EPA 430-R-09-004
INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS:
1990-2007
APRIL 15, 2009
U.S. Environmental Protection Agency
1200 Pennsylvania Ave., N.W.
Washington, DC 20460
U.S.A.
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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 2007, 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, 2009
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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,
Climate and Transportation Practice, including Don Robinson, Diana Pape, Susan Asam, Michael Grant, Robert
Lanza, Chris Steuer, Lauren Pederson, Kamala Jayaraman, Jeremy Scharfenberg, Mollie Averyt, Stacy Hetzel,
Hemant Mallya, Don Robinson, Sandy Seastream, Douglas Sechler, Ashaya Basnyat, Kristen Schell, Victoria
Thompson, Jean Kim, Mark Flugge, Tristan Kessler, Sarah Menassian, Katrin Moffroid, Veronica Kennedy, Aaron
Beaudette, Anna Chavis, Larry O'Rourke, Rubab Bhangu, Deborah Harris, Emily Rowan, Erin Gray, Roshni Rathi,
Lauren Smith, Nikhil Nadkarni, 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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Preface
The United States Environmental Protection Agency (EPA) prepares the official U.S. Inventory of Greenhouse Gas
Emissions and Sinks to comply with existing commitments under the United Nations Framework Convention on
Climate Change (UNFCCC). Under decision 3/CP.5 of the UNFCCC Conference of the Parties, national
inventories for UNFCCC Annex I parties should be provided to the UNFCCC Secretariat each year by April 15.
In an effort to engage the public and researchers across the country, the EPA has instituted an annual public review
and comment process for this document. The availability of the draft document is announced via Federal Register
Notice and is posted on the EPA web site. Copies are also mailed upon request. The public comment period is
generally limited to 30 days; however, comments received after the closure of the public comment period are
accepted and considered for the next edition of this annual report.
in
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Table of Contents
ACKNOWLEDGMENTS I
PREFACE Ill
TABLE OF CONTENTS V
LIST OF TABLES, FIGURES, AND BOXES VII
EXECUTIVE SUMMARY ES-1
Background Information ES-2
Recent Trends in U.S. Greenhouse Gas Emissions and Sinks ES-3
Overview of Sector Emissions and Trends ES-11
Other Information ES-14
1. INTRODUCTION 1-1
1.1. Background Information 1-2
1.2. Institutional Arrangements 1-8
1.3. Inventory Process 1-9
1.4. Methodology and Data Sources 1-10
1.5. Key Categories 1-11
1.6. Quality Assurance and Quality Control (QA/QC) 1-14
1.7. Uncertainty Analysis of Emission Estimates 1-15
1.8. Completeness 1-16
1.9. Organization of Report 1-16
2. TRENDS IN GREENHOUSE GAS EMISSIONS 2-1
2.1. Recent Trends in U.S. Greenhouse Gas Emissions 2-1
2.2. Emissions by Economic Sector 2-15
2.3. Indirect Greenhouse Gas Emissions (CO, NOX, NMVOCs, and SO2) 2-24
3. ENERGY 3-1
3.1. Fossil Fuel Combustion (IPCC Source Category 1A) 3-3
3.2. Carbon Emitted from Non-Energy Uses of Fossil Fuels (IPCC Source Category 1A) 3-28
3.3. Coal Mining (IPCC Source Category IBla) 3-33
3.4. Abandoned Underground Coal Mines (IPCC Source Category IBla) 3-35
3.5. Natural Gas Systems (IPCC Source Category lB2b) 3-39
3.6. Petroleum Systems (IPCC Source Category lB2a) 3-42
3.7. Incineration of Waste (IPCC Source Category 1A5) 3-47
3.8. Energy Sources of Indirect Greenhouse Gas Emissions 3-50
3.9. International Bunker Fuels (IPCC Source Category 1: Memo Items) 3-51
3.10. WoodBiomass andEthanol Consumption (IPCC Source Category 1A) 3-55
4. INDUSTRIAL PROCESSES 4-1
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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) 4-10
4.4. Soda Ash Production and Consumption (IPCC Source Category 2A4) 4-13
4.5. Ammonia Production (IPCC Source Category 2B1) and Urea Consumption 4-16
4.6. Nitric Acid Production (IPCC Source Category 2B2) 4-19
4.7. Adipic Acid Production (IPCC Source Category 2B3) 4-21
4.8. Silicon Carbide Production (IPCC Source Category 2B4) and Consumption 4-23
4.9. Petrochemical Production (IPCC Source Category 2B5) 4-25
4.10. Titanium Dioxide Production (IPCC Source Category 2B5) 4-28
4.11. Carbon Dioxide Consumption (IPCC Source Category 2B5) 4-30
4.12. Phosphoric Acid Production (IPCC Source Category 2B5) 4-32
4.13. Iron and Steel Production (IPCC Source Category 2C1) and Metallurgical Coke Production 4-35
4.14. Ferroalloy Production (IPCC Source Category 2C2) 4-43
4.15. Aluminum Production (IPCC Source Category 2C3) 4-45
4.16. Magnesium Production and Processing (IPCC Source Category 2C4) 4-49
4.17. Zinc Production (IPCC Source Category 2C5) 4-52
4.18. Lead Production (IPCC Source Category 2C5) 4-55
4.19. HCFC-22 Production (IPCC Source Category 2E1) 4-56
4.20. Substitution of Ozone Depleting Substances (IPCC Source Category 2F) 4-59
4.21. Semiconductor Manufacture (IPCC Source Category 2F6) 4-62
4.22. Electrical Transmission and Distribution (IPCC Source Category 2F7) 4-67
4.23. Industrial Sources of Indirect Greenhouse Gases 4-72
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 5-3
6. AGRICULTURE 6-1
6.1. Enteric Fermentation (IPCC Source Category 4A) 6-2
6.2. Manure Management (IPCC Source Category 4B) 6-6
6.3. Rice 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) 6-27
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 7-12
7.3. Land Converted to Forest Land (IPCC Source Category 5A2) 7-24
7.4. Cropland Remaining Cropland (IPCC Source Category 5B1) 7-24
vi Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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7.5. Land Converted to Cropland (IPCC Source Category 5B2) 7-35
7.6. Grassland Remaining Grassland (IPCC Source Category 5C1) 7-38
7.7. Land Converted to Grassland (IPCC Source Category 5C2) 7-42
7.8. Wetlands Remaining Wetlands 7-45
7.9. Settlements Remaining Settlements 7-49
7.10. Land Converted to Settlements (Source Category 5E2) 7-54
7.11. Other (IPCC Source Category 5G) 7-54
8. WASTE 8-1
8.1. Landfills (IPCC Source Category 6A1) 8-1
8.2. Wastewater Treatment (IPCC Source Category 6B) 8-5
8.3. Composting (IPCC Source Category 6D) 8-16
9. OTHER 9-1
10. RECALCULATIONS AND IMPROVEMENTS 10-1
11. REFERENCES 11-1
List of Tables, Figures, and Boxes
Tables
Table ES-1: Global Warming Potentials (100-Year Time Horizon) Used in this Report ES-3
Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq. or million metric tons CO2
Eq.) ES-4
Table ES-3: CO2 Emissions fromFossil Fuel Combustion by Fuel Consuming End-Use Sector (Tg CO2 Eq.)....ES-7
Table ES-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg CO2Eq.)ES-
11
Table ES- 5: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (Tg CO2Eq.) ES-13
Table ES-6. Emissions from Land Use, Land-Use Change, and Forestry (TgCO2Eq.) ES-13
Table ES-7: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (TgCO2Eq.) ES-14
Table ES-8: U.S Greenhouse Gas Emissions by Economic Sector with Electricity-Related Emissions Distributed
(TgC02Eq.) ES-15
Table ES-9: Recent Trends in Various U.S. Data (Index 1990 = 100) ES-16
Table ES-10: Emissions of NOX, CO,NMVOCs, and SO2 (Gg) ES-16
Table 1-1: Global Atmospheric Concentration, Rate of Concentration Change, and Atmospheric Lifetime (years) of
Selected Greenhouse Gases 1-2
Table 1 -2: Global Warming Potentials and Atmospheric Lifetimes (Years) Used in this Report 1-6
Table 1-3: Comparison of 100-Year GWPs 1-7
Table 1-4: Key Categories for the United States (1990-2007) 1-12
Table 1 -5. Estimated Overall Inventory Quantitative Uncertainty (Tg CO2 Eq. and Percent) 1-15
Table 1-6: IPCC Sector Descriptions 1-16
Table 1-7: List of Annexes 1-17
vn
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Table 2-1: Recent Trends inU.S. Greenhouse Gas Emissions and Sinks (Tg CO2Eq.) 2-3
Table 2-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg) 2-4
Table 2-3: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (TgCO2Eq.)... 2-7
Table 2-4: Emissions from Energy (TgCO2Eq.) 2-7
Table 2-5: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 2-8
Table 2-6: Emissions from Industrial Processes (TgCO2Eq.) 2-10
Table 2-7: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq.) 2-11
Table 2-8: Emissions from Agriculture (Tg CO2 Eq.) 2-12
Table 2-9: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (TgCO2Eq.) 2-13
Table 2-10: Emissions from Land Use, Land-Use Change, and Forestry (TgCO2Eq.) 2-13
Table 2-11: Emissions from Waste (Tg CO2 Eq.) 2-14
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq. and Percent of Total in
2007) 2-15
Table 2-13: Electricity Generation-Related Greenhouse Gas Emissions (Tg CO2Eq.) 2-17
Table 2-14: U.S Greenhouse Gas Emissions by Economic Sector and Gas with Electricity-Related Emissions
Distributed (TgCO2Eq.) and Percent of Total in 2007 2-18
Table 2-15: Transportation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 2-20
Table 2-16: Recent Trends in Various U.S. Data (Index 1990 = 100) 2-24
Table 2-17: Emissions of NOX, CO, NMVOCs, and SO2 (Gg) 2-25
Table 3-1: CO2, CH4, andN2O Emissions fromEnergy (Tg CO2 Eq.) 3-1
Table 3-2: CO2, CH4, and N2O Emissions from Energy (Gg) 3-2
Table 3-3: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion (Tg CO2 Eq.) 3-3
Table 3-4: CO2, CH4, and N2O Emissions fromFossil Fuel Combustion (Gg) 3-3
Table 3 -5: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq.) 3-3
Table 3-6: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors (Tg CO2
Eq. and Percent) 3-4
Table 3 -7: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion by Sector (Tg CO2 Eq.) 3-6
Table 3-8: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 3-7
Table 3-9: CO2 Emissions from Stationary Fossil Fuel Combustion (Tg CO2 Eq.) 3-8
Table 3-10: CH4 Emissions from Stationary Combustion (Tg CO2 Eq.) 3-8
Table 3-11: N2O Emissions from Stationary Combustion (Tg CO2 Eq.) 3-9
Table 3-12: CO2 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector (Tg CO2 Eq.)a 3-13
Table 3-13: CH4 Emissions fromMobile Combustion (Tg CO2 Eq.) 3-14
Table 3-14: N2O Emissions from Mobile Combustion (Tg CO2 Eq.) 3-15
Table 3-15: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg CO2 Eq./QBtu) 3-19
Table 3-16: Carbon Intensity from All Energy Consumption by Sector (Tg CO2 Eq./QBtu) 3-19
Table 3-17: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Energy-related Fossil Fuel
Combustion by Fuel Type and Sector (Tg CO2Eq. and Percent) 3-21
viii Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Table 3-18: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Energy-Related Stationary
Combustion, Including Biomass (Tg CO2Eq. and Percent) 3-24
Table 3-19. Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Mobile Sources (Tg CO2
Eq. and Percent) 3-26
Table 3 -20: CO2 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg CO2 Eq.) 3-28
Table 3-21: Adjusted Consumption of Fossil Fuels for Non-Energy Uses (TBtu) 3-29
Table 3-22: 2007 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions 3-30
Table 3-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Non-Energy Uses of Fossil Fuels
(Tg CO2 Eq. and Percent) 3-31
Table 3-24: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of Non-Energy Uses of Fossil Fuels
(Percent) 3-31
Table 3-25: CH4 Emissions from Coal Mining (Tg CO2 Eq.) 3-33
Table 3-26: CH4 Emissions from Coal Mining (Gg) 3-33
Table 3-27: Coal Production (Thousand Metric Tons) 3-34
Table 3-28: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Coal Mining (Tg CO2 Eq. and
Percent) 3-35
Table 3-29: CH4 Emissions from Abandoned Coal Mines (Tg CO2Eq.) 3-36
Table 3-30: CH4 Emissions from Abandoned Coal Mines (Gg) 3-36
Table 3-31: Number of gassy abandoned mines occurring in U.S. basins grouped by class according to post-
abandonment state 3-37
Table 3-32: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Abandoned Underground Coal
Mines (Tg CO2 Eq. and Percent) 3-39
Table 3-33. CH4 Emissions from Natural Gas Systems (Tg CO2 Eq.)* 3-40
Table 3-34. CH4 Emissions from Natural Gas Systems (Gg)* 3-40
Table 3-35. Non-combustion CO2 Emissions from Natural Gas Systems (TgCO2Eq.) 3-40
Table 3-36. Non-combustion CO2 Emissions from Natural Gas Systems (Gg) 3-40
Table 3-37: Tier 2 Quantitative Uncertainty Estimates for CH4 and Non-energy CO2 Emissions from Natural Gas
Systems (Tg CO2 Eq. and Percent) 3-41
Table 3-38: CH4 Emissions from Petroleum Systems (Tg CO2Eq.) 3-43
Table 3-39: CH4 Emissions from Petroleum Systems (Gg) 3-43
Table 3-40: CO2 Emissions from Petroleum Systems (Tg CO2Eq.) 3-44
Table 3-41: CO2 Emissions from Petroleum Systems (Gg) 3-44
Table 3-42: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petroleum Systems (Tg CO2 Eq. and
Percent) 3-45
Table 3-43: Potential Emissions from CO2 Capture and Transport (Tg CO2 Eq.) 3-46
Table 3-44: Potential Emissions from CO2 Capture and Transport (Gg) 3-47
Table 3-45: CO2 and N2O Emissions from the Incineration of Waste (TgCO2Eq.) 3-47
Table 3-46: CO2 and N2O Emissions from the Incineration of Waste (Gg) 3-47
Table 3-47: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted. Source: ArSova et al.
(2008) 3-48
IX
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Table 3-48: Tier 2 Quantitative Uncertainty Estimates for CO2 and N2O from the Incineration of Waste (Tg CO2 Eq.
and Percent) 3-49
Table 3-49: NOX, CO, and NMVOC Emissions from Energy-Related Activities (Gg) 3-50
Table 3 -50: CO2, CH4, and N2O Emissions from International Bunker Fuels (Tg CO2 Eq.) 3-52
Table 3-51: CO2, CH4 and N2O Emissions from International Bunker Fuels (Gg) 3-52
Table 3-52: Aviation Jet Fuel Consumption for International Transport (Million Gallons) 3-53
Table 3-53: Marine Fuel Consumption for International Transport (Million Gallons) 3-53
Table 3-54: CO2 Emissions from Wood Consumption by End-Use Sector (Tg CO2 Eq.) 3-55
Table 3-55: CO2 Emissions from Wood Consumption by End-Use Sector (Gg) 3-55
Table 3-56: CO2 Emissions fromEthanol Consumption (Tg CO2 Eq.) 3-55
Table 3-57: CO2 Emissions fromEthanol Consumption (Gg) 3-56
Table 3-58: Woody Biomass Consumption by Sector (Trillion Btu) 3-56
Table 3-59: Ethanol Consumption by Sector (Trillion Btu) 3-56
Table 4-1: Emissions from Industrial Processes (TgCO2Eq.) 4-1
Table 4-2: Emissions from Industrial Processes (Gg) 4-2
Table 4-3: CO2 Emissions from Cement Production (Tg CO2 Eq. and Gg) 4-4
Table 4-4: Clinker Production (Gg) 4-5
Table 4-5: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Cement Production (Tg CO2 Eq. and
Percent) 4-6
Table 4-6: CO2 Emissions from Lime Production (Tg CO2 Eq. and Gg) 4-7
Table 4-7: Potential, Recovered, and Net CO2 Emissions from Lime Production (Gg) 4-7
Table 4-8: High-Calcium- and Dolomitic-Quicklime, High-Calcium- and Dolomitic-Hydrated, and Dead-Burned-
Dolomite Lime Production (Gg) 4-8
Table 4-9: Adjusted Lime Production3 (Gg) 4-8
Table 4-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lime Production (Tg CO2 Eq. and
Percent) 4-10
Table 4-11: CO2 Emissions from Limestone & Dolomite Use (Tg CO2 Eq.) 4-10
Table 4-12: CO2 Emissions from Limestone & Dolomite Use (Gg) 4-11
Table 4-13: Limestone and Dolomite Consumption (Thousand Metric Tons) 4-12
Table 4-14: Dolomitic Magnesium Metal Production Capacity (Metric Tons) 4-12
Table 4-15: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Limestone and Dolomite Use (Tg
CO2 Eq. and Percent) 4-13
Table 4-16: CO2 Emissions from Soda Ash Production and Consumption (Tg CO2 Eq.) 4-14
Table 4-17: CO2 Emissions from Soda Ash Production and Consumption (Gg) 4-14
Table 4-18 : Soda Ash Production and Consumption (Gg) 4-15
Table 4-19 : Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Soda Ash Production and
Consumption (Tg CO2Eq. and Percent) 4-15
Table 4-20: CO2 Emissions from Ammonia Production and Urea Consumption (Tg CO2Eq.) 4-16
Table 4-21: CO2 Emissions from Ammonia Production and Urea Consumption (Gg) 4-17
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Table 4-22: Ammonia Production, Urea Production, Urea Net Imports, and Urea Exports (Gg) 4-18
Table 4-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ammonia Production and Urea
Consumption (Tg CO2 Eq. and Percent) 4-18
Table 4-24: N2O Emissions from Nitric Acid Production (Tg CO2 Eq. and Gg) 4-19
Table 4-25: Nitric Acid Production (Gg) 4-20
Table 4-26: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Nitric Acid Production (Tg CO2 Eq.
and Percent) 4-20
Table 4-27: N2O Emissions from Adipic Acid Production (Tg CO2 Eq. and Gg) 4-21
Table 4-28: Adipic Acid Production (Gg) 4-22
Table 4-29: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Adipic Acid Production (Tg CO2
Eq. and Percent) 4-23
Table 4-30: CO2 and CH4 Emissions from Silicon Carbide Production and Consumption (Tg CO2 Eq.) 4-23
Table 4-31: CO2 and CH4 Emissions from Silicon Carbide Production and Consumption (Gg) 4-23
Table 4-32: Production and Consumption of Silicon Carbide (Metric Tons) 4-24
Table 4-33: Tier 2 Quantitative Uncertainty Estimates for CH4 and CO2 Emissions from Silicon Carbide Production
and Consumption (Tg CO2Eq. and Percent) 4-24
Table 4-34: CO2 and CH4 Emissions from Petrochemical Production (Tg CO2 Eq.) 4-25
Table 4-35: CO2 and CH4 Emissions from Petrochemical Production (Gg) 4-25
Table 4-36: Production of Selected Petrochemicals (Thousand Metric Tons) 4-26
Table 4-37: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock (Secondary Feedstock)
Consumption (Thousand Metric Tons) 4-26
Table 4-38: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petrochemical Production and CO2
Emissions from Carbon Black Production (Tg CO2 Eq. and Percent) 4-27
Table 4-39: CO2 Emissions from Titanium Dioxide (Tg CO2 Eq. and Gg) 4-28
Table 4-40: Titanium Dioxide Production (Gg) 4-29
Table 4-41: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Titanium Dioxide Production (Tg
CO2 Eq. and Percent) 4-29
Table 4-42: CO2 Emissions from CO2 Consumption (Tg CO2 Eq. and Gg) 4-30
Table 4-43: CO2 Production (Gg CO2) and the Percent Used for Non-EOR Applications for Jackson Dome and
Bravo Dome 4-31
Table 4-44: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from CO2 Consumption (Tg CO2 Eq. and
Percent) 4-32
Table 4-45: CO2 Emissions from Phosphoric Acid Production (Tg CO2 Eq. and Gg) 4-33
Table 4-46: Phosphate Rock Domestic Production, Exports, and Imports (Gg) 4-33
Table 4-47: Chemical Composition of Phosphate Rock (percent by weight) 4-34
Table 4-48: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Phosphoric Acid Production (Tg
CO2 Eq. and Percent) 4-35
Table 4-49: CO2 and CH4 Emissions from Metallurgical Coke Production (Tg CO2 Eq.) 4-36
Table 4-50: CO2 and CH4 Emissions from Metallurgical Coke Production (Gg) 4-37
Table 4-51: CO2 Emissions from Iron and Steel Production (Tg CO2 Eq.) 4-37
XI
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Table 4-52: CO2 Emissions from Iron and Steel Production (Gg) 4-37
Table 4-53: CH4 Emissions from Iron and Steel Production (Tg CO2 Eq.) 4-37
Table 4-54: CH4 Emissions from Iron and Steel Production (Gg) 4-38
Table 4-55: Material Carbon Contents for Metallurgical Coke Production 4-38
Table 4-56: CH4 Emission Factor for Metallurgical Coke Production (g CH^/metric ton) 4-38
Table 4-57: Production and Consumption Data for the Calculation of CO2 and CH4 Emissions from Metallurgical
Coke Production (Thousand Metric Tons) 4-39
Table 4-58: Production and Consumption Data for the Calculation of CO2 Emissions from Metallurgical Coke
Production (million ft3) 4-39
Table 4-59: CO2 Emission Factors for Sinter Production and Direct Reduced Iron Production 4-39
Table 4-60: Material Carbon Contents for Iron and Steel Production 4-40
Table 4-61: CH4 Emission Factors for Sinter and Pig Iron Production 4-40
Table 4-62: Production and Consumption Data for the Calculation of CO2 and CH4 Emissions from Iron and Steel
Production (Thousand Metric Tons) 4-41
Table 4-63: Production and Consumption Data for the Calculation of CO2 Emissions from Iron and Steel
Production (million ft3 unless otherwise specified) 4-41
Table 4-64: Tier 2 Quantitative Uncertainty Estimates for CO2 and CH4 Emissions from Iron and Steel Production
(Tg. CO2 Eq. and Percent)3 4-42
Table 4-65: CO2 and CH4 Emissions from Ferroalloy Production (Tg CO2 Eq.) 4-43
Table 4-66: CO2 and CH4 Emissions from Ferroalloy Production (Gg) 4-44
Table 4-67: Production of Ferroalloys (Metric Tons) 4-44
Table 4-68: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ferroalloy Production (Tg CO2 Eq.
and Percent) 4-45
Table 4-69: CO2 Emissions from Aluminum Production (Tg CO2 Eq. andGg) 4-46
Table 4-70: PFC Emissions from Aluminum Production (Tg CO2 Eq.) 4-46
Table 4-71: PFC Emissions from Aluminum Production (Gg) 4-46
Table 4-72: Production of Primary Aluminum (Gg) 4-48
Table 4-73: Tier 2 Quantitative Uncertainty Estimates for CO2 and PFC Emissions from Aluminum Production (Tg
CO2 Eq. and Percent) 4-49
Table 4-74: SF6 Emissions from Magnesium Production and Processing (Tg CO2 Eq. and Gg) 4-50
Table 4-75: SF6 Emission Factors (kg SF6 per metric ton of magnesium) 4-50
Table 4-76: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Magnesium Production and
Processing (Tg CO2 Eq. and Percent) 4-52
Table 4-77: CO2 Emissions fromZinc Production (Tg CO2 Eq. and Gg) 4-53
Table 4-78: Zinc Production (Metric Tons) 4-54
Table 4-79: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Zinc Production (Tg CO2 Eq. and
Percent) 4-55
Table 4-80: CO2 Emissions from Lead Production (Tg CO2 Eq. and Gg) 4-55
Table 4-81: Lead Production (Metric Tons) 4-56
Table 4-82: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lead Production (Tg CO2 Eq. and
xii Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Percent) 4-56
Table 4-83: HFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and Gg) 4-57
Table 4-84: HCFC-22 Production (Gg) 4-58
Table 4-85: Quantitative Uncertainty Estimates for HFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and
Percent) 4-58
Table 4-86: Emissions of HFCs and PFCs from ODS Substitutes (TgCO2Eq.) 4-59
Table 4-87: Emissions of HFCs and PFCs from ODS Substitution (Mg) 4-59
Table 4-88: Emissions of HFCs and PFCs from ODS Substitutes (Tg CO2 Eq.) by Sector 4-60
Table 4-89: Tier 2 Quantitative Uncertainty Estimates for HFC and PFC Emissions from ODS Substitutes (Tg CO2
Eq. and Percent) 4-62
Table 4-90: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg CO2 Eq.) 4-63
Table 4-91: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Mg) 4-63
Table 4-92: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from Semiconductor
Manufacture (Tg CO2 Eq. and Percent) 4-67
Table 4-93: SF6 Emissions from Electric Power Systems and Electrical Equipment Manufacturers (Tg CO2 Eq.).. 4-
68
Table 4-94: SF6 Emissions from Electric Power Systems and Electrical Equipment Manufacturers (Gg) 4-68
Table 4-95: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Electrical Transmission and
Distribution (Tg CO2 Eq. and Percent) 4-71
Box 4-1: Potential Emission Estimates of HFCs, PFCs, and SF6 4-71
Table 4-96: 2007 Potential and Actual Emissions of HFCs, PFCs, and SF6 from Selected Sources (Tg CO2 Eq.) 4-72
Table 4-97: NOX, CO, and NMVOC Emissions from Industrial Processes (Gg) 4-73
Table 5-1: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq. and Gg) 5-1
Table 5-2: N2O Production (Gg) 5-1
Table 5-3: N2O Emissions fromN2O Product Usage (Tg CO2 Eq. and Gg) 5-1
Table 5-4: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions From N2O Product Usage (Tg CO2 Eq. and
Percent) 5-3
Table 5-5: Emissions of NOX, CO, and NMVOC from Solvent Use (Gg) 5-4
Table 6-1: Emissions from Agriculture (Tg CO2Eq.) 6-1
Table 6-2: Emissions from Agriculture (Gg) 6-1
Table 6-3: CH4 Emissions from Enteric Fermentation (Tg CO2Eq.) 6-2
Table 6-4: CH4 Emissions from Enteric Fermentation (Gg) 6-2
Table 6-5: Quantitative Uncertainty Estimates for CH4 Emissions from Enteric Fermentation (Tg CO2 Eq. and
Percent) 6-5
Table 6-6. CH4 and N2O Emissions from Manure Management (Tg CO2 Eq.) 6-7
Table 6-7. CH4 and N2O Emissions from Manure Management (Gg) 6-8
Table 6-8. Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O (Direct and Indirect) Emissions from Manure
Management (Tg CO2Eq. and Percent) 6-11
Table 6-9: CH4 Emissions from Rice Cultivation (Tg CO2 Eq.) 6-13
Table 6-10: CH4 Emissions fromRice Cultivation (Gg) 6-14
xiii
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Table 6-11: Rice Areas Harvested (Hectares) 6-14
Table 6-12: Ratooned Area as Percent of Primary Growth Area 6-15
Table 6-13: Non-USDA Data Sources for Rice Harvest Information 6-15
Table 6-14: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Rice Cultivation (Tg CO2 Eq. and
Percent) 6-16
Table 6-15: N2O Emissions from Agricultural Soils (Tg CO2Eq.) 6-17
Table 6-16: N2O Emissions from Agricultural Soils (Gg) 6-18
Table 6-17: Direct N2O Emissions from Agricultural Soils by Land Use Type and N Input Type (Tg CO2 Eq.)... 6-18
Table 6-18: Indirect N2O Emissions from all Land-Use Types (Tg CO2Eq.) 6-18
Table 6-19: Quantitative Uncertainty Estimates of N2O Emissions from Agricultural Soil Management in 2007 (Tg
CO2 Eq. and Percent) 6-25
Table 6-20: CH4 and N2O Emissions from Field Burning of Agricultural Residues (Tg CO2 Eq.) 6-28
Table 6-21: CH4, N2O, CO, and NOX Emissions from Field Burning of Agricultural Residues (Gg) 6-28
Table 6-22: Agricultural Crop Production (Gg of Product) 6-31
Table 6-23: Percent of Rice Area Burned by State 6-31
Table 6-24: Key Assumptions for Estimating Emissions from Field Burning of Agricultural Residues 6-31
Table 6-25: Greenhouse Gas Emission Ratios and Conversion Factors 6-31
Table 6-26: Tier 2 Uncertainty Estimates for CH4 and N2O Emissions from Field Burning of Agricultural Residues
(Tg CO2 Eq. and Percent) 6-32
Table 7-1: Net CO2 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.) 7-
1
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 CO2Eq.) 7-2
Table 7-4: Emissions from Land Use, Land-Use Change, and Forestry (Gg) 7-3
Table 7-5. Land use and land use change areas on managed land during the inventory reporting period (thousands of
hectares). The abbreviations are "F" for Forest Land, "C" for Cropland, "G" for Grassland, "W" for
Wetlands, "S" for Settlements, and "O" for Other Lands. Lands remaining in the same land use are
identified with the land use abbreviation given twice (e.g., "FF" is Forest Land Remaining Forest Land),
and land use change categories are identified with the previous land use abbreviation followed by the new
land use after conversion (e.g., "CF" is Cropland Converted to Forest Land) 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 7-14
Table 7-9: Estimates of CO2 (Tg/yr) emissions for the lower 48 states and Alaska1 7-15
Table 7-10: Tier 2 Quantitative Uncertainty Estimates for Net CO2 Flux from Forest Land Remaining Forest Land:
Changes in Forest C Stocks (Tg CO2Eq. and Percent) 7-19
Table 7-11: Estimated Non-CO2 Emissions from Forest Fires (TgCO2Eq.) for U.S. Forests1 7-21
Table 7-12: Estimated Non-CO2 Emissions from Forest Fires (Gg Gas) for U.S. Forests1 7-21
Table 7-13: Estimated Carbon Released from Forest Fires for U.S. Forests 7-21
Table 7-14: Tier 2 Quantitative Uncertainty Estimates of Non-CO2 Emissions from Forest Fires in Forest Land
Remaining Forest Land (Tg CO2Eq. and Percent) 7-22
xiv Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Table 7-15: N2O Fluxes from Soils in Forest Land Remaining Forest Land (Tg CO2 Eq. and Gg N2O) 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.) 7-25
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 Soil C Stock Changes occurring within Cropland Remaining
Cropland (Tg CO2 Eq. and Percent) 7-30
Table 7-20: Emissions from Liming of Agricultural Soils (TgCO2Eq.) 7-31
Table 7-21: Emissions from Liming of Agricultural Soils (TgC) 7-31
Table 7-22: Applied Minerals (Million Metric Tons) 7-32
Table 7-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Liming of Agricultural Soils (Tg
CO2 Eq. and Percent) 7-32
Table 7-24: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg CO2 Eq.) 7-33
Table 7-25: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C) 7-33
Table 7-26: Applied Urea (Million Metric Tons) 7-34
Table 7-27: Quantitative Uncertainty Estimates for CO2 Emissions from Urea Fertilization (Tg CO2 Eq. and Percent)
7-34
Table 7-28: Net CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg CO2 Eq.) 7-35
Table 7-29: Net CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg C) 7-35
Table 7-30: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Land Converted to
Cropland (Tg CO2 Eq. and Percent) 7-37
Table 7-31: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg CO2 Eq.) 7-38
Table 7-32: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg C) 7-38
Table 7-33: Quantitative Uncertainty Estimates for C Stock Changes occurring within Grassland Remaining
Grassland (Tg CO2 Eq. and Percent) 7-40
Table 7-34: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg CO2 Eq.) 7-42
Table 7-35: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C) 7-42
Table 7-36: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Land Converted to
Grassland (Tg CO2 Eq. and Percent) 7-44
Table 7-37: Emissions from Lands Undergoing Peat Extraction (Tg CO2 Eq.) 7-46
Table 7-38: Emissions from Lands Undergoing Peat Extraction (Gg) 7-46
Table 7-39: Peat Production of Lower 48 States (in thousands of Metric Tons) 7-47
Table 7-40: Peat Production of Alaska (in thousands of Cubic Meters) 7-47
Table 7-41: Tier-2 Quantitative Uncertainty Estimates for CO2 Emissions from Lands Undergoing Peat Extraction
7-48
Table 7-42: Net C Flux from Urban Trees (Tg CO2 Eq. and Tg C) 7-49
Table 7-43: C Stocks (Metric Tons C), Annual C Sequestration (Metric Tons C/yr), Tree Cover (Percent), and
Annual C Sequestration per Area of Tree Cover (kg C/m2cover-yr) for 15 U.S. Cities 7-51
Table 7-44: Tier 2 Quantitative Uncertainty Estimates for Net C Flux from Changes in C Stocks in Urban Trees (Tg
CO2 Eq. and Percent) 7-52
xv
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Table 7-45: N2O Fluxes from Soils in Settlements Remaining Settlements (Tg CO2 Eq. and Gg N2O) 7-53
Table 7-46: Quantitative Uncertainty Estimates of N2O Emissions from Soils in Settlements Remaining Settlements
(Tg CO2 Eq. and Percent) 7-54
Table 7-47: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg CO2Eq.) 7-55
Table 7-48: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C) 7-55
Table 7-49: Moisture Content (%), C Storage Factor, Proportion of Initial C Sequestered (%), Initial C Content (%),
and Half-Life (years) for Landfilled Yard Trimmings and Food Scraps in Landfills 7-57
Table 7-50: C Stocks in Yard Trimmings and Food Scraps in Landfills (Tg C) 7-57
Table 7-51: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Yard Trimmings and Food Scraps in
Landfills (Tg CO2 Eq. and Percent) 7-58
Table 8-1: Emissions from Waste (Tg CO2 Eq.) 8-1
Table 8-2: Emissions from Waste (Gg) 8-1
Table 8-3: CH4 Emissions from Landfills (Tg CO2 Eq.) 8-2
Table 8-4: CH4 Emissions from Landfills (Gg) 8-2
Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills (Tg CO2 Eq. and Percent) 8-
4
Table 8-6: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Tg CO2 Eq.) 8-6
Table 8-7: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Gg) 8-6
Table 8-8. U.S. Population (Millions) and Domestic Wastewater BOD5 Produced (Gg) 8-8
Table 8-9: Industrial Wastewater CH4 emissions by sector for 2007 8-9
Table 8-10: U.S. Pulp and Paper, Meat and Poultry, and Vegetables, Fruits and Juices Production (Tg) 8-9
Table 8-11: Variables Used to Calculate Percent Wastewater Treated Anaerobically by Industry 8-10
Table 8-12: Wastewater Flow (m3/ton) and BOD Production (g/L) for U.S. Vegetables, Fruits, and Juices Production
8-11
Table 8-13: U.S. Population (Millions), Available Protein [kg/(person-year)], and Protein Consumed (kg/person-
year) 8-14
Table 8-14: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Wastewater Treatment (Tg CO2 Eq.
and Percent) 8-14
Table 8-15: CH4 and N2O Emissions from Composting (Tg CO2 Eq.) 8-16
Table 8-16: CH4 and N2O Emissions from Composting (Gg) 8-17
Table 8-17: U.S. Waste Composted (Gg) 8-17
Table 8-18 : Tier 1 Quantitative Uncertainty Estimates for Emissions from Composting (Tg CO2 Eq. and Percent) 8-
17
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
CO2Eq.) 10-4
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
xvi Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Figure ES-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 1990 ES-4
Figure ES-4: 2007 Greenhouse Gas Emissions by Gas (percents based on Tg CO2 Eq.) ES-6
Figure ES-5: 2007 Sources of CO2 Emissions ES-7
Figure ES-6: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type ES-7
Figure ES-7: 2007 End-Use Sector Emissions of CO2, CH4, and N2O from Fossil Fuel Combustion ES-7
Figure ES-8: 2007 Sources of CH4 Emissions ES-9
Figure ES-9: 2007 Sources of N2O Emissions ES-10
Figure ES-10: 2007 Sources of HFCs, PFCs, and SF6 Emissions ES-10
Figure ES-11: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector ES-11
Figure ES-12: 2007 U.S. Energy Consumption by Energy Source ES-11
Figure ES-13: Emissions Allocated to Economic Sectors ES-14
Figure ES-14: Emissions with Electricity Distributed to Economic Sectors ES-15
Figure ES-15: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product ES-16
Figure ES-16: 2007 Key Categories ES-18
Figure 2-1: U.S. Greenhouse Gas Emissions by Gas 2-1
Figure 2-2: Annual Percent Change in U.S. Greenhouse Gas Emissions 2-1
Figure 2-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 1990 2-1
Figure 2-4: U.S. Greenhouse Gas Emissions by Chapter/IPCC Sector 2-7
Figure 2-5: 2007 Energy Chapter Greenhouse Gas Sources 2-7
Figure 2-6: 2007 U.S. Fossil CarbonFlows (Tg CO2 Eq.) 2-7
Figure 2-7: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 2-9
Figure 2-8: 2007 End-Use Sector Emissions of CO2, CH4, and N2O from Fossil Fuel Combustion 2-9
Figure 2-9: 2007 Industrial Processes Chapter Greenhouse Gas Sources 2-10
Figure 2-10: 2007 Agriculture Chapter Greenhouse Gas Sources 2-12
Figure 2-11: 2007 Waste Chapter Greenhouse Gas Sources 2-14
Figure 2-12: Emissions Allocated to Economic Sectors 2-15
Figure 2-13: Emissions with Electricity Distributed to Economic Sectors 2-18
Figure 2-14: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product 2-24
Figure 3-1: 2007 Energy Chapter Greenhouse Gas Sources 3-1
Figure 3-2: 2007 U.S. Fossil CarbonFlows (Tg CO2 Eq.) 3-1
Figure 3-3: 2007 U.S. Energy Consumption by Energy Source 3-5
Figure 3-4: U.S. Energy Consumption (QuadrillionBtu) 3-5
Figure 3-5: 2007 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 (1950-2007) 3-5
Figure 3 -7: Annual Deviations from Normal Cooling Degree Days for the United States (1950-2007) 3-5
Figure 3-8: Aggregate Nuclear and Hydroelectric Power Plant Capacity Factors in the United States (1974-2007) 3-
6
xvn
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Figure 3-9: Electricity Generation Retail Sales by End-Use Sector 3-10
Figure 3-10: Industrial Production Indices (Index 2002=100) 3-11
Figure 3-11: Sales-Weighted Fuel Economy of New Passenger Cars and Light-Duty Trucks, 1990-2007 3-12
Figure 3-12: Sales of New Passenger Cars and Light-Duty Trucks, 1990-2007 3-12
Figure 3-13: Mobile Source CH4 and N2O Emissions 3-14
Figure 3-14: U.S. Energy Consumption and Energy-Related CO2 Emissions Per Capita and Per Dollar GDP 3-20
Figure 4-1: 2007 Industrial Processes Chapter Greenhouse Gas Sources 4-1
Figure 6-1: 2007 Agriculture Chapter Greenhouse Gas Emission Sources 6-1
Figure 6-2: Agricultural Sources and Pathways of N that Result in N2O Emissions from Agricultural Soil
Management 6-17
Figure 6-3: Major Crops, Average Annual Direct N2O Emissions by State, Estimated Using the DAYCENT Model,
1990-2007 (TgCO2Eq./year) 6-19
Figure 6-4: Grasslands, Average Annual Direct N2O Emissions by State, Estimated Using the DAYCENT Model,
1990-2007 (TgCO2Eq./year) 6-19
Figure 6-5: Major Crops, Average Annual N Losses Leading to Indirect N2O Emissions by State, Estimated Using
the DAYCENT Model, 1990-2007 (Gg N/year) 6-19
Figure 6-6: Grasslands, Average Annual N Losses Leading to Indirect N2O Emissions, by State, Estimated Using the
DAYCENT Model, 1990-2007 (Gg N/year) 6-19
Figure 6-7: Comparison of Measured Emissions at Field Sites with Modeled Emissions Using the DAYCENT
Simulation Model 6-26
Figure 7-1. Percent of Total Land Area in the General Land Use Categories for 2007 7-5
Figure 7-2: Forest Sector Carbon Pools and Flows 7-12
Figure 7-3: Estimates of Net Annual Changes in C Stocks for Major C Pools 7-15
Figure 7-4: Average C Density in the Forest Tree Pool in the Conterminous United States, 2007 7-15
Figure 7-5: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007,
Cropland Remaining Cropland 7-26
Figure 7-6: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007,
Cropland Remaining Cropland 7-26
Figure 7-7: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007, Land
Converted to Cropland 7-35
Figure 7-8: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007, Land
Converted to Cropland 7-35
Figure 7-9: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007,
Grassland Remaining Grassland 7-39
Figure 7-10: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007,
Grassland Remaining Grassland 7-39
Figure 7-11: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007,
Land Converted to Grassland 7-43
Figure 7-12: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007,
Land Converted to Grassland 7-43
Figure 8-1: 2007 Waste Chapter Greenhouse Gas Sources 8-1
xviii Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Boxes
BoxES-1: Recalculations of Inventory Estimates ES-1
BoxES-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data ES-15
Box 1-1: The IPCC Fourth Assessment Report and Global Warming Potentials 1-7
Box 1-2: IPCC Reference Approach 1-11
Box 2-1: Methodology for Aggregating Emissions by Economic Sector 2-22
Box 2-2: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data 2-24
Box 2-3: Sources and Effects of Sulfur Dioxide 2-26
Box 3-1: Weather and Non-Fossil Energy Effects on CO2 from Fossil Fuel Combustion Trends 3-5
Box 3-2: Carbon Intensity of U.S. Energy Consumption 3-18
Box 3-3. Carbon Dioxide Transport, Injection, and Geological Storage 3-46
Box 4-1: Potential Emission Estimates of HFCs, PFCs, and SF6 4-71
Box 6-1. Tier 1 vs. Tier 3 Approach for Estimating N2O Emissions 6-20
Box 6-2: Comparison of Tier 2 U. S. Inventory Approach and IPCC (2006) Default Approach 6-29
Box 7-1: CO2 Emissions from Forest Fires 7-15
Box 7-2: Tier 3 Approach for Soil C Stocks Compared to Tier 1 or 2 Approaches 7-27
Box 8-1: Biogenic Emissions and Sinks of Carbon 8-5
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 2007. 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 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.
[BEGIN BOX]
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.
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(l)(a) of the United Nations Framework Convention on Climate Change (also identified in Article 12). Subsequent
decisions by the Conference of the Parties elaborated the role of Annex I Parties in preparing national inventories. See
.
4 See .
Executive Summary ES-1
-------
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 2006) 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.
[END BOX]
Background Information
Naturally occurring greenhouse gases include water vapor, carbon dioxide (CO2), methane (CH4), nitrous oxide
(N2O), and ozone (O3). Several classes of halogenated substances that contain fluorine, chlorine, or bromine are
also greenhouse gases, but they are, for the most part, solely a product of industrial activities. Chlorofluorocarbons
(CFCs) 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 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-CH4
volatile organic compounds (NMVOCs). Aerosols, which are extremely small particles or liquid droplets, such as
those produced by sulfur dioxide (SO2) or elemental carbon emissions, can also affect the absorptive characteristics
of the atmosphere.
Although the direct greenhouse gases CO2, CH4, and N2O occur naturally in the atmosphere, human activities have
changed their atmospheric concentrations. From the pre-industrial era (i.e., ending about 1750) to 2005,
concentrations of these greenhouse gases have increased globally by 36, 148, and 18 percent, respectively (IPCC
2007).
Beginning in the 1950s, the use of CFCs and other stratospheric ozone depleting substances (ODS) increased by
nearly 10 percent per year until the mid-1980s, when international concern about ozone depletion led to the entry
into force of the Montreal Protocol. Since then, the production of ODS is being phased out. In recent years, use of
ODS substitutes such as HFCs and PFCs has grown as they begin to be phased in as replacements for CFCs and
HCFCs. Accordingly, atmospheric concentrations of these substitutes have been growing (IPCC 2007).
Global Warming Potentials
Gases in the atmosphere can contribute to the greenhouse effect both directly and indirectly. Direct effects occur
when the gas itself absorbs radiation. Indirect radiative forcing occurs when chemical transformations of the
substance produce other greenhouse gases, when a gas influences the atmospheric lifetimes of other gases, and/or
when a gas affects atmospheric processes that alter the radiative 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
5 Emissions estimates of CFCs, HCFCs, halons and other ozone-depleting substances are included in the annexes of the
Inventory report for informational purposes.
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.
ES-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 (or million metric tons) of CO2 equivalent (Tg CO2 Eq.).7'8 All gases
in this Executive Summary are presented in units of Tg CO2 Eq.
The UNFCCC reporting guidelines for national inventories were updated in 2006,9 but continue to require the use of
GWPs from the IPCC Second Assessment Report (SAR) (IPCC 1996). This requirement ensures that current
estimates of aggregate greenhouse gas emissions for 1990 to 2007 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 Chapter 1 and, in more detail, in Annex 6.1 of this report. The GWP values used in this
report are listed below in Table ES-1.
Table ES-1: Global Warming Potentials (100-Year Time Horizon) Used in this Report
Gas GWP
CO2 1
CH4* 21
N2O 310
HFC-23 11,700
HFC-32 650
HFC-125 2,800
HFC-134a 1,300
HFC-143a 3,800
HFC-152a 140
HFC-227ea 2,900
HFC-236fa 6,300
HFC-4310mee 1,300
CF4 6,500
C2F6 9,200
C4F10 7,000
C6F14 7,400
SF6 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 CO2 is not included.
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).
Recent Trends in U.S. Greenhouse Gas Emissions and Sinks
In 2007, total U.S. greenhouse gas emissions were 7,150.1 TgCO2Eq. Overall, total U.S. emissions have risen by
17 percent from 1990 to 2007. Emissions rose from 2006 to 2007, increasing by 1.4 percent (99.0 Tg CO2 Eq.).
The following factors were primary contributors to this increase: (1) cooler winter and warmer summer conditions in
2007 than in 2006 increased the demand for heating fuels and contributed to the increase in the demand for
electricity, (2) increased consumption of fossil fuels to generate electricity and (3) a significant decrease (14.2
7 Carbon comprises 12/44fts of carbon dioxide by weight.
8 One teragram is equal to 1012 grams or one million metric tons.
9 See .
Executive Summary ES-3
-------
percent) in hydropower generation used to meet this demand.
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 2007.
Figure ES-1: U.S. Greenhouse Gas Emissions by Gas
Figure ES-2: Annual Percent Change in U.S. Greenhouse Gas Emissions
Figure ES-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 1990
Table ES-2: Recent Trends in U.S
Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
US Territories
Non-Energy Use of Fuels
Iron and Steel Production &
Metallurgical Coke Production
Cement Production
Natural Gas Systems
Incineration of Waste
Lime Production
Ammonia Production and Urea
Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and
Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production and
Consumption
Land Use, Land-Use Change,
and Forestry (Sink)"
Biomass — Wood
International Bunker Fuelsb
Greenhouse
1990
5,076.7
4,708.9
1,809.7
1,484.5
834.2
337.7
214.5
28.3
117.0
109.8
33.3
33.7
10.9
11.5
16.8
7.1
5.1
6.8
4.1
2.2
1.2
1.4
2.2
1.5
1.0
0.9
0.4
0.3
0.4
(841.4)
215.2
114.3
Gas Emissions
1995
5,407.9
5,013.9
1,938.9
1,598.7
862.6
354.4
224.4
35.0
137.5
103.1
36.8
33.8
15.7
13.3
17.8
7.0
6.7
5.7
4.3
2.8
.5
.4
2.0
.5
.0
.0
0.3
0.3
0.3
(851.0)
229.1
101.6
and Sinks (Tg
2000
5,955.2
5,561.5
2,283.2
1,800.3
844.6
370.4
226.9
36.2
144.5
95.1
41.2
29.4
17.5
14.1
16.4
7.5
5.1
6.1
4.2
3.0
1.8
1.4
1.9
1.4
1.2
1.1
0.3
0.3
0.2
(717.5)
218.1
99.0
COa Eq. or million metric
2005
6,090.8
5,723.5
2,381.0
1,881.5
828.0
358.0
221.8
53.2
138.1
73.2
45.9
29.5
19.5
14.4
12.8
7.9
6.8
4.1
4.2
2.8
1.8
1.3
1.4
1.4
1.1
0.5
0.3
0.3
0.2
(1,122.7)
208.9
111.5
2006
6,014.9
5,635.4
2,327.3
1,880.9
844.5
321.9
206.0
54.8
145.1
76.1
46.6
29.5
19.8
15.1
12.3
7.9
8.0
3.8
4.2
2.6
1.9
1.7
1.5
1.2
0.9
0.5
0.3
0.3
0.2
(1,050.5)
209.9
110.5
tons CO2
2007
6,103.4
5,735.8
2,397.2
1,887.4
845.4
340.6
214.4
50.8
133.9
77.4
44.5
28.7
20.8
14.6
13.8
8.0
6.2
4.3
4.1
2.6
1.9
1.9
1.6
1.2
1.0
0.5
0.3
0.3
0.2
(1,062.6)
209.8
108.8
ES-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Biomass — Ethanof
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Forest Land Remaining Forest
Land
Petroleum Systems
Wastewater Treatment
Stationary Combustion
Rice Cultivation
Abandoned Underground Coal
Mines
Mobile Combustion
Composting
Petrochemical Production
Field Burning of Agricultural
Residues
Iron and Steel Production &
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and
Consumption
International Bunker Fuelsb
N2O
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Adipic Acid Production
Wastewater Treatment
N2O from Product Uses
Forest Land Remaining Forest
Land
Composting
Settlements Remaining
Settlements
Field Burning of Agricultural
Residues
Incineration of Waste
Wetlands Remaining Wetlands
International Bunker Fuelsb
HFCs
Substitution of Ozone Depleting
Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and
Distribution
4.2
616.6
133.2
149.2
129.6
84.1
30.4
4.6
33.9
23.5
7.4
7.1
6.0
4.7
0.3
0.9
0.7
1.0
+
+
0.2
315.0
200.3
43.7
20.0
12.1
12.8
15.3
3.7
4.4
0.5
0.4
1.0
0.4
0.5
+
1.1
36.9
0.3
36.4
0.2
20.8
18.5
2.2
32.8
26.8
7.7
615.8
143.6
144.3
132.6
67.1
34.5
6.1
32.0
24.8
7.1
7.6
8.2
4.3
0.7
1.1
0.7
1.0
+
+
0.1
334.1
202.3
53.7
22.3
12.9
13.3
17.3
4.0
4.6
0.8
0.8
1.2
0.4
0.5
+
0.9
61.8
28.5
33.0
0.3
15.6
11.8
3.8
28.1
21.6
9.2
591.1
134.4
122.3
130.8
60.5
37.9
20.6
30.3
25.2
6.6
7.5
7.4
3.4
1.3
1.2
0.8
0.9
+
+
0.1
329.2
204.5
52.8
21.9
14.0
14.5
6.2
4.5
4.9
2.4
1.4
1.2
0.5
0.4
+
0.9
100.1
71.2
28.6
0.3
13.5
8.6
4.9
19.2
15.1
22.6
561.7
136.0
127.8
106.3
57.1
41.8
14.2
28.3
24.3
6.7
6.8
5.6
2.5
1.6
1.1
0.9
0.7
+
+
0.1
315.9
210.6
36.7
18.6
14.2
14.8
5.9
4.8
4.4
1.8
1.7
1.5
0.5
0.4
+
1.0
116.1
100.0
15.8
0.2
6.2
3.0
3.2
17.9
14.0
30.5
582.0
138.2
130.4
104.8
58.4
41.9
31.3
28.3
24.5
6.3
5.9
5.5
2.4
1.6
1.0
0.8
0.7
+
+
0.1
312.1
208.4
33.5
18.2
14.6
14.5
5.9
4.8
4.4
3.5
1.8
1.5
0.5
0.4
+
1.0
119.1
105.0
13.8
0.3
6.0
2.5
3.5
17.0
13.2
38.0
585.3
139.0
132.9
104.7
57.6
44.0
29.0
28.8
24.4
6.6
6.2
5.7
2.3
1.7
1.0
0.9
0.7
+
+
0.1
311.9
207.9
30.1
21.7
14.7
14.7
5.9
4.9
4.4
3.3
1.8
1.6
0.5
0.4
+
1.0
125.5
108.3
17.0
0.3
7.5
3.8
3.6
16.5
12.7
Executive Summary ES-5
-------
Magnesium Production and
Processing
Semiconductor Manufacture
Total
Net Emissions (Sources and
Sinks)
5.4
0.5
6,098.7
5,257.3
5.6
0.9
6,463.3
5,612.3
3.0
1.1
7,008.2
6,290.7
2.9
1.0
7,108.6
5,985.9
2.9
1.0
7,051.1
6,000.6
3.0
0.8
7,150.1
6,087.5
+ Does not exceed 0.05 Tg CO2 Eq.
a Parentheses indicate negative values or sequestration. The net CO2 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.
0 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.
Figure ES-4 illustrates the relative contribution of the direct greenhouse gases to total U.S. emissions in 2007. The
primary greenhouse gas emitted by human activities in the United States was CO2, representing approximately 85.4
percent of total greenhouse gas emissions. The largest source of CO2, and of overall greenhouse gas emissions, was
fossil fuel combustion. CH4 emissions, which have declined 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 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 as a by-product of primary aluminum production
and from semiconductor manufacturing.
Figure ES-4: 2007 Greenhouse Gas Emissions by Gas (percents based on Tg CO2 Eq.)
Overall, from 1990 to 2007, total emissions of CO2 increased by 1,026.7 Tg CO2 Eq. (20.2 percent), while CH4 and
N2O emissions decreased by 31.2 Tg CO2 Eq. (5.1 percent) and 3.1 Tg CO2 Eq. (1.0 percent), respectively. During
the same period, aggregate weighted emissions of HFCs, PFCs, and SF6 rose by 59.0 Tg CO2 Eq. (65.2 percent).
From 1990 to 2007, HFCs increased by 88.6 Tg CO2 Eq. (240.0 percent), PFCs decreased by 13.3 Tg CO2 Eq. (64.0
percent), and SF6 decreased by 16.3 Tg CO2 Eq. (49.8 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 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 14.9 percent of
total emissions in 2007. 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 94.0 percent of CO2 emissions in 2007. Globally, approximately 29,195 Tg of CO2 were
added to the atmosphere through the combustion of fossil fuels in 2006, 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
10 Global CO2 emissions from fossil fuel combustion were taken from Energy Information Administration International Energy
Annual 2006 (EIA 2008b).
ES-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
land to agricultural or urban use) or can act as a sink for CO2 (e.g., through net additions to forest biomass).
Figure ES- 5: 2007 Sources of CO2 Emissions
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 2007. Emissions of CO2 from fossil fuel combustion increased at an
average annual rate of 1.3 percent from 1990 to 2007. The fundamental factors influencing this trend include (1) a
generally growing domestic economy over the last 17 years, and (2) significant overall growth in emissions from
electricity generation and transportation activities. Between 1990 and 2007, CO2 emissions from fossil fuel
combustion increased from 4,708.9 Tg CO2 Eq. to 5,735.8 Tg CO2 Eq. —a 21.8 percent total increase over the
eighteen-year period. From 2006 to 2007, these emissions increased by 100.4 Tg CO2 Eq. (1.8 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.
Figure ES- 6: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type
Figure ES- 7: 2007 End-Use Sector Emissions of CO2, CH4, and N2O from Fossil Fuel Combustion
The five major fuel consuming sectors contributing to CO2 emissions from fossil fuel combustion are electricity
generation, transportation, industrial, residential, and commercial. CO2 emissions are produced by the electricity
generation sector as they consume fossil fuel to provide electricity to one of the other four sectors, or "end-use"
sectors. For the discussion below, electricity generation emissions have been distributed to each end-use sector on
the basis of each sector's share of aggregate electricity consumption. This method of distributing emissions assumes
that each end-use sector consumes electricity that is generated from the national average mix of fuels according to
their carbon intensity. Emissions from electricity generation are also addressed separately after the end-use sectors
have been discussed.
Note that emissions from U.S. territories are calculated separately due to a lack of specific consumption data for the
individual end-use sectors.
Figure ES- 6, Figure ES- 7, and Table ES-3 summarize CO2 emissions from fossil fuel combustion by end-use
sector.
Table ES-3: CO2 Emissions from Fossil Fuel Combustion by Fuel Consuming End-Use Sector (Tg CO2 Eq.)
End-Use Sector 1990 1995 2000 2005 2006 2007
Transportation
Combustion
Electricity
Industrial
Combustion
Electricity
Residential
Combustion
1,487.5
1,484.5
3.0
1,516.8
834.2
682.6
927.1
337.7
1,601.7
1,598.7
3.0
1,575.5
862.6
712.9
993.3
354.4
1,803.7
1,800.3
3.4
1,629.6
844.6
785.0
1,128.2
370.4
1,886.2
1,881.5
4.7
1,558.5
828.0
730.5
1,207.2
358.0
1,885.4
1,880.9
4.5
1,550.7
844.5
706.2
1,145.9
321.9
1,892.2
1,887.4
4.8
1,553.4
845.4
708.0
1,198.0
340.6
Executive Summary ES-7
-------
Electricity
Commercial
Combustion
Electricity
U.S. Territories3
Total
Electricity Generation
589.4
749.2
214.5
534.7
28.3
4,708.9
1,809.7
638.8
808.5
224.4
584.1
35.0
5,013.9
1,938.9
757.9
963.8
226.9
736.8
36.2
5,561.5
2,283.2
849.2
1,018.4
221.8
796.6
53.2
5,723.5
2,381.0
824.1
998.6
206.0
792.5
54.8
5,635.4
2,327.3
857.4
1,041.4
214.4
827.1
50.8
5,735.8
2,397.2
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.
a Fuel consumption by U.S. territories (i.e., American Samoa, Guam, Puerto Rico, U.S. Virgin Islands, Wake Island, and other
U.S. Pacific Islands) is included in this report.
Transportation End-Use Sector. Transportation activities (excluding international bunker fuels) accounted for 33
percent of CO2 emissions from fossil fuel combustion in 2007.11 Virtually all of the energy consumed in this end-
use sector came from petroleum products. Nearly 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 27 percent of CO2 from
fossil fuel combustion in 2007. 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 21
and 18 percent, respectively, of CO2 emissions from fossil fuel combustion in 2007. 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 42 percent of the CO2 from fossil fuel combustion in 2007. The type of
fuel combusted by electricity generators has a significant effect on their emissions. For example, some electricity is
generated with low CO2 emitting energy technologies, particularly non-fossil options such as nuclear, hydroelectric,
or geothermal energy. However, electricity generators rely on coal for over half of their total energy requirements
and accounted for 94 percent of all coal consumed for energy in the United States in 2007. 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 16.9 Tg CO2 Eq. (14.5 percent) from
1990 through 2007. Emissions from non-energy uses of fossil fuels were 133.9 Tg CO2 Eq. in 2007, which
constituted 2.2 percent of total national CO2 emissions, approximately the same proportion as in 1990.
• CO2 emissions from iron and steel production and metallurgical coke production increased slightly from
2006 to 2007 (1.3 Tg CO2 Eq.), but have decreased by 29.5 percent to 77.4 Tg CO2 Eq. from 1990 through
2007, due to restructuring of the industry, technological improvements, and increased scrap utilization.
• In 2007, CO2 emissions from cement production decreased slightly by 2.0 Tg CO2 Eq. (4.4 percent) from
2006 to 2007. 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 2006. Overall,
from 1990 to 2007, emissions from cement production increased by 34 percent, an increase of 11.2 Tg CO2
Eq.
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 2007.
ES-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
• CO2 emissions from incineration of waste (20.8 Tg CO2 Eq. in 2007) increased by 9.8 Tg CO2 Eq. (90
percent) from 1990 through 2007, as the volume of plastics and other fossil carbon-containing materials in
the waste stream grew.
• Net CO2 sequestration from Land Use, Land-Use Change, and Forestry increased by 221.1 Tg CO2 Eq. (26
percent) from 1990 through 2007. 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.
Methane Emissions
According to the IPCC, CH4 is more than 20 times as effective as CO2 at trapping heat in the atmosphere. Over the
last two hundred and fifty years, the concentration of CH4 in the atmosphere increased by 148 percent (IPCC 2007).
Anthropogenic sources of CH4 include landfills, natural gas and petroleum systems, agricultural activities, coal
mining, wastewater treatment, stationary and mobile combustion, and certain industrial processes (see Figure ES- 8).
Figure ES- 8: 2007 Sources of CH4 Emissions
Some significant trends in U.S. emissions of CH4 include the following:
• Enteric Fermentation is the largest anthropogenic source of CH4 emissions in the United States. In 2007,
enteric fermentation CH4 emissions were 139.0 Tg CO2 Eq. (approximately 24 percent of total CH4
emissions), which represents an increase of 5.8 Tg CO2 Eq., or 4.3 percent, since 1990.
• Landfills are the second largest anthropogenic source of CH4 emissions in the United States, accounting for
approximately 23 percent of total CH4 emissions (132.9 Tg CO2 Eq.) in 2007. From 1990 to 2007, net CH4
emissions from landfills decreased by 16.3 Tg CO2 Eq. (11 percent), with small increases occurring in
some interim years, including 2007. This downward trend in overall emissions is the result of increases in
the amount of landfill gas collected and combusted,12 which has more than offset the additional CH4
emissions resulting from an increase in the amount of municipal solid waste landfilled.
• CH4 emissions from natural gas systems were 104.7 Tg CO2 Eq. in 2007; emissions have declined by 24.9
Tg CO2 Eq. (19 percent) since 1990. This decline has been due to improvements in technology and
management practices, as well as some replacement of old equipment.
• In 2007, CH4 emissions from coal mining were 57.6 Tg CO2 Eq., a 0.8 Tg CO2 Eq. (1.3 percent) decrease
over 2006 emission levels. The overall decline of 26.4 Tg CO2 Eq. (31 percent) from 1990 results from the
mining of less gassy coal from underground mines and the increased use of CH4 collected from
degasification systems.
• CH4 emissions from manure management increased by 44.7 percent for CH4, from 30.4 Tg CO2 Eq. in
1990 to 44.0 Tg CO2 Eq. in 2007. The majority of this increase was from swine and dairy cow manure,
since the general trend in manure management is one of increasing use of liquid systems, which tends to
produce greater CH4 emissions. The increase in liquid systems is the combined result of a shift to larger
facilities, and to facilities in the West and Southwest, all of which tend to use liquid systems. Also, new
regulations limiting the application of manure nutrients have shifted manure management practices at
smaller dairies from daily spread to manure managed and stored on site.
Nitrous Oxide Emissions
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
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.
Executive Summary ES-9
-------
lower than CO2 emissions, N2O is approximately 300 times more powerful than CO2 at trapping heat in the
atmosphere. Since 1750, the global atmospheric concentration of N2O has risen by approximately 18 percent (IPCC
2007). The main anthropogenic activities producing N2O in the United States are agricultural soil management, fuel
combustion in motor vehicles, nitric acid production, stationary fuel combustion, manure management, and adipic
acid production (see Figure ES-9).
Figure ES-9: 2007 Sources of N2O Emissions
Some significant trends in U.S. emissions of N2O include the following:
• Agricultural soils produced approximately 67 percent of N2O emissions in the United States in 2007.
Estimated emissions from this source in 2007 were 207.9 Tg CO2 Eq. Annual N2O emissions from
agricultural soils fluctuated between 1990 and 2007, although overall emissions were 3.8. percent higher in
2007 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 2007, N2O emissions from mobile combustion were 30.1 Tg CO2 Eq. (approximately 10 percent of U.S.
N2O emissions). From 1990 to 2007, N2O emissions from mobile combustion decreased by 31 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 2007, and have decreased significantly
since 1996 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 1.2 Tg CO2 Eq. annually since 1998.
HFC, RFC, 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. HFCs and PFCs do not deplete
the stratospheric ozone layer, and are therefore acceptable alternatives under the Montreal Protocol.
These compounds, however, along with SF6, are potent greenhouse gases. In addition to having high global
warming potentials, SF6 and PFCs have extremely long atmospheric lifetimes, resulting in their essentially
irreversible accumulation in the atmosphere once emitted. Sulfur hexafluoride is the most potent greenhouse gas the
IPCC has evaluated.
Other emissive sources of these gases include HCFC-22 production, electrical transmission and distribution systems,
semiconductor manufacturing, aluminum production, and magnesium production and processing (see Figure ES-10).
Figure ES-10: 2007 Sources of HFCs, PFCs, and SF6 Emissions
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 108.3 Tg CO2 Eq. in 2007. 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 53 percent (19.4 Tg CO2 Eq.) from 1990
through 2007, due to a steady decline in the emission rate of HFC-23 (i.e., the amount of HFC-23 emitted
ES-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
per kilogram of HCFC-22 manufactured) 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 53 percent (14.1 Tg
CO2 Eq.) from 1990 to 2007, primarily because of higher purchase prices for SF6 and efforts by industry to
reduce emissions.
• PFC emissions from aluminum production decreased by 79 percent (14.7 Tg CO2 Eq.) from 1990 to 2007,
due to both industry emission reduction efforts and lower domestic aluminum production.
Overview of Sector Emissions and Trends
In accordance with the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories
(IPCC/UNEP/OECD/IEA 1997), and the 2003 UNFCCC Guidelines on Reporting and Review (UNFCCC 2003),
Figure ES-11 and Table ES-4 aggregate emissions and sinks by these chapters. Emissions of all gases can be
summed from each source category from Intergovernmental Panel on Climate Change (IPCC) guidance. Over the
eighteen-year period of 1990 to 2007, total emissions in the Energy, Industrial Processes, and Agriculture sectors
climbed by 976.7 Tg CO2 Eq. (19 percent), 28.5 Tg CO2 Eq. (9 percent), and 28.9 Tg CO2 Eq. (8 percent),
respectively. Emissions decreased in the Waste and Solvent and Other Product Use sectors by 11.5 Tg CO2 Eq. (6
percent) and less than 0.1 Tg CO2 Eq. (0.4 percent), respectively. Over the same period, estimates of net C
sequestration in the Land Use, Land-Use Change, and Forestry sector increased by 192.5 Tg CO2 Eq. (23 percent).
Figure ES-11: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector
Table ES-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg CO2 Eq.)
Chapter/IPCC Sector 1990 1995 2000 2005 2006 2007
Energy
Industrial Processes
Solvent and Other Product Use
Agriculture
Land Use, Land-Use Change, and
Forestry (Emissions)
Waste
Total Emissions
Net CO2 Flux from Land Use, Land-
Use Change, and Forestry (Sinks)*
Net Emissions (Sources and Sinks)
5,193.6
325.2
4.4
384.2
14.2
177.1
6,098.7
(841.4)
5,257.3
5,520.1
345.8
4.6
402.0
16.2
174.7
6,463.3
(851.0)
5,612.3
6,059.9
356.3
4.9
399.4
33.0
154.6
7,008.2
(717.5)
6,290.7
6,169.2 6,084.4
337.6 343.9
4.4 4.4
410.8 410.3
26.4 45.1
160.2 163.0
7,108.6 7,051.1
(1,122.7) (1,050.5)
5,985.9 6,000.6
6,170.3
353.8
4.4
413.1
42.9
165.6
7,150.1
(1,062.6)
6,087.5
' The net CO2 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.
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 2007. In 2007,
approximately 85 percent of the energy consumed in the United States (on a Btu basis) was produced through the
combustion of fossil fuels. The remaining 15 percent came from other energy sources such as hydropower, biomass,
nuclear, wind, and solar energy (see Figure ES-12). Energy-related activities are also responsible for CH4 and N2O
emissions (35 percent and 14 percent of total U.S. emissions of each gas, respectively). Overall, emission sources in
the Energy chapter account for a combined 86.3 percent of total U.S. greenhouse gas emissions in 2007.
Figure ES-12: 2007 U.S. Energy Consumption by Energy Source
Executive Summary ES-11
-------
Industrial Processes
The Industrial Processes chapter contains by-product or fugitive emissions of greenhouse gases from industrial
processes not directly related to energy activities such as fossil fuel combustion. For example, industrial processes
can chemically transform raw materials, which often release waste gases such as CO2, CH4, and N2O. These
processes include iron and steel production and metallurgical coke production, cement production, ammonia
production and urea consumption, lime manufacture, limestone and dolomite use (e.g., flux stone, flue gas
desulfurization, and glass manufacturing), soda ash manufacture 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 Process chapter account for 4.9 percent of U.S. greenhouse gas emissions in 2007.
Solvent and Other Product Use
The Solvent and Other Product Use chapter contains greenhouse gas emissions that are produced as a by-product of
various solvent and other product uses. In the United States, emissions from N2O from product uses, the only source
of greenhouse gas emissions from this sector, accounted for less than 0.1 percent of total U.S. anthropogenic
greenhouse gas emissions on a carbon equivalent basis in 2007.
Agriculture
The Agricultural chapter contains anthropogenic emissions from agricultural activities (except fuel combustion,
which is addressed in the Energy chapter, and agricultural CO2 fluxes, which are addressed in the Land Use, Land-
Use Change, and Forestry Chapter). Agricultural activities contribute directly to emissions of greenhouse gases
through a variety of processes, including the following source categories: enteric fermentation in domestic livestock,
livestock manure management, rice cultivation, agricultural soil management, and field burning of agricultural
residues. CH4 and N2O were the primary greenhouse gases emitted by agricultural activities. CH4 emissions from
enteric fermentation and manure management represented about 24 percent and 8 percent of total CH4 emissions
from anthropogenic activities, respectively, in 2007. Agricultural soil management activities such as fertilizer
application and other cropping practices were the largest source of U.S. N2O emissions in 2007, accounting for 67
percent. In 2007, emission sources accounted for in the Agricultural chapters were responsible for 6 percent of total
U.S. greenhouse gas emissions.
Land Use, Land-Use Change, and Forestry
The Land Use, Land-Use Change, and Forestry chapter contains emissions of CH4 and N2O, and emissions and
removals of CO2 from forest management, other land-use activities, and land-use change. Forest management
practices, tree planting in urban areas, the management of agricultural soils, and the landfilling of yard trimmings
and food scraps have resulted in a net uptake (sequestration) of C in the United States. Forests (including
vegetation, soils, and harvested wood) accounted for approximately 86 percent of total 2007 net CO2 flux, urban
trees accounted for 9 percent, mineral and organic soil carbon stock changes accounted for 4 percent, and landfilled
yard trimmings and food scraps accounted for 1 percent of the total net flux in 2007. 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 2007 resulted in a net C sequestration of 1,062.6 Tg CO2 Eq. (Table ES- 5). This represents an
offset of approximately 17.4 percent of total U.S. CO2 emissions, or 14.9 percent of total greenhouse gas emissions
in 2007. Between 1990 and 2007, total land use, land-use change, and forestry net C flux resulted in a 26.3 percent
increase in CO2 sequestration, primarily due to an increase in the rate of net C accumulation in forest C stocks,
ES-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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.
Table ES- 5: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (Tg CO2 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)
1990
(661.1)
(29.4)
2.2
(46.7)
(22.3)
(60.6)
(23.5)
1995
(686.6)
(22.9)
2.9
(36.4)
(22.5)
(71.5)
(13.9)
2000
(512.6)
(30.2)
2.4
(51.4)
(32.0)
(82.4)
(11.3)
2005
(975.7)
(18.3)
5.9
(4.6)
(26.7)
(93.3)
(10.2)
2006
(900.3)
(19.1)
5.9
(4.6)
(26.7)
(95.5)
(10.4)
2007
(910.1)
(19.7)
5.9
(4.7)
(26.7)
(97.6)
(9.8)
Total (841.4) (851.0) (717.5) (1,122.7) (1,050.5) (1,062.6)
Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
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 2007, and increase of 13 percent relative to 1990. The application of synthetic fertilizers to forest and
settlement soils in 2007 resulted in direct N2O emissions of 1.6 Tg CO2 Eq. Direct N2O emissions from fertilizer
application increased by approximately 61 percent between 1990 and 2007. Non-CO2 emissions from forest fires in
2007 resulted in CH4 emissions of 29.0 Tg CO2 Eq., and in N2O emissions of 2.9 Tg CO2 Eq. CO2 and N2O
emissions from peatlands totaled 1.0 Tg CO2 Eq. and less than 0.01 Tg CO2 Eq. in 2007, respectively.
Table ES-6. Emissions from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.)
Source Category
C02
Cropland Remaining Cropland:
Liming of Agricultural Soils
Cropland Remaining Cropland:
Urea Fertilization
Wetlands Remaining Wetlands:
Peatlands Remaining Peatlands
CH4
Forest Land Remaining Forest Land:
Forest Fires
N2O
Forest Land Remaining Forest Land:
Forest Fires
Forest Land Remaining Forest Land:
Forest Soils
Wetlands Remaining Wetlands:
Peatlands Remaining Peatlands
Settlements Remaining Settlements:
Settlement Soils
1990
8.1
4.7
2.4
1.0
4.6
4.6
1.5
0.5
0.0
+
1.0
1995
8.1
4.4
2.7
1.0
6.1
6.1
2.0
0.6
0.1
+
1.2
2000
8.8
4.3
3.2
1.2
20.6
20.6
3.6
2.1
0.3
+
1.2
2005
8.9
4.3
3.5
1.1
14.2
14.2
3.3
1.4
0.3
+
1.5
2006
8.8
4.2
3.7
0.9
31.3
31.3
5.0
3.2
0.3
+
1.5
2007
9.0
4.1
4.0
1.0
29.0
29.0
4.9
2.9
0.3
+
1.6
Total 14.2 16.2 33.0 26.4 45.1 42.9
+ Less than 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
Waste
The Waste chapter contains emissions from waste management activities (except incineration of waste, which is
addressed in the Energy chapter). Landfills were the largest source of anthropogenic CH4 emissions in the Waste
Executive Summary ES-13
-------
chapter, accounting for 23 percent of total U.S. CH4 emissions.13 Additionally, wastewater treatment accounts for 4
percent of U.S. CH4 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 grew from 1990 to 2007, and resulted in emissions of 1.7 Tg CO2 Eq. and 1.8 Tg CO2 Eq.,
respectively. Overall, in 2007, emission sources accounted for in the Waste chapter generated 2.3 percent of total
U.S. greenhouse gas emissions.
Other Information
Emissions by Economic Sector
Throughout the Inventory of U.S. Greenhouse Gas Emissions and Sinks report, emission estimates are grouped into
six sectors (i.e., chapters) defined by the IPCC: Energy; Industrial Processes; Solvent Use; Agriculture; Land Use,
Land-Use Change, and Forestry; and Waste. While it is important to use this characterization for consistency with
UNFCCC reporting guidelines, it is also useful to allocate emissions into more commonly used sectoral categories.
This section reports emissions by the following economic sectors: Residential, Commercial, Industry,
Transportation, Electricity Generation, Agriculture, and U.S. Territories.
Table ES-7 summarizes emissions from each of these sectors, and Figure ES-13 shows the trend in emissions by
sector from 1990 to 2007.
Figure ES-13: Emissions Allocated to Economic Sectors
Table ES-7: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq.)
Implied Sectors
Electric Power Industry
Transportation
Industry
Agriculture
Commercial
Residential
U.S. Territories
Total Emissions
Land Use, Land-Use Change, and
Forestry (Sinks)
Net Emissions (Sources and Sinks)
1990
1,859.1
1,543.6
1,496.0
428.5
392.9
344.5
34.1
6,098.7
(841.4)
5,257.3
1995
1,989.0
1,685.2
1,524.5
453.7
401.0
368.8
41.1
6,463.3
(851.0)
5,612.3
2000
2,329.3
1,919.7
1,467.5
470.2
388.2
386.0
47.3
7,008.2
(717.5)
6,290.7
2005
2,429.4
1,998.9
1,364.9
482.6
401.8
370.5
60.5
7,108.6
(1,122.7)
5,985.9
2006
2,375.5
1,994.4
1,388.4
502.9
392.6
334.9
62.3
7,051.1
(1,050.5)
6,000.6
2007
2,445.1
1,995.2
1,386.3
502.8
407.6
355.3
57.7
7,150.1
(1,062.6)
6,087.5
Note: Totals may not sum due to independent rounding. Emissions include CO2, CH4, N2O, HFCs, PFCs, and SF6.
See Table 2-12 for more detailed data.
Using this categorization, emissions from electricity generation accounted for the largest portion (34 percent) of
U.S. greenhouse gas emissions in 2007. Transportation activities, in aggregate, accounted for the second largest
portion (28 percent). Emissions from industry accounted for 20 percent of U.S. greenhouse gas emissions in 2007.
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 18 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 7 percent of U.S. emissions; unlike other economic sectors, agricultural sector emissions were
dominated by N2O emissions from agricultural soil management and CH4 emissions from enteric fermentation,
13 Landfills also store carbon, due to incomplete degradation of organic materials such as wood products and yard trimmings, as
described in the Land-Use, Land-Use Change, and Forestry chapter of the Inventory report.
ES-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
rather than CO2 from fossil fuel combustion. The commercial sector accounted for about 6 percent of emissions,
while U.S. territories accounted for approximately 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.
Electricity is ultimately consumed in the economic sectors described above. Table ES-8 presents greenhouse gas
emissions from economic sectors with emissions related to electricity generation distributed into end-use categories
(i.e., emissions from electricity generation are allocated to the economic sectors in which the electricity is
consumed). To distribute electricity emissions among end-use sectors, emissions from the source categories
assigned to electricity generation were allocated to the residential, commercial, industry, transportation, and
agriculture economic sectors according to retail sales of electricity.14 These source categories include CO2 from
fossil fuel combustion and the use of limestone and dolomite for flue gas desulfurization, CO2 and N2O from
incineration of waste, CH4 and N2O from stationary sources, and SF6 from electrical transmission and distribution
systems.
When emissions from electricity are distributed among these sectors, industry accounts for the largest share of U.S.
greenhouse gas emissions (30 percent) in 2007. 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 2007.
Table ES-8: U.S Greenhouse Gas Emissions by Economic Sector with Electricity-Related Emissions Distributed
(TgC02Eq.)
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
2,166.5
1,546.7
942.2
950.0
459.2
34.1
6,098.7
(841.4)
5,257.3
1995
2,219.8
1,688.3
1,000.2
1,024.2
489.7
41.1
6,463.3
(851.0)
5,612.3
2000
2,235.5
1,923.2
1,140.0
1,159.2
503.2
47.3
7,008.2
(717.5)
6,290.7
2005
2,081.2
2,003.6
1,214.6
1,237.0
511.7
60.5
7,108.6
(1,122.7)
5,985.9
2006
2,082.3
1,999.0
1,201.5
1,176.1
530.0
62.3
7,051.1
(1,050.5)
6,000.6
2007
2,081.2
2,000.1
1,251.2
1,229.8
530.1
57.7
7,150.1
(1,062.6)
6,087.5
See Table 2-14 for more detailed data.
Figure ES-14: Emissions with Electricity Distributed to Economic Sectors
[BEGIN BOX]
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
14 Emissions were not distributed to U.S. territories, since the electricity generation sector only includes emissions related to the
generation of electricity in the 50 states and the District of Columbia.
Executive Summary ES-15
-------
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 2007; (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
Population"1
Greenhouse Gas Emissions6
1990
100
100
100
100
100
100
1995
113
112
107
108
107
106
2000
138
127
117
117
113
115
2005
155
134
119
119
118
117
2006
159
135
117
118
119
115
2007
162
137
119
120
120
117
Growth
Rate3
2.9%
1.9%
1.1%
1.1%
1.1%
0.9%
Average annual growth rate
b Gross Domestic Product in chained 2000 dollars (BEA 2008)
0 Energy content-weighted values (EIA 2008a)
d U.S. Census Bureau (2008)
e GWP-weighted values
Figure ES-15: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product
Source: BEA (2008), U.S. Census Bureau (2008), and emission estimates in this report.
[END BOX]
Indirect Greenhouse Gases (CO, NOX, NMVOCs, and S02)
The reporting requirements of the UNFCCC request that information be provided on indirect greenhouse gases,
which include CO, NOX, NMVOCs, and SO2. These gases do not have a direct global warming effect, but indirectly
affect terrestrial radiation absorption by influencing the formation and destruction of tropospheric and stratospheric
ozone, or, in the case of SO2, by affecting the absorptive characteristics of the atmosphere. Additionally, some of
these gases may react with other chemical compounds in the atmosphere to form compounds that are greenhouse
gases.
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.
Table ES-10: Emissions of NOX, CO, NMVOCs, and SO2 (Gg)
Gas/Activity
1990
1995
2000
2005
2006
2007
15 See .
16 NOX and CO emission estimates from field burning of agricultural residues were estimated separately, and therefore not taken
from EPA (2008).
ES-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
NOX
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Oil and Gas Activities
Incineration of Waste
Agricultural Burning
Solvent Use
Waste
CO
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel Combustion
Industrial Processes
Incineration of Waste
Agricultural Burning
Oil and Gas Activities
Waste
Solvent Use
NMVOCs
Mobile Fossil Fuel Combustion
Solvent Use
Industrial Processes
Stationary Fossil Fuel Combustion
Oil and Gas Activities
Incineration of Waste
Waste
Agricultural Burning
S02
Stationary Fossil Fuel Combustion
Industrial Processes
Mobile Fossil Fuel Combustion
Oil and Gas Activities
Incineration of Waste
Waste
Solvent Use
Agricultural Burning
21,450
10,920
9,689
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
21,070
10,622
9,619
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
19,004
10,310
7,802
626
111
114
35
o
J
2
92,776
83,559
4,340
2,216
1,670
792
146
8
45
15,227
7,229
4,384
1,773
1,077
388
257
119
NA
14,830
12,849
1,031
632
287
29
1
1
NA
15,612
8,757
5,857
534
321
98
39
5
2
71,672
62,519
4,778
1,744
1,439
860
324
7
2
14,562
6,292
3,881
2,035
1,450
545
243
115
NA
13,348
11,641
852
600
233
22
1
0
NA
14,701
8,271
5,445
527
316
98
38
5
2
67,453
58,322
4,792
1,743
1,438
825
323
7
2
14,129
5,954
3,867
1,950
1,470
535
239
113
NA
12,259
10,650
845
520
221
22
1
0
NA
14,250
7,831
5,445
520
314
97
37
5
2
63,875
54,678
4,792
1,743
1,438
892
323
7
2
13,747
5,672
3,855
1,878
1,470
526
234
111
NA
11,725
10,211
839
442
210
22
1
0
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.
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 2007 emission estimates for the key categories as defined by a level analysis (i.e., the
17 See Chapter 7 "Methodological Choice and Recalculation" in IPCC (2000).
Executive Summary ES-17
-------
contribution of each source or sink category to the total inventory level). The UNFCCC reporting guidelines request
that key category analyses be reported at an appropriate level of disaggregation, which may lead to source and sink
category names which differ from those used elsewhere in the inventory report. For more information regarding key
categories, see section 1.5 and Annex 1 of the inventory report.
Figure ES-16: 2007 Key Categories
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 emissions are generated increases the uncertainty associated with the estimates
presented. Acquiring a better understanding of the uncertainty associated with inventory estimates is an important
step in helping to prioritize future work and improve the overall quality of 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.
ES-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
I MFCs, PFCs, & SF.
Nitrous Oxide
Methane
i Carbon Dioxide
6,099 6,054 6,156
6,288
6,395 6,463
6 673 6,727 6,769 6,822 7.008 6,896 6,942 6,981 7,065 7,109 7,051 7,150
8,000
7,000
6,000 -
5,000
S
tf 4,000
u
.01
I- 3,000 -
2,000
1,000 -
0
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure ES-1: U.S. Greenhouse Gas Emissions by Gas
3.2%
2.7%
1.4%
-0.8%
-2% J
-1.6%
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure ES-2: Annual Percent Change in U.S. Greenhouse Gas Emissions
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure ES-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 1990
-------
MFCs, PFCs, & SF6
N2O
CH4
2.1%
4.4%
8.2%
CO,
85.4%
Figure ES-4: 2007 Greenhouse Gas Emissions by Gas (percents based on Tg CO2 Eq.)
Fossil Fuel Combustion
Non-Energy Use of Fuels
Iron and Steel Production & Metallurgical Coke Production
Cement Production
Natural Gas Systems
Incineration of Waste
Lime Production
Ammonia Production and Urea Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production and Consumption
5,735.8
C02 as a Portion
of all Emissions
< 0.5
< 0.5
< 0.5
0 25
50 75 100
Tg CO2 Eq.
125 150
Figure ES-5: 2007 Sources of CO2 Emissions
-------
i Natural Gas
Petroleum
I Coal
Figure ES-6: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type
Note: Electricity generation also includes emissions of less than 0.5 Tg CO 2 Eq. from geothermal-based electricity generation.
2,500 -,
2,000 -
ff 1-500
| 1,000 ^
500
0 J
From Electricity Consumption
• From Direct Fossil Fuel Combustion
8-
Figure ES-7: 2007 End-Use Sector Emissions of CO2, CH4, and N2O from Fossil Fuel Combustion
-------
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Forest Land Remaining Forest Land
Petroleum Systems
Wastewater Treatment
Stationary Combustion |
Rice Cultivation |
Abandoned Underground Coal Mines |
Mobile Combustion |
Composting
Petrochemical Production
Field Burning of Agricultural Residues
Iron and Steel Production & Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and Consumption
CH4 as a Portion
of all Emissions
8.2%
0
< 0.5
< 0.5
Figure ES-8: 2007 Sources of CH4 Emissions
so 100
Tg CO2 Eq.
150
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Adipic Acid Production ^^B
Wastewater Treatment ^^|
N2O from Product Uses ^B
Forest Land Remaining Forest Land ^|
Composting |
Settlements Remaining Settlements |
Field Burning of Agricultural Residues | < 0.5
Incineration of Waste | < 0.5
Wetlands Remaining Wetlands < 0.5
207.9
10
N2O as a Portion
of all Emissions
4.4%
20 30
Tg CO2 Eq.
40
Figure ES-9: 2007 Sources of N2O Emissions
-------
Substitution of Ozone Depleting Substances
HCFC-22 Production
Electrical Transmission and Distribution
Semiconductor Manufacture
Aluminum Production
108.3
MFCs, PFCs, and SF6 as a Portion
of all Emissions
2.1%
Magnesium Production and Processing
10
20 30
TgCO2Eq.
Figure ES-1 0: 2007 Sources of MFCs, PFCs, and SF6 Emissions
40
50
o
p
7,500
7,000
6,500
6,000
5,500
5,000
4,500
4,000 -
3,500 -
3,000 -
2,500
2,000
1,500
1,000
500
0
(500) -
(1,000) -
(1,500) -
Industrial Processes
Waste
LULUCF (sources)
Energy
Note: Relatively smaller amounts of GWP-weighted emissions are also emitted from the Solvent and Other Product Use
sectors
Figure ES-11: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector
-------
7% Renewable
8% Nuclear
22% Natural Gas
22% Coal
39% Petroleum
Figure ES-12: 2007 U.S. Energy Consumption by Energy Source
Electricity
Generation
Transportation
Industry
Agriculture
Commercial
Residential
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure ES-13: Emissions Allocated to Economic Sectors
Note: Does not include U.S. Territories.
-------
2,500 -,
2,000 -
o
1,000 -
500 -
Industrial
Transportation
Residential (gray)
Commercial (black)
Agriculture
O'-trMco'^-Ln^orxeoaiO'-irMCQ'^-Ln^orx
cncncncncncncncncncnoooooooo
22222222228°°°°°°°
Figure ES-14: Emissions with Electricity Distributed to Economic Sectors
Note: Does not include U.S. Territories.
170 -i
160 -
150 -
140 -
130 -
120 -
110 -
100 -
90 -
80 -
70 -
Real GDP
Population
Emissions per capita
Emissions per $GDP
O*-H(Nr*l^J-L/l^DrxCQCT*
8 S
a a
-------
C02 Emissions from Stationary Combustion - Coal
Mobile Combustion: Road & Other
C02 Emissions from Stationary Combustion - Gas
C02 Emissions from Stationary Combustion - Oil
Mobile Combustion: Aviation |
Direct N20 Emissions from Agricultural Soil Management ^1
CH4 Emissions from Enteric Fermentation |
C02 Emissions from Non-Energy Use of Fuels ^1
CH4 Emissions from Landfills ^|
Emissions from Substitutes for Ozone Depleting Substances |
Fugitive Emissions from Natural Gas Systems ^|
C02 Emissions from Iron & Steel and Metallurgical Coke Prod. |
Fugitive Emissions from Coal Mining |
Mobile Combustion: Marine |
C02 Emissions from Cement Production |
CH4 Emissions from Manure Management |
Indirect N20 Emissions from Applied Nitrogen |
Fugitive Emissions from Petroleum Systems
C02 Emissions from Natural Gas Systems
Non-C02 Emissions from Stationary Combustion
Key Categories as a Portion of all
Emissions
200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200
TgC02Eq.
Figure ES-16: 2007 Key Categories
Notes: For a complete discussion of the key source analysis, see Annex 1.
Black bars indicate a Tier 1 level assessment key category.
Gray bars indicate a Tier 2 level assessment key category.
-------
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 2007. 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.18 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."19'20
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.. ."21 The United States views this report as an opportunity
to fulfill these commitments under the UNFCCC.
In 1988, preceding the creation of the UNFCCC, the World Meteorological Organization (WMO) and the United
Nations Environment Programme (UNEP) jointly established the Intergovernmental Panel on Climate Change
(IPCC). The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis the scientific,
technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced
climate change, its potential impacts and options for adaptation and mitigation (IPCC 2003). Under Working Group
1 of the IPCC, nearly 140 scientists and national experts from more than thirty countries collaborated in the creation
of the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997) to
ensure that the emission inventories submitted to the UNFCCC are consistent and comparable between nations. The
IPCC accepted the Revised 1996 IPCC Guidelines at its Twelfth Session (Mexico City, September 11-13, 1996).
This report presents information in accordance with these guidelines. In addition, this Inventory is in accordance
with the IPCC Good Practice Guidance and Uncertainty Management 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 includes 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).
18 See the section below entitled Global Warming Potentials for an explanation of GWP values.
19 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).
20 Article 2 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate
Change. See . (UNEP/WMO 2000)
21 Article 4(l)(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
-------
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.22 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 ofradiatively
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.23 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).
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.
Table 1-1: Global Atmospheric Concentration, Rate of Concentration Change, and Atmospheric Lifetime (years) of
Selected Greenhouse Gases
Atmospheric Variable CO2 CH4 N2O SFg CF4
22 For more on the science of climate change, see NRC (2001).
23 Emissions 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: 1990-2007
-------
Pre-industrial atmospheric
concentration
Atmospheric concentration3
Rate of concentration change
Atmospheric lifetime0
278 ppm
379 ppm
1.4ppm/yr
50-200d
0.715 ppm
1.774 ppm
0.005 ppm/yra
12e
0.270 ppm
0.3 19 ppm
0.26%/yr
114e
Oppt
5.6 ppt
Linearb
3,200
40 ppt
74 ppt
Linearb
>50,000
Source: Pre-industrial atmospheric concentrations, current atmospheric concentrations, and rate of concentration changes for all
gases are from IPCC (2007).
a The growth rate for atmospheric CELt 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).
d No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes.
e This lifetime has been defined as an "adjustment time" that takes into account the indirect effect of the gas on its own residence
time.
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).2425 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 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. CH4 is also emitted during the
production and distribution of natural gas and petroleum, and is released as a by-product of coal mining and
incomplete fossil fuel combustion. Atmospheric concentrations of CH4 have increased by about 143 percent since
1750, from a pre-industrial value of about 722 ppb to 1,774 ppb in 2005, although the rate of increase has been
declining. The IPCC has estimated that slightly more than half of the current CH4 flux to the atmosphere is
anthropogenic, from human activities such as agriculture, fossil fuel use, and waste disposal (IPCC 2007).
24 The pre-industrial period is considered as the time preceding the year 1750 (IPCC 2001).
25 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
-------
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 incineration; and biomass burning. The atmospheric concentration of N2O has increased by 18
percent since 1750, from a pre-industrial value of about 270 ppb to 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,26 where it shields the Earth from harmful levels of
ultraviolet radiation, and at lower concentrations in the troposphere,27 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
[HFCs]) result in stratospheric ozone depletion and are therefore controlled under the Montreal Protocol on
Substances that Deplete the Ozone Layer. Although CFCs and HCFCs include potent global warming gases, their
net radiative forcing effect on the atmosphere is reduced because they cause stratospheric ozone depletion, which
itself is an important greenhouse gas in addition to shielding the Earth from harmful levels of ultraviolet radiation.
Under the Montreal Protocol, the United States phased out the production and importation of halons by 1994 and of
CFCs by 1996. Under the Copenhagen Amendments to the Protocol, a cap was placed on the production and
importation of HCFCs by non-Article 528 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 by-product of the HCFC-22 manufacturing process. Currently, they have a small
aggregate radiative forcing impact, but it is anticipated that their contribution to overall radiative forcing will
26 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.
27 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.
28 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 often
additional years in the phase-out of ozone depleting substances.
1-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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.29 Additionally, NOX emissions from aircraft are also likely to
decrease CH4 concentrations, thus having a negative radiative forcing effect (IPCC 1999). Nitrogen oxides are
created from lightning, soil microbial activity, biomass burning (both natural and anthropogenic fires) fuel
combustion, and, in the stratosphere, from the photo-degradation of N2O. Concentrations of NOX are both relatively
short-lived in the atmosphere and spatially variable.
Nonmethane Volatile Organic Compounds (NMVOCs). Non-CH4 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
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
carbonaceous30 aerosols (e.g., black carbon, organic carbon) from transportation, coal combustion, cement
manufacturing, waste incineration, and biomass burning.
The net effect of aerosols on radiative forcing is believed to be negative (i.e., net cooling effect on the climate),
although because they remain in the atmosphere for only days to weeks, their concentrations respond rapidly to
changes in emissions.31 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).
29 NOX emissions injected higher in the stratosphere, primarily from fuel combustion emissions from high altitude supersonic
aircraft, can lead to stratospheric ozone depletion.
30 Carbonaceous aerosols are aerosols that are comprised mainly of organic substances and forms of black carbon (or soot)
(IPCC 2001).
31 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).
Introduction 1-5
-------
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 absorbs radiation. Indirect radiative forcing occurs when chemical
transformations involving the original gas produce a gas or gases that are greenhouse gases, or when a gas
influences other radiatively important processes such as the atmospheric lifetimes of other gases. The reference gas
used is CO2, and therefore GWP weighted emissions are measured in teragrams of CO2 equivalent (Tg CO2 Eq.)32
The relationship between gigagrams (Gg) of a gas and Tg CO2 Eq. can be expressed as follows:
/ „ \
Tg CO 2 Eq = (Gg of gas) x (GWP);
.1,000 GgJ
where,
Tg CO2 Eq. = Teragrams of CO2 Equivalents
Gg = Gigagrams (equivalent to a thousand metric tons)
GWP = Global Warming Potential
Tg = Teragrams
GWP values allow for a comparison of the impacts of emissions and reductions of different gases. According to the
IPCC, GWPs typically have an uncertainty of ±35 percent. The parties to the UNFCCC have also agreed to use
GWPs based upon a 100-year time horizon although other time horizon values are available.
Greenhouse gas emissions and removals should be presented on a gas-by-gas basis in units of mass... In
addition, consistent with decision 2/CP.3, Parties should report aggregate emissions and removals of
greenhouse gases, expressed in 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.33
Greenhouse gases with relatively long atmospheric lifetimes (e.g., CO2, CH4, N2O, HFCs, PFCs, and SF6) tend to be
evenly distributed throughout the atmosphere, and consequently global average concentrations can be determined.
The short-lived gases such as water vapor, carbon monoxide, tropospheric ozone, ozone precursors (e.g., NOX, 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.
Table 1-2: Global Warming Potentials and Atmospheric Lifetimes (Years) Used in this Report
Gas Atmospheric Lifetime GWP"
CO2 50-200 1
32 Carbon comprises 12/44ths of carbon dioxide by weight.
33 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: 1990-2007
-------
CH4b
N2O
HFC-23
HFC-32
HFC-125
HFC-134a
HFC-143a
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
CF4
C2F6
C4F10
CsF^
SF6
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
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 CO2 is not included.
[BEGIN BOX]
Box 1-1: The IPCC Fourth Assessment Report and Global Warming Potentials
In 2007, the IPCC published its Fourth Assessment Report (AR4), which provided an updated and more
comprehensive scientific assessment of climate change. Within this report, the GWPs of several gases were revised
relative to the SAR and the IPCC's Third Assessment Report (TAR) (IPCC 2001). Thus the GWPs used in this
report have been updated twice by the IPCC; although the SAR GWPs are used throughout this report, it is
interesting to review the changes to the GWPs and the impact such improved understanding has on the total GWP-
weighted emissions of the United States. Since the SAR and TAR, the IPCC has applied an improved calculation of
CO2 radiative forcing and an improved CO2 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*
N2O
HFC-23
HFC-32
HFC-125
HFC-134a
HFC-143a
HFC-152a
HFC-227ea
HFC-236fa
HFC-4310mee
SAR
1
21
310
11,700
650
2,800
1,300
3,800
140
2,900
6,300
1,300
TAR
1
23
296
12,000
550
3,400
1,300
4,300
120
3,500
9,400
1,500
AR4
1
25
298
14,800
675
3,500
1,430
4,470
124
3,220
9,810
1,640
Change from
SAR
TAR
NC
2
(14)
300
(100)
600
NC
500
(20)
600
3,100
200
AR4
0
4
(12)
3,100
25
700
130
670
(16)
320
3,510
340
Introduction 1-7
-------
CF4
C2F6
C4FJO
C6F14
SF6
6,500
9,200
7,000
7,400
23,900
5,700
11,900
8,600
9,000
22,200
7,390
12,200
8,860
9,300
22,800
(800)
2,700
1,600
1,600
(1,700)
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 CO2 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 inventories34 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.
[END BOX]
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.
34
See.
1-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 throughout the NIR and CRF tables.
Emission calculations for individual sources are the responsibility of individual source leads, who are most familiar
with each source category and the unique characteristics of its emissions profile. The individual source leads
determine the most appropriate methodology and collect the best activity data to use in the emission calculations,
based upon their expertise in the source category, as well as coordinating with researchers and contractors familiar
with the sources. A multi-stage process for collecting information from the individual source leads and producing
the Inventory is undertaken annually to compile all information and data.
Methodology Development, Data Collection, and Emissions and Sink Estimation
Source leads at EPA collect input data and, as necessary, evaluate or develop the estimation methodology for the
individual source categories. For most source categories, the methodology for the previous year is applied to the
new "current" year of the Inventory, and inventory analysts collect any new data or update data that have changed
from the previous year. If estimates for a new source category are being developed for the first time, or if the
methodology is changing for an existing source category (e.g., the United States is implementing a higher Tiered
approach for that source category), then the source category lead will develop a new methodology, gather the most
appropriate activity data and emission factors (or in some cases direct emission measurements) for the entire time
series, and conduct a special source-specific peer review process involving relevant experts from industry,
government, and universities.
Once the methodology is in place and the data are collected, the individual source leads calculate emissions and sink
estimates. The source leads then update or create the relevant text and accompanying annexes for the Inventory.
Source leads are also responsible for completing the relevant sectoral background tables of the Common Reporting
Format, conducting quality assurance and quality control (QA/QC) checks, and uncertainty analyses.
Summary Spreadsheet Compilation and Data Storage
The inventory coordinator at EPA collects the source categories' descriptive text and Annexes, and also aggregates
the emission estimates into a summary spreadsheet that links the individual source category spreadsheets together.
This summary sheet contains all of the essential data in one central location, in formats commonly used in the
Inventory document. In addition to the data from each source category, national trend and related data are also
gathered in the summary sheet for use in the Executive Summary, Introduction, and Recent Trends sections of the
Inventory report. Electronic copies of each year's summary spreadsheet, which contains all the emission and sink
estimates for the United States, are kept on a central server at EPA under the jurisdiction of the Inventory
coordinator.
National Inventory Report Preparation
The NIR is compiled from the sections developed by each individual source lead. In addition, the inventory
coordinator prepares a brief overview of each chapter that summarizes the emissions from all sources discussed in
the chapters. The inventory coordinator then carries out a key category analysis for the Inventory, consistent with
the IPCC Good Practice Guidance, IPCC Good Practice Guidance for Land Use, Land Use Change and Forestry,
and in accordance with the reporting requirements of the UNFCCC. Also at this time, the Introduction, Executive
Summary, and Recent Trends sections are drafted, to reflect the trends for the most recent year of the current
Inventory. The analysis of trends necessitates gathering supplemental data, including weather and temperature
conditions, economic activity and gross domestic product, population, atmospheric conditions, and the annual
consumption of electricity, energy, and fossil fuels. Changes in these data are used to explain the trends observed in
greenhouse gas emissions in the United States. Furthermore, specific factors that affect individual sectors are
researched and discussed. Many of the factors that affect emissions are included in the Inventory document as
separate analyses or side discussions in boxes within the text. Text boxes are also created to examine the data
aggregated in different ways than in the remainder of the document, such as a focus on transportation activities or
emissions from electricity generation. The document is prepared to match the specification of the UNFCCC
reporting guidelines for National Inventory Reports.
Introduction 1-9
-------
Common Reporting Format Table Compilation
The CRF tables are compiled from individual tables completed by each individual source lead, which contain source
emissions and activity data. The inventory coordinator integrates the source data into the UNFCCC's "CRF
Reporter" for the United States, assuring consistency across all sectoral tables. The summary reports for emissions,
methods, and emission factors used, the overview tables for completeness and quality of estimates, the recalculation
tables, the notation key completion tables, and the emission trends tables are then completed by the inventory
coordinator. Internal automated quality checks on the CRF Reporter, as well as reviews by the source leads, are
completed for the entire time series of CRF tables before submission.
QA/QC and Uncertainty
QA/QC and uncertainty analyses are supervised by the QA/QC and Uncertainty coordinators, who have general
oversight over the implementation of the QA/QC plan and the overall uncertainty analysis for the Inventory (see
sections on QA/QC and Uncertainty, below). These coordinators work closely with the source leads to ensure that a
consistent QA/QC plan and uncertainty analysis is implemented across all inventory sources. The inventory QA/QC
plan, detailed in a following section, is consistent with the quality assurance procedures outlined by EPA and IPCC.
Expert and Public Review Periods
During the Expert Review period, a first draft of the document is sent to a select list of technical experts outside of
EPA. The purpose of the Expert Review is to encourage feedback on the methodological and data sources used in
the current Inventory, especially for sources which have experienced any changes since the previous Inventory.
Once comments are received and addressed, a second draft of the document is released for public review by
publishing a notice in the U.S. Federal Register and posting the document on the EPA Web site. The Public Review
period allows for a 30 day comment period and is open to the entire U.S. public.
Final Submittal to UNFCCC and Document Printing
After the final revisions to incorporate any comments from the Expert Review and Public Review periods, EPA
prepares the final National Inventory Report and the accompanying Common Reporting Format Reporter database.
The U.S. Department of State sends the official submission of the U.S. Inventory to the UNFCCC. The document is
then formatted for printing, posted online, printed by the U.S. Government Printing Office, and made available for
the public.
1.4. Methodology and Data Sources
Emissions of greenhouse gases from various source and sink categories have been estimated using methodologies
that are consistent with the Revised 1996 IPCC 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 fora
variety of source categories, and many of these methodologies continue to be improved and refined as new research
and data become available. This report uses the IPCC methodologies when applicable, and supplements them with
other available methodologies and data where possible. Choices made regarding the methodologies and data
sources used are provided in conjunction with the discussion of each source category in the main body of the report.
Complete documentation is provided in the annexes on the detailed methodologies and data sources utilized in the
calculation of each source category.
[BEGIN BOX]
1-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Box 1-2: IPCC Reference Approach
The UNFCCC reporting guidelines require countries to complete a "top-down" reference approach for estimating
CO2 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.
[END BOX]
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."35
By definition, key categories include those sources that have the greatest contribution to the absolute level of
national emissions. In addition, when an entire time series of emission estimates is prepared, a thorough
investigation of key categories must also account for the influence of trends and uncertainties of individual source
and sink categories. This analysis culls out source and sink categories that diverge from the overall trend in national
emissions. Finally, a qualitative evaluation of key categories is performed to capture any categories that were not
identified in 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. Following the Tier 1 approach, a Tier 2 approach, as defined in the IPCC's Good Practice
Guidance (IPCC 2000), was then implemented to identify any additional key categories not already identified in the
Tier 1 assessment. This analysis, which includes each source cateogories' uncertainty assessments in its calculations,
was also performed twice to include or exclude LULUCF sources.
In addition to conducting Tier 1 and 2 level and trend assessments, a qualitative assessment of the source categories,
as described in the IPCC's Good Practice Guidance (IPCC 2000), was conducted to capture any key categories that
were not identified by either quantitative method. One additional key category, international bunker fuels, was
identified using this qualitative assessment. International bunker fuels are fuels consumed for aviation or marine
international transport activities, and emissions from these fuels are reported separately from totals in accordance
with IPCC guidelines. If these emissions were included in the totals, bunker fuels would qualify as a key category
according to the Tier 1 approach. The amount of uncertainty associated with estimation of emissions from
international bunker fuels also supports the qualification of this source category as key, which would qualify it as a
key category according to the Tier 2 approach.
Table 1-4 presents the key categories for the United States (including and excluding LULUCF categories) using
emissions and uncertainty data in this report, and ranked according to their sector and global warming potential-
weighted emissions in 2007. The table also indicates the criteria used in identifying these categories (i.e., level,
trend, Tier 1, Tier 2, and/or qualitative assessments). Annex 1 of this report provides additional information
35 See Chapter 7 "Methodological Choice and Recalculation" in IPCC (2000).
Introduction 1-11
-------
regarding the key categories in the United States and the methodologies used to identify them.
