An emissions inventory that identifies and quantifies a country's primary anthropogenic1 sources and sinks of greenhouse gases is essential for addressing climate change. The Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 (U.S. EPA/OAP 2006c) 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 contributions of different emission sources and greenhouse gases to climate change.

In 1992, the United States signed and ratified the UNFCCC. 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….”2 The United States views the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 (U.S. EPA/OPA 2006b) as an opportunity to fulfill these commitments.

This chapter summarizes the latest information on U.S. anthropogenic greenhouse gas emission trends from 1990 through 2004. 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 Intergovernmental Panel on Climate Change (IPCC) 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 the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 is consistent with the UNFCCC guidelines for inventory reporting.3 For most source categories, the IPCC methodologies were expanded, resulting in a more comprehensive and detailed estimate of emissions.

Naturally occurring greenhouse gases include water vapor, carbon dioxide (CO 2 ), methane (CH 4 ), nitrous oxide (N 2 O), and ozone (O 3 ). 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 (ODS), 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.4 Some other fluorine-containing halogenated substances—hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF 6 )—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 (NO x ), and nonmethane volatile organic compounds (NMVOCs). Aerosols, which are extremely small particles or liquid droplets, such as those produced by sulfur dioxide (SO 2 ) or elemental carbon emissions, can also affect the absorptive characteristics of the atmosphere.

Although the direct greenhouse gases CO 2 , CH 4 , and N 2 O 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 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 2001a).

RECENT TRENDS IN U.S. GREENHOUSE GAS EMISSIONS AND SINKS

In 2004, total U.S. greenhouse gas emissions were 7,074.4 Tg CO 2 Eq. Overall, total U.S. emissions rose by 15.8 percent from 1990 through 2004, while the U.S. gross domestic product increased by 51 percent over the same period (U.S. DOC/BEA 2006a). Emissions rose from 2003 through 2004, increasing by 1.7 percent (115.3 Tg CO 2 Eq.). The following factors were primary contributors to this increase: (1) robust economic growth in 2004, leading to increased demand for electricity and fossil fuels; (2) expanding industrial production in energy-intensive industries, also increasing demand for electricity and fossil fuels; and (3) increased travel, leading to higher rates of consumption of petroleum fuels.

Figures 3-1 through 3-3 illustrate the overall trends in total U.S. emissions by gas, annual changes, and absolute change since 1990. Table 3-2 provides a detailed summary of U.S. greenhouse gas emissions and sinks from 1990 through 2004.

Figure 3-4 illustrates the relative contribution of the direct greenhouse gases to total U.S. emissions in 2004. The primary greenhouse gas emitted by human activities in the United States was CO 2 , representing approximately 85 percent of total greenhouse gas emissions. The largest source of CO 2 , and of overall greenhouse gas emissions, was fossil fuel combustion. CH 4 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 N 2 O emissions. The emissions of ODS substitutes 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 SF 6 emissions, while PFC emissions resulted from semiconductor manufacturing and as a by-product of primary aluminum production.

Overall, from 1990 through 2004, total emissions of CO 2 increased by 982.7 Tg CO 2 Eq. (20 percent), while CH 4 and N 2 O emissions decreased by 61.3 Tg CO 2 Eq. (10 percent) and 8.2 Tg CO 2 Eq. (2 percent), respectively. During the same period, aggregate weighted emissions of HFCs, PFCs, and SF 6 rose by 52.2 Tg CO 2 Eq. (58 percent). Despite being emitted in smaller quantities relative to the other principal greenhouse gases, emissions of HFCs, PFCs, and SF 6 are significant because many of them have extremely high GWPs and, in the cases of PFCs and SF 6 , 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 percent of total emissions in 2004. The following sections describe each gas's 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 CO 2 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 CO 2 have risen about 35 percent (IPCC 2001a; Hofmann 2004), principally due to the combustion of fossil fuels. Within the United States , fuel combustion accounted for 94 percent of CO 2 emissions in 2004 (Figure 3-5 and Table 3-3). Globally, approximately 25,575 Tg of CO 2 were added to the atmosphere through the combustion of fossil fuels in 2002, of which the United States accounted for about 23 percent.6 Changes in land use and forestry practices can also emit CO 2 (e.g., through conversion of forest land to agricultural or urban use) or can act as a sink for CO 2 (e.g., through net additions to forest biomass)

As the largest source of U.S. greenhouse gas emissions, CO 2 from fossil fuel combustion has accounted for approximately 80 percent of GWP-weighted emissions since 1990, growing slowly from 77 percent of total GWP-weighted emissions in 1990 to 80 percent in 2004. Emissions of CO 2 from fossil fuel combustion increased at an average annual rate of 1.3 percent from 1990 through 2004. The fundamental factors influencing this trend include a generally growing domestic economy over the last 14 years, and significant growth in emissions from transportation activities and electricity generation. Between 1990 and 2004, CO 2 emissions from fossil fuel combustion increased from 4,696.6 Tg CO 2 Eq. to 5,656.6 Tg CO 2 Eq.—a 20 percent total increase over the 14-year period. Historically, changes in emissions from fossil fuel combustion have been the dominant factor affecting U.S. emission trends.

