
0.1
+
+
0.7
0.4
+
0.1
0.1
1.3
0.2
0.2
0.1
0.8
0.9
+
0.1
0.3
0.4
3.7
+
0.2
0.5
2.9
0.1
+
0.1
+
+
Total
7.4
7.1
6.6
6.6
6.2
6.5
6.7
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
fossil fuel combustion, approximately 42 percent in 2008.
Methane and N2O from electricity generation represented 8
and 25 percent of emissions from fossil fuel combustion in
2008, respectively. Electricity was consumed primarily in
the residential, commercial, and industrial end-use sectors
for lighting, heating, electric motors, appliances, electronics,
and air conditioning (see Figure 3-9).
The electric power industry includes all power producers,
consisting of both regulated utilities and nonutilities (e.g.
independent power producers, qualifying cogenerators, and
other small power producers). For the underlying energy data
used in this chapter, the Energy Information Administration
(EIA) places electric power generation into three functional
categories: the electric power sector, the commercial sector,
and the industrial sector. The electric power sector consists
of electric utilities and independent power producers whose
Figure 3-9
Electricity Generation Retail Sales by
End-Use Sector (1974-2008)
1,600 -,
1,400 -
1,200 -
1,000 -
800-
600-
400-
Residential
Industrial
sssiililslssl
Note: Trie transportation end-use sector consumes minor quanties of electricity.
Energy 3-11
-------
Table 3-11: N20 Emissions from Stationary Combustion (Tg C02 Eq.)
Sector/Fuel Type
1990
1995
2000
2005
2006
2007
2008
10.1
9.6
0.1
0.2
0.2
2.8
0.5
0.5
0.2
1.5
0.3
0.1
0.1
0.1
0.9
0.2
0.1
0.6
0.1
0.1
Electricity Generation
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial
Coal
Fuel Oil
Natural Gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
8.1
7.6
0.2
0.1
0.2
3.2
0.8
0.5
0.2
1.7
0.4
0.1
0.2
0.1
0.1
1.1
0.3
"
°J
8.6
8.1
0.1
0.1
0.1
3.3
0.7
0.4
0.3
1.9
0.4
0.1
0.1
0.1
0.1
1.0
0.2
0.1
0.6
0.1
0.1
10.0
9.4
0.2
0.2
0.2
3.2
0.6
0.4
0.3
1.9
0.3
0.1
0.1
0.1
0.9
0.3
0.2
0.5
0.1
+
0.1
+
10.3
9.7
0.2
0.2
0.2
3.0
0.6
0.5
0.2
1.7
0.3
0.1
0.1
0.1
0.9
0.3
0.1
0.5
0.1
0.1
10.1
9.5
0.1
0.2
0.2
3.2
0.6
0.6
0.2
1.8
0.3
0.1
0.1
0.1
0.8
0.2
0.1
0.5
0.1
0.1
10.2
9.7
0.1
0.2
0.2
3.0
0.5
0.6
0.2
1.7
0.3
0.1
0.1
0.1
0.9
0.2
0.1
0.5
0.1
0.1
Total
12.8
13.3
14.5
14.7
14.5
14.6
14.2
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
primary business is the production of electricity,11 while the
other sectors consist of those producers that indicate their
primary business is something other than the production of
electricity.
The industrial, residential, and commercial end-use
sectors, as presented in Table 3-8, were reliant on electricity
for meeting energy needs. The residential and commercial
end-use sectors were especially reliant on electricity
consumption for lighting, heating, air conditioning, and
operating appliances. Electricity sales to the residential and
commercial end-use sectors in 2008 decreased approximately
0.9 percent and increased 1.2 percent, respectively. The
trend in the commercial sector can largely be attributed to
11 Utilities primarily generate power for the U.S. electric grid for sale
to retail customers. Nonutilities produce electricity for their own
use, to sell to large consumers, or to sell on the wholesale electricity
market (e.g., to utilities for distribution and resale to customers).
slightly positive growth in the U.S. economy that was largely
concentrated in non-industrial activity that takes place in the
commercial sector. Furthermore, unless entire facilities such
as shopping centers are closed, their lights and electronic
equipment remain on, even if there are fewer customers using
them. The decrease in the residential sector is attributed to a
decrease in air conditioning-related electricity consumption
in the residential and commercial sectors that occurred as a
result of the cooler summer compared to 2007. In 2008, the
amount of electricity generated (in kWh) decreased by 1.3
percent from the previous year. This decline was due to the
economic downturn, an increase in the cost of electricity,
and more favorable weather conditions compared to 2007.
As a result, CO2 emissions from the electric power sector
decreased by 2.0 percent as the consumption of coal, natural
gas, and petroleum for electricity generation decreased.
Consumption of coal, natural gas, and petroleum for
3-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
electricity generation, decreased by 1.3 percent, 2.6 percent,
and 29.5 percent, respectively, in 2008. While in 2008 the
amount of electricity generated decreased by 1.3 percent,
CO2 emissions from the electric power sector decreased by
2.0 percent. The decrease in C intensity of the electricity
supply (see Table 3-16) was the result of a decrease in fossil
fuels consumed to generate electricity and an increase in
renewable generation of 7 percent spurred by a 51 percent
increase in wind-generated electricity.
Industrial Sector
The industrial sector accounted for 15 percent of CO2
emissions from fossil fuel combustion, 15 percent of CH4
emissions from fossil fuel combustion, and 7 percent of N2O
emissions from fossil fuel combustion. Carbon dioxide, CH4,
and N2O emissions resulted from the direct consumption of
fossil fuels for steam and process heat production.
The industrial sector, per the underlying energy
consumption data from EIA, includes activities such as
manufacturing, construction, mining, and agriculture. The
largest of these activities in terms of energy consumption
is manufacturing, of which six industries—Petroleum
Refineries, Chemicals, Primary Metals, Paper, Food, and
Nonmetallic Mineral Products—represent the vast majority
of the energy use (EIA 2009a and EIA 2005).
Figure 3-10
Industrial Production Indices (Index 2002=100)
120
110
100
90
80
70
60
120
110
100
90
80
120
110
100
90
80
70
120-,
110-
100-
90-
80-
Total excluding Computers,
Communications Equipment,
and Semiconductors
Foods
Stone, Clay & Glass Products
^^
Chemicals
Primary Metals^,
Petroleum Refineries
gSSŁSŁgŁg?S85gSSSS
In theory, emissions from the industrial sector should
be highly correlated with economic growth and industrial
output, but heating of industrial buildings and agricultural
energy consumption are also affected by weather conditions.12
In addition, structural changes within the U.S. economy
that lead to shifts in industrial output away from energy-
intensive manufacturing products to less energy-intensive
products (e.g., from steel to computer equipment) also have
a significant effect on industrial emissions.
From 2007 to 2008, total industrial production and
manufacturing output decreased by 2.2 and 3.1 percent,
respectively (FRB 2008). Over this period, output increased
for Food and Petroleum Refineries, and decreased for
Chemicals, Paper, Primary Metals, and Nonmetallic Mineral
Products (see Figure 3-10).
Despite the growth in industrial output (56 percent)
and the overall U.S. economy (66 percent) from 1990 to
2008, CO2 emissions from fossil fuel combustion in the
industrial sector decreased by 3.1percent over that time. A
number of factors are believed to have caused this disparity
between growth in industrial output and decrease in industrial
emissions, including: (1) more rapid growth in output
from less energy-intensive industries relative to traditional
manufacturing industries, and (2) energy-intensive industries
such as steel are employing new methods, such as electric
arc furnaces, that are less carbon intensive than the older
methods. In 2008, CO2, CH4, andN2O emissions from fossil
fuel combustion and electricity use within the industrial end-
use sector totaled 1,518.2 Tg CO2 Eq., or approximately 3.9
percent below 2007 emissions.
Residential and Commercial Sectors
The residential and commercial sectors accounted for
an average 6 and 4 percent of CO2 emissions from fossil
fuel combustion, 43 and 10 percent of CH4 emissions from
fossil fuel combustion, and 2 and 1 percent of N2O emissions
from fossil fuel combustion, respectively. Emissions from
these sectors were largely due to the direct consumption of
natural gas and petroleum products, primarily for heating and
12 Some commercial customers are large enough to obtain
an industrial price for natural gas and/or electricity and are
consequently grouped with the industrial end-use sector in U.S.
energy statistics. These misclassifications of large commercial
customers likely cause the industrial end-use sector to appear to
be more sensitive to weather conditions.
Energy 3-13
-------
cooking needs. Coal consumption was a minor component
of energy use in both of these end-use sectors. In 2008, CO2,
CH4, and N2O emissions from fossil fuel combustion and
electricity use within the residential and commercial end-
use sectors were 1,193.0 Tg CO2 Eq. and 1,049.9 Tg CO2
Eq., respectively. Total CO2, CH4, and N2O emissions from
the residential sector decreased by 1.1 percent in 2008, with
emissions in 2008 from the commercial sector 0.6 percent
higher than in 2007.
Emissions from the residential and commercial
sectors have generally been increasing since 1990, and
are often correlated with short-term fluctuations in energy
consumption caused by weather conditions, rather than
prevailing economic conditions. In the long-term, both
sectors are also affected by population growth, regional
migration trends, and changes in housing and building
attributes (e.g., size and insulation).
Emissions from natural gas consumption represent
over 77 percent of the direct fossil fuel CO2 emissions from
each of these sectors. In 2008, natural gas CO2 emissions
from the residential and commercial sectors increased by
3.2 percent and 3.5 percent, respectively. The increase in
natural gas emissions in both sectors is a result of cooler
winter conditions in the United States compared to 2007.
U.S. Territories
Emissions from U.S. territories are based on the fuel
consumption in American Samoa, Guam, Puerto Rico, U.S.
Virgin Islands, Wake Island, and other U.S. Pacific Islands.
As described the Methodology section for CO2 from fossil
fuel combustion, this data is collected separately from the
sectoral-level data available for the general calculations. As
sectoral information is not available for U.S. Territories, CO2,
CH4, and N2O emissions are not presented for U.S. Territories
in the tables above, though the emissions will include some
transportation and mobile combustion sources.
Transportation and Mobile Combustion
This discussion of transportation emissions follows the
alternative method of presenting combustion emissions by
allocating emissions associated with electricity generation to
the transportation end-use sector, as presented in Table 3-8.
For direct emissions from transportation (i.e., not including
electricity consumption), please see Table 3-7.
Transportation End-Use Sector
The transportation end-use sector accounted for 1,818.1
Tg CO2 Eq. in 2008, which represented 32 percent of CO2
emissions, 24 percent of CH4 emissions, and 65 percent of
N2O emissions from fossil fuel combustion, respectively.
Fuel purchased in the U.S. for international aircraft and
marine travel accounted for an additional 135.2 Tg CO2 in
2008; these emissions are recorded as international bunkers
and are not included in U.S. totals according to UNFCCC
reporting protocols. Among domestic transportation sources,
light duty vehicles (including passenger cars and light-duty
trucks) represented 62 percent of CO2 emissions, medium-
and heavy-duty trucks 22 percent, commercial aircraft 7
percent, and other sources 9 percent. See Table 3-12 for a
detailed breakdown of CO2 emissions by mode and fuel type.
From 1990 to 2008, transportation emissions rose by 22
percent due, in large part, to increased demand for travel and
the stagnation of fuel efficiency across the U.S. vehicle fleet.
The number of vehicle miles traveled by light-duty motor
vehicles (passenger cars and light-duty trucks) increased
37 percent from 1990 to 2008, as a result of a confluence
of factors including population growth, economic growth,
urban sprawl, and low fuel prices over much of this period.
A similar set of social and economic trends has led to a
significant increase in air travel and freight transportation
by both air and road modes during the time series.
From 2007 to 2008, CO2 emissions from transportation
sources declined 6 percent (the largest annual change in
either absolute or percentage terms recorded between
1990 and 2008). The decrease in emissions can largely be
attributed to the decline in economic activity during 2008 and
the increased price of transportation fuels. Modes such as
medium- and heavy-duty trucks were significantly impacted
by the decline in freight transport. Similarly, increased jet
fuel prices were a factor in the 12 percent drop in commercial
aircraft emissions since 2007.
Almost all of the energy consumed for transportation
was supplied by petroleum-based products, with more than
half being related to gasoline consumption in automobiles
and other highway vehicles. Other fuel uses, especially diesel
fuel for freight trucks and jet fuel for aircraft, accounted for
the remainder. The primary driver of transportation-related
emissions was CO2 from fossil fuel combustion, which
increased by 20 percent from 1990 to 2008. This rise in
CO2 emissions, combined with an increase in HFCs from
3-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-12: C02 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector (Tg C02 Eq.)a
Fuel/Vehicle Type
Gasoline
Passenger Cars
Light-Duty Trucks
Medium- and Heavy-Duty Trucksb
Buses
Motorcycles
Recreational Boats
Distillate Fuel Oil (Diesel)
Passenger Cars
Light-Duty Trucks
Medium- and Heavy-Duty Trucksb
Buses
Rail
Recreational Boats
Ships and Other Boats
International Bunker Fuels0
Jet Fuel
Commercial Aircraft
Military Aircraft
General Aviation Aircraft
International Bunker Fuels0
Aviation Gasoline
General Aviation Aircraft
Residual Fuel Oil
Ships and Other Boatsd
International Bunker Fuels c- d
Natural Gas
Passenger Cars
Light-Duty Trucks
Buses
Pipeline
LPG
Light-Duty Trucks
Medium- and Heavy-Duty Trucksb
Buses
Electricity
Rail
Total
Total (Including Bunkers)0
1990
983.6
621. sl
309.0 1
38.7 1
0.3 1
12.41
262.9
7.9 1
11. 5|
190.5
8.0 1
35.5
2.0 1
7.5l
11.7
176.2
135.4
34.4
6.4l
46.4M
31 1
3.1
22.6
22.6
53.7
36.0
1
1
1
36.0
1.3|
0.5
0.8
3.0 1
3.0
1,488.8
1,600.7
1995 2000 2005
1,041.7 1,135.1 1,187.3
598.7 640.5 657.8
391.0 446.3 478.6
35.9
0.4
1.8
14.0
324.2
7.8
14.9
238.4
8.7
39.6
2.3
12.4
9.3
170.9
141.6
23.9
5.4
51.2
2.7
2.7
29.1
29.1
39.3
38.4
0.1
+
0.1
38.2
1.0
0.5
0.5
»
36.0 34.9
0.4l 0.4
1.sl 1.6
10.1 14.1
402.5 458.1
3.7 4.2
20.1 25.8
309.6 360.6
10.2 10.6
42.1 45.6
2j| 3.1
14.1 8.1
6.3 9.4
199.8 193.5
169.2 161.2
21.1
9.5
58.8
2.5
2.5
33.3
33.3
33.3
35.6
-
-
0.4
35.2
0.7
0.4
0.2
+
3.4
18.1
14.1
57.5
2.4
2.4
19.3
19.3
43.6
33.1
-
-
0.8
32.3
1.6
1.2
0.4
+
4.7
3.1 3.4 4.7
1,611.0 1,813.0 1,900.1
1,710.9 1,911.5 2,010.6
2006
1,177.6
634.6
491.2
35.5
0.4
1.9
14.0
470.3
4.1
26.8
370.1
10.8
47.8
3.2
7.5
8.8
168.8
137.1
16.4
15.3
75.3
2.3
2.3
23.0
23.0
45.0
33.1
-
-
0.8
32.3
1.6
1.1
0.4
+
4.5
4.5
1,881.2
2,010.3
2007
1,180.4
628.3
499.8
36.0
0.4
2.1
13.9
476.3
4.1
27.3
376.1
10.8
46.6
3.3
8.2
8.2
169.3
138.1
16.1
15.0
73.2
2.2
2.2
29.0
29.0
45.6
35.3
-
-
1.0
34.3
1.3
0.9
0.4
5.0
5.0
1,898.8
2,025.8
2008
1,129.4
593.6
486.1
33.7
0.4
2.1
13.5
441.9
3.9
26.7
354.5
10.3
43.2
0.9
2.2
9.0
153.6
122.2
16.3
15.1
77.0
2.0
2.0
21.4
21.4
49.2
35.8
-
-
1.0
34.9
1.2
0.9
0.3
4.7
4.7
1,789.9
1,925.1
+ Less than 0.05 Tg C02Eq.
- Unreported or zero.
"This table does not include emissions from non-transportation mobile sources, such as agricultural equipment and construction/mining equipment; it also
does not include emissions associated with electricity consumption by pipelines or lubricants used in transportation.
Includes medium- and heavy-duty trucks over 8,500 Ibs.
c Official estimates exclude emissions from the combustion of both aviation and marine international bunker fuels; however, estimates including
international bunker fuel-related emissions are presented for informational purposes.
d Fluctuations in emission estimates from the combustion of residual fuel oil are associated with fluctuations in reported fuel consumption and may reflect
data collection problems.
Note: Totals may not sum due to independent rounding.
Energy 3-15
-------
Figure 3-11
Sales-Weighted Fuel Economy of New Passenger Cars
and Light-Duty Trucks, 1990-2008
Figure 3-12
25-
24-
23-
22~
21-
20
ll
18-
17-
16
15-
~ ooooooooo
close to zero emissions in 1990 to 64.9 Tg CO2 Eq. in 2008,
led to an increase in overall emissions from transportation
activities of 22 percent.
Fossil Fuel Combustion C02 Emissions from
Transportation
Domestic transportation CO2 emissions increased by 20
percent (301.1 Tg) between 1990 and 2008, an annualized
increase of 1.1 percent. The 6 percent decline in emissions
between 2007 and 2008 reversed a trend of generally
increasing emission since 2001. Almost all of the energy
consumed by the transportation sector is petroleum-based,
including motor gasoline, diesel fuel, jet fuel, and residual
oil. Transportation sources also produce CH4 and N2O;
these emissions are included in Table 3-13 and Table 3-14
in the "Mobile Combustion" Section. Annex 3.2 presents
total emissions from all transportation and mobile sources,
including CO2, N2O, CH4, and HFCs.
Carbon dioxide emissions from passenger cars and
light-duty trucks totaled 1,111.2 Tg in 2008, an increase
of 17 percent (160.9 Tg) from 1990. CO2 emissions from
passenger cars and light-duty trucks peaked at 1,183.9
Tg in 2004, and since then have declined about 6 percent.
Over the 1990s through early this decade, growth in vehicle
travel substantially outweighed improvements in vehicle
fuel economy; however, the rate of vehicle miles traveled
(VMT) growth slowed considerably starting in 2005 (and
declined rapidly in 2008) while average vehicle fuel economy
increased. Among new vehicles sold annually, average fuel
economy gradually declined from 1990 to 2004 (Figure
3-11), reflecting substantial growth in sales of light-duty
Sales of New Passenger Cars and
Light-Duty Trucks, 1990-2008
10,000-,
\
2,000 J
0)0)0)3)010)0)0)
liiiiiiii
CVJCVJCVJCVJCVJCVJCVJCVJCVJ
trucks—in particular, growth in the market share of sport
utility vehicles—relative to passenger cars (Figure 3-12).
New vehicle fuel economy improved beginning in 2005,
largely due to higher light-duty truck fuel economy standards,
which have risen each year since 2005. The overall increase
in fuel economy is also due to a slightly lower light-duty
truck market share, which peaked in 2004 at 52 percent and
declined to 49 percent in 2008.
General aviation aircraft CO2 emissions increased by 80
percent (7.6 Tg) from 1990 to 2008, representing the largest
percentage increase of any transportation mode. Among
on-road vehicles, CO2 emissions from medium- and heavy-
duty trucks13 increased by the largest percentage, 69 percent
(158.5 Tg) from 1990 to 2008. This increase was largely
due to a substantial increase in truck freight movement, as
medium- and heavy-duty truck VMT increased by 54 percent.
Carbon dioxide from the domestic operation of commercial
aircraft decreased by 10 percent (13.2 Tg) from 1990 to
2008 in large part because of the economic recession in
2008. Additionally, commercial aircraft have substantially
improved operational efficiency because of a growing
percentage of seats occupied per flight, improvements in the
fuel efficiency of new aircraft, and the accelerated retirement
of older, less fuel efficient aircraft. Across all categories of
aviation,14 CO2 emissions decreased by 13.3 percent (23.8
13 Includes "medium- and heavy-duty trucks" fueled by gasoline.
diesel and LPG.
14 Includes consumption of jet fuel and aviation gasoline. Does not
include aircraft bunkers, which are not accounted for in national
emission totals.
3-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Tg) between 1990 and 2008. This includes a 53 percent (18.2
Tg) decrease in emissions from domestic military operations.
For further information on all greenhouse gas emissions from
transportation sources, please refer to Annex 3.2.
Fossil Fuel Combustion CH4 and N20 Emissions from
Mobile Sources
Mobile combustion includes emissions of CH4 and
N2O from all transportation sources identified in the U.S.
Inventory with the exception of pipelines, which are
stationary; mobile sources also include non-transportation
sources such as construction/mining equipment, agricultural
equipment, vehicles used off-road, and other sources (e.g.,
snowmobiles, lawnmowers, etc.). Annex 3.2 includes a
summary of all emissions from both transportation and
mobile sources. Table 3-13 and Table 3-14 provide CH4 and
N2O emission estimates in Tg CO2 Eq.15
Mobile combustion was responsible for a small portion
of national CH4 emissions (0.4 percent) but was the second
largest source of U.S. N2O emissions (8 percent). From 1990
to 2008, mobile source CH4 emissions declined by 56 percent,
to 2.0 Tg CO2 Eq. (97 Gg), due largely to control technologies
employed in on-road vehicles since the mid-1990s to reduce
CO, NOX, NMVOC, and CH4 emissions. Mobile source
emissions of N2O decreased by 40 percent, to 26.1 Tg CO2
Eq. (84 Gg). Earlier generation control technologies initially
resulted in higher N2O emissions, causing a 26 percent
increase in N2O emissions from mobile sources between
1990 and 1998. Improvements in later-generation emission
control technologies have reduced N2O output, resulting in
a 53 percent decrease in mobile source N2O emissions from
1998to2008 (Figure3-13). Overall,CH4andN2Oemissions
were predominantly from gasoline-fueled passenger cars and
light-duty trucks.
Table 3-13: CH4 Emissions from Mobile Combustion (Tg C02 Eq.)
Fuel/Vehicle Type3
Gasoline On-Road
Passenger Cars
Light-Duty Trucks
Medium- and Heavy-Duty Trucks and Buses
Motorcycles
Diesel On-Road
Passenger Cars
Light-Duty Trucks
Medium- and Heavy-Duty Trucks and Buses
Alternative Fuel On-Road
Non-Road
Ships and Other Boats
Rail
Aircraft
Agricultural Equipment b
Construction/Mining Equipment0
Otherd
1990
4.2
2.6
1.4l
0.2l
+ 1
+ 1
+ 1
+ 1
+ 1
+ 1
0.4l
+ 1
0.1
0.2l
0.1 1
0+
1995 2000 2005
3.8 2.8 1.9
2.1
1.4
0.2
+
+
+
+
+
+
0.5
+
0.1
0.1
0.1
0.1
0.1
1.6
1.1
0.1
+
+
+
+
+
+
0.5
+
0.1
0.2
0.1
0.1
0.1
1.1
0.7
0.1
+
+
+
+
+
+
0.6
+
0.1
0.2
0.1
0.1
0.1
2006
1.7
1.0
0.6
0.1
+
+
+
+
+
0.1
0.6
+
0.1
0.1
0.1
0.1
0.1
2007
1.6
0.9
0.6
0.1
+
+
+
+
+
0.1
0.6
+
0.1
0.1
0.1
0.1
0.1
2008
1.4
0.8
0.6
0.1
+
+
+
+
+
0.1
0.5
+
0.1
0.1
0.1
0.1
0.1
Total
4.7
4.3
3.4
2.5
2.3
2.2
2.0
+ Less than 0.05 Tg C02Eq.
a See Annex 3.2 for definitions of on-road vehicle types.
b Includes equipment, such as tractors and combines, as well as fuel consumption from trucks that are used off-road in agriculture.
c Includes equipment, such as cranes, dumpers, and excavators, as well as fuel consumption from trucks that are used off-road in construction.
d "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment,
commercial equipment, and industrial equipment, as well as fuel consumption from trucks that are used off-road for commercial/industrial purposes.
Note: Totals may not sum due to independent rounding.
15 See Annex 3.2 for a complete time series of emission estimates
for 1990 through 2008.
Energy 3-17
-------
Table 3-14: N20 Emissions from Mobile Combustion (Tg C02 Eq.)
Fuel/Vehicle Type3
1990
1995
2000
2005
2006
2007
2008
Gasoline On-Road 40.1
Passenger Cars 25.4
Light-Duty Trucks 14.1
Medium- and Heavy-Duty Trucks and Buses 0.6
Motorcycles +
Diesel On-Road 0.2
Passenger Cars +
Light-Duty Trucks +
Medium- and Heavy-Duty Trucks and Buses 0.2
Alternative Fuel On-Road 0.1
Non-Road 3.6
Ships and Boats 0.6
Rail 0.3
Aircraft 1.7
Agricultural Equipment 0.2
Construction/Mining Equipment0 0.3
Otherd 0.4
32.1
17.7
13.6
0.8
+
0.3
0.3
0.2
4.3
0.6
0.4
1.9
0.4
0.5
0.6
29.0
15.7
12.5
0.7
+
0.3
0.3
0.2
4.2
0.7
0.4
1.6
0.4
0.5
0.6
25.5
13.7
11.1
0.7
+
0.3
0.3
0.2
4.3
0.8
0.4
1.6
0.4
0.5
0.6
21.8
11.7
9.5
0.6
+
0.3
0.3
0.2
3.8
0.5
0.3
1.5
0.4
0.5
0.6
Total
43.9
54.0
53.2
36.9
33.6
30.3
26.1
+ Less than 0.05 Tg C02Eq.
a See Annex 3.2 for definitions of on-road vehicle types.
b Includes equipment, such as tractors and combines, as well as fuel consumption from trucks that are used off-road in agriculture.
c Includes equipment, such as cranes, dumpers, and excavators, as well as fuel consumption from trucks that are used off-road in construction.
d "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment,
commercial equipment, and industrial equipment, as well as fuel consumption from trucks that are used off-road for commercial/industrial purposes.
Note: Totals may not sum due to independent rounding.
Figure 3-13
Mobile Source CHa and N?0 Emissions
60
50
40
30
20
10
0-
CH,
CO2 from Fossil Fuel Combustion
Methodology
The methodology used by the United States for
estimating CO2 emissions from fossil fuel combustion is
conceptually similar to the approach recommended by
the IPCC for countries that intend to develop detailed,
sectoral-based emission estimates (IPCC 2006). A detailed
description of the U.S. methodology is presented in Annex
2.1, and is characterized by the following steps:
1. Determine total fuel consumption by fuel type and
sector. Total fossil fuel consumption for each year is
estimated by aggregating consumption data by end-
use sector (e.g., commercial, industrial, etc.), primary
fuel type (e.g., coal, petroleum, gas), and secondary
fuel category (e.g., motor gasoline, distillate fuel oil,
etc.). Fuel consumption data for the United States
were obtained directly from the Energy Information
Administration (EIA) of the U.S. Department of Energy
(DOE), primarily from the Monthly Energy Review and
3-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
published supplemental tables on petroleum product
detail (El A 2009b). The El A does not include territories
in its national energy statistics, so fuel consumption data
for territories were collected separately from Grillot
(2009).16
For consistency of reporting, the IPCC has recommended
that countries report energy data using the International
Energy Agency (IEA) reporting convention and/or IEA
data. Data in the IEA format are presented "top down"—
that is, energy consumption for fuel types and categories
are estimated from energy production data (accounting
for imports, exports, stock changes, and losses).
The resulting quantities are referred to as "apparent
consumption." The data collected in the United States
by EIA on an annual basis and used in this inventory
are predominantly from mid-stream or conversion
energy consumers such as refiners and electric power
generators. These annual surveys are supplemented
with end-use energy consumption surveys, such as the
Manufacturing Energy Consumption Survey, that are
conducted on a periodic basis (every 4 years). These
consumption data sets help inform the annual surveys
to arrive at the national total and sectoral breakdowns
for that total.17
It is also important to note that U.S. fossil fuel energy
statistics are generally presented using gross calorific
values (GCV) (i.e., higher heating values). Fuel
consumption activity data presented here have not been
adjusted to correspond to international standards, which
are to report energy statistics in terms of net calorific
values (NCV) (i.e., lower heating values).18
2. Subtract uses accounted for in the Industrial Processes
chapter. Portions of the fuel consumption data for seven
fuel categories—coking coal, distillate fuel, industrial
16 Fuel consumption by U.S. territories (i.e., American Samoa,
Guam, Puerto Rico, U.S. Virgin Islands, Wake Island, and other U.S.
Pacific Islands) is included in this report and contributed emissions
of43TgCO2Eq. in 2008.
17 See IPCC Reference Approach for estimating CO2 emissions
from fossil fuel combustion in Annex 4 for a comparison of U.S.
estimates using top-down and bottom-up approaches.
18 A crude convention to convert between gross and net calorific
values is to multiply the heat content of solid and liquid fossil fuels
by 0.95 and gaseous fuels by 0.9 to account for the water content of
the fuels. Biomass-based fuels in U.S. energy statistics, however,
are generally presented using net calorific values.
other coal, petroleum coke, natural gas, residual fuel
oil, and other oil—were reallocated to the industrial
processes chapter, as they were consumed during
non-energy related industrial activity. To make these
adjustments, additional data were collected from AISI
(2004 through 2009), Coffeyville (2009), Corathers
(2009), U.S. Census Bureau (2009), EIA (2009g), EIA
(2001), Smith, G. (2007), USGS (2009a), USGS (1995,
1998, 2000 through 2002, 2009), USGS (1991 through
2009), USGS (2009b), USGS (1991 through 2008), and
USGS (1994 through 2009).19
3. Adjust for biofuels, conversion offossilfuels, and exports
of CO2. Fossil fuel consumption estimates are adjusted
downward to exclude (1) fuels with biogenic origins;
(2) fuels created from other fossil fuels; and (3) exports
of CO2. Fuels with biogenic origins are assumed to
result in no net CO2 emissions, and must be subtracted
from fuel consumption estimates, which includes
ethanol added to motor gasoline. Synthetic natural
gas is created from industrial coal, and is currently
included in EIA statistics for both coal and natural gas.
Therefore, synthetic natural gas is subtracted from
energy consumption statistics.20 Since October 2000,
the Dakota Gasification Plant has been exporting CO2
to Canada by pipeline. Since this CO2 is not emitted
to the atmosphere in the United States, energy used to
produce this CO2 is subtracted from energy consumption
statistics. To make these adjustments, additional data
for ethanol were collected from EIA (2009b) and data
for synthetic natural gas were collected from EIA
(2009e), and data for CO2 exports were collected from
the Dakota Gasification Company (2006), Fitzpatrick
(2002), Erickson (2003), and EIA (2007b).
4. Adjust Sectoral Allocation of Distillate Fuel Oil and
Motor Gasoline. EPA had conducted a separate
bottom-up analysis of transportation fuel consumption
based on the Federal Highway Administration's
(FHWA) VMT that indicated that the amount of
distillate and motor gasoline consumption allocated
19 See sections on Iron and Steel Production and Metallurgical
Coke Production, Ammonia Production and Urea Consumption,
Petrochemical Production, Titanium Dioxide Production, Ferroalloy
Production, Aluminum Production, and Silicon Carbide Production
and Consumption in the Industrial Processes chapter.
20 These adjustments are explained in greater detail in Annex 2.1.
Energy 3-19
-------
to the transportation sector in the EIA statistics should
be adjusted. Therefore, for these estimates, the
transportation sector's distillate fuel and motor gasoline
consumption was adjusted upward to match the value
obtained from the bottom-up analysis based on VMT.
As the total distillate and motor gasoline consumption
estimate from EIA are considered to be accurate at the
national level, the distillate consumption totals for the
residential, commercial, and industrial sectors were
adjusted downward proportionately. The data sources
used in the bottom-up analysis of transportation fuel
consumption include AAR (2008 through 2009), Benson
(2002 through 2004), DOE (1993 through 2009), EIA
(2009a), EIA (1991 through 2009), EPA (2009), and
FHWA (1996 through 2009).
5. Adjust for fuels consumed for non-energy uses. U.S.
aggregate energy statistics include consumption of fossil
fuels for non-energy purposes. These are fossil fuels
that are manufactured into plastics, asphalt, lubricants,
or other products. Depending on the end-use, this can
result in storage of some or all of the C contained in the
fuel for a period of time. As the emission pathways of
C used for non-energy purposes are vastly different than
fuel combustion (since the C in these fuels ends up in
products instead of being combusted), these emissions
are estimated separately in the Carbon Emitted and
Stored in Products from Non-Energy Uses of Fossil
Fuels section in this chapter. Therefore, the amount of
fuels used for non-energy purposes was subtracted from
total fuel consumption. Data on non-fuel consumption
was provided by EIA (2009b).
6. Subtract consumption of international bunker fuels.
According to the UNFCCC reporting guidelines
emissions from international transport activities,
or bunker fuels, should not be included in national
totals. U.S. energy consumption statistics include
these bunker fuels (e.g., distillate fuel oil, residual
fuel oil, and jet fuel) as part of consumption by the
transportation end-use sector, however, so emissions
from international transport activities were calculated
separately following the same procedures used for
emissions from consumption of all fossil fuels (i.e.,
estimation of consumption, and determination of C
content). The Office of the Under Secretary of Defense
(Installations and Environment) and the Defense
Energy Support Center (Defense Logistics Agency) of
the U.S. Department of Defense (DoD) (DESC 2009)
supplied data on military jet fuel and marine fuel use.
Commercial jet fuel use was obtained from FAA (2006
and 2008); residual and distillate fuel use for civilian
marine bunkers was obtained from DOC (1991 through
2009) for 1990 through 2001,2007 and 2008, and DHS
(2008) for 2003 through 2006. Consumption of these
fuels was subtracted from the corresponding fuels in the
transportation end-use sector. Estimates of international
bunker fuel emissions for the United States are discussed
in detail later in the International Bunker Fuels section
of this chapter.
7. Determine the total C content of fuels consumed. Total
C was estimated by multiplying the amount of fuel
consumed by the amount of C in each fuel. This total
C estimate defines the maximum amount of C that could
potentially be released to the atmosphere if all of the
C in each fuel was converted to CO2. The C content
coefficients used by the United States were obtained
from El As Emissions of Greenhouse Gases in the
United States 2007 (EIA 2009c), and an EPA analysis of
C content coefficients used in the mandatory reporting
rule (EPA 2010). A discussion of the methodology used
to develop the C content coefficients are presented in
Annexes 2. land 2.2.
8. Estimate C02 Emissions. Total CO2 emissions are the
product of the adjusted energy consumption (from the
previous methodology steps 1 through 6), the C content
of the fuels consumed, and the fraction of C that is
oxidized. The fraction oxidized was assumed to be 100
percent for petroleum, coal, and natural gas based on
guidance in IPCC (2006) (see Annex 2.1).
9. Allocate transportation emissions by vehicle type. This
report provides a more detailed accounting of emissions
from transportation because it is such a large consumer
of fossil fuels in the United States. For fuel types other
than jet fuel, fuel consumption data by vehicle type and
transportation mode were used to allocate emissions by
fuel type calculated for the transportation end-use sector.
• For on-road vehicles, annual estimates of
combined motor gasoline and diesel fuel
consumption by vehicle category were
obtained from FHWA (1996 through 2009);
for each vehicle category, the percent
3-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Box 3-2: Carbon Intensity of U.S. Energy Consumption
Fossil fuels are the dominant source of energy in the United States, and C02 is emitted as a product from their combustion. Useful
energy, however, is generated in the United States from many other sources that do not emit C02 in the energy conversion process, such as
renewable (i.e., hydropower, biofuels, geothermal, solar, and wind) and nuclear sources.
Energy-related C02 emissions can be reduced by not only lowering total energy consumption (e.g., through conservation measures)
but also by lowering the C intensity of the energy sources employed (e.g., fuel switching from coal to natural gas). The amount of C emitted
from the combustion of fossil fuels is dependent upon the C content of the fuel and the fraction of that C that is oxidized. Fossil fuels vary
in their average C content, ranging from about 53 Tg C02 Eq./QBtu for natural gas to upwards of 95 Tg C02 Eq./QBtu for coal and petroleum
coke. In general, the C content per unit of energy of fossil fuels is the highest for coal products, followed by petroleum, and then natural gas.
Other sources of energy, however, may be directly or indirectly C neutral (i.e., zero Tg C02 Eq./Btu). Energy generated from nuclear and many
renewable sources do not result in direct emissions of C02. Biofuels such as wood and ethanol are also considered to be C neutral; although
these fuels do emit C02, in the long run the C02 emitted from biomass consumption does not increase atmospheric C02 concentrations if the
biogenic C emitted is offset by the growth of new biomass. The overall C intensity of the U.S. economy is thus dependent upon the quantity
and combination of fuels and other energy sources employed to meet demand.
Table 3-15 provides a time series of the C intensity for each sector of the U.S. economy. The time series incorporates only the energy
consumed from the direct combustion of fossil fuels in each sector. For example, the C intensity for the residential sector does not include
the energy from or emissions related to the consumption of electricity for lighting or wood for heat. Looking only at this direct consumption
of fossil fuels, the residential sector exhibited the lowest C intensity, which is related to the large percentage of its energy derived from natural
gas for heating. The C intensity of the commercial sector has predominantly declined since 1990 as commercial businesses shift away
from petroleum to natural gas. The industrial sector was more dependent on petroleum and coal than either the residential or commercial
sectors, and thus had higher C intensities over this period. The C intensity of the transportation sector was closely related to the C content
of petroleum products (e.g., motor gasoline and jet fuel, both around 70 Tg C02 Eq./EJ), which were the primary sources of energy. Lastly,
the electricity generation sector had the highest C intensity due to its heavy reliance on coal for generating electricity.
Table 3-15: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg C02 Eq./QBtu)
Sector 1990 1995 2000 2005 2006 2007 2008
Residential3 57.4 56.6 56.7 56.6 56.5 56.3 56.0
Commercial3 59.2 57.7 57.1 57.5 57.2 57.0 56.7
Industrial3 64.2 63.0 62.7 64.2 64.5 64.1 63.6
Transportation3 71.1 71.0 71.3 71.4 71.6 71.9 71.6
Electricity Generation" 87.3 86.5 86.2 85.8 85.4 84.7 84.9
U.S. Territories0 73.0 73J 72.5 73.6 73.7 73.8 73.7
All Sectors' 73.0 72.4 73.0 73.5 73.5 73.3 73.1
a Does not include electricity or renewable energy consumption.
b Does not include electricity produced using nuclear or renewable energy.
c Does not include nuclear or renewable energy consumption.
Note: Excludes non-energy fuel use emissions and consumption.
In contrast to Table 3-15, Table 3-16 presents C intensity values that incorporate energy consumed from all sources (i.e., fossil fuels,
renewables, and nuclear). In addition, the emissions related to the generation of electricity have been attributed to both electricity generation
and the end-use sectors in which that electricity was eventually consumed. This table, therefore, provides a more complete picture of the
actual C intensity of each end-use sector per unit of energy consumed. The transportation end-use sector in Table 3-16 emerges as the most
C intensive when all sources of energy are included, due to its almost complete reliance on petroleum products and relatively minor amount
of biomass-based fuels used, such as ethanol. The "other end-use sectors" (i.e., residential, commercial, and industrial) use significant
quantities of biofuels such as wood, thereby lowering the overall C intensity. The C intensity of the electricity generation sector differs greatly
from the scenario in Table 3-15, where only the energy consumed from the direct combustion of fossil fuels was included. This difference
is due almost entirely to the inclusion of electricity generation from nuclear and hydropower sources, which do not emit C02.
Energy 3-21
-------
Box 3-2: Carbon Intensity of U.S. Energy Consumption (continued)
Table 3-16: Carbon Intensity from All Energy Consumption by Sector (Tg C02 Eq./QBtu)
Sector
Transportation3
Other End-Use Sectors3' b
Electricity Generation0
All Sectors11
1990
70.9
57.8
59.4
61.4
1995
70.6
56.6
58.2!
60.4 1
2000
70.9
58.0
60.3
61.6
2005
70.5
58.5
60.5
62.0
2006
70.3
57.8
59.4
61.5
2007
70.2
57.8
59.7
61.4
2008
69.2
57.2
59.1
60.6
3 Includes electricity (from fossil fuel, nuclear, and renewable sources) and direct renewable energy consumption.
b Other End-Use Sectors includes the residential, commercial, and industrial sectors.
c Includes electricity generation from nuclear and renewable sources.
d Includes nuclear and renewable energy consumption.
Note: Excludes non-energy fuel use emissions and consumption.
By comparing the values in Table 3-15 and Table 3-16, a few
observations can be made. The use of renewable and nuclear energy
sources has resulted in a significantly lower C intensity of the U.S.
economy. Over the nineteen-year period of 1990 through 2008, however,
the C intensity of U.S. energy consumption has been fairly constant, as
the proportion of renewable and nuclear energy technologies have not
changed significantly. Per capita energy consumption has fluctuated, but
is now roughly equivalent to levels in 1990 (see Figure 3-14). Due to a
general shift from a manufacturing-based economy to a service-based
economy, as well as overall increases in efficiency, energy consumption
and energy-related C02 emissions per dollar of gross domestic product
(GDP) have both declined since 1990 (BEA 2009).
Carbon intensity estimates were developed using nuclear and
renewable energy data from EIA (2009a), EPA (2010), and fossil fuel
consumption data as discussed above and presented in Annex 2.1.
Figure 3-14
U.S. Energy Consumption and Energy-Related C02
Emissions Per Capita and Per Dollar GDP
Energy
Consumption/
Capita
C0z/Capita
OT— CMOO**l«lOr— COCnOT— CMOO**l«lOr— CO
gasoline, diesel, and other (e.g., CNG, LPG)
fuel consumption are estimated using data
from DOE (1993 through 2009).
For non-road vehicles, activity data were
obtained from AAR (2008 through 2009),
APTA (2007 through 2009), BEA (1991
through 2009), Benson (2002 through 2004),
DOE (1993 through 2008), DESC (2009),
DOC (1991 through 2009), DOT (1991
through 2009), EIA (2009a), EIA (2009d),
EIA (2007a), EIA (2002), EIA (1991
through 2009), EPA (2009), FAA (2008), and
Gaffney (2007).
For jet fuel used by aircraft, CO2 emissions
were calculated directly based on reported
consumption of fuel as reported by EIA,
and allocated to commercial aircraft using
flight-specific fuel consumption data from
the Federal Aviation Administration's
(FAA) Aviation Environmental Design
Tool (AEDT) (FAA 2010).21 Allocation to
domestic general aviation was made using
FAA Aerospace Forecast data, and allocation
21 Data for inventory years 2000 through 2005 were developed
using the FAA's System for assessing Aviation's Global Emissions
(SAGE) model. That tool has been subsequently replaced by the
Aviation Environmental Design Tool (AEDT), which calculates
noise in addition to aircraft fuel burn and emissions for all
commercial flights globally in a given year. Data for inventory
years 2006-2008 were developed using AEDT. The AEDT model
dynamically models aircraft performance in space and time to
produce fuel burn, emissions and noise. Full flight gate-to-gate
analyses are possible for study sizes ranging from a single flight at
an airport to scenarios at the regional, national, and global levels.
AEDT is currently used by the U.S. government to consider the
interdependencies between aircraft-related fuel burn, noise and
emissions.
3-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
to domestic military uses was made using
DoD data (see Annex 3.7).
Heat contents and densities were obtained from EIA
(2009a) and USAF (1998).22
Uncertainty and Time-Series Consistency
For estimates of CO2 from fossil fuel combustion, the
amount of CO2 emitted is directly related to the amount
of fuel consumed, the fraction of the fuel that is oxidized,
and the carbon content of the fuel. Therefore, a careful
accounting of fossil fuel consumption by fuel type, average
carbon contents of fossil fuels consumed, and production
of fossil fuel-based products with long-term carbon storage
should yield an accurate estimate of CO2 emissions.
Nevertheless, there are uncertainties in the consumption
data, carbon content of fuels and products, and carbon
oxidation efficiencies. For example, given the same primary
fuel type (e.g., coal, petroleum, or natural gas), the amount
of carbon contained in the fuel per unit of useful energy can
vary. For the United States, however, the impact of these
uncertainties on overall CO2 emission estimates is believed
to be relatively small. See, for example, Marland and Pippin
(1990).
Although statistics of total fossil fuel and other energy
consumption are relatively accurate, the allocation of this
consumption to individual end-use sectors (i.e., residential,
commercial, industrial, and transportation) is less certain. For
example, for some fuels the sectoral allocations are based on
price rates (i.e., tariffs), but a commercial establishment may
be able to negotiate an industrial rate or a small industrial
establishment may end up paying an industrial rate, leading
to a misallocation of emissions. Also, the deregulation of
the natural gas industry and the more recent deregulation of
the electric power industry have likely led to some minor
problems in collecting accurate energy statistics as firms in
these industries have undergone significant restructuring.
To calculate the total CO2 emission estimate from
energy-related fossil fuel combustion, the amount of fuel used
22 For a more detailed description of the data sources used for
the analysis of the transportation end use sector see the Mobile
Combustion (excluding CO2) and International Bunker Fuels
sections of the Energy chapter, Annex 3.2, and Annex 3.7.
in these non-energy production processes were subtracted
from the total fossil fuel consumption for 2008. The amount
of CO2 emissions resulting from non-energy related fossil
fuel use has been calculated separately and reported in the
Carbon Emitted from Non-Energy Uses of Fossil Fuels
section of this report. These factors all contribute to the
uncertainty in the CO2 estimates. Detailed discussions on
the uncertainties associated with C emitted from Non-Energy
Uses of Fossil Fuels can be found within that section of this
chapter.
Various sources of uncertainty surround the estimation
of emissions from international bunker fuels, which are
subtracted from the U.S. totals (see the detailed discussions
on these uncertainties provided in the International Bunker
Fuels section of this chapter). Another source of uncertainty
is fuel consumption by U.S. territories. The United States
does not collect energy statistics for its territories at the
same level of detail as for the fifty states and the District of
Columbia. Therefore, estimating both emissions and bunker
fuel consumption by these territories is difficult.
Uncertainties in the emission estimates presented above
also result from the data used to allocate CO2 emissions from
the transportation end-use sector to individual vehicle types
and transport modes. In many cases, bottom-up estimates
of fuel consumption by vehicle type do not match aggregate
fuel-type estimates from EIA. Further research is planned to
improve the allocation into detailed transportation end-use
sector emissions.
The uncertainty analysis was performed by primary fuel
type for each end-use sector, using the IPCC-recommended
Tier 2 uncertainty estimation methodology, Monte Carlo
Simulation technique, with @RISK software. For this
uncertainty estimation, the inventory estimation model for
CO2 from fossil fuel combustion was integrated with the
relevant variables from the inventory estimation model for
International Bunker Fuels, to realistically characterize
the interaction (or endogenous correlation) between the
variables of these two models. About 150 input variables
were modeled for CO2 from energy-related Fossil Fuel
Combustion (including about 10 for non-energy fuel
consumption and about 20 for International Bunker Fuels).
Energy 3-23
-------
In developing the uncertainty estimation model, uniform
distributions were assumed for all activity-related input
variables and emission factors, based on the SAIC/EIA
(2001) report.23 Triangular distributions were assigned for
the oxidization factors (or combustion efficiencies). The
uncertainty ranges were assigned to the input variables
based on the data reported in SAIC/EIA (2001) and on
conversations with various agency personnel.24
The uncertainty ranges for the activity-related input
variables were typically asymmetric around their inventory
estimates; the uncertainty ranges for the emissions factors
were symmetric. Bias (or systematic uncertainties)
associated with these variables accounted for much of the
uncertainties associated with these variables (SAIC/EIA
2001).25 For purposes of this uncertainty analysis, each
input variable was simulated 10,000 times through Monte
Carlo Sampling.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 3-17. Fossil fuel combustion CO2
emissions in 2008 were estimated to be between 5,481.0 and
5,875.0 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 1 percent below to 6 percent above the
2008 emission estimate of 5,572.8 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
23 SAIC/EIA (2001) characterizes the underlying probability density
function for the input variables as a combination of uniform and
normal distributions (the former to represent the bias component
and the latter to represent the random component). However, for
purposes of the current uncertainty analysis, it was determined
that uniform distribution was more appropriate to characterize the
probability density function underlying each of these variables.
24 In the SAIC/EIA (2001) report, the quantitative uncertainty
estimates were developed for each of the three major fossil fuels
used within each end-use sector; the variations within the sub-fuel
types within each end-use sector were not modeled. However, for
purposes of assigning uncertainty estimates to the sub-fuel type
categories within each end-use sector in the current uncertainty
analysis, SAIC/EIA (2001)-reported uncertainty estimates were
extrapolated.
25 Although, in general, random uncertainties are the main focus
of statistical uncertainty analysis, when the uncertainty estimates
are elicited from experts, their estimates include both random and
systematic uncertainties. Hence, both these types of uncertainties
are represented in this uncertainty analysis.
QA/QC and Verification
A source-specific QA/QC plan for CO2 from fossil fuel
combustion was developed and implemented. This effort
included a Tier 1 analysis, as well as portions of a Tier 2
analysis. The Tier 2 procedures that were implemented
involved checks specifically focusing on the activity data
and methodology used for estimating CO2 emissions from
fossil fuel combustion in the United States. Emission totals
for the different sectors and fuels were compared and trends
were investigated to determine whether any corrective actions
were needed. Minor corrective actions were taken.
Recalculations Discussion
The Energy Information Administration (EIA 2009b)
updated energy consumption statistics across the time series.
These revisions primarily impacted the emission estimates
for 2006 and 2007. In addition, carbon content coefficients
were updated based on the EPA (2010) analysis. Overall,
these changes resulted in an average annual increase of 23.7
Tg CO2 Eq. (0.4 percent) in CO2 emissions from fossil fuel
combustion for the period 1990 through 2007.
Planned Improvements
An analysis is being undertaken to update the carbon
content factors for coal, as presented in the annexes of
this report. To reduce uncertainty of CO2 from fossil fuel
combustion estimates, efforts will be taken to work with
EIA and other agencies to improve the quality of the U.S.
territories data. This improvement is not all-inclusive, and
is part of an ongoing analysis and efforts to continually
improve the CO2 from fossil fuel combustion estimates. In
addition, further expert elicitation may be conducted to better
quantify the total uncertainty associated with emissions from
this source.
CH4 and N20 from Stationary
Combustion
Methodology
Methane and N2O emissions from stationary combustion
were estimated by multiplying fossil fuel and wood
consumption data by emission factors (by sector and
fuel type). National coal, natural gas, fuel oil, and wood
consumption data were grouped by sector: industrial,
3-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-17: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Energy-related Fossil Fuel Combustion
by Fuel Type and Sector (Tg C02 Eq. and Percent)
Fuel/Sector
2008 Emission Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Coalb
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Natural Gasb
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Petrol eumb
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Total (excluding Geothermal)b
Geothermal
Total (including Geothermal)bc
2,076.6
0.7
6.4
102.9
NE
1,962.6
4.1
1,227.0
264.7
169.7
393.6
35.8
361.6
1.6
2,268.8
77.3
43.4
322.9
1,749.4
38.9
36.9
5,572.4
0.4
5,572.8
Lower Bound
1,985.5
0.7
6.1
99.2
NE
1,864.6
3.6
1,236.8
257.5
165.1
402.9
34.9
351.6
1.4
2,157.1
73.2
41.3
276.5
1,651.5
37.2
34.1
5,480.6
NE
5,481.0
Upper Bound
2,249.6
0.8
7.4
120.2
NE
2,128.1
4.8
1,306.2
283.5
181.8
444.0
38.4
380.4
1.8
2,425.3
81.3
45.2
382.1
1,879.6
41.7
41.0
5,874.6
NE
5,875.0
Lower Bound
-4%
-5%
-4%
-4%
NA
-5%
-12%
+ 1%
-3%
-3%
+ 2%
-3%
-3%
-13%
-5%
-5%
-5%
-14%
-6%
-4%
-8%
-2%
NE
-1%
Upper Bound
+8%
+ 16%
+ 17%
+ 17%
NA
+8%
+ 19%
+6%
+7%
+7%
+ 13%
+7%
+5%
+ 17%
+7%
+5%
+4%
+ 18%
+7%
+7%
+ 11%
+5%
NE
+6%
NA (Not Applicable).
NE (Not Estimated).
3 Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
bThe low and high estimates for total emissions were calculated separately through simulations and, hence, the low and high emission estimates for the
sub-source categories do not sum to total emissions.
c Geothermal emissions added for reporting purposes, but an uncertainty analysis was not performed for C02 emissions from geothermal production.
commercial, residential, electricity generation, and U.S.
territories. For the CH4 and N2O estimates, fuel consumption
data for coal, natural gas, and fuel oil for the United States
were obtained from EIA's Monthly Energy Review and
unpublished supplemental tables on petroleum product detail
(EIA 2009a). Wood consumption data for the United States
was obtained from El A's Annual Energy Review (EIA 2009b).
Because the United States does not include territories in
its national energy statistics, fuel consumption data for
territories were provided separately by Grillot (2009).26 Fuel
consumption for the industrial sector was adjusted to subtract
out construction and agricultural use, which is reported
under mobile sources.27 Construction and agricultural fuel
use was obtained from EPA (2006). Estimates for wood
biomass consumption for fuel combustion do not include
26 U.S. territories data also include combustion from mobile
activities because data to allocate territories' energy use were
unavailable. For this reason, CH4 and N2O emissions from
combustion by U.S. territories are only included in the stationary
combustion totals.
27 Though emissions from construction and farm use occur due to
both stationary and mobile sources, detailed data was not available
to determine the magnitude from each. Currently, these emissions
are assumed to be predominantly from mobile sources.
Energy 3-25
-------
wood wastes, liquors, municipal solid waste, tires, etc. that
are reported as biomass by EIA.
Emission factors for the four end-use sectors were
provided by the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006). U.S. territories'
emission factors were estimated using the U.S. emission
factors for the primary sector in which each fuel was
combusted.
More detailed information on the methodology for
calculating emissions from stationary combustion, including
emission factors and activity data, is provided in Annex 3.1.
Uncertainty and Time-Series Consistency
Methane emission estimates from stationary sources
exhibit high uncertainty, primarily due to difficulties in
calculating emissions from wood combustion (i.e., fireplaces
and wood stoves). The estimates of CH4 and N2O emissions
presented are based on broad indicators of emissions (i.e., fuel
use multiplied by an aggregate emission factor for different
sectors), rather than specific emission processes (i.e., by
combustion technology and type of emission control).
An uncertainty analysis was performed by primary fuel
type for each end-use sector, using the IPCC-recommended
Tier 2 uncertainty estimation methodology, Monte Carlo
Simulation technique, with @RISK software.
The uncertainty estimation model for this source
category was developed by integrating the CH4 and N2O
stationary source inventory estimation models with the
model for CO2 from fossil fuel combustion to realistically
characterize the interaction (or endogenous correlation)
between the variables of these three models. About 90 input
variables were simulated for the uncertainty analysis of this
source category (about 55 from the CO2 emissions from fossil
fuel combustion inventory estimation model and about 35
from the stationary source inventory models).
In developing the uncertainty estimation model, uniform
distribution was assumed for all activity-related input
variables and N2O emission factors, based on the SAIC/
EIA (2001) report.28 For these variables, the uncertainty
ranges were assigned to the input variables based on the
data reported in SAIC/EIA (2001).29 However, the CH4
emission factors differ from those used by EIA. Since these
factors were obtained from IPCC/UNEP/OECD/IEA (1997),
uncertainty ranges were assigned based on IPCC default
uncertainty estimates (IPCC 2000).
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 3-18. Stationary combustion CH4
emissions in 2008 (including biomass) were estimated to be
between 4.3 and 15.6 Tg CO2 Eq. at a 95 percent confidence
level. This indicates a range of 36 percent below to 133
percent above the 2008 emission estimate of 6.7 Tg CO2 Eq.
Stationary combustion N2O emissions in 2008 (including
biomass) were estimated to be between 10.8 and 41.1 Tg
CO2 Eq. at a 95 percent confidence level. This indicates a
range of 24 percent below to 189 percent above the 2008
emissions estimate of 14.2 Tg CO2 Eq.
The uncertainties associated with the emission
estimates of CH4 and N2O are greater than those associated
with estimates of CO2 from fossil fuel combustion, which
mainly rely on the carbon content of the fuel combusted.
Uncertainties in both CH4 and N2O estimates are due to the
fact that emissions are estimated based on emission factors
representing only a limited subset of combustion conditions.
For the indirect greenhouse gases, uncertainties are partly
due to assumptions concerning combustion technology
types, age of equipment, emission factors used, and activity
data projections.
28 SAIC/EIA (2001) characterizes the underlying probability density
function for the input variables as a combination of uniform and
normal distributions (the former distribution to represent the bias
component and the latter to represent the random component).
However, for purposes of the current uncertainty analysis, it was
determined that uniform distribution was more appropriate to
characterize the probability density function underlying each of
these variables.
29 In the SAIC/EIA (2001) report, the quantitative uncertainty
estimates were developed for each of the three major fossil fuels
used within each end-use sector; the variations within the sub-fuel
types within each end-use sector were not modeled. However, for
purposes of assigning uncertainty estimates to the sub-fuel type
categories within each end-use sector in the current uncertainty
analysis, SAIC/EIA (2001)-reported uncertainty estimates were
extrapolated.
3-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-18: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 Emissions from Energy-Related Stationary
Combustion, Including Biomass (Tg C02 Eq. and Percent)
Source
Stationary Combustion
Stationary Combustion
2008 Emission Estimate Uncertainty Range Relative to Emission Estimate3
Gas (Tg C02 Eq.) (Tg C02 Eq.) (%)
CH4
N20
6.7
14.2
Lower Bound
4.3
10.8
Upper Bound
15.6
41.1
Lower Bound
-36%
-24%
Upper Bound
+ 133%
+ 189%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A source-specific QA/QC plan for stationary combustion
was developed and implemented. This effort included a
Tier 1 analysis, as well as portions of a Tier 2 analysis. The
Tier 2 procedures that were implemented involved checks
specifically focusing on the activity data and emission factor
sources and methodology used for estimating CH4, N2O, and
the indirect greenhouse gases from stationary combustion in
the United States. Emission totals for the different sectors
and fuels were compared and trends were investigated.
Recalculations Discussion
Historical CH4 and N2O emissions from stationary
sources (excluding CO2) were revised due to a couple of
changes. Slight changes to emission estimates for sectors
are due to revised data from El A (2009a). This revision is
explained in greater detail in the section on CO2 Emissions
from Fossil Fuel Combustion within this sector. Wood
consumption data from EIA (2009b) were revised for the
residential, industrial, and electric power sectors. The
combination of the methodological and historical data
changes resulted in an average annual increase of less than 0.1
Tg CO2 Eq. (0.1 percent) in CH4 emissions from stationary
combustion and an average annual decrease of less than 0.1
Tg CO2 Eq. (0.2 percent) in N2O emissions from stationary
combustion for the period 1990 through 2007. Details on the
emission trends through time are described in more detail in
the Methodology section, above.
Planned Improvements
Several items are being evaluated to improve the CH4
and N2O emission estimates from stationary combustion and
to reduce uncertainty. Efforts will be taken to work with
EIA and other agencies to improve the quality of the U.S.
territories data. Because these data are not broken out by
stationary and mobile uses, further research will be aimed at
trying to allocate consumption appropriately. In addition, the
uncertainty of biomass emissions will be further investigated
since it was expected that the exclusion of biomass from the
uncertainty estimates would reduce the uncertainty; and in
actuality the exclusion of biomass increases the uncertainty.
These improvements are not all-inclusive, but are part of an
ongoing analysis and efforts to continually improve these
stationary estimates.
CH4 and N20 from Mobile Combustion
Methodology
Estimates of CH4 and N2O emissions from mobile
combustion were calculated by multiplying emission factors
by measures of activity for each fuel and vehicle type (e.g.,
light-duty gasoline trucks). Activity data included VMT
for on-road vehicles and fuel consumption for non-road
mobile sources. The activity data and emission factors used
are described in the subsections that follow. A complete
discussion of the methodology used to estimate CH4 and N2O
emissions from mobile combustion and the emission factors
used in the calculations is provided in Annex 3.2.
On-Road Vehicles
Estimates of CH4 and N2O emissions from gasoline
and diesel on-road vehicles are based on VMT and emission
factors by vehicle type, fuel type, model year, and emission
control technology. Emission estimates for alternative fuel
Energy 3-27
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vehicles (AFVs)30 are based on VMT and emission factors
by vehicle and fuel type.
Emission factors for gasoline and diesel on-road vehicles
utilizing Tier 2 and low emission vehicle (LEV) technologies
were developed by ICE (2006b); all other gasoline and diesel
on-road vehicle emissions factors were developed by ICE
(2004). These factors were derived from EPA, California
Air Resources Board (CARB) and Environment Canada
laboratory test results of different vehicle and control
technology types. The EPA, CARB and Environment Canada
tests were designed following the Federal Test Procedure
(FTP), which covers three separate driving segments, since
vehicles emit varying amounts of GHGs depending on the
driving segment. These driving segments are: (1) a transient
driving cycle that includes cold start and running emissions;
(2) a cycle that represents running emissions only; and (3)
a transient driving cycle that includes hot start and running
emissions. For each test run, a bag was affixed to the tailpipe
of the vehicle and the exhaust was collected; the content of
this bag was then analyzed to determine quantities of gases
present. The emissions characteristics of segment 2 were
used to define running emissions, and subtracted from the
total FTP emissions to determine start emissions. These
were then recombined based upon the ratio of start to running
emissions for each vehicle class from MOBILE6.2, an EPA
emission factor model that predicts gram per mile emissions
of CO2, CO, HC, NOX, and PM from vehicles under various
conditions, to approximate average driving characteristics.31
Emission factors for AFVs were developed by ICE
(2006a) after examining Argonne National Laboratory's
GREET 1.7-Transportation Fuel Cycle Model (ANL 2006)
and Lipman and Delucchi (2002). These sources describe
AFV emission factors in terms of ratios to conventional
vehicle emission factors. Ratios of AFV to conventional
vehicle emissions factors were then applied to estimated
Tier 1 emissions factors from light-duty gasoline vehicles to
estimate light-duty AFVs. Emissions factors for heavy-duty
AFVs were developed in relation to gasoline heavy-duty
vehicles. A complete discussion of the data source and
methodology used to determine emission factors from AFVs
is provided in Annex 3.2.
Annual VMT data for 1990 through 2008 were obtained
from the Federal Highway Administration's (FHWA)
Highway Performance Monitoring System database as
reported in Highway Statistics (FHWA 1996 through 2009).
VMT estimates were then allocated from FHWA's vehicle
categories to fuel-specific vehicle categories using the
calculated shares of vehicle fuel use for each vehicle category
by fuel type reported in DOE (1993 through 2009) and
information on total motor vehicle fuel consumption by fuel
type from FHWA (1996 through 2009). VMT for AFVs were
taken from Browning (2003). The age distributions of the
U.S. vehicle fleet were obtained from EPA (2007c, 2000), and
the average annual age-specific vehicle mileage accumulation
of U.S. vehicles were obtained from EPA (2000).
Control technology and standards data for on-road
vehicles were obtained from EPA's Office of Transportation
and Air Quality (EPA 2007a, 2007b, 2000,1998, and 1997)
and Browning (2005). These technologies and standards are
defined in Annex 3.2, and were compiled from EPA (1993,
1994a, 1994b, 1998, 1999a) and IPCC/UNEP/OECD/IEA
(1997).
Non-Road Vehicles
To estimate emissions from non-road vehicles, fuel
consumption data were employed as a measure of activity,
and multiplied by fuel-specific emission factors (in grams of
N2O and CH4 per kilogram of fuel consumed).32 Activity data
were obtained from AAR (2008 through 2009), APTA (2007
through 2009), APTA (2006), BEA (1991 through 2005),
Benson (2002 through 2004), DHS (2008), DOC (1991
through 2008), DOE (1993 through 2009), DESC (2008),
DOT (1991 through 2009), EIA (2008a, 2007a, 2007b, 2002),
EIA (2007 through 2009), EIA (1991 through 2009), EPA
(2009), Esser (2003 through 2004), FAA (2010, 2009, and
2006), Gaffney (2007), and Whorton (2006 through 2009).
Emission factors for non-road modes were taken from IPCC/
UNEP/OECD/IEA (1997) and Browning (2009).
30 Alternative fuel and advanced technology vehicles are those that
can operate using a motor fuel other than gasoline or diesel. This
includes electric or other bi-fuel or dual-fuel vehicles that may be
partially powered by gasoline or diesel.
31 Additional information regarding the model can be found online
at http://www.epa.gov/OMS/m6.htm.
32 The consumption of international bunker fuels is not included in
these activity data, but is estimated separately under the International
Bunker Fuels source category.
3-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Uncertainty and Time-Series Consistency
A quantitative uncertainty analysis was conducted
for the on-road portion of the mobile source sector using
the IPCC-recommended Tier 2 uncertainty estimation
methodology, Monte Carlo simulation technique, using @
RISK software. The uncertainty analysis was performed on
2008 estimates of CH4 and N2O emissions, incorporating
probability distribution functions associated with the major
input variables. For the purposes of this analysis, the
uncertainty was modeled for the following two major sets
of input variables: (1) VMT data, by vehicle and fuel type,
and (2) emission factor data, by vehicle, fuel, and control
technology type.
Uncertainty analyses were not conducted for NOX, CO,
or NMVOC emissions. Emission factors for these gases have
been extensively researched since emissions of these gases
from motor vehicles are regulated in the United States, and
the uncertainty in these emission estimates is believed to be
relatively low. However, a much higher level of uncertainty
is associated with CH4 and N2O emission factors, because
emissions of these gases are not regulated in the United States
(and, therefore, there are not adequate emission test data),
and because, unlike CO2 emissions, the emission pathways
of CH4 and N2O are highly complex.
The results of the Tier 2 quantitative uncertainty analysis
for the mobile source CH4 and N2O emissions from on-road
vehicles are summarized in Table 3-19. As noted above, an
uncertainty analysis was not performed for CH4 and N2O
emissions from non-road vehicles. Mobile combustion CH4
emissions (from on-road vehicles) in 2008 were estimated to
be between 1.4 and 1.7 Tg CO2 Eq. at a 95 percent confidence
level. This indicates a range of 10 percent below to 11
percent above the corresponding 2008 emission estimate of
1.5 Tg CO2 Eq. Also at a 95 percent confidence level, mobile
combustion N2O emissions from on-road vehicles in 2008
were estimated to be between 18.3 and 26.3 Tg CO2 Eq.,
indicating a range of 18 percent below to 18 percent above
the corresponding 2008 emission estimate of 22.3 Tg CO2 Eq.
This uncertainty analysis is a continuation of a multi-
year process for developing quantitative uncertainty estimates
for this source category using the IPCC Tier 2 approach to
uncertainty analysis. As a result, as new information becomes
available, uncertainty characterization of input variables
may be improved and revised. For additional information
regarding uncertainty in emission estimates for CFLt and N2O
please refer to the Uncertainty Annex.
Methodological recalculations were applied to the entire
time-series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A source-specific QA/QC plan for mobile combustion
was developed and implemented. This plan is based on the
IPCC-recommended QA/QC Plan. The specific plan used
for mobile combustion was updated prior to collection and
analysis of this current year of data. This effort included a
Tier 1 analysis, as well as portions of a Tier 2 analysis. The
Tier 2 procedures focused on the emission factor and activity
data sources, as well as the methodology used for estimating
emissions. These procedures included a qualitative
assessment of the emissions estimates to determine whether
they appear consistent with the most recent activity data
and emission factors available. A comparison of historical
emissions between the current Inventory and the previous
Inventory was also conducted to ensure that the changes in
Table 3-19: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 Emissions from On-Road Sources
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"b
(TgC02Eq.) (%)
On-Road Sources
On-Road Sources
CH4
N20
1.5
22.3
Lower Bound
1.4
18.3
Upper Bound
1.7
26.3
Lower Bound
-10%
-18%
Upper Bound
+11%
+ 18%
"2008 emission estimates and the uncertainty ranges presented in this table correspond to on-road vehicles, comprising conventional and alternative fuel
vehicles. Because the uncertainty associated with the emissions from non-road vehicles were not estimated, they were excluded in the estimates reported
in this table.
b Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
Energy 3-29
-------
estimates were consistent with the changes in activity data
and emission factors.
Recalculations Discussion
In order to ensure that these estimates are continuously
improved, the calculation methodology is revised annually
based on comments from internal and external reviewers. A
number of adjustments were made to the methodologies used
in calculating emissions in the current Inventory relative to
the previous Inventory report.
Emission factors for CH4 and N2O from the consumption
of residual fuel oil and distillate fuel by ships and boats
have been updated across the time series (Browning 2009).
CH4 emission factors for residual and distillate fuel dropped
89 and 92 percent, respectively. N2O emission factors for
residual and distillate fuel increased 93 and 79 percent,
respectively.
Commercial jet fuel emissions for the years 2006 through
2007 are now calculated directly from jet fuel consumption
data from the FAA's AEDT (FAA 2010) database. Previously,
commercial aircraft activity data for 2006 through 2007 was
estimated by multiplying DOT (1991 through 2009) data
by the percentage difference between 2005 (the most recent
available SAGE data point) and the respective year.
As a result of these changes, estimates of CH4 emissions
were lower while N2O emissions were slightly higher relative
to the previous Inventory. CH4 emissions for 2007 decreased
the most, 3 percent (0.1 Tg CO2 Eq.). Nitrous oxide emissions
for 2000 increased 0.7 percent (0.4 Tg CO2 Eq.), the greatest
increase of any year relative to the previous Inventory.
Planned Improvements
While the data used for this report represent the most
accurate information available, five areas have been
identified that could potentially be improved in the short-
term given available resources.
1. Develop updated emissions factors for diesel vehicles,
motorcycle, and biodiesel vehicles. Previous emission
factors were based upon extrapolations from other
vehicle classes and new test data from Environment
Canada may allow for better estimation of emission
factors for these vehicles.
2. Develop new emission factors for non-road equipment.
The current inventory estimates for non-CO2 emissions
from non-road sources are based on emission factors
from IPCC guidelines published in 1996. Recent data
on non-road sources from Environment Canada and the
California Air Resources Board will be investigated in
order to assess the feasibility of developing new N2O
and CH4 emissions factors for non-road equipment.
3. Examine the feasibility of estimating aircraft N2O and
CH4 emissions by the number of takeoffs and landings,
instead of total fuel consumption. Various studies
have indicated that aircraft N2O and CH4 emissions
are more dependent on aircraft takeoffs and landings
than on total aircraft fuel consumption; however,
aircraft emissions are currently estimated from fuel
consumption data. FAA's SAGE and AEDT databases
contain detailed data on takeoffs and landings for each
calendar year starting in 2000, and could potentially be
used to conduct a Tier II analysis of aircraft emissions.
This methodology will require a detailed analysis of
the number of takeoffs and landings by aircraft type
on domestic trips, the development of procedures to
develop comparable estimates for years prior to 2000,
and the dynamic interaction of ambient air with aircraft
exhausts is developed. The feasibility of this approach
will be explored.
4. Develop improved estimates of domestic waterborne fuel
consumption. The inventory estimates for residual and
distillate fuel used by ships and boats is based in part
on data on bunker fuel use from the U.S. Department
of Commerce. The Department of Homeland Security
(DHS) maintains an electronic reporting system that
automatically registers monthly sales of bunker fuel
at ports, which may provide a more accurate and
comprehensive estimate of residual and distillate bunker
fuel use by reducing the amount of non-reporting. This
system has been used to collect data since 2002, and
these data could be incorporated into the development
of inventory figures. The DHS figures will need to be
reconciled with figures from the current sources of data
and a methodology will need to be developed to produce
updated estimates for prior years.
5. Continue to examine the use of EPA's MOVES model
in the development of the inventory estimates, including
use for uncertainty analysis. Although the Inventory
uses some of the underlying data from MOVES, such as
vehicle age distributions by model year, MOVES is not
3-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
used directly in calculating mobile source emissions. As
MOVES goes through additional testing and refinement,
the use of MOVES will be further explored.
3.2. Carbon Emitted from Non-
Energy Uses of Fossil Fuels (IPCC
Source Category 1 A)
In addition to being combusted for energy, fossil
fuels are also consumed for non-energy uses (NEU) in
the United States. The fuels used for these purposes are
diverse, including natural gas, liquefied petroleum gases
(LPG), asphalt (a viscous liquid mixture of heavy crude
oil distillates), petroleum coke (manufactured from heavy
oil), and coal coke (manufactured from coking coal). The
non-energy applications are equally diverse, and include
feedstocks for the manufacture of plastics, rubber, synthetic
fibers and other materials; reducing agents for the production
of various metals and inorganic products; and non-energy
products such as lubricants, waxes, and asphalt (IPCC 2006).
Carbon dioxide emissions arise from non-energy
uses via several pathways. Emissions may occur during
the manufacture of a product, as is the case in producing
plastics or rubber from fuel-derived feedstocks. Additionally,
emissions may occur during the product's lifetime, such as
during solvent use. Overall, throughout the time series and
across all uses, about 61 percent of the total C consumed for
non-energy purposes was stored in products, and not released
to the atmosphere; the remaining 39 percent was emitted.
There are several areas in which non-energy uses of
fossil fuels are closely related to other parts of the inventory.
For example, some of the NEU products release CO2 at the
end of their commercial life when they are combusted after
disposal; these emissions are reported separately within the
Energy chapter in the Incineration of Waste source category.
In addition, there is some overlap between fossil fuels
consumed for non-energy uses and the fossil-derived CO2
emissions accounted for in the Industrial Processes chapter,
especially for fuels used as reducing agents. To avoid double-
counting, the "raw" non-energy fuel consumption data
reported by El A are modified to account for these overlaps.
There are also net exports of petrochemicals that are not
completely accounted for in the EIA data, and the inventory
calculations also make adjustments to address the effect of
net exports on the mass of C in non-energy applications.
As shown in Table 3-20, fossil fuel emissions in 2008
from the non-energy uses of fossil fuels were 134.2 Tg CO2
Eq., which constituted approximately 2 percent of overall
fossil fuel emissions. In 2008, the consumption of fuels for
non-energy uses (after the adjustments described above) was
4,921.9 TBtu, an increase of 10 percent since 1990 (see Table
3-21). About 57.3 Tg of the C (210.3 Tg CO2 Eq.) in these
fuels was stored, while the remaining 36.6 Tg C (134.2 Tg
CO2 Eq.) was emitted.
Methodology
The first step in estimating C stored in products was to
determine the aggregate quantity of fossil fuels consumed
for non-energy uses. The C content of these feedstock
fuels is equivalent to potential emissions, or the product of
consumption and the fuel-specific C content values. Both
the non-energy fuel consumption and C content data were
supplied by the EIA (2009a) (see Annex 2.1). Consumption
of natural gas, LPG, pentanes plus, naphthas, other oils, and
special naphtha were adjusted to account for net exports of
these products that are not reflected in the raw data from EIA.
Consumption values for industrial coking coal, petroleum
coke, other oils, and natural gas in Table 3-21 and Table
3-22 have been adjusted to subtract non-energy uses that are
included in the source categories of the Industrial Processes
Table 3-20: C02 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg C02 Eq.)
Type
Potential Emissions
C Stored
Emissions as a % of Potential
Emissions
1990
313.5
193.9
38%
119.6
1995
354.8
211.8
40%
142.9
2000
389.4
243.3
38%
146.1
2005
379.3
242.8
36%
136.5
2006
383.6
242.2
37%
141.4
2007
374.7
239.4
36%
135.3
2008
344.5
210.3
39%
134.2
Energy 3-31
-------
chapter.33 Consumption values were also adjusted to subtract
net exports of intermediary chemicals.
For the remaining non-energy uses, the quantity of C
stored was estimated by multiplying the potential emissions
by a storage factor. For several fuel types—petrochemical
feedstocks (including natural gas for non-fertilizer uses, LPG,
pentanes plus, naphthas, other oils, still gas, special naphtha,
and industrial other coal), asphalt and road oil, lubricants,
and waxes—U.S. data on C stocks and flows were used to
develop C storage factors, calculated as the ratio of (a) the
C stored by the fuel's non-energy products to (b) the total
C content of the fuel consumed. A lifecycle approach was
used in the development of these factors in order to account
for losses in the production process and during use. Because
losses associated with municipal solid waste management
are handled separately in this sector under the Incineration
of Waste source category, the storage factors do not account
for losses at the disposal end of the life cycle. For industrial
coking coal and distillate fuel oil, storage factors were taken
from IPCC/UNEP/OECD/IEA (1997), which in turn draws
from Marland and Rotty (1984). For the remaining fuel
types (petroleum coke, miscellaneous products, and other
petroleum), IPCC does not provide guidance on storage
factors, and assumptions were made based on the potential
fate of C in the respective NEU products.
Table 3-21: Adjusted Consumption of Fossil Fuels for Non-Energy Uses (TBtu)
Sector/Fuel Type
Industry
Industrial Coking Coal
Industrial Other Coal
Natural Gas to Chemical Plants, Other Uses
Asphalt & Road Oil
LPG
Lubricants
Pentanes Plus
Naphtha (<401 ° F)
Other Oil (>401 °F)
Still Gas
Petroleum Coke
Special Naphtha
Distillate Fuel Oil
Waxes
Miscellaneous Products
Transportation
Lubricants
U.S. Territories
Lubricants
Other Petroleum (Misc. Prod.)
Total
1990
4,206.3
1
8.2 •
275.4
1,170.2
1,119.ll
186.3
77.5
325.9
661.4
21.3
82.1
100.7
7.0 1
33.3
137.8
176.0
176.0
86.7
0.7
86.0
4,469.0
1995
4,831.7
75.0
11.3
335.9
1,178.2
1,506.6
177.8
289.7
355.6
602.5
40.1
45.6
67.7
8.0 1
40.6
97.1
167.9
167.9
90.8
2.0 1
88.8
5,090.5
2000
5,259.0
82.2
12.4
421.5
1,275.7
1,606.8
189.9
229.3
593.7
527.0
12.6
49.7
94.4
11.7
33.1
119.2
179.4
179.4
165.5
16.4
149.1
5,603.8
2005
5,154.1
53.3
11.9
396.3
1,323.2
1,443.8
160.2
146.3
679.5
514.7
67.7
136.2
60.9
16.0
31.4
112.8
151.3
151.3
121.9
4.6
117.3
5,427.3
2006
5,178.1
73.8
12.4
405.7
1,261.2
1,488.4
156.1
105.5
618.0
573.3
57.2
177.9
68.9
17.5
26.1
136.0
147.4
147.4
134.5
7.3
127.2
5,460.0
2007
5,083.6
30.2
12.4
402.9
1,197.0
1,482.6
161.2
132.6
542.5
669.0
44.2
160.7
75.4
17.5
21.9
133.5
152.2
152.2
128.6
7.1
121.5
5,364.4
2008
4,672.2
50.1
12.4
383.1
1,012.0
1,409.2
149.6
114.8
467.2
599.0
47.3
165.6
83.2
17.5
19.1
142.0
141.3
141.3
108.4
6.1
102.3
4,921.9
+ Does not exceed 0.05 TBtu.
Note: To avoid double-counting, coal coke, petroleum coke, natural gas consumption, and other oils are adjusted for industrial process consumption
reported in the Industrial Processes sector. Natural gas, LPG, Pentanes Plus, Naphthas, Special Naphtha, and Other Oils are adjusted to account for exports
of chemical intermediates derived from these fuels. For residual oil (not shown in the table), all non-energy use is assumed to be consumed in C black
production, which is also reported in the Industrial Processes chapter.
Note: Totals may not sum due to independent rounding.
33 These source categories include Iron and Steel Production, Lead
Production, Zinc Production, Ammonia Manufacture, Carbon Black
Manufacture (included in Petrochemical Production), Titanium
Dioxide Production, Ferroalloy Production, Silicon Carbide
Production, and Aluminum Production.
3-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 3-22: 2008 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions
Adjusted
Non-Energy
Use3
Sector/Fuel Type (TBtu)
Industry
Industrial Coking Coal
Industrial Other Coal
Natural Gas to Chemical Plants
Asphalt & Road Oil
LPG
Lubricants
Pentanes Plus
Naphtha (<401°F)
Other Oil (>401°F)
Still Gas
Petroleum Coke
Special Naphtha
Distillate Fuel Oil
Waxes
Miscellaneous Products
Transportation
Lubricants
U.S. Territories
Lubricants
Other Petroleum (Misc. Prod.)
Total
4,672.2
50.1
12.4
383.1
1,012.0
1,409.2
149.6
114.8
467.2
599.0
47.3
165.6
83.2
17.5
19.1
142.0
141.3
141.3
108.4
6.1
102.3
4,921.9
Carbon
Content
Coefficient
(Tg C/QBtu)
-
31.00
25.82
14.47
20.55
17.06
20.20
19.10
18.55
20.17
17.51
27.85
19.74
20.17
19.80
20.31
-
20.20
-
20.20
20.00
-
Potential
Carbon
(TgC)
88.9
1.6
0.3
5.5
20.8
24.0
3.0
2.2
8.7
12.1
0.8
4.6
1.6
0.4
0.4
2.9
2.9
2.9
2.2
0.1
2.0
93.9
Storage
Factor
-
0.10
0.61
0.61
1.00
0.61
0.09
0.61
0.61
0.61
0.61
0.30
0.61
0.50
0.58
0.00
-
0.09
-
0.09
0.10
-
Carbon
Stored
(TgC)
56.9
0.2
0.2
3.4
20.8
14.7
0.3
1.3
5.3
7.4
0.5
1.4
1.0
0.2
0.2
0.0
0.3
0.3
0.2
0.0
0.2
57.3
Carbon
Emissions
(TgC)
32.1
1.4
0.1
2.2
+
9.3
2.7
0.9
3.4
4.7
0.3
3.2
0.6
0.2
0.2
2.9
2.6
2.6
2.0
0.1
1.8
36.6
Carbon
Emissions
(Tg C02 Eq.)
117.5
5.1
0.5
7.9
+
34.2
10.1
3.1
12.3
17.2
1.2
11.8
2.3
0.6
0.6
10.6
9.5
9.5
7.2
0.4
6.8
134.2
+ Does not exceed 0.05 Tg.
- Not applicable.
aTo avoid double counting, net exports have been deducted.
Note: Totals may not sum due to independent rounding.
Lastly, emissions were estimated by subtracting the
C stored from the potential emissions (see Table 3-20).
More detail on the methodology for calculating storage and
emissions from each of these sources is provided in Annex
2.3.
Where storage factors were calculated specifically for
the United States, data were obtained on (1) products such as
asphalt, plastics, synthetic rubber, synthetic fibers, cleansers
(soaps and detergents), pesticides, food additives, antifreeze
and deicers (glycols), and silicones; and (2) industrial
releases including volatile organic compound, solvent, and
non-combustion CO emissions, Toxics Release Inventory
(TRI) releases, hazardous waste incineration, and energy
recovery. Data were taken from a variety of industry sources,
government reports, and expert communications. Sources
include EPA reports and databases such as compilations
of air emission factors (EPA 2001), National Emissions
Inventory (NEI) Air Pollutant Emissions Trends Data (EPA
2008), Toxics Release Inventory, 1998 (2000b), Biennial
Reporting System (EPA 2004, 2007a), and pesticide sales
and use estimates (EPA 1998, 1999, 2002, 2004); the EIA
Manufacturer's Energy Consumption Survey (MECS) (EIA
1994,1997,2001,2005,2009b); the National Petrochemical
& Refiners Association (NPRA 2002); the U.S. Census
Bureau (1999, 2004); the American Plastics Council (APC
2007; Bank of Canada (2006); Financial Planning Association
(2006); INEGI (2006); the United States International Trade
Commission (2008); Gosselin, Smith, and Hodge (1984);
the Rubber Manufacturers' Association (RMA 2009a,b); the
International Institute of Synthetic Rubber Products (IISRP
2000, 2003); the Fiber Economics Bureau (FEE 2009); and
the American Chemistry Council (ACC 2009). Specific data
sources are listed in full detail in Annex 2.3.
Energy 3-33
-------
Uncertainty and Time-Series Consistency
An uncertainty analysis was conducted to quantify the
uncertainty surrounding the estimates of emissions and storage
factors from non-energy uses. This analysis, performed
using @RISK software and the IPCC-recommended Tier 2
methodology (Monte Carlo Simulation technique), provides
for the specification of probability density functions for key
variables within a computational structure that mirrors the
calculation of the inventory estimate. The results presented
below provide the 95 percent confidence interval, the range
of values within which emissions are likely to fall, for this
source category.
As noted above, the non-energy use analysis is based
on U.S.-specific storage factors for (1) feedstock materials
(natural gas, LPG, pentanes plus, naphthas, other oils, still
gas, special naphthas, and other industrial coal); (2) asphalt;
(3) lubricants; and (4) waxes. For the remaining fuel types
(the "other" category), the storage factors were taken directly
from the IPCC Guidelines for National Greenhouse Gas
Inventories, where available, and otherwise assumptions
were made based on the potential fate of carbon in the
respective NEU products. To characterize uncertainty, five
separate analyses were conducted, corresponding to each of
the five categories. In all cases, statistical analyses or expert
judgments of uncertainty were not available directly from
the information sources for all the activity variables; thus,
uncertainty estimates were determined using assumptions
based on source category knowledge.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 3-23 (emissions) and Table 3-24
(storage factors). Carbon emitted from non-energy uses of
fossil fuels in 2008 was estimated to be between 106.9 and
145.4 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 20 percent below to 8 percent above the
2008 emission estimate of 134.2 Tg CO2 Eq. The uncertainty
in the emission estimates is a function of uncertainty in both
the quantity of fuel used for non-energy purposes and the
storage factor.
In Table 3-24, feedstocks and asphalt contribute least
to overall storage factor uncertainty on a percentage basis.
Table 3-23: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Non-Energy Uses of Fossil Fuels
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Feedstocks
Asphalt
Lubricants
Waxes
Other
Total
C02
C02
C02
C02
C02
C02
78.7
0.00
20.0
0.6
34.9
134.2
Lower Bound
63.7
0.2
16.6
0.4
15.1
106.9
Upper Bound
95.5
0.8
23.3
0.9
35.6
145.4
Lower Bound
-19%
NA
-17%
-28%
-57%
-20%
Upper Bound
+21%
NA
+ 16%
+54%
+ 2%
+8%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
NA (Not Applicable).
Table 3-24: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of Non-Energy Uses of Fossil Fuels
(Percent)
Source
2008 Storage Factor
Gas (%)
Uncertainty Range Relative to Emission Estimate3
(%) (%, Relative)
Feedstocks
Asphalt
Lubricants
Waxes
Other
C02
C02
C02
C02
C02
61%
100%
9%
58%
17%
Lower Bound
59%
99%
4%
49%
18%
Upper Bound
63%
100%
17%
71%
63%
Lower Bound
-4%
-1%
-61%
-16%
+ 6%
Upper Bound
+ 3%
+ 0%
+88%
+22%
+279%
3 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval, as a percentage of the inventory value
(also expressed in percent terms).
3-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Although the feedstocks category—the largest use category
in terms of total carbon flows—appears to have tight
confidence limits, this is to some extent an artifact of the
way the uncertainty analysis was structured. As discussed
in Annex 2.3, the storage factor for feedstocks is based on
an analysis of six fates that result in long-term storage (e.g.,
plastics production), and eleven that result in emissions (e.g.,
volatile organic compound emissions). Rather than modeling
the total uncertainty around all of these fate processes, the
current analysis addresses only the storage fates, and assumes
that all C that is not stored is emitted. As the production
statistics that drive the storage values are relatively well-
characterized, this approach yields a result that is probably
biased toward understating uncertainty.
As is the case with the other uncertainty analyses
discussed throughout this document, the uncertainty
results above address only those factors that can be readily
quantified. More details on the uncertainty analysis are
provided in Annex 2.3.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A source-specific QA/QC plan for non-energy uses of
fossil fuels was developed and implemented. This effort
included a Tier 1 analysis, as well as portions of a Tier
2 analysis for non-energy uses involving petrochemical
feedstocks and for imports and exports. The Tier 2 procedures
that were implemented involved checks specifically focusing
on the activity data and methodology for estimating the fate
of C (in terms of storage and emissions) across the various
end-uses of fossil C. Emission and storage totals for the
different subcategories were compared, and trends across
the time series were analyzed to determine whether any
corrective actions were needed. Corrective actions were
taken to rectify minor errors and to improve the transparency
of the calculations, facilitating future QA/QC.
For petrochemical import and export data, special
attention was paid to NAICS numbers and titles to verify
that none had changed or been removed. Import and export
totals were compared for 2007 as well as their trends across
the time series.
Recalculations Discussion
The Rubber Manufacturers Association's Scrap Tire
Markets in the United States: 9th Biennial Edition (RMA
2009) began reporting the amount of scrap tires in each end
use market in thousands of tons (as opposed to millions of
tires as they had done previously). RMA also updated their
assumed weight for passenger and commercial scrap tires to
22.5 pounds and 110 pounds, respectively. As a result, the
percentage of rubber abraded during tire use for these two
categories was reduced from 20 percent for all tires to 10 and
8 percent for passenger and commercial tires, respectively.
These changes resulted in an average 73 percent reduction in
carbon black emissions and an average 68 percent reduction
in synthetic rubber carbon emissions per year across the
time series.
Additionally, the EIA Manufacturer's Energy
Consumption Survey (MECS) for 2006 was released in the
past year. MECS is only released once every four years and
contributes to approximately 26 percent (as a time-weighted
average) of the C accounted for in feedstocks. Updating the
energy recovery emissions with this new data resulted in
an average annual increase of 4 percent in emissions from
feedstocks for 2003 through 2007.
Planned Improvements
There are several improvements planned for the future:
• Improving the uncertainty analysis. Most of the input
parameter distributions are based on professional
judgment rather than rigorous statistical characterizations
of uncertainty.
• Better characterizing flows of fossil C. Additional
"fates" may be researched, including the fossil C load
in organic chemical wastewaters, plasticizers, adhesives,
films, paints, and coatings. There is also a need to further
clarify the treatment of fuel additives and backflows
(especially methyl tert-butyl ether, MTBE).
Finally, although U.S.-specific storage factors have been
developed for feedstocks, asphalt, lubricants, and waxes,
default values from IPCC are still used for two of the non-
energy fuel types (industrial coking coal and distillate oil),
and broad assumptions are being used for miscellaneous
products and other petroleum. Over the long term, there are
plans to improve these storage factors by conducting analyses
of C fate similar to those described in Annex 2.3.
Energy 3-35
-------
3.3. Incineration of Waste (IPCC
Source Category 1A1 a)
Incineration is used to manage about 7 to 19 percent of
the solid wastes generated in the United States, depending on
the source of the estimate and the scope of materials included
in the definition of solid waste (EPA 2000, Goldstein and
Matdes 2001, Kaufman et al. 2004, Simmons et al. 2006,
ArSova et al. 2008). In the context of this section, waste
includes all municipal solid waste (MS W) as well as tires. In
the United States, almost all incineration of MSW occurs at
waste-to-energy facilities where useful energy is recovered,
and thus emissions from waste incineration are accounted
for in the Energy chapter. Similarly, tires are combusted for
energy recovery in industrial and utility boilers. Incineration
of waste results in conversion of the organic inputs to CO2.
According to IPCC guidelines, when the CO2 emitted is of
fossil origin, it is counted as a net anthropogenic emission
of CO2 to the atmosphere. Thus, the emissions from waste
incineration are calculated by estimating the quantity of waste
combusted and the fraction of the waste that is C derived
from fossil sources.
Most of the organic materials in municipal solid wastes
are of biogenic origin (e.g., paper, yard trimmings), and
have their net C flows accounted for under the Land Use,
Land-Use Change, and Forestry chapter. However, some
components—plastics, synthetic rubber, synthetic fibers, and
carbon black—are of fossil origin. Plastics in the U.S. waste
stream are primarily in the form of containers, packaging,
and durable goods. Rubber is found in durable goods, such
as carpets, and in non-durable goods, such as clothing and
footwear. Fibers in municipal solid wastes are predominantly
from clothing and home furnishings. As noted above, tires
(which contain rubber and carbon black) are also considered
a "non-hazardous" waste and are included in the waste
incineration estimate, though waste disposal practices for
tires differ from municipal solid waste (viz., most incineration
occurs outside of MSW combustion facilities).
Approximately 26 million metric tons of waste was
incinerated in the United States in 2008 (EPA 2008). Carbon
dioxide emissions from incineration of waste rose 63 percent
since 1990, to an estimated 13.1 Tg CO2 Eq. (13,128 Gg) in
2008, as the volume of tires and other fossil C-containing
materials in waste increased (see Table 3-25 and Table 3-26).
Waste incineration is also a source of N2O and CH4 emissions
(De Soete 1993; IPCC 2006). Nitrous oxide emissions from
the incineration of waste were estimated to be 0.4 Tg CO2 Eq.
(1 Gg N2O) in 2008, and have not changed significantly since
1990. Methane emissions from the incineration of waste were
estimated to be less than 0.05 Tg CO2 Eq. (less than 0.5 Gg
CH4) in 2008, and have not changed significantly since 1990.
Methodology
Emissions of CO2 from the incineration of waste include
CO2 generated by the incineration of plastics, synthetic
fibers, and synthetic rubber, as well as the incineration of
synthetic rubber and carbon black in tires. These emissions
were estimated by multiplying the amount of each material
incinerated by the C content of the material and the fraction
oxidized (98 percent). Plastics incinerated in municipal solid
wastes were categorized into seven plastic resin types, each
material having a discrete C content. Similarly, synthetic
rubber is categorized into three product types, and synthetic
fibers were categorized into four product types, each having
a discrete C content. Scrap tires contain several types of
synthetic rubber, as well as carbon black. Each type of
synthetic rubber has a discrete C content, and carbon black
is 100 percent C. Emissions of CO2 were calculated based
on the amount of scrap tires used for fuel and the synthetic
rubber and carbon black content of tires.
More detail on the methodology for calculating
emissions from each of these waste incineration sources is
provided in Annex 3.6.
For each of the methods used to calculate CO2 emissions
from the incineration of waste, data on the quantity of product
combusted and the C content of the product are needed. For
plastics, synthetic rubber, and synthetic fibers, the amount of
specific materials discarded as municipal solid waste (i.e.,
the quantity generated minus the quantity recycled) was
taken from Municipal Solid Waste Generation, Recycling, and
Disposal in the United States: Facts and Figures (EPA 1999
through 2003, 2005 through 2009) and detailed unpublished
backup data for some years not shown in the reports
(Schneider 2007). The proportion of total waste discarded
that is incinerated was derived from data in BioCycle's "State
of Garbage in America" (ArSova et al. 2008). For synthetic
rubber and carbon black in scrap tires, information was
obtained from U.S. Scrap Tire Markets in the United States
2007 Edition (RMA 2009a). For 2008, synthetic rubber mass
3-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-25: C02 and N20 Emissions from the Incineration of Waste (Tg C02 Eq.)
Gas/Waste Product
C02
Plastics
Synthetic Rubber in Tires
Carbon Black in Tires
Synthetic Rubber in MSW
Synthetic Fibers
N20
CH4
Total
1990H
8.0
5.6
0.3 1
0.4
0.9 1
0.8
0.5
+
8.5
1995
11.5
6.3l
1
1.7|
1
1
0.5
+
11.9
2000
11.3
6.1 1
1
1
0.8 •
1
0.4
+
11.7
2005
12.6
6.9
1.6
2.0
0.9
1.2
0.4
+
13.0
2006
12.7
6.7
1.7
2.1
0.9
1.2
0.4
+
13.1
2007
13.3
7.0
1.8
2.3
1.0
1.2
0.4
+
13.7
2008
13.1
6.7
1.8
2.3
1.0
1.3
0.4
+
13.5
+ Does not exceed 0.05 Tg C02 Eq.
Table 3-26: C02 and N20 Emissions from the Incineration of Waste (Gg)
Gas/Waste Product
1990
1995
2000
2005
2006
2007
2008
C02
Plastics
Synthetic Rubber in Tires
Carbon Black in Tires
Synthetic Rubber in MSW
Synthetic Fibers
N20
CH4
8,049
5,588
308
385
932
838
11,461
6,323
1,356
1,696
973
1,115
1
11,270
6,104
1,454
1,818
847
1,046
1
| 1,454
11,818
1,046
I
12,616
6,919
1,599
1,958
946
1,194
1
12,684
6,722
1,712
2,113
928
1,208
1
13,289
6,966
1,823
2,268
1,021
1,212
1
13,128
6,739
1,823
2,268
1,031
1,268
1
+ Does not exceed 0.05 Tg C02 Eq.
in tires is assumed to be equal to that in 2007 due to a lack
of more recently available data.
Average C contents for the "Other" plastics category,
synthetic rubber in municipal solid wastes, and synthetic
fibers were calculated from 1998 production statistics, which
divide their respective markets by chemical compound.
Information about scrap tire composition was taken from
the Rubber Manufacturers' Association internet site (RMA
2009b).
The assumption that 98 percent of organic C is oxidized
(which applies to all waste incineration categories for CO2
emissions) was reported in EPA's life cycle analysis of
greenhouse gas emissions and sinks from management of
solid waste (EPA 2006).
Incineration of waste also results in emissions of N2O
and CH4. These emissions were calculated as a function of
the total estimated mass of waste incinerated and an emission
factor. As noted above, N2O and CH4 emissions are a
function of total waste incinerated in each year; for 1990
through 2006, these data were derived from the information
published in BioCycle (ArSova et al. 2008). Data on total
waste incinerated was not available for 2007 and 2008,
so this value was assumed to equal the most recent value
available (2006). Table 3-27 provides data on municipal
solid waste generation and percentage combusted for the
total waste stream. According to Covanta Energy (Bahor
2009) and confirmed by additional research based on IS WA
(ERC 2009), all municipal solid waste combustors in the
United States are continuously fed stoker units. The emission
factors of N2O and CH4 emissions per quantity of municipal
solid waste combusted are default emission factors for
this technology type and were taken from the IPCC 2006
Guidelines (IPCC 2006).
Uncertainty and Time-Series Consistency
A Tier 2 Monte Carlo analysis was performed to
determine the level of uncertainty surrounding the estimates
of CO2 emissions and N2O emissions from the incineration
of waste (given the very low emissions for CH4, no
uncertainty estimate was derived). IPCC Tier 2 analysis
allows the specification of probability density functions for
key variables within a computational structure that mirrors
Energy 3-37
-------
Table 3-27: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted
Year Waste Generated Waste Discarded Waste Incinerated Incinerated (% of Discards)
1990
^m
1995
^
2000
266,365,714
^^M
296,390,405
^^H
371,071,109
235,733,657
^m^m
216,364,996
^^m
252,328,354
30,632,057
m^m
29,639,040
^^m
25,974,978
a Assumed equal to 2006 value.
Source: Arsova et al. (2008).
10.3
2005
2006
2007
2008
363,274,720
374,686,965
374,686,965a
374,686,965a
259,559,787
267,526,493
267,526,493a
267,526,493a
29,973,520
25,853,401
25,853,401a
25,853,401a
10.0
9.7
9.7C
9.7C
the calculation of the inventory estimate. Uncertainty
estimates and distributions for waste generation variables
(i.e., plastics, synthetic rubber, and textiles generation) were
obtained through a conversation with one of the authors
of the Municipal Solid Waste in the United States reports.
Statistical analyses or expert judgments of uncertainty were
not available directly from the information sources for the
other variables; thus, uncertainty estimates for these variables
were determined using assumptions based on source category
knowledge and the known uncertainty estimates for the waste
generation variables.
The uncertainties in the waste incineration emission
estimates arise from both the assumptions applied to
the data and from the quality of the data. Key factors
include MSW incineration rate; fraction oxidized; missing
data on waste composition; average C content of waste
components; assumptions on the synthetic/biogenic C ratio;
and combustion conditions affecting N2O emissions. The
highest levels of uncertainty surround the variables that
are based on assumptions (e.g., percent of clothing and
footwear composed of synthetic rubber); the lowest levels
of uncertainty surround variables that were determined by
quantitative measurements (e.g., combustion efficiency, C
content of C black).
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 3-28. Waste incineration
CO2 emissions in 2008 were estimated to be between 10.8
and 14.4 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 18 percent below to 10 percent above
the 2008 emission estimate of 13.1 Tg CO2 Eq. Also at a 95
percent confidence level, waste incineration N2O emissions
in 2008 were estimated to be between 0.1 and 1.3 Tg CO2
Eq. This indicates a range of 66 percent below to 212 percent
above the 2008 emission estimate of 0.4 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A source-specific QA/QC plan was implemented for
incineration of waste. This effort included a Tier 1 analysis,
as well as portions of a Tier 2 analysis. The Tier 2 procedures
Table 3-28: Tier 2 Quantitative Uncertainty Estimates for C02 and N20 from the Incineration of Waste (Tg C02 Eq.
and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Incineration of Waste
Incineration of Waste
a Range of emission estimates
C02
N20
predicted
13.1
0.4
by Monte Carlo Stochastic
Lower Bound
10.8
0.1
Upper Bound
14.4
1.3
Lower Bound
-18%
-66%
Upper Bound
+ 10%
+212%
Simulation for a 95 percent confidence interval.
3-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
that were implemented involved checks specifically focusing
on the activity data and specifically focused on the emission
factor and activity data sources and methodology used for
estimating emissions from incineration of waste. Trends
across the time series were analyzed to determine whether
any corrective actions were needed. Actions were taken to
streamline the activity data throughout the incineration of
waste calculations.
Recalculations Discussion
Rather than basing the estimate of the percentage
discards combusted on data from MSW Facts and Figures,
as had been done in previous years, the percent of discards
combusted was updated with BioCycle's time series estimate.
This percentage was used along with discard values for
plastics, synthetic rubber, and synthetic fibers from EPA's
MSW Facts and Figures (the same data source as in previous
years) to estimate CO2 emissions. The change in the source
for percentage combusted was made because using BioCycle
data for discards is in line with other estimates in the
Inventory; BioCycle data are used to estimate CH4 emissions
from landfills and N2O emissions from waste incineration.
This change decreases CO2 emissions annually on average
by 32 percent across the time series for materials other than
tires (the estimate for tires is not affected by this change)
The Rubber Manufacturers Association changed their
reporting for the scrap tire market for 2007 (RMA 2009a).
Previously, the scrap tire market was profiled by end use in
millions of tires, assuming light duty scrap tires weighed 20
pounds and commercial scrap tires weighed 100 pounds.
RMA (2009a) estimates the scrap tire market by end use in
thousands of tons assuming that light duty scrap tires weigh
22.5 pounds and commercial scrap tires weigh 110 pounds
(RMA 2009b). Assuming that average scrap tire weights
and composition could change over time, previous Scrap
Tire Reports were analyzed to establish a time series for the
following factors for both light duty and commercial tires
that would affect CO2 emissions from scrap tire incineration:
• Scrap tire weight
• Tire composition: percent synthetic rubber
• Tire composition: percent carbon black
• Market composition: percent light duty tires
disposed
Where this data was not available in the time series the
missing values were linearly interpolated between bracketing
years' data or, for the ends of time series, set equal to the
closest year with reported data. This updated methodology
resulted in an average annual increase in CO2 emissions of
52 percent.
An emissions estimate for CH4 was also added according
to the 2006 IPCC Guidelines (IPCC 2006). Assuming that
all municipal solid waste combustors in the United States
use continuously fed stoker technology (Bahor 2009, ERC
2009), a default emissions factor from IPCC (2006) of 0.2
kg CH4/Gg waste was applied to BioCycle estimates for the
amount of waste combusted for all years in the Inventory.
Planned Improvements
Additional data sources for calculating the N2O and
CH4 emission factors for U.S. incineration of waste may be
investigated.
3.4. Coal Mining (IPCC Source
Category 1B1 a)
Three types of coal mining related activities release CH4
to the atmosphere: underground mining, surface mining, and
post-mining (i.e., coal-handling) activities. Underground
coal mines contribute the largest share of CH4 emissions.
In 2008, 137 gassy underground coal mines in the United
States employ ventilation systems to ensure that CH4
levels remain within safe concentrations. These systems
can exhaust significant amounts of CH4 to the atmosphere
in low concentrations. Additionally, 24 U.S. coal mines
supplement ventilation systems with degasification systems.
Degasification systems are wells drilled from the surface or
boreholes drilled inside the mine that remove large volumes
of CH4 before, during, or after mining. In 2008,13 coalmines
collected CH4 from degasification systems and utilized this
gas, thus reducing emissions to the atmosphere. Of these
mines, 12 coal mines sold CH4 to the natural gas pipeline
and one coal mine used CH4 from its degasification system
to heat mine ventilation air on site. In addition, one of the
coal mines that sold gas to pipelines also used CH4 to fuel a
thermal coal dryer. Surface coal mines also release CH4 as the
overburden is removed and the coal is exposed, but the level
of emissions is much lower than from underground mines.
Energy 3-39
-------
Finally, some of the CH4 retained in the coal after mining is
released during processing, storage, and transport of the coal.
Total CH4 emissions in 2008 were estimated to be
67.6 Tg CO2 Eq. (3,221 Gg), a decline of 20 percent since
1990 (see Table 3-29 and Table 3-30). Of this amount,
underground mines accounted for 67 percent, surface
mines accounted for 21 percent, and post-mining emissions
accounted for 12 percent. The decline in CH4 emissions
from underground mines from 1996 to 2002 was the result
of the reduction of overall coal production, the mining of less
gassy coal, and an increase in CH4 recovered and used. Since
that time, underground coal production and the associated
methane emissions have remained fairly level, while surface
coal production and its associated emissions have generally
increased.
Methodology
The methodology for estimating CH4 emissions from
coal mining consists of two parts. The first part involves
estimating CH4 emissions from underground mines. Because
Table 3-29: CH4 Emissions from Coal Mining (Tg C02 Eq.)
of the availability of ventilation system measurements,
underground mine emissions can be estimated on a mine-by-
mine basis and then summed to determine total emissions.
The second step involves estimating emissions from surface
mines and post-mining activities by multiplying basin-
specific coal production by basin-specific emission factors.
Underground mines. Total CH4 emitted from
underground mines was estimated as the sum of CH4
liberated from ventilation systems and CH4 liberated by
means of degasification systems, minus CH4 recovered and
used. The Mine Safety and Heath Administration (MSHA)
samples CH4 emissions from ventilation systems for all
mines with detectable34 CH4 concentrations. These mine-
by-mine measurements are used to estimate CH4 emissions
from ventilation systems.
Some of the higher-emitting underground mines also
use degasification systems (e.g., wells or boreholes) that
remove CH4 before, during, or after mining. This CH4 can
then be collected for use or vented to the atmosphere. Various
approaches were employed to estimate the quantity of CH4
Activity
1990
1995
2000
2005
2006
2007
2008
Underground Mining
Liberated
Recovered & Used
Surface Mining
Post-Mining (Underground)
Post-Mining (Surface)
62.3
67.9
(5.6)
12.0
7.7
2.0
159.0
(12.2)1
•
39.5
54.4
(14.9)
12.3
^
35.0
50.2
(15.1)
13.3
6.4
2.2
35.7
54.3
(18.6)
14.0
6.3
2.3
35.9
51.0
(15.1)
13.8
6.1
2.2
44.9
59.7
(14.8)
14.3
6.1
2.3
Total
84.1
67.1
60.5
56.9
58.3
58.1
67.6
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
Table 3-30: CH4 Emissions from Coal Mining (Gg)
Activity
Underground Mining
Liberated
Recovered & Used
Surface Mining
Post-Mining (Underground)
Post-Mining (Surface)
Total
1990
2,968
3,234
(266)
574
368
93
4,003
1995
2,226
2,808
(583)
548 1
330 1
89
3,193
2000
1,882
2,592
(710)
586
3181
95
2,881
2005
1,668
2,389
(721)
633
306
103
2,710
2006
1,701
2,587
(886)
668
299
109
2,776
2007
1,710
2,427
(717)
659
290
107
2,765
2008
2,138
2,844
(707)
681
292
111
3,221
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
34 MSHA records coal mine CH4 readings with concentrations of
greater than 50 ppm (parts per million) CH4. Readings below this
threshold are considered non-detectable.
