Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2005 

EPA 430-R-07-002, April 2007 

US Environmental Protection Agency 

Office of Atmospheric Programs 

Washington, DC 

Available at: 

http://www.epa.gov/climatechange/emissions/usgginv_archive.html
Executive Summary ES-

Executive Summary

A

n emissions inventory that identifies and quantifies a country’s primary anthropogenic1 sources and sinks of 

greenhouse gases is essential for addressing climate change. This inventory adheres to both (1) a comprehensive 

and detailed set of methodologies for estimating sources and sinks of anthropogenic greenhouse gases, and (2) a 

common and consistent mechanism that enables Parties to the United Nations Framework Convention on Climate Change 

(UNFCCC) to compare the relative contribution of different emission sources and greenhouse gases to climate change. 

In 1992, the United States signed and ratified the UNFCCC. As stated in Article 2 of the UNFCCC, “The ultimate 

objective of this Convention and any related legal instruments that the Conference of the Parties may adopt is to achieve, in 

accordance with the relevant provisions of the Convention, stabilization of greenhouse gas concentrations in the atmosphere 

at a level that would prevent dangerous anthropogenic interference with the climate system. Such a level should be achieved 

within a time-frame sufficient to allow ecosystems to adapt naturally to climate change, to ensure that food production is 

not threatened and to enable economic development to proceed in a sustainable manner.”2

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

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

Montreal Protocol, using comparable methodologies....”3 The United States views this report as an opportunity to fulfill 

these commitments.

This chapter summarizes the latest information on U.S. anthropogenic greenhouse gas emission trends from 1990 through 

2005. To ensure that the U.S. emissions inventory is comparable to those of other UNFCCC Parties, the estimates presented 

here were calculated using methodologies consistent with those recommended in the Revised 1996 IPCC Guidelines for 

National Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997), the IPCC Good Practice Guidance and Uncertainty 

Management in National Greenhouse Gas Inventories (IPCC 2000), and the IPCC Good Practice Guidance for Land 

Use, Land-Use Change, and Forestry (IPCC 2003). Additionally, the U.S. emission inventory has begun to incorporate 

new methodologies and data from the 2006 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC 2006). The 

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

Intergovernmental Panel on Climate Change (IPCC) methodologies were expanded, resulting in a more comprehensive 

and detailed estimate of emissions.

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>.
ES- Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

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 to the UNFCCC are not required to include these 

gases in their national greenhouse gas emission inventories.5

Some other fluorine-containing halogenated substances—

hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and 

sulfur hexafluoride (SF6)—do not deplete stratospheric ozone 

but are potent greenhouse gases. These latter substances are 

addressed by the UNFCCC and accounted for in national 

greenhouse gas emission inventories.

There are also several gases that do not have a direct 

global warming effect but indirectly affect terrestrial and/or 

solar radiation absorption by influencing the formation or 

destruction of greenhouse gases, including tropospheric and 

stratospheric ozone. These gases include carbon monoxide 

(CO), oxides of nitrogen (NOx), and non-CH4 volatile organic 

compounds (NMVOCs). Aerosols, which are extremely 

small particles or liquid droplets, such as those produced by 

sulfur dioxide (SO2) or elemental carbon emissions, can also 

affect the absorptive characteristics of the atmosphere.

Although the direct greenhouse gases CO2, CH4, and 

N2O occur naturally in the atmosphere, human activities 

have changed their atmospheric concentrations. From 

the pre-industrial era (i.e., ending about 1750) to 2004, 

concentrations of these greenhouse gases have increased 

globally by 35, 143, and 18 percent, respectively (IPCC 

2001, Hofmann 2004). 

Beginning in the 1950s, the use of CFCs and other 

stratospheric ozone depleting substances (ODS) increased 

by nearly 10 percent per year until the mid-1980s, when 

international concern about ozone depletion led to the 

entry into force of the Montreal Protocol. Since then, the 

production of ODS is being phased out. In recent years, use 

of ODS substitutes such as HFCs and PFCs has grown as 

they begin to be phased in as replacements for CFCs and 

HCFCs. Accordingly, atmospheric concentrations of these 

substitutes have been growing (IPCC 2001).

Global Warming Potentials

Gases in the atmosphere can contribute to the greenhouse 

effect both directly and indirectly. Direct effects occur when 

Box ES-1: Recalculations of Inventory Estimates

Each year, emission and sink estimates are recalculated and revised for all years in the Inventory of U.S. Greenhouse Gas Emissions and 

Sinks, as attempts are made to improve both the analyses themselves, through the use of better methods or data, and the overall usefulness 

of the report. In this effort, the United States follows the IPCC Good Practice Guidance (IPCC 2000), which states, regarding recalculations 

of the time series, “It is good practice to recalculate historic emissions when methods are changed or refined, when new source categories 

are included in the national inventory, or when errors in the estimates are identified and corrected.” In general, recalculations are made to the 

U.S. greenhouse gas emission estimates either to incorporate new methodologies or, most commonly, to update recent historical data.

In each Inventory report, the results of all methodology changes and historical data updates are presented in the “Recalculations 

and Improvements” chapter; detailed descriptions of each recalculation are contained within each source’s description contained in the 

report, if applicable. In general, when methodological changes have been implemented, the entire time series (in the case of the most 

recent inventory report, 1990 through 2004) 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.

5

 Emissions estimates of CFCs, HCFCs, halons and other ozone-depleting substances are included in the annexes of this report for informational 

purposes.
Executive Summary ES-

the gas itself absorbs radiation. Indirect radiative forcing 

occurs when chemical transformations of the substance 

produce other greenhouse gases, when a gas influences 

the atmospheric lifetimes of other gases, and/or when a 

gas affects atmospheric processes that alter the radiative 

balance of the earth (e.g., affect cloud formation or albedo).6

The IPCC developed the Global Warming Potential (GWP) 

concept to compare the ability of each greenhouse gas to trap 

heat in the atmosphere relative to another gas.

The GWP of a greenhouse gas is defined as the ratio of 

the time-integrated radiative forcing from the instantaneous 

release of 1 kilogram (kg) of a trace substance relative to 

that of 1 kg of a reference gas (IPCC 2001). Direct radiative 

effects occur when the gas itself is a greenhouse gas. The 

reference gas used is CO2, and therefore GWP-weighted 

emissions are measured in teragrams of CO2 equivalent (Tg 

CO2 Eq.).7 All gases in this Executive Summary are presented 

in units of Tg CO2 Eq. 

The UNFCCC reporting guidelines for national 

inventories were updated in 2002,8 but continue to require 

the use of GWPs from the IPCC Second Assessment Report 

(SAR) (IPCC 1996). This requirement ensures that current 

estimates of aggregate greenhouse gas emissions for 1990 

to 2005 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 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 of this report. The GWP values used in this report 

are listed below in Table ES-1.

Global warming potentials are not provided for CO, 

NOx, NMVOCs, SO2, and aerosols because there is no 

agreed-upon method to estimate the contribution of gases that 

are short-lived in the atmosphere, spatially variable, or have 

only indirect effects on radiative forcing (IPCC 1996).

ES.2. Recent Trends in U.S. 

Greenhouse Gas Emissions and 

Sinks 

In 2005, total U.S. greenhouse gas emissions were 

7,260.4 Tg CO2 Eq. Overall, total U.S. emissions have risen 

by 16.3 percent from 1990 to 2005, while the U.S. gross 

domestic product has increased by 55 percent over the same 

period (BEA 2006). Emissions rose from 2004 to 2005, 

increasing by 0.8 percent (56.7 Tg CO2 Eq.). The following 

factors were primary contributors to this increase: (1) strong 

economic growth in 2005, leading to increased demand for 

electricity and (2) an increase in the demand for electricity 

due to warmer summer conditions. These factors were 

moderated by decreasing demand for fuels due to warmer 

winter conditions and higher fuel prices. 

