Total emissions of the six main greenhouse gases in 2008 were equivalent to 6,957 million metric tons of carbon dioxide. The gases include carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride. Though overall emissions dropped in 2008, emissions are still 13.5 percent higher than they were in 1990.
The Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008 is the latest annual report that the United States has submitted to the Secretariat of the United Nations Framework Convention on Climate Change. The convention sets an overall framework for intergovernmental efforts to tackle the challenge posed by climate change. EPA prepares the annual report with experts from multiple federal agencies and after gathering comments from a broad range of stakeholders across the country.
The inventory tracks annual greenhouse gas emissions at the national level and presents historical emissions from 1990 to 2008. The inventory also calculates carbon dioxide emissions that are removed from the atmosphere by “sinks,” which occurs through the uptake of carbon by forests, vegetation and soils.
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 GWPweighted emissions are measured in teragrams (or million metric tons) of CO2 equivalent (Tg CO2 Eq.)7,8 All gases in this Executive Summary are presented in units of Tg CO2 Eq.
The UNFCCC reporting guidelines for national inventories were updated in 2006,9 but continue to require the use of GWPs from the IPCC Second Assessment Report (SAR) (IPCC 1996). This requirement ensures that current estimates of aggregate greenhouse gas emissions for 1990 to 2008 are consistent with estimates developed prior to the publication of the IPCC Third Assessment Report (TAR) and the IPCC Fourth Assessment Report (AR4).
Therefore, to comply with international reporting standards under the UNFCCC, official emission estimates are reported by the United States using SAR GWP values. All estimates are provided throughout the report in both CO2 equivalents and unweighted units. A comparison of emission values using the SAR GWPs versus the TAR and AR4 GWPs can be found in Chapter 1 and, in more detail, in Annex 6.1 of this report. The GWP values used in this report are listed below.
Global Warming Potentials (100-Year Time Horizon) Used in this Report
Source: IPCC (1996)
* The CH4 GWP includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor. The indirect effect due to the production of CO2 is not included.
Global warming potentials are not provided for CO, NOx, NMVOCs, SO2, and aerosols because there is no agreedupon method to estimate the contribution of gases that are short-lived in the atmosphere, spatially variable, or have only indirect effects on radiative forcing (IPCC 1996).
Recent Trends in U.S. Greenhouse Gas Emissions and Sinks
In 2008, total U.S. greenhouse gas emissions were 6,956.8 Tg CO2 Eq. Overall, total U.S. emissions have risen by approximately 14 percent from 1990 to 2008. Emissions declined from 2007 to 2008, decreasing by 2.9 percent (211.3 Tg CO2 Eq.). This decrease is primarily a result of a decrease in demand for transportation fuels associated with the record high costs of these fuels that occurred in 2008. Additionally, electricity demand declined in 2008 in part due to a significant increase in the cost of fuels used to generate electricity. In 2008, temperatures were cooler in the United States than in 2007, both in the summer and the winter. This lead to an increase in heating related energy demand in the winter, however, much of this increase was offset by a decrease in cooling related electricity demand in the summer.
The primary greenhouse gas emitted by human activities in the United States was CO2, representing approximately 85.1 percent of total greenhouse gas emissions. The largest source of CO2, and of overall greenhouse gas emissions, was fossil fuel combustion. CH4 emissions, which have declined by 5.5 percent since 1990, resulted primarily from enteric fermentation associated with domestic livestock, decomposition of wastes in landfills, and natural gas systems. Agricultural soil management and mobile source fuel combustion were the major sources of N2O emissions. Ozone depleting substance substitute emissions and emissions of HFC-23 during the production of HCFC-22 were the primary contributors to aggregate HFC emissions. PFC emissions resulted as a by-product of primary aluminum production and from semiconductor manufacturing, while electrical transmission and distribution systems accounted for most SF6 emissions.