Table 1-4: Key Categories for the United States (1990-2007)
IPCC Source Categories
Energy
CO2 Emissions from
Stationary Combustion -
Coal
Mobile Combustion: Road
& Other
CO2 Emissions from
Stationary Combustion -
Gas
CO2 Emissions from
Stationary Combustion -
Oil
Mobile Combustion:
Aviation
CO2 Emissions from Non-
Energy Use of Fuels
Mobile Combustion: Marine
CO2 Emissions from Natural
Gas Systems
CO2 Emissions from
Incineration of Waste
Fugitive Emissions from
Natural Gas Systems
Fugitive Emissions from
Coal Mining
Fugitive Emissions from
Petroleum Systems
Non-CO2 Emissions from
Stationary Combustion
Mobile Combustion: Road
& Other
Non-CO2 Emissions from
Stationary Combustion
International Bunker Fuels'3
Industrial Processes
CO2 Emissions from Iron
and Steel Production &
Metallurgical Coke
Production
CO2 Emissions from
Cement Production
CO2 Emissions from
Ammonia Production and
Urea Consumption
N2O Emissions from Adipic
Acid Production
Emissions from Substitutes
for Ozone Depleting
Substances
Gas
C02
C02
C02
C02
C02
C02
C02
C02
C02
cm
CH4
CH4
CH4
N20
N20
Several
C02
C02
C02
N20
Several
Tierl
Level Trend Level Trend
Without Without With With
LULUCF LULUCF LULUCF LULUCF
• • • •
• • • •
• • • •
• • • •
• • • •
• •
• •
• • • •
• •
• • • •
• • • •
• • • •
• • • •
• • • •
• •
• •
• •
• • • •
Tier 2
Level Trend Level Trend
Without Without With With
LULUCF LULUCF LULUCF LULUCF
• • • •
• • • •
• • •
• • • •
• •
• •
• • • •
• • • •
• • • •
• • • •
•
• •
• •
• • • •
• •
Quala
•
2007
Emissions
Tg C02 Eq.)
2,086.5
1,649.1
1,181.1
580.4
187.5
133.9
50.8
28.7
20.8
104.7
57.6
28.8
6.6
27.9
14.7
109.9
77.4
44.5
13.8
5.9
108.3
1-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
IPCC Source Categories
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
CH4 Emissions from Rice
Cultivation
Direct N2O Emissions from
Agricultural Soil
Management
Indirect N2O Emissions
from Applied Nitrogen
Waste
CH4 Emissions from
Landfills
CH4 Emissions from
Wastewater Treatment
Land Use, Land Use
Change, and Forestry
CO2 from Changes in Forest
Carbon Stocks
CO2 Emissions from Urban
Trees
CO2 Emissions from
Cropland Remaining
Cropland
CO2 Emissions from
Landfilled Yard Trimmings
and Food Scraps
CO2 Emissions from
Grassland Remaining
Grassland
CH4 Emissions from Forest
Fires
N2O Emissions from Forest
Fires
Subtotal Without
LULUCF
Total Emissions Without
LULUCF
Percent of Total Without
LULUCF
Subtotal With LULUCF
Total Emissions With
LULUCF
Gas
MFCs
SF6
PFCs
cm
CH4
CH4
N20
N20
CH4
CH4
C02
C02
C02
C02
C02
CH4
N20
Tierl
Level Trend Level Trend
Without Without With With
LULUCF LULUCF LULUCF LULUCF
• • • •
• •
• •
• • • •
• • • •
• • • •
• • • •
• • • •
• •
• •
•
•
• •
• •
Tier 2
Level Trend Level Trend
Without Without With With
LULUCF LULUCF LULUCF LULUCF
•
• •
• • • •
• •
• • • •
• • • •
• • • •
•
• •
• •
• •
• •
• •
• •
•
Quala
2007
Emissions
Tg C02 Eq.)
17.0
12.7
3.8
139.0
44.0
6.2
172.0
35.9
132.9
24.4
-910.1
-97.6
-11.6
-9.8
-4.7
29.0
2.9
6,972.3
7,107.2
98%
5,991.9
6,087.5
Introduction 1-13
-------
IPCC Source Categories
Percent of Total With
LULUCF
Gas
Tierl
Level Trend Level Trend
Without Without With With
LULUCF LULUCF LULUCF LULUCF
Tier 2
Level Trend Level Trend
Without Without With With
LULUCF LULUCF LULUCF LULUCF
Quala
2007
Emissions
Tg C02 Eq.)
98%
Qualitative criteria.
bEmissions from this source not included in totals.
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;
• 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
1-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 1996 IPCC
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.
Additional research in the following areas could help reduce uncertainty in the U.S. Inventory:
• Incorporating excluded emission sources. Quantitative estimates for some of the sources and sinks of
greenhouse gas emissions are not available at this time. In particular, emissions from some land-use
activities and industrial processes are not included in the inventory either because data are incomplete or
because methodologies do not exist for estimating emissions from these source categories. See Annex 5 of
this report for a discussion of the sources of greenhouse gas emissions and sinks excluded from this report.
• Improving the accuracy of emission factors. Further research is needed in some cases to improve the
accuracy of emission factors used to calculate emissions from a variety of sources. For example, the
accuracy of current emission factors applied to CH4 and N2O emissions from stationary and mobile
combustion is highly uncertain.
• Collecting detailed activity data. Although methodologies exist for estimating emissions for some sources,
problems arise in obtaining activity data at a level of detail in which aggregate emission factors can be
applied. For example, the ability to estimate emissions of SF6 from electrical transmission and distribution
is limited due to a lack of activity data regarding national SF6 consumption or average equipment leak
rates.
The overall uncertainty estimate for the U.S. greenhouse gas emissions inventory was developed using the IPCC
Tier 2 uncertainty estimation methodology. 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.
Table 1-5. Estimated Overall Inventory Quantitative Uncertainty (Tg CO2 Eq. and Percent)
Gas
2007 Emission
Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Standard
Estimate" Meanb Deviation
(Tg C02 Eq.) (%) (Tg C02 Eq.)
Lower Upper Lower Upper
Bound0 Bound0 Bound0 Bound0
CO2 6,103.4 5,974.9 6,390.0 -2% 5% 6,181.5 106.8
Introduction 1-15
-------
CH4
N2O
PFC, HFC & SF6d
Total
Net Emissions (Sources
and Sinks)
585.3
311.9
149.5
7,150.1
6,087.5
527.0
278.7
141.6
7,047.8
5,917.7
689.0
440.6
160.3
7,525.1
6,503.9
-10%
-11%
-5%
-1%
-3%
18%
41%
7%
5%
7%
599.3
352.4
148.1
7,281.3
6,205.6
41.3
42.8
4.7
121.9
150.1
Notes:
a The emission estimates correspond to 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.
0 The low and high estimates for total emissions were separately calculated through simulations and, hence, the low and high
emission estimates for the sub-source categories do not sum to total emissions.
d The overall uncertainty estimate did not take into account the uncertainty in the GWP values for CH4, N2O and high GWP gases
used in the inventory emission calculations for 2007.
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.
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 1996 IPCC 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.
Table 1-6: IPCC Sector Descriptions
Chapter/IPCC Sector Activities Included
Energy Emissions of all greenhouse gases resulting from stationary and
mobile energy activities including fuel combustion and fugitive fuel
emissions.
Industrial Processes By-product or fugitive emissions of greenhouse gases from
industrial processes not directly related to energy activities such as
fossil fuel combustion.
Solvent and Other Product Use Emissions, of primarily NMVOCs, resulting from the use of
solvents and N2O from product uses.
Agriculture Anthropogenic emissions from agricultural activities except fuel
combustion, which is addressed under Energy.
Land Use, Land-Use Change, Emissions and removals of CO2, CH4, and N2O from forest
and Forestry management, other land-use activities, and land-use change.
Waste Emissions from waste management activities.
Source: (IPCC/UNEP/OECD/IEA 1997)
1-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Within each chapter, emissions are identified by the anthropogenic activity that is the source or sink of the
greenhouse gas emissions being estimated (e.g., coal mining). Overall, the following organizational structure is
consistently applied throughout this report:
ChaptGlVIPCC S GCtO r: Overview of emission trends for each IPCC defined sector
Source CStegory. 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. For example, each energy consuming end-use sector (i.e.,
residential, commercial, industrial, and transportation), as well as the electricity generation sector, is described
individually. Additional information for certain source categories and other topics is also provided in several
Annexes listed in Table 1-7.
Table 1-7: List of Annexes
ANNEX 1 Key Category Analysis
ANNEX 2 Methodology and Data for Estimating CO2 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 CH4, 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
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 CO2 Emissions from Petroleum Systems
3.6. Methodology for Estimating CO2 and N2O Emissions from Incineration of Waste
3.7. Methodology for Estimating Emissions from International Bunker Fuels used by the U.S. Military
3.8. Methodology for Estimating HFC and PFC Emissions from Substitution of Ozone Depleting
Substances
3.9. Methodology for Estimating CH4 Emissions from Enteric Fermentation
3.10. Methodology for Estimating CH4 and N2O Emissions from Manure Management
3.11. Methodology for Estimating N2O Emissions from Agricultural Soil Management
3.12. Methodology for Estimating Net Carbon Stock Changes in Forest Lands Remaining Forest Lands
3.13. Methodology for Estimating Net Changes in Carbon Stocks in Mineral and Organic Soils on
Croplands and Grasslands
3.14. Methodology for Estimating CH4 Emissions from Landfills
ANNEX 4 IPCC Reference Approach for Estimating CO2 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
Introduction 1-17
-------
6.2. Ozone Depleting Substance Emissions
6.3. Sulfur Dioxide Emissions
6.4. Complete List of Source Categories
6.5. Constants, Units, and Conversions
6.6. Abbreviations
6.7. Chemical Formulas
ANNEX 7 Uncertainty
7.1. Overview
7.2. Methodology and Results
7.3. Planned Improvements
7.4. Additional Information on Uncertainty Analyses by Source
1-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
2. Trends in Greenhouse Gas Emissions
2.1. Recent Trends in U.S. Greenhouse Gas Emissions
In 2007, total U.S. greenhouse gas emissions were 7,150.1 teragrams of carbon dioxide equivalents (Tg CO2 Eq.).36
Overall, total U.S. emissions have risen by 17 percent from 1990 to 2007. Emissions increased from 2006 to 2007
by 1.4 percent (99.0 Tg CO2 Eq.). The following factors were primary contributors to this increase: (1) cooler
winter and warmer summer conditions in 2007 than in 2006 increased the demand for heating fuels and contributed
to the increase in the demand for electricity, (2) increased consumption of fossil fuels to generate electricity and (3)
a significant decrease (14.2 percent) in hydropower generation used to meet this demand.
Figure 2-1: U.S. Greenhouse Gas Emissions by Gas
Figure 2-2: Annual Percent Change in U.S. Greenhouse Gas Emissions
Figure 2-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 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 percent in 2007. Emissions from
this source category grew by 21.8 percent (1,026.9 Tg CO2 Eq.) from 1990 to 2007 and were responsible for most of
the increase in national emissions during this period. From 2006 to 2007, these emissions increased by 1.8 percent
(100.4 Tg CO2 Eq.). Historically, changes in emissions from fossil fuel combustion have been the dominant factor
affecting U.S. emission trends.
Changes in CO2 emissions from fossil fuel combustion are influenced by many long-term and short-term factors,
including population and economic growth, energy price fluctuations, technological changes, and seasonal
temperatures. On an annual basis, the overall consumption of fossil fuels in the United States generally fluctuates in
response to changes in general economic conditions, energy prices, weather, and the availability of non-fossil
alternatives. For example, in a year with increased consumption of goods and services, low fuel prices, severe
summer and winter weather conditions, nuclear plant closures, and lower precipitation feeding hydroelectric dams,
there would likely be proportionally greater fossil fuel consumption than in a year with poor economic performance,
high fuel prices, mild temperatures, and increased output from nuclear and hydroelectric plants.
In the longer-term, energy consumption patterns respond to changes that affect the scale of consumption (e.g.,
population, number of cars, and size of houses), the efficiency with which energy is used in equipment (e.g., cars,
power plants, steel mills, and light bulbs) and consumer behavior (e.g., walking, bicycling, or telecommuting to
work instead of driving).
Energy-related CO2 emissions also depend on the type of fuel or energy consumed and its carbon (C) intensity.
Producing a unit of heat or electricity using natural gas instead of coal, for example, can reduce the CO2 emissions
because of the lower C content of natural gas.
Emissions from fuel combustion increased in 2003 at about the average annual growth rate since 1990 (1.4 percent).
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
36 Estimates are presented in units of teragrams of carbon dioxide equivalent (Tg CO2 Eq.), which weight each gas by its global
warming potential, or GWP, value. (See section on global warming potentials, Executive Summary.)
Trends in Greenhouse Gas Emissions 2-1
-------
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 continued to increase at about the average annual growth rate since 1990. A
primary reason behind this trend was strong growth in the U.S. economy and industrial production, particularly 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 increased
by 1.3 percent and .5.9 percent in 2006, respectively, 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 a 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.
After experiencing a decrease from 2005 to 2006, emissions from fuel combustion grew from 2006 to 2007 at a rate
slightly higher than the average growth rate since 1990. There were a number of factors contributing to this
increase. Unfavorable weather conditions in both the winter and summer resulted in an increase in consumption of
heating fuels, as well as an increase in the demand for electricity. This demand for electricity was met with an
increase in coal consumption of 1.8 percent, and with an increase in natural gas consumption of 10.3 percent. This
increase in fossil fuel consumption, combined with a 14.2 percent decrease in hydropower generation from 2006 to
2007, resulted in an increase in emissions in 2007. The increase in emissions from the residential and commercial
sectors is a result of increased electricity consumption due to warmer summer conditions and cooler winter
conditions compared to 2006. In addition to these unfavorable weather conditions, electricity prices remained
relatively stable compared to 2006, and natural gas prices decreased slightly. Emissions from the industrial sector
increased slightly compared to 2006 as a result of a 1.7 percent increase in industrial production and the increase in
fossil fuels used for electricity generation. Despite an overall decrease in electricity generation from renewable
energy in 2007 driven by decreases in hydropower generation, wind and solar generation increased significantly.
Overall, from 1990 to 2007, total emissions of CO2 increased by 1,026.7 Tg CO2 Eq. (20.2 percent), while CH4 and
N2O emissions decreased by 31.2 Tg CO2 Eq. (5.1 percent) and 3.1 Tg CO2 Eq. (1 percent) respectively. During the
same period, aggregate weighted emissions of HFCs, PFCs, and SF6 rose by 59 Tg CO2 Eq. (65.2 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 14.9 percent
2-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
of total emissions in 2007.
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.
Table 2-1: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Iron and Steel Production &
Metallurgical Coke Production
Cement Production
Natural Gas Systems
Incineration of Waste
Lime Production
Ammonia Production and Urea
Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and
Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production and
Consumption
Land Use, Land-Use Change, and
Forestry (Sink)"
Biomass — Woocf
International Bunker Fuelsb
Biomass — Ethanof
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Forest Land Remaining Forest
Land
Petroleum Systems
Wastewater Treatment
Stationary Combustion
Rice Cultivation
1990
5,076.7
4,708.9
1,809.7
1,484.5
834.2
337.7
214.5
28.3
117.0
109.8
33.3
33.7
10.9
11.5
16.8
7.1
5.1
6.8
4.1
2.2
1.2
1.4
2.2
1.5
1.0
0.9
0.4
0.3
0.4
(841.4)
215.2
114.3
4.2
616.6
133.2
149.2
129.6
84.1
30.4
4.6
33.9
23.5
7.4
7.1
1995
5,407.9
5,013.9
1,938.9
1,598.7
862.6
354.4
224.4
35.0
137.5
103.1
36.8
33.8
15.7
13.3
17.8
7.0
6.7
5.7
4.3
2.8
.5
.4
2.0
.5
.0
.0
0.3
0.3
0.3
(851.0)
229.1
101.6
7.7
615.8
143.6
144.3
132.6
67.1
34.5
6.1
32.0
24.8
7.1
7.6
2000
5,955.2
5,561.5
2,283.2
1,800.3
844.6
370.4
226.9
36.2
144.5
95.1
41.2
29.4
17.5
14.1
16.4
7.5
5.1
6.1
4.2
3.0
.8
.4
.9
.4
.2
.1
0.3
0.3
0.2
(717.5)
218.1
99. C
9.2
591.1
134.4
122.3
130.8
60.5
37.9
20.6
30.3
25.2
6.6
7.5
2005
6,090.8
5,723.5
2,381.0
1,881.5
828.0
358.0
221.8
53.2
138.1
73.2
45.9
29.5
19.5
14.4
12.8
7.9
6.8
4.1
4.2
2.8
.8
o
.J
.4
.4
.1
0.5
0.3
0.3
0.2
(1,122.7)
208.9
111.5
22.6
561.7
136.0
127.8
106.3
57.1
41.8
14.2
28.3
24.3
6.7
6.8
2006
6,014.9
5,635.4
2,327.3
1,880.9
844.5
321.9
206.0
54.8
145.1
76.1
46.6
29.5
19.8
15.1
12.3
7.9
8.0
3.8
4.2
2.6
1.9
1.7
1.5
1.2
0.9
0.5
0.3
0.3
0.2
(1,050.5)
209.9
110.5
30.5
582.0
138.2
130.4
104.8
58.4
41.9
31.3
28.3
24.5
6.3
5.9
2007
6,103.4
5,735.8
2,397.2
1,887.4
845.4
340.6
214.4
50.8
133.9
77.4
44.5
28.7
20.8
14.6
13.8
8.0
6.2
4.3
4.1
2.6
.9
.9
.6
.2
.0
0.5
0.3
0.3
0.2
(1,062.6)
209.8
108.8
38.0
585.3
139.0
132.9
104.7
57.6
44.0
29.0
28.8
24.4
6.6
6.2
Trends in Greenhouse Gas Emissions
2-3
-------
Abandoned Underground Coal
Mines
Mobile Combustion
Composting
Petrochemical Production
Field Burning of Agricultural
Residues
Iron and Steel Production &
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and
Consumption
International Bunker Fuelsb
N20
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Adipic Acid Production
Wastewater Treatment
N2O from Product Uses
Forest Land Remaining Forest
Land
Composting
Settlements Remaining Settlements
Field Burning of Agricultural
Residues
Incineration of Waste
Wetlands Remaining Wetlands
International Bunker Fuelsb
HFCs
Substitution of Ozone Depleting
Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and
Distribution
Magnesium Production and
Processing
Semiconductor Manufacture
Total
Net Emissions (Sources and
Sinks)
6.0
4.7
0.3
0.9
0.7
1.0
0.2
315.0
200.3
43.7
20.0
12.1
12.8
15.3
3.7
4.4
0.5
0.4
1.0
0.4
0.5
1.1
36.9
0.3
36.4
0.2
20.8
18.5
2.2
32.8
26.8
5.4
0.5
6,098.7
5,257.3
8.2
4.3
0.7
1.1
0.7
1.0
0.1
334.1
202.3
53.7
22.3
12.9
13.3
17.3
4.0
4.6
0.8
0.8
1.2
0.4
0.5
0.9
61.8
28.5
33.0
0.3
15.6
11.8
3.8
28.1
21.6
5.6
0.9
6,463.3
5,612.3
7.4
3.4
1.3
1.2
0.8
0.9
0.1
329.2
204.5
52.8
21.9
14.0
14.5
6.2
4.5
4.9
2.4
1.4
1.2
0.5
0.4
0.9
100.1
71.2
28.6
0.3
13.5
8.6
4.9
19.2
15.1
3.0
1.1
7,008.2
6,290.7
5.6
2.5
1.6
1.1
0.9
0.7
0.1
315.9
210.6
36.7
18.6
14.2
14.8
5.9
4.8
4.4
1.8
1.7
1.5
0.5
0.4
1.0
116.1
100.0
15.8
0.2
6.2
3.0
3.2
17.9
14.0
2.9
1.0
7,108.6
5,985.9
5.5
2.4
1.6
1.0
0.8
0.7
0.1
312.1
208.4
33.5
18.2
14.6
14.5
5.9
4.8
4.4
3.5
1.8
1.5
0.5
0.4
1.0
119.1
105.0
13.8
0.3
6.0
2.5
3.5
17.0
13.2
2.9
1.0
7,051.1
6,000.6
5.7
2.3
1.7
1.0
0.9
0.7
0.1
311.9
207.9
30.1
21.7
14.7
14.7
5.9
4.9
4.4
3.3
1.8
1.6
0.5
0.4
1.0
125.5
108.3
17.0
0.3
7.5
3.8
3.6
16.5
12.7
3.0
0.8
7,150.1
6,087.5
+ Does not exceed 0.05 Tg CO2 Eq.
a The net CO2 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.
0 Small amounts of PFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
Table 2-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg)
2-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Gas/Source
CO2
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Iron and Steel Production &
Metallurgical Coke
Production
Cement Production
Natural Gas Systems
Incineration of Waste
Lime Production
Ammonia Production and Urea
Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and
Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production and
Consumption
Land Use, Land-Use Change,
and Forestry (Sink)"
Biomass — Woocf
International Bunker Fuelsb
Biomass — Ethanof
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Forest Land Remaining Forest
Land
Petroleum Systems
Wastewater Treatment
Stationary Combustion
Rice Cultivation
Abandoned Underground Coal
Mines
Mobile Combustion
Composting
1990
5,076,694
4,708,918
1,809,685
1,484,485
834,204
337,715
214,544
28,285
116,977
109,760
33,278
33,733
10,950
11,533
16,831
7,084
5,127
6,831
4,141
2,221
1,195
1,416
2,152
1,529
1,033
949
376
285
375
(841,430)
215,186
114,330
4,155
29,360
6,342
7,105
6,171
4,003
1,447
218
1,613
1,120
352
339
288
225
15
1995
5,407,885
5,013,910
1,938,862
1,598,668
862,557
354,443
224,400
34,978
137,460
103,116
36,847
33,810
15,712
13,325
17,796
7,049
6,651
5,659
4,304
2,750
1,526
1,422
2,036
1,513
1,018
1,013
341
298
329
(850,952)
229,091
101,620
7,683
29,325
6,837
6,871
6,314
3,193
1,642
293
1,524
1,183
340
363
392
207
35
2000
5,955,177
5,561,515
2,283,177
1,800,305
844,554
370,352
226,932
36,195
144,473
95,062
41,190
29,394
17,485
14,088
16,402
7,541
5,056
6,086
4,181
3,004
1,752
1,421
1,893
1,382
1,227
1,140
325
311
248
(717,506)
218,088
98,966
9,188
28,148
6,398
5,825
6,231
2,881
1,804
983
1,441
1,200
315
357
350
163
60
2005
6,090,838
5,723,477
2,381,002
1,881,470
828,008
358,036
221,761
53,201
138,070
73,190
45,910
29,463
19,532
14,379
12,849
7,854
6,768
4,142
4,228
2,804
1,755
1,321
1,392
1,386
1,079
465
287
266
219
(1,122,745)
208,927
111,487
22,554
26,748
6,474
6,088
5,062
2,719
1,991
676
1,346
1,159
318
326
265
121
75
2006
6,014,871
5,635,418
2,327,313
1,880,874
844,505
321,852
206,049
54,824
145,137
76,100
46,562
29,540
19,824
15,100
12,300
7,889
8,035
3,801
4,162
2,573
1,876
1,709
1,505
1,167
879
529
288
270
207
(1,050,541)
209,926
110,520
30,459
27,713
6,580
6,211
4,991
2,780
1,993
1,489
1,346
1,165
300
282
263
115
75
2007
6,103,408
5,735,789
2,397,191
1,887,403
845,416
340,625
214,351
50,803
133,910
77,370
44,525
28,680
20,786
14,595
13,786
8,007
6,182
4,251
4,140
2,636
,876
,867
,552
,166
,010
530
287
267
196
(1,062,566)
209,785
108,756
38,044
27,872
6,618
6,327
4,985
2,744
2,093
1,381
1,370
1,160
315
293
273
109
79
Trends in Greenhouse Gas Emissions
2-5
-------
Petrochemical Production
Field Burning of Agricultural
Residues
Iron and Steel Production &
Metallurgical Coke
Production
Ferroalloy Production
Silicon Carbide Production and
Consumption
International Bunker Fuelsb
N2O
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Adipic Acid Production
Wastewater Treatment
N2O from Product Uses
Forest Land Remaining Forest
Land
Composting
Settlements Remaining
Settlements
Field Burning of Agricultural
Residues
Incineration of Waste
Wetlands Remaining Wetlands
International Bunker Fuelsb
HFCs
Substitution of Ozone
Depleting Substances0
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and
Distribution
Magnesium Production and
Processing
Semiconductor Manufacture
41
33
46
1
1
5
1,016
646
141
64
39
41
49
12
14
2
1
o
J
1
2
+
3
M
M
3
+
M
M
M
1
1
+
+
52
32
47
1
1
6
1,078
653
173
72
42
43
56
13
15
2
o
J
4
1
1
+
3
M
M
3
+
M
M
M
1
1
+
+
59
38
44
1
1
6
1,062
660
170
71
45
47
20
14
16
8
4
4
1
1
+
3
M
M
2
+
M
M
M
1
1
+
+
51
41
34
+
+
7
1,019
679
118
60
46
48
19
15
14
6
6
5
2
1
+
3
M
M
1
+
M
M
M
1
1
+
+
48
39
35
+
+
7
1,007
672
108
59
47
47
19
15
14
11
6
5
2
1
+
3
M
M
1
+
M
M
M
1
1
+
+
48
42
33
+
+
7
1,006
671
97
70
47
47
19
16
14
11
6
5
2
1
+
3
M
M
1
+
M
M
M
1
1
+
+
+ Does not exceed 0.5 Gg.
M Mixture of multiple gases
a The net CO2 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.
0 Small amounts of PFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
Emissions of all gases can be summed from each source category from Intergovernmental Panel on Climate Change
(IPCC) guidance. Over the eighteen-year period of 1990 to 2007, total emissions in the Energy, Industrial
Processes, and Agriculture sectors grew by 976.7 Tg CO2 Eq. (19 percent), 28.5 Tg CO2 Eq. (9 percent), and 28.9
Tg CO2 Eq. (8 percent), respectively. Emissions decreased in the Waste and Solvent and Other Product Use sectors
by 11.5 Tg CO2 Eq. (6 percent) and less than 0.1 Tg CO2 Eq. (less than 0.4 percent), respectively. Over the same
2-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
period, estimates of net C sequestration in the Land Use, Land-Use Change, and Forestry sector increased by 192.5
Tg CO2 Eq. (23 percent).
Figure 2-4: U.S. Greenhouse Gas Emissions by Chapter/IPCC Sector
Table 2-3: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg CO2 Eq.)
Chapter/IPCC Sector 1990 1995 2000 2005 2006 2007
Energy
Industrial Processes
Solvent and Other Product Use
Agriculture
Land Use, Land-Use Change, and
Forestry (Emissions)
Waste
Total Emissions
Net CO2 Flux from Land Use, Land-Use
Change, and Forestry (Sinks)*
Net Emissions (Sources and Sinks)
5,193.6
325.2
4.4
384.2
14.2
177.1
6,098.7
(841.4)
5,257.3
5,520.1
345.8
4.6
402.0
16.2
174.7
6,463.3
(851.0)
5,612.3
6,059.9
356.3
4.9
399.4
33.0
154.6
7,008.2
(717.5)
6,290.7
6,169.2
337.6
4.4
410.8
26.4
160.2
7,108.6
(1,122.7)
5,985.9
6,084.4
343.9
4.4
410.3
45.1
163.0
7,051.1
(1,050.5)
6,000.6
6,170.3
353.8
4.4
413.1
42.9
165.6
7,150.1
(1,062.6)
6,087.5
The net CO2 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.
Note: Parentheses indicate negative values or sequestration.
Energy
Energy-related activities, primarily fossil fuel combustion, accounted for the vast majority of U.S. CO2 emissions for
the period of 1990 through 2007. In 2007, approximately 85 percent of the energy consumed in the United States
(on a Btu basis) was produced through the combustion of fossil fuels. The remaining 15 percent came from other
energy sources such as hydropower, biomass, nuclear, wind, and solar energy (see Figure 2-5 and Figure 2-6). A
discussion of specific trends related to CO2 as well as other greenhouse gas emissions from energy consumption is
presented in the Energy chapter. Energy-related activities are also responsible for CH4 and N2O emissions (35
percent and 14 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.
Figure 2-5: 2007 Energy Chapter Greenhouse Gas Sources
Figure 2-6: 2007 U.S. Fossil Carbon Flows (Tg CO2 Eq.)
Table 2-4: Emissions from Energy (Tg CO2 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
U.S. Territories
Non-Energy Use of Fuels
Natural Gas Systems
1990
4,871.0
4,708.9
1,809.7
1,484.5
834.2
337.7
214.5
28.3
117.0
33.7
1995
5,201.2
5,013.9
1,938.9
1,598.7
862.6
354.4
224.4
35.0
137.5
33.8
2000
5,753.2
5,561.5
2,283.2
1,800.3
844.6
370.4
226.9
36.2
144.5
29.4
2005
5,910.8
5,723.5
2,381.0
1,881.5
828.0
358.0
221.8
53.2
138.1
29.5
2006
5,830.2
5,635.4
2,327.3
1,880.9
844.5
321.9
206.0
54.8
145.1
29.5
2007
5,919.5
5,735.8
2,397.2
1,887.4
845.4
340.6
214.4
50.8
133.9
28.7
Trends in Greenhouse Gas Emissions 2-7
-------
Incineration of Waste
Petroleum Systems
Wood Biomass andEthanol
Consumption *
International Bunker Fuels*
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground
Coal Mines
Mobile Combustion
International Bunker Fuels*
N2O
Mobile Combustion
Stationary Combustion
Incineration of Waste
International Bunker Fuels*
Total
10.9
0.4
219.3
114.3
265.7
129.6
84.1
33.9
7.4
6.0
4.7
0.2
57.0
43.7
12.8
0.5
1.1
5,193.6
15.7
0.3
236. 8
101.6
251.4
132.6
67.1
32.0
7.1
8.2
4.3
0.1
67.5
53.7
13.3
0.5
0.9
5,520.1
17.5
0.3
227.3
99.0
239.0
130.8
60.5
30.3
6.6
7.4
3.4
0.1
67.7
52.8
14.5
0.4
0.9
6,059.9
19.5
0.3
231.5
111.5
206.5
106.3
57.1
28.3
6.7
5.6
2.5
0.1
51.9
36.7
14.8
0.4
1.0
6,169.2
19.8
0.3
240.4
110.5
205.7
104.8
58.4
28.3
6.3
5.5
2.4
0.1
48.5
33.5
14.5
0.4
1.0
6,084.4
20.8
0.3
247.8
108.8
205.7
104.7
57.6
28.8
6.6
5.7
2.3
0.1
45.2
30.1
14.7
0.4
1.0
6,170.3
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.
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 electric power sector comprises electricity-only and combined-heat-and-power (CHP) plants within the
NAICS 22 category whose primary business is to sell electricity, or electricity and heat, to the public (nonutility
power producers can be included in this sector as long as they meet they electric power sector definition). EIA
statistics for the industrial sector include fossil fuel consumption that occurs in the fields of manufacturing,
agriculture, mining, and construction. EIA's fuel consumption data for the transportation sector consists of all
vehicles whose primary purpose is transporting people and/or goods from one physical location to another. EIA's
fuel consumption data for the industrial sector consists of all facilities and equipment used for producing,
processing, or assembling goods (EIA includes generators that produce electricity and/or useful thermal output
primarily to support on-site industrial activities in this sector). EIA's fuel consumption data for the residential sector
consists of living quarters for private households. EIA's fuel consumption data for the commercial sector consists of
service-providing facilities and equipment from private and public organizations and businesses (EIA includes
generators that produce electricity and/or useful thermal output primarily to support the activities at commercial
establishments in this sector). Table 2-5, Figure 2-7, and Figure 2-8 summarize CO2 emissions from fossil fuel
combustion by end-use sector.
Table 2-5: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.)
End-Use Sector 1990 1995 2000 2005 2006 2007
Transportation
Combustion
Electricity
Industrial
Combustion
Electricity
Residential
Combustion
Electricity
Commercial
Combustion
1,487.5
1,484.5
3.0
1,516.8
834.2
682.6
927.1
337.7
589.4
749.2
214.5
1,601.7
1,598.7
3.0
1,575.5
862.6
712.9
993.3
354.4
638.8
808.5
224.4
1,803.7
1,800.3
3.4
1,629.6
844.6
785.0
1,128.2
370.4
757.9
963.8
226.9
1,886.2
1,881.5
4.7
1,558.5
828.0
730.5
1,207.2
358.0
849.2
1,018.4
221.8
1,885.4
1,880.9
4.5
1,550.7
844.5
706.2
1,145.9
321.9
824.1
998.6
206.0
1,892.2
1,887.4
4.8
1,553.4
845.4
708.0
1,198.0
340.6
857.4
1,041.4
214.4
2-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Electricity 534.7 584.1 736.8 796.6 792.5 827.1
U.S. Territories" 28.3 35.0 36.2 53.2 54.8 50.8
Total 4,708.9 5,013.9 5,561.5 5,723.5 5,635.4 5,735.8
Electricity Generation 1,809.7 1,938.9 2,283.2 2,381.0 2,327.3 2,397.2
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: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type
Figure 2-8: 2007 End-Use Sector Emissions of CO2, CH4, and N2O from Fossil Fuel Combustion
The main driver of emissions in the energy sector is CO2 from fossil fuel combustion. The transportation end-use
sector accounted for 1,892.2 Tg CO2 Eq. in 2007, or approximately 33 percent of total CO2 emissions from fossil
fuel combustion, the largest share of any end-use economic sector.37 The industrial end-use sector accounted for 27
percent of CO2 emissions from fossil fuel combustion. The residential and commercial end-use sectors accounted
for an average 21 and 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
eighteen-year period from 1990 through 2007 included the following:
• Total CO2 emissions from fossil fuel combustion increased from 4,708.9 Tg CO2 Eq. to 5,735.8 Tg CO2
Eq.—a 22 percent total increase over the eighteen-year period. From 2006 to 2007, these emissions
increased by 100.4 Tg CO2 Eq. (1.8 percent).
• CO2 emissions from non-energy use of fossil fuels have increased 16.9 Tg CO2 Eq. (14.5 percent) from
1990 through 2007. Emissions from non-energy uses of fossil fuels were 133.9 Tg CO2 Eq. in 2007, which
constituted 2.2 percent of total national CO2 emissions.
• CH4 emissions from natural gas systems were 104.7 Tg CO2 Eq. in 2007; emissions have declined by 24.9
Tg CO2 Eq. (19 percent) since 1990. This decline has been due to improvements in technology and
management practices, as well as some replacement of old equipment.
• CH4 emissions from coal mining were 57.6 Tg CO2 Eq. This decline of 26.4 Tg CO2 Eq. (31 percent) from
1990 results from the mining of less gassy coal from underground mines and the increased use of CH4
collected from degasification systems.
• In 2007, N2O emissions from mobile combustion were 30.1 Tg CO2 Eq. (approximately 10 percent of U.S.
N2O emissions). From 1990 to 2007, N2O emissions from mobile combustion decreased by 31 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.
• CO2 emissions from incineration of waste (20.8 Tg CO2 Eq. in 2007) increased by 9.8 Tg CO2 Eq. (90
percent) from 1990 through 2007, as the volume of plastics and other fossil carbon-containing materials in
municipal solid waste grew.
Industrial Processes
Emissions are produced as a by-product 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, CH4, and
N2O. These processes include iron and steel production and metallurgical coke production, cement production,
37 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
-------
ammonia production and urea application, lime manufacture, limestone and dolomite use (e.g., flux stone, flue gas
desulfurization, and glass manufacturing), soda ash manufacture 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 (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.
Figure 2-9: 2007 Industrial Processes Chapter Greenhouse Gas Sources
Table 2-6: Emissions from Industrial Processes (Tg CO2 Eq.)
Gas/Source
C02
Iron and Steel Production &
Metallurgical Coke Production
Cement Manufacture
Lime Manufacture
Ammonia Production & Urea
Application
Limestone and Dolomite Use
Aluminum Production
Soda Ash Manufacture and
Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and
Consumption
CH4
Petrochemical Production
Iron and Steel Production &
Metallurgical Coke Production
Ferroalloy Production
Silicon Carbide Production and
Consumption
N20
Nitric Acid Production
Adipic Acid Production
HFCs
Substitution of Ozone Depleting
Substances a
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and
Distribution
Magnesium Production and
Processing
1990
197.6
109.8
33.3
11.5
16.8
5.1
6.8
4.1
2.2
1.2
1.4
2.2
1.5
0.9
0.3
0.4
1.9
0.9
1.0
+
+
35.3
20.0
15.3
36.9
0.3
36.4
0.2
20.8
18.5
2.2
32.8
26.8
5.4
1995
198.6
103.1
36.8
13.3
17.8
6.7
5.7
4.3
2.8
1.5
1.4
2.0
1.5
1.0
0.3
0.3
2.1
1.1
1.0
+
+
39.6
22.3
17.3
61.8
28.5
33.0
0.3
15.6
11.8
3.8
28.1
21.6
5.6
2000
193.2
95.1
41.2
14.1
16.4
5.1
6.1
4.2
3.0
.8
.4
.9
.4
.1
0.3
0.2
2.2
1.2
0.9
+
+
28.1
21.9
6.2
100.1
71.2
28.6
0.3
13.5
8.6
4.9
19.2
15.1
3.0
2005
171.1
73.2
45.9
14.4
12.8
6.8
4.1
4.2
2.8
1.8
1.3
1.4
1.4
0.5
0.3
0.2
1.8
1.1
0.7
+
+
24.6
18.6
5.9
116.1
100.0
15.8
0.2
6.2
3.0
3.2
17.9
14.0
2.9
2006
175.9
76.1
46.6
15.1
12.3
8.0
3.8
4.2
2.6
1.9
1.7
1.5
1.2
0.5
0.3
0.2
1.7
1.0
0.7
+
+
24.2
18.2
5.9
119.1
105.0
13.8
0.3
6.0
2.5
3.5
17.0
13.2
2.9
2007
174.9
77.4
44.5
14.6
13.8
6.2
4.3
4.1
2.6
1.9
1.9
1.6
1.2
0.5
0.3
0.2
1.7
1.0
0.7
+
+
27.6
21.7
5.9
125.5
108.3
17.0
0.3
7.5
3.8
3.6
16.5
12.7
3.0
2-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Semiconductor Manufacture 0.5 0.9 LI LO LO 0.8
Total 325.2 345.8 356.3 337.6 343.9 353.8
+ Does not exceed 0.05 Tg CO2 Eq.
a Small amounts of PFC emissions also result from this source.
Note: Totals may not sum due to independent rounding.
Overall, emissions from industrial processes increased by 8.8 percent from 1990 to 2007 despite decreases in
emissions from several industrial processes, such as iron and steel production and metallurgical coke production,
aluminum production, HCFC-22 production, and electrical transmission and distribution. The increase in overall
emissions was driven by a rise in the emissions originating from cement manufacture and, primarily, the emissions
from the use of substitutes for ozone depleting substances. Significant trends in emissions from industrial processes
source categories over the eighteen-year period from 1990 through 2007 included the following:
• HFC emissions from ODS substitutes have been increasing from small amounts in 1990 to 108.3 Tg CO2
Eq. in 2007. 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 CH4 emissions from iron and steel production and metallurgical coke production increased by 1.6
percent to 78.1 Tg CO2 Eq. in 2007, but have declined overall by 32.6 Tg CO2 Eq. (29.5 percent) from
1990 through 2007, due to restructuring of the industry, technological improvements, and increased scrap
utilization.
• PFC emissions from aluminum production decreased by 79 percent (14.7 Tg CO2 Eq.) from 1990 to 2007,
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 2007, 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 1.2 Tg CO2 Eq. annually since 1998.
• CO2 emissions from ammonia production and urea application (13.8 Tg CO2 Eq. in 2007) have decreased
by 3.0 Tg CO2 Eq. (18 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 by-product 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 2007 (see Table 2-7).
Table 2-7: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq.)
Gas/Source
N2O
N2O 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
2005
4.4
4.4
4.4
2006
4.4
4.4
4.4
2007
4.4
4.4
4.4
In 2007, N2O emissions from product uses constituted 1 percent of U.S. N2O emissions. From 1990 to 2007,
emissions from this source category decreased by less than 0.5 percent, though slight increases occurred in
intermediate years.
Agriculture
Agricultural activities contribute directly to emissions of greenhouse gases through a variety of processes, including
the following source categories: enteric fermentation in domestic livestock, livestock manure management, rice
cultivation, agricultural soil management, and field burning of agricultural residues.
In 2007, agricultural activities were responsible for emissions of 413.1 Tg CO2 Eq., or 5.8 percent of total U.S.
Trends in Greenhouse Gas Emissions 2-11
-------
greenhouse gas emissions. CH4 and N2O were the primary greenhouse gases emitted by agricultural activities. CH4
emissions from enteric fermentation and manure management represented about 24 percent and 8 percent of total
CH4 emissions from anthropogenic activities, respectively, in 2007. Agricultural soil management activities, such as
fertilizer application and other cropping practices, were the largest source of U.S. N2O emissions in 2007,
accounting for 67 percent.
Figure 2-10: 2007 Agriculture Chapter Greenhouse Gas Sources
Table 2-8: Emissions from Agriculture (Tg CO2 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
1990
171.4
133.2
30.4
7.1
0.7
212.8
200.3
12.1
0.4
1995
186.3
143.6
34.5
7.6
0.7
215.6
202.3
12.9
0.4
2000
180.5
134.4
37.9
7.5
0.8
218.9
204.5
14.0
0.5
2005
185.5
136.0
41.8
6.8
0.9
225.3
210.6
14.2
0.5
2006
186.8
138.2
41.9
5.9
0.8
223.5
208.4
14.6
0.5
2007
190.0
139.0
44.0
6.2
0.9
223.1
207.9
14.7
0.5
Total 384.2 402.0 399.4 410.8 410.3 413.1
Note: Totals may not sum due to independent rounding.
Some significant trends in U.S. emissions from Agriculture include the following:
• Agricultural soils produced approximately 67 percent of N2O emissions in the United States in 2007.
Estimated emissions from this source in 2007 were 207.9 Tg CO2 Eq. Annual N2O emissions from
agricultural soils fluctuated between 1990 and 2007, although overall emissions were 3.8 percent higher in
2007 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.
• Enteric fermentation was the largest source of CH4 emissions in 2007, at 139.0 Tg CO2 Eq. Although
emissions from enteric fermentation have increased by 4 percent between 1990 and 2007, emissions
increased about 8 percent between 1990 and 1995 and decreased about 7 percent from 1995 to 2004,
mainly due to decreasing populations of both beef and dairy cattle and improved feed quality for feedlot
cattle. The last three years have shown an increase in emissions. During this timeframe, populations of
sheep have decreased 46 percent since 1990 while horse populations have increased over 80 percent,
mostly over the last 6 years. Goat and swine populations have increased 1 percent and 21 percent,
respectively, during this timeframe.
• Overall, emissions from manure management increased 38 percent between 1990 and 2007. This
encompassed an increase of 45 percent for CH4, from 30.4 Tg CO2 Eq. in 1990 to 44.0 Tg CO2 Eq. in 2007;
and an increase of 22 percent for N2O, from 12.1 Tg CO2 Eq. in 1990 to 14.7 Tg CO2 Eq. in 2007. The
majority of this increase was from swine and dairy cow manure, since the general trend in manure
management is one of increasing use of liquid systems, which tends to produce greater CH4 emissions.
Land Use, Land-Use Change, and Forestry
When humans alter the terrestrial biosphere through land use, changes in land use, and land management practices,
they also alter the background carbon fluxes between biomass, soils, and the atmosphere. Forest management
practices, tree planting in urban areas, the management of agricultural soils, and the landfilling of yard trimmings
and food scraps have resulted in an uptake (sequestration) of carbon in the United States, which offset about 14.9
2-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
percent of total U.S. greenhouse gas emissions in 2007. Forests (including vegetation, soils, and harvested wood)
accounted for approximately 86 percent of total 2007 net CO2 flux, urban trees accounted for 9 percent, mineral and
organic soil carbon stock changes accounted for 4 percent, and landfilled yard trimmings and food scraps accounted
for 1 percent of the total net flux in 2007. 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 from these soils through liming, urea fertilization, or
both. 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 2007 resulted in a net C sequestration of 1,062.6 Tg CO2 Eq.
(Table 2-9). This represents an offset of approximately 17.4 percent of total U.S. CO2 emissions, or 14.9 percent of
total greenhouse gas emissions in 2007. Between 1990 and 2007, total land use, land-use change, and forestry net C
flux resulted in a 26.3 percent increase in CO2 sequestration.
Table 2-9: Net CO2 Flux from Land Use, Land-Use Change, and Forestry (Tg CO2 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)
1990
(661.1)
(29.4)
2.2
(46.7)
(22.3)
(60.6)
(23.5)
1995
(686.6)
(22.9)
2.9
(36.4)
(22.5)
(71.5)
(13.9)
2000
(512.6)
(30.2)
2.4
(51.4)
(32.0)
(82.4)
(11.3)
2005
(975.7)
(18.3)
5.9
(4.6)
(26.7)
(93.3)
(10.2)
2006
(900.3)
(19.1)
5.9
(4.6)
(26.7)
(95.5)
(10.4)
2007
(910.1)
(19.7)
5.9
(4.7)
(26.7)
(97.6)
(9.8)
Total (841.4) (851.0) (717.5) (1,122.7) (1,050.5) (1,062.6)
Note: Totals may not sum due to independent rounding. Parentheses indicate net sequestration.
Land use, land-use change, and forestry source categories also resulted in emissions of CO2, CH4, and N2O that are
not included in the net CO2 flux estimates presented in Table 2-10. The application of crushed limestone and
dolomite to managed land (i.e., soil liming) and urea fertilization resulted in CO2 emissions of 8.0 Tg CO2 Eq. in
2007, an increase of 13 percent relative to 1990. Lands undergoing peat extraction resulted in CO2 emissions of 1.0
Tg CO2 Eq. (1,010 Gg), and N2O emissions of less than 0.01 Tg CO2 Eq. N2O emissions from the application of
synthetic fertilizers to forest soils have increased from 1990 to 0.3 Tg CO2 Eq. in 2007. Settlement soils in 2007
resulted in direct N2O emissions of 1.6 Tg CO2 Eq., a 61 percent increase relative to 1990. Non-CO2 emissions from
forest fires in 2007 resulted in CH4 emissions of 29 Tg CO2 Eq., and in N2O emissions of 2.9 Tg CO2 Eq. (Table
2-10).
Table 2-10: Emissions from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.)
Source Category 1990 1995 2000 2005 2006 2007
CO2 8.1 8.1 8.8 8.9 8.8 9.0
Cropland Remaining Cropland:
Liming of Agricultural Soils 4.7 4.4 4.3 4.3 4.2 4.1
Urea Fertilization 2.4 2.7 3.2 3.5 3.7 4.0
Wetlands Remaining Wetlands:
Peatlands Remaining Peatlands 1.0 1.0 1.2 1.1 0.9 1.0
CH4 4.6 6.1 20.6 14.2 31.3 29.0
Forest Land Remaining Forest Land:
Forest Fires 4.6 6.1 20.6 14.2 31.3 29.0
N2O 1.5 2.0 3.6 3.3 5.0 4.9
Forest Land Remaining Forest Land:
Forest Fires 0.5 0.6 2.1 1.4 3.2 2.9
Forest Soils 0.0 0.1 0.3 0.3 0.3 0.3
Wetlands Remaining Wetlands: Peatlands
Remaining Peatlands + + + + + +
Trends in Greenhouse Gas Emissions 2-13
-------
Settlements Remaining Settlements:
Settlement Soils LO L2 L2 1.5 1.5 1.6
Total 14.2 16.2 33.0 26.4 45.1 42.9
+ Less than 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
Other significant trends from 1990 to 2007 in land use, land-use change, and forestry emissions include:
• Net C sequestration by forest land has increased 38 percent. This is primarily due to increased forest
management and the effects of previous reforestation. The increase in intensive forest management
resulted in higher growth rates and higher biomass density. The tree planting and conservation efforts of
the 1970s and 1980s continue to have a significant impact on sequestration rates. Finally, the forested area
in the United States increased over the past 18 years, although only at an average rate of 0.25 percent per
year.
• Net sequestration of C by urban trees has increased by 61 percent over the period from 1990 to 2007. 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 58 percent since
1990. This is due in part to a decrease in the amount of yard trimmings and food scraps generated. In
addition, the proportion of yard trimmings and food scraps landfilled has decreased, as there has been a
significant rise in the number of municipal composting facilities in the United States.
Waste
Waste management and treatment activities are sources of greenhouse gas emissions (see Figure 2-11). In 2007,
landfills were the second largest source of anthropogenic CH4 emissions, accounting for 23 percent of total U.S.
CH4 emissions.38 Additionally, wastewater treatment accounts for 4 percent of U.S. CH4 emissions, and 2 percent of
N2O emissions. Emissions of CH4 and N2O from composting grew from 1990 to 2007, and resulted in emissions of
3.5 Tg CO2 Eq. in 2007. A summary of greenhouse gas emissions from the Waste chapter is presented in Table
2-11.
Figure 2-11: 2007 Waste Chapter Greenhouse Gas Sources
Overall, in 2007, waste activities generated emissions of 165.6 Tg CO2 Eq., or 2.3 percent of total U.S. greenhouse
gas emissions.
Table 2-11: Emissions from Waste (Tg CO2 Eq.)
Gas/Source
CH4
Landfills
Wastewater Treatment
Composting
N2O
Wastewater Treatment
Composting
Total
1990
173.0
149.2
23.5
0.3
4.0
3.7
0.4
177.1
1995
169.9
144.3
24.8
0.7
4.8
4.0
0.8
174.7
2000
148.8
122.3
25.2
1.3
5.8
4.5
1.4
154.6
2005
153.8
127.8
24.3
1.6
6.5
4.8
1.7
160.2
2006
156.5
130.4
24.5
1.6
6.6
4.8
1.8
163.0
2007
158.9
132.9
24.4
1.7
6.7
4.9
1.8
165.6
Note: Totals may not sum due to independent rounding.
Some significant trends in U.S. emissions from Waste include the following:
38 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.
2-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
• From 1990 to 2007, net CH4 emissions from landfills decreased by 16.3 Tg CO2 Eq. (11 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,39 which has more than offset the
additional CH4 emissions resulting from an increase in the amount of municipal solid waste landfilled.
• From 1990 to 2007, CH4 and N2O emissions from wastewater treatment increased by 0.8 Tg CO2 Eq. (4
percent) and 1.2 Tg CO2 Eq. (32 percent), respectively.
• CH4 and N2O emissions from composting each increased by less than 0.1 Tg CO2 Eq. (4 percent) from
2006 to 2007. Emissions from composting have been continually increasing since 1990, from 0.7 Tg CO2
Eq. to 3.5 Tg CO2 Eq. in 2007, 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 2007. Transportation activities, in aggregate, accounted for the second largest
portion (28 percent). Emissions from industry accounted for about 20 percent of U.S. greenhouse gas emissions in
2007. 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 18 percent of U.S. greenhouse gas emissions were contributed by the residential, agriculture, and
commercial sectors, plus emissions from U.S. territories. The residential sector accounted for 5 percent, and
primarily consisted of CO2 emissions from fossil fuel combustion. Activities related to agriculture accounted for
roughly 7 percent of U.S. emissions; unlike other economic sectors, agricultural sector emissions were dominated by
N2O emissions from agricultural soil management and 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 2007.
Figure 2-12: Emissions Allocated to Economic Sectors
Table 2-12: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq. and Percent of Total in
2007)
Sector/Source
Electric Power Industry
CO2 from Fossil Fuel Combustion
Incineration of Waste
Electrical Transmission and
Distribution
Stationary Combustion
1990
1,859.1
1,809.7
11.4
26.8
8.6
1995
1,989.0
1,938.9
16.2
21.6
9.1
2000
2,329.3
2,283.2
17.9
15.1
10.6
2005
2,429.4
2,381.0
19.9
14.0
11.0
2006
2,375.5
2,327.3
20.2
13.2
10.8
2007
2,445.1
2,397.2
21.2
12.7
11.0
Percent"
34.2%
33.5%
0.3%
0.2%
0.2%
39 The CO2 produced from combusted landfill CH4 at landfills is not counted in national inventories as it is considered part of the
natural C cycle of decomposition.
Trends in Greenhouse Gas Emissions 2-15
-------
Limestone and Dolomite Use
Transportation
CO2 from Fossil Fuel Combustion
Substitution of Ozone Depleting
Substances
Mobile Combustion
Non-Energy Use of Fuels
Industry
CO2 from Fossil Fuel Combustion
Natural Gas Systems
Non-Energy Use of Fuels
Iron and Steel & Metallurgical
Coke Production
Coal Mining
Cement Production
Petroleum Systems
Nitric Acid Production
HCFC-22 Production
Lime Production
Ammonia Production and Urea
Consumption
Aluminum Production
Substitution of Ozone Depleting
Substances
Adipic Acid Production
Abandoned Underground Coal
Mines
Semiconductor Manufacture
Stationary Combustion
N2O from Product Uses
Soda Ash Production and
Consumption
Petrochemical Production
Limestone and Dolomite Use
Magnesium Production and
Processing
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Mobile Combustion
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and
Consumption
Agriculture
N2O from Agricultural Soil
Management
Enteric Fermentation
Manure Management
CO2 from Fossil Fuel Combustion
CH4 and N2O from Forest Fires
Rice Cultivation
Liming of Agricultural Soils
Urea Fertilization
Field Burning of Agricultural
2.6
1,543.6
1,484.5
+
47.3
11.9
1,496.0
803.4
163.3
99.4
110.7
84.1
33.3
34.2
20.0
36.4
11.5
16.8
25.4
+
15.3
6.0
2.9
4.7
4.4
4.1
3.1
2.6
5.4
1.2
1.4
2.2
0.9
1.5
0.9
0.3
0.4
428.5
200.3
133.2
42.4
30.8
5.1
7.1
4.7
2.4
1.1
3.3
1,685.2
1,598.7
18.6
56.6
11.3
1,524.5
826.3
166.4
120.2
104.1
67.1
36.8
32.3
22.3
33.0
13.3
17.8
17.5
1.2
17.3
8.2
4.9
4.9
4.6
4.3
3.8
3.3
5.6
1.5
1.4
2.0
1.0
1.5
1.0
0.3
0.3
453.7
202.3
143.6
47.4
36.3
6.8
7.6
4.4
2.7
1.0
2.5
1,919.7
1,800.3
52.6
54.7
12.1
1,467.5
806.1
160.2
121.4
96.0
60.5
41.2
30.6
21.9
28.6
14.1
16.4
14.7
3.1
6.2
7.4
6.2
4.8
4.9
4.2
4.2
2.5
3.0
1.8
1.4
1.9
1.1
1.4
1.1
0.3
0.3
470.2
204.5
134.4
51.9
38.4
22.7
7.5
4.3
3.2
1.3
3.4
1,998.9
1,881.5
69.7
37.5
10.2
1,364.9
781.6
135.8
120.8
73.9
57.1
45.9
28.6
18.6
15.8
14.4
12.8
7.1
5.2
5.9
5.6
4.4
4.5
4.4
4.2
3.9
3.4
2.9
1.8
1.3
1.4
1.3
1.4
0.5
0.3
0.2
482.6
210.6
136.0
56.0
46.4
15.6
6.8
4.3
3.5
1.4
4.0
1,994.4
1,880.9
69.5
34.1
9.9
1,388.4
796.0
134.3
127.9
76.8
58.4
46.6
28.6
18.2
13.8
15.1
12.3
6.3
5.7
5.9
5.5
4.7
4.6
4.4
4.2
3.6
4.0
2.9
1.9
1.7
1.5
1.3
1.2
0.5
0.3
0.2
502.9
208.4
138.2
56.4
48.6
34.4
5.9
4.2
3.7
1.3
3.1
1,995.2
1,887.4
67.0
30.6
10.2
1,386.3
797.5
133.4
117.0
78.1
57.6
44.5
29.1
21.7
17.0
14.6
13.8
8.1
6.1
5.9
5.7
4.7
4.5
4.4
4.1
3.7
3.1
3.0
.9
.9
.6
.3
.2
0.5
0.3
0.2
502.8
207.9
139.0
58.7
47.9
31.9
6.2
4.1
4.0
1.4
+
27.9%
26.4%
0.9%
0.4%
0.1%
19.4%
11.2%
1.9%
1.6%
1.1%
0.8%
0.6%
0.4%
0.3%
0.2%
0.2%
0.2%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
0.1%
+
+
+
+
+
+
+
+
+
+
7.0%
2.9%
1.9%
0.8%
0.7%
0.4%
0.1%
0.1%
0.1%
+
2-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Residues
CO2 and N2O from Managed
Peatlands
Mobile Combustion
N2O from Forest Soils
Stationary Combustion
Commercial
CO2 from Fossil Fuel Combustion
Landfills
Substitution of Ozone Depleting
Substances
Wastewater Treatment
Human Sewage
Composting
Stationary Combustion
Residential
CO2 from Fossil Fuel Combustion
Substitution of Ozone Depleting
Substances
Stationary Combustion
Settlement Soil Fertilization
U.S. Territories
CO2 from Fossil Fuel Combustion
Total Emissions
Sinks
CO2 Flux from Forests
Urban Trees
CO2 Flux from Agricultural Soil
Carbon Stocks
Landfilled Yard Trimmings and
Food Scraps
1.0
0.3
+
+
392.9
214.5
149.2
+
23.5
3.7
0.7
1.2
344.5
337.7
0.3
5.5
1.0
34.1
34.1
6,098.7
(841.4)
(661.1)
(60.6)
(96.3)
(23.5)
1.0
0.4
0.1
+
401.0
224.4
144.3
0.7
24.8
4.0
1.5
1.3
368.8
354.4
8.1
5.0
1.2
41.1
41.1
6,463.3
(851.0)
(686.6)
(71.5)
(78.9)
(13.9)
1.2
0.4
0.3
+
388.2
226.9
122.3
5.5
25.2
4.5
2.6
1.2
386.0
370.4
10.1
4.3
1.2
47.3
47.3
7,008.2
(717.5)
(512.6)
(82.4)
(111.2)
(11.3)
1.1
0.5
0.3
+
401.8
221.8
127.8
18.5
24.3
4.8
3.3
1.2
370.5
358.0
6.5
4.5
1.5
60.5
60.5
7,108.6
(1,122.7)
(975.7)
(93.3)
(43.6)
(10.2)
0.9
0.5
0.3
+
392.6
206.0
130.4
22.4
24.5
4.8
3.3
1.1
334.9
321.9
7.5
4.0
1.5
62.3
62.3
7,051.1
(1,050.5)
(900.3)
(95.5)
(44.5)
(10.4)
1.0
0.5
0.3
+
407.6
214.4
132.9
26.6
24.4
4.9
3.5
1.2
355.3
340.6
8.6
4.4
1.6
57.7
57.7
7,150.1
(1,062.6)
(910.1)
(97.6)
(45.1)
(9.8)
+
+
+
+
5.7%
3.0%
1.9%
0.4%
0.3%
0.1%
+
+
5.0%
4.8%
0.1%
0.1%
+
0.8%
0.8%
100.0%
-14.9%
-12.7%
-1.4%
-0.6%
-0.1%
Net Emissions (Sources and
Sinks) 5,257.3 5,612.3 6,290.7 5,985.9 6,000.6 6,087.5 85.1%
Note: Includes all emissions of CO2, CH4, N2O, HFCs, PFCs, and SF6. Parentheses indicate negative values or sequestration.
Totals may not sum due to independent rounding.
ODS (Ozone Depleting Substances)
+ Does not exceed 0.05 Tg CO2 Eq. or 0.05 percent.
a Percent of total emissions for year 2007.
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 2007. 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 increased from 2006
to 2007 by 3 percent, primarily due to increased 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.
Table 2-13: Electricity Generation-Related Greenhouse Gas Emissions (Tg CO2 Eq.)
Gas/Fuel Type or Source 1990 1995 2000 2005 2006 2007
CO2 1,823.2 1,957.9 2,303.2 2,403.9 2,351.2 2,421.1
Trends in Greenhouse Gas Emissions 2-17
-------
CO2 from Fossil Fuel
Combustion
Coal
Natural Gas
Petroleum
Geothermal
Incineration of Waste
Limestone and Dolomite Use
CH4
Stationary Combustion*
N2O
Stationary Combustion*
Incineration of Waste
SF6
Electrical Transmission and
Distribution
Total
1,809.7
1,531.1
176.5
101.8
0.4
10.9
2.6
0.6
0.6
8.5
8.1
0.5
26.8
26.8
1,859.1
1,938.9
1,648.6
229.2
60.7
0.3
15.7
3.3
0.6
0.6
9.0
8.6
0.5
21.6
21.6
1,989.0
2,283.2
1,909.5
281.8
91.5
0.4
17.5
2.5
0.7
0.7
10.4
10.0
0.4
15.1
15.1
2,329.3
2,381.0
1,958.4
319.9
102.3
0.4
19.5
3.4
0.7
0.7
10.7
10.3
0.4
14.0
14.0
2,429.4
2,327.3
1,932.4
338.9
55.6
0.4
19.8
4.0
0.7
0.7
10.5
10.1
0.4
13.2
13.2
2,375.5
2,397.2
1,967.6
373.8
55.3
0.4
20.8
3.1
0.7
0.7
10.7
10.3
0.4
12.7
12.7
2,445.1
Note: Totals may not sum due to independent rounding.
* Includes only stationary combustion emissions related to the generation of electricity.
To distribute electricity emissions among economic end-use sectors, emissions from the source categories assigned
to the electricity generation sector were allocated to the residential, commercial, industry, transportation, and
agriculture economic sectors according to retail sales of electricity (EIA 2008a 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.40
When emissions from electricity are distributed among these sectors, industry accounts for the largest share of U.S.
greenhouse gas emissions (30 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
2007.
Figure 2-13: Emissions with Electricity Distributed to Economic Sectors
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 2007
Sector/Gas
Industry
Direct Emissions
C02
CH4
N2O
MFCs, PFCs, and SF6
Electricity-Related
C02
1990
2,166.5
1,496.0
1,097.9
291.1
43.6
63.3
670.6
657.6
1995
2,219.8
1,524.5
1,141.7
277.8
48.4
56.6
695.3
684.4
2000
2,235.5
1,467.5
1,118.3
262.5
37.2
49.6
767.9
759.3
2005
2,081.2
1,364.9
1,070.1
230.4
33.1
31.3
716.3
708.8
2006
2,082.3
1,388.4
1,095.8
230.2
32.8
29.6
693.8
686.7
2007
2,081.2
1,386.3
1,086.4
229.1
36.2
34.7
694.9
688.0
Percent"
29.1%
19.4%
15.2%
3.2%
0.5%
0.5%
9.7%
9.6%
40 Emissions were not distributed to U.S. territories, since the electricity generation sector only includes emissions related to the
generation of electricity in the 50 states and the District of Columbia.
2-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
CH4
N2O
SF6
Transportation
Direct Emissions
C02
CH4
N2O
HFCsb
Electricity -Related
CO2
CH4
N2O
SF6
Commercial
Direct Emissions
C02
CH4
N2O
MFCs
Electricity-Related
C02
CH4
N2O
SF6
Residential
Direct Emissions
C02
CH4
N2O
MFCs
Electricity-Related
CO2
CH4
N2O
SF6
Agriculture
Direct Emissions
C02
CH4
N2O
Electricity -Related
C02
CH4
N2O
SF6
U.S. Territories
Total
0.2
3.1
9.7
1,546.7
1,543.6
1,496.3
4.5
42.7
+
3.1
3.1
+
+
+
942.2
392.9
214.5
173.9
4.4
+
549.3
538.7
0.2
2.5
7.9
950.0
344.5
337.7
4.4
2.1
0.3
605.5
593.8
0.2
2.8
8.7
459.2
428.5
38.9
176.1
213.5
30.6
30.0
+
0.1
0.4
34.1
6,098.7
0.2
3.2
7.5
1,688.3
1,685.2
1,610.0
4.1
52.5
18.6
3.1
3.1
+
+
+
1,000.2
401.0
224.4
170.8
5.2
0.7
599.2
589.8
0.2
2.7
6.5
1,024.2
368.8
354.4
4.0
2.2
8.1
655.4
645.1
0.2
3.0
7.1
489.7
453.7
44.4
192.6
216.7
36.0
35.5
+
0.2
0.4
41.1
6,463.3
0.2
3.4
5.0
1,923.2
1,919.7
1,812.4
3.2
51.6
52.6
3.5
3.5
+
+
+
1,140.0
388.2
226.9
149.7
6.2
5.5
751.7
743.3
0.2
3.3
4.9
1,159.2
386.0
370.4
3.4
2.1
10.1
773.2
764.5
0.2
3.4
5.0
503.2
470.2
47.2
201.3
221.7
33.0
32.6
+
0.1
0.2
47.3
7,008.2
0.2
3.2
4.1
2,003.6
1,998.9
1,891.7
2.2
35.2
69.7
4.8
4.7
+
+
+
1,214.6
401.8
221.8
154.6
6.8
18.5
812.8
804.3
0.2
3.6
4.7
1,237.0
370.5
358.0
3.5
2.4
6.5
866.5
857.4
0.3
3.8
5.0
511.7
482.6
55.3
199.8
227.5
29.0
28.7
+
0.1
0.2
60.5
7,108.6
0.2
3.1
3.9
1,999.0
1,994.4
1,890.8
2.1
32.0
69.5
4.6
4.5
+
+
+
1,201.5
392.6
206.0
157.3
6.9
22.4
808.9
800.6
0.2
3.6
4.5
1,176.1
334.9
321.9
3.2
2.4
7.5
841.2
832.5
0.3
3.7
4.7
530.0
502.9
57.3
218.2
227.4
27.0
26.8
+
0.1
0.2
62.3
7,051.1
0.2
3.0
3.6
2,000.1
1,995.2
1,897.6
2.0
28.6
67.0
4.9
4.8
+
+
+
1,251.2
407.6
214.4
159.7
7.0
26.6
843.6
835.3
0.3
3.7
4.4
1,229.8
355.3
340.6
3.5
2.5
8.6
874.5
865.9
0.3
3.8
4.5
530.1
502.8
56.9
219.2
226.7
27.3
27.0
+
0.1
0.1
57.7
7,150.1
+
+
0.1%
28.0%
27.9%
26.5%
+
0.4%
0.9%
0.1%
0.1%
+
+
+
17.5%
5.7%
3.0%
2.2%
0.1%
0.4%
11.8%
11.7%
+
0.1%
0.1%
17.2%
5.0%
4.8%
+
+
0.1%
12.2%
12.1%
+
0.1%
0.1%
7.4%
7.0%
0.8%
3.1%
3.2%
0.4%
0.4%
+
+
+
0.8%
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 CO2 Eq. or 0.05 percent.
a Percent of total emissions for year 2007.
b Includes primarily HFC-134a.
Trends in Greenhouse Gas Emissions 2-19
-------
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 by-product of the non-energy-related
industrial process activities. The variety of activities producing these non-energy-related emissions, to name a few
includes fugitive CH4 emissions from coal mining, by-product CO2 emissions from cement manufacture, and HFC,
PFC, and SF6 by-product emissions from semiconductor manufacture. Overall, direct industry sector emissions
have declined since 1990, while electricity-related emissions have risen. In theory, emissions from the industrial
end-use sector should be highly correlated with economic growth and industrial output, but heating of industrial
buildings and agricultural energy consumption are also affected by weather conditions. In addition, structural
changes within the U.S. economy that lead to shifts in industrial output away from energy-intensive manufacturing
products to less energy-intensive products (e.g., from steel to computer equipment) also have a significant affect 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 2007. The largest sources of transportation GHGs in 2007 were
passenger cars (33 percent), light duty trucks, which include sport utility vehicles, pickup trucks, and minivans (28
percent), freight trucks (21 percent) and commercial aircraft (8 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 2007, transportation emissions rose by 29 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 40 percent from 1990 to 2007, as a result of a
confluence of factors including population growth, economic growth, urban sprawl, and low fuel prices over much
of this period. A similar set of social and economic trends has led to a significant increase in air travel and freight
transportation by both air and road modes during the time series.
Although average fuel economy over this period increased slightly due primarily to the retirement of older vehicles,
average fuel economy among new vehicles sold annually gradually declined from 1990 to 2004. The decline in new
vehicle fuel economy between 1990 and 2004 reflected the increasing market share of light duty trucks, which grew
from about one-fifth of new vehicle sales in the 1970s to slightly over half of the market by 2004. Increasing fuel
prices have since decreased the momentum of light duty truck sales, and average new vehicle fuel economy has
improved since 2005 as the market share of passenger cars increased. VMT growth among all passenger vehicles
has also been impacted, growing an average annual rate of 0.6 percent from 2004 to 2007, compared to an annual
rate of 2.6 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 29 percent from 1990 to
2007. This rise in CO2 emissions, combined with an increase in HFCs from virtually no emissions in 1990 to 67.0
Tg CO2 Eq. in 2007, led to an increase in overall emissions from transportation activities of 28 percent.
Table 2-15: Transportation-Related Greenhouse Gas Emissions (Tg CO2 Eq.)
Gas/Vehicle Type
Passenger Cars
C02
CH4
N2O
HFCs
Light-Duty Trucks
CO2
CH4
N2O
1990
656.9
628.8
2.6
25.4
+
336.2
320.7
1.4
14.1
1995
644.1
604.9
2.1
26.9
10.1
434.7
405.0
1.4
22.1
2000
694.6
643.5
1.6
25.2
24.3
508.3
466.2
1.1
22.4
2005
705.8
658.4
1.1
17.8
28.5
544.8
502.8
0.7
13.7
2006
678.3
634.4
1.0
15.7
27.2
557.1
515.5
0.7
12.6
2007
664.6
625.0
0.9
13.7
24.9
561.7
522.0
0.6
11.1
2-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
MFCs
Medium- and Heavy-
Duty Trucks
C02
CH4
N2O
MFCs
Buses
C02
CH4
N2O
MFCs
Motorcycles
C02
CH4
N2O
Commercial Aircraft"
C02
CH4
N2O
Other Aircraft11
C02
CH4
N2O
Ships and Boats0
C02
CH4
N2O
MFCs
Rail
C02
CH4
N2O
MFCs
Other Emissions from
Electricity Generation"1
Pipelines6
C02
Lubricants
C02
Total Transportation
International Bunker Fuel/
+
228.8
227.8
0.2
0.8
+
8.3
8.3
+
+
+
1.8
1.7
+
+
136.9
135.5
0.1
1.3
44.4
43.9
0.1
0.4
46.9
46.5
0.1
0.4
+
38.6
38.1
0.1
0.3
+
0.1
36.2
36.2
11.9
11.9
1,546.7
115.6
6.1
272.7
271.2
0.2
1.0
0.3
9.1
9.0
+
+
+
1.8
1.8
+
+
143.1
141.6
0.1
1.4
32.3
32.0
0.1
0.3
56.6
55.5
0.1
0.4
0.6
44.1
42.2
0.1
0.3
1.4
0.1
38.5
38.5
11.3
11.3
1,688.3
702.7
18.6
344.2
341.3
0.1
1.2
1.6
11.1
10.9
+
+
0.1
1.9
1.8
+
+
167.8
166.0
0.1
1.6
32.9
32.5
0.1
0.3
65.1
61.0
0.1
0.5
3.4
50.1
45.1
0.1
0.3
4.6
0.1
35.2
35.2
12.1
12.1
1,923.2
100.0
27.7
395.1
391.6
0.1
1.2
2.1
12.1
11.8
+
+
0.2
1.6
1.6
+
+
159.8
158.2
0.1
1.5
34.5
34.1
0.1
0.3
50.7
45.4
0.1
0.4
4.7
56.7
49.8
0.1
0.4
6.4
0.1
32.4
32.4
10.2
10.2
2,003.6
112.7
28.3
404.5
401.1
0.1
1.1
2.2
12.4
12.1
+
+
0.3
1.9
1.9
+
+
155.5
153.9
0.1
1.5
33.8
33.4
0.1
0.3
54.1
48.7
0.1
0.4
4.9
58.9
51.8
0.1
0.4
6.5
0.1
32.6
32.6
9.9
9.9
1,999.0
111.7
27.9
410.8
407.4
0.1
1.1
2.2
12.4
12.1
+
+
0.3
2.1
2.0
+
+
155.2
153.6
0.1
1.5
34.2
33.9
0.1
0.3
56.3
50.8
0.1
0.4
4.9
58.0
50.8
0.1
0.4
6.6
0.1
34.6
34.6
10.2
10.2
2,000.1
109.9
Note: Totals may not sum due to independent rounding. Passenger cars and light-duty trucks include vehicles typically used for
personal travel and less than 8500 Ibs; medium- and heavy-duty trucks include vehicles 8501 Ibs and above.
HFC emissions primarily reflect HFC-134a.
+ Does not exceed 0.05 Tg CO2 Eq.
a Consists of emissions from jet fuel consumed by domestic operations of commercial aircraft (no bunkers).
b Consists of emissions from jet fuel and aviation gasoline consumption by general aviation and military aircraft.
0 Fluctuations in emission estimates are associated with fluctuations in reported fuel consumption, and may reflect data collection
problems.
d Other emissions from electricity generation are a result of waste incineration (as the majority of municipal solid waste is
combusted in "trash-to-steam" electricity generation plants), electrical transmission and distribution, and a portion of limestone
and dolomite use (from pollution control equipment installed in electricity generation plants).
e CO2 estimates reflect natural gas used to power pipelines, but not electricity. While the operation of pipelines produces CH4 and
N2O, these emissions are not directly attributed to pipelines in the US Inventory.
f 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
2-21
-------
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 increases 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 2007, enteric fermentation was the largest
source of CH4 emissions in the U.S., and agricultural soil management was the largest source of N2O emissions in
the United States. This sector also includes small amounts of CO2 emissions from fossil fuel combustion by
motorized farm equipment like tractors.
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 34 percent of total U.S. emissions. Electricity generation also
accounted for the largest share of CO2 emissions from fossil fuel combustion, approximately 42 percent in 2007.
Electricity was consumed primarily in the residential, commercial, and industrial end-use sectors for lighting,
heating, electric motors, appliances, electronics, and air conditioning.
[BEGIN BOX]
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, CO2 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 N2O are also based on the EIA electric utility sector. Additional sources include CO2 and N2O from
waste incineration, as the majority of municipal solid waste is combusted in "trash-to-steam" electricity generation
plants. The Electricity Generation economic sector also includes SF6 from Electrical Transmission and Distribution,
and a portion of CO2 from Limestone and Dolomite Use (from pollution control equipment installed in electricity
generation plants).
In the Transportation economic sector, the CO2 emissions from the combustion of fossil fuels included in the EIA
2-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
transportation fuel consuming sector are apportioned to this economic sector (additional analyses and refinement of
the El A data is further explained in the Energy chapter of this report). Additional emissions are apportioned from
the CH4 and N2O 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, CO2 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 CO2 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 N2O 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 metallurgical coke production, and Ammonia Production.
Additionally, fugitive emissions from energy production sources, such as Natural Gas Systems, Coal Mining, and
Petroleum Systems are included in the Industry economic sector. A portion of CO2 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 CO2 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 EIA'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 CO2 emissions from fossil fuel combustion, and CH4 and N2O 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 N2O emissions from Agricultural Soils, CH4 from Enteric Fermentation (i.e., exhalation from the digestive tracts
of domesticated animals), CH4 and N2O from Manure Management, CH4 from Rice Cultivation, CO2 emissions
from liming of agricultural soils and urea application, and CH4 and N2O from Forest Fires. N2O 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 CO2 emissions from the combustion of fossil fuels reported for the
EIA residential sector. Stationary combustion emissions of CH4 and N2O 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. N2O
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 CO2 emissions from the combustion of fossil fuels reported in the
EIA commercial fuel consuming sector data. Stationary combustion emissions of CH4 and N2O are also based on the
EIA commercial sector. Substitutes of Ozone Depleting Substitutes 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 N2O from Wastewater Treatment and
Composting are included in this economic sector.
[END BOX]
Trends in Greenhouse Gas Emissions 2-23
-------
[BEGIN BOX]
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 2007; (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 Table 2-16).
Table 2-16: Recent Trends in Various U.S. Data (Index 1990 = 100)
Variable
GDPb
Electricity Consumption0
Fossil Fuel Consumption0
Energy Consumption0
Population"1
Greenhouse Gas Emissions6
1990
100
100
100
100
100
100
1995
113
112
107
108
107
106
2000
138
127
117
117
113
115
2005
155
134
119
119
118
117
2006
159
135
117
118
119
116
2007
162
137
119
120
120
117
Growth
Rate3
2.9%
1.9%
1.1%
1.1%
1.1%
0.9%
a Average annual growth rate
b Gross Domestic Product in chained 2000 dollars (BEA 2007)
0 Energy-content-weighted values (EIA 2008b)
d U.S. Census Bureau (2008)
e GWP-weighted values
Figure 2-14: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product
Source: BEA (2008), U.S. Census Bureau (2008), and emission estimates in this report.
[END BOX]
2.3. Indirect Greenhouse Gas Emissions (CO, NOX, NMVOCs, and
The reporting requirements of the UNFCCC41 request that information be provided on indirect greenhouse gases,
which include CO, NOX, NMVOCs, and SO2. These gases do not have a direct global warming effect, but indirectly
affect terrestrial radiation absorption by influencing the formation and destruction of tropospheric and stratospheric
ozone, or, in the case of SO2, by affecting the absorptive characteristics of the atmosphere. Additionally, some of
these gases may react with other chemical compounds in the atmosphere to form compounds that are greenhouse
gases. Carbon monoxide is produced when carbon-containing fuels are combusted incompletely. Nitrogen oxides
(i.e., NO and NO2) are created by lightning, fires, fossil fuel combustion, and in the stratosphere from N2O. Non-
CH4 volatile organic compounds—which include hundreds of organic compounds that participate in atmospheric
41 See .
2-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 CD's interaction with the hydroxyl radical—the major
atmospheric sink for CH4 emissions—to form CO2. Therefore, increased atmospheric concentrations of CO limit
the number of hydroxyl molecules (OH) available to destroy CH4.
Since 1970, the United States has published estimates of annual emissions of CO, NOX, NMVOCs, and SO2 (EPA
2005),42 which are regulated under the Clean Air Act. Table 2-17 shows that fuel combustion accounts for the
majority of emissions of these indirect greenhouse gases. Industrial processes—such as the manufacture of
chemical and allied products, metals processing, and industrial uses of solvents—are also significant sources of CO,
NOX, and NMVOCs.
Table 2-17: Emissions of NOX, CO, NMVOCs, and SO2 (Gg)
Gas/Activity
NOX
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel
Combustion
Industrial Processes
Oil and Gas Activities
Incineration of Waste
Agricultural Burning
Solvent Use
Waste
CO
Mobile Fossil Fuel Combustion
Stationary Fossil Fuel
Combustion
Industrial Processes
Incineration of Waste
Agricultural Burning
Oil and Gas Activities
Waste
Solvent Use
NMVOCs
Mobile Fossil Fuel Combustion
Solvent Use
Industrial Processes
Stationary Fossil Fuel
Combustion
Oil and Gas Activities
Incineration of Waste
Waste
Agricultural Burning
S02
Stationary Fossil Fuel
Combustion
1990
21,450
10,920
9,689
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
1995
21,070
10,622
9,619
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
2000
19,004
10,310
7,802
626
111
114
35
3
2
92,776
83,559
4,340
2,216
1,670
792
146
8
45
15,227
7,229
4,384
1,773
1,077
388
257
119
NA
14,830
12,849
2005
15,612
8,757
5,857
534
321
98
39
5
2
71,672
62,519
4,778
1,744
1,439
860
324
7
2
14,562
6,292
3,881
2,035
1,450
545
243
115
NA
13,348
11,641
2006
14,701
8,271
5,445
527
316
98
38
5
2
67,453
58,322
4,792
1,743
1,438
825
323
7
2
14,129
5,954
3,867
1,950
1,470
535
239
113
NA
12,259
10,650
2007
14,250
7,831
5,445
520
314
97
37
5
2
63,875
54,678
4,792
1,743
1,438
892
323
7
2
13,747
5,672
3,855
1,878
1,470
526
234
111
NA
11,725
10,211
42 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
2-25
-------
Industrial Processes
Mobile Fossil Fuel Combustion
Oil and Gas Activities
Incineration of Waste
Waste
Solvent Use
Agricultural Burning
1,307
793
390
38
0
0
NA
1,117
672
335
42
1
1
NA
1,031
632
287
29
1
1
NA
852
600
233
22
1
0
NA
845
520
221
22
1
0
NA
839
442
210
22
1
0
NA
Source: (EPA 2005) except for estimates from field burning of agricultural residues.
NA (Not Available)
Note: Totals may not sum due to independent rounding.
[BEGIN BOX]
Box 2-3: Sources and Effects of Sulfur Dioxide
Sulfur dioxide (SO2) 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 SO2 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 SO2 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 SO2 emissions in the Clean Air Act.
Electricity generation is the largest anthropogenic source of SO2 emissions in the United States, accounting for 87
percent in 2007. Coal combustion contributes nearly all of those emissions (approximately 92 percent). Sulfur
dioxide emissions have decreased in recent years, primarily as a result of electric power generators switching from
high-sulfur to low-sulfur coal and installing flue gas desulfurization equipment.
[END BOX]
2-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
I MFCs, PFCs, &SF6
Nitrous Oxide
Methane
i Carbon Dioxide
6,099 6,054 6,156 6,288 6,395 6,463
6,673
6/27 6769 6,822 7>008 6,896 6,942 6,981 7,065 7,109 7,051 7,150
8,000 -,
7,000 -
6,000 -
5,000 -
S
O 4,000 -
u
i5?
H 3,000 -
2,000 -
1,000
0 -
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure 2-1: U.S. Greenhouse Gas Emissions by Gas
4% -,
3% -
2% -
1% -
0%
-1% -
3.2%
2.7%
1.4%
-2% J
-0.8%
-1.6%
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure 2-2: Annual Percent Change in U.S. Greenhouse Gas Emissions
1,010 1,051
966 -L_ 952
1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure 2-3: Cumulative Change in Annual U.S. Greenhouse Gas Emissions Relative to 1990
-------
Industrial Processes
Waste
8
Agriculture
LULUCF (sources)
\
Energy
7,500
7,000 -
6,500
6,000
5,500
5,000 -
4,500
4,000
3,500
3,000 -
2,500
"~ 2,000
1,500 -
1,000
500
0
(500) -
(1,000) -
(1,500) -
T-lT-lT-lT-lT-lT-lT-lT-lT-lT-l(N
-------
NEU Emissions
3
Natural Gas Emissions
1,225
NEU Emissions 122
Non-Energy Use
Carbon Sequestered
227
Fossil Fuel Stock Non-Energy
Non-Energy Consumption Changes Use U.S.
Use imports U.S. 25 Territories
55 Territories °
51
Note: Totals may not sum due to independent rounding.
The "Balancing Item" above accounts for the statisticai imbalances
and unknowns in the reported data sets combined here.
2-6 2007 U.S. (Tg C02 Eq.)
-------
o
p
2,500 -i
2,000 -
1,500 -
1,000 -
500 -
0 -
Relative Contribution
by Fuel Type
I Natural Gas
Petroleum
I Coal
Figure 2-7: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type
Note: Electricity generation also includes emissions of less than 0.5 Tg CO 2 Eq. from geothermal-based electricity
generation.
2,500 -i
2,000
1,500 -
1,000 -
500 -
0
From Electricity
Consumption
i From Direct Fossil Fuel
Combustion
Figure 2-8: 2007 End-Use Sector Emissions of CO2, CH4, and N2O from Fossil Fuel Combustion
-------
Substitution of Ozone Depleting Substances
Iron and Steel Production & Metallurgical Coke Production
Cement Production
Nitric Acid Production
HCFC-22 Production
Lime Production
Ammonia Production and Urea Consumption
Electrical Transmission and Distribution
Aluminum Production
Limestone and Dolomite Use
Adipic Acid Production
Semiconductor Manufacture
Soda Ash Production and Consumption
Petrochemical Production
Magnesium Production and Processing
Titanium Dioxide Production
Carbon Dioxide Consumption |
Ferroalloy Production |
Phosphoric Acid Production |
Zinc Production |
Lead Production
Silicon Carbide Production and Consumption
<0.5
<0.5
Industrial Processes
as a Portion of all Emissions
4.9%
25
50 75
TgCO2Eq.
100
125
Figure 2-9: 2007 Industrial Processes Chapter Greenhouse Gas Sources
Agricultural Soil Management
Enteric Fermentation
Manure Management
Rice Cultivation I
Field Burning of Agricultural Residues
207.9
Agriculture as a Portion of all
Emissions
5.8%
©
0 50 100
TgCO2Eq.
Figure 2-10: 2007 Agriculture Chapter Greenhouse Gas Sources
150
-------
Landfills
Wastewater Treatment
Composting
Waste as a Portion of all Emissions
2.3%
I
20
40
60 80
TgCO2Eq.
Figure 2-11: 2007 Waste Chapter Greenhouse Gas Sources
100
120
140
Electricity
Generation
Transportation
Industry
Agriculture
Commercial
Residential
19901991 1992 1993199419951996199719981999 2000 2001 2002 2003 2004 2005 2006 2007
Figure 2-12: Emissions Allocated to Economic Sectors
Note: Does not include U.S. Territories.
-------
2,500 -|
2,000 -
3 1,500 1
o
ra 1,000
500 -
Industrial
Transportation
Residential (gray)
Commercial (black)
Agriculture
O)O)O)O)O)O)O)O)O)O)OOOOOOOO
O)O)O)O)O)O)O)O)O)O)OOOOOOOO
T-T-T-T-T-T-T-T-T-T-(N
-------
-------
3. Energy
Energy-related activities were the primary sources of U.S. anthropogenic greenhouse gas emissions, accounting for
86.3 percent of total emissions on a carbon dioxide (CO2) equivalent basis in 2007. This included 97, 35, and 14
percent of the nation's CO2, methane (CH4), 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 29,195 Tg of CO2 were added to the atmosphere
through the combustion of fossil fuels in 2006, of which the United States accounted for about 20 percent.43 Due to
their relative importance, fossil fuel combustion-related CO2 emissions are considered separately, and in more detail
than other energy-related emissions (see Figure 3-2). Fossil fuel combustion also emits CH4 and N2O, as well as
indirect greenhouse gases such as nitrogen oxides (NOX), carbon monoxide (CO), 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.
Figure 3-1: 2007 Energy Chapter Greenhouse Gas Sources
Figure 3-2: 2007 U.S. Fossil Carbon Flows (Tg CO2 Eq.)
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 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 6,170.3 Tg CO2 Eq. in 2007, an increase of 19 percent since 1990.
Table 3-1: CO2, CH4, and N2O Emissions from Energy (Tg CO2 Eq.)
Gas/Source
C02
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
1990
4,871.0
4,708.9
1,809.7
1,484.5
834.2
337.7
214.5
1995
5,201.2
5,013.9
1,938.9
1,598.7
862.6
354.4
224.4
2000
5,753.2
5,561.5
2,283.2
1,800.3
844.6
370.4
226.9
2005
5,910.8
5,723.5
2,381.0
1,881.5
828.0
358.0
221.8
2006
5,830.2
5,635.4
2,327.3
1,880.9
844.5
321.9
206.0
2007
5,919.5
5,735.8
2,397.2
1,887.4
845.4
340.6
214.4
43 Global CO2 emissions from fossil fuel combustion were taken from Energy Information Administration International Energy
Annual 2006 EIA (2008).
Energy 3-1
-------
U.S. Territories
Non-Energy Use of Fuels
Natural Gas Systems
Incineration of Waste
Petroleum Systems
Wood Biomass andEthanol
Consumption *
International Bunker Fuels*
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground
Coal Mines
Mobile Combustion
International Bunker Fuels*
N2O
Mobile Combustion
Stationary Combustion
Incineration of Waste
International Bunker Fuels*
Total
28.3
117.0
33.7
10.9
0.4
219.3
114.3
265.7
129.6
84.1
33.9
7.4
6.0
4.7
0.2
57.0
43.7
12.8
0.5
1.1
5,193.6
35.0
137.5
33.8
15.7
0.3
236. 8
101.6
251.4
132.6
67.1
32.0
7.1
8.2
4.3
0.1
67.5
53.7
13.3
0.5
0.9
5,520.1
36.2
144.5
29.4
17.5
0.3
227.3
99.0
239.0
130.8
60.5
30.3
6.6
7.4
3.4
0.1
67.7
52.8
14.5
0.4
0.9
6,059.9
53.2
138.1
29.5
19.5
0.3
231.5
111.5
206.5
106.3
57.1
28.3
6.7
5.6
2.5
0.1
51.9
36.7
14.8
0.4
1.0
6,169.2
54.8
145.1
29.5
19.8
0.3
240.4
110.5
205.7
104.8
58.4
28.3
6.3
5.5
2.4
0.1
48.5
33.5
14.5
0.4
1.0
6,084.4
50.8
133.9
28.7
20.8
0.3
247.8
108.8
205.7
104.7
57.6
28.8
6.6
5.7
2.3
0.1
45.2
30.1
14.7
0.4
1.0
6,170.3
* 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: CO2, CH4, and N2O Emissions from Energy (Gg)
Gas/Source
CO2
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Incineration of Waste
Petroleum Systems
Wood Biomass and Ethanol
Consumption *
International Bunker Fuels*
CH4
Natural Gas Systems
Coal Mining
Petroleum Systems
Stationary Combustion
Abandoned Underground
Coal Mines
Mobile Combustion
International Bunker Fuels*
N2O
Mobile Combustion
Stationary Combustion
Incineration of Waste
International Bunker Fuels*
1990
4,870,953
4,708,918
116,977
33,733
10,950
376
219,341
114,330
12,651
6,171
4,003
1,613
352
288
225
8
184
141
41
2
3
1995
5,201,233
5,013,910
137,460
33,810
15,712
341
236, 775
101,620
11,970
6,314
3,193
1,524
340
392
207
6
218
173
43
1
3
2000
5,753,192
5,561,515
144,473
29,394
17,485
325
227,276
98,966
11,381
6,231
2,881
1,441
315
350
163
6
219
170
47
1
3
* These values are presented for informational purposes only and are not included or are already
2005
5,910,830
5,723,477
138,070
29,463
19,532
287
231,481
111,487
9,832
5,062
2,719
1,346
318
265
121
7
167
118
48
1
3
2006
5,830,206
5,635,418
145,137
29,540
19,824
288
240,386
110,520
9,795
4,991
2,780
1,346
300
263
115
7
156
108
47
1
3
2007
5,919,452
5,735,789
133,910
28,680
20,786
287
247,829
108,756
9,796
4,985
2,744
1,370
315
273
109
7
146
97
47
1
3
accounted for in totals.
Note: Totals may not sum due to independent rounding.
3-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
3.1. Fossil Fuel Combustion (IPCC Source Category 1A)
Emissions from the combustion of fossil fuels for energy include the gases CO2, CH4, and N2O. Given that CO2 is
the primary gas emitted from fossil fuel combustion and represents the largest share of U.S. total emissions, CO2
emissions from fossil fuel combustion are discussed at the beginning of this section. Following that is a discussion
of emissions of all three gases from fossil fuel combustion presented by sectoral breakdowns. Methodologies for
estimating CO2 from fossil fuel combustion also differ from the estimation of CH4 and N2O emissions from
stationary combustion and mobile combustion. Thus, three separate descriptions of methodologies, uncertainties,
recalculations, and planned improvements are provided at the end of this section. Total CO2, CH4, and N2O
emissions from fossil fuel combustion are presented in Table 3-3 and Table 3-4.
Table 3-3: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion (Tg CO2 Eq.)
Gas
C02
CH4
N2O
Total
1990
4,708.9
12.1
56.5
4,777.6
1995
5,013.9
11.5
67.0
5,092.4
2000
5,561.5
10.0
67.4
5,638.9
2005
5,723.5
9.2
51.5
5,784.2
2006
5,635.4
8.7
48.1
5,692.2
2007
5,735.8
8.9
44.8
5,789.5
Note: Totals may not sum due to independent rounding.
Table 3-4: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion (Gg)
Gas
C02
CH4
N2O
1990
4,708,918
578
182
1995
5,013,910
547
216
2000
5,561,515
478
217
2005
5,723,477
439
166
2006
5,635,418
415
155
2007
5,735,789
424
145
Note: Totals may not sum due to independent rounding.
C02 from Fossil Fuel Combustion
CO2 is the primary gas emitted from fossil fuel combustion and represents the largest share of U.S. total greenhouse
gas emissions. CO2 emissions from fossil fuel combustion are presented in Table 3-5. In 2007, CO2 emissions from
fossil fuel combustion increased by 1.8 percent relative to the previous year. This increase is primarily a result of an
increase in electricity demand, combined with a significant decrease (14.2 percent) in hydropower generation used
to meet this demand. Additionally, cooler winter and warmer summer conditions in 2007 than in 2006 increased the
demand for heating fuels and contributed to the increase in the demand for electricity. In 2007, CO2 emissions from
fossil fuel combustion were 5,735.8 Tg CO2 Eq., or 22 percent above emissions in 1990 (see Table 3-5).44
Table 3-5: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq.)
Fuel/Sector 1990 1995 2000 2005 2006
2007
Coal
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Natural Gas
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Petroleum
1,695.9
2.9
11.8
149.5
NE
1,531.1
0.6
1,001.7
237.4
141.5
410.1
36.2
176.5
NO
2,010.9
1,801.9
1.7
11.1
139.6
NE
1,648.6
0.9
1,159.1
262.3
164.0
465.0
38.6
229.2
NO
2,052.6
2,046.4
1.0
8.2
126.8
NE
1,909.5
0.9
1,210.8
268.8
171.6
452.3
35.6
281.8
0.7
2,303.9
2,088.2
0.8
9.1
116.2
NE
1,958.4
3.7
1,161.4
262.0
163.1
381.8
33.2
319.9
1.3
2,473.5
2,057.2
0.5
6.2
114.1
NE
1,932.4
4.0
1,140.7
236.8
153.8
376.2
33.5
338.9
1.4
2,437.2
2,086.5
0.6
6.8
107.4
NE
1,967.6
4.1
1,216.5
256.9
163.4
385.6
35.4
373.8
1.4
2,432.4
44 An additional discussion of fossil fuel emission trends is presented in the Trends in U.S. Greenhouse Gas Emissions Chapter.
Energy 3-3
-------
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Geothermal*
Total
97.4
61.2
274.6
1,448.3
101.8
27.6
0.40
4,708.9
90.5
49.3
257.9
1,560.1
60.7
34.0
0.34
5,013.9
100.5
47.2
265.5
1,764.7
91.5
34.6
0.36
5,561.5
95.2
49.6
330.0
1,848.2
102.3
48.2
0.38
5,723.5
84.5
46.0
354.2
1,847.4
55.6
49.4
0.37
5,635.4
83.2
44.2
352.5
1,852.0
55.3
45.3
0.38
5,735.8
NE (Not estimated)
NO (Not occurring)
* Although not technically a fossil fuel, geothermal energy-related CO2 emissions are included for reporting purposes.
Note: Totals may not sum due to independent rounding.
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, size of houses, and number of houses),
the efficiency with which energy is used in equipment (e.g., cars, power plants, steel mills, and light bulbs), and
social planning and consumer behavior (e.g., walking, bicycling, or telecommuting to work instead of driving).
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.45 Producing a
unit of heat or electricity using natural gas instead of coal can reduce the CO2 emissions associated with energy
consumption, and using nuclear or renewable energy sources (e.g., wind) can essentially eliminate emissions (see
Box 3-2). Table 3-6 shows annual changes in emissions during the last five years for coal, petroleum, and natural
gas in selected sectors.
Table 3-6: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors (Tg CO2
Eq. and Percent)
Sector
Electricity Generation
Electricity Generation
Electricity Generation
Transportation a
Residential
Commercial
Industrial
Industrial
All Sectors b
Fuel Type
Coal
Natural Gas
Petroleum
Petroleum
Natural Gas
Natural Gas
Coal
Natural Gas
All Fuels b
2003 to
11.4
18.4
2.0
51.1
-13.7
-5.1
1.2
-17.8
64.4
2004
0.6%
6.6%
2.0%
2.9%
-4.9%
-2.9%
1.0%
-4.2%
1.1%
2004 to
40.8
Tin
2.2
19.9
-0.5
-5.7
-2.4
-28.3
54.2
2005
2.1%
7.6%
2.2%
1.1%
-0.2%
-3.4%
-2.0%
-6.9%
1.0%
2005
-26.0
19.0
-46.7
-0.8
-25.2
-9.3
-2.1
-5.6
-88.1
to 2006
-1.3%
5.9%
-45.6%
0.0%
-9.6%
-5.7%
-1.8%
-1.5%
-1.5%
2006 to
35.3
34.9
-0.3
4.6
20.1
9.6
-6.7
9.4
100.4
2007
1.8%
10.3%
-0.6%
0.2%
8.5%
6.2%
-5.9%
2.5%
1.8%
a Excludes emissions from International Bunker Fuels.
b Includes fuels and sectors not shown in table.
In the United States, 85 percent of the energy consumed in 2007 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 (7 percent), primarily
hydroelectric power and biofuels (EIA 2008a). Specifically, petroleum supplied the largest share of domestic
energy demands, accounting for an average of 42 percent of total fossil fuel based energy consumption in 2007.
45 Based on national aggregate carbon 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: 1990-2007
-------
Natural gas and coal followed in order of importance, accounting for 30 and 28 percent of total consumption,
respectively. Petroleum was consumed primarily in the transportation end-use sector, the vast majority of coal was
used in electricity generation, and natural gas was broadly consumed in all end-use sectors except transportation (see
Figure 3-5) (EIA 2008a).
Figure 3-3: 2007 U.S. Energy Consumption by Energy Source
Figure 3-4: U.S. Energy Consumption (Quadrillion Bru)
Figure 3-5: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type
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 CH4, CO, and NMVOCs.46 These other C containing non-CO2 gases are emitted as a by-product of
incomplete fuel combustion, but are, for the most part, eventually oxidized to CO2 in the atmosphere. Therefore, it
is assumed that all of the C in fossil fuels used to produce energy is eventually converted to atmospheric CO2.
[BEGIN BOX]
Box 3-1: Weather and Non-Fossil Energy Effects on CO2 from Fossil Fuel Combustion Trends
In 2007, weather conditions became much cooler in the winter and slightly warmer in the summer, compared to
2006. Although winter conditions were cooler in 2007 compared to 2006, the winter was warmer than normal, with
heating degree days in the United States 6 percent below normal (see Figure 3-6). Cooler winter conditions
compared to 2006 led to an increase in demand for heating fuels. Although summer conditions were slightly
warmer in 2007 compared to 2006, summer temperatures were substantially warmer than usual, with cooling degree
days 13 percent above normal (see Figure 3-7) (EIA 2008I).47 As a result, the demand for electricity increased due
to warmer summer conditions compared to 2006.
Figure 3-6: Annual Deviations from Normal Heating Degree Days for the United States (1950-2007)
Figure 3-7: Annual Deviations from Normal Cooling Degree Days for the United States (1950-2007)
Although no new U.S. nuclear power plants have been constructed in recent years, the utilization (i.e., capacity
factors48) of existing plants in 2007 remained high at just over 90 percent. Electricity output by hydroelectric power
46 See the sections entitled Stationary Combustion and Mobile Combustion in this chapter for information on non-CO2 gas
emissions from fossil fuel combustion.
47 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).
48 The capacity factor is defined as the ratio of the electrical energy produced by a generating unit for a given period of time to
Energy 3-5
-------
plants decreased in 2007 by approximately 14 percent. Electricity generated by nuclear plants in 2007 provided
almost 3 times as much of the energy consumed in the United States as hydroelectric plants (EIA 2008a). 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 (1974-2007)
[END BOX]
Fossil Fuel Combustion Emissions by Sector
In addition to the CO2 emitted from fossil fuel combustion, CH4 and N2O are emitted from stationary and mobile
combustion as well. Table 3-7 provides an overview of the CO2, CH4, and N2O emissions from fossil fuel
combustion by sector.
Table 3-7: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion by Sector (Tg CO2 Eq.)
End-Use Sector
Electricity Generation
C02
CH4
N2O
Transportation
C02
CH4
N2O
Industrial
C02
CH4
N2O
Residential
CO2
CH4
N2O
Commercial
C02
CH4
N2O
U.S. Territories*
Total
1990
1,818.3
1,809.7
0.6
8.1
1,532.9
1,484.5
4.7
43.7
838.9
834.2
1.5
3.2
343.2
337.7
4.4
1.1
215.8
214.5
0.9
0.4
28.4
4,777.6
1995
1,948.0
1,938.9
0.6
8.6
1,656.7
1,598.7
4.3
53.7
867.5
862.6
1.6
3.3
359.4
354.4
4.0
1.0
225.7
224.4
0.9
0.4
35.1
5,092.4
2000
2,293.8
2,283.2
0.7
10.0
1,856.5
1,800.3
3.4
52.8
849.4
844.6
1.6
3.2
374.7
370.4
3.4
0.9
228.2
226.9
0.9
0.3
36.3
5,638.9
2005
2,392.1
2,381.0
0.7
10.3
1,920.7
1,881.5
2.5
36.7
832.5
828.0
1.5
3.1
362.5
358.0
3.5
0.9
223.0
221.8
0.9
0.3
53.4
5,784.2
2006
2,338.1
2,327.3
0.7
10.1
1,916.8
1,880.9
2.4
33.5
849.2
844.5
1.5
3.2
325.9
321.9
3.2
0.8
207.2
206.0
0.8
0.3
55.0
5,692.2
2007
2,408.2
2,397.2
0.7
10.3
1,919.8
1,887.4
2.3
30.1
849.9
845.4
1.5
3.1
345.1
340.6
3.5
0.9
215.5
214.4
0.8
0.3
51.0
5,789.5
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.
* U.S. Territories are not apportioned by sector, and emissions are total greenhouse gas emissions from all fuel combustion
Other than CO2, gases emitted from stationary combustion include the greenhouse gases CH4 and N2O and the
indirect greenhouse gases NOX, CO, and NMVOCs. 49 CH4 and N2O emissions from stationary combustion sources
depend upon fuel characteristics, size and vintage, along with combustion technology, pollution control equipment,
ambient environmental conditions, and operation and maintenance practices. N2O emissions from stationary
the electrical energy that could have been produced at continuous full-power operation during the same period (EIA 2008a).
49 Sulfur dioxide (SO2) emissions from stationary combustion are addressed in Annex 6.3.
3-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 CH4 content of the fuel and combustion efficiency.
Mobile combustion produces greenhouse gases other than CO2, including CH4, N2O, and indirect greenhouse gases
including NOX, CO, and NMVOCs. As with stationary combustion, N2O and NOX emissions from mobile
combustion are closely related to fuel characteristics, air-fuel mixes, combustion temperatures, and the use of
pollution control equipment. N2O from mobile sources, in particular, can be formed by the catalytic processes used
to control NOX, CO, and hydrocarbon emissions. Carbon monoxide emissions from mobile combustion are
significantly affected by combustion efficiency and the presence of post-combustion emission controls. 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 CH4 content of the motor fuel, the amount of hydrocarbons passing uncombusted
through the engine, and any post-combustion control of hydrocarbon emissions (such as catalytic converters).
An alternative method of presenting combustion emissions is to allocate emissions associated with electricity
generation to the sectors in which it is used. Four end-use sectors were defined: industrial, transportation,
residential, and commercial. In the table below, electricity generation emissions have been distributed to each end-
use sector based upon the sector's share of national electricity consumption, with the exception of CH4 and N2O
from transportation.50 Emissions from U.S. territories are also calculated separately due to a lack of end-use-specific
consumption data. This method of distributing emissions assumes that each sector consumes electricity generated
from an equally carbon-intensive mix of fuels and other energy sources. Table 3-7 and Table 3-8 summarize CO2,
CH4, and N2O emissions from direct fossil fuel combustion and pro-rated electricity generation emissions from
electricity consumption by end-use sector. The following discussions for stationary combustion sources focus on
direct emissions, as presented in Table 3-7, while the discussion of transportation and mobile combustion sources
focus on the alternative method as presented in Table 3-8.
Table 3-8: CO2, CH4, and N2O Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.)
End-Use Sector
Transportation
C02
CH4
N2O
Industrial
CO2
CH4
N2O
Residential
C02
CH4
N2O
Commercial
C02
CH4
N2O
U.S. Territories*
Total
1990
1,536.0
1,487.5
4.7
43.7
1,524.7
1,516.8
1.7
6.2
935.4
927.1
4.6
3.7
753.0
749.2
1.0
2.8
28.4
4,777.6
1995
1,659.7
1,601.7
4.3
53.7
1,583.8
1,575.5
1.8
6.5
1,001.3
993.3
4.2
3.8
812.5
808.5
1.1
2.9
35.1
5,092.4
2000
1,860.0
1,803.7
3.4
52.8
1,638.1
1,629.6
1.8
6.7
1,136.1
1,128.2
3.6
4.2
968.5
963.8
1.1
3.6
36.3
5,638.9
2005
1,925.4
1,886.2
2.5
36.7
1,566.4
1,558.5
1.7
6.2
1,215.6
1,207.2
3.8
4.6
1,023.3
1,018.4
1.1
3.8
53.4
5,784.2
2006
1,921.3
1,885.4
2.4
33.6
1,558.7
1,550.7
1.7
6.2
1,153.8
1,145.9
3.4
4.4
1,003.4
998.6
1.1
3.7
55.0
5,692.2
2007
1,924.6
1,892.2
2.3
30.1
1,561.2
1,553.4
1.7
6.1
1,206.4
1,198.0
3.8
4.6
1,046.4
1,041.4
1.1
3.9
51.0
5,789.5
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.