6

 Albedo is a measure of the Earth’s reflectivity, and is defined as the fraction of the total solar radiation incident on a body that is reflected by it.

7

 Carbon constitutes 12/44ths of carbon dioxide by weight.

8

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

Table ES-1: Global Warming Potentials (100-Year Time 

Horizon) Used in This Report

Gas GWP

CO2 1

CH4

* 21

N2O 310

HFC-23 11,700

HFC-32 650

HFC-125 2,800

HFC-134a 1,300

HFC-143a 3,800

HFC-152a 140

HFC-227ea 2,900

HFC-236fa 6,300

HFC-4310mee 1,300

CF4 6,500

C2F6 9,200

C4F10 7,000

C6F14 7,400

SF6 23,900

Source: IPCC (1996)

* The CH4 GWP includes the direct effects and those indirect effects due 

to the production of tropospheric ozone and stratospheric water vapor. 

The indirect effect due to the production of CO2 is not included.
ES- Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

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 2005.

Figure ES-4 illustrates the relative contribution of the 

direct greenhouse gases to total U.S. emissions in 2005. 

The primary greenhouse gas emitted by human activities 

in the United States was CO2, representing approximately 

83.9 percent of total greenhouse gas emissions. The largest 

source of CO2, and of overall greenhouse gas emissions, 

was fossil fuel combustion. CH4 emissions, which have 

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.

Overall, from 1990 to 2005, total emissions of CO2

increased by 1,027.9 Tg CO2 Eq. (20.3 percent), while CH4

and N2O emissions decreased by 69.8 Tg CO2 Eq. (11.5 

percent) and 13.4 Tg CO2 Eq. (2.8 percent), respectively. 

During the same period, aggregate weighted emissions 

of HFCs, PFCs, and SF6 rose by 73.7 Tg CO2 Eq. (82.5 

Figure ES-1

U.S. Greenhouse Gas Emissions by Gas

0

1,000

2,000

3,000

4,000

5,000

6,000

7,000

8,000

HFCs, PFCs, & SF6

Nitrous Oxide

Methane

Carbon Dioxide

Tg CO2 Eq.

1990199119921993199419951996199719981999200020012002200320042005

Figure ES-2

Annual Percent Change in U.S. Greenhouse Gas Emissions

0.8%

4.0%

0.4%

0.9%

0.2%

3.1%

0.5% 0.6%

1.4%

0.8%

1.2%

2.5%

1.6%

-1.7%

-1.0% -0.9%

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

-2.0%

199119921993199419951996199719981999200020012002200320042005

Figure ES-3

Cumulative Change in U.S. Greenhouse Gas 

Emissions Relative to 1990

-100

0

100

200

300

400

500

600

700

800

900

1,000

1,100

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

200220032004

2005

Tg CO2 Eq.

Figure ES-4

2005 Greenhouse Gas Emissions by Gas 

(percents based on Tg CO2 Eq.)

HFCs, PFCs, & SF6

N2O

CH4

CO2 83.9%

7.4%

6.5%

2.2%
Executive Summary ES-5

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

Gas/Source 1990 1995 2000 2001 2002 2003 2004 2005

CO2 5,061.6 5,384.6 5,940.0 5,843.0 5,892.7 5,952.5 6,064.3 6,089.5 

Fossil Fuel Combustion 4,724.1 5,030.0 5,584.9 5,511.7 5,557.2 5,624.5 5,713.0 5,751.2 

Non-Energy Use of Fuels 117.3 133.2 141.0 131.4 135.3 131.3 150.2 142.4 

Cement Manufacture 33.3 36.8 41.2 41.4 42.9 43.1 45.6 45.9 

Iron and Steel Production 84.9 73.3 65.1 57.9 54.6 53.4 51.3 45.2 

Natural Gas Systems 33.7 33.8 29.4 28.8 29.6 28.4 28.2 28.2 

Municipal Solid Waste Combustion 10.9 15.7 17.9 18.3 18.5 19.5 20.1 20.9 

Ammonia Manufacture and Urea Application 19.3 20.5 19.6 16.7 17.8 16.2 16.9 16.3 

Lime Manufacture 11.3 12.8 13.3 12.9 12.3 13.0 13.7 13.7 

Limestone and Dolomite Use 5.5 7.4 6.0 5.7 5.9 4.7 6.7 7.4 

Soda Ash Manufacture and Consumption 4.1 4.3 4.2 4.1 4.1 4.1 4.2 4.2 

Aluminum Production 6.8 5.7 6.1 4.4 4.5 4.5 4.2 4.2 

Petrochemical Production 2.2 2.8 3.0 2.8 2.9 2.8 2.9 2.9 

Titanium Dioxide Production 1.3 1.7 1.9 1.9 2.0 2.0 2.3 1.9 

Ferroalloy Production 2.2 2.0 1.9 1.5 1.3 1.3 1.4 1.4 

Phosphoric Acid Production 1.5 1.5 1.4 1.3 1.3 1.4 1.4 1.4 

Carbon Dioxide Consumption 1.4 1.4 1.4 0.8 1.0 1.3 1.2 1.3 

Zinc Production 0.9 1.0 1.1 1.0 0.9 0.5 0.5 0.5 

Lead Production 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 

Silicon Carbide Production and Consumption 0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.2 

Land Use, Land-Use Change, and Forestry (Sink)a (712.8) (828.8) (756.7) (767.5) (811.9) (811.9) (824.8) (828.5)

International Bunker Fuelsb 113.7 100.6 101.1 97.6 89.1 83.7 97.2 97.2 

Wood Biomass and Ethanol Consumptionb 219.3 236.8 228.3 203.2 204.4 209.6 224.8 206.5 

CH4 609.1 598.7 563.7 547.7 549.7 549.2 540.3 539.3

Landfills 161.0 157.1 131.9 127.6 130.4 134.9 132.1 132.0 

Enteric Fermentation 115.7 120.6 113.5 112.5 112.6 113.0 110.5 112.1 

Natural Gas Systems 124.5 128.1 126.6 125.4 125.0 123.7 119.0 111.1 

Coal Mining 81.9 66.5 55.9 55.5 52.0 52.1 54.5 52.4 

Manure Management 30.9 35.1 38.7 40.1 41.1 40.5 39.7 41.3 

Petroleum Systems 34.4 31.1 27.8 27.4 26.8 25.8 25.4 28.5 

Wastewater Treatment 24.8 25.1 26.4 25.9 25.8 25.6 25.7 25.4 

Forest Land Remaining Forest Land 7.1 4.0 14.0 6.0 10.4 8.1 6.9 11.6 

Stationary Combustion 8.0 7.8 7.4 6.8 6.8 7.0 7.1 6.9 

Rice Cultivation 7.1 7.6 7.5 7.6 6.8 6.9 7.6 6.9 

Abandoned Underground Coal Mines 6.0 8.2 7.3 6.7 6.1 5.9 5.8 5.5 

Mobile Combustion 4.7 4.3 3.5 3.2 3.1 2.9 2.8 2.6 

Petrochemical Production 0.9 1.1 1.2 1.1 1.1 1.1 1.2 1.1 

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

Field Burning of Agricultural Residues 0.7 0.7 0.8 0.8 0.7 0.8 0.9 0.9 

Silicon Carbide Production and Consumption + + + + + + + + 

Ferroalloy Production + + + + + + + + 

International Bunker Fuelsb 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 

