EPA 430-R-05-003 

INVENTORY OF U.S. GREENHOUSE GAS EMISSIONS AND SINKS: 

1990 – 2003 

APRIL 15, 2005 

U.S. Environmental Protection Agency 

1200 Pennsylvania Ave., N.W. 

Washington, DC 20460 

U.S.A. 

 
[Inside Front Cover] 

HOW TO OBTAIN COPIES 

You can electronically download this document on the U.S. EPA's homepage at 

<http://www.epa.gov/globalwarming/publications/emissions>. 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 2003, 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. Lisa Hanle, Environmental Protection Agency, (202) 343-9434, hanle.lisa@epa.gov. 

For more information regarding climate change and greenhouse gas emissions, see the EPA web site at 

<http://www.epa.gov/globalwarming>. 

Released for printing: April 15, 2005 

[INSERT DISCUSSION OF COVER DESIGN] 
Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page i 

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 lead by Leif Hockstad and Lisa Hanle. Work on 

energy and waste sector methane emissions was directed by Elizabeth Scheehle, while work on agriculture sector 

emissions was directed by Tom Wirth and Joe Mangino. Tom Wirth led the preparation of the chapter on Land-Use 

Change and Forestry. Work on emissions of HFCs, PFCs, and SF6 was directed by Deborah Schafer and Dave 

Godwin. John Davies directed the work on mobile combustion. 

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, Randall Freed, and their staff at ICF Consulting’s Energy 

Policy and Programs Practice, including John Venezia, Leonard Crook, Diana Pape, Meg Walsh, Michael Grant, 

Beth Moore, Ravi Kantamaneni, Robert Lanza, Chris Steuer, Lauren Flynn, Kamala Jayaraman, Dan Lieberman, 

Jeremy Scharfenberg, Matt Stanberry, Rebecca LePrell, Philip Groth, Sarah Percy, Daniel Karney, Brian Gillis, 

Zachary Schaffer, Vineet Aggarwal, Lauren Pederson, and Toby Mandel for synthesizing this report and preparing 

many of the individual analyses. Eastern Research Group, Raven Ridge Resources, and Arcadis also provided 

significant analytical support. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ii 

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).1 Under decision 3/CP.5 of the UNFCCC Conference of the Parties, national 

inventories for UNFCCC Annex I parties should be provided to the UNFCCC Secretariat each year by April 15. 

In an effort to engage the public and researchers across the country, the EPA has instituted an annual public review 

and comment process for this document. The availability of the draft document is announced via Federal Register 

Notice and is posted on the EPA web site.2 Copies are also mailed upon request. The public comment period is 

generally limited to 30 days; however, comments received after the closure of the public comment period are 

accepted and considered for the next edition of this annual report. 

 

1 See Article 4(1)(a) of the United Nations Framework Convention on Climate Change <http://www.unfccc.int>. 

2 See <http://www.epa.gov/globalwarming/publications/emissions>. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page iii 

Table of Contents 

ACKNOWLEDGMENTS I

TABLE OF CONTENTS III

LIST OF TABLES, FIGURES, AND BOXES VI

Tables vi

Figures vi

Boxes xiv

EXECUTIVE SUMMARY ES-1

ES.1. Background Information ES-1

ES.2. Recent Trends in U.S. Greenhouse Gas Emissions and Sinks ES-3

ES.3. Overview of Sector Emissions and Trends ES-9

ES.4. Other Information ES-12

1. INTRODUCTION 1

1.1. Background Information 2

1.2. Institutional Arrangements 10

1.3. Inventory Process 10

1.4. Methodology and Data Sources 12

1.5. Key Sources 13

1.6. Quality Assurance and Quality Control 16

1.7. Uncertainty Analysis of Emission Estimates 17

1.8. Completeness 18

1.9. Organization of Report 18

2. TRENDS IN GREENHOUSE GAS EMISSIONS 21

2.1. Recent Trends in U.S. Greenhouse Gas Emissions 21

2.2. Emissions by Economic Sector 42

2.3. Ambient Air Pollutant Emissions 48

3. ENERGY 51

3.1. Carbon Dioxide Emissions from Fossil Fuel Combustion (IPCC Source Category 1A) 52

3.2. Carbon Emitted from Non-Energy Uses of Fossil Fuels (IPCC Source Category 1A) 69

3.3. Stationary Combustion (excluding CO2) (IPCC Source Category 1A) 75

3.4. Mobile Combustion (excluding CO2) (IPCC Source Category 1A) 81

3.5. Coal Mining (IPCC Source Category 1B1a) 92

3.6. Abandoned Underground Coal Mines (IPCC Source Category 1B1a) 95

3.7. Petroleum Systems (IPCC Source Category 1B2a) 99
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page iv 

3.8. Natural Gas Systems (IPCC Source Category 1B2b) 102

3.9. Municipal Solid Waste Combustion (IPCC Source Category 1A5) 106

3.10. Natural Gas Flaring and Ambient Air Pollutant Emissions from Oil and Gas Activities (IPCC Source 

Category 1B2) 111

3.11. International Bunker Fuels (IPCC Source Category 1: Memo Items) 113

3.12. Wood Biomass and Ethanol Consumption (IPCC Source Category 1A) 118

4. INDUSTRIAL PROCESSES 123

4.1. Iron and Steel Production (IPCC Source Category 2C1) 125

4.2. Cement Manufacture (IPCC Source Category 2A1) 129

4.3. Ammonia Manufacture and Urea Application (IPCC Source Category 2B1) 132

4.4. Lime Manufacture (IPCC Source Category 2A2) 136

4.5. Limestone and Dolomite Use (IPCC Source Category 2A3) 140

4.6. Soda Ash Manufacture and Consumption (IPCC Source Category 2A4) 143

4.7. Titanium Dioxide Production (IPCC Source Category 2B5) 146

4.8. Phosphoric Acid Production (IPCC Source Category 2A7) 148

4.9. Ferroalloy Production (IPCC Source Category 2C2) 151

4.10. Carbon Dioxide Consumption (IPCC Source Category 2B5) 153

4.11. Petrochemical Production (IPCC Source Category 2B5) 157

4.12. Silicon Carbide Production (IPCC Source Category 2B4) 160

4.13. Nitric Acid Production (IPCC Source Category 2B2) 161

4.14. Adipic Acid Production (IPCC Source Category 2B3) 163

4.15. Substitution of Ozone Depleting Substances (IPCC Source Category 2F) 166

4.16. HCFC-22 Production (IPCC Source Category 2E1) 168

4.17. Electrical Transmission and Distribution (IPCC Source Category 2F7) 170

4.18. Aluminum Production (IPCC Source Category 2C3) 174

4.19. Semiconductor Manufacture (IPCC Source Category 2F6) 179

4.20. Magnesium Production and Processing (IPCC Source Category 2C4) 182

4.21. Industrial Sources of Ambient Air Pollutants 186

5. SOLVENT AND OTHER PRODUCT USE 189

5.1. Nitrous Oxide Product Usage (IPCC Source Category 3D) 189

5.2. Ambient Air Pollutants from Solvent Use 192

6. AGRICULTURE 195

6.1. Enteric Fermentation (IPCC Source Category 4A) 196

6.2. Manure Management (IPCC Source Category 4B) 200

6.3. Rice Cultivation (IPCC Source Category 4C) 207

6.4. Agricultural Soil Management (IPCC Source Category 4D) 212
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page v 

6.5. Field Burning of Agricultural Residues (IPCC Source Category 4F) 222

7. LAND-USE CHANGE AND FORESTRY 229

7.1. Forest Land Remaining Forest Land 231

7.2. Land Converted to Forest Land (Source Category 5A2) 241

7.3. Croplands Remaining Croplands 241

7.4. Lands Converted to Croplands (Source Category 5B2) 249

7.5. Settlements Remaining Settlements 250

7.6. Lands Converted to Settlements (Source Category 5E2) 259

8. WASTE 261

8.1. Landfills (IPCC Source Category 6A1) 261

8.2. Wastewater Treatment (IPCC Source Category 6B) 265

8.3. Human Sewage (Domestic Wastewater) (IPCC Source Category 6B) 269

8.4. Waste Sources of Ambient Air Pollutants 272

9. OTHER 275

10. RECALCULATIONS AND IMPROVEMENTS 277

REFERENCES 281

 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page vi 

List of Tables, Figures, and Boxes 

Tables 

Table ES-1: Global Warming Potentials (100 Year Time Horizon) Used in this Report 3

Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.) 4

Table ES-3: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 7

Table ES-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg CO2 Eq.) 9

Table ES-5: Net CO2 Flux from Land-Use Change and Forestry (Tg CO2 Eq.) 11

Table ES-6: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq.) 12

Table ES-7: U.S Greenhouse Gas Emissions by Economic Sector and Gas with Electricity-Related Emissions 

Distributed (Tg CO2 Eq.) 13

Table ES-8: Recent Trends in Various U.S. Data (Index 1990 = 100) and Global Atmospheric CO2 Concentration

14

Table ES-9: Emissions of NOx, CO, NMVOCs, and SO2 (Gg) 14

Table 1-1: Global atmospheric concentration (ppm unless otherwise specified), rate of concentration change 

(ppb/year) and atmospheric lifetime (years) of selected greenhouse gases 3

Table 1-2: Global Warming Potentials and Atmospheric Lifetimes (Years) Used in this Report 7

Table 1-3: Comparison of 100 Year GWPs 8

Table 1-4: Effects on U.S. Greenhouse Gas Emission Trends Using IPCC SAR and TAR GWP Values (Tg CO2 

Eq.) 9

Table 1-5: Comparison of Emissions by Sector using IPCC SAR and TAR GWP Values (Tg CO2 Eq.) 9

Table 1-6: Key Source Categories for the United States (1990-2003) Based on Tier 1 Approach 15

Table 1-7: IPCC Sector Descriptions 18

Table 1-8: List of Annexes 19

Table 2-1: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors (Tg CO2

Eq. and Percent) 22

Table 2-2: Recent Trends in Various U.S. Data (Index 1990 = 100) and Global Atmospheric CO2 Concentration 24

Table 2-3: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.) 24

Table 2-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg) 26

Table 2-5: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg CO2 Eq.) 28

Table 2-6: Emissions from Energy (Tg CO2 Eq.) 28

Table 2-7: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 29

Table 2-8: Emissions from Industrial Processes (Tg CO2 Eq.) 33

Table 2-9: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq.) 37

Table 2-10: Emissions from Agriculture (Tg CO2 Eq.) 38

Table 2-11: Net CO2 Flux from Land-Use Change and Forestry (Tg CO2 Eq.) 39

Table 2-12: N2O Emissions from Land-Use Change and Forestry (Tg CO2 Eq.) 40
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page vii 

Table 2-13: Emissions from Waste (Tg CO2 Eq.) 41

Table 2-14: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq. and Percent of Total in 

2003) 42

Table 2-15: Electricity Generation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 44

Table 2-16: U.S Greenhouse Gas Emissions by “Economic Sector” and Gas with Electricity-Related Emissions 

Distributed (Tg CO2 Eq.) and Percent of Total in 2003 45

Table 2-17: Transportation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 47

Table 2-18: Emissions of NOx, CO, NMVOCs, and SO2 (Gg) 49

Table 3-1: Emissions from Energy (Tg CO2 Eq.) 51

Table 3-2: Emissions from Energy (Gg) 52

Table 3-3: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq.) 53

Table 3-4: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors (Tg CO2

Eq. and Percent) 54

Table 3-5: CO2 Emissions from International Bunker Fuels (Tg CO2 Eq.)* 56

Table 3-6: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 56

Table 3-7: CO2 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector (Tg CO2 Eq.) 57

Table 3-8: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg CO2 Eq./QBtu) 62

Table 3-9: Carbon Intensity from all Energy Consumption by Sector (Tg CO2 Eq./QBtu) 62

Table 3-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Energy-related Fossil Fuel 

Combustion by Fuel Type and Sector (Tg CO2 Eq. and Percent) 67

Table 3-11: CO2 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg CO2 Eq.) 69

Table 3-12: Adjusted Consumption of Fossil Fuels for Non-Energy Uses (TBtu) 71

Table 3-13: 2003 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions 71

Table 3-14: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Non-Energy Uses of Fossil Fuels 

(Tg CO2 Eq. and Percent) 73

Table 3-15: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of Non-Energy Uses of Fossil Fuels 

(Percent) 73

Table 3-16: CH4 Emissions from Stationary Combustion (Tg CO2 Eq.) 75

Table 3-17: N2O Emissions from Stationary Combustion (Tg CO2 Eq.) 76

Table 3-18: CH4 Emissions from Stationary Combustion (Gg) 77

Table 3-19: N2O Emissions from Stationary Combustion (Gg) 77

Table 3-20: NOx, CO, and NMVOC Emissions from Stationary Combustion in 2003 (Gg) 78

Table 3-21: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Energy-Related Stationary 

Combustion, Including Biomass (Tg CO2 Eq. and Percent) 80

Table 3-22: CH4 Emissions from Mobile Combustion (Tg CO2 Eq.) 82

Table 3-23: N2O Emissions from Mobile Combustion (Tg CO2 Eq.) 82

Table 3-24: CH4 Emissions from Mobile Combustion (Gg) 83

Table 3-25: N2O Emissions from Mobile Combustion (Gg) 83

Table 3-26: NOx Emissions from Mobile Combustion (Gg) 84
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page viii 

Table 3-27: CO Emissions from Mobile Combustion (Gg) 84

Table 3-28: NMVOC Emissions from Mobile Combustion (Gg) 85

Table 3-29: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Mobile Sources (Tg CO2

Eq. and Percent) 90

Table 3-30: CH4 Emissions from Coal Mining (Tg CO2 Eq.) 93

Table 3-31: CH4 Emissions from Coal Mining (Gg) 93

Table 3-32: Coal Production (Thousand Metric Tons) 94

Table 3-33: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Coal Mining (Tg CO2 Eq. and 

Percent) 94

Table 3-34: CH4 Emissions from Abandoned Coal Mines (Tg CO2 Eq.) 96

Table 3-35: CH4 Emissions from Abandoned Coal Mines (Gg) 96

Table 3-36: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Abandoned Underground Coal 

Mines (Tg CO2 Eq. and Percent) 98

Table 3-37: CH4 Emissions from Petroleum Systems (Tg CO2 Eq.) 100

Table 3-38: CH4 Emissions from Petroleum Systems (Gg) 100

Table 3-39: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petroleum Systems (Tg CO2 Eq. and 

Percent) 101

Table 3-40: CH4 Emissions from Natural Gas Systems (Tg CO2 Eq.)* 103

Table 3-41: CH4 Emissions from Natural Gas Systems (Gg)* 103

Table 3-42: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Natural Gas Systems (Tg CO2 Eq. 

and Percent) 104

Table 3-43: CO2 and N2O Emissions from Municipal Solid Waste Combustion (Tg CO2 Eq.) 107

Table 3-44: CO2 and N2O Emissions from Municipal Solid Waste Combustion (Gg) 107

Table 3-45: NOx, CO, and NMVOC Emissions from Municipal Solid Waste Combustion (Gg) 107

Table 3-46: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted 108

Table 3-47: Tier 2 Quantitative Uncertainty Estimates for CO2 and N2O from Municipal Solid Waste Combustion 

(Tg CO2 Eq. and Percent) 109

Table 3-48: U.S. Municipal Solid Waste Combusted, as Reported by EPA and BioCycle (Metric Tons) 110

Table 3-49: CO2 Emissions from On-Shore and Off-Shore Natural Gas Flaring (Tg CO2 Eq.) 111

Table 3-50: CO2 Emissions from On-Shore and Off-Shore Natural Gas Flaring (Gg) 111

Table 3-51: NOx, NMVOCs, and CO Emissions from Oil and Gas Activities (Gg) 111

Table 3-52: Total Natural Gas Reported Vented and Flared (Million Ft3) and Thermal Conversion Factor (Btu/Ft3)

112

Table 3-53: Volume Flared Offshore (MMcf) and Fraction Vented and Flared (Percent) 112

Table 3-54: Emissions from International Bunker Fuels (Tg CO2 Eq.) 114

Table 3-55: Emissions from International Bunker Fuels (Gg) 115

Table 3-56: Aviation Jet Fuel Consumption for International Transport (Million Gallons) 116

Table 3-57: Marine Fuel Consumption for International Transport (Million Gallons) 116

Table 3-58: CO2 Emissions from Wood Consumption by End-Use Sector (Tg CO2 Eq.) 118
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ix 

Table 3-59: CO2 Emissions from Wood Consumption by End-Use Sector (Gg) 118

Table 3-60: CO2 Emissions from Ethanol Consumption 119

Table 3-61: Woody Biomass Consumption by Sector (Trillion Btu) 119

Table 3-62: Ethanol Consumption 119

Table 3-63: CH4 Emissions from Non-Combustion Fossil Sources (Gg) 121

Table 3-64: Formation of CO2 through Atmospheric CH4 Oxidation (Tg CO2 Eq.) 121

Table 4-1: Emissions from Industrial Processes (Tg CO2 Eq.) 123

Table 4-2: Emissions from Industrial Processes (Gg) 124

Table 4-3: CO2 and CH4 Emissions from Iron and Steel Production (Tg CO2 Eq.) 126

Table 4-4: CO2 and CH4 Emissions from Iron and Steel Production (Gg) 126

Table 4-5: CH4 Emission Factors for Coal Coke, Sinter, and Pig Iron Production (g/kg) 127

Table 4-6: Production and Consumption Data for the Calculation of CO2 and CH4 Emissions from Iron and Steel 

Production (Thousand Metric Tons) 127

Table 4-7: Tier 2 Quantitative Uncertainty Estimates for CO2 and CH4 Emissions from Iron and Steel Production 

(Tg. CO2 Eq. and Percent) 128

Table 4-8: CO2 Emissions from Cement Production (Tg CO2 Eq. and Gg)* 129

Table 4-9: Cement Production (Gg) 131

Table 4-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Cement Manufacture (Tg CO2 Eq. 

and Percent) 131

Table 4-11: CO2 Emissions from Ammonia Manufacture and Urea Application (Tg CO2 Eq.) 132

Table 4-12: CO2 Emissions from Ammonia Manufacture and Urea Application (Gg) 133

Table 4-13: Ammonia Production (Gg) 133

Table 4-14: Urea Production (Gg) 134

Table 4-15: Urea Net Imports (Gg) 134

Table 4-16: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ammonia Manufacture and Urea 

Application (Tg CO2 Eq. and Percent) 136

Table 4-17: Net CO2 Emissions from Lime Manufacture (Tg CO2 Eq.) 137

Table 4-18: CO2 Emissions from Lime Manufacture (Gg) 137

Table 4-19: Lime Production and Lime Use for Sugar Refining and PCC (Gg) 138

Table 4-20: Hydrated Lime Production (Gg) 138

Table 4-21: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lime Manufacture (Tg CO2 Eq. and 

Percent) 140

Table 4-22: CO2 Emissions from Limestone & Dolomite Use (Tg CO2 Eq.) 140

Table 4-23: CO2 Emissions from Limestone & Dolomite Use (Gg) 140

Table 4-24: Limestone and Dolomite Consumption (Thousand Metric Tons) 142

Table 4-25: Dolomitic Magnesium Metal Production Capacity (Metric Tons) 142

Table 4-26: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Limestone and Dolomite Use (Tg 

CO2 Eq. and Percent) 143
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page x 

Table 4-27: CO2 Emissions from Soda Ash Manufacture and Consumption (Tg CO2 Eq.) 144

Table 4-28: CO2 Emissions from Soda Ash Manufacture and Consumption (Gg) 144

Table 4-29: Soda Ash Manufacture and Consumption (Gg) 145

Table 4-30: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Soda Ash Manufacture and 

Consumption (Tg CO2 Eq. and Percent) 145

Table 4-31: CO2 Emissions from Titanium Dioxide (Tg CO2 Eq. and Gg) 146

Table 4-32: Titanium Dioxide Production (Gg) 147

Table 4-33: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Titanium Dioxide Production (Tg 

CO2 Eq. and Percent) 147

Table 4-34: CO2 Emissions from Phosphoric Acid Production (Tg CO2 Eq. and Gg) 148

Table 4-35: Phosphate Rock Domestic Production, Exports, and Imports (Gg) 149

Table 4-36: Chemical Composition of Phosphate Rock (percent by weight) 150

Table 4-37: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Phosphoric Acid Production (Tg 

CO2 Eq. and Percent) 151

Table 4-38: CO2 Emissions from Ferroalloy Production (Tg CO2 Eq. and Gg) 151

Table 4-39: Production of Ferroalloys (Metric Tons) 152

Table 4-40: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ferroalloy Production (Tg CO2 Eq. 

and Percent) 153

Table 4-41: CO2 Emissions from Carbon Dioxide Consumption (Tg CO2 Eq. and Gg) 154

Table 4-42: Carbon Dioxide Consumption (Metric Tons) 155

Table 4-43: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Carbon Dioxide Consumption (Tg 

CO2 Eq. and Percent) 156

Table 4-44: CO2 and CH4 Emissions from Petrochemical Production (Tg CO2 Eq.) 157

Table 4-45: CO2 and CH4 Emissions from Petrochemical Production (Gg) 157

Table 4-46: Production of Selected Petrochemicals (Thousand Metric Tons) 158

Table 4-47: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock (Secondary Feedstock) 

Consumption (Thousand Metric Tons) 159

Table 4-48: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petrochemical Production and CO2

Emissions from Carbon Black Production (Tg CO2 Eq. and Percent) 159

Table 4-49: CH4 Emissions from Silicon Carbide Production (Tg CO2 Eq. and Gg) 160

Table 4-50: Production of Silicon Carbide (Metric Tons) 160

Table 4-51: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Silicon Carbide Production (Tg CO2

Eq. and Percent) 161

Table 4-52: N2O Emissions from Nitric Acid Production (Tg CO2 Eq. and Gg) 162

Table 4-53: Nitric Acid Production (Gg) 162

Table 4-54: Tier 1 Quantitative Uncertainty Estimates for N2O Emissions from Nitric Acid Production (Tg CO2 Eq. 

and Percent) 163

Table 4-55: N2O Emissions from Adipic Acid Production (Tg CO2 Eq. and Gg) 164

Table 4-56: Adipic Acid Production (Gg) 165
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page xi 

Table 4-57: Tier 1 Quantitative Uncertainty Estimates for N2O Emissions from Adipic Acid Production (Tg CO2

Eq. and Percent) 165

Table 4-58: Emissions of HFCs and PFCs from ODS Substitution (Tg CO2 Eq.) 166

Table 4-59: Emissions of HFCs and PFCs from ODS Substitution (Mg) 166

Table 4-60: Tier 2 Quantitative Uncertainty Estimates for HFC and PFC Emissions from ODS Substitution (Tg 

CO2 Eq. and Percent) 168

Table 4-61: HFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and Gg) 169

Table 4-62: HCFC-22 Production (Gg) 169

Table 4-63: Tier 1 Quantitative Uncertainty Estimates for HFC-23 Emissions from HCFC-22 Production (Tg CO2

Eq. and Percent) 170

Table 4-64: SF6 Emissions from Electric Power Systems and Original Equipment Manufactures (Tg CO2 Eq.) 170

Table 4-65: SF6 Emissions from Electric Power Systems and Original Equipment Manufacturers (Gg) 171

Table 4-66: Simulated Variables for Tier 2 Uncertainty Analysis 173

Table 4-67: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Electrical Transmission and 

Distribution (Tg CO2 Eq. and Percent) 174

Table 4-68: CO2 Emissions from Aluminum Production (Tg CO2 Eq. and Gg) 175

Table 4-69: PFC Emissions from Aluminum Production (Tg CO2 Eq.) 175

Table 4-70: PFC Emissions from Aluminum Production (Gg) 175

Table 4-71: Production of Primary Aluminum (Gg) 177

Table 4-72: Tier 2 Quantitative Uncertainty Estimates for PFC Emissions from Aluminum Production (Tg CO2 Eq. 

and Percent) 178

Table 4-73: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg CO2 Eq.) 179

Table 4-74: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Mg) 179

Table 4-75: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from Semiconductor 

Manufacture (Tg CO2 Eq. and Percent) 182

Table 4-76: SF6 Emissions from Magnesium Production and Processing (Tg CO2 Eq. and Gg) 182

Table 4-77: SF6 Emission Factors (kg SF6 per metric ton of magnesium) 183

Table 4-78: Simulated Variables for Tier 2 Uncertainty Analysis 184

Table 4-79: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Magnesium Production and 

Processing (Tg CO2 Eq. and Percent) 184

Table 4-80: 2003 Potential and Actual Emissions of HFCs, PFCs, and SF6 from Selected Sources (Tg CO2 Eq.) 186

Table 4-81: NOx, CO, and NMVOC Emissions from Industrial Processes (Gg) 186

Table 5-1: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq. and Gg) 189

Table 5-2: Ambient Air Pollutant Emissions from Solvent and Other Product Use (Gg) 189

Table 5-3: N2O Emissions from Nitrous Oxide Product Usage (Tg CO2 Eq. and Gg) 189

Table 5-4: N2O Production (Gg) 191

Table 5-5: Tier 1 Quantitative Uncertainty Estimates for N2O Emissions from Nitrous Oxide Product Usage (Tg 

CO2 Eq. and Percent) 191

Table 5-6: Emissions of NOx, CO, and NMVOC from Solvent Use (Gg) 192
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page xii 

Table 6-1: Emissions from Agriculture (Tg CO2 Eq.) 195

Table 6-2: Emissions from Agriculture (Gg) 195

Table 6-3: CH4 Emissions from Enteric Fermentation (Tg CO2 Eq.) 196

Table 6-4: CH4 Emissions from Enteric Fermentation (Gg) 197

Table 6-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Enteric Fermentation (Tg CO2 Eq. 

and Percent) 199

Table 6-6: CH4 and N2O Emissions from Manure Management (Tg CO2 Eq.) 201

Table 6-7: CH4 and N2O Emissions from Manure Management (Gg) 201

Table 6-8: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Manure Management (Tg 

CO2 Eq. and Percent) 204

Table 6-9: CH4 Emissions from Rice Cultivation (Tg CO2 Eq.) 208

Table 6-10: CH4 Emissions from Rice Cultivation (Gg CH4) 209

Table 6-11: Rice Areas Harvested (Hectares) 210

Table 6-12: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Rice Cultivation (Tg CO2 Eq. and 

Percent) 212

Table 6-13: N2O Emissions from Agricultural Soils (Tg CO2 Eq.) 213

Table 6-14: N2O Emissions from Agricultural Soils (Gg) 213

Table 6-15: Direct N2O Emissions from Agricultural Soils (Tg CO2 Eq.) 213

Table 6-16: Direct N2O Emissions from PRP Livestock Manure (Tg CO2 Eq.) 214

Table 6-17: Indirect N2O Emissions from all Land Use Types* (Tg CO2 Eq.) 214

Table 6-18: Tier 1 Quantitative Uncertainty Estimates of N2O Emissions from Agricultural Soil Management in 

2003 (Tg CO2 Eq. and Percent) 219

Table 6-19. Comparison of Direct Soil N2O Emission Estimates for IPCC versus Current Methodologies (Tg CO2

Eq.). 221

Table 6-20. Comparison of Indirect Soil N2O Emission Estimates for IPCC versus Current Methodologies (Tg CO2

Eq.) 221

Table 6-21. Comparison of Total Soil N2O Emission Estimates for IPCC versus Current Methodologies (Tg CO2

Eq.) 221

Table 6-22: Emissions from Field Burning of Agricultural Residues (Tg CO2 Eq.) 222

Table 6-23: Emissions from Field Burning of Agricultural Residues (Gg)* 223

Table 6-24: Agricultural Crop Production (Gg of Product) 225

Table 6-25: Percentage of Rice Area Burned by State 225

Table 6-26: Percentage of Rice Area Burned in California 225

Table 6-27: Key Assumptions for Estimating Emissions from Field Burning of Agricultural Residues 226

Table 6-28: Greenhouse Gas Emission Ratios 226

Table 6-29: Tier 1 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Field Burning of 

Agricultural Residues (Tg CO2 Eq. and Percent) 227

Table 7-1: Net CO2 Flux from Land-Use Change and Forestry (Tg CO2 Eq.) 230

Table 7-2: Net CO2 Flux from Land-Use Change and Forestry (Tg C) 230
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page xiii 

Table 7-3: Net N2O Emissions from Land-Use Change and Forestry (Tg CO2 Eq.) 231

Table 7-4: Net N2O Emissions from Land-Use Change and Forestry (Gg) 231

Table 7-5. Net Annual Changes in Carbon Stocks (Tg CO2 Eq. yr-1) in Forest and Harvested Wood Pools 233

Table 7-6. Net Annual Changes in Carbon Stocks (Tg C yr-1) in Forest and Harvested Wood Pools 233

Table 7-7. Carbon Stocks (Tg C) in Forest and Harvested Wood Pools 234

Table 7-8: Tier 2 Quantitative Uncertainty Estimates for CO2 Net Flux from Forest Land Remaining Forest Land: 

Changes in Forest Carbon Stocks (Tg CO2 Eq. and Percent) 237

Table 7-9. N2O Fluxes from Soils in Forests Remaining Forests (Tg CO2 Eq. and Gg) 239

Table 7-10: Tier 1 Quantitative Uncertainty Estimates of N2O Fluxes from Forest Soils (Tg CO2 Eq. and Percent)

240

Table 7-11: Net CO2 Flux from Agricultural Soils (Tg CO2 Eq.) 242

Table 7-13: Quantities of Applied Minerals (Thousand Metric Tons) 245

Table 7-14: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Mineral and Organic Agricultural Soil 

Carbon Stocks (Tg CO2 Eq. and Percent) 246

Table 7-15: Tier 1 Quantitative Uncertainty Estimates for CO2 Emissions from Liming of Agricultural Soils (Tg 

CO2 Eq. and Percent) 247

Table 7-16: Net Changes in Yard Trimming and Food Scrap Stocks (Tg CO2 Eq.) 250

Table 7-17: Net Changes in Yard Trimming and Food Scrap Stocks (Tg C) 250

Table 7-18: Moisture Content (%), Carbon Storage Factor, Initial Carbon Content (%), Proportion of Initial Carbon 

Sequestered (%), and Half-Life (years) for Landfilled Yard Trimmings and Food Scraps 252

Table 7-19: Carbon Stocks in Yard Trimmings and Food Scraps (Tg C) 252

Table 7-20: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Yard Trimmings and Food Scraps (Tg 

CO2 Eq. and Percent) 253

Table 7-21: Net C Flux from Urban Trees (Tg CO2 Eq. and Tg C) 255

Table 7-22: Carbon Stocks (Metric Tons C), Annual Carbon Sequestration (Metric Tons C/yr), Tree Cover 

(Percent), and Annual Carbon Sequestration per Area of Tree Cover (kg C/m2 cover-yr) for Ten U.S. Cities 256

Table 7-23: Tier 1 Quantitative Uncertainty Estimates for Net C Flux from Changes in Carbon Stocks in Urban 

Trees (Tg CO2 Eq. and Percent) 257

Table 7-24: N2O Fluxes from Soils in Settlements Remaining Settlements (Tg CO2 Eq.) 258

Table 7-25: Tier 1 Quantitative Uncertainty Estimates of N2O Emissions from Soils in Settlements Remaining 

Settlements (Tg CO2 Eq. and Percent) 259

Table 8-1: Emissions from Waste (Tg CO2 Eq.) 261

Table 8-2: Emissions from Waste (Gg) 261

Table 8-3: CH4 Emissions from Landfills (Tg CO2 Eq.) 262

Table 8-4: CH4 Emissions from Landfills (Gg) 262

Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills (Tg CO2 Eq. and Percent)

264

Table 8-6: CH4 Emissions from Domestic and Industrial Wastewater Treatment (Tg CO2 Eq.) 266

Table 8-7: CH4 Emissions from Domestic and Industrial Wastewater Treatment (Gg) 266

Table 8-8: U.S. Population (Millions) and Wastewater BOD5 Produced (Gg) 266
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page xiv 

Table 8-9: U.S. Pulp and Paper, Meat and Poultry, and Vegetables, Fruits and Juices Production (Tg) 267

Table 8-10: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Wastewater Treatment (Tg CO2 Eq. 

and Percent) 268

Table 8-11: N2O Emissions from Human Sewage (Tg CO2 Eq. and Gg) 269

Table 8-12: U.S. Population (Millions) and Average Protein Intake [kg/(person.year)] 271

Table 8-13: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Human Sewage (Tg CO2 Eq. and 

Percent) 272

Table 8-14: Emissions of NOx, CO, and NMVOC from Waste (Gg) 272

Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg CO2 Eq.) 278

Table 10-2: Revisions to Net Flux of CO2 to the Atmosphere from Land-Use Change and Forestry (Tg CO2 Eq.)

279

Figures 

Figure ES-1: U.S. Greenhouse Gas Emissions by Gas 3

Figure ES-2: Annual Percent Change in U.S. Greenhouse Gas Emissions 3

Figure ES-3: Cumulative Change in U.S. Greenhouse Gas Emissions Relative to 1990 3

Figure ES-4: 2003 Greenhouse Gas Emissions by Gas 5

Figure ES-5: 2003 Sources of CO2 6

Figure ES-6: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 6

Figure ES-7: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 6

Figure ES-8: 2003 U.S. Sources of CH4 8

Figure ES-9: 2003 U.S. Sources of N2O 8

Figure ES-10: 2003 U.S. Sources of HFCs, PFCs, and SF6 9

Figure ES-11: U.S. Greenhouse Gas Emissions by Chapter/IPCC Sector 9

Figure ES-12: 2003 U.S. Energy Consumption by Energy Source 10

Figure ES-13: Emissions Allocated to Economic Sectors 12

Figure ES-14: Emissions with Electricity Distributed to Economic Sectors 13

Figure ES-15: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product 14

Figure ES-16: 2003 Key Sources – Tier 1 Level Assessment 15

Figure 2-1: U.S. Greenhouse Gas Emissions by Gas 21

Figure 2-2: Annual Percent Change in U.S. Greenhouse Gas Emissions 21

Figure 2-3: Cumulative Change in U.S. Greenhouse Gas Emissions Relative to 1990 21

Figure 2-4: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product 24

Figure 2-5: U.S. Greenhouse Gas Emissions by Chapter/IPCC Sector 28

Figure 2-6: 2003 Energy Sector Greenhouse Gas Sources 28

Figure 2-7: 2003 U.S. Fossil Carbon Flows (Tg CO2 Eq.) 28

Figure 2-8: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 30

Figure 2-9: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 30
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page xv 

Figure 2-10: 2003 Industrial Processes Chapter Greenhouse Gas Sources 33

Figure 2-11: 2003 Agriculture Chapter Greenhouse Gas Sources 37

Figure 2-12: 2003 Waste Sector Greenhouse Gas Sources 40

Figure 2-13: Emissions Allocated to Economic Sectors 42

Figure 2-14: Emissions with Electricity Distributed to Economic Sectors 45

Figure 3-1: 2003 Energy Sector Greenhouse Gas Sources 51

Figure 3-2: 2003 U.S. Fossil Carbon Flows (Tg CO2 Eq.) 51

Figure 3-3: 2003 U.S. Energy Consumption by Energy Source 54

Figure 3-4: U.S. Energy Consumption (Quadrillion Btu) 54

Figure 3-5: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 54

Figure 3-6: Annual Deviations from Normal Heating Degree Days for the United States (1949-2003) 55

Figure 3-7: Annual Deviations from Normal Cooling Degree Days for the United States (1949-2003) 55

Figure 3-8: Aggregate Nuclear and Hydroelectric Power Plant Capacity Factors in the United States (1973-2003) 55

Figure 3-9: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 57

Figure 3-10: Motor Gasoline Retail Prices (Real) 57

Figure 3-11: Motor Vehicle Fuel Efficiency 57

Figure 3-12: Industrial Production Indexes (Index 1997=100) 59

Figure 3-13: Heating Degree Days 60

Figure 3-14: Cooling Degree Days 60

Figure 3-15: Electricity Generation Retail Sales by End-Use Sector 60

Figure 3-16: U.S. Energy Consumption and Energy-Related CO2 Emissions Per Capita and Per Dollar GDP 63

Figure 3-17: Mobile Source CH4 and N2O Emissions 82

Figure 4-1: 2003 Industrial Processes Chapter Greenhouse Gas Sources 123

Figure 6-1: 2003 Agriculture Chapter Greenhouse Gas Emission Sources 195

Figure 6-2: Direct N2O Emissions Pathways from Cropland and Grassland Soils, and Indirect N2O Emissions 

Pathways from All Sources. 213

Figure 7-1: Forest Sector Carbon Pools and Flows. 232

Figure 7-2: Estimates of Net Annual Changes in Carbon Stocks for Major Carbon Pools (Tg C yr-1) 234

Figure 7-3: Average Carbon Density in the Forest Tree Pool in the Conterminous U.S. During 2004. 234

Figure 7-4: Net Annual CO2 Flux, per Hectare, From Mineral Soils Under Agricultural Management, 1990-1992

243

Figure 7-5: Net Annual CO2 Flux, per Hectare, From Mineral Soils Under Agricultural Management, 1993-2003

243

Figure 7-6: Net Annual CO2 Flux, per Hectare, From Organic Soils Under Agricultural Management, 1990-1992

243

Figure 7-7: Net Annual CO2 Flux, per Hectare, From Organic Soils Under Agricultural Management, 1993-2003

243

Figure 8-1: 2003 Waste Chapter Greenhouse Gas Sources 261
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page xvi 

Boxes 

Box ES-1: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data 13

Box 1-1: The IPCC Third Assessment Report and Global Warming Potentials 8

Box 1-2: IPCC Good Practice Guidance 12

Box 2-1: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data 23

Box 2-2: Methodology for Aggregating Emissions by Economic Sector 48

Box 2-3: Sources and Effects of Sulfur Dioxide 50

Box 3-1: Weather and Non-Fossil Energy Effects on CO2 from Fossil Fuel Combustion Trends 55

Box 3-2: Carbon Intensity of U.S. Energy Consumption 61

Box 3-3: Biogenic Emissions and Sinks of Carbon 106

Box 3-4: Formation of CO2 through Atmospheric CH4 Oxidation 120

Box 4-1: Potential Emission Estimates of HFCs, PFCs, and SF6 185
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-1 

Executive Summary 

Central to any study of climate change is the development of an emissions inventory that identifies and quantifies a 

country's primary anthropogenic1 sources and sinks of greenhouse gases. This inventory adheres to both 1) a 

comprehensive and detailed methodology 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...is to achieve...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 2003. 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). The structure of this 

report is consistent with the UNFCCC guidelines for inventory reporting.4 For most source categories, the IPCC 

methodologies were expanded, resulting in a more comprehensive and detailed estimate of emissions. 

ES.1. Background Information 

Naturally occurring greenhouse gases include water vapor, carbon dioxide (CO2), methane (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 are not required to include these 

gases in their national greenhouse gas emission inventories.5 Some other fluorine-containing halogenated 

 

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 <http://unfccc.int>. 

3 Article 4(1)(a) of the United Nations Framework Convention on Climate Change (also identified in Article 12). Subsequent 

decisions by the Conference of the Parties elaborated the role of Annex I Parties in preparing national inventories. See 

<http://unfccc.int>. 

4 See <http://unfccc.int/resource/docs/cop8/08.pdf>. 

5 Emissions estimates of CFCs, HCFCs, halons and other ozone-depleting substances are included in this document for 

informational purposes. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-2 

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 other greenhouse gases, including 

tropospheric and stratospheric ozone. These gases include carbon monoxide (CO), oxides of nitrogen (NOx), and 

non-methane 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. Since the pre-industrial era (i.e., ending about 1750), concentrations of 

these greenhouse gases have increased by 31, 150, and 16 percent, respectively (IPCC 2001). 

Beginning in the 1950s, the use of CFCs and other stratospheric ozone depleting substances (ODSs) 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 ODSs 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 2001). 

Global Warming Potentials 

Gases in the atmosphere can contribute to the greenhouse effect both directly and indirectly. Direct effects occur 

when the gas itself absorbs radiation. Indirect radiative forcing occurs when chemical transformations of the 

substance produce other greenhouse gases, when a gas influences the atmospheric lifetimes of other gases, and/or 

when a gas affects atmospheric processes that alter the radiative balance of the earth (e.g., affect cloud formation or 

albedo).6 The IPCC developed the Global Warming Potential (GWP) concept to compare the ability of each 

greenhouse gas to trap heat in the atmosphere relative to another gas. 

The GWP of a greenhouse gas is defined as the ratio of the time-integrated radiative forcing from the instantaneous 

release of 1 kg of a trace substance relative to that of 1 kg of a reference gas (IPCC 2001). Direct radiative effects 

occur when the gas itself is a greenhouse gas. The reference gas used is CO2, and therefore GWP-weighted 

emissions are measured in teragrams of CO2 equivalent (Tg CO2 Eq.).7 All gases in this Executive Summary are 

presented in units of Tg CO2 Eq. The relationship between gigagrams (Gg) of a gas and Tg CO2 Eq. can be 

expressed as follows: 

( )( ) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = × ×

1,000Gg

Tg Tg CO2 Eq Gg of gas GWP

The UNFCCC reporting guidelines for national inventories were updated in 2002,8 but continue to require the use 

of GWPs from the IPCC Second Assessment Report (SAR). This requirement ensures that current estimates of 

aggregate greenhouse gas emissions for 1990 to 2003 are consistent with estimates developed prior to the 

publication of the IPCC Third Assessment Report (TAR). 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 

 

6 Albedo is a measure of the Earth’s reflectivity; see the Glossary (Annex 6.8) for definition. 

7 Carbon comprises 12/44ths of carbon dioxide by weight. 

8 See <http://unfccc.int/resource/docs/cop8/08.pdf>. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-3 

comparison of emission values using the SAR GWPs versus the TAR GWPs can be found in Chapter 1 and in more 

detail in Annex 6.1. 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 methane 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 agreedupon method to estimate the contribution of gases that are short-lived in the atmosphere, spatially variable, or have 

only indirect effects on radiative forcing (IPCC 1996). 

ES.2. Recent Trends in U.S. Greenhouse Gas Emissions and Sinks 

In 2003, total U.S. greenhouse gas emissions were 6,900.2 Tg CO2 Eq. Overall, total U.S. emissions have risen by 

13 percent from 1990 to 2003, while the U.S. gross domestic product has increased by 46 percent over the same 

period (BEA 2004). Emissions rose slightly from 2002 to 2003, increasing by 0.6 percent (42.2 Tg CO2 Eq.). The 

following factors were primary contributors to this increase: 1) moderate economic growth in 2003, leading to 

increased demand for electricity and fossil fuels, 2) increased natural gas prices, causing some electric power 

producers to switch to burning coal, and 3) a colder winter, which caused an increase in the use of heating fuels, 

primarily in the residential end-use sector. 

Figure ES-1 through Figure ES-3 illustrate the overall trends in total U.S. emissions by gas, annual changes, and 

absolute change since 1990. Table ES-2 provides a detailed summary of U.S. greenhouse gas emissions and sinks 

for 1990 through 2003. 

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 U.S. Greenhouse Gas Emissions Relative to 1990 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-4 

Table ES-2: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 5,009.6 5,580.0 5,607.2 5,678.0 5,858.2 5,744.8 5,796.8 5,841.5 

Fossil Fuel Combustion 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6 

Non-Energy Use of Fuels 108.0 120.3 135.4 141.6 124.7 120.1 118.8 118.0 

Iron and Steel Production 85.4 71.9 67.4 64.4 65.7 58.9 55.1 53.8 

Cement Manufacture 33.3 38.3 39.2 40.0 41.2 41.4 42.9 43.0 

Waste Combustion 10.9 17.8 17.1 17.6 18.0 18.8 18.8 18.8 

Ammonia Production and Urea 

Application 19.3 20.7 21.9 20.6 19.6 16.7 18.6 15.6 

Lime Manufacture 11.2 13.7 13.9 13.5 13.3 12.8 12.3 13.0 

Natural Gas Flaring 5.8 7.9 6.6 6.9 5.8 6.1 6.2 6.0 

Limestone and Dolomite Use 5.5 7.2 7.4 8.1 6.0 5.7 5.9 4.7 

Aluminum Production 6.3 5.6 5.8 5.9 5.7 4.1 4.2 4.2 

Soda Ash Manufacture and 

Consumption 4.1 4.4 4.3 4.2 4.2 4.1 4.1 4.1 

Petrochemical Production 2.2 2.9 3.0 3.1 3.0 2.8 2.9 2.8 

Titanium Dioxide Production 1.3 1.8 1.8 1.9 1.9 1.9 2.0 2.0 

Phosphoric Acid Production 1.5 1.5 1.6 1.5 1.4 1.3 1.3 1.4 

Ferroalloys 2.0 2.0 2.0 2.0 1.7 1.3 1.2 1.4 

Carbon Dioxide Consumption 0.9 0.8 0.9 0.8 1.0 0.8 1.0 1.3 

Land-Use Change and Forestry 

(Sinks)a

 (1,042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

International Bunker Fuelsb

 113.5 109.9 114.6 105.3 101.4 97.9 89.5 84.2 

Biomass Combustionb

 216.7 233.2 217.2 222.3 226.8 200.5 207.2 216.8 

CH4 605.3 579.5 569.1 557.3 554.2 546.8 542.5 545.0 

Landfills 172.2 147.4 138.5 134.0 130.7 126.2 126.8 131.2 

Natural Gas Systems 128.3 133.6 131.8 127.4 132.1 131.8 130.6 125.9 

Enteric Fermentation 117.9 118.3 116.7 116.8 115.6 114.5 114.6 115.0 

Coal Mining 81.9 62.6 62.8 58.9 56.2 55.6 52.4 53.8 

Manure Management 31.2 36.4 38.8 38.8 38.1 38.9 39.3 39.1 

Wastewater Treatment 24.8 31.7 32.6 33.6 34.3 34.7 35.8 36.8 

Petroleum Systems 20.0 18.8 18.5 17.8 17.6 17.4 17.1 17.1 

Rice Cultivation 7.1 7.5 7.9 8.3 7.5 7.6 6.8 6.9 

Stationary Sources 7.8 7.4 6.9 7.1 7.3 6.7 6.4 6.7 

Abandoned Coal Mines 6.1 8.1 7.2 7.3 7.7 6.9 6.4 6.4 

Mobile Sources 4.8 4.0 3.9 3.6 3.4 3.1 2.9 2.7 

Petrochemical Production 1.2 1.6 1.7 1.7 1.7 1.4 1.5 1.5 

Iron and Steel Production 1.3 1.3 1.2 1.2 1.2 1.1 1.0 1.0 

Agricultural Residue Burning 0.7 0.8 0.8 0.8 0.8 0.8 0.7 0.8 

Silicon Carbide Production + + + + + + + + 

International Bunker Fuelsb

 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 

N2O 382.0 396.3 407.8 382.1 401.9 385.8 380.5 376.7 

Agricultural Soil Management 253.0 252.0 267.7 243.4 263.9 257.1 252.6 253.5 

Mobile Sources 43.7 55.2 55.3 54.6 53.2 49.0 45.6 42.1 

Manure Management 16.3 17.3 17.4 17.4 17.8 18.0 17.9 17.5 

Human Sewage 13.0 14.7 15.0 15.4 15.6 15.6 15.7 15.9 

Nitric Acid 17.8 21.2 20.9 20.1 19.6 15.9 17.2 15.8 

Stationary Sources 12.3 13.5 13.4 13.5 14.0 13.5 13.5 13.8 

Settlements Remaining 

Settlements 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0 

Adipic Acid 15.2 10.3 6.0 5.5 6.0 4.9 5.9 6.0 

N2O Product Usage 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8 

Waste Combustion 0.4 0.4 0.3 0.3 0.4 0.4 0.5 0.5 

Agricultural Residue Burning 0.4 0.4 0.5 0.4 0.5 0.5 0.4 0.4 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-5 

Forest Land Remaining Forest 

Land 0.1 0.3 0.4 0.5 0.4 0.4 0.4 0.4 

International Bunker Fuelsb

 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 

HFCs, PFCs, and SF6 91.2 121.7 135.7 134.8 138.9 129.5 138.3 137.0 

Substitution of Ozone Depleting 

Substances 0.4 46.5 56.6 65.8 75.0 83.3 91.5 99.5 

Electrical Transmission and 

Distribution 29.2 21.7 17.1 16.4 15.6 15.4 14.7 14.1 

HCFC-22 Production 35.0 30.0 40.1 30.4 29.8 19.8 19.8 12.3 

Semiconductor Manufacture 2.9 6.3 7.1 7.2 6.3 4.5 4.4 4.3 

Aluminum Production 18.3 11.0 9.1 9.0 9.0 4.0 5.2 3.8 

Magnesium Production and 

Processing 5.4 6.3 5.8 6.0 3.2 2.6 2.6 3.0 

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2 

Net Emissions (Sources and 

Sinks) 

5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2 

+ Does not exceed 0.05 Tg CO2 Eq. a

 Sinks are only included in net emissions total, and are based partially on projected activity data. Parentheses indicate negative 

values (or sequestration). 

b

 Emissions from International Bunker Fuels and Biomass combustion are not included in totals. 

Note: Totals may not sum due to independent rounding. 

Figure ES-4 illustrates the relative contribution of the direct greenhouse gases to total U.S. emissions in 2003. The 

primary greenhouse gas emitted by human activities in the United States was CO2, representing approximately 85 

percent of total greenhouse gas emissions. The largest source of CO2, and of overall greenhouse gas emissions, was 

fossil fuel combustion. Methane emissions, which have steadily declined since 1990, resulted primarily from 

decomposition of wastes in landfills, natural gas systems, and enteric fermentation associated with domestic 

livestock. Agricultural soil management and mobile source fossil fuel combustion were the major sources of N2O 

emissions. The emissions of substitutes for ozone depleting substances and emissions of HFC-23 during the 

production of HCFC-22 were the primary contributors to aggregate HFC emissions. Electrical transmission and 

distribution systems accounted for most SF6 emissions, while PFC emissions resulted from semiconductor 

manufacturing and as a by-product of primary aluminum production. 

Figure ES-4: 2003 Greenhouse Gas Emissions by Gas 

Overall, from 1990 to 2003, total emissions of CO2 increased by 832.0 Tg CO2 Eq. (17 percent), while CH4 and 

N2O emissions decreased by 60.4 Tg CO2 Eq. (10 percent) and 5.2 Tg CO2 Eq. (1 percent), respectively. During the 

same period, aggregate weighted emissions of HFCs, PFCs, and SF6 rose by 45.8 Tg CO2 Eq. (50 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 12 percent of total emissions in 2003. 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, atmospheric concentrations of CO2 have risen about 31 percent (IPCC 

2001), principally due to the combustion of fossil fuels. Within the United States, fuel combustion accounted for 95 

percent of CO2 emissions in 2003. Globally, approximately 24,240 Tg of CO2 were added to the atmosphere 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-6 

through the combustion of fossil fuels in 2000, of which the United States accounted for about 23 percent.9 

Changes in land use and forestry practices can also emit CO2 (e.g., through conversion of forest land to agricultural 

or urban use) or can act as a sink for CO2 (e.g., through net additions to forest biomass). 

Figure ES-5: 2003 Sources of CO2

As the largest source of U.S. greenhouse gas emissions, CO2 from fossil fuel combustion has accounted for a nearly 

constant 80 percent of GWP weighted emissions since 1990. Emissions of CO2 from fossil fuel combustion 

increased at an average annual rate of 1.3 percent from 1990 to 2003. The fundamental factors influencing this 

trend include (1) a generally growing domestic economy over the last 13 years, and (2) significant growth in 

emissions from transportation activities and electricity generation. Between 1990 and 2003, CO2 emissions from 

fossil fuel combustion increased from 4,711.7 Tg CO2 Eq. to 5,551.6 Tg CO2 Eq.⎯an 18 percent total increase over 

the thirteen-year period. Historically, changes in emissions from fossil fuel combustion have been the dominant 

factor affecting U.S. emission trends. 

From 2002 to 2003, these emissions increased by 50.2 Tg CO2 Eq. (1 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 increase in 2003. The 

primary drivers behind this trend were the growing economy and the increase in U.S. housing stock. Use of nuclear 

and renewable fuels remained relatively stable. 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. 

Figure ES-6: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 

Figure ES-7: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 

The four major end-use sectors contributing to CO2 emissions from fossil fuel combustion are industrial, 

transportation, residential, and commercial. Electricity generation also emits CO2, although these emissions are 

produced as they consume fossil fuel to provide electricity to one of the four end-use sectors. For the discussion 

below, electricity generation emissions have been distributed to each end-use sector on the basis of each sector’s 

share of aggregate electricity consumption. This method of distributing emissions assumes that each end-use sector 

consumes electricity that is generated from the national average mix of fuels according to their carbon intensity. In 

reality, sources of electricity vary widely in carbon intensity. By assuming the same carbon intensity for each enduse sector's electricity consumption, for example, emissions attributed to the residential end-use sector may be 

underestimated, while emissions attributed to the industrial end-use sector may be overestimated. 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. 

 

9 Global CO2 emissions from fossil fuel combustion were taken from Marland et al. (2003) 

<http://cdiac.esd.ornl.gov/trends/emis/tre_glob.htm>. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-7 

Table ES-3: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 

End-Use Sector 1990 1997 1998 1999 2000 2001 2002 2003

Transportation 1,449.8 1,606.4 1,636.5 1,693.9 1,741.0 1,723.1 1,755.4 1,770.4

Combustion 1,446.8 1,603.3 1,633.4 1,690.8 1,737.7 1,719.7 1,752.3 1,767.2

Electricity 3.0 3.1 3.1 3.2 3.4 3.4 3.2 3.2

Industrial 1,553.9 1,703.0 1,668.5 1,651.2 1,684.4 1,587.4 1,579.0 1,572.9

Combustion 882.8 963.8 911.6 888.1 905.0 878.2 876.6 858.6

Electricity 671.1 739.2 757.0 763.1 779.4 709.3 702.4 714.3

Residential 924.8 1,040.7 1,044.4 1,063.5 1,124.2 1,116.2 1,145.0 1,168.9

Combustion 339.6 370.6 338.6 359.3 379.1 367.0 371.4 385.1

Electricity 585.3 670.2 705.8 704.2 745.0 749.2 773.6 783.8

Commercial 755.1 876.7 892.9 901.2 959.5 972.7 973.9 983.1

Combustion 224.2 237.2 219.7 222.3 235.2 226.7 230.0 234.0

Electricity 530.9 639.5 673.2 678.9 724.3 745.9 743.9 749.2

U.S. Territories 28.0 36.4 36.3 36.2 35.9 48.6 48.1 56.2

Total 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6

Electricity Generation 1,790.3 2,051.9 2,139.0 2,149.3 2,252.1 2,207.8 2,223.0 2,250.5 

Note: Totals may not sum due to independent rounding. Combustion-related emissions from electricity generation are allocated 

based on aggregate national electricity consumption by each end-use sector. 

Transportation End-Use Sector. Transportation activities (excluding international bunker fuels) accounted for 32 

percent of CO2 emissions from fossil fuel combustion in 2003.10 Virtually all of the energy consumed in this enduse sector came from petroleum products. Over 60 percent of the emissions resulted from gasoline consumption for 

personal vehicle use. The remaining emissions came from other transportation activities, including the combustion 

of diesel fuel in heavy-duty vehicles and jet fuel in aircraft. 

Industrial End-Use Sector. Industrial CO2 emissions, resulting both directly from the combustion of fossil fuels and 

indirectly from the generation of electricity that is consumed by industry, accounted for 28 percent of CO2 from 

fossil fuel combustion in 2003. About half of these emissions resulted from direct fossil fuel combustion to produce 

steam and/or heat for industrial processes. The other half of the 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 2003. Both sectors relied heavily on 

electricity for meeting energy demands, with 67 and 76 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 35 percent of 

U.S. energy from fossil fuels and emitted 41 percent of the CO2 from fossil fuel combustion in 2003. 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 93 percent of all coal consumed for energy in the United States in 2003. Consequently, changes 

in electricity demand have a significant impact on coal consumption and associated CO2 emissions. 

Other significant CO2 trends included the following: 

 

10 If emissions from international bunker fuels are included, the transportation end-use sector accounted for 33 percent of U.S. 

emissions from fossil fuel combustion in 2003. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-8 

● Carbon dioxide emissions from iron and steel production decreased to 53.8 Tg CO2 Eq. in 2003, and have 

declined by 31.7 Tg CO2 Eq. (37 percent) from 1990 through 2003, due to reduced domestic production of pig 

iron, sinter, and coal coke. 

● Carbon dioxide emissions from waste combustion (18.8 Tg CO2 Eq. in 2003) increased by 7.9 Tg CO2 Eq. (72 

percent) from 1990 through 2003, as the volume of plastics and other fossil carbon-containing materials in 

municipal solid waste grew. 

● Net CO2 sequestration from land-use change and forestry decreased by 214.0 Tg CO2 Eq. (21 percent) from 

1990 through 2003. This decline was primarily attributable to forest soils, a result of the slowed rate of forest 

area increases after 1997. 

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 150 percent (IPCC 2001). 

Experts believe that over half of this atmospheric increase was due to emissions from anthropogenic sources, such 

as 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: 2003 U.S. Sources of CH4

Some significant trends in U.S. emissions of CH4 included the following: 

● Landfills are the largest anthropogenic source of CH4 emissions in the United States. In 2003, landfill CH4

emissions were 131.2 Tg CO2 Eq. (approximately 24 percent of total CH4 emissions), which represents a 

decline of 41.1 Tg CO2 Eq., or 24 percent, since 1990. 

● Methane emissions from coal mining declined by 28.1 Tg CO2 Eq. (34 percent) from 1990 to 2003, as a result 

of the mining of less gassy coal from underground mines and the increased use of methane collected from 

degasification systems. 

Nitrous Oxide Emissions 

Nitrous oxide is produced by biological processes that occur in soil and water and by a variety of anthropogenic 

activities in the agricultural, energy-related, industrial, and waste management fields. While total N2O emissions 

are much lower than CO2 emissions, N2O is approximately 300 times more powerful than CO2 at trapping heat in 

the atmosphere. Since 1750, the atmospheric concentration of N2O has risen by approximately 16 percent (IPCC 

2001). The main anthropogenic activities producing N2O in the United States are agricultural soil management, fuel 

combustion in motor vehicles, manure management, nitric acid production, human sewage, and stationary fuel 

combustion (see Figure ES-9). 

Figure ES-9: 2003 U.S. Sources of N2O 

Some significant trends in U.S. emissions of N2O included the following: 

● 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 (253.5 Tg CO2 Eq.). 

● In 2003, N2O emissions from mobile combustion were 42.1 Tg CO2 Eq. (approximately 11 percent of U.S. N2O 

emissions). From 1990 to 2003, N2O emissions from mobile combustion decreased by 4 percent. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-9 

HFC, PFC, and SF6 Emissions 

HFCs and PFCs are families of synthetic chemicals that are being 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: 2003 U.S. Sources of HFCs, PFCs, and SF6

Some significant trends in U.S. HFC, PFC and SF6 emissions included the following: 

● Emissions resulting from the substitution of ozone depleting substances (e.g., CFCs) have been increasing from 

small amounts in 1990 to 99.5 Tg CO2 Eq. in 2003. Emissions from substitutes for ozone depleting substances 

are both the largest and the fastest growing source of HFC, PFC and SF6 emissions. 

● The increase in ODS emissions is offset substantially by decreases in emission of HFCs, PFCs, and SF6 from 

other sources. Emissions from aluminum production decreased by 79 percent (14.5 Tg CO2 Eq.) from 1990 to 

2003, due to both industry emission reduction efforts and lower domestic aluminum production. Emissions 

from the production of HCFC-22 decreased by 65 percent (22.6 Tg CO2 Eq.), due to a steady decline in the 

emission rate of HFC-23 (i.e., the amount of HFC-23 emitted per kilogram of HCFC-22 manufactured) and the 

use of thermal oxidation at some plants to reduce HFC-23 emissions. Emissions from electric power 

transmission and distribution systems decreased by 52 percent (15.1 Tg CO2 Eq.) from 1990 to 2003, primarily 

because of higher purchase prices for SF6 and efforts by industry to reduce emissions. 

ES.3. 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), 

this Inventory of U.S. Greenhouse Gas Emissions and Sinks is segregated into six sector-specific chapters. Figure 

ES-11 and Table ES-4 aggregate emissions and sinks by these chapters. 

Figure ES-11: U.S. Greenhouse Gas Emissions 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 1997 1998 1999 2000 2001 2002 2003

Energy 5,141.7 5,712.8 5,737.7 5,802.6 5,985.3 5,877.3 5,920.7 5,963.4 

Industrial Processes 299.9 327.1 334.9 329.2 332.1 304.7 315.4 308.6 

Solvent and Other Product Use 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8 

Agriculture 426.5 432.8 449.8 425.9 444.1 437.5 432.4 433.3 

Land-Use Change and Forestry (Emissions) 5.6 6.4 6.5 6.6 6.3 6.2 6.4 6.4 

Waste 210.1 193.7 186.0 183.1 180.6 176.5 178.3 183.8 

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2 

Land-Use Change and Forestry (Sinks) (1042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

Net Emissions (Sources and Sinks) 5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-10 

* Sinks are only included in net emissions total, and are based partially on projected activity data. 

Note: Totals may not sum due to independent rounding. 

Note: 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 2003. In 2003, 

approximately 86 percent of the energy consumed in the United States was produced through the combustion of 

fossil fuels. The remaining 14 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 (39 percent and 15 percent of total U.S. emissions, respectively). Overall, emission sources in the Energy 

chapter account for a combined 87 percent of total U.S. greenhouse gas emissions in 2003. 

Figure ES-12: 2003 U.S. Energy Consumption by Energy Source 

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. The processes 

include iron and steel production, cement manufacture, ammonia manufacture 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, aluminum production, petrochemical production, silicon carbide production, nitric acid production, 

and adipic acid production. Additionally, emissions from industrial processes release HFCs, PFCs and SF6. Overall, 

emission sources in the Industrial Process chapter account for 4.5 percent of U.S. greenhouse gas emissions in 2003. 

Solvent and Other Product Use 

The Solvent and Other Product Use chapter contains emissions Greenhouse gas emissions are produced as a byproduct of various solvent and other product uses. In the United States, emissions from 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 2003. 

Agriculture 

The Agricultural chapter contains anthropogenic emissions from agricultural activities (except fuel combustion, 

which is addressed in the Energy chapter). Agricultural activities contribute directly to emissions of greenhouse 

gases through a variety of processes, including the following source categories: enteric fermentation in domestic 

livestock, livestock manure management, rice cultivation, agricultural soil management, and field burning of 

agricultural residues. Methane and N2O were the primary greenhouse gases emitted by agricultural activities. 

Methane emissions from enteric fermentation and manure management represented about 21 percent and 7 percent 

of total CH4 emissions from anthropogenic activities, respectively in 2003. Agricultural soil management activities 

such as fertilizer application and other cropping practices were the largest source of U.S. N2O emissions in 2003, 

accounting for 67 percent. In 2003, emission sources accounted for in the Agricultural chapters were responsible 

for 6.3 percent of total U.S. greenhouse gas emissions. 

Land-Use Change and Forestry 

The Land-Use Change and Forestry chapter contains 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-11 

management of agricultural soils, and the landfilling of yard trimmings and food scraps have resulted in a net uptake 

(sequestration) of carbon in the United States. Forests (including vegetation, soils, and harvested wood) accounted 

for approximately 91 percent of total 2003 sequestration, urban trees accounted for 7 percent, agricultural soils 

(including mineral and organic soils and the application of lime) accounted for 1 percent, and landfilled yard 

trimmings and food scraps accounted for 1 percent of the total sequestration in 2003. 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 soils account for a net carbon sink that is approximately one and a third times larger than the sum of 

emissions from organic soils and liming. The mineral soil carbon sequestration is largely due to 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 2003 resulted in a net carbon sequestration of 828.0 Tg CO2

Eq. (Table ES-5). This represents an offset of approximately 14 percent of total U.S. CO2 emissions, or 12 percent 

of total gross greenhouse gas emissions in 2003. Total land use, land-use change, and forestry net carbon 

sequestration declined by approximately 21 percent between 1990 and 2003. This decline was primarily due to a 

decline in the rate of net carbon accumulation in forest carbon stocks. Annual carbon accumulation in landfilled 

yard trimmings and food scraps also slowed over this period, as did annual carbon accumulation in agricultural 

soils. As described above, the constant rate of carbon accumulation in urban trees is a reflection of limited 

underlying data (i.e., this rate represents an average for 1990 through 1999). 

Land use, land-use change, and forestry activities in 2003 also resulted in emissions of N2O (6.4 Tg CO2 Eq.). Total 

N2O emissions from the application of fertilizers to forests and settlements increased by approximately 14 percent 

between 1990 and 2003. 

Table ES-5: Net CO2 Flux from Land-Use Change and Forestry (Tg CO2 Eq.) 

Sink Category 1990 1997 1998 1999 2000 2001 2002 2003

Forest Land Remaining Forest Land (949.3) (851.0) (805.5) (751.7) (747.9) (750.9) (751.5) (752.7)

Changes in Forest Carbon Stocks (949.3) (851.0) (805.5) (751.7) (747.9) (750.9) (751.5) (752.7)

Cropland Remaining Cropland (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6)

Changes in Agricultural Soil Carbon 

Stocks (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6)

Settlements Remaining Settlements (84.7) (71.6) (71.2) (70.0) (68.9) (68.9) (68.8) (68.7)

Urban Trees (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7)

Landfilled Yard Trimmings and Food 

Scraps (26.0) (12.9) (12.5) (11.4) (10.2) (10.3) (10.2) (10.1)

Total (1,042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding. 

Waste 

The Waste chapter contains emissions from waste management activities (except waste incineration, which is 

addressed in the Energy chapter). Landfills were the largest source of anthropogenic CH4 emissions, accounting for 

24 percent of total U.S. CH4 emissions.11 Wastewater treatment systems are a potentially significant source of N2O 

emissions; however, methodologies are not currently available to develop a complete estimate. Nitrous oxide 

emissions from the treatment of the human sewage component of wastewater were estimated, however, using a 

 

11 Landfills also store carbon, due to incomplete degradation of organic materials such as wood products and yard trimmings, as 

described in the Land-Use Change and Forestry chapter. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-12 

simplified methodology. Overall, in 2003, emission sources accounted for in the Waste chapter generated 2.7 

percent of total U.S. greenhouse gas emissions. 

ES.4. Other Information 

Emissions by Economic Sector 

Throughout this report, emission estimates are grouped into six sectors (i.e., chapters) defined by the IPCC: 

Energy, Industrial Processes, Solvent Use, Agriculture, Land-Use 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, and Agriculture, and U.S. 

Territories. Table ES-6 summarizes emissions from each of these sectors, and Figure ES-13 shows the trend in 

emissions by sector from 1990 to 2003. 

Figure ES-13: Emissions Allocated to Economic Sectors 

Table ES-6: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq.) 

Economic Sector 1990 1997 1998 1999 2000 2001 2002 2003

Electric Power Industry 1,841.8 2,104.6 2,186.8 2,197.3 2,299.0 2,254.9 2,269.7 2,296.2

Transportation 1,506.8 1,693.0 1,728.7 1,790.0 1,839.6 1,819.8 1,851.6 1,864.4

Industry 1,446.1 1,509.1 1,470.6 1,427.9 1,431.8 1,371.0 1,365.7 1,331.4

Agriculture 473.3 492.0 508.4 486.9 495.3 488.6 485.6 486.4

Commercial 435.4 445.2 424.2 426.8 440.7 431.4 440.2 453.5

Residential 350.9 391.0 358.4 379.5 399.7 387.1 391.6 406.1

U.S. Territories 33.8 42.7 42.7 43.9 47.0 54.1 53.6 62.3

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2

Land-Use Change and Forestry 

Sinks (1,042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

Net Emissions (Sources and 

Sinks) 5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2

Note: Totals may not sum. Emissions include CO2, CH4, HFCs, PFCs, and SF6. 

See Table 2-14 for more detailed data. 

Using this categorization, emissions from electricity generation accounted for the largest portion (33 percent) of 

U.S. greenhouse gas emissions in 2003. Transportation activities, in aggregate, accounted for the second largest 

portion (27 percent). Emissions from industry accounted for 19 percent of U.S. greenhouse gas emissions in 2003. 

In contrast to electricity generation and transportation, emissions from industry have declined over the past decade, 

as structural changes have occurred in the U.S. economy (i.e., shifts from a manufacturing based to a service-based 

economy), fuel switching has occurred, and efficiency improvements have been made. The remaining 21 percent of 

U.S. greenhouse gas emissions were contributed by the residential, agriculture, and commercial economic sectors, 

plus emissions from U.S. Territories. Residences accounted for about 6 percent, and primarily consisted of CO2

emissions from fossil fuel combustion. Activities related to agriculture accounted for roughly 7 percent of U.S. 

emissions; these emissions were dominated by N2O emissions from agricultural soils instead of CO2 from fossil fuel 

combustion. The commercial sector accounted for about 7 percent of emissions, while U.S. territories accounted for 

1 percent. 

Carbon dioxide was also emitted and sequestered by a variety of activities related to forest management practices, 

tree planting in urban areas, the management of agricultural soils, and landfilling of yard trimmings. 

Electricity is ultimately consumed in the economic sectors described above. Table ES-7 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-13 

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.12 These source categories include CO2 from 

fossil fuel combustion and the use of limestone and dolomite for flue gas desulfurization, CO2 and N2O from waste 

combustion, 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 2003. Emissions from the residential and commercial sectors also 

increase substantially due to their relatively large share of electricity consumption (e.g., lighting, appliances, etc.). 

Transportation activities remain the second largest contributor to emissions. In all sectors except agriculture, CO2

accounts for more than 75 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 2003. 

Table ES-7: U.S Greenhouse Gas Emissions by Economic Sector and Gas with Electricity-Related Emissions 

Distributed (Tg CO2 Eq.) 

Economic Sector 1990 1997 1998 1999 2000 2001 2002 2003

Industry 2,075.7 2,247.3 2,223.2 2,190.1 2,207.7 2,074.0 2,062.9 2,040.1

Transportation 1,509.9 1,696.1 1,731.8 1,793.2 1,843.0 1,823.2 1,854.8 1,867.6

Commercial 981.6 1,083.8 1,093.5 1,104.9 1,161.8 1,170.6 1,178.5 1,196.8

Residential 953.0 1,060.3 1,060.0 1,082.9 1,141.4 1,129.6 1,159.5 1,183.7

Agriculture 534.1 547.4 568.6 537.3 552.3 555.5 548.8 549.8

U.S. Territories 33.8 42.7 42.7 43.9 47.0 54.1 53.6 62.3

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2

Land-Use Change and Forestry Sinks (1,042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

Net Emissions (Sources and Sinks) 5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2

See Table 2-16 for more detailed data. 

Figure ES-14: Emissions with Electricity Distributed to Economic Sectors 

 [BEGIN BOX] 

Box ES-1: Recent Trends in Various U.S. Greenhouse Gas Emissions-Related Data 

Total emissions can be compared to other economic and social indices to highlight changes over time. These 

comparisons include: 1) emissions per unit of aggregate energy consumption, because energy-related activities are 

the largest sources of emissions; 2) emissions per unit of fossil fuel consumption, because almost all energy-related 

emissions involve the combustion of fossil fuels; 3) emissions per unit of electricity consumption, because the 

electric power industry—utilities and nonutilities combined—was the largest source of U.S. greenhouse gas 

emissions in 2003; 4) emissions per unit of total gross domestic product as a measure of national economic activity; 

or 5) emissions per capita. 

Table ES-8 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 1.0 percent 

since 1990. This rate is 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 more slowly than national population since 1990 (see Figure ES-15). Overall, global atmospheric CO2

 

12 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-14 

concentrations⎯a function of many complex anthropogenic and natural processes⎯are increasing at 0.5 percent 

per year. 

Table ES-8: Recent Trends in Various U.S. Data (Index 1990 = 100) and Global Atmospheric CO2 Concentration 

Variable 1991 1997 1998 1999 2000 2001 2002 2003

Growth 

Ratef

Greenhouse Gas Emissionsa 99 110 110 111 114 112 113 113 1.0%

Energy Consumptionb 100 112 113 114 117 114 116 116 1.2%

Fossil Fuel Consumptionb 99 112 113 114 117 115 116 116 1.2%

Electricity Consumptionb

 102 117 121 124 128 125 129 130 2.1%

GDPc 100 122 127 133 138 139 142 146 3.0%

Populationd 101 109 110 112 113 114 115 116 1.1%

Atmospheric CO2

Concentratione 100 103 104 104 104 105 105 106 0.5% a

 GWP weighted values 

b

 Energy content weighted values (EIA 2004) 

c

 Gross Domestic Product in chained 2000 dollars (BEA 2004) 

d

 (U.S. Census Bureau 2004) 

e

 Mauna Loa Observatory, Hawaii (Keeling and Whorf 2004) 

f

 Average annual growth rate 

Figure ES-15: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product 

[END BOX] 

Ambient Air Pollutant Emissions 

In the United States, CO, NOx, NMVOCs, SO2 are referred to as "ambient air pollutants," and are regulated under 

the Clean Air Act in an effort to protect human health and the environment. These pollutants 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 pollutants 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 ambient air pollutants (EPA 2004).13 Table ES-9 shows that fuel combustion accounts for the 

majority of emissions of these 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-9: Emissions of NOx, CO, NMVOCs, and SO2 (Gg) 

Gas/Activity 1990 1997 1998 1999 2000 2001 2002 2003

NOx 22,860 22,284 21,964 20,530 20,288 19,414 18,850 18,573

Stationary Fossil Fuel Combustion 9,884 9,578 9,419 8,344 8,002 7,667 7,523 7,222

Mobile Fossil Fuel Combustion 12,134 11,768 11,592 11,300 11,395 10,823 10,389 10,418

Oil and Gas Activities 139 130 130 109 111 113 135 124

Waste Combustion 82 140 145 143 114 114 134 121

 

13 NOx and CO emission estimates from field burning of agricultural residues were estimated separately, and therefore not taken 

from EPA (2004). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-15 

Industrial Processes 591 629 637 595 626 656 630 648

Solvent Use 1 3 3 3 3 3 5 4

Agricultural Burning 28 34 35 34 35 35 33 33

Waste 0 3 3 3 2 2 2 2

CO 130,580 101,138 98,984 94,361 92,895 89,329 87,451 85,077

Stationary Fossil Fuel Combustion 4,999 3,927 3,927 5,024 4,340 4,377 4,020 4,454

Mobile Fossil Fuel Combustion 119,482 90,284 87,940 83,484 83,680 79,972 78,574 75,526

Oil and Gas Activities 302 333 332 145 146 147 116 125

Waste Combustion 978 2,668 2,826 2,725 1,670 1,672 1,672 1,674

Industrial Processes 4,124 3,153 3,163 2,156 2,217 2,339 2,308 2,431

Solvent Use 4 1 1 46 46 45 46 65

Agricultural Burning 689 767 789 767 790 770 707 794

Waste 1 5 5 13 8 8 8 8

NMVOCs 20,937 16,994 16,403 15,869 15,228 15,048 14,222 13,939

Stationary Fossil Fuel Combustion 912 1,016 1,016 1,045 1,077 1,080 926 1,007

Mobile Fossil Fuel Combustion 10,933 7,928 7,742 7,586 7,230 6,872 6,560 6,351

Oil and Gas Activities 555 442 440 414 389 400 340 345

Waste Combustion 222 313 326 302 257 258 281 263

Industrial Processes 2,426 2,038 2,047 1,813 1,773 1,769 1,725 1,711

Solvent Use 5,217 5,100 4,671 4,569 4,384 4,547 4,256 4,138

Agricultural Burning NA NA NA NA NA NA NA NA

Waste 673 157 161 140 119 122 133 125

SO2 20,936 17,091 17,189 15,917 14,829 14,452 13,928 14,463

Stationary Fossil Fuel Combustion 18,407 15,104 15,191 13,915 12,848 12,461 11,946 12,477

Mobile Fossil Fuel Combustion 793 659 665 704 632 624 631 634

Oil and Gas Activities 390 312 310 283 286 289 315 293

Waste Combustion 39 29 30 30 29 30 24 28

Industrial Processes 1,306 985 991 984 1,031 1,047 1,009 1,029

Solvent Use 0 1 1 1 1 1 2 2

Agricultural Burning NA NA NA NA NA NA NA NA

Waste 0 1 1 1 1 1 1 1

Source: (EPA 2004) except for estimates from field burning of agricultural residues. 

+ Does not exceed 0.5 Gg 

NA (Not Available) 

Note: Totals may not sum due to independent rounding. 

Figure ES-16: 2003 Key Sources – Tier 1 Level Assessment 

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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page ES-16 

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 and Uncertainty Management in National Greenhouse Gas Inventories (hereafter referred 

to as the IPCC Good Practice Guidance) and require that countries provide single point estimates of uncertainty for 

many 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. 
Figure ES-1: U.S. GHG Emissions by Gas

Figure ES-2: Annual Percent Change in U.S. Greenhouse Gas Emissions

Figure ES-3: Cumulative Change in U.S. Greenhouse Gas Emissions Relative to 1990

6,953 6,752

6,0886,038 6,121 6,251 6,3436,407 6,621 6,678 6,720 6,807 6,858

6,900

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Tg CO2 Eq.

HFCs, PFCs, & SF

Nitrous Oxide

Methane

Carbon Dioxide

6

1.0%

3.3%

0.9% 0.6% 0.5%

3.0%

0.8% 0.6%

1.5%

2.1%

1.4%

-2.1%

-0.8%

-3%

-2%

-1%

0%

1%

2%

3%

4%

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

33

163

255

319

533

589 632 664

865

719

770 812

-50

-100

0

100

200

300

400

500

600

700

800

900

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Tg CO2 Eq.
Figure ES-4: 2003 Greenhouse Gas Emissions by Gas

Figure ES-5: 2003 Sources of CO2

5,551.6

0 10 20 30 40 50 60

Carbon Dioxide Consumption

Ferroalloys

Phosphoric Acid Production

Titanium Dioxide Production

Soda Ash Manufacture and Consumption

Aluminum Production

Limestone and Dolomite Use

Natural Gas Flaring

Lime Manufacture

Ammonia Production and Urea Application

Waste Combustion

Cement Manufacture

Iron and Steel Production

Fossil Fuel Combustion

Tg CO2 Eq

CO2 as a Portion 

of all Emissions

84.7%

CO2

CH4

N2O

84.7%

7.9%

5.5%

HFCs, PFCs & SF6 2.0%
Figure ES-6: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type

Figure ES-7: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion

Note: Electricity generation also includes emissions of less than 1 Tg CO2 Eq. from geothermal-based 

electricity generation.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Residential

Commercial

Industrial

Transportation

U.S. Territories

Tg CO2 Eq

From Electricity

Consumption

From Direct Fossil

Fuel Combustion

0

500

1,000

1,500

2,000

Residential

Commercial

Industrial

Transportation

Electricity

Generation

U.S.

Territories

Tg CO2 Eq

Natural Gas

Petroleum

Coal

Relative Contribution by 

Fuel Type
Figure ES-8: 2003 Sources of CH4

Figure ES-9: 2003 Sources of N2O

0 20 40 60 80 100 120 140

Silicon Carbide Production

Agricultural Residue Burning

Iron and Steel Production

Petrochemical Production

Mobile Sources

Abandoned Coal Mines

Stationary Sources

Rice Cultivation

Petroleum Systems

Wastewater Treatment

Manure Management

Coal Mining

Enteric Fermentation

Natural Gas Systems

Landfills

Tg CO2 Eq.

<0.05

CH4 as a Portion 

of all Emissions

7.9%

<1.0

<1.0

0 10 20 30 40 50 60 70

Waste Combustion

Agricultural Residue Burning

N2O Product Usage

Adipic Acid

Stationary Sources

Nitric Acid

Human Sewage

Manure Management

Mobile Sources

Agricultural Soil Management

Tg CO2 Eq

253.5

N2O as a Portion 

of all Emissions

5.5%
Figure ES-10: 2003 Sources of HFCs, PFCs, and SF6

Figure ES-11: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector

(2,000)

(1,000)

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

19901991199219931994199519961997199819992000200120022003

Tg CO2 Eq.

Energy

Land-Use Change and Forestry (sink)

Industrial Processes

Agriculture

Waste

0 10 20 30 40 50 60 70 80 90 100

Magnesium Production

and Processing

Aluminum Production

Semiconductor

Manufacture

HCFC-22 Production

Electrical Transmission

and Distribution

Substitution of Ozone

Depleting Substances

Tg CO2 Eq

HFCs, PFCs, and SF6 

as a Portion 

of all Emissions

2.0%
Figure ES-12: 2003 U.S. Energy Consumption by Energy Source

39.1%

Petroleum

22.5%

Natural Gas

22.8% Coal

8.0% Nuclear

6.1% Renewable
Figure ES-13: Emissions Allocated to Economic Sectors

Note: Does not include U.S. territories.

Figure ES-14: Emissions with Electricity Distributed to Economic Sectors

Note: Does not include U.S. territories.

0

500

1,000

1,500

2,000

2,500

19901991199219931994199519961997199819992000200120022003

Tg CO2 Eq.

Transportation

Industrial

Residential

Agriculture

Commercial

0

500

1,000

1,500

2,000

2,500

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

(Tg CO2 Eq.)

Transportation

Electricity Generation

Industry

Residential

Agriculture

Commercial
Figure ES-15: U.S. Greenhouse Gas Emissions Per Capita 

and Per Dollar of Gross Domestic Product

Emissions per $GDP

Emissions per capita

Population

Real GDP

70

80

90

100

110

120

130

140

150

19901991199219931994199519961997199819992000200120022003

Index (1990 = 100)
Figure ES-16: 2003 Key Sources - Tier 1 Level Assessment

Note: For a complete discussion of the key source analysis see Annex 1.

0 200 400 600 800 1000 1200 1400 1600 1800 2000

N20 - Mobile Combustion: Road & Other

CO2 - Cement Production

CO2 - Iron and Steel Production

CH4 - Fugitive Emissions from Coal Mining and Handling

CO2 - Mobile Combustion: Marine

Indirect N2O Emissions from Nitrogen Used in Agriculture

Various - Substitutes for Ozone Depleting Substances

CH4 - Enteric Fermentation in Domestic Livestock

CO2 - Non-Energy Use of Fuels

CH4 - Fugitive Emissions from Natural Gas Operations

CH4 - Solid Waste Disposal Sites

Direct N2O Emissions from Agricultural Soils

CO2 - Mobile Combustion: Aviation

CO2 - Stationary Combustion - Oil

CO2 - Stationary Combustion - Gas

CO2 - Mobile Combustion: Road & Other

CO2 - Stationary Combustion - Coal

Tg CO2 Eq.

 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 1 

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 2003. A summary of these estimates is provided in Table 2-3 and Table 2-4 by gas 

and source category in the Trends in Greenhouse Gas Emissions chapter. The emission estimates in these tables are 

presented on both a full molecular mass basis and on a Global Warming Potential (GWP) weighted basis in order to 

show the relative contribution of each gas to global average radiative forcing.1 This report also discusses the 

methods and data used to calculate these emission estimates. 

In June of 1992, the United States signed, and later ratified in October, the United Nations Framework Convention 

on Climate Change (UNFCCC). As stated in Article 2 of the UNFCCC, ‘The ultimate objective of the UNFCCC is 

to achieve...stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent 

dangerous anthropogenic interference with the climate system. Such a level should be achieved within a time-frame 

sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is not threatened 

and to enable economic development to proceed in a sustainable manner.”2,3

Parties to the Convention, by ratifying, “shall develop, periodically update, publish and make available...national 

inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse gases not controlled by 

the Montreal Protocol, using comparable methodologies...”4 The United States views this report as an opportunity 

to fulfill these commitments under the UNFCCC. 

In 1988, preceding the creation of the UNFCCC, the World Meteorological Organization (WMO) and the United 

Nations Environment Programme (UNEP) jointly established the Intergovernmental Panel on Climate Change 

(IPCC). The charter of the IPCC is to assess available scientific information on climate change, assess the 

environmental and socio-economic impacts of climate change, and formulate response strategies (IPCC 1996). 

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, 11-13 September 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, which further expanded upon the methodologies in the Revised 1996 IPCC

Guidelines. The IPCC has also accepted the Good Practice Guidance for Land Use, Land-Use Change and 

Forestry at its Twenty-First Session (Vienna, November 3-7, 2003), as an elaboration of the Revised 1996 

Guidelines.

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 Reporting and Review (UNFCCC 2003). 

 

1 See the section below entitled Global Warming Potentials for an explanation of GWP values. 

2 The term “anthropogenic”, in this context, refers to greenhouse gas emissions and removals that are a direct result of human 

activities or are the result of natural processes that have been affected by human activities (IPCC/UNEP/OECD/IEA 1997). 

3 Article 2 of the Framework Convention on Climate Change published by the UNEP/WMO Information Unit on Climate 

Change. See <http://unfccc.int>. (UNEP/WMO 2000) 

4 Article 4(1)(a) of the United Nations Framework Convention on Climate Change (also identified in Article 12). Subsequent 

decisions by the Conference of the Parties elaborated the role of Annex I Parties in preparing national inventories. See 

<http://unfccc.int>. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 2 

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 1996). Changes in the 

atmospheric concentrations of these greenhouse gases can alter the balance of energy transfers between the 

atmosphere, space, land, and the oceans.5 A gauge of these changes is called radiative forcing, which is a simple 

measure of changes in the energy available to the Earth-atmosphere system (IPCC 1996). Holding everything else 

constant, increases in greenhouse gas concentrations in the atmosphere will produce positive radiative forcing (i.e., 

a net increase in the absorption of energy by the Earth). 

Climate change can be driven by changes in the atmospheric concentrations of a number of radiatively 

active gases and aerosols. We have clear evidence that human activities have affected concentrations, 

distributions and life cycles of these gases (IPCC 1996). 

Naturally occurring greenhouse gases include water vapor, CO2, methane (CH4), nitrous oxide (N2O), and ozone 

(O3). Several classes of halogenated substances that contain fluorine, chlorine, or bromine are also greenhouse 

gases, but they are, for the most part, solely a product of industrial activities. Chlorofluorocarbons (CFCs) and 

hydrochlorofluorocarbons (HCFCs) are halocarbons that contain chlorine, while halocarbons that contain bromine 

are referred to as bromofluorocarbons (i.e., halons). As stratospheric ozone depleting substances, CFCs, HCFCs, 

and halons are covered under the Montreal Protocol on Substances that Deplete the Ozone Layer. The UNFCCC 

defers to this earlier international treaty. Consequently, Parties are not required to include these gases in national 

greenhouse gas inventories.6 Some other fluorine-containing halogenated substances—hydrofluorocarbons (HFCs), 

perfluorocarbons (PFCs), and sulfur hexafluoride (SF6)—do not deplete stratospheric ozone but are potent 

greenhouse gases. These latter substances are addressed by the UNFCCC and accounted for in national greenhouse 

gas inventories. 

There are also several gases that, although they do not have a commonly agreed upon direct radiative forcing effect, 

do influence the global radiation budget. These tropospheric gases include carbon monoxide (CO), nitrogen dioxide 

(NO2), sulfur dioxide (SO2), and tropospheric (ground level) O3. Tropospheric ozone is formed by two precursor 

pollutants, volatile organic compounds (VOCs) and nitrogen oxides (NOx) in the presence of ultraviolet light 

(sunlight). Aerosols are extremely small particles or liquid droplets that are often composed of sulfur compounds, 

carbonaceous combustion products, crustal materials and other human induced pollutants. They can affect the 

absorptive characteristics of the atmosphere. Comparatively, however, the level of scientific understanding of 

aerosols is still very low (IPCC 2001). 

Carbon dioxide, CH4, and N2O are continuously emitted to and removed from the atmosphere by natural processes 

on Earth. Anthropogenic activities, however, can cause additional quantities of these and other greenhouse gases to 

be emitted or sequestered, thereby changing their global average atmospheric concentrations. Natural activities 

such as respiration by plants or animals and seasonal cycles of plant growth and decay are examples of processes 

that only cycle carbon or nitrogen between the atmosphere and organic biomass. Such processes, except when 

directly or indirectly perturbed out of equilibrium by anthropogenic activities, generally do not alter average 

atmospheric greenhouse gas concentrations over decadal timeframes. Climatic changes resulting from 

anthropogenic activities, however, could have positive or negative feedback effects on these natural systems. 

 

5 For more on the science of climate change, see NRC (2001). 

6 Emissions estimates of CFCs, HCFCs, halons and other ozone-depleting substances are included in this document for 

informational purposes. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 3 

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 (ppm unless otherwise specified), rate of concentration change 

(ppb/year) and atmospheric lifetime (years) of selected greenhouse gases 

Atmospheric Variable CO2 CH4 N2O SF6

a

 CF4

a

Pre-industrial atmospheric 

concentration 280 0.722 0.270 0 40 

Atmospheric concentrationb 372.3 1.729-1.843c 0.317-0.318c 4.7-4.8 80 

Rate of concentration changed

 1.5e 0.007e 0.0008 0.24 1.0 

Atmospheric Lifetime 50-200f

 12g 114g 3,200 >50,000 

Source: Current atmospheric concentrations for CO2, CH4, N2O, and SF6 are from Blasing and Jones (2003). All other data is 

from IPCC (2001). 

a

 Concentrations in parts per trillion (ppt) and rate of concentration change in ppt/year. b

 Concentration for SF6 was measured in 2001; concentration for CF4 was measured in 2000. Concentrations for all other gases 

were measured in 2002. 

c The low and high endpoints of the range represent concentrations from Tasmania, a mid-latitude Southern-Hemisphere site, and 

Ireland, a mid-latitude Northern-Hemisphere site, respectively. 

d

 Rate is calculated over the period 1990 to 1999. 

e

 Rate has fluctuated between 0.9 and 2.8 ppm per year for CO2 and between 0 and 0.013 ppm per year for CH4 over the period 

1990 to 1999. 

f

 No single lifetime can be defined for CO2 because of the different rates of uptake by different removal processes. g

 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. Carbon dioxide concentrations in the atmosphere increased from 

approximately 280 parts per million by volume (ppmv) in pre-industrial times to 372.3 ppmv in 2001, a 33 percent 

increase (IPCC 2001 and Blasing and Jones 2004).7,8 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. 

 

7 The pre-industrial period is considered as the time preceding the year 1750 (IPCC 2001). 

8 Carbon dioxide concentrations during the last 1,000 years of the pre-industrial period (i.e., 750-1750), a time of relative climate 

stability, fluctuated by about ±10 ppmv around 280 ppmv (IPCC 2001). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 4 

In its second assessment, the IPCC also stated that “[t]he increased amount of carbon dioxide [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. Methane is primarily produced through anaerobic decomposition of organic matter in biological systems. 

Agricultural processes such as wetland rice cultivation, enteric fermentation in animals, and the decomposition of 

animal wastes emit CH4, as does the decomposition of municipal solid wastes. Methane 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 150 percent since 

pre-industrial times, 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 2001). 

Methane is removed from the atmosphere through a reaction with the hydroxyl radical (OH) and is ultimately 

converted to CO2. Minor removal processes also include reaction with chlorine in the marine boundary layer, a soil 

sink, and stratospheric reactions. Increasing emissions of CH4 reduce the concentration of OH, a feedback that may 

increase the atmospheric lifetime of CH4 (IPCC 2001). 

Nitrous Oxide. Anthropogenic sources of N2O emissions include agricultural soils, especially production of 

nitrogen-fixing crops and forages, the use of synthetic and manure fertilizers, and manure deposition by livestock; 

fossil fuel combustion, especially from mobile combustion; adipic (nylon) and nitric acid production; wastewater 

treatment and waste combustion; and biomass burning. The atmospheric concentration of N2O has increased by 17 

percent since 1750, from a pre-industrial value of about 270 ppb to 314 ppb in 1998, a concentration that has not 

been exceeded during the last thousand years. Nitrous oxide is primarily removed from the atmosphere by the 

photolytic action of sunlight in the stratosphere (IPCC 2001). 

Ozone. Ozone is present in both the upper stratosphere,9 where it shields the Earth from harmful levels of 

ultraviolet radiation, and at lower concentrations in the troposphere,10 where it is the main component of 

anthropogenic photochemical “smog.” During the last two decades, emissions of anthropogenic chlorine and 

bromine-containing halocarbons, such as CFCs, have depleted stratospheric ozone concentrations. This loss of 

ozone in the stratosphere has resulted in negative radiative forcing, representing an indirect effect of anthropogenic 

emissions of chlorine and bromine compounds (IPCC 1996). The depletion of stratospheric ozone and its radiative 

forcing was expected to reach a maximum in about 2000 before starting to recover, with detection of such recovery 

not expected to occur much before 2010 (IPCC 2001). 

The past increase in tropospheric ozone, which is also a greenhouse gas, is estimated to provide the third largest 

increase in direct radiative forcing since the pre-industrial era, behind CO2 and CH4. Tropospheric ozone is 

produced from complex chemical reactions of volatile organic compounds mixing with NOx in the presence of 

sunlight. Ozone, CO, SO2, nitrogen dioxide (NO2) and particulate matter are included in the category referred to as 

“ambient air pollutants” in the United States under the Clean Air Act11 and its subsequent amendments. The 

tropospheric concentrations of ozone and these other pollutants are short-lived and, therefore, spatially variable. 

 

9 The stratosphere is the layer from the troposphere up to roughly 50 kilometers. In the lower regions the temperature is nearly 

constant but in the upper layer the temperature increases rapidly because of sunlight absorption by the ozone layer. The ozonelayer is the part of the stratosphere from 19 kilometers up to 48 kilometers where the concentration of ozone reaches up to 10 

parts per million. 

10 The troposphere is the layer from the ground up to 11 kilometers near the poles and up to 16 kilometers in equatorial regions 

(i.e., the lowest layer of the atmosphere where people live). It contains roughly 80 percent of the mass of all gases in the 

atmosphere and is the site for most weather processes, including most of the water vapor and clouds. 

11 [42 U.S.C § 7408, CAA § 108] 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 5 

Halocarbons, Perfluorocarbons, and Sulfur Hexafluoride. Halocarbons are, for the most part, man-made chemicals 

that have both direct and indirect radiative forcing effects. Halocarbons that contain chlorine (CFCs, HCFCs, 

methyl chloroform, and carbon tetrachloride) and bromine (halons, methyl bromide, and hydrobromofluorocarbons 

(HBFCs)) result in stratospheric ozone depletion and are therefore controlled under the Montreal Protocol on 

Substances that Deplete the Ozone Layer. Although CFCs and HCFCs include potent global warming gases, their 

net radiative forcing effect on the atmosphere is reduced because they cause stratospheric ozone depletion, which 

itself is an 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 512 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 for informational purposes. 

HFCs, PFCs, and SF6 are not ozone depleting substances, and therefore are not covered under the Montreal 

Protocol. They are, however, powerful greenhouse gases. HFCs are primarily used as replacements for ozone 

depleting substances but also emitted as a by-product of the HCFC-22 manufacturing process. Currently, they have 

a small aggregate radiative forcing impact, but it is anticipated that their contribution to overall radiative forcing 

will increase (IPCC 2001). PFCs and SF6 are predominantly emitted from various industrial processes including 

aluminum smelting, semiconductor manufacturing, electric power transmission and distribution, and magnesium 

casting. Currently, the radiative forcing impact of PFCs and SF6 is also small, but they have a significant growth 

rate, extremely long atmospheric lifetimes, and are strong absorbers of infrared radiation, and therefore have the 

potential to influence climate far into the future (IPCC 2001). 

Carbon Monoxide. Carbon monoxide has an indirect radiative forcing effect by elevating concentrations of CH4

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.13 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). Nonmethane 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 

 

12 Article 5 of the Montreal Protocol covers several groups of countries, especially developing countries, with low consumption 

rates of ozone depleting substances. Developing countries with per capita consumption of less than 0.3 kg of certain ozone 

depleting substances (weighted by their ozone depleting potential) receive financial assistance and a grace period of ten 

additional years in the phase-out of ozone depleting substances. 

13 NOx emissions injected higher in the stratosphere, primarily from fuel combustion emissions from high altitude supersonic 

aircraft, can lead to stratospheric ozone depletion. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 6 

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 

carbonaceous14 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.15 Locally, the negative radiative forcing effects of aerosols can offset the positive forcing of 

greenhouse gases (IPCC 1996). “However, the aerosol effects do not cancel the global-scale effects of the much 

longer-lived greenhouse gases, and significant climate changes can still result” (IPCC 1996). 

The IPCC’s Third Assessment Report notes that “the indirect radiative effect of aerosols is now understood to also 

encompass effects on ice and mixed-phase clouds, but the magnitude of any such indirect effect is not known, 

although it is likely to be positive” (IPCC 2001). Additionally, current research suggests that another constituent of 

aerosols, black carbon, may have a positive radiative forcing (Jacobson 2001). The primary anthropogenic emission 

sources of black carbon include diesel exhaust and open biomass burning. 

Global Warming Potentials 

A GWP 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 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 produces 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.)16 

The relationship between gigagrams (Gg) of a gas and Tg CO2 Eq. can be expressed as follows: 

( )( ) ⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛ = × ×

1,000 Gg

Tg Tg CO2 Eq Gg of gas GWP

where, 

 

14 Carbonaceous aerosols are aerosols that are comprised mainly of organic substances and forms of black carbon (or soot) 

(IPCC 2001). 

15 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). 

16 Carbon comprises 12/44ths of carbon dioxide by weight. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 7 

Tg CO2 Eq. = Teragrams of Carbon Dioxide Equivalent 

Gg = Gigagrams (equivalent to a thousand metric tons) 

GWP = Global Warming Potential 

Tg = Teragrams 

GWP values allow for a comparison of the impacts of emissions and reductions of different gases. According to the 

IPCC, GWPs typically have an uncertainty of ±35 percent. The parties to the UNFCCC have also agreed to use 

GWPs based upon a 100-year time horizon although other time horizon values are available. 

Greenhouse gas emissions and removals should be presented on a gas-by-gas basis in units of mass... In 

addition, consistent with decision 2/CP.3, Parties should report aggregate emissions and removals of 

greenhouse gases, expressed in 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.17

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 GWPa

CO2 50-200 1 

CH4

b 12±3 21 

N2O 120 310 

HFC-23 264 11,700 

HFC-32 5.6 650 

HFC-125 32.6 2,800 

HFC-134a 14.6 1,300 

HFC-143a 48.3 3,800 

HFC-152a 1.5 140 

HFC-227ea 36.5 2,900 

HFC-236fa 209 6,300 

HFC-4310mee 17.1 1,300 

CF4 50,000 6,500 

C2F6 10,000 9,200 

C4F10 2,600 7,000 

C6F14 3,200 7,400 

SF6 3,200 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. 

 

17 Framework Convention on Climate Change; <http://unfccc.int/resource/docs/cop8/08.pdf>; 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) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 8 

[Begin Box] 

Box 1-1: The IPCC Third Assessment Report and Global Warming Potentials 

In 2001, the IPCC published its Third Assessment Report (TAR), 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 IPCC’s Second Assessment Report (SAR), and new GWPs have been calculated for an expanded set 

of gases. Since the SAR, the IPCC has applied an improved calculation of CO2 radiative forcing and an improved 

CO2 response function (presented in WMO 1999). The GWPs are drawn from WMO (1999) and the SAR, with 

updates for those cases where significantly different new laboratory or radiative transfer results have been 

published. Additionally, the atmospheric lifetimes of some gases have been recalculated. Because the revised 

radiative forcing of CO2 is about 12 percent lower than that in the SAR, the GWPs of the other gases relative to CO2

tend to be larger, taking into account revisions in lifetimes. In addition, the values for radiative forcing and 

lifetimes have been calculated 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 SAR TAR Change 

CO2 1 1 NC NC 

CH4* 21 23 2 10% 

N2O 310 296 (14) (5%) 

HFC-23 11,700 12,000 300 3% 

HFC-32 650 550 (100) (15%) 

HFC-125 2,800 3,400 600 21% 

HFC-134a 1,300 1,300 NC NC 

HFC-143a 3,800 4,300 500 13% 

HFC-152a 140 120 (20) (14%) 

HFC-227ea 2,900 3,500 600 21% 

HFC-236fa 6,300 9,400 3,100 49% 

HFC-4310mee 1,300 1,500 200 15% 

CF4 6,500 5,700 (800) (12%) 

C2F6 9,200 11,900 2,700 29% 

C4F10 7,000 8,600 1,600 23% 

C6F14 7,400 9,000 1,600 22% 

SF6 23,900 22,200 (1,700) (7%) 

Source: (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 inventories18 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 2003 are consistent and comparable with estimates developed prior to 

the publication of the TAR. For informational purposes, emission estimates that use the updated GWPs are 

presented below and in even more detail in Annex 6.1. Overall, these revisions to GWP values do not have a 

significant effect on U.S. emission trends, as shown in Table 1-4. All estimates provided throughout this report are 

also presented in unweighted units. 

 

18 See <http://unfccc.int/resource/docs/cop8/08.pdf>. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 9 

Table 1-4: Effects on U.S. Greenhouse Gas Emission Trends Using IPCC SAR and TAR GWP Values (Tg CO2 

Eq.) 

Change from 1990 to 2003 Revisions to Annual Estimates

Gas SAR TAR 1990 2003

CO2 832.0 832.0 0 0 

CH4 (60.4) (66.1) 57.7 51.9

N2O (5.2) (5.0) (17.3) (17.0)

HFCs, PFCs, and SF6 45.8 45.5 (2.7) (2.9)

Total 812.1 806.3 37.7 31.9

Percent Change 13.3% 13.2% 0.6% 0.5%

Note: Parentheses indicate negative values. Totals may not sum due to independent rounding. 

Table 1-5 below shows a comparison of total emission estimates by sector using both the IPCC SAR and TAR 

GWP values. For most sectors, the change in emissions was minimal. The effect on emissions from waste was by 

far the greatest (8.3 percent in 2003), due the predominance of CH4 emissions in this sector. Emissions from all 

other sectors were comprised of mainly CO2 or a mix of gases, which moderated the effect of the changes. 

Table 1-5: Comparison of Emissions by Sector using IPCC SAR and TAR GWP Values (Tg CO2 Eq.) 

Sector 1990 1997 1998 1999 2000 2001 2002 2003

Energy 

SAR GWP (Used In Inventory) 5,141.7 5,712.8 5,737.3 5,802.6 5,985.3 5,877.3 5,920.7 5,963.4

TAR GWP 5,162.9 5,732.0 5,756.6 5,820.7 6,003.7 5,895.6 5,938.6 5,981.1

Difference (%) 0.4% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3% 0.3%

Industrial Processes 

SAR GWP (Used In Inventory) 299.9 327.1 334.9 329.2 332.1 304.7 315.4 308.6

TAR GWP 296.0 323.3 332.0 325.7 328.4 301.2 311.8 304.9

Difference (%) (1.3%) (1.1%) (0.9%) (1.1%) (1.1%) (1.1%) (1.1%) (1.2%)

Solvent and Other Product Use 

SAR GWP (Used In Inventory) 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8

TAR GWP 4.1 4.6 4.6 4.6 4.6 4.6 4.6 4.6

Difference (%) (4.5%) (4.5%) (4.5%) (4.5%) (4.5%) (4.5%) (4.5%) (4.5%)

Agriculture 

SAR GWP (Used In Inventory) 426.5 432.8 449.8 425.9 444.1 437.5 432.4 433.3

TAR GWP 429.2 436.1 452.6 429.7 446.8 440.4 435.6 436.4

Difference (%) 0.6% 0.8% 0.6% 0.9% 0.6% 0.7% 0.7% 0.7%

Land-Use Change and Forestry 

SAR GWP (Used In Inventory) (1,036.5) (923.6) (874.5) (819.5) (816.1) (820.7) (820.1) (821.6)

TAR GWP (1,036.7) (923.9) (874.8) (819.8) (816.4) (821.0) (820.4) (821.9)

Difference (%) + + + + + + + +

Waste 

SAR GWP (Used In Inventory) 210.1 193.7 186.0 183.1 180.6 176.5 178.3 183.8

TAR GWP 228.3 210.1 201.6 198.4 195.6 191.1 193.1 199.1

Difference (%) 8.7% 8.5% 8.4% 8.3% 8.3% 8.3% 8.3% 8.3%

Net Emissions (Sources and 

Sinks) 

 

SAR GWP (Used In Inventory) 5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2

TAR GWP 5,083.8 5,782.2 5,872.6 5,959.2 6,162.6 6,012.0 6,063.2 6,104.1

Difference (%) 0.7% 0.6% 0.6% 0.6% 0.5% 0.5% 0.5% 0.5%

NC (No change) 

+ Less than 0.05%. 

Note: Parentheses indicate negative values. Totals may not sum due to independent rounding. 

[End Box] 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 10 

1.2. Institutional Arrangements 

The U.S. Environmental Protection Agency (EPA), in cooperation with other U.S. government agencies, prepares 

the Inventory of U.S. Greenhouse Gas Emissions and Sinks. A wide range of agencies and individuals are involved 

in supplying data to, reviewing, or preparing portions of the U.S. Inventory—including federal and state 

government authorities, research and academic institutions, industry associations, and private consultants. 

Within EPA, the Office of Atmospheric Programs (OAP) is the lead office responsible for the emission calculations 

provided in the Inventory, as well as the completion of the National Inventory Report and the Common Reporting 

Format tables. The Office of Transportation and Air Quality (OTAQ) is also involved in calculating emissions for 

the Inventory. While the U.S. Department of State officially submits the annual Inventory to the UNFCCC, EPA’s 

OAP serves as the focal point for technical questions and comments on the U.S. Inventory. The staff of OAP and 

OTAQ coordinates the annual methodological choice, activity data collection, and emission calculations at the 

individual source category level. Within OAP, an Inventory coordinator compiles the entire Inventory into the 

proper reporting format for submission to the UNFCCC, and is responsible for the collection and consistency of 

cross-cutting issues in the Inventory. 

Several other government agencies contribute to the collection and analysis of the underlying activity data used in 

the Inventory calculations. Formal relationships exist between EPA and other U.S. agencies that provide official 

data for use in the Inventory. The U.S. Department of Energy’s Energy Information Administration provides 

national fuel consumption data and the U.S. Department of Defense provides military fuel consumption and bunker 

fuels. Informal relationships also exist with other U.S. agencies to provide activity data for use in EPA’s emission 

calculations. These include: the U.S. Department of Agriculture, the U.S. Geological Survey, the Federal Highway 

Administration, the Department of Transportation, the Bureau of Transportation Statistics, the Department of 

Commerce, the National Agricultural Statistics Service, and the Federal Aviation Administration. Academic and 

research centers also provide activity data and calculations to EPA, as well as individual companies participating in 

voluntary outreach efforts with EPA. Finally, the U.S. Department of State officially submits the Inventory to the 

UNFCCC each April. 

1.3. Inventory Process 

EPA has a decentralized approach to preparing the annual U.S. Inventory, which consists of a National Inventory 

Report (NIR) and Common Reporting Format (CRF) tables. The Inventory coordinator at EPA is responsible for 

compiling all emission estimates, and ensuring consistency and quality 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 11 

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 is 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 source analysis for the Inventory, consistent with the 

IPCC Good Practice Guidance, IPCC Good Practice Guidance for Land Use, Land Use Change and Forestry, and 

in accordance with the reporting requirements of the UNFCCC. Also at this time, the Introduction, Executive 

Summary, and Recent Trends sections are drafted, to reflect the trends for the most recent year of the current 

Inventory. The analysis of trends necessitates gathering supplemental data, including weather and temperature 

conditions, economic activity and gross domestic product, population, atmospheric conditions, and the annual 

consumption of electricity, energy, and fossil fuels. Changes in these data are used to explain the trends observed in 

greenhouse gas emissions in the United States. Furthermore, specific factors that affect individual sectors are 

researched and discussed. Many of the factors that affect emissions are included in the Inventory document as 

separate analyses or side discussions in boxes within the text. Text boxes are also created to examine the data 

aggregated in different ways than in the remainder of the document, such as a focus on transportation activities or 

emissions from electricity generation. The document is prepared to match the specification of the UNFCCC 

reporting guidelines for National Inventory Reports. 

Common Reporting Format Table Compilation 

The CRF tables are compiled from individual tables completed by each individual source lead, which contain source 

emissions and activity data. The Inventory coordinator integrates the source data into the complete CRF tables 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 tables, 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 coordinator, who has 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). The QA/QC coordinator works closely with the source leads to ensure 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 12 

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 Tables. 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 

general 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). To the extent possible, the present report relies on published activity and emission 

factor data. Depending on the emission source category, activity data can include fuel consumption or deliveries, 

vehicle-miles traveled, raw material processed, etc. Emission factors are factors that relate quantities of emissions 

to an activity. 

The IPCC methodologies provided in the Revised 1996 IPCC Guidelines represent baseline methodologies for a 

variety of source categories, and many of these methodologies continue to be improved and refined as new research 

and data become available. This report uses the IPCC methodologies when applicable, and supplements them with 

other available methodologies and data where possible. Choices made regarding the methodologies and data 

sources used are provided in conjunction with the discussion of each source category in the main body of the report. 

Complete documentation is provided in the annexes on the detailed methodologies and data sources utilized in the 

calculation of each source category. 

[Begin Text Box] 

Box 1-2: IPCC Good Practice Guidance 

In response to a request by Parties in 1998 to the United Nations Framework Convention on Climate Change 

(UNFCCC), the Intergovernmental Panel on Climate Change (IPCC) prepared and published a report on inventory 

good practice. The report, entitled Good Practice Guidance and Uncertainty Management in National Greenhouse 

Gas Inventories (IPCC Good Practice Guidance), was developed with extensive participation of experts from the 

United States and many other countries.19 It focuses on providing direction to countries to produce emission 

estimates that are as accurate and transparent as possible, with the least possible uncertainty. In addition, the IPCC 

Good Practice Guidance was designed as a tool to complement the methodologies suggested in the Revised 1996 

IPCC Guidelines for National Greenhouse Gas Inventories (IPCC Guidelines). 

 

19 See <http://www.ipcc-nggip.iges.or.jp/public/gp/gpgaum.htm> 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 13 

In order to obtain these goals, IPCC Good Practice Guidance gives specific guidance in the following areas: 

• Selection of the most appropriate estimation method, within the context of the IPCC Guidelines 

• Implementation of quality control and quality assurance measures 

• Proper assessment and documentation of data and information 

• Quantification of uncertainties for most source categories 

By providing such direction, the IPCC hopes to help countries provide inventories that are transparent, documented, 

and comparable. 

In addition, the IPCC accepted the Land Use, Land-Use Change, and Forestry (LULUCF) Good Practice Guidance

report in 2003, and the United States has implemented the new guidance in this Inventory submission. 

[End Box] 

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). 

The reference approach assumes that once carbon-based fuels are brought into a national economy, they are either 

saved in some way (e.g., stored in products, kept in fuel stocks, or left unoxidized in ash) or combusted, and 

therefore the carbon in them is oxidized and released into the atmosphere. Accounting for actual consumption of 

fuels at the sectoral or sub-national level is not required. 

1.5. Key Sources 

The IPCC’s Good Practice Guidance (IPCC 2000) defines a key source category as a “[source category] that is 

prioritized within the national inventory system because its estimate has a significant influence on a country’s total 

inventory of direct greenhouse gases in terms of the absolute level of emissions, the trend in emissions, or both.”20 

By definition, key source 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 source categories must also account for the influence of trends of individual source categories. 

Therefore, a trend assessment is conducted to identify source categories for which significant uncertainty in the 

estimate would have considerable effects on overall emission trends. This analysis culls out source categories that 

diverge from the overall trend in national emissions. Finally, a qualitative evaluation of key source 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 source categories for the United States. Using this approach, a number of key source categories were identified 

based on an assessment of their absolute emission level and/or trend in emissions. 

Due to the relative quantity of CO2 emissions from fossil fuel combustion—particularly from mobile combustion in 

road vehicles and stationary combustion of coal, gas, and oil—these sources contributed most to this year’s level 

assessment. Additionally, the following sources were identified as key sources based on the level assessments for 

each year (listed in descending order of their 2003 emissions): 

 

20 See chapter 7 “Methodological Choice and Recalculation” in IPCC (2000). 

< http://www.ipcc-nggip.iges.or.jp/public/gp/gpgaum.htm> 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 14 

• CO2 emissions from mobile combustion in the aviation sector; 

• Direct N2O from agricultural soils; 

• CH4 from solid waste disposal sites; 

• Fugitive emissions from natural gas operations; 

• CO2 from non-energy use of fuel; 

• CH4 from enteric fermentation in domestic livestock; 

• Indirect N2O emissions from nitrogen used in agriculture; 

• Fugitive emissions from coal mining; 

• CO2 emissions from iron and steel production; 

• CO2 emissions from cement production; and 

• N2O emissions from mobile combustion in road vehicles. 