Overall, from 1990 to 2008 total emissions of CO2 increased by 820.4 Tg CO2 Eq. (16.1 percent), while CH4 and N2O emissions decreased by 45.8 Tg CO2 Eq. (7.5 percent) and 4.1 Tg CO2 Eq. (1.3 percent), respectively. During the same period, aggregate weighted emissions of HFCs, PFCs, and SF6 rose by 59.4 Tg CO2 Eq. (65.9 percent).
From 1990 to 2008, HFCs increased by 90.0 Tg CO2 Eq. (243.7 percent), PFCs decreased by 14.1 Tg CO2 Eq. (67.8 percent), and SF6 decreased by 16.5 Tg CO2 Eq. (50.5 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 these gases 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 13.5 percent of total emissions in 2008. The following sections describe each gas’ contribution to total U.S. greenhouse gas emissions in more detail.
Carbon Dioxide Emissions
The global carbon cycle is made up of large carbon flows and reservoirs. Billions of tons of carbon in the form of CO2 are absorbed by oceans and living biomass (i.e., sinks) and are emitted to the atmosphere annually through natural processes (i.e., sources). When in equilibrium, carbon fluxes among these various reservoirs are roughly balanced. Since the Industrial Revolution (i.e., about 1750), global atmospheric concentrations of CO2 have risen about 36 percent (IPCC 2007), principally due to the combustion of fossil fuels. Within the United States, fossil fuel combustion accounted for 94.1 percent of CO2 emissions in 2008. Globally, approximately 30,377 Tg of CO2 were added to the atmosphere through the combustion of fossil fuels in 2008, of which the United States accounted for about 19 percent.
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). In addition to fossil-fuel combustion, several other sources emit significant quantities of CO2. These sources include, but are not limited to non-energy use of fuels, iron and steel production and cement production.
As the largest source of U.S. greenhouse gas emissions, CO2 from fossil fuel combustion has accounted for approximately 79 percent of GWP-weighted emissions since 1990, growing slowly from 77 percent of total GWP weighted emissions in 1990 to 80 percent in 2008. Emissions of CO2 from fossil fuel combustion increased at an average annual rate of 1 percent from 1990 to 2008. The fundamental factors influencing this trend include (1) a generally growing domestic economy over the last 19 years, and (2) significant overall growth in emissions from electricity generation and transportation activities.
Between 1990 and 2008, CO2 emissions from fossil fuel combustion increased from 4,735.7 Tg CO2 Eq. to 5,572.8 Tg CO2 Eq.—an 18 percent total increase over the nineteen-year period. From 2007 to 2008, these emissions decreased by 184.2 Tg CO2 Eq. (3.2 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 nonfossil 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 five major fuel consuming sectors contributing to CO2 emissions from fossil fuel combustion are electricity generation, transportation, industrial, residential, and commercial. CO2 emissions are produced by the electricity generation sector as they consume fossil fuel to provide electricity to one of the other four sectors, or “end-use” sectors. For the discussion below, electricity generation emissions have been distributed to each end-use sector on the basis of each sector’s share of aggregate electricity consumption. This method of distributing emissions assumes that each end-use sector consumes electricity that is generated from the national average mix of fuels according to their carbon intensity. Emissions from electricity generation are also addressed separately after the end-use sectors have been discussed.
Transportation End-Use Sector. Transportation activities (excluding international bunker fuels) accounted for 32 percent of CO2 emissions from fossil fuel combustion in 2008.11 Virtually all of the energy consumed in this end use sector came from petroleum products. Nearly 53 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 2008. Approximately 54 percent of these emissions resulted from direct fossil fuel combustion to produce steam and/or heat for industrial processes. The remaining emissions resulted from consuming electricity for motors, electric furnaces, ovens, lighting, and other applications.
Residential and Commercial End-Use Sectors. The residential and commercial end-use sectors accounted for 21 and 19 percent, respectively, of CO2 emissions from fossil fuel combustion in 2008. Both sectors relied heavily on electricity for meeting energy demands, with 71 and 79 percent, respectively, of their emissions attributable to electricity consumption for lighting, heating, cooling, and operating appliances. The remaining emissions were due to the consumption of natural gas and petroleum for heating and cooking.