N2O 482.0 484.2 499.8 502.5 479.2 459.8 445.2 468.6 

Agricultural Soil Management 366.9 353.4 376.8 389.0 366.1 350.2 338.8 365.1 

Mobile Combustion 43.7 53.7 53.2 49.7 47.1 43.8 41.2 38.0 

Nitric Acid Production 17.8 19.9 19.6 15.9 17.2 16.7 16.0 15.7 

Stationary Combustion 12.3 12.8 14.0 13.5 13.4 13.7 13.9 13.8 

Manure Management 8.6 9.0 9.6 9.8 9.7 9.3 9.4 9.5 

Wastewater Treatment 6.4 6.9 7.6 7.6 7.7 7.8 7.9 8.0 

Adipic Acid Production 15.2 17.2 6.0 4.9 5.9 6.2 5.7 6.0 

Settlements Remaining Settlements 5.1 5.5 5.6 5.5 5.6 5.8 6.0 5.8 

N2O Product Usage 4.3 4.5 4.8 4.8 4.3 4.3 4.3 4.3 

Forest Land Remaining Forest Land 0.8 0.6 1.7 1.0 1.4 1.2 1.1 1.5 

Field Burning of Agricultural Residues 0.4 0.4 0.5 0.5 0.4 0.4 0.5 0.5 

Municipal Solid Waste Combustion 0.5 0.5 0.4 0.4 0.4 0.4 0.4 0.4 

International Bunker Fuelsb 1.0 0.9 0.9 0.9 0.8 0.8 0.9 0.9 

HFCs, PFCs, and SF6 89.3 103.5 143.8 133.8 143.0 142.7 153.9 163.0 

Substitution of Ozone Depleting Substances 0.3 32.2 80.9 88.6 96.9 105.5 114.5 123.3 

HCFC-22 Production 35.0 27.0 29.8 19.8 19.8 12.3 15.6 16.5 

Electrical Transmission and Distribution 27.1 21.8 15.2 15.1 14.3 13.8 13.6 13.2 

Semiconductor Manufacture 2.9 5.0 6.3 4.5 4.4 4.3 4.7 4.3 

Aluminum Production 18.5 11.8 8.6 3.5 5.2 3.8 2.8 3.0 

Magnesium Production and Processing 5.4 5.6 3.0 2.4 2.4 2.9 2.6 2.7 

Total 6,242.0 6,571.0 7,147.2 7,027.0 7,064.6 7,104.2 7,203.7 7,260.4

Net Emissions (Sources and Sinks) 5,529.2 5,742.2 6,390.5 6,259.5 6,252.7 6,292.3 6,378.9 6,431.9 

+ Does not exceed 0.05 Tg CO2 Eq. a

 Parentheses indicate negative values or sequestration. The net CO2 flux total includes both emissions and sequestration, and constitutes a sink in the 

United States. Sinks are only included in net emissions total.

b

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

Note: Totals may not sum due to independent rounding. 
ES-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

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 11.4 percent of total emissions in 2005. 

The following sections describe each gas’ contribution to 

total U.S. greenhouse gas emissions in more detail. 

Carbon Dioxide Emissions

The global carbon cycle is made up of large carbon 

flows and reservoirs. Billions of tons of carbon in the form 

of CO2 are absorbed by oceans and living biomass (i.e., 

sinks) and are emitted to the atmosphere annually through 

natural processes (i.e., sources). When in equilibrium, 

carbon fluxes among these various reservoirs are roughly 

balanced. Since the Industrial Revolution (i.e., about 1750), 

global atmospheric concentrations of CO2 have risen about 

35 percent (IPCC 2001, Hofmann 2004), principally due 

to the combustion of fossil fuels. Within the United States, 

fuel combustion accounted for 94 percent of CO2 emissions 

in 2005. Globally, approximately 27,044 Tg of CO2 were 

added to the atmosphere through the combustion of fossil 

fuels in 2004, of which the United States accounted for about 

22 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).

U.S. anthropogenic sources of CO2 are shown in 

Figure ES-5. As the largest source of U.S. greenhouse gas 

emissions, CO2 from fossil fuel combustion has accounted 

for approximately 77 percent of GWP-weighted emissions 

since 1990, growing slowly from 76 percent of total 

GWP-weighted emissions in 1990 to 79 percent in 2005. 

Emissions of CO2 from fossil fuel combustion increased at 

an average annual rate of 1.3 percent from 1990 to 2005. 

The fundamental factors influencing this trend include (1) 

a generally growing domestic economy over the last 15 

years, and (2) significant overall growth in emissions from 

electricity generation and transportation activities. Between 

1990 and 2005, CO2 emissions from fossil fuel combustion 

increased from 4,724.1 Tg CO2 Eq. to 5,751.2 Tg CO2 Eq.—a 

21.7 percent total increase over the fifteen-year period. From 

2004 to 2005, these emissions increased by 38.2 Tg CO2 Eq. 

(0.7 percent). 

Historically, changes in emissions from fossil fuel 

combustion have been the dominant factor affecting U.S. 

emission trends. Changes in CO2 emissions from fossil fuel 

combustion are influenced by many long-term and short-term 

factors, including population and economic growth, energy 

price fluctuations, technological changes, and seasonal 

temperatures. On an annual basis, the overall consumption 

of fossil fuels in the United States generally fluctuates in 

response to changes in general economic conditions, energy 

prices, weather, and the availability of non-fossil alternatives. 

For example, in a year with increased consumption of 

goods and services, low fuel prices, severe summer and 

winter weather conditions, nuclear plant closures, and lower 

precipitation feeding hydroelectric dams, there would likely 

be proportionally greater fossil fuel consumption than a 

year with poor economic performance, high fuel prices, 

mild temperatures, and increased output from nuclear and 

hydroelectric plants.

The four major end-use sectors contributing to CO2

emissions from fossil fuel combustion are industrial, 

transportation, residential, and commercial. Electricity 

9

 Global CO2 emissions from fossil fuel combustion were taken from Energy Information Administration International Energy Annual 2004

(EIA 2006a).

Figure ES-5

2005 Sources of CO2

0 25 50 75 100 125 150 175

Tg CO2 Eq.

C02 as a Portion

of all Emissions

Silicon Carbide Production and Consumption

Lead Production

Zinc Production

Carbon Dioxide Consumption

Phosphoric Acid Production

Ferroalloy Production

Titanium Dioxide Production

Petrochemical Production

Aluminum Production

Soda Ash Manufacture and Consumption

Limestone and Dolomite Use

Lime Manufacture

Ammonia Manufacture and Urea Application

Municipal Solid Waste Combustion

Natural Gas Systems

Iron and Steel Production

Cement Manufacture

Non-Energy Use of Fuels

Fossil Fuel Combustion

5,751.2

83.9%

<0.5

<0.5

<0.5
Executive Summary ES-7

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. Emissions from 

electricity generation are also addressed separately after the 

end-use sectors have been discussed. 

Note that emissions from U.S. territories are calculated 

separately due to a lack of specific consumption data for the 

individual end-use sectors. 

Figure ES-6, Figure ES-7, and Table ES-3 summarize 

CO2 emissions from fossil fuel combustion by end-use 

sector.

Transportation End-Use Sector. Transportation activities 

(excluding international bunker fuels) accounted for 33 

percent of CO2 emissions from fossil fuel combustion in 

2005.10 Virtually all of the energy consumed in this end-use 

sector came from petroleum products. Over 60 percent of the 

emissions resulted from gasoline consumption for personal 

vehicle use. The remaining emissions came from other 

transportation activities, including the combustion of diesel 

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

Industrial End-Use Sector. Industrial CO2 emissions, 

resulting both directly from the combustion of fossil fuels and 

indirectly from the generation of electricity that is consumed 

by industry, accounted for 27 percent of CO2 from fossil fuel 

combustion in 2005. 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 2005. Both sectors relied heavily 

on electricity for meeting energy demands, with 70 and 

78 percent, respectively, of their emissions attributable to 

electricity consumption for lighting, heating, cooling, and 

operating appliances. The remaining emissions were due to 

the consumption of natural gas and petroleum for heating 

and cooking.