The remaining key sources identified under the level assessment varied by year. The following five source 

categories were determined to be key using the level assessment for only part of the complete time series (listed in 

descending order of their 2003 emissions): 

• HFC and PFC emissions from substitutes for ozone depleting substances (1996-2003); 

• CO2 emissions from mobile combustion in the marine sector (1990-1997, 1999-2003); 

• CH4 emissions from manure management (1990-2000); 

• CH4 emissions from wastewater handling (1995 and 1997); and 

• HFC-23 emissions from HCFC-22 manufacture (1990-1996, 1998). 

Although other sources have fluctuated by greater percentages since 1990, by virtue of their size, CO2 emissions 

from mobile combustion from road vehicles and stationary combustion of coal are the greatest contributors to the 

overall trend for 2003. The third largest contributor to the overall trend in 2003 is emissions from substitutes for 

ozone depleting substances (ODSs). These emissions have grown quickly with the phase out of ODSs under the 

Montreal Protocol. 

Two additional source categories with trends of note are fugitive emissions from coal mining and PFC emissions 

from aluminum manufacturing, which decreased from 1990 through 2003 by approximately 34 and 79 percent, 

respectively. Reductions in emissions from coal mining are primarily due to EPA’s voluntary coalbed methane 

capture program and the mining of less gassy coal than in previous years. PFC emissions have decreased primarily 

as a result of emission reduction activities by the aluminum industry. 

The remaining source categories that were identified as key sources based solely on a trend assessment are listed 

below (listed in descending order of their 2003 emissions). 

• CO2 emissions from waste incineration; 

• Fugitive emissions from oil operations; 

• CO2 emissions from ammonia production and urea application; 

• SF6 emissions from electrical equipment; and 

• N2O emissions from adipic acid production. 

In addition to conducting Tier 1 level and trend assessments, a qualitative assessment of the source categories, as 

described in the IPCC’s Good Practice Guidance (IPCC 2000), was conducted to capture any key sources that were 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 15 

not identified by either quantitative method. One additional key source was identified using this qualitative 

assessment. A brief discussion of the reasoning for the qualitative designation is provided below. 

• 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 source according to the Tier 1 

approach. The amount of uncertainty associated with estimation of emissions from international bunker fuels 

also supports the qualification of this source category as key. 

Table 1-6 presents the key source categories for the United States based on the Tier 1 approach using emissions data 

in this report, and ranked according to their sector and global warming potential-weighted emissions in 2003. The 

table also indicates the criteria used in identifying these source categories (i.e., level, trend, and/or qualitative 

assessments). Please see Annex 1 for additional information regarding the key source categories in the United 

States and the methodologies used to identify them. 

Table 1-6: Key Source Categories for the United States (1990-2003) Based on Tier 1 Approach 

IPCC Source Categories Gas Level Trend Quala

2003 Emissions 

(Tg CO2 Eq.)

Energy 

CO2 Emissions from Stationary Combustion - Coal CO2 ✓ ✓ 2,013.8 

Mobile Combustion: Road & Other CO2 ✓ ✓ 1,538.5 

CO2 Emissions from Stationary Combustion - Gas CO2 ✓ ✓ 1,134.9 

CO2 Emissions from Stationary Combustion - Oil CO2 ✓ ✓ 635.3 

Mobile Combustion: Aviation CO2 ✓ ✓ 171.3 

Fugitive Emissions from Natural Gas Operations CH4 ✓ ✓ 125.9 

CO2 Emissions from Non-Energy Use of Fuels CO2 ✓ 118.0 

International Bunker Fuelsb

 Several ✓ 85.1 

Mobile Combustion: Marine CO2 ✓ 57.5 

Fugitive Emissions from Coal Mining and Handling CH4 ✓ ✓ 53.8 

Mobile Combustion: Road & Other N2O ✓ ✓ 39.9 

Fugitive Emissions from Oil Operations CH4 ✓ 17.1 

Industrial Processes 

Emissions from Substitutes for Ozone Depleting Substances Several ✓ ✓ 99.5 

CO2 Emissions from Iron and Steel Production CO2 ✓ ✓ 53.8 

CO2 Emissions from Cement Production CO2 ✓ ✓ 43.0 

CO2 Emissions from Ammonia Production and Urea 

Application CO2 ✓ 15.6 

SF6 Emissions from Electrical Equipment SF6 ✓ 14.1 

HFC-23 Emissions from HCFC-22 Manufacture HFCs ✓ ✓ 12.3 

N2O Emissions from Adipic Acid Production N2O ✓ 6.0 

PFC Emissions from Aluminum Production PFCs ✓ 3.8 

Agriculture 

Direct N2O Emissions from Agricultural Soils N2O ✓ 155.3 

CH4 Emissions from Enteric Fermentation in Domestic 

Livestock CH4 ✓ ✓ 115.0 

Indirect N2O Emissions from Nitrogen Used in Agriculture N2O ✓ ✓ 98.2 

CH4 Emissions from Manure Management CH4 ✓ 39.1 

Waste 

CH4 Emissions from Solid Waste Disposal Sites CH4 ✓ ✓ 131.2 

CH4 Emissions from Wastewater Handling CH4 ✓ ✓ 36.8 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 16 

CO2 Emissions from Waste Incineration CO2 ✓ 18.8 

Subtotal of Key Source Emissions 6,833.5 

Total Emissions 6,900.2

Percent of Total 99.0% a Qualitative criteria. 

b Emissions from these sources not included in totals. 

Note: Sinks (e.g., LUCF, Landfill Carbon Storage) are not included in this analysis. The Tier 1 approach for identifying key 

source categories does not directly include an assessment of uncertainty in emission estimates. 

1.6. Quality Assurance and Quality Control 

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: 

• The plan includes specific detailed procedures (or protocols) and templates (or 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. 

• The plan includes 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. 

• The QC process includes both Tier 1 (general) and Tier 2 (source-specific) quality controls and checks, as 

recommended by IPCC Good Practice Guidance. 

• Investigations of secondary data quality and source-specific quality checks (Tier 2 QC) are considered 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. 

• The plan contains record-keeping provisions to track what 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. 

• The plan is designed so that QA/QC procedures are implemented throughout the whole inventory development 

process—from initial data collection, through preparation of the emission estimates, to publication of the 

Inventory. 

• The plan includes a schedule for multi-year implementation. 

• The plan promotes and involves 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. For example, the availability of new information or additional detail on 

techniques or procedures for checking the quality of data inputs or emission calculations could necessitate 

revising the procedures in the Procedures Manual or preparing a background paper expanding on procedures to 

be used. 

The quality checking and 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 17 

addition, quality assurance occurs at two stages—an expert review and a public review. While both phases can 

significantly contribute to inventory quality, the public review phase is also essential for promoting the openness of 

the inventory development process and the transparency of the inventory data and methods. 

QA/QC procedures guide 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. 

In addition, based on the national QA/QC plan for the Inventory, source-specific QA/QC plans have been developed 

for a limited 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 the current Inventory, sourcespecific plans have been developed and implemented for the majority of sources within the Energy and Industrial 

Process sectors. 

Throughout 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. 

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 Revised 1996 

IPCC Guidelines for National Greenhouse Gas Inventories (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 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 IPCC provides good practice guidance on two approaches—Tier 1 and Tier 2—to estimating uncertainty for 

individual source categories. The Tier 1 method is a spreadsheet-based analysis that estimates uncertainties by 

using the error propagation equation. The spreadsheet employs uncertainty ranges for activity data and emission 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 18 

factors consistent with the sectoral good practice guidance. The Tier 2 uncertainty estimation methodology employs 

the Monte Carlo Stochastic Simulation technique. The principle of Monte Carlo analysis is to select random values 

of emission factor and activity data from within their individual probability density functions, and to calculate the 

corresponding emission values. Tier 2 uncertainty analysis was applied wherever data and resources permitted. 

Consistent with the Good Practice Guidance, over a multi-year timeframe, the United States expects to continue to 

improve the uncertainty estimates presented in this report and add quantitative estimates of uncertainty where none 

currently exist. See Annex 7, Uncertainty, 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. 

While there are two types of estimation uncertainty, parameter uncertainty and model uncertainty, the Tier 1 and 

Tier 2 approaches were applied only to estimate parameter uncertainty of emission estimates. Parameter uncertainty 

refers to the uncertainty associated with quantifying the parameters used as inputs (e.g., activity data and emission 

factors) to the emission estimation models. Model uncertainty refers to the uncertainty associated with developing 

mathematical equations or models to characterize the emission and/or removal processes. Model uncertainties can 

be evaluated by comparing the model results with the results of other models that are developed to characterize the 

same emission generation process and through sensitivity analysis. Model uncertainties for some sources are 

identified, but not evaluated. 

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. 

1.8. Completeness 

This report, along with its accompanying CRF tables, 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 2003. 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. 

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 1-7. 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 1-7: 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 usage. 

Agriculture Anthropogenic emissions from agricultural activities except fuel 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 19 

combustion, which is addressed under Energy. 

Land-Use Change and Forestry Emissions and removals of CO2 from forest management, other landuse activities, and land-use change. 

Waste Emissions from waste management activities. 

Source: (IPCC/UNEP/OECD/IEA 1997) 

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: 

Chapter/IPCC Sector: Overview of emission trends for each IPCC defined sector 

Source category: Description of source pathway and emission trends. 

Methodology: Description of analytical methods employed to produce emission estimates and identification of 

data references, primarily for activity data and emission factors. 

Uncertainty: A discussion and quantification of the uncertainty in emission estimates and a discussion of timeseries 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 necessitating 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 relative to other sources 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 1-8. 

Table 1-8: List of Annexes 

ANNEX 1 Key Source 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 Stored in Products 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 Ambient Air Pollutants from Stationary Combustion 

3.2. Methodology for Estimating Emissions of CH4, N2O, and Ambient Air Pollutants 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 Emissions from Petroleum Systems 

3.6. Methodology for Estimating CO2 and N2O Emissions from Municipal Solid Waste Combustion 

3.7. Methodology for Estimating Emissions from International Bunker Fuels used by the U.S. Military 

3.8. Methodology for Estimating HFC and PFC Emissions from Substitution of Ozone Depleting Substances 

3.9. Methodology for Estimating 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 Carbon Stocks 

3.13. Methodology for Estimating Net Changes in Carbon Stocks in Mineral and Organic Soils 

3.14. Methodology for Estimating CH4 Emissions from Landfills 

ANNEX 4 IPCC Reference Approach for Estimating CO2 Emissions from Fossil Fuel Combustion 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 20 

ANNEX 5 Assessment of the Sources and Sinks of Greenhouse Gas Emissions Excluded 

ANNEX 6 Additional Information 

6.1. Global Warming Potential Values 

6.2. Ozone Depleting Substance Emissions 

6.3. Sulfur Dioxide Emissions 

6.4. Complete List of Source Categories 

6.5. Constants, Units, and Conversions 

6.6. Abbreviations 

6.7. Chemical Formulas 

6.8. Glossary 

ANNEX 7 Uncertainty 

7.1. Overview 

7.2. Methodology and Results 

7.3. Uncertainty Estimation as a Process 

7.4. Planned Improvements 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 21 

2. Trends in Greenhouse Gas Emissions 

2.1. Recent Trends in U.S. Greenhouse Gas Emissions 

In 2003, total U.S. greenhouse gas emissions were 6,900.2 teragrams of carbon dioxide equivalent (Tg CO2 Eq.)1

(13.3 percent above 1990 emissions). Emissions rose slightly from 2002 to 2003, increasing by 0.6 percent (42.2 

Tg CO2 Eq.). The following factors were primary contributors to this increase: 1) moderate economic growth in 

2003, leading to increased demand for electricity and fossil fuels, 2) increased natural gas prices, causing some 

electric power producers to switch to burning coal, and 3) a colder winter, which caused an increase in the use of 

heating fuels, primarily in the residential sector. (See the following section for an analysis of emission trends by 

general economic sectors.) Figure 2-1 through Figure 2-3 illustrate the overall trends in total U.S. emissions by gas, 

annual changes, and absolute changes since 1990. 

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 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 a nearly constant 80 percent of global warming potential (GWP) weighted emissions since 1990. 

Emissions from this source category grew by 17.8 percent (839.8 Tg CO2 Eq.) from 1990 to 2003 and were 

responsible for most of the increase in national emissions during this period. From 2002 to 2003, these emissions 

increased by 50.2 Tg CO2 Eq. (0.9 percent), the same rate as the source's average annual growth rate of 1.3 percent 

from 1990 through 2003. 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. 

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). 

 

1 Estimates are presented in units of teragrams of carbon dioxide equivalent (Tg CO2 Eq.), which weight each gas by its Global 

Warming Potential, or GWP, value. (See section on Global Warming Potentials, Chapter 1.) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 22 

Energy-related CO2 emissions also depend on the type of fuel or energy consumed and its carbon intensity. 

Producing a unit of heat or electricity using natural gas instead of coal, for example, can reduce the CO2 because of 

the lower carbon content of natural gas. Table 2-1 shows annual changes in emissions during the last six years for 

coal, petroleum, and natural gas in selected sectors. 

Table 2-1: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors (Tg CO2

Eq. and Percent) 

Sector Fuel Type 1999 to 2000 2000 to 2001 2001 to 2002 2002 to 2003 

Electricity Generation Coal 87.6 5% -62.6 -3% 22.2 1% 36.6 2% 

Electricity Generation Natural Gas 20.8 8% 8.4 3% 16.5 6% -28.0 -9% 

Electricity Generation Petroleum -5.6 -6% 9.8 11% -23.5 -23% 18.9 24% 

Transportationa

 Petroleum 47.0 3% -16.4 -1% 29.4 2% 16.6 1% 

Residential Natural Gas 13.9 5% -10.7 -4% 6.2 2% 11.5 4% 

Commercial Natural Gas 7.1 4% -7.9 -5% 4.2 3% 2.0 1% 

Industrial Coal 1.1 1% -4.4 -3% -7.9 -6% 0.8 1% 

Industrial Natural Gas 8.2 2% -38.5 -8% 9.5 2% -27.6 -6% 

All Sectorsb

 All Fuelsb 199.2 4% -97.1 -2% 53.5 1% 50.2 1% a

 Excludes emissions from International Bunker Fuels. b

 Includes fuels and sectors not shown in table. 

In 1999, the increase in emissions from fossil fuel combustion was driven largely by growth in petroleum 

consumption for transportation. In addition, residential and commercial heating fuel demand partially recovered as 

winter temperatures dropped relative to 1998, although temperatures were still warmer than normal.2 These 

increases were offset, in part, by a decline in emissions from electric power producers due primarily to: 1) an 

increase in net generation of electricity by nuclear plants which reduced demand from fossil fuel plants; and 2) 

moderated summer temperatures compared to the previous year—thereby reducing electricity demand for air 

conditioning. 

Emissions from fuel combustion increased considerably in 2000, due to several factors. The primary reason for the 

increase was the robust U.S. economy, which produced a high demand for fuels—especially for petroleum in the 

transportation sector—despite increases in the price of both natural gas and petroleum. Colder winter conditions 

relative to the previous year triggered a rise in residential and commercial demand for heating. Additionally, 

electricity generation became more carbon intensive as coal and natural gas consumption offset reduced hydropower 

output. 

In 2001, economic growth in the United States slowed considerably for the second time since 1990, contributing to 

a decrease in CO2 emissions from fossil fuel combustion, also for the second time since 1990. A significant 

reduction in industrial output contributed to weak economic growth, primarily in manufacturing, and led to lower 

emissions from the industrial sector. Several other factors also played a role in this decrease in emissions. Warmer 

winter conditions compared to 2000, along with higher natural gas prices, reduced demand for heating fuels. 

Additionally, nuclear facilities operated at a very high capacity, offsetting electricity produced from fossil fuels. 

Since there are no greenhouse gas emissions associated with electricity production from nuclear plants, this 

substitution reduces the overall carbon intensity of electricity generation. 

Emissions from fuel combustion resumed a modest growth in 2002, slightly less than the average annual growth rate 

since 1990. There were a number of reasons behind this increase. The U.S. economy experienced moderate 

growth, recovering from weak conditions in 2001. Prices for fuels remained at or below 2001 levels; the cost of 

natural gas, motor gasoline, and electricity were all lower–triggering an increase in demand for fuel. In addition, the 

United States experienced one of the hottest summers on record, causing a significant increase in electricity use in 

 

2 Normals are based on data from 1971 through 2000. Source: EIA (2004b) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 23 

the residential sector as the use of air-conditioners increased. Partially offsetting this increased consumption of 

fossil fuels, however, were increases in the use of nuclear and renewable fuels. Nuclear facilities operated at the 

highest capacity on record in 2002. Furthermore, there was a considerable increase in the use of hydroelectric 

power in 2002 after a very low output the previous year. 

Emissions from fuel combustion continued growing in 2003, at about the average annual growth rate since 1990. A 

number of factors played a major role in the magnitude of this increase. The U.S. economy experienced moderate 

growth from 2002, causing an increase in the demand for fuels. The price of natural gas escalated dramatically, 

causing some electric power producers to switch to coal, which remained at relatively stable prices. Colder winter 

conditions brought on more demand for heating fuels, primarily in the residential sector. Though a cooler summer 

partially offset demand for electricity as the use of air-conditioners decreased, electricity consumption continued to 

increase in 2003. The primary drivers behind this trend were the growing economy and the increase in U.S. housing 

stock. Use of nuclear and renewable fuels remained relatively stable. Nuclear capacity decreased slightly, and for 

the first time since 1997. Use of renewable fuels rose slightly due to increases in the use of hydroelectric power and 

biofuels. 

Other significant trends in emissions from additional source categories over the fourteen-year period from 1990 

through 2003 included the following: 

● Carbon dioxide emissions from waste combustion increased by 7.9 Tg CO2 Eq. (72 percent), as the volume of 

plastics and other fossil carbon-containing materials in municipal solid waste grew. 

● Net CO2 sequestration from land use change and forestry decreased by 214.0 Tg CO2 Eq. (21 percent), 

primarily due to a decline in the rate of net carbon accumulation in forest carbon stocks. This decline largely 

resulted from a decrease in the estimated rate of forest soil sequestration caused by a slowing rate of increase in 

forest area after 1997. 

● Methane (CH4) emissions from coal mining declined by 28.1 Tg CO2 Eq. (34 percent) from 1990 to 2003, as a 

result of the mining of less gassy coal from underground mines and the increased use of methane collected from 

degasification systems. 

● The increase in ODS emissions is offset substantially by decreases in emission of HFCs, PFCs, and SF6 from 

other sources. Emissions from aluminum production decreased by 79 percent (14.5 Tg CO2 Eq.) from 1990 to 

2003, due to both industry emission reduction efforts and lower domestic aluminum production. Emissions 

from the production of HCFC-22 decreased by 65 percent (22.6 Tg CO2 Eq.) from 1990 to 2003, due to a 

steady decline in the emission rate of HFC-23 (i.e., the amount of HFC-23 emitted per kilogram of HCFC-22 

manufactured) and the use of thermal oxidation at some plants to reduce HFC-23 emissions. Emissions from 

electric power transmission and distribution systems decreased by 52 percent (15.1 Tg CO2 Eq.) from 1990 to 

2003, primarily because of higher purchase prices for SF6 and efforts by industry to reduce emissions. 

Overall, from 1990 to 2003, total emissions of CO2 increased by 832.0 Tg CO2 Eq. (17 percent), while CH4 and 

N2O emissions decreased by 60.4 Tg CO2 Eq. (10 percent) and 5.2 Tg CO2 Eq. (just over 1 percent), respectively. 

During the same period, aggregate weighted emissions of HFCs, PFCs, and SF6 rose by 45.8 Tg CO2 Eq. (50 

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, 

which was estimated to be 12 percent of total emissions in 2003. 

[BEGIN BOX] 

Box 2-1: 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 24 

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 2003; 4) emissions per unit of total gross domestic product as a measure of national economic activity; 

or 5) emissions per capita. 

Table 2-2 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 1.0 percent 

since 1990. This rate is 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 more slowly than national population since 1990 (see Figure 2-4). Overall, global atmospheric CO2

concentrations⎯a function of many complex anthropogenic and natural processes⎯are increasing at 0.5 percent 

per year. 

Table 2-2: Recent Trends in Various U.S. Data (Index 1990 = 100) and Global Atmospheric CO2 Concentration 

Variable 1991 1997 1998 1999 2000 2001 2002 2003

Growth 

Ratef

Greenhouse Gas Emissionsa 99 110 110 111 114 112 113 113 1.0%

Energy Consumptionb 100 112 113 114 117 114 116 116 1.2%

Fossil Fuel Consumptionb 99 112 113 114 117 115 116 116 1.2%

Electricity Consumptionb

 102 117 121 124 128 125 129 130 2.1%

GDPc 100 122 127 133 138 139 142 146 3.0%

Populationd 101 109 110 112 113 114 115 116 1.1%

Atmospheric CO2 Concentratione 100 103 104 104 104 105 105 106 0.5% a

 GWP weighted values 

b

 Energy content weighted values (EIA 2004a) 

c

 Gross Domestic Product in chained 2000 dollars (BEA 2004) 

d

 (U.S. Census Bureau 2004) 

e

 Mauna Loa Observatory, Hawaii (Keeling and Whorf 2004) 

f

 Average annual growth rate 

Figure 2-4: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product 

Source: BEA (2004), U.S. Census Bureau (2004), and emission estimates in this report. 

[END BOX] 

As an alternative, emissions of all gases can be totaled for each of the IPCC sectors. Over the thirteen year period 

of 1990 to 2003, total emissions in the Energy, Industrial Processes, Agriculture, and Solvent and Other Product 

Use sectors climbed by 821.6Tg CO2 Eq. (16 percent), 8.7 Tg CO2 Eq. (3 percent), 6.8 Tg CO2 Eq. (2 percent), and 

0.5 Tg CO2 Eq. (11 percent), respectively, while emissions from the Waste sector decreased 26.3 Tg CO2 Eq. (13 

percent). Over the same period, estimates of net carbon sequestration in the Land-Use Change and Forestry sector 

declined by 214.8 Tg CO2 Eq. (21 percent). 

Table 2-3 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 2-4. Alternatively, emissions 

and sinks are aggregated by chapter in Table 2-5 and Figure 2-5. 

Table 2-3: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 5,009.6 5,580.0 5,607.2 5,678.0 5,858.2 5,744.8 5,796.8 5,841.5 

Fossil Fuel Combustion 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6 

Non-Energy Use of Fuels 108.0 120.3 135.4 141.6 124.7 120.1 118.8 118.0 

Iron and Steel Production 85.4 71.9 67.4 64.4 65.7 58.9 55.1 53.8 

Cement Manufacture 33.3 38.3 39.2 40.0 41.2 41.4 42.9 43.0 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 25 

Waste Combustion 10.9 17.8 17.1 17.6 18.0 18.8 18.8 18.8 

Ammonia Production and Urea 

Application 19.3 20.7 21.9 20.6 19.6 16.7 18.6 15.6 

Lime Manufacture 11.2 13.7 13.9 13.5 13.3 12.8 12.3 13.0 

Natural Gas Flaring 5.8 7.9 6.6 6.9 5.8 6.1 6.2 6.0 

Limestone and Dolomite Use 5.5 7.2 7.4 8.1 6.0 5.7 5.9 4.7 

Aluminum Production 6.3 5.6 5.8 5.9 5.7 4.1 4.2 4.2 

Soda Ash Manufacture and 

Consumption 4.1 4.4 4.3 4.2 4.2 4.1 4.1 4.1 

Petrochemical Production 2.2 2.9 3.0 3.1 3.0 2.8 2.9 2.8 

Titanium Dioxide Production 1.3 1.8 1.8 1.9 1.9 1.9 2.0 2.0 

Phosphoric Acid Production 1.5 1.5 1.6 1.5 1.4 1.3 1.3 1.4 

Ferroalloys 2.0 2.0 2.0 2.0 1.7 1.3 1.2 1.4 

Carbon Dioxide Consumption 0.9 0.8 0.9 0.8 1.0 0.8 1.0 1.3 

Land-Use Change and Forestry 

(Sinks)a

 (1,042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

International Bunker Fuelsb

 113.5 109.9 114.6 105.3 101.4 97.9 89.5 84.2 

Biomass Combustionb

 216.7 233.2 217.2 222.3 226.8 200.5 207.2 216.8 

CH4 605.3 579.5 569.1 557.3 554.2 546.8 542.5 545.0 

Landfills 172.2 147.4 138.5 134.0 130.7 126.2 126.8 131.2 

Natural Gas Systems 128.3 133.6 131.8 127.4 132.1 131.8 130.6 125.9 

Enteric Fermentation 117.9 118.3 116.7 116.8 115.6 114.5 114.6 115.0 

Coal Mining 81.9 62.6 62.8 58.9 56.2 55.6 52.4 53.8 

Manure Management 31.2 36.4 38.8 38.8 38.1 38.9 39.3 39.1 

Wastewater Treatment 24.8 31.7 32.6 33.6 34.3 34.7 35.8 36.8 

Petroleum Systems 20.0 18.8 18.5 17.8 17.6 17.4 17.1 17.1 

Rice Cultivation 7.1 7.5 7.9 8.3 7.5 7.6 6.8 6.9 

Stationary Sources 7.8 7.4 6.9 7.1 7.3 6.7 6.4 6.7 

Abandoned Coal Mines 6.1 8.1 7.2 7.3 7.7 6.9 6.4 6.4 

Mobile Sources 4.8 4.0 3.9 3.6 3.4 3.1 2.9 2.7 

Petrochemical Production 1.2 1.6 1.7 1.7 1.7 1.4 1.5 1.5 

Iron and Steel Production 1.3 1.3 1.2 1.2 1.2 1.1 1.0 1.0 

Agricultural Residue Burning 0.7 0.8 0.8 0.8 0.8 0.8 0.7 0.8 

Silicon Carbide Production + + + + + + + + 

International Bunker Fuelsb

 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 

N2O 382.0 396.3 407.8 382.1 401.9 385.8 380.5 376.7 

Agricultural Soil Management 253.0 252.0 267.7 243.4 263.9 257.1 252.6 253.5 

Mobile Sources 43.7 55.2 55.3 54.6 53.2 49.0 45.6 42.1 

Manure Management 16.3 17.3 17.4 17.4 17.8 18.0 17.9 17.5 

Human Sewage 13.0 14.7 15.0 15.4 15.6 15.6 15.7 15.9 

Nitric Acid 17.8 21.2 20.9 20.1 19.6 15.9 17.2 15.8 

Stationary Sources 12.3 13.5 13.4 13.5 14.0 13.5 13.5 13.8 

Settlements Remaining 

Settlements 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0 

Adipic Acid 15.2 10.3 6.0 5.5 6.0 4.9 5.9 6.0 

N2O Product Usage 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8 

Waste Combustion 0.4 0.4 0.3 0.3 0.4 0.4 0.5 0.5 

Agricultural Residue Burning 0.4 0.4 0.5 0.4 0.5 0.5 0.4 0.4 

Forest Land Remaining Forest 

Land 0.1 0.3 0.4 0.5 0.4 0.4 0.4 0.4 

International Bunker Fuelsb

 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 

HFCs, PFCs, and SF6 91.2 121.7 135.7 134.8 138.9 129.5 138.3 137.0 

Substitution of Ozone Depleting 

Substances 0.4 46.5 56.6 65.8 75.0 83.3 91.5 99.5 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 26 

Electrical Transmission and 

Distribution 29.2 21.7 17.1 16.4 15.6 15.4 14.7 14.1 

HCFC-22 Production 35.0 30.0 40.1 30.4 29.8 19.8 19.8 12.3 

Semiconductor Manufacture 2.9 6.3 7.1 7.2 6.3 4.5 4.4 4.3 

Aluminum Production 18.3 11.0 9.1 9.0 9.0 4.0 5.2 3.8 

Magnesium Production and 

Processing 5.4 6.3 5.8 6.0 3.2 2.6 2.6 3.0 

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2 

Net Emissions (Sources and 

Sinks) 

5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2 

+ Does not exceed 0.05 Tg CO2 Eq. a

 Sinks are only included in net emissions total, and are based partially on projected activity data. Parentheses indicate negative 

values (or sequestration). 

b

 Emissions from International Bunker Fuels and Biomass Combustion are not included in totals. 

Note: Totals may not sum due to independent rounding. 

Table 2-4: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 5,009,552 5,579,984 5,607,159 5,677,970 5,858,201 5,744,782 5,796,757 5,841,504 

Fossil Fuel Combustion 4,711,741 5,263,164 5,278,721 5,345,904 5,545,083 5,447,969 5,501,427 5,551,580 

Non-Energy Use of 

Fuels 107,965 120,301 135,352 141,583 124,714 120,104 118,811 118,001 

Iron and Steel 

Production 85,413 71,863 67,428 64,376 65,693 58,887 55,082 53,763 

Cement Manufacture 33,278 38,323 39,218 39,991 41,190 41,357 42,898 43,030 

Waste Combustion 10,919 17,761 17,094 17,632 17,979 18,781 18,781 18,781 

Ammonia Production 

and Urea Application 19,306 20,650 21,934 20,615 19,616 16,719 18,571 15,560 

Lime Manufacture 11,238 13,685 13,914 13,466 13,315 12,823 12,304 12,983 

Natural Gas Flaring 5,805 7,874 6,566 6,943 5,769 6,094 6,233 5,970 

Limestone and Dolomite 

Use 5,533 7,242 7,449 8,057 5,959 5,733 5,885 4,720 

Aluminum Production 6,315 5,621 5,792 5,895 5,723 4,114 4,220 4,219 

Soda Ash Manufacture 

and Consumption 4,141 4,354 4,325 4,217 4,181 4,147 4,139 4,082 

Petrochemical 

Production 2,221 2,919 3,015 3,054 3,004 2,787 2,857 2,777 

Titanium Dioxide 

Production 1,308 1,836 1,819 1,853 1,918 1,857 1,997 2,013 

Phosphoric Acid 

Production 1,529 1,544 1,593 1,539 1,382 1,264 1,338 1,382 

Ferroalloys 1,980 2,038 2,027 1,996 1,719 1,329 1,237 1,374 

Carbon Dioxide 

Consumption 860 808 912 849 957 818 978 1,267 

Land-Use Change and 

Forestry (Sinks)a

 (1,042,050) (930,011) (880,995) (826,106) (822,409) (826,879) (826,483) (828,046)

International Bunker 

Fuelsb

 113,503 109,858 114,557 105,294 101,404 97,865 89,489 84,193 

Biomass Combustion 216,702 233,243 217,201 222,340 226,765 200,477 207,249 216,813 

CH4 28,826 27,595 27,100 26,537 26,389 26,039 25,832 25,950 

Landfills 8,202 7,017 6,595 6,382 6,223 6,010 6,039 6,246 

Natural Gas Systems 6,112 6,363 6,276 6,066 6,289 6,277 6,221 5,998 

Enteric Fermentation 5,612 5,634 5,557 5,561 5,505 5,454 5,458 5,475 

Coal Mining 3,900 2,983 2,989 2,805 2,677 2,647 2,497 2,561 

Manure Management 1,485 1,733 1,850 1,846 1,813 1,853 1,873 1,864 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 27 

Wastewater Treatment 1,183 1,509 1,550 1,602 1,635 1,651 1,705 1,751 

Petroleum Systems 951 895 879 848 836 831 815 815 

Rice Cultivation 339 356 376 395 357 364 325 328 

Stationary Sources 373 351 328 338 349 318 305 319 

Abandoned Coal Mines 288 385 341 349 369 331 303 306 

Mobile Sources 228 193 185 172 161 147 138 128 

Petrochemical 

Production 56 78 80 81 80 68 72 72 

Iron and Steel 

Production 63 60 57 56 57 51 48 49 

Field Burning of 

Agricultural Residues 33 37 38 37 38 37 34 38 

Silicon Carbide 

Production 1 1 1 1 1 + + + 

International Bunker 

Fuelsb

 8 7 7 6 6 5 4 4 

N2O 1,232 1,278 1,315 1,233 1,297 1,245 1,228 1,215 

Agricultural Soil 

Management 816 813 864 785 851 829 815 818 

Mobile Sources 141 178 179 176 171 158 147 136 

Manure Management 52 56 56 56 57 58 58 57 

Human Sewage 42 47 48 50 50 50 51 51 

Nitric Acid 58 68 67 65 63 51 56 51 

Stationary Sources 40 44 43 43 45 43 44 45 

Settlements Remaining 

Settlements 18 20 20 20 19 19 19 19 

Adipic Acid 49 33 19 18 19 16 19 19 

N2O Product Usage 14 15 15 15 15 15 15 15 

Waste Combustion 1 1 1 1 1 1 1 1 

Field Burning of 

Agricultural Residues 1 1 1 1 1 1 1 1 

Forest Land Remaining 

Forest Land + 1 1 2 1 1 1 1 

International Bunker 

Fuelsb

 3 3 3 3 3 3 3 2 

HFCs, PFCs, and SF6 M M M M M M M M

Substitution of Ozone 

Depleting Substances M M M M M M M M

Electrical Transmission 

and Distributiond

 1 1 1 1 1 1 1 1 

HCFC-22 Productionc

 3 3 3 3 3 2 2 1 

Semiconductor 

Manufacture M M M M M M M M

Aluminum Production M M M M M M M M

Magnesium Production 

and Processingd

 + + + + + + + + 

SO2 20,936 17,091 17,189 15,917 14,829 14,452 13,928 14,463 

NOx 22,860 22,284 21,964 20,530 20,288 19,414 18,850 18,573 

CO 130,580 101,138 98,984 94,361 92,895 89,329 87,451 85,077 

NMVOCs 20,937 16,994 16,403 15,869 15,228 15,048 14,222 13,939 

+ Does not exceed 0.5 Gg. 

M Mixture of multiple gases 

a

 Sinks are not included in CO2 emissions total, and are based partially on projected activity data. b

 Emissions from International Bunker Fuels and Biomass Combustion are not included in totals. c

 HFC-23 emitted 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 28 

d

 SF6 emitted 

Note: Totals may not sum due to independent rounding. 

Note: Parentheses indicate negative values (or sequestration). 

Figure 2-5: U.S. Greenhouse Gas Emissions by Chapter/IPCC Sector 

Table 2-5: Recent Trends in U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector (Tg CO2 Eq.) 

Chapter/IPCC Sector 1990 1997 1998 1999 2000 2001 2002 2003

Energy 5,141.7 5,712.8 5,737.7 5,802.6 5,985.3 5,877.3 5,920.7 5,963.4 

Industrial Processes 299.9 327.1 334.9 329.2 332.1 304.7 315.4 308.6 

Solvent and Other Product Use 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8 

Agriculture 426.5 432.8 449.8 425.9 444.1 437.5 432.4 433.3 

Land-Use Change and Forestry (Emissions) 5.6 6.4 6.5 6.6 6.3 6.2 6.4 6.4 

Waste 210.1 193.7 186.0 183.1 180.6 176.5 178.3 183.8 

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2 

Land-Use Change and Forestry (Sinks) (1042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

Net Emissions (Sources and Sinks) 5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2 

* Sinks are only included in net emissions total, and are based partially on projected activity data. 

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 2003. In 2003, approximately 86 percent of the energy consumed in the United 

States was produced through the combustion of fossil fuels. The remaining 14 percent came from other energy 

sources such as hydropower, biomass, nuclear, wind, and solar energy (see Figure 2-6 and Figure 2-7). A 

discussion of specific trends related to CO2 as well as other greenhouse gas emissions from energy consumption is 

presented below. Energy related activities are also responsible for CH4 and N2O emissions (39 percent and 15 

percent of total U.S. emissions, respectively). Table 2-6 presents greenhouse gas emissions from the Energy sector, 

by source and gas. 

Figure 2-6: 2003 Energy Sector Greenhouse Gas Sources 

Figure 2-7: 2003 U.S. Fossil Carbon Flows (Tg CO2 Eq.) 

Table 2-6: Emissions from Energy (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 4,836.4 5,409.1 5,437.7 5,512.1 5,693.5 5,592.9 5,645.3 5,694.3

Fossil Fuel Combustion 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6

Non-Energy Use of Fuels 108.0 120.3 135.4 141.6 124.7 120.1 118.8 118.0

Waste Combustion 10.9 17.8 17.1 17.6 18.0 18.8 18.8 18.8

Natural Gas Flaring 5.8 7.9 6.6 6.9 5.8 6.1 6.2 6.0

Biomass-Wood* 212.5 226.3 209.5 214.3 217.6 190.8 195.8 201.0

International Bunker Fuels* 113.5 109.9 114.6 105.3 101.4 97.9 89.5 84.2

Biomass-Ethanol* 4.2 7.0 7.7 8.0 9.2 9.7 11.5 15.8

CH4 248.9 234.6 230.9 222.1 224.3 221.6 215.8 212.7

Natural Gas Systems 128.3 133.6 131.8 127.4 132.1 131.8 130.6 125.9

Coal Mining 81.9 62.6 62.8 58.9 56.2 55.6 52.4 53.8

Petroleum Systems 20.0 18.8 18.5 17.8 17.6 17.4 17.1 17.1
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 29 

Stationary Sources 7.8 7.4 6.9 7.1 7.3 6.7 6.4 6.7

Abandoned Coal Mines 6.1 8.1 7.2 7.3 7.7 6.9 6.4 6.4

Mobile Sources 4.8 4.0 3.9 3.6 3.4 3.1 2.9 2.7

International Bunker Fuels* 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1

N2O 56.4 69.1 69.1 68.4 67.5 62.8 59.6 56.4

Mobile Sources 43.7 55.2 55.3 54.6 53.2 49.0 45.6 42.1

Stationary Sources 12.3 13.5 13.4 13.5 14.0 13.5 13.5 13.8

Waste Combustion 0.4 0.4 0.3 0.3 0.4 0.4 0.5 0.5

International Bunker Fuels* 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8

Total 5,141.7 5,712.8 5,737.7 5,802.6 5,985.3 5,877.3 5,920.7 5,963.4

* 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. 

Fossil Fuel Combustion (5,551.6 Tg CO2 Eq.) 

As fossil fuels are combusted, the carbon stored in them is emitted almost entirely as CO2. The amount of carbon in 

fuels per unit of energy content varies significantly by fuel type. For example, coal contains the highest amount of 

carbon per unit of energy, while petroleum and natural gas have about 25 percent and 45 percent less carbon than 

coal, respectively. From 1990 through 2003, petroleum supplied the largest share of U.S. energy demands, 

accounting for an average of 39 percent of total energy consumption with natural gas and coal accounting for 24 

and 23 percent of total energy consumption, respectively. Petroleum was consumed primarily in the transportation 

end-use sector, the vast majority of coal was used by electric power generators, and natural gas was consumed 

largely in the industrial and residential end-use sectors. 

Emissions of CO2 from fossil fuel combustion increased at an average annual rate of 1.3 percent from 1990 to 2003. 

The fundamental factors influencing this trend include (1) a generally growing domestic economy over the last 13 

years, and (2) significant growth in emissions from transportation activities and electricity generation. Between 

1990 and 2003, CO2 emissions from fossil fuel combustion increased from 4,711.7 Tg CO2 Eq. to 5,551.6 Tg CO2

Eq.⎯an 18 percent total increase over the thirteen-year period. 

The four major end-use sectors contributing to CO2 emissions from fossil fuel combustion are industrial, 

transportation, residential, and commercial. Electricity generation also emits CO2, although these emissions are 

produced as they consume fossil fuel to provide electricity to one of the four end-use sectors. For the discussion 

below, electricity generation emissions have been distributed to each end-use sector on the basis of each sector’s 

share of aggregate electricity consumption. This method of distributing emissions assumes that each end-use sector 

consumes electricity that is generated from the national average mix of fuels according to their carbon intensity. In 

reality, sources of electricity vary widely in carbon intensity. By assuming the same carbon intensity for each enduse sector's electricity consumption, for example, emissions attributed to the residential end-use sector may be 

underestimated, while emissions attributed to the industrial end-use sector may be overestimated. 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. Table 2-7, Figure 2-8, and Figure 2-9 summarize CO2 emissions from fossil fuel 

combustion by end-use sector. 

Table 2-7: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 

End-Use Sector 1990 1997 1998 1999 2000 2001 2002 2003

Transportation 1,449.8 1,606.4 1,636.5 1,693.9 1,741.0 1,723.1 1,755.4 1,770.4

Combustion 1,446.8 1,603.3 1,633.4 1,690.8 1,737.7 1,719.7 1,752.3 1,767.2

Electricity 3.0 3.1 3.1 3.2 3.4 3.4 3.2 3.2

Industrial 1,553.9 1,703.0 1,668.5 1,651.2 1,684.4 1,587.4 1,579.0 1,572.9

Combustion 882.8 963.8 911.6 888.1 905.0 878.2 876.6 858.6

Electricity 671.1 739.2 757.0 763.1 779.4 709.3 702.4 714.3

Residential 924.8 1,040.7 1,044.4 1,063.5 1,124.2 1,116.2 1,145.0 1,168.9
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 30 

Combustion 339.6 370.6 338.6 359.3 379.1 367.0 371.4 385.1

Electricity 585.3 670.2 705.8 704.2 745.0 749.2 773.6 783.8

Commercial 755.1 876.7 892.9 901.2 959.5 972.7 973.9 983.1

Combustion 224.2 237.2 219.7 222.3 235.2 226.7 230.0 234.0

Electricity 530.9 639.5 673.2 678.9 724.3 745.9 743.9 749.2

U.S. Territories 28.0 36.4 36.3 36.2 35.9 48.6 48.1 56.2

Total 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6

Electricity Generation 1,790.3 2,051.9 2,139.0 2,149.3 2,252.1 2,207.8 2,223.0 2,250.5 

Note: Totals may not sum due to independent rounding. Combustion-related emissions from electricity generation are allocated 

based on aggregate national electricity consumption by each end-use sector. 

Figure 2-8: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type 

Figure 2-9: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 

Transportation End-Use Sector. Transportation activities (excluding international bunker fuels) accounted for 32 

percent of CO2 emissions from fossil fuel combustion in 2003.3 Virtually all of the energy consumed in this enduse sector came from petroleum products. Over 60 percent of the emissions resulted from gasoline consumption for 

personal vehicle use. The remaining emissions came from other transportation activities, including the combustion 

of diesel fuel in heavy-duty vehicles and jet fuel in aircraft. 

Industrial End-Use Sector. Industrial CO2 emissions, resulting both directly from the combustion of fossil fuels and 

indirectly from the generation of electricity that is consumed by industry, accounted for 28 percent of CO2 from 

fossil fuel combustion in 2003. About half of these emissions resulted from direct fossil fuel combustion to produce 

steam and/or heat for industrial processes. The other half of the 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 2003. Both sectors relied heavily on 

electricity for meeting energy demands, with 67 and 76 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 35 percent of 

U.S. energy from fossil fuels and emitted 41 percent of the CO2 from fossil fuel combustion in 2003. 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 93 percent of all coal consumed for energy in the United States in 2003. Consequently, changes 

in electricity demand have a significant impact on coal consumption and associated CO2 emissions. 

 

3 If emissions from international bunker fuels are included, the transportation end-use sector accounted for 33 percent of U.S. 

emissions from fossil fuel combustion in 2003. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 31 

Non-Energy Use of Fossil Fuels (118.0 Tg CO2 Eq.) 

In addition to being combusted for energy, fossil fuels are also consumed for non-energy uses (NEUs). Fuels are 

used in the industrial and transportation end-use sectors for a variety of NEUs, including application as solvents, 

lubricants, and waxes, or as raw materials in the manufacture of plastics, rubber, and synthetic fibers. Carbon 

dioxide emissions arise from non-energy uses via several pathways. Emissions may occur during the manufacture 

of a product, as is the case in producing plastics or rubber from fuel-derived feedstocks. Additionally, emissions 

may occur during the product’s lifetime, such as during solvent use. Where appropriate data and methodologies are 

available, NEUs of fossil fuels used for industrial processes are reported in the Industrial Processes sector. 

Emissions in 2003 for non-energy uses of fossil fuels were 118.0 Tg CO2 Eq., which constituted 2 percent of overall 

fossil fuel CO2 emissions and 2 percent of total national CO2 emissions, approximately the same proportion as in 

1990. 

Waste Combustion (19.2 Tg CO2 Eq.) 

Combustion is used to manage about 7 to 17 percent of the municipal solid wastes generated in the United States. 

The burning of garbage and non-hazardous solids, referred to as municipal solid waste, as well as the burning of 

hazardous waste, is usually performed to recover energy from the waste materials. Carbon dioxide and N2O 

emissions arise from the organic materials found in these wastes. Within municipal solid waste, many products 

contain carbon of biogenic origin (e.g., paper, yard trimmings), and the CO2 emissions from their combustion are 

accounted for under the Land Use Change and Forestry chapter. Several components of municipal solid waste, such 

as plastics, synthetic rubber, synthetic fibers, and carbon black, are of fossil fuel origin, and are included as sources 

of CO2 and N2O emissions. In 2003, CO2 emissions from waste combustion amounted to 18.8 Tg CO2 Eq., while 

N2O emissions amounted to 0.5 Tg CO2 Eq. 

Natural Gas Flaring (6.0 Tg CO2 Eq.) 

The flaring of natural gas from oil wells results in the release of CO2 emissions. Natural gas is flared from both onshore and off-shore oil wells to relieve rising pressure or to dispose of small quantities of gas that are not 

commercially marketable. In 2003, flaring accounted for approximately 0.1 percent of U.S. CO2 emissions. 

Natural Gas Systems (125.9 Tg CO2 Eq.) 

Methane is the major component of natural gas. Fugitive emissions of CH4 occur throughout the production, 

processing, transmission, and distribution of natural gas. Because natural gas is often found in conjunction with 

petroleum deposits, leakage from petroleum systems is also a source of emissions. Emissions vary greatly from 

facility to facility and are largely a function of operation and maintenance procedures and equipment conditions. In 

2003, CH4 emissions from U.S. natural gas systems accounted for approximately 23 percent of U.S. CH4 emissions. 

Coal Mining (53.8 Tg CO2 Eq.) 

Produced millions of years ago during the formation of coal, CH4 trapped within coal seams and surrounding rock 

strata is released when the coal is mined. The quantity of CH4 released to the atmosphere during coal mining 

operations depends primarily upon the type of coal and the method and rate of mining. 

Methane from surface mines is emitted directly to the atmosphere as the rock strata overlying the coal seam are 

removed. Because CH4 in underground mines is explosive at concentrations of 5 to 15 percent in air, most active 

underground mines are required to vent this methane, typically to the atmosphere. At some mines, CH4-recovery 

systems may supplement these ventilation systems. Recovery of CH4 in the United States has increased in recent 

years. During 2003, coal mining activities emitted 10 percent of U.S. CH4 emissions. From 1990 to 2003, 

emissions from this source decreased by 34 percent due to increased use of the CH4 collected by mine degasification 

systems and a general shift toward surface mining. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 32 

Petroleum Systems (17.1 Tg CO2 Eq.) 

Petroleum is often found in the same geological structures as natural gas, and the two are often retrieved together. 

Crude oil is saturated with many lighter hydrocarbons, including methane. When the oil is brought to the surface 

and processed, many of the dissolved lighter hydrocarbons (as well as water) are removed through a series of highpressure and low-pressure separators. The remaining hydrocarbons in the oil are emitted at various points along the 

system. Methane emissions from the components of petroleum systems generally occur as a result of system leaks, 

disruptions, and routine maintenance. In 2003, emissions from petroleum systems were just over 3 percent of U.S. 

CH4 emissions. 

Mobile Combustion (44.8 Tg CO2 Eq.) 

Mobile combustion results in N2O and CH4 emissions. Nitrous oxide is a product of the reaction that occurs 

between nitrogen and oxygen during fuel combustion. The quantity emitted varies according to the type of fuel, 

technology, and pollution control device used, as well as maintenance and operating practices. For example, some 

types of catalytic converters installed to reduce motor vehicle pollution can promote the formation of N2O. In 2003, 

N2O emissions from mobile combustion were 42.1 Tg CO2 Eq. (11 percent of U.S. N2O emissions). From 1990 to 

2003, N2O emissions from mobile combustion decreased by about 4 percent. 

In 2003, CH4 emissions were estimated to be 2.7 Tg CO2 Eq. The combustion of gasoline in highway vehicles was 

responsible for the majority of the CH4 emitted from mobile combustion. 

Stationary Combustion (20.5 Tg CO2 Eq.) 

Stationary combustion results in N2O and CH4 emissions. In 2003, N2O emissions from stationary combustion 

accounted for 13.8 Tg CO2 Eq. (4 percent of U.S. N2O emissions). From 1990 to 2003, N2O emissions from 

stationary combustion increased by 13 percent, due to increased fuel consumption. In 2003, CH4 emissions were 

6.7 Tg CO2 Eq. (1 percent of U.S. CH4 emissions). The majority of CH4 emissions from stationary combustion 

resulted from the burning of wood in the residential end-use sector. 

Abandoned Coal Mines (6.4 Tg CO2 Eq.) 

Coal mining activities result in the emission of CH4 into the atmosphere. However, the closure of a coal mine does 

not correspond with an immediate cessation in the release of emissions. 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. In 2003, the emissions from abandoned coal mines constituted 1 percent of U.S. CH4 emissions. 

CO2 from Biomass Combustion (216.8 Tg CO2 Eq.) 

Biomass refers to organically-based carbon fuels (as opposed to fossil-based). Biomass in the form of fuel wood 

and wood waste was used primarily in the industrial sector, while the transportation sector was the predominant 

user of biomass-based fuels, such as ethanol from corn and woody crops. 

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 carbon emitted is offset by the growth of new biomass. For 

example, fuel wood burned one year but re-grown the next only recycles carbon, rather than creating a net increase 

in total atmospheric carbon. Net carbon fluxes from changes in biogenic carbon reservoirs in wooded or croplands 

are accounted for in the estimates for the Land-Use Change and Forestry sector. 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. 

The consumption of wood biomass in the industrial, residential, electric power, and commercial end-use sectors 

accounted for 66, 17, 8, and 2 percent of gross CO2 emissions from biomass combustion, respectively. Ethanol 

consumption in the transportation end-use sector accounted for the remaining 7 percent. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 33 

International Bunker Fuels (85.1 Tg CO2 Eq.) 

Greenhouse gases emitted from the combustion of fuels used for international transport activities, termed 

international bunker fuels under the UNFCCC, include CO2, CH4, and N2O. Emissions from these activities 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. These decisions are reflected in the Revised 1996 IPCC Guidelines, 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). 

Two transport modes are addressed under the IPCC definition of international bunker fuels: aviation and marine. 

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. Emissions of CO2, CH4, and N2O from international bunker fuel combustion were 84.2, 0.1, 

and 0.8 Tg CO2 Eq. in 2003, respectively. 

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. The processes include iron and steel production, cement manufacture, ammonia manufacture 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, aluminum production, petrochemical production, silicon carbide production, nitric 

acid production, and adipic acid production (see Figure 2-10). Additionally, emissions from industrial processes 

release HFCs, PFCs and SF6. Table 2-8 presents greenhouse gas emissions from Industrial Processes by source 

category. 

Figure 2-10: 2003 Industrial Processes Chapter Greenhouse Gas Sources 

Table 2-8: Emissions from Industrial Processes (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 173.1 170.9 169.4 165.9 164.7 151.8 151.5 147.2

Iron and Steel Production 85.4 71.9 67.4 64.4 65.7 58.9 55.1 53.8

Cement Manufacture 33.3 38.3 39.2 40.0 41.2 41.4 42.9 43.0

Ammonia Manufacture & Urea Application 19.3 20.7 21.9 20.6 19.6 16.7 18.6 15.6

Lime Manufacture 11.2 13.7 13.9 13.5 13.3 12.8 12.3 13.0

Limestone and Dolomite Use 5.5 7.2 7.4 8.1 6.0 5.7 5.9 4.7

Aluminum Production 6.3 5.6 5.8 5.9 5.7 4.1 4.2 4.2

Soda Ash Manufacture and Consumption 4.1 4.4 4.3 4.2 4.2 4.1 4.1 4.1

Petrochemical Production 2.2 2.9 3.0 3.1 3.0 2.8 2.9 2.8

Titanium Dioxide Production 1.3 1.8 1.8 1.9 1.9 1.9 2.0 2.0

Phosphoric Acid Production 1.5 1.5 1.6 1.5 1.4 1.3 1.3 1.4

Ferroalloy Production 2.0 2.0 2.0 2.0 1.7 1.3 1.2 1.4

Carbon Dioxide Consumption 0.9 0.8 0.9 0.8 1.0 0.8 1.0 1.3

CH4 2.5 2.9 2.9 2.9 2.9 2.5 2.5 2.5 

Petrochemical Production 1.2 1.6 1.7 1.7 1.7 1.4 1.5 1.5

Iron and Steel Production 1.3 1.3 1.2 1.2 1.2 1.1 1.0 1.0

Silicon Carbide Production + + + + + + + + 

N2O 33.0 31.5 26.9 25.6 25.6 20.8 23.1 21.8

Nitric Acid Production 17.8 21.2 20.9 20.1 19.6 15.9 17.2 15.8
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 34 

Adipic Acid Production 15.2 10.3 6.0 5.5 6.0 4.9 5.9 6.0

HFCs, PFCs, and SF6 91.2 121.7 135.7 134.8 138.9 129.5 138.3 137.0

Substitution of Ozone Depleting 

Substances 

0.4 46.5 56.6 65.8 75.0 83.3 91.5 99.5

Electrical Transmission and Distribution 29.2 21.7 17.1 16.4 15.6 15.4 14.7 14.1

HCFC-22 Production 35.0 30.0 40.1 30.4 29.8 19.8 19.8 12.3

Aluminum Production 18.3 11.0 9.1 9.0 9.0 4.0 5.2 3.8

Semiconductor Manufacture 2.9 6.3 7.1 7.2 6.3 4.5 4.4 4.3

Magnesium Production and Processing 5.4 6.3 5.8 6.0 3.2 2.6 2.6 3.0

Total 299.9 327.1 334.9 329.2 332.1 304.7 315.4 308.6

+ Does not exceed 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. 

Iron and Steel Production (54.8 Tg CO2 Eq.) 

Pig iron is the product of combining iron oxide (i.e., iron ore) and sinter with metallurgical coke in a blast furnace. 

The pig iron production process, as well as the thermal processes used to create sinter and metallurgical coke, 

resulted in emissions of CO2 and CH4. In 2003, iron and steel production resulted in 1.0 Tg CO2 Eq. of CH4

emissions, with the majority of the emissions coming from the pig iron production process. The majority of CO2

emissions from iron and steel processes come from the production of coke for use in pig iron creation, with smaller 

amounts evolving from the removal of carbon from pig iron used to produce steel. CO2 emissions from iron and 

steel amounted to 53.8 Tg CO2 Eq. in 2003. From 1990 to 2003, emissions from this source decreased by 37 

percent. 

Cement Manufacture (43.0 Tg CO2 Eq.) 

Clinker is an intermediate product in the formation of finished Portland and masonry cement. Heating calcium 

carbonate (CaCO3) in a cement kiln forms lime and CO2. The lime combines with other materials to produce 

clinker, and the CO2 is released into the atmosphere. From 1990 to 2003, emissions from this source increased by 

29 percent. 

Ammonia Manufacture and Urea Application (15.6 Tg CO2 Eq.) 

In the United States, roughly 98 percent of synthetic ammonia is produced by catalytic steam reforming of natural 

gas, and the remainder is produced using naphtha (i.e., a petroleum fraction) or the electrolysis of brine at chlorine 

plants (EPA 1997). The two fossil fuel-based reactions produce carbon monoxide and hydrogen gas. This carbon 

monoxide is transformed into CO2 in the presence of a catalyst. The CO2 is generally released into the atmosphere, 

but some of the CO2, together with ammonia, is used as a raw material in the production of urea [CO(NH2)2], which 

is a type of nitrogenous fertilizer. The carbon in the urea that is produced and assumed to be subsequently applied 

to agricultural land as a nitrogenous fertilizer is ultimately released into the environment as CO2. 

Lime Manufacture (13.0 Tg CO2 Eq.) 

Lime is used in steel making, construction, flue gas desulfurization, and water and sewage treatment. It is 

manufactured by heating limestone (mostly calcium carbonate, CaCO3) in a kiln, creating quicklime (calcium oxide, 

CaO) and CO2, which is normally emitted to the atmosphere. 

Limestone and Dolomite Use (4.7 Tg CO2 Eq.) 

Limestone (CaCO3) and dolomite (CaMg(CO3)) are basic raw materials used in a wide variety of industries, 

including construction, agriculture, chemical, and metallurgy. For example, limestone can be used as a purifier in 

refining metals. In the case of iron ore, limestone heated in a blast furnace reacts with impurities in the iron ore and 

fuels, generating CO2 as a by-product. Limestone is also used in flue gas desulfurization systems to remove sulfur 

dioxide from the exhaust gases. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 35 

Aluminum Production (8.0 Tg CO2 Eq.) 

Aluminum production results in emissions of CO2, CF4 and C2F6. Carbon dioxide is emitted when alumina 

(aluminum oxide, Al2O3) is reduced to aluminum. The reduction of the alumina occurs through electrolysis in a 

molten bath of natural or synthetic cryolite. The reduction cells contain a carbon lining that serves as the cathode. 

Carbon is also contained in the anode, which can be a carbon mass of paste, coke briquettes, or prebaked carbon 

blocks from petroleum coke. During reduction, some of this carbon is oxidized and released to the atmosphere as 

CO2. In 2003, CO2 emissions from aluminum production amounted to 4.2 Tg CO2 Eq. 

During the production of primary aluminum, CF4 and C2F6 are emitted as intermittent by-products of the smelting 

process. These PFCs are formed when fluorine from the cryolite bath combines with carbon from the electrolyte 

anode. PFC emissions from aluminum production have decreased by 79 percent between 1990 and 2003 due to 

emission reduction efforts by the industry and falling domestic aluminum production. In 2003, CF4 and C2F6

emissions from aluminum production amounted to 3.8 Tg CO2 Eq. 

Soda Ash Manufacture and Consumption (4.1 Tg CO2 Eq.) 

Commercial soda ash (sodium carbonate, Na2CO3) is used in many consumer products, such as glass, soap and 

detergents, paper, textiles, and food. During the manufacturing of soda ash, some natural sources of sodium 

carbonate are heated and transformed into a crude soda ash, in which CO2 is generated as a by-product. In addition, 

CO2 is often released when the soda ash is consumed. 

Petrochemical Production (4.3 Tg CO2 Eq.) 

The production process for carbon black results in the release CO2 emissions to the atmosphere. Carbon black is a 

black powder generated by the incomplete combustion of an aromatic petroleum or coal-based feedstock 

production. The majority of carbon black produced in the United States is consumed by the tire industry, which 

adds it to rubber to increase strength and abrasion resistance. Small amounts of CH4 are also released during the 

production of five petrochemicals: carbon black, ethylene, ethylene dichloride, styrene, and methanol. These 

production processes resulted in emissions of 2.8 Tg CO2 Eq. of CO2 and 1.5 Tg CO2 Eq. of CH4 in 2003. 

Titanium Dioxide Production (2.0 Tg CO2 Eq.) 

Titanium dioxide (TiO2) is a metal oxide manufactured from titanium ore, and is principally used as a pigment. It is 

used in white paint and as a pigment in the manufacture of white paper, foods, and other products. Two processes, 

the chloride process and the sulfate process, are used for making TiO2. Carbon dioxide is emitted from the chloride 

process, which uses petroleum coke and chlorine as raw materials. 

Phosphoric Acid Production (1.4 Tg CO2 Eq.) 

Phosphoric acid is a basic raw material in the production of phosphate-based fertilizers. The phosphate rock 

consumed in the United States originates from both domestic mines, located primarily in Florida, North Carolina, 

Idaho, and Utah, and foreign mining operations in Morocco. The primary use of this material is as a basic 

component of a series of chemical reactions that lead to the production of phosphoric acid, as well as the byproducts CO2 and phosphogypsum. 

Ferroalloy Production (1.4 Tg CO2 Eq.) 

Carbon dioxide is 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. 

Carbon Dioxide Consumption (1.3 Tg CO2 Eq.) 

Many segments of the economy consume CO2, including food processing, beverage manufacturing, chemical 

processing, and a host of industrial and other miscellaneous applications. Carbon dioxide may be produced as a by-
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 36 

product from the production of certain chemicals (e.g., ammonia), from select natural gas wells, or by separating it 

from crude oil and natural gas. The majority of the CO2 used in these applications is eventually released to the 

atmosphere. 

Silicon Carbide Production (0.01 Tg CO2 Eq.) 

Small amounts of CH4 are released during the production of silicon carbide, a material used as an industrial 

abrasive. Silicon carbide (SiC) is made through a reaction of quartz (SiO2) and carbon (in the form of petroleum 

coke). Methane is produced during this reaction from volatile compounds in the petroleum coke. Methane 

emissions from silicon carbide production have declined significantly due a 67 percent decrease in silicon carbide 

production since 1990. In 2003, only two companies produced silicon carbide in the United States (one company 

produced abrasive-grade silicon carbide and the other produced a small quantity for heat-resistant products). 

Nitric Acid Production (15.8 Tg CO2 Eq.) 

Nitric acid production is an industrial source of N2O emissions. Used primarily to make synthetic commercial 

fertilizer, this raw material is also a major component in the production of adipic acid and explosives. 

Virtually all of the nitric acid manufactured in the United States is produced by the oxidation of ammonia, during 

which N2O is formed and emitted to the atmosphere. In 2003, N2O emissions from nitric acid production accounted 

for 4 percent of U.S. N2O emissions. From 1990 to 2003, emissions from this source category decreased by11 

percent with the trend in the time series closely tracking the changes in production. 

Adipic Acid Production (6.0 Tg CO2 Eq.) 

Most adipic acid produced in the United States is used to manufacture nylon 6,6. Adipic acid is also used to 

produce some low-temperature lubricants and to add a "tangy" flavor to foods. Nitrous oxide is emitted as a byproduct of the chemical synthesis of adipic acid. 

In 2003, U.S. adipic acid plants emitted 2 percent of U.S. N2O emissions. Even though adipic acid production has 

increased in recent years, by 1998 all three major adipic acid plants in the United States had voluntarily 

implemented N2O abatement technology. As a result, emissions have decreased by 60 percent since 1990. 

Substitution of Ozone Depleting Substances (99.5 Tg CO2 Eq.) 

The use and subsequent emissions of HFCs and PFCs as substitutes for ODSs have increased from small amounts in 

1990 to account for 73 percent of aggregate HFC, PFC, and SF6 emissions. This increase was in large part the 

result of efforts to phase-out CFCs and other ODSs in the United States, especially the introduction of HFC-134a as 

a CFC substitute in refrigeration and air-conditioning applications. In the short term, this trend is expected to 

continue, and will likely accelerate over the coming 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. 

HCFC-22 Production (12.3 Tg CO2 Eq.) 

HFC-23 is a by-product of the production of HCFC-22. Emissions from this source have decreased by 65 percent 

since 1990. The HFC-23 emission rate (i.e., the amount of HFC-23 emitted per kilogram of HCFC-22 

manufactured) has declined significantly since 1990, although production has been increasing. 

Electrical Transmission and Distribution Systems (14.1 Tg CO2 Eq.) 

The primary use of SF6 is as a dielectric in electrical transmission and distribution systems. Fugitive emissions of 

SF6 occur from leaks in and servicing of substations and circuit breakers, especially from older equipment. The gas 

can also be released during equipment manufacturing, installation, servicing, and disposal. Estimated emissions 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 37 

from this source decreased by 52 percent since 1990, primarily due to higher SF6 prices and industrial efforts to 

reduce emissions. 

Semiconductor Manufacture (4.3 Tg CO2 Eq.) 

The semiconductor industry uses combinations of HFCs, PFCs, SF6, and other gases for plasma etching and to clean 

chemical vapor deposition tools. Emissions from this source category have increased 49 percent since 1990 with 

the growth in the semiconductor industry and the rising intricacy of chip designs. However, the growth rate in 

emissions has slowed since 1997, and emissions actually declined between 1999 and 2003. This later reduction is 

due to the implementation of PFC emission reduction methods, such as process optimization. 

Magnesium Production (3.0 Tg CO2 Eq.) 

Sulfur hexafluoride is also used as a protective cover gas for the casting of molten magnesium. Emissions from 

primary magnesium production and magnesium casting have decreased by 45 percent since 1990. This decrease has 

primarily taken place since 1999, due to a decline in the quantity of magnesium die cast and the closure of a U.S. 

primary magnesium production facility. 

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 N2O Product Usage, the only source of greenhouse gas emissions from this sector, accounted 

for 4.8 Tg CO2 Eq. of N2O, or less than 0.1 percent of total U.S. anthropogenic greenhouse gas emissions on a 

carbon equivalent basis in 2003 (see Table 2-9). 

Table 2-9: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

N2O 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8

Nitrous Oxide Product Usage 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8

Total 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8

N2O Product Usage (4.8 Tg CO2 Eq.) 

Nitrous oxide is 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. In 

2003, N2O emissions from product usage constituted approximately 1 percent of U.S. N2O emissions. From 1990 to 

2003, emissions from this source category increased by 11 percent. 

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 2003, agricultural activities were responsible for emissions of 433.3 Tg CO2 Eq., or 6 percent of total U.S. 

greenhouse gas emissions. Methane and N2O were the primary greenhouse gases emitted by agricultural activities. 

Methane emissions from enteric fermentation and manure management represented about 21 percent and 7 percent 

of total CH4 emissions from anthropogenic activities, respectively in 2003. Agricultural soil management activities 

such as fertilizer application and other cropping practices were the largest source of U.S. N2O emissions in 2003, 

accounting for 67 percent. Table 2-10 presents emission estimates for the Agriculture sector. 

Figure 2-11: 2003 Agriculture Chapter Greenhouse Gas Sources 

 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 38 

Table 2-10: Emissions from Agriculture (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CH4 156.9 163.0 164.2 164.6 162.0 161.9 161.5 161.8 

Enteric Fermentation 117.9 118.3 116.7 116.8 115.6 114.5 114.6 115.0 

Manure Management 31.2 36.4 38.8 38.8 38.1 38.9 39.3 39.1 

Rice Cultivation 7.1 7.5 7.9 8.3 7.5 7.6 6.8 6.9 

Field Burning of Agricultural 

Residues 0.7 0.8 0.8 0.8 0.8 0.8 0.7 0.8 

N2O 269.6 269.8 285.6 261.3 282.1 275.6 270.9 271.5 

Agricultural Soil Management 253.0 252.0 267.7 243.4 263.9 257.1 252.6 253.5 

Manure Management 16.3 17.3 17.4 17.4 17.8 18.0 17.9 17.5 

Field Burning of Agricultural 

Residues 0.4 0.4 0.5 0.4 0.5 0.5 0.4 0.4 

Total 426.5 432.8 449.8 425.9 444.1 437.5 432.4 433.3 

Note: Totals may not sum due to independent rounding. 

Enteric Fermentation (115.0 Tg CO2 Eq.) 

During animal digestion, CH4 is produced through the process of enteric fermentation, in which microbes residing 

in animal digestive systems break down food. Ruminants, which include cattle, buffalo, sheep, and goats, have the 

highest CH4 emissions among all animal types because they have a rumen, or large fore-stomach, in which CH4-

producing fermentation occurs. Non-ruminant domestic animals, such as pigs and horses, have much lower CH4

emissions. In 2003, enteric fermentation was the source of about 21 percent of U.S. CH4 emissions, and more than 

71 percent of the CH4 emissions from agriculture. From 1990 to 2003, emissions from this source decreased by 2 

percent. Emissions from enteric fermentation have decreasing significantly since 1995, primarily due to declining 

dairy cow and beef cattle populations as a result of improved efficiency in milk and beef production. 

Manure Management (56.7 Tg CO2 Eq.) 

Both CH4 and N2O resulted from manure management. The decomposition of organic animal waste in an anaerobic 

environment produces CH4. The most important factor affecting the amount of CH4 produced is how the manure is 

managed, because certain types of storage and treatment systems promote an oxygen-free environment. In 

particular, liquid systems tend to encourage anaerobic conditions and produce significant quantities of CH4, whereas 

solid waste management approaches produce little or no CH4. Higher temperatures and moist climatic conditions 

also promote CH4 production. 

Emissions from manure management were 39.1 Tg CO2 Eq., or about 7 percent of U.S. CH4 emissions in 2003 and 

24 percent of the CH4 emissions from the agriculture sector. From 1990 to 2003, emissions from this source 

increased by 26 percent. The bulk of this increase was from swine and dairy cow manure, and is attributed to the 

shift in the composition of the swine and dairy industries towards larger facilities. Larger swine and dairy farms 

tend to use liquid management systems. 

Nitrous oxide is also produced as part of microbial nitrification and denitrification processes in managed and 

unmanaged manure. Emissions from unmanaged manure are accounted for within the agricultural soil management 

source category. Total N2O emissions from managed manure systems in 2003 accounted for 17.5 Tg CO2 Eq., or 5 

percent of U.S. N2O emissions. From 1990 to 2003, emissions from this source category increased by 8 percent, 

primarily due to increases in swine and poultry populations over the same time period. 

Rice Cultivation (6.9 Tg CO2 Eq.) 

Most of the world's rice, and all of the rice in the United States, is grown on flooded fields. When fields are 

flooded, anaerobic conditions develop and the organic matter in the soil decomposes, releasing CH4 to the 

atmosphere, primarily through the rice plants. In 2003, rice cultivation was the source of 1 percent of U.S. CH4

emissions, and about 4 percent of U.S. CH4 emissions from agriculture. Emission estimates from this source have 

decreased about 3 percent since 1990. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 39 

Field Burning of Agricultural Residues (1.2 Tg CO2 Eq.) 

Burning crop residue releases N2O and CH4. Because field burning is not a common debris clearing method used in 

the United States, it was responsible for only 0.1 percent of U.S. CH4 (0.8 Tg CO2 Eq.) and N2O (0.4 Tg CO2 Eq.) 

emissions in 2003. 

Agricultural Soil Management (253.5 Tg CO2 Eq.) 

Nitrous oxide is produced naturally in soils through microbial processes of nitrification and denitrification. A 

number of anthropogenic activities add to the amount of nitrogen available to be emitted as N2O by microbial 

processes. These activities may add nitrogen to soils either directly or indirectly. Direct additions occur through the 

application of synthetic and organic fertilizers; production of nitrogen-fixing crops and forages; the application of 

livestock manure, crop residues, and sewage sludge; cultivation of high-organic-content soils; and direct excretion 

by animals onto soil. Indirect additions result from volatilization and subsequent atmospheric deposition, and from 

leaching and surface run-off of some of the nitrogen applied to or deposited on soils as fertilizer, livestock manure, 

and sewage sludge. 

In 2003, agricultural soil management accounted for 67 percent of U.S. N2O emissions. From 1990 to 2003, 

emissions from this source increased slightly as fertilizer consumption, manure production, and production of 

nitrogen-fixing and other crops rose. Year-to-year fluctuations are largely a reflection of annual variations in 

climate, synthetic fertilizer consumption, and crop production. 

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 a net uptake (sequestration) of carbon in the United States, which offset about 12 

percent of total U.S. gross CO2 emissions in 2003. Forests (including vegetation, soils, and harvested wood) 

accounted for approximately 91 percent of total 2003 sequestration, urban trees accounted for 7 percent, agricultural 

soils (including mineral and organic soils and the application of lime) accounted for 1 percent, and landfilled yard 

trimmings and food scraps accounted for 1 percent of the total sequestration in 2003. 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 soils account for a net carbon sink that is approximately one and a third times larger than the sum of 

emissions from organic soils and liming. The mineral soil carbon sequestration is largely due to 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 2003 resulted in a net carbon sequestration of 828.0 Tg CO2

Eq. (Table ES-5). This represents an offset of approximately 14 percent of total U.S. CO2 emissions. Total land 

use, land-use change, and forestry net carbon sequestration declined by approximately 21 percent between 1990 and 

2003. This decline was primarily due to a decline in the rate of net carbon accumulation in forest carbon stocks. 

Annual carbon accumulation in landfilled yard trimmings and food scraps also slowed over this period, as did 

annual carbon accumulation in agricultural soils. As described above, the constant rate of carbon accumulation in 

urban trees is a reflection of limited underlying data (i.e., this rate represents an average for 1990 through 1999). 

Table 2-11: Net CO2 Flux from Land-Use Change and Forestry (Tg CO2 Eq.) 

Sink Category 1990 1997 1998 1999 2000 2001 2002 2003

Forest Land Remaining Forest Land (949.3) (851.0) (805.5) (751.7) (747.9) (750.9) (751.5) (752.7)

Changes in Forest Carbon Stocks (949.3) (851.0) (805.5) (751.7) (747.9) (750.9) (751.5) (752.7)

Cropland Remaining Cropland (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6)

Changes in Agricultural Soil Carbon (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6)
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 40 

Stocks 

Settlements Remaining Settlements (84.7) (71.6) (71.2) (70.0) (68.9) (68.9) (68.8) (68.7)

Urban Trees (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7)

Landfilled Yard Trimmings and Food 

Scraps (26.0) (12.9) (12.5) (11.4) (10.2) (10.3) (10.2) (10.1)

Total (1,042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0)

Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding. 

Land use, land-use change, and forestry activities in 2003 also resulted in emissions of N2O (6.4 Tg CO2 Eq., Table 

2-12). Total N2O emissions from the application of fertilizers to forests and settlements increased by approximately 

14 percent between 1990 and 2003. 

Table 2-12: N2O Emissions from Land-Use Change and Forestry (Tg CO2 Eq.) 

Sink Category 1990 1997 1998 1999 2000 2001 2002 2003 

Forest Land Remaining Forest Land 0.1 0.3 0.4 0.5 0.4 0.4 0.4 0.4 

N2O Fluxes from Soils 0.1 0.3 0.4 0.5 0.4 0.4 0.4 0.4 

Settlements Remaining Settlements 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0 

N2O Fluxes from Soils 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0 

Total 5.6 6.4 6.5 6.6 6.3 6.2 6.4 6.4 

Note: Totals may not sum due to independent rounding. 

Forest Land Remaining Forest Land (0.4 Tg CO2 Eq.) 

As with other agricultural applications, forests may be fertilized to stimulate growth rates. This relative magnitude 

of the impact of this practice is limited, however, because forests are generally only fertilized twice during their life 

cycles, and applications account for no more than one percent of total U.S. fertilizer applications annually. In terms 

of trends, however, N2O emissions from forest soils for 2003 were almost 7 times higher than in 1990, primarily the 

result of an increase in fertilized area of pine plantations in the southeastern U.S. This source accounts for 

approximately 0.1 percent of total U.S. N2O emissions. 

Settlements Remaining Settlements (6.0 Tg CO2 Eq.) 

Of the fertilizers applied to soils in the U.S., approximately 10 percent are applied to lawns, golf courses, and other 

landscaping within settled areas. In 2003, N2O emissions from settlement soils constituted approximately 1.6 

percent of total U.S N2O emissions. There has been an overall increase in emissions of 9 percent since 1990, a 

result of a general increase in the applications of synthetic fertilizers. 

Waste 

Waste management and treatment activities are sources of greenhouse gas emissions (see Figure 2-12). Landfills 

were the largest source of anthropogenic methane (CH4) emissions, accounting for 24 percent of total U.S. CH4

emissions.4 Wastewater treatment systems are a potentially significant source of nitrous oxide (N2O) emissions; 

however, methodologies are not currently available to develop a complete estimate. Nitrous oxide emissions from 

the treatment of the human sewage component of wastewater were estimated, however, using a simplified 

methodology. Nitrogen oxide (NOx), carbon monoxide (CO), and non-methane volatile organic compounds 

(NMVOCs) are also emitted by waste activities. A summary of greenhouse gas and ambient air pollutant emissions 

from the Waste sector is presented in Table 2-13. 

Figure 2-12: 2003 Waste Sector Greenhouse Gas Sources 

 

4 Landfills also store carbon, due to incomplete degradation of organic materials such as wood products and yard trimmings, as 

described in the Land-Use Change and Forestry chapter. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 41 

Overall, in 2003, waste activities generated emissions of 183.8 Tg CO2 Eq., or 3 percent of total U.S. greenhouse 

gas emissions. 

Table 2-13: Emissions from Waste (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CH4 197.1 179.0 171.0 167.7 165.0 160.9 162.6 167.9

Landfills 172.2 147.4 138.5 134.0 130.7 126.2 126.8 131.2

Wastewater Treatment 24.8 31.7 32.6 33.6 34.3 34.7 35.8 36.8

N2O 13.0 14.7 15.0 15.4 15.6 15.6 15.7 15.9

Human Sewage 13.0 14.7 15.0 15.4 15.6 15.6 15.7 15.9

Total 210.1 193.7 186.0 183.1 180.6 176.5 178.3 183.8

Note: Totals may not sum due to independent rounding. 

Landfills (131.2 Tg CO2 Eq.) 

Landfills are the largest anthropogenic source of CH4 emissions in the United States, accounting for approximately 

24 percent of total CH4 emissions in 2003. In an environment where the oxygen content is low or zero, anaerobic 

bacteria can decompose organic materials, such as yard waste, household waste, food waste, and paper, resulting in 

the generation of CH4 and biogenic CO2. Site-specific factors, such as waste composition, moisture, and landfill 

size, influence the level of methane generation. 

From 1990 to 2003, net CH4 emissions from landfills decreased by approximately 24 percent, with small increases 

occurring in some interim years. This downward trend in overall emissions is the result of increases in the amount 

of landfill gas collected and combusted by landfill operators, which has more than offset the additional CH4

emissions resulting from an increase in the amount of municipal solid waste landfilled. 

Wastewater Treatment (36.8 Tg CO2 Eq.) 

Wastewater from domestic sources (i.e., municipal sewage) and industrial sources is treated to remove soluble 

organic matter, suspended solids, pathogenic organisms and chemical contaminants. Soluble organic matter is 

generally removed using biological processes in which microorganisms consume the organic matter for maintenance 

and growth. Microorganisms can biodegrade soluble organic material in wastewater under aerobic or anaerobic 

conditions, with the latter condition producing CH4. During collection and treatment, wastewater may be 

accidentally or deliberately managed under anaerobic conditions. In addition, the sludge may be further 

biodegraded under aerobic or anaerobic conditions. Untreated wastewater may also produce CH4 if contained under 

anaerobic conditions. In 2003, wastewater treatment was the source of approximately 7 percent of U.S. CH4

emissions. 

Human Sewage (Domestic Wastewater) (15.9 Tg CO2 Eq.) 

Domestic human sewage is usually mixed with other household wastewater, which includes shower drains, sink 

drains, washing machine effluent, etc., and transported by a collection system to either a direct discharge, an on-site 

or decentralized or centralized wastewater treatment system. After processing, treated effluent may be discharged to 

a receiving water environment (e.g., river, lake, estuary, etc.), applied to soils, or disposed of below the surface. 

Nitrous oxide may be generated during both nitrification and denitrification of the nitrogen present, usually in the 

form of urea, ammonia, and proteins. Emissions of N2O from treated human sewage discharged into aquatic 

environments accounted for 4 percent of U.S. N2O emissions in 2003. From 1990 to 2003, emissions from this 

source category increased by 22 percent. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 42 

2.2. Emissions by Economic Sector 

Throughout this report, emission estimates are grouped into six sectors (i.e., chapters) defined by the IPCC: 

Energy, Industrial Processes, Solvent Use, Agriculture, Land-Use 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, and Agriculture, as well as 

U.S. Territories. Using this categorization, emissions from electricity generation accounted for the largest portion 

(33 percent) of U.S. greenhouse gas emissions in 2003. Transportation activities, in aggregate, accounted for the 

second largest portion (27 percent). Additional discussion and data on these two economic sectors is provided 

below. 

Emissions from industry accounted for 19 percent of U.S. greenhouse gas emissions in 2003. In contrast to 

electricity generation and transportation, emissions from industry have declined over the past decade, as structural 

changes have occurred in the U.S. economy (i.e., shifts from a manufacturing-based to a service-based economy), 

fuel switching has occurred, and efficiency improvements have been made. The residential, agriculture, commercial 

economic sectors, and U.S. territories, contributed the remaining 20 percent of emissions. The residential economic 

sector accounted for approximately 6 percent, and primarily consisted of CO2 emissions from fossil fuel 

combustion. Activities related to agriculture accounted for roughly 7 percent of U.S. emissions, but unlike all other 

economic sectors these emissions were dominated by non-CO2 emissions. The commercial sector accounted for 

about 7 percent of emissions, while U.S. territories accounted for 1 percent of total emissions. 

Carbon dioxide was also emitted and sequestered by a variety of activities related to forest management practices, 

tree planting in urban areas, the management of agricultural soils, and landfilling of yard trimmings. 

Table 2-14 presents a detailed breakdown of emissions from each of these economic sectors by source category, as 

they are defined in this report. Figure 2-13 shows the trend in emissions by sector from 1990 to 2003. 

Figure 2-13: Emissions Allocated to Economic Sectors 

Table 2-14: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq. and Percent of Total in 

2003) 

Sector/Source 1990 1997 1998 1999 2000 2001 2002 2003 Percenta

Electricity Generation 1,841.8 2,104.6 2,186.8 2,197.3 2,299.0 2,254.9 2,269.7 2,296.2 33.3%

CO2 from Fossil Fuel 

Combustion 1,790.3 2,051.9 2,139.0 2,149.3 2,252.1 2,207.8 2,223.0 2,250.5 32.6%

Stationary Combustiond

 8.1 9.3 9.5 9.5 10.0 9.7 9.8 10.0 0.1%

Electrical Transmission and 

Distributionc

 29.2 21.7 17.1 16.4 15.6 15.4 14.7 14.1 0.2%

Waste Combustionb

 11.3 18.1 17.4 18.0 18.3 19.2 19.2 19.2 0.3%

Limestone and Dolomite 

Use 2.8 3.6 3.7 4.0 3.0 2.9 2.9 2.4 +

Transportation 1,506.8 1,693.0 1,728.7 1,790.0 1,839.6 1,819.8 1,851.6 1,864.4 27.0%

CO2 from Fossil Fuel 

Combustion 1,446.8 1,603.3 1,633.4 1,690.8 1,737.7 1,719.7 1,752.3 1,767.2 25.6%

Mobile Combustiond

 48.1 58.7 58.7 57.7 56.1 51.6 48.1 44.3 0.6%

Substitution of ODSe

 + 19.4 24.4 29.3 33.8 37.4 40.4 42.7 0.6%

Non-Energy Use of Fuels 11.9 11.6 12.1 12.3 12.1 11.1 10.9 10.2 0.1%

Industry 1,446.1 1,509.1 1,470.6 1,427.9 1,431.8 1,371.0 1,365.7 1,331.4 19.3%

CO2 from Fossil Fuel 

Combustion 836.5 905.4 853.9 828.0 854.6 827.9 824.3 806.3 11.7%

Non-Energy Use of Fuels 90.4 102.6 117.0 121.8 101.7 103.7 102.6 101.9 1.5%

Stationary Combustion 5.3 5.9 5.5 5.5 5.6 5.2 5.3 5.3 0.1%
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 43 

Coal Mining 81.9 62.6 62.8 58.9 56.2 55.6 52.4 53.8 0.8%

Abandoned Coal Mines 6.1 8.1 7.2 7.3 7.7 6.9 6.4 6.4 0.1%

Natural Gas Systems 128.3 133.6 131.8 127.4 132.1 131.8 130.6 125.9 1.8%

Petroleum Systems 20.0 18.8 18.5 17.8 17.6 17.4 17.1 17.1 0.2%

Natural Gas Flaring 5.8 7.9 6.6 6.9 5.8 6.1 6.2 6.0 0.1%

Titanium Dioxide 

Production 1.3 1.8 1.8 1.9 1.9 1.9 2.0 2.0 +

Aluminum Productionh

 24.7 16.6 14.8 14.9 14.7 8.1 9.5 8.0 0.1%

Iron and Steel Productionf

 86.7 73.1 68.6 65.5 66.9 60.0 56.1 54.8 0.8%

Ferroalloys 2.0 2.0 2.0 2.0 1.7 1.3 1.2 1.4 +

Ammonia Manufacture 19.3 20.7 21.9 20.6 19.6 16.7 18.6 15.6 0.2%

Cement Manufacture 33.3 38.3 39.2 40.0 41.2 41.4 42.9 43.0 0.6%

Lime Manufacture 11.2 13.7 13.9 13.5 13.3 12.8 12.3 13.0 0.2%

Limestone and Dolomite 

Use 2.8 3.6 3.7 4.0 3.0 2.9 2.9 2.4 +

Soda Ash Manufacture and 

Consumption 4.1 4.4 4.3 4.2 4.2 4.1 4.1 4.1 0.1%

Carbon Dioxide 

Consumption 0.9 0.8 0.9 0.8 1.0 0.8 1.0 1.3 +

Petrochemical Production 3.4 4.6 4.7 4.8 4.7 4.2 4.4 4.3 0.1%

Silicon Carbide Production + + + + + + + + +

Phosphoric Acid Production 1.5 1.5 1.6 1.5 1.4 1.3 1.3 1.4 +

Adipic Acid 15.2 10.3 6.0 5.5 6.0 4.9 5.9 6.0 0.1%

Nitric Acid 17.8 21.2 20.9 20.1 19.6 15.9 17.2 15.8 0.2%

N2O Product Usage 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8 0.1%

HCFC-22 Productiong

 35.0 30.0 40.1 30.4 29.8 19.8 19.8 12.3 0.2%

Semiconductor 

Manufacturee

 2.9 6.3 7.1 7.2 6.3 4.5 4.4 4.3 0.1%

Magnesium Production and 

Processingc

 5.4 6.3 5.8 6.0 3.2 2.6 2.6 3.0 +

Substitution of ODSe

 0.1 4.1 5.1 6.4 7.4 8.4 9.7 11.2 0.2%

Agriculture 473.3 492.0 508.4 486.9 495.3 488.6 485.6 486.4 7.0%

CO2 from Fossil Fuel 

Combustion 46.3 58.4 57.6 60.0 50.4 50.3 52.3 52.2 0.8%

Stationary Combustiond

 + + + + + + + + +

Mobile Combustiond

 0.4 0.5 0.5 0.5 0.4 0.4 0.5 0.5 +

Enteric Fermentation 117.9 118.3 116.7 116.8 115.6 114.5 114.6 115.0 1.7%

Manure Managementd

 47.4 53.7 56.2 56.2 55.9 56.9 57.3 56.7 0.8%

Rice Cultivation 7.1 7.5 7.9 8.3 7.5 7.6 6.8 6.9 0.1%

Agricultural Residue 

Burningd

 1.1 1.2 1.2 1.2 1.2 1.2 1.1 1.2 +

Agricultural Soil 

Management 253.0 252.0 267.7 243.4 263.9 257.1 252.6 253.5 3.7%

Forest Soil Fertilization 0.1 0.3 0.4 0.5 0.4 0.4 0.4 0.4 +

Commercial 435.4 445.2 424.2 426.8 440.7 431.4 440.2 453.5 7.0%

CO2 from Fossil Fuel 

Combustion 224.2 237.2 219.7 222.3 235.2 226.7 230.0 234.0 3.4%

Stationary Combustiond

 1.1 1.1 1.1 1.1 1.2 1.0 1.1 1.1 +

Substitution of ODSd

 + 13.1 17.4 20.3 23.8 27.1 30.8 34.7 0.5%

Landfills 172.2 147.4 138.5 134.0 130.7 126.2 126.8 131.2 1.9%

Human Sewage 13.0 14.7 15.0 15.4 15.6 15.6 15.7 15.9 0.2%

Wastewater Treatment 24.8 31.7 32.6 33.6 34.3 34.7 35.8 36.8 0.5%

Residential 350.9 391.0 358.4 379.5 399.7 387.1 391.6 406.1 5.9%

CO2 from Fossil Fuel 

Combustion 339.6 370.6 338.6 359.3 379.1 367.0 371.4 385.1 5.6%
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 44 

Stationary Combustionc 5.5 4.4 4.0 4.3 4.5 4.0 3.6 3.9 0.1%

Substitution of ODSe 0.3 9.9 9.6 9.8 10.1 10.3 10.6 11.0 0.2%

Settlement Soil Fertilization 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0 0.1%

U.S. Territories 33.8 42.7 42.7 43.9 47.0 54.1 53.6 62.3 0.9%

CO2 from Fossil Fuel 

Combustion 33.8 42.7 42.7 43.9 47.0 54.1 53.6 62.3 0.9%

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2 100.0%

Sinks (1,042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0) -12.0%

Forests (949.3) (851.0) (805.5) (751.7) (747.9) (750.9) (751.5) (752.7) -10.9%

Urban Trees (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) -0.9%

Agricultural Soils (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6) -0.1%

Landfilled Yard Trimmings (26.0) (12.9) (12.5) (11.4) (10.2) (10.3) (10.2) (10.1) -0.1%

Net Emissions (Sources and 

Sinks) 5,046.1 5,747.5 5,838.8 5,926.1 6,130.8 5,980.1 6,031.6 6,072.2 

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%. 

- Not applicable. 

a

 Percent of total emissions for year 2003. 

b

 Includes both CO2 and N2O. c

 SF6 emitted. d

 Includes both CH4 and N2O. e

 May include a mixture of HFCs, PFCs, and SF6. f

 Includes both CH4 and CO2. g

 HFC-23 emitted. 

h

 Includes both CO2 and PFCs. 

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 33 percent of total U.S. greenhouse gas 

emissions in 2003. Emissions increased by 25 percent since 1990, as electricity demand grew and fossil fuels 

remained the dominant energy source for generation. The electricity generation sector in the United States is 

composed of traditional electric utilities as well as other entities, such as power marketers and nonutility 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 2-15 provides a detailed summary of emissions 

from electricity generation-related activities. 

Table 2-15: Electricity Generation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 

Gas/Fuel Type or Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 1,804.0 2,073.3 2,159.9 2,171.0 2,273.1 2,229.4 2,244.8 2,271.7 

CO2 from Fossil Fuel Combustion 1,790.3 2,051.9 2,139.0 2,149.3 2,252.1 2,207.8 2,223.0 2,250.5 

Coal 1,513.0 1,758.4 1,786.4 1,792.4 1,880.0 1,817.4 1,839.7 1,876.3 

Natural Gas 176.0 218.9 248.0 259.9 280.7 289.1 305.6 277.6 

Petroleum 101.0 74.3 104.3 96.7 91.0 100.9 77.4 96.3 

Geothermal 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 

Waste Combustion 10.9 17.8 17.1 17.6 18.0 18.8 18.8 18.8 

Limestone and Dolomite Use 2.8 3.6 3.7 4.0 3.0 2.9 2.9 2.4 

CH4 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.7 

Stationary Combustion* 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.7 

N2O 8.0 9.0 9.2 9.2 9.6 9.4 9.5 9.8 

Stationary Combustion* 7.6 8.6 8.9 8.9 9.3 9.0 9.1 9.3 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 45 

Waste Combustion 0.4 0.4 0.3 0.3 0.4 0.4 0.5 0.5 

SF6 29.2 21.7 17.1 16.4 15.6 15.4 14.7 14.1 

Electrical Transmission and 

Distribution 29.2 21.7 17.1 16.4 15.6 15.4 14.7 14.1 

Total 1,841.8 2,104.6 2,186.8 2,197.3 2,299.0 2,254.9 2,269.7 2,296.2 

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 2004a and Duffield 2004). These three 

source categories include CO2 from fossil fuel combustion, CH4 and N2O from stationary sources, and SF6 from 

electrical transmission and distribution systems.5

When emissions from electricity are distributed among these sectors, industry accounts for the largest share of U.S. 

greenhouse gas emissions (30 percent). Emissions from the residential and commercial sectors also increase 

substantially due to their relatively large share of electricity consumption. Transportation activities remain the 

second largest contributor to emissions. In all sectors except agriculture, CO2 accounts for more than 75 percent of 

greenhouse gas emissions, primarily from the combustion of fossil fuels. 

Table 2-16 presents a detailed breakdown of emissions from each of these economic sectors, with emissions from 

electricity generation distributed to them. Figure 2-14 shows the trend in these emissions by sector from 1990 to 

2003. 

Figure 2-14: Emissions with Electricity Distributed to Economic Sectors 

Table 2-16: U.S Greenhouse Gas Emissions by “Economic Sector” and Gas with Electricity-Related Emissions 

Distributed (Tg CO2 Eq.) and Percent of Total in 2003 

Sector/Gas 1990 1997 1998 1999 2000 2001 2002 2003 Percenta

Industry 2,075.7 2,247.3 2,223.2 2,190.1 2,207.7 2,074.0 2,062.9 2,040.1 29.6%

Direct Emissions 1,446.1 1,509.1 1,470.6 1,427.9 1,431.8 1,371.0 1,365.7 1,331.4 19.3%

CO2 1,103.0 1,183.2 1,143.2 1,118.6 1,123.7 1,086.7 1,081.7 1,059.0 15.3%

CH4 240.9 228.4 225.3 216.5 218.7 216.4 211.2 207.9 3.0%

N2O 40.5 39.8 34.9 33.6 33.7 28.7 31.1 29.8 0.4%

HFCs, PFCs, and SF6 61.7 57.7 67.2 59.1 55.6 39.3 41.8 34.6 0.5%

Electricity-Related 629.6 738.2 752.5 762.2 775.9 702.9 697.2 708.7 10.3%

CO2 616.7 727.2 743.3 753.1 767.2 695.0 689.5 701.2 10.2%

CH4 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 +

N2O 2.7 3.2 3.2 3.2 3.3 2.9 2.9 3.0 +

SF6 10.0 7.6 5.9 5.7 5.3 4.8 4.5 4.4 0.1%

Transportation 1,509.9 1,696.1 1,731.8 1,793.2 1,843.0 1,823.2 1,854.8 1,867.6 27.1%

Direct Emissions 1,506.8 1,693.0 1,728.7 1,790.0 1,839.6 1,819.8 1,851.6 1,864.4 27.0%

CO2 1,458.7 1,614.9 1,645.6 1,703.0 1,749.8 1,730.8 1,763.2 1,777.4 25.8%

CH4 4.4 3.5 3.4 3.1 2.9 2.6 2.4 2.2 +

N2O 43.7 55.2 55.3 54.6 53.2 49.0 45.6 42.1 0.6%

HFCsb

 + 19.4 24.4 29.3 33.8 37.4 40.4 42.7 0.6%

 

5 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 46 

Electricity-Related 3.1 3.1 3.1 3.1 3.3 3.4 3.1 3.2 +

CO2 3.0 3.0 3.0 3.1 3.3 3.3 3.1 3.2 +

CH4 + + + + + + + + +

N2O + + + + + + + + +

SF6 + + + + + + + + +

Commercial 981.6 1,083.8 1,093.5 1,104.9 1,161.8 1,170.6 1,178.5 1,196.8 17.3%

Direct Emissions 435.4 445.2 424.2 426.8 440.7 431.4 440.2 453.5 6.6%

CO2 224.2 237.2 219.7 222.3 235.2 226.7 230.0 234.0 3.4%

CH4 197.8 179.8 171.8 168.5 165.8 161.6 163.4 168.7 2.4%

N2O 13.4 15.0 15.3 15.8 15.9 16.0 16.0 16.2 0.2%

HFCs + 13.1 17.4 20.3 23.8 27.1 30.8 34.7 0.5%

Electricity-Related 546.2 638.7 669.3 678.1 721.1 739.2 738.3 743.3 10.8%

CO2 535.0 629.2 661.0 670.0 713.0 730.9 730.2 735.3 10.7%

CH4 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 +

N2O 2.4 2.7 2.8 2.8 3.0 3.1 3.1 3.2 +

SF6 8.7 6.6 5.2 5.1 4.9 5.1 4.8 4.6 0.1%

Residential 953.0 1,060.3 1,060.0 1,082.9 1,141.4 1,129.6 1,159.5 1,183.7 17.2%

Direct Emissions 350.9 391.0 358.4 379.5 399.7 387.1 391.6 406.1 5.9%

CO2 339.6 370.6 338.6 359.3 379.1 367.0 371.4 385.1 5.6%

CH4 4.4 3.5 3.1 3.4 3.5 3.1 2.8 3.1 +

N2O 6.6 7.1 7.0 7.1 6.9 6.7 6.8 6.9 0.1%

HFCs 0.3 9.9 9.6 9.8 10.1 10.3 10.6 11.0 0.2%

Electricity-Related 602.1 669.3 701.6 703.4 741.7 742.5 767.9 777.6 11.3%

CO2 589.7 659.3 693.0 695.0 733.3 734.1 759.4 769.3 11.2%

CH4 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 +

N2O 2.6 2.9 3.0 2.9 3.1 3.1 3.2 3.3 +

SF6 9.6 6.9 5.5 5.2 5.0 5.1 5.0 4.8 0.1%

Agriculture 534.1 547.4 568.6 537.3 552.3 555.5 548.8 549.8 8.0%

Direct Emissions 473.3 492.0 508.4 486.9 495.3 488.6 485.6 486.4 7.0%

CO2 46.3 58.4 57.6 60.0 50.4 50.3 52.3 52.2 0.8%

CH4 157.0 163.1 164.4 164.8 162.1 162.0 161.6 162.0 2.3%

N2O 270.0 270.5 286.3 262.1 282.8 276.3 271.7 272.2 3.9%

Electricity-Related 60.8 55.4 60.2 50.4 57.0 66.9 63.2 63.4 0.9%

CO2 59.6 54.6 59.5 49.8 56.3 66.2 62.5 62.7 0.9%

CH4 + + + + + + + + +

N2O 0.3 0.2 0.3 0.2 0.2 0.3 0.3 0.3 +

SF6 1.0 0.6 0.5 0.4 0.4 0.5 0.4 0.4 +

U.S. Territories 33.8 42.7 42.7 43.9 47.0 54.1 53.6 62.3 0.9%

CO2 33.8 42.7 42.7 43.9 47.0 54.1 53.6 62.3 0.9%

Total 6,088.1 6,677.5 6,719.7 6,752.2 6,953.2 6,806.9 6,858.1 6,900.2 100.0% 

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%. a

 Percents for year 2003. 

b Includes primarily HFC-134a. 

Transportation 

Transportation activities accounted for 27 percent of U.S. greenhouse gas emissions in 2003. Table 2-17 provides a 

detailed summary of greenhouse gas emissions from transportation-related activities. Total emissions in Table 2-17 

differ slightly from those shown in Table 2-16 primarily because the table below excludes a few minor nontransportation mobile sources, such as construction and industrial equipment. 

From 1990 to 2003, transportation emissions rose by 24 percent due, in part, to increased demand for travel and the 

stagnation of fuel efficiency across the U.S. vehicle fleet. Since the 1970s, the number of highway vehicles 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 47 

registered in the United States has increased faster than the overall population, according to the Federal Highway 

Administration (FHWA). Likewise, the number of miles driven (up 35 percent from 1990 to 2003) and the gallons 

of gasoline consumed each year in the United States have increased steadily since the 1980s, according to the 

FHWA and Energy Information Administration, respectively. These increases in motor vehicle usage are the result 

of a confluence of factors including population growth, economic growth, urban sprawl, low fuel prices, and 

increasing popularity of sport utility vehicles and other light-duty trucks that tend to have lower fuel efficiency. 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 1990s.

Almost all of the energy consumed for transportation was supplied by petroleum-based products, with nearly twothirds 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 22 percent from 1990 to 

2003. This rise in CO2 emissions, combined with an increase of 42.7 Tg CO2 Eq. in HFC emissions over the same 

period, led to an increase in overall emissions from transportation activities of 24 percent. 

Table 2-17: Transportation-Related Greenhouse Gas Emissions (Tg CO2 Eq.) 

Gas/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

CO2 1,461.7 1,618.0 1,648.7 1,706.2 1,753.1 1,734.2 1,766.4 1,780.7 

Passenger Cars 612.5 595.5 613.8 622.4 623.4 625.7 639.5 633.7 

Light-Duty Trucks 312.2 421.6 432.1 449.2 452.1 456.2 468.1 478.8 

Other Trucks 217.0 279.9 290.4 304.3 320.4 327.5 327.5 341.2 

Buses 7.8 9.1 9.3 10.4 10.2 9.6 9.1 8.9 

Aircrafta

 177.2 179.0 181.3 186.7 193.2 183.4 174.9 171.3 

Ships and Boats 49.2 38.7 32.4 42.3 63.1 42.7 57.2 57.5 

Locomotives 36.3 40.0 40.5 41.7 41.8 42.8 41.0 42.8 

Otherb 49.4 54.2 48.7 49.3 48.9 46.1 49.0 46.6 

International Bunker Fuelsc

 93.6 106.1 103.3 102.7 102.2 98.5 89.5 84.2 

CH4 4.6 3.8 3.7 3.4 3.2 2.9 2.7 2.4 

Passenger Cars 2.6 1.9 1.8 1.7 1.5 1.4 1.2 1.1 

Light-Duty Trucks 1.4 1.3 1.3 1.1 1.0 0.9 0.9 0.8 

Other Trucks and Buses 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 

Aircraft 0.2 0.2 0.1 0.2 0.2 0.1 0.1 0.1 

Ships and Boats 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Locomotives 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Motorcycles + + + + + + + + 

International Bunker Fuelsc

 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 

N2O 42.9 54.2 54.4 53.7 52.2 47.9 44.5 40.9 

Passenger Cars 25.5 26.7 26.7 25.9 24.7 23.1 21.6 19.9 

Light-Duty Trucks 14.1 23.7 23.7 23.6 23.0 20.6 18.6 16.8 

Other Trucks and Buses 0.9 1.4 1.6 1.7 1.7 1.7 1.8 1.8 

Aircraft 1.7 1.7 1.8 1.8 1.9 1.8 1.7 1.7 

Ships and Boats 0.4 0.3 0.3 0.3 0.5 0.3 0.5 0.5 

Locomotives 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Motorcycles + + + + + + + + 

International Bunker Fuelsc

 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 

HFCs + 19.4 24.4 29.3 33.8 37.4 40.4 42.7 

Mobile Air Conditionersd

 + 13.8 17.4 20.8 24.0 26.7 28.8 30.3 

Refrigerated Transport + 5.5 7.0 8.5 9.8 10.8 11.5 12.3 

Total 1,509.3 1,695.4 1,731.1 1,792.5 1,842.2 1,822.4 1,853.9 1,866.7

+ Does not exceed 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. a

 Aircraft emissions consist of emissions from all jet fuel (less bunker fuels) and aviation gas consumption. b

 "Other" CO2 emissions include motorcycles, pipelines, and lubricants. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 48 

c

 Emissions from International Bunker Fuels include emissions from both civilian and military activities, but are not included in 

totals. 

d

 Includes primarily HFC-134a. 

[BEGIN TEXT BOX] 

Box 2-2: Methodology for Aggregating Emissions by Economic Sector 

In order to aggregate emissions by economic sector, source category emission estimates were generated according 

to the methodologies outlined in the appropriate sections of this report. Those emissions were then simply 

reallocated into economic sectors. In most cases, the IPCC subcategories distinctly fit into an apparent economic 

sector category. Several exceptions exist, and the methodologies used to disaggregate these subcategories are 

described below: 

● Agricultural CO2 Emissions from Fossil Fuel Combustion, and non-CO2 emissions from Stationary and Mobile 

Combustion. Emissions from on-farm energy use were accounted for in the Energy chapter as part of the 

industrial and transportation end-use sectors. To calculate agricultural emissions related to fossil fuel 

combustion, energy consumption estimates were obtained from economic survey data from the U.S. 

Department of Agriculture (Duffield 2004) and fuel sales data (EIA 1991 through 2004). To avoid doublecounting, emission estimates of CO2 from fossil fuel combustion and non-CO2 from stationary and mobile 

sources were subtracted from the industrial economic sector, although some of these fuels may have been 

originally accounted for under the transportation end-use sector. 

● Landfills, Wastewater Treatment, and Human Sewage. CH4 emissions from landfills and wastewater treatment, 

as well as N2O emissions from human sewage, were allocated to the commercial sector. 

● Waste Combustion. CO2 and N2O emissions from waste combustion were allocated completely to the 

electricity generation sector since nearly all waste combustion occurs in waste-to-energy facilities. 

● Limestone and Dolomite Use. CO2 emissions from limestone and dolomite use are allocated to the electricity 

generation (50 percent) and industrial (50 percent) sectors, because 50 percent of the total emissions for this 

source are used in flue gas desulfurization. 

● Substitution of Ozone Depleting Substances. All greenhouse gas emissions resulting from the substitution of 

ozone depleting substances were placed in the industrial economic sector, with the exception of emissions from 

domestic, commercial, mobile and transport refrigeration/air-conditioning systems were placed in the 

residential, commercial, and transportation sectors, respectively. Emissions from non-MDI aerosols were 

attributed to the residential economic sector.

● Settlement Soil Fertilization, Forest Soil Fertilization. Emissions from settlement soil fertilization were 

allocated to the residential economic sector; forest soil fertilization was allocated to the agriculture economic 

sector. 

[END TEXT BOX] 

2.3. Ambient Air Pollutant Emissions 

In the United States, carbon monoxide (CO), nitrogen oxides (NOx), nonmethane volatile organic compounds 

(NMVOCs), and sulfur dioxide (SO2) are referred to as "ambient air pollutants," and are regulated under the Clean 

Air Act in an effort to protect human health and the environment. These pollutants do not have a direct global 

warming effect, but indirectly affect terrestrial radiation absorption by influencing the formation and destruction of 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 49 

tropospheric and stratospheric ozone, or, in the case of SO2, by affecting the absorptive characteristics of the 

atmosphere. Additionally, some of these pollutants 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 nitrous oxide (N2O). NMVOCs⎯which include hundreds of organic compounds that 

participate in atmospheric chemical reactions (i.e., propane, butane, xylene, toluene, ethane and many others)⎯are 

emitted primarily from transportation, industrial processes, and non-industrial consumption of organic solvents. In 

the United States, SO2 is primarily emitted from coal combustion for electric power generation and the metals 

industry. Sulfur-containing compounds emitted into the atmosphere tend t 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 

ambient air pollutant formation into greenhouse gases is carbon monoxide’s interaction with the hydroxyl 

radical⎯the major atmospheric sink for methane emissions⎯to form CO2. Therefore, increased atmospheric 

concentrations of CO limit the number of hydroxyl molecules (OH) available to destroy methane. 

Since 1970, the United States has published estimates of annual emissions of ambient air pollutants (EPA 2004).6 

Table 2-18 shows that fuel combustion accounts for the majority of emissions of these 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 2-18: Emissions of NOx, CO, NMVOCs, and SO2 (Gg) 

Gas/Activity 1990 1997 1998 1999 2000 2001 2002 2003

NOx 22,860 22,284 21,964 20,530 20,288 19,414 18,850 18,573

Stationary Fossil Fuel Combustion 9,884 9,578 9,419 8,344 8,002 7,667 7,523 7,222

Mobile Fossil Fuel Combustion 12,134 11,768 11,592 11,300 11,395 10,823 10,389 10,418

Oil and Gas Activities 139 130 130 109 111 113 135 124

Waste Combustion 82 140 145 143 114 114 134 121

Industrial Processes 591 629 637 595 626 656 630 648

Solvent Use 1 3 3 3 3 3 5 4

Field Burning of Agricultural 

Residues 28 34 35 34 35 35 33 33

Waste 0 3 3 3 2 2 2 2

CO 130,580 101,138 98,984 94,361 92,895 89,329 87,451 85,077

Stationary Fossil Fuel Combustion 4,999 3,927 3,927 5,024 4,340 4,377 4,020 4,454

Mobile Fossil Fuel Combustion 119,482 90,284 87,940 83,484 83,680 79,972 78,574 75,526

Oil and Gas Activities 302 333 332 145 146 147 116 125

Waste Combustion 978 2,668 2,826 2,725 1,670 1,672 1,672 1,674

Industrial Processes 4,124 3,153 3,163 2,156 2,217 2,339 2,308 2,431

Solvent Use 4 1 1 46 46 45 46 65

Field Burning of Agricultural 

Residues 689 767 789 767 790 770 707 794

Waste 1 5 5 13 8 8 8 8

NMVOCs 20,937 16,994 16,403 15,869 15,228 15,048 14,222 13,939

Stationary Fossil Fuel Combustion 912 1,016 1,016 1,045 1,077 1,080 926 1,007

Mobile Fossil Fuel Combustion 10,933 7,928 7,742 7,586 7,230 6,872 6,560 6,351

Oil and Gas Activities 555 442 440 414 389 400 340 345

 

6 NOx and CO emission estimates from field burning of agricultural residues were estimated separately, and therefore not taken 

from EPA (2004). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 50 

Waste Combustion 222 313 326 302 257 258 281 263

Industrial Processes 2,426 2,038 2,047 1,813 1,773 1,769 1,725 1,711

Solvent Use 5,217 5,100 4,671 4,569 4,384 4,547 4,256 4,138

Field Burning of Agricultural 

Residues NA NA NA NA NA NA NA NA

Waste 673 157 161 140 119 122 133 125

SO2 20,936 17,091 17,189 15,917 14,829 14,452 13,928 14,463

Stationary Fossil Fuel Combustion 18,407 15,104 15,191 13,915 12,848 12,461 11,946 12,477

Mobile Fossil Fuel Combustion 793 659 665 704 632 624 631 634

Oil and Gas Activities 390 312 310 283 286 289 315 293

Waste Combustion 39 29 30 30 29 30 24 28

Industrial Processes 1,306 985 991 984 1,031 1,047 1,009 1,029

Solvent Use 0 1 1 1 1 1 2 2

Field Burning of Agricultural 

Residues NA NA NA NA NA NA NA NA

Waste 0 1 1 1 1 1 1 1

Source: (EPA 2004) except for estimates from field burning of agricultural residues. 

+ Does not exceed 0.5 Gg 

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 69 

percent in 2003. 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] 
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 U.S. Greenhouse Gas Emissions Relative to 1990

1.0%

3.3%

0.9% 0.6% 0.5%

3.0%

0.8% 0.6%

-0.8%

-2.1%

1.4%

2.1%

1.5%

-3%

-2%

-1%

0%

1%

2%

3%

4%

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

33

163

255

319

533

589 632 664

865

719

770 812

-50

-100

0

100

200

300

400

500

600

700

800

900

1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Tg CO2 Eq.

6,900

6,720 6,807 6,858 6,678 6,407 6,621 6,343 6,121 6,251 6,038 6,088

6,752 6,953

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Tg CO2 Eq.

HFCs, PFCs, & SF

Nitrous Oxide

Methane

Carbon Dioxide

6
Figure 2-4: U.S. Greenhouse Gas Emissions Per Capita and Per Dollar of Gross Domestic Product

Figure 2-5: U.S. Greenhouse Gas Emissions and Sinks by Chapter/IPCC Sector

(2,000)

(1,000)

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

19901991199219931994199519961997199819992000200120022003

Tg CO2 Eq.

Energy

Land-Use Change and Forestry (sink)

Industrial Processes

Agriculture

Waste

Emissions per 

$GDP

Emissions per 

capita

Population

Real GDP

70

80

90

100

110

120

130

140

150

19901991199219931994199519961997199819992000200120022003

Index (1990 = 100)
Figure 2-6: 2003 Energy Sector Greenhouse Gas Sources

5,551.6

0 20 40 60 80 100 120 140

Natural Gas Flaring

Petroleum Systems

Waste Combustion

Stationary Sources

Coal Mining

Mobile Sources

Natural Gas Systems

Fossil Fuel Combustion

Tg CO2 Eq.

Energy as a Portion 

of all Emissions

86.6%

Figure 2-8: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type

Figure 2-9: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion

Note: Electricity generation also includes emissions of less than 1 Tg CO2 Eq. from geothermal-based 

electricity generation.

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Residential

Commercial

Industrial

Transportation

U.S. Territories

Tg CO2 Eq

From Electricity

Consumption

From Direct Fossil

Fuel Combustion

0

500

1,000

1,500

2,000

Residential

Commercial

Industrial

Transportation

Electricity

Generation

U.S.

Territories

Tg CO2 Eq

Natural Gas

Petroleum

Coal

Relative Contribution by 

Fuel Type
Figure 2-10: 2003 Industrial Processes Chapter Greenhouse Gas Sources

<0.05

0 10 20 30 40 50 60 70 80 90 100

Silicon Carbide Production

Carbon Dioxide Consumption

Ferroalloys

Phosphoric Acid Production

Titanium Dioxide Production

Magnesium Production and Processing

Soda Ash Manufacture and Consumption

Petrochemical Production

Semiconductor Manufacture

Limestone and Dolomite Use

Adipic Acid

Aluminum Production

HCFC-22 Production

Lime Manufacture

Electrical Transmission and Distribution

Ammonia Production and Urea Application

Nitric Acid

Cement Manufacture

Iron and Steel Production

Substitution of Ozone Depleting Substances

Tg CO2 Eq.

