v/EPA Inventory of U.S. Greenhouse Gas
Emissions and Sinks: 1990-2006
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INVENTORY OF U.S. GREENHOUSE GAS
EMISSIONS AND SINKS:
199O-2OO6
April 15,2008
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 fuel combustion and industrial process emissions was led by Leif Hockstad and Mausami Desai. Work on
methane emissions from the energy sector was directed by Lisa Hanle. Calculations for the waste sector were led by Melissa
Weitz. Tom Wirth directed work on the Agriculture chapter, and Kimberly Klunich directed work on the Land Use, Land-
Use Change, and Forestry chapter. Work on emissions of HFCs, PFCs, and SF6 was directed by Deborah Ottinger and Dave
Godwin. John Davies 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 and
Resources Practice, including Don Robinson, Diana Pape, Susan Asam, Michael Grant, Ravi Kantamaneni, Robert Lanza,
Chris Steuer, Lauren Pederson, Kamala Jayaraman, Jeremy Scharfenberg, Mollie Averyt, Sarah Shapiro, Nina Kshetry,
Pankaj Kumar, Stacy Hetzel, Brian Gillis, Zachary Schaffer, Vineet Aggarwal, Colin McGroarty, Hemant Mallya, Victoria
Thompson, Jean Kim, Tristan Kessler, Sarah Menassian, Katrin Moffroid, Veronica Kennedy, Joseph Aamidor, Aaron
Beaudette, Dylan Harrison-Atlas, Nikhil Nadkarni, Joseph Herr, and Toby Krasney 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).1 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.2 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.
1 See Article 4(l)(a) of the United Nations Framework Convention on Climate Change .
2 See .
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Table of Contents
Acknowledgments i
Table of Contents v
List of Tables, Figures, and Boxes viii
Tables viii
Figures xvii
Boxes xix
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 1-7
1.4. Methodology and Data Sources 1-10
1.5. Key Categories 1-11
1.6. Quality Assurance and Quality Control (QA/QC) 1-11
1.7. Uncertainty Analysis of Emission Estimates 1-13
1.8. Completeness 1-15
1.9. Organization of Report 1-15
2. Trends in Greenhouse Gas Emissions 2-1
2.1. Recent Trends in U.S. Greenhouse Gas Emissions 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. Carbon Dioxide Emissions from Fossil Fuel Combustion (IPCC Source Category 1A) 3-3
3.2. Carbon Emitted from Non-Energy Use of Fossil Fuels (IPCC Source Category 1A) 3-19
3.3. Stationary Combustion (excluding CO2) (IPCC Source Category 1A) 3-24
3.4. Mobile Combustion (excluding CO2) (IPCC Source Category 1A) 3-29
3.5. Coal Mining (IPCC Source Category IBla) 3-36
3.6. Abandoned Underground Coal Mines (IPCC Source Category IB la) 3-39
3.7. Natural Gas Systems (IPCC Source Category lB2b) 3-42
3.8. Petroleum Systems (IPCC Source Category lB2a) 3-46
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3.9. Municipal Solid Waste Combustion (IPCC Source Category 1A5) 3-50
3.10. Energy Sources of Indirect Greenhouse Gas Emissions 3-54
3.11. International Bunker Fuels (IPCC Source Category 1: Memo Items) 3-55
3.12. Wood Biomass and Ethanol Consumption (IPCC Source Category 1A) 3-60
4. Industrial Processes 4-1
4.1. Cement Production (IPCC Source Category 2A1) 4-4
4.2. Lime Production (IPCC Source Category 2A2) 4-6
4.3. Limestone and Dolomite Use (IPCC Source Category 2A3) 4-9
4.4. Soda Ash Production and Consumption (IPCC Source Category 2A4) 4-12
4.5. Ammonia Production (IPCC Source Category 2B1) and Urea Consumption 4-14
4.6. Nitric Acid Production (IPCC Source Category 2B2) 4-18
4.7. Adipic Acid Production (IPCC Source Category 2B3) 4-19
4.8. Silicon Carbide Production (IPCC Source Category 2B4) and Consumption 4-22
4.9. Petrochemical Production (IPCC Source Category 2B5) 4-23
4.10. Titanium Dioxide Production (IPCC Source Category 2B5) 4-26
4.11. Carbon Dioxide Consumption (IPCC Source Category 2B5) 4-28
4.12. Phosphoric Acid Production (IPCC Source Category 2B5) 4-30
4.13. Iron and Steel Production (IPCC Source Category 2C1) 4-33
4.14. Ferroalloy Production (IPCC Source Category 2C2) 4-37
4.15. Aluminum Production (IPCC Source Category 2C3) 4-39
4.16. Magnesium Production and Processing (IPCC Source Category 2C4) 4-43
4.17. Zinc Production (IPCC Source Category 2C5) 4-46
4.18. Lead Production (IPCC Source Category 2C5) 4-49
4.19. HCFC-22 Production (IPCC Source Category 2E1) 4-50
4.20. Substitution of Ozone Depleting Substances (IPCC Source Category 2F) 4-52
4.21. Semiconductor Manufacture (IPCC Source Category 2F6) 4-56
4.22. Electrical Transmission and Distribution (IPCC Source Category 2F7) 4-61
4.23. Industrial Sources of Indirect Greenhouse Gases 4-66
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. Puce Cultivation (IPCC Source Category 4C) 6-12
6.4. Agricultural Soil Management (IPCC Source Category 4D) 6-17
6.5. Field Burning of Agricultural Residues (IPCC Source Category 4F) 6-31
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7. Land Use, Land-Use Change, and Forestry 7-1
7.1. Representation of the U.S. Land Base 7-3
7.2. Forest Land Remaining Forest Land 7-11
7.3. Land Converted to Forest Land (IPCC Source Category 5A2) 7-24
7.4. Cropland Remaining Cropland (IPCC Source Category 5B1) 7-24
7.5. Land Converted to Cropland (IPCC Source Category 5B2) 7-37
7.6. Grassland Remaining Grassland (IPCC Source Category 5C1) 7-41
7.7. Land Converted to Grassland (IPCC Source Category 5C2) 7-46
7.8. Settlements Remaining Settlements 7-50
7.9. Land Converted to Settlements (Source Category 5E2) 7-55
7.10. Other (IPCC Source Category 5G) 7-56
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 CK4, N2O, and Indirect Greenhouse Gases from
Stationary Combustion
3.2. Methodology for Estimating Emissions of CH4, N2O, and Indirect Greenhouse Gases from Mobile Combustion
and Methodology for and Supplemental Information on Transportation-Related GHG Emissions
3.3. Methodology for Estimating CFL, Emissions from Coal Mining
3.4. Methodology for Estimating CH4 and CO2 Emissions from Natural Gas Systems
3.5. Methodology for Estimating CFL, and CO2 Emissions from Petroleum Systems
3.6. Methodology for Estimating CO2 and N2O Emissions from Municipal Solid Waste Combustion
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 CFL, Emissions from Enteric Fermentation
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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 CFL, 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
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.) ES-5
Table ES-3: CO2 Emissions from Fossil Fuel Combustion by
Fuel Consuming End-Use Sector (Tg CO2 Eq.) ES-9
Table ES-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by
Chapter/IPCC Sector (Tg CO2 Eq.) ES-13
Table ES-5: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) ES-15
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
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Table 1-4: Key Categories for the United States (1990-2006) Based on Tier 1 Approach 1-12
Table 1-5. Estimated Overall Inventory Uncertainty (Tg CO2 Eq. and Percent) 1-14
Table 1-6: IPCC Sector Descriptions 1-15
Table 1-7: List of Annexes 1-16
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
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 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.) 2-13
Table 2-8: Emissions from Agriculture (Tg CO2 Eq.) 2-14
Table 2-9: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) 2-15
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 2006) 2-18
Table 2-13: Electricity Generation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 2-20
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 2006 2-21
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) 2-28
Table 3-1: CO2, CFL,, and N2O Emissions from Energy (Tg CO2 Eq.) 3-2
Table 3-2: CO2, CFL,, and N2O Emissions from Energy (Gg) 3-3
Table 3-3: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq.) 3-4
Table 3-4: Annual Change in CO2 Emissions from Fossil Fuel Combustion for
Selected Fuels and Sectors (Tg CO2 Eq. and Percent) 3-5
Table 3-5: CO2 Emissions from International Bunker Fuels (Tg CO2 Eq.) 3-7
Table 3-6: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 3-8
Table 3-7: CO2 Emissions from Fossil Fuel Combustion in
Transportation End-Use Sector (Tg CO2 Eq.) 3-9
Table 3-8: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg CO2 Eq./QBtu) 3-15
Table 3-9: Carbon Intensity from all Energy Consumption by Sector (Tg CO2 Eq./QBtu) 3-16
Table 3-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Energy-Related Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq. and Percent) 3-18
Table 3-11: CO2 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg CO2 Eq.) 3-20
Table 3-12: Adjusted Consumption of Fossil Fuels for Non-Energy Use (TBtu) 3-21
Table 3-13: 2006 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions 3-22
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Table 3-14: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Non-Energy Uses of Fossil Fuels (Tg CO2 Eq. and Percent) 3-23
Table 3-15: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of
Non-Energy Uses of Fossil Fuels (Percent) 3-23
Table 3-16: CK4 Emissions from Stationary Combustion (Tg CO2 Eq.) 3-25
Table 3-17: N2O Emissions from Stationary Combustion (Tg CO2 Eq.) 3-26
Table 3-18: CH4 Emissions from Stationary Combustion (Gg) 3-27
Table 3-19: N2O Emissions from Stationary Combustion (Gg) 3-28
Table 3-20: Tier 2 Quantitative Uncertainty Estimates for CFLj and N2O Emissions from
Energy-Related Stationary Combustion, Including Biomass (Tg CO2 Eq. and Percent) 3-28
Table 3-21: CFLj Emissions from Mobile Combustion (Tg CO2 Eq.) 3-30
Table 3-22: N2O Emissions from Mobile Combustion (Tg CO2 Eq.) 3-30
Table 3-23: CH4 Emissions from Mobile Combustion (Gg) 3-31
Table 3-24: N2O Emissions from Mobile Combustion (Gg) 3-31
Table 3-25: Tier 2 Quantitative Uncertainty Estimates for CFLj and N2O Emissions from
Mobile Sources (Tg CO2 Eq. and Percent) 3-33
Table 3-26: CK4 Emissions from Coal Mining (Tg CO2 Eq.) 3-36
Table 3-27: CK4 Emissions from Coal Mining (Gg) 3-36
Table 3-28: Coal Production (Thousand Metric Tons) 3-37
Table 3-29: Tier 2 Quantitative Uncertainty Estimates for CFLj Emissions from Coal Mining
(Tg CO2 Eq. and Percent) 3-38
Table 3-30: CH4 Emissions from Abandoned Coal Mines (Tg CO2 Eq.) 3-39
Table 3-31: CFLj Emissions from Abandoned Coal Mines (Gg) 3-39
Table 3-32: Number of Gassy Abandoned Mines Occurring in U.S. Basins Grouped by
Class According to Post-abandonment State 3-41
Table 3-33: Tier 2 Quantitative Uncertainty Estimates for CFLj Emissions from
Abandoned Underground Coal Mines (Tg CO2 Eq. and Percent) 3-42
Table 3-34: CH4 Emissions from Natural Gas Systems (Tg CO2 Eq.) 3-43
Table 3-35: CK4 Emissions from Natural Gas Systems (Gg) 3-43
Table 3-36: Non-combustion CO2 Emissions from Natural Gas Systems (Tg CO2 Eq.) 3-43
Table 3-37: Non-combustion CO2 Emissions from Natural Gas Systems (Gg) 3-43
Table 3-38: Tier 2 Quantitative Uncertainty Estimates for CFLj and
Non-Combustion CO2 Emissions from Natural Gas Systems (Tg CO2 Eq. and Percent) 3-45
Table 3-39: CH4 Emissions from Petroleum Systems (Tg CO2 Eq.) 3-47
Table 3-40: CFLj Emissions from Petroleum Systems (Gg) 3-47
Table 3-41: CO2 Emissions from Petroleum Systems (Tg CO2 Eq.) 3-47
Table 3-42: CO2 Emissions from Petroleum Systems (Gg) 3-47
Table 3-43: Tier 2 Quantitative Uncertainty Estimates for CFLj Emissions from Petroleum Systems
(Tg CO2 Eq. and Percent) 3-49
Table 3-44: Potential Emissions from CO2 Capture and Transport (Tg CO2 Eq.) 3-51
Table 3-45: Potential Emissions from CO2 Capture and Transport (Gg) 3-51
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Table 3-46: CO2 and N2O Emissions from Municipal Solid Waste Combustion (Tg CO2 Eq.) 3-52
Table 3-47: CO2 and N2O Emissions from Municipal Solid Waste Combustion (Gg) 3-52
Table 3-48: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted 3-53
Table 3-49: Tier 2 Quantitative Uncertainty Estimates for CO2 and
N2O from Municipal Solid Waste Combustion (Tg CO2 Eq. and Percent) 3-54
Table 3-50: NOX, CO, and NMVOC Emissions from Energy-Related Activities (Gg) 3-55
Table 3-51: CO2, CH4, and N2O Emissions from International Bunker Fuels (Tg CO2 Eq.) 3-57
Table 3-52: CO2, CH4 and N2O Emissions from International Bunker Fuels (Gg) 3-57
Table 3-53: Aviation Jet Fuel Consumption for International Transport (Million Gallons) 3-58
Table 3-54: Marine Fuel Consumption for International Transport (Million Gallons) 3-58
Table 3-55: CO2 Emissions from Wood Consumption by End-Use Sector (Tg CO2 Eq.) 3-60
Table 3-56: CO2 Emissions from Wood Consumption by End-Use Sector (Gg) 3-60
Table 3-57: CO2 Emissions from Ethanol Consumption (Tg CO2 Eq.) 3-61
Table 3-58: CO2 Emissions from Ethanol Consumption (Gg) 3-61
Table 3-59: Woody Biomass Consumption by Sector (Trillion Btu) 3-61
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: High-Calcium- and Dolomitic-Quicklime, High-Calcium- and Dolomitic-Hydrated, and
Dead-Burned-Dolomite Lime Production (Gg) 4-7
Table 4-8: Adjusted Lime Production (Gg) 4-8
Table 4-9: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lime Production
(Tg CO2 Eq. and Percent) 4-9
Table 4-10: CO2 Emissions from Limestone & Dolomite Use (Tg CO2 Eq.) 4-9
Table 4-11: CO2 Emissions from Limestone & Dolomite Use (Gg) 4-10
Table 4-12: Limestone and Dolomite Consumption (Thousand Metric Tons) 4-11
Table 4-13: Dolomitic Magnesium Metal Production Capacity (Metric Tons) 4-11
Table 4-14: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Limestone and Dolomite Use (Tg CO2 Eq. and Percent) 4-12
Table 4-15: CO2 Emissions from Soda Ash Production and Consumption (Tg CO2 Eq.) 4-12
Table 4-16: CO2 Emissions from Soda Ash Production and Consumption (Gg) 4-13
Table 4-17: Soda Ash Production and Consumption (Gg) 4-13
Table 4-18: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Soda Ash Production and Consumption (Tg CO2 Eq. and Percent) 4-14
Table 4-19: CO2 Emissions from Ammonia Production and Urea Consumption (Tg CO2 Eq.) 4-15
Table 4-20: CO2 Emissions from Ammonia Production and Urea Consumption (Gg) 4-15
Table 4-21: Ammonia Production, Urea Production, Urea Net Imports, and Urea Exports (Gg) 4-16
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Table 4-22: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Ammonia Production and Urea Consumption (Tg CO2 Eq. and Percent) 4-17
Table 4-23: N2O Emissions from Nitric Acid Production (Tg CO2 Eq. and Gg) 4-18
Table 4-24: Nitric Acid Production (Gg) 4-18
Table 4-25: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions
From Nitric Acid Production (Tg CO2 Eq. and Percent) 4-19
Table 4-26: N2O Emissions from Adipic Acid Production (Tg CO2 Eq. and Gg) 4-20
Table 4-27: Adipic Acid Production (Gg) 4-21
Table 4-28: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from
Adipic Acid Production (Tg CO2 Eq. and Percent) 4-21
Table 4-29: CO2 and CK4 Emissions from Silicon Carbide Production and Consumption (Tg CO2 Eq.) 4-22
Table 4-30: CO2 and CK4 Emissions from Silicon Carbide Production and Consumption (Gg) 4-22
Table 4-31: Production and Consumption of Silicon Carbide (Metric Tons) 4-23
Table 4-32: Tier 2 Quantitative Uncertainty Estimates for CFLj and CO2 Emissions from
Silicon Carbide Production and Consumption (Tg CO2 Eq. and Percent) 4-23
Table 4-33: CO2 and CH4 Emissions from Petrochemical Production (Tg CO2 Eq.) 4-24
Table 4-34: CO2 and CK4 Emissions from Petrochemical Production (Gg) 4-24
Table 4-35: Production of Selected Petrochemicals (Thousand Metric Tons) 4-25
Table 4-36: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock (Secondary Feedstock)
Consumption (Thousand Metric Tons) 4-25
Table 4-37: Tier 2 Quantitative Uncertainty Estimates for CFLj Emissions from Petrochemical Production and
CO2 Emissions from Carbon Black Production (Tg CO2 Eq. and Percent) 4-26
Table 4-38: CO2 Emissions from Titanium Dioxide Production (Tg CO2 Eq. and Gg) 4-27
Table 4-39: Titanium Dioxide Production (Gg) 4-27
Table 4-41: CO2 Emissions from CO2 Consumption (Tg CO2 Eq. and Gg) 4-29
Table 4-42: CO2 Production (Gg CO2) and the Percent Used for Non-EOR Applications for
Jackson Dome and Bravo Dome 4-29
Table 4-43: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from CO2 Consumption
(Tg CO2 Eq. and Percent) 4-30
Table 4-44: CO2 Emissions from Phosphoric Acid Production (Tg CO2 Eq. and Gg) 4-31
Table 4-45: Phosphate Rock Domestic Production, Exports, and Imports (Gg) 4-31
Table 4-46: Chemical Composition of Phosphate Rock (percent by weight) 4-32
Table 4-47: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Phosphoric Acid Production (Tg CO2 Eq. and Percent) 4-33
Table 4-48: CO2 and CK4 Emissions from Iron and Steel Production (Tg CO2 Eq.) 4-34
Table 4-49: CO2 and CK4 Emissions from Iron and Steel Production (Gg) 4-34
Table 4-50: CH4 Emission Factors for Coal Coke, Sinter, and Pig Iron Production (g/kg) 4-35
Table 4-51: Production and Consumption Data for the Calculation of CO2 and CFLj Emissions from
Iron and Steel Production (Thousand Metric Tons) 4-36
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Table 4-52: Tier 2 Quantitative Uncertainty Estimates for CO2 and CH4 Emissions from
Iron and Steel Production (Tg. CO2 Eq. and Percent) 4-37
Table 4-53: CO2 and CFL, Emissions from Ferroalloy Production (Tg CO2 Eq.) 4-37
Table 4-54: CO2 and CH4 Emissions from Ferroalloy Production (Gg) 4-38
Table 4-55: Production of Ferroalloys (Metric Tons) 4-38
Table 4-56: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Ferroalloy Production (Tg CO2 Eq. and Percent) 4-39
Table 4-57: CO2 Emissions from Aluminum Production (Tg CO2 Eq. and Gg) 4-40
Table 4-58: PFC Emissions from Aluminum Production (Tg CO2 Eq.) 4-40
Table 4-59: PFC Emissions from Aluminum Production (Gg) 4-40
Table 4-60: Production of Primary Aluminum (Gg) 4-42
Table 4-61: Tier 2 Quantitative Uncertainty Estimates for CO2 and PFC Emissions from
Aluminum Production (Tg CO2 Eq. and Percent) 4-43
Table 4-62: SF6 Emissions from Magnesium Production and Processing (Tg CO2 Eq. and Gg) 4-43
Table 4-63: SF6 Emission Factors (kg SF6 per metric ton of Magnesium) 4-44
Table 4-64: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from
Magnesium Production and Processing (Tg CO2 Eq. and Percent) 4-45
Table 4-65: CO2 Emissions from Zinc Production (Tg CO2 Eq. and Gg) 4-46
Table 4-66: Zinc Production (Metric Tons) 4-48
Table 4-67: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Zinc Production (Tg CO2 Eq. and Percent) 4-48
Table 4-68: CO2 Emissions from Lead Production (Tg CO2 Eq. and Gg) 4-49
Table 4-69: Lead Production (Metric Tons) 4-49
Table 4-70: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from
Lead Production (Tg CO2 Eq. and Percent) 4-50
Table 4-71: FfFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and Gg) 4-51
Table 4-72: HCFC-22 Production (Gg) 4-51
Table 4-73: Quantitative Uncertainty Estimates for HFC-23 Emissions from
HCFC-22 Production (Tg CO2 Eq. and Percent) 4-52
Table 4-74: Emissions of HFCs and PFCs from ODS Substitutes (Tg CO2 Eq.) 4-53
Table 4-75: Emissions of HFCs and PFCs from ODS Substitutes (Mg) 4-53
Table 4-76: Emissions of HFCs and PFCs from ODS Substitutes (Tg CO2 Eq.) by Sector 4-53
Table 4-77: Tier 2 Quantitative Uncertainty Estimates for HFC and PFC Emissions from
ODS Substitutes (Tg CO2 Eq. and Percent) 4-56
Table 4-78: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg CO2 Eq.) 4-57
Table 4-80: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from
Semiconductor Manufacture (Tg CO2 Eq. and Percent) 4-60
Table 4-81: SF6 Emissions from Electric Power Systems and
Electrical Equipment Manufacturers (Tg CO2 Eq.) 4-61
Table 4-82: SF6 Emissions from Electric Power Systems and Electrical Equipment Manufacturers (Gg) .... 4-61
xiii
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Table 4-83: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from
Electrical Transmission and Distribution (Tg CO2 Eq. and Percent) 4-64
Table 4-84: 2006 Potential and Actual Emissions of HFCs, PFCs, and SF6 from
Selected Sources (Tg CO2 Eq.) 4-65
Table 4-85: NOX, CO, and NMVOC Emissions from Industrial Processes (Gg) 4-66
Table 5-1: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq. and Gg) 5-1
Table 5-2: N2O Emissions from N2O Product Uses (Tg CO2 Eq. and Gg) 5-2
Table 5-3: N2O Production (Gg) 5-2
Table 5-4: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from
N2O Product Uses (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: CFL, Emissions from Enteric Fermentation (Tg CO2 Eq.) 6-3
Table 6-4: CFLj Emissions from Enteric Fermentation (Gg) 6-3
Table 6-5: Quantitative Uncertainty Estimates for CH4 Emissions from
Enteric Fermentation (Tg CO2 Eq. and Percent) 6-5
Table 6-6: CK4 and N2O Emissions from Manure Management (Tg CO2 Eq.) 6-8
Table 6-7: CH4 and N2O Emissions from Manure Management (Gg) 6-9
Table 6-8: Tier 2 Quantitative Uncertainty Estimates for CFLj 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: CFLj Emissions from Puce Cultivation (Gg) 6-14
Table 6-11: Rice Areas Harvested (Hectares) 6-15
Table 6-12: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from
Rice Cultivation (Tg CO2 Eq. and Percent) 6-17
Table 6-13: N2O Emissions from Agricultural Soils (Tg CO2 Eq.) 6-19
Table 6-14: N2O Emissions from Agricultural Soils (Gg) 6-19
Table 6-15: Direct N2O Emissions from Agricultural Soils by Land Use and N Input (Tg CO2 Eq.) 6-19
Table 6-16: Indirect N2O Emissions from all Land-Use Types (Tg CO2 Eq.) 6-20
Table 6-17: Quantitative Uncertainty Estimates of N2O Emissions from
Agricultural Soil Management in 2006 (Tg CO2 Eq. and Percent) 6-29
Table 6-18: CH4 and N2O Emissions from Field Burning of Agricultural Residues (Tg CO2 Eq.) 6-31
Table 6-19: CH4, N2O, CO, and NOX Emissions from Field Burning of Agricultural Residues (Gg) 6-32
Table 6-20: Agricultural Crop Production (Gg of Product) 6-34
Table 6-21: Percent of Rice Area Burned by State 6-34
Table 6-22: Key Assumptions for Estimating Emissions from Field Burning of Agricultural Residues 6-34
Table 6-23: Greenhouse Gas Emission Ratios 6-35
Table 6-24: Tier 2 Uncertainty Estimates for CLLj and N2O Emissions from
Field Burning of Agricultural Residues (Tg CO2 Eq. and Percent) 6-35
xiv
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Table 7-1: Net CO2 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and
Forestry (Tg CO2 Eq.) 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: Non-CO2 Emissions from Land Use, Land-Use Change, and Forestry (Gg) 7-3
Table 7-5: Land Use Areas During the Inventory Reporting Period (Millions of Hectares) 7-4
Table 7-6: Net Annual Changes in C Stocks (Tg CO2/yr) in Forest and Harvested Wood Pools 7-14
Table 7-7: Net Annual Changes in C Stocks (Tg C/yr) in Forest and Harvested Wood Pools 7-14
Table 7-8: Forest Area (1000 ha) and C Stocks (Tg C) in Forest and Harvested Wood Pools 7-15
Table 7-9: Estimates of CO2 (Tg/yr) emissions for the lower 48 states and Alaska 7-16
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) 7-19
Table 7-11: Estimated Non-CO2 Emissions from Forest Fires (Tg CO2 Eq.) for U.S. Forests 7-21
Table 7-12: Estimated Non-CO2 Emissions from Forest Fires (Gg Gas) for U.S. Forests 7-21
Table 7-13: Estimated Carbon Released from Forest Fires for U.S. Forests 7-22
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) 7-22
Table 7-15: N2O Fluxes from Soils in Forest Land Remaining Forest Land (Tg CO2 Eq. and Gg) 7-23
Table 7-16: Quantitative Uncertainty Estimates of N2O Fluxes from Soils in
Forest Land Remaining Forest Land (Tg CO2 Eq. and Percent) 7-24
Table 7-17: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining Cropland (Tg CO2 Eq.) 7-26
Table 7-18: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining Cropland (Tg C) 7-26
Table 7-19: Quantitative Uncertainty Estimates for C Stock Changes occurring within
Cropland Remaining Cropland (Tg CO2 Eq. and Percent) 7-31
Table 7-20: Emissions from Liming of Agricultural Soils (Tg CO2 Eq.) 7-33
Table 7-21: Emissions from Liming of Agricultural Soils (Tg C) 7-33
Table 7-22: Applied Minerals (Million Metric Tons) 7-34
Table 7-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Liming of
Agricultural Soils (Tg CO2 Eq. and Percent) 7-35
Table 7-24: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg CO2 Eq.) 7-35
Table 7-25: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C) 7-36
Table 7-26: Applied Urea (Million Metric Tons) 7-36
Table 7-27: Quantitative Uncertainty Estimates for CO2 Emissions from Urea Fertilization
(Tg CO2 Eq. and Percent) 7-36
Table 7-28: Net CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg CO2 Eq.) 7-37
Table 7-29: Net CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg C) 7-37
Table 7-30: Quantitative Uncertainty Estimates for C Stock Changes occurring within
Land Converted to Cropland (Tg CO2 Eq. and Percent) 7-40
Table 7-31: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg CO2 Eq.) 7-42
XV
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Table 7-32: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg C) 7-42
Table 7-33: Quantitative Uncertainty Estimates for C Stock Changes occurring within
Grassland Remaining Grassland (Tg CO2 Eq. and Percent) 7-45
Table 7-34: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg CO2 Eq.) 7-47
Table 7-35: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C) 7-47
Table 7-36: Quantitative Uncertainty Estimates for C Stock Changes occurring within
Land Converted to Grassland (Tg CO2 Eq. and Percent) 7-50
Table 7-37: Net C Flux from Urban Trees (TgCO2Eq. andTgC) 7-50
Table 7-38: 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 cover-yr) for 15 U.S. Cities 7-52
Table 7-39: Tier 2 Quantitative Uncertainty Estimates for Net C Flux from Changes in C Stocks in
Urban Trees (Tg CO2 Eq. and Percent) 7-53
Table 7-40: N2O Fluxes from Soils in Settlements Remaining Settlements (Tg CO2 Eq. and Gg) 7-54
Table 7-41: Quantitative Uncertainty Estimates of N2O Emissions from Soils in
Settlements Remaining Settlements (Tg CO2 Eq. and Percent) 7-56
Table 7-42: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg CO2 Eq.) 7-57
Table 7-43: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C) 7-57
Table 7-44: 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 7-58
Table 7-45: C Stocks in Yard Trimmings and Food Scraps in Landfills (Tg C) 7-59
Table 7-46: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Yard Trimmings and
Food Scraps in Landfills (Tg CO2 Eq. and Percent) 7-60
Table 8-1: Emissions from Waste (Tg CO2 Eq.) 8-1
Table 8-2: Emissions from Waste (Gg) 8-2
Table 8-3: CLL, Emissions from Landfills (Tg CO2 Eq.) 8-3
Table 8-4: CLL, 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: CK4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Tg CO2 Eq.) .... 8-7
Table 8-7: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Gg) 8-8
Table 8-8: U.S. Population (Millions) and Domestic Wastewater BODS Produced (Gg) 8-10
Table 8-9: U.S. Pulp and Paper, Meat and Poultry, and Vegetables, Fruits and Juices Production (Tg) 8-10
Table 8-10: Wastewater How (m3/ton) and BOD Production (g/L) for
U.S. Vegetables, Fruits and Juices Production 8-12
Table 8-11: U.S. Population (Millions) and Average Protein Intake [kg/(person-year)] 8-14
Table 8-12: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from
Wastewater Treatment (Tg CO2 Eq. and Percent) 8-15
Table 8-13: CLL, and N2O Emissions from Composting (Tg CO2 Eq.) 8-17
Table 8-14: CH4 and N2O Emissions from Composting (Gg) 8-17
Table 8-15: U.S. Waste Composted (Gg) 8-18
xvi
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Table 8-16: Tier 1 Quantitative Uncertainty Estimates for Emissions from
Composting (Tg CO2 Eq. and Percent) 8-18
Table 8-17: Emissions of NOX, CO, andNMVOC from Waste (Gg) 8-19
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg CO2 Eq.) 10-3
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 U.S. Greenhouse Gas Emissions Relative to 1990 ES-4
Figure ES-4: 2006 Greenhouse Gas Emissions by Gas ES-4
Figure ES-5: 2006 Sources of CO2 Emissions ES-7
Figure ES-6: 2006 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type ES-8
Figure ES-7: 2006 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion ES-8
Figure ES-8: 2006 Sources of CFL, Emissions ES-10
Figure ES-9: 2006 Sources of N2O Emissions ES-11
Figure ES-10: 2006 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: 2006 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: 2006 Key Categories-Tier 1 Level Assessment ES-20
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 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: 2006 Energy Chapter Greenhouse Gas Emission Sources 2-8
Figure 2-6: 2006 U.S. Fossil Carbon Flows (Tg CO2 Eq.) 2-9
Figure 2-7: 2006 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 2-11
Figure 2-8: 2006 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 2-11
Figure 2-9: 2006 Industrial Processes Chapter Greenhouse Gas Emission Sources 2-12
Figure 2-10: 2006 Agriculture Chapter Greenhouse Gas Emission Sources 2-14
Figure 2-11: 2006 Waste Chapter Greenhouse Gas Emission Sources 2-16
Figure 2-12: Emissions Allocated to Economic Sectors 2-20
Figure 2-13: Emissions with Electricity Distributed to Economic Sectors 2-22
Figure 2-14: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product 2-27
Figure 3-1: 2006 Energy Chapter Greenhouse Gas Emission Sources 3-1
Figure 3-2: 2006 U.S. Fossil Carbon Hows (Tg CO2 Eq.) 3-2
Figure 3-3: 2006 U.S. Energy Consumption by Energy Source 3-5
xvii
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Figure 3-4: U.S. Energy Consumption (Quadrillion Btu) 3-5
Figure 3-5: 2006 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 3-5
Figure 3-6: Annual Deviations from Normal Heating Degree Days for the United States (1950-2006) 3-6
Figure 3-7: Annual Deviations from Normal Cooling Degree Days for the United States (1950-2006) 3-6
Figure 3-8: Aggregate Nuclear and Hydroelectric Power Plant Capacity Factors in the
United States (1974-2006) 3-7
Figure 3-9: 2006 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 3-8
Figure 3-10: Sales-Weighted Fuel Economy of New Automobiles and Light-Duty Trucks, 1990-2006 3-10
Figure 3-11: Sales of New Automobiles and Light-Duty Trucks, 1990-2006 3-10
Figure 3-12: Industrial Production Indices (Index 2002=100) 3-11
Figure 3-13: Electricity Generation Retail Sales by End-Use Sector 3-12
Figure 3-14: U.S. Energy Consumption and Energy-Related CO2 Emissions Per Capita and Per Dollar GDP 3-16
Figure 3-15: Mobile Source CK4 and N2O Emissions 3-29
Figure 4-1: 2006 Industrial Processes Chapter Greenhouse Gas Emission Sources 4-1
Figure 6-1: 2006 Agriculture Chapter Greenhouse Gas Emission Sources 6-1
Figure 6-2: Agricultural Sources and Pathways of N that Result in N2O Emissions 6-18
Figure 6-3: Major Crops, Average Annual Direct N2O Emissions Estimated Using the
DAYCENT Model, 1990-2006 (Tg CO2 Eq./state/year) 6-21
Figure 6-4: Grasslands, Average Annual Direct N2O Emissions Estimated Using the
DAYCENT Model, 1990-2006 (Tg CO2 Eq./state/year) 6-21
Figure 6-5: Major Crops, Average Annual N Losses Leading to Indirect N2O Emissions Using the
DAYCENT Model, 1990-2006 (Gg N/state/year) 6-22
Figure 6-6: Grasslands, Average Annual N Losses Leading to Indirect N2O Emissions Using the
DAYCENT Model, 1990-2006 (Gg N/state/year) 6-22
Figure 6-7: Comparison of Measured Emissions at Field Sites with Modeled Emissions Using the
DAYCENT Simulation Model 6-29
Figure 7-1: Percent of Total Land Area in Each Land-Use Category by State 7-5
Figure 7-2: Forest Sector Carbon Pools and Hows 7-12
Figure 7-3: Estimates of Net Annual Changes in Carbon Stocks for Major Carbon Pools 7-15
Figure 7-4: Average C Density in the Forest Tree Pool in the Conterminous United States, 2007 7-16
Figure 7-5: Total Net Annual CO2 Flux For Mineral Soils Under Agricultural Management within States,
1993-2006 Cropland Remaining Cropland 7-27
Figure 7-6: Total Net Annual CO2 Flux For Organic Soils Under Agricultural Management within States,
1993-2006 Cropland Remaining Cropland 7-27
Figure 7-7: Total Net Annual CO2 Flux For Mineral Soils Under Agricultural Management within States,
1993-2006 Land Converted to Cropland 7-39
Figure 7-8: Total Net Annual CO2 Flux For Organic Soils Under Agricultural Management within States,
1993-2006 Land Converted to Cropland 7-39
xviii
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Figure 7-9: Total Net Annual CO2 Flux For Mineral Soils Under Agricultural Management within States,
1993-2006 Grassland Remaining Grassland
Figure 7-10: Total Net Annual CO2 Flux For Organic Soils Under Agricultural Management within States,
1993-2006 Grassland Remaining Grassland
Figure 7-11: Total Net Annual CO2 Flux For Mineral Soils Under Agricultural Management within States,
1993-2006 Land Converted to Grassland
Figure 7-12: Total Net Annual CO2 Flux For Organic Soils Under Agricultural Management within States,
1993-2006 Land Converted to Grassland
Figure 8-1: 2006 Waste Chapter Greenhouse Gas Emission Sources
Boxes
Box 1-1
Box 1-2
Box 2-1
Box 2-2
Box 2-3
Box 3-1
Box 3-2
Box 3-3
Box 4-1
Box 6-1
Box 6-2
Box 7-1
Box 7-2
Box 8-1
7-42
7-43
7-47
7-48
8-1
1: Recalculations of Inventory Estimates
2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data
The IPCC Fourth Assessment Report and Global Warming Potentials
IPCC Reference Approach
Methodology for Aggregating Emissions by Economic Sector
Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data
Sources and Effects of Sulfur Dioxide
Weather and Non-Fossil Energy Effects on CO2 from Fossil Fuel Combustion Trends
Carbon Intensity of U.S. Energy Consumption
Carbon Dioxide Transport, Injection, and Geological Storage
Potential Emission Estimates of HFCs, PFCs, and SF6
Tier 1 vs. Tier 3 Approach for Estimating N2O Emissions
Comparison of Tier 2 U.S . Inventory Approach and IPCC (2006) Default Approach
CO2 Emissions from Forest Fires
Tier 3 Inventory for Soil C Stocks Compared to Tier 1 or 2 Approaches
Biogenic Emissions and Sinks of Carbon
ES-2
ES-18
1-8
1-10
2-23
2-27
2-29
3-6
3-15
3-51
4-65
6-24
6-33
7-16
7-29
8-6
xix
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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 this 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 2006. 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 1996 IPCC
Guidelines for National Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997), 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. emission Inventory has begun to
incorporate new methodologies and data from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC
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 .
Executive Summary ES-1
-------
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 2005) 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.
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.
ES.1. Background Information
Naturally occurring greenhouse gases include water
vapor, carbon dioxide (CO2), methane (CK4), 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 their national greenhouse gas emission inventories.5
Some other fluorine-containing halogenated substances —
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and
4 See .
5 Emission estimates of CFCs, HCFCs, halons and other ozone depleting
substances are included in the annexes of this report for informational
purposes.
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), and non-CFLj 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, CFLj, 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
ES-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
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 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 is a greenhouse gas. The
reference gas used is CO2, and therefore GWP-weighted
emissions are measured in teragrams of CO2 equivalents
(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 2002,9 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 2006 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 the 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
Table ES-1: Global Warming Potentials
(100-Year Time Horizon) Used in This Report
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^FIO
CeF-14
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.
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).
ES.2. Recent Trends in
U.S. Greenhouse Gas Emissions
and Sinks
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 12/44'hs of carbon dioxide by weight.
8 One teragram is equal to 1012 grams or one million metric tons.
9 See .
In 2006, total U.S. greenhouse gas emissions were
7,054.2 Tg CO2 Eq. Overall, total U.S. emissions have
risen by 14.7 percent from 1990 to 2006, while the U.S.
gross domestic product has increased by 59 percent over
the same period (BEA 2007). Emissions fell from 2005 to
2006, decreasing by 1.1 percent (75.7 Tg CO2 Eq.). The
following factors were primary contributors to this decrease:
(1) compared to 2005, 2006 had warmer winter conditions,
which decreased consumption of heating fuels, as well
as cooler summer conditions, which reduced demand for
Executive Summary ES-3
-------
Figure ES-1
U.S. Greenhouse Gas Emissions by Gas
8,000 -
7,000 -
6,000 -
5^5,000-
cT
m 4,000 -
3,000 -
2,000 -
1,000-
o-
MFCs, PFCs, & SF, Methane
Nitrous Oxide • Carbon Dioxide
q 1 § I §• | | I- §• £ 5
{Q" (O (O tO ™
iiiiliiiiiiiiilii
Figure ES-2
Annual Percent Change in U.S. Greenhouse Gas Emissions
4%-,
-1%
-2%
12.8%
•
siil •-
1.1%
It
-1.1%
i— CM co
electricity, (2) restraint on fuel consumption caused by rising
fuel prices, primarily in the transportation sector and (3)
increased use of natural gas and renewables in the electric
power sector.
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 2006.
Figure ES-4 illustrates the relative contribution of the
direct greenhouse gases to total U.S. emissions in 2006.
The primary greenhouse gas emitted by human activities
in the United States was CO2, representing approximately
84.8 percent of total greenhouse gas emissions. The largest
source of CO2, and of overall greenhouse gas emissions,
was fossil fuel combustion. CK4 emissions, which have
declined from 1990 levels, resulted primarily from
enteric fermentation associated with domestic livestock,
decomposition of wastes in landfills, and natural gas
systems. Agricultural soil management and mobile source
fossil fuel combustion were the major sources of N2O
emissions. The emissions of substitutes for ozone depleting
substances and emissions of HFC-23 during the production
of HCFC-22 were the primary contributors to aggregate
HFC emissions. Electrical transmission and distribution
systems accounted for most SF6 emissions, while PFC
emissions resulted from semiconductor manufacturing and
as a byproduct of primary aluminum production.
Figure ES-3
Cumulative Change in U.S. Greenhouse Gas
Emissions Relative to 1990
1,000
900
800
700
~ 600
500
400
300
200
100
0
-100
930
982
906
i- CM co
Figure ES-4
2006 Greenhouse Gas Emissions by Gas
MFCs, PFCs, & SF6
N20
CH4
CO,
2.1%
5.2%
7.9%
84.8%
ES-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table ES-2: 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
Cement Production
Natural Gas Systems
Municipal Solid Waste Combustion
Lime Production
Ammonia Production and
Urea Consumption
Limestone and Dolomite Use
Cropland Remaining Cropland
Soda Ash Production
and Consumption
Aluminum Production
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production
and Consumption
Land Use, Land-Use Change,
and Forestry (Sink)3
Biomass — Wood"
International Bunker Fuels'3
Biomass — Ethanolb
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Forest Land Remaining Forest Land
Wastewater Treatment
Stationary Combustion
Rice Cultivation
Abandoned Underground Coal Mines
Mobile Combustion
1990
5,068.5
4,724.1
1,809.6
1,485.1
844.9
340.1
216.1
28.3
117.2
86.2
33.3
33.7
10.9
12.0
16.9
5.5
7.1
4.1
6.8
2.2
1.2
1.4
2.2
1.5
0.9
0.4
0.3
0.4
(737.7)
215.2
113.7
4.2
606.1
126.9
149.6
124.7
84.1
31.0
33.9
4.5
23.0
7.4
7.1
6.0
4.7
1995
5,394.2
5,032.4
1,939.3
1,599.4
876.5
356.5
225.8
35.0
133.2
74.7
36.8
33.8
15.7
14.0
17.8
7.4
7.0
4.3
5.7
2.8
1.5
1.4
2.0
1.5
1.0
0.3
0.3
0.3
(775.3)
229.1
100.6
7.7
598.9
132.3
144.0
128.1
67.1
35.2
32.0
4.7
24.3
7.2
7.6
8.2
4.3
2000 2001 2002 2003 2004 2005 2006
5,939.7 5,846.2 5,908.6 5,952.7 6,038.2 6,074.3 5,983.1
5,577.1 5,507.4 5,564.8 5,617.0 5,681.4 5,731.0 5,637.9
2,282.3 2,244.3 2,253.7 2,283.1 2,314.9 2,380.2 2,328.2
1,798.2 1,775.6 1,828.9 1,807.6 1,856.4 1,869.8 1,856.0
860.3 852.5 854.8 856.0 857.7 847.3 862.2
372.1 363.6 360.5 382.9 368.3 358.5 326.5
228.0 222.3 222.8 236.5 230.6 221.9 210.1
36.2 49.0 44.0 51.0 53.5 53.2 54.9
141.4 131.9 135.9 131.8 148.9 139.1 138.0
66.6 59.2 55.9 54.7 52.8 46.6 49.1
41.2 41.4 42.9 43.1 45.6 45.9 45.7
29.4 28.8 29.6 28.4 28.1 29.5 28.5
17.5 18.0 18.5 19.1 20.1 20.7 20.9
14.9 14.3 13.7 14.5 15.2 15.1 15.8
16.4 13.3 14.2 12.5 13.2 12.8 12.4
6.0 5.7 5.9 4.8 6.7 7.4 8.6
7.5 7.8 8.5 8.3 7.6 7.9 8.0
4.2 4.1 4.1 4.1 4.2 4.2 4.2
6.1 4.4 4.5 4.5 4.2 4.2 3.9
3.0 2.8 2.9 2.8 2.9 2.8 2.6
1.8 1.7 1.8 1.8 2.1 1.8 1.9
1.4 0.8 1.0 1.3 1.2 1.3 1.6
1.9 1.5 1.3 1.3 1.4 1.4 1.5
1.4 1.3 1.3 1.4 1.4 1.4 1.2
1.1 1.0 0.9 0.5 0.5 0.5 0.5
0.3 0.3 0.3 0.3 0.3 0.3 0.3
0.3 0.3 0.3 0.3 0.3 0.3 0.3
0.2 0.2 0.2 0.2 0.2 0.2 0.2
(673. 6) (750.2) (826.8) (860.9) (873.7) (878.6) (883.7)
218.1 193.5 192.8 193.8 205.1 204.8 204.4
101.1 97.6 89.1 103.6 119.0 122.6 127.1
9.2 9.7 11.5 15.7 19.7 22.6 30.3
574.3 558.8 563.5 559.4 545.6 539.7 555.3
124.6 123.6 123.8 124.6 122.4 124.5 126.2
120.8 117.6 120.1 125.6 122.6 123.7 125.7
126.5 125.3 124.9 123.3 114.0 102.5 102.4
60.4 60.3 56.8 56.9 59.8 57.1 58.5
38.8 40.2 41.3 40.7 40.1 41.8 41.4
30.3 30.2 29.9 29.2 28.7 28.3 28.4
19.0 9.4 16.4 8.7 6.9 12.3 24.6
24.6 24.2 24.1 23.9 24.0 23.8 23.9
6.6 6.2 6.2 6.4 6.5 6.5 6.2
7.5 7.6 6.8 6.9 7.6 6.8 5.9
7.4 6.7 6.2 6.0 5.8 5.6 5.4
3.4 3.3 3.0 2.7 2.6 2.5 2.4
Executive Summary ES-5
-------
Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg C02 Eq.) (continued)
Gas/Source
Composting
Petrochemical Production
Iron and Steel Production
Field Burning of Agricultural Residues
Ferroalloy Production
Silicon Carbide Production
and Consumption
International Bunker Fuels"
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Stationary Combustion
Manure Management
Wastewater Treatment
AdipicAcid Production
N20 from Product Uses
Forest Land Remaining Forest Land
Composting
Settlements Remaining Settlements
Field Burning of Agricultural Residues
Municipal Solid Waste Combustion
International Bunker Fuels"
MFCs
Substitution of
Ozone Depleting Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Semiconductor Manufacture
Aluminum Production
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture
Total
Net Emissions (Sources and Sinks)
1990
0.3
0.9
1.3
0.7
+
+
0.2
383.4
269.4
43.5
17.0
12.8
12.1
6.3
15.3
4.4
0.5
0.4
1.0
0.4
0.5
1.0
36.9
0.3
36.4
0.2
20.8
2.2
18.5
32.7
26.7
5.4
0.5
6,148.3
5,410.6
1995
0.7
1.1
1.3
0.7
+
+
0.1
395.6
264.8
53.4
18.9
13.4
12.8
6.9
17.3
4.6
0.6
0.8
1.2
0.4
0.5
0.9
61.8
28.5
33.0
0.3
15.6
3.8
11.8
28.0
231.5
5.6
0.9
6,494.0
5,718.7
2000 2001 2002 2003 2004 2005 2006
1.3 1.3 1.3 1.5 1.6 1.6 1.6
1.2 1.1 1.1 1.1 1.2 1.1 1.0
1.2 1.1 1.0 1.0 1.0 1.0 0.9
0.8 0.8 0.7 0.8 0.9 0.9 0.8
+ + + + + + +
+ + + + + + +
0.1 0.1 0.1 0.1 0.1 0.2 0.2
385.9 392.9 376.1 356.6 353.5 370.1 367.9
262.1 277.0 262.0 247.3 246.9 265.2 265.0
52.5 49.9 45.9 42.3 39.7 36.3 33.1
18.6 15.1 16.4 15.4 15.2 15.8 15.6
14.6 14.1 14.0 14.3 14.6 14.8 14.5
13.7 14.0 14.0 13.6 13.8 13.9 14.3
7.6 7.8 7.6 7.7 7.8 8.0 8.1
6.2 5.1 6.1 6.3 5.9 5.9 5.9
4.9 4.9 4.4 4.4 4.4 4.4 4.4
2.2 1.3 2.0 1.2 1.1 1.6 2.8
1.4 1.4 1.4 1.6 1.7 1.7 1.8
1.2 1.4 1.5 1.5 1.6 1.5 1.5
0.5 0.5 0.4 0.4 0.5 0.5 0.5
0.4 0.4 0.4 0.4 0.4 0.4 0.4
0.9 0.9 0.8 0.9 1.1 1.1 1.1
100.1 97.9 106.3 104.5 116.6 121.4 124.5
71.2 78.0 85.0 92.0 99.1 105.4 110.4
28.6 19.7 21.1 12.3 17.2 15.8 13.8
0.3 0.2 0.2 0.2 0.2 0.2 0.3
13.5 7.0 8.7 7.1 6.1 6.2 6.0
4.9 3.5 3.5 3.3 3.3 3.2 3.6
8.6 3.5 5.2 3.8 2.8 3.0 2.5
19.1 18.7 18.0 18.1 18.0 18.2 17.3
15.1 15.0 14.4 13.8 13.9 14.0 13.2
3.0 2.9 2.9 3.4 3.2 3.3 3.2
1.1 0.7 0.7 0.8 0.8 1.0 1.0
7,032.6 6,921.3 6,981.2 6,998.2 7,078.0 7,129.9 7,054.2
6,359.0 6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
+ 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 PFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
Note: One teragram (Tg) equals one million metric tons.
Overall, from 1990 to 2006, total emissions of CO2
increased by 914.6 Tg CO2 Eq. (18.0 percent), while CH4
and N2O emissions decreased by 50.8 Tg CO2 Eq. (8.4
percent) and 15.5 Tg CO2 Eq. (4.0 percent), respectively.
During the same period, aggregate weighted emissions
of HFCs, PFCs, and SF6 rose by 57.6 Tg CO2 Eq. (63.7
percent). From 1990 to 2006, HFCs increased by 87.6 Tg
CO2 Eq. (237.3 percent), PFCs decreased by 14.7 Tg CO2
Eq. (70.9 percent), and SF6 decreased by 15.3 Tg CO2 Eq.
(47.0 percent). Despite being emitted in smaller quantities
ES-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
relative to the other principal greenhouse gases, emissions
of HFCs, PFCs, and SF6 are significant because many of
them 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 12.5 percent of total
emissions in 2006. The following sections describe each
gas' contribution to total U.S. greenhouse gas emissions in
more detail.
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, fuel combustion accounted for 94.2
percent of CO2 emissions in 2006. Globally, approximately
28,193 Tg of CO2 were added to the atmosphere through the
combustion of fossil fuels in 2005, of which the United States
accounted for about 20 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).
U.S. anthropogenic sources of CO2 are shown in
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 2006.
Emissions of CO2 from fossil fuel combustion increased at
an average annual rate of 1.1 percent from 1990 to 2006.
The fundamental factors influencing this trend include
(1) a generally growing domestic economy over the last 16
years, and (2) significant overall growth in emissions from
electricity generation and transportation activities. Between
Figure ES-5
2006 Sources of CO? Emissions
Fossil Fuel Combustion
Non-Energy Use of Fuels
Iron and Steel Production
Cement Production
Natural Gas Systems
Municipal Solid Waste Combustion
Lime Production
Ammonia Production and Urea Consumption
Limestone and Dolomite Use
Cropland Remaining Cropland
Soda Ash Production and Consumption
Aluminum Production
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production and Consumption
5,637.9
CO, as a Portion
of all Emissions
<0.5
<0.5
<0.5
0 25
75 100 125 150 175
TgCO,Eq.
10 Global CO2 emissions from fossil fuel combustion were taken from
Energy Information Administration International Energy Annual 2005
(EIA2007).
1990 and 2006, CO2 emissions from fossil fuel combustion
increased from 4,724.1 Tg CO2 Eq. to 5,637.9 Tg CO2 Eq. - a
19.3 percent total increase over the sixteen-year period. From
2005 to 2006, these emissions decreased by 93.1 Tg CO2
Eq. (1.6 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
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 four major fuel consuming end-use sectors
contributing to CO2 emissions from fossil fuel combustion
are industrial, transportation, residential, and commercial.
Electricity generation also emits CO2, although these
Executive Summary ES-7
-------
Figure ES-6
2006 C02 Emissions from Fossil Fuel
Combustion by Sector and Fuel Type
Natural Gas
Petroleum
I Coal
Relative Contribution
by Fuel Type
2,500 -i
2,000 -
1,500 -
' 1,000 -
500 -
0 -1
U.S. Commercial Residential Industrial Transportation Electricity
Territories Generation
Note: Electricity generation also includes emissions of less than 0.5 Tg C02 Eq. from geothermal-based
electricity generation.
Figure ES-7
2006 End-Use Sector Emissions of C02 from
Fossil Fuel Combustion
2,000 -
1,800 -
1,600 -
1,400 -
1,200 -
1,000-
800 -
600 -
400 -
200 -
o -J
From Electricity
Consumption
I From Direct Fossil
Fuel Combustion
U.S. Commercial Residential Industrial Transportation
Territories
emissions are produced as they consume fossil fuel to
provide electricity to one of the four 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 33
percent of CO2 emissions from fossil fuel combustion in
2006.n Virtually all of the energy consumed in this end-use
sector came from petroleum products. Over 60 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 28 percent of CO2 from fossil
fuel combustion in 2006. Just over half of these emissions
resulted from direct fossil fuel 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
20 and 18 percent, respectively, of CO2 emissions from
fossil fuel combustion in 2006. Both sectors relied heavily
on electricity for meeting energy demands, with 72 and
79 percent, respectively, of their emissions attributable to
electricity consumption for lighting, heating, cooling, and
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 36 percent of
U.S. energy from fossil fuels and emitted 41 percent of the
CO2 from fossil fuel combustion in 2006. The type of fuel
combusted by electricity generators has a significant effect
on their emissions. For example, some electricity is generated
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 2006.
ES-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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. Territories
Total
Electricity Generation
1990
1,488.1
1,485.1
3.0
1,527.5
844.9
682.5
929.5
340.1
589.4
750.8
216.1
534.7
28.3
4,724.1
1,809.6
1995
1,602.5
1,599.4
3.0
1,589.5
876.5
713.1
995.5
356.5
639.0
810.0
225.8
584.2
35.0
5,032.4
1,939.3
2000 2001 2002 2003 2004 2005 2006
1,801.6 1,779.2 1,832.3 1,811.8 1,860.9 1,874.5 1,861.0
1,798.2 1,775.6 1,828.9 1,807.6 1,856.4 1,869.8 1,856.0
3.4 3.6 3.4 4.2 4.5 4.7 4.9
1,645.1 1,583.9 1,572.5 1,592.1 1,596.8 1,579.6 1,567.1
860.3 852.5 854.8 856.0 857.7 847.3 862.2
784.7 731.4 717.7 736.1 739.0 732.3 704.9
1,129.7 1,121.8 1,145.6 1,178.3 1,173.1 1,206.4 1,151.9
372.1 363.6 360.5 382.9 368.3 358.5 326.5
757.6 758.1 785.1 795.4 804.9 847.9 825.4
964.6 973.5 970.3 983.8 997.1 1,017.3 1,003.0
228.0 222.3 222.8 236.5 230.6 221.9 210.1
736.6 751.1 747.5 747.3 766.5 795.4 792.9
36.2 49.0 44.0 51.0 53.5 53.2 54.9
5,577.1 5,507.4 5,564.8 5,617.0 5,681.4 5,731.0 5,637.9
2,282.3 2,244.3 2,253.7 2,283.1 2,314.9 2,380.2 2,328.2
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.
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 94
percent of all coal consumed for energy in the United States
in 2006. Consequently, changes in electricity demand have
a significant impact on coal consumption and associated
CO2 emissions.
Other significant CO2 trends included the following:
• CO2 emissions from non-energy use of fossil fuels have
increased 20.8 Tg CO2 Eq. (18 percent) from 1990
through 2006. Emissions from non-energy uses of fossil
fuels were 138.0 Tg CO2 Eq. in 2006, which constituted
2.4 percent of overall fossil fuel CO2 emissions and 2.3
percent of total national CO2 emissions, approximately
the same proportion as in 1990.
• CO2 emissions from iron and steel production increased
by 5.3 percent to 49.1 Tg CO2 Eq. in 2006, but have
declined overall by 37.1 Tg CO2 Eq. (43 percent)
from 1990 through 2006, due to restructuring of the
industry, technological improvements, and increased
scrap utilization.
• In 2006, CO2 emissions from cement production
decreased slightly by 0.2 Tg CO2 Eq. (0.4 percent) from
2005 to 2006. This decrease occurs despite the overall
increase over the time series. After falling in 1991 by two
percent from 1990 levels, cement production emissions
grew every year through 2005. Overall, from 1990 to
2006, emissions from cement production increased by
37 percent, an increase of 12.5 Tg CO2 Eq.
CO2 emissions from municipal solid waste combustion
(20.9 Tg CO2 Eq. in 2006) increased by 10.0 Tg CO2
Eq. (91 percent) from 1990 through 2006, as the volume
of plastics and other fossil carbon-containing materials
in municipal solid waste grew.
CO2 emissions from ammonia production and urea
consumption (12.4 Tg CO2 Eq. in 2006) have decreased
by 4.5 Tg CO2 Eq. (27 percent) since 1990. The decrease
in emissions from ammonia production and urea
consumption is associated with an overall decrease in
domestic ammonia production, and is due to several
factors including market fluctuations and high natural
gas prices.
Net CO2 sequestration from Land Use, Land-Use
Change, and Forestry increased by 146.0 Tg CO2 Eq.
(20 percent) from 1990 through 2006. 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. 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 CK4 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 CtLj emissions in the United States. In 2006, enteric
fermentation CH4 emissions were 126.2 Tg CO2 Eq.
(approximately 22.7 percent of total CFL, emissions),
which represents a decline of 0.7 Tg CO2 Eq., or 0.6
percent, since 1990. Despite this overall decline in
emissions, the last two years have shown a slight
increase in emissions.
• Landfills are the second largest anthropogenic source
of CH4 emissions in the United States, accounting for
approximately 22.6 percent of total CH4 emissions
(125.7 Tg CO2 Eq.) in 2006. From 1990 to 2006, net
CH4 emissions from landfills decreased by 23.9 Tg
CO2 Eq. (16 percent), with small increases occurring
in some interim years, including 2006. This downward
trend in overall emissions is the result of increases in
Figure ES-8
2006 Sources of CH* Emissions
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Forest Land Remaining Forest Land
Wastewater Treatment
Stationary Combustion
Rice Cultivation
Abandoned Coal Mines
Mobile Combustion
Composting
Petrocbemical Production
Iron and Steel Production
Field Burning of Agricultural Residues
Ferroalloy Production
Silicon Carbide Production and Consumption
<0.05
<0.05
0 20 40 60 80 100 120 140
Tg CO, Eq.
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.
• CH4 emissions from natural gas systems were 102.4
Tg CO2 Eq. in 2006; emissions have declined by 22.3
Tg CO2 Eq. (18 percent) since 1990. This decline
has been due to improvements in technology and
management practices, as well as some replacement of
old equipment.
• In 2006, CH4 emissions from coal mining were 58.5 Tg
CO2 Eq., a 1.4 Tg CO2 Eq. (2.5 percent) increase over
2005 emission levels. The overall decline of 25.6 Tg CO2
Eq. (30 percent) from 1990 results from the mining of less
gassy coal from underground mines and the increased use
of CFLj collected from degasification systems.
• CH4 emissions from manure management increased by
34 percent for CH^ from 31.0 Tg CO2 Eq. in 1990 to
41.4 Tg CO2 Eq. in 2006. 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 CK4
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
N2O 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
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.
ES-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Figure ES-9
2006 Sources of N,0 Emissions
265.0
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Stationary Combustion
Manure Management
Wastewater Treatment ^^
Adipic Acid Production ^J
N,0 from Product Uses ^|
Forest Land Remaining Forest Land |
Composting |
Settlements Remaining Settlements |
Field Burning of Agricultural Residues |
Municipal Solid Waste Combustion | <0.5
N20 as a Portion
of all Emissions
10
20 30
Tg CO, Eq.
40
50
in motor vehicles, nitric acid production, stationary fuel
combustion, manure management, and wastewater treatment
(see Figure ES-9).
Some significant trends in U.S. emissions of N2O include
the following:
• Agricultural soils produced approximately 72 percent of
N2O emissions in the United States in 2006. Estimated
emissions from this source in 2006 were 265.0 Tg
CO2 Eq. Annual N2O emissions from agricultural soils
fluctuated between 1990 and 2006, although overall
emissions were 1.6 percent lower in 2006 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 2006, N2O emissions from mobile combustion were
33.1 Tg CO2 Eq. (approximately 9 percent of U.S. N2O
emissions). From 1990 to 2006, N2O emissions from
mobile combustion decreased by 24 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.
• N2O emissions from adipic acid production were 5.9
Tg CO2 Eq. in 2006, and have decreased significantly
in recent years from the widespread installation of
pollution control measures. Emissions from adipic acid
production have decreased 61 percent since 1990, and
emissions from adipic acid production have fluctuated
by less than 1 Tg CO2 Eq. annually since 1998.
HFC, PFC, and SF6 Emissions
HFCs and PFCs are families of synthetic chemicals
that are used as alternatives to the ODSs, which are being
phased out under the Montreal Protocol and Clean Air
Act Amendments of 1990. FfFCs 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 ozone
depleting substances (e.g., CFCs) have been increasing
from small amounts in 1990 to 110.4 Tg CO2 Eq. in
2006. Emissions from substitutes for ozone depleting
substances 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 62 percent (22.6 Tg CO2 Eq.) from
1990 through 2006, due to a steady decline in the
emission rate of HFC-23 (i.e., the amount of HFC-23
Executive Summary ES-11
-------
Figure ES-10
Figure ES-11
2006 Sources of MFCs, PFCs, and SF6 Emissions
Substitution of Ozone
Depleting Substances
HCFC-22 Production I
Electrical Transmission I
and Distribution I
Semiconductor I
Manufacture I
Magnesium Production I
and Processing I
Aluminum Production I
MFCs, PFCs, and
SF6 as a Portion of
all Emissions
25
50 75
TgCO,Eq.
100
125
emitted per kilogram of HCFC-22 produced) and the
use of thermal oxidation at some plants to reduce
HFC-23 emissions.
• SF6 emissions from electric power transmission and
distribution systems decreased by 51 percent (13.5
Tg CO2 Eq.) from 1990 to 2006, primarily because of
higher purchase prices for SF6 and efforts by industry
to reduce emissions.
• PFC emissions from aluminum production decreased by
87 percent (16.1 Tg CO2 Eq.) from 1990 to 2006, due
to both industry emission reduction efforts and lower
domestic aluminum production.
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), the Inventory
of U.S. Greenhouse Gas Emissions and Sinks report is
segregated into six sector-specific chapters. 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 sixteen-year period
of 1990 to 2006, total emissions in the Energy, Industrial
Processes, and Agriculture sectors climbed by 873.0 Tg
CO2 Eq. (17 percent), 21.0 Tg CO2 Eq. (7 percent), and 6.6
U.S. Greenhouse Gas Emissions and Sinks
by Chapter/IPCC Sector
Industrial Processes
Agriculture
Waste
LULUCF (sources)
t
S
(1,000)J
Land Use, Land-Use Change and Forestry (sinks)
iiiiliiiiiiiiiiii
Note: Relatively smaller amounts of GWP-weighted emissions are also emitted from the Solvent and
Other Product Use sector.
Tg CO2 Eq. (1 percent), respectively. Emissions decreased
in the Waste and Solvent and Other Product Use sectors by
18.6 Tg CO2 Eq. (10 percent) and less than 0.1 Tg CO2 Eq.
(less than 1 percent), respectively. Over the same period,
estimates of net C sequestration in the Land Use, Land-Use
Change, and Forestry sector increased by 122.2 Tg CO2 Eq.
(17 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 2006. In 2006, approximately
83 percent of the energy consumed in the United States (on
a Btu basis) was produced through the combustion of fossil
fuels. The remaining 17 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 CFL, 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 86.1 percent of total U.S. greenhouse gas
emissions in 2006.
ES-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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)*
Net Emissions (Sources and Sinks)
1990
5,203.9
299.9
4.4
447.5
13.1
179.6
6,148.3
(737.7)
5,410.6
1995
5,529.6
315.7
4.6
453.8
13.6
176.8
6,494.0
(775.3)
5,718.7
2000 2001 2002 2003 2004 2005 2006
6,067.8 5,982.8 6,036.3 6,078.3 6,150.9 6,174.4 6,076.9
326.5 297.9 308.6 301.2 315.9 315.5 320.9
4.9 4.9 4.4 4.4 4.4 4.4 4.4
447.9 463.7 449.0 434.3 432.1 453.6 454.1
30.0 20.0 28.4 19.7 17.1 23.2 36.9
155.6 152.1 154.5 160.3 157.7 158.7 161.0
7,032.6 6,921.3 6,981.2 6,998.2 7,078.0 7,129.9 7,054.2
(673.6) (750.2) (826.8) (860.9) (873.7) (878.6) (883.7)
6,359.0 6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
* 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 ES-12
2006 U.S. Energy Consumption by Energy Source
Nuclear
Renewable
Natural Gas
Coal
Petroleum
22%
22%
39%
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, CFL,, and N2O. These
processes include iron and steel 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 use, 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 Processes chapter account for 4.5 percent of
U.S. greenhouse gas emissions in 2006.
Solvent and Other Product Use
The Solvent and Other Product Use chapter contains
greenhouse gas emissions that are produced as a byproduct 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 less
than 0.1 percent of total U.S. anthropogenic greenhouse gas
emissions on a carbon equivalent basis in 2006.
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
Executive Summary ES-13
-------
soil management, and field burning of agricultural residues.
CtLj and N2O were the primary greenhouse gases emitted
by agricultural activities. CH4 emissions from enteric
fermentation and manure management represented about
23 percent and 7 percent of total CH4 emissions from
anthropogenic activities, respectively, in 2006. Agricultural
soil management activities such as fertilizer application
and other cropping practices were the largest source of U.S.
N2O emissions in 2006, accounting for 72 percent. In 2006,
emission sources accounted for in the Agricultural chapters
were responsible for 6.4 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 CFL, 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 approximately 84 percent of
total 2006 net CO2 flux, urban trees accounted for 11 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 2006. 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 70 percent more C than is emitted through
these soils, liming, and urea fertilization, combined. 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
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 2006 resulted in a net C sequestration
of 883.7 Tg CO2 Eq. (Table ES-5). This represents an offset
of approximately 14.8 percent of total U.S. CO2 emissions,
or 12.5 percent of total greenhouse gas emissions in 2006.
Between 1990 and 2006, total land use, land-use change, and
forestry net C flux resulted in a 20 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. 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.
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., soil liming) and
urea fertilization resulted in CO2 emissions of 8.0 Tg CO2
Eq. in 2006, an increase of 13 percent relative to 1990. The
application of synthetic fertilizers to forest and settlement
soils in 2006 resulted in direct N2O emissions of 1.8 Tg
CO2 Eq. Direct N2O emissions from fertilizer application
increased by approximately 74 percent between 1990 and
2006. Non-CO2 emissions from forest fires in 2006 resulted
in CLLj emissions of 24.6 Tg CO2 Eq., and in N2O emissions
of 2.5 Tg C02 Eq.
ES-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
(621.7)
(30.1)
14.7
(1.9)
(14.3)
(60.6)
(23.9)
(737.7)
1995
(659.9)
(39.4)
9.4
16.6
(16.3)
(71.5)
(14.1)
(775.3)
2000 2001 2002 2003 2004 2005 2006
(550.7) (623.4) (697.3) (730.9) (741.4) (743.6) (745.1)
(38.4) (40.0) (40.3) (40.5) (40.9) (41.0) (41.8)
9.4 9.4 9.4 9.4 9.4 9.4 9.4
16.4 16.4 16.4 16.4 16.3 16.3 16.2
(16.3) (16.3) (16.3) (16.3) (16.3) (16.3) (16.3)
(82.4) (84.6) (86.8) (88.9) (91.1) (93.3) (95.5)
(11.5) (11.6) (11.8) (10.0) (9.6) (10.0) (10.5)
(673.6) (750.2) (826.8) (860.9) (873.7) (878.6) (883.7)
Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
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
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
Total
1990
7.1
7.1
4.5
4.5
1.5
0.5
0.1
1.0
13.1
1995
7.0
7.0
4.7
4.7
1.8
0.5
0.2
1.2
13.6
2000
7.5
7.5
19.0
19.0
3.5
1.9
0.3
1.2
30.0
2001
7.8
7.8
9.4
9.4
2.7
1.0
0.3
1.4
20.0
2002
8.5
8.5
16.4
16.4
3.5
1.7
0.3
1.5
28.4
2003
8.3
8.3
8.7
8.7
2.7
0.9
0.3
1.5
19.7
2004
7.6
7.6
6.9
6.9
2.6
0.7
0.3
1.6
17.1
2005
7.9
7.9
12.3
12.3
3.1
1.2
0.3
1.5
23.2
2006
8.0
8.0
24.6
24.6
4.3
2.5
0.3
1.5
36.9
Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
Waste
The Waste chapter contains emissions from waste
management activities (except waste incineration, which is
addressed in the Energy chapter). Landfills were the largest
source of anthropogenic CH4 emissions in the Waste chapter,
accounting for 23 percent of total U.S. CK4 emissions.13
Additionally, wastewater treatment accounts for 4 percent
of U.S. CtLj emissions. N2O emissions from the discharge
of wastewater treatment effluents into aquatic environments
were estimated, as were N2O emissions from the treatment
process itself. Emissions of CH4 and N2O from composting
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.
grew from 1990 to 2006, and resulted in emissions of 1.6 Tg
CO2 Eq. and 1.8 Tg CO2 Eq., respectively. Overall, in 2006,
emission sources accounted for in the Waste chapter generated
2.3 percent of total U.S. greenhouse gas emissions.
ES.4. Other Information
Emissions by Economic Sector
Throughout this 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
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
1990
1,859.1
1,544.1
1,460.3
506.8
396.9
346.9
34.1
6,148.3
(737.7)
5,410.6
1995
1,989.7
1,685.8
1,478.0
524.1
404.5
370.9
41.1
6,494.0
(775.3)
5,718.7
2000 2001 2002 2003 2004 2005 2006
2,328.9 2,290.9 2,300.4 2,329.4 2,363.4 2,430.0 2,377.8
1,917.5 1,895.8 1,948.5 1,925.9 1,975.4 1,987.2 1,969.5
1,432.9 1,384.3 1,384.9 1,375.5 1,388.9 1,354.3 1,371.5
528.0 533.4 529.3 498.0 499.2 521.3 533.6
390.3 383.0 388.1 410.2 404.6 400.4 394.6
387.7 379.3 376.6 399.6 385.5 376.0 344.8
47.3 54.5 53.3 59.7 61.0 60.5 62.4
7,032.6 6,921.3 6,981.2 6,998.2 7,078.0 7,129.9 7,054.2
(673.6) (750.2) (826.8) (860.9) (873.7) (878.6) (883.7)
6,359.0 6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
Note: Totals may not sum due to independent rounding. Emissions include C02, CH4, N20, MFCs, PFCs, and SF6. See Table 2-12 for more detailed data.
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 2006.
Using this categorization, emissions from electricity
generation accounted for the largest portion (34 percent)
of U.S. greenhouse gas emissions in 2006. Transportation
activities, in aggregate, accounted for the second largest
portion (28 percent). Emissions from industry accounted
for 19 percent of U.S. greenhouse gas emissions in 2006.
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 the residential, agriculture, and commercial
sectors, plus emissions from U.S. territories. The residential
sector accounted for about 5 percent, and primarily consisted
of CO2 emissions from fossil fuel combustion. Activities
related to agriculture accounted for roughly 8 percent of
U.S. emissions; unlike other economic sectors, agricultural
Figure ES-13
Emissions Allocated to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
0-
Electricity Generation
.^^
Transportation
Industry
'Agriculture
, Commercial
Residential
Note: Does not include U.S. territories.
sector emissions were dominated by N2O emissions from
agricultural soil management and CELj emissions from enteric
fermentation, rather than CO2 from fossil fuel combustion.
The commercial sector accounted for about 6 percent of
emissions, while U.S. territories accounted for 1 percent.
CO2 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 landfilling of yard trimmings.
ES-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
2,100.4
1,547.2
946.3
952.4
567.9
34.1
6,148.3
(737.7)
5,410.6
1995
2,141.1
1,688.9
1,003.8
1,026.5
592.5
41.1
6,494.0
(775.3)
5,718.7
2000
2,174.3
1,921.0
1,141.9
1,160.7
587.4
47.3
7,032.6
(673.6)
6,359.0
2001 2002 2003 2004 2005 2006
2,061.1 2,051.6 2,064.0 2,075.4 2,038.3 2,029.2
1,899.4 1,952.0 1,930.2 1,980.0 1,992.0 1,974.5
1,149.8 1,151.1 1,172.7 1,187.2 1,212.5 1,204.4
1,153.2 1,178.0 1,211.2 1,207.2 1,241.7 1,187.8
603.2 595.1 560.5 567.2 584.9 595.8
54.5 53.3 59.7 61.0 60.5 62.4
6,921.3 6,981.2 6,998.2 7,078.0 7,129.9 7,054.2
(750.2) (826.8) (860.9) (873.7) (878.6) (883.7)
6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
See Table 2-14 of this report for more detailed data.
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,
Figure ES-14
Emissions with Electricity Distributed
to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
0-
Industrial
S
Transportation
Residential
Commercial
Agriculture
Note: Does not include U.S. territories.
emissions from the source categories assigned to electricity
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 waste combustion, CH^ and N2O from
stationary sources, and SF6 from electrical transmission and
distribution systems.
When emissions from electricity are distributed among
these sectors, industry accounts for the largest share of U.S.
greenhouse gas emissions (29 percent) in 2006. 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
(28 percent). 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 2006.
14Emissions 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.
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 2006, (4) emissions per unit of total gross domestic product as a measure of national economic activity, or
(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 0.9 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 ES-15).
Table ES-9: Recent Trends in Various U.S. Data (Index 1990 = 100)
Variable
GDPb
Electricity Consumption0
Fossil Fuel Consumption0
Energy Consumption0
Populationd
Greenhouse Gas Emissions6
1990
100
100
100
100
100
100
1995
113
112
107
108
107
106
2000
138
127
117
116
113
114
2001
139
125
115
112
114
113
2002
141
128
116
115
115
114
2003
145
129
116
115
116
114
2004
150
131
119
118
117
115
2005
155
134
119
118
118
116
2006
159
135
117
117
119
115
Growth
Rate3
3.0%
1.9%
1.0%
1.0%
1.1%
0.9%
a Average annual growth rate
b Gross Domestic Product in chained 2000 dollars (BEA 2007)
c Energy content-weighted values (EIA 2007)
11 U.S. Census Bureau (2007)
e 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 (2007), U.S. Census Bureau (2007), and emission estimates in this report.
ES-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
Table ES-10: Emissions of NOX, CO, NMVOCs, and S02 (Gg)
Gas/Activity
N08
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Oil and Gas Activities
Municipal Solid Waste Combustion
Agricultural Burning
Solvent Use
Waste
CO
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Municipal Solid Waste Combustion
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
Municipal Solid Waste Combustion
Waste
Agricultural Burning
S02
Stationary Fossil Fuel Combustion
Industrial Processes
Mobile Fossil Fuel Combustion
Oil and Gas Activities
Municipal Solid Waste Combustion
Waste
Solvent Use
Agricultural Burning
1990
21,645
10,920
9,883
591
139
82
28
1
0
130,461
119,360
5,000
4,125
978
691
302
1
5
20,930
10,932
5,216
2,422
912
554
222
673
NA
20,935
18,407
1,307
793
390
38
0
0
NA
1995
21,272
10,622
9,821
607
100
88
29
3
1
109,032
97,630
5,383
3,959
1,073
663
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
2000 2001 2002 2003 2004 2005 2006
19,203 18,410 17,938 17,043 16,177 15,569 14,869
10,310 9,819 10,154 9,642 9,191 8,739 8,287
8,002 7,667 6,791 6,419 6,004 5,853 5,610
626 656 534 528 524 519 515
111 113 321 316 316 316 315
114 114 98 97 97 97 97
35 35 33 34 39 39 38
3355555
2222222
92,777 89,212 84,609 80,221 76,342 72,365 68,372
83,559 79,851 75,421 71,038 67,096 63,154 59,213
4,340 4,377 4,965 4,893 4,876 4,860 4,844
2,217 2,339 1,744 1,724 1,724 1,724 1,724
1,670 1,672 1,439 1,437 1,437 1,437 1,437
792 774 709 800 879 860 825
146 147 323 321 321 321 322
8877777
46 45 1 1 1 1 1
15,228 15,048 15,640 15,170 14,807 14,444 14,082
7,230 6,872 7,235 6,885 6,587 6,289 5,991
4,384 4,547 3,881 3,862 3,854 3,846 3,839
1,773 1,769 2,036 1,972 1,931 1,890 1,849
1,077 1,080 1,585 1,560 1,553 1,545 1,538
389 400 545 538 533 528 523
257 258 243 239 237 235 232
119 122 115 114 112 111 110
NA NA NA NA NA NA NA
14,829 14,452 13,403 13,631 13,232 13,114 12,258
12,848 12,461 11,613 11,956 11,625 11,573 10,784
1,031 1,047 850 804 800 797 793
632 624 683 621 564 508 451
286 289 233 226 220 213 207
29 30 23 22 22 22 22
1111111
1100000
NA NA NA NA NA NA NA
Source: (EPA 2008, 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.
15 See .
Executive Summary ES-19
-------
react with other chemical compounds in the atmosphere to
form compounds that are greenhouse gases.
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
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.
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.
Figure ES-16 presents 2006 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
Figure ES-16
2006 Key Categories-Tier 1 Level Assessment
CO2 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 |
C02 Emissions from Non-Energy Use of Fuels |
CH, Emissions from Enteric Fermentation |
CH, Emissions from Landfills |
Emissions from Substitutes for Ozone Depleting Substances |
Fugitive CH, Emissions from Natural Gas Systems |
Fugitive CH, Emissions from Coal Mining |
Indirect N20 Emissions from Applied Nitrogen |
C02 Emissions from Iron and Steel Production J
C02 Emissions from Cement Production J
C02 Emissions from Mobile Combustion: Marine |
CH, Emissions from Manure Management |
I I I I I I I I I I I I
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 source analysis see Annex 1.
16 NOX and CO emission estimates from field burning of agricultural residues
were estimated separately, and therefore not taken from EPA (2008).
17 See Chapter 7 "Methodological Choice and Recalculation" in IPCC (2000).
ES-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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.
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 emis sions
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 the Inventory. 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 2006. 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). Underworking 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
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 . (UNEP/WMO 2000)
4 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 .
Introduction 1-1
-------
guidelines. In addition, this Inventory is in accordance
with the IPCC Good Practice Guidance and Uncertainty
Management 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) 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: 1990-2006
-------
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 lifetime0
C02
278 ppm
379 ppm
1 .4 ppm/yr
50-200d
CH4
0.71 5 ppm
1.774 ppm
0.005 ppm/yra
12e
N20
0.270 ppm
0.31 9 ppm
0.26% yr
114e
SF6
Oppt
5.6 ppt
Linear"
3,200
CF4
40 ppt
74 ppt
Linear"
>50,000
Source: Pre-industrial atmospheric concentrations, current atmospheric concentrations, and rate of concentration changes for all gases are from IPCC (2007).
aThe growth rate for atmospheric CH4 has been decreasing from 1.4 ppb/yr in 1984 to less than 0 ppb/yr in 2001, 2004, and 2005.
b IPCC (2007) identifies the rate of concentration change for SF6 and CF4 as linear.
c Source: IPCC (1996).
11 No single lifetime can be defined for C02 because of the different rates of uptake by different removal processes.
eThis lifetime has been defined as an "adjustment time" that takes into account the indirect effect of the gas on its own residence time.
CO2, 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 (H2O). 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 affects 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. CO2 concentrations in the atmosphere
increased from approximately 280 parts per million by
volume (ppmv) in pre-industrial times to 379 ppmv in 2005,
a 35 percent increase (IPCC 2007 and Hofmann 2004).78
The IPCC definitively states that "the present atmospheric
CO2 increase is caused by anthropogenic 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
7The 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
-------
processes (e.g., cement production) also emit notable
quantities of CO2.
In its second assessment, the IPCC also stated that "[t]he
increased amount of CO2 [in the atmosphere] is leading
to climate change and will produce, on average, a global
warming of the earth's surface because of its enhanced
greenhouse effect—although the magnitude and significance
of the effects are not fully resolved" (IPCC 1996).
Methane. CH4 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. CH^ is also emitted during the
production and distribution of natural gas and petroleum,
and is released as a byproduct 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 1,774 ppb in 2005,
although the rate of increase has been declining. The IPCC
has estimated that slightly more than half of the current
CtLj flux to the atmosphere is anthropogenic, from human
activities such as agriculture, fossil fuel use, and waste
disposal (IPCC 2007).
CH4 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 combustion; 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 319
ppb in 2005, a concentration that has not been exceeded
during the last thousand years. N2O 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,9
where it shields the Earth from harmful levels of ultraviolet
radiation, and at lower concentrations in the troposphere,10
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,
with detection of such recovery not expected to occur much
before 2010 (IPCC 2001).
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
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
9 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.
10 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: 1990-2006
-------
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 511 countries 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 byproduct 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
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.12 Additionally,
NOX emissions from aircraft are 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-CFLj 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
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
"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.
12 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
-------
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 aerosols13 (e.g., black carbon, organic
carbon) from transportation, coal combustion, cement
production, 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.14 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, may have a positive radiative forcing (Jacobson
2001). 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
13 Carbonaceous aerosols are aerosols that are comprised mainly of organic
substances and forms of black carbon (or soot) (IPCC 2001).
14 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).
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 equivalents (Tg CO2 Eq.)15
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):
l.OOOGg
where,
Tg C02 Eq.
Gg
GWP
Tg
Teragrams of CO2 Equivalents
Gigagrams (equivalent to a
thousand metric tons)
Global Warming Potential
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 21
CP.3, Parties should report aggregate emissions
and removals of greenhouse gases, expressed in
CO2 equivalent terms at summary inventory level,
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.16
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
15 Carbon comprises 12/44'hs of carbon dioxide by weight.
16 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).
1-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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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
C4Fio
CeFi4
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: (IPCC 1996)
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.
determined. The short-lived gases such as water vapor,
carbon monoxide, tropospheric ozone, ozone precursors
(e.g., NOX and NMVOCs), 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 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
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
and quality and contractors familiar with the sources. A
multi-stage process for collecting information from the
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 useful 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
C4Fio
C6F14
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
TAR
NC
2
(14)
300
(100)
600
NC
500
(20)
600
3,100
200
(800)
2,700
1,600
1,600
(1,700)
from 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 inventories17 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 2006 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.