* U.S. Territories are not apportioned by sector, and emissions are total greenhouse gas emissions from all fuel combustion
sources.
50 Separate calculations were performed for transportation-related CH4 and N2O. The methodology used to calculate these
emissions are discussed in the mobile combustion section.
Energy 3-7
-------
Stationary Combustion
The direct combustion of fuels by stationary sources in the electricity generation, industrial, commercial, and
residential sectors represent the greatest share of U.S. greenhouse gas emissions. Table 3-9 presents CO2 emissions
from fossil fuel combustion by stationary sources. The CO2 emitted is closely linked to the type of fuel being
combusted in each sector (see Methodology section for CO2 from fossil fuel combustion). Other than CO2, gases
emitted from stationary combustion include the greenhouse gases CH4 and N2O. Table 3-10 and Table 3-11 present
CH4 and N2O emissions from the combustion of fuels in stationary sources. CH4 and N2O emissions from stationary
combustion sources depend upon fuel characteristics, size and vintage, along with combustion technology, pollution
control equipment, ambient environmental conditions, and operation and maintenance practices. 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 CH4 content of the fuel and combustion efficiency. Please refer to Table 3-7 for the
corresponding presentation of all direct emission sources of fuel combustion.
Table 3-9: CO2 Emissions from Stationary Fossil Fuel Combustion (Tg CO2 Eq.)
Sector/Fuel Type
Electricity Generation
Coal
Natural Gas
Fuel Oil
Geothermal
Industrial
Coal
Natural Gas
Fuel Oil
Commercial
Coal
Natural Gas
Fuel Oil
Residential
Coal
Natural Gas
Fuel Oil
U.S. Territories
Coal
Natural Gas
Fuel Oil
Total
1990
1,809.7
1,531.1
176.5
101.8
0.4
834.2
149.5
410.1
274.6
214.5
11.8
141.5
61.2
337.7
2.9
237.4
97.4
28.3
0.6
NO
27.6
4,708.9
1995
1,938.9
1,648.6
229.2
60.7
0.3
862.6
139.6
465.0
257.9
224.4
11.1
164.0
49.3
354.4
1.7
262.3
90.5
35.0
0.9
NO
34.0
5,013.9
* U.S. Territories are not apportioned by sector, and emissions are
presented in this table.
Table 3 -10: CH4 Emissions
Sector/Fuel Type
Electricity Generation
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial
Coal
from Stationary
1990
0.6
0.3
0.1
0.1
0.1
1.5
0.3
0.2
0.2
0.9
0.9
+
2000
2,283.2
1,909.5
281.8
91.5
0.4
844.6
126.8
452.3
265.5
226.9
8.2
171.6
47.2
370.4
1.0
268.8
100.5
36.2
0.9
0.7
34.6
5,561.5
from all fuel
2005
2,381.0
1,958.4
319.9
102.3
0.4
828.0
116.2
381.8
330.0
221.8
9.1
163.1
49.6
358.0
0.8
262.0
95.2
53.2
3.7
1.3
48.2
5,723.5
2006
2,327.3 2,
1,932.4 1,
338.9
55.6
0.4
844.5
114.1
376.2
354.2
206.0
6.2
153.8
46.0
321.9
0.5
236.8
84.5
54.8
4.0
1.4
49.4
5,635.4 5,
combustion sources (stationary
2007
397.2
967.6
373.8
55.3
0.4
845.4
107.4
385.6
352.5
214.4
6.8
163.4
44.2
340.6
0.6
256.9
83.2
50.8
4.1
1.4
45.3
735.8
and mobile) are
Combustion (Tg CO2 Eq.)
1995
0.6
0.4
+
0.1
0.1
1.6
0.3
0.1
0.2
1.0
0.9
+
2000
0.7
0.4
0.1
0.1
0.1
1.6
0.3
0.1
0.2
1.0
0.9
+
2005
0.7
0.4
0.1
0.1
0.1
1.5
0.3
0.2
0.1
0.9
0.9
+
2006
0.7
0.4
+
0.1
0.1
1.5
0.3
0.2
0.1
0.9
0.8
+
2007
0.7
0.4
+
0.1
0.1
1.5
0.2
0.2
0.1
0.9
0.8
+
3-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Fuel Oil
Natural Gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
Total
0.2
0.3
0.4
4.4
0.2
0.3
0.4
3.5
+
+
+
+
+
7.4
0.1
0.3
0.4
4.0
0.1
0.3
0.5
3.1
+
+
+
+
+
7.1
0.1
0.3
0.4
3.4
0.1
0.3
0.5
2.5
0.1
+
+
+
+
6.6
0.1
0.3
0.4
3.5
0.1
0.3
0.5
2.7
0.1
+
0.1
+
+
6.7
0.1
0.3
0.4
3.2
+
0.3
0.4
2.5
0.1
+
0.1
+
+
6.3
0.1
0.3
0.4
3.5
+
0.3
0.5
2.8
0.1
+
0.1
+
+
6.6
+ Does not exceed 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
Table 3-11: N2O Emissions from
Sector/Fuel Type
Electricity Generation
Coal
Fuel Oil
Natural Gas
Wood
Industrial
Coal
Fuel Oil
Natural Gas
Wood
Commercial
Coal
Fuel Oil
Natural Gas
Wood
Residential
Coal
Fuel Oil
Natural Gas
Wood
U.S. Territories
Coal
Fuel Oil
Natural Gas
Wood
Total
Stationary Combustion
1990
8.1
7.6
0.2
0.1
0.2
3.2
0.7
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
3.3
0.7
0.4
0.3
1.9
0.4
0.1
0.1
0.1
0.1
1.0
+
0.2
0.1
0.6
0.1
+
0.1
+
+
13.3
(TgC02Eq.)
2000
10.0
9.4
0.2
0.2
0.2
3.2
0.6
0.4
0.3
1.9
0.3
+
0.1
0.1
0.1
0.9
+
0.3
0.1
0.5
0.1
+
0.1
+
+
14.5
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
1.8
0.3
+
0.1
0.1
0.1
0.8
+
0.2
0.1
0.5
0.1
+
0.1
+
+
14.5
2007
10.3
9.7
0.1
0.2
0.2
3.1
0.5
0.6
0.2
1.7
0.3
+
0.1
0.1
0.1
0.9
+
0.2
0.1
0.5
0.1
+
0.1
+
+
14.7
+ Does not exceed 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
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. CH4 and N2O accounted
for a small portion of emissions from electricity generation, representing less than 0.1 percent and 0.4 percent,
respectively. Electricity generation also accounted for the largest share of CO2 emissions from fossil fuel
combustion, approximately 42 percent in 2007. CH4 and N2O from electricity generation represented 8 and 23
Energy 3-9
-------
percent of emissions from fossil fuel combustion in 2007. Electricity was consumed primarily in the residential,
commercial, and industrial end-use sectors for lighting, heating, electric motors, appliances, electronics, and air
conditioning (see Figure 3-9).
Figure 3-9: Electricity Generation Retail Sales by End-Use Sector
The electric power industry includes all power producers, consisting of both regulated utilities and nonutilities (e.g.
independent power producers, qualifying cogenerators, and other small power producers). For the underlying
energy data used in this chapter, the Energy Information Administration (EIA) places electric power generation into
three functional categories: the electric power sector, the commercial sector, and the industrial sector. The electric
power sector consists of electric utilities and independent power producers whose primary business is the production
of electricity,51 while the other sectors consist of those producers that indicate their primary business is something
other than the production of electricity.
The industrial, residential, and commercial end-use sectors, as presented in Table 3-8, were reliant on electricity for
meeting energy needs. The residential and commercial end-use sectors were especially reliant on electricity
consumption for lighting, heating, air conditioning, and operating appliances. Electricity sales to the residential and
commercial end-use sectors in 2007 increased about 3 percent in the residential and 3.3 percent in the commercial
sectors. The trend in the commercial sector can largely be attributed to the growing economy (2.0 percent), which
led to increased demand for electricity. The increase is also attributed to an increase in air conditioning-related
electricity consumption in the residential and commercial sectors that occurred as a result of the warmer summer
compared to 2006. In 2007, the amount of electricity generated (in kWh) increased by 2.1 percent from the previous
year. This growth is due to the growing economy, expanding industrial production, and warmer summer conditions
compared to 2006. As a result, CO2 emissions from the electric power sector increased by 3.0 percent as the
consumption of coal and natural gas for electricity generation increased. Coal and natural gas consumption for
electricity generation increased by 1.8 percent and 10.3 percent, respectively, in 2007, and nuclear power increased
by just over 2 percent. As a result of the significant increase in natural gas consumption, C intensity from direct
fossil fuel combustion decreased slightly overall in 2007 (see Table 3-15). 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 2007. Spurred by a 14.2-percent decrease in hydropower, total renewable electricity generation fell by
8.9 percent in 2007. However non-hydropower renewable generation grew by 6.8 percent, thus preventing an even
greater increase in emissions.
Industrial Sector
The industrial sector accounted for 15 percent of CO2 emissions from fossil fuel combustion, 17 percent of CH4
emissions from fossil fuel combustion, and 7 percent of N2O emissions from fossil fuel combustion. CO2, CH4, and
N2O emissions resulted from the direct consumption of fossil fuels for steam and process heat production.
The industrial sector, per the underlying energy consumption data from EIA, includes activities such as
manufacturing, construction, mining, and agriculture. The largest of these activities in terms of energy consumption
is manufacturing, of which six industries—Petroleum Refineries, Chemicals, Primary Metals, Paper, Food, and
Nonmetallic Mineral Products—represent the vast majority of the energy use (EIA 2008a and EIA 2005).
In theory, emissions from the industrial sector should be highly correlated with economic growth and industrial
output, but heating of industrial buildings and agricultural energy consumption are also affected by weather
conditions.52 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
51 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).
52 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.
3-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
equipment) also have a significant affect on industrial emissions.
From 2006 to 2007, total industrial production and manufacturing output increased by 1.7 and 1.8 percent,
respectively (FRB 2007). Over this period, output increased for Chemicals, and Food, but decreased for Petroleum
Refineries, Paper, Primary Metals, and Nonmetallic Mineral Products (see Figure 3-10).
Figure 3-10: Industrial Production Indices (Index 2002=100)
Despite the growth in industrial output (60 percent) and the overall U.S. economy (62 percent) from 1990 to 2007,
CO2 emissions from the industrial sector increased by only 1.3 percent over that time. A number of factors are
believed to have caused this disparity between rapid growth in industrial output and only minor growth in industrial
emissions, including: (1) more rapid growth in output from less energy-intensive industries relative to traditional
manufacturing industries, and (2) improvements in energy efficiency. In 2007, CO2, CH4, and N2O emissions from
fossil fuel combustion and electricity use within the industrial end-use sectors totaled 1,561.2 Tg CO2 Eq., or 0.2
percent above 2006 emissions.
Residential and Commercial Sectors
The residential and commercial sectors accounted for an average 6 and 4 percent of CO2 emissions from fossil fuel
combustion, 40 and 9 percent of CH4 emissions from fossil fuel combustion, and 2 and 1 percent of N2O emissions
from fossil fuel combustion, respectively. Emissions from these sectors were largely due to the direct consumption
of natural gas and petroleum products, primarily for heating and cooking needs. Coal consumption was a minor
component of energy use in both of these end-use sectors. In 2007, CO2, CH4, and N2O emissions from fossil fuel
combustion and electricity use within the residential and commercial end-use sectors were 1,206.4 Tg CO2 Eq. and
1,046.4 Tg CO2 Eq., respectively. Total CO2, CH4, and N2O emissions from the residential sector increased by 4.4
percent in 2007, with emissions in 2007 from the commercial sector 4.1 percent higher than in 2006.
Emissions from the residential and commercial sectors have generally been increasing since 1990, and are often
correlated with short-term fluctuations in energy consumption caused by weather conditions, rather than prevailing
economic conditions. In the long-term, both sectors are also affected by population growth, regional migration
trends, and changes in housing and building attributes (e.g., size and insulation).
Emissions from natural gas consumption represent over 75 and 76 percent of the direct fossil fuel CO2 emissions
from the residential and commercial sectors, respectively. In 2007, natural gas CO2 emissions increased by 8.5
percent and 6 percent, respectively, in each of these sectors. The increase in emissions in both sectors is a result of
cooler winter conditions in the United States compared to 2006.
U.S. Territories
Emissions from U.S. territories are based on the fuel consumption in American Samoa, Guam, Puerto Rico, U.S.
Virgin Islands, Wake Island, and other U.S. Pacific Islands. As described the Methodology section for CO2 from
fossil fuel combustion, this data is collected separately from the sectoral-level data available for the general
calculations. As sectoral information is not available for U.S. Territories, CO2, CH4, and N2O emissions are
presented in the tables above, though the emissions will include some transportation and mobile combustion sources.
Transportation and Mobile Combustion
This discussion of transportation emissions follows the alternative method of presenting combustion emissions by
allocating emissions associated with electricity generation to the transportation end-use sector, as presented in Table
3-8. For direct emissions from transportation (i.e., not including electricity consumption), please see Table 3-7.
Transportation End-Use Sector
The transportation end-use sector accounted for 1,924.6 Tg CO2 in 2007, which represented 33 percent of CO2
emissions from fossil fuel combustion, 26 percent of CH4 emissions from fossil fuel combustion, and 67 percent of
N2O emissions from fossil fuel combustion, respectively. Fuel purchased in the U.S. for international aircraft and
marine travel accounted for an additional 108.8 Tg CO2 in 2007; these emissions are recorded as international
Energy 3-11
-------
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 61 percent of
CO2 emissions, medium- and heavy-duty trucks 22 percent, commercial aircraft 8 percent, and other sources 10
percent. See Table 3-12 for a detailed breakdown of CO2 emissions by mode and fuel type.
From 1990 to 2007, transportation emissions rose by 29 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 40 percent from 1990 to 2007, 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.
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 29 percent from 1990 to
2007. This rise in CO2 emissions, combined with an increase in HFCs from virtually no emissions in 1990 to 67.0
Tg CO2 Eq. in 2007, led to an increase in overall emissions from transportation activities of 28 percent.
Fossil Fuel Combustion CO2 Emissions from Transportation
Domestic transportation CO2 emissions increased by 27 percent (404.7 Tg) between 1990 and 2007, an annualized
increase of 1.5 percent. Since 2005, the growth rate of emissions has slowed considerably; transportation CO2
emissions increased by just 0.3 percent in total between 2005 and 2007. Almost all of the energy consumed by the
transportation sector is petroleum-based, including motor gasoline, diesel fuel, jet fuel, and residual oil.
Transportation sources also produce CH4 and N2O; these emissions are included in Table 3-13 and Table 3-14 in the
"Mobile Combustion" Section. Annex 3.2 presents total emissions from all transportation and mobile sources,
including CO2, N2O, CH4, and HFCs.
Carbon dioxide emissions from passenger cars and light-duty trucks totaled 1,147.0 Tg in 2007, an increase of 21
percent (197.5 Tg)from 1990. CO2 emissions from passenger cars and light-duty trucks peaked at 1,181.3 Tgin
2004, and since then have declined about 3 percent. Over the 1990s through early this decade, growth in vehicle
travel substantially outweighed improvements in vehicle fuel economy; however, the rate of Vehicle Miles Traveled
(VMT) growth slowed considerably starting in 2005 while average vehicle fuel economy increased. Among new
vehicles sold annually, average fuel economy gradually declined from 1990 to 2004 (Figure 3-11), reflecting
substantial growth in sales of light-duty trucks—in particular, growth in the market share of sport utility vehicles—
relative to passenger cars (Figure 3-12). New vehicle fuel economy improved beginning in 2005, largely due to
higher light-duty truck fuel economy standards, which have risen each year since 2005. The overall increase in fuel
economy is also due to a slightly lower light-duty truck market share, which peaked in 2004 at 52 percent and
declined to 48 percent in 2007.
Figure 3-11: Sales-Weighted Fuel Economy of New Passenger Cars and Light-Duty Trucks, 1990-2007
Figure 3-12: Sales of New Passenger Cars and Light-Duty Trucks, 1990-2007
Medium- and heavy-duty truck53 CO2 emissions increased by 79 percent (179.9 Tg) from 1990 to 2007,
representing the largest percentage increase of any major transportation mode. This increase was largely due to a
substantial increase in truck freight movement, as medium- and heavy-duty truck VMT increased by 55 percent.
CO2 from the domestic operation of commercial aircraft increased by 13 percent (18.2 Tg) from 1990 to 2007, well
below the growth in travel activity. 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,
53Includes "medium- and heavy-duty trucks" fueled by gasoline, diesel and LPG.
3-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
and the accelerated retirement of older, less fuel efficient aircraft. Across all categories of aviation, 54 CO2
emissions increased by 5.1 percent (9.0 Tg CO2) between 1990 and 2007. This overall increase includes a 57
percent (18.6 Tg CO2) decrease in emissions from domestic military operations. For further information on all
greenhouse gas emissions from transportation sources, please refer to Annex 3.2.
Table 3-12: CO2 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector (Tg CO2 Eq.)a
Fuel/Vehicle Type 1990 1995 2000 2005 2006 2007
Gasoline 982.7 1,038.9 1,135.7
Passenger Cars 621.0 597.0 639.9
Light-Duty Trucks 308.9 389.9 446.0
Medium- and Heavy-Duty
Trucksb 38.7 35.8 36.0
Buses 0.3 0.4 0.4
Motorcycles 1.7 1.8 1.8
Recreational Boats 12.1 14.1 11.6
Distillate Fuel Oil (Diesel) 261.2 315.9 394.7
Passenger Cars 7.8 7.7 3.6
Light-Duty Trucks 11.3 14.7 19.8
Medium- and Heavy-Duty
Trucksb 188.3 234.9 305.1
Buses 7.9 8.6 10.1
Rail 35.1 39.2 41.7
Recreational Boats 1.9 2.3 2.7
Ships and Other Boats 8.8 8.6 11.7
International Bunker Fuelsc 11.6 9.2 6.3
Jet Fuel 176.2 170.9 196.1
Commercial Aircraft 135.5 141.6 166.0
Military Aircraft 34.4 23.9 20.7
General Aviation Aircraft 6.4 5.4 9.3
International Bunker Fuels ° 46.4 51.2 57.7
Aviation Gasoline 3.1 2.7 2.5
General Aviation Aircraft 3.1 2.7 2.5
Residual Fuel Oil 23.7 30.5 34.9
Ships and Other Boatsd 23.7 30.5 34.9
International Bunker Fuelsc d 56.4 41.2 35.0
Natural Gas 36.2 38.6 35.6
Passenger Cars + 0.1 +
Light-Duty Trucks + + +
Buses + 0.1 0.4
Pipeline 36.2 38.5 35.2
LPG 1.4 1.1 0.7
Light-Duty Trucks 0.5 0.5 0.4
Medium- and Heavy-Duty
Trucks'3 0.8 0.5 0.2
Buses + + +
Electricity 3.0 3.0 3.4
Rail 3.0 3.0 3.4
1181.1 1,169.7
654.2 630.3
476.0 487.9
34.7
0.4
1.6
14.2
453.0
4.2
25.5
356.5
10.6
45.1
3.1
8.0
9.3
189.9
158.2
17.8
13.9
56.4
2.4
2.4
20.2
20.2
45.8
33.2
0.8
32.4
1.7
1.3
0.4
4.7
4.7
35.3
0.4
1.9
14.0
464.7
4.1
26.4
365.4
10.9
47.3
3.2
7.4
8.7
185.0
153.9
16.1
15.0
54.6
2.3
2.3
24.1
24.1
47.2
33.5
0.8
32.6
1.6
1.2
0.5
4.5
4.5
1,166.7
620.9
493.9
35.6
0.4
2.0
13.8
470.6
4.1
26.9
371.3
10.9
46.0
o o
J.J
8.1
8.1
185.3
153.6
15.8
15.8
52.7
2.2
2.2
25.6
25.6
47.9
35.4
0.8
34.6
1.6
1.2
0.5
4.8
4.8
Total
1,487.5
1,601.7
1,803.7
1,886.2 1,885.4 1,892.2
Total (Including Bunkers) c
1,601.8
1,703.3
1,902.7
1,997.6 1,995.9 2,000.9
a This table does not include emissions from non-transportation mobile sources, such as agricultural equipment and
construction/mining equipment; it also does not include emissions associated with electricity consumption by pipelines or
lubricants used in transportation.
54 Includes consumption of jet fuel and aviation gasoline. Does not include aircraft bunkers, which are not accounted for in
national emission totals.
Energy 3-13
-------
b Includes medium- and heavy-duty trucks over 8,500 Ibs.
0 Official estimates exclude emissions from the combustion of both aviation and marine international bunker fuels; however,
estimates including international bunker fuel-related emissions are presented for informational purposes.
d Fluctuations in emission estimates from the combustion of residual fuel oil are associated with fluctuations in reported fuel
consumption and may reflect data collection problems.
Note: Totals may not sum due to independent rounding.
+ Less than 0.05 Tg CO2 Eq.
Fossil Fuel Combustion CFi4 and N2O Emissions from Mobile Sources
Mobile combustion includes emissions of CH4 and N2O from all transportation sources identified in the U.S.
inventory with the exception of pipelines, which are stationary; mobile sources also include non-transportation
sources such as construction/mining equipment, agricultural equipment, vehicles used off-road, and other sources
(e.g., snowmobiles, lawnmowers, etc.). Annex 3.2 includes a summary of all emissions from both transportation
and mobile sources. Table 3-13 and Table 3-14 provide CH4 and N2O emission estimates in Tg CO2 Eq.55
Mobile combustion was responsible for a small portion of national CH4 emissions (0.4 percent) but was the second
largest source of U.S. N2O emissions (10 percent). From 1990 to 2007, mobile source CH4 emissions declined by
52 percent, to 2.3 Tg CO2 Eq. (109 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 31
percent, to 30.1 Tg CO2 Eq. (97 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 45 percent
decrease in mobile source N2O emissions from 1998 to 2007 (Figure 3-13). Overall, CH4 and N2O emissions were
predominantly from gasoline-fueled passenger cars and light-duty trucks.
Figure 3-13: Mobile Source CH4 and N2O Emissions
Table 3-13: CH4 Emissions from Mobile Combustion (Tg CO2 Eq.)
Fuel Type/Vehicle Type3
Gasoline On-Road
Passenger Cars
Light-Duty Trucks
Medium- and Heavy -Duty
Trucks and Buses
Motorcycles
Diesel On-Road
Passenger Cars
Light-Duty Trucks
Medium- and Heavy -Duty
Trucks and Buses
Alternative Fuel On-Road
Non-Road
Ships and Other Boats
Rail
Agricultural Equipment13
Construction/Mining
Equipment0
Aircraft
Otherd
Total
1990 199f
4.2 3.5
2.6 2.
1.4 1.^
0.2 o.:
+ H
+ H
+ 4
+ 4
+ 4
+ -1
0.5 O.f
0.1 0.
0.1 0.
0.1 0.
+ 0.
0.2 0.
0.1 0.
4.7 4.:
5 2000
{ 2.8
[ 1.6
1 1.1
I 0.1
+
h +
+
+
+
h +
5 0.6
0.1
0.1
0.1
0.1
0.2
0.1
$ 3.4
2005 200(
1.9 1.'
1.1 l.(
0.7 0.(
0.1 0.
+ 4
+ H
+ 4
+ 4
+ 4
0.1 0.1
0.6 0.(
0.1 0.
0.1 0.
0.1 0.
0.1 0.
0.2 0.
0.1 0.
2.5 2.<
i 2007
7 1.6
) 0.9
5 0.6
[ 0.1
4
h -f
4
4
4
l 0.1
i 0.6
0.
0.
0.
0.
0.
0.
I 2.3
1 See Annex 3.2 for definitions of on-road vehicle types.
55 See Annex 3.2 for a complete time series of emission estimates for 1990 through 2007.
3-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
b Includes equipment, such as tractors and combines, as well as fuel consumption from trucks that are used off-road in
agriculture.
0 Includes equipment, such as cranes, dumpers, and excavators, as well as fuel consumption from trucks that are used off-road in
construction.
d "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad
equipment, airport equipment, commercial equipment, and industrial equipment, as well as fuel consumption from trucks that are
used off-road for commercial/industrial purposes.
Note: Totals may not sum due to independent rounding.
+ Less than 0.05 Tg CO2 Eq.
Table 3-14: N2O Emissions from Mobile Combustion (Tg CO2 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 Other Boats
Rail
Agricultural Equipment13
Construction/Mining
Equipment0
Aircraft
Otherd
Total
1990
40.1
25.4
14.1
0.6
+
0.2
+
+
0.2
0.1
3.4
0.4
0.3
0.2
0.3
1.7
0.4
43.7
1995
49.8
26.9
22.1
0.7
+
0.3
+
+
0.2
0.1
3.6
0.4
0.3
0.3
0.4
1.7
0.5
53.7
2000
48.4
25.2
22.4
0.9
+
0.3
+
+
0.3
0.1
4.0
0.5
0.3
0.3
0.4
1.9
0.5
52.8
2005
32.1
17.7
13.6
0.8
+
0.3
+
+
0.3
0.2
4.1
0.4
0.4
0.4
0.5
1.9
0.6
36.7
2006
29.0
15.7
12.5
0.7
+
0.3
+
+
0.3
0.2
4.1
0.4
0.4
0.4
0.5
1.8
0.6
33.5
2007
25.5
13.7
11.1
0.7
+
0.3
+
+
0.3
0.2
4.1
0.4
0.4
0.4
0.5
1.8
0.6
30.1
' See Annex 3.2 for definitions of on-road vehicle types.
b Includes equipment, such as tractors and combines, as well as fuel consumption from trucks that are used off-road in
agriculture.
0 Includes equipment, such as cranes, dumpers, and excavators, as well as fuel consumption from trucks that are used off-road in
construction.
d "Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad
equipment, airport equipment, commercial equipment, and industrial equipment, as well as fuel consumption from trucks that are
used off-road for commercial/industrial purposes.
Note: Totals may not sum due to independent rounding.
+ Less than 0.05 Tg CO2 Eq.
C02 from Fossil Fuel Combustion
Methodology
The methodology used by the United States for estimating CO2 emissions from fossil fuel combustion is
conceptually similar to the approach recommended by the IPCC for countries that intend to develop detailed,
sectoral-based emission estimates (IPCC 2006). A detailed description of the U.S. methodology is presented in
Annex 2.1, and is characterized by the following steps:
1. Determine total fuel consumption by fuel type and sector. Total fossil fuel consumption for each year is
estimated by aggregating consumption data by end-use sector (e.g., commercial, industrial, etc.), primary
fuel type (e.g., coal, petroleum, gas), and secondary fuel category (e.g., motor gasoline, distillate fuel oil,
etc.). Fuel consumption data for the United States were obtained directly from the Energy Information
Energy 3-15
-------
Administration (EIA) of the U.S. Department of Energy (DOE), primarily from the Monthly Energy
Review and published supplemental tables on petroleum product detail (EIA 2008b). The EIA does not
include territories in its national energy statistics, so fuel consumption data for territories were collected
separately from Grillot (2008).56
For consistency of reporting, the IPCC has recommended that countries report energy data using the
International Energy Agency (IEA) reporting convention and/or IEA data. Data in the IEA format are
presented "top down"—that is, energy consumption for fuel types and categories are estimated from energy
production data (accounting for imports, exports, stock changes, and losses). The resulting quantities are
referred to as "apparent consumption." The data collected in the United States by EIA on an annual basis
and used in this inventory are predominantly from mid-stream or conversion energy consumers such as
refiners and electric power generators. These annual surveys are supplemented with end-use energy
consumption surveys, such as the Manufacturing Energy Consumption Survey, that are conducted on a
periodic basis (every 4 years). These consumption data sets help inform the annual surveys to arrive at the
national total and sectoral breakdowns for that total.57
It is also important to note that U.S. fossil fuel energy statistics are generally presented using gross calorific
values (GCV) (i.e., higher heating values). Fuel consumption activity data presented here have not been
adjusted to correspond to international standards, which are to report energy statistics in terms of net
calorific values (NCV) (i.e., lower heating values).58
2. Subtract uses accounted for in the Industrial Processes chapter. Portions of the fuel consumption data for
seven fuel categories—coking coal, distillate fuel, industrial 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 2008), CVR Energy (2008), Corathers (2008), U.S. Census Bureau (2008), EIA
(2008g), EIA (2001), Smith, G. (2007), USGS (2008), USGS (1995, 1998, 2000 through 2002), USGS
(1995), USGS (1991a through 2007a), USGS (1991b through 2007b), USGS (1991 through 2005), and
USGS (1995 through 2006). 59
3. Adjust for biofuels, conversion of fossil fuels, and exports of CO 2. 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.60 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 (2008b) and data for synthetic natural gas
were collected from EIA (2008e), and data for CO2 exports were collected from the Dakota Gasification
Company (2006), Fitzpatrick (2002), Erickson (2003), and EIA (2006).
4. Adjust Sectoral Allocation of Distillate Fuel Oil and Motor Gasoline. EPA had conducted a separate
bottom-up analysis of transportation fuel consumption based on the Federal Highway Administration's
56 Fuel consumption by U.S. territories (i.e., American Samoa, Guam, Puerto Rico, U.S. Virgin Islands, Wake Island, and other
U. S. Pacific Islands) is included in this report and contributed emissions of 51 Tg CO2 Eq. in 2007.
57 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.
58 A crude convention to convert between gross and net calorific values is to multiply the heat content of solid and liquid fossil
fuels by 0.95 and gaseous fuels by 0.9 to account for the water content of the fuels. Biomass-based fuels in U.S. energy statistics,
however, are generally presented using net calorific values.
59 See sections on Iron and Steel Production and Metallurgical Coke Production, Ammonia Production and Urea Consumption,
Petrochemical Production, Titanium Dioxide Production, Ferroalloy Production, Aluminum Production, and Silicon Carbide
Production and Consumption in the Industrial Processes chapter.
60 These adjustments are explained in greater detail in Annex 2.1.
3-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
(FHWA) 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 upward to match the
value obtained from the bottom-up analysis based on VMT. As the total distillate consumption 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 (2008), Benson (2002 through 2004), DOE (1993 through 2008), EIA (2008a), EIA (1991
through 2005), EPA (2006), and FHWA (1996 through 2008).
5. Adjust for fuels consumed for non-energy uses. U.S. aggregate energy statistics include consumption of
fossil fuels for non-energy purposes. These are fossil fuels that are manufactured into plastics, asphalt,
lubricants, or other products. Depending on the end-use, this can result in storage of some or all of the C
contained in the fuel for a period of time. As the emission pathways of C used for non-energy purposes are
vastly different than fuel combustion (since the C in these fuels ends up in products instead of being
combusted), these emissions are estimated separately in the Carbon Emitted and Stored in Products from
Non-Energy Uses of Fossil Fuels section in this chapter. Therefore, the amount of fuels used for non-
energy purposes was subtracted from total fuel consumption. Data on non-fuel consumption was provided
by EIA (2008b).
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).61 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
2008) supplied data on military jet fuel and marine fuel use. Commercial jet fuel use was obtained from
FAA (2006); residual and distillate fuel use for civilian marine bunkers was obtained from DOC (1991
through 2008) for 1990 through 2001, and 2007, and DHS (2008) for 2003 through 2006. Consumption of
these fuels was subtracted from the corresponding fuels in the transportation end-use sector. Estimates of
international bunker fuel emissions for the United States are discussed in detail later in the International
Bunker Fuels section of this chapter.
7. Determine the total C content of fuels consumed. Total C was estimated by multiplying the amount of fuel
consumed by the amount of C in each fuel. This total C estimate defines the maximum amount of C that
could potentially be released to the atmosphere if all of the C in each fuel was converted to CO2. The C
content coefficients used by the United States were obtained from EIA's Emissions of Greenhouse Gases in
the United States 2007 (EIA 2008c) and EIA's Monthly Energy Review and published supplemental tables
on petroleum product detail EIA (EIA 2008b). 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
61 See International Bunker Fuels section in this chapter for a more detailed discussion.
Energy 3-17
-------
category were obtained from FHWA (1996 through 2008); for each vehicle category, the percent gasoline,
diesel, and other (e.g., CNG, LPG) fuel consumption are estimated using data from DOE (1993 through
2008).
• For non-road vehicles, activity data were obtained from AAR (2008), APTA (2007 through 2008), BEA
(1991 through 2008), Benson (2002 through 2004), DOE (1993 through 2008), DESC (2008), DOC (1991
through 2008), DOT (1991 through 2007), EIA (2008a), EIA (2008d), EIA (2007), EIA (2002), EIA (1991
through 2005), EPA (2006), FAA (2008), and Gaffney (2007).
• For jet fuel used by aircraft, CO2 emissions were calculated directly based on reported consumption of fuel
as reported by EIA, and allocated to commercial aircraft using flight-specific fuel consumption data from
the Federal Aviation Administration's (FAA) System for assessing Aviation's Global Emission (SAGE)
model. 62 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 (2008a) and USAF (1998).63
[BEGIN BOX]
Box 3-2: Carbon Intensity of U.S. Energy Consumption
Fossil fuels are the dominant source of energy in the United States, and CO2 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
CO2 in the energy conversion process, such as renewable (i.e., hydropower, biofuels, geothermal, solar, and wind)
and nuclear sources. 64
Energy-related CO2 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 CO2 Eq./QBtu for natural gas to upwards of 95 Tg CO2 Eq./QBru for coal and petroleum coke.65
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 CO2
Eq./Btu). Energy generated from nuclear and many renewable sources do not result in direct emissions of CO2.
Biofuels such as wood and ethanol are also considered to be C neutral; although these fuels do emit CO2, in the long
run the CO2 emitted from biomass consumption does not increase atmospheric CO2 concentrations if the biogenic C
emitted is offset by the growth of new biomass.66 The overall C intensity of the U.S. economy is thus dependent
upon the quantity and combination of fuels and other energy sources employed to meet demand.
Table 3-15 provides a time series of the C intensity for each sector of the U.S. economy. The time series
62 FAA's System for assessing Aviation's Global Emissions (SAGE) model develops aircraft fuel bum 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
.
63 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.
64 Small quantities of CO2, however, are released from some geologic formations tapped for geothermal energy. These
emissions are included with fossil fuel combustion emissions from the 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.
65 One exajoule (EJ) is equal to 1018 joules or 0.9478 QBtu.
66 Net carbon fluxes from changes in biogenic carbon reservoirs in wooded or croplands are accounted for in the estimates for
Land Use, Land-Use Change, and Forestry.
3-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 CO2 Eq./EJ), which were the primary sources of energy. Lastly, the electricity
generation sector had the highest C intensity due to its heavy reliance on coal for generating electricity.
Table 3-15: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg CO2 Eq./QBtu)
Sector 1990 1995 2000 2005 2006 2007
Residential a
Commercial a
Industrial a
Transportation a
Electricity Generation b
U.S. Territories'
All Sectors c
57.4
59.3
63.7
71.0
86.7
74.1
72.7
56.7
57.8
62.7
71.0
86.0
74.1
72.2
56.7
57.1
62.5
71.0
85.6
73.2
72.7
56.6
57.6
64.0
71.1
85.0
74.6
73.1
56.6
57.2
64.2
71.1
84.6
74.6
73.1
56.3
57.0
63.9
71.1
84.0
74.7
72.8
a Does not include electricity or renewable energy consumption.
b Does not include electricity produced using nuclear or renewable energy.
0 Does not include nuclear or renewable energy consumption.
Note: Excludes non-energy fuel use emissions and consumption.
In contrast to Table 3-15, Table 3-16 presents C intensity values that incorporate energy consumed from all sources
(i.e., fossil fuels, renewables, and nuclear). In addition, the emissions related to the generation of electricity have
been attributed to both electricity generation and the end-use sectors in which that electricity was eventually
consumed.67 This table, therefore, provides a more complete picture of the actual C intensity of each end-use sector
per unit of energy consumed. The transportation end-use sector in Table 3-16 emerges as the most C intensive when
all sources of energy are included, due to its almost complete reliance on petroleum products and relatively minor
amount of biomass-based fuels used, such as ethanol. The "other end-use sectors" (i.e., residential, commercial, and
industrial) use significant quantities of biofuels such as wood, thereby lowering the overall C intensity. The C
intensity of the electricity generation sector differs greatly from the scenario in Table 3-15, where only the energy
consumed from the direct combustion of fossil fuels was included. This difference is due almost entirely to the
inclusion of electricity generation from nuclear and hydropower sources, which do not emit CO2.
Table 3-16: Carbon Intensity from All Energy Consumption by Sector (Tg CO2 Eq./QBtu)
Sector
Transportation a
Other End-Use Sectors a'b
Electricity Generation °
All Sectors d
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
2005
70.1
58.1
59.9
61.6
2006
69.8
57.5
58.9
61.1
2007
69.4
57.5
59.3
61.0
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.
0 Includes electricity generation from nuclear and renewable sources.
d Includes nuclear and renewable energy consumption.
Note: Excludes non-energy fuel use emissions and consumption.
By comparing the values in Table 3-15 and Table 3-16, a few observations can be made. The use of renewable and
nuclear energy sources has resulted in a significantly lower C intensity of the U.S. economy. Over the eighteen-year
period of 1990 through 2007, however, the C intensity of U.S. energy consumption has been fairly constant, as the
67 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 3-19
-------
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 CO2 emissions per dollar of gross domestic product (GDP) have both
declined since 1990 (BEA 2008).
Figure 3-14: U.S. Energy Consumption and Energy-Related CO2 Emissions Per Capita and Per Dollar GDP
C intensity estimates were developed using nuclear and renewable energy data from EIA (2008a) and fossil fuel consumption
data as discussed above and presented in Annex 2.1.
[END BOX]
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 2007. The amount
of CO2 emissions resulting from non-energy related fossil fuel use has been calculated separately and reported in the
Carbon Emitted from Non-Energy Uses of Fossil Fuels section of this report. These factors all contribute to the
uncertainty in the CO2 estimates. Detailed discussions on the uncertainties associated with C emitted from Non-
Energy Uses of Fossil Fuels can be found within that section of this chapter.
Various sources of uncertainty surround the estimation of emissions from international bunker fuels, which are
subtracted from the U.S. totals (see the detailed discussions on these uncertainties provided in the International
Bunker Fuels section of this chapter). Another source of uncertainty is fuel consumption by U.S. territories. The
United States does not collect energy statistics for its territories at the same level of detail as for the fifty states and
the District of Columbia. Therefore, estimating both emissions and bunker fuel consumption by these territories is
difficult.
Uncertainties in the emission estimates presented above also result from the data used to allocate CO2 emissions
from the transportation end-use sector to individual vehicle types and transport modes. In many cases, bottom-up
estimates of fuel consumption by vehicle type do not match aggregate fuel-type estimates from EIA. Further
research is planned to improve the allocation into detailed transportation end-use sector emissions. 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.
3-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
The uncertainty analysis was performed by primary fuel type for each end-use sector, using the IPCC-recommended
Tier 2 uncertainty estimation methodology, Monte Carlo Simulation technique, with @RISK software. For this
uncertainty estimation, the inventory estimation model for CO2 from fossil fuel combustion was integrated with the
relevant variables from the inventory estimation model for International Bunker Fuels, to realistically characterize
the interaction (or endogenous correlation) between the variables of these two models. About 150 input variables
were modeled for CO2 from energy-related Fossil Fuel Combustion (including about 10 for non-energy fuel
consumption and about 20 for International Bunker Fuels).
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.68 Triangular distributions were assigned for
the oxidization factors (or combustion efficiencies). The uncertainty ranges were assigned to the input variables
based on the data reported in SAIC/EIA (2001) and on conversations with various agency personnel.69
The uncertainty ranges for the activity-related input variables were typically asymmetric around their inventory
estimates; the uncertainty ranges for the emissions factors were symmetric. Bias (or systematic uncertainties)
associated with these variables accounted for much of the uncertainties associated with these variables (SAIC/EIA
2001).70 For purposes of this uncertainty analysis, each input variable was simulated 10,000 times through Monte
Carlo Sampling.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-17. Fossil fuel combustion
CO2 emissions in 2007 were estimated to be between 5,622.3 and 6,029.3 Tg CO2 Eq. at a 95 percent confidence
level. This indicates a range of 2 percent below to 6 percent above the 2007 emission estimate of 5,735.8 Tg CO2
Eq.
Table 3-17: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Energy-related Fossil Fuel
Combustion by Fuel Type and Sector (Tg CO2 Eq. and Percent)
Fuel/Sector
2007 Emission Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate"
(TgC02Eq.)
Coalb
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Natural Gas b
Residential
Commercial
Industrial
Transportation
Electricity Generation
U.S. Territories
Petroleum b
2,086.5
0.6
6.8
107.4
NE
1,967.6
4.1
1,216.5
256.9
163.4
385.6
35.4
373.8
1.4
2,432.4
Lower
Bound
2,015.7
0.5
6.4
103.3
NE
1,890.6
3.6
1,226.2
249.7
158.9
396.1
34.4
363.1
1.2
2,279.1
Upper
Bound
2,284.1
0.7
7.8
125.4
NE
2,157.3
4.9
1,295.9
275.0
174.9
436.0
37.9
393.0
1.7
2,553.7
Lower
Bound
-3%
-6%
-5%
-4%
NA
-4%
-12%
+1%
-3%
-3%
+3%
-3%
-3%
-12%
-6%
Upper
Bound
+9%
+15%
+15%
+17%
NA
+10%
+19%
+7%
+7%
+7%
+13%
+7%
+5%
+17%
+5%
68 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.
69 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.
70 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.
Energy 3-21
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Residential
Commercial
Industrial
Transportation
Electric Utilities
U.S. Territories
Total (excluding
Geothermal) b
Geothermal
Total (including
Geothermal) b'c
83.2
44.2
352.5
1,852.0
55.3
45.3
5,735.4
0.4
5,735.8
78.8
42.1
306.4
1,710.8
53.3
41.8
5,621.9
NE
5,622.3
87.4
46.0
411.5
1,947.9
58.8
50.4
6,028.9
NE
6,029.3
-5%
-5%
-13%
-8%
-4%
-8%
-2%
NE
-2%
+5%
+4%
+17%
+5%
+6%
+11%
+5%
NE
+6%
NA (Not Applicable)
NE (Not Estimated)
a Range of emission estimates predicted by Monte Carlo 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.
0 Geothermal emissions added for reporting purposes, but an uncertainty analysis was not performed for CO2 emissions from
geothermal production.
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
Estimates of CO2 from the industrial sector have been revised for the years 1990 through 2006 to subtract for non-
energy related consumption of coal, distillate fuel, and natural gas used in iron and steel and metallurgical coke
production. A discussion of the methodology used to estimate non-energy related consumption is contained in the
Iron and Steel Production and Metallurgical Coke Production section of the Industrial Processes chapter. In addition,
the Energy Information Administration (EIA 2008b) updated energy consumption data for all years. These revisions
primarily impacted the emission estimates for 2006. Overall, these changes resulted in an average annual decrease
of 17 Tg CO2 Eq. (0.3 percent) in CO2 emissions from fossil fuel combustion for the period 1990 through 2006.
Planned Improvements
An analysis is being undertaken to update the carbon content factors for fossil fuels, as presented in the annexes of
this report. To reduce uncertainty of CO2 from fossil fuel combustion estimates, efforts will be taken to work with
EIA and other agencies to improve the quality of the U.S. territories data. This improvement is not all-inclusive, and
is part of an ongoing analysis and efforts to continually improve the CO2 from fossil fuel combustion estimates. In
addition, further expert elicitation may be conducted to better quantify the total uncertainty associated with
emissions from this source.
CH4 and N20 from Stationary Combustion
Methodology
CH4 and N2O emissions from stationary combustion were estimated by multiplying fossil fuel and wood
consumption data by emission factors (by sector and fuel type). National coal, natural gas, fuel oil, and wood
consumption data were grouped by sector: industrial, commercial, residential, electricity generation, and U.S.
territories. For the CH4 and N2O estimates, fuel consumption data for coal, natural gas, and fuel oil for the United
States were obtained from EIA's Monthly Energy Review and unpublished supplemental tables on petroleum
product detail (EIA 2008a). Wood consumption data for the United States was obtained from EIA's Annual Energy
3-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Review (EIA 2008b). Because the United States does not include territories in its national energy statistics, fuel
consumption data for territories were provided separately by Grillot (2008).71 Fuel consumption for the industrial
sector was adjusted to subtract out construction and agricultural use, which is reported under mobile sources.72
Construction and agricultural fuel use was obtained from EPA (2006). 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.
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 CH4 and N2O emissions
presented are based on broad indicators of emissions (i.e., fuel use multiplied by an aggregate emission factor for
different sectors), rather than specific emission processes (i.e., by combustion technology and type of emission
control).
An uncertainty analysis was performed by primary fuel type for each end-use sector, using the IPCC-recommended
Tier 2 uncertainty estimation methodology, Monte Carlo Simulation technique, with @RISK software.
The uncertainty estimation model for this source category was developed by integrating the CH4 and N2O stationary
source inventory estimation models with the model for CO2 from fossil fuel combustion to realistically characterize
the interaction (or endogenous correlation) between the variables of these three models. 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/EIA (2001) report.73 For these variables, the uncertainty
ranges were assigned to the input variables based on the data reported in SAIC/EIA (2001).74 However, the CH4
emission factors differ from those used by EIA. Since these factors were obtained from IPCC/UNEP/OECD/IEA
(1997), uncertainty ranges were assigned based on IPCC default uncertainty estimates (IPCC 2000).
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-18. Stationary combustion
CH4 emissions in 2007 (including biomass) were estimated to be between 4.3 and 15.1 Tg CO2 Eq. at a 95 percent
confidence level. This indicates a range of 34 percent below to 128 percent above the 2007 emission estimate of 6.6
Tg CO2 Eq.75 Stationary combustion N2O emissions in 2007 (including biomass) were estimated to be between 11.2
71 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.
72 Though emissions from construction and farm use occur due to both stationary and mobile sources, detailed data was not
available to determine the magnitude from each. Currently, these emissions are assumed to be predominantly from mobile
sources.
73 SAIC/EIA (2001) characterizes the underlying probability density function for the input variables as a combination of uniform
and normal distributions (the former distribution to represent the bias component and the latter to represent the random
component). However, for purposes of the current uncertainty analysis, it was determined that uniform distribution was more
appropriate to characterize the probability density function underlying each of these variables.
74 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.
75 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.
Energy 3-23
-------
and 42.1 Tg CO2 Eq. at a 95 percent confidence level. This indicates a range of 24 percent below to 187 percent
above the 2007 emissions estimate of 14.7 Tg CO2 Eq.
Table 3-18: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Energy-Related Stationary
Combustion, Including Biomass (Tg CO2 Eq. and Percent)
Source
Gas
2007 Emission
Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate"
(TgC02Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Stationary Combustion
Stationary Combustion
CH4
N2O
6.6
14.7
4.3 15.1 -34% +128%
11.2 42.1 -24% +187%
1 Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
The uncertainties associated with the emission estimates of CH4 and N2O are greater than those associated with
estimates of CO2 from fossil fuel combustion, which mainly rely on the carbon content of the fuel combusted.
Uncertainties in both CH4 and N2O estimates are due to the fact that emissions are estimated based on emission
factors representing only a limited subset of combustion conditions. For the indirect greenhouse gases, uncertainties
are partly due to assumptions concerning combustion technology types, age of equipment, emission factors used,
and activity data projections.
QA/QC and Verification
A source-specific QA/QC plan for stationary combustion was developed and implemented. This effort included a
Tier 1 analysis, as well as portions of a Tier 2 analysis. The Tier 2 procedures that were implemented involved
checks specifically focusing on the activity data and emission factor sources and methodology used for estimating
CH4, N2O, and the indirect greenhouse gases from stationary combustion in the United States. Emission totals for
the different sectors and fuels were compared and trends were investigated.
Recalculations Discussion
Historical CH4 and N2O emissions from stationary sources (excluding CO2) were revised due to a couple of changes.
Slight changes to emission estimates for sectors are due to revised data from EIA (2008a). 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 (2008b) were revised for the residential, industrial, and electric power sectors. The
combination of the methodological and historical data changes resulted in an average annual increase of less than
0.1 Tg CO2 Eq. (less than 0.1 percent) in CH4 emissions from stationary combustion and an average annual decrease
of less than 0.1 Tg CO2 Eq. (0.2 percent) in N2O emissions from stationary combustion for the period 1990 through
2006.
Planned Improvements
Several items are being evaluated to improve the CH4 and N2O emission estimates from stationary combustion and
to reduce uncertainty. Efforts will be taken to work with EIA and other agencies to improve the quality of the U.S.
territories data. Because these data are not broken out by stationary and mobile uses, further research will be aimed
at trying to allocate consumption appropriately. In addition, the uncertainty of biomass emissions will be further
investigated since it was expected that the exclusion of biomass from the uncertainty estimates would reduce the
uncertainty; and in actuality the exclusion of biomass increases the uncertainty. These improvements are not all-
inclusive, but are part of an ongoing analysis and efforts to continually improve these stationary estimates.
CH4 and N20 from Mobile Combustion
Methodology
Estimates of CH4 and N2O emissions from mobile combustion were calculated by multiplying emission factors by
measures of activity for each fuel and vehicle type (e.g., light-duty gasoline trucks). Activity data included vehicle
miles traveled (VMT) for on-road vehicles and fuel consumption for non-road mobile sources. The activity data and
3-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
emission factors used are described in the subsections that follow. A complete discussion of the methodology used
to estimate CH4 and N2O emissions from mobile combustion and the emission factors used in the calculations is
provided in Annex 3.2.
On-Road Vehicles
Estimates of CH4 and N2O emissions from gasoline and diesel on-road vehicles are based on VMT and emission
factors by vehicle type, fuel type, model year, and emission control technology. Emission estimates for alternative
fuel vehicles (AFVs)76 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 ICF (2006b); all other gasoline and diesel on-road vehicle emissions factors were
developed by ICF (2004). These factors were derived from EPA, California Air Resources Board (CARB) and
Environment Canada laboratory test results of different vehicle and control technology types. The EPA, CARB and
Environment Canada tests were designed following the Federal Test Procedure (FTP), which covers three separate
driving segments, since vehicles emit varying amounts of GHGs depending on the driving segment. These driving
segments are: (1) a transient driving cycle that includes cold start and running emissions, (2) a cycle that represents
running emissions only, and (3) a transient driving cycle that includes hot start and running emissions. For each test
run, a bag was affixed to the tailpipe of the vehicle and the exhaust was collected; the content of this bag was then
analyzed to determine quantities of gases present. The emissions characteristics of segment 2 were used to define
running emissions, and subtracted from the total FTP emissions to determine start emissions. These were then
recombined based upon the ratio of start to running emissions for each vehicle class from MOBILE6.2, an EPA
emission factor model that predicts gram per mile emissions of CO2, CO, HC, NOX, and PM from vehicles under
various conditions, to approximate average driving characteristics.77
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 2007 were obtained from the Federal Highway Administration's (FHWA)
Highway Performance Monitoring System database as reported in Highway Statistics (FHWA 1996 through 2008).
VMT estimates were then allocated from FHWA's vehicle categories to fuel-specific vehicle categories using the
calculated shares of vehicle fuel use for each vehicle category by fuel type reported in DOE (1993 through 2008)
and information on total motor vehicle fuel consumption by fuel type from FHWA (1996 through 2008). VMT for
AFVs were taken from Browning (2003). The age distributions of the U.S. vehicle fleet were obtained from EPA
(2007c, 2000), and the average annual age-specific vehicle mileage accumulation of U.S. vehicles were obtained
from EPA (2000).
Control technology and standards data for on-road vehicles were obtained from EPA's Office of Transportation and
Air Quality (EPA 2007a, 2007b, 2000, 1998, and 1997) and Browning (2005). These technologies and standards are
defined in Annex 3.2, and were compiled from EPA (1993, 1994a, 1994b, 1998, 1999a) and
IPCC/UNEP/OECD/IEA (1997).
Non-Road Vehicles
To estimate emissions from non-road vehicles, fuel consumption data were employed as a measure of activity, and
multiplied by fuel-specific emission factors (in grams of N2O and CH4 per kilogram of fuel consumed).78 Activity
76 Alternative fuel and advanced technology vehicles are those that can operate using a motor fuel other than gasoline or diesel.
This includes electric or other bi-fuel or dual-fuel vehicles that may be partially powered by gasoline or diesel.
77 Additional information regarding the model can be found online at http://www.epa.gov/OMS/m6.htm.
78 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 3-25
-------
data were obtained from AAR (2008), APTA (2007 through 2008), APTA (2006), BEA (1991 through 2005),
Benson (2002 through 2004), DHS (2008), DOC (1991 through 2008), DOE (1993 through 2008), DESC (2008),
DOT (1991 through 2008), EIA (2008b, 2007a, 2007b, 2002), EIA (2007 through 2008), EIA (1991 through 2007),
EPA (2006b), Esser (2003 through 2004), FAA (2008 and 2006), Gaffney (2007), 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 @PJSK
software. The uncertainty analysis was performed on 2007 estimates of CH4 and N2O emissions, incorporating
probability distribution functions associated with the major input variables. For the purposes of this analysis, the
uncertainty was modeled for the following two major sets of input variables: (1) 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 3-19. As noted above, an uncertainty analysis was not performed for CH4
and N2O emissions from non-road vehicles. Mobile combustion CH4 emissions (from on-road vehicles) in 2007
were estimated to be between 1.5 and 1.8 Tg CO2 Eq. at a 95 percent confidence level. This indicates a range of 8
percent below to 8 percent above the corresponding 2007 emission estimate of 1.7 Tg CO2 Eq. Also at a 95 percent
confidence level, mobile combustion N2O emissions from on-road vehicles in 2007 were estimated to be between
21.1 and 30.8 Tg CO2 Eq., indicating a range of 19 percent below to 19 percent above the corresponding 2007
emission estimate of 26.0 Tg CO2 Eq.
Table 3-19. Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Mobile Sources (Tg CO2
Eq. and Percent)
Source
On-Road Sources
On-Road Sources
Gas
CH4
N2O
2007 Emission
Estimate"
(Tg C02 Eq.)
1.7
26.0
Uncertainty Range Relative to Emission Estimatea'b
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
1.5 1.8 -8% 8%
21.1 30.8 -19% 19%
a 2007 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 in the estimates reported in this table.
b Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
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 CH4 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
3-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 relative to the previous Inventory report.
New estimates of VMT by alternative fueled vehicles are now calculated using an updated method. The original
VMT for alternative fuels was determined from energy use data obtained from EIA and projected. The new update
uses actual energy use for 2005 through 2007 and improved estimations for future years.
Several changes were also made in the calculation of emissions from non-road vehicles. Commercial aircraft
activity data for 1990 through 1999 is now calculated as the result of estimating DOT (1991 through 2008) data
based upon the average difference between FAA (2006) and DOT (1991 through 2008) datasets for the years 2000
through 2005. For 2006 and 2007 commercial aircraft activity data, DOT (1991 through 2008) data is multiplied by
the percentage difference between 2005 (the most recent available SAGE datapoint) and the respective year.
International jet fuel bunkers are now calculated by assigning the difference between the sum of domestic activity
data (in TBtu) and the EIA transportation jet fuel allotment to the jet fuel bunkers category. Previously, international
jet fuel bunkers were calculated based upon DOT (1991 through 2008) and BEA (1991 through 2005) data for the
years 1990 through 1999 and 2006 through 2007 and estimated by FAA (2006) for 2000 through 2005.
Categories of non-road sources for which activity data are supplied from EPA's NONROAD model (EPA 2006)
now include all Source Classification Codes available within the model, rather than a subset of all sources. This
change results in an increase in emissions estimates from farm equipment, construction equipment, and other non-
road sources.
As a result of these changes, average estimates of CH4 and N2O emissions from mobile combustion were slightly
higher relative to the previous inventory—showing an increase of no more than 2.5 percent in a given year—for the
period 1990 through 2007. The greatest increase in absolute terms, 0.48 Tg CO2 Eq. (1.4 percent), occurs with the
2006 N2O estimate.
Planned Improvements
1. 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.
2. Develop updated emissions factors for diesel vehicles, motorcycle, and biodiesel vehicles. Previous
emission factors were based upon extrapolations from other vehicle classes and new test data from
Environment Canada will allow for better estimation of emission factors for these vehicles.
3. Develop updated emissions factors for ships and boats. Prior emission factors were derived from AP-42 for
combustion of diesel and residual fuel. The new factors will take into account new data obtained from the
Swedish Methodology for Environmental Data.
4. Develop new emission factors for non-road equipment. The current inventory estimates for non-CO2
emissions from non-road sources are based on emission factors from IPCC guidelines published in 1996.
Recent data on non-road sources from Environment Canada and the California Air Resources Board will be
investigated in order to assess the feasibility of developing new N2O and CH4 emissions factors for non-
road equipment.
5. 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 foryears priorto 1999. The feasibility of this approach will
be explored.
Energy 3-27
-------
6. 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.
7. 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.
3.2. Carbon Emitted from Non-Energy Uses of Fossil Fuels (IPCC Source
Category 1A)
In addition to being combusted for energy, fossil fuels are also consumed for non-energy uses (NEU) in the United
States. The fuels used for these purposes are diverse, including natural gas, liquefied petroleum gases (LPG),
asphalt (a viscous liquid mixture of heavy crude oil distillates), petroleum coke (manufactured from heavy oil), and
coal coke (manufactured from coking coal). The non-energy applications are equally diverse, and include
feedstocks for the manufacture of plastics, rubber, synthetic fibers and other materials; reducing agents for the
production of various metals and inorganic products; and non-energy products such as lubricants, waxes, and asphalt
(IPCC 2006).
CO2 emissions arise from non-energy uses via several pathways. Emissions may occur during the manufacture of a
product, as is the case in producing plastics or rubber from fuel-derived feedstocks. Additionally, emissions may
occur during the product's lifetime, such as during solvent use. Overall, throughout the time series and across all
uses, about 63 percent of the total C consumed for non-energy purposes was stored in products, and not released to
the atmosphere; the remaining 37 percent was emitted.
There are several areas in which non-energy uses of fossil fuels are closely related to other parts of the inventory.
For example, some of the NEU products release CO2 at the end of their commercial life when they are combusted
after disposal; these emissions are reported separately within the Energy chapter in the Municipal Solid Waste
Combustion source category. In addition, there is some overlap between fossil fuels consumed for non-energy uses
and the fossil-derived CO2 emissions accounted for in the Industrial Processes chapter, especially for fuels used as
reducing agents. To avoid double-counting, the "raw" non-energy fuel consumption data reported by 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 3-20, fossil fuel emissions in 2007 from the non-energy uses of fossil fuels were 133.9 Tg CO2
Eq., which constituted approximately 2 percent of overall fossil fuel emissions. In 2007, the consumption of fuels
for non-energy uses (after the adjustments described above) was 5,219.2 TBtu, an increase of 16 percent since 1990
(see Table 3-21). About 62.0 Tg of the C (227.2 Tg CO2 Eq.) in these fuels was stored, while the remaining 36.5 Tg
C (133.9 Tg CO2 Eq.) was emitted. The proportion of C emitted as CO2 has remained about constant since 1990, at
about 37 to 40 percent of total non-energy consumption (see Table 3-20).
Table 3-20: CO2 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg CO2 Eq.)
Year 1990 1995 2000 2005 2006 2007
Potential Emissions 312.8 350.4 387.7 375.9 383.4 361.1
C Stored 195.8 213.0 243.2 237.8 238.3 227.2
Emissions as a % of Potential 37% 39% 37% 37% 38% 37%
Emissions 117.0 137.5 144.5 138.1 145.1 133.9
3-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Methodology
The first step in estimating C stored in products was to determine the aggregate quantity of fossil fuels consumed for
non-energy uses. The C content of these feedstock fuels is equivalent to potential emissions, or the product of
consumption and the fuel-specific C content values. Both the non-energy fuel consumption and C content data were
supplied by the EIA (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 3-21
and Table 3-22 have been adjusted to subtract non-energy uses that are included in the source categories of the
Industrial Processes chapter.79 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 Incineration of Waste source category, the storage factors do not account for losses at the
disposal end of the life cycle. For industrial coking coal and distillate fuel oil, storage factors were taken from
IPCC/UNEP/OECD/IEA (1997), which in turn draws from Marland and Rotty (1984). For the remaining fuel types
(petroleum coke, miscellaneous products, and other petroleum), IPCC does not provide guidance on storage factors,
and assumptions were made based on the potential fate of C in the respective NEU products.
Table 3-21: Adjusted Consumption of Fossil Fuels for Non-Energy Uses (TBtu)
Year
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
4,222.3
8.2
276.0
1,170.2
1,119.0
186.3
77.3
325.7
677.2
21.3
82.1
100.9
7.0
33.3
137.8
176.0
176.0
86.7
0.7
86.0
4,485.0
1995
4,804.4
75.0
11.3
330.4
1,178.2
1,484.7
177.8
285.3
350.6
612.7
40.1
45.5
66.9
8.0
40.6
97.1
167.9
167.9
90.8
2.0
88.8
5,063.1
2000
5,278.9
82.2
12.4
420.7
1,275.7
1,603.1
189.9
228.5
592.3
553.8
12.6
49.4
94.3
11.7
33.1
119.2
179.4
179.4
165.5
16.4
149.1
5,623.7
2005
5,153.4
53.3
11.9
390.0
1,323.2
1,440.9
160.2
145.9
678.2
518.3
67.7
147.2
60.8
11.7
31.4
112.8
151.3
151.3
107.7
5.2
102.4
5,412.4
2006
5,245.8
74.7
12.4
403.2
1,261.2
1,492.0
156.1
105.7
619.4
572.9
123.9
181.5
69.1
11.7
26.1
136.0
147.4
147.4
110.3
5.4
105.0
5,503.6
2007
4,966.4
33.0
12.4
396.0
1,197.0
1,483.2
161.0
132.4
543.3
511.7
88.4
165.4
75.6
11.7
21.9
133.5
152.0
152.0
100.9
4.9
96.0
5,219.2
+ Does not exceed 0.05 TBtu
79 These source categories include Iron and Steel Production, Lead Production, Zinc Production, Ammonia Manufacture, Carbon
Black Manufacture (included in Petrochemical Production), Titanium Dioxide Production, Ferroalloy Production, Silicon
Carbide Production, and Aluminum Production.
Energy 3-29
-------
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.
Table 3-22: 2007 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions
Sector/Fuel Type
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
Adjusted
Non-Energy
Use3
(TBtu)
4,966.4
33.0
12.4
396.0
1,197.0
1,483.2
161.0
132.4
543.3
511.7
88.4
165.4
75.6
11.7
21.9
133.5
152.0
152.0
100.9
4.9
96.0
5,219.2
Carbon
Content
Coefficient
(Tg C/QBtu)
_
31.00
25.63
14.47
20.62
16.76
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)
93.4
1.0
0.3
5.7
24.7
24.9
3.3
2.4
9.9
10.2
1.5
4.6
1.5
0.2
0.4
2.7
3.1
3.1
2.0
0.1
1.9
98.5
Storage
Factor
_
0.10
0.61
0.61
1.00
0.61
0.09
0.61
0.61
0.61
0.61
0.30
0.61
0.50
0.58
0.00
-
0.09
-
0.09
0.10
Carbon
Stored
(TgC)
61.5
0.1
0.2
3.5
24.7
15.3
0.3
1.5
6.0
6.3
1.0
1.4
0.9
0.1
0.3
0.0
0.3
0.3
0.2
0.0
0.2
62.0
Carbon
Emissions
(TgC)
31.9
0.9
0.1
2.2
+
9.6
3.0
0.9
3.8
3.9
0.6
3.2
0.6
0.1
0.2
2.7
2.8
2.8
1.8
0.1
1.73
36.5
Carbon
Emissions
(Tg C02 Eq.)
117.0
3.4
0.4
8.1
+
35.2
10.8
3.4
14.0
14.5
2.2
11.8
2.1
0.4
0.7
9.9
10.2
10.2
6.7
0.3
6.3
133.9
+ Does not exceed 0.05 Tg
- Not applicable.
aTo avoid double counting, exports have been deducted.
Note: Totals may not sum due to independent rounding.
Lastly, emissions were estimated by subtracting the C stored from the potential emissions (see Table 3-20). More
detail on the methodology for calculating storage and emissions from each of these sources is provided in Annex
2.3.
Where storage factors were calculated specifically for the United States, data were obtained on (1) products such as
asphalt, plastics, synthetic rubber, synthetic fibers, cleansers (soaps and detergents), pesticides, food additives,
antifreeze and deicers (glycols), and silicones; and (2) industrial releases including volatile organic compound,
solvent, and non-combustion CO emissions, Toxics Release Inventory (TRI) releases, hazardous waste incineration,
and energy recovery. Data were taken from a variety of industry sources, government reports, and expert
communications. Sources include EPA reports and databases such as compilations of air emission factors (EPA
1995, 2001), National Emissions Inventory (NEI) Air Pollutant Emissions Trends Data (EPA 2008), 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 (MECS) (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 (SPI2000); Bank of Canada (2006); Financial Planning
3-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Association (2006); INEGI (2006); Statistics Canada (2006); the United States International Trade Commission
(2006 through 2008); 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 (CMA 1999); and the American
Chemistry Council (ACC 2005 through 2008) 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 the five categories. In all cases,
statistical analyses or expert judgments of uncertainty were not available directly from the information sources for
all the activity variables; thus, uncertainty estimates were determined using assumptions based on source category
knowledge.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-23 (emissions) and Table 3-24
(storage factors). Carbon emitted from non-energy uses of fossil fuels in 2007 was estimated to be between 107.0
and 144.6 Tg CO2 Eq. at a 95 percent confidence level. This indicates a range of 20 percent below to 8 percent
above the 2007 emission estimate of 133.9 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.
Table 3-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Non-Energy Uses of Fossil Fuels
(Tg CO2 Eq. and Percent)
Source
2007
Emission
Estimate
Gas (Tg CO2 Eq.)
Uncertainty Range Relative to Emission Estimate"
(TgC02Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Feedstocks
Asphalt
Lubricants
Waxes
Other
Total
C02
C02
C02
C02
C02
C02
79.9
0.0
21.4
0.7
31.9
133.9
64.4
0.2
17.7
0.5
13.7
107.0
95.9
0.8
24.9
1.1
33.0
144.6
-19%
NA
-17%
-24%
-57%
-20%
+20%
NA
+16%
+64%
+3%
+8%
a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
NA (Not Applicable)
Table 3-24: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of Non-Energy Uses of Fossil Fuels
(Percent)
Source
2007 Storage
Gas Factor Uncertainty Range Relative to Emission Estimate"
(%) (%) (%, Relative)
Lower Upper Lower Upper
Bound Bound Bound Bound
Feedstocks
Asphalt
CO2 61% 59% 63% -4% +3%
CO2 100% 99% 100% -1% +0%
Energy 3-31
-------
Lubricants CO2 9% 4% 17% -57% +89%
Waxes CO2 58% 44% 70% -25% +20%
Other CO2 17% 17% 64% +2% +273%
a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval, as a
percentage of the inventory value (also expressed in percent terms).
In Table 3-24, 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 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 uses of fossil fuels was developed and implemented. This effort
included a Tier 1 analysis, as well as portions of a Tier 2 analysis for non-energy uses involving petrochemical
feedstocks and for imports and exports. The Tier 2 procedures that were implemented involved checks specifically
focusing on the activity data and methodology for estimating the fate of C (in terms of storage and emissions) across
the various end-uses of fossil C. Emission and storage totals for the different subcategories were compared, and
trends across the time series were analyzed to determine whether any corrective actions were needed. Corrective
actions were taken to rectify minor errors and to improve the transparency of the calculations, facilitating future
QA/QC.
For petrochemical import and export data, special attention was paid to NAICS numbers and titles to verify that
none had changed or been removed. Import and export totals were compared for 2007 as well as their trends across
the time series.
Recalculations Discussion
Non-energy end uses for petroleum coke (other than in the industrial processing sectors, where it is accounted for
separately) had not been identified in the past. Huurman (2006) suggests that in the Netherlands petroleum coke is
used in some pigments, and identifies its corresponding storage factor as 0.3. This year, it was assumed that
petroleum coke used for non-energy purposes (and not accounted for in the industrial processes chapter, viz., for
production of primary aluminum anodes, electric arc furnace anodes, titanium dioxide, ammonia, urea, and
ferroalloys) is used in pigments, with a storage factor of 0.3 (rather than the value of 0.5 used previously). This
resulted in an average 1.4% increase in NEU emissions across the time series.
3-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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, paints, and coatings. There is also a need to
further clarify the treatment of fuel additives and backflows (especially methyl tert-butyl ether, MTBE).
Finally, although U.S.-specific storage factors have been developed for feedstocks, asphalt, lubricants, and waxes,
default values from IPCC are still used for two of the non-energy fuel types (industrial coking coal and distillate oil),
and broad assumptions are being used for miscellaneous products and other petroleum. Over the long term, there
are plans to improve these storage factors by conducting analyses of C fate similar to those described in Annex 2.X.
3.3. Coal Mining (IPCC Source Category 1B1a)
Three types of coal mining related activities release CH4 to the atmosphere: underground mining, surface mining,
and post-mining (i.e., coal-handling) activities. Underground coal mines contribute the largest share of CH4
emissions. In 2007, 233 coal mines, (including all 131 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 2007, 15 coal
mines collected CH4 from degasification systems and utilized this gas, thus reducing emissions to the atmosphere.
Of these mines, 13 coal mines sold CH4 to the natural gas pipeline, one coal mine generated electricity, and one coal
mine used CH4 from its degasification system to heat mine ventilation air on site. On addition, one of the coal mines
that sold gas to pipelines also used CH4 to fuel a thermal coal dryer. Surface coal mines also release CH4 as the
overburden is removed and the coal is exposed, but the level of emissions is much lower than from underground
mines. 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 2007 were estimated to be 57.6 Tg CO2 Eq. (2,744 Gg), a decline of 31 percent since 1990
(see Table 3-25 and Table 3-26). Of this amount, underground mines accounted for 62 percent, surface mines
accounted for 24 percent, and post-mining emissions accounted for 15 percent. The decline in CH4 emissions from
underground mines from 1996 to 2002 was the result of the reduction of overall coal production, the mining of less
gassy coal, and an increase in CH4 recovered and used. Since that time, underground coal production and the
associated methane emissions have remained fairly level, while surface coal production and its associated emissions
have generally increased.
Table 3-25: CH4 Emissions from Coal Mining (Tg CO2 Eq.)
Activity
UG Mining
Liberated
Recovered & Used
Surface Mining
Post-Mining (UG)
Post-Mining (Surface)
Total
1990
62.3
67.9
(5.6)
12.0
7.7
2.0
84.1
1995
46.8
59.2
(12.4)
11.5
6.9
1.9
67.1
2000
39.5
54.4
(14.9)
12.3
6.7
2.0
60.5
2005
35.2
50.1
(14.9)
13.3
6.4
2.2
57.1
2006
35.8
54.5
(18.6)
14.0
6.3
2.3
58.4
2007
35.5
47.7
(12.3)
13.8
6.1
2.2
57.6
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
Table 3-26: CH4 Emissions from Coal Mining (Gg)
Activity 1990 1995 2000 2005 2006 2007
UG Mining 2,968 2,225 1,883 1,677 1,705 1,689
Energy 3-33
-------
Liberated
Recovered & Used
Surface Mining
Post-Mining (UG)
Post-Mining (Surface)
Total
3,234
(266)
574
368
93
4,003
2,817
(592)
548
330
89
3,193
2,593
(710)
586
318
95
2,881
2,387
(710)
633
306
103
2,719
2,593
(888)
668
298
109
2,780
2,273
(584)
659
290
107
2,744
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
Methodology
The methodology for estimating CH4 emissions from coal mining consists of two parts. The first part involves
estimating CH4 emissions from underground mines. Because of the availability of ventilation system measurements,
underground mine emissions can be estimated on a mine-by-mine basis and then summed to determine total
emissions. The second step involves estimating emissions from surface mines and post-mining activities by
multiplying basin-specific coal production by basin-specific emission factors.
Underground mines. Total CH4 emitted from underground mines was estimated as the sum of CH4 liberated from
ventilation systems and CH4 liberated by means of degasification systems, minus CH4 recovered and used. The
Mine Safety and Heath Administration (MSHA) samples CH4 emissions from ventilation systems for all mines with
detectable80 CH4 concentrations. These mine-by-mine measurements are used to estimate CH4 emissions from
ventilation systems.
Some of the higher-emitting underground mines also use degasification systems (e.g., wells or boreholes) that
remove CH4 before, during, or after mining. This CH4 can then be collected for use or vented to the atmosphere.
Various approaches were employed to estimate the quantity of CH4 collected by each of the twenty mines using
these systems, depending on available data. For example, some mines report to EPA the amount of CH4 liberated
from their degasification systems. For mines that sell recovered CH4 to a pipeline, pipeline sales data published by
state petroleum and natural gas agencies were used to estimate degasification emissions. For those mines for which
no other data are available, default recovery efficiency values were developed, depending on the type of
degasification system employed.
Finally, the amount of CH4 recovered by degasification systems and then used (i.e., not vented) was estimated. In
2007, 13 active coal mines sold recovered CH4 into the local gas pipeline networks, one used recovered CH4 to
generate electricity while one coal mine used recovered CH4 on site for heating. Emissions avoided for these
projects were estimated using gas sales data reported by various state agencies. For most mines with recovery
systems, companies and state agencies provided individual well production information, which was used to assign
gas sales to a particular year. For the few remaining mines, coal mine operators supplied information regarding the
number of years in advance of mining that gas recovery occurs.
Surface Mines and Post-Mining Emissions. Surface mining and post-mining CH4 emissions were estimated by
multiplying basin-specific coal production, obtained from the Energy Information Administration's Annual Coal
Report (see Table 3-27) (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 CH4 content of the
coal. Revised data on in situ CH4 content and emissions factors are taken from EPA (2005), EPA (1996), and
AAPG (1984). This calculation accounts for CH4 released from the strata surrounding the coal seam. For post-
mining emissions, the emission factor was assumed to be 32.5 percent of the average in situ CH4 content of coals
mined in the basin.
Table 3-27: Coal Production (Thousand Metric Tons)
Year Underground Surface Total
1990 384,250 546,818 931,068
1995 359,477 577,638 937,115
80 MSHA records coal mine CH4 readings with concentrations of greater than 50 ppm (parts per million) CELt. Readings below
this threshold are considered non-detectable.
3-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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2000 338,173 635,592 973,765
2005
2006
2007
334,404
325,703
319,145
691,460
728,459
720,035
1,025,864
1,054,162
1,039,179
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 CH4 recovered by
degasification systems are relatively certain because many coal mine operators provided information on individual
well gas sales and mined through dates. Many of the recovery estimates use data on wells within 100 feet of a
mined area. Uncertainty also exists concerning the radius of influence of each well. The number of wells counted,
and thus the avoided emissions, may vary if the drainage area is found to be larger or smaller than currently
estimated.
Compared to underground mines, there is considerably more uncertainty associated with surface mining and post-
mining emissions because of the difficulty in developing accurate emission factors from field measurements.
However, since underground emissions comprise the majority of total coal mining emissions, the uncertainty
associated with underground emissions is the primary factor that determines overall uncertainty. The results of the
Tier 2 quantitative uncertainty analysis are summarized in Table 3-28. Coal mining CH4 emissions in 2007 were
estimated to be between 48.6 and 71.2 Tg CO2 Eq. at a 95 percent confidence level. This indicates a range of 16
percent below to 24 percent above the 2007 emission estimate of 57.6 Tg CO2 Eq.
Table 3-28: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Coal Mining (Tg CO2 Eq. and
Percent)
Source
2007 Emission
Estimate
Gas (Tg CO2 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Coal Mining CH4 57.6 48.6 71.2 -16% 24%
a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
Recalculations Discussion
In 2007, calculations of emissions avoided at the four Jim Walters Resources (JWR) coal mines in Alabama were
performed using the previous EPA method. This was done in order to take a better documented approach and to
track the four coal mines individually rather than as a group. Emissions avoided calculations for any pre-drainage
wells at JWR coal mines are based on publicly-available data records from the Alabama State Oil & Gas Board.
Emission reductions are calculated for pre-drainage wells that are located inside the mine plan boundaries and are
declared "shut-in" by the O&G Board. The total production for a well is claimed in the year that the well was shut-
in and mined through.
3.4. Abandoned Underground Coal Mines (IPCC Source Category 1B1a)
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
Energy 3-35
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overburden fractures. As work stops within the mines, the CH4 liberation decreases but it does not stop completely.
Following an initial decline, abandoned mines can liberate CH4 at a near-steady rate over an extended period of
time, or, if flooded, produce gas for only a few years. The gas can migrate to the surface through the conduits
described above, particularly if they have not been sealed adequately. In addition, diffuse emissions can occur when
CH4 migrates to the surface through cracks and fissures in the strata overlying the coal mine. The following factors
influence abandoned mine emissions:
• Time since abandonment;
• Gas content and adsorption characteristics of coal;
• CH4 flow capacity of the mine;
• Mine flooding;
• Presence of vent holes; and
• Mine seals.
Gross abandoned mine CH4 emissions ranged from 6.0 to 9.1 Tg CO2 Eq. from 1990 through 2007, 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 2007, with only three closures in 2007. By 2007, gross abandoned mine emissions
increased to 9.0 Tg CO2 Eq. (see Table 3-29and Table 3-30). Gross emissions are reduced by CH4 recovered and
used at 27 mines, resulting in net emissions in 2007 of 5.7 Tg CO2 Eq.
Table 3-29: CH4 Emissions from Abandoned Coal Mines (Tg CO2 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
2005
7.0
1.4
5.6
2006
7.5
2.0
5.5
2007
9.0
o o
J.J
5.7
Note: Totals may not sum due to independent rounding.
Table 3-30: 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
2005
334
68
265
2006
359
96
263
2007
428
155
273
Note: Totals may not sum due to independent rounding.
Methodology
Estimating CH4 emissions from an abandoned coal mine requires predicting the emissions of a mine from the time
of abandonment through the inventory year of interest. The flow of CH4 from the coal to the mine void is primarily
dependent on the mine's emissions when active and the extent to which the mine is flooded or sealed. The CH4
emission rate before abandonment reflects the gas content of the coal, rate of coal mining, and the flow capacity of
the mine in much the same way as the initial rate of a water-free conventional gas well reflects the gas content of the
producing formation and the flow capacity of the well. A well or a mine which produces gas from a coal seam and
the surrounding strata will produce less gas through time as the reservoir of gas is depleted. Depletion of a reservoir
will follow a predictable pattern depending on the interplay of a variety of natural physical conditions imposed on
the reservoir. The depletion of a reservoir is commonly modeled by mathematical equations and mapped as a type
curve. Type curves which are referred to as decline curves have been developed for abandoned coal mines. Existing
data on abandoned mine emissions through time, although sparse, appear to fit the hyperbolic type of decline curve
used in forecasting production from natural gas wells.
3-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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In order to estimate CH4 emissions over time for a given mine, it is necessary to apply a decline function, initiated
upon abandonment, to that mine. In the analysis, mines were grouped by coal basin with the assumption that they
will generally have the same initial pressures, permeability and isotherm. As CH4 leaves the system, the reservoir
pressure, Pr, declines as described by the isotherm. The emission rate declines because the mine pressure (Pw) is
essentially constant at atmospheric pressure, for a vented mine, and the PI term is essentially constant at the
pressures of interest (atmospheric to 30 psia). A rate-time equation can be generated that can be used to predict
future emissions. This decline through time is hyperbolic in nature and can be empirically expressed as:
q = q1(
where,
q = Gas rate at time t in mmcf/d
qj = Initial gas rate at time zero (to) 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 = qie(-Dt)
where,
q = Gas flow rate at time t in mcf/d
q; = Initial gas flow rate at time zero (to) 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 * (i - (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 CH4 emissions. This same
relationship is assumed for abandoned mines. It was determined that 448 abandoned mines closing after 1972
produced emissions greater than 100 mcfd when active. Further, the status of 267 of the 448 mines (or 60 percent)
is known to be either: 1) vented to the atmosphere; 2) sealed to some degree (either earthen or concrete seals); or, 3)
flooded (enough to inhibit CH4 flow to the atmosphere). The remaining 40 percent of the mines were placed in one
of the three categories by applying a probability distribution analysis based on the known status of other mines
located in the same coal basin (EPA 2003).
Table 3-31: Number of gassy abandoned mines occurring in U.S. basins grouped by class according to post-
abandonment state
Basin Sealed Vented Flooded Total Known Unknown Total Mines
Energy 3-37
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Central Appl.
Illinois
Northern Appl.
Warrior Basin
Western Basins
Total
24
28
42
0
25
119
25
3
22
0
3
53
48
14
16
15
2
95
97
45
79
15
30
267
115
25
32
0
9
181
212
70
112
15
39
448
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 emissions rates during the 1970s (EPA 2003).
Abandoned mines emission estimates are based on all closed mines known to have active mine CH4 ventilation
emission rates greater than 100 mcfd at the time of abandonment. 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. CH4
degasification amounts were added to the quantity of CH4 ventilated for the total CH4 liberation rate for fifteen
mines that closed between 1992 and 2007. 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 2007, emission totals were downwardly adjusted to reflect abandoned mine CH4 emissions
avoided from those mines. The inventory totals were not adjusted for abandoned mine reductions in 1990 through
1992, because no data was reported for abandoned coal mining CH4 recovery projects during that time.
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 each mine, a methodological approach to estimating
emissions was used that generates a probability distribution of potential outcomes based on the most likely value and
the probable range of values for each parameter. The range of values is not meant to capture the extreme values, but
values that represent the highest and lowest quartile of the cumulative probability density function of each
parameter. Once the low, mid, and high values are selected, they are applied to a probability density function.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-32. Abandoned coal mines
CH4 emissions in 2007 were estimated to be between 4.6 and 7.1 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 19 percent below to 23 percent above the 2007 emission estimate of 5.7 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
3-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
uncertainty is associated with the unknown status mines (which account for 40 percent of the mines), with a ±53
percent uncertainty.
Table 3-32: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Abandoned Underground Coal
Mines (Tg CO2 Eq. and Percent)
Source
2007 Emission
Estimate
Gas (TgC02Eq.)
Uncertainty Range Relative to Emission
Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Abandoned Underground
CH
Coal Mines 4 5/7 4.6 7.1 -19% +23%
a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
3.5. 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 104.7 Tg CO2
Eq. (4,985 Gg) of CH4 in 2007, a 19 percent decrease over 1990 emissions (see Table 3-33 and Table 3-34), and
28.7 Tg CO2 Eq. (28,680 Gg) of non-combustion CO2 in 2007, a 15 percent decrease over 1990 emissions (see
Table 3-35 and Table 3-36). Improvements in management practices and technology, along with the replacement of
older equipment, have helped to stabilize emissions. Methane emissions decreased since 2006 despite an increase in
production and production wells due to a decrease in 73 offshore platforms and an increase of 25 percent in Natural
Gas STAR production sector emissions reductions.
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 CH4 and non-combustion CO2 emissions
are discussed.
Field Production. In this initial stage, wells are used to withdraw raw gas from underground formations. Emissions
arise from the wells themselves, gathering pipelines, and well-site gas treatment facilities such as dehydrators and
separators. Fugitive emissions and emissions from pneumatic devices account for the majority of CH4 emissions.
Flaring emissions account for the majority of the non-combustion CO2 emissions. Emissions from field production
accounted for approximately 21 percent of CH4 emissions and about 26 percent of non-combustion CO2 emissions
from natural gas systems in 2007.
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 CH4 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 CH4 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 power plants or chemical plants. Compressor station facilities, which contain large reciprocating and turbine
compressors, are used to move the gas throughout the United States transmission system. Fugitive CH4 emissions
from these compressor stations and from metering and regulating stations account for the majority of the emissions
from this stage. Pneumatic devices and engine uncombusted exhaust are also sources of CH4 emissions from
transmission facilities.
Natural gas is also injected and stored in underground formations, or liquefied and stored in above ground tanks,
during periods of low demand (e.g., summer), and withdrawn, processed, and distributed during periods of high
Energy 3-39
-------
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 39 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 1,190,000 miles of distribution mains in 2007, an increase from just over 944,000 miles in
1990 (OPS 2007b). Distribution system emissions, which account for approximately 28 percent of CH4 emissions
from natural gas systems and less than 1 percent of non-combustion CO2 emissions, result mainly from fugitive
emissions from gate stations and pipelines. An increased use of plastic piping, which has lower emissions than other
pipe materials, has reduced emissions from this stage. Distribution system CH4 emissions in 2007 were 11.4 percent
lower than 1990 levels.
Table 3-33. CH4 Emissions from Natural Gas Systems (Tg CO2 Eq.)*
Stage
Field Production
Processing
Transmission and Storage
Distribution
Total
1990
34.2
15.0
47.0
33.4
129.6
1995
38.7
15.1
46.4
32.4
132.6
2000
40.3
14.5
44.6
31.4
130.8
2005
26.4
11.6
39.1
29.3
106.3
2006
27.8
11.6
38.4
27.0
104.8
2007
22.4
12.3
40.4
29.6
104.7
*Inchiding CH4 emission reductions achieved by the Natural Gas STAR program and NESHAP regulations.
Note: Totals may not sum due to independent rounding.
Table 3-34. CH4 Emissions from Natural Gas Systems (Gg)*
Stage
Field Production
Processing
Transmission and Storage
Distribution
Total
1990
1,629
714
2,237
1,591
6,171
1995
1,842
717
2,212
1,543
6,314
2000
1,918
692
2,123
1,498
6,231
2005
1,256
550
1,862
1,393
5,062
2006
1,323
555
1,828
1,285
4,991
2007
1,066
584
1,926
1,409
4,985
*Inchiding 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. Non-combustion CO2 Emissions from Natural Gas Systems (Tg CO2 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
2005
7.6
21.7
0.1
29.5
2006
8.2
21.2
0.1
29.5
2007
7.4
21.2
0.1
28.7
Note: Totals may not sum due to independent rounding.
+ Emissions are less than 0.00 TCO2e
Table 3-36. Non-combustion CO2 Emissions from Natural Gas Systems (Gg)
Stage
Field Production
Processing
Transmission and Storage
Distribution
Total
1990
5,877
27,752
59
46
33,733
1995
9,084
24,621
61
45
33,810
2000
5,956
23,332
61
44
29,394
2005
7,625
21,736
61
41
29,463
2006
8,235
21,204
60
40
29,540
2007
7,389
21,189
61
41
28,680
Note: Totals may not sum due to independent rounding.
Methodology
The primary basis for estimates of CH4 and non-combustion-related CO2 emissions from the U.S. natural gas
3-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
industry is a detailed study by the Gas Research Institute and EPA (EPA/GRI1996). The EPA/GRI study developed
over 80 CH4 emission and activity factors to characterize emissions from the various components within the
operating stages of the U.S. natural gas system. The same activity factors were used to estimate both CH4 and non-
combustion CO2 emissions. However, the CH4 emission factors were adjusted for CO2 content when estimating
fugitive and vented non-combustion CO2 emissions. The EPA/GRI study was based on a combination of process
engineering studies and measurements at representative gas facilities. From this analysis, a 1992 emission estimate
was developed using the emission and activity factors, except where direct activity data was available (e.g., offshore
platform counts, processing plant counts, transmission pipeline miles, and distribution pipelines). For other years, a
set of industry activity factor drivers was developed that can be used to update activity factors. These drivers
include statistics on gas production, number of wells, system throughput, miles of various kinds of pipe, and other
statistics that characterize the changes in the U.S. natural gas system infrastructure and operations. See Annex 3.4
for more detailed information on the methodology and data used to calculate CH4 and non-combustion CO2
emissions from natural gas systems.
Activity factor data were taken from the following sources: American Gas Association (AGA 1991-1998); Minerals
and Management Service (MMS 2008a-d); Monthly Energy Review (EIA 20081); Natural Gas Liquids Reserves
Report (EIA 2005); Natural Gas Monthly (EIA 2008b,c,e); the Natural Gas STAR Program annual emissions
savings (EPA 2008); Oil and Gas Journal (OGJ 1997-2008); Office of Pipeline Safety (OPS 2008a-b) and other
Energy Information Administration publications (EIA 2001, 2004, 2008a,d); World Oil Magazine (2008a-b). Data
for estimating emissions from hydrocarbon production tanks were incorporated (EPA 1999). Coalbed CH4 well
activity factors were taken from the Wyoming Oil and Gas Conservation Commission (Wyoming 2008) and the
Alabama State Oil and Gas Board (Alabama 2008). Other state well data was taken from: American Association of
Petroleum Geologists (AAPG 2004); Brookhaven College (Brookhaven 2004); Kansas Geological Survey (Kansas
2008); Montana Board of Oil and Gas Conservation (Montana 2008); Oklahoma Geological Survey (Oklahoma
2008); Morgan Stanley (Morgan Stanley 2005); Rocky Mountain Production Report (Lippman (2003); New Mexico
Oil Conservation Division (New Mexico 2008a,b); Texas Railroad Commission (Texas 2008a-d); Utah Division of
Oil, Gas and Mining (Utah 2008). Emission factors were taken from EPA/GRI (1996). GTFs Unconventional
Natural Gas and Gas Composition Databases (GTI 2001) were used to adapt the CH4 emission factors into non-
combustion related CO2 emission factors. Additional information about CO2 content in transmission quality natural
gas was obtained via the internet from numerous U.S. transmission companies to help further develop the non-
combustion CO2 emission factors.
Uncertainty
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 are
summarized in Table 3-37. Natural gas systems CH4 emissions in 2007 were estimated to be between 79.7 and
150.2 Tg CO2 Eq. at a 95 percent confidence level. Natural gas systems non-energy CO2 emissions in 2007 were
estimated to be between 21.8 and 41.1 Tg CO2 Eq. at 95 percent confidence level.
Table 3-37: Tier 2 Quantitative Uncertainty Estimates for CH4 and Non-energy CO2 Emissions from Natural Gas
Systems (Tg CO2 Eq. and Percent)
2007 Emission
Estimate
Source Gas (Tg CO2 Eq.)c
Natural Gas Systems CH4 104.7
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower
Bound0
79.7
Upper
Bound0
150.2
Lower
Bound0
-24%
Upper
Bound0
+43%
Energy 3-41
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Natural Gas Systems'3 CO2 28.7 21.8 41.1 -24% +43%
a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
b An uncertainty analysis for the non-energy CO2 emissions was not performed. The relative uncertainty estimated (expressed as
a percent) from the CH4 uncertainty analysis was applied to the point estimate of non-energy CO2 emissions.
0 All reported values are rounded after calculation. As a result, lower and upper bounds may not be duplicable from other
rounded values as shown in table.
Recalculations Discussion
In the previous Inventory, all activity factors were estimated using base year activity factors and activity drivers
even if activity data was publicly available for all years in the time series. This was done to maintain consistency of
methodology across all sources. However, this resulted in discrepancy in the activity factors in outer years. This is
because activity data in the base year have been revised since the GRI activity factors were developed.
Additionally, the oil and gas industry has undergone changes that do not get reflected in the outer years, if the base
year activity factors are driving the entire time series.
Therefore, where direct activity data were available for activity factors, the activity factors were replaced with the
direct data for all years to adapt the natural gas inventory to publicly available data and adjust the current inventory
to better reflect emissions from these sources. Direct activity data are available for shallow water gas platforms,
deep water gas platforms, gas processing plants, transmission pipeline miles, distribution mains pipeline miles (by
pipeline material), and distribution services (by pipeline material). This substitution resulted in a 3.5 to 4 percent
increase in CH4 emissions in the inventory time series.
The second recalculation is a result of changing several base year (1992) activity factors to re-estimated EPA/GRI
(1996). Methane Emissions from the Natural Gas Industry report base year activity factors. The GRI study consists
of direct activity factors and derived activity factors. Direct activity factors refer to publicly available data, whereas
derived activity factors were obtained by extrapolating sample data collected from the surveys to national estimates
using direct factors such as gas production, gas throughput, etc. The base year derived activity factors were re-
estimated by updating the 1992 direct activity factor with the publicly available data discussed in the previous
paragraph.
All other recalculations are the result of updating the previous Inventory activity data with revised values.
Planned Improvements
Most of the activity factors and emission factors in the natural gas model are from the EPA/GRI (1996) study. A
study is currently underway to review selected emission factors in the natural gas industry, and as appropriate,
conduct measurement-based studies to develop updated emission factors to better reflect current national
circumstances. Results from these studies are expected in the next few years, and will be incorporated into the
inventory, pending a peer review.
3.6. 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, CH4 emissions are released to the atmosphere as fugitive
emissions, vented emissions, emissions from operational upsets, and emissions from fuel combustion. Fugitive and
vented CO2 emissions from petroleum systems are primarily associated with crude oil production and are negligible
in the transportation and refining operations. Combusted CO2 emissions are already accounted for in the Fossil Fuels
Combustion source category, and hence have not been taken into account in the Petroleum Systems source category.
Total CH4 and CO2 emissions from petroleum systems in 2007 were 28.8 Tg CO2 Eq. (1,370 Gg CH4) and 0.3 Tg
CO2 (287 Gg), respectively. Since 1990, CH4 emissions have declined by 15 percent, due to industry efforts to
reduce emissions and a decline in domestic oil production (see Table 3-38 and Table 3-39). CO2 emissions have
also declined by 24 percent since 1990 due to similar reasons (see Table 3-40 and Table 3-41).
Production Field Operations. Production field operations account for almost 98 percent of total CH4 emissions
from petroleum systems. Vented CH4 from field operations account for 91.5 percent of the emissions from the
production sector, unburned CH4 combustion emissions account for 5.2 percent, fugitive emissions are 3.2 percent,
and process upset emissions are slightly over two-tenths of a percent. The most dominant sources of emissions, in
3-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 CH4 that vents
with the exhaust. Emissions from chemical injection pumps are due to the 25 percent that use associated gas to
drive pneumatic pumps. The remaining five percent of the emissions are distributed among 26 additional activities
within the four categories: vented, fugitive, combustion and process upset emissions. For more detailed, source-
level data on CH4 emissions in production field operations, refer to Annex 3.5.
Vented CO2 associated with natural gas emissions from field operations account for 99 percent of the total CO2
emissions from this source category, 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 COa emissions from this source category, 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 62
percent of CH4 emissions from crude oil transportation. Fugitive emissions, almost entirely from floating roof tanks,
account for 19 percent. The remaining 19 percent is distributed among six additional sources within these two
categories. Emissions from pump engine drivers and heaters were not estimated due to lack of data.
Crude Oil Refining. Crude oil refining processes and systems account for slightly less than two percent of total CH4
emissions from the oil industry because most of the CH4 in crude oil is removed or escapes before the crude oil is
delivered to the refineries. There is an insignificant amount of CH4 in all refined products. Within refineries, vented
emissions account for about 87 percent of the emissions, while fugitive and combustion emissions account for
approximately six and seven percent, respectively. Refinery system blowdowns for maintenance and the process of
asphalt blowing—with air, to harden the asphalt—are the primary venting contributors. Most of the fugitive CH4
emissions from refineries are from leaks in the fuel gas system. Refinery combustion emissions include small
amounts of unburned CH4 in process heater stack emissions and unburned CH4 in engine exhausts and flares.
Table 3-38: CH4 Emissions from Petroleum Systems (Tg CO2 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
2005
27.6
8.3
2.8
1.5
14.5
0.4
0.1
0.6
28.3
2006
27.6
8.3
2.8
1.5
14.6
0.4
0.1
0.6
28.3
2007
28.1
8.4
2.8
1.5
15.0
0.4
0.1
0.6
28.8
Note: Totals may not sum due to independent rounding.
Table 3-39: CH4 Emissions from Petroleum Systems (Gg)
Activity
Production Field Operations
Pneumatic device venting
Tank venting
Combustion & process upsets
Misc. venting & fugitives
Wellhead fugitives
1990
1,581
489
179
88
799
26
1995
1,493
463
161
82
762
25
2000
1,408
428
154
76
727
22
2005
1,314
397
135
71
691
19
2006
1,314
396
135
71
694
17
2007
1,338
398
135
72
716
18
Energy 3-43
-------
Grade Oil Transportation 7 6 5 555
Refining 25 25 28 28 28 27
Total 1,613 1,524 1,441 1,346 1,346 1,370
Note: Totals may not sum due to independent rounding.
Table 3-40: CO2 Emissions from Petroleum Systems (Tg CO2 Eq.)
Activity
Production Field Operations
Pneumatic device venting
Tank venting
Misc. venting & fugitives
Wellhead fugitives
1990
0.4
0.3
1995
0.3
0.3
2000
0.3
0.3
2005
0.3
0.2
2006
0.3
0.2
2007
0.3
0.2
Total 0.4 0.3 0.3 0.3 0.3 0.3
+ Does not exceed 0.05 Tg CO2 Eq.
Table 3-41: CO2 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
2005
287
22
248
16
1
287
2006
288
22
249
16
1
288
2007
287
22
247
16
1
287
Note: Totals may not sum due to independent rounding.
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 three EPA studies (1996, 1999 and 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 2007. 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 2008a-c). For oil
storage tanks, the emissions factor was calculated from API TankCalc data as the total emissions per barrel of crude
charge (EPA 1999).
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.
Activity factors for the years 1990 through 2007 were collected from a wide variety of statistical resources. For
3-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
some years, complete activity factor data were not available. In such cases, one of three approaches was employed.
Where appropriate, the activity factor was calculated from related statistics using ratios developed for EPA (1996).
For example, EPA (1996) found that the number of heater treaters (a source of CH4 emissions) is related to both
number of producing wells and annual production. To estimate the activity factor for heater treaters, reported
statistics for wells and production were used, along with the ratios developed for EPA (1996). In other cases, the
activity factor was held constant from 1990 through 2007 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 2007, 1990 through 2008, 1995 through 2008a-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, 2008a-c), analysis of MMS data (EPA 2005, MMS 2004), the Oil & Gas Journal (OGJ 2008a,b), the
Interstate Oil and Gas Compact Commission (IOGCC 2008), and the United States Army Corps of Engineers (1995-
2008).
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 to a top-down approach. However, some uncertainty still remains.
Emission factors and activity factors are based on a combination of measurements, equipment design data,
engineering calculations and studies, surveys of selected facilities and statistical reporting. Statistical uncertainties
arise from natural variation in measurements, equipment types, operational variability and survey and statistical
methodologies. Published activity factors are not available every year for all 64 activities analyzed for petroleum
systems; therefore, some are estimated. Because of the dominance of the seven major sources, which account for
93.1 percent of the total methane emissions, the uncertainty surrounding these seven sources has been estimated
most rigorously, and serves as the basis for determining the overall uncertainty of petroleum systems emission
estimates.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-42. Because the top emission
sources have not changed from 2006, the relative uncertainty ranges computed for 2006 and published in the
previous Inventory were taken as valid and applied to the 2007 inventory emission estimates. Petroleum systems
CH4 emissions in 2007 were estimated to be between 20.7 and 70.2 Tg CO2 Eq., while CO2 emissions were
estimated to be between 0.2 and 0.7 Tg CO2 Eq. at a 95 percent confidence level. This indicates a range of 28
percent below to 144 percent above the 2007 emission estimates of 28.8 and 0.3 Tg CO2 Eq. for CH4 and CO2,
respectively.
Table 3-42: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petroleum Systems (Tg CO2 Eq. and
Percent)
2007 Emission Uncertainty Range Relative to Emission Estimate"
Estimate
Source Gas (Tg CO2 Eq.)b (Tg CO2 Eq.) (%)
Lower Upper Lower Upper
Boundb Boundb Boundb Boundb
Petroleum Systems CH4 28.8 20.7 70.2 -28% +144%
Petroleum Systems CO2 03 0.2 O7 -28% +144%
a Range of 2006 relative uncertainty predicted by Monte Carlo Simulation, based on 1995 base year activity factors, for a 95
percent confidence interval.
All reported values are rounded after calculation. As a result, lower and upper bounds may not be duplicable from other
Energy 3-45
-------
rounded values as shown in table.
Recalculations Discussion
All revisions were due to updating previous years' data with revised data from existing data sources.
Planned Improvements
As noted above, nearly all emission factors used in the development of the petroleum systems estimates were taken
from EPA (1995, 1996, 1999), with the remaining emission factors taken from EPA default values (EPA 2005) and
a consensus of industry peer review panels. These emission factors will be reviewed as part of future inventory
work. Results of this review and analysis will be incorporated into future inventories, as appropriate.
[BEGIN BOX]
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 CO2 is produced from both naturally-occurring CO2
reservoirs and from industrial sources such as natural gas processing plants and ammonia plants. In the current
Inventory, emissions from naturally-produced CO2 are estimated based on the application.
In the current Inventory report, the CO2 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 CO2 used in EOR
operations is assumed to be fully sequestered. Additionally, all anthropogenic CO2 emitted from natural gas
processing and ammonia plants is assumed to be emitted to the atmosphere, regardless of whether the CO2 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 CO2. 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 CO2 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 CO2 emissions estimates for geologic storage.
In October 2007, the U.S. EPA announced plans to develop regulations for geologic sequestration of CO2 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 CO2 capture, transport, injection and
geologic storage are not yet included in national totals. Preliminary estimates indicate that the amount of CO2
captured from industrial and natural sites, as well as fugitive emissions from pipelines is 40.0 Tg CO2 (40,044 Gg
CO2) (see Table 3-43). Site-specific monitoring and reporting data for CO2 injection sites (i.e., EOR operations)
were not readily available, therefore, these estimates assume all CO2 is emitted.
Table 3-43: Potential Emissions from CO2 Capture and Transport (Tg CO2 Eq.)
Year
Acid Gas Removal Plants
Naturally Occurring CO2
Ammonia Production Plants
Pipelines Transporting CO2
Total
1990
4.8
20.8
0.0
0.0
25.6
1995
3.7
22.5
0.7
0.0
26.9
2000
2.3
23.2
0.7
0.0
26.1
2005
6.0
28.3
0.7
0.0
34.9
2006
6.4
30.2
0.7
0.0
37.3
2007
6.3
33.1
0.7
0.0
40.0
3-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Table 3-44: Potential Emissions from CO2 Capture and Transport (Gg)
Year
Acid Gas Removal Plants
Naturally Occurring CO2
Ammonia Production Plants
Pipelines Transporting CO2
Total
1990
4,832
20,811
0
8
25,643
1995
3,672
22,547
676
8
26,896
2000
2,264
23,208
676
8
26,149
2005
5,992
8,267
676
7
34,935
2006
6,417
30,224
676
8
37,318
2007
6,282
33,086
676
8
40,044
[END BOX]
3.7. Incineration of Waste (IPCC Source Category 1A5)
Incineration is used to manage about 7 to 19 percent of the solid wastes generated in the United States, depending on
the source of the estimate and the scope of materials included in the definition of solid waste (EPA 2000b, Goldstein
and Matdes 2001, Kaufman et al. 2004a, Simmons et al. 2006, ArSova et al. 2008). In the context of this section,
waste includes all municipal solid waste (MSW) as well as tires. In the United States, almost all incineration of
MSW occurs at waste-to-energy facilities where useful energy is recovered, and thus emissions from waste
incineration are accounted for in the Energy chapter. Similarly, tires are combusted for energy recovery in industrial
and utility boilers. Incineration of waste results in conversion of the organic inputs to CO2. According to IPCC
guidelines, when the CO2 emitted is of fossil origin, it is counted as a net anthropogenic emission of CO2 to the
atmosphere. Thus, the emissions from waste incineration are calculated by estimating the quantity of waste
combusted and the fraction of the waste that is C derived from fossil sources.
Most of the organic materials in municipal solid wastes are of biogenic origin (e.g., paper, yard trimmings), and
have their net C flows accounted for under the Land Use, Land-Use Change, and Forestry chapter. However, some
components—plastics, synthetic rubber, synthetic fibers, and carbon black—are of fossil origin. Plastics in the U.S.
waste stream are primarily in the form of containers, packaging, and durable goods. Rubber is found in durable
goods, such as carpets, and in non-durable goods, such as clothing and footwear. Fibers in municipal solid wastes
are predominantly from clothing and home furnishings. As noted above, tires (which contain rubber and carbon
black) are also considered a "non-hazardous" waste and are included in the waste incineration estimate, though
waste disposal practices for tires differ from municipal solid waste (viz., most incineration occurs outside of MSW
combustion facilities).
Approximately 32 million metric tons of waste was incinerated in the United States in 2007 (EPA 2008). CO2
emissions from incineration of waste rose 91 percent since 1990, to an estimated 20.8 Tg CO2 Eq. (20,786 Gg) in
2007, as the volume of synthetic fibers and other fossil C-containing materials in waste increased (see Table 3-45
and Table 3-46). Waste incineration is also a source of N2O emissions (De Soete 1993). N2O emissions from the
incineration of waste were estimated to be 0.4 Tg CO2 Eq. (1 Gg N2O) in 2007, and have not changed significantly
since 1990.
Table 3-45: CO2 and N2O Emissions from the Incineration of Waste (Tg CO2 Eq.)