3-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
collected by each of the twenty mines using these systems,
depending on available data. For example, some mines report
to EPA the amount of CH4 liberated from their degasification
systems. For mines that sell recovered CH4 to a pipeline,
pipeline sales data published by state petroleum and natural
gas agencies were used to estimate degasification emissions.
For those mines for which no other data are available, default
recovery efficiency values were developed, depending on the
type of degasification system employed.
Finally, the amount of CH4 recovered by degasification
systems and then used (i.e., not vented) was estimated. In
2008,12 active coal mines sold recovered CH4 into the local
gas pipeline networks and one coal mine used recovered CH4
on site for heating. Emissions avoided for these projects
were estimated using gas sales data reported by various
state agencies. For most mines with recovery systems,
companies and state agencies provided individual well
production information, which was used to assign gas sales
to a particular year. For the few remaining mines, coal mine
operators supplied information regarding the number of years
in advance of mining that gas recovery occurs.
Surface Mines and Post-Mining Emissions. Surface
mining and post-mining CH4 emissions were estimated by
multiplying basin-specific coal production, obtained from the
Energy Information Administration's Annual Coal Report
(see Table 3-31) (EIA 2008), by basin-specific emission
factors. Surface mining emission factors were developed by
assuming that surface mines emit two times as much CH4
as the average in situ CH4 content of the coal. Revised data
on in situ CH4 content and emissions factors are taken from
EPA (2005), EPA (1996), and AAPG (1984). This calculation
Table 3-31: Coal Production (Thousand Metric Tons)
Year
Underground Surface
Total
1990
384,250
546,818
931,068
1995
359,477
577,638
937,115
2005
2006
2007
2008
334,404
325,703
319,145
323,938
691,460
728,459
720,035
737,845
1,025,864
1,054,162
1,039,179
1,061,782
accounts for CH4 released from the strata surrounding the
coal seam. For post-mining emissions, the emission factor
was assumed to be 32.5 percent of the average in situ CH4
content of coals mined in the basin.
Uncertainty and Time-Series Consistency
A quantitative uncertainty analysis was conducted for the
coal mining source category using the IPCC-recommended
Tier 2 uncertainty estimation methodology. Because emission
estimates from underground ventilation systems were based
on actual measurement data, uncertainty is relatively
low. A degree of imprecision was introduced because
the measurements used were not continuous but rather an
average of quarterly instantaneous readings. Additionally,
the measurement equipment used can be expected to have
resulted in an average of 10 percent overestimation of annual
CH4 emissions (Mutmansky and Wang 2000). Estimates of
CH4 recovered by degasification systems are relatively certain
because many coal mine operators provided information on
individual well gas sales and mined through dates. Many of
the recovery estimates use data on wells within 100 feet of
a mined area. Uncertainty also exists concerning the radius
of influence of each well. The number of wells counted, and
thus the avoided emissions, may vary if the drainage area is
found to be larger or smaller than currently estimated.
Compared to underground mines, there is considerably
more uncertainty associated with surface mining and post-
mining emissions because of the difficulty in developing
accurate emission factors from field measurements. However,
since underground emissions comprise the majority of total
coal mining emissions, the uncertainty associated with
underground emissions is the primary factor that determines
overall uncertainty. The results of the Tier 2 quantitative
uncertainty analysis are summarized in Table 3-32. Coal
mining CH4 emissions in 2008 were estimated to be between
57.7 and 82.6 Tg CO2 Eq. at a 95 percent confidence level.
This indicates a range of 15 percent increase below to 22
percent above the 2008 emission estimate of 67.6 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Energy 3-41
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Table 3-32: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Coal Mining (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Coal Mining
CH4
67.6
57.7
82.6
-15%
+22%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
In the previous Inventory, calculations of emissions
avoided at the two active Jim Walters Resources (JWR)
coal mines in Alabama were performed using the previous
method. This was done in order to take a better documented
approach and to track the four coal mines individually rather
than as a group. Emissions avoided calculations for any
pre-drainage wells at JWR coal mines are based on publicly-
available data records from the Alabama State Oil & Gas
Board. Emission reductions are calculated for pre-drainage
wells that are located inside the mine plan boundaries and are
declared "shut-in" by the O&G Board. The total production
for a well is claimed in the year that the well was shut-in
and mined through.
3.5. Abandoned Underground Coal
Mines (IPCC Source Category 1B1 a)
Underground coal mines contribute the largest share of
CH4 emissions, with active underground mines the leading
source of underground emissions. However, mines also
continue to release CH4 after closure. As mines mature
and coal seams are mined through, mines are closed and
abandoned. Many are sealed and some flood through
intrusion of groundwater or surface water into the void.
Shafts or portals are generally filled with gravel and capped
with a concrete seal, while vent pipes and boreholes are
plugged in a manner similar to oil and gas wells. Some
abandoned mines are vented to the atmosphere to prevent the
buildup of CH4 that may find its way to surface structures
through overburden fractures. As work stops within the
mines, the CH4 liberation decreases but it does not stop
completely. Following an initial decline, abandoned mines
can liberate CH4 at a near-steady rate over an extended period
of time, or, if flooded, produce gas for only a few years. The
gas can migrate to the surface through the conduits described
above, particularly if they have not been sealed adequately. In
addition, diffuse emissions can occur when CH4 migrates to
the surface through cracks and fissures in the strata overlying
the coal mine. The following factors influence abandoned
mine emissions:
• Time since abandonment;
• Gas content and adsorption characteristics of coal;
• CH4 flow capacity of the mine;
• Mine flooding;
• Presence of vent holes; and
• Mine seals.
Gross abandoned mine CH4 emissions ranged from
6.0 to 9.1 Tg CO2 Eq. from 1990 through 2008, varying, in
general, by less than 1 to approximately 19 percent from year
to year. Fluctuations were due mainly to the number of mines
closed during a given year as well as the magnitude of the
emissions from those mines when active. Gross abandoned
mine emissions peaked in 1996 (9.1 Tg CO2 Eq.) due to the
large number of mine closures from 1994 to 1996 (70 gassy
mines closed during the three-year period). In spite of this
rapid rise, abandoned mine emissions have been generally
on the decline since 1996. There were fewer than fifteen
gassy mine closures during each of the years from 1998
through 2008, with only five closures in 2008. By 2008,
gross abandoned mine emissions increased to 9.0 Tg CO2
Eq. (see Table 3-33 and Table 3-34). Gross emissions are
reduced by CH4 recovered and used at 31 mines, resulting
in net emissions in 2008 of 5.9 Tg CO2 Eq.
Methodology
Estimating CH4 emissions from an abandoned coal mine
requires predicting the emissions of a mine from the time of
abandonment through the inventory year of interest. The flow
of CH4 from the coal to the mine void is primarily dependent
on the mine's emissions when active and the extent to which
the mine is flooded or sealed. The CH4 emission rate before
3-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 3-33: CH4 Emissions from Abandoned Underground Coal Mines (Tg C02 Eq.)
Activity
Abandoned Underground Mines
Recovered & Used
Total
1990H
6.0
0.0 1
6.0 1
1995
8.9
0.7 •
8.2|
2000
8.9
1'5|
7.4
2005
7.0
1.4
5.6
2006
7.6
2.1
5.5
2007
8.9
3.3
5.7
2008
9.0
3.1
5.9
Note: Totals may not sum due to independent rounding.
Table 3-34: CH4 Emissions from Abandoned Underground Coal Mines (Gg)
Activity
Abandoned Underground Mines
Recovered & Used
Total
1990
288
0
288
1995
424
32
392
2000
422
72
350
2005
334
68
266
2006
364
100
264
2007
425
156
269
2008
429
148
281
Note: Totals may not sum due to independent rounding.
abandonment reflects the gas content of the coal, rate of coal
mining, and the flow capacity of the mine in much the same
way as the initial rate of a water-free conventional gas well
reflects the gas content of the producing formation and the
flow capacity of the well. A well or a mine which produces
gas from a coal seam and the surrounding strata will produce
less gas through time as the reservoir of gas is depleted.
Depletion of a reservoir will follow a predictable pattern
depending on the interplay of a variety of natural physical
conditions imposed on the reservoir. The depletion of a
reservoir is commonly modeled by mathematical equations
and mapped as a type curve. Type curves which are referred
to as decline curves have been developed for abandoned coal
mines. Existing data on abandoned mine emissions through
time, although sparse, appear to fit the hyperbolic type of
decline curve used in forecasting production from natural
gas wells.
In order to estimate CH4 emissions over time for a given
mine, it is necessary to apply a decline function, initiated
upon abandonment, to that mine. In the analysis, mines
were grouped by coal basin with the assumption that they
will generally have the same initial pressures, permeability
and isotherm. As CH4 leaves the system, the reservoir
pressure, Pr, declines as described by the isotherm. The
emission rate declines because the mine pressure (Pw) is
essentially constant at atmospheric pressure, for a vented
mine, and the PI term is essentially constant at the pressures
of interest (atmospheric to 30 psia). A rate-time equation
can be generated that can be used to predict future emissions.
This decline through time is hyperbolic in nature and can be
empirically expressed as:
q = q; (l+bDit)(-1/b)
where,
q = Gas rate at time t in thousand cubic feet per
day (mcfd)
q; = Initial gas rate at time zero (t0) in mcfd
b = The hyperbolic exponent, dimensionless
D; = Initial decline rate, 1/yr
t = Elapsed time from t0 (years)
This equation is applied to mines of various initial
emission rates that have similar initial pressures, permeability
and adsorption isotherms (EPA 2003).
The decline curves created to model the gas emission
rate of coal mines must account for factors that decrease
the rate of emission after mining activities cease, such as
sealing and flooding. Based on field measurement data, it
was assumed that most U.S. mines prone to flooding will
become completely flooded within eight years and therefore
no longer have any measurable CH4 emissions. Based on this
assumption, an average decline rate for flooding mines was
established by fitting a decline curve to emissions from field
measurements. An exponential equation was developed from
emissions data measured at eight abandoned mines known to
be filling with water located in two of the five basins. Using
a least squares, curve-fitting algorithm, emissions data were
matched to the exponential equation shown below. There
Energy 3-43
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was not enough data to establish basin-specific equations as
was done with the vented, non-flooding mines (EPA 2003).
q = q;e ™
where,
q = Gas flow rate at time t in mcfd
q; = Initial gas flow rate at time zero (t0) in mcfd
D = Decline rate, 1/yr
t = Elapsed time from t0 (years)
Seals have an inhibiting effect on the rate of flow of
CH4 into the atmosphere compared to the rate that would
be emitted if the mine had an open vent. The total volume
emitted will be the same, but will occur over a longer period.
The methodology, therefore, treats the emissions prediction
from a sealed mine similar to emissions from a vented
mine, but uses a lower initial rate depending on the degree
of sealing. The computational fluid dynamics simulator
was again used with the conceptual abandoned mine model
to predict the decline curve for inhibited flow. The percent
sealed is defined as 100 x (1 - (initial emissions from sealed
mine / emission rate at abandonment prior to sealing)).
Significant differences are seen between 50 percent, 80
percent and 95 percent closure. These decline curves were
therefore used as the high, middle, and low values for
emissions from sealed mines (EPA 2003).
For active coal mines, those mines producing over 100
mcfd account for 98 percent of all CUt emissions. This
same relationship is assumed for abandoned mines. It was
determined that 457 abandoned mines closing after 1972
produced emissions greater than 100 mcfd when active.
Further, the status of 271 of the 457 mines (or 60 percent)
is known to be either: (1) vented to the atmosphere; (2)
sealed to some degree (either earthen or concrete seals); or,
(3) flooded (enough to inhibit CH4 flow to the atmosphere).
The remaining 40 percent of the mines were placed in one
of the three categories by applying a probability distribution
analysis based on the known status of other mines located in
the same coal basin (EPA 2003).
Inputs to the decline equation require the average
emission rate and the date of abandonment. Generally this
data is available for mines abandoned after 1972; however,
such data are largely unknown for mines closed before 1972.
Information that is readily available such as coal production
by state and county are helpful, but do not provide enough
data to directly employ the methodology used to calculate
emissions from mines abandoned after 1971. It is assumed
that pre-1972 mines are governed by the same physical,
geologic, andhydrologic constraints that apply to post-1972
mines; thus, their emissions may be characterized by the
same decline curves.
During the 1970s, 78 percent of CH4 emissions from
coal mining came from seventeen counties in seven states.
In addition, mine closure dates were obtained for two states,
Colorado and Illinois, for the hundred year period extending
from 1900 through 1999. The data were used to establish a
frequency of mine closure histogram (by decade) and applied
to the other five states with gassy mine closures. As a result,
basin-specific decline curve equations were applied to 145
gassy coal mines estimated to have closed between 1920
and 1971 in the United States, representing 78 percent of the
emissions. State-specific, initial emission rates were used
based on average coal mine CH4 emissions rates during the
1970s (EPA 2003).
Abandoned mines emission estimates are based on all
closed mines known to have active mine CH4 ventilation
emission rates greater than 100 mcfd at the time of
abandonment; a list by region is shown in Table 3-35. For
example, for 1990 the analysis included 145 mines closed
before 1972 and 258 mines closed between 1972 and 1990.
Initial emission rates based on MSHA reports, time of
abandonment, and basin-specific decline curves influenced by
a number of factors were used to calculate annual emissions
for each mine in the database. Coal mine degasification
data are not available for years prior to 1990, thus the initial
emission rates used reflect ventilation emissions only for pre-
1990 closures. Methane degasification amounts were added
to the quantity of CH4 ventilated for the total CH4 liberation
rate for seventeen mines that closed between 1992 and 2008.
Since the sample of gassy mines (with active mine emissions
greater than 100 mcfd) is assumed to account for 78 percent
of the pre-1971 and 98 percent of the post-1971 abandoned
mine emissions, the modeled results were multiplied by 1.22
and 1.02 to account for all U.S. abandoned mine emissions.
From 1993 through 2008, emission totals were
downwardly adjusted to reflect abandoned mine CH4
emissions avoided from those mines. The inventory totals
were not adjusted for abandoned mine reductions in 1990
through 1992, because no data was reported for abandoned
coal mining CH4 recovery projects during that time.
3-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-35: Number of gassy abandoned mines occurring in U.S. basins grouped by class according to post-
abandonment state
Basin
Central Appalachia
Illinois
Northern Appalachia
Warrior Basin
Western Basins
Total
Sealed
25
30
42
0
26
123
Vented
25
3
22
0
3
53
Flooded
48
14
16
15
2
95
Total Known
98
47
80
15
31
271
Unknown
119
25
33
0
9
186
Total Mines
217
72
113
15
40
457
Uncertainty and Time-Series Consistency
A quantitative uncertainty analysis was conducted
to estimate the uncertainty surrounding the estimates
of emissions from abandoned underground coal mines.
The uncertainty analysis described below provides for
the specification of probability density functions for key
variables within a computational structure that mirrors the
calculation of the inventory estimate. The results provide
the range within which, with 95 percent certainty, emissions
from this source category are likely to fall.
As discussed above, the parameters for which values
must be estimated for each mine in order to predict its decline
curve are: (1) the coal's adsorption isotherm; (2) CH4 flow
capacity as expressed by permeability; and (3) pressure at
abandonment. Because these parameters are not available
for each mine, a methodological approach to estimating
emissions was used that generates a probability distribution
of potential outcomes based on the most likely value and
the probable range of values for each parameter. The range
of values is not meant to capture the extreme values, but
values that represent the highest and lowest quartile of the
cumulative probability density function of each parameter.
Once the low, mid, and high values are selected, they are
applied to a probability density function.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 3-36. Abandoned coal mines CH4
emissions in 2008 were estimated to be between 4.8 and 7.5
Tg CO2 Eq. at a 95 percent confidence level. This indicates
a range of 19 percent below to 27 percent above the 2008
emission estimate of 5.9 Tg CO2 Eq. One of the reasons
for the relatively narrow range is that mine-specific data is
used in the methodology. The largest degree of uncertainty
is associated with the unknown status mines (which account
for 40 percent of the mines), with a ±53 percent uncertainty.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
3.6. Natural Gas Systems (IPCC
Source Category 1B2b)
The U.S. natural gas system encompasses hundreds
of thousands of wells, hundreds of processing facilities,
and over a million miles of transmission and distribution
pipelines. Overall, natural gas systems emitted 96.4 Tg CO2
Eq. (4,591 Gg) of CH4 in 2008, a 26 percent decrease over
1990 emissions (see Table 3-37 and Table 3-38), and 30.0
Tg CO2 Eq. (29,973 Gg) of non-combustion CO2 in 2008,
a 20 percent decrease over 1990 emissions (see Table 3-39
and Table 3-40). Improvements in management practices and
technology, along with the replacement of older equipment,
have helped to stabilize emissions. Methane emissions
decreased since 2007 despite an increase in production and
production wells due to a decrease in emissions from offshore
platforms and an increase in Natural Gas STAR production
sector emissions reductions.
Methane and non-combustion CO2 emissions from
natural gas systems are generally process related, with normal
operations, routine maintenance, and system upsets being
the primary contributors. Emissions from normal operations
include: natural gas engines and turbine uncombusted
exhaust, bleed and discharge emissions from pneumatic
devices, and fugitive emissions from system components.
Routine maintenance emissions originate from pipelines,
equipment, and wells during repair and maintenance
activities. Pressure surge relief systems and accidents can
lead to system upset emissions. Below is a characterization
Energy 3-45
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Table 3-36: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Abandoned Underground Coal Mines
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Abandoned Underground Coal Mines CH4
5.9
4.8
7.5
-19%
+27%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
of the four major stages of the natural gas system. Each of
the stages is described and the different factors affecting CH4
and non-combustion CO2 emissions are discussed.
Field Production. In this initial stage, wells are used to
withdraw raw gas from underground formations. Emissions
arise from the wells themselves, gathering pipelines, and
well-site gas treatment facilities such as dehydrators and
separators. Fugitive emissions and emissions from pneumatic
devices account for the majority of CH4 emissions. Flaring
emissions account for the majority of the non-combustion
CO2 emissions. Emissions from field production accounted
for approximately 15 percent of CH4 emissions and about
28 percent of non-combustion CO2 emissions from natural
gas systems in 2008.
Processing. In this stage, natural gas liquids and various
other constituents from the raw gas are removed, resulting in
"pipeline quality" gas, which is injected into the transmission
system. Fugitive CF^ emissions from compressors, including
compressor seals, are the primary emission source from this
stage. The majority of non-combustion CO2 emissions come
from acid gas removal units, which are designed to remove
CO2 from natural gas. Processing plants account for about
13 percent of CH4 emissions and approximately 71 percent
of non-combustion CO2 emissions from natural gas systems.
Transmission and Storage. Natural gas transmission
involves high pressure, large diameter pipelines that transport
gas long distances from field production and processing
areas to distribution systems or large volume customers
such as power plants or chemical plants. Compressor station
facilities, which contain large reciprocating and turbine
compressors, are used to move the gas throughout the United
States transmission system. Fugitive CH4 emissions from
these compressor stations and from metering and regulating
stations account for the majority of the emissions from this
stage. Pneumatic devices and engine uncombusted exhaust
are also sources of CH4 emissions from transmission
facilities.
Natural gas is also injected and stored in underground
formations, or liquefied and stored in above ground tanks,
during periods of low demand (e.g., summer), and withdrawn,
processed, and distributed during periods of high demand
(e.g., winter). Compressors and dehydrators are the primary
contributors to emissions from these storage facilities.
Methane emissions from the transmission and storage sector
account for approximately 41 percent of emissions from
natural gas systems, while CO2 emissions from transmission
and storage account for less than 1 percent of the non-
combustion CO2 emissions from natural gas systems.
Distribution. Distribution pipelines take the high-
pressure gas from the transmission system at "city gate"
stations, reduce the pressure and distribute the gas through
primarily underground mains and service lines to individual
end users. There were over 1,188,000 miles of distribution
mains in 2008, an increase from just over 944,000 miles in
1990 (OPS 2008b). Distribution system emissions, which
account for approximately 31 percent of CH4 emissions
from natural gas systems and less than 1 percent of non-
combustion CO2 emissions, result mainly from fugitive
emissions from gate stations and pipelines. An increased
use of plastic piping, which has lower emissions than other
pipe materials, has reduced emissions from this stage.
Distribution system CH4 emissions in 2008 were 10.5 percent
lower than 1990 levels.
Methodology
The primary basis for estimates of CH4 and non-
combustion-related CO2 emissions from the U.S. natural
gas industry is a detailed study by the Gas Research Institute
and EPA (EPA/GRI1996). The EPA/GRI study developed
over 80 CH4 emission and activity factors to characterize
emissions from the various components within the operating
stages of the U.S. natural gas system. The same activity
factors were used to estimate both CH4 and non-combustion
CO2 emissions. However, the CH4 emission factors were
3-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 3-37: CH4 Emissions from Natural Gas Systems (Tg C02 Eq.)a
Stage 1990 1995 2000 1
Field Production 34.2 38.7 40.3
Processing 15.0 15.1 14.6
Transmission and Storage 46.9 46.4 44.4
Distribution 33.4 32.4 31.4
Total 129.5 132.6 130.7 |
Including CH4 emission reductions achieved by the Natural Gas STAR program and NESHAP regulations
Note: Totals may not sum due to independent rounding.
Table 3-38: CH4 Emissions from Natural Gas Systems (Gg) a
Stage 1990 1995 2000 1
Field Production 1,629 1,842 1,918
Processing 714 718 1 693
Transmission and Storage 2,235 2,211 2,115
Distribution 1,591 1,543 1,497
Total 6,169 6,313 6,223 ]
Including CH4 emission reductions achieved by the Natural Gas STAR program and NESHAP regulations
Note: Totals may not sum due to independent rounding.
Table 3-39: Non-combustion C02 Emissions from Natural Gas Systems (Tg C02
Stage 1990 1995 2000 1
Field Production 9.5 17.5J 6.0 1
Processing 27.8 24.6 23.3
Transmission and Storage 0.1 1 0.1 1 0.1 1
Distribution + + +1
Total 37.3 42.2 29.4 ]
+ Emissions are less than 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Table 3-40: Non-combustion C02 Emissions from Natural Gas Systems (Gg)
Stage 1990 1995 2000 1
Field Production 9,461 17,523 5,956
Processing 27,752 24,621 23,333
Transmission and Storage 59 1 61 1 61 1
Distribution 46 45 44 1
Total 37,317 42,249 29,394 ]
2005
23.9
11.4
39.0
29.3
103.6
2005
1,139
545
1,855
1,395
4,935
Eq.)
2005
7.6
21.7
0.1
+
29.5
2005
7,633
21,736
61
41
29,472
2006
25.0
11.6
38.2
28.3
103.1
2006
1,189
552
1,820
1,346
4,907
2006
8.2
21.2
0.1
+
29.5
2006
8,221
21,204
60
40
29,526
2007
18.4
12.0
39.6
29.4
99.5
2007
877
573
1,886
1,402
4,738
2007
9.5
21.2
0.1
+
30.8
2007
9,525
21,188
61
41
30,816
2008
14.1
13.0
39.4
29.9
96.4
2008
674
617
1,877
1,424
4,591
2008
8.5
21.4
0.1
+
30.0
2008
8,495
21,375
61
42
29,973
Note: Totals may not sum due to independent rounding.
adjusted for CO2 content when estimating fugitive and vented
non-combustion CO2 emissions. The EPA/GRI study was
based on a combination of process engineering studies and
measurements at representative gas facilities. From this
analysis, a 1992 emission estimate was developed using the
emission and activity factors, except where direct activity
data was available (e.g., offshore platform counts, processing
plant counts, transmission pipeline miles, and distribution
pipelines). For other years, a set of industry activity factor
drivers was developed that can be used to update activity
factors. These drivers include statistics on gas production,
number of wells, system throughput, miles of various kinds
Energy 3-47
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of pipe, and other statistics that characterize the changes in
the U.S. natural gas system infrastructure and operations. See
Annex 3.4 for more detailed information on the methodology
and data used to calculate CH4 and non-combustion CO2
emissions from natural gas systems.
Activity factor data were taken from the following
sources: American Gas Association (AGA 1991-1998);
Minerals and Management Service (MMS 2009a-d);
Monthly Energy Review (ElA 2009f); Natural Gas Liquids
Reserves Report (EIA 2005); Natural Gas Monthly
(EIA 2009b,c,e); the Natural Gas STAR Program annual
emissions savings (EPA 2009); Oil and Gas Journal (OGJ
1997-2009); Office of Pipeline Safety (OPS 2009a-b);
Federal Energy Regulatory Commission (FERC 2009)
and other Energy Information Administration publications
(EIA 2001, 2004,2009a,d); World Oil Magazine (2009a-b).
Data for estimating emissions from hydrocarbon production
tanks were incorporated (EPA 1999). Coalbed CH4 well
activity factors were taken from the Wyoming Oil and
Gas Conservation Commission (Wyoming 2009) and the
Alabama State Oil and Gas Board (Alabama 2009). Other
state well data was taken from: American Association of
Petroleum Geologists (AAPG 2004); Brookhaven College
(Brookhaven 2004); Kansas Geological Survey (Kansas
2009); Montana Board of Oil and Gas Conservation
(Montana 2009); Oklahoma Geological Survey (Oklahoma
2009); Morgan Stanley (Morgan Stanley 2005); Rocky
Mountain Production Report (Lippman 2003); New Mexico
Oil Conservation Division (New Mexico 2009,2005); Texas
Railroad Commission (Texas 2009a-d); Utah Division of Oil,
Gas and Mining (Utah 2009). Emission factors were taken
from EPA/GRI (1996). GTFs Unconventional Natural Gas
and Gas Composition Databases (GTI 2001) were used to
adapt the CH4 emission factors into non-combustion related
CO2 emission factors. Additional information about CO2
content in transmission quality natural gas was obtained via
the internet from numerous U.S. transmission companies
to help further develop the non-combustion CO2 emission
factors.
Uncertainty and Time-Series Consistency
A quantitative uncertainty analysis was conducted to
determine the level of uncertainty surrounding estimates
of emissions from natural gas systems. Performed using
@RISK software and the IPCC-recommended Tier 2
methodology (Monte Carlo Simulation technique), this
analysis provides for the specification of probability density
functions for key variables within a computational structure
that mirrors the calculation of the inventory estimate. The @
RISK model utilizes 1992 (base year) emissions to quantify
the uncertainty associated with the emissions estimates.
The top ten emission sources for the year 2008 are the same
as 2007. The overall emissions change for the base year
between the 2007 and 2008 inventories are minimal (0.002
percent) and as a result the change in associated uncertainty
for the natural gas systems is going to be negligible. Due
to the negligible change, the uncertainty associated with
emissions estimates for 2008 is assumed to be the same as
in the 2007 iteration.
The results presented provide with 95 percent certainty
the range within which emissions from this source category
are likely to fall for the year 2008. The heterogeneous
nature of the natural gas industry makes it difficult to sample
facilities that are completely representative of the entire
industry. Because of this, scaling up from model facilities
introduces a degree of uncertainty. Additionally, highly
variable emission rates were measured among many system
components, making the calculated average emission rates
uncertain. The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 3-41. Natural gas systems
CH4 emissions in 2008 were estimated to be between 73.4
and 138.3 Tg CO2 Eq. at a 95 percent confidence level.
Natural gas systems non-energy CO2 emissions in 2008
were estimated to be between 22.8 and 43.0 Tg CO2 Eq. at
95 percent confidence level.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification Discussion
A number of potential data sources were investigated to
improve selected emission factors in the natural gas industry.
First, alternative data sources to improve transmission
segment compressor station equipment activity factors and
the emission factor for condensate tank flashing emissions
were investigated. Neither datasets provided data adequate
to update the current inventory factors.
3-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 3-41: Tier 2 Quantitative Uncertainty Estimates for CH4 and Non-energy C02 Emissions from Natural Gas
Systems (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Natural Gas Systems
Natural Gas Systems'1
CH4
C02
96.4
30.0
Lower Bound0
73.4
22.8
Upper Bound0
138.3
43.0
Lower Bound0
-24%
-24%
Upper Bound0
+43%
+43%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
b An uncertainty analysis for the non-energy C02 emissions was not performed. The relative uncertainty estimated (expressed as a percent) from the CH4
uncertainty analysis was applied to the point estimate of non-energy C02 emissions.
c All reported values are rounded after calculation. As a result, lower and upper bounds may not be duplicable from other rounded values as shown in table.
The Federal Energy Regulatory Commission (FERC)
Form 2 collects data from operators of compressor stations
on interstate natural gas transmission pipelines. This data
provides the number of interstate compressor stations as well
as installed compression capacity. A U.S .EIA representative
aggregated data from each submission and also contacted
several compressor stations on intrastate pipelines to gather
similar data to that which is available through FERC Form
2 for interstate stations. The U.S. reviewed this data for
inclusion in the Inventory; however, this data set is not
complete, missing the majority of intrastate compressor
station data. Therefore, the current activity data source was
not changed.
Recalculations Discussion
Most recalculations are the result of updating the
previous Inventory activity data with revised values, with
only one exception. Changes in CO2 emissions from Natural
Gas Systems are mostly the result of updating the previous
Inventory activity data with revised values. In addition, the
data source for the number of liquefied natural gas (LNG)
import terminals was changed to Federal Energy Regulatory
Commission reported data (FERC 2009) to provide a more
accurate and current emissions estimate from LNG import
terminals. Overall, these changes resulted in an average
annual increase of 3.0 Tg CO2 Eq. (9 percent) in CO2
emissions from Natural Gas Systems for the period 1990
through 2007.
Planned Improvements
Emission reductions reported to Natural Gas STAR are
deducted from the total sector emissions each year in the
natural gas systems inventory model to estimate emissions.
Originally, these reductions countered only a small portion
of the total sector emissions; however, it has increased
rapidly in recent years. Natural Gas STAR Partners may
use their own site-specific emission factors and therefore do
not necessarily apply the average emission factors assumed
in the national Inventory when estimating their emission
reductions. As a result, the Natural Gas STAR reported
reductions, in some cases, are exceeding the emission
estimated for those particular sources. A study will be
initiated to identify sources that have the most discrepancy
due to the Inventory emissions being exceeded by reported
Natural Gas STAR reductions. Natural gas well venting due
to unconventional well completions and workovers, as well
as conventional gas well blowdowns to unload liquids have
already been identified as sources for which Natural Gas
STAR reported reductions are significantly larger than the
estimated inventory emissions. Improvements to the emission
factors and/or the way in which reductions are accounted for
from the sources identified in the study will be investigated.
Separately, a larger study is currently underway to update
selected emission factors used in the national Inventory. Most
of the activity factors and emission factors in the natural gas
model are from the EPA/GRI (1996) study. The current study
seeks to review selected emission factors in the natural gas
industry, and as appropriate, conduct measurement-based
studies to develop updated emission factors to better reflect
current national circumstances. Results from these studies
are expected in the next few years, and will be incorporated
into the Inventory, pending a peer review.
A study prepared for the Texas Environmental Research
Consortium measured emissions rates from several oil and
condensate tanks in Texas (TERC 2009). This data was
plotted and compared to other estimation methods, such
as the Vasquez-Beggs correlation and the current inventory
emission factor. Because of the limited dataset and
Energy 3-49
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unexpected jumps in data points which can be attributed to
non-flashing emission affects, the United States decided that
further investigation would be necessary before updating the
inventory emission factor. A similar study for TCEQ was
recently released for public comment and the United States
will review the results of this additional study for the next
inventory update cycle.
3.7. Petroleum Systems (IPCC
Source Category 1B2a)
Methane emissions from petroleum systems are
primarily associated with crude oil production, transportation,
and refining operations. During each of these activities,
CH4 emissions are released to the atmosphere as fugitive
emissions, vented emissions, emissions from operational
upsets, and emissions from fuel combustion. Fugitive and
vented CO2 emissions from petroleum systems are primarily
associated with crude oil production and refining operations
but are negligible in transportation operations. Combusted
CO2 emissions from fuels are already accounted for in the
Fossil Fuels Combustion source category, and hence have
not been taken into account in the Petroleum Systems source
category. Total CH4 and CO2 emissions from petroleum
systems in 2008 were 29.1 Tg CO2 Eq. (1,384 Gg CH4)
and 0.5 Tg CO2 (453 Gg), respectively. Since 1990, CH4
emissions have declined by 14 percent, due to industry efforts
to reduce emissions and a decline in domestic oil production
(see Table 3-42 and Table 3-43). Carbon dioxide emissions
have also declined by 19 percent since 1990 due to similar
reasons (see Table 3-44 and Table 3-45).
Production Field Operations. Production field
operations account for almost 98 percent of total CH4
emissions from petroleum systems. Vented CH4 from field
operations account for 90.9 percent of the emissions from
the production sector, unburned CH4 combustion emissions
account for 5.3 percent, fugitive emissions are 3.6 percent,
and process upset emissions are slightly over two-tenths of a
percent. The most dominant sources of emissions, in order of
magnitude, are shallow water offshore oil platforms, natural-
gas-powered pneumatic devices (low bleed and high bleed),
field storage tanks, gas engines, chemical injection pumps
and deep water offshore platforms. These seven sources
alone emit about 95 percent of the production field operations
emissions. Offshore platform emissions are a combination
of fugitive, vented, and unburned fuel combustion emissions
from all equipment housed on oil platforms producing oil
and associated gas. Emissions from high and low-bleed
pneumatics occur when pressurized gas that is used for
control devices is bled to the atmosphere as they cycle
open and closed to modulate the system. Emissions from
storage tanks occur when the CH4 entrained in crude oil
under pressure volatilizes once the crude oil is put into
storage tanks at atmospheric pressure. Emissions from gas
engines are due to unburned CH4 that vents with the exhaust.
Emissions from chemical injection pumps are due to the 25
percent that use associated gas to drive pneumatic pumps.
The remaining five percent of the emissions are distributed
among 26 additional activities within the four categories:
vented, fugitive, combustion and process upset emissions.
For more detailed, source-level data on CH4 emissions in
production field operations, refer to Annex 3.5.
Vented CO2 associated with natural gas emissions from
field operations account for almost 99 percent of the total
CO2 emissions from this source category, while fugitive and
process upsets together account for about 1 percent of the
emissions. The most dominant sources of vented emissions
are field storage tanks, pneumatic devices (high bleed and low
bleed), shallow water offshore oil platforms, and chemical
injection pumps. These five sources together account for
98.4 percent of the non-combustion CO2 emissions from
this source category, while the remaining 0.6 percent of the
emissions is distributed among 24 additional activities within
the three categories: vented, fugitive and process upsets.
Crude Oil Transportation. Crude oil transportation
activities account for less than one half of one percent of
total CH4 emissions from the oil industry. Venting from tanks
and marine vessel loading operations accounts for 61 percent
of CH4 emissions from crude oil transportation. Fugitive
emissions, almost entirely from floating roof tanks, account
for 19 percent. The remaining 20 percent is distributed among
six additional sources within these two categories. Emissions
from pump engine drivers and heaters were not estimated
due to lack of data.
Crude Oil Refining. Crude oil refining processes and
systems account for slightly less than two percent of total
CH4 emissions from the oil industry because most of the
CH4 in crude oil is removed or escapes before the crude oil
is delivered to the refineries. There is an insignificant amount
of CH4 in all refined products. Within refineries, vented
3-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 3-42: CH4 Emissions from Petroleum Systems (Tg C02 Eq.)
Stage 1990 1995 2000 2005 2006 2007 2008
Production Field Operations 33.2 31.3 29.5 27.5 27.5 28.1 28.4
Pneumatic Device Venting 10.3 9.7 9.01 8.3 8.3 8.4 8.7
TankVenting 3.81 3.41 3.2! 2.8 2.8 2.9 2.8
Combustion & Process Upsets 1.91 1.71 1.61 1.5 1.5 1.5 1.6
Misc. Venting & Fugitives 16.8 16.0 15.3 14.5 14.5 15.0 14.8
Wellhead Fugitives 0.51 0.51 0.5 0.4 0.4 0.4 0.5
Crude Oil Transportation 0.1 0.1 0.1 0.1 0.1 0.1 0.1
Refining 0.5 0.5 0.6 0.6 0.6 0.6 0.5
Total 33.9 32.0 30.2 28.2 28.2 28.8 29.1
Note: Totals may not sum due to independent rounding.
Table 3-43: CH4 Emissions from Petroleum Systems (Gg)
Stage 1990 1995 2000 2005 2006 2007 2008
Production Field Operations 1,581 1,492 1,406 1,312 1,311 1,340 1,354
Pneumatic Device Venting 4891 463 4281 397 396 398 415
TankVenting 179J 161 15sl 134 134 137 135
Combustion & Process Upsets 881 82! 761 71 71 72 75
Misc. Venting & Fugitives 7991 7621 7261 690 693 714 707
Wellhead Fugitives 261 251 221 19 17 20 23
Crude Oil Transportation 71 61 51 5 5 5 5
Refining 25 26 28 28 28 27 25
Total 1,613 1,524 1,439 1,344 1,344 1,372 1,384
Note: Totals may not sum due to independent rounding.
Table 3-44: C02 Emissions from Petroleum Systems (Tg C02 Eq.)
Stage 1990 1995 2000 2005 2006 2007 2008
Production Field Operations 0.4 0.3 0.3 0.3 0.3 0.3 0.3
Pneumatic Device Venting +1 +1 +1 + + + +
TankVenting 0.31 0.31 0.31 0.2 0.2 0.3 0.2
Misc. Venting & Fugitives +1 +1 +1 + + + +
Wellhead Fugitives +1 +1 +1 + + + +
Crude Refining 0.2 0.2 0.2 0.2 0.2 0.2 0.2
Total 0.6 0.5 0.5 0.5 0.5 0.5 0.5
+ Does not exceed 0.05 Tg C02 Eq.
Table 3-45: C02 Emissions from Petroleum Systems (Gg)
Stage 1990 1995 2000 2005 2006 2007 2008
Production Field Operations 376 341 323 285 285 292 288
Pneumatic Device Venting 271 261 241 22 22 22 23
TankVenting 3281 2961 281 246 246 252 247
Misc. Venting & Fugitives isl isl 17J 16 16 16 16
Wellhead Fugitives ll ll ll 1 1 1 1
Crude Refining 180 187 211 205 203 182 165
Total 555 528 534 490 488 474 453
Note: Totals may not sum due to independent rounding.
Energy 3-51
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emissions account for about 86 percent of the emissions,
while both fugitive and combustion emissions account
for approximately seven percent each. Refinery system
blowdowns for maintenance and the process of asphalt
blowing—with air, to harden the asphalt—are the primary
venting contributors. Most of the fugitive CH4 emissions
from refineries are from leaks in the fuel gas system. Refinery
combustion emissions include small amounts of unburned
CH4 in process heater stack emissions and unburned CH4 in
engine exhausts and flares.
Asphalt blowing from crude oil refining accounts for
36 percent of the total non-combustion CO2 emissions in
petroleum systems.
Methodology
The methodology for estimating CH4 emissions
from petroleum systems is a bottom-up approach, based
on comprehensive studies of CH4 emissions from U.S.
petroleum systems (EPA 1996, EPA 1999). These studies
combined emission estimates from 64 activities occurring
in petroleum systems from the oil wellhead through crude
oil refining, including 33 activities for crude oil production
field operations, 11 for crude oil transportation activities,
and 20 for refining operations. Annex 3.5 provides greater
detail on the emission estimates for these 64 activities. The
estimates of CH4 emissions from petroleum systems do
not include emissions downstream of oil refineries because
these emissions are very small compared to CH4 emissions
upstream of oil refineries.
The methodology for estimating CH4 emissions from
the 64 oil industry activities employs emission factors
initially developed by EPA (1999). Activity factors for the
years 1990 through 2008 were collected from a wide variety
of statistical resources. Emissions are estimated for each
activity by multiplying emission factors (e.g., emission rate
per equipment item or per activity) by their corresponding
activity factor (e.g., equipment count or frequency of
activity). EPA (1999) provides emission factors for all
activities except those related to offshore oil production and
field storage tanks. For offshore oil production, two emission
factors were calculated using data collected over a one-year
period for all federal offshore platforms (EPA 2005, MMS
2004). One emission factor is for oil platforms in shallow
water, and one emission factor is for oil platforms in deep
water. Emission factors are held constant for the period
1990 through 2008. The number of platforms in shallow
water and the number of platforms in deep water are used
as activity factors and are taken from Minerals Management
Service statistics (MMS 2009a-c). For oil storage tanks, the
emissions factor was calculated from API TankCalc data as
the total emissions per barrel of crude charge (EPA 1999).
For some years, complete activity factor data were
not available. In such cases, one of three approaches was
employed. Where appropriate, the activity factor was
calculated from related statistics using ratios developed for
EPA (1996). For example, EPA (1996) found that the number
of heater treaters (a source of CH4 emissions) is related to
both number of producing wells and annual production.
To estimate the activity factor for heater treaters, reported
statistics for wells and production were used, along with the
ratios developed for EPA (1996). In other cases, the activity
factor was held constant from 1990 through 2008 based on
EPA (1999). Lastly, the previous year's data were used when
data for the current year were unavailable. The CH4 and
CO2 sources in the production sector share common activity
factors. See Annex 3.5 for additional detail.
Among the more important references used to obtain
activity factors are the Energy Information Administration
annual and monthly reports (EIA 1990 through 2008, 1990
through 2009, 1995 through 2009a-b), Methane Emissions
from the Natural Gas Industry by the Gas Research Institute
and EPA (EPA/GRI 1996a-d), Estimates of Methane
Emissions from the U.S. Oil Industry (EPA 1999), consensus
of industry peer review panels, MMS reports (MMS 2005,
2009a-c), analysis of MMS data (EPA 2005, MMS 2004),
the Oil & Gas Journal (OGJ 2009a,b), the Interstate Oil and
Gas Compact Commission (IOGCC 2008), and the United
States Army Corps of Engineers (1995-2008).
The methodology for estimating CO2 emissions from
petroleum systems combines vented, fugitive and process
upset emissions sources from 29 activities for crude oil
production field operations and one activity from petroleum
refining. Emissions are estimated for each activity by
multiplying emission factors by their corresponding activity
factors. The emission factors for CO2 are estimated by
multiplying the CH4 emission factors by a conversion factor,
which is the ratio of CO2 content and methane content
in produced associated gas. The only exceptions to this
methodology are the emission factors for crude oil storage
tanks, which are obtained from API TankCalc simulation
3-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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runs, and the emission factor for asphalt blowing, which
was derived using the methodology and sample data from
API (2004).
Uncertainty and Time-Series Consistency
This section describes the analysis conducted to quantify
uncertainty associated with the estimates of emissions from
petroleum systems. Performed using @RISK software and
the IPCC-recommended Tier 2 methodology (Monte Carlo
Simulation technique), the method employed provides for
the specification of probability density functions for key
variables within a computational structure that mirrors the
calculation of the inventory estimate. The results provide
the range within which, with 95 percent certainty, emissions
from this source category are likely to fall.
The detailed, bottom-up inventory analysis used to
evaluate U.S. petroleum systems reduces the uncertainty
related to the CH4 emission estimates in comparison to
a top-down approach. However, some uncertainty still
remains. Emission factors and activity factors are based
on a combination of measurements, equipment design data,
engineering calculations and studies, surveys of selected
facilities and statistical reporting. Statistical uncertainties
arise from natural variation in measurements, equipment
types, operational variability and survey and statistical
methodologies. Published activity factors are not available
every year for all 64 activities analyzed for petroleum
systems; therefore, some are estimated. Because of the
dominance of the seven major sources, which account for
92.9 percent of the total methane emissions, the uncertainty
surrounding these seven sources has been estimated most
rigorously, and serves as the basis for determining the overall
uncertainty of petroleum systems emission estimates.
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 3-46. Because the top
emission sources have not changed from 2006 and revised
base-year data has by less than 0.03 percent, the relative
uncertainty ranges computed for 2006 and published in the
previous Inventory were taken as valid and applied to the
2008 inventory emission estimates. Petroleum systems CH4
emissions in 2008 were estimated to be between 20.9 and
71.0 Tg CO2 Eq., while CO2 emissions were estimated to be
between 0.3 and 1.1 Tg CO2 Eq. at a 95 percent confidence
level. This indicates a range of 28 percent below to 144
percent above the 2008 emission estimates of 29.1 and 0.5
Tg CO2 Eq. for CH4 and CO2, respectively.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification Discussion
The United States investigated potential inventory
improvements to the crude oil storage tank flashing emission
factor and the potential inclusion of non-energy CO2 emission
sources from maintenance (such as welding and cutting),
incineration of waste materials and byproducts from thermal
oxidizers, decoking processes for tubing in thermal crackers
and heater or boiler tubes, delayed and flexi-coking, and
catalyst regeneration for refinery units.
The U.S. EIA collects information in form EIA-810
on all refinery fuel usage; however, the report does not
distinguish for which activities the fuel was consumed.
Therefore, estimates for fuels consumed for petroleum
refining, for sources such as maintenance, are included in
Table 3-46: Tier 2 Quantitative Uncertainty Estimates for CH4 and C02 Emissions from Petroleum Systems
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Petroleum Systems
Petroleum Systems
CH4
C02
29.1
0.5
Lower Boundb
20.9
0.3
Upper Boundb
71.0
1.1
Lower Boundb
-28%
-28%
Upper Boundb
+ 144%
+ 144%
3 Range of 2006 relative uncertainty predicted by Monte Carlo Stochastic Simulation, based on 1995 base year activity factors, for a 95 percent confidence
interval.
b All reported values are rounded after calculation. As a result, lower and upper bounds may not be duplicable from other rounded values as shown in table.
Energy 3-53
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the industrial sector of the Fossil Fuel Combustion chapter;
with the available data it was not possible to report this
source separately or move the fuel consumed by refineries
to the Petroleum Systems source category. Similarly, other
sources such as incineration of waste and byproducts, and
coke burn, are accounted for the Fossil Fuel Combustion
source category or other source categories.
Recalculations Discussion
Most revisions were due to updating previous years' data
with revised data from existing data sources. Well completion
venting, well drilling, and offshore platform activity factors
were updated from existing data sources from 1990 onward.
Activity factors for average stripper well production were
revised from 1993 onward with a new data source, IOGCC
(2008). Previously, the entire time series was assumed to
be constant with average stripper well production of 2.2
barrels of oil per day. IOGCC (2008) provides improved
estimated data for each year starting in 1993; 1990 through
1992 are still assumed to be an average of 2.2 barrels of oil
per day. Updating the activity factor for average stripper
well production resulted in an increase of emissions of 1.3
Gg from 2007 levels.
Non-combusted CO2 emissions from asphalt blowing
were included for the first time in this inventory. The activity
factor for asphalt blowing are the same as those already used
CH4 emission calculations. However, a new CO2 emissions
factor for asphalt blowing was derived from API (2004).
Carbon dioxide emissions from asphalt blowing resulted in
an increase in total CO2 emissions from petroleum systems
of 0.18 Tg CO2Eq. in 2007, or 38 percent of total emissions
from petroleum systems.
The total effect of the recalculations was to increase
emissions by 2 Gg of CH4 and 166 Gg of CO2 in 2007.
Planned Improvements
As noted above, nearly all emission factors used in the
development of the petroleum systems estimates were taken
from EPA (1995,1996,1999), with the remaining emission
factors taken from EPA default values (EPA 2005) and a
consensus of industry peer review panels. These emission
factors will be reviewed as part of future inventory work.
Results of this review and analysis will be incorporated into
future inventories, as appropriate.
Data available in a recently released TCEQ study (as
discussed in the QA/QC and Verification section) will be
reviewed and considered as a potential update to the emission
factor from crude oil storage tank flashing emissions.
A study prepared for the Texas Environmental Research
Consortium measured emissions rates from several oil
and condensate tanks in Texas (TERC 2009). This data
was plotted and compared to other estimation methods,
such as the Vasquez-Beggs correlation and the current
inventory emission factor. Because of the limited dataset
and unexpected jumps in data points that can be attributed
to non-flashing emission affects, the further investigation is
necessary before updating the inventory emission factor. A
similar study for Texas Council on Environmental Quality
has recently been released for public comment. The results of
this additional study will be reviewed for the next inventory
update cycle.
3.8. Energy Sources of Indirect
Greenhouse Gas Emissions
In addition to the main greenhouse gases addressed
above, many energy-related activities generate emissions of
indirect greenhouse gases. Total emissions of nitrogen oxides
(NOX), carbon monoxide (CO), and non-CH4 volatile organic
compounds (NMVOCs) from energy-related activities from
1990 to 2008 are reported in Table 3-49.
Methodology
These emission estimates were obtained from preliminary
data (EPA 2009), and disaggregated based on EPA (2003),
which, in its final iteration, will be published on the National
Emission Inventory (NEI) Air Pollutant Emission Trends
web site. Emissions were calculated either for individual
categories or for many categories combined, using basic
activity data (e.g., the amount of raw material processed)
as an indicator of emissions. National activity data were
collected for individual categories from various agencies.
Depending on the category, these basic activity data may
include data on production, fuel deliveries, raw material
processed, etc.
Activity data were used in conjunction with emission
factors, which together relate the quantity of emissions to the
activity. Emission factors are generally available from the
3-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Box 3-3: Carbon Dioxide Transport, Injection, and Geological Storage
Carbon dioxide is produced, captured, transported, and used for Enhanced Oil Recovery (EOR) as well as commercial and non-EOR
industrial applications. This C02 is produced from both naturally-occurring C02 reservoirs and from industrial sources such as natural
gas processing plants and ammonia plants. In the current Inventory, emissions from naturally-produced C02 are estimated based on
the application.
In the current Inventory, the C02 that is used in non-EOR industrial and commercial applications (e.g., food processing, chemical
production) is assumed to be emitted to the atmosphere during its industrial use. These emissions are discussed in the Carbon Dioxide
Consumption section. The naturally-occurring C02 used in EOR operations is assumed to be fully sequestered. Additionally, all anthropogenic
C02 emitted from natural gas processing and ammonia plants is assumed to be emitted to the atmosphere, regardless of whether the C02 is
captured or not. These emissions are currently included in the Natural Gas Systems and the Ammonia Production sections of the Inventory,
respectively.
IPCC (2006) includes, for the first time, methodological guidance to estimate emissions from the capture, transport, injection, and
geological storage of C02. The methodology is based on the principle that the carbon capture and storage system should be handled in a
complete and consistent manner across the entire Energy sector. The approach accounts for C02 captured at natural and industrial sites as
well as emissions from capture, transport, and use. For storage specifically, a Tier 3 methodology is outlined for estimating and reporting
emissions based on site-specific evaluations. However, IPCC (2006) notes that if a national regulatory process exists, emissions information
available through that process may support development of C02 emissions estimates for geologic storage.
In October 2007, the U.S. EPA announced plans to develop regulations for geologic sequestration of C02 under the EPA Underground
Injection Control Program. Given that the regulatory process is in its early phases, and site-specific emissions estimates are not yet available,
emissions estimates from C02 capture, transport, injection and geologic storage are not yet included in national totals. Preliminary estimates
indicate that the amount of C02 captured from industrial and natural sites, as well as fugitive emissions from pipelines is 44.2 Tg C02 (44,175
Gg C02) (see Table 3-47 and Table 3-48). Site-specific monitoring and reporting data for C02 injection sites (i.e., EOR operations) were not
readily available; therefore, these estimates assume all C02 is emitted.
Table 3-47: Potential Emissions from C02 Capture and Transport (Tg C02 Eq.)
Stage
Acid Gas Removal Plants
Naturally Occurring C02
Ammonia Production Plants
Pipelines Transporting C02
Total
1990
4.8
20.8
0.0
0.0
25.6 1
1995
3.7
22.5
0.7 1
0.0
26.9
2000
2.3
23.2
0.7 1
0.0
26.1
2005
5.8
28.3
0.7
0.0
34.7
2006
6.2
30.2
0.7
0.0
37.1
2007
6.1
33.1
0.7
0.0
39.9
2008
6.7
36.1
1.4
0.0
44.2
Table 3-48: Potential Emissions from C02 Capture and Transport (Gg)
Stage
Acid Gas Removal Plants
Naturally Occurring C02
Ammonia Production Plants
Pipelines Transporting C02
Total
1990
4,832
20,811
1
8
25,643
1995
3,672
22,547
676
8
26,896
2000
2,264
23,208
676 1
8
26,149
2005
5,798
28,267
676
7
34,742
2006
6,224
30,224
676
8
37,124
2007
6,088
33,086
676
9
39,851
2008
6,726
36,096
1,353
9
44,175
'S compilation of'Air Pollutant Emission Factors, AP-42 Uncertainty and Time-Series Consistency
(EPA 1997). The EPA currently derives the overall emission Uncertainties in these estimates are partly due to the
control efficiency of a source category from a variety of accuracy of the emission factors used and accurate estimates
information sources, including published reports, the 1985 of activity data A quantitative uncertainty analysis was not
National Acid Precipitation and Assessment Program performed
Emissions Inventory, and other EPA databases.
Energy 3-55
-------
Table 3-49: NOX, CO, and NMVOC Emissions from Energy-Related Activities (Gg)
Gas/Source
NOX
Mobile Combustion
Stationary Combustion
Oil and Gas Activities
Incineration of Waste
International Bunker Fuels3
CO
Mobile Combustion
Stationary Combustion
Incineration of Waste
Oil and Gas Activities
International Bunker Fuels3
NMVOCs
Mobile Combustion
Stationary Combustion
Oil and Gas Activities
Incineration of Waste
International Bunker Fuels3
1990
21,106
10,862
10,023
139
82l
2,020
125,640
119,360
5,000
978
3021
130
12,620
10,932
9121
5541
222
61
1995
20,586
10,536
9,862
100
88
1,566
104,402
97,630
5,383
1,073
316
124
10,538
8,745
973
5821
237
50
2000
18,477
10,199
8,053
111
114
1,344
89,714
83,559
4,340
1,670
146 1
128
8,952
7,229
1,077
388 1
257
45
2005
15,319
9,012
5,858
321
129
1,705
69,062
62,692
4,649
1,403
318
133
7,798
6,330
716
510
241
54
2006
14,473
8,488
5,545
319
121
1,796
65,399
58,972
4,695
1,412
319
162
7,702
6,037
918
510
238
59
2007
13,829
7,965
5,432
318
114
1,789
61,739
55,253
4,744
1,421
320
159
7,604
5,742
1,120
509
234
58
2008
13,012
7,441
5,148
318
106
1,923
58,078
51,533
4,792
1,430
322
167
7,507
5,447
1,321
509
230
62
a These values are presented for informational purposes only and are not included or are already accounted for in totals.
Note: Totals may not sum due to independent rounding.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
3.9. International Bunker Fuels (IPCC
Source Category 1: Memo Items)
Emissions resulting from the combustion of fuels used
for international transport activities, termed international
bunker fuels under the UNFCCC, are currently not included
in national emission totals, but are reported separately
based upon location of fuel sales. The decision to report
emissions from international bunker fuels separately, instead
of allocating them to a particular country, was made by the
Intergovernmental Negotiating Committee in establishing
the Framework Convention on Climate Change.35 These
decisions are reflected in the Revised 1996 IPCC Guidelines,
as well as the 2006 IPCC GLs, in which countries are
requested to report emissions from ships or aircraft that
depart from their ports with fuel purchased within national
boundaries and are engaged in international transport
separately from national totals (IPCC/UNEP/OECD/IEA
1997).36
Greenhouse gases emitted from the combustion of
international bunker fuels, like other fossil fuels, include CO2,
CH4 and N2O. Two transport modes are addressed under the
IPCC definition of international bunker fuels: aviation and
marine.37 Emissions from ground transport activities—by
road vehicles and trains—even when crossing international
borders are allocated to the country where the fuel was loaded
into the vehicle and, therefore, are not counted as bunker
fuel emissions.
35 See report of the Intergovernmental Negotiating Committee for a
Framework Convention on Climate Change on the work of its ninth
session, held at Geneva from 7 to 18 February 1994 (A/AC.237/55,
annex I, para. Ic).
36 Note that the definition of international bunker fuels used by the
UNFCCC differs from that used by the International Civil Aviation
Organization.
37 Most emission related international aviation and marine
regulations are under the rubric of the International Civil Aviation
Organization (ICAO) or the International Maritime Organization
(IMO), which develop international codes, recommendations, and
conventions, such as the International Convention of the Prevention
of Pollution from Ships (MARPOL).
3-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
The IPCC Guidelines distinguish between different
modes of air traffic. Civil aviation comprises aircraft used for
the commercial transport of passengers and freight, military
aviation comprises aircraft under the control of national
armed forces, and general aviation applies to recreational and
small corporate aircraft. The IPCC Guidelines further define
international bunker fuel use from civil aviation as the fuel
combusted for civil (e.g., commercial) aviation purposes by
aircraft arriving or departing on international flight segments.
However, as mentioned above, and in keeping with the IPCC
Guidelines, only the fuel purchased in the United States and
used by aircraft taking-off (i.e., departing) from the United
States are reported here. The standard fuel used for civil
aviation is kerosene-type jet fuel, while the typical fuel used
for general aviation is aviation gasoline.38
Emissions of CO2 from aircraft are essentially a function
of fuel use. Methane and N2O emissions also depend upon
engine characteristics, flight conditions, and flight phase
(i.e., take-off, climb, cruise, decent, and landing). Methane
is the product of incomplete combustion and occur mainly
during the landing and take-off phases. In jet engines, N2O is
primarily produced by the oxidation of atmospheric nitrogen,
and the majority of emissions occur during the cruise
phase. International marine bunkers comprise emissions
from fuels burned by ocean-going ships of all flags that are
engaged in international transport. Ocean-going ships are
generally classified as cargo and passenger carrying, military
(i.e., Navy), fishing, and miscellaneous support ships (e.g.,
tugboats). For the purpose of estimating greenhouse gas
emissions, international bunker fuels are solely related to
cargo and passenger carrying vessels, which is the largest of
the four categories, and military vessels. Two main types of
fuels are used on sea-going vessels: distillate diesel fuel and
residual fuel oil. Carbon dioxide is the primary greenhouse
gas emitted from marine shipping.
Overall, aggregate greenhouse gas emissions in 2008
from the combustion of international bunker fuels from
both aviation and marine activities were 136.6 Tg CO2 Eq.,
or twenty-one percent above emissions in 1990 (see Table
3-50 and Table 3-51). Emissions from both international
flights and international shipping voyages departing from the
United States have increased by 66 percent and decreased by
11 percent, respectively, since 1990. The majority of these
emissions were in the form of CO2; however, small amounts
of CH4 and N2O were also emitted.
Methodology
Emissions of CO2 were estimated by applying C content
and fraction oxidized factors to fuel consumption activity
data. This approach is analogous to that described under
CO2 from Fossil Fuel Combustion. C content and fraction
oxidized factors for jet fuel, distillate fuel oil, and residual
fuel oil were taken directly from EIA and are presented in
Annex 2.1, Annex 2.2, and Annex 3.7 of this Inventory.
Density conversions were taken from Chevron (2000), ASTM
(1989), and USAF (1998). Heat content for distillate fuel
oil and residual fuel oil were taken from EIA (2009) and
USAF (1998), and heat content for jet fuel was taken from
EIA (2009). A complete description of the methodology
and a listing of the various factors employed can be found
in Annex 2.1. See Annex 3.7 for a specific discussion
on the methodology used for estimating emissions from
international bunker fuel use by the U.S. military.
Emission estimates for CH4 and N2O were calculated
by multiplying emission factors by measures of fuel
consumption by fuel type and mode. Emission factors used
in the calculations of CH4 and N2O emissions were obtained
from the Revised 1996 IPCC Guidelines (IPCC/UNEP/
OECD/IEA 1997). For aircraft emissions, the following
values, in units of grams of pollutant per kilogram of fuel
consumed (g/kg), were employed: 0.09 for CH4 and 0.1
for N2O For marine vessels consuming either distillate
diesel or residual fuel oil the following values (g/MJ), were
employed: 0.32 for CH4 and 0.08 for N2O. Activity data
for aviation included solely jet fuel consumption statistics,
while the marine mode included both distillate diesel and
residual fuel oil.
Activity data on aircraft fuel consumption for inventory
years 2000 through 2005 were developed using the FAAs
System for assessing Aviation's Global Emissions (SAGE)
model (FAA 2006). That tool has been subsequently replaced
by the Aviation Environmental Design Tool (AEDT), which
calculates noise in addition to aircraft fuel burn and emissions
for flights globally in a given year (FAA 2010). Data for
inventory years 2006 through 2008 were developed using
AEDT.
38 Naphtha-type jet fuel was used in the past by the military in
turbojet and turboprop aircraft engines.
Energy 3-57
-------
Table 3-50: C02, CH4, and N20 Emissions from International Bunker Fuels (Tg C02 Eq.)
Gas/Mode 1990 1995 2000 2005 2006 2007 2008
C02 111.8 99.8 98.5 110.5 129.1 127.1 135.2
Aviation 46.4 51.2 58.8 57.5 75.3 73.2 77.0
Marine 65.4 48.6 39.7 53.0 53.8 53.9 58.2
CH4 0.2l 0.11 0.1 0.1 0.2 0.2 0.2
Aviation +1 +1 +1 + + + +
Marine 0.11 0.11 0.11 0.1 0.1 0.1 0.1
N2o 1.11 o.gl o.gl 1.0 1.2 1.2 1.2
Aviation 0.51 0.61 0.61 0.6 0.8 0.8 0.8
Marine 05 04 03 04 04 04 05
Total 113.0 100.9 99.5 111.7 130.5 128.4 136.6
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions.
Table 3-51: C02, CH4, and N20 Emissions from International Bunker Fuels (Gg)
Gas/Mode 1990 1995 2000 2005 2006 2007 2008
C02 111,828 99,817 98,482 110,505 129,104 127,054 135,226
Aviation 46,399 51,228 58,785 57,492 75,272 73,198 77,029
Marine 65,429 48,589 39,697 53,014 53,832 53,856 58,196
CH4 8I6I6I 7878
Aviation 2! 2! 2! 2 2 2 2
Marine 71 51 4! 5 5 5 6
N20 3I 3I 3I 3444
Aviation 2! 2! 2! 2 2 2 3
Marine 2 1 1 1 1 1 1_
Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions.
International aviation bunker fuel consumption from Logistics Agency (DESC 2009). Together, the data allow the
1990-2008 was calculated by assigning the difference quantity of fuel used in military international operations to
between the sum of domestic activity data (in Tbtu) from be estimated. Densities for each jet fuel type were obtained
SAGE and the AEDT, and the reported EIA transportation from a report from the U.S. Air Force (USAF 1998). Final
jet fuel consumption to the international bunker fuel category jet fuel consumption estimates are presented in Table 3-52.
for jet fuel from EIA (2009). Data on U.S. Department See Annex 3.7 for additional discussion of military data.
of Defense (DoD) aviation bunker fuels and total jet fuel Activity data on distillate diesel and residual fuel oil
consumed by the U.S. military was supplied by the Office consumption by cargo or passenger carrying marine vessels
of the Under Secretary of Defense (Installations and departing from TJ.S. ports were taken from unpublished
Environment), DoD. Estimates of the percentage of each data collected by me Foreign Trade Division of the U.S.
Service's total operations that were international operations Department of Commerce's Bureau of the Census (DOC
were developed by DoD. Military aviation bunkers included 1991 mrough 2009) for 1990 through 2001j 2007j and 2008)
international operations, operations conducted from mA the Department of Homeland Security's Bunker Report
naval vessels at sea, and operations conducted from U.S. for 2003 through 2006 (DHS 2008) Fuel consumption
installations principally over international water in direct data for 2002 was interpoiated due to inconsistencies in
support of military operations at sea. Military aviation reported fud consumption data. Activity data on distillate
bunker fuel emissions were estimated using military fuel diesel consumption by miiitary vesseis departing from U.S.
and operations data synthesized from unpublished data by ports were provided by DESC (2009). The total amount of
the Defense Energy Support Center, under DoD's Defense
3-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-52: Aviation Jet Fuel Consumption for International Transport (Million Gallons)
Nationality
U.S. and Foreign Carriers
U.S. Military
Total
1990
4,934
862
5,796
1995
5,465
581
6,046
2000
6,157
480
6,638
2005
6,022
462
6,484
2006
7,884
400
8,284
2007
7,667
410
8,078
2008
8,068
386
8,455
Note: Totals may not sum due to independent rounding.
Table 3-53: Marine Fuel Consumption for International Transport (Million Gallons)
Fuel Type
Residual Fuel Oil
Distillate Diesel Fuel & Other
U.S. Military Naval Fuels
Total
1990
4,781
617
522
5,920
1995
3,495
5731
3341
4,402
2000
2,967
290
329
3,586
2005
3,881
444
471
4,796
2006
4,004
446
414
4,864
2007
4,059
358
444
4,861
2008
4,373
445
437
5,254
Note: Totals may not sum due to independent rounding.
fuel provided to naval vessels was reduced by 13 percent to
account for fuel used while the vessels were not-underway
(i.e., in port). Data on the percentage of steaming hours
underway versus not-underway were provided by the U.S.
Navy. These fuel consumption estimates are presented in
Table 3-53.
Uncertainty and Time-Series Consistency
Emission estimates related to the consumption
of international bunker fuels are subject to the same
uncertainties as those from domestic aviation and marine
mobile combustion emissions; however, additional
uncertainties result from the difficulty in collecting accurate
fuel consumption activity data for international transport
activities separate from domestic transport activities.39
For example, smaller aircraft on shorter routes often carry
sufficient fuel to complete several flight segments without
refueling in order to minimize time spent at the airport gate or
take advantage of lower fuel prices at particular airports. This
practice, called tankering, when done on international flights,
complicates the use of fuel sales data for estimating bunker
fuel emissions. Tankering is less common with the type of
large, long-range aircraft that make many international flights
from the United States, however. Similar practices occur in
the marine shipping industry where fuel costs represent a
significant portion of overall operating costs and fuel prices
39 See uncertainty discussions under Carbon Dioxide Emissions
from Fossil Fuel Combustion.
vary from port to port, leading to some tankering from ports
with low fuel costs.
Uncertainties exist with regard to the total fuel used by
military aircraft and ships, and in the activity data on military
operations and training that were used to estimate percentages
of total fuel use reported as bunker fuel emissions. Total
aircraft and ship fuel use estimates were developed from
DoD records, which document fuel sold to the Navy and
Air Force from the Defense Logistics Agency. These data
may slightly over or under estimate actual total fuel use in
aircraft and ships because each Service may have procured
fuel from, and/or may have sold to, traded with, and/or given
fuel to other ships, aircraft, governments, or other entities.
There are uncertainties in aircraft operations and training
activity data. Estimates for the quantity of fuel actually used
in Navy and Air Force flying activities reported as bunker
fuel emissions had to be estimated based on a combination
of available data and expert judgment. Estimates of marine
bunker fuel emissions were based on Navy vessel steaming
hour data, which reports fuel used while underway and fuel
used while not underway. This approach does not capture
some voyages that would be classified as domestic for a
commercial vessel. Conversely, emissions from fuel used
while not underway preceding an international voyage are
reported as domestic rather than international as would be
done for a commercial vessel. There is uncertainty associated
with ground fuel estimates for 1997 through 2001. Small
fuel quantities may have been used in vehicles or equipment
other than that which was assumed for each fuel type.
Energy 3-59
-------
There are also uncertainties in fuel end-uses by fuel-
type, emissions factors, fuel densities, diesel fuel sulfur
content, aircraft and vessel engine characteristics and fuel
efficiencies, and the methodology used to back-calculate
the data set to 1990 using the original set from 1995. The
data were adjusted for trends in fuel use based on a closely
correlating, but not matching, data set. All assumptions used
to develop the estimate were based on process knowledge,
Department and military Service data, and expert judgments.
The magnitude of the potential errors related to the various
uncertainties has not been calculated, but is believed to be
small. The uncertainties associated with future military
bunker fuel emission estimates could be reduced through
additional data collection.
Although aggregate fuel consumption data have been
used to estimate emissions from aviation, the recommended
method for estimating emissions of gases other than CO2 in
the Revised 1996 IPCC Guidelines is to use data by specific
aircraft type (IPCC/UNEP/OECD/IEA 1997). The IPCC
also recommends that cruise altitude emissions be estimated
separately using fuel consumption data, while landing and
take-off (LTO) cycle data be used to estimate near-ground
level emissions of gases other than CO2.40
There is also concern as to the reliability of the existing
DOC (1991 through 2009) data on marine vessel fuel
consumption reported at U.S. customs stations due to the
significant degree of inter-annual variation.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
40 U.S. aviation emission estimates for CO, NOX, and NMVOCs
are reported by EPA's National Emission Inventory (NEI) Air
Pollutant Emission Trends web site, and reported under the Mobile
Combustion section. It should be noted that these estimates are based
solely upon LTO cycles and consequently only capture near ground-
level emissions, which are more relevant for air quality evaluations.
These estimates also include both domestic and international flights.
Therefore, estimates reported under the Mobile Combustion section
overestimate IPCC-defined domestic CO, NOX, and NMVOC
emissions by including landing and take-off (LTO) cycles by
aircraft on international flights, but underestimate because they do
not include emissions from aircraft on domestic flight segments at
cruising altitudes. The estimates in Mobile Combustion are also
likely to include emissions from ocean-going vessels departing
from U.S. ports on international voyages.
QA/QC and Verification
A source-specific QA/QC plan for international bunker
fuels was developed and implemented. This effort included
a Tier 1 analysis, as well as portions of a Tier 2 analysis.
The Tier 2 procedures that were implemented involved
checks specifically focusing on the activity data and emission
factor sources and methodology used for estimating CO2,
CH4, and N2O from international bunker fuels in the United
States. Emission totals for the different sectors and fuels
were compared and trends were investigated. No corrective
actions were necessary.
Recalculations Discussion
Slight changes to emission estimates are due to
revisions made to historical activity data for aviation jet
fuel consumption using the FAA's AEDT. These historical
data changes resulted in changes to the emission estimates
for 1990 through 2007 relative to the previous Inventory,
which averaged to an annual increase in emissions from
international bunker fuels of 0.9 Tg CO2 Eq. (0.9 percent) in
CO2 emissions, an annual increase of less than 0.1 Tg CO2
Eq. (0.9 percent) in CH4 emissions, and an annual increase
of less than 0.1 Tg CO2 Eq. (2 percent) in N2O emissions.
3.10. Wood Biomass and Ethanol
Consumption (IPCC Source
Category 1 A)
The combustion of biomass fuels such as wood,
charcoal, and wood waste and biomass-based fuels such as
ethanol from corn and woody crops generates CO2. However,
in the long run the CO2 emitted from biomass consumption
does not increase atmospheric CO2 concentrations, assuming
that the biogenic C emitted is offset by the uptake of CO2 that
results from the growth of new biomass. As a result, CO2
emissions from biomass combustion have been estimated
separately from fossil fuel-based emissions and are not
included in the U.S. totals. Net C fluxes from changes in
biogenic C reservoirs in wooded or crop lands are accounted
for in the Land Use, Land-Use Change, and Forestry chapter.
In 2008, total CO2 emissions from the burning of woody
biomass in the industrial, residential, commercial, and
electricity generation sectors were approximately 198.4 Tg
CO2 Eq. (198,417 Gg) (see Table 3-54 and Table 3-55). As
the largest consumer of woody biomass, the industrial sector
3-60 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 3-54: C02 Emissions from Wood Consumption by End-Use Sector (Tg C02 Eq.)
End-Use Sector
Industrial
Residential
Commercial
Electricity Generation
Total
1990
135.3
59.8
6.8 1
13.3
215.2
1995
155.1
53.6
7.5
12.gl
229.1
2000
153.6
43.3
7.4 1
13.9
218.1
2005
136.3
44.3
7.2
19.1
206.9
2006
142.2
40.2
6.7
18.7
207.9
2007
136.8
44.3
7.2
19.2
207.4
2008
121.8
50.5
7.5
18.6
198.4
Note: Totals may not sum due to independent rounding.
Table 3-55: C02 Emissions from Wood Consumption by End-Use Sector (Gg)
End-Use Sector
Industrial
Residential
Commercial
Electricity Generation
Total
1990
135,348
59,808
6,779
13,252
215,186
1995
155,075
53,621
7,463
12,932
229,091
2000
153,559
43,309
7,370
13,851
218,088
2005
136,269
44,340
7,182
19,074
206,865
2006
142,
40,
6,
18,
207,
226
215
675
748
864
2007
136,767
44,340
7,159
19,175
207,441
2008
121,821
50,527
7,453
18,615
198,417
Note: Totals may not sum due to independent rounding.
was responsible for 61 percent of the CO2 emissions from this
source. Emissions from this sector decreased from 2007 to
2008 due to a corresponding decrease in wood consumption.
The residential sector was the second largest emitter,
constituting 25 percent of the total, while the commercial and
electricity generation sectors accounted for the remainder.
Biomass-derived fuel consumption in the United States
consisted primarily of ethanol use in the transportation
sector. Ethanol is primarily produced from corn grown
in the Midwest, and was used mostly in the Midwest and
South. Pure ethanol can be combusted, or it can be mixed
with gasoline as a supplement or octane-enhancing agent.
The most common mixture is a 90 percent gasoline, 10
percent ethanol blend known as gasohol. Ethanol and ethanol
blends are often used to fuel public transport vehicles such
as buses, or centrally fueled fleet vehicles. These fuels burn
cleaner than gasoline (i.e., lower in NOX and hydrocarbon
emissions), and have been employed in urban areas with poor
air quality. However, because ethanol is a hydrocarbon fuel,
its combustion emits CO2.
In 2008, the United States consumed an estimated
809 trillion Btu of ethanol, and as a result, produced
approximately 53.3 Tg CO2 Eq. (53,346 Gg) (see Table 3-56
and Table 3-57) of CO2 emissions. Ethanol production and
consumption has grown steadily every year since 1990, with
the exception of 1996 due to short corn supplies and high
prices in that year.
Methodology
Woody biomass emissions were estimated by applying
two EIA gross heat contents (Lindstrom 2006) to U.S.
consumption data (EIA 2009) (see Table 3-58), provided in
energy units for the industrial, residential, commercial, and
electric generation sectors. One heat content (16.953114
MMBtu/MT wood and wood waste) was applied to the
industrial sector's consumption, while the other heat content
(15.432359 MMBtu/MT wood and wood waste) was applied
to the consumption data for the other sectors. An EIA
emission factor of 0.434 MT C/MT wood (Lindstrom 2006)
was then applied to the resulting quantities of woody biomass
to obtain CO2 emission estimates. It was assumed that the
woody biomass contains black liquor and other wood wastes,
has a moisture content of 12 percent, and is converted into
CO2 with 100 percent efficiency. The emissions from ethanol
consumption were calculated by applying an EIA emission
factor of 17.99 Tg C/QBtu (Lindstrom 2006) to U.S. ethanol
consumption estimates that were provided in energy units
(EIA 2009) (see Table 3-59).
Uncertainty and Time-Series Consistency
It is assumed that the combustion efficiency for
woody biomass is 100 percent, which is believed to be an
overestimate of the efficiency of wood combustion processes
in the United States. Decreasing the combustion efficiency
would decrease emission estimates. Additionally, the heat
Energy 3-61
-------
Table 3-56: C02 Emissions from Ethanol Consumption (Tg C02 Eq.)
End-Use Sector
1990
1995
2000
2005
2006
2007
2008
Transportation
Industrial
Commercial
4.1
Total
4.2
- :;
77 9.2
22.0
0.5
0.1
29.7
0.7
0.1
37.5
0.7
0.1
52.2
0.9
0.2
22.6
30.5
38.3
53.3
+ Does not exceed 0.05 Tg C02 Eq.
Table 3-57: C02 Emissions from Ethanol Consumption (Gg)
End-Use Sector
Transportation
Industrial
Commercial
Total
1990
4,066
55
33
4,155
1995
7,570
104
9
7,683
2000
9,077
85 1
25
9,188
2005
22,034
460
59
22,554
2006
29,725
651
84
30,459
2007
37,470
662
132
38,264
2008
52,238
923
184
53,346
content applied to the consumption of woody biomass in
the residential, commercial, and electric power sectors is
unlikely to be a completely accurate representation of the
heat content for all the different types of woody biomass
consumed within these sectors. Emission estimates from
ethanol production are more certain than estimates from
woody biomass consumption due to better activity data
collection methods and uniform combustion techniques.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Recalculations Discussion
Wood consumption values were revised for 2005 through
2007 based on updated information from EIA's Annual
Energy Review (EIA 2009). EIA (2009) also reported minor
changes in wood consumption for all sectors in 2007. This
adjustment of historical data for wood biomass consumption
resulted in an average annual decrease in emissions from
wood biomass consumption of 0.3Tg CO2 Eq. (about 0.2
percent) from 1990 through 2007. Slight adjustments were
made to ethanol consumption based on updated information
from EIA (2009), which slightly decreased estimates for
ethanol consumed. As a result of these adjustments, average
annual emissions from ethanol consumption decreased by
less than 0.1 Tg CO2 Eq. (less than 0.1 percent) relative to
the previous Inventory.
Table 3-58: Woody Biomass Consumption by Sector (Trillion Btu)
End-Use Sector
Industrial
Residential
Commercial
Electricity Generation
Total
1990
1,442
580
66 1
129
2,216
1995
1,652
520 1
72 1
125
2,370
2000
1,636
420 1
71 1
134
2,262
2005
1,452
430
70
185
2,136
2006
1,515
390
65
182
2,152
2007
1,457
430
69
186
2,142
2008
1,298
490
72
181
2,041
Table 3-59: Ethanol Consumption by Sector (Trillion Btu)
End-Use Sector
Transportation
Industrial
Commercial
Total
1990
61.7
0.8
0.5
63.0
1995
114.8
i.el
0.1
116.5
2000
137.6
1.3l
0.4
139.3
2005
334.1
7.0
0.9
342.0
2006
450.7
9.9
1.3
461.9
2007
568.2
10.0
2.0
580.2
2008
792.1
14.0
2.8
808.9
3-62 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
4. Industrial Processes
Greenhouse gas emissions are produced as the by-products of various non-energy-related industrial activities.
That is, these emissions are produced from an industrial process itself and are not directly a result of energy
consumed during the process. For example, raw materials can be chemically transformed from one state to another.
This transformation can result in the release of greenhouse gases such as carbon dioxide (CO2), methane (CH4), or nitrous
oxide (N2O). The processes addressed in this chapter include iron and steel production and metallurgical coke production,
cement production, lime production, ammonia production and urea consumption, limestone and dolomite consumption
(e.g., flux stone, flue gas desulfurization, and glass manufacturing), soda ash production and use, aluminum production,
titanium dioxide production, CO2 consumption, ferroalloy production, phosphoric acid production, zinc production, lead
production, petrochemical production, silicon carbide
production and consumption, nitric acid production, and Figure 4-1
adipic acid production (see Figure 4-1).
In addition to the three greenhouse gases listed
above, there are also industrial sources of man-made
fluorinated compounds called hydrofluorocarbons (HFCs),
perfluorocarbons (PFCs), and sulfur hexafluoride (SF6).
The present contribution of these gases to the radiative
forcing effect of all anthropogenic greenhouse gases is
small; however, because of their extremely long lifetimes,
many of them will continue to accumulate in the atmosphere
as long as emissions continue. In addition, many of these
gases have high global warming potentials; SF6 is the most
potent greenhouse gas the Intergovernmental Panel on
Climate Change (IPCC) has evaluated. Usage of HFCs for
the substitution of ozone depleting substances is growing
rapidly, as they are the primary substitutes for ozone
depleting substances (ODSs), which are being phased-out
under the Montreal Protocol on Substances that Deplete the
Ozone Layer. In addition to their use as ODS substitutes,
HFCs, PFCs, SF6, and other fluorinated compounds are
employed and emitted by a number of other industrial
sources in the United States. These industries include
aluminum production, HCFC-22 production, semiconductor
manufacture, electric power transmission and distribution, o 25 50 75 100 125
and magnesium metal production and processing. Tg co2 Eq.
2008 Industrial Processes Chapter
Greenhouse Gas Emission Sources
Substitution of Ozone Depleting Substances
Iron and Steel Production &
Metallurgical Coke Production
Cement Production
Nitric Acid Production |
Lime Production |
HCFC-22 Production |
Electrical Transmission and Distribution |
Ammonia Production and Urea Consumption |
Aluminum Production I
Limestone and Dolomite Use |
Semiconductor Manufacture |
Petrochemical Production |
Soda Ash Production and Consumption |
Adipic Acid Production |
Magnesium Production and Processing |
Titanium Dioxide Production |
Carbon Dioxide Consumption |
Ferroalloy Production I
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
Industrial Processes
as a Portion of
all Emissions
<0.5
<0.5
<0.5
Industrial Processes 4-1
-------
In 2008, industrial processes generated emissions of
334.5 teragrams of CO2 equivalent (Tg CO2 Eq.), or 5 percent
of total U.S. greenhouse gas emissions. CO2 emissions from
all industrial processes were 162.1 Tg CO2 Eq. (162,111
Gg) in 2008, or 3 percent of total U.S. CO2 emissions. CH4
emissions from industrial processes resulted in emissions of
approximately 1.6 Tg CO2 Eq. (75 Gg) in 2008, which was
less than 1 percent of U.S. CH4 emissions. N2O emissions
from adipic acid and nitric acid production were 21.1 Tg
CO2 Eq. (68 Gg) in 2008, or 7 percent of total U.S. N2O
emissions. In 2008 combined emissions of HFCs, PFCs
and SF6 totaled 149.7 Tg CO2 Eq. Overall, emissions from
industrial processes increased by 5 percent from 1990 to
2008 despite decreases in emissions from several industrial
processes, such as iron and steel production and metallurgical
coke production, ammonia production and urea consumption,
and adipic acid production. The increase in overall emissions
was primarily driven by a rise in the emissions from the use
of substitutes for ozone depleting substances.
Table 4-1 summarizes emissions for the Industrial
Processes chapter in units of Tg CO2 Eq., while unweighted
native gas emissions in Gg are provided in Table 4-2. The
source descriptions that follow in the chapter are presented
in the order as reported to the UNFCCC in the common
reporting format tables, corresponding generally to: mineral
products, chemical production, metal production, and
emissions from the uses of HFCs, PFCs, and SF6.
Methodological recalculations were applied to the entire
time-series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification Procedures
Tier 1 quality assurance and quality control procedures
have been performed for all industrial process sources. For
industrial process sources of CO2 and CH4 emissions, a
detailed plan was developed and implemented. This plan
was based on U.S. strategy, but was tailored to include
specific procedures recommended for these sources. Two
types of checks were performed using this plan (1) general,
or Tier 1, procedures that focus on annual procedures and
checks to be used when gathering, maintaining, handling,
documenting, checking and archiving the data, supporting
documents, and files, and (2) source-category specific, or
Tier 2, procedures that focus on procedures and checks
of the emission factors, activity data, and methodologies
used for estimating emissions from the relevant Industrial
Processes sources. Examples of these procedures include,
among others, checks to ensure that activity data and emission
estimates are consistent with historical trends; that, where
possible, consistent and reputable data sources are used
across sources; that interpolation or extrapolation techniques
are consistent across sources; and that common datasets and
factors are used where applicable.
The general method employed to estimate emissions
for industrial processes, as recommended by the IPCC,
involves multiplying production data (or activity data) for
each process by an emission factor per unit of production.
The uncertainty in the emission estimates is therefore
generally a function of a combination of the uncertainties
surrounding the production and emission factor variables.
Uncertainty of activity data and the associated probability
density functions for industrial processes CO2 sources were
estimated based on expert assessment of available qualitative
and quantitative information. Uncertainty estimates and
probability density functions for the emission factors used
to calculate emissions from this source were devised based
on IPCC recommendations.
Activity data is obtained through a survey of
manufacturers conducted by various organizations (specified
within each source); the uncertainty of the activity data is a
function of the reliability of plant-level production data and is
influenced by the completeness of the survey response. The
emission factors used were either derived using calculations
that assume precise and efficient chemical reactions, or
were based upon empirical data in published references.
As a result, uncertainties in the emission coefficients can
be attributed to, among other things, inefficiencies in the
chemical reactions associated with each production process
or to the use of empirically-derived emission factors that
are biased; therefore, they may not represent U.S. national
averages. Additional assumptions are described within each
source.
The uncertainty analysis performed to quantify
uncertainties associated with the 2008 inventory estimates
from industrial processes continues a multi-year process
for developing credible quantitative uncertainty estimates
for these source categories using the IPCC Tier 2 approach.
As the process continues, the type and the characteristics of
4-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-1: Emissions from Industrial Processes (Tg C02 Eq.)
Gas/Source
C02
Iron and Steel Production and Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Cement Production
Lime Production
Ammonia Production & Urea Consumption
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
CH4
Petrochemical Production
Iron and Steel Production and Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and Consumption
N20
Nitric Acid Production
Adipic Acid Production
MFCs
Substitution of Ozone Depleting Substances3
HCFC-22 Manufacture
Semiconductor Manufacturing MFCs
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacturing SF6
PFCs
Aluminum Production
Semiconductor Manufacturing PFCs
Total
1990
191.5
102.6
97.?
5.sl
33.3
11.5
16.8 1
1
6.8
4.1
3.3l
1
1
2.2 •
1
0.9 •
0.3
0.4l
i.g|
o.gl
1.0!
7.01
+ 1
+ 1
+ 1
34.7
18.9
15.81
36.9
0.3 1
36.4
0.2l
32.6
26.6
5.4l
0.5 1
20.8
18.5
2.2
318.3
1995 2000 2005
192.6 187.7
95.7 88.1
90.7 83.7
5.01 4.4
36.8 41.2
13.3 14.1
17.8
6.7
5.7
4.3
4.1
1.5
1.4
2.0
1.5
1.0
0.3
0.3
2.1
1.1
1.0
7.0
+
+
+
38.6
21.0
17.6
62.2
16.4
5.1
6.1
4.2
4.5
1.8
1.4
1.9
1.4
1.1
0.3
0.2
2.2
1.2
0.9
0.9
+
+
+
167.0
67.7
63.9
3.8
45.9
14.4
12.8
6.8
4.1
4.2
4.2
1.8
1.3
1.4
1.4
0.5
0.3
0.2
1.8
1.1
0.7
0.7
+
+
+
26.3 22.6
20.7 17.6
5.5 5.0
103.2 119.3
29.0 74.3 103.2
33.0 28.6 15.8
0.3 1 0.3 0.2
27.9 19.1 17.8
21.4 15.0 14.0
5.6 1 3.0 1 2.9
0.9 1.1 1 1.0
15.6 13.5 6.2
11.8 8.6 1 3.0
3.8 4.9 3.2
339.1 351.9 334.7
2006
171.5
70.5
66.9
3.7
46.6
15.1
12.3
8.0
3.8
4.2
3.8
1.8
1.7
1.5
1.2
0.5
0.3
0.2
1.7
1.0
0.7
0.7
+
+
+
21.5
17.2
4.3
121.8
107.7
13.8
0.3
17.0
13.2
2.9
1.0
6.0
2.5
3.5
339.7
2007
174.0
72.8
69.0
3.8
45.2
14.6
14.0
7.7
4.3
4.1
3.9
1.9
1.9
1.6
1.2
0.4
0.3
0.2
1.7
1.0
0.7
0.7
+
+
+
24.2
20.5
3.7
127.4
110.1
17.0
0.3
16.1
12.7
2.6
0.8
7.5
3.8
3.6
350.9
2008
162.1
69.0
63.7
5.3
41.1
14.3
11.8
6.6
4.5
4.1
3.4
1.8
1.8
1.6
1.2
0.4
0.3
0.2
1.6
0.9
0.6
0.6
+
+
+
21.1
19.0
2.0
126.9
113.0
13.6
0.3
16.1
13.1
2.0
1.1
6.7
2.7
4.0
334.5
+ Does not exceed 0.05 Tg C02 Eq.
a Small amounts of RFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
the actual probability density functions underlying the input
variables are identified and better characterized (resulting in
development of more reliable inputs for the model, including
accurate characterization of correlation between variables),
based primarily on expert judgment. Accordingly, the
quantitative uncertainty estimates reported in this section
should be considered illustrative and as iterations of
ongoing efforts to produce accurate uncertainty estimates.
Industrial Processes 4-3
-------
Table 4-2: Emissions from Industrial Processes (Gg)
Gas/Source
C02
Iron and Steel Production and Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Cement Production
Lime Production
Ammonia Production & Urea Consumption
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
CH4
Petrochemical Production
Iron and Steel Production and Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and Consumption
N20
Nitric Acid Production
Adipic Acid Production
MFCs
Substitution of Ozone Depleting Substances3
HCFC-22 Production
Semiconductor Manufacturing MFCs
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacturing SF6
PFCs
Aluminum Production
Semiconductor Manufacturing PFCs
1990
191,498
102,564
97,066
5498
33,278
11,533
16,831
5,127
6,831
4,141
3,311
1,195
1,416
2,152
1,529
929 1
285
375
88
41 1
46 1
46
1
1
1
112
1995 2000 2005
192,580 187,715 167,034
95,748 88,110 1 67,731
90,77?! 83,728 63,882
5037 4,387 3,849
36,847 41,190 45,910
13,325 14,088 14,379
17,796 16,402 12,849
6,683 5,056 6,768
5,659 6,086 4,142
4,304 4,181 4,228
4,101 4,479l
1,526 1,752
1,422 1,421
2,036 1,893
1,513 1,382
993 1,115
298
329
100
52
47
47
+
1
1
125
68
57
M
M
3
+
1
1
+
+
M
M
M
311
248
104
59
44
44
+
1
1
85
67
18
M
M
2
+
1
1
+
+
M
M
M
4,181
1 1,755
1,321
1,392
1,386
506
266
219
86
51
34
34
+
+
+
73
57
16
M
M
1
+
1
1
+
+
M
M
M
2006
171,543
70,539
66,857
3,682
46,562
15,100
12,300
8,035
3,801
4,162
3,837
1,836
1,709
1,505
1,167
513
270
207
83
48
35
35
+
+
+
70
56
14
M
M
1
+
1
1
+
+
M
M
M
2007
174,005
72,802
68,996
3,806
45,229
14,595
13,968
7,702
4,251
4,140
3,931
1,930
1,867
1,552
1,166
411
267
196
82
48
33
33
+
+
+
78
66
12
M
M
1
+
1
1
+
+
M
M
M
2008
162,111
69,010
63,729
5287
41,147
14,330
11,755
6,617
4,477
4,111
3,449
1,809
1,780
1,599
1,187
402
264
175
75
43
31
37
+
+
+
68
61
7
M
M
1
+
1
1
+
+
M
M
M
+ Does not exceed 0.5 Gg.
M (Mixture of gases).
a Small amounts of RFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
The correlation among data used for estimating emissions
for different sources can influence the uncertainty analysis
of each individual source. While the uncertainty analysis
recognizes very significant connections among sources, a
more comprehensive approach that accounts for all linkages
will be identified as the uncertainty analysis moves forward.
4-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
4.1. Cement Production (IPCC
Source Category 2A1)
Cement production is an energy- and raw-material-
intensive process that results in the generation of CO2 from
both the energy consumed in making the cement and the
chemical process itself.1 Cement is produced in 37 states
and Puerto Rico. Carbon dioxide emitted from the chemical
process of cement production is the second largest source of
industrial CO2 emissions in the United States.
During the cement production process, calcium carbonate
(CaCO3) is heated in a cement kiln at a temperature of about
1,450°C (2,400°F) to form lime (i.e., calcium oxide or CaO)
and CO2 in a process known as calcination or calcining. A
very small amount of carbonates other than CaCO3 and non-
carbonates are also present in the raw material; however, for
calculation purposes all of the raw material is assumed to be
CaCO3. Next, the lime is combined with silica-containing
materials to produce clinker (an intermediate product), with
the earlier by-product CO2 being released to the atmosphere.
The clinker is then allowed to cool, mixed with a small
amount of gypsum, and potentially other materials (e.g.,
slag) and used to make portland cement.2
In 2008, U.S. clinker production—including Puerto
Rico—totaled 79,572 thousand metric tons (USGS 2009b).
The resulting emissions of CO2 from 2008 cement production
were estimated to be 41.1 Tg CO2 Eq. (41,147 Gg) (see
Table 4-3).
After falling in 1991 by two percent from 1990 levels,
cement production emissions grew every year through 2006,
and then decreased from 2006 to 2008. Overall, from 1990 to
2008, emissions increased by 24 percent. Cement continues
to be a critical component of the construction industry;
therefore, the availability of public construction funding,
as well as overall economic growth, have had considerable
influence on cement production.
1 The CO2 emissions related to the consumption of energy for cement
manufacture are accounted for under CO2 from Fossil Fuel Combustion
in the Energy chapter.
2 Approximately six percent of total clinker production is used to produce
masonry cement, which is produced using plasticizers (e.g., ground
limestone, lime) and portland cement. CO2 emissions that result from the
production of lime used to create masonry cement are included in the Lime
Manufacture source category (van Oss 2008c).
Table 4-3: C02 Emissions from Cement Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
33.3
33,278
2005
2006
2007
2008
45.9
46.6
45.2
41.1
45,910
46,562
45,229
41,147
Methodology
Carbon dioxide emissions from cement production
are created by the chemical reaction of carbon-containing
minerals (i.e., calcining limestone) in the cement kiln. While
in the kiln, limestone is broken down into CO2 and lime with
the CO2 released to the atmosphere. The quantity of CO2
emitted during cement production is directly proportional to
the lime content of the clinker. During calcination, each mole
of CaCO3 (i.e., limestone) heated in the clinker kiln forms
one mole of lime (CaO) and one mole of CO2:
CaCO3 + heat -> CaO + CO2
Carbon dioxide emissions were estimated by applying
an emission factor, in tons of CO2 released per ton of clinker
produced, to the total amount of clinker produced. The
emission factor used in this analysis is the product of the
average lime fraction for clinker of 65 percent (van Oss
2008c) and a constant reflecting the mass of CO2 released
per unit of lime. This calculation yields an emission factor
of 0.51 tons of CO2 per ton of clinker produced, which was
determined as follows:
EFclinker = 0.65 CaO x
44.01 g/mole CO2
56.08 g/mole CaO
= 0.51 tons CO2/ton clinker
During clinker production, some of the clinker precursor
materials remain in the kiln as non-calcinated, partially
calcinated, or fully calcinated cement kiln dust (CKD). The
emissions attributable to the calcinated portion of the CKD
are not accounted for by the clinker emission factor. The
IPCC recommends that these additional CKD CO2 emissions
should be estimated as two percent of the CO2 emissions
Industrial Processes 4-5
-------
Table 4-4: Clinker Production (Gg)
Year
Clinker
2005
2006
2007
2008
88,783
90,045
87,466
79,572
calculated from clinker production.3 Total cement production
emissions were calculated by adding the emissions from
clinker production to the emissions assigned to CKD (IPCC
2006).4
The 1990 through 2007 activity data for clinker
production (see Table 4-4) were obtained through the
USGS Mineral Yearbook: Cement (US Bureau of Mines
1990 through 1993, USGS 1995 through 2009a). The 2008
activity data were obtained through the USGS Mineral
Industry Survey (2009b). The data were compiled by USGS
through questionnaires sent to domestic clinker and cement
manufacturing plants.
that all calcium-containing raw materials are CaCO3 when a
small percentage likely consists of other carbonate and non-
carbonate raw materials. The lime content of clinker varies
from 60 to 67 percent (van Oss 2008b). CKD loss can range
from 1.5 to 8 percent depending upon plant specifications.
Additionally, some amount of CO2 is reabsorbed when the
cement is used for construction. As cement reacts with water,
alkaline substances such as calcium hydroxide are formed.
During this curing process, these compounds may react with
CO2 in the atmosphere to create calcium carbonate. This
reaction only occurs in roughly the outer 0.2 inches of surface
area. Because the amount of CO2 reabsorbed is thought to
be minimal, it was not estimated.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-5. Cement Production CO2
emissions were estimated to be between 35.8 and 46.7 Tg
CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 13 percent below and 13 percent
above the emission estimate of 41.2 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Uncertainty and Time-Series Consistency P|anned improvements
The uncertainties contained in these estimates are
primarily due to uncertainties in the lime content of clinker
and in the percentage of CKD recycled inside the cement
kiln. Uncertainty is also associated with the assumption
Future improvements to the cement source category
involve continued research into emission factors for clinker
production and CKD. Research has been conducted into the
accuracy and appropriateness of default emission factors
Table 4-5: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Cement Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Cement Production
CO,
41.2
35.8
46.7
-13%
+ 13%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
3 Default IPCC clinker and CKD emission factors were verified through
expert consultation with the Portland Cement Association (PCA 2008) and
van Oss (2008a).
4 The 2 percent CO2 addition associated with CKD is included in the
emission estimate for completeness. The cement emission estimate also
includes an assumption that all raw material is limestone (CaCO3) when
in fact a small percentage is likely composed of non-carbonate materials.
Together these assumptions may result in a small emission overestimate
(van Oss 2008c).
4-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
and reporting methodology used by other organizations. As
these methodologies continue to develop, the cement source
category will be updated with any improvements to IPCC
assumptions for clinker and CKD emissions.
4.2. Lime Production (IPCC Source
Category 2A2)
Lime is an important manufactured product with many
industrial, chemical, and environmental applications. Its
major uses are in steel making, flue gas desulfurization (FGD)
systems at coal-fired electric power plants, construction,
and water purification. For U.S. operations, the term
"lime" actually refers to a variety of chemical compounds.
These include calcium oxide (CaO), or high-calcium
quicklime; calcium hydroxide (Ca(OH)2), or hydrated lime;
dolomitic quicklime ([CaOMgO]); and dolomitic hydrate
([Ca(OH)2'MgO] or [Ca(OH)2'Mg(OH)2]).
Lime production involves three main processes: stone
preparation, calcination, and hydration. CO2 is generated
during the calcination stage, when limestone—mostly
calcium carbonate (CaCO3)—is roasted at high temperatures
in a kiln to produce CaO and CO2. The CO2 is given off as
a gas and is normally emitted to the atmosphere. Some of
the CO2 generated during the production process, however,
is recovered at some facilities for use in sugar refining and
precipitated calcium carbonate (PCC) production.5 In certain
additional applications, lime reabsorbs CO2 during use.
Lime production in the United States—including Puerto
Rico—was reported to be 19,838 thousand metric tons
in 2008 (USGS 2009b). This resulted in estimated CO2
emissions of 14.3 Tg CO2 Eq. (or 14,330 Gg) (see Table 4-6
and Table 4-7).
The contemporary lime market is approximately
distributed across five end-use categories as follows:
metallurgical uses, 36 percent; environmental uses, 30
percent; chemical and industrial uses, 22 percent; construction
uses, 10 percent; and refractory dolomite, 1 percent. In the
construction sector, lime is used to improve durability in
5 PCC is obtained from the reaction of CO2 with calcium hydroxide. It
is used as a filler and/or coating in the paper, food, and plastic industries.
Table 4-6: C02 Emissions from Lime Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
11.5
11,533
2005
2006
2007
2008
14.4
15.1
14.6
14.3
14,379
15,100
14,595
14,330
Table 4-7: Potential, Recovered, and Net C02 Emissions
from Lime Production (Gg)
Year
Potential
Recovered3 Net Emissions
1990
12,004
471
11,533
2005
2006
2007
2008
15,131
15,825
15,264
14,977
752
725
669
647
14,379
15,100
14,595
14,330
a For sugar refining and PCC production.
Note: Totals may not sum due to independent rounding.
plaster, stucco, and mortars, as well as to stabilize soils. In
2008, the amount of lime used for construction decreased by
14 percent from 2007 levels (USGS 2009b).
Lime production in 2008 decreased by 2 percent
compared to 2007, owing mostly to a downturn in major
markets including construction and steel (USGS 2009b).
Overall, from 1990 to 2008, lime production has increased
by 24 percent (USGS 1992 through 2007 & 2009a, USGS
2009b). Annual consumption for industrial and chemical
and metallurgical consumption decreased slightly or were
essentially unchanged (USGS 2009b). The environmental
sector exhibited a slight increase in lime use consumption
compared with 2007 levels due to the nearly 6 percent
increase in consumption for flue gas desulfurization (USGS
2009b).
Industrial Processes 4-7
-------
Methodology
During the calcination stage of lime production, CO2 is
given off as a gas and normally exits the system with the stack
gas. To calculate emissions, the amounts of high-calcium and
dolomitic lime produced were multiplied by their respective
emission factors. The emission factor is the product of a
constant reflecting the mass of CO2 released per unit of lime
and the average calcium plus magnesium oxide (CaO + MgO)
content for lime (95 percent for both types of lime) (IPCC
2006). The emission factors were calculated as follows:
For high-calcium lime:
[(44.01 g/mole CO2) + (56.08 g/mole CaO)] x
(0.95 CaO/lime) = 0.75 g CO2/g lime
For dolomitic lime:
[(88.02 g/mole CO2) + (96.39 g/mole CaO)] x
(0.95 CaO/lime) = 0.87 g CO2/g lime
Production was adjusted to remove the mass of
chemically combined water found in hydrated lime,
determined according to the molecular weight ratios of H2O
to (Ca(OH)2 and [Ca(OH)2'Mg(OH)2]) (IPCC 2000). These
factors set the chemically combined water content to 24.3
percent for high-calcium hydrated lime, and 27.2 percent for
dolomitic hydrated lime.
Lime emission estimates were multiplied by a factor of
1.02 to account for lime kiln dust (LKD), which is produced
as a by-product during the production of lime (IPCC 2006).
Lime emission estimates were further adjusted to
account for PCC producers and sugar refineries that recover
CO2 emitted by lime production facilities for use as an input
into production or refining processes. For CO2 recovery by
sugar refineries, lime consumption estimates from USGS
were multiplied by a CO2 recovery factor to determine
the total amount of CO2 recovered from lime production
facilities. According to industry surveys, sugar refineries
use captured CO2 for 100 percent of their CO2 input (Lutter
2009). CO2 recovery by PCC producers was determined by
multiplying estimates for the percentage CO2 of production
weight for PCC production at lime plants by a CO2 recovery
factor based on the amount of purchased CO2 by PCC
manufacturers (Prillaman 2008 and 2009). As data were
only available starting in 2007, CO2 recovery for the period
1990 through 2006 was extrapolated by determining a ratio
of PCC production at lime facilities to lime consumption for
PCC (USGS 2002 through 2007 & 2009a, USGS 2009b).
Lime production data (high-calcium- and dolomitic-
quicklime, high-calcium- and dolomitic-hydrated, and dead-
burned dolomite) for 1990 through 2008 (see Table 4-8) were
obtained from USGS (1992 through 2007 & 2009a, 2009b).
Natural hydraulic lime, which is produced from CaO and
hydraulic calcium silicates, is not produced in the United
States (USGS 2009b). Total lime production was adjusted to
account for the water content of hydrated lime by converting
hydrate to oxide equivalent, based on recommendations
from the IPCC Good Practice Guidance and is presented
in Table 4-9 (USGS 1992 through 2007 & 2009a, USGS
2009b, IPCC 2000). The CaO and CaO'MgO contents of
lime were obtained from the IPCC (IPCC 2006). Since data
for the individual lime types (high calcium and dolomitic)
was not provided prior to 1997, total lime production for
1990 through 1996 was calculated according to the three
year distribution from 1997 to 1999. Lime consumed by PCC
producers and sugar refineries were obtained from USGS
(1992 through 2007 & 2009a, 2009b).
Table 4-8: High-Calcium- and Dolomitic-Quicklime, High-Calcium- and Dolomitic-Hydrated,
and Dead-Burned-Dolomite Lime Production (Gg)
Year
High-Calcium
Quicklime
Dolomitic
Quicklime
High-Calcium
Hydrated
Dolomitic
Hydrated
Dead-Burned
Dolomite
1990
11,166
2,234
1,781
319
2005
2006
2007
2008
14,100
15,000
14,700
14,900
2,990
2,950
2,700
2,310
2,220
2,370
2,240
2,070
474
409
352
358
200
200
200
200
4-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-9: Adjusted Lime Production3 (Gg)
Year
High-Calcium
Dolomitic
1990
12,514
2,809
2005
2006
2007
2008
15,781
16,794
16,396
16,467
3,535
3,448
3,156
2,771
a Minus water content of hydrated lime.
Uncertainty and Time-Series Consistency
The uncertainties contained in these estimates can be
attributed to slight differences in the chemical composition
of these products and recovery rates for sugar refineries and
PCC manufacturers located at lime plants. Although the
methodology accounts for various formulations of lime, it
does not account for the trace impurities found in lime, such
as iron oxide, alumina, and silica. Due to differences in the
limestone used as a raw material, a rigid specification of lime
material is impossible. As a result, few plants produce lime
with exactly the same properties.
In addition, a portion of the CO2 emitted during lime
production will actually be reabsorbed when the lime is
consumed. As noted above, lime has many different chemical,
industrial, environmental, and construction applications. In
many processes, CO2 reacts with the lime to create calcium
carbonate (e.g., water softening). CO2 reabsorption rates
vary, however, depending on the application. For example,
100 percent of the lime used to produce precipitated calcium
carbonate reacts with CO2; whereas most of the lime used in
steel making reacts with impurities such as silica, sulfur, and
aluminum compounds. A detailed accounting of lime use
in the United States and further research into the associated
processes are required to quantify the amount of CO2 that
is reabsorbed.6
6 Representatives of the National Lime Association estimate that CO2
reabsorption that occurs from the use of lime may offset as much
as a quarter of the CO2 emissions from calcination (Males 2003).
In some cases, lime is generated from calcium carbonate
by-products at pulp mills and water treatment plants.7 The
lime generated by these processes is not included in the
USGS data for commercial lime consumption. In the pulping
industry, mostly using the Kraft (sulfate) pulping process,
lime is consumed in order to causticize a process liquor
(green liquor) composed of sodium carbonate and sodium
sulfide. The green liquor results from the dilution of the smelt
created by combustion of the black liquor where biogenic C
is present from the wood. Kraft mills recover the calcium
carbonate "mud" after the causticizing operation and calcine
it back into lime—thereby generating CO2—for reuse in the
pulping process. Although this re-generation of lime could
be considered a lime manufacturing process, the CO2 emitted
during this process is mostly biogenic in origin, and therefore
is not included in Inventory totals (Miner and Upton 2002).
In the case of water treatment plants, lime is used in the
softening process. Some large water treatment plants may
recover their waste calcium carbonate and calcine it into
quicklime for reuse in the softening process. Further research
is necessary to determine the degree to which lime recycling
is practiced by water treatment plants in the United States.
Uncertainties also remain surrounding recovery rates
used for sugar refining and PCC production. The recovery
rate for sugar refineries is based on two sugar beet processing
and refining facilities located in California that use 100
percent recovered CO2 from lime plants (Lutter 2009). This
analysis assumes that all sugar refineries located on-site at
lime plants also use 100 percent recovered CO2. The recovery
rate for PCC producers located on-site at lime plants is based
on the 2008 value for PCC manufactured at commercial lime
plants, given by the National Lime Association (Prillaman
2009).
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-10. Lime CO2 emissions were
estimated to be between 13.2 and 15.6 Tg CO2 Eq. at the
95 percent confidence level. This indicates a range of
approximately 8 percent below and 9 percent above the
emission estimate of 14.3 Tg CO2 Eq.
7 Some carbide producers may also regenerate lime from their calcium
hydroxide by-products, which does not result in emissions of CO2. In
making calcium carbide, quicklime is mixed with coke and heated in electric
furnaces. The regeneration of lime in this process is done using a waste
calcium hydroxide (hydrated lime) [CaC2 + 2H2O -> C2H2 + Ca(OH)2],
not calcium carbonate [CaCO3]. Thus, the calcium hydroxide is heated
in the kiln to simply expel the water [Ca(OH)2 + heat —> CaO + H2O] and
no CO, is released.