17 See .
throughout the NIR and CRF tables. Emission calculations individual source leads determine the most appropriate
for individual sources are the responsibility of individual methodology and collect the best activity data to use in
source leads, who are most familiar with each source category the emission calculations, based upon their expertise in the
and the unique characteristics of its emissions profile. The source category, as well as coordinating with researchers
1-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
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
Introduction 1-9
-------
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 CRE Reporter, as well as reviews by
the source leads, are completed for the entire time series of
CRE 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
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.
1-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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.
1.5. 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."18 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 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 either 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.
In addition to conducting Tier 1 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.
Table 1-4 presents the key categories for the United
States based on the Tier 1 approach (including and excluding
LULUCF categories) using emissions data in this report,
and ranked according to their sector and GWP-weighted
emissions in 2006. The table also indicates the criteria
used in identifying these categories (i.e., level, trend, 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.
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.
In particular, key attributes of the QA/QC plan
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;
18 See Chapter 7 "Methodological Choice and Recalculation" in IPCC
(2000).
Introduction 1-11
-------
Table 1-4: Key Categories for the United States (1990-2006) Based on Tier 1 Approach
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
C02 Emissions from Municipal Solid Waste Combustion
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
International Bunker Fuels"
Industrial Processes
C02 Emissions from Iron and Steel Production
C02 Emissions from Cement Production
C02 Emissions from Ammonia Production and Urea Application
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
Waste
CH4 Emissions from Landfills
Land Use, Land Use Change, and Forestry
C02 Emissions from Forest Land Remaining Forest Land
C02 Emissions from Settlements Remaining Settlements
C02 Emissions from Cropland Remaining Cropland
C02 Emissions from Grassland Remaining Grassland
C02 Emissions from Landfilled Yard Trimmings and Food Scraps
C02 Emissions from Land Converted to Cropland
CH4 Emissions from Forest Land Remaining Forest Land
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
C02
C02
C02
C02
C02
C02
CH4
CH4
CH4
N20
Several
C02
C02
C02
N20
Several
MFCs
SF6
PFCs
CH4
CH4
N20
N20
CH4
C02
C02
C02
C02
C02
C02
CH4
Level Trend Level Trend 2006
Without Without With With Emissions
LULUCF LULUCF LULUCF LULUCF dual" (Tg C02 Eq.)
/ / / / 2,065.3
/ / / / 1,643.0
/ / / 1,121.9
/ / / / 594.3
/ / / / 170.6
/ / 138.0
/ / / / 42.4
/ / / / 28.5
/ / 20.9
/ / / / 102.4
/ / / / 58.5
/ / / / 28.4
/ / / / 31.1
/ 128.4
/ / / / 49.1
/ / / / 45.7
/ / 12.4
/ / 5.9
/ / / / 110.4
/ / / / H Q Q
V V V V lO.O
/ / 13.2
/ / 2.5
/ / / / 126.2
/ / / 41.4
/ / / / 214.7
/ / / / 50.3
/ / / / 125.7
/ / (745.1)
/ / (95.5)
/ / (33.8)
/ 16.2
/ (10.5)
/ 9.4
/ 24.6
6,807.6
7,017.3
97.0%
5,972.8
6,170.5
96.8%
'Qualitative criteria.
b Emissions from this source not included in totals.
Note: The Tier 1 approach for identifying key source categories does not directly include assessment of uncertainty in emissions estimates.
Note: Parentheses indicate negative values or sequestration. The net C02 flux total includes both emissions and sequestration, and constitutes a sink in
the United States.
1-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
• 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
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 1 QA/
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
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 and cement
processing, 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 7996IPCC Guidelines (IPCC/UNEP/
OECD/IEA 1997) and require that countries provide single
point estimates of uncertainty 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.
Introduction 1-13
-------
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 CF^ 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. An estimate of the overall
quantitative uncertainty is 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. Consistent with the IPCC Good Practice Guidance,
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
uncertainties associated with 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.
Table 1-5. Estimated Overall Inventory Uncertainty (Tg C02 Eq. and Percent)
2006 Emission
Estimate
Gas (Tg C02 Eq.)
C02
CH4
N20
RFC, HFC&SF6d
Total
Net Emissions
(Sources and Sinks)
5,983.1
555.3
367.9
147.9
7,054.2
6,170.5
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound0
5,884.9
508.4
352.2
145.7
6,999.6
6,059.0
Upper Bound0 Lower Bound0
6,288.6 -2%
658.2 -8%
449.3 -4%
166.3 -1%
7,439.0 -1%
6,615.0 -2%
Upper Bound0
5%
19%
22%
12%
5%
7%
Standard
Mean" Deviation
(Tg C02 Eq.)
6,082.2
576.6
398.4
156.1
7,213.3
6,332.3
105.5
38.9
25.0
5.2
115.2
143.3
a Range of emission estimates for a 95 percent confidence interval.
b 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.
c The 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.
11 The overall uncertainty estimate 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 2006.
1-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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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 2006. 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 made, 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 foil owing organizational structure is consistently
applied throughout this report:
Chapter/IPCC Sector: Overview of emission trends for each
IPCC defined 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 and 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.
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.
Byproduct or fugitive emissions of greenhouse gases from industrial processes not
directly related to energy activities such as fossil fuel combustion.
Emissions, primarily of 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 1-15
-------
For example, each energy consuming end-use sector (i.e., individually. Additional information for certain source
residential, commercial, industrial, and transportation), categories and other topics is also provided in several
as well as the electricity generation sector, is described Annexes listed in Table 1-7.
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 Municipal Solid Waste Combustion
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
1-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
2. Trends in Greenhouse Gas
Emissions
2.1. Recent Trends in U.S. Greenhouse Gas Emissions
In 2006, total U.S. greenhouse gas emissions were 7,054.2 teragrams of carbon dioxide equivalents (Tg CO2 Eq.).1
Overall, total U.S. emissions have risen by 14.7 percent from 1990 to 2006, while the U.S. gross domestic product
has increased by 59 percent over the same period (BEA 2007). Emissions decreased from 2005 to 2006 by 1.1
percent (75.7 Tg CO2 Eq.). The following factors were primary contributors to this decrease: (1) compared to 2005, 2006
had warmer winter conditions, which decreased consumption of heating fuels, as well as cooler summer conditions, which
reduced demand for electricity, (2) restraint on fuel consumption caused by rising fuel prices, primarily in the transportation
sector and (3) increased use of natural gas and renewables in the electric power sector. Figure 2-1 through Figure 2-3 illustrate
the overall trends in total U.S. emissions by gas,2 annual changes, and absolute changes since 1990.
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
Figure 2-1 percentin 2006. Emissions from this source category grew by
19.3 percent (913.8Tg CO2 Eq.) from 1990 to 2006 and were
responsible for most of the increase in national emissions
during this period. From 2005 to 2006, these emissions
decreased by 1.6 percent (93.1 Tg CO2 Eq.). Historically,
changes in emissions from fossil fuel combustion have been
the dominant factor affecting U.S. emission trends.
U.S. Greenhouse Gas Emissions by Gas
MFCs, PFCs, & SFt
Nitrous Oxide
8,000 -
7,000 -
6,000 -
S 5,000 -
o™
m 4,000 -
3,000 -
2,000 -
1,000-
o-
Methane
I Carbon Dioxide
CO T- eo =0
a g s. si s
i— CM co
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.
1 Estimates are presented in units of teragrams of carbon dioxide equivalents (Tg CO2 Eq.), which weight each gas by its global warming potential, or
GWI^ value. (See section on global warming potentials, Executive Summary.)
2 See the following section for an analysis of emission trends by general U.S. economic sector.
Trends in Greenhouse Gas Emissions 2-1
-------
Figure 2-2
Annual Percent Change in U.S. Greenhouse Gas Emissions
Figure 2-3
4%-,
-1%
-2%
12.8%
•
.••I I-
1.1%
It
-1.1%
i— CM co
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.
After emissions significantly decreased in 2001 due to
the economic slowdown, emissions from fuel combustion
resumed modest growth in 2002, slightly less than the average
annual growth rate since 1990. There were a number of
reasons behind this increase. The U.S. economy experienced
moderate growth, recovering from weak economic conditions
in 2001. Prices for fuels remained at or below 2001 levels; the
cost of natural gas, motor gasoline, and electricity were all
lower—triggering an increase in demand for fuel. In addition,
the United States experienced one of the hottest summers on
Cumulative Change in U.S. Greenhouse Gas
Emissions Relative to 1990
1,000
900
800
S" 700
o~ 600
m 500
400
300
200
100
0
-100
930
982
-42
833 850
906
i— CM co
record, causing a significant increase in electricity use in the
residential sector as the use of air-conditioners increased.
Partially offsetting this increased consumption of fossil fuels,
however, were increases in the use of nuclear and renewable
fuels. Nuclear facilities operated at the highest capacity
on record in 2002. Furthermore, there was a considerable
increase in the use of hydroelectric power in 2002 after a
very low output the previous year.
Emissions from fuel combustion continued growing in
2003, at about the average annual growth rate since 1990. A
number of factors played a major role in the magnitude of this
increase. The U.S. economy experienced moderate growth
from 2002, causing an increase in the demand for fuels. The
price of natural gas escalated dramatically, causing some
electric power producers to switch to coal, which remained
at relatively stable prices. Colder winter conditions brought
on more demand for heating fuels, primarily in the residential
sector. Though a cooler summer partially offset demand for
electricity as the use of air-conditioners decreased, electricity
consumption continued to increase in 2003. The primary
drivers behind this trend were the growing economy and the
increase in U.S. housing stock. Nuclear capacity decreased
slightly, for the first time since 1997. Use of renewable fuels
rose slightly due to increases in the use of hydroelectric
power and biofuels.
From 2003 to 2004, these emissions increased at a rate
slightly higher than the average growth rate since 1990. A
number of factors played a major role in the magnitude of this
increase. A primary reason behind this trend was strong growth
in the U.S. economy and industrial production, particularly
2-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
in energy-intensive industries, causing an increase in the
demand for electricity and fossil fuels. Demand for travel was
also higher, causing an increase in petroleum consumed for
transportation. In contrast, the warmer winter conditions led
to decreases in demand for heating fuels, principally natural
gas, in both the residential and commercial sectors. Moreover,
much of the increased electricity demanded was generated by
natural gas combustion and nuclear power, which moderated
the increase in CO2 emissions from electricity generation.
Use of renewable fuels rose very slightly due to increases
in the use biofuels.
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 electricity emissions increased
among all end-use sectors, the fuels used to generate
electricity 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 primarily in the electricity generation,
transportation, residential, and commercial sectors due to a
number of factors. The decrease in emissions from electricity
generation is a result of a smaller share of electricity 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 6.4 percent, respectively,
in 2006, and nuclear power increased by less than 1
percent. The transportation decrease is 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 less than one percent in
2006. The decrease in emissions from the residential sector
is primarily a result of decreased electricity consumption
due to increases in the price of electricity, and warmer
winter weather conditions. The increase in emissions in
the industrial sector is a result of increased emissions from
fossil fuel combustion for this sector. A moderate increase
in the industrial sector is 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 wind.
Overall, from 1990 to 2006, total emissions of CO2
increased by 914.6 Tg CO2 Eq. (18 percent), while CFL, and
N2O emissions decreased by 50.8 Tg CO2 Eq. (8 percent)
and 15.5 Tg CO2 Eq. (4 percent) respectively. During the
same period, aggregate weighted emissions of HFCs, PFCs,
and SF6 rose by 57.6 Tg CO2 Eq. (64 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 12 percent of total emissions in 2006.
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.
Emissions of all gases can be summed from each
source category from Intergovernmental Panel on Climate
Change (IPCC) guidance. Over the sixteen-year period
of 1990 to 2006, total emissions in the Energy, Industrial
Processes, and Agriculture sectors grew by 873.0 Tg CO2
Eq. (17 percent), 21.0 Tg CO2 Eq. (7 percent), and 6.6 Tg
CO2 Eq. (1 percent), respectively. Emissions decreased
in the Waste and Solvent and Other Product Use sectors
by 18.6 Tg CO2 Eq. (10 percent) and less than 0.02 Tg
CO2 Eq. (less than 1 percent), respectively. Over the same
period, estimates of net C sequestration in the Land Use,
Land-Use Change, and Forestry sector increased by 122.2
Tg CO2 Eq. (17 percent).
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
Cement Production
Natural Gas Systems
Municipal Solid Waste Combustion
Lime Production
Ammonia Production
and Urea Consumption
Limestone and Dolomite Use
Cropland Remaining Cropland
Soda Ash Production and Consumption
Aluminum Production
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production
and Consumption
Land Use, Land-Use Change,
and Forestry (Sink)3
Wood Biomass and Ethanol Consumption13
International Bunker Fuels'3
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Forest Land Remaining Forest Land
Wastewater Treatment
Stationary Combustion
Rice Cultivation
1990
5,068.5
4,724.1
1,809.6
1,485.1
844.9
340.1
216.1
28.3
117.2
86.2
33.3
33.7
10.9
12.0
16.9
5.5
7.1
4.1
6.8
2.2
1.2
1.4
2.2
1.5
0.9
0.4
0.3
0.4
(737.7)
219.3
113.7
606.1
126.9
149.6
124.7
84.1
31.0
33.9
4.5
23.0
7.4
7.1
1995
5,394.2
5,032.4
1,939.3
1,599.4
876.5
356.5
225.8
35.0
133.2
74.7
36.8
33.8
15.7
14.0
17.8
7.4
7.0
4.3
5.7
2.8
1.5
1.4
2.0
1.5
1.0
0.3
0.3
0.3
(775.3)
236.8
100.6
598.9
132.3
144.0
128.1
67.1
35.2
32.0
4.7
24.3
7.2
7.6
2000 2001 2002 2003 2004 2005 2006
5,939.7 5,846.2 5,908.6 5,952.7 6,038.2 6,074.3 5,983.1
5,577.1 5,507.4 5,564.8 5,617.0 5,681.4 5,731.0 5,637.9
2,282.3 2,244.3 2,253.7 2,283.1 2,314.9 2,380.2 2,328.2
1,798.2 1,775.6 1,828.9 1,807.6 1,856.4 1,869.8 1,856.0
860.3 852.5 854.8 856.0 857.7 847.3 862.2
372.1 363.6 360.5 382.9 368.3 358.5 326.5
228.0 222.3 222.8 236.5 230.6 221.9 210.1
36.2 49.0 44.0 51.0 53.5 53.2 54.9
141.4 131.9 135.9 131.8 148.9 139.1 138.0
66.6 59.2 55.9 54.7 52.8 46.6 49.1
41.2 41.4 42.9 43.1 45.6 45.9 45.7
29.4 28.8 29.6 28.4 28.1 29.5 28.5
17.5 18.0 18.5 19.1 20.1 20.7 20.9
14.9 14.3 13.7 14.5 15.2 15.1 15.8
16.4 13.3 14.2 12.5 13.2 12.8 12.4
6.0 5.7 5.9 4.8 6.7 7.4 8.6
7.5 7.8 8.5 8.3 7.6 7.9 8.0
4.2 4.1 4.1 4.1 4.2 4.2 4.2
6.1 4.4 4.5 4.5 4.2 4.2 3.9
3.0 2.8 2.9 2.8 2.9 2.8 2.6
1.8 1.7 1.8 1.8 2.1 1.8 1.9
1.4 0.8 1.0 1.3 1.2 1.3 1.6
1.9 1.5 1.3 1.3 1.4 1.4 1.5
1.4 1.3 1.3 1.4 1.4 1.4 1.2
1.1 1.0 0.9 0.5 0.5 0.5 0.5
0.3 0.3 0.3 0.3 0.3 0.3 0.3
0.3 0.3 0.3 0.3 0.3 0.3 0.3
0.2 0.2 0.2 0.2 0.2 0.2 0.2
(673. 6) (750.2) (826.8) (860.9) (873.7) (878.6) (883.7)
227.3 203.2 204.4 209.5 224.8 227.4 234.7
101.1 97.6 89.1 103.6 119.0 122.6 127.1
574.3 558.8 563.5 559.4 545.6 539.7 555.3
124.6 123.6 123.8 124.6 122.4 124.5 126.2
120.8 117.6 120.1 125.6 122.6 123.7 125.7
126.5 125.3 124.9 123.3 114.0 102.5 102.4
60.4 60.3 56.8 56.9 59.8 57.1 58.5
38.8 40.2 41.3 40.7 40.1 41.8 41.4
30.3 30.2 29.9 29.2 28.7 28.3 28.4
19.0 9.4 16.4 8.7 6.9 12.3 24.6
24.6 24.2 24.1 23.9 24.0 23.8 23.9
6.6 6.2 6.2 6.4 6.5 6.5 6.2
7.5 7.6 6.8 6.9 7.6 6.8 5.9
2-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 2-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg C02 Eq.) (continued)
Gas/Source
Abandoned Underground Coal Mines
Mobile Combustion
Composting
Petrochemical Production
Iron and Steel Production
Field Burning of Agricultural Residues
Ferroalloy Production
Silicon Carbide Production
and Consumption
International Bunker Fuels b
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Stationary Combustion
Manure Management
Wastewater Treatment
Adipic Acid Production
N20 from Product Uses
Forest Land Remaining Forest Land
Composting
Settlements Remaining Settlements
Field Burning of Agricultural Residues
Municipal Solid Waste Combustion
International Bunker Fuels b
MFCs
Substitution of Ozone Depleting
Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Semiconductor Manufacture
Aluminum Production
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture
Total
Net Emissions (Sources and Sinks)
1990
6.0
4.7
0.3
0.9
1.3
0.7
+
+
0.2
383.4
269.4
43.5
17.0
12.8
12.1
6.3
15.3
4.4
0.5
0.4
1.0
0.4
0.5
1.0
36.9
0.3
36.4
0.2
20.8
2.2
18.5
32.7
26.7
5.4
0.5
6,148.3
5,410.6
1995
8.2
4.3
0.7
1.1
1.3
0.7
+
+
0.1
395.6
264.8
53.4
18.9
13.4
12.8
6.9
17.3
4.6
0.6
0.8
1.2
0.4
0.5
0.9
61.8
28.5
33.0
0.3
15.6
3.8
11.8
28.0
21.5
5.6
0.9
6,494.0
5,718.7
2000 2001 2002 2003 2004 2005 2006
7.4 6.7 6.2 6.0 5.8 5.6 5.4
3.4 3.3 3.0 2.7 2.6 2.5 2.4
1.3 1.3 1.3 1.5 1.6 1.6 1.6
1.2 1.1 1.1 1.1 1.2 1.1 1.0
1.2 1.1 1.0 1.0 1.0 1.0 0.9
0.8 0.8 0.7 0.8 0.9 0.9 0.8
+ + + + + + +
+ + + + + + +
0.1 0.1 0.1 0.1 0.1 0.2 0.2
385.9 392.9 376.1 356.6 353.5 370.1 367.9
262.1 277.0 262.0 247.3 246.9 265.2 265.0
52.5 49.9 45.9 42.3 39.7 36.3 33.1
18.6 15.1 16.4 15.4 15.2 15.8 15.6
14.6 14.1 14.0 14.3 14.6 14.8 14.5
13.7 14.0 14.0 13.6 13.8 13.9 14.3
7.6 7.8 7.6 7.7 7.8 8.0 8.1
6.2 5.1 6.1 6.3 5.9 5.9 5.9
4.9 4.9 4.4 4.4 4.4 4.4 4.4
2.2 1.3 2.0 1.2 1.1 1.6 2.8
1.4 1.4 1.4 1.6 1.7 1.7 1.8
1.2 1.4 1.5 1.5 1.6 1.5 1.5
0.5 0.5 0.4 0.4 0.5 0.5 0.5
0.4 0.4 0.4 0.4 0.4 0.4 0.4
0.9 0.9 0.8 0.9 1.1 1.1 1.1
100.1 97.9 106.3 104.5 116.6 121.4 124.5
71.2 78.0 85.0 92.0 99.1 105.4 110.4
28.6 19.7 21.1 12.3 17.2 15.8 13.8
0.3 0.2 0.2 0.2 0.2 0.2 0.3
13.5 7.0 8.7 7.1 6.1 6.2 6.0
4.9 3.5 3.5 3.3 3.3 3.2 3.6
8.6 3.5 5.2 3.8 2.8 3.0 2.5
19.1 18.7 18.0 18.1 18.0 18.2 17.3
15.1 15.0 14.4 13.8 13.9 14.0 13.2
3.0 2.9 2.9 3.4 3.2 3.3 3.2
1.1 0.7 0.7 0.8 0.8 1.0 1.0
7,032.6 6,921.3 6,981.2 6,998.2 7,078.0 7,129.9 7,054.2
6,359.0 6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
+ Does not exceed 0.05 Tg C02 Eq.
aThe 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 Consumption are not included in totals.
c Small amounts of PFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
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
Cement Production
Natural Gas Systems
Municipal Solid Waste Combustion
Lime Production
Ammonia Production and
Urea Consumption
Limestone and Dolomite Use
Cropland Remaining Cropland
Soda Ash Production
and Consumption
Aluminum Production
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production
and Consumption
Land Use, Land-Use Change,
and Forestry (Sink)3
Wood Biomass and
Ethanol Consumption13
International Bunker Fuels'3
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Petroleum Systems
Forest Land Remaining Forest Land
Wastewater Treatment
Stationary Combustion
Rice Cultivation
Abandoned Underground
Coal Mines
1990
5,068,472
4,724,146
1,809,614
1,485,057
844,937
340,109
216,144
28,285
117,170
86,220
33,278
33,729
10,950
12,004
16,889
5,533
7,084
4,141
6,831
2,221
1,195
1,416
2,152
1,529
949
376
285
375
(737,677)
219,341
113,683
28,861
6,044
7,124
5,937
4,003
1,474
1,612
213
1,096
353
339
288
2000
5,939,726
5,577,072
2,282,278
1,798,164
860,325
372,083
228,027
36,195
141,427
66,609
41,190
29,390
17,518
14,872
16,402
5,960
7,541
4,181
6,086
3,004
1,752
1,421
1,893
1,382
1,140
325
311
248
(673,608)
227,276
101,125
27,346
5,933
5,751
6,024
2,874
1,847
1,442
904
1,173
316
357
350
2001
5,846,151
5,507,406
2,244,279
1,775,636
852,494
363,629
222,341
49,027
131,887
59,249
41,357
28,793
17,971
14,261
13,305
5,733
7,825
4,147
4,381
2,787
1,697
829
1,459
1,264
986
325
291
199
(750,191)
203, 163
97,563
26,608
5,886
5,598
5,968
2,874
1,915
1,436
448
1,150
295
364
319
2002
5,908,568
5,564,795
2,253,729
1,828,910
854,822
360,492
222,828
44,014
135,857
55,938
42,898
29,629
18,458
13,652
14,194
5,885
8,549
4,139
4,490
2,857
1,824
989
1,349
1,338
937
320
286
183
(826,758)
204,351
89,101
26,832
5,896
5,720
5,946
2,707
1,964
1,422
780
1,148
295
325
293
2003
5,952,650
5,617,047
2,283,069
1,807,591
856,042
382,864
236,452
51,030
131,772
54,744
43,082
28,445
19,058
14,458
12,488
4,753
8,260
4,111
4,503
2,777
1,839
1,311
1,305
1,382
507
316
289
202
(860,912)
209,537
103,583
26,637
5,931
5,981
5,874
2,709
1,938
1,390
416
1,140
306
328
284
2004
6,038,211
5,681,363
2,314,907
1,856,373
857,722
368,258
230,617
53,486
148,931
52,771
45,603
28,122
20,097
15,154
13,241
6,702
7,555
4,205
4,231
2,895
2,064
1,198
1,419
1,395
477
302
263
224
(873,660)
224,825
118,975
25,979
5,828
5,838
5,426
2,846
1,908
1,368
330
1,141
311
360
276
2005
6,074,306
5,731,045
2,380,222
1,869,848
847,328
358,515
221,921
53,213
139,057
46,627
45,910
29,462
20,673
15,131
12,817
7,397
7,854
4,228
4,207
2,804
1,755
1,321
1,392
1,386
465
287
266
219
(878,605)
227,366
122,580
25,698
5,928
5,890
4,880
2,717
1,988
1,346
586
1,131
308
326
265
2006
5,983,108
5,637,931
2,328,153
1,856,047
862,187
326,522
210,140
54,882
137,980
49,119
45,739
28,504
20,922
15,825
12,376
8,615
8,012
4,162
3,923
2,573
1,876
1,579
1,505
1,167
529
293
270
207
(883,665)
234,726
127,097
26,442
6,010
5,985
4,877
2,784
1,972
1,354
1,169
1,136
296
282
257
2-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 2-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg) (continued)
Gas/Source
Mobile Combustion
Composting
Petrochemical Production
Iron and Steel Production
Field Burning of Agricultural
Residues
Ferroalloy Production
Silicon Carbide Production
and Consumption
International Bunker Fuels"
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Stationary Combustion
Manure Management
Wastewater Treatment
Adipic Acid Production
N20 from Product Uses
Forest Land Remaining Forest Land
Composting
Settlements Remaining Settlements
Field Burning of Agricultural
Residues
Municipal Solid Waste Combustion
International Bunker Fuels'1
MFCs
Substitution of Ozone Depleting
Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Semiconductor Manufacture
Aluminum Production
SF6
Electrical Transmission
and Distribution
Magnesium Production
and Processing
Semiconductor Manufacture
1990
224
15
41
63
33
1
1
8
1,237
869
140
55
41
39
20
49
14
2
1
3
1
2
3
M
M
3
+
M
M
M
1
1
+
+
2000
162
60
58
58
38
1
1
6
1,245
845
169
60
47
44
24
20
16
7
4
4
1
1
3
M
M
2
+
M
M
M
1
1
+
+
2001
157
60
51
51
37
+
+
5
1,267
894
161
49
46
45
25
16
16
4
5
5
1
1
3
M
M
2
+
M
M
M
1
1
+
+
2002
141
61
52
48
34
+
+
4
1,213
845
148
53
45
45
25
20
14
6
5
5
1
1
3
M
M
2
+
M
M
M
1
1
+
+
2003
131
69
51
49
38
+
+
6
1,150
798
137
50
46
44
25
20
14
4
5
5
1
1
3
M
M
1
+
M
M
M
1
1
+
+
2004
126
74
55
50
42
+
+
7
1,140
796
128
49
47
44
25
19
14
3
6
5
2
1
3
M
M
1
+
M
M
M
1
1
+
+
2005
119
75
51
45
41
+
+
7
1,194
855
117
51
48
45
26
19
14
5
6
5
2
1
4
M
M
1
+
M
M
M
1
1
+
+
2006
112
75
48
45
39
+
+
7
1,187
855
107
50
47
46
26
19
14
9
6
5
2
1
4
M
M
1
+
M
M
M
1
1
+
+
+ Does not exceed 0.5 Gg.
M Mixture of multiple gases.
aThe 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 Consumption are not included in totals.
c Small amounts of PFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
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)*
Net Emissions (Sources and Sinks)
1990
5,203.9
299.9
4.4
447.5
13.1
179.6
6,148.3
(737.7)
5,410.6
1995
5,529.6
315.7
4.6
453.8
13.6
176.8
6,494.0
(775.3)
5,718.7
2000 2001 2002 2003 2004 2005 2006
6,067.8 5,982.8 6,036.3 6,078.3 6,150.9 6,174.4 6,076.9
326.5 297.9 308.6 301.2 315.9 315.5 320.9
4.9 4.9 4.4 4.4 4.4 4.4 4.4
447.9 463.7 449.0 434.3 432.1 453.6 454.1
30.0 20.0 28.4 19.7 17.1 23.2 36.9
155.6 152.1 154.5 160.3 157.7 158.7 161.0
7,032.6 6,921.3 6,981.2 6,998.2 7,078.0 7,129.9 7,054.2
(673.6) (750.2) (826.8) (860.9) (873.7) (878.6) (883.7)
6,359.0 6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
* 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
Agriculture
Waste
LULUCF (sources)
Land Use, Land-Use Change and Forestry sinks
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 2006. In 2006,
approximately 83 percent of the energy consumed in the
United States (on a Btu basis) was produced through the
combustion of fossil fuels. The remaining 17 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
Figure 2-5
2006 Energy Chapter Greenhouse Gas Emission Sources
5,637.9
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Coal Mining
Mobile Combustion
Petroleum Systems
Municipal Solid Waste
Combustion
Stationary Combustion
Abandoned Underground
Energy as a Portion
of all Emissions
Coal Mines
25
50 75 100
Tg C02 Eq.
125 150
also responsible for CH^ and N2O emissions (37 percent and
13 percent of total U.S. emissions of each gas, respectively).
Table 2-4 presents greenhouse gas emissions from the Energy
chapter, by source and gas.
CO2 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). EIA's fuel consumption data for the electricity
2-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Figure 2-6
2006 U.S. Fossil Carbon Flows (Tg C02 Eq.)
NEU Emissions 121
Note: Totals may nol sum duelo 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
generation sector consists of privately and publicly owned
establishments that generate, transmit, distribute, or sell
electricity primarily for use by the public and that meet
EIA's definition of an electric utility (EIA does not include
non-utility power producers in this sector). 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 1,861.0 Tg CO2 Eq. in 2006, or
approximately 33 percent of total CO2 emissions from fossil
fuel combustion, the largest share of any end-use economic
sector.3 The industrial end-use sector accounted for 28 percent
of CO2 emissions from fossil fuel combustion. The residential
and commercial end-use sectors accounted for an average 20
and 18 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 72 and 79 percent
of emissions from the residential and commercial end-use
sectors, respectively. Significant trends in emissions from
energy source categories over the sixteen-year period from
1990 through 2006 included the following:
3 Note that electricity generation is the largest emitter of CO2 when electricity
is not distributed among end-use sectors.
Trends in Greenhouse Gas Emissions 2-9
-------
Table 2-4: Emissions from Energy (Tg C02 Eq.)
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Natural Gas Systems
Municipal Solid Waste Combustion
Petroleum Systems
Biomass—Wood*
International Bunker Fuels*
Biomass—Ethanol*
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
International Bunker Fuels*
N20
Mobile Combustion
Stationary Combustion
Municipal Solid Waste Combustion
International Bunker Fuels*
4,886.4
4,724.1
1,809.6
1,473.5
849.9
344.4
218.5
28.3
117.2
33.7
10.9
0.4
215.2
113.7
4.2
260.7
124.7
84.1
33.9
7.4
6.0
4.7
0.2
56.8
43.5
12.8
0.5
1.0
5,215.5
5,032.4
1,939.3
1,590.2
880.6
359.9
227.5
35.0
133.2
33.8
15.7
0.3
229.1
100.6
7.7
246.8
128.1
67.1
32.0
7.2
8.2
4.3
0.1
67.3
53.4
13.4
0.5
0.9
2000
5,765.7
5,577.1
2,283.1
1,801.5
858.8
385.0
237.6
51.0
141.4
29.4
17.5
0.3
218.1
101.1
9.2
234.5
126.5
60.4
30.3
6.6
7.4
3.4
0.1
67.5
52.5
14.6
0.4
0.9
6,067.8
2001
5,686.4
5,507.4
2,314.9
1,849.3
861.0
370.8
231.9
53.5
131.9
28.8
18.0
0.3
193.5
97.6
9.7
232.0
125.3
60.3
30.2
6.2
6.7
3.3
0.1
64.4
49.9
14.1
0.4
0.9
5,982.8
2002
5,749.1
5,564.8
2,380.2
1,862.6
850.9
360.9
223.2
53.2
135.9
29.6
18.5
0.3
192.8
89.1
11.5
226.9
124.9
56.8
29.9
6.2
6.2
3.0
0.1
60.4
45.9
14.0
0.4
0.8
6,036.3
2003
5,796.6
5,617.0
2,328.2
1,848.7
866.1
328.7
211.4
54.9
131.8
28.4
19.1
0.3
193.8
103.6
15.7
224.6
123.3
56.9
29.2
6.4
6.0
2.7
0.1
57.1
42.3
14.3
0.4
0.9
6,078.3
2004
5,878.8
5,681.4
2,283.1
1,801.5
858.8
385.0
237.6
51.0
148.9
28.1
20.1
0.3
205.1
119.0
19.7
217.4
114.0
59.8
28.7
6.5
5.8
2.6
0.1
54.7
39.7
14.6
0.4
1.1
6,150.9
2005
5,920.5
5,731.0
2,314.9
1,849.3
861.0
370.8
231.9
53.5
139.1
29.5
20.7
0.3
204.8
122.6
22.6
202.3
102.5
57.1
28.3
6.5
5.6
2.5
0.2
51.5
36.3
14.8
0.4
1.1
6,174.4
2006
5,825.6
5,637.9
2,380.2
1,862.6
850.9
360.9
223.2
53.2
138.0
28.5
20.9
0.3
204.4
127.1
30.3
203.3
102.4
58.5
28.4
6.2
5.4
2.4
0.2
48.0
33.1
14.5
0.4
1.1
6,076.9
* These values are presented for informational purposes only and are not included in totals or are already accounted for in other source categories.
Note: Totals may not sum due to independent rounding.
Total CO2 emissions from fossil fuel combustion
increased from 4,724.1 Tg CO2 Eq. to 5,637.9 Tg CO2
Eq. —a 19.3 percent total increase over the sixteen-year
period. From 2005 to 2006, these emissions decreased
by 93.1 Tg CO2 Eq. (1.6 percent).
CO2 emissions from non-energy use of fossil fuels have
increased 20.8 Tg CO2 Eq. (18 percent) from 1990
through 2006. Emissions from non-energy uses of fossil
fuels were 138.0 Tg CO2 Eq. in 2006, which constituted
2.4 percent of overall fossil fuel CO2 emissions and 2.3
percent of total national CO2 emissions, approximately
the same proportion as in 1990.
CtLj emissions from natural gas systems were 102.4 Tg
CO2 Eq. in 2006; emissions have declined by 22.3 Tg CO2
Eq. (18 percent) since 1990. This decline has been due to
improvements in technology and management practices,
as well as some replacement of old equipment.
CFLj emissions from coal mining were 58.5 Tg CO2
Eq. This decline of 25.6 Tg CO2 Eq. (30 percent)
from 1990 results from the mining of less gassy coal
from underground mines and the increased use of CK4
collected from degasification systems.
In 2006, N2O emissions from mobile combustion were
33.1 Tg CO2 Eq. (approximately 9 percent of U. S. N2O
emissions). From 1990 to 2006, N2O emissions from
mobile combustion decreased by 24 percent. However,
from 1990 to 1998 emissions increased by 26 percent,
due to control technologies that reduced NOX emissions
2-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 2-5: C02 Emissions from Fossil Fuel Combustion by 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
1,488.1
1,485.1
3.0
1,527.5
844.9
682.5
929.5
340.1
589.4
750.8
216.1
534.7
28.3
4,724.1
1,809.6
1995
1,602.5
1,599.4
3.0
1,589.5
876.5
713.1
995.5
356.5
639.0
810.0
225.8
584.2
35.0
5,032.4
1,939.3
2000 2001 2002 2003 2004 2005 2006
1,801.6 1,779.2 1,832.3 1,811.8 1,860.9 1,874.5 1,861.0
1,798.2 1,775.6 1,828.9 1,807.6 1,856.4 1,869.8 1,856.0
3.4 3.6 3.4 4.2 4.5 4.7 4.9
1,645.1 1,583.9 1,572.5 1,592.1 1,596.8 1,579.6 1,567.1
860.3 852.5 854.8 856.0 857.7 847.3 862.2
784.7 731.4 717.7 736.1 739.0 732.3 704.9
1,129.7 1,121.8 1,145.6 1,178.3 1,173.1 1,206.4 1,151.9
372.1 363.6 360.5 382.9 368.3 358.5 326.5
757.6 758.1 785.1 795.4 804.9 847.9 825.4
964.6 973.5 970.3 983.8 997.1 1,017.3 1,003.0
228.0 222.3 222.8 236.5 230.6 221.9 210.1
736.6 751.1 747.5 747.3 766.5 795.4 792.9
36.2 49.0 44.0 51.0 53.5 53.2 54.9
5,577.1 5,507.4 5,564.8 5,617.0 5,681.4 5,731.0 5,637.9
2,282.3 2,244.3 2,253.7 2,283.1 2,314.9 2,380.2 2,328.2
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 2-7
Figure 2-8
2,500 -i
2,000 -
1,500 -
3
2006 C02 Emissions from Fossil Fuel
Combustion by Sector and Fuel Type
Natural Gas
Petroleum
I Coal
Relative Contribution
by Fuel Type
' 1,000 -
500 -
0 -1
U.S. Commercial Residential Industrial Transportation Electricity
Territories Generation
Note: Electricity generation also includes emissions of less than 0.5 Tg C02 Eq. from geothermal-l
electricity generation.
2006 End-Use Sector Emissions of C02 from
Fossil Fuel Combustion
S
2,000 -
1,800 -
1,600 -
1,400 -
1,200 -
,1,000 -
800 -
600 -
400 -
200 -
o -J
From Electricity
Consumption
| From Direct Fossil
Fuel Combustion
U.S. Commercial Residential Industrial Transportation
Territories
while increasing N2O emissions. Since 1998, newer
control technologies have led to a steady decline in N2O
from this source.
CO2 emissions from municipal solid waste combustion
(20.9 Tg CO2 Eq. in 2006) increased by 10.0 Tg CO2
Eq. (91 percent) from 1990 through 2006, as the volume
of plastics and other fossil carbon-containing materials
in municipal solid waste grew.
Industrial Processes
Emissions are produced as a byproduct of many non-
energy-related industrial process activities. For example,
industrial processes can chemically transform raw materials,
which often release waste gases such as CO2, CH^, and N2O.
These processes include iron and steel 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 use, titanium dioxide production,
Trends in Greenhouse Gas Emissions 2-11
-------
Figure 2-9
2006 Industrial Processes Chapter
Greenhouse Gas Emission Sources
Substitution of Ozone Depleting Substances
Iron and Steel Production
Cement Production
Lime Production ^f
Nitric Acid Production ^f
HCFC-22 Production B
Electrical Transmission and Distribution ^|
Ammonia Production and Urea Consumption B
Limestone and Dolomite Use |
Aluminum Production |
Adipic Acid Production |
Semiconductor Manufacture |
Soda Asb Production and Consumption |
Petrocbemical Production |
Magnesium Production and Processing |
Titanium Dioxide Production |
Carbon Dioxide Consumption |
Ferroalloy Production |
Pbospboric Acid Production |
Zinc Production |
Lead Production | <0.5
Silicon Carbide Production and Consumption | <0.5
Industrial Processes
as a Portion of
all Emissions
0
25
50 75
Tg CO, Eq.
100 125
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). Additionally, emissions from
industrial processes release HFCs, PFCs and SF6. Table 2-6
presents greenhouse gas emissions from industrial processes
by source category.
Overall, emissions from industrial processes increased
by 7.0 percent from 1990 to 2006 despite decreases in
emissions from several industrial processes, such as iron
and steel, 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 production and, primarily, the
emissions from the use of substitutes for ozone depleting
substances. Significant trends in emissions from industrial
processes source categories over the sixteen-year period from
1990 through 2006 included the following:
• HFC emissions from ODS substitutes have been
increasing from small amounts in 1990 to 110.4Tg CO2
Eq. in 2006. 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.
• CO2 and CFLj emissions from iron and steel production
increased by 5.2 percent to 50.1 Tg CO2 Eq. in 2006,
but have declined overall by 37.5 Tg CO2 Eq. (42.8
percent) from 1990 through 2006, due to restructuring of
the industry, technological improvements, and increased
scrap utilization.
• PFC emissions from aluminum production decreased by
87 percent (16.1 Tg CO2 Eq.) from 1990 to 2006, due
to both industry emission reduction efforts and lower
domestic aluminum production.
• N2O emissions from adipic acid production were 5.9
Tg CO2 Eq. in 2006, and have decreased significantly
in recent years from the widespread installation of
pollution control measures. Emissions from adipic acid
production have decreased 61 percent since 1990, and
emissions from adipic acid production have fluctuated
by less than 1 Tg CO2 Eq. annually since 1998.
• CO2 emissions from ammonia production and urea
consumption (12.4 Tg CO2 Eq. in 2006) have decreased
by 4.5 Tg CO2 Eq. (27 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.
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 2006 (see Table 2-7).
• In 2006, N2O emissions from product uses constituted
1 percent of U.S. N2O emissions. From 1990 to 2006,
emissions from this source category decreased by less
than 1 percent, though slight increases occurred in
intermediate years.
2-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 2-6: Emissions from Industrial Processes (Tg C02 Eq.)
Gas/Source
C02
Iron and Steel Production
Cement Production
Lime Production
Ammonia Production & Urea Consumption
Limestone and Dolomite Use
Soda Ash Production and Consumption
Aluminum Production
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
Ferroalloy Production
Silicon Carbide Production and Consumption
N20
Nitric Acid Production
Adipic Acid Production
MFCs
Substitution of Ozone Depleting Substances3
HCFC-22 Production
Semiconductor Manufacture
PFCs
Semiconductor Manufacture
Aluminum Production
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture
Total
1990
175
86
33
12
16
5
4
6
2
1
1
2
1
0
0
0
2
0
1
32
17
15
36
0
36
0
20
2
18
32
26
5
0
299
0
2
3
0
9
5
1
8
2
2
4
2
5
9
3
4
2
9
3
•f
•f
3
0
3
9
3
4
2
8
2
5
7
7
4
5
9
1995
171.6
74.7
36.8
14.0
17.8
7.4
4.3
5.7
2.8
1.5
1.4
2.0
1.5
1.0
0.3
0.3
2.4
1.1
1.3
+
+
36.2
18.9
17.3
61.8
28.5
33.0
0.3
15.6
3.8
11.8
28.0
21.5
5.6
0.9
315.7
2000
166.5
66.6
41.2
14.9
16.4
6.0
4.2
6.1
3.0
1.8
1.4
1.9
1.4
1.1
0.3
0.2
2.5
1.2
1.2
+
+
24.8
18.6
6.2
100.1
71.2
28.6
0.3
13.5
4.9
8.6
19.1
15.1
3.0
1.1
326.5
2001
151
59
41
14
13
5
4
9
2
4
3
3
7
1
4.4
2
1
0
1
1
1
0
0
2
1
1
20
15
5
97
78
19
0
7
3
3
18
15
2
0
297
8
7
8
5
3
0
3
2
2
1
1
+
+
2
1
1
9
0
7
2
0
5
5
7
0
9
7
9
2002
151.0
55.9
42.9
13.7
14.2
5.9
4.1
4.5
2.9
1.8
1.0
1.3
1.3
0.9
0.3
0.2
2.1
1.1
1.0
+
+
22.4
16.4
6.1
106.3
85.0
21.1
0.2
8.7
3.5
5.2
18.0
14.4
2.9
0.7
308.6
2003
147.8
54.7
43.1
14.5
12.5
4.8
4.1
4.5
2.8
1.8
1.3
1.3
1.4
0.5
0.3
0.2
2.1
1.1
1.0
+
+
21.7
15.4
6.3
104.5
92.0
12.3
0.2
7.1
3.3
3.8
18.1
13.8
3.4
0.8
301.2
2004
151.8
52.8
45.6
15.2
13.2
6.7
4.2
4.2
2.9
2.1
1.2
1.4
1.4
0.5
0.3
0.2
2.2
1.2
1.0
+
+
21.2
15.2
5.9
116.6
99.1
17.2
0.2
6.1
3.3
2.8
18.0
13.9
3.2
0.8
315.9
2005
145.9
46.6
45.9
15.1
12.8
7.4
4.2
4.2
2.8
1.8
1.3
1.4
1.4
0.5
0.3
0.2
2.0
1.1
1.0
+
+
21.7
15.8
5.9
121.4
105.4
15.8
0.2
6.2
3.2
3.0
18.2
14.0
3.3
1.0
315.5
2006
149.5
49.1
45.7
15.8
12.4
8.6
4.2
3.9
2.6
1.9
1.6
1.5
1.2
0.5
0.3
0.2
2.0
1.0
0.9
+
+
21.6
15.6
5.9
124.5
110.4
13.8
0.3
6.0
3.6
2.5
17.3
13.2
3.2
1.0
320.9
+ 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.
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
4.6
2000
4.9
4.9
4.9
2001
4.9
4.9
4.9
2002
4.4
4.4
4.4
2003
4.4
4.4
4.4
2004
4.4
4.4
4.4
2005
4.4
4.4
4.4
2006
4.4
4.4
4.4
Trends in Greenhouse Gas Emissions 2-13
-------
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 (see Figure 2-10).
In 2006, agricultural activities were responsible for
emissions of 454.1 Tg CO2 Eq., or 6.4 percent of total U.S.
greenhouse gas emissions (see Table 2-8). CH4 and N2O
were the primary greenhouse gases emitted by agricultural
activities. CR4 emissions from enteric fermentation and
manure management represented about 23 percent and 7
percent of total CH4 emissions from anthropogenic activities,
respectively, in 2006. Agricultural soil management
activities, such as fertilizer application and other cropping
Figure 2-10
2006 Agriculture Chapter Greenhouse Gas
Emission Sources
265.0
Agricultural Soil Management
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of
Agricultural Residues
Agriculture
as a Portion of
all Emissions
50
100 150
Tg CO, Eq.
200
Table 2-8: Emissions from Agriculture (Tg C02 Eq.)
practices, were the largest source of U.S. N2O emissions in
2006, accounting for 72 percent.
Some significant trends in U.S. emissions from
Agriculture include the following:
• Agricultural soils produced approximately 72 percent of
N2O emissions in the United States in 2006. Estimated
emissions from this source in 2006 were 265.0 Tg CO2 Eq.
Annual N2O emissions from agricultural soils fluctuated
between 1990 and 2006, although overall emissions were
1.6 percent lower in 2006 than in 1990.
• Enteric fermentation was the largest source of CH4
emissions in 2006, at 126.2 Tg CO2 Eq. Although
emissions from enteric fermentation have decreased by
less than 1 percent between 1990 and 2006, emissions
increased about 2 percent between 1990 and 1994
and decreased 8 percent 1995 to 2004, mainly due to
decreasing populations of both beef and dairy cattle
and improved feed quality for feedlot cattle. The last
two years have shown an increase in emissions. During
this timeframe, populations of sheep have decreased
45 percent since 1990 while horse populations have
increased over 80 percent, mostly over the last 5 years.
Goat and swine populations have increased 1 percent
and 14 percent, respectively, during this timeframe.
• Overall, emissions from manure management increased
29 percent between 1990 and 2006. This encompassed
an increase of 34 percent for CH4, from 31.0 Tg
CO2 Eq. in 1990 to 41.4 Tg CO2 Eq. in 2006; and an
increase of 18 percent for N2O, from 12.1 Tg CO2 Eq.
in 1990 to 14.3 Tg CO2 Eq. in 2006. The majority of
this increase was from swine and dairy cow manure,
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
165.7
126.9
31.0
7.1
0.7
281.8
269.4
12.1
0.4
447.5
1995
175.8
132.3
35.2
7.6
0.7
278.0
264.8
12.8
0.4
453.8
2000 2001 2002 2003 2004 2005 2006
171.7 172.2 172.6 173.0 170.9 174.0 174.4
124.6 123.6 123.8 124.6 122.4 124.5 126.2
38.8 40.2 41.3 40.7 40.1 41.8 41.4
7.5 7.6 6.8 6.9 7.6 6.8 5.9
0.8 0.8 0.7 0.8 0.9 0.9 0.8
276.3 291.5 276.4 261.3 261.2 279.6 279.8
262.1 277.0 262.0 247.3 246.9 265.2 265.0
13.7 14.0 14.0 13.6 13.8 13.9 14.3
0.5 0.5 0.4 0.4 0.5 0.5 0.5
447.9 463.7 449.0 434.3 432.1 453.6 454.1
Note: Totals may not sum due to independent rounding.
2-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 13 percent of total U.S. greenhouse gas emissions
in 2006. Forests (including vegetation, soils, and harvested
wood) accounted for approximately 84 percent of total 2006
net CO2 flux, urban trees accounted for 11 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 2006. 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 70 percent
more C than is emitted through these soils, liming, and urea
fertilization, combined. 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
2006 resulted in a net C flux of -883.7 Tg CO2 Eq. (Table
2-9). This represents an offset of approximately 14.8
percent of total U.S. CO2 emissions, or 12.5 percent of
total greenhouse gas emissions in 2006. Between 1990 and
2006, total land use, land-use change, and forestry net C flux
resulted in a 20 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
2-9. The application of crushed limestone and dolomite to
managed land (i.e., soil liming) and urea fertilization resulted
in CO2 emissions of 8.0 Tg CO2 Eq. in 2006, and increase
of 13 percent relative to 1990. The application of synthetic
fertilizers to forest and settlement soils in 2006 resulted in
direct N2O emissions of 1.8 Tg CO2 Eq. Direct N2O emissions
from fertilizer application increased by approximately 74
percent between 1990 and 2006. Emissions of CH4 and
N2O from forest fires fluctuate widely from year to year, but
overall increased by 449 percent between 1990 and 2006
(Table 2-10).
Other significant trends from 1990 to 2006 in land use,
land-use change, and forestry emissions include:
• Net C sequestration by forest land has increased 20
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
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
(621.7)
(30.1)
14.7
(1.9)
(14.3)
(60.6)
(23.9)
(737.7)
1995
(659.9)
(39.4)
9.4
16.6
(16.3)
(71.5)
(14.1)
(775.3)
2000
(550.7)
(38.4)
9.4
16.4
(16.3)
(82.4)
(11.5)
(673.6)
2001
(623.4)
(40.0)
9.4
16.4
(16.3)
(84.6)
(11.6)
(750.2)
2002
(697.3)
(40.3)
9.4
16.4
(16.3)
(86.8)
(11.8)
(826.8)
2003
(730.9)
(40.5)
9.4
16.4
(16.3)
(88.9)
(10.0)
(860.9)
2004
(741.4)
(40.9)
9.4
16.3
(16.3)
(91.1)
(9.6)
(873.7)
2005
(743.6)
(41.0)
9.4
16.3
(16.3)
(93.3)
(10.0)
(878.6)
2006
(745.1)
(41.8)
9.4
16.2
(16.3)
(95.5)
(10.5)
(883.7)
Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
Trends in Greenhouse Gas Emissions 2-15
-------
Table 2-10: Emissions from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Source Category
C02
Cropland Remaining Cropland: Liming of
Agricultural Soils & Urea Fertilization
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
Total
1990
7.1
7.1
4.5
4.5
1.5
0.5
0.1
1.0
13.1
1995
7.0
7.0
4.7
4.7
1.8
0.5
0.2
1.2
13.6
2000
7.5
7.5
19.0
19.0
3.5
1.9
0.3
1.2
30.0
2001
7.8
7.8
9.4
9.4
2.7
1.0
0.3
1.4
20.0
2002
8.5
8.5
16.4
16.4
3.5
1.7
0.3
1.5
28.4
2003
8.3
8.3
8.7
8.7
2.7
0.9
0.3
1.5
19.7
2004
7.6
7.6
6.9
6.9
2.6
0.7
0.3
1.6
17.1
2005
7.9
7.9
12.3
12.3
3.1
1.2
0.3
1.5
23.2
2006
8.0
8.0
24.6
24.6
4.3
2.5
0.3
1.5
36.9
Note: Totals may not sum due to independent rounding.
increased over the past 16 years, although only at an
average rate of 0.1 percent per year.
• Net sequestration of C by urban trees has increased
by 57 percent over this sixteen-year period. 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 56 percent. 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.
Waste
Waste management and treatment activities are sources
of greenhouse gas emissions (see Figure 2-11). In 2006,
landfills were the second largest source of anthropogenic
CtLj emissions, accounting for 23 percent of total U.S. CH4
emissions.4 Additionally, wastewater treatment accounts
for 4 percent of U.S. CK4 emissions, and 2 percent of N2O
emissions. Emissions of CH4 and N2O from composting grew
from 1990 to 2006, and resulted in emissions of 3.3 Tg CO2
Eq. A summary of greenhouse gas emissions from the Waste
chapter is presented in Table 2-11.
Figure 2-11
2006 Waste Chapter Greenhouse Gas Emission Sources
Landfills
Composting
50 100
Tg C02 Eq.
150
Overall, in 2006, waste activities generated emissions
of 161.OTgCO2Eq., 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 2006, net CF^ emissions from landfills
decreased by 23.9 Tg CO2 Eq. (16 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 CFLj emissions
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.
2-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 2-11: Emissions from Waste (Tg C02 Eq.)
Gas/Source
CH4
Landfills
Wastewater Treatment
Composting
N20
Wastewater Treatment
Composting
Total
1990
172.9
149.6
23.0
0.3
6.6
6.3
0.4
179.6
1995
169.1
144.0
24.3
0.7
7.7
6.9
0.8
176.8
2000 2001 2002 2003 2004 2005 2006
146.7 143.0 145.5 151.0 148.1 149.0 151.1
120.8 117.6 120.1 125.6 122.6 123.7 125.7
24.6 24.2 24.1 23.9 24.0 23.8 23.9
1.3 1.3 1.3 1.5 1.6 1.6 1.6
8.9 9.2 9.0 9.3 9.6 9.7 9.9
7.6 7.8 7.6 7.7 7.8 8.0 8.1
1.4 1.4 1.4 1.6 1.7 1.7 1.8
155.6 152.1 154.5 160.3 157.7 158.7 161.0
Note: Totals may not sum due to independent rounding.
resulting from an increase in the amount of municipal
solid waste landfilled.
• From 1990 to 2006, CH4 and N2O emissions from
wastewater treatment increased by 0.8 Tg CO2 Eq. (4
percent) and 1.8 Tg CO2 Eq. (29 percent), respectively.
• CH4 and N2O emissions from composting each
increased by less than 0.1 Tg CO2 Eq. (1 percent) from
2005 to 2006. Emissions from composting have been
continually increasing since 1990, from 0.7 Tg CO2
Eq. to 3.3 Tg CO2 Eq. in 2006, a 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
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 (34 percent)
of U.S. greenhouse gas emissions in 2006. Transportation
activities, in aggregate, accounted for the second largest
portion (28 percent). Emissions from industry accounted for
about 19 percent of U.S. greenhouse gas emissions in 2006.
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 8
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, 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.
CO2 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 landfilling of yard trimmings.
Table 2-12 presents a detailed breakdown of emissions
from each of these economic sectors by source category, as
they are defined in this report. Figure 2-12 shows the trend
in emissions by sector from 1990 to 2006.
Trends in Greenhouse Gas Emissions 2-17
-------
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors
(Tg C02 Eq. and Percent of Total in 2006)
Sector/Source
Electric Power Industry
C02 from Fossil Fuel Combustion
Municipal Solid Waste Combustion
Electrical Transmission
and Distribution
Stationary Combustion
Limestone and Dolomite Use
Transportation
C02 from Fossil Fuel Combustion
Substitution of Ozone Depleting
Substances
Mobile Combustion
Non-Energy Use of Fuels
Industry
C02 from Fossil Fuel Combustion
Natural Gas Systems
Non-Energy Use of Fuels
Coal Mining
Iron and Steel Production
Cement Production
Petroleum Systems
Lime Production
Nitric Acid Production
HCFC-22 Production
Ammonia Production and
Urea Consumption
Aluminum Production
Adipic Acid Production
Substitution of Ozone
Depleting Substances
Abandoned Underground Coal Mines
Semiconductor Manufacture
Stationary Combustion
N20 from Product Uses
Limestone and Dolomite Use
Soda Ash Production
and Consumption
Petrochemical Production
Magnesium Production
and Processing
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Mobile Combustion
Zinc Production
Lead Production
Silicon Carbide Production
and Consumption
1990
1,859.1
1,809.6
11.4
26.7
8.6
2.8
1,544.1
1,485.1
+
47.2
11.9
1,460.3
798.2
158.4
99.6
84.1
87.5
33.3
34.2
12.0
17.0
36.4
16.9
25.4
15.3
+
6.0
2.9
4.7
4.4
2.8
4.1
3.1
5.4
1.2
1.4
2.2
1.5
0.6
0.9
0.3
0.4
1995
1,989.7
1,939.3
16.2
21.5
9.1
3.7
1,685.8
1,599.4
18.6
56.5
11.3
1,478.0
819.1
161.9
115.9
67.1
76.0
36.8
32.3
14.0
18.9
33.0
17.8
17.5
17.3
1.2
8.2
5.0
5.0
4.6
3.7
4.3
3.8
5.6
1.5
1.4
2.0
1.5
0.7
1.0
0.3
0.3
2000
2,328
2,282
17
15
10
3
1,917
1,798
52
54
12
1,432
809
155
118
60
67
41
30
14
18
28
16
14
6
3
7
6
4
4
3
4
4
3
1
1
1
1
0
1
0
0
9
3
9
1
6
0
5
2
6
7
1
9
4
9
4
4
8
2
6
9
6
6
4
7
2
1
4
3
9
9
0
2
2
0
8
4
9
4
8
1
3
3
2001
2,290.9
2,244.3
18.4
15.0
10.4
2.9
1,895.8
1,775.6
57.2
51.9
11.1
1,384.3
801.8
154.1
115.5
60.3
60.3
41.4
30.5
14.3
15.1
19.7
13.3
7.8
5.1
3.1
6.7
4.5
4.6
4.9
2.9
4.1
3.9
2.9
1.7
0.8
1.5
1.3
0.9
1.0
0.3
0.2
2002
2,300.4
2,253.7
18.9
14.4
10.4
2.9
1,948.5
1,828.9
61.1
47.5
10.9
1,384.9
801.9
154.5
115.8
56.8
57.0
42.9
30.2
13.7
16.4
21.1
14.2
9.7
6.1
3.7
6.2
4.3
4.4
4.4
2.9
4.1
4.0
2.9
1.8
1.0
1.4
1.3
0.9
0.9
0.3
0.2
2003
2,329.4
2,283.1
19.5
13.8
10.7
2.4
1,925.9
1,807.6
64.4
43.8
10.1
1,375.5
811.0
151.8
113.2
56.9
55.8
43.1
29.5
14.5
15.4
12.3
12.5
8.3
6.3
4.4
6.0
4.3
4.3
4.4
2.4
4.1
3.9
3.4
1.8
1.3
1.3
1.4
0.9
0.5
0.3
0.2
2004
2,363.4
2,314.9
20.5
13.9
10.8
3.4
1,975.4
1,856.4
67.8
40.9
10.2
1,388.9
806.6
142.1
131.4
59.8
53.8
45.6
29.0
15.2
15.2
17.2
13.2
7.1
5.9
4.8
5.8
4.3
4.6
4.4
3.4
4.2
4.1
3.2
2.1
1.2
1.4
1.4
1.0
0.5
0.3
0.2
2005
2,430.0
2,380.2
21.1
14.0
11.0
3.7
1,987.2
1,869.8
69.7
37.5
10.2
1,354.3
801.8
132.0
121.8
57.1
47.6
45.9
28.6
15.1
15.8
15.8
12.8
7.2
5.9
5.2
5.6
4.4
4.5
4.4
3.7
4.2
3.9
3.3
1.8
1.3
1.4
1.4
1.0
0.5
0.3
0.2
2006
2,377.8
2,328.2
21.3
13.2
10.8
4.3
1,969.5
1,856.0
69.5
34.1
9.9
1,371.5
818.6
130.9
120.8
58.5
50.1
45.7
28.7
15.8
15.6
13.8
12.4
6.4
5.9
5.7
5.4
4.8
4.6
4.4
4.3
4.2
3.6
3.2
1.9
1.6
1.5
1.2
1.0
0.5
0.3
0.2
Percent3
33.7%
33.0%
0.3%
0.2%
0.2%
0.1%
27.9%
26.3%
1.0%
0.5%
0.1%
19.4%
11.6%
1.9%
1.7%
0.8%
0.7%
0.6%
0.4%
0.2%
0.2%
0.2%
0.2%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
+
+
+
+
+
+
+
+
+
2-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors
(Tg C02 Eq. and Percent of Total in 2006) (continued)
Sector/Source
Agriculture
N20 from Agricultural Soil
Management
Enteric Fermentation
Manure Management
C02 from Fossil Fuel Combustion
CH4 and N20 from Forest Fires
Rice Cultivation
Liming of Agricultural Soils
Urea Fertilization
Field Burning of Agricultural
Residues
Mobile Combustion
N20 from Forest Soils
Stationary Combustion
Commercial
C02 from Fossil Fuel Combustion
Stationary Combustion
Substitution of Ozone Depleting
Substances
Landfills
Human Sewage
Wastewater Treatment
Composting
Residential
C02 from Fossil Fuel Combustion
Stationary Combustion
Substitution of Ozone
Depleting Substances
Settlement Soil Fertilization
U.S. Territories
C02 from Fossil Fuel 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
Net Emissions (Sources and Sinks)
1990
506.8
269.4
126.9
43.0
46.8
4.9
7.1
4.7
2.4
1.1
0.4
0.1
+
396.9
216.1
149.6
23.0
+
6.3
0.7
1.2
346.9
340.1
0.3
5.5
1.0
34.1
34.1
6,148.3
(737.68)
(621.69)
(60.65)
(31.47)
(23.87)
5,410.6
5,410.6
1995
524.1
264.8
132.3
48.0
57.3
5.2
7.6
4.4
2.7
1.0
0.5
0.2
+
404.5
225.8
144.0
24.3
0.7
6.9
1.5
1.3
370.9
356.5
8.1
5.0
1.2
41.1
41.1
6,494.0
(775.32)
(659.92)
(71.53)
(29.74)
(14.13)
5,718.7
5,718.7
2000 2001 2002 2003 2004 2005 2006
528.0 533.4 529.3 498.0 499.2 521.3 533.6
262.1 277.0 262.0 247.3 246.9 265.2 265.0
124.6 123.6 123.8 124.6 122.4 124.5 126.2
52.5 54.2 55.2 54.3 53.9 55.7 55.7
50.9 50.7 52.9 45.0 51.1 45.5 43.6
20.9 10.4 18.0 9.6 7.6 13.6 27.0
7.5 7.6 6.8 6.9 7.6 6.8 5.9
4.3 4.4 5.0 4.6 3.9 4.3 4.4
3.2 3.4 3.6 3.7 3.7 3.5 3.6
1.3 1.2 1.1 1.2 1.4 1.4 1.3
0.4 0.4 0.5 0.4 0.4 0.4 0.4
0.3 0.3 0.3 0.3 0.3 0.3 0.3
+ + + + + + +
390.3 383.0 388.1 410.2 404.6 400.4 394.6
228.0 222.3 222.8 236.5 230.6 221.9 210.1
120.8 117.6 120.1 125.6 122.6 123.7 125.7
24.6 24.2 24.1 23.9 24.0 23.8 23.9
5.5 7.4 9.6 12.1 15.0 18.5 22.4
7.6 7.8 7.6 7.7 7.8 8.0 8.1
2.6 2.7 2.7 3.1 3.3 3.3 3.3
1.2 1.2 1.2 1.3 1.3 1.2 1.2
387.7 379.3 376.6 399.6 385.5 376.0 344.8
372.1 363.6 360.5 382.9 368.3 358.5 326.5
10.1 10.3 10.7 11.0 11.4 11.9 12.9
4.3 3.9 4.0 4.2 4.3 4.2 3.9
1.2 1.4 1.5 1.5 1.6 1.5 1.5
47.3 54.5 53.3 59.7 61.0 60.5 62.4
47.3 54.5 53.3 59.7 61.0 60.5 62.4
7,032.6 6,921.3 6,981.2 6,998.2 7,078.0 7,129.9 7,054.2
(673.61) (750.19) (826.76) (860.91) (873.66) (878.61) (883.66)
(550.73) (623.43) (697.28) (730.93) (741.42) (743.60) (745.09)
(82.41) (84.59) (86.77) (88.94) (91.12) (93.30) (95.47)
(28.96) (30.60) (30.91) (31.07) (31.51) (31.71) (32.56)
(11.50) (11.57) (11.80) (9.96) (9.61) (10.00) (10.54)
6,359.0 6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
6,359.0 6,171.1 6,154.4 6,137.3 6,204.3 6,251.3 6,170.5
Percent3
7.6%
3.8%
1.8%
0.8%
0.6%
0.4%
0.1%
0.1%
0.1%
+
+
+
+
5.6%
3.0%
1.8%
0.3%
0.3%
0.1%
+
+
4.9%
4.6%
0.2%
0.1%
+
0.9%
0.9%
100.0%
(12.5%)
(10.6%)
(1.4%)
(0.5%)
(0.1%)
87.5%
87.5%
Note: Includes all emissions of C02, CH4, N20, MFCs, PFCs, and SF6
independent rounding.
ODS (Ozone Depleting Substances)
+ Does not exceed 0.05 Tg C02 Eq. or 0.05%.
a Percent of total emissions for year 2006.
Parentheses indicate negative values or sequestration. Totals may not sum due to
Trends in Greenhouse Gas Emissions 2-19
-------
Figure 2-12
Emissions Allocated to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
0-
Electricity Generation
^"*+
Transportation
Industry
^Agriculture
.Commercial
Residential
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
34 percent of total U.S. greenhouse gas emissions in 2006.
Emissions increased by 28 percent since 1990, as electricity
demand grew and fossil fuels remained the dominant
energy source for generation. Electricity generation-related
emissions decreased from 2005 to 2006 by 2 percent,
primarily due to reduced 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.
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 2006c and Duffield 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
Table 2-13: Electricity Generation-Related Greenhouse Gas Emissions (Tg C02 Eq.)
Gas/Fuel Type or Source
C02
C02from Fossil Fuel Combustion
Coal
Natural Gas
Petroleum
Geothermal
Municipal Solid Waste Combustion
Limestone and Dolomite Use
CH4
Stationary Combustion*
N20
Stationary Combustion*
Municipal Solid Waste Combustion
SF6
Electrical Transmission and Distribution
Total
1990
1,823.3
1,809.6
1,531.3
176.2
101.8
0.4
10.9
2.8
0.6
0.6
8.5
8.1
0.5
26.7
26.7
1,859.1
1995
1,958.6
1,939.3
1,648.7
229.5
60.7
0.3
15.7
3.7
0.6
0.6
9.0
8.6
0.5
21.5
21.5
1,989.7
2000
2,302.8
2,282.3
1,909.6
280.9
91.5
0.4
17.5
3.0
0.7
0.7
10.4
10.0
0.4
15.1
15.1
2,328.9
2001
2,265.1
2,244.3
1,852.3
289.6
102.0
0.4
18.0
2.9
0.7
0.7
10.1
9.7
0.4
15.0
15.0
2,290.9
2002
2,275.1
2,253.7
1,868.3
306.0
79.1
0.4
18.5
2.9
0.7
0.7
10.1
9.7
0.4
14.4
14.4
2,300.4
2003
2,304.5
2,283.1
1,906.2
278.3
98.1
0.4
19.1
2.4
0.7
0.7
10.4
10.0
0.4
13.8
13.8
2,329.4
2004
2,338.4
2,314.9
1,917.6
296.8
100.1
0.4
20.1
3.4
0.7
0.7
10.5
10.0
0.4
13.9
13.9
2,363.4
2005
2,404.6
2,380.2
1,958.4
319.1
102.3
0.4
20.7
3.7
0.7
0.7
10.7
10.3
0.4
14.0
14.0
2,430.0
2006
2,353.4
2,328.2
1,932.4
339.6
55.7
0.4
20.9
4.3
0.7
0.7
10.5
10.1
0.4
13.2
13.2
2,377.8
Note: Totals may not sum due to independent rounding.
* Includes only stationary combustion emissions related to the generation of electricity.
6Emissions 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.
2-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
28 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 2006.
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 2006
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
1990
2,100.4
1,460.3
1,070.1
286.5
40.4
63.3
640.1
627.7
0.2
2.9
9.2
1,547.2
1,544.1
1,496.9
4.5
42.67
+
3.1
3.1
+
+
+
946.3
396.9
216.1
173.8
7.0
+
549.3
538.7
0.2
2.5
7.9
19S
2,141
1,478
1,103
273
44
56
663
652
0
3
7
1,688
1,685
1,610
4
52.4
18
3
3
1,003
404
225
170
8
0
599
590
0
2
6
5
1
0
0
6
8
6
1
8
2
0
2
9
8
7
1
6
6
1
1
•f
•f
•f
8
5
8
0
0
7
4
0
2
7
5
2000
2,174.3
1,432.9
1,091.3
258.3
33.7
49.6
741.4
733.1
0.2
3.3
4.8
1,921.0
1,917.5
1,810.2
3.2
51.53
52.57
3.49
3.4
+
+
+
1,141.9
390.3
228.0
147.6
9.3
5.5
751.6
743.2
0.2
3.3
4.9
2001
2,061.1
1,384.3
1,066.4
255.4
28.9
33.6
676.8
669.2
0.2
3.0
4.4
1,899.4
1,895.8
1,786.7
3.1
48.80
57.20
3.69
3.6
+
+
+
1,149.8
383.0
222.3
143.8
9.5
7.4
766.7
758.1
0.2
3.4
5.0
2002
2,051.6
1,384.9
1,065.7
251.3
30.6
37.3
666.7
659.4
0.2
2.9
4.2
1,952.0
1,948.5
1,839.8
2.7
44.82
61.13
3.49
3.5
+
+
+
1,151.1
388.1
222.8
146.4
9.4
9.6
763.0
754.6
0.2
3.4
4.8
2003
2,064.0
1,375.5
1,069.6
247.8
29.8
28.2
688.5
681.1
0.2
3.1
4.1
1,930.2
1,925.9
1,817.7
2.5
41.26
64.41
4.33
4.3
+
+
+
1,172.7
410.2
236.5
151.9
9.7
12.1
762.5
754.4
0.2
3.4
4.5
2004
2,075.4
1,388.9
1,086.5
240.5
29.5
32.4
686.5
679.2
0.2
3.0
4.0
1,980.0
1,975.4
1,866.6
2.4
38.51
67.84
4.59
4.5
+
+
+
1,187.2
404.6
230.6
149.0
9.9
15.0
782.6
774.3
0.2
3.5
4.6
2005
2,038.3
1,354.3
1,065.8
226.7
30.0
31.7
683.9
676.8
0.2
3.0
3.9
1,992.0
1,987.2
1,880.0
2.3
35.20
69.74
4.78
4.7
+
+
+
1,212.5
400.4
221.9
149.9
10.1
18.5
812.0
803.5
0.2
3.6
4.7
2006
2,029.2
1,371.5
1,084.6
227.1
29.9
30.0
657.7
650.9
0.2
2.9
3.6
1,974.5
1,969.5
1,865.9
2.1
31.96
69.46
5.03
5.0
+
+
+
1,204.4
394.6
210.1
152.0
10.2
22.4
809.8
801.5
0.2
3.6
4.5
Percent3
28.5%
19.2%
15.2%
3.2%
0.4%
0.4%
9.2%
9.1%
+
+
0.1%
27.7%
27.6%
26.2%
+
0.4%
1.0%
0.1%
0.1%
+
+
+
16.9%
5.5%
2.9%
2.1%
0.1%
0.3%
11.4%
11.2%
+
0.1%
0.1%
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 2006 (continued)
Sector/Gas
Residential
Direct Emissions
C02
CH4
N20
MFCs
Electricity-Related
C02
CH4
N20
SF6
Agriculture
Direct Emissions
C02
CH4
N20
Electricity-Related
C02
CH4
N20
SF6
U.S. Territories
Total
1990
952
346
340
4
2
0
605
593
0
2
8
567
506
53
170
282
61
60
0
0
34
6,148
4
9
1
4
1
3
5
8
2
8
7
9
8
8
3
6
2
0
f
3
9
1
3
1995
1,026.5
370.9
356.5
4.0
2.2
8.1
655.6
645.4
0.2
3.0
7.1
592.5
524.1
64.4
180.7
279.0
68.5
67.4
+
0.3
0.7
41.1
6,494.0
2000
1,160.7
387.7
372.1
3.4
2.1
10.1
773.0
764.4
0.2
3.4
5.0
587.4
528.0
58.4
190.8
278.8
59.4
58.7
+
0.3
0.4
47.3
7,032.6
2001
1,153.2
379.3
363.6
3.1
2.3
10.3
773.9
765.2
0.2
3.4
5.1
603.2
533.4
58.5
181.8
293.1
69.8
69.0
+
0.3
0.5
54.5
6,921.3
2002
1,178.0
376.6
360.5
3.1
2.3
10.7
801.4
792.6
0.2
3.5
5.0
595.1
529.3
61.4
189.1
278.8
65.8
65.1
+
0.3
0.4
53.3
6,981.2
2003
1,211.2
399.6
382.9
3.3
2.4
11.0
811.6
802.9
0.2
3.6
4.8
560.5
498.0
53.3
181.8
262.9
62.5
61.9
+
0.3
0.4
59.7
6,998.2
2004
1,207.2
385.5
368.3
3.3
2.5
11.4
821.7
813.0
0.2
3.6
4.8
567.2
499.2
58.7
178.0
262.6
68.1
67.3
+
0.3
0.4
61.0
7,078.0
2005
1,241.7
376.0
358.5
3.3
2.4
11.9
865.6
856.6
0.3
3.8
5.0
584.9
521.3
53.4
186.4
281.5
63.6
63.0
+
0.3
0.4
60.5
7,129.9
2006
1,187.8
344.8
326.5
3.1
2.3
12.9
843.0
834.4
0.3
3.7
4.7
595.8
533.6
51.6
199.1
282.9
62.3
61.6
+
0.3
0.3
62.4
7,054.2
Percent3
16.7%
4.8%
4.6%
+
+
0.2%
11.8%
11.7%
+
0.1%
0.1%
8.4%
7.5%
0.7%
2.8%
4.0%
0.9%
0.9%
+
+
+
0.9%
100.0%
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.
+ Does not exceed 0.05 Tg C02 Eq. or 0.05 percent.
a Percent of total emissions for year 2006.
b Includes primarily HFC-134a.
Figure 2-13
Emissions with Electricity Distributed to Economic Sectors
2,500 -
2,000 -
1,500-
1,000-
500-
0-
Industrial
•• •»
Transportation
Residential
Commercial
Agriculture
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,
byproduct CO2 emissions from cement production,
and HFC, PFC, and SF6 byproduct 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
2-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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.
• In the Electricity Generation economic sector, C02 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 Municipal Solid Waste Combustion, 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).
In the Transportation economic sector, the C02 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 the CH4 and N20 from Mobile Combustion, based
on the EIA transportation sector. Substitutes of Ozone Depleting Substitutes are apportioned based on their specific end-uses
within the source category, with emissions from transportation refrigeration/air-conditioning systems to this economic sector.
Finally, C02 emissions from Non-Energy Uses of Fossil Fuels identified as lubricants for transportation vehicles are included in the
Transportation economic sector.
• For the Industry economic sector, the C02 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 of Ozone Depleting Substitutes are apportioned based on their specific end-uses
within the source category, with most emissions falling within the Industry economic sector (minus emissions from the other
economic sectors). 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
from the actual process to make the material, not from fuels to power the plant) from such activities as cement production, iron
and steel 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 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.
• As agriculture equipment is included in ElA's industrial fuel consuming sector surveys, additional data is used to extract 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 (all data is removed from the Industrial economic
sector, to avoid double-counting). The other emission sources included in this economic sector are intuitive for the agriculture
sectors, such as 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. N20 emissions from the application of fertilizers to tree
plantations (termed "forest land" by the IPCC) are also included in the Agriculture economic sector.
• The Residential 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
of Ozone Depleting Substitutes are apportioned based on their specific end-uses within the source category, with emissions from
residential air-conditioning systems to this economic sector. N20 emissions from the application of fertilizers to developed land
(termed "settlements" by the IPCC) are also included in the Residential economic sector.
The Commercial 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 of Ozone Depleting Substances are apportioned based on their specific end-uses within the source category, with
emissions from commercial refrigeration/air-conditioning systems to this economic sector. Public works sources including direct
CH4 from Landfills and CH4 and N20 from Wastewater Treatment and Composting are included in this economic sector.
Trends in Greenhouse Gas Emissions 2-23
-------
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 28 percent of U.S. greenhouse gas emissions in 2006. The
largest sources of transportation GHGs in 2006 were passenger
cars (34 percent), light duty trucks, which include sport utility
vehicles, pickup trucks, and mini vans (28 percent), freight
trucks (20 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 2006, transportation emissions rose by 28
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
39 percent from 1990 to 2006, 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 reflects an increasing market
share of light duty trucks, which have grown from about
one-fifth of new vehicle sales in the 1970s to slightly over
Table 2-15: Transportation-Related Greenhouse Gas Emissions (Tg C02 Eq.)
Gas/Vehicle Type
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
1990
656.9
628.8
2.6
25.4
+
336.2
320.7
1.4
14.1
+
228.6
227.8
+
0.8
+
8.5
8.3
0.2
+
+
1.8
1.7
+
+
1995
644.1
604.9
2.1
26.9
10.1
434.7
405.0
1.4
22.1
6.1
272.5
271.2
+
1.0
0.3
9.3
9.0
0.2
+
+
1.8
1.8
+
+
2000 2001 2002 2003 2004 2005 2006
694.6 699.1 713.7 692.4 689.5 705.8 678.4
643.5 647.9 662.6 642.1 640.0 658.4 634.5
1.6 1.5 1.4 1.3 1.2 1.1 1.0
25.2 23.8 22.5 21.0 19.5 17.8 15.6
24.3 25.9 27.2 28.0 28.8 28.5 27.2
508.1 513.3 525.1 560.4 583.0 544.0 556.6
466.0 470.3 483.2 518.8 540.8 501.9 514.9
1.1 1.1 0.9 0.8 0.7 0.7 0.7
22.4 21.3 18.5 16.6 15.3 13.7 12.7
18.6 20.6 22.5 24.2 26.1 27.7 28.3
344.3 343.6 357.9 354.4 367.4 395.2 404.6
341.5 340.6 354.8 351.2 364.1 391.9 401.3
+ + + + + + +
1.2 1.2 1.2 1.3 1.2 1.2 1.1
1.6 1.7 1.8 1.9 2.1 2.1 2.2
11.2 10.3 10.0 10.8 15.1 12.1 12.5
10.9 10.0 9.6 10.5 14.7 11.8 12.1
0.1 0.1 0.1 0.1 0.1 0.1 0.1
+ + + + + + +
0.1 0.2 0.2 0.2 0.2 0.2 0.3
1.9 1.7 1.7 1.7 1.8 1.6 1.9
1.8 1.7 1.7 1.6 1.7 1.6 1.9
+ + + + + + +
+ + + + + + +
2-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 2-15: Transportation-Related Greenhouse Gas Emissions (Tg C02 Eq.) (continued)
Gas/Vehicle Type
Commercial Aircraft — Domestic3
C02
CH4
N20
Other Aircraft — Domestic"
C02
CH4
N20
Ships and Boats — Domestic0
C02
CH4
N20
MFCs
Rail
C02
CH4
N20
MFCs
Pipelines"
C02
Lubricants
C02
1990
138.1
136.7
0.1
1.3
43.8
43.3
0.1
0.4
47.0
46.5
0.1
0.4
+
38.5
38.1
0.1
0.3
+
36.1
36.1
11.9
11.9
Other Transportation
(Unspecified)6 +
Total Transportation
International Bunker Fuels'
1,547.2
114.8
1995
144.6
143.1
0.1
1.4
31.9
31.5
0.1
0.3
56.6
55.5
0.1
0.4
0.6
44.0
42.2
0.1
0.3
1.4
38.2
38.2
11.3
11.3
+
1,688.9
101.6
2000 2001 2002 2003 2004 2005 2006
165.9 154.4 147.6 145.4 144.4 152.0 143.6
164.2 152.9 146.1 143.9 142.9 150.4 142.1
0.1 0.1 0.1 0.1 0.1 0.1 0.1
1.6 1.5 1.4 1.4 1.4 1.5 1.4
32.6 34.1 32.2 31.1 34.5 31.1 28.8
32.2 33.7 31.9 30.8 34.1 30.8 28.5
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.3 0.3 0.3 0.3 0.3 0.3 0.3
65.1 47.4 65.4 38.3 47.1 50.8 47.7
61.0 43.2 60.8 33.6 42.1 45.6 42.4
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.5 0.3 0.5 0.3 0.4 0.4 0.4
3.4 3.7 4.0 4.3 4.6 4.7 4.9
50.1 50.8 50.7 52.8 55.8 56.6 57.9
45.1 45.4 44.9 46.6 49.2 49.8 51.0
0.1 0.1 0.1 0.1 0.1 0.1 0.1
0.3 0.3 0.3 0.3 0.4 0.4 0.4
4.6 5.0 5.4 5.8 6.1 6.4 6.5
35.2 33.6 36.6 32.7 31.2 32.3 32.4
35.2 33.6 36.6 32.7 31.2 32.3 32.4
12.1 11.1 10.9 10.1 10.2 10.2 9.9
12.1 11.1 10.9 10.1 10.2 10.2 9.9
+ + +0.1 0.1 0.2 0.2
1,921.0 1,899.4 1,952.0 1,930.2 1,980.0 1,992.0 1,974.5
102.2 98.6 90.0 104.6 120.2 123.8 128.4
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding. Emissions estimates for passenger cars, light-duty trucks and heavy-duty trucks are calculated
using fuel consumption data from FHWA's Highway Statistics, which used an updated methodology to develop the 2006 estimates. In the most recent
Highway Statistics, FHWA also updated 2005 fuel consumption estimates, but did not revise other prior years. This causes some discontinuity in the
emissions estimates between 2004 and 2005.
Passenger cars and light-duty trucks include vehicles typically used for personal travel and less than 8,500 Ibs; medium- and heavy-duty trucks include
vehicles 8,501 Ibs and above.
HFC emissions primarily reflect HFC-134a.
aConsists 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.
11C02 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 U.S. Inventory.
e Combination of gases; balancing item for transportation emissions not specifically identified in table but which are included in transportation economic
sector emissions identified in Table 2-14.
'Emissions from International Bunker Fuels include emissions from both civilian and military activities; these emissions are not included in the
transportation totals.
Trends in Greenhouse Gas Emissions 2-25
-------
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 improved in 2005 and
2006 as the market share of passenger cars increased. VMT
growth among all passenger vehicles has also been impacted,
growing an average annual rate of 0.8 percent from 2004 to
2006, compared to an annual rate of 2.7 percent over the
period 1990 to 2004.
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 25 percent from 1990 to 2006. This rise in
CO2 emissions, combined with an increase in HFCs from
virtually no emissions in 1990 to 69.5 Tg CO2 Eq. in 2006,
led to an increase in overall emissions from transportation
activities of 28 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 2006, enteric fermentation was the largest
source of CIL, emissions in the United States, and agricultural
soil management was the largest source of N2O emissions
in the U.S. This sector also includes small amounts of CO2
emissions from fossil fuel combustion by motorized farm
equipment like tractors.