Electricity Generation. The United States relies on 

electricity to meet a significant portion of its energy demands, 

especially for lighting, electric motors, heating, and air 

conditioning. Electricity generators consumed 36 percent of 

U.S. energy from fossil fuels and emitted 41 percent of the 

10 If emissions from international bunker fuels are included, the transportation end-use sector accounted for 35 percent of U.S. emissions from fossil fuel 

combustion in 2005.

Figure ES-6

2005 CO2 Emissions from Fossil Fuel 

Combustion by Sector and Fuel Type

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

electricity generation.

0

500

1,000

1,500

2,000

2,500 Natural Gas

Petroleum

Coal

Relative Contribution

by Fuel Type

Tg CO2 Eq.

U.S. 

Territories

Residential Commercial Industrial Transportation Electricity

Generation

Figure ES-7

2005 End-Use Sector Emissions of CO2 from 

Fossil Fuel Combustion

0

500

1,000

1,500

2,000 From Electricity

Consumption

From Direct Fossil

Fuel Combustion

Tg CO2 Eq.

Transportation U.S.

Territories

Residential Commercial Industrial
ES-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

CO2 from fossil fuel combustion in 2005. 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 nonfossil 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 2005. Consequently, changes in electricity demand have 

a significant impact on coal consumption and associated 

CO2 emissions.

Other significant CO2 trends included the following: 

• CO2 emissions from non-energy use of fossil fuels have 

increased 25.1 Tg CO2 Eq. (21 percent) from 1990 

through 2005. Emissions from non-energy uses of fossil 

fuels were 142.4 Tg CO2 Eq. in 2005, which constituted 

2.5 percent of overall fossil fuel CO2 emissions and 2.3 

percent of total national CO2 emissions, approximately 

the same proportion as in 1990. 

• CO2 emissions from cement production increased to 45.9 

Tg CO2 Eq. in 2005, a 38 percent increase in emissions 

since 1990. Emissions mirror growth in the construction 

industry. In contrast to many other manufacturing 

sectors, demand for domestic cement remains strong 

because it is not cost-effective to transport cement far 

from its point of manufacture.

• CO2 emissions from iron and steel production decreased 

to 45.2 Tg CO2 Eq. in 2005, and have declined by 39.6 

Tg CO2 Eq. (47 percent) from 1990 through 2005, 

due to restructuring of the industry, technological 

improvements, and increased scrap utilization.

• CO2 emissions from municipal solid waste combustion 

(20.9 Tg CO2 Eq. in 2005) increased by 10.0 Tg CO2

Eq. (91 percent) from 1990 through 2005, as the volume 

of plastics and other fossil carbon-containing materials 

in municipal solid waste grew.

• Net CO2 sequestration from Land Use, Land-Use 

Change, and Forestry increased by 115.7 Tg CO2 Eq. 

(16 percent) from 1990 through 2005. This increase was 

primarily due to an increase in the rate of net carbon 

accumulation in forest carbon stocks, particularly in 

aboveground and belowground tree biomass. Annual 

carbon accumulation in landfilled yard trimmings and 

food scraps slowed over this period, while the rate of 

carbon accumulation in urban trees increased.

Methane Emissions

According to the IPCC, CH4 is more than 20 times as 

effective as CO2 at trapping heat in the atmosphere. Over the 

last two hundred and fifty years, the concentration of CH4

in the atmosphere increased by 143 percent (IPCC 2001, 

Hofmann 2004). Anthropogenic sources of CH4 include 

landfills, natural gas and petroleum systems, agricultural 

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

End-Use Sector 1990 1995 2000 2001 2002 2003 2004 2005

Transportation 1,467.0 1,593.3 1,787.8 1,761.5 1,815.7 1,814.8 1,868.9 1,897.9

Combustion 1,464.0 1,590.2 1,784.4 1,758.2 1,812.3 1,810.5 1,864.5 1,892.8

Electricity 3.0 3.0 3.4 3.3 3.4 4.3 4.4 5.2

Industrial 1,539.8 1,595.8 1,660.1 1,596.6 1,575.5 1,595.1 1,615.2 1,575.2

Combustion 857.1 882.7 875.0 869.9 857.7 858.3 875.6 840.1

Electricity 682.7 713.1 785.1 726.7 717.8 736.8 739.6 735.1

Residential 929.9 995.4 1,131.5 1,124.8 1,147.9 1,179.1 1,175.9 1,208.7

Combustion 340.3 356.4 373.5 363.9 362.4 383.8 369.9 358.7

Electricity 589.6 639.0 758.0 760.9 785.5 795.3 806.0 849.9

Commercial 759.2 810.6 969.3 979.7 973.8 984.2 999.1 1,016.8

Combustion 224.3 226.4 232.3 225.1 225.7 236.6 233.3 225.8

Electricity 534.9 584.2 736.9 754.6 748.0 747.6 765.8 791.0

U.S. Territories 28.3 35.0 36.2 49.0 44.3 51.3 54.0 52.5

Total 4,724.1 5,030.0 5,584.9 5,511.7 5,557.2 5,624.5 5,713.0 5,751.2

Electricity Generation 1,810.2 1,939.3 2,283.5 2,245.5 2,254.7 2,284.0 2,315.8 2,381.2

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

electricity consumption by each end-use sector.
Executive Summary ES-9

activities, coal mining, wastewater treatment, stationary 

and mobile combustion, and certain industrial processes 

(see Figure ES-8).

Some significant trends in U.S. emissions of CH4 include 

the following: 

• Landfills are the largest anthropogenic source of CH4

emissions in the United States. In 2005, landfill CH4

emissions were 132.0 Tg CO2 Eq. (approximately 24 

percent of total CH4 emissions), which represents a 

decline of 29.0 Tg CO2 Eq., or 18 percent, since 1990. 

Although the amount of solid waste landfilled each year 

continues to grow, the amount of CH4 captured and 

burned at landfills has increased dramatically, countering 

this trend.11

• In 2005, CH4 emissions from coal mining were 52.4 Tg 

CO2 Eq. This decline of 29.5 Tg CO2 Eq. (36 percent) 

from 1990 results from the mining of less gassy coal 

from underground mines and the increased use of CH4

collected from degasification systems.

• CH4 emissions from natural gas systems were 111.1 

Tg CO2 Eq. in 2005; emissions have declined by 13.3 

Tg CO2 Eq. (11 percent) since 1990. This decline 

has been due to improvements in technology and 

management practices, as well as some replacement of 

old equipment.

• CH4 emissions from manure management were 41.3 

Tg CO2 Eq. in 2005. From 1990 to 2005, emissions 

from this source increased by 10.4 Tg CO2 Eq. (34 

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 toward 

larger facilities. Larger swine and dairy farms tend to use 

liquid management systems, where the decomposition 

of materials in the manure tends to produce CH4.

Nitrous Oxide Emissions

N2O is produced by biological processes that occur in 

soil and water and by a variety of anthropogenic activities 

in the agricultural, energy-related, industrial, and waste 

management fields. While total N2O emissions are much 

lower than CO2 emissions, N2O is approximately 300 times 

more powerful than CO2 at trapping heat in the atmosphere. 

Since 1750, the global atmospheric concentration of N2O has 

risen by approximately 18 percent (IPCC 2001, Hofmann 

2004). The main anthropogenic activities producing N2O

in the United States are agricultural soil management, fuel 

combustion in motor vehicles, manure management, nitric 

acid production, wastewater treatment, and stationary fuel 

combustion (see Figure ES-9).

Some significant trends in U.S. emissions of N2O include 

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 78 percent (365.1 Tg CO2 Eq.) of 2005 emissions. 

N2O emissions from this source have not shown any 

significant long-term trend, as they are highly sensitive 

to the amount of N applied to soils, which has not 

changed significantly over the time-period.