Industrial Processes 4-9
-------
Table 4-10: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Lime Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Lime Production
CO,
14.3
13.2
15.6
-8%
+ 9%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Planned Improvements
Future improvements to the lime source category involve
continued research into CO2 recovery associated with lime
use during sugar refining and precipitate calcium carbonate
(PCC) production. Currently, two sugar refining facilities
in California have been identified to capture CO2 produced
in lime kilns located on the same site as the sugar refinery
(Lutter, 2009). Currently, data on CO2 production by these
lime facilities is unavailable. Future work will include
research to determine the number of sugar refineries that
employ the carbonation technique, the percentage of these
that use captured CO2 from lime production facilities, and the
amount of CO2 recovered per unit of lime production. Future
research will also aim to improve estimates of CO2 recovered
as part of the PCC production process using estimates of PCC
production and CO2 inputs rather than lime consumption by
PCC facilities.
4.3. Limestone and Dolomite Use
(IPCC Source Category 2A3)
Limestone (CaCO3) and dolomite (CaCO3MgCO3)8
are basic raw materials used by a wide variety of industries,
including construction, agriculture, chemical, metallurgy,
glass production, and environmental pollution control.
Limestone is widely distributed throughout the world in
deposits of varying sizes and degrees of purity. Large
deposits of limestone occur in nearly every state in the United
States, and significant quantities are extracted for industrial
applications. For some of these applications, limestone is
sufficiently heated during the process and generates CO2
as a byproduct. Examples of such applications include
limestone used as a flux or purifier in metallurgical furnaces,
as a sorbent in flue gas desulfurization systems for utility and
industrial plants, or as a raw material in glass manufacturing
and magnesium production.
In 2008, approximately 14,795 thousand metric tons of
limestone and 1,283 thousand metric tons of dolomite were
consumed during production for these emissive applications.
Overall, usage of limestone and dolomite resulted in
aggregate CO2 emissions of 6.6 Tg CO2 Eq. (6,617 Gg)
(see Table 4-11 and Table 4-12). Overall, emissions have
increased 31 percent overall from 1990 through 2008.
Methodology
Carbon dioxide emissions were calculated by multiplying
the quantity of limestone or dolomite consumed by the
average C content, 12.0 percent for limestone and 13.0
Table 4-11: C02 Emissions from Limestone & Dolomite Use (Tg C02 Eq.)
Activity
Flux Stone
Glass Making
FGD
Magnesium Production
Other Miscellaneous Uses
Total
1990
2.6
0.2l
1
0.8
5.1
1995
3.2
0.5 1
1
1.2
6.7
2000
2.1 1
0.4l
1
0.7
5.1
2005
2.7
0.4
3.0
0.0
0.7
6.8
2006
4.5
0.7
2.1
0.0
0.7
8.0
2007
2.0
0.3
3.2
0.0
2.2
7.7
2008
1.0
0.4
3.8
0.0
1.5
6.6
Notes: Totals may not sum due to independent rounding. "Other miscellaneous uses" include chemical stone, mine dusting or acid water treatment,
acid neutralization, and sugar refining.
8 Limestone and dolomite are collectively referred to as limestone by the
industry, and intermediate varieties are seldom distinguished.
4-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-12: C02 Emissions from Limestone & Dolomite Use (Gg)
Activity
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
Flue Gas Desulfurization
Magnesium Production
Other Miscellaneous Uses
Total
1990
2,593
2,304
289
2171
isgl
28 1
1,433
64l
819
5,127
1995
3,198
2,027
1,171
525 1
421
1031
1,719
73
1,168
6,683
2000
2,104
1,374
730 1
371
371 1
1
1,787
73
722
5,056
2005
2,650
1,096
1,554
425
405
20
2,975
0
718
6,768
2006
4,492
1,917
2,575
747
717
31
2,061
0
735
8,035
2007
1,959
1,270
689
333
333
0
3,179
0
2,231
7,702
2008
974
568
407
387
387
0
3,801
0
1,455
6,617
Notes: Totals may not sum due to independent rounding. "Other miscellaneous uses" include chemical stone, mine dusting or acid water treatment,
acid neutralization, and sugar refining.
percent for dolomite (based on stoichiometry), and converting However, the plant utilizing the dolomitic process ceased
this value to CO2. This methodology was used for flux its operations prior to the end of 2001, so beginning in 2002
stone, glass manufacturing, flue gas desulfurization systems, there were no emissions from this particular sub-use.
chemical stone, mine dusting or acid water treatment, acid Consumption data for 1990 through 2008 of limestone
neutralization, and sugar refining and then converting to CO2 md doiomite used for flux stone, giass manufacturing, flue
using a molecular weight ratio. Flux stone used during the gas desuifurization systems, chemical stone, mine dusting or
production of iron and steel was deducted from the Limestone add water treatment5 acid neutralization, and sugar refining
and Dolomite Use estimate and attributed to the Iron and (see Table 4_13) were obtained from the USGS Minerals
Steel Production estimate. Yearbook: Crushed Stone Annual Report (1995 through
Traditionally, the production of magnesium metal was 2007a, 2009b, 2010) and the U.S. Burea of Mines (1991 &
the only other significant use of limestone and dolomite that 1993a). The production capacity data for 1990 through 2008
produced CO2 emissions. At the start of 2001, there were of dolomitic magnesium metal (see Table 4-14) also came
two magnesium production plants operating in the United from the USGS (1995 through 2007b, 2008, 2009a) and
States and they used different production methods. One the U.S. Bureau of Mines (1990 through 1993b). The last
plant produced magnesium metal using a dolomitic process plant in the United States that used the dolomitic production
that resulted in the release of CO2 emissions, while the other process for magnesium metal closed in 2001. The USGS
plant produced magnesium from magnesium chloride using does not mention this process in the 2008 Minerals Yearbook:
a CO2-emissions-free process called electrolytic reduction. Magnesium; therefore, it is assumed that this process
Table 4-13: Limestone and Dolomite Consumption (Thousand Metric Tons)
Activity
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
Flue Gas Desulfurization
Other Miscellaneous Uses
Total
1990
6,737
5,804
9331
489 1
430 1
59l
3,258 1
1,835
12,31 9 |
1995
8,586
5,734
2,852
1,174
958 1
216
3,908
2,654
16,321
2000
6,283
4,151
2,132
843 1
843
0
4,061
1,640
12,826
2005
7,022
3,165
3,857
962
920
43
6,761
1,632
16,377
2006
11,030
5,208
5,822
1,693
1,629
64
4,683
1,671
19,078
2007
5,305
3,477
1,827
757
757
0
7,225
5,057
18,344
2008
3,253
1,970
1,283
879
879
0
8,639
3,307
16,077
Notes: "Other miscellaneous uses" includes chemical stone, mine dusting or acid water treatment, acid neutralization, and sugar refining. Zero values for
limestone and dolomite consumption for glass making result during years when the USGS reports that no limestone or dolomite are consumed for this use.
Industrial Processes 4-11
-------
Table 4-14: Dolomitic Magnesium Metal Production
Capacity (Metric Tons)
Year
Production Capacity
1990
35,000
2005
2006
2007
2008
40,000
^m
40,000
•
0
0
0
0
Note: Production capacity for 2002, 2003, 2004, 2005, 2006, and 2007
amounts to zero because the last U.S. production plant employing the
dolomitic process shut down mid-2001 (USGS 2002b through 2008b).
continues to be non-existent in the United States (USGS
2009a). During 1990 and 1992, the USGS did not conduct
a detailed survey of limestone and dolomite consumption by
end-use. Consumption for 1990 was estimated by applying
the 1991 percentages of total limestone and dolomite use
constituted by the individual limestone and dolomite uses
to 1990 total use. Similarly, the 1992 consumption figures
were approximated by applying an average of the 1991
and 1993 percentages of total limestone and dolomite use
constituted by the individual limestone and dolomite uses
to the 1992 total.
Additionally, each year the USGS withholds data
on certain limestone and dolomite end-uses due to
confidentiality agreements regarding company proprietary
data. For the purposes of this analysis, emissive end-uses
that contained withheld data were estimated using one of
the following techniques: (1) the value for all the withheld
data points for limestone or dolomite use was distributed
evenly to all withheld end-uses; (2) the average percent of
total limestone or dolomite for the withheld end-use in the
preceding and succeeding years; or (3) the average fraction
of total limestone or dolomite for the end-use over the entire
time period.
There is a large quantity of crushed stone reported to the
USGS under the category "unspecified uses." A portion of
this consumption is believed to be limestone or dolomite used
for emissive end uses. The quantity listed for "unspecified
uses" was, therefore, allocated to each reported end-use
according to each end uses fraction of total consumption in
that year.9
Uncertainty and Time Series Consistency
The uncertainty levels presented in this section arise
in part due to variations in the chemical composition of
limestone. In addition to calcium carbonate, limestone may
contain smaller amounts of magnesia, silica, and sulfur,
among other minerals. The exact specifications for limestone
or dolomite used as flux stone vary with the pyrometallurgical
process and the kind of ore processed. Similarly, the quality
of the limestone used for glass manufacturing will depend
on the type of glass being manufactured.
The estimates below also account for uncertainty
associated with activity data. Large fluctuations in reported
consumption exist, reflecting year-to-year changes in the
number of survey responders. The uncertainty resulting from
a shifting survey population is exacerbated by the gaps in
the time series of reports. The accuracy of distribution by
end use is also uncertain because this value is reported by
the manufacturer and not the end user. Additionally, there
is significant inherent uncertainty associated with estimating
withheld data points for specific end uses of limestone and
dolomite. The uncertainty of the estimates for limestone
used in glass making is especially high; however, since glass
making accounts for a small percent of consumption, its
contribution to the overall emissions estimate is low. Lastly,
much of the limestone consumed in the United States is
reported as "other unspecified uses;" therefore, it is difficult
to accurately allocate this unspecified quantity to the correct
end-uses.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-15. Limestone and Dolomite Use
CO2 emissions were estimated to be between 6.4 and 7.9 Tg
CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 3 percent below and 20 percent
above the emission estimate of 6.6 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
This approach was recommended by USGS.
4-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-15: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Limestone and Dolomite Use
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Limestone and Dolomite
Use CO?
6.6
6.4
7.9
-3%
+20%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations
Beginning in 2007, USGS began reporting a single
value for "unspecified uses." A portion of this consumption
is believed to be limestone or dolomite used for emissive
end uses. The quantity listed for "unspecified uses" was,
therefore, allocated to each reported end-use in 2007
according to each end-use's fraction of total consumption in
that year. This recalculation resulted in an increase in 2007
emissions by 25 percent relative to the previous Inventory.
Additionally, the dolomitic magnesium metal production
capacity value for the years 1992 through 1995 was updated
to reflect data from USGS (1995 through 1996) and the U.S.
Bureau of Mines (1992 through 1993b).
Planned Improvements
Future improvements to the limestone and dolomite
source category involve research into the availability of
limestone and dolomite end-use data. If sufficient data are
available, limestone and dolomite used as process materials
in source categories included in future inventories (e.g., glass
production, other process use of carbonates) may be removed
from this section and will be reported under the appropriate
source categories. Additionally, future improvements include
revisiting the methodology to distribute withheld data across
emissive end-uses for all years to improve consistency of
calculations.
4.4. Soda Ash Production and
Consumption (IPCC Source Category
2A4)
Soda ash (sodium carbonate, Na2CO3) is a white
crystalline solid that is readily soluble in water and strongly
alkaline. Commercial soda ash is used as a raw material in a
variety of industrial processes and in many familiar consumer
products such as glass, soap and detergents, paper, textiles,
and food. It is used primarily as an alkali, either in glass
manufacturing or simply as a material that reacts with and
neutralizes acids or acidic substances. Internationally, two
types of soda ash are produced, natural and synthetic. The
United States produces only natural soda ash and is second
only to China in total soda ash production. Trona is the
principal ore from which natural soda ash is made.
Only two states produce natural soda ash: Wyoming and
California. Of these two states, only net emissions of CO2
from Wyoming were calculated due to specifics regarding
the production processes employed in the state.10 During
10 In California, soda ash is manufactured using sodium carbonate bearing
brines instead of trona ore. To extract the sodium carbonate, the complex
brines are first treated with CO2 in carbonation towers to convert the
sodium carbonate into sodium bicarbonate, which then precipitates from
the brine solution. The precipitated sodium bicarbonate is then calcined
back into sodium carbonate. Although CO2 is generated as a by-product.
the CO2 is recovered and recycled for use in the carbonation stage and is
not emitted. A third state, Colorado, produced soda ash until the plant was
idled in 2004. The lone producer of sodium bicarbonate no longer mines
trona in the state. For a brief time, NaHCO3 was produced using soda ash
feedstocks mined in Wyoming and shipped to Colorado. Because the trona
is mined in Wyoming, the production numbers given by the USGS included
the feedstocks mined in Wyoming and shipped to Colorado. In this way, the
sodium bicarbonate production that took place in Colorado was accounted
for in the Wyoming numbers.
Industrial Processes 4-13
-------
Table 4-16: C02 Emissions from Soda Ash Production
and Consumption (Tg C02 Eq.)
Year
Production Consumption
Total
1990
1.4
2.7
2005
2006
2007
2008
1.7
1.6
1.7
1.7
2.6
2.5
2.5
2.4
4.2
4.2
4.1
4.1
Note: Totals may not sum due to independent rounding.
Table 4-17: C02 Emissions from Soda Ash Production
and Consumption (Gg)
Year
Production Consumption
Total
1990
1,431
2,710
4,141
2005
2006
2007
2008
1,655
1,626
1,675
1,733
2,573
2,536
2,465
2,378
4,228
4,162
4,140
4,111
Note: Totals may not sum due to independent rounding.
the production process used in Wyoming, trona ore is treated
to produce soda ash. CO2 is generated as a byproduct of
this reaction, and is eventually emitted into the atmosphere.
In addition, CO2 may also be released when soda ash is
consumed.
In 2008, CO2 emissions from the production of soda ash
from trona were approximately 1.7 Tg CO2 Eq. (1,733 Gg).
Soda ash consumption in the United States generated 2.4
Tg CO2 Eq. (2,378 Gg) in 2008. Total emissions from soda
ash production and consumption in 2008 were 4.1 Tg CO2
Eq. (4,111 Gg) (see Table 4-16 and Table 4-17). Emissions
have remained relatively constant with some fluctuations
since 1990. These fluctuations were strongly related to
the behavior of the export market and the U.S. economy.
Emissions in 2008 decreased by approximately 0.7 percent
from the previous year, and have also decreased overall by
0.7 percent since 1990.
The United States represents about one-fourth of total
world soda ash output. Based on final 2007 reported data,
the estimated distribution of soda ash by end-use in 2008 was
glass making, 49 percent; chemical production, 30 percent;
soap and detergent manufacturing, 8 percent; distributors, 5
percent; flue gas desulfurization, 2 percent; water treatment,
2 percent; pulp and paper production, 2 percent; and
miscellaneous, 3 percent (USGS 2009).
Although the United States continues to be a major
supplier of world soda ash, China, which surpassed the
United States in soda ash production in 2003, is the world's
leading producer. While Chinese soda ash production
appears to be stabilizing, U.S. competition in Asian markets
is expected to continue. Despite this competition, U.S. soda
ash production is expected to increase by about 0.5 percent
annually (USGS 2008).
Methodology
During the production process, trona ore is calcined in a
rotary kiln and chemically transformed into a crude soda ash
that requires further processing. CO2 and water are generated
as by-products of the calcination process. CO2 emissions
from the calcination of trona can be estimated based on the
following chemical reaction:
2(Na3(CO3)(HCO3>2H2O) HX 3Na2CO3 + 5H2O + CO2
[trona]
[soda ash]
Based on this formula, approximately 10.27 metric tons
of trona are required to generate one metric ton of CO2, or
an emission factor of 0.097 metric tons CO2 per metric ton
trona (IPCC 2006). Thus, the 17.8 million metric tons of
trona mined in 2008 for soda ash production (USGS 2008)
resulted in CO2 emissions of approximately 1.7 Tg CO2 Eq.
(1,733 Gg).
Table 4-18: Soda Ash Production and Consumption (Gg)
Year
Production3
Consumption
1990
14,700
6,530
^m
6,500
^m
6,390
2005
2006
2007
2008
17,000
16,700
17,200
17,800
6,200
6,110
5,940
5,730
a Soda ash produced from trona ore only.
4-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-19: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Soda Ash Production and Consumption
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Soda Ash Production
and Consumption
CO,
4.1
4.0
4.9
-2%
+20%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Once produced, most soda ash is consumed in glass
and chemical production, with minor amounts in soap and
detergents, pulp and paper, flue gas desulfurization and water
treatment. As soda ash is consumed for these purposes,
additional CO2 is usually emitted. In these applications, it
is assumed that one mole of C is released for every mole of
soda ash used. Thus, approximately 0.113 metric tons of C
(or 0.415 metric tons of CO2) are released for every metric
ton of soda ash consumed.
The activity data for trona production and soda ash
consumption (see Table 4-18) were taken from USGS (1994
through 2008). Soda ash production and consumption data
were collected by the USGS from voluntary surveys of the
U.S. soda ash industry.
Uncertainty and Time-Series Consistency
Emission estimates from soda ash production have
relatively low associated uncertainty levels in that reliable
and accurate data sources are available for the emission
factor and activity data. The primary source of uncertainty,
however, results from the fact that emissions from
soda ash consumption are dependent upon the type of
processing employed by each end-use. Specific information
characterizing the emissions from each end-use is limited.
Therefore, there is uncertainty surrounding the emission
factors from the consumption of soda ash.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-19. Soda Ash Production and
Consumption CO2 emissions were estimated to be between
4.0 and 4.9 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 2 percent below and
20 percent above the emission estimate of 4.1 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Future inventories are anticipated to estimate emissions
from glass production and other use of carbonates. These
inventories will extract soda ash consumed for glass
production and other use of carbonates from the current
soda ash consumption emission estimates and include them
under those sources.
4.5. Ammonia Production
(IPCC Source Category 2B1)
and Urea Consumption
Emissions of CO2 occur during the production of
synthetic ammonia, primarily through the use of natural gas
as a feedstock. The natural gas-based, naphtha-based, and
petroleum coke-based processes produce CO2 and hydrogen
(H2), the latter of which is used in the production of ammonia.
One N production plant located in Kansas is producing
ammonia from petroleum coke feedstock. In some plants
the CO2 produced is captured and used to produce urea. The
brine electrolysis process for production of ammonia does
not lead to process-based CO2 emissions.
There are five principal process steps in synthetic
ammonia production from natural gas feedstock. The primary
reforming step converts CH4 to CO2, carbon monoxide (CO),
and H2 in the presence of a catalyst. Only 30 to 40 percent
of the CH4 feedstock to the primary reformer is converted
to CO and CO2. The secondary reforming step converts the
remaining CH4 feedstock to CO and CO2. The CO in the
process gas from the secondary reforming step (representing
approximately 15 percent of the process gas) is converted to
CO2 in the presence of a catalyst, water, and air in the shift
Industrial Processes 4-15
-------
conversion step. Carbon dioxide is removed from the process
gas by the shift conversion process, and the hydrogen gas is
combined with the nitrogen (N2) gas in the process gas during
the ammonia synthesis step to produce ammonia. The CO2 is
included in a waste gas stream with other process impurities
and is absorbed by a scrubber solution. In regenerating the
scrubber solution, CO2 is released.
The conversion process for conventional steam reforming
of CH4, including primary and secondary reforming and the
shift conversion processes, is approximately as follows:
(catalyst)
0.88CH4 + 1.26Air + 1.24H2O -» 0.88CO2 + N2 + 3H2
N2 + 3H2 -» 2NH3
To produce synthetic ammonia from petroleum coke,
the petroleum coke is gasified and converted to CO2 and H2.
These gases are separated, and the H2 is used as a feedstock
to the ammonia production process, where it is reacted with
N2 to form ammonia.
Not all of the CO2 produced in the production of
ammonia is emitted directly to the atmosphere. Both
ammonia and CO2 are used as raw materials in the production
of urea [CO(NH2)2], which is another type of nitrogenous
fertilizer that contains C as well as N. The chemical reaction
that produces urea is:
2NH3 + CO2 HX NH2COONH4 HX CO(NH2)2 + H2O
Urea is consumed for a variety of uses, including as a
nitrogenous fertilizer, in urea-formaldehyde resins, and as a
deicing agent (TIG 2002). The C in the consumed urea is
assumed to be released into the environment as CO2 during
use. Therefore, the CO2 produced by ammonia production
that is subsequently used in the production of urea is still
emitted during urea consumption. The majority of CO2
emissions associated with urea consumption are those
that result from its use as a fertilizer. These emissions are
accounted for in the Cropland Remaining Cropland section
of the Land Use, Land-Use Change, and Forestry chapter.
Carbon dioxide emissions associated with other uses of urea
are accounted for in this chapter. Net emissions of CO2 from
ammonia production in 2008 were 11.8TgCO2Eq. (11,755
Gg), and are summarized in Table 4-20 and Table 4-21.
Emissions of CO2 from urea consumed for non-fertilizer
purposes in 2008 totaled 3.9 Tg CO2 Eq. (3,871 Gg), and are
summarized in Table 4-20 and Table 4-21. The decrease in
ammonia production in recent years is due to several factors,
including market fluctuations and high natural gas prices.
Ammonia production relies on natural gas as both a feedstock
and a fuel, and as such, domestic producers are competing
with imports from countries with lower gas prices. If natural
gas prices remain high, it is likely that domestically produced
ammonia will continue to decrease with increasing ammonia
imports (EEA 2004).
Table 4-20: C02 Emissions from Ammonia Production and Urea Consumption (Tg C02 Eq.)
Source
Ammonia Production
Urea Consumption3
Total
1990
13.0
3.8
16.8
1995
13.5
4.3
17.8
2000
12.2
4.2
16.4|
2005
9.2
3.7
12.8
2006
8.8
3.5
12.3
2007
9.1
4.9
14.0
2008
7.9
3.9
11.8
3 Urea Consumption is for non-fertilizer purposes only. Urea consumed as a fertilizer is accounted for in the Land Use, Land-Use Change, and Forestry chapter.
Note: Totals may not sum due to independent rounding.
Table 4-21: C02 Emissions from Ammonia Production and Urea Consumption (Gg)
Source
Ammonia Production
Urea Consumption3
Total
1990
13,047
3,784
16,831
1995
13,541
4,255
17,796
2000
12,172
4,231
16,402
2005
9,196
3,653
12,849
2006
8,781
3,519
12,300
2007
9,074
4,894
13,968
2008
7,885
3,871
11,755
3 Urea Consumption is for non-fertilizer purposes only. Urea consumed as a fertilizer is accounted for in the Land Use, Land-Use Change, and Forestry chapter.
Note: Totals may not sum due to independent rounding.
4-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Methodology
The calculation methodology for non-combustion
CO2 emissions from production of nitrogenous fertilizers
from natural gas feedstock is based on a CO2 emission
factor published by the European Fertilizer Manufacturers
Association (EFMA). The selected EFMA factor is based
on ammonia production technologies that are similar to
those employed in the U.S. The CO2 emission factor (1.2
metric tons CO2/metric ton NH3) is applied to the percent
of total annual domestic ammonia production from natural
gas feedstock. Emissions from fuels consumed for energy
purposes during the production of ammonia are accounted
for in the Energy chapter. Emissions of CO2 from ammonia
production are then adjusted to account for the use of some
of the CO2 produced from ammonia production as a raw
material in the production of urea. For each ton of urea
produced, 8.8 of every 12 tons of CO2 are consumed and
6.8 of every 12 tons of ammonia are consumed (IPCC
2006, EFMA 2000). The CO2 emissions reported for
ammonia production are therefore reduced by a factor of
0.73 multiplied by total annual domestic urea production.
Total CO2 emissions resulting from nitrogenous fertilizer
production do not change as a result of this calculation,
but some of the CO2 emissions are attributed to ammonia
production and some of the CO2 emissions are attributed to
urea consumption. Those CO2 emissions that result from the
use of urea as a fertilizer are accounted for in the Land Use,
Land-Use Change, and Forestry chapter.
The total amount of urea consumed for non-agricultural
purposes is estimated by deducting the quantity of urea
fertilizer applied to agricultural lands, which is obtained
directly from the Land Use, Land-Use Change, and Forestry
Chapter and is reported in Table 4-22, from total U.S.
production. Total urea production is estimated based on the
amount of urea produced plus the sum of net urea imports
and exports. Carbon dioxide emissions associated with urea
that is used for non-fertilizer purposes are estimated using a
factor of 0.73 tons of CO2 per ton of urea consumed.
All ammonia production and subsequent urea production
are assumed to be from the same process—conventional
catalytic reforming of natural gas feedstock, with the
exception of ammonia production from petroleum coke
feedstock at one plant located in Kansas. The CO2 emission
factor for production of ammonia from petroleum coke is
based on plant specific data, wherein all C contained in the
petroleum coke feedstock that is not used for urea production
is assumed to be emitted to the atmosphere as CO2 (Bark
2004). Ammonia and urea are assumed to be manufactured
in the same manufacturing complex, as both the raw materials
needed for urea production are produced by the ammonia
production process. The CO2 emission factor (3.57 metric
tons CO2/metric ton NH3) is applied to the percent of total
annual domestic ammonia production from petroleum coke
feedstock.
The emission factor of 1.2 metric ton CO2/metric ton
NH3 for production of ammonia from natural gas feedstock
was taken from the EFMA Best Available Techniques
publication, Production of Ammonia (EFMA 1995). The
EFMA reported an emission factor range of 1.15 to 1.30
metric ton CO2/metric ton NH3, with 1.2 metric ton CO2/
metric ton NH3 as a typical value. Technologies (e.g.,
catalytic reforming process) associated with this factor are
found to closely resemble those employed in the U.S. for
use of natural gas as a feedstock. The EFMA reference also
Table 4-22: Ammonia Production, Urea Production, Urea Net Imports, and Urea Exports (Gg)
Year
Ammonia
Production
Urea Production
Urea Applied
as Fertilizer
Urea Imports
Urea Exports
854
881
2000
14,342
6,910
4,382
3,904
663
2005
2006
2007
2008
10,143
9,962
10,393
9,571
5,270
5,410
5,590
5,240
4,779
4,985
5,191
5,191
5,026
5,029
6,546
5,459
536
656
271
230
Industrial Processes 4-17
-------
indicates that more than 99 percent of the CH4 feedstock to
the catalytic reforming process is ultimately converted to
CO2. The emission factor of 3.57 metric ton CO2/metric
ton NH3 for production of ammonia from petroleum coke
feedstock was developed from plant-specific ammonia
production data and petroleum coke feedstock utilization
data for the ammonia plant located in Kansas (Bark 2004).
As noted earlier, emissions from fuels consumed for energy
purposes during the production of ammonia are accounted
for in the Energy chapter. Ammonia production data (see
Table 4-22) was obtained from Coffeyville Resources
(Coffeyville 2005, 2006, 2007a, 2007b, 2009) and the
Census Bureau of the U.S. Department of Commerce (U.S.
Census Bureau 1991 through 1994, 1998 through 2009) as
reported in Current Industrial Reports Fertilizer Materials
and Related Products annual and quarterly reports. Urea-
ammonia nitrate production was obtained from Coffeyville
Resources (Coffeyville 2005, 2006, 2007a, 2007b, 2009).
Urea production data for 1990 through 2008 were obtained
from the Minerals Yearbook: Nitrogen (USGS 1994 through
2010). Import data for urea were obtained from the U.S.
Census Bureau Current Industrial Reports Fertilizer
Materials and Related Products annual and quarterly reports
for 1997 through 2008 (U.S. Census Bureau 1998 through
2009), The Fertilizer Institute (TFI 2002) for 1993 through
1996, and the United States International Trade Commission
Interactive Tariff and Trade DataWeb (U.S. ITC 2002) for
1990 through 1992 (see Table 4-22). Urea export data for
1990 through 2008 were taken from U.S. Fertilizer Import/
Exports from USDA Economic Research Service Data Sets
(U.S. Department of Agriculture 2009).
Uncertainty and Time-Series Consistency
The uncertainties presented in this section are primarily
due to how accurately the emission factor used represents
an average across all ammonia plants using natural gas
feedstock. Uncertainties are also associated with natural gas
feedstock consumption data for the U.S. ammonia industry
as a whole, the assumption that all ammonia production and
subsequent urea production was from the same process—
conventional catalytic reforming of natural gas feedstock,
with the exception of one ammonia production plant located
in Kansas that is manufacturing ammonia from petroleum
coke feedstock. It is also assumed that ammonia and urea
are produced at collocated plants from the same natural gas
raw material.
Such recovery may or may not affect the overall estimate
of CO2 emissions depending upon the end use to which the
recovered CO2 is applied. Further research is required to
determine whether byproduct CO2 is being recovered from
other ammonia production plants for application to end uses
that are not accounted for elsewhere.
Additional uncertainty is associated with the estimate
of urea consumed for non-fertilizer purposes. Emissions
associated with this consumption are reported in this
source category, while those associated with consumption
as fertilizer are reported in Cropland Remaining Cropland
section of the Land Use, Land-Use Change, and Forestry
chapter. The amount of urea used for non-fertilizer purposes
is estimated based on estimates of urea production, net urea
imports, and the amount of urea used as fertilizer. There is
uncertainty associated with the accuracy of these estimates
as well as the fact that each estimate is obtained from a
different data source.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-23. Ammonia Production
and Urea Consumption CO2 emissions were estimated to
be between 10.4 and 13.1 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of approximately
11 percent below and 11 percent above the emission estimate
of 11.8TgCO2Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
Table 4-23: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Ammonia Production and
Urea Consumption (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Ammonia Production
and Urea Consumption
CO,
11.8
10.4
13.1
-11%
+ 11%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Planned improvements to the Ammonia Production
and Urea Consumption source category include updating
emission factors to include both fuel and feedstock CO2
emissions and incorporating CO2 capture and storage.
Methodologies will also be updated if additional ammonia-
production plants are found to use hydrocarbons other than
natural gas for ammonia production. Additional efforts will
be made to find consistent data sources for urea consumption
and to report emissions from this consumption appropriately
as defined by the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006).
4.6. Nitric Acid Production (IPCC
Source Category 2B2)
Nitric acid (HNO3) is an inorganic compound used
primarily to make synthetic commercial fertilizers. It is
also a major component in the production of adipic acid—a
feedstock for nylon—and explosives. Virtually all of the
nitric acid produced in the United States is manufactured
by the catalytic oxidation of ammonia (EPA 1997). During
this reaction, N2O is formed as a by-product and is released
from reactor vents into the atmosphere.
Currently, the nitric acid industry controls for emissions
of NO and NO2 (i.e., NOX). As such, the industry in the United
States uses a combination of non-selective catalytic reduction
(NSCR) and selective catalytic reduction (SCR) technologies.
In the process of destroying NOX, NSCR systems are also
very effective at destroying N2O. However, NSCR units are
generally not preferred in modern plants because of high
energy costs and associated high gas temperatures. NSCRs
were widely installed in nitric plants built between 1971 and
1977. Approximately 17 percent of nitric acid plants use
NSCR and they represent 7.6 percent of estimated national
production (EPA 2010). The remaining 92.4 percent of
production occurs using SCR or extended absorption, neither
of which is known to reduce N9O emissions.
Table 4-24: N20 Emissions from Nitric Acid Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
18.9
61
2005
2006
2007
2008
17.6
17.2
20.5
19.0
57
56
66
61
Nitrous oxide emissions from this source were estimated
to be 19.0 Tg CO2 Eq. (61 Gg) in 2008 (see Table 4-24).
Emissions from nitric acid production have increased by 0.7
percent since 1990, with the trend in the time series closely
tracking the changes in production. Emissions decreased 7.4
percent between 2007 and 2008. Emissions have decreased
by 15.3 percent since 1997, the highest year of production
in the time series.
Methodology
Nitrous oxide emissions were calculated by multiplying
nitric acid production by the amount of N2O emitted per unit
of nitric acid produced. The emission factor was determined
as a weighted average of two known emission factors: 2 kg
N2O/metric ton HNO3 produced at plants using non-selective
catalytic reduction (NSCR) systems and 9 kg N2O/metric ton
HNO3 produced at plants not equipped with NSCR (IPCC
2006). In the process of destroying NOX, NSCR systems
destroy 80 to 90 percent of the N2O, which is accounted
for in the emission factor of 2 kg N2O/metric ton HNO3.
Approximately 17 percent of HNO3 plants in the United
States are equipped with NSCR representing 7.6 percent
of estimated national production (EPA 2010). Hence, the
emission factor is equal to (9 x 0.924) + (2 x 0.076) = 8.5
kg N2O per metric ton HNO3.
Nitric acid production data for 1990 through 2002 were
obtained from the U.S. Census Bureau, Current Industrial
Reports (2006). Production data for 2003 were obtained from
the U.S. Census Bureau, Current Industrial Reports (2008).
Production data for 2004 through 2008 were obtained from
Industrial Processes 4-19
-------
Table 4-25: Nitric Acid Production (Gg)
Year
Gg
1990
7,195
2005
2006
2007
2008
6,711
6,572
7,827
7,245
the U.S. Census Bureau, Current Industrial Reports (2009)
(see Table 4-25).
Uncertainty and Time-Series Consistency
The overall uncertainty associated with the 2008
N2O emissions estimate from nitric acid production
was calculated using the IPCC Guidelines for National
Greenhouse Gas Inventories (2006) Tier 2 methodology.
Uncertainty associated with the parameters used to estimate
N2O emissions included that of production data, the share
of U.S. nitric acid production attributable to each emission
abatement technology, and the emission factors applied to
each abatement technology type.
The results of this Tier 2 quantitative uncertainty analysis
are summarized in Table 4-26. Nitrous oxide emissions
from nitric acid production were estimated to be between
11.3 and 27.5 Tg CO2 Eq. at the 95 percent confidence
level. This indicates a range of approximately 41 percent
below to 45 percent above the 2008 emissions estimate of
19.0 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Recalculations Discussion
Historical estimates for N2O emissions from nitric
acid production have been revised relative to the previous
inventory based on updated information from US EPA
(2010) on abatement technologies in use and based on
revised production data published by the U.S. Census Bureau
(2009). The previous Inventory assumed that approximately
5 percent of facilities accounting for less than 1 percent of
national production were equipped with NSCR systems (EPA
2008). The current Inventory assumes that approximately
17 percent of facilities accounting for roughly 8 percent of
national production were equipped with NSCR systems (EPA
2010). This change resulted in a decrease in the weighted
average emission factor of 0.5 kg N2O/metric ton HNO3
(5.5 percent). Additionally, national nitric acid production
values for 2006 and 2007 have been updated relative to the
previous Inventory. Revised production in 2006 resulted in
a negligible decrease in emissions of less than 0.01 Tg CO2
Eq. (0.01 percent). Revised production in 2007 resulted
in a small increase in emissions of 0.01 Tg CO2 Eq. (0.06
percent). Overall, changes relative to the previous Inventory
resulted in an average annual decrease in emissions of 1.1
Tg CO2 Eq. (5.5 percent) for the period 1990 through 2007.
4.7. Adipic Acid Production (IPCC
Source Category 2B3)
Adipic acid production is an anthropogenic source of
N2O emissions. Worldwide, few adipic acid plants exist.
The United States and Europe are the major producers. In
2008, the United States had two companies in three locations
accounting for 34 percent of world production (VA DEQ
2009; CW 2007). Eight European producers account for a
combined 38 percent of world production (CW 2007). Adipic
acid is a white crystalline solid used in the manufacture
of synthetic fibers, plastics, coatings, urethane foams,
elastomers, and synthetic lubricants. Commercially, it is the
Table 4-26: Tier 2 Quantitative Uncertainty Estimates for N20 Emissions from Nitric Acid Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Nitric Acid Production
N,0
19.0
11.3
27.5
-41%
+45%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-27: N20 Emissions from Adipic Acid Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
15.8
51
2005
2006
2007
2008
5.0
4.3
3.7
2.0
16
14
12
7
most important of the aliphatic dicarboxylic acids, which are
used to manufacture polyesters. 84 percent of all adipic acid
produced in the United States is used in the production of
nylon 6,6; nine percent is used in the production of polyester
polyols; four percent is used in the production of plasticizers;
and the remaining four percent is accounted for by other uses,
including unsaturated polyester resins and food applications
(ICIS 2007). Food grade adipic acid is used to provide some
foods with a "tangy" flavor (Thiemens and Trogler 1991).
Adipic acid is produced through a two-stage process
during which N2O is generated in the second stage. The
first stage of manufacturing usually involves the oxidation of
cyclohexane to form a cyclohexanone/cyclohexanol mixture.
The second stage involves oxidizing this mixture with nitric
acid to produce adipic acid. N2O is generated as a by-product
of the nitric acid oxidation stage and is emitted in the waste
gas stream (Thiemens and Trogler 1991). Process emissions
from the production of adipic acid vary with the types of
technologies and level of emission controls employed by a
facility. In 1990, two of the three major adipic acid-producing
plants had N2O abatement technologies in place and, as of
1998, the three major adipic acid production facilities had
control systems in place (Reimer et al. 1999).n One small
plant, which last operated in April 2006 and represented
approximately two percent of production, did not control for
N2O (VA DEQ 2009; ICIS 2007; VA DEQ 2006).
Nitrous oxide emissions from adipic acid production
were estimated to be 2.0 Tg CO2 Eq. (7 Gg) in 2008 (see
Table 4-27). National adipic acid production has increased
by approximately 34 percent over the period of 1990 through
11 During 1997, the N2O emission controls installed by the third plant
operated for approximately a quarter of the year.
2008, to roughly one million metric tons. Over the same
period, emissions have been reduced by 87 percent due to
the widespread installation of pollution control measures in
the late 1990s and because the smallest of the four facilities
ceased production of adipic acid in April 2006 (VA DEQ
2009).
Methodology
Due to confidential business information, plant names
are not provided in this section. The four adipic acid-
producing plants will henceforth be referred to as Plants 1
through 4.
For Plants 1 and 2, 1990 to 2008 emission estimates
were obtained directly from the plant engineer and account
for reductions due to control systems in place at these
plants during the time series (Desai 2010). These estimates
were based on continuous emissions monitoring equipment
installed at the two facilities. For Plants 3 and 4, N2O
emissions were calculated by multiplying adipic acid
production by an emission factor (i.e., N2O emitted per unit of
adipic acid produced) and adjusting for the percentage of N2O
released as a result of plant-specific emission controls. On
the basis of experiments, the overall reaction stoichiometry
for N2O production in the preparation of adipic acid was
estimated at approximately 0.3 metric tons of N2O per metric
ton of product (IPCC 2006). Emissions are estimated using
the following equation:
N2O emissions = {production of adipic acid
[metric tons (MT) of adipic acid]} x
(0.3 MT N2O MT adipic acid) x
[1-(N2O destruction factor x abatement system utility factor)]
The "N2O destruction factor" represents the percentage
of N2O emissions that are destroyed by the installed abatement
technology. The "abatement system utility factor" represents
the percentage of time that the abatement equipment operates
during the annual production period. Overall, in the United
States, two of the plants employ catalytic destruction (Plants
1 and 2), one plant employs thermal destruction (Plant 3),
and the smallest plant used no N2O abatement equipment
(Plant 4). For Plant 3, which uses thermal destruction and
for which no reported plant-specific emissions are available,
the N2O abatement system destruction factor is assumed to
be 98.5 percent, and the abatement system utility factor is
assumed to be 97 percent (IPCC 2006).
Industrial Processes 4-21
-------
From 1990 to 2003, plant-specific production data were
estimated for Plant 3 where direct emission measurements
were not available. In order to calculate plant-specific
production for this plant, national adipic acid production
was allocated to the plant level using the ratio of known
plant capacity to total national capacity for all U.S. plants.
The estimated plant production for this plant was then used
for calculating emissions as described above. For 2004 and
2006, actual plant production data were obtained and used
for emission calculations (CW 2007; CW 2005). For 2005,
interpolated national production was used for calculating
emissions. Updated production data were not available for
Plant 3 in 2007 or 2008; therefore, production values in 2007
and 2008 were proxied using 2006 data.
For Plant 4, which last operated in April 2006 (VA DEQ
2009), plant-specific production data were obtained across
the timeseries from 1990 through 2008 (VADEQ 2010). The
plant-specific production data were then used for calculating
emissions as described above.
National adipic acid production data (see Table 4-28)
for 1990 through 2002 were obtained from the American
Chemistry Council (ACC 2003). Production for 2004 and
2006 were obtained from Chemical Week, Product Focus:
Adipic Acid (CW 2005, 2007). National production for
2003 was calculated through linear interpolation between
2002 and 2004 reported national production data. 2005
national production was proxied using 2004 reported national
production. National production in 2006 represents the sum
of annual production at Plants 1, 2, and 3 and 3.5 months
of production at Plant 4, resulting in estimated national
production of 989 Gg (VA DEQ, 2009; CW 2005, 2007).
Updated national production data were not available for 2007
or 2008; therefore, national production in 2007 and 2008
represents the sum of production at the three plants still in
operation (Plants 1, 2, and 3) resulting in estimated national
production of 985 Gg in 2007 and 2008, respectively (VA
DEQ, 2009; CW 2005, 2007).
Plant capacities for 1990 through 1994 were obtained
from Chemical and Engineering News, "Facts and Figures"
and "Production of Top 50 Chemicals" (C&EN 1992 through
1995). Plant capacities for 1995 and 1996 were kept the
same as 1994 data. The 1997 plant capacities were taken
from Chemical Market Reporter "Chemical Profile: Adipic
Acid" (CMR 1998). The 1998 plant capacities for all four
Table 4-28: Adipic Acid Production (Gg)
Year
Gg
2005
2006
2007
2008
1,002
990
985
985
plants and 1999 plant capacities for three of the plants were
obtained from Chemical Week, "Product Focus: Adipic Acid/
Adiponitrile" (CW 1999). Plant capacities for 2000 for three
of the plants were updated using Chemical Market Reporter,
"Chemical Profile: Adipic Acid" (CMR 2001). For 2001
through 2005, the plant capacities for three plants were kept
the same as the year 2000 capacities. Plant capacity for 1999
to 2005 for the one remaining plant was kept the same as
1998. For 2004 to 2008, although some plant capacity data
are available (CW 1999, CMR 2001, ICIS 2007), they are
not used to calculate plant-specific production for these years
because plant-specific production data for 2004 and 2006 are
also available and are used in our calculations instead (CW
2005, CW 2007).
Uncertainty and Time-Series Consistency
The overall uncertainty associated with the 2008 N2O
emission estimate from adipic acid production was calculated
using the IPCC Guidelines for National Greenhouse
Gas Inventories (2006) Tier 2 methodology. Uncertainty
associated with the parameters used to estimate N2O
emissions included that of company specific production
data, emission factors for abated and unabated emissions,
and company-specific historical emissions estimates.
The results of this Tier 2 quantitative uncertainty analysis
are summarized in Table 4-29. Nitrous oxide emissions from
adipic acid production were estimated to be between 1.3
and 2.9 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of approximately 37 percent below to 41
percent above the 2008 emission estimate of 2.0 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
4-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-29: Tier 2 Quantitative Uncertainty Estimates for N20 Emissions from Adipic Acid Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Adipic Acid Production
N,0
2.0
1.3
2.9
-37%
+41%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Improvement efforts will be focused on obtaining direct
measurement data from facilities. If they become available,
cross verification with top-down approaches will provide a
useful Tier 2 level QC check. Also, additional information
on the actual performance of the latest catalytic and thermal
abatement equipment at plants with continuous emission
monitoring may support the re-evaluation of current default
abatement values.
4.8. Silicon Carbide Production
(IPCC Source Category 2B4) and
Consumption
Carbon dioxide and CH4 are emitted from the
production12 of silicon carbide (SiC), a material used as an
industrial abrasive. To make SiC, quartz (SiO2) is reacted
with C in the form of petroleum coke. A portion (about 35
percent) of the C contained in the petroleum coke is retained
in the SiC. The remaining C is emitted as CO2, CH4, or CO.
Carbon dioxide is also emitted from the consumption of
SiC for metallurgical and other non-abrasive applications.
The USGS reports that a portion (approximately 50 percent)
of SiC is used in metallurgical and other non-abrasive
applications, primarily in iron and steel production (USGS
2006).
12 Silicon carbide is produced for both abrasive and metallurgical
applications in the United States. Production for metallurgical applications
is not available and therefore both CH4 and CO2 estimates are based solely
upon production estimates of silicon carbide for abrasive applications.
Carbon dioxide from SiC production and consumption
in 2008 were 0.2 Tg CO2 Eq. (175 Gg) (USGS 2009).
Approximately 52 percent of these emissions resulted
from SiC production while the remainder results from SiC
consumption. CH4 emissions from SiC production in 2008
were 0.01 Tg CO2 Eq. CH4 (0.4 Gg) (see Table 4-30 and
Table 4-31).
Methodology
Emissions of CO2 and CH4 from the production of SiC
were calculated by multiplying annual SiC production by
the emission factors (2.62 metric tons CO2/metric ton SiC
for CO2 and 11.6 kg CH^metric ton SiC for CH4) provided
by the 2006 IPCC Guidelines for National Greenhouse Gas
Inventories (IPCC 2006).
Emissions of CO2 from silicon carbide consumption
were calculated by multiplying the annual SiC consumption
(production plus net imports) by the percent used in
metallurgical and other non-abrasive uses (50 percent)
(USGS 2006). The total SiC consumed in metallurgical and
other non-abrasive uses was multiplied by the C content of
SiC (31.5 percent), which was determined according to the
molecular weight ratio of SiC.
Production data for 1990 through 2008 were obtained
from the Minerals Yearbook: Manufactured Abrasives
(USGS 1991a through 2005a, 2007) and the 2009 Mineral
Commodity Summary: Manufactured Abrasives (USGS
2009). Silicon carbide consumption by major end use was
obtained from the Minerals Yearbook: Silicon (USGS 1991b
through 2005b) (see Table 4-32) for years 1990 through
2004 and from the USGS Minerals Commodity Specialist
for 2005 and 2006 (Corathers 2006, 2007). Silicon carbide
consumption by major end us data for 2008 are proxied using
2007 data due to unavailability of data at time of publication.
Net imports for the entire time series were obtained from the
U.S. Census Bureau (2005 through 2009).
Industrial Processes 4-23
-------
Table 4-30: C02 and CH4 Emissions from Silicon Carbide Production and Consumption (Tg C02 Eq.)
Gas
C02
CH4
Total
1990
0.4
+
0.4
1995
0.3
+
0.3
2000
0.2
+ 1
0.3
2005
0.2
+
0.2
2006
0.2
+
0.2
2007
0.2
+
0.2
2008
0.2
+
0.2
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Table 4-31: C02 and CH4 Emissions from Silicon Carbide Production and Consumption (Gg)
Gas
C02
CH4
1990
375
1
1995
329
1
2000
248
\m
2005
219
+
2006
207
+
2007
196
+
2008
175
+
Does not exceed 0.5 Gg.
Uncertainty and Time-Series Consistency
There is uncertainty associated with the emission factors
used because they are based on stoichiometry as opposed to
monitoring of actual SiC production plants. An alternative
would be to calculate emissions based on the quantity
of petroleum coke used during the production process
rather than on the amount of silicon carbide produced.
However, these data were not available. For CH4, there is
Table 4-32: Production and Consumption of Silicon
Carbide (Metric Tons)
Year
Production
Consumption
1990
105,000
172,465
2005
2006
2007
2008
35,000
35,000
35,000
35,000
220,149
199,937
179,741
144,928
also uncertainty associated with the hydrogen-containing
volatile compounds in the petroleum coke (IPCC 2006).
There is also some uncertainty associated with production,
net imports, and consumption data as well as the percent of
total consumption that is attributed to metallurgical and other
non-abrasive uses.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-33. Silicon carbide production
and consumption CO2 emissions were estimated to be
between 9 percent below and 9 percent above the emission
estimate of 0.2 Tg CO2 Eq. at the 95 percent confidence
level. Silicon carbide production CH4 emissions were
estimated to be between 9 percent below and 9 percent above
the emission estimate of 0.01 Tg CO2 Eq. at the 95 percent
confidence level.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 4-33: Tier 2 Quantitative Uncertainty Estimates for CH4 and C02 Emissions from Silicon Carbide Production
and Consumption (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Silicon Carbide Production
and Consumption
Silicon Carbide Production
C02 0.18
CH4 +
Lower Bound Upper Bound Lower Bound
0.16 0.19 -9%
+ + -9%
Upper Bound
+ 9%
+ 9%
+ Does not exceed 0.05 Tg C02 Eq. or 0.5 Gg.
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Planned Improvements
Future improvements to the carbide production source
category include continued research to determine if calcium
carbide production and consumption data are available for
the United States. If these data are available, calcium carbide
emission estimates will be included in this source category.
4.9. Petrochemical Production
(IPCC Source Category 2B5)
The production of some petrochemicals results in
the release of small amounts of CH4 and CO2 emissions.
Petrochemicals are chemicals isolated or derived from
petroleum or natural gas. Methane emissions are presented
here from the production of carbon black, ethylene, ethylene
dichloride, and methanol, while CO2 emissions are presented
here for only carbon black production. The CO2 emissions
from petrochemical processes other than carbon black are
currently included in the Carbon Stored in Products from
Non-Energy Uses of Fossil Fuels Section of the Energy
chapter. The CO2 from carbon black production is included
here to allow for the direct reporting of CO2 emissions from
the process and direct accounting of the feedstocks used in
the process.
Carbon black is an intense black powder generated
by the incomplete combustion of an aromatic petroleum
or coal-based feedstock. Most carbon black produced in
the United States is added to rubber to impart strength and
abrasion resistance, and the tire industry is by far the largest
consumer. Ethylene is consumed in the production processes
of the plastics industry including polymers such as high, low,
and linear low density polyethylene (HOPE, LDPE, LLDPE),
polyvinyl chloride (PVC), ethylene dichloride, ethylene
oxide, and ethylbenzene. Ethylene dichloride is one of the
first manufactured chlorinated hydrocarbons with reported
production as early as 1795. In addition to being an important
intermediate in the synthesis of chlorinated hydrocarbons,
ethylene dichloride is used as an industrial solvent and as a
fuel additive. Methanol is an alternative transportation fuel
as well as a principle ingredient in windshield wiper fluid,
paints, solvents, refrigerants, and disinfectants. In addition,
methanol-based acetic acid is used in making PET plastics
and polyester fibers.
Emissions of CO2 and CH4 from petrochemical
production in 2008 were 3.4 Tg CO2 Eq. (3,449 Gg) and 0.9
Tg CO2 Eq. (43 Gg), respectively (see Table 4-34 and Table
4-35), totaling 4.4 Tg CO2 Eq. There has been an overall
increase in CO2 emissions from carbon black production of
four percent since 1990. CH4 emissions from petrochemical
production increased by approximately six percent since
1990.
Methodology
Emissions of CH4 were calculated by multiplying annual
estimates of chemical production by the appropriate emission
factor, as follows: 11 kg CH4/metric ton carbon black, 1 kg
CH4/metric ton ethylene, 0.4 kg CH^metric ton ethylene
dichloride,13 and 2 kg CH4/metric ton methanol. Although
the production of other chemicals may also result in CH4
emissions, insufficient data were available to estimate their
emissions.
Table 4-34: C02 and CH4 Emissions from Petrochemical Production (Tg C02 Eq.)
Gas
C02
CH4
Total
1990
3.3
0.9
4.2
1995
...
5.2 |
2000
...
5.7
2005
4.2
1.1
5.3
2006
3.8
1.0
4.8
2007
3.9
1.0
4.9
2008
3.4
0.9
4.4
ible 4-35: C02 and C
Gas
C02
CH4
H4 Emissions from PetrocH
1990
3,311
41
lemicai Proeniqflpii^jjT^ factor obtained from IPCC/UNEP/OECD/IEA (199?),
1995
4,101
52
2000
4,479
59
2005
4,181
51
2006
3,837
48
2007
3,931
48
2008
3,449
43
Industrial Processes 4-25
-------
Emission factors were taken from the Revised 1996
IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). Annual
production data (see Table 4-36) were obtained from the
American Chemistry Council's Guide to the Business of
Chemistry (ACC 2002, 2003, 2005 through 2009) and the
International Carbon Black Association (Johnson 2003,2005
through 2009).
Almost all carbon black in the United States is produced
from petroleum-based or coal-based feedstocks using the
"furnace black" process (European IPPC Bureau 2004).
The furnace black process is a partial combustion process in
which a portion of the carbon black feedstock is combusted to
provide energy to the process. Carbon black is also produced
in the United States by the thermal cracking of acetylene-
containing feedstocks ("acetylene black process") and by
the thermal cracking of other hydrocarbons ("thermal black
process"). One U.S carbon black plant produces carbon
black using the thermal black process, and one U.S. carbon
black plant produces carbon black using the acetylene black
process (The Innovation Group 2004).
The furnace black process produces carbon black from
"carbon black feedstock" (also referred to as "carbon black
oil"), which is a heavy aromatic oil that may be derived
as a byproduct of either the petroleum refining process or
the metallurgical (coal) coke production process. For the
production of both petroleum-derived and coal-derived
carbon black, the "primary feedstock" (i.e., carbon black
feedstock) is injected into a furnace that is heated by a
"secondary feedstock" (generally natural gas). Both the
natural gas secondary feedstock and a portion of the carbon
black feedstock are oxidized to provide heat to the production
process and pyrolyze the remaining carbon black feedstock to
carbon black. The "tail gas" from the furnace black process
contains CO2, carbon monoxide, sulfur compounds, CH4,
and non-CH4 volatile organic compounds. A portion of the
tail gas is generally burned for energy recovery to heat the
downstream carbon black product dryers. The remaining
tail gas may also be burned for energy recovery, flared, or
vented uncontrolled to the atmosphere.
The calculation of the C lost during the production
process is the basis for determining the amount of CO2
released during the process. The C content of national carbon
black production is subtracted from the total amount of C
contained in primary and secondary carbon black feedstock
to find the amount of C lost during the production process.
It is assumed that the C lost in this process is emitted to the
atmosphere as either CH4 or CO2. The C content of the
CH4 emissions, estimated as described above, is subtracted
from the total C lost in the process to calculate the amount
of C emitted as CO2. The total amount of primary and
secondary carbon black feedstock consumed in the process
(see Table 4-37) is estimated using a primary feedstock
consumption factor and a secondary feedstock consumption
factor estimated from U.S. Census Bureau (1999,2004, and
2007) data. The average carbon black feedstock consumption
factor for U.S. carbon black production is 1.69 metric tons of
carbon black feedstock consumed per metric ton of carbon
black produced. The average natural gas consumption factor
for U.S. carbon black production is 321 normal cubic meters
of natural gas consumed per metric ton of carbon black
produced. The amount of C contained in the primary and
secondary feedstocks is calculated by applying the respective
C contents of the feedstocks to the respective levels of
feedstock consumption (EIA 2003, 2004).
For the purposes of emissions estimation, 100 percent
of the primary carbon black feedstock is assumed to be
derived from petroleum refining byproducts. Carbon black
feedstock derived from metallurgical (coal) coke production
(e.g., creosote oil) is also used for carbon black production;
however, no data are available concerning the annual
consumption of coal-derived carbon black feedstock. Carbon
black feedstock derived from petroleum refining byproducts
is assumed to be 89 percent elemental C (Srivastava et al.
1999). It is assumed that 100 percent of the tail gas produced
from the carbon black production process is combusted
Table 4-36: Production of Selected Petrochemicals (Thousand Metric Tons)
Chemical
Carbon Black
Ethylene
Ethylene Dichloride
Methanol
1990
1,307
16,541
6,282
3,785
1995
1,619
21,214
7,829
4,992
2000
1,769
24,970
9,866
5,221
2005
1,651
23,954
11,260
2,336
2006
1,515
25,000
9,736
1,123
2007
1,552
25,392
9,566
1,068
2008
1,362
22,539
8,981
1,136
4-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-37: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock (Secondary Feedstock)
Consumption (Thousand Metric Tons)
Activity
Primary Feedstock
Secondary Feedstock
1990
2,213
284
1995
2,741
352
2000
2,993
384
2005
2,794
359
2006
2,564
329
2007
2,627
337
2008
2,305
296
and that none of the tail gas is vented to the atmosphere
uncontrolled. The furnace black process is assumed to be
the only process used for the production of carbon black
because of the lack of data concerning the relatively small
amount of carbon black produced using the acetylene black
and thermal black processes. The carbon black produced
from the furnace black process is assumed to be 97 percent
elemental C (Othmer et al. 1992).
Uncertainty and Time-Series Consistency
The CH4 emission factors used for petrochemical
production are based on a limited number of studies. Using
plant-specific factors instead of average factors could increase
the accuracy of the emission estimates; however, such data
were not available. There may also be other significant
sources of CH4 arising from petrochemical production
activities that have not been included in these estimates.
The results of the quantitative uncertainty analysis for
the CO2 emissions from carbon black production calculation
are based on feedstock consumption, import and export data,
and carbon black production data. The composition of carbon
black feedstock varies depending upon the specific refinery
production process, and therefore the assumption that carbon
black feedstock is 89 percent C gives rise to uncertainty.
Also, no data are available concerning the consumption
of coal-derived carbon black feedstock, so CO2 emissions
from the utilization of coal-based feedstock are not included
in the emission estimate. In addition, other data sources
indicate that the amount of petroleum-based feedstock used
in carbon black production may be underreported by the
U.S. Census Bureau. Finally, the amount of carbon black
produced from the thermal black process and acetylene black
process, although estimated to be a small percentage of the
total production, is not known. Therefore, there is some
uncertainty associated with the assumption that all of the
carbon black is produced using the furnace black process.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-38. Petrochemical production
CO2 emissions were estimated to be between 2.5 and 4.6 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 27 percent below to 32 percent above
the emission estimate of 3.4 Tg CO2 Eq. Petrochemical
production CH4 emissions were estimated to be between
0.6 and 1.2 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 30 percent below to
30 percent above the emission estimate of 0.9 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Recalculations Discussion
Carbon black feedstock consumption and natural
gas consumption were updated to include 2006 data. The
inclusion of this data changed the carbon black feedstock
average consumption factor from 1.43 metric tons of carbon
black feedstock consumed per metric ton of carbon black
Table 4-38: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petrochemical Production
and C02 Emissions from Carbon Black Production (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Petrochemical Production
Petrochemical Production
C02
CH4
3.4
0.9
Lower Bound
2.5
0.6
Upper Bound
4.6
1.2
Lower Bound
-27%
-30%
Upper Bound
+32%
+30%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-27
-------
produced to 1.69 metric tons of carbon black feedstock
consumed per metric ton of carbon black produced and
natural gas feedstock average consumption factor from
341 normal cubic meters of natural gas consumed per
metric ton of carbon black produced to 321 normal cubic
meters of natural gas consumed per metric ton of carbon
black produced. The change in these factors increased CO2
emissions by approximately 50 percent across the time series
relative to the previous Inventory.
Planned Improvements
Future improvements to the petrochemicals source
category include research into the use of acrylonitrile in the
United States, revisions to the carbon black CH4 and CO2
emission factors, and research into process and feedstock
data to obtain Tier 2 emission estimates from the production
of methanol, ethylene, propylene, ethylene dichloride, and
ethylene oxide.
4.10. Titanium Dioxide Production
(IPCC Source Category 2B5)
Titanium dioxide (TiO2) is a metal oxide manufactured
from titanium ore, and is principally used as a pigment.
Titanium dioxide is a principal ingredient in white paint,
and is also used as a pigment in the manufacture of white
paper, foods, and other products. There are two processes
for making TiO2: the chloride process and the sulfate process.
The chloride process uses petroleum coke and chlorine as
raw materials and emits process-related CO2. The sulfate
process does not use petroleum coke or other forms of C as
a raw material and does not emit CO2.
The chloride process is based on the following chemical
reactions:
2FeTiO3 + 7C12 + 3C -» 2TiCi4 + 2FeCl3 + 3CO2
2TiCl4 + 2O2 -» 2TiO2 + 4C12
The C in the first chemical reaction is provided by
petroleum coke, which is oxidized in the presence of the
chlorine and FeTiO3 (the Ti-containing ore) to form CO2.
The majority of U.S. TiO2 was produced in the United
States through the chloride process, and a special grade of
"calcined" petroleum coke is manufactured specifically for
this purpose.
Table 4-39: C02 Emissions from Titanium Dioxide
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
1.2
1,195
2005
2006
2007
2008
1.8
1.8
1.9
1.8
1,755
1,836
1,930
1,809
Emissions of CO2 in 2008 were 1.8 Tg CO2 Eq. (1,809
Gg), which represents an increase of 51 percent since 1990
(see Table 4-39).
Methodology
Emissions of CO2 from TiO2 production were calculated
by multiplying annual TiO2 production by chloride-process-
specific emission factors.
Data were obtained for the total amount of TiO2
produced each year. For years previous to 2004, it was
assumed that TiO2 was produced using the chloride process
and the sulfate process in the same ratio as the ratio of the
total U.S. production capacity for each process. As of 2004,
the last remaining sulfate-process plant in the United States
had closed; therefore, 100 percent of post-2004 production
uses the chloride process (USGS 2005). An emission factor
of 0.4 metric tons C/metric ton TiO2 was applied to the
estimated chloride-process production. It was assumed that
all TiO2 produced using the chloride process was produced
using petroleum coke, although some TiO2 may have been
produced with graphite or other C inputs. The amount of
petroleum coke consumed annually in TiO2 production
was calculated based on the assumption that the calcined
petroleum coke used in the process is 98.4 percent C and
1.6 percent inert materials (Nelson 1969).
The emission factor for the TiO2 chloride process
was taken from the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006). Titanium dioxide
production data and the percentage of total TiO2 production
capacity that is chloride process for 1990 through 2007 (see
Table 4-40) were obtained through the Minerals Yearbook:
Titanium Annual Report (USGS 1991 through 2008). The
4-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-40: Titanium Dioxide Production (Gg)
Year
Gg
2005
2006
2007
2008
1,310
1,370
1,440
1,350
2008 production value is from the 2009 Mineral Commodity
Summary Report (USGS 2009). Because the 2008 capacity
value was unavailable at the time of publication, the 2007
capacity value was used. Percentage chloride-process data
were not available for 1990 through 1993, and data from the
1994 USGS Minerals Yearbook were used for these years.
Because a sulfate-process plant closed in September 2001,
the chloride-process percentage for 2001 was estimated based
on a discussion with Joseph Gambogi (2002). By 2002, only
one sulfate plant remained online in the United States and
this plant closed in 2004 (USGS 2005).
Uncertainty and Time-Series Consistency
Although some TiO2 may be produced using graphite or
other C inputs, information and data regarding these practices
were not available. Titanium dioxide produced using graphite
inputs, for example, may generate differing amounts of CO2
per unit of TiO2 produced as compared to that generated
through the use of petroleum coke in production. While the
most accurate method to estimate emissions would be to
base calculations on the amount of reducing agent used in
each process rather than on the amount of TiO2 produced,
sufficient data were not available to do so.
Also, annual TiO2 is not reported by USGS by the type
of production process used (chloride or sulfate). Only the
percentage of total production capacity by process is reported.
The percent of total TiO2 production capacity that was
attributed to the chloride process was multiplied by total TiO2
production to estimate the amount of TiO2 produced using
the chloride process (since, as of 2004, the last remaining
sulfate-process plant in the United States closed). This
assumes that the chloride-process plants and sulfate-process
plants operate at the same level of utilization. Finally,
the emission factor was applied uniformly to all chloride-
process production, and no data were available to account
for differences in production efficiency among chloride-
process plants. In calculating the amount of petroleum coke
consumed in chloride-process TiO2 production, literature
data were used for petroleum coke composition. Certain
grades of petroleum coke are manufactured specifically for
use in the TiO2 chloride process; however, this composition
information was not available.
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 4-41. Titanium dioxide
consumption CO2 emissions were estimated to be between
1.6 and 2.0 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 12 percent below and
13 percent above the emission estimate of 1.8 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Future improvements to TiO2 production methodology
include researching the significance of titanium-slag
production in electric furnaces and synthetic-rutile
production using the Becher process in the United States.
Significant use of these production processes will be included
in future estimates.
Table 4-41: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Titanium Dioxide Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Titanium Dioxide Production CO?
1.E
1.6
2.0
-12%
+ 13%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-29
-------
4.11. Carbon Dioxide Consumption
(IPCC Source Category 2B5)
Carbon dioxide is used for a variety of commercial
applications, including food processing, chemical production,
carbonated beverage production, and refrigeration, and is also
used in petroleum production for enhanced oil recovery
(EOR). Carbon dioxide used for EOR is injected into the
underground reservoirs to increase the reservoir pressure to
enable additional petroleum to be produced.
For the most part, CO2 used in non-EOR applications will
eventually be released to the atmosphere, and for the purposes
of this analysis CO2 used in commercial applications other
than EOR is assumed to be emitted to the atmosphere. CO2
used in EOR applications is discussed in the Energy Chapter
under "Carbon Capture and Storage, including Enhanced Oil
Recovery" and is not discussed in this section.
Carbon dioxide is produced from naturally occurring
CO2 reservoirs, as a by-product from the energy and industrial
production processes (e.g., ammonia production, fossil
fuel combustion, ethanol production), and as a by-product
from the production of crude oil and natural gas, which
contain naturally occurring CO2 as a component. Only CO2
produced from naturally occurring CO2 reservoirs and used
in industrial applications other than EOR is included in this
analysis. Neither by-product CO2 generated from energy
nor industrial production processes nor CO2 separated from
crude oil and natural gas are included in this analysis for a
number of reasons. Carbon dioxide captured from biogenic
sources (e.g., ethanol production plants) is not included in
the Inventory. Carbon dioxide captured from crude oil and
gas production is used in EOR applications and is therefore
reported in the Energy Chapter. Any CO2 captured from
industrial or energy production processes (e.g., ammonia
plants, fossil fuel combustion) and used in non-EOR
applications is assumed to be emitted to the atmosphere.
The CO2 emissions from such capture and use are therefore
accounted for under Ammonia Production, Fossil Fuel
Combustion, or other appropriate source category.14
Carbon dioxide is produced as a by-product of crude oil
and natural gas production. This CO2 is separated from the
crude oil and natural gas using gas processing equipment,
and may be emitted directly to the atmosphere, or captured
and reinjected into underground formations, used for EOR,
or sold for other commercial uses. A further discussion of
CO2 used in EOR is described in the Energy Chapter under
the text box titled "Carbon Dioxide Transport, Injection,
and Geological Storage." The only CO2 consumption that
is accounted for in this analysis is CO2 produced from
naturally-occurring CO2 reservoirs that is used in commercial
applications other than EOR.
There are currently two facilities, one in Mississippi and
one in New Mexico, producing CO2 from naturally occurring
CO2 reservoirs for use in both EOR and in other commercial
applications (e.g., chemical manufacturing, food production).
There are other naturally occurring CO2 reservoirs, mostly
located in the western U.S. Facilities are producing CO2 from
these natural reservoirs, but they are only producing CO2 for
EOR applications, not for other commercial applications
(Allis et al. 2000). Carbon dioxide production from these
facilities is discussed in the Energy Chapter.
In 2008, the amount of CO2 produced by the Mississippi
and New Mexico facilities for commercial applications and
subsequently emitted to the atmosphere were 1.8 Tg CO2
Eq. (1,780 Gg) (see Table 4-42). This amount represents a
decrease of 5 percent from the previous year and an increase
of 26 percent since 1990. This increase was due to an in
increase in production at the Mississippi facility, despite
the decrease in the percent of the facility's total reported
production that was used for commercial applications.
Table 4-42: C02 Emissions from C02 Consumption
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
2005
2006
2007
2008
1.3
1.7
1.9
1.8
1,321
1,709
1,867
1,780
14 There are currently four known electric power plants operating in the U.S.
that capture CO2 for use as food-grade CO2 or other industrial processes;
however, insufficient data prevents estimating emissions from these activities
as part of Carbon Dioxide Consumption.
4-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Methodology
Carbon dioxide emission estimates for 1990 through
2008 were based on production data for the two facilities
currently producing CO2 from naturally-occurring CO2
reservoirs for use in non-EOR applications. Some of the
CO2 produced by these facilities is used for EOR and some
is used in other commercial applications (e.g., chemical
manufacturing, food production). It is assumed that
100 percent of the CO2 production used in commercial
applications other than EOR is eventually released into the
atmosphere.
Carbon dioxide production data for the Jackson Dome,
Mississippi facility and the percentage of total production
that was used for EOR and in non-EOR applications were
obtained from the Advanced Resources Institute (ARI
2006, 2007) for 1990 to 2000 and from the Annual Reports
for Denbury Resources (Denbury Resources 2002 through
2009) for 2001 to 2007 (see Table 4-43). Denbury Resources
reported the average CO2 production in units of MMCF CO2
per day for 2001 through 2008 and reported the percentage of
the total average annual production that was used for EOR.
Carbon dioxide production data for the Bravo Dome, New
Mexico facility were obtained from the Advanced Resources
International, Inc. (ARI 1990 - 2009). The percentage of
total production that was used for EOR and in non-EOR
applications were obtained from the New Mexico Bureau of
Geology and Mineral Resources (Broadhead 2003 and New
Mexico Bureau of Geology and Mineral Resources 2006).
Uncertainty and Time-Series Consistency
Uncertainty is associated with the number of facilities
that are currently producing CO2 from naturally occurring
CO2 reservoirs for commercial uses other than EOR, and for
which the CO2 emissions are not accounted for elsewhere.
Research indicates that there are only two such facilities,
which are in New Mexico and Mississippi; however,
additional facilities may exist that have not been identified.
In addition, it is possible that CO2 recovery exists in particular
production and end-use sectors that are not accounted for
elsewhere. Such recovery may or may not affect the overall
estimate of CO2 emissions from that sector depending upon
the end use to which the recovered CO2 is applied. Further
research is required to determine whether CO2 is being
recovered from other facilities for application to end uses
that are not accounted for elsewhere.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-44. Carbon dioxide consumption
CO2 emissions were estimated to be between 1.4 and 2.3 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 24 percent below to 27 percent above
the emission estimate of 1.8 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Future improvements to the Carbon Dioxide
Consumption source category include research into CO2
capture for industrial purposes at electric power plants.
Currently, four plants have been identified that capture CO2
for these purposes, but insufficient data prevents including
them in the current emission estimate.
Table 4-43: C02 Production (Gg C02) and the Percent Used for Non-EOR Applications for Jackson Dome
and Bravo Dome
Year
Jackson Dome C02
Production (Gg)
Jackson Dome % Used
for Non-EOR
Bravo Dome C02
Production (Gg)
Bravo Dome % Used
for Non-EOR
1990
1,353
100%
6,301
2005
2006
2007
2008
4,678
6,610
9,529
12,312
27%
25%
19%
14%
5,799
5,613
5,605
5,605
1%
1%
1%
1%
Industrial Processes 4-31
-------
Table 4-44: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from C02 Consumption
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
C02 Consumption
CO,
1.8
1.4
2.3
-24%
+27%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4.12. Phosphoric Acid Production
(IPCC Source Category 2B5)
Phosphoric acid (H3PO4) is a basic raw material in the
production of phosphate-based fertilizers. Phosphate rock
is mined in Florida, North Carolina, Idaho, Utah, and other
areas of the United States and is used primarily as a raw
material for phosphoric acid production. The production of
phosphoric acid from phosphate rock produces byproduct
gypsum (CaSO4-2H2O), referred to as phosphogypsum.
The composition of natural phosphate rock varies
depending upon the location where it is mined. Natural
phosphate rock mined in the United States generally contains
inorganic C in the form of calcium carbonate (limestone) and
also may contain organic C. The chemical composition of
phosphate rock (francolite) mined in Florida is:
Ca10.x.y Nax Mgy (PO4)6.x(CO3)xF2+0.4x
The calcium carbonate component of the phosphate rock
is integral to the phosphate rock chemistry. Phosphate rock
can also contain organic C that is physically incorporated
into the mined rock but is not an integral component of the
phosphate rock chemistry. Phosphoric acid production from
natural phosphate rock is a source of CO2 emissions, due to
the chemical reaction of the inorganic C (calcium carbonate)
component of the phosphate rock.
The phosphoric acid production process involves
chemical reaction of the calcium phosphate (Ca3(PO4)2)
component of the phosphate rock with sulfuric acid (H2SO4)
and recirculated phosphoric acid (H3PO4) (EFMA2000). The
primary chemical reactions for the production of phosphoric
acid from phosphate rock are:
Ca3(PO4)2 + 4H3PO4 HX 3Ca(H2PO4)2
3Ca(H2PO4)2 + 3H2SO4 + 6H2O HX
3CaSO4 • 6H2O + 6H3PO4
The limestone (CaCO3) component of the phosphate rock
reacts with the sulfuric acid in the phosphoric acid production
process to produce calcium sulfate (phosphogypsum) and
CO2. The chemical reaction for the limestone-sulfuric acid
reaction is:
CaCO3 + H2SO4 + H2O HX CaSO4 • 2H2O + CO2
Total marketable phosphate rock production in 2008 was
30.2 million metric tons (USGS 2009). Approximately 87
percent of domestic phosphate rock production was mined in
Florida and North Carolina, while approximately 13 percent
of production was mined in Idaho and Utah. Data on the
2008 imports of phosphate rock were unavailable at the time
of publication. The 2007 value of 2.7 million metric tons
of crude phosphate rock was therefore assumed as the 2008
value. The vast majority, 99 percent, of imported phosphate
rock is sourced from Morocco (USGS 2005). Marketable
phosphate rock production, including domestic production
and imports for consumption, increased by approximately 4
percent between 2007 and 2008. However, over the 1990 to
2008 period, production has decreased by 24 percent. Total
CO2 emissions from phosphoric acid production were 1.2 Tg
CO2 Eq. (1,187 Gg) in 2008 (see Table 4-45).
Table 4-45: C02 Emissions from Phosphoric Acid
Production (Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
1.5
1,529
2005
2006
2007
2008
1.4
1.2
1.2
1.2
1,386
1,167
1,166
1,187
4-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Methodology
Carbon dioxide emissions from production of phosphoric
acid from phosphate rock are calculated by multiplying the
average amount of calcium carbonate contained in the natural
phosphate rock by the amount of phosphate rock that is used
annually to produce phosphoric acid, accounting for domestic
production and net imports for consumption.
The CO2 emissions calculation methodology is based on
the assumption that all of the inorganic C (calcium carbonate)
content of the phosphate rock reacts to CO2 in the phosphoric
acid production process and is emitted with the stack gas. The
methodology also assumes that none of the organic C content
of the phosphate rock is converted to CO2 and that all of the
organic C content remains in the phosphoric acid product.
From 1993 to 2004, the USGS Mineral Yearbook:
Phosphate Rock disaggregated phosphate rock mined
annually in Florida and North Carolina from phosphate
rock mined annually in Idaho and Utah, and reported the
annual amounts of phosphate rock exported and imported
for consumption (see Table 4-46). For the years 1990,1991,
1992, 2005, 2006, and 2007 only nationally aggregated
mining data was reported by USGS. For these years, the
breakdown of phosphate rock mined in Florida and North
Carolina, and the amount mined in Idaho and Utah, are
approximated using 1993 to 2004 data. Data for domestic
production of phosphate rock, exports of phosphate rock
(primarily from Florida and North Carolina), and imports
of phosphate rock for consumption for 1990 through 2007
were obtained from USGS Minerals Yearbook: Phosphate
/tod: (USGS 1994 through 2008). From 2004 through 2007,
the USGS reported no exports of phosphate rock from U.S.
producers (USGS 2005 through 2008). Since 2008 data were
unavailable at the time of publication, the 2007 values and
assumptions outlined above were used as approximates for
2008. The 2008 value for phosphate rock production was
available (USGS 2009).
The carbonate content of phosphate rock varies
depending upon where the material is mined. Composition
data for domestically mined and imported phosphate rock
were provided by the Florida Institute of Phosphate Research
(FIPR 2003). Phosphate rock mined in Florida contains
approximately 1 percent inorganic C, and phosphate rock
imported from Morocco contains approximately 1.46 percent
inorganic C. Calcined phosphate rock mined in North
Carolina and Idaho contains approximately 0.41 percent
and 0.27 percent inorganic C, respectively (see Table 4-47).
Carbonate content data for phosphate rock mined
in Florida are used to calculate the CO2 emissions from
consumption of phosphate rock mined in Florida and North
Carolina (87 percent of domestic production) and carbonate
content data for phosphate rock mined in Morocco are used
to calculate CO2 emissions from consumption of imported
phosphate rock. The CO2 emissions calculation is based
on the assumption that all of the domestic production of
phosphate rock is used in uncalcined form. As of 2006,
the USGS noted that one phosphate rock producer in Idaho
produces calcined phosphate rock; however, no production
data were available for this single producer (USGS 2006).
Carbonate content data for uncalcined phosphate rock mined
in Idaho and Utah (13 percent of domestic production) were
not available, and carbonate content was therefore estimated
from the carbonate
Uncertainty and Time-Series Consistency
Phosphate rock production data used in the emission
calculations were developed by the USGS through monthly
and semiannual voluntary surveys of the active phosphate
rock mines during 2008. For previous years in the time
Table 4-46: Phosphate Rock Domestic Production, Exports, and Imports (Gg)
Location
U.S. Production3
FL&NC
ID&UT
Exports— FL & NC
Imports — Morocco
Total U.S. Consumption
1990
49,800
42,4941
7,306
6,240
451
44,011
1995
43,720
38,100
5,620
2,760
1,800
42,760
2000
37,370
31,900
5,470
29g|
1,930
39,001
2005
36,100
31,227
4,874
2,630
38,730
2006
30,100
26,037
4,064
2,420
32,520
2007
29,700
25,691
4,010
2,670
32,370
2008
30,200
26,123
4,077
2,750
32,950
-Assumed equal to zero.
3 USGS does not disaggregate production data regionally (FL&NC and ID &UT) for 1990,2005,2006, and 2007. Data for those years are estimated based on
the remaining time series distribution.
Industrial Processes 4-33
-------
Table 4-47: Chemical Composition of Phosphate Rock (Percent by Weight)
Composition Central Florida
Total Carbon (as C)
Inorganic Carbon (as C)
Organic Carbon (as C)
Inorganic Carbon (as C02)
-Assumed equal to zero.
Source: FIPR (2003).
1.60
1.00
0.60
3.67
North Florida
1.76
0.93
0.83
3.43
North Carolina
(calcined)
0.76
0.41
0.35
1.50
Idaho
(calcined)
0.60
0.27
-
1.00
Morocco
1.56
1.46
0.10
5.00
series, USGS provided the data disaggregated regionally;
however, beginning in 2006 only total U.S. phosphate rock
production were reported. Regional production for 2007 was
estimated based on regional production data from previous
years and multiplied by regionally-specific emission factors.
Regional production for 2008 was not yet available at the
time of publication. There is uncertainty associated with the
degree to which the estimated 2007 regional production data
represents actual production in those regions. Total U.S.
phosphate rock production data are not considered to be a
significant source of uncertainty because all the domestic
phosphate rock producers report their annual production
to the USGS. Data for exports of phosphate rock used in
the emission calculation are reported by phosphate rock
producers and are not considered to be a significant source of
uncertainty. Data for imports for consumption are based on
international trade data collected by the U.S. Census Bureau.
These U.S. government economic data are not considered to
be a significant source of uncertainty.
An additional source of uncertainty in the calculation
of CO2 emissions from phosphoric acid production is the
carbonate composition of phosphate rock; the composition
of phosphate rock varies depending upon where the material
is mined, and may also vary over time. Another source of
uncertainty is the disposition of the organic C content of the
phosphate rock. A representative of the FIPR indicated that
in the phosphoric acid production process, the organic C
content of the mined phosphate rock generally remains in the
phosphoric acid product, which is what produces the color
of the phosphoric acid product (FIPR 2003a). Organic C is
therefore not included in the calculation of CO2 emissions
from phosphoric acid production.
A third source of uncertainty is the assumption that all
domestically-produced phosphate rock is used in phosphoric
acid production and used without first being calcined.
Calcination of the phosphate rock would result in conversion
of some of the organic C in the phosphate rock into CO2.
However, according to the USGS, only one producer in
Idaho is currently calcining phosphate rock, and no data were
available concerning the annual production of this single
producer (USGS 2005). For available years, total production
of phosphate rock in Utah and Idaho combined amounts to
approximately 13 percent of total domestic production on
average (USGS 1994 through 2005).
Finally, USGS indicated that approximately 7 percent
of domestically-produced phosphate rock is used to
manufacture elemental phosphorus and other phosphorus-
based chemicals, rather than phosphoric acid (USGS 2006).
According to USGS, there is only one domestic producer of
elemental phosphorus, in Idaho, and no data were available
concerning the annual production of this single producer.
Elemental phosphorus is produced by reducing phosphate
rock with coal coke, and it is therefore assumed that 100
percent of the carbonate content of the phosphate rock will
be converted to CO2 in the elemental phosphorus production
process. The calculation for CO2 emissions is based on the
assumption that phosphate rock consumption, for purposes
other than phosphoric acid production, results in CO2
emissions from 100 percent of the inorganic C content in
phosphate rock, but none from the organic C content.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-48. Phosphoric acid production
CO2 emissions were estimated to be between 1.0 and 1.4 Tg
CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 18 percent below and 19 percent
above the emission estimate of 1.2 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
4-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 4-48: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Phosphoric Acid Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Phosphoric Acid Production C02
1.2
1.0
1.4
-18%
+ 19%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Planned Improvements
Currently, data sources for the carbonate content of
the phosphate rock are limited. If additional data sources
are found, this information will be incorporated into future
estimates.
4.13. Iron and Steel Production
(IPCC Source Category 2C1) and
Metallurgical Coke Production
The production of iron and steel is an energy-intensive
process that also generates process-related emissions of CO2
and CH4. In the United States, steel is produced through
both primary and secondary processes. Historically, primary
production—based on the use of a basic oxygen furnace
(EOF) with pig iron as the primary feedstock—has been the
dominant method. But secondary production through the use
of electric arc furnaces (EAFs) has increased significantly in
recent years due to the economic advantages steel recycling,
which has been driven by the increased availability of scrap
steel. Total production of crude steel in the United States
increased steadily from approximately 47,116,000 tons in
2001 to 62,835,000 tons in 2007. But due to the decrease
in demand caused by the global economic downturn, crude
steel production in the United States decreased to 58,191,000
tons in 2008.
Metallurgical coke is used widely during the production
of iron and steel. The production of metallurgical coke from
coking coal occurs both on-site at "integrated" iron and steel
plants and off-site at "merchant" coke plants. Metallurgical
coke is produced by heating coking coal in a coke oven
in a low-oxygen environment. The process drives off the
volatile components of the coking coal and produces coal
(metallurgical) coke. Carbon containing byproducts of the
metallurgical coke manufacturing process include coke oven
gas, coal tar, coke breeze (small-grade coke oven coke with
particle size <5mm) and light oil. Coke oven gas is recovered
and used for underfiring the coke ovens and within the iron
and steel mill. Small amounts of coke oven gas are also sold
as synthetic natural gas outside of iron and steel mills (and
are accounted for in the Energy chapter). Coal tar is used as
a raw material to produce anodes used for primary aluminum
production, electric arc furnace (EAF) steel production, and
other electrolytic processes, and also is used in the production
of other coal tar products. Light oil is sold to petroleum
refiners who use the material as an additive for gasoline.
The metallurgical coke production process produces CO2
emissions and fugitive CH4 emissions.
Iron is produced by first reducing iron oxide (iron ore)
with metallurgical coke in a blast furnace to produce pig iron
(impure or crude iron containing about 3 to 5 percent carbon
by weight). Inputs to the blast furnace include natural gas,
fuel oil, and coke oven gas. The carbon in the metallurgical
coke used in the blast furnace combines with oxides in the
iron ore in a reducing atmosphere to produce blast furnace
gas containing carbon monoxide (CO) and CO2. The CO is
then converted and emitted as CO2 when combusted to either
pre-heat the blast air used in the blast furnace or for other
purposes at the steel mill. Iron may be introduced into the
blast furnace in the form of raw iron ore, pellets (9-16mm
iron-containing spheres), briquettes, or sinter. Pig iron is used
as a raw material in the production of steel, which contains
about 1 percent carbon by weight. Pig iron is also used as a
raw material in the production of iron products in foundries.
The pig iron production process produces CO2 emissions
and fugitive CH4 emissions.
Iron can also be produced through the direct reduction
process; wherein, iron ore is reduced to metallic iron in the
solid state at process temperatures less than 1000°C. Direct
reduced iron production results in process emissions of CO2
and emissions of CH4 through the consumption of natural
gas used during the reduction process.
Industrial Processes 4-35
-------
Sintering is a thermal process by which fine iron-
bearing particles, such as air emission control system dust,
are baked, which causes the material to agglomerate into
roughly one-inch pellets that are then recharged into the blast
furnace for pig iron production. Iron ore particles may also
be formed into larger pellets or briquettes by mechanical
means, and then agglomerated by heating. The agglomerate
is then crushed and screened to produce an iron-bearing
feed that is charged into the blast furnace. The sintering
process produces CO2 and fugitive CH4 emissions through
the consumption of carbonaceous inputs (e.g., coke breeze)
during the sintering process.
Steel is produced from varying levels of pig iron and
scrap steel in specialized EOF and EAF steel-making
furnaces. Carbon inputs to EOF steel-making furnaces
include pig iron and scrap steel as well as natural gas, fuel oil,
and fluxes (e.g., limestone, dolomite). In a EOF, the carbon
in iron and scrap steel combines with high-purity oxygen to
reduce the carbon content of the metal to the amount desired
for the specified grade of steel. EAFs use carbon electrodes,
charge carbon and other materials (e.g., natural gas) to aid
in melting metal inputs (primarily recycled scrap steel),
which are refined and alloyed to produce the desired grade of
steel. Carbon dioxide emissions occur in BOFs through the
reduction process. In EAFs, CO2 emissions result primarily
from the consumption of carbon electrodes and also from the
consumption of supplemental materials used to augment the
melting process.
In addition to the production processes mentioned above,
CO2 is also generated at iron and steel mills through the
consumption of process by-products (e.g., blast furnace gas,
coke oven gas) used for various purposes including heating,
annealing, and electricity generation.15 Process by-products
sold for use as synthetic natural gas are deducted and reported
in the Energy chapter (emissions associated with natural gas
and fuel oil consumption for these purposes are reported in
the Energy chapter).
15 Emissions resulting from fuel consumption for the generation of
electricity are reported in the Energy chapter. Some integrated iron and
steel mills have on-site electricity generation for which fuel is used. Data
are not available concerning the amounts and types of fuels used in iron
and steel mills to generate electricity. Therefore all of the fuel consumption
reported at iron and steel mills is assumed to be used within the iron and
steel mills for purposes other than electricity consumption, and the amounts
of any fuels actually used to produce electricity at iron and steel mills are
not subtracted from the electricity production emissions value used in the
Energy chapter, therefore some double-counting of electricity-related CO2
emissions may occur.
The majority of CO2 emissions from the iron and steel
production process come from the use of metallurgical coke
in the production of pig iron and from the consumption of
other process by-products at the iron and steel mill, with
smaller amounts evolving from the use of flux and from the
removal of carbon from pig iron used to produce steel. Some
carbon is also stored in the finished iron and steel products.
According to the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006), the production
of metallurgical coke from coking coal is considered to be
an energy use of fossil fuel and the use of coke in iron and
steel production is considered to be an industrial process
source. Therefore, the Guidelines suggest that emissions
from the production of metallurgical coke should be reported
separately in the Energy source, while emissions from coke
consumption in iron and steel production should be reported
in the industrial process source. However, the approaches and
emission estimates for both metallurgical coke production
and iron and steel production are both presented here
because the activity data used to estimate emissions from
metallurgical coke production have significant overlap with
activity data used to estimate iron and steel production
emissions. Further, some by-products (e.g., coke oven gas)
of the metallurgical coke production process are consumed
during iron and steel production, and some by-products of the
iron and steel production process (e.g., blast furnace gas) are
consumed during metallurgical coke production. Emissions
associated with the consumption of these by-products are
attributed to point of consumption. As an example, CO2
emissions associated with the combustion of coke oven
gas in the blast furnace during pig iron production are
attributed to pig iron production. Emissions associated with
fuel consumption downstream of the iron and steelmaking
furnaces, such as natural gas used for heating and annealing
purposes, are reported in the Energy chapter.
Metallurgical Coke Production
Emissions of CO2 and CH4 from metallurgical coke
production in 2008 were 5.3 Tg CO2 Eq. (5,281 Gg) and
less than 0.05 Tg CO2 Eq. (less than 0.5 Gg), respectively
(see Table 4-49 and Table 4-50), totaling 5.3 Tg CO2 Eq.
Emissions increased in 2008, but have decreased overall
since 1990. In 2008, domestic coke production decreased
by 3.4 percent and has decreased overall since 1990. Coke
production in 2008 was 25 percent lower than in 2000 and 43
percent below 1990. Overall, emissions from metallurgical
4-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-49: C02 and CH4 Emissions from Metallurgical Coke Production (Tg C02 Eq.)
Gas
C02
CH4
Total
1990
5.5
+
5.5
1995
5.0
+
5.0 |
2000
4.4
+ 1
4.4
2005
3.8
+
3.8
2006
3.7
+
3.7
2007
3.8
+
3.8
2008
5.3
+
5.3
+ Does not exceed 0.05 Tg C02 Eq.
Table 4-50: C02 and CH4 Emissions from Metallurgical Coke Production (Gg)
Gas 1990 1995 2000 2005 2006 2007 2008
C02 5,498 5,037 4,381 3,849 3,682 3,806 5,281
ChJ4 + + + + + + +
+ Does not exceed 0.5 Gg.
coke production have declined by 4 percent (0.2 Tg CO2 Eq.) include emissions from the consumption of carbonaceous
from 1990 to 2008. materials in the blast furnace, EAF, and EOF as well as
blast furnace gas and coke oven gas consumption for other
Iron and Steel Production activities at the steel mill.
Emissions of CO2 and CH4 from iron and steel
In 2008, domestic production of pig iron decreased by 7
production in 2008 were 63.7 Tg CO2 Eq. (63,729 Gg) and
percent. Overall, domestic pig iron production has declined
0.6 Tg CO2 Eq. (30.8 Gg), respectively (see Table 4-51
since the 1990s. Pig iron production in 2008 was 30 percent
through Table 4-54), totaling 64.4 Tg CO? Eq. Emissions
lower than in 2000 and 32 percent below 1990. While CO2
decreased in 2008 and have decreased overall since 1990 due
emissions from steel production have increased by 2 percent
to restructuring of the industry, technological improvements,
(0.1 Tg CO2 Eq.) since 1990, overall CO2 emissions from
and increased scrap steel utilization. CO2 emission estimates
Table 4-51: C02 Emissions from Iron and Steel Production (Tg C02 Eq.)
Process
Sinter Production
Iron Production
Steel Production
Other Activities3
Total
1990
2.4l
47.9
7.5
39.3
97.1
1995
2.5
38.8
8.6 1
40.9 •
90.7 1
2000 •
2.2
33.8
7.9
39.9
83.7 1
2005
1.7
19.6
8.5
34.2
63.9
2006
1.4
23.9
8.9
32.6
66.9
2007
1.4
27.3
9.4
31.0
69.0
2008
1.3
25.7
7.6
29.1
63.7
a Includes emissions from blast furnace gas and coke oven gas combustion for activities at the steel mill other than consumption in blast furnace,
EAFs.orBOFs.
Note: Totals may not sum due to independent rounding.
Table 4-52: C02 Emissions from Iron and Steel Production (Gg)
Process
Sinter Production
Iron Production
Steel Production
Other Activities3
Total
1990
2,448
47,886
7,476
39,256
97,066
1995
2,512
38,791
8,557
40,850
90,711
2000
2,158
33,808
7,885
39,877
83,728
2005
1,663
19,576
8,491
34,152
63,882
2006
1,418
23,931
8,925
32,583
66,857
2007
1,383
27,265
9,384
30,964
68,996
2008
1,299
25,699
7,594
29,137
63,729
a Includes emissions from blast furnace gas and coke oven gas combustion for activities at the steel mill other than consumption in blast furnace,
EAFs.orBOFs.
Note: Totals may not sum due to independent rounding.
Industrial Processes 4-37
-------
Table 4-53: CH4 Emissions from Iron and Steel Production (Tg C02 Eq.)
Process
1990
1995
2000
2005
2006
2007
2008
Sinter Production
Iron Production
0.9
+
0.7
+
0.7
+
0.7
0.6
Total
1.0
1.0
0.9
0.7
0.7
0.7
0.6
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Table 4-54: CH4 Emissions from Iron and Steel Production (Gg)
Process
Sinter Production
Iron Production
Total
1990
0.9
44.7
45.6
1995
0.9
45.8 1
46.7
2000
•„,
43.8 |
2005
0.6
33.5
34.1
2006
0.5
34.1
34.6
2007
0.5
32.7
33.2
2008
0.4
30.4
30.8
Note: Totals may not sum due to independent rounding.
iron and steel production have declined by 34 percent (33.3
Tg CO2 Eq.) from 1990 to 2008.
Methodology
Emission estimates presented in this chapter are based on
the methodologies provided by the 2006IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC 2006), which
call for a mass balance accounting of the carbonaceous inputs
and outputs during the iron and steel production process and
the metallurgical coke production process.
Metallurgical Coke Production
Coking coal is used to manufacture metallurgical
(coal) coke that is used primarily as a reducing agent in
the production of iron and steel, but is also used in the
production of other metals including lead and zinc (see Lead
Production and Zinc Production in this chapter). Emissions
associated with producing metallurgical coke from coking
coal are estimated and reported separately from emissions
that result from the iron and steel production process. To
estimate emission from metallurgical coke production, a Tier
2 method provided by the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006) was utilized. The
amount of carbon contained in materials produced during
the metallurgical coke production process (i.e., coke, coke
breeze, coke oven gas, and coal tar) is deducted from the
amount of carbon contained in materials consumed during the
metallurgical coke production process (i.e., natural gas, blast
furnace gas, coking coal). Light oil, which is produced during
the metallurgical coke production process, is excluded from
the deductions due to data limitations. The amount of carbon
contained in these materials is calculated by multiplying the
material-specific carbon content by the amount of material
consumed or produced (see Table 4-55). The amount of coal
tar produced was approximated using a production factor
of 0.03 tons of coal tar per ton of coking coal consumed.
The amount of coke breeze produced was approximated
using a production factor of 0.075 tons of coke breeze per
ton of coking coal consumed. Data on the consumption of
carbonaceous materials (other than coking coal) as well as
coke oven gas production were available for integrated steel
mills only (i.e., steel mills with co-located coke plants).
Therefore, carbonaceous material (other than coking coal)
consumption and coke oven gas production were excluded
from emission estimates for merchant coke plants. Carbon
contained in coke oven gas used for coke-oven undernring
was not included in the deductions to avoid double-counting.
Table 4-55: Material Carbon Contents for
Metallurgical Coke Production
Material
kg C/kg
Coal Tar
Coke
Coke Breeze
Coking Coal
0.62
0.83
0.83
0.73
Material
kg C/GJ
Coke Oven Gas
Blast Furnace Gas
12.1
70.8
Source: IPCC (2006), Table 4.3. Coke Oven Gas and Blast Furnace Gas,
Table 1.3.
4-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-56: Production and Consumption Data for the Calculation of C02 and CH4 Emissions from Metallurgical
Coke Production (Thousand Metric Tons)
Source/Activity Data
Metallurgical Coke Production
Coking Coal Consumption at Coke Plants
Coke Production at Coke Plants
Coke Breeze Production
Coal Tar Production
1990
35,269
25,054
1,879
752
1995
1,616
646
2000
26,254
18,877
1,416
566
2005
21,259
15,167
1,138
455
2006
20,827
14,882
1,116
446
2007
20,607
14,698
1,102
441
2008
20,477
14,194
1,065
426
Table 4-57: Production and Consumption Data for the Calculation of C02 Emissions from Metallurgical
Coke Production (million ft3)
Source/Activity Data
Metallurgical Coke Production
Coke Oven Gas Production3
Natural Gas Consumption
Blast Furnace Gas Consumption
1990
250,767
599
24,602
1995
166,750 1
184
29,423
2000
149,4771
180
26,075
2005
114,213
2,996
4,460
2006
114,386
3,277
5,505
2007
109,912
3,309
5,144
2008
103,191
3,134
4,829
"Includes coke oven gas used for purposes other than coke oven underfiring only.
The production processes for metallurgical coke
production results in fugitive emissions of CH4, which are
emitted via leaks in the production equipment rather than
through the emission stacks or vents of the production plants.
The fugitive emissions were calculated by applying Tier
1 emission factors (0.1 g CH4 per metric ton) taken from
the 2006 IPCC Guidelines for National Greenhouse Gas
Inventories (IPCC 2006) for metallurgical coke production.
Data relating to the mass of coking coal consumed at
metallurgical coke plants and the mass of metallurgical
coke produced at coke plants were taken from the Energy
Information Administration (EIA), Quarterly Coal Report
October through December (EIA 1998 through 2004) and
January through March (EIA 2006,2007, 2008a, 2009) (see
Table 4-56). Data on the volume of natural gas consumption,
blast furnace gas consumption, and coke oven gas production
for metallurgical coke production at integrated steel mills
were obtained from the American Iron and Steel Institute
(AISI), Annual Statistical Report (AISI2004 through 2009)
and through personal communications with AISI (2008b)
(see Table 4-57). The factor for the quantity of coal tar
produced per ton of coking coal consumed was provided
by AISI (2008b). The factor for the quantity of coke breeze
produced per ton of coking coal consumed was obtained
through Table 2-1 of the report Energy and Environmental
Profile of the U.S. Iron and Steel Industry (DOE 2000). Data
on natural gas consumption and coke oven gas production at
merchant coke plants were not available and were excluded
from the emission estimate. Carbon contents for coking coal,
metallurgical coke, coal tar, coke oven gas, and blast furnace
gas were provided by the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006). The C content
for coke breeze was assumed to equal the C content of coke.
Iron and Steel Production
Emissions of CO2 from sinter production and direct
reduced iron production were estimated by multiplying
total national sinter production and the total national direct
reduced iron production by Tier 1 CO2 emission factors (see
Table 4-58). Because estimates of sinter production and
direct reduced iron production were not available, production
was assumed to equal consumption.
To estimate emissions from pig iron production in the
blast furnace, the amount of C contained in the produced pig
iron and blast furnace gas were deducted from the amount of
C contained in inputs (i.e., metallurgical coke, sinter, natural
ore, pellets, natural gas, fuel oil, coke oven gas, direct coal
injection). The C contained in the pig iron, blast furnace
gas, and blast furnace inputs was estimated by multiplying
the material-specific carbon content by each material type
(see Table 4-59). Carbon in blast furnace gas used to pre-
heat the blast furnace air is combusted to form CO2 during
this process.
Industrial Processes 4-39
-------
Table 4-58: C02 Emission Factors for Sinter Production
and Direct Reduced Iron Production
Material Produced
Metric Ton C02/Metric Ton
Sinter
Direct Reduced Iron
0.2
0.7
Source: IPCC (2006), Table 4.1.
Emissions from steel production in EAFs were estimated
by deducting the C contained in the steel produced from
the carbon contained in the EAF anode, charge carbon, and
scrap steel added to the EAF. Small amounts of C from
direct reduced iron, pig iron, and flux additions to the EAFs
were also included in the EAF calculation. For BOFs,
estimates of C contained in EOF steel were deducted from
carbon contained in inputs such as natural gas, coke oven
gas, fluxes, and pig iron. In each case, the C was calculated
by multiplying material-specific carbon contents by each
material type (see Table 4-59). For EAFs, the amount of EAF
anode consumed was approximated by multiplying total EAF
steel production by the amount of EAF anode consumed per
metric ton of steel produced (0.002 metric tons EAF anode
per metric ton steel produced (AISI 2008b)). The amount
of flux (e.g., limestone and dolomite) used during steel
manufacture was deducted from the Limestone and Dolomite
Use source category to avoid double-counting.
Carbon dioxide emissions from the consumption of blast
furnace gas and coke oven gas for other activities occurring
at the steel mill were estimated by multiplying the amount of
these materials consumed for these purposes by the material-
specific C content (see Table 4-59).
Carbon dioxide emissions associated with the sinter
production, direct reduced iron production, pig iron
production, steel production, and other steel mill activities
were summed to calculate the total CO2 emissions from iron
and steel production (see Table 4-51 and Table 4-52).
The production processes for sinter and pig iron result in
fugitive emissions of CH4, which are emitted via leaks in the
production equipment rather than through the emission stacks
or vents of the production plants. The fugitive emissions
were calculated by applying Tier 1 emission factors taken
from the 2006 IPCC Guidelines for National Greenhouse
Gas Inventories (IPCC 2006) for sinter production and the
1995 IPCC Guidelines (IPCC/UNEP/OECD/IEA1995) (see
Table 4-60) for pig iron production. The production of direct
reduced iron also results in emissions of CH4 through the
consumption of fossil fuels (e.g., natural gas); however, these
emissions estimates are excluded due to data limitations.
Sinter consumption and direct reduced iron consumption
data were obtained from AISI's Annual Statistical
Report (AISI 2004 through 2009) and through personal
communications with AISI (2008b) (see Table 4-61). Data
on direct reduced iron consumed in EAFs were not available
for the years 1990,1991,1999, 2006, 2007 and 2008. EAF
direct reduced iron consumption in 1990 and 1991 were
assumed to equal consumption in 1992, and consumption in
1999 was assumed to equal the average of 1998 and 2000.
EAF consumption in 2006, 2007, and 2008 were calculated
by multiplying the total DRI consumption for all furnaces as
provided in the 2008 AISI Annual Statistical Report by the
EAF share of total DRI consumption in 2005 (the most recent
year that data was available for EAF vs. EOF consumption
of DRI). Data on direct reduced iron consumed in BOFs
were not available for the years 1990 through 1994, 1999,
2006,2007 and 2008. EOF direct reduced iron consumption
in 1990 through 1994 was assumed to equal consumption
in 1995, and consumption in 1999 was assumed to equal
Table 4-59: Material Carbon Contents for Iron
and Steel Production
Material
Coke
Direct Reduced Iron
Dolomite
EAF Carbon Electrodes
EAF Charge Carbon
Limestone
Pig Iron
Steel
Material
Coke Oven Gas
Blast Furnace Gas
kg C/kg
0.83
0.02
0.13
0.82
0.83
0.12
0.04
0.01
kg C/GJ
12.1
70.8
Source: IPCC (2006), Table 4.3. Coke Oven Gas and Blast Furnace Gas,
Table 1.3.
Table 4-60: CH4 Emission Factors for Sinter and
Pig Iron Production
Material Produced
Factor
Unit
Pig Iron
Sinter
0.9
0.07
g CH^/kg
kg CHymetric ton
Source: Sinter (IPCC 2006, Table 4.2), Pig Iron (IPCC/UNEP/OECD/IEA
1995, Table 2.2).
4-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
the average of 1998 and 2000. EOF consumption in 2006,
2007, and 2008 were calculated by multiplying the total
DRI consumption for all furnaces as provided in the 2008
AISI Annual Statistical Report by the EOF share of total
DRI consumption in 2005 (the most recent year that data
was available for EAF vs. EOF consumption of DRI). The
Tier 1 CO2 emission factors for sinter production and direct
reduced iron production were obtained through the 2006
IPCC Guidelines for National Greenhouse Gas Inventories
(IPCC 2006). Data for pig iron production, coke, natural
gas, fuel oil, sinter, and pellets consumed in the blast furnace;
pig iron production; and blast furnace gas produced at the
iron and steel mill and used in the metallurgical coke ovens
and other steel mill activities were obtained from AISFs
Annual Statistical Report (AISI 2004 through 2009) and
through personal communications with AISI (2008b) (see
Table 4-62). Data for EAF steel production, flux, EAF
charge carbon, direct reduced iron, pig iron, scrap steel, and
natural gas consumption as well as EAF steel production
were obtained from AISFs Annual Statistical Report (AISI
2004 through 2009) and through personal communications
with AISI (2008b). The factor for the quantity of EAF anode
consumed per ton of EAF steel produced was provided by
AISI (AISI 2008b). Data for EOF steel production, flux,
direct reduced iron, pig iron, scrap steel, natural gas, natural
ore, pellet sinter consumption as well as EOF steel production
were obtained from AISFs Annual Statistical Report (AISI
2004 through 2009) and through personal communications
with AISI (2008b). Because data on pig iron consumption
and scrap steel consumption in BOFs and EAFs were not
available for 2006, 2007, and 2008, values for these years
were calculated by multiplying the total pig iron and scrap
steel consumption for all furnaces as provided in the 2008
AISI Annual Statistical Reportby the EOF and EAF shares of
total pig iron and scrap consumption in 2005 (the most recent
year that data was available for EAF vs. EOF consumption of
pig iron and scrap steel). Because pig iron consumption in
EAFs was also not available in 2003 and 2004, the average
of 2002 and 2005 pig iron consumption data were used. Data
on coke oven gas and blast furnace gas consumed at the iron
and steel mill other than in the EAF, EOF, or blast furnace
were obtained from AISFs Annual Statistical Report (AISI
2004 through 2009) and through personal communications
with AISI (2008b). Data on blast furnace gas and coke
oven gas sold for use as synthetic natural gas were obtained
through EIAs Natural Gas Annual 2007 (EIA 2008b).
As 2008 data were not available, 2007 data were used. C
contents for direct reduced iron, EAF carbon electrodes,
Table 4-61: Production and Consumption Data for the Calculation of C02 and CH4 Emissions from
Iron and Steel Production (Thousand Metric Tons)
Source/Activity Data
Sinter Production
Sinter Production
Direct Reduced Iron Production
Direct Reduced Iron Production
Pig Iron Production
Coke Consumption
Pig Iron Production
Direct Injection Coal Consumption
EAF Steel Production
EAF Anode and Charge Carbon Consumption
Scrap Steel Consumption
Flux Consumption
EAF Steel Production
BOF Steel Production
Pig Iron Consumption
Scrap Steel Consumption
Flux Consumption
BOF Steel Production
Blast Furnace Gas Production 1
1990
12
24:
,239
936
49,669
1
35
33
46
14
43
,439
,485
,743
319
,511
,564
,548
576
,973
,380 1
1995
12,562
989
22,198
50,891
1,509
77
39,010
267
38,472
49,896
15,967
1,259
56,721
,559,795
2000
1
10,
1,
19,
47,
3,
43,
47,
46,
14,
53,
,524,
788
914
215
888
012
96
001
654
860
993
969
978
965
891
2005
I*
1
13,
37:
2:
r
37:
1
32:
11:
1
1,299:
315
633
832
222
573
127
558
695
194
115
612
582
705
980
2006
7,088
1,497
14,684
37,904
2,526
1,245
38,033
671
56,071
32,638
11,759
610
42,119
1,236,526
2007
6,914
2,087
15,039
36,337
2,734
1,214
40,845
567
57,004
33,773
12,628
408
41,099
1,173,588
2008
6,497
1,769
14,251
33,730
2,578
1,109
38,414
680
52,791
29,322
11,877
431
39,105
1,104,674
Industrial Processes 4-41
-------
Table 4-62: Production and Consumption Data for the Calculation of C02 Emissions from Iron and Steel Production
(million ft3 unless otherwise specified)
Source/Activity Data
1990
1995
2000
2005
2006
2007
2008
Pig Iron Production
Natural Gas Consumption 56,273 106,5141
Fuel Oil Consumption (thousand gallons) 163,397 108,196
Coke Oven Gas Consumption 22,033 10,097
Blast Furnace Gas Production 1,439,380 1,559,795
EAF Steel Production
Natural Gas Consumption 9,604 11,026
BOF Steel Production
Natural Gas Consumption 6,301 16,546
Coke Oven Gas Consumption 3,851 1,284
Other Activities
Coke Oven Gas Consumption 224,8831 155,369
Blast Furnace Gas Consumption 1,414,778 1,530,372
91,798 59,844 58,344 56,112 53,349
120,921 16,170 87,702 84,498 55,552
13,702 16,557 16,649 16,239 15,336
,524,891 1,299,980 1,236,526 1,173,588 1,104,674
13,717 14,959 16,070 16,337 15,130
6,143 5,026 5,827 11,740 -4,304b
6401 524 559 525 528
135,135 97,132 97,178 93,148 87,327
,498,816 1,295,520 1,231,021 1,168,444 1,099,845
a Includes blast furnace gas used for purposes other than in the blast furnace only.
b EPA is continuing to investigate this value.