Industrial 

Processes 

as a Portion of all 

Emissions

4.5%
Figure 2-11: 2003 Agriculture Chapter GHG Sources

Figure 2-12: 2003 Waste Chapter Greenhouse Gas Sources

1.2

0 50 100 150 200 250

Agricultural Residue

Burning

Rice Cultivation

Manure

Management

Enteric Fermentation

Agricultural Soil

Management

Tg CO2 Eq

Agriculture as a 

Portion of all 

Emissions

6.3%

0 20 40 60 80 100 120 140

Human Sewage

Wastewater Treatment

Landfills

Tg CO2 Eq

Waste as a Portion of all 

Emissions

2.7%
Figure 2-13: Emissions Allocated to Economic Sectors

Figure 2-14: Emissions with Electricity Distributed to Economic Sectors

0

500

1,000

1,500

2,000

2,500

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Year

(Tg CO2 Eq.)

Transportation

Electricity Generation

Industry

Residential 

Agriculture

Commercial

0

500

1,000

1,500

2,000

2,500

19901991199219931994199519961997199819992000200120022003

Tg CO2 Eq.

Transportation

Industrial

Residential

Agriculture

Commercial
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 51 

3. Energy 

Energy-related activities were the primary sources of U.S. anthropogenic greenhouse gas emissions, accounting for 

86 percent of total emissions on a carbon equivalent basis in 2003. This included 97, 39, and 15 percent of the 

nation's carbon dioxide (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 carbon 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 24,240 Tg CO2 were added to the atmosphere 

through the combustion of fossil fuels in 2000, of which the United States accounted for about 23 percent.1 Due to 

the relative importance of fossil fuel combustion-related CO2 emissions, they are considered separately, and in more 

detail than other energy-related emissions (see Figure 3-2). Fossil fuel combustion also emits CH4 and N2O, as well 

as ambient air pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), and non-methane 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 ambient air pollutant 

emissions. 

Figure 3-1: 2003 Energy Sector Greenhouse Gas Sources 

Figure 3-2: 2003 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. Carbon dioxide emissions from 

these activities, however, are not included in national emissions totals because biomass fuels are of biogenic origin. 

It is assumed that the carbon 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 carbon cycle are accounted for within the Land-Use Change and Forestry sector. 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 energyrelated activities were 5,963.4 Tg CO2 Eq. in 2003, an increase of 16 percent since 1990. 

Table 3-1: Emissions from Energy (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 4,836.4 5,409.1 5,437.7 5,512.1 5,693.5 5,592.9 5,645.3 5,694.3

Fossil Fuel Combustion 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6

Non-Energy Use of Fuels 108.0 120.3 135.4 141.6 124.7 120.1 118.8 118.0

Waste Combustion 10.9 17.8 17.1 17.6 18.0 18.8 18.8 18.8

 

1 Global CO2 emissions from fossil fuel combustion were taken from Marland et al. (2003) 

<http://cdiac.esd.ornl.gov/trends/emis/meth_reg.htm>. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 52 

Natural Gas Flaring 5.8 7.9 6.6 6.9 5.8 6.1 6.2 6.0

Biomass-Wood* 212.5 226.3 209.5 214.3 217.6 190.8 195.8 201.0

International Bunker Fuels* 113.5 109.9 114.6 105.3 101.4 97.9 89.5 84.2

Biomass-Ethanol* 4.2 7.0 7.7 8.0 9.2 9.7 11.5 15.8

CH4 248.9 234.6 230.9 222.1 224.3 221.6 215.8 212.7

Natural Gas Systems 128.3 133.6 131.8 127.4 132.1 131.8 130.6 125.9

Coal Mining 81.9 62.6 62.8 58.9 56.2 55.6 52.4 53.8

Petroleum Systems 20.0 18.8 18.5 17.8 17.6 17.4 17.1 17.1

Stationary Sources 7.8 7.4 6.9 7.1 7.3 6.7 6.4 6.7

Mobile Sources 4.8 4.0 3.9 3.6 3.4 3.1 2.9 2.7

Abandoned Coal Mines 6.1 8.1 7.2 7.3 7.7 6.9 6.4 6.4

International Bunker Fuels* 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1

N2O 56.4 69.1 69.1 68.4 67.5 62.8 59.6 56.4

Mobile Sources 43.7 55.2 55.3 54.6 53.2 49.0 45.6 42.1

Stationary Sources 12.3 13.5 13.4 13.5 14.0 13.5 13.5 13.8

Waste Combustion 0.4 0.4 0.3 0.3 0.4 0.4 0.5 0.5

International Bunker Fuels* 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8

Total 5,141.7 5,712.8 5,737.7 5,802.6 5,985.3 5,877.3 5,920.7 5,963.4

* 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: Emissions from Energy (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 4,836,430 5,409,100 5,437,734 5,512,062 5,693,544 5,592,947 5,645,251 5,694,332

Fossil Fuel Combustion 4,711,741 5,263,164 5,278,721 5,345,904 5,545,083 5,447,969 5,501,427 5,551,580

Non-Energy Use of Fuels 107,964.9 120,300.6 135,352.1 141,582.9 124,713.8 120,104.1 118,810.6 118,001.4

Waste Combustion 10,919 17,761 17,094 17,632 17,979 18,781 18,781 18,781

Natural Gas Flaring 5,805 7,874 6,566 6,943 5,769 6,094 6,233 5,970

Biomass-Wood* 212,547 226,265 209,490 214,323 217,577 190,776 195,776 201,042

International Bunker Fuels* 113,503 109,858 114,557 105,294 101,404 97,865 89,489 84,193

Biomass-Ethanol* 4,155 6,978 7,711 8,017 9,188 9,701 11,473 15,771

CH4 11,852 11,170 10,997 10,577 10,680 10,551 10,279 10,126

Natural Gas Systems 6,112 6,363 6,276 6,066 6,289 6,277 6,221 5,998

Coal Mining 3,900 2,983 2,989 2,805 2,677 2,647 2,497 2,561

Petroleum Systems 951 895 879 848 836 831 815 815

Stationary Sources 373 351 328 338 349 318 305 319

Mobile Sources 228 193 185 172 161 147 138 128

Abandoned Coal Mines 288 385 341 349 369 331 303 306

International Bunker Fuels* 8 7 7 6 6 5 4 4

N2O 182 223 223 221 218 203 192 182

Mobile Combustion 141 178 179 176 171 158 147 136

Stationary Combustion 40 44 43 43 45 43 44 45

Waste Combustion 1 1 1 1 1 1 1 1

International Bunker Fuels* 3 3 3 3 3 3 3 2

* 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. 

3.1. Carbon Dioxide Emissions from Fossil Fuel Combustion (IPCC Source 

Category 1A) 

Carbon dioxide emissions from fossil fuel combustion in 2003 increased 0.9 percent from the previous year. This 

increase is primarily a result of increased demand for fuels due to a growing economy, fuel-switching from natural 

gas to coal in the electric power sector, and increased use of heating fuels in the residential sector caused by a colder 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 53 

winter. In 2003, CO2 emissions from fossil fuel combustion were 5,551.6 Tg CO2 Eq., or 18 percent above 

emissions in 1990 (see Table 3-3).2

Table 3-3: CO2 Emissions from Fossil Fuel Combustion by Fuel Type and Sector (Tg CO2 Eq.) 

Fuel/Sector 1990 1997 1998 1999 2000 2001 2002 2003

Coal 1,680.9 1,920.2 1,936.5 1,938.2 2,025.6 1,959.2 1,973.1 2,013.8 

Residential 2.4 1.5 1.2 1.3 1.1 1.1 1.0 1.1 

Commercial 12.1 12.2 8.7 9.7 8.6 9.2 8.6 9.3 

Industrial 152.6 147.2 139.2 133.8 135.0 130.6 122.7 123.4 

Transportation NE NE NE NE NE NE NE NE

Electricity Generation 1,513.0 1,758.4 1,786.4 1,792.4 1,880.0 1,817.4 1,839.7 1,876.3 

U.S. Territories 0.6 1.0 1.0 0.9 0.9 0.9 1.2 3.6 

Natural Gas 1,009.5 1,193.7 1,169.9 1,173.6 1,224.1 1,174.4 1,214.0 1,170.3 

Residential 238.8 270.2 246.5 256.5 270.3 259.7 265.9 277.3 

Commercial 142.6 174.3 163.5 165.2 172.4 164.5 168.7 170.7 

Industrial 416.3 489.1 476.7 456.4 464.6 426.0 435.5 407.9 

Transportation 35.9 41.1 35.1 35.6 35.5 33.9 37.1 35.4 

Electricity Generation 176.0 218.9 248.0 259.9 280.7 289.1 305.6 277.6 

U.S. Territories NO NO NO NO 0.7 1.2 1.2 1.4 

Petroleum 2,020.9 2,148.9 2,172.0 2,233.7 2,295.0 2,314.0 2,314.0 2,367.1 

Residential 98.3 98.9 90.9 101.5 107.7 106.2 104.5 106.7

Commercial 69.5 50.7 47.5 47.3 54.2 53.1 52.7 53.9 

Industrial 313.9 327.4 295.6 297.8 305.5 321.6 318.4 327.3 

Transportation 1,410.9 1,562.2 1,598.3 1,655.2 1,702.2 1,685.8 1,715.2 1,731.8 

Electricity Generation 101.0 74.3 104.3 96.7 91.0 100.9 77.4 96.3 

U.S. Territories 27.4 35.5 35.4 35.2 34.3 46.5 45.7 51.2 

Geothermal* 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.3 

Total 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6 

NE (Not estimated) 

NO (Not occurring) 

+ Does not exceed 0.05 Tg CO2 Eq. 

* 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, and size of houses), the efficiency with 

which energy is used in equipment (e.g., cars, power plants, steel mills, and light bulbs), and social planning and 

consumer behavior (e.g., walking, bicycling, or telecommuting to work instead of driving). 

Carbon dioxide emissions also depend on the source of energy and its carbon intensity. The amount of carbon in 

fuels varies significantly by fuel type. For example, coal contains the highest amount of carbon per unit of useful 

 

2 An additional discussion of fossil fuel emission trends is presented in the Trends in U.S. Greenhouse Gas Emissions Chapter. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 54 

energy. Petroleum has roughly 75 percent of the carbon per unit of energy as coal, and natural gas has only about 

55 percent.3 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 3-4 shows annual changes in emissions during the last six years for coal, 

petroleum, and natural gas in selected sectors. 

Table 3-4: Annual Change in CO2 Emissions from Fossil Fuel Combustion for Selected Fuels and Sectors (Tg CO2

Eq. and Percent) 

Sector Fuel Type 1999 to 2000 2000 to 2001 2001 to 2002 2002 to 2003 

Electricity Generation Coal 87.6 5% -62.6 -3% 22.2 1% 36.6 2% 

Electricity Generation Natural Gas 20.8 8% 8.4 3% 16.5 6% -28.0 -9% 

Electricity Generation Petroleum -5.6 -6% 9.8 11% -23.5 -23% 18.9 24% 

Transportationa

 Petroleum 47.0 3% -16.4 -1% 29.4 2% 16.6 1% 

Residential Natural Gas 13.9 5% -10.7 -4% 6.2 2% 11.5 4% 

Commercial Natural Gas 7.1 4% -7.9 -5% 4.2 3% 2.0 1% 

Industrial Coal 1.1 1% -4.4 -3% -7.9 -6% 0.8 1% 

Industrial Natural Gas 8.2 2% -38.5 -8% 9.5 2% -27.6 -6% 

All Sectorsb

 All Fuelsb 199.2 4% -97.1 -2% 53.5 1% 50.2 1% a

 Excludes emissions from International Bunker Fuels. b

 Includes fuels and sectors not shown in table. 

In the United States, 86 percent of the energy consumed in 2003 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 (6 percent), primarily 

hydroelectric power and biofuels (EIA 2004a). Specifically, petroleum supplied the largest share of domestic 

energy demands, accounting for an average of 39 percent of total energy consumption from 1990 through 2003. 

Natural gas and coal followed in order of importance, accounting for 24 and 23 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 2004a). 

Figure 3-3: 2003 U.S. Energy Consumption by Energy Source 

Figure 3-4: U.S. Energy Consumption (Quadrillion Btu) 

Figure 3-5: 2003 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 carbon stored in the fuels is oxidized and emitted as CO2 and smaller amounts of other 

gases, including CH4, CO, and NMVOCs.4 These other carbon containing non-CO2 gases are emitted as a byproduct of incomplete fuel combustion, but are, for the most part, eventually oxidized to CO2 in the atmosphere. 

Therefore, except for the soot and ash left behind during the combustion process, all the carbon in fossil fuels used 

to produce energy is eventually converted to atmospheric CO2. 

 

3 Based on national aggregate carbon content of all coal, natural gas, and petroleum fuels combusted in the United States. 

4 See the sections entitled Stationary Combustion and Mobile Combustion in this chapter for information on non-CO2 gas 

emissions from fossil fuel combustion. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 55 

[BEGIN BOX] 

Box 3-1: Weather and Non-Fossil Energy Effects on CO2 from Fossil Fuel Combustion Trends 

In 2003, weather conditions became cooler in both the winter and summer. Heating degree days in the United 

States were 2 percent below normal (see Figure 3-6), while cooling degree days in 2003 were 5 percent above 

normal (see Figure 3-7) (EIA 2004f).5 Winter conditions were colder in 2003 than in 2002, which, coupled with a 

1.2 percent increase in the U.S. housing stock (EIA 2004f), led to an increase in demand for heating fuels, despite 

escalating fuel prices for heating fuels. Though the summer of 2003 was cooler than the near record heat of 2002, 

demand for electricity increased most likely due to the growing economy and increase in housing stock. 

Figure 3-6: Annual Deviations from Normal Heating Degree Days for the United States (1949-2003) 

Figure 3-7: Annual Deviations from Normal Cooling Degree Days for the United States (1949-2003) 

Although no new U.S. nuclear power plants have been constructed in recent years, the utilization (i.e., capacity 

factors6) of existing plants remained high, at 88 percent in 2003. This utilization level actually represents a 2 

percent decrease in electricity output by nuclear plants, down from a record high of 90 percent in 2002. Electricity 

output by hydroelectric power plants increased in 2003 by approximately 4 percent. Nevertheless, electricity 

generated by nuclear plants in 2003 provided almost 3 times as much of the energy consumed in the United States 

as hydroelectric plants (EIA 2004a). 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 (1973-2003) 

[END BOX] 

For the purpose of international reporting, the IPCC (IPCC/UNEP/OECD/IEA 1997) recommends that particular 

adjustments be made to national fuel consumption statistics. Certain fossil fuels can be manufactured into plastics, 

asphalt, lubricants, or other products. A portion of the carbon consumed for these non-energy products can be 

stored (i.e., sequestered) indefinitely. To account for the fact that the carbon in these fuels ends up in products 

instead of being combusted (i.e., oxidized and released into the atmosphere), consumption of fuels for non-energy 

purposes is estimated and subtracted from total fuel consumption estimates. Emissions from non-energy uses of 

fuels are estimated in the Carbon Emitted and Stored in Products from Non-Energy Uses of Fossil Fuels section in 

this chapter.

 

5 Degree days are relative measurements of outdoor air temperature. Heating degree days are deviations of the mean daily 

temperature below 65° F, while cooling degree days are deviations of the mean daily temperature above 65° F. Heating degree 

days have a considerably greater affect on energy demand and related emissions than do cooling degree days. Excludes Alaska 

and Hawaii. Normals are based on data from 1971 through 2000. The variation in these normals during this time period was 

±10 percent and ±14 percent for heating and cooling degree days, respectively (99 percent confidence interval). 

6 The capacity factor is defined as the ratio of the electrical energy produced by a generating unit for a given period of time to 

the electrical energy that could have been produced at continuous full-power operation during the same period (EIA 2004a).
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 56 

According to the UNFCCC reporting guidelines, CO2 emissions from the consumption of fossil fuels for aviation 

and marine international transport activities (i.e., international bunker fuels) should be reported separately, and not 

included in national emission totals. Estimates of international bunker fuel emissions for the United States are 

provided in Table 3-5. 

Table 3-5: CO2 Emissions from International Bunker Fuels (Tg CO2 Eq.)* 

Vehicle Mode 1990 1997 1998 1999 2000 2001 2002 2003 

Aviation 46.2 55.9 56.7 58.9 60.5 59.4 61.8 59.6 

Marine 67.3 54.0 57.9 46.4 40.9 38.5 27.7 24.6 

Total 113.5 109.9 114.6 105.3 101.4 97.9 89.5 84.2 

* See International Bunker Fuels section for additional detail. 

Note: Totals may not sum due to independent rounding. 

End-Use Sector Consumption 

An alternative method of presenting CO2 emissions is to allocate emissions associated with electricity generation to 

the sectors in which it is used. Four end-use sectors were defined: industrial, transportation, residential, and 

commercial.7 For the discussion below, electricity generation emissions have been distributed to each end-use 

sector based upon the sector’s share of national electricity consumption. This method of distributing emissions 

assumes that each sector consumes electricity generated from an equally carbon-intensive mix of fuels and other 

energy sources. In reality, sources of electricity vary widely in carbon intensity (e.g., coal versus wind power). By 

giving equal carbon-intensity weight to each sector's electricity consumption, emissions attributed to one end-use 

sector may be somewhat overestimated, while emissions attributed to another end-use sector may be slightly 

underestimated. After the end-use sectors are discussed, emissions from electricity generation are addressed 

separately. Emissions from U.S. territories are also calculated separately due to a lack of end-use-specific 

consumption data. Table 3-6 and Figure 3-9 summarize CO2 emissions from direct fossil fuel combustion and prorated electricity generation emissions from electricity consumption by end-use sector. 

Table 3-6: CO2 Emissions from Fossil Fuel Combustion by End-Use Sector (Tg CO2 Eq.) 

End-Use Sector 1990 1997 1998 1999 2000 2001 2002 2003

Transportation 1,449.8 1,606.4 1,636.5 1,693.9 1,741.0 1,723.1 1,755.4 1,770.4

Combustion 1,446.8 1,603.3 1,633.4 1,690.8 1,737.7 1,719.7 1,752.3 1,767.2

Electricity 3.0 3.1 3.1 3.2 3.4 3.4 3.2 3.2

Industrial 1,553.9 1,703.0 1,668.5 1,651.2 1,684.4 1,587.4 1,579.0 1,572.9

Combustion 882.8 963.8 911.6 888.1 905.0 878.2 876.6 858.6

Electricity 671.1 739.2 757.0 763.1 779.4 709.3 702.4 714.3

Residential 924.8 1,040.7 1,044.4 1,063.5 1,124.2 1,116.2 1,145.0 1,168.9

Combustion 339.6 370.6 338.6 359.3 379.1 367.0 371.4 385.1

Electricity 585.3 670.2 705.8 704.2 745.0 749.2 773.6 783.8

Commercial 755.1 876.7 892.9 901.2 959.5 972.7 973.9 983.1

Combustion 224.2 237.2 219.7 222.3 235.2 226.7 230.0 234.0

Electricity 530.9 639.5 673.2 678.9 724.3 745.9 743.9 749.2

U.S. Territories 28.0 36.4 36.3 36.2 35.9 48.6 48.1 56.2

Total 4,711.7 5,263.2 5,278.7 5,345.9 5,545.1 5,448.0 5,501.4 5,551.6

Electricity Generation 1,790.3 2,051.9 2,139.0 2,149.3 2,252.1 2,207.8 2,223.0 2,250.5 

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. 

 

7 See Glossary (Annex 6.8) for more detailed definitions of the industrial, residential, commercial, and transportation end-use 

sector, as well as electricity generation. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 57 

Figure 3-9: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion 

Transportation End-Use Sector 

Using this allocation method, the transportation end-use sector accounted for the largest share (approximately 32 

percent) of CO2 emissions from fossil fuel combustion.8 Almost all of the energy consumed in the transportation 

sector was petroleum-based, with nearly two-thirds being 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.9

Carbon dioxide emissions from fossil fuel combustion for transportation increased by 22 percent from 1990 to 2003, 

to 1,770.4 Tg CO2 Eq. The growth in transportation end-use sector emissions has been relatively steady, excluding 

a 4 percent single year increase in 1999 and slight decreases in 1991 and 2001. Like overall energy demand, 

transportation fuel demand is a function of many short and long-term factors. In the short term only minor 

adjustments can generally be made through consumer behavior (e.g., not driving as far for summer vacation). 

However, long-term adjustments such as vehicle purchase choices, transport mode choice and access (i.e., trains 

versus planes), and urban planning can have a significant impact on fuel demand. 

In 2003, CO2 emissions from the transportation sector increased by approximately 1 percent. A 12 percent increase 

in the price of motor gasoline in 2003 tempered the effects of the growing economy10 on demand for vehicle fuel 

(see Figure 3-10). 

Since 1990, travel activity in the United States has grown more rapidly than population, with a 16 percent increase 

in vehicle miles traveled per capita. In the meantime, improvements in the average fuel efficiency of the U.S. 

vehicle fleet stagnated after increasing steadily since 1976 (FHWA 1996 through 2004). The average miles per 

gallon achieved by the U.S. vehicle fleet has remained fairly constant since 1991. This trend is due, in part, to the 

increasing dominance of new motor vehicle sales by less fuel-efficient light-duty trucks and sport-utility vehicles 

(see Figure 3-11). 

Figure 3-10: Motor Gasoline Retail Prices (Real) 

Figure 3-11: Motor Vehicle Fuel Efficiency 

Table 3-7 provides a detailed breakdown of CO2 emissions by fuel category and vehicle type for the transportation 

end-use sector. Fifty-nine percent of the emissions from this end-use sector in 2003 were the result of the 

combustion of motor gasoline in passenger cars and light-duty trucks. Diesel highway vehicles and jet aircraft were 

also significant contributors, accounting for 18 and 12 percent of CO2 emissions from the transportation end-use 

sector, respectively.11 For information on transportation-related CO2 emissions from agriculture and construction 

equipment, other off-road equipment, and recreational vehicles, please refer to Table 3-36 in Annex 3.2. 

Table 3-7: CO2 Emissions from Fossil Fuel Combustion in Transportation End-Use Sector (Tg CO2 Eq.) 

Fuel/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline 955.2 1,042.5 1,072.9 1,099.9 1,105.9 1,111.2 1,138.7 1,143.7 

 

8 Note that electricity generation is actually the largest emitter of CO2 when electricity is not distributed among end-use sectors. 

9 See Glossary (Annex 6.8) for a more detailed definition of the transportation end-use sector. 

10 Gross domestic product increased 3.1 percent between 2002 and 2003 (BEA 2004).

11 These percentages include emissions from bunker fuels.
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 58 

Automobiles 605.1 589.8 608.6 618.4 620.1 622.3 636.1 630.2 

Light-Duty Trucks 301.0 406.1 416.3 432.6 435.3 438.7 450.9 460.9 

Other Trucks 37.7 33.3 34.6 35.7 37.3 37.1 38.7 39.6 

Buses 0.3 0.4 0.4 0.4 0.4 0.4 0.3 0.3 

Motorcycles 1.7 1.7 1.8 1.8 1.8 1.6 1.6 1.6 

Boats (Recreational) 9.4 11.2 11.2 11.1 11.2 11.2 11.1 11.0 

Distillate Fuel Oil (Diesel) 265.1 338.4 348.4 362.2 374.0 383.2 378.1 392.6 

Automobiles 7.4 5.6 5.2 4.0 3.4 3.5 3.4 3.4 

Light-Duty Trucks 10.7 15.1 15.4 16.3 16.6 17.2 16.9 17.6 

Other Trucks 178.4 246.2 255.2 268.1 282.7 289.9 288.3 301.1 

Buses 7.5 8.6 8.7 9.7 9.4 8.8 8.2 8.0 

Locomotives 33.3 37.0 37.4 38.5 38.4 39.4 37.9 39.6 

Ships & Boats 16.3 16.9 15.0 17.4 17.3 19.1 18.4 17.0 

Ships (Bunkers) 11.4 9.1 11.5 8.2 6.2 5.2 5.1 6.0 

Jet Fuel 220.4 232.1 235.6 242.9 251.2 240.4 234.4 228.6 

Commercial Aircraft 117.2 128.5 126.3 136.4 140.6 132.8 121.7 122.8 

Military Aircraft 34.8 21.0 21.5 20.6 21.0 22.8 20.4 20.5 

General Aviation Aircraft 6.3 6.1 7.7 9.2 9.2 9.0 9.3 9.4 

Other Aircraftb

 15.9 20.6 23.4 17.8 19.9 16.4 21.2 16.3 

Aircraft (Bunkers) 46.2 55.9 56.7 58.9 60.5 59.4 61.8 59.6 

Aviation Gasoline 3.1 2.7 2.4 2.7 2.5 2.4 2.3 2.2 

General Aviation Aircraft 3.1 2.7 2.4 2.7 2.5 2.4 2.3 2.2 

Residual Fuel Oil 79.3 55.5 52.6 51.9 69.2 45.7 50.4 48.1 

Ships & Boatsc

 23.4 10.6 6.2 13.7 34.6 12.4 27.7 29.5 

Ships (Bunkers)c

 55.8 44.9 46.4 38.2 34.6 33.2 22.6 18.6 

Natural Gas 35.9 41.1 35.1 35.6 35.5 33.9 37.1 35.4 

Automobiles + + + + + + + + 

Light Trucks + + + + + + + + 

Buses + 0.2 0.2 0.3 0.4 0.5 0.6 0.6 

Pipeline 35.9 40.9 34.9 35.3 35.0 33.4 36.4 34.8 

LPG 1.4 0.8 1.0 0.8 0.7 0.8 0.9 0.8 

Light Trucks 0.5 0.4 0.4 0.3 0.3 0.3 0.3 0.3 

Other Trucks 0.8 0.4 0.6 0.5 0.4 0.5 0.5 0.5 

Buses + + + + + + + + 

Electricity 3.0 3.1 3.1 3.2 3.4 3.4 3.2 3.2 

Rail 3.0 3.1 3.1 3.2 3.4 3.4 3.2 3.2 

Total (Including Bunkers)d 1,563.3 1,716.2 1,751.1 1,799.2 1,842.5 1,821.0 1,844.9 1,854.6 

Total (Excluding Bunkers)d 1,449.8 1,606.4 1,636.5 1,693.9 1,741.0 1,723.1 1,755.4 1,770.4 

Note: Totals may not sum due to independent rounding. a

 Emissions are no longer allocated to gasoline and diesel consumption from agriculture and construction, and electricity 

consumption from pipelines. This based on recognition that EIA statistics account for these activities in the industrial sector. b

 This category represents all other jet fuel consumption, and may include some small commercial aircraft and jet fuel used for 

heating oil. 

c

 Fluctuations in emission estimates from the combustion of residual fuel oil are currently unexplained, but may be related to data 

collection problems. 

d

 Official estimates exclude emissions from the combustion of both aviation and marine international bunker fuels; however, 

estimates including international bunker fuel-related emissions are presented for informational purposes. 

+ Less than 0.05 Tg CO2 Eq. 

Industrial End-Use Sector 

The industrial end-use sector accounted for 28 percent of CO2 emissions from fossil fuel combustion. On average, 

55 percent of these emissions resulted from the direct consumption of fossil fuels for steam and process heat 

production. The remaining 45 percent was associated with their consumption of electricity for uses such as motors, 

electric furnaces, ovens, and lighting. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 59 

The industrial end-use sector includes activities such as manufacturing, construction, mining, and agriculture.12 

The largest of these activities in terms of energy consumption is manufacturing, which was estimated in 1998 to 

have accounted for about 84 percent of industrial energy consumption (EIA 2001a). Just six industries—Petroleum, 

Chemicals, Primary Metals, Paper, Food, and Stone, Clay, and Glass products—represent 83 percent of total 

manufacturing energy use. 

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 is also affected by weather 

conditions.13 In addition, structural changes within the U.S. economy that lead to shifts in industrial output away 

from energy intensive manufacturing products to less energy intensive products (e.g., from steel to computer 

equipment) also have a significant affect on industrial emissions. 

From 2002 to 2003, total industrial production and manufacturing output increased slightly, by 0.2 and 0.1 percent, 

respectively (FRB 2004). Also from 2002 to 2003, output increased for the Petroleum Refinery, Nonmetallic 

Mineral Product, and Chemical industries, but declined for the Primary Metal, Food, and Paper industries (see 

Figure 3-12). 

Figure 3-12: Industrial Production Indexes (Index 1997=100) 

Despite the growth in industrial output (44 percent) and the overall U.S. economy (46 percent) from 1990 to 2003, 

emissions from the industrial end-use sector increased only slightly (by 1 percent). The reasons for the disparity 

between rapid growth in industrial output and stagnant growth in industrial emissions are not entirely clear. It is 

likely, though, that several factors have influenced industrial emission trends, including: 1) more rapid growth in 

output from less energy-intensive industries relative to traditional manufacturing industries, 2) improvements in 

energy efficiency; and 3) a lowering of the carbon intensity of fossil fuel consumption as industry shifts from its 

historical reliance on coal and coke to heavier usage of natural gas. In 2003, carbon dioxide emissions from fossil 

fuel combustion and electricity use within the industrial end-use sectors were 1,572.9 Tg CO2 Eq., or 0.4 percent 

below 2002 emissions. 

Residential and Commercial End-Use Sectors 

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 67 and 76 percent of emissions from the residential and commercial end-use sectors, respectively. The 

remaining emissions were largely due to the direct consumption of natural gas and petroleum products, primarily for 

heating and cooking needs. Coal consumption was a minor component of energy use in both of these end-use 

sectors. In 2003, CO2 emissions from fossil fuel combustion and electricity use within the residential and 

commercial end-use sectors were 1,168.9 Tg CO2 Eq. and 983.1 Tg CO2 Eq., respectively. 

Emissions from the residential and commercial sectors have generally been increasing since 1990, and are often 

correlated with short-term fluctuations in energy consumption caused by weather conditions, rather than prevailing 

economic conditions (see Table 3-6). In the long-term, both end-use sectors are also affected by population growth, 

regional migration trends, and changes in housing and building attributes (e.g., size and insulation). 

 

12 See Glossary (Annex 6.8) for a more detailed definition of the industrial end-use sector. 

13 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 60 

Emissions from natural gas consumption represent over 70 percent of the direct (not including electricity) fossil fuel 

emissions from the residential and commercial sectors. In 2003, these emissions increased by 4 and 1 percent, 

respectively, in each of these sectors. Colder winter conditions in the United States (see Figure 3-13) and an 

increasing U.S. housing stock led to higher demand for natural gas, despite drastic increases in natural gas prices 

(66 percent).

Figure 3-13: Heating Degree Days14 

Electricity sales to the residential and commercial end-use sectors in 2003 increased by 1 and 0.4 percent, 

respectively. This trend can largely be attributed to the growing economy (3.1 percent) and increase in U.S. 

housing stock (1.2 percent), which led to increased demand for electricity. Increased consumption due to these 

factors was somewhat offset by decreases in air conditioning-related electricity consumption expected with the 

cooler summer (see Figure 3-14), and increases in electricity prices (1 and 2 percent higher to the residential and 

commercial sectors, respectively). Electricity-related emissions in both sectors rose with increasing consumption 

and the higher carbon intensity of electricity generation. Total emissions from the residential sector increased by 

2.1 percent in 2003, with emission from the commercial sector 0.9 percent higher than in 2002.

Figure 3-14: Cooling Degree Days15 

Electricity Generation 

The process of generating electricity is the single largest source of CO2 emissions in the United States (39 percent). 

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 3-15). Electricity generation also 

accounted for the largest share of CO2 emissions from fossil fuel combustion, approximately 41 percent in 2003. 

Figure 3-15: 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). While 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). However, the electric power industry in the United States has undergone significant changes 

as both federal and state government agencies have modified regulations to create a more competitive market for 

electricity generation. These changes have led to the growth of nonutility power producers, including the sale of 

generating capacity by electric utilities to nonutilities. As a result, the Department of Energy no longer categorizes 

electric power generation into these ownership groups, and instead uses 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, while the other sectors 

consist of those producers that indicate their primary business is other than the production of electricity. 

In 2003, the amount of electricity generated decreased very slightly, by 0.3 percent. Although total U.S. electricity 

use actually increased by 1 percent, net generation declined due to increased net imports and reduced losses of 

 

14 Degree days are relative measurements of outdoor air temperature. Heating degree days are deviations of the mean daily 

temperature below 65° F. Excludes Alaska and Hawaii. Normals are based on data from 1971 through 2000. 

15 Degree days are relative measurements of outdoor air temperature. Cooling degree days are deviations of the mean daily 

temperature above 65° F. Excludes Alaska and Hawaii. Normals are based on data from 1971 through 2000.
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 61 

electricity.16 However, CO2 emissions increased by 1.2 percent, as escalating natural gas prices caused power 

producers to switch from natural gas to coal, a more carbon-intensive fuel. Coal consumption for electricity 

generation increased by 2.0 percent in 2003, while natural gas consumption decreased by 9.2 percent. As a result of 

this shift, carbon intensity from energy consumption for electricity generation increased in 2003 (see Table 3-9).

Coal is consumed primarily by the electric power sector in the United States, which accounted for 93 percent of 

total coal consumption for energy purposes in 2003. Electricity generation by nuclear and renewable resources 

remained relatively stable, increasing 1 percent in 2003.

[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, can be generated from many other sources that do not emit CO2 in the energy 

conversion process. In the United States, useful energy is also produced from renewable (i.e., hydropower, 

biofuels, geothermal, solar, and wind) and nuclear sources.17

Energy-related CO2 emissions can be reduced by not only lowering total energy consumption (e.g., through 

conservation measures) but also by lowering the carbon intensity of the energy sources employed (e.g., fuel 

switching from coal to natural gas). The amount of carbon emitted from the combustion of fossil fuels is dependent 

upon the carbon content of the fuel and the fraction of that carbon that is oxidized.18 Fossil fuels vary in their 

average carbon content, ranging from about 53 Tg CO2 Eq./QBtu for natural gas to upwards of 95 Tg CO2 Eq./QBtu 

for coal and petroleum coke.19 In general, the carbon 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 carbon 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 carbon 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 carbon emitted is offset by the growth of new biomass.20 The 

overall carbon intensity of the U.S. economy is thus dependent upon the quantity and combination of fuels and other 

energy sources employed to meet demand. 

Table 3-8 provides a time series of the carbon intensity for each sector of the U.S. economy. The time series 

incorporates only the energy consumed from the direct combustion of fossil fuels in each sector. For example, the 

carbon 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 carbon intensity, which is related to the large percentage of its energy derived from 

 

16 EIA statistics track net generation, imports, exports, and compare these with the end use of electricity. The difference 

between these values is classified as “T&D losses and Unaccounted for”. T&D losses refer to electricity lost during the 

transmission and distribution of electricity from the source to the end user. There also exists a small amount of electricity not 

accounted for due to data collection time frame differences and nonsampling error. 

17 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. 

18 Generally, more than 97 percent of the carbon in fossil fuel is oxidized to CO2 with most carbon combustion technologies 

used in the United States. 

19 One exajoule (EJ) is equal to 1018 joules or 0.9478 QBtu. 

20 Net carbon fluxes from changes in biogenic carbon reservoirs in wooded or croplands are accounted for in the estimates for 

Land-Use Change and Forestry. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 62 

natural gas for heating. The carbon 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 carbon intensities over this 

period. The carbon intensity of the transportation sector was closely related to the carbon 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 carbon intensity due to its heavy reliance on coal for 

generating electricity.

Table 3-8: Carbon Intensity from Direct Fossil Fuel Combustion by Sector (Tg CO2 Eq./QBtu) 

Sector 1990 1997 1998 1999 2000 2001 2002 2003

Residentiala 57.0 56.5 56.5 56.6 56.6 56.7 56.6 56.5 

Commerciala 59.3 57.4 57.1 57.1 57.2 57.3 57.2 57.3 

Industriala 66.1 65.8 65.2 65.1 65.1 64.7 64.5 65.0 

Transportationa 70.3 70.1 70.1 70.2 70.3 70.3 70.3 70.3 

Electricity Generationb 85.8 85.9 85.2 84.9 84.7 84.3 84.1 84.8 

U.S. Territoriesc 73.3 73.3 73.2 73.0 72.5 72.9 73.0 73.5 

All Sectorsc 72.6 72.5 72.6 72.5 72.5 72.3 72.2 72.5 a

 Does not include electricity or renewable energy consumption. b

 Does not include electricity produced using nuclear or renewable energy. 

c

 Does not include nuclear or renewable energy consumption. 

Note: Excludes non-energy fuel use emissions and consumption. 

In contrast to Table 3-8, Table 3-9 presents carbon intensity values that incorporate energy consumed from all 

sources (i.e., fossil fuels, renewables, and nuclear). In addition, the emissions related to the generation of electricity 

have been attributed to both electricity generation and the end-use sectors in which that electricity was eventually 

consumed.21 This table, therefore, provides a more complete picture of the actual carbon intensity of each end-use 

sector per unit of energy consumed. The transportation end-use sector in Table 3-9 emerges as the most carbon 

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 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 carbon 

intensity. The carbon intensity of the electricity generation sector differs greatly from the scenario in Table 3-8, 

where only the energy consumed from the direct combustion of fossil fuels was included. This difference is due 

almost entirely to the inclusion of electricity generation from nuclear and hydropower sources, which do not emit 

CO2. 

Table 3-9: Carbon Intensity from all Energy Consumption by Sector (Tg CO2 Eq./QBtu) 

Sector 1990 1997 1998 1999 2000 2001 2002 2003 

Transportationa 70.1 69.8 69.8 69.8 69.9 69.8 69.8 69.6 

Other End-Use Sectorsa, b 57.8 57.8 57.9 57.3 58.0 58.2 57.5 58.0 

Electricity Generationc 58.4 58.9 59.1 58.2 59.2 59.3 58.3 59.0 

All Sectorsd 61.1 61.0 61.1 60.8 61.3 61.5 60.9 61.2 a

 Includes electricity (from fossil fuel, nuclear, and renewable sources) and direct renewable energy consumption. b

 Other End-Use Sectors includes the residential, commercial, and industrial sectors. c

 Includes electricity generation from nuclear and renewable sources. d

 Includes nuclear and renewable energy consumption. 

Note: Excludes non-energy fuel use emissions and consumption. 

 

21 In other words, the emissions from the generation of electricity are intentionally double counted by attributing them both to 

electricity generation and the end-use sector in which electricity consumption occurred. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 63 

By comparing the values in Table 3-8 and Table 3-9, a few observations can be made. The use of renewable and 

nuclear energy sources has resulted in a significantly lower carbon intensity of the U.S. economy. Over the 

thirteen-year period of 1990 through 2003, however, the carbon intensity of U.S. energy consumption has been 

fairly constant, as the proportion of renewable and nuclear energy technologies has not changed significantly. 

The carbon intensity of total energy consumption in the United States has remained fairly constant. Per capita 

energy consumption has fluctuated, but has generally demonstrated a constant overall trend since 1990 (see Figure 

3-16). 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. 

Figure 3-16: U.S. Energy Consumption and Energy-Related CO2 Emissions Per Capita and Per Dollar GDP 

Carbon intensity estimates were developed using nuclear and renewable energy data from EIA (2004a) and fossil 

fuel consumption data as discussed above and presented in Annex 2.1. 

[END BOX] 

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/UNEP/OECD/IEA 1997). A detailed description of the U.S. methodology 

is presented in Annex 2.1, and is characterized by the following steps: 

1. Determine total fuel consumption by fuel type and sector. Total fossil fuel consumption for each year is 

estimated by aggregating consumption data by end-use sector (e.g., commercial, industrial, etc.), primary fuel 

type (e.g., coal, petroleum, gas), and secondary fuel category (e.g., motor gasoline, distillate fuel oil, etc.). Fuel 

consumption data for the United States were obtained directly from the Energy Information Administration 

(EIA) of the U.S. Department of Energy (DOE), primarily from the Monthly Energy Review and unpublished 

supplemental tables on petroleum product detail (EIA 2004b). The United States does not include territories in 

its national energy statistics, so fuel consumption data for territories were collected separately from Grillot 

(2004).22 

 For consistency of reporting, the IPCC has recommended that countries report energy data using the 

International Energy Agency (IEA) reporting convention and/or IEA data. Data in the IEA format are 

presented "top down"⎯that is, energy consumption for fuel types and categories are estimated from energy 

production data (accounting for imports, exports, stock changes, and losses). The resulting quantities are 

referred to as "apparent consumption." The data collected in the United States by EIA, and used in this 

inventory, are, instead, "bottom up" in nature. In other words, they are collected through surveys at the point of 

delivery or use and aggregated to determine national totals.23

 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 

 

22 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 56 Tg CO2 Eq. in 2003. 

23 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 64 

adjusted to correspond to international standard, which are to report energy statistics in terms of net calorific 

values (NCV) (i.e., lower heating values).24

2. Subtract uses accounted for in the Industrial Processes chapter. Portions of the fuel consumption data for five 

fuel categories—coking 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 Gambogi (2004), EFMA (1995), U.S. Census Bureau 

(1991 through 1994), U.S. Census Bureau (1998 through 2003), U.S. Census Bureau (2003), U.S. Census 

Bureau (2004a), EIA (2000 through 2004), EIA (2001b), USGS (2003 through 2004), USGS (1998 through 

2002), USGS (1995), USGS (1995 through 2004), USGS (1991 through 1994), USGS (1991 through 2003), 

U.S. International Trade Commission (2004a), U.S. International Trade Commission (2004b), and Onder and 

Bagdoyan (1993).25

3. Adjust for biofuels, conversion of fossil fuels, and exports of CO2. Fossil fuel consumption estimates are 

adjusted downward to exclude 1) fuels with biogenic origins, 2) fuels created from other fossil fuels, and 3) 

exports of CO2. Fuels with biogenic origins are assumed to result in no net CO2 emissions, and must be 

subtracted from fuel consumption estimates. 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.26 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 (2004b) and data for synthetic natural gas were collected from EIA (2004e), 

and data for CO2 exports were collected from the Dakota Gasification Company (2003), Fitzpatrick (2002), and 

Erickson (2003). 

4. Adjust for fuels consumed for non-energy uses. U.S. aggregate energy statistics include consumption of fossil 

fuels for non-energy purposes. Depending on the end-use, this can result in storage of some or all of the carbon 

contained in the fuel for a period of time. As the emission pathways of carbon used for non-energy purposes are 

vastly different than fuel combustion, these emissions are estimated separately in the Carbon Emitted and 

Stored in Products from Non-Energy 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 (2004b). 

5. 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, determination of carbon content, and adjustment for the fraction of 

carbon not oxidized).27 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 

2004) supplied data on military jet fuel use. Commercial jet fuel use was obtained from BEA (1991 through 

 

24 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. 

25 See sections on Iron and Steel Production, Ammonia Manufacture, Petrochemical Production, Titanium Dioxide Production, 

Ferroalloy Production, and Aluminum Production in the Industrial Processes chapter. 

26 These adjustments are explained in greater detail in Annex 2.1. 

27 See International Bunker Fuels section in this chapter for a more detailed discussion. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 65 

2004) and DOT (1991 through 2004); residual and distillate fuel use for civilian marine bunkers was obtained 

from DOC (1991 through 2004). Consumption of these fuels was subtracted from the corresponding fuels in 

the transportation end-use sector. Estimates of international bunker fuel emissions are discussed further in the 

section entitled International Bunker Fuels. 

6. Determine the total carbon content of fuels consumed. Total carbon was estimated by multiplying the amount 

of fuel consumed by the amount of carbon in each fuel. This total carbon estimate defines the maximum 

amount of carbon that could potentially be released to the atmosphere if all of the carbon in each fuel was 

converted to CO2. The carbon content coefficients used by the United States were obtained from EIA’s 

Emissions of Greenhouse Gases in the United States 2003 (EIA 2004c) and EIA’s Monthly Energy Review and 

unpublished supplemental tables on petroleum product detail EIA (EIA 2004b). They are presented in Annexes 

2.1 and 2.2. 

7. Adjust for carbon that does not oxidize during combustion. Because combustion processes are not 100 percent 

efficient, some of the carbon contained in fuels is not emitted to the atmosphere. Rather, it remains behind as 

soot and ash. The estimated amount of carbon not oxidized due to inefficiencies during the combustion process 

was assumed to be 1 percent for petroleum and coal and 0.5 percent for natural gas (see Annex 2.1). 

Unoxidized or partially oxidized organic (i.e., carbon containing) combustion products were assumed to have 

eventually oxidized to CO2 in the atmosphere.28 IPCC provided fraction oxidized values for petroleum and 

natural gas (IPCC/UNEP/OECD/IEA 1997). Bechtel (1993) provided the fraction oxidation value for coal. 

8. 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.29 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 jet fuel, CO2 emissions were 

calculated directly based on reported consumption of fuel. For highway vehicles, annual estimates of combined 

motor gasoline and diesel fuel consumption by vehicle category were obtained from FHWA (1996 through 

2004); for each vehicle category, the percent gasoline, diesel, and other (e.g., CNG, LPG) fuel consumption are 

estimated using data from DOE (1993 through 2004). For non-highway vehicles, activity data were obtained 

from AAR (2004), BEA (1991 through 2004), Benson (2002 through 2004), DOE (1993 through 2004), DESC 

(2004), DOC (1991 through 2004), DOT (1991 through 2004), EIA (2002a), EIA (2002b), EIA (2004a), EIA 

(2004b), EIA (2003 through 2004), EIA (1991 through 2004), EPA (2004c), and FAA (2004). Heat contents 

and densities were obtained from EIA (2004a) and USAF (1998).30 The difference between total U.S. jet fuel 

consumption (as reported by EIA) and civilian air carrier consumption for both domestic and international 

flights (as reported by DOT and BEA) plus military jet fuel consumption is reported as “other” under the jet 

fuel category in Table 3-7, and includes such fuel uses as blending with heating oils and fuel used for chartered 

aircraft flights. 

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. 

 

28 See Indirect CO2 from CH4 Oxidation section in this chapter for a more detailed discussion. 

29 Electricity generation is not considered a final end-use sector, because energy is consumed primarily to provide electricity to 

the other sectors. 

30 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 66 

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 fuels used in 

these non-energy production processes were subtracted from the total fossil fuel consumption for 2003. 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. Additionally, inefficiencies in the 

combustion process, which can result in ash or soot remaining unoxidized for long periods, were also assumed. 

These factors all contribute to the uncertainty in the CO2 estimates. Detailed discussions on the uncertainties 

associated with Carbon 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. 

The uncertainty analysis was performed by primary fuel type for each end-use sector, using the IPCC-recommended 

Tier 2 uncertainty estimation methodology, Monte Carlo Simulation technique, with @RISK software. For this 

uncertainty estimation, the inventory estimation model for CO2 from fossil fuel combustion was integrated with the 

relevant inventory variables from the inventory estimation model for International Bunker Fuels, to realistically 

characterize the interaction (or endogenous correlation) between the variables of these two models. About 150 

input variables were modeled for CO2 from energy-related Fossil Fuel Combustion (including about 10 for nonenergy 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.31 Triangular distributions were assigned for 

 

31 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 67 

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.32 

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).33 For purposes of this uncertainty analysis, each input variable was simulated 10,000 times through Monte 

Carlo Sampling. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-10. Fossil fuel combustion 

CO2 emissions in 2003 were estimated to be between 5,474.3 and 5,863.3 Tg CO2 Eq. at a 95 percent confidence 

level (or in 19 out of 20 Monte Carlo Simulations). This indicates a range of 1 percent below to 6 percent above the 

2003 emission estimate of 5,551.6 Tg CO2 Eq. 

Table 3-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Energy-related Fossil Fuel 

Combustion by Fuel Type and Sector (Tg CO2 Eq. and Percent)

Fuel/Sector 

2003 Emission 

Estimate 

(Tg CO2 Eq.) 

Uncertainty Range Relative to Emission Estimatea

(Tg CO2 Eq.) (%) 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Coalb 2,013.8 1,962.0 2,220.5 -3% +10% 

Residential 1.1 1.1 1.3 -5% +16% 

Commercial 9.3 8.9 10.8 -4% +16% 

Industrial 123.4 120.0 144.1 -3% +17% 

Transportation NE NE NE NA NA 

Electricity Generation 1,876.3 1815.7 2,072.5 -3% +10% 

U.S. Territories 3.6 3.2 4.3 -12% +20% 

Natural Gasb

 1,170.3 1,161.0 1,229.9 -1% +5% 

Residential 277.3 270.5 297.7 -2% +7% 

Commercial 170.7 166.5 183.2 -2% +7% 

Industrial 407.9 396.6 439.0 -3% +8% 

Transportation 35.4 34.5 38.0 -2% +7% 

Electricity Generation 277.6 270.5 292.8 -3% +5% 

U.S. Territories 1.4 1.2 1.7 -12% +17% 

Petroleumb

 2,367.1 2,256.9 2,518.5 -5% +6% 

Residential 106.7 101.7 112.5 -5% +6% 

Commercial 53.9 51.6 56.7 -4% +5% 

Industrial 327.3 283.4 386.0 -13% +18% 

Transportation 1,731.8 1,627.9 1,857.2 -6% +7% 

Electric Utilities 96.3 93.8 101.8 -3% +6% 

U.S. Territories 51.2 47.6 57.2 -7% +12% 

Total (excluding 

Geothermal)b

 5,551.2 5,474.0 5,863.0 -1% +6% 

 

32 In the SAIC/EIA (2001) report, the quantitative uncertainty estimates were developed for each of the three major fossil fuels 

used within each end-use sector; the variations within the sub-fuel types within each end-use sector were not modeled. However,

for purposes of assigning uncertainty estimates to the sub-fuel type categories within each end-use sector in the current 

uncertainty analysis, SAIC/EIA (2001)-reported uncertainty estimates were extrapolated. 

33 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 68 

Geothermal 0.3 NE NE NE NE 

Total (including 

Geothermal)b,c 5,551.6 5,474.3 5,863.3 -1% +6% 

NA (Not Applicable) 

NE (Not Estimated) 

Notes:

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. c

 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 

A major change this year was the decision to report emissions from fuels used for non-energy purposes separately 

from fuel combustion emission estimates. Previously, the carbon stored from non-energy uses was subtracted from 

total carbon in fuels. However, this method resulted in non-energy emissions being reported within the emission 

estimates for fuel combustion. This year, these emissions are reported separately in the section entitled “Carbon 

Emitted and Stored in Products from Non-Energy Uses of Fossil Fuels.” 

Bunker fuel consumption estimates are now subtracted out from total fuel consumption, instead of subtracting the 

total carbon in bunker fuels from total potential carbon. This change in methodology does not have any affect on 

emission estimates, though it allows for clearer and more transparent emission calculations. 

Adjustments are now made to the consumption data for residual oil and other oils (>401 deg F) to subtract 

consumption for carbon black production, for which emissions are estimated in the Petrochemical Production 

section of the Industrial Processes chapter. 

EIA statistics report consumption for non-energy use for a number of fuels to be roughly equivalent to total use of 

these fuels, though for certain years these data series did not match exactly. After discussions with EIA, it was 

assumed that 100 percent of the use of these fuels should be assumed to be for non-energy purposes. These fuels 

are asphalt & road oil, lubricants, naphtha, other oil (>401 deg. F), special naphtha, waxes, and miscellaneous 

products. 

The Energy Information Administration (EIA 2004a) updated energy consumption data for all years. These 

revisions primarily impacted the emission estimates for 2002.

The combination of the methodological and historical data changes, as well as changes in the estimates of Carbon 

Emitted and Stored in Products from Non-Energy Uses of Fossil Fuels and International Bunker Fuels (which affect 

emissions from this source) resulted in an average annual decrease of 115.8 Tg CO2 Eq. (2.2 percent) in CO2

emissions from fossil fuel combustion for the period 1990 through 2002. This decrease is largely a result of 

reporting emissions from non-energy uses separately this year. 

Planned Improvements 

Several items are being evaluated to improve the estimates of CO2 emissions from fossil fuel combustion and to 

reduce uncertainty: 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 69 

• Currently, the IPCC guidelines recommend a default factor of 0.99 to represent the fraction of carbon in fossil 

fuels that is oxidized to CO2 during the fuel combustion of petroleum, though national experts are encouraged 

to improve upon this assumption if better data is available. As a result, carbon mass balances for light-duty 

gasoline cars and trucks have been analyzed to assess what would be the most appropriate carbon oxidation 

fraction for these vehicles. The analysis, currently under peer review, suggests that the amount of unoxidized 

carbon is insignificant compared to the gaseous carbon fraction, and that 1.00 should be used to represent the 

oxidized carbon fraction in future inventories for gasoline fueled light-duty vehicles. Upon further peer review, 

the revised factor is expected to be used in future inventories. A further examination into diesel fueled vehicles 

is also planned. 

• The 0.99 oxidation factor for coal will be further investigated in order to verify or revise this value. 

• Efforts will be taken to work with EIA and other agencies to improve the quality of the U.S. territories data. 

These improvements are not all-inclusive, but are part of an ongoing analysis and efforts to continually improve the 

CO2 from fossil fuel combustion estimates. 

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). These fuels 

are used in the industrial and transportation end-use sectors and are quite diverse, including natural gas, liquid 

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 fuel uses are 

equally diverse, and include application as solvents, lubricants, and waxes, or as raw materials in the manufacture of 

plastics, rubber, and synthetic fibers. 

Carbon dioxide emissions arise from non-energy uses via several pathways. Emissions may occur during the 

manufacture of a product, as is the case in producing plastics or rubber from fuel-derived feedstocks. Additionally, 

emissions may occur during the product’s lifetime, such as during solvent use. Overall, throughout the time series 

and across all uses, about 65 percent of the total carbon consumed for non-energy purposes is stored in products, 

and not released to the atmosphere; the remaining 35 percent is emitted. 

There are several areas in which non-energy uses of fossil fuels relates closely 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; these 

emissions are reported separately within this sector in the 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 sector. To avoid double-counting, the “raw” non-energy fuel consumption data 

reported by EIA are modified to account for these overlaps, resulting in the adjusted consumption values shown in 

Table 3-12. There are also net exports of petrochemicals that are not completely accounted for in the EIA data, and 

these affect the mass of carbon in non-energy applications; the effects of these adjustments are also shown in Table 

3-11. 

As shown in Table 3-11, fossil fuel emissions in 2003 from the non-energy uses of fossil fuels were 118.0 Tg CO2

Eq., which constituted 2 percent of overall fossil fuel emissions, approximately the same proportion as in 1990. In 

2003, the consumption of fuels for non-energy uses (after the adjustments described above) was 5,264 TBtu, an 

increase of 24 percent since 1990 (see Table 3-12). About 66.1 Tg of the C (242.5 Tg CO2 Eq.) in these fuels was 

stored, while the remaining 32.2 Tg C (118.0 Tg CO2 Eq.) was emitted. The proportion of C emitted has remained 

about constant since 1990, at about 31 to 35 percent of total non-energy consumption (see Table 3-13). 

Table 3-11: CO2 Emissions from Non-Energy Use Fossil Fuel Consumption (Tg CO2 Eq.) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 70 

Year 1990 1997 1998 1999 2000 2001 2002 2003 

Potential Emissions 299.3 348.3 373.3 395.0 365.8 357.8 360.9 360.5 

Carbon Stored 191.4 228.0 238.0 253.4 241.1 237.7 242.1 242.5 

Emissions 108.0 120.3 135.4 141.6 124.7 120.1 118.8 118.0 

Methodology 

The first step in estimating carbon stored in products was to determine the aggregate quantity of fossil fuels 

consumed for non-energy uses. The carbon content of these feedstock fuels is equivalent to potential emissions, or 

the product of consumption and the fuel-specific carbon content values. Both the non-energy fuel consumption and 

carbon content data were supplied by the EIA (2003) (see Annex 2.1). Consumption of natural gas, LPG, pentanes 

plus, naphthas, other oils, and special naphtha were adjusted to account for net exports of these products that are not 

reflected in the raw data from EIA. Consumption values for industrial coking coal, petroleum coke, other oils, and 

natural gas in Table 3-12 and Table 3-13, have been adjusted to subtract non-energy uses that are included in the 

source categories of the Industrial Processes sector.34

For the remaining non-energy uses, the amount of C stored was estimated by multiplying the potential emissions by 

a storage factor. For several fuel types—petrochemical feedstocks (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 Waste Combustion 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 the 

IPCC Guidelines for National Greenhouse Gas Inventories, 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 carbon in the respective NEU 

products. 

 

34 These source categories include Iron and Steel Production, Ammonia Manufacture, Carbon Black Manufacture (included in 

Petrochemical Production), Titanium Dioxide Production, Ferroalloy Production, and Aluminum Production. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 71 

Table 3-12: Adjusted Consumption of Fossil Fuels for Non-Energy Uses (TBtu) 

Year 1990 1997 1998 1999 2000 2001 2002 2003

Industry 3,993.2 4,794.6 5,127.6 5,398.1 4,985.1 4,933.3 5,016.4 5,023.4

Industrial Coking Coal 20.5 27.6 15.4 6.3 14.8 23.7 7.0 3.0

Industrial Other Coal 8.2 11.2 10.4 11.1 12.4 11.3 12.0 11.9

Natural Gas to Chemical Plants, 

Other Uses 

244.1 323.7 376.3 390.3 388.9 394.0 402.0 401.0

Asphalt & Road Oil 1,170.2 1,223.6 1,262.6 1,324.4 1,275.7 1,256.9 1,240.0 1,217.4

LPG 1,001.9 1,440.9 1,568.1 1,651.2 1,497.2 1,483.9 1,550.4 1,478.4

Lubricants 186.3 182.3 190.8 192.8 189.9 174.0 171.9 160.8

Pentanes Plus 69.9 260.9 185.2 239.0 214.7 193.2 164.6 162.4

Naphtha (<401 deg. F) 294.3 467.8 529.1 458.6 556.5 473.2 559.5 588.4

Other Oil (>401 deg. F) 615.4 608.8 593.8 619.5 515.5 506.1 460.9 530.0

Still Gas 21.3 2.1 0.0 16.1 12.6 35.8 57.8 133.0

Petroleum Coke 90.7 29.9 124.7 193.9 53.9 132.8 113.6 90.9

Special Naphtha 92.1 63.8 98.1 133.9 89.1 75.5 98.7 77.5

Distillate Fuel Oil 7.0 10.4 11.7 11.7 11.7 11.7 11.7 11.7

Waxes 33.3 43.7 42.4 37.4 33.1 36.3 32.2 31.0

Miscellaneous Products 137.9 97.8 119.0 111.9 119.3 124.9 134.2 126.0

Transportation 176.0 172.1 180.2 182.1 179.4 164.3 162.4 151.8

Lubricants 176.0 172.1 180.2 182.1 179.4 164.3 162.4 151.8

U.S. Territories 86.7 92.5 94.8 114.5 165.5 80.3 80.6 88.7

Lubricants 0.7 2.5 1.3 1.4 16.4 0.0 0.0 0.0

Other Petroleum (Misc. Prod.) 86.0 90.0 93.5 113.1 149.1 80.3 80.5 88.7

Total 4,255.9 5,059.3 5,402.5 5,694.6 5,330.0 5,177.9 5,259.4 5,263.9

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 carbon black production, which is also reported in the 

Industrial Processes sector. 

- Not applicable. 

Note: Totals may not sum due to independent rounding. 

Table 3-13: 2003 Adjusted Non-Energy Use Fossil Fuel Consumption, Storage, and Emissions 

Sector/Fuel Type Adjusted

Consumption

Carbon 

Content 

Storage 

Factor

Carbon

Stored

Carbon 

Emissions 

Carbon

Emissions

 (TBtu) (Tg C) (Tg C) (Tg C) (Tg CO2 Eq.)

Industry 5,023.0 93.5 - 65.7 27.8 101.8

Industrial Coking Coal 3.0 0.1 0.75 0.1 0.0 0.1

Industrial Other Coal 11.9 0.3 0.65 0.2 0.1 0.4

Natural Gas to Chemical Plants 401.0 5.8 0.65 3.8 2.0 7.4

Asphalt & Road Oil 1,217.4 25.1 1.00 25.1 0.0 0.0

LPG 1,478.4 24.9 0.65 16.2 8.7 31.9

Lubricants 160.8 3.3 0.09 0.3 3.0 10.8

Pentanes Plus 162.4 3.0 0.65 1.9 1.0 3.8

Naphtha (<401 deg. F) 588.4 10.7 0.65 6.9 3.7 13.7

Other Oil (>401 deg. F) 530.0 10.6 0.65 6.9 3.7 13.5

Still Gas 133.0 2.3 0.65 1.5 0.8 3.0

Petroleum Coke 90.9 2.5 0.50 1.3 1.3 4.6

Special Naphtha 77.5 1.5 0.65 1.0 0.5 2.0

Distillate Fuel Oil 11.7 0.2 0.50 0.1 0.1 0.4

Waxes 31.0 0.6 0.58 0.4 0.3 0.9

Miscellaneous Products 126.0 2.5 0.00 0.0 2.5 9.3

Transportation 151.8 3.1 - 0.3 2.8 10.2

Lubricants 151.8 3.1 0.09 0.3 2.8 10.2
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 72 

U.S. Territories 88.7 1.8 - 0.2 1.6 5.9

Lubricants 0.0 0.0 0.09 0.0 0.0 0.0

Other Petroleum (Misc. Prod.) 88.7 1.8 0.10 0.2 1.60 5.9

Total 5,263.9 98.3 66.1 32.2 118.0

a

 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 carbon black production, which is also reported in the 

Industrial Processes sector. 

- Not applicable. 

Note: Totals may not sum due to independent rounding. 

Lastly, emissions were estimated by subtracting the carbon stored from the potential emissions (see Table 3-11). 

More detail on the methodology for calculating storage and emissions from each of these sources is provided in 

Annex 2.3. 

Where storage factors were calculated specifically for the United States, data were obtained on fuel products such as 

asphalt, plastics, synthetic rubber, synthetic fibers, carbon black, personal cleansers, pesticides, and solvents, and 

industrial releases including VOC, solvent, and non-combustion CO emissions, TRI releases, refinery wastewater, 

hazardous waste incineration, and energy recovery. Data were taken from a variety of industry sources, government 

reports, and expert communications. Sources include EPA’s compilations of air emission factors (EPA 1995, 

2001), National Air Quality and Emissions Trends Report data (EPA 2004a), Toxics Release Inventory, 1998

(2000a), Biennial Reporting System data (EPA 2004b), pesticide sales and use estimates (EPA 1998, 1999, 2002) 

and hazardous waste data (EPA 2004b); the EIA Manufacturer’s Energy Consumption Survey (MECS) (EIA 1994, 

1997, 2001b); 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. Bureau 

of the Census (1999, 2003); the American Plastics Council (APC 2000, 2001, 2003; Eldredge-Roebuck 2000); the 

Society of the Plastics Industry (SPI 2000); the Rubber Manufacturers’ Association (RMA 2002; STMC 2003); the 

International Institute of Synthetic Rubber Products (IISRP 2000); the Fiber Economics Bureau (FEB 2001); the; 

Material Safety Data Sheets (Miller 1999); and the Chemical Manufacturer’s Association (CMA 1999); Specific 

data sources are listed in full detail in Annex 2.3. 

Uncertainty 

A Tier 2 Monte Carlo analysis was performed using @RISK software to determine the level of uncertainty 

surrounding the estimates of emissions and storage factors from non-energy uses. The Tier 2 analysis was 

performed to allow the specification of probability density functions for key variables, within a computational 

structure that mirrors the calculation of the inventory estimate. 

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 3-14 (emissions) and Table 3-15 

(storage factors). Carbon emitted from non-energy uses of fossil fuels in 2003 was estimated to be between 97.5 

and 130.9 Tg CO2 Eq. at a 95 percent confidence level (or in 19 out of 20 Monte Carlo Simulations). This indicates 

a range of 17 percent below to 11 percent above the 2003 emission estimate of 118.0 Tg CO2 Eq. The uncertainty 

in the emission estimates are a function of uncertainty in both the quantity of fuel used for non-energy purposes and 

the storage factor. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 73 

Table 3-14: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Non-Energy Uses of Fossil Fuels 

(Tg CO2 Eq. and Percent) 

Source Gas 

2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Feedstocks CO2 75.3 61.0 90.5 -19% +20% 

Asphalt CO2 0.0 0.2 0.9 NA NA 

Lubricants CO2 21.1 17.4 24.4 -17% +16% 

Waxes CO2 0.9 0.7 1.5 -24% +54% 

Other CO2 20.9 8.9 23.2 -57% +11% 

Total CO2 118.0 97.5 130.9 -17% +11% a

Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval. 

NA (Not Applicable) 

Table 3-15: Tier 2 Quantitative Uncertainty Estimates for Storage Factors of Non-Energy Uses of Fossil Fuels 

(Percent) 

Source Gas 

2003 Storage 

Factor Uncertainty Range Relative to Inventory Factora

 (%) (%) (%, Relative) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Feedstocks CO2 65% 63% 67% -3% +3% 

Asphalt CO2 100% 99% 100% -1% +0% 

Lubricants CO2 9% 4% 18% -58% +90% 

Waxes CO2 58% 44% 69% -24% +19% 

Other CO2 24% 18% 67% -24% +180% 

Total CO2 65% 63% 67% -3% +3% a

Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval. 

In Table 3-15, feedstocks and asphalt contribute least to overall storage factor uncertainty. 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 five fates that result in long-term storage (e.g., plastics production), 

and ten that result in emissions (e.g., volatile organic compound emissions). Rather than modeling the total 

uncertainty around all 15 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 wellcharacterized, 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. 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 carbon. Emission and storage totals for the different subcategories were compared, and trends across the time 

series were carefully 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 74 

Recalculations Discussion 

This year’s methodology reflects several refinements and improvements. First and most fundamentally, this year 

the NEU analysis is presented as its own source category. In the past, the NEU component of this category was 

described in the context of a calculation sequence that first determined potential emissions (based on total fuel 

consumption for all purposes), and then deducted (1) C not oxidized (e.g., in ash) and (2) C put into long-term 

storage as a result of NEUs. In this context, the focus of the NEU section was previously on carbon storage rather 

than emissions. This year, the presentation within this source category emphasizes emissions, though the storage 

factors (i.e., fraction of C stored rather than emitted) are still presented to facilitate comparison with IPCC guidance. 

Several substantive changes also appear in this year’s inventory. First, a U.S.-specific storage factor (58 percent) 

was developed for waxes. Previously, the IPCC storage factor for wax (100 percent stored) had been used. 

In addition, the methodology for calculating emissions and storage for feedstocks has been revised in several ways. 

First, three additional fuel types have been added as inputs to the system covered by the feedstocks mass balance 

calculations: other industrial coal, still gas, and special naphthas. The other significant improvement for feedstocks 

is that the estimates of U.S. plastics consumption have been revised: in previous years, the consumption data for 

some of the plastic resins in the dataset included consumption in Canada and Mexico. By adjusting the data to 

reflect U.S. (rather than North American) consumption for those resins, the accuracy of the estimate for feedstocks 

has improved. In addition, several minor adjustments were made (e.g., the calculations for storage now include 

synthetic rubber in durable and non-durable goods other than tires, and the calculations for emissions include losses 

due to abrasion/ oxidation of rubber from tires). 

As noted in the beginning of the section, there are several areas where the boundaries of the NEU analysis adjoin 

the systems covered within the Industrial Processes sector. Carbon black production has been removed from the 

NEU analysis, as it is now addressed as a subcategory of IP-petrochemical emissions (if it was retained in NEU, 

there would be double-counting). As a result of this change, residual oil—which is an input to carbon black 

production, and had been among the fuels covered in the NEU section in previous years—no longer appears in the 

list of fuels in this section. A portion of other oils is also assumed to be used in the production of carbon black, and 

therefore other oils consumption is adjusted to account for the amount of fuel used in the C black calculation. 

The storage factor for miscellaneous products also changed. In prior years, a value of 100 percent was used; in this 

year’s inventory, based on a review of the specific applications of these products, there is now assumed to be no 

storage at all from these products. 

There have been several updates to the data used to calculate storage factors, not only by adding information for 

2003 (where available) but also for updating data sets for earlier years. For example, the results reflect new data for 

hazardous waste incineration (1999 through 2001) and fiber production (2001 through 2002). 

Planned Improvements 

There are several improvements planned for the future: 

• Collecting additional information on energy recovery from Manufacturing Energy Consumption Surveys. An 

effort is planned to carefully examine the “microdata” from these surveys to determine the nature and quantity 

of materials initially identified as being destined for “non-energy use” that are actually combusted for energy 

recovery. 

• 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 carbon. Additional “fates” may be researched, including: the fossil carbon 

load in organic chemical wastewaters; an expanded import and export analysis (i.e., evaluating additional 

commodities); and improving the characterization of cleansers (to exclude any potential biogenic carbon 

sources). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 75 

Finally, although U.S.-specific storage factors have been developed for feedstocks, asphalt, lubricants, and waxes, 

default values from IPCC are still used for two of the non-energy fuel types (industrial coking coal and distillate 

oil), and broad assumptions are being used for the remaining fuels (petroleum coke, miscellaneous products, and 

other petroleum). Over the long term, there are plans to improve these storage factors by conducting analyses of C 

fate similar to those described in Annex 2.3. 

3.3. Stationary Combustion (excluding CO2) (IPCC Source Category 1A) 

Stationary combustion encompasses all fuel combustion activities from fixed sources (versus mobile combustion). 

Other than CO2, which was addressed in the previous section, gases from stationary combustion include the 

greenhouse gases CH4 and N2O and the ambient air pollutants NOx, CO, and NMVOCs.35 Emissions of these gases 

from stationary combustion sources depend upon fuel characteristics, size and vintage, along with combustion 

technology, pollution control equipment, and ambient environmental conditions. Emissions also vary with 

operation and maintenance practices. 

Nitrous oxide and NOx 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. Carbon monoxide 

emissions from stationary combustion are generally a function of the efficiency of combustion; they are highest 

when less oxygen is present in the air-fuel mixture than is necessary for complete combustion. These conditions are 

most likely to occur during start-up, shutdown and during fuel switching (e.g., the switching of coal grades at a 

coal-burning electric utility plant). Methane and NMVOC emissions from stationary combustion are primarily a 

function of the CH4 and NMVOC content of the fuel and combustion efficiency. 

Emissions of CH4 decreased 15 percent overall to 6.7 Tg CO2 Eq. (319 Gg) in 2003. This decrease in CH4

emissions was primarily due to lower wood consumption in the residential sector. Conversely, N2O emissions rose 

13 percent since 1990 to 13.8 Tg CO2 Eq. (45 Gg) in 2003. The largest source of N2O emissions was coal 

combustion by electricity generators, which alone accounted for 63 percent of total N2O emissions from stationary 

combustion in 2003. Overall, however, stationary combustion is a small source of CH4 and N2O in the United 

States. 

In contrast, stationary combustion is a significant source of NOx emissions, though a smaller source of CO and 

NMVOCs. In 2003, emissions of NOx from stationary combustion represented 39 percent of national NOx

emissions, while CO and NMVOC emissions from stationary combustion contributed approximately 5 and 7 

percent, respectively, to the national totals. From 1990 to 2003, emissions of NOx and CO from stationary 

combustion decreased by 27 and 11 percent, respectively, and emissions of NMVOCs increased by 10 percent. 

The decrease in NOx emissions from 1990 to 2003 are mainly due to decreased emissions from electric power. The 

decrease in CO and increase in NMVOC emissions over this time period can largely be attributed to apparent 

changes in residential wood use, which is the most significant source of these pollutants from stationary 

combustion. Table 3-16 through Table 3-19 provide CH4 and N2O emission estimates from stationary combustion 

by sector and fuel type. Estimates of NOx, CO, and NMVOC emissions in 2003 are given in Table 3-20.36

Table 3-16: CH4 Emissions from Stationary Combustion (Tg CO2 Eq.) 

Sector/Fuel Type 1990 1997 1998 1999 2000 2001 2002 2003 

Electric Power 0.6 0.6 0.7 0.7 0.7 0.7 0.7 0.7 

Coal 0.3 0.4 0.4 0.4 0.4 0.4 0.4 0.4 

Fuel Oil 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Natural gas 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

 

35 Sulfur dioxide (SO2) emissions from stationary combustion are addressed in Annex 6.3. 

36 See Annex 3.1 for a complete time series of ambient air pollutant emission estimates for 1990 through 2003. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 76 

Wood 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Industrial 2.1 2.4 2.3 2.2 2.3 2.1 2.1 2.1 

Coal 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Fuel Oil 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Natural gas 0.8 0.9 0.9 0.9 0.9 0.8 0.8 0.8 

Wood 0.9 1.0 1.0 1.0 1.0 0.9 0.9 0.9 

Commercial 0.7 0.8 0.8 0.8 0.8 0.7 0.7 0.8 

Coal + + + + + + + + 

Fuel Oil 0.2 0.1 0.1 0.1 0.2 0.2 0.2 0.2 

Natural gas 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Wood 0.2 0.3 0.3 0.3 0.3 0.2 0.3 0.3 

Residential 4.4 3.5 3.1 3.4 3.5 3.1 2.8 3.1 

Coal 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Fuel Oil 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Natural Gas 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 

Wood 3.5 2.6 2.3 2.5 2.6 2.2 1.9 2.1 

U.S. Territories + 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Coal + + + + + + + + 

Fuel Oil + 0.1 0.1 0.1 + 0.1 0.1 0.1 

Natural Gas + + + + + + + + 

Wood + + + + + + + + 

Total 7.8 7.4 6.9 7.1 7.3 6.7 6.4 6.7 

+ Does not exceed 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. 

Table 3-17: N2O Emissions from Stationary Combustion (Tg CO2 Eq.) 

Sector/Fuel Type 1990 1997 1998 1999 2000 2001 2002 2003 

Electric Power 7.6 8.6 8.9 8.9 9.3 9.0 9.1 9.3 

Coal 7.1 8.2 8.3 8.3 8.7 8.5 8.6 8.7 

Fuel Oil 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 

Natural Gas 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 

Wood 0.2 0.2 0.2 0.2 0.2 0.1 0.2 0.2 

Industrial 3.2 3.5 3.3 3.3 3.3 3.1 3.2 3.2 

Coal 0.7 0.7 0.7 0.6 0.6 0.6 0.6 0.6 

Fuel Oil 0.5 0.5 0.5 0.5 0.5 0.6 0.6 0.6 

Natural Gas 0.2 0.3 0.3 0.3 0.3 0.2 0.2 0.2 

Wood 1.7 2.0 1.9 1.9 1.9 1.7 1.8 1.8 

Commercial 0.4 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Coal 0.1 0.1 + + + + + + 

Fuel Oil 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Natural Gas 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Wood + 0.1 0.1 0.1 0.1 + + + 

Residential 1.1 0.9 0.8 0.9 1.0 0.9 0.8 0.9 

Coal + + + + + + + + 

Fuel Oil 0.3 0.3 0.2 0.3 0.3 0.3 0.3 0.3 

Natural Gas 0.1 0.2 0.1 0.1 0.2 0.1 0.1 0.2 

Wood 0.7 0.5 0.5 0.5 0.5 0.4 0.4 0.4 

U.S. Territories 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Coal + + + + + + + + 

Fuel Oil 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Natural Gas + + + + + + + + 

Wood + + + + + + + + 

Total 12.3 13.5 13.4 13.5 14.0 13.5 13.5 13.8 

+ Does not exceed 0.05 Tg CO2 Eq. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 77 

Note: Totals may not sum due to independent rounding. 

Table 3-18: CH4 Emissions from Stationary Combustion (Gg) 

Sector/Fuel Type 1990 1997 1998 1999 2000 2001 2002 2003 

Electric Power 27 29 31 31 32 32 32 33 

Coal 16 19 19 19 20 19 20 20 

Fuel Oil 4 3 4 4 3 4 3 4 

Natural Gas 3 4 4 5 5 5 5 5 

Wood 4 4 4 4 4 4 4 5 

Industrial 101 115 108 107 108 100 102 100 

Coal 17 16 15 14 15 14 13 13 

Fuel Oil 6 6 5 5 6 6 6 6 

Natural Gas 37 44 43 41 42 38 39 37 

Wood 41 49 46 46 47 41 44 43 

Commercial 35 38 36 37 39 35 35 36 

Coal 1 1 1 1 1 1 1 1 

Fuel Oil 10 7 7 7 8 7 7 8 

Natural Gas 13 16 15 15 16 15 15 15 

Wood 11 14 14 15 15 12 12 12 

Residential 209 167 150 160 167 148 132 146 

Coal 8 5 4 4 4 4 3 4 

Fuel Oil 14 14 13 15 16 15 15 15 

Natural Gas 21 24 22 23 24 23 24 25 

Wood 166 123 110 118 123 105 89 102 

U.S. Territories 2 2 2 2 2 3 3 4 

Coal + + + + + + + + 

Fuel Oil 2 2 2 2 2 3 3 3 

Natural Gas + + + + + + + + 

Wood + + + + + + + + 

Total 373 351 328 338 349 318 305 319 

+ Does not exceed 0.5 Gg 

Note: Totals may not sum due to independent rounding. 

Table 3-19: N2O Emissions from Stationary Combustion (Gg) 

Sector/Fuel Type 1990 1997 1998 1999 2000 2001 2002 2003 

Electricity Generation 24 28 29 29 30 29 29 30 

Coal 23 26 27 27 28 27 28 28 

Fuel Oil 1 1 1 1 1 1 1 1 

Natural Gas + + + + 1 1 1 + 

Wood + 1 1 1 1 + 1 1 

Industrial 10 11 11 11 11 10 10 10 

Coal 2 2 2 2 2 2 2 2 

Fuel Oil 2 2 2 2 2 2 2 2 

Natural Gas 1 1 1 1 1 1 1 1 

Wood 5 7 6 6 6 5 6 6 

Commercial 1 1 1 1 1 1 1 1 

Coal + + + + + + + + 

Fuel Oil 1 + + + + + + + 

Natural Gas + + + + + + + + 

Wood + + + + + + + + 

Residential 4 3 3 3 3 3 3 3 

Coal + + + + + + + + 

Fuel Oil 1 1 1 1 1 1 1 1 

Natural Gas + + + + + + + + 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 78 

Wood 2 2 1 2 2 1 1 1 

U.S. Territories + + + + + + + + 

Coal + + + + + + + + 

Fuel Oil + + + + + + + + 

Natural Gas + + + + + + + + 

Wood + + + + + + + + 

Total 40 44 43 43 45 43 44 45 

+ Does not exceed 0.5 Gg 

Note: Totals may not sum due to independent rounding. 

Table 3-20: NOx, CO, and NMVOC Emissions from Stationary Combustion in 2003 (Gg) 

Sector/Fuel Type NOx CO NMVOC

Electric Generation 4,045 480 50 

Coal 3,447 240 24 

Fuel Oil 135 30 4 

Natural gas 301 101 11 

Wood 34 NA NA

Other Fuelsa

 NA 35 2 

Internal Combustion 127 73 10 

Industrial 2,516 1,249 154 

Coal 522 138 10 

Fuel Oil 154 50 8 

Natural gas 920 402 52 

Wood NA NA NA

Other Fuelsa

 116 353 28 

Internal Combustion 803 307 55 

Commercial/Institutional 244 149 29 

Coal 19 13 1 

Fuel Oil 49 17 3 

Natural gas 155 82 13 

Wood NA NA NA

Other Fuelsa

 21 37 11 

Residential 417 2,575 773 

Coalb

 NA NA NA

Fuel Oilb

 NA NA NA

Natural Gasb

 NA NA NA

Wood 20 2,358 748 

Other Fuels 397 217 25 

Total 7,222 4,454 1,007 

NA (Not Available) 

a

 Includes LPG, waste oil, coke oven gas, and coke (EPA 2003), (EPA 2004a). b

 Residential coal, fuel oil, and natural gas emissions are included in “Other Fuels” (EPA 2003), (EPA 2004a). 

Note: Totals may not sum due to independent rounding. See Annex 3.1 for emissions in 1990 through 2003. 

Methodology 

Methane and N2O emissions were estimated by multiplying fossil fuel and wood consumption data by emission 

factors (by sector and fuel type). National coal, natural gas, fuel oil, and wood consumption data were grouped by 

sector: industrial, commercial, residential, electric power, and U.S. territories. For the CH4 and N2O estimates, fuel 

consumption data for the United States were obtained from EIA’s Monthly Energy Review and unpublished 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 79 

supplemental tables on petroleum product detail (EIA 2004). Because the United States does not include territories 

in its national energy statistics, fuel consumption data for territories were provided separately by the Grillot 

(2004).37 Fuel consumption for the industrial sector was adjusted to subtract out construction and agricultural use, 

which is reported under mobile sources.38 Construction and agricultural fuel use was obtained from EPA (2004b). 

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 Revised 1996 IPCC Guidelines for National 

Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997). U.S. territories’ emission factors were estimated 

using the U.S. emission factors for the primary sector in which each fuel was combusted. 

Emission estimates for NOx, CO, and NMVOCs in this section were obtained from preliminary data (EPA 2004a) 

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. The major categories included in this section are those 

reported in EPA (2003) and EPA (2004a): coal, fuel oil, natural gas, wood, other fuels (including LPG, coke, coke 

oven gas, and others), and stationary internal combustion. The EPA estimates emissions of NOx, CO, and 

NMVOCs by sector and fuel source using a "bottom-up" estimating procedure. In other words, emissions were 

calculated either for individual sources (e.g., industrial boilers) or for multiple sources combined, using basic 

activity data as indicators of emissions. Depending on the source category, these basic activity data may include 

fuel consumption, fuel deliveries, tons of refuse burned, raw material processed, etc. 

The overall emission control efficiency of a source category was derived from published reports, the 1985 National 

Acid Precipitation and Assessment Program (NAPAP) emissions inventory, and other EPA databases. The U.S. 

approach for estimating emissions of NOx, CO, and NMVOCs from stationary combustion, as described above, is 

consistent with the methodology recommended by the IPCC (IPCC/UNEP/OECD/IEA 1997). 

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 

Methane emission estimates from stationary sources exhibit high uncertainty, primarily due to difficulties in 

calculating emissions from wood combustion (i.e., fireplaces and wood stoves). The estimates of CH4 and N2O 

emissions presented are based on broad indicators of emissions (i.e., fuel use multiplied by an aggregate emission 

factor for different sectors), rather than specific emission processes (i.e., by combustion technology and type of 

emission control). 

An uncertainty analysis was performed by primary fuel type for each end-use sector, using the IPCC-recommended 

Tier 2 uncertainty estimation methodology, Monte Carlo Simulation technique, with @RISK software.

The uncertainty estimation model for this source category was developed by integrating the CH4 and N2O stationary 

source inventory estimation models with the model for CO2 from fossil fuel combustion to realistically characterize 

the interaction (or endogenous correlation) between the variables of these three models. 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). 

 

37 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. 

38 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 80 

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.39 For these variables, the uncertainty 

ranges were assigned to the input variables based on the data reported in SAIC/EIA (2001).40 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 Good Practice 

Guidance 2000). 

The uncertainty ranges for the activity-related input variables and N2O emission factors were typically asymmetric 

around their inventory estimates. The uncertainty ranges for the non-biomass-related CH4 emission factors were 

symmetric around their inventory estimates; for biomass, they were asymmetric around their emission factor 

estimates. Bias (or systematic uncertainties) associated with these variables accounted for much of the uncertainty 

associated with the activity data and N2O emission factor variables.41 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 3-21. Stationary combustion 

CH4 emissions in 2003 (including biomass) were estimated to be between 4.8 and 13.4 Tg CO2 Eq. at a 95 percent 

confidence level (or in 19 out of 20 Monte Carlo Simulations). This indicates a range of 28 percent below to 99 

percent above the 2003 emission estimate of 6.7 Tg CO2 Eq.42 Stationary combustion N2O emissions in 2003 

(including biomass) were estimated to be between 10.9 and 39.5 Tg CO2 Eq. at a 95 percent confidence level (or in 

19 out of 20 Monte Carlo Simulations). This indicates a range of 22 percent below to 184 percent above the 2003 

emissions estimate of 13.8 Tg CO2 Eq. 

Table 3-21: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Energy-Related Stationary 

Combustion, Including Biomass (Tg CO2 Eq. and Percent) 

Source Gas 

2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Stationary Combustion CH4 6.7 4.8 13.4 -28% +99% 

Stationary Combustion N2O 13.8 10.9 39.5 -22% +184% a

 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 ambient air pollutants, uncertainties are 

 

39 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. 

40 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. 

41 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.

42 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 81 

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 ambient air pollutants from stationary combustion in the United States. Emission totals for the 

different sectors and fuels were compared and trends were investigated. 

A few corrective actions were taken. It was determined that emissions from construction and farm use had been 

previously estimated in both this source category (in the industrial sector) and for mobile sources. To avoid doublecounting, these emissions are now reported only under mobile sources. Therefore, it was necessary to subtract out 

energy consumption from construction and agriculture from the industrial sector. Also, a small error was corrected 

in the conversion of natural gas and wood energy consumption from gross calorific value to net calorific value. 

Recalculations Discussion 

Historical CH4 and N2O emissions from stationary sources (excluding CO2) were revised due to several changes. 

First, industrial sector energy consumption was adjusted downward to avoid double-counting of emissions from 

construction and agriculture. Second, the conversion of natural gas and wood energy consumption from gross 

calorific value to net calorific value was corrected. These adjustments have been explained in the previous section. 

Third, slight changes to emission estimates for the other sectors are due to revised data from EIA (2004). This latter 

revision is explained in greater detail in the section on CO2 Emissions from Fossil Fuel Combustion within this 

sector. The combination of the methodological and historical data changes resulted in an average annual decrease 

of 0.4 Tg CO2 Eq. (5.2 percent) in CH4 emissions from stationary combustion and an average annual decrease of 0.4 

Tg CO2 Eq. (3.2 percent) in N2O emissions from stationary combustion for the period 1990 through 2002. 

Planned Improvements 

Several items are being evaluated to improve the CH4 and N2O emission estimates from stationary source 

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. Currently, the exclusion of biomass increases the uncertainty, although it 

was expected to reduce the uncertainty. These improvements are not all-inclusive, but are part of an ongoing 

analysis and efforts to continually improve these stationary estimates. 

3.4. Mobile Combustion (excluding CO2) (IPCC Source Category 1A) 

Mobile combustion emits greenhouse gases other than CO2, including CH4, N2O, and the ambient air pollutants 

NOx, CO, and NMVOCs. While air conditioners and refrigerated units in vehicles also emit hydrofluorocarbons 

(HFCs), these gases are covered in Chapter 3, Industrial Processes, under the section entitled Substitution of Ozone 

Depleting Substances. As with stationary combustion, N2O and NOx emissions are closely related to fuel 

characteristics, air-fuel mixes, combustion temperatures, as well as usage of pollution control equipment. Nitrous 

oxide, in particular, can be formed by the catalytic processes used to control NOx, CO, and hydrocarbon emissions. 

Carbon monoxide emissions from mobile combustion are significantly affected by combustion efficiency and the 

presence of post-combustion emission controls. Carbon monoxide emissions are highest when air-fuel mixtures 

have less oxygen than required for complete combustion. These emissions occur especially in idle, low speed, and 

cold start conditions. Methane and NMVOC emissions from motor vehicles are a function of the CH4 content of the 

motor fuel, the amount of hydrocarbons passing uncombusted through the engine, and any post-combustion control 

of hydrocarbon emissions, such as catalytic converters. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 82 

Emissions from mobile combustion were estimated by transport mode (e.g., highway, air, rail), fuel type (e.g. motor 

gasoline, diesel fuel, jet fuel), and vehicle type (e.g. passenger cars, light-duty trucks). Road transport accounted for 

the majority of mobile source fuel consumption, and hence, the majority of mobile combustion emissions. Table 

3-22 and Table 3-23 provide CH4 and N2O emission estimates, respectively, in Tg CO2 Eq.; Table 3-24 and Table 

3-25 present these estimates in Gg of each gas. Estimates of NOx, CO, and NMVOC emissions are given in Table 

3-26 through Table 3-28.43

Mobile combustion was responsible for a small portion of national CH4 emissions (0.5 percent) but was the second 

largest source of N2O (11.2 percent) in the United States. From 1990 to 2003, CH4 emissions declined by 44 

percent, to 2.7 Tg CO2 Eq. (128 Gg), due largely to control technologies employed on highway vehicles in the 

United States that reduce CO, NOx, NMVOC, and CH4 emissions. The same technologies, however, resulted in 

higher N2O emissions, causing a 27 percent increase in N2O emissions from mobile sources between 1990 and 

1998. Nitrous oxide emissions have subsequently declined 24 percent as improvements in the emission control 

technologies installed on new vehicles have reduced emission rates of both NOx and N2O per vehicle mile traveled. 

As a result, N2O emissions in 2003 were 4 percent lower than in 1990, at 42.1 Tg CO2 Eq. (136 Gg) (see Figure 

3-17). Overall, CH4 and N2O emissions were predominantly from gasoline-fueled passenger cars and light-duty 

gasoline trucks. 

Figure 3-17: Mobile Source CH4 and N2O Emissions 

Table 3-22: CH4 Emissions from Mobile Combustion (Tg CO2 Eq.) 

Fuel Type/Vehicle Typea

 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline Highway 4.3 3.5 3.3 3.0 2.7 2.4 2.2 2.0 

Passenger Cars 2.6 1.9 1.8 1.7 1.5 1.4 1.2 1.1 

Light-Duty Trucks 1.4 1.3 1.3 1.1 1.0 0.9 0.8 0.8 

Heavy-Duty Vehicles 0.3 0.2 0.2 0.2 0.1 0.1 0.1 0.1 

Motorcycles + + + + + + + + 

Diesel Highway + + + + + + + + 

Passenger Cars + + + + + + + + 

Light-Duty Trucks + + + + + + + + 

Heavy-Duty Vehicles + + + + + + + + 

Alternative Fuel Highway + 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Non-Highway 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 

Ships and Boats 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Locomotives 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Farm Equipment 0.2 0.2 0.1 0.2 0.2 0.1 0.1 0.1 

Construction Equipment 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Aircraft + 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Otherb + + + + + + 0.1 0.1 

Total 4.8 4.0 3.9 3.6 3.4 3.1 2.9 2.7 

+ Less than 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. a See Annex 3.2 for definitions of highway vehicle types. 

b

 “Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad 

equipment, airport equipment, commercial equipment, and industrial equipment. 

Table 3-23: N2O Emissions from Mobile Combustion (Tg CO2 Eq.) 

Fuel Type/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline Highway 40.3 51.5 51.7 50.8 49.1 45.0 41.6 38.0 

 

43 See Annex 3.2 for a complete time series of emission estimates for 1990 through 2003. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 83 

Passenger Cars 25.5 26.7 26.7 25.9 24.7 23.1 21.6 19.8 

Light-Duty Trucks 14.1 23.7 23.7 23.6 23.0 20.6 18.6 16.7 

Heavy-Duty Vehicles 0.7 1.1 1.3 1.3 1.3 1.4 1.4 1.4 

Motorcycles + + + + + + + + 

Diesel Highway 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Passenger Cars + + + + + + + + 

Light-Duty Trucks + + + + + + + + 

Heavy-Duty Vehicles 0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.3 

Alternative Fuel Highway + 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Non-Highway 3.2 3.3 3.3 3.4 3.7 3.5 3.6 3.6 

Ships and Boats 0.4 0.3 0.3 0.3 0.5 0.3 0.5 0.5 

Locomotives 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Farm Equipment 1.7 1.7 1.8 1.8 1.9 1.8 1.7 1.7 

Construction Equipment 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Aircraft 0.3 0.4 0.4 0.4 0.4 0.4 0.4 0.5 

Other* 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Total 43.7 55.2 55.3 54.6 53.2 49.0 45.6 42.1 

+ Less than 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. 

*“Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad 

equipment, airport equipment, commercial equipment, and industrial equipment. 

Table 3-24: CH4 Emissions from Mobile Combustion (Gg) 

Fuel Type/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline Highway 205 166 158 143 130 116 105 95 

Passenger Cars 125 91 87 82 73 65 59 53 

Light-Duty Trucks 65 64 60 52 49 44 40 38 

Heavy-Duty Vehicles 13 10 10 9 7 6 5 5 

Motorcycles 1 1 1 1 1 1 1 1 

Diesel Highway 1 1 1 1 1 1 1 1 

Passenger Cars + + + + + + + + 

Light-Duty Trucks + + + + + + + + 

Heavy-Duty Vehicles 1 1 1 1 1 1 1 1 

Alternative Fuel Highway 1 3 3 4 4 5 5 6 

Non-Highway 22 23 23 24 26 25 26 26 

Ships and Boats 4 3 3 3 5 3 4 4 

Locomotives 3 3 3 3 3 3 3 3

Farm Equipment 7 7 7 7 7 7 7 7 

Construction Equipment 4 5 5 5 5 6 6 6 

Aircraft 2 3 3 3 3 3 3 3 

Other* 2 2 2 2 2 2 2 2 

Total 228 193 185 172 161 147 138 128 

+ Less than 0.5 Gg 

Note: Totals may not sum due to independent rounding. 

* “Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad 

equipment, airport equipment, commercial equipment, and industrial equipment. 

Table 3-25: N2O Emissions from Mobile Combustion (Gg) 

Fuel Type/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline Highway 130 166 167 164 158 145 134 123

Passenger Cars 82 86 86 84 80 74 70 64

Light-Duty Trucks 46 76 76 76 74 66 60 54

Heavy-Duty Vehicles 2 4 4 4 4 4 5 5

Motorcycles + + + + + + + + 

Diesel Highway 1 1 1 1 1 1 1 1

Passenger Cars + + + + + + + + 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 84 

Light-Duty Trucks + + + + + + + + 

Heavy-Duty Vehicles 1 1 1 1 1 1 1 1

Alternative Fuel Highway + + + + + + + + 

Non-Highway 10 11 11 11 12 11 12 12

Ships and Boats 1 1 1 1 2 1 2 1

Locomotives 1 1 1 1 1 1 1 1

Farm Equipment 6 6 6 6 6 6 6 5

Construction Equipment 1 1 1 1 1 1 1 1

Aircraft 1 1 1 1 1 1 1 1

Other* 1 1 1 1 1 1 1 1

Total 141 178 179 176 171 158 147 136 

+ Less than 0.5 Gg 

Note: Totals may not sum due to independent rounding. 

* “Other" includes snowmobiles and other recreational equipment, logging equipment, lawn and garden equipment, railroad 

equipment, airport equipment, commercial equipment, and industrial equipment.

Mobile sources comprise the single largest source category of CO, NOx, and NMVOC emissions in the United 

States. In 2003, mobile combustion contributed 89 percent of CO emissions, 56 percent of NOx emissions, and 46 

percent of NMVOC emissions. Since 1990, emissions of NMVOCs from mobile combustion decreased by 42 

percent, CO emissions decreased 37 percent, and emissions of NOx decreased by 14 percent.

Table 3-26: NOx Emissions from Mobile Combustion (Gg) 

Fuel Type/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline Highway 5,746 4,268 4,090 3,924 3,812 3,715 3,519 3,527 

Passenger Cars 3,847 2,447 2,316 2,158 2,084 2,027 1,920 1,924 

Light-Duty Trucks 1,364 1,334 1,294 1,268 1,303 1,285 1,217 1,220 

Heavy-Duty Vehicles 515 475 467 485 411 390 369 370 

Motorcycles 20 13 13 13 13 14 13 13 

Diesel Highway 2,956 3,708 3,729 3,671 3,803 3,338 3,162 3,169 

Passenger Cars 39 13 11 10 7 6 6 6 

Light-Duty Trucks 20 10 9 8 6 5 5 5 

Heavy-Duty Vehicles 2,897 3,685 3,709 3,653 3,791 3,326 3,151 3,158 

Alternative Fuel Highwaya IE IE IE IE IE IE IE IE

Non-Highway 3,432 3,792 3,772 3,705 3,780 3,770 3,707 3,722 

Ships and Boats 953 963 919 818 966 971 954 958 

Locomotives 857 962 973 910 908 907 891 895 

Farm Equipment 63 75 83 84 80 73 72 72 

Construction Equipment 437 487 487 497 484 480 472 474 

Aircraftb

 641 708 706 765 697 690 678 681 

Otherc

 480 597 604 632 645 650 639 642 

Total 12,134 11,768 11,592 11,300 11,395 10,823 10,389 10,418 

IE (Included Elsewhere) 

Note: Totals may not sum due to independent rounding. a

 NOx emissions from alternative fuel highway vehicles are included under gasoline and diesel highway vehicles. b

 Aircraft estimates include only emissions related to landing and take-off (LTO) cycles, and therefore do not include cruise 

altitude emissions. 

c

 "Other" includes gasoline- and diesel-powered recreational, industrial, lawn and garden, light commercial, logging, airport 

service, and other equipment. 

Table 3-27: CO Emissions from Mobile Combustion (Gg) 

Fuel Type/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline Highway 98,328 67,509 65,246 61,210 60,657 56,716 55,541 52,544 

Passenger Cars 60,757 36,825 35,686 32,921 32,867 31,600 30,945 29,275 

Light-Duty Trucks 29,237 25,748 24,754 23,343 24,532 22,574 22,107 20,914 

Heavy-Duty Vehicles 8,093 4,787 4,642 4,782 3,104 2,411 2,361 2,234 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 85 

Motorcycles 240 150 163 164 154 131 129 122 

Diesel Highway 1,696 1,301 1,202 1,122 1,088 869 851 805 

Passenger Cars 35 13 10 10 7 6 6 5 

Light-Duty Trucks 22 13 12 9 6 5 5 5 

Heavy-Duty Vehicles 1,639 1,276 1,179 1,103 1,075 858 840 795 

Alternative Fuel Highwaya IE IE IE IE IE IE IE IE

Non-Highway 19,459 21,474 21,493 21,152 21,935 22,387 22,181 22,177 

Ships and Boats 1,679 1,948 1,943 2,121 1,946 1,952 1,934 1,934 

Locomotives 85 89 83 98 90 90 89 89 

Farm Equipment 217 250 274 285 245 233 231 231 

Construction Equipment 582 636 633 630 626 621 615 615 

Aircraftb

 1,090 1,098 1,081 1,074 1,047 1,041 1,032 1,031 

Otherc

 15,807 17,453 17,478 16,943 17,981 18,449 18,280 18,276 

Total 119,482 90,284 87,940 83,484 83,680 79,972 78,574 75,526 

IE = Included Elsewhere 

Note: Totals may not sum due to independent rounding. a

 CO emissions from alternative fuel highway vehicles are included under gasoline and diesel highway vehicles. b

 Aircraft estimates include only emissions related to landing and take-off (LTO) cycles, and therefore do not include cruise 

altitude emissions. 

c

 "Other" includes gasoline- and diesel-powered recreational, industrial, lawn and garden, light commercial, logging, airport 

service, and other equipment. 

Table 3-28: NMVOC Emissions from Mobile Combustion (Gg) 

Fuel Type/Vehicle Type 1990 1997 1998 1999 2000 2001 2002 2003

Gasoline Highway 8,110 5,167 5,067 4,924 4,615 4,285 3,931 3,832 

Passenger Cars 5,120 2,928 2,895 2,810 2,610 2,393 2,195 2,140 

Light-Duty Trucks 2,374 1,882 1,812 1,734 1,750 1,664 1,527 1,488 

Heavy-Duty Vehicles 575 336 335 351 232 206 189 184 

Motorcycles 42 22 25 28 23 22 20 20 

Diesel Highway 406 263 249 230 216 207 190 185 

Passenger Cars 16 6 5 5 3 3 3 3 

Light-Duty Trucks 14 8 7 6 4 4 3 3 

Heavy-Duty Vehicles 377 249 237 219 209 201 184 179 

Alternative Fuel Highwaya IE IE IE IE IE IE IE IE

Non-Highway 2,416 2,498 2,427 2,432 2,398 2,379 2,438 2,333 

Ships and Boats 608 766 763 769 744 730 748 716 

Locomotives 33 35 33 38 35 35 36 35 

Farm Equipment 28 32 35 38 24 19 20 19 

Construction Equipment 85 83 81 81 76 72 74 71 

Aircraftb 149 142 137 141 130 125 128 123 

Otherc 1,513 1,441 1,378 1,366 1,390 1,397 1,432 1,370 

Total 10,933 7,928 7,742 7,586 7,230 6,872 6,560 6,351 

IE (Included Elsewhere) 

Note: Totals may not sum due to independent rounding. a

 NMVOC emissions from alternative fuel highway vehicles are included under gasoline and diesel highway vehicles. b

 Aircraft estimates include only emissions related to landing and take-off (LTO) cycles, and therefore do not include cruise 

altitude emissions. 

c

 "Other" includes gasoline- and diesel-powered recreational, industrial, lawn and garden, light commercial, logging, airport 

service, and other equipment. 

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). Depending upon the category, 

activity data included such information as fuel consumption, and vehicle miles traveled (VMT). The activity data 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 86 

and emission factors used are described in the subsections that follow. A complete discussion of the methodology 

used to estimate emissions from mobile combustion and the emission factors used in the calculations is provided in 

Annex 3.2. 

EPA (2003) provided emissions estimates of NOx, CO, and NMVOCs for eight categories of highway vehicles,44

aircraft, and seven categories of non-highway vehicles.45 These emission estimates were provided from preliminary 

EPA data, which, in its final iteration, will be published on the National Emission Inventory (NEI) Air Pollutant 

Emission Trends web site. The methodology used to develop these estimates can be found on EPA’s Air Pollutant 

Emission Trends website, at <http://www.epa.gov/ttn/chief/trends/index.html>. 

Highway Vehicles 

Emission estimates for gasoline and diesel highway vehicles were based on VMT and emission factors by vehicle 

type, fuel type, model year, and control technology. Emissions from alternative fuel vehicles (AFVs)46 were based 

on VMT by vehicle and fuel type. 

Emission factors for gasoline and diesel highway vehicles were developed by ICF (2004). These factors were based 

on EPA and California Air Resources Board (CARB) laboratory test results of different vehicle and control 

technology types. The EPA and CARB 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 emission characteristics of 

segment 2 was 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 to approximate average driving characteristics. 

Emission factors for AFVs were developed after consulting a number of sources, including Argonne National 

Laboratory’s GREET 1.5–Transportation Fuel Cycle Model (Wang 1999), Lipman and Delucchi (2002), the 

Auto/Oil Air Quality Improvement Research Program (CRC 1997), the California Air Resources Board (Brasil and 

McMahon 1999), and the University of California Riverside (Norbeck, et al., 1998). The primary approach taken 

was to calculate CH4 emissions from actual test data and determine N2O emissions from NOx emissions from the 

same tests. While the formation of N2O is highly dependent on the type of catalyst used and the catalyst 

temperature, tailpipe N2O is likely to increase as engine out NOx emissions increase. Thus, as a first approximation, 

the NOx to N2O emission ratio will likely be constant for a given emission control group. 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 2003 were obtained from the Federal Highway Administration’s (FHWA) 

Highway Performance Monitoring System database as reported in Highway Statistics (FHWA 1996 through 2004). 

VMT was then allocated from FHWA’s vehicle categories to fuel-specific vehicle categories based on estimates of 

fuel consumption by fuel type for each vehicle category, developed using information on shares of vehicle fuel use 

for each vehicle category by fuel type reported in DOE (2004) and information on total motor vehicle fuel 

 

44 These categories included: gasoline passenger cars, diesel passenger cars, light-duty gasoline trucks less than 6,000 pounds in 

weight, light-duty gasoline trucks between 6,000 and 8,500 pounds in weight, light-duty diesel trucks, heavy-duty gasoline 

trucks and buses, heavy-duty diesel trucks and buses, and motorcycles. 

45 These categories included: locomotives, marine vessels, farm equipment, construction equipment, other off-highway liquid 

fuel (e.g. recreational vehicles and lawn and garden equipment), and other off-highway gaseous fuel (e.g., other off-highway 

equipment running on compressed natural gas). 

46 Alternative fuel and advanced technology vehicles are those that can operate using a motor fuel other than gasoline or diesel.

This includes electric or other bifuel or dual fuel vehicles that may be partially powered by gasoline or diesel. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 87 

consumption by fuel type from FHWA (1996 to 2004). VMT for AFVs were taken from Browning (2003). The age 

distributions of the U.S. vehicle fleet were obtained from EPA (2004b) and EPA (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 highway vehicles were obtained from the EPA’s Office of Transportation 

and Air Quality (EPA 2004a, 2004d, 2000, 1998, and 1997). These technologies and standards are defined in 

Annex 3.2, and were compiled from EPA (1993), EPA (1994a), EPA (1994b), EPA (1998), EPA (1999), and 

IPCC/UNEP/OECD/IEA (1997). 

Preliminary estimates for NOx, CO, and NMVOCs were obtained from EPA (2004e) and disaggregated based on 

EPA (2003), which, in its final iteration, will be published on the National Emission Inventory (NEI) Air Pollutant 

Emission Trends web site. 

Non-Highway 

Fuel consumption data were employed as a measure of activity for non-highway vehicles, and then fuel-specific 

emission factors were applied.47 Activity data were obtained from AAR (2004), BEA (1991 through 2004), Benson 

(2002 through 2004), DOE (1993 through 2004), DESC (2004), DOC (1991 through 2004), DOT (1991 through 

2004), EIA (2002a), EIA (2002b), EIA (2004a), EIA (2004b), EIA (2003 through 2004), EIA (1991 through 2004), 

EPA (2004c), and FAA (2004). Emission factors for non-highway modes were taken from IPCC/UNEP/OECD/IEA 

(1997).

Uncertainty 

This section discusses the uncertainty of the emissions estimates for CH4 and N2O. Uncertainty was analyzed 

separately for highway vehicles and non-highway vehicles, due to differences in their characteristics and their 

contributions to total mobile source emissions. 

Uncertainty analyses were not conducted for CO, NOx, or NMVOC emissions. Emission factors for these gases 

have been extensively researched, since these gases are regulated emissions from motor vehicles in the United 

States, and the uncertainty of these emissions estimates is believed to be relatively low. A much higher level of 

uncertainty is associated with CH4 and N2O emission factors, since emissions of these gases are not regulated in the 

United States, and unlike CO2 emissions, the emission pathways of CH4 and N2O are also highly complex. 

Highway Vehicles

A quantitative uncertainty analysis was conducted for the highway portion of the mobile source sector using the 

IPCC-recommended Tier 2 uncertainty estimation methodology, Monte Carlo Simulation technique, using @RISK 

software. The uncertainty analysis was performed on 2003 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 mile traveled (VMT) data, 

by vehicle and fuel type and (2) emission factor data, by vehicle, fuel, and control technology type. 

Mobile combustion emissions of CH4 and N2O per vehicle mile traveled vary significantly due to fuel type and 

composition, technology type, operating speeds and conditions, type of emission control equipment, equipment age, 

and operating and maintenance practices. The primary activity data, VMT, are collected and analyzed each year by 

government agencies. 

 

47 The consumption of international bunker fuels is not included in these activity data, but is estimated separately under the 

International Bunker Fuels source category. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 88 

To determine the uncertainty associated with the activity data used in the calculations of CH4 and N2O emissions, 

the agencies and the experts that supply the data were contacted. Because few of these sources were able to provide 

quantitative estimates of uncertainty, expert quantitative judgments were used to assess the uncertainty associated 

with the activity data. 

The emission factors for highway vehicles used in the Inventory were obtained from ICF (2004). These factors 

were based on laboratory testing of vehicles. While the controlled testing environment simulates real driving 

conditions, emission results from such testing can only approximate real world conditions and emissions. For some 

vehicle and control technology types, the testing did not yield statistically significant results within the 95 percent 

confidence interval, requiring expert judgments to be used in developing the emission factors. In those cases, the 

emission factors were developed based on comparisons of fuel consumption between similar vehicle and control 

technology categories. 

The estimates of VMT for highway vehicles by vehicle type in the United States were provided by FHWA (1996 

through 2004), and were generated though the cooperation of FHWA and state and local governments. These 

estimates are subject to several possible sources of error, such as unregistered vehicles, unreported fuel sales to 

avoid fuel taxes, differences in achieved versus estimated fuel economy, and measurement and estimation errors. 

These VMT were apportioned by fuel type, and then allocated to individual model years using temporal profiles of 

both the vehicle fleet by age and vehicle usage by model year in the United States provided by EPA (2004b) and 

EPA (2000). While the uncertainty associated with total U.S. VMT is believed to be low, the uncertainty within 

individual source categories was assumed to be higher given uncertainties associated with apportioning total VMT 

into individual vehicle categories, by technology type, and equipment age. The uncertainty of individual estimates 

was assumed to relate to the magnitude of estimated VMT (i.e., it was assumed smaller sources had greater 

percentage uncertainty). A further source of uncertainty occurs since FHWA and EPA use different definitions of 

vehicle type and estimates of VMT by vehicle type (provided by FHWA) are broken down by fuel type using EPA 

vehicle categories. 

A total of 69 highway data input variables were simulated through Monte Carlo Simulation technique using @RISK 

software. Variables included VMT and emission factors for individual vehicle categories and technologies. In 

developing the uncertainty estimation model, a normal distribution was assumed for all activity-related input 

variables (e.g., VMT) except in the case of buses, in which a triangular distribution was used. The dependencies 

and other correlations among the activity data were incorporated into the model to ensure consistency in the model 

specification and simulation. Emission factors were assigned uniform distributions, with upper and lower bounds 

assigned to input variables based on 97.5 percent confidence intervals of laboratory test data. In cases where data 

did not yield statistically significant results within the 95 percent confidence interval, estimates of upper and lower 

bounds were made using expert judgment. The bounds for the emission factor-related input variables were typically 

asymmetrical around their inventory estimates. Bias (or systematic uncertainties) associated with the emission 

factors was incorporated into the analysis when expert judgments were applied to the laboratory test results in 

determining the uncertainty characteristics and/or the bounds of the emission factors.48 The results of this analysis 

are reported in the section below, titled Quantitative Estimates of Uncertainty. 

Non-Highway 

Emissions from non-highway vehicles are a small portion of total emissions from mobiles sources, representing 20 

percent of CH4 emissions from mobile sources and 9 percent of N2O emissions from mobile sources in 2003. Since 

they comprise a small share of mobile source emissions, even large uncertainties in these estimates would have a 

relatively small impact on the total emission estimate for mobile sources. As a result, a quantitative analysis of 

uncertainty of emissions from non-highway vehicles has not been performed. However, sources of uncertainty for 

 

48 Random uncertainties are the main focus of statistical uncertainty analysis. Uncertainty estimates elicited from experts 

include both random and systematic uncertainty. Hence, both these types of uncertainty are represented in this uncertainty 

analysis.
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 89 

non-highway vehicles are being investigated by examining the underlying uncertainty of emission factors and fuel 

consumption data. 

Overall, a significant amount of uncertainty is associated with the emission estimates for non-road sources. A 

primary cause is a large degree of uncertainty surrounding emission factors. The IPCC Good Practice Guidance 

and Uncertainty Management in National Greenhouse Gas Inventories reports that CH4 emissions from aviation 

and marine sources may be uncertain by a factor of two, while N2O emissions may be uncertain by an order of 

magnitude for marine sources and several orders of magnitude for aviation. No information is provided on the 

uncertainty of emission factors for other non-highway sources. 

Fuel consumption data have a lower uncertainty than emission factors, though large uncertainties do exist for 

individual sources. Fuel consumption for off-highway vehicles (i.e., equipment used for agriculture, construction, 

lawn and garden, railroad, airport ground support, etc., as well as recreational vehicles) was generated by EPA’s 

NONROAD model (EPA 2004c). This model estimates fuel consumption based on estimated equipment/vehicle 

use (in hours) and average fuel consumed per hour of use. Since the fuel estimates are not based upon documented 

fuel sales or consumption, a fair degree of uncertainty accompanies these estimates.

Distillate consumption for ships and boats was obtained from sales estimates from EIA’s Fuel Oil and Kerosene 

Sales (EIA 1991 through 2004). The estimates for distillate consumption have associated uncertainty, as EIA’s 

estimates are based on sales to economic sectors, and it can be difficult to determine how much of the fuel sold in 

each sector is used by mobile or stationary sources and to further attribute this consumption to specific final users. 

For example, some fuel purchased by the marine sector may be used for operating heavy equipment or even 

generators, instead of being used entirely by ships and boats. This distinction between mobile and stationary fuel 

users is not made by EIA. 