Gas/Waste Product
C02
Plastics
Synthetic Rubber in Tires
Carbon Black in Tires
Synthetic Rubber in MSW
Synthetic Fibers
N2O
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
17.5
11.8
0.9
1.2
1.6
2.0
0.4
17.9
2005
19.5
12.8
1.2
1.6
1.8
2.2
0.4
19.9
2006
19.8
12.9
1.2
1.6
1.8
2.3
0.4
20.2
2007
20.8
13.6
1.2
1.6
2.0
2.4
0.4
21.2
Table 3-46: CO2 and N2O Emissions from the Incineration of Waste (Gg)
Energy 3-47
-------
Gas/Waste Product
CO2
Plastics
Synthetic Rubber in Tires
Carbon Black in Tires
Synthetic Rubber in MSW
Synthetic Fibers
N2O
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
17,485
11,766
893
1,167
1,636
2,023
1
2005
19,532
12,782
1,207
1,579
1,752
2,212
1
2006
19,824
12,920
1,207
1,579
1,788
2,330
1
2007
20,786
13,622
1,207
1,579
2,000
2,378
1
Methodology
Emissions of CO2 from the incineration of waste include CO2 generated by the incineration of plastics, synthetic
fibers, and synthetic rubber, as well as the incineration of synthetic rubber and carbon black in tires. These
emissions were estimated by multiplying the amount of each material incinerated by the C content of the material
and the fraction oxidized (98 percent). Plastics incinerated in municipal solid wastes were categorized into seven
plastic resin types, each material having a discrete C content. Similarly, synthetic rubber is categorized into three
product types, and synthetic fibers were categorized into four product types, each having a discrete C content. Scrap
tires contain several types of synthetic rubber, as well as carbon black. Each type of synthetic rubber has a discrete
C content, and carbon black is 100 percent C. Emissions of CO2 were calculated based on the 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 incineration sources is provided
in Annex 3.6.
For each of the methods used to calculate CO2 emissions from the incineration of waste, data on the quantity of
product combusted and the C content of the product are needed. For plastics, synthetic rubber, and synthetic fibers,
the amount of material in municipal solid wastes and its portion incinerated were taken from the Characterization of
Municipal Solid Waste in the United States (EPA 2000b, 2002, 2003, 2005a, 2006b, 2007, 2008) 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 and 2007, 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 waste incineration categories for CO2
emissions) was reported in EPA's life cycle analysis of greenhouse gas emissions and sinks from management of
solid waste (EPA 2006a).
Incineration of waste also results in emissions of N2O. These emissions were calculated as a function of the total
estimated mass of waste incinerated 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 incinerated in
each year; for 1990 through 2006, these data were derived from the information published in BioCycle (ArSova et
al. 2008). Data on total waste incinerated was not available for 2007, so this value was assumed to equal the most
recent value available (2006). Table 3-47provides data on municipal solid waste generation and percentage
combusted 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).
Table 3-47: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted. Source: ArSova et al.
(2008).
Year Waste Generation Incinerated (%)
1990 266,365,714 11.5
1995 296,390,405 10.0
3-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
2000
2001
2002
2003
2004
2005
2006
2007
371,071,109
353,086,962"
335,102,816
343,482,645b
351,862,474
363,274,720
374,686,965
374,686,965°
7.0
7.4a
7.7
7.6b
7.4
7.2%
6.9%
6.9%c
a Interpolated between 2000 and 2002 values.
b Interpolated between 2002 and 2004 values.
0 Assumed equal to 2006 value.
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 the incineration of waste. 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 incineration emission estimates arise from both the assumptions applied to the data
and from the quality of the data. Key factors include MSW incineration rate; fraction oxidized; missing data on
waste composition; average C content of waste components; assumptions on the synthetic/biogenic C ratio; and
combustion conditions affecting N2O emissions. The highest levels of uncertainty surround the variables that are
based on assumptions (e.g., percent of clothing and footwear composed of synthetic rubber); the lowest levels of
uncertainty surround variables that were determined by quantitative measurements (e.g., combustion efficiency, C
content of C black).
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-48. Waste incineration CO2
emissions in 2007 were estimated to be between 15.2 and 25.0 Tg CO2 Eq. at a 95 percent confidence level. This
indicates a range of 27 percent below to 20 percent above the 2007 emission estimate of 20.8 Tg CO2 Eq. Also at a
95 percent confidence level, Waste incineration N2O emissions in 2007 were estimated to be between 0.1 and 1.2 Tg
CO2 Eq. This indicates a range of 71 percent below to 191 percent above the 2007 emission estimate of 0.4 Tg CO2
Eq.
Table 3-48: Tier 2 Quantitative Uncertainty Estimates for CO2 and N2O from the Incineration of Waste (Tg CO2 Eq.
and Percent)
2007 Emission
Estimate
Source Gas (Tg CO2 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Incineration of Waste CO2 20.8
Incineration of Waste N2O 0.4
15.2 25.0 -27% 20%
0.1 1.2 -71% 191%
a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.
QA/QC and Verification
A source-specific QA/QC plan was implemented for incineration of waste. This effort included a Tier 1 analysis, as
well as portions of a Tier 2 analysis. The Tier 2 procedures that were implemented involved checks specifically
focusing on the activity data and specifically focused on the emission factor and activity data sources and
methodology used for estimating emissions from incineration of waste. Trends across the time series were analyzed
Energy 3-49
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to determine whether any corrective actions were needed. Actions were taken to streamline the activity data
throughout the incineration of waste calculations.
Recalculations Discussion
This emissions source was previously known as Municipal Solid Waste Combustion.
Planned Improvements
Additional data sources for calculating an N2O emission factor for U.S. incineration of waste may be investigated.
In conjunction with its efforts to develop methods for reporting GHG emissions from various sources, the use of
new techniques using radiochemistry methods to directly measure the fossil C content of flue gas from the
incineration of waste may also be investigated.
3.8. Energy Sources of Indirect Greenhouse Gas Emissions
In addition to the main greenhouse gases addressed above, many energy-related activities generate emissions of
indirect greenhouse gases. Total emissions of nitrogen oxides (NOX), carbon monoxide (CO), and non-CH4 volatile
organic compounds (NMVOCs) from energy-related activities from 1990 to 2007 are reported in Table 3-49.
Table 3-49: NOX, CO, and NMVOC Emissions from Energy-Related Activities (Gg)
Gas/Source
NOX
Mobile Combustion
Stationary Combustion
Oil and Gas Activities
Incineration of Waste
International Bunker Fuels*
CO
Mobile Combustion
Stationary Combustion
Incineration of Waste
Oil and Gas Activities
International Bunker Fuels*
NMVOCs
Mobile Combustion
Stationary Combustion
Oil and Gas Activities
Incineration of Waste
International Bunker Fuels*
1990
20,829
10,920
9,689
139
82
2,020
125,640
119,360
5,000
978
302
130
12,620
10,932
912
554
222
61
1995
20,429
10,622
9,619
100
88
1,566
104,402
97,630
5,383
1,073
316
124
10,538
8,745
973
582
237
50
2000
18,338
10,310
7,802
111
114
1,344
89,714
83,559
4,340
1,670
146
128
8,952
7,229
1,077
388
257
45
2005
15,033
8,757
5,857
321
98
1,705
69,060
62,519
4,778
1,439
324
133
8,530
6,292
1,450
545
243
54
2006
14,129
8,271
5,445
316
98
1,719
64,876
58,322
4,792
1,438
323
130
8,198
5,954
1,470
535
239
54
2007
13,687
7,831
5,445
314
97
1,712
61,231
54,678
4,792
1,438
323
127
7,903
5,672
1,470
526
234
54
* These values are presented for informational purposes only and are not included in totals.
Note: Totals may not sum due to independent rounding.
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.
3-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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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.
3.9. International Bunker Fuels (IPCC Source Category 1: Memo Items)
Emissions resulting from the combustion of fuels used for international transport activities, termed international
bunker fuels under the UNFCCC, are currently not included in national emission totals, but are reported separately
based upon location of fuel sales. The decision to report emissions from international bunker fuels separately,
instead of allocating them to a particular country, was made by the Intergovernmental Negotiating Committee in
establishing the Framework Convention on Climate Change.81 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).82
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.83 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.
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.84
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 2007 from the combustion of international bunker fuels from both
aviation and marine activities were 109.9 Tg CO2 Eq., or five percent below emissions in 1990 (see Table 3-50 and
Table 3-51). Although emissions from international flights departing from the United States have increased (14
percent), emissions from international shipping voyages 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
81 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).
82 Note that the definition of international bunker fuels used by the UNFCCC differs from that used by the International Civil
Aviation Organization.
83 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).
84 Naphtha-type jet fuel was used in the past by the military in turbojet and turboprop aircraft engines.
Energy 3-51
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were also emitted.
Table 3-50: CO2, CH4, and N2O Emissions from International Bunker Fuels (Tg CO2 Eq.)
Gas/Mode 1990 1995 2000 2005 2006 2007
C02
Aviation
Marine
CH4
Aviation
Marine
N2O
Aviation
Marine
Total
114.3
46.4
68.0
0.2
+
0.1
1.1
0.5
0.5
115.6
101.6
51.2
50.4
0.1
+
0.1
0.9
0.6
0.4
102.7
99.0
57.7
41.3
0.1
+
0.1
0.9
0.6
0.3
100.0
111.5
56.4
55.1
0.1
+
0.1
1.0
0.6
0.4
112.7
110.5
54.6
56.0
0.1
+
0.1
1.0
0.6
0.4
111.7
108.8
52.7
56.0
0.1
+
0.1
1.0
0.6
0.4
109.9
+ Does not exceed 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions.
Table 3-51: CO2, CH4 and N2O Emissions from International Bunker Fuels (Gg)
Gas/Mode 1990 1995 2000 2005 2006 2007
CO2
Aviation
Marine
CH4
Aviation
Marine
N2O
Aviation
Marine
114,330
46,378
67,952
8
2
7
3
2
2
101,620
51,196
50,425
6
2
5
3
2
1
98,966
57,694
41,272
6
2
4
3
2
1
111,487
56,424
55,063
7
2
5
3
2
1
110,520
54,564
55,956
7
2
5
3
2
1
108,756
52,740
56,016
7
2
5
3
2
1
Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions.
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
(2008) and USAF (1998), and heat content for jet fuel was taken from EIA (2008). 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 derived from FAA's System for assessing Aviation Global
Emissions (SAGE) Model (FAA 2006). International aviation bunker fuel consumption from 1990-2007 was
calculated by assigning the difference between the sum of domestic activity data (in Tbtu) from SAGE and the
reported EIA transportation jet fuel consumption to the international bunker fuel category for jet fuel from EIA
(2008). 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.
3-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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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 2008). 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 3-52. 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 2008) for 1990 through 2001, and 2007, and
the Department of Homeland Security's Bunker Report for 2003 through 2006 (DHS 2008). Fuel consumption data
for 2002 was interpolated due to inconsistencies in reported fuel consumption data. Activity data on distillate diesel
consumption by military vessels departing from U.S. ports were provided by DESC (2008). 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 3-52.
Table 3-52: Aviation Jet Fuel Consumption for International Transport (Million Gallons)
Nationality
U.S. and Foreign Carriers
U.S. Military
Total
1990
4,932
862
5,794
1995
5,462
581
6,043
2000
6,158
480
6,638
2005
6,022
462
6,484
2006
5,823
400
6,223
2007
5,629
410
6,039
Note: Totals may not sum due to independent rounding.
Table 3-53: Marine Fuel Consumption for International Transport (Million Gallons)
Fuel Type 1990 1995 2000 2005 2006 2007
Residual Fuel Oil
Distillate Diesel Fuel &
Other
U.S. Military Naval Fuels
4,781
617
522
3,495
573
334
2,967
290
329
3,881
444
471
4,004
446
414
4,059
358
444
Total 5,920 4,402 3,586 4,796 4,864 4,861
Note: Totals may not sum due to independent rounding.
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.85 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.
Uncertainties exist with regard to the total fuel used by military aircraft and ships, and in the activity data on military
operations and training that were used to estimate percentages of total fuel use reported as bunker fuel emissions.
Total aircraft and ship fuel use estimates were developed from DoD records, which document fuel sold to the Navy
and Air Force from the Defense Logistics Agency. These data may slightly over or under estimate actual total fuel
use in aircraft and ships because each Service may have procured fuel from, and/or may have sold to, traded with,
85
See uncertainty discussions under Carbon Dioxide Emissions from Fossil Fuel Combustion.
Energy 3-53
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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 1996 IPCC Guidelines is to use data by
specific aircraft type (IPCC/UNEP/OECD/IEA 1997). The IPCC also recommends that cruise altitude emissions be
estimated separately using fuel consumption data, while landing and take-off (LTO) cycle data be used to estimate
near-ground level emissions of gases other than CO2.86
There is also concern as to the reliability of the existing DOC (1991 through 2008) 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. International jet fuel bunkers are
now calculated in tandem with the domestic jet fuel estimates. EPA performs the analysis for domestic activity data
(in Tbtu), as described in the CO2 from fossil fuel combustion section, and, using that calculated total for domestic
in comparison with EIA's total consumption activity data, assigns the remainder to the jet fuel bunkers consumption.
The previous method for international jet fuel bunkers were calculated based upon DOT (1991 through 2008) and
BEA (1991 through 2005) data for the years 1990-1999 and 2006-2007 and estimated by FAA (2006) for 2000-
2005. That data is still collected and used to quality assure the new method. The new method is understood to
reduce the uncertainty of the domestic emissions calculation, as it relies on one dataset, rather than the multiple
datasets that were used in the previous method for international jet fuel bunkers. Distillate and residual fuel oil
consumption by cargo or passenger carrying marine vessels from 2003 through 2006 was revised using DHS (2008),
and 2002 distillate and residual fuel oil consumption was interpolated to adjust inconsistencies in reported fuel
86 U.S. aviation emission estimates for CO, NOX, and NMVOCs are reported by EPA's National Emission Inventory (NET) 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.
3-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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consumption data. These historical data changes resulted in changes to the emission estimates for 1990 through
2006, which averaged to an annual increase in emissions from international bunker fuels of 6.6 Tg CO2 Eq. (7.0
percent) in CO2 emissions, an annual increase of less than 0.1 Tg CO2 Eq. (14 percent) in CH4 emissions, and an
annual increase of 0.1 Tg CO2 Eq. (12 percent) in N2O emissions.
3.10. 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 2007, total CO2 emissions from the burning of woody biomass in the industrial, residential, commercial, and
electricity generation sectors were approximately 209.8 Tg CO2 Eq. (209,785 Gg) (see Table 3-54 and Table 3-55).
As the largest consumer of woody biomass, the industrial sector was responsible for 65 percent of the CO2 emissions
from this source. The residential sector was the second largest emitter, constituting 23 percent of the total, while the
commercial and electricity generation sectors accounted for the remainder.
Table 3-54: CO2 Emissions from Wood Consumption by End-Use Sector (Tg CO2 Eq.)
End-Use Sector 1990 1995 2000 2005 2006 2007
Industrial
Residential
Commercial
Electricity Generation
Total
135.3
59.8
6.8
13.3
215.2
155.1
53.6
7.5
12.9
229.1
153.6
43.3
7.4
13.9
218.1
136.3
46.4
7.2
19.1
208.9
142.2
42.3
6.7
18.7
209.9
136.7
47.4
6.7
18.9
209.8
Note: Totals may not sum due to independent rounding.
Table 3-55: CO2 Emissions from Wood Consumption by End-Use Sector (Gg)
End-Use Sector
Industrial
Residential
Commercial
Electricity Generation
Total
1990
135,348
59,808
6,779
13,252
215,186
1995
155,075
53,621
7,463
12,932
229,091
2000
153,559
43,309
7,370
13,851
218,088
2005
136,269
46,402
7,182
19,074
208,927
2006
142,226
42,278
6,675
18,748
209,926
2007
136,729
47,434
6,675
18,947
209,785
Note: Totals may not sum due to independent rounding.
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 2007, the United States consumed an estimated 577 trillion Btu of ethanol, and as a result, produced
approximately 38.0 Tg CO2 Eq. (38,044 Gg) (see Table 3-56 and Table 3-57 ) of CO2 emissions. Ethanol
production and consumption has grown steadily every year since 1990, with the exception of 1996 due to short corn
supplies and high prices in that year.
Table 3-56: CO2 Emissions from Ethanol Consumption (Tg CO2 Eq.)
End-Use Sector
Transportation
Industrial
Commercial
1990
4.1
0.1
+
1995
7.6
0.1
+
2000
9.1
0.1
+
2005
22.0
0.5
0.1
2006
29.8
0.6
0.1
2007
37.2
0.8
0.1
Energy 3-55
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Total 4.2 7/7 9.2 22.6 30.5 38.0
+ Does not exceed 0.05 Tg CO2 Eq.
Table 3-57: CO2 Emissions from Ethanol Consumption (Gg)
End-Use Sector
Transportation
Industrial
Commercial
Total
1990
4,066
55
33
4,155
1995
7,570
104
9
7,683
2000
9,077
85
25
9,188
2005
22,034
460
59
22,554
2006
29,758
622
80
30,459
2007
37,168
111
100
38,044
Methodology
Woody biomass emissions were estimated by applying two EIA gross heat contents (Lindstrom 2006) to U.S.
consumption data (EIA 2008) (see Table 3-58), provided in energy units for the industrial, residential, commercial,
and electric generation sectors. One heat content (16.953114 MMBtu/MT wood and wood waste) was applied to the
industrial sector's consumption, while the other heat content (15.432359 MMBtu/MT wood and wood waste) was
applied to the consumption data for the other sectors. An EIA emission factor of 0.434 MT C/MT wood (Lindstrom
2006) was then applied to the resulting quantities of woody biomass to obtain CO2 emission estimates. It was
assumed that the woody biomass contains black liquor and other wood wastes, has a moisture content of 12 percent,
and is converted into CO2 with 100 percent efficiency. The emissions from ethanol consumption were calculated by
applying an EIA emission factor of 17.99 Tg C/QBtu (Lindstrom 2006) to U.S. ethanol consumption estimates that
were provided in energy units (EIA 2008) (see Table 3-59).
Table 3-58: 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
2005
1,452
450
70
185
2,156
2006
1,515
410
65
182
2,172
2007
1,457
460
65
184
2,165
Table 3-59: Ethanol Consumption by Sector (Trillion Btu)
End-Use Sector
Transportation
Industrial
Commercial
Total
1990
61.7
0.8
0.5
63.0
1995
114.8
1.6
0.1
116.5
2000
137.6
1.3
0.4
139.3
2005
334.1
7.0
0.9
342.0
2006
451.2
9.4
1.2
461.9
2007
563.6
11.8
1.5
576.9
Uncertainty
It is assumed that the combustion efficiency for woody biomass is 100 percent, which is believed to be an
overestimate of the efficiency of wood combustion processes in the United States. Decreasing the combustion
efficiency would 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
Wood consumption values were revised in 2001 through 2003, and 2005 through 2006 based on updated
information from EIA's Annual Energy Review (EIA 2008). EIA (2008) also reported minor changes in wood
consumption for all sectors in 2006. This adjustment of historical data for wood biomass consumption resulted in an
average annual increase in emissions from wood biomass consumption of 0.6 Tg CO2 Eq. (0.3 percent) from 1990
3-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
through 2006. Slight adjustments were made to ethanol consumption based on updated information from EIA
(2008), which slightly decreased estimates for ethanol consumed. As a result of these adjustments, average annual
emissions from ethanol consumption decreased by less than 0.1 Tg CO2 Eq. (less than 0.1 percent).
Energy 3-57
-------
Fossil Fuel Combustion
Non-Energy Use of Fuels
Natural Gas Systems
Coal Mining
Mobile Combustion
Petroleum Systems
Stationary Combustion •
Incineration of Waste I
Abandoned Underground Coal Mines I
Energy as a Portion
of all Emissions
5,735.8
0 25 50 75 100
Tg C02 Eq.
Figure 3-1: 2007 Energy Chapter Greenhouse Gas Sources
125 150
-------
NEU Emissions
3
Natural Gas Emissions
1,225
NEU Emissions 122
Non-Energy Use
Carbon Sequestered
227
Fossil Fuel Stock Non-Energy
Non-Energy Consumption Changes Use U.S.
Use imports U.S. 25 Territories
55 Territories °
51
Note: Totals may not sum due to independent rounding.
The "Balancing Item" above accounts for the statisticai imbalances
and unknowns in the reported data sets combined here.
3-2 2007 U.S. (Tg C02 Eq.)
-------
7% Renewable
Nuclear
22% Natural Gas
22% Coal
39% Petroleum
Figure 3-3: 2007 U.S. Energy Consumption by Energy Source
-------
120 -]
100 -
O- 80 -
E.
60 -
40 -
20 -
Total Energy
Fossil Fuels
Renewable & Nuclear
0 J
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure 3-4: U.S. Energy Consumption (Quadrillion Btu)
Note: Expressed as gross calorific values.
2,500 -|
. 2,000 -
a-
L1J
O 1,500 -
(_)
P 1,000 -
500 -
0 -
Relative Contribution
by Fuel Type
i Natural Gas
Petroleum
I Coal
E
5
•!=
i/i 2
=i'E
£
Figure 3-5: 2007 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type
Note: The electricity generation sector also includes emissions of less than 0.5 Tg CO 2 Eq. from geothermal-based electricity
-------
15 -|
10 -
Normal
(4,524 Heating Degree Days)
-15 J
8 m m m £n
-------
1,600
1,400 -
1,200 -
_c
c 1,000
3
* 800 -\
600
400 J
Industrial
Figure 3-9: Electric Generation Retail Sales by End-Use Sector
Note: The transportation end-use sector consumes minor quanties of electricity.
(N (N (N (N
-------
Total excluding
'Computers, Communications
Equipment, and
Semiconductors
Paper
120
110
100
90
80
70
60
120
110
100
90
80
120
110
100
90
80
70
120
110
100
90
80
o •«~i r\i ro ^~ LD ^D i^ co CTI o •«~i r\i ro ^~ LTI ^D i^
2222222222§§§§§§§§
Figure 3-10: Industrial Production Indexes (Index 2002=100)
.2
01
CD
25 -i
24 -
23 -
22 -
21 -
20
19 -
18 -
17 -
16
15
Model Year
Figure 3-11: Sales-Weighted Fuel Economy of New Passenger Cars and Light-Duty Trucks, 1990-2007
-------
12,000 -,
10,000 -
? 8,000 -
p
| 6,000 -
| 4,000 -
2,000 -
0 -
Passenger Cars
Linht-Dutv Trucks
-12: Sales of New Passenger Cars and Light-Duty Trucks, 1990-2007
N,O
CH4
Figure 3-13: Mobile Source CH4 and N2O Emissions
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007
Figure 3-14: U.S. Energy Consumption and Energy-Related CO2 Emissions
Per Capita and Per Dollar GDP
-------
-------
4. Industrial Processes
Greenhouse gas emissions are produced as the by-products of various non-energy-related industrial activities. That
is, these emissions are produced from an industrial process itself and are not directly a result of energy consumed
during the process. For example, raw materials can be chemically transformed from one state to another. This
transformation can result in the release of greenhouse gases such as carbon dioxide (CO2), methane (CH4), or nitrous
oxide (N2O). The processes addressed in this chapter include iron and steel production, 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 use, aluminum production, titanium dioxide
production, CO2 consumption, ferroalloy production, phosphoric acid production, zinc production, lead production,
petrochemical production, silicon carbide production and consumption, nitric acid production, and adipic acid
production (see Figure 4-1).
Figure 4-1: 2007 Industrial Processes Chapter Greenhouse Gas Sources
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 2007, industrial processes generated emissions of 353.8 teragrams of CO2 equivalent (Tg CO2 Eq.), or 5 percent
of total U.S. greenhouse gas emissions. CO2 emissions from all industrial processes were 174.9 Tg CO2 Eq.
(174,939 Gg) in 2007, or 3 percent of total U.S. CO2 emissions. CH4 emissions from industrial processes resulted in
emissions of approximately 1.7 Tg CO2 Eq. (82 Gg) in 2007, which was less than 1 percent of U.S. CH4 emissions.
N2O emissions from adipic acid and nitric acid production were 27.6 Tg CO2 Eq. (89 Gg) in 2007, or 9 percent of
total U.S. N2O emissions. In 2007 combined emissions of HFCs, PFCs and SF6 totaled 149.5 Tg CO2 Eq. Overall,
emissions from industrial processes increased by 9 percent from 1990 to 2007 despite decreases in emissions from
several industrial processes, such as cement production, lime production, limestone and dolomite use, soda ash
production and consumption, and electrical transmission and distribution. The increase in overall emissions was
driven by a rise in the emissions originating from HCFC-22 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.
Table 4-1: Emissions from Industrial Processes (Tg CO2 Eq.)
Gas/Source
CO2
Iron and Steel Production and Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Cement Production
Ammonia Production & Urea Consumption
Lime Production
1990
197.6
109.8
104.3
5.5
33.3
16.8
11.5
1995
198.6
103.1
98.1
5.0
36.8
17.8
13.3
2000
193.2
95.1
90.7
4.4
41.2
16.4
14.1
2005
171.1
73.2
69.3
3.8
45.9
12.8
14.4
2006
175.9
76.1
72.4
3.7
46.6
12.3
15.1
2007
174.9
77.4
73.6
3.8
44.5
13.8
14.6
Industrial Processes 4-1
-------
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
CH4
Petrochemical Production
Iron and Steel Production and Metallurgical
Coke Production
Iron and Steel Production
Metallurgical Coke Production
Ferroalloy Production
5
6
4
2
1
1
2
1
0
0
0
1
0
1
i
.1
.8
.1
.2
.2
.4
.2
.5
.9
.3
.4
.9
.9
.0
.0
+
+
6.7
5.7
4.3
2.8
1.5
1.4
2.0
1.5
1.0
0.3
0.3
2.1
1.1
1.0
1.0
+
+
5.1
6.1
4.2
3.0
.8
.4
.9
.4
.1
0.3
0.2
2.2
1.2
0.9
0.9
+
+
6
4
4
2
1
1
1
1
0
0
0
1
1
0
.8
.1
.2
.8
.8
.3
.4
.4
.5
.3
.2
.8
.1
.7
0.7
+
+
8.0
3.8
4.2
2.6
1.9
1.7
1.5
1.2
0.5
0.3
0.2
1.7
1.0
0.7
0.7
+
+
6.2
4.3
4.1
2.6
1.9
1.9
1.6
1.2
0.5
0.3
0.2
1.7
1.0
0.7
0.7
+
+
Silicon Carbide Production and Consumption + + + + + +
N2O
Nitric Acid Production
Adipic Acid Production
HFCs
Substitution of Ozone Depleting Substances3
HCFC-22 Manufacture
Semiconductor Manufacturing HFCs
PFCs
Aluminum Production
Semiconductor Manufacturing PFCs
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacturing SF6
Total
+ Does not exceed 0.05 Tg CO2 Eq.
35
20
15
36
0
36
0
20
18
2
32
26
5
0
325
.3
.0
.3
.9
.3
.4
.2
.8
.5
.2
.8
.8
.4
.5
.2
39.6
22.3
17.3
61.8
28.5
33.0
0.3
15.6
11.8
3.8
28.1
21.6
5.6
0.9
345.8
28.1
21.9
6.2
100.1
71.2
28.6
0.3
13.5
8.6
4.9
19.2
15.1
3.0
1.1
356.3
24
.6
18.6
5
116
100
15
0
6
3
3
17
14
2
1
337
.9
.1
.0
.8
.2
.2
.0
.2
.9
.0
.9
.0
.6
24.2
18.2
5.9
119.1
105.0
13.8
0.3
6.0
2.5
3.5
17.1
13.2
2.9
1.0
343.9
27.6
21.7
5.9
125.5
108.3
17.0
0.3
7.5
3.8
3.7
16.5
12.7
3.0
0.8
353.8
Note: Totals may not sum due to independent rounding.
a Small amounts of PFC emissions also result from this
source
Table 4-2: Emissions from Industrial Processes (Gg)
Gas/Source
CO2
Iron and Steel Production and
Metallurgical Coke Production
Iron and Steel Production
Metallurgical Coke Production
Cement Production
Ammonia Production & Urea
Consumption
Lime Production
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and
Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
1990
197,623
109,760
104,262
5,498
33,278
16,831
11,533
5,127
6,831
4,141
2,221
1,195
1,416
1995
198
103
,584
,116
98,078
5
36
17
13
6
5
4
2
1
1
,037
,847
,796
,325
,651
,659
,304
,750
,526
,422
2000
193,217
95,062
90,680
4,381
41,190
16,402
14,088
5,056
6,086
4,181
3,004
1,752
1,421
2005
171,075
73,190
69,341
3,849
45,910
12,849
14,379
6,768
4,142
4,228
2,804
1,755
1,321
2006
175,897
76,100
72,418
3,682
46,562
12,300
15,100
8,035
3,801
4,162
2,573
1,876
1,709
2007
174,939
77,370
73,564
3,806
44,525
13,786
14,595
6,182
4,251
4,140
2,636
1,876
1,867
4-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Ferroalloy Production 2,152 2,036 1,893 1,392 1,505 1,552
Phosphoric Acid Production 1,529 1,513 1,382 1,386 1,167 1,166
Zinc Production 949 1,013 1,140 465 529 530
Lead Production 285 298 311 266 270 267
Silicon Carbide Production and
Consumption 375 329 248 219 207 196
CH4 88 100 104 86 83 82
Petrochemical Production 41 52 59 51 48 48
Iron and Steel Production and 46 47 44 34 35 33
Metallurgical Coke Production
Iron and Steel Production 46 47 44 34 35 33
Metallurgical Coke Production + + + + + +
Ferroalloy Production 1 1 1 + + +
Silicon Carbide Production and
Consumption 1 1 1 + + +
N2O 114 128 91 79 78 89
Nitric Acid Production 64 72 71 60 59 70
Adipic Acid Production 49 56 20 19 19 19
HFCs M M M M M M
Substitution of Ozone Depleting
Substances3 M M M M M M
HCFC-22 Production + 3 3 111
Semiconductor Manufacturing HFCs + + + + + +
PFCs M M M M M M
Aluminum Production M M M M M M
Semiconductor Manufacturing PFCs M M M M M M
SF6 111111
Electrical Transmission and
Distribution + 1 1 111
Magnesium Production and
Processing + + + + + +
Semiconductor Manufacturing SF6 + + + + + +_
+ Does not exceed 0.5 Gg
M (Mixture of gases)
Note: Totals may not sum due to independent rounding.
a Small amounts of PFC emissions also result from this source.
QA/QC and Verification Procedures
Tier 1 quality assurance and quality control procedures have been performed for all industrial process sources. For
industrial process sources of CO2 and CH4 emissions, a detailed planned 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
Industrial Processes 4-3
-------
qualitative and quantitative information. Uncertainty estimates and probability density functions for the emission
factors used to calculate emissions from this source were devised based on IPCC recommendations.
Activity data is obtained through a survey of manufacturers conducted by various organizations (specified within
each source); the uncertainty of the activity data is a function of the reliability of plant-level production data and is
influenced by the completeness of the survey response. The emission factors used were either derived using
calculations that assume precise and efficient chemical reactions, or were based upon empirical data in published
references. As a result, uncertainties in the emission coefficients can be attributed to, among other things,
inefficiencies in the chemical reactions associated with each production process or to the use of empirically-derived
emission factors that are biased; therefore, they may not represent U.S. national averages. Additional assumptions
are described within each source.
The uncertainty analysis performed to quantify uncertainties associated with the 2007 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. 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.
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 itself87 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.
During the cement production process, calcium carbonate (CaCO3) is heated in a cement kiln at a temperature of
about 1,450°C (2,400°F) to form lime (i.e., calcium oxide or CaO) and CO2 in a process known as calcination or
calcining. A very small amount of carbonates other than CaCO3 and non-carbonates are also present in the raw
material; however, for calculation purposes all of the raw material is assumed to be CaCO3. Next, the lime is
combined with silica-containing materials to produce clinker (an intermediate product), with the earlier by-product
CO2 being released to the atmosphere. The clinker is then allowed to cool, mixed with a small amount of gypsum,
and potentially other materials (e.g., slag) and used to make portland cement.88
In 2007, U.S. clinker production—including Puerto Rico—totaled 86,106 thousand metric tons (vanOss 2008b).
The resulting emissions of CO2 from 2007 cement production were estimated to be 44.5 Tg CO2 Eq. (44,525 Gg)
(see Table 4-3).
Table 4-3: CO2 Emissions from Cement Production (Tg CO2 Eq. and Gg)
Year
1990
1995
2000
Tg C02 Eq.
33.3
36.8
41.2
Gg
33,278
36,847
41,190
87 The CO2 emissions related to the consumption of energy for cement manufacture are accounted for under CO2 from Fossil
Fuel Combustion in the Energy chapter.
88 Approximately six percent of total clinker production is used to produce masonry cement, which is produced using plasticizers
(e.g., ground limestone, lime) and portland cement. CO2 emissions that result from the production of lime used to create masonry
cement are included in the Lime Manufacture source category (van Oss 2008c).
4-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
2005 45.9 45,910
2006 46.6 46,562
2007 44.5 44,525
After falling in 1991 by two percent from 1990 levels, cement production emissions grew every year through 2006,
and then decreased slightly from 2006 to 2007. Overall, from 1990 to 2007, emissions increased by 34 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.
Methodology
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 CaCOs (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 2008c) and a constant reflecting the mass of CO2 released per unit of
lime. This calculation yields an emission factor of 0.51 tons of CO2 per ton of clinker produced, which was
determined as follows:
EF = 0.65 CaO x
Clinker
56.08 g/moleCaO
44.01g/moleCO
= 0.51 tons CO /ton clinker
2
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.89 Total
cement production emissions were calculated by adding the emissions from clinker production to the emissions
assigned to CKD (IPCC 2006).90
The 1990 through 2007 activity data for clinker production (see Table 4-4) were obtained through a personal
communication with Hendrik van Oss (van Oss 2008b) of the USGS and through the USGS Mineral Yearbook:
Cement (US Bureau of Mines 1990 through 1993, USGS 1995 through 2006). The data were compiled by USGS
through questionnaires sent to domestic clinker and cement manufacturing plants.
Table 4-4: Clinker Production (Gg)
Year Clinker
1990 64,355
1995 71,257
2000 79,656
2005 88,783
2006 90,045
89 Default IPCC clinker and CKD emission factors were verified through expert consultation with the Portland Cement
Association (PCA 2008) and van Oss (2008a).
90 The 2 percent CO2 addition associated with CKD is included in the emission estimate for completeness. The cement emission
estimate also includes an assumption that all raw material is limestone (CaCO3) when in fact a small percentage is likely
composed of non-carbonate materials. Together these assumptions may result in a small emission overestimate (van Oss 2008c).
Industrial Processes 4-5
-------
2007 86,106
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 2008b). CKD loss can
range from 1.5 to 8 percent depending upon plant specifications. Additionally, some amount of CO2 is reabsorbed
when the cement is used for construction. As cement reacts with water, alkaline substances such as calcium
hydroxide are formed. During this curing process, these compounds may react with CO2 in the atmosphere to create
calcium carbonate. This reaction only occurs in roughly the outer 0.2 inches of surface area. Because the amount of
CO2 reabsorbed is thought to be minimal, it was not estimated.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-5. Cement Production CO2
emissions were estimated to be between 38.8 and 50.5 Tg CO2 Eq. at the 95 percent confidence level. This indicates
a range of approximately 13 percent below and 13 percent above the emission estimate of 44.5 Tg CO2 Eq.
Table 4-5: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Cement Production (Tg CO2 Eq. and
Percent)
2007 Emission
Source Gas Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate"
(TgC02Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Cement Production CO2 44.5
38.8 50.5 -13% +13%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations
Estimates of CO2 emissions from cement production were revised for 2006 to reflect updates to the clinker
production data for that year.
Planned Improvements
Future improvements to the cement source category involve continued research into emission factors for clinker
production and CKD. Research has been conducted into the accuracy and appropriateness of default emission
factors and reporting methodology used by other organizations. As these methodologies continue to develop, the
cement source category will be updated with any improvements to IPCC assumptions for clinker and CKD
emissions.
4.2. Lime Production (IPCC Source Category 2A2)
Lime is an important manufactured product with many industrial, chemical, and environmental applications. Its
major uses are in steel making, flue gas desulfurization (FGD) systems at coal-fired electric power plants,
construction, and water purification. For U.S. operations, the term "lime" actually refers to a variety of chemical
compounds. These include calcium oxide (CaO), or high-calcium quicklime; calcium hydroxide (Ca(OH)2), or
hydrated lime; dolomitic quicklime ([CaOMgO]); and dolomitic hydrate ([Ca(OH)2«MgO] or
[Ca(OH)2-Mg(OH)2]).
Lime production involves three main processes: stone preparation, calcination, and hydration. CO2 is generated
during the calcination stage, when limestone—mostly calcium carbonate (CaCO3)—is roasted at high temperatures
in a kiln to produce CaO and CO2. The CO2 is given off as a gas and is normally emitted to the atmosphere. Some
of the CO2 generated during the production process, however, is recovered at some facilities for use in sugar refining
4-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
and precipitated calcium carbonate (PCC) production.91 In certain additional applications, lime reabsorbs CO2
during use.
Lime production in the United States—including Puerto Rico—was reported to be 20,192 thousand metric tons in
2007 (USGS 2008). This resulted in estimated CO2 emissions of 14.6 Tg CO2 Eq. (or 14,595 Gg) (see Table 4-6
and Table 4-7).
Table 4-6: CO2 Emissions from Lime Production (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 11.5 11,533
1995 13.3 13,325
2000 14.1 14,088
2005 14.4 14,379
2006 15.1 15,100
2007 14.6 14,595
Table 4-7: Potential, Recovered, and Net CO2 Emissions from Lime Production (Gg)
Year Potential Recovered* Net Emissions
1990 12,004 471 11,533
1995 14,019 694 13,325
2000 14,872 784 14,088
2005
2006
2007
15,131
15,825
15,264
752
725
669
14,379
15,100
14,595
For sugar refining and PCC production.
Note: Totals may not sum due to rounding
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, 22 percent; construction uses, 12 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 2007, the amount of lime used for construction decreased by 8 percent from
2006 levels. This is most likely a result of increased prices for lime and the downturn in new home construction;
wherein, total construction spending decreased by 3 percent and residential construction spending decreased by
nearly 18 percent compared with that of 2006 (USGS 2008).
Lime production in 2007 decreased by 4 percent compared to 2006, owing to a downturn in major markets including
construction, mining, and steel (USGS 2008). Overall, from 1990 to 2007, lime production has increased by 28
percent. Annual consumption for industrial and chemical, and environmental lime consumption decreased by 1
percent and 4 percent, respectively (USGS 2008). The decrease in environmental production for environmental uses
is attributed to a decrease in lime consumption for drinking water treatment, sludge treatment, and utility
powerplant market for flue gas desulfurization (USGS 2008). Lime production also decreased for metallurgical
consumption, owing to a shift in steel production from basic oxygen furnaces (EOF) to electric arc furnaces (EAF).
EAFs use iron and steel scrap as their primary iron source which contains fewer impurities and requires less than
one-half of the lime per ton of steel produced than pig iron used by BOFs (USGS 2008).
91 PCC is obtained from the reaction of CO2 with calcium hydroxide. It is used as a filler and/or coating in the paper, food, and
plastic industries.
Industrial Processes 4-7
-------
Methodology
During the calcination stage of lime production, CO2 is given off as a gas and normally exits the system with the
stack gas. To calculate emissions, the amounts of high-calcium and dolomitic lime produced were multiplied by
their respective emission factors. The emission factor is the product of a constant reflecting the mass of CO2
released per unit of lime and the average calcium plus magnesium oxide (CaO + MgO) content for lime (95 percent
for both types of lime) (IPCC 2006). The emission factors were calculated as follows:
For high-calcium lime:
[(44.01 g/mole CO2) - (56.08 g/mole CaO)] x (0.95 CaO/lime) = 0.75 g CO2/g lime
For dolomitic lime:
[(88.02 g/mole CO2) - (96.39 g/mole CaO)] x (0.95 CaO/lime) = 0.87 g CO2/g lime
Production was adjusted to remove the mass of chemically combined water found in hydrated lime, determined
according to the molecular weight ratios of H2O to (Ca(OH)2 and [Ca(OH)2«Mg(OH)2]) (IPCC 2000). These factors
set the chemically combined water content to 24.3 percent for high-calcium hydrated lime, and 27.3 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 by-product during the production of lime (IPCC 2006).
Lime emission estimates were further adjusted to account for PCC producers and sugar refineries that recover CO2
emitted by lime production facilities and use the captured CO2 as an input into production or refining processes. For
CO2 recovery by sugar refineries, lime consumption estimates from USGS were multiplied by a CO2 recovery factor
to determine the total amount of CO2 recovered from lime production facilities. According to industry surveys,
sugar refineries use captured CO2 for 100 percent of their CO2 input (Lutter 2008). CO2 recovery by PCC producers
was determined by multiplying estimates for the percentage CO2 of production weight for PCC production at lime
plants, by a CO2 recovery factor of 93 percent for 2007 (Prillaman 2008). As data were only available for 2007,
CO2 recovery for the period 1990 through 2006 were extrapolated by determining a ratio of PCC production at lime
facilities to lime consumption for PCC (USGS 2002 through 2007, 2008).
Lime production data (high-calcium- and dolomitic-quicklime, high-calcium- and dolomitic-hydrated, and dead-
burned dolomite) for 1990 through 2007 (see Table 4-8) 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 2008). Total lime production was adjusted to account for the water content of hydrated lime by converting
hydrate to oxide equivalent, based on recommendations from the IPCC Good Practice Guidance and is presented in
Table 4 9 (USGS 1992 through 2007, IPCC 2000). The CaO and CaO'MgO contents of lime were obtained from
the IPCC (IPCC 2006). Since data for the individual lime types (high calcium and dolomitic) was not provided prior
to 1997, total lime production for 1990 through 1996 was calculated according to the three year distribution from
1997 to 1999. Lime consumed by PCC producers and sugar refineries were obtained from USGS (1992 through
2007).
Table 4-8: High-Calcium- and Dolomitic-Quicklime, High-Calcium- and Dolomitic-Hydrated, and Dead-Burned-
Dolomite Lime Production (Gg)
Year
1990
1995
2000
High-Calcium
Quicklime
11,166
13,165
14,300
Dolomitic
Quicklime
2,234
2,635
3,000
High-Calcium
Hydrated
1,781
2,027
1,550
Dolomitic
Hydrated
319
363
421
Dead-Burned
Dolomite
342
308
200
2005
2006
2007
14,100
15,000
14,700
2,990
2,950
2,700
2,220
2,370
2,240
474
409
352
200
200
200
Table 4-9: Adjusted Lime Production3 (Gg)
4-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Year High-Calcium Dolomitic
1990 12,514 2,809
1995 14,700 3,207
2000 15,473 3,506
2005 15,781 3,535
2006 16,794 3,448
2007 16,396 3,156
a Minus water content of hydrated lime
Uncertainty
The uncertainties contained in these estimates can be attributed to slight differences in the chemical composition of
these products and recovery rates for sugar refineries and PCC manufacturers located at lime plants. Although the
methodology accounts for various formulations of lime, it does not account for the trace impurities found in lime,
such as iron oxide, alumina, and silica. Due to differences in the limestone used as a raw material, a rigid
specification of lime material is impossible. As a result, few plants produce lime with exactly the same properties.
In addition, a portion of the CO2 emitted during lime production will actually be reabsorbed when the lime is
consumed. As noted above, lime has many different chemical, industrial, environmental, and construction
applications. In many processes, CO2 reacts with the lime to create calcium carbonate (e.g., water softening). CO2
reabsorption rates vary, however, depending on the application. For example, 100 percent of the lime used to
produce precipitated calcium carbonate reacts with CO2; whereas most of the lime used in steel making reacts with
impurities such as silica, sulfur, and aluminum compounds. A detailed accounting of lime use in the United States
and further research into the associated processes are required to quantify the amount of CO2 that is reabsorbed. 92
In some cases, lime is generated from calcium carbonate by-products at pulp mills and water treatment plants. 93
The lime generated by these processes is not included in the USGS data for commercial lime consumption. In the
pulping industry, mostly using the Kraft (sulfate) pulping process, lime is consumed in order to causticize a process
liquor (green liquor) composed of sodium carbonate and sodium sulfide. The green liquor results from the dilution
of the smelt created by combustion of the black liquor where biogenic C is present from the wood. Kraft mills
recover the calcium carbonate "mud" after the causticizing operation and calcine it back into lime—thereby
generating CO2—for reuse in the pulping process. Although this re-generation of lime could be considered a lime
manufacturing process, the CO2 emitted during this process is mostly biogenic in origin, and therefore is not
included in Inventory totals (Miner and Upton 2002).
In the case of water treatment plants, lime is used in the softening process. Some large water treatment plants may
recover their waste calcium carbonate and calcine it into quicklime for reuse in the softening process. Further
research is necessary to determine the degree to which lime recycling is practiced by water treatment plants in the
United States.
Uncertainties also remain surrounding recovery rates used for sugar refining and PCC production. The recovery rate
for sugar refineries is based on two sugar beet processing and refining facilities located in California that use 100
percent recovered CO2 from lime plants (Lutter 2008). This analysis assumes that all sugar refineries located on-site
at lime plants also use 100 percent recovered CO2. The recovery rate for PCC producers located on-site at lime
plants is based on the 2007 value for PCC manufactured at commercial lime plants, given by the National Lime
92 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).
93 Some carbide producers may also regenerate lime from their calcium hydroxide by-products, which does not result in
emissions of CO2. In making calcium carbide, quicklime is mixed with coke and heated in electric furnaces. Lhe 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]. Lhus, the calcium hydroxide is heated in the kiln to simply expel the water [Ca(OH)2 + heat —> CaO + H2O]
and no CO2 is released.
Industrial Processes 4-9
-------
Association (Prillaman 2008).
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-10. Lime CO2 emissions were
estimated to be between 13.5 and 15.9 Tg CO2 Eq. at the 95 percent confidence level. This indicates a range of
approximately 8 percent below and 9 percent above the emission estimate of 14.6 Tg CO2 Eq.
Table 4-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lime Production (Tg CO2 Eq. and
Percent)
Source
Gas
2007 Emission
Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound Upper Bound
Lime Production
CO?
14.6
13.5
15.9
-8%
+9%
1 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 years 1990 through 2006 to include estimates of
CO2 recovery from PCC production and sugar refining. On average, these revisions resulted in an annual decrease
in emissions of approximately 13 percent.
Planned Improvements
Future improvements to the lime source category involve continued research into CO2 recovery associated with lime
use during sugar refining and precipitate calcium carbonate (PCC) production. Currently, two sugar refining
facilities in California have been identified to capture CO2 produced in lime kilns located on the same site as the
sugar refinery (Lutter, 2008). Currently, data on CO2 production by these lime facilities is unavailable. Future work
will include research to determine the number of sugar refineries that employ the carbonation technique, the
percentage of these that use captured CO2 from lime production facilities, and the amount of CO2 recovered per unit
of lime production. Future research will also aim to improve estimates of CO2 recovered as part of the PCC
production process using estimates of PCC production and CO2 inputs rather than lime consumption by PCC
facilities.
4.3. Limestone and Dolomite Use (IPCC Source Category 2A3)
Limestone (CaCO3) and dolomite (CaCO3MgCO3)94 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 by-product. 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 2007, approximately 13,075 thousand metric tons of limestone and 1,827 thousand metric tons of dolomite were
consumed during production for these applications. Overall, usage of limestone and dolomite resulted in aggregate
CO2 emissions of 6.2 Tg CO2 Eq. (6,182 Gg) (see Table 4-11 and Table 4-12). Emissions in 2007 decreased 23
percent from the previous year and have increased 21 percent overall from 1990 through 2007.
Table 4-11: CO2
Activity
Flux Stone
Glass Making
Emissions from Limestone & Dolomite Use (Tg CO2 Eq.)
1990
2.611H!
0.21111!
1995i
II 3.2!
I 0.5 1
! 2000
1 2.1!
1 0.4
2005 2006
2.7 4.5
0.4 0.7
2007
2.0
0.3
94 Limestone and dolomite are collectively referred to as limestone by the industry, and intermediate varieties are seldom
distinguished.
4-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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FGD 1.4 1.7 1.8 3.0 2.1 3.2
Magnesium Production 0.1 0.0 0.1 0.0 0.0 0.0
Other Miscellaneous Uses 0.8 L2 0.7 0.7 0.7 0.7
Total 5.1 6/7 5.1 6.8 8.0 6.2
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.
Table 4-12: CO2 Emissions from Limestone & Dolomite Use (Gg)
Activity
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
FGD
Magnesium Production
Other Miscellaneous Uses
Total
1990
2,593
2,304
289
217
189
28
1,433
64
819
5,127
1995
3,198
2,027
1,171
525
421
103
1,719
41
1,168
6,651
2000
2,104
1,374
730
371
371
0
1,787
73
722
5,056
2005
2,650
1,096
1,554
425
405
20
2,975
0
718
6,768
2006
4,492
1,917
2,575
747
717
31
2,061
0
735
8,035
2007
1,959
1,270
689
333
333
0
3,179
0
711
6,182
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. Flux stone used during the production of iron and steel
was deducted from the Limestone and Dolomite Use estimate and attributed to the Iron and Steel Production
estimate.
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 2007 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 4-13) were obtained from the USGS Minerals Yearbook: Crushed Stone Annual Report (USGS
1993, 1995a through 2007a, 2008a). The production capacity data for 1990 through 2007 of dolomitic magnesium
metal (see Table 4-14) also came from the USGS (1995b through 2007b, 2008b). 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 2007 Minerals Yearbook: Magnesium; therefore, it is assumed that this process continues to be non-
existent in the United States (USGS 2008b). 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
Industrial Processes 4-11
-------
withheld data were estimated using one of the following techniques: (1) the value for all the withheld data points for
limestone or dolomite use was distributed evenly to all withheld end-uses; (2) the average percent of total limestone
or dolomite for the withheld end-use in the preceding and succeeding years; or (3) the average fraction of total
limestone or dolomite for the end-use over the entire time period.
There is a large quantity of crushed stone reported to the USGS under the category "unspecified uses." A portion of
this consumption is believed to be limestone or dolomite used for emissive end uses. The quantity listed for
"unspecified uses" was, therefore, allocated to each reported end-use according to each end uses fraction of total
consumption in that year.95
Table 4-13: Limestone and Dolomite Consumption (Thousand Metric Tons)
Activity
Flux Stone
Limestone
Dolomite
Glass Making
Limestone
Dolomite
FGD
Other Miscellaneous
Uses
Total
1990
6,737
5,804
933
489
430
59
3,258
1,835
12,319
1995
8,586
5,734
2,852
1,174
958
216
3,908
2,654
16,321
2000
6,283
4,151
2,132
843
843
0
4,061
1,640
12,826
2005
7,022
3,165
3,857
962
920
43
6,761
1,632
16,377
2006
11,030
5,208
5,822
1,693
1,629
64
4,683
1,671
19,078
2007
5,305
3,477
1,827
757
757
0
7,225
1,616
14,903
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 4-14: Dolomitic Magnesium Metal Production Capacity (Metric Tons)
Year Production Capacity
1990 35,000
1995 22,222
2000 40,000
2005 0
2006 0
2007 0
Note: Production capacity for 2002, 2003, 2004, 2005, 2006, and 2007 amounts to zero because the last U.S. production plant
employing the dolomitic process shut down mid-2001 (USGS 2002b through 2008b).
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
95This approach was recommended by USGS.
4-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
end uses of limestone and dolomite. The uncertainty of the estimates for limestone used in glass making is
especially high; however, since glass making accounts for a small percent of consumption, its contribution to the
overall emissions estimate is low. Lastly, much of the limestone consumed in the United States is reported as "other
unspecified uses;" therefore, it is difficult to accurately allocate this unspecified quantity to the correct end-uses.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-15. Limestone and Dolomite
Use CO2 emissions were estimated to be between 5.4 and 7.2 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of approximately 12 percent below and 16 percent above the emission estimate of 6.2 Tg CO2 Eq.
Table 4-15: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Limestone and Dolomite Use (Tg
CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Lower
Bound
Upper
Bound
Lower
Bound
Upper
Bound
Limestone and Dolomite CO2 6.2 5.4 7.2 -12% +16%
Use
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Estimates of CO2 emissions from Limestone and Dolomite Use have been revised for the entire time series to
accommodate minor revisions to the "unspecified uses" of limestone and dolomite identified by the USGS. On
average, these revisions resulted in an annual decrease in emissions of 0.1 percent. Additionally, limestone and
dolomite consumption data were updated to attribute emissions from limestone and dolomite used for iron and steel
production to the Iron and Steel Production estimate. On average, this resulted in an additional decrease in emissions
of 10 percent.
Planned Improvements
Future improvements to the limestone and dolomite source category involve research into the availability of
limestone and dolomite end-use data. If sufficient data are available, limestone and dolomite used as process
materials in source categories included in future inventories (e.g., glass production, other process use of carbonates)
may be removed from this section and will be reported under the appropriate source categories.
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 D 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.96 During
96 In California, soda ash is manufactured using sodium carbonate-bearing brines instead of trona ore. To extract the sodium
carbonate, the complex brines are first treated with CO2 in carbonation towers to convert the sodium carbonate into sodium
bicarbonate, which then precipitates from the brine solution. The precipitated sodium bicarbonate is then calcined back into
sodium carbonate. Although CO2 is generated as a by-product, the CO2 is recovered and recycled for use in the carbonation stage
and is not emitted. A third state, Colorado, produced soda ash until the plant was idled in 2004. The lone producer of sodium
bicarbonate no longer mines trona in the state. For a brief time, NaHCO3 was produced using soda ash feedstocks mined in
Wyoming and shipped to Colorado. Because the trona is mined in Wyoming, the production numbers given by the USGS
Industrial Processes 4-13
-------
the production process used in Wyoming, trona ore is treated to produce soda ash. CO2 is generated as a by-product
of this reaction, and is eventually emitted into the atmosphere. In addition, CO2 may also be released when soda ash
is consumed.
In 2007, CO2 emissions from the production of soda ash from trona were approximately 1.7 Tg CO2 Eq. (1,675 Gg).
Soda ash consumption in the United States generated 2.5 Tg CO2 Eq. (2,465 Gg) in 2007. Total emissions from
soda ash production and consumption in 2007 were 4.1 Tg CO2 Eq. (4,140 Gg) (see Table 4-16 and Table 4-17).
Emissions have fluctuated since 1990. These fluctuations were strongly related to the behavior of the export market
and the U.S. economy. Emissions in 2007 decreased by approximately 0.5 percent from the previous year, and have
decreased overall by less than 0.5 percent since 1990.
Table 4-16: CO2 Emissions from Soda Ash Production and Consumption (Tg CO2 Eq.)
Year Production Consumption Total
1990 1.4 2.7 4.1
1995 1.6 2.7 4.3
2000 1.5 2.7 4.2
2005 1.7 2.6 4.2
2006 1.6 2.5 4.2
2007 1.7 2.5 4.1
Note: Totals may not sum due to independent rounding.
Table 4-17: CO2 Emissions from Soda Ash Production and Consumption (Gg)
Year Production Consumption Total
1990 1,431 2,710 4,141
1995 1,607 2,698 4,304
2000 1,529 2,652 4,181
2005 1,655 2,573 4,228
2006 1,626 2,536 4,162
2007 1,675 2,465 4,140
Note: Totals may not sum due to independent rounding.
The United States represents about one-fourth of total world soda ash output. The approximate distribution of soda
ash by end-use in 2007 was glass making, 49 percent; chemical production, 30 percent; soap and detergent
manufacturing, 8 percent; distributors, 5 percent; flue gas desulfurization, 2 percent; water treatment, 2 percent; pulp
and paper production, 2 percent; and miscellaneous, 3 percent (USGS 2008).
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 by-products of the calcination process. CO2
emissions from the calcination of trona can be estimated based on the following chemical reaction:
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.
4-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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2(Na3(C03)(HC03)'2H20) -> 3Na2CO3 + 5H2O + CO2
[trona] [soda ash]
Based on this formula, approximately 10.27 metric tons of trona are required to generate one metric ton of CO2, or
an emission factor of 0.097 metric tons CO2 per metric ton trona (IPCC 2006). Thus, the 17.2 million metric tons of
trona mined in 2007 for soda ash production (USGS 2008) resulted in CO2 emissions of approximately 1.7 Tg CO2
Eq. (1,675 Gg).
Once produced, most soda ash is consumed in glass and chemical production, with minor amounts in soap and
detergents, pulp and paper, flue gas desulfurization and water treatment. As soda ash is consumed for these
purposes, additional CO2 is usually emitted. In these applications, it is assumed that one mole of C is released for
every mole of soda ash used. Thus, approximately 0.113 metric tons of C (or 0.415 metric tons of CO2) are released
for every metric ton of soda ash consumed.
The activity data for trona production and soda ash consumption (see Table 4-18) were taken from USGS (1994
through 2008). Soda ash production and consumption data were collected by the USGS from voluntary surveys of
the U.S. soda ash industry.
Table 4-18 : Soda Ash Production and Consumption (Gg)
Year Production' Consumption
1990 14,700 6,530
1995 16,500 6,500
2000 15,700 6,390
2005 17,000 6,200
2006 16,700 6,110
2007 17,200 5,940
* Soda ash produced from trona ore only.
Uncertainty
Emission estimates from soda ash production have relatively low associated uncertainty levels in that reliable and
accurate data sources are available for the emission factor and activity data. The primary source of uncertainty,
however, results from the fact that emissions from soda ash consumption are dependent upon the type of processing
employed by each end-use. Specific information characterizing the emissions from each end-use is limited.
Therefore, there is uncertainty surrounding the emission factors from the consumption of soda ash.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-19. Soda Ash Production and
Consumption CO2 emissions were estimated to be between 3.8 and 4.4 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 4.1 Tg
C02Eq.
Table 4-19 : Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Soda Ash Production and
Consumption (Tg CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Lower
Bound
Upper
Bound
Lower
Bound
Upper
Bound
Soda Ash Production
and Consumption CO2 4.1 3.8 4.4 -7% +7%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-15
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Planned Improvements
Future inventories are anticipated to estimate emissions from glass production and other use of carbonates. These
inventories will extract soda ash consumed for glass production and other use of carbonates from the current soda
ash consumption emission estimates and include them under those sources.
4.5. Ammonia Production (IPCC Source Category 2B1) and Urea Consumption
Emissions of CO2 occur during the production of synthetic ammonia, primarily through the use of natural gas as a
feedstock. The natural gas-based, naphtha-based, and petroleum coke-based processes produce CO2 and hydrogen
(H2), the latter of which is used in the production of ammonia. One N production plant located in Kansas is
producing ammonia from petroleum coke feedstock. In some plants the CO2 produced is captured and used to
produce urea. The brine electrolysis process for production of ammonia does not lead to process-based CO2
emissions.
There are five principal process steps in synthetic ammonia production from natural gas feedstock. The primary
reforming step converts CH4 to CO2, carbon monoxide (CO), and H2 in the presence of a catalyst. Only 30 to 40
percent of the CH4 feedstock to the primary reformer is converted to CO and CO2. The secondary reforming step
converts the remaining CH4 feedstock to CO and CO2. The CO in the process gas from the secondary reforming
step (representing approximately 15 percent of the process gas) is converted to CO2 in the presence of a catalyst,
water, and air in the shift 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 CH4, including primary and secondary reforming and
the shift conversion processes, is approximately as follows:
(catalyst)
0.88 CH4 + 1.26 Air + 1.24 H2O > 0.88 CO2 + N2 + 3 H2
N2 + 3 H2 -» 2 NH3
To produce synthetic ammonia from petroleum coke, the petroleum coke is gasified and converted to CO2 and H2.
These gases are separated, and the H2 is used as a feedstock to the ammonia production process, where it is reacted
with N2 to form ammonia.
Not all of the CO2 produced in the production of ammonia is emitted directly to the atmosphere. Both ammonia and
CO2 are used as raw materials in the production of urea [CO(NH2)2], which is another type of nitrogenous fertilizer
that contains C as well as N. The chemical reaction that produces urea is:
2 NH3 + CO2 -» NH2COONH4 -» CO(NH2)2 + H2O
Urea is consumed for a variety of uses, including as a nitrogenous fertilizer, in urea-formaldehyde resins, and as a
deicing agent (TIG 2002). The C in the consumed urea is assumed to be released into the environment as CO2
during use. Therefore, the CO2 produced by ammonia production that is subsequently used in the production of urea
is still emitted during urea consumption. The majority of CO2 emissions associated with urea consumption are those
that result from its use as a fertilizer. These emissions are accounted for in the Cropland Remaining Cropland
section of the Land Use, Land-Use Change, and Forestry chapter. CO2 emissions associated with other uses of urea
are accounted for in this chapter. Net emissions of CO2 from ammonia production in 2007 were 13.8 Tg CO2 Eq.
(13,786 Gg), and are summarized in Table 4-20 and Table 4-21. Emissions of CO2 from urea consumed for non-
fertilizer purposes in 2007 totaled 4.7 Tg CO2 Eq. (4,750 Gg), and are summarized in Table 4-20 and Table 4-21.
The decrease in ammonia production in recent years is due to several factors, including market fluctuations and high
natural gas prices. Ammonia production relies on natural gas as both a feedstock and a fuel, and as such, domestic
producers are competing with imports from countries with lower gas prices. If natural gas prices remain high, it is
likely that domestically produced ammonia will continue to decrease with increasing ammonia imports (EEA 2004).
Table 4-20: CO2 Emissions from Ammonia Production and Urea Consumption (Tg CO2 Eq.)
Source 1990 1995 2000 2005 2006 2007
Ammonia Production 13.0 13.5 12.2 9.2 8.8 9.0
4-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Urea Consumption3 3.8 4.3 4.2 3.7 3.5 4.7
Total 16.8 17.8 16.4 12.8 12.3 13.8
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 4-21: CO2 Emissions from Ammonia Production and Urea Consumption (Gg)
Source
Ammonia Production
Urea Consumption3
Total
1990
13,047
3,784
16,831
1995
13,541
4,255
17,796
2000
12,172
4,231
16,402
2005
9,196
3,653
12,849
2006
8,781
3,519
12,300
2007
9,036
4,750
13,786
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.
Methodology
The calculation methodology for non-combustion CO2 emissions from production of nitrogenous fertilizers from
natural gas feedstock is based on a CO2 emission factor published by the European Fertilizer Manufacturers
Association (EFMA). The selected EFMA factor is based on ammonia production technologies that are similar to
those employed in the U.S. The CO2 emission factor (1.2 metric tons CO2/metric ton NH3) is applied to the percent
of total annual domestic ammonia production from natural gas feedstock. Emissions from fuels consumed for
energy purposes during the production of ammonia are accounted for in the Energy chapter. Emissions of CO2 from
ammonia production are then adjusted to account for the use of some of the CO2 produced from ammonia
production as a raw material in the production of urea. For each ton of urea produced, 8.8 of every 12 tons of CO2
are consumed and 6.8 of every 12 tons of ammonia are consumed. The CO2 emissions reported for ammonia
production are therefore reduced by a factor of 0.73 multiplied by total annual domestic urea production. Total CO2
emissions resulting from nitrogenous fertilizer production do not change as a result of this calculation, but some of
the CO2 emissions are attributed to ammonia production and some of the CO2 emissions are attributed to urea
consumption. Those CO2 emissions that result from the use of urea as a fertilizer are accounted for in the Land Use,
Land-Use Change, and Forestry chapter.
The total amount of urea consumed for non-agricultural purposes is estimated by deducting the quantity of urea
fertilizer applied to agricultural lands, which is obtained directly from the Land Use, Land-Use Change, and Forestry
Chapter and is reported in Table 4-22, from the total U.S. production Total urea production is estimated based on
the amount of urea produced plus the sum of net urea imports and exports CO2 emissions associated with urea that
is used for non-fertilizer purposes are estimated using a factor of 0.73 tons of CO2 per ton of urea consumed..
All ammonia production and subsequent urea production are assumed to be from the same process—conventional
catalytic reforming of natural gas feedstock, with the exception of ammonia production from petroleum coke
feedstock at one plant located in Kansas. The CO2 emission factor for production of ammonia from petroleum coke
is based on plant specific data, wherein all C contained in the petroleum coke feedstock that is not used for urea
production is assumed to be emitted to the atmosphere as CO2 (Bark 2004). Ammonia and urea are assumed to be
manufactured in the same manufacturing complex, as both the raw materials needed for urea production are
produced by the ammonia production process. The CO2 emission factor (3.57 metric tons CO2/metric ton NH3) is
applied to the percent of total annual domestic ammonia production from petroleum coke feedstock.
The emission factor of 1.2 metric ton CO2/metric ton NH3 for production of ammonia from natural gas feedstock
was taken from the EFMA Best Available Techniques publication, Production of Ammonia (EFMA 1995). The
EFMA reported an emission factor range of 1.15 to 1.30 metric ton CO2/metric tonNH3, with 1.2 metric ton
CO2/metric ton NH3 as a typical value. Technologies (e.g., catalytic reforming process) associated with this factor
are found to closely resemble those employed in the U.S. for use of natural gas as a feedstock. The EFMA reference
also indicates that more than 99 percent of the CH4 feedstock to the catalytic reforming process is ultimately
converted to CO2. The emission factor of 3.57 metric ton CO2/metric ton NH3 for production of ammonia from
petroleum coke feedstock was developed from plant-specific ammonia production data and petroleum coke
feedstock utilization data for the ammonia plant located in Kansas (Bark 2004). As noted earlier, emissions from
fuels consumed for energy purposes during the production of ammonia are accounted for in the Energy chapter.
Industrial Processes 4-17
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Ammonia production data (see Table 4-22) was obtained from Coffeyville Resources (Coffeyville 2005, 2006,
2007a, 2007b) 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 Coffeyville Resources (Coffeyville
2005, 2006, 2007a). Urea production data for 1990 through 2007 were obtained from the Minerals Yearbook:
Nitrogen (USGS 1994 through 2007). Import data for urea were obtained from the U.S. Census Bureau Current
Industrial Reports Fertilizer Materials and Related Products annual and quarterly reports for 1997 through 2007
(U.S. Census Bureau 1998 through 2007), The Fertilizer Institute (TFI2002) for 1993 through 1996, and the United
States International Trade Commission Interactive Tariff and Trade DataWeb (U.S. ITC 2002) for 1990 through
1992 (see Table 4-22). Urea export data for 1990 through 2007 were taken from U.S. Fertilizer Import/Exports from
USDA Economic Research Service Data Sets (U.S. Department of Agriculture 2008).
Table 4-22: Ammonia Production, Urea Production, Urea Net Imports, and Urea Exports (Gg)
Year Ammonia Production Urea Production Urea Applied Urea Imports Urea Exports
as Fertilizer
1990
1995
2000
2005
2006
2007
15,425
15,788
14,342
10,143
9,962
10,386
7,450
7,370
6,910
5,270
5,410
5,630
3,296
3,623
4,382
4,779
4,985
5,389
1,860
2,936
3,904
5,026
5,029
6,546
854
881
663
536
656
310
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. Uncertainties are also associated with natural gas
feedstock consumption data for the U.S. ammonia industry as a whole, the assumption that all ammonia production
and subsequent urea production was from the same process—conventional catalytic reforming of natural gas
feedstock, with the exception of one ammonia production plant located in Kansas that is manufacturing ammonia
from petroleum coke feedstock. It is also assumed that ammonia and urea are produced at collocated plants from the
same natural gas raw material.
Such recovery may or may not affect the overall estimate of CO2 emissions depending upon the end use to which the
recovered CO2 is applied. Further research is required to determine whether byproduct CO2 is being recovered from
other ammonia production plants for application to end uses that are not accounted for elsewhere.
Additional uncertainty is associated with the estimate of urea consumed for non-fertilizer purposes. Emissions
associated with this consumption are reported in this source category, while those associated with consumption as
fertilizer are reported in Cropland Remaining Cropland section of the Land Use, Land-Use Change, and Forestry
chapter. The amount of urea used for non-fertilizer purposes is estimated based on estimates of urea production, net
urea imports, and the amount of urea used as fertilizer. There is uncertainty associated with the accuracy of these
estimates as well as the fact that each estimate is obtained from a different data source.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-23. Ammonia Production and
Urea Consumption CO2 emissions were estimated to be between 12.1 and 15.2 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of approximately 12 percent below and 11 percent above the emission
estimate of 13.8 Tg CO2 Eq.
Table 4-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ammonia Production and Urea
Consumption (Tg CO2 Eq. and Percent)
Source
2007 Emission
Gas Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper
Lower Upper
4-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Bound Bound Bound Bound
Ammonia Production
and Urea Consumption CO2 13.8 12.1 15.2 -12% 11%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Urea export data were revised for 1990 through 2006 using the U.S. Department of Agriculture's Economic
Research Service Data Set for U.S. Fertilizer Exports. These data were used because the previous data source
discontinued publication of urea export data. On average, revisions to the exported urea dataset resulted in a
decrease in annual emission estimates of less than one percent. Urea production data were revised for 1990 through
2006.. These data were used in place of estimating urea production based on quantity of urea applied to agricultural
lands and an estimated percent of urea consumed for agricultural purposes. On average, the new data resulted in a
decrease in annual emission estimates of less than half of one percent.
Planned Improvements
Planned improvements to the Ammonia Production and Urea Consumption source category include updating
emission factors to include both fuel and feedstock CO2 emissions and incorporating CO2 capture and storage.
Methodologies will also be updated if additional ammonia-production plants are found to use hydrocarbons other
than natural gas for ammonia production. Additional efforts will be made to find consistent data sources for urea
consumption and to report emissions from this consumption appropriately as defined by the 2006IPCC Guidelines
for National Greenhouse Gas Inventories (IPCC 2006).
4.6. Nitric Acid Production (IPCC Source Category 2B2)
Nitric acid (HNO3) is an inorganic compound used primarily to make synthetic commercial fertilizers. It is also a
major component in the production of adipic acid—a feedstock for nylon—and explosives. Virtually all of the nitric
acid produced in the United States is manufactured by the catalytic oxidation of ammonia (EPA 1997). During this
reaction, N2O is formed as a by-product and is released from reactor vents into the atmosphere.
Currently, the nitric acid industry controls for emissions of NO and NO2 (i.e., NOX). As such, the industry 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. Less than 5 percent of nitric acid plants use
NSCR and they represent 0.6 percent of estimated national production (EPA 2008). The remaining 95 percent of the
facilities use SCR or extended absorption, neither of which is known to reduce N2O emissions.
N2O emissions from this source were estimated to be 21.7 Tg CO2 Eq. (70 Gg) in 2007 (see Table 4-24). Emissions
from nitric acid production have increased by 8.5 percent since 1990, with the trend in the time series closely
tracking the changes in production. Emissions increased 19 percent between 2006 and 2007, which resulted from an
increase in nitric acid production driven by increased synthetic fertilizer demand by farmers taking advantage of
high grain prices by expanding crop planting (ICIS 2008). Emissions have decreased by 8.8 percent since 1997, the
highest year of production in the time series.
Table 4-24: N2O Emissions from Nitric Acid Production (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 20.0 64
1995 22.3 72
2000 21.9 71
2005 18.6 60
2006 18.2 59
2007 21.7 70
Industrial Processes 4-19
-------
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 produced
at plants using non-selective catalytic reduction (NSCR) systems and 9 kg N2O/metric ton HNO3 produced at plants
not equipped with NSCR (IPCC 2006). In the process of destroying NOX, NSCR systems destroy 80 to 90 percent
of the N2O, which is accounted for in the emission factor of 2 kg N2O/metric ton HNO3. Less than 5 percent of
HNO3 plants in the United States are equipped with NSCR representing 0.6 percent of estimated national production
(EPA 2008). Hence, the emission factor is equal to (9 x 0.994) + (2 x 0.006) = 9.0 kg N2O per metric ton HNO3.
Nitric acid production data for 1990 through 2002 were obtained from the U.S. Census Bureau, Current Industrial
Reports (2006), and for 2003 through 2007 from the U.S. Census Bureau, Current Industrial Reports (2008) (see
Table 4-25).
Table 4-25: Nitric Acid Production (Gg)
Year Gg
1990 7,195
1995 8,019
2000 7,900
2005 6,711
2006 6,573
2007 7,823
Uncertainty
The overall uncertainty associated with the 2007 N2O emissions estimate from nitric acid production was calculated
using the IPCC Guidelines for National Greenhouse Gas Inventories (2006) Tier 2 methodology. Uncertainty
associated with the parameters used to estimate N2O emissions included that of production data, the share of U.S.
nitric acid production attributable to each emission abatement technology, and the emission factors applied to each
abatement technology type.
The results of this Tier 2 quantitative uncertainty analysis are summarized in Table 4-26. N2O emissions from nitric
acid production were estimated to be between 12.7 and 31.3 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of approximately 42 percent below to 44 percent above the 2007 emissions estimate of 21.7 Tg
CO2 Eq.
Table 4-26: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Nitric Acid Production (Tg CO2 Eq.
and Percent)
2007 Emission
Source Gas Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate"
(TgC02Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Nitric Acid Production N2O 21.7
12.7 31.3 -42% +44%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Changes to the weighted N2O emission factor resulted in an increase in emissions across the time series. The
weighted N2O emission factor was previously based on the percentage of facilities equipped and not equipped with
NSCR systems. The emission factor used for the current estimate is based on the percentage of HNO3 produced at
plants with NCSR systems and HNO3 produced at plants without NSCR systems. Additionally, the nitric acid
4-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
production value for 2006 has also been updated relative to the previous Inventory based on revised production data
published by the U.S. Census Bureau (2008). Revised production data reduced emissions for 2006 by 0.2 Tg CO2
Eq. (1.0 percent). Overall, these changes resulted in an average annual increase in N2O emissions of 3.1 Tg CO2 Eq.
(17.8 percent) for the period 1990 through 2006 relative to the previous inventory.
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 and Trogler 1991).
Adipic acid is produced through a two-stage process during which N2O is generated in the second stage. The first
stage of manufacturing usually involves the oxidation of cyclohexane to form a cyclohexanone/cyclohexanol
mixture. The second stage involves oxidizing this mixture with nitric acid to produce adipic acid. N2O is generated
as a by-product of the nitric acid oxidation stage and is emitted in the waste gas stream (Thiemens and Trogler
1991). Process emissions from the production of adipic acid vary with the types of technologies and level of
emission controls employed by a facility. In 1990, two of the three major adipic acid-producing plants had N2O
abatement technologies in place and, as of 1998, the three major adipic acid production facilities had control systems
in place (Reimer et al. 1999). 97 Only one small plant, representing approximately two percent of production, does
not control for N2O (ICIS 2007; VA DEQ 2006).
N2O emissions from adipic acid production were estimated to be 5.9 Tg CO2 Eq. (19 Gg) in 2007 (see Table 4-27).
National adipic acid production has increased by approximately 26 percent over the period of 1990 through 2007, to
approximately one million metric tons. Over the same period, emissions have been reduced by 61 percent due to the
widespread installation of pollution control measures in the late 1990s.
Table 4-27: N2O Emissions from Adipic Acid Production (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 15.3 49
1995 17.3 56
2000 6.2 20
2005 5.9 19
2006 5.9 19
2007 5.9 19
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 2007 were unavailable. Emission estimates for 2003 and 2004 were calculated by
applying 4.4 and 4.2 percent national production growth rates, respectively. Emission estimates for 2005 to 2007
were kept the same as 2004. National production for 2003 was calculated through linear interpolation between 2002
and 2004 reported national production data. 2005 national production was calculated through linear interpolation
97During 1997, the N2O emission controls installed by the third plant operated for approximately a quarter of the year.
Industrial Processes 4-21
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between 2004 and 2006 reported national production. 2007 national production was kept the same as 2006. 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 [metric tons {MT} of adipic acid]) x (0.3 MT N2O / MT adipic acid) x
(1 - [N2O destruction factor x abatement system utility factor])
The "N2O destruction factor" represents the percentage of N2O emissions that are destroyed by the installed
abatement technology. The "abatement system utility factor" represents the percentage of time that the abatement
equipment operates during the annual production period. Overall, in the United States, two of the plants employ
catalytic destruction, 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. For 2007, production was kept the
same as 2006, as described above.
National adipic acid production data (see Table 4-28) for 1990 through 2002 were obtained from the American
Chemistry Council (ACC 2003). Production for 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 2007, 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).
Table 4-28: Adipic Acid Production (Gg)
Year Gg
1990 735
1995 830
2000 925
2005 1,002
2006 1,002
2007 1,002
Uncertainty
The overall uncertainty associated with the 2007 N2O emission estimate from adipic acid production was calculated
using the IPCC Guidelines for National Greenhouse Gas Inventories (2006) Tier 2 methodology. Uncertainty
4-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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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 4-29. N2O emissions from
adipic acid production were estimated to be between 4.9 and 7.1 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 18 percent below to 20 percent above the 2007 emission estimate of 5.9 Tg
CO2 Eq.
Table 4-29: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Adipic Acid Production (Tg CO2
Eq. and Percent)
2007 Emission
Source Gas Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Adipic Acid Production N2O 5.9
4.9 7.1 -18% 20%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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 production98 of silicon carbide (SiC), a material used as an industrial abrasive.
To make SiC, quartz (SiO2) is reacted with C in the form of petroleum coke. A portion (about 35 percent) of the C
contained in the petroleum coke is retained in the SiC. The remaining C is emitted as CO2, CH4, or CO.
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 2007 were 0.2 Tg CO2 Eq. (196 Gg). Approximately 47 percent of
these emissions resulted from SiC production while the remainder results from SiC consumption. CH4 emissions
from SiC production in 2007 were 0.01 Tg CO2 Eq. CH4 (0.4 Gg) (see Table 4-30 and Table 4-31).
Table 4-30: CO2 and CH4 Emissions from Silicon Carbide Production and Consumption (Tg CO2 Eq.)
Year
C02
CH4
Total
1990
0.4
+
0.4
1995
0.3
+
0.3
2000
0.2
+
0.3
2005
0.2
+
0.2
2006
0.2
+
0.2
2007
0.2
+
0.2
+ Does not exceed 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
Table 4-31: CO2 and CH4 Emissions from Silicon Carbide Production and Consumption (Gg)
Year 1990 1995 2000 2005 2006 2007
CO2 375 329 248 219 207 196
CH4 1 1 1 + + +_
+ Does not exceed 0.5 Gg.
98 Silicon carbide is produced for both abrasive and metallurgical applications in the United States. Production for metallurgical
applications is not available and therefore both CELt and CO2 estimates are based solely upon production estimates of silicon
carbide for abrasive applications.
Industrial Processes 4-23
-------
Methodology
Emissions of CO2 and CH4 from the production of SiC were calculated by multiplying annual SiC production by the
emission factors (2.62 metric tons CO2/metric ton SiC for CO2 and 11.6 kg CH^metric ton SiC for CH4) provided
by the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006).
Emissions of CO2 from silicon carbide consumption were calculated by multiplying the annual SiC consumption
(production plus net imports) by the percent used in metallurgical and other non-abrasive uses (50 percent) (USGS
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 2007 were obtained from the Minerals Yearbook: Manufactured Abrasives (USGS
1991a through 2005a, 2006). Silicon carbide consumption by major end use was obtained from the Minerals
Yearbook: Silicon (USGS 1991b through 2005b) (see Table 4-32) foryears 1990 through 2004 and from the USGS
Minerals Commodity Specialist for 2005 and 2006 (Corathers 2006, 2007). Silicon carbide consumption by major
end us data for 2007 are proxied using 2006 data due to unavailability of data at time of publication. Net imports for
the entire time series were obtained from the U.S. Census Bureau (2005 through 2008).
Table 4-32: Production and Consumption of Silicon Carbide (Metric Tons)
Year
1990
1995
2000
2005
2006
2007
Production
105,000
75,400
45,000
35,000
35,000
35,000
Consumption
172,465
227,395
225,070
220,149
199,937
179,741
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 CH4, 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 4-33. 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.
Table 4-33: Tier 2 Quantitative Uncertainty Estimates for CH4 and CO2 Emissions from Silicon Carbide Production
and Consumption (Tg CO2 Eq. and Percent)
Source
Silicon Carbide Production
and Consumption
Silicon Carbide Production
Gas
C02
CH4
2007 Emission
Estimate
(Tg C02 Eq.)
0.2
+
Uncertainty Range Relative
(Tg C02 Eq.)
Lower Upper
Bound Bound
0.18 0.22
+ +
to Emission Estimate"
(%)
Lower Upper
Bound Bound
-10% +10%
-9% +10%
4-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
+ Does not exceed 0.05 Tg CO2 Eq. or 0.5 Gg.
Recalculations Discussion
Estimates of CO2 emissions from silicon carbide consumption were revised for all years due to the availability of
more precise import and export data from the United States International Trade Commission. On average, these
revisions resulted in a decrease in annual emissions of less than 1 percent.
Planned Improvements
Future improvements to the carbide production source category include continued research to determine if calcium
carbide production and consumption data are available for the United States. If these data are available, calcium
carbide emission estimates will be included in this source category.
4.9. Petrochemical Production (IPCC Source Category 2B5)
The production of some petrochemicals results in the release of small amounts of CH4 and CO2 emissions.
Petrochemicals are chemicals isolated or derived from petroleum or natural gas. 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 intense 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 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 2007 were 2.6 Tg CO2 Eq. (2,636 Gg) and 1.0 Tg CO2
Eq. (48 Gg), respectively (see Table 4-34 and Table 4-35), totaling 3.7 Tg CO2 Eq. Emissions of CO2 from C black
production remained constant at 2.6 Tg CO2 Eq. (2,573 Gg) in 2006 and 2007. There has been an overall increase in
CO2 emissions from C black production of 18 percent since 1990. CH4 emissions from petrochemical production
increased by approximately 17 percent since 1990.
Table 4-34: CO2 and CH4 Emissions from Petrochemical Production (Tg CO2 Eq.)
Year
CO2
CH4
Total
1990
2.2
0.9
3.1
1995
2.8
1.1
3.8
2000
3.0
1.2
4.2
2005
2.8
1.1
3.9
2006
2.6
1.0
3.6
2007
2.6
1.0
3.7
Table 4-35: CO2 and CH4 Emissions from Petrochemical Production (Gg)
Year 1990 1995 2000 2005 2006 2007
CO2
CH4
2,221
41
2,750
52
3,004
59
2,804
51
2,573
48
2,636
48
Methodology
Emissions of CH4 were calculated by multiplying annual estimates of chemical production by the appropriate
Industrial Processes 4-25
-------
emission factor, as follows: 11 kg CH4/metric ton C black, 1 kg CH4/metric ton ethylene, 0.4 kg CH4/metric ton
ethylene dichloride," and 2 kg CHVmetric ton methanol. Although the production of other chemicals may also
result in CH4 emissions, insufficient data were available to estimate their emissions.
Emission factors were taken from the Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). Annual
production data (see Table 4-36) were obtained from the American Chemistry Council's Guide to the Business of
Chemistry (ACC 2002, 2003, 2005 through 2008) and the International Carbon Black Association (Johnson 2003,
2005 through 2008).
Table 4-36: Production of Selected Petrochemicals (Thousand Metric Tons)
Chemical
Carbon Black
Ethylene
Ethylene Dichloride
Methanol
1990
1,307
16,541
6,282
3,785
1995
1,619
21,214
7,829
4,992
2000
1,769
24,970
9,866
5,221
2005
1,651
23,954
11,260
2,336
2006
1,515
25,000
9,736
1,123
2007
1,552
25,392
9,566
1,068
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 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, CH4, and non-CH4
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 CH4 or CO2. The C content of the CH4
emissions, estimated as described above, is subtracted from the total C lost in the process to calculate the amount of
C emitted as CO2. The total amount of primary and secondary C black feedstock consumed in the process (see
Table 4-37) is estimated using a primary feedstock consumption factor and a secondary feedstock consumption
factor estimated from U.S. Census Bureau (1999 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).
Table 4-37: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock (Secondary Feedstock)
Consumption (Thousand Metric Tons)
Activity 1990 1995 2000 2005 2006 2007
Primary Feedstock 1,864 2,308 2,521 2,353 2,159 2,212
99 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).
4-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Secondary Feedstock 302 374 408 381 350 358
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
etal. 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 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 4-38. Petrochemical production
CO2 emissions were estimated to be between 1.7 and 3.7 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of approximately 34 percent below to 40 percent above the emission estimate of 2.6 Tg CO2 Eq.
Petrochemical production CH4 emissions were estimated to be between 0.7 and 1.3 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of approximately 31 percent below to 31 percent above the emission
estimate of 1.0 Tg CO2 Eq.
Table 4-38: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petrochemical Production and CO2
Emissions from Carbon Black Production (Tg CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate
(TgC02Eq.)
Petrochemical Production CO2 2.6
Petrochemical Production CH4 1.0
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower
Bound
1.7
0.7
Upper
Bound
3.7
1.3
Lower
Bound
-34%
-31%
Upper
Bound
+40%
+31%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Estimates of CH4 emissions from petrochemical production were revised to account for small changes in ethylene,
ethylene dichloride, and methanol production for years 1990 through 2006. On average, these revisions resulted in
an annual increase in CH4 emissions of approximately 1.5 percent.
Industrial Processes 4-27
-------
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 manufactured from titanium ore, and is principally used as a pigment.
Titanium dioxide is a principal ingredient in white paint, and is also used as a pigment in the manufacture of white
paper, foods, and other products. There are two processes for making TiO2: the chloride process and the sulfate
process. The chloride process uses petroleum coke and chlorine as raw materials and emits process-related CO2.
The sulfate process does not use petroleum coke or other forms of C as a raw material and does not emit CO2.
The chloride process is based on the following chemical reactions:
2 FeTiOs + 7 C12 + 3 C -» 2 TiCL, + 2 FeCl3 + 3 CO2
2 TiCl4 + 2 O2 -> 2 TiO2 + 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 process, and a special grade of "calcined" petroleum coke is manufactured specifically for this
purpose.
Emissions of CO2 in 2007 were 1.9 Tg CO2 Eq. (1,876 Gg), which represents an increase of 57 percent since 1990
(see Table 4-39).
Table 4-3 9: CO2 Emissions from Titanium Dioxide (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 1.2 1,195
1995 1.5 1,526
2000 1.8 1,752
2005 1.8 1,755
2006 1.9 1,876
2007 1.9 1,876
Methodology
Emissions of CO2 from TiO2 production were calculated by multiplying annual TiO2 production by chloride-
process-specific emission factors.
Data were obtained for the total amount of TiO2 produced each year. For years previous to 2004, it was assumed
that TiO2 was produced using the chloride process and the sulfate process in the same ratio as the ratio of the total
U.S. production capacity for each process. As of 2004, the last remaining sulfate-process plant in the United States
had closed. 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 the calcined petroleum coke used in the process is
98.4 percent C and 1.6 percent inert materials (Nelson 1969).
The emission factor for the TiO2 chloride process was taken from the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006). Titanium dioxide production data and the percentage of total TiO2
production capacity that is chloride process for 1990 through 2006 (see Table 4-40) were obtained through the
4-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Minerals Yearbook: Titanium Annual Report (USGS 1991 through 2008). Because 2007 production and capacity
data were unavailable, 2006 production data were used. Percentage chloride-process data were not available for
1990 through 1993, and data from the 1994 USGS Minerals Yearbook were used for these years. Because a sulfate-
process plant closed in September 2001, the chloride-process percentage for 2001 was estimated based on a
discussion with Joseph Gambogi (2002). By 2002, only one sulfate plant remained online in the United States and
this plant closed in 2004 (USGS 2005).
Table 4-40: Titanium Dioxide Production (Gg)
Year Gg
1990 979
1995 1,250
2000 1,400
2005 1,310
2006 1,400
2007 1,400
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 CO2per unit of TiO2 produced as compared to that generated through the use of petroleum coke in
production. While the most accurate method to estimate emissions would be to base calculations on the amount of
reducing agent used in each process rather than on the amount of TiO2 produced, sufficient data were not available
to do so.
Also, annual TiO2 is not reported by USGS by the type of production process used (chloride or sulfate). Only the
percentage of total production capacity by process is reported. The percent of total TiO2 production capacity that
was attributed to the chloride process was multiplied by total TiO2 production to estimate the amount of TiO2
produced using the chloride process (since, as of 2004, the last remaining sulfate-process plant in the United States
closed). This assumes that the chloride-process plants and sulfate-process plants operate at the same level of
utilization. Finally, the emission factor was applied uniformly to all chloride-process production, and no data were
available to account for differences in production efficiency among chloride-process plants. In calculating the
amount of petroleum coke consumed in chloride-process TiO2 production, literature data were used for petroleum
coke composition. Certain grades of petroleum coke are manufactured specifically for use in the TiO2 chloride
process; however, this composition information was not available.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-41. Titanium dioxide
consumption CO2 emissions were estimated to be between 1.6 and 2.1 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 C02 Eq.
Table 4-41: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Titanium Dioxide Production (Tg
CO2 Eq. and Percent)
Source
Gas
2007 Emission
Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate"
(TgC02Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Titanium Dioxide Production
C02
1.9
1.6 2.1 -12% +13%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Industrial Processes 4-29
-------
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 by-product from the energy and industrial production
processes (e.g., ammonia production, fossil fuel combustion, ethanol production), and as a by-product from the
production of crude oil and natural gas, which contain naturally occurring CO2 as a component. Only CO2 produced
from naturally occurring CO2 reservoirs and used in industrial applications other than EOR is included in this
analysis. Neither by-product CO2 generated from energy nor industrial production processes nor CO2 separated
from crude oil and natural gas are included in this analysis for a number of reasons. 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.
100
CO2 is produced as a by-product of crude oil and natural gas production. This CO2 is separated from the crude oil
and natural gas using gas processing equipment, and may be emitted directly to the atmosphere, or captured and
reinjected into underground formations, used for EOR, or sold for other commercial uses. A further discussion of
CO2 used in EOR is described in the Energy Chapter under the text box titled "Carbon Dioxide Transport, Injection,
and Geological Storage." The only CO2 consumption that is accounted for in this analysis is CO2 produced from
naturally-occurring CO2 reservoirs that is used in commercial applications other than EOR.
There are currently two facilities, one in Mississippi and one in New Mexico, producing CO2 from naturally
occurring CO2 reservoirs for use in both EOR and in other commercial applications (e.g., chemical manufacturing,
food production). There are other naturally occurring CO2 reservoirs, mostly located in the western U.S. Facilities
are producing CO2 from these natural reservoirs, but they are only producing CO2 for EOR applications, not for
other commercial applications (Allis et al. 2000). CO2 production from these facilities is discussed in the Energy
Chapter.
In 2007, the amount of CO2 produced by the Mississippi and New Mexico facilities for commercial applications and
subsequently emitted to the atmosphere were 1.9 Tg CO2Eq. (1,867 Gg) (see Table 4-42). This amount represents
an increase of 9 percent from the previous year and an increase of 32 percent since 1990. This increase was due to
an in increase in production at the Mississippi facility, despite the decrease in the percent of the facility's total
reported production that was used for commercial applications.
Table 4-42: CO2 Emissions from CO2 Consumption (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 1.4 1,416
1°° There are currently four known electric power plants operating in the U.S. that capture CO2 for use as food-grade CO2 or
other industrial processes; however, insufficient data prevents estimating emissions from these activities as part of Carbon
Dioxide Consumption.
4-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
1995 1.4 1,422
2000 1.4 1,421
2005 1.3 1,321
2006 1.7 1,709
2007 L9 1,867
Methodology
CO2 emission estimates for 1990 through 2007 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 (ARI2006, 2007) for
1990 to 2000 and from the Annual Reports for Denbury Resources (Denbury Resources 2002 through 2007) for
2001 to 2007 (see Table 4-43). Denbury Resources reported the average CO2 production in units of MMCF CO2 per
day for 2001 through 2007 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. (Codec 2008). The percentage of total production that was used for EOR and in non-EOR
applications were obtained from the New Mexico Bureau of Geology and Mineral Resources (Broadhead 2003 and
New Mexico Bureau of Geology and Mineral Resources 2006).
Table 4-43: CO2 Production (Gg CO2) and the Percent Used for Non-EOR Applications for Jackson Dome and
Bravo Dome
Year Jackson Dome CO2 Jackson Dome % Bravo Dome CO2 Bravo Dome % Used
Production (Gg) Used for Non-EOR Production (Gg) for Non-EOR
1990 1,353 100% 6,301 1%
1995 1,353 100% 6,862 1%
2000 1,353 100% 6,834 1%
2005
2006
2007
4,677
6,610
9,529
27%
25%
19%
5,799
5,613
5,605
1%
1%
1%
Uncertainty
Uncertainty is associated with the number of facilities that are currently producing CO2from naturally occurring
CO2 reservoirs for commercial uses other than EOR, and for which the CO2 emissions are not accounted for
elsewhere. Research indicates that there are only two such facilities, which are in New Mexico and Mississippi;
however, additional facilities may exist that have not been identified. In addition, it is possible that CO2 recovery
exists in particular production and end-use sectors that are not accounted for elsewhere. Such recovery may or may
not affect the overall estimate of CO2 emissions from that sector depending upon the end use to which the recovered
CO2 is applied. Further research is required to determine whether CO2 is being recovered from other facilities for
application to end uses that are not accounted for elsewhere.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-44. CO2 consumption CO2
emissions were estimated to be between 1.5 and 2.3 Tg CO2 Eq. at the 95 percent confidence level. This indicates a
range of approximately 18 percent below to 22 percent above the emission estimate of 1.9 TgCO2Eq.
Industrial Processes 4-31
-------
Table 4-44: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from CO2 Consumption (Tg CO2 Eq. and
Percent)
Source
CO2 Consumption
Gas
C02
2007 Emission
Estimate
(Tg C02 Eq.)
1.9
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Bound Upper Bound Lower Bound
1.5 2.3 -18%
Upper Bound
22%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Estimates of CO2 emissions from CO2 Consumption have been revised for 2006 based on revised CO2 production
data from Jackson Dome. The revision resulted in an increase in emissions of approximately 8 percent for 2006.
Planned Improvements
Future improvements to the Carbon Dioxide Consumption source category include research into CO2 capture for
industrial purposes at electric power plants. Currently, four plants have been identified that capture CO2 for these
purposes, but insufficient data prevents including them in the current emission estimate.
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+o.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 -> 3CaSO4 6H2O + 6H3PO4
The limestone (CaCO3) component of the phosphate rock reacts with the sulfuric acid in the phosphoric acid
production process to produce calcium sulfate (phosphogypsum) and CO2. The chemical reaction for the limestone-
sulfuric acid reaction is:
CaCO3 + H2SO4 + H2O -> CaSO4 2H2O + CO2
Total marketable phosphate rock production in 2007 was 29.7 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, 2.7 million metric tons of crude phosphate rock was imported
for consumption in 2007. The vast majority, 99 percent, of imported phosphate rock is sourced from Morocco
(USGS 2005). Marketable phosphate rock production, including domestic production and imports for consumption,
decreased by less than 1 percent between 2006 and 2007. However, over the 1990 to 2007 period, production has
decreased by 26 percent. Total CO2 emissions from phosphoric acid production were 1.2 Tg CO2 Eq. (1,166 Gg) in
2007 (see Table 4-45).
4-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Table 4-45: CO2 Emissions from Phosphoric Acid Production (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 1.5 1,529
1995 1.5 1,513
2000 1.4 1,382
2005 1.4 1,386
2006 1.2 1,167
2007 1.2 1,166
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 USGSMineral Yearbook: Phosphate Rock disaggregated phosphate rock mined annually in
Florida and North Carolina from phosphate rock mined annually in Idaho and Utah, and reported the annual
amounts of phosphate rock exported and imported for consumption (see Table 4-46). For the years 1990, 1991,
1992, 2005, 2006, and 2007 only nationally aggregated mining data was reported by USGS. For these years, the
breakdown of phosphate rock mined in Florida and North Carolina, and the amount mined in Idaho and Utah, are
approximated using 1993 to 2004 data. Data for domestic production of phosphate rock, exports of phosphate rock
(primarily from Florida and North Carolina), and imports of phosphate rock for consumption for 1990 through 2007
were obtained from USGS Minerals Yearbook: Phosphate Rock (USGS 1994 through 2008). From 2004-2007, the
USGS reported no exports of phosphate rock from U.S. producers (USGS 2005 through 2008).
The carbonate content of phosphate rock varies depending upon where the material is mined. Composition data for
domestically mined and imported phosphate rock were provided by the Florida Institute of Phosphate Research
(FIPR 2003). Phosphate rock mined in Florida contains approximately 1 percent inorganic C, and phosphate rock
imported from Morocco contains approximately 1.46 percent inorganic C. Calcined phosphate rock mined in North
Carolina and Idaho contains approximately 0.41 percent and 0.27 percent inorganic C, respectively (see Table 4-47).
Carbonate content data for phosphate rock mined in Florida are used to calculate the CO2 emissions from
consumption of phosphate rock mined in Florida and North Carolina (87 percent of domestic production) and
carbonate content data for phosphate rock mined in Morocco are used to calculate CO2 emissions from consumption
of imported phosphate rock. The CO2 emissions calculation is based on the assumption that all of the domestic
production of phosphate rock is used in uncalcined form. As of 2006, the USGS noted that one phosphate rock
producer in Idaho produces calcined phosphate rock; however, no production data were available for this single
producer (USGS 2006). Carbonate content data for uncalcined phosphate rock mined in Idaho and Utah (13 percent
of domestic production) were not available, and carbonate content was therefore estimated from the carbonate
content data for calcined phosphate rock mined in Idaho.
Table 4-46: 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
37,370
31,900
5,470
299
1,930
39,001
2005
36,100
31,227
4,874
2,630
38,730
2006
30,100
26,037
4,064
2,420
32,520
2007
29,700
25,691
4,010
2,670
32,370
Industrial Processes 4-33
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a USGS does not disaggregate production data regionally (FL & NC and ID & UT) for 1990, 2005,2006, and 2007. Data for
those years are estimated based on the remaining time series distribution.
- Assumed equal to zero.
Table 4-47: Chemical Composition of Phosphate Rock (percent by weight)
Composition
Total Carbon (as C)
Inorganic Carbon (as C)
Organic Carbon (as C)
Inorganic Carbon (as CO2)
Central
Florida
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.
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 2007. For previous years in the time series,
USGS provided the data disaggregated regionally; however, beginning in 2006 only total U.S. phosphate rock
production were reported. Regional production for 2007 was estimated based on regional production data from
previous years and multiplied by regionally-specific emission factors. There is uncertainty associated with the
degree to which the estimated 2007 regional production data represents actual production in those regions. Total
U.S. phosphate rock production data are not considered to be a significant source of uncertainty because all the
domestic phosphate rock producers report their annual production to the USGS. Data for exports of phosphate rock
used in the emission calculation are reported by phosphate rock producers and are not considered to be a significant
source of uncertainty. Data for imports for consumption are based on international trade data collected by the U.S.
Census Bureau. These U.S. government economic data are not considered to be a significant source of uncertainty.
An additional source of uncertainty in the calculation of CO2 emissions from phosphoric acid production is the
carbonate composition of phosphate rock; the composition of phosphate rock varies depending upon where the
material is mined, and may also vary over time. Another source of uncertainty is the disposition of the organic C
content of the phosphate rock. A representative of the FIPR indicated that in the phosphoric acid production
process, the organic C content of the mined phosphate rock generally remains in the phosphoric acid product, which
is what produces the color of the phosphoric acid product (FIPR 2003a). Organic C is therefore not included in the
calculation of CO2 emissions from phosphoric acid production.
A third source of uncertainty is the assumption that all domestically-produced phosphate rock is used in phosphoric
acid production and used without first being calcined. Calcination of the phosphate rock would result in conversion
of some of the organic C in the phosphate rock into CO2. However, according to the USGS, only one producer in
Idaho is currently calcining phosphate rock, and no data were available concerning the annual production of this
single producer (USGS 2005). For available years, total production of phosphate rock in Utah and Idaho combined
amounts to approximately 13 percent of total domestic production on average (USGS 1994 through 2005).
Finally, USGS indicated that approximately 7 percent of domestically-produced phosphate rock is used to
manufacture elemental phosphorus and other phosphorus-based chemicals, rather than phosphoric acid (USGS
2006). According to USGS, there is only one domestic producer of elemental phosphorus, in Idaho, and no data
were available concerning the annual production of this single producer. Elemental phosphorus is produced by
reducing phosphate rock with coal coke, and it is therefore assumed that 100 percent of the carbonate content of the
phosphate rock will be converted to CO2 in the elemental phosphorus production process. The calculation for CO2
emissions is based on the assumption that phosphate rock consumption, for purposes other than phosphoric acid
production, results in CO2 emissions from 100 percent of the inorganic C content in phosphate rock, but none from
the organic C content.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-48. Phosphoric acid
production CO2 emissions were estimated to be between 1.0 and 1.4 Tg CO2 Eq. at the 95 percent confidence level.
This indicates a range of approximately 18 percent below and 18 percent above the emission estimate of 1.2 Tg CO2
Eq.
4-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Table 4-48: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Phosphoric Acid Production (Tg
CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower
Bound Bound Bound Upper Bound
Phosphoric Acid Production CO2 1.2
1.0 1.4 -18% +18%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Planned Improvements
Currently, data sources for the carbonate content of the phosphate rock are limited. If additional data sources are
found, this information will be incorporated into future estimates.
4.13. Iron and Steel Production (IPCC Source Category 2C1) and Metallurgical
Coke Production
The production of iron and steel is an energy-intensive process that also generates process-related emissions of CO2
and CH4. Metallurgical coke, which is manufactured using coking coal as a raw material, is used widely during the
production of iron and steel. According to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories
(IPCC 2006), the production of metallurgical coke from coking coal is considered to be an energy use of fossil fuel
and the production of iron and steel is considered to be an industrial process source, so emissions from these are
reported separately. Emission estimates presented in this chapter are based on the methodologies provided by the
2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006), which call for a mass balance
accounting of the carbonaceous inputs and outputs during the iron and steel production process and the metallurgical
coke production process. The methodologies also call for reporting emissions from metallurgical coke production in
the Energy sector; however, the approaches and emission estimates for both metallurgical coke production and iron
and steel production are presented separately here because the activity data used to estimate emissions from
metallurgical coke production have significant overlap with activity data used to estimate iron and steel production
emissions. Further, some by-products (e.g., coke oven gas) of the metallurgical coke production process are
consumed during iron and steel production, and some by-products of the iron and steel production process (e.g.,
blast furnace gas) are consumed during metallurgical coke production. Emissions associated with the consumption
of these by-products are attributed to point of consumption. As an example, CO2 emissions associated with the
combustion of coke oven gas in the blast furnace during pig iron production are attributed to pig iron production.
Emissions associated with fuel consumption downstream of the iron and steelmaking furnaces, such as natural gas
used for heating and annealing purposes, are reported in the Energy chapter.
The production of metallurgical coke from coking coal occurs both on-site at "integrated" iron and steel plants and
off-site at "merchant" coke plants. Metallurgical coke is produced by heating coking coal in a coke oven in a low-
oxygen environment. The process drives off the volatile components of the coking coal and produces coal
(metallurgical) coke. Coke oven gas, coal tar, coke breeze (small-grade coke oven coke with particle size <5mm)
and light oil are carbon-containing by-products of the metallurgical coke manufacturing process. Coke oven gas is
recovered and used for underfiring the coke ovens and within the iron and steel mill. Small amounts of coke oven
gas are also sold as synthetic natural gas outside of the iron and steel mills and are accounted for in the Energy
chapter. Coal tar is used as a raw material to produce anodes used for primary aluminum production, electric arc
furnace (EAF) steel production, and other electrolytic processes, and also used in the production of other coal tar
products. Light oil is sold to petroleum refiners who use the material as an additive for gasoline. The metallurgical
coke production process produces CO2 emissions and fugitive CH4 emissions.
Iron is produced by first reducing iron oxide (iron ore) with metallurgical coke in a blast furnace to produce pig iron
(impure or crude iron containing about 3 to 5 percent carbon by weight). Inputs to the blast furnace include natural
gas, fuel oil, and coke oven gas. The carbon in the metallurgical coke used in the blast furnace combines with
oxides in the iron ore in a reducing atmosphere to produce blast furnace gas containing carbon monoxide (CO) and
CO2. The CO is then converted and emitted as CO2 when combusted to either pre-heat the blast air used in the blast
furnace or for other purposes at the steel mill. Iron may be introduced into the blast furnace in the form of raw iron
Industrial Processes 4-35
-------
ore, pellets (9-16mm iron-containing spheres), briquettes, or sinter. Pig iron is used as a raw material in the
production of steel, which contains about 1 percent carbon by weight. Pig iron is also used as a raw material in the
production of iron products in foundries. The pig iron production process produces CO2 emissions and fugitive CH4
emissions.
Iron can also be produced through the direct reduction process; wherein, iron ore is reduced to metallic iron in the
solid state at process temperatures less than 1000°C. Direct reduced iron production results in process emissions of
CO2 and emissions of CH4 through the consumption of natural gas used during the reduction process.
Sintering is a thermal process by which fine iron-bearing particles, such as air emission control system dust, are
baked, which causes the material to agglomerate into roughly one-inch pellets that are then recharged into the blast
furnace for pig iron production. Iron ore particles may also be formed into larger pellets or briquettes by mechanical
means, and then agglomerated by heating. The agglomerate is then crushed and screened to produce an iron-bearing
feed that is charged into the blast furnace. The sintering process produces CO2 and fugitive CH4 emissions through
the consumption of carbonaceous inputs (e.g., coke breeze) during the sintering process.
Steel is produced from pig iron in a variety of specialized steel-making furnaces, including EAFs and basic oxygen
furnaces (BOFs). Carbon inputs to steel-making furnaces include pig iron and scrap steel as well as natural gas, fuel
oil, and fluxes (e.g., limestone, dolomite). In a EOF, the carbon in iron and scrap steel combines with high-purity
oxygen to reduce the carbon content of the metal to the amount desired for the specified grade of steel. EAFs use
carbon electrodes, charge carbon and other materials (e.g., natural gas) to aid in melting metal inputs (primarily
recycled scrap steel), which are refined an alloyed to produce the desired grade of steel. CO2 emissions occur in
BOFs occur through the reduction process. In EAFs, CO2 emissions result primarily from the consumption of
carbon electrodes and also from the consumption of supplemental materials used to augment the melting process.