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 33 percent
of total U.S. emissions. Electricity generation also accounted
for the largest share of CO2 emissions from fossil fuel
combustion, approximately 41 percent in 2006. Electricity
was consumed primarily in the residential, commercial,
and industrial end-use sectors for lighting, heating, electric
motors, appliances, electronics, and air conditioning.
2-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 non-utilities combined—was the largest source of U.S.
greenhouse gas emissions in 2006, (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
Greenhouse Gas Emissions6
1990
100
100
100
100
100
100
1995
113
112
107
108
107
106
2000
138
127
117
116
113
114
2001
139
125
115
112
114
113
2002
141
128
116
115
115
114
2003
145
129
116
115
116
114
2004
150
131
119
118
117
115
2005
155
134
119
118
118
116
2006
159
135
117
117
119
115
Growth
Rate3
3.0%
1.9%
1.0%
1.0%
1.1%
0.9%
'Average annual growth rate
"Gross Domestic Product in chained 2000 dollars (BEA 2007)
c Energy-content-weighted values (EIA 2007b)
11 U.S. Census Bureau (2007)
eGWP-weighted values
Figure 2-14
Source: BEA (2007), U.S. Census Bureau (2007), and emission estimates in the this report.
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,
Table 2-17: Emissions of NOX, CO, NMVOCs, and S02 (Gg)
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
Gas/Activity
NO,
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Oil and Gas Activities
Municipal Solid Waste Combustion
Agricultural Burning
Solvent Use
Waste
CO
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Municipal Solid Waste Combustion
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
Municipal Solid Waste Combustion
Waste
Agricultural Burning
S02
Stationary Fossil Fuel Combustion
Industrial Processes
Mobile Fossil Fuel Combustion
Oil and Gas Activities
Municipal Solid Waste Combustion
Waste
Solvent Use
Agricultural Burning
1990
21,645
10,920
9,883
591
139
82
28
1
0
130,461
119,360
5,000
4,125
978
691
302
1
5
20,930
10,932
5,216
2,422
912
554
222
673
NA
20,935
18,407
1,307
793
390
38
0
0
NA
1995
21,272
10,622
9,821
607
100
88
29
3
1
109,032
97,630
5,383
3,959
1,073
663
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
2000 2001 2002 2003 2004 2005 2006
19,203 18,410 17,938 17,043 16,177 15,569 14,869
10,310 9,819 10,154 9,642 9,191 8,739 8,287
8,002 7,667 6,791 6,419 6,004 5,853 5,610
626 656 534 528 524 519 515
111 113 321 316 316 316 315
114 114 98 97 97 97 97
35 35 33 34 39 39 38
3355555
2222222
92,777 89,212 84,609 80,221 76,342 72,365 68,372
83,559 79,851 75,421 71,038 67,096 63,154 59,213
4,340 4,377 4,965 4,893 4,876 4,860 4,844
2,217 2,339 1,744 1,724 1,724 1,724 1,724
1,670 1,672 1,439 1,437 1,437 1,437 1,437
792 774 709 800 879 860 825
146 147 323 321 321 321 322
8877777
46 45 1 1 1 1 1
15,228 15,048 15,640 15,170 14,807 14,444 14,082
7,230 6,872 7,235 6,885 6,587 6,289 5,991
4,384 4,547 3,881 3,862 3,854 3,846 3,839
1,773 1,769 2,036 1,972 1,931 1,890 1,849
1,077 1,080 1,585 1,560 1,553 1,545 1,538
389 400 545 538 533 528 523
257 258 243 239 237 235 232
119 122 115 114 112 111 110
NA NA NA NA NA NA NA
14,829 14,452 13,403 13,631 13,232 13,114 12,258
12,848 12,461 11,613 11,956 11,625 11,573 10,784
1,031 1,047 850 804 800 797 793
632 624 683 621 564 508 451
286 289 233 226 220 213 207
29 30 23 22 22 22 22
1111111
1100000
NA NA NA NA NA NA NA
These emission estimates were obtained from preliminary data (EPA 2008) and 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.
7 See .
2-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 71 percent in
2006. 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.
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 CK4 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
2005) ,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 production of chemical and allied
products, metals processing, and industrial uses of solvents —
are also significant sources of CO, NOX, and NMVOCs.
8 Estimates from preliminary data (EPA 2008) and disaggregated based
on EPA (2003). NOX and CO emission estimates from field burning of
agricultural residues were estimated separately, and therefore not taken
from EPA (2008).
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 percent of total emissions on a carbon dioxide (CO2) equivalent basis in 2006. This included 97, 37,
and 13 percent of the nation's CO2, methane (CFLj, and nitrous oxide (N2O) emissions, respectively. Energy-
related CO2 emissions alone constituted 83 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
(4 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 28,193 Tg of CO2 were added to the atmosphere through
the combustion of fossil fuels in 2005, of which the United States accounted for about 20 percent.1; Due to the relative
importance of fossil fuel combustion-related CO2 emissions, they 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), and non-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 these indirect greenhouse gas emissions.
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. It is assumed that the
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 gases from the combustion
Figure 3-1
2006 Energy Chapter Greenhouse Gas Emission Sources
5,637.9
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Coal Mining
Mobile Combustion
Petroleum Systems
Municipal Solid Waste
Combustion
Stationary Combustion
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 Annual 2005
EIA (2007).
Energy 3-1
-------
Figure 3-2
2006 U.S. Fossil Carbon Flows (Tg C02 Eq.)
Fossil Fuel
Energy Exports
329
NEU Emissions 121
Non-Energy Use
Carbon Sequestered
240
Fossil Fuel
Combustion Residual
(Not Oxidized Fraction)
53
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
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
Municipal Solid Waste Combustion
Petroleum Systems
Wood Biomass and Ethanol
Consumption3
International Bunker Fuels a
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground Coal Mines
Mobile Combustion
International Bunker Fuels3
1990
4,886.4
4,724.1
1,809.6
1,485.1
844.9
340.1
216.1
28.3
117.2
33.7
10.9
0.4
219.3
113.7
260.8
124.7
84.1
33.9
7.4
6.0
4.7
0.2
1995
5,215.5
5,032.4
1,939.3
1,599.4
876.5
356.5
225.8
35.0
133.2
33.8
15.7
0.3
236.8
100.6
246.8
128.1
67.1
32.0
7.2
8.2
4.3
0.1
2000 2001 2002 2003 2004 2005 2006
5,765.7 5,686.4 5,749.1 5,796.6 5,878.8 5,920.5 5,825.6
5,577.1 5,507.4 5,564.8 5,617.0 5,681.4 5,731.0 5,637.9
2,282.3 2,244.3 2,253.7 2,283.1 2,314.9 2,380.2 2,328.2
1,798.2 1,775.6 1,828.9 1,807.6 1,856.4 1,869.8 1,856.0
860.3 852.5 854.8 856.0 857.7 847.3 862.2
372.1 363.6 360.5 382.9 368.3 358.5 326.5
228.0 222.3 222.8 236.5 230.6 221.9 210.1
36.2 49.0 44.0 51.0 53.5 53.2 54.9
141.4 131.9 135.9 131.8 148.9 139.1 138.0
29.4 28.8 29.6 28.4 28.1 29.5 28.5
17.5 18.0 18.5 19.1 20.1 20.7 20.9
0.3 0.3 0.3 0.3 0.3 0.3 0.3
227.3 203.2 204.4 209.5 224.8 227.4 234.7
101.1 97.6 89.1 103.6 119.0 122.6 127.1
234.5 232.0 226.9 224.6 217.4 202.4 203.3
126.5 125.3 124.9 123.3 114.0 102.5 102.4
60.4 60.3 56.8 56.9 59.8 57.1 58.5
30.3 30.2 29.9 29.2 28.7 28.3 28.4
6.7 6.2 6.2 6.4 6.6 6.5 6.2
7.4 6.7 6.2 6.0 5.8 5.6 5.4
3.4 3.3 3.0 2.7 2.6 2.5 2.4
0.1 0.1 0.1 0.1 0.1 0.2 0.2
3-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-1: C02, CH4, and N20 Emissions from Energy (Tg C02 Eq.) (continued)
Gas/Source
N20
Mobile Combustion
Stationary Combustion
Municipal Solid Waste Combustion
International Bunker Fuels3
Total
1990
56.8
43.5
12.8
0.5
1.0
5,203.9
1995
67.3
53.4
13.4
0.5
0.9
5,529.6
2000
67.5
52.5
14.6
0.4
0.9
6,067.8
2001
64.4
49.9
14.1
0.4
0.9
5,982.8
2002
60.4
45.9
14.0
0.4
0.8
6,036.3
2003
57.1
42.3
14.4
0.4
0.9
6,078.3
2004
54.7
39.7
14.6
0.4
1.1
6,150.9
2005
51.5
36.3
14.8
0.4
1.1
6,174.4
2006
48.0
33.1
14.5
0.4
1.1
6,076.9
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
Municipal Solid Waste
Combustion
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
International Bunker Fuels3
N20
Mobile Combustion
Stationary Combustion
Municipal Solid Waste
Combustion
International Bunker Fuels3
1990
4,886,370
4,724,146
117,170
33,729
10,950
376
219,341
113,683
12,417
5,937
4,003
1,612
353
288
224
8
183
140
41
2
3
1995 2000 2001 2002 2003 2004 2005 2006
5,215,509 5,765,732 5,686,382 5,749,059 5,796,639 5,878,815 5,920,526 5,825,631
5,032,416 5,577,072 5,507,406 5,564,795 5,617,047 5,681,363 5,731,045 5,637,931
133,234 141,427 131,887 135,857 131,772 148,931 139,057 137,980
33,806 29,390 28,793 29,629 28,445 28,122 29,462 28,504
15,712 17,518 17,971 18,458 19,058 20,097 20,673 20,922
341 325 325 320 316 302 287 293
236,775 227,276 203, 763 204,35? 209,537 224,825 227,366 234,726
700,627 707,725 97,563 89,707 703,583 778,975 722,580 727,097
11,754 11,168 11,048 10,804 10,693 10,353 9,636 9,679
6,098 6,024 5,968 5,946 5,874 5,426 4,880 4,877
3,193 2,874 2,874 2,707 2,709 2,846 2,717 2,784
1,524 1,442 1,436 1,422 1,390 1,368 1,346 1,354
341 316 295 295 306 311 308 296
392 350 319 293 284 276 265 257
205 162 157 141 131 126 119 112
6 6546777
217 218 208 195 184 176 166 155
172 169 161 148 137 128 117 107
43 47 46 45 46 47 48 47
1 1111111
3 3333344
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.
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 6,076.9 Tg CO2 Eq. in 2006, an increase of 17 percent
since 1990.
3.1. Carbon Dioxide Emissions from
Fossil Fuel Combustion (IPCC Source
Category 1 A)
CO2 emissions from fossil fuel combustion in 2006
decreased by 1.6 percent from the previous year. This decrease
is primarily a result of the restraint on fuel consumption caused
Energy 3-3
-------
by rising fuel prices, primarily in the transportation sector, an
increase in the cost of electricity, and decreases in the cost
of natural gas. Additionally, warmer winter conditions in
2006 decreased the demand for heating fuels. In 2006, CO2
emissions from fossil fuel combustion were 5,637.9 Tg CO2
Eq., or 19 percent above emissions in 1990 (see Table 3-3).2
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
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, and size 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).
CO2 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.3 Producing a unit of
Table 3-3: 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
1,698.9
2.9
11.8
152.3
NE
1,531.3
0.6
1,011.5
239.8
143.1
416.3
36.1
176.2
NO
2,013.3
97.4
61.2
276.3
1,449.0
101.8
27.6
0.40
4,724.1
1995
1,805.4
1.7
11.1
143.0
NE
1,648.7
0.9
1,172.0
264.4
165.4
474.4
38.4
229.5
NO
2,054.7
90.5
49.3
259.0
1,561.0
60.7
34.0
0.34
5,032.4
2000 2001 2002 2003 2004 2005 2006
2,053.2 1,996.3 2,002.8 2,043.1 2,058.4 2,094.1 2,065.3
1.0 1.0 1.1 1.2 1.2 0.9 0.6
8.2 8.4 8.4 7.9 9.7 9.2 6.2
133.5 133.5 123.4 124.0 126.2 122.0 122.0
NE NE NE NE NE NE NE
1,909.6 1,852.3 1,868.3 1,906.2 1,917.6 1,958.4 1,932.4
0.9 1.0 1.7 3.8 3.6 3.7 4.1
1,220.5 1,175.3 1,227.6 1,200.3 1,180.1 1,173.9 1,155.1
270.6 260.3 265.0 277.5 264.5 262.7 237.5
172.7 165.1 171.0 176.7 170.0 163.2 154.1
460.0 425.0 447.2 433.0 415.5 394.5 389.3
35.7 34.1 37.2 33.4 32.0 33.2 33.2
280.9 289.6 306.0 278.3 296.8 319.1 339.6
0.7 1.2 1.2 1.4 1.3 1.3 1.4
2,303.0 2,335.5 2,334.0 2,373.3 2,442.5 2,462.7 2,417.1
100.5 102.2 94.4 104.2 102.5 95.0 88.5
47.2 48.8 43.4 51.8 50.9 49.6 49.8
266.8 294.0 284.3 299.1 316.0 330.9 350.9
1,762.5 1,741.6 1,791.7 1,774.2 1,824.4 1,836.7 1,822.8
91.5 102.0 79.1 98.1 100.1 102.3 55.7
34.6 46.8 41.1 45.8 48.6 48.2 49.4
0.36 0.35 0.37 0.37 0.38 0.38 0.38
5,577.1 5,507.4 5,564.8 5,617.0 5,681.4 5,731.0 5,637.9
NE (Not estimated)
NO (Not occurring)
a 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.
2 An additional discussion of fossil fuel emission trends is presented in the
Trends in U.S. Greenhouse Gas Emissions Chapter.
3 Based on national aggregate C content of all coal, natural gas, and
petroleum fuels combusted in the United States.
3-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-4: Annual Change in C02 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors
(Tg C02 Eq. and Percent)
Sector
Electricity Generation
Electricity Generation
Electricity Generation
Transportation3
Residential
Commercial
Industrial
Industrial
All Sectors"
Fuel Type
Coal
Natural Gas
Petroleum
Petroleum
Natural Gas
Natural Gas
Coal
Natural Gas
All Fuels"
2002
38.0
-27.6
19.0
-17.5
12.5
5.7
0.6
-14.2
52.3
to 2003
2.0%
-9.0%
24.0%
-1.0%
4.7%
3.3%
0.5%
-3.2%
0.9%
2003 to 2004
11.4
18.5
2.0
50.2
-12.9
-6.7
2.3
-17.5
64.3
0.6%
6.6%
2.0%
2.8%
-4.7%
-3.8%
1.8%
-4.0%
1.1%
2004 to 2005
40.8
22.3
2.2
12.3
-1.9
-6.8
-4.3
-21.0
49.7
2.1%
7.5%
2.2%
0.7%
-0.7%
-4.0%
-3.4%
-5.1%
0.9%
2005(02006
-26.0
20.5
-46.6
-13.9
-25.2
-9.1
0.1
-5.2
-93.1
-1.3%
6.4%
-45.5%
-0.8%
-9.6%
-5.6%
0.1%
-1.3%
-1.6%
a Excludes emissions from International Bunker Fuels.
b Includes fuels and sectors not shown in table.
Figure 3-3
Figure 3-5
2006 U.S. Energy Consumption by Energy Source
Nuclear
Renewable
Natural Gas
Coal
Petroleum
22%
22%
39%
Figure 3-4
U.S. Energy Consumption (Quadrillion Btu)
120-1
100-
.2
S
60-
40-
20-
o-1
Total Energy
Fossil Fuels
Renewable & Nuclear
010101010101010101010000000
Note: Expressed as gross calorific values.
2006 C02 Emissions from Fossil Fuel
Combustion by Sector and Fuel Type
2,500 -|
2,000 -
1,500 -
8
Natural Gas
Petroleum
I Coal
Relative Contribution
by Fuel Type
1,000 -
500 -
0 -1
U.S. Commercial Residential Industrial Transportation Electricity
Territories Generation
Note: Electricity generation also includes emissions of less than 0.5 Tg C02 Eq. from geothermal-based
electricity generation.
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 3-4
shows annual changes in emissions during the last five years
for coal, petroleum, and natural gas in selected sectors.
In the United States, 82 percent of the energy consumed
in 2006 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 (8 percent) and by a variety of renewable
energy sources (9 percent), primarily hydroelectric power
and biofuels (ElA 2007a). Specifically, petroleum supplied
the largest share of domestic energy demands, accounting
Energy 3-5
-------
Box 3-1: Weather and Non-Fossil Energy Effects on C02 from Fossil Fuel Combustion Trends
In 2006, weather conditions became warmer in the winter and slightly cooler in the summer, compared to 2005. The winter was
significantly warmer than usual, with heating degree days in the United States 12 percent below normal (see Figure 3-6). Warmer winter
conditions led to a decrease in demand for heating fuels. Summer temperatures were substantially warmer than usual, with cooling degree
days 10 percent above normal (see Figure 3-7) (EIA 2007f),4 however the demand for electricity only increased slightly due to the cooler
summer conditions compared to 2005.
Figure 3-6
Annual Deviations from Normal Heating Degree Days for the United States (1950-2006)
=-12-1
§1 9-
VI 6-
»E 3 -
-12 -
-15 -
Normal (4,524 Heating Degree Days)
99% Confidence
|,- — -
ssssssss
s s
§ s s s
Note: Climatological normal data are highlighted. Statistical confidence interval for "normal" climatology period of 1971 through 2000.
Figure 3-7
Annual Deviations from Normal Cooling Degree Days for the United States (1950-2006)
_ 2°
= 1 15
99% Confidence
LJl
I
1 Normal (1,242 Cooling Degree Days)
Normal (1,242 Co
_ CM *t CO 0- _ - - _____ _____ ___
ir}ir}ir}ir}mi£>i£>i£>i£>i£>r— r— r— r— r— cococococ
Note: Climatological normal data are highlighted. Statistical confidence interval for "normal" climatology period of 1971 through 2000.
S S
S S S S
4 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).
for an average of 43 percent of total fossil fuel based energy
consumption in 2006. Coal and natural gas followed in
order of importance, each accounting for 28 percent of total
consumption. 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) (EIA 2007a).
Fossil fuels are generally combusted for the purpose of
producing energy for useful heat and work. During the
combustion process, the C stored in the fuels is oxidized
and emitted as CO2 and smaller amounts of other gases,
including CH^ CO, and NMVOCs.5 These other C containing
non-CO2 gases are emitted as a byproduct of incomplete fuel
combustion, but are, for the most part, eventually oxidized
to CO2 in the atmosphere. Therefore, it is assumed that all
the C in fossil fuels used to produce energy is eventually
converted to atmospheric CO2.
5 See the sections entitled Stationary Combustion and Mobile Combustion
in this chapter for information on non-CO2 gas emissions from fossil fuel
combustion.
3-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Box 3-1: Weather and Non-Fossil Energy Effects on C02 from Fossil Fuel Combustion Trends (continued)
Although no new U.S. nuclear power plants have been
constructed in recent years, the utilization (i.e., capacity factors6)
of existing plants in 2006 remained high at just under 90 percent.
Electricity output by hydroelectric power plants increased in 2006 by
approximately 7 percent. Electricity generated by nuclear plants in
2006 provided almost three times as much of the energy consumed
in the United States as hydroelectric plants (EIA 2007a). Aggregate
nuclear and hydroelectric power plant capacity factors since 1973
are shown in Figure 3-8.
Figure 3-8
Aggregate Nuclear and Hydroelectric Power Plant
Capacity Factors in the United States (1974-2006)
80-
70-
60-
J::
I. 30-
3
20-
10-
0-
6 The capacity factor is defined as the ratio of the electrical energy produced by a generating unit for a given period of time to the electrical energy that could
have been produced at continuous full-power operation during the same period (EIA 2007a).
For the purpose of international reporting, the
Intergovernmental Panel on Climate Change (IPCC)
(IPCC/UNEP/OECD/IEA 1997) recommends that particular
adjustments be made to national fuel consumption statistics.
Certain fossil fuels can be manufactured into plastics, asphalt,
lubricants, or other products. A portion of the C consumed for
these non-energy products can be stored (i.e., sequestered)
indefinitely. To account for the fact that the C in these fuels
ends up in products instead of being combusted (i.e., oxidized
and released into the atmosphere), consumption of fuels for
non-energy purposes is estimated and subtracted from total
fuel consumption estimates. Emissions from non-energy use
of fuels are estimated in the Carbon Emitted and Stored in
Products from Non-Energy Use of Fossil Fuels section in
this chapter.
According to the UNFCCC reporting guidelines, CO2
emissions from the consumption of fossil fuels for aviation
and marine international transport activities (i.e., international
bunker fuels) should be reported separately, and not included
in national emission totals. Estimates of international bunker
fuel emissions for the United States are provided in Table
3-5, and explained in detail later in the chapter.
End-Use Sector Consumption
An alternative method of presenting CO2 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. For the discussion below, electricity generation
emissions have been distributed to each end-use sector based
upon the sector's share of national electricity consumption.
This method of distributing emissions assumes that each
sector consumes electricity generated from an equally
carbon-intensive mix of fuels and other energy sources.
After the end-use sectors are discussed, emissions from
electricity generation are addressed separately. Emissions
from U.S. territories are also calculated separately due to
a lack of end-use-specific consumption data. Table 3-6 and
Table 3-5: C02 Emissions from International Bunker Fuels (Tg C02 Eq.)a
Vehicle Mode
Aviation
Marine
Total
1990
45.7
68.0
113.7
1995
50.2
50.4
100.6
2000
59.9
41.3
101.1
2001
58.7
38.9
97.6
2002
61.1
28.0
89.1
2003
58.8
44.8
103.6
2004
64.9
54.1
119.0
2005
67.5
55.1
122.6
2006
71.1
56.0
127.1
a See International Bunker Fuels section for additional detail.
Note: Totals may not sum due to independent rounding.
Energy 3-7
-------
Table 3-6: C02 Emissions from Fossil Fuel Combustion by 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
1,488.1
1,485.1
3.0
1,527.5
844.9
682.5
929.5
340.1
589.4
750.8
216.1
534.7
28.3
4,724.1
1,809.6
1995
1,602.5
1,599.4
3.0
1,589.5
876.5
713.1
995.5
356.5
639.0
810.0
225.8
584.2
35.0
5,032.4
1,939.3
2000 2001 2002 2003 2004 2005 2006
1,801.6 1,779.2 1,832.3 1,811.8 1,860.9 1,874.5 1,861.0
1,798.2 1,775.6 1,828.9 1,807.6 1,856.4 1,869.8 1,856.0
3.4 3.6 3.4 4.2 4.5 4.7 4.9
1,645.1 1,583.9 1,572.5 1,592.1 1,596.8 1,579.6 1,567.1
860.3 852.5 854.8 856.0 857.7 847.3 862.2
784.7 731.4 717.7 736.1 739.0 732.3 704.9
1,129.7 1,121.8 1,145.6 1,178.3 1,173.1 1,206.4 1,151.9
372.1 363.6 360.5 382.9 368.3 358.5 326.5
757.6 758.1 785.1 795.4 804.9 847.9 825.4
964.6 973.5 970.3 983.8 997.1 1,017.3 1,003.0
228.0 222.3 222.8 236.5 230.6 221.9 210.1
736.6 751.1 747.5 747.3 766.5 795.4 792.9
36.2 49.0 44.0 51.0 53.5 53.2 54.9
5,577.1 5,507.4 5,564.8 5,617.0 5,681.4 5,731.0 5,637.9
2,282.3 2,244.3 2,253.7 2,283.1 2,314.9 2,380.2 2,328.2
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.
Figure 3-9
2006 End-Use Sector Emissions of C02 from
Fossil Fuel Combustion
2,000 -
1,800 -
1,600 -
1,400 -
,1,200 -
, 1,000 -
800 -
600 -
400 -
200 -
o -J
From Electricity
Consumption
I From Direct Fossil
Fuel Combustion
U.S. Commercial Residential Industrial Transportation
Territories
Figure 3-9 summarize CO2 emissions from direct fossil fuel
combustion and pro-rated electricity generation emissions
from electricity consumption by end-use sector.
Transportation End-Use Sector
The transportation end-use sector accounted for 1,861.0
Tg CO2 in 2006, representing 33 percent of total CO2
emissions from fossil fuel combustion; the largest share of
any end-use economic sector.7 Fuel purchased in the United
States for international aircraft and marine travel accounted
for an additional 127.1 Tg CO2 in 2006; 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, domestic commercial aircraft 7.6 percent, and
other sources just over 8 percent. (See Table 3-7 for a detailed
breakdown of CO2 emissions by mode and fuel type.)
Domestic transportation CO2 emissions increased by
almost 25 percent (372.9 Tg) between 1990 and 2006, an
annualized increase of 1.5 percent. From 2005 to 2006
transportation CO2 emissions decreased by 0.7 percent.
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 CFLj and N2O; these emissions are included
beginning in Table 3-21 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 1,151.3 Tg in 2006, an increase of
7 Note that electricity generation is the largest emitter of CO2 when electricity
is not distributed among end-use sectors.
3-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-7: 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 Trucks"
Buses
Motorcycles
Recreational Boats
Distillate Fuel Oil (Diesel)
Passenger Cars
Light-Duty Trucks
Medium- and Heavy-Duty Trucks"
Buses
Rail
Recreational Boats
Ships and Other Boats
Ships (Bunkers)
Jet Fuel
Commercial Aircraft-Domestic
Military Aircraft
General Aviation Aircraft
Aircraft (Bunkers)
Aviation Gasoline
General Aviation Aircraft
Residual Fuel Oil
Ships and Other Boats0
Ships (Bunkers)0
Natural Gas
Passenger Cars
Light-Duty Trucks
Buses
Pipeline
LPG
Light-Duty Trucks
Medium- and Heavy-Duty Trucks"
Buses
Electricity
Rail
Total (Including Bunkers)"
Total (Excluding Bunkers)"
1990
982.8
621.0
308.9
38.7
0.3
1.7
12.1
272.7
7.8
11.3
188.3
7.9
35.1
1.9
8.8
11.6
222.6
136.7
33.9
6.3
45.7
3.1
3.1
80.1
23.7
56.4
36.1
+
+
+
36.1
1.4
0.5
0.8
+
3.0
3.0
1,601.8
1,488.1
1995
1,038.9
597.0
389.9
35.8
0.4
1.8
14.1
325.1
7.7
14.7
234.9
8.6
39.2
2.3
8.6
9.2
222.1
143.1
23.5
5.3
50.2
2.7
2.7
71.7
30.5
41.2
38.4
0.1
+
0.1
38.2
1.1
0.5
0.5
+
3.0
3.0
1,703.1
1,602.5
2000 2001 2002 2003 2004 2005 2006
1,135.7 1,145.4 1,172.3 1,176.5 1,194.8 1,181.2 1,170.0
639.9 644.2 658.9 638.0 635.8 654.2 630.4
446.0 449.4 461.3 491.5 511.6 476.0 488.0
36.0 35.0 35.5 30.6 30.9 34.7 35.2
0.4 0.4 0.3 0.3 0.4 0.4 0.4
1.8 1.7 1.7 1.6 1.7 1.6 1.9
11.6 14.6 14.6 14.5 14.4 14.3 14.1
401.0 401.6 415.1 421.8 447.2 462.2 472.1
3.6 3.6 3.7 4.2 4.3 4.2 4.1
19.8 20.6 21.6 26.9 28.8 25.5 26.4
305.1 305.1 318.8 320.0 332.5 356.5 365.4
10.1 9.2 8.7 9.4 13.4 10.6 10.9
41.7 41.8 41.5 42.4 44.7 45.1 46.0
2.7 2.8 2.9 3.0 3.0 3.1 3.205
11.7 13.2 12.8 8.3 10.0 7.9 7.4
6.3 5.3 5.1 7.6 10.5 9.3 8.7
253.8 242.8 236.8 231.5 239.8 246.3 239.5
164.2 152.9 146.1 143.9 142.9 150.4 142.1
20.5 22.5 20.4 19.9 20.4 16.9 14.8
9.2 8.8 9.1 8.8 11.5 11.4 11.4
59.9 58.7 61.1 58.8 64.9 67.5 71.1
2.5 2.4 2.3 2.1 2.2 2.4 2.3
2.5 2.4 2.3 2.1 2.2 2.4 2.3
69.9 46.1 53.3 45.0 58.3 66.0 64.9
34.9 12.6 30.5 7.8 14.7 20.2 17.7
35.0 33.6 22.8 37.2 43.6 45.8 47.2
35.7 34.1 37.2 33.4 32.0 33.2 33.2
+ + + + + + +
+ + + + + + +
0.4 0.5 0.6 0.7 0.8 0.8 0.8
35.2 33.6 36.6 32.7 31.2 32.3 32.4
0.7 0.8 0.8 1.0 1.1 1.1 1.1
0.3 0.3 0.3 0.4 0.4 0.4 0.4
0.4 0.5 0.5 0.6 0.7 0.6 0.6
+ + + + + + +
3.4 3.6 3.4 4.2 4.5 4.7 4.9
3.4 3.6 3.4 4.2 4.5 4.7 4.9
1,902.7 1,876.8 1,921.4 1,915.4 1,979.8 1,997.1 1,988.1
1,801.6 1,779.2 1,832.3 1,811.8 1,860.9 1,874.5 1,861.0
Note: Totals may not sum due to independent rounding. Emissions estimates for passenger cars, light-duty trucks, and heavy-duty trucks are calculated
using fuel consumption data from FHWA's Highway Statistics, which used an updated methodology to develop the 2005 and 2006 estimates This causes
some discontinuity in the emissions estimates for gasoline and diesel on-road vehicles between 2004 and 2005.
aThis 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.
b Includes medium- and heavy-duty trucks over 8,500 Ibs.
c 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.
11 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.
+ Less than 0.05 Tg C02 Eq.
Energy 3-9
-------
21 percent (200.1 Tg) from 1990. The increase in light-duty
CO2 emissions is due primarily to the growth in vehicle
travel, which substantially outweighed improvements in
vehicle fuel economy. Light-duty vehicle miles traveled
(VMT) increased 39 percent from 1990 to 2006; average
vehicle fuel economy increased from 18.9 miles per gallon
(mpg) in 1990 to 20.4 mpg in 2006, primarily reflecting
the retirement of older vehicles. Among new vehicles sold
annually, average fuel economy gradually declined from
1990 to 2006 (Figure 3-10), reflecting substantial growth in
sales of light-duty trucks relative to passenger cars (Figure
3-11). Average new vehicle fuel economy improved in 2005
and 2006 as the market share of passenger cars increased in
response to rising fuel prices.
Figure 3-10
Sales-Weighted Fuel Economy of New Automobiles
and Light-Duty Trucks, 1990-2006
25-
24-
23-
I 2
8 21-
| 20-
Medium- and heavy-duty truck8 CO2 emissions
increased by 76 percent (173.4 Tg) from 1990 to 2006,
representing the largest percentage increase of any major
transportation mode. Fuel economy for the medium- and
heavy-duty truck fleet did not significantly improve over this
period, and most likely declined from levels recorded in the
late 1990s. Meanwhile, medium- and heavy-duty truck VMT
increased by 52 percent. CO2 from the domestic operation
of commercial aircraft increased by 4 percent (5.4 Tg)
from 1990 to 2006, well below the growth in travel activity
(passenger miles traveled grew by 69 percent from 1990 to
2005, the most recent year of available data). The operational
efficiency of commercial aircraft improved substantially
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 aviation9, CO2 emissions decreased
by approximately 5.2 percent (9.4 Tg CO2) between 1990 and
2006. This decline reflects a 56 percent decrease in emissions
from domestic military operations, which more than offset a
small increase in domestic commercial and general aviation
emissions. For further information on all greenhouse gas
emissions from transportation sources, please refer to Table
A-108 in Annex 3.2.
Industrial End-Use Sector
The industrial end-use sector accounted for 28 percent
of CO2 emissions from fossil fuel combustion. On average,
55 percent of these emissions resulted from the direct
consumption of fossil fuels for steam and process heat
production. The remaining 45 percent was associated with
their consumption of electricity for uses such as motors,
electric furnaces, ovens, and lighting.
The industrial end-use 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
8 Includes "medium- and heavy-duty trucks" fueled by gasoline, diesel
and LPG.
'Includes consumption of jet fuel and aviation gasoline. Does not include
aircraft bunkers, which are not accounted for in national emission totals.
3-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
refineries, chemicals, primary metals, paper, food, and
nonmetallic mineral products—represent the vast majority
of the energy use (EIA 2007a and EIA 2005).
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.10 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 affect on industrial emissions.
From 2005 to 2006, total industrial production and
manufacturing output increased by 3.9 and 4.9 percent,
respectively (FRB 2006). Over this period, output increased
for petroleum refineries, chemicals, primary metals, food,
and nonmetallic mineral products, but decreased slightly for
paper (see Figure 3-12).
Figure 3-12
Industrial Production Indices (Index 2002=100)
120
110
100
110
100
120-,
110-
100-
90-
80-
70-I
120-|
110-
100-
90-
Total
Industrial
Index
Total excluding Computers,
Communications Equipment,
and Semiconductors
Foods
Stone, Clay & Glass Products
Chemicals
Primary Metals
Petroleum Refineries
§ 5 g s s §
10 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.
Despite the growth in industrial output (62 percent) and
the overall U.S. economy (59 percent) from 1990 to 2006,
CO2 emissions from the industrial end-use sector increased
by only 3.0 percent over that time. A number of factors are
believed to have caused this disparity between rapid growth
in industrial output and decrease in industrial emissions,
including: (l)more rapid growth in output fromless energy-
intensive industries relative to traditional manufacturing
industries and (2) improvements in energy efficiency. In
2006, CO2 emissions from fossil fuel combustion and
electricity use within the industrial end-use sectors were
1,571.0 Tg CO2 Eq., or 0.8 percent below 2005 emissions.
Residential and Commercial End-Use Sectors
The residential and commercial end-use sectors
accounted for an average 20 and 18 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 72 and 79 percent of emissions from the residential
and commercial end-use sectors, respectively. The remaining
emissions were largely due to the direct consumption of
natural gas and petroleum products, primarily for heating and
cooking needs. Coal consumption was a minor component
of energy use in both of these end-use sectors. In 2006, CO2
emissions from fossil fuel combustion and electricity use
within the residential and commercial end-use sectors were
1,151.9 Tg CO2 Eq. and 1,003.0 Tg CO2 Eq., respectively.
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 (see Table 3-6). In the long-
term, both end-use 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
72 and 73 percent of the direct (not including electricity)
fossil fuel emissions from the residential and commercial
sectors, respectively. In 2006, natural gas emissions
decreased by 10 percent and 6 percent, respectively, in each
of these sectors. The decrease in emissions in both sectors is
a result of warmer conditions in the United States.
Energy 3-11
-------
Electricity sales to the residential and commercial
end-use sectors in 2006 decreased less than 1 percent in
the residential sector and increased by 2 percent in the
commercial sector. The trend in the commercial sector can
largely be attributed to the growing economy (2.9 percent),
which led to increased demand for electricity. Increased
consumption due to the growing economy was somewhat
offset by decreased air conditioning-related electricity
consumption in the residential sector with the cooler summer
compared to 2005, and increases in electricity prices.
Electricity-related emissions in both the residential and
commercial sectors decreased due to decreased consumption;
total emissions from the residential sector decreased by 8.9
percent in 2006, with emissions from the commercial sector
5.3 percent lower than in 2005.
Electricity Generation
The process of generating electricity is the single largest
source of CO2 emissions in the United States, representing
39 percent of total CO2 emissions from all CO2 emissions
sources across the United States. Electricity generation
also accounted for the largest share of CO2 emissions from
fossil fuel combustion, approximately 41 percent in 2006.
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 3-13).
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
Figure 3-13
Electricity Generation Retail Sales by End-Use Sector
1,600 -.
1,400 -
1,200 -
1,000 -
800-
600-
400-
Residential.
Industrial
Commercial
CDOOOCM^-CDOOOCM^-CDOOOCM^-CD
cncncncncncncncncncncncncj)^^^^
Note: The transportation end-use sector consumes minor quanties of electricity.
energy data used in this chapter, the Energy Information
Administration (EIA) categorizes 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 primary business is the production
of electricity,11 while the other sectors consist of those
producers that indicate their primary business is other than
the production of electricity.
In 2006, the amount of electricity generated (in kWh)
increased by 0.1 percent. This growth is due to the growing
economy, expanding industrial production, and warmer
summer conditions. However, CO2 emissions decreased by
2.2 percent, as a smaller share of electricity was 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 6.4 percent, respectively, in
2006, and nuclear power increased by less than 1 percent.
As a result of the decrease in coal consumption, C intensity
from direct fossil fuel combustion decreased slightly overall
in 2006 (see Table 3-8). Coal is consumed primarily by the
electric power sector in the United States, which accounted
for 94 percent of total coal consumption for energy purposes
in 2006. The amount of electricity generated from renewables
increased by 8.1 percent in 2006.
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
"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).
3-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
obtained directly from the El A of the U.S. Department
of Energy (DOE), primarily from the Monthly Energy
Review and published supplemental tables on petroleum
product detail (EIA 2007b). The United States does
not include territories in its national energy statistics,
so fuel consumption data for territories were collected
separately from Grillot (2007).12
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, and used in this Inventory, are,
instead, "bottom up" in nature. In other words, they are
collected through surveys at the point of delivery or use
and aggregated to determine national totals.13
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 standard, which
are to report energy statistics in terms of net calorific
values (NCV) (i.e., lower heating values).14
2. Subtract uses accounted for in the Industrial Processes
chapter. Portions of the fuel consumption data for six
fuel categories — coking coal, industrial 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 (1995 through
2007), Gambogi (2007), Coffeyville Resources Nitrogen
12Fuel 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 of 55 Tg CO2
Eq. in 2006.
13 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.
14A 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.
Fertilizers, EEC (2007), Corathers (2007), U.S. Census
Bureau (2007), EIA (2007h), EIA (2001), Smith, G.
(2007), USGS (1998 through 2002), USGS (1995),
USGS (1991a through 2006a), USGS (1991b through
2006b), USGS (1991 through 2005), and USGS (1995
through 2006).15
3. Adjust for biofuels, conversion offossilfuels, and exports
ofCO2. 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. These fuels include
ethanol added to motor gasoline and biomass gas used
as natural gas. 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.16
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 and biogas were
collected from EIA (2007b) and data for synthetic natural
gas were collected from EIA (2007e), and data for CO2
exports were collected from the Dakota Gasification
Company (2006), Fitzpatrick (2002), Erickson (2003),
EIA (2006), and EIA (2007e).
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 FHWA Vehicle Miles Traveled (VMT) that
indicated that the amount of distillate and motor gasoline
consumption allocated 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 higher to
match the value obtained from the bottom-up analysis
based on VMT. As the total distillate consumption
15 See sections on Iron and Steel Production, Ammonia Production,
Petrochemical Production, Titanium Dioxide Production, Ferroalloy
Production, Aluminum Production, and Silicon Carbide Production in the
Industrial Processes chapter.
16 These adjustments are explained in greater detail in Annex 2.1.
Energy 3-13
-------
estimate from EIA is considered to be accurate at the
national level, the distillate consumption totals for the
residential, commercial, and industrial sectors were
adjusted downward proportionately. Similarly, as the
total motor gasoline consumption estimate is considered
to be accurate at the national level, the motor gasoline
consumption totals for commercial and industrial sectors
were adjusted downward proportionately. The data
sources used in the bottom-up analysis of transportation
fuel consumption include AAR (2007), Benson (2002
through 2004), DOE (1993 through 2005), EIA (2007a),
EIA (1991 through 2005), EPA (2004), and FHWA (1996
through 2006).
5. Adjust for fuels consumed for non-energy use. U.S.
aggregate energy statistics include consumption of
fossil fuels for non-energy purposes. 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, these emissions are
estimated separately in the Carbon Emitted and Stored
in Products from Non-Energy Use 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 (2007b).
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).17 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 2007) supplied
data on military j et fuel and marine fuel use. Commercial
jet fuel use was obtained from BEA (1991 through 2007)
and DOT (1991 through 2007); residual and distillate
fuel use for civilian marine bunkers was obtained from
DOC (1991 through 2007) for 1990 through 2002,
and DHS (2008) for 2003 through 2008. Consumption
of these fuels was subtracted from the corresponding
fuels in the transportation end-use sector. Estimates of
international bunker fuel emissions are discussed further
in the section entitled International Bunker Fuels.
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 EIA's Emissions of Greenhouse Gases in the United
States 2006 (EIA 2007c) and EIA's Monthly Energy
Review and published supplemental tables on petroleum
product detail EIA (EIA 2007b). They are presented in
Annexes 2.1 and 2.2.
8. Estimate CO2 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
2006); for each vehicle category, the percent gasoline,
diesel, and other (e.g., CNG, LPG) fuel consumption are
estimated using data from DOE (1993 through 2005).
17 See International Bunker Fuels section in this chapter for a more detailed
discussion.
3-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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.18
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.19 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., 0 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.20 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 3-8 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 3-8: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg C02 Eq./QBtu)
Sector
Residential3
Commercial3
Industrial3
Transportation3
Electricity Generation"
U.S. Territories0
All Sectors"
1990
57.3
59.2
63.7
71.0
86.7
74.1
72.7
1995
56.6
57.8
62.7
71.0
86.0
74.1
72.2
2000
56.7
57.1
62.6
71.0
85.6
73.2
72.7
2001
56.9
57.4
63.5
71.0
85.2
73.6
72.7
2002
56.6
57.0
62.8
71.0
85.0
73.7
72.5
2003
56.8
57.3
63.2
71.0
85.7
74.0
72.7
2004
56.9
57.6
63.6
71.0
85.4
74.6
72.9
2005
56.6
57.6
64.0
71.1
85.0
74.6
73.1
2006
56.7
57.5
64.2
71.1
84.6
74.6
73.0
aDoes 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 3-8, Table 3-9 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.21 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 3-9 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
18 Small quantities of C02, however, are released from some geologic formations tapped for geothermal energy. These emissions are included with fossil
fuel combustion emissions from electricity generation. Carbon dioxide emissions may also be generated from upstream activities (e.g., manufacture of the
equipment) associated with fossil fuel and renewable energy activities, but are not accounted for here.
19 One exajoule (EJ) is equal to 1018 joules or 0.9478 QBtu.
20 Net C fluxes from changes in biogenic C reservoirs in wooded or croplands are accounted for in the estimates for Land Use, Land-Use Change,
and Forestry.
21 In other words, the emissions from the generation of electricity are intentionally double counted by attributing them both to electricity generation and the
end-use sector in which electricity consumption occurred.
Energy 3-15
-------
Box 3-2: Carbon Intensity of U.S. Energy Consumption (continued)
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 3-8, 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.
Table 3-9: Carbon Intensity from all Energy Consumption by Sector (Tg C02 Eq./QBtu)
Sector
Transportation3
Other End-Use Sectors3' b
Electricity Generation0
All Sectors"
1990
70.8
57.5
59.0
61.1
1995
70.6
56.4
57.9
60.3
2000
70.6
57.7
59.9
61.4
2001
70.5
58.4
60.2
61.8
2002
70.5
57.6
59.2
61.3
2003
70.3
58.0
59.8
61.5
2004
70.2
58.0
59.6
61.5
2005
70.1
58.2
59.9
61.6
2006
70.0
57.5
58.8
61.1
a 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 3-8 and Table 3-9, 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 sixteen-
year period of 1990 through 2006, 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 2007).
C intensity estimates were developed using nuclear and renewable
energy data from EIA (2007a) 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
C02/Capita
For non-road vehicles, activity data were obtained
from AAR (2007), APIA (2007 and 2006), BEA (1991
through 2007), Benson (2002 through 2004), DOE
(1993 through 2005), DESC (2007), DOC (1991 through
2007), DOT (1991 through 2007), EIA (2007a), EIA
(2007d), EIA (2007g), EIA (2002), EIA (1991 through
2005), EPA (2004), FAA (2005), 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) System for
assessing Aviation's Global Emission (SAGE) model.22
Allocation to domestic general aviation was made
using FAA Aerospace Forecast data, and allocation to
domestic military uses was made using DoD data (see
Annex 3.7).
Heat contents and densities were obtained from EIA
(2007a) and USAF (1998).23
22FAA's System for assessing Aviation's Global Emissions (SAGE)
model develops aircraft fuel burn and emissions for all commercial flights
globally in a given year. The SAGE model dynamically models aircraft
performance, fuel burn, and emissions, and is based on actual flight-by-
flight aircraft movements. See .
23 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.
3-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Uncertainty
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 in these non-energy production processes were
subtracted from the total fossil fuel consumption for 2006.
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 Use
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 Use 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. In particular, residual fuel consumption
data for marine vessels are highly uncertain, as shown by the
large fluctuations in emissions that do not mimic changes in
other variables such as shipping ton miles.
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 inventory 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).
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.24 Triangular distributions were assigned for
the oxidization factors (or combustion efficiencies). The
24 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.
Energy 3-17
-------
uncertainty ranges were assigned to the input variables
based on the data reported in SAIC/EIA (2001) and on
conversations with various agency personnel.25
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 uncertainty
associated with these variables (SAIC/EIA 2001).26 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 3-10. Fossil fuel combustion CO2
emissions in 2006 were estimated to be between 5,542.9 and
5,944.7 Tg CO2 Eq. at a 95 percent confidence level. This
Table 3-10: 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
2006 Emission Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
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
Total (excluding Geothermal)"
Geothermal
Total (including Geothermal)116
2,065.3
0.6
6.2
122.0
NE
1,932.4
4.1
1,155.1
237.5
154.1
389.3
33.2
339.6
1.4
2,417.1
88.5
49.8
350.9
1,822.8
55.7
49.4
5,637.6
0.4
5,637.9
Lower Bound
1,996.8
0.5
5.9
117.3
NE
1,856.4
3.6
1,164.8
230.8
149.7
399.5
32.3
329.6
1.2
2,282.9
83.8
47.5
305.2
1,700.5
53.3
45.6
5,542.5
NE
5,542.9
Upper Bound
2,262.1
0.7
7.1
142.4
NE
2,119.9
4.9
1,231.2
254.2
164.9
440.0
35.5
356.9
1.6
2,554.7
92.8
51.8
407.1
1,940.0
59.7
54.9
5,944.3
NE
5,944.7
Lower Bound
-3%
-6%
-5%
-4%
NA
-4%
-12%
+ 1%
-3%
-3%
+ 3%
OO/
-o/o
-3%
-12%
-6%
-5%
-5%
-13%
-7%
-4%
-8%
-2%
NE
-2%
Upper Bound
+10%
+ 15%
+ 15%
+ 17%
NA
+ 10%
+ 19%
+7%
+7%
+7%
+ 13%
+7%
+5%
+ 17%
+6%
+5%
+4%
+ 16%
+6%
+7%
+ 11%
+5%
NE
+5%
NA (Not Applicable)
NE (Not Estimated)
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
b The 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.
Note: Totals may not sum due to independent rounding.
25 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.
26 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.
3-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
indicates a range of 2 percent below to 5 percent above the
2006 emission estimate of 5,637.9 Tg CO2 Eq.
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 2007b)
updated energy consumption data for all years. These
revisions primarily impacted the emission estimates for
2005. Overall, these changes resulted in an average annual
increase of 0.8 Tg CO2 Eq. (less than 0.1 percent) in CO2
emissions from fossil fuel combustion for the period 1990
through 2005.
Planned Improvements
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.
3.2. Carbon Emitted from Non-
Energy Use of Fossil Fuels (IPCC
Source Category 1 A)
In addition to being combusted for energy, fossil
fuels are also consumed for non-energy use (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).
CO2 emissions arise from non-energy use 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
62 percent of the total C consumed for non-energy purposes
was stored in products, and not released to the atmosphere;
the remaining 38 percent was emitted.
There are several areas in which non-energy use 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 Municipal Solid Waste Combustion source
category. In addition, there is some overlap between fossil
fuels consumed for non-energy use 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 EIA 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 these
affect the mass of C in non-energy applications.
As shown in Table 3-11, fossil fuel emissions in 2006
from the non-energy use of fossil fuels were 138.0 Tg CO2
Eq., which constituted approximately 2 percent of overall
fossil fuel emissions, approximately the same proportion as
in 1990. In 2006, the consumption of fuels for non-energy
use (after the adjustments described above) was 5,417.8
TBtu, an increase of 21 percent since 1990 (see Table 3-12).
About 65.4 Tg of the C (239.6 Tg CO2 Eq.) in these fuels
was stored, while the remaining 37.6 Tg C (138.0 Tg CO2
Eq.) was emitted. The proportion of C emitted as CO2 has
remained about constant since 1990, at about 36 to 40 percent
of total non-energy consumption (see Table 3-11).
Energy 3-19
-------
Table 3-11: C02 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg C02 Eq.)
Emissions/Storage
Potential Emissions
C Stored
Emissions as a % of Potential
Emissions
1990
312.6
195.5
37%
117.2
1995
346.8
213.5
38%
133.2
2000
385.5
244.1
37%
141.4
2001
364.8
232.9
36%
131.9
2002
368.4
232.6
37%
135.9
2003
356.3
224.5
37%
131.8
2004
394.9
246.0
38%
148.9
2005
382.2
243.1
36%
139.1
2006
377.6
239.6
37%
138.0
Note: Totals may not sum due to independent rounding.
Methodology
The first step in estimating C stored in products was to
determine the aggregate quantity of fossil fuels consumed
for non-energy use. 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 (2007) (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 3-12 and Table
3-13 have been adjusted to subtract non-energy uses that are
included in the source categories of the Industrial Processes
chapter.27 Consumption values were also adjusted to subtract
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 Municipal Solid
Waste Combustion source category, the storage factors do not
27 These source categories include Iron and Steel Production, Lead
Production, Zinc Production, Ammonia Production, Carbon Black
Production (included in Petrochemical Production), Titanium Dioxide
Production, Ferroalloy Production, Silicon Carbide Production, and
Aluminum Production.
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 draw s 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.
Lastly, emissions were estimated by subtracting the C
stored from the potential emissions (see Table 3-11). 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 1995,2001), National Air Quality
and Emissions Trends Report (EPA 2006a), Toxics Release
Inventory, 1998 (2000a), Biennial Reporting System (EPA
2004a, 2006b, 2007), and pesticide sales and use estimates
(EPA 1998, 1999, 2002, 2004b); the EIA Manufacturer's
Energy Consumption Survey (MEGS) (EIA 1994, 1997,
2001, 2005); the National Petrochemical & Refiners
Association (NPRA 2001); the National Asphalt Pavement
Association (Connolly 2000); the Emissions Inventory
Improvement Program (EIIP 1998, 1999); the U.S. Census
Bureau (1999, 2003, 2004); the American Plastics Council
(APC 2000, 2001, 2003, 2005, 2006; Eldredge-Roebuck
2000); the Society of the Plastics Industry (SPI 2000);
Bank of Canada (2006); Financial Planning Association
3-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-12: Adjusted Consumption of Fossil Fuels for Non-Energy Use (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
4,221.2
0.0
8.2
276.0
1,170.2
1,119.0
186.3
77.3
325.7
677.2
21.3
81.0
100.9
7.0
33.3
137.8
176.0
176.0
86.7
0.7
86.0
4,483.9
1995
4,771.8
43.9
11.3
330.4
1,178.2
1,484.7
177.8
285.3
350.6
612.7
40.1
44.1
66.9
8.0
40.6
97.1
167.9
167.9
90.8
2.0
88.8
5,030.6
2000 2001 2002 2003 2004 2005 2006
5,261.1 5,044.4 5,032.4 4,865.3 5,308.4 5,210.0 5,160.5
62.8 25.6 46.5 72.1 214.7 109.7 85.9
12.4 11.3 12.0 11.9 11.9 11.9 12.4
421.1 407.8 364.6 352.0 360.2 390.3 403.2
1,275.7 1,256.9 1,239.9 1,219.5 1,303.8 1,323.2 1,225.6
1,604.6 1,539.0 1,565.4 1,437.8 1,436.7 1,442.0 1,491.8
189.9 174.0 171.9 159.0 161.0 160.2 130.6
228.7 199.8 166.1 158.3 156.5 146.0 105.1
592.8 489.4 564.2 573.4 687.9 678.6 592.9
554.3 525.9 456.2 501.0 547.8 518.7 573.4
12.6 35.8 57.8 59.0 63.5 67.7 122.3
47.8 128.1 110.2 76.9 161.3 145.0 178.7
94.4 77.9 99.5 75.7 47.2 60.9 68.7
11.7 11.7 11.7 11.7 11.7 11.7 11.7
33.1 36.3 32.2 31.0 30.8 31.4 25.2
119.2 124.9 134.2 126.0 113.4 112.8 133.2
179.4 164.3 162.4 150.1 152.1 151.3 147.0
179.4 164.3 162.4 150.1 152.1 151.3 147.0
165.5 80.3 138.6 127.9 110.8 107.6 110.3
16.4 + 1.5 9.3 5.1 5.2 5.4
149.1 80.3 137.2 118.6 105.7 102.4 104.9
5,605.9 5,289.0 5,333.4 5,143.4 5,571.3 5,468.9 5,417.8
+ 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.
(2006); INEGI (2006); Statistics Canada (2006); the United
States International Trade Commission (2006, 2007); the
Pesticide Action Network (PAN 2002); Gosselin, Smith,
and Hodge (1984); the Rubber Manufacturers' Association
(RMA 2002, 2006; STMC 2003); the International Institute
of Synthetic Rubber Products (IISRP 2000,2003); the Fiber
Economics Bureau (FEE 2001, 2003, 2005 through 2007);
the Material Safety Data Sheets (Miller 1999); the Chemical
Manufacturer's Association (CMA1999); and the American
Chemistry Council (ACC 2005 through 2007) Specific data
sources are listed in full detail in Annex 2.3.
Uncertainty
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
Energy 3-21
-------
Table 3-13: 2006 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions
Adjusted Carbon
Non-Energy Content
Usea Coefficient
Sector/Fuel Type (TBtu) (Tg C/QBtu)
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
5,160.5
85.9
12.4
403.2
1,225.6
1,491.8
130.6
105.1
592.9
573.4
122.3
178.7
68.7
11.7
25.2
133.2
147.0
147.0
110.3
5.4
104.9
5,417.8
-
31.00
25.63
14.47
20.62
16.78
20.24
18.24
18.14
19.95
17.51
27.85
19.86
19.95
19.81
20.33
-
20.24
-
20.24
20.00
Potential
Carbon
(TgC)
97.8
2.7
0.3
5.8
25.3
25.0
2.6
1.9
10.8
11.4
2.1
5.0
1.4
0.2
0.5
2.7
3.0
3.0
2.2
0.1
2.1
103.0
Storage
Factor
-
0.10
0.62
0.62
1.00
0.62
0.09
0.62
0.62
0.62
0.62
0.50
0.62
0.50
0.58
0.00
-
0.09
-
0.09
0.10
Carbon Carbon Carbon
Stored Emissions Emissions
(TgC) (TgC) (Tg C02 Eq.)
64.9
0.3
0.2
3.6
25.3
15.4
0.2
1.2
6.6
7.0
1.3
2.5
0.8
0.1
0.3
0.0
0.3
0.3
0.2
+
0.2
65.4
32.9
2.4
0.1
2.2
0.0
9.6
2.4
0.7
4.1
4.4
0.8
2.5
0.5
0.1
0.2
2.7
2.7
2.7
2.0
0.1
1.89
37.6
120.8
8.8
0.4
8.2
0.0
35.3
8.8
2.7
15.2
16.1
3.0
9.1
1.9
0.4
0.8
9.9
9.9
9.9
7.3
0.4
6.9
138.0
+ Does not exceed 0.05 TBtu
- Not applicable.
aTo avoid double counting, exports have been deducted.
Note: Totals may not sum due to independent rounding.
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 3-14 (emissions) and Table 3-15
(storage factors). Carbon emitted from non-energy use of
fossil fuels in 2006 was estimated to be between 110.2 and
150.3 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 20 percent below to 9 percent above the
2006 emission estimate of 138.0 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 3-15, feedstocks and asphalt contribute least
to overall storage factor uncertainty on a percentage basis.
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
3-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-14: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Non-Energy Uses of Fossil Fuels
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Feedstocks
Asphalt
Lubricants
Waxes
Other
Total
C02
C02
C02
C02
C02
C02
83.0
0.0
19.1
0.8
35.2
138.0
Lower Bound
66.4
0.1
15.7
0.6
16.5
110.2
Upper Bound
99.5
0.7
22.1
1.2
38.3
150.3
Lower Bound
-20%
NA
-17%
-23%
-53%
-20%
Upper Bound
+20%
NA
+ 16%
+59%
+ 9%
+9%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
NA (Not Applicable)
Note: Totals may not sum due to independent rounding.
Table 3-15: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of Non-Energy Uses of Fossil Fuels
(Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Feedstocks
Asphalt
Lubricants
Waxes
Other
C02
C02
C02
C02
C02
62%
100%
9%
58%
24%
Lower Bound
59%
99%
4%
44%
20%
Upper Bound
64%
100%
18%
69%
64%
Lower Bound
-4%
-1%
-58%
-24%
-17%
Upper Bound
+ 3%
+ 0%
+89%
+ 19%
+ 162%
a 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).
quantified. More details on the uncertainty analysis are
provided in Annex 2.3.
QA/QC and Verification
A source-specific QA/QC plan for non-energy use 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 2006 as well as their trends across
the time series.
Planned Improvements
There are several improvements planned for the
future:
• Future updates in line with the 2006 IPCC Guidelines.
These changes could affect both the non-energy use and
industrial processes sections.
• 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,
Energy 3-23
-------
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 the remaining
fuels (petroleum coke, 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.
3.3. Stationary Combustion
(excluding C02) (IPCC Source
Category 1 A)
Stationary combustion encompasses all fuel combustion
activities from fixed sources (versus mobile combustion).
Other than CO2, which was addressed in the previous section,
gases from stationary combustion include the greenhouse
gases CH4 and N2O and the indirect greenhouse gases NOX,
CO, andNMVOCs.28 Emissions of these gases from stationary
combustion sources depend upon fuel characteristics, size
and vintage, along with combustion technology, pollution
control equipment, and ambient environmental conditions.
Emissions also vary with operation and maintenance
practices.
N2O 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. CH4 emissions from stationary combustion
are primarily a function of the CH^ content of the fuel and
combustion efficiency.
Emissions of CFLj decreased 16 percent overall since
1990 to 6.2 Tg CO2 Eq. (296 Gg) in 2006. This decrease in
CFLj emissions was primarily due to lower wood consumption
in the residential sector. Conversely, N2O emissions rose 13
percent since 1990 to 14.5 Tg CO2 Eq. (47 Gg) in 2006. The
largest source of N2O emissions was coal combustion by
electricity generators, which alone accounted for 66 percent
of total N2O emissions from stationary combustion in 2006.
Overall, however, stationary combustion is a small source
of CH4 and N2O in the United States.
Table 3-16 and Table 3-17 provide CH4 and N2O
emission estimates in Tg CO2 Eq.; Table 3-18 and Table 3-19
present these estimates in Gg of each gas.
Methodology
CH4 and N2O emissions 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, commercial, residential, electricity generation,
and U.S. territories. For the CFLj and N2O estimates, fuel
consumption data for coal, natural gas, fuel oil for the United
States were obtained from EIA's Monthly Energy Review
and unpublished supplemental tables on petroleum product
detail (ElA 2007a). Wood consumption data for the United
States was obtained from EIA's Annual Energy Review (EIA
2007b). Because the United States does not include territories
in its national energy statistics, fuel consumption data for
territories were provided separately by Grillot (2007).29 Fuel
consumption for the industrial sector was adjusted to subtract
out construction and agricultural use, which is reported
under mobile sources.30 Construction and agricultural fuel
use was obtained from EPA (2004). Estimates for wood
biomass consumption for fuel combustion do not include
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.
28 Sulfur dioxide (SO2) emissions from stationary combustion are addressed
in Annex 6.3.
29 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.
30 Though emissions from construction and farm use occur due to both
stationary and mobile sources, detailed data were not available to determine
the magnitude from each. Currently, these emissions are assumed to be
predominantly from mobile sources.
3-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-16: CH4 Emissions from Stationary Combustion (Tg C02 Eq.)
Electric Power
Coal
Fuel Oil
Natural gas
Wood
Industrial
Coal
Fuel Oil
Natural gas
Wood
Commercial/Institutional
Coal
Fuel Oil
Natural gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
Total
0.6
0.3
0.1
0.1
0.1
1.5
0.3
0.2
0.2
0.9
0.9
+
0.2
0.3
0.4
4.4
0.2
0.3
0.5
3.5
+
+
+
+
+
7.4
0.6
0.4
+
0.1
0.1
1.6
0.3
0.2
0.2
1.0
0.9
+
0.1
0.3
0.4
4.0
0.1
0.3
0.5
3.1
+
+
+
+
+
7.2
2000
0.7
0.4
0.1
0.1
0.1
1.6
0.3
0.2
0.2
1.0
0.9
+
0.1
0.3
0.4
3.4
0.1
0.3
0.5
2.5
0.1
+
+
+
+
6.6
2001
0.7
0.4
0.1
0.1
0.1
1.5
0.3
0.2
0.2
0.9
0.9
+
0.1
0.3
0.4
3.1
0.1
0.3
0.5
2.2
0.1
+
0.1
+
+
6.2
2002
0.7
0.4
0.1
0.1
0.1
1.4
0.3
0.2
0.2
0.8
0.9
+
0.1
0.3
0.4
3.1
0.1
0.3
0.5
2.3
0.1
+
0.1
+
+
6.2
2003
0.7
0.4
0.1
0.1
0.1
1.4
0.3
0.2
0.2
0.8
0.9
+
0.2
0.3
0.4
3.3
0.1
0.3
0.5
2.4
0.1
+
0.1
+
+
6.4
2004
0.7
0.4
0.1
0.1
0.1
1.5
0.3
0.2
0.2
0.9
0.9
+
0.1
0.3
0.4
3.3
0.1
0.3
0.5
2.5
0.1
+
0.1
+
+
6.5
2005
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
3.3
0.1
0.3
0.5
2.5
0.1
+
0.1
+
+
6.5
2006
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
+
+
6.2
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Uncertainty
CH4 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 CtLj 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. A total of 115
input variables were simulated for the uncertainty analysis of
this source category (85 from the CO2 emissions from fossil
fuel combustion inventory estimation model and 30 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/
El A (2001) report.31 For these variables, the uncertainty
ranges were assigned to the input variables based on the
31SAIC/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.
Energy 3-25
-------
Table 3-17: N20 Emissions from Stationary Combustion (Tg C02 Eq.)
Sector/Fuel Type
Electric Power
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial/Institutional
Coal
Fuel Oil
Natural Gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
Total
1990
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
0.1
0.7
0.1
+
0.1
+
+
12.8
1995
8.6
8.1
0.1
0.1
0.1
3.4
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
+
+
13.4
2000
10.0
9.4
0.2
0.2
0.2
3.3
0.7
0.5
0.3
1.9
0.3
+
0.1
0.1
0.1
0.9
+
0.3
0.2
0.5
0.1
+
0.1
+
+
14.6
2001
9.7
9.1
0.2
0.2
0.1
3.1
0.7
0.5
0.2
1.7
0.3
+
0.1
0.1
0.1
0.9
+
0.3
0.1
0.4
0.1
+
0.1
+
+
14.1
2002
9.7
9.2
0.2
0.2
0.2
3.0
0.6
0.5
0.2
1.6
0.3
+
0.1
0.1
0.1
0.9
+
0.3
0.1
0.4
0.1
+
0.1
+
+
14.0
2003
10.0
9.4
0.2
0.2
0.2
3.0
0.6
0.5
0.2
1.6
0.4
+
0.1
0.1
0.1
0.9
+
0.3
0.2
0.5
0.1
+
0.1
+
+
14.3
2004
10.0
9.5
0.2
0.2
0.2
3.1
0.6
0.5
0.2
1.7
0.4
+
0.1
0.1
0.1
0.9
+
0.3
0.1
0.5
0.1
+
0.1
+
+
14.6
2005
10.3
9.7
0.2
0.2
0.2
3.1
0.6
0.6
0.2
1.7
0.3
+
0.1
0.1
0.1
0.9
+
0.3
0.1
0.5
0.1
+
0.1
+
+
14.8
2006
10.1
9.5
0.1
0.2
0.2
3.2
0.6
0.6
0.2
1.7
0.3
+
0.1
0.1
0.1
0.8
+
0.2
0.1
0.5
0.1
+
0.1
+
+
14.5
+ Does not exceed 0.05 Tg C02
Note: Totals may not sum due to
Eq.
independent rounding.
data reported in SAIC/EIA (2001).32 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 3-20. Stationary combustion CH4
emissions in 2006 (including biomass) were estimated to be
between 4.3 and 13.4 Tg CO2 Eq. at a 95 percent confidence
level. This indicates a range of 31 percent below to 116
32 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.
percent above the 2006 emission estimate of 6.2 Tg CO2 Eq.33
Stationary combustion N2O emissions in 2006 (including
biomass) were estimated to be between 11.0 and 42.2 Tg
CO2 Eq. at a 95 percent confidence level. This indicates a
range of 24 percent below to 190 percent above the 2006
emissions estimate of 14.5 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 CH^ and N2O estimates are due to the
fact that emissions are estimated based on emission factors
representing only a limited subset of combustion conditions.
33 The low emission estimates reported in this section have been rounded
down to the nearest integer values and the high emission estimates have
been rounded up to the nearest integer values.
3-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-18: CH4 Emissions from Stationary Combustion (Gg)
Electric Power
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial/Institutional
Coal
Fuel Oil
Natural Gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
Total
27
16
4
3
4
73
16
9
7
41
41
1
8
13
19
210
9
14
21
165
2
+
2
+
+
353
27
18
2
4
4
78
15
7
8
47
43
1
7
15
21
190
5
13
24
148
2
+
2
+
+
341
2000
33
20
3
5
4
76
14
7
8
47
43
1
7
15
20
162
3
15
24
120
2
+
2
+
+
316
2001
32
20
4
5
4
71
14
8
8
41
41
1
7
15
19
147
3
15
23
105
3
+
3
+
+
295
2002
32
20
3
5
4
69
13
8
8
40
42
1
6
15
20
149
4
14
24
108
3
+
3
+
+
295
2003
34
20
4
5
5
68
13
8
8
39
44
1
7
16
20
158
4
15
25
114
3
+
3
+
+
306
2004
34
20
4
5
5
71
13
9
7
42
43
1
7
15
20
159
4
15
24
117
3
+
3
+
+
311
2005
35
21
4
6
5
71
13
9
7
41
42
1
7
15
20
157
3
14
24
117
3
+
3
+
+
308
2006
34
21
2
6
5
72
13
10
7
42
40
1
7
14
18
147
2
13
21
111
3
+
3
+
+
296
+ Does not exceed 0.5 Gg
Note: Totals may not sum due to independent rounding.
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.
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 CK4, 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 several changes.
One of the most significant changes was implementing
stationary combustion emission factors updated in
IPCC (2006). As a result, N2O emission factors for coal
consumption in all sectors, and CtLj emission factors for
industrial petroleum and natural gas were revised. Slight
changes to emission estimates for sectors are also due to
revised data from EIA (2007a). 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 (2007b) were revised for the residential,
industrial, and electric power sectors. The combination of
the methodological and historical data changes resulted in an
average annual decrease of 0.6 Tg CO2 Eq. (8.3 percent) in
Energy 3-27
-------
Table 3-19: N20 Emissions from Stationary Combustion (Gg)
Sector/Fuel Type
Electric Power
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial/Institutional
Coal
Fuel Oil
Natural Gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
Total
1990
26
24
1
+
+
10
2
2
1
5
1
+
+
+
+
4
+
1
+
2
+
+
+
+
+
41
1995
28
26
+
+
+
11
2
1
1
6
1
+
+
+
+
3
+
1
+
2
+
+
+
+
+
43
2000
32
30
1
1
1
11
2
1
1
6
1
+
+
+
+
3
+
1
+
2
+
+
+
+
+
47
2001
31
29
1
1
+
10
2
2
1
5
1
+
+
+
+
3
+
1
+
1
+
+
+
+
+
46
2002
31
30
1
1
1
10
2
2
1
5
1
+
+
+
+
3
+
1
+
1
+
+
+
+
+
45
2003
32
30
1
+
1
10
2
2
1
5
1
+
+
+
+
3
+
1
+
2
+
+
+
+
+
46
2004
32
31
1
1
1
10
2
2
1
6
1
+
+
+
+
3
+
1
+
2
+
+
+
+
+
47
2005
33
31
1
1
1
10
2
2
1
6
1
+
+
+
+
3
+
1
+
2
+
+
+
+
+
48
2006
32
31
+
1
1
10
2
2
1
6
1
+
+
+
+
3
+
1
+
1
+
+
+
+
+
47
+ Does not exceed 0.5 Gg
Note: Totals may not sum due to independent rounding.
Table 3-20: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 Emissions from Energy-Related Stationary
Combustion, Including Biomass (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Stationary Combustion
Stationary Combustion
CH4
N20
6.2
14.5
Lower Bound
4.3
11.0
Upper Bound
13.4
42.2
Lower Bound
-31%
-24%
Upper Bound
+ 116%
+ 190%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
CH4 emissions from stationary combustion and an average
annual increase of 0.6 Tg CO2 Eq. (4.9 percent) in N2O
emissions from stationary combustion for the period 1990
through 2005.
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
El A and other agencies to improve the quality of the U.S.
3-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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.
3.4. Mobile Combustion (excluding
C02) (IPCC Source Category 1 A)
Mobile combustion produces greenhouse gases other
than CO2, including CH4, N2O, and indirect greenhouse
gases including NOX, CO, and NMVOCs. Mobile combustion
includes 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 that
have utility associated with their movement but do not have
a primary purpose of transporting people or goods (e.g.,
snowmobiles, riding lawnmowers, etc.). Annex 3.2 includes
a summary of all emissions from both transportation and
mobile sources.
As with stationary combustion, N2O and NOX emissions
are closely related to fuel characteristics, air-fuel mixes,
combustion temperatures, and the use of pollution control
equipment. N2O, 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. CO 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. CH4 and NMVOC emissions from motor
vehicles are a function of the CK4 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).
Table 3-21 and Table 3-22 provide CH4 and N2O
emission estimates in Tg CO2 Eq.; Table 3-23 and Table 3-24
present these estimates in Gg of each gas.34
Mobile combustion was responsible for a small portion
of national CK4 emissions (0.4 percent) but was the second
largest source of U.S. N2O emissions (9 percent). From
1990 to 2006, mobile source CFLj emissions declined by 50
percent, to 2.4 Tg CO2 Eq. (112 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 24 percent,
to 33.1 Tg CO2 Eq (107 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 40 percent decrease in mobile source
N2O emissions from 1998 to 2006 (seeFigure 3-15). Overall,
CH4 and N2O emissions were predominantly from gasoline-
fueled passenger cars and light-duty trucks.
Figure 3-15
60-
50-
40-
30-
20-
10-
0-
Mobile Source CHA and N,0 Emissions
N20
CH,
i- CM co
34 See Annex 3.2 for a complete time series of emission estimates for 1990
through 2006.
Energy 3-29
-------
Table 3-21: CH4 Emissions from Mobile Combustion (Tg C02 Eq.)
Fuel Type/Vehicle Typea
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 Boats-Domestic
Rail
Agricultural Equipment
Construction/Mining Equipment
Aircraft-Domestic
Other"
Total
1990
4.2
2.6
1.4
0.2
+
+
+
+
+
+
0.5
0.1
0.1
0.1
+
0.2
+
4.7
1995
3.8
2.1
1.4
0.2
+
+
+
+
+
+
0.5
0.1
0.1
0.1
0.1
0.1
+
4.3
2000
2.8
1.6
1.1
0.1
+
+
+
+
+
+
0.5
0.1
0.1
0.1
0.1
0.2
+
3.4
2001
2.7
1.5
1.1
0.1
+
+
+
+
+
+
0.5
0.1
0.1
0.1
0.1
0.1
+
3.3
2002
2.4
1.4
0.9
0.1
+
+
+
+
+
+
0.5
0.1
0.1
0.1
0.1
0.1
0.1
3.0
2003
2.2
1.3
0.8
0.1
+
+
+
+
+
+
0.5
0.1
0.1
0.1
0.1
0.1
0.1
2.7
2004
2.0
1.2
0.7
0.1
+
+
+
+
+
0.1
0.5
0.1
0.1
0.1
0.1
0.1
0.1
2.6
2005
1.9
1.1
0.7
0.1
+
+
+
+
+
0.1
0.6
0.1
0.1
0.1
0.1
0.1
0.1
2.5
2006
1.7
1.0
0.7
0.1
+
+
+
+
+
0.1
0.6
0.1
0.1
0.1
0.1
0.1
0.1
2.4
+ Less than 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
aSee Annex 3.2 for definitions of on-road vehicle types.
b "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment,
commercial equipment, and industrial equipment.
Table 3-22: N20 Emissions from Mobile Combustion (Tg C02 Eq.)
Fuel Type/Vehicle Type
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 Boats-Domestic
Rail
Agricultural Equipment
Construction/Mining Equipment
Aircraft-Domestic
Other3
Total
1990
40.1
25.4
14.1
0.6
+
0.2
+
+
0.2
0.1
3.2
0.4
0.3
0.2
0.3
1.7
0.2
43.5
1995
49.8
26.9
22.1
0.7
+
0.3
+
+
0.2
0.1
3.3
0.4
0.3
0.3
0.4
1.7
0.3
53.4
2000
48.4
25.2
22.4
0.9
+
0.3
+
+
0.3
0.1
3.7
0.5
0.3
0.3
0.4
1.9
0.3
52.5
2001
45.9
23.8
21.2
0.9
+
0.3
+
+
0.3
0.1
3.6
0.3
0.3
0.3
0.4
1.8
0.3
49.9
2002
41.8
22.5
18.5
0.9
+
0.3
+
+
0.3
0.1
3.6
0.5
0.3
0.3
0.5
1.7
0.3
45.9
2003
38.4
21.0
16.5
0.9
+
0.3
+
+
0.3
0.2
3.4
0.3
0.3
0.3
0.5
1.7
0.3
42.3
2004
35.6
19.5
15.3
0.8
+
0.3
+
+
0.3
0.2
3.6
0.4
0.3
0.4
0.5
1.7
0.4
39.7
2005
32.1
17.7
13.6
0.8
+
0.3
+
+
0.3
0.2
3.7
0.4
0.4
0.4
0.5
1.7
0.4
36.3
2006
29.0
15.6
12.6
0.7
+
0.3
+
+
0.3
0.2
3.6
0.4
0.4
0.4
0.5
1.6
0.4
33.1
+ Less than 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
a "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment,
commercial equipment, and industrial equipment.
3-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-23: CH4 Emissions from Mobile Combustion (Gg)
Fuel Type/Vehicle Type
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 Boats-Domestic
Rail
Agricultural Equipment
Construction/Mining Equipment
Aircraft-Domestic
Other3
Total
1990
201
125
65
10
1
1
+
+
1
+
22
3
3
4
2
7
2
224
1995
180
101
69
9
1
1
+
+
1
+
24
4
3
5
3
7
2
205
2000
134
77
50
5
1
1
+
+
1
1
26
5
3
5
3
7
2
162
2001
129
72
52
5
1
1
+
+
1
2
25
3
3
6
3
7
2
157
2002
112
66
41
4
1
1
+
+
1
2
26
4
3
6
3
7
2
141
2003
103
61
38
4
1
1
+
+
1
2
24
3
3
6
3
6
3
131
2004
96
56
35
4
1
1
+
+
1
3
26
3
4
6
3
7
3
126
2005
88
52
32
3
1
1
+
+
1
3
27
4
4
7
4
7
3
119
2006
82
47
31
3
1
1
+
+
1
3
27
3
4
7
4
7
3
112
+ Less than 0.5 Gg
Note: Totals may not sum due to independent rounding.
a "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment,
commercial equipment, and industrial equipment.
Table 3-24: N20 Emissions from Mobile Combustion (Gg)
Fuel Type/Vehicle Type
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 Boats-Domestic
Rail
Agricultural Equipment
Construction/Mining Equipment
Aircraft-Domestic
Other3
Total
1990
129
82
45
2
+
1
+
+
1
+
10
1
1
1
1
6
1
140
1995
161
87
71
2
+
1
+
+
1
+
11
1
1
1
1
5
1
172
2000
156
81
72
3
+
1
+
+
1
+
12
2
1
1
1
6
1
169
2001
148
77
69
3
+
1
+
+
1
+
11
1
1
1
1
6
1
161
2002
135
72
60
3
+
1
+
+
1
+
12
2
1
1
1
6
1
148
2003
124
68
53
3
+
1
+
+
1
1
11
1
1
1
2
5
1
137
2004
115
63
49
3
+
1
+
+
1
1
12
1
1
1
2
6
1
128
2005
104
57
44
2
+
1
+
+
1
1
12
1
1
1
2
6
1
117
2006
93
50
41
2
+
1
+
+
1
1
12
1
1
1
2
5
1
107
+ Less than 0.5 Gg
Note: Totals may not sum due to independent rounding.
a "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad equipment, airport equipment,
commercial equipment, and industrial equipment.
Energy 3-31
-------
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
vehicle miles traveled (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.
EPA (2008, 2005, 2003) provides emission estimates
of NOX, CO, and NMVOCs for eight categories of on-
road vehicles,35 aircraft, and seven categories of non-road
vehicles.36 These emission estimates primarily reflect EPA
data, which, in final iteration, will be published on the
National Emission Inventory (NEI) Air Pollutant Emission
Trends web site. The methodology used to develop these
estimates can be found on EPA's Air Pollutant Emission
Trends website, at .
On-Road Vehicles
Estimates of CK4 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
vehicles (AFVs)37 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
35 Categories include: gasoline passenger cars, diesel passenger cars, light-
duty gasoline trucks less than 6,000 pounds in weight, light-duty gasoline
trucks between 6,000 and 8,500 pounds in weight, light-duty diesel trucks,
heavy-duty gasoline trucks and buses, heavy-duty diesel trucks and buses,
and motorcycles.
36 Categories include: locomotives, marine vessels, farm equipment,
construction equipment, other non-road liquid fuel (e.g. recreational vehicles
and lawn and garden equipment), and other non-road gaseous fuel (e.g.,
other non-road equipment running on compressed natural gas).
37 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 bifuel or dual fuel vehicles that may be partially powered
by gasoline or diesel.
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.38
Emission factors for AFVs were developed by ICF
(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 2006 were obtained
from the Federal Highway Administration's (FHWA)
Highway Performance Monitoring System database as
reported in Highway Statistics (FHWA 1996 through 2007).
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
38 Additional information regarding the model can be found online at .
3-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
by fuel type reported in DOE (1993 through 2007) and
information on total motor vehicle fuel consumption by fuel
type from FHWA (1996 through 2007). 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).
These emission estimates were obtained from preliminary
data (EPA 2008), 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.
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 CH^ per kilogram of fuel consumed).39 Activity
data were obtained from AAR (2007), APTA (2007 and
2006), BEA (1991 through 2005), Benson (2002 through
2004), DHS (2008), DOC (1991 through 2007), DOE (1993
through 2007), DESC (2007), DOT (1991 through 2007), EIA
(2007a, 2007b, 2007d, 2002), EIA (1991 through 2007), EPA
(2006b), Esser (2003 through 2004), FAA (2007 and 2006),
Gaffney (2007), Lou (2008), and Whorton (2006 through
2007). Emission factors for non-road modes were taken from
IPCC/UNEP/OECD/IEA (1997).
Uncertainty
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
2006 estimates of CtLj 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) vehicle miles traveled (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 3-25. As noted above, an
Table 3-25: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 Emissions from Mobile Sources
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate3
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate1"1
(Tg C02 Eq.) (%)
On-Road Sources
On-Road Sources
CH4
N20
1.8
29.5
Lower Bound
1.7
23.8
Upper Bound
1.9
35.2
Lower Bound
-6%
-19%
Upper Bound
+ 6%
+ 19%
a The 2006 emission estimates and the uncertainty range 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 from the estimates
reported in this table.
b Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
39 The consumption of international bunker fuels is not included in these
activity data, but is estimated separately under the International Bunker
Fuels source category.
Energy 3-33
-------
uncertainty analysis was not performed for CK4 and N2O
emissions from non-road vehicles. Mobile combustion CH4
emissions (from on-road vehicles) in 2006 were estimated to
be between 1.7 and 1.9 Tg CO2 Eq. at a 95 percent confidence
level. This indicates a range of 6 percent below to 6 percent
above the corresponding 2006 emission estimate of 1.8
Tg CO2 Eq. Also at a 95 percent confidence level, mobile
combustion N2O emissions from on-road vehicles in 2006
were estimated to be between 23.8 and 35.2 Tg CO2 Eq.,
indicating a range of 19 percent below to 19 percent above the
corresponding 2006 emission estimate of 29.5 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 CK4 and N2O
please refer to the Uncertainty Annex.
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
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.
Carbon dioxide emissions from gasoline-powered on-
road sources are now calculated directly using "bottom-up"
fuel sales data; this methodology is similar to the bottom-
up calculation of CO2 from transportation diesel sources
implemented beginning with the 1990-2004 Inventory
report. On-road gasoline fuel sales estimates come from
FHWA's Highway Statistics (FHWA 1996 through 2007).
The ethanol component of these fuel sales is subtracted to
yield a fossil-only estimate, which is used to calculate CO2 for
gasoline-powered passenger cars, light-duty trucks, medium-
and heavy-duty trucks, and buses. To preserve EIA's estimate
of total gasoline consumption across all sectors, adjustments
were made to estimated gasoline consumption by recreational
boats, the commercial sector and the industrial sector. EIA
estimates of transportation sector fuel consumption continue
to serve as the foundation of inventory estimates for other fuel
types (jet fuel, aviation gasoline, residual fuel, natural gas,
LPG and natural gas). CO2 from these fuels continues to be
apportioned to individual modes using bottom-up data from
BTS, FAA, and DOE's Transportation Energy Data Book.
Vehicle age distributions for 1999 to the present were
revised based on new data obtained from EPA's MOVES
model (EPA 2007c). Diesel fractions for light trucks and
medium- and heavy-duty trucks and buses were updated
based on data obtained from the Transportation Energy
Data Book (DOE 1993 through 2007) for 1998 through
2003, which increased emissions from diesel vehicles
and reduced emissions from gasoline vehicles. Updates
were made to alternative fuel vehicle (AFV) vehicle miles
traveled (VMT) numbers based on new activity data (ICF
2006a) and biodiesel was also added as a vehicle category
under alternative fuel vehicles. VMT and fuel consumption
estimates for on-road vehicles were also revised for 2005
based on updated data from FHWA's Highway Statistics
(FHWA 1996 through 2007).