EAF charge carbon, limestone, dolomite, pig iron, and steel
were provided by the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006). The C contents
for natural gas, fuel oil, and direct injection coal as well as
the heat contents for the same fuels were provided by EIA
(1992, 2008b, 2009). Heat contents for coke oven gas and
blast furnace gas were provided in Table 2-2 of the report
Energy and Environmental Profile of the U.S. Iron and Steel
Industry (DOE 2000).
Uncertainty and Time-Series Consistency
The estimates of CO2 and CH4 emissions from
metallurgical coke production are based on material
production and consumption data and average carbon
contents. Uncertainty is associated with the total U.S.
coking coal consumption, total U.S. coke production and
materials consumed during this process. Data for coking
coal consumption and metallurgical coke production are from
different data sources (EIA) than data for other carbonaceous
materials consumed at coke plants (AISI), which does not
include data for merchant coke plants. There is uncertainty
associated with the fact that coal tar and coke breeze
production were estimated based on coke production because
coal tar and coke breeze production data were not available.
Since merchant coke plant data is not included in the estimate
of other carbonaceous materials consumed at coke plants,
the mass balance equation for CO2 from metallurgical coke
production cannot be reasonably completed. Therefore,
for the purpose of this analysis, uncertainty parameters are
applied to primary data inputs to the calculation (i.e, coking
coal consumption and metallurgical coke production) only.
The estimates of CO2 emissions from iron and
steel production are based on material production and
consumption data and average carbon contents. There
is uncertainty associated with the assumption that direct
reduced iron and sinter consumption are equal to production.
There is uncertainty associated with the assumption that
all coal used for purposes other than coking coal is for
direct injection coal. Some of this coal may be used for
electricity generation. There is also uncertainty associated
with the carbon contents for pellets, sinter, and natural ore,
which are assumed to equal the carbon contents of direct
reduced iron. For EAF steel production there is uncertainty
associated with the amount of EAF anode and charge carbon
consumed due to inconsistent data throughout the timeseries.
Uncertainty is also associated with the use of process gases
such as blast furnace gas and coke oven gas. Data are not
available to differentiate between the use of these gases for
processes at the steel mill versus for energy generation (e.g.,
electricity and steam generation); therefore, all consumption
is attributed to iron and steel production. These data and
carbon contents produce a relatively accurate estimate of
CO2 emissions. However, there are uncertainties associated
with each.
4-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-63: Tier 2 Quantitative Uncertainty Estimates for C02 and CH4 Emissions from Iron and Steel Production
(Tg C02 Eq. and Percent)3
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate11
(TgC02Eq.) ' (%)
Metallurgical Coke Production
Metallurgical Coke Production
Iron and Steel Production
Iron and Steel Production
C02
CH4
C02
CH4
5.3
+
63.7
0.6
Lower Bound
2.8
+
62.3
0.5
Upper Bound
20.6
+
76.5
0.8
Lower Bound
-47%
-21%
-2%
-21%
Upper Bound
+289%
+23%
+20%
+22%
+ Does not exceed 0.05 Tg C02 Eq.
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
For the purposes of the CH4 calculation from iron and
steel production it is assumed that all of the CH4 escapes as
fugitive emissions and that none of the CH4 is captured in
stacks or vents. Additionally, the CO2 emissions calculation
is not corrected by subtracting the C content of the CH4,
which means there may be a slight double counting of C as
both CO2 and CH4.
For both the CO2 and CH4 calculations for iron and steel
production, it is assumed that the uncertainty associated with
metallurgical coke production does not impact iron and steel
production.
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 4-63 for metallurgical coke
production and iron and steel production. Metallurgical Coke
Production CO2 emissions were estimated to be between
1.3 and 9.3 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 47 percent below
and 289 percent above the emission estimate of 5.3 Tg CO2
Eq. Metallurgical Coke Production CH4 emissions were
estimated to be less than 0.05 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of approximately 21
percent below and 23 percent above the emission estimate
of less than 0.05 Tg CO2 Eq. Iron and Steel Production CO2
emissions were estimated to be between 62.3 and 76.3 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 2 percent below and 20 percent above
the emission estimate of 63.7 Tg CO2 Eq. Iron and Steel
Production CH4 emissions were estimated to be between 0.5
Tg CO2 Eq. and 0.8 Tg CO2 Eq. at the 95 percent confidence
level. This indicates a range of approximately 21 percent
below and 22 percent above the emission estimate of 0.6
Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Plans for improvements to the Iron and Steel Production
source category include attributing emissions estimates for
the production of metallurgical coke to the Energy chapter
as well as identifying the amount of carbonaceous materials,
other than coking coal, consumed at merchant coke plants.
Additional improvements include identifying the amount of
coal used for direct injection and the amount of coke breeze,
coal tar, and light oil produced during coke production.
Efforts will also be made to identify inputs for preparing
Tier 2 estimates for sinter and direct reduced iron production,
as well as identifying information to better characterize
emissions from the use of process gases and fuels within the
Energy and Industrial Processes chapters.
Recalculations Discussion
In last year's Inventory, pig iron consumption for BOFs
was being counted twice as a process input. This was the
result of an incorrect interpretation of two tables in the AISI
Annual Statistical Yearbook. This issue has been corrected
and decreased the 1990 through 2007 emissions from iron
and steel production by an average of 8 percent per year
relative to the previous Inventory.
Industrial Processes 4-43
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4.14. Ferroalloy Production (IPCC
Source Category 2C2)
Carbon dioxide and CH4 are emitted from the production
of several ferroalloys. Ferroalloys are composites of iron and
other elements such as silicon, manganese, and chromium.
When incorporated in alloy steels, ferroalloys are used to
alter the material properties of the steel. Estimates from
two types of ferrosilicon (25 to 55 percent and 56 to 95
percent silicon), silicon metal (about 98 percent silicon), and
miscellaneous alloys (36 to 65 percent silicon) have been
calculated. Emissions from the production of ferrochromium
and ferromanganese are not included here because of the
small number of manufacturers of these materials in the
United States. Subsequently, government information
disclosure rules prevent the publication of production data
for these production facilities.
Similar to emissions from the production of iron and
steel, CO2 is emitted when metallurgical coke is oxidized
during a high-temperature reaction with iron and the selected
alloying element. Due to the strong reducing environment,
CO is initially produced, and eventually oxidized to CO2.
A representative reaction equation for the production of 50
percent ferrosilicon is given below:
Fe2O3 + 2SiO2 + 1C -» 2FeSi + 7CO
While most of the C contained in the process materials
is released to the atmosphere as CO2, a percentage is also
released as CH4 and other volatiles. The amount of CH4
that is released is dependent on furnace efficiency, operation
technique, and control technology.
Emissions of CO2 from ferroalloy production in 2008
were 1.6 Tg CO2 Eq. (1,599 Gg) (see Table 4-64 and Table
4-65), which is a three percent increase from the previous
year and a 26 percent reduction since 1990. Emissions of
CH4 from ferroalloy production in 2008 were 0.01 Tg CO2
Eq. (0.465 Gg), which is a four percent increase from the
previous year and a 32 percent decrease since 1990.
Methodology
Emissions of CO2 and CH4 from ferroalloy production
were calculated by multiplying annual ferroalloy production
by material-specific emission factors. Emission factors taken
from the 2006 IPCC Guidelines for National Greenhouse
Gas Inventories (IPCC 2006) were applied to ferroalloy
production. For ferrosilicon alloys containing 25 to 55
percent silicon and miscellaneous alloys (including primarily
magnesium-ferrosilicon, but also including other silicon
alloys) containing 32 to 65 percent silicon, an emission factor
for 45 percent silicon was applied for CO2 (2.5 metric tons
CO2/metric ton of alloy produced) and an emission factor
for 65 percent silicon was applied for CH4 (1 kg CH4/metric
ton of alloy produced). Additionally, for ferrosilicon alloys
containing 56 to 95 percent silicon, an emission factor for
75 percent silicon ferrosilicon was applied for both CO2
and CH4 (4 metric tons CO2/metric ton alloy produced and
1 kg CH4/metric ton of alloy produced, respectively). The
emission factors for silicon metal equaled 5 metric tons CO2/
metric ton metal produced and 1.2 kg CH4/metric ton metal
produced. It was assumed that 100 percent of the ferroalloy
production was produced using petroleum coke using an
electric arc furnace process (IPCC 2006), although some
Table 4-64: C02 and CH4 Emissions from Ferroalloy Production (Tg C02 Eq.)
Gas
1990
1995
2000
2005
2006
2007
2008
C02
CH4
Total
1.4
1.5
1.6
1.6
2.0
1.9
1.4
1.5
1.6
1.6
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Table 4-65: C02 and CH4 Emissions from Ferroalloy Production (Gg)
Gas
1990
1995
2000
2005
2006
2007
2008
C02
CH4
2,152
1
2,036
1
1,893
1
1,392 1,505
1,552
1,599
+ Does not exceed 0.5 Gg.
4-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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ferroalloys may have been produced with coking coal, wood,
other biomass, or graphite C inputs. The amount of petroleum
coke consumed in ferroalloy production was calculated
assuming that the petroleum coke used is 90 percent C and
10 percent inert material.
Ferroalloy production data for 1990 through 2008
(see Table 4-66) were obtained from the USGS through
personal communications with the USGS Silicon Commodity
Specialist (Corathers 2009) and through the Minerals
Yearbook: Silicon Annual Report (USGS 1991 through
2009). Because USGS does not provide estimates of silicon
metal production for 2006-2008, 2005 production data
are used. Until 1999, the USGS reported production of
ferrosilicon containing 25 to 55 percent silicon separately
from production of miscellaneous alloys containing 32 to 65
percent silicon; beginning in 1999, the USGS reported these
as a single category (see Table 4-66). The composition data
for petroleum coke was obtained from Onder and Bagdoyan
(1993).
Uncertainty and Time-Series Consistency
Although some ferroalloys may be produced using
wood or other biomass as a C source, information and data
regarding these practices were not available. Emissions from
ferroalloys produced with wood or other biomass would not
be counted under this source because wood-based C is of
biogenic origin.16 Even though emissions from ferroalloys
produced with coking coal or graphite inputs would be
counted in national trends, they may be generated with
varying amounts of CO2 per unit of ferroalloy produced.
The most accurate method for these estimates would be to
base calculations on the amount of reducing agent used in
the process, rather than the amount of ferroalloys produced.
These data, however, were not available.
Emissions of CH4 from ferroalloy production will vary
depending on furnace specifics, such as type, operation
technique, and control technology. Higher heating
temperatures and techniques such as sprinkle charging will
reduce CUt emissions; however, specific furnace information
was not available or included in the CH4 emission estimates.
Also, annual ferroalloy production is now reported by
the USGS in three broad categories: ferroalloys containing
25 to 55 percent silicon (including miscellaneous alloys),
ferroalloys containing 56 to 95 percent silicon, and silicon
metal. It was assumed that the IPCC emission factors apply
to all of the ferroalloy production processes, including
miscellaneous alloys. Finally, production data for silvery
pig iron (alloys containing less than 25 percent silicon) are
not reported by the USGS to avoid disclosing company
proprietary data. Emissions from this production category,
therefore, were not estimated.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-67. Ferroalloy production CO2
emissions were estimated to be between 1.4 and 1.8 Tg CO2
Eq. at the 95 percent confidence level. This indicates a range
of approximately 12 percent below and 13 percent above the
emission estimate of 1.6 Tg CO2 Eq. Ferroalloy production
CH4 emissions were estimated to be between a range of
approximately 12 percent below and 13 percent above the
emission estimate of 0.01 Tg CO2 Eq.
Table 4-66: Production of Ferroalloys (Metric Tons)
Year
Ferrosilicon
25%-55%
Ferrosilicon
56%-95%
Silicon Metal
Misc. Alloys
32%-65%
1990
321,385
109,566
145,744
72,442
2005
2006
2007
2008
123,000
164,000
180,000
193,000
86,100
88,700
90,600
94,000
148,000
148,000
148,000
148,000
NA
NA
NA
NA
NA (Not Available).
16 Emissions and sinks of biogenic carbon are accounted for in the Land
Use, Land-Use Change, and Forestry chapter.
Industrial Processes 4-45
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Table 4-67: Tier 2 Quantitative Uncertainty Estimates for C02 and CH4 Emissions from Ferroalloy Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Ferroalloy Production
Ferroalloy Production
C02
CH4
1.6
Lower Bound Upper Bound Lower Bound
1.4 1.8 -12%
+ + -12%
Upper Bound
+ 13%
+ 13%
+ Does not exceed 0.05 Tg C02 Eq.
3 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Future improvements to the ferroalloy production
source category include research into the data availability
for ferroalloys other than ferrosilicon and silicon metal. If
data are available, emissions will be estimated for those
ferroalloys. Additionally, research will be conducted to
determine whether data are available concerning raw material
consumption (e.g., coal coke, limestone and dolomite
flux, etc.) for inclusion in ferroalloy production emission
estimates.
4.15. Aluminum Production (IPCC
Source Category 2C3)
Aluminum is a light-weight, malleable, and corrosion-
resistant metal that is used in many manufactured products,
including aircraft, automobiles, bicycles, and kitchen
utensils. As of last reporting, the United States was the fourth
largest producer of primary aluminum, with approximately
seven percent of the world total (USGS 2009). The United
States was also a major importer of primary aluminum. The
production of primary aluminum—in addition to consuming
large quantities of electricity—results in process-related
emissions of CO2 and two perfluorocarbons (PFCs):
perfluoromethane (CF4) and perfluoroethane (C2F6).
Carbon dioxide is emitted during the aluminum smelting
process when alumina (aluminum oxide, A12O3) is reduced
to aluminum using the Hall-Heroult reduction process. The
reduction of the alumina occurs through electrolysis in a
molten bath of natural or synthetic cryolite (Na3AlF6). The
reduction cells contain a C lining that serves as the cathode.
Carbon is also contained in the anode, which can be a C
mass of paste, coke briquettes, or prebaked C blocks from
petroleum coke. During reduction, most of this C is oxidized
and released to the atmosphere as CO2.
Process emissions of CO2 from aluminum production
were estimated to be 4.5 Tg CO2 Eq. (4,477 Gg) in 2008
(see Table 4-68). The C anodes consumed during aluminum
production consist of petroleum coke and, to a minor extent,
coal tar pitch. The petroleum coke portion of the total CO2
process emissions from aluminum production is considered
to be a non-energy use of petroleum coke, and is accounted
for here and not under the CO2 from Fossil Fuel Combustion
source category of the Energy sector. Similarly, the coal tar
pitch portion of these CO2 process emissions is accounted
for here rather than in the Iron and Steel source category of
the Industrial Processes sector.
In addition to CO2 emissions, the aluminum production
industry is also a source of PFC emissions. During the
smelting process, when the alumina ore content of the
electrolytic bath falls below critical levels required for
electrolysis, rapid voltage increases occur, which are termed
"anode effects." These anode effects cause carbon from the
anode and fluorine from the dissociated molten cryolite bath
to combine, thereby producing fugitive emissions of CF4 and
C2F6. In general, the magnitude of emissions for a given
smelter and level of production depends on the frequency
and duration of these anode effects. As the frequency and
duration of the anode effects increase, emissions increase.
Since 1990, emissions of CF4 and C2F6 have declined by
86 percent and 82 percent, respectively, to 2.2 Tg CO2 Eq. of
CF4 (0.34 Gg) and 0.49 Tg CO2 Eq. of C2F6 (0.054 Gg) in
2008, as shown in Table 4-69 and Table 4-70. This decline is
due both to reductions in domestic aluminum production and
4-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
to actions taken by aluminum smelting companies to reduce
the frequency and duration of anode effects. (Note, however,
that production increased and the frequency and duration of
anode effects decreased in 2008 compared to 2007. In 2007,
higher emissions (and emission rate) were primarily due to
increased anode effects at a relatively emissive smelter.)
Since 1990, aluminum production has declined by 34 percent,
while the combined CF4 and C2F6 emission rate (per metric
ton of aluminum produced) has been reduced by 78 percent.
In 2008, U.S. primary aluminum production totaled
approximately 2.7 million metric tons, a 4 percent increase
from 2007 production levels (USAA 2009). In 2008, six
companies managed production at 14 operational primary
aluminum smelters. Four smelters were temporarily idled
and one smelter that was idle since 2000 was demolished
(USGS 2009). During the first half of 2008, U.S. primary
aluminum production increased (1.4 million metric tons
during January-June as compared to 1.2 million metric tons
for the same period in 2007; USGS 2008). However, in the
second half of the year, Columbia Falls Aluminum Company
shut two potlines and operated at 25 percent capacity from
July (USGS 2008b). In October, production was curtailed
at the Alcoa Inc. smelter in Rockdale, TX (Alcoa Inc. 2008)
as a result of uncompetitive power supply to the smelter
and overall market conditions. And in December, Century
Aluminum Co. announced the immediate curtailment of one
potline at its Ravenswood, WV smelter and possible future
curtailment of 100 percent of plant operations as a result of
the decline in aluminum prices leading to monthly losses
(Century Aluminum Co. 2008).
For 2009, total production during January-June was
0.9 million metric tons compared to 1.4 million metric tons
for the same period in 2008, a 33 percent decrease (USGS
2009b). Based on this decrease in production, process CO2
and PFC emissions are likely to decrease over this period
in 2009 given no significant changes in process controls at
operational facilities.
Methodology
Carbon dioxide emissions released during aluminum
production were estimated using the combined application
of process-specific emissions estimates modeling with
individual partner reported data. These estimates are based
on information gathered by EPA's Voluntary Aluminum
Industrial Partnership (VAIP) program.
Table 4-68: C02 Emissions from Aluminum Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
6.8
6,831
2005
2006
2007
2008
4.1
3.8
4.3
4.5
4,142
3,801
4,251
4,477
Table 4-69: PFC Emissions from Aluminum Production
(Tg C02 Eq.)
Year
CF4
Total
1990
15.9
2.7
2005
2006
2007
2008
Note: Totals may not sum due to independent rounding.
Table 4-70: PFC Emissions from Aluminum Production
(Gg)
Year
CF4
C2F6
2005
2006
2007
2008
+ Does not exceed 0.05 Gg.
Most of the CO2 emissions released during aluminum
production occur during the electrolysis reaction of the C
anode, as described by the following reaction:
2A12O3 + 3C -> 4A1 + 3CO2
Industrial Processes 4-47
-------
For prebake smelter technologies, CO2 is also emitted
during the anode baking process. These emissions can
account for approximately 10 percent of total process CO2
emissions from prebake smelters.
Depending on the availability of smelter-specific data,
the CO2 emitted from electrolysis at each smelter was
estimated from: (1) the smelter's annual anode consumption;
(2) the smelter's annual aluminum production and rate of
anode consumption (per ton of aluminum produced) for
previous and /or following years; or (3) the smelter's annual
aluminum production and IPCC default CO2 emission
factors. The first approach tracks the consumption and carbon
content of the anode, assuming that all carbon in the anode
is converted to CO2. Sulfur, ash, and other impurities in the
anode are subtracted from the anode consumption to arrive
at a carbon consumption figure. This approach corresponds
to either the IPCC Tier 2 or Tier 3 method, depending on
whether smelter-specific data on anode impurities are used.
The second approach interpolates smelter-specific anode
consumption rates to estimate emissions during years for
which anode consumption data are not available. This
avoids substantial errors and discontinuities that could be
introduced by reverting to Tier 1 methods for those years.
The last approach corresponds to the IPCC Tier 1 method
(2006) and is used in the absence of present or historic anode
consumption data.
The equations used to estimate CO2 emissions in the
Tier 2 and 3 methods vary depending on smelter type (IPCC
2006) For Prebake cells, the process formula accounts for
various parameters, including net anode consumption, and
the sulfur, ash, and impurity content of the baked anode. For
anode baking emissions, the formula accounts for packing
coke consumption, the sulfur and ash content of the packing
coke, as well as the pitch content and weight of baked anodes
produced. For S0derberg cells, the process formula accounts
for the weight of paste consumed per metric ton of aluminum
produced, and pitch properties, including sulfur, hydrogen,
and ash content.
Through the VAIP, anode consumption (and some
anode impurity) data have been reported for 1990, 2000,
2003, 2004, 2005, 2006, 2007, and 2008. Where available,
smelter-specific process data reported under the VAIP were
used; however, if the data were incomplete or unavailable,
information was supplemented using industry average values
recommended by IPCC (2006). Smelter-specific CO2 process
data were provided by 18 of the 23 operating smelters in
1990 and 2000, by 14 out of 16 operating smelters in 2003
and 2004, 14 out of 15 operating smelters in 2005, 13 out
of 14 operating smelters in 2006, and 5 out of 14 operating
smelters in, 2007 and 2008. For years where CO2 process
data were not reported by these companies, estimates were
developed through linear interpolation, and/or assuming
industry default values.
In the absence of any previous smelter specific process
data (i.e., 1 out of 14 smelters in 2006, 2007, and 2008, 1
out of 15 smelters in 2005, and 5 out of 23 smelters between
1990 and 2003), CO2 emission estimates were estimated
using Tier 1 S0derberg and/or Prebake emission factors
(metric ton of CO2 per metric ton of aluminum produced)
from IPCC (2006).
Aluminum production data for 13 out of 14 operating
smelters were reported under the VAIP in 2008. Between
1990 and 2007, production data were provided by 21 of the
23 U.S. smelters that operated during at least part of that
period. For the non-reporting smelters, production was
estimated based on the difference between reporting smelters
and national aluminum production levels (US AA 2009), with
allocation to specific smelters based on reported production
capacities (USGS 2002, 2009c).
PFC emissions from aluminum production were
estimated using a per-unit production emission factor that
is expressed as a function of operating parameters (anode
effect frequency and duration), as follows:
PFC (CF4 or C2F6) kg/metric ton Al =
S x Anode Effect Minutes/Cell-Day
where,
S = Slope coefficient (kg PFC/metric ton
Al)/(Anode Effect Minute/Cell-Day)
Anode Effect
Minutes/
Cell-Day = Anode Effect Frequency/Cell-Day x
Anode Effect Duration (Minutes)
This approach corresponds to either the Tier 3 or the
Tier 2 approach in the 2006 IPCC Guidelines, depending
upon whether the slope-coefficient is smelter-specific (Tier
3) or technology-specific (Tier 2). For 1990 through 2008,
smelter-specific slope coefficients were available and were
used for smelters representing between 30 and 94 percent of
U.S. primary aluminum production. The percentage changed
from year to year as some smelters closed or changed hands
4-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
and as the production at remaining smelters fluctuated.
For smelters that did not report smelter-specific slope
coefficients, IPCC technology-specific slope coefficients
were applied (IPCC 2000,2006). The slope coefficients were
combined with smelter-specific anode effect data collected
by aluminum companies and reported under the VAIP, to
estimate emission factors over time. For 1990 through 2008,
smelter-specific anode effect data were available for smelters
representing between 80 and 100 percent of U.S. primary
aluminum production. Where smelter-specific anode effect
data were not available, industry averages were used.
For all smelters, emission factors were multiplied by
annual production to estimate annual emissions at the smelter
level. For 1990 through 2008, smelter-specific production
data were available for smelters representing between 30 and
100 percent of U. S. primary aluminum production. (For the
years after 2000, this percentage was near the high end of the
range.) Production at non-reporting smelters was estimated
by calculating the difference between the production reported
under VAIP and the total U.S. production supplied by
USGS or USAA and then allocating this difference to non-
reporting smelters in proportion to their production capacity.
Emissions were then aggregated across smelters to estimate
national emissions.
National primary aluminum production data for 2008
were obtained via USAA (USAA 2009). For 1990 through
2001, and 2006 (see Table 4-71) data were obtained from
USGS,MineralIndustry Surveys: Aluminum Annual Report
(USGS 1995, 1998, 2000, 2001, 2002, 2007). For 2002
through 2005, and 2007 national aluminum production
data were obtained from the United States Aluminum
Association's Primary Aluminum Statistics (USAA 2004,
2005, 2006, 2008).
Uncertainty and Time Series Consistency
The overall uncertainties associated with the 2008 CO2,
CF4, and C2F6 emission estimates were calculated using
Approach 2, as defined by IPCC (2006). For CO2, uncertainty
was assigned to each of the parameters used to estimate CO2
emissions. Uncertainty surrounding reported production
data was assumed to be 1 percent (IPCC 2006). For
additional variables, such as net C consumption, and sulfur
Table 4-71: Production of Primary Aluminum (Gg)
Year
Gg
2005
2006
2007
2008
2,478
2,284
2,560
2,659
and ash content in baked anodes, estimates for uncertainties
associated with reported and default data were obtained
from IPCC (2006). A Monte Carlo analysis was applied to
estimate the overall uncertainty of the CO2 emission estimate
for the U.S. aluminum industry as a whole, and the results
are provided below.
To estimate the uncertainty associated with emissions
of CF4 and C2F6, the uncertainties associated with three
variables were estimated for each smelter: (1) the quantity of
aluminum produced; (2) the anode effect minutes per cell day
(which may be reported directly or calculated as the product
of anode effect frequency and anode effect duration); and,
(3) the smelter- or technology-specific slope coefficient. A
Monte Carlo analysis was then applied to estimate the overall
uncertainty of the emission estimate for each smelter and for
the U.S. aluminum industry as a whole.
The results of this quantitative uncertainty analysis are
summarized in Table 4-72. Aluminum production-related
CO2 emissions were estimated to be between 4.3 and 4.6 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 4 percent below to 4 percent above
the emission estimate of 4.5 Tg CO2 Eq. Also, production-
related CF4 emissions were estimated to be between 2.0
and 2.4 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 9 percent below
to 9 percent above the emission estimate of 2.2 Tg CO2
Eq. Finally, aluminum production-related C2F6 emissions
were estimated to be between 0.4 and 0.5 Tg CO2 Eq. at
the 95 percent confidence level. This indicates a range of
approximately 11 percent below to 12 percent above the
emission estimate of 0.5 Tg CO2 Eq.
Industrial Processes 4-49
-------
Table 4-72: Tier 2 Quantitative Uncertainty Estimates for C02 and PFC Emissions from Aluminum Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Aluminum Production
Aluminum Production
Aluminum Production
C02
CF4
C2F6
4.5
2.2
0.5
Lower Bound
4.3
2.0
0.4
Upper Bound
4.6
2.4
0.5
Lower Bound
-4%
-9%
-11%
Upper Bound
+4%
+ 9%
+ 12%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
The 2008 emission estimate was developed using either
company-wide or site-specific PFC slope coefficients for all
but 1 of the 14 operating smelters where default IPCC (2006)
slope data was used. In some cases, where smelters are owned
by one company, data have been reported on a company-wide
basis as totals or weighted averages. Consequently, in the
Monte Carlo analysis, uncertainties in anode effect minutes
per cell day, slope coefficients, and aluminum production
have been applied to the company as a whole and not to
each smelter. This probably overestimates the uncertainty
associated with the cumulative emissions from these smelters,
because errors that were in fact independent were treated as if
they were correlated. It is therefore likely that uncertainties
calculated above for the total U.S. 2008 emission estimates
for CF4 and C2F6 are also overestimated.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
4.16. Magnesium Production and
Processing (IPCC Source Category
2C4)
The magnesium metal production and casting industry
uses sulfur hexafluoride (SF6) as a cover gas to prevent the
rapid oxidation of molten magnesium in the presence of air.
Sulfur hexafluoride has been used in this application around
the world for more than twenty-five years. A dilute gaseous
mixture of SF6 with dry air and/or CO2 is blown over molten
magnesium metal to induce and stabilize the formation of
a protective crust. A small portion of the SF6 reacts with
the magnesium to form a thin molecular film of mostly
magnesium oxide and magnesium fluoride. The amount
of SF6 reacting in magnesium production and processing is
considered to be negligible and thus all SF6 used is assumed
to be emitted into the atmosphere. Although alternative cover
gases, such as AM-cover™ (containing HFC-134a), Novec™
612 and dilute SO2 systems can be used, most companies
in the United States are still using traditional SF6 cover gas
systems.
The magnesium industry emitted 2.0 Tg CO2 Eq. (0.1
Gg) of SF6 in 2008, representing a decrease of approximately
23 percent from 2007 emissions (See Table 4-73). The
decrease may be attributed to die casting facilities closing
in the United States over the past year due to reduced
demand from the American auto industry and other industrial
sectors (USGS 2008a). Although the price of magnesium
on the international level fell in 2008 because of weakening
economies, the tight import market in the U.S. prevented
domestic magnesium prices from declining on the same scale,
which also lead to losses in the industry and expansion plan
delays in the U.S. magnesium sector (USGS 2008a).
Methodology
Emission estimates for the magnesium industry
incorporate information provided by industry participants
in EPA's SF6 Emission Reduction Partnership for the
Magnesium Industry. The Partnership started in 1999 and,
currently, participating companies represent 100 percent
of U.S. primary and secondary production and 90 percent
of the casting sector production (i.e., die, sand, permanent
mold, wrought, and anode casting). Absolute emissions for
1999 through 2008 from primary production, secondary
production (i.e., recycling), and die casting were generally
4-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 4-73: SF6 Emissions from Magnesium Production
and Processing (Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
5.5
0.2
2005
2006
2007
2008
2.9
2.9
2.6
2.0
0.1
0.1
0.1
0.1
reported by Partnership participants. Partners reported their
SF6 consumption, which was assumed to be equivalent to
emissions. When a Partner did not report emissions, they
were estimated based on the metal processed and emission
rate reported by that partner in previous and (if available)
subsequent years. Where data for subsequent years was
not available, metal production and emissions rates were
extrapolated based on the trend shown by partners reporting
in the current and previous years. When it was determined
a Partner is no longer in production, their metal production
and emissions rates were set to zero if no activity information
was available; in one case a partner that closed mid-year was
estimated to have produced 50 percent of the metal from the
prior year.
Emission factors for 2002 to 2006 for sand casting
activities were also acquired through the Partnership. For
2007 and 2008, the sand casting partner did not report and
the reported emission factor from 2005 was utilized as
being representative of the industry. The 1999 through 2008
emissions from casting operations (other than die) were
estimated by multiplying emission factors (kg SF6 per metric
ton of Mg produced or processed) by the amount of metal
produced or consumed. The emission factors for casting
activities are provided below in Table 4-74. The emission
factors for primary production, secondary production and
sand casting are withheld to protect company-specific
production information. However, the emission factor for
primary production has not risen above the average 1995
partner value of 1.1 kg SF6 per metric ton.
Die casting emissions for 1999 through 2008, which
accounted for 17 to 52 percent of all SF6 emissions from the
U.S. magnesium industry during this period, were estimated
based on information supplied by industry partners. From
2000 to 2008, partners accounted for all U.S. die casting that
was tracked by USGS. In 1999, partners did not account
for all die casting tracked by USGS, and, therefore, it was
necessary to estimate the emissions of die casters who were
not partners. Die casters who were not partners were assumed
to be similar to partners who cast small parts. Due to process
requirements, these casters consume larger quantities of SF6
per metric ton of processed magnesium than casters that
process large parts. Consequently, emission estimates from
this group of die casters were developed using an average
emission factor of 5.2 kg SF6 per metric ton of magnesium.
The emission factors for the other industry sectors (i.e.,
permanent mold, wrought, and anode casting) were based
on discussions with industry representatives.
Data used to develop SF6 emission estimates were
provided by the Magnesium Partnership participants and
the USGS. U.S. magnesium metal production (primary
and secondary) and consumption (casting) data from 1990
through 2008 were available from the USGS (USGS 2002,
2003, 2005, 2006, 2007, 2008b, 2009a). Emission factors
from 1990 through 1998 were based on a number of sources.
Emission factors for primary production were available from
U.S. primary producers for 1994 and 1995, and an emission
factor for die casting of 4.1 kg per metric ton was available
for the mid-1990s from an international survey (Gjestland
& Magers 1996).
To estimate emissions for 1990 through 1998, industry
emission factors were multiplied by the corresponding metal
Table 4-74: SF6 Emission Factors (kg SF6 per metric ton
of Magnesium)
Year
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
Die
Casting
2.14a
0.72
0.72
0.71
0.81
0.81
0.79
0.86
0.67
1.15
Permanent
Mold Wrought
2 1
2 1
2 1
2 1
2 1
2 1
2 1
2 1
2 1
2 1
Anodes
1
1
1
1
1
1
1
1
1
1
a Weighted average that includes an estimated emission factor of 5.2 kg
SF6 per metric ton of magnesium for die casters that do not participate
in the Partnership.
Industrial Processes 4-51
-------
production and consumption (casting) statistics from USGS.
The primary production emission factors were 1.2 kg per
metric ton for 1990 through 1993, and 1.1 kg per metric
ton for 1994 through 1997. For die casting, an emission
factor of 4.1 kg per metric ton was used for the period 1990
through 1996. For 1996 through 1998, the emission factors
for primary production and die casting were assumed to
decline linearly to the level estimated based on partner reports
in 1999. This assumption is consistent with the trend in SF6
sales to the magnesium sector that is reported in the RAND
survey of major SF6 manufacturers, which shows a decline of
70 percent from 1996 to 1999 (RAND 2002). Sand casting
emission factors for 2002 through 2007 were provided by
the Magnesium Partnership participants, and 1990 through
2001 emission factors for this process were assumed to have
been the same as the 2002 emission factor. The emission
factor for secondary production from 1990 through 1998
was assumed to be constant at the 1999 average partner
value. The emission factors for the other processes (i.e.,
permanent mold, wrought, and anode casting), about which
less is known, were assumed to remain constant at levels
defined in Table 4-72.
Uncertainty and Time Series Consistency
To estimate the uncertainty surrounding the estimated
2008 SF6 emissions from magnesium production and
processing, the uncertainties associated with three variables
were estimated (1) emissions reported by magnesium
producers and processors that participate in the SF6
Emission Reduction Partnership; (2) emissions estimated for
magnesium producers and processors that participate in the
Partnership but did not report this year; and (3) emissions
estimated for magnesium producers and processors that do
not participate in the Partnership. An uncertainty of 5 percent
was assigned to the data reported by each participant in the
Partnership. If partners did not report emissions data during
the current reporting year, SF6 emissions data were estimated
using available emission factor and production information
reported in prior years; the extrapolation was based on the
average trend for partners reporting in the current reporting
year and the year prior. The uncertainty associated with the
SF6 usage estimate generated from the extrapolated emission
factor and production information was estimated to be 30
percent for each year of extrapolation. The lone sand casting
partner did not report in the past two reporting years and its
activity and emission factor were held constant at 2005 levels
due to a reporting anomaly in 2006 because of malfunctions
at the facility. The uncertainty associated with the SF6
usage for the sand casting partner was 43 percent. For those
industry processes that are not represented in Partnership,
such as permanent mold and wrought casting, SF6 emissions
were estimated using production and consumption statistics
reported by USGS and estimated process-specific emission
factors (see Table 4-74). The uncertainties associated with
the emission factors and USGS-reported statistics were
assumed to be 75 percent and 25 percent, respectively.
Emissions associated with sand casting activities utilized a
partner-reported emission factor with an uncertainty of 75
percent. In general, where precise quantitative information
was not available on the uncertainty of a parameter, a
conservative (upper-bound) value was used.
Additional uncertainties exist in these estimates that
are not addressed in this methodology, such as the basic
assumption that SF6 neither reacts nor decomposes during
use. The melt surface reactions and high temperatures
associated with molten magnesium could potentially
cause some gas degradation. Recent measurement studies
have identified SF6 cover gas degradation in die casting
applications on the order of 20 percent (Bartos et al. 2007).
Sulfur hexafluoride may also be used as a cover gas for the
casting of molten aluminum with high magnesium content;
however, the extent to which this technique is used in the
United States is unknown.
The results of this Tier 2 quantitative uncertainty analysis
are summarized in Table 4-75. Sulfur hexafluoride emissions
associated with magnesium production and processing
were estimated to be between 1.9 and 2.1 Tg CO2 Eq. at
the 95 percent confidence level. This indicates a range
of approximately 6 percent below to 5 percent above the
2008 emission estimate of 2.0 Tg CO2 Eq. The uncertainty
estimates for 2008 are lower relative to the 2007 reporting
year which is likely due to a significant decrease in reported
sand casting activity.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
4-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-75: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Magnesium Production and Processing
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Magnesium Production SF6
2.0
1.9
2.1
-6%
+ 5%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
The USGS revised the volume of metal produced from
the magnesium sand casting sector for the 2007 reporting
year. This revision, which amounted to approximately 2,200
MT less metal produced from magnesium sand casting
(USGS 2009a), is reflected in the current Inventory report.
This revision resulted in an approximate decrease in 2007
emissions by 0.37 Tg CO2 Eq. relative to the previous
Inventory report.
Planned Improvements
Cover gas research conducted by the EPA over the
last decade has found that SF6 used for magnesium melt
protection can have degradation rates on the order of 20
percent in die casting applications (Bartos et al. 2007).
Current emission estimates assume (per the 2006 IPCC
Guidelines, IPCC 2006) that all SF6 utilized is emitted to
the atmosphere. Additional research may lead to a revision
of IPCC Guidelines to reflect this phenomenon and until
such time, developments in this sector will be monitored for
possible application to the inventory methodology. Another
issue that will be addressed in future inventories is the
likely adoption of alternate cover gases by U.S. magnesium
producers and processors. These cover gases, which include
AM-cover™ (containing HFC-134a) andNovec™ 612, have
lower GWPs than SF6, and tend to quickly decompose during
their exposure to the molten metal. Magnesium producers
and processors have already begun using these cover gases
for 2006 through 2008 in a limited fashion; because the
amounts being used by companies on the whole are low
enough that they have a minor effect on the overall emissions
from the industry, these emissions are only being monitored
and recorded at this time.
4.17. Zinc Production (IPCC Source
Category 2C5)
Zinc production in the United States consists of both
primary and secondary processes. Primary production
techniques used in the United States are the electrothermic
and electrolytic process while secondary techniques used
in the United States include a range of metallurgical,
hydrometallurgical, and pyrometallurgical processes.
Worldwide primary zinc production also employs a
pyrometallurgical process using the Imperial Smelting
Furnace process; however, this process is not used in the
United States (Sjardin 2003). Of the primary and secondary
processes used in the United States, the electrothermic
process results in non-energy CO2 emissions, as does the
Waelz Kiln process—a technique used to produce secondary
zinc from electric-arc furnace (EAF) dust (Viklund-White
2000).
During the electrothermic zinc production process,
roasted zinc concentrate and, when available, secondary
zinc products enter a sinter feed where they are burned to
remove impurities before entering an electric retort furnace.
Metallurgical coke added to the electric retort furnace reduces
the zinc oxides and produces vaporized zinc, which is then
captured in a vacuum condenser. This reduction process
produces non-energy CO2 emissions (Sjardin 2003). The
electrolytic zinc production process does not produce non-
energy CO2 emissions.
In the Waelz Kiln process, EAF dust, which is captured
during the recycling of galvanized steel, enters a kiln along
with a reducing agent—often metallurgical coke. When kiln
temperatures reach approximately 1100-1200°C, zinc fumes
are produced, which are combusted with air entering the kiln.
This combustion forms zinc oxide, which is collected in a
baghouse or electrostatic precipitator, and is then leached
to remove chloride and fluoride. Through this process,
Industrial Processes 4-53
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Table 4-76: C02 Emissions from Zinc Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
0.9
929
2005
2006
2007
2008
0.5
0.5
0.4
0.4
506
513
411
402
approximately 0.33 ton of zinc is produced for every ton of
EAF dust treated (Viklund-White 2000).
In 2008, U.S. primary and secondary zinc production
totaled 440,000 metric tons (Nyrstar 2009, USGS 2009a).
The resulting emissions of CO2 from zinc production in 2008
were estimated to be 0.4 Tg CO2 Eq. (402 Gg) (see Table
4-76). All 2008 CO2 emissions result from secondary zinc
production.
After a gradual increase in total emissions from 1990 to
2000, largely due to an increase in secondary zinc production,
emissions have decreased in recent years due to the closing of
an electrothermic-process zinc plant in Monaca, PA (USGS
2004). In 2008, emissions decreased 2 percent from 2007
and decreased by 57 percent since 1990.
Methodology
Non-energy CO2 emissions from zinc production result
from those processes that use metallurgical coke or other
C-based materials as reductants. Sjardin (2003) provides an
emission factor of 0.43 metric tons CO2/ton zinc produced for
emissive zinc production processes; however, this emission
factor is based on the Imperial Smelting Furnace production
process. Because the Imperial Smelting Furnace production
process is not used in the United States, emission factors
specific to those emissive zinc production processes used
in the United States, which consist of the electro-thermic
and Waelz Kiln processes, were needed. Due to the limited
amount of information available for these electro-thermic
processes, only Waelz Kiln process-specific emission factors
were developed. These emission factors were applied to
both the Waelz Kiln process and the electro-thermic zinc
production processes. A Waelz Kiln emission factor based
on the amount of zinc produced was developed based on
the amount of metallurgical coke consumed for non-energy
purposes per ton of zinc produced, 1.19 metric tons coke/
metric ton zinc produced (Viklund-White 2000), and the
following equation:
The USGS disaggregates total U.S. primary zinc
production capacity into zinc produced using the
electrothermic process and zinc produced using the
electrolytic process; however, the USGS does not report
the amount of zinc produced using each process, only the
total zinc production capacity of the zinc plants using each
process. The total electro-thermic zinc production capacity
is divided by total primary zinc production capacity to
EFW,
Waelz Kiln
= 1.19
0.85
3.67
3.70
metric tons coke
metric tons zinc
metric tons C
metric tons coke
metric tons CO2
metric tons C
metric tons CO2
metric tons zinc
estimate the percent of primary zinc produced using the
electro-thermic process. This percent is then multiplied by
total primary zinc production to estimate the amount of zinc
produced using the electro-thermic process, and the resulting
value is multiplied by the Waelz Kiln process emission factor
to obtain total CO2 emissions for primary zinc production.
According to the USGS, the only remaining plant producing
primary zinc using the electro-thermic process closed in 2003
(USGS 2004). Therefore, CO2 emissions for primary zinc
production are reported only for years 1990 through 2002.
In the United States, secondary zinc is produced through
either the electro-thermic or Waelz Kiln process. In 1997,
the Horsehead Corporation plant, located in Monaca, PA,
produced 47,174 metric tons of secondary zinc using the
electro-thermic process (Queneau et al. 1998). This is the
only plant in the United States that uses the electro-thermic
process to produce secondary zinc, which, in 1997, accounted
for 12 percent of total secondary zinc production. This
percentage was applied to all years within the time series
up until the Monaca plant's closure in 2003 (USGS 2004) to
estimate the total amount of secondary zinc produced using
the electro-thermic process. This value is then multiplied by
the Waelz Kiln process emission factor to obtain total CO2
4-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
emissions for secondary zinc produced using the electro-
thermic process.
U.S. secondary zinc is also produced by processing
recycled EAF dust in a Waelz Kiln furnace. Due to the
complexities of recovering zinc from recycled EAF dust, an
emission factor based on the amount of EAF dust consumed
rather than the amount of secondary zinc produced is believed
to represent actual CO2 emissions from the process more
accurately (Stuart 2005). An emission factor based on the
amount of EAF dust consumed was developed based on the
amount of metallurgical coke consumed per ton of EAF
dust consumed, 0.4 metric tons coke/metric ton EAF dust
consumed (Viklund-White 2000), and the following equation:
The Horsehead Corporation plant, located in Palmerton,
PA, is the only large plant in the United States that produces
secondary zinc by recycling EAF dust (Stuart 2005). In
2003, this plant consumed 408,240 metric tons of EAF dust,
producing 137,169 metric tons of secondary zinc (Recycling
Today 2005). This zinc production accounted for 34 percent
of total secondary zinc produced in 2003. This percentage
Table 4-77: Zinc Production (Metric Tons)
EF
EAF Dust
= 0.4
0.84
3.67
1.23
metric tons coke
metric tons EAF dust
metric tons C
metric tons coke
metric tons CO2
metric tons C
metric tons CO2
metric tons EAF dust
was applied to the USGS data for total secondary zinc
production for all years within the time series to estimate
the total amount of secondary zinc produced by consuming
recycled EAF dust in a Waelz Kiln furnace. This value is
multiplied by the Waelz Kiln process emission factor for
EAF dust to obtain total CO2 emissions.
The 1990 through 2007 activity data for primary and
secondary zinc production (see Table 4-77) were obtained
through the USGS Mineral Yearbook: Zinc (USGS 1994
through 2009b). Preliminary data for 2008 primary
production were obtained from the annual report for the
company operating the only primary zinc refinery in the
U.S. (Nyrstar 2009, Tolcin 2009). Preliminary data for 2008
production from scrap was obtained from the USGS Mineral
Commodity Summary for Zinc (USGS 2009a). Because data
for 2008 secondary zinc production were unavailable, 2007
data were used.
Year
Primary
Secondary
2005
2006
2007
2008
191,120
113,000
121,000
125,000
397,000
402,000
322,000
315,000
Uncertainty and Time-Series Consistency
The uncertainties contained in these estimates are two-
fold, relating to activity data and emission factors used.
First, there are uncertainties associated with the percent
of total zinc production, both primary and secondary, that
is attributed to the electro-thermic and Waelz Kiln emissive
zinc production processes. For primary zinc production,
the amount of zinc produced annually using the electro-
thermic process is estimated from the percent of primary-zinc
production capacity that electro-thermic production capacity
constitutes for each year of the time series. This assumes
that each zinc plant is operating at the same percentage of
total production capacity, which may not be the case and
this calculation could either overestimate or underestimate
the percentage of the total primary zinc production that is
produced using the electro-thermic process. The amount of
secondary zinc produced using the electro-thermic process is
estimated from the percent of total secondary zinc production
that this process accounted for during a single year, 2003.
The amount of secondary zinc produced using the Waelz
Kiln process is estimated from the percent of total secondary
zinc production this process accounted for during a single
year, 1997. This calculation could either overestimate or
underestimate the percentage of the total secondary zinc
production that is produced using the electro-thermic or
Waelz Kiln processes. Therefore, there is uncertainty
associated with the fact that percents of total production
data estimated from production capacity, rather than actual
production data, are used for emission estimates.
Second, there are uncertainties associated with the
emission factors used to estimate CO2 emissions from the
primary and secondary production processes. Because the
only published emission factors are based on the Imperial
Industrial Processes 4-55
-------
Table 4-78: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Zinc Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Zinc Production
CO,
0.4
0.3
0.5
-22%
+24%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Smelting Furnace, which is not used in the United States,
country-specific emission factors were developed for the
Waelz Kiln zinc production process. Data limitations
prevented the development of emission factors for the
electro-thermic process. Therefore, emission factors for the
Waelz Kiln process were applied to both electro-thermic and
Waelz Kiln production processes. Furthermore, the Waelz
Kiln emission factors are based on materials balances for
metallurgical coke and EAF dust consumed during zinc
production provided by Viklund-White (2000). Therefore,
the accuracy of these emission factors depend upon the
accuracy of these materials balances.
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 4-78. Zinc production
CO2 emissions were estimated to be between 0.3 and 0.5 Tg
CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 22 percent below and 24 percent
above the emission estimate of 0.4 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Recalculations Discussion
The data for secondary zinc production from 2003
through 2007 were revised in the 2007 USGS Mineral
Yearbook: Zinc. As the revised production data were greater
than the data used in the previous Inventory, this change
resulted in increased emissions for these years. These
revisions also affected emissions across the time series as the
2003 data are used to establish the "percent of total secondary
zinc production" represented by secondary zinc production
from EAF dust. As this percentage is applied to data in all
years to complete the emission estimates, emissions for
all years subsequently decreased, relative to the previous
Inventory. The overall effect of the two revisions was to
increase average annual CO2 emissions by 3.3% from 1990
to 2007.
Planned Improvements
A future improvement will be to update the assumptions
used to estimate the amount of secondary zinc production
from EAF dust in the United States, which will affect the
CO2 emission estimates for zinc. Secondary zinc production
from EAF dust is currently estimated by extrapolating a 2005
published production number by overall zinc production
trends from USGS. However, there has been production
information published in years since 2005 that can be used
in place of the extraploted estimates.
4.18. Lead Production (IPCC Source
Category 2C5)
Lead production in the United States consists of both
primary and secondary processes—both of which emit CO2
(Sjardin 2003). Primary lead production, in the form of
direct smelting, mostly occurs at plants located in Alaska
and Missouri, though to a lesser extent in Idaho, Montana,
and Washington. Secondary production largely involves the
recycling of lead acid batteries at approximately 15 separate
smelters located in 11 states (USGS 2010). Secondary lead
production has increased in the United States over the past
decade while primary lead production has decreased. In 2008,
secondary lead production accounted for approximately 89
percent of total lead production (USGS 2010).
Primary production of lead through the direct smelting
of lead concentrate produces CO2 emissions as the lead
concentrates are reduced in a furnace using metallurgical
coke (Sjardin 2003). U.S. primary lead production increased
by 10 percent from 2007 to 2008 and has decreased by 67
percent since 1990 (USGS 2010, USGS 1995).
Secondary lead production, primarily from the recycling
of lead-acid batteries, accounted for approximately 89
4-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 4-79: C02 Emissions from Lead Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
0.3
285
2005
2006
2007
2008
0.3
0.3
0.3
0.3
266
270
267
264
percent of total refined lead production in the United States in
2008 (USGS 2010). Similar to primary lead production, CO2
emissions result when a reducing agent, usually metallurgical
coke, is added to the smelter to aid in the reduction process
(Sjardin 2003). U.S. secondary lead production decreased
from 2007 to 2008 by 3 percent, and has increased by 25
percent since 1990 (USGS 2010, USGS 1995).
At last reporting, the United States was the third largest
mine producer of lead in the world, behind China and
Australia, accounting for 11 percent of world production in
2008 (USGS 2010). In 2008, U.S. primary and secondary
lead production totaled 1,150,000 metric tons (USGS 2010).
The resulting emissions of CO2 from 2008 production were
estimated to be 0.3 Tg CO2 Eq. (264 Gg) (see Table 4-79).
The majority of 2008 lead production is from secondary
processes, which account for 87 percent of total 2008 CO2
emissions.
After a gradual increase in total emissions from 1990 to
2000, total emissions have decreased by eight percent since
1990, largely due to a decrease in primary production (67
percent since 1990) and a transition within the United States
from primary lead production to secondary lead production,
which is less emissions intensive than primary production,
although the sharp decrease leveled off in 2005 (USGS 2010,
Smith 2007).
Methodology
Non-energy CO2 emissions from lead production result
from primary and secondary production processes that use
metallurgical coke or other C-based materials as reductants.
For primary lead production using direct smelting, Sjardin
(2003) and the IPCC (2006) provide an emission factor
of 0.25 metric tons CO2/ton lead. For secondary lead
production, Sjardin (2003) and IPCC (2006) provide an
emission factor of 0.2 metric tons CO2/ton lead produced.
Both factors are multiplied by total U.S. primary and
secondary lead production, respectively, to estimate CO2
emissions.
The 1990 through 2008 activity data for primary and
secondary lead production (see Table 4-80) were obtained
through the USGS Mineral Yearbook: Lead (USGS 1994
through 2010) for 1990-2008.
Table 4-80: Lead Production (Metric Tons)
Year
Primary
Secondary
386,000
^M
374,000
922,000
^H
1,020,000
2000
^m
2005
2006
2007
2008
341,000
^M
143,000
153,000
123,000
135,000
1,130,000
^^m
1,150,000
1,160,000
1,180,000
1,150,000
Uncertainty and Time-Series Consistency
Uncertainty associated with lead production relates
to the emission factors and activity data used. The direct
smelting emission factor used in primary production is taken
from Sjardin (2003) who averages the values provided by
three other studies (Dutrizac et al. 2000, Morris et al. 1983,
Ullman 1997). For secondary production, Sjardin (2003)
reduces this factor by 50 percent and adds a CO2 emission
factor associated with battery treatment. The applicability
of these emission factors to plants in the United States
is uncertain. There is also a smaller level of uncertainty
associated with the accuracy of primary and secondary
production data provided by the USGS.
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 4-81. Lead production
CO2 emissions were estimated to be between 0.2 and 0.3 Tg
CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 12 percent below and 22 percent
above the emission estimate of 0.3 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
Industrial Processes 4-57
-------
Table 4-81: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Lead Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Lead Production
CO,
0.3
0.2
0.3
-12%
+22%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
are described in more detail in the Methodology section,
above.
4.19. HCFC-22 Production (IPCC
Source Category 2E1)
Trifluoromethane (HFC-23 or CHF3) is generated as a
by-product during the manufacture of chlorodifluoromethane
(HCFC-22), which is primarily employed in refrigeration
and air conditioning systems and as a chemical feedstock
for manufacturing synthetic polymers. Between 1990 and
2000, U.S. production of HCFC-22 increased significantly
as HCFC-22 replaced chlorofluorocarbons (CFCs) in many
applications. Since 2000, U.S. production has fluctuated
but has generally remained above 1990 levels. Because
HCFC-22 depletes stratospheric ozone, its production for
non-feedstock uses is scheduled to be phased out by 2020
under the U.S. Clean Air Act.17 Feedstock production,
however, is permitted to continue indefinitely.
HCFC-22 is produced by the reaction of chloroform
(CHC13) and hydrogen fluoride (HF) in the presence of a
catalyst, SbCl5. The reaction of the catalyst and HF produces
SbClxFy, (where x + y = 5), which reacts with chlorinated
hydrocarbons to replace chlorine atoms with fluorine. The
HF and chloroform are introduced by submerged piping
into a continuous-flow reactor that contains the catalyst in a
hydrocarbon mixture of chloroform and partially fluorinated
intermediates. The vapors leaving the reactor contain HCFC-
21 (CHC12F), HCFC-22 (CHC1F2), HFC-23 (CHF3), HC1,
chloroform, and HF. The under-fluorinated intermediates
(HCFC-21) and chloroform are then condensed and returned
to the reactor, along with residual catalyst, to undergo further
fluorination. The final vapors leaving the condenser are
primarily HCFC-22, HFC-23, HC1 and residual HF. The HC1
is recovered as a useful byproduct, and the HF is removed.
Once separated from HCFC-22, the HFC-23 may be released
to the atmosphere, recaptured for use in a limited number of
applications, or destroyed.
Emissions of HFC-23 in 2008 were estimated to be
13.6 Tg CO2 Eq. (1.2 Gg) (Table 4-82). This quantity
represents a 20 percent decrease from 2007 emissions and
a 63 percent decline from 1990 emissions. The dencrease
from 2007 emissions was caused by a 22 percent decrease in
HCFC-22 production and a 3 percent increase in the HFC-
23 emission rate. The decline from 1990 emissions is due
to a 59 percent decrease in the HFC-23 emission rate since
1990. The decrease is primarily attributable to four factors:
(1) five plants that did not capture and destroy the HFC-23
generated have ceased production of HCFC-22 since 1990;
(2) one plant that captures and destroys the HFC-23 generated
began to produce HCFC-22; (3) one plant implemented and
documented a process change that reduced the amount of
HFC-23 generated; and (4) the same plant began recovering
HFC-23, primarily for destruction and secondarily for sale.
Three HCFC-22 production plants operated in the United
States in 2006, two of which used thermal oxidation to
significantly lower their HFC-23 emissions.
Table 4-82: HFC-23 Emissions from HCFC-22
Production (Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
36.4
17 As construed, interpreted, and applied in the terms and conditions of the
Montreal Protocol on Substances that Deplete the Ozone Layer [42 U.S.C.
§7671m(b), CAA §614].
2005
2006
2007
2008
15.8
13.8
17.0
13.6
4-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Methodology
To estimate their emissions of HFC-23, five of the eight
HCFC-22 plants that have operated in the U.S. since 1990
use (or, for those plants that have closed, used) methods
comparable to the Tier 3 methods in the 2006 IPCC
Guidelines (IPCC 2006). The other three plants, the last of
which closed in 1993, used methods comparable to the Tier 1
method in the 2006 IPCC Guidelines. Emissions from these
three plants have been recalculated using the recommended
emission factor for unoptimized plants operating before
1995 (0.04 kg HCFC-23/kg HCFC-22 produced). (This
recalculation was reflected in the 1990 through 2006
inventory submission.)
The five plants that have operated since 1994 measured
concentrations of HFC-23 to estimate their emissions of HFC-
23. Plants using thermal oxidation to abate their HFC-23
emissions monitor the performance of their oxidizers to verify
that the HFC-23 is almost completely destroyed. Plants that
release (or historically have released) some of their byproduct
HFC-23 periodically measure HFC-23 concentrations in the
output stream using gas chromatography. This information
is combined with information on quantities of products (e.g.,
HCFC-22) to estimate HFC-23 emissions.
In most years, including 2009, an industry association
aggregates and reports to EPA country-level estimates of
HCFC-22 production and HFC-23 emissions (ARAP 1997,
1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
2008, 2009). However, in 1997 and 2008, EPA (through a
contractor) performed comprehensive reviews of plant-level
estimates of HFC-23 emissions and HCFC-22 production
(RTI 1997; RTI 2008). These reviews enabled EPA to
review, update, and where necessary, correct U.S. totals,
and also to perform plant-level uncertainty analyses (Monte-
Table 4-83: HCFC-22 Production (Gg)
Year
Gg
2000
2005
2006
2007
2008
156
154
162
126
Carlo simulations) for 1990, 1995, 2000, 2005, and 2006.
Estimates of annual U.S. HCFC-22 production are presented
in Table 4-83.
Uncertainty and Time Series Consistency
The uncertainty analysis presented in this section was
based on a plant-level Monte Carlo simulation for 2006. The
Monte Carlo analysis used estimates of the uncertainties
in the individual variables in each plant's estimating
procedure. This analysis was based on the generation of
10,000 random samples of model inputs from the probability
density functions for each input. A normal probability
density function was assumed for all measurements and
biases except the equipment leak estimates for one plant;
a log-normal probability density function was used for this
plant's equipment leak estimates. The simulation for 2006
yielded a 95-percent confidence interval for U.S. emissions
of 6.8 percent below to 9.6 percent above the reported total.
Because EPA did not have access to plant-level
emissions data for 2008, the relative errors yielded by the
Monte Carlo simulation for 2006 were applied to the U.S.
emission estimate for 2008. The resulting estimates of
absolute uncertainty are likely to be accurate because (1) the
methods used by the three plants to estimate their emissions
are not believed to have changed significantly since 2006;
(2) the distribution of emissions among the plants is not
believed to have changed significantly since 2006 (one plant
continues to dominate emissions); and (3) the country-level
relative errors yielded by the Monte Carlo simulations for
2005 and 2006 were very similar, implying that these errors
are not sensitive to small, year-to-year changes.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-84. HFC-23 emissions from
HCFC-22 production were estimated to be between 12.7 and
14.9 Tg CO2 Eq. at the 95-percent confidence level. This
indicates a range of approximately 7 percent below and 10
percent above the emission estimate of 13.60 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Industrial Processes 4-59
-------
Table 4-84: Quantitative Uncertainty Estimates for HFC-23 Emissions from HCFC-22 Production
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
HCFC-22 Production
HFC-23
13.6
12.7
14.9
-7%
+ 10%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4.20. Substitution of Ozone Depleting
Substances (IPCC Source Category
2F)
HFCs and PFCs are used as alternatives to several classes
of ODSs that are being phased out under the terms of the
Montreal Protocol and the Clean Air Act Amendments of
1990.18 Ozone depleting substances—chlorofluorocarbons
(CFCs), halons, carbon tetrachloride, methyl chloroform, and
hydrochlorofluorocarbons (HCFCs)—are used in a variety
of industrial applications including refrigeration and air
conditioning equipment, solvent cleaning, foam production,
sterilization, fire extinguishing, and aerosols. Although HFCs
and PFCs are not harmful to the stratospheric ozone layer,
they are potent greenhouse gases. Emission estimates for
HFCs and PFCs used as substitutes for ODSs are provided
in Table 4-85 and Table 4-86.
In 1990 and 1991, the only significant emissions of
HFCs and PFCs as substitutes to ODSs were relatively
small amounts of HFC-152a—used as an aerosol propellant
and also a component of the refrigerant blend R-500
used in chillers—and HFC-134a in refrigeration end-
uses. Beginning in 1992, HFC-134a was used in growing
amounts as a refrigerant in motor vehicle air-conditioners
and in refrigerant blends such as R-404A.19 In 1993, the
use of HFCs in foam production began, and in 1994 these
compounds also found applications as solvents. In 1995,
ODS substitutes for halons entered widespread use in the
United States as halon production was phased-out.
The use and subsequent emissions of HFCs and PFCs
as ODS substitutes has been increasing from small amounts
in 1990 to 113.0 Tg CO2 Eq. in 2008. This increase was in
large part the result of efforts to phase out CFCs and other
ODSs in the United States. In the short term, this trend is
expected to continue, and will likely accelerate over the next
decade as HCFCs, which are interim substitutes in many
applications, are themselves phased-out under the provisions
of the Copenhagen Amendments to the Montreal Protocol.
Improvements in the technologies associated with the use
of these gases and the introduction of alternative gases and
technologies, however, may help to offset this anticipated
increase in emissions.
Table 4-87 presents emissions of HFCs and PFCs as
ODS substitutes by end-use sector for 1990 through 2008.
The end-use sectors that contributed the most toward
emissions of HFCs and PFCs as ODS substitutes in 2008
include refrigeration and air-conditioning (101.7 Tg CO2
Eq., or approximately 90 percent), aerosols (6.4 Tg CO2 Eq.,
or approximately 6 percent), and foams (2.8 Tg CO2 Eq., or
approximately 2 percent). Within the refrigeration and air-
conditioning end-use sector, motor vehicle air-conditioning
was the highest emitting end-use (50.7 Tg CO2 Eq.), followed
by refrigerated transport and retail food. Each of the end-use
sectors is described in more detail below.
Refrigeration/Air Conditioning
The refrigeration and air-conditioning sector includes
a wide variety of equipment types that have historically
used CFCs or HCFCs. End-uses within this sector include
motor vehicle air-conditioning, retail food refrigeration,
refrigerated transport (e.g., ship holds, truck trailers, railway
freight cars), household refrigeration, residential and small
commercial air-conditioning/heat pumps, chillers (large
comfort cooling), cold storage facilities, and industrial
process refrigeration (e.g., systems used in food processing,
chemical, petrochemical, pharmaceutical, oil and gas,
and metallurgical industries). As the ODS phaseout is
18 [42 U.S.C § 7671, CAA § 601].
19 R-404A contains HFC-125, HFC-143a, and HFC-134a.
4-60 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-85: Emissions of MFCs and PFCs from ODS Substitutes (Tg C02 Eq.)
Gas
HFC-23
HFC-32
HFC-125
HFC-134a
HFC-143a
HFC-236fa
CF4
Others3
Total
1990 |
0.3
0.3
1995|
1.6
29.0
2000
4.0
74.3
2005
+
0.4
10.3
73.7
12.2
0.8
+
5.9
103.2
2006
+
0.6
12.3
73.4
14.4
0.8
+
6.2
107.7
2007
+
0.9
14.7
70.5
16.7
0.9
+
6.5
110.1
2008
+
1.2
17.7
67.3
19.2
0.9
+
6.8
113.0
+ Does not exceed 0.05 Tg C02 Eq.
a Others include HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee, and PFC/PFPEs, the latter being a proxy for a diverse collection of PFCs and
perfluoropolyethers (PFPEs) employed for solvent applications. For estimating purposes, the GWP value used for PFC/PFPEs was based upon C6F14.
Note: Totals may not sum due to independent rounding.
Table 4-86: Emissions of HFCs
Gas
HFC-23
HFC-32
HFC-125
HFC-134a
HFC-143a
HFC-236fa
CF4
Others3
and PFCs from ODS
1990 •
+
M|
M (Mixture of Gases)
+ Does not exceed 0.5 Mg.
aOthers include HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee
perfluoropolyethers (PFPEs) employed for solvent applications.
Substitution
1995H
+
291
19,875
1321
36 1
M
(Mg)
2000
1
44l
1,873
46,379
1,089
85 1
ll
M
2005
1
562
3,675
56,675
3,200
125
2
M
2006
1
913
4,394
56,484
3,782
131
2
M
2007
1
1,325
5,253
54,210
4,402
136
2
M
2008
2
1,801
6,310
51,750
5,044
141
2
M
C4F10, and PFC/PFPEs, the latter being a proxy for a diverse collection of PFCs and
Table 4-87: Emissions of MFCs and PFCs from ODS Substitutes (Tg C02 Eq.) by Sector
Sector
1990
1995
2000
2005 2006
2007
2008
Refrigeration/Air-conditioning
Aerosols
Foams
Solvents
Fire Protection
0.3
119.8 m.t
8.11 10.1
0.2l 0.31
0.91 2.11
+ 1 0.2
93.3
5.9
2.2
1.3
0.5
97.4
6.1
2.4
1.3
0.6
99.3
6.2
2.6
1.3
0.7
101.7
6.4
2.8
1.3
0.7
Total
0.3
29.0
74.3
103.2
107.7
110.1
113.0
+ Does not exceed 0.05 Tg C02 Eq.
taking effect, most equipment is being or will eventually
be retrofitted or replaced to use HFC-based substitutes.
Common HFCs in use today in refrigeration/air-conditioning
equipment are HFC-134a,R-410A,20R-404A,andR-507A.21
These HFCs are emitted to the atmosphere during equipment
20 R-410A contains HFC-32 and HFC-125.
21 R-507 contains HFC-125 and HFC-143a.
Industrial Processes 4-61
-------
manufacture and operation (as a result of component failure,
leaks, and purges), as well as at servicing and disposal events.
Aerosols
Aerosol propellants are used in metered dose inhalers
(MDIs) and a variety of personal care products and technical/
specialty products (e.g., duster sprays and safety horns).
Many pharmaceutical companies that produce MDIs—a
type of inhaled therapy used to treat asthma and chronic
obstructive pulmonary disease—have committed to replace
the use of CFCs with HFC-propellant alternatives. The
earliest ozone-friendly MDIs were produced with HFC-134a,
but eventually, the industry expects to use HFC-227ea as well.
Conversely, since the use of CFC propellants was banned in
1978, most consumer aerosol products have not transitioned
to HFCs, but to "not-in-kind" technologies, such as solid
roll-on deodorants and finger-pump sprays. The transition
away from ODS in specialty aerosol products has also led
to the introduction of non-fluorocarbon alternatives (e.g.,
hydrocarbon propellants) in certain applications, in addition
to HFC-134a or HFC-152a. These propellants are released
into the atmosphere as the aerosol products are used.
Foams
CFCs and HCFCs have traditionally been used as foam
blowing agents to produce polyurethane (PU), polystyrene,
polyolefin, and phenolic foams, which are used in a wide
variety of products and applications. Since the Montreal
Protocol, flexible PU foams as well as other types of
foam, such as polystyrene sheet, polyolefin, and phenolic
foam, have transitioned almost completely away from
fluorocompounds, into alternatives such as CO2, methylene
chloride, and hydrocarbons. The majority of rigid PU foams
have transitioned to HFCs—primarily HFC-134a and HFC-
245fa. Today, these HFCs are used to produce polyurethane
appliance foam, PU commercial refrigeration, PU spray, and
PU panel foams—used in refrigerators, vending machines,
roofing, wall insulation, garage doors, and cold storage
applications. In addition, HFC-152a is used to produce
polystyrene sheet/board foam, which is used in food
packaging and building insulation. Emissions of blowing
agents occur when the foam is manufactured as well as
during the foam lifetime and at foam disposal, depending on
the particular foam type.
Solvents
CFCs, methyl chloroform (1,1,1-trichloroethane or
TCA), and to a lesser extent carbon tetrachloride (CC14)
were historically used as solvents in a wide range of cleaning
applications, including precision, electronics, and metal
cleaning. Since their phaseout, metal cleaning end-use
applications have primarily transitioned to non-fluorocarbon
solvents and not-in-kind processes. The precision and
electronics cleaning end-uses have transitioned in part to
high-GWP gases, due to their high reliability, excellent
compatibility, good stability, low toxicity, and selective
solvency. These applications rely on HFC-4310mee, HFC-
365mfc, HFC-245fa, and to a lesser extent, PFCs. Electronics
cleaning involves removing flux residue that remains after
a soldering operation for printed circuit boards and other
contamination-sensitive electronics applications. Precision
cleaning may apply to either electronic components or to
metal surfaces, and is characterized by products, such as disk
drives, gyroscopes, and optical components, that require a
high level of cleanliness and generally have complex shapes,
small clearances, and other cleaning challenges. The use of
solvents yields fugitive emissions of these HFCs and PFCs.
Fire Protection
Fire protection applications include portable fire
extinguishers ("streaming" applications) that originally used
halon 1211, and total flooding applications that originally
used halon 1301, as well as some halon 2402. Since the
production and sale of halons were banned in the United
States in 1994, the halon replacement agent of choice in the
streaming sector has been dry chemical, although HFC-236ea
is also used to a limited extent. In the total flooding sector,
HFC-227ea has emerged as the primary replacement for
halon 1301 in applications that require clean agents. Other
HFCs, such as HFC-23, HFC-236fa, and HFC-125, are used
in smaller amounts. The majority of HFC-227ea in total
flooding systems is used to protect essential electronics, as
well as in civil aviation, military mobile weapons systems,
oil/gas/other process industries, and merchant shipping. As
fire protection equipment is tested or deployed, emissions of
these HFCs occur.
4-62 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Methodology
A detailed Vintaging Model of ODS-containing
equipment and products was used to estimate the actual—
versus potential—emissions of various ODS substitutes,
including HFCs and PFCs. The name of the model refers to
the fact that the model tracks the use and emissions of various
compounds for the annual "vintages" of new equipment
that enter service in each end-use. This Vintaging Model
predicts ODS and ODS substitute use in the United States
based on modeled estimates of the quantity of equipment
or products sold each year containing these chemicals and
the amount of the chemical required to manufacture and/or
maintain equipment and products over time. Emissions for
each end-use were estimated by applying annual leak rates
and release profiles, which account for the lag in emissions
from equipment as they leak over time. By aggregating the
data for more than 50 different end-uses, the model produces
estimates of annual use and emissions of each compound.
Further information on the Vintaging Model is contained in
Annex 3.8.
Uncertainty and Time-Series Consistency
Given that emissions of ODS substitutes occur from
thousands of different kinds of equipment and from millions
of point and mobile sources throughout the United States,
emission estimates must be made using analytical tools such
as the Vintaging Model or the methods outlined in IPCC
(2006). Though the model is more comprehensive than the
IPCC default methodology, significant uncertainties still
exist with regard to the levels of equipment sales, equipment
characteristics, and end-use emissions profiles that were used
to estimate annual emissions for the various compounds.
The Vintaging Model estimates emissions from over 50
end-uses. The uncertainty analysis, however, quantifies the
level of uncertainty associated with the aggregate emissions
resulting from the top 17 end-uses, comprising over 95
percent of the total emissions, and 8 other end-uses. These
25 end-uses comprise 97 percent of the total emissions. In
an effort to improve the uncertainty analysis, additional
end-uses are added annually, with the intention that over
time uncertainty for all emissions from the Vintaging Model
will be fully characterized. This year, one new end-use was
included in the uncertainty estimate— extruded polystyrene
sheet foam. Any end-uses included in previous years'
uncertainty analysis were included in the current uncertainty
analysis, whether or not those end-uses were included in the
top 95 percent of emissions from ODS substitutes.
In order to calculate uncertainty, functional forms were
developed to simplify some of the complex "vintaging"
aspects of some end-use sectors, especially with respect to
refrigeration and air-conditioning, and to a lesser degree,
fire extinguishing. These sectors calculate emissions based
on the entire lifetime of equipment, not just equipment put
into commission in the current year, thereby necessitating
simplifying equations. The functional forms used variables
that included growth rates, emission factors, transition from
ODSs, change in charge size as a result of the transition,
disposal quantities, disposal emission rates, and either
stock for the current year or original ODS consumption.
Uncertainty was estimated around each variable within the
functional forms based on expert judgment, and a Monte
Carlo analysis was performed. The most significant sources
of uncertainty for this source category include the emission
factors for retail food equipment and refrigerated transport,
as well as the percent of non-MDI aerosol propellant that
is HFC-152a.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-88. Substitution of ozone
depleting substances HFC and PFC emissions were estimated
to be between 110.8 and 127.7 Tg CO2 Eq. at the 95 percent
Table 4-88: Tier 2 Quantitative Uncertainty Estimates for HFC and PFC Emissions from ODS Substitutes
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)a
Uncertainty Range Relative to Emission Estimate11
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Substitution of Ozone HFCs and
Depleting Substances PFCs 110.0
107.4
124.3
-2%
+ 13%
"2008 Emission estimates and the uncertainty range presented in this table correspond to aerosols, foams, solvents, fire extinguishing agents, and
refrigerants, but not for other remaining categories. Therefore, because the uncertainty associated with emissions from "other" ODS substitutes was not
estimated, they were exclude in the estimates reported in this table.
b Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-63
-------
confidence level. This indicates a range of approximately 2
percent below to 13 percent above the emission estimate of
113.0 Tg CO2 Eq. These estimates exclude about 3 percent
(or 3.0 Tg CO2 Eq.) emissions from 37 end-uses within this
source category, for which quantitative uncertainty estimates
were not developed.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Recalculations Discussion
An extensive review of growth rates and charge sizes for
mobile air conditioning units resulted in updated assumptions
for the Vintaging Model. These changes resulted in an
average annual net increase of 1.7 Tg CO2 Eq. (2.4 percent)
in HFC and PEC emissions from the substitution of ozone
depleting substances for the period 1990 through 2007
relative to the previous Inventory.
4.21. Semiconductor Manufacture
(IPCC Source Category 2F6)
The semiconductor industry uses multiple long-lived
fluorinated gases in plasma etching and plasma enhanced
chemical vapor deposition (PECVD) processes to produce
semiconductor products. The gases most commonly employed
are trifluoromethane (HFC-23 or CHF3), perfluoromethane
(CF4), perfluoroethane (C2F6), nitrogen trifluoride (NF3),
and sulfur hexafluoride (SF6), although other compounds
such as perfluoropropane (C3F8) and perfluorocyclobutane
(c-C4F8) are also used. The exact combination of compounds
is specific to the process employed.
A single 300 mm silicon wafer that yields between
400 to 500 semiconductor products (devices or chips) may
require as many as 100 distinct fluorinated-gas-using process
steps, principally to deposit and pattern dielectric films.
Plasma etching (or patterning) of dielectric films, such as
silicon dioxide and silicon nitride, is performed to provide
pathways for conducting material to connect individual
circuit components in each device. The patterning process
uses plasma-generated fluorine atoms, which chemically
react with exposed dielectric film to selectively remove the
desired portions of the film. The material removed as well
as undissociated fluorinated gases flow into waste streams
and, unless emission abatement systems are employed, into
the atmosphere. PECVD chambers, used for depositing
dielectric films, are cleaned periodically using fluorinated and
other gases. During the cleaning cycle the gas is converted to
fluorine atoms in plasma, which etches away residual material
from chamber walls, electrodes, and chamber hardware.
Undissociated fluorinated gases and other products pass
from the chamber to waste streams and, unless abatement
systems are employed, into the atmosphere. In addition to
emissions of unreacted gases, some fluorinated compounds
can also be transformed in the plasma processes into different
fluorinated compounds which are then exhausted, unless
abated, into the atmosphere. For example, when C2F6 is
used in cleaning or etching, CF4 is generated and emitted
as a process by-product. Besides dielectric film etching
and PECVD chamber cleaning, much smaller quantities
of fluorinated gases are used to etch polysilicon films and
refractory metal films like tungsten.
For 2008, total weighted emissions of all fluorinated
greenhouse gases by the U.S. semiconductor industry were
estimated to be 5.4 Tg CO2 Eq. Combined emissions of all
fluorinated greenhouse gases are presented in Table 4-89
and Table 4-90 below for years 1990, 1995, 2000 and the
period 2005 to 2008. The rapid growth of this industry and
the increasing complexity (growing number of layers)22 of
semiconductor products led to an increase in emissions of
150 percent between 1990 and 1999, when emissions peaked
at 7.2 Tg CO2 Eq. The emissions growth rate began to slow
after 1998, and emissions declined by 26 percent between
1999 and 2008. Together, industrial growth and adoption of
emissions reduction technologies, including but not limited
to abatement technologies, resulted in a net increase in
emissions of 84 percent between 1990 and 2008.
Methodology
Emissions are based on Partner reported emissions
data received through the EPA's PFC Reduction/Climate
Partnership and the EPA's PFC Emissions Vintage Model
(PEVM), a model which estimates industry emissions in
22 Complexity is a term denoting the circuit required to connect the active
circuit elements (transistors) on a chip. Increasing miniaturization, for the
same chip size, leads to increasing transistor density, which, in turn, requires
more complex interconnections between those transistors. This increasing
complexity is manifested by increasing the levels (i.e., layers) of wiring,
with each wiring layer requiring fluorinated gas usage for its manufacture.
4-64 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 4-89: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg C02 Eq.)
Gas
CF4
C2F6
CsFs
c-C4F8
HFC-23
SF6
NF3a
Total
1995
4.9
2000
6.2
2005
1.1
2.0
0.0
0.1
0.2
1.0
0.4
2006
4.4
1.2
2.2
0.0
0.1
0.3
1.0
0.7
4.7
2007
1.3
2.3
0.0
0.1
0.3
0.8
0.5
4.7
2008
1.3
2.6
0.0
0.1
0.3
1.1
0.4
5.4
aNF3 emissions are presented for informational purposes, using the AR4 GWP of 17,200, and are not included in totals.
Note: Totals may not sum due to independent rounding.
Table 4-90: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Mg)
Gas
CF4
C2F6
CsFs
c-C4F8
HFC-23
SF6
NF3
1990
115
160
1
22
3
1995
193
2721
1
37
3
2000
281
321
1
23
45 1
11
2005
168
216
5
13
18
40
26
2006
181
240
5
13
22
40
40
2007
195
246
6
7
22
34
30
2008
199
285
4
7
25
44
21
the absence of emission control strategies (Burton and
Beizaie 2001).23 The availability and applicability of Partner
data differs across the 1990 through 2008 time series.
Consequently, emissions from semiconductor manufacturing
were estimated using four distinct methods, one each for the
periods 1990 through 1994,1995 through 1999,2000 through
2006, and 2007 through 2008.
1990 through 1994
From 1990 through 1994, Partnership data was
unavailable and emissions were modeled using the PEVM
(Burton and Beizaie 2001).24 1990 to 1994 emissions are
23 A Partner refers to a participant in the U.S. EPA PFC Reduction/Climate
Partnership for the Semiconductor Industry. Through a Memorandum of
Understanding (MoU) with the EPA, Partners voluntarily report their PFC
emissions to the EPA by way of a third party, which aggregates the emissions.
24 Various versions of the PEVM exist to reflect changing industrial
practices. From 1990 to 1994 emissions estimates are from PEVM vl.O.
completed in September 1998. The emission factor used to estimate 1990
to 1994 emissions is an average of the 1995 and 1996 emissions factors.
which were derived from Partner reported data for those years.
assumed to be uncontrolled, since reduction strategies such
as chemical substitution and abatement were yet developed.
PEVM is based on the recognition that PFC emissions
from semiconductor manufacturing vary with (1) the number
of layers that comprise different kinds of semiconductor
devices, including both silicon wafer and metal interconnect
layers, and (2) silicon consumption (i.e., the area of
semiconductors produced) for each kind of device. The
product of these two quantities, Total Manufactured Layer
Area (TML A), constitutes the activity data for semiconductor
manufacturing. PEVM also incorporates an emission factor
that expresses emissions per unit of layer-area. Emissions
are estimated by multiplying TMLA by this emission factor.
PEVM incorporates information on the two attributes
of semiconductor devices that affect the number of layers:
(1) linewidth technology (the smallest manufactured feature
Industrial Processes 4-65
-------
size),25 and (2) product type (discrete, memory or logic).26
For each linewidth technology, a weighted average number
of layers is estimated using VLSI product-specific worldwide
silicon demand data in conjunction with complexity factors
(i.e., the number of layers per Integrated Circuit (1C)) specific
to product type (Burton and Beizaie 2001, ITRS 2007).
PEVM derives historical consumption of silicon (i.e., square
inches) by linewidth technology from published data on
annual wafer starts and average wafer size (VLSI Research,
Inc. 2009).
The emission factor in PEVM is the average of four
historical emission factors, each derived by dividing the
total annual emissions reported by the Partners for each of
the four years between 1996 and 1999 by the total TMLA
estimated for the Partners in each of those years. Over this
period, the emission factors varied relatively little (i.e., the
relative standard deviation for the average was 5 percent).
Since Partners are believed not to have applied significant
emission reduction measures before 2000, the resulting
average emission factor reflects uncontrolled emissions.
The emission factor is used to estimate world uncontrolled
emissions using publicly available data on world silicon
consumption.
1995 through 1999
For 1995 through 1999, total U.S. emissions were
extrapolated from the total annual emissions reported by the
Partners (1995 through 1999). Partner-reported emissions
are considered more representative (e.g., in terms of capacity
utilization in a given year) than PEVM estimated emissions,
and are used to generate total U.S. emissions when applicable.
25By decreasing features of 1C components, more components can be
manufactured per device, which increases its functionality. However, as those
individual components shrink it requires more layers to interconnect them
to achieve the functionality. For example, a microprocessor manufactured
with the smallest feature sizes (65 nm) might contain as many as 1 billion
transistors and require as many as 11 layers of component interconnects
to achieve functionality while a device manufactured with 130 nm feature
size might contain a few hundred million transistors and require 8 layers
of component interconnects (ITRS 2007).
26 Memory devices manufactured with the same feature sizes as
microprocessors (a logic device) require approximately one-half the number
of interconnect layers, whereas discrete devices require only a silicon base
layer and no interconnect layers (ITRS 2007). Since discrete devices did
not start using PFCs appreciably until 2004, they are only accounted for in
the PEVM emissions estimates from 2004 onwards.
The emissions reported by the Partners were divided by the
ratio of the total capacity of the plants operated by the
Partners and the total capacity of all of the semiconductor
plants in the United States; this ratio represents the share
of capacity attributable to the Partnership. This method
assumes that Partners and non-Partners have identical
capacity utilizations and distributions of manufacturing
technologies. Plant capacity data is contained in the World
Fab Forecast (WFF) database and its predecessors, which is
updated quarterly (Semiconductor Equipment and Materials
Industry 2009).
2000 through 2006
The emission estimate for the years 2000 through
2006—the period during which Partners began the
consequential application of PFC-reduction measures—was
estimated using a combination of Partner reported emissions
and PEVM modeled emissions. The emissions reported
by Partners for each year were accepted as the quantity
emitted from the share of the industry represented by those
Partners. Remaining emissions, those from non-Partners,
were estimated using PEVM and the method described
above. This is because non-Partners are assumed not to
have implemented any PFC-reduction measures, and PEVM
models emissions without such measures. The portion
of the U.S. total attributed to non-Partners is obtained by
multiplying PEVM's total U.S. emissions figure by the
non-Partner share of U. S. total silicon capacity for each
year as described above.27>28 Annual updates to PEVM
reflect published figures for actual silicon consumption from
VLSI Research, Inc., revisions and additions to the world
population of semiconductor manufacturing plants, and
changes in 1C fabrication practices within the semiconductor
27 This approach assumes that the distribution of linewidth technologies is
the same between Partners and non-Partners. As discussed in the description
of the method used to estimate 2007 emissions, this is not always the case.
28 Generally 5 percent or less of the fields needed to estimate TMLA shares
are missing values in the World Fab Watch databases. In the 2007 World
Fab Watch database used to generate the 2006 non-Partner TMLA capacity
share, these missing values were replaced with the corresponding mean
TMLA across fabs manufacturing similar classes of products. However,
the impact of replacing missing values on the non-Partner TMLA capacity
share was inconsequential.
4-66 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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industry (see, ITRS, 2007 and Semiconductor Equipment
and Materials Industry 2009).29'30-31
2007 through 2008
For the years 2007 and 2008, emissions were also
estimated using a combination of Partner reported emissions
and PEVM modeled emissions; however, two improvements
were made to the estimation method employed for the
previous years in the time series. First, the 2007 and 2008
emission estimates account for the fact that Partners and
non-Partners employ different distributions of manufacturing
technologies, with the Partners using manufacturing
technologies with greater transistor densities and therefore
greater numbers of layers.32 Second, the scope of the 2007
and 2008 estimates is expanded relative to the estimates
for the years 2000 through 2006 to include emissions from
Research and Development fabs. This was feasible through
29 Special attention was given to the manufacturing capacity of plants that
use wafers with 300 mm diameters because the actual capacity of these
plants is ramped up to design capacity, typically over a 2-3 year period. To
prevent overstating estimates of partner-capacity shares from plants using
300 mm wafers, design capacities contained in WFW were replaced with
estimates of actual installed capacities for 2004 published by Citigroup
Smith Barney (2005). Without this correction, the partner share of capacity
would be overstated, by approximately 5 percentage points. For perspective,
approximately 95 percent of all new capacity additions in 2004 used 300
mm wafers and by year-end those plants, on average, could operate at
approximately 70 percent of the design capacity. For 2005, actual installed
capacities were estimated using an entry in the World Fab Watch database
(April 2006 Edition) called "wafers/month, 8-inch equivalent", which
denoted the actual installed capacity instead of the fully-ramped capacity.
For 2006, actual installed capacities of new fabs were estimated using an
average monthly ramp rate of 1100 wafer starts per month (wspm) derived
from various sources such as semiconductor fabtech, industry analysts,
and articles in the trade press. The monthly ramp rate was applied from
the first-quarter of silicon volume (FQSV) to determine the average design
capacity over the 2006 period.
30 In 2006, the industry trend in co-ownership of manufacturing facilities
continued. Several manufacturers, who are Partners, now operate fabs with
other manufacturers, who in some cases are also Partners and in other cases
not Partners. Special attention was given to this occurrence when estimating
the Partner and non-Partner shares of U.S. manufacturing capacity.
31 Two versions of PEVM are used to model non-Partner emissions during
this period. For the years 2000 to 2003 PEVM v3.2.0506.0507 was used to
estimate non-Partner emissions. During this time, discrete devices did not use
PFCs during manufacturing and therefore only memory and logic devices
were modeled in the PEVM v3.2.0506.0507. From 2004 onwards, discrete
device fabrication started to use PFCs, hence PEVM v4.0.0701.0701, the
first version of PEVM to account for PFC emissions from discrete devices,
was used to estimate non-Partner emissions for this time period.
32EPA considered applying this change to years before 2007, but found
that it would be difficult due to the large amount of data (i.e., technology-
specific global and non-Partner TMLA) that would have to be examined
and manipulated for each year. This effort did not appear to be justified
given the relatively small impact of the improvement on the total estimate
for 2007 and the fact that the impact of the improvement would likely be
lower for earlier years because the estimated share of emissions accounted
for by non-Partners is growing as Partners continue to implement emission-
reduction efforts.
the use of more detailed data published in the World Fab
Forecast. PEVM databases are updated annually as described
above. The published world average capacity utilization for
2007 and 2008 was used for production fabs while in 2008
for R&D fabs a 20 percent figure was assumed (SIA 2009).
In addition, publicly available actual utilization data
was used to account for differences in fab utilization for
manufacturers of discrete and 1C products for the emissions
of 2008 for non-partner. PEVM estimates were adjusted
using technology weighted capacity shares that reflect
relative influence of different utilization.
Gas-Specific Emissions
Two different approaches were also used to estimate
the distribution of emissions of specific fluorinated gases.
Before 1999, when there was no consequential adoption
of fluorinated-gas-reducing measures, a fixed distribution
of fluorinated-gas-use was assumed to apply to the entire
U.S. industry. This distribution was based upon the average
fluorinated-gas purchases by semiconductor manufacturers
during this period and the application of IPCC default
emission factors for each gas (Burton and Beizaie 2001).
For the 2000 through 2008 period, the 1990 through 1999
distribution was assumed to apply to the non-Partners.
Partners, however, began reporting gas-specific emissions
during this period. Thus, gas-specific emissions for 2000
through 2008 were estimated by adding the emissions
reported by the Partners to those estimated for the non-
Partners.
Data Sources
Partners estimate their emissions using a range of
methods. For 2008, it is assumed that most Partners
used a method at least as accurate as the IPCC's Tier 2a
Methodology, recommended in the IPCC Guidelines for
National Greenhouse Inventories (2006). The Partners with
relatively high emissions use leading-edge manufacturing
technology, the newest process equipment. When purchased,
this equipment is supplied with fluorinated-gas emission
factors, measured using industry standard guidelines
(International Sematech 2006). The larger emitting Partners
likely use these process-specific emission factors instead of
the somewhat less representative default emission factors
provided in the IPCC guidelines. Data used to develop
Industrial Processes 4-67
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emission estimates are attributed in part to estimates provided
by the members of the Partnership, and in part from data
obtained from PEVM estimates. Estimates of operating
plant capacities and characteristics for Partners and non-
Partners were derived from the Semiconductor Equipment
and Materials Industry (SEMI) World Fab Forecast (formerly
World Fab Watch) database (1996 through 2009). Actual
world capacity utilizations for 2008 were obtained from
Semiconductor International Capacity Statistics (SICAS).
Estimates of silicon consumed by linewidth from 1990
through 2008 were derived from information from VLSI
Research (2009), and the number of layers per linewidth
was obtained from International Technology Roadmap for
Semiconductors: 2006 Update (Burton and Beizaie 2001,
ITRS 2007, ITRS 2008).
Uncertainty and Time Series Consistency
A quantitative uncertainty analysis of this source
category was performed using the IPCC-recommended Tier
2 uncertainty estimation methodology, the Monte Carlo
Stochastic Simulation technique. The equation used to
estimate uncertainty is:
U.S. emissions = Z Partnership gas-specific submittals +
[(non-Partner share of world TMLA) x
(PEVM Emission Factor x World TMLA)]
The Monte Carlo analysis results presented below relied
on estimates of uncertainty attributed to the four quantities on
the right side of the equation. Estimates of uncertainty for
the four quantities were in turn developed using the estimated
uncertainties associated with the individual inputs to each
quantity, error propagation analysis, Monte Carlo simulation,
and expert judgment. The relative uncertainty associated
with World TMLA estimate in 2008 is about ±10 percent,
based on the uncertainty estimate obtained from discussions
with VLSI, Inc. For the share of World layer-weighted
silicon capacity accounted for by non-Partners, a relative
uncertainty of ±8 percent was estimated based on a separate
Monte Carlo simulation to account for the random occurrence
of missing data in the World Fab Watch database. For the
aggregate PFC emissions data supplied to the partnership,
a relative uncertainty of ±50 percent was estimated for
each gas-specific PFC emissions value reported by an
individual Partner, and error propagation techniques were
used to estimate uncertainty for total Partnership gas-specific
submittals.33 A relative error of approximately 10 percent
was estimated for the PEVM emission factor, based on the
standard deviation of the 1996 to 1999 emission factors.34 All
estimates of uncertainties are given at 95-percent confidence
intervals.
In developing estimates of uncertainty, consideration
was also given to the nature and magnitude of the potential
bias that World activity data (i.e., World TMLA) might
have in its estimates of the number of layers associated with
devices manufactured at each technology node. The result
of a brief analysis indicated that U.S. TMLA overstates the
average number of layers across all product categories and
all manufacturing technologies for 2004 by 0.12 layers or 2.9
percent. The same upward bias is assumed for World TMLA,
and is represented in the uncertainty analysis by deducting
the absolute bias value from the World activity estimate when
it is incorporated into the Monte Carlo analysis.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-91. The emissions estimate for
total U.S. PFC emissions from semiconductor manufacturing
Table 4-91: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from Semiconductor
Manufacture (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)a
Uncertainty Range Relative to Emission Estimate11
(TgC02Eq.) (%)
Semiconductor
Manufacture
HFC, PFC,
and SF6
Lower Bound0
5.7 5.2
Upper Bound0
6.2
Lower Bound
-9%
Upper Bound
+ 9%
3 Because the uncertainty analysis covered all emissions (including NF3), the emission estimate presented here does not match that shown in Table 4-90.
b Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
c Absolute lower and upper bounds were calculated using the corresponding lower and upper bounds in percentages.
33 Error propagation resulted in Partnership gas-specific uncertainties ranging
from 17 to 27 percent.
34 The average of 1996 to 1999 emission factor is used to derive the PEVM
emission factor.
4-68 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
were estimated to be between 5.2 and 6.2 Tg CO2 Eq. at a
95 percent confidence level. This range represents 9 percent
below to 9 percent above the 2008 emission estimate of
5.7 Tg CO2 Eq. This range and the associated percentages
apply to the estimate of total emissions rather than those of
individual gases. Uncertainties associated with individual
gases will be somewhat higher than the aggregate, but were
not explicitly modeled.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
With the exception of possible future updates to emission
factors, the method to estimate non-Partner related emissions
(i.e., PEVM) is not expected to change. Future improvements
to the national emission estimates will primarily be associated
with determining the portion of national emissions to attribute
to Partner report totals (about 80 percent in recent years)
and improvements in estimates of non-Partner totals. As
the nature of the Partner reports change through time and
industry-wide reduction efforts increase, consideration will
be given to what emission reduction efforts—if any—are
likely to be occurring at non-Partner facilities. Currently,
none are assumed to occur.
Another point of consideration for future national
emissions estimates is the inclusion of PFC emissions from
heat transfer fluid (HTF) loss to the atmosphere and the
production of photovoltaic cells (PVs). Heat transfer fluids,
of which some are liquid perfluorinated compounds, are used
during testing of semiconductor devices and, increasingly, are
used to manage heat during the manufacture of semiconductor
devices. Evaporation of these fluids is a source of emissions
(EPA 2006). PFCs are also used during manufacture of PV
cells that use silicon technology, specifically, crystalline,
poly crystalline and amorphous silicon technologies. PV
manufacture is growing in the United States, and therefore
may be expected to constitute a growing share of U.S. PFC
emissions from the electronics sector.
4.22. Electrical Transmission and
Distribution (IPCC Source Category
2F7)
The largest use of SF6, both in the United States and
internationally, is as an electrical insulator and interrupter in
equipment that transmits and distributes electricity (RAND
2004). The gas has been employed by the electric power
industry in the United States since the 1950s because of its
dielectric strength and arc-quenching characteristics. It is
used in gas-insulated substations, circuit breakers, and other
switchgear. Sulfur hexafluoride has replaced flammable
insulating oils in many applications and allows for more
compact substations in dense urban areas.
Fugitive emissions of SF6 can escape from gas-insulated
substations and switchgear through seals, especially from
older equipment. The gas can also be released during
equipment manufacturing, installation, servicing, and
disposal. Emissions of SF6 from equipment manufacturing
and from electrical transmission and distribution systems
were estimated to be 13.1 Tg CO2 Eq. (0.5 Gg) in 2008. This
quantity represents a 51 percent decrease from the estimate
for 1990 (see Table 4-92 and Table 4-93). This decrease is
believed to have two causes: a sharp increase in the price
of SF6 during the 1990s and a growing awareness of the
environmental impact of SF6 emissions through programs
such as EPA's SF6 Emission Reduction Partnership for
Electric Power Systems.
Table 4-92: SF6 Emissions from Electric Power Systems
and Electrical Equipment Manufacturers (Tg C02 Eq.)
Year
Electric Power
Systems
Electrical Equipment
Manufacturers
Total
1990
1995
26.3
20.9
0.3
0.5
26.6
21.4
2000
14.3
0.7
15.0
2005
2006
2007
2008
13.1
12.4
12.0
11.9
0.8
0.8
0.7
1.1
14.0
13.2
12.7
13.1
Industrial Processes 4-69
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Table 4-93: SF6 Emissions from Electric Power Systems
and Electrical Equipment Manufacturers (Gg)
Year
Emissions
1990
1.1
2005
2006
2007
2008
0.6
0.6
0.5
0.5
Methodology
The estimates of emissions from Electric Transmission
and Distribution are comprised of emissions from electric
power systems and emissions from the manufacture of
electrical equipment. The methodologies for estimating both
sets of emissions are described below.
1999 through 2008 Emissions from Electric Power Systems
Emissions from electric power systems from 1999 to
2008 were estimated based on: (1) reporting from utilities
participating in EPA's SF6 Emission Reduction Partnership
for Electric Power Systems (Partners), which began in
1999; and, (2) the relationship between emissions and
utilities' transmission miles as reported in the 2001, 2004
and 2007 Utility Data Institute (UDI) Directories of Electric
Power Producers and Distributors (UDI 2001, 2004, 2007).
(Transmission miles are defined as the miles of lines carrying
voltages above 34.5 kV.) Over the period from 1999 to 2008,
Partner utilities, which for inventory purposes are denned
as utilities that either currently are or previously have been
part of the Partnership, represented between 42 percent and
47 percent of total U.S. transmission miles. For each year,
the emissions reported by or estimated for Partner utilities
were added to the emissions estimated for utilities that have
never participated in the Partnership (i.e., non-Partners).35
Partner utilities estimated their emissions using a Tier
3 utility-level mass balance approach (IPCC 2006). If a
Partner utility did not provide data for a particular year,
emissions were interpolated between years for which data
were available or extrapolated based on Partner-specific
transmission mile growth rates. In 2008, non-reporting
Partners accounted for approximately 9 percent of the total
emissions attributed to Partner utilities.
Emissions from non-Partners in every year since 1999
were estimated using the results of a regression analysis
that showed that the emissions from reporting utilities were
most strongly correlated with their transmission miles. The
results of this analysis are not surprising given that, in the
United States, SF6 is contained primarily in transmission
equipment rated at or above 34.5 kV. The equations were
developed based on the 1999 SF6 emissions reported by 42
Partner utilities (representing approximately 23 percent of
U.S. transmission miles) and 2000 transmission mileage data
obtained from the 2001 UDI Directory of Electric Power
Producers and Distributors (UDI 2001). Two equations
were developed, one for small and one for large utilities
(i.e., with fewer or more than 10,000 transmission miles,
respectively). The distinction between utility sizes was made
because the regression analysis showed that the relationship
between emissions and transmission miles differed for small
and large transmission networks. The same equations were
used to estimate non-Partner emissions in 1999 and every
year thereafter because non-Partners were assumed not to
have implemented any changes that would have resulted in
reduced emissions since 1999.
The regression equations are:
Non-Partner small utilities (less than 10,000 transmission
miles, in kilograms):
Emissions (kg) = 0.89 x Transmission Miles
Non-Partner large utilities (more than 10,000 transmission
miles, in kilograms):
Emissions (kg) = 0.58 x Transmission Miles
Data on transmission miles for each non-Partner utility
for the years 2000, 2003, and 2006 were obtained from the
2001, 2004, and 2007 UDI Directories of Electric Power
Producers and Distributors, respectively (UDI 2001, 2004,
2007). The U.S. transmission system grew by over 22,000
miles between 2000 and 2003 and by over 55,000 miles
between 2003 and 2006. These periodic increases are
assumed to have occurred gradually, therefore transmission
mileage was assumed to increase at an annual rate of 1.2
percent between 2000 and 2003 and 2.8 percent between
2003 and 2006. Transmission miles in 2008 were then
35 Partners in EPA's SF6 Emission Reduction Partnership reduced their
emissions by approximately 61% from 1999 to 2008.
4-70 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
extrapolated from 2006 based on the 2.8 percent annual
growth rate.
As a final step, total electric power system emissions
were determined for each year by summing the Partner
reported and estimated emissions (reported data was available
through the EPA's SF6 Emission Reduction Partnership for
Electric Power Systems) and the non-Partner emissions
(determined using the 1999 regression equations).
1990 through 1998 Emissions from Electric Power Systems
Because most participating utilities reported emissions
only for 1999 through 2008, modeling was used to estimate
SF6 emissions from electric power systems for the years 1990
through 1998. To perform this modeling, U.S. emissions
were assumed to follow the same trajectory as global
emissions from this source during the 1990 to 1999 period.
To estimate global emissions, the RAND survey of global
SF6 sales were used, together with the following equation
for estimating emissions, which is derived from the mass-
balance equation for chemical emissions (Volume 3, Equation
7.3) in the IPCC Guidelines for National Greenhouse Gas
Inventories (IPCC 2006).36 (Although equation 7.3 of the
IPCC Guidelines appears in the discussion of substitutes for
ozone-depleting substances, it is applicable to emissions from
any long-lived pressurized equipment that is periodically
serviced during its lifetime.)
Emissions (kilograms SF6) = SF6 purchased to refill
existing equipment (kilograms) + SF6 nameplate capacity
of retiring equipment (kilograms)37
Note that the above equation holds whether the gas
from retiring equipment is released or recaptured; if the gas
is recaptured, it is used to refill existing equipment, thereby
lowering the amount of SF6 purchased by utilities for this
purpose.
Gas purchases by utilities and equipment manufacturers
from 1961 through 2003 are available from the RAND
(2004) survey. To estimate the quantity of SF6 released or
recovered from retiring equipment, the nameplate capacity
36Ideally, sales to utilities in the U.S. between 1990 and 1999 would be
used as a model. However, this information was not available. There were
only two U.S. manufacturers of SF6 during this time period, so it would not
have been possible to conceal sensitive sales information by aggregation.
37 Nameplate capacity is defined as the amount of SF6 within fully charged
electrical equipment.
of retiring equipment in a given year was assumed to equal
81.2 percent of the amount of gas purchased by electrical
equipment manufacturers 40 years previous (e.g., in 2000,
the nameplate capacity of retiring equipment was assumed
to equal 81.2 percent of the gas purchased in 1960). The
remaining 18.8 percent was assumed to have been emitted
at the time of manufacture. The 18.8 percent emission factor
is an average of IPCC default SF6 emission rates for Europe
and Japan for 1995 (IPCC 2006). The 40-year lifetime for
electrical equipment is also based on IPCC (2006). The
results of the two components of the above equation were
then summed to yield estimates of global SF6 emissions from
1990 through 1999.
U.S. emissions between 1990 and 1999 are assumed to
follow the same trajectory as global emissions during this
period. To estimate U.S. emissions, global emissions for each
year from 1990 through 1998 were divided by the estimated
global emissions from 1999. The result was a time series of
factors that express each year's global emissions as a multiple
of 1999 global emissions. Historical U.S. emissions were
estimated by multiplying the factor for each respective year
by the estimated U.S. emissions of SF6 from electric power
systems in 1999 (estimated to be 15.0 Tg CO2 Eq.).
Two factors may affect the relationship between the
RAND sales trends and actual global emission trends. One is
utilities' inventories of SF6 in storage containers. When SF6
prices rise, utilities are likely to deplete internal inventories
before purchasing new SF6 at the higher price, in which case
SF6 sales will fall more quickly than emissions. On the other
hand, when SF6 prices fall, utilities are likely to purchase
more SF6 to rebuild inventories, in which case sales will
rise more quickly than emissions. This effect was accounted
for by applying 3-year smoothing to utility SF6 sales data.
The other factor that may affect the relationship between
the RAND sales trends and actual global emissions is the
level of imports from and exports to Russia and China. SF6
production in these countries is not included in the RAND
survey and is not accounted for in any another manner by
RAND. However, atmospheric studies confirm that the
downward trend in estimated global emissions between 1995
and 1998 was real (see the Uncertainty discussion below).
Industrial Processes 4-71
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1990 through 2008 Emissions from Manufacture of
Electrical Equipment
The 1990 to 2008 emission estimates for original
equipment manufacturers (OEMs) were derived by assuming
that manufacturing emissions equal 10 percent of the quantity
of SF6 provided with new equipment. The quantity of SF6
provided with new equipment was estimated based on
statistics compiled by the National Electrical Manufacturers
Association (NEMA). These statistics were provided for
1990 to 2000; the quantities of SF6 provided with new
equipment for 2001 to 2008 were estimated using Partner
reported data and the total industry SF6 nameplate capacity
estimate (136.3 Tg CO2 Eq. in 2008). Specifically, the ratio
of new nameplate capacity to total nameplate capacity of a
subset of Partners for which new nameplate capacity data
was available from 1999 to 2008 was calculated. This ratio
was then multiplied by the total industry nameplate capacity
estimate to derive the amount of SF6 provided with new
equipment for the entire industry. The 10 percent emission
rate is the average of the "ideal" and "realistic" manufacturing
emission rates (4 percent and 17 percent, respectively)
identified in a paper prepared under the auspices of the
International Council on Large Electric Systems (CIGRE)
in February 2002 (O'Connell et al. 2002).
Uncertainty and Time-Series Consistency
To estimate the uncertainty associated with emissions
of SF6 from Electric Transmission and Distribution,
uncertainties associated with three quantities were estimated:
(1) emissions from Partners; (2) emissions from non-
Partners; and (3) emissions from manufacturers of electrical
equipment. A Monte Carlo analysis was then applied to
estimate the overall uncertainty of the emissions estimate.
Total emissions from the SF6 Emission Reduction
Partnership include emissions from both reporting and non-
reporting Partners. For reporting Partners, individual Partner-
reported SF6 data was assumed to have an uncertainty of 10
percent. Based on a Monte Carlo analysis, the cumulative
uncertainty of all Partner reported data was estimated to be
3.6 percent. The uncertainty associated with extrapolated
or interpolated emissions from non-reporting Partners was
assumed to be 20 percent.
There are two sources of uncertainty associated with
the regression equations used to estimate emissions in 2008
from non-Partners: (1) uncertainty in the coefficients (as
defined by the regression standard error estimate), and (2) the
uncertainty in total transmission miles for non-Partners. In
addition, there is uncertainty associated with the assumption
that the emission factor used for non-Partner utilities (which
accounted for approximately 58 percent of U.S. transmission
miles in 2008) will remain at levels defined by Partners who
reported in 1999. However, the last source of uncertainty
was not modeled.
Uncertainties were also estimated regarding (1) the
quantity of SF6 supplied with equipment by equipment
manufacturers, which is projected from Partner provided
nameplate capacity data and industry SF6 nameplate capacity
estimates, and (2) the manufacturers' SF6 emissions rate.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-94. Electrical Transmission and
Distribution SF6 emissions were estimated to be between
10.1 and 16.2 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 23 percent below and
24 percent above the emission estimate of 13.1 Tg CO2 Eq.
In addition to the uncertainty quantified above, there
is uncertainty associated with using global SF6 sales data
to estimate U.S. emission trends from 1990 through 1999.
However, the trend in global emissions implied by sales of
SF6 appears to reflect the trend in global emissions implied
by changing SF6 concentrations in the atmosphere. That
is, emissions based on global sales declined by 29 percent
between 1995 and 1998, and emissions based on atmospheric
measurements declined by 27 percent over the same period.
Table 4-94: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Electrical Transmission and
Distribution (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Electrical Transmission
and Distribution
SFB
13.1
10.1
16.2
-23%
+24%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-72 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Box 4-1: Potential Emission Estimates of MFCs, PFCs, and SF6
Emissions of MFCs, PFCs and SF6 from industrial processes can be estimated in two ways, either as potential emissions or as actual
emissions. Emission estimates in this chapter are "actual emissions," which are defined by the Revised 1996IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997) as estimates that take into account the time lag between consumption and
emissions. In contrast, "potential emissions" are defined to be equal to the amount of a chemical consumed in a country, minus the amount
of a chemical recovered for destruction or export in the year of consideration. Potential emissions will generally be greater for a given year
than actual emissions, since some amount of chemical consumed will be stored in products or equipment and will not be emitted to the
atmosphere until a later date, if ever. Although actual emissions are considered to be the more accurate estimation approach for a single
year, estimates of potential emissions are provided for informational purposes.