Electricity Generation. The United States relies on electricity to meet a significant portion of its energy demands, especially for lighting, electric motors, heating, and air conditioning. Electricity generators consumed 37 percent of U.S. energy from fossil fuels and emitted 42 percent of the CO2 from fossil fuel combustion in 2008. The type of fuel combusted by electricity generators has a significant effect on their emissions. For example, some electricity is generated with low CO2 emitting energy technologies, particularly non-fossil options such as nuclear, hydroelectric, or geothermal energy. However, electricity generators rely on coal for over half of their total energy requirements and accounted for 95 percent of all coal consumed for energy in the United States in 2008. 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 14.6 Tg CO2 Eq. (12.2 percent) from 1990 through 2008. Emissions from non-energy uses of fossil fuels were 134.2 Tg CO2 Eq. in 2008, which constituted 2.3 percent of total national CO2 emissions, approximately the same proportion as in 1990.
*CO2 emissions from iron and steel production and metallurgical coke production decreased from 2007 to 2008 (3.8 Tg CO2 Eq.), continuing a trend of decreasing emissions from 1990 through 2008 of 33 percent. This decline is due to the restructuring of the industry, technological improvements, and increased scrap utilization.
*In 2008, CO2 emissions from cement production decreased by 4.1 Tg CO2 Eq. (9.0 percent) from 2007. After decreasing in 1991 by two percent from 1990 levels, cement production emissions grew every year through 2006; emissions decreased in the last two years. Overall, from 1990 to 2008, emissions from cement production increased by 24 percent, an increase of 7.9 Tg CO2 Eq.
*Net CO2 flux from Land Use, Land-Use Change, and Forestry increased by 30.9 Tg CO2 Eq. (3 percent) from 1990 through 2008. 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, and harvested wood pools. Annual carbon accumulation in landfilled yard trimmings and food scraps slowed over this period, while the rate of carbon accumulation in urban trees increased.
According to the IPCC, CH4 is more than 20 times as effective as CO2 at trapping heat in the atmosphere. Over the last two hundred and fifty years, the concentration of CH4 in the atmosphere increased by 148 percent (IPCC 2007). Anthropogenic sources of CH4 include landfills, natural gas and petroleum systems, agricultural activities, coal mining, wastewater treatment, stationary and mobile combustion, and certain industrial processes.
Some significant trends in U.S. emissions of CH4 include the following:
• Enteric Fermentation is the largest anthropogenic source of CH4 emissions in the United States. In 2008, enteric fermentation CH4 emissions were 140.8 Tg CO2 Eq. (25 percent of total CH4 emissions), which represents an increase of 8.5 Tg CO2 Eq. (6.4 percent) since 1990.
• Landfills are the second largest anthropogenic source of CH4 emissions in the United States, accounting for 22 percent of total CH4 emissions (126.3 Tg CO2 Eq.) in 2008. From 1990 to 2008, net CH4 emissions from landfills decreased by 23.0 Tg CO2 Eq. (15 percent), with small increases occurring in some interim years. This downward trend in overall emissions is the result of increases in the amount of landfill gas collected and combusted,12 which has more than offset the additional CH4 emissions resulting from an increase in the amount of municipal solid waste landfilled.
• CH4 emissions from natural gas systems were 96.4 Tg CO2 Eq. in 2008; emissions have declined by 33.1 Tg CO2 Eq. (26 percent) since 1990. This decline is due to improvements in technology and management practices, as well as some replacement of old equipment.
• In 2008, CH4 emissions from coal mining were 67.6 Tg CO2 Eq., a 9.6 Tg CO2 Eq. (16 percent) increase over 2007 emission levels. The overall decline of 16.4 Tg CO2 Eq. (20 percent) from 1990 results from the mining of less gassy coal from underground mines and the increased use of CH4 collected from degasification systems.