In addition to the production processes mentioned above, CO2 is also generated at iron and steel mills through the
consumption of process by-products (e.g., blast furnace gas, coke oven gas) used for various purposes including
heating, annealing, and electricity generation.101 Process by-products sold for use as synthetic natural gas are
deducted and reported in the Energy chapter. Emissions associated with natural gas and fuel oil consumption for
these purposes are reported in the Energy chapter.
The majority of CO2 emissions from the iron and steel production process come from the use of metallurgical coke
in the production of pig iron and from the consumption of other process by-products at the iron and steel mill, with
smaller amounts evolving from the use of flux and from the removal of carbon from pig iron used to produce steel.
Some carbon is also stored in the finished iron and steel products.
Metallurgical Coke Production
Emissions of CO2 and CH4 from metallurgical coke production in 2007 were 3.8 Tg CO2 Eq. (3,806 Gg) and less
than 0.05 Tg CO2 Eq. (less than 0.5 Gg), respectively (see Table 4-49 and Table 4-50), totaling 3.8 Tg CO2 Eq.
Emissions increased in 2007, but have decreased overall since 1990. In 2007, domestic coke production decreased
by 1.2 percent and has decreased overall since 1990. Coke production in 2007 was 22 percent lower than in 2000
and 41 percent below 1990. Overall, emissions from metallurgical coke production have declined by 31 percent (1.7
Tg CO2 Eq.) from 1990 to 2007.
Table 4-49: CO2 and CH4 Emissions from Metallurgical Coke Production (Tg CO2 Eq.)
Year 1990 1
CO2 5.5 1
CH4 +|
1995
5.0
+
! 2000
| 4.4
+
! 2005 2006 2007
j 3.8 3.7 3.8
| + + +
Total
5.51
5.01
4.411
3.8
3.7
3.8
+ Does not exceed 0.05 Tg CO2 Eq.
101 Emissions resulting from fuel consumption for the generation of electricity are reported in the Energy chapter. Some
integrated iron and steel mills have on-site electricity generation for which fuel is used. Data are not available concerning the
amounts and types of fuels used in iron and steel mills to generate electricity. Therefore all of the fuel consumption reported at
iron and steel mills is assumed to be used within the iron and steel mills for purposes other than electricity consumption, and the
amounts of any fuels actually used to produce electricity at iron and steel mills are not subtracted from the electricity production
emissions value used in the Energy chapter, therefore some double-counting of electricity-related CO2 emissions may occur.
4-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Table 4-50: CO2 and CH4 Emissions from Metallurgical Coke Production (Gg)
Year 1990 1995 2000 2005 2006 2007
CO2 5,498 5,037 4,381 3,849 3,682 3,806
CH4 + + + + + +
+ Does not exceed 0.5 Gg
Iron and Steel Production
Emissions of CO2 and CH4 from iron and steel production in 2007 were 73.6 Tg CO2 Eq. (73,564 Gg) and 0.7 Tg
CO2 Eq. (33.2 Gg), respectively (see Table 4-51, Table 4-52, Table 4-53, and Table 4-54), totaling 74.3 Tg CO2 Eq.
Emissions increased in 2007, but have decreased overall since 1990 due to restructuring of the industry,
technological improvements, and increased scrap utilization. CO2 emission estimates include emissions from the
consumption of carbonaceous materials in the blast furnace, EAF, and EOF as well as blast furnace gas and coke
oven gas consumption for other activities at the steel mill.
In 2007, domestic production of pig iron decreased by 4 percent. Overall, domestic pig iron production has declined
since the 1990s. Pig iron production in 2007 was 24 percent lower than in 2000 and 26 percent below 1990. CO2
emissions from steel production have decreased by 3 percent (4 Tg CO2 Eq.) since 1990. Overall, CO2 emissions
from iron and steel production have declined by 29 percent (30.7 Tg CO2 Eq.) from 1990 to 2007.
Table 4-51: CO2 Emissions from Iron and Steel Production (Tg CO2 Eq.)
Year
Sinter Production
Iron Production
Steel Production
Other Activities3
Total
1990
2.4
47.9
14.7
39.3
104.3
1995
2.5
38.8
15.9
40.9
98.1
2000
2.2
33.8
14.8
39.9
90.7
2005
1.7
19.6
14.0
34.2
69.3
2006
1.4
24.0
14.4
32.6
72.4
2007
1.4
26.9
14.3
31.0
73.6
Note: Totals may not sum due to independent rounding.
a Includes emissions from blast furnace gas and coke oven gas combustion for activities at the steel mill other than consumption
in blast furnace, EAFs, or BOFs.
Table 4-52: CO2 Emissions from Iron and Steel Production (Gg)
Year
Sinter Production
Iron Production
Steel Production
Other Activities a
Total
1990
2,448
47,886
14,672
39,256
104,262
1995
2,512
38,791
15,925
40,850
98,078
2000
2,158
33,808
14,837
39,877
90,680
2005
1,663
19,576
13,950
34,152
69,341
2006
1,418
24,026
14,392
32,583
72,418
2007
1,383
26,948
14,270
30,964
73,564
Note: Totals may not sum due to independent rounding.
a Includes emissions from blast furnace gas and coke oven gas combustion for activities at the steel mill other than consumption
in blast furnace, EAFs, or BOFs.
Table 4-53: CH4 Emissions from Iron and Steel Production (Tg CO2 Eq.)
Year 1990 1995 2000 2005 2006 2007
Sinter Production + + + + + +
Iron Production 0.9 LO 0.9 0.7 0.7 0.7
Total LO LO 0.9 0/7 0.7 0.7
+ Does not exceed 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
Industrial Processes 4-37
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Table 4-54: CH4 Emissions from Iron and Steel Production (Gg)
Year 1990 1995 2000 2005 2006 2007
Sinter Production 0.9 0.9 0.7 0.6 0.5 0.5
Iron Production 44.7 45.8 43.1 33.5 34.1 32.7
Total 45.6 46.7 43.8 34.1 34.6 33.2
Note: Totals may not sum due to independent rounding.
Methodology
Metallurgical Coke Production
Coking coal is used to manufacture metallurgical (coal) coke that is used primarily as a reducing agent in the
production of iron and steel, but is also used in the production of other metals including lead and zinc (see Lead
Production and Zinc Production in this chapter). Emissions associated with producing metallurgical coke from
coking coal are estimated and reported separately from emissions that result from the iron and steel production
process. To estimate emission from metallurgical coke production, a Tier 2 method provided by the 2006IPCC
Guidelines for National Greenhouse Gas Inventories (IPCC 2006) was utilized. The amount of carbon contained in
materials produced during the metallurgical coke production process (i.e., coke, coke breeze, coke oven gas, and
coal tar) is deducted from the amount of carbon contained in materials consumed during the metallurgical coke
production process (i.e., natural gas, blast furnace gas, coking coal). Light oil, which is produced during the
metallurgical coke production process, is excluded from the deductions due to data limitations. The amount of
carbon contained in these materials is calculated by multiplying the material-specific carbon content by the amount
of material consumed or produced (see Table 4-55). The amount of coal tar produced was approximated using a
production factor of 0.03 tons of coal per ton of coking coal consumed. The amount of coke breeze produced was
approximated using a production factor of 0.075 tons of coke breeze per ton of coking coal consumed. Data on the
consumption of carbonaceous materials (other than coking coal) as well as coke oven gas production were available
for integrated steel mills only (i.e., steel mills with co-located coke plants). Therefore, carbonaceous material (other
than coking coal) consumption and coke oven gas production were excluded from emission estimates for merchant
coke plants. Carbon contained in coke oven gas used for coke-oven underfiring was not included in the deductions
to avoid double-counting.
Table 4-55: Material Carbon Contents for Metallurgical Coke Production
Material kg C/kg
Coal Tar 0.62
Coke 0.83
Coking Coal 0.73
Material kg C/GJ
Coke Oven Gas 12.1
Blast Furnace Gas 70.8
Source: IPCC 2006, Table 4.3. Coke Oven Gas and Blast Furnace Gas, Table 1.3.
The production processes for metallurgical coke production results in fugitive emissions of CH4, which are emitted
via leaks in the production equipment rather than through the emission stacks or vents of the production plants. The
fugitive emissions were calculated by applying Tier 1 emission factors taken from the 2006 IPCC Guidelines for
National Greenhouse Gas Inventories (IPCC 2006) for metallurgical coke production (see Table 4-56).
Table 4-56: CH4 Emission Factor for Metallurgical Coke Production (g CH^metric ton)
Material Produced g CH4/metric ton
Metallurgical Coke (XI
Source: IPCC 2006, Table 4.2
Data relating to the amount of coking coal consumed at metallurgical coke plants and the amount of metallurgical
coke produced at coke plants 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, 2008a) (see
Table 4-57). Data on natural gas consumption, blast furnace gas consumption, and coke oven gas production for
4-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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metallurgical coke production at integrated steel mills were obtained from the American Iron and Steel Institute
(AISI), Annual Statistical Report (AISI 2004 through 2008a) and through personal communications with AISI
(2008b) (see Table 4-58). The factor for the quantity of coal tar produced per ton of coking coal consumed was
provided by AISI (2008b). The factor for the quantity of coke breeze produced per ton of coking coal consumed
was obtained through Table 2-1 of the report Energy and Environmental Profile of the U.S. Iron and Steel Industry
(DOE 2000). Data on natural gas consumption and coke oven gas production at merchant coke plants were not
available and were excluded from the emission estimate. Carbon contents for coking coal, metallurgical coke, coal
tar, coke oven gas, and blast furnace gas were provided by the 2006IPCC Guidelines for National Greenhouse Gas
Inventories (IPCC 2006). The carbon content for coke breeze was assumed to equal the carbon content of coke.
Table 4-57: Production and Consumption Data for the Calculation of CO2 and CH4 Emissions from Metallurgical
Coke Production (Thousand Metric Tons)
Source/Activity Data
Metallurgical Coke Production
Coal Consumption at Coke
Plants
Coke Production at Coke Plants
Coal Tar and Coke Breeze
Production
1990
35,269
25,054
2,631
1995
29,948
21,545
2,262
2000
26,254
18,877
1,982
2005
21,259
15,167
1,593
2006
20,827
14,882
1,563
2007
20,607
14,698
1,543
Table 4-58: Production and Consumption Data for the Calculation of CO2 Emissions from Metallurgical Coke
Production (million ft3)
Source/Activity Data 1990 1995 2000 2005 2006 2007
Metallurgical Coke
Production
Coke Oven Gas Production3 250,767 166,750 149,477 114,213 114,386 109,912
Natural Gas Consumption 599 184 180 2,996 3,277 3,309
Blast Furnace Gas 24,602 29,423 26,075 4,460 5,505 5,144
Consumption
a Includes coke oven gas used for purposes other than coke oven underfiring only.
Iron and Steel Production
Emissions of CO2 from sinter production and direct reduced iron production were estimated by multiplying total
national sinter production and the total national direct reduced iron production by Tier 1 CO2 emission factors (see
Table 4-59). Because estimates of sinter production and direct reduced iron production were not available,
production was assumed to equal consumption.
Table 4-59: CO2 Emission Factors for Sinter Production and Direct Reduced Iron Production
Material Produced Metric Ton
CO2/Metric Ton
Sinter 0.2
Direct Reduced Iron 0.7
Source: IPCC 2006, Table 4.1.
To estimate emissions from pig iron production in the blast furnace, the amount of carbon contained in the produced
pig iron and blast furnace gas were deducted from the amount of carbon contained in inputs (i.e., metallurgical coke,
sinter, natural ore, pellets, natural gas, fuel oil, coke oven gas, direct coal injection). The carbon contained in the pig
iron, blast furnace gas, and blast furnace inputs was estimated by multiplying the material-specific carbon content by
each material type (see Table 4-60). Carbon in blast furnace gas used to pre-heat the blast furnace air is combusted
to form CO2 during this process.
Emissions from steel production in EAFs were estimated by deducting the carbon contained in the steel produced
from the carbon contained in the EAF anode, charge carbon, and scrap steel added to the EAF. Small amounts of
carbon from direct reduced iron, pig iron, and flux additions to the EAFs were also included in the EAF calculation.
For BOFs, estimates of carbon contained in EOF steel were deducted from carbon contained in inputs such as
Industrial Processes 4-39
-------
natural gas, coke oven gas, fluxes, and pig iron. In each case, the carbon was calculated by multiplying material-
specific carbon contents by each material type (see Table 4-60). For EAFs, the amount of EAF anode consumed
was approximated by multiplying total EAF steel production by the amount of EAF anode consumed per metric ton
of steel produced (0.002 metric ton EAF anode per metric ton steel produced). The amount of flux (e.g., limestone
and dolomite) used during steel manufacture was deducted from the Limestone and Dolomite Use source category to
avoid double-counting.
CO2 emissions from the consumption of blast furnace gas and coke oven gas for other activities occurring at the
steel mill were estimated by multiplying the amount of these materials consumed for these purposes by the material-
specific C content (see Table 4-60).
CO2 emissions associated with the sinter production, direct reduced iron production, pig iron production, steel
production, and other steel mill activities were summed to calculate the total CO2 emissions from iron and steel
production (see Table 4-51 and Table 4-52).
Table 4-60: Material Carbon Contents for Iron and Steel Production
Material
Coke
Direct Reduced Iron
Dolomite
EAF Carbon Electrodes
EAF Charge Carbon
Limestone
Pig Iron
Steel
Material
Coke Oven Gas
Blast Furnace Gas
kg C/kg
0.83
0.02
0.13
0.82
0.83
0.12
0.04
0.01
kg C/GJ
12.1
70.8
Source: IPCC 2006, Table 4.3. Coke Oven Gas and Blast Furnace Gas, Table 1.3.
The production processes for sinter and pig iron result in fugitive emissions of CH4, which are emitted via leaks in
the production equipment rather than through the emission stacks or vents of the production plants. The fugitive
emissions were calculated by applying Tier 1 emission factors taken from the 2006 IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006) for sinter production and the 1995 IPCC Guidelines
(IPCC/UNEP/OECD/IEA 1995) (see Table 4-61) for pig iron production. The production of direct reduced iron
also results in emissions of CH4 through the consumption of fossil fuels (e.g., natural gas); however, these emissions
estimates are excluded due to data limitations.
Table 4-61: CH4 Emission Factors for Sinter and Pig Iron Production
Material Produced Factor Unit
Pig Iron 0.9 g CH4/kg
Sinter 0.07 kg CH4/metric ton
Source: Sinter (IPCC 2006, Table 4.2), Pig Iron (IPCC/UNEP/OECD/IEA 1995, Table 2.2)
Sinter consumption and direct reduced iron consumption data were obtained from AISI's Annual Statistical Report
(AISI 2004 through 2008a) and through personal communications with AISI (2008b) (see Table 4-62). Data on
direct reduced iron consumed in EAFs were not available for the years 1990, 1991, 1999, 2006, and 2007. EAF
direct reduced iron consumption in 1990 and 1991 was assumed to equal consumption in 1992, consumption in 1999
was assumed to equal the average of 1998 and 2000, and consumption in 2006 and 2007 was assumed to equal
consumption in 2005. Data on direct reduced iron consumed in BOFs were not available for the years 1990 through
1994, 1999, 2006, and 2007. EOF direct reduced iron consumption in 1990 through 1994 was assumed to equal
consumption in 1995, consumption in 1999 was assumed to equal the average of 1998 and 2000, and consumption
in 2006 and 2007 was assumed to equal consumption in 2005. The Tier 1 CO2 emission factors for sinter production
and direct reduced iron production were obtained through the 2006 IPCC Guidelines for National Greenhouse Gas
Inventories (IPCC 2006). Data for pig iron production, coke, natural gas, fuel oil, sinter, and pellets consumed in
the blast furnace; pig iron production; and blast furnace gas produced at the iron and steel mill and used in the
metallurgical coke ovens and other steel mill activities were obtained from AISI's Annual Statistical Report (AISI
4-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
2004 through 2008a) and through personal communications with AISI (2008b) (see Table 4-63). Data for EAF steel
production, flux, EAF charge carbon, direct reduced iron, pig iron, scrap steel, and natural gas consumption as well
as EAF steel production were obtained from AISFs Annual Statistical Report (AISI 2004 through 2008a) and
through personal communications with AISI (2008b). The factor for the quantity of EAF anode consumed per ton
of EAF steel produced was provided by AISI (AISI 2008b). Data for EOF steel production, flux, direct reduced
iron, pig iron, scrap steel, natural gas, natural ore, pellet sinter consumption as well as EOF steel production were
obtained from AISI's Annual Statistical Report (AISI 2004 through 2008a) and through personal communications
with AISI (2008b). Because data on pig iron consumption and scrap steel consumption in BOFs and EAFs were not
available for 2006 and 2007, 2005 data were used. Because pig iron consumption in EAFs was also not available in
2003 and 2004, the average of 2002 and 2005 pig iron consumption data were used. Data on coke oven gas and
blast furnace gas consumed at the iron and steel mill other than in the EAF, EOF, or blast furnace were obtained
from AISI's Annual Statistical Report (AISI 2004 through 2008a) and through personal communications with AISI
(2008b). Data on blast furnace gas and coke oven gas sold for use as synthetic natural gas were obtained through
EIA's Natural Gas Annual 2007 (EIA 2008b). C contents for direct reduced iron, EAF carbon electrodes, EAF
charge carbon, limestone, dolomite, pig iron, and steel were provided by the 2006IPCC Guidelines for National
Greenhouse Gas Inventories (IPCC 2006). The C contents for natural gas, fuel oil, and direct injection coal as well
as the heat contents for the same fuels were provided by EIA (2008b). Heat contents for coke oven gas and blast
furnace gas were provided in Table 2-2 of the report Energy and Environmental Profile of the U.S. Iron and Steel
Industry (DOE 2000).
Table 4-62: Production and Consumption Data for the Calculation of CO2 and CH4 Emissions from Iron and Steel
Production (Thousand Metric Tons)
Source/Activity Data
Sinter Production
Sinter Production
Direct Reduced Iron
Production
Direct Reduced Iron Production
Pig Iron Production
Coke Consumption
Pig Iron Production
Direct Injection Coal
Consumption
EAF Steel Production
EAF Anode and Charge Carbon
Consumption
Scrap Steel Consumption
Flux Consumption
EAF Steel Production
EOF Steel Production
Pig Iron Consumption
Scrap Steel Consumption
Flux Consumption
EOF Steel Production
Blast Furnace Gas Production3
1990
12,239
936
24,946
49,669
1,485
67
35,743
319
33,511
46,564
14,548
576
43,973
1,439,380
1995
12,562
989
22,198
50,891
1,509
77
39,010
267
38,472
49,896
15,967
1,259
56,721
1,559,795
2000
10,788
1,914
19,215
47,888
3,012
96
43,001
654
47,860
46,993
14,969
978
53,965
1,524,891
2005
8,315
1,633
13,832
37,222
2,573
104
37,558
695
52,194
32,115
11,612
582
42,705
1,299,980
2006
7,088
1,633
14,684
37,904
2,526
112
37,558
671
56,071
32,115
11,612
610
42,119
1,236,526
2007
6,914
1,633
15,039
36,337
2,734
114
37,558
567
57,004
32,115
11,612
408
41,099
1,173,588
Table 4-63: Production and Consumption Data for the Calculation of CO2 Emissions from Iron and Steel
Production (million ft3 unless otherwise specified)
Source/Activity Data
Pig Iron Production
Natural Gas Consumption
Fuel Oil Consumption
(thousand gallons)
Coke Oven Gas Consumption
EAF Steel Production
Natural Gas Consumption
1990
56,273
163,397
22,033
9,604
1995
106,514
108,196
10,097
11,026
2000
91,798
120,921
13,702
13,717
2005
59,844
16,170
16,557
14,959
2006
58,344
87,702
16,649
16,070
2007
56,112
84,498
16,239
16,337
Industrial Processes 4-41
-------
EOF Steel Production
Natural Gas Consumption 6,301
Coke Oven Gas Consumption 3,851
Other Activities
Coke Oven Gas Consumption 224,883
Blast Furnace Gas 1,414,778
Consumption
16,546 6,143
1,284 640
155,369 135,135
1,530,372 1,498,816
5,026
524
97,132
1,295,520
5,827
559
97,178
1,231,021
11,740
525
93,148
1,168,444
a Includes blast furnace gas used for purposes other than in the blast furnace only.
Uncertainty
The estimates of CO2 emissions from metallurgical coke production are based on material production and
consumption data and average carbon contents. Uncertainty is associated with the total U.S. coking coal
consumption, total U.S. coke production and materials consumed during this process. Data for coking coal
consumption and metallurgical coke production are from different data sources (EIA) than data for other
carbonaceous materials consumed at coke plants (AISI), which does not include data for merchant coke plants.
There is uncertainty associated with the fact that coal tar and coke breeze production were estimated based on coke
production because coal tar and coke breeze production data were not available.
The estimates of CO2 emissions from iron and steel production are based on material production and consumption
data and average carbon contents. There is uncertainty associated with the assumption that direct reduced iron and
sinter consumption are equal to production. There is uncertainty associated with the assumption that all coal used
for purposes other than coking coal is for direct injection coal. Some of this coal may be used for electricity
generation. There is also uncertainty associated with the carbon contents for pellets, sinter, and natural ore, which
are assumed to equal the carbon contents of direct reduced iron. For EAF steel production there is uncertainty
associated with the amount of EAF anode and charge carbon consumed due to inconsistent data throughout the
timeseries. Uncertainty is also associated with the use of process gases such as blast furnace gas and coke oven gas.
Data are not available to differentiate between the use of these gases for processes at the steel mill versus for energy
generation (e.g., electricity and steam generation); therefore, all consumption is attributed to iron and steel
production. These data and carbon contents produce a relatively accurate estimate of CO2 emissions. However,
there are uncertainties associated with each.
For the purposes of the CH4 calculation it is assumed that all of the CH4 escapes as fugitive emissions and that none
of the CH4 is captured in stacks or vents and that. Additionally, the CO2 emissions calculation is not corrected by
subtracting the C content of the CH4, which means there may be a slight double counting of C as both CO2 and CH4.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-64 for iron and steel
production. Iron and Steel Production CO2 emissions were estimated to be between 57.0 and 87.9 Tg CO2 Eq. at the
95 percent confidence level. This indicates a range of approximately 22 percent below and 20 percent above the
emission estimate of 73.6 Tg CO2 Eq. Iron and Steel Production CH4 emissions were estimated to be between 0.6
Tg CO2 Eq. and 0.8 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 0.7 Tg CO2 Eq.
Table 4-64: Tier 2 Quantitative Uncertainty Estimates for CO2 and CH4 Emissions from Iron and Steel Production
(Tg. CO2 Eq. and Percent)3
Source
Gas
2007
Emission
Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate1"
(TgC02Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Iron and Steel Production
Iron and Steel Production
CO2
CH4
73.6
0.7
57.0 87.9 -22% +20%
0.6 0.8 -8% 8%
"• The emission estimates and the uncertainty range presented in this table correspond to iron and steel production only.
Uncertainty associated with emissions from metallurgical coke production were not estimated due to data limitations and were
excluded from the uncertainty estimates presented in this table.
b Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Recalculations Discussion
Estimates of CO2 from iron and steel production have been revised for the years 1990 through 2006 to adhere to the
methods presented in the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006). Previously
the estimates focused primarily on the consumption of coking coal to produce metallurgical coke and the
consumption of metallurgical coke, carbon anodes, and scrap steel to produce iron and steel. The revised estimates
differentiate between emissions associated with metallurgical coke production and those associated with iron and
steel production and include CO2 emissions from the consumption of other materials such as natural gas, fuel oil,
flux (e.g. limestone and dolomite use), direction injection goal, sinter, pellets, and natural ore during the iron and
steel production process as well as the metallurgical coke production process. Currently, CO2 emissions from iron
and steel production are reported separately from CO2 emissions from the metallurgical coke production. On
average, revisions to the Iron and Steel Production estimate resulted in an annual increase of CO2 emissions of 26.1
Tg CO2 Eq. (40.7 percent).
Estimates of CH4 emissions from iron and steel production have been revised based on revisions to the CH4
emission factor from sinter production and to report emissions from metallurgical coke production separately. On
average, revisions to the Iron and Steel Production estimate resulted in an annual decrease of CH4 emissions of 0.3
Tg CO2 Eq. (24.6 percent).
Planned Improvements
Plans for improvements to the Iron and Steel Production source category include attributing emissions estimates for
the production of metallurgical coke to the Energy chapter as well as identifying the amount of carbonaceous
materials, other than coking coal, consumed at merchant coke plants. Additional improvements include identifying
the amount of coal used for direct injection and the amount of coke breeze, coal tar, and light oil produced during
coke production. Efforts will also be made to identify inputs for preparing Tier 2 estimates for sinter and direct
reduced iron production, as well as identifying information to better characterize emissions from the use of process
gases and fuels within the Energy and Industrial Processes chapters.
4.14. Ferroalloy Production (IPCC Source Category 2C2)
CO2 and CH4 are emitted from the production of several ferroalloys. Ferroalloys are composites of iron and other
elements such as silicon, manganese, and chromium. When incorporated in alloy steels, ferroalloys are used to alter
the material properties of the steel. Estimates from two types of ferrosilicon (25 to 55 percent and 56 to 95 percent
silicon), silicon metal (about 98 percent silicon), and miscellaneous alloys (36 to 65 percent silicon) have been
calculated. Emissions from the production of ferrochromium and ferromanganese are not included here because of
the small number of manufacturers of these materials in the United States. Subsequently, government information
disclosure rules prevent the publication of production data for these production facilities.
Similar to emissions from the production of iron and steel, CO2 is emitted when metallurgical coke is oxidized
during a high-temperature reaction with iron and the selected alloying element. Due to the strong reducing
environment, CO is initially produced, and eventually oxidized to CO2. A representative reaction equation for the
production of 50 percent ferrosilicon is given below:
Fe2O3 +2SiO2 + 7C HX 2FeSi + 7CO
While most of the C contained in the process materials is released to the atmosphere as CO2, a percentage is also
released as CH4 and other volatiles. The amount of CH4 that is released is dependent on furnace efficiency,
operation technique, and control technology.
Emissions of CO2 from ferroalloy production in 2007 were 1.6 Tg CO2 Eq. (1,552 Gg) (see Error! Reference
source not found, and Table 4-66), which is an three percent increase from the previous year and a 28 percent
reduction since 1990. Emissions of CH4 from ferroalloy production in 2007 were 0.01 Tg CO2 Eq. (0.448 Gg),
which is an 3 percent increase from the previous year and a 28 percent decrease since 1990.
Table 4-65: CO2 and CH4 Emissions from Ferroalloy Production (Tg CO2 Eq.)
Year 1990 1995 2000 2005 2006 2007
CO2 2.2 2.0 1.9 1.4 1.5 1.6
CH4 + + + + + +
Industrial Processes 4-43
-------
Total 2.2 2.0 1.9 1.4 1.5 1.6
+ Does not exceed 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
Table 4-66: CO2 and CH4 Emissions from Ferroalloy Production (Gg)
Year
C02
CH4
1990
2,152
1
1995
2,036
1
2000
1,893
1
2005
1,392
+
2006
1,505
+
2007
1,552
+
Methodology
Emissions of CO2 and CH4 from ferroalloy production were calculated by multiplying annual ferroalloy production
by material-specific emission factors. Emission factors taken from the 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
CH4 (1 kg CHVmetric ton of alloy produced). Additionally, for ferrosilicon alloys containing 56 to 95 percent
silicon, an emission factor for 75 percent silicon ferrosilicon was applied for both CO2 and CH4 (4 metric tons
CO2/metric ton alloy produced and 1 kg CH4/metric ton of alloy produced, respectively). The emission factors for
silicon metal equaled 5 metric tons CO2/metric ton metal produced and 1.2 kg CH4/metric ton metal produced. It
was assumed that 100 percent of the ferroalloy production was produced using petroleum coke using an electric arc
furnace process (IPCC 2006), although some 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 2007 (see Table 4-67) were obtained from the USGS through personal
communications with the USGS Silicon Commodity Specialist (Corathers 2008) and through the Minerals
Yearbook: Silicon Annual Report (USGS 1991 through 2007). Because USGS does not provide estimates of silicon
metal production for 2006 and 2007, 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 4-67). The
composition data for petroleum coke was obtained from Onder and Bagdoyan (1993).
Table 4-67: Production of Ferroalloys (Metric Tons)
Year
1990
1995
2000
2005
2006
2007
Ferrosilicon
25%-55%
321,385
184,000
229,000
123,000
164,000
180,000
Ferrosilicon
56%-95%
109,566
128,000
100,000
86,100
88,700
90,600
Silicon Metal
145,744
163,000
184,000
148,000
148,000
148,000
Misc. Alloys
32-65%
72,442
99,500
NA
NA
NA
NA
NA (Not Available)
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
4-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
would not be counted under this source because wood-based C is of biogenic origin.102 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 4-68. Ferroalloy production CO2
emissions were estimated to be between 1.4 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 1.6 Tg CO2 Eq. Ferroalloy
production CH4 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.
Table 4-68: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ferroalloy Production (Tg CO2 Eq.
and Percent)
2007 Emission
Source Gas Estimate
(Tg C02 Eq.)
Ferroalloy Production CO2 1.6
Ferroalloy Production CH4 +
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower
Bound Bound Bound
1.4 1.7 -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 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 seven percent of the world total (USGS 2008). 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
102
Emissions and sinks of biogenic carbon are accounted for in the Land Use, Land-Use Change, and Forestry chapter.
Industrial Processes 4-45
-------
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 4.3 Tg CO2 Eq. (4,251 Gg) in 2007 (see
Table 4-69). 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.
Table 4-69: CO2 Emissions from Aluminum Production (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 6.8 6,831
1995 5.7 5,659
2000 6.1 6,086
2005 4.1 4,142
2006 3.8 3,801
2007 4.3 4,251
In addition to CO2 emissions, the aluminum production industry is also a source of PFC emissions. During the
smelting process, when the alumina ore content of the electrolytic bath falls below critical levels required for
electrolysis, rapid voltage increases occur, which are termed "anode effects." These anode effects cause carbon
from the anode and fluorine from the dissociated molten cryolite bath to combine, thereby producing fugitive
emissions of CF4 and C2F6. In general, the magnitude of emissions for a given smelter and level of production
depends on the frequency and duration of these anode effects. As the frequency and duration of the anode effects
increase, emissions increase.
Since 1990, emissions of CF4 and C2F6 have declined by 80 percent and 76 percent, respectively, to 3.2 Tg CO2 Eq.
of CF4 (0.5 Gg) and 0.64 Tg CO2 Eq. of C2F6 (0.07 Gg) in 2007, as shown in Table 4-70 and Table 4-71. 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. (Note, however, that production and the
frequency and duration of anode effects increased in 2007 compared to 2006.) Since 1990, aluminum production
has declined by 37 percent, while the combined CF4 and C2F6 emission rate (per metric ton of aluminum produced)
has been reduced by 67 percent.
Table 4-70: PFC Emissions from Aluminum Production (Tg CO2 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
2005 2.5 0.4 3.0
2006 2.1 0.4 2.5
2007 3.2 0.6 3.8
Note: Totals may not sum due to independent rounding.
Table 4-71: PFC Emissions from Aluminum Production (Gg)
Year CF4 C2F6
1990 2.4 0.3
1995 1.6 0.2
4-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
2000 1.2 0.1
2005 0.4 +
2006 0.3 +
2007 0.5 0.1
+ Does not exceed 0.05 Gg.
In 2007, U.S. primary aluminum production totaled approximately 2.6 million metric tons, a 12 percent increase
from 2006 production levels. In December 2006, production resumed at the 265,000-t/y smelter in Hannibal, OH,
owned by Ormet Corp (USGS 2007). In 2007, Columbia Falls Aluminum Co. announced it was restarting
additional potlines (USAA 2007), and Alcoa Intalco Works reported increased production from a re-energized
potline at their Ferndale operation (Alcoa Inc. 2007).
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 based on
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:
2A12O3 + 3C -> 4A1 + 3CO2
For prebake smelter technologies, CO2 is also emitted during the anode baking process. These emissions can
account for approximately 10 percent of total process CO2 emissions from prebake smelters.
Depending on the availability of smelter-specific data, the CO2 emitted from electrolysis at each smelter was
estimated from: 1) the smelter's annual anode consumption, 2) the smelter's annual aluminum production and rate of
anode consumption (per ton of aluminum produced) for previous and /or following years, or, 3) the smelter's annual
aluminum production and IPCC default CO2 emission factors. The first approach tracks the consumption and
carbon content of the anode, assuming that all carbon in the anode is converted to CO2. Sulfur, ash, and other
impurities in the anode are subtracted from the anode consumption to arrive at a carbon consumption figure. This
approach corresponds to either the IPCC Tier 2 or Tier 3 method, depending on whether smelter-specific data on
anode impurities are used. The second approach interpolates smelter-specific anode consumption rates to estimate
emissions during years for which anode consumption data are not available. This avoids substantial errors and
discontinuities that could be introduced by reverting to Tier 1 methods for those years. The last approach
corresponds to the IPCC Tier 1 method (2006) and is used in the absence of present or historic anode consumption
data.
The equations used to estimate CO2 emissions in the Tier 2 and 3 methods vary depending on smelter type (IPCC
2006) For Prebake cells, the process formula accounts for various parameters, including net anode consumption,
and the sulfur, ash, and impurity content of the baked anode. For anode baking emissions, the formula accounts for
packing coke consumption, the sulfur and ash content of the packing coke, as well as the pitch content and weight of
baked anodes produced. For Sederberg cells, the process formula accounts for the weight of paste consumed per
metric ton of aluminum produced, and pitch properties, including sulfur, hydrogen, and ash content.
Through the VAIP, anode consumption (and some anode impurity) data have been reported for 1990, 2000, 2003,
2004, 2005, 2006, and 2007. 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
13 out of 14 operating smelters in 2006 and 2007. 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 14 smelters in 2007 and 2006, 1 out of 15 smelters
in 2005, and 5 out of 23 smelters between 1990 and 2003), CO2 emission estimates were estimated using Tier 1
Sederberg and/or Prebake emission factors (metric ton of CO2 per metric ton of aluminum produced) from IPCC
Industrial Processes 4-47
-------
(2006).
Aluminum production data for 13 out of 14 operating smelters were reported under the VAIP in 2007. Between
1990 and 2006, 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 (USAA 2008), 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 minutes/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 2007,
smelter-specific slope coefficients were available and were used for smelters representing between 30 and 94
percent of U.S. primary aluminum production. The percentage changed from year to year as some smelters closed
or changed hands and as the production at remaining smelters fluctuated. For smelters that did not report smelter-
specific slope coefficients, IPCC technology-specific slope coefficients were applied (IPCC 2000, 2006). The slope
coefficients were combined with smelter-specific anode effect data collected by aluminum companies and reported
under the VAIP, to estimate emission factors over time. For 1990 through 2007, 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 2007, smelter-specific production data were available for smelters representing between 30
and 100 percent of U.S. primary aluminum production. (For the years after 2000, this percentage was near the high
end of the range.) Production at non-reporting smelters was estimated by calculating the difference between the
production reported under VAIP and the total U.S. production supplied by USGS or USAA and then allocating this
difference to non-reporting smelters in proportion to their production capacity. Emissions were then aggregated
across smelters to estimate national emissions.
National primary aluminum production data for 2007 were obtained via USAA (USAA 2008). For 1990 through
2001, and 2006 (see Table 4-72) data were obtained from USGS, Mineral Industry Surveys: Aluminum Annual
Report (USGS 1995, 1998, 2000, 2001, 2002, 2007). 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 4-72: Production of Primary Aluminum (Gg)
Year Gg
1990 4,048
1995 3,375
2000 3,668
2005 2,478
2006 2,284
2007 2,560
Uncertainty
The overall uncertainties associated with the 2007 CO2, CF4, and C2F6 emission estimates were calculated using
Approach 2, as defined by IPCC (2006). For CO2, uncertainty was assigned to each of the parameters used to
4-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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estimate CO2 emissions. Uncertainty surrounding reported production data was assumed to be 1 percent (IPCC
2006). For additional variables, such as net C consumption, and sulfur and ash content in baked anodes, estimates
for uncertainties associated with reported and default data were obtained from IPCC (2006). A Monte Carlo
analysis was applied to estimate the overall uncertainty of the CO2 emission estimate for the U.S. aluminum industry
as a whole, and the results are provided below.
To estimate the uncertainty associated with emissions of CF4 and C2F6, the uncertainties associated with three
variables were estimated for each smelter: (1) the quantity of aluminum produced, (2) the anode effect minutes per
cell day (which may be reported directly or calculated as the product of anode effect frequency and anode effect
duration), and, (3) the smelter- or technology-specific slope coefficient. A Monte Carlo analysis was then applied to
estimate the overall uncertainty of the emission estimate for each smelter and for the U.S. aluminum industry as a
whole.
The results of this quantitative uncertainty analysis are summarized in Table 4-73. Aluminum production-related
CO2 emissions were estimated to be between 4.1 and 4.4 Tg CO2 Eq. at the 95 percent confidence level. This
indicates a range of approximately 4 percent below to 4 percent above the emission estimate of 4.3 Tg CO2 Eq.
Also, production-related CF4 emissions were estimated to be between 2.9 and 3.5 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of approximately 10 percent below to 9 percent above the emission estimate
of 3.2 Tg CO2 Eq. Finally, aluminum production-related C2F6 emissions were estimated to be between 0.5 and 0.8
Tg CO2 Eq. at the 95 percent confidence level. This indicates a range of approximately 27 percent below to 32
percent above the emission estimate of 0.6 Tg CO2 Eq.
Table 4-73: Tier 2 Quantitative Uncertainty Estimates for CO2 and PFC Emissions from Aluminum Production (Tg
CO2 Eq. and Percent)
Source Gas 2007 Emission Uncertainty Range Relative to 2007 Emission Estimate"
Estimate
(Tg C02 Eq.) (%)
Aluminum Production
Aluminum Production
Aluminum Production
CO2
CF4
C2F6
(Tg C02 Eq.)
4.3
3.2
0.6
Lower
Bound
4.1
2.9
0.5
Upper
Bound
4.4
3.5
0.8
Lower
Bound
-4%
-10%
-27%
Upper
Bound
+4%
+9%
+32%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
The 2007 emission estimate was developed using site-specific PFC slope coefficients for all but 1 of the 14
operating smelters where default IPCC (2006) slope data was used.
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 (MacNeal et al. 1990, Gariepy and
Dube 1992, Ko et al. 1993, Ten Eyck and Lukens 1996, Zurecki 1996).
Recalculations Discussion
There were no recalculations in the historical time series for this source category.
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-five years.
Industrial Processes 4-49
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The magnesium industry emitted 3.0 Tg CO2 Eq. (0.1 Gg) of SF6 in 2007, representing an increase of approximately
4 percent from 2006 emissions (see Table 4-74). The increase is attributed to higher production by the sand casting
sector in 2007 (USGS 2008a). Counter to the increase in production from sand casting, a combination of high
magnesium prices and reduced demand from the American auto industry has adversely impacted die casting
operations in the United States (USGS 2008b).
Table 4-74: SF6 Emissions from Magnesium Production and Processing (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 5.4 0.2
1995 5.6 0.2
2000 3.0 0.1
2005 2.9 0.1
2006 2.9 0.1
2007 3.0 0.1
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 2007 from primary production, secondary production (i.e., recycling), and die casting were generally
reported by Partnership participants. Partners reported their SF6 consumption, which was assumed to be equivalent
to emissions. When a partner did not report emissions, they were estimated based on the metal processed and
emission rate reported by that partner in previous and (if available) subsequent years. Where data for subsequent
years was not available, metal production and emissions rates were extrapolated based on the trend shown by
partners reporting in the current and previous years.
Emission factors for 2002 to 2006 for sand casting activities were also acquired through the Partnership. For 2007,
the sand casting partner did not report and the reported emission factor from 2005 was utilized as being
representative of the industry. The 1999 through 2007 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 4-75. 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 2007, which accounted for 19 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 2007, partners accounted for all U.S. die casting that was tracked by USGS. In 1999, partners did not
account for all die casting tracked by USGS, and, therefore, it was necessary to estimate the emissions of die casters
who were not partners. Die casters who were not partners were assumed to be similar to partners who cast small
parts. Due to process requirements, these casters consume larger quantities of SF6 per metric ton of processed
magnesium than casters that process large parts. Consequently, emission estimates from this group of die casters
were developed using an average emission factor of 5.2 kg SF6 per metric ton of magnesium. The emission factors
for the other industry sectors (i.e., permanent mold, wrought, and anode casting) were based on discussions with
industry representatives.
Table 4-75: SF6 Emission Factors (kg SF6 per metric ton of magnesium)
Year Die Casting Permanent Mold Wrought Anodes
1999
2000
2001
2002
2.14a
0.72
0.72
0.71
2
2
2
2
1 1
1 1
1 1
1 1
4-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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2003
2004
2005
2006
2007
0.81
0.81
0.76
0.86
0.67
2
2
2
2
2
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.
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 2007 were available from the USGS (USGS 2002, 2003, 2005, 2006, 2007, 2008a). 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 metric ton for 1990 through 1993, and 1.1 kg per metric ton for 1994 through 1997. For die casting, an emission
factor of 4.1 kg per metric ton was used for the period 1990 through 1996. For 1996 through 1998, the emission
factors for primary production and die casting were assumed to decline linearly to the level estimated based on
partner reports in 1999. This assumption is consistent with the trend in SF6 sales to the magnesium sector that is
reported in the RAND survey of major SF6 manufacturers, which shows a decline of 70 percent from 1996 to 1999
(RAND 2002). Sand casting emission factors for 2002 through 2007 were provided by the Magnesium Partnership
participants, and 1990 through 2001 emission factors for this process were assumed to have been the same as the
2002 emission factor. The emission factor for secondary production from 1990 through 1998 was assumed to be
constant at the 1999 average partner value. The emission factors for the other processes (i.e., permanent mold,
wrought, and anode casting), about which less is known, were assumed to remain constant at levels defined in Table
4-75.
Uncertainty
To estimate the uncertainty surrounding the estimated 2007 SF6 emissions from magnesium production and
processing, the uncertainties associated with three variables were estimated (1) emissions reported by magnesium
producers and processors that participate in the SF6 Emission Reduction Partnership, (2) emissions estimated for
magnesium producers and processors that participate in the Partnership but did not report this year, and (3)
emissions estimated for magnesium producers and processors that do not participate in the Partnership. An
uncertainty of 5 percent was assigned to the data reported by each participant in the Partnership. If partners did not
report emissions data during the current reporting year, SF6 emissions data were estimated using available emission
factor and production information reported in prior years; the extrapolation was based on the average trend for
partners reporting in the current reporting year and the year prior. The uncertainty associated with the SF6 usage
estimate generated from the extrapolated emission factor and production information was estimated to be 30 percent;
the lone sand casting partner did not report in the current reporting year and its activity and emission factor was held
constant at 2006 and 2005 levels, respectively, and given an uncertainty of 30 percent. For those industry processes
that are not represented in Partnership, such as permanent mold and wrought casting, SF6 emissions were estimated
using production and consumption statistics reported by USGS and estimated process-specific emission factors (see
Table 4-75). The uncertainties associated with the emission factors and USGS-reported statistics were assumed to
be 75 percent and 25 percent, respectively. Emissions associated with sand casting activities utilized a partner-
reported emission factor with an uncertainty of 75 percent. In general, where precise quantitative information was
not available on the uncertainty of a parameter, a conservative (upper-bound) value was used.
Additional uncertainties exist in these estimates, such as the basic assumption that SF6 neither reacts nor
decomposes during use. The melt surface reactions and high temperatures associated with molten magnesium could
potentially cause some gas degradation. Recent measurement studies have identified SF6 cover gas degradation in
die casting applications on the order of 20 percent (Bartos et al. 2007). Sulfur hexafluoride may also be used as a
cover gas for the casting of molten aluminum with high magnesium content; however, the extent to which this
technique is used in the United States is unknown.
Industrial Processes 4-51
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The results of this Tier 2 quantitative uncertainty analysis are summarized in Table 4-76. SF6 emissions associated
with magnesium production and processing were estimated to be between 2.6 and 3.4 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of approximately 12 percent below to 13 percent above the 2007 emission
estimate of 3.0 Tg CO2 Eq.
Table 4-76: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Magnesium Production and
Processing (Tg CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate
(TgC02Eq.)
Uncertainty Range Relative to Emission Estimate"
(TgC02Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Magnesium Production SF6 3.0
2.6 3.4 -12% +13%
1 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Recalculations Discussion
Newly reported historical data from a secondary remelt partner led to revised SF6 emission estimates in the years
2001 to 2006; the new data resulted in an average decrease of 0.3 Tg CO2 Eq. in emissions for the 2004 to 2006
period, or about 10 percent of total emissions.
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 the 2006 IPCC
Guidelines, 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 for 2006 and 2007 in a limited fashion; because the amounts are currently negligible these emissions are
only being monitored and recorded at this time.
4.17. Zinc Production (IPCC Source Category 2C5)
Zinc production in the United States consists of both primary and secondary processes. Primary production
techniques used in the United States are the electrothermic and electrolytic process while secondary techniques used
in the United States include a range of metallurgical, hydrometallurgical, and pyrometallurgical processes.
Worldwide primary zinc production also employs a pyrometallurgical process using the Imperial Smelting Furnace
process; however, this process is not used in the United States (Sjardin 2003). Of the primary and secondary
processes used in the United States, the electrothermic process results in non-energy CO2 emissions, as does the
Waelz Kiln process—a technique used to produce secondary zinc from electric-arc furnace (EAF) dust (Viklund-
White 2000).
During the electrothermic zinc production process, roasted zinc concentrate and, when available, secondary zinc
products enter a sinter feed where they are burned to remove impurities before entering an electric retort furnace.
Metallurgical coke added to the electric retort furnace reduces the zinc oxides and produces vaporized zinc, which is
then captured in a vacuum condenser. This reduction process produces non-energy CO2 emissions (Sjardin 2003).
The electrolytic zinc production process does not produce non-energy CO2 emissions.
In the Waelz Kiln process, EAF dust, which is captured during the recycling of galvanized steel, enters a kiln along
with a reducing agent—often metallurgical coke. When kiln temperatures reach approximately 1100-1200°C, zinc
fumes are produced, which are combusted with air entering the kiln. This combustion forms zinc oxide, which is
collected in a baghouse or electrostatic precipitator, and is then leached to remove chloride and fluoride. Through
this process, approximately 0.33 ton of zinc is produced for every ton of EAF dust treated (Viklund-White 2000).
4-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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In 2007, U.S. primary and secondary zinc production totaled 519,221 metric tons (Tokin 2009). The resulting
emissions of CO2 from zinc production in 2007 were estimated to be 0.5 Tg CO2 Eq. (530 Gg) (see Table 4-77). All
2007 CO2 emissions result from secondary zinc production.
Table 4-77: CO2 Emissions from Zinc Production (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 0.9 949
1995 1.0 1,013
2000 1.1 1,140
2005 0.5 465
2006 0.5 529
2007 0.5 530
After a gradual increase in total emissions from 1990 to 2000, largely due to an increase in secondary zinc
production, emissions have decreased in recent years due to the closing of an electrothermic-process zinc plant in
Monaca, PA (USGS 2004). Emissions for 2007, which are nearly half that of 1990 (44 percent), remained constant
from 2006 due to the use of proxied data for secondary zinc production.
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:
1.19 metric tons coke 0.84 metric tons C 3.67 metric tons CO2 3.66 metric tons CO2
f^f^ — x x —
Waelz Kiln metric tons zinc metric ton coke metric ton C metric ton 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.
Industrial Processes 4-53
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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:
0.4 metric tons coke 0.84 metric tons C 3.67 metric tons CO2 1.23 metric tons CO2
EF = x x =
EAF Dust metric tons EAF dust metric ton coke metric ton C metric ton 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 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 4-78) were obtained
through the USGS Mineral Yearbook: Zinc (USGS 1994 through 2008). Preliminary data for 2007 primary
production and production from scrap were obtained from the USGS Mineral Commodity Specialist (Tolcin 2009).
Because data for 2007 secondary zinc production were unavailable, 2006 data were used.
Table 4-78: Zinc Production (Metric Tons)
Year Primary Secondary
1990 262,704 341,400
1995 231,840 353,000
2000 227,800 440,000
2005 191,120 349,000
2006 113,000 397,000
2007 121,221 398,000
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
4-54 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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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 4-79. 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 4-79: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Zinc Production (Tg CO2 Eq. and
Percent)
2007 Emission
Source Gas Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Zinc Production CO2 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.
4.18. Lead Production (IPCC Source Category 2C5)
Lead production in the United States consists of both primary and secondary processes—both of which emit CO2
(Sjardin 2003). Primary lead production, in the form of direct smelting, mostly occurs at plants located in Alaska
and Missouri, though to a lesser extent in Idaho, Montana, and Washington. Secondary production largely involves
the recycling of lead acid batteries at approximately 18 separate smelters located in 11 states (USGS 2008 and
2009). Secondary lead production has increased in the United States over the past decade while primary lead
production has decreased. In 2007, secondary lead production accounted for approximately 91 percent of total lead
production (USGS 2009).
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
decreased by 20 percent from 2006 to 2007 and has decreased by 68 percent since 1990 (USGS 2009, USGS 1995).
At last reporting, approximately 93 percent of refined lead production is produced primarily from scrapped lead acid
batteries (USGS 2009). Similar to primary lead production, CO2 emissions result when a reducing agent, usually
metallurgical coke, is added to the smelter to aid in the reduction process (Sjardin 2003). U.S. secondary lead
production decreased from 2006 to 2007 by 2 percent, and has increased by 28 percent since 1990 (USGS 2009,
USGS 1995).
At last reporting, the United States was the third largest mine producer of lead in the world, behind China and
Australia, accounting for 12 percent of world production in 2007 (USGS 2009). In 2007, U.S. primary and
secondary lead production totaled 1,303,000 metric tons (USGS 2009). The resulting emissions of CO2 from 2007
production were estimated to be 0.3 Tg CO2 Eq. (267 Gg) (see Table 4-80). The majority of 2007 lead production is
from secondary processes, which account for 88 percent of total 2007 CO2 emissions.
Table 4-80: CO2 Emissions from Lead Production (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 0.3 285
1995 0.3 298
2000 0.3 311
2005 0.3 266
2006 0.3 270
2007 0.3 267
Industrial Processes 4-55
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After a gradual increase in total emissions from 1990 to 2000, total emissions have decreased by six percent since
1990, largely due to a decrease in primary production (68 percent since 1990) and a transition within the United
States from primary lead production to secondary lead production, which is less emissive than primary production,
although the sharp decrease leveled off in 2005 (USGS 2009, 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 2007 activity data for primary and secondary lead production (see Table 4-81) were obtained
through the USGS Mineral Yearbook: Lead (USGS 1994 through 2009).
Table 4-81: Lead Production (Metric Tons)
Year Primary Secondary
1990 404,000 922,000
1995 374,000 1,020,000
2000 341,000 1,130,000
2005 143,000 1,150,000
2006 153,000 1,160,000
2007 123,000 1,180,000
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 4-82. 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 17 percent above the emission estimate of 0.3 Tg CO2 Eq.
Table 4-82: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lead Production (Tg CO2 Eq. and
Percent)
2007 Emission
Source Gas Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Lead Production CO2 0.3
0.2 0.3 -16% +17%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
4.19. HCFC-22 Production (IPCC Source Category 2E1)
Trifluoromethane (HFC-23 or CHF3) is generated as a by-product during the manufacture of chlorodifluoromethane
(HCFC-22), which is primarily employed in refrigeration and air conditioning systems and as a chemical feedstock
for manufacturing synthetic polymers. Between 1990 and 2000, U.S. production of HCFC-22 increased
4-56 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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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.103 Feedstock
production, however, is permitted to continue indefinitely.
HCFC-22 is produced by the reaction of chloroform (CHC13) and hydrogen fluoride (HF) in the presence of a
catalyst, SbCl5. The reaction of the catalyst and HF produces SbClxFy, (where x + y = 5), which reacts with
chlorinated hydrocarbons to replace chlorine atoms with fluorine. The HF and chloroform are introduced by
submerged piping into a continuous-flow reactor that contains the catalyst in a hydrocarbon mixture of chloroform
and partially fluorinated intermediates. The vapors leaving the reactor contain HCFC-21 (CHC12F), HCFC-22
(CHC1F2), HFC-23 (CHF3), HC1, chloroform, and HF. The under-fluorinated intermediates (HCFC-21) and
chloroform are then condensed and returned to the reactor, along with residual catalyst, to undergo further
fluorination. The final vapors leaving the condenser are primarily HCFC-22, HFC-23, HC1 and residual HF. The
HC1 is recovered as a useful byproduct, and the HF is removed. Once separated from HCFC-22, the HFC-23 may
be released to the atmosphere, recaptured for use in a limited number of applications, or destroyed.
Emissions of HFC-23 in 2007 were estimated to be 17.0 Tg CO2 Eq. (1.2 Gg) (Table 4-83). This quantity represents
a 23 percent increase from 2006 emissions and a 53 percent decline from 1990 emissions. The increase from 2006
emissions was caused by a 5 percent increase in HCFC-22 production and a 17 percent increase in the HFC-23
emission rate. The decline from 1990 emissions is due to a 60 percent decrease in the HFC-23 emission rate since
1990. The decrease is 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.
Table 4-83: HFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and Gg)
Year TgCO2Eq. Gg
1990 36.4 +
1995 33.0 3
2000 28.6 3
2005 15.8 1
2006 13.8 1
2007 17.0 1
Methodology
To estimate their emissions of HFC-23, five of the eight HCFC-22 plants that have operated in the U.S. since 1990
use (or, for those plants that have closed, used) methods comparable to the Tier 3 methods in the 2006 IPCC
Guidelines (IPCC 2006). The other three plants, the last of which closed in 1993, used methods comparable to the
Tier 1 method in the 2006 IPCC Guidelines. Emissions from these three plants have been recalculated using the
recommended emission factor for unoptimized plants operating before 1995 (0.04 kg HCFC-23/kg HCFC-22
produced). (This recalculation was reflected in the 1990 through 2006 inventory submission.)
The five plants that have operated since 1994 measured concentrations of HFC-23 to estimate their emissions of
HFC-23. Plants using thermal oxidation to abate their HFC-23 emissions monitor the performance of their oxidizers
to verify that the HFC-23 is almost completely destroyed. Plants that release (or historically have released) some of
their byproduct HFC-23 periodically measure HFC-23 concentrations in the output stream using gas
chromatography. This information is combined with information on quantities of products (e.g., HCFC-22) to
103 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]
Industrial Processes 4-57
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estimate HFC-23 emissions.
In most years, including 2008, an industry association aggregates and reports to EPA country-level estimates of
HCFC-22 production and HFC-23 emissions (ARAP 1997, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,
2008). However, in 1997 and 2008, EPA (through a contractor) performed comprehensive reviews of plant-level
estimates of HFC-23 emissions and HCFC-22 production (RTI1997; RTI2008). These reviews enabled EPA to
review, update, and where necessary, correct U.S. totals, and also to perform plant-level uncertainty analyses
(Monte-Carlo simulations) for 1990, 1995, 2000, 2005, and 2006. Estimates of annual U.S. HCFC-22 production
are presented in Table 4-84.
Table 4-84: HCFC-22 Production (Gg)
Year Gg
1990 139
1995 155
2000 186
2005 156
2006 154
2007 162
Uncertainty
The uncertainty analysis presented in this section was based on a plant-level Monte Carlo simulation for 2006. The
Monte Carlo analysis used estimates of the uncertainties in the individual variables in each plant's estimating
procedure. This analysis was based on the generation of 10,000 random samples of model inputs from the
probability density functions for each input. A normal probability density function was assumed for all
measurements and biases except the equipment leak estimates for one plant; a log-normal probability density
function was used for this plant's equipment leak estimates. The simulation for 2006 yielded a 95-percent
confidence interval for U.S. emissions of 6.8 percent below to 9.6 percent above the reported total.
Because EPA did not have access to plant-level emissions data for 2007, the relative errors yielded by the Monte
Carlo simulation for 2006 were applied to the U.S. emission estimate for 2007. The resulting estimates of absolute
uncertainty are likely to be accurate because (1) the methods used by the three plants to estimate their emissions are
not believed to have changed significantly since 2006, (2) the distribution of emissions among the plants is not
believed to have changed significantly since 2006 (one plant continues to dominate emissions), and (3) the country-
level relative errors yielded by the Monte Carlo simulations for 2005 and 2006 were very similar, implying that
these errors are not sensitive to small, year-to-year changes.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-85. HFC-23 emissions from
HCFC-22 production were estimated to be between 15.8 and 18.6 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 17.0 Tg CO2
Eq.
Table 4-85: Quantitative Uncertainty Estimates for HFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and
Percent)
2007 Emission
Source Gas Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
HCFC-22 Production HFC-23 17.0
15.8 18.6 -7% +10%
1 Range of emissions reflects a 95 percent confidence interval.
4-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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4.20. Substitution of Ozone Depleting Substances (IPCC Source Category 2F)
Hydrofluorocarbons (HFCs) and perfluorocarbons (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 1990.104 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
4-86 and Table 4-87.
Table 4-86: Emissions of HFCs and PFCs from OPS Substitutes (Tg CO2 Eq.)
Gas 1990
HFC-23 +
HFC-32 +
HFC-125 +
HFC-134a +
HFC-143a +
HFC-236fa +
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
2005
+
0.4
10.3
70.5
12.2
0.8
+
5.9
100.0
2006
+
0.6
12.3
70.7
14.4
0.8
+
6.2
105.0
2007
+
0.9
14.7
68.6
16.7
0.9
+
6.5
108.3
+ Does not exceed 0.05 Tg CO2 Eq.
* Others include HFC-152a, HFC-227ea, HFC-245fa, HFC-431 Omee, 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 4-87: Emissions of HFCs and PFCs from OPS Substitution (Mg)
Gas 1990
HFC-23 +
HFC-32 +
HFC-125 +
HFC-134a +
HFC-143a +
HFC-236fa +
CF4 +
Others* M
1995
+
+
291
19,537
132
36
+
M
2000
1
44
1,873
44,011
1,089
85
1
M
2005
1
562
3,675
54,226
3,200
125
2
M
2006
1
913
4,394
54,362
3,782
131
2
M
2007
1
1,325
5,253
52,782
4,402
136
2
M
M (Mixture of Gases)
+ Does not exceed 0.5 Mg
* Others include HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee, C4F!0, and PFC/PFPEs, the latter being a proxy for a
diverse collection of PFCs and perfluoropoly ethers (PFPEs) employed for solvent applications.
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 suchasR-404A.105 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.
The use and subsequent emissions of HFCs and PFCs as ODS substitutes has been increasing from small amounts in
1990 to 108.3 Tg CO2 Eq. in 2007. This increase was in large part the result of efforts to phase out CFCs and other
104 [42 U.S.C § 7671, CAA § 601]
105 R.4Q4A contains HFC-125, HFC-143a, and HFC-134a.
Industrial Processes 4-59
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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 4-88 presents HFCs and PFCs emissions by end-use sector for 1990 through 2007. The end-use sectors that
contributed the most toward emissions of HFCs and PFCs as ODS substitutes in 2007 include refrigeration and air-
conditioning (97.5 Tg CO2 Eq., or approximately 90 percent), aerosols (6.2 Tg CO2 Eq., or approximately 6
percent), and foams (2.6 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 (52.9 Tg CO2 Eq.), followed by
refrigerated transport and retail food. Each of the end-use sectors is described in more detail below.
Table 4-88: Emissions of HFCs and PFCs from ODS Substitutes (Tg CO2 Eq.) by Sector
Gas 1990 1995 2000 2005 2006 2007
Refrigeration/ Air Conditioning
Aerosols
Foams
Solvents
Fire Protection
Total
+ 19.3
+ 8.1
+ +
+ 0.9
+ +
+ 28.5
58.6
10.1
+
2.1
+
71.2
90.1
5.9
2.2
1.3
0.5
100.0
94.6
6.1
2.4
1.3
0.6
105.0
97.5
6.2
2.6
1.3
0.7
108.3
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
4-60 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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cold storage applications. In addition, HFC-152a is used to produce polystyrene sheet/board foam, which is used in
food packaging and building insulation. Emissions of blowing agents occur when the foam is manufactured as well
as during the foam lifetime and at foam disposal, depending on the particular foam type.
Solvents
CFCs, methyl chloroform (1,1,1-trichloroethane or TCA), and to a lesser extent carbon tetrachloride (CC14) were
historically used as solvents in a wide range of cleaning applications, including precision, electronics, and metal
cleaning. Since their phaseout, metal cleaning end-use applications have primarily transitioned to non-fluorocarbon
solvents and not-in-kind processes. The precision and electronics cleaning end-uses have transitioned in part to high-
GWP gases, due to their high reliability, excellent compatibility, good stability, low toxicity, and selective solvency.
These applications rely on HFC-4310mee, HFC-365mfc, HFC-245fa, and to a lesser extent, PFCs. Electronics
cleaning involves removing flux residue that remains after a soldering operation for printed circuit boards and other
contamination-sensitive electronics applications. Precision cleaning may apply to either electronic components or to
metal surfaces, and is characterized by products, such as disk drives, gyroscopes, and optical components, that
require a high level of cleanliness and generally have complex shapes, small clearances, and other cleaning
challenges. The use of solvents yields fugitive emissions of these HFCs and PFCs.
Fire Protection
Fire protection applications include portable fire extinguishers ("streaming" applications) that originally used halon
1211, and total flooding applications that originally used halon 1301, as well as some halon 2402. Since the
production and sale of halons were banned in the United States in 1994, the halon replacement agent of choice in the
streaming sector has been dry chemical, although HFC-236ea is also used to a limited extent. In the total flooding
sector, HFC-227ea has emerged as the primary replacement for halon 1301 in applications that require clean agents.
Other HFCs, such as HFC-23, HFC-236fa, and HFC-125, are used in smaller amounts. The majority of HFC-227ea
in total flooding systems is used to protect essential electronics, as well as in civil aviation, military mobile weapons
systems, oil/gas/other process industries, and merchant shipping. As fire protection equipment is tested or
deployed, emissions of these HFCs 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, two new end-use were included in the uncertainty estimate—polyurethane
flexible integral skin foam and residential unitary air conditioners. Any end-uses included in previous years'
Industrial Processes 4-61
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uncertainty analysis were included in the current uncertainty analysis, whether or not those end-uses were included
in the top 97 percent of emissions from ODS Substitutes.
In order to calculate uncertainty, functional forms were developed to simplify some of the complex "vintaging"
aspects of some end-use sectors, especially with respect to refrigeration and air-conditioning, and to a lesser degree,
fire extinguishing. These sectors calculate emissions based on the entire lifetime of equipment, not just equipment
put into commission in the current year, thereby necessitating simplifying equations. The functional forms used
variables that included growth rates, emission factors, transition from ODSs, change in charge size as a result of the
transition, disposal quantities, disposal emission rates, and either stock for the current year or original ODS
consumption. Uncertainty was estimated around each variable within the functional forms based on expert
judgment, and a Monte Carlo analysis was performed. The most significant sources of uncertainty for this source
category include the emission factors for mobile air-conditioning and refrigerated transport, as well as the percent of
non-MDI aerosol propellant that is HFC-152a.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-89. Substitution of ozone
depleting substances HFC and PFC emissions were estimated to be between 97.5 and 115.2 Tg CO2 Eq. at the 95
percent confidence level. This indicates a range of approximately 8 percent below to 9 percent above the emission
estimate of 105.9 Tg CO2 Eq.
Table 4-89: Tier 2 Quantitative Uncertainty Estimates for HFC and PFC Emissions from ODS Substitutes (Tg CO2
Eq. and Percent)
2007 Emission
Source Gases Estimate
(Tg C02 Eq.)a
Substitution of Ozone
Depleting HFCs and
Substances PFCs 105.9
Uncertainty Range Relative to Emission Estimate1"
(Tg C02 Eq.) (%)
Lower
Bound
97.5
Upper
Bound
115.2
Lower
Bound
-8%
Upper
Bound
+9%
a 2007 Emission estimates and the uncertainty range presented in this table correspond to aerosols, foams, solvents, fire
extinguishing agents, and refrigerants, but not for other remaining categories. Therefore, because the uncertainty associated with
emissions from "other" ODS substitutes was not estimated, they were exclude in the estimates reported in this table.
b Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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 1.2Tg CO2 Eq. (1.2 percent) in HFC and PFC emissions from the
substitution of ozone depleting substances for the period 1990 through 2007. The primary change was a revision in
the non-MDI aerosol sector, where a fraction of the market formerly assumed to use HFC-134a (with a GWP of
1,300) was discovered to be transitioning more quickly to HFC-152a (with a GWP of 140).
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
4-62 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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waste streams and, unless emission abatement systems are employed, into the atmosphere. PECVD chambers, used
for depositing dielectric films, are cleaned periodically using fluorinated and other gases. During the cleaning cycle
the gas is converted to fluorine atoms in plasma, which etches away residual material from chamber walls,
electrodes, and chamber hardware. Undissociated fluorinated gases and other products pass from the chamber to
waste streams and, unless abatement systems are employed, into the atmosphere. In addition to emissions of
unreacted gases, some fluorinated compounds can also be transformed in the plasma processes into different
fluorinated compounds which are then exhausted, unless abated, into the atmosphere. For example, when C2F6 is
used in cleaning or etching, CF4 is generated and emitted as a process by-product. Besides dielectric film etching
and PECVD chamber cleaning, much smaller quantities of fluorinated gases are used to etch poly silicon films and
refractory metal films like tungsten.
For 2007, total weighted emissions of all fluorinated greenhouse gases by the U.S. semiconductor industry were
estimated to be 4.7 Tg CO2 Eq. Combined emissions of all fluorinated greenhouse gases are presented in Table 4-90
and Table 4-9 Ibelow for years 1990, 1995, 2000 and the period 2005 to 2007. The rapid growth of this industry and
the increasing complexity (growing number of layers)106 of semiconductor products led to an increase in emissions
of 150 percent between 1990 and 1999, when emissions peaked at 7.2 Tg CO2 Eq. The emissions growth rate began
to slow after 1998, and emissions declined by 35 percent between 1999 and 2007. Together, industrial growth and
adoption of emissions reduction technologies, including but not limited to abatement technologies, resulted in a net
increase in emissions of 63 percent between 1990 and 2007.
Table 4-90: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg CO2 Eq.)
Year 1990 1995 2000 2005 2006 2007
CF4
C2F6
C3F8
C4F8
HFC-23
SF6
NFS*
Total
0.7
1.5
0.0
0.0
0.2
0.5
0.0
2.9
1.3
2.5
0.0
0.0
0.3
0.9
0.1
4.9
1.8
3.0
0.1
0.0
0.3
1.1
0.2
6.2
1.1
2.0
0.0
0.1
0.2
1.0
0.4
4.4
1.2
2.2
0.0
0.1
0.3
1.0
0.7
4.7
1.3
2.3
0.0
0.1
0.3
0.8
0.5
4.7
Note: Totals may not sum due to independent rounding.
' NF3 emissions are presented for informational purposes, using the AR4 GWP of 17,200, and are not included in totals.
Table 4-91: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Mg)
Year
CF4
C2F6
C3F8
C4F8
HFC-23
SF6
NF3
1990
115
160
0
0
15
22
3
1995
193
272
0
0
25
37
3
2000
281
321
18
0
23
45
11
2005
168
216
5
13
18
40
26
2006
181
240
5
13
22
40
40
2007
195
246
6
7
22
34
30
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).107 The availability and applicability of
106 Complexity is a term denoting the circuit required to connect the active circuit elements (transistors) on a chip. Increasing
miniaturization, for the same chip size, leads to increasing transistor density, which, in turn, requires more complex
interconnections between those transistors. This increasing complexity is manifested by increasing the levels (i.e., layers) of
wiring, with each wiring layer requiring fluorinated gas usage for its manufacture.
107 A Partner refers to a participant in the U.S. EPA PFC Reduction/Climate Partnership for the Semiconductor Industry.
Through a Memorandum of Understanding (MoU) with the EPA, Partners voluntarily report their PFC emissions to the EPA by
Industrial Processes 4-63
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Partner data differs across the 1990 through 2007 time series. Consequently, emissions from semiconductor
manufacturing were estimated using four distinct methods, one each for the periods 1990 through 1994, 1995
through 1999, 2000 through 2006, and 2007.
1990 through 1994
From 1990 through 1994, Partnership data was unavailable and emissions were modeled using the PEVM (Burton
and Beizaie 2001).108 1990 to 1994 emissions are assumed to be uncontrolled, since reduction strategies such as
chemical substitution and abatement were yet developed.
PEVM is based on the recognition that PFC emissions from semiconductor manufacturing vary with (1) the number
of layers that comprise different kinds of semiconductor devices, including both silicon wafer and metal
interconnect layers, and (2) silicon consumption (i.e., the area of semiconductors produced) for each kind of device.
The product of these two quantities, Total Manufactured Layer Area (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 manufactured feature size), 109 and (2) product type (discrete, memory or
logic). *10 For each linewidth technology, a weighted average number of layers is estimated using VLSI product-
specific worldwide silicon demand data in conjunction with complexity factors (i.e., the number of layers per
Integrated Circuit (1C)) specific to product type (Burton and Beizaie 2001, ITRS 2007). PEVM derives historical
consumption of silicon (i.e., square inches) by linewidth technology from published data on annual wafer starts and
average wafer size (VLSI Research, Inc. 2007).
The emission factor in PEVM is the average of four historical emission factors, each derived by dividing the total
annual emissions reported by the Partners for each of the four years between 1996 and 1999 by the total TMLA
estimated for the Partners in each of those years. Over this period, the emission factors varied relatively little (i.e.,
the relative standard deviation for the average was 5 percent). Since Partners are believed not to have applied
significant emission reduction measures before 2000, the resulting average emission factor reflects uncontrolled
emissions. The emission factor is used to estimate world uncontrolled emissions using publicly available data on
world silicon consumption.
1995 through 1999
For 1995 through 1999, total U.S. emissions were extrapolated from the total annual emissions reported by the
Partners (1995 through 1999). Partner-reported emissions are considered more representative (e.g., in terms of
capacity utilization in a given year) than PEVM estimated emissions, and are used to generate total U.S. emissions
when applicable. The emissions reported by the Partners were divided by the ratio of the total capacity of the plants
operated by the Partners and the total capacity of all of the semiconductor plants in the United States; this ratio
represents the share of capacity attributable to the Partnership. This method assumes that Partners and non-Partners
have identical capacity utilizations and distributions of manufacturing technologies. Plant capacity data is contained
in the World Fab Forecast (WFF) database and its predecessors, which is updated quarterly (Semiconductor
way of a third party, which aggregates the emissions.
108 Various versions of the PEVM exist to reflect changing industrial practices. From 1990 to 1994 emissions estimates are from
PEVM vl.O, completed in September 1998. The emission factor used to estimate 1990 to 1994 emissions is an average of the
1995 and 1996 emissions factors, which were derived from Partner reported data for those years.
109 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).
110 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.
4-64 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Equipment and Materials Industry 2007).
2000 through 2006
The emission estimate for the years 2000 through 2006—the period during which Partners began the consequential
application of PFC-reduction measures—was estimated using a combination of Partner reported emissions and
PEVM modeled emissions. The emissions reported by Partners for each year were accepted as the quantity emitted
from the share of the industry represented by those Partners. Remaining emissions, those from non-Partners, were
estimated using PEVM and the method described above. This is because non-Partners are assumed not to have
implemented any PFC-reduction measures, and PEVM models emissions without such measures. The portion of the
U.S. total attributed to non-Partners is obtained by multiplying PEVM's total U.S. emissions figure by the non-
Partner share of U. S. total silicon capacity for each year as described above.111'112 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, ITRS, 2007 and Semiconductor Equipment and Materials Industry 2008).113'114115
2007
For the year 2007, emissions were also estimated using a combination of Partner reported emissions and PEVM
modeled emissions; however, two improvements were made to the estimation method employed for the previous
years in the time series. First, the 2007 emission estimates account for the fact that Partners and non-Partners
employ different distributions of manufacturing technologies, with the Partners using manufacturing technologies
with greater transistor densities and therefore greater numbers of layers. Had the method used to estimate the 2000
through 2006 emissions (described above) been employed, the emissions estimated for 2007 would have been 1.5
percent higher because the estimate of uncontrolled non-Partner emissions would have been overstated by 2.5
percent.116
111 This approach assumes that the distribution of linewidth technologies is the same between Partners and non-Partners. As
discussed in the description of the method used to estimate 2007 emissions, this is not always the case.
112 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.
113 Special attention was given to the manufacturing capacity of plants that use wafers with 300 mm diameters because the actual
capacity of these plants is ramped up to design capacity, typically over a 2-3 year period. To prevent overstating estimates of
partner-capacity shares from plants using 300 mm wafers, design capacities contained in WFW were replaced with estimates of
actual installed capacities for 2004 published by Citigroup Smith Barney (2005). Without this correction, the partner share of
capacity would be overstated, by approximately 5 percentage points. For perspective, approximately 95 percent of all new
capacity additions in 2004 used 300 mm wafers and by year-end those plants, on average, could operate at approximately 70
percent of the design capacity. For 2005, actual installed capacities were estimated using an entry in the World Fab Watch
database (April 2006 Edition) called "wafers/month, 8-inch equivalent", which denoted the actual installed capacity instead of the
fully-ramped capacity. For 2006, actual installed capacities of new fabs were estimated using an average monthly ramp rate of
1100 wafer starts per month (wspm) derived from various sources such as semiconductor fabtech, industry analysts, and articles
in the trade press. The monthly ramp rate was applied from the first-quarter of silicon volume (FQS V) to determine the average
design capacity over the 2006 period.
114 In 2006, the industry trend in co-ownership of manufacturing facilities continued. Several manufacturers, who are Partners,
now operate fabs with other manufacturers, who in some cases are also Partners and in other cases not Partners. Special attention
was given to this occurrence when estimating the Partner and non-Partner shares of U.S. manufacturing capacity.
115 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.
116 EPA considered applying this change to years before 2007, but found that it would be difficult due to the large amount of
data (i.e., technology-specific global and non-Partner TMLA) that would have to be examined and manipulated for each year.
This effort did not appear to be justified given the relatively small impact of the improvement on the total estimate for 2007 and
the fact that the impact of the improvement would likely be lower for earlier years because the estimated share of emissions
Industrial Processes 4-65
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Second, the scope of the 2007 estimate is expanded relative to the estimates for the years 2000 through 2006 to
include emissions from Research and Development fabs. This was feasible through the use of more detailed data
published in the World Fab Forecast. PEVM databases are updated annually as described above. The published
world average capacity utilization for 2007 was used for production fabs while for R&D fabs, a 20 percent figure
was assumed. Inclusion of R&D fabs increased the estimated emissions by less than one percent.
Gas-Specific Emissions
Two different approaches were also used to estimate the distribution of emissions of specific fluorinated gases.
Before 1999, when there was no consequential adoption of fluorinated-gas-reducing measures, a fixed distribution
of fluorinated-gas-use was assumed to apply to the entire U.S. industry. This distribution was based upon the
average fluorinated-gas purchases by semiconductor manufacturers during this period and the application of IPCC
default emission factors for each gas (Burton and Beizaie 2001). For the 2000 through 2007 period, the 1990
through 1999 distribution was assumed to apply to the non-Partners. Partners, however, began reporting gas-
specific emissions during this period. Thus, gas-specific emissions for 2000 through 2007 were estimated by adding
the emissions reported by the Partners to those estimated for the non-Partners.
Data Sources
Partners estimate their emissions using a range of methods. For 2007, it is assumed that most Partners used a
method at least as accurate as the IPCC's Tier 2a Methodology, recommended in the IPCC Guidelines for National
Greenhouse Inventories (2006). The Partners with relatively high emissions use leading-edge manufacturing
technology, the newest process equipment. When purchased, this equipment is supplied with fluorinated-gas
emission factors, measured using industry standard guidelines (International Sematech 2006). The larger emitting
Partners likely use these process-specific emission factors instead of the somewhat less representative default
emission factors provided in the IPCC guidelines. Data used to develop emission estimates are attributed in part to
estimates provided by the members of the Partnership, and in part from data obtained from PEVM estimates.
Estimates of operating plant capacities and characteristics for Partners and non-Partners were derived from the
Semiconductor Equipment and Materials Industry (SEMI) World Fab Forecast (formerly World Fab Watch)
database (1996 through 2008). Estimates of world average capacity utilizations for 2007 were obtained from
Semiconductor International Capacity Statistics (SICAS). Estimates of silicon consumed by linewidth from 1990
through 2007 were derived from information from VLSI Research (2008), and the number of layers per linewidth
was obtained from International Technology Roadmap for Semiconductors: 2006 Update (Burton and Beizaie 2001,
ITRS 2007, ITRS 2008).
Uncertainty
A quantitative uncertainty analysis of this source category was performed using the IPCC-recommended Tier 2
uncertainty estimation methodology, the Monte Carlo Stochastic Simulation technique. The equation used to
estimate uncertainty is:
U.S. emissions = ^Partnership gas-specific submittals + [(non-Partner share of World TMLA) x (PEVM Emission
Factor x World TMLA)]
The Monte Carlo analysis results presented below relied on estimates of uncertainty attributed to the four quantities
on the right side of the equation. Estimates of uncertainty for the four quantities were in turn developed using the
estimated uncertainties associated with the individual inputs to each quantity, error propagation analysis, Monte
Carlo simulation and expert judgment. The relative uncertainty associated with World TMLA estimate in 2007 is
±9 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.117 A relative
accounted for by non-Partners is growing as Partners continue to implement emission-reduction efforts.
117 Error propagation resulted in Partnership gas-specific uncertainties ranging from 18 to 36 percent
4-66 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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error of approximately 10 percent was estimated for the PEVM emission factor, based on the standard deviation of
the 1996 to 1999 emission factors.118 All estimates of uncertainties are given at 95-percent confidence intervals.
In developing estimates of uncertainty, consideration was also given to the nature and magnitude of the potential
bias that World activity data (i.e., World TMLA) might have in its estimates of the number of layers associated with
devices manufactured at each technology node. The result of a brief analysis indicated that U.S. TMLA overstates
the average number of layers across all product categories and all manufacturing technologies for 2004 by 0.12
layers or 2.9 percent. The same upward bias is assumed for World TMLA, and is represented in the uncertainty
analysis by deducting the absolute bias value from the World activity estimate when it is incorporated into the
Monte Carlo analysis.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-92. The emissions estimate for
total U.S. PFC emissions from semiconductor manufacturing were estimated to be between 4.7 and 5.7 Tg CO2 Eq.
at a 95 percent confidence level. This range represents 9 percent below to 9 percent above the 2007 emission
estimate of 5.2 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 4-92: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from Semiconductor
Manufacture (Tg CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate" Uncertainty Range Relative to Emission Estimate1"
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Semiconductor HFC, PFC,
Manufacture and SF6 5.2
Lower
Bound0
4.7
Upper
Bound0
5.7
Lower
Bound
-9%
Upper
Bound
9%
a Because the uncertainty analysis covered all emissions (including NF3), the emission estimate presented here does not match
that shown in Table 4-90.
b Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
0 Absolute lower and upper bounds were calculated using the corresponding lower and upper bounds in percentages.
Planned Improvements
With the exception of possible future updates to emission factors, the method to estimate non-Partner related
emissions (i.e., PEVM) is not expected to change. Future improvements to the national emission estimates will
primarily be associated with determining the portion of national emissions to attribute to Partner report totals (about
80 percent in recent years) and improvements in estimates of non-Partner totals. As the nature of the Partner reports
change through time and industry-wide reduction efforts increase, consideration will be given to what emission
reduction efforts—if any—are likely to be occurring at non-Partner facilities. Currently, none are assumed to occur.
Another point of consideration for future national emissions estimates is the inclusion of PFC emissions from heat
transfer fluid (HTF) loss to the atmosphere and the production of photovoltaic cells (PVs). Heat transfer fluids, of
which some are liquid perfluorinated compounds, are used during testing of semiconductor devices and,
increasingly, are used to manage heat during the manufacture of semiconductor devices. Evaporation of these fluids
is a source of emissions (EPA 2006). PFCs are also used during manufacture of PV cells that use silicon
technology, specifically, crystalline, polycrystalline and amorphous silicon technologies. PV manufacture is
growing in the United States, and therefore may be expected to constitute a growing share of U.S. PFC emissions
from the electronics sector.
4.22. Electrical Transmission and Distribution (IPCC Source Category 2F7)
T 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
118 The average of 1996 to 1999 emission factor is used to derive the PEVM emission factor.
Industrial Processes 4-67
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industry in the United States since the 1950s because of its dielectric strength and arc-quenching characteristics. It
is used in gas-insulated substations, circuit breakers, and other switchgear. Sulfur hexafluoride has replaced
flammable insulating oils in many applications and allows for more compact substations in dense urban areas.
Fugitive emissions of SF6 can escape from gas-insulated substations and switchgear through seals, especially from
older equipment. The gas can also be released during equipment manufacturing, installation, servicing, and
disposal. Emissions of SF6 from equipment manufacturing and from electrical transmission and distribution systems
were estimated to be 12.7 Tg CO2 Eq. (0.5 Gg) in 2007. This quantity represents a 53 percent decrease from the
estimate for 1990 (see Table 4-93 and Table 4-94). This decrease is believed to have two causes: a sharp increase in
the price of SF6 during the 1990s and a growing awareness of the environmental impact of SF6 emissions through
programs such as EPA's SF6 Emission Reduction Partnership for Electric Power Systems.
Table 4-93: SF6 Emissions from Electric Power Systems and Electrical Equipment Manufacturers (Tg CO2 Eq.)
Year Electric Power Electrical Equipment Total
Systems Manufacturers
1990 26.5 0.3
1995 21.0 0.5 21.6
2000 14.4 0.7 15.1
2005 13.2 0.8 14.0
2006 12.4 0.8 13.2
2007 12.0 0/7 12.7
Table 4-94: SF6 Emissions from Electric Power Systems and Electrical Equipment Manufacturers (Gg)
Year Emissions
1990 1.1
1995 0.9
2000 0.6
2005 0.6
2006 0.6
2007 0.5
Methodology
The estimates of emissions from electric transmission and distribution are comprised of emissions from electric
power systems and emissions from the manufacture of electrical equipment. The methodologies for estimating both
sets of emissions are described below.
1999 through 2007 Emissions from Electric Power Systems
Emissions from electric power systems from 1999 to 2007 were estimated based on: (1) reporting from utilities
participating in EPA's SF6 Emission Reduction Partnership for Electric Power Systems (partners), which began in
1999; and, (2) the relationship between emissions and utilities' transmission miles as reported in the 2001, 2004 and
2007 Utility Data Institute (UDI) Directories of Electric Power Producers and Distributors (UDI2001, 2004, 2007).
(Transmission miles are defined as the miles of lines carrying voltages above 34.5 kV.) Over the period from 1999
to 2007, 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 47 percent of total U.S. transmission miles.
For each year, the emissions reported by or estimated for partner utilities were added to the emissions estimated for
4-68 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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utilities that have never participated in the Partnership (i.e., non-partners).119
Partner utilities estimated their emissions using a Tier 3 utility-level mass balance approach (IPCC 2006). If a
partner utility did not provide data for a particular year, emissions were interpolated between years for which data
were available or extrapolated based on partner-specific transmission mile growth rates. In 2007, non-reporting
partners accounted for approximately 8 percent of the total emissions attributed to partner utilities.
Emissions from non-partners in every year since 1999 were estimated using the results of a regression analysis that
showed that the emissions from reporting utilities were most strongly correlated with their transmission miles. The
results of this analysis are not surprising given that, in the United States, SF6 is contained primarily in transmission
equipment rated at or above 34.5 kV. The equations were developed based on the 1999 SF6 emissions reported by
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 (UDI2001). Two
equations were developed, one for small and one for large utilities (i.e., with fewer or more than 10,000 transmission
miles, respectively). The distinction between utility sizes was made because the regression analysis showed that the
relationship between emissions and transmission miles differed for small and large transmission networks. The
same equations were used to estimate non-partner emissions in 1999 and every year thereafter because non-partners
were assumed not to have implemented any changes that would have resulted in reduced emissions since 1999.
The regression equations are:
Non-partner small utilities (fewer than 10,000 transmission miles, in kilograms):
Emissions (kg) = 0.89 x Transmission Miles
Non-partner large utilities (more than 10,000 transmission miles, in kilograms):
Emissions (kg) = 0.58 x Transmission Miles
Data on transmission miles for each non-partner utility for the years 2000, 2003 and 2006 were obtained from the
2001, 2004 and 2007 UDI Directories of Electric Power Producers and Distributors, respectively (UDI 2001, 2004,
2007). The U.S. transmission system grew by over 22,000 miles between 2000 and 2003 and by over 55,000 miles
between 2003 and 2006. These periodic increases are assumed to have occurred gradually, therefore transmission
mileage were assumed to increase at an annual rate of 1.2 percent between 2000 and 2003 and 2.8 percent between
2003 and 2006. Transmission miles in 2007 were then extrapolated from 2006 based on the 2.8 percent growth rate.
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 2007, 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 global120 emissions from this source during the 1990 to
1999 period. To estimate global emissions, the PxAND 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-lived pressurized equipment that is periodically serviced
during its lifetime.)
Emissions (kilograms SF6) = SF6 purchased to refill existing equipment (kilograms) + nameplate capacity121 of
retiring equipment (kilograms)
119 Partners in EPA's SF6 Emission Reduction Partnership reduced their emissions by approximately 54% from 1999 to 2007.
120 Ideally, sales to utilities in the U.S. between 1990 and 1999 would be used as a model. However, this information was not
available. There are only two U.S. manufacturers of SF& so sensitive sales information is not concealed by aggregation.
121 Nameplate capacity is defined as the amount of SF6 within fully charged electrical equipment.
Industrial Processes 4-69
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Note that the above equation holds whether the gas from retiring equipment is released or recaptured; if the gas is
recaptured, it is used to refill existing equipment, thereby lowering the amount of SF6 purchased by utilities for this
purpose.
Gas purchases by utilities and equipment manufacturers from 1961 through 2003 are available from the RAND
(2004) survey. To estimate the quantity of SF6 released or recovered from retiring equipment, the nameplate
capacity of retiring equipment in a given year was assumed to equal 81.2 percent of the amount of gas purchased by
electrical equipment manufacturers 40 years previous (e.g., in 2000, the nameplate capacity of retiring equipment
was assumed to equal 81.2 percent of the gas purchased in 1960). The remaining 18.8 percent was assumed to have
been emitted at the time of manufacture. The 18.8 percent emission factor is an average of IPCC default SF6
emission rates for Europe and Japan for 1995 (IPCC 2006). The 40-year lifetime for electrical equipment is also
based on IPCC (2006). The results of the two components of the above equation were then summed to yield
estimates of global SF6 emissions from 1990 through 1999.
U.S. emissions between 1990 and 1999 are assumed to follow the same trajectory as global emissions during this
period. To estimate U.S. emissions, global emissions for each year from 1990 through 1998 were divided by the
estimated global emissions from 1999. The result was a time series of factors that express each year's global
emissions as a multiple of 1999 global emissions. Historical U.S. emissions were estimated by multiplying the
factor for each respective year by the estimated U.S. emissions of SF6 from electric power systems in 1999
(estimated to be 15.1 Tg CO2 Eq.).
Two factors may affect the relationship between the RAND sales trends and actual global emission trends. One is
utilities' inventories of SF6 in storage containers. When SF6 prices rise, utilities are likely to deplete internal
inventories before purchasing new SF6 at the higher price, in which case SF6 sales will fall more quickly than
emissions. On the other hand, when SF6 prices fall, utilities are likely to purchase more SF6 to rebuild inventories,
in which case sales will rise more quickly than emissions. This effect was accounted for by applying 3-year
smoothing to utility SF6 sales data. The other factor that may affect the relationship between the RAND sales trends
and actual global emissions is the level of imports from and exports to Russia and China. SF6 production in these
countries is not included in the RAND survey and is not accounted for in any another manner by RAND. However,
atmospheric studies confirm that the downward trend in estimated global emissions between 1995 and 1998 was real
(see the Uncertainty discussion below).
1990 through 2007 Emissions from Manufacture of Electrical Equipment
The 1990 to 2007 emission estimates for original equipment manufacturers (OEMs) were derived by assuming that
manufacturing emissions equal 10 percent of the quantity of SF6 provided with new equipment. The quantity of SF6
provided with new equipment was estimated based on statistics compiled by the National Electrical Manufacturers
Association (NEMA). These statistics were provided for 1990 to 2000; the quantities of SF6 provided with new
equipment for 2001 to 2007 were estimated using partner reported data and the total industry SF6 nameplate capacity
estimate (131.8 Tg CO2 Eq. in 2007). 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 2007 was calculated. This
ratio was then multiplied by the total industry nameplate capacity estimate to derive the amount of SF6 provided
with new equipment for the entire industry. The 10 percent emission rate is the average of the "ideal" and "realistic"
manufacturing emission rates (4 percent and 17 percent, respectively) identified in a paper prepared under the
auspices of the International Council on Large Electric Systems (CIGRE) in February 2002 (O'Connell et al. 2002).
Uncertainty
To estimate the uncertainty associated with emissions of SF6 from electric transmission and distribution,
uncertainties associated with three quantities were estimated: (1) emissions from partners, (2) emissions from non-
partners, and (3) emissions from manufacturers of electrical equipment. A Monte Carlo analysis was then applied to
estimate the overall uncertainty of the emissions estimate.
Total emissions from the SF6 Emission Reduction Partnership include emissions from both reporting and non-
reporting partners. For reporting partners, individual partner-reported SF6 data was assumed to have an uncertainty
of 10 percent. Based on a Monte Carlo analysis, the cumulative uncertainty of all partner reported data was
estimated to be 3.6 percent. The uncertainty associated with extrapolated or interpolated emissions from non-
reporting partners was assumed to be 20 percent.
4-70 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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There are two sources of uncertainty associated with the regression equations used to estimate emissions in 2007
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 2007) 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 supplied with 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 4-95. Electrical Transmission
and Distribution SF6 emissions were estimated to be between 10.0 and 15.5 Tg CO2 Eq. at the 95 percent confidence
level. This indicates a range of approximately 21 percent below and 22 percent above the emission estimate of 12.7
Tg CO2 Eq.
Table 4-95: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Electrical Transmission and
Distribution (Tg CO2 Eq. and Percent)
Source
2007 Emission
Gas Estimate
(TgC02Eq.)
Uncertainty Range Relative to 2007 Emission Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Electrical Transmission
and Distribution SFg 12.7 10.0 15.5 -21% +22%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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.
Several pieces of evidence indicate that U.S. SF6 emissions were reduced as global emissions were reduced. First,
the decreases in sales and emissions coincided with a sharp increase in the price of SF6 that occurred in the mid-
1990s and that affected the United States as well as the rest of the world. A representative from Dilo, a major
manufacturer of SF6 recycling equipment, stated that most U.S. utilities began recycling rather than venting SF6
within two years of the price rise. Finally, the emissions reported by the one U.S. utility that reported 1990 through
1999 emissions to EPA showed a downward trend beginning in the mid-1990s.
Recalculations Discussion
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 interpolated and extrapolated non-reported partner data; and 3) a
revised regression equation coefficient for non-partner small utilities (fewer than 10,000 transmission miles). The
new regression coefficient resulted from a revised 1999 emission estimate from a Partner of EPA's SF6 Emission
Reduction Partnership. This new emission estimate changed the regression coefficient from 0.88 to 0.89. Based on
the revisions listed above, SF6 emissions from electric transmission and distribution increased between 0.04 to 1.02
percent for each year from 1990 through 2006.
[BEGIN BOX]
Box 4-1: Potential Emission Estimates of HFCs, PFCs, and SF6
Industrial Processes 4-71
-------
Emissions of HFCs, 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 1996 IPCC 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:
• By-product emissions. Some emissions do not result from the consumption or use of a chemical, but are
the unintended by-products 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 4-96 presents potential emission estimates for HFCs and PFCs from the substitution of ozone depleting
substances, HFCs, PFCs, and SF6 from semiconductor manufacture, and SF6 from magnesium production and
processing and electrical transmission and distribution.122 Potential emissions associated with the substitution for
ozone depleting substances were calculated using the EPA's Vintaging Model. Estimates of HFCs, PFCs, and SF6
consumed by semiconductor manufacture were developed by dividing chemical-by-chemical emissions by the
appropriate chemical-specific emission factors from the IPCC Good Practice Guidance (Tier 2c). Estimates of CF4
consumption were adjusted to account for the conversion of other chemicals into CF4 during the semiconductor
manufacturing process, again using the default factors from the IPCC Good Practice Guidance. Potential SF6
emissions estimates for electrical transmission and distribution were developed using U.S. utility purchases of SF6
for electrical equipment. From 1999 through 2007, estimates were obtained from reports submitted by participants
in EPA's SF6 Emission Reduction Partnership for Electric Power Systems. U.S. utility purchases of SF6 for
electrical equipment from 1990 through 1998 were backcasted based on world sales of SF6 to utilities. Purchases of
SF6 by utilities were added to SF6 purchases by electrical equipment manufacturers to obtain total SF6 purchases by
the electrical equipment sector.
Table 4-96: 2007 Potential and Actual Emissions of HFCs, PFCs, and SF6 from Selected Sources (Tg CO2 Eq.)
Source
Substitution of Ozone Depleting Substances
Aluminum Production
HCFC-22 Production
Semiconductor Manufacture
Magnesium Production and Processing
Electrical Transmission and Distribution
Potential
185.5
7.6
3.0
20.9
Actual
108.3
3.8
17.0
4.7
3.0
12.7
- Not applicable.
[END BOX]
4.23. Industrial Sources of Indirect Greenhouse Gases
In addition to the main greenhouse gases addressed above, many industrial processes generate emissions of indirect
122
See Annex 5 for a discussion of sources of SF6 emissions excluded from the actual emissions estimates in this report.
4-72 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 2007 are reported in Table 4-97.
Table 4-97: NOX, CO, and NMVOC Emissions from Industrial Processes (Gg)
Gas/Source
NOX
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
435
95
81
14
2
2,216
1,175
537
327
153
23
1,773
1,067
412
230
61
3
2005
534
389
64
63
17
2
1,744
895
445
258
107
39
2035
1346
401
226
42
20
2006
527
382
64
63
17
2
1,743
895
444
258
107
40
1950
1280
388
221
42
19
2007
520
375
64
63
17
2
1,743
894
444
258
107
40
1878
1228
376
216
42
17
* 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.
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.
Industrial Processes 4-73
-------
Substitution of Ozone Depleting Substances
Iron and Steel Production & Metallurgical Coke Production
Cement Production
Nitric Acid Production
HCFC-22 Production
Lime Production |
Ammonia Production and Urea Consumption |
Electrical Transmission and Distribution |
Aluminum Production |
Limestone and Dolomite Use |
Adipic Acid Production |
Semiconductor Manufacture |
Soda Ash Production and Consumption |
Petrochemical Production |
Magnesium Production and Processing |
Titanium Dioxide Production |
Carbon Dioxide Consumption |
Ferroalloy Production |
Phosphoric Acid Production |
Zinc Production
Lead Production
Silicon Carbide Production and Consumption
<0.5
<0.5
Industrial Processes
as a Portion of all Emissions
4.9%
25
50 75
TgCO2Eq.
100
125
Figure 4-1: 2007 Industrial Processes Chapter Greenhouse Gas Sources
-------
5. Solvent and Other Product Use
Greenhouse gas emissions are produced as a by-product of various solvent and other product uses. In the United
States, emissions from Nitrous Oxide (N2O) Product Usage, the only source of greenhouse gas emissions from this
sector, accounted for less than 0.1 percent of total U.S. anthropogenic greenhouse gas emissions on a carbon
equivalent basis in 2007 (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: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq. and Gg)
Gas/Source 1990 1995 2000 2005 2006 2007
N2O from Product Uses
Tg CO2 Eq.
Gg
4.4
14
4.6
15
4.9
16
4.4
14
4.4
14
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 (Airgas 2007; FTC 2001). 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 2007 was approximately 15 Gg (Table 5-2).
Table 5-2: N2O Production (Gg)
Year Gg
1990 16
1995 17
2000 17
2005 15
2006 15
2007 15
N2O emissions were 4.4 Tg CO2 Eq. (14 Gg) in 2007 (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).
Table 5-3: N2O Emissions from N2O Product Usage (Tg CO2 Eq. and Gg)
Year Tg CO2 Eq. Gg
1990 4.4 14
Solvent and Other Product Use 5-1
-------
1995 4.6 15
2000 4.9 16
2005 4.4 14
2006 4.4 14
2007 4A 14
Methodology
Emissions from N2O product usage were calculated by first multiplying the total amount of N2O produced in the
United States by the share of the total quantity of N2O attributed to each end use. This value was then multiplied by
the associated emission rate for each end use. After the emissions were calculated for each end use, they were added
together to obtain a total estimate of N2O product usage emissions. Emissions were determined using the following
equation:
N2O Product Usage Emissions = Zi [Total U.S. Production of N2O] x [Share of Total Quantity of N2O Usage by
Sector i] x [Emissions Rate for Sector i]
where,
i = Sector.
The share of total quantity of N2O usage by end use represents the share of national N2O produced that is used by
the specific subcategory (i.e., anesthesia, food processing, etc.). In 2007, the medical/dental industry used an
estimated 89.5 percent of total N2O produced, followed by food processing propellants at 6.5 percent. All other
categories combined used the remainder of the N2O produced. This subcategory breakdown has changed only
slightly over the past decade. For instance, the small share of N2O usage in the production of sodium azide has
declined significantly during the decade of the 1990s. Due to the lack of information on the specific time period of
the phase-out in this market subcategory, most of the N2O usage for sodium azide production is assumed to have
ceased after 1996, with the majority of its small share of the market assigned to the larger medical/dental
consumption subcategory (Heydorn 1997). The N2O was allocated across the following categories: medical
applications, food processing propellant, and sodium azide production (pre-1996). A usage emissions rate was then
applied for each sector to estimate the amount of N2O emitted.
Only the medical/dental and food propellant subcategories were estimated to release emissions into the atmosphere,
and therefore these subcategories were the only usage subcategories with emission rates. For the medical/dental
subcategory, due to the poor solubility of N2O in blood and other tissues, none of the N2O is assumed to be
metabolized during anesthesia and quickly leaves the body in exhaled breath. Therefore, an emission factor of 100
percent was used for this subcategory (IPCC 2006). For N2O used as a propellant in pressurized and aerosol food
products, none of the N2O is reacted during the process and all of the N2O is emitted to the atmosphere, resulting in
an emission factor of 100 percent for this subcategory (IPCC 2006). For the remaining subcategories, all of the N2O
is consumed/reacted during the process, and therefore the emission rate was considered to be zero percent (Tupman
2002).
The 1990 through 1992 N2O production data were obtained from SRI Consulting's Nitrous Oxide, North America
report (Heydorn 1997). N2O production data for 1993 through 1995 were not available. Production data for 1996
was specified as a range in two data sources (Heydorn 1997, Tupman 2002). In particular, for 1996, Heydorn
(1997) estimates N2O production to range between 13.6 and 18.1 thousand metric tons. Tupman (2003) provided a
narrower range (i.e., 15.9 to 18.1 thousand metric tons) for 1996 that falls within the production bounds described by
Heydorn (1997). Tupman (2003) data are considered more industry-specific and current. Therefore, the midpoint of
the narrower production range was used to estimate N2O emissions for years 1993 through 2001 (Tupman 2003).
The 2002 and 2003 N2O production data were obtained from the Compressed Gas Association Nitrous Oxide Fact
Sheet and Nitrous Oxide Abuse Hotline (CGA 2002, 2003). These data were also provided as a range. For
example, in 2003, CGA (2003) estimates N2O production to range between 13.6 and 15.9 thousand metric tons. Due
to unavailable data, production for 2004, 2005, 2006, and 2007 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
5-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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, 2006, and 2007 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 2006 IPCC
Guidelines.
Uncertainty
The overall uncertainty associated with the 2007 N2O emission estimate from N2O product usage was calculated
using the IPCC Guidelines for National Greenhouse Gas Inventories (2006) Tier 2 methodology. Uncertainty
associated with the parameters used to estimate N2O emissions included that of production data, total market share
of each end use, and the emission factors applied to each end use, respectively.
The results of this Tier 2 quantitative uncertainty analysis are summarized in Table 5-4. N2O emissions from N2O
product usage were estimated to be between 4.3 and 4.5 Tg CO2 Eq. at the 95 percent confidence level (or in 19 out
of 20 Monte Carlo Stochastic Simulations). This indicates a range of approximately 2 percent below to 2 percent
above the 2007 emissions estimate of 4.4 Tg CO2 Eq.
Table 5-4: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions From N2O Product Usage (Tg CO2 Eq. and
Percent)
Source Gas 2007 Emission
Estimate
(TgC02Eq.)
N2O Product Usage N2O 4.4
Uncertainty Range Relative to Emission Estimate"
(Tg
Lower
Bound
4.3
C02Eq.)
Upper
Bound
4.5
(o/
/O
Lower
Bound
-2%
)
Upper
Bound
+2%
_ _ _ _ _
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Planned Improvements
Planned improvements include a continued evaluation of alternative production statistics for cross verification and a
reassessment of subcategory usage to accurately represent the latest trends in the product usage, and investigation of
production and use cycles and the potential need to incorporate a time lag between production and ultimate product
use and resulting release of N2O. Additionally, planned improvements include considering imports and exports of
N2O for product uses.
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).123 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 by-products, such as
carbon monoxide (CO) and nitrogen oxides (NOX), are also reported with this source category. In the United States,
emissions from solvents are primarily the result of solvent evaporation, whereby the lighter hydrocarbon molecules
in the solvents escape into the atmosphere. The evaporation process varies depending on different solvent uses and
solvent types. The major categories of solvent uses include: degreasing, graphic arts, surface coating, other
industrial uses of solvents (i.e., electronics, etc.), dry cleaning, and non-industrial uses (i.e., uses of paint thinner,
etc.).
123 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.
Solvent and Other Product Use 5-3
-------
Total emissions of NOX, NMVOCs, and CO from 1990 to 2007 are reported in Table 5-5.
Table 5-5: Emissions of NOX, CO, and NMVOC from Solvent Use (Gg)
Activity 1990 1995 2000 2005 2006 2007
NOX 1 3 3 555
Surface Coating 1 2 3 555
Graphic Arts + 1 + + + +
Degreasing + + + + + +
Dry Cleaning + + + + + +
Other Industrial Processes3 + + + + + +
Non-Industrial Processes'3 + + + + + +
Other
CO
Surface Coating
Other Industrial Processes3
Dry Cleaning
Degreasing
Graphic Arts
Non-Industrial Processes'3
Other
NMVOCs
Surface Coating
Non-Industrial Processes'3
Degreasing
Dry Cleaning
Graphic Arts
Other Industrial Processes3
Other
NA
5
+
4
+
+
+
+
NA
5,216
2,289
1,724
675
195
249
85
+
+
5
1
3
1
+
+
+
NA
5,609
2,432
1,858
716
209
307
87
+
+
45
45
+
+
+
+
+
+
4,384
1,766
1,676
316
265
222
98
40
+
2
2
+
+
+
+
+
+
3,881
1,590
1,457
283
232
195
88
36
+
2
2
+
+
+
+
+
+
3,867
1,584
1,452
282
231
194
88
36
+
2
2
+
+
+
+
+
+
3,855
1,579
1,447
281
230
194
88
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.
Methodology
Emissions were calculated by aggregating solvent use data based on information relating to solvent uses from
different applications such as degreasing, graphic arts, etc. Emission factors for each consumption category were
then applied to the data to estimate emissions. For example, emissions from surface coatings were mostly due to
solvent evaporation as the coatings solidify. By applying the appropriate solvent-specific emission factors to the
amount of solvents used for surface coatings, an estimate of emissions was obtained. Emissions of CO and NOX
result primarily from thermal and catalytic incineration of solvent-laden gas streams from painting booths, printing
operations, and oven exhaust.
These emission estimates were obtained from preliminary data (EPA 2009), and disaggregated based on EPA
(2003), which, in its final iteration, will be published on the National Emission Inventory (NEI) Air Pollutant
Emission Trends web site. Emissions were calculated either for individual categories or for many categories
combined, using basic activity data (e.g., the amount of solvent purchased) as an indicator of emissions. National
activity data were collected for individual applications from various agencies.
Activity data were used in conjunction with emission factors, which together relate the quantity of emissions to the
activity. Emission factors are generally available from the EPA's Compilation of Air Pollutant Emission Factors,
AP-42 (EPA 1997). The EPA currently derives the overall emission control efficiency of a source category from a
variety of information sources, including published reports, the 1985 National Acid Precipitation and Assessment
Program emissions inventory, and other EPA databases.
5-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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
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.
Figure 6-1: 2007 Agriculture Chapter Greenhouse Gas Emission Sources
In 2007, the Agricultural sector was responsible for emissions of 413.1 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 24 percent and 8 percent of total CH4 emissions from anthropogenic activities,
respectively. Of all domestic animal types, beef and dairy cattle were by far the largest emitters of CH4. Rice
cultivation and field burning of agricultural residues were minor sources of CH4. Agricultural soil management
activities such as fertilizer application and other cropping practices were the largest source of U.S. N2O emissions,
accounting for 67 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 2007, CH4
emissions from agricultural activities increased by 11 percent, while N2O emissions fluctuated from year to year, but
overall increased by 5 percent.
Table 6-1: Emissions from Agriculture (Tg CO2 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
171.4
133.2
30.4
7.1
0.7
212.8
200.3
12.1
0.4
384.2
1995
186.3
143.6
34.5
7.6
0.7
215.6
202.3
12.9
0.4
402.0
2000
180.5
134.4
37.9
7.5
0.8
218.9
204.5
14.0
0.5
399.4
2005
185.5
136.0
41.8
6.8
0.9
225.5
210.6
14.2
0.5
410.8
2006
186.8
138.2
41.9
5.9
0.8
223.5
208.4
14.6
0.5
410.3
2007
190.0
139.0
44.0
6.2
0.9
223.1
207.9
14.7
0.5
413.1
Note: Totals may not sum due to independent rounding.