Separate estimates of potential emissions were not made for industrial processes that fall into the following categories:
• Byproduct emissions. Some emissions do not result from the consumption or use of a chemical, but are the unintended byproducts
of another process. For such emissions, which include emissions of CF4 and C2F6 from aluminum production and of HFC-23 from
HCFC-22 production, the distinction between potential and actual emissions is not relevant.
• Potential emissions that equal actual emissions. For some sources, such as magnesium production and processing, no delay
between consumption and emission is assumed and, consequently, no destruction of the chemical takes place. In this case, actual
emissions equal potential emissions.
Table 4-95 presents potential emission estimates for MFCs and PFCs from the substitution of ozone depleting substances, MFCs,
PFCs, and SF6 from semiconductor manufacture, and SF6 from magnesium production and processing and electrical transmission and
distribution.38 Potential emissions associated with the substitution for ozone depleting substances were calculated using the EPA's Vintaging
Model. Estimates of MFCs, PFCs, and SF6 consumed by semiconductor manufacture were developed by dividing chemical-by-chemical
emissions by the appropriate chemical-specific emission factors from the IPCC Good Practice Guidance (Tier 2c). Estimates of CF4
consumption were adjusted to account for the conversion of other chemicals into CF4 during the semiconductor manufacturing process,
again using the default factors from the IPCC Good Practice Guidance. Potential SF6 emissions estimates for electrical transmission and
distribution were developed using U.S. utility purchases of SF6 for electrical equipment. From 1999 through 2007, estimates were obtained
from reports submitted by participants in EPA's SF6 Emission Reduction Partnership for Electric Power Systems. U.S. utility purchases of
SF6 for electrical equipment from 1990 through 1998 were backcasted based on world sales of SF6 to utilities. Purchases of SF6 by utilities
were added to SF6 purchases by electrical equipment manufacturers to obtain total SF6 purchases by the electrical equipment sector.
Table 4-95: 2008 Potential and Actual Emissions of MFCs, PFCs, and SF6
from Selected Sources (Tg C02 Eq.)
Source
Substitution of Ozone Depleting Substances
Aluminum Production
HCFC-22 Production
Semiconductor Manufacture
Magnesium Production and Processing
Electrical Transmission and Distribution
Potential
187.3
-
-
3.6
2.0
30.0
Actual
113.0
2.7
13.6
5.4
2.0
13.1
-Not applicable.
38 See Annex 5 for a discussion of sources of SF6 emissions excluded from the actual emissions estimates in this report.
Several pieces of evidence indicate that U.S. SF6 U.S. utilities began recycling rather than venting SF6 within
emissions were reduced as global emissions were reduced, two years of the price rise. Finally, the emissions reported
First, the decreases in sales and emissions coincided with by the one U.S. utility that reported 1990 through 1999
a sharp increase in the price of SF6 that occurred in the emissions to EPA showed a downward trend beginning in
mid-1990s and that affected the United States as well as the mid-1990s.
the rest of the world. A representative from DILO, a major Methodological recalculations were applied to the entire
manufacturer of SF6 recycling equipment, stated that most time series to ensure time.series consistency from 1990
Industrial Processes 4-73
-------
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Recalculations Discussion
Sulfur hexafluoride emission estimates for the period
1990 through 2007 were updated based on (1) new data from
EPA's SF6 Emission Reduction Partnership; (2) revisions to
interpolated and extrapolated non-reported Partner data; and
(3) a revised regression equation coefficient for non-Partner
small utilities (fewer than 10,000 transmission miles). The
new regression coefficient resulted from a revised 1999
emission estimate from a Partner of EPA's SF6 Emission
Reduction Partnership. This new emission estimate changed
the regression coefficient slightly from 0.888 to 0.890 kg
of emissions per transmission mile. Based on the revisions
listed above, SF6 emissions from electric transmission and
distribution decreased between 0.04 to 0.73 percent for each
year from 1990 through 2005 and increased by 0.10 and 0.15
percent for 2006 and 2007, respectively.
4.23. Industrial Sources of Indirect
Greenhouse Gases
In addition to the main greenhouse gases addressed
above, many industrial processes generate emissions of
indirect greenhouse gases. Total emissions of nitrogen
oxides (NOX), carbon monoxide (CO), and non-CH4 volatile
organic compounds (NMVOCs) from non-energy industrial
processes from 1990 to 2008 are reported in Table 4-96.
Methodology
These emission estimates were obtained from preliminary
data (EPA 2009), and disaggregated based on EPA (2003),
which, in its final iteration, will be published on the National
Emission Inventory (NEI) Air Pollutant Emission Trends
web site. Emissions were calculated either for individual
categories or for many categories combined, using basic
activity data (e.g., the amount of raw material processed)
as an indicator of emissions. National activity data were
collected for individual categories from various agencies.
Table 4-96: NOX, CO, and NMVOC Emissions from Industrial Processes (Gg)
Gas/Source
1990
1995
2000
2005
2006
2007
2008
NOX
Other Industrial Processes
Chemical & Allied Product Manufacturing
Metals Processing
Storage and Transport
Miscellaneous3
CO
Metals Processing
Other Industrial Processes
Chemical & Allied Product Manufacturing
Storage and Transport
Miscellaneous3
NMVOCs
Storage and Transport
Other Industrial Processes
Chemical & Allied Product Manufacturing
Metals Processing
Miscellaneous3
591
3431
152
i
4,125
2,395
4871
1,073
691
101
2,422
1,352
3641
5751
111
20
607
362
143
89
3,959
2,159
566
1,110
23
102
2,642
1,499
408
599
113
23
626
435
95
81
14
2
2,216
1,175
537
327
153
23
1,773
1,067
412
230
61
3
569
437
55
60
15
2
1,555
752
484
189
97
32
1,997
1,308
415
213
44
17
553
418
57
61
15
2
1,597
788
474
206
100
30
1,933
1,266
398
211
44
14
537
398
59
62
16
2
1,640
824
464
223
103
27
1,869
1,224
383
210
43
10
520
379
61
62
16
2
1,682
859
454
240
104
25
1,804
1,182
367
207
42
7
"Miscellaneous includes the following categories: catastrophic/accidental release, other combustion
It does not include agricultural fires or slash/prescribed burning, which are accounted for under the
Note: Totals may not sum due to independent rounding.
health services, cooling towers, and fugitive dust.
Field Burning of Agricultural Residues source.
4-74 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Depending on the category, these basic activity data may
include data on production, fuel deliveries, raw material
processed, etc.
Activity data were used in conjunction with emission
factors, which together relate the quantity of emissions to the
activity. Emission factors are generally available from the
EPA's Compilation of Air Pollutant Emission Factors, AP-42
(EPA 1997). The EPA currently derives the overall emission
control efficiency of a source category from a variety of
information sources, including published reports, the 1985
National Acid Precipitation and Assessment Program
Emissions Inventory, and other EPA databases.
Uncertainty and Time-Series Consistency
Uncertainties in these estimates are partly due to the
accuracy of the emission factors used and accurate estimates
of activity data. A quantitative uncertainty analysis was not
performed.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Industrial Processes 4-75
-------
5. Solvent and Other Product Use
Greenhouse gas emissions are produced as a by-product of various solvent and other product uses. In the United
States, emissions from Nitrous Oxide (N2O) Product Usage, the only source of greenhouse gas emissions from
this sector, accounted for less than 0.1 percent of total U.S. anthropogenic greenhouse gas emissions on a carbon
equivalent basis in 2008 (see Table 5-1). Indirect greenhouse gas emissions also result from solvent and other product use,
and are presented in Table 5-5 in gigagrams (Gg).
Table 5-1: N20 Emissions from Solvent and Other Product Use (Tg C02 Eq. and Gg)
Gas/Source
N20 from Product Uses
Tg C02 Eq.
Gg
1990
4.4l
14
1995
4.6
15
2000
4.9 1
16
2005
4.4
14
2006
4.4
14
2007
4.4
14
2008
4.4
14
5.1 Nitrous Oxide from Product Uses (IPCC Source Category 3D)
Nitrous oxide is a clear, colorless, oxidizing liquefied gas, with a slightly sweet odor. Two companies operate a total
of five N2O production facilities in the United States (Airgas 2007, FTC 2001). Nitrous oxide is primarily used in carrier
gases with oxygen to administer more potent inhalation anesthetics for general anesthesia and as an anesthetic in various
dental and veterinary applications. As such, it is used to treat short-term pain, for sedation in minor elective surgeries, and
as an induction anesthetic. The second main use of N2O is as a propellant in pressure and aerosol products, the largest
application being pressure-packaged whipped cream. Small quantities of N2O also are used in the following applications:
• Oxidizing agent and etchant used in semiconductor manufacturing;
• Oxidizing agent used, with acetylene, in atomic absorption spectrometry;
• Production of sodium azide, which is used to inflate airbags;
• Fuel oxidant in auto racing; and
• Oxidizing agent in blowtorches used by jewelers and others (Heydorn 1997).
Production of N2O in 2008 was approximately 15 Gg (Table 5-2). Nitrous oxide emissions were 4.4 Tg CO2 Eq. (14 Gg)
in 2008 (Table 5-3). Production of N2O stabilized during the 1990s because medical markets had found other substitutes
for anesthetics, and more medical procedures were being performed on an outpatient basis using local anesthetics that do
not require N2O. The use of N2O as a propellant for whipped cream has also stabilized due to the increased popularity of
cream products packaged in reusable plastic tubs (Heydorn 1997).
Solvent and Other Product Use 5-1
-------
Table 5-2: N20 Production (Gg)
Year
Gg
2005
2006
2007
2008
Table 5-3: N20 Emissions from N20 Product Usage
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
4.4
14
2005
2006
2007
2008
4.4
4.4
4.4
4.4
14
14
14
14
Methodology
Emissions from N2O product usage were calculated
by first multiplying the total amount of N2O produced in
the United States by the share of the total quantity of N2O
attributed to each end use. This value was then multiplied
by the associated emission rate for each end use. After the
emissions were calculated for each end use, they were added
together to obtain a total estimate of N2O product usage
emissions. Emissions were determined using the following
equation:
N2O Product Usage Emissions =
Zi [Total U.S. Production of N2O] x
[Share of Total Quantity of N2O Usage by Sector i] x
[Emissions Rate for Sector i]
where,
i = Sector.
The share of total quantity of N2O usage by end use
represents the share of national N2O produced that is used
by the specific subcategory (i.e., anesthesia, food processing,
etc.). In 2008, the medical/dental industry used an estimated
89.5 percent of total N2O produced, followed by food
processing propellants at 6.5 percent. All other categories
combined used the remainder of the N2O produced. This
subcategory breakdown has changed only slightly over the
past decade. For instance, the small share of N2O usage in the
production of sodium azide has declined significantly during
the decade of the 1990s. Due to the lack of information
on the specific time period of the phase-out in this market
subcategory, most of the N2O usage for sodium azide
production is assumed to have ceased after 1996, with the
majority of its small share of the market assigned to the larger
medical/dental consumption subcategory (Heydorn 1997).
The N2O was allocated across the following categories:
medical applications, food processing propellant, and sodium
azide production (pre-1996). A usage emissions rate was
then applied for each sector to estimate the amount of N2O
emitted.
Only the medical/dental and food propellant
subcategories were estimated to release emissions into the
atmosphere, and therefore these subcategories were the only
usage subcategories with emission rates. For the medical/
dental subcategory, due to the poor solubility of N2O in
blood and other tissues, none of the N2O is assumed to be
metabolized during anesthesia and quickly leaves the body in
exhaled breath. Therefore, an emission factor of 100 percent
was used for this subcategory (IPCC 2006). For N2O used
as a propellant in pressurized and aerosol food products,
none of the N2O is reacted during the process and all of the
N2O is emitted to the atmosphere, resulting in an emission
factor of 100 percent for this subcategory (IPCC 2006). For
the remaining subcategories, all of the N2O is consumed/
reacted during the process, and therefore the emission rate
was considered to be zero percent (Tupman 2002).
The 1990 through 1992 N2O production data were
obtained from SRI Consulting's Nitrous Oxide, North
America report (Heydorn 1997). N2O production data for
1993 through 1995 were not available. Production data for
1996 was specified as a range in two data sources (Heydorn
1997, Tupman 2002). In particular, for 1996, Heydorn (1997)
5-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
estimates N2O production to range between 13.6 and 18.1
thousand metric tons. Tupman (2003) provided a narrower
range (i.e., 15.9 to 18.1 thousand metric tons) for 1996 that
falls within the production bounds described by Heydorn
(1997). Tupman (2003) data are considered more industry-
specific and current. Therefore, the midpoint of the narrower
production range was used to estimate N2O emissions for
years 1993 through 2001 (Tupman 2003). The 2002 and 2003
N2O production data were obtained from the Compressed
Gas Association Nitrous Oxide Fact Sheet and Nitrous Oxide
Abuse Hotline (CGA 2002, 2003). These data were also
provided as a range. For example, in 2003, CGA (2003)
estimates N2O production to range between 13.6 and 15.9
thousand metric tons. Due to unavailable data, production
for years 2004 through 2008 were held at the 2003 value.
The 1996 share of the total quantity of N2O used by
each subcategory was obtained from SRI Consulting's
Nitrous Oxide, North America report (Heydorn 1997). The
1990 through 1995 share of total quantity of N2O used by
each subcategory was kept the same as the 1996 number
provided by SRI Consulting. The 1997 through 2001share
of total quantity of N2O usage by sector was obtained from
communication with a N2O industry expert (Tupman 2002).
The 2002 and 2003 share of total quantity of N2O usage
by sector was obtained from CGA (2002, 2003). Due to
unavailable data, the share of total quantity of N2O usage data
for years 2004 through 2008 was assumed to equal the 2003
value. The emissions rate for the food processing propellant
industry was obtained from SRI Consulting's Nitrous Oxide,
North America report (Heydorn 1997), and confirmed by a
N2O industry expert (Tupman 2002). The emissions rate for
all other subcategories was obtained from communication
with a N2O industry expert (Tupman 2002). The emissions
rate for the medical/dental subcategory was obtained from
the 2006IPCC Guidelines.
Uncertainty and Time-Series Consistency
The overall uncertainty associated with the 2008 N2O
emission estimate from N2O product usage was calculated
using the IPCC Guidelines for National Greenhouse
Gas Inventories (2006) Tier 2 methodology. Uncertainty
associated with the parameters used to estimate N2O
emissions included that of production data, total market
share of each end use, and the emission factors applied to
each end use, respectively.
The results of this Tier 2 quantitative uncertainty analysis
are summarized in Table 5-4. Nitrous oxide emissions from
N2O product usage were estimated to be between 4.3 and 4.5
Tg CO2 Eq. at the 95 percent confidence level (or in 19 out
of 20 Monte Carlo Stochastic Simulations). This indicates
a range of approximately 2 percent below to 2 percent above
the 2007 emissions estimate of 4.4 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Planned Improvements
Planned improvements include a continued evaluation
of alternative production statistics for cross verification and
a reassessment of subcategory usage to accurately represent
the latest trends in the product usage, and investigation
of production and use cycles and the potential need to
incorporate a time lag between production and ultimate
product use and resulting release of N2O. Additionally,
planned improvements include considering imports and
exports of N2O for product uses.
Table 5-4: Tier 2 Quantitative Uncertainty Estimates for N20 Emissions From N20 Product Usage
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
N20 Product Usage
N,0
4.4
4.3
4.5
-2%
+ 2%
3 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Solvent and Other Product Use 5-3
-------
5.2 Indirect Greenhouse Gas
Emissions from Solvent Use
The use of solvents and other chemical products
can result in emissions of various ozone precursors
(i.e., indirect greenhouse gases).1 Non-methane volatile
organic compounds (NMVOCs), commonly referred to as
"hydrocarbons," are the primary gases emitted from most
processes employing organic or petroleum based solvents.
As some of industrial applications also employ thermal
incineration as a control technology, combustion by-products,
such as carbon monoxide (CO) and nitrogen oxides (NOX),
are also reported with this source category. In the United
States, emissions from solvents are primarily the result
of solvent evaporation, whereby the lighter hydrocarbon
molecules in the solvents escape into the atmosphere. The
evaporation process varies depending on different solvent
uses and solvent types. The major categories of solvent
uses include: degreasing, graphic arts, surface coating,
other industrial uses of solvents (i.e., electronics, etc.), dry
cleaning, and non-industrial uses (i.e., uses of paint thinner,
etc.).
Total emissions of NOX, NMVOCs, and CO from 1990
to 2008 are reported in Table 5-5.
Table 5-5: Emissions of NOX, CO, and NMVOC from Solvent Use (Gg)
Activity
1990
1995
2000
2005
2006
2007
2008
NOX
Surface Coating
Graphic Arts
Degreasing
Dry Cleaning
Other Industrial Processes3
Non-Industrial Processes'1
Other
CO
Surface Coating
Other Industrial Processes3
Dry Cleaning
Degreasing
Graphic Arts
Non-Industrial Processes'1
Other
NMVOCs
Surface Coating
Non-Industrial Processes'1
Degreasing
Dry Cleaning
Graphic Arts
Other Industrial Processes3
Other
1333
1
+
+
+
+
+
NA
5
+
4
+
+
+
+
NA
5,216
2,289
1,724
675
2
1
+
+
+
+
+
5
1
3
1
+
+
+
NA
5,609
2,432
1,858
716
195 209
249 1 307
85 1 87
3
+
+
+
+
+
+
45
45
+
+
+
+
+
+
4,384
1,766
1,676
316
265
222
98
3
+
+
+
+
+
+
2
2
+
+
+
+
+
+
3,851
1,578
1,446
280
230
194
88
+ + 40 36
4
4
+
+
+
+
+
+
2
2
+
+
+
+
+
+
3,846
1,575
1,444
280
230
193
88
36
4
4
+
+
+
+
+
+
2
2
+
+
+
+
+
+
3,839
1,573
1,441
280
229
193
87
36
4
4
+
+
+
+
+
+
2
2
+
+
+
+
+
+
3,834
1,571
1,439
279
229
193
87
36
+ Does not exceed 0.5 Gg.
"Includes rubber and plastics manufacturing, and other miscellaneous applications.
b Includes cutback asphalt, pesticide application adhesives, consumer solvents, and other miscellaneous applications.
Note: Totals may not sum due to independent rounding.
1 Solvent usage in the United States also results in the emission of
small amounts of hydrofluorocarbons (HFCs) and hydrofluoroethers
(HFEs), which are included under Substitution of Ozone Depleting
Substances in the Industrial Processes chapter.
5-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Methodology
Emissions were calculated by aggregating solvent use
data based on information relating to solvent uses from
different applications such as degreasing, graphic arts, etc.
Emission factors for each consumption category were then
applied to the data to estimate emissions. For example,
emissions from surface coatings were mostly due to solvent
evaporation as the coatings solidify. By applying the
appropriate solvent-specific emission factors to the amount of
solvents used for surface coatings, an estimate of emissions
was obtained. Emissions of CO and NOX result primarily
from thermal and catalytic incineration of solvent-laden
gas streams from painting booths, printing operations, and
oven exhaust.
These emission estimates were obtained from preliminary
data (EPA 2009), and disaggregated based on EPA (2003),
which, in its final iteration, will be published on the National
Emission Inventory (NEI) Air Pollutant Emission Trends
web site. Emissions were calculated either for individual
categories or for many categories combined, using basic
activity data (e.g., the amount of solvent purchased) as an
indicator of emissions. National activity data were collected
for individual applications from various agencies.
Activity data were used in conjunction with emission
factors, which together relate the quantity of emissions to the
activity. Emission factors are generally available from the
EPA's Compilation of Air Pollutant Emission Factors, AP-42
(EPA 1997). The EPA currently derives the overall emission
control efficiency of a source category from a variety of
information sources, including published reports, the 1985
National Acid Precipitation and Assessment Program
Emissions Inventory, and other EPA databases.
Uncertainty and Time-Series Consistency
Uncertainties in these estimates are partly due to the
accuracy of the emission factors used and the reliability
of correlations between activity data and actual emissions.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Solvent and Other Product Use 5-5
-------
6. Agriculture
Figure 6-1
Agricultural activities contribute directly to emissions of greenhouse gases through a variety of processes. This
chapter provides an assessment of non-carbon-dioxide emissions from the following source categories: enteric
fermentation in domestic livestock, livestock manure management, rice cultivation, agricultural soil management,
and field burning of agricultural residues (see Figure 6-1). Carbon dioxide (CO2) emissions and removals from agriculture-
related land-use activities, such as conversion of grassland
to cultivated land, are presented in the Land Use, Land-Use
Change, and Forestry chapter. Carbon dioxide emissions from
on-farm energy use are accounted for in the Energy chapter.
In 2008, the Agricultural sector was responsible for
emissions of 427.5 teragrams of CO2 equivalents (Tg CO2
Eq.), or 6 percent of total U.S. greenhouse gas emissions.
Methane (CH4) and nitrous oxide (N2O) were the primary
greenhouse gases emitted by agricultural activities. Methane
emissions from enteric fermentation and manure management
represent about 25 percent and 8 percent of total CH4
emissions from anthropogenic activities, respectively. Of all
domestic animal types, beef and dairy cattle were by far the
largest emitters of CH4. Rice cultivation and field burning of
agricultural residues were minor sources of CH4. Agricultural
soil management activities such as fertilizer application and
2008 Agriculture Chapter Greenhouse Gas
Emission Sources
Agricultural Soil Management
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of
Agricultural Residues
Agriculture
as a Portion of
all Emissions
50
100
Tg C02 Eq.
150
Table 6-1: Emissions from Agriculture (Tg C02 Eq.)
Gas/Source
CH4
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of Agricultural Residues
N20
Agricultural Soil Management
Manure Management
Field Burning of Agricultural Residues
Total
1990
169.6
132.4
29.3
7.1 1
0.8 1
218.3
203.5
14.4
0.4
387.8
1995
185.9
143.7
33.9
7 1
07l
221.8
205.9
15.5
0.4
407.7
2000
183.7
136.8
38.6
0.9
227.2
210.1
16.7
0.5
410.9
2005
186.7
136.7
42.2
6.8
0.9
233.0
215.8
16.6
0.5
419.7
2006
188.1
139.0
42.3
5.9
0.9
229.1
211.2
17.3
0.5
417.2
2007
194.2
141.2
45.9
6.2
1.0
228.8
211.0
17.3
0.5
423.0
2008
194.0
140.8
45.0
7.2
1.0
233.5
215.9
17.1
0.5
427.5
Note: Totals may not sum due to independent rounding.
Agriculture 6-1
-------
Table 6-2: Emissions from Agriculture (Gg)
Gas/Source
CH4
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of Agricultural Residues
N20
Agricultural Soil Management
Manure Management
Field Burning of Agricultural Residues
1990
8,074
6,303
1,395l
3391
36 1
704
656 1
47 1
1
1995
8,854
6,844
1,612
363
35 1
7151
6641
50 1
1
2000
8,749
6,513
1,837
3571
42l
733
678 1
54|
2
2005
8,890
6,509
2,011
326
44
752
696
54
2
2006
8,959
6,619
2,015
282
43
739
681
56
2
2007
9,246
6,723
2,183
295
46
738
681
56
2
2008
9,239
6,707
2,144
343
46
753
696
55
2
Note: Totals may not sum due to independent rounding.
other cropping practices were the largest source of U.S. N2O
emissions, accounting for 68 percent. Manure management
and field burning of agricultural residues were also small
sources of N2O emissions.
Table 6-1 and Table 6-2 present emission estimates
for the Agriculture sector. Between 1990 and 2008, CH4
emissions from agricultural activities increased by 14.4
percent, while N2O emissions fluctuated from year to year,
but overall increased by 7.0 percent.
6.1. Enteric Fermentation (IPCC
Source Category 4A)
Methane is produced as part of normal digestive
processes in animals. During digestion, microbes resident
in an animal's digestive system ferment food consumed by
the animal. This microbial fermentation process, referred to
as enteric fermentation, produces CH4 as a byproduct, which
can be exhaled or eructated by the animal. The amount of
CH4 produced and emitted by an individual animal depends
primarily upon the animal's digestive system, and the amount
and type of feed it consumes.
Ruminant animals (e.g., cattle, buffalo, sheep, goats, and
camels) are the major emitters of CH4 because of their unique
digestive system. Ruminants possess a rumen, or large "fore-
stomach," in which microbial fermentation breaks down the
feed they consume into products that can be absorbed and
metabolized. The microbial fermentation that occurs in the
rumen enables them to digest coarse plant material that non-
ruminant animals can not. Ruminant animals, consequently,
have the highest CH4 emissions among all animal types.
Non-ruminant animals (e.g., swine, horses, and mules)
also produce CH4 emissions through enteric fermentation,
although this microbial fermentation occurs in the large
intestine. These non-ruminants emit significantly less CH4
on a per-animal basis than ruminants because the capacity
of the large intestine to produce CH4 is lower.
In addition to the type of digestive system, an animal's
feed quality and feed intake also affects CH4 emissions. In
general, lower feed quality and/or higher feed intake leads to
higher CH4 emissions. Feed intake is positively correlated
to animal size, growth rate, and production (e.g., milk
production, wool growth, pregnancy, or work). Therefore,
feed intake varies among animal types as well as among
different management practices for individual animal types
(e.g., animals in feedlots or grazing on pasture).
Methane emission estimates from enteric fermentation
are provided in Table 6-3 and Table 6-4. Total livestock
CH4 emissions in 2008 were 140.8 Tg CO2 Eq. (6,707 Gg).
Beef cattle remain the largest contributor of CH4 emissions
from enteric fermentation, accounting for 72 percent in
2008. Emissions from dairy cattle in 2008 accounted for
23 percent, and the remaining emissions were from horses,
sheep, swine, and goats.
From 1990 to 2008, emissions from enteric fermentation
have increased by 6.4 percent. Generally, emissions decreased
from 1996 to 2003, though with a slight increase in 2002.
This trend was mainly due to decreasing populations of both
beef and dairy cattle and increased digestibility of feed for
feedlot cattle. Emissions increased from 2004 through 2007,
as both dairy and beef populations have undergone increases
and the literature for dairy cow diets indicated a trend toward
6-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 6-3: CH4 Emissions from Enteric Fermentation (Tg C02 Eq.)
Livestock Type
Beef Cattle
Dairy Cattle
Horses
Sheep
Swine
Goats
Total
1990 •
94.5
32.0
i.gl
...
132.4 |
1995
107.7
30.5
1
1.5
1.9
0.2
143.7
2000
100.6
30.9
2.0 1
1.2
1
0.3
136.8
2005
99.3
30.6
3.5
1.0
1.9
0.3
136.7
2006
100.9
31.3
3.6
1.0
1.9
0.3
139.0
2007
101.6
32.7
3.6
1.0
2.1
0.3
141.2
2008
100.8
33.1
3.6
1.0
2.1
0.3
140.8
Note: Totals may not sum due to independent rounding.
Table 6-4: CH4 Emissions from Enteric Fermentation (Gg)
Livestock Type
1990
1995
2000
2005
2006
2007
2008
Beef Cattle
Dairy Cattle
Horses
Sheep
Swine
Goats
4,502
1,526
91
91
81
13
11,452 1,471
921 941
721 56
88 • 88 •
4,731
1,459
166
49
92
13
4,803
1,490
171
50
93
13
4,837
1,555
171
49
98
13
4,799
1,576
171
48
101
13
Total
6,303
6,844
6,513
6,509
6,619
6,723 6,707
Note: Totals may not sum due to independent rounding.
a decrease in feed digestibility for those years. Emissions
decreased again in 2008 as beef cattle populations decreased
again. During the timeframe of this analysis, populations
of sheep have decreased 48 percent since 1990 while horse
populations have increased over 85 percent, mostly since
1999. Goat and swine populations have increased 1 percent
and 25 percent, respectively, during this timeframe.
Methodology
Livestock emission estimate methodologies fall into two
categories: cattle and other domesticated animals. Cattle, due
to their large population, large size, and particular digestive
characteristics, account for the majority of CH4 emissions
from livestock in the United States. A more detailed
methodology (i.e., IPCC Tier 2) was therefore applied to
estimate emissions for all cattle except for bulls. Emission
estimates for other domesticated animals (horses, sheep,
swine, goats, and bulls) were handled using a less detailed
approach (i.e., IPCC Tier 1).
While the large diversity of animal management
practices cannot be precisely characterized and evaluated,
significant scientific literature exists that provides the
necessary data to estimate cattle emissions using the IPCC
Tier 2 approach. The Cattle Enteric Fermentation Model
(CEFM), developed by EPA and used to estimate cattle
CH4 emissions from enteric fermentation, incorporates this
information and other analyses of livestock population,
feeding practices, and production characteristics.
National cattle population statistics were disaggregated
into the following cattle sub-populations:
Dairy Cattle
• Calves
• Heifer Replacements
• Cows
Beef Cattle
• Calves
• Heifer Replacements
• Heifer and Steer Stackers
• Animals in Feedlots (Heifers and Steers)
• Cows
• Bulls
Agriculture 6-3
-------
Calf birthrates, end of year population statistics, detailed
feedlot placement information, and slaughter weight data
were used to create a transition matrix that models cohorts of
individual animal types and their specific emission profiles.
The key variables tracked for each of the cattle population
categories are described in Annex 3.9. These variables
include performance factors such as pregnancy and lactation
as well as average weights and weight gain. Annual cattle
population data were obtained from the U.S. Department
of Agriculture's (USDA) National Agricultural Statistics
Service Quick Stats database (USDA 2009).
Diet characteristics were estimated by region for U.S.
dairy, beef, and feedlot cattle. These estimates were used
to calculate Digestible Energy (DE) values (expressed as
the percent of gross energy intake digested by the animal)
and CH4 conversion rates (Ym) (expressed as the fraction of
gross energy converted to CH4) for each population category.
The IPCC recommends Ym values of 3.0+/-1.0 percent for
feedlot cattle and 6.5+/-1.0 percent for other well-fed cattle
consuming temperate-climate feed types (IPCC 2006).
Given the availability of detailed diet information for
different regions and animal types in the United States, DE
and Ym values unique to the United States were developed,
rather than using the recommended IPCC values. The diet
characterizations and estimation of DE and Ym values were
based on information from state agricultural extension
specialists, a review of published forage quality studies
and scientific literature, expert opinion, and modeling of
animal physiology. The diet characteristics for dairy cattle
were based on Donovan (1999) and an extensive review
of nearly 20 years of literature. Dairy replacement heifer
diet assumptions were based on the observed relationship
in the literature between dairy cow and dairy heifer diet
characteristics. The diet assumptions for beef cattle were
derived from NRC (2000). For feedlot animals, the DE and
Ym values used for 1990 were recommended by Johnson
(1999). Values for DE and Ym for 1991 through 1999 were
linearly extrapolated based on the 1990 and 2000 data. DE
and Ym values for 2000 onwards were based on survey data in
Galyean and Gleghorn (2001) and Vasconcelos and Galyean
(2007). For grazing beef cattle, DE values were based on
diet information in NRC (2000) and Ym values were based
on Johnson (2002). Weight and weight gains for cattle were
estimated from Enns (2008), Patton et al. (2008), Lippke
et al. (2000), Pinchack et al., (2004), Platter et al. (2003),
Skogerboe et al. (2000), and expert opinion. See Annex 3.9
for more details on the method used to characterize cattle
diets and weights in the United States.
To estimate CH4 emissions from all cattle types except
bulls and calves younger than 7 months,1 the population
was divided into state, age, sub-type (i.e., dairy cows and
replacements, beef cows and replacements, heifer and steer
stackers, and heifer and steer in feedlots), and production
(i.e., pregnant, lactating) groupings to more fully capture
differences in CH4 emissions from these animal types. The
transition matrix was used to simulate the age and weight
structure of each sub-type on a monthly basis, to more
accurately reflect the fluctuations that occur throughout the
year. Cattle diet characteristics were then used in conjunction
with Tier 2 equations from IPCC (2006) to produce CH4
emission factors for the following cattle types: dairy
cows, beef cows, dairy replacements, beef replacements,
steer stackers, heifer stackers, steer feedlot animals, and
heifer feedlot animals. To estimate emissions from cattle,
population data from the transition matrix were multiplied
by the calculated emission factor for each cattle type. More
details are provided in Annex 3.9.
Emission estimates for other animal types were based on
average emission factors representative of entire populations
of each animal type. Methane emissions from these animals
accounted for a minor portion of total CH4 emissions from
livestock in the United States from 1990 through 2008. Also,
the variability in emission factors for each of these other
animal types (e.g., variability by age, production system, and
feeding practice within each animal type) is less than that
for cattle. Annual livestock population data for these other
livestock types, except horses and goats, as well as feedlot
placement information were obtained for all years from
1 Emissions from bulls are estimated using a Tier 1 approach because it is
assumed there is minimal variation in population and diets; because calves
younger than 7 months consume mainly milk and the IPCC recommends the
use of methane conversion factor of zero for all juveniles consuming only
milk, this results in no methane emissions from this subcategory of cattle.
6-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
the U.S. Department of Agriculture's National Agricultural
Statistics Service (USDA 2009). Horse population data were
obtained from the FAOSTAT database (FAO 2009), because
USDA does not estimate U.S. horse populations annually.
Goat population data were obtained for 1992,1997, and 2002
(USDA 2009); these data were interpolated and extrapolated
to derive estimates for the other years. Methane emissions
from sheep, goats, swine, and horses were estimated by
using emission factors utilized in Crutzen et al. (1986, cited
in IPCC 2006). These emission factors are representative of
typical animal sizes, feed intakes, and feed characteristics in
developed countries. The methodology is the same as that
recommended by IPCC (2006).
See Annex 3.9 for more detailed information on the
methodology and data used to calculate CH4 emissions from
enteric fermentation.
Uncertainty and Time-Series Consistency
Quantitative uncertainty analysis for this source
category was performed through the IPCC-recommended
Tier 2 uncertainty estimation methodology, Monte Carlo
Stochastic Simulation technique as described in ICF (2003).
These uncertainty estimates were developed for the 1990
through 2001 Inventory report. No significant changes
occurred in the method of data collection, data estimation
methodology, or other factors that influence the uncertainty
ranges around the 2008 activity data and emission factor
input variables used in the current submission. Consequently,
these uncertainty estimates were directly applied to the 2008
emission estimates.
A total of 185 primary input variables (177 for cattle and
8 for non-cattle) were identified as key input variables for
the uncertainty analysis. A normal distribution was assumed
for almost all activity- and emission factor-related input
variables. Triangular distributions were assigned to three
input variables (specifically, cow-birth ratios for the three
most recent years included in the 2001 model run) to capture
the fact that these variables can not be negative. For some key
input variables, the uncertainty ranges around their estimates
(used for inventory estimation) were collected from published
documents and other public sources; others were based on
expert opinion and our best estimates. In addition, both
endogenous and exogenous correlations between selected
primary input variables were modeled. The exogenous
correlation coefficients between the probability distributions
of selected activity-related variables were developed through
expert judgment.
The uncertainty ranges associated with the activity data-
related input variables were plus or minus 10 percent or lower.
However, for many emission factor-related input variables,
the lower- and/or the upper-bound uncertainty estimates were
over 20 percent. The results of the quantitative uncertainty
analysis (Table 6-5) indicate that, on average, the emission
estimate range of this source is approximately 125.3 to 166.2
Tg CO2 Eq., calculated as 11 percent below and 18 percent
above the actual 2008 emission estimate of 140.8 Tg CO2 Eq.
Among the individual cattle sub-source categories, beef cattle
account for the largest amount of CH4 emissions as well as
the largest degree of uncertainty in the inventory emission
estimates. Among non-cattle, horses account for the largest
degree of uncertainty in the inventory emission estimates
because there is a higher degree of uncertainty among the
FAO population estimates used for horses than for the USDA
population estimates used for swine, goats, and sheep.
Methodological recalculations were applied to the entire
time series to ensure time series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section.
Table 6-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Enteric Fermentation
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3 b
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Enteric Fermentation
CH,
140.8
125.3
166.2
-11%
+ 18%
3 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
b Note that the relative uncertainty range was estimated with respect to the 2001 emission estimates submitted in 2003 and applied to the 2008 estimates.
Agriculture 6-5
-------
QA/QC and Verification
In order to ensure the quality of the emission estimates
from enteric fermentation, the IPCC Tier 1 and Tier 2
Quality Assurance/Quality Control (QA/QC) procedures
were implemented consistent with the U.S. QA/QC plan.
Tier 2 QA procedures included independent peer review of
emission estimates. As described below, particular emphasis
this year was placed on revising CEFM diet assumptions and
additional modifications of the stocker population estimates
in the transition matrix, which required further QA/QC to
ensure consistency of estimates generated by the updated
model.
Recalculations Discussion
There were several modifications to the estimates
relative to the previous Inventory that had an effect on
emission estimates, including the following:
• Four models to predict CH4 production from cattle
(two mechanistic, and two empirical) were evaluated
to determine appropriate Ym and DE values for use in
the analysis. The results are described in Kebreab et al.
(2008). In addition to the model evaluation, separate
research was conducted to update the assumptions used
for cattle diet components for feedlot and dairy cattle.
An extensive literature review was performed on dairy
diets and nearly 250 diets were analyzed to derive the
current DE and Ym estimates for dairy. In addition,
feedlot diets were updated based on current survey data
from Galyean and Gleghorn (2001) and Vasconcelos and
Galyean (2007).
• Further modifications were made to the feedlot placement
methodology for reconciling the USDA placement data
and the estimated populations of stockers available
for placement. In cases where there are discrepancies
between the USDA estimated placements by weight
class and the calculated animals available by weight,
the model pulls available stockers from one higher
weight category if available. If there are still not enough
animals to fulfill requirements, the model pulls animals
from the next lower category. In the current time series,
this method was able to ensure that total placement
data matched USDA estimates, and no shortfalls have
occurred. In the previous Inventory, additional animals
were only added to the 700-800 Ibs category.
• Bull populations are no longer averaged between January
and July. It was determined that there is a greater degree
of uncertainty in the July estimates; therefore they are
no longer used, and bull populations are based solely
on January estimates.
• The USDA published revised population estimates that
affected historical emissions estimated for swine and
sheep in 2007. The FAO published revised population
estimates for horses for 2006 and 2007. In addition, some
historical population estimates for certain beef and dairy
populations were also updated as a result of changes in
USDA inputs.
As a result of these changes, dairy cattle emissions
decreased an average of 8.7 Gg (0.5 percent) per year and beef
cattle emissions increased an average of 49 Gg (1.0 percent)
per year over the entire time series relative to the previous
Inventory. Historical emission estimates for 2007 increased
by 0.6 percent for swine and decreased by 0.7 percent for
sheep as a result of the USDA revisions described above.
Horse emission estimates for 2006 and 2007 increased by
approximately 3 percent from the revisions in the FAO data.
Planned Improvements
Continued research and regular updates are necessary
to maintain a current model of cattle diet characterization,
feedlot placement data, rates of weight gain and calving,
among other data inputs. Ongoing revisions could include
some of the following options:
• Reviewing and updating the diet assumptions for
foraging beef cattle;
• Estimating bull emissions using the IPCC Tier 2
approach;
• Updating input variables that are from older data sources,
such as beef births by month and beef cow lactation
rates;
• The possible breakout of other animal types (i.e., sheep,
swine, goats, horses) from national estimates to state-
level estimates; and
• Including bison in the estimates for other domesticated
animals.
In addition, recent changes that have been implemented
to the CEFM warrant an assessment of the current uncertainty
analysis, therefore a revision of the quantitative uncertainty
6-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
surrounding emission estimates from this source category
will be initiated.
6.2. Manure Management (IPCC
Source Category 4B)
The management of livestock manure can produce
anthropogenic CH4 and N2O emissions. Methane is produced
by the anaerobic decomposition of manure. Direct N2O
emissions are produced as part of the N cycle through the
nitrification and denitrification of the organic N in livestock
manure and urine.2 Indirect N2O emissions are produced
as result of the volatilization of N as NH3 and NOX and
runoff and leaching of N during treatment, storage and
transportation.
When livestock or poultry manure are stored or treated in
systems that promote anaerobic conditions (e.g., as a liquid/
slurry in lagoons, ponds, tanks, or pits), the decomposition
of materials in the manure tends to produce CH4. When
manure is handled as a solid (e.g., in stacks or drylots)
or deposited on pasture, range, or paddock lands, it tends
to decompose aerobically and produce little or no CH4.
Ambient temperature, moisture, and manure storage or
residency time affect the amount of CH4 produced because
they influence the growth of the bacteria responsible for CH4
formation. For non-liquid-based manure systems, moist
conditions (which are a function of rainfall and humidity)
can promote CH4 production. Manure composition, which
varies by animal diet, growth rate, and type, including the
animal's digestive system, also affects the amount of CH4
produced. In general, the greater the energy content of the
feed, the greater the potential for CH4 emissions. However,
some higher energy feeds also are more digestible than
lower quality forages, which can result in less overall waste
excreted from the animal.
The production of direct N2O emissions from livestock
manure depends on the composition of the manure and urine,
the type of bacteria involved in the process, and the amount
of oxygen and liquid in the manure system. For direct
N2O emissions to occur, the manure must first be handled
2 Direct and indirect N2O emissions from manure and urine spread onto
fields either directly as daily spread or after it is removed from manure
management systems (e.g., lagoon, pit, etc.) and from livestock manure
and urine deposited on pasture, range, or paddock lands are accounted for
and discussed in the Agricultural Soil Management source category within
the Agriculture sector.
aerobically where ammonia (NH3) or organic N is converted
to nitrates and nitrites (nitrification), and then handled
anaerobically where the nitrates and nitrites are reduced to
nitrogen gas (N2), with intermediate production of N2O and
nitric oxide (NO) (denitrification) (Groffman et al. 2000).
These emissions are most likely to occur in dry manure
handling systems that have aerobic conditions, but that also
contain pockets of anaerobic conditions due to saturation.
A very small portion of the total N excreted is expected to
convert to N2O in the waste management system (WMS).
Indirect N2O emissions are produced when N is lost from
the system through volatilization (as NH3 or NOX) or through
runoff and leaching. The vast majority of volatilization losses
from these operations are NH3. Although there are also
some small losses of NOX, there are no quantified estimates
available for use, so losses due to volatilization are only based
on NH3 loss factors. Runoff losses would be expected from
operations that house animals or store manure in a manner
that is exposed to weather. Runoff losses are also specific to
the type of animal housed on the operation due to differences
in manure characteristics. Little information is known
about leaching from manure management systems as most
research focuses on leaching from land application systems.
Since leaching losses are expected to be minimal, leaching
losses are coupled with runoff losses and the runoff/leaching
estimate does not include any leaching losses.
Estimates of CH4 emissions in 2008 were 45 Tg CO2
Eq. (2,144 Gg), 54 percent higher than in 1990. Emissions
increased on average by 0.9 Tg CO2 Eq. (2.5 percent)
annually over this period. The majority of this increase was
from swine and dairy cow manure, where emissions increased
50 and 91 percent, respectively. Although the majority of
manure in the United States is handled as a solid, producing
little CH4, the general trend in manure management,
particularly for dairy and swine (which are both shifting
towards larger facilities), is one of increasing use of liquid
systems. Also, new regulations limiting the application of
manure nutrients have shifted manure management practices
at smaller dairies from daily spread to manure managed and
stored on site. Although national dairy animal populations
have been generally decreasing, some states have seen
increases in their dairy populations as the industry becomes
more concentrated in certain areas of the country. These
areas of concentration, such as California, New Mexico, and
Idaho, tend to utilize more liquid-based systems to manage
Agriculture 6-7
-------
Table 6-6: CH4 and N20 Emissions from Manure Management (Tg C02 Eq.)
Gas/Animal Type
1990
1995
2000
2005
2006
2007
2008
CH4a
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
N20b
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
29.3
10.2
2.6 1
13.1 1
0.1 1
+ 1
2.8
0.5
14.4
5.0 1
6.3
1.2l
0.1
I
0.2
33.9 38.6 42.2
11.8
2.8
16.0
0.1
+
2.7
0.4
15.5
5.2
6.8
1.4
0.2
+
1.6
15.3
2.6
17.5
0.1
+
2.6
0.5
16.7
5.3
7.7
1.4
0.3
,+7
17.3
2.5
18.9
0.1
+
2.7
0.8
16.6
5.4
7.2
1.5
0.3
+
1.7
0.2 0.2 0.4
42.3
17.5
2.6
18.5
0.1
+
2.7
0.9
17.3
5.6
7.7
1.5
0.4
+
1.8
0.4
45.9
19.5
2.6
20.1
0.1
+
2.7
0.9
17.3
5.6
7.6
1.6
0.3
+
1.8
0.4
45.0
19.4
2.5
19.6
0.1
+
2.6
0.8
17.1
5.5
7.4
1.7
0.3
+
1.8
0.4
Total
43.7
49.4
55.3
58.8
59.6
+ Less than 0.5 Gg.
a Includes CH4 emission reductions due to anaerobic digestion.
b Includes both direct and indirect N20 emissions.
Note: Totals may not sum due to independent rounding.
63.2
62.1
+ Less than 0.05 Tg C02Eq.
3 Includes CH4 emission reductions due to anaerobic digestion.
b Includes both direct and indirect N20 emissions.
Note: Totals may not sum due to independent rounding.
Table 6-7: CH4 and N20 Emissions from Manure Management (Gg)
Gas/Animal Type
CH4a
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
N20b
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
1990
1,395
485 1
126
624
,
1995
1,612
561
133
7641
1
128
21 1
50
1
22 •
1
1
+
1
1
2000 2005
1,837
727
125
832
4
1
126
22
54
17
25
5
1
+
5
2,011
822
120
899
4
1
127
39
54
17
23
5
1
+
6
1 1
2006
2,015
835
125
882
4
1
128
41
56
18
25
5
1
+
6
1
2007
2,183
929
122
957
4
1
131
41
56
18
24
5
1
+
6
1
2008
2,144
925
117
932
4
1
125
39
55
18
24
5
1
+
6
1
(flush or scrape) and store manure. Thus the shift toward
larger facilities is translated into an increasing use of liquid
manure management systems, which have higher potential
CH4 emissions than dry systems. This shift was accounted
for by incorporating state and WMS-specinc CH4 conversion
factor (MCF) values in combination with the 1992,1997, and
2002 farm-size distribution data reported in the Census of
Agriculture (USDA 2005). Methane emissions from horses
6-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
have nearly doubled since 1990 (an 82 percent increase from
1990 to 2008); however, this is due to population increases
rather than changes in manure management practices.
Overall, horses contribute only 2 percent of CH4 emissions
from animal manure management. From 2007 to 2008, there
was a 2 percent decrease in total CH4 emissions, due to minor
shifts in the animal populations and the resultant effects on
manure management system allocations.
In 2008, total N2O emissions were estimated to be 17.11
Tg CO2 Eq. (55 Gg); in 1990, emissions were 14.43 Tg CO2
Eq. (47 Gg). These values include both direct and indirect
N2O emissions from manure management. N2O emissions
have remained fairly steady since 1990. Small changes
in N2O emissions from individual animal groups exhibit
the same trends as the animal group populations, with the
overall net effect that N2O emissions showed a 19 percent
increase from 1990 to 2008 and a 1 percent decrease from
2007 through 2008.
Table 6-6 and Table 6-7 provide estimates of CH4
and N2O emissions from manure management by animal
category.
Methodology
The methodologies presented in IPCC (2006) form the
basis of the CH4 and N2O emission estimates for each animal
type. This section presents a summary of the methodologies
used to estimate CH4 and N2O emissions from manure
management for this Inventory. See Annex 3.10 for more
detailed information on the methodology and data used to
calculate CH4 and N2O emissions from manure management.
Methane Calculation Methods
The following inputs were used in the calculation of
CH4 emissions:
• Animal population data (by animal type and state);
• Typical animal mass (TAM) data (by animal type);
• Portion of manure managed in each waste management
system (WMS), by state and animal type;
• Volatile solids (VS) production rate (by animal type and
state or United States);
• Methane producing potential (B0) of the volatile solids
(by animal type); and
• Methane conversion factors (MCE), the extent to which
the CH4 producing potential is realized for each type
of WMS (by state and manure management system,
including the impacts of any biogas collection efforts).
Methane emissions were estimated by first determining
activity data, including animal population, TAM, WMS
usage, and waste characteristics. The activity data sources
are described below:
• Annual animal population data for 1990 through 2008
for all livestock types, except horses and goats were
obtained from the USDA National Agricultural Statistics
Service (NASS). For cattle, the USDA populations
were utilized in conjunction with birth rates, detailed
feedlot placement information, and slaughter weight
data to create the transition matrix in the Cattle Enteric
Fermentation Model (CEFM) that models cohorts of
individual animal types and their specific emission
profiles. The key variables tracked for each of the
cattle population categories are described in section
6.1, Enteric Fermentation and in more detail in Annex
3.9, Methodology for Estimating CH4 Emissions from
Enteric Fermentation. Horse population data were
obtained from the Food and Agriculture Organization
(FAO) FAOSTAT database (FAO 2009). Goat population
data for 1992, 1997, and 2002 were obtained from the
Census of Agriculture (USDA 2005).
• The TAM is an annual average weight which was
obtained for each animal type from information
in USDA's Agricultural Waste Management Field
Handbook (USDA 1996a), the American Society of
Agricultural Engineers, StandardD384.1 (ASAE1999)
and others (EPA 1992 and Safley 2000).
• WMS usage was estimated for swine and dairy cattle
for different farm size categories using data from
USDA (USDA 1996b, 1998b, 2000a) and EPA (ERG
2000a, EPA 2002a, 2002b). For beef cattle and poultry,
manure management system usage data were not tied
to farm size but were based on other data sources (ERG
2000a, USDA 2000b, UEP 1999). For other animal
types, manure management system usage was based on
previous estimates (EPA 1992).
• VS production rates for all cattle except for bulls and
calves were calculated for each state and animal type
in the Cattle Enteric Fermentation Model (CEFM),
which is described in section 6.1, Enteric Fermentation
and in more detail in Annex 3.9, Methodology for
Agriculture 6-9
-------
Estimating CH4 Emissions from Enteric Fermentation.
VS production rates for all other animals were
determined using data from USDA's Agricultural Waste
Management Field Handbook (USDA 1996a) and data
from the American Society of Agricultural Engineers,
Standard D384.1 (ASAE 1999).
• The maximum CH4 producing capacity of the VS (B0)
was determined for each animal type based on literature
values (Morris 1976, Bryant et al, 1976, Hashimoto
1981, Hashimoto 1984, EPA 1992, Hill 1982, and Hill
1984).
• MCFs for dry systems were set equal to default IPCC
factors based on state climate for each year (IPCC
2006). MCFs for liquid/slurry, anaerobic lagoon,
and deep pit systems were calculated based on the
forecast performance of biological systems relative to
temperature changes as predicted in the van't Hoff-
Arrhenius equation which is consistent with IPCC 2006
Tier 2 methodology.
• Anaerobic digestion system data were obtained from the
EPA AgSTAR Program, including information presented
in the AgSTAR Digest (EPA 2000, 2003b, 2006). AD
emissions were calculated based on estimated methane
production and collection and destruction efficiency
assumptions (ERG 2008).
To estimate CH4 emissions, EPA first calculated the
annual amount of VS (kg per year) from manure that is
excreted in each WMS for each animal type, state, and year.
This calculation multiplied the animal population (head)
by the VS excretion rate (kg VS per 1,000 kg animal mass
per day), the TAM (kg animal mass per head) divided by
1,000, the WMS distribution (percent), and the number of
days per year.
The estimated amount of VS managed in each WMS
was used to estimate the CH4 emissions (kg CH4 per year)
from each WMS. The amount of VS (kg per year) were
multiplied by the maximum CH4 producing capacity of
the VS (B0) (m3 CH4 per kg VS), the MCF for that WMS
(percent), and the densitry of methane (kg CH4 per m3 CH4).
The CH4 emissions for each WMS, state, and animal type
were summed to determine the total U.S. CH4 emissions.
Nitrous Oxide Calculation Methods
The following inputs were used in the calculation of
direct and indirect N2O emissions:
• Animal population data (by animal type and state);
• TAM data (by animal type);
• Portion of manure managed in each WMS (by state and
animal type);
• Total Kjeldahl N excretion rate (Nex);
• Direct N2O emission factor (EFWMS);
• Indirect N2O emission factor for volitalization
v-^ volitalization/ >
• Indirect N2O emission factor for runoff and leaching
(EFranoff/leach);
• Fraction of N loss from volitalization of ammonia and
NOX (Fracgas); and
• Fraction of N loss from runoff and leaching
(Fracmnoff/leach).
Nitrous oxide emissions were estimated by first
determining activity data, including animal population,
TAM, WMS usage, and waste characteristics. The activity
data sources (except for population, TAM, and WMS, which
were described above) are described below:
• Nex rates for all cattle except for bull and calves
were calculated for each state and animal type in the
Cattle Enteric Fermentation Model (CEFM), which is
described in section 6.1, Enteric Fermentation and in
more detail in Annex 3.9, Methodology for Estimating
CH4 Emissions from Enteric Fermentation. Nex rates
for all other animals were determined using data
from USDA's Agricultural Waste Management Field
Handbook (USDA 1996a) and data from the American
Society of Agricultural Engineers, Standard D384.1
(ASAE 1999).
• All N2O emissions factors (direct and indirect) were
taken from IPCC (IPCC 2006).
• Country-specific estimates were developed for the
fraction of N loss from volatilization (Fracgas) and runoff
and leaching (Fracranoff/leach). Fracgas values were based
on WMS-specific volatilization values as estimated from
U.S. EPA's National Emission Inventory - Ammonia
Emissions from Animal Agriculture Operations (EPA
2005). Fracranoff/leaching values were based on regional
6-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
cattle runoff data from EPA's Office of Water (EPA
2002b; see Table A-9 in Annex 3.1).
To estimate N2O emissions, first, the amount of N
excreted (kg per year) in manure in each WMS for each
animal type, state, and year was calculated. The population
(head) for each state and animal was multiplied by TAM (kg
animal mass per head) divided by 1,000, the N excretion
rate (Nex, in kg N per 1000 kg animal mass per day), WMS
distribution (percent), and the number of days per year.
Direct N2O emissions were calculated by multiplying the
amount of Nex (kg per year) in each WMS by the N2O direct
emission factor for that WMS (EFwMs, in kg N2O-N per kg N)
and the conversion factor of N2O-N to N2O. These emissions
were summed over state, animal and WMS to determine the
total direct N2O emissions (kg of N2O per year).
Then, indirect N2O emissions from volatilization (kg
N2O per year) were calculated by multiplying the amount of
N excreted (kg per year) in each WMS by the fraction of N
lost through volatilization (Fracgas) divided by 100, and the
emission factor for volatilization (EFvolatilization in kg N2O per
kg N), and the conversion factor of N2O-N to N2O. Next,
indirect N2O emissions from runoff and leaching (kg N2O
per year) were calculated by multiplying the amount of N
excreted (kg per year) in each WMS by the fraction of N lost
through runoff and leaching (Fracranoff/leach) divided by 100,
and the emission factor for runoff and leaching (EFrunoff/leach
in kg N2O per kg N), and the conversion factor of N2O-N
to N2O. The indirect N2O emissions from volatilization and
runoff and leaching were summed to determine the total
indirect N2O emissions.
The direct and indirect N2O emissions were summed to
determine total N2O emissions (kg N2O per year).
Uncertainty and Time-Series Consistency
An analysis was conducted for the manure management
emission estimates presented in this Inventory to determine
the uncertainty associated with estimating CH4 and N2O
emissions from livestock manure management. The
quantitative uncertainty analysis for this source category
was first performed in 2002 through the IPCC-recommended
Tier 2 uncertainty estimation methodology, the Monte Carlo
Stochastic Simulation technique. The uncertainty analysis
was developed based on the methods used to estimate CH4
and N2O emissions from manure management systems.
A normal probability distribution was assumed for each
source data category. The series of equations used were
condensed into a single equation for each animal type and
state. The equations for each animal group contained four
to five variables around which the uncertainty analysis was
performed for each state.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 6-8. Manure management CH4
emissions in 2008 were estimated to be between 36.9 and
54.0 Tg CO2 Eq. at a 95 percent confidence level, which
indicates a range of 18 percent below to 20 percent above the
actual 2008 emission estimate of 45.0 Tg CO2 Eq. At the 95
percent confidence level, N2O emissions were estimated to
be between 14.4 and 21.2 Tg CO2 Eq. (or approximately 16
percent below and 24 percent above the actual 2008 emission
estimate of 17.1 Tg CO2 Eq.).
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 6-8: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 (Direct and Indirect) Emissions from
Manure Management (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"b
(TgC02Eq.) (%)
Manure Management
Manure Management
CH4
N20
45.0
17.1
Lower Bound
36.9
14.4
Upper Bound
54.0
21.2
Lower Bound
-18%
-16%
Upper Bound
+20%
+24%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
b Note that the relative uncertainty range was estimated with respect to the 2001 emission estimates submitted in 2003 and applied to the 2008 estimates.
Agriculture 6-11
-------
QA/QC and Verification
Tier 1 and Tier 2 QA/QC activities were conducted
consistent with the U.S. QA/QC plan. Tier 2 activities
focused on comparing estimates for the previous and current
inventories for N2O emissions from managed systems and
CH4 emissions from livestock manure. All errors identified
were corrected. Order of magnitude checks were also
conducted, and corrections made where needed. Manure N
data were checked by comparing state-level data with bottom
up estimates derived at the county level and summed to the
state level. Similarly, a comparison was made by animal and
WMS type for the full time series, between national level
estimates for N excreted and the sum of county estimates
for the full time series.
Recalculations Discussion
The Cattle Enteric Fermentation Model (CEFM)
produces volatile solids data for cattle that are used in
the manure management inventory. The CEFM team
implemented changes to the estimated diet characteristics for
feedlot and dairy cattle, as well as other minor data updates,
which created changes in VS data and changes in the amount
of CH4 estimated for manure management. These updates
decreased historical emissions through 1995, after which
emissions increased (except for 2003); total emissions for
the time series remained approximately the same (within
0.5 Tg CO2 Eq.) (See section 6.1, Enteric Fermentation).
For the current Inventory, cattle population data from the
CEFM were incorporated. The incorporation of these data
and updated VS data changed the estimated CH4 emissions
from manure management for cattle relative to the previous
report. With these changes, CH4 emission estimates from
manure management systems are slightly higher than
reported in the previous Inventory for beef and lower for
dairy cattle. Over the inventory years of 1990 through 2007,
the annual CH4 emission estimates differ from those of the
previous Inventory report by less than 5 percent.
For the current Inventory, Nex data from the CEFM was
incorporated. Due to the population and Nex changes, N2O
emission estimates from manure management systems have
increased for all years in the current Inventory as compared
to the previous Inventory report. Overall the total emission
estimates from manure management for the current Inventory
increased by an average of 18 percent, as compared to the
previous report.
Planned Improvements
In future Inventories, the manure management inventory
will be updated to reflect changes in the Cattle Enteric
Fermentation Model (CEFM). Additional steps will be
taken to complete the harmonization of animal populations
and characteristics between the manure management and
enteric fermentation source categories. Specifically, the
TAM estimates will be evaluated and updated so that total VS
and N excretion estimates from the CEFM can be utilized.
The manure management emission estimates will be
updated to ensure that the dairy heifer WMS distribution is
consistent between the CH4 and N2O inventories.
An updated version of the USDA Agricultural Waste
Management Field Handbook is expected to be available in
the next year. This reference will be reviewed to determine
if updates should be made to any of the activity data.
The emission estimates only take into account anaerobic
digestion systems for dairy and swine operations. Data
from the AgSTAR Program will be reviewed and anaerobic
digestions systems that exist for other animals types will be
incorporated.
An examination of new research on B0 values for dairy
and swine will be undertaken and applied to future emission
estimates.
The uncertainty analysis will be updated in the future to
more accurately assess uncertainty of emission calculations.
This update is necessary due to the extensive changes in
emission calculation methodology which began with the 1990
through 2006 Inventory, including estimation of emissions at
the WMS level and the use of new calculations and variables
for indirect N2O emissions.
6.3. Rice Cultivation (IPCC Source
Category 4C)
Most of the world's rice, and all rice in the United
States, is grown on flooded fields. When fields are flooded,
aerobic decomposition of organic material gradually depletes
most of the oxygen present in the soil, causing anaerobic
soil conditions. Once the environment becomes anaerobic,
CH4 is produced through anaerobic decomposition of soil
organic matter by methanogenic bacteria. As much as 60
to 90 percent of the CH4 produced is oxidized by aerobic
methanotrophic bacteria in the soil (some oxygen remains
6-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
at the interfaces of soil and water, and soil and root system)
(Holzapfel-Pschorn et al. 1985, Sass et al. 1990). Some of
the CH4 is also leached away as dissolved CH4 in floodwater
that percolates from the field. The remaining un-oxidized
CH4 is transported from the submerged soil to the atmosphere
primarily by diffusive transport through the rice plants.
Minor amounts of CH4 also escape from the soil via diffusion
and bubbling through floodwaters.
The water management system under which rice is
grown is one of the most important factors affecting CH4
emissions. Upland rice fields are not flooded, and therefore
are not believed to produce CH4. In deepwater rice fields
(i.e., fields with flooding depths greater than one meter),
the lower stems and roots of the rice plants are dead, so
the primary CH4 transport pathway to the atmosphere is
blocked. The quantities of CH4 released from deepwater
fields, therefore, are believed to be significantly less than
the quantities released from areas with shallower flooding
depths. Some flooded fields are drained periodically during
the growing season, either intentionally or accidentally. If
water is drained and soils are allowed to dry sufficiently,
CH4 emissions decrease or stop entirely. This is due to soil
aeration, which not only causes existing soil CH4 to oxidize
but also inhibits further CH4 production in soils. All rice
in the United States is grown under continuously flooded
conditions; none is grown under deepwater conditions. Mid-
season drainage does not occur except by accident (e.g., due
to levee breach).
Other factors that influence CH4 emissions from flooded
rice fields include fertilization practices (especially the use of
organic fertilizers), soil temperature, soil type, rice variety,
and cultivation practices (e.g., tillage, seeding, and weeding
practices). The factors that determine the amount of organic
material available to decompose (i.e., organic fertilizer use,
soil type, rice variety,3 and cultivation practices) are the most
important variables influencing the amount of CH4 emitted
over the growing season; the total amount of CH4 released
depends primarily on the amount of organic substrate
available. Soil temperature is known to be an important
factor regulating the activity of methanogenic bacteria, and
therefore the rate of CH4 production. However, although
temperature controls the amount of time it takes to convert
a given amount of organic material to CH4, that time is short
relative to a growing season, so the dependence of total
emissions over an entire growing season on soil temperature
is weak. The application of synthetic fertilizers has also
been found to influence CH4 emissions; in particular, both
nitrate and sulfate fertilizers (e.g., ammonium nitrate and
ammonium sulfate) appear to inhibit CH4 formation.
Rice is cultivated in eight states: Arkansas, California,
Florida, Louisiana, Mississippi, Missouri, Oklahoma, and
Texas.4 Soil types, rice varieties, and cultivation practices
for rice vary from state to state, and even from farm to farm.
However, most rice farmers apply organic fertilizers in the
form of residue from the previous rice crop, which is left
standing, disked, or rolled into the fields. Most farmers
also apply synthetic fertilizer to their fields, usually urea.
Nitrate and sulfate fertilizers are not commonly used in rice
cultivation in the United States. In addition, the climatic
conditions of southwest Louisiana, Texas, and Florida often
allow for a second, or ratoon, rice crop. Ratoon crops are
much less common or non-existent in Arkansas, California,
Mississippi, Missouri, Oklahoma, and northern areas of
Louisiana. Methane emissions from ratoon crops have been
found to be considerably higher than those from the primary
crop. This second rice crop is produced from regrowth of
the stubble after the first crop has been harvested. Because
the first crop's stubble is left behind in ratooned fields, and
there is no time delay between cropping seasons (which
would allow the stubble to decay aerobically), the amount of
organic material that is available for anaerobic decomposition
is considerably higher than with the first (i.e., primary) crop.
Rice cultivation is a small source of CH4 in the United
States (Table 6-9 and Table 6-10). In 2008, CH4 emissions
from rice cultivation were 7.2 Tg CO2 Eq. (343 Gg).
Although annual emissions fluctuated unevenly between
the years 1990 and 2008, ranging from an annual decrease
of 14 percent to an annual increase of 17 percent, there was
an overall decrease of 14 percent between 1990 and 2007,
due to an overall decrease in primary crop area.5 However,
emissions levels increased by 16 percent in 2008 due to an
increase in rice crop area in all states except Florida and
3 The roots of rice plants shed organic material, which is referred to as
"root exudate." The amount of root exudate produced by a rice plant over
a growing season varies among rice varieties.
4 A very small amount of rice is grown on about 20 acres in South Carolina;
however, this amount was determined to be too insignificant to warrant
inclusion in national emission estimates.
5 The 14 percent decrease occurred between 2005 and 2006; the 17 percent
increase happened between 1993 and 1994.
Agriculture 6-13
-------
Table 6-9: CH4 Emissions from Rice Cultivation (Tg C02 Eq.)
State
1990
1995
2000
2005
2006
2007
2008
Primary
Arkansas
California
Florida
Louisiana
Mississippi
Missouri
Oklahoma
Texas
Ratoon
Arkansas
Florida
Louisiana
Texas
5.1
2.1
0.7
1.0
0.4
•
0.6
2.1
1.1
0.9
Total
7.1
5.1
2.5
0.9
+
0.6
0.3
0.4
+
0.3
0.9
0.5
0.4
7.5
6.8
5.9
4.9
2.4
1.0
+
0.7
0.3
0.3
0.0
0.3
1.3
0.9
0.3
6.2
5.3
2.5
0.9
+
0.8
0.4
0.4
+
0.3
1.9
1.2
0.6
7.2
+ Less than 0.05 Tg C02Eq.
Note: Totals may not sum due to independent rounding.
Table 6-10: CH4 Emissions from Rice Cultivation (Gg)
State
1990
1995
2000
2005
2006
2007
2008
Primary
Arkansas
California
Florida
Louisiana
Mississippi
Missouri
Oklahoma
Texas
Ratoon
Arkansas
Florida
Louisiana
Texas
241
102
34
46
i
30
981
+J
52
45
265
114
40
2
48
24
27
98 1
54
40
260
120
47
2
18
"
«1
34
287
139
45
1
45
22
18
+
17
39
1
+
22
17
241
119
44
1
29
16
18
+
13
41
+
1
22
18
235
113
45
1
32
16
15
0
12
60
+
1
42
16
254
119
44
1
39
19
17
+
15
89
+
1
59
29
Total
339
363
357
326
282
295
343
+ Less than 0.5 Gg
Note: Totals may not sum due to independent rounding.
California, and especially due to an increase in the ratoon
crop in Louisiana and Texas. The factors that affect the rice
acreage in any year vary from state to state, although the price
of rice relative to competing crops is the primary controlling
variable in most states.
Methodology
IPCC (2006) recommends using harvested rice areas,
area-based daily emission factors (i.e., amount of CH4 emitted
per day per unit harvested area), and length of growing season
to estimate annual CH4 emissions from rice cultivation. This
Inventory uses the recommended methodology and employs
Tier 2 U.S.-specific emission factors derived from rice field
measurements. State-specific and daily emission factors were
6-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 6-11: Rice Areas Harvested (Hectares)
State/Crop
Arkansas
Primary
Ratoon3
California
Florida
Primary
Ratoon
Louisiana
Primary
Ratoon
Mississippi
Missouri
Oklahoma
Texas
Primary
Ratoon
Total Primary
Total Ratoon
Total
1990
485,633
0
159,854
4,978
2,489
220,558 1
66,168
101,1741
32,376
617
142,8571
57,143
1,148,047
125,799
1,273,847
1995
542,291
ol
188,1831
9,713
4,856
230,676 1
69,203
116,5521
45,326
3641
128,6931
51,477
1,261,796
125,536
1,387,333
2000
570,619
ol
221,773
7,801
3,193
194,253
77,701
88,223
68,393
283
86,605
43,302
1,237,951
124,197
1,362,148
2005
661,675
662
212,869
4,565
0
212,465
27,620
106,435
86,605
271
81,344
21,963
1,366,228
50,245
1,416,473
2006
566,572
6
211,655
4,575
1,295
139,620
27,924
76,487
86,605
17
60,704
23,675
1,146,235
52,899
1,199,135
2007
536,220
5
215,702
6,242
1,873
152,975
53,541
76,487
72,036
0
58,681
21,125
1,118,343
76,544
1,194,887
2008
564,549
6
209,227
5,463
1,639
187,778
75,111
92,675
80,534
77
69,607
36,892
1,209,911
113,648
1,323,559
11 Arkansas ratooning occurred only in 1998,1999, and 2005 through 2008.
Note: Totals may not sum due to independent rounding.
Table 6-12: Ratooned Area as Percent of Primary Growth Area
State
Arkansas
Florida
Louisiana
Texas
1990 1997 1998
0% +
50%
30%
40%
1999
+
65%
2000
41%
40%
50%
2001
60%
30%
40%
2002
0%
54%
15%
37%
2003
100%
35%
38%
2004
77%
30%
35%
2005
0.1%
0%
13%
27%
2006
+
28%
20%
39%
2007
+
30%
35%
36%
2008
+
30%
40%
53%
+ Indicates ratooning rate less than 0.1 percent.
not available, however, so average U.S. seasonal emission
factors were used. Seasonal emissions have been found to
be much higher for ratooned crops than for primary crops,
so emissions from ratooned and primary areas are estimated
separately using emission factors that are representative of
the particular growing season. This approach is consistent
with IPCC (2006).
The harvested rice areas for the primary and ratoon crops
in each state are presented in Table 6-11, and the area of
ratoon crop area as a percent of primary crop area is shown
in Table 6-12. Primary crop areas for 1990 through 2008
for all states except Florida and Oklahoma were taken from
U.S. Department of Agriculture's Field Crops Final Estimates
1987-1992 (USDA 1994), Field Crops Final Estimates
1992-1997 (USDA 1998), Field Crops Final Estimates
1997-2002 (USDA 2003), and Crop Production Summary
(USDA 2005 through 2009). Source data for non-USDA
sources of primary and ratoon harvest areas are shown in
Table 6-13. California, Mississippi, Missouri, and Oklahoma
have not ratooned rice over the period 1990 through 2008
(Guethle 1999, 2000, 2001a, 2002 through 2008; Lee 2003
through 2007; Mutters 2002 through 2005; Street 1999
through 2003; Walker 2005, 2007, 2008).
To determine what CH4 emission factors should be used
for the primary and ratoon crops, CH4 flux information from
rice field measurements in the United States was collected.
Experiments that involved atypical or nonrepresentative
management practices (e.g., the application of nitrate
or sulfate fertilizers, or other substances believed to
suppress CH4 formation), as well as experiments in which
Agriculture 6-15
-------
Table 6-13: Non-USDA Data Sources for Rice Harvest Information
State/Crop 1990
1999 2000 2001 2002 2003 2004 2005 2006 2007 2008
Arkansas
Ratoon
Florida
Primary
Ratoon
Louisiana
Ratoon
Oklahoma
Primary
Texas
Ratoon
Scheuneman
(1999b, 1999c, 2000, 2001 a)
Scheuneman
(1999a)
Bollich (2000)
Wilson (2002-2009)
Deren
(2002)
Deren
(2002)
Kirstein
(2003, 2006)
Kirstein Cantens
(2003-2004) (2005)
Linscombe (1999, 2001 a, 2002 through 2009)
l_66
(2003-2007)
Gonzales
(2006-2009)
Gonzales
(2006-2009)
Anderson
(2008-2009)
Klosterboer
(1999-2003)
Stansel
(2004-2005)
Texas Ag Experiment Station (2006-2009)
measurements were not made over an entire flooding season
or floodwaters were drained mid-season, were excluded
from the analysis. The remaining experimental results6 were
then sorted by season (i.e., primary and ratoon) and type
of fertilizer amendment (i.e., no fertilizer added, organic
fertilizer added, and synthetic and organic fertilizer added).
The experimental results from primary crops with added
synthetic and organic fertilizer (Bossio et al. 1999; Cicerone
et al. 1992; Sass et al. 1991a, 1991b) were averaged to derive
an emission factor for the primary crop, and the experimental
results from ratoon crops with added synthetic fertilizer
(Lindau and Bollich 1993, Lindau et al. 1995) were averaged
to derive an emission factor for the ratoon crop. The resultant
emission factor for the primary crop is 210 kg CH4/hectare-
season, and the resultant emission factor for the ratoon crop
is 780 kg CH4/hectare-season.
Uncertainty and Time-Series Consistency
The largest uncertainty in the calculation of CH4
emissions from rice cultivation is associated with the
emission factors. Seasonal emissions, derived from field
measurements in the United States, vary by more than one
order of magnitude. This inherent variability is due to
differences in cultivation practices, particularly fertilizer type,
6 In some of these remaining experiments, measurements from individual
plots were excluded from the analysis because of the aforementioned
reasons. In addition, one measurement from the ratooned fields (i.e., the
flux of 1,490 kg CH4/hectare-season in Lindau and Bollich 1993) was
excluded, because this emission rate is unusually high compared to other
flux measurements in the United States, as well as IPCC (2006) default
emission factors.
amount, and mode of application; differences in cultivar type;
and differences in soil and climatic conditions. A portion of
this variability is accounted for by separating primary from
ratooned areas. However, even within a cropping season or
a given management regime, measured emissions may vary
significantly. Of the experiments used to derive the emission
factors applied here, primary emissions ranged from 22 to
479 kg CH4/hectare-season and ratoon emissions ranged
from 481 to 1,490 kg CH4/hectare-season. The uncertainty
distributions around the primary and ratoon emission factors
were derived using the distributions of the relevant primary
or ratoon emission factors available in the literature and
described above. Variability about the rice emission factor
means was not normally distributed for either primary or
ratooned crops, but rather skewed, with a tail trailing to the
right of the mean. A lognormal statistical distribution was,
therefore, applied in the Tier 2 Monte Carlo analysis.
Other sources of uncertainty include the primary rice-
cropped area for each state, percent of rice-cropped area that
is ratooned, and the extent to which flooding outside of the
normal rice season is practiced. Expert judgment was used
to estimate the uncertainty associated with primary rice-
cropped area for each state at 1 to 5 percent, and a normal
distribution was assumed. Uncertainties were applied to
ratooned area by state, based on the level of reporting
performed by the state. No uncertainties were calculated
for the practice of flooding outside of the normal rice season
because CH4 flux measurements have not been undertaken
over a sufficient geographic range or under a broad enough
6-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 6-14: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Rice Cultivation
Manure Management (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Rice Cultivation
CH4
7.2
2.6
17.5
-64%
+ 143%
3 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
range of representative conditions to account for this source
in the emission estimates or its associated uncertainty.
To quantify the uncertainties for emissions from rice
cultivation, a Monte Carlo (Tier 2) uncertainty analysis
was performed using the information provided above. The
results of the Tier 2 quantitative uncertainty analysis are
summarized in Table 6-14. Rice cultivation CH4 emissions
in 2008 were estimated to be between 2.6 and 17.5 Tg CO2
Eq. at a 95 percent confidence level, which indicates a range
of 64 percent below to 143 percent above the actual 2008
emission estimate of 7.2 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A source-specific QA/QC plan for rice cultivation was
developed and implemented. This effort included a Tier 1
analysis, as well as portions of a Tier 2 analysis. The Tier 2
procedures focused on comparing trends across years, states,
and cropping seasons to attempt to identify any outliers or
inconsistencies. No problems were found.
Planned Improvements
A possible future improvement is to create region-
specific emission factors for rice cultivation. The current
methodology uses a nationwide average emission factor,
derived from several studies done in a number of states.
The prospective improvement would take the same studies
and average them by region, presumably resulting in more
spatially-specific emission factors.
Agriculture 6-17
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6.4. Agricultural Soil Management
(IPCC Source Category 4D)
Nitrous oxide is produced naturally in soils through the
microbial processes of nitrification and denitrification.7 A
number of agricultural activities increase mineral nitrogen
(N) availability in soils, thereby increasing the amount
available for nitrification and denitrification, and ultimately
the amount of N2O emitted. These activities increase soil
mineral N either directly or indirectly (see Figure 6-2). Direct
increases occur through a variety of management practices
that add or lead to greater release of mineral N to the soil,
including fertilization; application of managed livestock
manure and other organic materials such as sewage sludge;
deposition of manure on soils by domesticated animals in
pastures, rangelands, and paddocks (PRP) (i.e., by grazing
animals and other animals whose manure is not managed);
production of N-fixing crops and forages; retention of crop
residues; and drainage and cultivation of organic cropland
soils (i.e., soils with a high organic matter content, otherwise
known as histosols).8 Other agricultural soil management
activities, including irrigation, drainage, tillage practices,
and fallowing of land, can influence N mineralization in
soils and thereby affect direct emissions. Mineral N is
also made available in soils through decomposition of
soil organic matter and plant litter, as well as asymbiotic
fixation of N from the atmosphere,9 which are influenced
by agricultural management through impacts on moisture
7 Nitrification and denitrification are driven by the activity of microorganisms
in soils. Nitrification is the aerobic microbial oxidation of ammonium
(NH4+) to nitrate (NO3), and denitrification is the anaerobic microbial
reduction of nitrate to N2. Nitrous oxide is a gaseous intermediate product in
the reaction sequence of denitrification, which leaks from microbial cells into
the soil and then into the atmosphere. Nitrous oxide is also produced during
nitrification, although by a less well-understood mechanism (Nevison 2000).
8 Drainage and cultivation of organic soils in former wetlands enhances
mineralization of N-rich organic matter, thereby increasing N2O emissions
from these soils.
9 Asymbiotic N fixation is the fixation of atmospheric N2 by bacteria living
in soils that do not have a direct relationship with plants.
and temperature regimes in soils. These additional sources
of mineral N are included at the recommendation of IPCC
(2006) for complete accounting of management impacts on
greenhouse gas emissions, as discussed in the Methodology
section. Indirect emissions of N2O occur through two
pathways: (1) volatilization and subsequent atmospheric
deposition of applied/mineralized N,10 and (2) surface runoff
and leaching of applied/mineralized N into groundwater
and surface water. Direct emissions from agricultural lands
(i.e., croplands and grasslands) are included in this section,
while direct emissions from forest lands and settlements are
presented in the Land Use, Land-Use Change, and Forestry
chapter. However, indirect N2O emissions from all land-use
types (cropland, grassland, forest lands, and settlements) are
reported in this section.
Agricultural soils produce the majority of N2O emissions
in the United States. Estimated emissions from this source in
2008 were 215.9 Tg CO2 Eq. (696 Gg N2O) (see Table 6-15
and Table 6-16). Annual N2O emissions from agricultural
soils fluctuated between 1990 and 2008, although overall
emissions were 6 percent higher in 2008 than in 1990.
Year-to-year fluctuations are largely a reflection of annual
variation in weather patterns, synthetic fertilizer use, and
crop production. On average, cropland accounted for
approximately 69 percent of total direct emissions, while
grassland accounted for approximately 31 percent. These
percentages are about the same for indirect emissions
since forest lands and settlements account for such a small
percentage of total indirect emissions. Estimated direct and
indirect N2O emissions by sub-source category are shown in
Table 6-17 and Table 6-18.
10 These processes entail volatilization of applied or mineralized N as NH3
and NOX, transformation of these gases within the atmosphere (or upon
deposition), and deposition of the N primarily in the form of particulate
NH4+, nitric acid (HNO3), and NOX.
6-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Figure 6-2
Sources and Pathways of N that Result in N20 Emissions from Agricultural Soil Management
N Volatilization
Synthetic N Fertilizers
Synthetic N fettilizet applied to soil
Organic
Amendments
,metcisl fettilizets
animal manute, compost,
sewage sludge, tankage, etc.)
Urine and Dung from
Grazing Animals
Manute deposited on pastute, tange,
and paddock
Includes above- and belowg
residues for all crops (
Fixing) and from perennial fora
crops and pastures following i
Mineralization of
Soil Organic Matter
Includes N convetted to minetal ft
upon decomposition of soil otganic
Asymbiotic Fixation
Histosol
Cultivation
This graphic illustrates the sources and pathways of nitrogen that result
in direct and indirect N20 emissions from soils using the methodologies
described in this Inventory. Emission pathways are shown with arrows.
On the lower right-hand side is a cut-away view of a representative
section of a managed soil; histosol cultivation is represented here.
Agriculture 6-19
-------
Table 6-15: N20 Emissions from Agricultural Soils (Tg C02 Eq.)
Activity
Direct
Cropland
Grassland
Indirect (All Land-Use Types)
Cropland
Grassland
Forest Land
Settlements
Total
+ Less than 0.05 Tg C02Eq.
1990
156.7
103.0
53.7
46.7
36.0
10.4
+
0.3
203.5|
1995
161.8
109.8
51.9
44.2
33.9
91
01!
0.5
205.9
2000 1
165.8
115.6
50.21
44.3
35.7 1
8.0 1
01!
0.5 1
210.1 |
2005
170.5
117.9
52.6
45.4
35.4
9.3
0.1
0.6
215.8
2006
166.0
114.7
51.3
45.2
35.3
9.2
0.1
0.6
211.2
2007
167.2
116.7
50.5
43.8
34.1
9.0
0.1
0.6
211.0
2008
170.4
118.3
52.1
45.5
35.1
9.6
0.1
0.6
215.9
Table 6-16: N20 Emissions from Agricultural Soils (Gg)
Activity
Direct
Cropland
Grassland
Indirect (All Land-Use Types)
Cropland
Grassland
Forest Land
Settlements
Total
+ Less than 0.5 Gg N20
1990
506
332
173
151
116
33 1
_
656
Table 6-17: Direct N20 Emissions from Agricultural
Activity
Cropland
Mineral Soils
Synthetic Fertilizer
Organic Amendments3
Residue Nb
Mineralization and Asymbiotic
Organic Soils
Grassland
Synthetic Fertilizer
PRP Manure
Managed Manure0
Sewage Sludge
Residue Nd
Mineralization and Asymbiotic
Total
a Organic amendment inputs include
1990
103.0
156.7
managed manure amendments
1995
522
3541
1681
1431
iog|
31 1
2
664 |
Soils by Land
1995
109.8
106.9
39.8
"1
48.6
2.9 1
51.9
4.1 1
10.8
0.7 1
0.3 1
11.1
24.8
161.8
2000
535
3731
1621
1431
1151
26 1
1
1
678
Use Type and
2000
115.6
112.7
39.0
11.2
7.8
54.7
2.9 1
50.2
3.8 1
10.3
1
0.4|
10.4
24.6 H
165.8
2005
550
380
170
146
114
30
+
2
696
2006
535
370
165
146
114
30
+
2
681
N Input Type (Tg
2005
117.9
115.0
41.4
11.4
7.5
54.7
2.9
52.6
4.0
10.5
0.8
0.5
11.1
25.6
170.5
2006
114.7
111.8
39.4
11.6
7.5
53.3
2.9
51.3
4.0
10.4
0.8
0.5
10.8
24.8
166.0
2007
539
376
163
141
110
29
+
2
681
C02 Eq.)
2007
116.7
113.8
40.3
11.8
7.5
54.2
2.9
50.5
3.9
10.3
0.8
0.5
10.7
24.4
167.2
, daily spread manure amendments, and commercial organic fertilizers (i.e.,
2008
550
382
168
147
113
31
+
2
696
2008
118.3
115.4
40.8
11.7
7.8
55.1
2.9
52.1
4.0
10.4
0.8
0.5
11.0
25.4
170.4
dried
blood, dried manure, tankage, compost, and other).
b Cropland residue N inputs include N in unharvested legumes as well as crop residue N.
c Accounts for managed manure and
d Grassland residue N inputs include
daily spread manure amendments that are applied
to grassland soils.
N in ungrazed legumes as well as ungrazed grass residue N.
6-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 6-18: Indirect N20 Emissions from all Land-Use Types (Tg C02 Eq.)
Activity
1990
1995
2000
2005
2006
2007
2008
Cropland
Volatilization & Aim.
Surface Leaching &
Grassland
Volatilization & Aim.
Surface Leaching &
Forest Land
Volatilization & Aim.
Surface Leaching &
Settlements
Volatilization &Atm.
Surface Leaching &
Deposition
Run-Off
Deposition
Run-Off
Deposition
Run-Off
Deposition
Run-Off
36.0
10.5
25.6
10.4
5.6
4.8
0.3
0.1
0.2
33.9
11.7
22.2
9.7
5.6
4.1
"
0.5
0.2
0.3
35.7
11.9
23.8
8.0
5.1
2.9
"
0.1
0.5
0.2
0.3
35.4
11.7
23.6
9.3
5.3
4.0
0.1
+
0.1
0.6
0.2
0.4
35.3
12.9
22.4
9.2
5.3
3.9
0.1
+
0.1
0.6
0.2
0.4
34.1
11.3
22.7
9.0
5.2
3.8
0.1
+
0.1
0.6
0.2
0.4
35.1
12.0
23.1
9.6
5.2
4.4
0.1
+
0.1
0.6
0.2
0.4
Total
46.7
44.2
44.3
45.4
45.2
43.8
45.5
Less than 0.05 Tg C02Eq.
Figure 6-3 through Figure 6-6 show regional patterns
in direct N2O emissions, and also show N losses from
volatilization, leaching, and runoff that lead to indirect N2O
emissions. Average annual emissions and N losses from
croplands that produce major crops and from grasslands are
shown for each state. Direct N2O emissions from croplands
tend to be high in the Corn Belt (Illinois, Iowa, Indiana,
Ohio, southern Minnesota, southern Wisconsin, and eastern
Nebraska), where a large portion of the land is used for
growing highly fertilized corn and N-flxing soybean crops.
Direct emissions are also high in Missouri, Kansas, and
Texas, primarily from irrigated cropping in western Texas,
dryland wheat in Kansas, and hay cropping in eastern Texas
and Missouri. Direct emissions are low in many parts of
the eastern United States because a small portion of land is
cultivated, and also low in many western states where rainfall
and access to irrigation water are limited.
Direct emissions (Tg CO2 Eq./state/year) from grasslands
are highest in the central and western United States (Figure
6-4) where a high proportion of the land is used for cattle
grazing. Some areas in the Great Lake states, the Northeast,
and Southeast have moderate to low emissions even though
emissions from these areas tend to be high on a per unit area
basis, because the total amount of grazed land is much lower
than states in the central and western United States.
Indirect emissions from croplands and grasslands
(Figure 6-5 and Figure 6-6) show patterns similar to direct
emissions, because the factors that control direct emissions
(N inputs, weather, soil type) also influence indirect
emissions. However, there are some exceptions, because
the processes that contribute to indirect emissions (NO3~
leaching, N volatilization) do not respond in exactly the
same manner as the processes that control direct emissions
(nitrification and denitrification). For example, coarser-
textured soils facilitate relatively high indirect emissions in
Florida grasslands due to high rates of N volatilization and
NO3" leaching, even though they have only moderate rates
of direct N2O emissions.
Agriculture 6-21
-------
Figure 6-3
Major Crops, Average Annual Direct N20 Emissions Estimated Using the DAYCENT Model,
1990-2008 (Tg C02 Eq./year)
Figure 6-4
Grasslands, Average Annual Direct N20 Emissions Estimated Using the DAYCENT Model,
1990-2008 (TgC02 Eq./year)
6-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Figure 6-5
Major Crops, Average Annual N Losses Leading to Indirect N20 Emissions
Estimated Using the DAYCENT Model, 1990-2008 (Gg N/year)
Gg N/state/year
< 1
1-25
• 25-50
D 50-100
D 100-200
D 200-400
• >400
Figure 6-6
Grasslands, Average Annual N Losses Leading to Indirect N20 Emissions
Estimated Using the DAYCENT Model, 1990-2008 (Gg N/year)
Agriculture 6-23
-------
Methodology
The 2006 IPCC Guidelines (IPCC 2006) divide the
Agricultural Soil Management source category into four
components: (1) direct emissions due to N additions to
cropland and grassland mineral soils, including synthetic
fertilizers, sewage sludge applications, crop residues, organic
amendments, and biological N fixation associated with
planting of legumes on cropland and grassland soils; (2)
direct emissions from drainage and cultivation of organic
cropland soils; (3) direct emissions from soils due to the
deposition of manure by livestock on PRP grasslands; and
(4) indirect emissions from soils and water due to N additions
and manure deposition to soils that lead to volatilization,
leaching, or runoff of N and subsequent conversion to N2O.
The United States has adopted recommendations from
IPCC (2006) on methods for agricultural soil management.
These recommendations include (1) estimating the
contribution of N from crop residues to indirect soil N2O
emissions; (2) adopting a revised emission factor for direct
N2O emissions to the extent that Tier 1 methods are used in
the Inventory (described later in this section); (3) removing
double counting of emissions from N-fixing crops associated
with the biological N fixation and crop residue N input
categories; (4) using revised crop residue statistics to compute
N inputs to soils based on harvest yield data to the extent that
Tier 1 methods are used in the Inventory; (5) accounting for
indirect as well as direct emissions from N made available
via mineralization of soil organic matter and litter, in addition
to asymbiotic fixation11 (i.e., computing total emissions from
managed land); and (6) reporting all emissions from managed
lands, largely because management affects all processes
leading to soil N2O emissions. One recommendation from
IPCC (2006) has not been adopted: accounting for emissions
from pasture renewal, which involves occasional plowing to
improve forage production. This practice is not common in
the United States, and is not estimated.
The methodology used to estimate emissions from
agricultural soil management in the United States is based
on a combination of IPCC Tier 1 and 3 approaches. A Tier
3, process-based model (DAYCENT) was used to estimate
direct emissions from major crops on mineral (i.e., non-
organic) soils; as well as most of the direct emissions from
grasslands. The Tier 3 approach has been specifically
designed and tested to estimate N2O emissions in the
United States, accounting for more of the environmental
and management influences on soil N2O emissions than the
IPCC Tier 1 method (see Box 6-1 for further elaboration).
The Tier 1 IPCC (2006) methodology was used to estimate
(1) direct emissions from non-major crops on mineral soils
(e.g., barley, oats, vegetables, and other crops); (2) the portion
of the grassland direct emissions that were not estimated with
the Tier 3 DAYCENT model (i.e., federal grasslands); and
Box 6-1: Tier 1 vs. Tier 3 Approach for Estimating N20 Emissions
The IPCC (2006) Tier 1 approach is based on multiplying activity data on different N inputs (e.g., synthetic fertilizer, manure, N fixation,
etc.) by the appropriate default IPCC emission factors to estimate N20 emissions on an input-by-input basis. The Tier 1 approach requires a
minimal amount of activity data, readily available in most countries (e.g., total N applied to crops); calculations are simple; and the methodology
is highly transparent. In contrast, the Tier 3 approach developed for this Inventory employs a process-based model (i.e., DAYCENT) that
represents the interaction of N inputs and the environmental conditions at specific locations. Consequently, the Tier 3 approach is likely
to produce more accurate estimates; it accounts more comprehensively for land-use and management impacts and their interaction with
environmental factors (i.e., weather patterns and soil characteristics), which may enhance or dampen anthropogenic influences. However,
the Tier 3 approach requires more refined activity data (e.g., crop-specific N amendment rates), additional data inputs (e.g., daily weather,
soil types, etc.), and considerable computational resources and programming expertise. The Tier 3 methodology is less transparent, and
thus it is critical to evaluate the output of Tier 3 methods against measured data in order to demonstrate the adequacy of the method for
estimating emissions (IPCC 2006). Another important difference between the Tier 1 and Tier 3 approaches relates to assumptions regarding
N cycling. Tier 1 assumes that N added to a system is subject to N20 emissions only during that year and cannot be stored in soils and
contribute to N20 emissions in subsequent years. This is a simplifying assumption that is likely to create bias in estimated N20 emissions
for a specific year. In contrast, the process-based model used in the Tier 3 approach includes such legacy effects when N added to soils is
re-mineralized from soil organic matter and emitted as N20 during subsequent years.
11 N inputs from asymbiotic N fixation are not directly addressed in 2006
IPCC Guidelines, but are a component of the total emissions from managed
lands and are included in the Tier 3 approach developed for this source.
6-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
(3) direct emissions from drainage and cultivation of organic
cropland soils. Indirect emissions were also estimated with
a combination of DAYCENT and the IPCC Tier 1 method.
In past Inventories, attempts were made to subtract
"background" emissions that would presumably occur if
the lands were not managed. However, this approach is
likely to be inaccurate for estimating the anthropogenic
influence on soil N2O emissions. Moreover, if background
emissions could be measured or modeled based on processes
unaffected by anthropogenic activity, they would be a very
small portion of the total emissions, due to the high inputs
of N to agricultural soils from fertilization and legume
cropping. Given the recommendation from IPCC (2006)
and the influence of management on all processes leading
to N2O emissions from soils in agricultural systems, the
decision was made to report total emissions from managed
lands for this source category. Annex 3.11 provides more
detailed information on the methodologies and data used to
calculate N2O emissions from each component.
Direct N20 Emissions from Cropland Soils
Major Crop Types on Mineral Cropland Soils
The DAYCENT ecosystem model (Del Grosso et al.
2001, Parton et al. 1998) was used to estimate direct N2O
emissions from mineral cropland soils that are managed for
production of major crops—specifically corn, soybeans,
wheat, alfalfa hay, other hay, sorghum, and cotton—
representing approximately 90 percent of total croplands in
the United States. For these croplands, DAYCENT was used
to simulate crop growth, soil organic matter decomposition,
greenhouse gas fluxes, and key biogeochemical processes
affecting N2O emissions, and the simulations were driven
by model input data generated from daily weather records
(Thornton et al. 1997, 2000; Thornton and Running 1999),
land management surveys (see citations below), and soil
physical properties determined from national soil surveys
(Soil Survey Staff 2005). Note that the influence of land-
use change on soil N2O emissions was not addressed in this
analysis, but is a planned improvement.
DAYCENT simulations were conducted for each major
crop at the county scale in the United States. Simulating
N2O emissions at the county scale was facilitated by soil and
weather data that were available for every county with more
than 100 acres of agricultural land, and by land management
data (e.g., timing of planting, harvesting, intensity of
cultivation) that were available at the agricultural-region level
as defined by the Agricultural Sector Model (McCarl et al.
1993). ASM has 63 agricultural regions in the contiguous
United States. Most regions correspond to one state, except
for those states with greater heterogeneity in agricultural
practices; in such cases, more than one region is assigned
to a state. While cropping systems were simulated for each
county, the results best represent emissions at regional
(i.e., state) and national levels due to the regional scale of
management data, which include model parameters that
determined the influence of management activities on soil
N2O emissions (e.g., when crops were planted/harvested).
Nitrous oxide emissions from managed agricultural lands
are the result of interactions among anthropogenic activities
(e.g., N fertilization, manure application, tillage) and other
driving variables, such as weather and soil characteristics.
These factors influence key processes associated with N
dynamics in the soil profile, including immobilization of N by
soil microbial organisms, decomposition of organic matter,
plant uptake, leaching, runoff, and volatilization, as well as
the processes leading to N2O production (nitrification and
denitrification). It is not possible to partition N2O emissions
into each anthropogenic activity directly from model outputs
due to the complexity of the interactions (e.g., N2O emissions
from synthetic fertilizer applications cannot be distinguished
from those resulting from manure applications). To
approximate emissions by activity, the amount of mineral N
added to the soil for each of these sources was determined
and then divided by the total amount of mineral N that was
made available in the soil according to the DAYCENT model.
The percentages were then multiplied by the total of direct
N2O emissions in order to approximate the portion attributed
to key practices. This approach is only an approximation
because it assumes that all N made available in soil has an
equal probability of being released as N2O, regardless of its
source, which is unlikely to be the case (Delgado et al., 2009).
However, this approach allows for further disaggregation of
emissions by source of N, which is valuable for reporting
purposes and is analogous to the reporting associated with
the IPCC (2006) Tier 1 method, in that it associates portions
of the total soil N2O emissions with individual sources of N.
DAYCENT was used to estimate direct N2O emissions
due to mineral N available from: (1) the application of
Agriculture 6-25
-------
synthetic fertilizers; (2) the application of livestock manure;
(3) the retention of crop residues (i.e., leaving residues in the
field after harvest instead of burning or collecting residues);
and (4) mineralization of soil organic matter and litter, in
addition to asymbiotic fixation. Note that commercial organic
fertilizers are addressed with the Tier 1 method because
county-level application data would be needed to simulate
applications in the DAYCENT, and currently data are only
available at the national scale. The third and fourth sources
are generated internally by the DAYCENT model. For the
first two practices, annual changes in soil mineral N due to
anthropogenic activity were obtained or derived from the
following sources:
• Crop-specific N-fertilization rates: Data sources for
fertilization rates include Alexander and Smith (1990),
Anonymous (1924), Battaglin and Goolsby (1994),
Engle and Makela (1947), ERS (1994,2003), Fraps and
Asbury (1931), Ibach and Adams (1967), Ibach et al.
(1964), NFA (1946), NRIAI (2003), Ross and Mehring
(1938), Skinner (1931), Smalley et al. (1939), Taylor
(1994), USDA (1966, 1957, 1954, 1946). Information
on fertilizer use and rates by crop type for different
regions of the United States were obtained primarily
from the USDA Economic Research Service Cropping
Practices Survey (ERS 1997) with additional data
from other sources, including the National Agricultural
Statistics Service (NASS 1992, 1999, 2004).
• Managed manure production and application to
croplands and grasslands: Manure N amendments
and daily spread manure N amendments applied to
croplands and grasslands (not including PRP manure)
were determined using USDA Manure N Management
Databases for 1997 (Kellogg et al. 2000; Edmonds et
al. 2003). Amendment data for 1997 were scaled to
estimate values for other years based on the availability
of managed manure N for application to soils in 1997
relative to other years. The amount of available N from
managed manure for each livestock type was calculated
as described in the Manure Management section (Section
6.2) and Annex 3.10.
• Retention of crop residue, N mineralization from soil
organic matter, and asymbiotic N fixation from the
atmosphere: The IPCC approach considers crop residue
N and N mineralized from soil organic matter as activity
data. However, they are not treated as activity data in
DAYCENT simulations because residue production, N
fixation, mineralization of N from soil organic matter,
and asymbiotic fixation are internally generated by the
model. In other words, DAYCENT accounts for the
influence of N fixation, mineralization of N from soil
organic matter, and retention of crop residue on N2O
emissions, but these are not model inputs.
• Historical and modern crop rotation and management
information (e.g., timing and type of cultivation, timing
of planting/harvest, etc.): These activity data were
derived from Hurd (1930, 1929), Latta (1938), Iowa
State College Staff Members (1946), Bogue (1963),
Hurt (1994), USDA (2000a) as extracted by Eve (2001)
and revised by Ogle (2002), CTIC (1998), Piper et al.
(1924), Hardies and Hume (1927), Holmes (1902,1929),
Spillman (1902, 1905, 1907, 1908), Chilcott (1910),
Smith (1911), Kezer (ca. 1917), Hargreaves (1993), ERS
(2002), Warren (1911), Langston et al. (1922), Russell
et al. (1922), Elliott and Tapp (1928), Elliott (1933),
Ellsworth (1929), Garey (1929), Hodges et al. (1930),
Bonnen and Elliott (1931), Brenner et al. (2002,2001),
and Smith et al. (2002). Approximately 3 percent of
the crop residues were assumed to be burned based on
state inventory data (ILENR 1993, Oregon Department
of Energy 1995, Noller 1996, Wisconsin Department
of Natural Resources 1993, and Cibrowski 1996), and
therefore did not contribute to soil N2O emissions.
DAYCENT simulations produced per-area estimates
of N2O emissions (g N2O-N/m2) for major crops in each
county, which were multiplied by the cropland areas in each
county to obtain county-scale emission estimates. Cropland
area data were from NASS (USDA 2009a,b). The emission
estimates by reported crop areas in the county were scaled
to the regions, and the national estimate was calculated by
summing results across all regions. DAYCENT is sensitive
to interannual variability in weather patterns and other
controlling variables, so emissions associated with individual
activities vary through time even if the management practices
remain the same (e.g., if N fertilization remains the same
for two years). In contrast, Tier 1 methods do not capture
this variability and rather have a linear, monotonic response
that depends solely on management practices. DAYCENT's
ability to capture these interactions between management and
environmental conditions produces more accurate estimates
of N2O emissions than the Tier 1 method.
6-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Non-Major Crop Types on Mineral Cropland Soils
The IPCC (2006) Tier 1 methodology was used to
estimate direct N2O emissions for mineral cropland soils
that are managed for production of non-major crop types,
including barley, oats, tobacco, sugarcane, sugar beets,
sunflowers, millet, rice, peanuts, and other crops that were
not included in the DAYCENT simulations. Estimates of
direct N2O emissions from N applications to non-major crop
types were based on mineral soil N that was made available
from the following practices: (1) the application of synthetic
commercial fertilizers; (2) application of managed manure
and non-manure commercial organic fertilizers;12 and (3)
the retention of above- and below-ground crop residues in
agricultural fields (i.e., crop biomass that is not harvested).
Non-manure organic amendments were not included in
the DAYCENT simulations because county-level data
were not available. Consequently, non-manure organic
amendments, as well as manure amendments not included
in the DAYCENT simulations, were included in the Tier
1 analysis. The influence of land-use change on soil N2O
emissions from non-major crops has not been addressed in
this analysis, but is a planned improvement. The following
sources were used to derive activity data:
• A process-of-elimination approach was used to estimate
synthetic N fertilizer additions for non-major crops,
because little information exists on their fertilizer
application rates. The total amount of fertilizer used
on farms has been estimated by the USGS from sales
records (Ruddy et al. 2006), and these data were
aggregated to obtain state-level N additions to farms.
After subtracting the portion of fertilizer applied to
major crops and grasslands (see sections on Major
Crops and Grasslands for information on data sources),
the remainder of the total fertilizer used on farms was
assumed to be applied to non-major crops.
• A process-of-elimination approach was used to estimate
manure N additions for non-major crops, because little
information exists on application rates for these crops.
The amount of manure N applied to major crops and
grasslands was subtracted from total manure N available
12 Commercial organic fertilizers include dried blood, tankage, compost,
and other; dried manure and sewage sludge that are used as commercial
fertilizer have been excluded to avoid double counting. The dried manure
N is counted with the non-commercial manure applications, and sewage
sludge is assumed to be applied only to grasslands.
for land application (see sections on Major Crops and
Grasslands for information on data sources), and this
difference was assumed to be applied to non-major
crops.
• Non-manure, non-sewage-sludge commercial organic
fertilizer additions were based on organic fertilizer
consumption statistics, which were converted to units of
N using average organic fertilizer N content (TVA1991
through 1994; AAPFCO 1995 through 2008). Manure
and sewage sludge components were subtracted from
total commercial organic fertilizers to avoid double
counting.
• Crop residue N was derived by combining amounts
of above- and below-ground biomass, which were
determined based on crop production yield statistics
(USDA 1994, 1998, 2003, 2005, 2006, 2008, 2009a),
dry matter fractions (IPCC 2006), linear equations to
estimate above-ground biomass given dry matter crop
yields from harvest (IPCC 2006), ratios of below-to-
above-ground biomass (IPCC 2006), and N contents
of the residues (IPCC 2006). Approximately 3 percent
of the crop residues were burned and therefore did
not contribute to soil N2O emissions, based on state
inventory data (ILENR 1993, Oregon Department of
Energy 1995, Noller 1996, Wisconsin Department of
Natural Resources 1993, and Cibrowski 1996).
The total increase in soil mineral N from applied
fertilizers and crop residues was multiplied by the IPCC
(2006) default emission factor to derive an estimate of direct
N2O emissions from non-major crop types.
Drainage and Cultivation of Organic Cropland Soils
The IPCC (2006) Tier 1 methods were used to estimate
direct N2O emissions due to drainage and cultivation of
organic soils at a state scale. State-scale estimates of the
total area of drained and cultivated organic soils were
obtained from the National Resources Inventory (NRI)
(USDA 2000a, as extracted by Eve 2001 and amended by
Ogle 2002). Temperature data from Daly et al. (1994,1998)
were used to subdivide areas into temperate and tropical
climates using the climate classification from IPCC (2006).
Data were available for 1982, 1992 and 1997. To estimate
annual emissions, the total temperate area was multiplied by
the IPCC default emission factor for temperate regions, and
the total sub-tropical area was multiplied by the average of
Agriculture 6-27
-------
the IPCC default emission factors for temperate and tropical
regions (IPCC 2006).
Direct N20 Emissions from Grassland Soils
As with N2O from croplands, the Tier 3 process-based
DAYCENT model and Tier 1 method described in IPCC
(2006) were combined to estimate emissions from grasslands.
Grasslands include pastures and rangelands used for grass
forage production, where the primary use is livestock grazing.
Rangelands are typically extensive areas of native grasslands
that are not intensively managed, while pastures are often
seeded grasslands, possibly following tree removal, which
may or may not be improved with practices such as irrigation
and interseeding legumes.
DAYCENT was used to simulate county-scale N2O
emissions from non-federal grasslands resulting from manure
deposited by livestock directly onto pastures and rangelands
(i.e., PRP manure), N fixation from legume seeding, managed
manure amendments (i.e., manure other than PRP manure),
and synthetic fertilizer application. Other N inputs were
simulated within the DAYCENT framework, including
N input from mineralization due to decomposition of soil
organic matter and N inputs from senesced grass litter, as
well as asymbiotic fixation of N from the atmosphere. The
simulations used the same weather, soil, and synthetic N
fertilizer data as discussed under the section for Major
Crop Types on Mineral Cropland Soils. Managed manure
N amendments to grasslands were estimated from Edmonds
et al. (2003) and adjusted for annual variation using data on
the availability of managed manure N for application to soils,
according to methods described in the Manure Management
section (Section 6.2) and Annex 3.10. Biological N fixation
is simulated within DAYCENT and therefore was not an
input to the model.
Manure N deposition from grazing animals (i.e., PRP
manure) was an input to the DAYCENT model (see Annex
3.10), and included approximately 91 percent of total PRP
manure. The remainder of the PRP manure N excretions in
each county was assumed to be excreted on federal grasslands
(i.e., DAYCENT simulations were only conducted for non-
federal grasslands), and the N2O emissions were estimated
using the IPCC (2006) Tier 1 method with IPCC default
emission factors. The amounts of PRP manure N applied on
non-federal and federal grasslands in each county were based
on the proportion of non-federal grassland area according
to data from the NRI (USDA 2000a), relative to the area of
federal grasslands from the National Land Cover Dataset
(Vogelman et al. 2001).
Sewage sludge was assumed to be applied on grasslands
because of the heavy metal content and other pollutants in
human waste that limit its use as an amendment to croplands.
Sewage sludge application was estimated from data compiled
by EPA (1993,1999,2003), McFarland (2001), andNEBRA
(2007). Sewage sludge data on soil amendments on
agricultural lands were only available at the national scale,
and it was not possible to associate application with specific
soil conditions and weather at the county scale. Therefore,
DAYCENT could not be used to simulate the influence of
sewage sludge amendments on N2O emissions from grassland
soils, and consequently, emissions from sewage sludge were
estimated using the IPCC (2006) Tier 1 method.
DAYCENT simulations produced per-area estimates of
N2O emissions (g N2O-N/m2) for pasture and rangelands,
which were multiplied by the reported pasture and rangeland
areas in each county. Grassland area data were obtained
from the NRI (USDA 2000a). The 1997 NRI area data for
pastures and rangeland were aggregated to the county level
to estimate the grassland areas for 1995 to 2007, and the
1992 NRI pasture and rangeland data were aggregated to
the county level to estimate areas from 1990 to 1994. The
county estimates were scaled to the 63 agricultural regions,
and the national estimate was calculated by summing results
across all regions. Tier 1 estimates of N2O emissions for the
PRP manure N deposited on non-federal lands and applied
sewage sludge N were produced by multiplying the N input
by the appropriate emission factor.