From 2003 through 2004, emissions from fossil fuel combustion increased by 85.5 Tg CO 2 Eq. (1.5 percent). A number of factors played a major role in the magnitude of this increase. Strong growth in the U.S. economy and industrial production, particularly in energy-intensive industries, caused an increase in the demand for electricity and fossil fuels. Demand for travel was also higher, causing an increase in petroleum consumed for transportation. In contrast, the warmer winter conditions led to decreases in demand for heating fuels in the residential and commercial sectors. Moreover, much of the increased electricity demanded was generated by natural gas consumption and nuclear power, rather than by more carbon-intensive coal, moderating the increase in CO 2 emissions from electricity generation. Use of renewable fuels rose very slightly, due to increases in the use of biofuels. Figures 3-6 and 3-7 summarize CO 2 emissions from fossil fuel combustion by sector and fuel type and by end-use sector.

Other significant CO 2 trends included the following:

• CO 2 emissions from iron and steel production decreased to 51.3 Tg CO 2 Eq. in 2004, and declined by 33.7 Tg CO 2 Eq. (40 percent) from 1990 through 2004, due to reduced domestic production of pig iron, sinter, and coal coke.

• CO 2 emissions from cement production increased to 45.6 Tg CO 2 Eq. in 2004, a 37 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.

• CO 2 emissions from waste combustion (19.4 Tg CO 2 Eq. in 2004) increased by 8.4 Tg CO 2 Eq. (77 percent) from 1990 through 2004, as the volume of plastics and other fossil carbon-containing materials in municipal solid waste grew.

• Net CO 2 sequestration from land use, land-use change, and forestry decreased by 130.3 Tg CO 2 Eq. (14 percent) from 1990 through 2004. 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, while the rate of carbon accumulation in agricultural soils and urban trees increased.

Methane Emissions

According to the IPCC, CH 4 is more than 20 times as effective as CO 2 at trapping heat in the atmosphere. Over the last 250 years, the concentration of CH 4 in the atmosphere increased by 143 percent (IPCC 2001a; Hofmann 2004). Anthropogenic emission sources of CH 4 include landfills, natural gas and petroleum systems, agricultural activities, coal mining, wastewater treatment, stationary and mobile combustion, and certain industrial processes (Figure 3-8 and Table 3-4).

Some significant trends in U.S. emissions of CH 4 include the following:

• Landfills are the largest anthropogenic source of CH 4 emissions in the United States . In 2004, landfill CH 4 emissions were 140.9 Tg CO 2 Eq. (approximately 25 percent of total CH 4 emissions), which represents a decline of 31.4 Tg CO 2 Eq., or 18 percent, since 1990. Although the amount of solid waste landfilled each year continues to climb, the amount of CH 4 captured and burned at landfills has increased dramatically, countering this trend.

• CH 4 emissions from natural gas systems were 118.8 Tg CO 2 Eq. in 2004; emissions have declined by 7.9 Tg CO 2 Eq. (6 percent) since 1990. This decline has been due to improvements in technology and management practices, as well as some replacement of old equipment.

• Enteric fermentation was also a significant source of CH 4 , accounting for 112.6 Tg CO 2 Eq. in 2004. This amount has declined by 5.3 Tg CO 2 Eq. (4 percent) since 1990, and by 10.4 Tg CO 2 Eq. (8 percent) from a high in 1995. 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.

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 N 2 O emissions are much lower than CO 2 emissions, N 2 O is approximately 300 times more powerful than CO 2 at trapping heat in the atmosphere. Since 1750, the global atmospheric concentration of N 2 O has risen by approximately 18 percent (IPCC 2001a; Hofmann 2004). The main anthropogenic activities producing N 2 O in the United States are agricultural soil management, fuel combustion in motor vehicles, manure management, nitric acid production, human sewage, and stationary fuel combustion (Figure 3-9 and Table 3-5).

Some significant trends in U.S. emissions of N 2 O include the following:

• Agricultural soil management activities, such as fertilizer application and other cropping practices, were the largest source of U.S. N 2 O emissions, accounting for 68 percent (261.5 Tg CO 2 Eq.) of 2004 emissions. N 2 O emissions from this source have not shown any significant long-term trend, as they are highly sensitive to such factors as temperature and precipitation, which have generally outweighed changes in the amount of nitrogen applied to soils.