Several changes were also made in the calculation of
emissions from non-road vehicles. Similar to the previous
Inventory, commercial aircraft energy consumption
estimates for 2000-2005 come from the Federal Aviation
Administration's (FAA) System for Assessing Aviation's
Global Emissions (SAGE) database (FAA 2006). Aviation
estimates were developed without the availability of 2006
data from the Federal Aviation Administration's (FAA)
System for Assessing Aviation's Global Emissions (SAGE)
3-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
database. Estimates for 1990-1999 were calculated using fuel
consumption estimates from the Bureau of Transportation
Statistics (DOT 1991 through 2007) adjusted based on
the 2000-2005 data. For 2006, an estimate—similar to
the method used for 1990-1999—was derived using a
combination of data from BTS and SAGE data. Class II and
III railroad diesel use estimates are now obtained from the
American Short Line and Regional Railroad Association,
with new data for 2002, 2004, and 2006 (Whorton 2006
through 2007).
As a result of these changes, average estimates of CH4
and N2O emissions from mobile combustion were slightly
lower relative to the previous Inventory—showing a decrease
of nomorethan0.7TgCO2Eq. (2.0percent) each year—for
the period 1990 through 2000. Larger decreases in estimates
occurred for years 2002 to 2005 when comparing the current
inventory estimates with the previous Inventory's estimates.
The greatest decrease, 1.5 Tg CO2 Eq. (4 percent), occurs
with the 2005 N2O estimate. Estimates for the year 2001 are
the exception, as these estimates increased from the previous
Inventory's estimates by 0.07 Tg CO2 Eq. for CH4 and 0.15
Tg CO2 Eq. for N2O.
Planned Improvements
While the data used for this report represent the most
accurate information available, six areas have been identified
that could potentially be improved in the short-term given
available resources.
1. Develop new emission factors for non-road sources.
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 CLLj emissions factors for non-road equipment.
2. 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 database
contains detailed data on takeoffs and landings for each
calendar year starting in 1999, 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 and development of procedures to
develop comparable estimates for years prior to 1999.
The feasibility of this approach will be explored.
3. Develop improved estimates of domestic waterborne fuel
consumption. The inventory estimates for residual 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 should
provide a more accurate and comprehensive estimate
of residual 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.
4. Develop improved estimates of domestic aviation fuel
consumption. The inventory calculation of domestic
jet fuel consumption is derived by first estimating
international aviation bunker fuel consumption and
subtracting this value from EIA's estimate of total jet fuel
consumption. Aviation bunker fuel estimates involve a
number of uncertainties, including the lack of specific
data on the amount of total jet fuel consumption to
allocate to international bunkers. As mentioned above,
FAA's SAGE database contains detailed data of domestic
operations of aircraft and associated fuel consumption,
and could potentially be used for a direct calculation of
commercial aviation fuel consumption similar to the
bottom-up approaches currently used for transportation
diesel sources and on-road gasoline.
5. Improve the process of apportioning VMT by vehicle
type to each fuel type. The current inventory process for
estimating VMT by vehicle/fuel type category involves
apportioning VMT on the basis of fuel consumption.
While this is a reasonable simplification, this approach
implicitly assumes the same average fuel economy
for gasoline and diesel vehicles. A more accurate
apportionment of VMT by fuel type for light-duty trucks
Energy 3-35
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and medium/heavy-duty trucks could potentially be
developed using data on vehicle travel from the Vehicle
Inventory and Use Survey (U.S. Census Bureau 2000)
and other publications, or using VMT breakdowns by
vehicle/fuel type combinations from the MOBILE6 or
MOVES models.
6. 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
used directly in calculating mobile source emissions. As
MOVES goes through additional testing and refinement,
the use of MOVES will be further explored.
3.5. 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 CH^ emissions. All 120
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, 20 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 2006, 13 coal mines collected CK4 from
degasification systems and sold this gas to a pipeline, thus
reducing emissions to the atmosphere. In addition, one coal
mine used CH4 from its degasification system to heat mine
ventilation air on site. Two of the coal mines that sold gas
to pipelines also used CH4 to generate electricity or fuel a
thermal coal dryer. Surface coal mines also release CH^ as
the overburden is removed and the coal is exposed, but the
level of emissions is much lower than from underground
mines. 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 2006 were estimated to be
58.5 Tg CO2 Eq. (2,784 Gg), a decline of 30 percent since
1990 (see Table 3-26 and Table 3-27). Of this amount,
underground mines accounted for 61 percent, surface
Table 3-26: CH4 Emissions from Coal Mining (Tg C02 Eq.)
Activity
Underground Mining
Liberated
Recovered & Used
Surface Mining
Post-Mining (Underground)
Post-Mining (Surface)
Total
1990
62.3
67.9
(5.6)
12.0
7.7
2.0
84.1
1995
46.7
59.1
(12.4)
11.5
6.9
1.9
67.1
2000 2001 2002 2003 2004 2005 2006
39.4 38.2 35.5 36.0 38.1 35.2 35.9
55.0 55.5 54.7 53.0 53.2 52.3 54.6
(15.6) (17.2) (19.2) (17.0) (15.1) (17.1) (18.7)
12.3 13.2 12.8 12.4 12.9 13.3 14.0
6.7 6.8 6.4 6.4 6.6 6.4 6.3
2.0 2.1 2.1 2.0 2.1 2.2 2.3
60.4 60.3 56.8 56.9 59.8 57.1 58.5
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
Table 3-27: 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
2,816
(591)
548
330
89
3,193
2000
1,875
2,619
(744)
586
318
95
2,874
2001
1,820
2,641
(821)
627
325
102
2,874
2002
1,692
2,605
(913)
610
305
99
2,707
2003
1,715
2,522
(807)
592
306
96
2,709
2004
1,813
2,534
(721)
616
317
100
2,846
2005
1,675
2,491
(815)
633
306
103
2,717
2006
1,709
2,599
(891)
668
298
109
2,784
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
3-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
mines accounted for 24 percent, and post-mining emissions
accounted for 15 percent. The decline in CH^ 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 steadily increased.
Methodology
The methodology for estimating CH^ emissions from
coal mining consists of two parts. The first part involves
estimating CH4 emissions from underground mines. Because
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.
Undergro und mines. Total CH4 emitted from underground
mines was estimated as the sum of CFLj liberated from
ventilation systems and CH4 liberated by means of
degasification systems, minus CK4 recovered and used. The
Mine Safety and Heath Administration (MSHA) samples
CFLj emissions from ventilation systems for all mines with
detectable40 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 CH^ 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
collected by each of the twenty mines using these systems,
depending on available data. For example, some mines report
to EPA the amount of CF^ liberated from their degasification
systems. For mines that sell recovered CK4 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
2006, thirteen active coal mines sold recovered CH4 into
the local gas pipeline networks, while one coal mine used
recovered CH4 on site. 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. In the case
of Jim Walter Resources (JWR), the emissions avoided data
was taken from the 1605b reports that the mining company
has been filing with the Department of Energy (DOE) since
1991 as part of their Voluntary Reporting Program.
Surface Mines and Post-Mining Emissions. Surface
mining and post-mining CFLj emissions were estimated by
multiplying basin-specific coal production, obtained from the
Energy Information Administration's Annual Coal Report
(see Table 3-28) (EIA 2006), 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 CFLj 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
accounts for CF^ 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.
Table 3-28: Coal Production (Thousand Metric Tons)
Year Underground Surface
Total
1990
1995
384,250
359,477
546,818
577,638
931,068
937,115
40 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.
2000
2001
2002
2003
2004
2005
2006
338,173
345,305
324,219
320,047
333,449
334,404
325,703
635,592
676,142
667,619
651,251
674,551
691,460
728,459
973,765
1,021,446
991,838
971,297
1,008,000
1,025,864
1,054,162
Energy 3-37
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Uncertainty
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
CtLj 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 3-29. Coal
mining CH4 emissions in 2006 were estimated to be between
53.3 and 76.3 Tg CO2 Eq. at a 95 percent confidence level.
This indicates a range of 9 percent below to 30 percent above
the 2006 emission estimate of 58.5 Tg CO2 Eq.
Recalculations Discussion
In 2006, recalculations of emissions avoided at three JWR
coal mines in Alabama were performed as the mining company
provided copies of their 1605b reports that they had been
filing with DOE. These reports cover the years 1991 through
2005. The 2006 report has not yet been filed, however JWR
provided the 2006 data. In previous inventories, emissions
avoided calculations for any pre-drainage wells at JWR coal
mines were based on publicly-available data records from the
Alabama State Oil & Gas Board. Emission reductions were
calculated for pre-drainage wells located inside the mine plan
boundaries and were declared "shut-in" by the O&G Board.
The total production for a well was claimed in the year that
the well was shut-in and mined through.
Secondly, the gas content values assigned to each
coal basin in the surface mine emissions component of the
Inventory were changed to reflect recent work carried out
by U.S. EPA (EPA 2005). This change impacted the reported
emissions attributed to surface mining operations (active and
post mining) for the full time-series, resulting in increased
emission estimates for surface and post-surface mining
operations for all reported years.
Finally, the conversion factor used previously to convert
from mmcf of CH4 to Gg of CELj was 52,150. The conversion
factor used in the natural gas emission estimates is 51,921.
In order to ensure consistency of emission estimates across
the Inventory, the conversion factor for the active mines
emissions was changed to 51,921. This change impacted all
estimates for the values previously calculated and reported
for Gg of CH4 emitted, and for Tg of CO2 Eq. emitted. The
difference between these factors is 0.44 percent.
Table 3-29: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Coal Mining (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Coal Mining
CH,
58.5
53.3
76.3
-9%
+30%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
3-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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3.6. 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
CK4 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 2006, 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 2006, with only one closure in 2006. By 2006, gross
abandoned mine emissions declined to 6.8 Tg CO2 Eq. (see
Table 3-30 and Table 3-31). Gross emissions are reduced
by methane recovered and used at 20 mines, resulting in net
emissions in 2006 of 5.4 Tg CO2Eq.
Methodology
Estimating CF^ 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 CFLj 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
abandonment reflects the gas content of the coal, rate of coal
mining, and the flow capacity of the mine in much the same
Table 3-30: CH4 Emissions from Abandoned Coal Mines (Tg C02 Eq.)
Activity
Abandoned Underground Mines
Recovered & Used
Total
1990
6.0
0.0
6.0
1995
8.9
0.7
8.2
2000
8.9
1.5
7.4
2001
8.2
1.5
6.7
2002
7.7
1.6
6.2
2003
7.5
1.5
6.0
2004
7.3
1.5
5.8
2005
7.0
1.4
5.6
2006
6.8
1.4
5.4
Note: Totals may not sum due to independent rounding.
Table 3-31: CH4 Emissions from Abandoned Coal Mines (Gg)
Activity
Abandoned Underground Mines
Recovered & Used
Total
1990
288
0
288
1995
424
32
392
2000
422
72
350
2001
389
70
319
2002
368
75
293
2003
356
72
284
2004
347
71
276
2005
333
68
265
2006
322
65
257
Note: Totals may not sum due to independent rounding.
Energy 3-39
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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 CtLj 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). Arate-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:
where,
q = Gas rate at time t in mmcf/d
q; = Initial gas rate at time zero (t0) in million cubic
feet per day mmcfd
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
was not enough data to establish basin-specific equations as
was done with the vented, non-flooding mines (EPA 2003).
q = q^-DO
where,
q = Gas flow rate at time t in mcf/d
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 CFLj emissions. This
same relationship is assumed for abandoned mines. It was
determined that 441 abandoned mines closing after 1972
produced emissions greater than 100 mcfd when active.
Further, the status of 264 of the 441 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
3-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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, and hydrologic 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 emission rates during
the 1970s (EPA 2003).
Abandoned mines emission estimates are based on all
closed mines known to have active mine CK4 ventilation
emission rates greater than 100 mcfd at the time of
abandonment; a list by region is shown in Table 3-32. 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. CFLj degasification amounts were added to the
quantity of CH4 ventilated for the total CH4 liberation rate for
fifteen mines that closed between 1992 and 2006. 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 2006, emission totals were
downwardly adjusted to reflect abandoned mine CH4
emissions avoided from those mines. The emission estimates
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.
Uncertainty
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
Table 3-32: Number of Gassy Abandoned Mines Occurring in U.S. Basins Grouped by Class According to
Post-abandonment State
Basin
Central Appl.
Illinois
Northern Appl.
Warrior Basin
Western Basins
Total
Sealed
23
28
41
0
25
117
Vented
25
3
22
0
3
53
Flooded
48
14
16
14
2
94
Total Known
96
45
79
14
30
264
Unknown
111
25
32
0
9
177
Total Mines
207
70
111
14
39
441
Energy 3-41
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Table 3-33: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Abandoned Underground Coal Mines
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Abandoned Underground
Coal Mines CH4
5.4
4.5
6.4
-17%
+ 19%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
each mine, the methodology for 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 3-33. Abandoned coal
mines CK4 emissions in 2006 were estimated to be between
4.5 and 6.4 Tg CO2 Eq. at a 95 percent confidence level.
This indicates a range of 17 percent below to 19 percent
above the 2006 emission estimate of 5.4 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 +50
percent uncertainty.
3.7. 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 102.4Tg CO2
Eq. (4,877 Gg) of CF^ in 2006, an 18 percent decrease over
1990 emissions (see Table 3-34 and Table 3-35), and 28.5
Tg CO2 Eq. (28,504 Gg) of non-combustion CO2 in 2006,
a 15 percent decrease over 1990 emissions (see Table 3-36
and Table 3-37). Improvements in management practices and
technology, along with the replacement of older equipment,
have helped to stabilize emissions.
CH4 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
of the four major stages of the natural gas system. Each of
the stages is described and the different factors affecting CFLj
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 CK4 emissions. Flaring
emissions account for the majority of the non-combustion
CO2 emissions. Emissions from field production accounted
for approximately 27 percent of CFLj emissions and about
25 percent of non-combustion CO2 emissions from natural
gas systems in 2006.
3-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-34: CH4 Emissions from Natural Gas Systems (Tg C02 Eq.)a
Stage
Field Production
Processing
Transmission and Storage
Distribution
Total
1990
32.7
14.9
46.3
30.8
124.7
1995
37.2
14.9
45.8
30.1
128.1
2000
38.8
14.6
43.8
29.3
126.5
2001
41.5
14.7
40.7
28.5
125.3
2002
42.5
14.2
42.4
25.8
124.9
2003
40.1
13.6
42.8
26.9
123.3
2004
32.9
13.4
40.9
26.8
114.0
2005
25.0
11.8
38.5
27.2
102.5
2006
27.6
11.9
38.2
24.7
102.4
a Including CH4 emission reductions achieved by the Natural Gas STAR program and NESHAP regulations.
Note: Totals may not sum due to independent rounding.
Table 3-35: CH4 Emissions from Natural Gas Systems (Gg)a
Stage
Field Production
Processing
Transmission and Storage
Distribution
Total
1990
1,555
707
2,206
1,468
5,937
1995
1,774
711
2,181
1,432
6,098
2000
1,849
693
2,087
1,395
6,024
2001
1,976
699
1,936
1,356
5,968
2002
2,024
675
2,019
1,228
5,946
2003
1,909
646
2,039
1,279
5,874
2004
1,566
639
1,947
1,275
5,426
2005
1,190
560
1,834
1,296
4,880
2006
1,317
568
1,817
1,176
4,877
a Including CH4 emission reductions achieved by the Natural Gas STAR program and NESHAP regulations.
Note: Totals may not sum due to independent rounding.
Table 3-36: Non-combustion C02 Emissions from Natural Gas Systems (Tg C02 Eq.)
Stage
Field Production
Processing
Transmission and Storage
Distribution
Total
1990
5.9
27.8
0.1
+
33.7
1995
9.1
24.6
0.1
+
33.8
2000
6.0
23.3
0.1
+
29.4
2001
6.3
22.4
0.1
+
28.8
2002
6.5
23.1
0.1
+
29.6
2003
6.3
22.0
0.1
+
28.4
2004
6.2
21.8
0.1
+
28.1
2005
7.6
21.7
0.1
+
29.5
2006
7.2
21.2
0.1
+
28.5
Note: Totals may not sum due to independent rounding.
Table 3-37: Non-combustion C02 Emissions from Natural Gas Systems (Gg)
Stage
Field Production
Processing
Transmission and Storage
Distribution
Total
1990
5,876
27,752
58
42
33,729
1995
9,083
24,621
60
42
33,806
2000 2001 2002 2003 2004 2005 2006
5,956 6,307 6,463 6,342 6,242 7,627 7,203
23,333 22,387 23,066 22,002 21,780 21,736 21,204
60 59 61 61 61 60 59
41 40 40 40 40 39 37
29,390 28,793 29,629 28,445 28,122 29,462 28,504
Note: Totals may not sum due to independent rounding.
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 CtLj 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 12
percent of CK4 emissions and approximately 74 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
Energy 3-43
-------
power plants or chemical plants. Compressor station facilities,
which contain large reciprocating and turbine compres sors , 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
j 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. CH4
emissions from the transmission and storage sector account
for approximately 37 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 1,250,000 miles of distribution
mains in 2006, an increase from just over 947,000 miles in
1990 (OPS 2007b). Distribution system emissions, which
account for approximately 24 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.41 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 2006 were 20 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/GPJ 1996). The EPA/GPJ 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
41 The percentages of total emissions from each stage may not sum to 100
percent due to independent rounding.
factors were used to estimate both CFL, and non-combustion
CO2 emissions. However, the CFL, emission factors were
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. 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 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 through
1998); Minerals and Management Service (MMS 2007a-e);
Monthly Energy Review (ElA 20071); Natural Gas Liquids
Reserves Report (EIA 2005); Natural Gas Monthly (EIA
2007b,c,e); the Natural Gas STAR Program annual emissions
savings (EPA 2007); Oil and Gas Journal (OGJ 1997 through
2007); Office of Pipeline Safety (OPS 2007a-b) and other
Energy Information Administration publications (EIA 2001,
2004, 2007a,d); World Oil Magazine (2007a-b). Data for
estimating emissions from hydrocarbon production tanks is
incorporated (EPA 1999). Coalbed CH4 well activity factors
were taken from the Wyoming Oil and Gas Conservation
Commission (Wyoming 2007) and the Alabama State Oil
and Gas Board (Alabama 2007). Other state well data was
taken from: American Association of Petroleum Geologists
(AAPG 2004); Brookhaven College (Brookhaven 2004);
Kansas Geological Survey (Kansas 2007); Montana Board
of Oil and Gas Conservation (Montana 2007); Oklahoma
Geological Survey (Oklahoma 2007); Morgan Stanley
(Morgan Stanley 2005); Rocky Mountain Production Report
(Lippman (2003); New Mexico Oil Conservation Division
(New Mexico 2007a,b); Texas Railroad Commission (Texas
2007a-d); Utah Division of Oil, Gas and Mining (Utah 2007).
Emission factors were taken from EPA/GRI (1996). GTI's
Unconventional Natural Gas and Gas Composition Databases
(GTI2001) were used to adapt the CK4 emission factors into
non-combustion related CO2 emission factors. Additional
3-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
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 results presented
below provide with 95 percent certainty the range within
which emissions from this source category are likely to fall.
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 is summarized in Table 3-38.
Natural gas systems CK4 emissions in 2006 were estimated
to be between 79.1 and 148.4 Tg CO2 Eq. at a 95 percent
confidence level. Natural gas systems non-combustion CO2
emissions in 2006 were estimated to be between 22.0 and
41.3 Tg CO2 Eq. at 95 percent confidence level.
Recalculations Discussion
Offshore oil and gas platform counts are driven by the
percentage of total platforms that are located on oil and gas
fields, respectively, as identified by MMS. This percentage
can be calculated from MMS (2007a,b,e) for years 2003
onward. For year 1992, the estimate was provided by MMS
by direct communication. The oil platform count for years
1990, 1991, and 1993 through 2002 are driven by a linear
projection based on known platform counts in 1992 and
2003. A miscalculation in year 2003 for oil platform count
was re-estimated, which caused the entire time series prior
to year 2003 (except 1992) to change. This change resulted
in a reallocation of platform counts to the natural gas and
petroleum models. The total number of offshore platforms,
both oil and gas, used as drivers remained the same. The
effects of this recalculation are most significant in 1990,
with an absolute difference of 31 Gg; less significant changes
occurred in all subsequent years.
The second recalculation is a result of changing several
base year (1992) activity factor and emissions factor data
to the exact values from the EPA/GPJ 1996 report. These
changes were small and the effects were an increase of less
than 1 percent in 1992 CH4 emissions.
A third recalculation is the result of updating previous
years' activity and Natural Gas STAR reduction values
with revised data. This is especially evident in 2005, where
the revised reductions data reports an additional 443 Gg of
reductions totaled from all four sectors; decreasing total 2005
emission estimates by nearly 9.5 percent.
QA/QC and Verification Discussion
ATier 2 QA/QC analysis was undertaken to examine why
emissions from small reciprocating compressors are lower
in the production sector than in other sectors of the natural
gas industry. The emission factor for these compressors is
based on EPA/GPJ 1996. Background information from
EPA/GPJ 1996, along with information from the Natural Gas
STAR program was analyzed and it was determined that the
Table 3-38: Tier 2 Quantitative Uncertainty Estimates for CH4 and Non-Combustion C02 Emissions from Natural Gas
Systems (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Natural Gas Systems
Natural Gas Systems"
CH4
C02
102.4
28.5
Lower Bound
79.1
22.0
Upper Bound
148.4
41.3
Lower Bound
-23%
-23%
Upper Bound
+45%
+45%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
b An uncertainty analysis for the non-combustion 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-combustion C02 emissions.
Energy 3-45
-------
emission factor for small compressors in the eastern United
States (U.S. East) was significantly lower than the emission
factor developed for the western area of the country (U.S.
West). Details of the emission factor development revealed
that the U.S. East emission factor in EPA/GRI1996 does not
include fugitives from compressor seals and pressure relief
valves. Experience from the Natural Gas STAR Program
demonstrates that seal leakage from rod packing is the largest
source of fugitive emissions from reciprocating compressors.
To account for compressor seal leakage, the U.S. West
emission factor was used for the entire United States for 1990
through 2006. These updated emission factors are an interim
improvement and further research is underway to compare
these updated emissions factors with recent vendor data.
Planned Improvements
Currently, activity factors for most sources in the natural
gas inventory are dependent on EPA/GRI 1996 estimates of
activity data in base year 1992. The activity factors for all
years other than the base year are estimated from the base
year activity data and are driven by an appropriate activity
driver. However, in some instances activity data are directly
available from published sources and there is no need to
derive the current year activity data through the use of drivers.
Research is underway to determine the feasibility of using
published activity data, where available, and whether this
would impact any of the emission factors currently used.
Separately, work has been initiated to update select
emission factors from the earlier study. Where relevant, these
emission factors will be incorporated into the Inventory when
they become available.
As noted above, additional research will be undertaken
to evaluate, and as necessary refine, the emission factor for
small reciprocating compressors in the U.S. West region.
3.8. Petroleum Systems (IPCC
Source Category 1B2a)
CH4 emissions from petroleum systems are primarily
associated with crude oil production, transportation,
and refining operations. During each of these activities,
CtLj 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 are negligible in
the transportation and refining operations. Combusted CO2
emissions are already accounted for in the Fossil Fuels
Combustion calculations previously discussed, and hence
have not been taken into account in this Inventory. Total CFLj
and CO2 emissions from petroleum systems in 2006 were
28.4 Tg CO2 Eq. (1,354 Gg CFLO and 0.3 Tg CO2 (293 Gg)
respectively. Since 1990, CH4 emissions have declined by
16 percent, due to industry efforts to reduce emissions and
a decline in domestic oil production (see Table 3-39, Table
3-40, Table 3-41, and Table 3-42). CO2 emissions have also
declined by 22 percent since 1990 due to similar reasons.
Production Field Operations. Production field operations
account for over 97 percent of total CH4 emissions from
petroleum systems. Vented CFLj from field operations account
for over 91 percent of the emissions from the production
sector, unburned CFLj combustion emissions account for 5.2
percent, fugitive emissions are 3.4 percent, and process upset
emissions, slightly over one-tenth of a percent. The most
dominant sources of emissions, in the 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 over 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 CFLj 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 methane emissions in production field operations
refer to Annex 3.5.
3-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-39: CH4 Emissions from Petroleum Systems (Tg C02 Eq.)
Activity
Production Field Operations
Pneumatic device venting
Tank venting
Combustion & process upsets
Misc. venting & fugitives
Wellhead fugitives
Crude Oil Transportation
Refining
Total
1990
33.2
10.3
3.8
1.9
16.8
0.5
0.1
0.5
33.9
1995
31.3
9.7
3.4
1.7
16.0
0.5
0.1
0.5
32.0
2000
29.6
9.0
3.2
1.6
15.3
0.5
0.1
0.6
30.3
2001
29.5
8.9
3.2
1.6
15.3
0.5
0.1
0.6
30.2
2002
29.2
8.9
3.2
1.6
15.1
0.5
0.1
0.6
29.9
2003
28.5
8.7
3.1
1.5
14.7
0.5
0.1
0.6
29.2
2004
28.0
8.6
3.0
1.5
14.5
0.4
0.1
0.6
28.7
2005
27.6
8.3
2.8
1.5
14.5
0.4
0.1
0.6
28.3
2006
27.7
8.4
2.9
1.5
14.5
0.4
0.1
0.6
28.4
Note: Totals may not sum due to independent rounding.
Table 3-40: CH4 Emissions from Petroleum Systems (Gg)
Activity
Production Field Operations
Pneumatic device venting
Tank venting
Combustion & process upsets
Misc. venting & fugitives
Wellhead fugitives
Crude Oil Transportation
Refining
Total
1990
1,581
489
179
88
799
26
7
25
1,612
1995
1,493
463
161
82
762
25
6
25
1,524
2000
1,409
428
154
76
728
22
5
28
1,442
2001
1,404
425
154
75
727
22
5
27
1,436
2002
1,390
424
151
75
717
23
5
27
1,422
2003
1,357
412
150
73
701
22
5
27
1,390
2004
1,335
408
142
72
692
21
5
28
1,368
2005
1,314
397
135
71
691
20
5
28
1,346
2006
1,321
399
138
72
693
20
5
28
1,354
Note: Totals may not sum due to independent rounding.
Table 3-41: C02 Emissions from Petroleum Systems (Tg C02 Eq.)
Activity
Production Field Operations
Pneumatic device venting
Tank venting
Misc. venting & fugitives
Wellhead fugitives
Total
1990
0.4
+
0.3
+
+
0.4
1995
0.3
+
0.3
+
+
0.3
2000
0.3
+
0.3
+
+
0.3
2001
0.3
+
0.3
+
+
0.3
2002
0.3
+
0.3
+
+
0.3
2003
0.3
+
0.3
+
+
0.3
2004
0.3
+
0.3
+
+
0.3
2005
0.3
+
0.2
+
+
0.3
2006
0.3
+
0.3
+
+
0.3
+ Does not exceed 0.05 Tg C02 Eq.
Table 3-42: C02 Emissions from Petroleum Systems (Gg)
Activity
Production Field Operations
Pneumatic device venting
Tank venting
Misc. venting & fugitives
Wellhead fugitives
Total
1990
376
27
328
18
1
376
1995
341
26
296
18
1
341
2000
325
24
283
17
1
325
2001
325
24
283
17
1
325
2002
320
24
278
16
1
320
2003
316
23
276
16
1
316
2004
302
23
262
16
1
302
2005
287
22
248
16
1
287
2006
293
22
253
16
1
293
Note: Totals may not sum due to independent rounding.
Energy 3-47
-------
Vented CO2 associated with natural gas emissions
from field operations account for 99 percent of the total
CO2 emissions, while fugitive and process upsets together
account for 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 98.5 percent of the
non-combustion CO2 emissions while the remaining 1.5
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 64 percent
of CH4 emissions from crude oil transportation. Fugitive
emissions, almost entirely from floating roof tanks, account
for 19 percent. The remaining 17 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 over 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 CtLj in all refined products. Within refineries, vented
emissions account for about 87 percent of the emissions,
while fugitive and combustion emissions account for
approximately six and seven percent, respectively. Refinery
system blow downs for maintenance and the process of asphalt
blowing—with air, to harden the asphalt—are the primary
venting contributors. Most of the fugitive CFLj 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.
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) and activity factors that are based
on two EPA studies (1996,1999) andEPA(2005). 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). The report provides emission factors
and activity 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 2006. 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 2007a-c).
For oil storage tanks, the emission factor was calculated
from API TankCalc data as the total emissions per barrel of
crude charge (EPA 1999).
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. 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 runs.
3-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Activity factors for years 1990 through 2006 were
collected from a wide variety of statistical resources. 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 CFLj 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 2006 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.
Nearly all emission factors were taken from EPA (1995,
1996, 1999). The remaining emission factors were taken
from EPA default values in (EPA 2005) and the consensus
of industry peer review panels.
Among the more important references used to obtain
activity factors are the Energy Information Administration
annual and monthly reports (EIA 1990 through 2006, 1990
through 2007, 1995 through 2007a-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 2001,
2007a-c), analysis of MMS data (EPA 2005, MMS 2004),
the Oil & Gas Journal (OGJ 2007a,b), the Interstate Oil and
Gas Compact Commission (IOGCC 2007), and the United
States Army Corps of Engineers (1995-2005).
Uncertainty
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 with
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 93.0 percent of
the total 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 3-43. Petroleum systems CFL,
emissions in 2006 were estimated to be between 20.5 and
69.3 Tg CO2 Eq., while CO2 emissions were estimated to be
Table 3-43: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petroleum Systems
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Petroleum Systems
Petroleum Systems
CH4
C02
28.4
0.3
Lower Bound
20.5
0.2
Upper Bound
69.3
0.7
Lower Bound
-28%
-28%
Upper Bound
+ 144%
+ 144%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Energy 3-49
-------
between 0.2 and 0.7 Tg CO2 Eq. at a 95 percent confidence 2 percent or less; while between 1993 and 2004 the total
level. This indicates a range of 28 percent below to 144 emission estimates increased by up to 13 percent from the
percent above the 2006 emission estimates of 28.4 and 0.3 previous inventory's estimates. This increase is largely due
Tg CO2 Eq. for CK4 and CO2, respectively. to the recalculation of the oil platform activity driver.
Recalculations Discussion
Estimates of vented, fugitive, and process upset CO2
emissions from the production sector were incorporated into
the petroleum systems emission estimates for 1990 through
2006. CO2 emissions were estimated using the methane
emission sources' activity factors and drivers for the
corresponding CO2 emission sources. The emission factors
for CO2 were estimated by multiplying the methane emission
factors by a conversion factor, which is the ratio of CO2
content divided by CH4 content in produced associated gas.
The only exceptions to this methodology are the emission
factors for crude oil storage tanks, which were estimated
from API TankCalc simulation runs. CO2 emissions from
the production sector account for 293 Gg of CO2 in the year
2006. CO2 emissions from the transportation and refining
sectors are assumed to be negligible. Combustion emissions
are already accounted for in the Fossils Fuels Combustion
section of this chapter.
In addition, two types of activity factor and seven types of
activity driver revisions were made for the current Inventory.
All revisions but one were due to updating previous years' data
with revised data from existing data sources. The one exception
to the general revisions was the recalculation of an activity
driver for oil platforms. Offshore oil and gas platform counts
are driven by the percentage of total platforms that are located
on oil and gas fields, respectively, as identified by MMS. This
percentage can be calculated from MMS (2007a-c) for years
2003 onward. For year 1992, the estimate was provided by
MMS by direct communication. The oil platform counts for
years 1990,1991, and 1993 through 2002 are driven by a linear
projection based on known platform counts in 1992 and 2003.
A miscalculation in year 2003 was re-estimated, causing the
entire time series prior to year 2003 (except 1992) to change.
This change resulted in a reallocation of platform counts to
the natural gas and petroleum models. The total number of
offshore platforms, both oil and gas, used as drivers remained
the same.
Overall changes resulted in a decrease in total emissions
of approximately 0.22 Tg CO2 Eq. (0.8 percent) for year
2005. For 1990 and 1991, total emissions decreased by
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 in (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.
3.9. Municipal Solid Waste
Combustion (IPCC Source Category
1A5)
Combustion is used to manage about 7 to 17 percent
of the municipal solid wastes (MSW) 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 2000b, Goldstein and Matdes 2001, Kaufman
et al. 2004a, Simmons et al. 2006). Almost all combustion
of municipal solid wastes in the United States occurs at
waste-to-energy facilities where useful energy is recovered,
and thus emissions from waste combustion are accounted
for in the Energy chapter. Combustion of municipal solid
wastes 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
combustion 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
3-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 report, 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
report, 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 39.0 Tg C02 (39,041
Gg C02) (see Table 3-44). 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 3-44: Potential Emissions from C02 Capture and Transport (Tg C02 Eq.)
Source
Acid Gas Removal Plants
Naturally Occurring C02
Ammonia Production Plants
Pipelines Transporting C02
Total
1990
4.8
15.1
0.0
0.0
20.0
1995
3.7
22.7
0.7
0.0
26.9
2000
2.3
23.1
0.7
0.0
26.1
2001
2.9
23.4
0.7
0.0
27.0
2002
2.9
23.0
0.7
0.0
26.6
2003
3.0
24.4
0.7
0.0
28.1
2004
3.7
27.0
0.7
0.0
31.4
2005
6.0
28.2
0.7
0.0
34.9
2006
7.0
31.4
0.7
0.0
39.0
Table 3-45: Potential Emissions from C02 Capture and Transport (Gg)
Source
Acid Gas Removal Plants
Naturally Occurring C02
Ammonia Production Plants
Pipelines Transporting C02
Total
1990
4,832
15,129
0
8
19,969
1995
3,672
22,547
676
8
26,904
2000 2001 2002 2003 2004 2005 2006
2,264 2,894 2,943 2,993 3,719 5,992 6,997
23,149 23,442 22,967 24,395 27,002 28,192 31,359
676 676 676 676 676 676 676
8888778
26,098 27,020 26,595 28,073 31,405 34,868 39,041
Energy 3-51
-------
municipal solid wastes are predominantly from clothing and
home furnishings. Tires (which contain rubber and carbon
black) are also considered a "non-hazardous" waste and are
included in the municipal solid waste combustion estimate,
though waste disposal practices for tires differ from the rest of
municipal solid waste (viz., most combustion occurs outside
of MSW combustion facilities).
Over 31 million metric tons of municipal solid wastes
were combusted in the United States in 2006 (EPA 2007).
CO2 emissions from combustion of municipal solid wastes
rose 91 percent since 1990, to an estimated 20.9 Tg CO2
Eq. (20,922 Gg) in 2006, as the volume of synthetic fibers
and other fossil C-containing materials in MSW increased
(see Table 3-46 and Table 3-47). Waste combustion is also
a source of N2O emissions (De Soete 1993). N2O emissions
from municipal solid waste combustion were estimated to be
0.4 Tg CO2 Eq. (1 Gg N2O) in 2006, and have not changed
significantly since 1990.
Methodology
Emissions of CO2 from MSW combustion include
CO2 generated by the combustion of plastics, synthetic
fibers, and synthetic rubber, as well as the combustion of
synthetic rubber and carbon black in tires. These emissions
were estimated by multiplying the amount of each material
combusted by the C content of the material and the fraction
oxidized (98 percent). Plastics combusted 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 number of scrap tires used for fuel and the synthetic
rubber and carbon black content of the tires.
More detail on the methodology for calculating
emissions from each of these waste combustion sources is
provided in Annex 3.6.
For each of the methods used to calculate CO2 emissions
from municipal solid waste combustion, 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 material in municipal solid wastes and its
Table 3-46: C02 and N20 Emissions from Municipal Solid Waste Combustion (Tg C02 Eq.)
Gas/Waste Product
C02
Plastics
Synthetic Rubber in Tires
Carbon Black in Tires
Synthetic Rubber in MSW
Synthetic Fibers
N20
Total
1990
10.9
8.0
0.2
0.2
1.3
1.2
0.5
11.4
1995
15.7
10.3
0.8
1.1
1.6
1.8
0.5
16.2
2000 2001 2002 2003 2004 2005 2006
17.5 18.0 18.5 19.1 20.1 20.7 20.9
11.8 12.1 12.3 12.7 13.4 13.7 13.7
0.9 0.9 1.0 1.0 1.1 1.2 1.2
1.2 1.2 1.2 1.3 1.4 1.6 1.6
1.6 1.7 1.8 1.8 1.9 1.9 1.9
2.0 2.1 2.2 2.3 2.3 2.4 2.5
0.4 0.4 0.4 0.4 0.4 0.4 0.4
17.9 18.4 18.9 19.5 20.5 21.1 21.3
Note: Totals may not sum due to independent rounding.
Table 3-47: C02 and N20 Emissions from Municipal Solid Waste Combustion (Gg)
Gas/Waste Product
C02
Plastics
Synthetic Rubber in Tires
Carbon Black in Tires
Synthetic Rubber in MSW
Synthetic Fibers
N20
1990
10,950
7,976
191
249
1,334
1,200
2
1995
15,712
10,347
841
1,099
1,596
1,830
1
2000
17,518
11,791
893
1,167
1,640
2,027
1
2001 2002 2003 2004 2005 2006
17,971 18,458 19,058 20,097 20,673 20,922
12,094 12,316 12,657 13,356 13,662 13,746
895 952 1,010 1,108 1,207 1,207
1,170 1,245 1,320 1,449 1,579 1,579
1,721 1,760 1,815 1,871 1,873 1,902
2,090 2,185 2,257 2,312 2,352 2,489
111111
Note: Totals may not sum due to independent rounding.
3-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
portion combusted were taken from the Characterization
of Municipal Solid Waste in the United States (EPA 2000b,
2002, 2003, 2005a, 2006b, 2007) and detailed unpublished
backup data for some years not shown in the reports
(Schneider 2007). For synthetic rubber and carbon black in
scrap tires, information was obtained from U.S. Scrap Tire
Markets in the United States 2005 Edition (RMA 2006) and
Scrap Tires, Facts and Figures (STMC 2000 through 2003,
2006). For 2006, synthetic rubber data is set equal to 2005
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 Scrap Tire
Management Council's internet site (STMC 2006).
The assumption that 98 percent of organic C is oxidized
(which applies to all municipal solid waste combustion
categories for CO2 emissions) was reported in EPA's life
cycle analysis of greenhouse gas emissions and sinks from
management of solid waste (EPA 2006a).
Combustion of municipal solid waste also results in
emissions of N2O. These emissions were calculated as a
function of the total estimated mass of municipal solid waste
combusted and an emission factor. The N2O emission estimates
are based on different data sources than the CO2 emission
estimates. As noted above, N2O emissions are a function of
total waste combusted in each year; for 1990 through 2004,
these data were derived from the information published
Table 3-48: Municipal Solid Waste Generation (Metric
Tons) and Percent Combusted
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Waste Generation
266,365,714
296,390,405
371,071,109
353,086,962a
335,102,816
343,482,645"
351,862,474
351,862,474C
351,86,2474C
Combusted (%)
11.5
10.0
7.0
7.4a
7.7
7.6b
7.4
7.4C
7.4C
aInterpolated between 2000 and 2002 values.
b Interpolated between 2002 and 2004 values.
c Assumed equal to 2004 value.
in BioCycle (Simmons et al. 2006). Data on total waste
combusted was not available for 2005 and 2006, so the values
for 2005 and 2006 were assumed to equal the most recent value
available (2004). Table 3-48 provides data on municipal solid
waste generation and percentage combustion for the total waste
stream. The emission factor of N2O emissions per quantity of
municipal solid waste combusted is an average of values from
IPCC's Good Practice Guidance (2000).
Uncertainty
A Tier 2 Monte Carlo analysis was performed to determine
the level of uncertainty surrounding the estimates of CO2
emissions and N2O emissions from municipal solid waste
combustion. IPCC Tier 2 analysis allows the specification
of probability density functions for key variables within a
computational structure that mirrors 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 combustion emission
estimates arise from both the assumptions applied to
the data and from the quality of the data. Key factors
include MSW combustion rate; fraction oxidized; missing
data on MSW composition; average C content of MSW
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 3-49. Municipal solid waste
combustion CO2 emissions in 2006 were estimated to be
between 16.8 and 23.7 Tg CO2 Eq. at a 95 percent confidence
level. This indicates a range of 20 percent below to 13
percent above the 2006 emission estimate of 20.9 Tg CO2
Energy 3-53
-------
Table 3-49: Tier 2 Quantitative Uncertainty Estimates for C02 and N20 from Municipal Solid Waste Combustion
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Municipal Solid Waste Combustion C02 20.9
Municipal Solid Waste Combustion N20 0.4
16.8
0.1
23.7
1.1
-20%
-66%
+ 13%
+ 184%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Eq. Also at a 95 percent confidence level, municipal solid
waste combustion N2O emissions in 2006 were estimated to
be between 0.1 and 1.1 Tg CO2 Eq. This indicates a range
of 66 percent below to 184 percent above the 2006 emission
estimate of 0.4 Tg CO2 Eq.
QA/QC and Verification
A source-specific QA/QC plan was implemented for
MSW Combustion. 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 specifically focused on the emission
factor and activity data sources and methodology used for
estimating emissions from MSW combustion. Trends across
the time series were analyzed to determine whether any
corrective actions were needed.
Planned Improvements
Additional data sources for calculating an N2O emission
factor for U.S. MSW combustion will be investigated for
future drafts. In addition, the use of new techniques using
radiocarbon dating to directly measure biogenic C content
of MSW combustion flue gas will also be investigated.
Additional data sources for calculating an N2O emission
factor for U.S. MSW combustion will be investigated for
future drafts. In addition, the use of new techniques using
radiocarbon dating to directly measure biogenic C content
of MSW combustion flue gas will also be investigated.
Furthermore, efforts have been initiated to reconcile
differences in the separate data sources used for the CO2 and
N2O emission calculations
3.10. 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-CELj volatile organic
compounds (NMVOCs) from energy-related activities from
1990 to 2006 are reported in Table 3-50.
Methodology
These emission estimates were obtained from
preliminary data (EPA 2008), 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
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
3-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 3-50: NO,, CO, and NMVOC Emissions from Energy-Related Activities (Gg)
Gas/Source
NO,
Mobile Combustion
Stationary Combustion
Oil and Gas Activities
Municipal Solid Waste Combustion
International Bunker Fuels3
CO
Mobile Combustion
Stationary Combustion
Municipal Solid Waste Combustion
Oil and Gas Activities
International Bunker Fuels3
NMVOCs
Mobile Combustion
Stationary Combustion
Oil and Gas Activities
Municipal Solid Waste Combustion
International Bunker Fuels3
1990
21,024
10,920
9,883
139
82
1,985
125,640
119,360
5,000
302
978
115
12,620
10,932
912
554
222
59
1995
20,631
10,622
9,821
100
88
1,540
104,402
97,630
5,383
316
1,073
113
10,538
8,745
973
582
237
48
2000 2001 2002 2003 2004 2005 2006
18,537 17,714 17,364 16,474 15,607 15,005 14,309
10,310 9,819 10,154 9,642 9,191 8,739 8,287
8,002 7,667 6,791 6,419 6,004 5,853 5,610
111 113 321 316 316 316 315
114 114 98 97 97 97 97
1,334 1,266 988 900 1,190 1,190 1,731
89,715 86,046 82,148 77,689 73,731 69,773 65,815
83,559 79,851 75,421 71,038 67,096 63,154 59,213
4,340 4,377 4,965 4,893 4,876 4,860 4,844
146 147 323 321 321 321 322
1,670 1,672 1,439 1,437 1,437 1,437 1,437
124 120 118 112 128 133 150
8,953 8,610 9,608 9,223 8,910 8,597 8,284
7,230 6,872 7,235 6,885 6,587 6,289 5,991
1,077 1,080 1,585 1,560 1,553 1,545 1,538
389 400 545 538 533 528 523
257 258 243 239 237 235 232
44 42 35 32 41 41 56
a These values are presented for informational purposes only and are not included in totals.
Note: Totals may not sum due to independent rounding.
National Acid Precipitation and Assessment Program
emissions inventory, and other EPA databases.
Uncertainty
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.
3.11. 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.42 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)43
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.44 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.
42 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).
43 Note that the definition of international bunker fuels used by the UNFCCC
differs from that used by the International Civil Aviation Organization.
44 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).
Energy 3-55
-------
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.45
Emissions of CO2 from aircraft are essentially a function
of fuel use. CH4 and N2O emissions also depend upon engine
characteristics, flight conditions, and flight phase (i.e., take-
off, climb, cruise, decent, and landing). CH4 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. CO2 is the primary greenhouse gas emitted
from marine shipping.
Overall, aggregate greenhouse gas emissions in 2006
from the combustion of international bunker fuels from
both aviation and marine activities were 128.4 Tg CO2 Eq.,
or 12 percent above emissions in 1990 (see Table 3-51 and
Table 3-52). Although emissions from international flights
departing from the United States have increased significantly
(56 percent), emissions from international shipping voyages
45 Naphtha-type jet fuel was used in the past by the military in turbojet and
turboprop aircraft engines.
departing the United States have decreased by 18 percent
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 (2007) and
USAF (1998), and heat content for jet fuel was taken from
EIA (2007). 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 were collected
from three government agencies. Jet fuel consumed by U.S.
flag air carriers for international flight segments was supplied
by the Bureau of Transportation Statistics (DOT 1991 through
2006). It was assumed that 50 percent of the fuel used by U.S.
flagged carriers for international flights —both departing and
arriving in the United States—was purchased domestically
for flights departing from the United States. In other words,
3-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Table 3-51: C02, CH4, and N20 Emissions from International Bunker Fuels (Tg C02 Eq.)
Gas/Mode
C02
Aviation
Marine
CH4
Aviation
Marine
N20
Aviation
Marine
Total
1990
113.7
45.7
68.0
0.2
+
0.1
1.0
0.5
0.5
114.8
1995
100.6
50.2
50.4
0.1
+
0.1
0.9
0.5
0.4
101.6
2000
101.1
59.9
41.3
0.1
+
0.1
0.9
0.6
0.3
102.2
2001 2002 2003 2004 2005 2006
97.6 89.1 103.6 119.0 122.6 127.1
58.7 61.1 58.8 64.9 67.5 71.1
38.9 28.0 44.8 54.1 55.1 56.0
0.1 0.1 0.1 0.1 0.1 0.2
+ + + + + +
0.1 0.1 0.1 0.1 0.1 0.1
0.9 0.8 0.8 1.1 1.1 1.1
0.6 0.6 0.6 0.6 0.7 0.7
0.3 0.2 0.3 0.4 0.4 0.4
98.6 90.0 104.6 120.2 123.8 128.4
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions.
Table 3-52: C02, CH4 and N20 Emissions from International Bunker Fuels (Gg)
Gas/Mode
C02
Aviation
Marine
CH4
Aviation
Marine
N20
Aviation
Marine
1990
113,683
45,731
67,952
8
1
7
3
1
2
1995
100,627
50,202
50,425
6
1
5
3
2
1
2000 2001 2002 2003 2004 2005 2006
101,125 97,563 89,101 103,583 118,975 122,580 127,097
59,853 58,696 61,120 58,806 64,891 67,517 71,141
41,272 38,866 27,981 44,777 54,084 55,063 55,956
6546777
2222222
4434555
3333344
2222222
1111111
Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions.
only one-half of the total annual fuel consumption estimate
was used in the calculations. Data on jet fuel expenditures
by foreign flagged carriers departing U.S. airports was taken
from unpublished data collected by the Bureau of Economic
Analysis (BEA) under the U.S. Department of Commerce
(BEA 1991 through 2006). Approximate average fuel prices
paid by air carriers for aircraft on international flights was
taken from DOT (1991 through 2006) and used to convert
the BEA expenditure data to gallons of fuel consumed. Data
on U. S. Department of Defense (DoD) aviation bunker fuels
and total jet fuel consumed by the U. S. military was supplied
by the Office of the Under Secretary of Defense (Installations
and Environment), DoD. Estimates of the percentage of each
Service's total operations that were international operations
were developed by DoD. Military aviation bunkers included
international operations, operations conducted from
naval vessels at sea, and operations conducted from U.S.
installations principally over international water in direct
support of military operations at sea. Military aviation
bunker fuel emissions were estimated using military fuel
and operations data synthesized from unpublished data by
the Defense Energy Support Center, under DoD's Defense
Logistics Agency (DESC 2007). Together, the data allow the
quantity of fuel used in military international operations to
be estimated. Densities for each jet fuel type were obtained
from a report from the U.S. Air Force (USAF 1998). Final jet
fuel consumption estimates are presented in Table 3-53. See
Annex 3.7 for additional discussion of military data.
Activity data on distillate diesel and residual fuel oil
consumption by cargo or passenger carrying marine vessels
departing from U.S. ports were taken from unpublished
data collected by the Foreign Trade Division of the U.S.
Department of Commerce's Bureau of the Census (DOC 1991
through 2007) for 1990 through 2002, and the Department of
Homeland Security's Bunker Report for 2003 through 2006
(DHS 2008). Activity data on distillate diesel consumption
Energy 3-57
-------
Table 3-53: Aviation Jet Fuel Consumption for International Transport (Million Gallons)
Nationality
U.S. Carriers
Foreign Carriers
U.S. Military
Total
1990
1,954
2,051
862
4,867
1995
2,221
2,544
581
5,347
2000
2,737
3,162
480
6,380
2001
2,619
3,113
524
6,255
2002
2,495
3,537
482
6,515
2003
2,418
3,377
473
6,268
2004
2,466
3,953
498
6,917
2005
2,760
3,975
462
7,198
2006
2,914
4,272
400
7,586
Note: Totals may not sum due to independent rounding.
Table 3-54: 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
573
334
4,402
2000
2,967
290
329
3,586
2001
2,846
204
318
3,368
2002
1,937
158
348
2,443
2003
3,152
290
459
3,901
2004
3,695
505
530
4,730
2005
3,881
444
471
4,796
2006
4,004
446
414
4,864
Note: Totals may not sum due to independent rounding.
by military vessels departing from U.S. ports were provided
by DESC (2007). The total amount of 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 3-54.
Uncertainty
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.46
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
vary from port to port, leading to some tankering from ports
with low fuel costs.
Particularly for aviation, the DOT (1991 through
2007) international flight segment fuel data used for U.S.
flagged carriers does not include smaller air carriers and
unfortunately defines flights departing to Canada and some
flights to Mexico as domestic instead of international. As
for the BEA (1991 through 2007) data on foreign flagged
carriers, there is some uncertainty as to the average fuel price,
and to the completeness of the data. It was also not possible
to determine what portion of fuel purchased by foreign
carriers at U.S. airports was actually used on domestic flight
segments; this error, however, is believed to be small.47
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
46 See uncertainty discussions under Carbon Dioxide Emissions from Fossil
Fuel Combustion.
47 Although foreign flagged air carriers are prevented from providing
domestic flight services in the United States, passengers may be collected
from multiple airports before an aircraft actually departs on its international
flight segment. Emissions from these earlier domestic flight segments should
be classified as domestic, not international, according to the IPCC.
3-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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.
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 1996IPCC 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 CO2.48
There is also concern as to the reliability of the existing
DOC (1991 through 2007) data on marine vessel fuel
consumption reported at U.S. customs stations due to the
significant degree of inter-annual variation.
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
Historical activity data for aviation was revised for both
U.S. and foreign carriers. In addition, distillate and residual
fuel oil consumption by cargo or passenger carrying marine
vessels from 2003 through 2006 was revised using DHS
(2008). These historical data changes resulted in changes
to the emission estimates for 1990 through 2005, which
averaged to an annual decrease in emissions relative to the
previous Inventory from international bunker fuels of 4.3 Tg
CO2 Eq. (4.7 percent) in CO2 emissions, an annual decrease
of less than 0.1 Tg CO2 Eq. (8 percent) in CK4 emissions,
and annual decrease of less than 0.1 Tg CO2 Eq. (4 percent)
in N,O emissions.
48U.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.
Energy 3-59
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3.12. Wood Biomass and Ethanol
Consumption (IPCC Source Category
1A)
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 2006, total CO2 emissions from the burning of
woody biomass in the industrial, residential, commercial,
and electricity generation sectors were approximately
204.4 Tg C02 Eq. (204,435 Gg) (see Table 3-55 and
Table 3-56). As the largest consumer of woody biomass,
the industrial sector was responsible for 67 percent of the
CO9 emissions from this source. The residential sector
was the second largest emitter, constituting 20 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 2006, the United States consumed an estimated
459 trillion Btu of ethanol, and as a result, produced
approximately 30.3 Tg CO2 Eq. (30,291 Gg) (see Table 3-57
and Table 3-58) 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.
Table 3-55: 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
13.3
215.2
1995
155.1
53.6
7.5
12.9
229.1
2000
153.6
43.3
7.4
13.9
218.1
2001
135.4
38.2
6.9
13.0
193.5
2002
131.1
39.2
7.1
15.5
192.8
2003
128.0
41.2
7.4
17.3
193.8
2004
138.5
42.3
7.3
17.0
205.1
2005
136.3
42.3
7.2
19.1
204.8
2006
137.9
40.2
6.7
19.6
204.4
Note: Totals may not sum due to independent rounding.
Table 3-56: 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 2000 2001 2002 2003 2004 2005 2006
155,075 153,559 135,415 131,079 127,970 138,522 136,269 137,929
53,621 43,309 38,153 39,184 41,247 42,278 42,278 40,215
7,463 7,370 6,887 7,080 7,366 7,252 7,191 6,685
12,932 13,851 13,034 15,487 17,250 17,034 19,704 19,606
229,091 218,088 193,489 192,830 193,833 205,086 204,812 204,435
Note: Totals may not sum due to independent rounding.
3-60 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Table 3-57: C02 Emissions from Ethanol Consumption (Tg C02 Eq.)
End-Use Sector
Transportation
Industrial
Commercial
Total
1990
4.1
0.1
+
4.2
1995
7.6
0.1
+
7.7
2000
9.1
0.1
+
9.2
2001
9.5
0.2
+
9.7
2002
11.3
0.2
+
11.5
2003
15.4
0.3
0.1
15.7
2004
19.3
0.4
0.1
19.7
2005
22.0
0.5
0.1
22.6
2006
29.6
0.6
0.1
30.3
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Table 3-58: 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
9,078
85
25
9,188
2001
9,479
172
22
9,673
2002
11,280
209
31
11,520
2003
15,353
296
55
15,704
2004
19,267
418
55
19,740
2005
22,014
478
62
22,554
2006
29,566
641
84
30,291
Note: Totals may not sum due to independent rounding.
Methodology
Woody biomass emissions were estimated by applying
two EIA gross heat contents (Lindstrom 2006) to U.S.
consumption data (EIA 2007) (see Table 3-59), 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 2007) (see Table 3-60).
Uncertainty
It is assumed that the combustion efficiency for
woody biomass is 100 percent, which is believed to be an
Table 3-59: Woody Biomass Consumption by Sector (Trillion Btu)
End-Use Sector
Industrial
Residential
Commercial
Electricity Generation
Total
1990
1,442
580
66
129
2,216
1995
1,652
520
72
125
2,370
2000
1,636
420
71
134
2,262
2001
1,443
370
67
126
2,006
2002
1,396
380
69
150
1,995
2003
1,363
400
71
167
2,002
2004
1,476
410
70
165
2,121
2005
1,452
410
70
185
2,116
2006
1,469
390
65
190
2,114
Table 3-60: Ethanol Consumption (Trillion Btu)
End-Use Sector
Transportation
Industrial
Commercial
Total
1990
61.7
0.8
0.5
63.0
1995
114.8
1.6
0.1
116.5
2000
137.7
1.3
0.4
139.3
2001
143.7
2.6
0.3
146.7
2002
171.0
3.2
0.5
174.7
2003
232.8
4.5
0.8
238.1
2004
292.1
6.3
0.8
299.3
2005
333.8
7.2
0.9
342.0
2006
448.3
9.7
1.3
459.3
Energy 3-61
-------
overestimate of the efficiency of wood combustion processes
in the United States. Decreasing the combustion efficiency
would increase emission estimates. Additionally, the heat
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.
Recalculations Discussion
Residential wood consumption values were revised in
1997, 1999, and 2000 based on updated information from
EIA's Annual Energy Review (EIA 2007). EIA (2007) also
reported minor changes in wood consumption for all sectors
in 2005. This adjustment of historical data for wood biomass
consumption resulted in an average annual increase in
emissions from wood biomass consumption of 1.1 Tg CO2
Eq. (0.6 percent) from 1990 through 2005. Industrial and
commercial sector ethanol consumption is now estimated in
EIA (2007), which slightly decreased estimates for ethanol
consumed in the transportation sector in all years. As a result
of these adjustments, average annual emissions from ethanol
consumption in the transportation sector decreased by 0.2
Tg CO2 Eq. (1.8 percent).
3-62 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
4. Industrial Processes
Figure 4-1
2006 Industrial Processes Chapter
Greenhouse Gas Emission Sources
Greenhouse gas emissions are produced as the byproducts 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, cement production, lime
production, ammonia production and urea consumption, limestone and dolomite use (e.g., flux stone, flue gas desulfurization,
and glass manufacturing), soda ash production and consumption, 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 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, and magnesium metal
production and processing.
In 2006, industrial processes generated emissions
of 320.9 teragrams of CO2 equivalents (Tg CO2 Eq.), or
5 percent of total U.S. greenhouse gas emissions. CO2
Substitution of Ozone Depleting Substances
Iron and Steel Production
Cement Production
Lime Production
Nitric Acid Production
HCFC-22 Production
Electrical Transmission and Distribution
Ammonia Production and Urea Consumption
Limestone and Dolomite Use
Aluminum Production
Adipic Acid Production
Semiconductor Manufacture
Soda Asb Production and Consumption
Petrocbemical Production
Magnesium Production and Processing
Titanium Dioxide Production |
Carbon Dioxide Consumption |
Ferroalloy Production |
Pbospboric Acid Production |
Zinc Production I
Lead Production
Silicon Carbide Production and Consumption
Industrial Processes
as a Portion of
all Emissions
<0.5
<0.5
0
50 75
TgCO,Eq.
100 125
Industrial Processes 4-1
-------
emissions from all industrial processes were 149.5 Tg CO2
Eq. (149,465 Gg) in 2006, or 2 percent of total U.S. CO2
emissions. CH4 emissions from industrial processes resulted
in emissions of approximately 2.0 Tg CO2 Eq. (94 Gg) in
2006, which was less than 1 percent of U.S. CH4 emissions.
N2O emissions from adipic acid and nitric acid production
were 21.6 Tg CO2 Eq. (70 Gg) in 2006, or 4 percent of total
U.S. N2O emissions. In 2006, combined emissions of HFCs,
PFCs and SF6 totaled 147.9 Tg CO2 Eq. Overall, emissions
from industrial processes increased by 7.0 percent from 1990
to 2006 despite decreases in emissions from several industrial
processes, such as iron and steel, 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 production
and, primarily, 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.
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 datais 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 2006 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 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.
4-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-1: Emissions from Industrial Processes (Tg C02 Eq.)
Gas/Source
C02
Iron and Steel Production
Cement Production
Lime Production
Ammonia Production & Urea Consumption
Limestone and Dolomite Use
Soda Ash Production and Consumption
Aluminum Production
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
Ferroalloy Production
Silicon Carbide Production and Consumption
N20
Nitric Acid Production
Adipic Acid Production
MFCs
Substitution of Ozone Depleting Substances3
HCFC-22 Production
Semiconductor Manufacture
PFCs
Semiconductor Manufacture
Aluminum Production
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture
Total
1990
175
86
33
12
16
5
4
6
2
1
1
2
1
0
0
0
2
0
1
32
17
15
36
0
36
0
20
2
18
32
26
5
0
299
0
2
3
0
9
5
1
8
2
2
4
2
5
9
3
4
2
9
3
•f
•f
3
0
3
9
3
4
2
8
2
5
7
7
4
5
9
1995
171.6
74.7
36.8
14.0
17.8
7.4
4.3
5.7
2.8
1.5
1.4
2.0
1.5
1.0
0.3
0.3
2.4
1.1
1.3
+
+
36.2
18.9
17.3
61.8
28.5
33.0
0.3
15.6
3.8
11.8
28.0
21.5
5.6
0.9
315.7
2000
166.5
66.6
41.2
14.9
16.4
6.0
4.2
6.1
3.0
1.8
1.4
1.9
1.4
1.1
0.3
0.2
2.5
1.2
1.2
+
+
24.8
18.6
6.2
100.1
71.2
28.6
0.3
13.5
4.9
8.6
19.1
15.1
3.0
1.1
326.5
2001
151
59
41
14
13
5
4
9
2
4
3
3
7
1
4.4
2
1
0
1
1
1
0
0
2
1
1
20
15
5
97
78
19
0
7
3
3
18
15
2
0
297
8
7
8
5
3
0
3
2
2
1
1
+
+
2
1
1
9
0
7
2
0
5
5
7
0
9
7
9
2002
151.0
55.9
42.9
13.7
14.2
5.9
4.1
4.5
2.9
1.8
1.0
1.3
1.3
0.9
0.3
0.2
2.1
1.1
1.0
+
+
22.4
16.4
6.1
106.3
85.0
21.1
0.2
8.7
3.5
5.2
18.0
14.4
2.9
0.7
308.6
2003
147.8
54.7
43.1
14.5
12.5
4.8
4.1
4.5
2.8
1.8
1.3
1.3
1.4
0.5
0.3
0.2
2.1
1.1
1.0
+
+
21.7
15.4
6.3
104.4
92.0
12.3
0.2
7.1
3.3
3.8
18.1
13.8
3.4
0.8
301.2
2004
151.8
52.8
45.6
15.2
13.2
6.7
4.2
4.2
2.9
2.1
1.2
1.4
1.4
0.5
0.3
0.2
2.2
1.2
1.0
+
+
21.2
15.2
5.9
116.6
99.1
17.2
0.2
6.1
3.3
2.8
18.0
13.9
3.2
0.8
315.9
2005
145.9
46.6
45.9
15.1
12.8
7.4
4.2
4.2
2.8
1.8
1.3
1.4
1.4
0.5
0.3
0.2
2.0
1.1
1.0
+
+
21.7
15.8
5.9
121.4
105.4
15.8
0.2
6.2
3.2
3.0
18.2
14.0
3.3
1.0
315.5
2006
149.5
49.1
45.7
15.8
12.4
8.6
4.2
3.9
2.6
1.9
1.6
1.5
1.2
0.5
0.3
0.2
2.0
1.0
0.9
+
+
21.6
15.6
5.9
124.5
110.4
13.8
0.3
6.1
3.6
2.5
17.3
13.2
3.2
1.0
320.9
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
a Small amounts of RFC emissions also result from this source.
Accordingly, the quantitative uncertainty estimates reported
in this section should be considered illustrative and as
iterations of ongoing efforts to produce accurate uncertainty
estimates. 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.
Industrial Processes 4-3
-------
Table 4-2: Emissions from Industrial Processes (Gg)
Gas/Source
C02
Iron and Steel Production
Cement Production
Lime Production
Ammonia Production & Urea Consumption
Limestone and Dolomite Use
Soda Ash Production and Consumption
Aluminum Production
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
Ferroalloy Production
Silicon Carbide Production and Consumption
N20
Nitric Acid Production
AdipicAcid Production
MFCs
Substitution of Ozone Depleting Substances3
HCFC-22 Production
Semiconductor Manufacture
PFCs
Semiconductor Manufacture
Aluminum Production
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture
1990
175,018
86,220
33,278
12,004
16,889
5,533
4,141
6,831
2,221
1,195
1,416
2,152
1,529
949
285
375
106
41
63
1
1
104
55
49
M
M
3
+
M
M
M
1
1
+
+
1995
171,600
74,729
36,847
14,019
17,796
7,359
4,304
5,659
2,750
1,526
1,422
2,036
1,513
1,013
298
329
116
52
62
1
1
117
61
56
M
M
3
+
M
M
M
1
1
+
+
2000 2001 2002 2003 2004 2005 2006
166,452 151,944 150,960 147,752 151,841 145,926 149,465
66,609 59,249 55,938 54,744 52,771 46,627 49,119
41,190 41,357 42,898 43,082 45,603 45,910 45,739
14,872 14,261 13,652 14,458 15,154 15,131 15,825
16,402 13,305 14,194 12,488 13,241 12,817 12,376
5,960 5,733 5,885 4,753 6,702 7,397 8,615
4,181 4,147 4,139 4,111 4,205 4,228 4,162
6,086 4,381 4,490 4,503 4,231 4,207 3,923
3,004 2,787 2,857 2,777 2,895 2,804 2,573
1,752 1,697 1,824 1,839 2,064 1,755 1,876
1,421 829 989 1,311 1,198 1,321 1,579
1,893 1,459 1,349 1,305 1,419 1,392 1,505
1,382 1,264 1,338 1,382 1,395 1,386 1,167
1,140 986 937 507 477 465 529
311 291 286 289 263 266 270
248 199 183 202 224 219 207
117 103 101 101 106 97 94
58 51 52 51 55 51 48
58 51 48 49 50 45 45
1 + + + + + +
1 + + + + + +
80 65 72 70 68 70 70
60 49 53 50 49 51 50
20 16 20 20 19 19 19
M M M M M M M
M M M M M M M
2221111
+ + + + + + +
M M M M M M M
M M M M M M M
M M M M M M M
1111111
1111111
+ + + + + + +
+ + + + + + +
+ Does not exceed 0.5 Gg
M (Mixture of gases)
Note: Totals may not sum due to independent rounding.
a Small amounts of RFC emissions also result from this source.
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. CO2 emitted from the chemical process of
cement production is the second largest source of industrial
CO2 emissions in the United States.
1 The CO2 emissions related to the consumption of energy for cement
production are accounted for under CO2 from Fossil Fuel Combustion in
the Energy chapter.
4-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
During the cement production proces s, 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 byproduct 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 2006, U.S. clinker production—including Puerto
Rico —totaled 88,453 thousand metric tons (van Oss
2007). The resulting emissions of CO2 from 2006 cement
production were estimated to be 45.7 Tg CO2 Eq. (45,739
Gg) (see Table 4-3).
After falling in 1991 by two percent from 1990 levels,
cement production emissions grew every year through 2005,
and then decreased slightly from 2005 to 2006. Overall, from
1990 to 2006, emissions increased by 37 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.
Table 4-3: C02 Emissions from Cement Production
(Tg C02 Eq. and Gg)
Methodology
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
33.3
36.8
41.2
41.4
42.9
43.1
45.6
45.9
45.7
Gg
33,278
36,847
41,190
41,357
42,898
43,082
45,603
45,910
45,739
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
Production source category (van Oss 2008).
CO2 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
CO2 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 2008) 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:
EFainker = 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 calculated
from clinker production. Total cement production emissions
were calculated by adding the emissions from clinker
production to the emissions assigned to CKD (IPCC 2006).3
The 1990 through 2006 activity data for clinker
production (see Table 4-4) were obtained through a personal
communication with Hendrik van Oss (van Oss 2007) of the
USGS and through the USGS Mineral Yearbook: Cement
(USGS 1993 through 2006). The data were compiled by
3 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 2008).
Industrial Processes 4-5
-------
Table 4-4: Clinker Production (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Clinker
64,355
71,257
79,656
79,979
82,959
83,315
88,190
88,783
88,453
USGS through questionnaires sent to domestic clinker and
cement production plants.
Uncertainty
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
that all calcium-containing raw material is 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 2008). 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 39.8 and 52.0 Tg
CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 13 percent below and 14 percent
above the emission estimate of 45.7 Tg CO2 Eq.
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. In certain
applications, lime reabsorbs CO2 during use.
Lime production in the United States—including Puerto
Rico—was reported to be 20,929 thousand metric tons in
2006 (USGS 2007). This resulted in estimated CO2 emissions
of 15.8 Tg CO2 Eq. (or 15,825 Gg) (see Table 4-6).
The contemporary lime market is distributed across five
end-use categories as follows: metallurgical uses, 36 percent;
environmental uses, 29 percent; chemical and industrial uses,
21 percent; construction uses, 13 percent; and refractory
dolomite, 1 percent. In the construction sector, lime is used
to improve durability in plaster, stucco, and mortars, as well
as to stabilize soils. In 2006, the amount of lime used for
Table 4-5: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Cement Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Cement Production
CO,
45.7
39.8
52.0
-13%
+ 14%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-6: C02 Emissions from Lime Production
(Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
12.0
14.0
14.9
14.3
13.7
14.5
15.2
15.1
15.8
Gg
12,004
14,019
14,872
14,261
13,652
14,458
15,154
15,131
15,825
construction decreased slightly from 2005 levels, most likely
as a result of increased prices for lime and the downturn in
new home construction (USGS 2007).
Lime production in 2006 slightly increased over 2005,
the fourth annual increase in production after four years of
decline. Overall, from 1990 to 2006, lime production has
increased by 32 percent. Annual consumption for industrial
and chemical, and environmental lime consumption increased
by 8 percent and 7 percent, respectively (USGS 2007). The
increase in environmental production for environmental
uses is attributed in part to growth in demand for flue gas
desulfurization technologies, particularly at incineration
plants, and wastewater treatment (USGS 2007).
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 27.3 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 byproduct during the production of lime (IPCC 2006).
Lime production data (high-calcium- and dolomitic-
quicklime, high-calcium- and dolomitic-hydrated, and dead-
burned dolomite) for 1990 through 2006 (see Table 4-7) were
obtained from USGS (1992 through 2007). Natural hydraulic
lime, which is produced from CaO and hydraulic calcium
silicates, is not produced in the United States (USGS 2006).
Table 4-7: High-Calcium- and Dolomitic-Quicklime, High-Calcium- and Dolomitic-Hydrated,
and Dead-Burned-Dolomite Lime Production (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
High-Calcium
Quicklime
11,166
13,165
14,300
13,600
13,400
13,900
14,200
14,100
15,000
Dolomitic
Quicklime
2,234
2,635
3,000
2,580
2,420
2,460
3,020
2,990
2,950
High-Calcium
Hydrated
1,781
2,027
1,550
2,030
1,500
2,140
2,140
2,220
2,370
Dolomitic
Hydrated
319
363
421
447
431
464
421
474
409
Dead-Burned
Dolomite
342
308
200
200
200
200
200
200
200
Industrial Processes 4-7
-------
Table 4-8: Adjusted Lime Production3 (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
High-Calcium
12,514
14,700
15,473
15,137
14,536
15,520
15,820
15,781
16,794
Dolomitic
2,809
3,207
3,506
3,105
2,934
2,998
3,526
3,535
3,448
a Minus water content of hydrated lime
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 4-8 (USGS 1992
through 2007, IPCC 2000). The CaO and CaOMgO 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.
Uncertainty
The uncertainties contained in these estimates can be
attributed to slight differences in the chemical composition
of these products. 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 manufacture 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.4
In some cases, lime is generated from calcium carbonate
byproducts at pulp mills and water treatment plants.5 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 production process, the CO2 emitted
during this process is mostly biogenic in origin, and therefore
is not included in inventory totals.6
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.
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 4-9. Lime CO2 emissions
were estimated to be between 14.6 and 17.1 Tg CO2 Eq.
at the 95 percent confidence level. This indicates a range
of approximately 8 percent below and 8 percent above the
emission estimate of 15.8 Tg CO2 Eq.
4 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).
5 Some carbide producers may also regenerate lime from their calcium
hydroxide byproducts, 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
CO2 is released.
6 Based on comments submitted by and personal communication with
Dr. Sergio F. Galeano, Georgia-Pacific Corporation.
4-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-9: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Lime Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Lime Production
CO,
15.8
14.6
17.1
+8%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Estimates of CO2 emissions from lime production were
revised for all years in the time series to remove estimates
of CO2 recovery associated with lime use during sugar
refining and precipitate calcium carbonate (PCC) production.
Currently, research does not indicate that CO2 used in these
processes stems from CO2 captured during lime production.
Additional research is needed to determine if lime production
plants in the US capture CO2 as well as to determine the fates
of precipitates formed during the sugar refining process. This
change resulted in an average annual emission increase of
9.5 percent.
4.3. Limestone and Dolomite Use
(IPCC Source Category 2A3)
Limestone (CaCO3) and dolomite (CaCO3MgCO3)7
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 2006, approximately 13,192 thousand metric tons of
limestone and 5,886 thousand metric tons of dolomite were
consumed during production for these applications. Overall,
usage of limestone and dolomite resulted in aggregate CO2
emissions of 8.6 Tg CO2 Eq. (8,615 Gg) (see Table 4-10 and
Table 4-11). Emissions in 2006 increased 17 percent from
the previous year and have increased 56 percent overall from
1990 through 2006.
Table 4-10: C02 Emissions from Limestone & Dolomite Use (Tg C02 Eq.)
Activity
Flux Stone
Glass Making
FGD
Magnesium Production
Other Miscellaneous Uses
Total
1990
3.0
0.2
1.4
0.1
0.8
5.5
1995
4.0
0.5
1.7
0.0
1.1
7.4
2000
2.8
0.4
1.8
0.1
0.9
6.0
2001
2.5
0.1
2.6
0.1
0.5
5.7
2002
2.4
0.1
2.8
0.0
0.7
5.9
2003
2.1
0.3
1.9
0.0
0.4
4.8
2004
4.1
0.4
1.9
0.0
0.4
6.7
2005
3.3
0.4
3.0
0.0
0.7
7.4
2006
5.1
0.7
2.1
0.0
0.7
8.6
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.
7 Limestone and dolomite are collectively referred to as limestone by the
industry, and intermediate varieties are seldom distinguished.
Industrial Processes 4-9
-------
Table 4-11: C02 Emissions from Limestone & Dolomite Use (Gg)
Activity
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
FGD
Magnesium Production
Other Miscellaneous Uses
Total
1990
2,999
2,554
446
217
189
28
1,433
64
819
5,533
1995
4,004
3,077
927
533
410
122
1,663
41
1,119
7,359
2000 2001 2002 2003 2004 2005 2006
2,830 2,514 2,405 2,081 4,112 3,265 5,072
1,810 1,640 1,330 913 2,023 1,398 2,291
1,020 874 1,075 1,168 2,088 1,867 2,781
368 113 61 339 350 427 747
368 113 61 339 350 406 717
0 0 0 0 0 21 31
1,774 2,551 2,766 1,950 1,871 2,985 2,061
73 53 0 0 0 0 0
916 501 652 383 369 721 735
5,960 5,733 5,885 4,753 6,702 7,397 8,615
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.
Methodology
CO2 emissions were calculated by multiplying the
quantity of limestone or dolomite consumed by the average
C content, approximately 12.0 percent for limestone and
13.2 percent for dolomite (based on stoichiometry), and
converting this value to CO2. This methodology was used
for flux stone, glass manufacturing, flue gas desulfurization
systems, chemical stone, mine dusting or acid water
treatment, acid neutralization, and sugar refining and then
converting to CO2 using a molecular weight ratio.
Traditionally, the production of magnesium metal was
the only other significant use of limestone and dolomite that
produced CO2 emissions. At the start of 2001, there were
two magnesium production plants operating in the United
States and they used different production methods. One plant
produced magnesium metal using a dolomitic process that
resulted in the release of CO2 emissions, while the other
plant produced magnesium from magnesium chloride using
a CO2-emissions-free process called electrolytic reduction.
However, the plant utilizing the dolomitic process ceased
its operations prior to the end of 2001, so beginning in 2002
there were no emissions from this particular sub-use.
Consumption data for 1990 through 2006 of limestone
and dolomite used for flux stone, glass manufacturing, flue
gas desulfurization systems, chemical stone, mine dusting or
acid water treatment, acid neutralization, and sugar refining
(see Table 4-12) were obtained from the USGS Minerals
Yearbook: Crushed Stone Annual Report (USGS 1993,
1995a, 1996a through 2007a). The production capacity
data for 1990 through 2006 of dolomitic magnesium metal
(see Table 4-13) also came from the USGS (1995b, 1996b
through 2007b). The last plant in the United States that
used the dolomitic production process for magnesium metal
closed in 2001. The USGS does not mention this process
in the 2006 Minerals Yearbook: Magnesium; therefore, it
is assumed that this process continues to be non-existent in
the United States (USGS 2007b). 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.
Finally, there is a large quantity of crushed stone
reported to the USGS under the category "unspecified uses."
4-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-12: Limestone and Dolomite Consumption (Thousand Metric Tons)
Activity
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
FGD
Other Miscellaneous Uses
Total
1990
6,738
5,804
933
489
430
59
3,258
1,835
12,319
1995
8,935
6,995
1,941
1,189
933
256
3,779
2,543
16,445
2000 2001 2002 2003 2004 2005 2006
6,249 5,558 5,275 4,521 8,971 7,086 11,030
4,114 3,727 3,023 2,075 4,599 3,176 5,208
2,135 1,831 2,252 2,446 4,373 3,910 5,822
836 258 139 771 796 966 1,693
836 258 139 771 796 923 1,629
0 0 0 0 0 43 64
4,031 5,798 6,286 4,432 4,253 6,785 4,683
2,081 1,138 1,483 870 840 1,638 1,671
13,197 12,751 13,183 10,594 14,859 16,475 19,078
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.
Table 4-13: Dolomitic Magnesium Metal Production
Capacity (Metric Tons)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Production Capacity
35,000
22,222
40,000
29,167
0
0
0
0
0
Note: Production capacity for 2002, 2003, 2004, 2005, and 2006
amounts to zero because the last U.S. production plant employing the
dolomitic process shut down mid-2001 (USGS 2002b through 2007b).
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.8
Uncertainty
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 4-14. Limestone and Dolomite Use
CO2 emissions were estimated to be between 8.0 and 9.2 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 7 percent below and 7 percent above
the emission estimate of 8.6 Tg CO2 Eq.
8 This approach was recommended by USGS.
Industrial Processes 4-11
-------
Table 4-14: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Limestone and Dolomite Use
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Limestone and
Dolomite Use
CO?
8.6
8.0
9.2
-7%
+ 7%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Emission estimates for 2003 were revised to reflect
updated limestone production data. This change resulted in
a less than one percent increase in 2003 emissions.
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 to be included in future inventories (e.g.,
glass production, other process use of carbonates) may be
removed and the emission estimates included there.
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.9 During 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 2006, CO2 emissions from the production of soda ash
from trona were approximately 1.6 Tg CO2 Eq. (1,626 Gg).
Soda ash consumption in the United States generated 2.5
Tg CO2 Eq. (2,536 Gg) in 2006. Total emissions from soda
Table 4-15: C02 Emissions from Soda Ash Production
and Consumption (Tg C02 Eq.)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Production
1.4
1.6
1.5
1.5
1.5
1.5
1.6
1.7
1.6
Consumption
2.7
2.7
2.7
2.6
2.7
2.6
2.6
2.6
2.5
Total
4.1
4.3
4.2
4.1
4.1
4.1
4.2
4.2
4.2
Note: Totals may not sum due to independent rounding.
9 In California, soda ash is produced 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 byproduct, 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.
4-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-16: C02 Emissions from Soda Ash Production
and Consumption (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Production
1,431
1,607
1,529
1,500
1,470
1,509
1,607
1,655
1,626
Consumption
2,710
2,698
2,652
2,648
2,668
2,602
2,598
2,573
2,536
Total
4,141
4,304
4,181
4,147
4,139
4,111
4,205
4,228
4,162
Table 4-17: Soda Ash Production and
Consumption (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Production*
14,700
16,500
15,700
15,400
15,100
15,500
16,500
17,000
16,700
Consumption
6,530
6,500
6,390
6,380
6,430
6,270
6,260
6,200
6,110
Note: Totals may not sum due to independent rounding.
' Soda ash produced from trona ore only.
ash production and consumption in 2006 were 4.2 Tg CO2
Eq. (4,162 Gg) (see Table 4-15 and Table 4-16). Emissions
have fluctuated since 1990. These fluctuations were strongly
related to the behavior of the export market and the U.S.
economy. Emissions in 2006 decreased by approximately 1.6
percent from the previous year, and have increased overall
by approximately 0.5 percent since 1990.
The United States represents about one-fourth of total
world soda ash output. The approximate distribution of
soda ash by end-use in 2006 was glass making, 50 percent;
chemical production, 29 percent; soap and detergent
manufacturing, 9 percent; distributors, 4 percent; flue gas
desulfurization, 2 percent; water treatment, 2 percent; pulp
and paper production, 1 percent; and miscellaneous, 3 percent
(USGS 2007).
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 over the next five years (USGS 2006).
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 byproducts 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) -» 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 16.7 million metric tons of
trona mined in 2006 for soda ash production (USGS 2007)
resulted in CO2 emissions of approximately 1.6 Tg CO2 Eq.
(1,626 Gg).
Once manufactured, 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 4-17) were taken from USGS (1994
through 2007). Soda ash production and consumption data
were collected by the USGS from voluntary surveys of the
U.S. soda ash industry.
Uncertainty
Emission estimates from soda ash production have
relatively low associated uncertainty levels in that
reliable and accurate data sources are available for the
Industrial Processes 4-13
-------
Table 4-18: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Soda Ash Production and
Consumption (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Soda Ash Production
and Consumption
CO?
4.2
3.9
4.5
-7%
+ 7%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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 4-18. Soda Ash Production and
Consumption CO2 emissions were estimated to be between
3.9 and 4.5 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 7 percent below and
7 percent above the emission estimate of 4.2 Tg CO2 Eq.
Planned Improvements
Future Inventories may include estimates of 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 nitrogen 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 CH^ 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 CH^ 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
conversion step. CO2 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 CtLj, including primary and secondary reforming and the
shift conversion processes, is approximately as follows:
(catalyst)
0.88 CH4 + 1.26 Air + 1.24H2O -» 0.88 CO2 + N2 + 3H2
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
4-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
fertilizer that contains C as well as N. The chemical reaction
that produces urea is:
2 NH3 + CO2 -» NH2COONH4 -» CO(NH2)2 + H2O
Urea is consumed in 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.
CO2 emissions associated with other uses of urea are
accounted for in this chapter. Net emissions of CO2 from
ammonia production in 2006 were 11.8TgCO2Eq. (11,832
Gg), and are summarized in Table 4-19 and Table 4-20.
Emissions of CO2 from urea consumed for non-fertilizer
purposes in 2006 totaled 0.5 Tg CO2 Eq. (543 Gg), and are
summarized in Table 4-19 and Table 4-20. 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).
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 United States. 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. 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.
Approximately 87 percent (TIG 2002) of urea consumed
in the United States is consumed as a nitrogenous fertilizer
Table 4-19: C02 Emissions from Ammonia Production and Urea Consumption (Tg C02 Eq.)