Total Direct N20 Emissions from Cropland and Grassland
Soils
Annual direct emissions from major and non-major
crops on mineral cropland soils, from drainage and
cultivation of organic cropland soils, and from grassland soils
were summed to obtain the total direct N2O emissions from
agricultural soil management (see Table 6-15 and Table 6-16).
Indirect N20 Emissions from Managed Soils of all Land-Use
Types
This section describes the methods used for estimating
indirect soil N2O emissions from all land-use types (i.e.,
croplands, grasslands, forest lands, and settlements). Indirect
N2O emissions occur when mineral N made available through
6-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
anthropogenic activity is transported from the soil either
in gaseous or aqueous forms and later converted into N2O.
There are two pathways leading to indirect emissions. The
first pathway results from volatilization of N as NOX and
NH3 following application of synthetic fertilizer, organic
amendments (e.g., manure, sewage sludge), and deposition
of PRP manure. N made available from mineralization of
soil organic matter and asymbiotic fixation also contributes
to volatilized N emissions. Volatilized N can be returned to
soils through atmospheric deposition, and a portion of the
deposited N is emitted to the atmosphere as N2O. The second
pathway occurs via leaching and runoff of soil N (primarily
in the form of NO3") that was made available through
anthropogenic activity on managed lands, mineralization
of soil organic matter, and asymbiotic fixation. The nitrate
is subject to denitrification in water bodies, which leads to
N2O emissions. Regardless of the eventual location of the
indirect N2O emissions, the emissions are assigned to the
original source of the N for reporting purposes, which here
includes croplands, grasslands, forest lands, and settlements.
Indirect N20 Emissions from Atmospheric Deposition
of Volatilized N from Managed Soils
As in the direct emissions calculation, the Tier 3
DAYCENT model and IPCC (2006) Tier 1 methods were
combined to estimate the amount of N that was transported
from croplands, grasslands, forest lands, and settlements
through volatilization and eventually emitted as N2O.
DAYCENT was used to estimate N volatilization for
land areas whose direct emissions were simulated with
DAYCENT (i.e., major croplands and most grasslands). The
N inputs included are the same as described for direct N2O
emissions in the sections on major crops and grasslands.
N volatilization for all other areas was estimated using the
Tier 1 method and default IPCC fractions for N subject to
volatilization (i.e., N inputs on non-major croplands, PRP
manure N excretion on federal grasslands, sewage sludge
application on grasslands). The Tier 1 method and default
fractions were also used to estimate N subject to volatilization
from N inputs on settlements and forest lands (see the Land
Use, Land-Use Change, and Forestry chapter). With both the
DAYCENT and Tier 1 approaches, the IPCC (2006) default
emission factor was used to estimate indirect N2O emissions
associated with the amount of volatilized N (Table 6-18).
Indirect N20 Emissions from Leaching/Runoff
As with the calculations of indirect emissions from
volatilized N, the Tier 3 DAYCENT model and IPCC (2006)
Tier 1 method were combined to estimate the amount of N
that was transported from croplands, grasslands, forest lands,
and settlements through leaching and surface runoff into
water bodies, and eventually emitted as N2O. DAYCENT
was used to simulate the amount of N transported from lands
used to produce major crops and most grasslands. N transport
from all other areas was estimated using the Tier 1 method
and the IPCC (2006) default factor for the proportion of N
subject to leaching and runoff. This N transport estimate
includes N applications on croplands that produce non-
major crops, sewage sludge amendments on grasslands, PRP
manure N excreted on federal grasslands, and N inputs on
settlements and forest lands. For both the DAYCENT and
IPCC (2006) Tier 1 methods, nitrate leaching was assumed
to be an insignificant source of indirect N2O in cropland
and grassland systems in arid regions as discussed in IPCC
(2006). In the United States, the threshold for significant
nitrate leaching is based on the potential evapotranspiration
(PET) and rainfall amount, similar to IPCC (2006), and is
assumed to be negligible in regions where the amount of
precipitation plus irrigation does not exceed 80 percent of
PET. With both the DAYCENT and Tier 1 approaches, the
IPCC (2006) default emission factor was used to estimate
indirect N2O emissions associated with N losses through
leaching and runoff (Table 6-18).
Uncertainty and Time-Series Consistency
Uncertainty was estimated for each of the following
five components of N2O emissions from agricultural soil
management: (1) direct emissions calculated by DAYCENT;
(2) the components of indirect emissions (N volatilized
and leached or runoff) calculated by DAYCENT; (3) direct
emissions calculated with the IPCC (2006) Tier 1 method;
(4) the components of indirect emissions (N volatilized and
leached or runoff) calculated with the IPCC (2006) Tier 1
method; and (5) indirect emissions calculated with the IPCC
(2006) Tier 1 method. Uncertainty in direct emissions, which
account for the majority of N2O emissions from agricultural
management, as well as the components of indirect emissions
calculated by DAYCENT were estimated with a Monte
Carlo Analysis, addressing uncertainties in model inputs and
Agriculture 6-29
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Table 6-19: Quantitative Uncertainty Estimates of N20 Emissions from Agricultural Soil Management in 2008
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Direct Soil N20 Emissions N20
Indirect Soil N20 Emissions N20
170.4
45.5
129.7
23.8
278.4
110.2
-24%
-48%
+63%
+ 142%
Note: Due to lack of data, uncertainties in areas for major crops, managed manure N production, PRP manure N production, other organic fertilizer
amendments, indirect losses of N in the DAYCENT simulations, and sewage sludge amendments to soils are currently treated as certain; these sources of
uncertainty will be included in future Inventories.
structure (i.e., algorithms and parameterization) (Del Grosso
et al., 2009). Uncertainties in direct emissions calculated
with the IPCC (2006) Tier 1 method, the proportion of
volatilization and leaching or runoff estimated with the IPCC
(2006) Tier 1 method, and indirect N2O emissions were
estimated with a simple error propagation approach (IPCC
2006). Additional details on the uncertainty methods are
provided in Annex 3.11.
Uncertainties from the Tier 1 and Tier 3 (i.e., DAYCENT)
estimates were combined using simple error propagation
(IPCC 2006), and the results are summarized in Table
6-19. Agricultural direct soil N2O emissions in 2008 were
estimated to be between 129.7 and 278.4 Tg CO2 Eq. at a 95
percent confidence level. This indicates a range of 24 percent
below and 63 percent above the 2008 emission estimate of
170.4 Tg CO2 Eq. The indirect soil N2O emissions in 2008
were estimated to range from 23.6 to 110.4 Tg CO2 Eq. at
a 95 percent confidence level, indicating an uncertainty of
48 percent below and 142 percent above the 2008 emission
estimate of 45.5 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
For quality control, DAYCENT results for N2O
emissions and NO3" leaching were compared with field data
representing various cropped/grazed systems, soil types,
and climate patterns (Del Grosso et al. 2005, Del Grosso et
al. 2008), and further evaluated by comparing to emission
estimates produced using the IPCC (2006) Tier 1 method
for the same sites. Nitrous oxide measurement data were
available for 11 sites in the United States and one in Canada,
representing 30 different combinations of fertilizer treatments
and cultivation practices. DAYCENT estimates of N2O
emissions were closer to measured values at all sites except
for Colorado dryland cropping (Figure 6-7). In general, IPCC
Tier 1 methodology tends to over-estimate emissions when
observed values are low and under-estimate emissions when
Figure 6-7
Comparison of Measured Emissions at Field Sites and Modeled Emissions Using the DAYCENT Simulation Model
40 -
30 -
,
10 -
I Measured
DAYCENT
I IPCC
.1
I ,1
CO CO NE NE Ml Corn/
Dryland Dryland Dryland Grass Soy/
Wheat Cropping Wheat Alfalfa
TN CO CO CO
Corn Irrigated Irrigated Grass
Corn Corn/
Barley
ll.-.l
Ontario PA PA Average
Corn Crop Grass
6-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
observed values are high, while DAYCENT estimates are less
biased. This is not surprising because DAYCENT accounts
for site-level factors (weather, soil type) that influence N2O
emissions. Nitrate leaching data were available for three sites
in the United States representing nine different combinations
of fertilizer amendments. Linear regressions of simulated
vs. observed emission and leaching data yielded correlation
coefficients of 0.89 and 0.94 for annual N2O emissions and
NO3" leaching, respectively. This comparison demonstrates
that DAYCENT provides relatively high predictive capability
for N2O emissions and NO3" leaching, and is an improvement
over the IPCC Tier 1 method (see additional information in
Annex 3.11).
Spreadsheets containing input data and probability
distribution functions required for DAYCENT simulations of
major croplands and grasslands and unit conversion factors
were checked, as were the program scripts that were used
to run the Monte Carlo uncertainty analysis. Several errors
were identified following re-organization of the calculation
spreadsheets, and corrective actions have been taken. In
particular, some of the links between spreadsheets were
missing or needed to be modified. Spreadsheets containing
input data, emission factors, and calculations required for
the Tier 1 approach were checked and no errors were found.
Recalculations Discussion
Several revisions were made in the Agricultural Soil
Management Section for the current Inventory.
First, NO3" leaching to groundwater and flow into
streams does not occur in more arid regions according to the
IPCC (2006). In the previous Inventory, it was assumed that
NO3" leaching was not significant in soils with precipitation
input that did not exceed potential evapotranspitation, except
in soils that were irrigated. Quality control measures revealed
that NO3" leaching was under-estimated using this criterion;
for example, a large portion of Iowa and some counties in
other parts of the central and eastern United States were
assumed to have no leaching during some years with this
criterion. Several studies have shown that significant NO3"
leaching occurs annually in these regions (Jaynes et al.,
2001; David et al., 2009) so the threshold was revised to
better reflect U.S. conditions. Specifically, the criterion was
modified so that NO3~ leaching would be estimated and lead
to indirect N2O emissions for soils with a precipitation input
that was equal to or greater than 80 percent of the potential
evapotranspiration, in addition to irrigated soils. Second, in
the previous Inventory, the leaching criterion was not applied
for lands estimated using Tier 1 methodology. For this year's
Inventory, NO3" leaching was assumed to occur in states
where the area weighted mean precipitation plus irrigation
input was equal to or greater than 80 percent of the potential
evapotranspiration.
Third, the N2O emission factor for PRP manure
associated with horses, sheep and goats was revised to 0.01
in accordance with guidance from IPCC (2006). Previously
the emission factor of 0.02, which is for manure from cattle,
swine, and poultry, had been used for all livestock. Fourth,
the methodology to calculate livestock manure N was
changed such that total manure N added to soils increased by
approximately 5 percent (see Chapter 6, Section 6.2 "Manure
Management" for details).
The recalculations increased emissions from agricultural
soil management by about 2 percent on average over the time
series relative to the previous Inventory.
Planned Improvements
A key improvement is underway for Agricultural Soil
Management to incorporate more land-use survey data from
the NRI (USDA 2000a) into the DAYCENT simulation
analysis, beyond the area estimates for rangeland and pasture
that are currently used to estimate emissions from grasslands.
NRI has a record of land-use activities since 1979 for all
U.S. agricultural land, which is estimated at about 386 Mha.
NASS is used as the basis for land-use records in the current
Inventory, and there are three major disadvantages to this.
First, most crops are grown in rotation with other crops
(e.g., corn-soybean), but NASS data provide no information
regarding rotation histories. In contrast, NRI is designed to
track rotation histories, which is important because emissions
from any particular year can be influenced by the crop that
was grown the previous year. Second, NASS does not conduct
a complete survey of cropland area each year, leading to gaps
in the land base. NRI provides a complete history of cropland
areas for four out of every five years from 1979 to 1997,
and then every year after 1998. Third, the current Inventory
based on NASS does not quantify the influence of land-use
change on emissions, which can be addressed using the NRI
survey records. NRI also provides additional information on
pasture land management that can be incorporated into the
analysis (particularly the use of irrigation). Using NRI data
Agriculture 6-31
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will also make the Agricultural Soil Management methods
more consistent with the methods used to estimate C stock
changes for agricultural soils. The structure of model input
files that contain land management data will need to be
extensively revised to facilitate use of the annualized NRI
data. This improvement is planned to take place over the
next several years.
Other planned improvements are minor but will lead
to more accurate estimates, including updating DAYMET
weather data for more recent years following the release of
new data and using a rice-crop-specific emission factor for
N amendments to rice areas.
6.5. Field Burning of Agricultural
Residues (IPCC Source Category 4F)
Farming activities produce large quantities of agricultural
crop residues, and farmers use or dispose of these residues in
a variety of ways. For example, agricultural residues can be
left on or plowed into the field; composted and then applied
to soils; landfilled; or burned in the field. Alternatively, they
can be collected and used as fuel, animal bedding material,
supplemental animal feed, or construction material. Field
burning of crop residues is not considered a net source
of CO2, because the C released to the atmosphere as CO2
during burning is assumed to be reabsorbed during the
next growing season. Crop residue burning is, however, a
net source of CH4, N2O, CO, and NOX, which are released
during combustion.
Field burning is not a common method of agricultural
residue disposal in the United States. The primary crop types
whose residues are typically burned in the United States are
wheat, rice, sugarcane, corn, barley, soybeans, and peanuts. It
is assumed that 3 percent of the residue for each of these crops
is burned each year, except for rice and sugarcane.13 In 2008,
CH4 and N2O emissions from field burning were 1.0 Tg CO2
Eq. (46 Gg) and 0.5 Tg. CO2 Eq. (2 Gg), respectively. Annual
emissions from this source over the period 1990 to 2008 have
remained relatively constant, averaging approximately 0.9
Tg CO2 Eq. (41 Gg) of CH4 and 0.5 Tg CO2 Eq. (1 Gg) of
N2O (see Table 6-20 and Table 6-21).
Table 6-20: CH4 and N20 Emissions from Field Burning of Agricultural Residues (Tg C02 Eq.)
Gas/Crop Type
CH4
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
N20
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
Total
+ Less than 0.05 Tg C02Eq.
Note: Totals may not sum due to
1990
0.8
0.3 1
0.4 1
+ 1
0.2l
+
1.2
independent rounding.
1995 2000
0.7 0.9
0.1
0.1
0.1
0.3
0.2
+
0.4
+
•
+
0.2
+ 1
0.1
+
0.3
+ +
1.1 1.4
13 The fractions of rice
are significantly higher
discussion below).
2005
0.9
0.1
0.1
0.1
0.4
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.5
straw and
than those
2006
0.9
0.1
0.1
0.1
0.4
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.4
2007
1.0
0.1
0.1
0.1
0.5
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.5
2008
1.0
0.1
0.1
0.1
0.4
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.5
sugarcane residue burned each year
for other crops (see "Methodology"
6-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Table 6-21: CH4, N20, CO, and NOX Emissions from Field Burning of Agricultural Residues (Gg)
Gas/Crop Type
1990
1995
2000
2005
2006
2007
2008
CH4
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
N20
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
CO
NOX
36
1
13|
1
?!
+ 1
ll
+ 1
+ 1
+ 1
+ 1
+ 1
ll
+ 1
766
30
35
5
4
5
13
1
8
+
1
+
+
+
+
+
1
+
745
42
5
4
6
17
1
10
+
2
+
+
+
+
+
1
+
888
44
5
5
4
19
+
11
+
2
+
+
+
+
+
1
+
930
30 37 40
43
4
4
5
18
+
12
+
2
+
+
+
+
+
1
+
905
40
46
5
4
5
22
+
10
+
2
+
+
+
+
+
1
+
960
38
46
6
3
5
21
+
11
+
2
+
+
+
+
+
1
+
970
40
+ Less than 0.5 Gg
Note: Totals may not sum due to independent rounding.
Methodology
The Tier 2 methodology used for estimating greenhouse
gas emissions from field burning of agricultural residues in
the United States is consistent with IPCC (2006) (for more
details, see Box 6-2). In order to estimate the amounts of
C and nitrogen (N) released during burning, the following
equation was used:14
C or N released = S over all crop types
(Crop Production x Residue/Crop Ratio x
Dry Matter Fraction x Fraction of Residue Burned x
Burning Efficiency x Combustion Efficiency x
Fraction of C or N)
where,
Crop Production
Residue/Crop
Ratio
Fraction of
Residue Burned
Annual production of crop
inGg
Amount of residue produced
per unit of crop production
Amount of residue that is
burned per unit of total
residue
Dry Matter Fraction =
Fraction of C or N =
Burning Efficiency =
Combustion
Efficiency =
Amount of dry matter per
unit of biomass
Amount of C or N per unit
of dry matter
The proportion of prefire
fuel biomass consumed15
The proportion of C or N
released with respect to the
total amount of C or N avail-
able in the burned material,
respectively15
The amount C or N released was used in the following
equation to determine the CH4, CO, N2O andNOx emissions
from the field burning of agricultural residues:
CH4 and CO, or N2O and NOX Emissions from Field
Burning of Agricultural Residues = (C or N Released) x
(Emissions Ratio for C or N) x (Conversion Factor)
where,
Emissions Ratio = g CH4-C or CO-C/g C
released, or g N2O-N or
NOy-N/g N released
14 As is explained later in this section, the fraction of rice residues burned
varies among states, so these equations were applied at the state level for rice.
These equations were applied at the national level for all other crop types.
15 In IPCC/UNEP/OECD/IEA (1997), the equation for C or N released
contains the variable 'fraction oxidized in burning.' This variable is
equivalent to (burning efficiency x combustion efficiency).
Agriculture 6-33
-------
Conversion Factor = conversion, by molecular
weight ratio, of CH4-C to
C (16/12), or CO-C to
C (28/12), orN2O-Nto
N (44/28), or NOX-N to
N (30/14)
The types of crop residues burned in the United States
were determined from various state-level greenhouse gas
emission inventories (ILENR 1993, Oregon Department of
Energy 1995, Wisconsin Department of Natural Resources
1993) and publications on agricultural burning in the United
States (Jenkins et al. 1992, Turn et al. 1997, EPA 1992).
Crop production data for all crops except rice in Florida
and Oklahoma were taken from the USDA's Field Crops,
Final Estimates 1987-1992,1992-1997,1997-2002 (USDA
1994, 1998, 2003), and Crop Production Summary (USDA
2005 through 2009). Rice production data for Florida and
Oklahoma, which are not collected by USDA, were estimated
separately. Average primary and ratoon crop yields for
Florida (Schueneman and Deren 2002) were applied to
Florida acreages (Schueneman 1999b, 2001; Deren 2002;
Kirstein 2003, 2004; Cantens 2004, 2005; Gonzalez 2007a,
2008, 2009), and crop yields for Arkansas (USDA 1994,
1998, 2003, 2005 through 2009) were applied to Oklahoma
acreages16 (Lee 2003 through 2006; Anderson 2008, 2009).
The production data for the crop types whose residues are
burned are presented in Table 6-22.
The percentage of crop residue burned was assumed
to be 3 percent for all crops in all years, except rice and
sugarcane, based on state inventory data (ILENR 1993,
Oregon Department of Energy 1995, Noller 1996, Wisconsin
Department of Natural Resources 1993, and Cibrowski
1996). Estimates of the percentage of rice residue burned
were derived from state-level estimates of the percentage
of rice area burned each year, which were multiplied by
state-level annual rice production statistics. The annual
percentages of rice area burned in each state were obtained
from agricultural extension agents in each state and reports
of the California Air Resources Board (Anonymous 2006;
Bollich 2000; Buehring 2009; California Air Resources
Board 1999, 2001; Cantens 2005; Deren 2002; Fife 1999;
Guethle 2007, 2008,2009; Klosterboer 1999a, 1999b, 2000
through 2003; Lancero 2006 through 2009; Lee 2005 through
16 Rice production yield data are not available for Oklahoma, so the Arkansas
values are used as a proxy.
Box 6-2: Comparison of Tier 2 U.S. Inventory Approach
and IPCC (2006) Default Approach
This Inventory calculates emissions from Burning of
Agricultural Residues using a Tier 2 methodology that is based on
IPCC/UNEP/OECD/IEA (1997) and incorporates crop- and country-
specific emission factors and variables. The equation used in this
Inventory varies slightly in form from the one presented in the IPCC
(2006) guidelines, but both equations rely on the same underlying
variables. The IPCC (2006) equation was developed to be broadly
applicable to all types of biomass burning, and, thus, is not specific
to agricultural residues. IPCC (2006) default factors are provided
only for four crops (wheat, corn, rice, and sugarcane), while this
Inventory analyzes emissions from seven crops. A comparison of
the methods and factors used in (1) the current Inventory and (2)
the default IPCC (2006) approach was undertaken to determine
the magnitude of the difference in overall estimates resulting from
the two approaches. Since the default IPCC (2006) approach calls
for area burned data that are currently unavailable for the United
States, estimates of area burned were developed using USDA data
on area harvested for each crop multiplied by the estimated fraction
of residue burned for that crop (see Table 6-24).
The IPCC (2006) default approach resulted in 20 percent
higher emissions of CH4 and 42 percent higher emissions of N20
than the current estimates in this Inventory. It is reasonable to
maintain the current methodology, since the IPCC (2006) defaults
are only available for four crops and are worldwide average
estimates, while current inventory estimates are based on U.S.-
specific, crop-specific, published data.
2007; Lindberg 2002 through 2005; Linscombe 1999a,
1999b, 2001 through 2009; Najita 2000, 2001; Sacramento
Valley Basinwide Air Pollution Control Council 2005,2007;
Schueneman 1999a, 1999b, 2001; Stansel 2004,2005; Street
2001 through 2003; Texas Agricultural Experiment Station
2006 through 2009; Walker 2004 through 2008; Wilson 2003
through 2007,2009) (see Table 6-23). The estimates provided
for Florida remained constant over the entire 1990 through
2008 period. While the estimates for all other states varied
over the time series, estimates for Missouri remained constant
through 2005, dropped in 2006, and remained constant near
the 2006 value in 2007 and 2008. For California, the annual
percentages of rice area burned in the Sacramento Valley are
assumed to be representative of burning in the entire state,
because the Sacramento Valley accounts for over 95 percent
of the rice acreage in California (Fife 1999). These values
generally declined between 1990 and 2008 because of a
legislated reduction in rice straw burning (Lindberg 2002),
6-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 6-22: Agricultural Crop Production (Gg of Product)
Crop
Wheat
Rice
Sugarcane
Corn3
Barley
Soybeans
Peanuts
a Corn for grain
1990
74,292
7,114l
25,525
201,5341
9,192
52,416
1,635
(i.e., excludes corn for silage).
1995
59,404
7,947
27,922
187,970
7,824
59,174
1,570
2000
60,641
8,705
32,762
251,8541
6,919
75,055
1,481
2005
57,280
10,150
24,137
282,311
4,613
83,368
2,209
2006
49,316
8,813
26,820
267,598
3,923
86,770
1,571
2007
55,821
9,033
27,187
331,177
4,575
72,859
1,666
2008
68,026
9,272
27,842
307,386
5,214
80,536
2,335
Table 6-23: Percent of Rice Area Burned by State
State
Arkansas
California
Florida3
Louisiana
Mississippi
Missouri
Oklahoma
Texas
1990
13%
75%
0%
6%
10%
18%
90%
1%
1995
13%
59%
0%
6%l
10%
18%!
90%
1%
2000 •
13%
27%
0%
5% !
40%
18%!
90%
0%
2005
22%
16%
0%
3%
23%
18%
94%
0%
2006
27%
10%
0%
5%
25%
3%
0%
0%
2007
20%
16%
0%
5%
24%
3%
0%
0%
2008
20%
11%
0%
5%
25%
3%
91%
0%
"Although rice is cultivated in Florida, crop residue burning is illegal.
Table 6-24: Key Assumptions for Estimating Emissions from Field Burning of Agricultural Residues
Crop
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
Residue/
Crop Ratio
1.3
1.4
0.2
1.0
1.2
2.1
1.0
Fraction of
Residue Burned
0.03
Variable
0.95
0.03
0.03
0.03
0.03
Dry Matter
Fraction
0.93
0.91
0.62
0.91
0.93
0.87
0.86
C
Fraction
0.4428
0.3806
0.4235
0.4478
0.4485
0.4500
0.4500
N
Fraction
0.0062
0.0072
0.0040
0.0058
0.0077
0.0230
0.0106
Burning
Efficiency
0.93
0.93
0.81
0.93
0.93
0.93
0.93
Combustion
Efficiency
0.88
0.88
0.68
0.88
0.88
0.88
0.88
although there was a slight increase from 2004 to 2005 and
from 2006 to 2007 (see Table 6-23). Estimates for percent
of sugarcane burned were obtained from Ashman (2008).
All residue/crop product mass ratios except sugarcane
were obtained from Strehler and Stiitzle (1987). The datum
for sugarcane is from Kinoshita (1988). Residue dry matter
contents for all crops except soybeans, peanuts were obtained
from Turn et al. (1997). Soybean dry matter content was
obtained from Strehler and Stiitzle (1987). Peanut dry matter
content was obtained through personal communications
with Jen Ketzis (1999), who accessed Cornell University's
Department of Animal Science's computer model, Cornell
Net Carbohydrate and Protein System. The residue C
contents and N contents for all crops except soybeans and
peanuts are from Turn et al. (1997). The residue C content
for soybeans and peanuts is the IPCC default (IPCC/UNEP/
OECD/IEA 1997). The N content of soybeans is from
Barnard and Kristoferson (1985). The N content of peanuts
is from Ketzis (1999). These data are listed in Table 6-24.
The burning efficiency was assumed to be 93 percent, and the
combustion efficiency was assumed to be 88 percent, for all
crop types, except sugarcane (EPA 1994). For sugarcane, the
burning efficiency was assumed to be 81 percent (Kinoshita
Agriculture 6-35
-------
Table 6-25: Greenhouse Gas Emission Ratios and
Conversion Factors
Gas
CH4:C
CO:C
N20:N
NOX:N
Emission Ratio
0.0053
0.0603
0.007b
0.121b
Conversion Factor
16/12
28/12
44/28
30/14
a Mass of C compound released (units of C) relative to mass of total C
released from burning (units of C).
b Mass of N compound released (units of N) relative to mass of total N
released from burning (units of N).
1988) and the combustion efficiency was assumed to be 68
percent (Turn et al. 1997). Emission ratios and conversion
factors for all gases (see Table 6-25) were taken from the
Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA
1997).
Uncertainty and Time-Series Consistency
A significant source of uncertainty in the calculation
of non-CO2 emissions from field burning of agricultural
residues is in the estimates of the fraction of residue of each
crop type burned each year. Data on the fraction burned,
as well as the gross amount of residue burned each year,
are not collected at either the national or state level. In
addition, burning practices are highly variable among crops
and among states. The fractions of residue burned used in
these calculations were based upon information collected by
state agencies and in published literature. Based on expert
judgment, uncertainty in the fraction of crop residue burned
ranged from zero to 100 percent, depending on the state and
crop type.
The results of the Tier 2 Monte Carlo uncertainty
analysis are summarized in Table 6-26. CH4 emissions
from field burning of agricultural residues in 2008 were
estimated to be between 0.3 and 1.8 Tg CO2 Eq. at a 95
percent confidence level. This indicates a range of 68 percent
below and 88 percent above the 2008 emission estimate of
1.0 Tg CO2 Eq. Also at the 95 percent confidence level, N2O
emissions were estimated to be between 0.2 and 1.0 Tg CO2
Eq. (or approximately 71 percent below and 83 percent above
the 2008 emission estimate of 0.5 Tg CO2 Eq.).
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/WC and Verification
A source-specific QA/QC plan for field burning of
agricultural residues was implemented. This effort included
a Tier 1 analysis, as well as portions of a Tier 2 analysis.
The Tier 2 procedures focused on comparing trends across
years, states, and crops to attempt to identify any outliers or
inconsistencies. No problems were found.
Recalculation Discussion
The crop production data for 2007 were updated using
data from USDA (2009). In addition, sugarcane-specific
information for residue/crop ratio, fraction residue burned,
dry matter fraction, burning efficiency, and combustion
efficiency were obtained. Although some of these factors
are specific to Hawaii, it was felt that they better represented
the common practice in the United States than the factors
previously in use. These changes resulted in an average
increase in the CH4 emission estimates of 11 percent across
the time series, and an average increase in the N2O emission
estimate of 4 percent across the time series, relative to the
previous Inventory.
Table 6-26: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 Emissions from Field Burning of
Agricultural Residues (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Field Burning of
Agricultural Residues
Field Burning of
Agricultural Residues
CH4
N20
1.0
0.5
Lower Bound
0.3
0.2
Upper Bound Lower Bound
1.8 -68%
1.0 -71%
Upper Bound
+88%
+83%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
6-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
IrnprOVBITIBntS Preliminary research on agricultural burning in the
The estimated 3 percent of crop residue burned for all United States indicates that residues from several additional
crops, except rice and sugarcane, is based on data gathered cr°P tyPes (e-§-> §rass for seed> blueberries, and fruit and nut
from several state greenhouse gas inventories. This fraction f668) &K burned- Whether sufficient information exists for
is the most statistically significant input to the emissions inclusion of these additional crop types in future Inventories
equation, and an important area for future improvement. is being investigated. The extent of recent state crop-burning
More crop- and state-specific information on the fraction regulations is also being investigated.
burned will be investigated by literature review and/or by
contacting state departments of agriculture.
Agriculture 6-37
-------
7. Land Use, Land-Use Change,
and Forestry
This chapter provides an assessment of the net greenhouse gas flux1 resulting from the uses and changes in land
types and forests in the United States. The Intergovernmental Panel on Climate Change 2006 Guidelines for
National Greenhouse Gas Inventories (IPCC 2006) recommends reporting fluxes according to changes within
and conversions between certain land-use types, termed forest land, cropland, grassland, and settlements (as well as
wetlands). The greenhouse gas flux from Forest Land Remaining Forest Land is reported using estimates of changes in
forest carbon (C) stocks, non-carbon dioxide (CO2) emissions from forest fires, and the application of synthetic fertilizers
to forest soils. The greenhouse gas flux reported in this chapter from agricultural lands (i.e., cropland and grassland)
includes changes in organic C stocks in mineral and organic soils due to land use and management, and emissions of CO2
due to the application of crushed limestone and dolomite to managed land (i.e., soil liming) and urea fertilization. Fluxes
are reported for four agricultural land use/land-use change categories: Cropland Remaining Cropland, Land Converted
to Cropland, Grassland Remaining Grassland, and Land Converted to Grassland. Fluxes resulting from Settlements
Remaining Settlements include those from urban trees and soil fertilization. Landfilled yard trimmings and food scraps
are accounted for separately under Other.
The estimates in this chapter, with the exception of CO2 fluxes from wood products and urban trees, and CO2 emissions
from liming and urea fertilization, are based on activity data collected at multiple-year intervals, which are in the form of
forest, land-use, and municipal solid waste surveys. Carbon dioxide fluxes from forest C stocks (except the wood product
components) and from agricultural soils (except the liming component) are calculated on an average annual basis from
data collected in intervals ranging from 1 to 10 years. The resulting annual averages are applied to years between surveys.
Calculations of non-CO2 emissions from forest fires are based on forest CO2 flux data. For the landfilled yard trimmings
and food scraps source, periodic solid waste survey data were interpolated so that annual storage estimates could be derived.
This flux has been applied to the entire time series, and periodic U.S. census data on changes in urban area have been used
to develop annual estimates of CO2 flux.
Land use, land-use change, and forestry activities in 2008 resulted in a net C sequestration of 940.3 Tg CO2 Eq. (256.5 Tg
C) (Table 7-1 and Table 7-2). This represents an offset of approximately 13.5 percent of total U.S. CO2 emissions. Total land
use, land-use change, and forestry net C sequestration2 increased by approximately 3.4 percent between 1990 and 2008. This
increase was primarily due to an increase in the rate of net C accumulation in forest C stocks. Net C accumulation in Forest
Land Remaining Forest Land, Land Converted to Grassland, and Settlements Remaining Settlements increased, while net C
1 The term "flux" is used here to encompass both emissions of greenhouse gases to the atmosphere, and removal of C from the atmosphere. Removal of
C from the atmosphere is also referred to as "carbon sequestration."
2 Carbon sequestration estimates are net figures. The C stock in a given pool fluctuates due to both gains and losses. When losses exceed gains, the C
stock decreases, and the pool acts as a source. When gains exceed losses, the C stock increases, and the pool acts as a sink. This is also referred to as
net C sequestration.
Land Use, Land-Use Change, and Forestry 7-1
-------
Table 7-1: Net C02 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Sink Category
Forest Land Remaining Forest Land3
Cropland Remaining Cropland
Land Converted to Cropland
Grassland Remaining Grassland
Land Converted to Grassland
Settlements Remaining Settlements'1
Other (Landfilled Yard Trimmings
and Food Scraps)
Total
1990
(729.8)
(29.4)
2.2
(52.0)
(19.8)
(57.1)
(23.5)
(909.4)
1995
(692.6)
(22.9)
2.9 1
(26.7)
(22.3)
(67.3)
(13.9)
(842.9)
2000
(467.7)
(30.2)
4 1
(52.6)
(27.3)
(77.5)1
(11.3)
(664.2)
2005
(806.6)
(18.3)
5.9
(9.0)
(24.6)
(87.8)
(10.1)
(950.4)
2006
(812.5)
(19.1)
5.9
(8.9)
(24.5)
(89.8)
(10.3)
(959.2)
2007
(806.9)
(19.7)
5.9
(8.8)
(24.3)
(91.9)
(9.8)
(955.4)
2008
(791.9)
(18.1)
5.9
(8.7)
(24.2)
(93.9)
(9.5)
(940.3)
3 Estimates include C stock changes on both Forest Land Remaining Forest Land and Land Converted to Forest Land.
b Estimates include C stock changes on both Settlements Remaining Settlements and Land Converted to Settlements.
Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
Table 7-2: Net C02 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and Forestry (Tg C)
Sink Category
Forest Land Remaining Forest Land3
Cropland Remaining Cropland
Land Converted to Cropland
Grassland Remaining Grassland
Land Converted to Grassland
Settlements Remaining Settlements'1
Other (Landfilled Yard Trimmings
and Food Scraps)
Total
1990
(199.0)
(8.0)
0.6 1
(14.2)
(5.4)
(15.6)
(6.4)
(248.0)
1995
(188.9)
(6.3)
0.8
(7.3)
(6.1)
(18.4)
(3.8)
(229.9)
2000
(127.6)
(8.2)
0.6 1
(14.3)
(7.4)
(21.1)1
(3.1)
(181.2)
2005
(220.0)
(5.0)
1.6
(2.5)
(6.7)
(23.9)
(2.8)
(259.2)
2006
(221.6)
(5.2)
1.6
(2.4)
(6.7)
(24.5)
(2.8)
(261.6)
2007
(220.1)
(5.4)
1.6
(2.4)
(6.6)
(25.1)
(2.7)
(260.6)
2008
(216.0)
(4.9)
1.6
(2.4)
(6.6)
(25.6)
(2.6)
(256.5)
3 Estimates include C stock changes on both Forest Land Remaining Forest Land and Land Converted to Forest Land.
b Estimates include C stock changes on both Settlements Remaining Settlements and Land Converted to Settlements.
Note: 1 Tg C = 1 teragram C = 1 million metric tons C. Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
accumulation in Cropland Remaining Cropland, Grassland
Remaining Grassland, and landfilled yard trimmings and
food scraps slowed over this period. Emissions from Land
Converted to Cropland increased between 1990 and 2008.
Emissions from Land Use, Land-Use Change, and
Forestry are shown in Table 7-3 and Table 7-4. Liming of
agricultural soils and urea fertilization in 2008 resulted in
CO2 emissions of 3.8 Tg CO2 Eq. (3,831 Gg) and 3.8 Tg
CO2 Eq. (3,807 Gg), respectively. Lands undergoing peat
extraction (i.e., Peatlands Remaining Peatlands) resulted
in CO2 emissions of 0.9 Tg CO2 Eq. (941 Gg), and N2O
emissions of less than 0.01 Tg CO2 Eq. The application of
synthetic fertilizers to forest soils in 2008 resulted in direct
N2O emissions of 0.4 Tg CO2 Eq. (1 Gg). Direct N2O
emissions from fertilizer application to forest soils have
increased by 455 percent since 1990, but still account for a
relatively small portion of overall emissions. Additionally,
direct N2O emissions from fertilizer application to settlement
soils in 2008 accounted for 1.6 Tg CO2 Eq. (5 Gg) in 2008.
This represents an increase of 61 percent since 1990. Forest
fires in 2008 resulted in methane (CH4) emissions of 11.9
Tg CO2 Eq. (568 Gg), and in N2O emissions of 9.7 Tg CO2
Eq. (31 Gg).
7-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 7-3: Emissions from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Source Category 1990 1995 2000 2005 2006 2007 2008
C02
Cropland Remaining Cropland: Liming of
Agricultural Soils
Urea Fertilization
Wetlands Remaining Wetlands: Peatlands
Remaining Peatlands
CH4
Forest Land Remaining Forest Land:
Forest Fires
N20
Forest Land Remaining Forest Land:
Forest Fires
Forest Land Remaining Forest Land:
Forest Soils3
Settlements Remaining Settlements:
Settlement Soils"
Wetlands Remaining Wetlands: Peatlands
Remaining Peatlands
„
,7
2.4
1.0
3.2
32
3,
2.6
0,
„
4.4
27
1.0
4.3
,3
4.9
3.5
0.2
1
8.8
4.3
3.2
1.2
14.3
14.3
13.2
11.7
0.4
1.1
8.9
4.3
3.5
1.1
9.8
9.8
9.8
8.0
0.4
1.5
8.8
4.2
3.7
0.9
21.6
21.6
19.5
17.6
0.4
1.5
9.3
4.5
3.8
1.0
20.0
20.0
18.3
16.3
0.4
1.6
8.6
3.8
3.8
0.9
11.9
11.9
11.7
9.7
0.4
1.6
Total
15.0
17.2
36.3
28.6
49.8
47.6
Table 7-4: Emissions from Land Use, Land-Use Change, and Forestry (Gg)
Source Category
1995
2000
C02
Cropland Remaining Cropland: Liming of
Agricultural Soils
Urea Fertilization
Wetlands Remaining Wetlands: Peatlands
Remaining Peatlands
CH4
Forest Land Remaining Forest Land:
Forest Fires
N20
Forest Land Remaining Forest Land:
Forest Fires
Forest Land Remaining Forest Land:
Forest Soils3
Settlements Remaining Settlements:
Settlement Soils"
Wetlands Remaining Wetlands: Peatlands
Remaining Peatlands
2005
2006
2007
8,933
4,349
3,504
1,079
467
467
32
26
1
5
8,754
4,220
3,656
879
1,027
1,027
63
57
1
5
9,331
4,512
3,807
1,012
953
953
59
53
1
5
32.2
+ Less than 0.05 Tg C02Eq.
a Estimates include emissions from N fertilizer additions on both Forest Land Remaining Forest Land, and Land Converted to Forest Land,
but not from land-use conversion.
b Estimates include emissions from N fertilizer additions on both Settlements Remaining Settlements, and Land Converted to Settlements,
but not from land-use conversion.
Note: These estimates include direct emissions only. Indirect N20 emissions are reported in the Agriculture chapter.
Totals may not sum due to independent rounding.
2008
8,579
3,831
3,807
941
568
568
38
31
1
5
+ Less than 0.5 Gg.
a Estimates include emissions from N fertilizer additions on both Forest Land Remaining Forest Land, and Land Converted to Forest Land,
but not from land-use conversion.
b Estimates include emissions from N fertilizer additions on both Settlements Remaining Settlements, and Land Converted to Settlements,
but not from land-use conversion.
Note: These estimates include direct emissions only. Indirect N20 emissions are reported in the Agriculture chapter.
Totals may not sum due to independent rounding.
Land Use, Land-Use Change, and Forestry 7-3
-------
7.1. Representation of the U.S.
Land Base
A national land-use categorization system that is
consistent and complete both temporally and spatially is
needed in order to assess land use and land-use change
status and the associated greenhouse gas fluxes over the
inventory time series. This system should be consistent with
IPCC (2006), such that all countries reporting on national
greenhouse gas fluxes to the UNFCCC should (1) describe the
methods and definitions used to determine areas of managed
and unmanaged lands in the country; (2) describe and apply
a consistent set of definitions for land-use categories over
the entire national land base and time series associated with
the greenhouse gas inventory, such that increases in the land
areas within particular land-use categories are balanced
by decreases in the land areas of other categories; and (3)
account for greenhouse gas fluxes on all managed lands.
The implementation of such a system helps to ensure that
estimates of greenhouse gas fluxes are as accurate as possible.
This section of the Inventory has been developed in order to
comply with this guidance.
Multiple databases are used to track land management
in the United States, are also used as the basis to classify
U.S. land area into the six IPCC land-use categories (i.e.,
Forest Land Remaining Forest Land, Cropland Remaining
Cropland, Grassland Remaining Grassland, Wetlands
Remaining Wetlands, Settlements Remaining Settlements
and Other Land Remaining Other Land) and thirty land-use
change categories (e.g., Cropland Converted to Forest Land,
Grassland Converted to Forest Land, Wetlands Converted
to Forest Land, Settlements Converted to Forest Land,
Other Land Converted to Forest Land)3 (IPCC 2006). The
primary databases are the U.S. Department of Agriculture
(USDA) National Resources Inventory (NRI)4 and the USDA
Forest Service (USFS) Forest Inventory and Analysis (FIA)5
Database. The U.S. Geological Survey (USGS) National
Land Cover Dataset (NLCD)6 is also used to identify land
uses in regions that were not included in the NRI or FIA. The
total land area included in the U.S. Inventory is 786 million
hectares, and this entire land base is considered managed.7 In
2008 the United States had a total of 273 million hectares of
Forest Land (a 4.1 percent increase since 1990), 163 million
hectares of Cropland (down 4.4 percent since 1990), 259
million hectares of Grassland (down 4.3 percent since 1990),
27 million hectares of Wetlands (down 3.8 percent since
1990), 49 million hectares of Settlements (up 24.4 percent
since 1990), and 14 million hectares of Other Land. It is
important to note that the land base formally classified for
the Inventory (see Table 7-5) is considered managed. Much
of the unmanaged area in the United States (see definition
later in this section) occurs in Alaska. Alaska is not formally
included in the current land representation, but there is a
planned improvement underway to include this portion of
the United States in future Inventories. In addition, wetlands
are not differentiated between managed and unmanaged,
although some wetlands would be unmanaged according
to the U.S. definition (see definition later in this section).
Future improvements will include a differentiation between
managed and unmanaged wetlands.
Dominant land uses vary by region, largely due to
climate patterns, soil types, geology, proximity to coastal
regions, and historical settlement patterns, although all
land-uses occur within each of the fifty states (Figure 7-1).
Forest Land tends to be more common in the eastern states,
mountainous regions of the western United States, and
Alaska. Cropland is concentrated in the mid-continent
region of the United States, and Grassland is more common
in the western United States. Wetlands are fairly ubiquitous
throughout the United States, though they are more common
in the upper Midwest and eastern portions of the country.
Settlements are more concentrated along the coastal margins
and in the eastern states.
3 Land-use category definitions are provided in the Methodology section.
4 NRI data is available at .
6 NLCD data is available at .
7 The current land representation does not include areas from Alaska or
U.S. territories, but there are planned improvements to include these regions
in future reports.
7-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 7-5: Land Use, Land-Use Change, and Forestry on Managed Land (Thousands of Hectares)
Land Use, Land-Use Change Categories
1990
1995
2000
2005
2006
2007
2008
Total Forest Land 262,625
Forest Land Remaining Forest Land 255,922
Cropland Converted to Forest Land 1,267
Grassland Converted to Forest Land 4,883
Wetlands Converted to Forest Land 63
Settlements Converted to Forest Land 101
Other Lands Converted to Forest Land 390
Total Cropland 170,661
Cropland Remaining Cropland 155,462
Forest Land Converted to Cropland 1,105
Grassland Converted to Cropland 13,298
Wetlands Converted to Cropland 163
Settlements Converted to Cropland 470
Other Lands Converted to Cropland 162
Total Grassland 270,658
Grassland Remaining Grassland 261,096
Forest Land Converted to Grassland 1,459
Cropland Converted to Grassland 7,489
Wetlands Converted to Grassland 230
Settlements Converted to Grassland 129
Other Lands Converted to Grassland 255
Total Wetlands 27,821
Wetlands Remaining Wetlands 27,213
Forest Land Converted to Wetlands 137
Cropland Converted to Wetlands 134
Grassland Converted to Wetlands 286
Settlements Converted to Wetlands +
Other Lands Converted to Wetlands 51
Total Settlements 39,559
Settlements Remaining Settlements 34,783
Forest Lands Converted to Settlements 1,842
Cropland Converted to Settlements 1,373
Grassland Converted to Settlements 1,498
Wetlands Converted to Settlements 3
Other Lands Converted to Settlements 60
Total Other Land 14,519
Other Lands Remaining Other Lands 13,531
Forest Land Converted to Other Lands 193
Cropland Converted to Other Lands 279
Grassland Converted to Other Lands 458
Wetlands Converted to Other Lands 55
Settlements Converted to Other Lands 3
1,890
18,056
185
7161
265,753
254,772
1,890
8,056
134
185
716
168,484
149,337
1,289
16,517
249
869
223
266,081
253,167
2,067
9,868
348
227
404
27,508
26,607
249
225
401
2,067 3,057
9,868 13,325
3481 376
2271 2551
I 404 657
25
43,356
34,383
3,561
2,518
2,756
9
1281
14,663
12,974
321
385
888
88
7
321 506
13851 4401
8881 1,086
881 1151
7m 11!
268,247
252,529
2,793
11,354
202
268
1,101
164,391
143,995
1,101
17,834
264
886
311
263,601
245,932
3,057
13,325
376
255
657
27,552
26,144
380
347
635
1
43
47,565
34,062
5,480
3,599
4,183
29
212
14,489
12,331
506
440
1,086
115
11
271,169
255,309
2,975
11,111
205
303
1,266
163,182
145,521
805
15,513
234
825
283
260,694
243,862
2,802
12,712
339
255
724
27,206
25,728
403
352
675
3
44
49,249
34,977
5,872
3,672
4,479
32
217
14,345
12,103
559
499
1,058
114
12
271,889
255,997
2,981
11,136
206
303
1,266
163,170
145,510
805
15,512
234
825
283
260,173
243,411
2,794
12,652
338
254
724
27,055
25,584
401
350
673
3
44
49,241
34,969
5,872
3,672
4,479
32
217
14,317
12,076
559
499
1,057
114
12
272,609
256,686
2,986
11,160
207
304
1,266
163,159
145,500
804
15,512
234
825
283
259,653
242,960
2,786
12,592
337
253
724
26,903
25,439
399
348
671
3
44
49,233
34,961
5,872
3,672
4,479
32
217
14,289
12,049
559
499
1,057
114
12
273,329
257,375
2,992
11,184
207
305
1,266
163,147
145,489
804
15,512
234
825
283
259,132
242,509
2,778
12,532
336
252
724
26,751
25,293
397
345
669
3
44
49,225
34,953
5,872
3,672
4,478
32
217
14,261
12,022
559
499
1,056
113
12
Grand Total
785,845 785,845
785,845
785,845 785,845 785,845 785,845
+ Does not exceed one thousand hectares.
Note: All land areas reported in this table are considered managed. A planned improvement is underway to deal with an exception for wetlands which
includes both managed and unmanaged lands based on the definitions for the current U.S. Land Representation Assessment. In addition, U.S. Territories
have not been classified into land uses and are not included in the U.S. Land Representation Assessment. See Planned Improvements for discussion on
plans to include Alaska and territories in future Inventories.
Land Use, Land-Use Change, and Forestry 7-5
-------
Figure 7-1
Percent of Total Land Area in the General Land Use Categories for 2008
Croplands
Grasslands
Wetlands
<10%
Forest Lands
Settlements
r\
Other Lands
31%-50% B>50%
Note: Land use/land-use change categories were aggregated into the 6 general land-use categories based on the current use in 2008.
7-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Methodology
IPCC Approaches for Representing Land Areas
IPCC (2006) describes three approaches for representing
land areas. Approach 1 provides data on the total area for
each individual land-use category, but does not provide
detailed information on changes of area between categories
and is not spatially explicit other than at the national or
regional level. With Approach 1, total net conversions
between categories can be detected, but not the individual
changes between the land-use categories that led to those
net changes. Approach 2 introduces tracking of individual
land-use changes between the categories (e.g., Forest Land to
Cropland, Cropland to Forest Land, Grassland to Cropland,
etc.), using surveys or other forms of data that do not provide
location data on specific parcels of land. Approach 3 extends
Approach 2 by providing location data on specific parcels
of land, such as maps, along with the land-use history. The
three approaches are not presented as hierarchical tiers and
are not mutually exclusive.
According to IPCC (2006), the approach or mix of
approaches selected by an inventory agency should reflect
calculation needs and national circumstances. For this
analysis, the NRI, FIA, and the NLCD have been combined
to provide a complete representation of land use for managed
lands. These data sources are described in more detail later
in this section. All of these datasets have a spatially-explicit
time series of land-use data, and therefore Approach 3
is used to provide a full representation of land use in the
U.S. Inventory. Lands are treated as remaining in the same
category (e.g., Cropland Remaining Cropland) if a land-use
change has not occurred in the last 20 years. Otherwise, the
land is classified in a land-use-change category based on
the current use and most recent use before conversion to
the current use (e.g., Cropland Converted to Forest Land).
Definitions of Land Use in the United States
Managed and Unmanaged Land
The U.S. definitions of managed and unmanaged lands
are similar to the basic IPCC (2006) definition of managed
land, but with some additional elaboration to reflect national
circumstances. Based on the following definitions, most
lands in the United States are classified as managed:
• Managed Land: Land is considered managed
if direct human intervention has influenced its
condition. Direct intervention includes altering or
maintaining the condition of the land to produce
commercial or non-commercial products or services;
to serve as transportation corridors or locations for
buildings, landfills, or other developed areas for
commercial or non-commercial purposes; to extract
resources or facilitate acquisition of resources; or to
provide social functions for personal, community
or societal objectives. Managed land also includes
legal protection of lands (e.g., wilderness, preserves,
parks, etc.) for conservation purposes (i.e., meets
societal objectives).8
• Unmanaged Land: All other land is considered
unmanaged. Unmanaged land is largely comprised
of areas inaccessible to human intervention due
to the remoteness of the locations, or lands with
essentially no development interest or protection
due to limited personal, commercial or social value.
Though these lands may be influenced indirectly by
human actions such as atmospheric deposition of
chemical species produced in industry, they are not
influenced by a direct human intervention.9
Land-Use Categories
As with the definition of managed lands, IPCC (2006)
provides general non-prescriptive definitions for the six
main land-use categories: Forest Land, Cropland, Grassland,
Wetlands, Settlements and Other Land. In order to reflect
U.S. circumstances, country-specific definitions have been
developed, based predominantly on criteria used in the land-
use surveys for the United States. Specifically, the definition
8 Wetlands are an exception to this general definition, because these lands, as
specified by IPCC (2006), are only considered managed if they are created
through human activity, such as dam construction, or the water level is
artificially altered by human activity. Distinguishing between managed and
unmanaged wetlands is difficult, however, due to limited data availability.
Wetlands are not characterized by use within the NRI. Therefore, unless
wetlands are managed for cropland or grassland, it is not possible to know
if they are artificially created or if the water table is managed based on the
use of NRI data. See the Planned Improvements section of the Inventory
for work being done to refine the Wetland area estimates.
9 There will be some areas that qualify as Forest Land or Grassland according
to the land use criteria, but are classified as unmanaged land due to the
remoteness of their location.
Land Use, Land-Use Change, and Forestry 7-7
-------
of Forest Land is based on the FIA definition of forest,10 while
definitions of Cropland, Grassland, and Settlements are based
on the NRI.11 The definitions for Other Land and Wetlands
are based on the IPCC (2006) definitions for these categories.
• Forest Land: A land-use category that includes areas
at least 36.6 m wide and 0.4 ha in size with at least 10
percent cover (or equivalent stocking) by live trees
of any size, including land that formerly had such
tree cover and that will be naturally or artificially
regenerated. Forest land includes transition zones,
such as areas between forest and non-forest lands
that have at least 10 percent cover (or equivalent
stocking) with live trees and forest areas adjacent to
urban and built-up lands. Roadside, streamside, and
shelterbelt strips of trees must have a crown width
of at least 36.6 m and continuous length of at least
110.6 m to qualify as forest land. Unimproved roads
and trails, streams, and clearings in forest areas are
classified as forest if they are less than 36.6 m wide
or 0.4 ha in size, otherwise they are excluded from
Forest Land and classified as Settlements. Tree-
covered areas in agricultural production settings,
such as fruit orchards, or tree-covered areas in urban
settings, such as city parks, are not considered forest
land (Smith et al. 2009). NOTE: This definition
applies to all U.S. lands and territories. However,
at this time, data may be limited or based solely
on remote sensing in some areas, such as western
Texas, western Oklahoma, and interior Alaska.
• Cropland: A land-use category that includes areas
used for the production of adapted crops for harvest,
this category includes both cultivated and non-
cultivated lands.12 Cultivated crops include row
crops or close-grown crops and also hay or pasture
in rotation with cultivated crops. Non-cultivated
cropland includes continuous hay, perennial
crops (e.g., orchards) and horticultural cropland.
Cropland also includes land with alley cropping and
windbreaks,13 as well as lands in temporary fallow
or enrolled in conservation reserve programs (i.e.,
set-asides).14 Roads through Cropland, including
interstate highways, state highways, other paved
roads, gravel roads, dirt roads, and railroads are
excluded from Cropland area estimates and are,
instead, classified as Settlements.
• Grassland: A land-use category on which the
plant cover is composed principally of grasses,
grass-like plants, forbs, or shrubs suitable for
grazing and browsing, and includes both pastures
and native rangelands.15 This includes areas where
practices such as clearing, burning, chaining, and/
or chemicals are applied to maintain the grass
vegetation. Savannas, some wetlands and deserts,
in addition to tundra are considered Grassland.16
Woody plant communities of low forbs and shrubs,
such as mesquite, chaparral, mountain shrub, and
pinyon-juniper, are also classified as Grassland
if they do not meet the criteria for Forest Land.
Grassland includes land managed with agroforestry
practices such as silvipasture and windbreaks,
assuming the stand or woodlot does not meet the
criteria for Forest Land. Roads through Grassland,
including interstate highways, state highways, other
paved roads, gravel roads, dirt roads, and railroads
are excluded from Grassland area estimates and are,
instead, classified as Settlements.
• Wetlands: A land-use category that includes land
covered or saturated by water for all or part of
the year. Managed Wetlands are those where
the water level is artificially changed, or were
created by human activity. Certain areas that
fall under the managed Wetlands definition are
10 See .
11 See .
12 A minor portion of Cropland occurs on federal lands, and is not currently
included in the C stock change inventory. A planned improvement is
underway to include these areas in future C inventories.
13 Currently, there is no data source to account for biomass C stock change
associated with woody plant growth and losses in alley cropping systems
and windbreaks in cropping systems, although these areas are included in
the cropland land base.
14 A set-aside is cropland that has been taken out of active cropping and
converted to some type of vegetative cover, including, for example, native
grasses or trees.
15 Grasslands on federal lands are included in the managed land base, but
C stock changes are not estimated on these lands. Federal grassland areas
have been assumed to have negligible changes in C due to limited land
use and management change, but planned improvements are underway to
further investigate this issue and include these areas in future C inventories.
16 IPCC (2006) guidelines do not include provisions to separate desert and
tundra as land categories.
7-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
covered in other areas of the IPCC guidance and/
or the inventory, including Cropland (e.g., rice
cultivation), Grassland, and Forest Land (including
drained or undrained forested wetlands).
• Settlements: A land-use category representing
developed areas consisting of units of 0.25 acres
(0.1 ha) or more that includes residential, industrial,
commercial, and institutional land; construction
sites; public administrative sites; railroad yards;
cemeteries; airports; golf courses; sanitary landfills;
sewage treatment plants; water control structures
and spillways; parks within urban and built-
up areas; and highways, railroads, and other
transportation facilities. Also included are tracts
of less than 10 acres (4.05 ha) that may meet the
definitions for Forest Land, Cropland, Grassland,
or Other Land but are completely surrounded by
urban or built-up land, and so are included in the
settlement category. Rural transportation corridors
located within other land uses (e.g., Forest Land,
Cropland) are also included in Settlements.
• Other Land: A land-use category that includes
bare soil, rock, ice, non-settlement transportation
corridors, and all land areas that do not fall into any
of the other five land-use categories. It allows the
total of identified land areas to match the managed
national area.
Land-Use Data Sources: Description
and Application to U.S. Land Area
Classification
U.S. Land-Use Data Sources
The three main data sources for land area and use data
in the United States are the NRI, FIA, and the NLCD. For
the Inventory, the NRI is the official source of data on all
land uses on non-federal lands (except forest land), and is
also used as the resource to determine the total land base
for the conterminous United States and Hawaii. The NRI is
conducted by the USDA Natural Resources Conservation
Service and is designed to assess soil, water, and related
environmental resources on non-federal lands. The NRI
has a stratified multi-stage sampling design, where primary
sample units are stratified on the basis of county and township
boundaries defined by the U.S. Public Land Survey (Nusser
and Goebel 1997). Within a primary sample unit (typically
a 160-acre (64.75 ha) square quarter-section), three sample
points are selected according to a restricted randomization
procedure. Each point in the survey is assigned an area
weight (expansion factor) based on other known areas and
land-use information (Nusser and Goebel 1997). The NRI
survey utilizes data derived from remote sensing imagery and
site visits in order to provide detailed information on land
use and management, particularly for croplands, and is used
as the basis to account for C stock changes in agricultural
lands (except federal Grasslands). The NRI survey was
conducted every 5 years between 1982 and 1997, but shifted
to annualized data collection in 1998. This Inventory
incorporates data through 2003 from the NRI.
The FIA program, conducted by the USFS, is the official
source of data on Forest Land area and management data
for the Inventory. FIA engages in a hierarchical system of
sampling, with sampling categorized as Phases 1 through 3,
in which sample points for phases are subsets of the previous
phase. Phase 1 refers to collection of remotely-sensed data
(either aerial photographs or satellite imagery) primarily
to classify land into forest or non-forest and to identify
landscape patterns like fragmentation and urbanization.
Phase 2 is the collection of field data on a network of ground
plots that enable classification and summarization of area,
tree, and other attributes associated with forest land uses.
Phase 3 plots are a subset of Phase 2 plots where data on
indicators of forest health are measured. Data from all three
phases are also used to estimate C stock changes for forest
land. Historically, FIA inventory surveys had been conducted
periodically, with all plots in a state being measured at a
frequency of every 5 to 14 years. A new national plot design
and annual sampling design was introduced by FIA about
ten years ago. Most states, though, have only recently been
brought into this system. Annualized sampling means that a
portion of plots throughout each state is sampled each year,
with the goal of measuring all plots once every 5 years. See
Annex 3.12 to see the specific survey data available by state.
The most recent year of available data varies state by state
(2002 through 2008).
Though NRI provides land-area data for both federal
and non-federal lands, it only includes land-use data on
non-federal lands, and FIA only records data for forest
Land Use, Land-Use Change, and Forestry 7-9
-------
land.17 Consequently, major gaps exist when the datasets
are combined, such as federal grassland operated by the
Bureau of Land Management (BLM), USDA, and National
Park Service, as well as most of Alaska.18 Consequently, the
NLCD is used as a supplementary database to account for
land use on federal lands that are not included in the NRI
and FIA databases. The NLCD is a land-cover classification
scheme, available for 1992 and 2001, that has been applied
over the conterminous United States. The 2001 product also
provides land use data that has been used for Hawaii federal
lands. For this analysis, the NLCD Retrofit Land Cover
Change Product was used in order to represent both land use
and land-use change for federal lands in the conterminous
United States. It is based primarily on Landsat Thematic
Mapper imagery. The NLCD contains 21 categories of
land-cover information, which have been aggregated into
the IPCC land-use categories, and the data are available at a
spatial resolution of 30 meters. The federal land portion of
the NLCD was extracted from the dataset using the federal
land area boundary map from the National Atlas.19 This map
represents federal land boundaries in 2005, so as part of the
analysis, the federal land area was adjusted annually based on
the NRI federal land area estimates (i.e., land is periodically
transferred between federal and non-federal ownership).
Consequently, the portion of the land base categorized with
NLCD data varied from year to year, corresponding to an
increase or decrease in the federal land base. The NLCD is
strictly a source of land-cover information, however, and
does not provide the necessary site conditions, crop types,
and management information from which to estimate C stock
changes on those lands.
Another step in the analysis is to address gaps as
well as overlaps in the representation of the U.S. land
base between the Agricultural Carbon Stock Inventory
(Cropland Remaining Cropland, Land Converted to
Cropland, Grassland Remaining Grassland, Land Converted
to Grassland) and Forest Land Carbon Stock Inventory
(Forest Land Remaining Forest Land and Land Converted
to Forest Land), which are based on the NRI and FIA
17 FIA does collect some data on non-forest land use, but these are held in
regional databases versus the national database. The status of these data
is being investigated.
18 The survey programs also do not include U.S. Territories with the
exception of non-federal lands in Puerto Rico, which are included in the NRI
survey. Furthermore, NLCD does not include coverage for U.S. Territories.
19 See .
7-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
a time series of land-use change data or land management
information, which is critical for conducting emission
inventories and is provided from the NRI and FIA surveys.
Consequently, the U.S. Census Survey was not adopted as
the official land area estimate for the Inventory. Rather, the
NRI data were adopted because this database provides full
coverage of land area for the conterminous United States and
Hawaii. Regardless, the total difference between the U.S.
Census Survey and the data sources used in the Inventory
is about 25 million hectares for the total land base of about
786 million hectares currently included in the Inventory, or a
3.1 percent difference. Much of this difference is associated
with open waters in coastal regions and the Great Lakes. NRI
does not include as much of the area of open waters in these
regions as the U.S. Census Survey.
Approach for Combining Data Sources
The managed land base in the United States has been
classified into the six IPCC land-use categories using
definitions21 developed to meet national circumstances, while
adhering to IPCC (2006). In practice, the land was initially
classified into a variety of land-use categories using the NRI,
FIA and NLCD, and then aggregated into the thirty-six broad
land use and land-use-change categories identified in IPCC
(2006). Details on the approach used to combine data sources
for each land use are described below as are the gaps that
will be reconciled as part of ongoing planned improvements:
• Forest Land: Both non-federal and federal forest
lands in both the continental United States and
coastal Alaska are covered by FIA. FIA is used
as the basis for both Forest Land area data as well
as to estimate C stocks and fluxes on Forest Land.
Interior Alaska is not currently surveyed by FIA,
but NLCD has a new product for Alaska that will
be incorporated into the assessment as a planned
improvement for future reports. Forest Lands
in U.S. territories are currently excluded from
the analysis, but FIA surveys are currently being
conducted on U.S. territories and will become
available in the future. NRI is being used in the
current report to provide Forest Land areas on
non-federal lands in Hawaii. Currently, federal
forest land in Hawaii is evaluated with the 2001
Definitions are provided in the previous section.
NLCD, but FIA data will be collected in Hawaii
in the future.
• Cropland: Cropland is classified using the NRI,
which covers all non-federal lands within 49 states
(excluding Alaska), including state and local
government-owned land as well as tribal lands. NRI
is used as the basis for both Cropland area data as
well as to estimate C stocks and fluxes on Cropland.
Croplands in U.S. territories are excluded from both
NRI data collection and the NLCD. NLCD has a
new product for Alaska that will be incorporated
into the assessment as a planned improvement for
future reports.
• Grassland: Grassland on non-federal lands is
classified using the NRI within 49 states (excluding
Alaska), including state and local government-
owned land as well as tribal lands. NRI is used
as the basis for both Grassland area data as well
as to estimate C stocks and fluxes on Grassland.
U.S. territories are excluded from both NRI data
collection and the current release of the NLCD
product. Grassland on federal Bureau of Land
Management lands, Department of Defense lands,
National Parks and within USFS lands are covered
by the NLCD In addition, federal and non-federal
grasslands in Alaska are currently excluded from the
analysis, but NLCD has a new product for Alaska
that will be incorporated into the assessment for
future reports.
• Wetlands: NRI captures wetlands on non-federal
lands within 49 states (excluding Alaska), while
federal wetlands are covered by the NLCD. Alaska
and U.S. territories are excluded. This currently
includes both managed and unmanaged wetlands
as no database has yet been applied to make this
distinction. See Planned Improvements for details.
• Settlements: The NRI captures non-federal
settlement area in 49 states (excluding Alaska). If
areas of Forest Land or Grassland under 10 acres
(4.05 ha) are contained within settlements or urban
areas, they are classified as Settlements (urban) in
the NRI database. If these parcels exceed the 10
acre (4.05 ha) threshold and are Grassland, they will
be classified as such by NRI. Regardless of size,
Land Use, Land-Use Change, and Forestry 7-11
-------
a forested area is classified as non-forest by FIA if
it is located within an urban area. Settlements on
federal lands are covered by NLCD. Settlements
in U.S. territories are currently excluded from NRI
and NLCD. NLCD has a new product for Alaska
that will be incorporated into the assessment as a
planned improvement for future reports.
• Other Land: Any land not falling into the other
five land categories and, therefore, categorized as
Other Land is classified using the NRI for non-
federal areas in the 49 states (excluding Alaska)
and NLCD for the federal lands. Other land in U.S.
territories is excluded from the NLCD. NLCD has
a new product for Alaska that will be incorporated
into the assessment as a planned improvement for
future reports.
Some lands can be classified into one or more categories
due to multiple uses that meet the criteria of more than one
definition. However, a ranking has been developed for
assignment priority in these cases. The ranking process is
initiated by distinguishing between managed and unmanaged
lands. The managed lands are then assigned, from highest
to lowest priority, in the following manner:
Settlements > Cropland > Forest Land > Grassland >
Wetlands > Other Land
Settlements are given the highest assignment priority
because they are extremely heterogeneous with a mosaic
of patches that include buildings, infrastructure and travel
corridors, but also open grass areas, forest patches, riparian
areas, and gardens. The latter examples could be classified as
Grassland, Forest Land, Wetlands, and Cropland, respectively,
but when located in close proximity to settlement areas they
tend to be managed in a unique manner compared to non-
settlement areas. Consequently, these areas are assigned to
the Settlements land-use category. Cropland is given the
second assignment priority, because cropping practices tend
to dominate management activities on areas used to produce
food, forage or fiber. The consequence of this ranking is that
crops in rotation with grass will be classified as Cropland,
and land with woody plant cover that is used to produce
crops (e.g., orchards) is classified as Cropland, even though
these areas may meet the definitions of Grassland or Forest
Land, respectively. Similarly, Wetlands that are used for rice
production are considered Croplands. Forest Land occurs
next in the priority assignment because traditional forestry
practices tend to be the focus of the management activity in
areas with woody plant cover that are not croplands (e.g.,
orchards) or settlements (e.g., housing subdivisions with
significant tree cover). Grassland occurs next in the ranking,
while Wetlands and Other Land complete the list.
The assignment priority does not reflect the level of
importance for reporting greenhouse gas emissions and
removals on managed land, but is intended to classify all areas
into a single land use. Currently, the IPCC does not make
provisions in the guidelines for assigning land to multiple
uses. For example, a Wetland is classified as Forest Land
if the area has sufficient tree cover to meet the stocking and
stand size requirements. Similarly, Wetlands are classified
as Cropland if they are used to produce a crop, such as rice.
In either case, emissions from Wetlands are included in the
Inventory if human interventions are influencing emissions
from Wetlands, in accordance with the guidance provided
in IPCC (2006).
Recalculations Discussion
One major revision was made in the current Inventory
for land representation; the time series for Forest Land was
updated with a new release of data from the FIA. The updated
time series also influenced the time series for Grassland and
Wetlands, which are adjusted in the process of combining
FIA data with NRI and NLCD (see previous section entitled
"U.S. Land-Use Data Sources" for more information on the
process of combining these datasets).
Planned Improvements
Area data by land-use category are not estimated for
major portions of Alaska or any of the U.S. territories. A key
planned improvement is to incorporate land-use data from
these areas into the Inventory. For Alaska, a new NLCD 2001
data product will be used to cover those land areas presently
omitted. Fortunately, most of the managed land in the United
States is included in the current land-use statistics, but a
complete accounting is a key goal for the near future. Data
sources will also be evaluated for representing land use on
federal and non-federal lands in U.S. territories.
Additional work will be done to reconcile differences in
Forest Land estimates between the NRI and FIA, evaluating
the assumption that the majority of discrepancies in Forest
Land areas are associated with an over- or under-estimation
of Grassland and Wetland area. In some regions of the United
7-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
States, a discrepancy in Forest Land areas between NRI and
FIA may be associated with an over- or under-prediction of
other land uses.
There are also other databases that may need to be
reconciled with the NRI and NLCD datasets, particularly
for Settlements and Wetlands. Urban area estimates, used
to produce C stock and flux estimates from urban trees,
are currently based on population data (1990 and 2000
U.S. Census data). Using the population statistics, "urban
clusters" are defined as areas with more than 500 people
per square mile. The USFS is currently moving ahead with
an urban forest inventory program so that urban forest area
estimates will be consistent with FIA forest area estimates
outside of urban areas, which would be expected to reduce
omissions and overlap of forest area estimates along urban
boundary areas. For Wetlands, the Army Corps of Engineers
National Inventory of Dams (NID) (ACE 2005) and the
U.S. Fish and Wildlife Service National Wetlands Inventory
(NWI)22 databases are being evaluated and will be compared
against the NRI and NLCD. The NID and NWI may be
used to refine Wetlands area estimates for the U.S. Land
Representation assessment, including disaggregation of
managed and unmanaged Wetlands.
7.2. Forest Land Remaining
Forest Land
Changes in Forest Carbon Stocks
(IPCC Source Category 5A1)
For estimating C stocks or stock change (flux), C in
forest ecosystems can be divided into the following five
storage pools (IPCC 2003):
• Aboveground biomass, which includes all living
biomass above the soil including stem, stump,
branches, bark, seeds, and foliage. This category
includes live understory.
• Belowground biomass, which includes all living
biomass of coarse living roots greater than 2 mm
diameter.
• Dead wood, which includes all non-living woody
biomass either standing, lying on the ground (but
not including litter), or in the soil.
• Litter, which includes the litter, fumic, and humic
layers, and all non-living biomass with a diameter
less than 7.5 cm at transect intersection, lying on
the ground.
• Soil organic C (SOC), including all organic material
in soil to a depth of 1 meter but excluding the coarse
roots of the aboveground pools.
In addition, there are two harvested wood pools
necessary for estimating C flux:
• Harvested wood products in use, and
• Harvested wood products in solid waste disposal
sites (SWDS).
Carbon is continuously cycled among these storage
pools and between forest ecosystems and the atmosphere as a
result of biological processes in forests (e.g., photosynthesis,
respiration, growth, mortality, decomposition, and
disturbances such as fires or pest outbreaks) and anthropogenic
activities (e.g., harvesting, thinning, clearing, and replanting).
As trees photosynthesize and grow, C is removed from the
atmosphere and stored in living tree biomass. As trees die
and otherwise deposit litter and debris on the forest floor,
C is released to the atmosphere or transferred to the soil by
organisms that facilitate decomposition.
The net change in forest C is not equivalent to the net flux
between forests and the atmosphere because timber harvests
do not cause an immediate flux of C of all vegetation C to
the atmosphere. Instead, harvesting transfers a portion of
the C stored in wood to a "product pool." Once in a product
pool, the C is emitted over time as CO2 when the wood
product combusts or decays. The rate of emission varies
considerably among different product pools. For example, if
timber is harvested to produce energy, combustion releases C
immediately. Conversely, if timber is harvested and used as
lumber in a house, it may be many decades or even centuries
before the lumber decays and C is released to the atmosphere.
If wood products are disposed of in SWDS, the C contained
in the wood may be released many years or decades later, or
may be stored almost permanently in the SWDS.
See .
Land Use, Land-Use Change, and Forestry 7-13
-------
This section quantifies the net changes in C stocks in
the five forest C pools and two harvested wood pools. The
net change in stocks for each pool is estimated, and then the
changes in stocks are summed over all pools to estimate total
net flux. The focus on C implies that all C-based greenhouse
gases are included, and the focus on stock change suggests
that specific ecosystem fluxes do not need to be separately
itemized in this report. Disturbances from forest fires and
pest outbreaks are implicitly included in the net changes.
For instance, an inventory conducted after fire counts only
the trees that are left. The change between inventories thus
accounts for the C changes due to fires; however, it may
not be possible to attribute the changes to the disturbance
specifically. The IPCC (2003) recommends reporting C
stocks according to several land-use types and conversions,
specifically Forest Land Remaining Forest Land and Land
Converted to Forest Land. Currently, consistent datasets are
just becoming available for the conterminous United States to
allow forest land conversions and forest land remaining forest
land to be identified, and research is ongoing to properly
use that information based on research results. Thus, net
changes in all forest-related land, including non-forest land
converted to forest and forests converted to non-forest, are
reported here.
Forest C storage pools, and the flows between them via
emissions, sequestration, and transfers, are shown in Figure
7-2. In the figure, boxes represent forest C storage pools and
arrows represent flows between storage pools or between
storage pools and the atmosphere. Note that the boxes are
not identical to the storage pools identified in this chapter.
The storage pools identified in this chapter have been refined
in this graphic to better illustrate the processes that result in
transfers of C from one pool to another, and emissions to as
well as uptake from the atmosphere.
Approximately 33 percent (304 million hectares) of
the U.S. land area is forested (Smith et al. 2009). The
current forest inventory includes 270 million hectares in
the conterminous 48 states (USDA Forest Service 2009a,
2009b) that are considered managed and are included in
this Inventory. The additional 34 million hectares of forest
land are located in Alaska and Hawaii. Of this forest area
outside the conterminous United States, 6.2 million hectares
of southeast and south central Alaskan forest are inventoried
and are included here. Survey data are not yet available from
Figure 7-2
Forest Sector Carbon Pools and Flows
Combustion from
forest fires (carbon
" dioxide, methane)
Combustion from forest fires
(carbon dioxide, methane)
Harvest
Processing/ Residue
Consumption
Soil Organic
Material
Decompostion Methane
Flaring
and
Utilization
Legend
Carbon Pool
Carbon transfer or flux
Combustion
Source: Heath etal. 2003
7-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Hawaii. While Hawaii and U.S. territories have relatively
small areas of forest land and will thus probably not influence
the overall C budget substantially, these regions will be
added to the C budget as sufficient data become available.
Agroforestry systems are also not currently accounted for
in the Inventory, since they are not explicitly inventoried by
either the Forest Inventory and Analysis (FIA) program of
the U.S. Department of Agriculture (USDA) Forest Service
or the National Resources Inventory (NRI) of the USDA
Natural Resources Conservation Service (Perry et al. 2005).
Sixty-eight percent of U.S. forests (208 million
hectares) are classified as timberland, meaning they meet
minimum levels of productivity. Nine percent of Alaska
forests overall and 81 percent of forests in the conterminous
United States are classified as timberlands. Of the remaining
nontimberland forests, 30 million hectares are reserved forest
lands (withdrawn by law from management for production
of wood products) and 66 million hectares are lower
productivity forest lands (Smith et al. 2009). Historically,
the timberlands in the conterminous 48 states have been more
frequently or intensively surveyed than other forest lands.
Forest land area declined by approximately 10 million
hectares over the period from the early 1960s to the late
1980s. Since then, forest area has increased by about 12
million hectares. Current trends in forest area represent
average annual change of less than 0.2 percent. Given
the low rate of change in U.S. forest land area, the major
influences on the current net C flux from forest land are
management activities and the ongoing impacts of previous
land-use changes. These activities affect the net flux of C
by altering the amount of C stored in forest ecosystems. For
example, intensified management of forests that leads to
an increased rate of growth increases the eventual biomass
density of the forest, thereby increasing the uptake of C.23
Though harvesting forests removes much of the aboveground
C, on average the volume of annual net growth nationwide is
about 32 percent higher than the volume of annual removals
(USDA Forest Service 2009d). The reversion of cropland to
forest land increases C storage in biomass, forest floor, and
soils. The net effects of forest management and the effects
of land-use change involving forest land are captured in the
estimates of C stocks and fluxes presented in this chapter.
In the United States, improved forest management
practices, the regeneration of previously cleared forest areas,
and timber harvesting and use have resulted in net uptake
(i.e., net sequestration) of C each year from 1990 through
2008. The rate of forest clearing begun in the 17th century
following European settlement had slowed by the late 19th
century. Through the later part of the 20th century many
areas of previously forested land in the United States were
allowed to revert to forests or were actively reforested. The
impacts of these land-use changes still influence C fluxes
Table 7-6: Net Annual Changes in C Stocks (Tg C02/yr) in Forest and Harvested Wood Pools
Carbon Pool
Forest
Aboveground Biomass
Belowground Biomass
Dead Wood
Litter
Soil Organic Carbon
Harvested Wood
Products in use
SWDS
Total Net Flux
1990H
(598.1)
(377.7)1
(74.5)
(29.4)
(46.5)
(70.0)
(131.8)
(64.8)
(67.0)
(729.8) •
1995
(574.2)
(398.3)
(79.3)
(31.0)
(28.3)
(37.2)
(118.4)
(55.2)
(63.2)
(692.6)
2000
(354.8)
(309.3)
(61.7)
(15.8)
L.U.,
(112.9)
(47.0)
(65.9)
(467.7)
2005
(701.2)
(397.2)
(78.8)
(23.4)
(55.9)
(145.9)
(105.4)
(45.4)
(59.9)
(806.6)
2006
(703.9)
(397.2)
(78.8)
(26.2)
(55.9)
(145.9)
(108.6)
(45.1)
(63.4)
(812.5)
2007
(703.9)
(397.2)
(78.8)
(26.2)
(55.9)
(145.9)
(103.0)
(39.1)
(63.8)
(806.9)
2008
(703.9)
(397.2)
(78.8)
(26.2)
(55.9)
(145.9)
(88.0)
(24.4)
(63.6)
(791.9)
Note: Forest C stocks do not include forest stocks in U.S. territories, Hawaii, a portion of managed forests in Alaska, or trees on non-forest land (e.g.,
urban trees, agroforestry systems). Parentheses indicate net C sequestration (i.e., a net removal of C from the atmosphere). Total net flux is an estimate
of the actual net flux between the total forest C pool and the atmosphere. Forest area estimates are based on interpolation and extrapolation of inventory
data as described in the text and in Annex 3.12. Harvested wood estimates are based on results from annual surveys and models. Totals may not sum
due to independent rounding.
23 The term "biomass density" refers to the mass of live vegetation per unit
area. It is usually measured on a dry-weight basis. Dry biomass is 50
percent C by weight.
Land Use, Land-Use Change, and Forestry 7-15
-------
Table 7-7: Net Annual Changes in C Stocks (Tg C/yr) in Forest and Harvested Wood Pools
Carbon Pool
Forest
Aboveground Biomass
Belowground Biomass
Dead Wood
Litter
Soil Organic Carbon
Harvested Wood
Products in use
SWDS
Total Net Flux
1990
(163.1)
(103.0)
(20.3)
(8.0)
(12.7)1
(19.1)1
(35.9)
(17.7)
(18.3)
(199.0)|
1995
(156.6)
(108.6)
(21.6)
(8.5)
(7.7)
(10.1)1
(32.3)
(15.1)1
(17.2)
(188.9)
2000
(96.8)
(84.4)
(16.8)
(4.3)
0.9 •
,8
(30.8)
(12.8)
(18.0)
(127.6)
2005
(191.2)
(108.3)
(21.5)
(6.4)
(15.2)
(39.8)
(28.7)
(12.4)
(16.3)
(220.0)
2006
(192.0)
(108.3)
(21.5)
(7.1)
(15.2)
(39.8)
(29.6)
(12.3)
(17.3)
(221.6)
2007
(192.0)
(108.3)
(21.5)
(7.1)
(15.2)
(39.8)
(28.1)
(10.7)
(17.4)
(220.1)
2008
(192.0)
(108.3)
(21.5)
(7.1)
(15.2)
(39.8)
(24.0)
(6.7)
(17.3)
(216.0)
Note: Forest C stocks do not include forest stocks in U.S. territories, Hawaii, a portion of managed lands in Alaska, or trees on non-forest land (e.g.,
urban trees, agroforestry systems). Parentheses indicate net C sequestration (i.e., a net removal of C from the atmosphere). Total net flux is an estimate
of the actual net flux between the total forest C pool and the atmosphere. Harvested wood estimates are based on results from annual surveys and
models. Totals may not sum due to independent rounding.
Table 7-8: Forest Area (1000 ha) and C Stocks (Tg C) in Forest and Harvested Wood Pools
Forest Area (1000 ha)
Carbon Pools (Tg C) Forest
Aboveground Biomass
Belowground Biomass
Dead Wood
Litter
Soil Organic C
Harvested Wood
Products in Use
SWDS
Total C Stock
1990
267,986
42,540
15,027
2,986
2,949
4,755
16,823
1,859
1,231
628
44,399
1995
271,194
43,332
15,550
3,089
2,990
4,812
16,890
2,029
1,311
718
45,361
2000
273,767
43,973
16,030
3,185
3,025
4,831
16,902
2,187
1,382
805
46,161
2005
276,796
44,762
16,529
3,284
3,053
4,880
17,016
2,325
1,436
890
47,087
2006
277,536
44,953
16,637
3,305
3,059
4,895
17,056
2,354
1,448
906
47,307
2007
278,276
45,145
16,745
3,327
3,066
4,910
17,096
2,383
1,460
923
47,528
2008
279,016
45,337
16,854
3,348
3,073
4,925
17,136
2,412
1,471
941
47,748
2009
279,756
45,529
16,962
3,370
3,080
4,941
17,176
2,436
1,478
958
47,964
Note: Forest area estimates include portions of managed forests in Alaska for which survey data are available. Forest C stocks do not include forest
stocks in U.S. territories, Hawaii, a large portion of Alaska, or trees on non-forest land (e.g., urban trees, agroforestry systems). Wood product
stocks include exports, even if the logs are processed in other countries, and exclude imports. Forest area estimates are based on interpolation and
extrapolation of inventory data as described in Smith et al. (2010) and in Annex 3.12. Harvested wood estimates are based on results from annual
surveys and models. Totals may not sum due to independent rounding. Inventories are assumed to represent stocks as of January 1 of the inventory
year. Flux is the net annual change in stock. Thus, an estimate of flux for 2006 requires estimates of C stocks for 2006 and 2007.
from these forest lands. More recently, the 1970s and 1980s harvested wood are transferred to long-term storage pools
saw a resurgence of federally-sponsored forest management rather than being released rapidly to the atmosphere (Skog
programs (e.g., the Forestry Incentive Program) and soil and Nicholson 1998, Skog 2008). The size of these long-
conservation programs (e.g., the Conservation Reserve term C storage pools has increased during the last century.
Program), which have focused on tree planting, improving Changes in C stocks in U.S. forests and harvested wood
timber management activities, combating soil erosion, and were estimated to account for net sequestration of 792 Tg
converting marginal cropland to forests. In addition to cc,2 Eq. (216 Tg C) in 2008 (Table 7-6, Table 7-7, Table
forest regeneration and management, forest harvests have 7.8j md Figure 7.3) In addition to me net accumulation
also affected net C fluxes. Because most of the timber of c in harvested wood poolSj sequestration is a reflection
harvested from U.S. forests is used in wood products, and of net forest growth and increasing forest area over this
many discarded wood products are disposed of in SWDS period Overall average c in forest ecOsystem biomass
rather than by incineration, significant quantities of C in (aboveground md belowground) increased from 67 to 73 Mg
7-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
C/ha between 1990 and 2009 (see Annex 3 -12 for average C
densities by specific regions and forest types). Continuous,
regular annual surveys are not available over the period for
each state; therefore, estimates for non-survey years were
derived by interpolation between known data points. Survey
years vary from state to state, and national estimates are a
composite of individual state surveys. Therefore, changes
in sequestration over the interval 1990 to 2008 are the result
of the sequences of new inventories for each state. C in
forest ecosystem biomass had the greatest effect on total
change through increases in C density and total forest land.
Management practices that increase C stocks on forest land,
as well as afforestation and reforestation efforts, influence
the trends of increased C densities in forests and increased
forest land in the United States.
Stock estimates for forest and harvested wood C storage
pools are presented in Table 7-8. Together, the aboveground
live and forest soil pools account for a large proportion of
total forest C stocks. C stocks in all non-soil pools increased
over time. Therefore, C sequestration was greater than C
emissions from forests, as discussed above. Figure 7-4
shows county-average C densities for live trees on forest land,
including both above- and belowground biomass.
Figure 7-3
Estimates of Net Annual Changes in Carbon Stocks
for Major Carbon Pools
50 -,
O 1— CM CO
O O O O
s s s s
•d-mu3i--.es
o o o o o
s s s s s
Figure 7-4
Average C Density in the Forest Tree Pool in the Conterminous United States, 2008
Live Tree
Mg C02 Eq./ha
• 1-200
D 201^400
• over 400
Land Use, Land-Use Change, and Forestry 7-17
-------
Box 7-1: C02 Emissions from Forest Fires
Table 7-9: Estimates of C02 (Tg/yr) Emissions for the Lower 48 States
and Alaska3
Year
1990
C02 Emitted from C02 Emitted from C02 Emitted from
Wildfires in Prescribed Fires in Wildfires in Total C02
Lower 48 States Lower 48 States Alaska Emitted
(Tg/yr) (Tg/yr) (Tg/yr) (Tg/yr)
42.1
50.7
As stated previously, the forest
inventory approach implicitly accounts
for emissions due to disturbances such as
forest fires, because only C remaining in
the forest is estimated. Net C stock change
is estimated by subtracting consecutive C
stock estimates. A disturbance removes
C from the forest. The inventory data on
which net C stock estimates are based
already reflect this C loss. Therefore,
estimates of net annual changes in C stocks
for U.S. forestland already account for C02
emissions from forest fires occurring in the
lower 48 states as well as in the proportion
of Alaska's managed forest land captured
in this Inventory. Because it is of interest
to quantify the magnitude of C02 emissions
from fire disturbance, these estimates are
being highlighted here, using the full extent
of available data. Non-C02 greenhouse gas emissions from forest fires are also quantified in a separate section below.
The IPCC (2003) methodology and IPCC (2006) default combustion factor for wildfire were employed to estimate C02 emissions from
forest fires. Carbon dioxide emissions for wildfires and prescribed fires in the lower 48 states and wildfires in Alaska in 2008 were estimated
to be 189.7 Tg C02/yr. This amount is masked in the estimate of net annual forest carbon stock change for 2008, however, because this
net estimate accounts for the amount sequestered minus any emissions.
2005
2006
2007
2008
131.0
313.6
284.1
168.9
24.8
29.3
34.0
20.8
+ 155.9
+ 342.9
+ 318.0
+ 189.7
+ Does not exceed 0.05 Tg C02 Eq.
a Note that these emissions have already been accounted for in the estimates of net annual
changes in C stocks, which account for the amount sequestered minus any emissions.
Methodology and Data Sources
The methodology described herein is consistent with
IPCC (2003, 2006) and IPCC/UNEP/OECD/IEA (1997).
Forest ecosystem C stocks and net annual C stock change
are determined according to stock-difference methods, which
involve applying C estimation factors to forest inventory
data and interpolating between successive inventory-based
estimates of C stocks. Harvested wood C estimates are based
on factors such as the allocation of wood to various primary
and end-use products as well as half-life (the time at which
half of amount placed in use will have been discarded from
use) and expected disposition (e.g., product pool, SWDS,
combustion). An overview of the different methodologies
and data sources used to estimate the C in forest ecosystems
or harvested wood products is provided here. See Annex
3.12 for details and additional information related to the
methods and data.
Forest Ecosystem Carbon from Forest Inventory
Forest ecosystem stock and flux estimates are based on
the stock-difference method and calculations for all estimates
are in units of C. Separate estimates are made for the five
IPCC C storage pools described above. All estimates are
based on data collected from the extensive array of permanent
forest inventory plots in the United States as well as models
employed to fill gaps in field data. Carbon conversion factors
are applied at the disaggregated level of each inventory plot
and then appropriately expanded to population estimates. A
combination of tiers as outlined by IPCC (2006) is used. The
Tier 3 biomass C values are from forest inventory tree-level
data. The Tier 2 dead organic and soil C pools are based on
empirical or process models from the inventory data. All
carbon conversion factors are specific to regions or individual
states within the United States, which are further classified
according to characteristic forest types within each region.
The first step in developing forest ecosystem estimates
is to identify useful inventory data and resolve any
inconsistencies among datasets. Forest inventory data
were obtained from the USDA Forest Service FIA program
(Prayer and Furnival 1999, USDA Forest Service 2009b).
Inventories include data collected on permanent inventory
7-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
plots on forest lands24 and are organized as a number of
separate datasets, each representing a complete inventory,
or survey, of an individual state at a specified time. Some
of the more recent annual inventories reported for some
states include "moving averages" which means that a
portion—but not all—of the previous year's Inventory is
updated each year (USDA Forest Service 2009d). Forest C
calculations are organized according to these state surveys,
and the frequency of surveys varies by state. All available
data sets are identified for each state starting with pre-1990
data, and all unique surveys are included.25 Since C stock
change is based on differences between successive surveys
within each state, accurate estimates of net C flux thus depend
on consistent representation of forest land between these
successive inventories. In order to achieve this consistency
from 1990 to the present, states are sometimes subdivided
into sub-state areas where the sum of sub-state inventories
produces the best whole-state representation of C change as
discussed in Smith et al. (2007).
The principal FIA datasets employed are freely available
for download at USDA Forest Service (2009b) as the
Forest Inventory and Analysis Database (FIADB) Version
4.0. However, to achieve consistent representation (spatial
and temporal), two other general sources of past FIA data
are included as necessary. First, older FIA plot- and tree-
level data—not in the current FIADB format—are used if
available. Second, Resources Planning Act Assessment
(RPA) databases, which are periodic, plot-level only,
summaries of state inventories, are used mostly to provide
the data at or before 1990. See USDA Forest Service (2009a)
for information on current and older data as well as additional
FIA Program features. A detailed list of the specific inventory
data used in this Inventory is in Annex 3.12.
Forest C stocks are estimated from inventory data by a
collection of conversion factors and models referred to as
FORCARB2 (Birdsey and Heath 1995, Birdsey and Heath
2001, Heath et al. 2003, Smith et al. 2004a), which have
24 Forest land in the United States includes land that is at least 10 percent
stocked with trees of any size. Timberland is the most productive type of
forest land, which is on unreserved land and is producing or capable of
producing crops of industrial wood.
25 Accurate estimates of C stock change based on annual inventory data
require that only one full cycle of inventory data (collected over a 5- to
10-year period) be included. This process ensures that each sample plot
measurement is used only once in the estimates of net stock change (see
recalculations section in this Chapter and detailed discussion in Annex 3.12).
been formalized in an FIADB-to-carbon calculator (Smith
et al. 2010). The conversion factors and model coefficients
are categorized by region and forest type, and forest C stock
estimates are calculated from application of these factors at
the scale of FIA inventory plots. The results are estimates
of C density (Mg C per hectare) for six forest ecosystem
pools: live trees, standing dead trees, understory vegetation,
down dead wood, forest floor, and soil organic matter. The
six carbon pools used in the FIADB-to-carbon calculator are
aggregated to the 5 carbon pools defined by IPCC (2006):
aboveground biomass, belowground biomass, dead wood,
litter, and soil organic matter. All non-soil pools except
forest floor are separated into aboveground and belowground
components. The live tree and understory C pools are pooled
as biomass, and standing dead trees and down dead wood
are pooled as dead wood, in accordance with IPCC (2006).
Once plot-level C stocks are calculated as C densities
on Forest Land Remaining Forest Land for the five IPCC
(2006) reporting pools, the stocks are expanded to population
estimates according to methods appropriate to the respective
inventory data (for example, see USDA Forest Service
(2008)). These expanded C stock estimates are summed to
state or sub-state total C stocks. Annualized estimates of C
stocks are developed by using available FIA inventory data
and interpolating or extrapolating to assign a C stock to
each year in the 1990-2009 time series. Flux, or net annual
stock change, is estimated by calculating the difference
between two successive years and applying the appropriate
sign convention; net increases in ecosystem C are identified
as negative flux. By convention, inventories are assigned
to represent stocks as of January 1 of the inventory year; an
estimate of flux for 1996 requires estimates of C stocks for
1996 and 1997, for example. Additional discussion of the
use of FIA inventory data and the C conversion process is
in Annex 3.12.
Carbon in Biomass
Live tree C pools include aboveground and belowground
(coarse root) biomass of live trees with diameter at breast
height (d.b.h.) of at least 2.54 cm at 1.37 m above the
forest floor. Separate estimates are made for full-tree
and aboveground-only biomass in order to estimate the
belowground component. If inventory plots include data
on individual trees, tree C is based on Jenkins et al. (2003)
and is a function of species and diameter. Some inventory
Land Use, Land-Use Change, and Forestry 7-19
-------
data do not provide measurements of individual trees; tree
C in these plots is estimated from plot-level volume of
merchantable wood, or growing-stock volume, of live trees,
which is calculated from updates of Smith et al. (2003).
These biomass conversion and expansion factors (BCEFs)
are applied to about 5 percent of the inventory records, all of
which are pre-1998 data. Some inventory data, particularly
some of the older datasets, may not include sufficient
information to calculate tree C because of incomplete or
missing tree or volume data; C estimates for these plots are
based on averages from similar, but more complete, inventory
data. This applies to an additional 3 percent of inventory
records, which represent older (pre-1998) non-timberlands.
Understory vegetation is a minor component of biomass,
which is defined as all biomass of undergrowth plants in a
forest, including woody shrubs and trees less than 2.54 cm
d.b.h. In this Inventory, it is assumed that 10 percent of total
understory C mass is belowground. Estimates of C density
are based on information in Birdsey (1996). Understory
frequently represents over 1 percent of C in biomass, but its
contribution rarely exceeds 2 percent of the total.
Carbon in Dead Organic Matter
Dead organic matter is initially calculated as three
separate pools with C stocks modeled from inventory data.
Estimates are specific to regions and forest types within each
region, and stratification of forest land for dead organic matter
calculations is identical to that used for biomass through the
state and sub-state use of FIA data as discussed above. The
two components of dead wood—standing dead trees and
down dead wood—are estimated separately. The standing
dead tree C pools include aboveground and belowground
(coarse root) mass and include trees of at least 2.54 cm d.b.h.
Calculations are BCEF-like factors based on updates of Smith
et al. (2003). Down dead wood is defined as pieces of dead
wood greater than 7.5 cm diameter, at transect intersection,
that are not attached to live or standing dead trees. Down
dead wood includes stumps and roots of harvested trees.
Ratios of down dead wood to live tree are used to estimate
this quantity. Litter C is the pool of organic C (also known
as duff, humus, and fine woody debris) above the mineral
soil and includes woody fragments with diameters of up
to 7.5 cm. Estimates are based on equations of Smith and
Heath (2002).
Carbon in Forest Soil
Soil organic C (SOC) includes all organic material in
soil to a depth of 1 meter but excludes the coarse roots of the
biomass or dead wood pools. Estimates of SOC are based
on the national STATSGO spatial database (USDA 1991),
which includes region and soil type information. SOC
determination is based on the general approach described
by Amichev and Galbraith (2004). Links to FIA inventory
data were developed with the assistance of the USDA Forest
Service FIA Geospatial Service Center by overlaying FIA
forest inventory plots on the soil C map. This method
produced mean SOC densities stratified by region and forest
type group. It did not provide separate estimates for mineral
or organic soils but instead weighted their contribution to the
overall average based on the relative amount of each within
forest land. Thus, forest SOC is a function of species and
location, and net change also depends on these two factors
as total forest area changes. In this respect, SOC provides a
country-specific reference stock for 1990-present, but it does
not reflect effects of past land use.
Harvested Wood Carbon
Estimates of the harvested wood product (HWP)
contribution to forest C sinks and emissions (hereafter called
"HWP Contribution") are based on methods described in Skog
(2008) using the WOODCARB II model. These methods
are based on IPCC (2006) guidance for estimating HWP C.
IPCC (2006) provides methods that allow Parties to report
HWP Contribution using one of several different accounting
approaches: production, stock change and atmospheric flow,
as well as a default method that assumes there is no change in
HWP C stocks (see Annex 3.12 for more details about each
approach). The United States uses the production accounting
approach to report HWP Contribution. Under the production
approach, C in exported wood is estimated as if it remains in
the United States, and C in imported wood is not included in
inventory estimates. Though reported U.S. HWP estimates
are based on the production approach, estimates resulting
from use of the two alternative approaches, the stock change
and atmospheric flow approaches, are also presented for
comparison (see Annex 3.12). Annual estimates of change
are calculated by tracking the additions to and removals from
the pool of products held in end uses (i.e., products in use
such as housing or publications) and the pool of products
held in solid waste disposal sites (SWDS).
7-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Solidwood products added to pools include lumber and
panels. End-use categories for solidwood include single
and multifamily housing, alteration and repair of housing,
and other end-uses. There is one product category and one
end-use category for paper. Additions to and removals from
pools are tracked beginning in 1900, with the exception that
additions of softwood lumber to housing begins in 1800.
Solidwood and paper product production and trade data are
from USDA Forest Service and other sources (Hair and Ulrich
1963; Hair 1958; USDC Bureau of Census; 1976; Ulrich,
1985,1989; Steer 1948; AF&PA2006a2006b; Howard 2003,
2007). Estimates for disposal of products reflect the change
over time in the fraction of products discarded to SWDS (as
opposed to burning or recycling) and the fraction of SWDS
that are in sanitary landfills versus dumps.
There are five annual HWP variables that are used in
varying combinations to estimate HWP Contribution using
any one of the three main approaches listed above. These are:
(1A) annual change of C in wood and paper
products in use in the United States;
(IB) annual change of C in wood and paper products
in SWDS in the United States;
(2 A) annual change of C in wood and paper products
in use in the United States and other countries where the
wood came from trees harvested in the United States;
(2B) annual change of C in wood and paper products
in SWDS in the United States and other countries where
the wood came from trees harvested in the United States;
(3) C in imports of wood, pulp, and paper to the
United States,;
(4) C in exports of wood, pulp and paper from the
United States; and
(5) C in annual harvest of wood from forests in the
United States.
The sum of variables 2A and 2B yields the estimate
for HWP Contribution under the production accounting
approach. A key assumption for estimating these variables
is that products exported from the United States and held in
pools in other countries have the same half lives for products
in use, the same percentage of discarded products going to
SWDS, and the same decay rates in SWDS as they would
in the United States.
Uncertainty and Time-series consistency
A quantitative uncertainty analysis placed bounds
on current flux for forest ecosystems as well as carbon in
harvested wood products through Monte Carlo simulation
of the Methods described above and probabilistic sampling
of carbon conversion factors and inventory data. See Annex
3.12 for additional information. The 2008 flux estimate for
forest C stocks is estimated to be between -651 and -935
Tg CO2 Eq. at a 95 percent confidence level (see Table
7-10). This includes a range of -567 to -845 Tg CO2 Eq.
in forest ecosystems and -67 to -110 Tg CO2 Eq. for HWP.
The smaller range of relative uncertainty (that is, in terms
of percentage) for the total relative to the two separate
components occurs in part simply because the mean total
estimate is larger and in part because there is no correlation
between the two which would cause the uncertainty range
to change in a major way.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 7-10: Tier 2 Quantitative Uncertainty Estimates for Net C02 Flux from Forest Land Remaining Forest Land:
Changes in Forest C Stocks (Tg C02 Eq. and Percent)
Source
Forest Ecosystem
Harvested Wood Products
Total Forest
Gas
C02
C02
C02
2008 Flux Estimate
(Tg C02 Eq.)
(703.9)
(88.0)
(791.9)
Uncertainty Range Relative to Flux Estimate3
(TgC02Eq.) (%)
Lower Bound
(845.5)
(109.8)
(934.7)
Upper Bound
(566.8)
(67.2)
(651.2)
Lower Bound
-20%
-25%
-18%
Upper Bound
+ 19%
+24%
+18%
a Range of flux estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Note: Parentheses indicate negative values or net sequestration.
Land Use, Land-Use Change, and Forestry 7-21
-------
QA/QC and Verification
As discussed above, the FIA program has conducted
consistent forest surveys based on extensive statistically
based sampling of most of the forest land in the conterminous
United States, dating back to 1952. The main purpose of the
FIA program has been to estimate areas, volume of growing
stock, and timber products output and utilization factors.
The FIA program includes numerous quality assurance and
quality control (QA/QC) procedures, including calibration
among field crews, duplicate surveys of some plots, and
systematic checking of recorded data. Because of the
statistically based sampling, the large number of survey plots,
and the quality of the data, the survey databases developed
by the FIA program form a strong foundation for C stock
estimates. Field sampling protocols, summary data, and
detailed inventory databases are archived and are publicly
available on the Internet (USDA Forest Service 2009c).
Many key calculations for estimating current forest C
stocks based on FIA data are based on coefficients from
the FORCARB2 model (see additional discussion in the
Methodology section above and in Annex 3.12). The model
has been used for many years to produce national assessments
of forest C stocks and stock changes. General quality
control procedures were used in performing calculations to
estimate C stocks based on survey data. For example, the
derived C datasets, which include inventory variables such as
areas and volumes, were compared with standard inventory
summaries such as Resources Planning Act (RPA) Forest
Resource Tables or selected population estimates generated
from the FIA Database (FIADB), which are available at an
FIA Internet site (USDA Forest Service 2009d). Agreement
between the C datasets and the original inventories is
important to verify accuracy of the data used. Finally, C stock
estimates were compared with previous inventory report
estimates to ensure that any differences could be explained
by either new data or revised calculation methods (see the
"Recalculations" discussion below).
Estimates of the HWP variables and the HWP
contribution under the production accounting approach use
data from U.S. Census and USDA Forest Service surveys
of production and trade. Factors to convert wood and paper
to units C are based on estimates by industry and Forest
Service published sources. The WOODCARBII model uses
estimation methods suggested by IPCC (2006). Estimates
of annual C change in solidwood and paper products in use
were calibrated to meet two independent criteria. The first
criterion is that the WOODCARB II model estimate of C
in houses standing in 2001 needs to match an independent
estimate of C in housing based on U.S. Census and USDA
Forest Service survey data. Meeting the first criterion
resulted in an estimated half life of about 80 years for single
family housing built in the 1920s, which is confirmed by
other U.S. Census data on housing. The second criterion is
that the WOODCARB II model estimate of wood and paper
being discarded to SWDS needs to match EPA estimates
of discards each year over the period 1990 to 2000. These
criteria help reduce uncertainty in estimates of annual change
in C in products in use in the United States and, to a lesser
degree, reduce uncertainty in estimates of annual change
in C in products made from wood harvested in the United
States. In addition, WOODCARB II landfill decay rates have
been validated by making sure that estimates of methane
emissions from landfills based on EPA data are reasonable in
comparison with methane estimates based on WOODCARB
II landfill decay rates.
Recalculations Discussion
The basic models used to estimate forest ecosystem and
HWP C stocks and change are largely unchanged from the
previous Inventory (Smith et al. 2007, Skog 2008). Most
of the state-level estimates for 1990-present are relatively
similar to the values previously reported (EPA 2009).
However, changes in methodology and additions to the
underlying FIA data have driven some changes in estimates
across the time series. Most states have added new inventory
data or modified some of the information in previously
existing surveys, and the FIADB format changed to version
4.0 (USDA Forest Service 2009b). In particular, western
Texas forestlands were not previously included. This year we
are able to include 19.5 million hectares of forest, of which
1.1 million hectares is timberland. Thus, 80 percent of Texas
forests are included in carbon stock estimates, and 20 percent
of the forest land is included in the stock change estimate.
The important change in methodology for this year's
Inventory is in the selection and use of unique available
surveys from the remeasured annual inventories for some
states. Most eastern states have completed the first full
cycle of annualized inventories and are providing annual
updates to the state's forest inventory with each year's
remeasurements, such that one plot's measurements are
7-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
included in subsequent year's annual updates. Thus, annually
updated estimates of forest C stocks are accurate because they
give an accurate representation of the static C stock in that
year. However, estimates of stock change cannot utilize all
the annually updated inventory measurements as provided
in the FIADB as there is redundancy in the data used to
generate the annual updates of C stock (Smith and Heath in
preparation). To remedy this situation, the C stock and stock
change calculations used only the unique annual inventory
data available for download in the FIADB. Specifically, the
survey summaries included were the most recent version
of the first full annualized inventory as well as the second
annualized inventory cycle survey if it had at least 50 percent
of the plots measured. Annex 3.12 provides a list of the
specific surveys used here, and Smith et al. (2010) and Smith
and Heath (in press) provide further details. In addition, an
average estimate of logging residue was incorporated into the
down dead wood carbon calculations to explicitly account
for down dead wood following harvest on lands that were
reforested. Specific documentation is given in Annex 3.12.
Planned Improvements
The ongoing annual surveys by the FIA Program will
improve precision of forest C estimates as new state surveys
become available (USDA Forest Service 2009a). The annual
surveys will eventually include all states. To date, four
states are not yet reporting any data from the annualized
sampling design of FIA: Hawaii, Oklahoma, New Mexico
and Wyoming. Estimates for these states are currently based
on older, periodic data. Hawaii and U.S. territories will also
be included when appropriate forest C data are available. In
addition, the more intensive sampling of down dead wood,
litter, and soil organic C on some of the permanent FIA plots
continues and will substantially improve resolution of C pools
at the plot level for all U.S. forest land when this information
becomes available, unproved resolution, incorporating more
of Alaska's forests, and using annualized sampling data as
it becomes available for those states currently not reporting
are planned for future reporting.
As more information becomes available about historical
land use, the ongoing effects of changes in land use and forest
management will be better accounted for in estimates of soil
C (Birdsey and Lewis 2003, Woodbury et al. 2006, Woodbury
et al. 2007). Currently, soil C estimates are based on the
assumption that soil C density depends only on broad forest
type group, not on land-use history, but long-term residual
effects on soil and forest floor C stocks are likely after land-
use change. Estimates of such effects depend on identifying
past land use changes associated with forest lands.
Similarly, agroforestry practices, such as windbreaks
or riparian forest buffers along waterways, are not currently
accounted for in the Inventory. In order to properly account
for the C stocks and fluxes associated with agroforestry,
research will be needed that provides the basis and tools for
including these plantings in a nation-wide inventory, as well
as the means for entity-level reporting.
Non-C02 Emissions from Forest Fires
Emissions of non-CO2 gases from forest fires were
estimated using the default IPCC (2003) methodology
incorporating default IPCC (2006) emissions factors and
combustion factor for wildfires. Emissions from this source
Table 7-11: Estimated Non-C02 Emissions from Forest Fires (Tg C02 Eq.) for U.S. Forests3
Gas
CH4
N20
Total
1990
3.2
2.6
5.8
1995
4.3
3.5
7.7
2000
14.3
11.7
26.0
2005
9.8
8.0
17.8
2006
21.6
17.6
39.2
2007
20.0
16.3
36.3
2008
11.9
9.7
21.7
a Calculated based on C emission estimates in Changes in Forest Carbon Stocks and default factors in IPCC (2003, 2006).
Table 7-12: Estimated Non-C02 Emissions from Forest Fires (Gg) for U.S. Forests3
Gas
CH4
N20
1990
152
8
1995
203
11
2000
681
38
2005
467
26
2006
1,027
57
2007
953
53
2008
568
31
a Calculated based on C emission estimates in Changes in Forest Carbon Stocks and default factors in IPCC (2003, 2006).
Land Use, Land-Use Change, and Forestry 7-23
-------
in 2008 were estimated to be 11.9 Tg CO2 Eq. of CH4 and
9.7 Tg CO2 Eq. of N2O, as shown in Table 7-11 and Table
7-12. The estimates of non-CO2 emissions from forest fires
account for wildfires in the lower 48 states and Alaska as
well as prescribed fires in the lower 48 states.