• In 2004, N 2 O emissions from mobile combustion were 42.8 Tg CO 2 Eq. (approximately 11 percent of U.S. N 2 O emissions). From 1990 through 2004, N 2 O emissions from mobile combustion decreased by 1 percent. However, from 1990 through 1998, emissions increased by 26 percent, due to control technologies that reduced NO x emissions while increasing N 2 O emissions. Since 1998, newer control technologies have led to a steady decline in N 2 O emissions from this source.

HFC, PFC, and SF 6 Emissions

HFCs and PFCs are families of synthetic chemicals that are being used as alternatives to 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 SF 6 , are potent greenhouse gases. In addition to having high GWPs, SF 6 and PFCs have extremely long atmospheric lifetimes, resulting in their essentially irreversible accumulation in the atmosphere once emitted. SF 6 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 (Figure 3-10 and Table 3-6).

Some significant trends in U.S. HFC, PFC, and SF 6 emissions include the following:

• Emissions resulting from the substitution of ODSs (e.g., CFCs) have been increasing from small amounts in 1990 to 103.3 Tg CO 2 Eq. in 2004. Emissions from ODS substitutes are both the largest and the fastest growing source of HFC, PFC, and SF 6 emissions. These emissions have been increasing as phase-outs required under the Montreal Protocol come into effect, especially after 1994, when full market penetration was made for the first generation of new technologies featuring ODS substitutes.

• The increase in ODS substitute emissions is offset substantially by decreases in emissions of HFCs, PFCs, and SF 6 from other sources. Emissions from aluminum production decreased by 85 percent (15.6 Tg CO 2 Eq.) from 1990 through 2004, due to both industry emission reduction efforts and lower domestic aluminum production.

• Emissions from the production of HCFC-22 decreased by 55 percent (19.4 Tg CO 2 Eq.) from 1990 through 2004, 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 (14.8 Tg CO 2 Eq.) from 1990 through 2004, primarily because of higher purchase prices for SF 6 and efforts by industry to reduce emissions.

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), the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2004 (U.S. EPA/OAP 2006a) is segregated into six sector-specific chapters. Figure 3-11 and Table 3-7 aggregate emissions and sinks by these chapters.

Energy

The Energy sector 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. CO 2 emissions from 1990 through 2004. In 2004, 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 (Figure 3-12). Energy-related activities are also responsible for CH 4 and N 2 O emissions (39 percent and 15 percent of total U.S. emissions of each gas, respectively). Overall, emission sources in the Energy sector accounted for a combined 86 percent of total U.S. greenhouse gas emissions in 2004.

Industrial Processes

The Industrial Processes sector 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 CO 2 , CH 4 , and N 2 O. The processes 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, CO 2 consumption, aluminum production, petrochemical production, silicon carbide production, nitric acid production, and adipic acid production. Additionally, emissions from industrial processes release HFCs, PFCs, and SF 6 . Overall, emission sources in the Industrial Process sector accounted for 4.5 percent of U.S. greenhouse gas emissions in 2004.

Solvent and Other Product Use

The Solvent and Other Product Use sector contains greenhouse gas emissions that are produced as a by-product of various solvent and other product uses. In 2004, U.S. emissions from N 2 O 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.

Agriculture

The Agriculture sector contains anthropogenic emissions from agricultural activities (except fuel combustion, which is addressed in the Energy sector). 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. CH 4 and N 2 O were the primary greenhouse gases emitted by agricultural activities. In 2004, CH 4 emissions from enteric fermentation and manure management represented about 20 percent and 7 percent of total CH 4 emissions from anthropogenic activities, respectively. Agricultural soil management activities, such as fertilizer application and other cropping practices, were the largest source of U.S. N 2 O emissions in 2004, accounting for 68 percent. In 2004, emission sources accounted for in the Agriculture sector were responsible for 6.2 percent of total U.S. greenhouse gas emissions.

Land Use, Land-Use Change, and Forestry

The Land Use, Land-Use Change, and Forestry sector contains emissions and removals of CO 2 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 carbon in the United States . Forests (including vegetation, soils, and harvested wood) accounted for approximately 82 percent of total 2004 sequestration; urban trees accounted for 11 percent; agricultural soils (including mineral and organic soils and the application of lime) accounted for 6 percent; and landfilled yard trimmings and food scraps accounted for 1 percent of the total sequestration in 2004. 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 almost two times larger than the sum of emissions from organic soils and liming. The mineral soil carbon 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 2004 resulted in a net carbon sequestration of 780.1 Tg CO 2 Eq. (Table 3-7). This represents an offset of approximately 13 percent of total U.S. CO 2 emissions, or 11 percent of total greenhouse gas emissions in 2004. Total land use, land-use change, and forestry net carbon sequestration declined by approximately 14 percent from 1990 through 2004, which contributed to an increase in net U.S. emissions (all sources and sinks) of 21 percent from 1990 through 2004. This decline was primarily due to a decline in the rate of net carbon accumulation in forest carbon stocks, as forests mature. Annual carbon accumulation in landfilled yard trimmings and food scraps and agricultural soils also slowed over this period. However, the rate of annual carbon accumulation increased in both agricultural soils and urban trees.