Table 6-2: Emissions from Agriculture (Gg)
Gas/Source 1990 1995 2000 2005 2006 2007
CH4 8,161 8,873 8,597 8,833 8,894 9,047
Enteric Fermentation 6,342 6,837 6,398 6,474 6,580 6,618
Manure Management 1,447 1,642 1,804 1,991 1,993 2,093
Rice Cultivation 339 363 357 326 282 293
Field Burning of
Agricultural Residues 33 32 38 41 39 42
N2O 686 696 706 727 721 720
Agricultural Soil
Management 646 653 660 679 672 671
Agriculture 6-1
-------
Manure Management 39 42
Field Burning of
Agricultural Residues 1 1_
Note: Totals may not sum due to independent rounding.
45
1
46
2
47
2
47
2
6.1. Enteric Fermentation (IPCC Source Category 4A)
CH4 is produced as part of normal digestive processes in animals. During digestion, microbes resident in an
animal's digestive system ferment food consumed by the animal. This microbial fermentation process, referred to as
enteric fermentation, produces CH4 as a byproduct, which can be exhaled or eructated by the animal. The amount of
CH4 produced and emitted by an individual animal depends primarily upon the animal's digestive system, and the
amount and type of feed it consumes.
Ruminant animals (e.g., cattle, buffalo, sheep, goats, and camels) are the major emitters of CH4 because of their
unique digestive system. Ruminants possess a rumen, or large "fore-stomach," in which microbial fermentation
breaks down the feed they consume into products that can be absorbed and metabolized. The microbial
fermentation that occurs in the rumen enables them to digest coarse plant material that non-ruminant animals can
not. Ruminant animals, consequently, have the highest CH4 emissions among all animal types.
Non-ruminant animals (e.g., swine, horses, and mules) also produce CH4 emissions through enteric fermentation,
although this microbial fermentation occurs in the large intestine. These non-ruminants emit significantly less CH4
on a per-animal basis than ruminants because the capacity of the large intestine to produce CH4 is lower.
In addition to the type of digestive system, an animal's feed quality and feed intake also affects CH4 emissions. In
general, lower feed quality and/or higher feed intake leads to higher CH4 emissions. Feed intake is positively
correlated to animal size, growth rate, and production (e.g., milk production, wool growth, pregnancy, or work).
Therefore, feed intake varies among animal types as well as among different management practices for individual
animal types (e.g., animals in feedlots or grazing on pasture).
CH4 emission estimates from enteric fermentation are provided in Table 6-3 and Table 6-4. Total livestock CH4
emissions in 2007 were 139.0 TgCO2Eq. (6,618 Gg). Beef cattle remain the largest contributor of CH4 emissions
from enteric fermentation, accounting for 72 percent in 2007. Emissions from dairy cattle in 2007 accounted for 23
percent, and the remaining emissions were from horses, sheep, swine, and goats.
From 1990 to 2007, emissions from enteric fermentation have increased by 4.3 percent. Generally, emissions
decreased from 1995 to 2004, though with slight increases in 2002 and 2003. This trend was mainly due to
decreasing populations of both beef and dairy cattle and increased digestibility of feed for feedlot cattle. Emissions
have increased from 2004 through 2007, as both dairy and beef populations have undergone increases. During the
timeframe of this analysis, populations of sheep have decreased 46 percent since 1990 while horse populations have
increased over 80 percent, mostly since 1999. Goat and swine populations have increased 1 percent and 21 percent,
respectively, during this timeframe.
Table 6-3: CH4 Emissions from Enteric Fermentation (Tg CO2 Eq.)
Livestock Type
Beef Cattle
Dairy Cattle
Horses
Sheep
Swine
Goats
Total
1990
94.6
32.8
1.9
1.9
1.7
0.3
133.2
1995
106.7
31.3
1.9
1.5
1.9
0.2
143.6
2000
98.8
30.2
2.0
1.2
1.9
0.3
134.4
2005
98.4
30.8
3.5
1.0
1.9
0.3
136.0
2006
100.0
31.4
3.5
1.0
1.9
0.3
138.2
2007
100.2
31.9
3.5
1.0
2.0
0.3
139.0
Note: Totals may not sum due to independent rounding.
Table 6-4: CH4 Emissions from Enteric Fermentation (Gg)
Livestock Type
1990
1995
2000
2005
2006
2007
Beef Cattle
Dairy Cattle
Horses
4,504
1,563
91
5,082
1,490
92
4,707
1,440
94
4,687
1,468
166
4,762
1,497
166
4,772
1,521
166
6-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Sheep
Swine
Goats
Total
91
81
13
6,342
72
88
12
6,837
56
88
12
6,398
49
92
13
6,474
50
93
13
6,580
49
98
13
6,618
Note: Totals may not sum due to independent rounding.
Methodology
Livestock emission estimate methodologies fall into two categories: cattle and other domesticated animals. Cattle,
due to their large population, large size, and particular digestive characteristics, account for the majority of CH4
emissions from livestock in the United States. A more detailed methodology (i.e., IPCC Tier 2) was therefore
applied to estimate emissions for all cattle except for bulls. Emission estimates for other domesticated animals
(horses, sheep, swine, goats, and bulls) were handled using a less detailed approach (i.e., IPCC Tier 1).
While the large diversity of animal management practices cannot be precisely characterized and evaluated,
significant scientific literature exists that provides the necessary data to estimate cattle emissions using the IPCC
Tier 2 approach. The Cattle Enteric Fermentation Model (CEFM), developed by EPA and used to estimate cattle
CH4 emissions from enteric fermentation, incorporates this information and other analyses of livestock population,
feeding practices, and production characteristics.
National cattle population statistics were disaggregated into the following cattle sub-populations:
• Dairy Cattle
• Calves
• Heifer Replacements
• Cows
• Beef Cattle
• Calves
• Heifer Replacements
• Heifer and Steer Stackers
• Animals in Feedlots (Heifers and Steers)
• Cows
• Bulls
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 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 2008).
Diet characteristics were estimated by region for U.S. dairy, beef, and feedlot cattle. These estimates were used to
calculate Digestible Energy (DE) values (expressed as the percent of gross energy intake digested by the animal) and
CH4 conversion rates (Ym) (expressed as the fraction of gross energy converted to CH4) for each population
category. The IPCC recommends Ym values of 3.0+1.0 percent for feedlot cattle and 6.5+1.0 percent for other well-
fed cattle consuming temperate-climate feed types (IPCC 2006). Given the availability of detailed diet information
for different regions and animal types in the United States, DE and Ym values unique to the United States were
developed, rather than using the recommended IPCC values. The diet characterizations and estimation of DE and
Ym values were based on information from state agricultural extension specialists, a review of published forage
quality studies, 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
Agriculture 6-3
-------
calculated from diet characteristics using a model simulating ruminant digestion in growing and/or lactating cattle
(Donovan and Baldwin 1999). Values from EPA (1993) were used for dairy replacement heifers. For feedlot
animals, DE and Ym values recommended by Johnson (1999) were used. For grazing beef cattle, DE values were
based on diet information in NRC (2000) and Ym values were based on Johnson (2002). Weight and weight gains
for cattle were estimated from Enns (2008), Patton et al. (2008), Lippke et al. (2000), Pinchack et al., (2004), Platter
et al. (2003), Skogerboe et al. (2000), and expert opinion. See Annex 3.9 for more details on the method used to
characterize cattle diets and weights in the United States.
To estimate CH4 emissions from all cattle types except bulls and calves younger than 7 months,124 the population
was divided into state, age, sub-type (i.e., dairy cows and replacements, beef cows and replacements, heifer and
steer stackers, and heifer and steer in feedlots), and production (i.e., pregnant, lactating) groupings to more fully
capture differences in CH4 emissions from these animal types. The transition matrix was used to simulate the age
and weight structure of each sub-type on a monthly basis, to more accurately reflect the fluctuations that occur
throughout the year. Cattle diet characteristics were then used in conjunction with Tier 2 equations from IPCC
(2006) to produce CH4 emission factors for the following cattle types: dairy cows, beef cows, dairy replacements,
beef replacements, steer stackers, heifer stackers, steer feedlot animals, and heifer feedlot animals. To estimate
emissions from cattle, population data from the transition matrix were multiplied by the calculated emission factor
for each cattle type. More details are provided in Annex 3.9.
Emission estimates for other animal types were based on average emission factors representative of entire
populations of each animal type. CH4 emissions from these animals accounted for a minor portion of total CH4
emissions from livestock in the United States from 1990 through 2007. 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 2008). Horse population data were obtained from the
FAOSTAT database (FAO 2008), because USDA does not estimate U.S. horse populations annually. Goat
population data were obtained for 1992, 1997, and 2002 (USDA 2008); these data were interpolated and
extrapolated to derive estimates for the other years. CH4 emissions from sheep, goats, swine, and horses were
estimated by using emission factors utilized in Crutzen et al. (1986, cited in IPCC 2006). These emission factors are
representative of typical animal sizes, feed intakes, and feed characteristics in developed countries. The
methodology is the same as that recommended by IPCC (2006).
See Annex 3.9 for more detailed information on the methodology and data used to calculate CH4 emissions from
enteric fermentation.
Uncertainty
Quantitative uncertainty analysis for this source category was performed through the IPCC-recommended Tier 2
uncertainty estimation methodology, Monte Carlo Stochastic Simulation technique as described in ICF (2003).
These uncertainty estimates were developed for the 1990 through 2001 Inventory report. No significant changes
occurred in the method of data collection, data estimation methodology, or other factors that influence the
uncertainty ranges around the 2007 activity data and emission factor input variables used in the current submission.
Consequently, these uncertainty estimates were directly applied to the 2007 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
124 Emissions from bulls are estimated using a Tier 1 approach because it is assumed there is minimal variation in population and
diets; because calves younger than 7 months consume mainly milk and the IPCC recommends the use of methane conversion
factor of zero for all juveniles consuming only milk, this results in no methane emissions from this subcategory of cattle.
6-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 123.7 to 164.0 Tg CO2 Eq., calculated as 11
percent below and 18 percent above the actual 2007 emission estimate of 139.0 Tg CO2 Eq. Among the individual
cattle sub-source categories, beef cattle account for the largest amount of CH4 emissions as well as the largest degree
of uncertainty in the inventory emission estimates. Among non-cattle, horses account for the largest degree of
uncertainty in the inventory emission estimates because there is a higher degree of uncertainty among the FAO
population estimates used for horses than for the USDA population estimates used for swine, goats, and sheep.
Table 6-5: Quantitative Uncertainty Estimates for CH4 Emissions from Enteric Fermentation (Tg CO2 Eq. and
Percent)
Source Gas 2007 Emission
Estimate
(TgC02Eq.)
Enteric Fermentation CH4 139.0
Uncertainty Range Relative to Emission Estimate3' b
(Tg C02 Eq.) (%)
Lower
Bound
123.7
Upper
Bound
164.0
Lower
Bound
-11%
Upper
Bound
+18%
Range of emissions estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Note that the relative uncertainty range •
2003 and applied to the 2007 estimates.
b Note that the relative uncertainty range was estimated with respect to the 2001 emission estimates submitted in
QA/QC and Verification
In order to ensure the quality of the emission estimates from enteric fermentation, the IPCC Tier 1 and Tier 2
Quality Assurance/Quality Control (QA/QC) procedures were implemented consistent with the U.S. QA/QC plan.
Tier 2 QA procedures included independent peer review of emission estimates. As described below, particular
emphasis this year was placed on revising CEFM weight assumptions and modifications of the stacker population
estimates in the transition matrix, which required further QA/QC to ensure consistency of estimates generated by the
updated model.
Recalculations Discussion
There were several modifications to the estimates relative to the previous Inventory that had an effect on emission
estimates, including the following:
• During the QA/QC process, it was noted that a portion of the steer and heifer populations that were held aside
(e.g., not eligible to be placed in feedlots) to establish the stacker population for the following January were
inadvertently left out of the emissions calculations. These heifer and steer stacker populations are now included.
• An additional adjustment was made to the CEFM to allow feedlot placements for the 700-800 Ibs category to
use excess animals from the over 800 Ibs category if insufficient animals are available to place in a given month
at 700-800 Ibs. This process reduced the discrepancy in the model between actual placement numbers by weight
category from USDA and available animals within the transition matrix.
• Calf weight at 7 months was adjusted to be equal for all months, as current research indicated that evidence was
not sufficient to suggest that calf weight at weaning differs by birth month.
• Mature weight for beef cows was revised based on annual data collected from 1989 through 2007, as was
replacement weight at 15 and 24 months.
• Mature weight for dairy cows was adjusted to 1,550 for all years, and replacement weight at 15 and 24 months
was adjusted accordingly.
• Monthly weight gain for stackers was increased to 1.83 Ibs per day starting in 2000, and a linear function was
used to determine adjustments from previous estimates between 1989 and 2000.
Agriculture 6-5
-------
• Bulls were added to the CEFM calculations for the first time, as previously they had been calculated separately,
however the estimates are still carried out with the Tier 1 approach, so this change did not result in any changes
in emissions from previous years.
• The USDA published revised population estimates that affected historical emissions estimated for swine in
2006. 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 65 Gg (4.6 percent) per year and beef
cattle increased an average of 423 Gg (9.7 percent) per year over the entire time series relative to the previous
Inventory. Historical emission estimates for swine in 2006 increased by less than one half of one percent as a result
of the USDA revisions described above.
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.
In addition to the diet characteristics research discussed above several revisions will be investigated, including:
• Estimating bull emissions using the IPCC Tier 2 approach;
• updating input variables that are from older data sources, such as beef births by month and beef cow lactation
rates;
• Continue to evaluate and improve the CEFM handling of the differences between the USDA feedlot placement
data by weight category and the number of animals that are available for placement by weight class according to
the CEFM transition matrix.
• the possible breakout of other animal types (i.e., sheep, swine, goats, horses) from national estimates to state-
level estimates; and
• including bison in the estimates for other domesticated animals.
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 CH4 and N2O emissions. Methane is produced by the anaerobic
decomposition of manure. Direct N2O emissions are produced as part of the N cycle through the nitrification and
denitrification of the organic N in livestock manure and urine.125 Indirect N2O emissions are produced as result of
the volatilization of N as ammonia (NH3) and oxides of nitrogen (NOX) and runoff and leaching of N during
treatment, storage and transportation.
When livestock or poultry manure are stored or treated in systems that promote anaerobic conditions (e.g., as a
125 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.
6-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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liquid/slurry in lagoons, ponds, tanks, or pits), the decomposition of materials in the manure tends to produce CH4.
When manure is handled as a solid (e.g., in stacks or drylots) or deposited on pasture, range, or paddock lands, it
tends to decompose aerobically and produce little or no CH4. Ambient temperature, moisture, and manure storage
or residency time affect the amount of CH4 produced because they influence the growth of the bacteria responsible
for CH4 formation. For non-liquid-based manure systems, moist conditions (which are a function of rainfall and
humidity) can promote CH4 production. Manure composition, which varies by animal diet, growth rate, and type,
including the animal's digestive system, also affects the amount of CH4 produced. In general, the greater the energy
content of the feed, the greater the potential for CH4 emissions. However, some higher energy feeds also are more
digestible than lower quality forages, which can result in less overall waste excreted from the animal.
The production of direct N2O emissions from livestock manure depends on the composition of the manure and urine,
the type of bacteria involved in the process, and the amount of oxygen and liquid in the manure system. For direct
N2O emissions to occur, the manure must first be handled aerobically where NH3 or organic N is converted to
nitrates and nitrites (nitrification), and then handled anaerobically where the nitrates and nitrites are reduced to
nitrogen gas (N2), with intermediate production of N2O and nitric oxide (NO) (denitrification) (Groffman et al.
2000). These emissions are most likely to occur in dry manure handling systems that have aerobic conditions, but
that also contain pockets of anaerobic conditions due to saturation. A very small portion of the total N excreted is
expected to convert to N2O in the waste management system (WMS). Indirect N2O emissions are produced when N
is lost from the system through volatilization (as NH3 or NOX) or through runoff and leaching. The vast majority of
volatilization losses from these operations are NH3. Although there are also some small losses of NOX, there are no
quantified estimates available for use, so losses due to volatilization are only based on NH3 loss factors. Runoff
losses would be expected from operations that house animals or store manure in a manner that results in exposure to
weather. Runoff losses are also specific to the type of animal housed on the operation due to differences in manure
characteristics. Little information is known about leaching from manure management systems as most research
focuses on leaching from land application systems. Since leaching losses are expected to be minimal, leaching
losses are coupled with runoff losses and the runoff/leaching estimate does not include any leaching losses.
Estimates of CH4 emissions in 2007 were 44.0 Tg CO2 Eq. (2,093 Gg), 45 percent higher than in 1990. Emissions
increased on average by 0.8 Tg CO2 Eq. (2.5 percent) annually over this period. The majority of this increase was
from swine and dairy cow manure, where emissions increased 51 and 60 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 2007); 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 2006 to 2007, there was a 5 percent increase in total CH4
emissions, due to minor shifts in the animal populations and the resultant effects on manure management system
allocations.
In 2007, total N2O emissions were estimated to be 14.7 Tg CO2 Eq. (47 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 a 22
percent increase from 1990 to 2007 and a 1 percent increase from 2006 through 2007.
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 N2O Emissions from Manure Management (Tg CO2 Eq.)
Gas/Animal Type 1990 1995 2000 2005 2006 2007
Agriculture 6-7
-------
CH41
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
N202
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
Total
30.4
11.3
2.6
13.1
0.1
+
2.8
0.5
12.1
3.5
5.5
1.2
0.1
+
1.5
0.2
42.5
34.5
12.5
2.6
16.0
0.1
+
2.7
0.4
12.9
3.5
6.0
1.4
0.2
+
1.6
0.2
47.4
37.9
14.7
2.5
17.5
0.1
+
2.6
0.5
14.0
3.6
6.7
1.4
0.3
+
1.7
0.2
51.9
41.8
17.2
2.4
18.6
0.1
+
2.7
0.8
14.2
3.7
6.5
1.5
0.3
+
1.7
0.4
56.0
41.9
17.5
2.5
18.3
0.1
+
2.7
0.8
14.6
3.8
6.7
1.5
0.4
+
1.8
0.4
56.4
44.0
18.1
2.4
19.7
0.1
+
2.7
0.8
14.7
3.9
6.7
1.6
0.3
+
1.8
0.4
58.7
+ Does not exceed 0.05 Tg CO2 Eq.
Note: Totals may not sum due to independent rounding.
'includes CH4 emission reductions due to CH4 collection and combustion by anaerobic digestion utilization systems.
Includes both direct and indirect N2O emissions.
Table 6-7. CH4 and N2O Emissions from Manure Management (Gg)
Gas/Animal Type
CH41
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
N2O2
Dairy Cattle
Beef Cattle
Swine
Sheep
Goats
Poultry
Horses
1990
1,447
538
124
624
7
1
131
22
39
11
18
4
+
+
5
1
1995
1,642
597
125
764
5
1
128
21
42
11
19
5
1
+
5
1
2000
1,804
701
118
832
4
1
126
22
45
12
22
5
1
+
5
1
2005
1,991
820
114
887
4
1
127
39
46
12
21
5
1
+
6
1
2006
1,993
833
119
870
4
1
128
39
47
12
22
5
1
+
6
1
2007
2,093
863
116
940
4
1
130
39
47
13
22
5
1
+
6
1
+ Less than 0.5 Gg.
Note: Totals may not sum due to independent rounding.
'includes CH4 emission reductions due to CH4 collection and combustion by anaerobic digestion utilization systems.
Includes both direct and indirect N2O emissions.
Methodology
The methodologies presented in IPCC (2006) form the basis of the CH4 and N2O emission estimates for each animal
type. This section presents a summary of the methodologies used to estimate CH4 and N2O emissions from manure
management for this inventory. See Annex 3.10 for more detailed information on the methodology and data used to
calculate CH4 and N2O emissions from manure management.
Methane Calculation Methods
• The following inputs were used in the calculation of CH4 emissions:
6-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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• Animal population data (by animal type and state);
• Typical Animal Mass (TAM) data (by animal type);
• Portion of manure managed in each Waste Management System (WMS), by state and animal type;
• Volatile solids (VS) production rate (by animal type and state or U.S.);
• CH4 producing potential (Bo) of the volatile solids (by animal type);
• Methane Conversion Factors (MCF), representing the extent to which the CH4 producing potential is realized
for each type of WMS (by state and manure management system, including the impacts of any biogas
collection/utilization efforts).
CH4 emissions were estimated by first determining activity data, including animal population, typical animal mass
(TAM), WMS usage, and waste characteristics. The activity data sources are described below:
• Annual animal population data for 1990 through 2007 for all livestock types, except horses and goats were
obtained from the USDA National Agricultural Statistics Service (NASS). Horse population data were obtained
from the Food and Agriculture Organization (FAO) FAOSTAT database (FAO 2008). Goat population data for
1992, 1997, and 2002 were obtained from the Census of Agriculture (USDA 2005).
• The TAM is an annual average weight which was obtained for each animal type from information in USDA's
Agricultural Waste Management Field Handbook (USDA 1996a), the American Society of Agricultural
Engineers, Standard D384.1 (ASAE 1999) and others (EPA 1992, Shuyler2000, and Safley 2000).
• WMS usage was estimated for swine and dairy cattle for different farm size categories using data from USDA
(USDA 1996b, 1998, 2000a) and EPA (ERG 2000a, EPA 2002a, 2002b). For beef cattle and poultry, manure
management system usage data were not tied to farm size but were based on other data sources (ERG 2000a,
USDA 2000b, UEP 1999). For other animal types, manure management system usage was based on previous
estimates (EPA 1992).
• VS production rates for all cattle except for bull and calves were calculated for each state and animal type in the
Cattle Enteric Fermentation Model (CEFM), which is described in section 6.1, Enteric Fermentation. VS
production rates for all other animals were determined using data from USDA's Agricultural Waste
Management Field Handbook (USDA 1996a) and data from the American Society of Agricultural Engineers,
Standard D384.1 (ASAE 1999).
• The maximum CH4 producing capacity of the VS (Bo) was determined for each animal type based on literature
values (Morris 1976, Bryant et al, 1976, Hashimoto 1981, Haskimoto 1984, EPA 1992, Hill 1982, and Hill
1984).
• MCFs for dry systems were set equal to default IPCC factors based on state climate for each year (IPCC 2006).
MCFs for liquid/slurry, anaerobic lagoon, and deep pit systems were calculated based on the forecast
performance of biological systems relative to temperature changes as predicted in the van't Hoff-Arrhenius
equation which is consistent with IPCC 2006 Tier 2 methodology.
• Anaerobic digestion system data were obtained from the EPA AgSTAR Program, including information
presented in the AgSTAR Digest (EPA 2000, 2003b, 2006).
• Emissions from anaerobic digestion systems were estimated based on the methodology described in EPA's
Climate Leaders Greenhouse Gas Inventory Protocol Offset Project Methodology for Project Types Managing
Manure with Biogas Recovery Systems (EPA 2008).
To estimate CH4 emissions, first the annual amount of VS (kg per year) from manure that is excreted in each WMS
for each animal type, state, and year was calculated. This calculation multiplied the animal population (head) by the
VS excretion rate (kg VS per 1,000 kg animal mass per day), the TAM (kg animal mass per head) divided by 1,000,
the WMS distribution (percent), and the number of days per year.
The estimated amount of VS managed in each WMS was used to estimate the CH4 emissions (kg CH4 per year)
from each WMS. The amount of VS (kg per year) was multiplied by the maximum CH4 producing capacity of the
VS (B0) (m3 CH4 per kg VS), the MCF for that WMS (percent), and the density of CH4 (kg CH4 per m3 CH4).
Agriculture 6-9
-------
For anaerobic digestion systems, the maximum CH4 producing capacity of the VS (B0) (m3 CH4 per kg VS) was
multiplied by an estimated CH4 production value (percent), assumed values of the system collection efficiency (CE)
(percent), an assumed value of the system destruction efficiency (DE) (percent), and the density of CH4 (kg CH4 per
m3 CH4) (ERG 2008). Anaerobic digestion systems were assumed to produce 90 percent of the maximum CH4
producing capacity of the VS (B0). The CH4 CE of covered lagoon systems was estimated to be 75 percent, and the
CH4 CE of complete mix and plug flow anaerobic digestion systems was assumed to be 99 percent (EPA 2008).
Any CH4 that was not collected was assumed to be emitted as leakage. A DE from flaring or burning in an engine is
estimated to be 98 percent; therefore, the amount of CH4 that would not be flared or combusted and would be
emitted is 2 percent (EPA 2008).
The CH4 emissions for each WMS (including anaerobic digestion systems), state, and animal type were summed to
determine the total U.S. CH4 emissions from manure management.
Nitrous Oxide Calculation Methods
The following inputs were used in the calculation of direct and indirect N2O emissions:
• Animal population data (by animal type and state);
• TAM data (by animal type);
• Portion of manure managed in each WMS (by state and animal type);
• Total Kjeldahl N excretion rate (Nex);
• Direct N2O emission factor (EFWMS);
• Indirect N2O emission factor for volatilization (EFvoiltailzatlon) ;
• Indirect N2O emission factor for runoff and leaching (EF^o^-/^^);
• Fraction of N loss from volatilization of ammonia and NOX (Fracgas);
• Fraction of N loss from runoff and leaching
N2O emissions were estimated by first determining activity data, including animal population, typical animal mass
(TAM), WMS usage, and waste characteristics. The activity data sources (except for population, TAM, and WMS,
which were described above) are described below:
• N excretion rates from the USD A Agricultural Waste Management Field Handbook (USD A 1 996a) were used
for all animal types except sheep, goats, and horses. Data from the American Society of Agricultural Engineers
(ASAE1999) were used for these animal types.
• All N2O emissions factors (direct and indirect) were from IPCC (IPCC 2006).
• Country-specific estimates for the fraction of N loss from volatilization (Fracgas) and runoff and leaching
(FraCrunoff/ieach) were developed. Fracgas values were based on WMS-specific volatilization values as estimated
from U.S. EPA's National Emission Inventory - Ammonia Emissions from Animal Agriculture Operations
(EPA 2005). FraCrunoff/ieachmg values were based on regional cattle runoff data from EPA's Office of Water (EPA
2002b; see Annex 3.1).
To estimate N2O emissions, first the amount of Nexcreted(kg per year) in manure in each WMS for each animal type,
state, and year was calculated. The population (head) for each state and animal was multiplied by TAM (kg animal
mass per head) divided by 1,000, the N excretion rate (Nex, in kg N per 1000 kg animal mass per day), WMS
distribution (percent), and the number of days per year.
Direct N2O emissions were calculated by multiplying the amount of Nexcreted (kg per year) in each WMS by the N2O
direct emission factor for that WMS (EFwMS, in kg N2O-N per kg N) and the conversion factor of N2O-N to N2O.
These emissions were summed over state, animal and WMS to determine the total direct N2O emissions (kg of N2O
per year).
Then, indirect N2O emissions from volatilization (kg N2O per year) were calculated by multiplying the amount of N
6-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
excreted (kg per year) in each WMS by the fraction of N lost through volatilization (Fracgas) divided by 100, and the
emission factor for volatilization (EFvoiatlilzatlon, in kg N2O per kg N), and the conversion factor of N2O-N to N2O.
Next, indirect N2O emissions from runoff and leaching (kg N2O per year) were calculated by multiplying the
amount of N excreted (kg per year) in each WMS by the fraction of N lost through runoff and leaching
(FraCrunoff/ieach) divided by 100, and the emission factor for runoff and leaching (EF^noff/i^^, in kg N2O per kg N), and
the conversion factor of N2O-N to N2O. The indirect N2O emissions from volatilization and runoff and leaching
were summed to determine the total indirect N2O emissions.
The direct and indirect N2O emissions were summed to determine total N2O emissions (kg N2O per year).
Uncertainty
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 CH4 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 CH4 and N2O emissions from manure management systems. A normal probability
distribution was assumed for each source data category. The series of equations used were condensed into a single
equation for each animal type and state. The equations for each animal group contained four to five variables
around which the uncertainty analysis was performed for each state. No significant changes occurred in the
methods, data or other factors that influence the uncertainty ranges around the 2007 activity data. Consequently,
these uncertainty estimates were directly applied to the 2007 emission estimates.
The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 6-8. Manure management CH4
emissions in 2007 were estimated to be between 36.0 and 52.8 Tg CO2 Eq. at a 95 percent confidence level, which
indicates a range of 18 percent below to 20 percent above the actual 2007 emission estimate of 44.0 Tg CO2 Eq. At
the 95 percent confidence level, N2O emissions were estimated to be between 12.3 and 18.2 Tg CO2 Eq. (or
approximately 16 percent below and 24 percent above the actual 2007 emission estimate of 14.7 Tg CO2 Eq.).
Table 6-8. Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O (Direct and Indirect) Emissions from Manure
Management (Tg CO2 Eq. and Percent)
Source
Manure Management
Manure Management
Gas
CH4
N2O
2007 Emission
Estimate
(Tg C02 Eq.)
44.0
14.7
Uncertainty Range Relative
(Tg C02 Eq.)
Lower Upper
Bound Bound
36.0 52.8
12.3 18.2
to Emission Estimate"
Lower Upper
Bound Bound
-18% +20%
-16% +24%
aRange 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 CH4 and N2O emissions from manure
management. All errors identified were corrected. Order of magnitude checks were also conducted, and corrections
made where needed. Manure N data were checked by comparing state-level data with bottom up estimates derived
at the county level and summed to the state level. Similarly, a comparison was made by animal and WMS type for
the full time series, between national level estimates for N excreted and the sum of county estimates for the full time
series.
Recalculations Discussion
For the current Inventory, anaerobic digester systems were incorporated into the WMS distributions in the CH4
estimates using the existing WMS distributions and EPA AgSTAR data. Emissions for anaerobic digestion systems
were also calculated using an assumed CH4 production rate, collection efficiency, and combustion efficiency (ERG
2008).
Agriculture 6-11
-------
Using the APHIS 2001 Sheep report, the WMS distribution for sheep was updated. The APHIS report presents
regional percentages of sheep and lambs that are primarily managed in open range/pasture, fenced range/pasture,
farms, or feedlots in 2001 (USDA 2003). WMS data for sheep were previously obtained from USDA NASS sheep
report for years 1990 through 1993 (USDA 1994). The WMS data for years 1994 through 2000 were calculated
assuming a linear progression from 1993 to 2001. Due to lack of additional data, data for years 2002 and beyond
were assumed to be the same as 2001.
The Cattle Enteric Fermentation Model (CEFM) produces volatile solids data for cattle that are used in the manure
management estimates. The CEFM team implemented methodological changes to the VS estimation, which created
changes in VS data and changes in the amount of methane estimated for manure management (see Section 6.1,
Enteric Fermentation).
With these recalculations, CH4 emission estimates from manure management systems are slightly higher than
reported in the previous Inventory for swine and slightly lower for dairy cattle. On average, annual CH4 emission
estimates are less than those of the previous Inventory by 1.7 percent.
N2O emission estimates from manure management systems have increased for all years for beef cattle and since
1994 for sheep in the current Inventory as compared to the previous Inventory due to the recalculations. Overall the
total emission estimates for the current Inventory increased by 1.2 percent, relative to the previous Inventory.
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 emission estimates and
CEFM are using the same data sources and variables where appropriate.
An updated version of the USDA Agricultural Waste Management Field Handbook became available in March
2008. This reference will be reviewed to determine if updates should be made to any of the inventory activity data.
The current inventory estimates take into account anaerobic digestion systems for only dairy and swine operations.
Data from the AgSTAR Program will also be reviewed and anaerobic digestions systems that exist for other animal
types will be incorporated.
The uncertainty analysis will be updated in the future to more accurately assess uncertainty of emission calculations.
This update is necessary due to 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.
6.3. Rice Cultivation (IPCC Source Category 4C)
Most of the world's rice, and all rice in the United States, is grown on flooded fields. When fields are flooded,
aerobic decomposition of organic material gradually depletes most of the oxygen present in the soil, causing
anaerobic soil conditions. Once the environment becomes anaerobic, CH4 is produced through anaerobic
decomposition of soil organic matter by methanogenic bacteria. As much as 60 to 90 percent of the CH4 produced is
oxidized by aerobic methanotrophic bacteria in the soil (some oxygen remains at the interfaces of soil and water, and
soil and root system) (Holzapfel-Pschorn et al. 1985, Sass et al. 1990). Some of the CH4 is also leached away as
dissolved CH4 in floodwater that percolates from the field. The remaining un-oxidized CH4 is transported from the
submerged soil to the atmosphere primarily by diffusive transport through the rice plants. Minor amounts of CH4
also escape from the soil via diffusion and bubbling through floodwaters.
The water management system under which rice is grown is one of the most important factors affecting CH4
emissions. Upland rice fields are not flooded, and therefore are not believed to produce CH4. In deepwater rice
fields (i.e., fields with flooding depths greater than one meter), the lower stems and roots of the rice plants are dead,
so the primary CH4 transport pathway to the atmosphere is blocked. The quantities of CH4 released from deepwater
fields, therefore, are believed to be significantly less than the quantities released from areas with shallower flooding
depths. Some flooded fields are drained periodically during the growing season, either intentionally or accidentally.
If water is drained and soils are allowed to dry sufficiently, CH4 emissions decrease or stop entirely. This is due to
soil aeration, which not only causes existing soil CH4 to oxidize but also inhibits further CH4 production in soils.
All rice in the United States is grown under continuously flooded conditions; none is grown under deepwater
conditions. Mid-season drainage does not occur except by accident (e.g., due to levee breach).
6-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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,126 and cultivation practices) are the most important variables influencing the
amount of CH4 emitted over the growing season; the total amount of CH4 released depends primarily on the amount
of organic substrate available. Soil temperature is known to be an important factor regulating the activity of
methanogenic bacteria, and therefore the rate of CH4 production. However, although temperature controls the
amount of time it takes to convert a given amount of organic material to CH4, that time is short relative to a growing
season, so the dependence of total emissions over an entire growing season on soil temperature is weak. The
application of synthetic fertilizers has also been found to influence CH4 emissions; in particular, both nitrate and
sulfate fertilizers (e.g., ammonium nitrate and ammonium sulfate) appear to inhibit CH4 formation.
Rice is cultivated in seven states: Arkansas, California, Florida, Louisiana, Mississippi, Missouri, and Texas.127
Until 2006, rice was also cultivated in Oklahoma, but as of 2007 rice cultivation in the state ceased (Anderson
2008). Soil types, rice varieties, and cultivation practices for rice vary from state to state, and even from farm to
farm. However, most rice farmers apply organic fertilizers in the form of residue from the previous rice crop, which
is left standing, disked, or rolled into the fields. Most farmers also apply synthetic fertilizer to their fields, usually
urea. Nitrate and sulfate fertilizers are not commonly used in rice cultivation in the United States. In addition, the
climatic conditions of southwest Louisiana, Texas, and Florida often allow for a second, or ratoon, rice crop. Ratoon
crops are much less common or non-existent in Arkansas, California, Mississippi, Missouri, Oklahoma, and northern
areas of Louisiana. CH4 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 2007, CH4 emissions
from rice cultivation were 6.2 Tg CO2 Eq. (293 Gg). Although annual emissions fluctuated unevenly between the
years 1990 and 2007, ranging from an annual decrease of 14 percent to an annual increase of 17 percent, there was
an overall decrease of 14 percent over the seventeen-year period, due to an overall decrease in primary crop area.128
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.
Table 6-9: CH4 Emissions from Rice Cultivation (Tg CO2 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
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
2007
4.9
2.4
1.0
+
0.7
0.3
0.3
0.0
0.3
1.2
+
+
0.9
0.3
6.2
126 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.
127 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.
128 The 14 percent decrease occurred between 2005 and 2006; the 17 percent increase happened between 1993 and 1994.
Agriculture 6-13
-------
+ Less than 0.05 Tg CO2 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
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
2007
234
113
45
1
32
16
15
+
12
59
+
1
42
16
293
+ Lessthan0.5Gg
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).
The harvested rice areas for the primary and ratoon crops in each state are presented in Table 6-11, and the area of
ratoon crop area as a percent of primary crop area is shown in Table 6-12. Primary crop areas for 1990 through
2007 for all states except Florida and Oklahoma were taken from U.S. Department of Agriculture's Field Crops
Final Estimates 1987-1992 (USDA 1994), Field Crops Final Estimates 1992-1997 (USDA 1998), Field Crops Final
Estimates 1997-2002 (USDA 2003), and Crop Production Summary (USDA 2005 through 2008). Source data for
non-USDA sources of primary and ratoon harvest areas are shown in Table 6-13. California, Mississippi, Missouri,
and Oklahoma have not ratooned rice over the period 1990 through 2007 (Guethle 1999, 2000, 2001a, 2002 through
2008; Lee 2003 through 2007; Mutters 2002 through 2005; Street 1999 through 2003; Walker 2005, 2007, 2008).
Table 6-11: Rice Areas Harvested (Hectares)
State/Crop
Arkansas
Primary
Ratoon3
California
Florida
Primary
Ratoon
Louisiana
Primary
Ratoon
Mississippi
1990
485,633
0
159,854
4,978
2,489
220,558
66,168
101,174
1995
542,291
0
188,183
9,713
4,856
230,676
69,203
116,552
2000
570,619
0
221,773
7,801
3,193
194,253
77,701
88,223
2005
661,675
662
212,869
4,565
0
212,465
27,620
106,435
2006
566,572
6
211,655
4,575
1,295
139,620
27,924
76,487
2007
536,220
5
215,702
4,199
840
152,975
53,541
76,487
6-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Missouri
Oklahoma
Texas
Primary
Ratoon
Total Primary
Total Ratoon
Total
32,376
617
142,857
57,143
1,148,047
125,799
1,273,847
45,326
364
128,693
51,477
1,261,796
125,536
1,387,333
68,393
283
86,605
43,302
1,237,951
124,197
1,362,148
86,605
271
81,344
21,963
1,366,228
50,245
1,416,473
86,605
17
60,704
23,675
1,146,235
52,899
1,199,135
72,036
0
58,681
21,125
1,116,299
75,511
1,191,810
a Arkansas ratooning occurred only in 1998, 1999,2005, and 2006 and was assumed to occur in 2007.
Note: Totals may not sum due to independent rounding.
Table 6-12: Ratooned Area as Percent of Primary Growth Area
State
Arkansas
Florida
Louisiana
Texas
1990 1997 1998
0% +
50%
30%
40%
1999
65%
2000
41%
40%
50%
2001
60%
30%
40%
2002
0%
54%
15%
37%
2003
100%
35%
38%
2004
77%
30%
35%
2005
0.1%
0%
13%
27%
2006
28%
20%
39%
2007
20%
35%
36%
+ Indicates ratooning rate less than 0.1 percent.
Table 6-13: Non-USD A Data Sources for Rice Harvest Information
State/Crop
Ratoon
Primary
Ratoon
Ratoon
Oklahoma
Primary
1990 1999 2000
Scheuneman (1999b,
1999c, 2000, 2001a)
Scheuneman (1999a)
___________
2001 2002 2003 2004
Arkansas
Wilson (2002 - 2007)
Florida
Deren Kirstein (2003, 2006)
(2002)
Deren Kirstein (2003 Cantens
(2002) - 2004) (2005)
Louisiana
Linscombe (1999, 2001a, 2002
____________
2005 2006
Q^jJ^^O^
Gonzales (2006
_________
2007
_____
- 2008)
Anderson
(2008)
Ratoon
Texas
2003) Stansel (2004 -
2005)
Texas Ag Experiment
Station (2006 - 2008)
To determine what CH4 emission factors should be used for the primary and ratoon crops, CH4 flux information
from rice field measurements in the United States was collected. Experiments that involved atypical or
nonrepresentative management practices (e.g., the application of nitrate or sulfate fertilizers, or other substances
believed to suppress CH4 formation), as well as experiments in which measurements were not made over an entire
flooding season or floodwaters were drained mid-season, were excluded from the analysis. The remaining
experimental results129 were then sorted by season (i.e., primary and ratoon) and type of fertilizer amendment (i.e.,
no fertilizer added, organic fertilizer added, and synthetic and organic fertilizer added). The experimental results
from primary crops with added synthetic and organic fertilizer (Bossio et al. 1999; Cicerone et al. 1992; Sass et al.
1991a, 1991b) were averaged to derive an emission factor for the primary crop, and the experimental results from
ratoon crops with added synthetic fertilizer (Lindau and Bollich 1993, Lindau et al. 1995) were averaged to derive
an emission factor for the ratoon crop. The resultant emission factor for the primary crop is 210 kg CH^hectare-
129 In some of these remaining experiments, measurements from individual plots were excluded from the analysis because of the
aforementioned reasons. In addition, one measurement from the ratooned fields (i.e., the flux of 1,490 kg CH4/hectare-season in
Lindau and Bollich 1993) was excluded, because this emission rate is unusually high compared to other flux measurements in the
United States, as well as IPCC (2006) default emission factors.
Agriculture 6-15
-------
season, and the resultant emission factor for the ratoon crop is 780 kg CH^/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 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 CH^hectare-season
and ratoon emissions ranged from 481 to 1,490 kg CH^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 6-14. Rice cultivation CH4 emissions in 2007 were estimated to be between 2.1 and 16.3 Tg
CO2 Eq. at a 95 percent confidence level, which indicates a range of 66 percent below to 164 percent above the
actual 2007 emission estimate of 6.2 Tg CO2 Eq.
Table 6-14: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Rice Cultivation (Tg CO2 Eq. and
Percent)
Source Gas 2007 Emission
Estimate
(Tg C02 Eq.)
Rice Cultivation CH4 6.2
Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (%)
Lower
Bound
2.1
Upper
Bound
16.3
Lower
Bound
-66%
Upper
Bound
+164%
a 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.
Planned Improvements
A possible future improvement is to create region-specific emission factors for rice cultivation. The current
methodology uses a nationwide average emission factor, derived from several studies done in a number of states.
The prospective improvement would take the same studies and average them by region, presumably resulting in
more spatially-specific emission factors.
6-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
6.4. Agricultural Soil Management (IPCC Source Category 4D)
130
Nitrous oxide is produced naturally in soils through the microbial processes of nitrification and denitrification. 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).131 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, which are influenced by agricultural management through impacts on moisture and temperature
regimes in soils. These additional sources of mineral N are included at the recommendation of IPCC (2006) for
complete accounting of management impacts on greenhouse gas emissions, as discussed in the Methodology
section.132 Indirect emissions of N2O occur through two pathways: (1) volatilization and subsequent atmospheric
deposition of applied/mineralized N,133 and (2) surface runoff and leaching of applied/mineralized N into
groundwater and surface water. Direct emissions from agricultural lands (i.e., croplands and grasslands) are
included in this section, while direct emissions from forest lands and settlements are presented in the Land Use,
Land-Use Change, and Forestry chapter. However, indirect N2O emissions from all land-use types (cropland,
grassland, forest lands, and settlements) are reported in this section.
Figure 6-2: Agricultural Sources and Pathways of N that Result in N2O Emissions from Agricultural Soil
Management
Agricultural soils produce the majority of N2O emissions in the United States. Estimated emissions from this source
in 2007 were 207.9 Tg CO2 Eq. (671 Gg N2O) (see Table 6-15 and Table 6-16). Annual N2O emissions from
agricultural soils fluctuated between 1990 and 2007, although overall emissions were 3.8 percent higher in 2007
than in 1990. Year-to-year fluctuations are largely a reflection of annual variation in weather patterns, synthetic
fertilizer use, and crop production. On average, cropland accounted for approximately 69 percent of total direct
emissions, while grassland accounted for approximately 31 percent. These percentages are about the same for
indirect emissions since forest lands and settlements account for such a small percentage of total indirect emissions.
Estimated direct and indirect N2O emissions by sub-source category are shown in Table 6-17 and Table 6-18.
Table 6-15: N2O Emissions from Agricultural Soils (Tg CO2 Eq.)
Activity
Direct
Cropland
Grassland
1990
158.9
106.3
52.5
1995
165.8
114.2
51.6
2000
169.2
119.4
49.9
2005
174.4
122.2
52.1
2006
170.7
119.9
50.8
2007
172.0
121.9
50.1
130 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).
131 Drainage and cultivation of organic soils in former wetlands enhances mineralization of N-rich organic matter, thereby
enhancing N2O emissions from these soils.
132 Asymbiotic N fixation is the fixation of atmospheric N2 by bacteria living in soils that do not have a direct relationship with
plants.
133 These processes entail volatilization of applied or mineralized N as NH3 andNOx, 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),andNOx.
Agriculture 6-17
-------
Indirect (All Land-Use Types) 41.5
Cropland 29.1
Grassland 12.0
Forest Land +
Settlements 0.4
36.5
24.8
11.2
+
0.5
35.3
25.6
9.1
0.1
0.5
36.3
25.0
10.5
0.1
0.6
37.7
26.7
10.3
0.1
0.6
35.9
24.9
10.3
0.1
0.6
Total
200.3
202.3
204.5
210.6
208.4
207.9
+ Less than 0.05 Tg CO2 Eq.
Table 6-16: N2O Emissions from Agricultural Soils (Gg)
Activity
Direct
Cropland
Grassland
Indirect (All Land-Use Types)
Cropland
Grassland
Forest Land
Settlements
Total
1990
512
343
169
134
94
39
+
1
646
1995
535
368
167
118
80
36
+
2
653
2000
546
385
161
114
82
29
+
2
660
2005
562
394
168
117
81
34
+
2
679
2006
551
387
164
122
86
33
+
2
672
2007
555
393
162
116
80
33
+
2
671
Lessthan0.5GgN,O
Table 6-17: Direct N2O Emissions from Agricultural Soils by Land Use Type and N Input Type (Tg CO2 Eq.)
Activity 1990 1995 2000 2005 2006 2007
Cropland
Mineral Soils
Synthetic Fertilizer
Organic Amendments3
Residue Nb
Mineralization and
Asymbiotic Fixation
Organic Soils
Grassland
Synthetic Fertilizer
PRP Manure
Managed Manure0
Sewage Sludge
Residue Nd
Mineralization and
Asymbiotic Fixation
Total
106.3
103.5
41.0
7.6
7.0
47.8
2.9
52.5
1.0
10.3
1.0
0.3
12.0
27.9
158.9
114.2
111.3
46.6
8.3
7.7
48.7
2.9
51.6
1.0
10.9
0.9
0.3
11.9
26.6
165.8
119.4
116.5
45.4
8.8
7.7
54.6
2.9
49.9
0.9
10.2
0.9
0.4
11.1
26.3
169.2
122.2
119.3
48.3
9.2
7.5
54.3
2.9
52.1
0.9
10.7
1.0
0.5
11.8
27.3
174.4
119.9
117.0
46.5
9.3
7.6
53.7
2.9
50.8
0.9
10.5
1.0
0.5
11.5
26.4
170.7
121.9
119.0
47.3
9.8
7.6
54.4
2.9
50.1
0.9
10.4
1.0
0.5
11.3
26.0
172.0
1 Organic amendment inputs include managed manure amendments, daily spread manure amendments, and commercial organic
fertilizers (i.e., dried blood, dried manure, tankage, compost, and other).
b Cropland residue N inputs include N in unharvested legumes as well as crop residue N.
0 Accounts for managed manure and daily spread manure amendments that are applied to grassland soils.
d Grassland residue N inputs include N in ungrazed legumes as well as ungrazed grass residue N
Table 6-18: Indirect N2O Emissions from all Land-Use Types (Tg CO2 Eq.)
Activity
Cropland
Volatilization & Atm. Deposition
Surface Leaching &
Grassland
fc Run-Off
Volatilization & Atm. Deposition
Surface Leaching &
Forest Land
fc Run-Off
1990
29.1
7.8
21.3
12.0
5.6
6.4
1995
24.8
8.9
15.9
11.2
5.6
5.6
2000
25.6
9.0
16.6
9.1
5.0
4.0
0.1
2005
25.0
9.2
15.8
10.5
5.2
5.3
0.1
2006
26.7
10.1
16.6
10.3
5.2
5.1
0.1
2007
24.9
8.9
16.0
10.3
5.3
5.0
0.1
6-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Volatilization & Atm. Deposition + + + + + +
Surface Leaching &
Settlements
fc Run-Off
Volatilization & Atm. Deposition
Surface Leaching &
Total
fc Run-Off
+
0.4
0.1
0.2
41.5
+
0.5
0.2
0.3
36.5
0.1
0.5
0.2
0.3
35.3
0.1
0.6
0.2
0.4
36.3
0.1
0.6
0.2
0.4
37.7
0.1
0.6
0.2
0.4
35.9
+ Less than 0.05 Tg CO2 Eq.
Figure 6-3 through Figure 6-6 show regional patterns in direct N2O emissions, and also show N losses from
volatilization, leaching, and runoff that lead to indirect N2O emissions. Average annual emissions and N losses
from croplands that produce major crops and from grasslands are shown for each state. Direct N2O emissions from
croplands tend to be high in the Corn Belt (Illinois, Iowa, Indiana, Ohio, southern Minnesota, and eastern Nebraska),
where a large portion of the land is used for growing highly fertilized corn and N-fixing soybean crops. Direct
emissions are also high in North Dakota, Kansas, and Texas, primarily from irrigated cropping and dryland wheat.
Direct emissions are low in many parts of the eastern United States because a small portion of land is cultivated, and
also low in many western states where rainfall and access to irrigation water are limited.
Direct emissions (Tg CO2 Eq./state/year) from grasslands are highest in the central and western United States
(Figure 6-4) where a high proportion of the land is used for cattle grazing. Some areas in the Great Lake states, the
Northeast, and Southeast have moderate emissions even though emissions from these areas tend to be high on a per
unit area basis, because the total amount of grazed land is much lower than states in the central and western United
States.
Indirect emissions from croplands and grasslands (Figure 6-5 and Figure 6-6) show patterns similar to direct
emissions, because the factors that control direct emissions (N inputs, weather, soil type) also influence indirect
emissions. However, there are some exceptions, because the processes that contribute to indirect emissions (NO3~
leaching, N volatilization) do not respond in exactly the same manner as the processes that control direct emissions
(nitrification and denitrification). For example, coarser-textured soils facilitate relatively high indirect emissions in
Florida grasslands due to high rates of N volatilization and NO3" leaching, even though they have only moderate
rates of direct N9O emissions.
Figure 6-3: Major Crops, Average Annual Direct N2O Emissions by State, Estimated Using the DAYCENT Model,
1990-2007 (Tg CO2 Eq./year)
Figure 6-4: Grasslands, Average Annual Direct N2O Emissions by State, Estimated Using the DAYCENT Model,
1990-2007 (Tg CO2 Eq./year)
Figure 6-5: Major Crops, Average Annual N Losses Leading to Indirect N2O Emissions by State, Estimated Using
the DAYCENT Model, 1990-2007 (Gg N/year)
Figure 6-6: Grasslands, Average Annual N Losses Leading to Indirect N2O Emissions, by State, Estimated Using the
DAYCENT Model, 1990-2007 (Gg N/year)
Methodology
The 2006IPCC Guidelines (IPCC 2006) divide the Agricultural Soil Management source category into four
components: (1) direct emissions due to N additions to cropland and grassland mineral soils, including synthetic
fertilizers, sewage sludge applications, crop residues, organic amendments, and biological nitrogen fixation
associated with planting of legumes on cropland and grassland soils; (2) direct emissions from drainage and
cultivation of organic cropland soils; (3) direct emissions from soils due to the deposition of manure by livestock on
PRP grasslands; and (4) indirect emissions from soils and water due to N additions and manure deposition to soils
Agriculture 6-19
-------
that lead to volatilization, leaching, or runoff of N and subsequent conversion to N2O.
The United States has adopted recommendations from IPCC (2006) on methods for agricultural soil management.
These recommendations include (1) estimating the contribution of N from crop residues to indirect soil N2O
emissions; (2) adopting a revised emission factor for direct N2O emissions to the extent that Tier 1 methods are used
in the Inventory (described later in this section); (3) removing double counting of emissions from N-fixing crops
associated with the biological N fixation and crop residue N input categories; (4) using revised crop residue statistics
to compute N inputs to soils based on harvest yield data; (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 fixation134 (i.e.,
computing total emissions from managed land); (6) reporting all emissions from managed lands, largely because
management affects all processes leading to soil N2O emissions. One recommendation from IPCC (2006) has not
been adopted: accounting for emissions from pasture renewal, which involves occasional plowing to improve forage
production. This practice is not common in the United States, and is not estimated.
The methodology used to estimate emissions from agricultural soil management in the United States is based on a
combination of IPCC Tier 1 and 3 approaches. A Tier 3, process-based model (DAYCENT) was used to estimate
direct emissions from major crops on mineral (i.e., non-organic) soils; as well as most of the direct emissions from
grasslands. The Tier 3 approach has been specifically designed and tested to estimate N2O emissions in the United
States, accounting for more of the environmental and management influences on soil N2O emissions than the IPCC
Tier 1 method (see Box 6-1 for further elaboration). The Tier 1 IPCC (2006) methodology was used to estimate (1)
direct emissions from non-major crops on mineral soils (e.g., barley, oats, vegetables, and other crops), (2) the
portion of the grassland direct emissions that were not estimated with the Tier 3 DAYCENT model (i.e., federal
grasslands), and (3) direct emissions from drainage and cultivation of organic cropland soils. Indirect emissions
were also estimated with a combination of DAYCENT and the IPCC Tier 1 method.
In past 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 and legume cropping. Given the recommendation from IPCC
(2006) and the influence of management on all processes leading to N2O emissions from soils in agricultural
systems, the decision was made to report total emissions from managed lands for this source category. Annex 3.11
provides more detailed information on the methodologies and data used to calculate N2O emissions from each
component.
[BEGIN BOX]
Box 6-1. Tier 1 vs. Tier 3 Approach for Estimating N2O Emissions
The IPCC (2006) Tier 1 approach is based on multiplying activity data on different N inputs (e.g., synthetic
fertilizer, manure, N fixation, etc.) by the appropriate default IPCC emission factors to estimate N2O emissions on a
input-by-input basis. The Tier 1 approach requires a minimal amount of activity data, readily available in most
countries (e.g., total N applied to crops); calculations are simple; and the methodology is highly transparent. In
contrast, the Tier 3 approach developed for this Inventory employs a process-based model (i.e., DAYCENT) that
represents the interaction of N inputs and the environmental conditions at specific locations. Consequently, the Tier
3 approach is likely to produce more accurate estimates; it accounts more comprehensively for land-use and
management impacts and their interaction with environmental factors (i.e., weather patterns and soil characteristics),
which may enhance or dampen anthropogenic influences. However, the Tier 3 approach requires more refined
activity data (e.g., crop-specific N amendment rates), additional data inputs (e.g., daily weather, soil types, etc.), and
considerable computational resources and programming expertise. The Tier 3 methodology is less transparent, and
134 N inputs from asymbiotic N fixation are not directly addressed in 2006 IPCC Guidelines, but are a component of the total
emissions from managed lands and are included in the Tier 3 approach developed for this source.
6-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
thus it is critical to evaluate the output of Tier 3 methods against measured data in order to demonstrate the
adequacy of the method for estimating emissions (IPCC 2006). Another important difference between the Tier 1
and Tier 3 approaches relates to assumptions regarding N cycling. Tier 1 assumes that N added to a system is
subject to N2O emissions only during that year and cannot be stored in soils and contribute to N2O emissions in
subsequent years. This is a simplifying assumption that is likely to create bias in estimated N2O emissions for a
specific year. In contrast, the process-based model used in the Tier 3 approach includes such legacy effects when N
added to soils is re-mineralized from soil organic matter and emitted as N2O during subsequent years.
[END BOX]
Direct N2O Emissions from Cropland Soils
Major Crop Types on Mineral Cropland Soils
The DAYCENT ecosystem model (Del Grosso et al. 2001, Parton et al. 1998) was used to estimate direct N2O
emissions from mineral cropland soils that are managed for production of major crops—specifically corn, soybeans,
wheat, alfalfa hay, other hay, sorghum, and cotton—representing approximately 90 percent of total croplands in the
United States. For these croplands, DAYCENT was used to simulate crop growth, soil organic matter
decomposition, greenhouse gas fluxes, and key biogeochemical processes affecting N2O emissions, and the
simulations were driven by model input data generated from daily weather records (Thornton et al. 1997, 2000;
Thornton and Running 1999), land management surveys (see citations below), and soil physical properties
determined from national soil surveys (Soil Survey Staff 2005). Note that the influence of land-use change on soil
N2O emissions was not addressed in this analysis, but is a planned improvement.
DAYCENT simulations were conducted for each major crop at the county scale in the United States. Simulating
N2O emissions at the county scale was facilitated by soil and weather data that were available for every county with
more than 100 acres of agricultural land, and by land management data (e.g., timing of planting, harvesting, intensity
of cultivation) that were available at the agricultural-region level as defined by the Agricultural Sector Model
(McCarl et al. 1993). ASM has 63 agricultural regions in the contiguous United States. Most regions correspond to
one state, except for those states with greater heterogeneity in agricultural practices; in such cases, more than one
region is assigned to a state. While cropping systems were simulated for each county, the results best represent
emissions at regional (i.e., state) and national levels due to the regional scale of management data, which include
model parameters that determined the influence of management activities on soil N2O emissions (e.g., when crops
were planted/harvested).
Nitrous oxide emissions from managed agricultural lands are the result of interactions among anthropogenic
activities (e.g., N fertilization, manure application, tillage) and other driving variables, such as weather and soil
characteristics. These factors influence key processes associated with N dynamics in the soil profile, including
immobilization of N by soil microbial organisms, decomposition of organic matter, plant uptake, leaching, runoff,
and volatilization, as well as the processes leading to N2O production (nitrification and denitrification). It is not
possible to partition N2O emissions by anthropogenic activity directly from model outputs due to the complexity of
the interactions (e.g., N2O emissions from synthetic fertilizer applications cannot be distinguished from those
resulting from manure applications). To approximate emissions by activity, the amount of mineral N added to the
soil for each of these sources was determined and then divided by the total amount of mineral N that was made
available in the soil according to the DAYCENT model. The percentages were then multiplied by the total of direct
N2O emissions in order to approximate the portion attributed to key practices. This approach is only an
approximation because it assumes that all N made available in soil has an equal probability of being released as
N2O, regardless of its source, which is unlikely to be the case. However, this approach allows for further
disaggregation of emissions by source of N, which is valuable for reporting purposes and is analogous to the
reporting associated with the IPCC (2006) Tier 1 method, in that it associates portions of the total soil N2O
emissions with individual sources of N.
DAYCENT was used to estimate direct N2O emissions due to mineral N available from: (1) the application of
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
Agriculture 6-21
-------
litter, in addition to asymbiotic fixation. Note that commercial organic fertilizers are addressed with the Tier 1
method because county-level application data would be needed to simulate applications in the DAYCENT, and
currently data are only available at the national scale. The third and fourth sources are generated internally by the
DAYCENT model. For the first two practices, annual changes in soil mineral N due to anthropogenic activity were
obtained or derived from the following sources:
• Crop-specific N-fertilization rates: Data sources for fertilization rates include Alexander and Smith (1990),
Anonymous (1924), Battaglin and Goolsby (1994), Engle and Makela (1947), ERS (1994, 2003), Fraps and
Asbury (1931), Ibach and Adams (1967), Ibach et al. (1964), NFA (1946), NRIAI (2003), Ross and Mehring
(1938), Skinner (1931), Smalley et al. (1939), Taylor (1994), USDA (1966, 1957, 1954, 1946). Information on
fertilizer use and rates by crop type for different regions of the United States were obtained primarily from the
USDA Economic Research Service Cropping Practices Survey (ERS 1997) with additional data from other
sources, including the National Agricultural Statistics Service (NASS 1992, 1999, 2004).
• Managed manure production and application to croplands and grasslands: Manure N amendments and daily
spread manure N amendments applied to croplands and grasslands (not including PRP manure) were
determined using USDA Manure N Management Databases for 1997 (Kellogg et al. 2000; Edmonds et al.
2003). Amendment data for 1997 were scaled to estimate values for other years based on the availability of
managed manure N for application to soils in 1997 relative to other years. The amount of available nitrogen
from managed manure for each livestock type was calculated as described in the Manure Management section
(Section 6.2) and annex (Annex 3.10).
• Retention of crop residue, N mineralization from soil organic matter, and asymbiotic N fixation from the
atmosphere: The IPCC approach considers crop residue N and N mineralized from soil organic matter as
activity data. However, they are not treated as activity data in DAYCENT simulations because residue
production, N fixation, mineralization of N from soil organic matter, and asymbiotic fixation are internally
generated by the model. In other words, DAYCENT accounts for the influence of N fixation, mineralization of
N from soil organic matter, and retention of crop residue on N2O emissions, but these are not model inputs.
• Historical and modern crop rotation and management information (e.g., timing and type of cultivation, timing of
planting/harvest, etc.): These activity data were derived from Kurd (1930, 1929), Latta (1938), Iowa State
College Staff Members (1946), Bogue (1963), Hurt (1994), USDA (2000a) as extracted by Eve (2001) and
revised by Ogle (2002), CTIC (1998), Piper et al. (1924), Hardies and Hume (1927), Holmes (1902, 1929),
Spillman (1902, 1905, 1907, 1908), Chilcott (1910), Smith (1911), Kezer (ca. 1917), Hargreaves (1993), ERS
(2002), Warren (1911), Langston et al. (1922), Russell et al. (1922), Elliott and Tapp (1928), Elliott (1933),
Ellsworth (1929), Garey (1929), Hodges et al. (1930), Bonnen and Elliott (1931), Brenner et al. (2002, 2001),
and Smith et al. (2002). . Approximately 3 percent of the crop residues were assumed to be burned based on
state inventory data (ILENR 1993, Oregon Department of Energy 1995, Noller 1996, Wisconsin Department of
Natural Resources 1993, and Cibrowski 1996), and therefore did not contribute to soil N2O emissions.
DAYCENT simulations produced per-area estimates of N2O emissions (g N2O-N/m2) for major crops in each
county, which were multiplied by the cropland areas in each county to obtain county-scale emission estimates.
Cropland area data were from NASS (USDA 2008a,b). The emission estimates by reported crop areas in the county
were scaled to the regions, and the national estimate was calculated by summing results across all regions.
DAYCENT is sensitive to interannual variability in weather patterns and other controlling variables, so emissions
associated with individual activities vary through time even if the management practices remain the same (e.g., if N
fertilization remains the same for two years). In contrast, Tier 1 methods do not capture this variability and rather
have a linear, monotonic response that depends solely on management practices. DAYCENT's ability to capture
these interactions between management and environmental conditions produces more accurate estimates of N2O
emissions than the Tier 1 method.
Non-Major Crop Types on Mineral Cropland Soils
The IPCC (2006) Tier 1 methodology was used to estimate direct N2O emissions for mineral cropland soils that are
managed for production of non-major crop types, including barley, oats, tobacco, sugarcane, sugar beets,
sunflowers, millet, rice, peanuts, and other crops that were not included in the DAYCENT simulations. Estimates of
direct N2O emissions from N applications to non-major crop types were based on mineral soil N that was made
available from the following practices: (1) the application of synthetic commercial fertilizers, (2) application of
6-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
managed manure and non-manure commercial organic fertilizers,135 and (3) the retention of above- and below-
ground crop residues in agricultural fields (i.e., crop biomass that is not harvested). Non-manure organic
amendments were not included in the DAYCENT simulations because county-level data were not available.
Consequently, non-manure organic amendments, as well as manure amendments not included in the DAYCENT
simulations, were included in the Tier 1 analysis. The influence of land-use change on soil N2O emissions from
non-major crops has not been addressed in this analysis, but is a planned improvement. The following sources were
used to derive activity data:
• A process-of-elimination approach was used to estimate synthetic N fertilizer additions for non-major crops,
because little information exists on their fertilizer application rates. The total amount of fertilizer used on farms
has been estimated by the USGS from sales records (Ruddy et al. 2006), and these data were aggregated to
obtain state-level N additions to farms. After subtracting the portion of fertilizer applied to major crops and
grasslands (see sections on Major Crops and Grasslands for information on data sources), the remainder of the
total fertilizer used on farms was assumed to be applied to non-major crops.
• A process-of-elimination approach was used to estimate manure N additions for non-major crops, because little
information exists on application rates for these crops. The amount of manure N applied to major crops and
grasslands was subtracted from total manure N available for land application (see sections on Major Crops and
Grasslands for information on data sources), and this difference was assumed to be applied to non-major crops.
• Non-manure, non-sewage-sludge commercial organic fertilizer additions were based on organic fertilizer
consumption statistics, which were converted to units of N using average organic fertilizer N content (TVA
1991 through 1994; AAPFCO 1995 through 2008). Manure and sewage sludge components were subtracted
from total commercial organic fertilizers to avoid double counting.
• Crop residue N was derived by combining amounts of above- and below-ground biomass, which were
determined based on crop production yield statistics (USDA 1994, 1998, 2003, 2005, 2006, 2008a), dry matter
fractions (IPCC 2006), linear equations to estimate above-ground biomass given dry matter crop yields from
harvest (IPCC 2006), ratios of below-to-above-ground biomass (IPCC 2006), and N contents of the residues
(IPCC 2006). Approximately 3 percent of the crop residues were burned and therefore did not contribute to soil
N2O emissions, based on state inventory data (ILENR 1993, Oregon Department of Energy 1995, Noller 1996,
Wisconsin Department of Natural Resources 1993, and Cibrowski 1996).
The total increase in soil mineral N from applied fertilizers and crop residues was multiplied by the IPCC (2006)
default emission factor to derive an estimate of direct N2O emissions from non-major crop types.
Drainage and Cultivation of Organic Cropland Soils
The IPCC (2006) Tier 1 methods were used to estimate direct N2O emissions due to drainage and cultivation of
organic soils at a state scale. State-scale estimates of the total area of drained and cultivated organic soils were
obtained from the National Resources Inventory (NRI) (USDA 2000a, as extracted by Eve 2001 and amended by
Ogle 2002). Temperature data from Daly et al. (1994, 1998) were used to subdivide areas into temperate and
tropical climates using the climate classification from IPCC (2006). Data were available for 1982, 1992 and 1997.
To estimate annual emissions, the total temperate area was multiplied by the IPCC default emission factor for
temperate regions, and the total sub-tropical area was multiplied by the average of the IPCC default emission factors
for temperate and tropical regions (IPCC 2006).
Direct N2O Emissions from Grassland Soils
As with N2O from croplands, the Tier 3 process-based DAYCENT model and Tier 1 method described in IPCC
(2006) were combined to estimate emissions from grasslands. Grasslands include pastures and rangelands used for
grass forage production, where the primary use is livestock grazing. Rangelands are typically extensive areas of
native grasslands that are not intensively managed, while pastures are often seeded grasslands, possibly following
tree removal, which may or may not be improved with practices such as irrigation and interseeding legumes.
135 Commercial organic fertilizers include dried blood, tankage, compost, and other; dried manure and sewage sludge that are
used as commercial fertilizer have been excluded to avoid double counting. The dried manure N is counted with the non-
commercial manure applications, and sewage sludge is assumed to be applied only to grasslands.
Agriculture 6-23
-------
DAYCENT was used to simulate county-scale N2O emissions from non-federal grasslands resulting from manure
deposited by livestock directly onto pastures and rangelands (i.e., PRP manure), N fixation from legume seeding,
managed manure amendments (i.e., manure other than PRP manure), and synthetic fertilizer application.). Other N
inputs were simulated within the DAYCENT framework, including N input from mineralization due to
decomposition of soil organic matter and N inputs from senesced grass litter, as well as asymbiotic fixation of N
from the atmosphere. The simulations used the same weather, soil, and synthetic N fertilizer data as discussed under
the section for Major Crop Types on Mineral Cropland Soils. Managed manure N amendments to grasslands were
estimated from Edmonds et al. (2003) and adjusted for annual variation using data on the availability of managed
manure N for application to soils, according to methods described in the Manure Management section (Section 6.2)
and annex (Annex 3.10). Biological N fixation is simulated within DAYCENT and therefore was not an input to the
model.
Manure N deposition from grazing animals (i.e., PRP manure) was an input to the DAYCENT model (see Annex
3.10), and included approximately 91 percent of total PRP manure. The remainder of the PRP manure N excretions
in each county was assumed to be excreted on federal grasslands (i.e., DAYCENT simulations were only conducted
for non-federal grasslands), and the N2O emissions were estimated using the IPCC (2006) Tier 1 method with IPCC
default emission factors. The amounts of PRP manure N applied on non-federal and federal grasslands in each
county were based on the proportion of non-federal grassland area according to data from the NRI (USDA 2000a),
relative to the area of federal grasslands from the National Land Cover Dataset (Vogelman et al. 2001).
Sewage sludge was assumed to be applied on grasslands because of the heavy metal content and other pollutants in
human waste that limit its use as an amendment to croplands. Sewage sludge application was estimated from data
compiled by EPA (1993, 1999, 2003), McFarland (2001), and NEBRA (2007). Sewage sludge data on soil
amendments on agricultural lands were only available at the national scale, and it was not possible to associate
application with specific soil conditions and weather at the county scale. Therefore, DAYCENT could not be used
to simulate the influence of sewage sludge amendments on N2O emissions from grassland soils, and consequently,
emissions from sewage sludge were estimated using the IPCC (2006) Tier 1 method.
DAYCENT simulations produced per-area estimates of N2O emissions (g N2O-N/m2) for pasture and rangelands,
which were multiplied by the reported pasture and rangeland areas in each county. Grassland area data were
obtained from the NRI (USDA 2000a). The 1997 NRI area data for pastures and rangeland were aggregated to the
county level to estimate the grassland areas for 1995 to 2007, and the 1992 NRI pasture and rangeland data were
aggregated to the county level to estimate areas from 1990 to 1994. The county estimates were scaled to the 63
agricultural regions, and the national estimate was calculated by summing results across all regions. Tier 1 estimates
of N2O emissions for the PRP manure N applied to non-federal lands and sewage sludge N were produced by
multiplying the N input by the appropriate emission factor.
Total Direct N2O Emissions from Cropland and Grassland Soils
Annual direct emissions from major and non-major crops on mineral cropland soils, from drainage and cultivation of
organic cropland soils, and from grassland soils were summed to obtain the total direct N2O emissions from
agricultural soil management (see Table 6-15 and Table 6-16).
Indirect N2O 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, organic amendments (e.g., manure, sewage sludge),
and deposition of PRP manure. N made available from mineralization of soil organic matter and asymbiotic fixation
also contributes to volatilized N emissions. Volatilized N can be returned to soils through atmospheric deposition,
and a portion 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, and asymbiotic fixation. The nitrate is subject to denitrification in water
bodies, which leads to N2O emissions. Regardless of the eventual location of the indirect N2O emissions, the
emissions are assigned to the original source of the N for reporting purposes, which here includes croplands,
grasslands, forest lands, and settlements.
6-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Indirect N2O Emissions from Atmospheric Deposition of Volatilized N from Managed Soils
Similarly to the direct emissions calculation, the Tier 3 DAYCENT model and IPCC (2006) Tier 1 methods were
combined to estimate the amount of N that was transported from croplands, grasslands, forest lands, and settlements
through volatilization, and eventually emitted as N2O. DAYCENT was used to estimate N volatilization for land
areas whose direct emissions were simulated with DAYCENT (i.e., major croplands and most grasslands). The N
inputs included are the same as described for direct N2O emissions in the sections on major crops and grasslands.
The Tier 1 method and default IPCC fractions for N subject to volatilization were used for areas and N applications
that were not simulated with DAYCENT (i.e., N inputs on non-major croplands, PRP manure N excretion on federal
grasslands, sewage sludge application on grasslands). The Tier 1 method and default fractions were also used to
estimate N subject to volatilization from N inputs on settlements and forest lands (see the Land Use, Land-Use
Change, and Forestry chapter). With the DAYCENT and Tier 1 approaches, the IPCC (2006) default emission factor
was used to estimate indirect N2O emissions associated with the amount of volatilized N (Table 6-18).
Indirect N2O from Leaching/Runoff
As with the calculations of indirect emissions from volatilized N, the Tier 3 DAYCENT model and IPCC (2006)
Tier 1 method were combined to estimate the amount of N that was transported from croplands, grasslands, forest
lands, and settlements through leaching and surface runoff into water bodies, and eventually emitted as N2O.
DAYCENT was used to simulate the amount of N transported from lands used to produce major crops and most
grasslands. N transport from all other areas was estimated using the Tier 1 method and the IPCC (2006) default
factor for the proportion of N subject to leaching and runoff. This N transport estimate includes N applications on
croplands that produce non-major crops, sewage sludge amendments on grasslands, PRP manure N excreted on
federal grasslands, and N inputs on settlements and forest lands. For both the DAYCENT and IPCC (2006) Tier 1
methods, nitrate leaching was assumed to be an insignificant source of indirect N2O in cropland and grassland
systems where the amount of precipitation plus irrigation did not exceed the potential evapotranspiration, as
recommended by IPCC (2006). With both the DAYCENT and Tier 1 approaches, the IPCC (2006) default emission
factor was used to estimate indirect N2O emissions associated with N losses through leaching and runoff (Table
6-18).
Uncertainty
Uncertainty was estimated for each of the following five components of N2O emissions from agricultural soil
management: (1) direct emissions calculated by DAYCENT, (2) the components of indirect emissions (N
volatilized and leached or runoff) calculated by DAYCENT (3) direct emissions calculated with the IPCC (2006)
Tier 1 method, (4) the components of indirect emissions (N volatilized and leached or runoff) calculated with the
IPCC (2006) Tier 1 method, and (5) indirect emissions calculated with the IPCC (2006) Tier 1 method. Uncertainty
in direct emissions, which account for the majority of N2O emissions from agricultural management, as well as the
components of indirect emissions calculated by DAYCENT were estimated with a Monte Carlo Analysis,
addressing uncertainties in model inputs and structure (i.e., algorithms and parameterization). Uncertainties in direct
emissions calculated with the IPCC (2006) Tier 1 method, the proportion of volatilization and leaching or runoff
estimated with the IPCC (2006) Tier 1 method, and indirect N2O emissions were estimated with a simple error
propagation approach (IPCC 2006). Additional details on the uncertainty methods are provided in Annex 3.11.
Uncertainties from the Tier 1 and Tier 3 (i.e., DAYCENT) estimates were combined using simple error propagation
(IPCC 2006), and the results are summarized in Table 6-19. Agricultural direct soil N2O emissions in 2007 were
estimated to be between 126.2 and 265.2 Tg CO2 Eq. at a 95 percent confidence level. This indicates a range of 27
percent below and 54 percent above the 2007 emission estimate of 172.0 Tg CO2 Eq. The indirect soil N2O
emissions in 2007 were estimated to range from 20.5 to 84.8 Tg CO2 Eq. at a 95 percent confidence level, indicating
an uncertainty of 43 percent below and 136 percent above the 2007 emission estimate of 35.9 Tg CO2 Eq.
Table 6-19: Quantitative Uncertainty Estimates of N2O Emissions from Agricultural Soil Management in 2007 (Tg
CO2 Eq. and Percent)
2007 Emission Uncertainty Range Relative to Emission
Source Gas Estimate Estimate
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Lower
Bound
Upper
Bound
Lower Upper
Bound Bound
Agriculture 6-25
-------
Direct Soil N2O Emissions N2O 172.0 126.2 265.2 -27% +54%
Indirect Soil N2O Emissions N2O 35.9 20.6 84.8 -43% +136%
Note: Due to lack of data, uncertainties in areas for major crops, managed manure N production, PRP manure N production, other
organic fertilizer amendments, indirect losses of N in the DAYCENT simulations, and sewage sludge amendments to soils are
currently treated as certain; these sources of uncertainty will be included in future Inventories.
QA/QC and Verification
For quality control, DAYCENT results for N2O emissions and NO3" leaching were compared with field data
representing various cropped/grazed systems, soil types, and climate patterns (Del Grosso et al. 2005, Del Grosso et
al. 2008), and further evaluated by comparing to emission estimates produced using the IPCC (2006) Tier 1 method
for the same sites. N2O measurement data were available for 11 sites in the United States and one in Canada,
representing 30 different combinations of fertilizer treatments and cultivation practices. DAYCENT estimates of
N2O emissions were closer to measured values at all sites except for Colorado dryland cropping (Figure 6-7). In
general, IPCC Tier 1 methodology tends to over-estimate emissions when observed values are low and under-
estimate emissions when 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 in the United States representing nine different combinations of
fertilizer amendments. Linear regressions of simulated vs. observed emission and leaching data yielded correlation
coefficients of 0.89 and 0.94 for annual N2O emissions and NO3" leaching, respectively. This comparison
demonstrates that DAYCENT provides relatively high predictive capability for N2O emissions and NO3" leaching,
and is an improvement over the IPCC Tier 1 method (see additional information in Annex 3.11).
Figure 6-7: Comparison of Measured Emissions at Field Sites with Modeled Emissions Using the DAYCENT
Simulation Model
Spreadsheets containing input data and probability distribution functions 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. Several errors were identified following re-organization of the
calculation spreadsheets, and corrective actions have been taken. In particular, some of the links between
spreadsheets were missing or needed to be modified. Spreadsheets containing input data, emission factors, and
calculations required for the Tier 1 approach were checked and no errors were found.
Recalculations Discussion
Several revisions were made in the Agricultural Soil Management Section for the current Inventory.
First, a new version of the DAYCENT model was made operational for the Inventory. This version of DAYCENT
has several improvements, including elimination of the influence of labile (i.e., easily decomposable by microbes) C
availability on surface litter denitrification rates, incorporation of precipitation events as a controlling variable on
surface litter denitrification, and allowing the wettest soil layer within the rooting zone to control plant transpiration.
Second, given a new operational version of DAYCENT, the structural uncertainty in the model was re-evaluated and
estimates were revised from the previous Inventory. In the current application, residual error from the linear mixed-
effect model was also included as a component of the structural uncertainty, and this led to a larger uncertainty in
the N2O emission estimates from DAYCENT. This component was not addressed in the previous Inventory because
it was considered measurement error. However, some of the residual error is likely associated with the structure of
the model. In addition, structural uncertainty was evaluated in the grassland predictions from DAYCENT, which
had not been included in the previous Inventory.
Third, PRP manure N deposition on non-federal grasslands was estimated from county-level grazing animal
population data, instead of using estimates of N deposition computed internally in the DAYCENT model. Quality
control on the previous Inventory suggested that DAYCENT over-estimated PRP manure N deposition in some
states. This improvement ensures that the data on PRP manure N in the DAYCENT model simulations is consistent
with N excretion data from the Manure Management section of this Inventory.
6-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Fourth, nitrate leaching was assumed to be an insignificant source of indirect N2O in cropland and grassland systems
where the amount of precipitation plus irrigation did not exceed the potential evapotranspiration, as recommended
by IPCC (2006). These areas are typically semi-arid to arid, and nitrate leaching to groundwater is a relatively
uncommon event. Adopting this recommendation reduced indirect N2O emissions.
The recalculations associated with these changes reduced emissions by about 23 percent on average, primarily due
to the new operational version of DAYCENT, revised structural uncertainty associated with the model, and reduced
impact of N leaching on indirect N2O emissions in arid and semi-arid regions. Earlier versions of DAYCENT
tended to over-estimate emissions above 6 g N2O/m2, and although these emissions were adjusted using the
structural uncertainty estimator, there was considerable uncertainty in those adjustments. The new operational
version of DAYCENT does not overestimate N2O emissions for the majority of crops, with the exception of small
grains.
Including residual error from the linear mixed-effect model as a component of the structural uncertainty and
addressing structural uncertainty in the grassland predictions from DAYCENT resulted in wider 95 percent
confidence intervals compared to the previous Inventory. Of these changes, including structural uncertainty in the
grassland predictions from DAYCENT was responsible for most of the increase in uncertainty.
Planned Improvements
Several improvements are planned for the Agricultural Soil Management sector. The first improvement is to
incorporate more land-use survey data from the NRI (USDA 2000a) into the DAYCENT simulation analysis,
beyond the area estimates for rangeland and pasture that are currently used to estimate emissions from grasslands.
NRI has a record of land-use activities since 1979 for all U.S. agricultural land, which is estimated at about 386
Mha. NASS is used as the basis for land-use records in the current Inventory, and there are three major
disadvantages to this. First, most crops are grown in rotation with other crops (e.g., corn-soybean), but NASS data
provide no information regarding rotation histories. In contrast, NRI is designed to track rotation histories, which is
important because emissions from any particular year can be influenced by the crop that was grown the previous
year. Second, NASS does not conduct a complete survey of cropland area each year, leading to gaps in the land
base. NRI provides a complete history of cropland areas for four out of every five years from 1979 to 1997, and
then every year after 1998. Third, the current Inventory based on NASS does not quantify the influence of land-use
change on emissions, which can be addressed using the NRI survey records. NRI also provides additional
information on pasture land management that can be incorporated into the analysis (particularly the use of
irrigation). Using NRI data will also make the Agricultural Soil Management methods more consistent with the
methods used to estimate C stock changes for agricultural soils. The structure of model input files that contain land
management data will need to be extensively revised to facilitate use of the annualized NRI data. This improvement
is planned to take place over the next several years.
Other planned improvements are minor but will lead to more accurate estimates, including updating DAYMET
weather data for more recent years, setting the PRP emission factor for horse, sheep and goats to 0.01 in accordance
with guidance from IPCC (2006) and using a rice-crop-specific EF for N amendments to rice areas.
6.5. Field Burning of Agricultural Residues (IPCC Source Category 4F)
Farming activities produce large quantities of agricultural crop residues, and farmers use or dispose of these residues
in a variety of ways. For example, agricultural residues can be left on or plowed into the field; composted and then
applied to soils; landfilled; or burned in the field. Alternatively, they can be collected and used as fuel, animal
bedding material, supplemental animal feed, or construction material. Field burning of crop residues is not
considered a net source of CO2, because the C released to the atmosphere as CO2 during burning is assumed to be
reabsorbed during the next growing season. Crop residue burning is, however, a net source of CH4, N2O, CO, and
NOX, which are released during combustion.
Field burning is not a common method of agricultural residue disposal in the United States. The primary crop types
whose residues are typically burned in the United States are wheat, rice, sugarcane, corn, barley, soybeans, and
peanuts. It is assumed that 3 percent of the residue for each of these crops is burned each year, except for rice.136 In
136 The fraction of rice straw burned each year is significantly higher than that for other crops (see "Methodology" discussion
Agriculture 6-27
-------
2007, CH4 and N2O emissions from field burning were 0.9 Tg CO2 Eq. (42 Gg) and 0.5 Tg. CO2 Eq. (2 Gg),
respectively. Annual emissions from this source over the period 1990 to 2007 have remained relatively constant,
averaging approximately 0.8 Tg CO2 Eq. (37 Gg) of CH4 and 0.4 Tg CO2 Eq. (1 Gg) of N2O (see Table 6-20 and
Table 6-21).
Table 6-20: CH4 and N2O Emissions from Field Burning of Agricultural Residues (Tg CO2 Eq.)
Gas/Crop Type
CH4
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
N2O
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
Total
+ Less than 0.05 Tg
1990
0.7
0.1
0.1
+
0.3
+
0.1
+
0.4
+
+
+
0.1
+
0.2
+
1.1
C02 Eq.
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
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
2007
0.9
0.1
0.1
+
0.5
+
0.2
+
0.5
+
+
+
0.1
+
0.2
+
1.4
Note: Totals may not sum due to independent rounding.
Table 6-21: CH4,
Gas/Crop Type
CH4
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
N2O
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
CO
NOX
N2O, CO, andNOx
1990
33
7
4
1
13
1
7
+
1
+
+
+
+
+
1
+
691
28
Emissions from Field Burning of Agricultural
1995
32
5
4
1
13
1
8
+
1
+
+
+
+
+
1
+
663
29
2000
38
5
4
1
17
1
10
+
1
+
+
+
+
+
1
+
792
35
Residues
2005 2006
41
5
5
1
19
+
11
+
2
+
+
+
+
+
1
+
860
39
39
4
4
1
18
+
12
+
2
+
+
+
+
+
1
+
825
38
(Gg)
2007
42
5
4
1
22
+
9
+
2
+
+
+
+
+
1
+
892
37
+ Less than 0.5 Gg
Note: Totals may not sum due to independent rounding.
below).
6-28 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Methodology
The Tier 2 methodology used for estimating greenhouse gas emissions from field burning of agricultural residues in
the United States is consistent with IPCC (2006) (for more details, see Box 6 2). In order to estimate the amounts of
C and nitrogen (N) released during burning, the following equation was used:137
C or N released = £ over all crop types (Crop Production x Residue/Crop Ratio x Dry Matter Fraction x Fraction of
Residue Burned x Burning Efficiency x Combustion Efficiency x Fraction of C or N)
where,
Crop Production = Annual production of crop in Gg
Residue/Crop Ratio = Amount of residue produced per unit of crop production
Fraction of Residue Burned = Amount of residue that is burned per unit of total residue
Dry Matter Fraction = Amount of dry matter per unit of biomass
Fraction of C or N = Amount of C or N per unit of dry matter
Burning Efficiency = The proportion of prefire fuel biomass consumed138
Combustion Efficiency = The proportion of C or N released with respect to the total amount of C
or N available in the burned material, respectively138
The amount C or N released was used in the following equation to determine the CH4, CO, N2O and NOX emissions
from the field burning of agricultural residues:
CH4 and CO, or N2O and NOX Emissions from Field Burning of Agricultural Residues = (C or N Released) x
(Emissions Ratio for C or N) x (Conversion Factor)
where,
Emissions Ratio = g CH4-C or CO-C/g C released, or g N2O-N or NOx-N/g N released
Conversion Factor = conversion, by molecular weight ratio, of CH4-C to C (16/12), or CO-C to C (28/12),
or N2O-N to N (44/28), or NOX-N to N (30/14)
The types of crop residues burned in the United States were determined from various state-level greenhouse gas
emission inventories (ILENR 1993, Oregon Department of Energy 1995, Wisconsin Department of Natural
Resources 1993) and publications on agricultural burning in the United States (Jenkins et al. 1992, Turn et al. 1997,
EPA 1992).
[BEGIN BOX]
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
137 As is explained later in this section, the fraction of rice residues burned varies among states, so these equations were applied
at the state level for rice. These equations were applied at the national level for all other crop types.
13 8 In IPCC/UNEP/OECD/IEA (1997), the equation for C or N released contains the variable ' fraction oxidized in burning.'
This variable is equivalent to (burning efficiency x combustion efficiency).
Agriculture 6-29
-------
IPCC (2006) approach was undertaken to determine the magnitude of the difference in overall estimates resulting
from the two approaches. Since the default IPCC (2006) approach calls for area burned data that are currently
unavailable for the United States, estimates of area burned were developed using USDA data on area harvested for
each crop multiplied by the estimated fraction of residue burned for that crop (see Table 6-24).
The IPCC (2006) default approach resulted in 19 percent higher emissions of CH4 and 35 percent higher emissions
of N2O than the current estimates in this Inventory. It is reasonable to maintain the current methodology, since the
IPCC (2006) defaults are only available for four crops and are worldwide average estimates, while current inventory
estimates are based on U.S.-specific, crop-specific, published data.
[END BOX]
Crop production data for all crops except rice in Florida and Oklahoma were taken from the USDA's Field Crops,
Final Estimates 1987-1992, 1992-1997, 1997-2002 (USDA 1994, 1998, 2003), and Crop Production Summary
(USDA 2005 through 2008). Rice production data for Florida and Oklahoma, which are not collected by USDA,
were estimated separately. Average primary and ratoon crop yields for Florida (Schueneman and Deren 2002) were
applied to Florida acreages (Schueneman 1999b, 2001; Deren 2002; Kirstein 2003, 2004; Cantens 2004, 2005;
Gonzalez 2007a, 2008), and crop yields for Arkansas (USDA 1994, 1998, 2003, 2005, 2006) were applied to
Oklahoma acreages139 (Lee 2003 through 2006; Anderson 2008). The production data for the crop types whose
residues are burned are presented in Table 6-22.
The percentage of crop residue burned was assumed to be 3 percent for all crops in all years, except rice, 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,
2008; Klosterboer 1999a, 1999b, 2000 through 2003; Lancero 2006 through 2008; Lee 2005 through 2007; Lindberg
2002 through 2005; Linscombe 1999a, 1999b, 2001 through 2008; Najita 2000, 2001; Sacramento Valley Basinwide
Air Pollution Control Council 2005, 2007; Schueneman 1999a, 1999b, 2001; Stansel 2004, 2005; Street 2001
through 2003; Texas Agricultural Experiment Station 2006 through 2008; Walker 2004 through 2008; Wilson 2003
through 2007) (see Table 6-23). The estimates provided for Florida remained constant over the entire 1990 through
2007 period, while the estimates for all other states varied over the time series, except for Missouri, which remained
constant through 2005, dropped in 2006 and remained constant at the 2006 value in 2007. 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 2007 because of a legislated reduction in rice straw burning
(Lindberg 2002), although there was a slight increase from 2004 to 2005 and from 2006 to 2007 (see Table 6-23).
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 et al. (1997). Soybean dry matter content was obtained from Strehler and Stutzle
(1987). Peanut dry matter content was obtained through personal communications with Jen Ketzis (1999), who
accessed Cornell University's Department of Animal Science's computer model, Cornell Net Carbohydrate and
Protein System. The residue C contents and N contents for all crops except soybeans and peanuts are from Turn et
al. (1997). The residue C content for soybeans and peanuts is the IPCC default (IPCC/UNEP/OECD/IEA 1997).
The N content of soybeans is from Barnard and Kristoferson (1985). The N content of peanuts is from Ketzis
(1999). These data are listed in Table 6-24. The burning efficiency was assumed to be 93 percent, and the
combustion efficiency was assumed to be 88 percent, for all crop types (EPA 1994). Emission ratios and conversion
factors for all gases (see Table 6-25) were taken from the Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA
1997).
139
Rice production yield data are not available for Oklahoma, so the Arkansas values are used as a proxy.
6-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Table 6-22: Agricultural Crop Production (Gg of Product)
Crop
Wheat
Rice
Sugarcane
Corn3
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
60,641
8,705
32,762
251,854
6,919
75,055
1,481
2005
57,280
10,150
24,137
282,311
4,613
83,368
2,209
2006
49,316
8,813
26,820
267,598
3,923
86,770
1,571
2007
56,247
8,979
27,972
332,092
4,612
70,358
1,697
1 Corn for grain (i.e., excludes corn for silage).
Table 6-23: Percent of Rice Area Burned by State
State
Arkansas
California
Florida3
Louisiana
Mississippi
Missouri
Oklahoma
Texas
1990
13%
75%
0%
6%
10%
18%
90%
1%
1995
13%
59%
0%
6%
10%
18%
90%
1%
2000
13%
27%
0%
5%
40%
18%
90%
0%
2005
22%
16%
0%
3%
23%
18%
94%
0%
2006
27%
10%
0%
5%
25%
3%
0%
0%
2007
27%
16%
0%
5%
24%
3%
0%
0%
aAlthough rice is cultivated in Florida, crop residue burning is illegal.
Table 6-24:
Crop
Wheat
Rice
Sugarcane
Corn
Barley
Soybeans
Peanuts
Table 6-25:
Gas
CH4:C
CO:C
N2O:N
NOX:N
Key Assumptions for Estimating Emissions from Field Burning of Agricultural Residues
Residue/Crop
Ratio
1.3
1.4
0.8
1.0
1.2
2.1
1.0
Greenhouse Gas
Emission Ratio
0.005 a
0.0603
0.007b
0.121b
Fraction of
Residue Burned
0.03
Variable
0.03
0.03
0.03
0.03
0.03
Emission Ratios and
Conversion
Factor
16/12
28/12
44/28
30/14
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
Conversion Factors
* 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.
Agriculture 6-31
-------
The results of the Tier 2 Monte Carlo uncertainty analysis are summarized in Table 6-26. CH4 emissions from field
burning of agricultural residues in 2007 were estimated to be between 0.2 and 1.7 Tg CO2 Eq. at a 95 percent
confidence level. This indicates a range of 73 percent below and 94 percent above the 2007 emission estimate of 0.9
Tg CO2 Eq. Also at the 95 percent confidence level, N2O emissions were estimated to be between 0.1 and 0.9 Tg
CO2 Eq. (or approximately 73 percent below and 85 percent above the 2007 emission estimate of 0.5 Tg CO2 Eq.).
Table 6-26: Tier 2 Uncertainty Estimates for CH4 and N2O Emissions from Field Burning of Agricultural Residues
(Tg CO2 Eq. and Percent)
Source Gas 2007 Emission Uncertainty Range Relative to Emission
Estimate Estimate"
(TgC02Eq.) (TgC02Eq.) (%)
Field Burning of Agricultural Residues CH4 0.9
Field Burning of Agricultural Residues N2O 0.5
Lower
Bound
0.2
0.1
Upper
Bound
1.7
0.9
Lower
Bound
-73%
-73%
Upper
Bound
+94%
+85%
aRange 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 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 2006 and 2007 were updated using data from USDA (2008). This change resulted in
an increase in the CH4 emission estimate for 2006 of 0.01 percent, and an increase in the N2O emission estimate for
2006 of 0.002 percent, relative to the previous Inventory.
Planned Improvements
The estimated 3 percent of crop residue burned for all crops, except rice, is based on data gathered from several state
greenhouse gas inventories. This fraction is the most statistically significant input to the emissions equation, and an
important area for future improvement. More crop- and state-specific information on the fraction burned will be
investigated by literature review and/or by contacting state departments of agriculture.
Preliminary research on agricultural burning in the United States indicates that residues from several additional crop
types (e.g., grass for seed, blueberries, and fruit and nut trees) are burned. Whether sufficient information exists for
inclusion of these additional crop types in future Inventories is being investigated. The extent of recent state crop-
burning regulations is also being investigated.
6-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Agricultural Soil Management
Enteric Fermentation
Manure Management
Rice Cultivation I
Field Burning of Agricultural Residues
Agriculture as a Portion of
all Emissions
5.8%
0
0 50 100 150 200 250
TgC02Eq.
Figure 6-1: 2007 Agriculture Chapter Greenhouse Gas Sources
-------
Figure 6-2
Sources and Pathways of N that Result in N20 Emissions from Agricultural Soil Management
Synthetic N Fertilizers
Synthetic N fertilizer applied to soil
Organic
Amendments
Includes both commercial and
non-co,mmercisl fertilizers (i.e.,
animal manure,compost,
sewage sludge, tankage, etc.
N Volatilization
and Deposition
Urine and Dunq from
Grazing Animals
Manure deposited on pasture, range,
and paddock
Includes above- and belowground
residues for a II crops (non-N and N-
fixing (and from perennial forage
crops and pastures following renewa
Mineralization of
Soil Organic Matter
Includes N converted to mineral form
upon decomposition of soil organic
Asymbiotic Fixation
Fixation of atmospheric N2 by bacteria
living in soils that do not have a direct
relationship with plants
This graphic illustrates the sources and pathways of nitrogen that result
in direct and indirect N20 emissions from soils using the methodologies
described in this Inventory. Emission pathways are shown with arrows.
On the lower right-hand side is a cut-away view of a representative
section of a managed soil; histosol cultivation is represented here.
-------
Figure 6-3
Major Crops, Average Annual Direct N20 Emissions by State, Estimated Using the DAYCENT Model,
1990-2007 (Tg CO. EqYyear)
TgC02Eq./year
<0.25
D 0.25-0.5
• 0.5-1
1-2
2-5
5-10
> 10
-------
Figure 6-4
Grasslands, Average Annual Direct N20 Emissions by State, Estimated Using the DAYCENT Model,
1990-2007 (Tg CO. EqYyear)
-------
Figure 6-5
Major Crops, Average Annual N Losses Leading to Indirect N20 Emissions by State,
Estimated Using the DAYCENT Model, 1990-2007 (Gg N/year)
-------
Figure 6-6
Grasslands, Average Annual N Losses Leading to Indirect N20 Emissions by State,
Estimated Using the DAYCENT Model, 1990-2007 (Gg N/year)
-------
Figure 6-7
Comparison of Measured Emissions at Field Sites with Modeled Emissions
Using the DAYCENT Simulation Model
CO
T3
O)
O
40
35
30
25
20
15
10
5
0
D measured
DAYCENT
• IPCC
0°
0°
-------
-------
7. Land Use, Land-Use Change, and Forestry
This chapter provides an assessment of the net greenhouse gas flux140 resulting from the uses and changes in land
types and forests in the United States. The Intergovernmental Panel on Climate Change 2006 Guidelines for
National Greenhouse Gas Inventories (IPCC 2006) recommends reporting fluxes according to changes within and
conversions between certain land-use types, termed forest land, cropland, grassland, and settlements (as well as
wetlands). The greenhouse gas flux from Forest Land Remaining Forest Land is reported using estimates of
changes in forest carbon (C) stocks, non-carbon dioxide (CO2) emissions from forest fires, and the application of
synthetic fertilizers to forest soils. The greenhouse gas flux reported in this chapter from agricultural lands (i.e.,
cropland and grassland) includes changes in organic C stocks in mineral and organic soils due to land use and
management, and emissions of CO2 due to the application of crushed limestone and dolomite to managed land (i.e.,
soil liming) and urea fertilization. Fluxes are reported for four agricultural land use/land-use change categories:
Cropland Remaining Cropland, Land Converted to Cropland, Grassland Remaining Grassland, and Land
Converted to Grassland. Fluxes resulting from Settlements Remaining Settlements include those from urban trees
and soil fertilization. Landfilled yard trimmings and food scraps are accounted for separately under Other.
The estimates in this chapter, with the exception of CO2 fluxes from wood products and urban trees, and CO2
emissions from liming and urea fertilization, are based on activity data collected at multiple-year intervals, which
are in the form of forest, land-use, and municipal solid waste surveys. 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.
For the landfilled yard trimmings and food scraps source, periodic solid waste survey data were interpolated so that
annual storage estimates could be derived. This flux has been applied to the entire time series, and periodic U.S.
census data on changes in urban area have been used to develop annual estimates of CO2 flux.
Land use, land-use change, and forestry activities in 2007 resulted in a net C sequestration of 1,062.6 Tg CO2 Eq.
(289.8 Tg C) (Table 7-1 and Table 7-2). This represents an offset of approximately 17.4 percent of total U.S. CO2
emissions. Total land use, land-use change, and forestry net C sequestration141 increased by approximately 26
percent between 1990 and 2007. This increase was primarily due to an increase in the rate of net C accumulation in
forest C stocks. Net C accumulation in Forest Land Remaining Forest Land, Land Converted to Grassland, and
Settlements Remaining Settlements increased, while net C accumulation in Cropland Remaining Cropland,
Grassland Remaining Grassland, and landfilled yard trimmings and food scraps slowed over this period. Emissions
from Land Converted to Cropland increased between 1990 and 2007.
Table 7-1: Net CO2 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.)
Sink Category
Forest Land Remaining Forest
Land1
Cropland Remaining Cropland
Land Converted to Cropland
Grassland Remaining Grassland
Land Converted to Grassland
Settlements Remaining
Settlements2
Other (Landfilled Yard
Trimmings and Food Scraps)
Total
1990
(661.1)
(29.4)
2.2
(46.7)
(22.3)
(60.6)
(23.5)
(841.4)
1995
(686.6)
(22.9)
2.9
(36.4)
(22.5)
(71.5)
(13.9)
(851.0)
2000
(512.6)
(30.2)
2.4
(51.4)
(32.0)
(82.4)
(11.3)
(717.5)
2005
(975.7)
(18.3)
5.9
(4.6)
(26.7)
(93.3)
(10.2)
(1,122.7)
2006
(900.3)
(19.1)
5.9
(4.6)
(26.7)
(95.5)
(10.4)
(1,050.5)
2007
(910.1)
(19.7)
5.9
(4.7)
(26.7)
(97.6)
(9.8)
(1,062.6)
Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding.
1 Estimates include C stock changes on both Forest Land Remaining Forest Land and Land Converted to Forest Land.
140 jjjg 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."
141 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.
Land Use, Land-Use Change, and Forestry 7-1
-------
2 Estimates include C stock changes on both Settlements Remaining Settlements and Land Converted to Settlements.
Table 7-2: Net CO2 Flux from Carbon Stock Changes in Land Use, Land-Use Change, and Forestry (Tg C)
Sink Category 1990 1995 2000 2005 2006 2007
Forest Land Remaining
Forest Land1
Cropland Remaining
Cropland
Land Converted to
Cropland
Grassland Remaining
Grassland
Land Converted to
Grassland
Settlements Remaining
Settlements2
Other (Landfilled Yard
Trimmings and Food
Scraps)
Total
(180.3)
(8.0)
0.6
(12.7)
(6.1)
(16.5)
(6.4)
(229.5)
(187.2)
(6.3)
0.8
(9.9)
(6.1)
(19.5)
(3.8)
(232.1)
(139.8)
(8.2)
0.6
(14.0)
(8.7)
(22.5)
(3.1)
(195.7)
(266.1)
(5.0)
1.6
(1.3)
(7.3)
(25.4)
(2.8)
(306.2)
(245.5)
(5.2)
1.6
(1.3)
(7.3)
(26.0)
(2.8)
(286.5)
(248.2)
(5.4)
1.6
(1.3)
(7.3)
(26.6)
(2.7)
(289.8)
Note: 1 Tg C = 1 teragram C = 1 million metric tons C. Parentheses indicate net sequestration. Totals may not sum due to
independent rounding.
1 Estimates include C stock changes on both Forest Land Remaining Forest Land and Land Converted to Forest Land.
2 Estimates include C stock changes on both Settlements Remaining Settlements and Land Converted to Settlements.
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 2007 resulted in CO2 emissions of 4.1 Tg CO2 Eq. (4,055 Gg) and 4.0 Tg
CO2 Eq. (3,952 Gg), respectively. Lands undergoing peat extraction (i.e., peatlands remaining peatlands) resulted
in CO2 emissions of 1.0 Tg CO2 Eq. (1,010 Gg), and N2O emissions of less than 0.01 Tg CO2 Eq. The application
of synthetic fertilizers to forest and settlement soils in 2007 resulted in direct N2O emissions of 1.9 Tg CO2 Eq. (6
Gg). Direct N2O emissions from fertilizer application to forest soils have increased by a multiple of 6.7 since 1990,
but still account for a relatively small portion of overall emissions at 0.3 Tg CO2 Eq. (1 Gg) in 2007. Forest fires in
2007 resulted in methane (CH4) emissions of 29.0 Tg CO2 Eq. (1,381 Gg), and inN2O emissions of 2.9 Tg CO2 Eq.
(9 Gg).
Table 7-3: Emissions from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.)
Source Category 1990 1995 2000 2005 2006 2007
CO2 8.1 8.1 8.8 8.9 8.8 9.0
Cropland Remaining Cropland:
Liming of Agricultural Soils 4.7 4.4 4.3 4.3 4.2 4.1
Urea Fertilization 2.4 2.7 3.2 3.5 3.7 4.0
Wetlands Remaining Wetlands:
Peatlands Remaining Peatlands 1.0 1.0 1.2 1.1 0.9 1.0
CH4 4.6 6.1 20.6 14.2 31.3 29.0
Forest Land Remaining Forest
Land: Forest Fires 4.6 6.1 20.6 14.2 31.3 29.0
N2O 1.5 2.0 3.6 3.3 5.0 4.9
Forest Land Remaining Forest
Land: Forest Fires 0.5 0.6 2.1 1.4 3.2 2.9
Forest Land Remaining Forest
Land: Forest Soils1 0.0 0.1 0.3 0.3 0.3 0.3
Settlements Remaining
Settlements: Settlement Soils2 1.0 1.2 1.2 1.5 1.5 1.6
Wetlands Remaining Wetlands:
Peatlands Remaining Peatlands + + + + + +_
Total 14.2 16.2 33.0 26.4 45.1 42.9
7-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
+ Less than 0.01 Tg CO2 Eq.
Note: These estimates include direct emissions only. Indirect N2O emissions are reported in the Agriculture chapter. Totals may
not sum due to independent rounding.
1 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.
2 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: Emissions from Land Use, Land-Use Change, and Forestry (Gg)
Source Category 1990 1995 2000 2005 2006 2007
CO2 8,117 8,067 8,768 8,933 8,768 9,018
Cropland Remaining Cropland:
Liming of Agricultural Soils 4,667 4,392 4,328 4,349 4,233 4,055
Urea Fertilization 2,417 2,657 3,214 3,504 3,656 3,952
Wetlands Remaining Wetlands:
Peatlands Remaining Peatlands 1,033 1,018 1,227 1,079 879 1,010
CH4 218 293 983 676 1,489 1,381
Forest Land Remaining Forest
Land: Forest Fires 218 293 983 676 1,489 1,381
N2O 5 6 12 11 16 16
Forest Land Remaining Forest
Land: Forest Fires 2 2 7 5 10 9
Forest Land Remaining Forest
Land: Forest Soils1 0 0 1 111
Settlements Remaining Settlements:
Settlement Soils2 3 4 4 555
Wetlands Remaining Wetlands:
Peatlands Remaining Peatlands + + + + + j_
+ Less than 0.05 Gg
Note: These estimates include direct emissions only. Indirect N2O emissions are reported in the Agriculture chapter. Totals may
not sum due to independent rounding.
1 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.
2 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 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 six IPCC land-use categories (i.e., Forest Land Remaining Forest Land, Cropland
Remaining Cropland, Grassland Remaining Grassland, Wetlands Remaining Wetlands, Settlements Remaining
Settlements and Other Land Remaining Other Land) and thirty land-use change categories (e.g., Cropland Converted
to Forest Land, Grassland Converted to Forest Land, Wetlands Converted to Forest Land, Settlements Converted to
Land Use, Land-Use Change, and Forestry 7-3
-------
Forest Land, Other Land Converted to Forest Lands)142 (IPCC 2006). The primary databases are the U.S.