Table 7-13: Estimated Carbon Released from
Forest Fires for U.S. Forests
Methodology
The IPCC (2003) Tier 2 default methodology was used
to calculate non-CO2 emissions from forest fires. However,
more up-to-date default emission factors from IPCC (2006)
were incorporated into the methodology and were converted
into gas-specific emission ratios. Estimates for CH4 and N2O
emissions were calculated by multiplying the total estimated
CO2 emitted from forest burned by the gas-specific emissions
ratios. CO2 emitted was estimated by multiplying total carbon
emitted (Table 7-13) by the C to CO2 conversion factor of
44/12 and by 92.8 percent which is the estimated proportion
of C emitted as CO2 (Smith 2008a). The equations used were:
CH4 Emissions = (C released) x 92.8% x (44/12) x
(CH4 to CO2 emission ratio)
N2O Emissions = (C released) x 92.8% x (44/12) x
(N2O to CO2 emission ratio)
Estimates for C emitted from forest fires are the same
estimates used to generate estimates of CO2 presented earlier
in Box 7-1. Estimates for C emitted include emissions from
wildfires in both Alaska and the lower 48 states as well as
emissions from prescribed fires in the lower 48 states only
(based on expert judgment that prescribed fires only occur in
the lower 48 states) (Smith 2008a). The IPCC (2006) default
combustion factor of 0.45 for "all 'other' temperate forests"
was applied in estimating C emitted from both wildfires
and prescribed fires. See the explanation in Annex 3.12 for
more details on the methodology used to estimate C emitted
from forest fires.
Year
C Emitted (Tg/yr)
1990
14.9
2005
2006
2007
2008
45.8
100.8
93.5
55.8
Uncertainty and Time-Series Consistency
Non-CO2 gases emitted from forest fires depend on
several variables, including: forest area for Alaska and the
lower 48 states; average carbon densities for wildfires in
Alaska, wildfires in the lower 48, and prescribed fires in
the lower 48; emission ratios; and combustion factor values
(proportion of biomass consumed by fire). To quantify
the uncertainties for emissions from forest fires, a Monte
Carlo (Tier 2) uncertainty analysis was performed using
information about the uncertainty surrounding each of these
variables. The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 7-14.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 7-14: Quantitative Uncertainty Estimates of Non-C02 Emissions from Forest Fires in Forest Land Remaining
Forest Land (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(TgC02Eq.) (%)
Non-C02 Emissions
from Forest Fires
Non-C02 Emissions
from Forest Fires
CH4
N20
11.9
9.7
Lower Bound
3.3
2.8
Upper Bound
30.2
24.7
Lower Bound
-73%
-71%
Upper Bound
+ 153%
+ 154%
7-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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QA/QC and Verification
Tier 1 and Tier 2 QA/QC activities were conducted
consistent with the U.S. QA/QC plan. Source-specific quality
control measures for forest fires included checking input
data, documentation, and calculations to ensure data were
properly handled through the inventory process. Errors that
were found during this process were corrected as necessary.
Recalculations Discussion
Non-CO2 emissions were recalculated using the 2006
IPCC default emission factors for CH4 and N2O relative to
the previous Inventory. These default emission factors were
converted to CH4 to CO2 and N2O to CO2 emission ratios
and then multiplied by CO2 emissions to estimate CH4 and
N2O emissions. The previous 2003 IPCC methodology
provides emission ratios that are multiplied by total carbon
emitted. Updating to the 2006 IPCC emission factors results
in estimates for CH4 emissions decreasing by a factor of
approximately one third between methods (from 18.7 with
2003 factors to 11.9 Tg CO2 Eq. with 2006 factors for 2008).
In contrast, the update causes the estimated values for N2O
emissions to increase by a factor of approximately four (from
1.9 with 2003 factors to 9.7 Tg CO2 Eq. with 2006 factors
for 2008). Due to the similar magnitudes of the decrease in
the CH4 estimates and the increase in the N2O estimates, the
total estimates for non-CO2 emissions from forest fire remain
relatively consistent between methods.
Planned Improvements
The default combustion factor of 0.45 from IPCC (2006)
was applied in estimating C emitted from both wildfires and
prescribed fires. Additional research into the availability
of a combustion factor specific to prescribed fires will be
conducted.
Direct N20 Fluxes from Forest Soils
(IPCC Source Category 5A1)
Of the synthetic N fertilizers applied to soils in the
United States, no more than one percent is applied to forest
soils. Application rates are similar to those occurring on
cropped soils, but in any given year, only a small proportion
of total forested land receives N fertilizer. This is because
forests are typically fertilized only twice during their
approximately 40-year growth cycle (once at planting and
Table 7-15: N20 Fluxes from Soils in Forest Land
Remaining Forest Land (Tg C02 Eq. and Gg N20)
Year
Tg C02 Eq.
Gg
1990
0.1
0.2
2005
2006
2007
2008
0.4
0.4
0.4
0.4
1.2
1.2
1.2
1.2
Note: These estimates include direct N20 emissions from N fertilizer
additions only. Indirect N20 emissions from fertilizer additions
are reported in the Agriculture chapter. These estimates include
emissions both from Forest Land Remaining Forest Land and from
Land Converted to Forest Land.
once approximately 20 years later). Thus, although the rate
of N fertilizer application for the area of forests that receives
N fertilizer in any given year is relatively high, average
annual applications, inferred by dividing all forest land that
may undergo N fertilization at some point during its growing
cycle by the amount of N fertilizer added to these forests in
a given year, is quite low. Direct N2O emissions from forest
soils in 2008 were 0.4 Tg CO2 Eq. (1 Gg). Emissions have
increased by 455 percent from 1990 to 2008 as a result of
an increase in the area of N fertilized pine plantations in
the southeastern United States and Douglas-fir timberland
in western Washington and Oregon. Total forest soil N2O
emissions are summarized in Table 7-15.
Methodology
The IPCC Tier 1 approach was used to estimate N2O
from soils within Forest Land Remaining Forest Land.
According to U.S. Forest Service statistics for 1996 (USDA
Forest Service 2001), approximately 75 percent of trees
planted were for timber, and about 60 percent of national
total harvested forest area is in the southeastern United States.
Although southeastern pine plantations represent the majority
of fertilized forests in the United States, this Inventory
also accounted for N fertilizer application to commercial
Douglas-fir stands in western Oregon and Washington.
For the Southeast, estimates of direct N2O emissions from
fertilizer applications to forests were based on the area of
pine plantations receiving fertilizer in the southeastern United
States and estimated application rates (Albaugh et al., 2007).
Land Use, Land-Use Change, and Forestry 7-25
-------
Not accounting for fertilizer applied to non-pine plantations
is justified because fertilization is routine for pine forests
but rare for hardwoods (Binkley et al. 1995). For each year,
the area of pine receiving N fertilizer was multiplied by the
weighted average of the reported range of N fertilization rates
(121 Ibs. N per acre). Area data for pine plantations receiving
fertilizer in the Southeast were not available for 2005,2006,
2007 and 2008, so data from 2004 were used for these years.
For commercial forests in Oregon and Washington, only
fertilizer applied to Douglas-fir was accounted for, because
the vast majority (~95 percent) of the total fertilizer applied
to forests in this region is applied to Douglas-fir (Briggs,
2007). Estimates of total Douglas-fir area and the portion
of fertilized area were multiplied to obtain annual area
estimates of fertilized Douglas-fir stands and these were
multiplied by the typical rate used in this region (200 Ibs. N
per acre) to estimate annual N additions (Briggs 2007). The
total N applied to forests was multiplied by the IPCC (2006)
default emission factor of 1 percent to estimate direct N2O
emissions. The volatilization and leaching/runoff fractions,
calculated according to the IPCC default factors of 10 percent
and 30 percent, respectively, were included with all sources
of indirect emissions in the Agricultural Soil Management
source category of the Agriculture chapter.
Uncertainty and Time-Series Consistency
The amount of N2O emitted from forests depends not
only on N inputs, but also on a large number of variables,
including organic C availability, oxygen gas partial pressure,
soil moisture content, pH, temperature, and tree planting/
harvesting cycles. The effect of the combined interaction of
these variables on N2O flux is complex and highly uncertain.
IPCC (2006) does not incorporate any of these variables into
the default methodology and only accounts for variations
in estimated fertilizer application rates and estimated areas
of forested land receiving N fertilizer. All forest soils are
treated equivalently under this methodology. Furthermore,
only synthetic N fertilizers are captured, so applications
of organic N fertilizers are not estimated. However, the
total quantity of organic N inputs to soils is included in the
Agricultural Soil Management and Settlements Remaining
Settlements sections.
Uncertainties exist in the fertilization rates, annual
area of forest lands receiving fertilizer, and the emission
factors. Fertilization rates were assigned a default level26
of uncertainty at ±50 percent, and area receiving fertilizer
was assigned a ±20 percent according to expert knowledge
(Binkley 2004). IPCC (2006) provided estimates for the
uncertainty associated with direct N2O emission factor
for synthetic N fertilizer application to soils. Quantitative
uncertainty of this source category was estimated through
the IPCC-recommended Tier 2 uncertainty estimation
methodology. The uncertainty ranges around the 2005
activity data and emission factor input variables were directly
applied to the 2008 emissions estimates. The results of the
quantitative uncertainty analysis are summarized in Table
7-16. N2O fluxes from soils were estimated to be between
0.1 and 1.1 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 59 percent below and 211 percent above
the 2008 emission estimate of 0.4 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 7-16: Quantitative Uncertainty Estimates of N20 Fluxes from Soils in Forest Land Remaining Forest Land
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(TgC02Eq.) ' (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Forest Land Remaining Forest Land:
N20 Fluxes from Soils N20
0.4
0.1
1.1
-59%
+211%
Note: This estimate includes direct N20 emissions from N fertilizer additions to both Forest Land Remaining Forest Land and Land Converted to Forest Land.
26 Uncertainty is unknown for the fertilization rates so a conservative value
of ±50% was used in the analysis.
7-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Recalculations Discussion
The improvement in this Inventory was to account for
N fertilizer applications to commercial Douglas-fir forests
in western Oregon and Washington, requiring a recalculation
for the time series. This resulted in an annual increase in
emissions of approximately 24 percent compared to the
previous Inventory.
Planned Improvements
State-level area data will be acquired for southeastern
pine plantations and northwestern Douglas-fir forests
receiving fertilizer to estimate soil N2O emission by state
and provide information about regional variation in emission
patterns.
7.3. Land Converted to Forest Land
(IPCC Source Category 5A2)
Land-use change is constantly occurring, and areas
under a number of differing land-use types are converted to
forest each year, just as forest land is converted to other uses.
However, the magnitude of these changes is not currently
known. Given the paucity of available land-use information
relevant to this particular IPCC source category, it is not
possible to separate CO2 or N2O fluxes on Land Converted
to Forest Land from fluxes on Forest Land Remaining Forest
Land at this time.
7.4. Cropland Remaining Cropland
(IPCC Source Category 5B1)
Mineral and Organic Soil Carbon
Stock Changes
Soils contain both organic and inorganic forms of C, but
soil organic C (SOC) stocks are the main source and sink
for atmospheric CO2 in most soils. Changes in inorganic C
stocks are typically minor. In addition, soil organic C is the
dominant organic C pool in cropland ecosystems, because
biomass and dead organic matter have considerably less
C and those pools are relatively ephemeral. IPCC (2006)
recommends reporting changes in soil organic C stocks due
to agricultural land-use and management activities on mineral
and organic soils.27
Typical well-drained mineral soils contain from 1 to 6
percent organic C by weight, although mineral soils that are
saturated with water for substantial periods during the year
may contain significantly more C (NRCS 1999). Conversion
of mineral soils from their native state to agricultural uses can
cause as much as half of the SOC to be decomposed and the
C lost to the atmosphere. The rate and ultimate magnitude of
C loss will depend on pre-conversion conditions, conversion
method and subsequent management practices, climate,
and soil type. In the tropics, 40 to 60 percent of the C
loss generally occurs within the first 10 years following
conversion; C stocks continue to decline in subsequent
decades but at a much slower rate. In temperate regions, C
loss can continue for several decades, reducing stocks by 20
to 40 percent of native C levels. Eventually, the soil can reach
a new equilibrium that reflects a balance between C inputs
(e.g., decayed plant matter, roots, and organic amendments
such as manure and crop residues) and C loss through
microbial decomposition of organic matter. However, land
use, management, and other conditions may change before
the new equilibrium is reached. The quantity and quality
of organic matter inputs and their rate of decomposition
are determined by the combined interaction of climate, soil
properties, and land use. Land use and agricultural practices
such as clearing, drainage, tillage, planting, grazing, crop
residue management, fertilization, and flooding can modify
both organic matter inputs and decomposition, and thereby
result in a net flux of C to or from the pool of soil C.
Organic soils, also referred to as histosols, include all
soils with more than 12 to 20 percent organic C by weight,
depending on clay content (NRCS 1999, Brady and Weil
1999). The organic layer of these soils can be very deep
(i.e., several meters), forming under inundated conditions
in which minimal decomposition of plant residue occurs.
When organic soils are prepared for crop production, they
are drained and tilled, leading to aeration of the soil, which
accelerates the rate of decomposition and CO2 emissions.
Because of the depth and richness of the organic layers, C loss
from drained organic soils can continue over long periods of
time. The rate of CO2 emissions varies depending on climate
and composition (i.e., decomposability) of the organic matter.
27 Carbon dioxide emissions associated with liming are also estimated but
are included in a separate section of the report.
Land Use, Land-Use Change, and Forestry 7-27
-------
Table 7-17: Net C02 Flux from Soil C Stock Changes in Cropland Remaining Cropland (Tg C02 Eq.)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(56.8)
27.4
(29.4)
1995
(50.6)
27.7
(22.9)
2000
(57.9)
27.7
(30.2)
2005
(45.9)
27.7
(18.3)
2006
(46.8)
27.7
(19.1)
2007
(47.3)
27.7
(19.7)
2008
(45.7)
27.7
(18.1)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections. All other values
are based on historical data only. Totals may not sum due to independent rounding.
Table 7-18: Net C02 Flux from Soil C Stock Changes in Cropland Remaining Cropland (Tg C)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(15.5)
7.5
(8.0)
1995
(13.8)
7.5
(6.3)
2000
(15.8)
7.5
(8.2)
2005
(12.5)
7.5
(5.0)
2006
(12.8)
7.5
(5.2)
2007
(12.9)
7.5
(5.4)
2008
(12.5)
7.5
(4.9)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections. All other values
are based on historical data only. Totals may not sum due to independent rounding.
Also, the use of organic soils for annual crop production
leads to higher C loss rates than drainage of organic soils
in grassland or forests, due to deeper drainage and more
intensive management practices in cropland (Armentano and
Verhoeven 1990, as cited in IPCC/UNEP/OECD/EA1997).
Carbon losses are estimated from drained organic soils under
both grassland and cropland management in this Inventory.
Cropland Remaining Cropland includes all cropland
in an inventory year that had been cropland for the last 20
years28 according to the USDA NRI land-use survey (USDA-
NRCS 2000). The Inventory includes all privately-owned
croplands in the conterminous United States and Hawaii,
but there is a minor amount of cropland on federal lands,
which is not currently included in the estimation of C stock
changes. It is important to note that these areas are part of
the managed land base for the United States, as described
in Section 7.1, and plans are being made to include federal
croplands in future C inventories.
The area of Cropland Remaining Cropland changes
through time as land is converted to or from cropland
management. CO2 emissions and removals29 due to
changes in mineral soil C stocks are estimated using a Tier
3 approach for the majority of annual crops. A Tier 2IPCC
method is used for the remaining crops (vegetables, tobacco,
28 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began, and consequently the classifications
were based on less than 20 years from 1990 to 2001.
29 Note that removals occur through crop and forage uptake of CO2
perennial/horticultural crops, and rice) not included in the
Tier 3 method. In addition, a Tier 2 method is used for
very gravelly, cobbly, or shaley soils (i.e., classified as soils
that have greater than 35 percent of soil volume comprised
of gravel, cobbles, or shale) and for additional changes in
mineral soil C stocks that were not addressed with the Tier
2 or 3 approaches (i.e., change in C stocks after 2003 due
to Conservation Reserve Program enrollment). Emissions
from organic soils are estimated using a Tier 2 IPCC method.
Of the two sub-source categories, land-use and land
management of mineral soils was the most important
component of total net C stock change between 1990 and
2008 (see Table 7-17 and Table 7-18). In 2008, mineral
soils were estimated to remove 45.7 Tg CO2 Eq. (12.5 Tg
C). This rate of C storage in mineral soils represented about
a 20 percent decrease in the rate since the initial reporting
year of 1990. Emissions from organic soils were 27.7 Tg
CO2 Eq. (7.5 Tg C) in 2008. In total, U.S. agricultural soils
in Cropland Remaining Cropland removed approximately
18.1 Tg CO2 Eq. (4.9 Tg C) in 2008.
The net reduction in soil C accumulation over the time
series (39 percent for 2008, relative to 1990) was largely
due to the declining influence of annual cropland enrolled
in the Conservation Reserve Program, which began in the
late 1980s. However, there were still positive increases
in C stocks from land enrolled in the reserve program, as
well as intensification of crop production by limiting the
use of bare-summer fallow in semi-arid regions, increased
7-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Figure 7-5
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States,
2008, Cropland Remaining Cropland
Note: Values greater than zero represent emissions, and values less than zero represent sequestration. Map accounts for fluxes associated with the
Tier 2 and 3 inventory computations. See Methodology for additional details.
Figure 7-6
Total Net Annual C02 Flux For Organic Soils Under Agricultural Management within States,
2008, Cropland Remaining Cropland
o
Note: Values greater than zero represent emissions.
Tg C02 Eq./year
>2
1 to 2
D0.5to1
D0.1to0.5
DO to 0.1
El No organic soils
Land Use, Land-Use Change, and Forestry 7-29
-------
hay production, and adoption of conservation tillage (i.e.,
reduced- and no-till practices).
The spatial variability in annual CO2 flux associated with
C stock changes in mineral and organic soils is displayed
in Figure 7-5 and Figure 7-6. The highest rates of net C
accumulation in mineral soils occurred in the Midwest, which
is the area with the largest amounts of cropland managed
with conservation tillage. Rates were also high in the
Great Plains due to enrollment in the Conservation Reserve
Program. Emission rates from drained organic soils were
highest along the southeastern coastal region, in the northeast
central United States surrounding the Great Lakes, and along
the central and northern portions of the West Coast.
Methodology
The following section includes a description of the
methodology used to estimate changes in soil C stocks due
to: (1) agricultural land-use and management activities on
mineral soils; and (2) agricultural land-use and management
activities on organic soils for Cropland Remaining Cropland.
Soil C stock changes were estimated for Cropland
Remaining Cropland (as well as agricultural land falling
into the IPCC categories Land Converted to Cropland,
Grassland Remaining Grassland, and Land Converted to
Grassland) according to land-use histories recorded in the
USD A National Resources Inventory (NRI) survey (USDA-
NRCS 2000). The NRI is a statistically-based sample of all
non-federal land, and includes approximately 260,000 points
in agricultural land for the conterminous United States and
Hawaii.30 Each point is associated with an "expansion factor"
that allows scaling of C stock changes from NRI points to
the entire country (i.e., each expansion factor represents the
amount of area with the same land-use/management history
as the sample point). Land-use and some management
information (e.g., crop type, soil attributes, and irrigation)
were originally collected for each NRI point on a 5-year
cycle beginning in 1982. For cropland, data were collected
for 4 out of 5 years in the cycle (i.e., 1979-1982,1984-1987,
1989-1992, and 1994-1997). However, the NRI program
began collecting annual data in 1998, and data are currently
available through 2003. NRI points were classified as
Cropland Remaining Cropland in a given year between 1990
and 2008 if the land use had been cropland for 20 years.31
Cropland includes all land used to produce food and fiber, or
forage that is harvested and used as feed (e.g., hay and silage).
Mineral Soil Carbon Stock Changes
An IPCC Tier 3 model-based approach was applied to
estimate C stock changes for mineral soils used to produce
a majority of annual crops in the United States (Ogle et al.
2009). The remaining crops on mineral soils were estimated
using an IPCC Tier 2 method (Ogle et al. 2003), including
vegetables, tobacco, perennial/horticultural crops, rice, and
crops rotated with these crops. The Tier 2 method was
also used for very gravelly, cobbly, or shaley soils (greater
than 35 percent by volume). Mineral SOC stocks were
estimated using a Tier 2 method for these areas, because
the Century model used for the Tier 3 method has not been
fully tested to address its adequacy for estimating C stock
changes associated with certain crops and rotations, as well
as cobbly, gravelly, or shaley soils. An additional stock
change calculation was made for mineral soils using Tier 2
emission factors, accounting for enrollment patterns in the
Conservation Reserve Program after 2003, which was not
addressed by the Tier 3 methods.
Further elaboration on the methodology and data used
to estimate stock changes from mineral soils are described
below and in Annex 3.13.
Tier 3 Approach
Mineral SOC stocks and stock changes were estimated
using the Century biogeochemical model (Parton et al. 1987,
1988, 1994; Metherell et al. 1993), which simulates the
dynamics of C and other elements in cropland, grassland,
forest, and savanna ecosystems. It uses monthly weather
data as an input, along with information about soil physical
properties. Input data on land use and management are
specified at monthly resolution and include land-use type,
crop/forage type, and management activities (e.g., planting,
harvesting, fertilization, manure amendments, tillage,
irrigation, residue removal, grazing, and fire). The model
computes net primary productivity and C additions to
soil, soil temperature, and water dynamics, in addition to
turnover, stabilization, and mineralization of soil organic
30 NRI points were classified as agricultural if under grassland or cropland
management between 1990 and 2003.
31 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification prior to
2002 was based on less than 20 years of recorded land-use history for the
time series.
7-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Box 7-2: Tier 3 Inventory for Soil C Stocks Compared to Tier 1 or 2 Approaches
A Tier 3 model-based approach is used to inventory soil C stock changes on the majority of agricultural land with mineral soils. This
approach entails several fundamental differences compared to the IPCC Tier 1 or 2 methods, which are based on a classification of land
areas into a number of discrete classes based on a highly aggregated classification of climate, soil, and management (i.e., only six climate
regions, seven soil types and eleven management systems occur in U.S. agricultural land under the IPCC classification). Input variables
to the Tier 3 model, including climate, soils, and management activities (e.g., fertilization, crop species, tillage, etc.), are represented in
considerably more detail both temporally and spatially, and exhibit multi-dimensional interactions through the more complex model structure
compared with the IPCC Tier 1 or 2 approach. The spatial resolution of the analysis is also finer in the Tier 3 method compared to the lower
tier methods as implemented in the United States for previous Inventories (e.g., 3,037 counties versus 181 Major Land Resource Areas
(MLRAs), respectively).
In the Century model, soil C dynamics (and C02 emissions and uptake) are treated as continuous variables, which change on a monthly
time step. Carbon emissions and removals are an outcome of plant production and decomposition processes, which are simulated in the
model structure. Thus, changes in soil C stocks are influenced by not only changes in land use and management but also inter-annual climate
variability and secondary feedbacks between management activities, climate, and soils as they affect primary production and decomposition.
This latter characteristic constitutes one of the greatest differences between the methods, and forms the basis for a more complete accounting
of soil C stock changes in the Tier 3 approach compared with Tier 2 methodology.
Because the Tier 3 model simulates a continuous time period rather than the equilibrium step change used in the IPCC methodology
(Tier 1 and 2), the Tier 3 model addresses the delayed response of soils to management and land-use changes. Delayed responses can
occur due to variable weather patterns and other environmental constraints that interact with land use and management and affect the time
frame over which stock changes occur. Moreover, the Tier 3 method also accounts for the overall effect of increasing yields and, hence,
C input to soils that have taken place across management systems and crop types within the United States. Productivity has increased by
1 to 2 percent annually over the past 4 to 5 decades for most major crops in the United States (Reilly and Fuglie 1998), which is believed
to have led to increases in cropland soil C stocks (e.g., Allmaras et al. 2000). This is a major difference from the IPCC-based Tier 1 and 2
approaches, in which trends in soil C stocks only capture discrete changes in management and/or land use, rather than a longer term trend
such as gradual increases in crop productivity.
matter C and nutrient (N, K, S) elements. This method is
more accurate than the Tier 1 and 2 approaches provided
by the IPCC, because the simulation model treats changes
as continuous over time rather than the simplified discrete
changes represented in the default method (see Box 7-2 for
additional information). National estimates were obtained
by simulating historical land-use and management patterns
as recorded in the USDA National Resources Inventory
(NRI) survey.
Additional sources of activity data were used to
supplement the land-use information from NRI. The
Conservation Technology Information Center (CTIC 1998)
provided annual data on tillage activity at the county level
since 1989, with adjustments for long-term adoption of no-till
agriculture (Towery 2001). Information on fertilizer use and
rates by crop type for different regions of the United States
were obtained primarily from the USDA Economic Research
Service Cropping Practices Survey (ERS 1997) with
additional data from other sources, including the National
Agricultural Statistics Service (NASS 1992, 1999, 2004).
Frequency and rates of manure application to cropland during
1997 were estimated from data compiled by the USDA
Natural Resources Conservation Service (Edmonds et al.
2003), and then adjusted using county-level estimates of
manure available for application in other years. Specifically,
county-scale ratios of manure available for application to
soils in other years relative to 1997 were used to adjust
the area amended with manure (see Annex 3.13 for further
details). Greater availability of managed manure N relative
to 1997 was, thus, assumed to increase the area amended with
manure, while reduced availability of manure N relative to
1997 was assumed to reduce the amended area. The amount
of manure produced by each livestock type was calculated for
managed and unmanaged waste management systems based
on methods described in the Manure Management section
(Section 6.2) and annex (Annex 3.10).
Manure amendments were an input to the Century
Model based on manure N available for application from
all managed or unmanaged systems except pasture/range/
Land Use, Land-Use Change, and Forestry 7-31
-------
paddock.32 Data on the county-level N available for
application were estimated for managed systems based on
the total amount of N excreted in manure minus N losses
during storage and transport, and including the addition of
N from bedding materials. Nitrogen losses include direct
nitrous oxide emissions, volatilization of ammonia and
NOx, runoff and leaching, and poultry manure used as a feed
supplement. More information on these losses is available in
the description of the Manure Management source category.
For unmanaged systems, it is assumed that no N losses or
additions occur prior to the application of manure to the soil.
Monthly weather data were used as an input in the model
simulations, based on an aggregation of gridded weather data
to the county scale from the Parameter-elevation Regressions
on Independent Slopes Model (PRISM) database (Daly et
al. 1994). Soil attributes, which were obtained from an
NRI database, were assigned based on field visits and soil
series descriptions. Each NRI point was run 100 times as
part of the uncertainty assessment, yielding a total of over
18 million simulation runs for the analysis. Carbon stock
estimates from Century were adjusted using a structural
uncertainty estimator accounting for uncertainty in model
algorithms and parameter values (Ogle et al. 2007,2009). C
stocks and 95 percent confidence intervals were estimated for
each year between 1990 and 2003, but C stock changes from
2004 to 2008 were assumed to be similar to 2003 because
no additional activity data are currently available from the
NRI for the latter years.
Tier 2 Approach
In the IPCC Tier 2 method, data on climate, soil types,
land-use, and land management activity were used to classify
land area to apply appropriate stock change factors. MLRAs
formed the base spatial unit for mapping climate regions in
the United States; each MLRA represents a geographic unit
with relatively similar soils, climate, water resources, and
land uses (NRCS 1981). MLRAs were classified into climate
regions according to the IPCC categories using the PRISM
climate database of Daly et al. (1994).
Reference C stocks were estimated using the National
Soil Survey Characterization Database (NRCS 1997) with
cultivated cropland as the reference condition, rather than
32 Pasture/Range/Paddock manure additions to soils are addressed in
the Grassland Remaining Grassland and Land Converted to Grassland
categories.
native vegetation as used in IPCC (2003,2006). Changing the
reference condition was necessary because soil measurements
under agricultural management are much more common and
easily identified in the National Soil Survey Characterization
Database (NRCS 1997) than those that are not considered
cultivated cropland.
U.S.-specific stock change factors were derived from
published literature to determine the impact of management
practices on SOC storage, including changes in tillage,
cropping rotations and intensification, and land-use change
between cultivated and uncultivated conditions (Ogle et al.
2003, Ogle et al. 2006). U.S. factors associated with organic
matter amendments were not estimated because there were
an insufficient number of studies to analyze those impacts.
Instead, factors from IPCC (2003) were used to estimate the
effect of those activities. Euliss and Gleason (2002) provided
the data for computing the change in SOC storage resulting
from restoration of wetland enrolled in the Conservation
Reserve Program.
Activity data were primarily based on the historical
land-use/management patterns recorded in the NRI. Each
NRI point was classified by land use, soil type, climate
region (using PRISM data, Daly et al. 1994) and management
condition. Classification of cropland area by tillage practice
was based on data from the Conservation Tillage Information
Center (CTIC 1998, Towery 2001) as described above.
Activity data on wetland restoration of Conservation Reserve
Program land were obtained from Euliss and Gleason (2002).
Manure N amendments over the inventory time period
were based on application rates and areas amended with
manure N from Edmonds et al. (2003), in addition to the
managed manure production data discussed in the previous
methodology subsection on the Tier 3 analysis for mineral
soils.
Combining information from these data sources, SOC
stocks for mineral soils were estimated 50,000 times for 1982,
1992, and 1997, using a Monte Carlo simulation approach
and the probability distribution functions for U.S.-specific
stock change factors, reference C stocks, and land-use activity
data (Ogle et al. 2002, Ogle et al. 2003). The annual C flux for
1990 through 1992 was determined by calculating the average
annual change in stocks between 1982 and 1992; annual C
flux for 1993 through 2008 was determined by calculating
the average annual change in stocks between 1992 and 1997.
7-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Additional Mineral C Stock Change
Annual C flux estimates for mineral soils between 1990
and 2008 were adjusted to account for additional C stock
changes associated with gains or losses in soil C after 2003
due to changes in Conservation Reserve Program enrollment.
The change in enrollment acreage relative to 2003 was based
on data from USDA-FSA (2007) for 2004 through 2008,
and the differences in mineral soil areas were multiplied
by 0.5 metric tons C per hectare per year to estimate the
net effect on soil C stocks. The stock change rate is based
on estimations using the IPCC method (see Annex 3.13 for
further discussion).
Organic Soil Carbon Stock Changes
Annual C emissions from drained organic soils in
Cropland Remaining Cropland were estimated using the
Tier 2 method provided in IPCC (2003, 2006), with U.S.-
specific C loss rates (Ogle et al. 2003) rather than default
IPCC rates. The final estimates included a measure of
uncertainty as determined from the Monte Carlo simulation
with 50,000 iterations. Emissions were based on the 1992
and 1997 Cropland Remaining Cropland areas from the 1997
National Resources Inventory (USDA-NRCS 2000). The
annual flux estimated for 1992 was applied to 1990 through
1992, and the annual flux estimated for 1997 was applied to
1993 through 2008.
Uncertainty and Time-Series Consistency
Uncertainty associated with the Cropland Remaining
Cropland land-use category was addressed for changes in
agricultural soil C stocks (including both mineral and organic
soils). Uncertainty estimates are presented in Table 7-19 for
mineral soil C stocks and organic soil C stocks disaggregated
to the level of the inventory methodology employed (i.e.,
Tier 2 and Tier 3). Uncertainty for the portions of the
Inventory estimated with Tier 2 and 3 approaches was derived
using a Monte Carlo approach (see Annex 3.13 for further
discussion). A combined uncertainty estimate for changes in
soil C stocks is also included. Uncertainty estimates from
each component were combined using the error propagation
equation in accordance with IPCC (2006). The combined
uncertainty was calculated by taking the square root of the
sum of the squares of the standard deviations of the uncertain
quantities. More details on how the individual uncertainties
were developed are in Annex 3.13. The combined uncertainty
for soil C stocks in Cropland Remaining Cropland ranged
from 166 percent below to 161 percent above the 2008 stock
change estimate of -18.1 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 7-19: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Cropland Remaining
Cropland (Tg C02 Eq. and Percent)
Source
Mineral Soil C Stocks: Cropland Remaining Cropland,
Tier 3 Inventory Methodology
Mineral Soil C Stocks: Cropland Remaining Cropland,
Tier 2 Inventory Methodology
Mineral Soil C Stocks: Cropland Remaining Cropland
(Change in CRP enrollment relative to 2003)
Organic Soil C Stocks: Cropland Remaining Cropland,
Tier 2 Inventory Methodology
2008 Flux Estimate
(Tg C02 Eq.)
(42.3)
(3.0)
(0.4)
27.7
Uncertainty Range Relative to Flux Estimate
(Tg C02
Lower
Bound
(69.6)
(6.9)
(0.6)
15.8
Eq.)
Upper
Bound
(15.1)
0.8
(0.2)
36.9
(
Lower
Bound
-64%
-127%
-50%
-43%
%)
Upper
Bound
+64%
+ 128%
+50%
+33%
Combined Uncertainty for Flux Associated with
Agricultural Soil Carbon Stock Change in Cropland
Remaining Cropland (18-1)
(48.0)
11.0
-166% +161%
Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
Land Use, Land-Use Change, and Forestry 7-33
-------
QA/QC and Verification
Quality control measures included checking input data,
model scripts, and results to ensure data were properly
handled throughout the inventory process. As discussed
in the uncertainty section, results were compared to field
measurements, and a statistical relationship was developed
to assess uncertainties in the model's predictive capability.
The comparisons included over 40 long-term experiments,
representing about 800 combinations of management
treatments across all of the sites (Ogle et al. 2007). Inventory
reporting forms and text were reviewed and revised as needed
to correct transcription errors.
Planned Improvements
The first improvement is to update the Tier 2 inventory
analysis with the latest annual National Resources Inventory
(NRI) data. While the land base for the Tier 3 approach uses
the latest available data from the NRI, the Tier 2 portion of the
Inventory has not updated and is based on the Revised 1997
NRI data product (USDA-NRCS 2000). This improvement
will extend the time series of the land use data from 1997
through 2003 for the Tier 2 portion of the Inventory.
The second improvement is to incorporate remote
sensing in the analysis for estimation of crop and forage
production. Specifically, the Enhanced Vegetation Index
(EVI) product that is derived from MODIS satellite imagery
is being used to refine the production estimation for the
Tier 3 assessment framework. EVI reflects changes in
plant "greenness" over the growing season and can be used
to compute production based on the light use efficiency
of the crop or forage (Potter et al. 1993). In the current
framework, production is simulated based on the weather
data, soil characteristics, and the genetic potential of the
crop. While this method produces reasonable results, remote
sensing can be used to refine the productivity estimates and
reduce biases in crop production and subsequent C input to
soil systems. It is anticipated that precision in the Tier 3
assessment framework will be increased by 25 percent or
more with the new method.
CO2 Emissions from Agricultural
Liming
IPCC (2006) recommends reporting CO2 emissions from
lime additions (in the form of crushed limestone (CaCO3)
and dolomite (CaMg(CO3)2) to agricultural soils. Limestone
and dolomite are added by land managers to ameliorate
acidification. When these compounds come in contact with
acid soils, they degrade, thereby generating CO2. The rate
and ultimate magnitude of degradation of applied limestone
and dolomite depends on the soil conditions, climate regime,
and the type of mineral applied. Emissions from liming have
Table 7-20: C02 Emissions from Liming of Agricultural Soils (Tg C02 Eq.)
Source
1990
1995
2000
2005
2006
2007
2008
Liming of Agricultural Soils3
4.7
4.4
4.3
4.3
4.2
4.5
3.8
a Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to Grassland, and Settlements
Remaining Settlements.
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
Table 7-21: C02 Emissions from Liming of Agricultural Soils (Tg C)
Source
1990
1995
2000
2005
2006
2007
2008
Liming of Agricultural Soils3
1.3
1.2
1.2
1.2
1.2
1.2
1.0
3 Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to Grassland, and Settlements
Remaining Settlements.
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
7-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
fluctuated over the past sixteen years, ranging from 3.8 Tg
CO2 Eq. to 5.0 Tg CO2 Eq. In 2008, liming of agricultural
soils in the United States resulted in emissions of 3.8 Tg CO2
Eq. (1.0 Tg C), representing about a 18 percent decrease in
emissions since 1990 (see Table 7-20 and Table 7-21). The
trend is driven entirely by the amount of lime and dolomite
estimated to have been applied to soils over the time period.
Methodology
Carbon dioxide emissions from degradation of limestone
and dolomite applied to agricultural soils were estimated
using a Tier 2 methodology consistent with IPCC (2006). The
annual amounts of limestone and dolomite applied (see Table
7-22) were multiplied by CO2 emission factors from West and
McBride (2005). These emission factors (0.059 metric ton C/
metric ton limestone, 0.064 metric ton C/metric ton dolomite)
are lower than the IPCC default emission factors because they
account for the portion of agricultural lime that may leach
through the soil and travel by rivers to the ocean (West and
McBride 2005). This analysis of lime dissolution is based
on liming occurring in the Mississippi River basin, where the
vastmajority of all U.S. liming takes place (West 2008). U.S.
liming that does not occur in the Mississippi River basin tends
to occur under similar soil and rainfall regimes, and, thus,
the emission factor is appropriate for use across the United
States (West 2008). The annual application rates of limestone
and dolomite were derived from estimates and industry
statistics provided in the Minerals Yearbook and Mineral
Industry Surveys (Tepordei 1993 through 2006; Willett
2007a,b;USGS 2007,2008). To develop these data, the U.S.
Geological Survey (USGS; U.S. Bureau of Mines prior to
1997) obtained production and use information by surveying
crushed stone manufacturers. Because some manufacturers
were reluctant to provide information, the estimates of total
crushed limestone and dolomite production and use were
divided into three components: (1) production by end-use,
as reported by manufacturers (i.e., "specified" production);
(2) production reported by manufacturers without end-uses
specified (i.e., "unspecified" production); and (3) estimated
additional production by manufacturers who did not respond
to the survey (i.e., "estimated" production).
The "unspecified" and "estimated" amounts of crushed
limestone and dolomite applied to agricultural soils were
calculated by multiplying the percentage of total "specified"
limestone and dolomite production applied to agricultural
soils by the total amounts of "unspecified" and "estimated"
limestone and dolomite production. In other words, the
proportion of total "unspecified" and "estimated" crushed
limestone and dolomite that was applied to agricultural
soils (as opposed to other uses of the stone) was assumed
to be proportionate to the amount of "specified" crushed
limestone and dolomite that was applied to agricultural
soils. In addition, data were not available for 1990, 1992,
and 2008 on the fractions of total crushed stone production
that were limestone and dolomite, and on the fractions of
limestone and dolomite production that were applied to
soils. To estimate the 1990 and 1992 data, a set of average
fractions were calculated using the 1991 and 1993 data.
These average fractions were applied to the quantity of
"total crushed stone produced or used" reported for 1990
and 1992 in the 1994 Minerals Yearbook (Tepordei 1996).
To estimate 2008 data, the previous year's fractions were
applied to a 2008 estimate of total crushed stone presented
in the USGS Mineral Industry Surveys: Crushed Stone and
Sand and Gravel in the First Quarter of 2009 (USGS 2009).
The primary source for limestone and dolomite activity
data is the Minerals Yearbook, published by the Bureau
of Mines through 1994 and by the USGS from 1995 to
the present. In 1994, the "Crushed Stone" chapter in the
Minerals Yearbook began rounding (to the nearest thousand
metric tons) quantities for total crushed stone produced or
used. It then reported revised (rounded) quantities for each
Table 7-22: Applied Minerals (Million Metric Tons)
Source
Limestone
Dolomite
1990
19.01
2.36
1995
17.30
2.77
2000
15.86
3.81
2005
18.09
1.85
2006
16.54
2.73
2007
17.77
2.84
2008
15.09
2.41
Note: These numbers represent amounts applied to Cropland Remaining Cropland, Land Converted to Cropland, Grassland Remaining Grassland,
Land Converted to Grassland, and Settlements Remaining Settlements. Shaded areas indicate values based on a combination of historical data and
projections. All other values are based on historical data only.
Land Use, Land-Use Change, and Forestry 7-35
-------
of the years from 1990 to 1993. In order to minimize the
inconsistencies in the activity data, these revised production
numbers have been used in all of the subsequent calculations.
Since limestone and dolomite activity data are also available
at the state level, the national-level estimates reported here
were broken out by state, although state-level estimates are
not reported here.
Uncertainty and Time-Series Consistency
Uncertainty regarding limestone and dolomite activity
data inputs was estimated at ±15 percent and assumed to
be uniformly distributed around the inventory estimate
(Tepordei 2003b). Analysis of the uncertainty associated
with the emission factors included the following: the fraction
of agricultural lime dissolved by nitric acid versus the fraction
that reacts with carbonic acid, and the portion of bicarbonate
that leaches through the soil and is transported to the ocean.
Uncertainty regarding the time associated with leaching and
transport was not accounted for, but should not change the
uncertainty associated with CO2 emissions (West 2005). The
uncertainties associated with the fraction of agricultural lime
dissolved by nitric acid and the portion of bicarbonate that
leaches through the soil were each modeled as a smoothed
triangular distribution between ranges of zero percent to 100
percent. The uncertainty surrounding these two components
largely drives the overall uncertainty estimates reported
below. More information on the uncertainty estimates
for Liming of Agricultural Soils is contained within the
Uncertainty Annex.
A Monte Carlo (Tier 2) uncertainty analysis was applied
to estimate the uncertainty of CO2 emissions from liming.
The results of the Tier 2 quantitative uncertainty analysis are
summarized in Table 7-23. CO2 emissions from Liming of
Agricultural Soils in 2008 were estimated to be between 0.1
and 7.7 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of 97 percent below to 102 percent above
the 2008 emission estimate of 3.8 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A QA/QC analysis was performed for data gathering and
input, documentation, and calculation. The QA/QC analysis
did not reveal any inaccuracies or incorrect input values.
Recalculations Discussion
Several adjustments were made in the current Inventory
to improve the results. The quantity of applied minerals
reported in the previous Inventory for 2007 has been revised;
the updated activity data for 2007 are approximately 1,570
thousand metric tons greater than the data used last year.
Consequently, the reported emissions resulting from liming
in 2007 increased by about 11 percent. In the previous
Inventory, to estimate 2007 data, the previous year's fractions
were applied to a 2007 estimate of total crushed stone
presented in the USGS Mineral Industry Surveys: Crushed
Stone and Sand and Gravel in the First Quarter of 2008
(USGS 2008). Since publication of the previous Inventory,
the Minerals Yearbook has published actual quantities
of crushed stone sold or used by producers in the United
States in 2007. These values have replaced those used in
the previous Inventory to calculate the quantity of minerals
applied to soil and the emissions from liming.
Table 7-23: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Liming of Agricultural Soils
(Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Liming of Agricultural Soils'1
C02
30
.0
0.1
7.7
-97%
+ 102%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
b Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to Grassland, and
Settlements Remaining Settlements.
7-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 7-24: C02 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C02 Eq.)
Source
1990
1995
2000
2005
2006
2007
2008
Urea Fertilization3
2.4
2.7
3.2
3.5
3.7
3.8
Table 7-25: C02 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C)
Source
1990
1995
2000
2005
2006
2007
3.8
3 Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to Grassland,
Settlements Remaining Settlements, and Forest Land Remaining Forest Land.
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
2008
Urea Fertilization3
0.7
0.7
0.9
1.0
1.0
1.0
1.0
3 Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to Grassland,
Settlements Remaining Settlements, and Forest Land Remaining Forest Land.
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
C02 Emissions from Urea Fertilization
The use of urea (CO(NH2)2) as fertilizer leads to
emissions of CO2 that was fixed during the industrial
production process. Urea in the presence of water and urease
enzymes is converted into ammonium (NH4+), hydroxyl
ion (OH"), and bicarbonate (HCO3~). The bicarbonate then
evolves into CO2 and water. Emissions from urea fertilization
in the United States totaled 3.8 Tg CO2 Eq. (1.0 Tg C) in 2008
(Table 7-24 and Table 7-25). Emissions from urea
Methodology
Carbon dioxide emissions from the application of urea
to agricultural soils were estimated using the IPCC (2006)
Tier 1 methodology. The annual amounts of urea fertilizer
applied (see Table 7-26) were derived from state-level
fertilizer sales data provided in Commercial Fertilizers (TVA
1991, 1992, 1993, 1994; AAPFCO 1995 through 2008)
and were multiplied by the default IPCC (2006) emission
factor of 0.20, which is equal to the C content of urea on
an atomic weight basis. Because fertilizer sales data are
reported in fertilizer years (July through June), a calculation
was performed to convert the data to calendar years (January
through December). According to historic monthly fertilizer
Table 7-26: Applied Urea (Million Metric Tons)
use data (TVA 1992b), 65 percent of total fertilizer used
in any fertilizer year is applied between January through
June of that calendar year, and 35 percent of total fertilizer
used in any fertilizer year is applied between July through
December of the previous calendar year. Fertilizer sales data
for the 2008 and 2009 fertilizer years were not available
in time for publication. Accordingly, urea application in
the 2008 fertilizer year was assumed to be equal to that
of the 2007 fertilizer year. Since 2009 fertilizer year data
were not available, July through December 2008 fertilizer
consumption was assumed to be equal to July through
December 2007 fertilizer consumption. State-level estimates
of CO2 emissions from the application of urea to agricultural
soils were summed to estimate total emissions for the entire
United States.
Uncertainty and Time-Series Consistency
Uncertainty estimates are presented in Table 7-27 for
Urea Fertilization. A Tier 2 Monte Carlo analysis was
completed. The largest source of uncertainty was the default
emission factor, which assumes that 100 percent of the C
applied to soils is ultimately emitted into the environment
as CO2. This factor does not incorporate the possibility that
some of the C may be retained in the soil. The emission
1990
1995
2000
2005
2006
2007
2008
Urea Fertilization3
3.30
3.62
4.38
4.78
4.98
5.19
5.19
3 Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to Grassland,
Settlements Remaining Settlements, and Forest Land Remaining Forest Land.
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
Land Use, Land-Use Change, and Forestry 7-37
-------
Table 7-27: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Urea Fertilization (Tg C02 Eq. and Percent)
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Urea Fertilization
CO,
30
.0
2.2
3.9
-43%
+ 3%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Note: These numbers represent amounts applied to all agricultural land, including Land Converted to Cropland, Grassland Remaining Grassland,
Land Converted to Grassland, Settlements Remaining Settlements, and Forest Land Remaining Forest Land.
estimate is, thus, likely to be high. In addition, each urea
consumption data point has an associated uncertainty.
Urea for non-fertilizer use, such as aircraft deicing, may be
included in consumption totals; it was determined through
personal communication with Fertilizer Regulatory Program
Coordinator, David L. Terry (2007), however, that this
amount is most likely very small. Research into aircraft
deicing practices also confirmed that urea is used minimally
in the industry; a 1992 survey found a known annual usage
of approximately 2,000 tons of urea for deicing; this would
constitute 0.06 percent of the 1992 consumption of urea
(EPA 2000). Similarly, surveys conducted from 2002 to
2005 indicate that total urea use for deicing at U.S. airports
is estimated to be 3,740 MT per year, or less than 0.07
percent of the fertilizer total for 2007 (Me 2009). Lastly,
there is uncertainty surrounding the assumptions behind the
calculation that converts fertilizer years to calendar years.
CO2 emissions from urea fertilization of agricultural soils in
2008 were estimated to be between 2.2 and 3.9 Tg CO2 Eq.
at the 95 percent confidence level. This indicates a range
of 43 percent below to 3 percent above the 2008 emission
estimate of 3.8 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A QA/QC analysis was performed for data gathering and
input, documentation, and calculation. Inventory reporting
forms and text were reviewed. No errors were found.
Recalculations Discussion
July to December 2007 urea application data were
updated with assumptions for fertilizer year 2008, and the
2007 emission estimate was revised accordingly. The activity
data decreased about 200,000 metric tons for this year and
this change resulted in an approximately 4 percent decrease
in emissions in 2007 relative to the previous Inventory. In
the previous Inventory, the application for this period was
calculated based on application during July to December
2006.
Planned Improvements
The primary planned improvement is to investigate using
a Tier 2 or Tier 3 approach, which would utilize country-
specific information to estimate a more precise emission
factor.
7.5. Land Converted to Cropland
(IPCC Source Category 5B2)
Land Converted to Cropland includes all cropland in an
inventory year that had been another land use at any point
during the previous 20 years33 according to the USDA NRI
land-use survey (USDA-NRCS 2000). Consequently, lands
are retained in this category for 20 years as recommended
by the IPCC guidelines (IPCC 2006) unless there is another
land-use change. The Inventory includes all privately-owned
croplands in the conterminous United States and Hawaii,
but there is a minor amount of cropland on federal lands,
which is not currently included in the estimation of C stock
33 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began, and consequently the classifications
were based on less than 20 years from 1990 to 2001.
7-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 7-28: Net C02 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg C02 Eq.)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(0.3)
2.4
2.2
1995
0.3
2.6
2.9
2000
(0.3)
2.6
2.4
2005
3.3
2.6
5.9
2006
3.3
2.6
5.9
2007
3.3
2.6
5.9
2008
3.3
2.6
5.9
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections. All
other values are based on historical data only. Totals may not sum due to independent rounding.
Table 7-29: Net C02 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg C)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections. All
other values are based on historical data only. Totals may not sum due to independent rounding.
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(0.1)
0.7
0.6 |
1995
Ł
0.8
2000
(0.1)
0.7
0.6
2005
0.9
0.7
1.6
2006
0.9
0.7
1.6
2007
0.9
0.7
1.6
2008
0.9
0.7
1.6
changes. It is important to note that these areas are part of
the managed land base for the United States, as described in
Section 7.1, and plans are being made to include these areas
in future C inventories.
Background on agricultural C stock changes is
provided in Cropland Remaining Cropland and will only
be summarized here for Land Converted to Cropland. Soils
are the largest pool of C in agricultural land, and also have
the greatest potential for storage or release of C, because
biomass and dead organic matter C pools are relatively
small and ephemeral compared with soils. The IPCC (2006)
recommends reporting changes in soil organic C stocks due
to: (1) agricultural land-use and management activities on
mineral soils, and (2) agricultural land-use and management
activities on organic soils.34
Land-use and management of mineral soils in Land
Converted to Cropland generally led to relatively small
increases in soil C during the 1990s but the pattern changed
to small losses of C through the latter part of the time series
(Table 7-28 and Table 7-29). The total rate of change in soil
C stocks was 5.9 Tg CO2 Eq. (1.6 Tg C) in 2008. Mineral
soils were estimated to lose 3.3 Tg CO2 Eq. (0.9 Tg C) in
2008, while drainage and cultivation of organic soils led to
annual losses of 2.6 Tg CO2 Eq. (0.7 Tg C) in 2008.
34 Carbon dioxide emissions associated with liming are also estimated but
included in a separate section of the report.
The spatial variability in annual CO2 flux associated
with C stock changes in mineral and organic soils for Land
Converted to Cropland is displayed in Figure 7-7 and Figure
7-8. While a large portion of the United States had net losses
of soil C for Land Converted to Cropland, there were some
notable areas with net C accumulation in the Great Plains,
Midwest, mid-Atlantic states. These areas were gaining C
following conversion, because the land had been brought into
hay production, including grass and legume hay, leading to
enhanced plant production relative to the previous land use,
and thus higher C input to the soil. Emissions from organic
soils were largest in California, Florida, and the upper
Midwest, which coincided with largest concentrations of
cultivated organic soils in the United States.
Methodology
The following section includes a brief description of the
methodology used to estimate changes in soil C stocks due to
agricultural land-use and management activities on mineral
and organic soils for Land Converted to Cropland. Further
elaboration on the methodologies and data used to estimate
stock changes for mineral and organic soils are provided in
the Cropland Remaining Cropland section and Annex 3.13.
Soil C stock changes were estimated for Land Converted
to Cropland according to land-use histories recorded in
the USDA NRI survey (USDA-NRCS 2000). Land-use
and some management information (e.g., crop type, soil
Land Use, Land-Use Change, and Forestry 7-39
-------
Figure 7-7
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States,
2008, Land Converted to Cropland
Tg C02Eq./year
D> 0
D-0.1 toO
D-0.5 to-0.1
• -1to-0.5
Note: Values greater than zero represent emissions, and values less than zero represent sequestration. Map accounts for fluxes associated with the
Tier 2 and 3 Inventory computations. See Methodology for additional details.
Figure 7-8
Total Net Annual C02 Flux For Organic Soils Under Agricultural Management within States,
2008, Land Converted to Cropland
Note: Values greater than zero represent emissions.
Tg C02Eq./year
• 0.5 to 1
D 0.1 to 0.5
Do to 0.1
EH No organic soils
7-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
attributes, and irrigation) were originally collected for each
NRI point on a 5-year cycle beginning in 1982. However,
the NRI program initiated annual data collection in 1998,
and the annual data are currently available through 2003.
NRI points were classified as Land Converted to Cropland
in a given year between 1990 and 2008 if the land use was
cropland but had been another use during the previous 20
years. Cropland includes all land used to produce food or
fiber, or forage that is harvested and used as feed (e.g., hay
and silage).
Mineral Soil Carbon Stock Changes
A Tier 3 model-based approach was applied to estimate
C stock changes for soils on Land Converted to Cropland
used to produce a majority of all crops (Ogle et al. 2009). Soil
C stock changes on the remaining soils were estimated with
the IPCC Tier 2 method (Ogle et al. 2003), including land
used to produce vegetable, tobacco, perennial/horticultural
crops, and rice; land on very gravelly, cobbly, or shaley soils
(greater than 35 percent by volume); and land converted from
forest or federal ownership.35
Tier 3 Approach
Mineral SOC stocks and stock changes were estimated
using the Century biogeochemical model for the Tier 3
methods. National estimates were obtained by using the
model to simulate historical land-use change patterns as
recorded in the USDA National Resources Inventory (USDA-
NRCS 2000). The methods used for Land Converted to
Cropland are the same as those described in the Tier 3 portion
of Cropland Remaining Cropland section for mineral soils
(see Cropland Remaining Cropland Tier 3 methods section
and Annex 3.13 for additional information).
Tier 2 Approach
For the mineral soils not included in the Tier 3 analysis,
SOC stock changes were estimated using a Tier 2 Approach
for Land Converted to Cropland as described in the Tier 2
portion of Cropland Remaining Cropland section for mineral
35 .Federal land is not a land use, but rather an ownership designation that
is treated as forest or nominal grassland for purposes of these calculations.
The specific use for federal lands is not identified in the NRI survey (USDA-
NRCS 2000).
soils (see Cropland Remaining Cropland Tier 2 methods
section for additional information).
Organic Soil Carbon Stock Changes
Annual C emissions from drained organic soils in Land
Converted to Cropland were estimated using the Tier 2
method provided in IPCC (2003, 2006), with U.S.-specific
C loss rates (Ogle et al. 2003) rather than default IPCC rates.
The final estimates included a measure of uncertainty as
determined from the Monte Carlo simulation with 50,000
iterations. Emissions were based on the 1992 and 1997
Land Converted to Cropland areas from the 1997 National
Resources Inventory (USDA-NRCS 2000). The annual
flux estimated for 1992 was applied to 1990 through 1992,
and the annual flux estimated for 1997 was applied to 1993
through 2008.
Uncertainty and Time-Series Consistency
Uncertainty analysis for mineral soil C stock changes
using the Tier 3 and Tier 2 approaches were based on the
same method described for Cropland Remaining Cropland,
except that the uncertainty inherent in the structure of the
Century model was not addressed. The uncertainty for
annual C emission estimates from drained organic soils in
Land Converted to Cropland was estimated using the Tier 2
approach, as described in the Cropland Remaining Cropland
section.
Uncertainty estimates are presented in Table 7-30 for
each subsource (i.e., mineral soil C stocks and organic
soil C stocks) disaggregated to the level of the inventory
methodology employed (i.e., Tier 2 and Tier 3). Uncertainty
for the portions of the Inventory estimated with Tier 2 and 3
approaches was derived using a Monte Carlo approach (see
Annex 3.13 for further discussion). A combined uncertainty
estimate for changes in agricultural soil C stocks is also
included. Uncertainty estimates from each component were
combined using the error propagation equation in accordance
with IPCC (2006), i.e., by taking the square root of the sum
of the squares of the standard deviations of the uncertain
quantities. The combined uncertainty for soil C stocks in
Land Converted to Cropland was estimated to be 40 percent
below and 36 percent above the inventory estimate of 5.9
Tg C02 Eq.
Land Use, Land-Use Change, and Forestry 7-41
-------
Table 7-30: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Land Converted to
Cropland (Tg C02 Eq. and Percent)
Source
Mineral Soil C Stocks: Land Converted to Cropland,
Tier 3 Inventory Methodology
Mineral Soil C Stocks: Land Converted to Cropland,
Tier 2 Inventory Methodology
Organic Soil C Stocks: Land Converted to Cropland,
Tier 2 Inventory Methodology
2008 Flux Estimate
(Tg C02 Eq.)
(0.8)
4.1
2.6
Uncertainty Range Relative to Flux Estimate
(Tg
Lower
Bound
(1.5)
2.3
1.2
C02 Eq.)
Upper
Bound
(0.1)
5.8
3.7
("'
Lower
Bound
-84%
-44%
-53%
6)
Upper
Bound
+84%
+41%
+41%
Combined Uncertainty for Flux Associated with
Soil Carbon Stock Change in Land Converted
to Cropland
5.9
3.5
8.1
-40%
+36%
Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
See QA/QC and Verification section under Cropland
Remaining Cropland.
Planned Improvements
The empirically-based uncertainty estimator described
in the Cropland Remaining Cropland section for the Tier 3
approach has not been developed to estimate uncertainties
related to the structure of the Century model for Land
Converted to Cropland, but this is a planned improvement.
This improvement will produce a more rigorous assessment
of uncertainty. See Planned Improvements section under
Cropland Remaining Cropland for additional planned
improvements.
7.6. Grassland Remaining Grassland
(IPCC Source Category 5C1)
Grassland Remaining Grassland includes all grassland
in an inventory year that had been grassland for the
previous 20 years36 according to the USDA NRI land use
survey (USDA-NRCS 2000). The Inventory includes all
privately-owned grasslands in the conterminous United
States and Hawaii, but does not address changes in C stocks
for grasslands on federal lands. It is important to note that
these areas are part of the managed land base for the United
States, as described in Section 7.1. While federal grasslands
probably have minimal changes in land management and
C stocks, plans are being made to further evaluate and
potentially include these areas in future C inventories.
Background on agricultural C stock changes is provided
in the Cropland Remaining Cropland section and will only
be summarized here for Grassland Remaining Grassland.
Soils are the largest pool of C in agricultural land, and also
have the greatest potential for storage or release of C, because
biomass and dead organic matter C pools are relatively
small and ephemeral compared to soils. IPCC (2006)
recommends reporting changes in soil organic C stocks due
to: (1) agricultural land-use and management activities on
mineral soils, and (2) agricultural land-use and management
activities on organic soils.37
Land-use and management of mineral soils in Grassland
Remaining Grassland increased soil C, while organic soils
lost relatively small amounts of C in each year 1990 through
2008. Due to the pattern for mineral soils, the overall trend
was a gain in soil C over the time series although the rates
varied from year to year, with a net removal of 8.7 Tg CO2
Eq. (1.3 Tg C) in 2008 (Table 7-31 and Table 7-32). There
36 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began, and consequently the classifcations
were based on less than 20 years from 1990 to 2001.
37 Carbon dioxide missions associated with liming are also estimated but
included in a separate section of the report.
7-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 7-31: Net C02 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg C02 Eq.)
Soil Type
1990
1995
2000
2005
2006
2007
2008
Mineral Soils
Organic Soils
(55.9)
3.9
(30.4)
3.7 •
(56.3)
3.7
(12.7)
3.7
(12.6)
3.7
(12.5)
3.7
(12.4)
3.7
Total Net Flux
(52.0)
(26.7)
(52.6)
(9.0)
(8.9) (8.8)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections.
All other values are based on historical data only. Totals may not sum due to independent rounding.
(8.7)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections.
All other values are based on historical data only. Totals may not sum due to independent rounding.
Table 7-32: Net C02 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg C)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(15.2)
1.1
(14.2)
1995
(8.3)
1.0
(7.3)|
2000
(15.4)
1.0
(14.3)
2005
(3.5)
1.0
(2.5)
2006
(3.4)
1.0
(2.4)
2007
(3.4)
1.0
(2.4)
2008
(3.4)
1.0
(2.4)
was considerable variation over the time series driven by
variability in weather patterns and associated interaction with
land management activity. The change rates on per hectare
basis were small, however, even in the years with larger total
changes in stocks. Overall, flux rates declined by 43.3 Tg
CO2 Eq. (11.9 Tg C) when comparing the net change in soil
C for 1990 and 2008.
The spatial variability in annual CO2 flux associated with
C stock changes in mineral and organic soils is displayed in
Figure 7-9 and Figure 7-10.
Methodology
The following section includes a brief description of the
methodology used to estimate changes in soil C stocks due to
agricultural land-use and management activities on mineral
and organic soils for Grassland Remaining Grassland.
Further elaboration on the methodologies and data used to
estimate stock changes from mineral and organic soils are
provided in the Cropland Remaining Cropland section and
Annex 3.13.
Soil C stock changes were estimated for Grassland
Remaining Grassland according to land-use histories
recorded in the USDA NRI survey (USDA-NRCS 2000).
Land-use and some management information (e.g., crop
type, soil attributes, and irrigation) were originally collected
for each NRI point on a 5-year cycle beginning in 1982.
However, the NRI program initiated annual data collection
in 1998, and the annual data are currently available through
2003. NRI points were classified as Grassland Remaining
Grassland in a given year between 1990 and 2008 if the land
use had been grassland for 20 years. Grassland includes
pasture and rangeland used for grass forage production,
where the primary use is livestock grazing. Rangelands
are typically extensive areas of native grassland that are
not intensively managed, while pastures are often seeded
grassland, possibly following tree removal, that may or
may not be improved with practices such as irrigation and
interseeding legumes.
Mineral Soil Carbon Stock Changes
An IPCC Tier 3 model-based approach was applied to
estimate C stock changes for most mineral soils in Grassland
Remaining Grassland. The C stock changes for the remaining
soils were estimated with an IPCC Tier 2 method (Ogle et
al. 2003), including gravelly, cobbly, or shaley soils (greater
than 35 percent by volume) and additional stock changes
associated with sewage sludge amendments.
Tier 3 Approach
Mineral soil organic C stocks and stock changes for
Grassland Remaining Grassland were estimated using the
Century biogeochemical model, as described in Cropland
Remaining Cropland. Historical land-use and management
patterns were used in the Century simulations as recorded in
\heUSDANationalResourcesInventory (NRI) survey, with
Land Use, Land-Use Change, and Forestry 7-43
-------
Figure 7-9
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States,
2008, Grassland Remaining Grassland
o
Note: Values greater than zero represent emissions, and values less than zero represent sequestration. Map accounts for fluxes associated with the
Tier 2 and 3 inventory computations. See Methodology for additional details.
Figure 7-10
Total Net Annual C02 Flux For Organic Soils Under Agricultural Management within States,
2008, Grassland Remaining Grassland
Note: Values greater than zero represent emissions.
Tg C02 Eq./year
lto2
0.5 to 1
DO.1 to 0.5
DO to 0.1
Q No organic soils
7-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
supplemental information on fertilizer use and rates from
the USDA Economic Research Service Cropping Practices
Survey (ERS 1997) and National Agricultural Statistics
Service (NASS 1992, 1999, 2004). Frequency and rates of
manure application to grassland during 1997 were estimated
from data compiled by the USDA Natural Resources
Conservation Service (Edmonds, et al. 2003), and then
adjusted using county-level estimates of manure available
for application in other years. Specifically, county-scale
ratios of manure available for application to soils in other
years relative to 1997 were used to adjust the area amended
with manure (see Annex 3.13 for further details). Greater
availability of managed manure N relative to 1997 was,
thus, assumed to increase the area amended with manure,
while reduced availability of manure N relative to 1997 was
assumed to reduce the amended area.
The amount of manure produced by each livestock
type was calculated for managed and unmanaged waste
management systems based on methods described in the
Manure Management Section (Section 6.2) and Annex
(Annex 3.10). In contrast to manure amendments, pasture/
range/paddock (PRP) manure N deposition was estimated
internally in the Century model, as part of the grassland
system simulations (i.e., PRP manure deposition was not
an external input into the model). See the Tier 3 methods
in Cropland Remaining Cropland section for additional
discussion on the Tier 3 methodology for mineral soils.
Tier 2 Approach
The Tier 2 approach is based on the same methods
described in the Tier 2 portion of Cropland Remaining
Cropland section for mineral soils (see Cropland Remaining
Cropland Tier 2 methods section and Annex 3.13 for
additional information).
Additional Mineral C Stock Change Calculations
Annual C flux estimates for mineral soils between 1990
and 2008 were adjusted to account for additional C stock
changes associated with sewage sludge amendments using
a Tier 2 method. Estimates of the amounts of sewage sludge
N applied to agricultural land were derived from national
data on sewage sludge generation, disposition, and nitrogen
content. Total sewage sludge generation data for 1988,1996,
and 1998, in dry mass units, were obtained from an EPA
report (EPA 1999) and estimates for 2004 were obtained
from an independent national biosolids survey (NEBRA
2007). These values were linearly interpolated to estimate
values for the intervening years. N application rates from
Kellogg et al. (2000) were used to determine the amount of
area receiving sludge amendments. Although sewage sludge
can be added to land managed for other land uses, it was
assumed that agricultural amendments occur in grassland.
Cropland is assumed to rarely be amended with sewage
sludge due to the high metal content and other pollutants
in human waste. The soil C storage rate was estimated at
0.38 metric tons C per hectare per year for sewage sludge
amendments to grassland. The stock change rate is based on
country-specific factors and the IPCC default method (see
Annex 3.13 for further discussion).
Organic Soil Carbon Stock Changes
Annual C emissions from drained organic soils in
Grassland Remaining Grassland were estimated using the
Tier 2 method provided in IPCC (2003,2006), which utilizes
U.S.-specific C loss rates (Ogle et al. 2003) rather than
default IPCC rates. Emissions were based on the 1992 and
1997 Grassland Remaining Grassland areas from the 1997
National Resources Inventory (USDA-NRCS 2000). The
annual flux estimated for 1992 was applied to 1990 through
1992, and the annual flux estimated for 1997 was applied to
1993 through 2008.
Uncertainty and Time-Series Consistency
Uncertainty estimates are presented in Table 7-33 for
each subsource (i.e., mineral soil C stocks and organic
soil C stocks) disaggregated to the level of the inventory
methodology employed (i.e., Tier 2 and Tier 3). Uncertainty
for the portions of the Inventory estimated with Tier 2 and 3
approaches was derived using a Monte Carlo approach (see
Annex 3.13 for further discussion). A combined uncertainty
estimate for changes in agricultural soil C stocks is also
included. Uncertainty estimates from each component were
combined using the error propagation equation in accordance
with IPCC (2006), i.e., by taking the square root of the sum
of the squares of the standard deviations of the uncertain
quantities. The combined uncertainty for soil C stocks in
Grassland Remaining Grassland was estimated to be 29
Land Use, Land-Use Change, and Forestry 7-45
-------
Table 7-33: Tier 2 Quantitative Uncertainty Estimates for C Stock Changes occurring within Grassland Remaining
Grassland (Tg C02 Eq. and Percent)
2008 Flux Estimate Uncertainty Range Relative to Flux Estimate
Source (Tg C02 Eq.) (Tg C02 Eq.) (%)
Mineral Soil C Stocks Grassland Remaining Grassland,
Tier 3 Methodology
Mineral Soil C Stocks: Grassland Remaining Grassland,
Tier 2 Methodology
Mineral Soil C Stocks: Grassland Remaining Grassland,
Tier 2 Methodology (Change in Soil C due to Sewage
Sludge Amendments)
Organic Soil C Stocks: Grassland Remaining Grassland,
Tier 2 Methodology
(11.0)
(0.2)
(1.2)
3.7
Lower
Bound
(11.2)
(0.3)
(1.8)
1.2
Upper
Bound
(10.8)
0.0
(0.6)
5.5
Lower
Bound
-2%
-89%
-50%
-66%
Upper
Bound
+2%
+ 127%
+50%
+49%
Combined Uncertainty for Flux Associated with
Agricultural Soil Carbon Stock Change in Grassland
Remaining Grassland
(8.7)
(11.2)
(6.8)
-29% +22%
Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
percent below and 22 percent above the inventory estimate
of-8.7TgCO2Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Uncertainties in Mineral Soil Carbon Stock Changes
The uncertainty analysis for Grassland Remaining
Grassland using the Tier 3 approach and Tier 2 approach
were based on the same method described for Cropland
Remaining Cropland, except that the uncertainty inherent in
the structure of the Century model was not addressed. See
the Tier 3 approach for mineral soils under the Cropland
Remaining Cropland section for additional discussion.
A ±50 percent uncertainty was assumed for additional
adjustments to the soil C stocks between 1990 and 2008
to account for additional C stock changes associated with
amending grassland soils with sewage sludge.
Uncertainties in Soil Carbon Stock Changes for
Organic Soils
Uncertainty in C emissions from organic soils was
estimated using country-specific factors and a Monte Carlo
analysis. Probability distribution functions for emission
factors were derived from a synthesis of 10 studies, and
combined with uncertainties in the NRI land use and
management data for organic soils in the Monte Carlo
analysis. See the Tier 2 section under minerals soils of
Cropland Remaining Cropland for additional discussion.
QA/QC and Verification
Quality control measures included checking input data,
model scripts, and results to ensure data were properly
handled through the inventory process. No additional errors
were found in this Inventory.
Recalculations Discussion
The estimated area of grasslands changed across the time
series relative to the previous Inventory due to revisions in the
forest land definition. This adjustment reduced the area of
grassland in the United States because woodlands previously
designated as grassland are now considered forest land. The
revised areas altered the estimated soil C stock changes in
Grassland Remaining Grassland by an average of 1.2 Tg
CO2 eq. or 3 percent over the time series from 1990 to 2007,
relative to the previous Inventory.
Planned Improvements
The empirically based uncertainty estimator described
in the Cropland Remaining Cropland section for the Tier 3
approach has not been developed to estimate uncertainties in
Century model results for Grassland Remaining Grassland,
7-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
but this is a planned improvement for the Inventory. This
improvement will produce a more rigorous assessment
of uncertainty. See Planned Improvements section under
Cropland Remaining Cropland for additional planned
improvements.
7.7. Land Converted to Grassland
(IPCC Source Category 5C2)
Land Converted to Grassland includes all grassland in an
inventory year that had been in another land use at any point
during the previous 20 years38 according to the USDA NRI
land-use survey (USDA-NRCS 2000). Consequently, lands
are retained in this category for 20 years as recommended
by IPCC (2006) unless there is another land use change.
The Inventory includes all privately-owned grasslands in the
conterminous United States and Hawaii, but does not address
changes in C stocks for grasslands on federal lands. It is
important to note that these areas are part of the managed land
base for the United States, as described in Section 7.1. Land
use can lead to significant changes in C stocks, and plans are
being made to include these areas in future C inventories.
Background on agricultural C stock changes is
provided in Cropland Remaining Cropland and will only be
summarized here for Land Converted to Grassland. Soils
are the largest pool of C in agricultural land, and also have
the greatest potential for storage or release of C, because
biomass and dead organic matter C pools are relatively
small and ephemeral compared with soils. IPCC (2006)
recommend reporting changes in soil organic C stocks due
to: (1) agricultural land-use and management activities on
mineral soils, and (2) agricultural land-use and management
activities on organic soils.39
Land-use and management of mineral soils in Land
Converted to Grassland led to an increase in soil C stocks
from 1990 through 2008, which was largely due to annual
cropland conversion to pasture (see Table 7-34 and Table
7-35). For example, the stock change rates were estimated
to remove 20.3 Tg CO2 Eq./yr (5.5 Tg C) and 25.1 Tg
CO2 Eq./yr (6.8 Tg C) from mineral soils in 1990 and
2008, respectively. Drainage of organic soils for grazing
management led to losses varying from 0.5 to 0.9 Tg CO2
Eq./yr (0.1 to 0.2 TgC).
The spatial variability in annual CO2 flux associated with
C stock changes in mineral soils is displayed in Figure 7-11
and Figure 7-12. Soil C stock increased in most states for
Land Converted to Grassland. The largest gains were in the
South-Central region, Midwest, and northern Great Plains.
The patterns were driven by conversion of annual cropland
into continuous pasture. Emissions from organic soils
were largest in California, Florida, and the upper Midwest,
coinciding with largest concentrations of organic soils in
the United States that are used for agricultural production.
Methodology
38 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began, and consequently the classifcations
were based on less than 20 years from 1990 to 2001.
39 Carbon dioxide emissions associated with liming are also estimated but
included in a separate section of the report.
This section includes a brief description of the
methodology used to estimate changes in soil C stocks
due to agricultural land-use and management activities on
mineral soils for Land Converted to Grassland. Biomass C
stock changes are not explicitly included in this category but
losses of associated with conversion of forest to grassland are
included in the Forest Land Remaining Forest Land section.
Further elaboration on the methodologies and data used to
estimate stock changes from mineral and organic soils are
provided in the Cropland Remaining Cropland section and
Annex 3.13.
Soil C stock changes were estimated for Land Converted
to Grassland according to land-use histories recorded in
the USDA NRI survey (USDA-NRCS 2000). Land-use
and some management information (e.g., crop type, soil
attributes, and irrigation) were originally collected for each
NRI point on a 5-year cycle beginning in 1982. However,
the NRI program initiated annual data collection in 1998,
and the annual data are currently available through 2003.
NRI points were classified as Land Converted to Grassland
in a given year between 1990 and 2008 if the land use was
grassland, but had been another use in the previous 20
years. Grassland includes pasture and rangeland used for
grass forage production, where the primary use is livestock
grazing. Rangeland typically includes extensive areas of
native grassland that are not intensively managed, while
pastures are often seeded grassland, possibly following tree
Land Use, Land-Use Change, and Forestry 7-47
-------
Table 7-34: Net C02 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C02 Eq.)
Soil Type
Mineral Soils3' b
Organic Soils
Total Net Fluxb
1990
(20.3)
0.5
(19.8)
1995
(23.2)
0.9
(22.3)
2000
(28.1)
0.9
(27.3)
2005
(25.5)
0.9
(24.6)
2006
(25.4)
0.9
(24.5)
2007
(25.2)
0.9
(24.3)
2008
(25.1)
0.9
(24.2)
3 Stock changes due to application of sewage sludge are reported in Grassland Remaining Grassland.
b Preliminary estimates that will be finalized after public review period following completion of quality control measures.
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections. All other values
are based on historical data only. Totals may not sum due to independent rounding.
Table 7-35: Net C02 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C)
Soil Type
Mineral Soils3' b
Organic Soils
Total Net Fluxb
1990
(5.5)
0.1
(5.4)
1995
(6.3)
0.2
(6.1)
2000
(7.7)
0.2
(7.4)
2005
(7.0)
0.2
(6.7)
2006
(6.9)
0.2
(6.7)
2007
(6.9)
0.2
(6.6)
2008
(6.8)
0.2
(6.6)
3 Stock changes due to application of sewage sludge are reported in Grassland Remaining Grassland.
b Preliminary estimates that will be finalized after public review period following completion of quality control measures.
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and projections. All other values
are based on historical data only. Totals may not sum due to independent rounding.
removal, that may or may not be improved with practices
such as irrigation and interseeding legumes.
Mineral Soil Carbon Stock Changes
An IPCC Tier 3 model-based approach was applied to
estimate C stock changes for Land Converted to Grassland on
most mineral soils. C stock changes on the remaining soils
were estimated with an IPCC Tier 2 approach (Ogle et al.
2003), including prior cropland used to produce vegetables,
tobacco, perennial/horticultural crops, and rice; land areas
with very gravelly, cobbly, or shaley soils (greater than 35
percent by volume); and land converted from forest or federal
ownership.40 A Tier 2 approach was also used to estimate
additional changes in mineral soil C stocks due to sewage
sludge amendments. However, stock changes associated with
sewage sludge amendments are reported in the Grassland
Remaining Grassland section.
Tier 3 Approach
Mineral SOC stocks and stock changes were estimated
using the Century biogeochemical model as described for
40 Federal land is not a land use, but rather an ownership designation that
is treated as forest or nominal grassland for purposes of these calculations.
The specific use for federal lands is not identified in the NRI survey (USDA-
NRCS 2000).
Grassland Remaining Grassland. Historical land-use and
management patterns were used in the Century simulations
as recorded in the NRI survey, with supplemental information
on fertilizer use and rates from the USDA Economic
Research Service Cropping Practices Survey (ERS 1997)
and the National Agricultural Statistics Service (NASS 1992,
1999, 2004) (see Grassland Remaining Grassland Tier 3
methods section for additional information).
Tier 2 Approach
The Tier 2 approach used for Land Converted to
Grassland on mineral soils is the same as described for
Cropland Remaining Cropland (See Cropland Remaining
Cropland Tier 2 Approach and Annex 3.13 for additional
information).
Organic Soil Carbon Stock Changes
Annual C emissions from drained organic soils in Land
Converted to Grassland were estimated using the Tier 2
method provided in IPCC (2003,2006), which utilizes U.S .-
specific C loss rates (Ogle et al. 2003) rather than default
IPCC rates. Emissions were based on the 1992 and 1997
Land Converted to Grassland areas from the 1997 National
Resources Inventory (USDA-NRCS 2000). The annual
flux estimated for 1992 was applied to 1990 through 1992,
7-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Figure 7-11
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States,
2008, Land Converted to Grassland
Tg C02Eq./year
D-0.1 to 0
D-0.5 to -0.1
D-1 to -0.5
G-2to-1
Note: Values greater than zero represent emissions, and values less than zero represent sequestration. Map accounts for fluxes associated with the
Tier 2 and 3 inventory computations. See Methodology for additional details.
Figure 7-12
Total Net Annual C02 Flux For Organic Soils Under Agricultural Management within States,
2008, Land Converted to Grassland
o
Note: Values greater than zero represent emissions.
Tg C02 Eq./year
• o.5 to 1
D 0.1 to 0.5
Do to 0.1
CH No organic soils
Land Use, Land-Use Change, and Forestry 7-49
-------
and the annual flux estimated for 1997 was applied to 1993
through 2008.
Uncertainty and Time-Series Consistency
Uncertainty analysis for mineral soil C stock changes
using the Tier 3 and Tier 2 approaches were based on the
same method described in Cropland Remaining Cropland,
except that the uncertainty inherent in the structure of the
Century model was not addressed. The uncertainty or
annual C emission estimates from drained organic soils
in Land Converted to Grassland was estimated using the
Tier 2 approach, as described in the Cropland Remaining
Cropland section.
Uncertainty estimates are presented in Table 7-36 for
each subsource (i.e., mineral soil C stocks and organic
soil C stocks), disaggregated to the level of the inventory
methodology employed (i.e., Tier 2 and Tier 3). Uncertainty
for the portions of the Inventory estimated with Tier 2 and 3
approaches was derived using a Monte Carlo approach (see
Annex 3.13 for further discussion). A combined uncertainty
estimate for changes in agricultural soil C stocks is also
included. Uncertainty estimates from each component were
combined using the error propagation equation in accordance
with IPCC (2006), (i.e., by taking the square root of the sum
of the squares of the standard deviations of the uncertain
quantities). The combined uncertainty for soil C stocks in
Land Converted to Grassland ranged from 9 percent below
to 10 percent above the 2008 estimate of -24.2 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
See the QA/QC and Verification section under Grassland
Remaining Grassland.
Recalculations Discussion
The estimated area of grasslands changed across the
time series relative to the previous Inventory due to revisions
in the forest land definition. This adjustment reduced the
area of grassland in the United States because woodlands
previously designated as grassland are now considered forest
land. The revised areas altered the estimated soil C stock
changes in Land Converted to Grassland by an average of
2.0 Tg CO2 eq. or 8 percent over the time series from 1990
to 2007, relative to the previous Inventory.
Planned Improvements
The empirically-based uncertainty estimator described
in the Cropland Remaining Cropland section for the Tier 3
approach has not been developed to estimate uncertainties
in Century model results for Land Converted to Grassland,
but this is a planned improvement for the Inventory. This
improvement will produce a more rigorous assessment
of uncertainty. See Planned Improvements section under
Cropland Remaining Cropland for additional planned
improvement.
Table 7-36: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Land Converted to
Grassland (Tg C02 Eq. and Percent)
2008 Flux Estimate Uncertainty Range Relative to Flux Estimate
Source (Tg C02 Eq.) (Tg C02 Eq.) (%)
Mineral Soil C Stocks: Land Converted to Grassland,
Tier 3 Inventory Methodology
Mineral Soil C Stocks: Land Converted to Grassland,
Tier 2 Inventory Methodology
Organic Soil C Stocks: Land Converted to Grassland,
Tier 2 Inventory Methodology
(20.1)
(5.0)
0.9
Lower
Bound
(20.7)
(7.0)
0.2
Upper
Bound
(19.5)
(2.8)
1.8
Lower
Bound
-3%
-39%
-76%
Upper
Bound
+ 3%
+43%
+ 104%
Combined Uncertainty for Flux Associated with
Agricultural Soil Carbon Stocks in Land Converted
to Grassland
(24.2)
(26.4)
(21.8)
-9%
+10%
Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
7-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
7.8. Wetlands Remaining Wetlands
Peatlands Remaining Peatlands
Emissions from Managed Peatlands
Managed peatlands are peatlands which have been
cleared and drained for the production of peat. The
production cycle of a managed peatland has three phases:
land conversion in preparation for peat extraction (e.g.,
draining, and clearing surface biomass), extraction (which
results in the emissions reported under Peatlands Remaining
Peatlands), and abandonment, restoration or conversion of
the land to another use.
Carbon dioxide emissions from the removal of biomass
and the decay of drained peat constitute the major greenhouse
gas flux from managed peatlands. Managed peatlands may
also emit CH4 and N2O. The natural production of CH4 is
largely reduced but not entirely shut down when peatlands are
drained in preparation for peat extraction (Strack et al., 2004);
however, methane emissions are assumed to be insignificant
under Tier 1 (IPCC, 2006). Nitrous oxide emissions from
managed peatlands depend on site fertility. In addition,
abandoned and restored peatlands continue to release GHG
emissions, and at present no methodology is provided by
IPCC (2006) to estimate GHG emissions or removals from
restored peatlands. This Inventory estimates both CO2 and
N2O emissions from Peatlands Remaining Peatlands in
accordance with Tier 1 IPCC (2006) guidelines.
C02 and N20 Emissions from Peatlands Remaining
Peatlands
IPCC (2006) recommends reporting CO2 and N2O
emissions from lands undergoing active peat extraction
(i.e., Peatlands Remaining Peatlands) as part of the estimate
for emissions from managed wetlands. Peatlands occur in
wetland areas where plant biomass has sunk to the bottom
of water bodies and water-logged areas and exhausted the
oxygen supply below the water surface during the course of
decay. Due to these anaerobic conditions, much of the plant
matter does not decompose but instead forms layers of peat
over decades and centuries. In the United States, peat is
extracted for horticulture and landscaping growing media,
and for a wide variety of industrial, personal care, and other
products. It has not been used for fuel in the United States
for many decades. Peat is harvested from two types of peat
deposits in the United States: sphagnum bogs in northern
states and wetlands in states further south. The peat from
sphagnum bogs in northern states, which is nutrient poor,
is generally corrected for acidity and mixed with fertilizer.
Production from more southerly states is relatively coarse
(i.e., fibrous) but nutrient rich.
IPCC (2006) recommends considering both on-site and
off-site emissions when estimating CO2 emissions from
Peatlands Remaining Peatlands using the Tier 1 approach.
Current methodologies estimate only on-site N2O emissions,
since off-site N2O estimates are complicated by the risk of
double-counting emissions from nitrogen fertilizers added
to horticultural peat. On-site emissions from managed
peatlands occur as the land is cleared of vegetation and
the underlying peat is exposed to sun and weather. As this
occurs, some peat deposit is lost and CO2 is emitted from
the oxidation of the peat. On-site N2O is emitted during
draining depending on site fertility and if the deposit contains
significant amounts of organic nitrogen in inactive form.
Draining land in preparation for peat extraction allows
bacteria to convert the nitrogen into nitrates which leach to
the surface where they are reduced to N2O.
Off-site CO2 emissions from managed peatlands occur
from the horticultural and landscaping use of peat. Carbon
dioxide emissions occur as the nutrient-poor (but now
fertilizer-enriched) peat is used in bedding plants, other
greenhouse and plant nursery production, and by consumers,
and as nutrient-rich (but relatively coarse) peat is used
directly in landscaping, athletic fields, golf courses, and plant
nurseries. Most of the CO2 emissions from peat occur off-
site, as the peat is processed and sold to firms which, in the
United States, use it predominately for horticultural purposes.
The magnitude of the CO2 emitted from peat depends on
whether the peat has been extracted from nutrient-rich or
nutrient-poor peat deposits.
Total emissions from Peatlands Remaining Peatlands
were estimated to be 0.9 Tg CO2 Eq. in 2008 (see Table 7-37)
comprising 0.9 Tg CO2 Eq. (941 Gg) of CO2 and 0.005 Tg
CO2Eq. (0.016 Gg) of N2O.
Total emissions from Peatlands Remaining Peatlands
have fluctuated between 0.9 and 1.2 Tg CO2 Eq. across the
time series with a decreasing trend from 1990 until 1994
followed by an increasing trend through 2000. Since 2000,
Land Use, Land-Use Change, and Forestry 7-51
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Table 7-37: Emissions from Lands Undergoing Peat Extraction (Tg C02 Eq.)
Gas
C02
N20
Total
1990
1:
1.0
1995
1:
1.0
2000
1+2
1.2
2005
1.1
+
1.1
2006
0.9
+
0.9
2007
1.0
+
1.0
2008
0.9
+
0.9
+ Does not exceed 0.05 Tg C02 Eq.
Note: These numbers are based on U.S. production data in accordance with Tier 1 guidelines, which does not take into account imports,
exports and stockpiles (i.e., apparent consumption).
Table 7-38: Emissions from Lands Undergoing Peat Extraction (Gg)
Gas
1990
1995
2000
2005
2006
2007
C02
N20
1,033
+
879
+
1,012
+
+ Does not exceed 0.5 Gg.
Note: These numbers are based on U.S. production data in accordance with Tier 1 guidelines, which does not take into account imports,
exports and stockpiles (i.e., apparent consumption).
2008
941
+
total emissions show a decreasing trend until 2006 followed
by a leveling off in recent years. Carbon dioxide emissions
from Peatlands Remaining Peatlands have fluctuated between
0.9 and 1.2 Tg CO2 across the time series and drive the trends
in total emissions. Nitrous oxide emissions remained close
to zero across the time series with a decreasing trend from
1990 until 1995 followed by an increasing trend through
2002. Nitrous oxide emissions show a decreasing trend
between 2002 and 2006 followed by a leveling off in recent
years. (See Table 7-37 and Table 7-38).
Methodology
Off-site C02 Emissions
Carbon dioxide emissions from domestic peat production
were estimated using a Tier 1 methodology consistent with
IPCC (2006). Off-site CO2 emissions from Peatlands
Remaining Peatlands were calculated by apportioning the
annual weight of peat produced in the United States (Table
7-39) into peat extracted from nutrient-rich deposits and
peat extracted from nutrient-poor deposits using annual
percentage by weight figures. These nutrient-rich and
nutrient-poor production values were then multiplied by
the appropriate default carbon fraction conversion factor
taken from IPCC (2006) in order to obtain off-site emission
estimates. Both annual percentages of peat type by weight
and domestic peat production data were sourced from
estimates and industry statistics provided in the Minerals
Yearbook and Mineral Commodity Summaries from the U.S.
Geological Survey (USGS 1991-2009). To develop these
data, the U.S. Geological Survey (USGS; U.S. Bureau of
Mines prior to 1997) obtained production and use information
by surveying domestic peat producers. The USGS often
receives a response to the survey from most of the smaller
peat producers, but fewer of the larger ones. For example,
of the four active operations producing 23,000 or more
metric tons per year, two did not respond to the survey in
2007. As a result, the USGS estimates production from
the non-respondent peat producers based on responses to
previous surveys (responses from 2004 and 2005, in the case
above) or other sources. Estimates were made separately for
Alaska, because the state conducts its own mineral survey
and reports peat production by volume, rather than by weight
(Table 7-40). However, volume production data were used
to calculate off-site CO2 emissions from Alaska applying the
same methodology but with volume-specific carbon fraction
conversion factors from IPCC (2006).41
The apparent consumption of peat, which includes
production plus imports minus exports plus the decrease
in stockpiles, in the United States is over two-and-a-half
times the amount of domestic peat production. Therefore,
off-site CO9 emissions from the use of all horticultural
41 Peat produced from Alaska was assumed to be nutrient-poor; as is the case
in Canada, "where deposits of high-quality [but nutrient-poor] sphagnum
moss are extensive" (USGS 2008).
7-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 7-39: Peat Production of Lower 48 States (in thousands of metric tons)
Type of Deposit
Nutrient-Rich
Nutrient-Poor
Total Production
1990
595.1
55.4
692.0
1995
531.4
116.6
648.0
2000
728.6
63.4
792.0
2005
657.6
27.4
685.0
2006
529.0
22.0
551.0
2007
581.0
54.0
635.0
2008
562.7
52.3
615.0
Sources: Minerals Yearbook: Peat (1990-2007 Reports), Mineral Commodity Summaries: Peat (1996-2008 Reports), and Mineral
commodity summaries 2010 (2010 Report). United States Geological Survey.
Table 7-40: Peat Production of Alaska (in thousands of cubic meters)
1990
1995
2000
2005
2006
2007
2008
Total Production
49.7
26.8
27.2
47.8
50.8
52.3
7.2
Sources: Alaska's Mineral Industry (1992-2007) Reports and Alaska's mineral industry 2008: A summary. Division of Geological & Geophysical
Surveys, Alaska Department of Natural Resources.
peat within the United States are not accounted for using
the Tier 1 approach. The United States has increasingly
imported peat from Canada for horticultural purposes; in
2008, imports of sphagnum moss (nutrient-poor) peat from
Canada represented 98 percent of total U.S. peat imports
(USGS 2009). Most peat produced in the United States
is reed-sedge peat, generally from southern states, which
is classified as nutrient-rich by IPCC (2006). Higher-tier
calculations of CO2 emissions from apparent consumption
would involve consideration of the percentages of peat types
stockpiled (nutrient-rich versus nutrient-poor) as well as the
percentages of peat types imported and exported.
On-site C02 Emissions
IPCC (2006) suggests basing the calculation of on-site
emissions estimates on the area of peatlands managed for
peat extraction differentiated by the nutrient type of the
deposit (rich versus poor). Information on the area of land
managed for peat extraction is currently not available for
the United States, but in accordance with IPCC (2006), an
average production rate for the industry was applied to derive
an area estimate. In a mature industrialized peat industry,
such as exists in the United States and Canada, the vacuum
method42 can extract up to 100 metric tons per hectare per
year (Cleary et al. 2005 as cited in IPCC 2006). The area of
land managed for peat extraction in the United States was
42 The vacuum method is one type of extraction that annually "mills" or
breaks up the surface of the peat into particles, which then dry during the
summer months. The air-dried peat particles are then collected by vacuum
harvesters and transported from the area to stockpiles (IPCC 2006).
estimated using nutrient-rich and nutrient-poor production
data and the assumption that 100 metric tons of peat are
extracted from a single hectare in a single year. The annual
land area estimates were then multiplied by the appropriate
nutrient-rich or nutrient-poor IPCC (2006) default emission
factor in order to calculate on-site CO2 emission estimates.
Production data are not available by weight for Alaska. In
order to calculate on-site emissions resulting from Peatlands
Remaining Peatlands in Alaska, the production data by
volume were converted to weight using annual average peat
bulk density values, and then converted to land area estimates
using the same assumption that a single hectare yields 100
metric tons. The IPCC (2006) on-site emissions equation also
includes a term which accounts for emissions resulting from
the change in carbon stocks that occurs during the clearing
of vegetation prior to peat extraction. Area data on land
undergoing conversion to peatlands for peat extraction is also
unavailable for the United States. However, USGS records
show that the number of active operations in the United States
has been declining since 1990; therefore, it seems reasonable
to assume that no new areas are being cleared of vegetation
for managed peat extraction. Other changes in carbon stocks
in living biomass on managed peatlands are also assumed to
be zero under the Tier 1 methodology (IPCC 2006).
On-site N20 Emissions
IPCC (2006) suggests basing the calculation of on-
site N2O emissions estimates on the area of nutrient-rich
peatlands managed for peat extraction. These area data are
not available directly for the United States, but the on-site
Land Use, Land-Use Change, and Forestry 7-53
-------
CO2 emissions methodology above details the calculation
of area data from production data. In order to estimate N2O
emissions, the area of nutrient rich Peatlands Remaining
Peatlands was multiplied by the appropriate default emission
factor taken from IPCC (2006).
Uncertainty and Time-Series Consistency
The uncertainty associated with peat production data was
estimated to be ± 25 percent (Apodaca 2008) and assumed
to be normally distributed. The uncertainty associated with
peat production data stems from the fact that the USGS
receives data from the smaller peat producers but estimates
production from some larger peat distributors. This same
uncertainty and distribution was assumed for the peat type
production percentages. The uncertainty associated with
the Alaskan reported production data was assumed to be the
same as the lower 48 states, or ± 25 percent with a normal
distribution. It should be noted that the Alaskan Department
of Natural Resources estimate that around half of producers
do not respond to their survey with peat production data;
therefore, the production numbers reported are likely to
underestimate Alaska peat production (Szumigala2008). The
uncertainty associated with the average bulk density values
was estimated to be ± 25 percent with a normal distribution
(Apodaca 2008). IPCC (2006) gives uncertainty values for
the emissions factors for the area of peat deposits managed for
peat extraction based on the range of underlying data used to
determine the emissions factors. The uncertainty associated
with the emission factors was assumed to be triangularly
distributed. The uncertainty values surrounding the carbon
fractions were based on IPCC (2006) and the uncertainty
was assumed to be uniformly distributed. Based on these
values and distributions, a Monte Carlo (Tier 2) uncertainty
analysis was applied to estimate the uncertainty of CO2 and
N2O emissions from Peatlands Remaining Peatlands. The
results of the Tier 2 quantitative uncertainty analysis are
summarized in Table 7-41. CO2 emissions from Peatlands
Remaining Peatlands in 2008 were estimated to be between
0.66 and 1.26 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of 30 percent below to 34 percent
above the 2008 emission estimate of 0.94 Tg CO2 Eq. N2O
emissions from Peatlands Remaining Peatlands in 2008
were estimated to be between 0.001 and 0.007 Tg CO2 Eq.
at the 95 percent confidence level. This indicates a range
of 74 percent below to 36 percent above the 2008 emission
estimate of 0.005 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A QA/QC analysis was performed for data gathering and
input, documentation, and calculation. The QA/QC analysis
did not reveal any inaccuracies or incorrect input values.
Recalculations Discussion
This is only the second year that emissions from
Peatlands Remaining Peatlands are included in the Inventory
of U.S. Greenhouse Gas Emissions and Sinks. A revised
2007 estimate of peat production by volume for Alaska was
reported in late 2008 (DGGS 2008). Updating the 2007
data with this revised estimate led to an incremental but not
significant change in the 2007 emission estimates. In 2008,
the preliminary peat production estimate for Alaska was
approximately 7,220 cubic meters (9,444 cubic yards) for
the year compared with 52,270 cubic meters (68,367 cubic
Table 7-41: Tier-2 Quantitative Uncertainty Estimates for C02 and N20 Emissions from Peatlands
Remaining Peatlands
Source
2008 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (%)
Peatlands Remaining
Peatlands
Peatlands Remaining
Peatlands
C02
N20
Lower Bound
0.9 0.7
Upper Bound
1.3
Lower Bound
-30%
-74%
Upper Bound
+34%
+36%
+ Does not exceed 0.05 Tg C02 Eq.
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
7-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
yards) produced in 2007. The peat sector in Alaska suffered
what appears to be a loss of production during the year, but
this is likely primarily due to "an obvious reporting shortfall"
in the sector (Szumigala et al. 2009).
Planned Improvements
In order to further improve estimates of CO2 and N2O
emissions from Peatlands Remaining Peatlands, future
efforts will consider options for obtaining better data on
the quantity of peat harvested per hectare and the total area
undergoing peat extraction.
7.9. Settlements Remaining
Settlements
Table 7-42: Net C Flux from Urban Trees
(Tg C02 Eq. and Tg C)
Changes in Carbon Stocks in Urban
Trees (IPCC Source Category 5E1)
Urban forests constitute a significant portion of the
total U.S. tree canopy cover (Dwyer et al. 2000). Urban
areas (cities, towns, and villages) are estimated to cover
over 4.4 percent of the United States (Nowak et al. 2005).
With an average tree canopy cover of 27 percent, urban areas
account for approximately 3 percent of total tree cover in
the continental United States (Nowak et al. 2001). Trees in
urban areas of the United States were estimated to account
for an average annual net sequestration of 75.5 Tg CO2 Eq.
(20.6 Tg C) over the period from 1990 through 2008. Total
sequestration increased by 65 percent between 1990 and
2008 due to increases in urban land area. Data on C storage
and urban tree coverage were collected since the early 1990s
and have been applied to the entire time series in this report.
Annual estimates of CO2 flux (Table 7-42) were developed
based on periodic (1990 and 2000) U.S. Census data on urban
area. Net C flux from urban trees in 2008 was estimated to
be -93.9 Tg CO2 Eq. (-25.6 Tg C).
Net C flux from urban trees is proportionately greater
on an area basis than that of forests. This trend is primarily
the result of different net growth rates in urban areas versus
forests—urban trees often grow faster than forest trees
because of the relatively open structure of the urban forest
(Nowak and Crane 2002). Also, areas in each case are
accounted for differently. Because urban areas contain less
tree coverage than forest areas, the C storage per hectare of
Year
Tg C02 Eq.
TgC
1990
(57.1)
(15.6)
2005
2006
2007
2008
(87.8)
(89.8)
(91.9)
(93.9)
(23.9)
(24.5)
(25.1)
(25.6)
Note: Parentheses indicate net sequestration.
land is in fact smaller for urban areas. However, urban tree
reporting occurs on a per unit tree cover basis (tree canopy
area), rather than total land area. Areas covered by urban
trees, therefore, appear to have a greater C density than do
forested areas (Nowak and Crane 2002).
Methodology
Methods for quantifying urban tree biomass, C
sequestration, and C emissions from tree mortality and
decomposition were taken directly from Nowak and Crane
(2002) and Nowak (1994). In general, the methodology used
by Nowak and Crane (2002) to estimate net C sequestration
in urban trees followed three steps. First, field data from 14
cities were used to generate allometric estimates of biomass
from measured tree dimensions. Second, estimates of tree
growth and biomass increment were generated from published
literature and adjusted for tree condition and land-use class
to generate estimates of gross C sequestration in urban
trees. Third, estimates of C emissions due to mortality and
decomposition were subtracted from gross C sequestration
values to derive estimates of net C sequestration. Estimates
for these cities were then used to estimate urban forest C
sequestration in the U.S. by using urban area estimates
from U.S. Census data and urban tree cover estimates from
remote sensing data, an approach consistent with Nowak
and Crane (2002).
This approach is also consistent with the default IPCC
methodology in IPCC (2006), although sufficient data are not
yet available to determine interannual gains and losses in C
stocks in the living biomass of urban trees. Annual changes
in net C flux from urban trees are based solely on changes
in total urban area in the United States. Most of the field
Land Use, Land-Use Change, and Forestry 7-55
-------
data used to develop the methodology of Nowak et al. were
analyzed using the U.S. Forest Service's Urban Forest Effects
(UFORE) model. UFORE is a computer model that uses
standardized field data from random plots in each city and
local air pollution and meteorological data to quantify urban
forest structure, values of the urban forest, and environmental
effects, including total C stored and annual C sequestration.
UFORE was used with field data from a stratified random
sample of plots in each city to quantify the characteristics
of the urban forest. (Nowak et al. 2007a).
In order to generate the allometric relationships between
tree dimensions and tree biomass, Nowak and Crane (2002)
and Nowak (1994,2007c, 2009) collected field measurements
in a number of U.S. cities between 1989 and 2002. For
a sample of trees in each of the cities in Table 7-43, data
including tree measurements of stem diameter, tree height,
crown height and crown width, and information on location,
species, and canopy condition were collected. The data
for each tree were converted into C storage by applying
allometric equations to estimate aboveground biomass, a
root-to-shoot ratio to convert aboveground biomass estimates
to whole tree biomass, moisture content, a C content of 50
percent (dry weight basis), and an adjustment factor of 0.8 to
account for urban trees having less aboveground biomass for
a given stem diameter than predicted by allometric equations
based on forest trees (Nowak 1994). C storage estimates for
deciduous trees include only carbon stored in wood. These
calculations were then used to develop an allometric equation
relating tree dimensions to C storage for each species of tree,
encompassing a range of diameters.
Tree growth was estimated using annual height growth
and diameter growth rates for specific land uses and diameter
classes. Growth calculations were adjusted by a factor to
account for tree condition (fair to excellent, poor, critical,
dying, or dead). For each tree, the difference in carbon
storage estimates between year 1 and year (x + 1) gave
the gross amount of C sequestered. These annual gross C
sequestration rates for each species (or genus), diameter class,
and land-use condition (e.g., parks, transportation, vacant,
golf courses) were then scaled up to city estimates using tree
population information.
Gross C emissions result from tree death and removals.
Estimates of gross C emissions from urban trees were derived
by applying estimates of annual mortality and condition, and
assumptions about whether dead trees were removed from
the site to the total C stock estimate for each city. Estimates
of annual mortality rates by diameter class and condition
class were derived from a study of street-tree mortality
(Nowak 1986). Different decomposition rates were applied
to dead trees left standing compared with those removed
from the site. For removed trees, different rates were
applied to the removed/aboveground biomass in contrast to
the belowground biomass. The estimated annual gross C
emission rates for each species (or genus), diameter class,
and condition class were then scaled up to city estimates
using tree population information.
The field data for 13 of the 14 cities are described
in Nowak and Crane (2002), Nowak et al. (2007a), and
references cited therein. Data for the remaining city,
Chicago, were taken from unpublished results (Nowak 2009).
The allometric equations applied to the field data for each tree
were taken from the scientific literature (see Nowak 1994,
Nowak et al. 2002), but if no allometric equation could be
found for the particular species, the average result for the
genus was used. The adjustment (0.8) to account for less
live tree biomass in urban trees was based on information in
Nowak (1994). A root-to-shoot ratio of 0.26 was taken from
Cairns et al. (1997), and species- or genus-specific moisture
contents were taken from various literature sources (see
Nowak 1994). Tree growth rates were taken from existing
literature. Average diameter growth was based on the
following sources: estimates for trees in forest stands came
from Smith and Shirley (1984); estimates for trees on land
uses with a park-like structure came from deVries (1987);
and estimates for more open-grown trees came from Nowak
(1994). Formulas from Fleming (1988) formed the basis for
average height growth calculations. As described above,
growth rates were adjusted to account for tree condition.
Growth factors for Atlanta, Boston, Freehold, Jersey City,
Moorestown, New York, Philadelphia, and Woodbridge were
adjusted based on the typical growth conditions of different
land-use categories (e.g., forest stands, park-like stands).
Growth factors for the more recent studies in Baltimore,
Chicago, Minneapolis, San Francisco, Syracuse, and
Washington were adjusted using an updated methodology
based on the condition of each individual tree, which is
determined using tree competition factors (depending on
whether it is open grown or suppressed) (Nowak 2007b).
7-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Assumptions for which dead trees would be removed
versus left standing were developed specific to each land
use and were based on expert judgment of the authors.
Decomposition rates were based on literature estimates
(Nowak and Crane 2002).
National annual net C sequestration by urban trees was
calculated based on estimates of gross and net sequestration
for each of the 14 cities (Table 7-43), as well as urban area
and urban tree cover data for the United States. This method
was described in Nowak and Crane (2002) and has been
updated to incorporate U.S. Census data. Net annual C
sequestration estimates were derived for these 14 cities by
subtracting the net annual emission estimates from the gross
annual sequestration estimates. The urban area estimates
were based on 1990 and 2000 U.S. Census data. The 1990
U.S. Census defined urban land as "urbanized areas," which
included land with a population density greater than 1,000
people per square mile, and adjacent "urban places," which
had predefined political boundaries and a population total
greater than 2,500. In 2000, the U.S. Census replaced the
"urban places" category with a new category of urban land
called an "urban cluster," which included areas with more
than 500 people per square mile. Urban land area increased
by approximately 36 percent from 1990 to 2000; Nowak
et al. (2005) estimate that the changes in the definition of
urban land are responsible for approximately 20 percent
of the total reported increase in urban land area from 1990
to 2000. Under both 1990 and 2000 definitions, the urban
category encompasses most cities, towns, and villages (i.e.,
it includes both urban and suburban areas). The gross and
net annual C sequestration values for each city were divided
by each city's area of tree cover to determine the average
annual sequestration rates per unit of tree area for each city.
The median value for gross sequestration per unit area of
tree cover (0.29 kg C/m2-yr) was then multiplied by the
estimate of national urban tree cover area to estimate national
annual gross sequestration, per the methods of Nowak and
Crane (2002). To estimate national annual net sequestration,
the estimate of national annual gross sequestration was
multiplied by the average of the ratios of net to gross
sequestration (0.72) for those cities that had both estimates.
The urban tree cover estimates for each of the 14 cities and
the United States were obtained from Dwyer et al. (2000),
Nowak et al. (2002), Nowak (2007a), and Nowak (2009). The
urban area estimates were taken from Nowak et al. (2005).
Table 7-43: C Stocks (Metric Tons C), Annual C Sequestration (Metric Tons C/yr), Tree Cover (Percent),
and Annual C Sequestration per Area of Tree Cover (kg C/m2-yr) for 14 U.S. Cities
City
Atlanta, GA
Baltimore, MD
Boston, MA
Chicago, IL
Freehold, NJ
Jersey City, NJ
Minneapolis, MN
Moorestown, NJ
New York, NY
Philadelphia, PA
San Francisco, CA
Syracuse, NY
Washington, DC
Woodbridge, NJ
Carbon
Stocks
1,219,256
541,589
289,392
649,000
18,144
19,051
226,796
106,141
1,224,699
480,808
175,994
156,943
477,179
145,150
Gross Annual
Sequestration
42,093
14,696
9,525
22,800
494
807
8,074
3,411
38,374
14,606
4,627
4,917
14,696
5,044
Net Annual
Sequestration
32,169
9,261
6,966
16,100
318
577
4,265
2,577
20,786
10,530
4,152
4,270
11,661
3,663
Tree
Cover
36.7%
21.0%
22.3%
17.2%
34.4%
11.5%
26.4%
28.0%
20.9%
15.7%
11.9%
23.1%
28.6%
29.5%
Gross Annual
Sequestration
per Area of
Tree Cover
0.34
0.35
0.30
0.22
0.28
0.18
0.20
0.32
0.23
0.27
0.33
0.33
0.32
0.28
Median: 0.29
Net Annual
Sequestration
per Area of
Tree Cover
0.26
0.22
0.22
0.16
0.18
0.13
0.11
0.24
0.12
0.02
0.29
0.29
0.26
0.21
Net:Gross
Annual
Sequestration
Ratio
0.76
0.63
0.73
0.71
0.64
0.71
0.53
0.76
0.54
0.72
0.90
0.87
0.79
0.73
Mean: 0.72
NA = not analyzed.
Sources: Nowak and Crane (2002) and Nowak (2007a,c), Nowak (2009).