Land use, land-use change, and forestry activities in 2004 also resulted in emissions of N 2 O (6.8 Tg CO 2 Eq.). Total N 2 O emissions from the application of fertilizers to forests and settlements increased by approximately 20 percent from 1990 through 2004.

Waste

The Waste sector contains emissions from waste management activities (except waste incineration, which is addressed in the Energy sector). Landfills were the largest source of anthropogenic CH 4 emissions, accounting for 25 percent of total U.S. CH 4 emissions.7 Additionally, wastewater treatment accounts for 7 percent of U.S. CH 4 emissions. N 2 O emissions from the discharge of wastewater treatment effluents into aquatic environments were estimated, as were N 2 O emissions from the treatment process itself, using a simplified methodology. Wastewater treatment systems are a potentially significant source of N 2 O emissions; however, methodologies are not currently available to develop a complete estimate. N 2 O emissions from the treatment of the human sewage component of wastewater were estimated, however, using a simplified methodology. Overall, in 2004, emission sources accounted for in the Waste sector generated 2.7 percent of total U.S. greenhouse gas emissions.

EMISSIONS BY ECONOMIC SECTOR

Emission estimates, for the purposes of inventory reports, are grouped into six sectors 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 3-8 summarizes emissions from each of these sectors, and Figure 3-13 shows emission trends by sector from 1990 through 2004.

Using this categorization, emissions from electricity generation accounted for the largest portion (33 percent) of U.S. greenhouse gas emissions in 2004; 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 2004. In contrast to electricity generation and transportation, emissions from industry have in general declined over the past decade, although there was an increase in industrial emissions in 2004 (up 3 percent from 2003 levels). The long-term decline in these emissions has been due to structural changes in the U.S. economy (i.e., shifts from a manufacturing-based to a service-based economy), fuel switching, and efficiency improvements.

The remaining 20 percent of U.S. greenhouse gas emissions were contributed by the residential, agriculture, and commercial sectors, plus emissions from U.S. territories. The residential sector accounted for about 6 percent, and primarily consisted of CO 2 emissions from fossil fuel combustion. Activities related to agriculture accounted for roughly 7 percent of U.S. emissions; unlike other economic sectors, agriculture sector emissions were dominated by N 2 O emissions from agricultural soil management and CH 4 emissions from enteric fermentation, rather than CO 2 from fossil fuel combustion. The commercial sector accounted for about 7 percent of emissions, while U.S. territories accounted for 1 percent.

CO 2 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 3-9 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.8 These source categories include CO 2 from fossil fuel combustion and the use of limestone and dolomite for flue gas desulfurization, CO 2 and N 2 O from waste combustion, CH 4 and N 2 O from stationary sources, and SF 6 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 2004. 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 (lighting, appliances, etc.). Transportation activities remain the second largest contributor to total U.S. emissions (28 percent). In all sectors except agriculture, CO 2 accounts for more than 80 percent of greenhouse gas emissions, primarily from the combustion of fossil fuels. Figure 3-14 shows the trend in these emissions by sector from 1990 through 2004.

INDIRECT GREENHOUSE GASES

The reporting requirements of the UNFCCC9 request that information be provided on indirect greenhouse gases, which include CO, NO x , NMVOCs, and SO 2 . These gases do not have a direct global warming effect, but indirectly affect terrestrial radiation absorption by influencing the formation and destruction of tropospheric and stratospheric ozone, or, in the case of SO 2 , 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, NO x , NMVOCs, and SO 2 (U.S. EPA 2005),10 which are regulated under the Clean Air Act. Table 3-10 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, NO x , and NMVOCs.

_________________________________

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 affected by human activities (IPCC/UNEP/OECD/IEA 1997).

2 Article 4(1)(a) of the UNFCCC (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 .

3 See .

4 Emission estimates of CFCs, HCFCs, halons, and other ODS are included in the annexes of the Inventory report for informational purposes.

5 See .

6 Global CO2 emissions from fossil fuel combustion were taken from Marland et al. 2005 .

7 Landfills also store carbon, resulting from incomplete degradation of organic materials, such as wood products and yard trimmings, as described in the Land Use, Land-Use Change, and Forestry chapter of the national Inventory report.

8 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 .

9 See .

10 NOx and CO emission estimates from field burning of agricultural residues were estimated separately, and therefore were not taken from U.S. EPA 2005.

[This is a mobile copy of 3. Greenhouse Gas Inventory ]