• In 2005, N2O emissions from mobile combustion were 

38.0 Tg CO2 Eq. (approximately 8 percent of U.S. N2O 

emissions). From 1990 to 2005, N2O emissions from 

mobile combustion decreased by 13 percent. However, 

from 1990 to 1998 emissions increased by 10 percent, 

due to control technologies that reduced NOx emissions 

while increasing N2O emissions. Since 1998, newer 

11 The CO2 produced from combusted landfill CH4 is not counted in national inventories as it is considered part of the natural C cycle of decomposition.

Figure ES-8

2005 Sources of CH4

CH4 as a Portion

of all Emissions

0 20 40 60 80 100 120 140

<0.05

<0.05

Tg CO2 Eq.

Ferroalloy Production

Silicon Carbide Production and Consumption

Field Burning of Agricultural Residues

Iron and Steel Production

Petrochemical Production

Mobile Combustion

Abandoned Underground Coal Mines

Rice Cultivation

Stationary Combustion

Forest Land Remaining Forest Land

Wastewater Treatment

Petroleum Systems

Manure Management

Coal Mining

Natural Gas Systems

Enteric Fermentation

Landfills

7.4%
ES-0 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

control technologies have led to a steady decline in N2O 

from this source.

HFC, PFC, and SF6 Emissions

HFCs and PFCs are families of synthetic chemicals 

that are used as alternatives to the ODSs, which are being 

phased out under the Montreal Protocol and Clean Air 

Act Amendments of 1990. HFCs and PFCs do not deplete 

the stratospheric ozone layer, and are therefore acceptable 

alternatives under the Montreal Protocol.

These compounds, however, along with SF6, are 

potent greenhouse gases. In addition to having high 

global warming potentials, SF6 and PFCs have extremely 

long atmospheric lifetimes, resulting in their essentially 

irreversible accumulation in the atmosphere once emitted. 

Sulfur hexafluoride is the most potent greenhouse gas the 

IPCC has evaluated.

Other emissive sources of these gases include HCFC-22 

production, electrical transmission and distribution systems, 

semiconductor manufacturing, aluminum production, and 

magnesium production and processing (see Figure ES-10).

Some significant trends in U.S. HFC, PFC, and SF6

emissions include the following:

• Emissions resulting from the substitution of ozone 

depleting substances (e.g., CFCs) have been increasing 

from small amounts in 1990 to 123.3 Tg CO2 Eq. in 

2005. Emissions from substitutes for ozone depleting 

substances are both the largest and the fastest 

growing source of HFC, PFC, and SF6 emissions. 

Figure ES-10

2005 Sources of HFCs, PFCs, and SF6

Tg CO2 Eq.

HFCs, PFCs, and

SF6 as a Portion of

all Emissions

0 25 50 75 100 125

Magnesium Production

and Processing

Aluminum Production

Semiconductor

Manufacture

Electrical Transmission

and Distribution

HCFC-22 Production

Substitution of Ozone

Depleting Substances

2.2%

Figure ES-9

2005 Sources of N2O

0 10 20 30 40 50

N20 as a Portion

of all Emissions

Field Burning of Agricultural Residues

Municipal Solid Waste Combustion

Forest Land Remaining Forest Land

N2O Product Usage

Settlements Remaining Settlements

Adipic Acid Production

Wastewater Treatment

Manure Management

Stationary Combustion

Nitric Acid Production

Mobile Combustion

Agricultural Soil Management

6.5%

365.1

Tg CO2 Eq.

These emissions have been increasing as phase-outs 

required under the Montreal Protocol come into effect, 

especially after 1994 when full market penetration 

was made for the first generation of new technologies 

featuring ODS substitutes.

• The increase in ODS substitute emissions is offset 

substantially by decreases in emission of HFCs, 

PFCs, and SF6 from other sources. Emissions from 

aluminum production decreased by 84 percent (15.6 

Tg CO2 Eq.) from 1990 to 2005, due to both industry 

emission reduction efforts and lower domestic aluminum 

production. 

• Emissions from the production of HCFC-22 decreased 

by 53 percent (18.4 Tg CO2 Eq.) from 1990 through 

2005, 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 51 percent (13.9 

Tg CO2 Eq.) from 1990 to 2005, 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 
Executive Summary ES-

Reporting and Review (UNFCCC 2003), the Inventory of U.S. 

Greenhouse Gas Emissions and Sinks report is segregated 

into six sector-specific chapters. Figure ES-11 and Table ES-4 

aggregate emissions and sinks by these chapters.

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 2005. In 2005, approximately 

86 percent of the energy consumed in the United States (on 

a Btu basis) 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 (38 percent and 

11 percent of total U.S. emissions of each gas, respectively). 

Overall, emission sources in the Energy chapter account 

for a combined 85 percent of total U.S. greenhouse gas 

emissions in 2005.

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 

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

Chapter/IPCC Sector 1990 1995 2000 2001 2002 2003 2004 2005

Energy 5,202.2 5,525.8 6,069.2 5,978.9 6,021.4 6,079.1 6,181.7 6,201.9 

Industrial Processes 300.1 314.8 338.7 309.6 320.2 316.4 330.6 333.6

Solvent and Other Product Use 4.3 4.5 4.8 4.8 4.3 4.3 4.3 4.3 

Agriculture 530.3 526.8 547.4 560.3 537.4 521.1 507.4 536.3

Land Use, Land-Use Change,

and Forestry (Non-CO2 Emissions) 13.0 10.1 21.3 12.4 17.4 15.0 13.9 18.9

Waste 192.2 189.1 165.9 161.1 163.9 168.4 165.7 165.4

Total 6,242.0 6,571.0 7,147.2 7,027.0 7,064.6 7,104.2 7,203.7 7,260.4

Net CO2 Flux from Land Use, 

Land-Use Change, and Forestry* (712.8) (828.8) (756.7) (767.5) (811.9) (811.9) (824.8) (828.5)

Net Emissions (Sources and Sinks) 5,529.2 5,742.2 6,390.5 6,259.5 6,252.7 6,292.3 6,378.9 6,431.9

* The net CO2 flux total includes both emissions and sequestration, and constitutes a sink in the United States. Sinks are only included in net emissions total. 

Note: Totals may not sum due to independent rounding. Parentheses indicate negative values or sequestration.

Figure ES-12

2005 U.S. Energy Consumption by Energy Source

Petroleum

Coal

Natural Gas

Nuclear

Renewable

40%

23%

23%

8%

6%

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

Land Use, Land-Use Change, and Forestry (net CO2 flux)

Waste Industrial Processes LULUCF (non-CO2)

Tg CO2 Eq.

1990199119921993199419951996199719981999200020012002200320042005

Agriculture

Energy

Note: Relatively smaller amounts of GWP-weighted emissions are also emitted from the Solvent and 

Other Product Use sector.
ES- Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

include iron and steel production, lead and zinc 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 accounted for 4.6 percent of U.S. greenhouse gas 

emissions in 2005.

Solvent and Other Product Use

The Solvent and Other Product Use chapter contains 

greenhouse gas emissions that are produced as a by-product 

of various solvent and other product uses. In the United 

States, emissions from N2O 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 2005. 

Agriculture

The Agriculture chapter contains anthropogenic 

emissions from agricultural activities (except fuel combustion, 

which is addressed in the Energy chapter, and agricultural 

CO2 fluxes, which are addressed in the Land Use, LandUse Change, and Forestry Chapter). Agricultural activities 

contribute directly to emissions of greenhouse gases through 

a variety of processes, including the following source 

categories: enteric fermentation in domestic livestock, 

livestock manure management, rice cultivation, agricultural 

soil management, and field burning of agricultural residues. 

CH4 and N2O were the primary greenhouse gases emitted 

by agricultural activities. CH4 emissions from enteric 

fermentation and manure management represented about 

21 percent and 8 percent of total CH4 emissions from 

anthropogenic activities, respectively, in 2005. Agricultural 

soil management activities such as fertilizer application 

and other cropping practices were the largest source of U.S. 