• CH4 emissions from manure management increased by 54 percent since 1990, from 29.3 Tg CO2 Eq. in 1990 to 45.0 Tg CO2 Eq. in 2008. The majority of this increase was from swine and dairy cow manure, since the general trend in manure management is one of increasing use of liquid systems, which tends to produce greater CH4 emissions. The increase in liquid systems is the combined result of a shift to larger facilities, and to facilities in the West and Southwest, all of which tend to use liquid systems. Also, new regulations limiting the application of manure nutrients have shifted manure management practices at smaller dairies from daily spread to manure managed and stored on site.
Nitrous Oxide Emissions
N2O is produced by biological processes that occur in soil and water and by a variety of anthropogenic activities in the agricultural, energy-related, industrial, and waste management fields. While total N2O emissions are much lower than CO2 emissions, N2O is approximately 300 times more powerful than CO2 at trapping heat in the atmosphere. Since 1750, the global atmospheric concentration of N2O has risen by approximately 18 percent (IPCC 2007). The main anthropogenic activities producing N2O in the United States are agricultural soil management, fuel combustion in motor vehicles, nitric acid production, stationary fuel combustion, manure management, and adipic acid production.
Some significant trends in U.S. emissions of N2O include the following:
• Agricultural soils accounted for approximately 68 percent of N2O emissions in the United States in 2008. Estimated emissions from this source in 2008 were 215.9 Tg CO2 Eq. Annual N2O emissions from agricultural soils fluctuated between 1990 and 2008, although overall emissions were 6.1 percent higher in 2008 than in 1990. N2O emissions from this source have not shown any significant long-term trend, as they are highly sensitive to the amount of N applied to soils (which has not changed significantly over the time-period), and to weather patterns and crop type.
• In 2008, N2O emissions from mobile combustion were 26.1 Tg CO2 Eq. (approximately 8 percent of U.S. N2O emissions). From 1990 to 2008, N2O emissions from mobile combustion decreased by 40 percent. However, from 1990 to 1998 emissions increased by 26 percent, due to control technologies that reduced NOx emissions while increasing N2O emissions. Since 1998, newer control technologies have led to a steady decline in N2O from this source.
• N2O emissions from adipic acid production were 2.0 Tg CO2 Eq. in 2008, and have decreased significantly since 1996 from the widespread installation of pollution control measures. Emissions from adipic acid production have decreased by 87 percent since 1990, and emissions from adipic acid production have remained consistently lower than pre-1996 levels since 1998.
HFC, PFC, and SF6 Emissions
HFCs and PFCs are families of synthetic chemicals that are used as alternatives to ODS, 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.
Some significant trends in U.S. HFC, PFC, and SF6 emissions include the following:
• Emissions resulting from the substitution of ODS (e.g., CFCs) have been consistently increasing, from small amounts in 1990 to 113.0 Tg CO2 Eq. in 2008. Emissions from ODS substitutes are both the largest and the fastest growing source of HFC, PFC, and SF6 emissions. These emissions have been increasing as phase-outs required under the Montreal Protocol come into effect, especially after 1994, when full market penetration was made for the first generation of new technologies featuring ODS substitutes.
HFC emissions from the production of HCFC-22 decreased by 63 percent (22.8 Tg CO2 Eq.) from 1990 through 2008, 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.
• SF6 emissions from electric power transmission and distribution systems decreased by 51 percent (13.6 Tg CO2 Eq.) from 1990 to 2008, primarily because of higher purchase prices for SF6 and efforts by industry to reduce emissions.
• PFC emissions from aluminum production decreased by 85 percent (15.8 Tg CO2 Eq.) from 1990 to 2008, due to both industry emission reduction efforts and lower domestic aluminum production.