EIA does provide coefficients of variation to estimate sampling error, which occurs due to the fact that these 

estimates are based on a sample set. However, as EIA points out, these coefficients do not take into account all the 

sources of potential bias, which includes incomplete information, misinterpretation of survey questions, and other 

factors that may cause estimates of fuel sales to be different from actual sales. In addition, diesel for ships and boats 

is adjusted for bunker fuel consumption, which introduces an additional (and much higher) level of uncertainty. 

Domestic consumption for residual fuel consumption by ships and boats is obtained from EIA (2004a). These 

estimates fluctuate widely from year to year. Such fluctuations cannot be fully explained without further analysis of 

the underlying activity data sets and, as such, the estimates are believed to be highly uncertain. The estimate of 

domestic consumption is then adjusted downward to account for international bunker fuels, which represents the 

primary use of residual fuel by ships and boats. As the international bunker fuel data are considered to have a 

moderate level of uncertainty,49 the overall uncertainty of the domestic ships and boats estimate for residual fuel 

consumption is considered high. 

Domestic jet fuel and aviation gasoline consumption data are obtained from EIA (2004a). Like diesel and residual 

marine fuel consumption, jet fuel consumption for aviation is adjusted downward to account for international 

bunker fuels. The international bunker fuel estimates introduce a significant amount of uncertainty. Additionally, 

all jet fuel consumption in the transportation sector is assumed to be consumed by aircraft. Some fuel purchased by 

airlines is not used in aircraft, but instead used to power auxiliary power units, in ground equipment, and to test 

engines. Some jet fuel may also be used for other purposes such as blending with diesel fuel or heating oil. 

In calculating CH4 emissions from aircraft, an average emission factor is applied to total jet fuel consumption. This 

average emission factor takes into account the fact that CH4 emissions occur only during the landing and take-off 

 

49 This is discussed in the section on International Bunker Fuels. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 90 

(LTO) cycles, with no CH4 being emitted during the cruise cycle. However, a better approach would be to apply 

emission factors based on the number of LTO cycles. 

Finally, U.S. aircraft emission estimates for CO, NOx, and NMVOCs from EPA (2004e and 2003) are based on 

LTO cycles and only estimate near ground-level emissions, which are more relevant for air quality evaluations. 

These estimates also include both domestic and international flights. Therefore, estimates presented here may 

overestimate IPCC-defined domestic CO, NOx, and NMVOC emissions by including LTO cycles by aircraft on 

international flights but underestimate total emissions because they exclude emissions from aircraft on domestic 

flight segments at cruising altitudes. 

Quantitative Estimates of Uncertainty 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-29. Mobile combustion CH4

emissions in 2003 were estimated to be between 2.5 and 2.8 Tg CO2 Eq. at a 95 percent confidence level (or in 19 

out of 20 Monte Carlo Simulations). This indicates a range of 9 percent below to 4 percent above the 2003 

emission estimate of 2.7 Tg CO2 Eq. Also at a 95 percent confidence level, mobile combustion N2O emissions in 

2003 were estimated to be between 35.2 and 52.8 Tg CO2 Eq., indicating a range of 16 percent below to 26 percent 

above the 2003 emission estimate of 42.1 Tg CO2 Eq. 

Table 3-29: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Mobile Sources (Tg CO2

Eq. and Percent) 

Source Gas 

2003 

Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Mobile Sources CH4 2.7 2.5 2.8 -9% +4% 

Mobile Sources N2O 42.1 35.2 52.8 -16% +26% a

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 credible quantitative uncertainty 

estimates for this source category using the IPCC Tier 2 approach to uncertainty analysis. In the upcoming years, 

the type and the characteristics of the actual probability density functions underlying the input variables will be 

identified and more credibly characterized. Accordingly, the quantitative uncertainty estimates reported in this 

section should be considered as preliminary and illustrative. 

QA/QC and Verification 

A source-specific QA/QC plan for mobile 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 focused on the emission factor and 

activity data sources, as well as the methodology used for estimating emissions. Because significant changes were 

made to highway vehicle emission factors and non-highway fuel consumption estimates (see Recalculations 

Discussion below), QA/QC efforts were focused in particular on emissions impacted by these changes. These 

procedures included a qualitative assessment of the emission estimates to determine whether they appear consistent 

with the most recent activity data and emission factors available. A comparison of historical emissions between this 

year’s and last year’s Inventories was also conducted, and was qualitatively assessed to ensure that the changes in 

estimates were consistent with the changes in activity data and emission factors. 

Recalculations Discussion 

In order to ensure the highest quality estimates, the methodology is continuously revised based on comments from 

internal and external reviewers. This year, adjustments were made to emission factors and activity data to 

incorporate new research and additional data sources. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 91 

The most significant changes in this year’s Inventory are the revisions to the highway emission factors for CH4 and 

N2O. The previous emission factors had been derived using methodology similar to that outlined in 

IPCC/UNEP/OECD/IEA (1997). However, preliminary tests suggested that these emission factors may not have 

been representative of actual emissions. EPA sponsored laboratory testing that resulted in the new emission factors 

used in this report. The emission factors changed notably, ranging from a decrease of 97 percent to an increase of 

114 percent for individual vehicle/fuel type/technology types; the combined effect of these changes in emission 

factors significantly lowered CH4 and N2O emissions from highway vehicles. Given the large contribution of 

highway vehicles to the mobile totals, these new emission factors were major drivers behind the changes in CH4 and 

N2O estimates. However, because these emission factors do not impact CO2 emissions, their impact is less 

significant when considering total mobile source emissions of all GHGs. 

Additionally, the methodology for estimating VMT by vehicle/fuel type category (e.g., LDGV, LDDV, LDGT, 

LDDT, etc.) was revised. The purpose of this revision was to ensure that the VMT estimates for individual vehicle 

categories match up with VMT estimates by vehicle type from FHWA (1996 through 2004), which is the 

recognized source of national VMT estimates. The new methodology apportions FHWA’s estimate of VMT for 

each vehicle category (e.g., light-duty vehicles, light-duty trucks, buses, heavy-duty trucks) to fuel-based vehicle 

categories based on estimates of gasoline and diesel fuel consumption for each vehicle category, drawn from DOE 

(2004). This change had a small impact on emissions. 

The vehicle age distribution values for highway vehicles were also revised. Previously, the Inventory relied on one 

vehicle age distribution across 25 model years from EPA (2000), which was applied for each year. For the current 

Inventory, that same distribution was applied for years 1990 through 1998, and annually variable vehicle age 

distributions were applied for years 1999 through 2003 based on data obtained from EPA’s MOVES model (EPA 

2004b). The annually variable age distributions more accurately reflect changes in the vehicle stock, and include 31 

model years. 

Emissions from gasoline- and diesel-electric hybrid vehicles are now included under gasoline and diesel vehicles 

instead of alternative fuel/advanced technology category, since these vehicles only run on traditional motor 

gasoline. This change did not have an impact on total emissions. 

EPA’s NONROAD model is now used as the primary data source for fuel consumption by off-highway equipment 

(e.g., construction, agricultural, lawn and garden equipment), rather than data from FHWA’s Highway Statistics 

(FHWA 1996 through 2004), EIA’s Fuel Oil and Kerosene Sales (EIA 1991 through 2004), and several other 

sources. The NONROAD model is also used to estimate fuel consumption by recreational boats, rather than data 

from FHWA’s Highway Statistics. Some of the individual sources used in the previous Inventory had not been 

updated for several years, while others had updated their methodologies, resulting in large variations in estimates 

from year to year. Use of the NONROAD Model for all of these estimates is believed to provide more accurate, upto-date, and consistent estimates. 

Other changes include minor revisions to historical aircraft fuel consumption estimates in FAA (2004) and rail 

consumption provided from Benson (2004). These revisions were not significant, and the overall impact on 

emissions estimates is small. 

Overall, these changes resulted in an average annual decrease of 0.6 Tg CO2 Eq. (14 percent) in CH4 emissions from 

mobile combustion and an average annual decrease of 6.2 Tg CO2 Eq. (11 percent) in N2O emissions from mobile 

combustion for the period 1990 through 2002. 

Planned Improvements 

While the data used for this report represent the most accurate information available, three areas have been 

identified that could potentially be improved in the short term given resource availability: 

1) Reconcile Fuel Consumption Estimates used for Calculating N2O/CH4 and CO2 – Estimates of transportation fuel 

consumption by fuel type from EIA are used as the basis for estimating CO2 emissions from the transportation 

sector. These estimates are then apportioned to mode and vehicle category based on “bottom up” estimates of fuel 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 92 

consumption from sources such as FHWA’s Highway Statistics (FHWA 1996 through 2004) and DOE’s 

Transportation Energy Data Book (DOE 1993 through 2004). These sources are also used to develop N2O and CH4

estimates. The EIA fuel consumption estimates, however, differ from the estimates derived using “bottom up” 

sources. For certain vehicle categories this leads to CO2 emissions trends that conflict with those of the “bottom up” 

sources. Potential improvements include reconciling fuel consumption estimates from EIA and other data sources, 

and revising the current process of allocating CO2 emissions to reflect trends from the more detailed vehicle 

category estimates of fuel consumption. 

2) Improve consideration of emissions from trucks used off-road – Some light- and heavy-duty trucks travel for a 

portion of their mileage off-road. N2O and CH4 estimates for highway vehicles are developed based on vehicle 

mileage data from FHWA’s Highway Statistics, which in turn, are drawn from the Highway Performance 

Monitoring System (HPMS). These emissions estimates do not address travel by trucks off-road. Gasoline fuel 

consumed by trucks used off-road for construction, agriculture, and other industrial/commercial uses is reported in 

Highway Statistics, and is included as part of the non-road agriculture and construction categories. However, diesel 

fuel consumed by trucks used off-road is not addressed in the current Inventory, and further work should be 

conducted to develop estimates of off-road truck use of diesel fuel. In addition, default emission factors from IPCC 

are applied to the off-highway modes. As a result, the emissions factors for agricultural equipment are applied both 

to equipment and trucks used in agriculture, and emissions factors for construction equipment are applied both to 

equipment and trucks used in construction. Emission estimates would be improved through an investigation of 

more appropriate emission factors for off-road trucks. 

3) Improve estimation of VMT by vehicle/fuel type category – The current Inventory process for estimating VMT by 

vehicle/fuel type category involves apportioning VMT by vehicle type to each fuel type on the basis of fuel 

consumption. While this is a reasonable simplification, this approach implicitly assumes the same average fuel 

economy for gasoline and diesel vehicles. A more accurate apportionment for VMT by fuel type for light-duty 

trucks and medium/heavy-duty trucks could potentially be developed using data on vehicle travel from the Vehicle 

Inventory and Use Survey and other publications, or using VMT breakdowns by vehicle/fuel type combinations 

from the MOBILE6 or MOVES models. These sources should be investigated in order to develop a more robust 

apportionment method. 

3.5. 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. All 101 gassy underground coal mines 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, twenty-one 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 2003, ten coal mines collected CH4 from degasification 

systems and sold this gas to a pipeline, thus reducing emissions to the atmosphere. In addition, one coal mine used 

CH4 from its degasification system to heat mine ventilation air on site. 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 2003 were estimated to be 53.8 Tg CO2 Eq. (2,561 Gg), a decline of 34 percent since 1990 

(see Table 3-30 and Table 3-31). Of this amount, underground mines accounted for 70 percent, surface mines 

accounted for 16 percent, and post-mining emissions accounted for 14 percent. In 1993, CH4 generated from 

underground mining dropped, primarily due to labor strikes at many large underground mines. In 1994 and 1995, 

CH4 emissions increased due to resumed production at high emitting mines after the labor strike. 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. CH4 emissions increased 

slightly in 2003 due to additional gas drainage being vented to the atmosphere and a reduction in CH4 recovery. 

Surface mine emissions and post-mining emissions remained relatively constant from 1990 to 2003. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 93 

Table 3-30: CH4 Emissions from Coal Mining (Tg CO2 Eq.) 

Activity 1990 1996 1997 1998 1999 2000 2001 2002 2003

Underground Mining 62.1 45.3 44.3 44.4 41.6 39.4 38.1 35.8 37.6

Liberated 67.6 59.8 55.7 58.6 54.4 54.0 54.2 53.3 53.6

Recovered & Used (5.6) (14.5) (11.4) (14.2) (12.7) (14.6) (16.1) (17.5) (16.1)

Surface Mining 10.4 9.2 9.3 9.4 9.0 8.8 9.2 8.8 8.4

Post-Mining (Underground) 7.7 7.2 7.4 7.4 6.8 6.7 6.8 6.4 6.4

Post-Mining (Surface) 1.7 1.5 1.5 1.5 1.5 1.4 1.5 1.4 1.4

Total 81.9 63.2 62.6 62.8 58.9 56.2 55.6 52.4 53.8

Note: Totals may not sum due to independent rounding. 

Table 3-31: CH4 Emissions from Coal Mining (Gg) 

Activity 1990 1996 1997 1998 1999 2000 2001 2002 2003

Underground Mining 2,956 2,158 2,111 2,117 1,982 1,876 1,816 1,705 1,788

Liberated 3,220 2,850 2,654 2,791 2,589 2,573 2,580 2,538 2,554

Recovered & Used (265) (692) (543) (674) (607) (697) (765) (833) (766)

Surface Mining 497 438 445 448 428 417 438 420 402

Post-Mining (Underground) 367 341 354 352 325 317 323 304 305

Post-Mining (Surface) 81 71 72 73 69 68 71 68 65

Total 3,900 3,008 2,983 2,989 2,805 2,677 2,647 2,497 2,561

Note: Totals may not sum due to independent rounding. 

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 

detectable50 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-one 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. This 

calculation was complicated by the fact that most CH4 is not recovered and used during the same year in which the 

particular coal seam is mined. In 2003, ten active coal mines sold recovered CH4 into the local gas pipeline 

 

50 MSHA records coal mine methane readings with concentrations of greater than 50 ppm (parts per million) methane. Readings 

below this threshold are considered non-detectable. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 94 

networks, while one coal mine used recovered CH4 on site. Emissions avoided for these projects were estimated 

using gas sales data reported by various state agencies. For most mines with recovery systems, companies and state 

agencies provided individual well production information, which was used to assign gas sales to a particular year. 

For the few remaining mines, coal mine operators supplied information regarding the number of years in advance of 

mining that gas recovery occurs. 

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 Coal Industry 

Annual (see Table 3-32) (EIA 2003), 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 (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 3-32: Coal Production (Thousand Metric Tons) 

Year Underground Surface Total 

1990 384,250 546,818 931,068 

1991 368,635 532,656 901,291 

1992 368,627 534,290 902,917 

1993 318,478 539,214 857,692 

1994 362,065 575,529 937,594 

1995 359,477 577,638 937,115 

1996 371,816 593,315 965,131 

1997 381,620 607,163 988,783 

1998 378,964 634,864 1,013,828 

1999 355,433 642,877 998,310 

2000 338,173 635,592 973,765 

2001 345,305 676,142 1,021,446 

2002 324,219 667,619 991,838 

2003 320,047 651,251 971,297 

Uncertainty 

The emission estimates from underground ventilation systems were based on actual measurement data, which are 

believed to have relatively low uncertainty. A degree of imprecision was introduced because the measurements 

were not continuous but rather an average of quarterly instantaneous readings. Additionally, the measurement 

equipment used possibly resulted in an average of 10 percent overestimation of annual CH4 emissions (Mutmansky 

and Wang 2000). Estimates of CH4 liberated and recovered by degasification systems are also 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. A level of uncertainty currently 

exists concerning the radius of influence of each well. The number of wells counted, and thus the avoided 

emissions, may increase if the drainage area is found to be larger than currently estimated. 

Compared to underground mines, there is considerably more uncertainty associated with surface mining and postmining 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 3-33. Coal mining CH4 emissions in 2003 were 

estimated to be between 51.9 and 55.7 Tg CO2 Eq. at a 95 percent confidence level (or in 19 out of 20 Monte Carlo 

Simulations). This indicates a range of 4 percent below to 4 percent above the 2003 emission estimate of 53.8 Tg 

CO2 Eq. 

Table 3-33: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Coal Mining (Tg CO2 Eq. and 

Percent)
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 95 

Source Gas 

2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Coal Mining CH4 53.8 51.9 55.7 -4% +4% a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

In-situ gas content is the principal variable used to determine post-mining methane emissions of mined coal. 

Previously, in-situ values used were based on average CH4 content values summarized in Exhibit 3-4 of the U.S. 

EPA publication, EPA/400/9-90/008; Methane Emissions From Coal Mining, Issues and Opportunities, September 

1990. The original source of information is derived from three primary sources: 1986 USBM Circular 9067, Results 

of the Direct Method Determination of the Gas Contents of U.S. Coal Basins, 1983 U.S. DOE Report 

(DOE/METC/83-76), Methane Recovery from Coalbeds: A Potential Energy Source, and a series of 1986-88 Gas 

Research Institute Topical Reports called A Geologic Assessment of Natural Gas from Coal Seams. No data was 

available for eight of the coal mining states and therefore default values from other coal basins were assigned to 

those states. 

Since Circular 9067 contained only a portion of the gas content data compiled by USBM, the complete dataset, 

published in 1996 Evaluation and Analysis of Gas Content and Coal Properties of Major Coal Bearing Regions of 

the United States, EPA/600/R-96-065, is now the basis of new in-situ gas content value. In addition, gas content 

data from the U.S. DOE Methane Recovery from Coalbed Projects (MRCP), which was the original source of data 

for the GRI Topical Reports noted above, was utilized. (Condensed versions of the original MRCP reports for 13 

U.S. coal basins are compiled in Coalbed Methane Resources of the United States, AAPG Studies in Geology Series 

#17, published in 1984). 

The compiled gas content data for each of the coal basins was sorted by depth to determine in-situ values for surface 

and underground mines, separately. Overburden depths of surface mines were analyzed using Keystone Coal 

Industry Manuals from 1991 through 2003 and found that the maximum depth was 250 feet. Therefore, gas content 

data from samples taken less than 250 feet deep were assigned to surface mines and the samples collected from 

deeper depths to underground mines. The combination of these changes resulted in an average annual decrease of 

0.2 Tg CO2 Eq. (0.3 percent) in CH4 emissions from coal mining for the period 1990 through 2002. 

Planned Improvements 

To reduce the uncertainty associated with the radius of influence of each well, the appropriate drainage radius will 

be investigated for future inventories. Since the number of wells counted may increase if the drainage area is found 

to be larger than currently estimated, additional mines may be included in future estimates of recovery. 

3.6. Abandoned Underground Coal Mines (IPCC Source Category 1B1a) 

All underground and surface coal mining liberates CH4 as part of the normal mining operations. The amount of 

CH4 liberated depends on the amount that resides in the coal (“in situ”) and surrounding strata when mining occurs. 

The in-situ CH4 content depends upon the amount of CH4 created during the coal formation (i.e., coalification) 

process, and the geologic characteristics of the coal seams. During coalification, more deeply buried deposits tend 

to generate more CH4 and retain more of the gas after uplift to minable depths. Deep underground coal seams 

generally have higher CH4 contents than shallow coal seams or surface deposits. 

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 close and are 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 96 

concrete seal, while vent pipes and boreholes are plugged in a manner similar to oil and gas wells. Some abandoned 

mines are vented to the atmosphere to prevent the buildup of CH4 that may find its way to surface structures through 

overburden fractures. As work stops within the mines, the CH4 liberation decreases but it does not stop completely. 

Following an initial decline, abandoned mines can liberate CH4 at a near-steady rate over an extended period of 

time, or, if flooded, produce gas for only a few years. The gas can migrate to the surface through the conduits 

described above, particularly if they have not been sealed adequately. In addition, diffuse emissions can occur 

when CH4 migrates to the surface through cracks and fissures in the strata overlying the coal mine. The following 

factors influence abandoned mine emissions: 

• Time since abandonment; 

• Gas content and adsorption characteristics of coal; 

• Methane flow capacity of the mine; 

• Mine flooding; 

• Presence of vent holes; and 

• Mine seals. 

Gross abandoned mine methane emissions ranged from 6.1 to 9.6 Tg CO2 Eq. from 1990 through 2003, varying as 

much as 1.3 Tg CO2 Eq. 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. Abandoned mine emissions 

peaked in 1996 (8.6 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 (with slight increases in 1999 and 2000). There were fewer than thirteen gassy mine closures 

during each of the years from 1998 through 2003. By 2003, abandoned mine emissions were reduced to 6.4 Tg CO2 

Eq. (see Table 3-34 and Table 3-35). 

Table 3-34: CH4 Emissions from Abandoned Coal Mines (Tg CO2 Eq.) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003

Abandoned Underground Mines 6.1 9.6 8.8 9.0 9.3 8.5 8.0 7.9

Recovered & Used 0 1.5 1.7 1.6 1.5 1.5 1.6 1.5

Total 6.1 8.1 7.1 7.3 7.7 6.9 6.4 6.4

Note: Totals may not sum due to independent rounding. 

Table 3-35: CH4 Emissions from Abandoned Coal Mines (Gg) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003

Abandoned Underground Mines 288 458 421 426 441 403 380 377

Recovered & Used - 74 80 78 73 73 77 72

Total 288 385 341 349 369 331 303 306

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 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. Existing data on abandoned mine emissions through time, 

although sparse, appear to fit the hyperbolic type of decline curve used in forecasting production from natural gas 

wells. 

In order to estimate CH4 emissions over time for a given mine, it is necessary to apply a decline function, initiated 

upon abandonment, to that mine. In the analysis, mines were grouped by coal basin with the assumption that they 

will generally have the same initial pressures, permeability and isotherm. As CH4 leaves the system, the reservoir 

pressure, Pr, declines as described by the isotherm. The emission rate declines because the mine pressure (Pw) is 

essentially constant at atmospheric pressure, for a vented mine, and the PI term is essentially constant at the 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 97 

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 = qi(1+bDit)(-1/b) 

Where: 

• q is the gas rate at time t in mcf/d 

• qi is the initial gas rate at time zero (to) in million cubic feet per day (mcfd) 

• b is the hyperbolic exponent, dimensionless 

• Di is the initial decline rate, 1/yr 

• t is elapsed time from to in 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 are also affected by both sealing and flooding. Based on field measurement data, it was assumed 

that most U.S. mines prone to flooding will become completely flooded within 8 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 is the gas flow rate at time t in mcf/d 

• qi is the initial gas flow rate at time zero (to) in mcfd 

• D is the decline rate, 1/yr 

• t is elapsed time from to in 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 × (1 - initial emissions from sealed mine / emission rate at abandonment prior 

to sealing). Significant differences are seen between 50 percent, 80 percent and 95 percent closure. These decline 

curves were therefore used as the high, middle, and low values for emissions from sealed mines (EPA 2003). 

For active coal mines, those mines producing over 100 mcfd account for 98 percent of all CH4 emissions. This 

same relationship is assumed for abandoned mines. It was determined that 434 abandoned mines closing after 1972 

produced emissions greater than 100 mcfd when active. Further, the status of 256 of the 434 mines (or 59 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 methane flow to the atmosphere). The remaining 41 percent of the mines were placed in 

one of the three categories by applying a probability distribution analysis based on the known status of other mines 

located in the same coal basin (EPA 2003). 

Inputs to the decline equation require the average emission rate and the date of abandonment. Generally this data is 

available for mines abandoned after 1972; however, such data are largely unknown for mines closed before 1972. 

Information that is readily available such as coal production by state and county are helpful, but do not provide 

enough data to directly employ the methodology used to calculate emissions from mines abandoned after 1971. It is 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 98 

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, throughout the 20th century. The 

data was 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 methane 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 259 mines closed between 1972 and 1990. Initial emission rates based on MSHA 

reports, time of abandonment, and basin-specific decline curves influenced by a number of factors were used to 

calculate annual emissions for each mine in the database. Coal mine degasification data are not available for years 

prior to 1990, thus the initial emission rates used reflect ventilation emissions only for pre-1990 closures. Methane 

degasification amounts were added to ventilation data for the total CH4 liberation rate for fourteen mines that closed 

between 1992 and 2003. 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. Once the 1991 

through 2003 totals were calculated, they 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 methane recovery projects during that time. 

Uncertainty 

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, an approach 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 the parameter. Once the low, mid, and high values 

are selected, they are applied to a probability density function. 

The emission estimates from underground ventilation systems were based on actual measurement data, which are 

believed to have relatively low uncertainty. A degree of imprecision was introduced because the measurements 

were not continuous, but rather an average of quarterly instantaneous readings. Additionally, the measurement 

equipment used possibly resulted in an average of 10 percent overestimation of annual CH4 emissions (Mutmansky 

and Wang 2000). Estimates of CH4 liberated and recovered by degasification systems are also relatively certain 

because many coal mine operators provided information on individual well gas sales and mined through dates. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 3-36. Abandoned coal mines 

CH4 emissions in 2003 were estimated to be between 5.4 and 7.8 Tg CO2 Eq. at a 95 percent confidence level (or in 

19 out of 20 Monte Carlo Simulations). This indicates a range of 16 percent below to 22 percent above the 2003 

emission estimate of 6.4 Tg CO2 Eq. One of the reasons for the relatively narrow range is that mine-specific data is 

used in the methodology. The largest degree of uncertainty is associated with the unknown status mines (which 

account for 41 percent of the mines), with a ±50 percent uncertainty. 

Table 3-36: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Abandoned Underground Coal 

Mines (Tg CO2 Eq. and Percent)

Source Gas 

2003 

Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 99 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Abandoned Coal Mines CH4 6.4 5.4 7.8 -16% +22% a

Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval. 

QA/QC and Verification 

As part of a Tier 2 analysis, the United States undertook an effort to verify the model results used in the U.S. 

Inventory with field measurements. Field measurements were used to test the accuracy of the mathematical decline 

curves to be used for basin-specific emissions estimates. A series of field measurements were conducted at 

abandoned mine vent locations across the United States. Between November 1998 and February 2000, EPA 

recorded measurements at five mines that were not flooded. Measurements were recorded at two abandoned mines 

located in Ohio and Virginia continuously for 6 to 12 hours. As the methodology was finalized, EPA measured 

emissions from three additional mines located in Illinois and Colorado. These measurements were recorded hourly 

for 3 to 4 days and were normalized to average barometric pressures. Prior to these measurements, EPA’s Office of 

Research and Development initiated a field research program in the early 1990s. Data for 21 abandoned mines 

located throughout the Northern and Central Appalachian, Black Warrior, and Illinois Basins were collected using 

similar techniques. 

Measurements for all field data recorded were plotted against predicted emissions as part of the two studies from 

1991 through 2000. Emission rates from nine of the ten mines that were measured fall very close to the predicted 

mid-case decline rate for their respective basins. For the exponential decline curve fit to the flooding mines, six of 

nine measurements fall within a 95 percent predictive confidence interval of the mean. 

Of the abandoned mines in the database, only about 13 percent of the mines maintain vents to the atmosphere. 

Therefore, it is difficult to obtain field data. Additional field measurements, however, would be beneficial to further 

calibrate the equations defined above. Furthermore, it would be useful to extend measurements of diffuse emissions 

from sealed mines, since they comprise 43 percent of total mines. 

Recalculations Discussion 

In 2003, all methane emissions from abandoned coal mines were recalculated using: 1) an updated mine list based 

on 1985 and 1988 USBM records (adding 41 mines to the inventory), 2) updated coal seam permeabilities based on 

a 2004 EPA study, and 3) revised closure dates for 43 of the mines based on MSHA’s data retrieval system. The 

combination of these changes resulted in an average annual increase of 2.8 Tg CO2 Eq. (64.5 percent) in CH4

emissions from abandoned coal mines for the period 1990 through 2002. 

3.7. Petroleum Systems (IPCC Source Category 1B2a) 

Methane emissions from petroleum systems are primarily associated with crude oil production, transportation, and 

refining operations. During each of these activities, CH4 is released to the atmosphere as fugitive emissions, vented 

emissions, emissions from operational upsets, and emissions from fuel combustion. Total CH4 emissions from 

petroleum systems in 2003 were 17.1 Tg CO2 Eq. (815 Gg). Since 1990, emissions declined due to a decline in 

domestic oil production and industry efforts to make emissions reductions (see Table 3-37 and Table 3-38). The 

various sources of emissions are detailed below. 

Production Field Operations. Production field operations account for over 95 percent of total CH4 emissions from 

petroleum systems. Vented CH4 from field operations account for approximately 83 percent of the emissions from 

the production sector, fugitive emissions account for six percent, combustion emissions ten percent, and process 

upset emissions barely one percent. The most dominant sources of vented emissions are field storage tanks, naturalgas-powered pneumatic devices (low bleed, high bleed, and chemical injection pumps). These four sources alone 

emit 79 percent of the production field operations emissions. 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 high and low-bleed pneumatics occur when pressurized gas that is used for control devices is bled 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 100 

to the atmosphere as they cycle up and down to modulate the system. Emissions from chemical injection pumps 

occur as high-pressure gas that is used to drive the pumps is vented to the atmosphere. Two additional large 

sources, oil well heads and gas engines, together account for 12 percent of emissions from the production sector. 

The remaining nine percent of the emissions are distributed among 33 additional activities within these four 

categories. 

Crude Oil Transportation. Crude oil transportation activities account for less than one percent of total CH4

emissions from the oil industry. Venting from tanks and marine vessel loading operations accounts for 65 percent 

of CH4 emissions from crude oil transportation. Fugitive emissions, almost entirely from floating roof tanks, 

account for 18 percent. The remaining 17 percent is distributed among 4 additional sources within these two 

categories. 

Crude Oil Refining. Crude oil refining processes and systems account for only three 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 methane 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 

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 from unburned CH4 in engine exhausts and flares. 

Table 3-37: CH4 Emissions from Petroleum Systems (Tg CO2 Eq.) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003

Production Field Operations 19.3 18.1 17.8 17.1 16.9 16.8 16.4 16.4 

 Pneumatic device venting 11.5 10.8 10.6 10.3 10.0 10.0 9.8 9.8 

 Tank Venting 3.8 3.4 3.4 3.2 3.2 3.2 3.2 3.2 

 Combustion & process upsets 2.2 2.0 2.0 1.9 1.9 1.9 1.8 1.8 

 Misc. venting & fugitives 1.4 1.3 1.3 1.3 1.2 1.2 1.2 1.2 

 Wellhead fugitives 0.5 0.5 0.5 0.5 0.5 0.5 0.4 0.4 

Crude Oil Transportation 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Refining 0.5 0.6 0.6 0.6 0.6 0.6 0.6 0.6 

Total estimated emissions 20.0 18.8 18.5 17.8 17.6 17.4 17.1 17.1 

Table 3-38: CH4 Emissions from Petroleum Systems (Gg) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003

Production Field Operations 920 862 846 815 803 799 783 782 

 Pneumatic device venting 545 515 504 488 478 475 465 465 

 Tank Venting 179 164 162 153 154 154 151 151 

 Combustion & process upsets 103 96 94 90 89 89 87 86 

 Misc. venting & fugitives 66 62 61 60 59 59 58 57 

 Wellhead fugitives 26 25 25 24 22 22 21 21 

Crude Oil Transportation 7 6 6 6 5 5 5 5 

Refining 25 27 27 27 28 27 27 27 

Total estimated emissions 951 895 879 848 836 831 815 815 

Methodology 

The methodology for estimating CH4 emissions from petroleum systems is a bottom-up approach, based on a 

comprehensive study of CH4 emissions from U.S. petroleum systems (EPA 1999, Radian 1996e). These studies 

combined emission estimates from 70 activities occurring in petroleum systems from the oil wellhead through crude 

oil refining, including 39 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 70 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 101 

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 70 oil industry activities employs emission factors initially 

developed by EPA (1999) and activity factors that are based on EPA (1999) and Radian (1996e) studies. 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. For offshore 

oil production, an emission factor was calculated by dividing an emission estimate from the Minerals Management 

Service (MMS) by the number of platforms (MMS 2004b). Emission factors were held constant for the period 1990 

through 2003. 

Activity factors for years 1990 through 2003 were collected from a wide variety of statistical resources. For some 

years, complete activity factor data were not available. In such cases, one of three approaches was employed. 

Where appropriate, the activity factor was calculated from related statistics using ratios developed for Radian 

(1996e). For example, Radian (1996e) 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 Radian (1996e). In other 

cases, the activity factor was held constant from 1990 through 2003 based on EPA (1999). Lastly, the previous 

year’s data were used when data for the current year were unavailable. See Annex 3.5 for additional detail. 

Nearly all emission factors were taken from Radian (1996e) and EPA (1999). The remaining emission factors were 

taken from the following sources: EPA default values, MMS reports (MMS 1995), the Exploration and Production 

(E&P) Tank model (DB Robinson Research Ltd. 1997), 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 1990-2003, 1995-2003a-b), the API Basic Petroleum Data Book (API 2003), 

Methane Emissions from the Natural Gas Industry by the Gas Research Institute and EPA (Radian 1996a-d),

consensus of industry peer review panels, MMS reports (MMS 1995, 2000, 2004a-b), and the Oil & Gas Journal 

(OGJ 2003a-b). Forecasts of activity factors for petroleum systems were developed using production and refining 

capacity data from the EIA Annual Energy Outlook (EIA 2004) as well as offshore activity projections from MMS 

(2001 and 2004a). Annex 3.5 provides a complete list of references.

Uncertainty 

The detailed, bottom-up inventory analysis used to evaluate U.S. petroleum systems reduces the uncertainty related 

to the CH4 emission estimates in comparison with a top-down approach. However, some uncertainty still remains. 

Emission factors and activity factors are based on a combination of measurements, equipment design data, 

engineering calculations and studies, surveys of selected facilities and statistical reporting. Statistical uncertainties 

arise from natural variation in measurements, equipment types, operational variability and survey and statistical 

methodologies. Published activity factors are not available every year for all 70 activities analyzed for petroleum 

systems; therefore, some are estimated. Because of the dominance of six major sources, which account for 90 

percent of the total emissions, the uncertainty surrounding these six 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 3-39. Petroleum systems CH4

emissions in 2003 were estimated to be between 11.9 and 51.4 Tg CO2 Eq. at a 95 percent confidence level (or in 19 

out of 20 Monte Carlo Simulations). This indicates a range of 30 percent below to 200 percent above the 2003 

emission estimate of 17.1 Tg CO2 Eq. 

Table 3-39: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petroleum Systems (Tg CO2 Eq. and 

Percent)

Source Gas 

2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 102 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Petroleum Systems CH4 17.1 11.9 51.4 -30% +200% a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

Estimates of CH4 from petroleum systems contain two changes with respect to previous inventories. First, the 

emission factor for CH4 emissions from oil tanks in the production sector was modified to remove venting from 

condensate tanks and only account for venting from crude oil tanks. The previous methodology included an 

emissions factor that was averaged from the API E&P Tank Calc runs on both oil and condensate tanks. The new 

calculation is averaged from API E&P Tank Calc runs (DB Robinson Research Ltd. 1997) for API gravity of 44 

degrees and below. The adjustment has been made so that vented emissions from condensate tanks can be moved 

into the emissions from natural gas systems, where they are more relevant. The second change was the 

development of a new activity factor for offshore Gulf of Mexico platform venting in the production sector. 

Previously, the activity factor was obtained through MMS, the total number of Gulf of Mexico platforms, and an 

assumption of what percentage were oil producing platforms. The number of Gulf of Mexico platforms, platform 

data, and field data is now available on an annual basis from MMS (2000, 2004b, 2004c). From the field and 

platform data, a new estimate was developed for the percentage of oil producing platforms. This change results in 

the model reflecting the trend towards more natural gas production than oil production in the Gulf of Mexico. A 

change in the Gulf of Mexico platform activity resulted in an indirect change in the emissions factor that is 

calculated from the known base-year emissions value. 

The combination of these changes resulted in an average annual decrease of 7.1 Tg CO2 Eq. (27.4 percent) in CH4

emissions from petroleum systems for the period 1990 through 2002. Oil tank venting accounted for virtually all of 

the decrease while Gulf of Mexico platform venting and fugitives remained relatively unchanged. 

Planned Improvements 

Several improvements to the emission estimates are being evaluated that fine-tune and better track changes in 

emissions. These include, but are not limited to, some activity factors that are also accounted for in the Natural Gas 

STAR Program emission reductions, some emission factors for consistency between emission estimates from 

Petroleum Systems and Natural Gas Systems, and new data from recent studies that bear on both emission factors 

and activity factors. The growing body of data in the Natural Gas STAR Program, coupled with an increasing 

number of oil and gas companies doing internal greenhouse gas emissions inventories, provides an opportunity to 

reevaluate emission and activity factors, as well as the methodology currently used to project emissions from the 

base year. 

3.8. 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 125.9 Tg CO2

Eq. (5,998 Gg) of CH4 in 2003, a slight decrease over 1990 emissions (see Table 3-40 and Table 3-41). 

Improvements in management practices and technology, along with the replacement of older equipment, have 

helped to stabilize emissions (EPA 2002). 

Methane 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 combusting engines and turbine 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 emissions are discussed. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 103 

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 emissions. 

Emissions from field production accounted for approximately 34 percent of CH4 emissions from natural gas systems 

in 2003. 

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 emissions from compressors, 

including compressor seals, are the primary emission source from this stage. Processing plants account for about 12 

percent of CH4 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 

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 exhaust are also sources of emissions from transmission 

facilities. 

Natural gas is also injected and stored in underground formations, or liquefied and stored in above ground tanks, 

during periods of low demand (e.g., summer), and withdrawn, processed, and distributed during periods of high 

demand (e.g., winter). Compressors and dehydrators are the primary contributors to emissions from these storage 

facilities. Methane emissions from transmission and storage sector account for approximately 32 percent of 

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 978,000 miles of distribution mains in 2003, an increase from just over 789,500 miles in 

1990 (OPS 2002a). Distribution system emissions, which account for approximately 22 percent of emissions from 

natural gas systems, result mainly from fugitive emissions from gate stations and non-plastic piping (cast iron, 

steel).51 An increased use of plastic piping, which has lower emissions than other pipe materials, has reduced the 

growth in emissions from this stage. Distribution system emissions in 2003 were 10 percent lower than 1990 levels. 

Table 3-40: CH4 Emissions from Natural Gas Systems (Tg CO2 Eq.)* 

Stage 1990 1997 1998 1999 2000 2001 2002 2003 

Field Production 36.3 40.4 41.2 37.5 41.0 44.5 44.5 43.4

Processing 14.8 14.9 14.7 14.6 14.9 15.1 14.6 14.5

Transmission and Storage 46.8 46.1 44.7 43.4 43.4 40.2 42.0 40.6

Distribution 30.5 32.4 31.2 31.9 32.8 32.1 29.5 27.4

Total 128.3 133.6 131.8 127.4 132.1 131.8 130.6 125.9 

*Including CH4 emission reductions achieved by the Natural Gas STAR program. 

Note: Totals may not sum due to independent rounding. 

Table 3-41: CH4 Emissions from Natural Gas Systems (Gg)* 

Stage 1990 1997 1998 1999 2000 2001 2002 2003

Field Production 1,731 1,921 1,962 1,785 1,953 2,117 2,121 2,068

Processing 704 708 702 696 708 717 693 691 

Transmission and Storage 2,226 2,193 2,127 2,068 2,067 1,914 2,002 1,933

Distribution 1,450 1,541 1,484 1,517 1,560 1,530 1,405 1,305

 

51 The percentages of total emissions from each stage may not add to 100 because of independent rounding. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 104 

Total 6,112 6,363 6,276 6,066 6,289 6,277 6,221 5,998

*Including CH4 emission reductions achieved by the Natural Gas STAR program. 

Note: Totals may not sum due to independent rounding. 

Methodology 

The basis for estimates of CH4 emissions from the U.S. natural gas industry is a detailed study by the Gas Research 

Institute and EPA (EPA/GRI 1996). The EPA/GRI study developed over 100 emission and activity factors to 

characterize emissions from the various components within the operating stages of the U.S. natural gas system. The 

study was based on a combination of process engineering studies and measurements at representative gas facilities. 

From this analysis, a 1992 emission estimate was developed using the emission and activity factors. For other 

years, a set of industry activity factor drivers was developed that can be used to update activity factors. These 

drivers include statistics on gas production, number of wells, system throughput, miles of various kinds of pipe, and 

other statistics that characterize the changes in the U.S. natural gas system infrastructure and operations. 

See Annex 3.4 for more detailed information on the methodology and data used to calculate CH4 emissions from 

natural gas systems. 

Activity factor data were taken from the following sources: American Gas Association (AGA 1991-1998); 

American Petroleum Institute (API 2002, 2003); Annual Energy Review (EIA 2002f); Historical Natural Gas 

Annual (EIA 2003e); Minerals and Management Service (MMS 1998, 1999, 2000, 2001, 2002a-b, 2003, 2004a-d); 

Monthly Energy Review (EIA 2004d); Natural Gas Annual (EIA 1993, 1996a, 1997a, 1998a-b, 2001a, 2002a, 

2003a); Natural Gas Liquids Reserves Report (EIA 1996b, 1997b, 1998c, 1999, 2000, 2001c, 2002d); Natural Gas 

Monthly (EIA 2001b, 2002b-c, 2003b-d, 2004a-c,); the Natural Gas STAR Program annual emissions savings (EPA 

2004); Oil and Gas Journal (OGJ 1999 - 2004); Office of Pipeline Safety (OPS 2004a-b) other Energy Information 

Administration publications (EIA 2002e, 2004e-f). The Gas Systems Analysis model was used to aid in collecting 

data for non-associated and associated wells (GSAM 1997). Data from a program for estimating emissions from 

hydrocarbon production tanks is incorporated (DB Robinson Research Ltd. 1997). Coalbed Methane well activity 

factors were taken from the Wyoming Oil and Gas Conservation Commission (Wyoming 2004) and the Alabama 

State Oil and Gas Board (Alabama 2004). Other state well data was taken from : American Association of 

Petroleum Geologists (AAPG 2204); Brookhaven College (Brookhaven 2004); Kansas Geological Survey (Kansas 

2004); Rocky Mountain Production Report (Lippman (2003); Montana Board of Oil and Gas Conservation 

(Montana 2004); Oklahoma Geological Survey (Oklahoma 2004); Utah Division of Oil, Gas and Mining (Utah 

2004). Emissions factors were taken from EPA/GRI (1996). 

Uncertainty 

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 3-42. Natural gas systems CH4 emissions in 2003 were estimated to be between 87.1 and 

166.7 Tg CO2 Eq. at a 95 percent confidence level (or in 19 out of 20 Monte Carlo Simulations). This indicates a 

range of 31 percent below to 32 percent above the 2003 emission estimate of 125.9 Tg CO2 Eq. 

Table 3-42: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Natural Gas Systems (Tg CO2 Eq. 

and Percent)

Source Gas 

2003 

Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Natural Gas CH4 125.9 87.1 166.7 -31% +32% 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 105 

Systems 

a

 Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

Emissions with Natural Gas STAR Program reductions were updated using new Gas STAR emissions reduction 

data from the iSTAR database. Gas STAR reductions reported retroactively for the years 1990 to 1992 are assumed 

to be characterized in the GRI/EPA 1996 study for the base year 1992. Therefore, to avoid double counting, those 

emissions reductions are not counted in the inventory. Gas STAR reductions are also subject to sunsetting rules that 

prevent perpetual crediting of all reductions. The inventory assumes that once an emissions reducing technology is 

put in place, it will continue to provide emissions savings and thus the sunsetting rule is not be implemented in the 

inventory. 

Three new sources of emissions in the production sector of the natural gas systems were added this year: 

• Gas condensate stored in tanks vents methane and other hydrocarbons to the atmosphere. Emissions from 

condensate tanks can be divided into two categories: tanks with control devices such as vapor recovery 

units or flares, and tanks that have no control devices. Condensate tanks contributed an estimated 52.3 Gg 

of methane emissions that represents 0.87 percent of total methane emissions from natural gas systems in 

2003. In 1990, Condensate tanks were estimated to contribute 39.9 Gg of emissions or about 0.65 percent 

of total emissions from natural gas systems. 

• Another source added to the production sector for the 2003 reporting year was unconventional gas well 

fugitives. Wells in this source were previously treated as conventional wells in the rest of U.S. (western) 

geographic region. Unconventional well fugitives include fugitive emissions from coal bed CH4 and shale 

wells. From 1990 through 2003, unconventional well fugitives accounted for a small amount of emissions 

from natural gas systems. 

• The production sector also had flaring from offshore Gulf of Mexico operations added, using a 2 percent 

uncombusted hydrocarbon factor. This source contributed to less than 0.1 percent to the emissions from 

the entire time series. A second Gulf of Mexico source added to the production sector was Gulf of Mexico 

offshore well venting. This contributed about 2.4 percent of the 2003 emissions and about 2.6 percent in 

1990. 

The combination of these methodological and historical data changes resulted in an average annual increase of 6.7 

Tg CO2 Eq. (5.3 percent) in CH4 emissions from natural gas systems for the period 1990 through 2002. 

Planned Improvements 

Several improvements to the emission estimates are being evaluated that fine-tune and better track changes in 

emissions. These include, but are not limited to, some activity factors that are also accounted for in the Natural Gas 

STAR Program emission reductions, some emission factors for consistency between emission estimates from the 

Petroleum Systems and Natural Gas Systems source categories, and new data from recent studies that bear on both 

emission factors and activity factors. The growing body of data in the Natural Gas STAR Program, coupled with an 

increasing number of oil and gas companies doing internal greenhouse gas emissions inventories, provides an 

opportunity to reevaluate emission and activity factors, as well as the methodology currently used to project 

emissions from the base year. Two improvements that are of particular note are in the production and processing 

sector. Recent data has suggested that the emission factor for well clean-ups in the production sector is low, and 

additional data sources are being investigated to produce a more robust emission factor. In the processing sector, 

improvements are being considered across the entire sector based on studies completed and near completion at five 

or more processing plants. These studies suggest the need for changing several emission factors as well as adding 

some additional sources within the processing sector. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 106 

3.9. Municipal Solid Waste Combustion (IPCC Source Category 1A5) 

Combustion is used to manage about 7 to 17 percent of the municipal solid wastes 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 

2000c, Goldstein and Matdes 2001, Kaufman et al. 2004). Almost all combustion of municipal solid wastes in the 

United States occurs at waste-to-energy facilities where energy is recovered, and thus emissions from waste 

combustion are accounted for in the Energy chapter. Combustion of municipal solid wastes results in conversion of 

the organic inputs to CO2. According to the IPCC Guidelines, when the CO2 emitted is of fossil origin, it is counted 

as a net anthropogenic emission of CO2 to the atmosphere. Thus, the emissions from waste combustion are 

calculated by estimating the quantity of waste combusted and the fraction of the waste that is carbon 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 carbon flows accounted for under the Land-Use Change and Forestry chapter (see Box 3-3). 

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. Tires (which contain rubber and 

carbon black) are also considered a “non-hazardous” waste and are included in the municipal solid waste 

combustion estimate, though waste disposal practices for tires differ from the rest of municipal solid waste. 

[Begin Text Box] 

Box 3-3: Biogenic Emissions and Sinks of Carbon 

For many countries, CO2 emissions from the combustion or degradation of biogenic materials are important because 

of the significant amount of energy they derive from biomass (e.g., burning fuelwood). The fate of biogenic 

materials is also important when evaluating waste management emissions (e.g., the decomposition of paper). The 

carbon contained in paper was originally stored in trees during photosynthesis. Under natural conditions, this 

material would eventually degrade and cycle back to the atmosphere as CO2. The quantity of carbon that these 

degradation processes cycle through the Earth’s atmosphere, waters, soils, and biota is much greater than the 

quantity added by anthropogenic greenhouse gas sources. However, the focus of the UNFCCC is on emissions 

resulting from human activities and subject to human control, because it is these emissions that have the potential to 

alter the climate by disrupting the natural balances in carbon's biogeochemical cycle, and enhancing the 

atmosphere’s natural greenhouse effect. 

Carbon dioxide emissions from the combustion or decomposition of biogenic materials (e.g., paper, wood products, 

and yard trimmings) grown on a sustainable basis are considered to mimic the closed loop of the natural carbon 

cycle⎯that is, they return to the atmosphere CO2 that was originally removed by photosynthesis. However, 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. 

The removal of carbon from the natural cycling of carbon between the atmosphere and biogenic materials⎯which 

occurs when wastes of biogenic origin are deposited in landfills⎯sequesters carbon. When wastes of sustainable, 

biogenic origin are landfilled, and do not completely decompose, the carbon that remains is effectively removed 

from the global carbon cycle. Landfilling of forest products, yard trimmings, and food scraps resulted in net longterm storage of 10.1 Tg CO2 Eq. in 2003, as described in the Land-Use Change and Forestry chapter. 

[End Box] 

Approximately 24 million metric tons of municipal solid wastes were combusted in the United States in 2003. 

Carbon dioxide emissions from combustion of municipal solid wastes rose 72 percent since 1990, to an estimated 

18.8 Tg CO2 Eq. (18,781 Gg) in 2003, as the volume of plastics and other fossil carbon-containing materials in 

MSW increased (see Table 3-43 and Table 3-44). Waste combustion is also a source of N2O emissions (De Soete 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 107 

1993). Nitrous oxide emissions from municipal solid waste combustion were estimated to be 0.5 Tg CO2 Eq. (1 Gg) 

in 2003, and have not changed significantly since 1990. 

Table 3-43: CO2 and N2O Emissions from Municipal Solid Waste Combustion (Tg CO2 Eq.) 

Gas/Waste Product 1990 1997 1998 1999 2000 2001 2002 2003 

CO2 10.9 17.8 17.1 17.6 18.0 18.8 18.8 18.8 

Plastics 8.0 11.9 11.4 12.0 12.1 12.7 12.7 12.7 

Synthetic Rubber in Tires 0.2 0.9 0.9 0.9 0.9 0.9 0.9 0.9 

Carbon Black in Tires 0.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 

Synthetic Rubber in MSW 1.3 1.7 1.6 1.6 1.7 1.8 1.8 1.8 

Synthetic Fibers 1.2 2.1 2.0 2.0 2.1 2.2 2.2 2.2 

N2O 0.4 0.4 0.3 0.3 0.4 0.5 0.5 0.5 

Total 11.3 18.1 17.4 18.0 18.3 19.2 19.2 19.2 

Table 3-44: CO2 and N2O Emissions from Municipal Solid Waste Combustion (Gg) 

Gas/Waste Product 1990 1997 1998 1999 2000 2001 2002 2003 

CO2 10,919 17,761 17,094 17,632 17,979 18,781 18,781 18,781 

Plastics 7,953 11,914 11,427 11,950 12,145 12,718 12,718 12,718 

Synthetic Rubber in Tires 191 891 887 890 893 895 895 895 

Carbon Black in Tires 249 1,165 1,160 1,164 1,167 1,170 1,170 1,170 

Synthetic Rubber in MSW 1,330 1,725 1,627 1,612 1,689 1,810 1,810 1,810 

Synthetic Fibers 1,196 2,065 1,992 2,016 2,086 2,187 2,187 2,187 

N2O 1 1 1 1 1 1 1 1 

Ambient air pollutants are also emitted during waste incineration and open burning, as shown in Table 3-45. These 

emissions are a relatively small portion of the overall ambient air pollutant emissions, comprising less than 5 percent 

for each gas over the entire time series. 

Table 3-45: NOx, CO, and NMVOC Emissions from Municipal Solid Waste Combustion (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003 

NOx 82 140 145 143 114 114 134 121 

Waste Incineration 44 48 49 48 38 38 45 41 

Open Burning 38 92 96 95 76 76 89 80 

CO 978 2,668 2,826 2,725 1,670 1,672 1,672 1,674 

Waste Incineration 337 68 69 66 40 41 41 41 

Open Burning 641 2,600 2,757 2,659 1,630 1,631 1,631 1,633 

NMVOCs 222 313 326 302 257 258 281 263 

Waste Incineration 44 23 23 19 15 16 18 16 

Open Burning 178 290 303 284 242 242 264 246 

Note: Totals may not sum due to independent rounding. 

Methodology 

Emissions of CO2 from MSW combustion include CO2 generated by the combustion of plastics, synthetic fibers, 

and synthetic rubber, as well as the combustion of synthetic rubber and carbon black in tires. These emissions were 

calculated by multiplying the amount of each material combusted by the carbon content of the material and the 

fraction oxidized (98 percent). Plastics combusted in municipal solid wastes were categorized into seven plastic 

resin types, each material having a discrete carbon content. Similarly, synthetic rubber is categorized into three 

product types, and synthetic fibers were categorized into four product types, each having a discrete carbon content. 

Scrap tires contain several types of synthetic rubber, as well as carbon black. Each type of synthetic rubber has a 

discrete carbon content, and carbon black is 100 percent carbon. Emissions of CO2 were calculated based on the 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 108 

number of scrap tires used for fuel and the synthetic rubber and carbon black content of the tires. More detail on the 

methodology for calculating emissions from each of these waste combustion sources is provided in Annex 3.6. 

For each of the methods used to calculate CO2 emissions from municipal solid waste combustion, data on the 

quantity of product combusted and the carbon content of the product are needed. For plastics, synthetic rubber, and 

synthetic fibers, the amount of material in municipal solid wastes and its portion combusted were taken from the 

Characterization of Municipal Solid Waste in the United States (EPA 2000c, 2002a, 2003). For synthetic rubber 

and carbon black in scrap tires, this information was provided by the U.S. Scrap Tire Markets 2001 (RMA 2002) 

and Scrap Tires, Facts and Figures (STMC 2000, 2001, 2002, 2003). Data were not available for 2002 or 2003, so 

the values for these years were assumed to equal the value for 2001. 

Average carbon 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. For synthetic rubber in scrap tires information about scrap tire composition was taken from the Scrap 

Tire Management Council’s internet site (STMC 2003). 

The assumption that 98 percent of organic carbon is oxidized (which applies to all municipal solid waste 

combustion categories for CO2 emissions) was reported in the EPA’s life cycle analysis of greenhouse gas 

emissions and sinks from management of solid waste (EPA 2002b). 

Combustion of municipal solid waste also results in emissions of N2O. These emissions were calculated as a 

function of the total estimated mass of municipal solid waste combusted and an emission factor. The N2O emission 

estimates are based on different data sources. As noted above, N2O emissions are a function of total waste 

combusted in each year; for 1990 through 2002, these data were derived from the information published in BioCycle

(Kaufman et al 2004). As for the activity data for CO2 emissions, data on total waste combusted was not available 

for 2003, so the value for this year was assumed to equal the most recent value available (2002). Table 3-46 

provides data on municipal solid waste generation and percentage combustion for the total waste stream. The 

emission factor of N2O emissions per quantity of municipal solid waste combusted is an average of values from 

IPCC’s Good Practice Guidance (2000). 

Table 3-46: Municipal Solid Waste Generation (Metric Tons) and Percent Combusted 

Year Waste Generation Combusted (%)

1990 266,365,714 11.5 

1991 254,628,360 10.0 

1992 264,668,342 11.0 

1993 278,388,835 10.0 

1994 292,915,829 10.0 

1995 296,390,405 10.0 

1996 297,071,712 10.0 

1997 308,870,755 9.0 

1998 339,865,243 7.5 

1999 347,089,277 7.0 

2000 371,071,109 7.0 

2001 404,002,786a

 7.4a

2002 436,934,464 7.7 

2003 436,934,464b

 7.7b

a

 Interpolated between 2000 and 2002 values. 

b

Assumed equal to 2002 value. 

EPA (2003) provided emission estimates for NOx, CO, and NMVOCs from waste incineration and open burning, 

which were determined using industry published production data and applying average emission factors. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 109 

Uncertainty 

A Tier 2 Monte Carlo analysis was performed to determine the level of uncertainty surrounding the estimates of 

CO2 emissions and N2O emissions from municipal solid waste combustion. IPCC Tier 2 analysis allows the 

specification of probability density functions for key variables within a computational structure that mirrors the 

calculation of the inventory estimate. Uncertainty estimates and distributions for waste generation variables (i.e., 

plastics, synthetic rubber, and textiles generation) were obtained through a conversation with one of the authors of 

the Municipal Solid Waste in the United States reports. Statistical analyses or expert judgments of uncertainty were 

not available directly from the information sources for the other variables; thus, uncertainty estimates for these 

variables were determined using assumptions based on source category knowledge and the known uncertainty 

estimates for the waste generation variables. The 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, 

carbon content of carbon black). 

The results of the 2003 uncertainty analysis of CO2 emissions are the same as the 2002 results, given that the data 

for the two years is identical (no data updated for 2003 were available). The results of the Tier 2 quantitative 

uncertainty analysis are summarized in Table 3-47. Municipal solid waste combustion CO2 emissions in 2003 were 

estimated to be between 15.2 and 21.6 Tg CO2 Eq. at a 95 percent confidence level (or in 19 out of 20 Monte Carlo 

Simulations). This indicates a range of 19 percent below to 15 percent above the 2003 emission estimate of 18.8 Tg 

CO2 Eq. Also at a 95 percent confidence level, municipal solid waste combustion N2O emissions in 2003 were 

estimated to be between 0.13 and 1.34 Tg CO2 Eq. This indicates a range of 71 percent below to 192 percent above 

the 2003 emission estimate of 0.5 Tg CO2 Eq. 

Table 3-47: Tier 2 Quantitative Uncertainty Estimates for CO2 and N2O from Municipal Solid Waste Combustion 

(Tg CO2 Eq. and Percent)

Source Gas 

2003 Emission 

Estimate 

Uncertainty Range Relative to 

Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Municipal Solid Waste Combustion CO2 18.8 15.2 21.6 -19% +15% 

Municipal Solid Waste Combustion N2O 0.5 0.13 1.34 -71% +192% a Range of emission estimates predicted by Monte Carlo Simulation for a 95 percent confidence interval.

The uncertainties in the waste combustion emission estimates arise from both the assumptions applied to the data 

and from the quality of the data. 

• MSW Combustion Rate. A source of uncertainty affecting both fossil CO2 and N2O emissions is the estimate of 

the MSW combustion rate. The EPA (2000c, 2002a, 2003) estimates of materials generated, discarded, and 

combusted carry considerable uncertainty associated with the material flows methodology used to generate 

them. Similarly, the BioCycle (Glenn 1999, Goldstein and Matdes 2000, Goldstein and Matdes 2001, Kaufman 

et al. 2004) estimate of total waste combustion⎯ used for the N2O emissions estimate⎯is based on a survey of 

state officials, who use differing definitions of solid waste and who draw from a variety of sources of varying 

reliability and accuracy. The survey methodology changed significantly and thus the results reported for 2002 

are not directly comparable to the earlier results (Kaufman et al. 2004), introducing further uncertainty. Despite 

the differences in methodology and data sources, the two references⎯ the EPA’s Office of Solid Waste (EPA 

2000a, 2002b, 2003) and the BioCycle series⎯provide estimates of total solid waste combusted that are 

relatively consistent (see Table 3-48). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 110 

Table 3-48: U.S. Municipal Solid Waste Combusted, as Reported by EPA and BioCycle (Metric Tons) 

Year EPA BioCycle 

1990 28,855,809 30,632,057 

1991 27,773,783 25,462,836 

1992 29,568,442 29,113,518 

1993 28,696,188 27,838,884 

1994 29,532,844 29,291,583 

1995 32,182,194 29,639,040 

1996 32,831,450 29,707,171 

1997 33,597,844 27,798,368 

1998 31,205,358 25,489,893 

1999 30,859,134 24,296,249 

2000 30,512,946 25,974,978 

2001 30,569,746 29,694,205a

2002 NA 33,643,954 

2003 NA NA 

NA (Not Available) 

a

 Interpolated between 2000 and 2002 values. 

• Fraction Oxidized. Another source of uncertainty for the CO2 emissions estimate is fraction oxidized. 

Municipal waste combustors vary considerably in their efficiency as a function of waste type, moisture content, 

combustion conditions, and other factors. Despite this variability in oxidation rates, a value of 98 percent was 

assumed for this analysis. 

• Missing Data on Municipal Solid Waste Composition. Disposal rates have been interpolated when there is an 

incomplete interval within a time series. Where data are not available for years at the end of a time series 

(1990, 2003), they are set equal to the most recent years for which estimates are available. 

• Average Carbon Contents. Average carbon contents were applied to the mass of “Other” plastics combusted, 

synthetic rubber in tires and municipal solid waste, and synthetic fibers. These average values were estimated 

from the average carbon content of the known products recently produced. The true carbon content of the 

combusted waste may differ from this estimate depending on differences in the chemical formulation between 

the known and unspecified materials, and differences between the composition of the material disposed and that 

produced. For rubber, this uncertainty is probably small since the major elastomers’ carbon contents range 

from 77 to 91 percent; for plastics, where carbon contents range from 29 to 92 percent, it may be more 

significant. Overall, this is a small source of uncertainty. 

• Synthetic/Biogenic Assumptions. A portion of the fiber and rubber in municipal solid waste is biogenic in 

origin. Assumptions have been made concerning the allocation between synthetic and biogenic materials based 

primarily on expert judgment. 

• Combustion Conditions Affecting N2O Emissions. Because insufficient data exist to provide detailed estimates 

of N2O emissions for individual combustion facilities, the estimates presented exhibit high uncertainty. The 

emission factor for N2O from municipal solid waste combustion facilities used in the analysis is an average of 

default values used to estimate N2O emissions from facilities worldwide (Johnke 1999, UK: Environment 

Agency 1999, Yasuda 1993). These factors span an order of magnitude, reflecting considerable variability in 

the processes from site to site. Due to a lack of information on the control of N2O emissions from MSW 

combustion facilities in the United States, the estimate of zero percent for N2O emissions control removal 

efficiency also exhibits uncertainty. 

Recalculations Discussion 

The N2O emissions estimates for 2001 and 2002 are slightly different from those reported in last year’s inventory 

because newly-available data (Kaufman et al. 2004) were used for the tonnage of waste burned in municipal solid 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 111 

waste combustion for those years. The change resulted in 2001 and 2002 N2O emissions from municipal solid 

waste combustion that increased by less than 0.1 Tg CO2 Eq. (14.7 percent and 26.4 percent, respectively). 

The NOx, CO, and NMVOC emissions estimates for 1999 through 2002 also vary slightly from last year’s inventory 

due to revised criteria pollutant data from EPA. The change constituted less than a 1687 Gg decrease in NOx, CO, 

and NMVOC emissions in 2002. 

3.10. Natural Gas Flaring and Ambient Air Pollutant Emissions from Oil and Gas 

Activities (IPCC Source Category 1B2) 

The flaring of natural gas from on- and off-shore oil wells is a small source of CO2. In addition, oil and gas 

activities also release small amounts of NOx, CO, and NMVOCs. This source accounts for only a small proportion 

of overall emissions of each of these gases. Emissions of NOx, and CO from petroleum and natural gas production 

activities were both less than 1 percent of national totals, while NMVOC and SO2 emissions were roughly 2 percent 

of national totals. 

The flaring (i.e. combustion) and venting of natural gas during petroleum production result in the release of CO2

and CH4 emissions, respectively. Barns and Edmonds (1990) noted that of total reported U.S. venting and flaring, 

approximately 20 percent may be vented, with the remaining 80 percent flared, but it is now believed that flaring 

accounts for an even greater proportion. Studies indicate that the percentage of natural gas that is flared from offshore U.S. production is considerably lower (approximately 30 percent in 2003), due in part to differences in the 

legislation governing on- and off-shore natural gas production. Methane emissions from venting are accounted for 

in the Petroleum Systems source category. For 2003, total CO2 emissions from flaring activities were estimated to 

be 5.9 Tg CO2 Eq. (5,970 Gg), an increase of 3 percent from 1990 levels. On-shore flaring activities accounted for 

5.7 Tg CO2 Eq. (5,743 Gg), or 96 percent, of the total flaring emissions, while off-shore flaring constituted 0.2 Tg 

CO2 Eq. (227 Gg), or 4 percent, of the total (see Table 3-49). 

Table 3-49: CO2 Emissions from On-Shore and Off-Shore Natural Gas Flaring (Tg CO2 Eq.) 

 1990 1997 1998 1999 2000 2001 2002 2003 

On-Shore Flaring 5.5 7.6 6.3 6.7 5.5 5.9 6.0 5.7 

Off-Shore Flaring 0.3 0.3 0.3 0.3 0.2 0.2 0.2 0.2 

Total Flaring 5.8 7.9 6.6 7.0 5.8 6.1 6.2 5.9 

Note: Totals may not sum due to independent rounding. 

Table 3-50: CO2 Emissions from On-Shore and Off-Shore Natural Gas Flaring (Gg) 

 1990 1997 1998 1999 2000 2001 2002 2003 

On-Shore Flaring 5,514 7,565 6,250 6,679 5,525 5,858 6,006 5,743 

Off-Shore Flaring 296 309 316 264 244 236 227 227 

Total Flaring 5,810 7,874 6,566 6,943 5,769 6,094 6,233 5,970 

Note: Totals may not sum due to independent rounding. 

In addition, oil and gas activities, including production, transportation, and storage, result in the release of small 

amounts of NOx, CO, and NMVOCs. Ambient air pollutant emissions from this source from 1990 to 2003 are 

presented below (see Table 3-51). 

Table 3-51: NOx, NMVOCs, and CO Emissions from Oil and Gas Activities (Gg) 

Year NOx CO NMVOCs 

1990 139 302 555 

1996 126 321 433 

1997 130 333 442 

1998 130 332 440 

1999 109 145 414 

2000 111 146 389 

2001 113 147 400 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 112 

2002 135 116 340 

2003 124 125 345 

Methodology 

Estimates of CO2 emissions from on- and off-shore natural gas flaring were prepared using an emission factor of 

54.71 Tg CO2 Eq./QBtu of flared gas, and an assumed flaring efficiency of 100 percent. Ambient air pollutant 

emission estimates for NOx, CO, and NMVOCs were determined using industry-published production data and 

applying average emission factors. 

Total on-shore natural gas vented and flared was taken from EIA’s Natural Gas Annual (EIA 2004); however, there 

is a discrepancy in the time series. One facility in Wyoming had been incorrectly reporting CO2 vented as CH4. 

EIA noted and corrected these data in the Natural Gas Annual 2000 (EIA 2001) for the years 1998 and 1999 only. 

Data for 1990 through 1997 were adjusted by assuming a proportionate share of CO2 in the flare gas for those years 

as for 1998 and 1999. The adjusted values are provided in Table 3-52. It was assumed that all reported vented and 

flared gas was flared. This assumption is consistent with that used by EIA in preparing their emission estimates, 

under the assumption that many states require flaring of natural gas (EIA 2000b). The emission and thermal 

conversion factors were also provided by EIA (2001) and are included in Table 3-52. 

The total off-shore natural gas vented and flared was obtained from the Minerals Management Service’s OGOR-B 

reports (MMS 2003). The percentage of natural gas flared was estimated using data from a 1993 air quality study 

and emissions inventory of the Gulf of Mexico (MOADS) and a 2000 emissions inventory conducted for the Breton 

National Wilderness Area Management Plan (BOADS). See Table 3-53 

Emission estimates for NOx, CO, and NMVOCs from petroleum refining, petroleum product storage and transfer, 

and petroleum marketing operations were obtained from preliminary data (EPA 2003), which, in its final iteration, 

will be published on the National Emission Inventory (NEI) Air Pollutant Emission Trends web site. Included are 

gasoline, crude oil and distillate fuel oil storage and transfer operations, gasoline bulk terminal and bulk plants 

operations, and retail gasoline service stations operations. 

Table 3-52: Total Natural Gas Reported Vented and Flared (Million Ft3

) and Thermal Conversion Factor (Btu/Ft3) 

Year Vented and 

Flared (original) 

Vented and Flared 

(revised)* 

Thermal 

Conversion 

Factor 

1990 150,415 91,130 1,105 

1991 169,909 92,207 1,108 

1992 167,519 83,363 1,110 

1993 226,743 108,238 1,106 

1994 228,336 109,493 1,105 

1995 283,739 144,265 1,106 

1996 272,117 135,709 1,109 

1997 256,351 124,918 1,107 

1998 103,019 103,019 1,109 

1999 110,285 110,285 1,107 

2000 91,232 91,232 1,107 

2001 96,913 96,913 1,105 

2002 99,173 99,173 1,107 

2003 94,929 94,929 1,106 *

 Wyoming venting and flaring estimates were revised. See text for further explanation. 

Table 3-53: Volume Flared Offshore (MMcf) and Fraction Vented and Flared (Percent) 

 1990 1997 1998 1999 2000 2001 2002 2003

Total Gulf of Mexico (GOM) 

Vented & Flared (MMcf) 13,610 15,440 16,280 14,057 12,971 12,990 12,487 12,487
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 113 

Estimated Flaring Fraction of GOM 

Vented & Flared 36% 33% 32% 31% 31% 30% 30% 30%

Total 4,900 5,095 5,210 4,358 4,021 3,897 3,746 3,746

Uncertainty 

Uncertainties in CO2 emission estimates primarily arise from assumptions concerning the flaring efficiency and the 

correction factor applied to 1990 through 1997 venting and flaring data. Uncertainties in ambient air pollutant 

emission estimates are partly due to the accuracy of the emission factors used and projections of growth. 

Recalculations Discussion 

The historical data for natural gas flaring was adjusted slightly, which resulted in an average annual increase in CO2

emissions from flaring of 0.1 Tg CO2 Eq. (2.3 percent) for the period 1990 through 2002. 

3.11. International Bunker Fuels (IPCC Source Category 1: Memo Items) 

Emissions resulting from the combustion of fuels used for international transport activities, termed international 

bunker fuels under the UNFCCC, are currently not included in national emission totals, but are reported separately 

based upon location of fuel sales. The decision to report emissions from international bunker fuels separately, 

instead of allocating them to a particular country, was made by the Intergovernmental Negotiating Committee in 

establishing the Framework Convention on Climate Change.52 These decisions are reflected in the Revised 1996 

IPCC Guidelines, 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).53

Greenhouse gases emitted from the combustion of international bunker fuels, like other fossil fuels, include CO2, 

CH4, N2O, CO, NOx, NMVOCs, particulate matter, and SO2.

54 Two transport modes are addressed under the IPCC 

definition of international bunker fuels: aviation and marine.55 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., 

 

52 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. 1c). 

53 Note that the definition of international bunker fuels used by the UNFCCC differs from that used by the International Civil 

Aviation Organization. 

54 Sulfur dioxide emissions from jet aircraft and marine vessels, although not estimated here, are mainly determined by the 

sulfur content of the fuel. In the United States, jet fuel, distillate diesel fuel, and residual fuel oil average sulfur contents of 0.05, 

0.3, and 2.3 percent, respectively. These percentages are generally lower than global averages.

55 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). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 114 

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.56

Emissions of CO2 from aircraft are essentially a function of fuel use. Methane, N2O, CO, NOx, and NMVOC 

emissions also depend upon engine characteristics, flight conditions, and flight phase (i.e., take-off, climb, cruise, 

decent, and landing). Methane, CO, and NMVOCs are the product of incomplete combustion and occur mainly 

during the landing and take-off phases. In jet engines, N2O and NOx are primarily produced by the oxidation of 

atmospheric nitrogen, and the majority of emissions occur during the cruise phase. The impact of NOx on 

atmospheric chemistry depends on the altitude of the actual emission. The cruising altitude of supersonic aircraft, 

near or in the ozone layer, is higher than that of subsonic aircraft. At this higher altitude, NOx emissions contribute 

to stratospheric ozone depletion.57 At the cruising altitudes of subsonic aircraft, however, NOx emissions contribute 

to the formation of tropospheric ozone. At these lower altitudes, the positive radiative forcing effect of ozone has 

enhanced the anthropogenic greenhouse gas forcing.58 The vast majority of aircraft NOx emissions occur at these 

lower cruising altitudes of commercial subsonic aircraft (NASA 1996).59 

International marine bunkers comprise emissions from fuels burned by ocean-going ships of all flags that are 

engaged in international transport. Ocean-going ships are generally classified as cargo and passenger carrying, 

military (i.e., Navy), fishing, and miscellaneous support ships (e.g., tugboats). For the purpose of estimating 

greenhouse gas emissions, international bunker fuels are solely related to cargo and passenger carrying vessels, 

which is the largest of the four categories, and military vessels. Two main types of fuels are used on sea-going 

vessels: distillate diesel fuel and residual fuel oil. Carbon dioxide is the primary greenhouse gas emitted from 

marine shipping. In comparison to aviation, the atmospheric impacts of NOx from shipping are relatively minor, as 

the emissions occur at ground level. 

Overall, aggregate greenhouse gas emissions in 2003 from the combustion of international bunker fuels from both 

aviation and marine activities were 85.1 Tg CO2 Eq., or 26 percent below emissions in 1990 (see Table 3-54). 

Although emissions from international flights departing from the United States have increased significantly (29 

percent), emissions from international shipping voyages departing the United States have decreased by 63 percent 

since 1990. The majority of these emissions were in the form of CO2; however, small amounts of CH4 and N2O 

were also emitted. Emissions of NOx by aircraft during idle, take-off, landing and at cruising altitudes are of 

primary concern because of their effects on ground-level ozone formation (see Table 3-55). 

Table 3-54: Emissions from International Bunker Fuels (Tg CO2 Eq.) 

Gas/Mode 1990 1997 1998 1999 2000 2001 2002 2003 

CO2 113.5 109.9 114.6 105.3 101.4 97.9 89.5 84.2 

Aviation 46.2 55.9 56.7 58.9 60.5 59.4 61.8 59.6 

Marine 67.3 54.0 57.9 46.4 40.9 38.5 27.7 24.6 

CH4 0.2 0.1 0.2 0.1 0.1 0.1 0.1 0.1 

Aviation + + + + + + + + 

Marine 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

N2O 1.0 1.0 1.0 0.9 0.9 0.9 0.8 0.8 

Aviation 0.5 0.5 0.6 0.6 0.6 0.6 0.6 0.6 

Marine 0.5 0.4 0.4 0.4 0.3 0.3 0.2 0.2 

Total 114.6 111.0 115.7 106.4 102.4 98.9 90.4 85.1 

+ Does not exceed 0.05 Tg CO2 Eq. 

 

56 Naphtha-type jet fuel was used in the past by the military in turbojet and turboprop aircraft engines. 

57 Currently there are only around a dozen civilian supersonic aircraft in service around the world that fly at these altitudes, 

however. 

58 However, at this lower altitude, ozone does little to shield the earth from ultraviolet radiation. 

59 Cruise altitudes for civilian subsonic aircraft generally range from 8.2 to 12.5 km (27,000 to 41,000 feet). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 115 

Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions. 

Table 3-55: Emissions from International Bunker Fuels (Gg) 

Gas/Mode 1990 1997 1998 1999 2000 2001 2002 2003 

CO2 113,503 109,858 114,557 105,294 101,404 97,865 89,489 84,193 

Aviation 46,230 55,899 56,657 58,865 60,545 59,388 61,787 59,558 

Marine 67,272 53,960 57,900 46,429 40,859 38,477 27,701 24,635 

CH4 8 7 7 6 6 5 4 4 

Aviation 1 2 2 2 2 2 2 2 

Marine 7 5 6 5 4 4 3 2 

N2O 3 3 3 3 3 3 3 2 

Aviation 1 2 2 2 2 2 2 2 

Marine 2 1 1 1 1 1 1 1 

CO 115 124 127 124 124 120 118 113 

Aviation 76 92 93 97 100 98 102 98 

Marine 39 32 34 27 24 23 16 15 

NOx 1,985 1,668 1,778 1,478 1,334 1,266 988 900 

Aviation 182 221 224 233 240 235 245 236 

Marine 1,803 1,446 1,554 1,245 1,095 1,031 743 664 

NMVOC 59 52 55 48 44 42 35 32 

Aviation 11 14 14 15 15 15 15 15 

Marine 48 38 41 33 29 27 20 18 

Note: Totals may not sum due to independent rounding. Includes aircraft cruise altitude emissions. 

Methodology 

Emissions of CO2 were estimated by applying of carbon content and fraction oxidized factors to fuel consumption 

activity data. This approach is analogous to that described under CO2 from Fossil Fuel Combustion. Carbon 

content and fraction oxidized factors for jet fuel, distillate fuel oil, and residual fuel oil were taken directly from the 

EIA and are presented in Annex 2.1, Annex 2.2, and Annex 3.7. Heat content and density conversions were taken 

from EIA (2004) and USAF (1998). 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, N2O, CO, NOx, and NMVOCs were calculated by multiplying emission factors by 

measures of fuel consumption by fuel type and mode. Emission factors used in the calculations of CH4, N2O, CO, 

NOx, and NMVOC 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, 0.1 for N2O, 5.2 for CO, 12.5 for NOx, and 0.78 for NMVOCs. For marine 

vessels consuming either distillate diesel or residual fuel oil the following values, in the same units, except where 

noted, were employed: 0.32 for CH4, 0.08 for N2O, 1.9 for CO, 87 for NOx, and 0.052 g/MJ for NMVOCs. Activity 

data for aviation included solely jet fuel consumption statistics, while the marine mode included both distillate 

diesel and residual fuel oil. 

Activity data on aircraft fuel consumption were collected from three government agencies. Jet fuel consumed by 

U.S. flag air carriers for international flight segments was supplied by the Bureau of Transportation Statistics (DOT 

1991 through 2004). It was assumed that 50 percent of the fuel used by U.S. flagged carriers for international 

flights⎯both departing and arriving in the United States⎯was purchased domestically for flights departing from 

the United States. In other words, only one-half of the total annual fuel consumption estimate was used in the 

calculations. Data on jet fuel expenditures by foreign flagged carriers departing U.S. airports was taken from 

unpublished data collected by the Bureau of Economic Analysis (BEA) under the U.S. Department of Commerce 

(BEA 1991 through 2004). Approximate average fuel prices paid by air carriers for aircraft on international flights 

was taken from DOT (1991 through 2004) and used to convert the BEA expenditure data to gallons of fuel 

consumed. Data on U.S. Department of Defense (DoD) aviation bunker fuels and total jet fuel consumed by the 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 116 

U.S. military was supplied by the Office of the Under Secretary of Defense (Installations and Environment), DoD. 

Estimates of the percentage of each Services’ 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 2004). 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 3-56. 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 2004). Activity data on distillate diesel 

consumption by military vessels departing from U.S. ports were provided by DESC (2004). The total amount of 

fuel provided to naval vessels was reduced by 13 percent to account for fuel used while the vessels were notunderway (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 3-57. 

Table 3-56: Aviation Jet Fuel Consumption for International Transport (Million Gallons) 

Nationality 1990 1997 1998 1999 2000 2001 2002 2003

U.S. Carriers 1,954 2,457 2,462 2,625 2,737 2,619 2,495 2,418

Foreign Carriers 2,051 2,939 3,009 3,093 3,166 3,118 3,537 3,388

U.S. Military 862 496 502 488 480 524 482 473

Total 4,867 5,892 5,973 6,206 6,384 6,261 6,515 6,280

Note: Totals may not sum due to independent rounding. 

Table 3-57: Marine Fuel Consumption for International Transport (Million Gallons) 

Fuel Type 1990 1997 1998 1999 2000 2001 2002 2003

Residual Fuel Oil 4,781 3,843 3,974 3,272 2,967 2,846 1,937 1,597

Distillate Diesel Fuel & Other 617 421 627 308 290 204 158 137

U.S. Military Naval Fuels 522 484 518 511 329 318 348 459

Total 5,920 4,748 5,119 4,091 3,586 3,368 2,443 2,193

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.60 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. 

 

60 See uncertainty discussions under Carbon Dioxide Emissions from Fossil Fuel Combustion. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 117 

Particularly for aviation, the DOT (1991 through 2004) international flight segment fuel data used for U.S. flagged 

carriers does not include smaller air carriers and unfortunately defines flights departing to Canada and some flights 

to Mexico as domestic instead of international. As for the BEA (1991 through 2004) data on foreign flagged 

carriers, there is some uncertainty as to the average fuel price, and to the completeness of the data. It was also not 

possible to determine what portion of fuel purchased by foreign carriers at U.S. airports was actually used on 

domestic flight segments; this error, however, is believed to be small.61 

Uncertainties exist with regard to the total fuel used by military aircraft and ships, and in the activity data on 

military operations and training that were used to estimate percentages of total fuel use reported as bunker fuel 

emissions. Total aircraft and ship fuel use estimates were developed from DoD records, which document fuel sold 

to the Navy and Air Force from the Defense Logistics Agency. These data may slightly over or under estimate 

actual total fuel use in aircraft and ships because each Service may have procured fuel from, and/or may have sold 

to, traded with, and/or given fuel to other ships, aircraft, governments, or other entities. There are uncertainties in 

aircraft operations and training activity data. Estimates for the quantity of fuel actually used in Navy and Air Force 

flying activities reported as bunker fuel emissions had to be estimated based on a combination of available data and 

expert judgment. Estimates of marine bunker fuel emissions were based on Navy vessel steaming hour data, which 

reports fuel used while underway and fuel used while not underway. This approach does not capture some voyages 

that would be classified as domestic for a commercial vessel. Conversely, emissions from fuel used while not 

underway preceding an international voyage are reported as domestic rather than international as would be done for 

a commercial vessel. There is uncertainty associated with ground fuel estimates for 1997 through 2001. Small fuel 

quantities may have been used in vehicles or equipment other than that which was assumed for each fuel type. 

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 Component 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.

62 

There is also concern as to the reliability of the existing DOC (1991 through 2004) data on marine vessel fuel 

consumption reported at U.S. customs stations due to the significant degree of inter-annual variation. 

 

61 Although foreign flagged air carriers are prevented from providing domestic flight services in the United States, passengers 

may be collected from multiple airports before an aircraft actually departs on its international flight segment. Emissions from 

these earlier domestic flight segments should be classified as domestic, not international, according to the IPCC. 

62 U.S. aviation emission estimates for CO, NOx, and NMVOCs are reported by EPA’s National Emission Inventory (NEI) Air 

Pollutant Emission Trends web site, and reported under the Mobile Combustion section. It should be noted that these estimates 

are based solely upon LTO cycles and consequently only capture near ground-level emissions, which are more relevant for air 

quality evaluations. These estimates also include both domestic and international flights. Therefore, estimates reported under 

the Mobile Combustion section overestimate IPCC-defined domestic CO, NOx, and NMVOC emissions by including landing and 

take-off (LTO) cycles by aircraft on international flights, but underestimate because they do not include emissions from aircraft 

on domestic flight segments at cruising altitudes. The estimates in Mobile Combustion are also likely to include emissions from 

ocean-going vessels departing from U.S. ports on international voyages. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 118 

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 slightly revised for both U.S. and foreign carriers. These changes were due 

to revisions to international fuel cost for foreign carriers and international jet fuel consumption for U.S. carriers, 

provided by DOT (1991 through 2004). These historical data changes resulted in minimal changes to the emission 

estimates for 1990 through 2002, which averaged to an annual increase in emissions from international bunker fuels 

of less than 0.1 Tg CO2 Eq. (0.1 percent) in CO2 emissions, annual increase of less than 0.1 Tg CO2 Eq. (less than 

0.1 percent) in CH4 emissions, and annual increase of less than 0.1 Tg CO2 Eq. (0.1 percent) in N2O emissions. 

3.12. Wood Biomass and Ethanol Consumption (IPCC Source Category 1A) 

The combustion of biomass fuels⎯such as wood, charcoal, and wood waste⎯and biomass-based fuels⎯such as 

ethanol from corn and woody crops⎯generates CO2. However, in the long run the CO2 emitted from biomass 

consumption does not increase atmospheric CO2 concentrations, assuming the biogenic carbon emitted is offset by 

the uptake of CO2 resulting 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 carbon 

fluxes from changes in biogenic carbon reservoirs in wooded or crop lands are accounted for in the Land-Use 

Change and Forestry chapter. 

In 2003, total CO2 emissions from the burning of woody biomass in the industrial, residential, commercial, and 

electricity generation sectors were approximately 201.0 Tg CO2 Eq. (201,042 Gg) (see Table 3-58 and Table 3-59). 

As the largest consumer of woody biomass, the industrial sector was responsible for 71 percent of the CO2

emissions from this source. The residential sector was the second largest emitter, constituting 18 percent of the 

total, while the commercial and electricity generation sectors accounted for the remainder. 

Table 3-58: CO2 Emissions from Wood Consumption by End-Use Sector (Tg CO2 Eq.) 

End-Use Sector 1990 1997 1998 1999 2000 2001 2002 2003

Industrial 135.3 162.4 150.5 152.0 153.6 135.4 143.7 143.1

Residential 59.9 44.6 39.9 42.7 44.7 38.2 32.3 37.0

Commercial 4.0 5.0 5.0 5.4 5.5 4.2 4.3 4.4

Electricity Generation 13.3 14.1 14.1 14.2 13.9 13.0 15.5 16.6

Total 212.5 226.3 209.5 214.3 217.6 190.8 195.8 201.0

Note: Totals may not sum due to independent rounding. 

Table 3-59: CO2 Emissions from Wood Consumption by End-Use Sector (Gg) 

End-Use Sector 1990 1996 1997 1998 1999 2000 2001 2002

Industrial 135,347 162,447 150,510 152,019 153,559 135,413 143,694 143,084

Residential 59,911 44,650 39,920 42,677 44,685 38,153 32,276 37,019

Commercial 4,037 5,042 4,963 5,394 5,481 4,175 4,319 4,369

Electricity Generation 13,252 14,126 14,097 14,233 13,851 13,034 15,487 16,570

Total 212,547 226,265 209,490 214,323 217,577 190,776 195,775 201,042

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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 119 

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 2003, the United States consumed an estimated 239 trillion Btus of ethanol, and as a result, produced 

approximately 15.8 Tg CO2 Eq. (15,771 Gg) (see Table 3-60) 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 3-60: CO2 Emissions from Ethanol Consumption 

Year Tg CO2 Eq. Gg 

1990 4.2 4,155 

1997 7.0 6,978 

1998 7.7 7,711 

1999 8.0 8,017 

2000 9.2 9,188 

2001 9.7 9,701 

2002 11.5 11,473 

2003 15.8 15,771 

Methodology 

Woody biomass emissions were estimated by taking U.S. consumption data (EIA 2004) (see Table 3-61), provided 

in energy units for the industrial, residential, commercial, and electric generation sectors, and applying two EIA 

gross heat contents (Lindstrom 2003). One heat content (16.953114 MMBtu/MT Wood & Wood Waste) was 

applied to the industrial sector’s consumption, while the other heat content (15.432359 MMBtu/MT Wood & Wood 

Waste) was applied to the consumption data for the other sectors. An EIA emission factor of 0.434 MT C/MT 

Wood (Lindstrom 2003) was then applied to the resulting quantities of woody biomass to obtain CO2 emissions 

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 carbon dioxide with 100 percent efficiency. The emissions from 

ethanol consumption were calculated by applying an EIA emission factor of 17.99 Tg C/QBtu (Lindstrom 2003) to 

U.S. ethanol consumption data that were provided in energy units (EIA 2004) (see Table 3-62). 

Table 3-61: Woody Biomass Consumption by Sector (Trillion Btu)

Year Industrial Residential Commercial Electricity Generation 

1990 1,442 581 39 129 

1997 1,731 433 49 137 

1998 1,603 387 48 137 

1999 1,620 414 52 138 

2000 1,636 433 53 134 

2001 1,443 370 40 126 

2002 1,531 313 42 150 

2003 1,524 359 42 161 

Table 3-62: Ethanol Consumption 

Year Trillion Btu 

1990 63 

1997 106 

1998 117 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 120 

1999 122 

2000 139 

2001 147 

2002 174 

2003 239 

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 

The historical data for wood biomass consumption was adjusted slightly, which resulted in an average annual 

decrease in emissions from wood biomass and ethanol consumption of 0.3 Tg CO2 Eq. (0.1 percent) from 1990 

through 2002. 

[BEGIN BOX] 

Box 3-4: Formation of CO2 through Atmospheric CH4 Oxidation 

Methane emitted to the atmosphere will eventually oxidize into CO2, which remains in the atmosphere for up to 200 

years. The global warming potential (GWP) of CH4, however, does not account for the radiative forcing effects of 

the CO2 formation that results from this CH4 oxidation. The IPCC Guidelines for Greenhouse Gas Inventories

(IPCC/UNEP/OECD/IEA 1997) do not explicitly recommend a procedure for accounting for oxidized CH4, but 

some of the resulting CO2 is, in practice, included in the inventory estimates because of the intentional “doublecounting” structure for estimating CO2 emissions from the combustion of fossil fuels. According to the IPCC 

Guidelines, countries should estimate emissions of CH4, CO, and NMVOCs from fossil fuel combustion, but also 

assume that these compounds eventually oxidize to CO2 in the atmosphere. This is accomplished by using CO2

emission factors that do not factor out carbon in the fuel that is released as in the form of CH4, CO, and NMVOC 

molecules. Therefore, the carbon in fossil fuel is intentionally double counted, as an atom in a CH4 molecule and as 

an atom in a CO2 molecule.63 While this approach does account for the full radiative forcing effect of fossil fuelrelated greenhouse gas emissions, the timing is not accurate because it may take up to 12 years for the CH4 to 

oxidize and form CO2. 

There is no similar IPCC approach to account for the oxidation of CH4 emitted from sources other than fossil fuel 

combustion (e.g., landfills, livestock, and coal mining). Methane from biological systems contains carbon that is 

part of a rapidly cycling biological system, and therefore any carbon created from oxidized CH4 from these sources 

 

63 It is assumed that 100 percent of the CH4 emissions from combustion sources are accounted for in the overall carbon 

emissions calculated as CO2 for sources using emission factors and carbon mass balances. However, it may be the case for some 

types of combustion sources that the oxidation factors used for calculating CO2 emissions do not accurately account for the full 

mass of carbon emitted in gaseous form (i.e., partially oxidized or still in hydrocarbon form). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 121 

is matched with carbon removed from the atmosphere by biological systems⎯likely during the same or subsequent 

year. Thus, there are no additional radiative forcing effects from the oxidation of CH4 from biological systems. For 

example, the carbon content of CH4 from enteric fermentation is derived from plant matter, which itself was created 

through the conversion of atmospheric CO2 to organic compounds. 

The remaining anthropogenic sources of CH4 (e.g., fugitive emissions from coal mining and natural gas systems, 

industrial process emissions) do increase the long-term CO2 burden in the atmosphere, and this effect is not captured 

in the inventory. The following tables provide estimates of the equivalent CO2 production that results from the 

atmospheric oxidation of CH4 from these remaining sources. The estimates for CH4 emissions are gathered from the 

respective sections of this report, and are presented in Table 3-63. The CO2 estimates are summarized in Table 

3-64. 

Table 3-63: CH4 Emissions from Non-Combustion Fossil Sources (Gg) 

Source 1990 1997 1998 1999 2000 2001 2002 2003 

Coal Mining 3,900 2,983 2,989 2,805 2,677 2,647 2,497 2,561 

Abandoned Coal Mines 288 385 341 349 369 331 303 306 

Natural Gas Systems 6,112 6,363 6,276 6,066 6,289 6,277 6,221 5,998 

Petroleum Systems 951 895 879 848 836 831 815 815 

Petrochemical Production 56 78 80 81 80 68 72 72 

Silicon Carbide 

Production 

1 1 1 1 1 + + + 

Iron and Steel Production 63 60 57 56 57 51 48 49 

Total 11,371 10,765 10,622 10,205 10,308 10,206 9,956 9,801 

Note: These emissions are accounted for under their respective source categories. Totals may not sum due to independent 

rounding. 

Table 3-64: Formation of CO2 through Atmospheric CH4 Oxidation (Tg CO2 Eq.)

Source 1990 1997 1998 1999 2000 2001 2002 2003 

Coal Mining 10.7 8.2 8.2 7.7 7.4 7.3 6.9 7.0 

Abandoned Coal Mines 0.8 1.1 0.9 1.0 1.0 0.9 0.8 0.8 

Natural Gas Systems 16.8 17.5 17.3 16.7 17.3 17.3 17.1 16.5 

Petroleum Systems 2.6 2.5 2.4 2.3 2.3 2.3 2.2 2.2 

Petrochemical Production 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 

Silicon Carbide Production + + + + + + + + 

Iron and Steel Production 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.1 

Total 31.3 29.6 29.2 28.1 28.3 28.1 27.4 27.0 

Note: Totals may not sum due to independent rounding. 

+ Does not exceed 0.05 Tg CO2 Eq. 

The estimates of CO2 formation are calculated by applying a factor of 44/16, which is the ratio of molecular weight 

of CO2 to the molecular weight of CH4. For the purposes of the calculation, it is assumed that CH4 is oxidized to 

CO2 in the same year that it is emitted. As discussed above, this is a simplification, because the average 

atmospheric lifetime of CH4 is approximately 12 years. 

Carbon dioxide formation can also result from the oxidation of CO and NMVOCs. However, the resulting increase 

of CO2 in the atmosphere is explicitly included in the mass balance used in calculating the storage and emissions 

from non-energy uses of fossil fuels, with the carbon components of CO and NMVOC counted as CO2 emissions in 

the mass balance.64 

[END BOX] 

 

64 See Annex 2.3 for a more detailed discussion on accounting for indirect emissions from CO and NMVOCs. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 122 
Figure 3-1: 2003 Energy Sector Greenhouse Gas Sources

5,551.6

0 20 40 60 80 100 120 140

Natural Gas Flaring

Petroleum Systems

Waste Combustion

Stationary Sources

Coal Mining

Mobile Sources

Natural Gas Systems

Fossil Fuel Combustion

Tg CO2 Eq.

Energy as a Portion 

of all Emissions

86.6%

Figure 3-3: 2003 U.S. Energy Consumption by Energy Source

Figure 3-4: U.S. Energy Consumption (Quadrillion Btu)

Note: Expressed as gross calorific values.

6.1% Renewable

8.0% Nuclear

22.8% Coal

39.1%

Petroleum

22.5%

Natural Gas

Fossil Fuels

Total Energy

Renewable & Nuclear

0

20

40

60

80

100

120

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Energy Consumption (QBtu)
Figure 3-5: 2003 CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type

Note: The electricity generation sector also includes emissions of less than 0.01 Tg CO2 Eq. 

 from geothermal-based electricity generation

0

500

1,000

1,500

2,000

Residential

Commercial

Industrial

Transportation

Electricity

Generation

U.S.

Territories

Tg CO2 Eq

Natural Gas

Petroleum

Coal

Relative Contribution by 

Fuel Type
Figure 3-6. Annual Deviations from Normal Heating Degree Days for the United States (1949-2003)

Note: Climatological normal data are highlighted. 

 Statistical confidence interval for "normal" climatology period of 1961 through 1990.

Figure 3-7: Annual Deviations from Normal Cooling Degree Days for the United States (1949-2003)

Note: Climatological normal data are highlighted. 

 Statistical confidence interval for "normal" climatology period of 1961 through 1990.

99% Confidence

-15

-10

-5

0

5

10

15

1950 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002

Index Normal = 100

(Deviation from Normal)

Normal 

(4,576 Heating Degree Days)

99% Confidence

-20

-10

0

10

20

1950 1954 1958 1962 1966 1970 1974 1978 1982 1986 1990 1994 1998 2002

Index Normal = 100

(Deviation from Normal)

N l
Figure 3-8: Aggregate Nuclear and Hydroelectric Power Plant Capacity Factors in the United States (1973-2003)

Figure 3-9: 2003 End-Use Sector Emissions of CO2 from Fossil Fuel Combustion

Figure 3-10: Motor Gasoline Retail Prices (Real)

Cents per Gallon

Cents per VMT

75

80

85

90

95

100

105

110

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Index (1990 = 100)

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Residential

Commercial

Industrial

Transportation

U.S. Territories

Tg CO2 Eq

From Electricity

Consumption

From Direct Fossil

Fuel Combustion

Nuclear

Hydro

0

20

40

60

80

100

1973197519771979198119831985198719891991199319951997199920012003

Capacity Factor
Figure 3-11: Motor Vehicle Fuel Efficiency

Passenger Cars

All Motor Vehicles

14

16

18

20

22

24

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Miles per Gallon
Figure 3-12: Industrial Production Indexes (Index 1997=100)

Primary 

Metals

Petroleum 

Refineries

70

90

110

19901991199219931994199519961997199819992000200120022003

Paper

Foods

80

100

120

70

90

110

130 Total excluding Computers, 

Communications Equip., and 

Semiconductors

Total Industrial 

Index

Chemicals

70

90

110

Stone, Clay & Glass 

Products
Figure 3-13: Heating Degree Days

Note: Excludes Alaska and Hawaii

Figure 3-14: Cooling Degree Days

Note: Excludes Alaska and Hawaii

Figure 3-15: Electric Generation Retail Sales by End-Use Sector

Note: The transportation end-use sector consumes minor quanties of electricity.

Residential

Commercial

Industrial

400

600

800

1,000

1,200

1,400

1973 1978 1983 1988 1993 1998 2003

Billion Kwh

80

90

100

110

120

19901991199219931994199519961997199819992000200120022003

Index (Normal = 100)

Normal 

(5,424 Heating Degree Days)

80

90

100

110

120

19901991199219931994199519961997199819992000200120022003

Index (Normal = 100)

Normal 

(1,215 Cooling Degree Days)
Figure 3-16: U.S. Energy Consumption and Energy-Related CO2 Emissions 

Per Capita and Per Dollar GDP

Figure 3-17: Mobile Source CH4 and N2O Emissions

CH4

N2O

0

10

20

30

40

50

60

19901991199219931994199519961997199819992000200120022003

Tg CO2 Eq.

CO2/Energy 

Consumption

CO2/capita

CO2/$GDP

75

79

83

87

91

95

99

103

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

Index (1990 = 100)

Energy 

Consumption/$GDP

Energy 

Consumption/

capita
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 123 

4. Industrial Processes 

Greenhouse gas emissions are produced as a by-product 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 

manufacture, ammonia manufacture 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, aluminum production, 

petrochemical production, silicon carbide production, nitric acid production, and adipic acid production (see Figure 

4-1). 

Figure 4-1: 2003 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 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 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 2003, industrial processes generated emissions of 308.6 teragrams of CO2 equivalent (Tg CO2 Eq.), or 4.5 

percent of total U.S. greenhouse gas emissions. Carbon dioxide emissions from all industrial processes were 147.2 

Tg CO2 Eq. (147,172 Gg) in 2003. This amount accounted for only 2.5 percent of national CO2 emissions. 

Methane emissions from petrochemical, silicon carbide, and iron and steel production resulted in emissions of 

approximately 2.5 Tg CO2 Eq. (121 Gg) in 2003, which was 0.5 percent of U.S. CH4 emissions. Nitrous oxide 

emissions from adipic acid and nitric acid production were 21.8 Tg CO2 Eq. (71 Gg) in 2003, or 5.8 percent of total 

U.S. N2O emissions. In 2003, combined emissions of HFCs, PFCs and SF6 totaled 137.0 Tg CO2 Eq. Overall, 

emissions from industrial processes increased by 2.9 percent from 1990 to 2003 despite decreases in emissions from 

several industrial processes, such as iron and steel, electrical transmission and distribution, HCFC-22 production, 

and aluminum production. 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. 

Table 4-1 summarizes emissions for the Industrial Processes chapter in units of Tg CO2 Eq., while unweighted 

native gas emissions in gigagrams (Gg) are provided in Table 4-2. 

Table 4-1: Emissions from Industrial Processes (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 173.1 170.9 169.4 165.9 164.7 151.8 151.5 147.2

Iron and Steel Production 85.4 71.9 67.4 64.4 65.7 58.9 55.1 53.8

Cement Manufacture 33.3 38.3 39.2 40.0 41.2 41.4 42.9 43.0

Ammonia Manufacture & Urea Application 19.3 20.7 21.9 20.6 19.6 16.7 18.6 15.6

Lime Manufacture 11.2 13.7 13.9 13.5 13.3 12.8 12.3 13.0

Limestone and Dolomite Use 5.5 7.2 7.4 8.1 6.0 5.7 5.9 4.7

Aluminum Production 6.3 5.6 5.8 5.9 5.7 4.1 4.2 4.2
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 124 

Soda Ash Manufacture and Consumption 4.1 4.4 4.3 4.2 4.2 4.1 4.1 4.1

Petrochemical Production 2.2 2.9 3.0 3.1 3.0 2.8 2.9 2.8

Titanium Dioxide Production 1.3 1.8 1.8 1.9 1.9 1.9 2.0 2.0

Phosphoric Acid Production 1.5 1.5 1.6 1.5 1.4 1.3 1.3 1.4

Ferroalloy Production 2.0 2.0 2.0 2.0 1.7 1.3 1.2 1.4

Carbon Dioxide Consumption 0.9 0.8 0.9 0.8 1.0 0.8 1.0 1.3

CH4 2.5 2.9 2.9 2.9 2.9 2.5 2.5 2.5

Petrochemical Production 1.2 1.6 1.7 1.7 1.7 1.4 1.5 1.5

Iron and Steel Production 1.3 1.3 1.2 1.2 1.2 1.1 1.0 1.0

Silicon Carbide Production + + + + + + + +

N2O 33.0 31.5 26.9 25.6 25.6 20.8 23.1 21.8

Nitric Acid Production 17.8 21.2 20.9 20.1 19.6 15.9 17.2 15.8

Adipic Acid Production 15.2 10.3 6.0 5.5 6.0 4.9 5.9 6.0

HFCs, PFCs, and SF6 91.2 121.7 135.7 134.8 138.9 129.5 138.3 137.0

Substitution of Ozone Depleting 

Substances 

0.4 46.5 56.6 65.8 75.0 83.3 91.5 99.5

HCFC-22 Production 35.0 30.0 40.1 30.4 29.8 19.8 19.8 12.3

Electrical Transmission and Distribution 29.2 21.7 17.1 16.4 15.6 15.4 14.7 14.1

Aluminum Production 18.3 11.0 9.1 9.0 9.0 4.0 5.2 3.8

Semiconductor Manufacture 2.9 6.3 7.1 7.2 6.3 4.5 4.4 4.3

Magnesium Production and Processing 5.4 6.3 5.8 6.0 3.2 2.6 2.6 3.0

Total 299.9 327.1 334.9 329.2 332.1 304.7 315.4 308.6

+ Does not exceed 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. 

Table 4-2: Emissions from Industrial Processes (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CO2 173,122 170,884 169,425 165,908 164,657 151,835 151,506 147,172

Iron and Steel Production 85,413 71,863 67,428 64,376 65,693 58,887 55,082 53,763

Cement Manufacture 33,278 38,323 39,218 39,991 41,190 41,357 42,898 43,030

Ammonia Manufacture & Urea 

Application 

19,306 20,650 21,934 20,615 19,616 16,719 18,571 15,560

Lime Manufacture 11,238 13,685 13,914 13,466 13,315 12,823 12,304 12,983

Limestone and Dolomite Use 5,533 7,242 7,449 8,057 5,959 5,733 5,885 4,720

Aluminum Production 6,315 5,621 5,792 5,895 5,723 4,114 4,220 4,219

Soda Ash Manufacture and Consumption 4,141 4,354 4,325 4,217 4,181 4,147 4,139 4,082

Petrochemical Production 2,221 2,919 3,015 3,054 3,004 2,787 2,857 2,777

Titanium Dioxide Production 1,308 1,836 1,819 1,853 1,918 1,857 1,997 2,013

Phosphoric Acid Production 1,529 1,544 1,593 1,539 1,382 1,264 1,338 1,382

Ferroalloy Production 1,980 2,038 2,027 1,996 1,719 1,329 1,237 1,374

Carbon Dioxide Consumption 860 808 912 849 957 818 978 1,267

CH4 120 139 138 138 138 119 120 121

Petrochemical Production 56 78 80 81 80 68 72 72

Iron and Steel Production 63 60 57 56 57 51 48 49

Silicon Carbide Production 1 1 1 1 1 + + + 

N2O 107 102 87 83 83 67 75 70

Nitric Acid Production 58 68 67 65 63 51 56 51

Adipic Acid Production 49 33 19 18 19 16 19 19

HFCs, PFCs, and SF6 M M M M M M M M

Substitution of Ozone Depleting 

Substances 

M M M M M M M M

HCFC-22 Productiona

 3 3 3 3 3 2 2 1

Electrical Transmission and Distributionb 1 1 1 1 1 1 1 1

Aluminum Production M M M M M M M M
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 125 

Semiconductor Manufacture M M M M M M M M

Magnesium Production and Processingb

 + + + + + + + +

NOx 591 629 637 595 626 656 630 648

CO 4,124 3,153 3,163 2,156 2,217 2,339 2,308 2,431

NMVOCs 2,426 2,038 2,047 1,813 1,773 1,769 1,725 1,711

+ Does not exceed 0.5 Gg 

M (Mixture of gases) 

a

 HFC-23 emitted 

b

 SF6 emitted 

Note: Totals may not sum due to independent rounding. 

In order to ensure the quality of the emission estimates from industrial processes, Tier 1 procedures and checks have 

been performed on all industrial process sources. If performed, Tier 2 procedures focused on the emission factor 

and activity data sources and methodology used for estimating emissions procedures, and will be described within 

the QA/QC and Verification Discussion of that source description. In addition to the national QA/QC plan, a more 

detailed plan was developed specifically for the CO2 and CH4 industrial processes sources. This plan was based on 

the U.S. strategy, but was tailored to include specific procedures recommended for these sources. 

The general method employed to estimate emissions for industrial processes, as recommended by the 

Intergovernmental Panel on Climate Change (IPCC), involves multiplying production data (or activity data) for 

each process by an emission factor per unit of production. The uncertainty of the emission estimates is therefore 

generally a function of a combination of the uncertainties surrounding the production and emission factor variables. 

Uncertainty of activity data and the associated probability density functions for industrial processes CO2 sources 

were estimated based on expert assessment of available qualitative and quantitative information. Uncertainty 

estimates and probability density functions for the emission factors used to calculate emissions from this source 

were devised based on IPCC recommendations. 

Activity data is obtained through a survey of manufacturers conducted by various organizations (specified within 

each source); the uncertainty of the activity data is a function of the reliability of plant-level production data and is 

influenced by the completeness of the survey response. The emission factors used were either derived using 

calculations that assume precise and efficient chemical reactions, or were based upon empirical data in published 

references. As a result, uncertainties in the emission coefficients can be attributed to, among other things, 

inefficiencies in the chemical reactions associated with each production process or to the use of empirically-derived 

emission factors that are biased; therefore, they may not represent U.S. national averages. Additional assumptions 

are described within each source. 

The uncertainty analysis performed to quantify uncertainties associated with the 2003 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 elicitation. 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. Iron and Steel Production (IPCC Source Category 2C1) 

In addition to being an energy intensive process, the production of iron and steel also generates process-related 

emissions of CO2 and CH4. Iron is produced by first reducing iron oxide (iron ore) with metallurgical coke in a 

blast furnace to produce pig iron (impure iron containing about 3 to 5 percent carbon by weight). Metallurgical 

coke is manufactured in a coke plant using coking coal as a raw material. Iron may be introduced into the blast 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 126 

furnace in the form of raw iron ore, pellets, briquettes, or sinter. Pig iron (containing about 0.4 percent carbon by 

weight) is used as a raw material in the production of steel. Pig iron is also used as a raw material in the production 

of iron products in foundries. The pig iron production process produces CO2 emissions and fugitive CH4 emissions. 

The production of metallurgical coke from coking coal and the consumption of the metallurgical coke used as a 

reducing agent in the blast furnace are considered in the inventory to be non-energy (industrial) processes, not 

energy (combustion) processes. Coal 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 coke. Coke 

oven gas and coal tar are carbon by-products of the coke manufacturing process. Coke oven gas is generally burned 

as a fuel within the steel mill. Coal tar is used as a raw material to produce anodes used for primary aluminum 

production and other electrolytic processes, and also used in the production of other coal tar products. The coke 

production process produces CO2 emissions and fugitive CH4 emissions. 

Sintering is a thermal process by which fine iron-bearing particles, such as air emission control system dust, are 

baked, which causes the material to agglomerate into roughly one-inch pellets that are then recharged into the blast 

furnace for pig iron production. Iron ore particles may also be formed into larger pellets or briquettes by 

mechanical means, and then agglomerated by heating prior to being charged into the blast furnace. The sintering 

process produces CO2 emissions and fugitive CH4 emissions. 

The metallurgical coke is a reducing agent in the blast furnace. Carbon dioxide is produced as the metallurgical 

coke used in the blast furnace process is oxidized. Steel (containing less than 2 percent carbon by weight) is 

produced from pig iron in a variety of specialized steel making furnaces. The majority of CO2 emissions from the 

iron and steel process come from the use of coke in the production of pig iron, with smaller amounts evolving from 

the removal of carbon from pig iron used to produce steel. Some carbon is also stored in the finished iron and steel 

products. 

Emissions of CO2 and CH4 from iron and steel production in 2003 were 53.8 Tg CO2 Eq. (53,763 Gg) and 1.0 Tg 

CO2 Eq. (48.7 Gg), respectively (see Table 4-3 and Table 4-4). Emissions have fluctuated significantly from 1990 

to 2003 due to changes in domestic economic conditions and changes in product imports and exports. In 2003, 

domestic production of pig iron and coal coke increased by 2.2 and 2.4 percent, respectively. Despite these 

increases, domestic pig iron and coke production have declined since the 1990s. Pig iron production in 2003 was 

15 percent lower than in 2000 and 19 percent below 1995 levels. Coke production in 2003 was 17 percent lower 

than in 2000 and 38 percent below 1990 levels. A slowdown in the domestic and worldwide economy and the 

availability of low-priced imports limit growth in domestic production (USGS 2002). 

Table 4-3: CO2 and CH4 Emissions from Iron and Steel Production (Tg CO2 Eq.) 

Year 1990 1997 1998 1999 2000 2001 2002 2003

CO2 85.4 71.9 67.4 64.4 65.7 58.9 55.1 53.8

CH4 1.3 1.3 1.2 1.2 1.2 1.1 1.0 1.0

Total 86.7 73.1 68.6 65.5 66.9 60.1 56.1 54.8

Table 4-4: CO2 and CH4 Emissions from Iron and Steel Production (Gg) 

Year 1990 1997 1998 1999 2000 2001 2002 2003

CO2 85,413 71,863 67,428 64,376 65,693 58,887 55,082 53,763

CH4 63 60 57 56 58 51 48 49

Methodology 

Since coke is consumed as a reducing agent during the manufacture of pig iron, the corresponding quantity of coal 

consumed during coking operations was identified. This quantity of coal is considered a non-energy use. Data 

were also collected on the amount of imported coke consumed in the blast furnace process. These data were 

converted to their energy equivalents. The carbon content of the combusted coal and imported coke was estimated 

by multiplying their energy consumption by material specific carbon-content coefficients. The carbon content 

coefficients used are presented in Annex 2.1. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 127 

Emissions from the re-use of scrap steel and imported pig iron in the steel production process were calculated by 

assuming that all the associated carbon-content of these materials are released during combustion. Steel has an 

associated carbon-content of approximately 0.4 percent, while pig iron is assumed to contain 4 percent carbon by 

weight. 

Emissions from carbon anodes, used during the production of steel in electric arc furnaces (EAF), were also 

estimated. Emissions of CO2 were calculated by multiplying the annual production of steel in electric arc furnaces 

by an emission factor (4.4 kg CO2/ton steelEAF). It was assumed that the carbon anodes used in the production of 

steel in electric arc furnaces are composed of 80 percent petroleum coke and 20 percent coal tar pitch (DOE 1997). 

Since coal tar pitch is a by-product of the coking process and its carbon-related emissions have already been 

accounted for earlier in the iron and steel emissions calculation as part of the coking process, the emission factor 

was reduced by 20 percent to avoid double counting. Additionally, emissions from the coal tar pitch component of 

carbon anodes consumed during the production of aluminum, which are accounted for in the aluminum production 

section of this chapter, have been subtracted from the total coal tar emissions that were calculated above. 

Carbon storage was accounted for by assuming that all domestically manufactured steel had a carbon content of 0.4 

percent. Furthermore, any pig iron that was not consumed during steel production, but fabricated into finished iron 

products, was assumed to have a by-weight carbon content of 4 percent. 

The production processes for coal coke, 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 emission factors taken from the 1996 IPCC Guidelines

(IPCC/UNEP/OECD/IEA 1997) (see Table 4-5) to annual domestic production data for coal coke, sinter, and pig 

iron. 