N2O emissions in 2005, accounting for 78 percent. In 2005, 

emission sources accounted for in the Agriculture chapter 

were responsible for 7.4 percent of total U.S. greenhouse 

gas emissions.

Land Use, Land-Use Change, and Forestry 

The Land Use, Land-Use Change, and Forestry chapter 

contains emissions of CH4 and N2O, and emissions and 

removals of CO2 from forest management, other land-use 

activities, and land-use change. Forest management practices, 

tree planting in urban areas, the management of agricultural 

soils, and the landfilling of yard trimmings and food scraps 

have resulted in a net uptake (sequestration) of C in the United 

States. Forests (including vegetation, soils, and harvested 

wood) accounted for approximately 84 percent of total 

2005 sequestration, urban trees accounted for 11 percent, 

agricultural soils (including mineral and organic soils and the 

application of lime) accounted for 2 percent, and landfilled 

yard trimmings and food scraps accounted for 1 percent of 

the total sequestration in 2005. 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 C sink that is almost two times larger 

than the sum of emissions from organic soils and liming. The 

mineral soil C sequestration is largely due to the conversion 

of cropland to permanent pastures and hay production, a 

reduction in summer fallow areas in semi-arid areas, an 

increase in the adoption of conservation tillage practices, 

and an increase in the amounts of organic fertilizers (i.e., 

manure and sewage sludge) applied to agriculture lands. The 

landfilled yard trimmings and food scraps net sequestration is 

due to the long-term accumulation of yard trimming carbon 

and food scraps in landfills. Land use, land-use change, and 

forestry activities in 2005 resulted in a net C sequestration of 

828.5 Tg CO2 Eq. (Table ES-5). This represents an offset of 

approximately 13.6 percent of total U.S. CO2 emissions, or 

11.4 percent of total greenhouse gas emissions in 2005. Total 

land use, land-use change, and forestry net C sequestration 

increased by approximately 16 percent between 1990 and 

2005, primarily due to an increase in the rate of net C 

accumulation in forest C stocks, particularly in aboveground 

and belowground tree biomass. Annual C accumulation in 

landfilled yard trimmings and food scraps slowed over this 
Executive Summary ES-

period, while the rate of annual C accumulation increased 

in urban trees. Net U.S. emissions (all sources and sinks) 

increased by 16.4 percent from 1990 to 2005. 

Non-CO2 emissions from Land Use, Land-Use Change, 

and Forestry are shown in Table ES-6. The application of 

synthetic fertilizers to forest and settlement soils in 2005 

resulted in direct N2O emissions of 6.2 Tg CO2 Eq. Direct 

N2O emissions from fertilizer application increased by 

approximately 19 percent between 1990 and 2005. NonCO2 emissions from forest fires in 2005 resulted in CH4

emissions of 11.6 Tg CO2 Eq., and in N2O emissions of 1.2 

Tg CO2 Eq.

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.12 Additionally, 

wastewater treatment accounts for just under 5 percent of 

U.S. CH4 emissions. N2O emissions from the discharge of 

wastewater treatment effluents into aquatic environments 

were estimated, as were N2O emissions from the treatment 

process itself. Overall, in 2005, emission sources accounted 

for in the Waste chapter generated 2.3 percent of total U.S. 

greenhouse gas emissions.

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

Category 1990 1995 2000 2001 2002 2003 2004 2005

Forest Land Remaining Forest Land (598.5) (717.5) (638.7) (645.7) (688.1) (687.0) (697.3) (698.7)

Changes in Forest Carbon Stocks (598.5) (717.5) (638.7) (645.7) (688.1) (687.0) (697.3) (698.7)

Cropland Remaining Cropland (28.1) (37.4) (36.5) (38.0) (37.8) (38.3) (39.4) (39.4)

Changes in Agricultural Soil Carbon 

Stocks and Liming Emissions (28.1) (37.4) (36.5) (38.0) (37.8) (38.3) (39.4) (39.4)

Land Converted to Cropland 8.7 7.2 7.2 7.2 7.2 7.2 7.2 7.2

Changes in Agricultural Soil 

Carbon Stocks 8.7 7.2 7.2 7.2 7.2 7.2 7.2 7.2 

Grassland Remaining Grassland 0.1 16.4 16.3 16.2 16.2 16.2 16.1 16.1

Changes in Agricultural Soil 

Carbon Stocks 0.1 16.4 16.3 16.2 16.2 16.2 16.1 16.1 

Land Converted to Grassland (14.6) (16.3) (16.3) (16.3) (16.3) (16.3) (16.3) (16.3)

Changes in Agricultural Soil 

Carbon Stocks (14.6) (16.3) (16.3) (16.3) (16.3) (16.3) (16.3) (16.3)

Settlements Remaining Settlements (57.5) (67.8) (78.2) (80.2) (82.3) (84.4) (86.4) (88.5)

Urban Trees (57.5) (67.8) (78.2) (80.2) (82.3) (84.4) (86.4) (88.5)

Other (22.8) (13.3) (10.5) (10.6) (10.8) (9.3) (8.7) (8.8)

Landfilled Yard Trimmings and 

Food Scraps (22.8) (13.3) (10.5) (10.6) (10.8) (9.3) (8.7) (8.8)

Total (712.8) (828.8) (756.7) (767.5) (811.9) (811.9) (824.8) (828.5)

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

Category 1990 1995 2000 2001 2002 2003 2004 2005

Forest Land Remaining Forest Land 7.8 4.5 15.7 6.9 11.8 9.2 8.0 13.1

CH4 Emissions from Forest Fires 7.1 4.0 14.0 6.0 10.4 8.1 6.9 11.6

N2O Emissions from Forest Fires 0.7 0.4 1.4 0.6 1.1 0.8 0.7 1.2

N2O Emissions from Soils 0.1 0.2 0.3 0.3 0.3 0.3 0.3 0.3

Settlements Remaining Settlements 5.1 5.5 5.6 5.5 5.6 5.8 6.0 5.8

N2O Emissions from Soils 5.1 5.5 5.6 5.5 5.6 5.8 6.0 5.8

Total 13.0 10.1 21.3 12.4 17.4 15.0 13.9 18.9 

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

Table ES-6: Non-CO2 Emissions from Land Use, Land-Use Change, and Forestry (Tg CO2 Eq.)

12 Landfills also store carbon, due to incomplete degradation of organic materials such as wood products and yard trimmings, as described in the LandUse, Land-Use Change, and Forestry chapter.
ES- Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

ES.4. Other Information

Emissions by Economic Sector

Throughout this report, emission estimates are grouped 

into six sectors (i.e., chapters) defined by the IPCC: Energy, 

Industrial Processes, Solvent Use, Agriculture, Land Use, 

Land-Use Change, and Forestry, and Waste. While it is 

important to use this characterization for consistency with 

UNFCCC reporting guidelines, it is also useful to allocate 

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-7 summarizes emissions from each of these 

sectors, and Figure ES-13 shows the trend in emissions by 

sector from 1990 to 2005.

Using this categorization, emissions from electricity 

generation accounted for the largest portion (33 percent) 

of U.S. greenhouse gas emissions in 2005. Transportation 

activities, in aggregate, accounted for the second largest 

portion (28 percent). Emissions from industry accounted 

for 19 percent of U.S. greenhouse gas emissions in 2005. 

In contrast to electricity generation and transportation, 

emissions from industry have in general declined over the 

past decade. The long-term decline in these emissions has 

been due to structural changes in the U.S. economy (i.e., shifts 

from a manufacturing-based to a service-based economy), 

fuel switching, and energy efficiency improvements. The 

remaining 20 percent of U.S. greenhouse gas emissions 

were contributed by the residential, agriculture, and 

Table ES-7: U.S. Greenhouse Gas Emissions Allocated to Economic Sectors (Tg CO2 Eq.)