Department of Agriculture (USDA) National Resources Inventory (NRI)143 and the USDA Forest Service (USFS)
Forest Inventory and Analysis (FIA)144 Database. The U.S. Geological Survey (USGS) National Land Cover
Dataset (NLCD)145 is also used to identify land uses in regions that were not included in the NRI or FIA. The total
land area included in the U.S. inventory is 786 million hectares, and this entire land base is considered managed.146
In 1990, the United States had a total of 244 million hectares of Forest Land, 171 million hectares of Cropland, 288
million hectares of Grassland, 28 million hectares of Wetlands, 40 million hectares of Settlements, and 14 million
hectares in the Other Land147 category (Table 7-5). By 2007, the total area in Forest Land had increased by 3.7
percent to 253 million hectares, Cropland had declined by 4.0 percent to 163 million hectares, Grassland declined by
3.5 percent to 278 million hectares, Wetlands decreased by 2.4 percent to 28 million hectares, Settlements increased
by 22.6 percent to 49 million hectares, and Other Land remained at about 14 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.
Table 7-5. Land use and land use change areas on managed land during the inventory reporting period (thousands of
hectares). The abbreviations are "F" for Forest Land, "C" for Cropland, "G" for Grassland, "W" for Wetlands, "S"
for Settlements, and "O" for Other Lands. Lands remaining in the same land use are identified with the land use
abbreviation given twice (e.g., "FF" is Forest Land Remaining Forest Land), and land use change categories are
identified with the previous land use abbreviation followed by the new land use after conversion (e.g., "CF" is
Cropland Converted to Forest Land).
Land Use, Land Use
Change Categories 1990 1995 2001 2005 2006 2007
Total Forest Land
FF
CF
GF
WF
SF
OF
Total Cropland
CC
FC
GC
we
sc
oc
Total Grassland
GG
FG
CG
243,160
238,088
1,147
3,401
58
98
368
170,677
155,478
1,105
13,298
163
470
162
289,333
279,318
1,514
7,873
246,363
237,767
1,804
5,802
125
179
686
168,501
149,353
1,289
16,517
249
869
223
284,622
270,985
2,129
10,506
248,993
235,855
2,842
8,691
193
278
1,135
163,914
143,816
1,027
17,623
267
889
293
281,748
262,679
3,136
14,585
251,441
238,335
2,863
8,574
192
288
1,188
163,236
145,573
806
15,514
234
825
283
279,282
261,555
2,858
13,517
252,252
239,111
2,871
8,600
193
289
1,188
163,195
145,533
805
15,513
234
825
283
278,762
261,105
2,846
13,463
252,927
239,755
2,878
8,623
194
290
1,188
163,183
145,522
805
15,513
234
825
283
278,273
260,676
2,837
13,415
142 Land-use category definitions are provided in the Methodology section.
143 NRI data is available at .
144 FIA data is available at .
145 NLCD data is available at .
146 The current land representation does not include areas from Alaska, U.S. territories or federal lands in Hawaii, but there are
planned improvements to include these regions in future reports.
147 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.
7-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
WG
SG
OG
Total Wetlands
WW
FW
CW
GW
SW
OW
Total Settlements
SS
FS
CS
GS
WS
OS
Total Other Land
OO
FO
CO
GO
WO
so
Grand Total148
233
133
262
28,545
27,892
140
139
322
<1
51
39,548
34,772
1,842
1,373
1,498
3
60
14,425
13,437
193
279
458
55
3
785,687
352
237
413
28,266
27,298
253
233
456
<1
25
43,351
34,378
3,561
2,518
2,756
9
128
14,584
12,895
321
385
888
88
7
785,687
359
276
712
28,456
26,907
406
371
726
3
43
48,160
33,999
5,777
3,738
4,397
31
218
14,427
12,171
545
473
1,105
123
11
785,698
345
270
738
28,151
26,591
415
363
736
3
43
49,285
35,011
5,873
3,673
4,479
32
217
14,304
12,061
560
499
1,058
114
12
785,698
344
269
735
27,960
26,408
412
360
734
3
43
49,255
34,982
5,873
3,673
4,479
32
217
14,275
12,033
559
499
1,057
114
12
785,698
343
268
734
27,817
26,272
409
358
732
3
43
49,248
34,975
5,873
3,672
4,479
32
217
14,250
12,009
559
499
1,057
114
12
785,698
Note: Managed and unmanaged lands are not differentiated in the current U.S. land representation assessment. In addition, U.S.
Territories along with federal lands in Hawaii have not been classified into land uses and are not included in the U.S. land
representation assessment. See planned improvements for discussion on plans to include Alaska, territories and federal lands in
Hawaii in future inventory reports.
Figure 7-1. Percent of Total Land Area in the General Land Use Categories for 2007
Methodology
IPCC Approaches for Representing Land Areas
IPCC (2006) describes three approaches for representing land areas. Approach 1 provides data on the total area for
each individual land-use category, but does not provide detailed information on changes of area between categories
and is not spatially explicit other than at the national or regional level. With Approach 1, total net conversions
between categories can be detected, but not the individual changes between the land-use categories that led to those
net changes. Approach 2 introduces tracking of individual land-use changes between the categories (e.g., forest land
to cropland, cropland to forest land, grassland to cropland, etc.), using surveys or other forms of data that do not
provide location data on specific parcels of land. Approach 3 extends Approach 2 by providing location data on
specific parcels of land, such as maps, along with the land use history. The three approaches are not presented as
hierarchical tiers and are not mutually exclusive.
According to IPCC (2006), the approach or mix of approaches selected by an inventory agency should reflect the
148 The total land changes over time because there is a net transfer of land from federal to non-federal ownership in Hawaii.
Federal lands in Hawaii are not currently included in the US Land Representation, leading to a change in the land base over time.
There is a planned improvement to include land use data for federal lands in Hawaii, which will resolve the issue with a changing
land base over time. In addition, area data for Hawaii are currently only available through 1997 leading to no change in the
federal land base after 1997.
Land Use, Land-Use Change, and Forestry 7-5
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calculation needs and national circumstances. For this analysis, the NRI, FIA, and the NLCD have been combined
to provide a complete representation of land use for managed lands. These data sources are described in more detail
later in this section. All of these datasets have a spatially-explicit time series of land use data, and therefore
Approach 3 is used to provide a full representation of land use in the U.S. Inventory. Lands are treated as remaining
in the same category (e.g., Cropland Remaining Cropland) if a land use change has not occurred in the last 20 years.
Otherwise, the land is classified in a land use change category based on the current use and most recent use before
conversion to the current use (e.g., Cropland Converted to Forest Land).
Definitions of Land Use in the United States
Managed and Unmanaged Land
The U.S. definitions of managed and unmanaged lands are similar to the basic IPCC (2006) definition of managed
land, but with some additional elaboration to reflect national circumstances. Based on the following definitions,
most lands in the United States are classified as managed:
• Managed Land: Land is considered managed if direct human intervention has influenced its condition.
Direct intervention includes altering or maintaining the condition of the land to produce commercial or
non-commercial products or services; to serve as transportation corridors or locations for buildings,
landfills, or other developed areas for commercial or non-commercial purposes; to extract resources or
facilitate acquisition of resources; or to provide social functions for personal, community or societal
objectives. Managed land also includes legal protection of lands (e.g., wilderness, preserves, parks, etc.)
for conservation purposes (i.e., meets societal objectives).149
• Unmanaged Land: All other land is considered unmanaged. Unmanaged land is largely comprised of areas
inaccessible to human intervention due to the remoteness of the locations, or lands with essentially no
development interest or protection due to limited personal, commercial or social value. Though these lands
may be influenced indirectly by human actions such as atmospheric deposition of chemical species
produced in industry, they are not influenced by a direct human intervention.150
Land-Use Categories
As with the definition of managed lands, IPCC (2006) provides general non-prescriptive definitions for the six main
land-use categories: Forest Land, Cropland, Grassland, Wetlands, Settlements and Other Land. In order to reflect
U.S. circumstances, country-specific definitions have been developed, based predominantly on criteria used in the
land-use surveys for the United States. Specifically, the definition of Forest Land is based on the FIA definition of
forest,151 while definitions of Cropland, Grassland, and Settlements are based on the NRI.152 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 stocked153 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
149 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.
150 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.
151 See .
152 See .
153 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.
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Forest Land, however, are extracted from forest area estimates and included in Settlements. 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.154 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,155 as well as lands in temporary fallow or enrolled in conservation
reserve programs (i.e., set-asides156). Roads through Cropland, including interstate highways, state
highways, other paved roads, gravel roads, dirt roads, and railroads are excluded from Cropland area
estimates and are, instead, classified as Settlements.
• Grassland: A land-use category on which the plant cover is composed principally of grasses, grass-like
plants, forbs or shrubs suitable for grazing and browsing, and includes both pastures and native rangelands.
This includes areas where practices such as clearing, burning, chaining, and/or chemicals are applied to
maintain the grass vegetation. Savannas, some wetlands and deserts, in addition to tundra are considered
Grassland.157 Woody plant communities of low forbs and shrubs, such as mesquite, chaparral, mountain
shrub, and pinyon-juniper, are also classified as Grassland if they do not meet the criteria for Forest Land.
Grassland includes land managed with agroforestry practices such as silvipasture and windbreaks,
assuming the stand or woodlot does not meet the criteria for Forest Land. Roads through Grassland,
including interstate highways, state highways, other paved roads, gravel roads, dirt roads, and railroads are
excluded from Grassland area estimates and are, instead, classified as Settlements.
• Wetlands: A land-use category that includes land covered or saturated by water for all or part of the year.
Managed Wetlands are those where the water level is artificially changed, or were created by human
activity. Certain areas that fall under the managed Wetlands definition are covered in other areas of the
IPCC guidance and/or the inventory, including Cropland (e.g., rice cultivation), Grassland, and Forest Land
(including drained or undrained forested wetlands).
• Settlements: A land-use category representing developed areas consisting of units of 0.25 acres (0.1 ha) or
more that includes residential, industrial, commercial, and institutional land; construction sites; public
administrative sites; railroad yards; cemeteries; airports; golf courses; sanitary landfills; sewage treatment
plants; water control structures and spillways; parks within urban and built-up areas; and highways,
railroads, and other transportation facilities. Also included are tracts of less than 10 acres (4.05 ha) that may
meet the definitions for Forest Land, Cropland, Grassland, or Other Land but are completely surrounded by
urban or built-up land, and so are included in the settlement category. Rural transportation corridors
located within other land uses (e.g., Forest Land, Cropland) are also included in Settlements.
• Other Land: A land-use category that includes bare soil, rock, ice, non-settlement transportation corridors,
and all land areas that do not fall into any of the other five land-use categories. It allows the total of
identified land areas to match the managed national area.
154 These subcategory definitions are fully described in the Forest Land Remaining Forest Land section.
155 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.
156 A set-aside is cropland that has been taken out of active cropping and converted to some type of vegetative cover, including,
for example, native grasses or trees.
157 IPCC guidelines (2006) do not include provisions to separate desert and tundra as land categories.
Land Use, Land-Use Change, and Forestry 7-7
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Land Use Data Sources: Description and Application to U.S. Land Area Classification
U.S. Land Use Data Sources
The three main data sources for land area and use data in the United States are the NRI, FIA, and the NLCD. For
the Inventory, the NRI is the official source of data on all land uses on non-federal lands (except forest land), and is
also used as the resource to determine the total land base for the conterminous United States and Hawaii. The NRI is
conducted by the USDA Natural Resources Conservation Service and is designed to assess soil, water, and related
environmental resources on non-federal lands. The NRI has a stratified multi-stage sampling design, where primary
sample units are stratified on the basis of county and township boundaries defined by the U.S. Public Land Survey
(Nusser and Goebel 1997). Within a primary sample unit (typically a 160-acre (64.75 ha) square quarter-section),
three sample points are selected according to a restricted randomization procedure. Each point in the survey is
assigned an area weight (expansion factor) based on other known areas and land-use information (Nusser and
Goebel 1997). The NRI survey utilizes data derived from remote sensing imagery and site visits in order to provide
detailed information on land use and management, particularly for croplands, and is used as the basis to account for
C stock changes in agricultural lands (except federal Grasslands). The NRI survey was conducted every 5 years
between 1982 and 1997, but shifted to annualized data collection in 1998. This Inventory incorporates data through
2003 from the NRI.
The FIA program, conducted by the USFS, is the official source of data on Forest Land area and management data
for the Inventory. FIA engages in a hierarchical system of sampling, with sampling categorized as Phases 1 through
3, in which sample points for phases are subsets of the previous phase. Phase 1 refers to collection of remotely-
sensed data (either aerial photographs or satellite imagery) primarily to classify land into forest or non-forest and to
identify landscape patterns like fragmentation and urbanization. Phase 2 is the collection of field data on a network
of ground plots that enable classification and summarization of area, tree, and other attributes associated with forest
land uses. Phase 3 plots are a subset of Phase 2 plots where data on indicators of forest health are measured. Data
from all three phases are also used to estimate C stock changes for forest land. Historically, FIA inventory surveys
had been conducted periodically, with all plots in a state being measured at a frequency of every 5 to 14 years. A
new national plot design and annual sampling design was introduced by FIA about ten years ago. Most states,
though, have only recently been brought into this system. Annualized sampling means that a portion of plots
throughout each state is sampled each year, with the goal of measuring all plots once every 5 years. See Annex 3.12
to see the specific survey data available by state. The most recent year of available data varies state by state (2002
through 2007).
Though NRI provides land area data for both federal and non-federal lands, it only includes land use data on non-
federal lands, and FIA only records data for forest land.158 Consequently, major gaps exist when the datasets are
combined, such as federal grassland operated by the Bureau of Land Management (BLM), USDA, and National
Park Service, as well as most of Alaska.159 Consequently, the NLCD is used as a supplementary database to
account for land use on federal lands that are not included in the NRI and FIA databases. The NLCD is a land cover
classification scheme, available for 1992 and 2001, that has been applied over the conterminous United States. For
this analysis, the NLCD Retrofit Land Cover Change Product was used in order to represent both land use and land
use change for federal lands. It is based primarily on Landsat Thematic Mapper imagery. The NLCD contains 21
categories of land cover information, which have been aggregated into the IPCC land-use categories, and the data
are available at a spatial resolution of 30 meters. The federal land portion of the NLCD was extracted from the
dataset using the federal land area boundary map from the National Atlas. 16° This map represents federal land
boundaries in 2005, so as part of the analysis, the federal land area was adjusted annually based on the NRI federal
land area estimates (i.e., land is periodically transferred between federal and non-federal ownership). Consequently,
the portion of the land base categorized with NLCD data varied from year to year, corresponding to an increase or
decrease in the federal land base. The NLCD is strictly a source of land cover information, however, and does not
provide the necessary site conditions, crop types and management information from which to estimate C stock
158 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.
159 The survey programs also do not include U.S. Territories with the exception of non-federal lands in Puerto Rico, which are
included in the NRI survey. Furthermore, NLCD does not include coverage for U.S. Territories.
160
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changes on those lands.
Another step in the analysis is to address gaps, as well as overlaps in the representation of the U.S. land base
between the Agricultural Carbon Stock Inventory (Cropland Remaining Cropland, Land Converted to Cropland,
Grassland Remaining Grassland, Land Converted to Grassland) and Forest Land Carbon Stock Inventory (Forest
Land Remaining Forest Land and Land Converted to Forest Land), which are based on the NRI and FIA databases,
respectively. NRI 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 defining and classifying Forest Land on federal lands. Moreover, dependence exists between the Forest
Land area and the amount of land designated as other land uses in both the NRI as well as the NLCD, such as the
amount of Grassland, Cropland and Wetland, relative to the Forest Land area. This results in inconsistencies among
the three databases for estimated Forest Land area, as well as for the area estimates for other land use categories.
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 of Forest Land for non-
federal and federal land, respectively. 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. The adjustments were based on
the proportion of land within each of these land use categories at the state-level, (i.e., a higher proportion of
grassland led to a larger adjustment in grassland area and vice versa).
As part of Quality Control/Quality Assurance, the land base derived from the NRI, FIA and NLCD was compared to
the U.S. Census Survey.161 The U.S. Census Bureau gathers data on the U.S. population and economy, and has a
database of land areas for the country. The land area estimates from the U.S. Census Bureau differ from those
provided by the land use surveys used in 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. Consequently, the U.S. Census Survey was not adopted as the official land area estimates for the
Inventory. Rather the NRI data were adopted given that this database provides full coverage of land area for the
conterminous United States and Hawaii. Regardless, the total difference between the U.S. Census Survey and the
data sources used in the Inventory is about 25 million hectares for the total land base of about 785 million hectares
currently included in the Inventory, or a 3.1 percent difference. Much of this difference is associated with open
waters in coastal regions and the Great Lakes. NRI does not include as much of the area of open waters in these
regions as the U.S. Census Survey.
Approach for Combining Data Sources
The managed land base in the United States has been classified into the six IPCC land-use categories using
definitions162 developed to meet national circumstances, while adhering to IPCC (2006). In practice, the land was
initially classified into a variety of land-use categories using the NRI, FIA and NLCD, and then aggregated into the
thirty-six broad land use and land use change categories identified in IPCC (2006). Details on the approach used to
combine data sources for each land use are described below as are the gaps that will be reconciled as part of ongoing
planned improvements:
• Forest Land: Both non-federal and federal forest lands on both the continental United States and coastal
Alaska are covered by FIA. FIA is used as the basis for both Forest Land area data as well as to estimate C
stocks and fluxes on Forest Land. Interior Alaska is not currently surveyed by FIA, but NLCD has a new
product for Alaska that will be incorporated into the assessment as a planned improvement for future
reports. Forest land in U.S. territories are currently excluded from the analysis, but FIA surveys are
161 See .
162 Definitions are provided in the previous section.
Land Use, Land-Use Change, and Forestry 7-9
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currently being conducted on U.S. territories and will become available in the future. NRI is being used in
the current report to provide Forest Land areas on non-federal lands in Hawaii. Federal forest land in
Hawaii is currently excluded, but FIA data will be collected in Hawaii in the future.
• Cropland: Cropland is classified using the NRI, which covers all non-federal lands, within 49 states
(excluding Alaska), including state and local government-owned land as well as tribal lands. NRI is used
as the basis for both Cropland area data as well as to estimate C stocks and fluxes on Cropland. Cropland
in U. S. territories are excluded from both NRI data collection and the NLCD. NLCD has a new product for
Alaska that will be incorporated into the assessment as a planned improvement for future reports.
• Grassland: Grassland on non-federal lands is classified using the NRI within 49 states (excluding Alaska),
including state and local government-owned land as well as tribal lands. NRI is used as the basis for both
Grassland area data as well as to estimate C stocks and fluxes on Grassland. U.S. territories are excluded
from both NRI data collection and the current release of the NLCD product. Grassland on federal Bureau
of Land Management lands, Department of Defense lands, National Parks and within USFS lands are
covered by the NLCD, with the exception of federal grasslands in Hawaii, which will be added as a
planned improvement in the future. In addition, federal and non-federal grasslands in Alaska are currently
excluded from the analysis, but NLCD has a new product for Alaska that will be incorporated into the
assessment for future reports.
• Wetlands: NRI captures wetlands on non-federal lands within 49 states (excluding Alaska), while federal
wetlands are covered by the NLCD, with the exception of federal lands in Hawaii, which will be added as a
planned improvement in the future. Alaska and U.S. territories are excluded. This currently includes both
managed and unmanaged wetlands as no database has yet been applied to make this distinction. See
Planned Improvements for details.
• Settlements: The NRI captures non-federal settlement area in 49 states (excluding Alaska). If areas of
Forest Land or Grassland under 10 acres (4.05 ha) are contained within settlements or urban areas, they are
classified as Settlements (urban) in the NRI database. If these parcels exceed the 10 acre (4.05 ha)
threshold and are grassland, they will be classified as such by NRI. Regardless of size, a forested area is
classified as nonforest by FIA if it is located within an urban area. Settlements on federal lands are covered
by NLCD, with the exception of federal lands in Hawaii, which will be added as a planned improvement in
the future. Settlements in U.S. territories are currently excluded from NRI and NLCD. NLCD has a new
product for Alaska that will be incorporated into the assessment as a planned improvement for future
reports.
• Other Land: Any land not falling into the other five land categories and, therefore, categorized as Other
Land is classified using the NRI for non-federal areas in the 49 states (excluding Alaska) and NLCD for the
federal lands, with the exception of federal lands in Hawaii, which will be added as a planned improvement
in the future. Other land in U. S. territories is excluded from the NLCD. NLCD has a new product for
Alaska that will be incorporated into the assessment as a planned improvement for future reports.
Some lands can be classified into one or more categories due to multiple uses that meet the criteria of more than one
definition. However, a ranking has been developed for assignment priority in these cases. The ranking process is
initiated by distinguishing between managed and unmanaged lands. The managed lands are then assigned, from
highest to lowest priority, in the following manner:
Settlements > Cropland > Forest Land > Grassland > Wetlands > Other Land
Settlements are given the highest assignment priority because they are extremely heterogeneous with a mosaic of
patches that include buildings, infrastructure and travel corridors, but also open grass areas, forest patches, riparian
areas, and gardens. The latter examples could be classified as Grassland, Forest Land, Wetlands, and Cropland,
respectively, but when located in close proximity to settlement areas they tend to be managed in a unique manner
compared to non-settlement areas. Consequently, these areas are assigned to the Settlements land-use category.
Cropland is given the second assignment priority, because cropping practices tend to dominate management
activities on areas used to produce food, forage or fiber. The consequence of this ranking is that crops in rotation
with grass will be classified as Cropland, and land with woody plant cover that is used to produce crops (e.g.,
orchards) is classified as Cropland, even though these areas may meet the definitions of Grassland or Forest Land,
respectively. Similarly, Wetlands that are used for rice production are considered Croplands. Forest Land occurs
7-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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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 greenhouse gas emissions and removals on managed
land, but is intended to classify all areas into a single land use. Currently, the IPCC does not make provisions in the
guidelines for assigning land to multiple uses. For example, a Wetland is classified as Forest Land if the area has
sufficient tree cover to meet the stocking and stand size requirements. Similarly, Wetlands are classified as
Cropland if they are used to produce a crop, such as rice. In either case, emissions from Wetlands are included in
the Inventory if human interventions are influencing emissions from Wetlands, in accordance with the guidance
provided in IPCC (2006).
Recalculations/Revisions
Three major revisions were made in the current Inventory for land representation.
• First, land uses were further disaggregated by land use and land use change categories as recommended by
IPCC (2006), which was possible with the new NLCD Retrofit Product in combination with the NRI data.
This change provides additional information on land use trends in the United States, and is expected to
improve estimation of greenhouse gas emissions and transparency of the report.
• Second, rural transportation corridors were re-classified as Settlements, instead of including these areas in
the Other Land category. Transportation corridors are managed in a manner more similar to land use
practices typically associated with Settlements, and therefore more aligned with this land use category.
• Finally, the NRI was adopted as the official land area estimate for the U.S. Inventory. This change led to a
decline in the managed land base for the United States because the NRI does not include some of the open
water areas in the Great Lakes and ocean coastal regions. Currently, there is no estimation of greenhouse
gas emissions associated with open waters of these regions from the perspective of land use, and so this
change has no consequences on the estimates of anthropogenic greenhouse gas emissions for the Inventory.
Planned Improvements
Area data by land-use category are not estimated for major portions of Alaska, federal lands in Hawaii, or 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 also be evaluated for
representing land use on federal lands in Hawaii and federal and non-federal lands in U.S. territories.
Additional work will be done to reconcile differences in Forest Land estimates between the NRI and FIA, evaluating
the assumption that the majority of discrepancies in Forest Land areas are associated with an over- or under-
estimation of Grassland and Wetland area. In some regions of the United States, a discrepancy in Forest Land areas
between NRI and FIA may be associated with an over- or under-prediction of other land uses.
There are also other databases that may need to be reconciled with the NRI and NLCD datasets, particularly for
Settlements and Wetlands. Urban area estimates, used to produce C stock and flux estimates from urban trees, are
currently based on population data (1990 and 2000 U.S. Census data). Using the population statistics, "urban
clusters" are defined as areas with more than 500 people per square mile. The USFS is currently moving ahead with
an urban forest inventory program so that urban forest area estimates will be consistent with FIA forest area
estimates outside of urban areas, which would be expected to reduce omissions and overlap of forest area estimates
along urban boundary areas. For Wetlands, the Army Corps of Engineers National Inventory of Dams (NID) (ACE
2005) and the U.S. Fish and Wildlife Service National Wetlands Inventory (NWI)163 databases are being evaluated
and will be compared against the NRI and NLCD. The NID and NWI may be used to refine wetland area estimates
for the U.S. Land Representation assessment, including disaggregation of managed and unmanaged wetlands.
163
http://www.fws.gov/nwi/
Land Use, Land-Use Change, and Forestry 7-11
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7.2. Forest Land Remaining Forest Land
Changes in Forest Carbon Stocks (IPCC Source Category 5A1)
For estimating C stocks or stock change (flux), C in forest ecosystems can be divided into the following five storage
pools (IPCC 2003):
• Aboveground biomass, which includes all living biomass above the soil including stem, stump, branches, bark,
seeds, and foliage. This category includes live understory.
• Belowground biomass, which includes all living biomass of coarse living roots greater than 2 mm diameter.
• Dead wood, which includes all non-living woody biomass either standing, lying on the ground (but not
including litter), or in the soil.
• Litter, which includes the litter, fumic, and humic layers, and all non-living biomass with a diameter less than
7.5 cm at transect intersection, lying on the ground.
• Soil organic C (SOC), including all organic material in soil to a depth of 1 meter but excluding the coarse roots
of the aboveground pools.
In addition, there are two harvested wood pools necessary for estimating C flux:
• Harvested wood products in use.
• 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 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 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.
Figure 7-2: Forest Sector Carbon Pools and Flows
7-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Approximately 33 percent (304 million hectares) of the U.S. land area is forested (Smith et al. 2008). The current
forest inventory includes 250 million hectares in the conterminous 48 states (USDA Forest Service 2008a, 2008b)
that are considered managed and are included in this inventory. The additional forest lands are located in Alaska and
Hawaii. This inventory includes approximately 3.8 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 becomes
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-eight percent of U.S. forests (208 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 81 percent of forests
in the conterminous United States are classified as timberlands. Of the remaining nontimberland forests, 30 million
hectares are reserved forest lands (withdrawn by law from management for production of wood products) and 66
million hectares are lower productivity forest lands (Smith et al. 2008). 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 8 million hectares. Current trends in forest area represent average
annual change of less than 0.2 percent. Given the low rate of change in U. S. forest land area, the major influences
on the current net C flux from forest land are management activities and the ongoing impacts of previous land-use
changes. These activities affect the net flux of C by altering the amount of C stored in forest ecosystems. For
example, intensified management of forests that leads to an increased rate of growth increases the eventual biomass
density of the forest, thereby increasing the uptake of C.164 Though harvesting forests removes much of the
aboveground C, there is a positive growth to harvest ratio on U.S. timberlands (AF&PA 2001). 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 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 2008). 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 910.1 Tg
CO2Eq. (248.2 Tg C) in 2007 (Table 7-6, Table 7-7, and Table 7-8). 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 70 to 76 Mg C/ha
between 1990 and 2008 (see Annex 3-12 for average C densities by specific regions and forest types). Continuous,
regular annual surveys are not available over the period for each state; therefore, estimates for non-survey years
164 jjjg 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
-------
were derived by interpolation between known data points. Survey years vary from state to state, and national
estimates are a composite of individual state surveys. Therefore, changes in sequestration over the interval 1990 to
2007 are the result of the sequences of new inventories for each state. C in forest ecosystem biomass had the
greatest effect on total change through increases in C density and total forest land. Management practices that
increase C stocks on forest land, as well as afforestation and reforestation efforts, influence the trends of increased C
densities in forests and increased forest land in the United States.
Table 7-6. Net Annual Changes in C Stocks (Tg CO2/yr) in Forest and Harvested Wood Pools
Carbon Pool 1990 1995 2000 2005 2006 2007
Forest
Aboveground Biomass
Belowground Biomass
Dead Wood
Litter
Soil Organic Carbon
Harvested Wood
Products in use
SWDS
Total Net Flux
(529.3)
(321.5)
(61.8)
(15.4)
(67.8)
(62.8)
(131.8)
(64.8)
(67.0)
(661.1)
(568.2)
(390.9)
(78.2)
(27.3)
(37.2)
(34.6)
(118.4)
(55.2)
(63.2)
(686.6)
(399.7)
(352.1)
(71.5)
(18.2)
(14.8)
56.9
(112.9)
(47.0)
(65.9)
(512.6)
(871.7)
(469.4)
(93.3)
(39.4)
(79.6)
(190.1)
(103.9)
(44.1)
(59.8)
(975.7)
(791.7)
(442.7)
(88.9)
(35.6)
(68.7)
(155.9)
(108.6)
(45.2)
(63.3)
(900.3)
(809.6)
(452.4)
(90.7)
(36.8)
(70.8)
(158.9)
(100.4)
(36.9)
(63.5)
(910.1)
Note: Forest C stocks do not include forest stocks in U.S. territories, Hawaii, a large portion of Alaska, western Texas 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 1990 1995 2000 2005 2006 2007
Forest
Aboveground Biomass
Belowground Biomass
Dead Wood
Litter
Soil Organic C
Harvested Wood
Products in Use
SWDS
Total Net Flux
(144.3)
(87.7)
(16.8)
(4.2)
(18.5)
(17.1)
(35.9)
(17.7)
(18.3)
(180.3)
(155.0)
(106.6)
(21.3)
(7.4)
(10.1)
(9.4)
(32.3)
(15.1)
(17.2)
(187.2)
(109.0)
(96.0)
(19.5)
(5.0)
(4.0)
15.5
(30.8)
(12.8)
(18.0)
(139.8)
(237.7)
(128.0)
(25.5)
(10.7)
(21.7)
(51.9)
(28.3)
(12.0)
(16.3)
(266.1)
(215.9)
(120.7)
(24.2)
(9.7)
(18.7)
(42.5)
(29.6)
(12.3)
(17.3)
(245.5)
(220.8)
(123.4)
(24.7)
(10.0)
(19.3)
(43.3)
(27.4)
(10.1)
(17.3)
(248.2)
Note: Forest C stocks do not include forest stocks in U.S. territories, Hawaii, a large portion of Alaska, western Texas 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.
Stock estimates for forest and harvested wood C storage pools are presented in Table 7-8. Together, the
aboveground live and forest soil pools account for a large proportion of total forest C stocks. C stocks in all non-soil
pools increased over time. Therefore, C sequestration was greater than C emissions from forests, as discussed
above. Figure 7-4 shows county-average C densities for live trees on forest land, including both above- and
belowground biomass.
Table 7-8. Forest area (1000 ha) and C Stocks (Tg C) in Forest and Harvested Wood Pools
1990 1995 2000 2005 2006 2007 2008
Forest Area (1000 ha) 245,697 249,240 251,732 255,358 256,227 257,001 257,787
Carbon Pools (Tg C)
Forest 40,011 40,762 41,475 42,488 42,726 42,942 43,163
Aboveground Biomass 14,378 14,845 15,365 15,974 16,102 16,222 16,346
Belowground Biomass 2,860 2,950 3,055 3,177 3,203 3,227 3,252
7-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Dead Wood
Litter
Soil Organic C
Harvested Wood
Products in Use
SWDS
Total C Stock
2,541
4,558
15,675
1,783
1,193
590
41,794
2,567
4,651
15,748
1,963
1,280
683
42,724
2,597
4,690
15,767
2,124
1,355
769
43,599
2,640
4,772
15,925
2,271
1,413
857
44,759
2,651
4,794
15,977
2,296
1,423
873
45,023
2,660
4,813
16,019
2,325
1,436
890
45,267
2,670
4,832
16,063
2,353
1,446
907
45,515
Forest area estimates include portions of Alaska. Forest C stocks do not include forest stocks in U.S. territories, Hawaii, western
Texas, 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, in preparation) 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.
Figure 7-3: Estimates of Net Annual Changes in C Stocks for Major C Pools
Figure 7-4: Average C Density in the Forest Tree Pool in the Conterminous United States, 2007
[BEGIN BOX]
Box 7-1: CO2 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.
forestland already account for CO2 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 CO2 emissions from fire disturbance, these estimates are being highlighted here, using the full extent
of available data. Non-CO2 greenhouse gas emissions from forest fires are also quantified in a separate section
below.
The IPCC (2003) methodology and IPCC (2006) default combustion factor for wildfire were employed to estimate
CO2 emissions from forest fires. CO2 emissions for wildfires and prescribed fires in the lower 48 states and wildfires
in Alaska in 2007 were estimated to be 293.7 Tg CO2/yr. This amount is masked in the estimate of net annual forest
carbon stock change for 2007, however, because this net estimate accounts for the amount sequestered minus any
emissions.
Table 7-9: Estimates of CO2 (Tg/yr) emissions for the lower 48 states and Alaska1
Year
CO2 emitted
from Wildfires in
Lower 48 States
(Tg/yr)
CO2 emitted
from Prescribed
Fires in Lower 48
States (Tg/yr)
CO2 emitted
from Wildfires in
Alaska (Tg/yr)
Total CO2
emitted (Tg/yr)
1990
38.6
46.4
1995
2000
53.6
207.0
8.6
2.0
62.3
209.0
2005
2006
120.9
289.5
22.9
27.0
Land Use, Land-Use Change, and Forestry 7-15
-------
2007 262.3 31.4 + 293.7
+ Does not exceed 0.05 Tg CO2 Eq.
1 Note that these emissions have already been accounted for in the estimates of net annual changes in C stocks, which account for
the amount sequestered minus any emissions.
[END BOX]
Methodology and Data Sources
The methodology described herein is consistent with IPCC (2003, 2006) and IPCC/UNEP/OECD/IEA (1997).
Forest ecosystem C stocks and net annual C stock change are determined according to stock-difference methods,
which involve applying C estimation factors to forest inventory data and interpolating between successive
inventory-based estimates of C stocks. Harvested wood C estimates are based on factors such as the allocation of
wood to various primary and end-use products as well as half-life (the time at which half of amount placed in use
will have been discarded from use) and expected disposition (e.g., product pool, SWDS, combustion). An overview
of the different methodologies and data sources used to estimate the C in forest ecosystems or harvested wood
products is provided here. See Annex 3.12 for details and additional information related to the methods and data.
Forest Ecosystem Carbon from Forest Inventory
Forest ecosystem stock and flux estimates are based on the stock-difference method and calculations for all
estimates are in units of C. Separate estimates are made for the five IPCC C storage pools described above. All
estimates are based on data collected from the extensive array of permanent forest inventory plots in the United
States as well as models employed to fill gaps in field data. Carbon conversion factors are applied at the
disaggregated level of each inventory plot and then appropriately expanded to population estimates. A combination
of tiers as outlined by Eggleston et al. (2006) is used. The Tier 3 biomass C values are from forest inventory tree-
level data. The Tier 2 dead organic and soil C pools are based on empirical or process models from the inventory
data. All carbon conversion factors are specific to regions or individual states within the U.S., which are further
classified according to characteristic forest types within each region.
The first step in developing forest ecosystem estimates is to identify useful inventory data and resolve any
inconsistencies among datasets. Forest inventory data were obtained from the USDA Forest Service FIA program
(Prayer and Furnival 1999, USDA Forest Service 2008a). Inventories include data collected on permanent inventory
plots on forest lands165 and are organized as a number of separate datasets, each representing a complete inventory,
or survey, of an individual state at a specified time. Some of the more recent annual inventories reported for some
states include "moving averages" which means that a portion - but not all - of the previous year's inventory is
updated each year (USDA Forest Service 2008d). Forest C calculations are organized according to these state
surveys, and the frequency of surveys varies by state. All available data sets are identified for each state starting
with pre-1990 data where possible and including all surveys since then. Since C stock change is based on
differences between successive surveys within each state, accurate estimates of net C flux thus depend on consistent
representation of forest land between these successive inventories. In order to achieve this consistency from 1990 to
the present, state-level data are sometimes subdivided in cases where the sum of sub-state inventories produces the
best whole-state represention of C change as discussed in Smith et al. (2007).
The principal FIA datasets employed are freely available for download at USDA Forest Service (2008b) as the
Forest Inventory and Analysis Database (FIADB) Version 3.0. However, to achieve consistent representation
(spatial and temporal), two other general sources of past FIA data are included as necessary. First, older FIA plot-
and tree-level data—not in the current FIADB format—are used if available. Second, Resources Planning Act
Assessment (RPA) databases, which are periodic, plot-level only, summaries of state inventories, are used mostly to
provide the data at or before 1990. See USDA Forest Service (2008a) for information on current and older data as
well as additional FIA Program features. A detailed list of the specific inventory data used in this inventory is in
Annex 3.12.
165 porest land m me 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.
7-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 FIADB-to-carbon calculator (Smith et al. 2007, In preparation). The conversion factors and
model coefficients are categorized by region and forest type, and forest C stock estimates are calculated from
application of these factors at the scale of FIA inventory plots. The results are estimates of C density (Mg C per
hectare) for six forest ecosystem pools: live trees, standing dead trees, understory vegetation, down dead wood,
forest floor, and soil organic matter. The six carbon pools used in the FIADB-to-carbon calculator are aggregated to
the 5 carbon pools defined by IPCC (2006): aboveground biomass, belowground biomass, dead wood, litter and soil
organic matter. All non-soil pools except forest floor are separated into aboveground and belowground components.
The live tree and understory C pools are pooled as biomass, and standing dead trees and down dead wood are pooled
as dead wood, in accordance with IPCC (2006).
Once plot-level C stocks are calculated as C densities on Forest Land Remaining Forest LandTor the five IPCC
(2006) reporting pools, the stocks are expanded to population estimates according to methods appropriate to the
respective inventory data (for example, see USDA Forest Service (2008d)). These expanded C stock estimates are
summed to state or sub-state total C stocks. Annualized estimates of C stocks are developed by using available FIA
inventory data and interpolating or extrapolating to assign a C stock to each year in the 1990-2008 time series. Flux,
or net annual stock change, is estimated by calculating the difference between two successive years and applying the
appropriate sign convention; net increases in ecosystem C are identified as negative flux. By convention,
inventories are assigned to represent stocks as of January 1 of the inventory year; an estimate of flux for 1996
requires estimates of C stocks for 1996 and 1997, for example. For this Inventory, 2008 stock and 2007 flux are
based on extrapolation of the average of the most recent three years of interpolated flux in the time series.
Additional discussion of the use of FIA inventory data and the C conversion process is in Annex 3.12.
Carbon in Biomass
Live tree C pools include aboveground and belowground (coarse root) biomass of live trees with diameter at
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 C in these plots is estimated from plot-level
volume of merchantable wood, or growing-stock volume, of live trees, which is calculated from updates of Smith et
al. (2003). These biomass conversion and expansion factors (BCEFs) are applied to about 5 percent of the inventory
records, all of which are pre-1998 data. Some inventory data, particularly some of the older datasets, may not
include sufficient information to calculate tree C because of incomplete or missing tree or volume data; C estimates
for these plots are based on averages from similar, but more complete, inventory data. This applies to an additional
3 percent of inventory records, which represent older (pre-1998) non-timberlands.
Understory vegetation is a minor component of biomass, which is defined as all biomass of undergrowth plants in a
forest, including woody shrubs and trees less than 2.54 cm d.b.h. In this inventory, it is assumed that 10 percent of
total understory C mass is belowground. Estimates of C density are based on information in Birdsey (1996).
Understory frequently represents over 1 percent of C in biomass, but its contribution rarely exceeds 2 percent of the
total.
Carbon in Dead Organic Matter
Dead organic matter is initially calculated as three separate pools with C stocks modeled from inventory data.
Estimates are specific to regions and forest types within each region, and stratification of forest land for dead
organic matter calculations is identical to that used for biomass through the state and sub-state use of FIA data as
discussed above. The two components of dead wood—standing dead trees and down dead wood—are estimated
separately. The standing dead tree C pools include aboveground and belowground (coarse root) mass and include
trees of at least 2.54 cm d.b.h. Calculations are BCEF-like factors based on updates of Smith et al. (2003). Down
dead wood is defined as pieces of dead wood greater than 7.5 cm diameter, at transect intersection, that are not
attached to live or standing dead trees. Down dead wood includes stumps and roots of harvested trees. Ratios of
down dead wood to live tree are used to estimate this quantity. Litter C is the pool of organic C (also known as duff,
humus, and fine woody debris) above the mineral soil and includes woody fragments with diameters of up to 7.5 cm.
Estimates are based on equations of Smith and Heath (2002).
Land Use, Land-Use Change, and Forestry 7-17
-------
Carbon in Forest Soil
Soil organic C (SOC) includes all organic material in soil to a depth of 1 meter but excludes the coarse roots of the
biomass or dead wood pools. Estimates of SOC are based on the national STATSGO spatial database (USDA
1991), which includes region and soil type information. SOC determination is based on the general approach
described by Amichev and Galbraith (2004). Links to FIA inventory data were developed with the assistance of the
USDA Forest Service FIA Geospatial Service Center by overlaying FIA forest inventory plots on the soil C map.
This method produced mean SOC densities stratified by region and forest type group. It did not provide separate
estimates for mineral or organic soils but instead weighted their contribution to the overall average based on the
relative amount of each within forest land. Thus, forest SOC is a function of species and location, and net change
also depends on these two factors as total forest area changes. In this respect, SOC provides a country-specific
reference stock for 1990-present, but it does not reflect effects of past land use.
Harvested Wood Carbon
Estimates of the harvested wood product (HWP) contribution to forest C sinks and emissions (hereafter called
"HWP Contribution") are based on methods described in Skog (2008) using the WOODCARB II model. These
methods are based on IPCC (2006) guidance for estimating HWP C. IPCC (2006) provides methods that allow
Parties to report HWP Contribution using one of several different accounting approaches: production, stock change
and atmospheric flow, as well as a default method that assumes there is no change in HWP C stocks (see Annex 3.12
for more details about each approach). The United States uses the production accounting approach to report HWP
Contribution. Under the production approach, C in exported wood is estimated as if it remains in the United States,
and C in imported wood is not included in inventory estimates. Though reported U.S. HWP estimates are based on
the production approach, estimates resulting from use of the two alternative approaches, the stock change and
atmospheric flow approaches, are also presented for comparison (see Annex 3.12). Annual estimates of change are
calculated by tracking the additions to and removals from the pool of products held in end uses (i.e., products in use
such as housing or publications) and the pool of products held in solid waste disposal sites (SWDS).
Solidwood products added to pools include lumber and panels. End-use categories for solidwood include single and
multifamily housing, alteration and repair of housing, and other end-uses. There is one product category and one
end-use category for paper. Additions to and removals from pools are tracked beginning in 1900, with the exception
that additions of softwood lumber to housing begins in 1800. Solidwood and paper product production and trade
data are from USDA Forest Service and other sources (Hair and Ulrich 1963; Hair 1958; USDC Bureau of Census;
1976; Ulrich, 1985, 1989; Steer 1948; AF&PA 2006a 2006b; Howard 2003, 2007). Estimates for disposal of
products reflect the change over time in the fraction of products discarded to SWDS (as opposed to burning or
recycling) and the fraction of SWDS that are in sanitary landfills versus dumps.
There are 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:
1 A) 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 2 A 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.
7-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Uncertainty
A quantitative uncertainty analysis placed bounds on current flux for forest ecosystems as well as carbon in
harvested wood products through Monte Carlo simulation of the Methods described above and probabilistic
sampling of carbon conversion factors and inventory data. See Annex 3.12 for additional information. The 2007
flux estimate for forest C stocks is estimated to be between -736 and -1083 Tg CO2 Eq. at a 95 percent confidence
level. This includes a range of -638 to -981 Tg CO2 Eq. in forest ecosystems and -76 to -127 Tg CO2 Eq. for HWP.
The relatively smaller range of uncertainty, in terms of percentage, for the total relative to the two separate parts in
because the total is based on summing the two independent uncertain parts.
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)
Source
Forest Ecosystem
Harvested Wood Products
Total Forest
Gas
C02
C02
C02
2007 Flux
Estimate
(Tg C02 Eq.)
(809.6)
(100.4)
(910.1)
Uncertainty Range Relative to Flux Estimate"
(Tg C02 Eq.) (%)
Lower
Bound
(980.9)
(127.0)
(1,083.1)
Upper
Bound
(637.5)
(76.2)
(735.6)
Lower
Bound
-21%
-26%
-19%
Upper
Bound
21%
24%
19%
Note: Parentheses indicate negative values or net sequestration.
aRange of flux estimates predicted by Monte Carlo stochastic simulation for a 95 percent confidence interval.
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
2008c).
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 2008d). 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
Land Use, Land-Use Change, and Forestry 7-19
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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 basic models used to estimate forest ecosystem and HWP C stocks and change are largely unchanged from the
previous Inventory (Smith et al. 2007, Skog 2008). Most of the estimates for 1990-present are relatively similar to
the values previously reported (EPA 2008). However, changes in underlying FIA data have driven some changes in
estimates across the time series. Most states have added new inventory data or modified some of the information in
previously existing surveys and the FIADB format changed to version 3.0 (USDA Forest Service 2008b). The
change to FIADB 3.0 resulted in three broad changes to the carbon calculation methods of Smith et al. (2007),
affecting: 1) expansion of plot-level carbon to total carbon stocks; 2) the more complete use of the moving averages;
and 3) the method of extrapolating stock and stock change, which is related to the use of the moving averages. See
Smith et al. (2007, In preparation) for further discussion. The plot-level carbon conversion process is essentially
unchanged. However, the process for expanding carbon to the totals used for determining net stock change is
modified somewhat from Smith et al. (2007) according to methods described in the current FIADB user's guide
(USDA Forest Service 2008d, Smith et al., in preparation).
The increasing number of annual inventory reports from moving averages (USDA Forest Service 2008b), especially
in the eastern U.S., are incorporated into this year's inventory (see Annex 3.12). The newly available annual
inventory data necessitated the second broad update to the carbon calculator, which was to incorporate th use of all
of these annual data summaries. Their use was minimized in previous forest carbon inventories (Smith et al. 2007,
In preparation). Moving averages have the potential for greater inter-annual variability in stocks for some states,
which in turn can have an even greater effect on carbon change because of the greater sensitivity in change (Smith et
al. 2007). That is, a very small change in stock can have a tremendous effect on stock change, which is based on a
small difference between two very large values. The use of the moving averages and the related sensitivity of stock
change led to the third general change in the calculator, which is the method of extrapolation applied. Extrapolated
values are based on short-term trends rather than being subject to a single year.
Most of these changes in data sources or methodology had only minor effects on estimates for 1990-present. A
notable exception is the spike in net annual changes in C stocks for forest ecosystem C after 2000; this spike occurs
in all five forest ecosystem pools to different degrees. Carbon change estimates generated for 2002 through 2006 are
notably greater than the corresponding values from the previous inventory. At the same time, the three previous
years (1999, 2000, and 2001) show a slight decrease relative to values reported in the 1990-2006 Inventory. This
combined effect is largely associated with forest areas reported by surveys over that interval and is a product of the
interpolated stock differences from the FIADB. Comparing the relative rates of change in area versus overall C
density for all forest carbon pools for 1990-2007 illustrates that the rate of change for carbon density is relatively
steady, but the rate of change for area fluctuates considerably. Extrapolated portions of the 1990-to-present
estimates are subject to change as new data become available and they generally include greater uncertainty.
However, most of the increased carbon sequestration over 2000-2003 is based on interpolation between stocks
because only 6 percent of the carbon change reported for 2003 is based on extrapolated values.
The uncertainty analysis for forest ecosystem carbon stock change has been revised. It is now possible to estimate
sampling errors associated with each of the specific carbon pools reported here; this has been incorporated into the
current uncertainty analysis (see Annex 3.12)
Planned Improvements
The ongoing annual surveys by the FIA Program will improve precision of forest C estimates as new state surveys
become available (USDA Forest Service 2008a). The annual surveys will eventually include all states. To date,
four states are not yet reporting any data from the annualized sampling design of FIA: Hawaii, Oklahoma, New
Mexico and Wyoming. Estimates for these states are currently based on older, periodic data. Hawaii and U.S.
territories will also be included when appropriate forest C data are available. In addition, the more intensive
sampling of down dead wood, litter, and soil organic C on some of the permanent FIA plots continues and will
substantially improve resolution of C pools at the plot level for all U.S. forest land when this information becomes
available. 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.
7-20 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
As more information becomes available about historical land use, the ongoing effects of changes in land use and
forest management will be better accounted for in estimates of soil C (Birdsey and Lewis 2003, Woodbury et al.
2006, Woodbury et al. 2007). Currently, soil C estimates are based on the assumption that soil C density depends
only on broad forest type group, not on land-use history, but long-term residual effects on soil and forest floor C
stocks are likely after land-use change. Estimates of such effects depend on identifying past land use changes
associated with forest lands.
Similarly, agroforestry practices, such as windbreaks or riparian forest buffers along waterways, are not currently
accounted for in the inventory. In order to properly account for the C stocks and fluxes associated with agroforestry,
research will be needed that provides the basis and tools for including these plantings in a nation-wide inventory, as
well as the means for entity-level reporting.
Non-C02 Emissions from Forest Fires
Emissions of non-CO2 gases from forest fires were estimated using the default IPCC (2003) methodology and
default IPCC (2006) combustion factor for wildfires. Emissions from this source in 2007 were estimated to be 29.0
Tg CO2 Eq. of CH4 and 2.9 Tg CO2 Eq. of N2O, as shown in Table 7-11 and Table 7-12. The estimates of non-CO2
emissions from forest fires account for wildfires in the lower 48 states and Alaska as well as prescribed fires in the
lower 48 states.
Table 7-11: Estimated Non-CO2 Emissions from Forest Fires (Tg CO2 Eq.) for U.S. Forests1
Gas 1990 1995 2000 2005 2006 2007
CH4 4.6 6.1 20.6 14.2 31.3 29.0
N2O 0.5 0.6 2.1 L4 3.2 2.9
Total 5.1 6.8 22.7 15.6 34.4 31.9
1 Calculated based on C emission estimates in Changes in Forest Carbon Stock? and default factors in IPCC (2003, 2006).
Table 7-12: Estimated Non-CO2 Emissions from Forest Fires (Gg Gas) for U.S. Forests1
Gas
CH4
N2O
1990
218
2
1995
293
2
2000
983
7
2005
676
5
2006
1,489
10
2007
1,381
9
Calculated based on C emission estimates in Changes in Forest Carbon Stocks and default factors in IPCC (2003, 2006).
Methodology
The IPCC (2003) Tier 2 default methodology was used to calculate non-CO2 emissions from forest fires. Estimates
for CH4 emissions were calculated by multiplying the total estimated C emitted (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 are the same estimates used to generate estimates of CO2 emissions from
forest fires, presented earlier in Box 7-1. Estimates for C emitted include emissions from wildfires in both Alaska
and the lower 48 states as well as emissions from prescribed fires in the lower 48 states only (based on expert
judgment that prescribed fires only occur in the lower 48 states) (Smith 2008a). The IPCC (2006) default
combustion factor of 0.45 for "all 'other' temperate forests" was applied in estimating C emitted from both wildfires
and prescribed fires. See the explanation in Annex 3.12 for more details on the methodology used to estimate C
emitted from forest fires.
Table 7-13: Estimated Carbon Released from Forest Fires for U.S. Forests
Year C Emitted (Tg/yr)
1990 13.6
1995 18.3
Land Use, Land-Use Change, and Forestry 7-21
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2000
61.4
2005
2006
2007
42.3
93.0
86.3
Uncertainty
Non-CO2 gases emitted from forest fires depend on several variables, including: forest area for Alaska and the lower
48 states; average carbon densities for wildfires in Alaska, wildfires in the lower 48, and prescribed fires in the
lower 48; emission ratios; and combustion factor values (proportion of biomass consumed by fire). To quantify the
uncertainties for emissions from forest fires, a Monte Carlo (Tier 2) uncertainty analysis was performed using
information about the uncertainty surrounding each of these variables. The results of the Tier 2 quantitative
uncertainty analysis are summarized in Table 7-14.
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)
2007 Emission Uncertainty Range Relative to Emission
Source Gas Estimate Estimate
(TgC02Eq.) (TgC02Eq.) (%)
Non-CO2 Emissions from Forest Fires CH4 29.0
N2O 2.9
Lower
Bound
7.7
0.8
Upper
Bound
73.9
7.4
Lower
Bound
-73%
-73%
Upper
Bound
155%
152%
QA/QC and Verification
Tier 1 and Tier 2 QA/QC activities were conducted consistent with the U.S. QA/QC plan. Source-specific quality
control measures for forest fires included checking input data, documentation, and calculations to ensure data were
properly handled through the inventory process. Errors that were found during this process were corrected as
necessary.
Recalculations Discussion
Based on new data from the FIA National Program, average carbon density for Alaska was updated from 331 Mg/ha
to 179 Mg/ha and for the lower 48 states from 89 Mg/ha to 91 Mg/ha. The previous value of 331 Mg/ha for Alaska
was from a much smaller subset of Alaskan forest. The updated density values correspond directly to the forestland
that the U.S. Forest Service uses to report carbon. Emissions from prescribed fires were included in the totals this
year. Reported area burned for prescribed fires was taken from the National Interagency Fire Center and an average
carbon density value of 30 Mg/ha was used based on expert judgment within the U.S. Forest Service. The IPCC
(2006) default combustion factor for "all 'other' temperate forests" of 0.45 was used in place of the 0.40 factor
previously used. Data for land area under wildland fire protection for the year 2006 was obtained from the National
Association of State Foresters State Forestry Statistics 2006 Report. This affected emission estimates across the
time series. See explanation in Annex 3.12 for more details on the methodology used to estimate land area under
wildland fire protection for the time series. Based on expert judgment, new uncertainty parameters were applied,
including updated uncertainty percentages and distributions surrounding the variables used in estimating emissions.
These changes resulted in a wider uncertainty range relative to the previous inventory.
Planned Improvements
The default combustion factor of 0.45 from IPCC (2006) was applied in estimating C emitted from both wildfires
and prescribed fires. Additional research into the availability of a combustion factor specific to prescribed fires will
be conducted.
7-22 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Direct N20 Fluxes from Forest Soils (IPCC Source Category 5A1)
Of the synthetic N fertilizers applied to soils in the United States, no more than one percent is applied to forest soils.
Application rates are similar to those occurring on cropped soils, but in any given year, only a small proportion of
total forested land receives N fertilizer. This is because forests are typically fertilized only twice during their
approximately 40-year growth cycle (once at planting and 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 a multiple of 5.7 from 1990 to 2007. 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.
Table 7-15: N2O Fluxes from Soils in Forest Land Remaining Forest Land (Tg CO2 Eq. and Gg N2O)
Year Tg CO2 Eq. Gg
1990 0.0 0.2
1995 0.1 0.4
2000 0.3 1.1
2005 0.3 1.0
2006 0.3 1.0
2007 0.3 1.0
Note: These estimates include direct N2O emissions from N fertilizer additions only. Indirect N2O 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.
Methodology
The IPCC Tier 1 approach was used to estimate N2O from soils within Forest Land Remaining Forest Land.
According to U.S. Forest Service statistics for 1996 (USDA Forest Service 2001), approximately 75 percent of trees
planted were for timber, and about 60 percent of national total harvested forest area is in the southeastern United
States. 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
(Albaugh et al., 2007). Not accounting for fertilizer applied to non-pine plantations is justified because fertilization
is routine for pine forests but rare for hardwoods (Binkley et al. 1995). For each year, the area of pine receiving N
fertilizer was multiplied by the weighted average of the reported range of N fertilization rates (121 Ibs. N per acre).
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. Area data for pine plantations receiving fertilizer in the Southeast
were not available for 2005, 2006 and 2007, so data from 2004 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, oxygen gas partial pressure, soil moisture content, pH, temperature, and tree
planting/harvesting cycles. The effect of the combined interaction of these variables on N2O flux is complex and
highly uncertain. IPCC (2006) does not incorporate any of these variables into the default methodology and only
accounts for variations in estimated fertilizer application rates and estimated areas of forested land receiving N
fertilizer. All forest soils are treated equivalently under this methodology. Furthermore, only synthetic N fertilizers
are captured, so applications of organic N fertilizers are not estimated. However, the total quantity of organic N
inputs to soils is included in the Agricultural Soil Management and Settlements Remaining Settlements sections.
Land Use, Land-Use Change, and Forestry 7-23
-------
Uncertainties exist in the fertilization rates, annual area of forest lands receiving fertilizer, and the emission factors.
Fertilization rates were assigned a default level166 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 2007 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 1.0 Tg CO2 Eq. at a 95 percent
confidence level. This indicates a range of 59 percent below and 211 percent above the 2007 emission estimate of
0.3 Tg C02 Eq.
Table 7-16: Quantitative Uncertainty Estimates of N2O Fluxes from Soils in Forest Land Remaining Forest Land
(Tg CO2 Eq. and Percent)
2007 Emission Uncertainty Range Relative to Emission
Source Gas Estimate Estimate
(TgC02Eq.) (TgC02Eq.) (%)
Lower
Bound
Upper
Bound
Lower
Bound
Upper
Bound
Forest Land Remaining Forest Land:
N2O Fluxes from Soils N2O 0.3 0.1 1.0 -59% +211%
Note: This estimate includes direct N2O emissions from N fertilizer additions to both Forest Land Remaining Forest Land and
Land Converted to Forest Land.
Recalculations Discussion
Number of acres fertilized and fertilizer application rate data for plantations in the southeastern United States
receiving N fertilizer were updated based on Albaugh et al. (2007) from the previous Inventory. This resulted in a
small decrease (less than 10 percent on average) in emissions compared to the previous Inventory.
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 and sink
for atmospheric CO2 in most soils. Changes in inorganic C stocks are typically minor. In addition, soil organic C is
the dominant organic C pool in cropland ecosystems, because biomass and dead organic matter have considerably
less C and those pools are relatively ephemeral. IPCC (2006) recommends reporting changes in soil organic C
stocks due to agricultural land-use and management activities on mineral and organic soils.167
Typical well-drained mineral soils contain from 1 to 6 percent organic C by weight, although mineral soils that are
166 Uncertainty is unknown for the fertilization rates so a conservative value of ±50% was used in the analysis.
167 CO2 emissions associated with liming are also estimated but are included in a separate section of the report.
7-24 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 of 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/IEA 1997). C losses are estimated from drained organic soils under both grassland and
cropland management in this Inventory.
Cropland Remaining Cropland includes all cropland in an inventory year that had been cropland for the last 20
years168 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. CO2 emissions and removals169 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 2003 due
to Conservation Reserve Program enrollment). Emissions from organic soils are estimated using a Tier 2 IPCC
method.
Of the two sub-source categories, land-use and land management of mineral soils was the most important
component of total net C stock change between 1990 and 2007 (see Table 7-17 and Table 7-18). In 2007, mineral
soils were estimated to remove 47.3 Tg CO2 Eq. (12.9 Tg C). This rate of C storage in mineral soils represented
about a 17 percent decrease in the rate since the initial reporting year of 1990. Emissions from organic soils were
27.7 Tg CO2 Eq. (7.5 Tg C) in 2007. In total, U.S. agricultural soils in Cropland Remaining Cropland removed
approximately 19.7 Tg CO2 Eq. (5.4 Tg C) in 2007.
Table 7-17: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining Cropland (Tg CO2 Eq.)
Soil Type
Mineral Soils*
Organic Soils
Total Net Flux*
1990
(56.8)
27.4
(29.4)
1995
(50.6)
27.7
(22.9)
2000
(57.9)
27.7
(30.2)
2005
(45.9)
27.7
(18.3)
2006
(46.8)
27.7
(19.1)
2007
(47.3)
27.7
(19.7)
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.
* Preliminary estimates that will be finalized after public review period following completion of quality control measures.
168 NRI points were classified according to land-use history records starting in 1982 when the NRI survey began, and
consequently the classifications were based on less than 20 years from 1990 to 2001.
169 Note that removals occur through crop and forage uptake of CO2 into biomass C that is later incorporated into soil pools.
Land Use, Land-Use Change, and Forestry 7-25
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Table 7-18: Net CO2 Flux from Soil C Stock Changes in Cropland Remaining, Cropland (Tg C)
Soil Type
Mineral Soils*
Organic Soils
Total Net Flux*
1990
(15.5)
7.5
(8.0)
1995
(13.8)
7.5
(6.3)
2000
(15.8)
7.5
(8.2)
2005
(12.5)
7.5
(5.0)
2006
(12.8)
7.5
(5.2)
2007
(12.9)
7.5
(5.4)
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.
* Preliminary estimates that will be finalized after public review period following completion of quality control measures.
The net reduction in soil carbon accumulation over the time series (33 percent for 2007, relative to 1990) was
largely due to the declining influence of annual cropland enrolled in the Conservation Reserve Program, which
began in the late 1980s. However, there were still positive increases in C stocks from land enrolled in the reserve
program, as well as intensification of crop production by limiting the use of bare-summer fallow in semi-arid
regions, increased hay production, and adoption of conservation tillage (i.e., reduced- and no-till practices).
The spatial variability in annual CO2 flux associated with C stock changes in mineral and organic soils is displayed
in Figure 7-5 and Figure 7-6. The highest rates of net C accumulation in mineral soils occurred in the Midwest,
which is the area with the largest amounts of cropland managed with conservation tillage. Rates were also high in
the Great Plains due to enrollment in the Conservation Reserve Program. Emission rates from drained organic soils
were highest along the southeastern coastal region, in the northeast central United States surrounding the Great
Lakes, and along the central and northern portions of the West Coast.
Figure 7-5: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007,
Cropland Remaining Cropland
Figure 7-6: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007,
Cropland Remaining Cropland
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 approximately 260,000 points in
agricultural land for the conterminous United States and Hawaii.170 Each point is associated with an "expansion
factor" that allows scaling of C stock changes from NRI points to the entire country (i.e., each expansion factor
represents the amount of area with the same land-use/management history as the sample point). Land-use and some
management information (e.g., crop type, soil attributes, and irrigation) were originally collected for each NRI point
on a 5-year cycle beginning in 1982. For cropland, data were collected for 4 out of 5 years in the cycle (i.e., 1979-
1982, 1984-1987, 1989-1992, and 1994-1997). However, the NRI program began collecting annual data in 1998,
and data are currently available through 2003. NRI points were classified as Cropland Remaining Cropland in a
given year between 1990 and 2007 if the land use had been cropland for 20 years.171 Cropland includes all land
used to produce food and fiber, or forage that is harvested and used as feed (e.g., hay and silage).
170 NRI points were classified as agricultural if under grassland or cropland management between 1990 and 2003.
171 NRI points were classified according to land-use history records starting in 1982 when the NRI survey began. Therefore, the
classification prior to 2002 was based on less than 20 years of recorded land-use history for the time series.
7-26 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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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, perennial/horticultural crops, rice, and crops rotated
with these crops. The Tier 2 method was also used for very gravelly, cobbly or shaley soils (greater than 35 percent
by volume). Mineral SOC stocks were estimated using a Tier 2 method for these areas, because the Century model
used for the Tier 3 method has not been fully tested to address its adequacy for estimating C stock changes
associated with certain crops and rotations, as well as cobbly, gravelly or shaley soils. An additional stock change
calculation was made for mineral soils using Tier 2 emission factors, accounting for enrollment patterns in the
Conservation Reserve Program after 2003, which was not addressed by the Tier 3 methods.
Further elaboration on the methodology and data used to estimate stock changes from mineral soils are described
below and in Annex 3.13.
Tier 3 Approach
Mineral SOC stocks and stock changes were estimated using the Century biogeochemical model (Parton et al. 1987,
1988, 1994; Metherell et al. 1993), which simulates the dynamics of C and other elements in cropland, grassland,
forest, and savanna ecosystems. It uses monthly weather data as an input, along with information about soil physical
properties. Input data on land use and management are specified at monthly resolution and include land-use type,
crop/forage type and management activities (e.g., planting, harvesting, fertilization, manure amendments, tillage,
irrigation, residue removal, grazing, and fire). The model computes net primary productivity and C additions to soil,
soil temperature, and water dynamics, in addition to turnover, stabilization, and mineralization of soil organic 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.
[BEGIN BOX]
Box 7-2: Tier 3 Approach for Soil C Stocks Compared to Tier 1 or 2 Approaches
A Tier 3 model-based approach is used to inventory soil C stock changes on the majority of agricultural land with
mineral soils. This approach entails several fundamental differences compared to the IPCC Tier 1 or 2 methods,
which are based on a classification of land areas into a number of discrete classes based on a highly aggregated
classification of climate, soil, and management (i.e., only six climate regions, seven soil types and eleven
management systems occur in U.S. agricultural land under the IPCC classification). Input variables to the Tier 3
model, including climate, soils, and management activities (e.g., fertilization, crop species, tillage, etc.), are
represented in considerably more detail both temporally and spatially, and exhibit multi-dimensional interactions
through the more complex model structure compared with the IPCC Tier 1 or 2 approach. The spatial resolution of
the analysis is also finer in the Tier 3 method compared to the lower tier methods as implemented in the United
States for previous Inventories (e.g., 3,037 counties versus 181 Major Land Resource Areas (MLRAs),
respectively).
In the Century model, soil C dynamics (and CO2 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
Land Use, Land-Use Change, and Forestry 7-27
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IPCC methodology (Tier 1 and 2), the Tier 3 model addresses the delayed response of the soil to management and
land-use changes. Delayed responses can occur due to variable weather patterns and other environmental
constraints that interact with land use and management and affect the time frame over which stock changes occur.
Moreover, the Tier 3 method also accounts for the overall effect of increasing yields and, hence, C input to soils that
have taken place across management systems and crop types within the United States. Productivity has increased by
1 to 2 percent annually over the past 4 to 5 decades for most major crops in the United States (Reilly and Fuglie
1998), which is believed to have led to increases in cropland soil C stocks (e.g., Allmaras et al. 2000). This is a
major difference from the IPCC-based Tier 1 and 2 approaches, in which trends in soil C stocks only capture
discrete changes in management and/or land use, rather than a longer term trend such as gradual increases in crop
productivity.
[END BOX]
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 USD A 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 USD A Natural Resources Conservation Service (Edmonds et
al. 2003), and then adjusted using county-level estimates of manure available for application in other years.
Specifically, county-scale ratios of manure available for application to soils in other years relative to 1997 were used
to adjust the area amended with manure (see Annex 3.13 for further details). Greater availability of managed
manure N relative to 1997 was, thus, assumed to increase the area amended with manure, while reduced availability
of manure N relative to 1997 was assumed to reduce the amended area. The amount of manure produced by each
livestock type was calculated for managed and unmanaged waste management systems based on methods described
in the Manure Management section (Section 6.2) and annex (Annex 3.10).
Manure amendments were an input to the Century Model based on manure N available for application from all
managed or unmanaged systems except Pasture/Range/Paddock.172 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, runoff and leaching, and poultry manure used as a feed supplement. More
information on these losses is available in the description of the Manure Management source category. 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 were used as an input in the model simulations, based on an aggregation of gridded weather
data to the county scale from the Parameter-elevation Regressions on Independent Slopes Model (PRISM) database
(Daly et al. 1994). Soil attributes, which were obtained from an NRI database, were assigned based on field visits
and soil series descriptions. Each NRI point was run 100 times as part of the uncertainty assessment, yielding a total
of over 18 million simulation runs for the analysis. 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). C
stocks and 95 percent confidence intervals were estimated for each year between 1990 and 2003, but C stock
changes from 2004 to 2007 were assumed to be similar to 2003 because no additional activity data are currently
available from the NRI for the latter years.
Tier 2 Approach
In the IPCC Tier 2 method, data on climate, soil types, land-use, and land management activity were used to classify
land area to apply appropriate stock change factors. MLRAs formed the base spatial unit for mapping climate
172 Pasture/Range/Paddock manure additions to soils are addressed in the Grassland Remaining Grassland and Land Converted
to Grassland categories.
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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). 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
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 2007 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 2007 were adjusted to account for additional C stock
changes associated with gains or losses in soil C after 2003 due to changes in Conservation Reserve Program
enrollment. The change in enrollment acreage relative to 2003 was based on data from USDA-FSA (2007) for 2004
through 2007, 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 2007.
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 to the level of the inventory methodology
employed (i.e., Tier 2 and Tier 3). Uncertainty for the portions of the Inventory estimated with Tier 2 and 3
Land Use, Land-Use Change, and Forestry 7-29
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approaches was derived using a Monte Carlo approach (see Annex 3.13 for further discussion). A combined
uncertainty estimate for changes in soil C stocks is also included. Uncertainty estimates from each component were
combined using the error propagation equation in accordance with IPCC (2006). The combined uncertainty was
calculated by taking the square root of the sum of the squares of the standard deviations of the uncertain quantities.
More details on how the individual uncertainties were developed are in Annex 3.13. The combined uncertainty for
soil C stocks in Cropland Remaining Cropland ranged from 152 percent below to 148 percent above the 2007 stock
change estimate of -19.7 Tg CO2 Eq.
Table 7-19: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Cropland Remaining
Cropland (Tg CO2 Eq. and Percent)
Source
2007 Flux
Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Flux
Estimate
(TgC02Eq.) (%)
Mineral Soil C Stocks: Cropland Remaining
Cropland, Tier 3 Inventory Methodology
Mineral Soil C Stocks: Cropland Remaining
Cropland, Tier 2 Inventory Methodology
Mineral Soil C Stocks: Cropland Remaining
Cropland (Change in CRP enrollment relative to
2003)
Organic Soil C Stocks: Cropland Remaining
Cropland, Tier 2 Inventory Methodology
(42.3)
(3.0)
(2.0)
27.7
Lower
Bound
(69.6)
(6.9)
(3.0)
15.8
Upper
Bound
(15.1)
0.8
(1.0)
36.9
Lower
Bound
-64%
-127%
-50%
-43%
Upper
Bound
+64%
+128%
+50%
+33%
Combined Uncertainty for Flux associated with
Agricultural Soil Carbon Stock Change in
Cropland Remaining Cropland
(19.7)
(49.6)
9.4
-152% +148%
QA/QC and Verification
Quality control measures included checking input data, model scripts, and results to ensure data were properly
handled throughout the inventory process. Several errors were found in the implementation of the new annual NRI
data, mostly involving problems in scheduling crops and practices with the more detailed histories; corrective
actions were taken to deal with the errors. As discussed in the uncertainty section, results were compared to field
measurements, and a statistical relationship was developed to assess uncertainties in the model's predictive
capability. The comparisons included over 40 long-term experiments, representing about 800 combinations of
management treatments across all of the sites (Ogle et al. 2007). Inventory reporting forms and text were reviewed
and revised as needed to correct transcription errors.
Recalculations Discussion
Annual survey data from the USDA National Resources Inventory (NRI) were incorporated into this year's
estimates. This resulted in several changes to the inventory methods:
First, the availability of new data extended the time series of activity data beyond 1997 to 2003.173 In previous
Inventories, activity data were only available through 1997, and so subsequent years were treated as the same land
use practice occurring in 1997.
Second, annual area data, rather than area data that had been collected in 5-year increments, were used to estimate
soil C stock changes, leading to more accurate estimates.
Third, each NRI point was simulated separately, instead of simulating clusters of points that had common cropping
173 Note that the new NRI data were only used in the Tier 3 estimates. The Tier 2 estimates still use data from the 1997 National
Resources Inventory, but will be updated in the future.
7-30 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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rotation histories and soil characteristics in a county. More importantly, the exact cropping histories were simulated,
instead of generalized cropping rotations (e.g., wheat-fallow, corn-soybean).
Overall, the recalculations resulted in an average annual decline in soil organic C stocks of 12.5 Tg CO2 Eq. for the
period 1990 through 2006, compared to the previous Inventory. Uncertainties were also higher in this year's
Inventory because soil C stock changes were estimated for each year from new annual NRI data. Previous
Inventories took an average of changes over 5-year periods.
C02 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 3.8 Tg CO2Eq. to 5.0 Tg CO2Eq. In 2007, liming of agricultural soils in the United States resulted in
emissions of 4.1 Tg CO2 Eq. (1.1 Tg C), representing about a 13 percent decrease in emissions since 1990 (see
Table 7-20 and Table 7-21). The trend is driven entirely by the amount of lime and dolomite estimated to have been
applied to soils over the time period.
Table 7-20: Emissions from Liming of Agricultural Soils (Tg CO2 Eq.)
Source 1990 1995 2000 2005 2006 2007
Liming of Soils1 4/7 4.4 4.3 4.3 4.2 4.1
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on
historical data only.
1 Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to
Grassland, and Settlements Remaining Settlements.
Table 7-21: Emissions from Liming of Agricultural Soils (Tg C)
Source 1990 1995 2000 2005 2006 2007
Liming of Soils1 L3 L2 L2 L2 L2 1.1
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on
historical data only.
1 Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to
Grassland, and Settlements Remaining Settlements.
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). This analysis of lime dissolution is based on liming occurring in the
Mississippi River basin, where the vast majority of all U.S. liming takes place (West 2008). U.S. liming that does
not occur in the Mississippi River basin tends to occur under similar soil and rainfall regimes, and, thus, the
emission factor is appropriate for use across the United States (West 2008). The annual application rates of
limestone and dolomite were derived from estimates and industry statistics provided in the Minerals Yearbook and
Mineral Industry Surveys (Tepordei 1993 through 2006; Willett 2007a, b; USGS 2007, 2008). To develop these
data, the U.S. Geological Survey (USGS; U.S. Bureau of Mines prior to 1997) obtained production and use
information by surveying crushed stone manufacturers. Because some manufacturers were reluctant to provide
information, the estimates of total crushed limestone and dolomite production and use were divided into three
components: (1) production by end-use, as reported by manufacturers (i.e., "specified" production); (2) production
reported by manufacturers without end-uses specified (i.e., "unspecified" production); and (3) estimated additional
production by manufacturers who did not respond to the survey (i.e., "estimated" production).
The "unspecified" and "estimated" amounts of crushed limestone and dolomite applied to agricultural soils were
Land Use, Land-Use Change, and Forestry 7-31
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calculated by multiplying the percentage of total "specified" limestone and dolomite production applied to
agricultural soils by the total amounts of "unspecified" and "estimated" limestone and dolomite production. In other
words, the proportion of total "unspecified" and "estimated" crushed limestone and dolomite that was applied to
agricultural soils (as opposed to other uses of the stone) was assumed to be proportionate to the amount of
"specified" crushed limestone and dolomite that was applied to agricultural soils. In addition, data were not
available for 1990, 1992, and 2007 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 2007 data, the previous year's fractions were applied to a 2007
estimate of total crushed stone presented in the USGS Mineral Industry Surveys: Crushed Stone and Sand and
Gravel in the First Quarter of 2008 (USGS 2008).
The primary source for limestone and dolomite activity data is the Minerals Yearbook, published by the Bureau of
Mines through 1994 and by the USGS from 1995 to the present. In 1994, the "Crushed Stone" chapter in the
Minerals Yearbook began rounding (to the nearest thousand metric tons) quantities for total crushed stone produced
or used. It then reported revised (rounded) quantities for each of the years from 1990 to 1993. In order to minimize
the inconsistencies in the activity data, these revised production numbers have been used in all of the subsequent
calculations. Since limestone and dolomite activity data are also available at the state level, the national-level
estimates reported here were broken out by state, although state-level estimates are not reported here.
Table 7-22: Applied Minerals (Million Metric Tons)
Mineral
Limestone
Dolomite
1990
19.01
2.36
1995
17.30
2.77
2000
15.86
3.81
2005
18.09
1.85
2006
17.1
2.24
2007
16.4
2.14
Note: These numbers represent amounts applied to Cropland Remaining Cropland, Land Converted to Cropland, Grassland
Remaining Grassland, Land Converted to Grassland, and Settlements Remaining Settlements.
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 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 2007 were estimated to be between 0.22 and 8.30 Tg CO2 Eq. at the 95 percent
confidence level. This indicates a range of 95 percent below to 105 percent above the 2007 emission estimate of
4.05 Tg CO2 Eq.
Table 7-23: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Liming of Agricultural Soils (Tg
CO2 Eq. and Percent)
2007 Emission
Source Estimate
Gas (Tg CO2 Eq.)
Uncertainty Range Relative to Emissions
Estimate"
(Tg CO2 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Liming of Agricultural Soils1 CO2 4.1
0.2 8.3 -95% 105%
aRange of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
1 Also includes emissions from liming on Land Converted to Cropland, Grassland Remaining Grassland, Land Converted to
7-32 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Grassland, and Settlements Remaining Settlements.
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 2006 has been revised. Consequently, the reported emissions resulting from
liming in 2006 have also changed. In the previous Inventory, 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). 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 2006. These values have replaced those used in the previous Inventory to calculate the quantity of minerals
applied to soil and the emissions from liming.
C02 Emissions from Urea Fertilization
The use of urea (CO(NH2)2) as fertilizer leads to emissions of CO2 that was fixed during the industrial production
process. Urea in the presence of water and urease enzymes is converted into ammonium (NH4+), hydroxyl ion (OH"
), and bicarbonate (HCO3~). The bicarbonate then evolves into CO2 and water. Emissions from urea fertilization in
the United States totaled 4.0 Tg CO2 Eq. (1.1 Tg C) in 2007 (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 4.0 Tg CO2 Eq.
Table 7-24: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg CO2 Eq.)
Source 1990 1995 2000 2005 2006 2007
Urea Fertilization1 2.4 2/7 3.2 3.5 3/7 4.0
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on
historical data only.
1 Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, Land
Converted to Grassland, Settlements Remaining Settlements, and Forest Land Remaining Forest Land.
Table 7-25: CO2 Emissions from Urea Fertilization in Cropland Remaining Cropland (Tg C)
Source 1990 1995 2000 2005 2006 2007
Urea Fertilization1 0/7 0/7 0.9 1.0 1.0 1.1
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on
historical data only.
1 Also includes emissions from urea fertilization on Land Converted to Cropland, Grassland Remaining Grassland, Land
Converted to Grassland, Settlements Remaining Settlements, and Forest Land Remaining Forest Land.
Methodology
Carbon dioxide emissions from the application of urea to agricultural soils were estimated using the IPCC (2006)
Tier 1 methodology. The annual amounts of urea fertilizer applied (see Table 7-26) were derived from state-level
fertilizer sales data provided in Commercial Fertilizers (TVA 1991, 1992, 1993, 1994; AAPFCO 1995 through
2008) and were multiplied by the default IPCC (2006) emission factor of 0.20, which is equal to the C content of
urea on an atomic weight basis. Because fertilizer sales data are reported in fertilizer years (July through June), a
calculation was performed to convert the data to calendar years (January through December). According to historic
monthly fertilizer 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 3 5 percent of total fertilizer used in any fertilizer year is applied
between July through December of the previous calendar year. Fertilizer sales data for the 2008 fertilizer year were
not available in time for publication. Accordingly, July through December 2007 fertilizer consumption was
estimated by calculating the percent change in urea use from January through June 2006 to July through December
2006. This percent change was then multiplied by the January through June 2007 data to estimate July through
Land Use, Land-Use Change, and Forestry 7-33
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December 2007 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.
Table 7-26: Applied Urea (Million Metric Tons)
1990 1995 2000 2005 2006 2007
Urea Fertilizer1 3.30 3.62 4.38 4.78 4.98 5.39
Note: Shaded areas indicate values based on a combination of historical data and projections. All other values are based on
historical data only.
'These numbers represent amounts applied to all agricultural land, including Land Converted to Cropland, Grassland Remaining
Grassland, Land Converted to Grassland, Settlements Remaining Settlements, and Forest Land Remaining Forest Land.
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, such as aircraft deicing, may
be included in consumption totals; it was determined through personal communication with Fertilizer Regulatory
Program Coordinator David L. Terry (2007), however, that this amount is most likely very small. Research into
aircraft deicing practices also confirmed that urea is used minimally in the industry; a 1992 survey found a known
annual usage of approximately 2,000 tons of urea for deicing; this would constitute 0.06 percent of the 1992
consumption of urea (EPA 2000). Similarly, surveys conducted from 2002 to 2005 indicate that total urea use for
deicing at U.S. airports is estimated to be 3,740 MT per year, or less than 0.07 percent of the fertilizer total for 2007
(Itle 2009). Lastly, there is uncertainty surrounding the assumptions behind the calculation that converts fertilizer
years to calendar years. CO2 emissions from urea fertilization of agricultural soils in 2007 were estimated to be
between 2.3 and 4.1 Tg CO2 Eq. at the 95 percent confidence level. This indicates a range of 43 percent below to 4
percent above the 2006 emission estimate of 4.0 Tg CO2 Eq.
Table 7-27: Quantitative Uncertainty Estimates for CO2 Emissions from Urea Fertilization (Tg CO2 Eq. and Percent)
2007 Emission
Estimate Uncertainty Range Relative to Emissions Estimate"
Source Gas (Tg CO2 Eq.) (Tg CO2 Eq.) (%)
Urea Fertilization
C02
4.0
Lower
Bound
2.3
Upper
Bound
4.1
Lower
Bound
-43%
Upper
Bound
+4%
aRange of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Note: These numbers represent amounts applied to all agricultural land, including Land Converted to Cropland, Grassland
Remaining Grassland, Land Converted to Grassland, Settlements Remaining Settlements, and Forest Land Remaining Forest
Land.
QA/QC and Verification
A QA/QC analysis was performed for data gathering and input, documentation, and calculation. Inventory reporting
forms and text were reviewed. No errors were found.
Recalculations Discussion
July to December 2006 urea application was updated with newly available data for fertilizer year 2007, and the 2006
emission estimate was revised accordingly. (In the previous Inventory, the application for this period was calculated
based on application during July to December 2005.) No other recalculations were needed, and the rest of the time
series remains the same as estimated in the previous Inventory.
Planned Improvements
The primary planned improvement is to investigate using a Tier 2 or Tier 3 approach, which would utilize country-
specific information to estimate a more precise emission factor.
7-34 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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7.5. Land Converted to Cropland (IPCC Source Category 5B2)
Land Converted to Cropland includes all cropland in an inventory year that had been another land use at any point
during the previous 20 years174 according to the USDA NRI land use survey (USDA-NRCS 2000). Consequently,
lands are retained in this category for 20 years as recommended by the IPCC guidelines (IPCC 2006) unless there is
another land-use change. Background on agricultural C stock changes is provided in Cropland Remaining Cropland
and will only be summarized here for Land Converted to Cropland. Soils are the largest pool of C in agricultural
land, and also have the greatest potential for storage or release of C, because biomass and dead organic matter C
pools are relatively small and ephemeral compared with soils. The IPCC (2006) recommends reporting changes in
soil organic C stocks due to: (1) agricultural land-use and management activities on mineral soils, and (2)
agricultural land-use and management activities on organic soils.175
Land-use and management of mineral soils in Land Converted to Cropland led 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 5.9 Tg CO2 Eq. (1.6 Tg C) in 2007. Mineral soils were estimated to lose 3.3 Tg
CO2 Eq. (0.9 Tg C) in 2007, while drainage and cultivation of organic soils led to annual losses of 2.6 Tg CO2 Eq.
(0.7 Tg C) in 2007.
Table 7-28: Net CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg CO2 Eq.)
Soil Type 1990 1995 2000 2005 2006 2007
Mineral Soils (0.3) 0.3 (0.3) 3.3 3.3 3.3
Organic Soils 2A 2.6 2.6 2.6 2.6 2.6
Total Net Flux 2.2 2.9 2.4 5.9 5.9 5.9
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 CO2 Flux from Soil C Stock Changes in Land Converted to Cropland (Tg C)
Soil Type 1990 1995 2000 2005 2006 2007
Mineral Soils (0.1) 0.1 (0.1) 0.9 0.9 0.9
Organic Soils 0/7 0/7 0/7 0.7 0.7 0.7
Total Net Flux 0.6 0.8 0.6 1.6 1.6 1.6
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.
The spatial variability in annual CO2 flux associated with C stock changes in mineral and organic soils for Land
Converted to Cropland is displayed in Figure 7-7 and Figure 7-8. While a large portion of the United States had net
losses in soil C for Land Converted to Cropland, there were some notable areas with net C accumulation in the
Great Plains, Midwest, mid-Atlantic states. These areas were gaining C following conversion, because the land had
been brought into hay production, including grass and legume hay, leading to enhanced plant production relative to
the previous land use, and thus higher C input to the soil. Emissions from organic soils were largest in California,
Florida and the upper Midwest, which coincided with largest concentrations of cultivated organic soils in the United
States.
Figure 7-7: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007,
Land Converted to Cropland
Figure 7-8: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007, Land
Converted to Cropland
174 NRI points were classified according to land-use history records starting in 1982 when the NRI survey began, and
consequently the classifications were based on less than 20 years from 1990 to 2001.
175 CO2 emissions associated with liming are also estimated but included in a separate section of the report.
Land Use, Land-Use Change, and Forestry 7-35
-------
Methodology
The following section includes a brief description of the methodology used to estimate changes in soil C stocks due
to agricultural land-use and management activities on mineral and organic soils for Land Converted to Cropland.
Further elaboration on the methodologies and data used to estimate stock changes for mineral and organic soils are
provided in the Cropland Remaining Cropland section and Annex 3.13.
Soil C stock changes were estimated for Land Converted to Cropland according to land-use histories recorded in the
USDA NRI survey (USDA-NRCS 2000). Land-use and some management information (e.g., crop type, soil
attributes, and irrigation) were originally collected for each NRI point on a 5-year cycle beginning in 1982.
However, the NRI program initiated annual data collection in 1998, and the annual data are currently available
through 2003. NRI points were classified as Land Converted to Cropland in a given year between 1990 and 2007 if
the land use was cropland but had been another use during the previous 20 years.176 Cropland includes all land used
to produce food or fiber, or forage that is harvested and used as feed (e.g., hay and silage).
Mineral Soil Carbon Stock Changes
A Tier 3 model-based approach was 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.177
Tier 3 Approach
Mineral SOC stocks and stock changes were estimated using the Century biogeochemical model for the Tier 3
methods. National estimates were obtained by using the model to simulate historical land-use change patterns as
recorded in the USDA National Resources Inventory (USDA-NRCS 2000). The methods used for Land Converted
to Cropland are the same as those described in the Tier 3 portion of Cropland Remaining Cropland section for
mineral soils (see Cropland Remaining Cropland Tier 3 methods section and Annex 3.13 for additional
information).
Tier 2 Approach
For the mineral soils not included in the Tier 3 analysis, SOC stock changes were estimated using a Tier 2 Approach
for Land Converted to Cropland as described in the Tier 2 portion of Cropland Remaining Cropland section for
mineral 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 2007.
176 NRI points were classified according to land-use history records starting in 1982 when the NRI survey began. Therefore, the
classification prior to 2002 was based on less than 20 years of recorded land-use history for the time series.
177 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).
7-36 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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). Uncertainty for
the portions of the Inventory estimated with Tier 2 and 3 approaches was derived using a Monte Carlo approach (see
Annex 3.13 for further discussion). A combined uncertainty estimate for changes in agricultural soil C stocks is also
included. Uncertainty estimates from each component were combined using the error propagation equation in
accordance with IPCC (2006), i.e., by taking the square root of the sum of the squares of the standard deviations of
the uncertain quantities. The combined uncertainty for soil C stocks in Land Converted to Cropland was estimated
to be 40 percent below and 36 percent above the inventory estimate of 5.9 Tg CO2 Eq.
Table 7-30: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Land Converted to
Cropland (Tg CO2 Eq. and Percent)
Source
2007 Flux Uncertainty Range Relative to Flux
Estimate Estimate
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
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
(0.8)
4.1
2.6
(1.5)
2.3
1.2
(0.1)
5.8
3.7
-84%
-44%
-53%
+84%
+41%
+41%
Combined Uncertainty for Flux associated
with Soil Carbon Stock Change in Land
Converted to Cropland
5.9
3.5
8.1
-40%
+36%
QA/QC and Verification
See QA/QC and Verification section under Cropland Remaining Cropland.
Recalculations Discussion
Annual survey data from the USDA National Resources Inventory (NPJ) were incorporated into the current
Inventory. This resulted in several changes to the inventory methods:
First, the availability of new data extended the time series of activity data beyond 1997 to 2003.178 In previous
Inventories, activity data were only available through 1997, and so subsequent years were treated as the same land
use practice occurring in 1997.
Second, annual area data, rather than area data that had been collected in 5-year increments, were used to estimate
soil C stock changes, leading to more accurate estimates.
Third, each NRI point was simulated separately, instead of simulating clusters of points that had common land
use/cropping rotation histories and edaphic characteristics in a county. More importantly, the exact cropping
histories were simulated, instead of generalized cropping rotations (e.g., wheat-fallow, corn-soybean).
178 Note that the new NRI data were only used in the Tier 3 inventory. The Tier 2 portion of the inventory still used data from
the 1997 National Resources Inventory, but will be updated in the future.
Land Use, Land-Use Change, and Forestry 7-37
-------
Fourth, NRI area data were reconciled with the forest area estimates in the Forest Inventory and Analysis (FIA)
dataset, and were incorporated into the estimation of soil C stock changes. The reconciliation led to adjustments in
the grassland areas in the NRI dataset, including Land Converted to Cropland119(i.e., Grassland and Wetlands
Converted to Cropland).
Overall, these recalculations resulted in an average annual increase in soil C stocks of 8.5 Tg CO2 Eq. for soil C
stock changes in Land Converted to Cropland over the time series from 1990 through 2006, 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 the Century model for Land
Converted to Cropland, but this is a planned improvement. This improvement will produce a more rigorous
assessment of uncertainty. See Planned Improvements section under Cropland Remaining Cropland for additional
planned improvements.
7.6. Grassland Remaining Grassland (IPCC Source Category 5C1)
Grassland Remaining Grassland includes all grassland in an inventory year that had been grassland for the previous
20 years180 according to the USDA NRI land use survey (USDA-NRCS 2000). Background on agricultural C stock
changes is provided in the Cropland Remaining Cropland section and will only be summarized here for Grassland
Remaining Grassland. Soils are the largest pool of C in agricultural land, and also have the greatest potential for
storage or release of C, because biomass and dead organic matter C pools are relatively small and ephemeral
compared to soils. IPCC (2006) recommends reporting changes in soil organic C stocks due to: (1) agricultural
land-use and management activities on mineral soils, and (2) agricultural land-use and management activities on
organic soils.181
Land-use and management of mineral soils in Grassland Remaining Grassland increased soil C, while organic soils
lost relatively small amounts of C in each year 1990 through 2007. Due to the pattern for mineral soils, the overall
trend were gains in soil C over the time series although the rates varied from year to year, with a net removal of 4.7
Tg CO2 Eq. (5.4 Tg C) in 2007. However, there was considerable variation driven by variability in weather patterns
over the time series. Overall, flux rates declined by 42.1 Tg CO2 Eq. (11.5 Tg C) when comparing the net change in
soil C for 1990 and 2007.
Table 7-31: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg CO2 Eq.)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(50.6)
3.9
(46.7)
1995
(40.1)
3.7
(36.4)
2000
(55.1)
3.7
(51.4)
2005
(8.3)
3.7
(4.6)
2006
(8.3)
3.7
(4.6)
2007
(8.4)
3.7
(4.7)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and
projections. All other values are based on historical data only. Totals may not sum due to independent rounding.
Table 7-32: Net CO2 Flux from Soil C Stock Changes in Grassland Remaining Grassland (Tg C)
Soil Type
Mineral Soils
Organic Soils
Total Net Flux
1990
(13.8)
1.1
(12.7)
1995
(10.9)
1.0
(9.9)
2000
(15.0)
1.0
(14.0)
2005
(2.3)
1.0
(1.3)
2006
(2.3)
1.0
(1.3)
2007
(2.3)
1.0
(1.3)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and
179 NRI area data for forest lands was adjusted the match the forest area estimates in the Forest Inventory and Analysis dataset.
In order to maintain the same total area, the area data for grasslands and wetlands in the NRI were adjusted to offset the increase
or decrease in the forest land area (see section on Representation of U.S. Land Base for more information).
180 NRI points were classified according to land-use history records starting in 1982 when the NRI survey began, and
consequently the classifcations were based on less than 20 years from 1990 to 2001.
181 CO2 emissions associated with liming are also estimated but included in a separate section of the report.
7-38 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
projections. All other values are based on historical data only. Totals may not sum due to independent rounding.
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 gained soil organic C in several regions during 2007, including the
Northeast, Midwest, Southwest and far western states; although these were relatively small increases in C on a per-
hectare basis. 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.
Figure 7-9: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007,
Grassland Remaining Grassland
Figure 7-10: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007,
Grassland Remaining Grassland
Methodology
The following section includes a brief description of the methodology used to estimate changes in soil C stocks due
to agricultural land-use and management activities on mineral and organic soils for Grassland Remaining
Grassland. Further elaboration on the methodologies and data used to estimate stock changes from mineral and
organic soils for are provided in the Cropland Remaining Cropland section and Annex 3.13.
Soil C stock changes were estimated for Grassland Remaining Grassland according to land-use histories recorded in
the USDA NRI survey (USDA-NRCS 2000). Land-use and some management information (e.g., crop type, soil
attributes, and irrigation) were originally collected for each NRI point on a 5-year cycle beginning in 1982.
However, the NRI program initiated annual data collection in 1998, and the annual data are currently available
through 2003. NRI points were classified as Grassland Remaining Grassland in a given year between 1990 and
2007 if the land use had been grassland for 20 years.182 Grassland includes pasture and rangeland used for grass
forage production, where the primary use is livestock grazing. Rangelands are typically extensive areas of native
grassland that are not intensively managed, while pastures are often seeded grassland, possibly following tree
removal, that may or may not be improved with practices such as irrigation and interseeding legumes.
Mineral Soil Carbon Stock Changes
An IPCC Tier 3 model-based approach was used to estimate C stock changes for most mineral soils in Grassland
Remaining Grassland. The C stock changes for the remaining soils were estimated with an IPCC Tier 2 method
(Ogle et al. 2003), including gravelly, cobbly or shaley soils (greater than 35 percent by volume) and additional
stock changes associated with sewage sludge amendments.
Tier 3 Approach
Mineral soil organic C stocks and stock changes for Grassland Remaining Grassland were estimated using the
Century biogeochemical model, as described in Cropland Remaining Cropland. Historical land-use and
management patterns were used in the Century simulations as recorded in 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. Specifically, county-scale ratios of manure available
182 NRI points were classified according to land-use history records starting in 1982 when the NRI survey began. Therefore, the
classification prior to 2002 was based on less than 20 years of recorded land-use history for the time series.
Land Use, Land-Use Change, and Forestry 7-39
-------
for application to soils in other years relative to 1997 were used to adjust the area amended with manure (see Annex
3.13 for further details). Greater availability of managed manure N relative to 1997 was, thus, assumed to increase
the area amended with manure, while reduced availability of manure N relative to 1997 was assumed to reduce the
amended area.
The amount of manure produced by each livestock type was calculated for managed and unmanaged waste
management systems based on methods described in the Manure Management Section (Section 6.2) and Annex
(Annex 3.10). In contrast to manure amendments, Pasture/Range/Paddock (PRP) manure N deposition was
estimated internally in the Century model, as part of the grassland system simulations (i.e., PPJ3 manure deposition
was not an external input into the model). See the Tier 3 methods in Cropland Remaining Cropland section for
additional discussion on the Tier 3 methodology for mineral soils.
Tier 2 Approach
The Tier 2 approach is based on the same methods described in the Tier 2 portion of Cropland Remaining Cropland
section for mineral soils (see Cropland Remaining Cropland Tier 2 methods section and Annex 3.13 for additional
information).
Additional Mineral C Stock Change Calculations
Annual C flux estimates for mineral soils between 1990 and 2007 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 (NEBPxA 2007). These values were linearly interpolated to estimate values for the intervening years. N
application rates from Kellogg et al. (2000) were used to determine the amount of area receiving sludge
amendments. Although sewage sludge can be added to land managed for other land uses, it was assumed that
agricultural amendments occur in grassland. Cropland is assumed to rarely be amended with sewage sludge due to
the high metal content and other pollutants in human waste. The soil C storage rate was estimated at 0.38 metric
tons C per hectare per year for sewage sludge amendments to grassland. The stock change rate is based on country-
specific factors and the IPCC default method (see Annex 3.13 for further discussion).
Organic Soil Carbon Stock Changes
Annual C emissions from drained organic soils in Grassland Remaining Grassland were estimated using the Tier 2
method provided in IPCC (2003, 2006), which utilizes U.S.-specific C loss rates (Ogle et al. 2003) rather than
default IPCC rates. Emissions were based on the 1992 and 1997 Grassland Remaining Grassland areas from the
1997 National Resources Inventory (USDA-NRCS 2000). The annual flux estimated for 1992 was applied to 1990
through 1992, and the annual flux estimated for 1997 was applied to 1993 through 2007.
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). Uncertainty for
the portions of the Inventory estimated with Tier 2 and 3 approaches was derived using a Monte Carlo approach (see
Annex 3.13 for further discussion). A combined uncertainty estimate for changes in agricultural soil C stocks is also
included. Uncertainty estimates from each component were combined using the error propagation equation in
accordance with IPCC (2006), i.e., by taking the square root of the sum of the squares of the standard deviations of
the uncertain quantities. The combined uncertainty for soil C stocks in Grassland Remaining Grassland was,
estimated to be 54 percent below and 41 percent above the inventory estimate of -4.7 Tg CO2 Eq.
Table 7-33: Quantitative Uncertainty Estimates for C Stock Changes occurring within Grassland Remaining
Grassland (Tg CO2 Eq. and Percent)
2007 Flux Uncertainty Range Relative to Flux
Estimate Estimate
Source (Tg CO2 Eq.) (Tg CO2 Eq.) (%)
7-40 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Lower Upper Lower Upper
Bound Bound Bound Bound
Mineral Soil C Stocks Grassland Remaining
Grassland, Tier 3 Methodology
Mineral Soil C Stocks: Grassland Remaining
Grassland, Tier 2 Methodology
Mineral Soil C Stocks: Grassland Remaining
Grassland, Tier 2 Methodology (Change in Soil
C due to Sewage Sludge Amendments)
Organic Soil C Stocks: Grassland Remaining
Grassland, Tier 2 Methodology
(7.0)
(0.2)
(1.2)
3.7
(7.2)
(0.3)
(1.8)
1.2
(6.8)
0.0
(0.6)
5.5
-2%
-89%
-50%
-66%
+2%
+127%
+50%
+49%
Combined Uncertainty for Flux Associated
with Agricultural Soil Carbon Stock Change
in Grassland Remaining Grassland
(4.7)
(7.2)
(2.7)
-54% +41%
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 2007 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. Several errors were found in the implementation of the new annual NRI
data, mostly involving problems in scheduling crops and practices with the more detailed histories; corrective
actions were taken to deal with the errors.
Recalculations Discussion
Annual survey data from the USDA National Resources Inventory (NRI) were incorporated into this year's
Inventory. This resulted in several changes to the inventory methods:
First, the availability of new data extended the time series of activity data beyond 1997 to 2003. *83 In previous
Inventories, activity data were only available through 1997, and so subsequent years were treated as the same land
use practice occurring in 1997.
Second, annual area data, rather than area data that had been collected in 5-year increments, were used to estimate
soil C stock changes, leading to more accurate estimates.
Third, each NRI point was simulated separately, instead of simulating clusters of points that had common land use
histories and edaphic characteristics in a county.
183 Note that the new NRI data were only used in the Tier 3 estimates. The Tier 2 portion of the estimates still used data from
the 1997 National Resources Inventory, but will be updated in the future.
Land Use, Land-Use Change, and Forestry 7-41
-------
Fourth, NRI area data were reconciled with the forest area estimates in the Forest Inventory and Analysis (FIA)
dataset, and were incorporated into the estimation of soil C stock changes. The reconciliation led to adjustments in
the grassland areas in the NRI dataset, including Grassland Remaining Grassland.1^
Overall, the recalculations resulted in an average annual increase in soil C stocks of 31 Tg CO2 Eq. for the time
series over the period from 1990 through 2006, 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 Grassland includes all grassland in an inventory year that had been in another land use at any
point during the previous 20 years185 according to the USDA NRI land use survey (USDA-NRCS 2000).
Consequently, lands are retained in this category for 20 years as recommended by IPCC (2006) unless there is
another land use change. Background on agricultural C stock changes is provided in Cropland Remaining Cropland
and will only be summarized here for Land Converted to Grassland. Soils are the largest pool of C in agricultural
land, and also have the greatest potential for storage or release of C, because biomass and dead organic matter C
pools are relatively small and ephemeral compared with soils. IPCC (2006) recommend reporting changes in soil
organic C stocks due to: (1) agricultural land-use and management activities on mineral soils, and (2) agricultural
land-use and management activities on organic soils.186
Land-use and management of mineral soils in Land Converted to Grassland led to an increase in soil C stocks from
1990 through 2007, which was largely due to annual cropland conversion to pasture (see Table 7-34 and Table
7-35). For example, the stock change rates were estimated to remove 22.7 Tg CO2 Eq./yr (6.2 Tg C) and 27.6 Tg
CO2 Eq./yr (7.5 Tg C) from mineral soils in 1990 and 2007, respectively. Drainage of organic soils for grazing
management led to losses varying from 0.5 to 0.9 Tg CO2 Eq./yr (0.1 to 0.2 Tg C).
Table 7-34: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg CO2 Eq.)
Soil Type 1990 2000J ] 2005 2006 2007
Mineral Soils8 (22.7) (23.4)||| (32.8)| | (27.6) (27.6) (27.6)
Organic Soils 0.5 °-9ilJ 0.9J | 0.9 0.9 (X9_
Total Net Flux (22.3) (22.5)g (32.0)| | (26.7) (26.7) (26.7)
Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and
projections. All other values are based on historical data only. Totals may not sum due to independent rounding.
a Stock changes due to application of sewage sludge are reported in Grassland Remaining Grassland.
Table 7-35: Net CO2 Flux from Soil C Stock Changes for Land Converted to Grassland (Tg C)
Soil Type
Mineral Soils3
Organic Soils
Total Net Flux
199011
(6.2) 11
O.ljlif
(6.1)111
2000| !
(9.0)! !
0.21 I
(8.7)1 !
2005
(7.5)
0.2
(7.3)
2006
(7.5)
0.2
(7.3)
2007
(7.5)
0.2
(7.3)
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.
184 NRI area data for forest lands was adjusted the match the forest area estimates in the Forest Inventory and Analysis dataset.
In order to maintain the same total area, the area data for grasslands and wetlands in the NRI were adjusted to offset the increase
or decrease in the forest land area (see section on Representation of U.S. Land Base for more information).
185 NRI points were classified according to land-use history records starting in 1982 when the NRI survey began, and
consequently the classifcations were based on less than 20 years from 1990 to 2001.
186 CO2 emissions associated with liming are also estimated but included in a separate section of the report.
7-42 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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a Stock changes due to application of sewage sludge in Land Converted to Grassland are reported in Grassland Remaining
Grassland.
The spatial variability in annual CO2 flux associated with C stock changes in mineral soils is displayed in Figure
7-11 and Figure 7-12. Soil C stock increased in most states for Land Converted to Grassland. The largest gains
were in the South-Central region, Midwest and northern Great Plains. The patterns were driven by conversion of
annual cropland into continuous pasture. Emissions from organic soils were largest in California, Florida and the
upper Midwest, coinciding with largest concentrations of organic soils in the United States that are used for
agricultural production.
Figure 7-11: Total Net Annual CO2 Flux for Mineral Soils under Agricultural Management within States, 2007,
Land Converted to Grassland
Figure 7-12: Total Net Annual CO2 Flux for Organic Soils under Agricultural Management within States, 2007,
Land Converted to Grassland
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. Further
elaboration on the methodologies and data used to estimate stock changes from mineral and organic soils are
provided in the Cropland Remaining Cropland section and Annex 3.13.
Soil C stock changes were estimated for Land Converted to Grassland according to land-use histories recorded in
the USDA NRI survey (USDA-NRCS 2000). Land-use and some management information (e.g., crop type, soil
attributes, and irrigation) were originally collected for each NRI point on a 5-year cycle beginning in 1982.
However, the NRI program initiated annual data collection in 1998, and the annual data are currently available
through 2003. NRI points were classified as Land Converted to Grassland in a given year between 1990 and 2007 if
the land use was grassland, but had been another use in the previous 20 years. Grassland includes pasture and
rangeland used for grass forage production, where the primary use is livestock grazing. Rangeland typically
includes extensive areas of native grassland that are not intensively managed, while pastures are often seeded
grassland, possibly following tree removal, that may or may not be improved with practices such as irrigation and
interseeding legumes.
Mineral Soil Carbon Stock Changes
An IPCC Tier 3 model-based approach was 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.187 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
187 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).
Land Use, Land-Use Change, and Forestry 7-43
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simulations as recorded in the NRI survey, with supplemental information on fertilizer use and rates from the USDA
Economic Research Service Cropping Practices Survey (ERS 1997) and the National Agricultural Statistics Service
(NASS 1992, 1999, 2004) (see Grassland Remaining Grassland Tier 3 methods section for additional information).
Tier 2 Approach
The Tier 2 approach used for Land Converted to Grassland on mineral soils is the same as described for Cropland
Remaining Cropland (See Cropland Remaining Cropland Tier 2 Approach and Annex 3.13 for additional
information).
Organic Soil Carbon Stock Changes
Annual C emissions from drained organic soils in Land Converted to Grassland were estimated using the Tier 2
method provided in IPCC (2003, 2006), which utilizes U.S.-specific C loss rates (Ogle et al. 2003) rather than
default IPCC rates. Emissions were based on the 1992 and 1997 Land Converted to Grassland areas from the 1997
National Resources Inventory (USDA-NRCS 2000). The annual flux estimated for 1992 was applied to 1990
through 1992, and the annual flux estimated for 1997 was applied to 1993 through 2007.