Table 4-5: CH4 Emission Factors for Coal Coke, Sinter, and Pig Iron Production (g/kg) 

Material Produced g CH4/kg produced

Coal Coke 0.5 

Pig Iron 0.9 

Sinter 0.5 

Source: IPCC/UNEP/OECD/IEA 1997 

Data relating to the amount of coal consumed at coke plants, for the production of coke for domestic consumption 

in blast furnaces, as well as the quantity of coke imported for iron production were taken from the Energy 

Information Administration (EIA), Quarterly Coal Report January through December 2003 (EIA 2004); U.S. Coal 

Domestic and International Issues (EIA 2001); Mineral Yearbook: Iron and Steel (USGS 1993, 1995a, 1997, 1999, 

2000a, 2001a, 2002a) and the American Iron and Steel Institute (AISI), Annual Statistical Report (AISI 2001, 2002, 

2003, 2004). Scrap steel and imported pig iron consumption data for 1990 through 2003 were obtained from 

Annual Statistical Reports (AISI 1995, 2001, 2002, 2003, 2004) (see Table 4-6). Crude steel production, as well as 

pig iron use for purposes other than steel production, was also obtained from Annual Statistical Reports (AISI 1996, 

2001, 2002, 2004). Carbon content percentages for pig iron and crude steel and the CO2 emission factor for carbon 

anode emissions from steel production were obtained from IPCC Good Practice Guidance and Uncertainty 

Management (IPCC 2000). Aluminum production data for 1990 through 2003 were obtained from Mineral 

Industry Surveys: Aluminum Annual Report (USGS 1995b, 1998, 2000b, 2001b, 2002b, 2003, 2004a). Annual 

consumption of iron ore used in sinter production for 1990 through 2003 were obtained from the USGS Iron Ore 

yearbook (USGS 1994, 1995c, 1996, 1997b, 1998b, 1999b, 2000c, 2001c, 2002c, 2004b). The CO2 emission factor 

for carbon anode emissions from aluminum production was taken from the Revised 1996 IPCC Guidelines

(IPCC/UNEP/OECD/IEA 1997). Estimates for the composition of carbon anodes used during steel and aluminum 

production were obtained from Energy and Environmental Profile of the U.S. Aluminum Industry (DOE 1997). 

Table 4-6: Production and Consumption Data for the Calculation of CO2 and CH4 Emissions from Iron and Steel 

Production (Thousand Metric Tons) 

Gas/Activity Data 1990 1997 1998 1999 2000 2001 2002 2003

CO2 

Coal Consumption at Coke Plants 35,289 27,400 25,573 25,499 26,253 23,655 21,460 21,997
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 128 

Coke Consumption for Pig Iron 24,946 22,100 19,800 18,700 19,215 17,129 15,850 15,379

Domestic Pig Iron Production for Steel 49,061 48,676 47,470 45,677 47,399 41,740 39,600 40,487

Basic Oxygen Furnace Steel Production 56,227 55,386 54,146 52,364 53,964 47,359 45,463 45,873

Electric Arc Furnace Steel Production 33,517 43,098 44,513 45,063 47,859 42,743 46,124 47,803

CH4 

Coke Production 25,054 20,063 18,181 18,240 18,877 17,190 15,220 15,579

Iron Ore Consumption for Sinter 12,239 11,426 10,791 11,072 10,784 9,234 9,018 8,984

Domestic Pig Iron Production for Steel 49,061 48,676 47,470 45,677 47,399 41,740 39,600 40,487

Uncertainty 

The time series data for production of coal coke, sinter, pig iron, steel, and aluminum and import and export data 

upon which the calculations are based are considered to be consistent for the entire time series. The estimates of 

CO2 emissions from the production and utilization of coke are based on energy consumption data, average carbon 

contents, and the fraction of carbon oxidized. These data and factors produce a relatively accurate estimate of CO2

emissions. However, there are uncertainties associated with each of these factors. For example, carbon oxidation 

factors may vary depending on inefficiencies in the combustion process, where varying degrees of ash or soot can 

remain unoxidized. 

Simplifying assumptions were made concerning the composition of carbon anodes (80 percent petroleum coke and 

20 percent coal tar). For example, within the aluminum industry, the coal tar pitch content of anodes can vary from 

15 percent in prebaked anodes to 24 to 28 percent in Soderberg anode pastes (DOE 1997). An average value was 

assumed and applied to all carbon anodes utilized during aluminum and steel production. The assumption is also 

made that all coal tar used during anode production originates as a by-product of the domestic coking process. 

Similarly, it was assumed that all pig iron and crude steel have carbon contents of 4 percent and 0.4 percent, 

respectively. The carbon content of pig iron can vary between 3 and 5 percent, while crude steel can have a carbon 

content of up to 2 percent, although it is typically less than 1 percent (IPCC 2000). 

There is uncertainty in the most accurate CO2 emission factor for carbon anode consumption in aluminum 

production. Emissions vary depending on the specific technology used by each plant (Prebake or Soderberg). The 

Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997) provide CO2 emission factors for each technology 

type. Consistent with the assumptions used in the Aluminum Production source, it was assumed that production 

was split 80 percent prebake and 20 percent Soderberg for the whole time series. Similarly, the carbon anode 

emission factor for steel production can vary between 3.7 and 5.5 kg CO2/ton steel (IPCC 2000). For this analysis, 

the upper bound value was used. 

For the purposes of the CH4 calculation it is assumed that none of the CH4 is captured in stacks or vents and that all 

of the CH4 escapes as fugitive emissions. Additionally, the CO2 emissions calculation is not corrected by 

subtracting the carbon content of the CH4, which means there may be a slight double counting of carbon as both 

CO2 and CH4. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-7. Iron and Steel CO2

emissions were estimated to be between 32.0 and 76.4 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 41 percent below and 42 

percent above the emission estimate of 53.8 Tg CO2 Eq. Iron and Steel CH4 emissions were estimated to be 

between 0.9 Tg CO2 Eq. and 1.1 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 11 percent below and 11 percent above the 

emission estimate of 1.0 Tg CO2 Eq. 

Table 4-7: Tier 2 Quantitative Uncertainty Estimates for CO2 and CH4 Emissions from Iron and Steel Production 

(Tg. CO2 Eq. and Percent) 

Source Gas 2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea 

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 129 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Iron and Steel CO2 53.8 32.0 76.4 -41% +42% 

Iron and Steel CH4 1.0 0.9 1.1 -11% +11% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

Estimates of CO2 from iron and steel production increased due to revised production data found in American Iron 

and Steel Institute’s 2003 Annual Statistical Report (AISI 2004) and EIA’s 2003 Quarterly Coal Report (EIA 

2004). These changes resulted in an average increase of 0.2 Tg CO2 Eq. (0.5 percent) in CO2 emissions from iron 

and steel production for 2001 and 2002. 

Estimates of CH4 2002 were revised due to revised sinter production data provided by the U.S. Geological Survey’s

Iron and Steel Report 2003 (USGE 2004b). This change resulted in an increase of less than 0.1 Tg CO2 Eq. (0.8 

percent) in CH4 emissions from iron and steel production for 2002.

4.2. Cement Manufacture (IPCC Source Category 2A1) 

Cement manufacture is an energy and raw material intensive process that results in the generation of CO2 from both 

the energy consumed in making the cement and the chemical process itself.1 Cement production has accounted for 

about 2.4 percent of total global industrial and energy-related CO2 emissions, and the United States is the world’s 

third largest cement producer (IPCC 1997, USGS 2003). Cement is manufactured in nearly 40 states. Carbon 

dioxide emitted from the chemical process of cement production represents one of the largest sources 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 1,300o

C (2,400oF) to form lime (i.e., calcium oxide or CaO) and CO2. This process is known as calcination or 

calcining. 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 used to make portland cement. The production of masonry cement from portland 

cement requires additional lime and, thus, results in additional CO2 emissions. However, this additional lime is 

already accounted for in the Lime Manufacture source category in this chapter; therefore, the additional emissions 

from making masonry cement from clinker are not counted in this source category’s total. They are presented here 

for informational purposes only. 

In 2003, U.S. clinker production⎯including Puerto Rico⎯totaled 83,214 thousand metric tons (Van Oss 2004). 

The resulting emissions of CO2 from 2003 cement production were estimated to be 43.0 Tg CO2 Eq. (43,030 Gg) 

(see Table 4-8). Emissions from masonry production from clinker raw material are accounted for under Lime 

Manufacture. 

Table 4-8: CO2 Emissions from Cement Production (Tg CO2 Eq. and Gg)* 

Year Tg CO2 Eq. Gg 

1990 33.3 33,278 

1997 38.3 38,323 

1998 39.2 39,218 

1999 40.0 39,991 

 

1 The CO2 emissions related to the consumption of energy for cement manufacture are accounted for under CO2 from Fossil Fuel 

Combustion in the Energy chapter. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 130 

2000 41.2 41,190 

2001 41.4 41,357 

2002 42.9 42,898 

2003 43.0 43,030 

* Totals exclude CO2 emissions from making masonry cement from clinker, which are accounted for under Lime Manufacture. 

After falling in 1991 by two percent from 1990 levels, cement production emissions have grown every year since. 

Overall, from 1990 to 2003, emissions increased by 29 percent. Cement is 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 

Carbon dioxide emissions from cement manufacture are created by the chemical reaction of carbon-containing 

minerals (i.e., calcining limestone). While in the kiln, limestone is broken down into CO2 and lime with the CO2

released to the atmosphere. The quantity of the CO2 emitted during cement production is directly proportional to 

the lime content of the clinker. During calcination, each mole of CaCO3 (i.e., limestone) heated in the clinker kiln 

forms one mole of lime (CaO) and one mole of CO2: 

CaCO3 + heat → CaO + CO2

Carbon dioxide emissions were estimated by applying an emission factor, in tons of CO2 released per ton of clinker 

produced, to the total amount of clinker produced. The emission factor used in this analysis is the product of the 

average lime fraction for clinker of 64.6 percent (IPCC 2000) and a constant reflecting the mass of CO2 released per 

unit of lime. This calculation yields an emission factor of 0.507 tons of CO2 per ton of clinker produced, which was 

determined as follows: 

/ton clinker 2 0.507 tons CO

56.08 g/moleCaO

2 44.01 g/moleCO

0.646 CaO Clinker = × =

⎥

⎥

⎦

⎤

⎢

⎢

⎣

⎡

EF

During clinker production, some of the clinker precursor materials remain in the kiln as non-calcinated, partially 

calcinated, or fully calcinated cement kiln dust (CKD). The emissions attributable to the calcinated portion of the 

CKD are not accounted for by the clinker emission factor. The IPCC recommends that these additional CKD CO2

emissions should be estimated as two percent of the CO2 emissions calculated from clinker production. Total 

cement production emissions were calculated by adding the emissions from clinker production to the emissions 

assigned to CKD (IPCC 2000). 

Masonry cement requires additional lime over and above the lime used in clinker production. In particular, nonplasticizer additives such as lime, slag, and shale are added to the cement, increasing its weight by approximately 

five percent. Lime accounts for approximately 60 percent of this added weight. Thus, the additional lime is 

equivalent to roughly 2.86 percent of the starting amount of the product, since: 

0.6 × 0.05/(1 + 0.05) = 2.86% 

An emission factor for this added lime can then be calculated by multiplying this 2.86 percent by the molecular 

weight ratio of CO2 to CaO (0.785) to yield 0.0224 metric tons of additional CO2 emitted for every metric ton of 

masonry cement produced. 

As previously mentioned, the CO2 emissions from the additional lime added during masonry cement production are 

accounted for in the section on CO2 emissions from Lime Manufacture. Thus, the activity data for masonry cement 

production are shown in this chapter for informational purposes only, and are not included in the cement emission 

totals. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 131 

The 1990 through 2003 activity data for clinker and masonry cement production (see Table 4-9) was obtained 

through a personal communication with Hendrick Van Oss (Van Oss 2004) of the USGS and through the USGS 

Mineral Yearbook: Cement (USGS 1992 through 2003). Data for 2003 masonry cement production were 

unavailable and were assumed to equal 2002 data. The data were compiled by USGS through questionnaires sent to 

domestic clinker and cement manufacturing plants. 

Table 4-9: Cement Production (Gg) 

Year Clinker Masonry 

1990 64,355 3,209 

1991 62,918 2,856 

1992 63,415 3,093 

1993 66,957 2,975 

1994 69,786 3,283 

1995 71,257 3,603 

1996 71,706 3,469 

1997 74,112 3,634 

1998 75,842 3,989 

1999 77,337 4,375 

2000 79,656 4,332 

2001 79,979 4,450 

2002 82,959 4,449 

2003 83,214 4,449 

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 clinker kiln. There is also an uncertainty in the amount of lime added to 

masonry cement, but it is accounted for under the Lime Manufacture source category. The lime content of clinker 

varies from 64 to 66 percent. CKD loss can range from 1.5 to eight 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 4-10. Cement Manufacture CO2

emissions were estimated to be between 39.7 and 46.3 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 8 percent below and 8 percent 

above the emission estimate of 43.0 Tg CO2 Eq. 

Table 4-10: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Cement Manufacture (Tg CO2 Eq. 

and Percent) 

Source Gas 2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea 

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Cement 

Manufacture 

CO2 43.0 39.7 46.3 -8% +8% 

a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

QA/QC and Verification 

Based on the results of the Tier 2 uncertainty analysis conducted on the cement emissions estimate for the 2004 U.S. 

GHG Inventory, the United States decided to conduct Tier 2 QA procedures on two elements of the cement 

emissions estimate for the current inventory submission: the CaO content of clinker and emissions from production 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 132 

of CKD. The United States currently uses the IPCC default values for these input values and the current 

investigation seeks to determine whether these default values are reflective of U.S. circumstances. We are currently 

in the process of conducting a form of expert elicitation on these values. Preliminary results suggest that the default 

values seem appropriate for the U.S. cement industry however there may be small differences. We are currently 

investigating any differences and will include any results in future inventories, if appropriate. 

4.3. Ammonia Manufacture and Urea Application (IPCC Source Category 2B1) 

Emissions of CO2 occur during the production of synthetic ammonia, primarily through the use of natural gas as a 

feedstock. One ammonia production plant located in Kansas is producing ammonia from petroleum coke 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. 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 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. Carbon dioxide is removed from the process gas by the shift conversion 

process, and the hydrogen gas is combined with the nitrogen (N2) gas in the process gas during the ammonia 

synthesis step to produce ammonia. The CO2 is included in a waste gas stream with other process impurities and is 

absorbed by a scrubber solution. In regenerating the scrubber solution, CO2 is released. 

The conversion process for conventional steam reforming of CH4, including primary and secondary reforming and 

the shift conversion processes, is approximately as follows: 

 (catalyst) 

0.88 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 

carbon dioxide are used as raw materials in the production of urea [CO(NH2)2], which is another type of nitrogenous 

fertilizer that contains carbon as well as nitrogen. The chemical reaction that produces urea is: 

2 NH3 + CO2 → NH2COONH4 → CO(NH2)2 + H2O 

The carbon in the urea that is produced and assumed to be subsequently applied to agricultural land as a nitrogenous 

fertilizer is ultimately released into the environment as CO2; therefore, the CO2 produced by ammonia production 

and subsequently used in the production of urea does not change overall CO2 emissions. However, the CO2

emissions are allocated to the ammonia and urea production processes in accordance to the amount of ammonia and 

urea produced. 

Net emissions of CO2 from ammonia manufacture in 2003 were 9.1 Tg CO2 Eq. (9,097 Gg), and are summarized in 

Table 4-11 and Table 4-12. Emissions of CO2 from urea application in 2003 totaled 6.5 Tg CO2 Eq. (6,463Gg), and 

are summarized in Table 4-11 and Table 4-12. 

Table 4-11: CO2 Emissions from Ammonia Manufacture and Urea Application (Tg CO2 Eq.) 

Source 1990 1997 1998 1999 2000 2001 2002 2003
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 133 

Ammonia Manufacture 12.6 14.0 14.2 12.9 12.1 9.3 10.6 9.1

Urea Application 6.8 6.6 7.7 7.7 7.5 7.4 8.0 6.5

Total 19.3 20.7 21.9 20.6 19.6 16.7 18.6 15.6

Table 4-12: CO2 Emissions from Ammonia Manufacture and Urea Application (Gg) 

Source 1990 1997 1998 1999 2000 2001 2002 2003

Ammonia Manufacture 12,553 14,028 14,215 12,948 12,128 9,321 10,561 9,097

Urea Application 6,753 6,622 7,719 7,667 7,488 7,398 8,010 6,463

 

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 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 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, and that amount of CO2 emissions is allocated to 

urea fertilizer application. 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 application. 

The calculation of the total non-combustion CO2 emissions from nitrogenous fertilizers accounts for CO2 emissions 

from the application of imported and domestically produced urea. For each ton of imported urea applied, 0.73 tons 

of CO2 are emitted to the atmosphere. The amount of imported urea applied is calculated based on the net of urea 

imports and exports. 

All ammonia production and subsequent urea production are assumed to be from the same process—conventional 

catalytic reforming of natural gas feedstock, with the exception of ammonia production from petroleum coke 

feedstock at one plant located in Kansas. The CO2 emission factor for production of ammonia from petroleum coke 

is based on plant specific data, wherein all carbon 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 European Fertilizer Manufacturers Association Best Available Techniques publication, 

Production of Ammonia (EFMA 1995). The EFMA reported an emission factor range of 1.15 to 1.30 metric ton 

CO2/metric ton NH3, with 1.2 metric ton CO2/metric ton NH3 as a typical value. 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). Ammonia and urea production data (see Table 4-13 and 

Table 4-14, respectively) were obtained from the Census Bureau of the U.S. Department of Commerce (U.S. Census 

Bureau 1991 through 2004) as reported in Current Industrial Reports Fertilizer Materials and Related Products

annual and quarterly reports. Import and export data for 2003 were unavailable and were assumed to equal 2002 

data (see Table 4-15). These data were obtained from the U.S. Census Bureau Current Industrial Reports Fertilizer 

Materials and Related Products annual reports (U.S. Census Bureau) for 1997 through 2002, The Fertilizer Institute 

(TFI 2002) for 1993 through 1996, and the United States International Trade Commission Interactive Tariff and 

Trade DataWeb (U.S. ITC 2002) for 1990 through 1992. 

Table 4-13: Ammonia Production (Gg) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 134 

Year Gg 

1990 15,425 

1991 15,576 

1992 16,261 

1993 15,599 

1994 16,211 

1995 15,788 

1996 16,260 

1997 16,231 

1998 16,761 

1999 15,728 

2000 14,342 

2001 11,092 

2002 12,577 

2003 10,468 

Table 4-14: Urea Production (Gg) 

Year Gg 

1990 8,124 

1991 7,373 

1992 8,142 

1993 7,557 

1994 7,584 

1995 7,363 

1996 7,755 

1997 7,430 

1998 8,042 

1999 8,080 

2000 6,969 

2001 6,080 

2002 7,038 

2003 5,783 

Table 4-15: Urea Net Imports (Gg) 

Year Gg 

1990 1,086 

1991 648 

1992 656 

1993 2,305 

1994 2,249 

1995 2,055 

1996 1,051 

1997 1,600 

1998 2,483 

1999 2,374 

2000 3,241 

2001 4,008 

2002 3,884 

2003 3,030 

Uncertainty 

The uncertainties contained in these estimates are primarily due to how accurately the emission factor used 

represents an average across all ammonia plants using natural gas feedstock. The EFMA reported an emission 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 135 

factor range of 1.15 to 1.30 ton CO2/ton NH3, with 1.2 ton CO2/ton NH3 reported as a typical value. The actual 

emission factor depends upon the amount of air used in the ammonia production process, with 1.15 ton CO2/ton 

NH3 being the approximate stoichiometric minimum that is achievable for the conventional reforming process. By 

using natural gas consumption data for each ammonia plant, more accurate estimates of CO2 emissions from 

ammonia production could be calculated. However, these consumption data are often considered confidential. 

Also, natural gas is consumed at ammonia plants both as a feedstock to the reforming process and for generating 

process heat and steam. Natural gas consumption data, if available, would need to be divided into feedstock use 

(non-energy) and process heat and steam (fuel) use, as CO2 emissions from fuel use and non-energy use are 

calculated separately.2 

Natural gas feedstock consumption data for the U.S. ammonia industry as a whole is available from the Energy 

Information Administration (EIA) Manufacturers Energy Consumption Survey (MECS) for the years 1985, 1988, 

1991, 1994 and 1998 (EIA 1994; EIA 1998). These feedstock consumption data collectively correspond to an 

effective average emission factor of 1.0 ton CO2/ton NH3, which appears to be below the stoichiometric minimum 

that is achievable for the conventional steam reforming process. The EIA data for natural gas consumption for the 

years 1994 and 1998 correspond more closely to the CO2 emissions calculated using the EFMA emission factor than 

do data for previous years. The 1994 and 1998 data alone yield an effective emission factor of 1.1 ton CO2/ton 

NH3, corresponding to CO2 emissions estimates that are approximately 1.5 Tg CO2 Eq. below the estimates 

calculated using the EFMA emission factor of 1.2 ton CO2/ton NH3. Natural gas feedstock consumption data are 

not available from EIA for other years, and data for 1991 and previous years may underestimate feedstock natural 

gas consumption, and therefore the EFMA emission factor was used to estimate CO2 emissions from ammonia 

production, rather than EIA data. 

All ammonia production and subsequent urea production was assumed to be 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. Research indicates that there is only one U.S. plant 

that manufactures ammonia from petroleum coke. CO2 emissions from this plant are explicitly accounted for in the 

Inventory estimates. No data for ammonia plants using naphtha or other feedstocks other than natural gas have been 

identified. Therefore, all other CO2 emissions from ammonia plants are calculated using the emission factor for 

natural gas feedstock. However, actual emissions may differ because processes other than catalytic steam 

reformation and feedstocks other than natural gas may have been used for ammonia production. Urea is also used 

for other purposes than as a nitrogenous fertilizer. It was assumed that 100 percent of the urea production and net 

imports are used as fertilizer or in otherwise emissive uses. 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 from that sector depending upon the end 

use to which the recovered CO2 is applied. For example, research has identified one ammonia production plant that 

is recovering byproduct CO2 for use in EOR. Such CO2 would be assumed to remain sequestered [see the section of 

 

2 It appears that the IPCC emission factor for ammonia production of 1.5 ton CO2 per ton ammonia may include both CO2

emissions from the natural gas feedstock to the process and some CO2 emissions from the natural gas used to generate process 

heat and steam for the process. Table 2-5, Ammonia Production Emission Factors, in Volume 3 of the Revised 1996 IPCC 

Guidelines for National Greenhouse Gas Inventories Reference Manual (IPCC 1997) includes two emission factors, one reported 

for Norway and one reported for Canada. The footnotes to the table indicate that the factor for Norway does not include natural 

gas used as fuel but that it is unclear whether the factor for Canada includes natural gas used as fuel. However, the factors for 

Norway and Canada are nearly identical (1.5 and 1.6 tons CO2 per ton ammonia, respectively) and it is likely that if one value 

does not include fuel use, the other value also does not. For the conventional steam reforming process, however, the EFMA 

reports an emission factor range for feedstock CO2 of 1.15 to 1.30 ton per ton (with a typical value of 1.2 ton per ton) and an 

emission factor for fuel CO2 of 0.5 tons per ton. This corresponds to a total CO2 emission factor for the ammonia production 

process, including both feedstock CO2 and process heat CO2, of 1.7 ton per ton, which is closer to the emission factors reported 

in the IPCC 1996 Reference Guidelines than to the feedstock-only CO2 emission factor of 1.2 ton CO2 per ton ammonia reported 

by the EFMA. Because it appears that the emission factors cited in the IPCC Guidelines may actually include natural gas used as

fuel, we use the 1.2 tons/ton emission factor developed by the EFMA. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 136 

this chapter on Carbon Dioxide Consumption] however, time series data for the amount of CO2 recovered from this 

plant is not available and therefore all of the CO2 produced by this plant is assumed to be emitted to the atmosphere 

and allocated to Ammonia Manufacture. Further research is required to determine whether byproduct CO2 is being 

recovered from other ammonia production plants for application to end uses that are not accounted for elsewhere. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-16. Ammonia CO2 emissions 

were estimated to be between 7.7 and 10.4 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 15 percent below and 15 percent above the 

emission estimate of 9.1 Tg CO2 Eq. Urea CO2 emissions were estimated to be between 6.0 and 7.0 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 8 percent below and 8 percent above the emission estimate of 6.5 Tg CO2 Eq. 

Table 4-16: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ammonia Manufacture and Urea 

Application (Tg CO2 Eq. and Percent) 

Source Gas 

2003 Emission 

Estimate 

(Tg CO2 Eq.) 

Uncertainty Range Relative to Emission Estimatea

(Tg CO2 Eq.) (%) 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Ammonia CO2 9.1 7.7 10.4 -15% +15% 

Urea CO2 6.5 6.0 7.0 -8% +8% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

Estimates of CO2 emissions from ammonia manufacture for the years 2000, 2001, and 2002 were updated to reflect 

revisions made to the methodology to include ammonia manufactured from petroleum coke. The plant, located in 

Kansas, began ammonia production in 2000. This change resulted in an average annual increase in CO2 emissions 

from ammonia manufacture of 0.5 Tg CO2 Eq. (2.7 percent) for the years 2000 through 2002. 

Planned Improvements 

The United States recognizes that the Tier 2 methodology is preferred for estimating CO2 emissions from ammonia 

manufacture. Historically, efforts have been made to acquire feedstock data for this source category however the 

relevant data were not available. In addition to some of the future work noted in the Uncertainty section, additional 

planned improvements for this source category include developing a plan to determine the feasibility of acquiring 

the relevant data for the Tier 2 assessment. If successful, the results will be included in future inventory 

submissions. 

4.4. Lime Manufacture (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. Lime has historically ranked fifth in total production of all chemicals in the 

United States. 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 ([CaO•MgO]); 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. Carbon dioxide 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 137 

atmosphere. Some of the CO2 generated during the production process, however, is recovered at some facilities for 

use in sugar refining and precipitated calcium carbonate (PCC)3 production. It is also important to note that, for 

certain applications, lime reabsorbs CO2 during use (see Uncertainty, below). 

Lime production in the United States⎯including Puerto Rico⎯was reported to be 19,164 thousand metric tons in 

2003 (USGS 2004). This resulted in estimated CO2 emissions of 13.0 Tg CO2 Eq. (or 12,983 Gg) (see Table 4-17 

and Table 4-18). 

Table 4-17: Net CO2 Emissions from Lime Manufacture (Tg CO2 Eq.) 

Year Tg CO2 Eq. 

1990 11.2 

1997 13.7 

1998 13.9 

1999 13.5 

2000 13.3 

2001 12.8 

2002 12.3 

2003 13.0 

Table 4-18: CO2 Emissions from Lime Manufacture (Gg) 

Year Potential Recovered* Net Emissions 

1990 11,730 (493) 11,238 

1997 14,649 (964) 13,685 

1998 14,975 (1,061) 13,914 

1999 14,655 (1,188) 13,466 

2000 14,548 (1,233) 13,315 

2001 13,941 (1,118) 12,823 

2002 13,355 (1,051) 12,304 

2003 14,132 (1,149) 12,983 

* For sugar refining and precipitated calcium carbonate production. 

Note: Totals may not sum due to independent rounding. 

At the turn of the 20th Century, over 80 percent of lime consumed in the United States went for construction uses. 

The contemporary quicklime market is distributed across four end-use categories as follows: metallurgical uses, 35 

percent; environmental uses, 28 percent; chemical and industrial uses, 23 percent, construction uses, 13 percent; and 

refractory dolomite, one percent. In the construction sector, hydrated lime is still used to improve durability in 

plaster, stucco, and mortars. The use of hydrated lime for traditional building increased by nearly seven percent in 

2003 (USGS 2004). 

Lime production in 2003 increased by nearly seven percent from 2002, the first increase in production in five years. 

Overall, from 1990 to 2003, lime production has increased by 17 percent. The increase in production is attributed in 

part to growth in demand for environmental applications, especially flue gas desulfurization technologies. In 1993, 

EPA completed regulations under the Clean Air Act capping sulfur dioxide (SO2) emissions from electric utilities. 

Lime scrubbers’ high efficiencies and increasing affordability have allowed the flue gas desulfurization end-use to 

expand significantly over the years. Phase II of the Clean Air Act Amendments, which went into effect on January 

1, 2000, remains the driving force behind the growth in the flue gas desulfurization market (USGS 2003). 

 

3 Precipitated calcium carbonate is a specialty filler used in premium-quality coated and uncoated papers. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 138 

Methodology 

During the calcination stage of lime manufacture, 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). The emission factors were calculated as follows: 

For high-calcium lime: [(44.01 g/mole CO2) ÷ (56.08 g/mole CaO)] × (0.95 CaO/lime) = 0.75 g CO2/g lime 

For dolomitic lime: [(88.02 g/mole CO2) ÷ (96.39 g/mole CaO)] × (0.95 CaO/lime) = 0.87 g CO2/g lime 

Production is adjusted to remove the mass of chemically combined water found in hydrated lime, using the midpoint 

of default ranges provided by the IPCC Good Practice Guidance (IPCC 2000). These factors set the chemically 

combined water content to 27 percent for high-calcium hydrated lime, and 24 percent for dolomitic hydrated lime. 

Lime production in the United States was 19,164 thousand metric tons in 2003 (USGS 2004), resulting in potential 

CO2 emissions of 14.1 Tg CO2 Eq. Some of the CO2 generated during the production process, however, was 

recovered for use in sugar refining and precipitated calcium carbonate (PCC) production. Combined lime 

manufacture by these producers was 1,926 thousand metric tons in 2003. It was assumed that approximately 80 

percent of the CO2 involved in sugar refining and PCC was recovered, resulting in actual CO2 emissions of 13.0 Tg 

CO2 Eq. 

The activity data for lime manufacture and lime consumption by sugar refining and PCC production for 1990 

through 2003 (see Table 4-19) were obtained from USGS (1992 through 2004). Hydrated lime production is 

reported separately in Table 4-20. The CaO and CaO•MgO contents of lime were obtained from the IPCC Good 

Practice Guidance (IPCC 2000). 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. For sugar refining and PCC, it was assumed that 100 percent of lime manufacture 

and consumption was high-calcium, based on communication with the National Lime Association (Males 2003). 

Table 4-19: Lime Production and Lime Use for Sugar Refining and PCC (Gg) 

Year 

High-Calcium 

Productiona Dolomitic Productiona,b 

Use for Sugar Refining 

and PCC 

1990 12,947 2,895 826 

1991 12,840 2,838 964 

1992 13,307 2,925 1,023 

1993 13,741 3,024 1,279 

1994 14,274 3,116 1,374 

1995 15,193 3,305 1,503 

1996 15,856 3,434 1,429 

1997 16,120 3,552 1,616 

1998 16,750 3,423 1,779 

1999 16,110 3,598 1,992 

2000 15,850 3,621 2,067 

2001 15,630 3,227 1,874 

2002 14,900 3,051 1,762 

2003 16,040 3,124 1,926 

a

 Includes hydrated lime. 

b Includes dead-burned dolomite. 

Table 4-20: Hydrated Lime Production (Gg) 

Year High-Calcium Hydrate Dolomitic Hydrate 

1990 1,781 319 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 139 

1991 1,841 329 

1992 1,892 338 

1993 1,908 342 

1994 1,942 348 

1995 2,027 363 

1996 1,858 332 

1997 1,820 352 

1998 1,950 383 

1999 2,010 298 

2000 1,550 421 

2001 2,030 447 

2002 1,500 431 

2003 2,140 464 

Uncertainty 

The uncertainties contained in these estimates can be attributed to slight differences in the chemical composition of 

these products. Although the methodology accounts for various formulations of lime, it does not account for the 

trace impurities found in lime, such as iron oxide, alumina, and silica. Due to differences in the limestone used as a 

raw material, a rigid specification of lime material is impossible. As a result, few plants manufacture lime with 

exactly the same properties. 

In addition, a portion of the CO2 emitted during lime manufacture 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). 

Carbon dioxide reabsorption rates vary, however, depending on the application. For example, 100 percent of the 

lime used to produce precipitated calcium carbonate reacts with CO2; whereas most of the lime used in steel making 

reacts with impurities such as silica, sulfur, and aluminum compounds. A detailed accounting of lime use in the 

United States and further research into the associated processes are required to quantify the amount of CO2 that is 

reabsorbed.4 

In some cases, lime is generated from calcium carbonate by-products at pulp mills and water treatment plants.5 The 

lime generated by these processes is not included in the USGS data for commercial lime consumption. In the 

pulping industry, mostly using the Kraft (sulfate) pulping process, lime is consumed in order to causticize a process 

liquor (green liquor) composed of sodium carbonate and sodium sulfide. The green liquor results from the dilution 

of the smelt created by combustion of the black liquor where biogenic carbon is present from the wood. Kraft mills 

recover the calcium carbonate “mud” after the causticizing operation and most sulfate mills recover the waste 

calcium carbonate 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. 

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 

 

4 Representatives of the National Lime Association estimate that CO2 reabsorption that occurs from the use of lime may offset as 

much as a quarter of the CO2 emissions from calcination (Males 2003). 

5 Some carbide producers may also regenerate lime from their calcium hydroxide by-products, which does not result in 

emissions of CO2. In making calcium carbide, quicklime is mixed with coke and heated in electric furnaces. The regeneration of 

lime in this process is done using a waste calcium hydroxide (hydrated lime) [CaC2 + 2H2O → C2H2 + Ca(OH)2], not calcium 

carbonate [CaCO3]. Thus, the calcium hydroxide is heated in the kiln to simply expel the water [Ca(OH)2 + heat → CaO + H2O] 

and no CO2 is released. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 140 

research is necessary to determine the degree to which lime recycling is practiced by water treatment plants in the 

United States. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-21. Lime CO2 emissions were 

estimated to be between 12.0 and 14.1 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 8 percent below and 8 percent above the 

emission estimate of 13.0 Tg CO2 Eq. 

Table 4-21: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Lime Manufacture (Tg CO2 Eq. and 

Percent) 

Source Gas 2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Lime Manufacture CO2 13.0 12.0 14.1 -8% +8% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

4.5. Limestone and Dolomite Use (IPCC Source Category 2A3) 

Limestone (CaCO3) and dolomite (CaCO3MgCO3)6 are basic raw materials used by a wide variety of industries, 

including construction, agriculture, chemical, metallurgy, glass manufacture, 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 to 

generate 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 2003, approximately 8,074 thousand metric tons of limestone and 2,446 thousand metric tons of dolomite were 

consumed for these applications. Overall, usage of limestone and dolomite resulted in aggregate CO2 emissions of 

4.7 Tg CO2 Eq. (4,720 Gg) (see Table 4-22 and Table 4-23). Emissions in 2003 decreased 20 percent from the 

previous year and have decreased 15 percent overall from 1990 through 2003. 

Table 4-22: CO2 Emissions from Limestone & Dolomite Use (Tg CO2 Eq.) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003 

Flux Stone 3.0 5.0 5.1 6.0 2.8 2.5 2.4 2.1 

Glass Making 0.2 0.3 0.2 0 0.4 0.1 0.1 0.3 

FGD 1.4 1.4 1.2 1.2 1.8 2.6 2.8 1.9 

Magnesium Production 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.0 

Other Miscellaneous Uses 0.8 0.4 0.9 0.7 0.9 0.5 0.7 0.4 

Total 5.5 7.2 7.4 8.1 6.0 5.7 5.9 4.7 

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 4-23: CO2 Emissions from Limestone & Dolomite Use (Gg) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003

Flux Stone 2,999 5,023 5,132 6,030 2,829 2,514 2,405 2,072

 

6 Limestone and dolomite are collectively referred to as limestone by the industry, and intermediate varieties are seldom 

distinguished. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 141 

Limestone 2,554 3,963 4,297 4,265 1,810 1,640 1,330 904

Dolomite 446 1,060 835 1,765 1,020 874 1,075 1,168

Glass Making 217 319 157 0 368 113 61 337

Limestone 189 319 65 0 368 113 61 337

Dolomite 28 0 91 0 0 0 0 0

FGD 1,433 1,426 1,230 1,240 1,773 2,551 2,766 1,932

Magnesium Production 64 73 73 73 73 53 0 0

Other Miscellaneous Uses 819 401 858 713 915 501 652 380

Total 5,533 7,242 7,449 8,057 5,959 5,733 5,885 4,720

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 

Carbon dioxide emissions were calculated by multiplying the quantity of limestone or dolomite consumed by the 

average carbon content, approximately 12.0 percent for limestone and 13.2 percent for dolomite (based on 

stoichiometry). This assumes that all carbon is oxidized and released. This methodology was used for flux stone, 

glass manufacturing, flue gas desulfurization systems, chemical stone, mine dusting or acid water treatment, acid 

neutralization, and sugar refining and then converting to CO2 using a molecular weight ratio. 

Traditionally, the production of magnesium metal was the only other 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 2003 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 4-24) were obtained from personal communication with Valentine Tepordei of the USGS 

regarding data in the Minerals Yearbook: Crushed Stone Annual Report (Tepordei 2002, 2003, 2004 and USGS 

1993, 1995a, 1995b, 1996a, 1997a, 1998a, 1999a, 2000a, 2001a, 2002a, 2003a). The production capacity data for 

1990 through 2003 of dolomitic magnesium metal (see Table 4-25) also came from the USGS (1995c, 1996b, 

1997b, 1998b, 1999b, 2000b, 2001b, 2002b, 2003b, 2004). During 1990 and 1992, the USGS did not conduct a 

detailed survey of limestone and dolomite consumption by end-use. Consumption for 1990 was estimated by 

applying the 1991 percentages of total limestone and dolomite use constituted by the individual limestone and 

dolomite uses to 1990 total use. Similarly, the 1992 consumption figures were approximated by applying an 

average of the 1991 and 1993 percentages of total limestone and dolomite use constituted by the individual 

limestone and dolomite uses to the 1992 total. 

Additionally, each year the USGS withholds data on certain limestone and dolomite end-uses due to confidentiality 

agreements regarding company proprietary data. For the purposes of this analysis, emissive end-uses that contained 

withheld data were estimated using one of the following techniques: (1) the value for all the withheld data points for 

limestone or dolomite use was distributed evenly to all withheld end-uses; (2) the average percent of total limestone 

or dolomite for the withheld end-use in the preceding and succeeding years; or (3) the average fraction of total 

limestone or dolomite for the end-use over the entire time period. 

Finally, there is a large quantity of crushed stone reported to the USGS under the category “unspecified uses.” A 

portion of this consumption is believed to be limestone or dolomite used for emissive end uses. The quantity listed 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 142 

for “unspecified uses” was, therefore, allocated to each reported end-use according to each end uses fraction of total 

consumption in that year.7

Table 4-24: Limestone and Dolomite Consumption (Thousand Metric Tons) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003

Flux Stone 6,738 11,226 11,514 13,390 6,248 5,558 5,275 4,501

Limestone 5,804 9,007 9,767 9,694 4,113 3,727 3,023 2,055

Dolomite 933 2,219 1,748 3,696 2,135 1,831 2,252 2,466

Glass Making 489 725 340 0 836 258 139 765

Limestone 430 725 149 0 836 258 139 765

Dolomite 59 0 191 0 0 0 0 0

FGD 3,258 3,242 2,795 2,819 4,030 5,798 6,286 4,390

Other Miscellaneous Uses 1,835 898 1,933 1,620 2,080 1,138 1,483 863

Total 12,319 16,091 16,582 17,830 13,194 12,751 13,183 10,520

Note: "Other miscellaneous uses" includes chemical stone, mine dusting or acid water treatment, acid neutralization, and sugar

refining. 

Table 4-25: Dolomitic Magnesium Metal Production Capacity (Metric Tons) 

Year Production Capacity 

1990 35,000 

1991 35,000 

1992 14,909 

1993 12,964 

1994 21,111 

1995 22,222 

1996 40,000 

1997 40,000 

1998 40,000 

1999 40,000 

2000 40,000 

2001 29,167 

2002 0 

2003 0 

Note: Production capacity for 2002 and 2003 amount to zero because the last U.S. production plant employing the dolomitic 

process shut down mid-2001 (USGS 2002). 

Uncertainty 

Uncertainties in this estimate are due, in part, to variations in the chemical composition of limestone. In addition to 

calcium carbonate, limestone may contain smaller amounts of magnesia, silica, and sulfur. The exact specifications 

for limestone or dolomite used as flux stone vary with the pyrometallurgical process, the kind of ore processed, and 

the final use of the slag. Similarly, the quality of the limestone used for glass manufacturing will depend on the 

type of glass being manufactured. 

Uncertainties also exist in the activity data. 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 enduses. Also, some of the limestone reported as “limestone” is believed to actually be dolomite, which has a higher 

carbon content. Additionally, there is significant inherent uncertainty associated with estimating withheld data 

points for specific end uses of limestone and dolomite. Lastly, the uncertainty of the estimates for limestone used in 

 

7 This approach was recommended by USGS. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 143 

glass making is especially high. 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. However, since glass making accounts for a small percent of consumption, its 

contribution to the overall emissions estimate is low. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-26. Limestone and Dolomite 

Use CO2 emissions were estimated to be between 4.4 and 5.1 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 7 percent below and 8 

percent above the emission estimate of 4.7 Tg CO2 Eq. 

Table 4-26: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Limestone and Dolomite Use (Tg 

CO2 Eq. and Percent) 

Source Gas 2003 Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Limestone and Dolomite Use CO2 4.7 4.4 5.1 -7% +8% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

A spreadsheet error was corrected for the limestone and dolomite use emission estimate for 2002. The change 

resulted in an increase of less than 0.1 Tg CO2 Eq. (0.8 percent) in CO2 emissions from limestone and dolomite use 

for that year. 

4.6. Soda Ash Manufacture and Consumption (IPCC Source Category 2A4) 

Soda ash (sodium carbonate, Na2CO3) is a white crystalline solid that is readily soluble in water and strongly 

alkaline. Commercial soda ash is used as a raw material in a variety of industrial processes and in many familiar 

consumer products such as glass, soap and detergents, paper, textiles, and food. It is used primarily as an alkali, 

either in glass manufacturing or simply as a material that reacts with and neutralizes acids or acidic substances. 

Internationally, two types of soda ash are produced⎯natural and synthetic. The United States produces only natural 

soda ash and is the largest soda ash-producing country in the world. Trona is the principal ore from which natural 

soda ash is made. 

Only three states produce natural soda ash: Wyoming, California, and Colorado. Of these three states, only net 

emissions of CO2 from Wyoming were calculated. This difference is a result of the production processes employed 

in each state.8 During the production process used in Wyoming, trona ore is treated to produce soda ash. Carbon 

dioxide 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 2003, CO2 emissions from the manufacture of soda ash from trona were approximately 1.5 Tg CO2 Eq. (1,509 

Gg). Soda ash consumption in the United States generated 2.6 Tg CO2 Eq. (2,573 Gg) in 2003. Total emissions 

from soda ash manufacture in 2003 were 4.1 Tg CO2 Eq. (4,082 Gg) (see Table 4-27 and Table 4-28). Emissions 

 

8 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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 144 

have fluctuated since 1990. These fluctuations were strongly related to the behavior of the export market and the 

U.S. economy. Emissions in 2003 decreased by approximately 1 percent from the previous year, and have 

decreased overall by approximately 1 percent since 1990. 

Table 4-27: CO2 Emissions from Soda Ash Manufacture and Consumption (Tg CO2 Eq.) 

Year Manufacture Consumption Total 

1990 1.4 2.7 4.1 

1997 1.7 2.7 4.4 

1998 1.6 2.7 4.3 

1999 1.5 2.7 4.2 

2000 1.5 2.7 4.2 

2001 1.5 2.6 4.1 

2002 1.5 2.7 4.1 

2003 1.5 2.6 4.1 

Note: Totals may not sum due to independent rounding. 

Table 4-28: CO2 Emissions from Soda Ash Manufacture and Consumption (Gg) 

Year Manufacture Consumption Total 

1990 1,431 2,710 4,141 

1997 1,665 2,689 4,354 

1998 1,607 2,718 4,325 

1999 1,548 2,668 4,217 

2000 1,529 2,652 4,181 

2001 1,500 2,648 4,147 

2002 1,470 2,668 4,139 

2003 1,509 2,573 4,082 

Note: Totals may not sum due to independent rounding. 

The United States has the world’s largest deposits of trona and represents about one-third of total world soda ash 

output. The distribution of soda ash by end-use in 2003 was glass making, 49 percent; chemical production, 26 

percent; soap and detergent manufacturing, 11 percent; distributors, five percent; flue gas desulfurization, pulp and 

paper production, two percent each; water treatment, one percent; and miscellaneous, four percent (USGS 2004). 

Although the United States continues to be the major supplier of world soda ash, China’s soda ash manufacturing 

capacity is rapidly increasing and is expected to surpass that of the United States. This will likely cause greater 

competition in Asian markets in the future. The world market for soda ash is expected to grow 1.5 to 2 percent 

annually (USGS 2004). 

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. Carbon dioxide and water are generated as by-products of the calcination 

process. Carbon dioxide emissions from the calcination of trona can be estimated based on the following chemical 

reaction: 

2(Na3H(CO3)

2 ⋅2H2O) → 3Na2CO3 + 5H2O + CO2

 [trona] [soda ash] 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 145 

Based on this formula, approximately 10.27 metric tons of trona are required to generate one metric ton of CO2. 

Thus, the 15.5 million metric tons of trona mined in 2003 for soda ash production (USGS 2004) resulted in CO2

emissions of approximately 1.5 Tg CO2 Eq. (1,509 Gg). 

Once manufactured, most soda ash is consumed in glass and chemical production, with minor amounts in soap and 

detergents, pulp and paper, flue gas desulfurization and water treatment. As soda ash is consumed for these 

purposes, additional CO2 is usually emitted. In these applications, it is assumed that one mole of carbon is released 

for every mole of soda ash used. Thus, approximately 0.113 metric tons of carbon (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 4-29) were taken from USGS (1994 

through 2004). Soda ash manufacture and consumption data were collected by the USGS from voluntary surveys of 

the U.S. soda ash industry. 

Table 4-29: Soda Ash Manufacture and Consumption (Gg) 

Year Manufacture* Consumption

1990 14,700 6,530 

1991 14,700 6,280 

1992 14,900 6,320 

1993 14,500 6,280 

1994 14,600 6,260 

1995 16,500 6,500 

1996 16,300 6,390 

1997 17,100 6,480 

1998 16,500 6,550 

1999 15,900 6,430 

2000 15,700 6,390 

2001 15,400 6,380 

2002 15,100 6,430 

2003 15,500 6,200 

* Soda ash manufactured from trona ore only. 

Uncertainty 

Emission estimates from soda ash manufacture are considered to have low associated uncertainty. Both the 

emission factor and activity data are reliable. However, emissions from soda ash consumption are dependent upon 

the type of processing employed by each end-use. Specific information characterizing the emissions from each enduse 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 4-30. Soda Ash Manufacture 

and Consumption CO2 emissions were estimated to be between 3.9 and 4.2 Tg CO2 Eq. at the 95 percent confidence 

level (or in 19 out of 20 Monte Carlo Stochastic Simulations). This indicates a range of approximately 4 percent 

below and 4 percent above the emission estimate of 4.1 Tg CO2 Eq. 

Table 4-30: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Soda Ash Manufacture and 

Consumption (Tg CO2 Eq. and Percent) 

Source Gas 2003 Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Soda Ash Manufacture 

and Consumption CO2 4.1 3.9 4.2 -4% +4% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 146 

Planned Improvements 

Emissions from soda ash production in Colorado, which is produced using the nahcolite production process, will be 

investigated for inclusion in future inventories. 

4.7. 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 TiO2 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. Carbon dioxide is emitted from the chloride process, which 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 

carbon as a raw material and does not emit CO2. 

The chloride process is based on the following chemical reactions: 

2 FeTiO3 + 7 Cl2 + 3 C → 2 TiCl4 + 2 FeCl3 + 3 CO2

2 TiCl4 + 2 O2 → 2 TiO2 + 4 Cl2

The carbon 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 petroleum coke is manufactured specifically for this 

purpose. Emissions of CO2 from titanium dioxide production in 2003 were 2.0 Tg CO2 Eq. (2,013 Gg), an increase 

of less than one percent from the previous year and 54 percent from 1990, due to increasing production within the 

industry (see Table 4-31). 

Table 4-31: CO2 Emissions from Titanium Dioxide (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 1.3 1,308 

1997 1.8 1,836 

1998 1.8 1,819 

1999 1.9 1,853 

2000 1.9 1,918 

2001 1.9 1,857 

2002 2.0 1,997 

2003 2.0 2,013 

Methodology

Emissions of CO2 from titanium dioxide production were calculated by multiplying annual titanium dioxide 

production by chlorine process-specific emission factors. 

Data were obtained for the total amount of titanium dioxide produced each year, and it was assumed that 97 percent 

of the total production in 2003 was produced using the chloride process. It was assumed that titanium dioxide 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. 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 titanium dioxide produced using the chloride process was 

produced using petroleum coke, although some titanium dioxide may have been produced with graphite or other 

carbon inputs. The amount of petroleum coke consumed annually in titanium dioxide production was calculated 

based on the assumption that petroleum coke used in the process is 90 percent carbon and 10 percent inert materials. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 147 

The emission factor for the titanium dioxide chloride process was taken from the report, Everything You’ve Always 

Wanted to Know about Petroleum Coke (Onder and Bagdoyan 1993). Titanium dioxide production data for 1990 

through 2003 (see Table 4-32) were obtained from personal communication with Joseph Gambogi, USGS 

Commodity Specialist, of the USGS (Gambogi 2004) and through the Minerals Yearbook: Titanium Annual Report

(USGS 1991 through 2003). Data for the percentage of the total titanium dioxide production capacity that is 

chloride process for 1994 through 2002 were also taken from the USGS Minerals Yearbook and from Joseph 

Gambogi for 2003. 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. The composition data for petroleum coke 

were obtained from Onder and Bagdoyan (1993). 

Table 4-32: Titanium Dioxide Production (Gg) 

Year Gg 

1990 979 

1991 992 

1992 1,140 

1993 1,160 

1994 1,250 

1995 1,250 

1996 1,230 

1997 1,340 

1998 1,330 

1999 1,350 

2000 1,400 

2001 1,330 

2002 1,410 

2003 1,420 

Uncertainty 

Although some titanium dioxide may be produced using graphite or other carbon inputs, information and data 

regarding these practices were not available. Titanium dioxide produced using graphite inputs may generate 

differing amounts of CO2 per unit of titanium dioxide produced compared to the use of petroleum coke. The most 

accurate method for these estimates would be basing calculations on the amount of reducing agent used in the 

process, rather than the amount of titanium dioxide produced. These data were not available, however. 

Also, annual titanium production is not reported by USGS by the type of production process used (chloride or 

sulfate). Only the percentage of total production capacity is reported. It was assumed that titanium dioxide 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. 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 titanium dioxide production, literature data 

were used for petroleum coke composition. Certain grades of petroleum coke are manufactured specifically for use 

in the titanium dioxide chloride process; however, this composition information was not available. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-33. Titanium Dioxide 

Consumption CO2 emissions were estimated to be between 1.7 and 2.3 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 16 percent 

below and 16 percent above the emission estimate of 2.0 Tg CO2 Eq. 

Table 4-33: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Titanium Dioxide Production (Tg 

CO2 Eq. and Percent) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 148 

Source Gas 

2003 

Emission 

Estimate 

Uncertainty Range Relative to Emission Estimatea 

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Titanium Dioxide Production CO2 2.0 1.7 2.3 -16% +16% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

4.8. Phosphoric Acid Production (IPCC Source Category 2A7) 

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 carbon in the form of calcium carbonate 

(limestone) and also may contain organic carbon. The chemical composition of phosphate rock (francolite) mined 

in Florida is: 

Ca10-x-y Nax Mgy (PO4)6-x(CO3)xF2+0.4x

The calcium carbonate component of the phosphate rock is integral to the phosphate rock chemistry. Phosphate 

rock can also contain organic carbon 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 carbon (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 

1997). 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 carbon dioxide. The chemical reaction for the 

limestone–sulfuric acid reaction is: 

CaCO3 + H2SO4 + H2O → CaSO4 2H2O + CO2

Total marketable phosphate rock production in 2003 was 38.7 million metric tons. Approximately 86 percent of 

domestic phosphate rock production was mined in Florida and North Carolina, while approximately 14 percent of 

production was mined in Idaho and Utah. Florida alone represented more than 75 percent of domestic production. 

In addition, 2.4 million metric tons of crude phosphate rock was imported for consumption in 2003. Marketable 

phosphate rock production, including domestic production and imports for consumption, increased by 

approximately 3.7 percent between 2002 and 2003. However, over the 1990 to 2003 period, production decreased 

by 12 percent. The 35.3 million metric tons produced in 2001 was the lowest production level recorded since 1965 

and was driven by a worldwide decrease in demand for phosphate fertilizers. Total CO2 emissions from phosphoric 

acid production were 1.4 Tg CO2 Eq. (1,382 Gg) in 2003 (see Table 4-34). 

Table 4-34: CO2 Emissions from Phosphoric Acid Production (Tg CO2 Eq. and Gg) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 149 

Year Tg CO2 Eq. Gg 

1990 1.5 1,529 

1997 1.5 1,544 

1998 1.6 1,593 

1999 1.5 1,539 

2000 1.4 1,382 

2001 1.3 1,264 

2002 1.3 1,338 

2003 1.4 1,382 

Methodology 

Carbon dioxide emissions from production of phosphoric acid from phosphate rock is 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 USGS reports in the Minerals Yearbook, Phosphate Rock, the aggregate amount of phosphate rock mined 

annually in Florida and North Carolina and the aggregate amount of phosphate rock mined annually in Idaho and 

Utah, and reports the annual amounts of phosphate rock exported and imported for consumption (see Table 4-35). 

Data for domestic production of phosphate rock, exports of phosphate rock, and imports of phosphate rock for 

consumption for 1990 through 2003 were obtained from USGS Mineral Yearbook, Phosphate Rock (USGS 1994 

through 2004). 

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 3.5 percent inorganic carbon (as CO2), and 

phosphate rock imported from Morocco contains approximately 5 percent inorganic carbon (as CO2). Calcined 

phosphate rock mined in North Carolina and Idaho contains approximately 1.5 percent and 1.0 percent inorganic 

carbon (as CO2), respectively (see Table 4-36). 

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 (86 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. The USGS reported that one phosphate rock producer in 

Idaho is producing calcined phosphate rock; however, no production data were available for this single producer 

(USGS 2003). Carbonate content data for uncalcined phosphate rock mined in Idaho and Utah (14 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. 

The CO2 emissions calculation methodology is based on the assumption that all of the inorganic carbon (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 carbon content of the phosphate rock is 

converted to CO2 and that all of the organic carbon content remains in the phosphoric acid product. 

Table 4-35: Phosphate Rock Domestic Production, Exports, and Imports (Gg) 

Location/Year 1990 1997 1998 1999 2000 2001 2002 2003

U.S. Production 

FL & NC 42,494 36,604 38,000 35,900 31,900 28,100 29,800 31,300

ID & UT 7,306 5,496 5,640 5,540 5,470 4,730 4,920 5,100

Exports - FL & NC 6,240 335 378 272 299 9 62 64

Imports - Morocco 451 1,830 1,760 2,170 1,930 2,500 2,700 2,400

Total U.S. Consumption 44,011 43,595 45,022 43,338 39,001 35,321 37,358 38,746
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 150 

Source: USGS 2004, 2003, 2002, 2001, 2000, 1999, 1998, 1997, 1996, 1995. 

Table 4-36: Chemical Composition of Phosphate Rock (percent by weight) 

Composition 

Central 

Florida North Florida 

North Carolina 

(calcined) 

Idaho 

(calcined) Morocco 

Total Carbon (as C) 1.60 1.76 0.76 0.60 1.56 

Inorganic Carbon (as C) 1.0 0.93 0.41 0.27 1.46 

Organic Carbon (as C) 0.60 0.83 0.35 — 0.1 

Inorganic Carbon (as CO2) 3.67 3.43 1.50 1.0 5.0 

Source: FIPR 2003 

(—): Assumed equal to zero. 

Uncertainty 

Phosphate rock production data used in the emission calculations are developed by the USGS through monthly and 

semiannual voluntary surveys of the eleven companies that owned phosphate rock mines during 2003. The 

phosphate rock production data are not considered to be a significant source of uncertainty, because all eleven of the 

domestic phosphate rock producers are reporting their annual production to the USGS. Data for imports for 

consumption and exports of phosphate rock used in the emission calculation are based on international trade data 

collected by the U.S. Census Bureau. These U.S. government economic data are not considered to be a significant 

source of uncertainty. 

One source of potentially significant uncertainty in the calculation of CO2 emissions from phosphoric acid 

production is the data for 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. Only one set of data from the Florida 

Institute of Phosphate Research was available for the composition of phosphate rock mined domestically and 

imported, and data for uncalcined phosphate rock mined in North Carolina and Idaho were unavailable. Inorganic 

carbon content (as CO2) of phosphate rock could vary ±1 percent from the data included in Table 4-36, resulting in 

a variation in CO2 emissions of ±20 percent. Another source of uncertainty is the disposition of the organic carbon 

content of the phosphate rock. A representative of the FIPR indicated that in the phosphoric acid production 

process the organic carbon 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 carbon is therefore not 

included in the calculation of CO2 emissions from phosphoric acid production. However, if, for example, 50 

percent of the organic carbon content of the phosphate rock were to be emitted as CO2 in the phosphoric acid 

production process, the CO2 emission estimate would increase by on the order of 50 percent. 

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 carbon 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 2003). Total production of phosphate rock in Utah and Idaho combined amounts to 

approximately 14 percent of total domestic production in 2003. If it is assumed that 100 percent of the reported 

domestic production of phosphate rock for Idaho and Utah was first calcined, and it is assumed that 50 percent of 

the organic carbon content of the total production for Idaho and Utah was converted to CO2 in the calcination 

process, the CO2 emission estimate would increase on the order of 10 percent. 

Finally, USGS indicated that 5 percent of domestically-produced phosphate rock is used to manufacture elemental 

phosphorus and other phosphorus-based chemicals, rather than phosphoric acid (USGS 2003a). 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 carbon content in phosphate rock, but none from the organic 

carbon content. This phosphate rock, consumed for other purposes, constitutes approximately 5 percent of total 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 151 

phosphate rock consumption. If it were assumed that there are zero emissions from other uses of phosphate rock, 

CO2 emissions would fall 5 percent. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-37. Phosphoric acid 

production CO2 emissions were estimated to be between 1.1 and 1.6 Tg CO2 Eq. at the 95 percent confidence level 

(or in 19 out of 20 Monte Carlo Stochastic Simulations). This indicates a range of approximately 18 percent below 

and 18 percent above the emission estimate of 1.4 Tg CO2 Eq. 

Table 4-37: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Phosphoric Acid Production (Tg 

CO2 Eq. and Percent) 

Source Gas 2003 Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Phosphoric Acid Production CO2 1.4 1.1 1.6 -18% +18% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

The historical activity data used to calculate the emissions from phosphoric acid production were updated for the 

year 2002. The change resulted in a decrease of less than 0.1 Tg CO2 Eq. (less than 0.1 percent) in CO2 emissions 

from phosphoric acid production for that year. 

Planned Improvements 

The estimate of CO2 emissions from phosphoric acid production could be improved through collection of additional 

data. Additional data is being collected concerning the carbonate content of uncalcined phosphate rock mined in 

various locations in the United States. Additional research will also be conducted concerning the disposition of the 

organic carbon content of the phosphate rock in the phosphoric acid production process. Only a single producer of 

phosphate rock is calcining the product, and only a single producer is manufacturing elemental phosphorus. Annual 

production data for these single producers will probably remain unavailable. 

4.9. Ferroalloy Production (IPCC Source Category 2C2) 

Carbon dioxide is 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: 

Fe 2O3 + 2SiO2 + 7C → 2FeSi + 7CO

Emissions of CO2 from ferroalloy production in 2003 were 1.4 Tg CO2 Eq. (1,374 Gg) (see Table 4-38), an 11 

percent increase from the previous year and a 31 percent reduction since 1990. 

Table 4-38: CO2 Emissions from Ferroalloy Production (Tg CO2 Eq. and Gg) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 152 

Year Tg CO2 Eq. Gg 

1990 2.0 1,980 

1997 2.0 2,038 

1998 2.0 2,027 

1999 2.0 1,996 

2000 1.7 1,719 

2001 1.3 1,329 

2002 1.2 1,237 

2003 1.4 1,374 

Methodology 

Emissions of CO2 from ferroalloy production were calculated by multiplying annual ferroalloy production by 

material-specific emission factors. Emission factors taken from the Revised 1996 IPCC Guidelines

(IPCC/UNEP/OECD/IEA 1997) 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 50 percent silicon ferrosilicon (2.35 tons 

CO2/ton of alloy produced) was applied. Additionally, for ferrosilicon alloys containing 56 to 95 percent silicon, an 

emission factor for 75 percent silicon ferrosilicon (3.9 tons CO2 per ton alloy produced) was applied. The emission 

factor for silicon metal was assumed to be 4.3 tons CO2/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/UNEP/OECD/IEA 1997), although some ferroalloys may have been produced with coking coal, wood, other 

biomass, or graphite carbon inputs. The amount of petroleum coke consumed in ferroalloy production was 

calculated assuming that the petroleum coke used is 90 percent carbon and 10 percent inert material. 

Ferroalloy production data for 1990 through 2003 (see Table 4-39) were obtained from the USGS through personal 

communications with Lisa Corathers (2004), the Silicon Commodity Specialist, and through the Minerals 

Yearbook: Silicon Annual Report (USGS 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 

2003). 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 4-39). The composition data for petroleum coke was obtained from Onder and 

Bagdoyan (1993). 

Table 4-39: Production of Ferroalloys (Metric Tons) 

Year 

Ferrosilicon 

25%-55% 

Ferrosilicon

56%-95% Silicon Metal

Misc. Alloys 

(32-65%)

1990 321,385 109,566 145,744 72,442

1997 175,000 147,000 187,000 106,000

1998 162,000 147,000 195,000 99,800

1999 252,000 145,000 195,000 NA

2000 229,000 100,000 184,000 NA

2001 167,000 89,000 137,000 NA

2002 156,000 98,600 113,000 NA

2003 113,000 75,800 189,000 NA

NA (Not Available) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 153 

Uncertainty 

Although some ferroalloys may be produced using wood or other biomass as a carbon source, information and data 

regarding these practices were not available. Emissions from ferroalloys produced with wood or other biomass 

would not be counted under this source because wood-based carbon is of biogenic origin.9

 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. 

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 4-40. Ferroalloy Production CO2

emissions were estimated to be between 1.3 and 1.4 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 3 percent below and 3 percent 

above the emission estimate of 1.4 Tg CO2 Eq. 

Table 4-40: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Ferroalloy Production (Tg CO2 Eq. 

and Percent) 

Source Gas 2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Ferroalloy Production CO2 1.4 1.3 1.4 -3% +3% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

4.10. Carbon Dioxide Consumption (IPCC Source Category 2B5) 

Carbon dioxide (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). Carbon dioxide used for EOR is injected into the underground reservoirs to increase 

the reservoir pressure to enable additional petroleum to be produced. 

For the most part, CO2 used in non-EOR applications will eventually be released to the atmosphere, and for the 

purposes of this analysis CO2 used in commercial applications other than EOR is assumed to be emitted to the 

atmosphere. Carbon dioxide used in EOR applications is considered for the purposes of this analysis to remain 

sequestered in the underground formations into which the CO2 is injected. 

It is unclear to what extent the CO2 used for EOR will be re-released to the atmosphere. Carbon dioxide used in 

EOR applications is compressed at the CO2 production source, transported by pipeline to the EOR field, and 

injected into wellheads. Potential CO2 leakage pathways from CO2 production, transportation, and injection process 

include fugitive emissions from the compressors, pipeline equipment, and wellheads. Also, the CO2 used for EOR 

may show up at the wellhead after a few years of injection (Hangebrauk et al. 1992) or may be partially recovered 

 

9 Emissions and sinks of biogenic carbon are accounted for in the Land-Use Change and Forestry chapter. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 154 

as a component of crude oil produced from the wells (Denbury Resources 2003a). This CO2 may be recovered and 

re-injected into the wellhead or separated from the petroleum produced and vented to the atmosphere. More 

research is required to determine the amount of CO2 that may escape from EOR operations through leakage from 

equipment, as a component of the crude oil produced, or as leakage directly from the reservoir through geologic 

faults and fractures or through improperly plugged or improperly completed wells. For the purposes of this 

analysis, it is assumed that all of the CO2 produced for use in EOR applications is injected into reservoirs (i.e., there 

is no loss of CO2 to the atmosphere during CO2 production, transportation, or injection for EOR applications) and 

that all of the injected CO2 remains sequestered within the reservoirs. 

Carbon dioxide is produced from naturally occurring CO2 reservoirs, as a by-product from the energy and industrial 

production processes (e.g., ammonia production, fossil fuel combustion, ethanol production), and as a by-product 

from the production of crude oil and natural gas, which contain naturally occurring CO2 as a component. Carbon 

dioxide 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 or industrial production processes nor CO2

separated from crude oil and natural gas are included in this analysis for a number of reasons. 

Depending on the raw materials that are used, by-product CO2 generated during energy and industrial production 

processes may already be accounted for in the CO2 emission estimates from fossil fuel consumption (either from 

fossil fuel combustion or from non-energy uses of fossil fuels). For example, ammonia is primarily manufactured 

using natural gas as both a feedstock and energy source. Carbon dioxide emissions from natural gas combustion for 

ammonia production are accounted for in the CO2 from Fossil Fuel Combustion source category of the Energy 

sector and, therefore, are not included under Carbon Dioxide Consumption. Likewise, CO2 emissions from natural 

gas used as feedstock for ammonia production are accounted for in this chapter under the Ammonia Manufacture 

source category and, therefore, are not included here.10

Carbon dioxide is produced as a by-product of crude oil and natural gas production. This CO2 is separated from the 

crude oil and natural gas using gas processing equipment, and may be emitted directly to the atmosphere, or 

captured and reinjected into underground formations, used for EOR, or sold for other commercial uses. The amount 

of CO2 separated from crude oil and natural gas has not been estimated.11 Therefore, the only CO2 consumption 

that is accounted for in this analysis is CO2 produced from natural wells other than crude oil and natural gas wells 

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 natural 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, R. et al. 2000). In 2003, the amount of CO2 produced by the Mississippi and New 

Mexico facilities for commercial applications and subsequently emitted to the atmosphere were 1.3 Tg CO2 Eq. 

(1,267 Gg) (see Table 4-41). This amount represents an increase of 29 percent from the previous year and an 

increase of 47 percent from emissions in 1990. This increase was due to an increase in the Mississippi facility’s 

reported production for use in other commercial applications. 

Table 4-41: CO2 Emissions from Carbon Dioxide Consumption (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

 

10 One ammonia manufacturer located in Oklahoma is reportedly capturing approximately 35 MMCF/day (0.67 Tg/yr) of byproduct CO2 for use in EOR applications. According to the methodology used in this analysis, this amount of CO2 would be 

considered to be sequestered and not emitted to the atmosphere. However, time series data for the amount of CO2 captured from 

the ammonia plant for use in EOR applications are not available, and therefore all of the CO2 produced by the ammonia plant is 

assumed to be emitted to the atmosphere and is accounted for in this chapter under Ammonia Manufacture. 

11 The United States is in the process of developing a methodology to account for CO2 emissions from natural gas systems and 

petroleum systems for inclusion in future Inventory submissions. For more information see Annex 5. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 155 

1990 0.9 860 

1997 0.8 808 

1998 0.9 912 

1999 0.8 849 

2000 1.0 957 

2001 0.8 818 

2002 1.0 979 

2003 1.3 1,267 

Methodology 

Carbon dioxide emission estimates for 2001, 2002, and 2003 were based on production data for the two facilities 

currently producing CO2 from naturally-occurring CO2 reservoirs. 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). 

Carbon dioxide produced from these two facilities that was used for EOR is assumed to remain sequestered and is 

not included in the CO2 emissions totals. It is assumed that 100 percent of the CO2 production used in commercial 

applications other than EOR is eventually released into the atmosphere. 

Carbon dioxide production data for the Jackson Dome, Mississippi facility in 2001, 2002, and 2003 and the 

percentage of total production that was used for EOR and in non-EOR applications were obtained from the Annual 

Reports for Denbury Resources, the operator of the facility (Denbury Resources 2002, Denbury Resources 2003b, 

Denbury Resources 2004). Denbury Resources reported the average CO2 production in units of MMCF CO2 per 

day for 2001, 2002, and 2003, and reported the percentage of the total average annual production that was used for 

EOR. Carbon dioxide production data for the Bravo Dome, New Mexico facility were obtained from the New 

Mexico Bureau of Geology and Mineral Resources for the years 1990 through 2001 (Broadhead 2003). According 

to the New Mexico Bureau, the amount of CO2 produced from Bravo Dome for use in non-EOR applications is less 

than one percent of total production (Broadhead 2003). Production data for 2002 and 2003 were not available for 

Bravo Dome, so it is assumed that the production values for those years are equal to the 2001 value. 

Denbury Resources acquired the Jackson Dome facility in 2001 and CO2 production data for the Jackson Dome 

facility are not available for years prior to 2001. Therefore, for 1990 through 2000, CO2 emissions from CO2

consumption in commercial applications other than EOR are estimated based on the total annual domestic 

consumption of CO2 in commercial applications other than EOR in 2001 multiplied by the percentage of the total 

CO2 consumed in commercial applications other than EOR that originated from CO2 production at the Jackson 

Dome and Bravo Dome facilities in 2001. The total domestic commercial consumption of CO2 in commercial 

applications other than EOR as reported by the U.S. Census Bureau was about 11,414 thousand metric tons in 2001. 

The total non-EOR CO2 produced from the Jackson Dome and Bravo Dome natural reservoirs in 2001 was about 

820 thousand metric tons, corresponding to 7.2 percent of the total domestic non-EOR commercial CO2

consumption. This 7.2 percent factor was applied to the annual non-EOR commercial CO2 consumption data for the 

years 1990 through 2000 to estimate annual CO2 emissions from non-EOR commercial consumption of CO2

produced from naturally occurring CO2 reservoirs. The remaining 92.8 percent of the total annual non-EOR 

commercial CO2 consumption is assumed to be accounted for in the CO2 emission estimates from other categories 

(e.g., Ammonia Manufacture, CO2 from Fossil Fuel Combustion, Wood Biomass and Ethanol Consumption). 

Non-EOR commercial CO2 consumption data (see Table 4-42) for years 1991 and 1992 were obtained from 

Industry Report 1992 (U.S. Census 1993). Consumption data are not available for 1990, and therefore CO2

consumption data for 1990 is assumed to be equal to that for 1991. Consumption data for 1993 and 1994 were 

obtained from U.S. Census Bureau Manufacturing Profile, 1994 (U.S. Census 1995). Consumption data for 1996 

through 2003 were obtained from the U.S. Census Bureau’s Industry Report, 1996, 1998, 2000, 2002, and 2003

(U.S. Census 1997, 1999, 2001, 2003, 2005). 

Table 4-42: Carbon Dioxide Consumption (Metric Tons) 

Year Metric Tons 

1990 11,997,726 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 156 

1997 11,268,219 

1998 12,716,070 

1999 11,843,386 

2000 13,354,262 

2001 11,413,889 

2002 11,313,478 

2003 11,103,777 

Uncertainty 

Uncertainty exists in the assumption that 92.6 percent of the total domestic CO2 production for commercial 

consumption other than EOR from 1990 through 2000 came from energy and industrial production processes, while 

7.4 percent came from naturally occurring CO2 reservoirs. The allocation for these years is assumed to be the same 

allocation as for 2001, the last year for which data are available to calculate the allocation. 

Uncertainty also exists with respect to the number of facilities that are currently producing CO2 from naturally 

occurring 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. For example, research has identified one ammonia production facility that is recovering CO2 for use 

in EOR. Such CO2 would be assumed to remain sequestered, however, time series data for the amount of recovered 

is not available and therefore all of the CO2 produced by this plant is assumed to be emitted to the atmosphere and is 

allocated to Ammonia Manufacture. Recovery of CO2 from ammonia production facilities for use in EOR is further 

discussed in this chapter under Ammonia Manufacture. 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. 

Uncertainty also exists in the assumption that 100 percent of the CO2 used for EOR is sequestered. Operating 

experience with EOR systems indicates that 100 percent of the CO2 used in EOR applications does not remain 

sequestered, but rather that it may be emitted to the atmosphere as leakage from equipment and reservoirs or 

recovered as a component of the crude oil produced. Potential sources of CO2 emissions from EOR applications 

include leakage from equipment used to produce, transport, compress, and inject the CO2, leakage from equipment 

used to process the crude oil produced, separate the CO2 from the crude oil and recompress and recycle [reinject] 

the CO2 recovered from the crude oil. Other potential sources of CO2 emissions from EOR applications include 

leakage from the reservoir itself, either through migration of the injected CO2 beyond the boundaries of the 

reservoir, chemical interactions between the injected CO2 and the reservoir rock, and leakage via faults, fractures, 

oil and gas well bores, and water wells. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-43. Carbon dioxide 

consumption CO2 emissions were estimated to be between 1.2 and 1.3 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 5 percent 

below to 5 percent above the emission estimate of 1.3 Tg CO2 Eq. 

Table 4-43: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Carbon Dioxide Consumption (Tg 

CO2 Eq. and Percent) 

Source Gas 2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea 

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Carbon Dioxide Consumption CO2 1.3 1.2 1.3 -5% +5% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 157 

Recalculations Discussion 

Carbon dioxide consumption values were updated for 2001 for the Bravo Dome, and for both 2001 and 2002 for the 

Jackson Dome. For the Bravo Dome, updated values are based on new production data from the facility. For 

Jackson Dome, previous production data was based on fourth quarter reporting by Denbury Resources, which was 

annualized for the entire year. Updated production values are based on annual production numbers reported by 

Denbury Resources. For 2001, updated production values resulted in a 4 percent decrease in emissions, and for 

2002 updated production values resulting in a 23 percent decrease in emissions. Based on updated 2001 

consumption values for Jackson Dome, the percent of CO2 emissions from CO2 consumption in commercial 

applications other than EOR applied to years 1990 through 2000 decreased by 0.2 percent relative to the percent 

previously assumed. These changes resulted in an average annual decrease in CO2 emissions from CO2

consumption of less than 0.1 Tg CO2 Eq. (5.1 percent) for the years 1990 through 2002. 

4.11. Petrochemical Production (IPCC Source Category 2B5) 

The production of some petrochemicals results in the release of small amounts of CH4 and CO2 emissions. 

Petrochemicals are chemicals isolated or derived from petroleum or natural gas. Methane emissions are presented 

here from the production of carbon black, ethylene, ethylene dichloride, styrene, and methanol, while CO2

emissions are presented here for only carbon black production. The CO2 emissions from petrochemical processes 

other than carbon black are currently included in the Carbon Stored in Products from Non-Energy Uses of Fossil 

Fuels Section of the Energy chapter. The CO2 from carbon black production is included here to allow for the direct 

reporting of CO2 emissions from the process and direct accounting of the feedstocks used in the process. 

Carbon black is an intensely black powder generated by the incomplete combustion of an aromatic petroleum or 

coal-based feedstock. Most carbon black produced in the United States is added to rubber to impart strength and 

abrasion resistance, and the tire industry is by far the largest consumer. Ethylene is consumed in the production 

processes of the plastics industry including polymers such as high, low, and linear low density polyethylene (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. Styrene is a common precursor for many plastics, rubber, and resins. 

It can be found in many construction products, such as foam insulation, vinyl flooring, and epoxy adhesives. 

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. The United States produces close to one quarter of the world’s supply of methanol. 

Emissions of CO2 and CH4 from petrochemical production in 2003 were 2.8 Tg CO2 Eq. (2,777 Gg) and 1.5 Tg CO2

Eq. (72 Gg), respectively (see Table 4-44 and Table 4-45). While emissions of CO2 from carbon black production 

in 2003 decreased by three percent from the previous year, there has been an overall increase in CO2 emissions from 

carbon black production of 25 percent since 1990. Methane emissions from petrochemical production decreased by 

less than one percent from the previous year and increased 30 percent since 1990. 

Table 4-44: CO2 and CH4 Emissions from Petrochemical Production (Tg CO2 Eq.) 

Year 1990 1997 1998 1999 2000 2001 2002 2003

CO2 2.2 2.9 3.0 3.1 3.0 2.8 2.9 2.8

CH4 1.2 1.6 1.7 1.7 1.7 1.4 1.5 1.5

Total 3.4 4.6 4.7 4.8 4.7 4.2 4.4 4.3

Table 4-45: CO2 and CH4 Emissions from Petrochemical Production (Gg) 

Year 1990 1997 1998 1999 2000 2001 2002 2003

CO2 2,221 2,919 3,015 3,054 3,004 2,787 2,857 2,777

CH4 56 78 80 81 80 68 72 72
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 158 

Methodology 

Emissions of CH4 were calculated by multiplying annual estimates of chemical production by the appropriate 

emission factor, as follows: 11 kg CH4/metric ton carbon black, 1 kg CH4/metric ton ethylene, 0.4 kg CH4/metric 

ton ethylene dichloride,12 4 kg CH4/metric ton styrene, and 2 kg CH4/metric ton methanol. Although the production 

of other chemicals may also result in CH4 emissions, there were not sufficient data available to estimate their 

emissions. 

Emission factors were taken from the Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). Annual 

production data for 1990 (see Table 4-46) were obtained from the Chemical Manufacturer’s Association Statistical 

Handbook (CMA 1999). Production data for 1991 through 2003 were obtained from the American Chemistry 

Council’s Guide to the Business of Chemistry (2003). 

Table 4-46: Production of Selected Petrochemicals (Thousand Metric Tons) 

Chemical 1990 1997 1998 1999 2000 2001 2002 2003 

Carbon Black 1,307 1,719 1,775 1,798 1,769 1,641 1,682 1,635 

Ethylene 16,542 23,088 23,474 25,118 24,971 22,521 23,623 22,957 

Ethylene Dichloride 6,282 10,324 11,080 10,308 9,866 9,294 9,288 9,952 

Styrene 3,637 5,171 5,183 5,410 5,420 4,277 4,974 5,239 

Methanol 3,785 5,743 5,860 5,303 4,876 3,402 3,289 3,166 

Almost all carbon black in the United States is produced from petroleum-based or coal-based feedstocks using the 

“furnace black” process (European IPPC Bureau 2004). The furnace black process is a partial combustion process 

in which a portion of the carbon black feedstock is combusted to provide energy to the process. Carbon black is 

also produced in the United States by the thermal cracking of acetylene-containing feedstocks (“acetylene black 

process”) and by the thermal cracking of other hydrocarbons (“thermal black process”). One U.S. carbon black 

plant produces carbon black using the thermal black process, and one U.S. carbon black plant produces carbon 

black using the acetylene black process (The Innovation Group 2004). 

The furnace black process produces carbon black from “carbon black feedstock” (also referred to as “carbon black 

oil”), which is a heavy aromatic oil that may be derived as a byproduct of either the petroleum refining process or 

the metallurgical (coal) coke production process. For the production of both petroleum-derived and coal-derived 

carbon black, the “primary feedstock” (i.e., carbon black feedstock) is injected into a furnace that is heated by a 

“secondary feedstock” (generally natural gas). Both the natural gas secondary feedstock and a portion of the carbon 

black feedstock are oxidized to provide heat to the production process and pyrolyze the remaining carbon black 

feedstock to carbon black. The “tail gas” from the furnace black process contains CO2, carbon monoxide, sulfur 

compounds, CH4, and non-methane volatile organic compounds. A portion of the tail gas is generally burned for 

energy recovery to heat the downstream carbon black product dryers. The remaining tail gas may also be burned 

for energy recovery, flared, or vented uncontrolled to the atmosphere. 

The calculation of the carbon lost during the production process is the basis for determining the amount of CO2

released during the process. The carbon content of national carbon black production is subtracted from the total 

amount of carbon contained in primary and secondary carbon black feedstock to find the amount of carbon lost 

during the production process. It is assumed that the carbon lost in this process is emitted to the atmosphere as 

either CH4 or CO2. The carbon content of the CH4 emissions, estimated as described above, is subtracted from the 

total carbon lost in the process to calculate the amount of carbon emitted as CO2. The total amount of primary and 

secondary carbon black feedstock consumed in the process (see Table 4-47) 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 carbon black feedstock consumption factor for U.S. carbon black production is 1.43 metric 

 

12 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 (C2H4Cl2) rather than dichloroethylene (C2H2Cl2). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 159 

tons of carbon black feedstock consumed per metric ton of carbon black produced. The average natural gas 

consumption factor for U.S. carbon black production is 341 normal cubic meters of natural gas consumed per metric 

ton of carbon black produced. The amount of carbon contained in the primary and secondary feedstocks is 

calculated by applying the respective carbon contents of the feedstocks to the respective levels of feedstock 

consumption. 

Table 4-47: Carbon Black Feedstock (Primary Feedstock) and Natural Gas Feedstock (Secondary Feedstock) 

Consumption (Thousand Metric Tons) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003 

Primary Feedstock 1,864 2,450 2,530 2,563 2,521 2,339 2,398 2,331 

Secondary Feedstock 302 378 397 410 415 408 379 388 

For the purposes of emissions estimation, 100 percent of the primary carbon black feedstock is assumed to be 

derived from petroleum refining byproducts. Carbon black feedstock derived from metallurgical (coal) coke 

production (e.g., creosote oil) is also used for carbon black production; however, no data are available concerning 

the annual consumption of coal-derived carbon black feedstock. Carbon black feedstock derived from petroleum 

refining byproducts is assumed to be 89 percent elemental carbon (Srivastava et al. 1999). It is assumed that 100 

percent of the tail gas produced from the carbon 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 carbon black because of the lack of data concerning the relatively small amount of carbon black 

produced using the acetylene black and thermal black processes. The carbon black produced from the furnace black 

process is assumed to be 97 percent elemental carbon (Othmer et al. 1992). 

Uncertainty 

The CH4 emission factors used for petrochemical production are based on a limited number of studies. Using plantspecific factors instead of average factors could increase the accuracy of the emission estimates; however, such data 

were not available. There may also be other significant sources of CH4 arising from petrochemical production 

activities that have not been included in these estimates. 

The results of the quantitative uncertainty analysis for the CO2 emissions from carbon black production calculation 

are based on feedstock consumption, import and export data, and carbon black production data. The composition of 

carbon black feedstock varies depending upon the specific refinery production process, and therefore the 

assumption that carbon black feedstock is 89 percent carbon gives rise to uncertainty. Also, no data are available 

concerning the consumption of coal-derived carbon black feedstock, so CO2 emissions from the utilization of coalbased feedstock are not included in the emission estimate. In addition, other data sources indicate that the amount 

of petroleum-based feedstock used in carbon black production may be underreported by the U.S. Census Bureau. 

Finally, the amount of carbon black produced from the thermal black process and acetylene black process, although 

estimated to be a small percentage of the total production, is not known. Therefore, there is some uncertainty 

associated with the assumption that all of the carbon black is produced using the furnace black process. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-48. Petrochemical production 

CH4 emissions were estimated to be between 1.4 and 1.6 Tg CO2 Eq. at the 95 percent confidence level (or in 19 out 

of 20 Monte Carlo Stochastic Simulations). This indicates a range of approximately 7 percent below to 7 percent 

above the emission estimate of 1.5 Tg CO2 Eq. Petrochemical production CO2 emissions were estimated to be 

between 2.8 and 3.1 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 10 percent below to 10 percent above the emission estimate 

of 2.8Tg CO2 Eq.

Table 4-48: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Petrochemical Production and CO2

Emissions from Carbon Black Production (Tg CO2 Eq. and Percent) 

Source Gas 2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 160 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Petrochemical Production CH4 1.5 1.4 1.6 -7% +7% 

Petrochemical Production CO2 2.8 2.5 3.1 -10% +10% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

In previous inventories, CO2 emissions from carbon black production were not calculated and reported separately in 

the Industrial Processes sector, but were included in the Carbon Stored in Products from Non-Energy Uses of Fossil 

Fuels in the Energy sector. Although the CH4 emissions from petrochemical production did not change for 1990 

through 2002 compared to the previous Inventory, the addition of CO2 emissions from carbon black production 

caused a large increase in petrochemical production emissions for every year of the time series. Overall, the change 

resulted in an average annual increase of 2.7 Tg CO2 Eq. (183 percent) in combined CO2 and CH4 emissions from 

petrochemical production for the period 1990 through 2002. 

4.12. Silicon Carbide Production (IPCC Source Category 2B4) 

Methane is emitted from the production of silicon carbide, a material used as an industrial abrasive. To make 

silicon carbide (SiC), quartz (SiO2) is reacted with carbon in the form of petroleum coke. During this reaction, 

methane is produced from volatile compounds in the petroleum coke. While CO2 is also emitted from the 

production process, the requisite data were unavailable for these calculations. CO2 emissions associated with the 

use of petroleum coke in the silicon carbide process are accounted for in the Non-energy Uses of Fossil Fuels 

section in the Energy Chapter. Emissions of CH4 from silicon carbide production in 2003 were 0.4 Gg CH4 (0.01 

Tg CO2 Eq.) (see Table 4-49). 

Table 4-49: CH4 Emissions from Silicon Carbide Production (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 + 1 

1997 + 1 

1998 + 1 

1999 + 1 

2000 + 1 

2001 + + 

2002 + + 

2003 + + 

+ Does not exceed 0.05 Tg CO2 Eq. or 0.5 Gg 

Methodology 

Emissions of CH4 were calculated by multiplying annual silicon carbide production by an emission factor (11.6 kg 

CH4/metric ton silicon carbide). This emission factor was derived empirically from measurements taken at 

Norwegian silicon carbide plants (IPCC/UNEP/OECD/IEA 1997). 

Production data for 1990 through 2003 (see Table 4-50) were obtained from the Minerals Yearbook: Volume IMetals and Minerals, Manufactured Abrasives (USGS 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 

2000, 2001, 2002, 2003, 2004). 

Table 4-50: Production of Silicon Carbide (Metric Tons) 

Year Metric Tons 

1990 105,000 

1991 78,900 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 161 

1992 84,300 

1993 74,900 

1994 84,700 

1995 75,400 

1996 73,600 

1997 68,200 

1998 69,800 

1999 65,000 

2000 45,000 

2001 40,000 

2002 30,000 

2003 35,000 

Uncertainty 

The emission factor used for silicon carbide production was based on one study of Norwegian plants. The 

applicability of this factor to average U.S. practices at silicon carbide plants is uncertain. 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. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-51. Silicon carbide production 

CO2 emissions were estimated to be between 0.008 and 0.01Tg CO2 Eq. at the 95 percent confidence level (or in 19 

out of 20 Monte Carlo Stochastic Simulations). This indicates a range of approximately 10 percent below to 10 

percent above the emission estimate of 0.009 Tg CO2 Eq.

Table 4-51: Tier 2 Quantitative Uncertainty Estimates for CO2 Emissions from Silicon Carbide Production (Tg CO2

Eq. and Percent) 

Source Gas 2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Silicon Carbide Production CO2 + + + -10% +10% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

+ Does not exceed 0.05 Tg CO2 Eq. or 0.5 Gg. 

4.13. 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 NO and NO2 (i.e., NOx). As such, the industry uses a combination of 

non-selective catalytic reduction (NSCR) and selective catalytic reduction (SCR) technologies. In the process of 

destroying NOx, NSCR systems are also very effective at destroying N2O. However, NSCR units are generally not 

preferred in modern plants because of high energy costs and associated high gas temperatures. NSCRs were widely 

installed in nitric plants built between 1971 and 1977. Approximately 20 percent of nitric acid plants use NSCR 

(Choe et al. 1993). The remaining 80 percent use SCR or extended absorption, neither of which is known to reduce 

N2O emissions. 

Nitrous oxide emissions from this source were estimated to be 15.8 Tg CO2 Eq. (51.1 Gg) in 2003 (see Table 4-52). 

Emissions from nitric acid production have decreased by 11 percent since 1990, with the trend in the time series 

closely tracking the changes in production. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 162 

Table 4-52: N2O Emissions from Nitric Acid Production (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 17.8 58 

1997 21.2 68 

1998 20.9 67 

1999 20.1 65 

2000 19.6 63 

2001 15.9 51 

2002 17.2 56 

2003 15.8 51 

Methodology 

Nitrous oxide emissions were calculated by multiplying nitric acid production by the amount of N2O emitted per 

unit of nitric acid produced. The emission factor was determined as a weighted average of 2 kg N2O / metric ton 

HNO3 for plants using non-selective catalytic reduction (NSCR) systems and 9.5 kg N2O / metric ton HNO3 for 

plants not equipped with NSCR (Choe et al. 1993). In the process of destroying NOx, NSCR systems destroy 80 to 

90 percent of the N2O, which is accounted for in the emission factor of 2 kg N2O / metric ton HNO3. An estimated 

20 percent of HNO3 plants in the United States are equipped with NSCR (Choe et al. 1993). Hence, the emission 

factor is equal to (9.5 × 0.80) + (2 × 0.20) = 8 kg N2O per metric ton HNO3. 

Nitric acid production data for 1990 (see Table 4-53) was obtained from Chemical and Engineering News, “Facts 

and Figures” (C&EN 2001). Nitric acid production data for 1991 through 1992 (see Table 4-53) were obtained 

from Chemical and Engineering News, “Facts and Figures” (C&EN 2002). Nitric acid production data for 1993 

through 2003 were obtained from Chemical and Engineering News, “Facts and Figures” (C&EN 2004). The 

emission factor range was taken from Choe et al. (1993). 

Table 4-53: Nitric Acid Production (Gg) 

Year Gg 

1990 7,196 

1991 7,191 

1992 7,379 

1993 7,486 

1994 7,904 

1995 8,018 

1996 8,349 

1997 8,556 

1998 8,421 

1999 8,113 

2000 7,898 

2001 6,416 

2002 6,939 

2003 6,388 

Uncertainty 

The uncertainties contained in these estimates are primarily due to the current organization within the nitric acid 

industry. A significant degree of uncertainty exists in nitric acid production figures because nitric acid plants are 

often part of larger production facilities, such as fertilizer or explosives manufacturing. As a result, only a small 

quantity of nitric acid is sold on the market, making production quantities difficult to track. Emission factors are 

also difficult to determine because of the large number of plants using a diverse range of technologies. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 163 

The results of the Tier 1 quantitative uncertainly analysis are summarized in Table 4-54. Nitric acid production N2O 

emissions were estimated to be between 13.2 and 18.5 Tg CO2 Eq. at the 95 percent confidence level. This 

indicates a range of 17 percent above to below the 2003 emission estimate of 15.8 Tg CO2 Eq. 

Table 4-54: Tier 1 Quantitative Uncertainty Estimates for N2O Emissions from Nitric Acid Production (Tg CO2 Eq. 

and Percent) 

Source Gas 

Year 2003 

Emissions 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Emission Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

Nitric Acid Production N2O 15.8 17% 13.2 18.5 

Recalculations Discussion 

The nitric acid production values for all years 1993 through 2002 were updated using newly published figures 

(C&EN 2004). Published figures remained consistent for all years of the historical time series except 2002. The 

updated production data for 2002 resulted in an increase of 0.5 Tg CO2 Eq. (2.8 percent) in N2O emissions from 

nitric acid production for that year. 

Planned Improvements 

Planned improvements are focused on assessing the plant-by-plant implementation of NOx abatement technologies 

to more accurately match plant production capacities to appropriate emission factors, instead of using a national 

profiling of abatement implementation. Also, any large scale updates to abatement configurations would be useful 

in revising the national profile. 

4.14. 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 is the major producer, with three companies in four locations accounting for approximately one-third 

of world production. Adipic acid is a white crystalline solid used in the manufacture of synthetic fibers, coatings, 

plastics, urethane foams, elastomers, and synthetic lubricants. Commercially, it is the most important of the 

aliphatic dicarboxylic acids, which are used to manufacture polyesters. Approximately 90 percent of all adipic acid 

produced in the United States is used in the production of nylon 6,6 (CMR 2001). Food grade adipic acid is also 

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. Nitrous oxide 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.13 Only one small plant, representing approximately two percent of production, does not control 

for N2O (Reimer 1999). 

Nitrous oxide emissions from this adipic acid production were estimated to be 6.0 Tg CO2 Eq. (19.4 Gg) in 2003 

(see Table 4-55). 

 

13During 1997, the N2O emission controls installed by the third plant operated for approximately a quarter of the year. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 164 

Table 4-55: N2O Emissions from Adipic Acid Production (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 15.2 49 

1997 10.3 33 

1998 6.0 19 

1999 5.5 18 

2000 6.0 20 

2001 4.9 16 

2002 5.9 19 

2003 6.0 19 

National adipic acid production has increased by approximately 28 percent over the period of 1990 through 2003, to 

approximately one million metric tons. At the same time, emissions have been significantly reduced due to the 

widespread installation of pollution control measures. 

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. Measured 

estimates for 2003 were unavailable and, thus, were calculated by applying a 1.8 percent production growth rate 

representative of the industry (see discussion below on sources of production data). For the other two plants, N2O 

emissions were calculated by multiplying adipic acid production by an emission factor (i.e., N2O emitted per unit of 

adipic acid produced) and adjusting for the actual 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 MT of N2O per MT of product (Thiemens and Trogler 1991). 

Emissions are estimated using the following equation: 

N2O emissions = [production of adipic acid (MT of adipic acid)] × [0.3 MT N2O / mt adipic acid] × [1 – 

(N2O destruction factor × 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. The N2O abatement system destruction factor is assumed to be 95 percent for catalytic abatement and 

98 percent for thermal abatement (Reimer et al. 1999, Reimer 1999). For the one plant that uses thermal 

destruction and for which no reported plant-specific emissions are available, the abatement system utility factor is 

assumed to be 98 percent. 

In order to calculate emissions for the two plants where direct emissions measurements were not available, plantspecific production data needed to be estimated since it was unavailable due to reasons of confidentiality. 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. 

National adipic acid production data (see Table 4-56) for 1990 through 2002 were obtained from the American 

Chemistry Council (ACC 2003). Production Data for 2003 were estimated based on an abstract from a Chemical 

Economics Handbook report entitled “Adipic Acid” indicating that production will increase by an annual average of 

1.8 percent from year 2002 to 2006. Plant capacity data for 1990 through 1994 were obtained from Chemical and 

Engineering News, “Facts and Figures” and “Production of Top 50 Chemicals” (C&EN 1992, 1993, 1994, 1995). 

Plant capacity data for 1995 and 1996 were kept the same as 1994 data. The 1997 plant capacity data were taken 

from Chemical Market Reporter “Chemical Profile: Adipic Acid” (CMR 1998). The 1998 plant capacity data for 

all four plants and 1999 plant capacity data for three of the plants were obtained from Chemical Week, Product 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 165 

focus: adipic acid/adiponitrile (CW 1999). Plant capacity data for 2000 for three of the plants were updated using 

Chemical Market Reporter, “Chemical Profile: Adipic Acid” (CMR 2001). For 2001 through 2003, the plant 

capacities for these three plants were kept the same as the year 2000 capacities. Plant capacity data for 1999 to 

2003 for the one remaining plant was kept the same as 1998. 

Table 4-56: Adipic Acid Production (Gg) 

Year Gg 

1990 735 

1991 708 

1992 724 

1993 769 

1994 821 

1995 830 

1996 839 

1997 871 

1998 862 

 1999 907 

2000 925 

2001 835 

2002 921 

2003 937 

Uncertainty 

The emission factor for adipic acid was based on experiments (Thiemens and Trogler 1991) that attempt to replicate 

the industrial process and, thereby, measure the reaction stoichiometry for N2O production in the preparation of 

adipic acid. However, the extent to which the lab results are representative of actual industrial emission rates is not 

known. 

The allocation of national production data for the two facilities where direct emission measurements were 

unavailable creates a degree of uncertainty in the adipic acid production data as all plants are assumed to operate at 

equivalent utilization levels as represented by their capacities. Also, plant capacity reference data is inconsistently 

available from year to year, which can affect the uncertainty of the allocated production through the time series. 

A 5 percent uncertainty level was associated with the activity data available for the two plants that reported 

emissions. For the remaining two plants, a 20 percent uncertainty level was assumed for production. The emission 

factor uncertainty for each of these two plants was estimated separately to account for the differences in the use of 

abatement technologies. For the plant that uses no abatement technology, a 10 percent IPCC-default emission factor 

uncertainty was assumed appropriate. The abatement factor uncertainty used for the second plant was based on a 5 

percent IPCC estimate for the N2O destruction factor and an assumed 5 percent uncertainty in the abatement system 

utility factor (IPCC 2000). These two estimates result in an overall uncertainty associated with abatement potential 

of 7 percent. This abatement uncertainty, combined with the 10 percent IPCC default uncertainty value associated 

with the emissions factor for unabated emissions, results in an overall 12 percent emission factor uncertainty. 

Combining the reporting plants emissions uncertainty with the activity data uncertainty and the emission factor 

uncertainty for the remaining two plants yields an overall uncertainty for the inventory estimate equal to 9 percent 

of 2003 emissions (see Table 4-57). 

The results of the Tier 1 quantitative uncertainly analysis are summarized in Table 4-57. Adipic acid production 

N2O emissions were estimated to be between 5.5 and 6.5 Tg CO2 Eq. at the 95 percent confidence level. This 

indicates a range of 9 percent above to below the 2003 emission estimate of 6.0 Tg CO2 Eq. 

Table 4-57: Tier 1 Quantitative Uncertainty Estimates for N2O Emissions from Adipic Acid Production (Tg CO2

Eq. and Percent) 

Source Gas Year 2003 Uncertainty Uncertainty Range Relative to 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 166 

Emissions 

(Tg CO2 Eq.) 

(%) 2003 Emission Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

Adipic Acid Production N2O 6.0 9% 5.5 6.5 

QA/QC and Verification 

In addition to performing Tier 1 level QA/QC and verification, trends in the production of the synthetic nylon fibers 

industry were compared to trends in adipic acid production to confirm a reasonable agreement, since almost 90 

percent of the adipic acid produced in the United States is used in the production of nylon 6,6. 

Planned Improvements 

Improvement efforts will be focused on obtaining direct measurement data from the remaining two plants when and 

if they become available. If they become available, cross verification with top-down approaches will provide a 

useful Tier 2 level QA 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. 

4.15. Substitution of Ozone Depleting Substances (IPCC Source Category 2F) 

Hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs) are used as alternatives to several classes of ozonedepleting substances (ODSs) that are being phased out under the terms of the Montreal Protocol and the Clean Air 

Act Amendments of 1990.14 Ozone depleting substances⎯chlorofluorocarbons (CFCs), halons, carbon 

tetrachloride, methyl chloroform, and hydrochlorofluorocarbons (HCFCs)⎯are used in a variety of industrial 

applications including refrigeration and air conditioning equipment, solvent cleaning, foam production, sterilization, 

fire extinguishing, and aerosols. Although HFCs and PFCs, are not harmful to the stratospheric ozone layer, they 

are potent greenhouse gases. Emission estimates for HFCs and PFCs used as substitutes for ODSs are provided in 

Table 4-58 and Table 4-59. 

Table 4-58: Emissions of HFCs and PFCs from ODS Substitution (Tg CO2 Eq.) 

Gas 1990 1997 1998 1999 2000 2001 2002 2003 

HFC-23 + + + 0.1 0.1 0.1 0.1 0.1 

HFC-32 + 0.2 0.3 0.3 0.3 0.3 0.3 0.4 

HFC-125 + 7.0 8.8 10.0 11.2 12.3 13.4 14.7 

HFC-134a + 31.4 36.7 42.2 48.0 52.7 56.9 60.5 

HFC-143a + 3.5 5.2 6.6 8.2 10.1 12.2 14.6 

HFC-236fa + 0.1 0.4 0.9 1.4 1.8 2.1 2.3 

CF4 + + + + + + + + 

Others* 0.4 4.2 5.2 5.7 5.9 6.1 6.5 6.9 

Total 0.4 46.5 56.6 65.8 75.0 83.3 91.5 99.5 

+ Does not exceed 0.05 Tg CO2 Eq. 

* Others include HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee, and PFC/PFPEs, the latter being a proxy for a diverse 

collection of PFCs and perfluoropolyethers (PFPEs) employed for solvent applications. For estimating purposes, the GWP value 

used for PFC/PFPEs was based upon C6F14. 

Note: Totals may not sum due to independent rounding. 

Table 4-59: Emissions of HFCs and PFCs from ODS Substitution (Mg) 

 

14 [42 U.S.C § 7671, CAA § 601] 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 167 

Gas 1990 1997 1998 1999 2000 2001 2002 2003 

HFC-23 + 3 4 4 5 5 6 6 

HFC-32 + 289 430 439 441 459 492 541 

HFC-125 + 2,516 3,134 3,571 4,004 4,385 4,777 5,246 

HFC-134a + 24,136 28,202 32,491 36,888 40,512 43,798 46,559 

HFC-143a + 926 1,369 1,738 2,162 2,647 3,203 3,834 

HFC-236fa + 9 64 142 214 281 341 369 

CF4 + + 1 1 1 1 2 2 

Others* M M M M M M M M 

M (Mixture of Gases) 

+ Does not exceed 0.5 Mg 

* Others include HFC-152a, HFC-227ea, HFC-245fa, HFC-4310mee and PFC/PFPEs, the latter being a proxy for a diverse 

collection of PFCs and perfluoropolyethers (PFPEs) employed for solvent applications. 

In 1990 and 1991, the only significant emissions of HFCs and PFCs as substitutes to ODSs were relatively small 

amounts of HFC-152a⎯a component of the refrigerant blend R-500 used in chillers⎯and HFC-134a in 

refrigeration end-uses. Beginning in 1992, HFC-134a was used in growing amounts as a refrigerant in motor 

vehicle air-conditioners and in refrigerant blends such as R-404A.15 In 1993, the use of HFCs in foam production 

and as an aerosol propellant 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 phasedout. 

The use and subsequent emissions of HFCs and PFCs as ODS substitutes has been increasing from small amounts in 

1990 to 99.5 Tg CO2 Eq. in 2003. This increase was in large part the result of efforts to phase out CFCs and other 

ODSs in the United States. In the short term, this trend is expected to continue, and will likely accelerate over the 

next decade as HCFCs, which are interim substitutes in many applications, are themselves phased-out under the 

provisions of the Copenhagen Amendments to the Montreal Protocol. Improvements in the technologies associated 

with the use of these gases and the introduction of alternative gases and technologies, however, may help to offset 

this anticipated increase in emissions. 

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 40 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/UNEP/OECD/IEA (1997). Though the model is more 

comprehensive than the IPCC default methodology, significant uncertainties still exist with regard to the levels of 

 

15 R-404A contains HFC-125, HFC-143a, and HFC-134a. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 168 

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 40 end-uses, but the uncertainty estimation was performed on 

only the top 14 end-uses, which account for 95 percent of emissions from this source category. 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, which necessitated these 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 results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-60. Substitution of ozone 

depleting substances HFC and PFC emissions were estimated to be between 89.9 and 108.4 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 10 percent below to 10 percent above the emission estimate of 99.5 Tg CO2 Eq. 

Table 4-60: Tier 2 Quantitative Uncertainty Estimates for HFC and PFC Emissions from ODS Substitution (Tg 

CO2 Eq. and Percent) 

Source Gases 

2003 

Emission 

Estimate 

Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Substitution of Ozone 

Depleting Substances 

HFCs and 

PFCs 99.5 89.9 108.4 -10% +9% a

 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 increase of less than 0.1 Tg CO2 Eq. (4.1 percent) in HFC and PFCemissions from 

the substitution of ozone depleting substances for the period 1990 through 2002. 

4.16. 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 rose significantly as 

HCFC-22 replaced chlorofluorocarbons (CFCs) in many applications. Since 2000, however, U.S. production has 

declined to levels near those of the early to mid 1990s. 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.16 

Feedstock production, however, is permitted to continue indefinitely. 

 

16 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] 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 169 

HCFC-22 is produced by the reaction of chloroform (CHCl3) 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 (CHCl2F), HCFC-22 

(CHClF2), HFC-23 (CHF3), HCl, 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, HCl and residual HF. The 

HCl is recovered as a useful byproduct, and the HF is removed. Once separated from HCFC-22, the HFC-23 is 

generally vented to the atmosphere as an unwanted by-product, or may be captured for use in a limited number of 

applications. 

Emissions of HFC-23 in 2003 were estimated to be 12.3 Tg CO2 Eq. (1.1 Gg). This quantity represents a 38 percent 

decline from 2002 emissions and a 65 percent decline from 1990 emissions. Both declines are primarily due to the 

steady decline in the emission rate of HFC-23 (i.e., the amount of HFC-23 emitted per kilogram of HCFC-22 

manufactured). Three HCFC-22 production plants operated in the United States in 2003, two of which used thermal 

oxidation to significantly lower their HFC-23 emissions. 

Table 4-61: HFC-23 Emissions from HCFC-22 Production (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 35.0 3 

1997 30.0 3 

1998 40.1 3 

1999 30.4 3 

2000 29.8 3 

2001 19.8 2 

2002 19.8 2 

2003 12.3 1 

Methodology 

The methodology employed for estimating emissions is based upon measurements at individual HCFC-22 

production plants. 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. The other plants periodically measure HFC-23 

concentrations in the output stream using gas chromatography. This information is combined with information on 

quantities of critical feed components (e.g., HF) and/or products (HCFC-22) to estimate HFC-23 emissions using a 

material balance approach. HFC-23 concentrations are determined at the point the gas leaves the chemical reactor; 

therefore, estimates also include fugitive emissions. 

Production data and emission estimates were prepared in cooperation with the U.S. manufacturers of HCFC-22 

(ARAP 1997, 1999, 2000, 2001, 2002, 2003, 2004; RTI 1997). Annual estimates of U.S. HCFC-22 production are 

presented in Table 4-62. 

Table 4-62: HCFC-22 Production (Gg) 

Year Gg 

1990 138.9 

1991 142.7 

1992 149.6 

1993 132.4 

1994 146.8 

1995 154.7 

1996 166.1 

1997 164.5 

1998 182.8 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 170 

1999 165.5 

2000 186.9 

2001 152.4 

2002 144.2 

2003 138.0 

Uncertainty 

A high level of confidence has been attributed to the HFC-23 concentration data employed because measurements 

were conducted frequently and accounted for day-to-day and process variability. The results of the Tier 1 

quantitative uncertainly analysis are summarized in Table 4-63. HCFC-22 production HFC-23 emissions were 

estimated to be between 11.1 and 13.6 Tg CO2 Eq. at the 95 percent confidence level. This indicates a range of 10 

percent above and below the 2003 emission estimate of 12.3 Tg CO2 Eq. 

Table 4-63: Tier 1 Quantitative Uncertainty Estimates for HFC-23 Emissions from HCFC-22 Production (Tg CO2

Eq. and Percent) 

Source Gas 

Year 2003 

Emissions 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Emission Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

HCFC-22 Production HFC-23 12.3 10% 11.1 13.6 

Recalculations Discussion 

The historical time series was adjusted to fully reflect reports from the Alliance for Responsible Atmospheric 

Policy. These changes resulted in an average annual decrease of less than 0.1 Tg CO2 Eq. (0.01 percent) in HFC

emissions from HCFC-22 through the period 1990 through 2002. 

4.17. Electrical Transmission and Distribution (IPCC Source Category 2F7) 

The largest use of SF6, both in the United States and internationally, is as an electrical insulator and interrupter in 

equipment that transmits and distributes electricity (RAND 2002). The gas has been employed by the electric 

power industry in the United States since the 1950s because of its dielectric strength and arc-quenching 

characteristics. It is used in gas-insulated substations, circuit breakers, and other switchgear. Sulfur hexafluoride 

has replaced flammable insulating oils in many applications and allows for more compact substations in dense urban 

areas. 

Fugitive emissions of SF6 can escape from gas-insulated substations and switch gear through seals, especially from 

older equipment. The gas can also be released during equipment manufacturing, installation, servicing, and 

disposal. Emissions of SF6 from electrical transmission and distribution systems were estimated to be 14.1 Tg CO2

Eq. (0.6 Gg) in 2003. This quantity represents a 52 percent decrease from the estimate for 1990 (see Table 4-64 and 

Table 4-65). This decrease, which is reflected in the atmospheric record (Maiss and Brenninkmeijer 2000), is 

believed to be a response to increases in the price of SF6 during the 1990s and to growing awareness of the 

environmental impact of SF6 emissions, through programs such as the EPA’s SF6 Emission Reduction Partnership 

for Electric Power Systems. 

Table 4-64: SF6 Emissions from Electric Power Systems and Original Equipment Manufactures (Tg CO2 Eq.) 

Year Electric Power 

Systems 

Original Equipment 

Manufacturers 

Total 

1990 28.9 0.3 29.2 

1997 21.3 0.3 21.7 

1998 16.7 0.4 17.1 

1999 15.8 0.6 16.4 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 171 

2000 15.0 0.7 15.6 

2001 14.7 0.7 15.4 

2002 14.0 0.7 14.7 

2003 13.4 0.7 14.1 

Table 4-65: SF6 Emissions from Electric Power Systems and Original Equipment Manufacturers (Gg) 

Year Emissions 

1990 1.2 

1997 0.9 

1998 0.7 

1999 0.7 

2000 0.7 

2001 0.6 

2002 0.6 

2003 0.6 

Methodology 

The estimates of emissions from electric transmission and distribution are comprised of emissions from electric 

power systems and emissions from the manufacture of electrical equipment. The methodologies for estimating both 

sets of emissions are described below. 

1999 to 2003 Emissions from Electric Power Systems 

Emissions from electric power systems from 1999 to 2003 were estimated based on (1) reporting from utilities 

participating in EPA’s SF6 Emissions Reduction Partnership for Electric Power Systems, which began in 1999, and 

(2) utilities’ transmission miles as reported in the 2001 and 2004 Utility Data Institute (UDI) Directories of Electric 

Power Producers and Distributors (UDI 2001, 2004). (Transmission miles are defined as the miles of lines carrying 

voltages above 34.5 kV.) Between 1999 and 2003, participating utilities represented between 31 percent and 35 

percent of total U.S. transmission miles. The emissions reported by participating utilities each year were added to 

the emissions estimated for non-reporting utilities in that year. Emissions from non-reporting utilities were 

estimated using the results of a regression analysis that showed that the emissions of reporting utilities were most 

strongly correlated with their transmission miles. As described further below, the transmission miles of the various 

types of non-reporting utilities were multiplied by the appropriate regression coefficients, yielding an estimate of 

emissions. Transmission miles are clearly physically related to emissions, since in the United States, SF6 is 

contained primarily in transmission equipment rated at or above 34.5 kV. 

The regression equations reflect two distinctions among non-reporting utilities: (1) between small and large utilities 

(i.e., with less or more than 10,000 transmission miles, respectively), and (2) between utilities that do not participate 

in the SF6 Emission Reduction Partnership (non-partners) and those that participate but that have not reported in a 

given year (non-reporting partners). (Historically, these non-reporting partners have accounted for 5 percent or less 

of total estimated partner emissions.) The distinction between small and large utilities was made because the 

regression analysis showed that the relationship between emissions and transmission miles differed for small and 

large facilities. The distinction between non-partners and non-reporting partners was made because the emission 

trends of these two groups were believed to be different. Reporting partners have reduced their emission rates 

significantly since 1999. The emission trend of non-reporting partners was believed to be similar to that of the 

reporting partners, because all partners commit to reducing SF6 emissions through technically and economically 

feasible means. However, non-partners were assumed not to have implemented any changes that would have 

reduced emissions over time. 

To estimate emissions from non-partners in every year since 1999, the following regression equations were used. 

These equations were developed based on the 1999 SF6 emissions reported by 49 partner utilities (representing 

approximately 31 percent of U.S. transmission miles), and 2000 transmission mileage data obtained from the 2001 

UDI Directory of Electric Power Producers and Distributors (UDI 2001): 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 172 

Non-partner small utilities (less than 10,000 transmission miles, in kilograms): 

Emissions = 0.874 × Transmission Miles 

Non-partner large utilities (more than 10,000 transmission miles, in kilograms): 

Emissions = 0.558 × Transmission Miles 

To estimate emissions from non-reporting partners in each year, the regression equations based on the emissions 

reported by partners in that year were used. To estimate non-reporting partner emissions in 2003, the regression 

equations were based on the 2003 SF6 emissions reported by 51 partner utilities, and updated 2003 transmission 

mileage data obtained from the 2004 UDI Directory of Electric Power Producers and Distributors (UDI 2004). The 

resulting equations for 2003 are: 

Non-reporting partner small utilities (less than 10,000 transmission miles, 2003, in kilograms): 

Emissions = 0.398 × Transmission Miles 

Non-reporting partner large utilities (more than 10,000 transmission miles, 2003, in kilograms): 

Emissions = 0.387 × Transmission Miles 

As indicated from the 2001 and 2004 UDI Directories of Electric Power Producers and Distributors (UDI 2001, 

2004), the U.S. transmission system has grown by over 14,000 miles between 2000 and 2003. To reflect the fact 

that this increase probably occurred gradually, transmission mileage was assumed to increase exponentially at an 

annual rate of approximately 0.7 percent during the 2000 to 2003 time period. 