Implied Sectors 1990 1995 2000 2001 2002 2003 2004 2005

Electric Power Industry 1,859.7 1,989.5 2,329.9 2,292.0 2,300.7 2,330.2 2,363.4 2,429.8

Transportation 1,523.0 1,677.2 1,903.2 1,876.4 1,931.2 1,928.2 1,982.6 2,008.9

Industry 1,470.9 1,478.4 1,443.3 1,395.4 1,380.0 1,371.8 1,403.3 1,352.8

Agriculture 585.3 589.2 614.4 618.4 602.6 575.7 567.0 595.4

Commercial 417.8 420.5 415.5 406.6 413.7 433.5 432.6 431.4

Residential 351.3 375.1 393.6 383.6 382.7 404.8 391.6 380.7

U.S. Territories 34.1 41.1 47.3 54.5 53.6 60.0 63.2 61.5

Total Emissions 6,242.0 6,571.0 7,147.2 7,027.0 7,064.6 7,104.2 7,203.7 7,260.4

Land Use, Land-Use Change, and 

Forestry (Sinks) (712.8) (828.8) (756.7) (767.5) (811.9) (811.9) (824.8) (828.5)

Net Emissions (Sources and Sinks) 5,529.2 5,742.2 6,390.5 6,259.5 6,252.7 6,292.3 6,378.9 6,431.9

Note: Totals may not sum due to independent rounding. Emissions include CO2, CH4, N2O, HFCs, PFCs, and SF6.

See Table 2-14 for more detailed data.

Figure ES-13

Emissions Allocated to Economic Sectors

0

500

1,000

1,500

2,000

2,500

3,000

1990199119921993199419951996199719981999200020012002200320042005

Residential

Agriculture

Electricity Generation

Tg CO2 Eq.

Industry

Transportation

Commercial

Note: Does not include U.S. territories.

commercial sectors, plus emissions from U.S. territories. 

The residential sector accounted for about 5 percent, and 

primarily consisted of CO2 emissions from fossil fuel 

combustion. Activities related to agriculture accounted for 

roughly 8 percent of U.S. emissions; unlike other economic 

sectors, agricultural sector emissions were dominated by 

N2O emissions from agricultural soil management and CH4

emissions from enteric fermentation, rather than CO2 from 

fossil fuel combustion. The commercial sector accounted 

for about 6 percent of emissions, while U.S. territories 

accounted for 1 percent.

CO2 was also emitted and sequestered by a variety 

of activities related to forest management practices, tree 
Executive Summary ES-5

planting in urban areas, the management of agricultural soils, 

and landfilling of yard trimmings. 

Electricity is ultimately consumed in the economic 

sectors described above. Table ES-8 presents greenhouse 

gas emissions from economic sectors with emissions related 

to electricity generation distributed into end-use categories 

(i.e., emissions from electricity generation are allocated to 

the economic sectors in which the electricity is consumed). 

To distribute electricity emissions among end-use sectors, 

emissions from the source categories assigned to electricity 

generation were allocated to the residential, commercial, 

industry, transportation, and agriculture economic sectors 

according to retail sales of electricity.13 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 (28 percent) in 2005. Emissions 

from the residential and commercial sectors also increase 

substantially when emissions from electricity are included, 

due to their relatively large share of electricity consumption 

(e.g., lighting, appliances, etc.). Transportation activities 

remain the second largest contributor to total U.S. emissions 

(28 percent). In all sectors except agriculture, CO2 accounts 

for more than 80 percent of greenhouse gas emissions, 

primarily from the combustion of fossil fuels. Figure ES14 shows the trend in these emissions by sector from 1990 

to 2005.

Indirect Greenhouse Gases (CO, NOx, 

NMVOCs, and SO2)

The reporting requirements of the UNFCCC14 request 

that information be provided on indirect greenhouse gases, 

which include CO, NOx, NMVOCs, and SO2. These gases do 

not have a direct global warming effect, but indirectly affect 

13 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.

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

Table ES-8: U.S Greenhouse Gas Emissions by Economic Sector with Electricity-Related Emissions Distributed 

(Tg CO2 Eq.)

Implied Sectors 1990 1995 2000 2001 2002 2003 2004 2005

Industry 2,111.1 2,141.5 2,185.0 2,067.1 2,046.6 2,061.4 2,090.1 2,039.3

Transportation 1,526.1 1,680.3 1,906.7 1,879.8 1,934.7 1,932.5 1,987.1 2,014.2

Commercial 967.2 1,019.8 1,167.4 1,176.8 1,177.0 1,196.2 1,214.1 1,238.5

Residential 956.9 1,030.6 1,167.0 1,160.3 1,184.3 1,216.2 1,214.2 1,248.0

Agriculture 646.5 657.6 673.9 688.5 668.4 637.9 635.0 659.1

U.S. Territories 34.1 41.1 47.3 54.5 53.6 60.0 63.2 61.5

Total Emissions 6,242.0 6,571.0 7,147.2 7,027.0 7,064.6 7,104.2 7,203.7 7,260.4

Land Use, Land-Use Change, and 

Forestry (Sinks) (712.8) (828.8) (756.7) (767.5) (811.9) (811.9) (824.8) (828.5)

Net Emissions (Sources and Sinks) 5,529.2 5,742.2 6,390.5 6,259.5 6,252.7 6,292.3 6,378.9 6,431.9

See Table 2-16 of this report for more detailed data.

Figure ES-14

Emissions with Electricity Distributed 

to Economic Sectors

Note: Does not include U.S. territories.

Agriculture

Residential

Commercial

Transportation

Industrial

0

500

1,000

1,500

2,000

2,500

Tg CO2 Eq.

1990199119921993199419951996199719981999200020012002200320042005
ES-6 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

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 2005; (4) emissions per unit of total gross domestic product as a measure of national economic activity; or 

(5) emissions per capita. 

Table ES-9 provides data on various statistics related to U.S. greenhouse gas emissions normalized to 1990 as a baseline year. Greenhouse 

gas emissions in the United States have grown at an average annual rate of 1.0 percent since 1990. This rate is slightly slower than that for 

total energy or fossil fuel consumption and much slower than that for either electricity consumption or overall gross domestic product. Total 

U.S. greenhouse gas emissions have also grown slightly slower than national population since 1990 (see Figure ES-15). 

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

Table ES-9: Recent Trends in Various U.S. Data (Index 1990 = 100) 

Variable 1990 1995 2000 2001 2002 2003 2004 2005

Growth 

Ratea

GDPb 100 113 138 139 141 145 150 155 3.0%

Electricity Consumptionc 100 112 127 125 128 129 131 134 2.0%

Fossil Fuel Consumptionc 100 107 117 115 116 118 119 119 1.2%

Energy Consumptionc 100 108 117 114 116 117 119 118 1.1%

Populationd 100 107 113 114 115 116 117 118 1.1%

Greenhouse Gas Emissionse 100 105 115 113 113 114 115 116 1.0%

a

 Average annual growth rate

b

 Gross Domestic Product in chained 2000 dollars (BEA 2006)

c

 Energy-content-weighted values (EIA 2006b)

d

 U.S. Census Bureau (2006)

e

 GWP-weighted values

Figure ES-15 1990 1991

19921993199419951996199719981999200020012002200320042005

Emissions

per $GDP 

Emissions

per capita 

Population

Real GDP

70

80

90

100

150

160

140

130

120

110

Index (1990 = 100)

Source: BEA (2006), U.S. Census Bureau (2006), and emission estimates in this report.