Overview of Sector Emissions and Trends
In accordance with the Revised 1996 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC/UNEP/OECD/IEA 1997), and the 2003 UNFCCC Guidelines on Reporting and Review (UNFCCC 2003), Figure ES-11 and Table ES-4 aggregate emissions and sinks by these chapters. Emissions of all gases can be summed from each source category from Intergovernmental Panel on Climate Change (IPCC) guidance. Over the nineteen-year period of 1990 to 2008, total emissions in the Energy, Industrial Processes, and Agriculture sectors climbed by 775.0 Tg CO2 Eq. (15 percent), 16.2 Tg CO2 Eq. (5 percent), and 39.7 Tg CO2 Eq. (10 percent), respectively. Emissions decreased in the Waste and Solvent and Other Product Use sectors by 18.1 Tg CO2 Eq. (10 percent) and less than 0.1 Tg CO2 Eq. (0.4 percent), respectively. Over the same period, estimates of net C sequestration in the Land Use, Land-Use Change, and Forestry sector (magnitude of emissions plus CO2 flux from all LULUCF source categories) increased by 13.7 Tg CO2 Eq. (1.5 percent).
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 2008. In 2008, approximately 84 percent of the energy consumed in the United States (on a Btu basis) was produced through the combustion of fossil fuels. The remaining 16 percent came from other energy sources such as hydropower, biomass, nuclear, wind, and solar energy.
Energy-related activities are also responsible for CH4 and N2O emissions (37 percent and 13 percent of total U.S. emissions of each gas, respectively). Overall, emission sources in the Energy chapter account for a combined 86 percent of total U.S. greenhouse gas emissions in 2008.
The Industrial Processes chapter contains by-product or fugitive emissions of greenhouse gases from industrial processes not directly related to energy activities such as fossil fuel combustion. For example, industrial processes can chemically transform raw materials, which often release waste gases such as CO2, CH4, and N2O. These processes include iron and steel production and metallurgical coke production, cement production, ammonia production and urea consumption, lime production, limestone and dolomite use (e.g., flux stone, flue gas desulfurization, and glass manufacturing), soda ash production and consumption, titanium dioxide production, phosphoric acid production, ferroalloy production, CO2 consumption, silicon carbide production and consumption, aluminum production, petrochemical production, nitric acid production, adipic acid production, lead production, and zinc production. Additionally, emissions from industrial processes release HFCs, PFCs, and SF6. Overall, emission sources in the Industrial Process chapter account for 5 percent of U.S. greenhouse gas emissions in 2008.
Solvent and Other Product Use
The Solvent and Other Product Use chapter contains greenhouse gas emissions that are produced as a by-product of various solvent and other product uses. In the United States, emissions from N2O from product uses, the only source of greenhouse gas emissions from this sector, accounted for about 0.1 percent of total U.S. anthropogenic greenhouse gas emissions on a carbon equivalent basis in 2008.
The Agricultural chapter contains anthropogenic emissions from agricultural activities (except fuel combustion, which is addressed in the Energy chapter, and agricultural CO2 fluxes, which are addressed in the Land Use, Land-Use Change, and Forestry Chapter). Agricultural activities contribute directly to emissions of greenhouse gases through a variety of processes, including the following source categories: enteric fermentation in domestic livestock, livestock manure management, rice cultivation, agricultural soil management, and field burning of agricultural residues. CH4 and N2O were the primary greenhouse gases emitted by agricultural activities. CH4 emissions from enteric fermentation and manure management represented 25 percent and 8 percent of total CH4 emissions from anthropogenic activities, respectively, in 2008. Agricultural soil management activities such as fertilizer application and other cropping practices were the largest source of U.S. N2O emissions in 2008, accounting for 68 percent. In 2008, emission sources accounted for in the Agricultural chapters were responsible for 6.1 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 84 percent of total 2008 net CO2 flux, urban trees accounted for 10 percent, mineral and organic soil carbon stock changes accounted for 5 percent, and landfilled yard trimmings and food scraps accounted for 1 percent of the total net flux in 2008. The net forest sequestration is a result of net forest growth and increasing forest area, as well as a net accumulation of carbon stocks in harvested wood pools. The net sequestration in urban forests is a result of net tree growth in these areas. In agricultural soils, mineral and organic soils sequester approximately 5.9 times as much C as is emitted from these soils through liming and urea fertilization. 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 2008 resulted in a net C sequestration of 940.3 Tg CO2 Eq. This represents an offset of 16 percent of total U.S. CO2 emissions, or 14 percent of total greenhouse gas emissions in 2008. Between 1990 and 2008, total land use, land-use change, and forestry net C flux resulted in a 3.4 percent increase in CO2 sequestration, primarily due to an increase in the rate of net C accumulation in forest C stocks, particularly in aboveground and belowground tree biomass, and harvested wood pools. Annual C accumulation in landfilled yard trimmings and food scraps slowed over this period, while the rate of annual C accumulation increased in urban trees.