For each year, total emissions were then determined by summing the partner-reported emissions, the non-reporting 

partner emissions (determined with that year’s regression equation for the partners), and the non-partner emissions 

(determined using the 1999 regression equation). 

1990 to 1998 Emissions from Manufacture of Electric Equipment 

Because most participating utilities reported emissions only for 1999 through 2003, modeling SF6 emissions from 

electric power systems for the years 1990 through 1998 was necessary. To do so, it was assumed that during this 

period, U.S. emissions followed the same trajectory as global emissions from this source. To estimate global 

emissions, the RAND survey of global SF6 sales to electric utilities was used, together with the following equation, 

which is derived from the equation for emissions in the IPCC report, Good Practice Guidance and Uncertainty 

Management in National Greenhouse Gas Inventories (IPCC 2001): 

Emissions (kilograms) = SF6 purchased to refill existing equipment (kilograms) + nameplate capacity of 

retiring equipment (kilograms) 

Note that the above equation holds whether the gas from retiring equipment is released or recaptured; if the gas is 

recaptured, it is used to refill existing equipment, lowering the amount of SF6 purchased by utilities for this purpose. 

Sulfur hexaflouride purchased to refill existing equipment in a given year was assumed to be approximately equal to 

the SF6 purchased by utilities in that year. Gas purchases by utilities and equipment manufacturers from 1961 

through 2001 are available from the RAND (2002) survey. To estimate the quantity of SF6 released from retiring 

equipment, the nameplate capacity of retiring equipment in a given year was assumed to equal 77.5 percent of the 

amount of gas purchased by electrical equipment manufacturers 30 years previous (e.g., in 1990, the nameplate 

capacity of retiring equipment was assumed to equal 77.5 percent of the gas purchased in 1960). The remaining 

22.5 percent was assumed to have been emitted at the time of manufacture. The 22.5 percent emission rate is an 

average of IPCC SF6 emission rates for Europe and Japan for years before 1996 (IPCC 2001). The 30-year lifetime 

for electrical equipment is also drawn from IPCC (2001). The results of the two components of the above equation 

were then summed to yield estimates of global SF6 emissions from 1990 through 1998. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 173 

To estimate U.S. emissions for 1990 through 1998, estimated 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. To estimate historical U.S. emissions, the 

factor for each year was multiplied by the estimated U.S. emissions of SF6 from electric power systems in 1999 

(estimated to be 15.8 Tg CO2 Eq.). 

1990 to 2003 Emissions from Manufacture of Electrical Equipment 

The 1990 to 2003 emissions estimates for original equipment manufacturers (OEMs) were derived by assuming that 

manufacturing emissions equal 10 percent of the quantity of SF6 charged into new equipment. The quantity of SF6

charged into new equipment was estimated based on statistics compiled by the National Electrical Manufacturers 

Association (NEMA). These statistics were provided for 1990 to 2000; the quantities of SF6 charged into new 

equipment for 2001 to 2003 were assumed to equal that charged into equipment in 2000. 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 

For electric power systems, individual partner-reported SF6 data was assumed to have an uncertainty of 10 percent. 

This uncertainty was assumed to incorporate potential errors associated with the weighing of SF6 cylinders and the 

tracking of SF6 purchases and use. Using error propagation, the cumulative uncertainty of all partner-reported data 

was estimated to be 5 percent. 

There are two sources of uncertainty associated with the regression equations used to extrapolate U.S. emissions 

from participant reports: 1) uncertainty in the coefficients (as defined by the regression standard error estimate); and 

2) the uncertainty in total transmission miles for non-partners and non-reporting partners, which is assumed to be 10 

percent. In addition, there is also uncertainty in the basic assumption that all non-reporting partners will follow the 

trend defined by reporting partners in a specific year, as well as uncertainty that the emission factor used for utilities 

that were not participants, which accounted for approximately 65 percent of U.S. transmission miles, will remain at 

levels defined by partners who reported in 1999. However, neither of these uncertainties was modeled. 

For OEMs, uncertainty estimates are based on the assumption that SF6 statistics obtained from NEMA have an 

uncertainty of 10 percent. Additionally, the OEMs SF6 emissions rate has an uncertainty bounded by the proposed 

“actual” and “ideal” emission rates defined in O’Connell, et al. (2002). That is, the uncertainty in the emission rate 

is approximately 65 percent. 

A Monte Carlo analysis was applied to estimate the overall uncertainty of the emission estimate for SF6 from 

electrical transmission and distribution. For each defined parameter (i.e., equation coefficient, transmission 

mileage, and partner-specific SF6 emissions data for electric power systems; and SF6 emission rate and statistics for 

OEMs), random variables were selected from probability density functions, all assumed to have normal distributions 

about the mean. The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-67. Electrical 

Transmission and Distribution SF6 emissions were estimated to be between 12.3 and 16.1 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 13 percent below and 14 percent above the emission estimate of 14.1 Tg CO2 Eq. 

Table 4-66: Simulated Variables for Tier 2 Uncertainty Analysis 

Parameter 

Probability 

Distribution 

Uncertaintya

 

(%) 

Total Partner-Reported SF6 Data (kg SF6) Normal 5 

Coefficient – Small Utilities, Non-Partners Normal 11 

Coefficient – Large Utilities, Non-Partners Normal 21 

Coefficient – Small Utilities, Non-Reporting Partners Normal 21 

Coefficient – Large Utilities, Non-Reporting Partners Normal NAb

Transmission Miles – Small Utilities, Non-Partners Normal 10 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 174 

Transmission Miles – Large Utilities, Non-Partners Normal 10 

Transmission Miles – Small Utilities, Non-Reporting Partners Normal 10 

Transmission Miles – Large Utilities, Non-Reporting Partner Normal NAb

OEM SF6 Emission Rate (percent) Normal 65 

SF6 Charged to Equipment (kg SF6) Normal 10 a

 Reflects a 95 percent confidence interval. 

b

 Not applicable. In 2003, all large utility partners reported to the SF6 Emission Reduction Partnership. 

Table 4-67: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Electrical Transmission and 

Distribution (Tg CO2 Eq. and Percent) 

Uncertainty Range Relative to 2003 Emission Estimatea

Source Gas 

2003 

Emission 

Estimate 

(Tg CO2 Eq.) 

(Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Electrical Transmission 

and Distribution SF6 14.1 12.3 16.1 -13% +14% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

Electric power system emission estimates for the years 2000, 2001 and 2002 were recalculated based on a 

combination of additional historical partner data and the incorporation of more up-to-date transmission mileage 

data. For historical partner submissions, the regression equations for each respective year were updated and new 

extrapolations to non-reporting partners were made. Additionally, recalculations were conducted using updated 

transmission mileage data, which reflect the growth of the U.S. transmission system. Previously-reported 2001 and 

2002 emission estimates had utilized the 2001 UDI Directory of Electric Power Producers and Distributors (UDI 

2001) for transmission mileage data (i.e., 2000 data). As mentioned above, transmission mileage data for 2001 and 

2002 were adjusted to account for increases in transmission mileage during this period. These adjustments have 

been incorporated in non-reporting partner regression equation re-calculations, resulting in revised estimates of nonpartner and non-reporting partner emissions. The combination of these changes resulted in an average annual 

decrease of less than 0.1 Tg CO2 Eq. (0.2 percent) in SF6 emissions from electrical transmission and distribution for 

the period 2000 through 2002. 

4.18. 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. In 2003, the United States was the third largest 

producer of primary aluminum, with 10 percent of the world total (USGS 2004). The United States was also a 

major importer of primary aluminum. The production of primary aluminum⎯in addition to consuming large 

quantities of electricity⎯results in process-related emissions of CO2 and two perfluorocarbons (PFCs): 

perfluoromethane (CF4) and perfluoroethane (C2F6). 

Carbon dioxide is emitted during the aluminum smelting process when alumina (aluminum oxide, Al2O3) is reduced 

to aluminum using the Hall-Heroult reduction process. The reduction of the alumina occurs through electrolysis in 

a molten bath of natural or synthetic cryolite (Na3AlF6). The reduction cells contain a carbon lining that serves as 

the cathode. Carbon is also contained in the anode, which can be a carbon mass of paste, coke briquettes, or 

prebaked carbon blocks from petroleum coke. During reduction, most of this carbon is oxidized and released to the 

atmosphere as CO2. 

Process emissions of CO2 from aluminum production were estimated to be 4.2 Tg CO2 Eq. (4,219 Gg) in 2003 (see 

Table 4-68). The carbon 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 175 

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 4-68: CO2 Emissions from Aluminum Production (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 6.3 6,315 

1997 5.6 5,621 

1998 5.8 5,792 

1999 5.9 5,895 

2000 5.7 5,723 

2001 4.1 4,114 

2002 4.2 4,220 

2003 4.2 4,219 

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, termed “anode effects.” These anode effects cause carbon from the 

anode and fluorine from the dissociated molten cryolite bath to combine, thereby producing fugitive emissions of 

CF4 and C2F6. In general, the magnitude of emissions for a given level of production depends on the frequency and 

duration of these anode effects. As the frequency and duration of the anode effects increase, a corresponding rise in 

emission levels occurs. 

Emissions of PFCs from primary aluminum production are estimated to have declined 79 percent since 1990. Since 

1990, emissions of CF4 and C2F6 have declined 80 percent and 77 percent, respectively, to 3.3 Tg CO2 Eq. of CF4

(0.5 Gg) and 0.5 Tg CO2 Eq. of C2F6 (0.1 Gg) in 2003, as shown in Table 4-69 and Table 4-70. This decline was 

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. 

Table 4-69: PFC Emissions from Aluminum Production (Tg CO2 Eq.) 

Year CF4 C2F6 Total

1990 16.1 2.3 18.3

1997 9.8 1.2 11.0

1998 8.1 1.0 9.1

1999 8.0 0.9 9.0

2000 8.1 0.9 9.0

2001 3.5 0.5 4.0

2002 4.5 0.7 5.2

2003 3.3 0.5 3.8

Note: Totals may not sum due to independent rounding. 

Table 4-70: PFC Emissions from Aluminum Production (Gg) 

Year CF4 C2F6

1990 2.5 0.2 

1997 1.5 0.1 

1998 1.2 0.1 

1999 1.2 0.1 

2000 1.2 0.1 

2001 0.5 0.1 

2002 0.7 0.1 

2003 0.5 0.1 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 176 

U.S. primary aluminum production for 2003—totaling 2.7 million metric tons—remained similar to 2002 

production levels. Due to high electric power costs in various regions of the country, aluminum production has 

been curtailed at several U.S. smelters, which resulted in current production levels that were nearly 26 percent lower 

than 2000 levels in 2003. The transportation industry remained the largest domestic consumer of primary 

aluminum, accounting for about 35 percent of U.S. consumption (USGS 2004). 

Methodology 

Carbon dioxide is generated during alumina reduction to aluminum metal following the reaction below: 

2Al2O3 + 3C → 4Al + 3CO2

The CO2 emission factor employed was estimated from the production of primary aluminum metal and the carbon 

consumed by the process. Emissions vary depending on the specific technology used by each plant (e.g., Prebake or 

Soderberg). The Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997) provide CO2 emission factors for 

each technology type. During alumina reduction in a prebake anode cell process, approximately 1.5 metric tons of 

CO2 are emitted for each metric ton of aluminum produced (IPCC/UNEP/OECD/IEA 1997). Similarly, during 

alumina reduction in a Soderberg cell process, approximately 1.8 metric tons of CO2 are emitted per metric ton of 

aluminum produced (IPCC/UNEP/OECD/IEA 1997). Based on information gathered by EPA’s Voluntary 

Aluminum Industrial Partnership (VAIP) program, production was assumed to be split 80 percent prebake and 20 

percent Soderberg for the whole time series. 

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 × 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 × Anode Effect Duration (minutes) 

Smelter-specific slope coefficients that are based on field measurements yield the most accurate results. To estimate 

emissions between 1990 and 2002, smelter-specific coefficients were available and were used for 12 out of the 23 

U.S. smelters. To estimate 2003 emissions, smelter-specific coefficients were available and were used for 6 out of 

the 17 operating U.S. smelters. For the remaining 11 operating smelters, technology-specific slope coefficients 

from Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (IPCC 2001) 

were applied. The slope coefficients were combined with smelter-specific anode effect data, collected by aluminum 

companies and reported to the VAIP, to estimate emission factors over time. In 2003, smelter-specific anode effect 

data was available for 15 of the 17 operating smelters. Where smelter-specific anode effect data were not available 

(i.e., 2 out of 17 smelters in 2003, 2 out of 23 smelters between 1990 and 2002), industry averages were used. For 

all smelters, emission factors were multiplied by annual production to estimate annual emissions at the smelter level. 

In 2003, smelter-specific production data was available for 16 of the 17 operating smelters; production at the one 

remaining smelter was estimated based on national aluminum production and capacity data (USGS). Between 1990 

and 2003, production data has been provided by 21 of the 23 U.S. smelters. Emissions were then aggregated across 

smelters to estimate national emissions. The methodology used to estimate emissions is consistent with the 

methodologies recommended by the Good Practice Guidance and Uncertainty Management in National 

Greenhouse Gas Inventories (IPCC 2001). 

National primary aluminum production data for 1990 through 2001 (see Table 4-71) were obtained from USGS, 

Mineral Industry Surveys: Aluminum Annual Report (USGS 1995, 1998, 2000, 2001, 2002). For 2002 and 2003, 

national aluminum production data were obtained from the United States Aluminum Association’s Primary 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 177 

Aluminum Statistics (USAA 2004). The CO2 emission factors were taken from the Revised 1996 IPCC Guidelines

(IPCC/UNEP/OECD/IEA 1997). 

Table 4-71: Production of Primary Aluminum (Gg) 

Year Gg 

1990 4,048 

1991 4,121 

1992 4,042 

1993 3,695 

1994 3,299 

1995 3,375 

1996 3,577 

1997 3,603 

1998 3,713 

1999 3,779 

2000 3,668 

2001 2,637 

2002 2,705 

2003 2,705 

Uncertainty 

The overall uncertainty associated with the 2003 CO2, CF4, and C2F6 emission estimates were calculated using the 

IPCC Good Practice Guidance Tier 2 methodology. Uncertainty associated with the parameters used to estimate 

CO2 emissions included that associated with production data, with the share of U.S. aluminum production 

attributable to each smelter type, and with the emission factors applied to production data to calculate emissions. 

Uncertainty surrounding production data was assumed to be characterized as described below, and other variables 

were modeled assuming triangular distributions. Emission factors were determined through expert elicitation to be 

50 percent certain at a 95 percent confidence level, while the share of production attributed to smelter types were 

determined to be associated with a 25 percent uncertainty. A Monte Carlo analysis was applied to estimate the 

overall uncertainty of the emissions estimate for the U.S. aluminum industry as a whole and the results are provided 

below. 

In determining uncertainty associated with emissions of CF4 and C2F6, for each smelter, uncertainty associated with 

the quantity of aluminum produced and the frequency and duration of anode effects was estimated. A Monte Carlo 

analysis was then applied to estimate the overall uncertainty of the emissions estimate for each smelter and for the 

U.S. aluminum industry as a whole. Data on anode effect frequency and duration and production data are assumed 

to be characterized by a normal distribution. The uncertainty of aluminum production estimates was assumed to be 

1 percent or 25 percent, depending on whether a smelter’s production was reported or estimated. The uncertainty of 

the anode effect frequency was assumed to be 2 percent if the data was reported; however, if the data was estimated, 

the uncertainty ranged from 33 to 78 percent, depending on the smelter technology type. Similarly, the uncertainty 

in anode effect duration was assumed to be 5 percent for data that was reported, but between 28 and 70 percent for 

data that was estimated. The uncertainty ranges for estimated technology-specific anode effect frequency and 

duration are based on the standard deviation of reported anode-effect frequency and duration in the International 

Aluminum Institute’s anode effect survey (IAI 2000). 

Additionally, for CF4 and C2F6 emission estimates, uncertainties associated with slope coefficients were calculated. 

Data for the slope coefficients are assumed to be characterized by a normal distribution. For the three smelters that 

participated in the 2003 EPA-funded measurement study, CF4 and C2F6 slope coefficient uncertainties were 

calculated to be 10 percent. For the remaining smelters, given the limited uncertainty data on site-specific slope 

coefficients (i.e., those developed using IPCC Tier 3b methodology), the overall uncertainty associated with the 

slope coefficients is conservatively assumed to be similar to that given by the IPCC guidance for technologyspecific slope coefficients. Consequently, the uncertainty assigned to the slope coefficients ranged between 10 

percent and 35 percent, depending upon the gas and the smelter technology type. In general, where precise 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 178 

quantitative information was not available on the uncertainty of a parameter, a conservative (upper-bound) value 

was used. 

The results of this Tier 2 quantitative uncertainty analysis are summarized in Table 4-72. Aluminum production 

CO2 emissions were estimated to be between 2.8 and 5.9 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 34 percent below to 40 percent 

above the emission estimate of 4.2 Tg CO2 Eq. Also at the 95 percent confidence interval, aluminum production 

CF4 emissions were estimated to be between 2.9 and 3.7 Tg CO2 Eq. at the 95 percent confidence level. This 

indicates a range of approximately 11 percent below to 11 percent above the emission estimate of 3.3 Tg CO2 Eq. 

Finally aluminum production C2F6 emissions were estimated to be between 0.46 and 0.59 Tg CO2 Eq. at the 95 

percent confidence level. This indicates a range of approximately 12 percent below to 13 percent above the emission 

estimate of 0.5 Tg CO2 Eq.

Table 4-72: Tier 2 Quantitative Uncertainty Estimates for PFC Emissions from Aluminum Production (Tg CO2 Eq. 

and Percent) 

Uncertainty Range Relative to 2003 Emission Estimatea

Source Gas 

2003 

Emission 

Estimate 

(Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Aluminum Production CO2 4.2 2.8 5.9 -34% +40% 

Aluminum Production CF4 3.3 2.9 3.7 -11% +11% 

Aluminum Production C2F6 0.5 0.5 0.6 -12% +13% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Occasionally, SF6 may be used by the aluminum industry as a cover gas or a fluxing and degassing agent in 

experimental and specialized casting operations. In its application as a cover gas, SF6 is mixed with nitrogen or CO2

and injected above the surface of molten aluminum. As a fluxing and degassing agent, SF6 is mixed with argon, 

nitrogen, and/or chlorine and blown through molten aluminum. These practices are not employed extensively by 

primary aluminum producers and are generally isolated to secondary casting firms. The aluminum industry in the 

United States and Canada has been estimated to use 230 metric tons of SF6 per year (Maiss and Brenninkmeijer 

1998); however, this estimate is highly uncertain. 

Historically, SF6 from aluminum activities has 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). Emissions are 

considered to be insignificant, given that 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). 

Emissions of SF6 from aluminum fluxing and degassing have not been estimated. Uncertainties exist as to the 

quantity of SF6 used by the aluminum industry and its rate of destruction in its uses as a degassing agent or cover 

gas. 

Recalculations Discussion 

The smelter-specific emission factors used for estimating PFC emissions, as well as aluminum production levels, 

were revised to reflect recently-reported data concerning smelter operating parameters, as well as measurements 

conducted at three U.S. aluminum smelters. The measurements were part of an EPA-funded study to determine 

facility-specific slope coefficients. Consequently, these coefficients were used instead of IPCC defaults to calculate 

smelter-specific emission factors. These data were provided in cooperation with participants in the VAIP program. 

The combination of these changes resulted in an average annual increase of less than 0.1 Tg CO2 Eq. (0.2 percent) 

in PFC emissions from aluminum production for the period 1990 through 2002. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 179 

Carbon dioxide emission estimates from aluminum production for 2002 were updated to include aluminum 

production data from the USAA. Previous CO2 emission estimates for 2002 were based on aluminum production 

data from the USGS. This change resulted in a decrease in CO2 emissions from aluminum production of less than 

0.1 Tg CO2 Eq. (less than -0.1 percent) for 2002. 

4.19. Semiconductor Manufacture (IPCC Source Category 2F6) 

The semiconductor industry uses multiple long-lived fluorinated gases in plasma etching and plasma enhanced 

chemical vapor deposition (PECVD) processes to produce semiconductor products. The gases most commonly 

employed are trifluoromethane (HFC-23 or CHF3), perfluoromethane (CF4), perfluoroethane (C2F6), nitrogen 

trifluoride (NF3), and sulfur hexafluoride (SF6), although other compounds such as perfluoropropane (C3F8) and 

perfluorocyclobutane (c-C4F8) are also used. The exact combination of compounds is specific to the process 

employed. 

A single 300 mm silicon wafer that yields between 400 to 500 semiconductor products (devices or chips) may 

require as many as 100 distinct fluorinated-gas-using process steps, principally to deposit and pattern dielectric 

films. Plasma etching (or patterning) of dielectric films, such as silicon dioxide and silicon nitride, is performed to 

provide pathways for conducting material to connect individual circuit components in each device. The patterning 

process uses plasma-generated fluorine atoms, which chemically react with exposed dielectric film, to selectively 

remove the desired portions of the film. The material removed as well as undissociated fluorinated gases flow into 

waste streams and, unless emission abatement systems are employed, into the atmosphere. PECVD chambers, used 

for depositing dielectric films, are cleaned periodically using fluorinated and other gases. During the cleaning cycle 

the gas is converted to fluorine atoms in a plasma, which etches away residual material from chamber walls, 

electrodes, and chamber hardware. Undissociated fluorinated gases and other products pass from the chamber to 

waste streams and, unless abatement systems are employed, into the atmosphere. In addition to emissions of 

unreacted gases, some fluorinated compounds can also be transformed in the plasma processes into different 

fluorinated compounds which are then exhausted, unless abated, into the atmosphere. For example, when C2F6 is 

used in cleaning or etching, CF4 is generated and emitted as a process by-product. Besides dielectric film etching 

and PECVD chamber cleaning, much smaller quantities of fluorinated gases are used to etch polysilicon films and 

refractory metal films like tungsten. 

For 2003, total weighted emissions of all fluorinated greenhouse gases by the U.S. semiconductor industry were 

estimated to be 4.3 Tg CO2 Eq. Combined emissions of all fluorinated greenhouse gases are presented in Table 

4-73 and Table 4-74 below. The rapid growth of this industry and the increasing complexity of semiconductor 

products which use more PFCs in the production process have led to an increase in emissions of 48 percent since 

1990. The emissions growth rate began to slow after 1997, and emissions declined by 40 percent between 1999 and 

2003. This decline is due both to a drop in production (with a continuing decline in silicon consumption) and to the 

initial implementation of PFC emission reduction methods such as process optimization. 

Table 4-73: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Tg CO2 Eq.)

Year 1990 1997 1998 1999 2000 2001 2002 2003 

CF4 0.7 1.6 1.8 1.8 1.8 1.3 1.1 1.0 

C2F6 1.5 3.2 3.6 3.7 3.0 2.1 2.2 2.1 

C3F8 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.1 

C4F8 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.1 

HFC-23 0.2 0.4 0.4 0.4 0.3 0.2 0.2 0.2 

SF6 0.5 1.1 1.3 1.3 1.1 0.8 0.7 0.8 

NF3

* 0.0 0.1 0.1 0.1 0.1 0.1 0.3 0.2 

Total 2.9 6.3 7.1 7.2 6.3 4.5 4.4 4.3 

Note: Totals may not sum due to independent rounding. 

* NF3 emissions are presented for informational purposes, using a GWP of 8,000, and are not included in totals. 

Table 4-74: PFC, HFC, and SF6 Emissions from Semiconductor Manufacture (Mg)

Year 1990 1997 1998 1999 2000 2001 2002 2003 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 180 

CF4 115 245 277 281 281 202 175 161 

C2F6 160 347 391 397 324 231 244 228 

C3F8 0 0 0 0 17 14 9 13 

C4F8 0 0 0 0 0 0 5 8 

HFC-23 15 33 37 37 23 16 15 17 

SF6 22 48 54 55 46 31 28 35 

NF3 3 8 9 9 11 12 32 30 

Methodology 

Emissions from semiconductor manufacturing were estimated using three distinct methods, one each for the periods 

1990 through 1994, 1995 through 1999, and 2000 and beyond. For 1990 through 1994, emissions were estimated 

using the most recent version of EPA’s PFC Emissions Vintage Model (PEVM) (Burton & Beizaie 2001).17 PFC 

emissions per square centimeter of silicon increase as the number of layers in semiconductor devices increases. 

Thus, PEVM incorporates information on the two attributes of semiconductor devices that affect the number of 

layers: (1) linewidth technology (the smallest feature size, which decreases as layers increase), and (2) product type 

(memory vs. logic). PEVM derives historical consumption of silicon (i.e., square centimeters) by linewidth 

technology from published data on annual wafer starts and average wafer size (VLSI 2003a,b,c). 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 IC) specific to product 

type (International SEMATECH 1998-2003). The distribution of memory/logic devices ranges over the period 

covered from 52 percent logic devices in 1995 to 59 percent logic devices in 2000. These figures were used to 

determine emission factors that express emissions per average layer per unit of area of silicon consumed during 

product manufacture. The per-layer emission factor was based on the total annual emissions reported by 

participants in EPA’s PFC Reduction/Climate Partnership for the Semiconductor Industry in 1995 and later years. 

For 1995 through 1999, total U.S. emissions were extrapolated from the total annual emissions reported by the 

Partnership participants (Burton & Lieberman 2004). The emissions reported by the participants were divided by 

the ratio of the total layer-weighted capacity of the plants operated by the participants and the total layer-weighted 

capacity of all of the semiconductor plants in the United States; this ratio represents the share of layer-weighted 

capacity attributable to partnership participants. The layer-weighted capacity of a plant (or group of plants) consists 

of the silicon capacity of that plant multiplied by the number of layers used to fabricate products at that plant. This 

method assumes that participants and non-participants have similar capacity utilizations and per-layer emission 

factors. 

The U.S. estimate for the years 2000 through 2003—the period during which partners began the consequential 

application of PFC-reduction measures—used a different estimation method. The emissions reporting by 

Partnership participants for each year were accepted as the quantity emitted from the share of the industry 

represented by those Partners. Remaining emissions (those from non-partners), however, were estimated using 

PEVM and the method described above. (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 nonPartners is obtained by multiplying PEVM’s total U.S. figure by the non-partner share of total layer-weighted 

silicon capacity for each year (as described above). Annual updates to PEVM reflect published figures for actual 

silicon consumption from VLSI Research, Inc. as well as revisions and additions to the world population of 

semiconductor manufacturing plants. 

Two different approaches were also used to estimate the distribution of emissions of specific PFCs. Before 1999, 

when there was no consequential adoption of PFC-reducing measures, a fixed distribution was assumed to apply to 

the entire U.S. industry. This distribution was based upon the average PFC purchases by semiconductor 

 

17 The most recent version of this model is v.3.1.0306.0304r, completed in March 2004. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 181 

manufacturers during this period and the application of IPCC default emission factors for each gas. For the 2000 

through 2003 period, the 1990 through 1999 distribution was assumed to apply to the non-Partners. Partners, 

however, began to report gas-specific emissions during this period. Thus, gas specific emissions for 2000 through 

2003 were estimated by adding the emissions reported by the Partners to those estimated for the non-Partners. 

Partners estimate their emissions using a range of methods. For 2003, most participants cited a method at least as 

accurate as the IPCC’s Tier 2c Methodology, recommended in Good Practice Guidance and Uncertainty 

Management in National Greenhouse Gas Inventories (IPCC 2000). The partners with relatively high emissions 

typically use the more accurate IPCC 2b or 2a methods, multiplying estimates of their PFC consumption by processspecific emission factors that they have either measured or obtained from tool suppliers. 

Data used to develop emission estimates were prepared in cooperation with the Partnership. Estimates of operating 

plant capacities and characteristics for participants and non-participants were derived from the Semiconductor 

Equipment and Materials International (SEMI) World Fab Watch (formerly International Fabs on Disk) database 

(1996 to 2003). Estimates of silicon consumed by line-width from 1990 through 2003 were derived from 

information from VLSI Research (2003d), and the number of layers per line-width was obtained from International 

SEMATECH’s International Technology Roadmap: 1998 – 2003. 

Uncertainty 

Quantitative uncertainty of this source category was performed using the IPCC-recommended Tier 2 uncertainty 

estimation methodology, Monte Carlo Stochastic Simulation technique. Uncertainty is associated with the emission 

estimates reported by the Partners, with the estimated share of total layer-weighted silicon capacity in 2003 

attributable to the Partners, and with the total U.S. PFC emissions estimate as determined by PEVM. 

The Monte Carlo analysis presented below relied on estimates of uncertainty attributed to these three variables. 

Estimates of uncertainty for the three variables were in turn developed using the estimated uncertainties associated 

with the individual inputs to each variable, error propagation analysis, and expert judgment. For the first variable, 

the aggregate PFC emissions data supplied to the partnership, EPA estimated an uncertainty of ±10 percent 

(representing a 95 percent confidence interval). This value accounts for uncertainty in partners’ estimates of gasvolume usage, and was calculated using 2003 Partnership submittals. Through expert judgment and review of the 

emission reports submitted by companies under the Partnership agreement, the relative uncertainties were assumed 

to be the same for each submittal in 2003, equal to ±29 percent of the individual Partner’s reported value. Under 

that assumption, uncertainty propagated across all Partners resulted in a combined relative uncertainty equal to 

about 10 percent of the aggregate emissions reports under the Partnership. For the second variable, the share of 

U.S. layer-weighted silicon capacity accounted for by non-Partners, an uncertainty of ±36 percent was estimated 

based on the variability of the share over the period 1995 through 2003. 

For the third variable, the relative error associated with the PEVM estimate in 2003, EPA estimated an uncertainty 

of ±44 percent, using the calculus of error propagation and considering the aggregate average emission factor, 

world silicon consumption, the U.S. share of layer-weighted silicon capacity, and the number of layers. The 

uncertainty associated with the aggregate average emission factor was estimated to be 15 percent based on the 

variability exhibited by the emission factor from 1996 through 1999. The uncertainty associated with the U.S. 

share of capacity was estimated to be 10 percent based on information from the firm that compiled the database; the 

principal source of errors was determined to be incomplete e-mail and telephone surveys of manufacturers (SMA 

2003). The uncertainty associated with silicon consumption data was estimated to be 10 percent, based on the 

reliability of industry surveys of world silicon consumption by technology node. Finally, the uncertainty associated 

with the number of layers was estimated to be 39 percent. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 4-75. The emissions estimate for 

total U.S. PFC emissions from semiconductor manufacturing were estimated to be between 3.7 and 5.7 Tg CO2 Eq. 

at a 95 percent confidence level (or in 19 out of 20 Monte Carlo Stochastic Simulations). This indicates a range of 

20 percent below to 23 percent above the emissions estimate of 4.6 Tg CO2 Eq. It should be noted that this range 

and the associated percentages apply to the estimate of total emissions rather than those of individual gases. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 182 

Uncertainties associated with individual gases will be somewhat higher than the aggregate, but were not explicitly 

modeled. 

Table 4-75: Tier 2 Quantitative Uncertainty Estimates for HFC, PFC, and SF6 Emissions from Semiconductor 

Manufacture (Tg CO2 Eq. and Percent) 

Source Gas 

2003 

Emission 

Estimatea

Uncertainty Range 

Relative to Emission Estimateb

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Semiconductor 

Manufacture 

HFC, PFC, 

and SF6 4.6 3.7 5.7 -20% +23% a

 Because the uncertainty analysis covered all emissions (including NF3), the emission estimate presented here does not match 

that shown in Table 4-73.

b

 Range of emissions estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval.

 

Planned Improvements 

The method by which non-partner related emissions are estimated (i.e., PEVM) is not expected to change (with the 

exception of possible future updates to emission factors and added technology nodes). Future improvements to the 

national emission estimates will primarily be associated with determining the portion of national emissions to 

attribute to partner report totals (about 80 percent in recent years). As the nature of the partner reports change 

through time and industry-wide reduction efforts increase, consideration will be given to what emission reduction 

efforts—if any—are likely to be occurring at non-partner facilities (currently none are assumed to occur.) 

4.20. 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 

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 emitted into the atmosphere. Sulfur hexafluoride has been used in this application around the world 

for the last twenty years. It has largely replaced salt fluxes and SO2, which are more toxic and corrosive than SF6. 

The magnesium industry emitted 3.0 Tg CO2 Eq. (0.1 Gg) of SF6 in 2003 (see Table 4-76). This represents a 12 

percent increase from 2002. The increase is attributable to a 1.5 percent rise in production and casting levels and to 

a 10.5 percent increase in the weighted-average SF6 usage rate at these facilities. There are no significant plans for 

expansion of primary magnesium production in the United States, but demand for magnesium metal by U.S. casting 

companies has grown as auto manufacturers design more lightweight magnesium parts into vehicle models. In the 

last ten years, the quantity of magnesium used in North American-produced vehicles has doubled (USGS 2004a). 

Foreign magnesium producers are expected to meet the growing U.S. demand for primary magnesium (USGS 

2004a). 

Table 4-76: SF6 Emissions from Magnesium Production and Processing (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 5.4 0.2 

1997 6.3 0.3 

1998 5.8 0.2 

1999 6.0 0.3 

2000 3.2 0.1 

2001 2.6 0.1 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 183 

2002 2.6 0.1 

2003 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 production and over 80 percent of the casting sector 

(i.e., die, gravity, wrought, and anode casting). Emissions for 1999 through 2003 from primary production, some 

secondary production (i.e., recycling), and a large fraction of die casting were reported by participants. The 1999 

through 2003 emissions from the remaining secondary production and casting were estimated by multiplying 

industry emission factors (kg SF6 per metric ton of Mg produced or processed) by the amount of metal produced or 

consumed in the five major processes (other than primary production) that require SF6 melt protection: 1) secondary 

production; 2) die casting; 3) gravity casting; 4) wrought products; and 5) anodes. The emission factors are 

provided below in Table 4-77. Because only one primary producer existed in the United States in 2003, the 

emission factor for primary production is withheld to protect production information. However, the emission factor 

has not risen above the 1995 value of 1.1 kg SF6 per metric ton. 

Die casting emissions for 1999 through 2003, which accounted for 48 to 75 percent of all SF6 emissions from U.S. 

casting and recycling processes during this period, were estimated based on information supplied by industry 

partners. From 2000 to 2003, partners accounted for all U.S. die casting that was tracked by USGS. In 1999, 

partners did not account for all die casting tracked by USGS, and, therefore, it was necessary to estimate the 

emissions of die casters who were not partners. Die casters who were not partners were assumed to be similar to 

partners who cast small parts. Due to process requirements, these casters consume larger quantities of SF6 per 

metric ton of processed magnesium than casters that process large parts. Consequently, emissions estimates from 

this group of die casters were developed using an average emission factor of 5.2 kg SF6 per metric ton of 

magnesium. The emission factors for the other industry sectors (i.e., secondary production, gravity, wrought, and 

anode casting) were based on discussions with industry representatives. 

Table 4-77: SF6 Emission Factors (kg SF6 per metric ton of magnesium) 

Year Secondary Die Casting Gravity Wrought Anodes

1999 1 2.14a 2 11

2000 1 0.73 2 1 1

2001 1 0.77 2 1 1

2002 1 0.70 2 1 1

2003 1 0.84 2 1 1 a The 1999 factor is 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 these 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 2003 were available from the USGS (USGS 2002, 2003, 2004b). 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 1996. 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). The emission factors for the other processes (i.e., secondary production, and gravity, wrought, and 

anode casting), about which less is known, were assumed to remain constant at levels defined in Table 4-65. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 184 

Uncertainty 

An uncertainty of 5 percent was assigned to the SF6 emissions data reported by each participant in the SF6 Emission 

Reduction Partnership. These data have low uncertainty since they are prepared through facility-specific tracking of 

SF6 cylinder purchases, usage, and returns. 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. For example, to estimate 2003 emission factors, the average change in emission factor from 2002 to 

2003 for reporting partners was applied to the 2002 emission factor of the non-reporting partner. The uncertainty 

associated with the extrapolated emission factor was assumed to be 25 percent. For production data, if estimates 

were unavailable for the current reporting year, data from the last reported year was applied. 

For 2003, the uncertainty associated with this approach was assumed to be 30 percent. Between 1999 and 2003, 

non-reporting partners have accounted for between 0 and 17 percent of total estimated sector emissions. For those 

industry processes that are not represented in EPA’s partnership, such as gravity, anode, and wrought casting, SF6 

emissions were estimated using production and consumption statistics reported by USGS and an estimated processspecific emission factor (see Table 4-78). The uncertainty associated with USGS-reported statistics and emission 

factors were assumed to be 25 percent and 75 percent, respectively. In general, where precise quantitative 

information was not available on the uncertainty of a parameter, a conservative (upper-bound) value was used. 

Table 4-78: Simulated Variables for Tier 2 Uncertainty Analysis 

Parameter 

Probability 

Distribution 

Uncertaintya

(%) 

Partner-Reported SF6 Data (kg SF6) Normal 5 

SF6 Emission Factor for Non-Reporting Partners (kg SF6/metric ton Mg) Normal 25 

Production Data for Non-Reporting Partners (metric ton Mg) Normal 30 

USGS Production Data for Gravity, Anode, Wrought Casting and Secondary 

Production (metric ton Mg) Normal 25 

SF6 Emission Factor for Gravity, Anode, Wrought Casting and Secondary 

Production (kg SF6/metric ton Mg) Normal 75 a

 Reflects a 95 percent confidence interval. 

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 at 

hot-chambered die casting machines on the order of 10 percent (Bartos et al. 2003). As is the case for other sources 

of SF6 emissions, total SF6 consumption data for magnesium production and processing in the United States were 

not available. Sulfur hexafluoride may also be used as a cover gas for the casting of molten aluminum with high 

magnesium content; however, to what extent this technique is used in the United States is unknown. 

A Monte Carlo analysis was applied to estimate the overall uncertainty of the emission estimate for the U.S. 

magnesium industry. Random variables were selected from the probability density functions for each parameter, 

which were assumed to be characterized by normal distributions. In the cases of estimates developed from partners 

and non-reporting partners, probability density functions were applied to parameters (i.e., SF6 emissions data, 

emission factors and production data) at the facility-specific level. The results of the Tier 2 quantitative uncertainty 

analysis are summarized in Table 4-79. Magnesium production and processing SF6 emissions were estimated to be 

between 2.6 and 3.3 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 11 percent below to 13 percent above the emission estimate 

of 3.0 Tg CO2 Eq. 

Table 4-79: Tier 2 Quantitative Uncertainty Estimates for SF6 Emissions from Magnesium Production and 

Processing (Tg CO2 Eq. and Percent) 

Source Gas 

2003 

Emission 

Estimate 

Uncertainty Range Relative to 2003 Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 185 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Magnesium Production 

and Processing SF6 3.0 2.6 3.3 -11% +13% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

The emission estimates for 2000, 2001, and 2002 were adjusted upward slightly from the previously reported 

values. This revision reflects an update to historical data supplied by partnership participants and the USGS. The 

changes resulted in an average annual increase of less than 0.1 Tg CO2 Eq. (4.1 percent) in SF6 emissions from 

magnesium production and processing for the period 2000 through 2002. 

Planned Improvements 

As more work assessing the degree of cover gas degradation and associated byproducts is undertaken and 

published, results could potentially be used to refine the emission estimates, which currently assume (per IPCC 

Good Practice Guidance 2001) that all SF6 utilized is emitted to the atmosphere. EPA-funded measurements of SF6

in hot chamber die casting have indicated that the latter assumption may be incorrect, with observed SF6

degradation on the order of 10 percent (Bartos et al. 2003). More recent EPA-funded measurement studies have 

confirmed this observation for cold chamber die casting (EPA 2004). 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-CoverTM (containing HFC-134a) and NovecTM 612, have lower GWPs than SF6, and 

tend to quickly decompose during their exposure to the molten metal. Additionally, as more companies join the 

partnership, in particular those from sectors not currently represented, such as gravity and anode casting, emission 

factors will be refined to incorporate these additional data. 

[BEGIN BOX] 

Box 4-1: Potential Emission Estimates of HFCs, PFCs, and SF6 

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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 186 

Table 4-80 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.18 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 2003, estimates were obtained from reports submitted by participants 

in EPA’s SF6 Emission Reduction Program for Electric Power Systems. U.S. utility purchases of SF6 for electrical 

equipment from 1990 through 1998 were backcasted based on world sales of SF6 to utilities. Purchases of SF6 by 

utilities were added to SF6 purchases by electrical equipment manufacturers to obtain total SF6 purchases by the 

electrical equipment sector. 

Table 4-80: 2003 Potential and Actual Emissions of HFCs, PFCs, and SF6 from Selected Sources (Tg CO2 Eq.) 

Source Potential Actual

Substitution of Ozone Depleting Substances 181.0 99.5

Aluminum Production - 3.8

HCFC-22 Production - 12.3

Semiconductor Manufacture 6.6 4.3

Magnesium Production and Processing 3.0 3.0

Electrical Transmission and Distribution 21.8 14.1

- Not applicable. 

[END BOX] 

4.21. Industrial Sources of Ambient Air Pollutants 

In addition to the main greenhouse gases addressed above, many industrial processes generate emissions of ambient 

air pollutants. Total emissions of nitrogen oxides (NOx), carbon monoxide (CO), and nonmethane volatile organic 

compounds (NMVOCs) from non-energy industrial processes from 1990 to 2003 are reported in Table 4-81. 

Table 4-81: NOx, CO, and NMVOC Emissions from Industrial Processes (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

NOx 591 629 637 595 626 656 630 648

Chemical & Allied Product Manufacturing 152 115 117 93 95 97 95 92

Metals Processing 88 81 81 78 81 86 76 83

Storage and Transport 3 15 15 13 14 15 14 14

Other Industrial Processes 343 417 424 409 434 457 442 457

Miscellaneous* 5 1 1 2 2 1 3 2

CO 4,124 3,153 3,163 2,156 2,217 2,339 2,308 2,431

Chemical & Allied Product Manufacturing 1,074 971 981 317 327 338 306 299

Metals Processing 2,395 1,551 1,544 1,138 1,175 1,252 1,174 1,290

Storage and Transport 69 64 65 148 154 162 195 219

Other Industrial Processes 487 528 535 518 538 558 576 575

Miscellaneous* 101 38 38 35 23 30 57 49

 

18 See Annex 5 for a discussion of sources of SF6 emissions excluded from the actual emissions estimates in this report. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 187 

NMVOCs 2,426 2,038 2,047 1,813 1,773 1,769 1,725 1,711

Chemical & Allied Product Manufacturing 575 352 357 228 230 238 194 198

Metals Processing 111 71 71 60 61 65 62 65

Storage and Transport 1,356 1,205 1,204 1,122 1,067 1,082 1,093 1,069

Other Industrial Processes 364 397 402 398 412 381 369 374

Miscellaneous* 20 13 13 6 3 4 7 5

* 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 2004), 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.
Figure 4-1: 2003 Industrial Processes Chapter Greenhouse Gas Sources

<0.05

0 20 40 60 80 100 120

Silicon Carbide Production

Carbon Dioxide Consumption

Ferroalloys

Phosphoric Acid Production

Titanium Dioxide Production

Magnesium Production and Processing

Soda Ash Manufacture and Consumption

Petrochemical Production

Semiconductor Manufacture

Limestone and Dolomite Use

Adipic Acid

Aluminum Production

HCFC-22 Production

Lime Manufacture

Electrical Transmission and Distribution

Ammonia Production and Urea Application

Nitric Acid

Cement Manufacture

Iron and Steel Production

Substitution of Ozone Depleting Substances

Tg CO2 Eq.

Industrial Processes 

as a Portion of all 

Emissions

4.5%
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 189 

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 2003 (see Table 5-1). Ambient air pollutant emissions also result from solvent and other product 

use, and are presented in Table 5-2. 

Table 5-1: N2O Emissions from Solvent and Other Product Use (Tg CO2 Eq. and Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

Nitrous Oxide Product Usage 

Tg CO2 Eq. 4.3 4.8 4.8 4.8 4.8 4.8 4.8 4.8

Gg 13.9 15.4 15.4 15.4 15.4 15.4 15.4 15.4

Table 5-2: Ambient Air Pollutant Emissions from Solvent and Other Product Use (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

NOx 1 3 3 3 3 3 5 4 

CO 4 1 1 46 46 45 46 65 

NMVOCs 5,217 5,100 4,671 4,569 4,384 4,547 4,256 4,138 

5.1. Nitrous Oxide Product Usage (IPCC Source Category 3D) 

Nitrous oxide is a clear, colorless, oxidizing liquefied gas, with a slightly sweet odor. Nitrous oxide is produced by 

thermally decomposing ammonium nitrate (NH4NO3), a chemical commonly used in fertilizers and explosives. The 

decomposition creates steam (H2O) and N2O through a low-pressure, low-temperature (500°F) reaction. Once the 

steam is removed through condensation, the remaining N2O is purified, compressed, dried, and liquefied for storage 

and distribution. Two companies operate a total of five N2O production facilities in the United States (CGA 2002). 

Nitrous oxide is primarily used in carrier gases with oxygen to administer more potent inhalation anesthetics for 

general anesthesia and as an anesthetic in various dental and veterinary applications. As such, it is used to treat 

short-term pain, for sedation in minor elective surgeries and as an induction anesthetic. The second main use of 

N2O is as a propellant in pressure and aerosol products, the largest application being pressure-packaged whipped 

cream. Small quantities of N2O also are used in the following applications: 

• Oxidizing agent and etchant used in semiconductor manufacturing; 

• Oxidizing agent used, with acetylene, in atomic absorption spectrometry; 

• Production of sodium azide, which is used to inflate airbags; 

• Fuel oxidant in auto racing; and 

• Oxidizing agent in blowtorches used by jewelers and others (Heydorn 1997). 

Production of N2O in 2003 was approximately 17 Gg. Nitrous oxide emissions were 4.8 Tg CO2 Eq. (15.4 Gg) in 

2003 (see Table 5-3). Production of N2O has stabilized over the past decade because medical markets have found 

other substitutes for anesthetics, and more medical procedures are 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 Nitrous Oxide Product Usage (Tg CO2 Eq. and Gg) 

Year Tg CO2 Eq. Gg 

1990 4.3 13.9 

1997 4.8 15.4 

1998 4.8 15.4 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 190 

1999 4.8 15.4 

2000 4.8 15.4 

2001 4.8 15.4 

2002 4.8 15.4 

2003 4.8 15.4 

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 that is used by each sector. This value was then multiplied 

by the associated emissions rate for each sector. After the emissions were calculated for each sector, they were 

added together to obtain a total estimate of N2O product usage emissions. Emissions were determined using the 

following equation: 

Nitrous Oxide Product Usage Emissions = ∑i [Total U.S. Production of Nitrous Oxide] × [Share of Total 

Quantity of N2O Usage by Sector i] × [Emissions Rate for Sector i], where i = sector. 

The share of total quantity of N2O usage by subcategory represents the share of national N2O produced that is used 

by the specific subcategory (i.e., anesthesia, food processing, etc.). In 2002, and also assumed for 2003, the 

medical/dental industry used an estimated 86 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 (Tupman 2002). 

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. Once the N2O is allocated across these 

subcategories, a usage emissions rate is then applied for each sector to estimate the amount of N2O emitted. 

Only the medical/dental and food propellant subcategories are estimated to release emissions into the atmosphere, 

and therefore these subcategories are the only usage subcategories with emission rates. For the medical/dental 

subcategory, due to the poor solubility of N2O in blood and other tissues, approximately 97.5 percent of the N2O is 

not metabolized during anesthesia and quickly leaves the body in exhaled breath. Therefore, an emission factor of 

97.5 percent is used for this subcategory (Tupman 2002). For N2O used as a propellant in pressurized and aerosol 

food products, none of the N2O is reacted during the process and all of the N2O is emitted to the atmosphere, 

resulting in an emissions factor of 100 percent for this subcategory (Heydorn 1997). For the remaining 

subcategories, all of the N2O is consumed/reacted during the process, and therefore the emissions rate is considered 

to be zero percent (Tupman 2002). 

The 1990 through 1992 and 1996 N2O production data were obtained from SRI Consulting’s Nitrous Oxide, North 

America report (Heydorn 1997). These data were provided as a range. For example, in 1996, Heydorn (1997) 

estimates N2O production to range between 13.6 and 18.1 thousand metric tons. Tupman (2003) was able to 

provide a narrower range for 1996 that falls within the production bounds described by Heydorn (1997). These data 

are considered more industry specific and current. The midpoint of the narrower production range (15.9 to 18.1 

thousand metric tons) was used to estimate N2O emissions for years 1993 through 2002 (Tupman 2003). 

Production data for 2003 was assumed to equal 2002 data. 

The 1996 share of the total quantity of N2O used by each subcategory was obtained from SRI Consulting’s Nitrous 

Oxide, North America report (Heydorn 1997). The 1990 through 1995 share of total quantity of N2O used by each 

subcategory was kept the same as the 1996 number provided by SRI Consulting. The 1997 through 2002 share of 

total quantity of N2O usage by sector was obtained from communication with a N2O industry expert (Tupman 

2003). Share of total quantity of N2O usage data for 2003 was assumed to equal that of 2002. 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 substantiated by the Encyclopedia of Chemical Technology (Othmer 1990). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 191 

Table 5-4: N2O Production (Gg) 

Year Gg 

1990 16.3 

1991 15.9 

1992 15.0 

1993 17.0 

1994 17.0 

1995 17.0 

1996 17.0 

1997 17.0 

1998 17.0 

1999 17.0 

2000 17.0 

2001 17.0 

2002 17.0 

2003 17.0 

Uncertainty 

Since plant-specific N2O production data is confidential, emissions are based on national production statistics 

acquired as ranges through reports and interviews with industry experts Heydorn (1997) and Tupman (2002). 

Based on these ranges, the uncertainty associated with the production estimate that was used to develop industry 

emissions in 2003 was calculated. Information regarding the industry-specific use of N2O is confidential. Thus, the 

predicted share of the total quantities of N2O used by each subcategory is somewhat uncertain because they are also 

based on industry expert opinion. 

While the level of certainty differs by industry, the minimum and maximum market shares, expressed as a percent of 

N2O usage, are within 2 to 3 percentage points of the estimated market share. The emissions rate for the 

medical/dental industry, an estimate also based on industry opinion, carries an uncertainty level of 3 percent. 

Unquantified areas of uncertainty include the schedule of the market decline of sodium azide production. 

An uncertainty analysis, based on the Tier 1 methods found in IPCC’s Good Practice Guidance and Uncertainty 

Management in National Greenhouse Gas Inventories, was conducted for all inputs to the N2O Product Usage 

source category analysis, including activity data, subcategory shares of N2O consumption, and emission factors. 

The results of the Tier 1 quantitative uncertainly analysis are summarized in Table 5-5. N2O Product Usage N2O 

emissions were estimated to be between 4.4 and 5.1 Tg CO2 Eq. at the 95 percent confidence level. This indicates a 

range of 7 percent above and below the 2003 emission estimate of 4.8 Tg CO2 Eq. 

Table 5-5: Tier 1 Quantitative Uncertainty Estimates for N2O Emissions from Nitrous Oxide Product Usage (Tg 

CO2 Eq. and Percent) 

IPCC Source 

Category Gas 

Year 2003 

Emissions 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Emission Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

N2O Product Usage N2O 4.8 7% 4.4 5.1 

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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 192 

5.2. Ambient Air Pollutants from Solvent Use 

The use of solvents and other chemical products can result in emissions of various ozone precursors (i.e., ambient 

air pollutants).1 Nonmethane 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. Surface coatings 

accounted for approximately 41 percent of NMVOC emissions from solvent use in 2003, while “non-industrial”2

uses accounted for about 38 percent and degreasing applications for 7 percent. Overall, solvent use accounted for 

approximately 28 percent of total U.S. emissions of NMVOCs in 2003; NMVOC emissions from solvent use have 

decreased 21 percent since 1990. 

Although NMVOCs are not considered direct greenhouse gases, their role as precursors to the formation of 

ozone⎯which is a greenhouse gas⎯results in their inclusion in a greenhouse gas inventory. Emissions from 

solvent use have been reported separately by the United States to be consistent with the inventory reporting 

guidelines recommended by the IPCC. These guidelines identify solvent use as one of the major source categories 

for which countries should report emissions. In the United States, emissions from solvents are primarily the result 

of solvent evaporation, whereby the lighter hydrocarbon molecules in the solvents escape into the atmosphere. The 

evaporation process varies depending on different solvent uses and solvent types. The major categories of solvent 

uses include: degreasing, graphic arts, surface coating, other industrial uses of solvents (i.e., electronics, etc.), dry 

cleaning, and non-industrial uses (i.e., uses of paint thinner, etc.). 

Total emissions of NOx, NMVOCs, and CO from 1990 to 2003 are reported in Table 5-6. 

Table 5-6: Emissions of NOx, CO, and NMVOC from Solvent Use (Gg) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003 

NOx 1 3 3 3 3 3 5 4 

Degreasing + + + + + + + + 

Graphic Arts + 1 1 + + + + + 

Dry Cleaning + + + + + + + + 

Surface Coating 1 2 2 3 3 3 5 4 

Other Industrial Processesa

 + + + + + + + + 

Non-Industrial Processesb + + + + + + + + 

Other NA + + + + + + + 

CO 4 1 1 46 46 45 46 65 

Degreasing + + + + + + + + 

Graphic Arts + + + + + + + + 

Dry Cleaning + + + + + + + + 

Surface Coating + 1 1 46 46 45 46 65 

Other Industrial Processesa

 4 + + + + + + + 

Non-Industrial Processesb + + + + + + + + 

Other NA + + + + + + + 

NMVOCs 5,217 5,100 4,671 4,569 4,384 4,547 4,256 4,138 

Degreasing 675 566 337 363 316 331 310 301 

Graphic Arts 249 266 272 224 222 229 214 208 

Dry Cleaning 195 148 151 267 265 272 254 247 

 

1 Solvent usage in the United States also results in the emission of small amounts of hydrofluorocarbons (HFCs) and 

hydrofluoroethers (HFEs), which are included under Substitution of Ozone Depleting Substances in the Industrial Processes 

chapter. 

2 “Non-industrial” uses include cutback asphalt, pesticide application adhesives, consumer solvents, and other miscellaneous 

applications. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 193 

Surface Coating 2,289 2,228 1,989 1,865 1,767 1,863 1,744 1,695 

Other Industrial Processesa

 85 100 101 95 98 103 97 94 

Non-Industrial Processesb 1,724 1,790 1,818 1,714 1,676 1,707 1,598 1,554 

Other + 3 3 40 40 42 40 38 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 2004), 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 data bases. 

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. 

 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 195 

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 LandUse Change and Forestry sector. Carbon dioxide emissions from on-farm energy use are accounted in the Energy 

chapter. 

Figure 6-1: 2003 Agriculture Chapter Greenhouse Gas Emission Sources 

In 2003, the agricultural sector was responsible for emissions of 433.3 Tg CO2 Eq., or 6.3 percent of total U.S. 

greenhouse gas emissions. Methane (CH4) and nitrous oxide (N2O) were the primary greenhouse gases emitted by 

agricultural activities. Methane emissions from enteric fermentation and manure management represent about 21 

percent and 7 percent of total CH4 emissions from anthropogenic activities, respectively. Of all domestic animal 

types, beef and dairy cattle were by far the largest emitters of CH4. Rice cultivation and agricultural crop residue 

burning 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 2003, CH4

emissions from agricultural activities increased by 3.2 percent while N2O emissions increased by 0.7 percent. In 

addition to CH4 and N2O, field burning of agricultural residues was also a minor source of the ambient air pollutants 

carbon monoxide (CO) and nitrogen oxides (NOx). 

Table 6-1: Emissions from Agriculture (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CH4 156.9 163.0 164.2 164.6 162.0 161.9 161.5 161.8 

Enteric Fermentation 117.9 118.3 116.7 116.8 115.6 114.5 114.6 115.0 

Manure Management 31.2 36.4 38.8 38.8 38.1 38.9 39.3 39.1 

Rice Cultivation 7.1 7.5 7.9 8.3 7.5 7.6 6.8 6.9 

Field Burning of Agricultural 

Residues 0.7 0.8 0.8 0.8 0.8 0.8 0.7 0.8 

N2O 269.6 269.8 285.6 261.3 282.1 275.6 270.9 271.5 

Agricultural Soil Management 253.0 252.0 267.7 243.4 263.9 257.1 252.6 253.5 

Manure Management 16.3 17.3 17.4 17.4 17.8 18.0 17.9 17.5 

Field Burning of Agricultural 

Residues 0.4 0.4 0.5 0.4 0.5 0.5 0.4 0.4 

Total 426.5 432.8 449.8 425.9 444.1 437.5 432.4 433.3 

Note: Totals may not sum due to independent rounding. 

Table 6-2: Emissions from Agriculture (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CH4 7,470 7,760 7,821 7,838 7,713 7,708 7,689 7,705 

Enteric Fermentation 5,612 5,634 5,557 5,561 5,505 5,454 5,458 5,475 

Manure Management 1,485 1,733 1,850 1,846 1,813 1,853 1,873 1,864 

Rice Cultivation 339 356 376 395 357 364 325 328 

Field Burning of Agricultural 

Residues 33 37 38 37 38 37 34 38 

N2O 870 870 921 843 910 889 874 876 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 196 

Agricultural Soil Management 816 813 864 785 851 829 815 818 

Manure Management 52 56 56 56 57 58 58 57 

Field Burning of Agricultural 

Residues 1 1 1 1 1 1 1 1 

CO 689 767 789 767 790 770 706 794

NOx 28 34 35 34 35 35 33 33

Note: Totals may not sum due to independent rounding. 

6.1. Enteric Fermentation (IPCC Source Category 4A) 

Methane is produced as part of normal digestive processes in animals. During digestion, microbes resident in an 

animal’s digestive system ferment food consumed by the animal. This microbial fermentation process, referred to 

as enteric fermentation, produces CH4 as a by-product, which can be exhaled or eructated by the animal. The 

amount of CH4 produced and excreted by an individual animal depends primarily upon the animal's digestive 

system, and the amount and type of feed it consumes. 

Among domesticated animal types, 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 cannot. Ruminant animals, consequently, have the highest CH4 emissions among all animal 

types. 

Non-ruminant domesticated 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 affect CH4 emissions. In 

general, lower feed quality or higher feed intake lead to higher CH4 emissions. Feed intake is positively related 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. 

Methane emission estimates from enteric fermentation are provided in Table 6-3 and Table 6-4. Total livestock 

CH4 emissions in 2003 were 115 Tg CO2 Eq. (5,475 Gg), increasing very slightly since 2002 due to minor increases 

in some animal populations and dairy cow milk production in some regions. Beef cattle remain the largest 

contributor of CH4 emissions from enteric fermentation, accounting for 72 percent in 2003. Emissions from dairy 

cattle in 2003 accounted for 24 percent, and the remaining emissions were from horses, sheep, swine, and goats. 

From 1990 to 2003, emissions from enteric fermentation have decreased by 2 percent. Generally, emissions have 

been decreasing since 1995, mainly due to decreasing populations of both beef and dairy cattle and improved feed 

quality for feedlot cattle. During this timeframe, populations of sheep and goats have also decreased, while horse 

populations increased and the populations of swine fluctuated. 

Table 6-3: CH4 Emissions from Enteric Fermentation (Tg CO2 Eq.) 

Livestock Type 1990 1997 1998 1999 2000 2001 2002 2003

Beef Cattle 83.2 86.6 85.0 84.9 83.4 82.4 82.3 82.5

Dairy Cattle 28.9 26.4 26.3 26.6 27.0 26.9 27.1 27.3

Horses 1.9 2.0 2.0 2.0 2.0 2.0 2.0 2.0

Sheep 1.9 1.3 1.3 1.2 1.2 1.2 1.1 1.1

Swine 1.7 1.8 2.0 1.9 1.9 1.9 1.9 1.9

Goats 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2

Total 117.9 118.3 116.7 116.8 115.6 114.5 114.6 115.0

Note: Totals may not sum due to independent rounding. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 197 

Table 6-4: CH4 Emissions from Enteric Fermentation (Gg) 

Livestock Type 1990 1997 1998 1999 2000 2001 2002 2003

Beef Cattle 3,961 4,124 4,047 4,045 3,973 3,923 3,919 3,930

Dairy Cattle 1,375 1,255 1,251 1,265 1,283 1,282 1,290 1,300

Horses 91 93 94 93 94 95 95 95

Sheep 91 64 63 58 56 56 53 50

Swine 81 88 93 90 88 88 90 90

Goats 13 10 10 10 10 10 10 10

Total 5,612 5,634 5,557 5,561 5,505 5,454 5,458 5,475

Note: Totals may not sum due to independent rounding. 

Methodology 

Livestock emission estimates fall into two categories: cattle and other domesticated animals. Cattle, due to their 

large population, large size, and particular digestive characteristics, account for the majority of CH4 emissions from 

livestock in the United States. A more detailed methodology (i.e., IPCC Tier 2) was therefore applied to estimating 

emissions for all cattle except for bulls. Emission estimates for other domesticated animals (horses, sheep, swine, 

goats, and bulls) were handled using a less detailed approach (i.e., IPCC Tier 1). 

While the large diversity of animal management practices cannot be precisely characterized and evaluated, 

significant scientific literature exists that describes the quantity of CH4 produced by individual ruminant animals, 

particularly cattle. A detailed model that incorporates this information and other analyses of livestock population, 

feeding practices and production characteristics was used to estimate emissions from cattle populations. 

National cattle population statistics were disaggregated into the following cattle sub-populations: 

Dairy Cattle 

• Calves 

• Heifer Replacements 

• Cows 

Beef Cattle 

• Calves 

• Heifer Replacements 

• Heifer and Steer Stockers 

• 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 model 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 National Agricultural Statistics Service 

(1995a,b, 1999a,c,d,f, 2000a,c,d,f, 2001a,c,d,f, 2002a,c,d,f, 2003a,c,d,f, 2004a,c,d,f). 

Diet characteristics were estimated by region for U.S. dairy, beef, and feedlot cattle. These estimates were used to 

calculate Digestible Energy (DE) values and CH4 conversion rates (Ym) for each population category. The IPCC 

recommends Ym values of 3.5 to 4.5 percent for feedlot cattle and 5.5 to 6.5 percent for other well-fed cattle 

consuming temperate-climate feed types. 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 198 

opinion, and modeling of animal physiology. The diet characteristics for dairy cattle were from Donovan (1999), 

while beef cattle were derived from NRC (2000). DE and Ym for dairy cows were calculated from diet 

characteristics using a model simulating ruminant digestion in growing and/or lactating cattle (Donovan and 

Baldwin 1999). For feedlot animals, DE and Ym values recommended by Johnson (1999) were used. Values from 

EPA (1993) were used for dairy replacement heifers. For grazing beef cattle, DE values were based on diet 

information in NRC (2000) and Ym values were based on Johnson (2002). Weight data were estimated from 

Feedstuffs (1998), Western Dairyman (1998), and expert opinion. See Annex 3.9 for more details on the method 

used to characterize cattle diets in the United States. 

To estimate CH4 emissions from cattle, the population was divided into region, age, sub-type (e.g., calves, heifer 

replacements, cows, etc.), and production (i.e., pregnant, lactating, etc.) groupings to more fully capture differences 

in CH4 emissions from these animal types. Cattle diet characteristics were used to develop regional emission factors 

for each sub-category. Tier 2 equations from IPCC (2000) were used to produce CH4 emission factors for the 

following cattle types: dairy cows, beef cows, dairy replacements, beef replacements, steer stockers, heifer stockers, 

steer feedlot animals, and heifer feedlot animals. To estimate emissions from cattle, population data were multiplied 

by the emission factor for each cattle type. More details are provided in Annex 3.9. 

Emission estimates for other animal types were based on average emission factors representative of entire 

populations of each animal type. Methane emissions from these animals accounted for a minor portion of total CH4

emissions from livestock in the United States from 1990 through 2003. 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, as well as feedlot placement information were obtained from the U.S. Department of Agriculture’s National 

Agricultural Statistics Service (USDA 1994a-b, 1995a-b, 1998a-b, 1999a-c, 2000a-g, 2001a-f, 2002a-f, 2003a-f, 

2004a-f). Horse population data were obtained from the FAOSTAT database (FAO 2004), because USDA does not 

estimate U.S. horse populations annually. Goat population data were obtained from the Census of Agriculture 

(USDA 1999g). Methane emissions from sheep, goats, swine, and horses were estimated by using emission factors 

utilized in Crutzen et al. (1986, cited in IPCC/UNEP/OECD/IEA 1997). 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 (IPCC/UNEP/OECD/IEA 1997, IPCC 2000).

See Annex 3.9 for more detailed information on the methodology and data used to calculate CH4 emissions from 

enteric fermentation. 

Uncertainty 

Uncertainty estimates were developed for the emission estimates presented in EPA (2003). No significant changes 

occurred in the method of data collection, data estimation methodology, or other factors that influence the 

uncertainty ranges around the 2003 activity data and emission factor input variables. Consequently, the EPA (2003) 

uncertainty estimates were directly applied to the 2003 emission estimates. 

A total of 185 primary input variables (178 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. A triangular distribution was assigned for three input variables (specifically cow-birth ratios for the 

current and the past two years). For some key input variables, the uncertainty ranges around their estimates (used 

for inventory estimation) were collected from published documents and other public sources. In addition, both 

endogenous and exogenous correlations between selected primary input variables were modeled. The exogenous 

correlation coefficients between the probability distributions of selected activity-related variables were developed as 

educated estimates. 

The uncertainty ranges associated with the activity-related input variables were no larger in magnitude than plus or 

minus 10 percent. However, for many emission factor-related input variables, the lower- and/or upper-bound 

uncertainty estimates were over 20 percent. The results of the Tier 2 quantitative uncertainty analysis are 

summarized in Table 6-5. Enteric fermentation CH4 emissions in 2003 were estimated to be between 102.3 and 

135.7 Tg CO2 Eq. at a 95 percent confidence level (or in 19 out of 20 Monte Carlo Stochastic Simulations). This 

indicates a range of 11 percent below to 18 percent above the 2003 emission estimate of 115.0 Tg CO2 Eq. Among 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 199 

the individual sub-source categories, beef cattle accounts for the largest amount of CH4 emissions as well as the 

largest degree of uncertainty in the emission estimates. Consequently, the cattle sub-source categories together 

contribute to the largest degree of uncertainty to the estimates of CH4 emissions from livestock enteric fermentation. 

Among non-cattle, horses account for the largest degree of uncertainty in the emission estimates. 

Table 6-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Enteric Fermentation (Tg CO2 Eq. 

and Percent)

Source Gas 

2003 Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Enteric Fermentation CH4 115.0 102.3 135.7 -11% +18% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

QA/QC and Verification 

In order to ensure the quality of the emission estimates from enteric fermentation, the IPCC Tier 1 and Tier 2 

Quality Assurance/Quality Control (QA/QC) procedures were implemented consistent with the U.S. QA/QC plan. 

Tier 2 QA procedures included independent peer review of emission estimates. Particular emphasis was placed on 

cattle population and growth data, and on evaluating the effects of data updates as described in the recalculations 

discussion below. 

Recalculations Discussion 

While there were no changes in the methodologies used for estimating CH4 emissions from enteric fermentation, 

emissions were revised slightly due to changes in historical data. USDA published revised population estimates in 

2004 for some cattle statistics; these include population, livestock placements, and slaughter statistics for 2000, 

2001, and 2002. Emission estimates changed for these years for both beef and dairy cattle as a result of revised 

inputs that reflect USDA updates. 

The rate of weight gain for growing steers and heifers was increased for the modeling of 2000 through 2003. The 

model uses the weight gain data to estimate the number of cattle (steers and heifers) available to be placed into 

feedlots (by weight class). These estimates were compared to the USDA statistics on actual feedlot placements (by 

weight class). The updated USDA data show increases in feedlot placements in the heavy weight classes, and 

required an increase in the rate of weight gain in the modeled population in order to match the observed statistics. 

Additionally, the distribution of cattle by weight at the start of the year was adjusted to reflect the larger portion of 

heavier animals. 

In 2000, both beef and dairy cattle emissions changed less than 3 Gg (0.1 percent) as a result of the recalculations. 

In 2001, beef cattle CH4 emissions increased 12 Gg (0.3 percent), while dairy cattle emissions decreased 1 Gg (0.1 

percent). In 2002, beef cattle CH4 emissions increased 8 Gg (0.2 percent), while dairy cattle emissions increased 

less than 1 Gg (0.03 percent). For other livestock types, a slight upward revision in the swine population for 2002 

resulted in an increase in CH4 emissions of less than 1 Gg (0.06 percent) in that year. Overall, the changes resulted 

in an average annual increase of less than 0.1 Tg CO2 Eq. (0.04 percent) in CH4 emissions from enteric fermentation 

for the period 1990 through 2002. 

Planned Improvements 

The revised and updated USDA data discussed above highlight the need to re-examine several model inputs. 

Although the enteric fermentation model was constructed to identify the imbalances mentioned in the recalculations 

discussion, the current inventory presents the first effort to address such differences by making adjustments to 

model inputs. The updates are based both on expert opinion and on equations published by the American Society of 

Agricultural Engineers (ASAE) that predict weight versus age statistics for steers and imply growth rates larger than 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 200 

those used in previous years (ASAE 1999). In addition, in 2001, USDA reported increased rates of gains for 

yearlings (USDA 2001g). While these two sources provide support for the updates, further research is necessary to 

verify the changes and to understand what changes over time may be necessary in future inventory analyses. 

6.2. Manure Management (IPCC Source Category 4B) 

The management of livestock manure can produce anthropogenic CH4 and N2O emissions. Methane is produced by 

the anaerobic decomposition of manure. Nitrous oxide is produced as part of the nitrogen cycle through the 

nitrification and denitrification of the organic nitrogen in livestock manure and urine. 

When livestock or poultry manure are stored or treated in systems that promote anaerobic conditions (e.g., as a 

liquid/slurry in lagoons, ponds, tanks, or pits), the decomposition of materials in the manure tends to produce CH4. 

When manure is handled as a solid (e.g., in stacks or pits) or deposited on pasture, range, or paddock lands, it tends 

to decompose aerobically and produce little or no CH4. A number of other factors related to how the manure is 

handled also affect the amount of CH4 produced. 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 example, CH4 production generally increases with rising temperature and residency time. Also, for 

non-liquid-based manure systems, moist conditions (which are a function of rainfall and humidity) favor CH4

production. Although the majority of manure is handled as a solid, producing little CH4, the general trend in 

manure management, particularly for large dairy and swine producers, is one of increasing use of liquid systems. In 

addition, use of daily spread systems at smaller dairies is decreasing, due to new regulations limiting the application 

of manure nutrients, which has resulted in an increase of manure managed and stored on site at these smaller dairies. 

The composition of the manure also affects the amount of CH4 produced. Manure composition varies by animal 

type, including the animal’s digestive system and diet. In general, the greater the energy content of the feed, the 

greater the potential for CH4 emissions. For example, feedlot cattle fed a high-energy grain diet generate manure 

with a high CH4-producing capacity. Range cattle fed a low energy diet of forage material produce manure with 

about 50 percent of the CH4-producing potential of feedlot cattle manure. However, some higher energy feeds also 

are more digestible than lower quality forages, which can result in less overall waste excreted from the animal. 

Ultimately, a combination of diet types and the growth rate of the animals will affect the quantity and characteristics 

of the manure produced. 

A very small portion of the total nitrogen excreted is expected to convert to N2O in the waste management system. 

The production of N2O 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 N2O emissions to 

occur, the manure must first be handled aerobically where ammonia or organic nitrogen is converted to nitrates and 

nitrites (nitrification), and then handled anaerobically where the nitrates and nitrites are reduced to nitrogen gas 

(N2), with intermediate production of N2O and nitric oxide (NO) (denitrification) (Groffman et al. 2000). These 

emissions are most likely to occur in dry manure handling systems that have aerobic conditions, but that also 

contain pockets of anaerobic conditions due to saturation. For example, manure at cattle drylots is deposited on 

soil, oxidized to nitrite and nitrate, and has the potential to encounter saturated conditions following rain events. 

Certain N2O emissions are accounted for and discussed in the Agricultural Soil Management source category within 

the Agriculture sector. These are emissions from livestock manure and urine deposited on pasture, range, or 

paddock lands, as well as emissions from manure and urine that is spread onto fields either directly as “daily 

spread” or after it is removed from manure management systems (e.g., lagoon, pit, etc.). 

Table 6-6 and Table 6-7 provide estimates of CH4 and N2O emissions from manure management by animal 

category. Estimates for CH4 emissions in 2003 were 39.1 Tg CO2 Eq. (1,864 Gg), 25 percent higher than in 1990. 

The majority of this increase was from swine and dairy cow manure, where emissions increased 30 and 38 percent, 

respectively. The increase in emissions from these animal types is primarily attributed to shifts by the swine and 

dairy industries towards larger facilities. Larger swine and dairy farms tend to use liquid 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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 201 

accounted for by incorporating state-specific weighted CH4 conversion factor (MCF) values in combination with the 

1992 and 1997 farm-size distribution data reported in the Census of Agriculture (USDA 1999e). From 2002 to 

2003, there was a 0.5 percent decrease in CH4 emissions, due to minor shifts in the animal populations and the 

resultant effects on manure management system allocations. A description of the emission estimation methodology 

is provided in Annex 3.10. 

Total N2O emissions from manure management systems in 2003 were estimated to be 17.5 Tg CO2 Eq. (57 Gg). 

The 8 percent increase in N2O emissions from 1990 to 2003 can be partially attributed to a shift in the poultry 

industry away from the use of liquid manure management systems, in favor of litter-based systems and high-rise 

houses. In addition, there was an overall increase in the population of poultry and swine from 1990 to 2002, 

although swine populations periodically declined slightly throughout the time series. Nitrous oxide emissions 

showed a 2 percent decrease from 2002 to 2003, due to minor shifts in animal population. 

The population of beef cattle in feedlots increased over the period of 1990 to 2003, resulting in increased N2O 

emissions from this sub-category of cattle. Although dairy cow populations decreased overall for the period 1990 to 

2003, the population of dairies managing and storing manure on-site—as opposed to using pasture, range, or 

paddock or daily spread systems—increased. Over the same period, dairies also experienced a shift to more liquid 

manure management systems at large operations, which result in lower N2O emissions then dry systems. The net 

result is a slight decrease in dairy cattle N2O emissions over the period 1990 to 2003. As stated previously, N2O 

emissions from livestock manure deposited on pasture, range, or paddock land and manure immediately applied to 

land in daily spread systems are accounted for in the Agricultural Soil Management source category of the 

Agriculture sector. 

Table 6-6: CH4 and N2O Emissions from Manure Management (Tg CO2 Eq.) 

Gas/Animal 

Type 

1990 1997 1998 1999 2000 2001 2002 2003 

CH4 31.2 36.4 38.8 38.8 38.1 38.9 39.3 39.1 

Dairy Cattle 11.4 13.4 13.9 14.7 14.5 15.0 15.2 15.7 

Beef Cattle 3.2 3.2 3.1 3.1 3.1 3.1 3.1 3.1 

Swine 13.1 16.4 18.4 17.6 17.1 17.4 17.7 17.0 

Sheep 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Goats + + + + + + + + 

Poultry 2.7 2.7 2.7 2.6 2.6 2.7 2.7 2.7 

Horses 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 

N2O 16.3 17.3 17.4 17.4 17.8 18.0 17.9 17.5 

Dairy Cattle 4.3 4.0 3.9 4.0 4.0 3.9 3.9 3.9 

Beef Cattle 4.9 5.4 5.5 5.5 5.9 6.1 5.9 5.6 

Swine 0.4 0.4 0.5 0.4 0.4 0.4 0.4 0.4 

Sheep 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Goats + + + + + + + + 

Poultry 6.4 7.2 7.2 7.2 7.2 7.3 7.4 7.3 

Horses 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 

Total 47.4 53.7 56.2 56.2 55.9 57.0 57.3 56.7 

+ Does not exceed 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. 

Table 6-7: CH4 and N2O Emissions from Manure Management (Gg) 

Gas/Animal 

Type 

1990 1997 1998 1999 2000 2001 2002 2003 

CH4 1,485 1,733 1,850 1,846 1,813 1,853 1,873 1,864 

Dairy Cattle 545 639 662 700 692 715 722 748 

Beef Cattle 153 152 149 150 149 148 147 146 

Swine 622 780 874 837 812 826 843 808 

Sheep 9 6 6 6 5 5 5 5 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 202 

Goats 1 1 1 1 1 1 1 1 

Poultry 128 127 130 125 125 129 126 127 

Horses 27 28 28 28 28 29 29 29 

N2O 52 56 56 56 57 58 58 57 

Dairy Cattle 14 13 13 13 13 13 13 13 

Beef Cattle 16 17 18 18 19 20 19 18 

Swine 1 1 1 1 1 1 1 1 

Sheep + + + + + + + + 

Goats + + + + + + + + 

Poultry 21 23 23 23 23 24 24 24 

Horses 1 1 1 1 1 1 1 1 

+ Does not exceed 0.5 Gg. 

Note: Totals may not sum due to independent rounding. 

Methodology 

The methodologies presented in Good Practice Guidance and Uncertainty Management in National Greenhouse 

Gas Inventories (IPCC 2000) form the basis of the CH4 and N2O emission estimates for each animal type. The 

calculation of emissions requires the following information: 

• Animal population data (by animal type and state); 

• Amount of nitrogen produced (excretion rate by animal type times animal population); 

• Amount of volatile solids produced (excretion rate by animal type times animal population); 

• Methane producing potential of the volatile solids (by animal type); 

• Extent to which the CH4 producing potential is realized for each type of manure management system (by 

state and manure management system, including the impacts of any biogas collection efforts); 

• Portion of manure managed in each manure management system (by state and animal type); and 

• Portion of manure deposited on pasture, range, or paddock or used in daily spread systems. 

This section presents a summary of the methodologies used to estimate CH4 and N2O emissions from manure 

management for this inventory. See Annex 3.10 for more detailed information on the methodology and data used to 

calculate CH4 and N2O emissions from manure management. 

Both CH4 and N2O emissions were estimated by first determining activity data, including animal population, waste 

characteristics, and manure management system usage. For swine and dairy cattle, manure management system 

usage was determined for different farm size categories using data from USDA (USDA 1996b, 1998d, 2000h) and 

EPA (ERG 2000a, EPA 2001a, 2001b). For beef cattle and poultry, manure management system usage data was not 

tied to farm size (ERG 2000a, USDA 2000i, UEP 1999). For other animal types, manure management system usage 

was based on previous estimates (EPA 1992). 

Next, MCFs and N2O emission factors were determined for all manure management systems. MCFs for dry systems 

and N2O emission factors for all systems were set equal to default IPCC factors for temperate climates (IPCC 2000). 

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 (see Annex 

3.10 for detailed information on MCF derivations for liquid systems). The MCF calculations model the average 

monthly ambient temperature, a minimum system temperature, the carryover of volatile solids in the system from 

month to month due to long storage times exhibited by anaerobic lagoon systems, and a factor to account for 

management and design practices that result in the loss of volatile solids from lagoon systems. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 203 

For each animal group, the base emission factors were then weighted to incorporate the distribution of management 

systems used within each state and thereby to create an overall state-specific weighted emission factor. To calculate 

this weighted factor, the percent of manure for each animal group managed in a particular system in a state was 

multiplied by the emission factor for that system and state, and then summed for all manure management systems in 

the state. 

Methane emissions were estimated using the volatile solids (VS) production for all livestock. For poultry and swine 

animal groups, for example, volatile solids production was calculated using a national average volatile solids 

production rate from the Agricultural Waste Management Field Handbook (USDA 1996a), which was then 

multiplied by the average weight of the animal and the state-specific animal population. For most cattle groups, 

regional animal-specific volatile solids production rates that are related to the diet of the animal for each year of the 

inventory were used (Lieberman et al., 2004). The resulting volatile solids for each animal group was then 

multiplied by the maximum CH4 producing capacity of the waste (Bo) and the state-specific CH4 conversion factors. 

Nitrous oxide emissions were estimated by determining total Kjeldahl nitrogen (TKN)1 production for all livestock 

wastes using livestock population data and nitrogen excretion rates based on measurements of excreted manure. For 

each animal group, TKN production was calculated using a national average nitrogen excretion rate from the 

Agricultural Waste Management Field Handbook (USDA 1996a), which was then multiplied by the average weight 

of the animal and the state-specific animal population. State-specific weighted N2O emission factors specific to the 

type of manure management system were then applied to total nitrogen production to estimate N2O emissions. 

The data used to calculate the inventory estimates were based on a variety of sources. Animal population data for 

all livestock types, except horses and goats, were obtained from the U.S. Department of Agriculture’s National 

Agricultural Statistics Service (USDA 1994a-b, 1995a-b, 1998a-b, 1999a-c, 2000a-g, 2001a-f, 2002a-f, 2003a-f, 

2004a-f). Horse population data were obtained from the FAOSTAT database (FAO 2004), because USDA does not 

estimate U.S. horse populations annually. Goat population data were obtained from the Census of Agriculture 

(USDA 1999d). Information regarding poultry turnover (i.e., slaughter) rate was obtained from state Natural 

Resource Conservation Service (NRCS) personnel (Lange 2000). Dairy cow and swine population data by farm 

size for each state, used for the weighted MCF and emission factor calculations, were obtained from the Census of 

Agriculture, which is conducted every five years (USDA 1999e). 

Manure management system usage data for dairy and swine operations were obtained from USDA’s Centers for 

Epidemiology and Animal Health (USDA 1996b, 1998d, 2000h) for small operations and from preliminary 

estimates for EPA’s Office of Water regulatory effort for large operations (ERG 2000a; EPA 2001a, 2001b). Data 

for layers were obtained from a voluntary United Egg Producers’ survey (UEP 1999), previous EPA estimates (EPA 

1992), and USDA’s Animal Plant Health Inspection Service (USDA 2000i). Data for beef feedlots were also 

obtained from EPA’s Office of Water (ERG 2000a; EPA 2001a, 2001b). Manure management system usage data 

for other livestock were taken from previous estimates (EPA 1992). Data regarding the use of daily spread and 

pasture, range, or paddock systems for dairy cattle were obtained from personal communications with personnel 

from several organizations, and data provided by those personnel (Poe et al. 1999). These organizations include 

state NRCS offices, state extension services, state universities, USDA National Agriculture Statistics Service 

(NASS), and other experts (Deal 2000, Johnson 2000, Miller 2000, Stettler 2000, Sweeten 2000, and Wright 2000). 

Additional information regarding the percent of beef steer and heifers on feedlots was obtained from contacts with 

the national USDA office (Milton 2000). 

Methane conversion factors for liquid systems were calculated based on average ambient temperatures of the 

counties in which animal populations were located. The average county and state temperature data were obtained 

from the National Climate Data Center (NOAA 2004), and the county population data were calculated from statelevel population data from NASS and county-state distribution data from the 1992 and 1997 Census data (USDA 

1999e). County population distribution data for 1990 and 1991 were assumed to be the same as 1992; county 

 

1 Total Kjeldahl nitrogen is a measure of organically bound nitrogen and ammonia nitrogen. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 204 

population distribution data for 1998 through 2003 were assumed to be the same as 1997; and county population 

distribution data for 1993 through 1996 were extrapolated based on 1992 and 1997 data. 

The maximum CH4 producing capacity of the volatile solids, or Bo, was determined based on data collected in a 

literature review (ERG 2000b). Bo data were collected for each animal type for which emissions were estimated. 

Nitrogen excretion rate data from the USDA Agricultural Waste Management Field Handbook (USDA 1996a) were 

used for all livestock except sheep, goats, and horses. Data from the American Society of Agricultural Engineers 

(ASAE 1999) were used for these animal types. Volatile solids excretion rate data from the USDA Agricultural 

Waste Management Field Handbook (USDA 1996a) were used for swine, poultry, bulls, and calves not on feed. In 

addition, volatile solids production rates from Lieberman et al. (2004) were used for dairy and beef cows, heifers, 

and steer for each year of the inventory. Nitrous oxide emission factors and MCFs for dry systems were taken from 

Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (IPCC 2000). 

Uncertainty 

An analysis was conducted for the manure management emission estimates presented in EPA (2003) to determine 

the uncertainty associated with estimating N2O and CH4 emissions from livestock manure management. Because no 

substantial modifications were made to the inventory methodology since the development of these estimates, it is 

expected that this analysis is applicable to the uncertainty associated with the current manure management emission 

estimates. 

The EPA (2003) quantitative uncertainty analysis for this source category was performed through the IPCCrecommended Tier 2 uncertainty estimation methodology, Monte Carlo Stochastic Simulation technique. The 

uncertainty analysis was developed based on the methods used to estimate N2O and CH4 emissions from manure 

management systems. A normal probability distribution was assumed for each source data category. The series of 

equations used were condensed into a single equation for each animal type and state. The equations for each animal 

group contained four to five variables around which the uncertainty analysis was performed for each state. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 6-8. Manure management CH4

emissions in 2003 were estimated to be between 32.1 and 47.0 Tg CO2 Eq. at a 95 percent confidence level (or 19 of 

20 Monte Carlo Stochastic Simulations). This indicates a range of 18 percent below to 20 percent above the 2003 

emission estimate of 39.1 Tg CO2 Eq. At the 95 percent confidence level, N2O emissions were estimated to be 

between 14.7 and 21.7 Tg CO2 Eq. (or approximately 16 percent below and 24 percent above the 2003 emission 

estimate of 17.5 Tg CO2 Eq.). 

Table 6-8: Tier 2 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Manure Management (Tg 

CO2 Eq. and Percent)

Source Gas 

2003 Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Manure Management CH4 39.1 32.1 47.0 -18% +20% 

Manure Management N2O 17.5 14.7 21.7 -16% +24% a

Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

The primary factors that contribute to the uncertainty in emission estimates are a lack of information on the usage of 

various manure management systems in each regional location and the exact CH4 generating characteristics of each 

type of manure management system. Because of significant shifts in the swine and dairy sectors toward larger 

farms, it is believed that increasing amounts of manure are being managed in liquid manure management systems. 

The existing estimates reflect these shifts in the weighted MCFs based on the 1992 and 1997 farm-size data. 

However, the assumption of a direct relationship between farm size and liquid system usage may not apply in all 

cases and may vary based on geographic location. In addition, the CH4 generating characteristics of each manure 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 205 

management system type are based on relatively few laboratory and field measurements, and may not match the 

diversity of conditions under which manure is managed nationally. 

Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (IPCC 2000) 

published a default range of MCFs for anaerobic lagoon systems of 0 to 100 percent, which reflects the wide range 

in performance that may be achieved with these systems. There exist relatively few data points on which to 

determine country-specific MCFs for these systems. In the United States, many livestock waste treatment systems 

classified as anaerobic lagoons are actually holding ponds that are substantially organically overloaded and 

therefore not producing CH4 at the same rate as a properly designed lagoon. In addition, these systems may not be 

well operated, contributing to higher loading rates when sludge is allowed to enter the treatment portion of the 

lagoon or the lagoon volume is pumped too low to allow treatment to occur. Rather than setting the MCF for all 

anaerobic lagoon systems in the United States based on data available from optimized lagoon systems, a MCF 

methodology was developed that more closely matches observed system performance and accounts for the affect of 

temperature on system performance. 

However, there is uncertainty related to this methodology. The MCF methodology used in the inventory includes a 

factor to account for management and design practices that result in the loss of volatile solids from the management 

system. This factor is currently estimated based on data from anaerobic lagoons in temperate climates, and from 

only three systems. However, this methodology is intended to account for systems across a range of management 

practices. Future work in gathering measurement data from animal waste lagoon systems across the country will 

contribute to the verification and refinement of this methodology. It will also be evaluated whether lagoon 

temperatures differ substantially from ambient temperatures and whether the lower bound estimate of temperature 

established for lagoons and other liquid systems should be revised for use with this methodology. 

The IPCC provides a suggested MCF for poultry waste management operations of 1.5 percent. Additional study is 

needed in this area to determine if poultry high-rise houses promote sufficient aerobic conditions to warrant a lower 

MCF. 

The default N2O emission factors published in Good Practice Guidance and Uncertainty Management in National 

Greenhouse Gas Inventories (IPCC 2000) were derived using limited information. The IPCC factors are global 

averages; U.S.-specific emission factors may be significantly different. Manure and urine in anaerobic lagoons and 

liquid/slurry management systems produce CH4 at different rates, and would in all likelihood produce N2O at 

different rates, although a single N2O emission factor was used for both system types. In addition, there are little 

data available to determine the extent to which nitrification-denitrification occurs in animal waste management 

systems. Ammonia concentrations that are present in poultry and swine systems suggest that N2O emissions from 

these systems may be lower than predicted by the IPCC default factors. At this time, there are insufficient data 

available to develop U.S.-specific N2O emission factors; however, this is an area of on-going research, and warrants 

further study as more data become available. 

Uncertainty also exists with the maximum CH4 producing potential of volatile solids excreted by different animal 

groups (i.e., Bo). The Bo values used in the CH4 calculations are published values for U.S. animal waste. However, 

there are several studies that provide a range of Bo values for certain animals, including dairy and swine. The Bo

values chosen for dairy assign separate values for dairy cows and dairy heifers to better represent the feeding 

regimens of these animal groups. For example, dairy heifers do not receive an abundance of high energy feed and 

consequently, dairy heifer manure will not produce as much CH4 as manure from a milking cow. However, the data 

available for Bo values are sparse, and do not necessarily reflect the rapid changes that have occurred in this 

industry with respect to feed regimens. 

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 2002 and 2003 Inventories for N2O emissions from managed systems and CH4

emissions from livestock manure. All errors identified were corrected. Order of magnitude checks were also 

conducted, and corrections made where needed. Manure nitrogen data were quality assured by comparing state-level 

data with bottom up estimates derived at the county level and summed to the state level. Similarly, a comparison 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 206 

was made by animal and waste management system type for the full time series, between national level estimates for 

nitrogen excreted and the sum of county estimates for the full time series. Efforts also continue to transition various 

components of the manure management inventory into a database to facilitate current and future QA checks. 

Recalculations Discussion 

No changes have been incorporated into the overall methodology for the manure management emission estimates; 

however, changes were made to the calculation of CH4 emissions from sheep, goats, and horses. Changes were also 

made to address errors and updates in the population and waste management system data from previous inventory 

submittals. Additionally the population distribution of horses and poultry were adjusted, the typical animal mass for 

sheep was adjusted, and the temperature estimations were changed to reflect a refined methodology. Each of these 

changes is described in detail below. 

• Methane emission estimation from sheep, goats, and horses. The sheep, goats, and horses emission 

methodologies were changed to be consistent with the methodologies used for the other animal groups. 

Previously, the sheep, goat, and horse methane estimates were scaled based on population data and earlier 

estimates of methane emissions (EPA 1992). 

• Population. All USDA data from 1998 through the present year underwent review pursuant to USDA 

NASS annual review procedures. The population data in these years reflect some adjustments due to this 

review. For horses, state-level populations were estimated using the national FAO population data and the 

state distributions from the 1992 and 1997 Census of Agriculture. For poultry, populations for states 

reporting non-disclosed populations were estimated by distributing population values attributed to “other” 

states. 

• Waste management system. The waste management system data for poultry were adjusted based on more 

recent data. Previously, layers were estimated to be 99 percent managed (EPA 1992). More recent WMS 

data available from USDA's Animal Plant and Health Inspection Service Layers '99 study (USDA 2000i) 

and the United Egg Producers Study (UEP 1999) indicate that layers are 100 percent managed. Therefore, 

the layer WMS estimates have been updated accordingly. Also, the waste management system distribution 

for dairy cows was adjusted to correct rounding errors. 

• Typical animal mass. The typical animal mass for sheep were reevaluated and adjusted. Typical animal 

mass of sheep was adjusted from 27 kg to 68.6 kg (see Annex 3.10 for details). 

• Temperature data: Temperature data are not available for every county with animal populations. 

Previously, counties without temperature data were not accounted for in the estimate of average weighted 

temperature. This methodology was changed to use the state average temperature for counties without 

temperature data available. 

The combination of these changes resulted in an average annual increase of 0.1 Tg CO2 Eq. (0.3 percent) in CH4

emissions and an average annual increase of 0.1 Tg CO2 Eq. (0.4 percent) in N2O emissions from manure 

management for the period 1990 through 2002. 

Planned Improvements 

Currently, temperate zone MCFs are used for non-liquid waste management systems, including pasture, range, and 

paddock, daily spread, solid storage, and drylot operations. However, there are some states that have an annual 

average temperature that would fall below 15°C (i.e., “cool”). Therefore, CH4 emissions from certain non-liquid 

waste management systems may be overestimated; however, the difference is expected to be relatively small due to 

the low MCFs for all “dry” management systems. The use of both cool and temperate MCFs for non-liquid waste 

management systems will be investigated for future inventories. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 207 

Although an effort was made to introduce the variability in volatile solids production due to differences in diet for 

beef and dairy cows, heifers, and steer, further research is needed to confirm and track diet changes over time. A 

methodology to assess variability in swine volatile solids production would be useful in future inventory estimates. 

The American Society of Agricultural Engineers is publishing new standards for manure production characteristics 

in 2004. These data will be investigated and evaluated for incorporation into future estimates. 

The development of the National Ammonia Emissions Inventory for the United States (EPA 2004) used similar data 

sources to the current estimates of emissions from manure management, and through the course of development of 

the ammonia inventory, updated waste management distribution data were identified. Future estimates will attempt 

to reflect these updated data. 

The methodology to calculate MCFs for liquid systems will be examined to determine how to account for a 

maximum temperature in the liquid systems. Additionally, available research will be investigated to develop a 

relationship between ambient air temperature and temperature in liquid waste management systems in order to 

improve that relationship in the MCF methodology. 

Research will be initiated into the estimation and validation of the maximum CH4-producing capacity of animal 

manure (Bo), for the purpose of obtaining more accurate data to develop emission estimates. 

The 2002 Census of Agriculture became available in mid-2004. These data will be used to update assumptions that 

previously relied on the 1992 and 1997 Census of Agriculture. 

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 the oxygen present in the soil and floodwater, causing 

anaerobic conditions in the soil to develop. 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 (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 more shallow 

flooding depths. Some flooded fields are drained periodically during the growing season, either intentionally or 

accidentally. If water is drained and soils are allowed to dry sufficiently, CH4 emissions decrease or stop entirely. 

This is due to soil aeration, which not only causes existing soil CH4 to oxidize but also inhibits further CH4

production in soils. All rice in the United States is grown under continuously flooded conditions; none is grown 

under deepwater conditions. Mid-season drainage does not occur except by accident (e.g., due to levee breach). 

Other factors that influence CH4 emissions from flooded rice fields include fertilization practices (especially the use 

of organic fertilizers), soil temperature, soil type, rice variety, and cultivation practices (e.g., tillage, seeding and 

weeding practices). The factors that determine the amount of organic material that is available to decompose (i.e., 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 208 

organic fertilizer use, soil type, rice variety,2 and cultivation practices) are the most important variables influencing 

the amount of CH4 emitted over an entire growing season because the total amount of CH4 released depends 

primarily on the amount of organic substrate available. Soil temperature is known to be an important factor 

regulating the activity of methanogenic bacteria, and therefore the rate of CH4 production. However, although 

temperature controls the amount of time it takes to convert a given amount of organic material to CH4, that time is 

short relative to a growing season, so the dependence of total emissions over an entire growing season on soil 

temperature is weak. The application of synthetic fertilizers has also been found to influence CH4 emissions; in 

particular, both nitrate and sulfate fertilizers (e.g., ammonium nitrate, and ammonium sulfate) appear to inhibit CH4

formation. 

Rice is cultivated in eight states: Arkansas, California, Florida, Louisiana, Mississippi, Missouri, Oklahoma, and 

Texas. Soil types, rice varieties, and cultivation practices for rice vary from state to state, and even from farm to 

farm. However, most rice farmers utilize organic fertilizers in the form of rice residue from the previous crop, 

which is left standing, disked, or rolled into the fields. Most farmers also apply synthetic fertilizer to their fields, 

usually urea. Nitrate and sulfate fertilizers are not commonly used in rice cultivation in the United States. In 

addition, the climatic conditions of Arkansas, southwest Louisiana, Texas, and Florida allow for a second, or ratoon, 

rice crop. Methane emissions from ratoon crops have been found to be considerably higher than those from the 

primary crop. This second rice crop is produced from regrowth of the stubble after the first crop has been 

harvested. Because the first crop’s stubble is left behind in ratooned fields, and there is no time delay between 

cropping seasons (which would allow for the stubble to decay aerobically), the amount of organic material that is 

available for 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 2003, CH4 emissions 

from rice cultivation were 6.9 Tg CO2 Eq. (328 Gg). Although annual emissions fluctuated unevenly between the 

years 1990 and 2003, ranging from an annual decrease of 11 percent to an annual increase of 17 percent, there was 

an overall decrease of 3 percent over the thirteen-year period, due to an overall decrease in ratoon crop area.3 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. Price is the primary factor affecting rice area in 

Arkansas, as farmers will plant more of what is most lucrative amongst soybeans, rice, and cotton. Government 

support programs have also been influential by affecting the price received for a rice crop (Slaton 2001b, Mayhew 

1997). California rice area is primarily influenced by price and government programs, but is also affected by water 

availability (Mutters 2001). In Florida, rice acreage is largely a function of the price of rice relative to sugarcane 

and corn. Most rice in Florida is rotated with sugarcane, but sometimes it is more profitable for farmers to follow 

their sugarcane crop with sweet corn or more sugarcane instead of rice (Schueneman 1997, 2001b). In Louisiana, 

rice area is influenced by government support programs, the price of rice relative to cotton, soybeans, and corn, and 

in some years, weather (Saichuk 1997, Linscombe 2001b). For example, a drought in 2000 caused extensive 

saltwater intrusion along the Gulf Coast, making over 32,000 hectares unplantable. The dramatic decrease in 

ratooned area in Louisiana in 2002 was the result of hurricane damage to that state’s rice-cropped area. In 

Mississippi, rice is usually rotated with soybeans, but if soybean prices increase relative to rice prices, then some of 

the acreage that would have been planted in rice, is instead planted in soybeans (Street 1997, 2001). In Missouri, 

rice acreage is affected by weather (e.g., rain during the planting season may prevent the planting of rice), the price 

differential between rice and soybeans or cotton, and government support programs (Stevens 1997, Guethle 2001). 

In Oklahoma, the state having the smallest harvested rice area, rice acreage is limited to the areas in the state with 

the right type of land for rice cultivation. Acreage is limited to growers who can afford the equipment, labor, and 

land for this intensive crop (Lee 2003). Texas rice area is affected mainly by the price of rice, government support 

programs, and water availability (Klosterboer 1997, 2001b). 

Table 6-9: CH4 Emissions from Rice Cultivation (Tg CO2 Eq.) 

 

2 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. 

3 The 11 percent decrease occurred between 1992 and 1993; the 17 percent increase happened between 1993 and 1994. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 209 

State 1990 1997 1998 1999 2000 2001 2002 2003 

Primary 5.1 5.6 5.8 6.3 5.5 5.9 5.7 5.4 

Arkansas 2.1 2.5 2.7 2.9 2.5 2.9 2.7 2.6 

California 0.7 0.9 0.8 0.9 1.0 0.8 0.9 0.9 

Florida + + + + + + + + 

Louisiana 1.0 1.0 1.1 1.1 0.9 1.0 1.0 0.8 

Mississippi 0.4 0.4 0.5 0.6 0.4 0.5 0.5 0.4 

Missouri 0.1 0.2 0.3 0.3 0.3 0.4 0.3 0.3 

Oklahoma + + + + NA + + + 

Texas 0.6 0.5 0.5 0.5 0.4 0.4 0.4 0.3 

Ratoon 2.1 1.9 2.1 2.0 2.0 1.7 1.1 1.5 

Arkansas + + + + + + + + 

Florida + 0.1 0.1 0.1 0.1 + + + 

Louisiana 1.1 1.2 1.2 1.2 1.3 1.1 0.5 1.0 

Texas 0.9 0.7 0.8 0.7 0.7 0.6 0.5 0.5 

Total 7.1 7.5 7.9 8.3 7.5 7.6 6.8 6.9 

+ Less than 0.05 Tg CO2 Eq. 

NA (Not Available) 

Note: Totals may not sum due to independent rounding. 

Table 6-10: CH4 Emissions from Rice Cultivation (Gg CH4)

State 1990 1997 1998 1999 2000 2001 2002 2003 

Primary 241 265 279 300 260 283 274 255 

Arkansas 102 118 126 138 120 138 128 124 

California 34 44 39 43 47 40 45 43 

Florida 1 2 2 2 2 1 1 1 

Louisiana 46 50 53 52 41 46 45 38 

Mississippi 21 20 23 27 19 22 22 20 

Missouri 7 10 12 16 14 18 15 15 

Oklahoma + + + + NA + + + 

Texas 30 22 24 22 18 18 18 15 

Ratoon 98 91 98 95 97 81 52 73 

Arkansas + + + + + + + + 

Florida 2 3 3 4 2 2 2 2 

Louisiana 52 55 59 58 61 52 25 50 

Texas 45 33 36 33 34 27 24 22 

Total 339 356 376 395 357 364 325 328 

+ Less than 0.5 Gg 

NA (Not Available) 

Note: Totals may not sum due to independent rounding. 

Methodology 

The Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997) recommends utilizing harvested rice areas and 

area-based seasonally integrated emission factors (i.e., amount of CH4 emitted over a growing season per unit 

harvested area) to estimate annual CH4 emissions from rice cultivation. This methodology is followed with the use 

of U.S.-specific emission factors derived from rice field measurements. 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 Good Practice Guidance and Uncertainty Management in National Greenhouse Gas 

Inventories (IPCC 2000). 

The harvested rice areas for the primary and ratoon crops in each state are presented in Table 6-11. Primary crop 

areas for 1990 through 2003 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
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 210 

(USDA 1998), Crop Production 2000 Summary (USDA 2001), Crop Production 2001 Summary (USDA 2002), 

Crop Production 2002 Summary (USDA 2003), and Crop Production 2003 Summary (USDA 2004). Harvested 

rice areas in Florida, which are not reported by USDA, were obtained from Tom Schueneman (1999b, 1999c, 2000, 

2001a) and Arthur Kirstein (2003), Florida agricultural extension agents, Dr. Chris Deren (2002) of the Everglades 

Research and Education Centre at the University of Florida, and Gaston Cantens (2004), Vice President of 

Corporate Relations of the Florida Crystals Company. Harvested rice areas for Oklahoma, which also are not 

reported by USDA, were obtained from Danny Lee of the Oklahoma Farm Services Agency (Lee 2003, 2004). 

Acreages for the ratoon crops were derived from conversations with the agricultural extension agents in each state. 

In Arkansas, ratooning occurred only in 1998 and 1999, when the ratooned area was less than 1 percent of the 

primary area (Slaton 1999, 2000, 2001a). In Florida, the ratooned area was 50 percent of the primary area from 

1990 to 1998 (Schueneman 1999a), about 65 percent of the primary area in 1999 (Schueneman 2000), around 41 

percent of the primary area in 2000 (Schueneman 2001a), about 60 percent of the primary area in 2001(Deren 

2002), about 54 percent of the primary area in 2002 (Kirstein 2003) and about 100 percent of the primary area in 

2003 (Kirsetin 2004). In Louisiana, the percentage of the primary area that was ratooned was constant at 30 percent 

over the 1990 to 1999 period, increased to approximately 40 percent in 2000, returned to 30 percent in 2001, 

dropped to 15 percent in 2002, and rose to 35 percent in 2003 (Linscombe 1999a, 2001a, 2002, 2003, 2004 and 

Bollich 2000). In Texas, the percentage of the primary area that was ratooned was constant at 40 percent over the 

entire 1990 to 1999 period and in 2001, but increased to 50 percent in 2000 due to an early primary crop; it then 

decreased to 40 percent in 2001, 37 percent in 2002, and 38 percent in 2003 (Klosterboer 1999, 2000, 2001a, 2002, 

2003, Stansel 2004). 

Table 6-11: Rice Areas Harvested (Hectares) 

State/Crop 1990 1997 1998 1999 2000 2001 2002 2003

Arkansas 

 Primary 485,633 562,525 600,971 657,628 570,619 656,010 608,256 588,830

 Ratoon* NO NO 202 202 NO NO NO NO

California 159,854 208,822 185,350 204,371 221,773 190,611 213,679 205,180

Florida 

 Primary 4,978 7,689 8,094 7,229 7,801 4,562 5,077 2,315

 Ratoon 2,489 3,845 4,047 4,673 3,193 2,752 2,734 2,315

Louisiana 

 Primary 220,558 235,937 250,911 249,292 194,253 220,963 216,512 182,113

 Ratoon 66,168 70,781 75,273 74,788 77,701 66,289 32,477 63,739

Mississippi 101,174 96,317 108,458 130,716 88,223 102,388 102,388 94,699

Missouri 32,376 47,349 57,871 74,464 68,393 83,772 73,654 69,203

Oklahoma 617 12 19 220 NA 265 274 53

Texas 

 Primary 142,857 104,816 114,529 104,816 86,605 87,414 83,367 72,845

 Ratoon 57,143 41,926 45,811 41,926 43,302 34,966 30,846 27,681

Total 

Primary 

1,148,047 1,263,468 1,326,203 1,428,736 1,237,668 1,345,984 1,303,206 1,215,237

Total Ratoon 125,799 116,552 125,334 121,589 124,197 104,006 66,056 93,735

Total 1,273,847 1,380,020 1,451,536 1,550,325 1,361,864 1,449,991 1,369,262 1,308,972

* Arkansas ratooning occurred only in 1998 and 1999. 

NO (Not Occurring) 

NA (Not Available) 

Note: Totals may not sum due to independent rounding. 

To determine what seasonal 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 which involved atypical 

or nonrepresenative management practices (e.g., the application of nitrate or sulfate fertilizers, or other substances 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 211 

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 results4 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 

and 1991b) were averaged to derive an emission factor for the primary crop, and the experimental results from 

ratoon crops with added synthetic fertilizer (Lindau and Bollich 1993, Lindau et al. 1995) were averaged to derive 

an emission factor for the ratoon crop. The resultant emission factor for the primary crop is 210 kg CH4/hectareseason, and the resultant emission factor for the ratoon crop is 780 kg CH4/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 CH4/hectare-season 

and ratoon emissions ranged from 481 to 1,490 kg CH4/hectare-season. From these ranges, an uncertainty for the 

emission factors of 109 percent for primary crops and 65 percent for ratoon was calculated. In order to perform a 

Tier 2 Monte Carlo uncertainty analysis, some information regarding the statistical distribution of the uncertainty is 

required. Variability about the rice emission factor means were not normally distributed for either primary or 

ratooned crops, but rather skewed, with a tail trailing to the right of the mean, therefore a lognormal-type statistical 

distribution was applied. The bounds of the distribution were set at 0 (indicating that CH4 absorption was unlikely 

given this management system) and three times the emission factor. 

Uncertainty regarding primary cropping area is an additional consideration. Uncertainty associated with primary 

rice-cropped area for each state was obtained from expert judgment, and ranged from 1 percent to 5 percent of the 

mean area. A triangular distribution of uncertainty was assumed about the mean for areas, which was bounded at 

half and one and a half times the estimated area. 

Another source of uncertainty lies in the ratooned areas, which are not compiled regularly. Ratooning accounts for 

less than 8 percent of the total rice-cropped area, though it is responsible for a proportionately larger portion of 

emissions. Based on expert judgment, the uncertainty associated with ratooned areas is between 1 percent and 5 

percent. A triangular distribution of uncertainty was assumed, and bound at half and one and a half times the 

estimated proportion of ratooned area. 

A final source of uncertainty is in the practice of flooding outside of the normal rice season. According to 

agricultural extension agents, all of the rice-growing states practice this on some part of their rice acreage. 

Estimates of these areas range from 5 to 68 percent of the rice acreage. Fields are flooded for a variety of reasons: 

to provide habitat for waterfowl, to provide ponds for crawfish production, and to aid in rice straw decomposition. 

To date, however, CH4 flux measurements have not been undertaken over a sufficient geographic range or under 

representative conditions to account for this source or its associated uncertainty adequate for inclusion in the 

emission estimates or uncertainty evaluations presented here. 

 

4 In some of these remaining experiments, measurements from individual plots were excluded from the analysis because of the 

reasons just mentioned. In addition, one measurement from the ratooned fields (i.e., the flux of 2.041 g/m2

/day in Lindau and 

Bollich 1993) was excluded since this emission rate is unusually high compared to other flux measurements in the United States,

as well as in Europe and Asia (IPCC/UNEP/OECD/IEA 1997). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 212 

To quantify the uncertainties for emissions from rice cultivation, a Monte Carlo (Tier 2) uncertainty analysis was 

performed using the information provided above. The results of the Tier 2 quantitative uncertainty analysis are 

summarized in Table 6-12. Rice cultivation CH4 emissions in 2003 were estimated to be between 2.9 and 13.9 Tg 

CO2 Eq. at a 95 percent confidence level (or 19 of 20 Monte Carlo Stochastic Simulations). This indicates a range 

of 58 percent below to 101 percent above the 2003 emission estimate of 6.9 Tg CO2 Eq. 

Table 6-12: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Rice Cultivation (Tg CO2 Eq. and 

Percent) 

Source Gas 

2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Rice Cultivation CH4 6.9 2.9 13.9 -58% +101% a

Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Planned Improvements 

In performing a Monte Carlo-type uncertainty analysis, a higher level Tier 2 type emission mean is calculated 

incidentally. One would expect there to be a difference in the emission means calculated by these different 

methods, because under the IPCC default Tier 2 method used here to estimate CH4 emissions, the statistical 

distribution of all parameters (i.e., activity data and emission factors) is implicitly considered to be normal. As 

described above, that is not the case with the uncertainty analysis, which allows for several asymmetrical statistical 

distributions. Here, the lower and upper bounds have been reported, directly from the Monte Carlo analysis. 

However, the percentages for the upper and lower bounds of the range have been calculated based on the reported 

emission mean rather than that mean calculated by the Monte Carlo software (as is the case with all reported Tier 2 

analyses). Because that mean may represent an improvement to the current Tier 2 methodology, including the 

higher level Tier 2 estimate in future inventories is being investigated. 

6.4. Agricultural Soil Management (IPCC Source Category 4D) 

Nitrous oxide is produced naturally in soils through the microbial processes of nitrification and denitrification.5 A 

number of agricultural activities add nitrogen (N) to soils, thereby increasing the amount available for nitrification 

and denitrification, and ultimately the amount of nitrous oxide (N2O) emitted. These activities may add N to soils 

either directly or indirectly (see Figure 6-2). Direct additions occur through various soil management practices and 

from the deposition of manure on soils by animals on pasture, range, and paddock (PRP) (i.e., by animals whose 

manure is not managed). Soil management practices that add N to soils include fertilizer use, application of 

managed livestock manure and sewage sludge, production of N-fixing crops and forages, retention of crop residues, 

and cultivation of histosols (i.e., soils with a high organic matter content, otherwise known as organic soils).6 Only 

direct emissions from agricultural lands (i.e., croplands and grasslands), along with emissions from PRP manure are 

included in this section. The direct emissions from forest lands and settlements are presented within the LUCF 

sector. Indirect nitrous oxide emissions from all land use types resulting from N additions to croplands, grasslands, 

forestlands, and settlements are also included in this section. These indirect emissions occur through two 

 

5 Nitrification and denitrification are two processes within the N cycle that are brought about by certain 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). 

6 Cultivation of histosols does not, per se, “add” N to soils. Instead, the process of cultivation enhances mineralization of N-rich 

organic matter that is present in histosols, thereby enhancing N2O emissions from histosols. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 213 

mechanisms: 1) volatilization and subsequent atmospheric deposition of applied N;7 and 2) surface runoff and 

leaching of applied N into groundwater and surface water. Other agricultural soil management activities, such as 

irrigation, drainage, tillage practices, and fallowing of land, can affect fluxes of N2O (as well as other greenhouse 

gases) to and from soils and are partially accounted for in the analysis. 

Figure 6-2: Direct N2O Emissions Pathways from Cropland and Grassland Soils, and Indirect N2O Emissions 

Pathways from All Sources. 

Agricultural soils are responsible for the majority of U.S. N2O emissions. Estimated emissions from this source in 

2003 were 253.5 Tg CO2 Eq. (818 Gg N2O) (see Table 6-13 and Table 6-14). Annual agricultural soil management 

N2O emissions fluctuated between 1990 and 2003; however, overall emissions were 0.2% greater in 2003 than in 

1990. Year-to-year fluctuations are largely a reflection of annual variations in climate, synthetic fertilizer 

consumption, and crop production. 