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). Uncertainty for
the portions of the Inventory estimated with Tier 2 and 3 approaches was derived using a Monte Carlo approach (see
Annex 3.13 for further discussion). A combined uncertainty estimate for changes in agricultural soil C stocks is also
included. Uncertainty estimates from each component were combined using the error propagation equation in
accordance with IPCC (2006), (i.e., by taking the square root of the sum of the squares of the standard deviations of
the uncertain quantities). The combined uncertainty for soil C stocks in Land Converted to Grassland ranged from 8
percent below to 9 percent above the 2007 estimate of -26.7 Tg CO2 Eq.
Table 7-36: Quantitative Uncertainty Estimates for Soil C Stock Changes occurring within Land Converted to
Grassland (Tg CO2 Eq. and Percent)
Source
2007 Flux Uncertainty Range Relative to Flux
Estimate Estimate
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
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
(22.6)
(5.0)
0.9
(23.1)
(7.0)
0.2
(22.1)
(2.8)
1.8
-2%
-39%
-76%
+2%
+43%
+104%
Combined Uncertainty for Flux associated with
Agricultural Soil Carbon Stocks in Land
Converted to Grassland
(26.7)
(28.8) (24.3)
-8%
+9%
QA/QC and Verification
See the QA/QC and Verification section under Grassland Remaining Grassland.
7-44 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Recalculations Discussion
Annual survey data from the USDA National Resources Inventory (NRI) were incorporated into this year's
inventory. This resulted in several changes to the inventory methods:
First, the availability of new data extended the time series of activity data beyond 1997 to 2003.188 In previous
Inventories, activity data were only available through 1997, and so subsequent years were treated as the same land
use practice occurring in 1997.
Second, annual area data, rather than area data that had been collected in 5-year increments, were used to estimate
soil C stock changes, leading to more accurate estimates.
Third, each NRI point was simulated separately, instead of simulating clusters of points that had common land use
histories and edaphic characteristics in a county.
Fourth, NRI area data were reconciled with the forest area estimates in the Forest Inventory and Analysis (FIA)
dataset, and were incorporated into the estimation of soil C stock changes. The reconciliation led to adjustments in
the grassland areas in the NRI dataset, including Land Converted to Grassland.1^9
Overall, the recalculations resulted in an average annual increase in soil C stocks of 9.4 Tg CO2 Eq. for the time
series from 1990 through 2006, 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.
7.8. Wetlands Remaining Wetlands
Peatlands Remaining Peatlands
Emissions from Managed Peatlands
Managed peatlands are peatlands which have been cleared and drained for the production of peat. The production
cycle of a managed peatland has three phases: land conversion in preparation for peat extraction (e.g., draining, and
clearing surface biomass); extraction (which results in the emissions reported under Peatlands Remaining
Peatlands); and abandonment, restoration or conversion of the land to another use.
CO2 emissions from the removal of biomass and the decay of drained peat constitute the major greenhouse gas flux
from managed peatlands. Managed peatlands may also emit CH4 and N2O. The natural production of CH4 is largely
reduced but not entirely shut down when peatlands are drained in preparation for peat extraction (Strack et al.,
2004); however, methane emissions are assumed to be insignificant under Tier 1 (IPCC, 2006). N2O emissions from
managed peatlands depend on site fertility. In addition, abandoned and restored peatlands continue to release GHG
emissions, and at present no methodology is provided by IPCC (2006) to estimate GHG emissions or removals from
restored peatlands. This inventory estimates both CO2 and N2O emissions from lands undergoing peat extraction in
accordance with Tier 1 IPCC (2006) guidelines.
CO2 and N2O Emissions from Lands Undergoing Peat Extraction
IPCC (2006) recommends reporting CO2 and N2O emissions from lands undergoing peat extraction (i.e., Peatlands
188 Note that the new NRI data were only used in the Tier 3 inventory. The Tier 2 portion of the inventory still used data from
the 1997 National Resources Inventory, but will be updated in the future.
189 NRI area data for forest lands was adjusted the match the forest area estimates in the Forest Inventory and Analysis dataset.
In order to maintain the same total area, the area data for grasslands and wetlands in the NRI were adjusted to offset the increase
or decrease in the forest land area (see section on Representation of U.S. Land Base for more information).
Land Use, Land-Use Change, and Forestry 7-45
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Remaining Peatlands) as part of the estimate for emissions from managed wetlands. Peatlands occur in wetland
areas where plant biomass has sunk to the bottom of water bodies and water-logged areas and exhausted the oxygen
supply below the water surface during the course of decay. Due to these anaerobic conditions, much of the plant
matter does not decompose but instead forms layers of peat over the course of many decades and centuries. In the
United States, peat is extracted for horticulture and landscaping growing media, and for a wide variety of industrial,
personal care, and other products. It has not been used for fuel in the United States for many decades. Peat is
harvested from two types of peat deposits in the United States: sphagnum bogs in northern states and wetlands in
states further south. The peat from sphagnum bogs in northern states, which is nutrient poor, is generally corrected
for acidity and mixed with fertilizer. Production from more southerly states is relatively course but nutrient rich.
IPCC (2006) recommends considering both on-site and off-site emissions when estimating CO2 emissions from
lands undergoing peat extraction using the Tier 1 approach. Current methodologies estimate only on-site N2O
emissions, since off-site N2O estimates are complicated by the risk of double-counting emissions from nitrogen
fertilizers added to horticultural peat. On-site emissions from managed peatlands occur as the land is cleared of
vegetation and the underlying peat is exposed to sun and weather. As this occurs, some peat deposit is lost and CO2
is emitted from the oxidation of the peat. On-site N2O is emitted during draining depending on site fertility and if
the deposit contains significant amounts of organic nitrogen in inactive form. Draining land in preparation for peat
extraction allows bacteria to convert the nitrogen into nitrates which leach to the surface where they are reduced to
N2O.
Off-site CO2 emissions from managed peatlands occur from the horticultural and landscaping use of peat. CO2
emissions occur as the nutrient-poor (but now fertilizer-enriched) peat is used in bedding plants, other greenhouse
and plant nursery production, and by consumers, and as nutrient-rich (but relatively course) peat is used directly in
landscaping, athletic fields, golf courses, and plant nurseries. .Most of the CO2 emissions from peat occur off-site,
as the peat is processed and sold to firms which, in the United States, use it predominately for horticultural purposes.
The magnitude of the CO2 emitted from peat depends on whether the peat has been extracted from nutrient-rich or
nutrient-poor peat deposits.
Total emissions from lands undergoing peat extraction have fluctuated between 0.9 and 1.2 Tg CO2 Eq. across the
time series with a gentle decrease until 1996 followed by an increase though 2000. Since 2000, total emissions have
decreased with some fluctuations. CO2 emissions from lands undergoing peat extraction have fluctuated between
0.9 and 1.2 Tg CO2 in recent years and have driven the trends in total emissions. N2O emissions remained close to
zero in recent years, with a decreasing trend until 1995 followed by an overall increase with fluctuations until
around 2000. Since 2000, N2O emissions have fluctuated but shown an overall decrease.
Table 7-37: Emissions from Lands Undergoing Peat Extraction (Tg CO2 Eq.)
Gas
CO2
N2O
Total
1990
1.0
+
1.0
1995
1.0
+
1.0
2000
1.2
+
1.2
2005
1.1
+
1.1
2006
0.9
+
0.9
2007
1.0
+
1.0
+ Less than 0.01 Tg CO2 Eq.
Note: These numbers are based on U.S. production data in accordance with Tier 1 guidelines, which does not take into account
imports, exports and stockpiles (i.e., apparent consumption).
Table 7-38: Emissions from Lands Undergoing Peat Extraction (Gg)
Gas 1990 1995 2000 2005 2006 2007
CO2 1,033 1,018 1,227 1,079 879 1,010
N2O + + + + + +_
+ Less than 0.05 Gg
Note: These numbers are based on U.S. production data in accordance with Tier 1 guidelines, which does not take into account
imports, exports and stockpiles (i.e., apparent consumption).
Methodology
Off-site CO2 Emissions
CO2 emissions from domestic peat production were estimated using a Tier 1 methodology consistent with IPCC
7-46 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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(2006). Off-site CO2 emissions from lands undergoing peat extraction were calculated by apportioning the annual
weight of peat produced in the United States (Table 7-39) into peat extracted from nutrient-rich deposits and peat
extracted from nutrient-poor deposits using annual percentage by weight figures. These nutrient-rich and nutrient-
poor production values were then multiplied by the appropriate default carbon fraction conversion factor taken from
IPCC (2006) in order to obtain off-site emission estimates. Both annual percentages of peat type by weight and
domestic peat production data were sourced from estimates and industry statistics provided in the Minerals
Yearbook and. Mineral Industry Surveys from the U.S. Geological Survey (USGS 1990-2008). To develop these
data, the U.S. Geological Survey (USGS; U.S. Bureau of Mines prior to 1997) obtained production and use
information by surveying domestic peat producers. The USGS often receives a response to the survey from most of
the smaller peat producers, but fewer of the larger ones. For example, of the four active operations producing
23,000 or more metric tons per year, two did not respond to the survey in 2007. As a result, the USGS estimates
production from the nonrespondent peat producers based on responses to previous surveys (responses from 2004 and
2005, in the case above) or other sources. Estimates were made separately for Alaska, because the state conducts its
own mineral survey and reports peat production by volume, rather than by weight (Table 7-40). However, volume
production data was used to calculate off-site CO2 emissions from Alaska applying the same methodology but with
volume-specific carbon fraction conversion factors from IPCC (2006). 19°
The apparent consumption of peat, which includes production plus imports minus exports plus the decrease in
stockpiles, in the United States is over two-and-a-half times the amount of domestic peat production. Therefore, off-
site CO2 emissions from the use of all horticultural peat within the United States are not accounted for using the Tier
1 approach. The United States has increasingly imported peat from Canada for horticultural purposes; in 2007,
imports of sphagnum moss (nutrient-poor) peat from Canada represented 97 percent of total U.S. peat imports
(USGS 2008). Most peat produced in the United States is reed-sedge peat, generally from southern states, which is
classified as nutrient rich by IPCC (2006). Higher-tier calculations of CO2 emissions from apparent consumption
would involve consideration of the percentages of peat types stockpiled (nutrient rich versus nutrient poor) as well
as the percentages of peat types imported and exported.
Table 7-39: Peat Production of Lower 48 States (in thousands of Metric Tons)
Type of Deposit
Nutrient-Rich
Nutrient-Poor
Total Production
1990
595.1
55.4
692.0
1995
531.4
116.6
648.0
2000
728.6
63.4
792.0
2005
657.6
27.4
685.0
2006
529.0
22.0
551.0
2007
581.0
54.0
635.0
Source: USGS (1990-2008) Minerals Yearbook and Mineral Industry Surveys.
Table 7-40: Peat Production of Alaska (in thousands of Cubic Meters)
1990 1995 2000 2005 2006 2007
Total Production 49.7 26.8 27.2 47.8 50.8 51.0
Source: Szumigala, DJ. andR.A. Hughes (1990-2007) Alaska's Mineral Industry Reports. Alaska Department of Natural
Resources.
On-site CO2 Emissions
IPCC (2006) suggests basing the calculation of on-site emissions estimates on the area of peatlands managed for
peat extraction differentiated by the nutrient type of the deposit (rich versus poor). Information on the area of land
managed for peat extraction is currently not available for the United States, but in accordance with IPCC (2006), an
average production rate for the industry was applied to derive an area estimate. In a mature industrialized peat
industry, such as exists in the United States and Canada, the vacuum method191 can extract up to 100 metric ton per
hectare per year (Cleary 2005). The area of land managed for peat extraction in the United States was estimated
using nutrient-rich and nutrient-poor production data and the assumption that 100 metric ton of peat is extracted
190 Peat produced from Alaska was assumed to be nutrient poor; as is the case in Canada, "where deposits of high-quality [but
nutrient poor] sphagnum moss are extensive" (USGS 2008).
191 The vacuum method is one type of extraction that annually "mills" or breaks up the surface of the peat into particles, which
then dry during the summer months. The air-dried peat particles are then collected by vacuum harvesters and transported from
the area to stockpiles (IPCC 2006).
Land Use, Land-Use Change, and Forestry 7-47
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from a single hectare in a single year. The annual land area estimates were then multiplied by the appropriate
nutrient-rich or nutrient-poor IPCC (2006) default emission factor in order to calculate on-site CO2 emission
estimates. Production data is not available by weight for Alaska. In order to calculate on-site emissions resulting
from land undergoing peat extraction in Alaska, the production data by volume were converted to weight using
annual peat density values, and then converted to land area estimates using the assumption that a single hectare
yields 100 metric tons. The IPCC (2006) on-site emissions equation also includes a term which accounts for
emissions resulting from the change in carbon stocks that occurs during the clearing of vegetation prior to peat
extraction. Area data on land undergoing conversion to peatlands for peat extraction is also unavailable for the
United States. However, USGS records show that the number of active operations in the United States has been
declining since 1990. Since vacuum-harvested peatlands have an average lifespan of thirty-five years (Cleary 2005),
it seems reasonable to assume that no new areas are being cleared of vegetation for peat extraction. Other changes
in carbon stocks in living biomass on managed peat lands are also assumed to be zero under the Tier 1 methodology
(IPCC 2006).
On-site N2O Emissions
IPCC (2006) suggests basing the calculation of on-site N2O emissions estimates on the area of nutrient-rich
peatlands managed for peat extraction. These data are not available for the United States, but the on-site CO2
emissions methodology above details the calculation of area data from production data. In order to estimate N2O
emissions, the area of nutrient rich land undergoing peat extraction was multiplied by the appropriate default
emission factor taken from IPCC (2006).
Uncertainty
The uncertainty associated with peat production data was estimated to be ± 25 percent (Apodaca 2008) and assumed
to be normally distributed. The uncertainty associated with peat production data stems from the fact that the USGS
receives data from the smaller peat producers but estimates production from some larger peat distributors. This
same uncertainty and distribution was assumed for the peat type production percentages. The uncertainty associated
with the Alaskan reported production data was assumed to be the same as the lower 48 states, or ± 25 percent with a
normal distribution. It should be noted that the Alaskan Department of Natural Resources estimate that around half
of producers do not respond to their survey with peat production data; therefore, the production numbers reported
are likely to underestimate Alaska peat production. The uncertainty associated with the average bulk density values
was estimated to be ± 25 percent with a normal distribution (Apodaca 2008). IPCC (2006) gives uncertainty values
for the emissions factors for the area of peat deposits managed for peat extraction based on the range of underlying
data used to determine the emissions factors. The uncertainty associated with the emission factors was assumed to
be triangularly distributed. The uncertainty values surrounding the carbon fractions were based on IPCC (2006) and
the uncertainty was assumed to be uniformly distributed. Based on these values and distributions, a Monte Carlo
(Tier 2) uncertainty analysis was applied to estimate the uncertainty of CO2 and N2O emissions from land
undergoing peat extraction. The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 7-41.
CO2 emissions from lands undergoing peat extraction in 2007 were estimated to be between 0.70 and 1.30 Tg CO2
Eq. at the 95 percent confidence level. This indicates a range of -31 percent below to 29 percent above the 2007
emission estimate of 0.99 Tg CO2 Eq. N2O emissions from lands undergoing peat extraction in 2007 were estimated
to be between 0.001 and 0.007 Tg CO2 Eq. at the 95 percent confidence level. This indicates a range of -73 percent
below to 37 percent above the 2007 emission estimate of 0.005 Tg CO2 Eq.
Table 7-41: Tier-2 Quantitative Uncertainty Estimates for CO2 Emissions from Lands Undergoing Peat Extraction
2007 Emissions
Estimate Uncertainty Range Relative to Emissions Estimate"
Source Gas (Tg CO2 Eq.) (Tg CO2 Eq.) (%)
Lands Undergoing
Peat Extraction
CO2 1.0
N2O +
Lower Upper Lower
Bound Bound Bound
0.7 1.3 -31%
+ + -73%
Upper
Bound
29%
37%
+ Does not exceed 0.05 Tg CO2 Eq. or 0.5 Gg.
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
7-48 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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QA/QC and Verification
A QA/QC analysis was performed for data gathering and input, documentation, and calculation. The QA/QC
analysis did not reveal any inaccuracies or incorrect input values.
Recalculations Discussion
This is the first year that emissions from Lands Undergoing Peat Extraction are included in the Inventory of U.S.
Greenhouse Gas Emissions and Sinks.
Planned Improvements
In order to further improve estimates of CO2 and N2O emissions from lands undergoing peat extraction, future
efforts will consider options for obtaining better data on the quantity of peat harvested per hectare and the total area
undergoing peat extraction.
7.9. Settlements Remaining Settlements
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 79.1 Tg CO2 Eq. (22 Tg C) over the period from 1990 through
2007. Total sequestration increased by 61 percent between 1990 and 2007 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 (1990 and 2000) U.S. Census
data on urban area (Table 7-42). Net C flux from urban trees in 2007 was estimated to be -97.6 Tg CO2 Eq. (-26.6
TgC).
Net C flux from urban trees is proportionately greater on an area basis than that of forests. This trend is primarily
the result of different net growth rates in urban areas versus forests—urban trees often grow faster than forest trees
because of the relatively open structure of the urban forest (Nowak and Crane 2002). Also, areas in each case are
accounted for differently. Because urban areas contain less tree coverage than forest areas, the C storage per hectare
of 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).
Table 7-42: Net C Flux from Urban Trees (Tg CO2 Eq. and Tg C)
Year
1990
1995
2000
2005
2006
2007
Tg CO2 Eq.
(60.6)
(71.5)
(82.4)
(93.3)
(95.5)
(97.6)
TgC
(16.5)
(19.5)
(22.5)
(25.4)
(26.0)
(26.6)
Note: Parentheses indicate net sequestration.
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
Land Use, Land-Use Change, and Forestry 7-49
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consistent with the default IPCC methodology in IPCC (2006), although sufficient data are not yet available to
determine interannual gains and losses in C stocks in the living biomass of urban trees. Annual changes in net C
flux from urban trees are based solely on changes in total urban area in the United States.
Most of the field data were analyzed using the U.S. Forest Service's Urban Forest Effects (UFORE) model.192 The
UFORE model is a computer model that uses standardized field data from random plots in each city and local air
pollution and meteorological data to quantify urban forest structure, values of the urban forest, and environmental
effects, including total C stored and annual C sequestration (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 10 U.S. cities. Subsequent studies have developed estimates for 5 more cities,
resulting in estimates for the following 15 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. Field data was collected for a sample of trees
in each of the 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 data
for each tree was converted into C storage by applying allometric equations to estimate aboveground biomass, a
root-to-shoot ratio to convert aboveground biomass estimates to whole tree biomass, moisture contents, a C content
of 50 percent (dry weight basis), and an adjustment factor to account for urban trees having less aboveground
biomass than predicted by allometric equations based on forest trees. C storage estimates for deciduous trees were
structured to include only carbon stored in wood. These calculations were then used to form an estimation equation
for each species of tree, encompassing a range of diameters.
Tree growth was estimated using annual height growth and diameter growth rates for specific land uses and diameter
classes. Growth calculations were adjusted by a factor to account for tree condition (fair to excellent, poor, critical,
dying, or dead). For each tree, the difference in carbon storage estimates between year 1 and year x + 1 gave the
gross amount of C sequestered. These annual gross C sequestration rates for each species (or genus), diameter class,
and land-use condition (parks, transportation, vacant, golf courses, etc.) were then scaled up to city estimates using
tree population information.
Gross C emissions result from tree death and removals. These emissions were derived by applying estimates of
annual mortality and condition and assumptions about whether dead trees were removed from the site to the total C
stock estimate for each city. Estimates of annual mortality rates by diameter class and condition class were derived
from a study of street-tree mortality (Nowak 1986). Different decomposition rates were applied to dead trees left
standing compared with those removed from the site. For removed trees, different rates were applied to the
removed/aboveground biomass in contrast to the belowground biomass. The estimated annual gross C emission
rates for each species (or genus), diameter class, and condition class were then scaled up to city estimates using tree
population information.
The field data 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 applied to the field data for
each tree were taken from the scientific literature (see Nowak 1994, Nowak et al. 2002), but if no allometric
equation could be found for the particular species, the average result for the genus was used. The adjustment (0.8)
to account for less live tree biomass in urban trees was based on information in Nowak (1994). A root-to-shoot ratio
of 0.26 was taken from Cairns et al. (1997), and species- or genus-specific moisture contents were taken from
various literature sources (see Nowak 1994). Tree growth rates were taken from existing literature. Average
diameter growth was based on the following sources: estimates for trees in forest stands came from Smith and
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).
192
Oakland and Chicago estimates were based on prototypes to the UFORE model.
7-50 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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Assumptions for which dead trees would be removed versus left standing were developed specific to each land use
and were based on expert judgment of the authors. Decomposition rates were based on literature estimates (Nowak
and Crane 2002).
National annual net C sequestration by urban trees was calculated based on estimates of gross and net sequestration
from 13 of the 15 cities (Table 7-43), and urban area and urban tree cover data for the United States. Annual net C
sequestration estimates193 were derived for 13 cities by subtracting the annual gross emission estimates from the
annual gross sequestration estimates. The urban area estimates were based on 1990 and 2000 U.S. Census data.
The 1990 U.S. Census defined urban land as "urbanized areas," which included land with a population density
greater than 1,000 people per square mile, and adjacent "urban places," which had predefined political boundaries
and a population total greater than 2,500. In 2000, the U.S. Census replaced the "urban places" category with a new
category of urban land called an "urban cluster," which included areas with more than 500 people per square mile.
Urban land area 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).
Table 7-43: C Stocks (Metric Tons C), Annual C Sequestration (Metric Tons C/yr), Tree Cover (Percent), and
Annual C Sequestration per Area of Tree Cover (kg C/m2cover-yr) for 15 U.S. Cities
City
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
Woodbridge, NJ
Carbon Gross Annual
Stocks Sequestration
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
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
Gross Annual
Net Annual Tree Sequestration per
Sequestration Cover Area of Tree Cover
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
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%
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
NA = not analyzed.
Sources: Nowak and Crane (2002) and Nowak (2007a,c).
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 13 of 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
193
Net estimates were not available for two cities (Chicago and Oakland).
Land Use, Land-Use Change, and Forestry 7-51
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associated with estimates of gross and net C sequestration for 13 of 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 13 of the 15 U.S. cities, and uncertainty in these estimates increases as they are scaled up to
the national level.
Additional uncertainty is associated with the biomass equations, conversion factors, and decomposition assumptions
used to calculate C sequestration and emission estimates (Nowak et al. 2002). These results also exclude changes in
soil C stocks, and there may be some overlap between the urban tree C estimates and the forest tree C estimates.
Due to data limitations, urban soil flux is not quantified as part of this analysis, while reconciliation of urban tree
and forest tree estimates will be addressed through the land representation effort described at the beginning of this
chapter.
A Monte Carlo (Tier 2) uncertainty analysis was applied to estimate the overall uncertainty of the sequestration
estimate. The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 7-44. The net C flux
from changes in C stocks in urban trees in 2007 was estimated to be between -115.3 and -77.3 Tg CO2 Eq. at a 95
percent confidence level. This indicates a range of 18 percent below and 21 percent above the 2007 flux estimate of
-97.6 Tg CO2 Eq.
Table 7-44: Tier 2 Quantitative Uncertainty Estimates for Net C Flux from Changes in C Stocks in Urban Trees (Tg
CO2 Eq. and Percent)
Source
2007 Flux
Gas Estimate
(Tg C02 Eq.)
Uncertainty Range Relative to Flux Estimate
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Changes in C Stocks
in Urban Trees CO2 (97.6) (115.3) (77.3) -18% 21%
Note: Parentheses indicate negative values or net sequestration.
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).
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 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 USD A 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 future, 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
7-52 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
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lawns, golf courses, and other landscaping occurring within settlement areas. Application rates are lower than those
occurring on cropped soils, and, therefore, account for a smaller proportion of total U.S. soil N2O emissions per unit
area. In addition to synthetic N fertilizers, a portion of surface applied sewage sludge is applied to settlement areas.
In 2007, N2O emissions from this source were 1.6 Tg CO2 Eq. (5.2 Gg). There was an overall increase of 61 percent
over the period from 1990 through 2007 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 7-45.
Table 7-45: N2O Fluxes from Soils in Settlements Remaining Settlements (Tg CO2 Eq. and Gg N2O)
Year
1990
1995
2000
2005
2006
2007
Tg C02 Eq.
1.0
1.2
1.2
1.5
1.5
1.6
Gg
3.2
4.0
3.9
4.9
5.0
5.2
Note: These estimates include direct N2O emissions from N fertilizer additions only. Indirect N2O 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 (see Annex
3.11 for a detailed discussion of the methodology for estimating sewage sludge application).
Nitrogen applications to settlement soils are estimated using data compiled by the USGS (Ruddy et al. 2006). The
USGS estimated on-farm and non-farm fertilizer use based on sales records at the county level from 1982 through
2001 (Ruddy et al. 2006). Non-farm N fertilizer was assumed to be applied to settlements and forests and values for
2002 through 2007 were based on 2001 values adjusted for annual total N fertilizer sales in the United States.
Settlement application was calculated by subtracting forest application from total non-farm fertilizer use. Sewage
sludge applications were derived from national data on sewage sludge generation, disposition, and N content (see
Annex 3.11 for further detail). The total amount of N resulting from these sources was multiplied by the IPCC
default emission factor for applied N (1 percent) to estimate direct N2O emissions (IPCC 2006). The volatilized and
leached/runoff proportions, calculated with the IPCC default volatilization factors (10 or 20 percent, respectively,
for synthetic or organic N fertilizers) and leaching/runoff factor for wet areas (30 percent), were included with the
total N contributions to indirect emissions, as reported in the N2O Emissions from Agricultural Soil Management
source category of the Agriculture chapter.
Uncertainty
The amount of N2O emitted from settlements depends not only on N inputs, but also on a large number of variables,
including organic C availability, oxygen gas partial pressure, soil moisture content, pH, temperature, and
irrigation/watering practices. The effect of the combined interaction of these variables on N2O flux is complex and
highly uncertain. The IPCC default methodology does not incorporate any of these variables and only accounts for
variations in fertilizer N and sewage sludge application rates. All settlement soils are treated equivalently under this
methodology.
Uncertainties exist in both the fertilizer N and sewage sludge application rates in addition to the emission factors.
Land Use, Land-Use Change, and Forestry 7-53
-------
Uncertainty in fertilizer N application was assigned a default level194 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 2007 emission estimates. The results of the quantitative uncertainty analysis are
summarized in Table 7-46. N2O emissions from soils in Settlements Remaining Settlements in 2007 were estimated
to be between 0.8 and 4.2 Tg CO2 Eq. at a 95 percent confidence level. This indicates a range of 49 percent below
to 163 percent above the 2007 emission estimate of 1.6 Tg CO2 Eq.
Table 7-46: Quantitative Uncertainty Estimates of N2O Emissions from Soils in Settlements Remaining Settlements
(Tg CO2 Eq. and Percent)
2007 Uncertainty Range Relative to Emission
Source Gas Emissions Estimate
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Lower
Bound
Upper
Bound
Lower
Bound
Upper
Bound
Settlements Remaining Settlements'.
N2O Fluxes from Soils N2O L6 0.8 4,2 -49% +163%
Note: This estimate includes direct N2O emissions from N fertilizer additions to both Settlements Remaining Settlements and
from Land Converted to Settlements.
Recalculations Discussion
The total amount of fertilizer in non-agricultural uses has been estimated by the USGS for 1990 through 2001 on a
county scale from fertilizer sales data (Ruddy et al. 2006). In previous Inventories, data for 2001 was used for
subsequent years without adjustment. For subsequent years in the current Inventory (2002 though 2007), county-
level data on non-farm fertilizer use for 2001 were adjusted based on annual fluctuations in total U.S. fertilizer sales.
This change resulted in a small (less than 1 percent on average) increase in emissions relative to the previous
Inventory.
Planned Improvements
A minor improvement is to update the uncertainty analysis for direct emissions from settlements to be consistent
with the most recent activity data for this source.
7.10. Land Converted to Settlements (Source Category 5E2)
Land-use change is constantly occurring, and land under a number of uses undergoes urbanization in the United
States each year. However, data on the amount of land converted to settlements is currently lacking. Given the lack
of available information relevant to this particular IPCC source category, it is not possible to separate CO2 or N2O
fluxes on Land Converted to Settlements from fluxes on Settlements Remaining Settlements at this time.
7.11. Other (IPCC Source Category 5G)
Changes in Yard Trimming and Food Scrap Carbon Stocks in Landfills
In the United States, a significant change in C stocks results from the removal of yard trimmings (i.e., grass
clippings, leaves, and branches) and food scraps from settlements to be disposed in landfills. Yard trimmings and
food scraps account for a significant portion of the municipal waste stream, and a large fraction of the collected yard
194 No uncertainty is provided with the USGS application data (Ruddy et al. 2006) so a conservative ±50% was used in the
analysis.
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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 collected annually and the fraction that is landfilled have declined over the last
decade. In 1990, over 50 million metric tons (wet weight) of yard trimmings and food scraps were generated (i.e.,
put at the curb for collection to be taken to disposal sites or to composting facilities) (EPA 2008; Schneider 2007,
2008). Since then, programs banning or discouraging yard trimmings disposal have led to an increase in backyard
composting and the use of mulching mowers, and a consequent 7 percent decrease in the tonnage generated (i.e.,
collected for composting or disposal). At the same time, a dramatic increase in the number of municipal composting
facilities has reduced the proportion of collected yard trimmings that are discarded in landfills—from 72 percent in
1990 to 29 percent in 2007. The net effect of the reduction in generation and the increase in composting is a 62
percent decrease in the quantity of yard trimmings disposed in landfills since 1990.
Food scraps generation has grown by 52 percent since 1990, but the proportion of food scraps discarded in landfills
has decreased slightly from 81 percent in 1990 to 79 percent in 2007. Overall, the decrease in the yard trimmings
landfill disposal rate has more than compensated for the increase in food scrap disposal in landfills, and the net
result is a decrease in annual landfill carbon storage from 23.5 Tg CO2 Eq. in 1990 to 9.8 Tg CO2 Eq. in 2007 (Table
7-47 and Table 7-48).
Table 7-47: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg CO2 Eq.)
Carbon Pool 1990 1995 2000 2005 2006 2007
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
Total Net Flux
(21.2)
(1.9)
(9.7)
(9.7)
(2.2)
(23.5)
(12.5)
(0.8)
(6.0)
(5.8)
(1.4)
(13.9)
(8.2)
(0.4)
(4.0)
(3.7)
(3.1)
(11.3)
(6.6)
(0.4)
(3.3)
(3.0)
(3.5)
(10.2)
(6.8)
(0.5)
(3.3)
(3.0)
(3.6)
(10.4)
(6.3)
(0.4)
(3.1)
(2.8)
(3.5)
(9.8)
Note: Totals may not sum due to independent rounding.
Table 7-48: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C)
Carbon Pool
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
Total Net Flux
1990
(5.8)
(0.5)
(2.7)
(2.6)
(0.6)
(6.4)
1995
(3.4)
(0.2)
(1.6)
(1.6)
(0.4)
(3.8)
2000
(2.2)
(0.1)
(1.1)
(1.0)
(0.9)
(3.1)
2005
(1.8)
(0.1)
(0.9)
(0.8)
(1.0)
(2.8)
2006
(1.8)
(0.1)
(0.9)
(0.8)
(1.0)
(2.8)
2007
(1.7)
(0.1)
(0.8)
(0.8)
(0.9)
(2.7)
Note: Totals may not sum due to independent rounding.
Methodology
When wastes of sustainable, biogenic origin (such as yard trimmings and food scraps) are landfilled and do not
completely decompose, the C that remains is effectively removed from the global C cycle. Empirical evidence
indicates that yard trimmings and food scraps do not completely decompose in landfills (Barlaz 1998, 2005, 2008),
and thus the stock of carbon in landfills can increase, with the net effect being a net atmospheric removal of carbon.
Estimates of net C flux resulting from landfilled yard trimmings and food scraps were developed by estimating the
change in landfilled C stocks between inventory years, based on methodologies presented for the Land Use, Land-
Use Change and Forestry sector in IPCC (2003). C stock estimates were calculated by determining the mass of
landfilled C resulting from yard trimmings or food scraps discarded in a given year; adding the accumulated
landfilled C from previous years; and subtracting the portion of C landfilled in previous years that decomposed.
Land Use, Land-Use Change, and Forestry 7-55
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To determine the total landfilled C stocks for a given year, the following were estimated: 1) the composition of the
yard trimmings; 2) the mass of yard trimmings and food scraps discarded in landfills; 3) the C storage factor of the
landfilled yard trimmings and food scraps; and 4) the rate of decomposition of the degradable C. The composition
of yard trimmings was assumed to be 30 percent grass clippings, 40 percent leaves, and 30 percent branches on a
wet weight basis (Oshins and Block 2000). The yard trimmings were subdivided, because each component has its
own unique adjusted C storage factor and rate of decomposition. The mass of yard trimmings and food scraps
disposed of in landfills was estimated by multiplying the quantity of yard trimmings and food scraps discarded by
the proportion of discards managed in landfills. Data on discards (i.e., the amount generated minus the amount
diverted to centralized composting facilities) for both yard trimmings and food scraps were taken primarily from
Municipal Solid Waste Generation, Recycling, and Disposal in the United States: 2007 Facts and Figures (EPA
2008), which provides data for 1960, 1970, 1980, 1990, 2000, 2002, and 2004 through 2007. 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 EPA (2008) report does
not subdivide discards of individual materials into volumes landfilled and combusted, although it provides an
estimate of the proportion of overall waste stream discards managed in landfills and combustors (i.e., ranging from
92 percent and 8 percent respectively in 1984-86, to 67 percent and 33 percent in 1960).
The amount of C disposed of in landfills each year, starting in 1960, was estimated by converting the discarded
landfilled yard trimmings and food scraps from a wet weight to a dry weight basis, and then multiplying by the
initial (i.e., pre-decomposition) C content (as a fraction of dry weight). The dry weight of landfilled material was
calculated using dry weight to wet weight ratios (Tchobanoglous et al. 1993, cited by Barlaz 1998) and the initial C
contents and the C storage factors were determined by Barlaz (1998, 2005, 2008) (Table 7-49).
The amount of C remaining in the landfill for each subsequent year was tracked based on a simple model of C fate.
As demonstrated by Barlaz (1998, 2005, 2008), a portion of the initial C resists decomposition and is essentially
persistent in the landfill environment. Barlaz (1998, 2005, 2008) conducted a series of experiments designed to
measure biodegradation of yard trimmings, food scraps, and other materials, in conditions designed to promote
decomposition (i.e., by providing ample moisture and nutrients). After measuring the initial C content, the materials
were placed in sealed containers along with a "seed" containing methanogenic microbes from a landfill. Once
decomposition was complete, the yard trimmings and food scraps were re-analyzed for C content; the C remaining
in the solid sample can be expressed as a proportion of initial C (shown in the row labeled "CS" in Table 7-49).
The modeling approach applied to simulate U.S. landfill C flows builds on the findings of Barlaz (1998, 2005,
2008). The proportion of C stored is assumed to persist in landfills. The remaining portion is assumed to degrade,
resulting in emissions of CH4 and CO2 (the CH4 emissions resulting from decomposition of yard trimmings and food
scraps are accounted for in the Waste chapter). The degradable portion of the C is assumed to decay according to
first order kinetics. 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= E Wu x (1 - MQ) x ICQ x {[CS, x ICQ] + [(1 - (CS, x ICQ)) x e'k(t-n) ]}
where,
t = Year for which C stocks are being estimated (year),
i = Waste type for which C stocks are being estimated (grass, leaves, branches, food scraps),
LFQ>t = Stock of C in landfills in year t, for waste i (metric tons),
W, n = Mass of waste i disposed in landfills in year n (metric tons, wet weight),
n = Year in which the waste was disposed (year, where 1960 < n < t),
MQ = Moisture content of waste i (percent of water),
CSi = Proportion of initial C that is stored for waste i (percent),
ICQ = 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 (year-1).
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For a given year /, the total stock of C in landfills (TLFQ) is the sum of stocks across all four materials (grass,
leaves, branches, food scraps). The annual flux of C in landfills (Ft) for year / 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 / through the end of the inventory period (2007).
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 2007, 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 2007), the total landfill C from food scraps
in 2007 was 30.6 million metric tons. This value is then added to the C stock from grass, leaves, and branches to
calculate the total landfill C stock in 2007, yielding a value of 240.4 million metric tons (as shown in Table 7-50).
In exactly the same way total net flux is calculated for forest C and harvested wood products, the total net flux of
landfill C for yard trimmings and food scraps for a given year (Table 7-48) is the difference in the landfill C stock
for that year and the stock in the preceding year. For example, the net change in 2007 shown in Table 7-48 (2.7 Tg
C) is equal to the stock in 2007 (240.4 Tg C) minus the stock in 2006 (237.7 Tg C).
The C stocks calculated through this procedure are shown in Table 7-50.
Table 7-49: Moisture Content (%), C Storage Factor, Proportion of Initial C Sequestered (%), Initial C Content (%),
and Half-Life (years) for Landfilled Yard Trimmings and Food Scraps in Landfills
Yard Trimmings Food Scraps
Variable
Moisture Content (% H2O)
CS, proportion of initial C stored (%)
Initial C Content (%)
Half -life (years)
Grass
70
53
45
5
Leaves
30
85
46
20
Branches
10
77
49
23
70
16
51
4
Table 7-50: C Stocks in Yard Trimmings and Food Scraps in Landfills (Tg C)
Carbon Pool
Yard Trimmings
Grass
Leaves
Branches
Food Scraps
Total Carbon Stocks
1990
160.3
16.2
71.7
72.5
18.4
178.7
1995
183.5
18.0
82.5
83.1
20.9
204.4
2000
196.0
18.6
88.6
88.8
24.3
220.3
2005
206.2
19.2
93.6
93.4
28.7
234.9
2006
208.0
19.4
94.5
94.2
29.7
237.7
2007
209.7
19.5
95.3
94.9
30.6
240.4
Note: Totals may not sum due to independent rounding.
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 7-51. Total yard
trimmings and food scraps CO2 flux in 2007 was estimated to be between -17.9 and -5.5 Tg CO2 Eq. at a 95 percent
confidence level (or 19 of 20 Monte Carlo stochastic simulations). This indicates a range of -84 percent below to 44
percent above the 2007 flux estimate of -9.8 Tg CO2 Eq. More information on the uncertainty estimates for Yard
Trimmings and Food Scraps in Landfills is contained within the Uncertainty Annex.
Land Use, Land-Use Change, and Forestry 7-57
-------
Table 7-51: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Yard Trimmings and Food Scraps in
Landfills (Tg CO2 Eq. and Percent)
Source
2007 Flux
Estimate
Gas (Tg CO2 Eq.)
Uncertainty Range Relative to Flux Estimate"
(Tg C02 Eq.) (%)
Lower Upper Lower Upper
Bound Bound Bound Bound
Yard Trimmings and
Food Scraps CO2 (9.8) (17.9) (5.5) -84% +44%
aRange of flux estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
Note: Parentheses indicate negative values or net C sequestration.
QA/QC and Verification
A QA/QC analysis was performed for data gathering and input, documentation, and calculation.
Recalculations Discussion
The current Inventory uses updated data from Municipal Solid Waste Generation, Recycling, and Disposal in the
United States: 2007 Facts and Figures (EPA 2008), which provides updated data through 2007 including revisions
to the amount of food scraps generated in 2000 and 2004 through 2007. This update results in 4.6 and 0.5 percent
decreases in carbon storage on average across the timeseries for food scraps and yard trimmings, respectively. This
translates to an average 1.0% decrease in carbon storage on average across the timeseries for the entire source
category.
Planned Improvements
Future work is planned to develop improved estimates of the decay rates for the individual materials. Additional
analysis may also be performed to evaluate the potential contribution of inorganic C, primarily in the form of
carbonates, to landfill sequestration, as well as the consistency between the estimates of C storage described in this
chapter and the estimates of landfill CH4 emissions described in the Waste chapter.
7-58 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Figure 7-1
Percent of Total Land Area in the General Land Use Categories for 2007
Cropland
Grassland
Wetlands
Forest Land
Settlements
Other Land
< 10%
11%-30% • 31%-50% • > 50%
Note: Land use/land-use change categories were aggregated into the 6 general land use categories based on the current use in 2007.
-------
Figure 7-2
Forest Sector Carbon Pools and Flows
Combustion from
forest fires (carbon
dioxide, methane)
Combustion from forest fires
(carbon dioxide, methane)
Harvest
Processing/ Residue
/^^Consumption
* ^4
Soil Organic
Material
Methane
Flaring
and
Utilization
Legend
Carbon Pool
Carbon transfer or flux
Combustion
Source: Heath et al. 2003
X-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006
-------
50
I '
x -50
IE
| -100
ro
(_>
- -150
z
•§ -200
1/1
* -250
£
-300
-350
Soil
Harvested Wood
Forest, Nonsoil
Total Net Change
Figure 7-3: Estimates of Net Annual Changes in C Stocks for Major C Pools
-------
Figure 7-4
Average C Density in the Forest Tree Pool in the Conterminous U.S., 2008
Live Tree
Mg CO Eq./ha
B 1-200
• 201-400
• over 400
Note: This graphic shows country-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 for each state or substate.)
-------
Figure 7-5
Total Net Annual C02 Flux for Mineral Soils Under Agricultural
Management within States, 2007, Cropland Remaining Cropland
Tg CO2 Eq./year
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.
>o
-0.1 toO
-0.5 to -0.1
-1 to -0.5
-2 to -1
-------
Figure 7-6
Total Net Annual C02 Flux for Organic Soils Under Agricultural
Management within States, 2007, Cropland Remaining Cropland
Note: Values greater than zero represent emissions.
Tg CO2 Eq./year
^B>2
0.5 to 1
Q^ 0.1 to 0.5
CD 0 to 0.1
CD No organic soils
-------
Figure 7-7
Total Net Annual C02 Flux for Mineral Soils Under Agricultural
Management within States, 2007, Land Converted to 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.
Tg CO2 Eq./year
>o
-0.1 toO
-0.5 to -0.1
-1 to -0.5
-------
Figure 7-8
Total Net Annual C02 Flux for Organic Soils Under Agricultural
Management within States, 2007, Land Converted to Cropland
Note: Values greater than zero represent emissions.
Tg CO2 Eq./year
0.5 to 1
0.1 to 0.5
0 to 0.1
No organic soils
-------
Figure 7-9
Total Net Annual C02 Flux for Mineral Soils Under Agricultural
Management within States, 2007, Grassland Remaining Grassland
Tg CO2 Eq./year
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.
>0
-0.1 toO
-0.5 to-0.1
-1 to -0.5
-2 to -1
-------
Figure 7-10
Total Net Annual C02 Flux for Organic Soils Under Agricultural
Management within States, 2007, Grassland Remaining Grassland
Note: Values greater than zero represent emissions.
Tg CO2 Eq./year
CD 1 to 2
CD 0.5 to 1
CB 0.1 to 0.5
CD 0 to 0.1
DD No organic soils
-------
Figure 7-11
Total Net Annual C02 Flux for Mineral Soils Under Agricultural
Management within States, 2007, Land Converted to Grassland
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.
Tg CO2 Eq./year
Q^ -0.1 toO
QB -0.5 to-0.1
Q^ -1 to -0.5
I <-2
-------
Figure 7-12
Total Net Annual C02 Flux for Organic Soils Under Agricultural
Management within States, 2007, Land Converted to Grassland
Note: Values greater than zero represent emissions.
Tg CO2 Eq./year
CD 0.5 to 1
CB 0.1 to 0.5
QD 0 to 0.1
CD No organic soils
-------
8. Waste
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 2007, the second
largest contribution of any CH4 source in the United States. Additionally, wastewater treatment and composting of
organic waste accounted for approximately 4 percent and less than 1 percent of U.S. CH4 emissions, respectively.
Nitrous oxide (N2O) emissions from the discharge of wastewater treatment effluents into aquatic environments were
estimated, as were N2O emissions from the treatment process itself. N2O emissions from composting were also
estimated. Together, these waste activities account for approximately 2 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.
Figure 8-1: 2007 Waste Chapter Greenhouse Gas Sources
Overall, in 2007, waste activities generated emissions of 165.6 Tg CO2 Eq., or just over 2 percent of total U.S.
greenhouse gas emissions.
Table 8-1: Emissions from Waste (Tg CO2 Eq.)
Gas/Source
CH,
Landfills
Wastewater Lreatment
Composting
N20
Domestic Wastewater Lreatment
Composting
Total
1990
173.0
149.2
23.5
0.3
4.0
3.7
0.4
177.1
1995
169.9
144.3
24.8
0.7
4.8
4.0
0.8
174.7
2000
148.8
122.3
25.2
1.3
5.8
4.5
1.4
154.6
2005
153.8
127.8
24.3
1.6
6.5
4.8
1.7
160.2
2006
156.5
130.4
24.5
1.6
6.6
4.8
1.8
163.0
2007
158.9
132.9
24.4
1.7
6.7
4.9
1.8
165.6
Note: Lotals may not sum due to independent rounding.
Table 8-2: Emissions from Waste (Gg)
Gas/Source
CH,
Landfills
Wastewater Lreatment
Composting
N20
Domestic Wastewater Lreatment
Composting
1990
8,240
7,105
1,120
15
13
12
1
1995
8,089
6,871
1,183
35
16
13
3
2000
7,084
5,825
1,200
60
19
14
4
2005
7,322
6,088
1,159
75
21
15
6
2006
7,451
6,211
1,165
75
21
15
6
2007
7,566
6,327
1,160
79
22
16
6
Note: Lotals may not sum due to independent rounding.
8.1. Landfills (IPCC Source Category 6A1)
In 2007, landfill CH4 emissions were approximately 132.9 Tg CO2 Eq. (6,327 Gg of CH4), representing the second
largest source of CH4 emissions in the United States, behind enteric fermentation. Emissions from municipal solid
waste (MSW) landfills, which received about 64.5 percent of the total solid waste generated in the United States,
accounted for about 90 percent of total landfill emissions, while industrial landfills accounted for the remainder.
Approximately 1,800 operational landfills exist in the United States, with the largest landfills receiving most of the
waste and generating the majority of the CH4 (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 CH4-producing anaerobic bacteria convert the
Waste 8-1
-------
fermentation products into stabilized organic materials and biogas consisting of approximately 50 percent carbon
dioxide (CO2) and 50 percent CH4, by volume. Significant CH4 production typically begins one or two years after
waste disposal in a landfill and continues for 10 to 60 years or longer.
From 1990 to 2007, net CH4 emissions from landfills decreased by approximately 10 percent (see Table 8-3 and
Table 8-4). This net CH4 emissions decrease is the result of increases in the amount of landfill gas collected and
combusted, which has more than offset the additional CH4 generation resulting from an increase in the amount of
municipal solid waste landfilled over the past 17 years. Over the past 6 years, however, the net CH4 emissions have
slowly increased, but have remained relatively steady since 2005. While the amount of landfill gas collected and
combusted continues to increase every year, the rate of increase no longer exceeds that rate of additional CH4
generation resulting from an increase in the amount of municipal solid waste landfilled as the U.S. population
grows.
Methane emissions from landfills are a function of several factors, including: (1) the total amount of waste in MSW
landfills, which is related to total waste landfilled annually; (2) the characteristics of landfills receiving waste (i.e.,
composition of waste-in-place, size, climate); (3) the amount of CH4 that is recovered and either flared or used for
energy purposes; and (4) the amount of CH4 oxidized in landfills instead of being released into the atmosphere. The
estimated annual quantity of waste placed in MSW landfills increased from about 209 Tg in 1990 to 291 Tg in 2007,
an increase of 28 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 878 Gg of CH4 were recovered and
combusted (i.e., used for energy or flared) from landfills, while in 2007, 5,812 Gg CH4 was combusted. In 2007, an
estimated 59 new landfill gas-to-energy (LFGTE) projects and 55 new flares began operation, resulting in a 3
percent increase in the quantity of CH4 recovered and combusted from 2006 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 landfilled, however, may decline due to increased recycling
and composting practices. In addition, the quantity of CH4 that is recovered and either flared or used for energy
purposes is expected to continue to increase as a result of 1996 federal regulations that require large municipal solid
waste landfills to collect and combust landfill gas (see 40 CFR Part 60, Subpart Cc 2005 and 40 CFR Part 60,
Subpart WWW 2005), voluntary programs encouraging CH4 recovery and use such as EPA's Landfill Methane
Outreach Program (LMOP), and federal and state incentives that promote renewable energy (e.g. tax credits, low
interest loans, and Renewable Portfolio Standards).
Table 8-3: CH4 Emissions from Landfills (Tg CO2 Eq.)
Activity
MSW Landfills
Industrial Landfills
Recovered
Gas-to-Energy
Flared
Oxidized3
Total
1990
172.6
11.6
(13.3)
(5.1)
(16.6)
149.2
1995
191.8
12.9
(22.3)
(22.0)
(16.0)
144.3
2000
206.9
14.4
(49.3)
(36.2)
(13.6)
122.3
2005
241.2
15.3
(56.8)
(57.6)
(14.2)
127.8
2006
248.1
15.3
(59.2)
(59.3)
(14.6)
130.4
2007
254.2
15.4
(64.3)
(57.7)
(14.8)
132.9
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
554
(635)
(242)
(789)
7,105
1995
9,132
615
(1,064)
(1,048)
(763)
6,871
2000
9,854
687
(2,348)
(1,722)
(647)
5,825
2005
11,486
728
(2,707)
(2,743)
(676)
6,088
2006
11,813
730
(2,819)
(2,822)
(690)
6,211
2007
12,107
735
(3,062)
(2,750)
(703)
6,327
Note: Totals may not sum due to independent rounding. Parentheses indicate negative values.
a Includes oxidation at municipal and industrial landfills.
8-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
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 recovered and combusted, minus the CH4 oxidized before
being released into the atmosphere:
CH4jg0l1(j Waste = [CH4jMSW + CH4)lnSoild waste = CH4 emissions from solid waste
CH4 MSW = CH4 generation from municipal solid waste landfills,
CH4 md = CH4 generation from industrial landfills,
R = CH4 recovered and combusted, and
Ox = CH4 oxidized from MSW and industrial landfills before release to the atmosphere.
The methodology for estimating CH4 emissions from municipal solid waste landfills is based on the first order decay
model described by the Intergovernmental Panel on Climate Change (IPCC 2006). Values for the CH4 generation
potential (L0) and rate constant (k) were obtained from an analysis of CH4 recovery rates for a database of 52
landfills and from published studies of other landfills (RTI2004; EPA 1998; SWANA 1998; Peer, Thorneloe, and
Epperson 1993). The rate constant was found to increase with average annual rainfall; consequently, values of k
were developed for 3 ranges of rainfall. The annual quantity of waste placed in landfills was apportioned to the 3
ranges of rainfall based on the percent of the U.S. population in each of the 3 ranges, and historical census data were
used to account for the shift in population to more arid areas over time. For further information, see Annex 3.14.
National landfill waste generation and disposal data for 2007 was extrapolated based on BioCycle data and the U.S.
Census population from 2006. 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 Annex 3.14 for more details.
The estimated landfill gas recovered per year was based on updated data collected from vendors of flaring
equipment, a database of landfill gas-to-energy (LFGTE) projects compiled by LMOP (EPA 2008), and a database
maintained by the Energy Information Administration (EIA) for the voluntary reporting of greenhouse gases (EIA
2007). As the EIA database only included data through 2006, 2007 recovery for projects included in the EIA
database were assumed to be the same as in 2006. The three databases were carefully compared to identify landfills
that were in two or all three of the databases to avoid double-counting reductions. Based on the information
provided by the EIA and flare vendor databases, the CH4 combusted by flares in operation from 1990 to 2007 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
Waste 8-3
-------
had not been identified from the emissions reductions associated with flares.
A destruction efficiency of 99 percent was applied to CH4 recovered to estimate CH4 emissions avoided. The value
for efficiency was selected based on the range of efficiencies (98 to 100 percent) recommended for flares in EPA's
AP-42 Compilation of Air Pollutant Emission Factors, Chapter 2.4 (EPA 1998) efficiencies used to establish new
source performance standards (NSPS) for landfills, and in recommendations for closed flares used in LMOP.
Emissions from industrial landfills were estimated from activity data for industrial production (ERG 2008), waste
disposal factors, and the first order decay model. As over 99 percent of the organic waste placed in industrial
landfills originated from the food processing (meat, vegetables, fruits) and pulp and paper industries, estimates of
industrial landfill emissions focused on these two sectors (EPA 1993). The amount of CH4 oxidized by the landfill
cover at both municipal and industrial landfills was assumed to be ten percent of the CH4 generated that is not
recovered (IPCC 2006, Mancinelli and McKay 1985, Czepiel et al. 1996). To calculate net CH4 emissions, both
CH4 recovered and CH4 oxidized were subtracted from CH4 generated at municipal and industrial landfills.
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 CH4 generation introduces
uncertainty. Aside from uncertainty in estimating CH4 generation potential, uncertainty exists in the estimates of
oxidation by cover soils. There is also uncertainty in the estimates of methane that is recovered by flaring and
energy projects. The IPCC default value of 10 percent for uncertainty in recovery estimates was used in the
uncertainty analysis when metering was in place (for about 64 percent of the CH4 estimated to be recovered). For
flaring without metered recovery data (approximately 34 percent of the CH4 estimated to be recovered), a much
higher uncertainty of approximately 50 percent was used (e.g., when recovery was estimated as 50 percent of the
flare's design capacity).
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 CH4 emissions in
2007 were estimated to be between 80.6 and 176.2 Tg CO2 Eq., which indicates a range of 39 percent below to 33
percent above the 2007 emission estimate of 132.9 Tg CO2 Eq.
Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills (Tg CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate Uncertainty Range Relative to Emission Estimate"
(Tg C02 Eq.) (Tg C02 Eq.) (%)
Landfills
CH4
132.9
Lower
Bound
80.6
Upper
Bound
176.2
Lower
Bound
-39%
Upper
Bound
+33%
a Range of emission 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. 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
8-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
previously using the original, peer-reviewed IPCC waste model. All model input values were verified by secondary
QA/QC review.
Recalculations Discussion
In developing the current Inventory, the data that formed the basis of the industrial food processing waste DOC
values were re-analyzed. Based on the re-analysis of the available data for industrial food processing waste, the
DOC value for industrial food waste was revised from 0.29 to 0.26 (Coburn 2008). This decrease in food industries'
DOC value led to a slight decrease in CH4 generation and CH4 emissions from food industry landfills.
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.
[Begin Text Box]
Box 8-1: Biogenic Emissions and Sinks of Carbon
CO2 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 CO2 that was originally removed by photosynthesis. In contrast, CH4 emissions
from landfilled waste occur due to the man-made anaerobic conditions conducive to CH4 formation that exist in
landfills, and are consequently included in this inventory.
Depositing wastes of biogenic origin in landfills causes the removal of carbon from its natural cycle between the
atmosphere and biogenic materials. As empirical evidence shows, some of these wastes degrade very slowly in
landfills, and the carbon they contain is effectively sequestered in landfills over a period of time (Barlaz 1998,
2005). Estimates of carbon removals from landfilling of forest products, yard trimmings, and food scraps are further
described in the Land Use, Land-Use Change, and Forestry chapter, based on methods presented in IPCC (2003) and
IPCC (2006).
[End Box]
8.2. Wastewater Treatment (IPCC Source Category 6B)
Wastewater treatment processes can produce anthropogenic CH4, N2O, and in some cases, CO2, 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, 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 biodegrade soluble organic material in wastewater under
aerobic or anaerobic conditions, where the latter condition produces CH4. During collection and treatment,
Waste 8-5
-------
wastewater may be accidentally or deliberately managed under anaerobic conditions. In addition, the sludge may be
further biodegraded under aerobic or anaerobic conditions. The generation of N2O may also result from the
treatment of domestic wastewater during both nitrification and denitrification of the N present, usually in the form of
urea, ammonia, and proteins. These compounds are converted to nitrate (NO3) through the aerobic process of
nitrification. Denitrification occurs under anoxic conditions (without free oxygen), and involves the biological
conversion of nitrate into 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 CH4 generation potential of wastewater is the amount of degradable organic
material in the wastewater. Common parameters used to measure the organic component of the wastewater are the
Biochemical Oxygen Demand (BOD) and Chemical Oxygen Demand (COD). Under the same conditions,
wastewater with higher COD (or BOD) concentrations will generally yield more CH4 than wastewater with lower
COD (or BOD) concentrations. BOD represents the amount of oxygen that would be required to completely
consume the organic matter contained in the wastewater through aerobic decomposition processes, while COD
measures the total material available for chemical oxidation (both biodegradable and non-biodegradable). Because
BOD is an aerobic parameter, it is preferable to use COD to estimate CH4 production. The principal factor in
determining the N2O generation potential of wastewater is the amount of N in the wastewater.
In 2007, CH4 emissions from domestic wastewater treatment were 15.8 Tg CO2 Eq. (755 Gg). Emissions gradually
increased from 1990 through 1996, but have decreased since that time due to decreasing percentages of wastewater
being treated in anaerobic systems, including reduced use of on-site septic systems and central anaerobic treatment
systems. In 2007, CH4 emissions from industrial wastewater treatment were estimated to be 8.5 Tg CO2 Eq. (405
Gg). Industrial emission sources have increased across the time series through 1999 and then fluctuated up and
down with production changes associated with the treatment of wastewater from the pulp and paper manufacturing,
meat and poultry processing, fruit and vegetable processing, starch-based ethanol production, and petroleum refining
industries. Table 8-6 and Table 8-7 provide CH4 and N2O emission estimates from domestic and industrial
wastewater treatment. With respect to N2O, the United States identifies two distinct sources for N2O emissions from
domestic wastewater: emissions from centralized wastewater treatment processes, and emissions from effluent from
centralized treatment systems that has been discharged into aquatic environments. The 2007 emissions of N2O from
centralized wastewater treatment processes and from effluent were estimated to be 0.3 Tg CO2 Eq. (1 Gg) and 4.6
Tg CO2 Eq. (15 Gg), respectively. Total N2O emissions from domestic wastewater were estimated to be 4.9 Tg CO2
Eq. (16 Gg). 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 N2O Emissions from Domestic and Industrial Wastewater Treatment (Tg CO2 Eq.)
Activity
CH,
Domestic
Industrial
N20
Domestic
Total
1990
23.5
16.4
7.1
3.7
3.7
27.2
1995
24.8
16.9
8.0
4.0
4.0
28.9
2000
25.2
16.8
8.4
4.5
4.5
29.6
2005
24.3
16.2
8.2
4.8
4.8
29.1
2006
24.5
16.0
8.5
4.8
4.8
29.3
2007
24.4
15.8
8.5
4.9
4.9
29.2
Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, fruit and vegetable processing,
starch-based ethanol production, and petroleum refining industries.
Note: Totals may not sum due to independent rounding.
Table 8-7: CH4 and N2O Emissions from Domestic and Industrial Wastewater Treatment (Gg)
Activity
CH,
Domestic
Industrial*
N20
Domestic
1990
1,120
782
338
11.9
11.9
1995
1,183
804
380
13.0
13.0
2000
1,200
802
398
14.4
14.4
2005
1,159
770
389
15.3
15.3
2006
1,165
762
403
15.5
15.5
2007
1,160
755
405
15.7
15.7
* Industrial activity includes the pulp and paper manufacturing, meat and poultry processing, fruit and vegetable processing,
starch-based ethanol production, and petroleum refining industries.
Note: Totals may not sum due to independent rounding.
8-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Methodology
Domestic Wastewater CH4 Emission Estimates
Domestic wastewater CH4 emissions originate from both septic systems and from centralized treatment systems,
such as publicly owned treatment works (POTWs). Within these centralized systems, CH4 emissions can arise from
aerobic systems that are not well managed or that are designed to have periods of anaerobic activity (e.g.,
constructed wetlands), anaerobic systems (anaerobic lagoons and facultative lagoons), and from anaerobic digesters
when the captured biogas is not completely combusted. CH4 emissions from septic systems were estimated by
multiplying the total BOD5 produced in the United States by the percent of wastewater treated in septic systems (20
percent), the maximum CH4 producing capacity for domestic wastewater (0.60 kg CH4/kg BOD), and the CH4
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 CH4-
producing capacity of domestic wastewater (0.6), and the relative MCFs for aerobic (zero or 0.3) and anaerobic (0.8)
systems. CH4 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 CH4), and the destruction efficiency associated with burning the biogas in an energy/thermal
device (0.99). The methodological equations are:
Emissions from Septic Systems = A
= (% onsite) x (total BOD5 produced) x (B0) x (MCF-septic) x 1/10A6
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
= [(POTW_flow_AD)
Emissions from Anaerobic Digesters = D
(digester gas)/ (per capita flow)] x conversion to m3
ofCH4)x(l-DE)x l/lOA9
Total CH4 Emissions (Gg) = A + B + C + D
(FRAC_CH4) x (365.25) x (density
Where:
% onsite =
% collected =
% 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 =
B0 =
MCF-septic =
Flow to septic systems / total flow
Flow to POTWs / total flow
Flow to aerobic systems / total flow to POTWs
Flow 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
32.5 %
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 CH4-producing capacity for domestic wastewater (0.60 kg
CH4/kgBOD)
CH4 correction factor for septic systems (0.5)
Waste 8-7
-------
1/10A6 = Conversion factor, kg to Gg
MCF-aerobic_not_well_man. = CH4 correction factor for aerobic systems that are not well managed
(0.3)
MCF-anaerobic = CH4 correction factor for anaerobic systems (0.8)
DE = CH4 destruction efficiency from flaring or burning in engine (0.99 for
enclosed flares)
POTW_flow_AD = Wastewater influent flow to POTWs that have anaerobic digesters (gal)
digester gas = Cubic feet of digester gas produced per person per day (1.0
ft3/person/day) (Metcalf and Eddy 1991)
per capita flow = Wastewater flow to POTW per person per day (100 gal/person/day)
conversion to m3= Conversion factor, ft3 to m3 (0.0283)
FRAC_CH4 = Proportion CH4 in biogas (0.65)
density of CH4 = 662 (g CH4/m3 CH4)
1/10A9 = Conversion factor, g to Gg
U.S. population data were taken from the U.S. Census Bureau International Database (U.S. Census 2008a) and
include the populations of the United States, American Samoa, Guam, Northern Mariana Islands, Puerto Rico, and
the Virgin Islands. Table 8-8 presents U.S. population and total BOD5 produced for 1990 through 2007. 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 2008b), with data for intervening years obtained by linear interpolation. The
wastewater flow to aerobic and anaerobic systems, and the wastewater flow to POTWs that have anaerobic digesters
were obtained from the 1992, 1996, 2000, and 2004 Clean Watershed Needs Survey (EPA 1992, 1996, 2000, and
2004a). Data for intervening years were obtained by linear interpolation. The BOD5 production rate (0.09
kg/capita/day) for domestic wastewater was obtained from Metcalf and Eddy (1991 and 2003). The CH4 emission
factor (0.6 kg CH4/kg BOD5) and the MCFs were taken from IPCC (2006). The CH4 destruction efficiency, 99
percent, was selected based on the range of efficiencies (98 to 100 percent) recommended for flares in AP-42
Compilation of Air Pollutant Emission Factors, Chapter 2.4 (EPA 1998), efficiencies used to establish new source
performance standards (NSPS) for landfills, and in recommendations for closed flares used by the Landfill Methane
Outreach Program (LMOP). The cubic feet of digester gas produced per person per day (1.0 ft3/person/day) and the
proportion of CH4 in biogas (0.65) come from Metcalf and Eddy (1991). The wastewater flow to a POTW (100
gal/person/day) was taken from the Great Lakes-Upper Mississippi River Board of State and Provincial Public
Health and Environmental Managers, "Recommended Standards for Wastewater Facilities (Ten-State Standards)"
(2004).
Table 8-8. U.S. Population (Millions) and Domestic Wastewater BOD5 Produced (Gg)
Year
1990
1995
2000
2001
2002
2003
2004
2005
2006
2007
Population
254
271
287
289
292
295
297
300
303
306
BODS
8,350
8,895
9,419
9,509
9,597
9,685
9,774
9,864
9,954
10,043
Source: U.S. Census Bureau (2008a); Metcalf & Eddy 1991 and 2003.
Industrial Wastewater CH4 Emission Estimates
CH4 emissions estimates from industrial wastewater were developed according to the methodology described in
IPCC (2006). Industry categories that are likely to produce significant CH4 emissions from wastewater treatment
were identified. High volumes of wastewater generated and a high organic wastewater load were the main criteria.
The top five industries that meet these criteria are pulp and paper manufacturing; meat and poultry processing;
vegetables, fruits, and juices processing; starch-based ethanol production; and petroleum refining. Wastewater
8-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
treatment emissions for these sectors for 2007 are displayed in Table 8-9 below.
Table 8-9: Industrial Wastewater CH4 emissions by sector for 2007.
emissions (TgCO2e)
% of Industrial Wastewater CH4
Pulp & Paper
Meat & Poultry
Petroleum Refineries
Fruit & Vegetables
Ethanol Refineries
4.1
3.6
0.6
0.1
0.1
48%
43%
7%
1%
1%
Total
8.5
100%
Table 8-10 contains production data for these industries.
Table 8-10: U.S. Pulp and Paper, Meat and Poultry, and Vegetables, Fruits and Juices Production (Tg)
Year Meat Poultry
(Live Weight (Live Weight Vegetables, Petroleum
Pulp and Paper Killed) Killed) Fruits and Juices Ethanol Refining
1990
1995
2000
2001
2002
2003
2004
2005
2006
2007
128.9
140.9
142.8
134.3
132.7
131.9
136.4
131.4
137.4
135.9
27.3
30.8
32.1
31.6
32.7
32.3
31.2
31.4
32.5
33.4
14.6
18.9
22.2
22.8
23.5
23.7
24.4
25.1
25.5
26.0
38.7
46.9
50.9
45.0
47.7
44.7
47.8
42.7
43.5
43.5
2.7
4.2
4.9
5.3
6.4
8.4
10.2
11.7
14.5
19.4
702.4
735.6
795.2
794.9
794.4
804.2
821.5
818.6
826.7
827.6
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 Bo value used for all industries is the IPCC default value of
0.25 kg CH4/kg COD (IPCC 2006).
For each industry, the percent of plants in the industry that treat wastewater on site, the percent of plants that have a
primary treatment step prior to biological treatment, and the percent of plants that treat wastewater anaerobically
were defined. The percent of wastewater treated anaerobically onsite (TA) was estimated for both primary treatment
and secondary treatment. For plants that have primary treatment in place, an estimate of COD that is removed prior
to wastewater treatment in the anaerobic treatment units was incorporated.
The methodological equations are:
CH4 (industrial wastewater) = P x W x COD x TA x B0 x MCF
TA = [%Plants0 x %WWa,p x %CODP]
Where:
[%Plantsa x %WWa,s x %CODS] + [%Plantst x %WWa,t x %CODS
CH4 (industrial wastewater)
P
W
COD
s
TA
%Plants0
%WWap
%CODP
%Plantsa
%Plantst
= 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)
= Removal of COD as sludge prior to anaerobic treatment (kg COD/year)
= Percent of wastewater treated anaerobically on site
= percent of plants with onsite treatment
= percent of wastewater treated anaerobically in primary treatment
= percent of COD entering primary treatment
= percent of plants with anaerobic secondary treatment
= percent of plants with other secondary treatment
Waste 8-9
-------
%wwa,s
%wwat
%CODS
Bo
MCF
= percent of wastewater treated anaerobically in anaerobic secondary treatment
= percent of wastewater treated anaerobically in other secondary treatment
= percent of COD entering secondary treatment
= Maximum CH4 producing potential of industrial wastewater (default value of
0.25 kg CH4/kg COD)
= CH4 correction factor, indicating the extent to which the organic content
(measured as COD) degrades anaerobically
As described below, EPA used the values presented in Table 8-11 in the Inventory calculations.
Table 8-11: Variables Used to Calculate Percent Wastewater Treated Anaerobically by Industry
ana e
%TAP
%TAS
%Plants0
%Plantsa
%PlantSt
%wwap
%wwas
%wwa't
%CODP
%CODS
Pulp and
Paper
0
10.5
60
25
35
0
100
0
100
42
Meat
Processing
0
33
100
33
67
0
100
0
100
100
Poultry
Processing
0
25
100
25
75
0
100
0
100
100
Industry
Fruit/
Vegetable
Processing
0
5.5
11
5.5
5.5
0
100
0
100
100
Ethanol
Production
- Wet Mill
0
33.3
100
33.3
66.7
0
100
0
100
100
Ethanol
Production
-Dry Mill
0
75
100
75
25
0
100
0
100
100
Petroleum
Refining
0
100
100
100
0
0
100
0
100
100
Pulp and Paper. Wastewater treatment for the pulp and paper industry typically includes neutralization, screening,
sedimentation, and flotation/hydrocycloning to remove solids (World Bank 1999, Nemerow and Dasgupta 1991).
Secondary treatment (storage, settling, and biological treatment) mainly consists of lagooning. In determining the
percent that degrades anaerobically, both primary and secondary treatment were considered. In the United States,
primary treatment is focused on solids removal, equalization, neutralization, and color reduction (EPA 1993). The
vast majority of pulp and paper mills with on-site treatment systems use mechanical 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
1993). 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).
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 2007
(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 gram BOD/liter (EPA 1997b, EPA 1993, World Bank 1999).
Meat and Poultry Processing. The meat and poultry processing industry makes extensive use of anaerobic lagoons
in sequence with screening, fat traps and dissolved air flotation when treating wastewater on site. About 33 percent
of meat processing operations (EPA 2002) and 25 percent of poultry processing operations (U.S. Poultry 2006)
perform on-site treatment in anaerobic lagoons. The IPCC default Bo of 0.25 kg CH4/kg COD and default MCF of
0.8 for anaerobic lagoons were used to estimate the CH4 produced from these on-site treatment systems. Production
data, in carcass weight and live weight killed for the meat and poultry industry, were obtained from the USDA
8-10 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Agricultural Statistics Database and the Agricultural Statistics Annual Reports (USDA 2008a). Data collected by
EPA's Office of Water provided estimates for wastewater flows into anaerobic lagoons: 5.3 and 12.5 m3/metric ton
for meat and poultry production (live weight killed), respectively (EPA 2002). The loadings are 2.8 and 1.5 g
BOD/liter for meat and poultry, respectively.
Vegetables, Fruits, and Juices Processing. Treatment of wastewater from fruits, vegetables, and juices processing
includes screening, coagulation/settling, and biological treatment (lagooning). The flows are frequently seasonal,
and robust treatment systems are preferred for on-site treatment. Effluent is suitable for discharge to the sewer.
This industry is likely to use lagoons intended for aerobic operation, but the large seasonal loadings may develop
limited anaerobic zones. In addition, some anaerobic lagoons may also be used (Nemerow and Dasgupta 1991).
Consequently, 4.2 percent of these wastewater organics are assumed to degrade anaerobically. The IPCC default Bo
of 0.25 kg CHykg COD and default MCF of 0.8 for anaerobic treatment were used to estimate the CH4 produced
from these on-site treatment systems. The USDA National Agricultural Statistics Service (USDA 2008a) provided
production data for potatoes, other vegetables, citrus fruit, non-citrus fruit, and grapes processed for wine. Outflow
and BOD data, presented in Table 8-12, were obtained from EPA (1974) for potato, citrus fruit, and apple
processing, and from EPA (1975) for all other sectors.
Table 8-12: Wastewater Flow (nrVton) and BOD Production (g/L) for U.S. Vegetables, Fruits, and Juices Production
Commodity BOD
Wastewater Outflow (m3/ton) (g/L)
Vegetables
Potatoes
Other Vegetables
Fruit
Apples
Citrus
Non-citrus
Grapes (for wine)
10.27
8.81
3.66
10.11
12.42
2.783
1.765
0.808
1.371
0.317
1.204
1.831
Ethanol Production. Ethanol, or ethyl alcohol, is produced primarily for use as a fuel component, but is also used in
industrial applications and in the manufacture of beverage alcohol. Ethanol can be produced from the fermentation
of sugar-based feedstocks (e.g., molasses and beets), starch- or grain-based feedstocks (e.g., corn, sorghum, and
beverage waste), and cellulosic biomass feedstocks (e.g., agricultural wastes, wood, and bagasse). Ethanol can also
be produced synthetically from ethylene or hydrogen and carbon monoxide. However, synthetic ethanol comprises
only about 2 percent of ethanol production, and although the Department of Energy predicts cellulosic ethanol to
greatly increase in the coming years, currently it is only in an experimental stage in the United States. According to
the Renewable Fuels Association, 82 percent of ethanol production facilities use corn as the sole feedstock and 7
percent of facilities use a combination of corn and another starch-based feedstock. The fermentation of corn is the
principal ethanol production process in the United States and is expected to increase 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
Waste 8-11
-------
(ERG 2006). CH4 emissions were then estimated as follows:
Methane = [Production x Flow x COD x 3.785 x ([%Plants0 x %WWa,p x %CODP] + [%Plantsa x %WWa,s
x%CODs] + [%PlantSt x %WWa,t x %CODS]) x Bo x MCF x % Not Recovered] + [Production x Flow x 3.785 x
COD x ([%Plants0 x %WWa>p x %CODP] + [%Plantsa x %WWa,s x %CODS] + [%Plantst x %WW£U x %CODJ)
x
Bo x MCF x (% Recovered) x (1-DE)] x 1/10A9
Where:
Production = gallons ethanol produced (wet milling or dry milling)
Flow = gallons wastewater generated per gallon ethanol produced (1.25 dry milling,
10 wet milling)
COD = COD concentration in influent (3 g/1)
3.785 = conversion, gallons to liters
%Plants0 = percent of plants with onsite treatment (100%)
%WWa p = percent of wastewater treated anaerobically in primary treatment (0%)
%CODp = percent of COD entering primary treatment (100%)
%Plantsa = percent of plants with anaerobic secondary treatment (33.3% wet, 75% dry)
%Plantst = percent of plants with other secondary treatment (66.7% wet, 25% dry)
%WW3]S = percent of wastewater treated anaerobically in anaerobic secondary treatment
(100%)
%WW3]t = percent of wastewater treated anaerobically in other secondary treatment (0%)
%CODs = percent of COD entering secondary treatment (100%)
B0 = maximum methane producing capacity (0.25 g CH4/g COD)
MCF = methane conversion factor (0.8 for anaerobic systems)
% Recovered = percent of wastewater treated in system with emission recovery
% Not Recovered = 1 - percent of wastewater treated in system with emission recovery
DE = destruction efficiency of recovery system (99%)
1/10A9 = conversion factor, g to Gg
A time series of CH4 emissions for 1990 through 2007 was developed based on production data from the Renewable
Fuels Association (RFA 2005).
Petroleum Refining. Petroleum refining wastewater treatment operations produce CH4 emissions from anaerobic
wastewater treatment. The wastewater inventory section includes CH4 emissions from petroleum refining
wastewater treated on site under intended or unintended anaerobic conditions. Most facilities use aerated biological
systems, such as trickling filters or rotating biological contactors; these systems can also exhibit anaerobic
conditions that can result in the production of methane. Oil/water separators are used as a primary treatment
method; however, it is unlikely that any COD is removed in this step.
Available information from the industry was compiled. The wastewater generation rate, from CARD 2007 and
Timm 1985, was determined to be 35 gallons per barrel of finished product. An average COD value in the
wastewater was estimated at 0.45 kg/m3 (Benyahia et al.).
The equation used to calculate CH4 generation at petroleum refining wastewater treatment systems is presented
below:
Methane = Flow x COD x B0 x MCF
Where:
Flow = Annual flow treated through anaerobic treatment system (mVyear)
COD = COD loading in wastewater entering anaerobic treatment system (kg/m3)
B0 = maximum methane producing potential of industrial wastewater (default value of 0.25
kg CH4 /kg COD)
MCF = methane conversion factor (0.3)
8-12 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
A time series of CH4 emissions for 1990 through 2007 was developed based on production data from the Energy
Information Association (EIA 2008).
Domestic Wastewater N2O Emission Estimates
N2O emissions from domestic wastewater (wastewater treatment) were estimated using the IPCC (2006)
methodology, including calculations that take into account N removal with sewage sludge, non-consumption and
industrial wastewater N, and emissions from advanced centralized wastewater treatment plants:
• In the United States, a certain amount of N is removed with sewage sludge, which is applied to land, incinerated,
or landfilled (NSLUDGE). The N disposal into aquatic environments is reduced to account for the sewage sludge
application.
• The IPCC methodology uses annual, per capita protein consumption (kg protein/[person-year]). For this
inventory, the amount of protein available to be consumed is estimated based on per capita annual food
availability data and its protein content, and then adjusts that data using a factor to account for the fraction of
protein actually consumed.
• Small amounts of gaseous nitrogen oxides are formed as by-products in the conversion of nitrate to N gas in
anoxic biological treatment systems. Approximately 7 grams N2O is generated per capita per year if wastewater
treatment includes intentional nitrification and denitrification (Scheehle and Doom 2001) Analysis of the 2000
CWNS shows 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 grams N2O per capita per year.
With the modifications described above, N2O emissions from domestic wastewater were estimated using the
following methodology:
N2OTOTAL = N2OPLA
N20
2MT/DEMT
= [(USPOPND) x EF2
T = {[(USPOP X WWTP) - USp
1/10*9
M] X EFj } X
N20EFFLUENT = {
where,
N2OTOTAL =
N2OPLANT =
N2OMT/DEMT
N2O
- (0.9 x USPOPND)) x Protein
- NSLUDGE] x EF3 x 44/28} x l/lOA6
2wour MT/DEMT
N2O
2EFFLUENT
WWTP =
EFi =
EF2 =
Protein =
FNPR =
FNON-CON =
FTND-COM =
NSLUDGE =
EF3 =
44/28 =
Annual emissions of N2O (kg)
N2O emissions from centralized wastewater treatment plants (kg)
N2O emissions from centralized wastewater treatment plants with
nitrification/denitrification (kg)
N2O emissions from centralized wastewater treatment plants without
nitrification/denitrification (kg)
N2O emissions from wastewater effluent discharged to aquatic environments (kg)
U.S. population
U.S. population that is served by biological denitrification (from CWNS)
Fraction of population using WWTP (as opposed to septic systems)
Emission factor (3 .2 g N2O/person-year)
Emission factor (7 g N2O/person-year)
Annual per capita protein consumption (kg/person/year)
Fraction of N in protein, default = 0.16 (kg N/kg protein)
Factor for non-consumed protein added to wastewater (1.4)
Factor for industrial and commercial co-discharged protein into the sewer system (1.25)
N removed with sludge, kg N/yr
Emission factor (0.005 kg N2O -N/kg sewage-N produced)
Molecular weight ratio of N2O to N2
Waste 8-13
-------
U.S. population data were taken from the U.S. Census Bureau International Database (U.S. Census 2008a) 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 2008b). Data
for intervening years were obtained by linear interpolation. The emission factor (EFi) used to estimate emissions
from wastewater treatment was taken from IPCC (2006). Data on annual per capita protein intake were provided by
U.S. Department of Agriculture Economic Research Service (USDA 2008b). Protein consumption data for 2005
through 2007 were extrapolated from data for 1990 through 2004. Table 8-13 presents the data for U.S. population
and average protein intake. An emission factor to estimate emissions from effluent (EF3) has not been specifically
estimated for the United States, thus the default IPCC value (0.005 kg N2O-N/kg sewage-N produced) was applied.
The fraction of N in protein (0.16 kg N/kg protein) was also obtained from IPCC (2006). Sludge generation was
obtained from EPA (1999) for 1988, 1996, and 1998 and from Beecher et al. (2007) for 2004. Intervening years
were interpolated, and estimates for 2005 through 2007 were forecasted from the rest of the time series. An estimate
for the nitrogen removed as sludge (NSLUDGE) was obtained by determining the amount of sludge disposed by
incineration, by land application (agriculture or other), through surface disposal, in landfills, or through ocean
dumping. In 2007, 266 Tg N was removed with sludge.
Table 8-13: U.S. Population (Millions), Available Protein [kg/(person-year)], and Protein Consumed (kg/person-
year)
Year Population Available Protein Protein Consumed
1990 254 38.7 29.6
1995 271 39.8 30.4
2000
2001
2002
2003
2004
2005
2006
2007
287
289
292
295
297
300
303
306
41.3
42.0
40.9
40.9
41.3
41.7
41.9
42.1
31.6
32.1
31.3
31.3
31.6
32.1
32.1
32.2
Source: U.S. Census Bureau 2008a, USDA 2008b.
Uncertainty
The overall uncertainty associated with both the 2007 CH4 and N2O emissions estimates from wastewater treatment
and discharge was calculated using the IPCC Good Practice Guidance Tier 2 methodology (2000). Uncertainty
associated with the parameters used to estimate CH4 emissions include that of numerous input variables used to
model emissions from domestic wastewater, and wastewater from pulp and paper manufacture, meat and poultry
processing, fruits and vegetable processing, ethanol production, and petroleum refining. Uncertainty associated with
the parameters used to estimate N2O emissions include that of sewage sludge disposal, total U.S. population,
average protein consumed per person, fraction of N in protein, non-consumption nitrogen factor, emission factors
per capita and per mass of sewage-N, and for the percentage of total population using centralized wastewater
treatment plants.
The results of this Tier 2 quantitative uncertainty analysis are summarized in Table 8-14. CH4 emissions from
wastewater treatment were estimated to be between 15.1 and 36.3 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 38 percent below to
49 percent above the 2007 emissions estimate of 24.4 Tg CO2 Eq. N2O emissions from wastewater treatment were
estimated to be between 1.2 and 9.4 Tg CO2 Eq., which indicates a range of approximately 75 percent below to 94
percent above the actual 2007 emissions estimate of 4.9 Tg CO2 Eq.
Table 8-14: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Wastewater Treatment (Tg CO2 Eq.
and Percent)
Source 2007 Emission
Gas Estimate Uncertainty Range Relative to Emission Estimate3
(TgC02Eq.) (Tg C02 Eq.)
8-14 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Wastewater Treatment
Domestic
Industrial
Domestic Wastewater
Treatment
CH4
CH4
CH4
N20
24.4
15.8
8.5
4.9
Lower Bound
15.1
7.7
5.1
1.2
Upper Bound
36.3
27.0
13.1
9.4
Lower Bound
-38%
-51%
-40%
-75%
Upper
Bound
+49%
+70%
+54%
+94%
a Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.
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 estimates of CH4 emissions from industrial wastewater treatment increased across the time series as petroleum
refining wastewater treatment was added to the inventory. The addition of this industrial sector increased industrial
wastewater estimates by 9.0 to 9.8 percent across the time series.
For treatment of the fruit and vegetable processing industry, a factor to account for the removal of organics as sludge
prior to anaerobic treatment was added. Based on data collected by EPA (1975), BOD is typically reduced by 17 to
30 percent, so a removal rate of 23 percent was used in the inventory.
Finally, the calculations of the percent of industrial wastewater treated anaerobically (%TA) were revised. A general
calculation for each industry defines the percent of plants in the industry that treat wastewater on site, the percent of
plants that have a primary treatment step prior to biological treatment, and the percent of plants that treat wastewater
anaerobically. The %TA was estimated for both primary treatment and secondary treatment.
Overall, the CH4 emission estimates for wastewater treatment are on average 0.5 percent greater than the previous
inventory.
For N2O emissions from domestic wastewater, a major refinement to the calculation was the reestimation of per
capita protein consumption to account for the amount consumed, not simply all protein available for consumption.
In addition, the nitrous oxide emission calculation was updated. The (USPOp) component of the N2OEFFLUENT
equation was replaced with [USPOp - (0.9xUSPOpND)] to more accurately represent the nitrogen loading of
wastewater discharged to aquatic environments. By making that replacement, the N lost as N2O from centralized
treatment systems was subtracted from the estimate of nitrogen discharged to the environment to account for loss
from nitrification/denitrification systems. Overall, the N2O emissions estimates for wastewater treatment are on
average 41 percent lower than the previous Inventory.
Overall, emissions from wastewater treatment and discharge (CH4 and N2O) decreased by an average of
approximately 9 percent from the previous inventory.
Waste 8-15
-------
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 has been
further refined to differentiate aerobic systems with the potential to generate small amounts of CH4 (aerobic
lagoons) versus other types of aerobic systems, and to differentiate between anaerobic systems to allow for the use
of different MCFs for different types of anaerobic treatment systems. Currently it is assumed that all aerobic
systems are well managed and produce no CH4, all aerobic systems that have some anaerobic activity have an MCF
of 0.3, and that all anaerobic systems have an MCF of 0.8. Efforts to obtain better data reflecting emissions from
various types of municipal treatment systems are currently being pursued.
For the current Inventory, an attempt was made to refine the designation of unit operations from aerobic and
anaerobic to include an aerobic/anaerobic designation for some of the treatment systems that were previously
designated anaerobic. However, the available data are not sufficiently detailed across the time series to complete
this designation.
Other potential sources of CH4 and CO2 emissions from wastewater treatment at petroleum refineries will be
investigated. Also, available data on wastewater treatment emissions at organic chemical manufacturers 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 C 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 2007, the amount of material composted in the United States has increased from 3,810 Gg to 19,695
Gg, an increase of approximately 400 percent. Emissions of CH4 and N2O from composting have increased by the
same percentage (see Table 8-15 and Table 8-16). In 2007, CH4 emissions from composting were 1.7 Tg CO2 Eq.
(79 Gg), and N2O emissions from composting were 1.8 Tg CO2 Eq. (6 Gg). The wastes that are composted include
primarily yard trimmings (grass, leaves, and tree and brush trimmings) and food scraps from residences and
commercial establishments (such as grocery stores, restaurants, and school and factory cafeterias). The composting
waste quantities reported here do not include backyard composting. The growth in composting is attributable
primarily to two factors: (1) steady growth in population and residential housing and (2) state and local
governments started enacting legislation that discouraged the disposal of yard trimmings in landfills. In 1992, 11
states and the District of Columbia had legislation in effect that banned or discouraged disposal of yard trimmings in
landfills. In 2005, 21 states and the District of Columbia, representing about 50 percent of the nation's population,
had enacted such legislation (EPA 2006).
Table 8-15: CH4 and N2O Emissions from Composting (Tg CO2 Eq.)
8-16 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Activity
CH4
N2O
Total
1990
0.3
0.4
0.7
1995
0.7
0.8
1.5
2000
1.3
1.4
2.6
2005
1.6
1.7
3.3
2006
1.6
1.8
3.3
2007
1.7
1.8
3.5
Table 8-16: CH4 and N2O Emissions from Composting (Gg)
Activity 1990 1995 2000 2005 2006 2007
CH4 15 35 60 75 75 79
N20 1 34666
Methodology
CH4 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 8-15 and Table 8-16 were estimated using the IPCC default (Tier 1) methodology
(IPCC 2006), which is the product of an emission factor and the mass of organic waste composted (note: no CH4
recovery is expected to occur at composting operations):
£,. =MxEFt
where,
E! = CH4 or N2O emissions from composting, Gg CH4 or N2O,
M = mass of organic waste composted in Gg,
EF; = 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 8-17. Estimates of the quantity composted
for 1990 and 1995 were taken from the Characterization of Municipal Solid Waste in the United States: 1996
Update (Franklin Associates 1997); estimates of the quantity composted for 2000, 2005, 2006, and 2007 were taken
from EPA's Municipal Solid Waste In The United States: 2007 Facts and Figures (EPA 2008).
Table 8-17: U.S. Waste Composted (Gg)
Activity 1990 1995 2000 2005 2006 2007
Waste Composted 3,810 8,682 14,923 18,643 18,852 19,695
Source: Franklin Associates 1997 and EPA 2008.
Uncertainty
The estimated uncertainty from the 2006 IPCC Guidelines is ±50 percent for the Tier 1 methodology. Emissions
from composting in 2007 were estimated to be between 1.7 and 5.2 Tg CO2 Eq., which indicates a range of 50
percent below to 50 percent above the actual 2007 emission estimate of 3.5 Tg CO2 Eq. (see Table 8-18).
Table 8-18 : Tier 1 Quantitative Uncertainty Estimates for Emissions from Composting (Tg CO2 Eq. and Percent)
2007 Emission
Source Gas Estimate Uncertainty Range Relative to Emission Estimate
(Tg CO, Eq.) (Tg CO, Eq.) (%J
Lower Bound Upper Bound Lower Bound Upper Bound
Composting CH4, N2O 3.5 1/7 52 -50% +50%
Planned Improvements
For future Inventories, additional efforts will be made to improve the estimates of CH4 and N2O emissions from
Waste 8-17
-------
composting. For example, a literature search may be conducted to determine if emission factors specific to various
composting systems and composted materials are available.
8-18 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Landfills
Wastewater Treatment
Composting
I
Waste as a Portion of all
Emissions
2.3%
20 40 60 80
TgCO2Eq.
100 120 140
Figure 8-1: 2007 Waste Chapter Greenhouse Gas Sources
-------
-------
9. Other
The United States does not report any greenhouse gas emissions under the Intergovernmental Panel on Climate
Change (IPCC) "Other" sector.
Other 9-1
-------
-------
10. 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 net CO2 flux to the atmosphere, both relative to the previously published U.S.
Inventory (i.e., the 1990 through 2006 report). These tables present the magnitude of these changes in units of
teragrams of carbon dioxide equivalent (Tg CO2 Eq.).
The Recalculations Discussion section of each source presents the details of each recalculation. In general, when
methodological changes have been implemented, the entire time series (i.e., 1990 through 2006) 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 2006, 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.
• Net CO 2 Flux from Land Use, Land-Use Change, and Forestry. Changes in the Land Use, Land-Use Change,
and Forestry sector occurred primarily in calculations for forest and grassland C stock and flux estimates. The
most significant changes were in forest aboveground biomass and soil organic carbon within the Forest Land
Remaining Forest Land land-use category and in the Grassland Remaining Grassland land-use category. In the
estimation of forest C stocks within the Forest Land Remaining Forest Land land-use category, newly available
state data contributed to a decrease in the average flux. Changes in calculation methodology for state-level
estimates, particularly in the scaling up of plot-level stock estimates and in extrapolating C stock and stock
change, resulted in significant decreases in forest carbon flux. With regard to C stock recalculation for
Grassland Remaining Grassland, several changes to historical estimates resulted from the incorporation of
annual survey data from the USDA National Resources Inventory (NRI) in the 1990-2007 Inventory. These
changes included: (1) the availability of new data extended the time series of activity data beyond 1997 to 2003;
(2) annual area data were used to estimate soil C stock changes, rather than data collected in 5-year increments;
(3) each NRI point was simulated separately, instead of simultaneously; and, (4) NRI area data were reconciled
with Forest Inventory and Analysis (FIA) area data, which led to adjustments in the NRI dataset. Overall, these
changes, in combination with smaller adjustments in the other sources/sinks within the sector, resulted in an
average annual decrease in net flux of CO2 to the atmosphere from the Land Use, Land-Use Change, and
Forestry sector of 117.3 Tg CO2 Eq. (14.1 percent) for the period 1990 through 2006, as compared to estimates
presented in the previous Inventory.
• Agricultural Soil Management. Changes in the estimates of N2O emissions from Agricultural Soil Management
occurred primarily due to a new operational version of the D AYCENT model and revised structural uncertainty
associated with the model. Improvements to the D AYCENT model include elimination of the influence of
labile (i.e., easily decomposable by microbes) C availability on surface litter denitrification rates, incorporation
of precipitation events as a controlling variable on surface litter denitrification, and allowing the wettest soil
layer within the rooting zone to control plant transpiration. Overall, changes resulted in an average annual
decrease in N2O emissions from Agricultural Soil Management of 61.3 Tg CO2 Eq. (22.7 percent) for the period
1990 through 2006.
• Iron and Steel Production. Estimates of CO2 from iron and steel production have been revised to adhere to the
2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006). Previously the estimates
focused primarily on the consumption of coking coal to produce metallurgical coke and the consumption of
metallurgical coke, carbon anodes, and scrap steel to produce iron and steel. The revised estimates differentiate
Recalculations and Improvements 10-1
-------
between emissions associated with metallurgical coke production and those associated with iron and steel
production and include CO2 emissions from the consumption of other materials such as natural gas, fuel oil,
flux (e.g. limestone and dolomite use), direction injection goal, sinter, pellets, and natural ore during the iron
and steel production process as well as the metallurgical coke production process. Overall, changes to the Iron
and Steel Production estimate resulted in an average annual increase in CO2 emissions of 26.1 Tg CO2 Eq. (40.7
percent) for the period 1990 through 2006.
• Fossil Fuel Combustion. Estimates of CO2 from the industrial sector have been revised for the years 1990
through 2006 to subtract for non-energy related consumption of coal, distillate fuel, and natural gas used to
produce pig iron in iron and steel and metallurgical coke production. A discussion of the methodology used to
estimate non-energy related consumption is contained in the Iron and Steel Production and Metallurgical Coke
Production section of the Industrial Processes chapter. Additionally, the Energy Information Administration
(EIA 2008b) updated energy consumption data for all years. These revisions primarily impacted the emission
estimates for 2006. Overall, changes resulted in an average annual decrease in CO2 emissions from Fossil Fuel
Combustion of 17 Tg CO2 Eq. (0.3 percent) for the period 1990 through 2006.
• Enteric Fermentation. Changes in the estimates of CH4 emissions from Enteric Fermentation occurred as a
result of (1) including additional heifer and steer stacker populations, (2) adjusting the Cattle Enteric
Fermentation Model (CEFM) to allow feedlot placements for the 700-800 Ibs category to use excess animals
from the over 800 Ibs category if insufficient animals are available to place in a given month at 700-800 Ibs, (3)
adjusting animal weights used in calculations, (4) using revised USDA population estimates that affected
historical emissions estimated for swine in 2006, and (5) some historical population estimates for certain beef
and dairy populations were also updated as a result of changes in USDA inputs. Overall, changes resulted in an
average annual increase in CH4 emissions from Enteric Fermentation of 10.2 Tg CO2 Eq. (8.1 percent) from
1990 through 2006.
• Natural Gas Systems. Changes in the estimates of CH4 emissions from this source category resulted primarily
from the substitution of activity factors with direct data for all years to adapt the natural gas inventory to
publicly available data and adjust the current inventory to better reflect emissions from these sources. Overall,
changes resulted in an average annual increase in CH4 emissions from Natural Gas Systems of 4.3 Tg CO2 Eq.
(3.5 percent) for the period 1990 through 2006.
• Non-Energy Use of Fuels. Changes in CO2 emissions estimates from Non-Energy Use of Fuels resulted from
changes in assumptions pertaining to petroleum coke. Non-energy end uses for petroleum coke (other than in
the industrial processing sectors, where it is accounted for separately) had not been identified in the past. This
year, it was assumed that petroleum coke used for non-energy purposes (and not accounted for in the industrial
processes chapter, viz., for production of primary aluminum anodes, electric arc furnace anodes, titanium
dioxide, ammonia, urea, and ferroalloys) is used in pigments, with a storage factor of 0.3 (rather than the value
of 0.5 used previously). Overall, the changes resulted in an average annual increase in CO2 emissions from
Non-Energy Use of Fuels of 3.9 Tg CO2 Eq. (2.9 percent) for the period 1990 through 2006.
• Nitric Acid Production. Changes in the estimates of N2O emissions from Nitric Acid Production were mostly
due to adjusting the weighted N2O emission factor (kg N2O/metric ton HNO3), which resulted in an increase in
emissions across the time series. The weighted N2O emission factor was previously based on the percentage of
facilities equipped and not equipped with non-selective catalytic reduction (NSCR) systems. The emission
factor used for the current estimate is based on the percentage of HNO3 produced at plants with NSCR systems
and HNO3 produced at plants without NSCR systems. Overall, changes resulted in an average annual increase
in N2O emissions from Nitric Acid Production of 3.1 Tg CO2 Eq. (17.8 percent) for the period 1990 through
2006.
• Wastewater Treatment. Changes in N2O emissions estimates from domestic wastewater resulted primarily from
a major refinement to the calculation—per capita protein consumption was reestimated to account for the
amount consumed, not simply all protein available for consumption. In addition, the N2O emission calculation
was updated to more accurately represent the N loading of wastewater discharged to aquatic environments.
Overall, changes resulted in an average annual decrease in N2O emissions from Wastewater Treatment of 3.0
Tg CO2 Eq. (41.0 percent) for the period 1990 through 2006.
• Forest Land Remaining Forest Land. Changes in CH4 emissions from Forest Land Remaining Forest Land
10-2 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
resulted primarily from updated carbon density values, combustion factors, and the inclusion of prescribed fires.
The carbon density for Alaska was revised to reflect the entire area that the U.S. Forest Service uses to report
carbon, and the default IPCC combustion factor for forests was used to replace the previous combustion factor.
Emissions from prescribed fires in the U.S. were included in this year's estimates. Finally, data for land area
under wildland fire protection were updated. Overall, changes resulted in an average annual increase in CH4
emissions from Forest Land Remaining Forest Land of 1.8 Tg CO2 Eq. (20.0 percent) for the period 1990
through 2006.
Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg CO2 Eq.)
Gas/Source
CO2
Fossil Fuel Combustion
Electricity Generation
Transportation
Industrial
Residential
Commercial
US Territories
Non-Energy Use of Fuels
Iron and Steel Production & Metallurgical
Coke Production
Cement Production
Natural Gas Systems
Incineration of Waste
Lime Production
Ammonia Production and Urea
Consumption
Cropland Remaining Cropland
Limestone and Dolomite Use
Aluminum Production
Soda Ash Production and Consumption
Petrochemical Production
Titanium Dioxide Production
Carbon Dioxide Consumption
Ferroalloy Production
Phosphoric Acid Production
Wetlands Remaining Wetlands3
Zinc Production
Petroleum Systems
Lead Production
Silicon Carbide Production and
Consumption
Land Use, Land-Use Change, and Forestry
(Sink)b
Wood Biomass and Ethanol Consumption1*
International Bunker Fuelsb
CH4
Enteric Fermentation
Landfills
Natural Gas Systems
Coal Mining
Manure Management
Forest Land Remaining Forest Land
Petroleum Systems
Wastewater Treatment
Stationary Combustion
1990
8.2
(15.2)
0.1
(0.6)
(10.7)
(2.4)
(1.6)
NC
(0.2)
23.5
NC
+
NC
(0.5)
(0.1)
NC
(0.4)
NC
NC
NC
NC
NC
NC
NC
1.0
NC
+
NC
+
(103.8)
NC
0.6
10.5
6.3
(0.4)
4.9
NC
(0.6)
0.1
+
0.5
+
1995
13.7
(18.5)
(0.4)
(0.7)
(13.9)
(2.1)
(1.4)
NC
4.2
28.4
NC
+
NC
(0.7)
+
NC
(0.7)
NC
NC
NC
NC
NC
NC
NC
1.0
NC
+
NC
+
(75.6)
+
1.0
16.9
11.2
0.2
4.5
NC
(0.7)
1.5
+
0.5
+
2000
15.5
(15.6)
0.9
2.1
(15.8)
(1.7)
(1.1)
NC
3.0
28.5
NC
+
+
(0.8)
+
NC
(0.9)
NC
NC
NC
NC
NC
NC
NC
1.2
NC
+
NC
+
(43.9)
+
(2.2)
16.8
9.8
1.5
4.3
0.1
(0.9)
1.7
+
0.6
+
2004
9.9
(12.0)
0.4
4.0
(13.1)
(2.0)
(1.3)
NC
0.4
23.3
NC
+
(1.5)
(0.7)
+
NC
(0.8)
NC
NC
NC
NC
NC
NC
NC
1.2
NC
+
NC
+
(420.9)
+
(4.1)
17.2
11.4
3.6
4.0
(1.6)
(0.3)
(0.3)
+
0.6
+
2005
16.5
(7.6)
0.8
11.6
(19.3)
(0.5)
(0.2)
+
(1.0)
26.6
NC
+
(1.1)
(0.8)
+
NC
(0.6)
(0.1)
NC
NC
NC
NC
NC
NC
1.1
NC
+
NC
+
(244.1)
4.1
(11.1)
22.0
11.5
4.2
3.8
+
0.1
1.9
+
0.6
0.2
2006
31.8
(2.5)
(0.8)
24.8
(17.7)
(4.7)
(4.1)
(0.1)
7.2
27.0
0.8
1.0
(1.1)
(0.7)
(0.1)
(0.1)
(0.6)
(0.1)
NC
NC
NC
0.1
NC
NC
0.9
NC
+
NC
+
(166.9)
5.7
(16.6)
26.7
12.0
4.8
2.4
(0.1)
0.4
6.7
(0.1)
0.6
0.1
Recalculations and Improvements 10-3
-------
Rice Cultivation
Abandoned Underground Coal Mines
Mobile Combustion
Composting
Petrochemical Production
Field Burning of Agricultural Residues
Iron and Steel Production & Metallurgical
Coke Production
Ferroalloy Production
Silicon Carbide Production and
Consumption
International Bunker Fuelsb
N2O
Agricultural Soil Management
Mobile Combustion
Nitric Acid Production
Manure Management
Stationary Combustion
Adipic Acid Production
Wastewater Treatment
N2O from Product Uses
Forest Land Remaining Forest Land
Composting
Settlements Remaining Settlements
Field Burning of Agricultural Residues
Incineration of Waste
Wetlands Remaining Wetlands3
International Bunker Fuelsb
HFCs
Substitution of Ozone Depleting
Substances
HCFC-22 Production
Semiconductor Manufacture
PFCs
Aluminum Production
Semiconductor Manufacture
SF6
Electrical Transmission and Distribution
Magnesium Production and Processing
Semiconductor Manufacture
Net Change in Total Emissions0
Percent Change
NC
NC
+
NC
+
NC
(0.4)
NC
NC
+
(68.4)
(69.0)
0.2
3.0
+
+
NC
(2.6)
NC
+
NC
+
NC
NC
+
0.1
NC
NC
NC
NC
NC
NC
NC
0.1
0.1
NC
NC
(49.6)
-0.8%
NC
NC
+
NC
+
NC
(0.3)
NC
NC
+
(61.5)
(62.5)
0.2
3.4
0.1
+
NC
(2.8)
NC
0.1
NC
+
NC
NC
+
0.1
+
+
NC
+
+
NC
+
0.1
0.1
NC
+
(30.7)
-0.5%
NC
NC
+
NC
+
NC
(0.3)
NC
NC
+
(56.7)
(57.6)
0.3
3.3
0.3
(0.1)
NC
(3.1)
NC
0.2
NC
+
NC
NC
+
+
+
+
NC
+
+
NC
+
0.1
0.1
NC
+
(24.4)
-0.3%
NC
+
+
NC
0.1
NC
(0.2)
NC
NC
+
(35.7)
(35.7)
0.3
2.7
0.3
(0.1)
NC
(3.2)
NC
(0.1)
NC
+
NC
NC
+
+
(4.2)
(4.2)
NC
+
+
NC
+
(0.2)
0.1
(0.3)
+
(13.0)
-0.2%
NC
+
+
NC
+
NC
(0.2)
NC
NC
+
(54.1)
(54.6)
0.4
2.8
0.3
+
NC
(3.2)
NC
0.2
NC
+
NC
+
+
(0.1)
(5.3)
(5.3)
NC
+
+
+
+
(0.3)
0.1
(0.4)
+
(21.3)
-0.3%
NC
0.1
0.1
NC
+
+
(0.2)
NC
NC
+
(55.8)
(56.5)
0.5
2.6
0.3
+
NC
(3.3)
NC
0.6
NC
0.1
+
+
+
(0.1)
(5.5)
(5.5)
NC
+
+
NC
+
(0.3)
+
(0.3)
+
(3.1)
0.0%
+ Absolute value does not exceed 0.05 Tg CO2 Eq. or 0.05 percent.
NC (No Change)
a New source category relative to previous Inventory.
b Not included in emissions total.
0 Excludes net CO2 flux from Land Use, Land-Use Change, and Forestry, and emissions from International Bunker Fuels and
Wood Biomass and Ethanol Consumption.
Note: Lotals may not sum due to independent rounding.
Table 10-2: Revisions to Net Flux of CO2 to the Atmosphere from Land Use, Land-Use Change, and Forestry (Tg
C02Eq.)
Component: Net CO2 Flux From Land
Use, Land-Use Change, and Forestry
Forest Land Remaining Forest Land
Cropland Remaining Cropland
1990
(39.4)
0.7
1995
(26.7)
16.4
2000
38.1
8.2
2004
(408.7)
22.7
2005
(232.1)
22.1
2006
(155.2)
22.7
10-4 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2007
-------
Land Converted to Cropland
Grassland Remaining Grassland
Land Converted to Grassland
Settlements Remaining Settlements
Other
Net Change in Total Flux
Percent Change
(12.6)
(44.9)
(8.0)
NC
0.4
(103.8)
-14.1%
(6.4)
(53.0)
(6.2)
NC
0.2
(75.6)
-9.8%
(7.0)
(67.8)
(15.6)
NC
0.2
(43.9)
-6.5%
(3.4)
(20.9)
(10.4)
NC
(0.3)
(420.9)
-48.2%
(3.4)
(20.9)
(10.4)
NC
(0.2)
(244.1)
-27.8%
(3.4)
(20.9)
(10.4)
NC
0.2
(166.9)
-18.9%
NC (No Change)
Note: Numbers in parentheses indicate a decrease in estimated net flux of CO2 to the atmosphere, or an increase in net
sequestration.
Note: Totals may not sum due to independent rounding.
Recalculations and Improvements 10-5
-------
-------
11. References
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