The Waste chapter contains emissions from waste management activities (except incineration of waste, which is addressed in the Energy chapter). Landfills were the largest source of anthropogenic greenhouse gas emissions in the Waste chapter, accounting for just over 79 percent of this chapter’s emissions, and 22 percent of total U.S. CH4 emissions. Additionally, wastewater treatment accounts for 18 percent of Waste emissions, 4 percent of U.S. CH4 emissions, and 2 percent of U.S. N2O emissions. Emissions of CH4 and N2O from composting are also accounted for in this chapter; generating emissions of 1.7 Tg CO2 Eq. and 1.8 Tg CO2 Eq., respectively. Overall, emission sources accounted for in the Waste chapter generated 2.3 percent of total U.S. greenhouse gas emissions in 2008.
Emissions by Economic Sector
Throughout the Inventory of U.S. Greenhouse Gas Emissions and Sinks report, emission estimates are grouped into six sectors (i.e., chapters) defined by the IPCC: Energy; Industrial Processes; Solvent Use; Agriculture; Land Use, Land-Use Change, and Forestry; and Waste. While it is important to use this characterization for consistency with UNFCCC reporting guidelines, it is also useful to allocate emissions into more commonly used sectoral categories. This section reports emissions by the following economic sectors: Residential, Commercial, Industry, Transportation, Electricity Generation, Agriculture, and U.S. Territories.
Using this categorization, emissions from electricity generation accounted for the largest portion (35 percent) of U.S. greenhouse gas emissions in 2008. Transportation activities, in aggregate, accounted for the second largest portion (27 percent), while emissions from industry accounted for the third largest portion (19 percent) of U.S. greenhouse gas emissions in 2008. In contrast to electricity generation and transportation, emissions from industry have in general declined over the past decade. The long-term decline in these emissions has been due to structural changes in the U.S. economy (i.e., shifts from a manufacturing-based to a service-based economy), fuel switching, and energy efficiency improvements. The remaining 19 percent of U.S. greenhouse gas emissions were contributed by, in order of importance, the agriculture, commercial, and residential sectors, plus emissions from U.S. territories.
Activities related to agriculture accounted for 7 percent of U.S. emissions; unlike other economic sectors, agricultural sector emissions were dominated by N2O emissions from agricultural soil management and CH4 emissions from enteric fermentation. The commercial sector accounted for 6 percent of emissions while the residential sector accounted for 5 percent of emissions and U.S. territories accounted for 1 percent of emissions; emissions from these sectors primarily consisted of CO2 emissions from fossil fuel combustion.
CO2 was also emitted and sequestered by a variety of activities related to forest management practices, tree planting in urban areas, the management of agricultural soils, and landfilling of yard trimmings.
Electricity is ultimately consumed in the economic sectors described above. 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. These source categories include CO2 from fossil fuel combustion and the use of limestone and dolomite for flue gas desulfurization, CO2 and N2O from incineration of waste, CH4 and N2O from stationary sources, and SF6 from electrical transmission and distribution systems.
When emissions from electricity are distributed among these sectors, industry accounts for the largest share of U.S. greenhouse gas emissions (29 percent) in 2008. 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 (27 percent) despite the considerable decline in emissions from this sector during the past year. In all sectors except agriculture, CO2 accounts for more than 80 percent of greenhouse gas emissions, primarily from the combustion of fossil fuels.