Table 6-13: N2O Emissions from Agricultural Soils (Tg CO2 Eq.) 

1990 1997 1998 1999 2000 2001 2002 2003

Direct 140.4 155.9 158.6 151.1 156.3 154.5 159.9 155.3

Agricultural Soils 100.1 113.6 116.5 111.0 116.4 113.0 118.5 114.8

Pasture, Range & Paddock Livestock 

Manure 40.2 42.2 42.1 40.1 39.8 41.5 41.4 40.5

Indirect (All Land Use Types)* 112.6 96.2 109.1 92.3 107.6 102.6 92.7 98.2

Total 253.0 252.0 267.7 243.4 263.9 257.1 252.6 253.5

Note: Totals may not sum due to independent rounding. 

*Includes cropland, grassland, forest land and settlements. 

Table 6-14: N2O Emissions from Agricultural Soils (Gg) 

1990 1997 1998 1999 2000 2001 2002 2003

Direct 453 503 512 487 504 498 516 501

Agricultural Soils 323 367 376 358 376 365 382 370

Pasture, Range & Paddock Livestock 

Manure 130 136 136 129 129 134 134 131

Indirect (All Land Use Types)* 363 310 352 298 347 331 299 317

Total 816 813 864 785 851 829 815 818

Note: Totals may not sum due to independent rounding. 

*Includes cropland, grassland, forest land and settlements. 

Estimated direct and indirect N2O emissions by sub-source category are provided in Table 6-15, Table 6-16, and 

Table 6-17. 

Table 6-15: Direct N2O Emissions from Agricultural Soils (Tg CO2 Eq.) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003

Mineral Agricultural Soils 97.3 110.8 113.7 108.2 113.6 110.1 115.6 111.9

Histosol Cultivation 2.8 2.9 2.9 2.9 2.9 2.9 2.9 2.9

 

7 These processes entail volatilization of applied N as ammonia (NH3) and oxides of N (NOx), transformations of these gases 

within the atmosphere (or upon deposition), and deposition of the N primarily in the form of particulate ammonium (NH4), nitric 

acid (HNO3), and oxides of N. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 214 

Pasture, Range & Paddock Livestock 

Manure 40.2 42.2 42.1 40.1 39.8 41.5 41.4 40.5

Total 140.4 155.9 158.6 151.1 156.3 154.5 159.9 155.3

Note: Totals may not sum due to independent rounding. Excludes sewage sludge and livestock manure used as commercial 

fertilizers. 

Table 6-16: Direct N2O Emissions from PRP Livestock Manure (Tg CO2 Eq.) 

Animal Type 1990 1997 1998 1999 2000 2001 2002 2003 

Beef Cattle 34.9 37.8 37.6 35.7 35.5 37.1 37.0 36.1 

Dairy Cows 1.9 1.4 1.4 1.3 1.3 1.3 1.3 1.3 

Swine 0.5 0.2 0.2 0.2 0.2 0.2 0.2 0.2 

Sheep 0.4 0.3 0.3 0.3 0.3 0.3 0.2 0.2 

Goats 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 

Poultry 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Horses 2.2 2.3 2.3 2.3 2.3 2.3 2.3 2.3 

Total 40.2 42.2 42.1 40.1 39.8 41.5 41.4 40.5 

Table 6-17: Indirect N2O Emissions from all Land Use Types* (Tg CO2 Eq.) 

1990 1997 1998 1999 2000 2001 2002 2003 

Volatilization and Atm. Deposition 15.6 16.5 16.4 16.4 16.8 16.4 16.6 16.5 

Surface Leaching & Run-Off 97.1 79.6 92.7 75.9 90.8 86.3 76.1 81.8 

Total 112.6 96.2 109.1 92.3 107.6 102.6 92.7 98.2 

Note: Totals may not sum due to independent rounding. 

*Includes cropland, grassland, forest land and settlements. 

Methodology 

The methodology used to estimate emissions from agricultural soil management is consistent with the Tier 3 

approach of the Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997), as amended by the IPCC Good 

Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories (IPCC 2000) and Good 

Practice Guidance for Land Use, Land-Use Change and Forestry (IPCC 2003). Current methods divide this N2O 

source category into three components: 1) direct emissions from managed soils due to applied N and the cultivation 

of histosols; 2) direct emissions from soils due to the deposition of manure by livestock on PRP lands; and 3) 

indirect emissions from soils or water induced by additions of fertilizers, sewage sludge, and livestock manure (both 

managed and unmanaged) to soils of all land use types. 

Annex 3.11 provides more detailed information on the methodologies and data used to calculate N2O emissions 

from each of the components. 

The methodology applied in this Inventory is a hybrid approach for estimating N2O emissions from mineral 

agricultural soils. This involves using the process-based model DAYCENT to estimate emissions from major crops 

on mineral (i.e., non-histosol) soils, and the IPCC methodology for non-major crops on mineral soils, PRP manure, 

as well as all emissions from histosols. 

Direct N2O Emissions from Mineral Agricultural Soils 

Different methodologies were used in quantifying direct N2O emissions from mineral agricultural soils with major 

crop types and those with non-major crop types as described below. 

Major Crop Types 

The DAYCENT ecosystem model (Del Grosso et al. 2001, Parton et al. 1998) was used to estimate direct soil N2O 

emissions from mineral agricultural soils cropped with major crop types. DAYCENT has been parameterized to 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 215 

simulate most of the major cropping systems (corn, soybean, wheat, alfalfa hay, other hay, sorghum, and cotton) in 

the United States. These cropping systems simulated by DAYCENT represent approximately 90 percent of total 

cropped land in the United States. DAYCENT simulates crop growth, soil organic matter decomposition, 

greenhouse gas fluxes, N deposited by grazing animals, and other biogeochemical processes using daily climate 

data, land management information, and soil physical properties as model inputs. The scale of DAYCENT 

simulations is dictated by the scale of available input data. Soil and climate inputs were available for every county 

with more than 100 acres of agricultural land. Therefore, a single parameter value (e.g., maximum temperature for a 

particular day) is applied at the county-level for those variables. Land management data (e.g., timing of planting, 

harvesting, applying fertilizer, intensity of cultivation, rate of fertilizer application) were available at the agricultural 

region level as defined by the Agricultural Sector Model (McCarl el al. 1993). There are 63 regions in the 

contiguous United States; most states correspond to one region, except for states that are divided into two or more 

regions if there is sufficient variability in cropping practices within the state. Although various cropping systems 

were simulated for each county, the parameters controlling management activities (e.g., when crops were 

planted/harvested, amount of fertilizer added), did not change within an agricultural region. 

Nitrous oxide emissions estimated by DAYCENT account for N additions, crop type, irrigation, and other factors. 

However, because DAYCENT is a process-based model that simulates the N cycle, N2O emissions cannot be 

partitioned into the contribution of N2O from different N inputs (e.g., N2O emissions from synthetic fertilizer 

applications cannot be distinguished from those emissions resulting from manure applications). Therefore, it was 

not possible to separate out these individual contributors to N2O flux, as is suggested in the IPCC Guidelines. 

In addition to simulating N2O emissions from mineral agricultural soils cropped with major crop types, a 

DAYCENT simulation was performed of those same areas as though they were covered by native vegetation, so 

that anthropogenic emissions could be isolated from natural background emissions. Emissions from managed 

agricultural lands are the result of complex and interactive processes, practices, and inputs arising from 

anthropogenic intervention. Because removing inputs alone would not reflect the full anthropogenic greenhouse 

gas signature, managed soil emissions have been compared to those soils under native vegetation as a means of 

identifying the anthropogenic contribution. The reported estimates of emissions from managed soils therefore 

represent the difference between simulated emissions from native vegetation and emissions from cropland soils. 

Estimates of direct N2O emissions from N applications were based on the total amount of N applied to soils 

annually through the following practices: 1) the application of synthetic and organic commercial fertilizers, 2) the 

application of livestock manure through both daily spread operations and through the eventual application of 

manure that had been stored in manure management systems, 3) the application of sewage sludge, 4) the production 

of N-fixing crops and forages, and 5) the retention of crop residues (i.e., leaving residues in the field after harvest). 

For each of these practices, annual N applications were obtained from the following sources: 

• Crop-specific N-fertilization rates: 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). 

• Manure management information was obtained from Poe et al. (1999), Safley et al. (1992), and personal 

communications with agricultural experts (Anderson 2000, Deal 2000, Johnson 2000, Miller 2000, Milton 

2000, Stettler 2000, Sweeten 2000, Wright 2000). Livestock weight data were obtained from Safely 

(2000), USDA (1996, 1998d), and ASAE (1999); daily rates of N excretion from ASAE (1999) and USDA 

(1996). Comparisons of estimates of managed manure production (i.e. non-PRP manure) with estimates of 

the amount of manure actually consumed by soils showed that manure consumed by soils accounted for 

approximately one-third of managed manure production). Values for manure consumption (Kellogg et al. 

2000; Edmonds et al. 2003) were subtracted from values of managed manure production. Only consumed 

manure N was applied to agricultural soils. The remainder was assumed to have volatized during storage 

and transport. In contrast to the IPCC methodology that only considers volatilization of manure that was 

applied to soils, the manure that was assumed to volatilize during transport and storage was included in the 

volatilization component of indirect N2O emissions. Instead of assuming that 10 percent of synthetic and 

20 percent of organic N applied to soils is volatilized and 30 percent of applied N was leached/runoff as 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 216 

with IPCC methodology, volatilization and N leaching/runoff were internally calculated by the processbased model. 

• Sewage sludge: Bastian (2002); USDA (1998a); EPA (1993, 1999); Metcalf and Eddy (1991). 

• Nitrogen-fixing crops and forages and retention of crop residue. Using the IPCC approach, these are 

considered activity data. However, when using DAYCENT, they should not be considered activity data 

because they are internally generated by the model. In other words, DAYCENT accounts for the influence 

of N fixation 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.): Hurd (1930, 1929), Latta (1938), Iowa State College Staff Members 

(1946), Bogue (1963), Hurt (1994), USDA (2004), USDA (2000h), 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), Langtson et al. (1922), Russell et al. (1922), Elliot and Tapp (1928), Elliot 

(1933), Ellsworth (1929), Garey (1929), Holmes (1929), Hodges et al. (1930), Bonnen and Elliot (1931), 

Brenner et al. (2002, 2001), Smith et al. (2002). 

Applied N was subject to volatilization and leaching/runoff according to the climatic conditions, soil type and 

condition, crop type, and land management practices such as cultivation and irrigation, as simulated by DAYCENT. 

These amounts were then applied in the calculation of indirect emissions as described below. The remaining 

applied soil N was then added to the applied N from N-fixing crops and crop residues to yield total soil N additions 

for the DAYCENT simulation of direct N2O emissions from soils cropped with major crop types. Because the 

model is sensitive to actual interannual variability in those factors to which N2O emissions are sensitive (e.g., 

climate), emissions vary through time rather than demonstrate a linear, monotonic response. 

Non-Major Crop Types 

For lands cropped with non-major crop types, the IPCC emission factor methodology was used to estimate N2O 

emissions from mineral agricultural soils, as described below. 

Estimates of direct N2O emissions from N applications to non-major crop types were based on the amount of N 

applied to soils annually through the following practices: 1) the application of synthetic commercial fertilizers, 2) 

the production of N-fixing crops and forages, and 3) the retention of crop residues. No organics were considered 

here because 100 percent of these were assumed to be applied to crops simulated by DAYCENT. This assumption is 

reasonable because DAYCENT simulated the 6 major cropping systems (corn, hay, pasture, sorghum, soybean, 

wheat) that receive the vast majority (approximately 95 percent) of manure applications (Kellogg et al. 2000, 

Edmonds et al. 2003). 

Yearly synthetic fertilizer N additions to non-major crop types were calculated by process of elimination. For each 

year, fertilizer accounted for by the cropping systems simulated by DAYCENT (approximately 75 percent of the 

U.S. total), fertilizer estimated to be applied to forests (less than 1 percent of the U.S. total), and fertilizer estimated 

to be applied in settlements (approximately 10 percent of the U.S. total) were summed and subtracted from total 

fertilizer used in the United States. This difference was assumed to be applied to non-major crop types and 

accounted for approximately 15 percent of total N fertilizer used in the United States. Non-major crop types include 

fruits, nuts, and vegetables, which account for approximately 5 percent of U.S. N fertilizer use (TFI 2000) and other 

crops not simulated by DAYCENT (barley, oats, tobacco, sugar cane, sugar beets, sunflower, millet, peanuts, etc.) 

which account for approximately 10 percent of total U.S. fertilizer use. The non-volatilized proportion was 

obtained by reducing total applications by the default IPCC volatilization fraction (IPCC 1997, 2000). In addition 

to synthetic fertilizer-N applied to non-major crop types, N in soils due to the cultivation of non-major N-fixing 

crops (e.g., edible legumes) was included in these estimates. Finally, crop residue N retention was derived from 

information about which residues are typically left on the field, the fractions that remain, annual crop production, 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 217 

mass ratios of aboveground residue to crop product, and dry matter fractions and N contents of the residues. For 

each of these practices, annual N applications were obtained from the following sources:

• Mass ratios of aboveground residue to crop product, dry matter fractions, and N contents for N-fixing 

crops: Strehler and Stützle (1987), Barnard and Kristoferson (1985), Karkosh (2000), Ketzis (1999), 

IPCC/UNEP/OECD/IEA (1997). 

• Annual production statistics for crops whose residues are left on the field: USDA (1994a, 1998b, 2000i, 

2001a, 2002a, 2003a), Schueneman (1999, 2001), Deren (2002), Schueneman and Deren (2002), Cantens 

(2004), Lee (2003, 2004). 

• Aboveground residue to crop mass ratios, residue dry matter fractions, and residue N contents: Strehler and 

Stützle (1987), Turn et al. (1997), Ketzis (1999), Barnard and Kristoferson (1985), Karkosh (2000). 

The net amount of N remaining on the soil from applied fertilizer was added to the N from N-fixing crops and crop 

residues to yield total unvolatilized applied N, which was multiplied by the IPCC default emission factor to derive 

an estimate of cropland N2O emissions from non-major crop types. 

Total annual emissions from major crops and other crops were summed to obtain total emissions from cropped 

mineral soils (see Table 6-13 and Table 6-14). 

Direct N2O Emissions from Histosols 

Estimates of annual N2O emissions from histosol cultivation were based on estimates of the total U.S. acreage of 

histosols cultivated annually for each of two climatic zones: 1) temperate, and 2) sub-tropical. Histosol area was 

obtained from the Natural Resources Inventory (USDA 2000h, as extracted by Eve 2001, and revised by Ogle 

2002). 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. 

Total Direct N2O Emissions from Nitrogen Applications to Agricultural Soils 

Total annual N2O emissions from N applications to mineral agricultural soils and annual N2O emissions from 

histosol cultivation were then summed to estimate total direct N2O emissions from agricultural soils. 

Direct N2O Emissions from Pasture, Range, and Paddock Livestock Manure 

As with N2O from major row crops, dual methodologies incorporating the process-based simulation model 

DAYCENT and IPCC methods were applied in tandem to estimate total emissions from PRP manure. For 

DAYCENT simulations, annual county-level pasture area data were not available so county-level pasture area 

estimates from Kellogg et al. (2000) and Edmonds et al. (2003) were used. DAYCENT does not simulate paddocks 

and no county level area data for rangeland were available so IPCC methodology was used to estimate emissions 

from these sources. Because DAYCENT simulated only pastures and not paddocks or rangeland, the amount of 

manure accounted for by DAYCENT (manure N added to soil is an output variable in DAYCENT) was subtracted 

from annual estimates of total PRP manure and assumed that this manure contributed to emissions from paddocks 

and rangeland. 

Estimates of N2O emissions from PRP livestock manure are based on the amount of N in the manure that is 

deposited annually on soils by livestock on PRP. Estimates of annual manure N from these livestock were derived 

from animal population and weight statistics; information on the fraction of the total population of each animal type 

that is on pasture, range, or paddock; and annual N excretion rates for each animal type. The amount of manure N 

from each animal type was summed over all animal types to yield total PRP manure N. Nitrous oxide emissions 

resulting from manure deposited on pastures by livestock was simulated by DAYCENT in each county. The 

emissions were obtained by multiplying DAYCENT emissions (in g N2O-N m-2) by the total reported pasture area 

for each county, and summing across all counties to achieve a nationwide value. All of the manure accounted for by 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 218 

DAYCENT was assumed to come from cattle because DAYCENT has been parameterized to simulate cattle 

manure, and cattle are responsible for approximately 90 percent of total PRP manure. The PRP manure N from 

paddocks and rangeland not accounted for by DAYCENT in the pasture component was multiplied by the IPCC 

default emission factor to estimate N2O emissions from paddock and rangeland manure deposition. Emissions from 

the three types of PRP manure were summed to provide total national emissions from PRP manure in the United 

States. 

Indirect N2O Emissions from Managed Soils of All Land Use Types 

This section describes the method for estimating indirect N2O emissions from managed soils of all land use types 

(i.e., cropland, grassland, forest land and settlements). Indirect emissions of N2O are composed of two parts, which 

are estimated separately and then summed. These parts are 1) emissions resulting from volatilization of non-N2O 

gases (i.e., NOx and NH3) from synthetic fertilizer and manure additions to managed soils and from managed 

manure during storage, treatment and transport that are subsequently deposited onto other areas and eventually 

emitted to the atmosphere as N2O, and 2) leaching and runoff of N (in the form of NO3

-

) from all soils where N 

additions have been made that is eventually denitrified and emitted as N2O from a water body. Regardless of the 

original source or eventual land use type where these indirect N2O emissions actually occur, all indirect N2O 

emissions are accounted for in this section of the Inventory. 

A mix of approaches was used to obtain the necessary information required to estimate indirect N2O emissions. 

While DAYCENT simulates NOx and NH3 volatilization as well as NO3 leaching/runoff, it does not model their 

transport or subsequent off-site conversion to N2O. Therefore, DAYCENT was used to simulate N volatilization 

and leaching/runoff losses for major crop types. Volatilized and leached/runoff N from non-major crops, 

settlements and forest lands were obtained by applying the IPCC default fractions to total fertilizer applications to 

those crops and/or land areas. The volatilization and leaching/runoff components of indirect emissions for PRP 

manure were obtained by using a combination of DAYCENT generated outputs for manure deposited on 

pasturelands and applying IPCC defaults to manure deposited on paddocks and rangelands. Manure from managed 

systems assumed to be volatilized during storage, treatment and transport was included in the indirect emission 

calculations as well. In contrast to the IPCC approach that has been used in the past, DAYCENT simulations for 

major crops, where all managed manure is assumed to be applied, do not assume that 100 percent of the N in 

managed manure is available to be applied to soils. According to data in Kellogg et al. (2000) and Edmonds et al. 

(2003), more than 50 percent of the N in managed manure is lost to volatilization, spillage and leaching/runoff 

during storage, treatment and transport. Consequently, manure N applied to soils, based on data from Kellogg et al. 

(2000) and Edmonds et al. (2003), is subtracted from total managed manure N and assumed to volatilize during 

storage, treatment, and transport where it is then included in the volatilization component of indirect emissions. 

Results from this mix of approaches described above were then summed for the appropriate indirect N2O emission 

pathway as described below. 

Voltilized Indirect Emissions 

Volatilized N emissions for settlements, forest lands, PRP manure, major crops, non-major crops, and volatilized 

managed manure prior to land application were summed. The IPCC default emission factor for indirect N2O was 

applied to the total to give total indirect N2O emissions from N volatilization from soils of all land use types and 

volatilized managed manure. 

Leaching/Runoff Indirect Emissions 

The amounts of leached/runoff N from settlements, forest lands, PRP manure, major and non-major crop types 

were summed and multiplied by the IPCC default emission factor for leached/runoff N. 

Total Indirect Emissions from Volatilization and Leaching/Runoff 

Total indirect emissions from volatilization and from leaching/runoff were summed to estimate total indirect 

emissions of N2O from croplands (Table 6-17). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 219 

Uncertainty 

The DAYCENT biogeochemical ecosystem model was used to calculate N2O emissions from major crop types. 

There are two broad classes of uncertainty in such analyses: that inherent in the activity data and emission factors, 

and structural uncertainty inherent to the model used to estimate emissions. Consistent with the United States' 

uncertainty management plan, uncertainty inherent to the DAYCENT model was not quantified as part of 

the IPCC Tier 1 approach described below. 

Three types of approaches were taken for estimating different types of emissions in this chapter: 1) Direct 

emissions calculated by DAYCENT; 2) Direct emissions not calculated by DAYCENT; and 3) Indirect emissions. 

Uncertainty was estimated differently for each category. 

For direct emissions calculated by DAYCENT (99.3 of the total direct 155.3 Tg CO2 Eq.), uncertainty in national 

totals for N inputs and uncertainty in how N application rates change with crop type, year, and agricultural region 

contribute to total uncertainty in the N application activity data. Total uncertainty in N inputs was estimated at 20 

percent (Mosier 2004). Other activity data include climate data, for which uncertainty was estimated to be 19 

percent, and soil type, which was estimated to have an uncertainty of 12 percent (Del Grosso 2005a). Their 

combined uncertainty, according to the sum-of-squares method, is approximately 30.1 percent. To estimate the 

uncertainty associated with the effective emission factor, DAYCENT outputs were compared with N2O 

measurements from various cropped soils over the annual cycle (Del Grosso et al. in press). Through this method, 

the uncertainty associated with the effective emission factor was estimated at 57 percent (Del Grosso 2005b). 

Through the calculus of error propagation, overall uncertainty for direct emissions calculated by DAYCENT was 64 

percent. 

Direct N2O emissions not calculated by DAYCENT were assumed to maintain the 64 percent uncertainty. 

Finally, indirect emissions were calculated according to the default IPCC methodology, as has been performed in 

past Inventories. Consequently, the maximum uncertainty calculated for last year’s indirect N2O emissions from 

agricultural soil management of 286 percent (U.S. EPA 2004) was applied to conservatively address the uncertainty 

in indirect emissions here. 

The results of the Tier 1 quantitative uncertainty analysis are summarized in Table 6-18. Agricultural soil 

management N2O emissions in 2003 were estimated to be between 45.2 and 461.8 Tg CO2 Eq. at a 95 percent 

confidence level. This indicates a range of 82 percent above and below the 2003 emission estimate of 253.5 Tg 

CO2 Eq. 

Table 6-18: Tier 1 Quantitative Uncertainty Estimates of N2O Emissions from Agricultural Soil Management in 

2003 (Tg CO2 Eq. and Percent)

Source Gas 

2003 Emission 

Estimate 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

Emission Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

Agricultural Soil Management N2O 253.5 82% 45.2 461.8 

Recalculations Discussion 

Differences in the present report compared to previous years exist for two reasons: differences in sources and 

differences in methodologies. In previous Inventories, fertilizer applied to forests and settlements were included in 

the agricultural sector. For the current Inventory, for the direct emissions, these fertilizer additions were included in 

the LUCF sector, and therefore approximately 15 percent less synthetic fertilizer is counted in the agricultural sector 

than in previous Inventories. Also in previous Inventories, the default Tier 1 IPCC methodology was used to 

estimate emissions from this sector. That methodology relied solely on N inputs, and did not account for effects of 

climate, soil type, and other factors that influence N2O emissions. To account for some of these additional factors 

and increase confidence in estimates, a Tier 3 method, the DAYCENT ecosystem model, was used to account for 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 220 

N2O emissions from major cropping systems. Overall, the changes resulted in an average annual decrease of 31.2 

Tg CO2 Eq. (11 percent) in N2O emissions from agricultural soil management for the period 1990 through 2002. 

The IPCC emission factor methodology is an example of a Tier 1 approach. This approach is activity driven, i.e., 

total N from different sources (e.g. synthetic fertilizer, manure, N fixation, etc.) is used to estimate N2O from these 

sources. The Tier 3 approach in this case uses a process-based model (i.e., DAYCENT) and is area driven, i.e., it is 

necessary to know the annual area of major crop types and the N amendment rates for each of these crops. With the 

Tier 3 approach, emissions cannot be separated by N inputs because once N is in the plant/soil system, the model 

does not distinguish its source according to IPCC categorizations (e.g., whether the N2O emitted was synthetic 

fertilizer-derived or derived from manure). Because the Tier 3 approach was used for approximately 90 percent of 

fertilized soils in the United States, N2O emissions are not partitioned into the IPCC’s N-input categories, as has 

been done in the past. 

The Tier 3 approach requires some of the same activity data as the Tier 1 approach, plus additional information. 

Like the Tier 1 approach, the Tier 3 approach requires national totals for N amendments, but it also requires data on 

N amendment rates for different cropping systems. Consequently, the total amounts of N fertilizer and organic N 

additions were identical to previous years but assumptions regarding the fate of these amendments are different. For 

example, in previous years, 100 percent of managed manure was assumed to be applied to cropped soils, though 

here approximately 64 percent of manure N was lost to volatilization during transport and storage before it was 

applied to soil. This manure that was assumed to volatilize before soil application was included with indirect 

emissions, which is different than previous years. In addition to N amendments, the Tier 3 approach requires area 

data for different cropping systems. The Tier 3 approach distinguishes different cropping systems because crops 

vary in growth rates, fertilization rates, biomass N concentration, and timing for planting, harvesting, and 

cultivating. These crop system specific factors are important because they influence N availability in soil, which 

controls N2O emissions. 

An important difference between Tier 1 and Tier 3 approaches relates to assumptions regarding N cycling. Tier 1 

assumes that N added to a system in one year completely cycles during that year; e.g., N added as fertilizer or 

through fixation contributes to N2O emission for that year, but cannot be stored in soil or biomass and be recycled 

and contribute to N2O emission in subsequent years. In contrast, the process-based models used in the Tier 3 

approach include legacy effects such that N added to the system in one year may be taken up by vegetation and 

returned to the soil in organic form during that year, then re-mineralized and emitted as N2O during subsequent 

years. In addition to previous years’ fertilizer additions, other long-term management practices that affect current 

soil organic matter (SOM) levels (e.g., intensive cultivation, summer fallow) also affect current N2O emission, 

because in process based models, N from internal cycling (mineralization of SOM) contributes to N2O emission. 

Thus, while Tier 1 estimates are influenced only by the current year’s N inputs, Tier 3 emissions are also influenced 

by management in previous years. 

Another difference in methodologies is that the Tier 1 method assumes that 10 percent of synthetic fertilizer and 20 

percent of applied manure are volatilized, and 30 percent of applied N is leached or run-off. DAYCENT, however, 

calculates N volatilization and N leached and run-off internally based on specific climatic, environmental, and 

management conditions. 

Consideration of N-fixation highlights another difference in the approaches. In the Tier 1 approach, a certain 

portion of aboveground fixed N is assumed to be emitted as direct soil N2O. In the Tier 3 approach, N fixation is 

calculated by the model and fixed N can be harvested, lost as N2O, lost in some other form (e.g., leached NO3), or 

stored in the plant/soil system. 

The Tier 1 approach also assumes that only N from fertilizer and organic matter additions contributes to indirect 

N2O emissions whereas the Tier 3 approach assumes that once N is in the plant/soil system, it can be cycled and lost 

thorough various pathways, regardless of its source. Similar to N fixation, N deposited on soil by pasture and range 

animals and N added to soils from crop residue are simulated by DAYCENT. More N from manure was assumed to 

volatilize before application to soils and hence less N from manure was available for leaching than previous years. 

However, total N volatilization and leaching/runoff were both still higher than previous years. This is because IPCC 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 221 

methodology considers only N from synthetic and organic fertilizer to contribute to indirect emissions whereas 

other sources of N (e.g., fixation, crop residue) contribute to volatilization and N leaching/runoff in DAYCENT. 

The methodology used here estimated total N2O emissions to be approximately 5 to 10 percent less than estimates 

based on the IPCC methodology due to changes in the calculation method, as well as accounting for N2O from 

fertilization of forest and settlement soils within the LUCF sector. The current method estimates lower direct N2O 

emissions Table 6-19, but higher indirect N2O emissions (Table 6-20) than the IPCC method. Differences in total 

N2O emissions are shown in Table 6-21. Direct emissions were lower because of different assumptions regarding 

the cycling of fixed N and lower manure N applications to the major crop types under the current methodology 

compared with that used in the past. Indirect emissions, on the other hand, were larger because more contributors to 

N volatilization and leaching/runoff are accommodated by the simulation (by including crop residue applications, 

for example). Mean direct emissions from non-N fixing crops differed by approximately one percent, whereas 

direct emissions from N fixing crops were approximately 30 percent less with hybrid than IPCC methodology. 

Interestingly, total N fixation with the hybrid approach was only approximately two percent lower than with IPCC 

methodology and the implied emission factor for direct N2O emissions from fixation is approximately 0.9 percent 

using hybrid methodology; i.e., these DAYCENT simulations suggest that the 1.25 percent emissions factor used 

for direct N2O emissions from N fixation is too high. This is consistent with field data showing that IPCC 

methodology may overestimate N2O emissions from soybean and alfalfa cropping (Del Grosso et al. in press, 

Rochette et al. 2004). 

Table 6-19. Comparison of Direct Soil N2O Emission Estimates for IPCC versus Current Methodologies (Tg CO2

Eq.). 

Method 1990 1997 1998 1999 2000 2001 2002 2003

IPCC 191.2 214.9 216.1 213.9 213.0 213.2 210.1 205.8

Current Simulation* 146.0 162.3 165.1 157.7 162.6 160.7 166.3 161.7

Difference 45.2 52.6 51.0 56.2 50.4 52.5 43.8 44.1

* Unlike Table 6-13, emissions due to N applied to forest lands and settlements are included here, to be consistent with IPCC 

estimates used in previous reports. 

Table 6-20. Comparison of Indirect Soil N2O Emission Estimates for IPCC versus Current Methodologies (Tg CO2

Eq.) 

Method 1990 1997 1998 1999 2000 2001 2002 2003 

IPCC 72.6 79.0 78.8 78.8 77.4 76.0 77.2 77.3 

Current Simulation 112.6 96.2 109.1 92.3 107.6 102.6 92.7 98.2 

Difference -40.0 -17.2 -30.3 -13.5 -30.2 -26.6 -15.5 -20.9 

. 

Table 6-21. Comparison of Total Soil N2O Emission Estimates for IPCC versus Current Methodologies (Tg CO2

Eq.) 

Method 1990 1997 1998 1999 2000 2001 2002 2003

IPCC 263.8 293.9 294.9 292.8 290.4 289.2 287.2 283.1

Current Simulation* 258.6 258.4 274.2 250.0 270.2 263.3 259.0 259.9

Difference 5.2 35.4 20.7 42.7 20.2 25.9 28.2 23.2

* Unlike Table 6-13, emissions due to N applied to forest land and settlements are included here, to be consistent with IPCC 

estimates used in previous reports. 

Compared with the IPCC methodology used in the past, the current methodology shows a smaller increase in total 

N2O emissions from 1990 through 2003. The current methodology takes into account climate patterns as well as 

annual fluctuations in N additions. The linear regression between emissions estimated with the new method and 

time shows a trend toward increasing emissions of approximately 0.39 percent per year. During this time period, 

synthetic N fertilizer applications increased by nine percent, manure additions increased by 11 percent, and N 

fixation increased by about 17 percent. Soybean cropped area increased by 27 percent, corn area increased by six 

percent, and wheat area decreased by 20 percent. The increase in soybean area is largely responsible for the 

increase in fixation. Because total non-legume cropped area decreased and total fertilizer applied to major crops 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 222 

increased, the average rate of fertilizer applied to major crops increased by 32 percent from 1990 through 2003. 

The current method accounts for each of these variables plus the effects of climate variability, whereas the previous 

method accounted only for changes in fertilizer and manure additions. Climate interacts with N additions to control 

emissions with the new methodology. Total N additions from fertilizer are important with the IPCC methodology, 

while the current method accounts for total N additions, the area that receives the N are important, as well as 

environmental and management conditions. As a result, simulated N2O emission estimates may increase or decrease 

non-linearly, whereas emissions always increase linearly with N applications when using the IPCC methodology. 

Planned Improvements 

The presented uncertainty estimate is incomplete in that uncertainty in model activity data besides N inputs (county 

level weather and soil type) was not included. Because county level soil and climate data are applied across the 

entire county, within which a great deal of variability may occur, there is inherent uncertainty in assuming that soil 

type and climate do not vary within a county. Future estimates of uncertainty will include sensitivity analyses so 

that the response of model N2O output to variations in climate, soil type, and N inputs can be quantified. Also, a 

more appropriate methodology than Tier 1 will also be used in future uncertainty estimates. Future efforts at 

characterizing uncertainty will work toward the inclusion of all agricultural soil management subsource categories 

in a Monte Carlo style calculation. 

6.5. Field Burning of Agricultural Residues (IPCC Source Category 4F) 

Large quantities of agricultural crop residues are produced by farming activities. A variety of ways exist to dispose 

of these residues. For example, agricultural residues can be left on or plowed back 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, or supplemental animal feed. Field burning of crop residues is not considered a net source of 

CO2, because the carbon 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; therefore, emissions 

from this source are minor. The primary crop types whose residues are typically burned in the United States are 

wheat, rice, sugarcane, corn, barley, soybeans, and peanuts. Of these residues, less than 5 percent is burned each 

year, except for rice.8 Annual emissions from this source over the period 1990 through 2003 have remained 

relatively constant, averaging approximately 0.7 Tg CO2 Eq. (35 Gg) of CH4, 0.4 Tg CO2 Eq. (1 Gg) of N2O, 737 

Gg of CO, and 32 Gg of NOx (see Table 6-22 and Table 6-23). 

Table 6-22: Emissions from Field Burning of Agricultural Residues (Tg CO2 Eq.) 

Gas/Crop Type 1990 1997 1998 1999 2000 2001 2002 2003 

CH4 0.7 0.8 0.8 0.8 0.8 0.8 0.7 0.8 

Wheat 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Rice 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Sugarcane + + + + + + + + 

Corn 0.3 0.3 0.3 0.3 0.4 0.3 0.3 0.4 

Barley + + + + + + + + 

Soybeans 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 

Peanuts + + + + + + + + 

N2O 0.4 0.4 0.5 0.4 0.5 0.5 0.4 0.4 

Wheat + + + + + + + + 

 

8 The fraction of rice straw burned each year is significantly higher than that for other crops (see “Methodology” discussion 

below). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 223 

Rice + + + + + + + + 

Sugarcane + + + + + + + + 

Corn 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Barley + + + + + + + + 

Soybeans 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.2 

Peanuts + + + + + + + + 

Total 1.1 1.2 1.2 1.2 1.2 1.2 1.1 1.2 

+ Does not exceed 0.05 Tg CO2 Eq. 

Note: Totals may not sum due to independent rounding. 

Table 6-23: Emissions from Field Burning of Agricultural Residues (Gg)* 

Gas/Crop Type 1990 1997 1998 1999 2000 2001 2002 2003 

CH4 33 37 38 37 38 37 34 38 

Wheat 7 6 6 5 5 5 4 6 

Rice 4 3 3 4 4 4 3 5 

Sugarcane 1 1 1 1 1 1 1 1 

Corn 13 16 17 16 17 16 15 17 

Barley 1 1 1 + 1 + + 0 

Soybeans 7 10 10 10 10 11 10 9 

Peanuts + + + + + + + 0 

N2O 1 1 1 1 1 1 1 1 

Wheat + + + + + + + + 

Rice + + + + + + + + 

Sugarcane + + + + + + + + 

Corn + + + + + + + + 

Barley + + + + + + + + 

Soybeans 1 1 1 1 1 1 1 1 

Peanuts + + + + + + + + 

CO 689 767 789 767 790 770 706 794 

Wheat 137 124 128 115 112 98 81 117 

Rice 86 72 65 76 76 77 60 96 

Sugarcane 18 21 22 23 24 23 23 23 

Corn 282 328 347 336 353 338 320 360 

Barley 16 13 13 10 12 9 8 10 

Soybeans 148 207 211 204 212 222 211 186 

Peanuts 2 2 2 2 2 3 2 3 

NOx 28 34 35 34 35 35 33 33 

Wheat 4 3 3 3 3 3 2 3 

Rice 3 3 2 3 3 3 2 3 

Sugarcane + + + + + + + + 

Corn 7 8 8 8 8 8 8 9 

Barley 1 + + + + + + + 

Soybeans 14 20 20 19 20 21 20 18 

Peanuts + + + + + + + + 

* Full molecular weight basis. 

+ Does not exceed 0.5 Gg 

Note: Totals may not sum due to independent rounding. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 224 

Methodology 

The methodology for estimating greenhouse gas emissions from field burning of agricultural residues is consistent 

with the Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997)9. In order to estimate the amounts of 

carbon and nitrogen released during burning, the following equations were used:10

Carbon Released = (Annual Crop Production) × (Residue/Crop Product Ratio) 

× (Fraction of Residues Burned in situ) × (Dry Matter Content of the Residue) 

× (Burning Efficiency) × (Carbon Content of the Residue) × (Combustion Efficiency)11

Nitrogen Released = (Annual Crop Production) × (Residue/Crop Product Ratio) 

× (Fraction of Residues Burned in situ) × (Dry Matter Content of the Residue) 

× (Burning Efficiency) × (Nitrogen Content of the Residue) × (Combustion Efficiency) 

Emissions of CH4 and CO were calculated by multiplying the amount of carbon released by the appropriate IPCC 

default emission ratio (i.e., CH4-C/C or CO-C/C). Similarly, N2O and NOx emissions were calculated by 

multiplying the amount of nitrogen released by the appropriate IPCC default emission ratio (i.e., N2O-N/N or NOxN/N). 

The crop residues that are burned in the United States were determined from various state-level greenhouse gas 

emission inventories (ILENR 1993, Oregon Department of Energy 1995, Wisconsin Department of Natural 

Resources 1993) and publications on agricultural burning in the United States (Jenkins et al. 1992, Turn et al. 1997, 

EPA 1992). 

Crop production data for all crops except rice in Florida and Oklahoma were taken from the USDA’s Field Crops, 

Final Estimates 1987-1992, 1992-1997 (USDA 1994, 1998), Crop Production 1999 Summary (USDA 2000), Crop 

Production 2000 Summary (USDA 2001), Crop Production 2001 Summary (USDA 2002), Crop Production 2002 

Summary (USDA 2003) and Crop Production 2003 Summary (USDA 2004). Rice production data for Florida and 

Oklahoma, which are not collected by USDA, were estimated by applying average primary and ratoon crop yields 

for Florida (Schueneman and Deren 2002) to Florida acreages (Schueneman 1999b, 2001; Deren 2002; Kirstein 

2003, 2004; Cantens 2004) and Oklahoma acreages12 (Lee 2003, 2004). The production data for the crop types 

whose residues are burned are presented in Table 6-24. 

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 the 

agricultural extension agents in each state and reports of the California Air Resources Board (CARB) (Bollich 2000; 

Deren 2002; Guethle 1999, 2000, 2001, 2002, 2003, 2004; Fife 1999; California Air Resources Board 1999, 2001; 

 

9 The IPCC Good Practice Guidance (IPCC 2000) provided no updates the methodology for estimating field burning of 

agricultural residues. 

10 Note: 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. 

11 Burning Efficiency is defined as the fraction of dry biomass exposed to burning that actually burns. Combustion Efficiency is

defined as the fraction of carbon in the fire that is oxidized completely to CO2. In the methodology recommended by the IPCC, 

the “burning efficiency” is assumed to be contained in the “fraction of residues burned” factor. However, the number used here

to estimate the “fraction of residues burned” does not account for the fraction of exposed residue that does not burn. Therefore, a 

“burning efficiency factor” was added to the calculations. 

12 Rice production yield data are not available for Oklahoma so the Florida values are used as a proxy. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 225 

Klosterboer 1999a, 1999b, 2000, 2001, 2002, 2003; Lindberg 2002, 2003, 2004; Linscombe 1999a, 1999b, 2001, 

2002, 2003, 2004; Mutters 2002, 2003; Najita 2000, 2001; Schueneman 1999a, 1999b, 2001; Slaton 1999a, 1999b, 

2000; Stansel 2004; Street 1999a, 1999b, 2000, 2001, 2002, 2003; Walker 2004; Wilson 2001, 2002, 2003, 2004) 

(see Table 6-25 and Table 6-26). The estimates provided for Florida remained constant over the entire 1990 

through 2003 period, while the estimates for all other states varied over the time series. For California, the annual 

percents 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 declined between 1990 and 2003 because of a legislated reduction in rice straw burning (Lindberg 2002) (see 

Table 6-26). 

All residue/crop product mass ratios except sugarcane were obtained from Strehler and Stützle (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 

Stützle (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 carbon contents and nitrogen contents for all crops except soybeans and peanuts are 

from Turn et al. (1997). The residue carbon content for soybeans and peanuts is the IPCC default 

(IPCC/UNEP/OECD/IEA 1997). The nitrogen content of soybeans is from Barnard and Kristoferson (1985). The 

nitrogen content of peanuts is from Ketzis (1999). These data are listed in Table 6-27. The burning efficiency was 

assumed to be 93 percent, and the combustion efficiency was assumed to be 88 percent, for all crop types (EPA 

1994). Emission ratios for all gases (see Table 6-28) were taken from the Revised 1996 IPCC Guidelines

(IPCC/UNEP/OECD/IEA 1997). 

Table 6-24: Agricultural Crop Production (Gg of Product) 

Crop 1990 1997 1998 1999 2000 2001 2002 2003

Wheat 74,292 67,534 69,327 62,569 60,758 53,262 43,992 63,590

Rice 7,113 8,346 8,578 9,391 8,703 9,794 9,601 9,050

Sugarcane 25,525 28,766 30,896 32,023 32,762 31,377 32,597 31,178

Corn* 201,534 233,864 247,882 239,549 251,854 241,485 228,805 256,905

Barley 9,192 7,835 7,667 6,103 6,939 5,430 4,940 6,011

Soybeans 52,416 73,176 74,598 72,223 75,055 78,671 74,291 65,795

Peanuts 1,635 1,605 1,798 1,737 1,481 1,940 1,506 1,880

*Corn for grain (i.e., excludes corn for silage). 

Table 6-25: Percentage of Rice Area Burned by State 

State Percent Burned 

1990-1998 

Percent Burned

1999 

Percent Burned 

2000 

Percent Burned 

2001 

Percent Burned 

2002 

Percent Burned 

2003 

Arkansas 13 13 13 13 16 22 

California variablea 27 27 23 13 14 

Floridab 0 0 0 0 0 0 

Louisiana 6 0 5 4 3 3 

Mississippi 10 40 40 40 8 65 

Missouri 5 5 8 5 5 4 

Oklahoma 3 3 3 3 3 0 

Texas 1 2 0 0 0 0 

a Values provided in Table 6-26. 

b Although rice is cultivated in Florida, crop residue burning is illegal. Therefore, emissions remain 0 throughout the time series. 

Table 6-26: Percentage of Rice Area Burned in California 

Year California 

1990 75 

1991 75 

1992 66 

1993 60 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 226 

1994 69 

1995 59 

1996 63 

1997 34 

1998 33 

Table 6-27: Key Assumptions for Estimating Emissions from Field Burning of Agricultural Residues 

Crop Residue/Crop 

Ratio

Fraction of 

Residue Burned

Dry Matter 

Fraction 

Carbon 

Fraction

Nitrogen 

Fraction 

Burning 

Efficiency 

Combustion 

Efficiency 

Wheat 1.3 0.03 0.93 0.4428 0.0062 0.93 0.88 

Rice 1.4 variable 0.91 0.3806 0.0072 0.93 0.88 

Sugarcane 0.8 0.03 0.62 0.4235 0.0040 0.93 0.88 

Corn 1.0 0.03 0.91 0.4478 0.0058 0.93 0.88 

Barley 1.2 0.03 0.93 0.4485 0.0077 0.93 0.88 

Soybeans 2.1 0.03 0.87 0.4500 0.0230 0.93 0.88 

Peanuts 1.0 0.03 0.86 0.4500 0.0106 0.93 0.88 

Table 6-28: Greenhouse Gas Emission Ratios 

Gas Emission Ratio 

CH4

a

 0.005 

COa 0.060 

N2Ob

 0.007 

NOx

b

 0.121 a

 Mass of carbon compound released (units of C) relative to mass of total carbon released from burning (units of C). b

 Mass of nitrogen compound released (units of N) relative to mass of total nitrogen released from burning (units of N). 

Uncertainty 

One 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, as well as 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. 

Based on expert judgment, the uncertainty in production for all crops considered here is estimated to be 5 percent. 

Residue/crop product ratios can vary among cultivars. For all crops except sugarcane, generic residue/crop product 

ratios, rather than ratios specific to the United States, have been used. An uncertainty of 10 percent was applied to 

the residue/crop product ratios for all crops. 

Based on the range given for measurements of soybean dry matter fraction (Strehler and Stützle 1994), residue dry 

matter contents were assigned an uncertainty of 3.1 percent for all crop types. 

Burning and combustion efficiencies were assigned an uncertainty of 5 percent based on expert judgment. 

The N2O emission ratio was estimated to have an uncertainty of 28.6 percent based on the range reported in IPCC 

(2000). The uncertainty estimated for the CH4 emission ratio was 40 percent based on the range of ratios reported 

in IPCC (2000). 

The results of the Tier 1 quantitative uncertainty analysis are summarized in Table 6-29. Field burning of 

agricultural residues CH4 emissions in 2003 were estimated to be between 0.2 and 1.3 Tg CO2 Eq. at a 95 percent 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 227 

confidence level. This indicates a range of 69 percent above and below the 2003 emission estimate of 0.8 Tg CO2

Eq. Also at the 95 percent confidence level, N2O emissions were estimated to between 0.1 and 0.7 Tg CO2 Eq. (or 

approximately 68 percent above and below the 2003 emission estimate of 0.4 Tg CO2 Eq.). 

Table 6-29: Tier 1 Quantitative Uncertainty Estimates for CH4 and N2O Emissions from Field Burning of 

Agricultural Residues (Tg CO2 Eq. and Percent) 

Source Gas 

2003 

Emission 

Estimate 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Emission Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

Field Burning of 

Agricultural Residues CH4 0.8 69% 0.2 1.3 

Field Burning of 

Agricultural Residues N2O 0.4 68% 0.1 0.7 

Recalculations Discussion 

For the current Inventory, a transcription error was fixed for the 1998 rice production data for California from the 

USDA 2000 Crop Production Summary Report (2001). The change resulted in increases of less than 0.1 Tg CO2

Eq. (0.1 percent) in CH4 and N2O emissions from the field burning of agricultural residues for 1998. Additionally, 

the 2002 rice production data was updated from the USDA 2003 Crop Production Summary Report (2004). The 

change resulted in increases of less than 0.1 Tg CO2 Eq. (0.2 and 0.4 percent, respectively) in CH4 and N2O 

emissions from the field burning of agricultural residues for that year. 

s

Figure 6-1: 2003 Agriculture Chapter GHG Sources

1.2

0 50 100 150 200 250 300

Agricultural Residue

Burning

Rice Cultivation

Manure

Management

Enteric

Fermentation

Agricultural Soil

Management

Tg CO2 Eq

Agriculture as a 

Portion of all 

Emissions

6.3%
N Inputs 

Direct N 0 

Emissions 

2

Geophysical 

Processes 

Indirect N 0 

Emissions 

2

Groundwater

Deposition

Histosol 

Cultivation

Surface 

Water

Volatilization

Runoff

Leaching

 

N Flows:

Animal 

Wastes

Livestock 

& Poultry

Managed

Unmanaged

Sewage 

Sludge

Commercial 

Fertilizer 

Application

Crop 

Residues

N-fixing 

Crops

This graphic illustrates the sources and pathways of nitrogen that

result in direct and indirect N2O emissions from agricultural soils

in the United States. Sources of nitrogen applied to, or deposited

on, soils are respresented with arrows on the left-hand side of the

graphic. Emissions pathways are also 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.

Direct and Indirect N2O Emissions from

Agricultural Soils

Figure 6-2
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 229 

7. Land-Use Change and Forestry 

This chapter provides an assessment of the net greenhouse gas flux1 resulting from forest lands, croplands, and 

settlements. IPCC Good Practice Guidance for Land Use, Land-Use Change and Forestry (IPCC 2003) 

recommends reporting fluxes according to changes within and conversions between these land use types, as well as 

grassland and wetlands. However, consistent datasets are not available for the entire United States to allow results 

to be partitioned in this way. Therefore, greenhouse gas flux has been estimated for the following categories: 1) 

forest land remaining forest land 2) croplands remaining croplands, and 3) settlements remaining settlements. This 

categorization provides additional sources of information regarding N2O emissions by major land use type. 

It should be noted that other land-use and land-use change activities result in fluxes of non-CO2 greenhouse gases to 

and from soils that are not comprehensively accounted for currently. These fluxes include emissions of CH4 from 

managed forest soils, as well as CH4 emissions from artificially flooded lands, which result from activities such as 

dam construction. Aerobic (i.e., non-flooded) soils are a sink for CH4, so soil drainage can result in soils changing 

from a CH4 source to a CH4 sink, but if the drained soils are used for agriculture, fertilization and tillage disturbance 

can reduce the ability of soils to oxidize CH4. The non-CO2 emissions and sinks from these other land use and landuse change activities were not assessed due to scientific uncertainties about the greenhouse gas fluxes that result 

from these activities. 

The greenhouse gas flux from forest land remaining forest land is reported using estimates of changes in forest 

carbon stocks and the application of nitrous oxide (N2O) fertilizers to forest soils. Seven components of forest 

carbon stocks are analyzed: aboveground biomass, belowground biomass, dead wood, litter, soil organic carbon, 

harvested wood products in use, and harvested wood products in landfills. The estimated carbon dioxide (CO2) flux 

from each of these forest components was derived from U.S. forest inventory data, using methodologies that are 

consistent with LULUCF Good Practice Guidance (IPCC 2003) and the Revised 1996 IPCC Guidelines

(IPCC/UNEP/OECD/IEA 1997). In addition, this year, according to the new LULUCF Good Practice Guidance

(IPCC 2003), N2O emissions from fertilized forest soils are accounted for utilizing a default methodology. 

Croplands remaining croplands emission estimates are a reflection of the changes in agricultural soil carbon stocks 

on both cropland and grazing land since the necessary datasets were not available to separate cropland and grassland 

fluxes. Changes in agricultural soil carbon stocks include mineral and organic soil carbon stock changes due to use 

and management of cropland and grazing land, and emissions of CO2 due to the application of crushed limestone 

and dolomite to agricultural soils (i.e., soil liming). The methods used to estimate all three components of flux in 

agricultural soil carbon stocks are consistent with the Revised 1996 IPCC Guidelines and the LULUCF Good 

Practice Guidance (IPCC 2003). 

Fluxes resulting from settlements remaining settlements include landfilled yard trimmings and food scraps, urban 

trees, and soil N2O emissions from fertilization. Changes in yard trimming and food scrap carbon stocks in landfills 

are estimated using analysis of life-cycle greenhouse gas emissions and sinks associated with solid waste 

management (EPA 1998). Changes in carbon stocks in urban trees are estimated based on field measurements in 

ten U.S. cities and data on national urban tree cover, using a methodology consistent with the LULUCF Good 

Practice Guidance (IPCC 2003). Finally, this year, according to the new LULUCF Good Practice Guidance (IPCC 

2003), N2O emissions from fertilized settlement soils are accounted for according to a default methodology. Note 

that the chapter title “Land-Use Change and Forestry” has been used here to maintain consistency with the IPCC 

reporting structure for national greenhouse gas inventories; however, the chapter covers land-use activities, in 

addition to land-use change and forestry activities. Therefore, except in table titles, the term “land use, land-use 

change, and forestry” will be used in the remainder of this chapter. 

 

1 The term “flux” is used here to encompass both emissions of greenhouse gases to the atmosphere, and removal of carbon from 

the atmosphere. Removal of carbon from the atmosphere is also referred to as “carbon sequestration.” 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 230 

Unlike the assessments in other sectors, which are based on annual activity data, the flux estimates in this chapter, 

with the exception of those from wood products, urban trees, liming, and settlement and forest N2O emissions, are 

based on periodic activity data in the form of forest, land-use, and municipal solid waste surveys. Carbon dioxide 

fluxes from forest carbon 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. Because state surveys are collected at 

different times, using this data structure, the estimated CO2 fluxes from forest carbon stocks differ at the national 

level from year to year. Agricultural soil carbon flux calculations are constant over multi-year intervals, with large 

discontinuities between intervals; however, fluxes after 1997 are inconsistent due to the method of accounting for 

the application of manure and sewage sludge amendments to mineral soils after 1997. For the landfilled yard 

trimmings and food scraps source, periodic solid waste survey data were interpolated so that annual storage 

estimates could be derived. In addition, because the most recent national forest, land-use, and municipal solid waste 

surveys were completed prior to 2003, the estimates of CO2 flux from forests, agricultural soils, and landfilled yard 

trimmings and food scraps are based in part on modeled projections or extrapolation. Carbon dioxide flux from 

urban trees is based on neither annual data nor periodic survey data, but instead on data collected over the period 

1990 through 1999. This flux has been applied to the entire time series. 

Land use, land-use change, and forestry activities in 2003 resulted in a net carbon sequestration of 828 Tg CO2 Eq. 

(226 Tg C) (Table 7-1and Table 7-2). This represents an offset of approximately 14 percent of total U.S. CO2

emissions. Total land use, land-use change, and forestry net carbon sequestration declined by approximately 21 

percent between 1990 and 2003. This decline was primarily due to a decline in the rate of net carbon accumulation 

in forest carbon stocks. Annual carbon accumulation in landfilled yard trimmings and food scraps also slowed over 

this period, as did annual carbon accumulation in agricultural soils. As described above, the constant rate of carbon 

accumulation in urban trees is a reflection of limited underlying data (i.e., this rate represents an average for 1990 

through 1999). 

Table 7-1: Net CO2 Flux from Land-Use Change and Forestry (Tg CO2 Eq.) 

Sink Category 1990 1997 1998 1999 2000 2001 2002 2003 

Forest Land Remaining Forest Land (949.3) (851.0) (805.5) (751.7) (747.9) (750.9) (751.5) (752.7)

Changes in Forest Carbon Stocks (949.3) (851.0) (805.5) (751.7) (747.9) (750.9) (751.5) (752.7) 

Cropland Remaining Cropland (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6)

Changes in Agricultural Soil Carbon 

Stocks (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6) 

Settlements Remaining Settlements (84.7) (71.6) (71.2) (70.0) (68.9) (68.9) (68.8) (68.7)

Urban Trees (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) (58.7) 

Landfilled Yard Trimmings and Food 

Scraps (26.0) (12.9) (12.5) (11.4) (10.2) (10.3) (10.2) (10.1) 

Total (1042.0) (930.0) (881.0) (826.1) (822.4) (826.9) (826.5) (828.0) 

Note: Parentheses indicate net sequestration. Totals may not sum due to independent rounding. 

Table 7-2: Net CO2 Flux from Land-Use Change and Forestry (Tg C) 

Sink Category 1990 1997 1998 1999 2000 2001 2002 2003 

Forest Land Remaining Forest Land (259) (232) (220) (205) (204) (205) (205) (205)

Changes in Forest Carbon Stocks (259) (232) (220) (205) (204) (205) (205) (205) 

Cropland Remaining Cropland (2) (2) (1) (1) (2) (2) (2) (2)

Changes in Agricultural Soil Carbon 

Stocks (2) (2) (1) (1) (2) (2) (2) (2) 

Settlements Remaining Settlements (23) (20) (19) (19) (19) (19) (19) (19)

Urban Trees (16) (16) (16) (16) (16) (16) (16) (16) 

Landfilled Yard Trimmings and Food 

Scraps (7) (4) (3) (3) (3) (3) (3) (3) 

Total (284) (254) (240) (225) (224) (226) (225) (226) 

Note: 1 Tg C = 1 teragram carbon = 1 million metric tons carbon. Parentheses indicate net sequestration. Totals may not sum 

due to independent rounding. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 231 

Land use, land-use change, and forestry activities in 2003 resulted in a net flux of 6.4 Tg CO2 Eq. of N2O (20.7 Gg) 

(Table 7-3 and Table 7-4). Total N2O emissions from the application of fertilizers to forests and settlements 

increased by approximately 14 percent between 1990 and 2003. 

Table 7-3: Net N2O Emissions from Land-Use Change and Forestry (Tg CO2 Eq.) 

Sink Category 1990 1997 1998 1999 2000 2001 2002 2003 

Forest Land Remaining Forest Land 0.1 0.3 0.4 0.5 0.4 0.4 0.4 0.4 

N2O Fluxes from Soils 0.1 0.3 0.4 0.5 0.4 0.4 0.4 0.4 

Settlements Remaining Settlements 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0 

N2O Fluxes from Soils 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0 

Total 5.6 6.4 6.5 6.6 6.3 6.2 6.4 6.4 

Note: Totals may not sum due to independent rounding. 

Table 7-4: Net N2O Emissions from Land-Use Change and Forestry (Gg) 

Sink Category 1990 1997 1998 1999 2000 2001 2002 2003 

Forest Land Remaining Forest Land 0.2 1.0 1.1 1.5 1.1 1.3 1.3 1.3 

N2O Fluxes from Soils 0.2 1.0 1.1 1.5 1.1 1.3 1.3 1.3 

Settlements Remaining Settlements 17.9 19.8 19.8 19.9 19.3 18.7 19.4 19.4 

N2O Fluxes from Soils 17.9 19.8 19.8 19.9 19.3 18.7 19.4 19.4 

Total 18.1 20.7 20.9 21.4 20.4 20.0 20.7 20.7 

Note: Totals may not sum due to independent rounding. 

7.1. Forest Land Remaining Forest Land 

Changes in Forest Carbon Stocks (IPCC Source Category 5A1) 

For estimating carbon (C) stocks or stock change (flux), C in forest ecosystems can be divided into the following 

five storage pools (IPCC 2003): 

• Aboveground biomass, all living biomass above the soil including stem, stump, branches, bark, seeds, and 

foliage. This category includes live understory. 

• Belowground biomass, all living biomass of coarse living roots greater than 2 mm diameter. 

• Dead wood, including all non-living woody biomass either standing, lying on the ground (but not including 

litter), or in the soil. 

• Litter, including 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 carbon (SOC), including all organic material in soil to a depth of 1 meter but excluding the 

coarse roots of the above pools. 

In addition, there are two harvested wood pools also necessary for estimating C flux, which are: 

• Harvested wood products in use. 

• Harvested wood products in landfills. 

Carbon is continuously cycled among these storage pools and between forest ecosystems and the atmosphere as a 

result of biological processes in forests (e.g., photosynthesis, 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 age, 

they continue to accumulate C until they reach maturity, at which point they store a relatively constant amount of C. 

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 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 232 

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 landfills, the C contained in the wood may be released many years or decades later, or may be stored 

almost permanently in the landfill. 

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. Thus, the focus on C implies that all C-based greenhouse gases are included, and the focus on stock 

change suggests that specific ecosystem fluxes are not 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 counts the carbon changes due to fires; however, it may 

not be possible to attribute the changes to the disturbance specifically. The IPCC LULUCF Good Practice 

Guidance (IPCC 2003) recommends reporting C stocks according to several land use types and conversions, 

specifically forest land remaining forest land, nonforest land becoming forest, and forest becoming non-forest. 

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 

7-1. 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 7-1: Forest Sector Carbon Pools and Flows. 

Note: Boxes represent forest C storage pools and arrows represent flows between storage pools or between storage pools and the 

atmosphere. 

Approximately 33 percent (303 million hectares) of the U.S. land area is forested (Smith et al. 2004b). From the 

early 1970s to the early 1980s, forest land declined by approximately 2.4 million hectares. During the 1980s and 

1990s, forest area increased by about 3.7 million hectares. These net changes in forest area represent average 

annual fluctuations of only about 0.1 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 can increase both the rate of growth and the eventual biomass 

density2 of the forest, thereby increasing the uptake of C. Harvesting forests removes much of the aboveground C, 

but trees can grow on this area again and sequester C. 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 lands 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 

 

2 The term “biomass density” refers to the mass of vegetation per unit area. It is usually measured on a dry-weight basis. Dry 

biomass is about 50 percent carbon by weight. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 233 

through 2003. Due to improvements in U.S. agricultural productivity, the rate of forest clearing for crop cultivation 

and pasture slowed in the late 19th century, and by 1920 this practice had all but ceased. As farming expanded in 

the Midwest and West, large areas of previously cultivated land in the East were taken out of crop production, 

primarily between 1920 and 1950, and were allowed to revert to forests or were actively reforested. The impacts of 

these land-use changes still affect C fluxes from forests in the East. In addition, C fluxes from eastern forests have 

been affected by a trend toward managed growth on private land. Collectively, these changes have nearly doubled 

the biomass density in eastern forests since the early 1950s. 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 landfills 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). 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 an average annual net 

sequestration of 832 Tg CO2 Eq. (227 Tg C) over the period 1990 through 2003 (Table 7-5, Table 7-6, and Figure 

7-2). 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, particularly before 1997. The increase in forest sequestration is 

due more to an increasing C density per area than to the increase in area of forestland. Forestland in the 

conterminous United States was approximately 246, 250, and 251 million hectares for 1987, 1997, and 2002, 

respectively, only a 2 percent increase over the period (Smith et al. 2004b). Continuous, regular annual surveys are 

not available over the period for each state; therefore, estimates for non-survey years were derived by interpolation 

between known data points. Survey years vary from state to state. National estimates are a composite of individual 

state surveys. Total sequestration declined by 21 percent between 1990 and 2003. This decline was primarily due 

to a decline in the estimated rate of sequestration in forest soils. Inventory derived estimates of soil C stocks are 

based solely on forest area and type. Thus, changes in soil C over time are directly the result of changes in total 

forest area or changes in forest type from forest inventory data. 

Table 7-5. Net Annual Changes in Carbon Stocks (Tg CO2 Eq. yr-1) in Forest and Harvested Wood Pools 

Carbon Pool 1990 1997 1998 1999 2000 2001 2002 2003

Forest (739) (638) (599) (537) (537) (537) (537) (537)

Aboveground Biomass (396) (457) (437) (400) (400) (400) (400) (400)

Belowground Biomass (77) (89) (85) (78) (78) (78) (78) (78)

Dead Wood (74) (53) (51) (45) (45) (45) (45) (45)

Litter (67) (31) (28) (26) (26) (26) (26) (26)

Soil Organic Carbon (125) (8) 1 12 12 12 12 12 

Harvested Wood (210) (213) (206) (215) (211) (214) (214) (216)

Wood Products (48) (58) (52) (62) (59) (59) (59) (60)

Landfilled Wood (162) (155) (154) (153) (152) (155) (155) (155)

Total Net Flux (949) (851) (806) (752) (748) (751) (751) (753)

Note: 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 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 7-6. Net Annual Changes in Carbon Stocks (Tg C yr-1) in Forest and Harvested Wood Pools

Carbon Pool 1990 1997 1998 1999 2000 2001 2002 2003

Forest (202) (174) (163) (146) (146) (146) (146) (146)

Aboveground Biomass (108) (125) (119) (109) (109) (109) (109) (109)

Belowground Biomass (21) (24) (23) (21) (21) (21) (21) (21)

Dead Wood (20) (14) (14) (12) (12) (12) (12) (12)

Litter (18) (9) (8) (7) (7) (7) (7) (7)
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 234 

Soil Organic Carbon (34) (2) 0 3 3 3 3 3

Harvested Wood (57) (58) (56) (59) (57) (58) (58) (59)

Wood Products (13) (16) (14) (17) (16) (16) (16) (16)

Landfilled Wood (44) (42) (42) (42) (41) (42) (42) (42)

Total Net Flux (259) (232) (220) (205) (204) (205) (205) (205)

Note: 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 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. 

Stock estimates for forest and harvested wood C storage pools are presented in Table 7-7. Together, the 

aboveground live and forest soil pools account for a large proportion of total forest C stocks. C stocks in all nonsoil pools increased over time. Therefore, C sequestration was greater than C emissions from forests, as discussed 

above. Figure 7-3 shows the average carbon density in forests by state, estimated for 2004. 

Table 7-7. Carbon Stocks (Tg C) in Forest and Harvested Wood Pools 

Carbon Pool 1990 1997 1998 1999 2000 2001 2002 2003 2004

Forest 39,498 40,812 40,986 41,149 41,296 41,442 41,589 41,735 41,882

Aboveground Biomass 14,114 14,928 15,053 15,172 15,281 15,390 15,499 15,608 15,717

Belowground Biomass 2,805 2,963 2,987 3,011 3,032 3,053 3,074 3,095 3,117

Dead Wood 2,444 2,572 2,587 2,600 2,613 2,625 2,638 2,650 2,662

Litter 4,496 4,598 4,606 4,614 4,621 4,628 4,636 4,643 4,650

Soil Organic Carbon 15,640 15,750 15,752 15,752 15,749 15,745 15,742 15,738 15,735

Harvested Wood 1,915 2,307 2,365 2,421 2,480 2,537 2,595 2,654 2,713

Wood Products 1,134 1,232 1,248 1,262 1,279 1,295 1,311 1,327 1,344

Landfilled Wood 781 1,074 1,117 1,159 1,200 1,242 1,284 1,327 1,369

Total Carbon Stock 41,414 43,119 43,351 43,570 43,775 43,979 44,184 44,389 44,594

Note: Forest C stocks do not include forest stocks in Alaska, Hawaii, or U.S. territories, or trees on non-forest land (e.g., urban 

trees). Wood product stocks include exports, even if the logs are processed in other countries, and exclude imports. Forest 

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. 

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 2003 requires estimates of C stocks for 2003 and 2004. 

Figure 7-2: Estimates of Net Annual Changes in Carbon Stocks for Major Carbon Pools (Tg C yr-1) 

Note: Estimates for harvested wood are based on the same methodology and data as the previous U.S. Inventory (EPA 2004). 

Estimates for all pools are based on measured forest inventory data as described in the text. Total Net includes all forest pools: 

biomass, dead wood, litter, forest soils, wood products, and landfilled wood. 

Figure 7-3: Average Carbon Density in the Forest Tree Pool in the Conterminous U.S. During 2004. 

Methodology 

The methodology described herein is consistent with LULUCF Good Practice Guidance (IPCC 2003) and the 

Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). Estimates of net C flux from Land-Use Change 

and Forestry, including all pools except harvested wood, were derived from periodic and annualized inventories of 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 235 

forest stocks. Net changes in C stocks were interpolated between survey years. Carbon emissions from harvested 

wood were determined by accounting for the variable rate of decay of harvested wood according to its disposition 

(e.g., product pool, landfill, combustion).3 Different data sources were used to estimate the C stocks and stock 

change in (1) forests (aboveground and belowground biomass, dead wood, and litter), (2) forest soils, and (3) 

harvested wood products. Therefore, these pools are described separately below. 

Live Biomass, Dead Wood, and Litter Carbon 

The estimates of non-soil forest C stocks are based on data derived from forest inventory surveys. Forest survey 

data were obtained from the USDA Forest Service, Forest Inventory and Analysis (FIA) program (Frayer and 

Furnival 1999, Smith et al. 2001). Surveys provide estimates of the merchantable volume of wood and other 

variables that are used to estimate C stocks. Estimates of temporal change such as growth, mortality, harvests, or 

area change are derived from repeated surveys, which were conducted every 5 to 14 years, depending on the state. 

Historically, the FIA program did not conduct detailed surveys of all forest land, but instead focused on land 

capable of supporting timber production (timberland4). However, over time individual state surveys gradually 

started to include reserved and less productive forest lands. The C stock estimates provided here include all forest 

land, see Annex 3.12 for discussion of how past data gaps on these lands were filled. 

Temporal and spatial gaps in surveys were addressed with the new national plot design and annualized sampling 

(Miles et al. 2001, Alerich et al. 2004), which were recently introduced by FIA. Annualized sampling means that a 

portion of plots throughout each state is sampled each year, with the goal of measuring all plots once each 5 years. 

Sampling is designed such that partial inventory cycles provide usable, unbiased samples of forest inventory. Thus, 

many states have relatively recent partial inventories, yet not all states are currently surveyed this way. All 

annualized surveys initiated since 1998 have followed the new national plot design for all forestlands, including 

reserved and less productive lands. Inventories are assumed to represent stocks as of January 1 of the inventory 

year. 

For each periodic or annualized inventory in each state, each C pool was estimated using coefficients from the 

FORCARB2 model (Birdsey and Heath 1995, Birdsey and Heath 2001, Heath et al. 2003, Smith et al. 2004a). 

Estimates of C stocks made by the FORCARB2 coefficients at the plot level are organized somewhat differently 

than the standard IPCC pools reported in Table 7-7. However, the estimators are compatible with reorganizing the 

pools following IPCC LULUCF Good Practice Guidance (2003). For example, the biomass pools here include the 

FORCARB2 pools of live trees and understory vegetation, each of which are divided into aboveground versus 

belowground portions. Calculations for the tree portion of the aboveground biomass C pool were made using 

volume-to-biomass conversion factors for different types of forests as presented in Smith et al. (2003). Biomass 

was converted to C mass by dividing by two because dry biomass is approximately 50 percent C 

(IPCC/UNEP/OECD/IEA 1997). The other portion of aboveground biomass, live understory C, was estimated from 

inventory data using tables presented in Birdsey (1996). Litter C was estimated from inventory data using the 

equations presented in Smith and Heath (2002). Down dead wood was estimated using a FORCARB2 simulation 

and U.S. forest statistics (Smith et al. 2001). 

 

3 The wood product stock and flux estimates presented here use the production approach, meaning that they do not account for C 

stored in imported wood products, but do include C stored in exports, even if the logs are processed in other countries. This 

approach is used because it follows the precedent established in previous reports (Heath et al. 1996). 

4 Forest land in the U.S. 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. 

Productivity is at a minimum rate of 20 cubic feet of industrial wood per acre per year. The remaining portion of forest land is 

classified as either reserved forest land, which is forest land withdrawn from timber use by statute or regulation, or other forest 

land, which includes less productive forests on which timber is growing at a rate less than 20 cubic feet per acre per year. In 

2002, there were about 199 million hectares of timberland in the conterminous U.S., which represented 79 percent of all forest 

lands over the same area (Smith et al. 2004b). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 236 

Forest Soil Carbon

Estimates of soil organic carbon stocks are based solely on forest area and on average soil C density for each broad 

forest type group. Thus, any changes in soil C stocks are due to changes in total forest area or the distribution of 

forest types within that area. Estimates of the organic C content of soils are based on the national STATSGO spatial 

database (USDA 1991) and follow methods of Amichev and Galbraith (2004). These data were overlaid with FIA 

survey data to estimate soil C on forest lands by broad forest type group. 

Forest Carbon Stocks and Fluxes

The overall approach for determining forest C stock change was to estimate forest C stocks based on data from two 

forest surveys conducted several years apart. Carbon stocks were calculated separately for each state based on 

inventories available since 1990 and for the most recent inventory prior to 1990. Thus, the number of separate 

stock estimates for each state was one less than the number of available inventories. For each pool in each state in 

each year, C stocks were estimated by linear interpolation between survey years. Similarly, fluxes were estimated 

for each pool in each state by dividing the difference between two successive stocks by the number of intervening 

years between surveys. Note that inventories are assumed to represent stocks as of January 1 of the inventory year; 

thus, stocks in 1989 and 1993 can be used to estimate flux for 1989 through 1992, for example. Stocks and fluxes 

since the most recent survey were based on extrapolating estimates from the last two surveys. C stock and flux 

estimates for each pool were summed over all states to form estimates for the conterminous United States. Data 

sources and methods for estimating individual C pools are described more fully in Annex 3.12. 

Harvested Wood Carbon

Estimates of C stock changes in wood products and wood discarded in landfills were based on the methods 

described by Skog and Nicholson (1998). Carbon stocks in wood products in use and wood products stored in 

landfills were estimated from 1910 onward based on historical data from the USDA Forest Service (USDA 1964, 

Ulrich 1989, Howard 2001), and historical data as implemented in the framework underlying the North American 

Pulp and Paper (NAPAP, Ince 1994) and the Timber Assessment Market and the Aggregate Timberland Assessment 

System Timber Inventory models (TAMM/ATLAS, Haynes 2003, Mills and Kincaid 1992). Beginning with data 

on annual wood and paper production, the fate of C in harvested wood was tracked for each year from 1910 through 

2003, and included the change in C stocks in wood products, the change in C in landfills, and the amount of C 

emitted to the atmosphere (CO2 and CH4) both with and without energy recovery. To account for imports and 

exports, the production approach was used, meaning that C in exported wood was counted as if it remained in the 

United States, and C in imported wood was not counted. 

Uncertainty 

The forest survey data that underlie the forest C estimates are based on a statistical sample designed to represent the 

wide variety of growth conditions present over large territories. However, forest survey data that are currently 

available generally exclude timber stocks on most forest land in Alaska, Hawaii, and U.S. territories. For this 

reason, estimates have been developed only for the conterminous United States. Within the conterminous United 

States, the USDA Forest Service mandates that forest area data are accurate within 3 percent at the 67 percent 

confidence level (one standard error) per 405,000 ha of forest land (Miles et al. 2001). For larger areas, the 

uncertainty in area is concomitantly smaller. For volume data, the accuracy is targeted to be 5 percent for each 

28,300 m3

 at the same confidence level. An analysis of uncertainty in growing stock volume data for timber 

producing lands was undertaken for five states: Florida, Georgia, North Carolina, South Carolina, and Virginia 

(Phillips et al. 2000). Nearly all of the uncertainty was found to be due to sampling rather than the regression 

equations used to estimate volume from tree height and diameter. Standard errors for growing stock volume ranged 

from 1 to 2 percent for individual states and less than 1 percent for the 5-state region. However, the total standard 

error for the change in growing stock volume was estimated to be 12 to 139 percent for individual states, and 20 

percent for the 5-state region. The high relative uncertainty for growing stock volume change in some states was 

due to small net changes in growing stock volume. However, the uncertainty in volume change may be smaller than 

was found in this study because estimates from samples taken at different times on permanent survey plots are 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 237 

correlated, and such correlation reduces the uncertainty in estimates of changes in volume or C over time (Smith and 

Heath 2000). Based on these accuracy guidelines and these results for the Southeastern United States, forest area 

and volume data for the conterminous United States are expected to be reasonably accurate, although estimates of 

small changes in growing stock volume may have substantial uncertainty. 

In addition to uncertainty in growing stock volume, there is uncertainty associated with the estimates of C stocks in 

other ecosystem pools. Estimates for these pools are derived from extrapolations of site-specific studies to all forest 

land since survey data on these pools are not generally available. Such extrapolation introduces uncertainty because 

available studies may not adequately represent regional or national averages. Uncertainty may also arise due to (1) 

modeling errors, for example relying on coefficients or relationships that are not well known, and (2) errors in 

converting estimates from one reporting unit to another (Birdsey and Heath 1995). An important source of 

uncertainty is that there is little consensus from available data sets on the effect of land use change and forest 

management activities (such as harvest) on soil C stocks. For example, while Johnson and Curtis (2001) found little 

or no net change in soil C following harvest, on average, across a number of studies, many of the individual studies 

did exhibit differences. Heath and Smith (2000b) noted that the experimental design in a number of soil studies 

limited their usefulness for determining effects of harvesting on soil C. Because soil C stocks are large, estimates 

need to be very precise, since even small relative changes in soil C sum to large differences when integrated over 

large areas. The soil C stock and stock change estimates presented herein are based on the assumption that soil C 

density for each broad forest type group stays constant over time. As more information becomes available, the 

effects of land use and of changes in land use and forest management will be better accounted for in estimates of 

soil C (see “Planned Improvements,” below). 

Recent studies have begun to quantify the uncertainty in national-level forest C budgets based on the methods 

adopted here. Smith and Heath (2000) and Heath and Smith (2000a) report on an uncertainty analysis they 

conducted on C sequestration in privately owned timberlands throughout the conterminous United States. These 

studies are not directly comparable to the estimates in this chapter because they used an older version of the 

FORCARB model and are based on older data. However, the relative magnitudes of the uncertainties are 

informative. For the period 1990 through 1999, the true mean C flux was estimated to be within 15 percent of the 

reported mean at the 80 percent confidence level. The corresponding true mean C stock estimate for 2000 was 

within approximately 5 percent of the reported mean value at the 80 percent confidence level. The relatively greater 

uncertainty in flux estimates compared to stock estimates is roughly similar to that found for estimates of growing 

stock volume discussed above (Phillips et al. 2000). In both analyses, there are greater relative uncertainties 

associated with smaller estimates of flux. Uncertainty in the estimates presented in this inventory may be greater 

than those presented by Heath and Smith (2000a) for several reasons. Most importantly, their analysis did not 

include uncertainty in growing stock volume data or uncertainties in stocks and fluxes of C from harvested wood. 

The uncertainty analysis was performed using the IPCC-recommended Tier 2 uncertainty estimation methodology, 

Monte Carlo Simulation technique. The results of the Tier 2 quantitative uncertainty analysis are summarized in 

Table 7-8. The 2003 flux estimate for forest C stocks is estimated to be between (1,120.5) and (383.5) Tg CO2 Eq. 

at a 95 percent confidence level (or 19 of 20 Monte Carlo Stochastic Simulations). This indicates a range of 49 

percent below to 49 percent above the 2003 flux estimate of (752.7) Tg CO2 Eq. 

Table 7-8: Tier 2 Quantitative Uncertainty Estimates for CO2 Net Flux from Forest Land Remaining Forest Land: 

Changes in Forest Carbon Stocks (Tg CO2 Eq. and Percent) 

Source Gas 

2003 Flux 

Estimate Uncertainty Range Relative to 2003 Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 

Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Forests Remaining 

Forests: Changes in Forest 

Carbon Stocks CO2 (752.7) (1,120.5) (383.5) -49% +49% a

Range of flux estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 238 

QA/QC and Verification 

As discussed above, the USDA Forest Service Forest Inventory and Analysis program has conducted consistent 

forest surveys based on extensive statistically-based sampling of most of the forest land in the conterminous United 

States since 1952. The main purpose of the Forest Inventory and Analysis program has been to estimate areas, 

volume of growing stock, and timber products output and utilization factors. The Forest Inventory and Analysis 

program includes numerous quality assurance and quality control 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 Forest 

Inventory and Analysis 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 (FIA 

Homepage). 

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 Methods 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 FIADB, which are available at an FIA Internet site (FIA Database Retrieval System). 

Agreement between the C datasets and the original inventories is important to verify accuracy of the data used. 

Forest Inventory and Analysis data and some model projections are given in English units, but C stock estimates 

were developed using metric units. To avoid unit conversion errors, a standard conversion table in electronic form 

was used (Appendix B of Smith et al. 2001). Finally, C stock estimates were compared with previous inventory 

report estimates to assure that any differences could be explained by either new data or revised calculation methods 

(see the “Recalculations” discussion below). 

Recalculations Discussion 

The overall scheme for developing annualized estimates of C stocks based on the individual state surveys is similar 

to that presented in the previous Inventory (EPA 2003). Methods for estimating soil organic carbon are new for the 

current Inventory; differences are in the interpretation of STATSGO data and their relationship with FIA survey 

data as described by Amichev and Galbraith (2004)—see Annex 3.12 for additional information. Similar to that 

reported in the previous Inventory, estimates of forest C stocks and fluxes are based on forest inventory data from 

individual states rather than regions, and the data collected in states were assigned an average survey inventory plot 

date rather than simply assigned the year for which the database was compiled. However, the selection and 

compilation of survey data was implemented differently for the current Inventory, and there were some important 

differences in the underlying data. 

Three differences in methods can affect the non-soil forest C estimates. First, the selection of the datasets 

representing individual state surveys was independent of last year’s selections. Both RPA and the newer FIADB 

datasets were considered, whereas last year only RPA data were used. The RPA data represent specific 

compilations of survey data and include some older data not currently available in the FIADB. Using both ensured 

the most recent data were used, yet older data were available as needed. Inventory data—even older surveys—are 

occasionally modified so that RPA and FIADB data of ostensibly the same survey may have some slight differences 

that can affect the C estimates. This is likely to have a very minor effect on recalculation of C stocks. Another 

minor change in method is that fluxes were separately determined from the original survey data for each state for 

each pool; in contrast, last year stocks were interpolated and summed to a national total for each year before flux 

was calculated. Finally, separate stocks and fluxes were determined for National Forest lands where, in the past, 

independent surveys were conducted at distinctly different times. 

Pool definitions have changed for the current Inventory, as suggested by IPCC LULUCF Good Practice Guidance

(IPCC 2003). In previous Inventories, the pools were trees, understory, forest floor, down dead wood, and forest 

soils. The forest soil pool is now soil organic carbon; forest floor is called litter. The previous tree pool included 

both above- and belowground biomass and mass of standing dead trees. The mass of standing dead trees was added 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 239 

to the down dead wood pool and is called dead wood. The remainder of the tree pool, live biomass, as well as the 

understory pool, was split into above- and belowground portions. The aboveground tree and understory pools were 

summed into the aboveground biomass pool; the belowground portions of these pools were added to create the 

belowground biomass pools. 

Two changes in the use of data are also likely to affect the recalculation of C. The equations used to estimate tree C 

from forest inventory data have been revised slightly; the net effect is that total tree C (live plus standing dead trees, 

which are part of both the biomass and dead wood pools as summarized here) calculated for the 2002 RPA database 

(Smith et al. 2004b) was 0.3 percent greater with the new set of equations relative to those used last year. Perhaps 

the largest effect on C recalculations is that for the previous Inventory the final C stocks were modeled. This year, 

however, values are simply extrapolated. The principal reason for eliminating the projections was the difficulty in 

establishing projections consistent with the available forest inventory data. 

Overall, these changes resulted in an average annual increase of 103.3 Tg CO2 Eq. (16 percent) in forest carbon 

stocks for the period 1990 through 2002. 

Planned Improvements 

The Forest Inventory and Analysis program has adopted a new annualized design, such that a portion of each state 

will be surveyed each year (Gillespie 1999). The annualized survey also includes measuring attributes that are 

needed to estimate C in various pools, such as soil C and forest floor C, on a subset of the plots. During the next 

several years, the use of annual data, including new data on soil and forest floor C stocks, and new data on nontimberlands, will improve the precision and accuracy of estimates of forest C stocks and fluxes. 

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). Currently, soil C 

estimates are based on the assumption that soil C density depends only on broad forest type group, not on land use 

history. However, many forests in the Eastern United States are re-growing on abandoned agricultural land. During 

such regrowth, soil and forest floor C stocks often increase substantially over many years or even decades, 

especially on highly eroded agricultural land. In addition, with deforestation, soil C stocks often decrease over many 

years. A new methodology is being developed to account for these changes in soil C over time. This methodology 

includes estimates of area changes among land uses (especially forest and agriculture), estimates of the rate of soil C 

stock gain with afforestation, and estimates of the rate of soil C stock loss with deforestation over time. This topic 

is important because soil C stocks are large, and soil C flux estimates contribute substantially to total forest C flux, 

as shown in Table 7-6 and Figure 7-2. 

The estimates of C stored in harvested wood products are currently being revised using more detailed wood 

products production and use data, and more detailed parameters on disposition and decay of products. 

N2O Fluxes from Soils (IPCC Source Category 5.A.1) 

Of the 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 fertilizer. This is because forests are typically fertilized only twice during their 

approximately 40 year growth cycle (once at planting and once at approximately 20 years). Thus, although the rate 

of fertilizer application for the area of forests that receives fertilizer in any given year is relatively high, average 

annual applications, inferred by dividing all forest land by the amount of fertilizer added to forests in a given year, 

is quite low. Nitrous oxide (N2O) emissions from forest soils for 2003 were almost 7 times higher than the baseline 

year (1990). The trend toward increasing N2O emissions is a result of an increase in fertilized area of pine 

plantations in the southeastern United States. Total 2003 forest soil N2O emissions are roughly equivalent to 3.3 

percent of the total forest soil carbon flux, and 0.07 percent of the total sequestration in standing forests, and are 

summarized in Table 7-9. 

Table 7-9. N2O Fluxes from Soils in Forests Remaining Forests (Tg CO2 Eq. and Gg) 

Forests Remaining Forests: N2O Fluxes 1990 1997 1998 1999 2000 2001 2002 2003 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 240 

from Soils 

Tg CO2 Eq. 0.06 0.30 0.35 0.47 0.35 0.39 0.39 0.39 

Gg 0.19 0.96 1.14 1.50 1.14 1.26 1.26 1.26 

Methodology 

According to U.S. Forest Service statistics for 1996 (USDA Forest Service 2001), approximately 75 percent of 

trees planted for timber, and about 60 percent of national total harvested forest area are in the southeastern United 

States. Consequently, it was assumed that southeastern pine plantations represent the vast majority of fertilized 

forests in the United States. Therefore, estimates of direct N2O emissions from fertilizer applications to forests were 

based on the area of pine plantations receiving fertilizer in the southeastern United States and estimated application 

rates (North Carolina Sate Forest Nutrition Cooperative 2002). Not accounting for fertilizer applied to non-pine 

plantations is justified because fertilization is routine for pine forests but rare for hardwoods (Binkley et al. 1995). 

For each year, the area of pine receiving N fertilizer was multiplied by the midpoint of the reported range of N 

fertilization rates (150 lbs. 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; however, it should be 

expected that emissions from the small areas of fertilized forests in other regions would be insubstantial because the 

majority of trees planted and harvested for timber are in the southeastern United States (USDA Forest Service 

2001). Area data for pine plantations receiving fertilizer in the southeast were not available for 2002 and 2003, so 

data from 2001 were substituted for these years. The proportion of N additions that volatilized from forest soils was 

assumed to be 10 percent of total amendments, according to the IPCC’s default. The unvolatilized N applied to 

forests was then multiplied by the IPCC default emission factor of 1.25 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 sector. 

Uncertainty 

The amount of N2O emitted from forests depends not only on N inputs, but also on a large number of variables, 

including organic carbon availability, O2 partial pressure, soil moisture content, pH, temperature, and tree 

planting/harvesting cycles. The effect of the combined interaction of these variables on N2O flux is complex and 

highly uncertain. The IPCC default methodology used here does not incorporate any of these variables and only 

accounts for variations in estimated fertilizer application rates and estimated areas of forested land receiving 

fertilizer. All forest soils are treated equivalently under this methodology. Furthermore, only synthetic fertilizers 

are captured, so applications of organic fertilizers are not accounted for here. 

Uncertainties exist in the fertilizer application rates, the area of forested land receiving fertilizer, and the emission 

factors used to derive emission estimates. Uncertainty was calculated according to a modified IPCC Tier 1 

methodology. The 95 percent confidence interval of the IPCC default emission factor for synthetic fertilizer applied 

to soil, according to Chapter 4 of IPCC (2000), ranges from 0.25 to 6 percent. While a Tier 1 analysis should be 

generated from a symmetrical distribution of uncertainty around the emission factor, an asymmetrical distribution 

was imposed here to account for the fact that the emission used was not the mean of the range given by IPCC. 

Therefore, an upper bound of 480 percent and a lower bound of 80 percent were assigned to the emission factor. 

The higher uncertainty percentage is shown below, but the lower bound reflects a truncated distribution. The 

uncertainties in the area of forested land receiving fertilizer and fertilization rates were conservatively estimated to 

be ±54 percent (Binkley 2004). The results of the Tier 1 quantitative uncertainty analysis are summarized in Table 

7-10. N2O fluxes from soils were estimated to be between 0.01 and 2.3 Tg CO2 Eq. at a 95 percent confidence 

level. This indicates a range of 96 percent below and 483 percent above the 2003 emission estimate of 0.4 Tg CO2

Eq. 

Table 7-10: Tier 1 Quantitative Uncertainty Estimates of N2O Fluxes from Forest Soils (Tg CO2 Eq. and Percent) 

IPCC Source 

Category Gas 

Year 2003 

Emissions 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Emission Estimate 

(Tg CO2 Eq.) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 241 

 Lower Bound Upper Bound 

Forests Remaining 

Forests: N2O Fluxes 

from Soils N2O 0.4 96 to 483% 0.01 2.3 

Recalculations Discussion 

The current Inventory reports N2O emissions from soils in forested areas separately for the first time. In previous 

Inventories, N2O emissions from this source were implicitly included with N2O emissions from agricultural soils. 

The net effect of separating forest soils from agricultural soils for the current Inventory is to reduce emissions 

reported from the agricultural sector by a very small amount. However, because the methods for reporting that 

source have changed significantly this year, it is impossible to isolate the magnitude of change caused by this 

recalculation alone on the overall differences in N2O emissions from agricultural soils. The 2003 direct emission 

estimate for N2O from forest soils amounts to an offset of total forest carbon sequestration of approximately 0.5 

percent (including standing forests and wood products). 

7.2. Land Converted to Forest Land (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 lands are 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 quantify CO2 or N2O fluxes from land converted to forest land at this time. 

7.3. Croplands Remaining Croplands 

Changes in Agricultural Soil Carbon Stocks (IPCC Source Category 5B1) 

Soils contain both organic and inorganic forms of carbon (C) that contribute to the total soil carbon stock. It is the 

organic soil carbon (SOC) stocks in mineral and organic soils that may respond to management practices by 

producing or sequestering greenhouse gases. The IPCC methodology for estimating impacts of agricultural 

practices on soil C stocks (IPCC/UNEP/OECD/IEA 1997) is divided into three categories of land-use/landmanagement activities: 1) agricultural land-use and land-management activities on mineral soils; 2) agricultural 

land-use and land-management activities on organic soils; and 3) liming of soils. Nitrous oxide emissions from 

agricultural soils are presented within the Agriculture sector. 

Mineral soils contain comparatively low amounts of organic C, much of which is concentrated near the soil surface. 

Typical well-drained mineral surface soils contain from 1 to 6 percent organic C (by weight), although some 

mineral soils that experience long-term saturation during the year may contain significantly more C (NRCS 1999). 

Mineral subsoils contain even lower amounts of organic C (NRCS 1999, Brady and Weil 1999). When mineral 

soils undergo conversion from their native state to agricultural use, as much as half the SOC can be lost to the 

atmosphere. The rate and ultimate magnitude of C loss will depend on native vegetation, 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; after that, C stocks continue to decline but at a much slower 

rate. In temperate regions, C loss can continue for several decades. Eventually, the soil will reach a new 

equilibrium that reflects a balance between C accumulation from plant biomass and C loss through oxidation. Any 

changes in land-use or management practices that result in increased organic inputs or decreased oxidation of 

organic C (e.g., improved crop rotations, cover crops, application of organic amendments and manure, and 

reduction or elimination of tillage) will result in a net accumulation of SOC until a new equilibrium is achieved. 

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 is also typically 

extremely deep. Organic soils form under waterlogged conditions, in which minimal decomposition of plant 

residue occurs. When organic soils are cultivated, they are first drained which, together with tilling or mixing of the 

soil, aerates the soil, and thereby accelerates the rate of decomposition and CO2 generation. Because of the depth 

and richness of the organic layers, C loss from cultivated organic soils can continue over long periods of time. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 242 

When organic soils are disturbed for cultivation purposes, which invariably include drainage, the rate at which 

organic matter decomposes and CO2 emissions are generated, is determined primarily by climate, composition (i.e., 

decomposability) of the organic matter, and the specific land-use practices undertaken. The use of organic soils for 

annual crop production results in greater C loss than conversion to pasture or forests, due to deeper drainage and 

more intensive management practices (Armentano and Verhoeven 1990, as cited in IPCC/UNEP/OECD/IEA 1997). 

The last category of the IPCC methodology addresses emissions from lime (in the form of crushed limestone 

(CaCO3)) and dolomite (CaMg(CO3)2) additions to agricultural soils. Lime and dolomite are added by land 

managers to ameliorate acidification. When these compounds come in contact with acid soils, they degrade, thereby 

generating CO2. Complete degradation of applied limestone and dolomite could take several years, but it could also 

take significantly less time, depending on the soil conditions and the type of mineral applied. 

The estimates in this section include management impacts on mineral and organic soil C stocks for croplands and 

grasslands. Due to limited data availability, it is impossible to differentiate between the stock changes resulting 

from management practices on croplands or grasslands and those resulting from conversions between the two (i.e., 

statistics have been developed in a manner that only net changes in conversions are determined with no tracking of 

the total amount of land converted between cropland and grassland uses). 

Total SOC stock depends on the balance between inputs of organic material (e.g., decayed plant matter, roots, and 

organic amendments such as manure and crop residues) and loss of C through decomposition. 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. 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 soils. 

Of the three activities (those associated with mineral soils, organic soils, and liming of soils) land use and 

management of mineral soils was the most important component of total flux during the 1990 through 2003 period. 

C sequestration in mineral soils in 2003 was estimated to be approximately 51.7 Tg CO2 Eq. (14 Tg C), while 

emissions from organic soils were estimated to be 35.6 Tg CO2 Eq. (10 Tg C), and emissions from the practice of 

liming were estimated at 9.5 Tg CO2 Eq. (3 Tg C). Together, the three activities accounted for net sequestration of 

approximately 6.6 Tg CO2 Eq. (2 Tg C) in 2003. Total annual net CO2 flux was negative (i.e., net sequestration 

occurred) each year over the inventory period, although the net C storage in soils did decline by 18 percent between 

1990 and 2003. Net sequestration was largely due to annual cropland enrolled in the Conservation Reserve 

Program, cropland converted to permanent pastures and hay production, a reduction in the frequency of summerfallow use in semi-arid areas, and some increase in the adoption of conservation tillage (i.e., reduced and no till 

practices). The decline in net sequestration was attributed to two management practices. First, the amount of 

organic soils that were drained for agricultural production increased during the time period, thus leading to higher 

emissions. Second, manure production declined over the inventory period, particularly during the last few years, 

and this decline reduced the amount of crop and grazing lands receiving organic amendments, and thus there was a 

small decline in C sequestration attributed to this activity. 

The spatial variability in annual CO2 flux for mineral and organic soils is displayed in Figure 7-4 through Figure 

7-7. The highest rates of sequestration occur mostly in the southern and northern Great Plains, southern portions of 

the corn-belt in the Midwest, and the lower Mississippi River Valley. Sequestration rates are also relatively high in 

the southeastern United States. These regions either have high Conservation Reserve Program enrollment 

(particularly the Great Plains region), and/or have adopted conservation tillage at a higher rate than other regions of 

the country. The greatest organic soil emission rates are from drained peatlands 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. 

Table 7-11: Net CO2 Flux from Agricultural Soils (Tg CO2 Eq.)

Soil Type 1990 1997 1998 1999 2000 2001 2002 2003 

Mineral Soils (52.4) (51.7) (49.5) (48.9) (50.0) (51.6) (51.9) (51.7) 

Organic Soils 34.8 35.6 35.6 35.6 35.6 35.6 35.6 35.6 

Liming of Soils 9.5 8.7 9.6 9.1 8.8 9.0 10.1 9.5 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 243 

Total Net Flux (8.1) (7.4) (4.3) (4.3) (5.7) (7.1) (6.2) (6.6) 

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. Estimates include the change in C storage resulting from the

annual application of sewage sludge and the change in manure amendments or Conservation Reserve Program enrollment after 

1997. 

Table 7-12: Net Carbon Flux from Agricultural Soils (Tg C) 

Soil Type 1990 1997 1998 1999 2000 2001 2002 2003 

Mineral Soils (14.3) (14.1) (13.5) (13.3) (13.6) (14.1) (14.1) (14.1) 

Organic Soils 9.5 9.7 9.7 9.7 9.7 9.7 9.7 9.7 

Liming of Soils 2.6 2.4 2.6 2.5 2.4 2.4 2.8 2.6 

Total Net Flux (2.2) (2.0) (1.2) (1.2) (1.5) (1.9) (1.7) (1.8) 

Note: Parentheses indicate net sequestration. Shaded areas indicate values based on a combination of historical data and 

projections. All other values are based on historical data only. Estimates include the change in C storage resulting from the

annual application of sewage sludge and the change in manure amendments or Conservation Reserve Program enrollment after 

1997. 

 

Figure 7-4: Net Annual CO2 Flux, per Hectare, From Mineral Soils Under Agricultural Management, 1990-1992

Figure 7-5: Net Annual CO2 Flux, per Hectare, From Mineral Soils Under Agricultural Management, 1993-2003

Figure 7-6: Net Annual CO2 Flux, per Hectare, From Organic Soils Under Agricultural Management, 1990-1992

Figure 7-7: Net Annual CO2 Flux, per Hectare, From Organic Soils Under Agricultural Management, 1993-2003

The flux estimates presented here are restricted to CO2 fluxes associated with the use and management of 

agricultural soils. Agricultural soils are also important sources of other greenhouse gases, particularly N2O from 

application of fertilizers, manure, and crop residues and from cultivation of legumes, as well as methane (CH4) from 

flooded rice cultivation. These emissions are accounted for under the Agriculture sector.5 

Methodology 

The methodologies used to calculate net CO2 flux from use and management of mineral and organic soils and from 

liming follow the Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997, Ogle et al. 2002, Ogle et al. 

2003), except where noted below. Additional details on the methodology and data used to estimate flux from 

mineral and organic soils are described in Annex 3.13. 

Mineral and Organic Soils 

Mineral SOC stocks were estimated for 1982, 1992, and 1997, as well as CO2 emissions from organic soils, for the 

conterminous United States and Hawaii using U.S. data on climate, soil types, land use and land management 

activity data, reference C stocks (for agricultural soils rather than native soils) and field studies addressing 

management effects on SOC storage. National-scale data on land-use and management changes over time were 

obtained from the 1997 National Resources Inventory (USDA-NRCS 2000). The 1997 National Resources 

Inventory provides land use/management data and soils information for more than 400,000 locations in U.S. 

agricultural lands. Two other sources were used to supplement the land-use information from the 1997 National 

 

5 Nitrous oxide emissions from agricultural soils and methane emissions from rice fields are addressed under the Agricultural 

Soil Management and Rice Cultivation sections, respectively, of the Agriculture sector. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 244 

Resources Inventory. The Conservation Technology Information Center (CTIC 1998) provided data on tillage 

activity, with adjustments for long-term adoption of no-till agriculture (Towery 2001), and Euliss and Gleason 

(2002) provided activity data on wetland restoration of Conservation Reserve Program Lands. Manure N 

production was derived from USDA livestock population data (USDA 1994a,b; 1995a,b; 1998a,b; 1999a-e; 2000ag; 2001a-f; 2002a-f; 2003a-f), the FAOSTAT database (FAO 2003), and Lange (2000). Manure management 

information was obtained from Poe et al. (1999), Safley et al. (1992), and personal communications with 

agricultural experts (Anderson 2000, Deal 2000, Johnson 2000, Miller 2000, Milton 2000, Stettler 2000, Sweeten 

2000, Wright 2000). Livestock weight data were obtained from Safley (2000), USDA (1996, 1998c), and ASAE 

(1999); daily rates of N excretion from ASAE (1999) and USDA (1996); and information about the fraction of 

poultry litter used as a feed supplement from Carpenter (1992). 

For estimating the emissions from both mineral and organic soils, Major Land Resource Areas were used as the 

base spatial unit for mapping climate regions in the United States. Each Major Land Resource Area represents a 

geographic unit with relatively similar soils, climate, water resources, and land uses (NRCS 1981).6 Major Land 

Resource Areas were classified into climate zones according to the IPCC categories using the Parameter-Evaluation 

Regressions on Independent Slopes Model (PRISM) climate-mapping program of Daly et al. (1994). 

For mineral soils, 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 the 

Revised 1996 IPCC Guidelines (IPCC/UNEP/OECD/IEA 1997). 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 which are not considered cultivated 

cropland. U.S. management factors7 were derived from published literature to determine the impact of management 

practices on SOC storage, including changes in tillage, cropping rotations and intensification, land-use change 

between cultivated and uncultivated conditions, as well as C loss rates associated with drainage of organic soils 

under agricultural management (Ogle et al. 2003, Ogle et al. in review). U.S. management factors associated with 

organic matter amendments and improving grazing lands were not estimated because of few studies analyzing those 

impacts. Instead, IPCC factors from LULUCF Good Practice Guidance (IPCC 2003) formed the basis for 

quantifying the effect of those activities. Euliss and Gleason (2002) provided the data for computing the change in 

SOC storage resulting from restoration of Conservation Reserve Program lands (Olness et al. in press, Euliss et al. 

in prep). 

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 density functions for U.S.-

specific management factors, reference C stocks, and land-use activity data (Ogle et al. 2003, Ogle et al. 2002). The 

annual C flux for 1990 through 1992 was determined by calculating the annual change in stocks between 1982 and 

1992; annual C flux for 1993 through 2003 was determined by calculating the annual change in stocks between 

1992 and 1997. Annual C flux estimates for mineral soils between 1990 and 2003 were adjusted to account for 

additional C sequestration from sewage sludge applications, as well as gains or losses in C sequestration after 1997 

due to changes in Conservation Reserve Program enrollment and manure N production. For the entire inventory 

period, the amount of land amended with sewage sludge was estimated from N application data from the Soil 

Management section of the Agriculture chapter of this volume, and an assumed application rate derived from 

Kellogg et al. (2000). To estimate the impact of manure amendments after 1997, the change in manure N 

production was determined relative to the amount produced in 1997, and then similar to sewage sludge calculations, 

the production values were multiplied by the assumed application rate to determine the change in land area that was 

amended with manure. Carbon storage rate was estimated at 0.22 metric tons C per hectare per year for both the 

manure and sewage sludge amendments. To estimate the impact of enrollment in the Conservation Reserve 

 

6 The polygons displayed in Figure 6-5 through Figure 6-8 are the Major Land Resource Areas. 

7 Management factors have been derived from published literature to reflect changes in tillage, cropping rotations and 

intensification, land-use change between cultivated and uncultivated conditions, as well as drainage of organic soils. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 245 

Program after 1997, the change in enrollment acreage relative to 1997 was derived based on Barbarika (2004), and 

the differences in mineral soil areas were multiplied by 0.5 metric tons C per hectare per year. 

Annual C emission estimates from organic soils between 1990 and 2002 were derived using Revised 1996 IPCC 

Guidelines (IPCC/UNEP/OECD/IEA 1997), except that U.S.-specific C loss rates were used in the calculations 

rather than default IPCC rates (Ogle et al. 2003). Similar to 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 land 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 2003. 

Liming 

Carbon dioxide emissions from degradation of limestone and dolomite applied to agricultural soils were calculated 

by multiplying the annual amounts of limestone and dolomite applied (see Table 7-13) by CO2 emission factors 

(0.120 metric ton C/metric ton limestone, 0.130 metric ton C/metric ton dolomite) (IPCC 2003).8 These emission 

factors are based on the assumption that all of the C in these materials evolves as CO2 in the same year in which the 

minerals are applied. The annual application rates of limestone and dolomite were derived from estimates and 

industry statistics provided in the Minerals Yearbook and Mineral Industry Surveys (Tepordei 1993, 1994, 1995, 

1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004; USGS 2002, 2003, 2004). To develop these data, 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). 

To estimate the “unspecified” and “estimated” amounts of crushed limestone and dolomite applied to agricultural 

soils, the fractions of “unspecified” and “estimated” production that were applied to agricultural soils in a specific 

year were assumed to be equal to the fraction of “specified” production that was applied to agricultural soils in that 

same year. In addition, data were not available for 1990, 1992, and 2003 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 2003 data, the previous year’s 

fractions were applied to a 2003 estimate of total crushed stone presented in the USGS Mineral Industry Surveys: 

Crushed Stone and Sand and Gravel in the First Quarter of 2004 (USGS 2004). 

The primary source for limestone and dolomite activity data is the Minerals Yearbook, published by the Bureau of 

Mines through 1994 and by the U.S. Geological Survey from 1995 to the present. In 1994, the “Crushed Stone” 

chapter in Minerals Yearbook began rounding (to the nearest thousand) quantities for total crushed stone produced 

or used. It then reported revised (rounded) quantities for each of the years from 1990 to 1993. In order to minimize 

the inconsistencies in the activity data, these revised production numbers have been used in all of the subsequent 

calculations. 

Table 7-13: Quantities of Applied Minerals (Thousand Metric Tons) 

Mineral 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003

 

8 The default emission factor for dolomite provided in the Workbook volume of the Revised 1996 IPCC Guidelines

(IPCC/UNEP/OECD/IEA 1997) and the Good Practice Guidance for Land Use, Land-Use Change, and Forestry (IPCC 2003) is 

incorrect. The value provided is 0.122 metric ton carbon/metric ton of dolomite; the correct value is 0.130 metric ton 

carbon/metric ton of dolomite. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 246 

Limestone 19,012 20,312 17,984 15,609 16,686 17,297 17,479 16,539 14,882 16,894 15,863 16,097 20,449 19,163

D Dolomite 2,360 2,618 2,232 1,740 2,264 2,769 2,499 2,989 6,389 3,420 3,812 3,951 2,353 2,205

Uncertainty 

Uncertainties for mineral and organic soils were quantified using a Monte Carlo Approach by constructing 

probability distribution functions (PDF) for inputs to the IPCC equations, including management factors, C 

emission rates for organic soils, and land use and management activity data, and then simulating a range of values 

using the Monte Carlo framework (Ogle et al. 2003, Annex 3.13). Uncertainty estimates do not include sewage 

sludge impacts on SOC storage for any year in the inventory period, or contributions of changing manure 

management and enrollment in the Conservation Reserve Program after 1997. PDFs for management factors were 

derived from a synthesis of 91 published studies, which addressed the impact of management on SOC storage. 

Uncertainties in land use and management activity data were also derived from a statistical analysis. The National 

Resources Inventory (NRI) has a two-stage sampling design that allowed PDFs to be constructed assuming a 

multivariate normal distribution accounting for dependencies in activity data. PDFs for the tillage activity data, as 

provided by the Conservation Technology and Information Center, were constructed on a bivariate normal 

distribution with a log-ratio scale, accounting for the negative dependence among the proportions of land under 

conventional and conservation tillage practices. PDFs for the crop and grazing land area receiving manure 

amendments were based on the data sources given for manure amendments in the methodology section, and a 

statistical relationship between production and the amount of land area that is amended according to manure 

management information derived from the USDA Census of Agriculture (Edmonds et al. 2003). Lastly, enrollment 

in wetland restoration programs was estimated from contract agreements, but due to a lack of information, PDFs 

were constructed assuming a nominal ±50 percent uncertainty range. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 7-14. CO2 flux from mineral and 

organic agricultural soil carbon stocks in 2003 was estimated to be between -40.0 and +5.9 Tg CO2 Eq. at a 95 

percent confidence level (or 19 of 20 Monte Carlo Stochastic Simulations). This indicates a range of 148 percent 

below to 136 percent above the 2003 flux estimate of -16.1 Tg CO2 Eq. 

Table 7-14: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Mineral and Organic Agricultural Soil 

Carbon Stocks (Tg CO2 Eq. and Percent) 

Uncertainty Range Relative to Flux Estimatea

(Tg CO2 Eq.) (%) 

Source Gas 

2003 Flux 

Estimate 

(Tg CO2 Eq.) 

Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Mineral and Organic 

Soil Uncertainty CO2 (16.1) (40.0) 5.9 -148% +136% a

 Includes mineral and organic soils only; estimates do not include the change in C storage resulting from the annual application 

of sewage sludge, or the change in manure amendments or Conservation Reserve Program enrollment after 1997. b Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

The time-series calculations were consistent for each reporting year of the inventory in terms of methodology, with 

the only difference in reported values stemming from the changes in land use and management activities across U.S. 

agricultural lands. In addition, the same management factors (i.e., emission factors) were used each year for 

calculating the impact of land use and management on SOC stocks. There is no evidence that changing 

management practices has a quantitatively different impact on SOC stocks over the inventory period. For example, 

changing from conventional to no-till management in 1990 is assumed to have the same impact on soil C stocks 

over the course of this first year as it is over the course of each year in the 20 year period following the management 

change. 

Although the mineral and organic soil estimates have been improved during the last two years using a Monte Carlo 

approach with the incorporation of U.S.-specific reference C stocks management factor values, and a more 

comprehensive accounting of manure amendment impacts on SOC storage, several limitations remain in the 

analysis. First, minimal data exist on where and how much sewage sludge has been applied to U.S. agricultural 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 247 

lands and the accounting of this activity appears to be much more difficult than the related-activity of using manure 

to amend agricultural soils. Consequently, uncertainties have not been estimated for the change in SOC storage 

resulting from sludge applications. Second, due to the IPCC requirement that inventories include all land areas that 

are potentially subject to land-use change, the 1997 National Resources Inventory dataset includes some points 

designated as non-agricultural land-uses if the points were once categorized as agricultural land use and their 

designation changed during the period from 1992 to 1997. The non-agricultural land uses are urban, water, and 

miscellaneous non-cropland (e.g., roads and barren areas). The impact on SOC storage that results from converting 

cropland to non-agricultural uses is not well-understood, and therefore, those points were not included in the 

calculations for mineral soils (emissions from organic soils, however, were computed for those points in the years 

that they were designated as an agricultural use). Third, the current estimates may underestimate losses of C from 

organic soils because the 1997 National Resources Inventory was not designed as a soil survey and organic soils 

frequently occur as relatively small inclusions within major soil types. Lastly, this methodology does not take into 

account changes in SOC stocks due to pre-1982 land use and land-use change. 

Uncertainties in the estimates of emissions from liming result from both the methodology and the activity data. The 

IPCC method assumes that all inorganic C in the applied minerals evolves to CO2, and that this degradation occurs 

in the same year that the minerals are applied. However, recent research has shown that liming can either be a C 

source or a sink, depending upon weathering reactions, which are pH dependent (Hamilton et al. 2002). Moreover, 

it can take several years for agriculturally applied limestone and dolomite to degrade completely. However, 

application rates are fairly constant over the entire time series, so this latter assumption may not contribute 

significantly to overall uncertainty.

There are several sources of uncertainty in the limestone and dolomite activity data. When reporting data to the 

USGS (or U.S. Bureau of Mines), some producers do not distinguish between limestone and dolomite. In these 

cases, data are reported as limestone, so this reporting could lead to an overestimation of limestone and an 

underestimation of dolomite. In addition, the total quantity of crushed stone listed each year in the Minerals 

Yearbook excludes American Samoa, Guam, Puerto Rico, and the U.S. Virgin Islands. 

Uncertainty regarding lime applications was estimated at 15 percent (Tepordei 2003). While IPCC provides no 

uncertainty values for the emission factor from this subsource, there is uncertainty stemming primarily from the 

inherent assumption that all applications are converted to CO2 within the year of application. In fact, limestone may 

persist in the soil for 3 to 4 years following application (Nardozzi 2004), indicating that emissions may continue 

throughout that period of time. Taking this into account, the resulting uncertainty in the emission factor is estimated 

to be 75 percent. The preliminary results of the Tier 1 quantitative uncertainty analysis are summarized in Table 

7-15. Liming of agricultural soils CO2 emissions in 2003 were estimated to be between 2.2 and 10.9 Tg CO2 Eq. at 

the 95 percent confidence level. This indicates a range of 76 percent above and below the 2003 emission estimate 

of 9.5 Tg CO2 Eq. 

Table 7-15: Tier 1 Quantitative Uncertainty Estimates for CO2 Emissions from Liming of Agricultural Soils (Tg 

CO2 Eq. and Percent) 

IPCC Source 

Category Gas 

Year 2003 

Emissions 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Emission Estimate 

(Tg CO2 Eq.) 

 

Lower Bound 

Upper 

Bound*

Liming of 

Agricultural Soils CO2 9.5 76% 2.2 10.9 

* Because the current Inventory methodology assumes that all of the limestone and dolomite decomposes in the year of 

application, the emission factors could be significantly lower, but could not be higher. Consequently, the emissions estimate 

may only be higher due to the uncertainty in the application data. Therefore, while normally, Tier 1 analyses generate a 

symmetrical distribution of uncertainty around the emission estimate, an asymmetrical distribution was necessarily imposed here. 