Source
Ammonia Production
Urea Consumption3
Total
1990
16.5
0.4
16.9
1995
17.4
0.4
17.8
2000
15.9
0.5
16.4
2001
12.8
0.5
13.3
2002
13.7
0.5
14.2
2003
11.9
0.6
12.5
2004
12.7
0.5
13.2
2005
12.3
0.5
12.8
2006
11.8
0.5
12.4
Note: Totals may not sum due to independent rounding.
a 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.
Table 4-20: C02 Emissions from Ammonia Production and Urea Consumption (Gg)
Source
Ammonia Production
Urea Consumption3
Total
1990
16,528
361
16,889
1995
17,399
397
17,796
2000
15,922
480
16,402
2001
12,795
510
13,305
2002
13,660
534
14,194
2003
11,937
551
12,488
2004
12,695
546
13,241
2005
12,293
524
12,817
2006
11,832
543
12,376
Note: Totals may not sum due to independent rounding.
a 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.
Industrial Processes 4-15
-------
on agricultural lands. The total amount of urea consumed
is estimated based on this percent and 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 4-21. CO2 emissions
associated with the remaining urea are estimated using a
factor of 0.73 tons of CO2 per ton of urea consumed. Total
urea production is estimated based on the amount of urea
applied plus the sum of net urea imports and exports.
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 produced 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 United States for
use of natural gas as a feedstock. The EFMA reference also
indicates that more than 99 percent of the CFL, 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
4-21) was obtained from Coffey ville Resources (Coffey ville
2005, 2006, 2007) and the Census Bureau of the U.S.
Department of Commerce (U.S. Census Bureau 1991 through
1994, 1998 through 2007) as reported in Current Industrial
Reports Fertilizer Materials and Related Products annual
and quarterly reports. Urea-ammonia nitrate production
was obtained from Coffey ville Resources (Coffey ville 2005
through 2007). With the exception of 2006 urea export data,
import and export 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 2006 (U.S. Census Bureau 1998 through
2007), 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 4-21). Because the U.S. Census
Bureau did not report urea export data for 2006, 2005 data
were proxied.
Table 4-21: Ammonia Production, Urea Production, Urea Net Imports, and Urea Exports (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Ammonia Production
15,425
15,788
14,342
11,092
12,577
10,279
10,939
10,143
9,962
Urea Applied as Fertilizer
3,296
3,623
4,382
4,655
4,871
5,025
4,982
4,779
4,958
Urea Imports
1,860
2,936
3,904
4,800
3,840
4,973
4,935
5,026
5,029
Urea Exports
774
881
663
792
970
723
704
579
579
4-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Uncertainty
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, and the assumption that 87 percent of urea
consumed is as fertilizer. 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.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-22. Ammonia Production and
Urea Consumption CO2 emissions were estimated to
be between 11.1 and 13.8 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of approximately 10
percent below and 12 percent above the emission estimate
of 12.4TgC02Eq.
Recalculations Discussion
Estimates of CO2 emissions from ammonia production
and urea consumption were revised for all years to allocate
CO2 emissions associated with urea applied as fertilizer
to the Land Use, Land-Use Change, and Forestry chapter.
Revised estimates reflect a new methodology that estimates
urea production and consumption based on urea consumed as
fertilizer. Previous estimates of urea production are believed
to have overestimated actual urea production. On average,
this change resulted in a 19 percent decrease in emissions
for each year in the time series 1990 through 2005; however,
because CO2 captured during ammonia production to produce
urea is estimated based on the amount of urea produced,
emissions from ammonia production have increased.
Planned Improvements
Plans for 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 hydrocarbons other than natural gas for ammonia
production.
Table 4-22: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Ammonia Production and Urea
Consumption (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Ammonia Production
and Urea Consumption
CO?
12.4
11.1
13.8
-10%
+12%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-17
-------
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 produced by the
catalytic oxidation of ammonia (EPA 1997). During this
reaction, N2O is formed as a byproduct 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 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 20 percent of nitric acid plants
use NSCR (Choe et al. 1993). The remaining 80 percent use
SCR or extended absorption, neither of which is known to
reduce N2O emissions.
N2O emissions from this source were estimated to be
15.6 Tg CO2 Eq. (50 Gg) in 2006 (seeTable 4-23). Emissions
from nitric acid production have decreased by 7.8 percent
since 1990, with the trend in the time series closely tracking
the changes in production.
Table 4-23: N20 Emissions from Nitric Acid Production
(Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
17.0
18.9
18.6
15.1
16.4
15.4
15.2
15.8
15.6
Gg
55
61
60
49
53
50
49
51
50
Methodology
N2O 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 2 kg N2O/metric ton HNO3 for plants
using non-selective catalytic reduction (NSCR) systems
and 9 kg N2O/metric ton HNO3 for 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. An estimated 20 percent of HNO3 plants in the
United States are equipped with NSCR (Choe et al. 1993).
Hence, the emission factor is equal to (9 x 0.80) + (2 x 0.20)
= 7.6 kg N2O per metric ton HNO3.
Nitric acid production data for 1990 through 2004 was
obtained from the U.S. Census Bureau, Current Industrial
Reports (2006) and for 2005 through 2006, from the U.S.
Census Bureau, Current Industrial Reports (2007) (see
Table 4-24).
Uncertainty
The overall uncertainty associated with the 2006
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.
Table 4-24: Nitric Acid Production (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Gg
7,195
8,019
7,900
6,417
6,941
6,522
6,467
6,711
6,637
4-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-25: Tier 2 Quantitative Uncertainty Estimates for N20 Emissions From Nitric Acid Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Nitric Acid Production
N,0
15.6
9.4
22.1
-40%
+41%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
The results of this Tier 2 quantitative uncertainty analysis
are summarized in Table 4-25. N2O emissions from nitric acid
production were estimated to be between 9.4 and 22.1 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 40 percent below to 41 percent above
the 2006 emissions estimate of 15.6 Tg CO2 Eq.
Recalculations Discussion
The nitric acid production values for 2003 and 2005
have been updated relative to the previous Inventory based
on revised production data published by the U.S. Census
Bureau (2006, 2007). The updated production data for 2003
resulted in a decrease of 0.6 Tg CO2 Eq. (3.3 percent) in N2O
emissions relative to the previous Inventory. The updated
production data for 2005 resulted in an increase of 1.0 Tg CO2
Eq. (6.1 percent) in N2O emissions relative to the previous
Inventory. Minor changes in production data due to directly
citing U.S. Census Bureau reports in this Inventory resulted in
negligible changes in N2O emissions relative to the previous
Inventory (less than one-tenth of one percent) for all other
years in the time series, respectively. Additionally, the N2O
emission factor for plants not equipped with NSCR systems
has been updated based on IPCC Guidelines (2006), which
resulted in a slight decrease in emissions in each year of
the time series relative to the previous Inventory. Overall,
these changes resulted in an average annual decrease in N2O
emissions of 0.9 Tg CO2 Eq. (4.8 percent) for the period 1990
through 2005 relative to the previous Inventory.
Planned Improvements
Planned improvements are focused on assessing
the plant-by-plant implementation of NOX abatement
technologies to more accurately match plant production
capacities to appropriate emission factors, instead of using
a national profiling of abatement implementation.
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.
The United States has three companies in four locations
accounting for 34 percent of world production, and 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 most important of the
aliphatic dicarboxylic acids, which are used to manufacture
polyesters. Eighty-four percent of all adipic acid produced
in the United States is used in the production of nylon 6,6,
9 percent is used in the production of polyester polyols, 4
percent is used in the production of plasticizers, and the
remaining 4 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 andTrogler 1991).
Adipic acid is produced through a two-stage process
during which N2O is generated in the second stage. The
first stage of production 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 byproduct
of the nitric acid oxidation stage and is emitted in the waste
gas stream (Thiemens andTrogler 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
Industrial Processes 4-19
-------
Table 4-26: N20 Emissions from Adipic Acid Production
(Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
15.3
17.3
6.2
5.1
6.1
6.3
5.9
5.9
5.9
Gg
49
56
20
16
20
20
19
19
19
control systems in place.10 Only one small plant, representing
approximately two percent of production, does not control
for N2O (Reimer 1999).
N2O emissions from adipic acid production were
estimated to be 5.9 Tg CO2 Eq. (19 Gg) in 2006 (see Table
4-26). National adipic acid production has increased by
approximately 36 percent over the period of 1990 through
2006, to approximately one million metric tons. At the same
time, emissions have been reduced by 61 percent due to the
widespread installation of pollution control measures in the
late 1990s.
Methodology
For two production plants, 1990 to 2002 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 (Childs 2002,2003). These
estimates were based on continuous emissions monitoring
equipment installed at the two facilities. Reported emission
estimates for 2003 to 2006 were unavailable and, thus,
were calculated by applying 4.4, 4.2, 0.0, and 0.0 percent
national production growth rates, respectively. 2003 national
production was calculated through linear interpolation
between 2002 and 2004 reported national production data.
2005 national production was calculated through linear
interpolation between 2004 and 2006 reported national
production. Subsequently, the growth rates for 2004, 2005,
and 2006 were based on the change between the estimated
2003 production data and the reported 2004 production data,
the change between 2004 reported production data and the
estimated 2005 production data, and between the estimated
2005 production data and the reported 2006 production
data, respectively (see discussion below on sources of
production data). For the other two plants, 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 (MT) 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, one
plant employs thermal destruction, and the smallest plant
uses no N2O abatement equipment. For the one plant that
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).
For 1990 to 2003, plant-specific production data was
estimated where direct emission measurements were not
available. In order to calculate plant-specific production
for the two plants, national adipic acid production was
allocated to the plant level using the ratio of their known plant
capacities to total national capacity for all U.S. plants. The
estimated plant production for the two plants was then used
for calculating emissions as described above. For 2004 and
2006, actual plant production data were obtained for these
two plants and used for emission calculations. For 2005,
interpolated national production was used for calculating
emissions as described above.
10 During 1997, the N2O emission controls installed by the third plant
operated for approximately a quarter of the year.
4-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Table 4-27: Atlipic Acid Production (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Gg
735
830
925
835
921
961
1,002
1,002
1,002
National adipic acid production data (see Table 4-27)
for 1990 through 2002 were obtained from the American
Chemistry Council (ACC 2003). Production for 2003 was
estimated based on linear interpolation of 2002 and 2004
reported production. Production for 2004 and 2006 were
obtained from Chemical Week, Product Focus: Adipic Acid
(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 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 these 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 2006, although 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
The overall uncertainty associated with the 2006 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,
industry wide estimated production growth rates, 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 4-28. N2O emissions from adipic
acid production were estimated to be between 5.0 and 6.9 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 15 percent below to 16 percent above
the 2006 emission estimate of 5.9 Tg CO2 Eq.
Recalculations Discussion
The adipic acid production value for 2005 was
recalculated. In the 1990-2005 Inventory, 2005 production
was calculated by applying the annual production growth rate
from 2003 to 2004 of 4 percent to 2004 production. In this
Inventory, 2005 production was recalculated because 2006
production data is now available (CW 2007). 2005 production
was estimated through linear interpolation between 2004
and 2006 reported production data. The updated production
value for 2005 resulted in a decrease of 0.3 Tg CO2 Eq. (4.1
percent) in N2O emissions relative to the previous inventory.
Additionally, changes based on IPCC Guidelines (2006) to
the N2O destruction factor and abatement system utilization
factor for one plant resulted in an increase of between 0.1
and 0.2 Tg CO2 Eq. (0.8 to 3.2 percent) in N2O emissions in
each year of the historical time series, respectively.
Table 4-28: Tier 2 Quantitative Uncertainty Estimates for N20 Emissions from Adipic Acid Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Adipic Acid Production N20
5.9
5.0
6.9
-15%
+ 16%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-21
-------
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
CO2 and CH4 are emitted from the production 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, CK4, or CO.
CO2 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 2005a).
CO2 from SiC production and consumption in 2006
were 0.2 Tg CO2 Eq. (207 Gg). Approximately 44 percent
of these emissions resulted from SiC production while the
remainder result from SiC consumption. CH4 emissions from
SiC production in 2006 were 0.01 Tg CO2 Eq. CH^ (0.4 Gg)
(see Table 4-29 and Table 4-30).
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 CH4/metric ton SiC for CH^ 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 2005a). 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 2005 were obtained
from the Minerals Yearbook: Manufactured Abrasives
(USGS 1991a through 2005a, 2006). Production data for
2006 were obtained from a personal communication with
the USGS Minerals Commodity Specialist (Corathers 2007).
Silicon carbide consumption by major end use was obtained
from the Minerals Yearbook: Silicon (USGS 1991b through
2005b) (see Table 4-31) for years 1990 through 2004 and
from the USGS Minerals Commodity Specialist for 2005
and 2006 (Corathers 2006,2007). Net imports for the entire
time series were obtained from the U.S. Census Bureau
(2005 through 2007).
Table 4-29: C02 and CH4 Emissions from Silicon Carbide Production and Consumption (Tg C02 Eq.)
Year
C02
CH4
Total
1990
0.4
+
0.4
1995
0.3
+
0.3
2000
0.2
+
0.3
2001
0.2
+
0.2
2002
0.2
+
0.2
2003
0.2
+
0.2
2004
0.2
+
0.2
2005
0.2
+
0.2
2006
0.2
+
0.2
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Table 4-30: C02 and CH4 Emissions from Silicon Carbide Production and Consumption (Gg)
Year
C02
CH4
1990
375
1
1995
329
1
2000
248
1
2001
199
+
2002
183
+
2003
202
+
2004
224
+
2005
219
+
2006
207
+
+ Does not exceed 0.5 Gg.
4-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-31: Production and Consumption of Silicon
Carbide (Metric Tons)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Production
105,000
75,400
45,000
40,000
30,000
35,000
35,000
35,000
35,000
Consumption
172,464
227,397
225,280
162,142
180,956
191,289
229,692
220,150
199,938
Uncertainty
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 CFLj, there is
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 4-32. Silicon carbide
production and consumption CO2 emissions were estimated
to be between 10 percent below and 10 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 10
percent above the emission estimate of 0.01 Tg CO2 Eq. at
the 95 percent confidence level.
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 CK4 and CO2 emissions.
Petrochemicals are chemicals isolated or derived from
petroleum or natural gas. CH4 emissions are presented
here from the production of C black, ethylene, ethylene
dichloride, and methanol, while CO2 emissions are presented
here for only C black production. The CO2 emissions from
petrochemical processes other than C 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 C 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.
C black is an intensely black powder generated by
the incomplete combustion of an aromatic petroleum or
coal-based feedstock. Most C 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
Table 4-32: Tier 2 Quantitative Uncertainty Estimates for CH4 and C02 Emissions from Silicon Carbide Production
and Consumption (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Silicon Carbide Production
and Consumption
Silicon Carbide Production
and Consumption
C02
CH4
Lower Bound
0.2 0.2
Upper Bound
0.2
Lower Bound
-10%
-9%
Upper Bound
+10%
+ 10%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
+ Does not exceed 0.05 Tg C02 Eq. or 0.5 Gg.
Industrial Processes 4-23
-------
linear low density polyethylene (HDPE, 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 2006 were 2.6 Tg CO2 Eq. (2,573 Gg) and 1.0
Tg CO2 Eq. (48 Gg), respectively (see Table 4-33 and Table
4-34), totaling 3.6 Tg CO2 Eq. Emissions of CO2 from C
black production decreased from 2.8 Tg CO2 Eq. (2,805 Gg)
in 2005 to 2.6 Tg CO2 Eq. (2,573 Gg) in 2006. There has been
an overall increase in CO2 emissions from C black production
of 16 percent since 1990. CH^ emissions from petrochemical
production decreased by less than 1 percent from the previous
year and increased 16 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 C black, 1
kg CELj/metric ton ethylene, 0.4 kg CH4/metric ton ethylene
dichloride,11 and 2 kg CH4/metric ton methanol. Although
the production of other chemicals may also result in CH4
emissions, there were not sufficient data available to estimate
their emissions.
Emission factors were taken from the Revised 1996
IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). Annual
production data for 1990 (see Table 4-35) were obtained
from the Chemical Manufacturer's Association Statistical
Handbook (CMA 1999). Production data for 1991 through
2006 were obtained from the American Chemistry Council's
Guide to the Business of Chemistry (ACC 2002, 2003,
2005 through 2007) and the International Carbon Black
Association (Johnson 2003, 2005 through 2007).
Almost all C 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 C black feedstock is combusted
to provide energy to the process. C 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. C black plant produces C black using the
thermal black process, and one U.S. C black plant produces
C black using the acetylene black process (The Innovation
Group 2004).
The furnace black process produces C black from "C
black feedstock" (also referred to as "C 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
Table 4-33: C02 and CH4 Emissions from Petrochemical Production (Tg C02 Eq.)
Year
C02
CH4
Total
1990
2.2
0.9
3.1
1995
2.8
1.1
3.8
2000
3.0
1.2
4.2
2001
2.8
1.1
3.9
2002
2.9
1.1
4.0
2003
2.8
1.1
3.9
2004
2.9
1.2
4.1
2005
2.8
1.1
4.0
2006
2.6
1.0
3.6
Table 4-34: C02 and CH4 Emissions from Petrochemical Production (Gg)
Year
C02
CH4
1990
2,221
41
1995
2,750
52
2000
3,004
58
2001
2,787
51
2002
2,857
52
2003
2,777
51
2004
2,895
55
2005
2,804
51
2006
2,573
48
11 The emission factor obtained from IPCC/UNEP/OECD/IEA (1997), page
2.23 is assumed to have a misprint; the chemical identified should be ethylene
dichloride (C2H4C12) rather than dichloroethylene (C2H2C12).
4-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-35: Production of Selected Petrochemicals (Thousand Metric Tons)
Chemical
Carbon Black
Ethylene
Ethylene Dichloride
Methanol
1990
1,307
16,542
6,282
3,785
1995
1,619
21,215
7,829
4,992
2000
1,769
24,971
9,866
4,876
2001
1,641
22,521
9,294
3,402
2002
1,682
23,623
9,288
3,289
2003
1,635
22,957
9,952
3,166
2004
1,705
25,660
12,111
2,937
2005
1,651
23,954
11,261
2,336
2006
1,515
25,000
9,737
1,123
Table 4-36: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock (Secondary Feedstock)
Consumption (Thousand Metric Tons)
Activity
Primary Feedstock
Secondary Feedstock
1990
1,864
302
1995
2,308
374
2000
2,521
408
2001
2,339
379
2002
2,398
388
2003
2,331
377
2004
2,430
393
2005
2,353
381
2006
2,159
350
petroleum-derived and coal-derived C black, the "primary
feedstock" (i.e., C 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 C black feedstock are oxidized to provide heat to the
production process and pyrolyze the remaining C black
feedstock to C black. The "tail gas" from the furnace black
process contains CO2, carbon monoxide, sulfur compounds,
CtLj, and non-CtLj volatile organic compounds. A portion of
the tail gas is generally burned for energy recovery to heat
the downstream C 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 C
black production is subtracted from the total amount of C
contained in primary and secondary C 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 CtLj 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
C black feedstock consumed in the process (see Table 4-36)
is estimated using a primary feedstock consumption factor
and a secondary feedstock consumption factor estimated
from U.S. Census Bureau (1999 and 2004) data. The
average C black feedstock consumption factor for U.S. C
black production is 1.43 metric tons of C black feedstock
consumed per metric ton of C black produced. The average
natural gas consumption factor for U.S. C black production is
341 normal cubic meters of natural gas consumed per metric
ton of C 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 C black feedstock is assumed to be derived from
petroleum refining byproducts. C black feedstock derived
from metallurgical (coal) coke production (e.g., creosote
oil) is also used for C black production; however, no data
are available concerning the annual consumption of coal-
derived C black feedstock. C 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 C black production
process is combusted 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
C black because of the lack of data concerning the relatively
small amount of C black produced using the acetylene black
and thermal black processes. The C black produced from the
furnace black process is assumed to be 97 percent elemental
C (Othmer et al. 1992).
Uncertainty
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
Industrial Processes 4-25
-------
Table 4-37: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petrochemical Production and
C02 Emissions from Carbon Black Production (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Petrochemical Production
Petrochemical Production
C02
CH4
2.6
1.0
1.7
0.9
3.6
1.1
-35%
-9%
+39%
+ 9%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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 C black production calculation
are based on feedstock consumption, import and export
data, and C black production data. The composition of C
black feedstock varies depending upon the specific refinery
production process, and therefore the assumption that C
black feedstock is 89 percent C gives rise to uncertainty.
Also, no data are available concerning the consumption of
coal-derived C 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 C
black production may be underreported by the U.S. Census
Bureau. Finally, the amount of C 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 C black is produced using
the furnace black process.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-37. Petrochemical production
CO2 emissions were estimated to be between 1.7 and 3.6 Tg
CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 35 percent below to 39 percent
above the emission estimate of 2.6 Tg CO2 Eq. Petrochemical
production CFLj emissions were estimated to be between 0.9
and 1.1 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 1.0 Tg CO2 Eq.
Recalculations Discussion
Estimates of CO2 from carbon black have been revised
for 2005 to reflect new production data. The revision resulted
in a decrease in emissions of less than one percent.
Planned Improvements
Future improvements to the petrochemicals source
category include research into the use of acrylonitrile in
the United States, revisions to the C 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 produced 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:
2 FeTi03 + 7 C12 + 3 C -» 2 TiCl4 + 2 FeCl3 + 3 CO2
2 TiCl4 + 2 02 -» 2 Ti02 + 4 C12
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 proces s, and a special grade of petroleum
coke is manufactured specifically for this purpose.
Emissions of CO2 in 2006 were 1.9 Tg CO2 Eq. (1,876
Gg), an increase of 7 percent from the previous year and an
increase of 57 percent since 1990 (Table 4-38).
4-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-38: C02 Emissions from Titanium Dioxide
Production (Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
1.2
1.5
1.8
1.7
1.8
1.8
2.1
1.8
1.9
Gg
1,195
1,526
1,752
1,697
1,824
1,839
2,064
1,755
1,876
Table 4-39: Titanium Dioxide Production
(Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Gg
979
1,250
1,400
1,330
1,410
1,420
1,540
1,310
1,400
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 thatTiO2 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. As a result, all U.S. current TiO2
production results from 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
petroleum coke used in the process is 90 percent C and 10
percent inert materials.
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 2006 (see
Table 4-39) were obtained from a personal communication
with Joe Gambogi, USGS Commodity Specialist, of the
USGS (Gambogi 2007) and through the Minerals Yearbook:
Titanium Annual Report (USGS 1991 through 2005).
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). The composition data for petroleum
coke were obtained from Onder and Bagdoyan (1993).
Uncertainty
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-
Industrial Processes 4-27
-------
Table 4-40: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Titanium Dioxide Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Titanium Dioxide
Production
CO?
1.9
1.7
2.1
-12%
+13%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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 4-40. Titanium dioxide consumption
CO2 emissions were estimated to be between 1.7 and 2.1 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 1.9 Tg CO2 Eq.
Recalculations Discussion
Estimates of CO2 emissions from titanium dioxide
production were updated to reflect a revised chloride-process
emission factor provided by the 2006 IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC 2006). The
change in emission factor resulted in a decrease in emissions
of 8.6 percent for each year in the time series.
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.
4.11. Carbon Dioxide Consumption
(IPCC Source Category 2B5)
CO2 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). CO2
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.
CO2 is produced from naturally occurring CO2
reservoirs, as a byproduct from the energy and industrial
production processes (e.g., ammonia production, fossil
fuel combustion, ethanol production), and as a byproduct
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 byproduct 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. CO2 captured from biogenic sources
(e.g., ethanol production plants) is not included in the
inventory. CO2 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.
CO2 is produced as a byproduct of crude oil and natural
gas production. This CO2 is separated from the crude oil and
4-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 "Box 3-3 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
Table 4-41: C02 Emissions from C02 Consumption
(Tg C02 Eq. and Gg)
(Allis et al. 2000). CO2 production from these facilities is
discussed in the Energy Chapter.
In 2006, the amount of CO2 produced by the Mississippi
and New Mexico facilities for commercial applications and
subsequently emitted to the atmosphere were 1.6 Tg CO2
Eq. (1,579 Gg) (see Table 4-41). This amount represents
an increase of 17.9 percent from the previous year and
an increase of 9.9 percent from emissions in 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.
Methodology
CO2 emission estimates for 1990 through 2006 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.
CO2 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 2007) for
2001 to 2006 (see Table 4-42). Denbury Resources reported
the average CO2 production in units of MMCF CO2 per day
Table 4-42: C02 Production (Gg C02) and the Percent Used for Non-EOR Applications for Jackson Dome and
Bravo Dome
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
1.4
1.4
1.4
0.8
1.0
1.3
1.2
1.3
1.6
Gg
1,416
1,422
1,421
829
989
1,311
1,198
1,321
1,579
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Jackson Dome C02
Production (Gg)
1,353
1,353
1,353
1,624
2,010
3,286
4,214
4,678
7,615
Jackson Dome % Used
for Non-EOR
100%
100%
100%
47%
46%
38%
27%
27%
20%
Bravo Dome C02
Production (Gg)
6,301
6,862
6,834
6,627
6,420
6,213
6,006
5,799
5,613
Bravo Dome % Used
for Non-EOR
1%
1%
1%
1%
1%
1%
1%
1%
1%
Industrial Processes 4-29
-------
Table 4-43: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from C02 Consumption
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
C02 Consumption
CO,
1.6
1.3
2.0
-21%
+26%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
for 2001 through 2006 and reported the percentage of the
total average annual production that was used for EOR. CO2
production data for the Bravo Dome, New Mexico facility
were obtained from the Advanced Resources International,
Inc. (ARI 2006, 2007).
Uncertainty
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 4-43. CO2 consumption CO2
emissions were estimated to be between 1.3 and 2.0 Tg CO2
Eq. at the 95 percent confidence level. This indicates a range
of approximately 21 percent below to 26 percent above the
emission estimate of 1.6 Tg CO2 Eq.
Recalculations Discussion
Data for total Bravo Dome CO2 production were
updated for the entire time series based on new production
data from the facility. These changes resulted in an average
annual emission increase of less than one percent for 1990
through 2005.
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) (EFMA 2000). The
primary chemical reactions for the production of phosphoric
acid from phosphate rock are:
Ca3(PO4)2 + 4H3PO4 -» 3Ca(H2PO4)2
3Ca(H2PO4)2 + 3H2SO4 + 6H2O -»
3CaS04»6H20 + 6H3P04
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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 -» CaSO4»2H2O + CO2
Total marketable phosphate rock production in 2006
was 30.1 million metric tons. 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. In addition, 2.4
million metric tons of crude phosphate rock was imported for
consumption in 2006. Marketable phosphate rock production,
including domestic production and imports for consumption,
decreased by approximately 16 percent between 2005 and
2006. However, over the 1990 to 2006 period, production
has decreased by 26 percent. Total CO2 emissions from
phosphoric acid production were 1.2 Tg CO2 Eq. (1,167 Gg)
in 2006 (see Table 4-44).
Table 4-44: C02 Emissions from Phosphoric Acid
Production (Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
1.5
1.5
1.4
1.3
1.3
1.4
1.4
1.4
1.2
Gg
1,529
1,513
1,382
1,264
1,338
1,382
1,395
1,383
1,167
Methodology
CO2 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 4-45). For the years 1990,1991,
1992,2005, and 2006 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, and imports of phosphate
rock for consumption for 1990 through 2006 were obtained
from USGS Mineral Yearbook: Phosphate Rock (USGS
1994 through 2007. From 2004-2006, the USGS reported
no exports of phosphate rock from U.S. producers (USGS
2005 through 2007).
Table 4-45: Phosphate Rock Domestic Production, Exports, and Imports (Gg)
Location/Year
U.S. Production3
FL&NC
ID&UT
Exports— FL & NC
Imports — Morocco
Total U.S. Consumption
1990
49,800
42,494
7,306
6,240
451
44,011
1995
43,720
38,100
5,620
2,760
1,800
42,760
2000 2001 2002 2003 2004 2005 2006
37,370 32,830 34,720 36,410 36,530 36,000 30,100
31,900 28,100 29,800 31,300 31,600 31,140 26,037
5,470 4,730 4,920 5,110 4,930 4,860 4,064
299 9 62 64
1,930 2,500 2,700 2,400 2,500 2,630 2,420
39,001 35,321 37,358 38,746 39,030 38,630 32,520
a USGS does not disaggregate production data regionally (FL &NCand ID &UT) for 1990 and 2006. Data for those years are estimated based on the
remaining time series distribution.
- Assumed equal to zero.
Industrial Processes 4-31
-------
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 4-46).
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. At last reporting,
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 content data for calcined phosphate rock
mined in Idaho.
Uncertainty
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 2006. For previous years in the time
series, USGS provided the data disaggregated regionally;
however, for 2006 only total U.S. phosphate rock production
were reported. Regional production for 2006 was estimated
based on regional production data from previous years and
multiplied by regionally-specific emission factors. There is
uncertainty associated with the degree to which the estimated
2006 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 imports for
consumption and exports of phosphate rock used in the
emission calculation 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.
Table 4-46: 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)
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
Source: FIPR 2003
- Assumed equal to zero.
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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 produce
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 4-47. 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.
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)
In addition to being an energy intensive process, the
production of iron and steel also generates process-related
emissions of CO2 and CH4. 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 C by weight). Metallurgical coke is
manufactured using coking coal as a raw material. Iron may
be introduced into the blast furnace in the form of raw iron
ore, pellets, briquettes, or sinter. Pig iron is used as a raw
material in the production of steel, which contains about 4
percent C 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.
The production of metallurgical coke from coking coal and
the consumption of the metallurgical coke used as a reducing
agent in the blast furnace are considered in the inventory to
be non-energy (industrial) processes, not energy (combustion)
processes. 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. Coke oven gas and coal
tar are C containing byproducts of the coke manufacturing
process. Coke oven gas is generally burned as a fuel within
the steel mill. Coal tar is used as a raw material to produce
Table 4-47: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Phosphoric Acid Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Phosphoric Acid Production C02
1.2
1.0
1.4
-18%
+19%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-33
-------
anodes used for primary aluminum production and other
electrolytic processes, and also used in the production of other
coal tar products. The coke production process produces CO2
emissions and fugitive CH4 emissions.
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 prior to being charged into the
blast furnace. The sintering process produces CO2 emissions
and fugitive CH4 emissions.
The metallurgical coke is a reducing agent in the blast
furnace. CO2 is produced as the metallurgical coke used in the
blast furnace process is oxidized and the iron ore is reduced.
Steel is produced from pig iron in a variety of specialized
steel-making furnaces. The majority of CO2 emissions from
the iron and steel process come from the use of coke in the
production of pig iron, with smaller amounts evolving from
the removal of C from pig iron used to produce steel. Some
C is also stored in the finished iron and steel products.
Emissions of CO2 and CH4 from iron and steel
production in 2006 were 49.1 Tg CO2 Eq. (49,119 Gg) and
0.9 Tg CO2 Eq. (45 Gg), respectively (see Table 4-48 and
Table 4-49), totaling 50.1 Tg CO2 Eq. Emissions increased
in 2006 after declining steadily from 1990 to 2005 due to
restructuring of the industry, technological improvements,
and increased scrap utilization. Interannual fluctuations in
CO2 emissions per unit of steel produced result, in part,
because iron and steel emission estimates include emissions
associated with producing metallurgical coke. Metallurgical
coke emissions are included here because metallurgical coke
is primarily used to produce iron and steel; however, some
amounts are also used to produce other metals (e.g., lead,
zinc). In 2006, domestic production of pig iron increased
by 1.8 percent and coal coke production decreased by 1.9
percent. Overall, domestic pig iron and coke production have
declined since the 1990s. Pig iron production in 2006 was
21 percent lower than in 2000 and 23 percent below 1990
levels. Coke production in 2006 was 21 percent lower than
in 2000 and 41 percent below 1990 levels. Overall, emissions
from iron and steel productions have declined by 43 percent
(37.4 Tg CO2 Eq.) from 1990 to 2006.
Methodology
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 then attributed to the iron and steel sector. To
estimate emission from coke produced from coking coal the
amount of C contained in coke (calculated by multiplying
the amount of C contained in coke by the amount of coke
produced) is deducted from the amount of C contained in
the coking coal (calculated by multiplying the C content
of coking coal by the amount of coking coal consumed).
The amount of coking coal needed for these production
processes is deducted from coking coal amounts provided
in the Energy chapter to avoid double counting. Emissions
associated with the consumption of coke to produce pig
iron are also estimated. The C content of the coking coal
and coke consumed in these processes were estimated by
Table 4-48: C02 and CH4 Emissions from Iron and Steel Production (Tg C02 Eq.)
Year
C02
CH4
Total
1990
86.2
1.3
87.5
1995
74.7
1.3
76.0
2000
66.6
1.2
67.8
2001
59.2
1.1
60.3
2002
55.9
1.0
57.0
2003
54.7
1.0
55.8
2004
52.8
1.0
53.8
2005
46.6
1.0
47.6
2006
49.1
0.9
50.1
Table 4-49: C02 and CH4 Emissions from Iron and Steel Production (Gg)
Year
C02
CH4
1990
86,220
63
1995
74,729
62
2000
66,609
58
2001
59,249
51
2002
55,938
48
2003
54,744
49
2004
52,771
50
2005
46,627
45
2006
49,119
45
4-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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multiplying the energy consumption by material specific
C-content coefficients. The C content coefficients used are
presented in Annex 2.1.
Emissions from the reuse of scrap steel were also
estimated by assuming that all the associated C content
of the scrap steel, which has an associated C content of
approximately 0.5 percent, are released during the scrap
re-use process.
Lastly, emissions from C anodes, used during the
production of steel in electric arc furnaces (EAFs), were also
estimated. Emissions of CO2 were calculated by multiplying
the annual production of steel in EAFs by an emission factor
(4.4 kg CO2/ton steel^p). It was assumed that the C anodes
used in the production of steel in EAFs are composed of
80 percent petroleum coke and 20 percent coal tar pitch
(DOE 1997). Since coal tar pitch is a byproduct of the coke
production process and its C-related emissions have already
been accounted for earlier in the iron and steel emissions
calculation as part of the process, the emissions were reduced
by the amount of C in the coal tar pitch used in the anodes
to avoid double counting.
Emissions associated with the production of coke from
coking coal, pig iron production, the re-use of scrap steel,
and the consumption of C anodes during the production of
steel were summed.
Additionally, the coal tar pitch component of C anodes
consumed during the production of aluminum is accounted
for in the aluminum production section of this chapter. The
emissions were reduced by the amount of coal tar pitch
used in aluminum production to avoid double counting.
The amount of coal tar pitch consumed for processes other
than the aluminum production and as EAF anodes and net
imports of coal tar were also estimated. A storage factor was
applied to estimate emissions associated with other coal tar
pitch consumption and net imports.
C storage was accounted for by assuming that all
domestically produced steel had a C content of 0.1 percent.
Furthermore, any pig iron that was not consumed during
steel production, but fabricated into finished iron products,
was assumed to have a C content of 4 percent.
The potential CO2 emissions associated with C contained
in pig iron used for purposes other than iron and steel
production, stored in the steel product, stored as coal tar,
and attributed to C anode consumption during aluminum
Table 4-50: CH4 Emission Factors for Coal Coke,
Sinter, and Pig Iron Production (g/kg)
Material Produced
g CtVkg produced
Coal Coke
Pig Iron
Sinter
0.5
0.9
0.5
Source: IPCC/UNEP/OECD/IEA 1997.
production were summed and subtracted from the total
emissions estimated above.
The production processes for coal coke, sinter, and pig
iron result in fugitive emissions of CFL,, 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 emission factors taken
from the 1995 IPCC Guidelines (IPCC/UNEP/OECD/IEA
1995) (see Table 4-50) to annual domestic production data
for coal coke, sinter, and pig iron.
Data relating to the amount of coal consumed at
coke plants, and for the production of coke for domestic
consumption in blast furnaces, were taken from the Energy
Information Administration (EIA), Quarterly Coal Report
October through December (EIA 1998 through 2004a)
and January through March (EIA 2006a, 2007). Data on
total coke consumed for pig iron production were taken
from the American Iron and Steel Institute (AISI), Annual
Statistical Report (AISI 2001 through 2007). Scrap steel
consumption data for 1990 through 2005 were obtained
from Annual Statistical Report (AISI 1995, 2001 through
2006) (see Table 4-51). Because scrap steel consumption
data were unavailable for 2006, 2005 data were used.
Crude steel production, as well as pig iron use for purposes
other than steel production, was also obtained from Annual
Statistical Report (AISI 1996,2001 through 2007). C content
percentages for pig iron and the CO2 emission factor for C
anode emissions from steel production were obtained from
IPCC Good Practice Guidance (IPCC 2000). C content
percentages for crude steel were taken from USGS (2005a).
Data on the non-energy use of coking coal were obtained
from EIA's Emissions of Greenhouse Gases in the United
States (EIA 2004b, 2006b). Information on coal tar net
imports was determined using data from the U.S. Bureau
of the Census's U.S. International Trade Commission's
Trade Dataweb (U.S. Bureau of the Census 2007). Coal tar
Industrial Processes 4-35
-------
Table 4-51: Production and Consumption Data for the Calculation of C02 and CH4 Emissions from Iron and
Steel Production (Thousand Metric Tons)
Gas/Activity Data
C02
Coal Consumption at Coke Plants
Coke Consumption for Pig Iron
Basic Oxygen Furnace Steel Production
Electric Arc Furnace Steel Production
CH4
Coke Production
Iron Ore Consumption for Sinter
Domestic Pig Iron Production for Steel
1990
35,269
25,043
56,216
33,510
25,054
12,239
49,062
1995
29,948
22,288
56,721
38,472
21,545
12,575
50,233
2000 2001 2002 2003 2004 2005 2006
26,254 23,655 21,461 21,998 21,473 21,259 20,827
19,307 17,236 15,959 15,482 15,068 13,848 14,729
53,965 47,359 45,463 45,874 47,714 42,705 42,119
47,860 42,774 46,125 47,804 51,969 52,194 56,071
18,877 17,191 15,221 15,579 15,340 15,167 14,882
10,784 9,234 9,018 8,984 8,047 8,313 7,085
47,888 42,134 40,226 40,644 42,292 37,222 37,903
consumption for aluminum production data was estimated
based on information gathered by EPA's Voluntary Aluminum
Industrial Partnership (VAIP) program and data from USAA
Primary Aluminum Statistics (US AA 2004,2005,2006) (see
Aluminum Production in this chapter). Annual consumption
of iron ore used in sinter production for 1990 through 2005
was obtained from the USGS Iron Ore Yearbook (USGS
1994 through 2005b). Iron ore consumption for 2006 was
obtained from the USGS Minerals Commodity Specialist
(Jorgenson 2007). The CO2 emission factor for C anode
emissions from aluminum production was taken from the
Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA
1997). Estimates for the composition of C anodes used
during EAF steel and aluminum production were obtained
from Energy and Environmental Profile of the U.S. Aluminum
Industry (DOE 1997).
Uncertainty
The time series data sources for production of coal
coke, sinter, pig iron, steel, and aluminum upon which
the calculations are based are assumed to be consistent
for the entire time series. The estimates of CO2 emissions
from the production and utilization of coke are based on
consumption data, average C contents, and the fraction of C
oxidized. Uncertainty is associated with the total U.S. coke
consumption and coke consumed for pig iron production.
These data are provided by different data sources (EIA
and AISI) and comparisons between the two datasets
for net imports, production, and consumption identified
discrepancies; however, the data chosen are considered the
best available. These data and factors produce a relatively
accurate estimate of CO2 emissions. However, there are
uncertainties associated with each of these factors. For
example, C oxidation factors may vary depending on
inefficiencies in the combustion process, where varying
degrees of ash or soot can remain unoxidized.
Simplifying assumptions were made concerning the
composition of C anodes and the C contents of all pig iron
and crude steel. It was also assumed that all coal tar used
during anode production originates as a byproduct of the
domestic coking process. There is also uncertainty associated
with the total amount of coal tar products produced and with
the storage factor for coal tar. Uncertainty surrounding the
CO2 emission factor for C anode consumption in aluminum
production was also estimated.
For the purposes of the CH4 calculation it is assumed
that none of the CK4 is captured in stacks or vents and that
all of the CH4 escapes as fugitive emissions. 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 CFLj.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-52. Iron and Steel CO2 emissions
were estimated to be between 40.3 and 56.5 Tg CO2 Eq. at
the 95 percent confidence level. This indicates a range of
approximately 18 percent below and 15 percent above the
emission estimate of 49.1 Tg CO2 Eq. Iron and Steel CK4
emissions were estimated to be between 0.9 Tg CO2 Eq.
and 1.0 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 0.9 Tg CO2 Eq.
4-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Table 4-52: Tier 2 Quantitative Uncertainty Estimates for C02 and CH4 Emissions from Iron and Steel Production
(Tg. C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Iron and Steel Production
Iron and Steel Production
C02
CH4
49.1
0.9
Lower Bound
40.3
0.9
Upper Bound
56.5
1.0
Lower Bound
-18%
-8%
Upper Bound
+ 15%
+ 9%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Estimates of CO2 from iron and steel production have
been revised for the entire time series to reflect a revised
carbon content for crude steel. This revision resulted in an
average annual increase in emissions of 2 percent throughout
the time series.
Planned Improvements
Plans for improvements to the Iron and Steel source
category are to include methodologies outlined in the 2006
IPCC Guidelines for National Greenhouse Gas Inventories
(IPCC 2006). These methodologies involve the inclusion
of energy- and flux-related emissions in the iron and steel
emission estimates as well as emissions associated with
metallurgical coke production, sinter production, pellet
production, and direct reduced iron ore production in addition
to iron and steel production.
4.14. Ferroalloy Production (IPCC
Source Category 2C2)
CO2 and CtLj 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 + 2 SiO2 + 7C -» 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 CFLj and other volatiles. The amount of CK4 that
is released is dependent on furnace efficiency, operation
technique, and control technology.
Emissions of CO2 from ferroalloy production in 2006
were 1.5 Tg CO2 Eq. (1,505 Gg) (see Table 4-53 and Table
4-54), which is an eight percent increase from the previous
year and a 30 percent reduction since 1990. Emissions of CFLj
from ferroalloy production in 2006 were 0.01 Tg CO2 Eq.
(0.4 Gg), which is an 11 percent increase from the previous
year and a 37 percent decrease since 1990.
Table 4-53: C02 and CH4 Emissions from Ferroalloy Production (Tg C02 Eq.)
Year
C02
CH4
Total
1990
2.2
+
2.2
1995
2.0
+
2.0
2000
1.9
+
1.9
2001
1.5
+
1.5
2002
1.3
+
1.4
2003
1.3
+
1.3
2004
1.4
+
1.4
2005
1.4
+
1.4
2006
1.5
+
1.5
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Industrial Processes 4-37
-------
Table 4-54: C02 and CH4 Emissions from Ferroalloy Production (Gg)
Year
C02
CH4
1990
2,152
0.7
1995
2,036
0.6
2000
1,893
0.5
2001
1,459
0.4
2002
1,349
0.4
2003
1,305
0.4
2004
1,419
0.4
2005
1,392
0.4
2006
1,505
0.4
Methodology
Emissions of CO2 and CE^ from ferroalloy production
were calculated by multiplying annual ferroalloy production
by material-specific emission factors. Emission factors taken
from the 2006IPCC 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 CEL^ (1 kg CELj/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 CEE, (4 metric tons CO2/metric
ton alloy produced and 1 kg CEL/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 CELj/
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 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 2006 (see
Table 4-55) were obtained from the USGS through personal
communications with the USGS Silicon Commodity Specialist
(Corathers 2007) and through the Minerals Yearbook: Silicon
Annual Report (USGS 1991 through 2006). Because USGS
does not provide estimates of silicon metal production for
2006, 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
4-55). The composition data for petroleum coke was obtained
from Onder and Bagdoyan (1993).
Uncertainty
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
Table 4-55: Production of Ferroalloys (Metric Tons)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Ferrosilicon
25%-55%
321,385
184,000
229,000
167,000
156,000
115,000
120,000
123,000
164,000
Ferrosilicon
56%-95%
109,566
128,000
100,000
89,000
98,600
80,500
92,300
86,100
88,700
Silicon Metal
145,744
163,000
184,000
137,000
113,000
139,000
150,000
148,000
148,000
Misc. Alloys
32%-65%
72,442
99,500
NA
NA
NA
NA
NA
NA
NA
NA (Not Available)
4-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-56: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Ferroalloy Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Ferroalloy Production
Ferroalloy Production
C02
CH4
1.5
+
Lower Bound
1.3
+
Upper Bound
1.7
+
Lower Bound
-12%
-12%
Upper Bound
+ 12%
+ 12%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
+ Does not exceed 0.05 Tg C02 Eq.
be counted under this source because wood-based C is of
biogenic origin.12 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 CH4 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 4-56. Ferroalloy production CO2
emissions were estimated to be between 1.3 and 1.7 Tg CO2
Eq. at the 95 percent confidence level. This indicates a range
of approximately 12 percent below and 12 percent above the
emission estimate of 1.5 Tg CO2 Eq. Ferroalloy production
emissions were estimated to be between a range of
approximately 12 percent below and 12 percent above the
emission estimate of 0.01 Tg CO2 Eq.
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
eight percent of the world total (USGS 2006). 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).
CO2 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
12 Emissions and sinks of biogenic carbon are accounted for in the Land
Use, Land-Use Change, and Forestry chapter.
Industrial Processes 4-39
-------
molten bath of natural or synthetic cryolite (Na3AlF6). The
reduction cells contain a C lining that serves as the cathode. C
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 3.9 Tg CO2 Eq. (3,923 Gg) in 2006
(see Table 4-57). 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
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.
Table 4-57: C02 Emissions from Aluminum Production
(Tg C02 Eq. and Gg)
Year Tg C02 Eq. Gg
1990 6.8 6,831
1995 5.7 5,659
2000 6.1 6,086
2001 4.4 4,381
2002 4.5 4,490
2003 4.5 4,503
2004 4.2 4,231
2005 4.2 4,207
2006 3.9 3,923
Since 1990, emissions of CF4 and C2F6 have both
declined by 87 percent to 2. 1 Tg CO2 Eq. of CF4 (0.4 Gg) and
0.4 Tg CO 2 Eq. of C2F6 (0.04 Gg) in 2006, as shown in Table
4-58 and Table 4-59. This decline is due both to reductions
in domestic aluminum production and to actions taken by
aluminum smelting companies to reduce the frequency and
duration of anode effects. Since 1990, aluminum production
has declined by 44 percent, while the average CF4 and C2F6
emission rates (per metric ton of aluminum produced) have
each been reduced by 76 percent.
In 2006, U.S. primary aluminum production totaled
approximately 2.3 million metric tons, a slight decrease from
2005 production levels. Due to high electric power costs in
various regions of the country, aluminum production has been
curtailed at several U.S. smelters, which resulted in 2006
production levels that were approximately 40 percent lower
Table 4-58: PFC Emissions from Aluminum Production
(Tg C02 Eq.)
Year CF4 C2F6 Total
1990 15.9 2.7 18.5
1995 10.2 1.7 11.8
2000 7.8 0.8 8.6
2001 3.0 0.4 3.5
2002 4.6 0.7 5.2
2003 3.3 0.5 3.8
2004 2.4 0.4 2.8
2005 2.5 0.4 3.0
2006 2.1 0.4 2.5
Note: Totals may not sum due to independent rounding.
Table 4-59: PFC Emissions from Aluminum Production (Gg)
Year CF4 C2F6
1990 2.4 0.3
1995 1.6 0.2
2000 1.2 0.1
2001 0.5 +
2002 0.7 0.1
2003 0.5 0.1
2004 0.4 +
2005 0.4 +
2006 0.3 +
+ Does not exceed 0.05 Gg
4-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
than the levels in 1999, the year with the highest production
since 1995.
Methodology
CO2 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 achieved through
information gathered by EPA's Voluntary Aluminum
Industrial Partnership (VAIP) program.
Most of the CO2 emissions released during aluminum
production occur during the electrolysis reaction of the C
anode, as described by the following reaction.
2A1203 + 3C -» 4A1 + 3C02
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. The CO2 emission factor
employed was estimated from the production of primary
aluminum metal and the C consumed by the process.
Emissions vary depending on the specific technology
used by each plant (e.g., prebake or S0derberg). CO2
process emissions were estimated using the methodology
recommended by IPCC (2006).
The prebake process specific formula recommended by
IPCC (2006) accounts for various parameters, including net C
consumption, and the sulfur, ash, and impurity content of the
baked anode. For anode baking emissions, process formulas
account 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. The S0derberg 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, process data have been reported for
1990, 2000, 2003, 2004, 2005, and 2006. 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, and 12
out of 13 operating smelters in 2006. 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 smelter specific process data (i.e.,
1 out of 13 in 2006, 1 out of 15 smelters in 2005, and 5 out
of 23 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 12 out of 13 operating
smelters were reported under the VAIP in 2006. Between
1990 and 2005, 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 2006), with
allocation to specific smelters based on reported production
capacities (USGS 2002).
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 2006, smelter-
specific slope coefficients were available and were used for
smelters representing between 30 and 55 percent of U.S.
primary aluminum production. The percentage changed from
year to year as some smelters closed or changed hands and as
the production at remaining smelters fluctuated. For smelters
that did not report smelter-specific slope coefficients, IPCC
Industrial Processes 4-41
-------
technology-specific slope coefficients were applied (IPCC
2001, 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 2006, 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 2006, 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 and then by 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 2006
were obtained via communication with USGS (USGS,
2007). For 1990 through 2001 (see Table 4-60) data were
obtained from USGS, Mineral Industry Surveys: Aluminum
Annual Report (USGS 1995, 1998, 2000, 2001, 2002). For
2002 through 2005, national aluminum production data were
obtained from the United States Aluminum Association's
Primary Aluminum Statistics (USAA 2004, 2005, 2006).
Table 4-60: Production of Primary Aluminum (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Gg
4,048
3,375
3,668
2,637
2,705
2,704
2,517
2,478
2,284
Uncertainty
The overall uncertainties associated with the 2006
CO2, CF4, and C2F6 emission estimates were calculated
using a Tier 2 approach, 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 2 percent (IPCC
2006). For additional variables, such as net C consumption,
and sulfur 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 awhole,
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 or
company and for the U.S. aluminum industry as a whole.
The results of this quantitative uncertainty analysis are
summarized in Table 4-61. Aluminum production-related
CO2 emissions were estimated to be between 3.7 and 4.1 Tg
CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 5 percent below to 5 percent above
the emission estimate of 3.9 Tg CO2 Eq. Also, production-
related CF4 emissions were estimated to be between 1.9
and 2.3 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 2.1 Tg CO2
Eq. Finally, aluminum production-related C2F6 emissions
were estimated to be between 0.3 and 0.4 Tg CO2 Eq. at
the 95 percent confidence level. This indicates a range of
approximately 17 percent below to 17 percent above the
emission estimate of 0.4 Tg CO2 Eq.
The 2006 emission estimate was developed using IPCC
(2001) slope coefficients for 7 of the 8 operating smelters
without site-specific PFC measurements. If these slope
coefficients were revised to incorporate recent IPCC (2006)
4-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Table 4-61: Tier 2 Quantitative Uncertainty Estimates for C02 and PFC Emissions from Aluminum Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Aluminum Production
Aluminum Production
Aluminum Production
C02
CF4
C2F6
3.9
2.1
0.4
Lower Bound
3.7
1.9
0.3
Upper Bound
4.1
2.3
0.4
Lower Bound
-5%
-9%
-17%
Upper Bound
+5%
+9%
+ 17%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
slope data, overall PFC emission estimates for 2006 would
probably be on the order of 10 percent lower than current
estimates. Additionally, since these smelters are owned
by one company, data have been reported on a company-
wide basis as totals or weighted averages. Consequently,
uncertainties in anode effect minutes per cell day, slope
coefficients, and aluminum production have been applied to
the company as a whole, and not on a smelter-specific basis.
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. 2006 emission estimates for CF4 and
C2F6 are also overestimated.
This Inventory may slightly underestimate greenhouse
gas emissions from aluminum production and casting
because it does not account for the possible use of SF6 as a
cover gas or a fluxing and degassing agent in experimental
and specialized casting operations. The extent of such use in
the United States is not known. Historically, SF6 emissions
from aluminum activities have been omitted from estimates
of global SF6 emissions, with the explanation that any
emissions would be insignificant (Ko et al. 1993, Victor and
MacDonald 1998). The concentration of SF6 in the mixtures
is small and a portion of the SF6 is decomposed in the process
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.
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 assumed to be negligible and thus all SF6
used is assumed to be emitted into the atmosphere. Sulfur
hexafluoride has been used in this application around the
world for the last twenty years.
The magnesium industry emitted 3.2 Tg CO2 Eq. (0.1
Gg) of SF6 in 2006, representing a decrease of approximately
3 percent from 2005 emissions (see Table 4-62). The recent
closure of a production facility in Canada has resulted in
Table 4-62: SF6 Emissions from Magnesium Production
and Processing (Tg C02 Eq. and Gg)
(MacNealetal. 1990,GariepyandDube 1992,Koetal. 1993,
Ten Eyck and Lukens 1996, Zurecki 1996).
Recalculations Discussion
The 2005 emission estimates were updated to reflect
revised prebake smelter production data. This change has
resulted in a less than one percent increase in PFC and CO2
emissions for 2005.
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
5.4
5.6
3.0
2.9
2.9
3.4
3.2
3.3
3.2
Gg
0.2
0.2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
Industrial Processes 4-43
-------
supply pressures in North America for magnesium ingot
that may encourage the expansion of primary magnesium
production in the United States (USGS 2007a). The
automotive industry is continuing to work towards converting
components to magnesium for fuel efficiency gains. As a
result of this shift, magnesium die casting processing is
forecasted to grow by 3 percent for 2007 with another 4
percent gain in 2008 (NADCA 2007).
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 2006 from primary production, secondary production
(i.e., recycling), and die casting were generally reported
by Partnership participants. When a Partner did not report
emissions, they were estimated based on the metal processed
and emission rate reported by that Partner in previous years.
(The extrapolation was based on the trend shown by Partners
reporting in the current and previous years.) Emissions for
one Partner that is a secondary producer were estimated
based on the average emission factor for other Partners that
are secondary producers.
Emission factors for 2002 to 2006 for sand casting
activities were also acquired through the Partnership. The
1999 through 2006 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
4-63. 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 2006, which
accounted for 25 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 2006, 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, emissions 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 2006 were available from the USGS (USGS 2002,
2003, 2005, 2006, 2007b). 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
production and consumption (casting) statistics from USGS.
The primary production emission factors were 1.2 kg per
Table 4-63: SF6 Emission Factors (kg SF6 per metric ton
of Magnesium)
Die Permanent
Year Casting Mold Wrought Anodes
1999
2000
2001
2002
2003
2004
2005
2006
2.14a
0.72
0.72
0.71
0.81
0.81
0.76
0.86
2 1
2 1
2 1
2 1
2 1
2 1
2 1
2 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.
4-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 2006 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 4-63.
Uncertainty
To estimate the uncertainty of the estimated 2006 SF6
emissions from magnesium production and processing, EPA
estimated the uncertainties associated with three variables (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.
Additional uncertainties exist in these estimates,
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). As is the case for other
sources of SF6 emissions, total SF6 consumption data for
magnesium production and processing in the United States
were not available. Sulfur hexafluoride may also be used as
a cover gas for the casting of molten aluminum with high
magnesium content; however, to what extent this technique
is used in the United States is unknown.
The results of this Tier 2 quantitative uncertainty
analysis are summarized in Table 4-64. SF6 emissions
associated with magnesium production and processing
were estimated to be between 2.7 and 3.6 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 14 percent below to 14 percent above the
2006 emissions estimate of 3.2 Tg CO2 Eq.
Recalculations Discussion
Data from the USGS (USGS 2007b) slightly revised the
amount of magnesium processed in 2005 for the wrought,
sand and permanent mold casting sectors. In addition,
wrought production numbers for 1990 and 1992 were
revised to match historical USGS publications based on
a data review. Revisions were also made to the approach
for extrapolating and interpolating data for non-reporting
Partners in order to improve accuracy. Emission estimates
for secondary production activities by a non-reporting
Partner were added for the years 2001 through 2006. The
default historical emission factor for secondary production
from 1990 to 1998 was also revised to be held constant at
the 1999 Partner reported value. These changes resulted in
an average annual increase in SF6 emissions of 0.03 Tg CO2
Eq. (approximately 0.5 percent) for 1990 to 1998 and 0.6 Tg
CO2 Eq. (approximately 22 percent) for 2001 to 2005 relative
to the previous report.
Table 4-64: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Magnesium Production and Processing
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Magnesium Production
SFfi
3.2
2.7
3.6
-14%
+ 14%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-45
-------
Planned Improvements
As more work assessing the degree of cover gas
degradation and associated byproducts is undertaken and
published, results could potentially be used to refine the
emission estimates, which currently assume (per IPCC
Good Practice Guidance, IPCC 2006) that all SF6 utilized
is emitted to the atmosphere. EPA-funded measurements of
SF6 in die casting applications have indicated that the latter
assumption may be incorrect, with observed SF6 degradation
on the order of 20 percent (Bartos et al. 2007). 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) and Novec™ 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 during this inventory year in a limited fashion;
because the amounts are negligible these emissions are only
being monitored and recorded at this time. Additionally, as
more companies join the Partnership, in particular those
from sectors not currently represented, such as permanent
mold and anode casting, emission factors will be refined to
incorporate these additional data.
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 electro-thermic
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 electro-thermic 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 electro-thermic 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,
approximately 0.33 ton of zinc is produced for every ton of
EAF dust treated (Viklund-White 2000).
In 2006, U.S. primary and secondary zinc production
totaled 510,000 metric tons (USGS 2008 ). The resulting
emissions of CO2 from zinc production in 2006 were estimated
to be 0.5 Tg CO2 Eq. (529 Gg) (see Table 4-65). All 2006 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 electro-thermic-process zinc plant in Monaca, PA
(USGS 2004). In 2006, emissions, which are nearly half
those of 1990 (44 percent), increased by 14 percent over
2005 levels despite decreases in overall production due to
an increase in production from emissive secondary zinc
production processes.
Table 4-65: C02 Emissions from Zinc Production
(Tg C02 Eq. and Gg)
Year
Tg C02 Eq.
Gg
1990
1995
0.9
1.0
949
1,013
2000
2001
2002
2003
2004
2005
2006
1.1
1.0
0.9
0.5
0.5
0.5
0.5
1,140
986
937
507
477
465
529
4-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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:
PF _ 1.19 metric tons coke
i^ waelz mm- metric tons zinc X
0.84 metric tons C
• , x
metnc tons coke
3.67 metric tons CO2 _
metric tons C
3.66 metric tons CO2
metric tons zinc
The USGS disaggregates total U.S. primary zinc
production capacity into zinc produced using the electro-
thermic 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 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 13
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 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:
PF _ 0.4 metric tons coke
EAFDust~ metric tons EAF dust X
0.84 metric tons C
• \ x
metnc tons coke
3.67 metric tons CO2 _
metric tons C
1.23 metric tons CO2
metric tons EAF dust
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 36 percent
of total secondary zinc produced in 2003. This percentage
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
Industrial Processes 4-47
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Table 4-66: Zinc Production (Metric Tons)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Primary
262,704
231,840
227,800
203,000
181,800
186,900
188,200
191,120
113,000
Secondary
341,400
353,000
440,000
375,000
366,000
381,000
358,000
349,000
397,000
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 2006 activity data for primary and
secondary zinc production (see Table 4-66) were obtained
through the USGS Mineral Yearbook: Zinc (USGS 1994
through 2008).
Uncertainty
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
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 4-67. Zinc production CO2
emissions were estimated to be between 0.4 and 0.7 Tg CO2
Eq. at the 95 percent confidence level. This indicates a range
of approximately 21 percent below and 25 percent above the
emission estimate of 0.5 Tg CO2 Eq.
Table 4-67: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Zinc Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Zinc Production
CO,
0.5
0.4
0.7
-21%
+25%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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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 18 separate smelters located
in 11 states (USGS 2006). Secondary lead production has
increased in the United States over the past decade while
primary lead production has decreased. In 2006, secondary
lead production accounted for approximately 88 percent of
total lead production (Smith 2007, USGS 1995).
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 7 percent from 2005 to 2006 and has decreased by 62
percent since 1990 (Smith 2007, USGS 1995).
Approximately 92 percent of U.S. secondary lead is
produced by recycling lead acid batteries in either blast
furnaces or reverberatory furnaces (USGS 2006). The
remaining 8 percent of secondary lead is produced from lead
scrap. 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 increased by half a
percent from 2005 to 2006, and has increased by 25 percent
since 1990.
At last reporting, the United States was the third largest
mine producer of lead in the world, behind China and Australia,
accounting for 13 percent of world production in 2005 (USGS
2006). In 2006, U.S. primary and secondary lead production
totaled 1,313,000 metric tons (Smith 2007). The resulting
emissions of CO2 from 2006 production were estimated to
be 0.3 Tg CO2 Eq. (270 Gg) (see Table 4-68). The majority
of 2006 lead production is from secondary processes, which
account for 86 percent of total 2006 CO2 emissions.
After a gradual increase in total emissions from 1990 to
2000, total emissions have decreased by five percent since
1990, largely due to a decrease in primary production (62
percent since 1990) and a transition within the United States
Table 4-68: C02 Emissions from Lead Production
(Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
Gg
285
298
311
291
286
289
263
266
270
from primary lead production to secondary lead production,
which is less emissive than primary production, although
the sharp decrease leveled off in 2005 and even increased
slightly in 2006 (USGS 2006, 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 2006 activity data for primary and
secondary lead production (see Table 4-69) were obtained
Table 4-69: Lead Production (Metric Tons)
Year
Primary
Secondary
1990
1995
404,000
374,000
922,000
1,020,000
2000
2001
2002
2003
2004
2005
2006
341,000
290,000
262,000
245,000
148,000
143,000
153,000
1,130,000
1,090,000
1,100,000
1,140,000
1,127,000
1,154,000
1,160,000
Industrial Processes 4-49
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Table 4-70: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Lead Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Lead Production
CO,
0.3
0.2
0.3
-16%
+16%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
through the USGS Mineral Yearbook: Lead (USGS 1994
through 2008).
Uncertainty
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 4-70. 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 16 percent below and 16 percent above the
emission estimate of 0.3 Tg CO2 Eq.
Recalculations Discussion
Estimates of CO2 emissions from lead production were
revised for the 2001, 2002, 2004, 2005, and 2006 to reflect
updated secondary production activity (USGS 2008). This
change resulted in a less than 2 percent decrease in emissions
for 2001 and 2002, and a less than 2 percent increase in
emissions for 2004 and 2005.
4.19. HCFC-22 Production (IPCC
Source Category 2E1)
Trifluoromethane (HFC-23 or CHF3) is generated as a
byproduct during the production 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.13 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
13 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].
4-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-71: HFC-23 Emissions from HCFC-22
Production (Tg C02 Eq. and Gg)
Table 4-72: HCFC-22 Production (Gg)
Year
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
36.4
33.0
28.6
19.7
21.1
12.3
17.2
15.8
13.8
Gg
3
3
2
2
2
1
1
1
1
from HCFC-22, the HFC-23 is generally vented to the
atmosphere as an unwanted byproduct, but it is sometimes
captured for use in a limited number of applications.
Emissions of HFC-23 in 2006 were estimated to be
13.8 Tg CO2 Eq. (1.2 Gg) (Table 4-71). This quantity
represents a 13 percent decline from 2005 emissions and a
62 percent decline from 1990 emissions. Both declines are
primarily due to decreases in the HFC-23 emission rate.
These decreases are primarily attributable to four factors:
(a) five plants that did not capture and destroy the HFC-23
generated have ceased production of HCFC-22 since 1990,
(b) one plant that captures and destroys the HFC-23 generated
began to produce HCFC-22, (c) one plant implemented and
documented a process change that reduced the amount of
HFC-23 generated, and (d) 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.
Methodology
To estimate their emissions of HFC-23, five of the eight
HCFC-22 plants that have operated in the United States
since 1990 use (or, for those plants that have closed, used)
methods comparable to the Tier 3 methods in the 2006IPCC
Guidelines. 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
Gg
1990
1995
2000
2001
2002
2003
2004
2005
2006
139
155
186
152
149
138
155
156
154
factor for unoptimized plants operating before 1995 (0.04
kg HCFC-23/kg HCFC-22 produced).
The five plants that have operated since 1994 measure(d)
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.
Production data and emission estimates were prepared
in cooperation with the U.S. producers of HCFC-22 (APxAP
1997,1999,2000,2001,2002,2003,2004,2005,2006,2007;
RTI 1997; RTI 2008). Annual estimates of U.S. HCFC-22
production are presented in Table 4-72.
Uncertainty
The uncertainty analysis presented in this section was
based on a Monte Carlo simulation as described in the 2006
IPCC Guidelines for each plant 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.
Industrial Processes 4-51
-------
Table 4-73: Quantitative Uncertainty Estimates for HFC-23 Emissions from HCFC-22 Production
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
HCFC-22 Production
HFC-23
13.8
12.9
15.2
-7%
+ 10%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-73. HCFC-22 production HFC
emissions were estimated to be between 12.9 and 15.2 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 13.8 Tg CO2 Eq.
Recalculations
EPA recently completed a comprehensive review of
plant-level estimates of HFC-23 emissions and HCFC-22
production (RTI, 2008). This review resulted in generally
small adjustments to estimates of HCFC-22 production and
HFC-23 emissions. As noted above, the HFC-23 emissions
for three plants that operated in the early 1990s were re-
calculated to conform with the 2006IPCC Guidelines using
the Tier 1 emission factor of 0.04 kg HFC-23/kg HCFC-
22. This revision increased the estimated U.S. emissions
by 4 to 6 percent for 1990 to 1993. The largest adjustment
was for the year 1995, for which the HFC-23 emissions
estimate increased by 22 percent. This increase reflected
a correction made by one plant to its emissions estimate.
This calculation was documented in the plant's files and
made the plant's 1995 emission rate more consistent with
its emission rates for previous and following years. There
were also minor revisions (ranging from -4 percent to
+10 percent) to the emissions estimated for 2000, 2002,
2004 and 2005. These changes reflected revisions that
plants made to their estimates after they were submitted
to the Alliance for Responsible Atmospheric Policy, which
aggregates the emissions of the plants and sends the total
to EPA. Again, the revised estimates were documented in
the plants' files.
4.20. Substitution of Ozone
Depleting Substances (IPCC Source
Category 2F)
Hydrofluorocarbons (HFCs) andperfluorocarbons (PFCs)
are used as alternatives to several classes of ozone depleting
substances (ODSs) that are being phased out under the terms
of the Montreal Protocol and the Clean Air Act Amendments
of 1990.14 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 4-74 and Table 4-75.
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.15 In 1993,
the use of HFCs in foam production began, and in 1994
these compounds also found applications as solvents and
sterilants. In 1995, ODS substitutes for halons entered
widespread use in the United States as halon production
was phased-out.
14 [42 U.S.C § 7671, CAA § 601]
contains HFC-125, HFC-143a, and HFC-134a.
4-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-74: Emissions of MFCs and PFCs from ODS Substitutes (Tg C02 Eq.)
Gas 1990
HFC-23 +
HFC-32 +
HFC-125 +
HFC-134a +
HFC-143a +
HFC-236f3 +
CF4 +
Others* 0.3
Total 0.3
1995
+
+
0.8
25.4
0.5
0.2
+
1.6
28.5
2000
+
+
5.2
57.2
4.1
0.5
+
4.0
71.2
2001
+
0.1
6.0
62.0
5.4
0.6
+
3.9
78.0
2002
+
0.1
6.8
66.3
6.8
0.6
+
4.3
85.0
2003
+
0.2
7.8
70.0
8.3
0.7
+
4.9
92.0
2004
+
0.3
9.0
73.8
10.1
0.7
+
5.2
99.1
2005
+
0.4
10.3
76.3
12.2
0.8
+
5.4
105.4
2006
+
0.6
12.3
76.6
14.4
0.8
+
5.7
110.4
+ Does not exceed 0.05 Tg C02 Eq.
* Others include HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee, C4F10, 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 4-75: Emissions of MFCs and PFCs from ODS Substitutes (Mg)
Gas 1990
HFC-23 +
HFC-32 +
HFC-125 +
HFC-1343 +
HFC-1433 +
HFC-236f3 +
CF4 +
Others* M
1995
+
+
291
19,536
132
36
+
M
2000
1
44
1,873
44,001
1,089
85
1
M
2001
1
92
2,150
47,712
1,415
94
1
M
2002
1
166
2,442
51,016
1,781
103
1
M
2003
1
268
2,798
53,843
2,194
111
2
M
2004
1
400
3,220
56,787
2,654
118
2
M
2005
1
562
3,675
58,700
3,200
125
2
M
2006
1
913
4,395
58,923
3,782
131
2
M
M (Mixture of Gases)
+ Does not exceed 0.5 Mg
* Others include HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee, C4F10, and PFC/PFPEs, the latter being a proxy for a diverse collection of PFCs and
perfluoropolyethers (PFPEs) employed for solvent applications.
The use and subsequent emissions of HFCs and PFCs
as ODS substitutes has been increasing from small amounts
in 1990 to 110.4 Tg CO2 Eq. in 2006. 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 4-76 presents HFCs and PFCs emissions by end-
use sector for 1990 through 2006. The end-use sectors that
contributed the most toward emissions of HFCs and PFCs
Table 4-76: Emissions of HFCs and PFCs from ODS Substitutes (Tg C02 Eq.) by Sector
Gas 1990
Refrigeration/Air Conditioning +
Aerosols +
Fosms +
Solvents +
Fire Protection +
Total +
1995
19.3
8.1
+
0.9
+
28.5
2000
58.6
10.1
+
2.1
+
71.2
2001
65.3
10.3
+
1.8
+
78.0
2002
71.6
10.6
1.0
1.6
+
85.0
2003
111
10.8
1.8
1.3
+
92.0
2004
84.4
11.1
2.0
1.3
+
99.1
2005
90.1
11.3
2.2
1.3
0.5
105.4
2006
94.6
11.6
2.4
1.3
0.6
110.4
Industrial Processes 4-53
-------
as ODS substitutes in 2006 include refrigeration and air-
conditioning (94.6 Tg CO2 Eq., or approximately 86 percent),
aerosols (11.6TgCO2Eq.,or approximately 10 percent), and
foams (2.4 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
(55.8 Tg CO2 Eq.), followed by retail food and refrigerated
transport. 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 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, R-404A, and R-507A. These HFCs are
emitted to the atmosphere during equipment 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
4-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 byproducts, 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, andHFC-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 are released.
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
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 16 end-uses, comprising over 95
percent of the total emissions, and 5 other end-uses. 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—fire extinguishing
streaming agents. 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
Industrial Processes 4-55
-------
Table 4-77: Tier 2 Quantitative Uncertainty Estimates for HFC and PFC Emissions from ODS Substitutes
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Substitution of Ozone MFCs and
Depleting Substances PFCs
110.4
110.1
129.6
-0.3%
+17%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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
mobile air-conditioning and retail food refrigeration, as well
as the stock (MT) of retail food refrigerant.
The results of the Tier 2 quantitative uncertainty
analysis are summarized in Table 4-77. Substitution of ozone
depleting substances HFC and PFC emissions were estimated
to be between 110.1 and 129.6 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 0.3
percent below to 17 percent above the emission estimate of
110.4TgCO2Eq.
Recalculations Discussion
An extensive review of the chemical substitution trends,
market sizes, growth rates, and charge sizes, together with
input from industry representatives, resulted in updated
assumptions for the Vintaging Model. These changes resulted
in an average annual net decrease of 8.5 Tg CO2 Eq. (14
percent) in HFC and PFC emissions from the substitution of
ozone depleting substances for the period 1990 through 2005.
The refrigeration and air conditioning sector was the source
of the greatest change, with an average annual net decrease
of 10.2 Tg CO2 Eq. (15 percent) in emissions. This decrease
can be attributed to changes in the assumptions regarding the
quantity of emissions at end of life (disposal) across the entire
sector, based on revised assumptions considering input from
industry representatives, as well significant modification to
assumptions for chiller end uses, based on industry input.
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
4-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 4-78: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg C02 Eq.)
Gas
CF4
C2F6
C^FS
C4F8
HFC-23
SF6
NF3*
Total
1990
0.7
1.5
0.0
0.0
0.2
0.5
0.0
2.9
1995
1.3
2.5
0.0
0.0
0.3
0.9
0.0
5.0
2000
1.8
3.0
0.1
0.0
0.3
1.1
0.1
6.3
2001
1.3
2.1
0.1
0.0
0.2
0.7
0.1
4.5
2002
1.1
2.2
0.1
0.0
0.2
0.7
0.3
4.3
2003
1.0
2.1
0.1
0.1
0.2
0.8
0.2
4.3
2004
1.1
2.1
0.0
0.1
0.2
0.8
0.2
4.3
2005
1.1
2.0
0.0
0.1
0.2
1.0
0.2
4.4
2006
1.2
2.2
0.0
0.1
0.3
1.0
0.3
4.8
Note: Totals may not sum due to independent rounding.
* NF3 emissions are presented for informational purposes, using a GWP of 8,000, and are not included in totals.
Table 4-79: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Mg)
Gas
CF4
C2F6
C^FS
C4F8
HFC-23
SF6
NF3
1990
115
160
0
0
15
22
3
1995
193
272
0
0
25
38
6
2000
281
322
18
0
23
45
11
2001
202
230
14
0
15
31
12
2002
174
241
10
6
15
28
32
2003
161
227
14
9
16
35
30
2004
172
225
6
9
17
35
30
2005
169
217
5
13
18
41
26
2006
183
242
5
13
22
40
40
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 byproduct. 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 2006, total weighted emissions of all fluorinated
greenhouse gases by the U.S. semiconductor industry were
estimated to be 4.8 Tg CO2 Eq. Combined emissions of all
fluorinated greenhouse gases are presented in Table 4-78
and Table 4-79 for years 1990, 1995 and the period 2000 to
2006. The rapid growth of this industry and the increasing
complexity (growing number of layers) of semiconductor
products led to an increase in emissions of 149 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 34 percent between 1999
and 2006. Together, industrial growth and use of abatement
technologies resulted in a net increase in emissions of 64
percent between 1990 and 2006.
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
the absence of emission control strategies (Burton and
Beizaie 2001).16 The availability and applicability of Partner
data differs across the 1990 through 2006 time series.
Consequently, emissions from semiconductor manufacturing
were estimated using three distinct methods, one each for the
periods 1990 through 1994, 1995 through 1999, and 2000
and beyond.
16A Partner refers to a participant of 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.
Industrial Processes 4-57
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1990 through 1994
For 1990 through 1994, Partnership data was unavailable
and emissions were modeled using the PEVM (Burton and
Beizaie 2001).17 1990 to 1994 emissions are assumed to be
primarily uncontrolled, since reduction strategies such as
chemical substitution and abatement were not widespread
during this period.
PEVM is based on the assumption that PFC emissions
from semiconductor manufacturing vary with (1) the number
of layers on 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 (TMLA),
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 feature size),18 and
(2) product type (discrete, memory or logic).19 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 1C) specific to product type (Burton
and Beizaie 2001, ITRS 2007). PEVM derives historical
consumption of silicon (i.e., square inches) by linewidth
"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 emission factors, which
were derived from Partner reported data for those years.
18 By 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).
19 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.
technology from published data on annual wafer starts and
average wafer size (VLSI Research, 2007).
The emission factor in PEVM is the average of the
four historical emission factors derived by dividing the
total annual emissions reported by the Partners for each
year between 1996 and 1999 by the total TMLA estimated
for the Partners in each of those years. Since Partners are
not believed to have applied significant emission reduction
measures before 2000, the resulting average emission factor
does not reflect such measures.
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 accurate than PEVM estimated emissions,
and are used to generate total U.S. emissions when applicable.
The emissions reported by the Partners were divided by the
ratio of the total layer-weighted capacity of the plants operated
by the Partners and the total layer-weighted capacity of all
of the semiconductor plants in the United States; this ratio
represents the share of layer-weighted capacity attributable
to the Partnership. The layer-weighted capacity of a plant (or
group of plants) consists of the silicon capacity of that plant
multiplied by the estimated number of layers used to fabricate
products at that plant. This method assumes that Partners and
non-Partners have similar capacity utilizations and per-layer
emission factors. Plant capacity, linewidth technology, and
products manufactured information is contained in the World
Fab Watch (WFW) database, which is updated quarterly
(see for example, Semiconductor Equipment and Materials
Industry 2007).
2000 through 2006
The U.S. 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), however,
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
4-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
models emissions without such measures. The portion
of the U.S. total attributed to non-Partners is obtained by
multiplying PEVM's total world emissions figure by the
non-Partner share of total layer-weighted silicon capacity for
each year (as described above).20 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
industry (see Semiconductor Equipment and Materials
Industry 2007).21< 22<23
Gas-Specific Emissions
Two different approaches were also used to estimate
the distribution of emissions of specific PFCs. Before 1999,
when there was no consequential adoption of PFC-reducing
measures, a fixed distribution was assumed to apply to the
20 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.
21 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 was 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.
22 In 2006, the industry trend in co-owernship 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. layer-weighted
manufacturing capacity.
23 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.
entire U.S. industry. This distribution was based upon the
average PFC 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 2006 period, the 1990 through 1999
distribution was assumed to apply to the non-Partners.
Partners, however, began to report gas-specific emissions
during this period. Thus, gas specific emissions for 2000
through 2006 were estimated by adding the emissions
reported by the Partners to those estimated for the non-
Partners.24
Data Sources
Partners estimate their emissions using a range of
methods. For 2006, we assume that most Partners used
a method as least as accurate as the IPCC's Tier 2c
Methodology, recommended in the IPCC (2000), since that
has been their approach for the past several years. Although
some of the default emission factors have been updated in
the IPCC (2006) guidelines, as of the 2006 reporting year
Partners continue to use the IPCC (2000) default emission
factors.25 The Partners with relatively high emissions use
leading-edge manufacturing technology, the newest process
equipment. When purchased, this equipment is supplied with
PFC emission factors, measured using industry standard
guidelines (International Sematech, 2006). The larger
emitting Partners likely use these process-specific emission
factors instead of the less accurate default emission factors
provided in IPCC guidelines; however, the documentation
regarding Partner emissions is incomplete (Burton and
Kshetry, 2007).
Data used to develop emission estimates were prepared
in cooperation with the Partnership. Estimates of operating
plant capacities and characteristics for Partners and non-
Partners were derived from the Semiconductor Equipment
and Materials Industry (SEMI) World Fab Watch (formerly
International Fabs on Disk) database (1996 through 2007).
Estimates of silicon consumed by linewidth from 1990 through
2006 were derived from information from VLSI Research
24 In recent years, the Partnership started reporting gas-specific emissions using
GWP values from the Third Assessment Report (TAR), while in previous
years the values were taken from the Second Assessment Report (SAR). The
emissions reported here are restated using GWPs from the SAR.
25 Currently, the majority of Partners use the IPCC (2000) Tier 2c guidelines,
which most closely resemble the IPCC (2006) Tier 2a guidelines.
Industrial Processes 4-59
-------
(2007), and the number of layers per linewidth was obtained
from International Technology Roadmap for Semiconductors:
2006 Update (Burton and Beizaie 2001, ITRS 2007).
Uncertainty
A quantitative uncertainty analysis26 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 = ^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 the world TMLA estimate in 2006 is approximately
+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.27Arelative error of approximately 11 percentwas
estimated for the PEVM emission factor, based on the standard
deviation of the 1996 to 1999 emission factors.28
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 4-80. The emissions estimate for
total U.S. PFC emissions from semiconductor manufacturing
were estimated to be between 4.8 and 5.5 Tg CO2 Eq. at a 95
percent confidence level. This range represents 10 percent
below to 8 percent above the 2006 emission estimate of
5.1 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.
Table 4-80: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from Semiconductor
Manufacture (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate3
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Semiconductor
Manufacture
HFC, PFC,
and SF6
5.1
Lower Bound6
4.8
Upper Bound6
5.5
Lower Bound
-10%
Upper Bound
+ 8%
a Because the uncertainty analysis covered all emissions (including NF3), the emission estimate presented here does not match that shown in Table 4-78.
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.
26 All uncertainties listed in this section are 95 percent confidence
intervals.
27 Error propagation resulted in Partnership gas-specific uncertainties ranging
from 17 to 33 percent.
28The average of 1996 to 1999 emission factor is used to derive the PEVM
emission factor.
4-60 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Planned Improvements
The method to estimate non-Partner related emissions
(i.e., PEVM) is not expected to change (with the exception
of possible future updates to emission factors and added
technology nodes). 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 Partner emission reporting (e.g., adoption
of the IPCC (2006) guidelines). 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.)
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 switch gear 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 13.2 Tg CO2 Eq. (0.6 Gg) in 2006.
This quantity represents a 51 percent decrease from the
estimate for 1990 (see Table 4-81 and Table 4-82). This
decrease is believed to be a response to increases in the price
of SF6 during the 1990s and to a growing awareness of the
environmental impact of SF6 emissions, through programs
such as the EPA's SF6 Emission Reduction Partnership for
Electric Power Systems.
Table 4-81: SF6 Emissions from Electric Power Systems
and Electrical Equipment Manufacturers (Tg C02 Eq.)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Electric
Power
Systems
26.4
20.9
14.4
14.5
13.6
13.2
13.3
13.2
12.4
Electrical
Equipment
Manufacturers
0.3
0.5
0.7
0.6
0.8
0.7
0.7
0.8
0.8
Total
26.7
21.5
15.1
15.0
14.4
13.8
13.9
14.0
13.2
Table 4-82: SF6 Emissions from Electric Power Systems
and Electrical Equipment Manufacturers (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Emissions
1.1
0.9
0.6
0.6
0.6
0.6
0.6
0.6
0.6
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 2006 Emissions from Electric Power Systems
Emissions from electric power systems from 1999 to
2006 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
Industrial Processes 4-61
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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 2006,
Partner utilities, which for inventory purposes are defined
as utilities that either currently are or previously have been
part of the Partnership, represented between 42 percent and
46 percent of total U.S. transmission miles. For each year,
the emissions reported by Partner utilities were added to the
emissions estimated for utilities that have never participated
in the Partnership (i.e., non-Partners).
Emissions from Partner utilities were estimated using a
combination of reported data and, where reported data were
unavailable, interpolated or extrapolated data. If a Partner
utility did not provide data for a historical year, emissions
were interpolated between years for which data were available
or extrapolated based on Partner-specific transmission mile
growth rates. In 2006, non-reporting Partners account for
approximately 6 percent of the total emissions attributable
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 43
Partner utilities (representing approximately 24 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
less 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) = 0.88 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 were assumed to increase at an annual rate of 1.2
percent between 2000 and 2003 and 2.8 percent between
2003 and 2006.