Land Use, Land-Use Change, and Forestry 7-57
-------
Uncertainty and Time-Series Consistency
Uncertainty associated with changes in C stocks in urban
trees includes the uncertainty associated with urban area,
percent urban tree coverage, and estimates of gross and net
C sequestration for each of the 14 U.S. cities. A 10 percent
uncertainty was associated with urban area estimates while a
5 percent uncertainty was associated with percent urban tree
coverage. Both of these uncertainty estimates were based on
expert judgment. Uncertainty associated with estimates of
gross and net C sequestration for each of the 14 U.S. cities
was based on standard error estimates for each of the city-
level sequestration estimates reported by Nowak (2007c)
and Nowak (2009). These estimates are based on field data
collected in each of the 14 U.S. cities, and uncertainty in these
estimates increases as they are scaled up to the national level.
Additional uncertainty is associated with the biomass
equations, conversion factors, and decomposition assumptions
used to calculate C sequestration and emission estimates
(Nowak et al. 2002). These results also exclude changes in
soil C stocks, and there may be some overlap between the
urban tree C estimates and the forest tree C estimates. Due
to data limitations, urban soil flux is not quantified as part of
this analysis, while reconciliation of urban tree and forest tree
estimates will be addressed through the land-representation
effort described at the beginning of this chapter.
A Monte Carlo (Tier 2) uncertainty analysis was applied
to estimate the overall uncertainty of the sequestration
estimate. The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 7-44. The net C flux from
changes in C stocks in urban trees in 2008 was estimated
to be between -114.5 and -75.9 Tg CO2 Eq. at a 95 percent
confidence level. This indicates a range of 22 percent below
and 19 percent above the 2008 flux estimate of -93.9 Tg
CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
The net C flux resulting from urban trees was
predominately calculated using estimates of gross and net
C sequestration estimates for urban trees and urban tree
coverage area published in the literature. The validity of
these data for their use in this section of the inventory was
evaluated through correspondence established with an author
of the papers. Through this correspondence, the methods
used to collect the urban tree sequestration and area data were
further clarified and the use of these data in the inventory was
reviewed and validated (Nowak 2002a, 2007b).
Recalculations Discussion
Revised data for Chicago's urban forest, consisting of
complete data on carbon storage, sequestration rates, and tree
cover, was provided in Nowak (2009). Previous versions
of the Inventory incorporated only a gross sequestration
rate for Chicago, as reported in Nowak and Crane (2002).
The incorporation of the new Chicago data resulted in a
lower median gross sequestration value and a lower net
sequestration to gross sequestration ratio for the set of 14
cities. These changes resulted in changes in the estimates
of net annual C flux for urban trees for the time period 1990
through 2007. On average, estimates of net annual C flux for
urban trees increased by 4.7 Tg CO2 Eq. over the period from
1990 to 2007 compared to the previous report, representing
on average a 5.9 percent decrease in annual sequestration
over the same period.
Table 7-44: Tier 2 Quantitative Uncertainty Estimates for Net C Flux from Changes in C Stocks in Urban Trees
(Tg C02 Eq. and Percent)
2008 Flux Estimate
Source Gas (Tg C02 Eq.)
Uncertainty Range Relative to Flux Estimate
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Changes in C Stocks
in Urban Trees C02 (93.9)
(114.5) (75.9) -22% +19%
Note: Parentheses indicate net sequestration.
7-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
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Planned Improvements
A consistent representation of the managed land base
in the United States is being developed. A component of
this effort, which is discussed at the beginning of the Land
Use, Land-Use Change, and Forestry chapter, will involve
reconciling the overlap between urban forest and non-urban
forest greenhouse gas inventories. It is highly likely that
urban forest inventories are including areas also denned as
forest land under the Forest Inventory and Analysis (FIA)
program of the USDA Forest Service, resulting in "double-
counting" of these land areas in estimates of C stocks and
fluxes for the Inventory. Planned improvements to the FIA
program include the development of a long-term dataset
that will define urban area boundaries and make it possible
to identify what area is forested. Once those data become
available, they will be incorporated into estimates of net C
flux resulting from urban trees.
Urban forest data for additional cities are expected in
the near future, and the use of these data will further refine
the estimated median sequestration value. It may also be
possible to report C losses and gains separately in the future.
It is currently not possible, since existing studies estimate
rather than measure natality or mortality; net sequestration
estimates are based on assumptions about whether dead trees
are being removed, burned, or chipped. There is an effort
underway to develop long-term data on permanent plots in
at least two cities, which would allow for direct calculation
of C losses and gains from observed rather than estimated
natality and mortality of trees.
Direct N20 Fluxes from Settlement
Soils (IPCC Source Category 5E1)
Of the synthetic N fertilizers applied to soils in the
United States, approximately 2.5 percent are currently
applied to lawns, golf courses, and other landscaping
occurring within settlement areas. Application rates are lower
than those occurring on cropped soils, and, therefore, account
for a smaller proportion of total U.S. soil N2O emissions per
unit area. In addition to synthetic N fertilizers, a portion of
surface applied sewage sludge is applied to settlement areas.
In 2008, N2O emissions from this source were 1.6 Tg CO2
Eq. (5.1 Gg). There was an overall increase of 61 percent
over the period from 1990 through 2008 due to a general
Table 7-45: N20 Fluxes from Soils in Settlements
Remaining Settlements (Tg C02 Eq. and Gg N20)
Year
Tg C02 Eq.
Gg
1990
1.0
3.2
2005
2006
2007
2008
1.5
1.5
1.6
1.6
4.7
4.8
5.1
5.1
Note: These estimates include direct N20 emissions from N fertilizer
additions only. Indirect N20 emissions from fertilizer additions
are reported in the Agriculture chapter. These estimates include
emissions from both Settlements Remaining Settlements and from
Land Converted to Settlements.
increase in the application of synthetic N fertilizers to an
expanding settlement area. Interannual variability in these
emissions is directly attributable to interannual variability
in total synthetic fertilizer consumption and sewage sludge
applications in the United States. Emissions from this source
are summarized in Table 7-45.
Methodology
For soils within Settlements Remaining Settlements,
the IPCC Tier 1 approach was used to estimate soil N2O
emissions from synthetic N fertilizer and sewage sludge
additions. Estimates of direct N2O emissions from soils
in settlements were based on the amount of N in synthetic
commercial fertilizers applied to settlement soils, the amount
of N in sewage sludge applied to non-agricultural land and
surface disposal of sewage sludge (see Annex 3.11 for a
detailed discussion of the methodology for estimating sewage
sludge application).
Nitrogen applications to settlement soils are estimated
using data compiled by the USGS (Ruddy et al. 2006). The
USGS estimated on-farm and non-farm fertilizer use based
on sales records at the county level from 1982 through 2001
(Ruddy et al. 2006). Non-farm N fertilizer was assumed to be
applied to settlements and forests and values for 2002 through
2008 were based on 2001 values adjusted for annual total N
fertilizer sales in the United States. Settlement application
was calculated by subtracting forest application from total
non-farm fertilizer use. Sewage sludge applications were
derived from national data on sewage sludge generation,
Land Use, Land-Use Change, and Forestry 7-59
-------
disposition, and N content (see Annex 3.11 for further detail).
The total amount of N resulting from these sources was
multiplied by the IPCC default emission factor for applied
N (1 percent) to estimate direct N2O emissions (IPCC 2006).
The volatilized and leached/runoff proportions, calculated
with the IPCC default volatilization factors (10 or 20
percent, respectively, for synthetic or organic N fertilizers)
and leaching/runoff factor for wet areas (30 percent), were
included with the total N contributions to indirect emissions,
as reported in the N2O Emissions from Agricultural Soil
Management source category of the Agriculture chapter.
Uncertainty and Time-Series Consistency
The amount of N2O emitted from settlements depends
not only on N inputs, but also on a large number of variables,
including organic C availability, oxygen gas partial pressure,
soil moisture content, pH, temperature, and irrigation/
watering practices. The effect of the combined interaction of
these variables on N2O flux is complex and highly uncertain.
The IPCC default methodology does not incorporate any of
these variables and only accounts for variations in fertilizer
N and sewage sludge application rates. All settlement soils
are treated equivalently under this methodology.
Uncertainties exist in both the fertilizer N and sewage
sludge application rates in addition to the emission factors.
Uncertainty in fertilizer N application was assigned a
default level43 of ±50 percent. Uncertainty in the amounts
of sewage sludge applied to non-agricultural lands and used
in surface disposal was derived from variability in several
factors, including: (1) N content of sewage sludge; (2) total
sludge applied in 2000; (3) wastewater existing flow in
1996 and 2000; and (4) the sewage sludge disposal practice
distributions to non-agricultural land application and surface
disposal. Uncertainty in the emission factors was provided
by the IPCC (2006).
Quantitative uncertainty of this source category was
estimated through the IPCC-recommended Tier 2 uncertainty
estimation methodology. The uncertainty ranges around the
2005 activity data and emission factor input variables were
directly applied to the 2008 emission estimates. The results
of the quantitative uncertainty analysis are summarized in
Table 7-46. Nitrous oxide emissions from soils in Settlements
Remaining Settlements in 2008 were estimated to be between
0.8 and 4.2 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 49 percent below to 163 percent above
the 2008 emission estimate of 1.6 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Recalculations Discussion
The total amount of fertilizer applied in non-agricultural
lands (settlements and forest land) has been estimated by the
USGS for 1990 through 2001 on a county scale from fertilizer
sales data (Ruddy et al. 2006). In the previous Inventory,
N fertilizer applications and subsequent N2O emissions
was only estimated for southeastern forest plantations, but
the analysis was extended in this Inventory to include N
applications to commercial Douglas fir timberlands in Oregon
and Washington. Consequently, there was less estimated N
fertilizer for application to settlements. This change resulted
Table 7-46: Quantitative Uncertainty Estimates of N20 Emissions from Soils in Settlements Remaining Settlements
(Tg C02 Eq. and Percent)
Source
2008 Emissions
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Settlements Remaining Settlements:
N20 Fluxes from Soils N20
1.6
0.8
4.2
-49%
+ 163%
Note: This estimate includes direct N20 emissions from N fertilizer additions to both Settlements Remaining Settlements and from Land Converted
to Settlements.
43 No uncertainty is provided with the USGS application data (Ruddy et al.
2006) so a conservative ±50% was used in the analysis.
7-60 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
in an average change in N2O emission of less than 4 percent
relative to the previous Inventory.
Planned Improvements
A minor improvement is to update the uncertainty
analysis for direct emissions from settlements to be consistent
with the most recent activity data for this source.
7.10. Land Converted to Settlements
(Source Category 5E2)
Land-use change is constantly occurring, and land under
a number of uses undergoes urbanization in the United States
each year. However, data on the amount of land converted
to settlements is currently lacking. Given the lack of
available information relevant to this particular IPCC source
category, it is not possible to separate CO2 or N2O fluxes on
Land Converted to Settlements from fluxes on Settlements
Remaining Settlements at this time.
7.11. Other
(IPCC Source Category 5G)
Changes in Yard Trimming and Food
Scrap Carbon Stocks in Landfills
In the United States, a significant change in C stocks
results from the removal of yard trimmings (i.e., grass
clippings, leaves, and branches) and food scraps from
settlements to be disposed in landfills. Yard trimmings and
food scraps account for a significant portion of the municipal
waste stream, and a large fraction of the collected yard
trimmings and food scraps are discarded in landfills. C
contained in landfilled yard trimmings and food scraps can
be stored for very long periods.
Carbon storage estimates are associated with particular
land uses. For example, harvested wood products are
accounted for under Forest Land Remaining Forest Land
because these wood products are a component of the forest
ecosystem. The wood products serve as reservoirs to which
Table 7-47: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C02 Eq.)
Carbon Pool
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
1995
2000
(3.1)
2005
(6.6)
(0.4)
(3.3)
(2.9)
(3.5)
2006
(6.8)
(0.5)
(3.3)
(3.0)
(3.6)
2007
(6.3)
(0.4)
(3.1)
(2.8)
(3.5)
2008
(6.3)
(0.4)
(3.1)
(2.7)
(3.3)
Total Net Flux
(23.5)
(13.9)
(11.3)
(10.1) (10.3)
(9.8)
(9.5)
Note: Totals may not sum due to independent rounding.
Table 7-48: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C)
Carbon Pool
1990
1995
2000
2005
2006
2007
2008
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
(5.8)
(0.5)1
(2.7)
(2.6)
(0.6)
(3.4)
(0.2)
(1.6)
(1.6)
(0.4)
\e-.c.)
II
(1.0)
(2.2)
(0.1)
(1.1)
(1.0)
(0.9)
(1.8)
(0.1)
(0.9)
(0.8)
(1.0)
(1.8)
(0.1)
(0.9)
(0.8)
(1.0)
(1.7)
(0.1)
(0.8)
(0.8)
(0.9)
(1.7)
(0.1)
(0.8)
(0.7)
(0.9)
Total Net Flux
(6.4)
(3.8)
(3.1)
(2.8)
(2.8)
(2.7)
(2.6)
Note: Totals may not sum due to independent rounding.
Land Use, Land-Use Change, and Forestry 7-61
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C resulting from photosynthesis in trees is transferred, but
the removals in this case occur in the forest. C stock changes
in yard trimmings and food scraps are associated with
settlements, but removals in this case do not occur within
settlements. To address this complexity, yard trimming and
food scrap C storage is therefore reported under the "Other"
source category.
Both the amount of yard trimmings collected annually
and the fraction that is landfilled have declined over the last
decade. In 1990, over 50 million metric tons (wet weight)
of yard trimmings and food scraps were generated (i.e., put
at the curb for collection to be taken to disposal sites or to
composting facilities) (EPA 2009; Schneider 2007, 2008).
Since then, programs banning or discouraging yard trimmings
disposal have led to an increase in backyard composting
and the use of mulching mowers, and a consequent 6
percent decrease in the tonnage generated (i.e., collected
for composting or disposal). At the same time, a dramatic
increase in the number of municipal composting facilities has
reduced the proportion of collected yard trimmings that are
discarded in landfills—from 72 percent in 1990 to 29 percent
in 2008. The net effect of the reduction in generation and
the increase in composting is a 62 percent decrease in the
quantity of yard trimmings disposed in landfills since 1990.
Food scraps generation has grown by 53 percent since
1990, but the proportion of food scraps discarded in landfills
has decreased slightly from 81 percent in 1990 to 79 percent
in 2008. Overall, the decrease in the yard trimmings landfill
disposal rate has more than compensated for the increase in
food scrap disposal in landfills, and the net result is a decrease
in annual landfill carbon storage from 23.5 Tg CO2 Eq. in
1990 to 9.5 Tg CO2 Eq. in 2008 (Table 7-47 and Table 7-48).
Methodology
When wastes of sustainable, biogenic origin (such as
yard trimmings and food scraps) are landfilled and do not
completely decompose, the C that remains is effectively
removed from the global C cycle. Empirical evidence
indicates that yard trimmings and food scraps do not
completely decompose in landfills (Barlaz 1998, 2005,
2008), and thus the stock of carbon in landfills can increase,
with the net effect being a net atmospheric removal of
carbon. Estimates of net C flux resulting from landfilled yard
trimmings and food scraps were developed by estimating
the change in landfilled C stocks between inventory years,
based on methodologies presented for the Land Use, Land-
Use Change and Forestry sector in IPCC (2003). C stock
estimates were calculated by determining the mass of
landfilled C resulting from yard trimmings or food scraps
discarded in a given year; adding the accumulated landfilled
C from previous years; and subtracting the portion of C
landfilled in previous years that decomposed.
To determine the total landfilled C stocks for a given
year, the following were estimated: (1) the composition of
the yard trimmings; (2) the mass of yard trimmings and food
scraps discarded in landfills; (3) the C storage factor of the
landfilled yard trimmings and food scraps; and (4) the rate
of decomposition of the degradable C. The composition of
yard trimmings was assumed to be 30 percent grass clippings,
40 percent leaves, and 30 percent branches on a wet weight
basis (Oshins and Block 2000). The yard trimmings were
subdivided, because each component has its own unique
adjusted C storage factor and rate of decomposition. The
mass of yard trimmings and food scraps disposed of in
landfills was estimated by multiplying the quantity of yard
trimmings and food scraps discarded by the proportion of
discards managed in landfills. Data on discards (i.e., the
amount generated minus the amount diverted to centralized
composting facilities) for both yard trimmings and food
scraps were taken primarily from Municipal Solid Waste
Generation, Recycling, and Disposal in the United States:
2008 Facts and Figures (EPA 2009), which provides data
for 1960, 1970, 1980, 1990, 2000, 2003, and 2005 through
2008. To provide data for some of the missing years, detailed
backup data were obtained from Schneider (2007, 2008).
Remaining years in the time series for which data were not
provided were estimated using linear interpolation. The
EPA (2009) report does not subdivide discards of individual
materials into volumes landfilled and combusted, although it
provides an estimate of the proportion of overall waste stream
discards managed in landfills and combustors (i.e., ranging
from 92 percent and 8 percent respectively in 1984-1986 to
67 percent and 33 percent in 1960).
The amount of C disposed of in landfills each year,
starting in 1960, was estimated by converting the discarded
landfilled yard trimmings and food scraps from a wet weight
to a dry weight basis, and then multiplying by the initial (i.e.,
pre-decomposition) C content (as a fraction of dry weight).
The dry weight of landfilled material was calculated using
dry weight to wet weight ratios (Tchobanoglous et al. 1993,
7-62 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
cited by Barlaz 1998) and the initial C contents and the C
storage factors were determined by Barlaz (1998, 2005,
2008) (Table 7-49).
The amount of C remaining in the landfill for each
subsequent year was tracked based on a simple model of
C fate. As demonstrated by Barlaz (1998, 2005, 2008), a
portion of the initial C resists decomposition and is essentially
persistent in the landfill environment. Barlaz (1998, 2005,
2008) conducted a series of experiments designed to measure
biodegradation of yard trimmings, food scraps, and other
materials, in conditions designed to promote decomposition
(i.e., by providing ample moisture and nutrients). After
measuring the initial C content, the materials were placed
in sealed containers along with a "seed" containing
methanogenic microbes from a landfill. Once decomposition
was complete, the yard trimmings and food scraps were re-
analyzed for C content; the C remaining in the solid sample
can be expressed as a proportion of initial C (shown in the
row labeled "CS" in Table 7-49).
The modeling approach applied to simulate U.S. landfill
C flows builds on the findings of Barlaz (1998,2005,2008).
The proportion of C stored is assumed to persist in landfills.
The remaining portion is assumed to degrade, resulting in
emissions of CH4 and CO2 (the CH4 emissions resulting
from decomposition of yard trimmings and food scraps
are accounted for in the "Waste" chapter). The degradable
portion of the C is assumed to decay according to first order
kinetics. Default IPCC 2006 Guidelines values for first order
rate constants are used to derive half-lives for branches and
food scraps, while expert judgment was used to estimate
the half-lives of grass and leaves. Food scraps are assumed
to have a half-life of 3.7 years; grass is assumed to have a
half-life of 5 years; leaves are assumed to have a half-life
of 20 years; and branches are assumed to have a half-life of
23.1 years. The half-life of food scraps is consistent with
analysis for landfill CH4 in the "Waste" chapter.
For each of the four materials (grass, leaves, branches,
food scraps), the stock of C in landfills for any given year is
calculated according to the following formula:
LFC u = S Wijn x (1 - MQ) x ICC; x
n
{[CS; x ICQ] + [(1 - (CS; x ICQ)) x e-k(t-n)]}
where,
t = Year for which C stocks are being estimated
(year)
i = Waste type for which C stocks are being
estimated (grass, leaves, branches, food
scraps)
LFQ t = Stock of C in landfills in year t, for waste i
(metric tons)
W; n = Mass of waste i disposed in landfills in year
n (metric tons, wet weight)
n = Year in which the waste was disposed (year,
where 1960 < n < t)
MQ = Moisture content of waste; (percent of water)
CS; = Proportion of initial C that is stored for
waste i (percent)
ICC; = Initial C content of waste i (percent),
e = Natural logarithm
k = First order rate constant for waste i, which
is equal to 0.693 divided by the half-life for
decomposition (year1)
For a given year t, the total stock of C in landfills (TLFCt)
is the sum of stocks across all four materials (grass, leaves,
branches, food scraps). The annual flux of C in landfills (Ft)
for year t is calculated as the change in stock compared to
the preceding year:
Ft = TLFCt - TLFCt -1
Thus, the C placed in a landfill in year n is tracked for
each year t through the end of the inventory period (2008).
For example, disposal of food scraps in 1960 resulted in
depositing about 1,135,000 metric tons of C. Of this amount,
Table 7-49: Moisture Content (%), C Storage Factor, Proportion of Initial C Sequestered (%), Initial C Content (%),
and Half-Life (years) for Landfilled Yard Trimmings and Food Scraps in Landfills
Variable
Moisture Content (% H20)
CS, proportion of initial C stored (%)
Initial C Content (%)
Half-life (years)
Grass
70
53
45
5
Yard Trimmings
Leaves
30
85
46
20
Branches
10
77
49
23
Food Scraps
70
16
51
4
Land Use, Land-Use Change, and Forestry 7-63
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16 percent (179,000 metric tons) is persistent; the remaining
84 percent (956,000 metric tons) is degradable. By 1964,
more than half of the degradable portion (500,000 metric
tons) decomposes, leaving a total of 635,000 metric tons
(the persistent portion, plus the remainder of the degradable
portion).
Continuing the example, by 2008, the total food scraps
C originally disposed in 1960 had declined to 179,000 metric
tons (i.e., virtually all of the degradable C had decomposed).
By summing the C remaining from 1960 with the C remaining
from food scraps disposed in subsequent years (1961 through
2008), the total landfill C from food scraps in 2008 was 31.5
million metric tons. This value is then added to the C stock
from grass, leaves, and branches to calculate the total landfill
C stock in 2008, yielding a value of 243.0 million metric tons
(as shown in Table 7-50). In exactly the same way total net
flux is calculated for forest C and harvested wood products,
the total net flux of landfill C for yard trimmings and food
scraps for a given year (Table 7-48) is the difference in the
landfill C stock for that year and the stock in the preceding
year. For example, the net change in 2008 shown in Table
7-48 (2.6 Tg C) is equal to the stock in 2008 (243.0 Tg C)
minus the stock in 2007 (240.4 Tg C).
The C stocks calculated through this procedure are
shown in Table 7-50.
Uncertainty and Time-Series Consistency
The uncertainty analysis for landfllled yard trimmings
and food scraps includes an evaluation of the effects of
uncertainty for the following data and factors: disposal in
landfills per year (tons of C), initial C content, moisture
content, decomposition rate (half-life), and proportion of C
stored. The C storage landfill estimates are also a function of
the composition of the yard trimmings (i.e., the proportions
of grass, leaves and branches in the yard trimmings mixture).
There are respective uncertainties associated with each of
these factors.
A Monte Carlo (Tier 2) uncertainty analysis was applied
to estimate the overall uncertainty of the sequestration
estimate. The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 7-51. Total yard trimmings
and food scraps CO2 flux in 2008 was estimated to be between
-18.3 and -4.6 Tg CO2 Eq. at a 95 percent confidence level
(or 19 of 20 Monte Carlo stochastic simulations). This
indicates a range of 93 percent below to 51 percent above
the 2008 flux estimate of-9.5 Tg CO2 Eq. More information
on the uncertainty estimates for Yard Trimmings and Food
Scraps in Landfills is contained within the Uncertainty
Annex.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
Table 7-50: C Stocks in Yard Trimmings and Food Scraps in Landfills (Tg C)
Carbon Pool
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
Total Carbon Stocks
1990
160.3
16.2
71.7
72.5
18.4
178.7
1995
183.5
18.0
82.5
83.1
20.9
204.4
2000
196.0
18.6
88.6
88.8
24.31
220.3
2005
206.2
19.2
93.6
93.4
28.7
234.9
2006
208.0
19.4
94.5
94.2
29.7
237.7
2007
209.7
19.5
95.3
94.9
30.6
240.4
2008
211.4
19.6
96.2
95.7
31.5
243.0
Note: Totals may not sum due to independent rounding.
Table 7-51: Tier 2 Quantitative Uncertainty Estimates for C02 Flux from Yard Trimmings and Food Scraps in Landfills
(Tg C02 Eq. and Percent)
2008 Flux Estimate
Source Gas (Tg C02 Eq.)
Uncertainty Range Relative to Flux Estimate3
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Yard Trimmings
and Food Scraps C02 (9.5)
(18.3) (4.6) -93% +51%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Note: Parentheses indicate negative values or net C sequestration.
7-64 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A QA/QC analysis was performed for data gathering
and input, documentation, and calculation.
Recalculations Discussion
Input data were updated for the years: 2003, 2005, and
2006 based on the update values reported in Municipal Solid
Waste Generation, Recycling, and Disposal in the United
States: 2008 Facts and Figures (EPA 2009). As a result, C
storage estimates for the years 2003, 2005, and 2006 were
revised relative to the previous Inventory, resulting in an
annual average decrease in C stored of 0.2 percent. While
data inputs for the years 2004 and 2007 were not revised,
overall C storage in any given year is dependent on the
previous year's storage (as shown in the second equation
above), and so C storage estimates for those years were also
revised. This residual change decreased the overall C stored
in 2004 and 2007 by 0.1 percent on average compared to
previous Inventory.
Planned Improvements
Future work is planned to develop improved estimates
of the decay rates for the individual materials. Additional
analysis may also be performed to evaluate the potential
contribution of inorganic C, primarily in the form of
carbonates, to landfill sequestration, as well as the
consistency between the estimates of C storage described
in this chapter and the estimates of landfill CH4 emissions
described in the "Waste" chapter.
Land Use, Land-Use Change, and Forestry 7-65
-------
8. Waste
Waste management and treatment activities are sources of greenhouse gas emissions (see Figure 8-1). Landfills
accounted for approximately 22 percent of total U.S. anthropogenic methane (CH4) emissions in 2008, the
second largest contribution of any CH4 source in the United States. Additionally, wastewater treatment
and composting of organic waste accounted for approximately 4 percent and less than 1 percent of U.S. CH4 emissions,
respectively. Nitrous oxide (N2O) emissions from the discharge of wastewater treatment effluents into aquatic environments
were estimated, as were N2O emissions from the treatment process itself. Nitrous oxide emissions from composting were
also estimated. Together, these waste activities account for less than 2 percent of total U.S. N2O emissions. Nitrogen oxides
(NOX), carbon monoxide (CO), and non-CH4 volatile organic compounds (NMVOCs) are emitted by waste activities, and
are addressed separately at the end of this chapter. A summary of greenhouse gas emissions from the Waste chapter is
presented in Table 8-1 and Table 8-2.
Carbon dioxide, N2O, and CH4 emissions from the
incineration of waste are accounted for in the Energy sector
rather than in the Waste sector because almost all incineration
of municipal solid waste (MSW) in the United States occurs
at waste-to-energy facilities where useful energy is recovered.
Similarly, the Energy sector also includes an estimate of
emissions from burning waste tires because virtually all of the
combustion occurs in industrial and utility boilers that recover
energy. The incineration of waste in the United States in 2008
resulted in 13.1 Tg CO2 Eq. emissions, nearly half of which
is attributable to the combustion of plastics. For more details
on emissions from the incineration of waste, see Section 3-7.
Figure 8-1
2008 Waste Chapter Greenhouse Gas Emission Sources
Landfills
Wastewater
Treatment
Composting
20
60 8G
Tg COZ Eq.
10G
120
140
Table 8-1: Emissions from Waste (Tg C02 Eq.)
Gas/Source
CH4
Landfills
Wastewater Treatment
Composting
N20
Domestic Wastewater Treatment
Composting
Total
1990
173.2
149.3
23.5
0.3
4.0
•'
177.2
1995
169.6
144.1
24.8
0.7 1
174.5
2000
147.1
120.7
25.2
1
5.8
4.5 1
1.4
153.0
2005
151.5
125.6
24.3
1.6
6.5
4.7
1.7
158.0
2006
153.1
127.1
24.5
1.6
6.6
4.8
1.8
159.7
2007
152.5
126.5
24.4
1.7
6.7
4.9
1.8
159.3
2008
152.3
126.3
24.3
1.7
6.8
4.9
1.8
159.1
Note: Totals may not sum due to independent rounding.
Waste 8-1
-------
Table 8-2: Emissions from Waste (Gg)
Gas/Source
1990
1995
2000
2005
2006
2007
2008
CH4
Landfills
Wastewater Treatment
Composting
N20
Domestic Wastewater Treatment
Composting
8,246
7,111
1,120
15
"
1
16,860 5,747
1,183 1,199
351 601
I I
7,213
5,980
1,158
75
21
15
6
7,292
6,050
1,166
75
21
15
6
7,264
6,023
1,162
79
22
16
6
7,254
6,016
1,158
80
22
16
6
Note: Totals may not sum due to independent rounding.
Overall, in 2008, waste activities generated emissions
of 159.1 Tg CO2 Eq., or just over 2 percent of total U.S.
greenhouse gas emissions.
8.1 Landfills (IPCC Source
Category 6A1)
In 2008, landfill CH4 emissions were approximately
126.3 Tg CO2 Eq. (6,016 Gg of CH4), representing the second
largest source of CH4 emissions in the United States, behind
enteric fermentation. Emissions from municipal solid waste
(MSW) landfills, which received about 64.5 percent of the
total solid waste generated in the United States, accounted for
about 94 percent of total landfill emissions, while industrial
landfills accounted for the remainder. Approximately 1,800
operational landfills exist in the United States, with the
largest landfills receiving most of the waste and generating
the majority of the CH4 (BioCyde 2006, adjusted to include
missing data from five states).
After being placed in a landfill, waste (such as paper,
food scraps, and yard trimmings) is initially decomposed by
aerobic bacteria. After the oxygen has been depleted, the
remaining waste is available for consumption by anaerobic
bacteria, which break down organic matter into substances
such as cellulose, amino acids, and sugars. These substances
are further broken down through fermentation into gases and
short-chain organic compounds that form the substrates for
the growth of methanogenic bacteria. These CH4-producing
anaerobic bacteria convert the fermentation products
into stabilized organic materials and biogas consisting
of approximately 50 percent carbon dioxide (CO2) and
50 percent CH4, by volume. Significant CH4 production
typically begins one or two years after waste disposal in a
landfill and continues for 10 to 60 years or longer.
From 1990 to 2008, net CH4 emissions from landfills
decreased by approximately 15 percent (see Table 8-3 and
Table 8-4). This net CH4 emissions decrease is the result
of increases in the amount of landfill gas collected and
combusted, which has more than offset the additional CH4
generation resulting from an increase in the amount of
municipal solid waste landfilled over the past 19 years. Over
the past 7 years, however, the net CUt emissions have slowly
increased from 2001 to 2006, but have shown a decreasing
trend in the past two years. While the amount of landfill gas
collected and combusted continues to increase every year,
the rate of increase no longer exceeds that rate of additional
CH4 generation resulting from an increase in the amount of
municipal solid waste landfilled as the U.S. population grows.
Methane emissions from landfills are a function of
several factors, including: (1) the total amount of waste in
MSW landfills, which is related to total waste landfilled
annually; (2) the characteristics of landfills receiving waste
(i.e., composition of waste-in-place, size, climate); (3) the
amount of CH4 that is recovered and either flared or used
for energy purposes; and (4) the amount of CH4 oxidized in
landfills instead of being released into the atmosphere. The
estimated annual quantity of waste placed in MSW landfills
increased from about 209 Tg in 1990 to 294 Tg in 2008,
an increase of 41 percent (see Annex 3.14). During this
period, the estimated CH4 recovered and combusted from
MSW landfills increased as well. In 1990, for example,
approximately 871 Gg of CH4 were recovered and combusted
(i.e., used for energy or flared) from landfills, while in 2008,
6,451 Gg CH4 was combusted, resulting in a 5 percent
increase in the quantity of CH4 recovered and combusted
from 2007 levels. In 2008, an estimated 64 new landfill
gas-to-energy (LFGTE) projects and 41 new flares began
operation.
8-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 8-3: CH4 Emissions from Landfills (Tg C02 Eq.)
Activity
MSW Landfills
Industrial Landfills
Recovered
Gas-to-Energy
Flared
Oxidized3
Total
1990H
172.6
11.6
(13.2)
(5.1)
(16.6)
149.3 •
1995
191.8
12.9
(22.5)
(22.5)
(16.0)
144.1
2000
206.9
14.4
(49.4)1
(37.91)
(13.4)
120.7
2005
241.2
15.3
(56.6)
(60.3)
(14.0)
125.6
2006
248.1
15.3
(59.1)
(63.2)
(14.1)
127.1
2007
254.2
15.4
(63.8)
(65.3)
(14.1)
126.5
2008
260.3
15.6
(68.3)
(67.2)
(14.0)
126.3
"Includes oxidation at both municipal and industrial landfills.
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
Table 8-4: CH4 Emissions from Landfills (Gg)
Activity
MSW Landfills
Industrial Landfills
Recovered
Gas-to-Energy
Flared
Oxidized3
Total
1990
8,219
5541
(629) 1
(242)
(790)
7,111
1995
9,132
6151
(1,055)
(1,069)
(762)
6,860
2000
9,854
6871
(2,352)
(1,804)
(639)
5,747
2005
11,486
728
(2,696)
(2,874)
(664)
5,980
2006
11,813
730
(2,812)
(3,008)
(672)
6,050
2007
12,107
735
(3,038)
(3,111)
(669)
6,023
2008
12,395
741
(3,252)
(3,200)
(668)
6,016
"Includes oxidation at both municipal and industrial landfills.
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
Over the next several years, the total amount of
municipal solid waste generated is expected to increase as the
U.S. population continues to grow. The percentage of waste
landfilled, however, may decline due to increased recycling
and composting practices. In addition, the quantity of CH4
that is recovered and either flared or used for energy purposes
is expected to continue to increase as a result of 1996 federal
regulations that require large municipal solid waste landfills
to collect and combust landfill gas (see 40 CFR Part 60,
Subpart Cc 2005 and 40 CFR Part 60, Subpart WWW 2005),
voluntary programs encouraging CH4 recovery and use such
as EPA's Landfill Methane Outreach Program (LMOP), and
federal and state incentives that promote renewable energy
(e.g. tax credits, low interest loans, and Renewable Portfolio
Standards).
Methodology
A detailed description of the methodology used to
estimate CH4 emissions from landfills can be found in
Annex 3.14.
Methane emissions from landfills were estimated to
equal the CHj produced from municipal solid waste landfills,
plus the CH4 produced by industrial landfills, minus the CH4
recovered and combusted, minus the CH4 oxidized before
being released into the atmosphere:
CH4jSoiidWaste = [CH4MSW + CH4jind — R] — Ox
where,
CH4jSoiid waste = CH4 emissions from solid waste
CH4 MSW = CH4 generation from municipal solid
waste landfills
CH4 ind = CH4 generation from industrial landfills
R = CH4 recovered and combusted
Ox = CH4 oxidized from MSW and
industrial landfills before release to
the atmosphere
The methodology for estimating CH4 emissions from
municipal solid waste landfills is based on the first order
decay model described by the Intergovernmental Panel on
Climate Change (IPCC 2006). Values for the CH4 generation
potential (L0) and rate constant (k) were obtained from an
Waste 8-3
-------
analysis of CH4 recovery rates for a database of 52 landfills
and from published studies of other landfills (RTI 2004;
EPA 1998; SWANA 1998; Peer, Thorneloe, and Epperson
1993). The rate constant was found to increase with average
annual rainfall; consequently, values of k were developed for
3 ranges of rainfall. The annual quantity of waste placed in
landfills was apportioned to the 3 ranges of rainfall based on
the percent of the U.S. population in each of the 3 ranges,
and historical census data were used to account for the shift
in population to more arid areas over time. For further
information, see Annex 3.14.
National landfill waste generation and disposal data for
2007 and 2008 were extrapolated based on BioCycle data
and the U.S. Census population from 2008. Data for 1989
through 2006 were obtained from BioCycle (2006). Because
BioCycle does not account for waste generated in U.S.
territories, waste generation for the territories was estimated
using population data obtained from the U.S. Census Bureau
(2009) and national per capita solid waste generation from
BioCycle (2006). Estimates of the annual quantity of waste
landfilled for 1960 through 1988 were obtained from EPA's
Anthropogenic Methane Emissions in the United States,
Estimates for 1990: Report to Congress (EPA 1993) and an
extensive landfill survey by the EPA's Office of Solid Waste
in 1986 (EPA 1988). Although waste placed in landfills in
the 1940s and 1950s contributes very little to current CH4
generation, estimates for those years were included in the
first order decay model for completeness in accounting for
CH4 generation rates and are based on the population in
those years and the per capita rate for land disposal for the
1960s. For calculations in this Inventory, wastes landfilled
prior to 1980 were broken into two groups: wastes disposed
of in landfills (Methane Conversion Factor, MCF, of 1) and
those disposed of in dumps (MCF of 0.6). Please see Annex
3.14 for more details.
The estimated landfill gas recovered per year was
based on updated data collected from vendors of flaring
equipment, a database of landfill gas-to-energy (LFGTE)
projects compiled by LMOP (EPA 2008), and a database
maintained by the Energy Information Administration (EIA)
for the voluntary reporting of greenhouse gases (EIA 2007).
As the EIA database only included data through 2006,2007
and 2008 recovery for projects included in the EIA database
were assumed to be the same as in 2006. The three databases
were carefully compared to identify landfills that were in
two or all three of the databases to avoid double counting
reductions. Based on the information provided by the EIA
and flare vendor databases, the CH4 combusted by flares in
operation from 1990 to 2008 was estimated. This quantity
likely underestimates flaring because these databases do not
have information on all flares in operation. Additionally, the
EIA and LMOP databases provided data on landfill gas flow
and energy generation for landfills with LFGTE projects. If
a landfill in the EIA database was also in the LMOP and/or
the flare vendor database, the emissions avoided were based
on the EIA data because landfill owners or operators reported
the amount recovered based on measurements of gas flow
and concentration, and the reporting accounted for changes
over time. If both flare data and LMOP recovery data were
available for any of the remaining landfills (i.e., not in the
EIA database), then the emissions recovery was based on
the LMOP data, which provides reported landfill-specific
data on gas flow for direct use projects and project capacity
(i.e., megawatts) for electricity projects. The flare data, on
the other hand, only provided a range of landfill gas flow for
a given flare size. Given that each LFGTE project is likely
to also have a flare, double counting reductions from flares
and LFGTE projects in the LMOP database was avoided by
subtracting emissions reductions associated with LFGTE
projects for which a flare had not been identified from the
emissions reductions associated with flares.
A destruction efficiency of 99 percent was applied to
CH4 recovered to estimate CH4 emissions avoided. The
value for efficiency was selected based on the range of
efficiencies (98 to 100 percent) recommended for flares in
EPA's AP-42 Compilation of Air Pollutant Emission Factors,
Chapter 2.4 (EPA 1998) efficiencies used to establish new
source performance standards (NSPS) for landfills, and in
recommendations for closed flares used in LMOP.
Emissions from industrial landfills were estimated from
activity data for industrial production (ERG 2009), waste
disposal factors, and the first order decay model. As over
99 percent of the organic waste placed in industrial landfills
originated from the food processing (meat, vegetables,
fruits) and pulp and paper industries, estimates of industrial
landfill emissions focused on these two sectors (EPA 1993).
The amount of CH4 oxidized by the landfill cover at both
municipal and industrial landfills was assumed to be ten
percent of the CH4 generated that is not recovered (IPCC
2006, Mancinelli and McKay 1985, Czepiel et al. 1996). To
8-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
calculate net CH4 emissions, both CH4 recovered and CH4
oxidized were subtracted from CH4 generated at municipal
and industrial landfills.
Uncertainty and Time-Series Consistency
Several types of uncertainty are associated with the
estimates of CH4 emissions from landfills. The primary
uncertainty concerns the characterization of landfills.
Information is not available on two fundamental factors
affecting CH4 production: the amount and composition of
waste placed in every landfill for each year of its operation.
The approach used here assumes that the CH4 generation
potential and the rate of decay that produces CH4, as
determined from several studies of CH4 recovery at landfills,
are representative of U.S. landfills.
Additionally, the approach used to estimate the
contribution of industrial wastes to total CH4 generation
introduces uncertainty. Aside from uncertainty in estimating
CH4 generation potential, uncertainty exists in the estimates
of oxidation by cover soils. There is also uncertainty in
the estimates of methane that is recovered by flaring and
energy projects. The IPCC default value of 10 percent for
uncertainty in recovery estimates was used in the uncertainty
analysis when metering was in place (for about 64 percent
of the CH4 estimated to be recovered). For flaring without
metered recovery data (approximately 34 percent of the
CH4 estimated to be recovered), a much higher uncertainty
of approximately 50 percent was used (e.g., when recovery
was estimated as 50 percent of the flare's design capacity).
Nitrous oxide emissions from the application of sewage
sludge on landfills are not explicitly modeled as part of
greenhouse gas emissions from landfills. N2O emissions from
sewage sludge applied to landfills would be relatively small
because the microbial environment in landfills is not very
conducive to the nitrification and denitrification processes
that result in N2O emissions. Furthermore, the 2006 IPCC
Guidelines (IPCC 2006) did not include a methodology for
estimating N2O emissions from solid waste disposal sites
"because they are not significant." Therefore, any uncertainty
or bias caused by not including N2O emissions from landfills
is expected to be minimal.
The results of the IPCC Good Practice Guidance Tier
2 quantitative uncertainty analysis are summarized in Table
8-5. Landfill CH4 emissions in 2008 were estimated to be
between 71.5 and 172.6 Tg CO2 Eq., which indicates a range
of 43 percent below to 37 percent above the 2008 emission
estimate of 126.3 Tg CO2 Eq.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A QA/QC analysis was performed for data gathering and
input, documentation, and calculation. A primary focus of the
QA/QC checks was to ensure that CH4 recovery estimates
were not double-counted. Both manual and electronic checks
were made to ensure that emission avoidance from each
landfill was calculated in only one of the three databases.
The primary calculation spreadsheet is tailored from the
IPCC waste model and has been verified previously using the
original, peer-reviewed IPCC waste model. All model input
values were verified by secondary QA/QC review.
Recalculations Discussion
In developing the current Inventory, additional steps
were taken in order to further describe the uncertainty
surrounding CH4 emissions from landfills relative to the
previous Inventory. A separate Monte Carlo analysis for
MSW and industrial landfills was conducted to emphasize
Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills (Tg C02 Eq. and Percent)
Source Gas
2008 Emission Estimate Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound
Landfills CH4
MSW CH4
Industrial CH4
3 Range of emission estimates predicted
126.3
112.3
14.0
71.5 172.6
70.8 172.7
10.2 16.9
-43%
-37%
-27%
Upper Bound
+37%
+54%
+21%
by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Waste 8-5
-------
Box 8-1: Biogenic Emissions and Sinks of Carbon
Carbon dioxide emissions from the combustion or
decomposition of biogenic materials (e.g., paper, wood products,
and yard trimmings) grown on a sustainable basis are considered
to mimic the closed loop of the natural carbon cycle—that is,
they return to the atmosphere C02 that was originally removed by
photosynthesis. In contrast, CH4 emissions from landfilled waste
occur due to the man-made anaerobic conditions conducive to
CH4 formation that exist in landfills, and are consequently included
in this inventory.
Depositing wastes of biogenic origin in landfills causes the
removal of carbon from its natural cycle between the atmosphere
and biogenic materials. As empirical evidence shows, some of
these wastes degrade very slowly in landfills, and the carbon they
contain is effectively sequestered in landfills over a period of time
(Barlaz 1998,2005). Estimates of carbon removals from landfilling
of forest products, yard trimmings, and food scraps are further
described in the Land Use, Land-Use Change, and Forestry chapter,
based on methods presented in IPCC (2003) and IPCC (2006).
the greater amount of uncertainty surrounding industrial
waste data.
Planned Improvements
For future Inventories, additional efforts will be made
to improve the estimates of the amount of waste placed in
MSW landfills. Improvements to the flare database will be
investigated, and an effort will be made to identify additional
landfills that have flares.
8.2 Wastewater Treatment (IPCC
Source Category 6B)
Wastewater treatment processes can produce
anthropogenic CH4 and N2O emissions. Wastewater from
domestic1 and industrial sources is treated to remove soluble
organic matter, suspended solids, pathogenic organisms, and
chemical contaminants. Treatment may either occur on site,
most commonly through septic systems or package plants,
or off site at centralized treatment systems. Centralized
wastewater treatment systems may include a variety of
processes, ranging from lagooning to advanced tertiary
treatment technology for removing nutrients. In the United
1 Throughout the Inventory, emissions from domestic wastewater also
includes any commercial and industrial wastewater collected and co-treated
with domestic wastewater.
States, approximately 20 percent of domestic wastewater is
treated in septic systems or other on-site systems, while the
rest is collected and treated centrally (U.S. Census Bureau
2007).
Soluble organic matter is generally removed using
biological processes in which microorganisms consume
the organic matter for maintenance and growth. The
resulting biomass (sludge) is removed from the effluent
prior to discharge to the receiving stream. Microorganisms
can biodegrade soluble organic material in wastewater
under aerobic or anaerobic conditions, where the latter
condition produces CH4. During collection and treatment,
wastewater may be accidentally or deliberately managed
under anaerobic conditions. In addition, the sludge may be
further biodegraded under aerobic or anaerobic conditions.
The generation of N2O may also result from the treatment
of domestic wastewater during both nitrification and
denitrification of the N present, usually in the form of urea,
ammonia, and proteins. These compounds are converted to
nitrate (NO3) through the aerobic process of nitrification.
Denitrification occurs under anoxic conditions (without free
oxygen), and involves the biological conversion of nitrate
into dinitrogengas (N2). Nitrous oxide can be an intermediate
product of both processes, but is more often associated with
denitrification.
The principal factor in determining the CH4 generation
potential of wastewater is the amount of degradable organic
material in the wastewater. Common parameters used to
measure the organic component of the wastewater are the
biochemical oxygen demand (BOD) and chemical oxygen
demand (COD). Under the same conditions, wastewater
with higher COD (or BOD) concentrations will generally
yield more CH4 than wastewater with lower COD (or BOD)
concentrations. BOD represents the amount of oxygen that
would be required to completely consume the organic matter
contained in the wastewater through aerobic decomposition
processes, while COD measures the total material available
for chemical oxidation (both biodegradable and non-
biodegradable). Because BOD is an aerobic parameter, it
is preferable to use COD to estimate CH4 production. The
principal factor in determining the N2O generation potential
of wastewater is the amount of N in the wastewater.
In 2008, CH4 emissions from domestic wastewater
treatment were 15.7 Tg CO2 Eq. (749 Gg). Emissions
gradually increased from 1990 through 1997, but have
8-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 8-6: CH4 and N20 Emissions from Domestic and Industrial Wastewater Treatment (Tg C02 Eq.)
Gas/Activity
CH4
Domestic
Industrial3
N20
Domestic
Total
1990
23.5
16.4
7.1
3.7
3.7
27.2
1995
24.8
16.9
8.0 1
4.0 1
4.0
28.9
2000
25.2
16.8
8.4l
4.5 1
4.5
29.6 |
2005
24.3
16.2
8.2
4.7
4.7
29.0
2006
24.5
16.0
8.5
4.8
4.8
29.3
2007
24.4
15.9
8.5
4.9
4.9
29.3
2008
24.3
15.7
8.6
4.9
4.9
29.2
3 Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, fruit and vegetable processing, starch-based
ethanol production, and petroleum refining industries.
Note: Totals may not sum due to independent rounding.
Table 8-7: CH4 and N20 Emissions from Domestic and Industrial Wastewater Treatment (Gg)
a Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, fruit and vegetable processing, starch-based
ethanol production, and petroleum refining industries.
Note: Totals may not sum due to independent rounding.
Gas/Activity
CH4
Domestic
Industrial3
N20
Domestic
1990
1,120
782
338
11.9
11.9
1995
1,183
804
380
13.0
13.0
2000
1,199
801
398
14.4
14.4
2005
1,158
770
389
15.3
15.3
2006
1,166
763
403
15.5
15.5
2007
1,162
757
406
15.7
15.7
2008
1,158
749
409
15.9
15.9
decreased since that time due to decreasing percentages of
wastewater being treated in anaerobic systems, including
reduced use of on-site septic systems and central anaerobic
treatment systems. In 2008, CH4 emissions from industrial
wastewater treatment were estimated to be 8.6 Tg CO2 Eq.
(409 Gg). Industrial emission sources have increased across
the time series through 1999 and then fluctuated up and down
with production changes associated with the treatment of
wastewater from the pulp and paper manufacturing, meat and
poultry processing, fruit and vegetable processing, starch-
based ethanol production, and petroleum refining industries.
Table 8-6 and Table 8-7 provide CH4 and N2O emission
estimates from domestic and industrial wastewater treatment.
With respect to N2O, the United States identifies two distinct
sources for N2O emissions from domestic wastewater:
emissions from centralized wastewater treatment processes.
and emissions from effluent from centralized treatment
systems that has been discharged into aquatic environments.
The 2008 emissions of N2O from centralized wastewater
treatment processes and from effluent were estimated to
be 0.3 Tg CO2 Eq. (1 Gg) and 4.6 Tg CO2 Eq. (15 Gg),
respectively. Total N2O emissions from domestic wastewater
were estimated to be 4.9 Tg CO2 Eq. (16 Gg). Nitrous oxide
emissions from wastewater treatment processes gradually
increased across the time series as a result of increasing U.S.
population and protein consumption.
Methodology
Domestic Wastewater CH4 Emission Estimates
Domestic wastewater CH4 emissions originate from both
septic systems and from centralized treatment systems, such
as publicly owned treatment works (POTWs). Within these
centralized systems, CH4 emissions can arise from aerobic
systems that are not well managed or that are designed to have
periods of anaerobic activity (e.g., constructed wetlands),
anaerobic systems (anaerobic lagoons and facultative
lagoons), and from anaerobic digesters when the captured
biogas is not completely combusted. Methane emissions from
septic systems were estimated by multiplying the total 5-day
BOD (BOD5) produced in the United States by the percent
of wastewater treated in septic systems (20 percent), the
maximum CH4 producing capacity for domestic wastewater
(0.60 kg CUt/kg BOD), and the CH4 correction factor (MCF)
for septic systems (0.5). Methane emissions from POTWs
were estimated by multiplying the total BOD5 produced in the
United States by the percent of wastewater treated centrally
(80 percent), the relative percentage of wastewater treated
Waste 8-7
-------
by aerobic and anaerobic systems, the relative percentage of
wastewater facilities with primary treatment, the percentage
of BOD5 treated after primary treatment (67.5 percent), the
maximum CH4-producing capacity of domestic wastewater
(0.6), and the relative MCFs for aerobic (zero or 0.3) and
anaerobic (0.8) systems. Methane emissions from anaerobic
digesters were estimated by multiplying the amount of
biogas generated by wastewater sludge treated in anaerobic
digesters by the proportion of CH4 in digester biogas (0.65),
the density of CH4 (662 g QVm3 CH4), and the destruction
efficiency associated with burning the biogas in an energy/
thermal device (0.99). The methodological equations are:
Emissions from Septic Systems = A
= (% onsite) x (total BOD5 produced) x (B0) x
(MCF-septic) x 1/106
Emissions from Centrally Treated Aerobic Systems = B
= [(% collected) x (total BOD5 produced) x (% aerobic) x
(% aerobic w/out primary) + (% collected) x
(total BOD5 produced) x (% aerobic) x
(% aerobic w/primary) x
(1-% BOD removed in prim, treat.)] x
(% operations not well managed) x (B0) x
(MCF-aerobic_not_well_man.) x 1/106
Emissions from Centrally Treated Anaerobic Systems = C
= [(% collected) x (total BOD5 produced) x
(% anaerobic) x (% anaerobic w/out primary) +
(% collected) x (total BOD5 produced) x
(% anaerobic) x (% anaerobic w/primary) x
(1-%BOD removed in prim, treat.)] x (B0) x
(MCF-anaerobic) x 1/106
Emissions from Anaerobic Digesters = D
= [(POTW_flow_AD) x (digester gas)/(per capita flow)] x
conversion to m3 x (FRAC_CH4) x (365.25) x
(density of CH4) x (1-DE) x 1/109
Total CH4 Emissions (Gg) = A + B+C+D
where,
% onsite = Flow to septic systems/total flow
% collected = Flow to POTWs/total flow
% aerobic = Flow to aerobic systems/total flow to
POTWs
% anaerobic = Flow to anaerobic systems/total flow
to POTWs
% aerobic
w/out
primary
% aerobic
w/primary
%BOD
removed in
prim, treat.
% operations
not well
managed
% anaerobic
w/out
primary
% anaerobic
w/primary
Total BOD5
produced
Bn
MCF-septic =
Percent of aerobic systems that do not
employ primary treatment
Percent of aerobic systems that
employ primary treatment
= 32.5 %
1/106
MCF-aerobic_
not_
well man. :
MCF-
anaerobic
DE
POTW_
flow_AD
Percent of aerobic systems that are
not well managed and in which some
anaerobic degradation occurs
Percent of anaerobic systems that do
not employ primary treatment
Percent of anaerobic systems that
employ primary treatment
kg BOD/capita/day x
population x 365.25 days/yr
U.S.
Maximum CH4-producing capacity
for domestic wastewater (0.60 kg
CH4/kg BOD)
CH4 correction factor for septic
systems (0.5)
= Conversion factor, kg to Gg
CH4 correction factor for aerobic
systems that are not well managed
(0.3)
CH4 correction factor for anaerobic
systems (0.8)
CH4 destruction efficiency from
flaring or burning in engine (0.99 for
enclosed flares)
= Wastewater influent flow to POTWs
that have anaerobic digesters (gal)
digester gas = Cubic feet of digester gas produced
per person per day (1.0 ft3/person/
day) (Metcalf and Eddy 1991)
8-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
per capita
flow
conversion
torn3
FRAC_CH4
density of
CH4
1/109
Wastewater flow to POTW per person
per day (100 gal/person/day)
Conversion factor, ft3 to m3 (0.0283)
Proportion CH4 in biogas (0.65)
662 (g CH4/m3 CH4)
Conversion factor, g to Gg
U.S. population data were taken from the U.S. Census
Bureau International Database (U.S. Census 2009a) and
include the populations of the United States, American
Samoa, Guam, Northern Mariana Islands, Puerto Rico, and
the Virgin Islands. Table 8-8 presents U.S. population and
total BOD5 produced for 1990 through 2008. The proportions
of domestic wastewater treated onsite versus at centralized
treatment plants were based on data from the 1989, 1991,
1993, 1995, 1997, 1999, 2001, 2003, 2005, and 2007
American Housing Surveys conducted by the U.S. Census
Bureau (U.S. Census 2007), with data for intervening years
obtained by linear interpolation. The wastewater flow to
aerobic and anaerobic systems, and the wastewater flow to
POTWs that have anaerobic digesters were obtained from the
1992,1996, 2000, and 2004 Clean Watershed Needs Survey
(EPA 1992, 1996, 2000, and 2004a). Data for intervening
years were obtained by linear interpolation. The BOD5
production rate (0.09 kg/capita/day) for domestic wastewater
was obtained from Metcalf and Eddy (1991 and 2003).
Table 8-8: U.S. Population (Millions) and Domestic
Wastewater BOD5 Produced (Gg)
Year
Population
BODH
1990
254
2000
2001
2002
2003
2004
2005
2006
2007
2008
286
289
292
295
297
300
303
306
308
9,414
9,509
9,598
9,681
9,770
9,858
9,950
10,047
10,139
The CH4 emission factor (0.6 kg CH4/kg BOD5) and the
MCFs were taken from IPCC (2006). The CH4 destruction
efficiency, 99 percent, was selected based on the range of
efficiencies (98 to 100 percent) recommended for flares
in AP-42 Compilation of Air Pollutant Emission Factors,
Chapter 2.4 (EPA 1998), efficiencies used to establish new
source performance standards (NSPS) for landfills, and
in recommendations for closed flares used by the Landfill
Methane Outreach Program (LMOP). The cubic feet of
digester gas produced per person per day (1.0 ft3/person/
day) and the proportion of CH4 in biogas (0.65) come from
Metcalf and Eddy (1991). The wastewater flow to a POTW
(100 gal/person/day) was taken from the Great Lakes-Upper
Mississippi River Board of State and Provincial Public Health
and Environmental Managers, "Recommended Standards for
Wastewater Facilities (Ten-State Standards)" (2004).
Industrial Wastewater CH4 Emission Estimates
Methane emissions estimates from industrial wastewater
were developed according to the methodology described in
IPCC (2006). Industry categories that are likely to produce
significant CH4 emissions from wastewater treatment were
identified. High volumes of wastewater generated and a
high organic wastewater load were the main criteria. The
top five industries that meet these criteria are pulp and paper
manufacturing; meat and poultry processing; vegetables,
fruits, and juices processing; starch-based ethanol production;
and petroleum refining. Wastewater treatment emissions for
these sectors for 2008 are displayed in Table 8-9 below. Table
8-10 contains production data for these industries.
Methane emissions from these categories were estimated
by multiplying the annual product output by the average
outflow, the organics loading (in COD) in the outflow,
the percentage of organic loading assumed to degrade
Table 8-9: Industrial Wastewater CH4 Emissions by
Sector for 2008
Pulp & Paper
Meat & Poultry
Petroleum Refineries
Fruit & Vegetables
Ethanol Refineries
Total
CH4 Emissions
(Tg C02 Eq.)
4.0
3.7
0.6
0.1
0.1
8.6
% of Industrial
Wastewater CH4
47%
43%
7%
1%
1%
100%
Source: U.S. Census Bureau (2009a); Metcalf & Eddy 1991 and 2003.
Note: Totals may not sum due to independent rounding.
Waste 8-9
-------
Table 8-10: U.S. Pulp and Paper; Meat and Poultry; Vegetables, Fruits and Juices Production; and Fuels
Production (Tg)
Year
Pulp and Paper
Meat
(Live Weight Killed)
Poultry
(Live Weight Killed)
Vegetables,
Fruits and Juices
Ethanol
Petroleum
Refining
1990
128.9
27.3
14.6
38.7
702.4
2000
2001
2002
2003
2004
2005
2006
2006
2007
2008
142.8
134.3
132.7
131.9
136.4
131.4
137.4
135.9
134.5
134.5
32.1
31.6
32.7
32.3
31.2
31.4
32.5
33.4
34.4
34.4
22.2
22.8
23.5
23.7
24.4
25.1
25.5
26.0
26.5
26.5
50.9
45.0
47.7
44.8
47.8
42.9
42.9
44.8
45.8
45.8
4.9
5.3
6.4
8.4
10.2
11.7
14.5
19.4
26.9
26.9
795.2
794.9
794.4
804.2
821.5
818.6
826.7
827.6
829.0
829.0
anaerobically, and the emission factor. Ratios of BOD:COD
in various industrial wastewaters were obtained from EPA
(1997a) and used to estimate COD loadings. The B0 value
used for all industries is the IPCC default value of 0.25 kg
CH4/kg COD (IPCC 2006).
For each industry, the percent of plants in the industry
that treat wastewater on site, the percent of plants that have
a primary treatment step prior to biological treatment, and
the percent of plants that treat wastewater anaerobically were
denned. The percent of wastewater treated anaerobically
onsite (TA) was estimated for both primary treatment and
secondary treatment. For plants that have primary treatment
in place, an estimate of COD that is removed prior to
wastewater treatment in the anaerobic treatment units was
incorporated.
The methodological equations are:
CH4 (industrial wastewater) =
P x W x COD x TA x B0 x MCF
%TAp = [%Plants0 x %WWa_p x %CODp]
%TAS = [%Plantsa x %WWa,s x %CODJ +
[%Plantst x %WWa_t x %CODS]
where,
CH4
(industrial
wastewater) = Total CH4 emissions from industrial
wastewater (kg/year)
P = Industry output (metric tons/year)
W
COD
S
%TAp
%TAS
%Plants0
treatment
%WWa_p
%CODp
%Plantsa
%Plantst
%WWa_s
%wwa_t
= Wastewater generated (m3/metric ton
of product)
= Organics loading in wastewater
(kg /m3)
= Removal of COD as sludge prior to
anaerobic treatment (kg COD/year)
= Percent of wastewater treated
anaerobically on site in primary
treatment
= Percent of wastewater treated
anaerobically on site in secondary
treatment
= Percent of plants with onsite
= Percent of wastewater treated
anaerobically in primary treatment
= Percent of COD entering primary
treatment
= Percent of plants with anaerobic
secondary treatment
= Percent of plants with other
secondary treatment
= Percent of wastewater treated
anaerobically in anaerobic
secondary treatment
= Percent of wastewater treated
anaerobically in other secondary
treatment
8-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 8-11: Variables Used to Calculate Percent Wastewater Treated Anaerobically by Industry (%)
Variable
%TAp
"/oTAs
%Plants0
%Plantsa
%Plants,
%wwa,p
%wwa,s
%wwa,t
%CODp
%CODS
Pulp and
Paper
0
10.5
60
25
35
0
100
0
100
42
Meat
Processing
0
33
100
33
67
0
100
0
100
100
Poultry
Processing
0
25
100
25
75
0
100
0
100
100
Industry
Fruit/Vegetable
Processing
0
4.2
11
5.5
5.5
0
100
0
100
77
Ethanol
Production
—Wet Mill
0
33.3
100
33.3
66.7
0
100
0
100
100
Ethanol
Production
—Dry Mill
0
75
100
75
25
0
100
0
100
100
Petroleum
Refining
0
100
100
100
0
0
100
0
100
100
%CODS = Percent of COD entering secondary
treatment
B0 = Maximum CH4 producing potential of
industrial wastewater (default value
of0.25kgCH4/kgCOD)
MCF = CH4 correction factor, indicating
the extent to which the organic
content (measured as COD) degrades
anaerobically
As described below, the values presented in Table 8-11
were used in the inventory calculations.
Pulp and Paper. Wastewater treatment for the pulp and
paper industry typically includes neutralization, screening,
sedimentation, and flotation/hydrocycloning to remove
solids (World Bank 1999, Nemerow and Dasgupta 1991).
Secondary treatment (storage, settling, and biological
treatment) mainly consists of lagooning. In determining
the percent that degrades anaerobically, both primary and
secondary treatment were considered. In the United States,
primary treatment is focused on solids removal, equalization,
neutralization, and color reduction (EPA 1993). The vast
majority of pulp and paper mills with on-site treatment
systems use mechanical clariflers to remove suspended solids
from the wastewater. About 10 percent of pulp and paper
mills with treatment systems use settling ponds for primary
treatment and these are more likely to be located at mills that
do not perform secondary treatment (EPA 1993). However,
because the vast majority of primary treatment operations at
U.S. pulp and paper mills use mechanical clariflers, and less
than 10 percent of pulp and paper wastewater is managed in
primary settling ponds that are not expected to have anaerobic
conditions, negligible emissions are assumed to occur during
primary treatment.
Approximately 42 percent of the BOD passes on to
secondary treatment, which consists of activated sludge,
aerated stabilization basins, or non-aerated stabilization
basins. No anaerobic activity is assumed to occur in
activated sludge systems or aerated stabilization basins
(note: although IPCC recognizes that some CH4 can be
emitted from anaerobic pockets, they recommend an MCF
of zero). However, about 25 percent of the wastewater
treatment systems used in the United States are non-aerated
stabilization basins. These basins are typically 10 to 25
feet deep. These systems are classified as anaerobic deep
lagoons (MCF = 0.8).
A time series of CH4 emissions for 1990 through 2001
was developed based on production figures reported in the
Lockwood-Post Directory (Lockwood-Post 2002). Published
data from the American Forest and Paper Association, data
published by Paper Loop, and other published statistics were
used to estimate production for 2002 through 2008 (Pulp and
Paper 2005, 2006 and monthly reports from 2003 through
2008; Paper 360 2007). The overall wastewater outflow
was estimated to be 85 m3/metric ton, and the average BOD
concentrations in raw wastewater was estimated to be 0.4
gram BOD/liter (EPA 1997b, EPA 1993, World Bank 1999).
Meat and Poultry Processing. The meat and poultry
processing industry makes extensive use of anaerobic lagoons
in sequence with screening, fat traps and dissolved air
flotation when treating wastewater on site. About 33 percent
of meat processing operations (EPA 2002) and 25 percent of
Waste 8-11
-------
poultry processing operations (U.S. Poultry 2006) perform
on-site treatment in anaerobic lagoons. The IPCC default Bo
of 0.25 kg CH4/kg COD and default MCF of 0.8 for anaerobic
lagoons were used to estimate the CH4 produced from these
on-site treatment systems. Production data, in carcass weight
and live weight killed for the meat and poultry industry, were
obtained from the USDA Agricultural Statistics Database and
the Agricultural Statistics Annual Reports (USDA 2009a).
Data collected by EPA's Office of Water provided estimates
for wastewater flows into anaerobic lagoons: 5.3 and 12.5
m3/metric ton for meat and poultry production (live weight
killed), respectively (EPA 2002). The loadings are 2.8 and
1.5 g BOD/liter for meat and poultry, respectively.
Vegetables, Fruits, and Juices Processing. Treatment
of wastewater from fruits, vegetables, and juices processing
includes screening, coagulation/settling, and biological
treatment (lagooning). The flows are frequently seasonal, and
robust treatment systems are preferred for on-site treatment.
Effluent is suitable for discharge to the sewer. This industry
is likely to use lagoons intended for aerobic operation, but
the large seasonal loadings may develop limited anaerobic
zones. In addition, some anaerobic lagoons may also be
used (Nemerow and Dasgupta 1991). Consequently, 4.2
percent of these wastewater organics are assumed to degrade
anaerobically. The IPCC default Bo of 0.25 kg CH^kg COD
and default MCF of 0.8 for anaerobic treatment were used
to estimate the CH4 produced from these on-site treatment
systems. The USDA National Agricultural Statistics Service
(USDA 2009a) provided production data for potatoes, other
vegetables, citrus fruit, non-citrus fruit, and grapes processed
for wine. Outflow and BOD data, presented in Table 8-12,
were obtained from EPA (1974) for potato, citrus fruit, and
apple processing, and from EPA (1975) for all other sectors.
Ethanol Production. Ethanol, or ethyl alcohol, is
produced primarily for use as a fuel component, but is
also used in industrial applications and in the manufacture
of beverage alcohol. Ethanol can be produced from the
fermentation of sugar-based feedstocks (e.g., molasses and
beets), starch- or grain-based feedstocks (e.g., corn, sorghum,
and beverage waste), and cellulosic biomass feedstocks (e.g.,
agricultural wastes, wood, and bagasse). Ethanol can also be
produced synthetically from ethylene or hydrogen and carbon
monoxide. However, synthetic ethanol comprises only about
2 percent of ethanol production, and although the Department
of Energy predicts cellulosic ethanol to greatly increase in the
Table 8-12: Wastewater Flow (m3/ton) and BOD
Production (g/L) for U.S. Vegetables, Fruits and
Juices Production
PniT Wastewater Outflow BOD
Commodity (m3/ton) (g/L)
Vegetables
Potatoes
Other Vegetables
Fruit
Apples
Citrus
Non-citrus
Grapes (for wine)
10.27
8.77
3.66
10.11
12.42
2.783
1.765
0.805
1.371
0.317
1.204
1.831
coming years, currently it is only in an experimental stage
in the United States. According to the Renewable Fuels
Association, 82 percent of ethanol production facilities use
corn as the sole feedstock and 7 percent of facilities use a
combination of corn and another starch-based feedstock.
The fermentation of corn is the principal ethanol production
process in the United States and is expected to increase
through 2012, and potentially more; therefore, emissions
associated with wastewater treatment at starch-based ethanol
production facilities were estimated (ERG 2006).
Ethanol is produced from corn (or other starch-based
feedstocks) primarily by two methods: wet milling and dry
milling. Historically, the majority of ethanol was produced
by the wet milling process, but now the majority is produced
by the dry milling process. The wastewater generated
at ethanol production facilities is handled in a variety of
ways. Dry milling facilities often combine the resulting
evaporator condensate with other process wastewaters,
such as equipment wash water, scrubber water, and boiler
blowdown and anaerobically treat this wastewater using
various types of digesters. Wet milling facilities often treat
their steepwater condensate in anaerobic systems followed
by aerobic polishing systems. Wet milling facilities may
treat the stillage (or processed stillage) from the ethanol
fermentation/distillation process separately or together
with steepwater and/or wash water. Methane generated in
anaerobic digesters is commonly collected and either flared
or used as fuel in the ethanol production process (ERG 2006).
Available information was compiled from the industry
on wastewater generation rates, which ranged from 1.25
gallon per gallon ethanol produced (for dry milling) to
10 gallons per gallon ethanol produced (for wet milling)
8-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
(Ruocco 2006a,b; Merrick 1998; Donovan 1996; andNRBP
2001). COD concentrations were also found to be about
3 g/L (Ruocco 2006a; Merrick 1998; White and Johnson
2003). The amount of wastewater treated anaerobically was
estimated, along with how much of the CH4 is recovered
through the use of biomethanators (ERG 2006). Methane
emissions were then estimated as follows:
Methane = {Production x Flow x COD x 3.785 x
[(%Plants0 x %WWa,p x %CODp) +
(%Plantsa x %WWa,s x %CODS) +
(%Plantst x %WWa,t x %CODS)] x
B0 x MCF x % Not Recovered} +
{Production x Flow x 3.785 x COD x
[(%Plants0 x %WWa,p x %CODp) +
(%Plantsa x %WWa,s x %CODS) +
(%Plantst x %WWa,t x %CODS)] x
B0 x MCF x % Recovered x (1-DE)} x 1/109
where,
Production = Gallons ethanol produced
(wet milling or dry milling)
Flow = Gallons wastewater generated per
gallon ethanol produced (1.25 dry
milling, 10 wet milling)
COD = COD concentration in influent (3 g/1)
3.785 = Conversion, gallons to liters
%Plants0 = Percent of plants with onsite
treatment (100%)
%WWap = Percent of wastewater treated
anaerobically in primary treatment
(0%)
%CODp = Percent of COD entering primary
treatment (100%)
%Plantsa = Percent of plants with anaerobic
secondary treatment (33.3% wet,
75% dry)
%Plantst = Percent of plants with other
secondary treatment (66.7% wet,
25% dry)
%WWa s = Percent of wastewater treated
anaerobically in anaerobic
secondary treatment (100%)
%WWSit = Percent of wastewater treated
anaerobically in other secondary
treatment (0%)
%CODS = Percent of COD entering secondary
treatment (100%)
B0 = Maximum methane producing
capacity (0.25 g CH4/g COD)
MCF = Methane conversion factor (0.8 for
anaerobic systems)
% Recovered = Percent of wastewater treated in
system with emission recovery
%Not
Recovered =
DE
1/109
1 -percent of wastewater treated in
system with emission recovery
= Destruction efficiency of recovery
system (99%)
= Conversion factor, g to Gg
A time series of CH4 emissions for 1990 through 2008
was developed based on production data from the Renewable
Fuels Association (RFA 2009).
Petroleum Refining. Petroleum refining wastewater
treatment operations produce CH4 emissions from anaerobic
wastewater treatment. The wastewater inventory section
includes CH4 emissions from petroleum refining wastewater
treated on site under intended or unintended anaerobic
conditions. Most facilities use aerated biological systems,
such as trickling filters or rotating biological contactors; these
systems can also exhibit anaerobic conditions that can result
in the production of CH4. Oil/water separators are used as
a primary treatment method; however, it is unlikely that any
COD is removed in this step.
Available information from the industry was compiled.
The wastewater generation rate, from CARB 2007 and
Timm 1985, was determined to be 35 gallons per barrel of
finished product. An average COD value in the wastewater
was estimated at 0.45 kg/m3 (Benyahia et al.).
The equation used to calculate CH4 generation at
petroleum refining wastewater treatment systems is presented
below:
Methane = Flow x COD x B0 x MCF
Where:
Flow
COD
Annual flow treated through
anaerobic treatment system (mVyear)
COD loading in wastewater entering
anaerobic treatment system (kg/m3)
Waste 8-13
-------
B0 = Maximum methane producing
potential of industrial wastewater
(default value of 0.25 kg CH4 /
kg COD)
MCF = Methane conversion factor (0.3)
A time series of CH4 emissions for 1990 through 2008
was developed based on production data from the Energy
Information Association (ElA 2009).
Domestic Wastewater N20 Emission Estimates
N2O emissions from domestic wastewater (wastewater
treatment) were estimated using the IPCC (2006)
methodology, including calculations that take into account N
removal with sewage sludge, non-consumption and industrial
wastewater N, and emissions from advanced centralized
wastewater treatment plants:
• In the United States, a certain amount of N is removed
with sewage sludge, which is applied to land, incinerated,
or landfilled (NSLUDGE). The N disposal into aquatic
environments is reduced to account for the sewage sludge
application.
• The IPCC methodology uses annual, per capita protein
consumption (kg protein/[person-year]). For this
Inventory, the amount of protein available to be consumed
is estimated based on per capita annual food availability
data and its protein content, and then adjusts that data
using a factor to account for the fraction of protein
actually consumed.
• Small amounts of gaseous nitrogen oxides are formed
as byproducts in the conversion of nitrate to N gas in
anoxic biological treatment systems. Approximately
7 grams N2O is generated per capita per year if
wastewater treatment includes intentional nitrification
and denitrification (Scheehle and Doom 2001). Analysis
of the 2000 CWNS shows that plants with denitrification
as one of their unit operations serve a population of
2.4 million people. Based on an emission factor of 7
grams per capita per year, approximately 17.5 metric
tons of additional N2O may have been emitted via
denitrification in 2000. Similar analyses were completed
for each year in the Inventory using data from CWNS
on the amount of wastewater in centralized systems
treated in denitrification units. Plants without intentional
nitrification/denitrification are assumed to generate 3.2
grams N2O per capita per year.
With the modifications described above, N2O emissions
from domestic wastewater were estimated using the following
methodology:
N2OTOTAL = N2OPLANT + N2OEFFLUENT
N2OPLANT = N2ONrr/DENiT + N2OWour Nrr/DENrr
x EF x F.] x 1/10
N2OWOuTNroDENrr= {[(USPOP x WWTP) - USPOPND x
x EFj} x 1/109
x Protein x
N2OEFFLUENT = {[((USpop - (0.9 x
FNPR x F
NPR NON_CON
- N
SLUDGE
] x
EF3x 44/28 }xl/106
where,
N2OTOTAL
N2OPLANT
N9C
= Annual emissions of N2O (kg)
= N2O emissions from centralized
wastewater treatment plants (kg)
= N2O emissions from centralized
wastewater treatment plants with
nitrification/denitrification (kg)
N2OWOuTNroDENrr= N2O emissions from centralized
wastewater treatment plants
without nitrification/denitrification
(kg)
N2OEFFLUENT = N2O emissions from wastewater
effluent discharged to aquatic
environments (kg)
USpop = U.S. population
= U.S. population that is served by
biological denitrification (from
CWNS)
= Fraction of population using
WWTP (as opposed to septic
systems)
= Emission factor (3 .2 g N2O/person-
year) - plant with no intentional
denitrification
= Emission factor (7 g N2O/person-
year) - plant with intentional
denitrification
= Annual per capita protein
consumption (kg/person/year)
= Fraction of N in protein, default =
0.16 (kg N/kg protein)
WWTP
EFj
EF2
Protein
NPR
8-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 8-13: U.S. Population (Millions), Available Protein
[kg/(person-year)], and Protein Consumed
[kg/(person-year)j
Year
Population
Available
Protein
Protein
Consumed
1990
38.7
29.6
2000
2001
2002
2003
2004
2005
2006
2007
2008
286
289
292
294
297
300
303
306
308
41.3
42.0
40.9
40.9
41.3
41.7
41.9
42.1
42.2
31.6
32.1
31.3
31.3
31.6
32.1
32.1
32.2
32.4
Source: U.S. Census Bureau (2009a), USDA (2009b).
^NON-CON
FIND-COM
NSLUDGE
EF3
0.9
44/28
= Factor for non-consumed protein
added to wastewater (1.4)
= Factor for industrial and commercial
co-discharged protein into the
sewer system (1.25)
= N removed with sludge, kg N/yr
= Emission factor (0.005 kg N2O -N/
kg sewage-N produced) - from
effluent
= Amount of nitrogen removed by
denitrification systems
= Molecular weight ratio of N2O to
N2
U.S. population data were taken from the U.S. Census
Bureau International Database (U.S. Census 2009a) and
include the populations of the United States, American
Samoa, Guam, Northern Mariana Islands, Puerto Rico,
and the Virgin Islands. The fraction of the U.S. population
using wastewater treatment plants is based on data from the
1989,1991,1993,1995,1997,1999,2001, 2003, 2005, and
2007 American Housing Survey (U.S. Census 2007). Data
for intervening years were obtained by linear interpolation.
The emission factor (EFj) used to estimate emissions from
wastewater treatment was taken from IPCC (2006). Data
on annual per capita protein intake were provided by U.S.
Department of Agriculture Economic Research Service
(USDA 2009b). Protein consumption data for 2005 through
2008 were extrapolated from data for 1990 through 2004.
Table 8-13 presents the data for U.S. population and average
protein intake. An emission factor to estimate emissions from
effluent (EF3) has not been specifically estimated for the
United States, thus the default IPCC value (0.005 kg N2O-N/
kg sewage-N produced) was applied. The fraction of N in
protein (0.16 kg N/kg protein) was also obtained from IPCC
(2006). Sludge generation was obtained from EPA (1999)
for 1988,1996, and 1998 and from Beecher et al. (2007) for
2004. Intervening years were interpolated, and estimates
for 2005 through 2008 were forecasted from the rest of the
time series. An estimate for the nitrogen removed as sludge
(NSLUDGE) was obtained by determining the amount of sludge
disposed by incineration, by land application (agriculture or
other), through surface disposal, in landfills, or through ocean
dumping. In 2008, 269 Tg N was removed with sludge.
Uncertainty and Time-Series Consistency
The overall uncertainty associated with both the 2008
CH4 and N2O emissions estimates from wastewater treatment
and discharge was calculated using the IPCC Good Practice
Guidance Tier 2 methodology (2000). Uncertainty associated
with the parameters used to estimate CH4 emissions include
that of numerous input variables used to model emissions
from domestic wastewater, and wastewater from pulp and
paper manufacture, meat and poultry processing, fruits and
vegetable processing, ethanol production, and petroleum
refining. Uncertainty associated with the parameters used
to estimate N2O emissions include that of sewage sludge
disposal, total U.S. population, average protein consumed per
person, fraction of N in protein, non-consumption nitrogen
factor, emission factors per capita and per mass of sewage-N,
and for the percentage of total population using centralized
wastewater treatment plants.
The results of this Tier 2 quantitative uncertainty analysis
are summarized in Table 8-14. Methane emissions from
wastewater treatment were estimated to be between 15.3
and 35.8 Tg CO2 Eq. at the 95 percent confidence level (or
in 19 out of 20 Monte Carlo Stochastic Simulations). This
indicates a range of approximately 37 percent below to 47
percent above the 2008 emissions estimate of 24.3 Tg CO2
Eq. Nitrous oxide emissions from wastewater treatment
were estimated to be between 1.2 and 9.6 Tg CO2 Eq., which
indicates a range of approximately 77 percent below to 94
percent above the actual 2008 emissions estimate of 4.9 Tg
CO2 Eq.
Waste 8-15
-------
Table 8-14: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 Emissions from Wastewater Treatment
(Tg C02 Eq. and Percent)
Source
Wastewater Treatment
Domestic
Industrial
Domestic Wastewater
Treatment
2008 Emission Estimate Uncertainty Range Relative to Emission Estimate3
Gas (TgC02Eq.) (TgC02Eq.) (%)
CH4
CH4
CH4
N20
24.3
15.7
8.6
4.9
Lower Bound
15.3
7.7
5.2
1.2
Upper Bound
35.8
26.5
13.2
9.6
Lower Bound
-37%
-51%
-40%
-77%
Upper Bound
+47%
+69%
+54%
+94%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
QA/QC and Verification
A QA/QC analysis was performed on activity data,
documentation, and emission calculations. This effort
included a Tier 1 analysis, including the following checks:
• Checked for transcription errors in data input;
• Ensured references were specified for all activity data
used in the calculations;
• Checked a sample of each emission calculation used
for the source category;
• Checked that parameter and emission units were
correctly recorded and that appropriate conversion
factors were used;
• Checked for temporal consistency in time series input
data for each portion of the source category;
• Confirmed that estimates were calculated and
reported for all portions of the source category and
for all years;
• Investigated data gaps that affected emissions
estimates trends; and
• Compared estimates to previous estimates to identify
significant changes.
All transcription errors identified were corrected. The
QA/QC analysis did not reveal any systemic inaccuracies or
incorrect input values.
Planned Improvements Discussion
The methodology to estimate CH4 emissions from
domestic wastewater treatment currently utilizes estimates for
the percentage of centrally treated wastewater that is treated
by aerobic systems and anaerobic systems. These data come
from the 1992,1996, 2000, and 2004 CWNS. The question
of whether activity data for wastewater treatment systems are
sufficient across the timeline to further differentiate aerobic
systems with the potential to generate small amounts of CH4
(aerobic lagoons) versus other types of aerobic systems,
and to differentiate between anaerobic systems to allow for
the use of different MCFs for different types of anaerobic
treatment systems, continues to be explored. Currently, it
is assumed that all aerobic systems are well managed and
produce no CH4 and that all anaerobic systems have an MCF
of 0.8. Efforts to obtain better data reflecting emissions from
various types of municipal treatment systems are currently
being pursued.
Available data on wastewater treatment emissions
at organic chemical manufacturers was reviewed and
determined to be a potentially significant source for the
Inventory. Existing data for wastewater generation and
COD was used along with an estimate of the number of
plants treated wastewater anaerobically to estimate 631 Gg
of CH4 emitted. However, this estimate was performed with
data collected for EPA's Office of Water in the 1980s. A more
recent source of industry-level data that could be used to fully
construct a time series has not been identified. However, data
sources will continue to be investigated in order to include
emissions from wastewater treatment at organic chemical
manufacturers in future inventories.
8-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
A review of other industrial wastewater treatment sources
for those industries believed to discharge significant loads
of BOD and COD has begun. Food processing industries
have the highest potential for CH4 generation due to the
waste characteristics generated, and the greater likelihood
to treat the wastes anaerobically. However, in all cases there
is dated information available on U.S. treatment operations
for these industries. The canned and seafood processing
and miscellaneous foods and beverages industries (including
wineries and distilleries) will be specifically reviewed to
estimate their potential to generate CH4. Industry-level
emissions will be estimated assuming anaerobic treatment
systems are in place, and consider potential inclusion of these
categories in future years of the Inventory.
With respect to estimating N2O emissions, the default
emission factor for indirect N2O from wastewater effluent and
direct N2O from centralized wastewater treatment facilities
has a high uncertainty. Current research is being conducted
by the Water Environment Research Foundation to measure
N2O emissions from municipal treatment systems. Such
data will be reviewed as they are available to determine if
a country-specific N2O emission factor can or should be
developed.
In addition, the estimate of N entering municipal
treatment systems is under review. The factor that accounts
for non-sewage nitrogen in wastewater (bath, laundry,
kitchen, industrial components) also has a high uncertainty.
Obtaining data on the changes in average influent N
concentrations to centralized treatment systems over the
time series would improve the estimate of total N entering
the system, which would reduce or eliminate the need for
other factors for non-consumed protein or industrial flow.
The dataset previously provided by NACWA was reviewed
to determine if it was representative of the larger population
of centralized treatment plants for potential inclusion into
the inventory. However, this limited dataset did not represent
the number of systems by state and the service populations
served in the United States.
8.3 Composting (IPCC Source
Category 6D)
Composting of organic waste, such as food waste, garden
(yard) and park waste and sludge, is common in the United
States. Advantages of composting include reduced volume in
the waste material, stabilization of the waste, and destruction
of pathogens in the waste material. The end products of
composting, depending on its quality, can be recycled as
fertilizer and soil amendment, or be disposed in a landfill.
Composting is an aerobic process and a large fraction
of the degradable organic carbon in the waste material is
converted into CO2. Methane is formed in anaerobic sections
of the compost, but it is oxidized to a large extent in the
aerobic sections of the compost. Anaerobic sections are
created in composting piles when there is excessive moisture
or inadequate aeration (or mixing) of the compost pile. The
estimated CH4 released into the atmosphere ranges from
less than 1 percent to a few per cent of the initial C content
in the material (IPCC 2006). Composting can also produce
N2O emissions. The range of the estimated emissions varies
from less than 0.5 percent to 5 percent of the initial nitrogen
content of the material (IPCC 2006).
From 1990 to 2008, the amount of material composted
in the United States has increased from 3,810 Gg to 19,886
Gg, an increase of approximately 422 percent. Emissions of
CH4 and N2O from composting have increased by the same
percentage (see Table 8-15 and Table 8-16). In 2008, CH4
emissions from composting were 1.7 Tg CO2 Eq. (80 Gg),
and N2O emissions from composting were 1.8 Tg CO2 Eq. (6
Gg). The wastes that are composted include primarily yard
trimmings (grass, leaves, and tree and brush trimmings) and
food scraps from residences and commercial establishments
(such as grocery stores, restaurants, and school and factory
cafeterias). The composting waste quantities reported here do
not include backyard composting. The growth in composting
is attributable primarily to two factors: (1) steady growth in
population and residential housing, and (2) state and local
governments started enacting legislation that discouraged the
disposal of yard trimmings in landfills. In 1992,11 states and
the District of Columbia had legislation in effect that banned
or discouraged disposal of yard trimmings in landfills. In
2005, 21 states and the District of Columbia, representing
about 50 percent of the nation's population, had enacted such
legislation (EPA 2006).
Methodology
Methane and N2O emissions from composting depend
on factors such as the type of waste composted, the amount
and type of supporting material (such as wood chips and
Waste 8-17
-------
Table 8-15: CH4 and N20 Emissions from Composting (Tg C02 Eq.)
Gas
CH4
N20
Total
1990H
0.3
0.4l
0.7 1
1995
0.7
0.8
1.5 |
2000
1.3
1.4
2.6 |
2005
1.6
1.7
3.3
2006
1.6
1.8
3.3
2007
1.7
1.8
3.5
2008
1.7
1.8
3.5
Table 8-16: CH4 and N20 Emissions from Composting (Gg)
Gas
CH4
N20
1990
15
1
1995
35
3
2000
60
4
2005
75
6
2006
75
6
2007
79
6
2008
80
6
peat) used, temperature, moisture content and aeration during
the process.
The emissions shown in Table 8-15 and Table 8-16 were
estimated using the IPCC default (Tier 1) methodology (IPCC
2006), which is the product of an emission factor and the
mass of organic waste composted (note: no CH4 recovery is
expected to occur at composting operations):
Ef = Mx EFf
where,
Ej = CH4 or N2O emissions from composting,
Gg CH4 or N2O
M = mass of organic waste composted in Gg
EFj = emission factor for composting, 4 g CH4/kg
of waste treated (wet basis) and 0.3 g N2O/
kg of waste treated (wet basis)
i = designates either CH4 or N2O
Estimates of the quantity of waste composted (M) are
presented in Table 8-17. Estimates of the quantity composted
for 1990 and 1995 were taken from the Characterization of
Municipal Solid Waste in the United States: 1996 Update
(Franklin Associates 1997); estimates of the quantity
composted for 2000,2005,2006, and 2007 were taken from
EPA's Municipal Solid Waste In The United States: 2007
Facts and Figures (EPA 2008); estimates of the quantity
composted for 2008 were calculated using the 2007 quantity
composted.
Uncertainty and Time-Series Consistency
The estimated uncertainty from the 2006 IPCC
Guidelines is ±50 percent for the Tier 1 methodology.
Emissions from composting in 2008 were estimated to be
between 1.8 and 5.3 Tg CO2 Eq., which indicates a range
of 50 percent below to 50 percent above the actual 2008
emission estimate of 3.5 Tg CO2 Eq. (see Table 8-18).
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 8-17: U.S. Waste Composted (Gg)
Activity
1990
1995
2000
2005
2006
2007
2008
Waste Composted
3,810
8,682
14,923
18,643 18,852 19,695 19,886
Source: Franklin Associates (1997) and EPA (2008).
Table 8-18: Tier 1 Quantitative Uncertainty Estimates for Emissions from Composting (Tg C02 Eq. and Percent)
Source
2007 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Composting
CH4, N20
3.5
1.8
5.3
-50%
+50%
8-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Planned Improvements
For future Inventories, additional efforts will be made
to improve the estimates of CH4 and N2O emissions from
composting. For example, a literature search may be
conducted to determine if emission factors specific to various
composting systems and composted materials are available.
8.4 Waste Sources of Indirect
Greenhouse Gases
In addition to the main greenhouse gases addressed
above, waste generating and handling processes are also
sources of indirect greenhouse gas emissions. Total
emissions of NOX, CO, and NMVOCs from waste sources
for the years 1990 through 2008 are provided in Table 8-19.
Methodology
These emission estimates were obtained from preliminary
data (EPA 2009), and disaggregated based on EPA (2003),
which, in its final iteration, will be published on the National
Emission Inventory (NET) Air Pollutant Emission Trends web
site. Emission estimates of these gases were provided by
sector, using a "top down" estimating procedure-emissions
were calculated either for individual sources or for many
sources combined, using basic activity data (e.g., the amount
of raw material processed) as an indicator of emissions.
National activity data were collected for individual source
categories from various agencies. Depending on the source
category, these basic activity data may include data on
production, fuel deliveries, raw material processed, etc.
Activity data were used in conjunction with emission
factors, which relate the quantity of emissions to the activity.
Emission factors are generally available from the EPA's
Compilation of Air Pollutant Emission Factors, AP-42 (EPA
1997). The EPA currently derives the overall emission
control efficiency of a source category from a variety of
information sources, including published reports, the 1985
National Acid Precipitation and Assessment Program
Emissions Inventory, and other EPA databases.
Uncertainty and Time-Series Consistency
No quantitative estimates of uncertainty were calculated
for this source category. Uncertainties in these estimates,
however, are primarily due to the accuracy of the emission
factors used and accurate estimates of activity data.
Methodological recalculations were applied to the entire
time series to ensure time-series consistency from 1990
through 2008. Details on the emission trends through time
are described in more detail in the Methodology section,
above.
Table 8-19: Emissions of NOX, CO, and NMVOCs from Waste (Gg)
Gas/Source
1990
1995
2000
2005
2006
2007
2008
NOX
Landfills
Wastewater Treatment
Miscellaneous3
CO
Landfills
Wastewater Treatment
Miscellaneous3
NMVOCs
Wastewater Treatment
Miscellaneous3
Landfills
673
58
57
557
1
7311
681
61
602
.
•I
1
119
22
51
461
114
22
49
43
113
21
49
43
111
21
48
42
109
21
47
41
+ Does not exceed 0.5 Gg.
a Miscellaneous includes TSDFs (Treatment, Storage, and Disposal Facilities under the Resource Conservation and Recovery Act [42 U.S.C. §
6924, SWDA § 3004]) and other waste categories.
Note: Totals may not sum due to independent rounding.
Waste 8-19
-------
9. Other
T
I he United States does not report any greenhouse gas emissions under the Intergovernmental Panel on Climate
Change (IPCC) "Other" sector.
Other 9-1
-------
1O. Recalculations and
Improvements
Each year, emission and sink estimates are recalculated and revised for all years in the Inventory of U.S. Greenhouse
Gas Emissions and Sinks, as attempts are made to improve both the analyses themselves, through the use of
better methods or data, and the overall usefulness of the report. In this effort, the United States follows the
Intergovernmental Panel on Climate Change (IPCC) Good Practice Guidance, which states, "It is good practice to recalculate
historic emissions when methods are changed or refined, when new source categories are included in the national inventory,
or when errors in the estimates are identified and corrected" (IPCC 2000).
The results of all methodological changes and historical data updates are presented in this section; detailed descriptions
of each recalculation are contained within each source's description found in this report, if applicable. Table 10-1 summarizes
the quantitative effect of these changes on U.S. greenhouse gas emissions and Table 10-2 summarizes the quantitative effect
on net CO2 flux to the atmosphere, both relative to the previously published U.S. Inventory (i.e., the 1990 through 2007
report). These tables present the magnitude of these changes in units of teragrams of carbon dioxide equivalent (Tg CO2 Eq.).
The Recalculations Discussion section of each source presents the details of each recalculation. In general, when
methodological changes have been implemented, the entire time series (i.e., 1990 through 2007) has been recalculated to
reflect the change, per IPCC (2000). Changes in historical data are generally the result of changes in statistical data supplied
by other agencies.
The following emission sources, which are listed in descending order of absolute average annual change in emissions
between 1990 and 2007, underwent some of the most important methodological and historical data changes. A brief summary
of the recalculation and/or improvement undertaken is provided for each emission source.
• Land Use, Land-use Change, and Forestry. The time series for Forest Land area was updated with a new release of data
from the Forest Inventory and Analysis Program. Most eastern states have completed the first full cycle of annualized
inventories and are providing annual updates to the state's forest inventory, resulting in increased accuracy of the model
used to estimate Forest Land area. In addition, an average estimate of logging residue was incorporated into the down
dead wood carbon calculations to explicitly account for down dead wood following harvest on lands that were reforested.
These updates resulted in an average annual decrease in carbon storage of 8 percent. Also, changes in carbon stock in
urban trees from the incorporation of new data on the city of Chicago resulted in a lower median gross sequestration
value and a lower net sequestration to gross sequestration ratio for the set of 14 cities used to estimate total carbon
sequestration in urban trees. On average, the change in carbon stocks from urban trees led to a 5.9 percent decrease in
annual sequestration from 1990 through 2007. Overall, these changes resulted in an average annual increase in estimated
net flux of CO2 to the atmosphere of 77.6 Tg CO2 Eq. (7 percent) for the period 1990 through 2007 compared to the
previous Inventory's estimate of Forest Land net CO2 flux.
Recalculations and Improvements 10-1
-------
• Fossil Fuel Combustion. Changes in CO2 emissions
from Fossil Fuel Combustion resulted from updated
energy consumption statistics provided by the Energy
Information Administration. These revisions impacted
the emission estimates for the entire time series. Carbon
content coefficients were also updated based on an EPA
analysis. Overall, these changes resulted in an average
annual increase of 23.7 Tg CO2 Eq. (0.4 percent) in CO2
emissions from Fossil Fuel Combustion for the period
1990 through 2007.
• Iron and Steel Production & Metallurgical Coke
Production. In the previous Inventory, pig iron
consumption for basic oxygen furnaces was being
counted twice as a process input. This was the result of
an incorrect interpretation of two tables in the American
Iron and Steel Institute (AISI) Annual Statistical
Yearbook. This issue has been corrected and decreased
the 1990 through 2007 emissions from iron and steel
production by an average of 8 percent per year. Overall,
these changes resulted in an average annual decrease of
6.5 Tg CO2 Eq. (7.1 percent) in CO2 emissions from Iron
and Steel Production and Metallurgical Coke Production
for the period of 1990 through 2007.
• Forest Land Remaining Forest Land (N2O). Nitrogen
fertilizer applications to commercial Douglas-fir forests
in western Oregon and Washington were added to the
N2O Fluxes from Soils category, which resulted in an
average annual increase in emissions of approximately
24 percent compared to the previous Inventory.
Additionally, non-CO2 emissions were recalculated
using the 2006 IPCC default emission factors for CH4
and N2O. The update caused the estimated values for
N2O emissions to increase by a factor of approximately
four. Overall, these changes resulted in an average
annual increase of 5.7 Tg CO2 Eq. (282 percent) in N2O
emissions from Forest Land Remaining Forest Land for
the period 1990 through 2007.
• Incineration of Waste. Changes in CO2 emissions
from Incineration of Waste stem from two changes in
methodology. First, rather than basing the estimate of
the percentage discards combusted on data from MSW
Facts and Figures, as had been done in previous years,
EPA updated the percent of discards combusted with
Biocycle's time series estimate. The change in the source
for percentage combusted was made because using
Biocycle data for discards is in line with other estimates
in the Inventory; Biocycle data are used to estimate CH4
emissions from landfills and N2O emissions from waste
incineration. This change decreases CO2 emissions
annually on average by 32 percent for materials other
than tires (the estimate for tires is not affected by
this change). Additionally, the Rubber Manufacturers
Association changed their reporting for the scrap tire
market for 2007 and as a result, EPA had to adjust the
calculations for CO2 from scrap tire incineration with
updated data on scrap tire weight, tire composition,
and scrap tire market composition. This updated
methodology resulted in an average annual increase
in CO2 emissions from scrap tire incineration of 52
percent. Overall, these changes resulted in an average
annual decrease of 5.4 Tg CO2 Eq. (30 percent) in CO2
emissions from Incineration of Waste.
• Agricultural Soil Management. Several revisions were
made for the current Inventory that resulted in changes
in N2O emissions from Agricultural Soil Management.
In the previous Inventory, it was assumed that nitrate
leaching was not significant in soils with precipitation
input that did not exceed potential evapotranspiration,
except in soils that were irrigated. Quality control
measures revealed that nitrate leaching was under-
estimated using this criterion, so the threshold was
revised to better reflect U.S. conditions. Second, in
the previous Inventory, the leaching criterion was not
applied for lands estimated using Tier 1 methodology.
For this year's Inventory, nitrate leaching was assumed
to occur in states where the area weighted mean
precipitation plus irrigation input was equal to or greater
than 80 percent of the potential evapotranspiration.
Third, the emission factor for pasture/range/paddock
manure associated with horses, sheep and goats was
revised to 0.01 in accordance with guidance from IPCC
(2006). Fourth, the methodology to calculate livestock
manure N was changed such that total manure N added
to soils increased by approximately 5 percent. Overall,
these changes resulted in an average annual increase
of 4.4 Tg CO2 Eq. (2 percent) in N2O emissions from
Agricultural Soil Management.
10-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Forest Land Remaining Forest Land (CH4). Non-CO2
emissions were recalculated using the 2006IPCC default
emission factors for CH4 and N2O. Updating to the 2006
IPCC emission factors results in estimates for CH4
emissions decreasing by a factor of approximately one
third between methods. Overall, these changes resulted
in an average annual decrease of 3.8 Tg CO2 Eq. (24
percent) in CH4 emissions from Forest Land Remaining
Forest Land for the period 1990 through 2007.
Natural Gas Systems (C02). Changes in CO2
emissions from Natural Gas Systems are mostly the
result of updating the previous Inventory activity
data with revised values from the Federal Energy
Regulatory Commission. In addition, the data source
for the number of liquefied natural gas (LNG) import
terminals was changed to Federal Energy Regulatory
Commission-reported data to provide a more accurate
and current emissions estimate from LNG import
terminals. Overall, these changes resulted in an average
annual increase of 3.0 Tg CO2 Eq. (9 percent) in CO2
emissions from Natural Gas Systems for the period 1990
through 2007.
Manure Management (N20). For the current Inventory,
cattle population data and total Kjeldahl nitrogen
excretion rate (Nex) data were incorporated from the
Cattle Enteric Fermentation Model. Population and Nex
changes resulted in increases in N2O emissions across
the timeseries. Overall, these changes resulted in an
average annual increase of 2.4 Tg CO2 Eq. (18 percent)
in N2O emissions from Manure Management for the
period 1990 through 2007.
Non-Energy Use of Fuels. Changes in CO2 emissions
from Non-Energy Use of Fuels are the result of changes
to the scrap tire, carbon black, and synthetic rubber
carbon emissions and updates to the energy recovery
emission estimates. The Rubber Manufacturers
Association's Scrap Tire Markets in the United States:
9th Biennial Edition began reporting the amount of
scrap tires in each end use market in thousands of
tons (as opposed to millions of tires as they had done
previously). RMA also updated their assumed weight
for passenger and commercial scrap tires to 22.5
pounds and 110 pounds, respectively. As a result, the
percentage of rubber abraded during tire use for these
two categories was reduced from 20 percent for all
tires to 10 and 8 percent for passenger and commercial
tires, respectively. These updates resulted in an average
73 percent reduction in carbon black emissions and an
average 68 percent reduction in synthetic rubber carbon
emissions per year across the time series. Additionally,
energy recovery emissions were updated with new
Manufacturer's Energy Consumption Survey (MFCS)
data for 2006, released this past year by the Energy
Information Administration. This update resulted in an
average annual increase of 4 percent in emissions from
feedstocks for 2003 and 2007. Overall, these changes
resulted in an average annual increase of 1.8 Tg CO2 Eq.
(1.4 percent) CO2 emissions from Non-Energy Use of
Fuels for the period of 1990 through 2007.
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg C02 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
US Territories
Non-Energy Use of Fuels
Iron and Steel Production &
Metallurgical Coke Production
Cement Production
Natural Gas Systems
Lime Production
1990
24.1
26.8
1,1
1.3
11.2
1.4l
1
(0.4)
2.6 1
(7.2)
NC
3.6 1
NC
1995
(7.4)
NC!
8.4l
NC
2000 2005
22.0
31.9
13.7
9.2
7.7
0.9
0.7
(0.3)
1.7
(7.0)
NC
NC
17.6
29.9
21.1
13.9
(2.4)
0.4
(0.5)
(2.6)
(1.5)
(5.5)
NC
+
NC
2006
2.3
17.4
19.1
(4.2)
6.2
0.2
+
(3.9)
(3.8)
(5.6)
NC
+
NC
2007
16.8
21.2
15.6
6.3
(3.2)
1.1
3.0
(1.7)
1.4
(4.6)
0.7
2.1
NC
Recalculations and Improvements 10-3
-------
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg C02 Eq.) (continued)
Gas/Source
Incineration of Waste
Ammonia Production and Urea Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands3
Petroleum Systems
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
Land Use, Land-Use Change,
and Forestry (Sink)b
Wood Biomass and Ethanol Consumption3
International Bunker Fuels3
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Wastewater Treatment
Forest Land Remaining Forest Land
Rice Cultivation
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
Composting
Field Burning of Agricultural Residues
Petrochemical Production
Iron and Steel Production &
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and Consumption
Incineration of Wasteb
International Bunker Fuels3
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Forest Land Remaining Forest Land
Wastewater Treatment
1990 1995
(2.9) (4.3)
NC
NC
NC
NC
NC
1.1
NC
NC
NC
NC
NC
0.2
+
NC
NC
NC
NC!
+ l
NC
NC
1.4
NC
NC!
NC!
NC!
NC
Inol
(68. om s.im
NC A/cl
(2.5) (1.8)
(3.2) (2.6)
(0.8) 0.1 1
0.1 (0.2)
+ l +l
NC| NC|
(1.1) (0.6)
+ l +l
+ 1 +1
(1.4) (1.9)
NC NCI
+
NC
+
NC
0.1
NC
NC
NC
NC
+
+
7.3
3.1
0.2
(1.1)
2.4
+
2.2
NC
+ 1
NC
8.4
3.6
0.3 1
(1.2)
2.5 •
2.9
NC|
2000
(6.2)
NC
NC!
NC!
NC
NC
1
NC
NC
NC!
NC!
NC!
0.2l
1
NC
NC
53.3
A/cl
(0.5)
(5.1)
2.4|
(1.6)
(0.2)
(0.1)
0.7
1
+ •
(6.3)
NC
NC!
(0.1)
NC|
0.1 1
Ncl
Ncl
NC!
Ncl
+ 1
+ 1
16.4
5. el
(1.2)
2.7
1
2005
(6.9)
NC
NC
NC
NC
NC
1.4
NC
NC
NC
NC
NC
0.2
+
NC
NC
172.3
(2.1)
(1.0)
(8.5)
0.7
(2.3)
(2.7)
(0.2)
0.4
+
+
(4.4)
NC
(0.1)
+
+
NC
0.1
NC
NC
NC
NC
+
+
12.4
5.2
0.2
(1.0)
2.4
+
6.6
+
2006
(7.1)
NC
+
NC
NC
NC
1.3
+
NC
NC
NC
NC
0.2
+
NC
NC
91.4
(2.1)
18.6
(13.7)
0.8
(3.4)
(1.8)
(0.1)
0.5
(0.1)
+
(9.7)
NC
(0.1)
+
(0.1)
NC
0.1
NC
NC
NC
NC
+
+
17.4
2.8
0.1
(1.0)
2.7
+
14.5
+
2007
(7.5)
0.2
0.3
1.5
NC
NC
1.3
0.1
NC
NC
NC
+
0.2
(0.1)
NC
NC
107.2
(2.1)
18.3
(16.1)
2.2
(6.4)
(5.2)
0.4
1.9
+
0.1
(9.0)
+
(0.1)
(0.1)
(0.1)
NC
0.1
NC
NC
NC
NC
+
+
15.8
3.1
0.2
(1.2)
2.6
(0.1)
13.4
+
10-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008
-------
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg C02 Eq.) (continued)
Gas/Source
1990
1995
2000
2005
2006
2007
N20 from Product Uses
Adipic Acid Production
Composting
Settlements Remaining Settlements
Field Burning of Agricultural Residues
Incineration of Waste
Wetlands Remaining Wetlands
International Bunker Fuels3
MFCs
Substitution of Ozone Depleting Substances
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture
NC
0.5
NC
;
NC
NC
+
NC
NC
NC
NC
NC
NC
NC
0.3
NC!
..„_
NC
-
0.4 •
0.4
NC
NC!
NC
NC
NCl NC|
(0.2) (0.2)
(0.2) (0.2)
NC NC
NC NC
NC
(0.7)
NC
(0.1)
+
NC
NC
+
3.1
3.1
NC
NC
NC
NC
NC
(0.1)
(0.1)
NC
NC
(1.0)
NC
+
+
+
NC
+
3.2
3.2
NC
NC
NC
NC
NC
(0.1)
(0.1)
NC
NC NC
NC
(1.6)
NC
+
+
NC
NC
0.2
2.8
2.8
NC
NC
NC
NC
NC
+
+
NC
NC
NC
(2.2)
NC
+
+
NC
NC
0.2
1.9
1.9
NC
NC
NC
NC
NC
(0.4)
+
(0.4)
NC
Net Change in Total Emissions"
28.1
25.5
36.3
24.6
8.8
18.0
Percent Change
0.5%
0.4%
0.5%
0.3%
0.1%
0.3%
+ Absolute value does not exceed 0.05 Tg C02 Eq. or 0.05 percent.
NC (No Change)
3 Not included in emissions total.
b New source category relative to previous Inventory.
c Excludes net C02 flux from Land Use, Land-Use Change, and Forestry, and emissions from International Bunker Fuels and Wood Biomass and
Ethanol Consumption.
Note: Totals may not sum due to independent rounding.
Table 10-2: Revisions to Net Flux of C02 to the Atmosphere from Land Use, Land-Use Change,
and Forestry (Tg C02 Eq.)
Component: Net C02 Flux From Land Use, Land-
Use Change, and Forestry
Forest Land Remaining Forest Land
Cropland Remaining Cropland
Land Converted to Cropland
Grassland Remaining Grassland
Land Converted to Grassland
Settlements Remaining Settlements
Other
Net Change in Total Flux
Percent Change
1990
(68.8)
NC!
NC!
(5.3)1
25|
3.6 •
Ncl
(68.0)
-8.1 %|
1995
(6.0)
NC
NC
9.7
0.2l
4.2l
NC
8.1
1.0% |
2000
44.9
Ncl
Ncl
(1.2)
41
4.9 •
NC
53.3
7.4%
2005
169.1
NC
NC
(4.4)
2.1
5.5
+
172.3
15.4%
2006
87.8
NC
NC
(4.3)
2.2
5.7
+
91.4
8.7%
2007
103.2
NC
NC
(4.1)
2.4
5.8
+
107.2
10.1%
+ Absolute value does not exceed 0.05 Tg C02 Eq. or 0.05 percent.
NC (No Change)
Note: Numbers in parentheses indicate a decrease in estimated net flux of C02 to the atmosphere or an increase in net sequestration.
Note: Totals may not sum due to independent rounding.
Recalculations and Improvements 10-5
-------
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-------
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United States
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Agency
EPA 430-R-10-006 April 2010
Office of Atmospheric Programs (6207J)
Washington, DC 20460
Official Business
Penalty for Private Use
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