It is not currently possible to combine the results of this Tier 1 uncertainty analysis with those of the Tier 2 

uncertainty analyses for CO2 fluxes from mineral soils and histosols. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 248 

Recalculations Discussion 

The estimates of changes in agricultural SOC stocks have been modified in several ways. First, uncertainty in 

manure amendments was evaluated and incorporated into the general inventory calculations for agricultural soil C, 

using the Tier 2 IPCC methodology (IPCC/UNEP/OECD/IEA 1997). In previous Inventories, the change in SOC 

storage attributed to organic amendments was estimated as a post-analysis calculation using a simplistic activitybased approach. Incorporating amendment data into the IPCC analysis led to a decline in the estimated change for 

SOC storage attributed to manure management, relative to previous years. The activity-based approach assumed 

that all of the applied manure was leading to an increase in SOC, while the IPCC method only accounted for the 

changes in organic amendments from the baseline year to estimate the impact on SOC storage. In general, the IPCC 

approach assumes that impacts on SOC storage are manifested in the first 20 years following a management change, 

and consequently, this method does not account for smaller residual changes in SOC storage that can occur in later 

years. However, the activity-based approach also has limitations because it does not account for the possibility that 

a portion of the manure applied in a year is simply maintaining SOC storage from past applications. In fact, the 

production of manure has not changed to a large extent since 1982 according to USDA statistics (see Annex 3.13), 

and, therefore, many of the current amendments are only serving the purpose of maintaining past storage. Hence, 

incorporating manure management into the IPCC calculations has produced a more conservative estimate by 

reducing the potential for over-estimating the impact of longer-term manure amendments, but also does not capture 

the residual change in SOC storage occurring after the first 20 years. 

Two additional revisions have been incorporated into the analysis since the previous Inventory. As part of the 

revision, management factors provided in the IPCC LULUCF Good Practice Guidance (IPCC 2003) now form the 

basis to estimate management impacts for which U.S.-specific factors have not been derived (i.e., improving grazing 

lands and organic amendments). Previously, defaults factor values from the Revised 1996 IPCC Guidelines formed 

the basis for these calculations using the IPCC method, but greater disaggregation of management factors by climate 

in the IPCC LULUCF Good Practice Guidance was assumed to produce more realistic estimates of those 

management impacts, compared to the single global values provided in the Revised 1996 IPCC Guidelines. 

Lastly, emissions from organic soils have changed slightly from those reported in the previous inventory, as a result 

of revising the land area included in the emission calculation. In previous years, lands that were converted between 

agricultural and non-agricultural uses (i.e., urban, water, and miscellaneous non-cropland) were not included in the 

estimation of emissions from organic soils, and this led to a slight under-estimation of emissions from organic soils. 

Specifically, a small amount of land area under agricultural management in 1992 or 1997 was not included in the 

emission calculation because it had been converted from a non-agricultural use. 

Estimates of CO2 emissions from agricultural soil management have been revised due to methodological and 

historical data changes in the calculations of N from livestock that is applied to soils. These changes include 

corrections to: the typical animal mass value for beef cows and calves; the accounting of sheep in New England 

states; state broiler populations; and updated NASS animal population estimates for the years 1998 through 2001 

(NASS 2000). Additionally, the factor for converting short tons to metric tons was revised to include another 

significant digit, and the percent residue applied for rice in the year 2001 was corrected. In combination, these 

changes resulted in a minor effect on the agricultural soil C estimates with a reduction in the CO2 sink by less than 1 

percent. 

The quantity of applied minerals reported in the previous inventory for 2002 has been revised. Consequently, the 

reported emissions resulting from liming in 2002 have also changed. In the previous inventory, to estimate 2002 

data, the previous year’s fractions were applied to a 2002 estimate of total crushed stone presented in the USGS 

Mineral Industry Surveys: Crushed Stone and Sand and Gravel in the First Quarter of 2003 (USGS 2003). 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 2002. These values have replaced those used in the previous inventory 

to calculate the quantity of minerals applied to soil and the emissions from liming. 

Overall, these changes resulted in an average annual decrease of 14.2 Tg CO2 Eq. (67 percent) in agricultural soil 

carbon stocks for the period 1990 through 2002. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 249 

Planned Improvements 

A major planned improvement is currently underway that will enhance reporting of changes in agricultural soil C 

stocks and deals with an alternative inventory approach to better represent between-year variability in annual fluxes. 

This new annual activity-based inventory will use the Century ecosystem simulation model, which relies on actual 

climate, soil, and land use/management databases to estimate variation in fluxes. This inventory will provide a more 

robust accounting of C stock changes in U.S. agricultural lands than the more simplistic IPCC soil C accounting 

approach. This approach is likely to be used in the future for reporting of land use and management impacts on 

agricultural soil C stocks, and therefore a short description of this method compared to the IPCC approach is 

provided. 

The Century ecosystem model has been widely tested and found to be successful in simulating those processes 

affecting SOC storage (Metherell et al. 1993, Parton et al. 1994). Simulation modeling differs from the IPCC 

approach in that annual changes are computed dynamically as a function of inputs of C and N to soil (e.g., crop 

residues, manure) and C emissions from organic matter decomposition, which are governed by climate and soil 

factors as well as management practices. The model distinguishes between all major field crops (maize, wheat and 

other small grains, soybean, sorghum, cotton) as well as hay and pasture (grass, alfalfa, clover). Management 

variables include tillage, fertilization, irrigation, drainage, and manure addition. 

Input data are largely derived from the same sources as the IPCC-based method (i.e., climate variables come from 

the Parameter-Evaluation Regressions on Independent Slopes Models (PRISM) database; crop rotation, irrigation 

and soil characteristics from the National Resources Inventory (NRI); and tillage data from the Conservation 

Technology Information Center (CTIC)). In addition, the Century analysis uses detailed information on crop 

rotation-specific fertilization and tillage implements obtained from USDA’s Economic Research Service. The main 

difference between the methods is that the climate, soil, and management data serve as driving variables in the 

Century simulation, whereas in the IPCC approach these data are more highly aggregated and are used for 

classification purposes. In the Century-based analysis, land areas having less than 5 percent of total area in crop 

production are excluded and several less-dominant crops (e.g., vegetables, sugar beets and sugar cane, potatoes, 

tobacco, orchards, and vineyards), for which the model has not yet been parameterized, are not included. Thus, the 

total area included in the Century analysis (149 million hectares) will be smaller than the corresponding area of 

cropland (165 million hectares) included in the IPCC estimates. 

Preliminary results using the Century model suggest (as with the IPCC model) that U.S. cropland mineral soils 

(excluding organic soils) are currently acting as a C sink. The Century model estimates that U.S. cropland soils 

sequestered an average of approximately 77 Tg CO2 Eq. annually (21 Tg C/year) for 1992 through 1997. Organic 

soils (which contribute large C losses) have not yet been simulated by Century. 

As with the IPCC method, increases in mineral SOC stocks in the Century analysis are associated with reduced 

tillage, Conservation Reserve Program lands, reduced bare fallow and some increase in hay area. However, the 

Century analysis also includes the effect of a long-term trend in increasing residue inputs due to higher productivity 

on cropland in general, contributing to increasing SOC stocks. Work is underway to refine model input data and to 

estimate uncertainty for the dynamic model approach. 

Potential advantages of a dynamic simulation-based approach include the ability to use actual observed weather, 

observed annual crop yields, and more detailed soils and management information to drive the estimates of soil C 

change. This would facilitate annual estimates of SOC stock changes and CO2 emissions from soils that would 

better reflect interannual variability in cropland production and weather influences on C cycle processes. 

7.4. Lands Converted to Croplands (Source Category 5B2) 

Land-use change is constantly occurring, and areas under a number of differing land use types are converted to 

croplands each year, just as croplands are converted to other uses. However, while the C stocks for Land Converted 

to Cropland are included in the Croplands Remaining Croplands section, it is not possible to sub-divide it and 

determine the magnitude of the change at this time. Additionally, given the lack of available land use information 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 250 

relevant to this particular IPCC source category, it is not possible to quantify the N2O flux from Lands Converted to 

Cropland at this time. 

7.5. Settlements Remaining Settlements 

Changes in Yard Trimming and Food Scrap Carbon Stocks in Landfills (IPCC Source 

Category 5E1) 

As is the case with carbon in landfilled forest products, carbon contained in landfilled yard trimmings and food 

scraps can be stored for very long periods. In the United States, yard trimmings (i.e., grass clippings, leaves, and 

branches) and food scraps comprise a significant portion of the municipal waste stream, and a large fraction of the 

collected yard trimmings and food scraps are discarded in landfills. However, both the amount of yard trimmings 

and food scraps collected annually and the fraction that is landfilled have declined over the last decade. In 1990, 

nearly 51 million metric tons (wet weight) of yard trimmings and food scraps were generated (i.e., put at the curb 

for collection or taken to disposal or composting facilities) (EPA 2003). Since then, programs banning or 

discouraging disposal have led to an increase in backyard composting and the use of mulching mowers, and a 

consequent 20 percent decrease in the amount of yard trimmings collected. 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 34 percent in 2003. There is considerably less centralized 

composting of food scraps; generation has grown by 26 percent since 1990, though the proportion of food scraps 

discarded in landfills has decreased slightly from 81 percent in 1990 to 77 percent in 2003. Overall, there has been 

a decrease in the yard trimmings and food scrap landfill disposal rate, which has resulted in a decrease in the rate of 

landfill carbon storage to 10.1 Tg CO2 Eq. in 2003 from 26.0 Tg CO2 Eq. in 1990 (Table 7-16 and Table 7-17). 

Table 7-16: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg CO2 Eq.) 

Carbon Pool 1990 1996 1997 1998 1999 2000 2001 2002 2003

Yard Trimmings (23.2) (11.3) (10.4) (9.6) (8.5) (7.2) (7.4) (7.5) (7.5)

Grass (2.5) (1.0) (0.9) (0.8) (0.7) (0.6) (0.7) (0.7) (0.7)

Leaves (11.2) (5.9) (5.4) (5.1) (4.5) (4.0) (4.0) (4.0) (4.0)

Branches (9.6) (4.4) (4.0) (3.7) (3.2) (2.6) (2.7) (2.7) (2.8)

Food Scraps (2.8) (2.2) (2.6) (2.9) (2.9) (3.0) (2.9) (2.7) (2.6)

Total Net Flux (26.0) (13.5) (12.9) (12.5) (11.4) (10.2) (10.3) (10.2) (10.1)

Note: Totals may not sum due to independent rounding. 

Table 7-17: Net Changes in Yard Trimming and Food Scrap Stocks in Landfills (Tg C) 

Carbon Pool 1990 1996 1997 1998 1999 2000 2001 2002 2003

Yard Trimmings (6.3) (3.1) (2.8) (2.6) (2.3) (2.0) (2.0) (2.0) (2.0)

Grass (0.7) (0.3) (0.2) (0.2) (0.2) (0.2) (0.2) (0.2) (0.2)

Leaves (3.0) (1.6) (1.5) (1.4) (1.2) (1.1) (1.1) (1.1) (1.1)

Branches (2.6) (1.2) (1.1) (1.0) (0.9) (0.7) (0.7) (0.7) (0.8)

Food Scraps (0.8) (0.6) (0.7) (0.8) (0.8) (0.8) (0.8) (0.7) (0.7)

Total Net Flux (7.1) (3.7) (3.5) (3.4) (3.1) (2.8) (2.8) (2.8) (2.7)

Note: Totals may not sum due to independent rounding. 

Methodology 

Estimates of net carbon flux resulting from landfilled yard trimmings and food scraps were developed by estimating 

the change in landfilled carbon stocks between inventory years. Carbon stock estimates were calculated by 

determining the mass of landfilled carbon resulting from yard trimmings or food scraps discarded in a given year; 

adding the accumulated landfilled carbon from previous years; and subtracting the portion of carbon landfilled in 

previous years that decomposed. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 251 

To determine the total landfilled carbon 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 carbon storage 

factor of the landfilled yard trimmings and food scraps, and 4) the rate of decomposition of the degradable carbon. 

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 carbon 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 in the United States: 2001 Facts and Figures (EPA 2003). That report provides data 

for 1960, 1970, 1980, 1990, 1995, and 1999 through 2001. To provide data for some of the missing years in the 

1990 through 1999 period, two earlier reports were used (Characterization of Municipal Solid Waste in the United 

States: 1998 Update (EPA 1999), and Municipal Solid Waste in the United States: 2000 Facts and Figures (EPA 

2002)). Remaining years in the time series for which data were not provided were estimated using linear 

interpolation, except for 2002 and 2003, which was assumed to have the same discards as 2001. These reports do 

not subdivide discards of individual materials into volumes landfilled and combusted, although they provide an 

estimate of the proportion of overall wastestream discards managed in landfills and combustors (i.e., ranging from 

81 percent and 19 percent respectively in 1990, to 79 percent and 21 percent in 2001). 

The amount of carbon 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) carbon 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 carbon contents were determined by Barlaz (1998) (Table 7-18). 

The amount of carbon remaining in the landfill for each subsequent year was tracked based on a simple model of 

carbon fate. According to Barlaz (1998), a portion of the initial carbon resists decomposition and is essentially 

persistent in the landfill environment; the modeling approach applied here builds on his findings. Barlaz (1998) 

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 carbon 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 carbon content. The mass of carbon remaining, divided by the original dry weight of the 

material, was reported as the carbon storage factor (Table 7-18). 

For purposes of simulating U.S. landfill carbon flows, the carbon storage factors are divided by the initial carbon 

content to determine the proportion of initial carbon that does not decompose. The remaining portion is assumed to 

degrade (and results in emissions of CH4 and CO2). For example, for branches Barlaz (1998) reported the carbon 

storage factor as 38 percent (of dry weight), and the initial carbon content as 49 percent (of dry weight). Thus, the 

proportion of initial carbon that does not decompose is 77 percent (i.e., 0.38/0.49). The remaining 23 percent 

degrades. 

The degradable portion of the carbon is assumed to decay according to first order kinetics. Grass and food scraps 

are assumed to have a half-life of 5 years; leaves and branches are assumed to have a half-life of 20 years. 

For each of the four materials (grass, leaves, branches, food scraps), the stock of carbon in landfills for any given 

year is calculated according to the following formula: 

 LFC i,t = Σ Wi,n * (1 - MCi) * ICCi * {[CSFi / ICCi] + [(1 – (CSFi / ICCi )) * e-k*(t - n) ]} 

 n 

where, 

t = the year for which carbon stocks are being estimated, 

LFC i,t = the stock of carbon in landfills in year t, for waste i (grass, leaves, branches, food scraps) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 252 

Wi,n = the mass of waste i disposed in landfills in year n, in units of wet weight 

n = the year in which the waste was disposed, where 1960 < n < t 

MCi = moisture content of waste i, 

ICCi = the initial carbon content of waste i, 

CSFi = the carbon storage factor of waste i, 

e = the natural logarithm, and 

k = the first order rate constant for waste i, and is equal to 0.693 divided by the half-life for 

decomposition. 

For a given year t, the total stock of carbon in landfills (TLFCt) is the sum of stocks across all four materials. The 

annual flux of carbon in landfills (Ft) for year t is calculated as the change in stock compared to the preceding year: 

 Ft = TLFCt - TLFCt - 1 

Thus, the carbon placed in a landfill in year n is tracked for each year t through the end of the inventory period 

(2003). For example, disposal of food scraps in 1960 resulted in depositing about 1,140,000 metric tons of carbon. 

Of this amount, 16 percent (180,000 metric tons) is persistent; the remaining 84 percent (960,000 metric tons) is 

degradable. By 1965, half of the degradable portion (480,000 metric tons) decomposes, leaving a total of 660,000 

tonnes (the persistent portion, plus the remaining half of the degradable portion). 

Continuing the example, by 2003, the total food scraps carbon originally disposed in 1960 had declined to 181,000 

metric tons (i.e., virtually all of the degradable carbon had decomposed). By summing the carbon remaining from 

1960 with the carbon remaining from food scraps disposed in subsequent years (1961 through 2003), the total 

landfill carbon from food scraps in 2003 was 29.3 million metric tons. This value is then added to the carbon stock 

from grass, leaves, and branches to calculate the total landfill carbon stock in 2003, yielding a value of 241.6 

million metric tons (as shown in Table 7-19). In exactly the same way total net flux is calculated for forest carbon 

and harvested wood products, the total net flux of landfill carbon for yard trimmings and food scraps for a given 

year (Table 7-17) is the difference in the landfill carbon stock for a given year and the stock in the preceding year. 

For example, the net change in 2003 shown in Table 7-17 (2.7 Tg C) is equal to the stock in 2003 (241.6 Tg C) 

minus the stock in 2002 (238.9 Tg C). 

When applying the carbon storage factor data reported by Barlaz (1998), an adjustment was made to the reported 

value for leaves, because the carbon storage factor was higher than the initial carbon content. This anomalous 

result, probably due to errors in the laboratory measurements, was addressed by applying a mass balance 

calculation, and assuming that (a) the initial carbon content was correctly measured, and (b) the carbon storage 

factor was incorrect. The same experiment measured not only the persistence of carbon (i.e., the carbon storage 

factor), but also the yield of methane for each of the individual waste materials (Eleazer et al. 1997). The anaerobic 

decomposition process results in release of equal molar volumes of CH4 and CO2. Thus, to derive a more realistic 

estimate of the carbon storage factor for leaves, the carbon released in the form of methane during decomposition 

was multiplied by two (to include the loss of carbon through CO2, which is generated in approximately equal molar 

amounts as CH4), and then subtracted from the initial carbon content of the leaves. This estimate of carbon 

remaining was used to derive the carbon storage factor (0.46). 

Table 7-18: Moisture Content (%), Carbon Storage Factor, Initial Carbon Content (%), Proportion of Initial Carbon 

Sequestered (%), and Half-Life (years) for Landfilled Yard Trimmings and Food Scraps in Landfills 

 Yard Trimmings Food Scraps 

Variable Grass Leaves Branches 

Moisture Content (% H2O) 70 30 10 70 

CSF (kg C sequestered / dry kg waste) 0.32 0.46a 0.38 0.08 

Initial Carbon Content (%) 45 49 49 51 

Proportion of initial carbon sequestered (%) 71 94 77 16 

Half-life (years) 5 20 20 5 

a Adjusted using CH4 yields in Eleazer et al. (1997). 

Table 7-19: Carbon Stocks in Yard Trimmings and Food Scraps in Landfills (Tg C) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 253 

Carbon Pool 1990 1996 1997 1998 1999 2000 2001 2002 2003

Yard Trimmings 167.8 196.5 199.3 201.9 204.2 206.2 208.2 210.2 212.3

Grass 18.8 21.7 21.9 22.2 22.4 22.5 22.7 22.9 23.1

Leaves 78.7 92.9 94.3 95.7 97.0 98.0 99.1 100.2 101.3

Branches 70.3 81.9 83.0 84.0 84.9 85.6 86.4 87.1 87.9

Food Scraps 20.3 24.0 24.7 25.5 26.3 27.1 27.9 28.6 29.3

Total Carbon 

Stocks 188.1 220.5 224.0 227.4 230.5 233.3 236.1 238.9 241.6

Note: Totals may not sum due to independent rounding. 

Uncertainty 

The estimation of carbon storage in landfills is directly related to the following yard trimming and food scrap data 

and factors: disposal in landfills per year (tons of carbon), initial carbon content, moisture content, decomposition 

rate (half-life), and proportion of carbon stored. The carbon 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 uncertainties associated with each of these factors. 

The uncertainty ranges were assigned based on expert judgment and are assumed to be uniformly distributed around 

the inventory estimate (e.g., +10 percent), except for the values for decomposition rate, proportion of carbon stored, 

and moisture content for branches. 

The uncertainty ranges associated with the input variables for the proportion of grass and leaves in yard trimmings, 

as well as the initial carbon content and moisture content for grass, leaves, and food scraps (all expressed as 

percentages in the calculations for the inventory) were plus or minus 10 percent. For the moisture content of 

branches (where the inventory estimate is 10 percent), the uncertainty range was assumed to be 5 to 30 percent. 

The uncertainty ranges associated with the disposal of grass, leaves, branches, and food scraps were bound at 50 

percent to 150 percent times the inventory estimates. The half-life of grass and food scraps were assumed to range 

from 1 to 20 years, and the half-lives of leaves and branches were assumed to range from 5 to 30 years. Finally, the 

proportion of carbon stored in grass, leaves, branches, and food scraps was assumed to vary by the addition of 20 

percent or subtraction of 10 percent from the best estimate, with an upper bound of 100 percent and a lower bound 

of 0 percent. 

A Monte Carlo (Tier 2) uncertainty analysis was then applied to estimate the overall uncertainty of the sequestration 

estimate. The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 7-20. Total yard 

trimmings and food scraps CO2 flux in 2003 was estimated to be between -17.5 and -7.0 Tg CO2 Eq. at a 95 percent 

confidence level (or 19 of 20 Monte Carlo Stochastic Simulations). This indicates a range of 73 percent below to 31 

percent above the 2003 flux estimate of -10.1 Tg CO2 Eq. 

Table 7-20: Tier 2 Quantitative Uncertainty Estimates for CO2 Flux from Yard Trimmings and Food Scraps in 

Landfills (Tg CO2 Eq. and Percent) 

Source Gas 

2003 Flux 

Estimate Uncertainty Range Relative to Flux Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 

Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Yard Trimmings CO2 (7.5) (11.3) (4.4) -51% +41% 

Food Scraps CO2 (2.6) (8.9) (1.2) -246% +54% 

Total CO2 (10.1) (17.5) (7.0) -73% +31% a

Range of flux estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Negative values indicate net carbon storage (positive values denote emissions).

The uncertainty of the landfilled carbon storage estimate arises from the disposal data and the factors applied to the 

following data. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 254 

Disposal per Year (tons of carbon) 

A source of uncertainty affecting CO2 sequestration is the estimate of the tonnage of yard trimmings and food scraps 

which are disposed of in landfills each year. Of all the individual inputs tested for sensitivity in the uncertainty 

analysis, net carbon storage in landfills is most sensitive to the estimate of the food scrap disposal rate. The 

estimates for yard trimming and food scrap disposal in landfills are determined using data from the EPA (1999, 

2002, 2003) estimates of materials generated, discarded, and combusted, which carry considerable uncertainty 

associated with the wastestream sampling methodology used to generate them. 

Moisture Content and Initial Carbon Content 

Moisture content, and to a lesser extent carbon content, vary widely. Moisture content for a given sample of waste 

can be affected by the precipitation conditions when the waste is placed at the curb for collection, as well as the 

status and condition of the landfill cover. Carbon content (on a dry weight basis) is a function of the specific waste 

constituents (e.g., oak leaves versus pine needles or banana peels versus bacon grease), which in turn vary 

temporally, geographically, and demographically (i.e., characteristics of households in the wasteshed). 

Decomposition Rate 

Although several investigators have made estimates of the decomposition rate of mixed solid waste in a landfill 

environment, there are no known studies of decomposition rates for individual materials in actual landfills, and thus 

the inventory estimate is based on assumed values. The uncertainty analysis indicates that the results are sensitive 

to decomposition rates, especially the food scraps half-life, and thus the decomposition rates introduce considerable 

uncertainty into the analysis. 

Proportion of Carbon Stored 

The estimate of the proportion of carbon stored is based on a set of experiments measuring the amount of carbon 

persisting in conditions promoting decomposition. Because these experiments have only used conditions conducive 

to decomposition, they are more likely to underestimate than to overestimate carbon storage. Thus, the uncertainty 

analysis used asymmetrical values (up to 10 percent less storage, up to 20 percent more storage) as inputs. 

Several of the planned improvements to the analysis, described later in this section, are intended to reduce the 

uncertainty associated with these factors. 

Recalculations Discussion 

While conducting quality control procedures, it was found that the 1994 value for the amount of discards that are 

landfilled (or disposed of by means other than combustion) had been incorrectly recorded. The error was corrected, 

resulting in reduced carbon storage values by less than 1.0 Tg C for the years 1994 through 2003. Overall, this 

change, in combination with historical data revisions, resulted in an average annual decrease of 0.2 Tg CO2 Eq. (0.7 

percent) in carbon sequestration from yard trimming and food scraps over the period 1990 through 2002. 

Planned Improvements 

As noted above, the estimates presented in this section are driven by a small carbon storage factor data set, and 

some of these measurements (especially for leaves) deserve close scrutiny. There are ongoing efforts to conduct a 

re-analysis of the leaves experiment, using the same techniques as in the original experiments cited, and future work 

may evaluate the potential contribution of inorganic carbon to landfill sequestration and to assure consistency 

between the estimates of carbon storage described in this chapter and the estimates of landfill CH4 emissions 

described in the Waste chapter. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 255 

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), which cover 3.5 percent of the continental United States, are estimated to contain about 

3.8 billion trees. With an average tree canopy cover of 27.1 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 

continental United States were estimated by Nowak and Crane (2002) to account for an average annual net 

sequestration of 58.7 Tg CO2 Eq. (16 Tg C). These data were collected throughout the 1990s, and have been 

applied to the entire time series in this report (see Table 7-21). Annual estimates of CO2 flux have not been 

developed, but are believed to be relatively constant from 1990 through 2003. 

Net carbon 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 carbon 

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 carbon 

density than forested areas (Nowak and Crane 2002). 

Table 7-21: Net C Flux from Urban Trees (Tg CO2 Eq. and Tg C) 

Year Tg CO2 Eq. Tg C 

1990 (58.7) (16) 

1997 (58.7) (16) 

1998 (58.7) (16) 

1999 (58.7) (16) 

2000 (58.7) (16) 

2001 (58.7) (16) 

2002 (58.7) (16) 

2003 (58.7) (16) 

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 

consistent with the default IPCC methodology in the IPCC LULUCF Good Practice Guidance (IPCC 2003), 

although sufficient data are not yet available to determine interannual changes in carbon stocks in the living biomass 

of urban trees. 

Nowak and Crane (2002) developed estimates of annual gross carbon sequestration from tree growth and annual 

gross carbon emissions from decomposition for ten U.S. cities: Atlanta, GA; Baltimore, MD; Boston, MA; Chicago, 

IL; Jersey City, NJ; New York, NY; Oakland, CA; Philadelphia, PA; Sacramento, CA; and Syracuse, NY. The 

gross carbon sequestration estimates were derived from field data that were collected in these ten cities during the 

period from 1989 through 1999, including tree measurements of stem diameter, tree height, crown height, and 

crown width, and information on location, species, and canopy condition. The field data were converted to annual 

gross carbon sequestration rates for each species (or genus), diameter class, and land-use condition (forested, parklike, and open growth) by applying allometric equations, a root-to-shoot ratio, moisture contents, a carbon content 

of 50 percent (dry weight basis), an adjustment factor to account for smaller aboveground biomass volumes (given a 

particular diameter) in urban conditions compared to forests, an adjustment factor to account for tree condition (fair 

to excellent, poor, critical, dying, or dead), and annual diameter and height growth rates. The annual gross carbon 

sequestration rates for each species (or genus), diameter class, and land-use condition were then scaled up to city 

estimates using tree population information. The field data from the 10 cities, some of which are unpublished, are 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 256 

described in Nowak and Crane (2002) and references cited therein. The allometric equations were taken from the 

scientific literature (see Nowak 1994, Nowak et al. 2002), and the adjustments to account for smaller volumes in 

urban conditions were based on information in Nowak (1994). A root-to-shoot ratio of 0.26 was taken from Cairns 

et al. (1997), and species- or genus-specific moisture contents were taken from various literature sources (see 

Nowak 1994). Adjustment factors to account for tree condition were based on percent crown dieback (Nowak and 

Crane 2002). Tree growth rates were also 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. 

Annual gross carbon emission estimates were derived by applying estimates of annual mortality and condition, and 

assumptions about whether dead trees were removed from the site, to carbon stock estimates. These values were 

derived as intermediate steps in the sequestration calculations, and different decomposition rates were applied to 

dead trees left standing compared with those removed from the site. The annual gross carbon emission rates for 

each species (or genus), diameter class, and condition class were then scaled up to city estimates using tree 

population information. Estimates of annual mortality rates by diameter class and condition class were derived from 

a study of street-tree mortality (Nowak 1986). Assumptions about whether dead trees would be removed from the 

site were based on expert judgment of the authors. Decomposition rates were based on literature estimates (Nowak 

and Crane 2002). 

Annual net carbon sequestration estimates were derived for seven of the ten cities by subtracting the annual gross 

emission estimates from the annual gross sequestration estimates.9 National annual net carbon sequestration by 

urban trees was estimated from the city estimates of gross and net sequestration, and urban area and urban tree cover 

data for the contiguous United States. The urban areas are based on 1990 U.S. Census data, which define “urban 

land” as areas having a population density greater than 1,000 people per square mile and adjacent urban places, with 

predefined political boundaries, having a population total greater than 2,500. Therefore, urban encompasses most 

cities, towns, and villages (i.e., it includes both urban and suburban areas). The gross and net carbon 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 (0.30 kg C/m2

-year) was then 

multiplied by an estimate of national urban tree cover area (76,151 km2

) 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. The 

average of these ratios is 0.70. The urban tree cover area estimates for each of the 10 cities and the contiguous 

United States were obtained from Dwyer et al. (2000) and Nowak et al. (2001). 

Table 7-22: Carbon Stocks (Metric Tons C), Annual Carbon Sequestration (Metric Tons C/yr), Tree Cover 

(Percent), and Annual Carbon Sequestration per Area of Tree Cover (kg C/m2 cover-yr) for Ten U.S. Cities 

City

Carbon 

Stocks 

Gross Annual 

Sequestration

Net Annual 

Sequestration

Tree 

Cover

Gross Annual 

Sequestration per 

Area of Tree Cover 

Net Annual 

Sequestration per 

Area of Tree Cover

New York, NY 1,225,200 38,400 20,800 20.9 0.23 0.12

Atlanta, GA 1,220,200 42,100 32,200 36.7 0.34 0.26

Sacramento, CA 1,107,300 20,200 NA 13.0 0.66 NA

Chicago, IL 854,800 40,100 NA 11.0 0.61 NA

Baltimore, MD 528,700 14,800 10,800 25.2 0.28 0.20

Philadelphia, PA 481,000 14,600 10,700 15.7 0.27 0.20

Boston, MA 289,800 9,500 6,900 22.3 0.30 0.22

Syracuse, NY 148,300 4,700 3,500 24.4 0.30 0.22

Oakland, CA 145,800 NA NA 21.0 NA NA

 

9 Three cities did not have net estimates. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 257 

Jersey City, NJ 19,300 800 600 11.5 0.18 0.13

NA = not analyzed 

Uncertainty 

The only quantifiable uncertainty associated with changes in C stocks in urban trees was sampling, as reported by 

Nowak and Crane (2002). The average standard deviation for urban tree carbon storage was 27 percent of the mean 

carbon storage on an area basis. Additionally, a 5 percent uncertainty was associated with national urban tree 

covered area. These estimates are based on field data collected in ten U.S. cities, and uncertainty in these estimates 

increases as they are scaled up to the national level. 

There is additional uncertainty associated with the biomass equations, conversion factors, and decomposition 

assumptions used to calculate carbon sequestration and emission estimates (Nowak et al. 2002). These results also 

exclude changes in soil carbon stocks, and there may be some overlap between the urban tree carbon estimates and 

the forest tree carbon estimates. However, both the omission of urban soil carbon flux and the potential overlap 

with forest carbon are believed to be relatively minor (Nowak 2002). Because these are inestimable, they are not 

quantified as part of this analysis. 

The results of the Tier 1 quantitative uncertainty analysis are summarized in Table 7-23. Net C flux from changes 

in C stocks in urban trees was estimated to be between -80.3 and -37.0 Tg CO2 Eq. at a 95 percent confidence level. 

This indicates a range of 37 percent above and below the 2003 flux estimate of -58.7 Tg CO2 Eq. 

Table 7-23: Tier 1 Quantitative Uncertainty Estimates for Net C Flux from Changes in Carbon Stocks in Urban 

Trees (Tg CO2 Eq. and Percent) 

Source Gas 

2003 Flux 

Estimate 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Flux Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

Changes in C Stocks 

in Urban Trees CO2 (58.7) 37% (80.3) (37.0) 

Note: Parentheses indicate net sequestration. 

QA/QC and Verification 

The net carbon flux resulting from urban trees was calculated using estimates of gross and net carbon 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 the 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 2002). 

Planned Improvements 

Some sources indicate a reduction in urban tree coverage in the United States over the Inventory period of 

approximately 21 percent over the last 10 years (AF 2004). However, because the methods for making this 

assertion have not yet been made available and their definition of urban land is unclear, the veracity and potential 

application of this estimate cannot currently be established. Because the magnitude of the urban tree greenhouse 

gas sink in the United States is not insignificant, identifying changes in this sector is considered a priority and is 

being actively pursued for inclusion in future Inventories. Should this diminishment prove to be accurate, it could 

mean the urban tree sink estimates will need to be significantly revised. 

Changes in Soil Carbon Stocks (IPCC Source Category 5E1) 

Given the lack of available land use information relevant to this particular IPCC source category, it is not possible to 

quantify the CO2 flux from soils in Settlements Remaining Settlements at this time. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 258 

N2O Fluxes from Soils (IPCC Source Category 5E1)

Of the fertilizers applied to soils in the United States, approximately 10 percent are applied to lawns, golf courses, 

and other landscaping occurring within settled areas. Application rates are less than those occurring on cropped 

soils, and therefore account for a smaller proportion of total U.S. soil N2O emissions per unit area. In 2003, N2O 

emissions from this source were 6.0 Tg CO2 Eq. (19.4 Gg N2O). There was an overall increase of 9 percent over 

the thirteen year period due to a general increase in the application of synthetic fertilizers. Interannual variability in 

these emissions is directly attributable to interannual variability in total synthetic fertilizer consumption in the 

United States. 

Emissions from this source are summarized in Table 7-24. 

Table 7-24: N2O Fluxes from Soils in Settlements Remaining Settlements (Tg CO2 Eq.) 

Settlements Remaining Settlements: N2O 

Fluxes from Soils 1990 1997 1998 1999 2000 2001 2002 2003

Tg CO2 Eq. 5.5 6.1 6.1 6.2 6.0 5.8 6.0 6.0

Gg 17.9 19.8 19.8 19.9 19.3 18.7 19.4 19.4

Methodology 

Estimates of direct N2O emissions from soils in settlements were based on the amount of N applied to turf grass 

annually through the application of synthetic commercial fertilizers. Nitrogen applications to turf grass are assumed 

to be 10 percent of the total synthetic fertilizer used in the United States (Qian 2004). Total synthetic fertilizer 

applications were derived from fertilizer statistics (TVA 1991, 1992, 1993, 1994; AAPFCO 1995, 1996, 1997, 

1998, 1999, 2000b, 2002, 2003, 2004) and a recent AAPFCO database (AAPFCO 2000a). Unvolatilized N applied 

to turf grass was multiplied by the IPCC default emission factor (1.25 percent) to estimate direct N2O emissions. 

The volatilized and leached/runoff proportion, calculated with the IPCC default volatilization factor of 10 percent 

and 30 percent, respectively, for synthetic fertilizers, was included with the total N contributions to indirect 

emissions, as reported in the N2O Emissions from Agricultural Soil Management source category of the Agriculture 

sector. 

Uncertainty 

The amount of N2O emitted from settlements depends not only on N inputs, but also on a large number of variables, 

including organic carbon availability, O2 partial pressure, soil moisture content, pH, temperature, and 

irrigation/watering practices. The effect of the combined interaction of these variables on N2O flux is complex and 

highly uncertain. The IPCC default methodology used here does not incorporate any of these variables and only 

accounts for variations in national fertilizer application rates. All settlement soils are treated equivalently under this 

methodology. Furthermore, only synthetic fertilizers are captured, so applications of organic fertilizers are not 

accounted for here. Uncertainties exist in both the fertilizer application rates and the emission factors used to derive 

emission estimates. 

The 95 percent confidence interval for the IPCC’s default emission factor for synthetic fertilizer applied to soil 

ranges from 0.25 to 6 percent, according to Chapter 4 of IPCC (2000). While a Tier 1 analysis should be generated 

from a symmetrical distribution of uncertainty around the emission factor, an asymmetrical distribution was 

imposed here to account for the fact that the emission used was not the mean of the range given by IPCC. 

Therefore, an upper bound of 480 percent and a lower bound of 80 percent were assigned to the emission factor. 

The higher uncertainty percentage is shown below, but the lower bound reflects a truncated distribution. The 

uncertainty in the amount of synthetic fertilizer N applied to settlement soils was conservatively estimated to be 50 

percent (Qian 2004). The results of the Tier 1 quantitative uncertainty analysis are summarized in Table 7-25. N2O 

emissions from soils in settlements remaining settlements in 2003 were estimated to be between 0.3 and 35.1 Tg 

CO2 Eq. at a 95 percent confidence level. This indicates a range of 94 percent below to 483 percent above the 2003 

emission estimate of 6.0 Tg CO2 Eq. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 259 

Table 7-25: Tier 1 Quantitative Uncertainty Estimates of N2O Emissions from Soils in Settlements Remaining 

Settlements (Tg CO2 Eq. and Percent) 

IPCC Source 

Category Gas 

Year 2003 

Emissions 

(Tg CO2 Eq.) 

Uncertainty 

(%) 

Uncertainty Range Relative to 

2003 Emission Estimate 

(Tg CO2 Eq.) 

 Lower Bound Upper Bound 

Settlements Remaining 

Settlements: N2O 

Fluxes from Soils N2O 6.0 94 to 483% 0.3 35.1 

Recalculations Discussion 

The current Inventory presents N2O emissions from soils in settlements separately for the first time. Previously, 

N2O emissions from this source were included with N2O emissions from agricultural soils. The net effect of 

separating this source from agricultural soils is to reduce emissions reported from that source. However, because 

the methods for reporting that source category have changed significantly this year, it is not possible to isolate the 

magnitude of change caused by this recalculation alone on the overall differences in N2O emissions from 

agricultural soils. 

Planned Improvements 

The process-based model DAYCENT, which was used to estimate N2O emissions from cropped soils this year, 

could also be used to simulate emissions as well as volatilization and leaching/runoff from settlements. DAYCENT 

has been parameterized to simulate turf grass. State-level settlement area data is available from the National 

Resource Inventory. Future inventories will include DAYCENT simulations to estimate emissions from 

settlements. 

7.6. Lands 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 quantify CO2 or N2O 

fluxes from lands converted to settlements at this time. 

Land-Use Change and Fo
Figure 7-2: Estimates of Net Annual Changes in Carbon Stocks for Major Carbon Pools

-300

-250

-200

-150

-100

-50

0

50

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

Year

Forest Sector Net Carbon Flux (Tg/yr)

Soil

Total Net Change

Forest, nonsoil

Harvested Wood
7-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2002

3 The product estimates in this study use the “production approach” meaning that they do not account for carbon stored in imported wood prod

do include carbon stored in exports, even if the logs are processed in other countries (Heath et al. 1996). 

Note: Estimates are based on forest inventory data as described in the text.
Net Annual CO2 Flux, per Hectare, From Mineral Soils Under Agricultural Management, 1990-1992

metric ton CO2/ha

< -0.5

-0.5 to -0.25

-0.25 to -0.01

-0.01 to 0

> 0

This map shows the spatial variability in net annual carbon dioxide flux from mineral soils for the year 1990 through 1992.

The color assigned to each polygon represents the average annual flux per hectare for the area of managed mineral soils in that polygon.

Note: Values greater than zero represent 

emissions and values less than zero 

represent sequestration. Map accounts 

only for fluxes in agricultural soils and does 

not include SOC storage resulting from 

sewage sludge amendments in any year.

Figure 7-4

Net Annual CO2 Flux, per Hectare, From Mineral Soils Under Agricultural Management, 1990–1992
metric ton CO2/ha

< -0.3

-0.5 to -0.25

-0.25 to -0.01

-0.01 to 0

> 0

This map shows the spatial variability in net annual carbon dioxide flux from mineral soils for the year 1993 through 2003.

The color assigned to each polygon represents the average annual flux per hectare for the area of managed mineral soils in that polygon.

Note: Values greater than zero represent 

emissions and values less than zero 

represent sequestration. Map accounts 

only for fluxes in agricultural soils and does 

not include SOC storage resulting from sewage 

sludge amendments in any year, or CRP 

enrollment and manure management after 1997.

Figure 7-5

Net Annual CO2 Flux, per Hectare, From Mineral Soils Under Agricultural Management, 1993–2003
metric ton CO2/ha

no organic soils

0 to 5

5 to 25

>25

This map shows the spatial variability in net annual carbon dioxide flux from organic soils for the year 1990 through 1992.

The color assigned to each polygon represents the average annual flux per hectare for the area of managed organic soils in that polygon.

Figure 7-6

Net Annual CO2 Flux, per Hectare, From Organic Soils Under Agricultural Management, 1990–1992
metric ton CO2/ha

no organic soils

0 to 5

5 to 25

>25

This map shows the spatial variability in net annual carbon dioxide flux from organic soils for the year 1993 through 2003.

The color assigned to each polygon represents the average annual flux per hectare for the area of managed organic soils in that polygon.

Figure 7-7

Net Annual CO2 Flux, per Hectare, From Organic Soils Under Agricultural Management, 1993–2003

 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 261 

8. Waste 

Waste management and treatment activities are sources of greenhouse gas emissions (see Figure 8-1). Landfills 

were the largest source of anthropogenic methane (CH4) emissions, accounting for 24 percent of total U.S. CH4

emissions.1 Smaller amounts of CH4 are emitted from wastewater systems by bacteria used in various treatment 

processes. Wastewater treatment systems are also a potentially significant source of nitrous oxide (N2O) emissions; 

however, methodologies are not currently available to develop a complete estimate. Nitrous oxide emissions from 

the treatment of the human sewage component of wastewater were estimated, however, using a simplified 

methodology. Nitrogen oxide (NOx), carbon monoxide (CO), and non-methane volatile organic compounds 

(NMVOCs) are emitted by waste activities, and are addressed separately at the end of this chapter. A summary of 

greenhouse gas and ambient air pollutant emissions from the Waste chapter is presented in Table 8-1 and Table 8-2. 

Figure 8-1: 2003 Waste Chapter Greenhouse Gas Sources 

Overall, in 2003, waste activities generated emissions of 183.8 Tg CO2 Eq., or 2.6 percent of total U.S. greenhouse 

gas emissions. 

Table 8-1: Emissions from Waste (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CH4 197.1 179.0 171.0 167.7 165.0 160.9 162.6 167.9

Landfills 172.2 147.4 138.5 134.0 130.7 126.2 126.8 131.2

Wastewater Treatment 24.8 31.7 32.6 33.6 34.3 34.7 35.8 36.8

N2O 13.0 14.7 15.0 15.4 15.6 15.6 15.7 15.9

Human Sewage 13.0 14.7 15.0 15.4 15.6 15.6 15.7 15.9

Total 210.1 193.7 186.0 183.1 180.6 176.5 178.3 183.8

Note: Totals may not sum due to independent rounding. 

Table 8-2: Emissions from Waste (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003

CH4 9,385 8,526 8,145 7,984 7,858 7,660 7,744 7,997

Landfills 8,202 7,017 6,595 6,382 6,223 6,010 6,039 6,246

Wastewater Treatment 1,183 1,509 1,550 1,602 1,635 1,651 1,705 1,751

N2O 42 47 48 50 50 50 51 51

Human Sewage 42 47 48 50 50 50 51 51

NOx + 3 3 3 2 2 2 2

CO 1 5 5 13 8 8 8 8

NMVOCs 673 157 161 140 119 122 133 125

Note: Totals may not sum due to independent rounding. 

8.1. Landfills (IPCC Source Category 6A1) 

Landfills are the largest anthropogenic source of CH4 emissions in the United States. In 2003, landfill CH4

emissions were approximately 131 Tg CO2 Eq. (6,246 Gg). Emissions from municipal solid waste (MSW) landfills, 

which received about 61 percent of the total solid waste generated in the United States, accounted for about 94 

percent of total landfill emissions, while industrial landfills accounted for the remainder. Approximately 1,800 

 

1 Landfills also store carbon, due to incomplete degradation of organic materials such as wood products and yard trimmings, as 

described in the Land-Use Change and Forestry chapter. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 262 

operational landfills exist in the United States (BioCycle 2004), with the largest landfills receiving most of the waste 

and generating the majority of the CH4. 

After being placed in a landfill, biogenic waste (such as paper, food scraps, and yard trimmings) is initially digested 

by aerobic bacteria. After the oxygen has been depleted, the remaining waste is available for consumption by 

anaerobic bacteria, which can 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. Methane-producing anaerobic bacteria convert these 

fermentation products into stabilized organic materials and biogas consisting of approximately 50 percent carbon 

dioxide (CO2) and 50 percent CH4, by volume.2 Significant CH4 production typically begins one or two years after 

waste disposal in a landfill and may last from 10 to 60 years. 

From 1990 to 2003, net CH4 emissions from landfills decreased by approximately 24 percent (see Table 8-3 and 

Table 8-4), with small increases occurring in some interim years. This downward trend in overall emissions is the 

result of increases in the amount of landfill gas collected and combusted by landfill operators, which has more than 

offset the additional CH4 emissions resulting from an increase in the amount of municipal solid waste landfilled. 

Methane emissions from landfills are a function of several factors, including: (1) the total amount of municipal 

solid waste in landfills, which is related to total municipal solid 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 landfills increased from about 209 

Tg in 1990 to 279 Tg in 2003, an increase of 33 percent (see Annex 3.14). During this period, the estimated CH4

recovered and combusted from landfills increased as well. In 1990, for example, approximately 935 Gg of CH4

were recovered and combusted (i.e., used for energy or flared) from landfills. In 2003, the estimated quantity of 

CH4 recovered and combusted increased to 5,545 Gg. 

Over the next several years, the total amount of municipal solid waste generated is expected to increase slightly. 

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 increase, 

as a result of a 1996 federal regulation that requires large municipal solid waste landfills to collect and combust 

landfill gas (see 40 CFR Part 60, Subparts Cc 2002), and the Landfill Methane Outreach Program (LMOP), an EPA 

program that encourages voluntary CH4 recovery and use at landfills not affected by the regulation. 

Table 8-3: CH4 Emissions from Landfills (Tg CO2 Eq.) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003 

MSW Landfills 197.2 215.9 219.1 222.3 226.5 231.9 238.6 245.0 

Industrial Landfills 13.8 15.1 15.3 15.6 15.9 16.2 16.7 17.2 

Recovered 

Gas-to-Energy (14.0) (34.7) (42.4) (48.0) (51.9) (57.5) (59.1) (61.9) 

Flared (5.6) (32.6) (38.2) (41.0) (45.2) (50.5) (55.3) (54.6) 

Oxidizeda

 (19.1) (16.4) (15.4) (14.9) (14.5) (14.0) (14.1) (14.6) 

Total 172.2 147.4 138.5 134.0 130.7 126.2 126.8 131.2 

Note: Totals may not sum due to independent rounding. a Includes oxidation at both municipal and industrial landfills. 

Table 8-4: CH4 Emissions from Landfills (Gg) 

 

2 The percentage of CO2 in biogas released from a landfill may be smaller because some CO2 dissolves in landfill water 

(Bingemer and Crutzen 1987). Additionally, less than 1 percent of landfill gas is typically composed of non-methane volatile 

organic compounds (NMVOCs). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 263 

Activity 1990 1997 1998 1999 2000 2001 2002 2003 

MSW Landfills 9,391 10,279 10,435 10,588 10,785 11,045 11,364 11,669 

Industrial Landfills 657 720 730 741 755 773 795 817 

Recovered 

Gas-to-Energy (669) (1,652) (2,018) (2,287) (2,472) (2,738) (2,814) (2,946) 

Flared (266) (1,551) (1,821) (1,951) (2,154) (2,403) (2,635) (2,599) 

Oxidizeda

 (911) (780) (733) (709) (691) (668) (671) (694) 

Total 8,202 7,017 6,595 6,382 6,223 6,010 6,039 6,246 

Note: Totals may not sum due to independent rounding. a Includes oxidation at municipal and industrial landfills. 

Methodology 

Methane emissions from landfills were estimated to equal the CH4 produced from municipal solid waste landfills, 

minus the CH4 recovered and combusted, plus the CH4 produced by industrial landfills, minus the CH4 oxidized 

before being released into the atmosphere: 

CH4,Solid Waste = [(CH4,MSW – R) + CH4,ind]- Ox 

 Where, 

 CH4 , Solid Waste = CH4 emissions from solid waste 

 CH4,MSW = CH4 generation from municipal solid waste landfills, 

 R = CH4 recovered and combusted, 

 CH4,ind = CH4 generation from industrial landfills, 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 in the IPCC Good Practice Guidance and Uncertainty Management in National Greenhouse 

Gas Inventories and in a background paper prepared by Jensen and Papatti (2002). 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 (RTI 2004; EPA 1998; SWANA 1998; Peer, Thorneloe, and 

Epperson 1993). The rate constant was found to increase with average annual rainfall; consequently, values of k 

were developed for 3 ranges of rainfall. The annual quantity of waste placed in landfills was apportioned to the 3 

ranges of rainfall based on the percent of the U.S. population in each of the 3 ranges, and historical census data were 

used to account for the shift in population to more arid areas over time. For further information, see Annex 3.14. 

National landfill waste generation and disposal data for 1989 through 2003 were obtained from BioCycle (2004). 

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 (2004) and national per capita solid waste 

generation from BioCycle (2004). 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 methane 

generation rates and are based on the population in those years and the per capita rate for land disposal for the 

1960s. 

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 EPA’s Landfill Methane Outreach 

Program (LMOP), and a database maintained by the Energy Information Administration (EIA) for the voluntary 

reporting of greenhouse gases (EIA 2004). 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 2003 was estimated. 

This quantity likely underestimates flaring, because these databases do not have information on all flares in 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 264 

operation. Additionally, the EIA and LMOP databases provided data on landfill gas flow and energy generation for 

358 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 landfillspecific 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 was likely to also have had a flare, double counting reductions from flares and LFGTE projects in 

the LMOP database was avoided by subtracting emissions reductions associated with LFGTE projects for which a 

flare had not been identified from the emissions reductions associated with flares.3 

Emissions from industrial landfills were assumed to be equal to seven percent of the total CH4 emissions from 

municipal landfills (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 (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 for the waste placed in every 

landfill for each year of its operation—a fundamental factor that affects CH4 production. The heterogeneity of 

waste disposed in landfills is uncertain as well. 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 and reflects this heterogeneity. Also, the approach used to estimate the contribution 

of industrial non-hazardous wastes to total CH4 generation introduces uncertainty. Aside from uncertainty in 

estimating CH4 generation potential, uncertainty exists in the estimates of oxidation efficiency. 

The N2O emissions from application of sewage sludge on landfills are not explicitly modeled as part of greenhouse 

gas emissions from landfills. Nitrous oxide 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. The total nitrogen (N) in sewage sludge increased from 189 to 247 Gg total 

N between 1990 and 2002. The quantity of sewage sludge applied to landfills decreased from 28 to 11 percent from 

1990 to 2001 (EPA 1993). 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 8-5. Landfill CH4 emissions in 

2003 were estimated to be between 84.0 and 152.2 Tg CO2 Eq. at a 95 percent confidence level (or in 19 out of 20 

Monte Carlo Stochastic Simulations). This indicates a range of 36 percent below to 16 percent above the 2003 

emission estimate of 131.2 Tg CO2 Eq. 

Table 8-5: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Landfills (Tg CO2 Eq. and Percent) 

Source Gas 

2003 Emission 

Estimate Uncertainty Range Relative to Emission Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 

3 Due to the differences in referencing landfills and incomplete data on the national population of flares, matching flare vendor 

data with the LFGTE data was problematic and a flare could not be identified for each of the LFGTE projects. Because each 

LFGTE project likely has a flare, the aggregate estimate of emission reductions through flaring was reduced by the LFGTE 

projects for which a specific flare could not be identified. This approach eliminated the potential for double counting emissions 

reductions at landfills with both flares and a LFGTE project. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 265 

 Lower Bound Upper Bound Lower Bound Upper Bound 

Landfills CH4 131.2 84.0 152.2 -36% +16% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

The estimates for the current inventory are based on the first order decay model rather than the linear regression 

model for 30-year waste in place used for previous inventories. The first order decay model was recommended by 

U.S. landfill and landfill gas experts and is the preferred approach in the IPCC guidance. Data are now available to 

develop reliable estimates of the two parameters needed for the first order decay model: CH4 generation potential 

and the first order decay rate constant. In the past, these parameters were considered highly variable, thus making 

the first order decay methodology less desirable. 

In addition to the change in method, improved estimates of the annual quantity of waste placed in landfills were 

developed, particularly for the time period of 1960 through 1988. These two methodological improvements resulted 

in a reduction in the estimate of CH4 generation of approximately 20 percent for 1990 and 27 percent for 2002. 

About half of this reduction is due to the use of the first order decay model to estimate emissions, while the other 

half is due to the improved estimates of the annual quantity of waste placed in landfills. 

Another revision was incorporating data from the EIA to improve estimates of emissions avoided by LFGTE 

projects and flaring. The use of the EIA data resulted in a decrease of about 11 percent in CH4 emissions avoided 

(using 2002 as an example), primarily due to improved estimates for landfills with flares. Changes were also made 

to the LFGTE database used to estimate emissions avoided by these projects. The changes included corrections to 

megawatt capacity and gas flow rates, adding new projects that started in 2003, and accounting for projects that shut 

down. These changes had only a very small effect on emissions avoided by LFGTE projects. Overall, these 

changes resulted in an average annual decrease of 52.9 Tg CO2 Eq. (26 percent) in CH4 emissions from landfills for 

the period 1990 through 2002. 

Planned Improvements 

For the future inventories, efforts will be made to improve the estimates of CH4 generation at industrial landfills and 

estimates of oxidation, especially for landfills with gas recovery systems. Improvements to the flare database will 

be investigated, and an effort will be made to identify additional landfills that have flares. The parameters for the 

first order decay model will be re-evaluated as more data become available. 

8.2. Wastewater Treatment (IPCC Source Category 6B) 

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 off-site or on-site. 

For example, in the United States, approximately 25 percent of domestic wastewater is treated in septic systems or 

other on-site systems. 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 methane. During collection and treatment, wastewater may be accidentally or deliberately managed under anaerobic 

conditions. In addition, the sludge may be further biodegraded under aerobic or anaerobic conditions. 

The organic content, expressed in terms of either biochemical oxygen demand (BOD) or chemical oxygen demand 

(COD), determines the methane producing potential of wastewater. BOD represents the amount of oxygen that 

would be required to completely consume the organic matter contained in the wastewater through aerobic 

decomposition processes. COD refers to the amount of oxygen consumed under specified conditions in the 

oxidation of the organic and oxidizable inorganic matter and is a parameter typically used to characterize industrial 

wastewater. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 266 

In 2003, CH4 emissions from domestic wastewater treatment were estimated to be 19.8 Tg CO2 Eq. (944 Gg). 

Emissions have increased since 1990 in response to the increase in the U.S. human population. Also, the per capita 

organic wastewater loading has increased. Industrial emission sources include wastewater from the pulp and paper, 

meat and poultry processing, and the vegetables, fruits and juices processing industry.4 In 2003, CH4 emissions 

from industrial wastewater treatment were estimated to be 16.9 Tg CO2 Eq. (807 Gg). Table 8-6 and Table 8-7 

provide emission estimates from domestic and industrial wastewater treatment. 

Table 8-6: CH4 Emissions from Domestic and Industrial Wastewater Treatment (Tg CO2 Eq.) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003 

Domestic 11.4 15.8 16.5 17.1 17.8 18.5 19.2 19.8 

Industrial* 13.5 15.9 16.1 16.5 16.5 16.2 16.7 16.9 

Total 24.8 31.7 32.6 33.6 34.3 34.7 35.8 36.8 

* Industrial activity includes the pulp and paper, meat and poultry, and the vegetables, fruits and juices processing industry.

Note: Totals may not sum due to independent rounding. 

Table 8-7: CH4 Emissions from Domestic and Industrial Wastewater Treatment (Gg) 

Activity 1990 1997 1998 1999 2000 2001 2002 2003 

Domestic 543 751 783 815 848 880 912 944 

Industrial* 640 758 767 787 788 771 794 807 

Total 1,183 1,509 1,550 1,602 1,635 1,651 1,705 1,751 

* Industrial activity includes the pulp and paper, meat and poultry, and the vegetables, fruits and juices processing industry.

Note: Totals may not sum due to independent rounding. 

Methodology 

Domestic wastewater CH4 emissions were estimated using the default IPCC methodology. National population data 

for 1990 to 2003, used in the domestic wastewater emissions estimates, were based on data from the U.S. Census 

Bureau (2004). For BOD5 for domestic wastewater, two data points were available for 1991 and 2003 (Metcalf & 

Eddy 1990; Metcalf & Eddy 2003).5 The BOD loadings for intervening years were obtained by linear interpolation. 

The emission factor (0.6 kg CH4/kg BOD5) was taken from IPCC Good Practice Guidance (IPCC 2000). The 

percent of wastewater BOD5 that was anaerobically digested was assumed to be 16.25 percent. This value also 

accounts for U.S. septic systems and is based on expert judgment and on septic system usage data from EPA (1996). 

Table 8-8: U.S. Population (Millions) and Wastewater BOD5 Produced (Gg) 

Year Population BOD5

1990 254 5,566 

1997 277 7,706 

1998 280 8,032 

1999 283 8,363 

2000 287 8,695 

2001 289 9,021 

2002 292 9,351 

2003 295 9,685 

 

4 Industrial wastewater emissions from petroleum systems are included in the petroleum systems section in the Energy chapter. 

Other industrial sectors include organic chemicals, starch production, alcohol refining, creameries, and textiles, however 

emissions from these sectors are considered to be insignificant. 

5 BOD5 is the 5-day biochemical oxygen demand (BOD) measurement (Metcalf and Eddy 2003). 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 267 

Methane emissions estimates from industrial wastewater were developed according to the methodology described in 

the IPCC (2000). Industry categories that are likely to have significant CH4 emissions from their wastewater 

treatment were identified. High volumes of wastewater generated and a high organic COD wastewater load were 

the main criteria. The top three industries that met these criteria included pulp and paper manufacturing, meat and 

poultry packing, and vegetables, fruits and juices processing. 

Methane emissions from these categories were estimated by multiplying the annual product output (metric 

tons/year) by the average outflow (m3

/metric ton of output), the organics loading in the outflow (grams of organic 

BOD/m3

), the emission factor (grams CH4/grams BOD), and the percentage of organic BOD assumed to degrade 

anaerobically. In developing estimates for the vegetables, fruits, and juices category, COD was used instead of 

BOD, because no accurate BOD numbers were available. The emission factor used for pulp and paper as well as 

meat and poultry wastewater is 0.6 kg CH4/kg BOD5, whereas the emission factor for vegetables, fruits and juices 

category is 0.25 kg CH4/kg COD (IPCC 2000). The pertinent industry-specific parameters are specified below. 

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). The most 

important step is lagooning for storage, settling, and biological treatment (secondary treatment). 

In determining the percent that degraded anaerobically, both primary and secondary treatment were considered. 

Primary treatment lagoons are aerated to reduce anaerobic activity. However, the lagoons are large and zones of 

anaerobic activity may occur and, consequently, the primary lagoons are assumed to be 1.4 percent anaerobic. 

Approximately 42 percent of the BOD passes on to secondary treatment, which is less likely to be aerated (EPA 

1993). It was assumed that 25 percent of the BOD in secondary treatment lagoons degrades anaerobically, while 10 

percent passes through to be discharged with the effluent (EPA 1997a). Consequently, the overall percentage of 

wastewater organics that degrade anaerobically was determined to be 10.3 percent (i.e., 58% × 1.4% + 42% × 90% 

× 25%). A time series of CH4 emissions for post-1990 years was developed based on production figures reported in 

the Lockwood-Post Directory (Lockwood-Post 2002). The overall wastewater outflow was estimated to be 85 

m3

/metric ton, and the average BOD loading entering the secondary treatment lagoons was estimated to be 0.4 gram 

BOD/liter (EPA 1997b, EPA 1993, WorldBank 1999). 

The meat and poultry processing industry makes extensive use of anaerobic lagoons in sequence with screening, fat 

traps and dissolved air flotation. Production data, in carcass weight for the meat and poultry industry, were obtained 

from the U.S. Census (2004). EPA (2002) provided wastewater flows into the anaerobic lagoons of 7.9 and 16.6 

m3

/metric ton for meat and poultry production, respectively. The loadings are 2.8 and 1.5 g BOD/liter for meat and 

poultry, respectively, while 90 percent of organic BOD is believed to degrade anaerobically in the lagoon (EPA 

1997b). 

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, 5 percent of these wastewater 

organics are assumed to degrade anaerobically. The USDA National Agricultural Statistics Service (USDA 2004) 

provided production data for the fruits, vegetables, and juices processing sector. Outflow data for various 

subsectors (canned fruit, canned vegetables, frozen vegetables, fruit juices, jams, baby food) were obtained from 

World Bank (1999) and an average wastewater outflow of 5.6 m3

/metric ton was used. For the organics loading, a 

COD value of 5 g/liter was used (EPA 1997b). 

Table 8-9: U.S. Pulp and Paper, Meat and Poultry, and Vegetables, Fruits and Juices Production (Tg) 

Year Pulp and Paper 

Meat

(carcass weight) 

Poultry

(carcass weight)

Vegetables,

Fruits and Juices

1990 128.9 17.9 10.6 30.2

1991 129.2 18.5 11.2 31.3

1992 134.5 18.7 12.0 33.5

1993 134.1 18.9 12.3 34.1
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 268 

1994 139.3 19.2 13.2 37.3

1995 140.9 19.8 13.8 36.8

1996 140.3 19.8 14.5 36.4

1997 145.6 19.7 15.0 37.7

1998 144.0 20.5 15.1 36.5

1999 145.1 21.0 16.0 37.4

2000 142.8 21.0 16.4 38.9

2001 134.3 20.8 16.8 35.0

2002 137.5 21.5 17.3 36.7

2003 140.0 21.8 17.6 34.4

Uncertainty 

Significant uncertainties are associated with the industrial wastewater emission estimates. Wastewater outflows and 

organics loadings vary considerably for different plants and different sub-sectors (e.g., paper vs. board, poultry vs. 

beef, or baby food vs. juices). For pulp and paper industrial wastewater, five key variables were simulated: 

material output, with a standard deviation of 10 percent; wastewater outflow, with a standard deviation of 48 

percent; BOD5 concentration, with a standard deviation of 25 percent; percent anaerobically treated, with a standard 

deviation of 50 percent; and the BOD5 IPCC methane emission factor, with a standard deviation of 30 percent. For 

industrial wastewater from meat and poultry processing, five key variables were simulated: material output, with a 

standard deviation of 10 percent; wastewater outflow, with a standard deviation of 59 percent; BOD5 concentration, 

with a standard deviation of 51 percent; percent anaerobically treated, with a standard deviation of 10 percent; and 

the BOD5 IPCC methane emission factor, with a standard deviation of 30 percent. For industrial wastewater from 

processing fruit and vegetables, five variables were simulated: material output, with a standard deviation of 10 

percent; wastewater outflow, with a standard deviation of 55 percent; COD5 concentration, with a standard 

deviation of 60 percent; percent anaerobically treated, with a standard deviation of 50 percent; and the BOD5 IPCC 

methane emission factor, with a standard deviation of 30 percent. 

In summary, uncertainties for outflows are approximately 50 percent for the different source categories and are 

based on a standard deviation calculation for meat and poultry, and on expert judgment and the literature for the 

pulp and paper and the vegetables, fruits, and juices category (Nemerow and Dasgupta 1991; World Bank 1999). 

Uncertainties for organic loadings are based on the same approach and are estimated at 25, 50, and 60 percent for 

pulp and paper, meat and poultry, and fruits, vegetables and juices, respectively. The uncertainty associated with 

the degree in which anaerobic degradation occurs in treatment systems is estimated at 50 percent for the pulp and 

paper and vegetables, fruits, and juices categories, while this factor for the meat and poultry industry is 10 percent, 

because the flow data are from the entrance of the anaerobic lagoons. 

For domestic wastewater uncertainty, a normal probability distribution was assumed for the four key variables 

simulated: population, with a standard deviation of 5 percent; BOD5, with a standard deviation of 30 percent; 

percent of wastewater treated anaerobically, with a standard deviation of 25 percent; and the BOD5 IPCC CH4

emission factor, with a standard deviation of 30 percent. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 8-10. Wastewater treatment CH4

emissions in 2003 were estimated to be between 25.2 and 50.3 Tg CO2 Eq. at a 95 percent confidence level (or in 19 

out of 20 Monte Carlo Stochastic Simulations). This indicates a range of 32 percent below to 37 percent above the 

2003 emission estimate of 36.8 Tg CO2 Eq. 

Table 8-10: Tier 2 Quantitative Uncertainty Estimates for CH4 Emissions from Wastewater Treatment (Tg CO2 Eq. 

and Percent) 

Source Gas 

2003 Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 269 

Wastewater Treatment CH4 36.8 25.2 50.3 -32% +37% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

Recalculations Discussion 

Population estimates for the United States from 1990 through 2002 have been adjusted to include U.S. territories 

(i.e., American Samoa, Guam, Northern Mariana Islands, and the Virgin Islands). Emission estimates from 

previous years have only considered estimates of U.S. states and Puerto Rico. On average, updated U.S. population 

estimates increased total annual CH4 emissions from wastewater treatment by less than one percent. 

The time series for domestic wastewater has been updated due to an adjustment in the per capita BOD factor. The 

BOD factor changed from a constant value of 0.06 kg per capita per day to an increasing value from 0.06 in 1990 to 

0.09 kg per capita per day in 2003. Compared to 2002 estimates, methane emissions from domestic wastewater 

increased 37 percent. The time series for industrial wastewater changed because more detailed field data became 

available for the meat and poultry industry (EPA 2002) that include new flow and organic loading data. As a result, 

estimates for the meat and poultry industrial subcategories have been separated. Organic matter loading 

calculations are also now based on BOD rather than COD estimates. The new flow data reflect field measurements 

at the anaerobic lagoon inlet, as opposed to more general plant outflow data. Industrial CH4 emissions increased 14 

percent compared to 2002 estimates, as a result of the more detailed meat and poultry data and a slight increase in 

pulp and paper production. The total 2002 CH4 emissions from wastewater treatment increased by 25 percent 

compared to the emissions in the previous Inventory. Overall, these changes resulted in an average annual increase 

of 3.8 Tg CO2 Eq. (14 percent) in CH4 emissions from wastewater treatment for the period 1990 through 2002. 

Planned Improvements Discussion 

The Authors/Experts Meeting for the Preparation of 2006 IPCC National Greenhouse Gas Inventories Guidelines 

for the Wastewater Sector, held in November 2004, will likely generate improved methodological data. 

Improvements will be planned accordingly. 

8.3. Human Sewage (Domestic Wastewater) (IPCC Source Category 6B) 

Domestic human sewage is usually mixed with other household wastewater, which includes shower drains, sink 

drains, washing machine effluent, etc. and transported by a collection system to either a direct discharge, an on-site 

or decentralized wastewater treatment system, or a centralized wastewater treatment system. Decentralized 

wastewater treatment systems are septic systems and package plants. Centralized wastewater treatment systems 

may include a variety of processes, ranging from lagooning to advanced tertiary treatment technology for removing 

nutrients. Often, centralized wastewater treatment systems also treat certain flows of industrial, commercial, and 

institutional wastewater. After processing, treated effluent is discharged to a receiving water environment (e.g., 

river, lake, estuary, etc.), or applied to soils, or disposed of below the surface. 

Nitrous oxide may be generated during both nitrification and denitrification of the nitrogen present, usually in the 

form of urea, ammonia, and proteins. These compounds are converted to nitrate via nitrification, an aerobic process 

converting ammonia-nitrogen into nitrate (NO3

-

). Denitrification occurs under anoxic conditions (without free 

oxygen), and involves the biological conversion of nitrate into dinitrogen gas (N2). Nitrous oxide can be an 

intermediate product of both processes, but is more often associated with denitrification. 

The United States identifies two distinct sources for N2O emissions from domestic wastewater: emissions from 

centralized wastewater treatment processes; and emissions from effluent that has been discharged into aquatic 

environments. The 2003 emissions of N2O from wastewater treatment processes and from effluent were estimated 

to be 0.3 Tg CO2 Eq. (0.9 Gg) and 15.6 Tg CO2 Eq. (50 Gg), respectively. Total N2O emissions from domestic 

wastewater were estimated to be 15.9 Tg CO2 Eq. (51 Gg) (see Table 8-11). Emissions from wastewater treatment 

processes have gradually increased as a result of increasing U.S. population and protein consumption. 

Table 8-11: N2O Emissions from Human Sewage (Tg CO2 Eq. and Gg) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 270 

Year Tg CO2 Eq. Gg 

1990 13.0 42 

1997 14.7 47 

1998 15.0 48 

1999 15.4 50 

2000 15.6 50 

2001 15.6 50 

2002 15.7 51 

2003 15.9 51 

Methodology

The IPCC default methodology (IPCC/UNEP/OECD/IEA 1997) assumes that nitrogen disposal, and thus N2O 

emissions associated with land disposal, subsurface disposal, and domestic wastewater treatment are negligible and 

all nitrogen is discharged directly into aquatic environments. For the United States, N2O emissions from domestic 

wastewater (human sewage) were estimated using the IPCC methodology with three modifications: 

• In the United States, a certain amount of nitrogen is removed with the sewage sludge, which is land 

applied, incinerated or landfilled (Nsludge). The nitrogen disposal into aquatic environments is reduced to 

account for the sewage sludge application. 

o The IPCC methodology uses annual, per capita protein consumption (kg/year). This number is likely 

to underestimate the amount of protein entering the sewer or septic system. Food (waste) that is not 

consumed is often washed down the drain, as a result of the use of garbage disposals. Also, bath and 

laundry water can be expected to contribute to nitrogen loadings. A factor of 1.4 is introduced to 

account for non-consumption nitrogen.6 Furthermore, a significant quantity of industrial wastewater 

(nitrogen) is co-discharged with domestic wastewater. To account for this, a factor of 1.25 is 

introduced.7 In summary, a factor of 1.75 (1.4 × 1.25) is used to account for the extra nitrogen 

discharge from kitchen, bath, and laundry wastes, as well as industrial wastewater that is codischarged into sewers, based on Metcalf & Eddy (1991) and expert judgment. 

• Process emissions from wastewater treatment plants are not accounted for in the current IPCC 

methodology. To estimate N2O emissions from U.S. wastewater treatment plants, an overall emission 

factor (4 g N2O/person.year) was introduced. This emission factor is based on a factor of 3.2 g 

N2O/person.year (Czepiel 1995) multiplied by the 1.25 factor mentioned above, which adjusts for codischarged industrial nitrogen and is based on expert judgment. The nitrogen quantity associated with 

 

6 Metcalf & Eddy (1991) provides an indication of the nitrogen concentration of 40 mg Total Kjeldahl Nitrogen (TKN)/liter for 

average wastewater from residences, which includes bathwater, laundry, and the use of garbage disposals. According to the 

NEEDS Survey (1996), the total volume of wastewater generated in the US in 1996 was 32,175 million gallons per day (MGD), 

serving 189,710,899 people (72 percent of population, not including the septic system users). In 1996, the per capita TKN 

loading was: 40 [mg/l] × 32,175 × 106

 [gal/day] × 3.8 [l/gal] × 365 days/yr × 1/(189.7 × 106) × 10-6 = 9.4 [kg TKN/yr.person]. 

Average protein intake in 1996 was 41 kg/year (6.6 kg N/year), leading to a factor of 1.4 (9.4/6.6). 

7 The type, composition, and quantity of this co-discharged wastewater will vary greatly between municipalities. Metcalf & 

Eddy (1991) provide an indicative nitrogen loading of 20 to 85 mg TKN/liter (average 55) for combined residential and 

industrial wastewater, while residential wastewater loading was roughly estimated at 40 mg TKN/liter (see footnote 1). Until 

better data become available, the amount of N in wastewater was increased by 10 mg/l to account for industrial co-discharge 

(factor of 1.25.) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 271 

these emissions (NWWT) is calculated by multiplying the N2O emitted by (2 × 14)/44 and is subtracted 

from the total quantity of nitrogen that is ultimately disposed into the aquatic environment. 

With the modifications described above, N2O emissions from domestic wastewater were estimated using the IPCC 

default methodology (IPCC/UNEP/OECD/IEA 1997). This methodology is illustrated below: 

N2O(s) = (USPOP × 0.75 × EF1 × 10-3) + {[(Protein × 1.75 × FracNPR × USPOP) – NWWT – Nsludge] × EF2 × 44/28} 

where, 

N2O(s) = N2O emissions from domestic wastewater (“human sewage”) [kg/year] 

USPOP = U.S. population 

0.75 = Fraction of population using centralized wastewater treatment plants (as opposed to septic 

systems) 

EF1 = Emission factor (4 g N2O/person.year) expressing emissions from the centralized wastewater treatment plants

Protein = Annual per capita protein consumption [kg N/(person.year)]

1.75 = Fraction of non-consumption protein in domestic wastewater

FracNPR = Fraction of nitrogen in protein (i.e., 0.16 kg N/kg protein) 

NWWT = Quantity of wastewater nitrogen removed by wastewater treatment processes [(USPOP × 0.75 ×

EF1× 10-3) × 28/44] (kg N/year). 

Nsludge = Quantity of sewage sludge N not entering aquatic environments (kg N/year)

EF2 = Emission factor (kg N2O-N/kg sewage-N produced)

(

44/28) = Molecular weight ratio of N2O to N2. 

U.S. population data were taken from the U.S. Census Bureau (2003). The fraction of the U.S. population using 

wastewater treatment plants is from the NEEDS Survey (EPA 1996). The emission factor (EF1) to estimate 

emissions from wastewater treatment is based on Czepiel, et al. (1995). Data on annual per capita protein intake 

were provided by the United Nations Food and Agriculture Organization for the 1990 to 2002 time frame (FAO 

2004). Because data on protein intake were unavailable for 2003, the value of per capita protein consumption was 

extrapolated from previous years. Table 8-12 presents the data for U.S. population and average protein intake. An 

emission factor to estimate emissions from effluent (EF2) has not been specifically estimated for the United States, 

thus the default IPCC value (0.01 kg N2O-N/kg sewage-N produced) was applied. The fraction of nitrogen in 

protein (0.16 kg N/kg protein) was also obtained from IPCC/UNEP/OECD/IEA (1997). 

Table 8-12: U.S. Population (Millions) and Average Protein Intake [kg/(person.year)] 

Year Population Protein 

1990 254 39.2 

1997 277 40.9 

1998 280 41.2 

1999 283 42.0 

2000 287 41.9 

2001 289 41.8 

2002 292 41.6 

2003 295 41.8 

Uncertainty

Nitrous oxide emissions from wastewater treatment are estimated to be substantially less than emissions from 

effluent-surface water. Thus, this wastewater treatment subcategory was not considered in the uncertainty analysis. 

A triangular distribution was used to simulate land application of sludge as defined for the agricultural soil 

management sector. The same distribution was assumed for landfilled sludge. The means for the distributions were 

the estimates used for the 2003 inventory estimate. Based on professional judgment, the standard deviation for 

population was 5 percent, the standard deviation for per capita protein consumption was 5 percent, the standard 

deviation of the fraction of nitrogen in protein (FracNPR) was 2 percent, and the standard deviation of the nonconsumption protein in domestic wastewater was 25 percent based on professional judgment. The standard 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 272 

deviation emission factor for effluent is 80 percent based on the range provided in IPCC (1996). For the triangular 

distributions, the lower bound and upper estimates for the land applied and landfilled sludge were 50 percent below 

and above the respective estimates used for the 2003 inventory. A normal distribution was used to simulate five 

variables: population, per capita protein intake data, fraction of nitrogen in protein (FracNPR), non-consumption 

protein in domestic wastewater, and the IPCC emission factor. 

The results of the Tier 2 quantitative uncertainty analysis are summarized in Table 8-13. Human sewage N2O 

emissions in 2003 were estimated to be between 4.2 and 29.9 Tg CO2 Eq. at a 95 percent confidence level (or in 19 

out of 20 Monte Carlo Stochastic Simulations). This indicates a range of 74 percent below to 88 percent above the 

2003 emission estimate of 15.9 Tg CO2 Eq. 

Table 8-13: Tier 2 Quantitative Uncertainty Estimates for N2O Emissions from Human Sewage (Tg CO2 Eq. and 

Percent) 

Source Gas 

2003 Emission 

Estimate 

Uncertainty Range Relative to Emission 

Estimatea

 (Tg CO2 Eq.) (Tg CO2 Eq.) (%) 

 Lower 

Bound 

Upper 

Bound 

Lower 

Bound 

Upper 

Bound 

Human Sewage N2O 15.9 4.2 29.9 -74% +88% a

 Range of emission estimates predicted by Monte Carlo Stochastic Simulation for a 95 percent confidence interval. 

QA/QC and Verification 

An IPCC Tier 1 level QA/QC verification was performed. During the QA/QC process the values for Wastewater 

BOD produced per capita (kg/capita/day) were adjusted to be based on a calculated rather than estimated 

interpolated value between the reported 1990 and 2003 values. In addition, the documentation of references in the 

spreadsheets was improved. 

Planned Improvements 

The default emission factor for N2O from wastewater effluent has a high uncertainty. 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. Several parameters constituting this factor are 

based on references that have since been updated, including Needs Survey (1996) and Metcalf & Eddy (1991). The 

uncertainty associated with this factor can likely be reduced incorporating more recent data. 

Recalculations Discussion 

Population estimates for the U.S. from 1990 through 2002 have been adjusted to include U.S. territories (i.e., 

American Samoa, Guam, Northern Mariana Islands, and the Virgin Islands). Emission estimates from previous 

years have only considered estimates of U.S. states and Puerto Rico. Overall, the change resulted in an average 

annual increase of 0.2 Tg CO2 Eq. (1.7 percent) in N2O emissions from wastewater treatment processes and effluent 

for the period 1990 through 2002. 

8.4. Waste Sources of Ambient Air Pollutants 

In addition to the main greenhouse gases addressed above, waste generating and handling processes are also sources 

of ambient air pollutant emissions. Total emissions of NOx, CO, and NMVOCs from waste sources for the years 

1990 through 2003 are provided in Table 8-14. 

Table 8-14: Emissions of NOx, CO, and NMVOC from Waste (Gg) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002 2003 

NOx + 3 3 3 2 2 2 2 

Landfills + 2 2 3 2 2 2 2 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 273 

Wastewater Treatment + + + + + + + + 

Miscellaneousa + 1 1 + + + + + 

CO 1 5 5 13 8 8 8 8 

Landfills 1 5 5 12 7 7 7 7 

Wastewater Treatment + + + 1 1 1 1 1 

Miscellaneousa + + + + + + + + 

NMVOCs 673 157 161 140 119 122 133 125 

Landfills 58 32 33 27 23 23 25 24 

Wastewater Treatment 57 62 63 59 51 53 58 54 

Miscellaneousa

 558 64 65 54 46 46 51 47 a

 Miscellaneous includes TSDFs (Treatment, Storage, and Disposal Facilities under the Resource Conservation and Recovery 

Act [42 U.S.C. § 6924, SWDA § 3004]) and other waste categories. 

Note: Totals may not sum due to independent rounding. 

+ Does not exceed 0.5 Gg 

Methodology and Data Sources 

These emission estimates were obtained from preliminary data (EPA 2004), and disaggregated based on EPA 

(2003), which, in its final iteration, will be published on the National Emission Inventory (NEI) Air Pollutant 

Emission Trends web site. Emission estimates of these gases were provided by sector, using a “top down” 

estimating procedure⎯emissions were calculated either for individual sources or for many sources combined, using 

basic activity data (e.g., the amount of raw material processed) as an indicator of emissions. National activity data 

were collected for individual source categories from various agencies. Depending on the source category, these 

basic activity data may include data on production, fuel deliveries, raw material processed, etc. 

Activity data were used in conjunction with emission factors, which relate the quantity of emissions to the activity. 

Emission factors are generally available from the EPA’s Compilation of Air Pollutant Emission Factors, AP-42 

(EPA 1997). The EPA currently derives the overall emission control efficiency of a source category from a variety 

of information sources, including published reports, the 1985 National Acid Precipitation and Assessment Program 

emissions inventory, and other EPA databases. 

Uncertainty 

No quantitative estimates of uncertainty were calculated for this source category. Uncertainties in these estimates, 

however, are primarily due to the accuracy of the emission factors used and accurate estimates of activity data. 
Figure 8-1: 2003 Waste Chapter Greenhouse Gas Sources

0 20 40 60 80 100 120 140

Human Sewage

Wastewater

Treatment

Landfills

Tg CO2 Eq

Waste as a Portion 

of all Emissions

2.7%
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 275 

9. Other 

The United States does not report any greenhouse gas emissions under the “other” IPCC sector. 

 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 277 

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 

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 (IPCC 2000).” 

The results of all methodology changes and historical data updates are presented in this section; detailed 

descriptions of each recalculation are contained within each source’s description contained in this report, if 

applicable. Table 10-1 summarizes the quantitative effect of these changes on U.S. greenhouse gas emissions and 

Table 10-2 summarizes the quantitative effect on U.S. sinks, both relative to the previously published U.S. 

Inventory (i.e., the 1990 through 2002 report). These tables present the magnitude of these changes in units of Tg 

CO2 Eq. In addition to the changes summarized by the tables below, four new sources—CO2 emissions from nonenergy use of fossil fuels and petrochemical production and N2O emissions from settlements remaining settlements 

and forest land remaining forest land—have been added to the current Inventory. 

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 2002) 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. 

The following emission sources, which are listed in descending order of absolute average annual change in 

emissions from 1990 through 2002, 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. 

● CO2 from Fossil Fuel Combustion. The most important change in the calculation was the revision that removed 

emissions from the non-energy use of fuels, which allowed them to be reported separately in the Carbon 

Emitted from Non-Energy Uses of Fossil Fuels source category. Overall, this change, along with several other 

alterations, resulted in an average annual decrease of 115.8 Tg CO2 Eq. (2.2 percent) in CO2 emissions from 

fossil fuel combustion for the period 1990 through 2002. 

● Land-Use Change and Forestry. The most influential of the changes in the Land-Use Change and Forestry 

sector occurred in calculations for agricultural soil carbon stocks. These changes include: a new methodology 

for the evaluation and incorporation of uncertainty in manure amendments into the calculations for agricultural 

soil carbon, the use of new management factors provided in the IPCC LULUCF Good Practice Guidance

(IPCC 2003), and revision of the land area included in the emission calculation for organic soils. Overall, these 

changes, in combination with adjustments in the other sources/sinks, resulted in an average annual decrease in 

net flux of CO2 to the atmosphere from the land-use change and forestry sector of 89.0 Tg CO2 Eq. (12.1 

percent) for the period 1990 through 2002. 

● Landfills. Revisions to the emissions calculation incorporated the use of a first order decay model rather than a 

linear regression model for 30-year waste in place figures, improved estimates of the annual quantity of waste 

placed in landfills, and more accurate estimates of emissions avoided by landfill gas to energy (LFGTE) 

projects and flaring. Overall, changes resulted in an average annual decrease in CH4 emissions from landfills of 

52.9 Tg CO2 Eq. (26.0 percent) for the period 1990 through 2002. 

● Agricultural Soil Management. The emissions calculation changed to incorporate the reallocation of emissions 

from fertilizer applied to forests and urban areas from the agricultural soil management source category within 

the Agriculture sector to the Land-Use Change and Forestry sector and the use of a Tier 3 methodology 

utilizing the DAYCENT ecosystem model rather than the Tier 1 methodology used in the past. Overall, 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 278 

changes resulted in an average annual decrease in N2O emissions from agricultural soil management of 31.2 Tg 

CO2 Eq. (11.0 percent) for the period 1990 through 2002. 

● Petroleum Systems. The calculation of emissions was revised to incorporate a modified emission factor for 

CH4 emissions from oil tanks in the production sector and new activity factor for offshore Gulf of Mexico 

platform venting in the production sector. Overall, changes resulted in an average annual decrease in CH4

emissions from petroleum systems of 7.1 Tg CO2 Eq. (27.4 percent) for the period 1990 through 2002. 

● Natural Gas Systems. The emissions calculation was revised to incorporate new Gas STAR emissions 

reduction data and the addition of three new emission sources in the production sector: CH4 from gas 

condensate stored in tanks, unconventional gas well fugitives, and flaring from offshore Gulf of Mexico 

operations. Overall, changes resulted in an average annual increase in CH4 emissions from natural gas systems 

of 6.7 Tg CO2 Eq. (5.3 percent) for the period 1990 through 2002. 

● Mobile Combustion. The most significant changes to this source were revisions to the emission factors for CH4 

and N2O from highway vehicles, which were generated from EPA-sponsored laboratory vehicle testing. 

Revisions to these emission factors resulted in lower emission estimates for both CH4 and N2O emissions from 

highway vehicles. Overall, changes resulted in an average annual decrease in N2O emissions from mobile 

combustion of 6.2 Tg CO2 Eq. (10.9 percent) and an average annual decrease in CH4 emissions from mobile 

combustion of 0.6 Tg CO2 Eq. (13.6 percent ) for the period 1990 through 2002. 

● Wastewater Treatment. The most influential changes in the calculation were the use of an adjusted per capita 

BOD factor in the domestic wastewater emissions calculations and the use of more detailed field data for the 

meat and poultry industry in the industrial wastewater emissions calculations. Overall, changes resulted in an 

average annual increase in CH4 emissions from wastewater treatment of 3.8 Tg CO2 Eq. (14.0 percent) for the 

period 1990 through 2002. 

● Abandoned Coal Mines. The calculation of emissions was revised to incorporate an updated mine list, updated 

coal seam permeabilities, and revised closure dates for 43 of the mines. Overall, changes resulted in an average 

annual increase in CH4 emissions from abandoned coal mines of 2.8 Tg CO2 Eq. (64.5 percent) for the period 

1990 through 2002. 

Table 10-1: Revisions to U.S. Greenhouse Gas Emissions (Tg CO2 Eq.) 

Gas/Source 1990 1997 1998 1999 2000 2001 2002

CO2 7.2 2.3 4.7 1.7 (0.8) 13.0 14.4 

Fossil Fuel Combustion (102.9) (120.8) (133.7) (142.9) (128.5) (110.8) (109.5)

Non-Energy Use of Fuelsa 108.0 120.3 135.4 141.6 124.7 120.1 118.8 

Natural Gas Flaring + NC NC NC + 0.7 0.9 

Cement Manufacture NC NC NC NC NC NC NC

Lime Manufacture NC NC NC NC NC NC NC

Limestone and Dolomite Use NC NC NC NC NC NC +

Soda Ash Manufacture and Consumption NC NC NC NC NC NC NC

Carbon Dioxide Consumption + + + + + + (0.3)

Waste Combustion NC NC NC NC NC NC NC

Titanium Dioxide Production NC NC NC NC NC NC NC

Aluminum Production NC NC NC NC NC NC +

Iron and Steel Production + + + + + (0.2) 0.7 

Ferroalloys NC NC NC NC NC NC NC

Ammonia Manufacture & Urea Application NC NC NC NC + 0.5 0.9 

Petrochemical Productiona NC NC NC NC NC NC +

Phosphoric Acid Production 2.2 2.9 3.0 3.1 3.0 2.8 2.9 

Land-Use Change and Forestry (Sink) (84.2) (109.1) (175.2) (150.4) (132.3) (137.1) (135.8)

International Bunker Fuels (0.4) + (0.5) + + + 2.6 

Biomass Combustion NC NC NC NC NC (3.9) 0.2 

CH4 (37.3) (49.3) (51.0) (55.8) (60.3) (58.3) (55.6)

Stationary Sources (0.4) (0.4) (0.3) (0.4) (0.4) (0.6) (0.5)
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 279 

Mobile Sources (0.2) (0.6) (0.7) (0.9) (1.0) (1.2) (1.3)

Coal Mining + NC + + NC + 0.2 

Abandoned Coal Mines 2.7 2.5 2.4 2.9 3.3 2.7 2.2 

Natural Gas Systems 6.3 7.5 7.3 6.5 6.4 6.9 8.8 

Petroleum Systems (8.9) (6.7) (6.5) (5.9) (5.9) (6.0) (6.1)

Petrochemical Production NC NC NC NC NC NC NC

Silicon Carbide Production NC NC NC NC NC NC NC

Iron and Steel Production + + + + + + +

Enteric Fermentation + + + 0.2 (0.1) 0.2 0.2 

Manure Management 0.2 0.1 0.1 0.1 0.1 0.1 (0.1)

Rice Cultivation NC NC NC NC NC NC NC

Field Burning of Agricultural Residues NC NC + NC NC NC +

Landfills (37.7) (56.0) (58.1) (63.8) (68.6) (67.0) (66.2)

Wastewater Treatment 0.7 4.3 4.8 5.4 6.0 6.5 7.1 

International Bunker Fuels + + + + + + +

N2O (11.2) (40.0) (24.4) (46.2) (23.9) (31.4) (35.3)

Stationary Sources (0.4) (0.4) (0.4) (0.4) (0.4) (0.4) (0.5)

Mobile Sources (6.9) (5.1) (4.2) (4.0) (4.2) (6.1) (7.3)

Adipic Acid NC NC NC NC NC NC NC

Nitric Acid NC NC NC NC NC NC 0.5 

Manure Management 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

Agricultural Soil Management (9.8) (41.2) (26.5) (48.7) (25.9) (31.5) (34.8)

Field Burning of Agricultural Residues NC NC + NC NC NC +

Human Sewage 0.2 0.2 0.3 0.3 0.3 0.2 0.1 

N2O Product Usage NC NC NC NC NC NC NC

Waste Combustion NC NC NC NC NC 0.1 0.1 

Settlements Remaining Settlementsa 5.5 6.1 6.1 6.2 6.0 5.8 6.0 

Forest Land Remaining Forest Landa 0.1 0.3 0.4 0.5 0.4 0.4 0.4 

International Bunker Fuels + + + + + + +

HFCs, PFCs, and SF6 0.3 0.1 + + (0.2) (0.2) +

Substitution of Ozone Depleting Substances 0.1 0.1 + + (0.1) (0.1) (0.2)

Aluminum Production 0.2 + + + 0.1 + +

HCFC-22 Production NC + + + + + NC

Semiconductor Manufacture NC NC NC NC NC NC NC

Electrical Transmission and Distribution + + + + (0.2) (0.2) (0.1)

Magnesium Production and Processing NC NC NC NC + + 0.2 

Net Change in Total Emissionsb (41.0) (86.9) (70.7) (100.3) (85.2) (77.0) (76.5)

Percent Change (0.7%) (1.3%) (1.0%) (1.5%) (1.2%) (1.1%) (1.1%)

+ 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 Excludes sinks from land-use change and forestry, and emissions from international bunker fuels and biomass combustion. 

Note: Totals may not sum due to independent rounding. 

Table 10-2: Revisions to Net Flux of CO2 to the Atmosphere from Land-Use Change and Forestry (Tg CO2 Eq.) 

Component 1990 1997 1998 1999 2000 2001 2002 

Forest Land Remaining Forest 

Land 

(102.7) (120.9) (187.7) (163.3) (145.6) (150.7) (150.7) 

Cropland Remaining Cropland 18.5 11.9 12.6 13.0 13.4 13.6 15.0 

Settlements Remaining Settlements NC (0.1) (0.1) (0.1) (0.1) + + 

Net Change in Total Flux (84.2) (109.1) (175.2) (150.4) (132.3) (137.1) (135.8) 

Percent Change (8.8%) (13.3%) (24.8%) (22.2%) (19.2%) (19.9%) (19.7%) 

+ Absolute value does not exceed 0.05 Tg CO2 Eq. or 0.05 percent. 

NC (No Change) 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 280 

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. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 281 

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USGS (2003a) Minerals Yearbook: Crushed Stone Annual Report 2002. U.S. Geological Survey, Reston, VA. 

USGS (2003b) Minerals Yearbook: Magnesium Annual Report 2002. U.S. Geological Survey, Reston, VA. 

USGS (2002a) Minerals Yearbook: Crushed Stone Annual Report 2001. U.S. Geological Survey, Reston, VA. 

USGS (2002b) Minerals Yearbook: Magnesium Annual Report 2001. U.S. Geological Survey, Reston, VA. 

USGS (2001a) Minerals Yearbook: Crushed Stone Annual Report 2000. U.S. Geological Survey, Reston, VA. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 307 

USGS (2001b) Minerals Yearbook: Magnesium Annual Report 2000. U.S. Geological Survey, Reston, VA. 

USGS (2000a) Minerals Yearbook: Crushed Stone Annual Report 1999. U.S. Geological Survey, Reston, VA. 

USGS (2000b) Minerals Yearbook: Magnesium Annual Report 1999. U.S. Geological Survey, Reston, VA. 

USGS (1999a) Minerals Yearbook: Crushed Stone Annual Report 1998. U.S. Geological Survey, Reston, VA. 

USGS (1999b) Minerals Yearbook: Magnesium Annual Report 1998. U.S. Geological Survey, Reston, VA. 

USGS (1998a) Minerals Yearbook: Crushed Stone Annual Report 1997. U.S. Geological Survey, Reston, VA. 

USGS (1998b) Minerals Yearbook: Magnesium Annual Report 1997. U.S. Geological Survey, Reston, VA. 

USGS (1997a) Minerals Yearbook: Crushed Stone Annual Report 1996. U.S. Geological Survey, Reston, VA. 

USGS (1997b) Minerals Yearbook: Magnesium Annual Report 1996. U.S. Geological Survey, Reston, VA. 

USGS (1996a) Minerals Yearbook: Crushed Stone Annual Report 1995. U.S. Geological Survey, Reston, VA. 

USGS (1996b) Minerals Yearbook: Magnesium Annual Report 1995. U.S. Geological Survey, Reston, VA. 

USGS (1995a) Crushed Stone: Annual Report 1993. U.S. Geological Survey, U.S. Department of the Interior, 

formerly Bureau of Mines. Washington, DC. January. 

USGS (1995b) Crushed Stone: Annual Report 1994. U.S. Geological Survey, U.S. Department of the Interior, 

formerly Bureau of Mines. Washington, DC. 

USGS (1995c) Magnesium: Annual Report 1994. U.S. Geological Survey, U.S. Department of the Interior, 

formerly Bureau of Mines. Washington, DC. 

USGS (1993) Crushed Stone: Annual Report 1991. U.S. Geological Survey, U.S. Department of the Interior, 

formerly Bureau of Mines. Washington, DC. March. 

Soda Ash Manufacture and Consumption 

USGS (2004) Minerals Industry Surveys: Soda Ash 2003. U.S. Geological Survey, Reston, VA. 

USGS (2003) Minerals Yearbook: Soda Ash Annual Report 2002. U.S. Geological Survey, Reston, VA. 

USGS (2002) Minerals Yearbook: Soda Ash Annual Report 2001. U.S. Geological Survey, Reston, VA. 

USGS (2001) Minerals Yearbook: Soda Ash Annual Report 2000. U.S. Geological Survey, Reston, VA. 

USGS (2000) Minerals Yearbook: Soda Ash Annual Report 1999. U.S. Geological Survey, Reston, VA. 

USGS (1999) Minerals Yearbook: Soda Ash Annual Report 1998. U.S. Geological Survey, Reston, VA. 

USGS (1998) Minerals Yearbook: Soda Ash Annual Report 1997. U.S. Geological Survey, Reston, VA. 

USGS (1997) Minerals Yearbook: Soda Ash Annual Report 1996. U.S. Geological Survey, Reston, VA. 

USGS (1996) Minerals Yearbook: Soda Ash Annual Report 1995. U.S. Geological Survey, Reston, VA. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 308 

USGS (1995) Minerals Yearbook: Soda Ash Annual Report 1994. U.S. Geological Survey, Reston, VA. 

USGS (1994) Soda Ash: Annual Report 1993. U.S. Geological Survey, U.S. Department of the Interior, formerly 

Bureau of Mines. Washington, DC. July. 

Titanium Dioxide Production 

Gambogi, J.(2004). Telephone conversation between Rebecca LePrell of ICF Consulting and Joseph Gambogi, 

Commodity Specialist, U.S. Geological Survey, 27 August. 

Gambogi, J.(2002). Telephone conversation between Philip Groth of ICF Consulting and Joseph Gambogi, 

Commodity Specialist, U.S. Geological Survey, 13 November. 

Onder, H, and E.A. Bagdoyan (1993) Everything You’ve Always Wanted to Know about Petroleum Coke. Allis 

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USGS (2003) Mineral Yearbook: Titanium Annual Report 2002. U.S. Geological Survey, Reston, VA. 

USGS (2002) Mineral Yearbook: Titanium Annual Report 2001. U.S. Geological Survey, Reston, VA. 

USGS (2001) Mineral Yearbook: Titanium Annual Report 2000. U.S. Geological Survey, Reston, VA. 

USGS (2000) Mineral Yearbook: Titanium Annual Report 1999. U.S. Geological Survey, Reston, VA. 

USGS (1999) Minerals Yearbook: Titanium Annual Report 1998. U.S. Geological Survey, Reston, VA. 

USGS (1998) Minerals Yearbook: Titanium Annual Report 1997. U.S. Geological Survey, Reston, VA. 

USGS (1997) Minerals Yearbook: Titanium Annual Report 1996. U.S. Geological Survey, Reston, VA. 

USGS (1996) Minerals Yearbook: Titanium Annual Report 1995. U.S. Geological Survey, Reston, VA. 

USGS (1995) Minerals Yearbook: Titanium Annual Report 1994. U.S. Geological Survey, Reston, VA. 

USGS (1994) Minerals Yearbook: Titanium Annual Report 1993. U.S. Geological Survey, Reston, VA. 

USGS (1993) Minerals Yearbook: Titanium Annual Report 1992. U.S. Geological Survey, Reston, VA. 

USGS (1992) Minerals Yearbook: Titanium Annual Report 1991. U.S. Geological Survey, Reston, VA. 

USGS (1991) Minerals Yearbook: Titanium Annual Report 1990. U.S. Geological Survey, Reston, VA. 

Phosphoric Acid 

EFMA (2000) European Fertilizer Manufacturers Association Best Available Techniques for Pollution Prevention 

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FIPR (2003) Florida Institute of Phosphate Research, Analyses of Some Phosphate Rocks, facsimile from Mr. Gary 

Albarelli, FIPR, Bartow, Florida, to Mr. Robert Lanza, ICF Consulting, July 29, 2003 

FIPR (2003a) Florida Institute of Phosphate Research, personal communication of Mr. Michael Lloyd, Laboratory 

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Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 309 

USGS (2004) Minerals Yearbook. Phosphate Rock Annual Report 2003. U.S. Geological Survey, Reston, VA. 

USGS (2003) Electronic mail from Mr. Stephen M Jasinski, USGS Commodity Specialist, Phosphate Rock, 

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USGS (2003a) Electronic mail from Mr. Stephen M Jasinski, USGS Commodity Specialist, Phosphate Rock, 

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USGS (2002) Minerals Yearbook. Phosphate Rock Annual Report 2001. U.S. Geological Survey, Reston, VA. 

USGS (2001) Minerals Yearbook. Phosphate Rock Annual Report 2000. U.S. Geological Survey, Reston, VA. 

USGS (2000) Minerals Yearbook. Phosphate Rock Annual Report 1999. U.S. Geological Survey, Reston, VA. 

USGS (1999) Minerals Yearbook. Phosphate Rock Annual Report 1998. U.S. Geological Survey, Reston, VA. 

USGS (1998) Minerals Yearbook. Phosphate Rock Annual Report 1997. U.S. Geological Survey, Reston, VA. 

USGS (1997) Minerals Yearbook. Phosphate Rock Annual Report 1996. U.S. Geological Survey, Reston, VA. 

USGS (1996) Minerals Yearbook. Phosphate Rock Annual Report 1995. U.S. Geological Survey, Reston, VA. 

USGS (1995) Minerals Yearbook. Phosphate Rock Annual Report 1994. U.S. Geological Survey, Reston, VA. 

Ferroalloy Production 

Corathers, L. (2004) Telephone conversation between Christopher Steuer of ICF Consulting and Lisa Corathers, 

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IPCC/UNEP/OECD/IEA (1997) Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, Paris: 

Intergovernmental Panel on Climate Change, United Nations Environment Programme, Organization for Economic 

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Onder, H, and E.A. Bagdoyan (1993) Everything You’ve Always Wanted to Know about Petroleum Coke. Allis 

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USGS (2003) Minerals Yearbook: Silicon Annual Report 2002. U.S. Geological Survey, Reston, VA 

USGS (2002) Minerals Yearbook: Silicon Annual Report 2001. U.S. Geological Survey, Reston, VA. 

USGS (2001) Minerals Yearbook: Silicon Annual Report 2000. U.S. Geological Survey, Reston, VA. 

USGS (2000) Minerals Yearbook: Silicon Annual Report 1999. U.S. Geological Survey, Reston, VA. 

USGS (1999) Minerals Yearbook: Silicon Annual Report 1998. U.S. Geological Survey, Reston, VA. 

USGS (1998) Minerals Yearbook: Silicon Annual Report 1997. U.S. Geological Survey, Reston, VA. 

USGS (1997) Minerals Yearbook: Silicon Annual Report 1996. U.S. Geological Survey, Reston, VA. 

USGS (1996) Minerals Yearbook: Silicon Annual Report 1995. U.S. Geological Survey, Reston, VA. 

USGS (1995) Minerals Yearbook: Silicon Annual Report 1994. U.S. Geological Survey, Reston, VA. 
 

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USGS (1994) Silicon: Annual Report 1993. U.S. Geological Survey, U.S. Department of the Interior, formerly 

Bureau of Mines. Washington, DC. 

USGS (1993) Silicon: Annual Report 1992. U.S. Geological Survey, U.S. Department of the Interior, formerly 

Bureau of Mines. Washington, DC. 

USGS (1992) Silicon: Annual Report 1991. U.S. Geological Survey, U.S. Department of the Interior, formerly 

Bureau of Mines. Washington, DC. 

USGS (1991) Silicon: Annual Report 1990. U.S. Geological Survey, U.S. Department of the Interior, formerly 

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Carbon Dioxide Consumption 

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Denbury Resources Inc. (2003b) Annual Report, 2002, Page 14. 

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U.S. Census Bureau (2003). Current Industrial Reports Industrial Gases: 2002. 

U.S. Census Bureau (2001). Current Industrial Reports Industrial Gases: 2000. 

U.S. Census Bureau (1999). Current Industrial Reports Industrial Gases: 1998. 

U.S. Census Bureau (1997). Current Industrial Reports Industrial Gases: 1996. 

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Petrochemical Production 

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CMA (1999) U.S. Chemical Industry Statistical Handbook. Chemical Manufacturer’s Association. Washington, 

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EIA (2004) Annual Energy Review 2003. Energy Information Administration, U.S. Department of Energy, 

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European IPPC Bureau (2004) Draft Reference Document on Best Available Techniques in the Large Volumen 

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IPCC/UNEP/OECD/IEA (1997) Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories, Paris: 

Intergovernmental Panel on Climate Change, United Nations Environment Programme, Organization for Economic 

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Johnson, G. L. (2003) Personal communication between Caren Mintz of ICF Consulting and Greg Johnson of 

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Othmer, Kirk (1992) Carbon (Carbon Black). Volume 4. Page 1045. 

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U.S. Census Bureau (1999) 1997 Economic Census: Manufacturing - Industry Series: Carbon Black 

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Silicon Carbide Production 

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USGS (2003) Minerals Yearbook: Manufactured Abrasives Annual Report 2002. U.S. Geological Survey, Reston, 

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USGS (2001) Minerals Yearbook: Manufactured Abrasives Annual Report 2000. U.S. Geological Survey, Reston, 

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USGS (2000) Minerals Yearbook: Manufactured Abrasives Annual Report 1999. U.S. Geological Survey, Reston, 

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USGS (1998) Minerals Yearbook: Manufactured Abrasives Annual Report 1997. U.S. Geological Survey, Reston, 

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USGS (1997) Minerals Yearbook: Manufactured Abrasives Annual Report 1996. U.S. Geological Survey, Reston, 

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USGS (1996) Minerals Yearbook: Manufactured Abrasives Annual Report 1995. U.S. Geological Survey, Reston, 

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USGS (1995) Minerals Yearbook: Manufactured Abrasives Annual Report 1994. U.S. Geological Survey, Reston, 

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USGS (1992) Manufactured Abrasives: Annual Report 1991. U.S. Geological Survey, U.S. Department of the 

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USGS (1991) Manufactured Abrasives: Annual Report 1990. U.S. Geological Survey, U.S. Department of the 

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Nitric Acid Production 

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Adipic Acid Production 

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Substitution of Ozone Depleting Substances 

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Electrical Transmission and Distribution 

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USDA (2002d) Milk Production, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 320 

USDA (2002f) Cattle on Feed, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2001b) Hogs and Pigs, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2001d) Milk Production, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2001e) Sheep and Goats, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2000b) Hogs and Pigs, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 321 

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Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 325 

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USDA (2002a) Cattle, U.S. Department of Agriculture, National Agriculture Statistics Service, Washington, DC. 

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USDA (2002b) Cattle on Feed Cattle, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2002c) Hogs and Pig, U.S. Department of Agriculture, National Agriculture Statistics Service, Washington, 

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USDA (2002f) Sheep and Goats, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2001a) Cattle, U.S. Department of Agriculture, National Agriculture Statistics Service, Washington, DC. 

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USDA (2001b) Cattle on Feed Cattle, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2001c) Hogs and Pig, U.S. Department of Agriculture, National Agriculture Statistics Service, Washington, 

DC. March 30, 2001. Data also available from <http://www.nass.usda.gov:81/ipedb/>. 

USDA (2001d) Chicken and Eggs - 1999 Summary Cattle, U.S. Department of Agriculture, National Agriculture 

Statistics Service, Washington, DC. January. 

USDA (2001e) Poultry Production and Value - 1999 Summary, U.S. Department of Agriculture, National 

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USDA (2001f) Sheep and Goats, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2000b) Cattle on Feed Cattle, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 326 

USDA (2000c) Hogs and Pig, U.S. Department of Agriculture, National Agriculture Statistics Service, Washington, 

DC. March 24, 2000. Data also available from <http://www.nass.usda.gov:81/ipedb/>. 

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USDA (2000f) Sheep and Goats, U.S. Department of Agriculture, National Agriculture Statistics Service, 

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USDA (2000g) Chicken and Eggs - Final Estimates 1988-1993, U.S. Department of Agriculture, National 

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of Agriculture, Animal and Plant Health Inspection Service (APHIS), National Animal Health Monitoring System 

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USDA (1999b) Poultry Production and Value - Final Estimates 1994-97, U.S. Department of Agriculture, National 

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USDA (1998b) Chicken and Eggs - Final Estimates 1994-97, U.S. Department of Agriculture, National Agriculture 

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USDA (1998d) Re-aggregated data from the National Animal Health Monitoring System’s (NAHMS) Swine ’95 

study aggregated by Eric Bush of the U.S. Department of Agriculture, Centers for Epidemiology and Animal 

Health. 
 

Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2003 Page 327 

USDA (1996a) Agricultural Waste Management Field Handbook, National Engineering Handbook (NEH), Part 

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DC. June. 

USDA (1995a) Cattle - Final Estimates 1989-93, U.S. Department of Agriculture, National Agriculture Statistics 

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USDA (1995b) Poultry Production and Value - Final Estimates 1989-1993, U.S. Department of Agriculture, 

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USDA (1994a) Hogs and Pigs - Final Estimates 1988-92, U.S. Department of Agriculture, National Agriculture 

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College of Agriculture and Life Sciences, June 23, 2000. 

Rice Cultivation 

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with Louisiana State University Agriculture Center. May 17, 2000. 

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Gaston Cantens, Vice President of Corporate Relations, Florida Crystals Company. July 30, 2004. 

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Education Centre at the University of Florida. August 15, 2002. 

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Agronomy Specialist, Missouri Cooperative Extension Service. June 23, 2004. 

Guethle, D. (2003) Telephone conversation between Caren Mintz of ICF Consulting and David Guethle, 

Agronomy Specialist, Missouri Cooperative Extension Service. June 19, 2003. 

Guethle, D. (2002) Telephone conversation between Caren Mintz of ICF Consulting and David Guethle, 

Agronomy Specialist, Missouri Cooperative Extension Service. August 19, 2002. 

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Agronomy Specialist at Missouri Cooperative Extension Service. September 4, 2001. 

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Coordinator, Agricultural Economic Development Program, Palm Beach County Cooperative Extension Service, 

FL. June 30, 2004. 

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Coordinator, Agricultural Economic Development Program, Palm Beach County Cooperative Extension Service, 

FL. August 13, 2003. 

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retired Extension Agronomist, Texas A&M University. July 7, 2003. 

Klosterboer, A. (2002) Telephone conversation between Caren Mintz of ICF Consulting and Arlen Klosterboer, 

Extension Agronomist, Texas A&M University. August 19, 2002. 

Klosterboer, A. (2001a) Telephone conversation between Caren Mintz of ICF Consulting and Arlen Klosterboer, 

Extension Agronomist, Texas A & M University. August 6, 2001. 

Klosterboer, A. (2001b) Telephone conversation between Caren Mintz of ICF Consulting and Arlen Klosterboer, 

Extension Agronomist, Texas A & M University. October 8, 2001. 

Klosterboer, A. (2000) Telephone conversation between Payton Deeks of ICF Consulting and Arlen Klosterboer, 

Extension Agronomist, Texas A & M University. May 18, 2000. 

Klosterboer, A. (1999) Telephone conversation between Catherine Leining of ICF Consulting and Arlen 

Klosterboer, Extension Agronomist, Texas A & M University. June 10, 1999. 

Klosterboer, A. (1997) Telephone conversation between Holly Simpkins of ICF Incorporated and Arlen 

Klosterboer, Texas A & M University. December 1, 1997. 

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Agency, Stillwater, OK. July 23, 2004. 

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Danny Lee, OK Farm Service Agency, Stillwater, OK. July 2, 2003. 

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Professor with the Rice Research Station at Louisiana State University Agriculture Center. June 23, 2004. 

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Professor with the Rice Research Station at Louisiana State University Agriculture Center. June 10, 2003. 
 

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Linscombe, S. (2002) Telephone conversation between Caren Mintz of ICF Consulting and Steve Linscombe, 

Professor with the Rice Research Station at Louisiana State University Agriculture Center. August 21, 2002. 

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Research Agronomist, Rice Research Station in Crowley, LA. July 30– August 1, 2001. 

Linscombe, S. (2001b) Email correspondence between Caren Mintz of ICF Consulting and Steve Linscombe, 

Research Agronomist, Rice Research Station in Crowley, LA. October 4, 2001. 

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Linscombe, Research Agronomist, Rice Research Station in Crowley, LA. June 3, 1999. 

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Research Agronomist, Rice Research Station in Crowley, LA. August 9, 1999. 

Mayhew, W. (1997) Telephone conversation between Holly Simpkins of ICF Incorporated and Walter Mayhew, 

University of Arkansas, Little Rock. November 24, 1997. 

Mutters, C. (2004) Telephone conversation between Lauren Flinn of ICF Consulting and Mr. Cass Mutters, Rice 

Farm Advisor for Butte, Glen, and Tehama Counties. University of California Cooperative Extension Service. June 

25, 2004. 

Mutters, C. (2003) Telephone conversation between Caren Mintz of ICF Consulting and Mr. Cass Mutters, Rice 

Farm Advisor for Butte, Glen, and Tehama Counties. University of California Cooperative Extension Service. June 

23, 2003. 

Mutters, C. (2002) Telephone conversation between Caren Mintz of ICF Consulting and Mr. Cass Mutters, Rice 

Farm Advisor for Butte, Glen, and Tehama Counties. University of California Cooperative Extension Service. 

August 27, 2002. 

Mutters, C. (2001) Telephone conversation between Caren Mintz of ICF Consulting and Cass Mutters, Rice Farm 

Advisor for Butte, Glen, and Tehama Counties, University of California Cooperative Extension Service. September 

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Louisiana State University. November 24, 1997. 

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Palm Beach County Agricultural Extension Agent, Florida. July 30, 2001. 

Schueneman, T. (2001b) Telephone conversation between Caren Mintz of ICF Consulting and Tom Schueneman, 

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Palm Beach County Agricultural Extension Agent, Florida. May 16, 2000. 

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Schueneman, Palm Beach County Agricultural Extension Agent, Florida. June 7, 1999. 

Schueneman, T. (1999b) Telephone conversation between Payton Deeks of ICF Consulting and Tom Schueneman, 

Palm Beach County Agricultural Extension Agent, Florida. August 10, 1999. 

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Palm Beach County Agricultural Extension Agent, Florida. August 7, 1999. 

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Schueneman, County Extension Agent, Florida. November 7, 1997. 

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Agronomist - Rice, University of Arkansas Division of Agriculture Cooperative Extension Service. August 23, 

2001. 

Slaton, N. (2001b) Telephone conversation between Caren Mintz of ICF Consulting and Nathan Slaton, Extension 

Agronomist - Rice, University of Arkansas Division of Agriculture Cooperative Extension Service. October 3, 

2001. 

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Joe Street, Rice Specialist, Mississippi State University, Delta Research Center. June 19, 2003. 

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Joe Street, Rice Specialist, Mississippi State University, Delta Research Center. August 19, 2002. 

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