As a final step, total 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 2006, 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
1998 period. To estimate global emissions, the PsAND
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).
(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-
4-62 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
lived pressurized equipment that is periodically serviced
during its lifetime.)
Emissions (kilograms SF6) = SF6 purchased to refill
existing equipment (kilograms) + nameplate capacity29
of retiring equipment (kilograms)
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.
Sulfur hexafluoride purchased to refill existing
equipment in a given year was assumed to be approximately
equal to the SF6 purchased by utilities in that year. 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
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 1998.
U.S. emissions between 1990 and 1998 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 PvAND 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, but may have been significant during the 1990
through 1999 period. This factor was not accounted for;
however, atmospheric studies confirmed that the downward
trend in the estimated global emissions between 1995 and
1998 was real (see the Uncertainty discussion below).
1990 through 2006 Emissions from Manufacture of
Electrical Equipment
The 1990 to 2006 emissions estimates for original
equipment manufacturers (OEMs) were derived by assuming
that manufacturing emissions equal 10 percent of the
quantity of SF6 charged into new equipment. The quantity
of SF6 charged into 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 charged into new equipment for
2001 to 2006 were estimated using partner reported data and
the total industry SF6 nameplate capacity estimate (128.4Tg
CO2 Eq. in 2006). 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 2006 was calculated. This ratio was then multiplied
by the total industry nameplate capacity estimate to derive
the amount of SF6 charged into 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'Connelletal. 2002).
29 Nameplate capacity is defined as the amount of SF6 within fully charged
electrical equipment.
Industrial Processes 4-63
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Uncertainty
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
4.1 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 2006
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 2006) 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 the quantity
of SF6 charged into equipment by equipment manufacturers,
which is projected from Partner provided nameplate capacity
data and industry SF6 nameplate capacity estimates, and the
manufacturers' SF6 emissions rate.
The results of the Tier 2 quantitative uncertainty analysis
are summarized in Table 4-83. Electrical Transmission and
Distribution SF6 emissions were estimated to be between 11.1
and 15.4 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of approximately 16 percent below and 17
percent above the emission estimate of 13.2 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.
However, U.S. emission patterns may differ from global
emission patterns.
Recalculations Discussion
Relative to the previous Inventory report, SF6 emission
estimates for the period 1990 through 2006 were updated
based on (1) new data from EPA's SF6 Emission Reduction
Partnership, (2) revisions to the assumptions used in
interpolating and extrapolating non-reported Partner data, (3)
new information on transmission mile growth available in the
UDI2007 database, (4) removal of double counting between
parent companies and their subsidiaries in UDI databases, and
(5) revision in the methodology for estimating 2001 to 2006
OEM emissions. For the period 1999 through 2006, estimates
have been revised to incorporate additional data from new
Partners. Additionally, Partner estimates are now based on
Partner-specific transmission mile growth rates, obtained via
the UDI 2001,2004, and 2007 databases. Partner data and the
industry SF6 nameplate capacity estimates are now used to
estimate OEM emissions from 2001 onwards, since NEMA
data for these years does not exist. Based on the revisions
listed above, SF6 emissions from electric transmission and
distribution decreased from 1990 to 2000 and in 2003 and
Table 4-83: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Electrical Transmission and
Distribution (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Electrical Transmission
and Distribution SFfi
13.2
11.1
15.4
-16%
+17%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-64 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
increased in 2002, 2004 and 2005, compared to the 1990 to the 1990 to 2005 inventory estimates and this year's estimates
2005 Inventory. The magnitude of the differences between varied by year and ranged from 0 to 6 percent.
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 4-84 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.30
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 emission estimates for electrical transmission and distribution were developed
using U.S. utility purchases of SF6 for electrical equipment. From 1999 through 2006, estimates were obtained from reports submitted by
participants in EPA's SF6 Emission Reduction Program 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 4-84:2006 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
182.1
-
-
7.6
3.2
22.6
Actual
110.4
2.5
13.8
4.8
3.2
13.2
- Not applicable.
30 See Annex 5 for a discussion of sources of SF6 emissions excluded from the actual emissions estimates in this report.
Industrial Processes 4-65
-------
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 2006 are reported in Table 4-85.
Methodology
These emission estimates were obtained from preliminary
data (EPA 2008), 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
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
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.
Table 4-85: NO,, CO, and NMVOC Emissions from Industrial Processes (Gg)
Gas/Source
NO,
Other Industrial Processes
Chemical & Allied Product Manufacturing
Metals Processing
Storage and Transport
Miscellaneous*
CO
Metals Processing
Other Industrial Processes
Chemical & Allied Product Manufacturing
Storage and Transport
Miscellaneous*
NMVOCs
Storage and Transport
Other Industrial Processes
Chemical & Allied Product Manufacturing
Metals Processing
Miscellaneous*
1990
591
343
152
88
3
5
4,125
2,395
487
1,073
69
101
2,422
1,352
364
575
111
20
1995
607
362
143
89
5
8
3,959
2,159
566
1,110
23
102
2,642
1,499
408
599
113
23
2000
626
434
95
81
14
2
2,217
1,175
538
327
154
23
1,773
1,067
412
230
61
3
2001
656
457
97
86
15
1
2,339
1,252
558
338
162
30
1,769
1,082
381
238
65
4
2002
534
390
63
63
17
2
1,744
895
444
258
107
39
2,036
1,346
401
226
42
20
2003
528
385
61
63
17
1
1,724
895
444
257
107
22
1,972
1,304
394
223
42
10
2004
524
381
61
63
17
1
1,724
895
444
257
107
22
1,931
1,274
386
220
42
9
2005
519
377
61
63
17
1
1,724
895
443
257
107
22
1,890
1,245
379
217
42
8
2006
515
373
61
62
17
1
1,724
895
443
257
107
22
1,849
1,215
372
214
41
7
* Miscellaneous includes the following categories: catastrophic/accidental release, other combustion, health services, cooling towers, and fugitive dust.
It does not include agricultural fires or slash/prescribed burning, which are accounted for under the Field Burning of Agricultural Residues source.
Note: Totals may not sum due to independent rounding.
4-66 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
5. Solvent and Other Product Use
Greenhouse gas emissions are produced as a byproduct 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 2006 (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.
eg
1990
4.4
14
1995
4.6
15
2000
4.9
16
2001
4.9
16
2002
4.4
14
2003
4.4
14
2004
4.4
14
2005
4.4
14
2006
4.4
14
5.1. Nitrous Oxide from Product Uses (IPCC Source Category 3D)
N2O 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 (CGA 2003). N2O 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 2006 was approximately 15 Gg. N2O emissions were 4.4 Tg CO2 Eq. (14 Gg) in 2006 (see 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 Emissions from N20 Product Uses
(Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
4.4
4.6
4.9
4.9
4.4
4.4
4.4
4.4
4.4
Gg
14
15
16
16
14
14
14
14
14
Methodology
Emissions from N2O product uses 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 Uses Emissions =
2, [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 2006, 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 emissions 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 emissions 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) 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
Table 5-3: N20 Production (Gg)
Year
Gg
1990
1995
16
17
2000
2001
2002
2003
2004
2005
2006
17
17
15
15
15
15
15
5-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 (CGA2002,2003). 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 2004, 2005, and 2006 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 2001 share
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 2004,2005, and 2006 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
The overall uncertainty associated with the 2006 N2O
emission estimate from N2O product uses 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. N2O emissions from
N2O product uses 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 2006 emissions estimate of 4.4 Tg CO2 Eq.
Recalculations Discussion
The N2O emis sion factor for medical applications has been
updated relative to the previous Inventory based on the revised
IPCC Guidelines for National Greenhouse Gas Inventories
(2006). The updated emission factor resulted in an average
increase in N2O emissions from N2O product uses relative to
the previous Inventory for each year in the 1990 through 2005
time series of 0.1 Tg CO2 Eq. (2 percent), respectively.
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
Table 5-4: Tier 2 Quantitative Uncertainty Estimates for N20 Emissions From N20 Product Uses
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
N20 Product Usage
N,0
4.4
4.3
4.5
-2%
+ 2%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Solvent and Other Product Use 5-3
-------
incorporate a time lag between production and ultimate
product use and resulting release of N2O.
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-CH4 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 byproducts, 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 2006 are reported in Table 5-5.
Table 5-5: Emissions of NOX, CO, and NMVOC from Solvent Use (Gg)
Activity
N08
Surface Coating
Graphic Arts
Degreasing
Dry Cleaning
Other Industrial Processes3
Non-Industrial Processes"
Other
CO
Surface Coating
Other Industrial Processes3
Dry Cleaning
Degreasing
Graphic Arts
Non-Industrial Processes"
Other
NMVOCs
Surface Coating
Non-Industrial Processes"
Degreasing
Dry Cleaning
Graphic Arts
Other Industrial Processes3
Other
1990
1
1
+
+
+
+
+
NA
5
+
4
+
+
+
+
NA
5,216
2,289
1,724
675
195
249
85
+
1995
3
2
1
+
+
+
+
+
5
1
3
1
+
+
+
NA
5,609
2,432
1,858
716
209
307
87
+
2000
3
3
+
+
+
+
+
+
46
46
+
+
+
+
+
+
4,384
1,767
1,676
316
265
222
98
40
2001
3
3
+
+
+
+
+
+
45
45
+
+
+
+
+
+
4,547
1,863
1,707
331
272
229
103
42
2002
5
5
+
+
+
+
+
+
1
1
+
+
+
+
+
+
3,881
1,590
1,457
283
232
195
88
36
2003
5
5
+
+
+
+
+
+
1
1
+
+
+
+
+
+
3,862
1,582
1,450
281
231
194
88
36
2004
5
5
+
+
+
+
+
+
1
1
+
+
+
+
+
+
3,854
1,579
1,447
281
230
194
88
36
2005
5
5
+
+
+
+
+
+
1
1
+
+
+
+
+
+
3,846
1,576
1,444
280
230
193
88
36
2006
5
5
+
+
+
+
+
+
1
1
+
+
+
+
+
+
3,839
1,573
1,441
280
229
193
87
36
a 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.
+ Does not exceed 0.5 Gg.
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: 1990-2006
-------
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 2008), 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
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.
Solvent and Other Product Use 5-5
-------
6. Agriculture
Figure 6-1
2006 Agriculture Chapter Greenhouse Gas
Emission Sources
265.0
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. CO2 emissions from on-farm
energy use are accounted for in the Energy chapter.
In 2006, the Agricultural sector was responsible for
emissions of 454.1 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. CH4
emissions from enteric fermentation and manure management
represent about 23 percent and 7 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 CK4. Agricultural
soil management activities such as fertilizer application and
Agricultural Soil Management
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of
Agricultural Residues
Agriculture
as a Portion of
all Emissions
50
100 150
Tg C02 Eq.
200
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
165.7
126.9
31.0
7.1
0.7
281.8
269.4
12.1
0.4
447.5
1995
175.8
132.3
35.2
7.6
0.7
278.0
264.8
12.8
0.4
453.8
2000 2001 2002 2003 2004 2005 2006
171.7 172.2 172.6 173.0 170.9 174.0 174.4
124.6 123.6 123.8 124.6 122.4 124.5 126.2
38.8 40.2 41.3 40.7 40.1 41.8 41.4
7.5 7.6 6.8 6.9 7.6 6.8 5.9
0.8 0.8 0.7 0.8 0.9 0.9 0.8
276.3 291.5 276.4 261.3 261.2 279.6 279.8
262.1 277.0 262.0 247.3 246.9 265.2 265.0
13.7 14.0 14.0 13.6 13.8 13.9 14.3
0.5 0.5 0.4 0.4 0.5 0.5 0.5
447.9 463.7 449.0 434.3 432.1 453.6 454.1
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
7,890
6,044
1,474
339
33
909
869
39
1
1995
8,373
6,302
1,676
363
32
897
854
41
1
2000 2001 2002 2003 2004 2005 2006
8,174 8,201 8,219 8,236 8,138 8,284 8,304
5,933 5,886 5,896 5,931 5,828 5,928 6,010
1,847 1,915 1,964 1,938 1,908 1,988 1,972
357 364 325 328 360 326 282
38 37 34 38 42 41 39
891 940 892 843 842 902 902
845 894 845 798 796 855 855
44 45 45 44 44 45 46
1111222
Note: Totals may not sum due to independent rounding.
other cropping practices were the largest source of U. S . N2O
emissions, accounting for 72 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 2006, CH4
emissions from agricultural activities increased by 5 percent,
while N2O emissions fluctuated from year to year, but overall
decreased by less than 1 percent.
6.1. Enteric Fermentation (IPCC
Source Category 4A)
j 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 CtLj as a byproduct, which
can be exhaled or eructated by the animal. The amount of
CtLj 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 CtLj 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 cannot. Ruminant animals, consequently,
have the highest CtLj emissions among all animal types.
Non-ruminant animals (e.g., swine, horses, and mules)
also produce CtLj emissions through enteric fermentation,
although this microbial fermentation occurs in the large
intestine. These non-ruminants emit significantly less CtLj
on a per-animal basis than ruminants because the capacity
of the large intestine to produce CtLj is lower.
In addition to the type of digestive system, an animal's
feed quality and feed intake also affects CtLj emissions. In
general, lower feed quality and/or higher feed intake lead to
higher CtLj 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).
CH4 emission estimates from enteric fermentation are
provided in Table 6-3 and Table 6-4. Total livestock CtLj
emissions in 2006 were 126.2 Tg CO2 Eq. (6,010 Gg). Beef
cattle remain the largest contributor of CtLj emissions from
enteric fermentation, accounting for 71 percent in 2006.
Emissions from dairy cattle in 2006 accounted for 24 percent,
and the remaining emissions were from horses, sheep, swine,
and goats.
From 1990 to 2006, emissions from enteric fermentation
have decreased by less than 1 percent. Generally, emissions
have been decreasing since 1995 to 2004, mainly due to
decreasing populations of both beef and dairy cattle and
improved feed quality for feedlot cattle. The last two years
have shown an increase in emissions. During this timeframe,
populations of sheep have decreased 45 percent since 1990
while horse populations have increased over 80 percent,
6-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 6-3: CH4 Emissions from Enteric Fermentation (Tg C02 Eq.)
Livestock Type
Beef Cattle
Dairy Cattle
Horses
Sheep
Swine
Goats
Total
1990
89.9
31.2
1.9
1.9
1.7
0.3
126.9
1995
96.9
29.9
1.9
1.5
1.9
0.2
132.3
2000
90.4
28.9
2.0
1.2
1.9
0.3
124.6
2001
89.4
28.8
2.1
1.2
1.9
0.3
123.6
2002
89.3
29.0
2.3
1.1
1.9
0.3
123.8
2003
89.5
29.2
2.6
1.1
1.9
0.3
124.6
2004
87.2
28.9
3.0
1.0
1.9
0.3
122.4
2005
88.2
29.6
3.5
1.0
1.9
0.3
124.5
2006
89.2
30.3
3.5
1.0
1.9
0.3
126.2
Note: Totals may not sum due to independent rounding.
Table 6-4: CH4 Emissions from Enteric Fermentation (Gg)
Livestock Type
Beef Cattle
Dairy Cattle
Horses
Sheep
Swine
Goats
Total
1990
4,281
1,488
91
91
81
13
6,044
1995
4,616
1,422
92
72
88
12
6,302
2000
4,304
1,377
94
56
88
12
5,933
2001
4,257
1,374
99
55
88
12
5,886
2002
4,251
1,381
108
53
90
13
5,896
2003
4,260
1,393
126
51
90
13
5,931
2004
4,155
1,377
144
49
91
13
5,828
2005
4,198
1,411
166
49
92
13
5,928
2006
4,249
1,441
166
50
93
13
6,010
Note: Totals may not sum due to independent rounding.
mostly over the last 5 years. Goat and swine populations
have increased 1 percent and 14 percent, respectively, during
this timeframe.
Methodology
Livestock emission estimates 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 CJL, 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 describes the quantity of CH4
produced by individual ruminant animals, particularly cattle.
The Cattle Enteric Fermentation Model (CEFM), developed
by EPA to estimate cattle enteric CJL, emissions, incorporates
this information and other analyses of livestock population,
feeding practices and production characteristics were used
to estimate emissions from cattle populations.
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
Calf birth rates, 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
Agriculture 6-3
-------
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 2007).
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 CELj conversion rates (Ym) (expressed as the fraction of
gross energy converted to CFLj) 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, expert opinion, and modeling
of animal physiology. The diet characteristics for dairy cattle
were from Donovan (1999), while those for beef cattle were
derived from NRC (2000). DE and Ym for dairy cows were
calculated from diet characteristics using a model simulating
ruminant digestion in growing and/or lactating cattle (Donovan
and Baldwin 1999). For feedlot animals, DE and Ym values
recommended by Johnson (1999) were used. Values from EPA
(1993) were used for dairy replacement heifers. For grazing
beef cattle, DE values were based on diet information in
NRC (2000) and Ym values were based on Johnson (2002).
Weight data were estimated from Feedstuffs (1998), Western
Dairyman (1998), and expert opinion. See Annex 3.9 for more
details on the method used to characterize cattle diets 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 (e.g., dairy cows
and replacements, beef cows and replacements, heifer
and steer stackers, and heifer and steer in feedlots), and
production (e.g., 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 were multiplied by
the 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. CH4 emissions from these animals
accounted for a minor portion of total CK4 emissions from
livestock in the United States from 1990 through 2006. 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
the U.S. Department of Agriculture's National Agricultural
Statistics Service (USDA 2007). Horse population data were
obtained from the FAOSTAT database (FAO 2007), because
USDA does not estimate U.S. horse populations annually.
Goat population data were obtained for 1992,1997, and 2002
(USDA 2007); these data were interpolated and extrapolated
to derive estimates for the other years. CFLj 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.
1 Emissions from bulls are estimated using a Tier 1 approach because it is
assumed there is minimal variation in population and diets; calves younger
than 7 months are assumed to emit little or no CH4.
6-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Uncertainty
Quantitative uncertainty of 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 2006 activity
data and emission factor input variables used in the current
submission. Consequently, these uncertainty estimates were
directly applied to the 2006 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 112.3
to 148.9 Tg CO2 Eq., calculated as 11 percent below and 18
percent above the actual 2006 emission estimate of 126.2 Tg
CO2 Eq. Among the individual cattle sub-source categories,
beef cattle account for the largest amount of CFLj 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.
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. Particular emphasis was placed this year
on reviewing and implementing the revised IPCC Guidelines
(IPCC 2006). Additionally, as described below, this year the
CEFM was modified to allow generation of the estimates by
state, which required further QA/QC to ensure consistency
of estimates generated by the updated model.
Recalculations Discussion
There were several modifications that had an effect on
emission estimates, including:
• The Cfj (a coefficient used for calculating the net energy
required for maintenance) used for lactating cattle was
adjusted from 0.322 (previously used for all cattle) to
0.386, based on the revised IPCC equations (IPCC 2006).
This change had the effect of increasing the energy
requirement for maintenance of lactating cows and thus
increasing emissions for dairy cows by approximately
7 percent and beef cows by approximately 16 percent.
Table 6-5: Quantitative Uncertainty Estimates for CH4 Emissions from Enteric Fermentation (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate""
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Enteric Fermentation
CH4
126.2
112.3
148.9
-11%
+ 18%
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 2006 estimates.
Agriculture 6-5
-------
During the QA/QC process it was noted that the C
factor (a coefficient used in calculating the net energy
required for growth) of 0.8 was only being used for some
feedlot heifers, and all other cows and heifers were being
calculated using a C factor of 1.0. This has been updated
so that all cows and heifers use a C factor of 0.8 and all
steer use a C factor of 1.0, as stated in the revised IPCC
Guidelines (IPCC 2006). This change resulted in an
increase in emissions of between three and ten percent in
animal subcategories that experience weight gain (e.g.,
feedlot, replacement, and stacker animals), depending
on the subcategory.
The equation used to calculate the net energy of growth
(NEg), which is part of the gross energy equation, was
also updated to match the simplified equation provided in
the revised IPCC Guidelines (IPCC 2006). The equation
now reads:
Weight
NEg = 22.02:
CxMW
0.75
xWG1
Previously the equation used was:
NEg = 4.18 x 0.0635 x
478
0.89 Ix (Weight x 0.96) x
CxMW
(WGxO.92)1097
where,
NE0
= The net energy required for
growth, MJ/day
Weight = Average live body weight of the
animals in the population, kg
C = A coefficient that is 0.8 for females,
1.0 for steer, and 1.2 for bulls
MW = The mature weight of an adult
female in moderate condition, kg
WG = The average weight gain for ani-
mals in the population, kg/day
This change resulted in a decrease of less than one
half of one percent in animal subcategories that experience
growth (i.e., weight gain, including, feedlot, replacement,
and stacker animals).
• In the current Inventory, the CEFM, which was used
to calculate emissions from cattle enteric fermentation,
was updated to output results by individual state rather
than by regional groupings, during this process two
changes occurred. First, the averaging approach used
to calculate the step-up DE and Ym for feedlot animals
is based on an average of the feedlot and stacker diet
characteristics. Given that we changed the model to run
50 states rather than 7 regions, the final values for the
step-up diet characteristics changed slightly. Second,
the milk production numbers are now input at the state,
rather than regional level, which allows for data input at
a more detailed level. Both of these changes had a very
small effect on emissions compared to the additional
modifications, discussed above.
• Population estimates were revised by FAO for 2001
through 2005 for horses.
• The USDA published revised population estimates that
affected historical emissions estimated for swine in
2005. 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
increased an average of 99 Gg (7.6 percent) per year and beef
cattle increased an average of 435 Gg (11.1 percent) per year
over the entire time series. Historical emission estimates for
swine in 2005 increased by less than one half of one percent
as a result of the USDA revisions described above. Historical
emission estimates for horses increased by an average of 35
percent from 2001 through 2005.
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. Research is currently underway to
update the diet assumptions. There are a variety of models
available to predict methane production from cattle. Four
of these models (two mechanistic, and two empirical) are
being evaluated to determine appropriate Ym and DE values
for each cattle type and state. In addition to the model
evaluation, separate research is being conducted to update
the assumptions used for cattle diet components for each
animal type. At the conclusion of both of these updates, it
is anticipated that a peer-reviewed article will be published
and will serve as the basis for future emission estimates for
enteric fermentation.
6-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
In addition to the diet characteristics discussed above
several revisions will be investigated, including:
• The possible inclusion of bulls into the CEFM at a Tier
1 or 2 level;
• 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 from
national estimates to state-level estimates; and
• Including bison in the estimates for other domesticated
animals.
It is anticipated that these updates may result in
significant changes to some of the activity data used in
generating emissions. Additionally, since these revised inputs
will be state-specific and peer-reviewed, uncertainty ranges
around these variables will likely decrease. As a consequence,
the current uncertainty analysis will become outdated, and a
revision of the quantitative uncertainty 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 CF^ and N2O emissions. Methane is produced
by the anaerobic decomposition of manure. Direct N2O
emissions are produced as part of the nitrogen cycle through
the nitrification and denitrification of the organic nitrogen
in livestock manure and urine.2 Indirect N2O emissions are
produced as result of the volatilization of nitrogen as ammonia
(NH3) and nitrogen oxides (NOX) and runoff and leaching of
nitrogen 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 CFL,. 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
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 and produce little or no CFL^. 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 CFLj formation. For non-liquid-
based manure systems, moist conditions (which are a function
of rainfall and humidity) can promote CFLj production. Manure
composition, which varies by animal diet, growth rate, and
type, including the animal's digestive system, also affects the
amount of CK4 produced. In general, the greater the energy
content of the feed, the greater the potential for CFL, 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 aerobically where NH3 or organic nitrogen 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 nitrogen excreted
is expected to convert to N2O in the waste management
system (WMS). Indirect N2O emissions are produced when
nitrogen 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. 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.
Agriculture 6-7
-------
Estimates of CH4 emissions in 2006 were 41.4 Tg CO2
Eq. (1,972 Gg), 34 percent higher than in 1990. Emissions
increased on average by 0.6 Tg CO2 Eq. (2.0 percent)
annually over this period. The majority of this increase was
from swine and dairy cow manure, where emissions increased
34 and 49 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
(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-specific 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
have nearly doubled since 1990 (an 82 percent increase from
1990 to 2006); 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 2005 to 2006, there
was a 1 percent decrease in total CH4 emissions, due to minor
shifts in the animal populations and the resultant effects on
manure management system allocations and increased use
of anaerobic digesters.
In 2006, total N2O emissions were estimated to be 14.3
Tg CO2 Eq. (46 Gg); in 1990, emissions were 12.1 Tg CO2
Eq. (39 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 an 18 percent
increase from 1990 to 2006 and a 2.5 percent increase from
2005 through 2006.
Table 6-6 and Table 6-7 provide estimates of CH4 and N2O
emissions from manure management by animal category.
Table 6-6: CH4 and N20 Emissions from Manure Management (Tg C02 Eq.)
Gas/Animal Type
CH4a
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
N20"
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
Total
1990
31.0
12.0
2.5
13.1
0.1
+
2.8
0.5
12.1
3.5
5.5
1.2
0.1
+
1.5
0.2
43.0
1995
35.2
13.4
2.6
16.0
0.1
+
2.7
0.4
12.8
3.5
5.9
1.4
0.1
+
1.6
0.2
48.0
2000
38.8
15.8
2.4
17.4
0.1
+
2.6
0.5
13.7
3.6
6.7
1.4
0.1
+
1.7
0.2
52.5
2001
40.2
16.6
2.4
17.8
0.1
0.0
2.7
0.5
14.0
3.6
6.9
1.4
0.1
0.0
1.7
0.2
54.2
2002
41.3
17.3
2.4
18.3
0.1
+
2.7
0.5
14.0
3.7
6.7
1.5
0.1
+
1.7
0.3
55.2
2003
40.7
17.7
2.3
17.2
0.1
+
2.7
0.6
13.6
3.7
6.3
1.5
0.1
+
1.7
0.3
54.3
2004
40.1
17.2
2.3
17.1
0.1
+
2.6
0.7
13.8
3.7
6.5
1.5
0.1
+
1.7
0.3
53.9
2005
41.8
17.9
2.3
17.9
0.1
+
2.6
0.8
13.9
3.7
6.5
1.5
0.1
+
1.7
0.4
55.7
2006
41.4
17.9
2.5
17.5
0.1
+
2.7
0.8
14.3
3.8
6.7
1.5
0.1
+
1.8
0.4
55.7
+ Does not exceed 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
a Includes CH4 emission reductions due to anaerobic digestion.
b Includes both direct and indirect N20 emissions.
6-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
1,474
572
120
623
7
1
131
22
39
11
18
4
+
+
5
1
1995
1,676
638
121
762
5
1
128
21
41
11
19
5
+
+
5
1
2000
1,847
751
114
830
4
1
125
22
44
12
22
5
+
+
5
1
2001
1,915
792
117
849
4
1
129
23
45
12
22
5
+
+
5
1
2002
1,964
822
113
873
4
1
127
25
45
12
22
5
+
+
6
1
2003
1,938
844
112
821
4
1
127
29
44
12
20
5
+
+
6
1
2004
1,908
818
111
815
4
1
126
34
44
12
21
5
+
+
6
1
2005
1,988
854
112
853
4
1
126
39
45
12
21
5
+
+
6
1
2006
1,972
852
117
832
4
1
126
39
46
12
22
5
+
+
6
1
Note: Totals may not sum due to independent rounding.
a Includes CH4 emission reductions due to anaerobic digestion.
b Includes both direct and indirect N20 emissions.
+ Less than 0.5 Gg.
Methodology
The methodologies presented in IPCC (2006) form the
basis of the CFLj and N2O emission estimates for each animal
type. The calculation of emissions requires the following
information:
• Animal population data (by animal type and state);
• Amount of N produced (excretion rate by animal type
times animal population);
• Amount of volatile solids produced (excretion rate by
animal type times animal population);
• CFLj producing potential of the volatile solids (by animal
type);
• Extent to which the CFLj producing potential is realized
for each type of manure management system (by state
and manure management system, including the impacts
of any biogas collection efforts);
• Portion of manure managed in each manure management
system (by state and animal type); and
• Portion of manure deposited on pasture, range, or
paddock or used in daily spread systems.
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
CtLj and N2O emissions from manure management.
Both CH4 and N2O emissions were estimated by first
determining activity data, including animal population, waste
characteristics, and manure management system usage. For
swine and dairy cattle, manure management system usage
was determined for different farm size categories using data
from USDA (USDA 1996b, 1998b, 2000b) 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 2000c, UEP 1999). For other animal types, manure
management system usage was based on previous estimates
(EPA 1992).
MCFs and N2O emission factors were determined for
all manure management systems. MCFs for dry systems
were set equal to default IPCC factors based on each state's
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. The MCF calculations model the average monthly
ambient temperature, a minimum system temperature, the
carryover of volatile solids (VS) in the system from month
Agriculture 6-9
-------
to month due to long storage times exhibited by anaerobic
lagoon systems, and a factor to account for management and
design practices that result in the loss of VS from lagoon
systems. Direct N2O emission factors for all systems were
set equal to default IPCC factors (IPCC 2006). For indirect
N2O, the default indirect N2O emission factors suggested
by IPCC were used: 0.010 kg N2O-N/kg N for volatilization
and 0.0075 kg N2O-N/kg N for runoff/leaching. The amount
of nitrogen that is lost due to volatilization of NH3 and NOX
(FracGas) is based on WMS-specific volatilization values as
estimated from U.S. EPA's National Emission Inventory—
Ammonia Emissions from Animal Agriculture Operations
(EPA 2005). The amount of nitrogen that is lost due to runoff
and leaching (FracRmoff/Leaching) is based on regional cattle
runoff data from EPA's Office of Water (EPA 2002b).
CFLj emissions were estimated using the VS production
for livestock. For all cattle groups except bulls and calves,
regional animal-specific VS production rates that are related to
the diet of the animal for each year of the Inventory were used
(Pederson et al., 2007). For other animal groups, VS production
was calculated using a national average VS production rate
from the Agricultural Waste Management Field Handbook
(USDA 1996a), which was then multiplied by the average
weight of the animal and the state-specific animal population.
The resulting VS for each animal group were then multiplied
by the maximum CFLj producing capacity of the waste (B0)
and the state- and WMS-specific MCFs.
The maximum CFLj producing capacity of the VS, or B0,
was determined based on data collected in a literature review
(ERG 2000b). B0 data were collected for each animal type
for which emissions were estimated.
Anaerobic digester reductions for 1990-2005 were
estimated based on data from the EPA AgSTAR program,
including information presented in the AgSTAR Digest (EPA
2000,2003b, 2006). Anaerobic digestion reductions for 2006
were calculated based on data from an AgSTAR digester
inventory (ERG 2008).
Nitrogen excretion rates from the USDA Agricultural
Waste Management Field Handbook (USDA 1996a) were
used for all livestock except sheep, goats, and horses. Data
from the American Society of Agricultural Engineers (ASAE
1999) were used for these animal types.
Direct N2O emissions were estimated by determining
total Kjeldahl nitrogen (TKN)3 production for all livestock
wastes using a national average N excretion rate for
each animal group from USDA (1996a), which was then
multiplied by the average weight of the animal and the state-
specific animal population. State- and WMS-specific direct
N2O emission factors were then applied to total nitrogen
production to estimate direct N2O emissions.
Indirect N2O emissions were calculated by first
estimating the amount of nitrogen loss from volatilization
and runoff/leaching by multiplying the N excreted by
FracGas and FracRunoff/Leaching. The N losses were then
multiplied by the indirect N2O emission factors to estimate
indirect N,O emissions.
Uncertainty
An analysis was conducted for the manure management
emission estimates presented in EPA's Inventory of U.S.
Greenhouse Gas Emissions and Sinks: 1990-2001 (EPA
2003a, ERG 2003) to determine the uncertainty associated
with estimating CELj and N2O emissions from livestock manure
management. The quantitative uncertainty analysis for this
source category was 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 CELj 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 2006 were estimated to be between 34.0 and
49.7 Tg CO2 Eq. at a 95 percent confidence level, which
indicates a range of 18 percent below to 20 percent above the
actual 2006 emission estimate of 41.4 Tg CO2 Eq. At the 95
percent confidence level, N2O emissions were estimated to
be between 12.0 and 17.7 Tg CO2 Eq. (or approximately 16
percent below and 24 percent above the actual 2006 emission
estimate of 14.3 Tg CO2 Eq.).
3 Total Kjeldahl nitrogen is a measure of organically bound nitrogen and
ammonia nitrogen.
6-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Manure Management
Manure Management
CH4
N20
41.4
14.3
Lower Bound
34.0
12.0
Upper Bound
49.7
17.7
Lower Bound
-18%
-16%
Upper Bound
+20%
+24%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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 emissions4 from managed systems and
CtLj 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 nitrogen excreted and the sum of county
estimates for the full time series.
Recalculations Discussion
There was a major change in the N2O and CH4 emissions
calculations for the current Inventory. These emissions are
now calculated from the "bottom up" such that CFLj and N2O
are calculated for each animal group, manure management
system, and state. These values are then summed to calculate
the total greenhouse gas emissions from manure management
in the United States. This methodology differs from previous
Inventories which calculated state weighted average N2O
emission factors and methane conversion factors (MCFs).
Although this new methodology does not alter the overall
estimates of greenhouse gases associated with this section,
it now allows emissions to be viewed by animal type and
manure management system at the state and national level.
In the previous Inventory, dairy heifers and beef on feed
each had a separate WMS distribution for managed systems
and unmanaged systems. The managed WMS distribution was
4N2O emissions in the previous Inventory reflect only direct emissions
whereas the current N2O emissions include both direct and indirect emissions
from livestock manure management.
used to calculate a state average EF for managed systems. In
the current inventory methodology, dairy heifers and beef on
feed have one WMS distribution that represents managed and
unmanaged systems. For all animals, emissions are calculated
for each WMS using the EF for that system, and not using
a state average EF. This change in calculation methodology
results in a slightly different (less than one percent change)
emission estimate for these animal groups.
The Inventory now includes indirect N2O emissions
in the manure management sector associated with N losses
from volatilization of N as ammonia (NH3), nitrogen oxides
(NOX), and leaching and runoff, as recommended by IPCC
(2006). These indirect N2O emissions are added to the direct
N2O emissions to present a more complete picture of N2O
emissions from manure management.
The days per year used in N2O calculations was changed
from 365 to 365.25 to include leap years and to be consistent
with the CFLj inventory calculations.
Methane emission reductions from anaerobic digestion
for 2006 were calculated from an AgSTAR digester inventory
by summing the estimated emission reductions by animal
type (ERG 2008). Anaerobic digestion reductions in previous
years were based on data obtained from AgSTAR Digests
(EPA 2000, 2003b, 2006).
Errors were identified in the calculation of the sheep
WMS distribution; population values for other states were
incorrectly distributed in the calculations. Correcting this
error resulted in very small changes in N2O emissions
estimates from sheep.
Changes were made to the current calculations involving
animal population data. Animal population data were
updated to reflect the final estimates reports from USDA
NASS (USDA 1994a-b, 1995a-b, 1998a-b, 1999a-c, 2000a,
2004a-e, 2006a-c, 2007a-d). The population data may differ
from previous Inventories because some values changed due
Agriculture 6-11
-------
to USDA NASS review. For horses, state-level populations
were estimated using the national FAO population data (FAO
2007) and the state distributions from the 1992, 1997, and
2002 Census of Agriculture (USDA 2005). The FAO horse
population estimates for recent years increased dramatically
between the current and previous Inventories, resulting in a
much larger estimated horse population, and therefore greater
greenhouse gas emissions from this source category.
With these recalculations, CH4 emission estimates
from manure management systems are slightly higher than
reported in the previous Inventory for dairy cattle and swine,
as well as horses for years 2001 through 2005. On average,
annual CH4 emission estimates are more than those of the
previous Inventory by about one percent.
N2O emission estimates from manure management
systems have increased by approximately 30 percent for
all years of the current Inventory compared to the previous
Inventory due to the change in calculation methodology,
which incorporates direct and indirect N2O emissions. The
most significant changes in N2O emissions compared to the
previous Inventory occurred in the poultry and swine sectors,
whose emissions were approximately 70 percent higher due
to the inclusion of indirect N2O emissions.
Changes were made to the Cattle Enteric Fermentation
Model that produces the VS estimates for all cattle groups
except bulls and calves. Refer to the Recalculations section
in Enteric Fermentation to see specific changes made to
the model.
Planned Improvements
The Manure Management emission estimates will be
updated to reflect changes in the Cattle Enteric Fermentation
Model (CEFM). In addition, efforts will be made to ensure
that the manure management estimates and CEFM are using
the same data sources and variables where appropriate.
The American Society of Agricultural Engineers proposed
new standards for manure production characteristics in 2004
and finalized them in 2005. These data were investigated and
evaluated for incorporation into future estimates.
A method to better estimate anaerobic digester CH4
emission reductions will be investigated. This method would
include separating systems with anaerobic digesters from the
total animal population before estimating CH4 emissions,
and then estimating emissions from the digesters using the
amount of biogas/CFLj collected and a 99 percent destruction
efficiency.
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 in the current Inventory,
including estimation of emissions at the WMS level and
the use of new calculations and variables for indirect N2O
emissions.
The current methodology for calculating runoff for
indirect N2O emissions will be reevaluated. Currently,
runoff is estimated at all manure management systems based
on outdoor cattle operations. A new methodology may be
incorporated which takes into account more recent model
runs from EPA's Office of Water.
In order to improve the efficiency of MCF calculations,
MCFs will be calculated in a database instead of spreadsheets
in the next inventory. Calculating MCFs in a database
will also increase the overall efficiency of CFLj emission
estimates by linking directly to the database that calculates
CH4 estimates.
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 CFLj produced is oxidized by aerobic methanotrophic
bacteria in the soil (some oxygen remains 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 CFLj 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 CFLj also escape from the soil via diffusion and bubbling
through floodwaters.
6-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 CK4. 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,
CtLj emissions decrease or stop entirely. This is due to soil
aeration, which not only causes existing soil CH4 to oxidize
but also inhibits further CH/j 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,5 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 CtLj emissions; in particular, both
nitrate and sulfate fertilizers (e.g., ammonium nitrate and
ammonium sulfate) appear to inhibit CK4 formation.
Rice is cultivated in eight states: Arkansas, California,
Florida, Louisiana, Mississippi, Missouri, Oklahoma, and
Texas.6 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 Arkansas, southwest Louisiana, Texas, and
Florida allow for a second, or ratoon, rice crop. CLLj 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 2006, CH4 emissions
from rice cultivation were 5.9 Tg CO2 Eq. (282 Gg). Although
annual emissions fluctuated unevenly between the years 1990
and 2006, ranging from an annual decrease of 14 percent
to an annual increase of 17 percent, there was an overall
decrease of 17 percent over the sixteen-year period, due to
an overall decrease in primary crop area.7 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.
5 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.
6 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 emissions estimates.
7The 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
Primary
Arkansas
California
Florida
Louisiana
Mississippi
Missouri
Oklahoma
Texas
Ratoon
Arkansas
Florida
Louisiana
Texas
Total
1990
5.1
2.1
0.7
+
1.0
0.4
0.1
+
0.6
2.1
+
+
1.1
0.9
7.1
1995
5.6
2.4
0.8
+
1.0
0.5
0.2
+
0.6
2.1
+
0.1
1.1
0.8
7.6
2000
5.5
2.5
1.0
+
0.9
0.4
0.3
+
0.4
2.0
+
0.1
1.3
0.7
7.5
2001
5.9
2.9
0.8
+
1.0
0.5
0.4
+
0.4
1.7
+
+
1.1
0.6
7.6
2002
5.7
2.7
0.9
+
1.0
0.5
0.3
+
0.4
1.1
+
+
0.5
0.5
6.8
2003
5.4
2.6
0.9
+
0.8
0.4
0.3
+
0.3
1.5
+
+
1.0
0.5
6.9
2004
6.0
2.8
1.1
+
1.0
0.4
0.3
+
0.4
1.6
+
+
1.1
0.5
7.6
2005
6.0
2.9
0.9
+
0.9
0.5
0.4
+
0.4
0.8
+
+
0.5
0.4
6.8
2006
5.1
2.5
0.9
+
0.6
0.3
0.4
+
0.3
0.9
+
+
0.5
0.4
5.9
+ Less than 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Table 6-10: CH4 Emissions from Rice Cultivation (Gg)
State
Primary
Arkansas
California
Florida
Louisiana
Mississippi
Missouri
Oklahoma
Texas
Ratoon
Arkansas
Florida
Louisiana
Texas
Total
1990
241
102
34
1
46
21
7
+
30
98
+
2
52
45
339
1995
265
114
40
2
48
24
10
+
27
98
+
4
54
40
363
2000
260
120
47
2
41
19
14
+
18
97
+
2
61
34
357
2001
283
138
40
1
46
22
18
+
18
81
+
2
52
27
364
2002
274
128
45
1
45
22
15
+
18
52
+
2
25
24
325
2003
255
124
43
+
38
20
15
+
15
73
+
2
50
22
328
2004
283
132
50
1
45
20
17
+
19
77
+
2
50
24
360
2005
287
139
45
1
45
22
18
+
17
39
1
+
22
17
326
2006
241
119
44
1
29
16
18
+
13
41
+
1
22
18
282
+ Less than 0.5 Gg
Note: Totals may not sum due to independent rounding.
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
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).
6-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
The harvested rice areas for the primary and ratoon
crops in each state are presented in Table 6-11. Primary
crop areas for 1990 through 2006 for all states except
Honda 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 2007). Harvested rice areas in Florida, which are not
reported by USDA, were obtained from: Tom Schueneman
(1999b, 1999c, 2000, 2001a) and Arthur Kirstein (2003,
2006), Florida agricultural extension agents; Dr. Chris Deren
(2002) of the Everglades Research and Education Centre
at the University of Florida; Gaston Cantens (2004, 2005),
Vice President of Corporate Relations of the Honda Crystals
Company; and Rene Gonzalez (2007a), Plant Manager of
Sem-Chi Rice Company. Harvested rice areas for Oklahoma,
which also are not reported by USDA, were obtained from
Danny Lee of the Oklahoma Farm Services Agency (2003
through 2007). Acreages for the ratoon crops were derived
from conversations with the agricultural extension agents
in each state. In Arkansas, ratooning occurred only in 1998,
1999, 2005, and 2006, when the ratooned area was less
than 1 percent of the primary area (Slaton 1999 through
Table 6-11: Rice Areas Harvested (Hectares)
2001a; Wilson 2002 through 2007). In Horida, the ratooned
area was 50 percent of the primary area from 1990 to 1998
(Schueneman 1999a), about 65 percent of the primary area in
1999 (Schueneman 2000), around 41 percent of the primary
area in 2000 (Schueneman 2001a), about 60 percent of the
primary area in 2001 (Deren 2002), about 54 percent of the
primary area in 2002 (Kirstein 2003), about 100 percent of
the primary area in 2003 (Kirstein 2004), about 77 percent
of the primary area in 2004 (Cantens 2005), 0 percent of the
primary area in 2005 (there was no ratooning this year due to
Hurricane Wilma), and about 28 percent of the primary area
in 2006 (Gonzalez 2007a). In Louisiana, the percentage of
the primary area that was ratooned was constant at 30 percent
over the 1990 to 1999 period, increased to approximately 40
percent in 2000, returned to 30 percent in 2001, dropped to
15 percent in 2002, rose to 35 percent in 2003, returned to 30
percent in 2004, dropped to 13 percent in 2005 and increased
to 20 percent in 2006 (Linscombe 1999,2001a, 2002 through
2007; Bollich 2000). In Texas, the percentage of the primary
area that was ratooned was constant at 40 percent over the
1990 to 1999 period, increased to 50 percent in 2000 due to
an early primary crop, and then decreased to 40 percent in
2001, 37 percent in 2002,38 percent in 2003, 35 percent in
2004,27 percent in 2005 and increased to 39 percent in 2006
State/Crop
Arkansas
Primary
Ratoon*
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
66,168
101,174
32,376
617
142,857
57,143
1,148,047
125,799
1,273,847
1995
542,291
0
188,183
9,713
4,856
230,676
69,203
116,552
45,326
364
128,693
51,477
1,261,796
125,536
1,387,333
2000 2001 2002 2003 2004 2005 2006
570,619 656,010 608,256 588,830 629,300 661,675 566,572
0 0 0 0 0 662 6
221,773 190,611 213,679 205,180 238,770 212,869 211,655
7,801 4,562 5,077 2,369 3,755 4,565 4,575
3,193 2,752 2,734 2,369 2,899 0 1,295
194,253 220,963 216,512 182,113 215,702 212,465 139,620
77,701 66,289 32,477 63,739 64,711 27,620 27,924
88,223 102,388 102,388 94,699 94,699 106,435 76,487
68,393 83,772 73,654 69,203 78,915 86,605 86,605
283 265 274 53 158 271 17
86,605 87,414 83,367 72,845 88,223 81,344 60,704
43,302 34,966 30,846 27,681 30,878 21,963 23,675
1,237,951 1,345,984 1,303,206 1,215,291 1,349,523 1,366,228 1,146,235
124,197 104,006 66,056 93,790 98,488 50,245 52,899
1,362,148 1,449,991 1,369,262 1,309,081 1,448,011 1,416,473 1,199,135
*Arkansas ratooning occurred only in 1998,1999, 2005, and 2006.
Note: Totals may not sum due to independent rounding.
Agriculture 6-15
-------
(Klosterboer 1999, 2000, 2001a, 2002, 2003; Stansel 2004,
2005; Texas Agricultural Experiment Station 2006, 2007).
California, Mississippi, Missouri, and Oklahoma have not
ratooned rice over the period 1990 through 2006 (Guethle
1999, 2000, 2001a, 2002 through 2007; Lee 2003 through
2007; Mutters 2002 through 2005; Street 1999 through 2003;
Walker 2005, 2007).
To determine what CEL, 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 which involved atypical or nonrepresentative
management practices (e.g., the application of nitrate
or sulfate fertilizers, or other substances believed to
suppress CK4 formation), as well as experiments in which
measurements were not made over an entire flooding season
or floodwaters were drained mid-season, were excluded
from the analysis. The remaining experimental results8 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
(LindauandBollich 1993,Lindauetal. 1995) were averaged
to derive an emission factor for the ratoon crop. The resultant
emission factor for the primary crop is 210 kg CEL/hectare-
season, and the resultant emission factor for the ratoon crop
is 780 kg CEL/hectare-season.
Uncertainty
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
8 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 2.041 g/m2/day 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.
differences in cultivation practices, in particular, fertilizer
type, 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 CEL/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
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 6-12. Rice cultivation CEL, emissions
in 2006 were estimated to be between 2.1 and 12.8 Tg CO2
Eq. at a 95 percent confidence level, which indicates a range
of 65 percent below to 117 percent above the actual 2006
emission estimate of 5.9 Tg CO2 Eq.
6-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 6-12: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Rice Cultivation
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Rice Cultivation
CH,
5.9
2.1
12.8
-65%
+ 117%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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.
Recalculations Discussion
When compiling the previous Inventory, no data on
area harvested and percent of area ratooned in Florida were
available for 2005, and consequently 2004 data was held
constant. For the current Inventory, Gonzalez (2007a) was
able to provide data for 2005 as well as 2006, resulting in an
decrease of about 0.6 percent in the estimate for 2005.
6.4. Agricultural Soil Management
(IPCC Source Category 4D)
Nitrous oxide is produced naturally in soils through the
microbial processes of nitrification and denitrification.9 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).10 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.11 Indirect emissions of
N2O occur through two pathways: (1) volatilization and
subsequent atmospheric deposition of applied N,12 and (2)
surface runoff and leaching of applied 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 sources
(cropland, grassland, forest lands, settlements, and managed
manure) are reported in this chapter.
Agricultural soils produce the majority of N2O
emissions in the United States. Estimated emissions from
this source in 2006 were 265.0 Tg CO2 Eq. (855 Gg N2O)
(see Table 6-13 and Table 6-14). Annual N2O emissions
from agricultural soils fluctuated between 1990 and 2006,
although overall emissions were 1.6 percent lower in
9 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).
10 Drainage and cultivation of organic soils in former wetlands enhances
mineralization of N-rich organic matter, thereby enhancing N2O emissions
from these soils.
"Asymbiotic N fixation is the fixation of atmospheric N2 by bacteria living
in soils that do not have a direct relationship with plants.
12 These processes entail volatilization of applied 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 ammonium
(NH4), nitric acid (HNO3), and NOX.
Agriculture 6-17
-------
Figure 6-2
Agricultural Sources and Pathways of N that Result in N20 Emissions
N Volatilization
Synthetic N Fertilizers
N Inputs to
Managed Soils
Organic
Amendments
Includes both commercial and
non-commercial fertilizers (i.e,
animal manure, compost,
sewage sludge, tankage, etc)
N Volatilization
and Deposition
Urine and Dung from
Grazing Animals
Indirect N20
Emissions
Indudes above- and belowground
residues for all oops (non-N and N
crops and pastures following renewal
Mineralization of
Soil Organic Matter
Histosol
Cultivation
biomass burning are not currently
accounted for in the Inventory,
they are a potential source of N to
soils through volatilization and
deposition.
Storage and Management
of Livestock Manure
Includes non-NiO N emissions from storage and
management of manure used as fertilizer.
This graphic illustrates the sources and pathways of nitrogen that result In direct and Indirect N20 emissions from soils In the United States. Sources of nitrogen applied to, or deposited
on, soils are represented with arrows on the left-hand side of the graphic. Emission pathways are also shown with arrows. On the lower right-hand side Is a cut-away view of a
representative section of a manaaed soil: histosol cultivation is represented here.
2006 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 64 percent of total direct
emissions, while grassland accounted for approximately
36 percent. Estimated direct and indirect N2O emissions
by sub-source category are provided in Table 6-15 and
Table 6-16.
6-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 6-13: N20 Emissions from Agricultural Soils (Tg C02 Eq.)
Activity
Direct
Cropland
Grassland
Indirect (All Land-Use Types)
Cropland
Grassland
Forest Land
Settlements
Total
1990
218.3
130.9
87.4
51.1
30.1
20.6
+
0.3
269.4
1995
210.3
133.1
77.2
54.5
30.5
23.6
0.1
0.4
264.8
2000
216.0
142.0
74.0
46.0
28.4
17.1
0.1
0.4
262.1
2001
222.3
147.6
74.8
54.7
28.9
25.2
0.1
0.5
277.0
2002
217.7
137.1
80.6
44.3
24.8
18.9
0.1
0.5
262.0
2003
202.2
130.2
72.0
45.0
27.8
16.7
0.1
0.5
247.3
2004
208.6
136.1
72.5
38.3
21.6
16.1
0.1
0.5
246.9
2005
217.9
140.0
77.9
47.3
28.4
18.3
0.1
0.5
265.2
2006
214.7
138.9
75.8
50.3
30.2
19.5
0.1
0.5
265.0
+ Less than 0.05 Tg C02 Eq.
Table 6-14: N20 Emissions from Agricultural Soils (Gg)
Activity
Direct
Cropland
Grassland
Indirect (All Land-Use Types)
Cropland
Grassland
Forest Land
Settlements
Total
1990
704
422
282
165
97
67
+
1
869
1995
678
429
249
176
98
76
+
1
854
2000
697
458
239
149
92
55
+
1
845
2001
717
476
241
176
93
81
+
2
894
2002
702
442
260
143
80
61
+
2
845
2003
652
420
232
145
90
54
+
2
798
2004
673
439
234
124
70
52
+
2
796
2005
703
452
251
153
92
59
+
2
855
2006
693
448
244
162
97
63
+
2
855
+ Less than 0.5 Gg
Table 6-15: Direct N20 Emissions from Agricultural Soils by Land Use and N Input (Tg C02 Eq.)
Activity
Cropland
Mineral Soils
Synthetic Fertilizer
Organic Amendments3
Residue Nb
Other0
Organic Soils
Grassland
Synthetic Fertilizer
PRP Manure
Managed Manured
Sewage Sludge
Residue Nb
Other0
Total
1990
130.9
128.1
51.3
9.4
9.0
58.4
2.8
87.4
3.0
19.8
0.6
0.3
12.5
51.3
218.3
1995
133.1
130.3
55.3
10.1
9.6
55.2
2.9
77.2
2.6
18.4
0.5
0.3
11.4
44.0
210.3
2000
142.0
139.1
55.8
10.2
10.2
62.8
2.9
74.0
2.5
19.6
0.5
0.4
10.4
40.7
216.0
2001
147.6
144.7
57.2
11.1
9.7
66.8
2.9
74.8
2.6
18.5
0.5
0.4
10.9
41.8
222.3
2002
137.1
134.3
54.2
10.7
8.9
60.4
2.9
80.6
2.7
23.3
0.5
0.4
10.8
42.8
217.7
2003
130.2
127.4
50.4
10.0
10.4
56.5
2.9
72.0
2.5
19.2
0.5
0.4
10.3
39.2
202.2
2004
136.1
133.2
55.3
10.7
9.2
58.1
2.9
72.5
2.5
20.9
0.5
0.5
10.5
37.6
208.6
2005
140.0
137.1
53.6
10.4
9.6
63.6
2.9
77.9
2.5
18.9
0.5
0.5
11.2
44.2
217.9
2006
138.9
136.1
53.6
10.7
10.1
61.7
2.9
75.8
2.6
19.6
0.5
0.5
10.4
42.2
214.7
a Organic amendment inputs include managed manure amendments and other commercial organic fertilizer (i.e., dried blood, dried manure, tankage,
compost, and other).
b Residue N inputs include unharvested fixed N from legumes as well as crop residue N.
c Other N inputs include mineralization from decomposition of soil organic matter as well as asymbiotic fixation of N from the atmosphere.
11 Accounts for managed manure that is applied to grassland soils.
Agriculture 6-19
-------
Table 6-16: Indirect N20 Emissions from all Land-Use Types (Tg C02 Eq.)
Activity
Cropland
Volatilization and Atm. Deposition
Surface Leaching & Run-Off
Grassland
Volatilization and Atm. Deposition
Surface Leaching & Run-Off
Forest Land
Volatilization and Atm. Deposition
Surface Leaching & Run-Off
Settlements
Volatilization and Atm. Deposition
Surface Leaching & Run-Off
Total
1990
30.1
5.8
24.3
20.6
10.7
9.9
+
+
+
0.3
0.1
0.2
51.1
1995
30.5
6.1
24.4
23.6
10.2
13.4
0.1
+
+
0.4
0.1
0.3
54.5
2000
28.4
6.7
21.7
17.1
9.3
7.8
0.1
+
0.1
0.4
0.1
0.3
46.0
2001
28.9
6.1
22.8
25.2
9.4
15.8
0.1
+
0.1
0.5
0.2
0.3
54.7
2002
24.8
6.0
18.8
18.9
9.3
9.6
0.1
+
0.1
0.5
0.2
0.3
44.3
2003
27.8
6.4
21.4
16.7
9.4
7.2
0.1
+
0.1
0.5
0.2
0.3
45.0
2004
21.6
6.1
15.5
16.1
9.2
6.9
0.1
+
0.1
0.5
0.2
0.3
38.3
2005
28.4
6.6
21.8
18.3
10.1
8.2
0.1
+
0.1
0.5
0.2
0.3
47.3
2006
30.2
6.5
23.7
19.5
9.4
10.1
0.1
+
0.1
0.5
0.2
0.3
50.3
+ Less than 0.05 Tg C02 Eq.
Figure 6-3 through Figure 6-6 show regional patterns
in N2O emissions for direct sources and regional patterns of
N losses leading to indirect N2O emissions, respectively, for
major crops and grasslands across the United States. Direct
N2O emissions tend to be high in the Corn Belt (Illinois,
Iowa, Indiana, Ohio, southern Minnesota, and eastern
Nebraska). A large portion of the land in many of these states
is covered with highly fertilized corn and with N-fixing
soybean cropping. Emissions are also high in North Dakota,
Kansas, and Texas, primarily from irrigated cropping and
dryland wheat cropping. 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 in many states is
used for cattle grazing. Some areas in the Great Lake states,
the Northeast, and Southeast have moderate 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 for 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, coarse-textured soils facilitate
nitrification and moderate direct emissions in grasslands in
some southeastern states, but indirect emissions are relatively
high in Florida and Georgia grasslands due to high rates of N
volatilization and NO3 leaching in coarse-textured soils.
6-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Figure 6-3
Major Crops, Average Annual Direct N20 Emissions Estimated Using the DAYCENT Model,
1990-2006 (Tg C02 Eq./state/year)
Figure 6-4
Grasslands, Average Annual Direct N20 Emissions Estimated Using the DAYCENT Model,
1990-2006 (Tg C02 Eq./state/year)
Tg C02 Eq./state/year
D < 0.25
D 0.25-0.5
D 0.5-1
D2-5
• 5-10
D10-11.9
Agriculture 6-21
-------
Figure 6-5
Major Crops, Average Annual N Losses Leading to Indirect N20 Emissions Using the DAYCENT Model,
1990-2006 (Gg N/state/year)
Gg N/state/year
D< 10
D 10-20
D 20-50
D50-100
D 100-200
D 200-400
D 400-692.9
Figure 6-6
Grasslands, Average Annual N Losses Leading to Indirect N20 Emissions Using the DAYCENT Model,
1990-2006 (Gg N/state/year)
Gg N/state/year
D< 10
D 10-20
D 20-50
D 50-100
D 100-200
D 200-400
D 400-872.3
6-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Methodology
The Revised 1996 IPCC Guidelines (IPCC/UNEP/
OECD/IEA 1997) divide the Agricultural Soil Management
source category into three components: (1) direct emissions
from agricultural soils due to N additions to cropland and
grassland mineral soils, planting of legumes on cropland
and grassland soils, and drainage and cultivation of organic
cropland soils, (2) direct emissions from soils due to the
deposition of manure by livestock on PRP grasslands,
and (3) indirect emissions from soils and water due to
N additions and manure deposition to soils that leads to
volatilization, leaching, or runoff of N and subsequent
conversion to N2O. Moreover, the 2006 IPCC Guidelines
(IPCC 2006) recommend reporting total emissions from
managed lands, and, therefore, this chapter includes
estimates for direct emissions due to asymbiotic fixation of
N from the atmosphere13 and decomposition of soil organic
matter and litter.
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 is more refined for estimating
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 methodology was used
to estimate (1) direct emissions from non-major crops on
mineral soils, (2) the portion of the grassland direct emissions
that were not estimated with the Tier 3 DAY CENT model,
and (3) direct emissions from drainage and cultivation of
13 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
Inventory.
organic cropland soils. The Tier 1 approach was based on
the 2006 IPCC Guidelines (IPCC 2006). Indirect emissions
were also estimated with a combination of DAY CENT and
the IPCC Tier 1 method.
Several recommendations from IPCC (2006) have been
adopted that are considered improvements over previous
IPCC methods, including: (1) estimating the contribution
of N from crop residues to indirect soil N2O emissions, (2)
adopting a revised emission factor for direct N2O emissions,
(3) removing double counting of emissions from N-fixing
crops associated with the symbiotic and crop residue N
input categories, (4) using revised crop residue statistics
to compute N inputs to soils based on harvest yield data,
and (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 fixation (i.e.,
computing total emissions from managed land). IPCC (2006)
recommends reporting all emissions from managed lands,
largely because management affects all processes leading
to soil N2O emissions. Agronomic practices, particularly
tillage, have a pervasive impact on soil processes. In
past Inventory reports, 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. 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.
Agriculture 6-23
-------
Box 6-1: Tier 1 vs. Tier 3 Approach for Estimating N20 Emissions
The Tier 1 approach (IPCC 2006) is based on multiplying activity data on different N sources (e.g., synthetic fertilizer, manure, N fixation,
etc.) by the appropriate default IPCC emission factors to estimate N20 emissions on a source-by-source basis. The Tier 3 approach developed
forthis Inventory employs a process-based model (i.e., DAYCENT) that represents the interaction of N inputs and the environmental conditions
at specific locations. Consequently, it is necessary to know the amount of N inputs and also the conditions under which the anthropogenic
activity is increasing mineral N in a soil profile. 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. The Tier 3 approach is thought
to produce more accurate estimates; it accounts 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, 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 with
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 is mineralized from soil organic matter and emitted as N20 during subsequent years.
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. DAYCENT simulated 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).
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. However, land management data (e.g., timing of
planting, harvesting, intensity of cultivation) were only
available at the agricultural region level as defined by the
Agricultural Sector Model (McCarl et al. 1993). There are
63 agricultural regions in the contiguous United States,
and most states correspond to one region, except for those
states with greater heterogeneity in agricultural practices,
where there are further subdivisions. While several cropping
systems were simulated for each county in an agricultural
region with county-level weather and soils data, the model
parameters that determined the influence of management
activities on soil N2O emissions (e.g., when crops were
planted/harvested) did not differ among the counties in
an agricultural region. Consequently, the results will best
represent emissions at the regional (i.e., state) and national
levels due to the scale of management data.
Nitrous oxide emission estimates from DAYCENT are
influenced by N additions, crop type, irrigation, and other
factors in aggregate, and, therefore, it is not possible to
partition N2O emissions by anthropogenic activity directly
from model outputs (e.g., N2O emissions from synthetic
fertilizer applications cannot be distinguished from those
resulting from manure applications). 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). 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 N2O emissions
6-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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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. However, this approach allows for
further disaggregation by source of N, which is valuable for
reporting purposes and is similar to the IPCC (2006) Tier
1 method (which assumes the rate of direct N2O emissions
does not vary by source).
DAY CENT was used to estimate direct N2O emissions
due to mineral N available from: (1) the application of
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. This last source is
generated internally by the DAY CENT model. For the first
three practices, annual increases 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), NEA (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
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
nitrogen from managed manure for each livestock type
was calculated by first determining the population of
animals that were on feedlots or otherwise housed
in order to collect and manage the manure. Annual
animal population data for all livestock types, except
horses and goats, were obtained for all years from the
U.S. Department of Agriculture-National Agricultural
Statistics Service. Population data used for cattle, swine,
and sheep were downloaded from the USDA NASS
Population Estimates Database (USDA 2007a). Poultry
population data were obtained from USDA NASS
reports (USDA 1995a, 1995b, 1998a, 1999, 2004a,
2004b, 2006a, 2006b, 2007b, 2007c). Horse population
data were obtained from the FAOSTAT database (FAO
2007). Goat population data for 1992, 1997, and 2002
were obtained from the Census of Agriculture (USDA
2005); these data were interpolated and extrapolated
to derive estimates for the other years. Information
regarding the poultry turnover (i.e., slaughter) rate was
obtained from state Natural Resource Conservation
Service personnel (Lange 2000). Additional population
data for different farm size categories for dairy and swine
were obtained from the 1992,1997, and 2002 Census of
Agriculture (USDA 2005). Once the animal populations
for each livestock type and management system were
estimated, these populations were multiplied by a
typical animal mass constant (USDA 1996, ASAE
1999; NRC 2000, ERG 2003, EPA 1992, Safley 2000)
to derive total animal mass for each animal type in each
management system. Total Kjeldahl N14 excreted per
year for each livestock type and management system
was then calculated using daily rates of N excretion per
unit of animal mass (USDA 1996, ASAE 1999). The
annual amounts of Kjeldahl N were then summed over
all livestock types and management systems to derive
estimates of the annual managed manure N produced.
Nitrogen available for application was estimated for
managed systems based on the total amount of N
produced in manure minus N losses and including the
addition of N from bedding materials. Nitrogen losses
include direct nitrous oxide emissions, volatilization
of ammonia and NOX, and runoff and leaching; more
information on these losses is available in Annex 3.10,
Manure Management. Animal-specific bedding factors
14Total Kjeldahl N is a measure of organically bound N and ammonia N in
both solid and liquid wastes.
Agriculture 6-25
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were set equal to IPCC default factors (IPCC 2006). The
estimated amount of manure available for application
was adjusted for the small percent of poultry manure
used for cattle feed between 1990 and 2002 (Carpenter
1992, Carpenter and Starkey 2007). The remaining
manure N that was not applied to major crops and
grassland was assumed to be applied to non-major
crop types. Frequency and rates of manure application
to cropland during the inventory period were estimated
from data compiled by the USDA Natural Resources
Conservation Service for 1997 (Edmonds et al. 2003),
with adjustments based on managed manure N excretion
in other years of the Inventory.
• Retention of crop residue, N mineralization from
soil organic matter, and asymbiotic N fixation from
the atmosphere: The IPCC approach considers this
information as separate activity data. However, they
are not treated as separate activity data in DAY CENT
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. The total
input of N from these sources is determined during the
model simulations.
• 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 (20041), USDA (2000b) 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 etal. (1930),BonnenandElliott(1931),
Brenner et al. (2002, 2001), and Smith et al. (2002).
DAYCENT simulations produced per-area estimates
of N2O emissions (g N2O-N m 2) for major crops, which
were multiplied by the cropland area data to obtain county-
scale emission estimates. Cropland area data were from
NASS (USDA 2006g). 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 actual 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.
Non-Major Crop Types on Mineral Cropland Soils
The Tier 1 methodology (IPCC 2006) 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 which
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 other
commercial organic fertilizers,15 and (3) the retention
of above- and below-ground crop residues. Non-manure
organic amendments were not included in the DAYCENT
simulations because county-level data were not available
and this source of fertilizer is a very small portion of total
organic amendments. Consequently, non-manure organic
amendments, as well as manure amendments not included
in the DAYCENT simulations, were included in the Tier
1 analysis. The following sources were used to derive
activity data.
15 Other commercial organic fertilizers include manure applied to non-major
crops, dried blood, dried manure, tankage, compost, other, but excludes
sewage sludge that is used as commercial fertilizer.
6-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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• A process-of-elimination approach was used to estimate
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
on 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 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 organic fertilizer additions were based on
organic fertilizer consumption statistics, which were
converted to units of N using average organic fertilizer
N content (TVA 1991, 1992a, 1993, 1994; AAPFCO
1995 through 2000a, 2000b, 2002 through 2007).
• Crop residue N was derived by combining amounts
of above- and below-ground biomass, which were
determined based on crop production yield statistics
(USDA 1994a, 1998b, 2003, 2005i, 2006b, 2007),
dry matter fractions (IPCC 2006), linear equations to
estimate above-ground biomass given dry matter crop
yields (IPCC 2006), ratios of below-to-above-ground
biomass (IPCC 2006), and N contents of the residues
(IPCC 2006).
The total increase in soil mineral N from applied fertilizers
and crop residues was multiplied by the IPCC (2006) default
emission factor (IPCC 2006) to derive an estimate of direct
N2O emissions from non-major crop types.
Drainage and Cultivation of Organic Cropland Soils
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 Natural Resources Inventory (NRI) (USDA 2000b, as
extracted by Eve 2001 and amended by Ogle 2002), using
temperature and precipitation data from Daly et al. (1994,
1998) to subdivide areas into temperate and tropical climates.
Data were available for 1982, 1992 and 1997, which were
linearly interpolated and extrapolated to estimate areas for
the other years in the inventory time series. 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
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 the IPCC
(2006) guidelines 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 grasslands resulting from manure deposited
by livestock directly onto the pasture (i.e., PRP manure,
which is simulated internally within the model), N fixation
from legume seeding, managed manure amendments (i.e.,
manure other than PRP manure), and synthetic fertilizer
application. 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 Annex
3.11. Other N inputs were simulated within the DAYCENT
framework, including N input from mineralization due to
decomposition of soil organic matter and plant litter, as
well as asymbiotic fixation of N from the atmosphere and
atmospheric N deposition.
DAYCENT simulations produced per-area estimates of
N2O emissions (g N2O-N m 2) for pasture and rangelands,
which were multiplied by the reported pasture and rangeland
Agriculture 6-27
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areas in the county. Grassland area data were obtained
from the NRI (USDA 2000b). The 1997 NRI area data for
pastures and rangeland were aggregated to the county level
to estimate the grassland areas for 1995 to 2006, 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.
Manure N deposition from grazing animals is modeled
internally within DAY CENT. Comparisons with estimates of
total manure deposited on PRP (see Annex 3.11) showed that
DAY CENT accounted for approximately 73 percent of total
PRP manure. The remainder of the PRP manure N excretions
were assumed to be excreted on federal grasslands (i.e.,
DAY CENT simulations were only conducted for privately-
owned grasslands), and the N2O emissions were estimated
using the Tier 1 method with IPCC default emission factors
(IPCC 2006).
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), and NEBRA (2007). Sewage sludge data on soil
amendments in 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. Consequently, emissions from sewage sludge
were also estimated using the Tier 1 method with IPCC
default emission factors (IPCC 2006).
Total Direct N20 Emissions from Cropland and
Grassland Soils
Emission estimates from DAY CENT and the IPCC
method were summed to provide total national emissions
for grasslands in the United States. 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 total direct N2O
emissions from agricultural soil management (see Table 6-13
and Table 6-14).
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
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 or 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. Through atmospheric deposition,
volatilized N can be returned to soils, and a portion is emitted
to the atmosphere as N2O. The second pathway occurs via
leaching and runoff of soil N (primarily in the form of nitrate
[NO3 ]) that was made available through anthropogenic
activity on managed lands, mineralization of soil organic
matter, asymbiotic fixation, and atmospheric deposition.
The nitrate is subject to denitrification in water bodies,
which leads to additional 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
Similarly to the direct emissions calculation, several
approaches were combined to estimate the amount of applied
N that was transported from croplands, grasslands, forest
lands, and settlements, through volatilization. DAY CENT was
used to simulate the amount of N transported from land areas
whose direct emissions were simulated with DAY CENT (i.e.,
major croplands and most grasslands), while the Tier 1 method
was used for areas that were not simulated with DAYCENT
(i.e., non-major croplands, sewage sludge application on
grasslands, PRP manure N excretion on federal grasslands)
(IPCC 2006). The IPCC (2006) default emission factor was
used to estimate indirect N2O emissions associated with the
amount of volatilized N (Table 6-16).
6-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Indirect N20 from Leaching/Runoff
As in the calculations of indirect emissions from
volatilized N, several approaches were combined to estimate
the amount of applied N that was transported from croplands,
grasslands, forest lands, and settlements through leaching
and surface runoff into water bodies. DAY CENT was used
to simulate the amount of N transported from major cropland
types and most grasslands. N transport from all other areas
(i.e., non-major croplands, sewage sludge amendments on
grasslands, PRP manure N excreted on federal grasslands,
in addition to N inputs on settlements and forest lands) was
estimated using the IPCC (2006) default factors for the
amount of N subject to leaching and runoff from mineral
fertilizer, manure, above- and below-ground crop residues,
soil organic matter decomposition and asymbiotic fixation.
The IPCC (2006) default emission factor was used to estimate
indirect N2O emissions associated with N losses through
leaching and runoff (Table 6-16).
Uncertainty
Uncertainty was estimated differently for each of
the following four components of N2O emissions from
agricultural soil management: (1) direct emissions calculated
by DAY CENT, (2) the components of indirect emissions (N
volatilized and leached or runoff) calculated by DAY CENT
(3) direct emissions not calculated by DAY CENT, and (4)
indirect emissions not calculated by DAY CENT.
Uncertainties from the Tier 1 and Tier 3 estimates were
combined using simple error propagation (IPCC 2006), and the
results are summarized in Table 6-17. Agricultural direct soil
N2O emissions in 2006 were estimated to be between 191.7
and 238.9 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 11 percent below and 11 percent above the
2006 emission estimate of 214.7 Tg CO2 Eq. The indirect soil
N2O emissions in 2006 were estimated to range from 28.0 to
113.2 Tg CO2 Eq. at a 95 percent confidence level, indicating
an uncertainty of 44 percent below and 125 percent above the
2006 emission estimate of 50.3 Tg CO2 Eq.
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, soils types, and
climate patterns (Del Grosso et al. 2005). N2O measurement
data were available for seven sites in the United States and
one in Canada, representing 25 different combinations of
fertilizer treatments and cultivation practices. DAYCENT
estimates of N2O emissions were closer to measured values
at all sites except for Colorado irrigated corn (Figure 6-7).
In general, IPCC Tier 1 methodology tends to over-estimate
when observed values are low and under-estimate when
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. NO3 leaching data were available for three sites
Figure 6-7
Comparison of Measured Emissions at Field Sites with
Modeled Emissions Using the DAYCENT Simulation Model
60 -1
50 -
40 -
30 -
20 -
10 -
0 -1
I Measured
DAYCENT
I IPCC
.•.III
I
I
I
CO NE Ml corn/ TN corn CO CO Ontario
dryland dryland soy/ irrigated irrigated corn
wheat wheat alfalfa corn corn/
barley
Table 6-17: Quantitative Uncertainty Estimates of N20 Emissions from Agricultural Soil Management in 2006
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Direct Soil N20 Emissions
Indirect Soil N20 Emissions
N20
N20
214.7
50.3
191.7
28.0
238.9
113.2
-11%
-44%
+ 11%
+ 125%
Note: Due to lack of data, uncertainties in areas for major crops, managed manure N production and PRP manure N production are currently treated as certain.
Agriculture 6-29
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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 0.73 and 0.96 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 also an
improvement over the IPCC Tier 1 method (see additional
information in Annex 3.11).
Spreadsheets containing input data and PDFs required
for DAYCENT simulations of major croplands and
grasslands and unit conversion factors were checked, as well
as the program scripts that were used to run the Monte Carlo
uncertainty analysis. An error was identified in direct N2O
estimates from major crops. The units were not converted
correctly with the transfer of data between the DAYCENT
model and the structural uncertainty estimator, leading to an
over-estimation of direct N2O emissions from major crops.
The error has been resolved and corrected. Spreadsheets
containing input data and emission factors required for the
Tier 1 approach used for non-major crops and grasslands
not simulated by DAYCENT were checked and no errors
were found.
Recalculations Discussion
Revisions in the calculations for the Agricultural Soil
N2O emission estimates included (1) using state-level N data
for on-farm use to estimate synthetic N fertilizer application
on non-major crops, (2) including uncertainty in DAYCENT
outputs of N volatilization and N leaching/runoff in the
calculation of uncertainty for indirect emissions, (3) using a
default uncertainty of +50 percent for Tier 1 uncertainties that
were addressed in previous inventory, including crop yields
and organic fertilizers, (4) assuming that manure N available
for land application not accounted for by the DAYCENT
simulations was applied to non-major crop types, (5) revising
DAYCENT parameterization for sorghum, and (6) correcting
an error in the empirically-based uncertainty estimator.
In the previous Inventory, N fertilizer application to minor
crops was based on total N available for application after
subtracting the amount applied to major crops, settlements,
and forest lands. In the current Inventory, a USGS study
(Ruddy et al. 2006) provides data from sales records about
the on-farm use of fertilizers, which were used to estimate
the amount of N applied to non-major crops after subtracting
the amount estimated for major crops from the DAYCENT
simulations. Previously, it was assumed that 90 percent
of the synthetic N fertilizer used in the United States was
applied to agricultural soils whereas the on-farm-use data
raise the amount to 97 percent. In addition, after accounting
for the amount applied to major crops and grasslands in the
DAYCENT simulations, the current Inventory assumes that
all manure N available for agricultural land application is
applied to non-major crops. Due to these changes, direct N2O
emissions from non-major crops are approximately 83 percent
higher, on average, compared to the previous Inventory.
However, direct soil N2O emissions from major crops
reported in the 1990-2005 Inventory were over-estimated by
approximately a factor of 2 as a result of a unit conversion error
in the empirically-based uncertainty estimator. Because major
crops are the greatest source, total emission estimates are
approximately 27.5 percent lower, on average, than reported
in the 1990-2005 Inventory. The revised parameterization for
sorghum had a minor influence on the emission estimates.
Planned Improvements
Three major improvements are planned for the
Agricultural Soil Management sector. The first improvement
is to incorporate more land-use survey data from the NRI
(USDA 2000b) 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 1982 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 cropping survey. 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 does provide 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
6-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
(particularly the use of irrigation). Using NRI data will also
make the Agricultural Soil N2O methods more consistent with
the methods used to estimate C stock changes for agricultural
soils. However, the structure of model input files that contain
land management data will need to be extensively revised to
facilitate use of NRI data.
The second planned improvement is to further refine the
uncertainty analysis. New studies are being completed and
published evaluating agricultural management impacts on
soil N2O emissions, and these studies can be incorporated into
the empirical analysis, leading to a more robust assessment
of structural uncertainty in DAY CENT. Moreover, structural
uncertainty is currently only evaluated for emission estimates
in croplands, but structural uncertainty is likely to be
significant for grasslands as well, and it is anticipated that
the analysis of structural uncertainty could be expanded in
the near future to include grasslands. In addition, the Monte
Carlo analysis will be expanded to address uncertainties in
activity data related to crop- and grassland areas, as well
as irrigation and tillage histories. Currently, the land-area
statistics are treated as certain because the NASS data do
not include a measure of uncertainty. Incorporating land-use
survey data from the NRI will facilitate the assessment of
uncertainties in agricultural activity data.
The third planned improvement is to further evaluate
the application of manure to major and minor crops, as well
as N recovery and losses from manure management systems
and field application. Manure amendments are a key source
of N leading to N2O emissions so any further improvements
in this estimation will reduce uncertainties in the emission
estimates. We will also evaluate potential for change in
application rates over time due to regulation of confined
animal feeding operations; this will improve the emission
estimates and reduce uncertainty. Additional improvements
are minor but will lead to more accurate estimates, including
updating DAYMET weather for more recent years.
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 carbon released to the atmosphere as
Table 6-18: 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
1990
0.7
0.1
0.1
+
0.3
+
0.1
+
0.4
+
+
+
0.1
+
0.2
+
1.1
1995
0.7
0.1
0.1
+
0.3
+
0.2
+
0.4
+
+
+
0.1
+
0.2
+
1.0
2000
0.8
0.1
0.1
+
0.4
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.3
2001
0.8
0.1
0.1
+
0.3
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.2
2002
0.7
0.1
0.1
+
0.3
+
0.2
+
0.4
+
+
+
0.1
+
0.3
+
1.1
2003
0.8
0.1
0.1
+
0.4
+
0.2
+
0.4
+
+
+
0.1
+
0.2
+
1.2
2004
0.9
0.1
0.1
+
0.4
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.4
2005
0.9
0.1
0.1
+
0.4
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.4
2006
0.8
0.1
0.1
+
0.4
+
0.2
+
0.5
+
+
+
0.1
+
0.3
+
1.3
+ Less than 0.05 Tg C02 Eq.
Note: Totals may not sum due to independent rounding.
Agriculture 6-31
-------
Table 6-19: CH4, N20, CO, and NO, Emissions from Field Burning of Agricultural Residues (Gg)
Gas/Crop Type
CH4
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
N20
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
CO
N08
1990
33
7
4
1
13
1
7
+
1
+
+
+
+
+
1
+
691
28
1995
32
5
4
1
13
1
8
+
1
+
+
+
+
+
1
+
663
29
2000
38
5
4
1
17
1
10
+
1
+
+
+
+
+
1
+
792
35
2001
37
5
4
1
16
+
11
+
1
+
+
+
+
+
1
+
774
35
2002
34
4
3
1
15
+
10
+
1
+
+
+
+
+
1
+
709
33
2003
38
6
5
1
17
+
9
+
1
+
+
+
+
+
1
+
800
34
2004
42
5
4
1
20
+
11
+
2
+
+
+
+
+
1
+
879
39
2005
41
5
5
1
19
+
11
+
2
+
+
+
+
+
1
+
860
39
2006
39
4
4
1
18
+
12
+
2
+
+
+
+
+
1
+
825
38
+ Less than 0.5 Gg
Note: Totals may not sum due to independent rounding.
CO2 during burning is assumed to be reabsorbed during the
next growing season. Crop residue burning is, however, a
net source of CFL,, 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.
Less than 5 percent of the residue for each of these crops is
burned each year, except for rice.16 Annual emissions from
this source over the period 1990 to 2006 have remained
relatively constant, averaging approximately 0.8 Tg CO2
Eq. (36 Gg) of CFL, and 0.4 Tg CO2 Eq. (1 Gg) of N2O (see
Table 6-18 and Table 6-19).
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
carbon (C) and nitrogen (N) released during burning, the
following equation was used:17
GPL, and N2O Emissions from Field Burning of
Agricultural Residues = (Fraction of Residues Burned
In Situ) x (Mass of Fuel Available for Combustion) x
(Burning Efficiency) x (Emission Factor) x 10 3
where,
Burning Efficiency = The proportion of prefire
fuel biomass consumed
To calculate the mass of fuel available for combustion, the
following equation was used:
Mass of Fuel Available for Combustion =
(Annual Crop Production) x (Residue/Crop Product Ratio)
x (Dry Matter Content of the Residue)
To calculate the emission factor, the following equation
was used:
Emission Factor = (Combustion Efficiency)
x (C or N Content of the Residue) x (Emissions Ratio)
x (Conversion Factor) x 1,000
16 The fraction of rice straw burned each year is significantly higher than
that for other crops (see "Methodology" discussion below).
17As 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.
6-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
where,
Combustion = The proportion of CH4 or
Efficiency N2O released with respect
to the total amount of C or
N available in the burned
material, respectively
Emissions Ratio = g CH4-C/g C released or g
N2O-N/g N released
Conversion Factor = Molecular weight ratio of
CH4:C or N2O:N
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
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 6-22).
The IPCC (2006) default run resulted in 20 percent higher
emissions of CH4 and 36 percent higher emissions of N20 than
the current estimates in this Inventory. It was determined that 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.
(USDA 2005,2006,2007). 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),
and crop yields for Arkansas (USDA 1994, 1998, 2003,
2005,2006) were applied to Oklahoma acreages18 (Lee 2003
through 2006). The production data for the crop types whose
residues are burned are presented in Table 6-20.
The percentage of crop residue burned was assumed
to be 3 percent for all crops in all years, except rice, 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;
California Air Resources Board 1999, 2001; Cantens 2005;
Deren 2002; Fife 1999; Guethle 2007; Klosterboer 1999a,
1999b, 2000 through 2003; Lancero 2006, 2007; Lee 2005
through 2007; Lindberg 2002 through 2005; Linscombe
1999a, 1999b, 2001 through 2007; 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 2006,2007; Walker 2004 through 2007;
Wilson 2003 through 2007) (see Table 6-21). The estimates
provided for Florida remained constant over the entire 1990
through 2006 period, while the estimates for all other states
varied over the time series, except for Missouri, which
remained constant through 2005 and dropped in 2006. 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 2006 because of a legislated reduction in rice straw
18 Rice production yield data are not available for Oklahoma, so the Arkansas
values are used as a proxy.
Agriculture 6-33
-------
burning (Lindberg 2002), although there was a slight increase
from 2004 to 2005 (see Table 6-21).
All residue/crop product mass ratios except sugarcane
were obtained from Strehler and Stiitzle (1987). The datum
for sugarcane is from University of California (1977).