U.S. Greenhouse Gas Emissions Per Capita and 

Per Dollar of Gross Domestic Product
Executive Summary ES-7

Gas/Activity 1990 1995 2000 2001 2002 2003 2004 2005

NOx 21,645 21,272 19,203 18,410 18,141 17,327 16,466 15,965

Mobile Fossil Fuel Combustion 10,920 10,622 10,310 9,819 10,319 9,911 9,520 9,145

Stationary Fossil Fuel Combustion 9,883 9,821 8,002 7,667 6,837 6,428 5,952 5,824

Industrial Processes 591 607 626 656 532 533 534 535

Oil and Gas Activities 139 100 111 113 316 317 317 318

Municipal Solid Waste Combustion 82 88 114 114 97 98 98 98

Agricultural Burning 28 29 35 35 33 34 39 39

Solvent Use 1 3 3 3 5 5 5 5

Waste 0 1 2 2 2 2 2 2

CO 130,581 109,157 92,897 89,333 86,796 84,370 82,073 79,811

Mobile Fossil Fuel Combustion 119,480 97,755 83,680 79,972 77,382 74,756 72,269 69,915

Stationary Fossil Fuel Combustion 5,000 5,383 4,340 4,377 5,224 5,292 5,361 5,431

Municipal Solid Waste Combustion 978 1,073 1,670 1,672 1,440 1,457 1,475 1,493

Industrial Processes 4,125 3,959 2,217 2,339 1,710 1,730 1,751 1,772

Agricultural Burning 691 663 792 774 709 800 879 858

Oil and Gas Activities 302 316 146 147 323 327 331 335

Waste 1 2 8 8 7 7 7 7

Solvent Use 5 5 46 45 1 1 1 1

NMVOCs 20,930 19,520 15,228 15,048 14,968 14,672 14,391 14,123

Mobile Fossil Fuel Combustion 10,932 8,745 7,230 6,872 6,608 6,302 6,011 5,734

Solvent Use 5,216 5,609 4,384 4,547 3,911 3,916 3,921 3,926

Industrial Processes 2,422 2,642 1,773 1,769 1,811 1,813 1,815 1,818

Stationary Fossil Fuel Combustion 912 973 1,077 1,080 1,733 1,734 1,735 1,736

Oil and Gas Activities 554 582 389 400 546 547 547 548

Municipal Solid Waste Combustion 222 237 257 258 244 244 244 245

Waste 673 731 119 122 116 116 116 116

Agricultural Burning NA NA NA NA NA NA NA NA

SO2 20,935 16,891 14,829 14,452 13,541 13,648 13,328 13,271

Stationary Fossil Fuel Combustion 18,407 14,724 12,848 12,461 11,852 12,002 11,721 11,698

Industrial Processes 1,307 1,117 1,031 1,047 752 759 766 774

Mobile Fossil Fuel Combustion 793 672 632 624 681 628 579 535

Oil and Gas Activities 390 335 286 289 233 235 238 240

Municipal Solid Waste Combustion 38 42 29 30 23 23 23 23

Waste 0 1 1 1 1 1 1 1

Solvent Use 0 1 1 1 0 0 0 0

Agricultural Burning NA NA NA NA NA NA NA NA

Source: (EPA 2006, disaggregated based on EPA 2003) except for estimates from field burning of agricultural residues.

NA (Not Available)

Note: Totals may not sum due to independent rounding.

Table ES-10: Emissions of NOx, CO, NMVOCs, and SO2 (Gg)

terrestrial radiation absorption by influencing the formation 

and destruction of tropospheric and stratospheric ozone, or, in 

the case of SO2, by affecting the absorptive characteristics of 

the atmosphere. Additionally, some of these gases may react 

with other chemical compounds in the atmosphere to form 

compounds that are greenhouse gases.

Since 1970, the United States has published estimates 

of annual emissions of CO, NOx, NMVOCs, and SO2 (EPA 

2006),15 which are regulated under the Clean Air Act. Table 

ES-10 shows that fuel combustion accounts for the majority 

of emissions of these indirect greenhouse gases. Industrial 

processes—such as the manufacture of chemical and allied 

products, metals processing, and industrial uses of solvents—

are also significant sources of CO, NOx, and NMVOCs.

Key Categories

The IPCC’s Good Practice Guidance (IPCC 2000) 

defines a key category as a “[source or sink category] that 

is prioritized within the national inventory system because 

its estimate has a significant influence on a country’s total 

inventory of direct greenhouse gases in terms of the absolute 

level of emissions, the trend in emissions, or both.”16 By 

15 NOx and CO emission estimates from field burning of agricultural residues were estimated separately, and therefore not taken from EPA (2006).

16 See Chapter 7 “Methodological Choice and Recalculation” in IPCC (2000). <http://www.ipcc-nggip.iges.or.jp/public/gp/gpgaum.htm>.
ES-8 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 –2005

definition, key categories are sources or sinks that have the 

greatest contribution to the absolute overall level of national 

emissions in any of the years covered by the time series. In 

addition, when an entire time series of emission estimates 

is prepared, a thorough investigation of key categories 

must also account for the influence of trends of individual 

source and sink categories. Finally, a qualitative evaluation 

of key categories should be performed, in order to capture 

any key categories that were not identified in either of the 

quantitative analyses.

Figure ES-16 presents 2005 emission estimates for 

the key categories as defined by a level analysis (i.e., the 

contribution of each source or sink category to the total 

inventory level). The UNFCCC reporting guidelines 

request that key category analyses be reported at an 

appropriate level of disaggregation, which may lead to 

source and sink category names which differ from those 

used elsewhere in this report. For more information 

regarding key categories, see section 1.5 and Annex 1 of 

this report.

Quality Assurance and Quality Control 

(QA/QC)

The United States seeks to continually improve the 

quality, transparency, and credibility of the Inventory of 

U.S. Greenhouse Gas Emissions and Sinks. To assist in these 

efforts, the United States implemented a systematic approach 

to QA/QC. While QA/QC has always been an integral part 

of the U.S. national system for inventory development, the 

procedures followed for the current inventory have been 

formalized in accordance with the QA/QC plan and the 

UNFCCC reporting guidelines.

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 energyFigure ES-16

2005 Key Categories-Tier 1 Level Assessment

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000 2,200

CO2 Emissions from Iron and Steel Production

CO2 Emissions from Cement Manufacture

Fugitive CH4 Emissions from Coal Mining

Indirect N2O Emissions from Applied Nitrogen

CO2 Emissions from Mobile Combustion: Marine

Fugitive CH4 Emissions from Natural Gas Systems

CH4 Emissions from Enteric Fermentation

Emissions from Substitutes for Ozone Depleting Substances

CH4 Emissions from Landfills

CO2 Emissions from Non-Energy Use of Fuels

CO2 Emissions from Mobile Combustion: Aviation

Direct N2O Emissions from Agricultural Soil Management

CO2 Emissions from Stationary Combustion – Oil

CO2 Emissions from Stationary Combustion – Gas

CO2 Emissions from Mobile Combustion: Road & Other

CO2 Emissions from Stationary Combustion – Coal

Tg CO2 Eq.

Note: For a complete discussion of the key source analysis see Annex 1.
Executive Summary ES-9

related activities and cement processing, are considered 

to have low uncertainties. For some other categories 

of emissions, however, a lack of data or an incomplete 

understanding of how emissions are generated increases 

the uncertainty associated with the estimates presented. 

Acquiring a better understanding of the uncertainty 

associated with inventory estimates is an important step 

in helping to prioritize future work and improve the 

overall quality of the Inventory. Recognizing the benefit of 

conducting an uncertainty analysis, the UNFCCC reporting 

guidelines follow the recommendations of the IPCC Good 

Practice Guidance (IPCC 2000) and require that countries 

provide single estimates of uncertainty for source and sink 

categories.

Currently, a qualitative discussion of uncertainty is 

presented for all source and sink categories. Within the 

discussion of each emission source, specific factors affecting 

the uncertainty surrounding the estimates are discussed. Most 

sources also contain a quantitative uncertainty assessment, 

in accordance with UNFCCC reporting guidelines.