Residue dry matter contents for all crops except soybeans
and peanuts were obtained from Turn etal. (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
Table 6-20: Agricultural Crop Production (Gg of Product)
Protein System. The residue carbon contents and nitrogen
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 6-22. The
burning efficiency was assumed to be 93 percent, and the
combustion efficiency was assumed to be 88 percent, for
all crop types (EPA 1994). Emission ratios for all gases
(see Table 6-23) were taken from the Revised 1996 IPCC
Guidelines (IPCC/UNEP/OECD/IEA 1997).
Crop
Wheat
Rice
Sugarcane
Corn*
Barley
Soybeans
Peanuts
1990
74,292
7,114
25,525
201,534
9,192
52,416
1,635
1995
59,404
7,947
27,922
187,970
7,824
59,174
1,570
2000 2001 2002 2003 2004 2005 2006
60,641 53,001 43,705 63,814 58,738 57,280 49,316
8,705 9,794 9,601 9,084 10,565 10,150 8,813
32,762 31,377 32,253 30,715 26,320 24,137 26,752
251,854 241,377 227,767 256,278 299,914 282,311 267,598
6,919 5,407 4,940 6,059 6,091 4,613 3,920
75,055 78,671 75,010 66,778 85,013 83,368 86,770
1,481 1,940 1,506 1,880 1,945 2,209 1,576
*Corn for grain (i.e., excludes corn for silage).
Table 6-21: Percent of Rice Area Burned by State
State
Arkansas
California
Florida*
Louisiana
Mississippi
Missouri
Oklahoma
Texas
1990
13%
75%
0%
6%
10%
18%
90%
1%
1995
13%
59%
0%
6%
10%
18%
90%
1%
2000
13%
27%
0%
5%
40%
18%
90%
0%
2001
13%
23%
0%
4%
40%
18%
90%
0%
2002
16%
13%
0%
3%
8%
18%
90%
0%
2003
22%
14%
0%
3%
65%
18%
100%
0%
2004
17%
11%
0%
3%
23%
18%
88%
0%
2005
22%
16%
0%
3%
23%
18%
94%
0%
2006
27%
10%
0%
5%
25%
3%
0%
0%
* Although rice is cultivated in Florida, crop residue burning is illegal.
Table 6-22: 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
0.8
1.0
1.2
2.1
1.0
Fraction of
Residue Burned
0.03
Variable
0.03
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
0.93
0.93
0.93
0.93
0.93
Combustion
Efficiency
0.88
0.88
0.88
0.88
0.88
0.88
0.88
6-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 6-23: Greenhouse Gas Emission Ratios
Gas
Emission Ratio
coa
N20b
0.005
0.060
0.007
0.121
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).
Uncertainty
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 6-24. CH4 emissions from
field burning of agricultural residues in 2006 were estimated
to be between 0.3 and 1.5 Tg CO2 Eq. at a 95 percent
confidence level. This indicates a range of 65 percent below
and 79 percent above the 2006 emission estimate of 0.8
Tg CO2 Eq. Also at the 95 percent confidence level, N2O
emissions were estimated to be between 0.2 and 0.9 Tg CO2
Eq. (or approximately 64 percent below and 73 percent
above the 2006 emission estimate of 0.5 Tg CO2 Eq.).
QA/QC 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.
Recalculations Discussion
The crop production data for 2005 and 2006 were
updated using data from USDA (2007). This change resulted
in an increase in the CH4 emission estimate for 2005 of 0.2
percent, and a decrease in the N2O emission estimate for
2005 of 0.1 percent relative to the previous Inventory. In
addition, a more robust uncertainty analysis was run for
the current Inventory, taking into account shared variables
between the Field Burning of Agricultural Residues and
Rice Cultivation sources and correcting errors that were
identified in the uncertainty analysis undertaken for the
previous Inventory. These changes resulted in a greater
uncertainty range surrounding the 2006 estimates than
those presented in the previous Inventory for the 2005
emission estimates.
Table 6-24: Tier 2 Uncertainty Estimates for CH4 and N20 Emissions from Field Burning of Agricultural Residues
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Field Burning of Agricultural Residues CH4
Field Burning of Agricultural Residues N20
0.8
0.5
0.3
0.2
1.5
0.9
-65%
-64%
+ 79%
+ 73%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Agriculture 6-35
-------
Planned Improvements 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, is based on data gathered from several cr°P ^P68 (e-8" §rass for seed' blueberries, and fruit and
state greenhouse gas inventories. This fraction is the most nut trees) are burned- Whether sufficient information
statistically significant input to the emissions equation, and exists for inclusion of these additional crop types in future
an important area for future improvement. More crop- and inventories is being investigated. The extent of recent state
state-specific information on the fraction burned will be crop-burning regulations is also being investigated.
investigated by literature review and/or by contacting state
departments of agriculture.
6-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 (IPCC) Good Practice
Guidance for Land Use, Land-Use Change, and Forestry (IPCC 2003) 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. CO2 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. Agricultural mineral and organic soil C flux
calculations are based primarily on national surveys, so these results are largely constant over multi-year intervals, with
large discontinuities between intervals. For the landfilled yard trimmings and food scraps source, periodic solid waste survey
data were interpolated so that annual storage estimates could be derived. In addition, because the most recent national
forest, and land-use surveys were completed prior to 2005, the estimates of CO2 flux from forests, agricultural soils, and
landfilled yard trimmings and food scraps are based in part on extrapolation. CO2 flux from urban trees is based on neither
annual data nor periodic survey data, but instead on data collected over the period 1990 through 1999. 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 2006 resulted in a net C sequestration of 883.7 Tg CO2 Eq. (241.0
Tg C) (Table 7-1 and Table 7-2). This represents an offset of approximately 14.8 percent of total U.S. CO2 emissions. Total
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."
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"
Other (Landfilled Yard Trimmings
and Food Scraps)
Total
1990
(621.7)
(30.1)
14.7
(1.9)
(14.3)
(60.6)
(23.9)
(737.7)
1995
(659.9)
(39.4)
9.4
16.6
(16.3)
(71.5)
(14.1)
(775.3)
2000
(550.7)
(38.4)
9.4
16.4
(16.3)
(82.4)
(11.5)
(673.6)
2001
(623.4)
(40.0)
9.4
16.4
(16.3)
(84.6)
(11.6)
(750.2)
2002
(697.3)
(40.3)
9.4
16.4
(16.3)
(86.8)
(11.8)
(826.8)
2003
(730.9)
(40.5)
9.4
16.4
(16.3)
(88.9)
(10.0)
(860.9)
2004
(741.4)
(40.9)
9.4
16.3
(16.3)
(91.1)
(9.6)
(873.7)
2005
(743.6)
(41.0)
9.4
16.3
(16.3)
(93.3)
(10.0)
(878.6)
2006
(745.1)
(41.8)
9.4
16.2
(16.3)
(95.5)
(10.5)
(883.7)
Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
a 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.
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"
Other (Landfilled Yard Trimmings
and Food Scraps)
Total
1990
(169.6)
(8.2)
4.0
(0.5)
(3.9)
(16.5)
(6.5)
(201.2)
1995
(180.0)
(10.7)
2.6
4.5
(4.5)
(19.5)
(3.9)
(211.4)
2000
(150.2)
(10.5)
2.6
4.5
(4.5)
(22.5)
(3.1)
(183.7)
2001
(170.0)
(10.9)
2.6
4.5
(4.5)
(23.1)
(3.2)
(204.6)
2002
(190.2)
(11.0)
2.6
4.5
(4.5)
(23.7)
(3.2)
(225.5)
2003
(199.3)
(11.0)
2.6
4.5
(4.5)
(24.3)
(2.7)
(234.8)
2004
(202.2)
(11.1)
2.6
4.5
(4.5)
(24.9)
(2.6)
(238.3)
2005
(202.8)
(11.2)
2.6
4.4
(4.5)
(25.4)
(2.7)
(239.6)
2006
(203.2)
(11.4)
2.6
4.4
(4.5)
(26.0)
(2.9)
(241.0)
Note: 1 Tg C = 1 teragram C = 1 million metric tons C. Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
a 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.
land use, land-use change, and forestry net C sequestration2
increased by approximately 20 percent between 1990 and
2006. This increase was primarily due to an increase in
the rate of net C accumulation in forest C stocks. Net C
accumulation in Settlements Remaining Settlements, Land
Converted to Grassland, and Cropland Remaining Cropland
increased, while net C accumulation in landfilled yard
trimmings and food scraps slowed over this period. The
Grassland Remaining Grassland land-use category resulted
in a net C sink from 1990 through 1994 and then remained
a fairly constant emission source. Emissions from Land
Converted to Cropland declined between 1990 and 2006.
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 2006 resulted in
CO2 emissions of 8.0 Tg CO2 Eq. (8,012 Gg). The application
of synthetic fertilizers to forest and settlement soils in 2006
resulted in direct N2O emissions of 1.8 Tg CO2 Eq. (6 Gg).
Direct N2O emissions from fertilizer application increased
by approximately 174 percent between 1990 and 2006.
Forest fires in 2006 resulted in methane (CFLj) emissions of
24.6 Tg CO2 Eq. (1,169 Gg), and in N2O emissions of 2.5
Tg C02 Eq. (8 Gg).
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 act as a sink. This is also referred to as
net C sequestration.
7-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 7-3: Emissions from Land Use, Land-Use Change, and Forestry (Tg C02 Eq.)
Source Category
C02
Cropland Remaining Cropland: Liming
of Agricultural Soils & Urea Fertilization
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"
Total
1990
7.1
7.1
4.5
4.5
1.5
0.5
0.1
1.0
13.1
1995
7.0
7.0
4.7
4.7
1.8
0.5
0.2
1.2
13.6
2000
7.5
7.5
19.0
19.0
3.5
1.9
0.3
1.2
30.0
2001
7.8
7.8
9.4
9.4
2.7
1.0
0.3
1.4
20.0
2002
8.5
8.5
16.4
16.4
3.5
1.7
0.3
1.5
28.4
2003
8.3
8.3
8.7
8.7
2.7
0.9
0.3
1.5
19.7
2004
7.6
7.6
6.9
6.9
2.6
0.7
0.3
1.6
17.1
2005
7.9
7.9
12.3
12.3
3.1
1.2
0.3
1.5
23.2
2006
8.0
8.0
24.6
24.6
4.3
2.5
0.3
1.5
36.9
Note: These estimates include direct emissions only. Indirect N20 emissions are reported in the Agriculture chapter. Totals may not sum due to independent
rounding.
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.
Table 7-4: Non-C02 Emissions from Land Use, Land-Use Change, and Forestry (Gg)
Source Category
C02
Cropland Remaining Cropland: Liming
of Agricultural Soils & Urea Fertilization
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"
1990
7,084
7,084
213
213
5
1
0
3
1995
7,049
7,049
224
224
6
2
1
4
2000
7,541
7,541
904
904
11
6
1
4
2001
7,825
7,825
448
448
9
3
1
5
2002
8,549
8,549
780
780
11
5
1
5
2003
8,260
8,260
416
416
9
3
1
5
2004
7,555
7,555
330
330
8
2
1
5
2005
7,854
7,854
586
586
10
4
1
5
2006
8,012
8,012
1,169
1,169
14
8
1
5
Note: These estimates include direct emissions only. Indirect N20 emissions are reported in the Agriculture chapter. Totals may not sum due to independent
rounding.
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.
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
Land Use, Land-Use Change, and Forestry 7-3
-------
Table 7-5: Land Use Areas During the Inventory Reporting Period (Millions of Hectares)
Land Use
Forest Land
Cropland
Grassland
Wetlands
Settlements
Other Land
1990
243
169
301
32
32
28
1995
246
166
296
32
36
28
2000
249
163
296
31
40
25
2001
250
163
295
31
41
25
2002
250
162
295
31
41
25
2003
251
162
294
31
42
25
2004
251
162
294
31
42
25
2005
252
162
293
31
42
25
2006
252
162
293
31
42
25
Note: Unmanaged land is not currently estimated because the only land designated as unmanaged occurs in Alaska, which has not been included in the
current U.S. land representation assessment. See Planned Improvements for discussion on plans to include Alaska in future Inventory reports.
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 national greenhouse gas Inventory has
been developed in order to comply with this guidance.
Multiple databases are utilized to track land management
in the United States, which are also used as the basis to
categorize the land area into the following six IPCC land-use
categories:3 Forest Land, Cropland, Grassland, Wetlands,
Settlements and Other Land (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)
Database.5 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. In
1990, the United States had a total of 243 million hectares of
Forest Land, 169 million hectares of Cropland, 301 million
hectares of Grassland, 32 million hectares of Wetlands, 32
million hectares of Settlements, and 28 million hectares in
the Other Land7 category (Table 7-5). By 2006, the total area
in Forest Land had increased by 3.9 percent to 252 million
hectares, Cropland had declined by 4.0 percent to 162 million
hectares, Grassland declined by 2.8 percent to 293 million
hectares, Wetlands decreased by 4.8 percent to 31 million
hectares, Settlements increased by 32.2 percent to 42 million
3 Land-use category definitions are provided in the Methodology section.
4NRI data are available at .
5 FIA data are available at .
6NLCD data are available at .
7 Other Land is a miscellaneous category that includes lands that are not
classified into the other five land-use categories. It also allows the total of
identified land areas to match the national area.
hectares, and Other Land decreased by 11.1 percent to 25
million hectares.
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.
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.). Approach 3 extends Approach 2 by allowing each
land-use conversion to be tracked on a spatially explicit
basis. The three approaches are not presented as hierarchical
tiers and are not mutually exclusive.
7-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Figure 7-1
Percent of Total Land Area in Each Land-Use Category by State
Croplands
Grasslands
Wetlands
Forest Lands
Settlements
Other Lands
< 10%
D31%-50% •>50%
n
Land Use, Land-Use Change, and Forestry 7-5
-------
According to IPCC (2003), the approach or mix of
approaches selected by an inventory agency should reflect
the 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. The NRI and the FIA data surveys meet the
standards for Approach 3, but the data from NLCD that are
currently utilized only meet the standards for Approach I.8
Consequently, Approach 1 is being used to provide a full
representation of land use in the current Inventory. The United
States is pursuing an effort to analyze available data with the
intent of moving beyond Approach 1 in future Inventories.
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).9
8A new NLCD product is being developed that will meet the standards of
Approach 3 data, with explicit information on land cover change, opposed
to information based solely on land cover for individual years.
9 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.
• Unmanaged Land: All other land is considered
unmanaged. Unmanaged land largely comprises
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.10
Land-Use Categories
As with the definition of managed lands, IPCC (2003,
2006) provide 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 of Forest Land is based on the FIA definition
of forest,11 while definitions of Cropland, Grassland, and
Settlements are based on the NPJ.12 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 land
that is at least 10 percent stocked13 by forest trees
of any size, or land formerly having such tree cover,
and not currently developed for a non-forest use. The
minimum area for classification as Forest Land is one
acre (0.40 ha). Roadside, stream-side, and shelterbelt
strips of timber must be at least 120 feet (36.58 m) wide
to qualify as Forest Land. Unimproved roads and trails,
streams and other bodies of water, or natural clearings
in forested areas are classified as Forest Land, if less
than 120 feet (36.58 m) in width or one acre (0.40 ha)
in size. Improved roads within Forest Land, however,
are extracted from forest area estimates and included
10 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.
11 See .
12 See .
13 The percentage stocked refers to the degree of occupancy of land by trees,
measured either by basal area or number of trees by size and spacing or
both, compared to a stocking standard.
7-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
in Other Land. Grazed woodlands, fields reverting to
forest, and pastures that are not actively maintained are
included if the above qualifications are satisfied. Forest
Land consists of three main subcategories: timberland,
reserved forest land, and other forest land.14 Forest Land
also includes woodlands, which describes forest types
consisting primarily of species that have their diameter
measured at root collar, and for which there are no
site index equations, nor stocking guides. These may
include areas with degrees of stocking between 5 and
9.9 percent. The FIA regions with woodland areas are,
however, considering new definitions that should result
in all Forest Land meeting the minimum 10 percent
stocking threshold.
• 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. 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,15 as well as lands
in temporary fallow or enrolled in conservation reserve
programs (i.e., set-asides16). 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 Other Land.
• 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. 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,
14 These subcategory definitions are fully described in the Forest Land
Remaining Forest Land section.
15 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.
16A 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.
in addition to tundra are considered Grassland.17 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 Other Land.
• 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.
IPCC (2006) provides guidance under "Wetlands"
for managed peatlands and flooded lands, such as
reservoirs developed for hydroelectricity, irrigation, and
navigation. Certain areas that fall under the managed
Wetlands definition are 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 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 if they are surrounded by urban or built-up
areas. 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.
• 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
17 IPCC guidelines (2006) do not include provisions to separate desert and
tundra as land categories.
Land Use, Land-Use Change, and Forestry 7-7
-------
land areas to match the managed national area. It also
specifically includes roads through forests (excluding
unimproved roads/trails) and all types of roads through
Grassland and Cropland areas that are discernible using
aerial photography or remote sensing imagery (i.e.,
interstate highways, state highways, other paved roads,
gravel roads, dirt roads, and railroads).
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, F1A, and the NLCD. The NRI
is conducted by the USDA Natural Resources Conservation
Service and is designed to assess soil, water, and related
environmental resources on nonfederal 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. The
NRI survey was conducted every 5 years between 1982 and
1997, but shifted to annualized data collection in 1998.
The FIA program, conducted by the USFS, is used to
obtain forest area and management data 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.
Because NRI only includes land use information
for non-federal land, and the FIA only records for forest
land,18 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.19 Consequently, the NLCD is used
as a supplementary database to account for federal land areas
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. It is based primarily on Landsat Thematic
Mapper imagery. The NLCD contains 21 categories of land
cover information, which have been aggregated into the six
IPCC land-use categories, and the data are available at a
spatial resolution of 30 meters. 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.
Along with the incorporation of NLCD data, another
major step has been taken 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
18 FIA does collect some data on nonforest land use, but these are held in
regional databases versus the national database. The status of these data is
being investigated.
19The 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.
7-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Forest Land Carbon Stock Inventory (Forest Land Remaining
Forest Land and Land Converted to Forest Land), which
are based on the NRI and FIA databases, respectively. NRI,
which covers only non-federal land, and FIA have different
criteria for classifying forest land, leading to discrepancies
in the resulting estimates of forest land area on non-federal
land. Similarly, there are discrepancies between the NLCD
and FIA data for forest land on federal lands. Moreover,
dependence exists between the Forest Land area and the
amount of land designated as other land uses in the NRI
and NLCD, such as grassland, cropland and wetland, and
thus there are inconsistencies in the Forest Land definitions
among the three databases. FIA is the main database for
forest statistics, and consequently, the NRI and NLCD were
adjusted to achieve consistency with FIA estimates of Forest
Land. The adjustments were made at a state-scale, and it was
assumed that the majority of the discrepancy in forest area
was associated with an under- or over-prediction of grassland
and wetland area in the NRI and NLCD due to differences in
Forest Land definitions. Specifically, the Forest Land area for
a given state according to the NRI and NLCD was adjusted
to match the FIA estimates for non-federal and federal land,
respectively. Adjustments were allotted to grassland and
wetlands, based on the proportion of land within each of these
land-use categories at the state-level. A higher proportion of
grassland led to a larger adjustment in grassland area and vice
versa. In a second step, corresponding increases or decreases
were made in the area estimates of grassland and wetland
from the NRI and NLCD, in order to balance the change in
forest area, and therefore not change the overall amount of
managed land within an individual state.
There are minor differences between the U.S. Census
Survey20 land area estimates and the land use surveys derived
for the Inventory because of discrepancies in the reporting
approach for the census and the methods used in the NRI, FIA
and NLCD. The area estimates of land-use categories, based
on NRI, FIA and NLCD, are derived from remote sensing
data instead of the land survey approach used by the U.S.
Census Survey. More importantly, the U.S. Census Survey
does not provide 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. Regardless, the total difference between the U.S.
Census Survey and the data sources used in the Inventory is
relatively minor, estimated at about 6 million hectares for the
total land base of over 800 million hectares currently included
in the Inventory, or a 0.7 percent difference.
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 six broad land
uses identified in IPCC (2006). Details on the approach used
to combine data sources for each land use are described below
along with 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 and at this time the NLCD cannot be
used to classify land use in this region. FIA surveys are
currently being conducted on U.S. territories and will
become available in the future. FIA data will also be
collected in Hawaii in the future.
• Cropland: Cropland is classified using the NRI, which
covers all non-federal lands, within 49 states, 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. Cropland in Alaska and U.S. territories is
excluded from both NRI data collection and the NLCD.22
Though crops are grown on some federal lands, these
Cropland areas are considered minimal and are excluded
from the inventory.
• Grassland: Grassland on non-federal lands is classified
using the NRI within 49 states, 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.
0 See .
21 Definitions are provided in the previous section.
22 With the exception of non-federal cropland in Puerto Rico, which are
included in the NRI survey.
Land Use, Land-Use Change, and Forestry 7-9
-------
Alaska and U.S. territories are excluded from both NRI
data collection and the current release of the NLCD
product.23 Grassland on federal BLM lands, National
Parks and within USFS lands is covered by the NLCD.
Department of Defense grasslands are also included in
area estimates using the NLCD.
• Wetlands: NRI captures wetlands on non-federal lands
within 49 states, 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. 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. If within an urban area, a
forested area is classified as nonforestby FIA, regardless
of size. Settlements on federal lands are covered by
NLCD. Settlements in Alaska and U.S. territories are
currently excluded from NRI and NLCD.
• Other Land: Any land not falling into the other five land
categories and, therefore, categorized as Other Land is
classified using the NRI and NLCD. Other land in Alaska
and U.S. territories are excluded from the NLCD.
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
assigning 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
23 With the exception of non-federal grasslands in Puerto Rico, which are
included in the NRI survey.
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.
Priority does not reflect the level of importance for
reporting GHG 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).
Planned Improvements
Area data by land-use category are not estimated for
major portions of Alaska and any of the U.S. territories. A
key planned improvement is to incorporate land-use data
from these areas in the national greenhouse gas emissions
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
be evaluated for representing land use in U.S. Territories.
7-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Another planned improvement is to utilize Approach
3-type area data for the U.S. land base. A new NLCD product,
with spatially-explicit information on land-use change is
currently being developed and will qualify as Approach 3. By
using this new data product in combination with the existing
NRI and FIA databases, land-use statistics will be further
subdivided by land-use change categories as recommended
in IPCC (2006). This will include land remaining in a land-
use category and land converted to another land-use category
(e.g., Forest Land Remaining Forest Land, Cropland
Converted to Forest Land, Grassland Converted to Forest
Land). The additional subdivisions will provide more explicit
land-use change statistics than currently reported, and also
provide better accounting of emissions and stock changes
associated with land use activities.
Additional work will be done to reconcile differences in
Forest Land estimates between the NPJ 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
States, a discrepancy in Forest Land areas between NPJ 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 NPJ 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, current estimates using the NPJ and
NLCD databases will be compared and reconciled to the
extent possible with the Army Corps of Engineers National
Inventory of Dams (ACE 2005) which provides data on the
total surface area of reservoirs created by dams.
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.
• Below ground 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.
• Harvested wood products in solid waste disposal sites
(SWDS).
C 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
Land Use, Land-Use Change, and Forestry 7-11
-------
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 to the
atmosphere. Instead, harvesting transfers C 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.
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
trees 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 not available for the
entire United States to allow results to be partitioned in this
way. Instead, 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
Figure 7-2
Forest Sector Carbon Pools and Flows
Combustion from
forest fires (carbon
dioxide, methane)
Combustion from forest fires
(carbon dioxide, methane)
Processing
^^Consu
/ ^4
Legend
Carbon Pool
Carbon transfer or flux
Combustion
Source: Heath et al. 2003
7-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
boxes are not identical to the storage pools identified in this
chapter. The storage pools identified in this chapter have
been altered in this graphic to better illustrate the processes
that result in transfers of C from one pool to another, and
emissions to the atmosphere as well as uptake from the
atmosphere.
Approximately 33 percent (303 million hectares) of
the U.S. land area is forested (Smith et al. 2004b). The
current forest inventory includes 249 million hectares in the
conterminous 48 states (USDA Forest Service 2006b) that
are considered managed and are included in this inventory.
The additional forest lands are located in Alaska and
Hawaii. This inventory includes approximately 3.7 million
hectares of Alaska forest, which are in the southeast and
south central regions of Alaska and represent the majority
of the state's managed forest land. Survey data are not yet
available from Hawaii. While Hawaii and U.S. territories
have relatively small areas of forest land and will probably
not affect the overall C budget to a great degree, these
areas will be included as sufficient data become available.
Agroforestry systems are also not currently accounted for
in the Inventory, since they are not explicitly inventoried by
either of the two primary national natural resource inventory
programs: the Forest Inventory and Analysis (FIA) program
of the U.S. Department of Agriculture (USDA) Forest
Service and the National Resources Inventory (NRI) of
the USDA Natural Resources Conservation Service (Perry
et al. 2005).
Sixty-seven percent of U.S. forests (204 million
hectares) are classified as timberland, meaning they meet
minimum levels of productivity and are available for timber
harvest. Nine percent of Alaska forests and 79 percent of
forests in the conterminous United States are classified as
timberlands. Of the remaining nontimberland forests, 31
million hectares are reserved forest lands (withdrawn by
law from management for production of wood products)
and 68 million hectares are lower productivity forest
lands (Smith et al. 2004b). Historically, the timberlands in
the conterminous 48 states have been more frequently or
intensively surveyed than other forest lands.
Forest land 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 7
million hectares. Current trends in forest area represent
average annual change of only about 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.24
Net volume of growing stock on U.S. timberlands increased
by 36 percent from 1953 to 1997. Though harvesting forests
removes much of the aboveground C, there is a positive
growth to harvest ratio on U.S. timberlands (AF&PA2001).
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, as well as timber harvesting and use have resulted in
net uptake (i.e., net sequestration) of C each year from 1990
through 2006. 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 affect C fluxes
from these forest lands. More recently, the 1970s and 1980s
saw a resurgence of federally-sponsored forest management
programs (e.g., the Forestry Incentive Program) and soil
conservation programs (e.g., the Conservation Reserve
Program), which have focused on tree planting, improving
24The 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 7-13
-------
timber management activities, combating soil erosion,
and converting marginal cropland to forests. In addition
to forest regeneration and management, forest harvests
have also affected net C fluxes. Because most of the timber
harvested from U.S. forests is used in wood products, and
many discarded wood products are disposed of in SWDS
rather than by incineration, significant quantities of C in
harvested wood are transferred to long-term storage pools
rather than being released rapidly to the atmosphere (Skog
and Nicholson 1998, Skog in preparation). The size of
these long-term C storage pools has increased during the
last century.
Changes in C stocks in U.S. forests and harvested
wood were estimated to account for net sequestration of
745.1 Tg CO2 Eq. (203.2 Tg C) in 2006 (Table 7-6, Table
7-7, and Figure 7-3). In addition to the net accumulation
of C in harvested wood pools, sequestration is a reflection
of net forest growth and increasing forest area over this
period. Overall, average C in forest ecosystem biomass
(aboveground and belowground) increased from 71 to
75 Mg C/ha between 1990 and 2007 (see Table A-4 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
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
1990
(489.1)
(287.6)
(54.2)
(40.1)
(63.3)
(43.9)
(132.6)
(64.8)
(67.9)
(621.7)
1995
(540.5)
(318.4)
(62.4)
(57.5)
(34.9)
(67.5)
(119.4)
(55.2)
(64.1)
(659.9)
2000
(436.8)
(335.4)
(67.2)
(44.9)
(17.3)
28.0
(113.9)
(47.0)
(66.9)
(550.7)
2001
(529.0)
(367.7)
(73.7)
(50.0)
(36.3)
(1.3)
(94.5)
(31.9)
(62.6)
(623.4)
2002
(598.0)
(384.4)
(76.9)
(53.0)
(47.7)
(36.0)
(99.2)
(35.1)
(64.2)
(697.3)
2003
(635.1)
(406.5)
(80.9)
(56.9)
(56.2)
(34.5)
(95.9)
(35.4)
(60.4)
(730.9)
2004
(635.1)
(406.5)
(80.9)
(56.9)
(56.2)
(34.5)
(106.3)
(45.5)
(60.8)
(741.4)
2005
(635.1)
(406.5)
(80.9)
(56.9)
(56.2)
(34.5)
(108.5)
(47.3)
(61.2)
(743.6)
2006
(635.1)
(406.5)
(80.9)
(56.9)
(56.2)
(34.5)
(110.0)
(45.3)
(64.7)
(745.1)
Note: 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). 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.
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
(133.4)
(78.4)
(14.8)
(10.9)
(17.3)
(12.0)
(36.2)
(17.7)
(18.5)
(169.6)
1995
(147.4)
(86.8)
(17.0)
(15.7)
(9.5)
(18.4)
(32.6)
(15.1)
(17.5)
(180.0)
2000
(119.1)
(91.5)
(18.3)
(12.2)
(4.7)
7.6
(31.1)
(12.8)
(18.2)
(150.2)
2001
(144.3)
(100.3)
(20.1)
(13.6)
(9.9)
(0.4)
(25.8)
(8.7)
(17.1)
(170.0)
2002
(163.1)
(104.8)
(21.0)
(14.5)
(13.0)
(9.8)
(27.1)
(9.6)
(17.5)
(190.2)
2003
(173.2)
(110.9)
(22.1)
(15.5)
(15.3)
(9.4)
(26.1)
(9.7)
(16.5)
(199.3)
2004
(173.2)
(110.9)
(22.1)
(15.5)
(15.3)
(9.4)
(29.0)
(12.4)
(16.6)
(202.2)
2005
(173.2)
(110.9)
(22.1)
(15.5)
(15.3)
(9.4)
(29.6)
(12.9)
(16.7)
(202.8)
2006
(173.2)
(110.9)
(22.1)
(15.5)
(15.3)
(9.4)
(30.0)
(12.3)
(17.7)
(203.2)
Note: 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). 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.
7-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 7-8: Forest Area (1000 ha) and C Stocks (Tg C) in Forest and Harvested Wood Pools
1990
Forest Area (1000 ha)
Carbon Pools (Tg C)
Forest
Aboveground Biomass
Belowground Biomass
Dead Wood
Litter
Soil Organic Carbon
Harvested Wood
Products in Use
SWDS
Total Carbon Stock
245,799
40,106
14,547
2,896
2,453
4,557
15,652
1,862
1,231
631
41,968
1995
249,036
40,810
14,955
2,974
2,515
4,641
15,725
2,033
1,311
722
42,843
2000 2001 2002 2003 2004 2005 2006 2007
252,251 252,798 253,443 254,155 254,889 255,624 256,358 257,093
41,535 41,654 41,798 41,962 42,135 42,308 42,481 42,654
15,405 15,496 15,596 15,701 15,812 15,923 16,034 16,145
3,063 3,081 3,102 3,123 3,145 3,167 3,189 3,211
2,592 2,605 2,618 2,633 2,648 2,664 2,679 2,695
4,680 4,684 4,694 4,707 4,723 4,738 4,753 4,769
15,795 15,788 15,788 15,798 15,807 15,817 15,826 15,835
2,193 2,224 2,250 2,277 2,303 2,332 2,362 2,392
1,382 1,395 1,404 1,413 1,423 1,436 1,448 1,461
810 829 846 863 880 896 913 931
43,728 43,878 44,048 44,238 44,438 44,640 44,843 43,376
Note: Forest Area estimates include portions of Alaska, which represents an addition relative to previous versions of this table. 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. (2007) 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.
to 2006 are the result of the sequences of new inventories for
each state. Net annual sequestration increased by 20 percent
for 2006 relative to 1990. C in forest ecosystem biomass
had the greatest effect on total change. As discussed above,
this was due to increased 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 below-ground biomass.
Figure 7-3
Estimates of Net Annual Changes in Carbon Stocks
for Major Carbon Pools
50-,
I
3
-50
-100-
S -150-
-200-
-250 J
Soil
Harvested Wood
Total Net Change
Land Use, Land-Use Change, and Forestry 7-15
-------
Figure 7-4
Average C Density in the Forest Tree Pool in the Conterminous United States, 2007
Live Tree
Mg C02 Eq./ha
D1-200
D 201^100
n over 400
Note: This graphic shows county-average carbon densities for live trees on forestland, including both above- and belowground biomass. These data
are based on the most recent forest inventory survey in each state. (See Table A-3 for the most recent inventory year for each state or substate.)
Box 7-1: C02 Emissions from Forest Fires
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. forest
land 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 was employed to estimate C02
emissions from forest fires. C02 emissions for the lower 48 states and
Alaska in 2006 were estimated to be 267.9 Tg C02/yr. This amount is
masked in the estimate of net annual forest carbon stock change for
2006, however, because this net estimate accounts for the amount
sequestered minus any emissions.
Table 7-9: Estimates of C02 (Tg/yr) emissions for
the lower 48 states and Alaska3
Year
1990
1995
^^^H
2000
2001
2002
2003
2004
2005
2006
C02 Emitted in
the Lower 48
States (Tg/yr)
36.8
51.1
196.9
99.7
149.0
92.4
43.4
111.4
266.6
C02 Emitted
in Alaska
(Tg/yr)
12.0
0.2
10.3
3.0
29.7
3.0
32.1
22.9
1.3
Total C02
Emitted
(Tg/yr)
48.8
51.3
207.2
102.6
178.7
95.4
75.5
134.3
267.9
a Note that these emissions have already been accounted for in the
estimates of net annual changes in carbon stocks, which accounts
for the amount sequestered minus any emissions.
7-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Methodology
The methodology described herein is consistent
with IPCC (2003) and IPCC/UNEP/OECD/IEA (1997).
Estimates of net annual C stock change, or flux, of forest
ecosystems are derived from applying C estimation factors
to forest inventory data and interpolating between successive
inventory-based estimates of C stocks. C emissions from
harvested wood 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). Different data sources
are used to estimate the C stocks and stock change in forest
ecosystems or harvested wood products. See Annex 3.12
for details and additional information related to the methods
described below.
Forest Carbon Stocks and Fluxes
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 2006a).
Inventories include forest lands25 of the conterminous United
States and are organized as a number of separate datasets,
each representing a complete inventory, or survey, of an
individual state at a specified time. Forest C calculations
are organized according to these state surveys, and the
frequency of surveys varies by state. To calculate a C stock
change, at least two surveys are needed in each state. Thus,
the most recent surveys for each state are used as well as
all additional consistent inventory data back through 1990.
Because C flux is based on change between successive C
stocks, consistent representation of forest land in successive
inventories is necessary. In order to achieve accurate
representation of forests from 1990 to the present, state-
level data are sometimes subdivided or additional inventory
sources are used to produce the consistent state or sub-state
inventories.
The principal FIA datasets employed are freely available
for download at USDA Forest Service (2006b) as the Forest
25 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.
Inventory and Analysis Database (FIADB) Version 2.1.
These data are identified as "snapshot" files, also identified
as FISDB 2.1, and include detailed plot information,
including individual-tree data. 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 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. A detailed list of the
specific inventory data used in this Inventory is in Table
A-188ofAnnex3.12.
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
been formalized in an application referred to as the Carbon
Calculation Tool (CCT), (Smith et al. 2007). The conversion
factors and model coefficients are usually categorized by
region and forest type, and forest C stock estimates are
dependent on these particular sets of factors. Factors are
applied to the data at the scale of FIA inventory plots. The
results are estimates of C density (Mg per hectare) for the
various forest pools. C density for live trees, standing dead
trees, understory vegetation, down dead wood, forest floor,
and soil organic matter are estimated. All non-soil pools
except forest floor can be separated into aboveground and
below ground components. The live tree and understory C
pools are pooled as biomass in this Inventory. Similarly,
standing dead trees and down dead wood are pooled as dead
wood in this Inventory. Definitions of ecosystem pools and
the C conversion process follow, with additional information
in Annex 3.12.
Live Biomass, Dead Wood, and Litter Carbon
Live tree C pools include aboveground and below ground
(coarse root) biomass of live trees with diameter at diameter
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
data do not provide measurements of individual trees; tree
Land Use, Land-Use Change, and Forestry 7-17
-------
C in these plots is estimated from plot-level volume of
merchantable wood, or growing-stock volume, of live trees,
whichis calculated from updates of Smith etal. (2003). 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.
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).
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.
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).
Forest Soil C
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), and
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. Thus, SOC is defined by region and forest
type group.
C stocks and fluxes for Forest Land Remaining Forest
Land are reported in pools following IPCC (2006). Total forest
C stock and flux estimates start with the plot-level calculations
described above. The separate C densities are summed and
multiplied by the appropriate expansion factors to obtain
a C stock estimate for the plot. In turn, these are summed
to state or sub-state total C stocks. Annualized estimates
of C stocks are based on interpolating or extrapolating as
necessary to assign a C stock to each year. For example, the
C stock of Alabama for 2007 is an extrapolation of the two
most recent inventory datasets for that particular state, which
are from 1999 and 2003. Flux, or net annual stock change, is
simply the difference between two successive years with the
appropriate sign convention so that net increases in ecosystem
C are identified as negative flux. This methodological detail
accounts for the constant estimates of flux from the second
most recent Inventory to the present (see 2003 through 2006
on Table 7-6 as an example).
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 (in preparation) using the WOODCAPJ3 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
heldinSWDS.
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
7-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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&PA 2006a 2006b; Howard 2003 &
forthcoming). 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 5 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,
(2A) annual change of C in wood and paper product
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
The 2006 flux estimate for forest C stocks is estimated
to be between -579.0 and -913.2 Tg CO2 Eq. at a 95 percent
confidence level. This includes a range of -471.2 to -802.2
Tg CO2 Eq. in forest ecosystems and -85.5 to -136.8 Tg CO2
Eq. for HWP. The relatively smaller range of uncertainty,
in terms of percentage, for the total relative to the two
separate parts is because the total is based on summing the
two independent uncertain parts, as discussed above. More
information on the uncertainty estimates for Net CO2 Flux
from Forest Land Remaining Forest Land: Changes in Forest
C Stocks is contained within the Uncertainty Annex.
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 2006b).
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
2006 Flux Estimate
(Tg C02 Eq.)
(635.1)
(110.0)
(745.1)
Uncertainty Range Relative to Flux Estimate3
(Tg C02 Eq.) (%)
Lower Bound
(802.2)
(136.8)
(913.2)
Upper Bound
(471.2)
(85.5)
(579.0)
Lower Bound
-26%
-24%
-23%
Upper Bound
+26%
+22%
+22%
Note: Parentheses indicate negative values or net sequestration.
a Range of flux estimates predicted by Monte Carlo stochastic simulation for a 95 percent confidence interval.
Land Use, Land-Use Change, and Forestry 7-19
-------
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 2006b). 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 from
original units to C units are based on estimates by industry
and Forest Service published sources. The WOODCARB II
model uses estimation methods suggested by IPCC (2006).
Estimates of annual C change in solidwood and paper
products in use were verified by two independent criteria. The
first criteria 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 criteria 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 criteria 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
reduces uncertainty in estimates of annual change in C in
products made from wood harvested in the United States.
Recalculations Discussion
The overall process for developing annualized estimates
of forest ecosystem C stocks based on the individual state
surveys and the C conversion factors are identical to that
presented in the previous Inventory (Smith et al. 2007).
However, revised estimates of forest ecosystem C stock
increased by 3 percent for 1990 and 2005. Similarly, estimated
net stock change increased by 4 percent for 1990 and by 6
percent for 2005. The addition of newly available forest
inventory data as well as some refinements in previously
existing data were the principal factors contributing to these
changes. Inventory data changed for 31 of the 48 states
included in the previous Inventory. However, not all of the
changes are apparent in the list of inventory data used for
C estimates (Table A-186) because some changes involved
reclassification and recalculation of existing data. In addition,
a portion of Alaskan forest is included in this Inventory for
the first time. Carbon stock and change estimates for the
early 1990s are still sensitive to updates made over the last
year, which are primarily associated with the most recent
data per state, because 13 of the 49 states are still entirely
or partly based on two C stock estimates (Table A-186).
Thus, even an update for a 2006 C stock, for example, is
propagated throughout the interval when stock change is
linearly interpolated between the two stocks.
The basic model and data used to estimate HWP
contribution under the production approach are unchanged
since the previous Inventory (Skog in preparation). However,
minor modifications to some model coefficients resulted
in slight increases in estimated C sequestration so that net
annual additions to C in HWP increased by 0.5 and 5 percent
for 1990 and 2005, respectively, with an average increase of 3
percent across the sixteen years. Modifications to parameters
included: (1) shorter half-life for decay in dumps and (2)
separation of decay in dumps from decay in landfills.
Planned Improvements
The ongoing annual surveys by the FIA Program will
improve precision of forest C estimates as new state surveys
become available (Gillespie 1999). The annual surveys will
eventually include all states. To date, five states are not yet
reporting any data from the annualized sampling design of
FIA: Hawaii, Mississippi, New Mexico, Oklahoma, and
7-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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. Improved 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. However, long-
term residual effects on soil and forest floor C stocks are
likely after land-use change. Estimates of such effects are
being developed based on methods described by Woodbury
et al. (2007), and preliminary results demonstrate effects
on soil organic C and forest floor. Additional development
is required to link model results with: (1) the C change
methods used for this Inventory (Smith et al. 2007), and (2)
a consistent representation of the land base and land-use
change for the United States (See 7.1 Representation of the
U.S. Land Base in the National Greenhouse Gas Inventory
for more details).
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.
Emissions from this source in 2006 were estimated to be
24.6 Tg C02 Eq. of CH4 and 2.5 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 both the lower
48 states and Alaska.
Methodology
The IPCC (2003) Tier 2 default methodology was used
to calculate non-CO2 emissions from forest fires. Estimates
for CIL, emissions were calculated by multiplying the total
estimated C emitted (see Table 7-13) from forest burned by
gas-specific emissions ratios and conversion factors. N2O
emissions were calculated in the same manner, but were also
multiplied by an N-C ratio of 0.01 as recommended by IPCC
(2003). The equations used were:
CH4 Emissions = (C released) x (emission ratio) x 16/12
N2O Emissions = (C released) x (N/C ratio) x
(emission ratio) x 44/28
Estimates for C emitted from forest fires, presented
in Table 7-13, are the same estimates used to generate
estimates of CO2 emissions from forest fires, presented
Table 7-11: Estimated Non-C02 Emissions from Forest Fires (Tg C02 Eq.) for U.S. Forests3
Gas
CH4
N20
Total
1990
4.5
0.5
4.9
1995
4.7
0.5
5.2
2000
19.0
1.9
20.9
2001
9.4
1.0
10.4
2002
16.4
1.7
18.0
2003
8.7
0.9
9.6
2004
6.9
0.7
7.6
2005
12.3
1.2
13.6
2006
24.6
2.5
27.0
! Calculated based on C emission estimates in Changes in Forest Carbon Stocks and default factors in IPCC (2003).
Table 7-12: Estimated Non-C02 Emissions from Forest Fires (Gg Gas) for U.S. Forests3
Gas
CH4
N20
1990
213
1
1995
224
2
2000
904
6
2001
448
3
2002
780
5
2003
416
3
2004
330
2
2005
586
4
2006
1,169
8
! Calculated based on C emission estimates in Changes in Forest Carbon Stocks and default factors in IPCC (2003).
Land Use, Land-Use Change, and Forestry 7-21
-------
Table 7-13: Estimated Carbon Released from Forest
Fires for U.S. Forests
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
C Emitted (Tg/yr)
13.3
14.0
56.5
28.0
48.7
26.0
20.6
36.6
73.1
earlier in Box 7-1. See Table A-197 and explanation in
Annex 3.12 for more details on the methodology used to
estimate C emitted from forest fires.
Uncertainty
Non-CO2 gases emitted from forest fires depend on
several variables, including forest area and average C
density for forest land in both Alaska and the lower 48 states,
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.
QA/QC and Verification
Tier 1 and Tier 2 QA/QC activities were conducted
consistent with the U.S. QA/QC plan. The QA/QC plan
for forest fires followed the QA/QC plan implemented
for forest C. A source-specific QA/QC plan for forest fires
will be developed and implemented for the next Inventory.
Quality control measures 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
Average carbon density for Alaska was updated from
70 Mg/ha to 331 Mg/ha based on new data from the FIA
National Program. In addition, the static ratio used in the
previous Inventory to estimate the proportion of forestland
burned from data on total area burned was replaced with a
ratio that varied across the inventory time series. See Annex
3.12 for details and additional information related to the
methods described.
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
Table 7-14: Tier 2 Quantitative Uncertainty Estimates of Non-C02 Emissions from Forest Fires in Forest Land
Remaining Forest Land (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Non-C02 Emissions from Forest Fires
Non-C02 Emissions from Forest Fires
CH4
N20
24.6
2.5
7.7
0.8
42.1
4.4
-69%
-69%
+ 71%
+ 75%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
7-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Table 7-15: N20 Fluxes from Soils in Forest Land
Remaining Forest Land (Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
0.1
0.2
0.3
0.3
0.3
0.3
0.3
0.3
0.3
Gg
0.2
0.5
1.0
1.1
1.1
1.1
1.1
1.1
1.1
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 Forest Land Remaining Forest Land and from Land Converted
to Forest Land.
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
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. N2O emissions from forest soils are
estimated to have increased by amultiple of 5.5 from 1990 to
2006. The trend toward increasing N2O emissions is a result
of an increase in the area of N fertilized pine plantations in the
southeastern United States. 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 are in the southeastern United
States. It was assumed that southeastern pine plantations
represent the vast majority of fertilized forests in the United
States. Therefore, 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 (North Carolina
Sate Forest Nutrition Cooperative 2002). 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 midpoint
of the reported range of N fertilization rates (150 Ibs. N per
acre). Data for areas of forests receiving fertilizer outside the
southeastern United States were not available, so N additions
to non-southeastern forests are not included here. It should
be expected, however, that emissions from the small areas
of fertilized forests in other regions would not be substantial
because the majority of trees planted and harvested for timber
are in the southeastern United States (USDA Forest Service
2001). Area data for pine plantations receiving fertilizer in
the Southeast were not available for 2002,2003,2004,2005,
and 2006, so data from 2001 were used for these years. The
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
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, O2 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.
Land Use, Land-Use Change, and Forestry 7-23
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Table 7-16: Quantitative Uncertainty Estimates of N20 Fluxes from Soils in Forest Land Remaining Forest Land
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Forest Land Remaining Forest Land:
N20 Fluxes from Soils N20
0.3
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.
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 2006 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 2006 emission estimate of 0.3 Tg CO2 Eq.
Recalculations Discussion
No recalculations were performed for the time series.
Planned Improvements
State-level area data will be acquired for southeastern
pine plantations 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 or sink for
atmospheric CO2 in most soils. Changes in inorganic C stocks
are typically minor. 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/UNEP/OECD/IEA (1997) and
IPCC (2006) recommends reporting changes in soil organic C
26 Uncertainty is unknown for the fertilization rates so a conservative value
of ±50% was used in the analysis.
7-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
stocks due to agricultural land-use and management activities
on mineral soils and organic soils.27
Typical well-drained mineral soils contain from 1 to 6
percent organic C by weight, although some mineral soils
that are saturated with water for substantial periods during the
year may contain significantly more C (NRCS 1999). When
mineral soils undergo conversion from their native state to
agricultural uses, as much as half the SOC can be 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.
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/IEA1997).
C losses are estimated from drained organic soils under both
grassland and cropland management in this Inventory.
Cropland Remaining Cropland includes all cropland
in a year of the Inventory that had been cropland for the
last 20 years28 according to the USDA NRI land use survey
(USDA-NRCS 2000). Consequently, the area of Cropland
Remaining Cropland changes through time with land-use
change. For this area, 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 2
IPCC method is used for the remaining crops (vegetables,
tobacco, 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 1997 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
2006 (see Table 7-17 and Table 7-18). In 2006, mineral soils
were estimated to remove about 69.5 Tg CO2 Eq. (19.0 Tg
C). This rate of C storage in mineral soils represented about a
20 percent increase in the rate since the initial reporting year
of 1990. Emissions from organic soils were about 27.7 Tg
CO2 Eq. (7.5 Tg C) in 2006. In total, U.S. agricultural soils
in Cropland Remaining Cropland removed approximately
41.8 Tg CO2 Eq. (11.4 Tg C) in 2006.
27 CO2 emissions associated with liming are also estimated but included in
a separate section of the report.
28 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
29 Note that removals occur through crop and forage uptake of CO2 into
biomass C that is later incorporated into soils pools.
Land Use, Land-Use Change, and Forestry 7-25
-------
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
(57.5)
27.4
(30.1)
1995
(67.0)
27.7
(39.4)
2000
(66.1)
27.7
(38.4)
2001
(67.7)
27.7
(40.0)
2002
(68.0)
27.7
(40.3)
2003
(68.1)
27.7
(40.5)
2004
(68.5)
27.7
(40.9)
2005
(68.7)
27.7
(41.0)
2006
(69.5)
27.7
(41.8)
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.
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
(15.7)
7.5
(8.2)
1995
(18.3)
7.5
(10.7)
2000
(18.0)
7.5
(10.5)
2001
(18.5)
7.5
(10.9)
2002
(18.5)
7.5
(11.0)
2003
(18.6)
7.5
(11.0)
2004
(18.7)
7.5
(11.1)
2005
(18.7)
7.5
(11.2)
2006
(19.0)
7.5
(11.4)
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.
The net increase in soil C stocks (39 percent for 2006,
relative to 1990) was largely due to an increase in annual
cropland enrolled in the Conservation Reserve Program,
intensification of crop production by limiting the use of
bare-summer fallow in semi-arid regions, increased hay
production, and adoption of conservation tillage (i.e.,
reduced- and no-till practices). At present (2006), cropland
enrolled in the Conservation Reserve Program accounts
for 32 percent of the increase of C stocks for Cropland
Remaining Cropland on mineral soils (Table 7-18).
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 sequestration
in mineral soils occurred in the Midwest, where there were
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.
The estimates presented here are restricted to C
stock changes in agricultural soils. Agricultural soils
are also important sources of other greenhouse gases,
particularly N2O from application of fertilizers, manure,
and crop residues and from cultivation of legumes, as well
as CH4 from flooded rice cultivation. These emissions
are accounted for in the Agriculture chapter, along with
non-CO2 greenhouse gas emissions from field burning of
crop residues and CK4 and N2O emissions from livestock
digestion and manure management.
7-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Figure 7-5
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States, 1993-2006
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, 1993-2006
Cropland Remaining Cropland
o
Note: Values greater than zero represent emissions.
Tg C02 Eq./year
• lto2
D0.5to1
D0.1to0.5
DO to 0.1
O No organic soils
Land Use, Land-Use Change, and Forestry 7-27
-------
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 USDA
National Resources Inventory (NRI) survey (USDA-NRCS
2000). The NRI is a statistically-based sample of all non-
federal land, and includes ca. 400,000 points in agricultural
land of 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 collected
for each NRI point on a 5-year cycle beginning in 1982,
and were subdivided into four inventory time periods, 1980
through 1984, 1985 through 1989, 1990 through 1994, and
1995 through 2000.
NRI points were classified as Cropland Remaining
Cropland for an inventory time period (e.g., 1990 through
1994 and 1995 through 2000) if the land use had been
cropland for 20 years.31 Cropland includes all land used to
produce food or fiber, as well as 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 used to
estimate C stock changes for mineral soils used to produce a
majority of annual crops in the United States. The remaining
crops on mineral soils were estimated using an IPCC Tier
2 method (Ogle et al. 2003), including vegetables, tobacco,
30 NRI points were classified as agricultural if under grassland or cropland
management in f 992 and/or f 997.
31 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
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 1997, 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 input, along with information about soil physical
properties. Input data on land use and management can
be 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
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. Land-use and management activities were grouped
into inventory time periods (i.e., time "blocks") for 1980
through 1984, 1985 through 1989, 1990 through 1994, and
1995 through 2000, using NRI data from 1982, 1987,1992,
and 1997, respectively.
7-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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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 states 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 categorization scheme). 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. C 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 as an equilibrium step change used in the IPCC methodology
(Tier 1 and 2), the Tier 3 model addresses the delayed response of the soil to management and land-use changes, which 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 soil C stocks change only with discrete changes in management and/or land use, rather than a longer term trend such as gradual
increases in crop productivity.
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 of the Inventory. Specifically, county-scale ratios of
manure available 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 amount
of 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. Managed systems include feedlots or
other housing (which requires manure to be collected and
managed); unmanaged systems include daily spread, pasture,
range, and paddock systems. Annual animal population data
for all livestock types, except horses and goats, were obtained
for all years from the U.S. Department of Agriculture-
National Agricultural Statistics Service. Population data
used for cattle, swine, and sheep were downloaded from the
USDANASS Population Estimates Database (USDA2007a).
Poultry population data were obtained from USDA NASS
reports (USDA 1995a, 1995b, 1998a, 1999, 2004a, 2004b,
2006a, 2006b, 2007b, 2007c). Horse population data were
obtained from the FAOSTAT database (FAO 2007). Goat
population data for 1992, 1997, and 2002 were obtained
from the Census of Agriculture (USDA 2005); these data
were interpolated and extrapolated to derive estimates for
Land Use, Land-Use Change, and Forestry 7-29
-------
the other years. Information regarding poultry turnover (i.e.,
slaughter) rate was obtained from state Natural Resource
Conservation Service personnel (Lange 2000). Additional
population data for different farm size categories for dairy
and swine were obtained from the 1992, 1997, and 2002
Census of Agriculture (USDA 2005).
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/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 and including the
addition of N from bedding materials. N losses include direct
nitrous oxide emissions, volatilization of ammonia and NOX,
and runoff and leaching. More information on these losses
is available in the description of the Manure Management
source category. Animal-specific bedding factors were set
equal to IPCC default factors (IPCC 2006). For unmanaged
systems, it is assumed that no N losses or additions occur.
Monthly weather data, aggregated to county-scale from
the Parameter-elevation Regressions on Independent Slopes
Model (PRISM) database (Daly et al. 1994), were used as
an input in the model simulations. Soil attributes, which
were obtained from an NRI database, were assigned based
on field visits and soil series descriptions. Where more than
one inventory point was located in the same county (i.e.,
same weather) and had the same land-use/management
histories and soil type, data inputs to the model were identical
and, therefore, these points were clustered for simulation
purposes. For the 370,738 NRI points representing non-
federal cropland and grassland, there were a total of 170,279
clustered points that represent the unique combinations of
climate, soils, land use, and management in the modeled
data set. Each NRI cluster point was run 100 times as part
of the uncertainty assessment, yielding a total of over 14
million simulation runs for the analysis. C stock estimates
from Century were adjusted using a structural uncertainty
estimator accounting for uncertainty in model algorithms
and parameter values (Ogle et al. 2007). Mean changes in C
stocks and 95 percent confidence intervals were estimated
for 1990 to 1994 and 1995 to 2000 (see Uncertainty section
for more details). C stock changes from 2001 to 2006 were
assumed to be similar to the 1995 to 2000 block, because
no additional activity data are currently available from the
NRI for the later 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
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).33 U.S. factors associated with
organic matter amendments were not estimated because of
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.
Similar to the Tier 3 Century method, activity data
were primarily based on the historical land-use/management
32 Pasture/Range/Paddock manure additions to soils are addressed in
the Grassland Remaining Grassland and Land Converted to Grassland
categories.
33 Stock change factors have been derived from published literature to
reflect changes in tillage, cropping rotations and intensification, land-use
change between cultivated and uncultivated conditions, and drainage of
organic soils.
7-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 2006 was
determined by calculating the average annual change in
stocks between 1992 and 1997.
Additional Mineral C Stock Change
Annual C flux estimates for mineral soils between 1990
and 2006 were adjusted to account for additional C stock
changes associated with gains or losses in soil C after 1997
due to changes in Conservation Reserve Program enrollment.
The change in enrollment acreage relative to 1997 was based
on data from USDA-FSA (2007) for 1998 through 2006,
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.
Similar to the Tier 2 analysis for mineral soils, 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 2006.
Uncertainty
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
Table 7-19: Quantitative Uncertainty Estimates for C Stock Changes occurring within Cropland Remaining Cropland
(Tg C02 Eq. and Percent)
2006 Flux Estimate1
Source (Tg C02 Eq.)
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 1997)
Organic Soil C Stocks: Cropland Remaining Cropland,
Tier 2 Inventory Methodology
(64.0)
(3.0)
(2.5)
27.7
a Uncertainty Range Relative to Flux Estimate3
(Tg C02 Eq.) (%)
Lower
Bound
(74.1)
(6.9)
(3.7)
15.8
Upper
Bound
(53.5)
0.8
(1.2)
36.9
Lower
Bound
-16%
-127%
-50%
-43%
Upper
Bound
+ 16%
+ 128%
+50%
+33%
Combined Uncertainty for Flux associated with
Agricultural Soil Carbon Stock Change in Cropland
Remaining Cropland
(41.8)
(57.9)
(27.3)
-38%
+35%
! Flux estimates based on soil C stock changes.
Land Use, Land-Use Change, and Forestry 7-31
-------
to the level of the inventory methodology employed (i.e., Tier
2 and Tier 3). A combined uncertainty estimate for changes in
soil C stocks occurring within Cropland Remaining Cropland
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
appear later in this section. The combined uncertainty for soil
C stocks in Cropland Remaining Cropland ranged from 38
percent below and 35 percent above the 2006 stock change
estimate of -41.8 Tg CO2 Eq.
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. The manure
amendment records were not recorded correctly in a subset
of the Century model output; corrective actions were
taken to resolve this error. As discussed in the uncertainty
sections, 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.
Recalculations Discussion
Two changes were implemented in the current Inventory
that led to a change in the time series. First, there was a
modification in the land-use classification. The classification
is based on the land use in a specific year of the Inventory
and the previous 20 years. However, in the 1990 through
2005 Inventory, each point was only classified once based
on the entire NRI time series of the land-use history. This
approach led to incorrect classifications for the early 1990s.
For example, a NRI point may have been cropland in 1982,
1987 and 1992, but converted to grassland in 1997. In the
previous Inventory, the NRI point would be classified as Land
Converted to Grassland for the entire inventory from 1990
through 2005. This is incorrect for the early 1990s because
the point was Cropland Remaining Cropland during those
years. Second, the time series for manure N between 1990
and 2006, which was used to adjust manure applications
relative to 1997, was based on manure N available for
application rather than manure N production. Overall, the
recalculations resulted in an average annual decrease of 1.9
Tg CO2 Eq. for the period 1990 through 2005, compared to
the previous Inventory.
Planned Improvements
Several improvements are planned for the agricultural
soil C inventory. The first improvement is to incorporate
new land-use and management activity data from the NRI.
In the current Inventory, NRI data only provide land-use
and management statistics through 1997, but it is anticipated
that new statistics will be released in the coming year for
2000 through 2003. The new data will greatly improve the
accuracy of land-use and management influences on soil C
in the latter part of the time series.
The second improvement is to incorporate additional
crops into the Tier 3 approach. Currently, crops such as
vegetables, rice, perennial and horticultural crops have not
been fully implemented in the Century model application.
However, efforts are currently underway to further develop
the model application for simulating soil C dynamics in land
managed for production of these crops. This improvement is
expected to reduce uncertainties in the inventory results.
The third 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
7-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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.
The fourth improvement is to develop an automated
quality control system to evaluate the results from Century
model simulations. Currently, there are over 14 million
simulations, and it is not possible to manually review each
simulation. Results are aggregated and evaluated at larger
scales such as MLRAs and States. QA/QC at these larger
scales may not uncover errors at the scale of individual
NRI points, which is the scale at which the Century model
is used to simulate soil C dynamics. An automated system
would greatly improve QA/QC, performing checks on the
results from each simulation and identifying errors for
further refinements.
The final improvement is to further develop the
uncertainty analysis for the Tier 3 method by addressing the
uncertainty inherent in the Century model results for other
agricultural land (i.e., Grassland Remaining Grassland, Land
Converted to Grassland, and Land Converted to Cropland).
In addition, uncertainties need to be addressed in the
simulation of soil C stocks for the pre-NRI time period (i.e.,
before 1979). In the current analysis, inventory development
focused on uncertainties in the last two decades because the
management activity during the most recent time periods
will likely have the largest impact on current trends in soil C
storage. However, legacy effects of past management can also
have a significant effect on current C stock trends, as well as
trajectories of those C stocks in the near future. Therefore,
a planned improvement is to revise the Inventory to address
uncertainties in management activity prior to 1979, providing
a more rigorous accounting of uncertainties associated with
the Tier 3 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
fluctuated over the past sixteen years, ranging from 3.9 Tg
CO2 Eq. to 5.0 Tg CO2 Eq. In 2006, liming of agricultural
soils in the United States resulted in emissions of 4.4 Tg CO2
Eq. (1.2 Tg C), representing about a 6 percent decrease in
emissions since 1990 (see Table 7-20 and Table 7-21).
Table 7-20: Emissions from Liming of Agricultural Soils (Tg C02 Eq.)
Source
Liming of Soils3
1990
4.7
1995
4.4
2000
4.3
2001
4.4
2002
5.0
2003
4.6
2004
3.9
2005
4.3
2006
4.4
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
a Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, and Land Converted to Grassland.
Table 7-21: Emissions from Liming of Agricultural Soils (Tg C)
Source
Liming of Soils3
1990
1.3
1995
1.2
2000
1.2
2001
1.2
2002
1.4
2003
1.2
2004
1.1
2005
1.2
2006
1.2
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
a Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, and Land Converted to Grassland.
Land Use, Land-Use Change, and Forestry 7-33
-------
Methodology
CO2 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). 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 1993,1994,1995,1996,
1997,1998,1999,2000,2001,2002,2003,2004,2005,2006;
Willett 2007; USGS 2007). 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
Table 7-22: Applied Minerals (Million Metric Tons)
limestone and dolomite that was applied to agricultural
soils. In addition, data were not available for 1990, 1992,
and 2006 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 2006 data,
the previous year's fractions were applied to a 2006 estimate
of total crushed stone presented in the USGS Mineral
Industry Surveys: Crushed Stone and Sand and Gravel in
the First Quarter of 2007 (USGS 2007).
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) quantities
for total crushed stone produced or used. It then reported
revised (rounded) quantities for each 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 for the first time this year, but are not reported here.
Uncertainty
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
Mineral
Limestone
Dolomite
1990
19.01
2.36
1995
17.30
2.77
2000
15.86
3.81
2001
16.10
3.95
2002
20.45
2.35
2003
18.71
2.25
2004
15.50
2.33
2005
18.09
1.85
2006
18.20
1.87
Note: These numbers represent amounts applied to all agricultural land, including Land Converted to Cropland, Grassland Remaining Grassland, and Land
Converted to Grassland.
7-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 7-23: Tier 2 Quantitative Uncertainty Estimates for C02 Emissions from Liming of Agricultural Soils
(Tg C02 Eq. and Percent)
Source
Gas
2006 Emission Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Liming of Agricultural Soils"
CO,
4.4
0.2
8.5
-95%
+95%
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, and Land Converted to Grassland.
transport was not accounted for, but should not change the
uncertainty associated with CO2 emissions (West 2005). The
uncertainty 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 0 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 2006 were estimated to be between 0.2
and 8.5 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of 95 percent below to 95 percent above
the 2006 emission estimate of 4.4 Tg CO2 Eq.
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 2005 has been revised.
Consequently, the reported emissions resulting from liming
in 2005 have also changed. In the previous Inventory, to
estimate 2005 data, the previous year's fractions were
applied to a 2005 estimate of total crushed stone presented
in the USGS Mineral Industry Surveys: Crushed Stone and
Sand and Gravel in the First Quarter of 2006 (USGS 2006).
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 2005. These
values have replaced those used in the previous Inventory
to calculate the quantity of minerals applied to soil and the
emissions from liming.
CO2 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
Table 7-24: C02 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C02 Eq.)
Source
Urea Fertilization3
1990
2.4
1995
2.7
2000
3.2
2001
3.4
2002
3.6
2003
3.7
2004
3.7
2005
3.5
2006
3.6
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
a Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, and Land Converted to Grassland.
Land Use, Land-Use Change, and Forestry 7-35
-------
Table 7-25: C02 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C)
Source
Urea Fertilization3
1990
0.7
1995
0.7
2000
0.9
2001
0.9
2002
1.0
2003
1.0
2004
1.0
2005
1.0
2006
1.0
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
a Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, and Land Converted to Grassland.
evolves into CO2 and water. Emissions from urea fertilization
in the United States totaled 3.6 Tg CO2 Eq. (1.0 Tg C) in
2006 (Table 7-24 and Table 7-25). Emissions from urea
fertilization have fluctuated over the past sixteen years,
ranging from 2.3 Tg CO2 Eq. to 3.7 Tg CO2 Eq.
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, 1996, 1997,
1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006)
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
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
Table 7-26: Applied Urea (Million Metric Tons)
of the previous calendar year. Fertilizer use data for the 2007
fertilizer year were not available in time for publication, so
July through December 2006 fertilizer use was estimated
by calculating the percent change (increase or decrease) in
fertilizer use from January through June 2005 to July through
December 2005. This percent change was then multiplied by
the January through June 2006 data to estimate July through
December 2006 fertilizer use. 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
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
estimate is, thus, likely to be high. In addition, each urea
consumption data point has an associated uncertainty. Urea
for non-fertilizer use may be included in consumption totals;
it was determined through personal communication with
Source
Urea Fertilization3
1990
3.30
1995
3.62
2000
4.38
2001
4.66
2002
4.87
2003
5.02
2004
4.98
2005
4.78
2006
4.96
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on historical data only.
a Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, and Land Converted to Grassland.
Table 7-27: Quantitative Uncertainty Estimates for C02 Emissions from Urea Fertilization (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Urea Fertilization
C02
3.6
2.1
3.8
-43%
+3%
Note: These numbers represent amounts applied to all agricultural land, including Land Converted to Cropland, Grassland Remaining Grassland, and Land
Converted to Grassland.
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
7-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Fertilizer Regulatory Program Coordinator David L. Terry
(2007), however, that amount is most likely very small.
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 2006 were estimated to be between 2.1
and 3.8 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of 43 percent below to 3 percent above the
2006 emission estimate of 3.6 Tg CO2 Eq.
QA/QC and Verification
A QA/QC analysis was performed for data gathering
and input, documentation, and calculation. Minor errors
were found in these steps and corrective actions were taken,
including a data point that was incorrectly transcribed.
Inventory reporting forms and text were reviewed and revised
as needed to correct transcription errors.
Recalculations Discussion
Emissions from Urea production and application were
previously included in the Industrial Processes chapter. That
chapter has been modified to only include emissions from
Urea production.
Planned Improvements
Several improvements are planned for the urea
fertilization inventory. The first 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. The second improvement is to investigate
and quantify, if possible, the amount of urea that is currently
included in urea consumption totals, but is used for non-
agricultural practices such as deicing.
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 in the past
20 years34 according to the USDA NPJ land use survey
(USDA-NRCS 2000). Consequently, the area considered
in Land Converted to Cropland changes through time with
land-use change. Lands are retained in this category for
20 years as recommended by the IPCC guidelines (IPCC
2006) unless there is another land-use change. Background
on agricultural C stock changes is provided in Cropland
Remaining Cropland and will only be summarized here for
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
12.3
2.4
14.7
1995
6.7
2.6
9.4
2000
6.7
2.6
9.4
2001
6.7
2.6
9.4
2002
6.7
2.6
9.4
2003
6.7
2.6
9.4
2004
6.7
2.6
9.4
2005
6.7
2.6
9.4
2006
6.7
2.6
9.4
Note: 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)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
3.4
0.7
4.0
1995
1.8
0.7
2.6
2000
1.8
0.7
2.6
2001
1.8
0.7
2.6
2002
1.8
0.7
2.6
2003
1.8
0.7
2.6
2004
1.8
0.7
2.6
2005
1.8
0.7
2.6
2006
1.8
0.7
2.6
Note: 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.
34 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
Land Use, Land-Use Change, and Forestry 7-37
-------
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/UNEP/OECD/IEA (1997) and the IPCC
(2003, 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.35
Land use and management of mineral soils in Land
Converted to Croplandled to losses of soil C during the early
1990s but losses declined slightly 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 9.4 Tg CO2 Eq. (2.6 Tg C) in
2006. Emissions from mineral soils were estimated at 6.7 Tg
CO2 Eq. (1.8 Tg C) in 2006, while drainage and cultivation
of organic soils led to annual losses of 2.6 Tg CO2 Eq. (0.7
Tg C) in 2006.
The spatial variability in annual CO2 flux associated
with C stock changes in mineral and organic soils for Land
Converted to Croplandis displayed in Figure 7-7 and Figure
7-8. While a large portion of the United States had net
losses in soil C for Land Converted to Cropland, there were
some notable areas with sequestration in the Intermountain
West and central United States. These areas were gaining
C following conversion, because croplands were irrigated
or receiving higher fertilizer inputs relative to the previous
land use. 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.
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 attributes, and
irrigation) were collected for each NRI point on a 5-year
cycle beginning in 1982, and were subdivided into four
inventory time periods, 1980 through 1984, 1985 through
1989, 1990 through 1994 and 1995 through 2000. NRI
points were classified as Land Converted to Cropland for
an inventory time period (e.g., 1990 through 1994 and 1995
through 2000) if the land use was cropland in the respective
inventory time period but had been another use during the
previous 20 years.36 Cropland includes all land used to
produce food or fiber, as well as forage that is harvested and
used as feed (e.g., hay and silage). 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.
Mineral Soil Carbon Stock Changes
A Tier 3 model-based approach was used to estimate
C stock changes for soils on Land Converted to Cropland
used to produce a majority of all crops. 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.37
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 US DA 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
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
35 CO2 emissions associated with liming are also estimated but included in
a separate section of the report.
36 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
37 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).
7-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Figure 7-7
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States, 1993-2006
Land Converted to Cropland
Tg C02 Eq./year
D>0
D-0.1 toO
D-0.5 to-0.1
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 7-8
Total Net Annual C02 Flux For Organic Soils Under Agricultural Management within States, 1993-2006
Land Converted to Cropland
o
Tg C02 Eq./year
• 0.5 to 1
D 0.1 to 0.5
DO to 0.1
EH No organic soils
Note: Values greater than zero represent emissions.
Land Use, Land-Use Change, and Forestry 7-39
-------
for Land Converted to Cropland as described in the Tier 2
portion of Cropland Remaining Cropland Section for mineral
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 2006.
Uncertainty
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). A combined
uncertainty estimate for changes in agricultural soil C
stocks occurring within Land Converted to Cropland^ 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 25 percent
below and 22 percent above the inventory estimate of 9.4
TgC02Eq.
QA/QC and Verification
See QA/QC and Verification Section under Cropland
Remaining Cropland.
Recalculations Discussion
Two changes were implemented in the current
Inventory that led to a change in the time series. First,
there was a modification in the land-use classification. The
classification is based on the land use in a specific year of
the Inventory and the previous 20 years. However, in the
Table 7-30: Quantitative Uncertainty Estimates3 for C Stock Changes occurring within Land Converted to Cropland
(Tg C02 Eq. and Percent)
2006 Flux Estimate3 Uncertainty Range Relative to Flux Estimate3
Source (Tg C02 Eq.) (Tg C02 Eq.) (%)
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
Combined Uncertainty for Flux associated with Soil
Carbon Stock Change in Land Converted to Cropland
2.6
4.1
2.6
9.4
Lower
Bound
2.0
2.3
1.2
7.0
Upper
Bound
3.1
5.8
3.7
11.4
Lower
Bound
-21%
-44%
-53%
-25%
Upper
Bound
+ 21%
+41%
+41%
+ 22%
a Flux estimates based on soil C stock changes.
7-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
1990 through 2005 Inventory, each point was only classified
once based on the entire NRI time series of the land-use
history. This approach led to incorrect classifications for
the early 1990s. For example, a NRI point may have been
grassland in 1982,1987 and 1992, but converted to cropland
in 1997. In the previous Inventory, the NRI point would
be classified as Land Converted to Cropland for the entire
Inventory from 1990 through 2005. This is incorrect for the
early 1990s because the point was Grassland Remaining
Grassland during those years. Second, the time series for
manure N between 1990 through 2006, which was used
to adjust manure applications relative to 1997, was based
on manure N available for application rather than manure
N production. Overall, these recalculations resulted in an
average annual increase in emissions of 3.3 Tg CO2 Eq. for
soil C stock changes in Land Converted to Cropland over
the time series from 1990 through 2005, compared 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 related to the structure of 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 years38 according to the USDA NRI land use survey
(USDA-NRCS 2000). Consequently, the area considered
in Grassland Remaining Grassland changes through time
with land-use change. 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. The IPCC/UNEP/OECD/IEA (1997) and IPCC
(2003, 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 Grassland
Remaining Grassland increased soil C during the early
1990s, but this trend was reversed over the decade, with small
losses of C prevailing during the latter part of the time series.
Organic soils lost about the same amount of C in each year of
the Inventory. Due to the pattern for mineral soils, the overall
trend shifted from small increases in soil C during 1990 to
decreases in soil C during the latter years of the Inventory,
estimated at 16.2 Tg CO2 Eq. (4.4 Tg C) in 2006 (Table 7-31
and Table 7-32). Overall, flux rates changed by 18.1 Tg CO2
Eq. (4.9 Tg C) from 1990 to 2006.
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. Grassland is losing soil
organic C in the United States largely due to droughts that
are causing small losses of C on a per hectare basis, but are
occurring over a large land base. In areas with net gains in
soil organic C, sequestration was driven by irrigation and
seeding legumes. Similar to Cropland Remaining Cropland,
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.
38 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
39 CO2 emissions associated with liming are also estimated but included in
a separate section of the report.
Land Use, Land-Use Change, and Forestry 7-41
-------
Table 7-31: Net C02 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg C02 Eq.)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(5.7)
3.9
(1.9)
1995
12.9
3.7
16.6
2000
12.8
3.7
16.4
2001
12.7
3.7
16.4
2002
12.7
3.7
16.4
2003
12.7
3.7
16.4
2004
12.6
3.7
16.3
2005
12.6
3.7
16.3
2006
12.5
3.7
16.2
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
(1.6)
1.1
(0.5)
1995
3.5
1.0
4.5
2000
3.5
1.0
4.5
2001
3.5
1.0
4.5
2002
3.5
1.0
4.5
2003
3.5
1.0
4.5
2004
3.4
1.0
4.5
2005
3.4
1.0
4.4
2006
3.4
1.0
4.4
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.
Figure 7-9
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States, 1993-2006
Grassland Remaining Grassland
Tg C02 Eq./year
D>0
D-0.1 toO
D -0.5 to -0.1
D-Ko-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.
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.
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., irrigation, legume
pastures) were collected for each NRI point on a 5-year
cycle beginning in 1982, 1980 through 1984, 1985 through
7-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Figure 7-10
Total Net Annual C02 Flux For Organic Soils Under Agricultural Management within States, 1993-2006
Grassland Remaining Grassland
o
Tg C02 Eq./year
• Ko2
D0.5to1
D0.1to0.5
DO to 0.1
I | No organic soils
Note: Values greater than zero represent emissions.
1989,1990 through 1994 and 1995 through 2000. NRI points
were classified as Grassland Remaining Grassland for an
inventory time period (e.g., 1990 through 1994 and 1995
through 2000) if the land use was grassland in the inventory
time period and had been grassland for the previous 20
years.40 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. 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.
Mineral Soil Carbon Stock Changes
An IPCC Tier 3 model-based approach was used to
estimate C stock changes for most mineral soils in Grassland
40 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
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 the USDA National Resources Inventory (NRI) survey,
with 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 of the Inventory.
Specifically, county-scale ratios of manure available in other
Land Use, Land-Use Change, and Forestry 7-43
-------
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. Managed systems include feedlots or
other housing (which requires manure to be collected and
managed); unmanaged systems include daily spread, pasture,
range, and paddock systems. Annual animal population data
for all livestock types, except horses and goats, were obtained
for all years from the U.S. Department of Agriculture-
National Agricultural Statistics Service. Population data
used for cattle, swine, and sheep were downloaded from the
USDANASS Population Estimates Database (USDA2007a).
Poultry population data were obtained from USDA NASS
reports (USDA 1995a, 1995b, 1998a, 1999, 2004a, 2004b,
2006a, 2006b, 2007b, 2007c). Horse population data were
obtained from the FAOSTAT database (FAO 2007). Goat
population data for 1992, 1997, and 2002 were obtained
from the Census of Agriculture (USDA 2005); these data
were interpolated and extrapolated to derive estimates for
the other years. Information regarding poultry turnover (i.e.,
slaughter) rate was obtained from state Natural Resource
Conservation Service personnel (Lange 2000). Additional
population data for different farm size categories for dairy
and swine were obtained from the 1992, 1997, and 2002
Census of Agriculture (USDA 2005).
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). Manure
amendments were an input to the Century Model based on
manure N available for application from all other managed
or unmanaged systems. Data on the county-lev el N available
for application were estimated for managed systems based on
the total amount of N excreted in manure minus N losses and
including the addition of N from bedding materials. Nitrogen
losses include direct nitrous oxide emissions, volatilization
of ammonia and NOX, and runoff and leaching. More
information on these losses is available in the description of
the Manure Management source category. Animal-specific
bedding factors were set equal to IPCC default factors (IPCC
2006). For unmanaged systems, it is assumed that no N
losses or additions occur. 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 for additional
information).
Additional Mineral C Stock Change Calculations
Annual C flux estimates for mineral soils between 1990
and 2006 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 7997
7-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 2006.
Uncertainty
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). A combined
uncertainty estimate for changes in agricultural soil C stocks
occurring within Grassland Remaining Grassland is also
included. Uncertainty estimates from each component were
combined using the error propagation equation in accordance
with IPCC Guidelines (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 18 percent below and 15 percent above the inventory
estimate of 16.2 Tg CO2 Eq.
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 2006
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. The manure
amendment records were not recorded correctly in a subset
of the Century model output; corrective actions were taken
to resolve this error.
Recalculations Discussion
Two changes were implemented in the current Inventory
that led to a change in the time series. First, there was a
modification in the land-use classification. The classification
is based on the land use in a specific year of the Inventory and
the previous 20 years. However, in the previous Inventory,
Table 7-33: Quantitative Uncertainty Estimates3 for C Stock Changes occurring within Grassland Remaining
Grassland (Tg C02 Eq. and Percent)
2006 Flux Estimate'
Source (Tg C02 Eq.)
Mineral Soil C Stocks: Grassland Remaining Grassland,
Tier 3 Inventory Methodology
Mineral Soil C Stocks: Grassland Remaining Grassland,
Tier 2 Inventory Methodology
Mineral Soil C Stocks: Grassland Remaining Grassland
(Change in Soil C due to Sewage Sludge Amendments)
Organic Soil C Stocks: Grassland Remaining Grassland,
Tier 2 Inventory Methodology
Combined Uncertainty for Flux Associated with
Agricultural Soil Carbon Stock Change in Grassland
Remaining Grassland
13.9
(0.2)
(1.2)
3.7
16.2
' Uncertainty Range Relative to Flux Estimate3
(Tg C02 Eq.) (%)
Lower
Bound
12.5
(0.3)
(1.7)
1.2
13.4
Upper
Bound
15.3
0.0
(0.6)
5.5
18.6
Lower
Bound
-10%
-89%
-50%
-66%
-18%
Upper
Bound
+10%
+ 127%
+50%
+49%
+15%
a Flux estimates based on soil C stock changes.
Land Use, Land-Use Change, and Forestry 7-45
-------
each point was only classified once based on the entire NRI
time series of the land-use history. This approach led to
incorrect classifications for the early 1990s. For example, a
NRI point may have been grassland in 1982,1987 and 1992,
but converted to cropland in 1997. In the previous Inventory,
the NRI point would be classified as Land Converted to
Cropland for the entire Inventory from 1990 through 2005.
This is incorrect for the early 1990s because the point was
Grassland Remaining Grassland during those years. Second,
the time series for manure N between 1990 through 2006,
which was used to adjust manure applications relative to
1997, was based on manure N available for application
rather than manure N production. Overall, the recalculations
resulted in an average annual decrease in emissions of 0.5
Tg CO 2 Eq. for the time series over the period from 1990
through 2005, compared 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,
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 Grow/and includes all grassland in
an inventory year that had been in another land use during
the previous 20 years41 according to the USDA NRI land
use survey (USDA-NRCS 2000). Consequently, the area
of Land Converted to Grassland changes through time
with land-use change. Lands are retained in this category
for 20 years as recommended by IPCC (2006) unless there
is another land-use change. 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/UNEP/OECD/IEA (1997) and IPCC (2003,
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.42
Land use and management of mineral soils in Land
Converted to Grassland led to an increase in soil C stocks
from 1990 through 2006, which was largely caused by annual
cropland converted into pasture (see Table 7-34 and Table
7-35). Stock change rates over the time series varied from
-14.7 to -17.2 Tg CO2 Eq./yr (4 to 5 Tg C). 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 Tg C).
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
Southeast and Northwest, and the amount of sequestration
increased through the 1990s. 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, which coincides with largest
concentrations of organic soils in the United States that are
used for agricultural production.
Methodology
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.
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., legume pastures,
crop type, soil attributes, and irrigation) was collected for
each NRI point on a 5-year cycle beginning in 1982, and
was subdivided into four inventory time periods, 1980
through 1984, 1985 through 1989, 1990 through 1994 and
41 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
42 CO2 emissions associated with liming are also estimated but included in
a separate section of the report.
7-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 7-34: Net C02 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C02 Eq.)
Soil Type
Mineral Soils3
Organic Soils
Total Net Flux
1990
(14.7)
0.5
(14.3)
1995
(17.2)
0.9
(16.3)
2000
(17.2)
0.9
(16.3)
2001
(17.2)
0.9
(16.3)
2002
(17.2)
0.9
(16.3)
2003
(17.2)
0.9
(16.3)
2004
(17.2)
0.9
(16.3)
2005
(17.2)
0.9
(16.3)
2006
(17.2)
0.9
(16.3)
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.
a Stock changes due to application of sewage sludge are reported in Grassland Remaining Grassland.
Table 7-35: Net C02 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C)
Soil Type
Mineral Soils3
Organic Soils
Total Net Flux
1990
(4.0)
0.1
(3.9)
1995
(4.7)
0.2
(4.5)
2000
(4.7)
0.2
(4.5)
2001
(4.7)
0.2
(4.5)
2002
(4.7)
0.2
(4.5)
2003
(4.7)
0.2
(4.5)
2004
(4.7)
0.2
(4.5)
2005
(4.7)
0.2
(4.5)
2006
(4.7)
0.2
(4.5)
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.
a Stock changes due to application of sewage sludge in Land Converted to Grassland are reported in Grassland Remaining Grassland.
Figure 7-11
Total Net Annual C02 Flux For Mineral Soils Under Agricultural Management within States, 1993-2006
Land Converted to Grassland
Tg C02 Eq./year
D-0.1 toO
D-0.5 to-0.1
• -1to-0.5
D-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.
1995 through 2000. NRI points were classified as Land period but had been another use in the previous 20 years.'
Converted to Grassland for an inventory time period (e.g.,
1990 through 1994 and 1995 through 2000) if the land use
was grassland at the end of the respective inventory time
43 NRI points were classified according to land-use history records starting
in 1982 when the NRI survey began. Therefore, the classification was based
on less than 20 years of recorded land-use history for the time series from
1982 to 2001.
Land Use, Land-Use Change, and Forestry 7-47
-------
Figure 7-12
Total Net Annual C02 Flux For Organic Soils Under Agricultural Management within States, 1993-2006
Land Converted to Grassland
Tg C02 Eq./year
• 0.5 to 1
D 0.1 to 0.5
Do to 0.1
CH No organic soils
Note: Values greater than zero represent emissions.
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 removal,
that may or may not be improved with practices such as
irrigation and interseeding legumes. 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.
Mineral Soil Carbon Stock Changes
An IPCC Tier 3 model-based approach was used 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.44 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
Grassland Remaining Grassland. Historical land-use and
management patterns were used in the Century simulations
as recorded in the NPJ 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 GrasslandTier3 methods
section for additional information).
44 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).
7-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 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 7997 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 2006.
Uncertainty
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). A combined
uncertainty estimate for changes in agricultural soil C
stocks occurring within Land Converted to Grassland 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 13
percent below and 14 percent above the 2006 estimate of
16.3 Tg CO2 Eq.
QA/QC and Verification
See the QA/QC and Verification section under Grassland
Remaining Grassland.
Recalculations Discussion
Two changes were implemented in the current
Inventory that led to a change in the time series. First,
there was a modification in the land-use classification. The
classification is based on the land use in a specific year of
the Inventory and the previous 20 years. However, in the
1990 through 2005 Inventory, each point was only classified
once based on the entire NPJ time series of the land-use
history. This approach led to incorrect classifications for
the early 1990s. For example, a NPJ point may have been
cropland in 1982,1987 and 1992, but converted to grassland
in 1997. In the previous Inventory, the NPJ point would
be classified as Land Converted to Grasslandfor the entire
Inventory from 1990 through 2005. This is incorrect for the
early 1990s because the point was Cropland Remaining
Cropland during those years. Second, the time series for
manure N between 1990 through 2006, which was used
to adjust manure applications relative to 1997, was based
on manure N available for application rather than manure
N production. Overall, the recalculations resulted in an
average annual decrease in emissions of 0.1 Tg CO2 Eq.
for the time series from 1990 through 2005, compared 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
improvements.
Land Use, Land-Use Change, and Forestry 7-49
-------
Table 7-36: Quantitative Uncertainty Estimates3 for C Stock Changes occurring within Land Converted to Grassland
(Tg C02 Eq. and Percent)
2006 Flux Estimate
Source (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
Combined Uncertainty for Flux associated with
Agricultural Soil Carbon Stocks in Land Converted
to Grassland
(12.2)
(5.0)
0.9
(16.3)
a Uncertainty Range Relative to Flux Estimate3
(Tg C02 Eq.) (%)
Lower
Bound
(12.5)
(7.0)
0.2
(18.4)
Upper
Bound
(11.9)
(2.8)
1.8
(14.0)
Lower
Bound
-2%
-39%
-76%
-13%
Upper
Bound
+ 2%
+43%
+ 104%
+14%
a Flux estimates based on soil C stock changes.
7.8. Settlements Remaining
Settlements
Table 7-37: 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 78.1 Tg CO2 Eq.
(21 Tg C) over the period from 1990 through 2006. Total
sequestration increased by 57 percent between 1990 and
2006 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 were developed based on
periodic U.S. Census data on urban area (Table 7-37). Net
C flux from urban trees in 2006 was estimated to be -95.5
Tg CO2 Eq. (-26 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
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
(60.6)
(71.5)
(82.4)
(84.6)
(86.8)
(88.9)
(91.1)
(93.3)
(95.5)
TgC
(16.5)
(19.5)
(22.5)
(23.1)
(23.7)
(24.3)
(24.9)
(25.4)
(26.0)
Note: Parentheses indicate net sequestration.
(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
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. Urban trees, therefore,
appear to have a greater C density than forested areas (Nowak
and Crane 2002).
Methodology
The methodology used by Nowak and Crane (2002) is
based on average annual estimates of urban tree growth and
decomposition, which were derived from field measurements
and data from the scientific literature, urban area estimates
from U.S. Census data, and urban tree cover estimates
from remote sensing data. This approach is consistent with
7-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 data were analyzed using the U.S.
Forest Service's Urban Forest Effects (UFORE) model.45 The
UFORE model is a computer model that uses standardized
field data from random plots 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 (Nowak et al. 2007a).
Nowak and Crane (2002) developed estimates of annual
gross C sequestration from tree growth and annual gross C
emissions from decomposition for 15 U.S. cities: Atlanta,
GA; Baltimore, MD; Boston, MA; Chicago, IL; Freehold, NJ;
Jersey City, NJ; Minneapolis, MN; Moorestown, NJ; New
York, NY; Oakland, CA; Philadelphia, PA; San Francisco,
CA; Syracuse, NY; Washington, DC; and Woodbridge, NJ.
The gross C sequestration estimates were derived from field
data that were collected in these 15 cities during the period
from 1989 through 2006, including tree measurements of
stem diameter, tree height, crown height, and crown width,
and information on location, species, and canopy condition.
The field data were converted to annual gross C sequestration
rates for each species (or genus), diameter class, and land-use
condition (forested, park-like, and open growth) by applying
allometric equations, a root-to-shoot ratio, moisture contents,
a C content of 50 percent (dry weight basis), an adjustment
factor to account for smaller aboveground biomass volumes
(given a particular diameter) in urban conditions compared
to forests, an adjustment factor to account for tree condition
(fair to excellent, poor, critical, dying, or dead), and annual
diameter and height growth rates. The annual gross C
sequestration rates for each species (or genus), diameter
class, and land-use condition were then scaled up to city
estimates using tree population information. The field data
from the 15 cities, some of which are unpublished (Nowak
2007c), are described in Nowak and Crane (2002), Nowak
et al. (2007a), and references cited therein. The allometric
equations were taken from the scientific literature (see
Nowak 1994, Nowak et al. 2002), and the adjustments to
45 Oakland and Chicago estimates were based on prototypes to the UFORE
model.
account for smaller volumes in urban conditions were 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 Shifley (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.
Growth rates were adjusted to account for tree condition.
Growth factors for Atlanta, Boston, Chicago, Freehold,
Jersey City, Moorestown, New York, Oakland, 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, 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).
Annual gross C emission estimates were derived by
applying estimates of annual mortality and condition, and
assumptions about whether dead trees were removed from
the site, to C stock estimates. These values were derived
as intermediate steps in the sequestration calculations, and
different decomposition rates were applied to dead trees left
standing compared with those removed from the site. The
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. Estimates of
annual mortality rates by diameter class and condition class
were derived from a study of street-tree mortality (Nowak
1986). Assumptions about whether dead trees would be
removed from the site 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
from 13 of the 15 cities (Table 7-38), and urban area and
urban tree cover data for the United States. Annual net
C sequestration estimates were derived for 13 cities by
subtracting the annual gross emission estimates from the
Land Use, Land-Use Change, and Forestry 7-51
-------
Table 7-38: 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 cover-yr) for 15 U.S. Cities
City
Atlanta, GA
Baltimore, MD
Boston, MA
Chicago, IL
Freehold, NJ
Jersey City, NJ
Minneapolis, MN
Moorestown, NJ
NewYork, NY
Oakland, CA
Philadelphia, PA
San Francisco, CA
Syracuse, NY
Washington, DC
Woodbridge, NJ
NA = not analyzed.
Sources: Nowakand
Carbon
Stocks
1,219,256
541,589
289,392
NA
18,144
19,051
226,796
106,141
1,224,699
NA
480,808
175,994
156,943
477,179
145,150
Crane (2002) and
Gross Annual
Sequestration
42,093
14,696
9,525
NA
494
807
8,074
3,411
38,374
NA
14,606
4,627
4,917
14,696
5,044
Nowak(2007a,c).
Net Annual
Sequestration
32,169
9,261
6,966
NA
318
577
4,265
2,577
20,786
NA
10,530
4,152
4,270
11,661
3,663
Tree
Cover
36.7%
21.0%
22.3%
11.0%
34.4%
11.5%
26.4%
28.0%
20.9%
21.0%
15.7%
11.9%
23.1%
28.6%
29.5%
Gross Annual
Sequestration per
Area of Tree Cover
0.34
0.35
0.30
0.61
0.28
0.18
0.20
0.32
0.23
NA
0.27
0.33
0.33
0.32
0.28
Net Annual
Sequestration per
Area of Tree Cover
0.26
0.22
0.22
NA
0.18
0.13
0.11
0.24
0.12
NA
0.20
0.29
0.29
0.26
0.21
annual gross sequestration estimates.46 The urban areas are
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
has increased by approximately 36 percent from 1990 to
2000; Nowak et al. (2005) estimate that the changes in the
definition of urban land have resulted in approximately 20
percent of the total reported increase in urban land area
from 1990 to 2000. Under both 1990 and 2000 definitions,
urban encompasses most cities, towns, and villages (i.e., it
includes both urban and suburban areas). The gross and net
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 (0.31 kg C/m2-year)
was then multiplied by the estimate of national urban tree
cover area to estimate national annual gross sequestration.
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 for those
cities that had both estimates (0.72). The urban tree cover
estimates for each of the 15 cities and the United States were
obtained from Dwyer et al. (2000), Nowak et al. (2002), and
Nowak (2007a). The urban area estimates were taken from
Nowak et al. (2005).
Uncertainty
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 the 15 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 the 15 U.S. cities was
based on standard error estimates for each of the city-level
sequestration estimates reported by Nowak (2007c). These
estimates are based on field data collected in 15 U.S. cities,
6 Two cities did not have net estimates.
7-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 7-39. The net C flux from changes in
C stocks in urban trees was estimated to be between -112.1
and -76.5 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 17 percent below and 20 percent above
the 2006 flux estimate of -95.5 Tg CO2 Eq.
QA/QC and Verification
The net C flux resulting from urban trees was
calculated using estimates of gross and net C sequestration
estimates for urban trees and urban tree coverage area
found in 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
New data were added for six U.S. cities: Freehold, NJ;
Minneapolis, MN; Moorestown, NJ; San Francisco, CA;
Washington, DC; and Woodbridge, NJ. Datafor Sacramento,
CA was removed from the urban trees estimates because it
was analyzed using a different methodology. These changes
brought the total number of included cities to 15, providing
a better median estimate of net and gross sequestration
than the previous inventory estimate based on data from
10 U.S. cities.
There was also a slight change in the methodology for
adjusting urban tree growth rates to account for tree condition.
Some of the older studies used average growth rates based on
the typical growth conditions of different land-use categories.
In contrast, some of the newer studies adjust growth factors
based on the condition of the tree, which is determined using
tree competition factors (depending on whether it is open
grown or suppressed) for each individual tree. The cities that
use each of these methodologies are identified above in the
Methodology section. The difference that resulted from this
change in methodological approach is very small and likely
washes out on average (Nowak 2007b).
These changes resulted in changes in the estimates of
net annual C sequestration by urban trees for the time period
1990 through 2005. On average, estimates of net annual C
sequestration by urban trees increased by 5.3 percent over the
period from 1990 to 2005 relative to the previous report.
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
Table 7-39: Tier 2 Quantitative Uncertainty Estimates for Net C Flux from Changes in C Stocks in Urban Trees (Tg
C02 Eq. and Percent)
2006 Flux Estimate Uncertainty Range Relative to Flux Estimate
Source Gas (Tg C02 Eq.) (Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Changes in C Stocks in Urban Trees C02 (95.5)
(112.1) (76.5) -17% +20%
Note: Parentheses indicate negative values or net sequestration.
Land Use, Land-Use Change, and Forestry 7-53
-------
LULUCF 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 considered non-urban 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 is expected in
the near term, and the use of this 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 less
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 2006, N2O emissions from this source were 1.5 Tg CO2
Eq. (4.7 Gg). There was an overall increase of 48 percent
over the period from 1990 through 2006 due to a general
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 7-40.
Table 7-40: N20 Fluxes from Soils in Settlements
Remaining Settlements (Tg C02 Eq. and Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Tg C02 Eq.
1.0
1.2
1.2
1.4
1.5
1.5
1.6
1.5
1.5
Gg
3.2
3.9
4.0
4.6
4.7
4.9
5.0
4.8
4.7
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.
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 and the
amount of N in sewage sludge applied to non-agricultural
land and in surface disposal of sewage sludge.
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 2001
were used for 2002 through 2006. 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, 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
7-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
N2O Emissions from Agricultural Soil Management source
category of the Agriculture chapter.
Uncertainty
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, O2 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 level47 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 2006 emissions estimates. The results
of the quantitative uncertainty analysis are summarized
in Table 7-41. N2O emissions from soils in Settlements
Remaining Settlements in 2006 were estimated to be between
0.8 and 3.9 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 49 percent below to 163 percent above
the 2006 emission estimate of 1.5 Tg CO2 Eq.
47 No uncertainty is provided with the USGS application data (Ruddy et al.
2006) so a conservative ±50% was used in the analysis.
Recalculations Discussion
A new data source was used for N fertilization in the
current inventory. Instead of assuming settlement soils
receive 10 percent of total synthetic N fertilizer applied in the
United States, fertilization data were based on county-scale
non-farm application amounts from a database compiled
by the USGS (Ruddy et al. 2006). According to the USGS
data, approximately 1.7 percent of synthetic fertilizer N sold
was for non-farm use in 1990 and this gradually increased
to 3.1 percent in 2001. After subtracting forest application
from non-farm fertilizer use, this change resulted in a 75
percent decrease in the emission estimates for 2005 and an
average decrease of about 78 percent over the period from
1990 to 2005.
Planned Improvements
The key planned improvement is to estimate emissions
using the process-based DAY CENT model instead of the
IPCC default methodology. DAYCENT has been used to
estimate N2O emissions from agricultural soils, reducing
bias and improving precision in estimates for the cropland
and grassland soils. Applying the DAYCENT model is also
anticipated to reduce uncertainties in the estimated emissions
from settlement soils. In addition, this planned improvement
would incorporate state-level settlement area data from the
National Resource Inventory. Another 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.
7.9. 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.
Land Use, Land-Use Change, and Forestry 7-55
-------
Table 7-41: Quantitative Uncertainty Estimates of N20 Emissions from Soils in Settlements Remaining Settlements
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Settlements Remaining Settlements:
N20 Fluxes from Soils
N,0
1.5
0.8
3.9
+163%
Note: This estimate includes direct N20 emissions from N fertilizer additions to both Settlements Remaining Settlements and from Land Converted to
Settlements.
7.10. 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
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 and food scraps
collected annually and the fraction that is landfilled have
declined over the last decade. In 1990, over 51 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 2007; Schneider 2007, 2008). Since then, programs
banning or discouraging disposal have led to an increase
in backyard composting and the use of mulching mowers,
and a consequent 7 percent decrease in the amount of
yard trimmings 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 31 percent in 2006. The
net effect of the reduction in generation and the increase in
composting is a 60 percent decrease in the quantity of yard
trimmings disposed in landfills since 1990. Food scraps
generation has grown by 50 percent since 1990, but the
proportion of food scraps discarded in landfills has decreased
slightly from 81 percent in 1990 to 80 percent in 2006.
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 C storage from 23.9 Tg CO2 Eq. in 1990 to 10.5 Tg
C02 Eq. in 2006 (Table 7-42 and Table 7-43).
Methodology
As empirical evidence shows, the removal of C from the
natural cycling of C between the atmosphere and biogenic
materials, which occurs when wastes of biogenic origin
are deposited in landfills, sequesters C (Barlaz 1998, 2005,
2008). When wastes of sustainable, biogenic origin (such
as yard trimming and food scraps) are landfilled and do not
completely decompose, the C that remains is effectively
removed from the global C cycle. 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;
7-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 7-42: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C02 Eq.)
Carbon Pool
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
Total Net Flux
1990
(21.4)
(1.9)
(9.7)
(9.7)
(2.5)
(23.9)
1995
(12.6)
(0.8)
(6.0)
(5.8)
(1.6)
(14.1)
2000
(8.2)
(0.4)
(4.0)
(3.7)
(3.3)
(11.5)
2001
(8.5)
(0.5)
(4.1)
(3.8)
(3.1)
(11.6)
2002
(8.7)
(0.6)
(4.2)
(3.9)
(3.1)
(11.8)
2003
(7.1)
(0.4)
(3.5)
(3.2)
(2.9)
(10.0)
2004
(6.2)
(0.3)
(3.1)
(2.8)
(3.4)
(9.6)
2005
(6.5)
(0.4)
(3.2)
(2.9)
(3.5)
(10.0)
2006
(6.8)
(0.5)
(3.3)
(3.0)
(3.7)
(10.5)
Note: Totals may not sum due to independent rounding.
Table 7-43: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C)
Carbon Pool
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
Total Net Flux
1990
(5.8)
(0.5)
(2.7)
(2.6)
(0.7)
(6.5)
1995
(3.4)
(0.2)
(1.6)
(1.6)
(0.4)
(3.9)
2000
(2.2)
(0.1)
(1.1)
(1.0)
(0.9)
(3.1)
2001
(2.3)
(0.1)
(1.1)
(1.0)
(0.8)
(3.2)
2002
(2.4)
(0.2)
(1.1)
(1.1)
(0.8)
(3.2)
2003
(1.9)
(0.1)
(0.9)
(0.9)
(0.8)
(2.7)
2004
(1.7)
(0.1)
(0.8)
(0.8)
(0.9)
(2.6)
2005
(1.8)
(0.1)
(0.9)
(0.8)
(0.9)
(2.7)
2006
(1.9)
(0.1)
(0.9)
(0.8)
(1.0)
(2.9)
Note: Totals may not sum due to independent rounding.
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 adjusted by mass
balance, 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 ona 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: 2006 Facts and Figures (EPA 2007), which provides
data for 1960, 1970, 1980, 1990, 2000, 2002, and 2004
through 2006. To provide data for some of the missing years,
detailed backup data was obtained from Schneider (2007,
2008). Remaining years in the time series for which data were
not provided were estimated using linear interpolation. The
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 90
percent and 10 percent respectively in 1980, 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 adry 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, cited
by Barlaz 1998) and the initial C contents were determined
by Barlaz (1998, 2005, 2008) (Table 7-44).
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
Land Use, Land-Use Change, and Forestry 7-57
-------
Table 7-44: 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
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 7-44).
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 CFL, 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. 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 = £ Wu x (1 - MQ) x ICQ 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),
Win = 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 IKUnlKUt),
MQ = Moisture content of waste i (percent of water),
CSj = Proportion of initial C that is stored for
waste i (percent),
ICC; = Initial C content of waste i (percent),
e = Natural logarithm, and
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 (TLFQ)
is the sum of stocks across all four materials. The annual flux
of C in landfills (Ft) for year t is calculated as the change in
stock compared to the preceding year:
Ft = TLFQ - TLFQ _ 1
Thus, the C placed in a landfill in year n is tracked for
each year t through the end of the inventory period (2006). For
example, disposal of food scraps in 1960 resulted in depositing
about 1,135,000 metric tons of C. Of this amount, 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 2006, 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
2006), the total landfill C from food scraps in 2006 was 29.5
million metric tons. This value is then added to the C stock
7-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 7-45: 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
156.9
15.8
70.2
70.9
17.9
174.8
1995
180.2
17.7
81.0
81.6
20.6
200.8
2000
192.9
18.3
87.2
87.4
24.1
217.1
2001
195.2
18.5
88.3
88.4
25.0
220.2
2002
197.6
18.6
89.5
89.5
25.8
223.4
2003
199.5
18.8
90.4
90.4
26.6
226.2
2004
201.2
18.8
91.3
91.1
27.5
228.8
2005
203.0
19.0
92.1
91.9
28.5
231.5
2006
204.9
19.1
93.0
92.8
29.5
234.4
from grass, leaves, and branches to calculate the total landfill
C stock in 2006, yielding a value of 234.4 million metric tons
(as shown in Table 7-45). 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 7-43) is the difference in the
landfill C stock for a given year and the stock in the preceding
year. For example, the net change in 2006 shown in Table
7-43 (2.9 Tg C) is equal to the stock in 2006 (234.4 Tg C)
minus the stock in 2005 (231.5 Tg C).
When applying the C storage data reported by Barlaz
(1998,2005,2008), an adjustment was made to the reported
values so that a perfect mass balance on total C could be
attained for each of the materials. There are four principal
elements in the mass balance:
• Initial C content (ICC, measured),
• C output as CH4 (CH4-C, measured),
• C output as CO2 (CO2-C, not measured), and
• Residual stored C (CS, measured).
In a simple system where the only C fates are CH4,
CO2, and C storage, the following equation is used to attain
a mass balance:
CH4-C + C02-C + CS = ICC
The experiments by Barlaz and his colleagues (Barlaz
1998,2005; Eleazer et al. 1997) did not measure CO2 outputs
in experiments. However, if the only decomposition is
anaerobic, then CFf^C = CO2-C.48 Thus, the system should
be defined by:
2 x CH4-C + CS = ICC
The C outputs (= 2 x CH4-C + CS ) were less than 100
percent of the initial C mass for food scraps, leaves, grass,
and branches (75, 94, 86, and 90 percent, respectively). For
these materials, it was assumed that the unaccounted for C
had exited the experiment as CH4 and CO2, and no adjustment
was made to the measured value of CS. The resulting C stocks
are shown in Table 7-45.
Uncertainty
The uncertainty analysis for landfilled 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 7-46. Total yard trimmings
and food scraps CO2 flux in 2006 was estimated to be between
-19.1 and-6.0TgCO2Eq. at a 95 percent confidence level (or
19 of 20 Monte Carlo stochastic simulations). This indicates
a range of -82 percent below to 43 percent above the 2006
flux estimate of -10.5 Tg CO2 Eq. More information on the
uncertainty estimates for Yard Trimmings and Food Scraps
in Landfills is contained within the Uncertainty Annex.
48The molar ratio of CH4 to CO2 is 1:1 for carbohydrates (e.g., cellulose,
hemicellulose). For proteins as C3 2H5ON0 86, the molar ratio is 1.65 CH4 per
1.55 CO2 (Barlaz et al. 1989). Given the predominance of carbohydrates,
for all practical purposes, the overall ratio is 1:1.
Land Use, Land-Use Change, and Forestry 7-59
-------
Table 7-46: Tier 2 Quantitative Uncertainty Estimates for C02 Flux from Yard Trimmings and Food Scraps in Landfills
(Tg C02 Eq. and Percent)
2006 Flux Estimate
Source Gas (Tg C02 Eq.)
Uncertainty Range Relative to Flux Estimate3
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Yard Trimmings and Food Scraps C02 (10.5)
(19.1) (6.0) -82% +43%
Note: Parentheses indicate negative values or net C sequestration.
a Range of flux estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
QA/QC and Verification
A QA/QC analysis was performed for data gathering
and input, documentation, and calculation.
Recalculations Discussion
The half lives of branches and food scraps were updated
to be consistent with recommended values for food scraps
and woody materials provided in IPCC (2006) for analyzing
landfill CH4.
The current Inventory uses detailed unpublished backup
data (Schneider 2007, 2008) for some years not previously
shown in the MSW Facts and Figures reports (EPA 1999,
2003,2005,2005a, 2006,2007). These data included updated
generation, materials recovery, composting, combustion,
and discard data for 1960, 1970, 1980, and 1990 through
2006. These newly available data allowed several previous
interpolations to be replaced with the complete time series
of data used to create the MSW Facts and Figures reports
(EPA 1999, 2003, 2005, 2005a, 2006, 2007).
Additionally, updated experimental results from Barlaz
(2008) were incorporated. These data changed several
estimates for leaves: the initial C content (from 42 percent
to 46 percent), the proportion of initial C stored (from 72
percent to 85 percent), and the C output from CFLj, used
as a check on the mass balance, The proportion of initial
C stored for grass also changed (from 68 percent to 53
percent). These changes are the result of a re-interpretation
of the experimental results, which combined a sample of the
material being tested (e.g., leaves) with a sample of "seed"
material—decomposed refuse—containing microorganisms
capable of anaerobic decomposition. Because the seed
material also contained some organic C, the mass balance had
to be adjusted to net out the influence of the C from the seed.
The re-interpretation of the results accounts for differences
in the rates of decomposition of the seed along compared to
the seed plus the material being tested.
These changes resulted in an average 7 percent increase
in stocks across the time series and a 13 percent change in the
stocks for 2005 compared to the previous Inventory.
Planned Improvements
Future work may 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.
7-60 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
8. Waste
Figure 8-1
2006 Waste Chapter Greenhouse Gas Emission Sources
Waste management and treatment activities are sources of greenhouse gas emissions (see Figure 8-1). Landfills
accounted for approximately 23 percent of total U.S. anthropogenic methane (CH4) emissions in 2006,:
the second largest contribution of any CtLj 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. N2O emissions from composting were also
estimated. Together, these waste activities account for less
than 3 percent of total U.S. N2O emissions. Nitrogen oxide
(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.
Overall, in 2006, waste activities generated emissions
of 161.0 Tg CO2 Eq., or just over 2 percent of total U.S.
greenhouse gas emissions.
Landfills
Composting
100
150
Tg C02 Eq.
Table 8-1: Emissions from Waste (Tg C02 Eq.)
Gas/Source
CH4
Landfills
Wastewater Treatment
Composting
N20
Domestic Wastewater Treatment
Composting
Total
1990
172.9
149.6
23.0
0.3
6.6
6.3
0.4
179.6
1995
169.1
144.0
24.3
0.7
7.7
6.9
0.8
176.8
2000 2001 2002 2003 2004 2005 2006
146.7 143.0 145.5 151.0 148.1 149.0 151.1
120.8 117.6 120.1 125.6 122.6 123.7 125.7
24.6 24.2 24.1 23.9 24.0 23.8 23.9
1.3 1.3 1.3 1.5 1.6 1.6 1.6
8.9 9.2 9.0 9.3 9.6 9.7 9.9
7.6 7.8 7.6 7.7 7.8 8.0 8.1
1.4 1.4 1.4 1.6 1.7 1.7 1.8
155.6 152.1 154.5 160.3 157.7 158.7 161.0
Note: Totals may not sum due to independent rounding.
1 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.
Waste 8-1
-------
Table 8-2: Emissions from Waste (Gg)
Gas/Source
CH4
Landfills
Wastewater Treatment
Composting
N20
Domestic Wastewater Treatment
Composting
1990
8,235
7,124
1,096
15
21
20
1
1995
8,052
6,859
1,158
35
25
22
3
2000 2001 2002 2003 2004 2005 2006
6,983 6,808 6,928 7,190 7,053 7,096 7,197
5,751 5,598 5,720 5,981 5,838 5,890 5,985
1,173 1,150 1,148 1,140 1,141 1,131 1,136
60 60 61 69 74 75 75
29 30 29 30 31 31 32
24 25 25 25 25 26 26
4555666
Note: Totals may not sum due to independent rounding.
8.1. Landfills (IPCC Source
Category 6A1)
In 2006, landfill CH^ emissions were approximately
125.7 Tg CO2 Eq. (5,985 Gg of CH^, representing the second
largest source of CtLj emissions in the United States, behind
enteric fermentation. Emissions from municipal solid waste
(MSW) landfills, which received about 64 percent of the total
solid waste generated in the United States, accounted for
about 88 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 CELj (BioCycle 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 CH/j-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.2 Significant CK4 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 2006, net CELj emissions from landfills
decreased by approximately 16 percent (see Table 8-3 and
Table 8-4), 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,3 which has more than offset the additional
CH4 generation resulting from an increase in the amount of
municipal solid waste landfilled.
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 CELj 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 307 Tg in 2006, an increase of
47 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 888 Gg of CELj
were recovered and combusted (i.e., used for energy or flared)
from landfills, while in 2006,5,958 Gg CELj was combusted.
In 2006, an estimated 26 new landfill gas-to-energy (LFGTE)
projects and 41 new flares began operation, resulting in a
4.4 percent increase in the quantity of CH4 recovered and
combusted from 2005 levels.
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
2 The percentage of CO2 in biogas released from a landfill may be smaller
because some CO2 dissolves in landfill water (Bingemer and Crutzen 1987).
Additionally, less than 1 percent of landfill gas is typically composed of
non-CH4 volatile organic compounds (NMVOCs).
3 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.
8-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Table 8-3: CH4 Emissions from Landfills (Tg C02 Eq.)
Activity
MSW Landfills
Industrial Landfills
Recovered
Gas-to-Energy
Flared
Oxidized3
Total
1990
172.6
12.3
(13.6)
(5.1)
(16.6)
149.6
1995
191.8
13.6
(23.4)
(22.0)
(16.0)
144.0
2000
206.9
15.2
(51.3)
(36.7)
(13.4)
120.8
2001
211.4
15.5
(56.1)
(40.3)
(13.1)
117.6
2002
225.8
15.8
(57.2)
(44.9)
(14.0)
120.1
2003
225.8
15.8
(57.2)
(44.9)
(14.0)
125.6
2004
233.7
16.0
(60.6)
(52.7)
(13.6)
122.6
2005
241.2
16.1
(62.2)
(57.7)
(13.7)
123.7
2006
248.6
16.2
(65.3)
(59.8)
(14.0)
125.7
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
a Includes oxidation at both municipal and industrial landfills.
Table 8-4: CH4 Emissions from Landfills (Gg)
Activity
MSW Landfills
Industrial Landfills
Recovered
Gas-to-Energy
Flared
Oxidized3
Total
1990
8,219
585
(646)
(242)
(792)
7,124
1995
9,132
649
(1,113)
(1,047)
(762)
6,859
2000
9,854
725
(2,441)
(1,747)
(639)
5,751
2001
10,068
739
(2,670)
(1,917)
(622)
5,598
2002
10,367
746
(2,721)
(2,037)
(636)
5,720
2003
10,754
754
(2,723)
(2,140)
(665)
5,981
2004
11,127
760
(2,888)
(2,512)
(649)
5,838
2005
11,486
760
(2,961)
(2,748)
(654)
5,890
2006
11,838
770
(3,110)
(2,848)
(665)
5,985
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
a Includes oxidation at both municipal and industrial landfills.
landfilled, however, may decline due to increased recycling
and composting practices. In addition, the quantity of CH^ 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 CtLj 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.
CH4 emissions from landfills were estimated to equal
the CH4 produced from municipal solid waste landfills, plus
the CH4 produced by industrial landfills, minus the CH4
olid waste
MSW
recovered and combusted, minus the CtLj oxidized before
being released into the atmosphere:
CH^ Solid Waste = PH^MSW + CH4ind — R] — Ox
where,
j emissions from solid waste
j generation from municipal solid
waste landfills,
CtLj generation from industrial landfills,
R = CtLj recovered and combusted, and
Ox = GIL, oxidized from MSW and
industrial landfills before release to
the atmosphere.
The methodology for estimating CH^ 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 CH^ generation
potential (L0) and rate constant (k) were obtained from an
analysis of CH^ recovery rates for a database of 52 landfills
and from published studies of other landfills (RTI 2004;
Waste 8-3
-------
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
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 (2007) 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 in landfills (MCF of 1) and those disposed
in dumps (MCF of 0.6). Please see the Recalculations
Discussion section and 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 2007), and a database maintained
by the Energy Information Administration (EIA) for the
voluntary reporting of greenhouse gases (EIA 2007). 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 CFL, combusted by flares
in operation from 1990 to 2006 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, 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 CFL,
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 calculate net CK4 emissions,
both CFL, recovered and CH4 oxidized were subtracted from
CH4 generated at municipal and industrial landfills.
8-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
Uncertainty
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 CtLj generation
introduces uncertainty. Aside from uncertainty in estimating
CFL, 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 methane estimated to be recovered). For flaring
without metered recovery data (approximately 34 percent
of the methane 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).
N2O 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 Tier 2 quantitative uncertainty analysis
are summarized in Table 8-5. Landfill CFL, emissions in 2006
were estimated to be between 74.7 and 168.5 Tg CO2 Eq.,
which indicates a range of 41 percent below to 34 percent
above the 2006 emission estimate of 125.7 Tg CO2 Eq.
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 methane
recovery estimates were not double-counted. Both manual
and electronic checks were made to ensure that emission
avoidance from each landfill was calculated only in 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 1990 to 2006 Inventory report, the
proportion of waste disposed of in managed landfills versus
open dumps prior to 1980 was re-evaluated. Based on the
historical data presented by Minz et al. (2003), a timeline
was developed for the transition from the use of open dumps
for solid waste disposed to the use of managed landfills.
Based on this timeline, 6 percent of the waste that was land
disposed in 1940 was disposed of in managed landfills and
94 percent was managed in open dumps. Between 1940
and 1980, the fraction of waste land disposed transitioned
towards managed landfills until 100 percent of the waste
was disposed of in managed landfills in 1980. Although this
timeline was based primarily on information about MSW
disposal, a similar trend in disposal practices was expected
Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Landfills
CH,
125.7
74.7
168.5
-41%
+34%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Waste 8-5
-------
for industrial landfills; therefore, this same time line was
applied to the industrial landfills. For wastes disposed of
in dumps, a methane correction factor (MCF) of 0.6 was
used based on the recommended IPCC default value for
uncharacterized land disposal (IPCC 2006); this MCF is
equivalent to assuming 50 percent of the open dumps are
deep and 50 percent are shallow. The recommended IPCC
default value for the MCF for managed landfills of 1 was used
for the managed landfills (IPCC 2006). This recalculation
reduced the MSW landfill GIL, generation rate for the 1990 to
2005 time series by 5.6 percent, and it reduced the industrial
landfill CFLj generation rate for the 1990 to 2005 time series
by 1.8 percent.
Another significant recalculation, which affected
estimates of CH4 recovery, was associated with updating the
EIA, LMOP, and flare vendor databases. The estimates of
gas recovery by LFGTE projects for 1990 to 2005 increased
because more landfills with operational gas recovery projects
were identified and included in the LFGTE database.
However, many of these LFGTE projects did not have a
corresponding flare in the flare vendor database. The gas
recovery and combustion estimates from the flare database
were adjusted by deducting the recovery and combustion
estimates associated with these LFGTE projects with
unmatched flares from the flare combustion totals. This
results in a decrease in the estimates for flaring. For the 1990
to 2005 time series, the recalculation resulted in an average
increase of 1.4 percent in the amount of CFL, recovered and
destroyed by gas-to-energy projects and a net decrease of
0.5 percent in the estimated CFL, emissions.
Overall, these recalculations resulted in an average
decrease of 7.8 percent in emissions across the time series
relative to the previous Inventory.
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.
Box 8-1: Biogenic Emissions and Sinks of Carbon
C02 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
(Barlaz1998,2005). 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).
8.2. Wastewater Treatment (IPCC
Source Category 6B)
Wastewater treatment processes can produce
anthropogenic CK4 and N2O emissions. Wastewater from
domestic (municipal sewage) 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,4 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 States, approximately 21 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 2007b).
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
4 Package plants are treatment plants assembled in a factory, skid mounted,
and transported to the treatment site.
8-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 nitrogen
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 dinitrogen gas (N2).
N2O can be an intermediate product of both processes, but
is more often associated with denitrification.
The principal factor in determining the CH^ 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 CK4 production. The
principal factor in determining the N2O generation potential
of wastewater is the amount of N in the wastewater.
In 2006, CH4 emissions from domestic wastewater
treatment were 16.0 Tg CO2 Eq. (762 Gg). Emissions
gradually increased from 1990 through 1997, but have
decreased since 1998 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 2006, CH4 emissions from industrial
wastewater treatment were estimated to be 7.9 Tg CO2
Eq. (374 Gg). Industrial emission sources have increased
across the time series through 1999 and then fluctuated up
and down in keeping with production changes associated
with the treatment of wastewater from the pulp and paper
manufacturing, meat and poultry processing, fruit and
vegetable processing, and starch-based ethanol production
industries.5 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 2006 emissions of N2O from centralized
wastewater treatment processes and from effluent were
estimated to be 0.3 Tg CO2 Eq. (1 Gg) and 7.8 Tg CO2 Eq.
(25 Gg), respectively. Total N2O emissions from domestic
wastewater were estimated to be 8.1 Tg CO2 Eq. (26 Gg).
N2O emissions from wastewater treatment processes
gradually increased across the time series as a result of
increasing U.S. population and protein consumption.
Table 8-6: CH4 and N20 Emissions from Domestic and Industrial Wastewater Treatment (Tg C02 Eq.)
Activity
CH4
Domestic
Industrial*
N20
Domestic
Total
1990
23.0
16.4
6.6
6.3
6.3
29.3
1995
24.3
16.9
7.4
6.9
6.9
31.2
2000
24.6
16.8
7.8
7.6
7.6
32.2
2001
24.2
16.6
7.5
7.8
7.8
32.0
2002
24.1
16.5
7.6
7.6
7.6
31.7
2003
23.9
16.4
7.6
7.7
7.7
31.6
2004
24.0
16.3
7.7
7.8
7.8
31.8
2005
23.8
16.2
7.6
8.0
8.0
31.8
2006
23.9
16.0
7.9
8.1
8.1
32.0
* Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, fruit and vegetable processing, and starch-based ethanol
production industries.
Note: Totals may not sum due to independent rounding.
5 Other industrial sectors include organic chemicals, starch production,
alcohol refining, creameries, and textiles; however, emissions from these
sectors are considered to be insignificant.
Waste 8-7
-------
Table 8-7: CH4 and N20 Emissions from Domestic and Industrial Wastewater Treatment (Gg)
Activity
CH4
Domestic
Industrial*
N20
Domestic
1990
1,096
782
314
20
20
1995
1,158
804
355
22
22
2000
1,173
802
371
24
24
2001
1,150
792
358
25
25
2002
1,148
786
362
25
25
2003
1,140
780
360
25
25
2004
1,141
775
366
25
25
2005
1,131
770
361
26
26
2006
1,136
762
374
26
26
* Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, fruit and vegetable processing, and starch-based ethanol
production industries.
Note: Totals may not sum due to independent rounding.
Methodology
Domestic Wastewater CH4 Emission Estimates
Domestic wastewater CFLj emissions originate from
both septic systems and from centralized treatment systems,
such as publicly owned treatment works (POTWs). Within
these centralized systems, CK4 emissions can arise from
aerobic systems that are not well managed, anaerobic
systems (anaerobic lagoons and facultative lagoons), and
from anaerobic digesters when the captured biogas is not
completely combusted. CK4 emissions from septic systems
were estimated by multiplying the total BODS produced in
the United States by the percent of wastewater treated in
septic systems (21 percent), the maximum CFLj producing
capacity for domestic wastewater (0.60 kg CH4/kg BOD),
and the CFLj correction factor (MCF) for septic systems
(0.5). CH4 emissions from POTWs were estimated by
multiplying the total BOD5 produced in the United States
by the percent of wastewater treated centrally (79 percent),
the relative percentage of wastewater treated 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
CFLj-producing capacity of domestic wastewater (0.6), and
the relative MCFs for aerobic (zero or 0.3) and anaerobic
(0.8) systems. CK4 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 CH4/m3 CtLj), and the destruction efficiency
associated with burning the biogas in an energy/thermal
device (0.99).6 The methodological equations are:
Emissions from Septic Systems = A
= (% onsite) x (total BOD5 produced) x (B0) x
(MCF-septic)xl/10A6
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 1/10A6
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 1/10A6
Emissions from Anaerobic Digesters = D
= [(POTW_flow_AD) x (digester gas)/(per capita flow)] x
conversion to m3 x (FRAC_CH4) _ (365.25) x
(density of CF^) x (1-DE) x 1/10A9
Total CFLj Emissions (Gg) =A+B + C + D
where,
% onsite = How to septic systems/total flow
% collected = How to POTWs/total flow
6Anaerobic digesters at wastewater treatment plants generated 798 Gg
CH4 in 2006, 790 Gg of which was combusted in flares or energy devices
(assuming a 99% destruction efficiency).
8-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
% aerobic =
% anaerobic =
% 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
MCF-septic =
1/10A6
MCF-aerobic_
not_
well man. :
MCF-
anaerobic
DE
POTW_
flow AD
How to aerobic systems/total flow
to POTWs
How to anaerobic systems/total
flow to POTWs
Percent of aerobic systems that do
not employ primary treatment
Percent of aerobic systems that
employ primary treatment
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 U.S.
population x 365.25 days/yr
Maximum CELrproducing capacity
for domestic wastewater (0.60 kg
CIL/kg BOD)
GEL, correction factor for septic
systems (0.5)
= Conversion factor, kg to Gg
GEL, correction factor for aerobic
systems that are not well managed
(0.3)
; correction factor for anaerobic
systems (0.8)
CH/j 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 =
per capita
flow
conversion
torn3
Cubic feet of digester gas produced
per person per day (1.0 ft3/person/
day) (Metcalf and Eddy 1991)
Wastewater flow to POTW per
person per day (100 gal/person/day)
Conversion factor, ft3 to m3
(0.0283)
FRAC_CH4 = Proportion CH4 in biogas (0.65)
density of
CH4
1/10A9
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 2007a) and
include the populations of the United States, American
Samoa, Guam, Northern Mariana Islands, Puerto Puco, and
the Virgin Islands. Table 8-8 presents U.S. population and
total BOD5 produced for 1990 through 2006. 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, and 2005 American
Housing Surveys conducted by the U.S. Census Bureau (U.S.
Census 2007b), with data for intervening years obtained by
linear interpolation. The wastewater flow to aerobic systems
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).7 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). The CH4
emission factor (0.6 kg CH4/kg BOD5) and the MCFs were
taken from IPCC (2006). The GIL, 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
7 Aerobic and anaerobic treatment were determined based on unit processes
in use at the facilities. Because the list of unit processes became more
extensive in the 2000 and 2004 surveys, the criteria used to identify
aerobic and anaerobic treatment differ slightly across the time series.
Once facilities were identified as aerobic or anaerobic, they were separated
by whether or not they had anaerobic digestion in place. Once these
classifications were determined, the flows associated with facilities in
each category were summed.
Waste 8-9
-------
Table 8-8: U.S. Population (Millions) and Domestic
Wastewater BOD5 Produced (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Population
254
271
287
289
292
295
297
300
303
BOD6
8,350
8,895
9,419
9,509
9,597
9,685
9,774
9,864
9,954
Source: U.S. Census Bureau (2006a); Metcalf & Eddy 1991 and 2003.
standards (NSPS) for landfills, and in recommendations
for closed flares used by 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
CtLj emissions estimates from industrial wastewater
were developed according to the methodology described in
IPCC (2006). Industry categories that are likely to produce
significant CK4 emissions from wastewater treatment were
identified. High volumes of wastewater generated and a
high organic wastewater load were the main criteria. The
top four industries that meet these criteria are pulp and paper
manufacturing; meat and poultry processing; vegetables,
fruits, and juices processing; and starch-based ethanol
production. Table 8-9 contains production data for these
industries.
CH4 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 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). The methodological equation is:
CH4 (industrial wastewater) =
PxWxCODxTAxB0xMCF
where,
CH4
(industrial
wastewater) =
P
W
COD
TA
Total CH4 emissions from industrial
wastewater (kg/year)
Industry output (metric tons/year)
Wastewater generated (m3/metric
ton of product)
Organics loading in wastewater
(kg /m3)
Percent of wastewater treated
anaerobically on site
Maximum CIL, producing potential
of industrial wastewater (default
value of 0.25 kg CIL/kg COD)
Table 8-9: U.S. Pulp and Paper, Meat and Poultry, and Vegetables, Fruits and Juices Production (Tg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Pulp and Paper
128.9
140.9
142.8
134.3
132.7
131.9
136.4
131.4
137.4
Meat
(Live Weight Killed)
27.3
30.8
32.1
31.6
32.7
32.3
31.2
31.4
32.5
Poultry
(Live Weight Killed)
14.6
18.9
22.2
22.8
23.5
23.7
24.4
25.1
25.5
Vegetables,
Fruits and Juices
38.7
46.9
50.9
45.0
47.7
44.8
47.8
43.3
42.6
Ethanol
2.7
4.2
4.9
5.3
6.4
8.4
10.2
11.7
14.5
8-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
MCF = CH4 correction factor, indicating the
extent to which the organic content (mea-
sured as COD) degrades anaerobically
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 1993b). The vast majority of pulp and paper
mills with on-site treatment systems use mechanical clarifiers
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 1993b). However, because the vast majority
of primary treatment operations at U.S. pulp and paper mills
use mechanical clarifiers, 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). Therefore, the pulp and paper CH4 emission
calculation is:
Methane = Production x How x BOD x 42% x
COD:BOD Ratio x %TA x B0 x MCF
where,
Production = metric tons of pulp, paper, and
paperboard production
How = cubic meters of wastewater gener-
ated per ton production
BOD = BOD concentration in influent
(4000 mg/L)
42% = Percent of BOD entering secondary
treatment
COD:BOD = COD to BOD ratio (for pulp and
paper, COD:BOD = 2)
%TA = estimated percent of wastewater
treated anaerobically on site (25%)
B0 = maximum methane producing
capacity (0.25 mg CH4/mg COD)
MCF = methane conversion factor for
anaerobic deep lagoons (0.80)
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 2006 (Pulp and
Paper 2005, 2006 and monthly reports from 2003 through
2006; 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 g
BOD/liter (EPA 1997b, EPA 1993b, World Bank 1999).
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 poultry processing
operations (U.S. Poultry 2006) perform on-site treatment in
anaerobic lagoons. The IPCC default B0 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 2007a).
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.
Treatment of wastewater from fruits, vegetables, and
juices processing includes screening, coagulation/settling and
biological treatment (lagooning). The flows are frequently
Waste 8-11
-------
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, 5 percent of these wastewater organics are
assumed to degrade anaerobically. The IPCC default B0 of
0.25 kg CH4/kg COD and default MCF of 0.8 for anaerobic
treatment were used to estimate the CFL, produced from these
on-site treatment systems. The USDA National Agricultural
Statistics Service (USDA 2007a) provided production data
for potatoes, other vegetables, citrus fruit, non-citrus fruit,
and grapes processed for wine. Outflow and BOD data,
presented in Table 8-10, were obtained from EPA (1974)
for potato, citrus fruit, and apple processing, and from EPA
(1975) for all other sectors.
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 coming years,
currently it is only in an experimental stage in the United
Table 8-10: Wastewater Flow (m3/ton) and BOD
Production (g/L) for U.S. Vegetables, Fruits and Juices
Production
Commodity
Vegetables
Potatoes
Other Vegetables
Fruit
Apples
Citrus
Non-citrus
Grapes (for wine)
'aSteSonTtflOW
10.27
8.86
3.66
10.11
12.42
2.78
BOD
(g/L)
1.765
0.813
1.371
0.317
1.204
1.831
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 for at least the next
6 years, 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. CH4 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)
(Ruocco 2006a,b; Merrick 1998; Donovan 1996; and NRBP
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 methane is
recovered through the use of biomethanators (ERG 2006).
CH4 emissions were then estimated as follows:
Methane = [Production x How x COD x 3.785 x
% TA x B0 x MCF x % Not Recovered] +
[Production x How x 3.785 x COD x %TA x
B0 x MCF x (% Recovered) x (1-DE)] x 1/10A9
8-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
where,
Production
How
COD
3.785
%TA
B0
MCF
% Recovered:
%Not
Recovered :
DE
1/10A9
= gallons ethanol produced
(wet milling or dry milling)
= gallons wastewater generated per
gallon ethanol produced (1.25 dry
milling, 10 wet milling)
= COD concentration in influent
(3 g/1)
= conversion, gallons to liters
= percent of wastewater treated
anaerobically (for dry milling
operations, this value is estimated
separately for facilities using
biomethanators with 100% recovery
and facilities using other anaerobic
systems)
= maximum methane producing
capacity (0.25 g CH4/g COD)
= methane conversion factor (0.8 for
anaerobic systems)
percent of wastewater treated in
system with emission recovery
= 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 CFLj emissions for 1990 through 2006
was developed based on production data from the Renewable
Fuels Association (RFA 2006).
Domestic Wastewater N20 Emission Estimates
N2O emissions from domestic wastewater (wastewater
treatment) were estimated using thelPCC (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.8
The IPCC methodology uses annual, per capita protein
consumption (kg protein/ [person-year]). This number is
likely to underestimate the amount of protein entering
the sewer or septic system. Food (waste) that is not
consumed is often washed down the drain, as a result
of the use of garbage disposals. Also, bath and laundry
water can be expected to contribute to N loadings.
As a result, a factor of 1.4 for non-consumption N is
introduced for each year in the Inventory.9 Furthermore,
a significant quantity of industrial wastewater (N) is
co-discharged with domestic wastewater. To account
for this, a factor of 1.25 is used.10
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 g 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 there are 88 treatment plants in the United
States, serving a population of 2.6 million people, with
denitrification as one of their unit operations. Based on
an emission factor of 7 grams/capita/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 g N2O per capita per year.
8 The methodology for estimating the quantity of sewage sludge N not
entering aquatic environments is described in Annex 3.11.
9 Metcalf & Eddy (1991) provide a typical influent nitrogen concentration
of 40 mg/L Total Kjeldahl Nitrogen (TKN) for average wastewater from
residences, which includes bathwater, laundry, and the use of garbage
disposals. The factor for non-consumptive protein was estimated based
on wastewater treated in 1990, the percent of population serviced by
centralized treatment systems, and the per capita TKN loading, resulting
in a factor of 1.4.
10 The type, composition, and quantity of this co-discharged wastewater
vary greatly between municipalities. Metcalf & Eddy (1991) provide
a range of influent nitrogen concentrations of 20 to 85 mg/L TKN
(average 55) for combined residential and industrial wastewater, while
residential wastewater loading was roughly estimated at 40 mg TKN/L
(see previous footnote). Until better data become available, the amount
of N in wastewater is increased by 10 mg/L to account for industrial co-
discharge (factor of 1.25).
Waste 8-13
-------
With the modifications described above, N2O emissions
from domestic wastewater were estimated using the
following methodology:
N2OTOTAL = N2OPLANT + N2OEFFLUENT
N2OPLANT = N2ONIT/DENIT + N2OWOUT NIT/DENIT
N2ONIT/DENIT = [(USpopNo) x EF2 x FIND_COM] x 1/10A9
N2OWOUT NIT/DENIT = {[(USPOp X WWTP) - USPOpND X
N2OEFFLUENT = {[(USpop x Protein x FNPR x FNON_CON
FIND-COM) - NSLUDGE] x EF3 x 44/28} x 1/10*6
where,
N2OTOTAL
N2OPLANT
N2ONIT/DENIT
= Annual emissions of N2O (kg)
= N2O emissions from centralized
wastewater treatment plants (kg)
= N2O emissions from centralized
wastewater treatment plants with
nitrification/denitrification (kg)
N2OWOuT NIT/DENIT = N2O emissions from centralized
wastewater treatment plants
without nitrification/denitrifica-
tion (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)
WWTP
EF3
44/28
= Factor for industrial and commer-
cial 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)
= Molecular weight ratio of N2O to
N2
U.S. population data were taken from the U.S. Census
Bureau International Database (U.S. Census 2007a) 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, and
2005 American Housing Survey (U.S. Census 2007b). Data
for intervening years were obtained by linear interpolation.
The emission factor (EF:) 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
(ERS 2006b). Protein consumption data for 2005 and 2006
were extrapolated from data for 1990 through 2004. Table
8-11 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
Table 8-11: U.S. Population (Millions) and Average
Protein Intake [kg/(person-year)]
person-year)
EF2 = Emission factor (7 g N2O/
person-year)
Protein = Annual per capita protein con-
sumption (kg/person/year)
FNPR = Fraction of N in protein, default =
0.16 (kg N/kg protein)
FNON-CON = Factor for non-consumed protein
added to wastewater (1.4)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
Population
254
271
287
289
292
295
297
300
303
Protein
38.7
39.8
41.3
42.0
40.9
40.9
41.3
41.7
41.9
Source: U.S. Census Bureau 2006a, USDA ERS 2006b.
8-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
EPA (1999) for 1988, 1996, and 1998 and from Beecher et
al. (2007) for 2004. Intervening years were interpolated, and
estimates for 2005 and 2006 were forecasted from the rest
of the time series. An estimate for the N 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 2006, 263 Tg N was removed with sludge.
Uncertainty
The overall uncertainty associated with both the
2006 CH4 and N2O emission 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 production, meat
and poultry processing, fruits and vegetable processing,
and ethanol production. 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 N 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 8-12. CK4 emissions from
wastewater treatment were estimated to be between 15.0 and
35.2 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 48 percent
above the 2006 emissions estimate of 23.9 Tg CO2 Eq. N2O
emissions from wastewater treatment were estimated to be
between 1.8 and 16.2 Tg CO2 Eq., which indicates a range
of approximately 78 percent below to 100 percent above the
actual 2006 emissions estimate of 8.1 Tg CO2 Eq.
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.
Recalculations Discussion
The 2006 estimates for CH4 emissions from domestic
wastewater include one major methodological refinement
Table 8-12: Tier 2 Quantitative Uncertainty Estimates for CH4 and N20 Emissions from Wastewater Treatment
(Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate3
(Tg C02 Eq.) (%)
Wastewater Treatment
Domestic
Industrial
Domestic Wastewater
Treatment
CH4
CH4
CH4
N20
23.9
16.0
7.9
8.1
Lower Bound
15.0
7.9
4.5
1.8
Upper Bound
35.2
26.6
12.8
16.2
Lower Bound
-37%
-51%
-43%
-78%
Upper Bound
+48%
+66%
+62%
+100%
! Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Waste 8-15
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and one major data change. First, for centralized wastewater
treatment systems, CH^ emissions were estimated based on
the total BOD5 available for biological treatment rather than
the total BOD5 entering wastewater treatment plants. Metcalf
and Eddy (1991) estimate that 25-40 percent of BOD5 at
aerobic and anaerobic plants is removed through primary
sedimentation, meaning that not all of the BOD5 entering
treatment plants has the potential to generate methane during
biological treatment. This change resulted in a decrease of
methane emissions from centrally treated anaerobic systems
of 20 percent, and an overall reduction in methane emissions
of 4.5 to 5.5 percent. The major data adjustment for the
current Inventory estimates involved the adjustment of the
1995 AHS data (US Census Bureau 2007b) that indicates
the percent of wastewater treated onsite versus the percent
collected. The previous Inventory indicated a total percent
of wastewater treated onsite and collected of 97.6 percent
for 1995, while all other years had a total of approximately
99.5 percent. Reevaluation of the 1995 AHS data resulted
in an updated total percent of 99.5 percent.
For industrial wastewater, the 2006 estimates include a
change in calculation methodology for pulp and paper, and
the inclusion of wastewater emissions from U.S. starch-based
ethanol production. First, the types of primary treatment in
place at pulp and paper operations were evaluated and it
was concluded that due to the majority of operations using
mechanical clarifiers, negligible emissions of CH4 occur
during primary treatment. The estimate of BOD treated
anaerobically during secondary treatment was also updated
based on the number of operations expected to have non-
aerated stabilization basins. These systems were reclassified
as anaerobic deep lagoons, and CH4 emissions were revised.
These changes resulted in a decrease in emissions from pulp
and paper wastewater treatment of 18.5 percent across the
time series.
Next, emissions associated with ethanol production were
estimated, as described earlier. The addition of this industrial
sector increased industrial wastewater emission estimates by
0.4 to 0.9 percent across the time series.
How and BOD data for fruits and vegetable processing
wastewater were updated to reflect commodity-specific data,
which had minimal impact on the emissions. Overall, the CH4
emission estimates for wastewater treatment are on average
6 percent lower than the previous Inventory.
For N2O emissions from domestic wastewater, one
major data source adjustment was made along with two
minor changes to account for co-discharged industrial and
commercial wastewater and to update the values used for
the nitrogen composition of sludge. The current Inventory
utilizes protein consumption data from the U.S. Department of
Agriculture Economic Research Service (USDA 2006b). The
previous Inventory report used UN FAO protein consumption
data. The protein data changed on average approximately
one percent for each year in the time series. The current
Inventory estimates also apply a factor for co-discharged
industrial and commercial wastewater to the emission
factors for direct N2O emissions from centralized wastewater
treatment plants. This resulted in a N2O emission factor from
centralized treatment plants that have intentional nitrification/
denitrification unit operations of 8.75 g N2O/person-year
(7 g N2O/person-year x 1.25) and a N2O emission factor
from centralized wastewater treatment plants that do not
have intentional nitrification/denitrification unit operations
of 4 g N2O/person-year (3.2 g N2O/person-year x 1.25). In
addition, the nitrogen composition of sludge was updated to
3.9 percent, representing an average N composition, rather
than the previous value of 3.3 percent which represented a
median value. The sludge generation estimates across the
time series changed slightly based on the inclusion of a new
reference for sludge generation in 2004.
Overall, emissions from wastewater treatment and
discharge (CH4 and N2O) decreased by an average of
approximately 5 percent from the previous Inventory.
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 designation of systems as aerobic or anaerobic could
be further refined to differentiate aerobic systems with the
potential to generate small amounts of CF^ (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.
Currently it is assumed that all aerobic systems are well
managed and produce no CK4, and that all anaerobic systems
have an MCF of 0.8. Efforts to obtain better data reflecting
8-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
emissions from various types of municipal treatment systems
are currently being pursued.
Available data on wastewater treatment emissions at
petroleum refineries will be reviewed to determine if this
is a significant source to be included in future versions of
the Inventory.
With respect to estimating N2O emissions, the default
emission factor for N2O from wastewater effluent has a high
uncertainty. The IPCC recently updated this factor; however,
future research may identify new studies that include updated
data. 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 nitrogen 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. In addition there is uncertainty
associated with the N2O emission factors for direct emissions
from centralized wastewater treatment facilities. Efforts to
gain greater confidence in these emission factors are currently
being pursued.
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 carbon dioxide (CO2). Methane (CH4)
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 carbon content in the material (IPCC
2006). Composting can also produce emissions of nitrous
oxide (N2O). 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 2006, the amount of material composted
in the United States has increased from 3,810 Gg to 18,852
Gg, an increase of almost 400 percent. Emissions of CH4
and N2O from composting have increased by the same
percentage (see Table 8-13 and Table 8-14). In 2006, CH4
emissions from composting were 1.6 Tg CO2 Eq. (75 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
Table 8-13: CH4 and N20 Emissions from Composting (Tg C02 Eq.)
Activity
CH4
N20
Total
1990
0.3
0.4
0.7
1995
0.7
0.8
1.5
2000
1.3
1.4
2.6
2001
1.3
1.4
2.7
2002
1.3
1.4
2.7
2003
1.5
1.6
3.1
2004
1.6
1.7
3.3
2005
1.6
1.7
3.3
2006
1.6
1.8
3.3
Table 8-14: CH4 and N20 Emissions from Composting (Gg)
Activity
CH4
N20
1990
15
1
1995
35
3
2000
60
4
2001
60
5
2002
61
5
2003
69
5
2004
74
6
2005
75
6
2006
75
6
Waste 8-17
-------
2005, 21 states and the District of Columbia, representing
about 50 percent of the nation's population, had enacted such
legislation (EPA 2006).
Methodology
j 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 peat)
used, temperature, moisture content and aeration during the
process.
The emissions shown in Table 8- 13 and Table 8-14 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):
E^MxEF,
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), and
i = designates either CH4 or N2O.
Estimates of the quantity of waste composted (M) are
presented in Table 8-15. Estimates of the quantity composted
for 1990, 1995, 2001, and 2002 were taken from EPA's
Municipal Solid Waste Generation, Recycling, and Disposal
in the United States: Facts and Figures for 2003 (EPA2005);
estimates of the quantity composted for 2003 through 2005
were taken from EPA's Municipal Solid Waste In The United
States: 2005 Facts and Figures (EPA 2006). The quantity
composted estimate for 2006 was taken from the "2006
MSW Characterization Data Tables" associated with EPA's
Municipal Solid Waste In The United States: 2006 Facts and
Figures (EPA 2007).
Uncertainty
The estimated uncertainty from the 2006 IPCC
Guidelines is +50 percent for the Tier 1 methodology.
Emissions from composting in 2006 were estimated to be
between 1.7 and 5.0 Tg CO2 Eq., which indicates a range
of 50 percent below to 50 percent above the actual 2006
emission estimate of 3.3 Tg CO2 Eq. (see Table 8-16).
Recalculations Discussion
No recalculations were performed because this is the first
year that composting has been included in the Inventory.
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.
Table 8-15: U.S. Waste Composted (Gg)
Activity
Waste Composted
1990
3,810
1995
8,682
2000
14,923
2001
15,014
2002
15,187
2003
17,309
2004
18,570
2005
18,643
2006
18,852
Source: EPA 2005, EPA 2006, and EPA 2007.
Table 8-16: Tier 1 Quantitative Uncertainty Estimates for Emissions from Composting (Tg C02 Eq. and Percent)
Source
2006 Emission Estimate
Gas (Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Composting
CH4, N20
3.3
1.7
5.0
-50%
+50%
8-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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 2006 are provided in Table 8-17.
Methodology
These emission estimates were obtained from preliminary
data (EPA 2008), 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. 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
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.
Table 8-17: Emissions of NOX, CO, and NMVOC from Waste (Gg)
Gas/Source
NO,
Landfills
Wastewater Treatment
Miscellaneous3
CO
Landfills
Wastewater Treatment
Miscellaneous3
NMVOCs
Wastewater Treatment
Miscellaneous3
Landfills
1990
+
+
+
+
1
1
+
+
673
57
557
58
1995
1
1
+
1
2
2
+
1
731
61
602
68
2000
2
2
+
+
8
7
1
+
119
23
51
46
2001
2
2
+
+
8
7
1
+
122
23
53
46
2002
2
2
+
+
7
6
+
+
115
22
50
44
2003
2
2
+
+
7
6
+
+
114
22
49
43
2004
2
2
+
+
7
6
+
+
112
21
48
43
2005
2
2
+
+
7
6
+
+
111
21
48
42
2006
2
2
+
+
7
6
+
+
110
21
47
42
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.
+ Does not exceed 0.5 Gg.
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 (IPCC 2000), 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."
The results of all methodology changes and historical data updates are presented in this section; detailed descriptions of
each recalculation are contained within each source's description contained 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 U.S. sinks, both relative to the previously published U.S. Inventory (i.e., the 1990 through 2005 report). These tables
present the magnitude of these changes in units of teragrams of carbon dioxide equivalents (Tg CO2 Eq). In addition to the
changes summarized by the tables below, the following sources and gases were added to the current Inventory:
• Carbon dioxide (CO2) emissions from Cropland Remaining Cropland, which include CO2 emissions from
Agricultural Liming and Urea Fertilization;
• CO2 emissions from Petroleum Systems, which account for vented, fugitive, and process upset emissions sources
from 29 activities for crude oil production field operations; and
• Methane (CELO and nitrous oxide (N2O) emissions from Composting.
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 2005) 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 2005, 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.
• Agricultural Soil Management. Changes occurred as a result of incorporating state-level nitrogen (N) fertilizer application
data for on-farm use as opposed to regional data, revising assumptions of manure N availability for land application,
and revising DAY CENT parameterization for sorghum. Overall, changes resulted in an average annual decrease in N2O
emissions from Agricultural Soil Management of 102.1 Tg CO2 Eq. (27.5 percent) for the period 1990 through 2005.
Recalculations and Improvements 10-1
-------
• Net CO 2 Flux from Land Use, Land-Use Change, and
Forestry. Forest Land Remaining Forest Land is the
principal section contributing to the change in net CO2
flux from Land Use, Land-Use Change, and Forestry
sector. The addition of newly available forest inventory
data as well as some refinements in previously existing
data were the principal factors contributing to the
changes. Changes for the period 1990 through 2005,
as compared to the estimates presented in the previous
Inventory, are based on the cumulative effects of (1)
incorporating and updating state and sub-state inventory
data, and (2) including a portion of Alaskan forest for
the first time. Minor refinements to the harvested wood
product contribution included (1) shorter half-life for
decay in dumps and (2) separation of decay in dumps
from decay in landfills. Overall, these changes, in
combination with adjustments in the other sources/sinks
within the sector, resulted in an average annual increase
in net flux of CO2 to the atmosphere from the Land Use,
Land-Use Change, and Forestry sector of 20.1 Tg CO2
Eq. (2.5 percent) for the period 1990 through 2005.
• Landfills. For municipal solid waste landfills, changes
to historical data resulted from revising the proportion
of waste disposed of in managed landfills versus open
dumps prior to 1980 and from using the recommended
IPCC (2006) default value for uncharacterized
land disposal. Additionally, the Energy Information
Administration, Landfill Methane Outreach Program,
and flare vendor databases were updated, affecting
estimates of CFLj recovery. Overall, changes resulted
in an average annual decrease in CH4 emissions from
landfills of 11.4 Tg CO2 Eq. (7.8 percent) for the period
1990 through 2005.
• Enteric Fermentation. Changes in the estimates of CH4
emissions resulting from Enteric Fermentation occurred
as a result of (1) modifying the Cfi coefficient based on
the revised IPCC equations (IPCC 2006), (2) updating
the C factor in accordance with the revised IPCC
Guidelines (IPCC 2006), (3) revising the equation for
net energy of growth (NEg), (4) modifying the Cattle
Enteric Fermentation Model to output at the state level
and include more detailed data inputs, (5) incorporating
revised FAO horse population estimates for 2001
through 2005, and (6) including revised USDA estimates
of swine population for 2005. Overall, changes resulted
in an average annual increase in CH4 emissions from
Enteric Fermentation of 11.4 Tg CO2 Eq. (9.9 percent)
from 1990 through 2005.
• Substitution of Ozone Depleting Substances. An extensive
review of chemical substitution trends, market sizes,
growth rates, and charge sizes, together with input from
industry representatives, resulted in updated assumptions
for the Vintaging Model, which is used to calculate
emissions from this category. These changes resulted in
an average annual decrease in hydrofluorocarbon (HFC)
emissions from the Substitution of Ozone Depleting
Substances of 7.4 Tg CO2 Eq. (14.1 percent) for the period
1990 through 2005.
• Settlements Remaining Settlements. The data source
used for N fertilization was updated for N2O Emissions
from Settlement Soils. This fertilization data is based
on county-scale non-farm application amounts from a
USGS database. Overall, changes resulted in an average
annual decrease in N2O emissions from Settlements
Remaining Settlements of 4.4 Tg CO2 Eq. (78.1 percent)
for the period 1990 through 2005.
• International Bunker Fuels. Historical activity data for
aviation was revised for both U.S. and foreign carriers.
In addition, distillate and residual fuel oil consumption
by cargo or passenger carrying marine vessels from 2003
through 2006 was revised. Overall, changes resulted
in an average annual increase in CO2 emissions from
International Bunker Fuels of 4.2 Tg CO2 Eq. (4.5
percent) for the period 1990 through 2005.
• Manure Management. Several changes were made in
this section. First, a major change in the N2O emission
calculations is that emissions are now calculated from
the "bottom-up" such that emissions are calculated for
each animal group, manure management system, and
state. These values are then summed to calculate the total
greenhouse gas emissions from manure management
in the United States. Second, dairy heifers and beef on
feed now have one WMS distribution that represents
managed and unmanaged systems, and emissions are
calculated for each WMS using the EF for that system,
and not using a state average EF. Third, the Inventory
now includes indirect N2O emissions in the Manure
Management Sector associated with N losses from
volatilization of nitrogen as ammonia (NH3), nitrogen
oxides (NOX), and leaching and runoff. Fourth, the days
10-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
per year used in N2O calculations was changed from
365 to 365.25 to include leap years and to be consistent
with the CH4 inventory calculations. Fifth, changes
were also made to the current calculations involving
animal population data. Overall, the changes resulted
in an average annual increase in N2O emissions from
Manure Management of 4.0 Tg CO2 Eq. (43.1 percent)
for the period 1990 through 2005.
Coal Mining. Three changes were made across the Coal
Mining sector. First, recalculations of emissions avoided
at three JWR coal mines in Alabama were performed
as the mining company reported and filed data for
1991 through 2005; data was also provided for 2006.
Secondly, the gas content values assigned to each coal
basin in the surface mine emissions component of the
inventory were changed to reflect recent work carried
out by U.S. EPA. Third, the conversion factor used
to convert from mmcf of methane was updated to be
consistent across the Inventory. Overall, the changes
resulted in an average annual increase in CH4 emissions
from Coal Mining of 3.7 Tg CO2 Eq. (6.2 percent) for
the period 1990 through 2005.
Ammonia Manufacture and Urea Consumption. CO2
emissions estimates were revised for all years to
incorporate a new methodology that estimates urea
production and consumption based on urea consumed
as fertilizer. The new methodology allocated CO2
emissions associated with urea applied as fertilizer to
the Land Use, Land-Use Change, and Forestry chapter.
Overall, the changes resulted in an average annual
decrease in CO2 emissions from Ammonia Manufacture
and Urea Consumption of 3.0 Tg CO2 Eq. (15.8 percent)
for the period 1990 through 2005.
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg C02 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Cement Manufacture
Lime Manufacture
Limestone and Dolomite Use
Soda Ash Manufacture and Consumption
Carbon Dioxide Consumption
Municipal Solid Waste Combustion
Titanium Dioxide Production
Aluminum Production
Iron and Steel Production
Ferroalloy Production
Ammonia Manufacture and Urea Consumption
Phosphoric Acid Production
Petrochemical Production
Silicon Carbide Production and Consumption
Lead Production
Zinc Production
Cropland Remaining Cropland3
Petroleum Systems
Land-Use, Land-Use Change, and
Forestry (Sink)
International Bunker Fuels
Wood Biomass and Ethanol Consumption
1990
6.8
+
(0.1)
+
NC
0.7
NC
NC
+
NC
(0.1)
NC
1.3
NC
(2.4)
NC
NC
NC
NC
NC
7.1
0.4
(24.9)
+
+
1995
9.5
2.4
+
+
NC
1.2
NC
NC
+
NC
(0.1)
NC
1.4
NC
(2.7)
NC
NC
NC
NC
NC
7.0
0.3
53.5
+
+
2000
(0.2)
(7.8)
0.4
+
NC
1.5
NC
NC
+
(0.4)
(0.2)
NC
1.5
NC
(3.2)
NC
NC
NC
NC
NC
7.5
0.3
83.1
NC
(1.0)
2001
3.1
(4.3)
0.5
+
NC
1.4
NC
NC
+
(0.4)
(0.2)
NC
1.3
NC
(3.4)
NC
NC
NC
+
NC
7.8
0.3
17.3
NC
NC
2002
15.8
7.6
0.5
+
NC
1.3
NC
NC
+
(0.1)
(0.2)
NC
1.3
NC
(3.6)
NC
NC
NC
+
NC
8.5
0.3
(14.9)
NC
NC
2003
0.1
(7.5)
0.4
+
NC
1.4
+
NC
+
(0.4)
(0.2)
NC
1.4
NC
(3.7)
NC
NC
NC
NC
NC
8.3
0.3
(49.0)
19.9
(0.1)
2004
(26.1)
(31.7)
(1.3)
(0.1)
NC
1.4
NC
NC
+
+
(0.2)
NC
1.5
NC
(3.7)
NC
NC
NC
+
NC
7.6
0.3
(48.9)
21.8
NC
2005
(15.2)
(20.2)
(3.3)
1.3
NC
1.5
NC
NC
+
(0.2)
(0.2)
+
1.4
NC
(3.5)
+
(0.1)
NC
+
NC
7.9
0.3
(50.2)
25.4
20.9
Recalculations and Improvements 10-3
-------
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg C02 Eq.) (continued)
Gas/Source
CH4
Stationary Combustion
Mobile Combustion
Coal Mining
Abandoned Underground Coal Mines
Natural Gas Systems
Petroleum Systems
Petrochemical Production
Silicon Carbide Production and Consumption
Iron and Steel Production
Ferroalloy Production
Enteric Fermentation
Manure Management
Rice Cultivation
Field Burning of Agricultural Residues
Forest Land Remaining Forest Land
Landfills
Wastewater Treatment
Composting3
International Bunker Fuels
N20
Stationary Combustion
Mobile Combustion
AdipicAcid Production
Nitric Acid Production
Manure Management
Agricultural Soil Management
Field Burning of Agricultural Residues
Wastewater Treatment
N20 from Product Uses
Municipal Solid Waste Combustion
Settlements Remaining Settlements
Forest Land Remaining Forest Land
Composting3
International Bunker Fuels
MFCs, PFCs, and SF6
Substitution of Ozone Depleting Substances
Aluminum Production
HCFC-22 Production
Semiconductor Manufacture
Electrical Transmission and Distribution
Magnesium Production and Processing
Net Change in Total Emissions"
Percent Change
1990
(3.0)
(0.6)
+
2.2
+
0.2
(0.6)
NC
NC
NC
NC
11.2
0.1
NC
+
(2.6)
(11.4)
(1.8)
0.3
NC
(98.6)
0.5
(0.2)
0.1
(0.9)
3.4
(97.5)
+
(0.1)
0.1
NC
(4.1)
(0.3)
0.4
NC
1.0
+
NC
1.4
NC
(0.4)
+
(93.7)
-2.1%
1995
0.2
(0.7)
+
0.6
+
(0.1)
0.9
NC
NC
NC
NC
11.7
0.1
NC
+
0.7
(13.0)
(0.8)
0.7
NC
(88.6)
0.6
(0.2)
0.1
(1.0)
3.8
(88.6)
+
+
0.1
NC
(4.3)
0.1
0.8
NC
1.9
(3.7)
NC
5.9
+
(0.3)
+
(77.0)
-0.4%
2000
10.6
(0.7)
(0.1)
4.5
+
(0.1)
2.4
NC
NC
+
NC
11.1
0.1
NC
+
5.0
(11.1)
(1.8)
1.3
NC
(113.9)
0.6
(0.7)
0.2
(1.0)
4.1
(114.7)
+
+
0.1
NC
(4.4)
0.5
1.4
NC
(11.1)
(9.7)
NC
(1.2)
+
(0.1)
+
(114.6)
-0.5%
2001
11.1
(0.6)
0.1
4.8
+
(0.1)
2.8
NC
NC
+
NC
11.1
0.1
NC
+
3.4
(10.1)
(1.7)
1.3
NC
(109.6)
0.6
0.2
0.2
(0.8)
4.2
(112.0)
+
0.2
0.1
NC
(4.0)
0.3
1.4
NC
(10.2)
(10.6)
NC
(0.1)
+
(0.1)
0.5
(105.7)
-1.4%
2002
13.8
(0.6)
(0.1)
4.8
+
(0.1)
3.1
NC
NC
+
NC
11.2
0.1
NC
+
6.0
(10.3)
(1.7)
1.3
NC
(103.1)
0.6
(1.2)
0.2
(0.9)
4.3
(104.0)
+
+
0.1
NC
(4.1)
0.6
1.4
NC
(9.9)
(11.8)
NC
1.3
+
0.1
0.6
(83.4)
-1.6%
2003
10.1
(0.6)
(0.1)
4.8
+
(0.4)
3.4
NC
NC
+
NC
11.6
0.2
NC
+
0.7
(9.3)
(1.7)
1.5
+
(103.2)
0.6
(1.4)
0.2
(1.4)
4.3
(102.9)
+
(0.1)
0.1
NC
(4.3)
0.1
1.6
0.1
(13.0)
(13.5)
NC
+
+
+
0.6
(106.0)
-2.5%
2004
5.3
(0.6)
(0.1)
5.2
+
(5.1)
3.3
NC
NC
NC
NC
11.9
0.3
NC
+
+
(9.5)
(1.7)
1.6
+
(91.7)
0.6
(1.5)
0.2
(0.8)
4.4
(91.9)
+
(0.1)
0.1
NC
(4.4)
+
1.7
0.2
(13.2)
(15.3)
+
1.6
(0.4)
0.3
0.6
(125.7)
-2.7%
2005
0.4
(0.5)
(0.1)
4.7
+
(8.7)
(0.2)
NC
NC
NC
NC
12.4
0.5
+
+
0.7
(8.3)
(1.6)
1.6
+
(98.5)
1.0
(1.7)
(0.1)
0.1
4.4
(99.9)
+
+
0.1
NC
(4.3)
0.1
1.7
0.2
(17.2)
(18.0)
NC
(0.7)
0.1
0.7
0.6
(130.5)
-2.8%
+ Absolute value does not exceed 0.05 Tg C02 Eq. or 0.05 percent.
NC (No Change)
a New source category relative to previous inventory.
b 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.
10-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
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
(23.1)
(1.9)
6.1
(2.0)
0.3
(3.1)
(1.1)
(24.9)
-3.5%
1995
57.6
(2.0)
2.1
0.2
NC
(3.7)
(0.8)
53.5
6.5%
2000
87.9
(1.9)
2.1
0.2
NC
(4.3)
(1.0)
83.1
11.0%
2001
22.3
(2.0)
2.1
0.2
NC
(4.4)
(1.0)
17.3
2.3%
2002
(9.2)
(2.6)
2.1
0.2
NC
(4.5)
(1.0)
(14.9)
-1.8%
2003
(43.9)
(2.2)
2.1
0.2
NC
(4.6)
(0.6)
(49.0)
-6.0%
2004
(44.1)
(1.5)
2.1
0.2
NC
(4.7)
(0.9)
(48.9)
-5.9%
2005
(44.9)
(1.6)
2.1
0.2
NC
(4.8)
(1.2)
(50.2)
-6.1%
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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City of Aberdeen, South Dakota and Sarah Holman, ERG.
"South Dakota Ethanol Production Facilities - Heartland
Grain Fuels, LP" September 6, 2006.
Holman (2006e) Personal Communication. Ron Ash,
Nebraska Department of Environmental Quality, Water
Division, NPDES Program and Sarah Holman, ERG.
"Nebraska Ethanol Production Facilities - Chief Ethanol,
NPDES ID No. NE0114243." September 7, 2006.
Holman (20061) Personal Communication. Matt Gluckman,
U.S. EPA, Region 5, WN-16J and Sarah Holman, ERG.
"Region 5 Ethanol Production Facilities - New Energy Corp.
and MGPI." September 19, 2006.
Holman (2006g) Personal Communication. Dr. Ann Wilkie,
University of Florida, Soil and Water Science Department
and Sarah Holman, ERG. "Wastewater stillage from ethanol
production facilities." September 20, 2006.
Holman (2006h) Personal Communication. Mike Pring,
ERG and Sarah Holman, ERG. "Emissions from ethanol
production facilities." September 7, 2006.
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Gas Inventories. The National Greenhouse Gas Inventories
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Opportunities for the Corn Wet Milling Industry: An
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Paper and Allied Trades, Miller-Freeman Publications. San
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Merrick (1998) Wastewater Treatment Options for the
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Renewable Energy Laboratory (NREL). Merrick & Company.
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11 -48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
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Pulp and Paper (2005) "U.S. paper/board production rises
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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 web site 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 2006, 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 Ms. Melissa Weitz, Environmental Protection Agency, (202) 343-9897, weitz.melissa@epa.gov.
For more information regarding climate change and greenhouse gas emissions, see the EPA web site at .
Released for printing: April 15, 2008
Greenhouse Gases
The photos on the front and back cover of this report depict the types of greenhouse gases covered in the 1990-2006 Inventory.
This Inventory presents emissions of carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons, and sulfur
hexafluoride. Of these, carbon dioxide is emitted in the largest quantities in the United States, so three of the pictures below
depict sources and sinks of carbon dioxide, while sources of each of the other gases are represented in one picture each.
Hydrofluorocarbons (MFCs)
HFCs are a class of synthetic chemicals, primarily used as alternatives to ozone depleting substances being phased
out under the Montreal Protocol. The uses of HFCs as a substitute include refrigeration and air-conditioning, foam
blowing, aerosols, and solvents. In addition, some is emitted during the production of another fluorochemical,
HCFC-22, and from semiconductor manufacture. Emissions of this gas have increased 237 percent since 1990,
mostly due to the phaseout of ozone depleting substances over that period. HFCs generally have high global warming
potentials compared to the naturally occurring greenhouse gases (carbon dioxide, methane, and nitrous oxide).
Sulfur Hexafluoride
Sulfur hexafluoride (SF6) is an inert synthetic chemical with a long atmospheric lifetime and a very high
global warming potential, and is the most potent greenhouse gas the IPCC has evaluated. Because of its inert
properties, it is used in electrical transmission and distribution as an insulator and interrupter, as a cover gas
in magnesium production and processing, and in semiconductor manufacture. Emissions have decreased 47
percent since 1990.
Carbon Dioxide: Industrial Processes
Some industrial processes emit carbon dioxide as part of the process itself rather than from energy consumption.
The two largest industrial emitters of carbon dioxide are iron and steel production and cement production, each
accounting for 1 percent of all carbon dioxide emissions in 2006. Industrial emissions of this process-based
carbon dioxide have decreased 15 percent since 1990.
Nitrous Oxide
Nitrous oxide is approximately 310 times stronger than carbon dioxide at trapping heat, and is emitted from
a variety of sources. In the United States, the largest source of this gas is agricultural soil management,
responsible for approximately 72 percent of nitrous oxide emissions. Other significant sources include mobile
and stationary combustion, adipic acid production, waste water treatment, and manure management. Emissions
of nitrous oxide have decreased 4 percent since 1990.
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Carbon Dioxide: Fossil Fuel Combustion
Carbon dioxide from the combustion of fossil fuels is the largest source of carbon dioxide emissions in the
United States, accounting for 80 percent of all emissions in 2006. In order of decreasing size, the contributors to
these emissions were electricity generation, and the transportation, industry, and the residential and commercial
end-use sectors. Emissions from fossil fuel combustion have increased 19 percent since 1990.
Carbon Dioxide: Land Use, Land-Use Change, and Forestry
Forests and soils in the United States are a net sink for carbon dioxide, offsetting about 15 percent of
emissions in 2006. The sink has increased by about 20 percent since 1990. Soils can also be a source of
carbon dioxide: liming of agricultural soils and urea application to cropland both lead to a small amount
of carbon dioxide emissions.
Methane
Methane is a greenhouse gas that is approximately 21 times stronger than carbon dioxide and is emitted from
numerous sources in the United States. The four largest sources of methane are enteric fermentation in domestic
animals, landfills, natural gas systems, and coal mining. Since 1990, emissions of methane have decreased 8
percent in the United States.
Perfluorocarbons (PFCs)
A family of synthetic fluorinated chemicals, PFCs are used in semiconductor manufacture and also emitted
during the electrolysis phase of aluminum production. PFCs generally have long atmospheric lifetimes as well
as very high global warming potentials, though they are emitted in relatively small quantities. Since 1990,
PFC emissions have decreased 71 percent.
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