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Executive Summary Overview
U.S. emissions of greenhouse gases in 2002 totaled 6,862.0 million metric tons carbon dioxide equivalent, 0.5 percent more than in 2001 (6,828.9 million metric tons carbon dioxide equivalent). Although emissions of carbon dioxide and emissions of hydrofluorocarbons, perfluorocarbons, and sulfur hexafluoride grew by 0.8 percent and 6.2 percent, respectively, those increases were moderated by reductions in methane (-2.7 percent) and nitrous oxide (-1.1 percent) emissions, resulting in a relatively low 0.5-percent growth in total greenhouse gas emissions. U.S. greenhouse gas emissions have averaged 0.9-percent annual growth since 1990. Even though the U.S. economy grew by 2.4 percent in 2002 (slightly less than its 2.9-percent growth trend for the 1990-2002 period), growth in total greenhouse gas emissions was restrained somewhat due to a 1.1-percent reduction in output from the energy- and emissions-intensive manufacturing sector. Consequently, U.S. greenhouse gas intensity (greenhouse gas emissions per unit of real economic output) was 2.1 percent lower in 2002 than in 2001. From 1990 to 2002, U.S. greenhouse gas intensity has declined by 21.4 percent, or by an average of 2.0 percent per year. U.S. greenhouse gas emissions in 2002 were 11.5 percent higher than 1990 emissions (6,155.8 million metric tons carbon dioxide equivalent). Since 1990, U.S. emissions have increased more slowly than the average annual growth in population (1.2 percent), primary energy consumption (1.2 percent), electric power generation (2.0 percent), or gross domestic product (2.9 percent). While the annual growth rate in carbon dioxide emissions since 1990 (1.2 percent) has closely tracked annual growth in population (1.2 percent) and energy consumption (1.2 percent), the average annual rate of growth in total greenhouse gas emissions has been lower (0.9 percent) because of reductions in methane and nitrous oxide emissions since 1990. Growth rates in electricity generation (2.0 percent) and gross domestic product (2.9 percent), meanwhile, have outstripped the growth in total greenhouse gas emissions because of the increased electrification of energy use and rapid growth in non-greenhouse-gas-intensive segments of the economy. Table ES1 shows trends in emissions of the principal greenhouse gases, measured in million metric tons of gas. In Table ES2, the value shown for each gas is weighted by its global warming potential (GWP), which is a measure of “radiative forcing.” The GWP concept, developed by the Intergovernmental Panel on Climate Change (IPCC), provides a comparative measure of the impacts of different greenhouse gases on global warming relative to the global warming potential of carbon dioxide.1
In 2001, the IPCC Working Group I released its Third Assessment Report, Climate Change 2001: The Scientific Basis.2 Among other things, the Third Assessment Report updated a number of the GWP estimates that appeared in the IPCC’s Second Assessment Report.3 The GWPs published in the Third Assessment Report were used for the calculation of carbon dioxide equivalent emissions for this report. For a discussion of GWPs and a comparison of U.S. carbon dioxide equivalent emissions calculated using the GWPs from the IPCC’s Third and Second Assessment Reports. Generally, total U.S. carbon dioxide equivalent emissions are 0.6 percent higher when the GWPs from the Third Assessment Report are used. During 2002, 82.8 percent of total U.S. greenhouse gas emissions consisted of carbon dioxide from the combustion of fossil fuels such as coal, petroleum, and natural gas (after adjustments for U.S. territories and international bunker fuels). U.S. emissions trends are driven largely by trends in fossil energy consumption. In recent years, national energy consumption, like emissions, has grown relatively slowly, with year-to-year deviations from trend growth caused by weather-related phenomena, fluctuations in business cycles, changes in the fuel mix for electric power generation, and developments in domestic and international energy markets.
Other 2002 U.S. greenhouse gas emissions include carbon dioxide from non-combustion sources (1.7 percent of total U.S. greenhouse gas emissions), methane (8.9 percent), nitrous oxide (4.9 percent), and other gases (1.8 percent) (Figure ES1). Methane and nitrous oxide emissions are caused by the biological decomposition of various waste streams and fertilizer, fugitive emissions from chemical processes, fossil fuel production and combustion, and many smaller sources. The other gases include hydrofluorocarbons (HFCs), used primarily as refrigerants; perfluorocarbons (PFCs), released as fugitive emissions from aluminum smelting and also used in semiconductor manufacture; and sulfur hexafluoride (SF6), used as an insulator in utility-scale electrical equipment. This report, required by Section 1605(a) of the Energy Policy Act of 1992, provides estimates of U.S. emissions of greenhouse gases. The estimates are based on activity data and applied emissions factors, not on measured or metered emissions monitoring. Carbon Dioxide The preliminary estimate of U.S. carbon dioxide emissions from both energy consumption and industrial processes in 2002 is 5,795.6 million metric tons, which is 0.8 percent higher than in 2001 and accounts for 84.5 percent of total U.S. greenhouse gas emissions. U.S. carbon dioxide emissions have grown by an average of 1.2 percent annually since 1990. Although short-term changes in carbon dioxide emissions can result from temporary variations in weather, power generation fuel mixes, and the economy, in the longer term their growth is driven by population, income, and consumer choices of energy-using equipment, as well as the “carbon intensity” of energy use (carbon dioxide emissions per unit of energy consumed). Figure ES2 shows recent trends in some common indexes used to measure the carbon intensity of the U.S. economy. Carbon dioxide emissions per unit of GDP have continued to fall relative to 1990; by 2002, this measure was 17.7 percent lower than in 1990. Carbon dioxide emissions per capita were 0.1 percent above the 1990 level in 2001 and 0.1 percent below the 1990 level in 2002. Population growth and other factors resulted in increased aggregate carbon dioxide emissions per year from 1990 through 2002 (a total increase of 15.8 percent). Carbon dioxide emissions per unit of net electricity generation in 2002 were 1.6 percent lower than in 2001. Carbon dioxide emissions from the U.S. electric power sector (which includes utilities, independent power producers, and combined heat and power facilities whose primary business is the production and sale of electricity) in 2002 are estimated at 2,249.0 million metric tons, 1.0 percent higher than the 2001 level of 2,226.6 million metric tons. The 1-percent increase is less than the 2.7-percent increase in electricity generation during 2002 because of an increase in the average efficiency of natural-gas-fired power plants. Although total electricity generation from natural gas increased by 8.2 percent from 2001 to 2002, emissions from natural-gas-fired generators increased by only 3.4 percent, due to the improved efficiency of new natural-gas-fired combined-cycle generating units. Figure ES3 illustrates trends in carbon dioxide emissions by energy consumption sector. In general, emissions have increased steadily at the sectoral level since 1990. Average annual growth rates in carbon dioxide emissions by sector during the 1990-2002 period were 2.2 percent for the commercial sector, 1.9 percent for the residential sector, and 1.4 percent for the transportation sector. Industrial sector carbon dioxide emissions, after peaking in 1997, have continued to fall and are now below 1990 levels. In the residential sector, total carbon dioxide emissions were up by 2.8 percent, from 1,160.8 million metric tons in 2001 to 1,193.0 million metric tons in 2002. The increase is attributed mainly to a 1.2-percent rise in natural gas use due to colder winter weather that increased heating degree-days by 1.4 percent. Emissions attributable to purchased electricity also rose by 3.3 percent, which also contributed to the overall increase in residential carbon dioxide emissions. Since 1990, residential carbon dioxide emissions have grown by an average of 1.9 percent annually. Carbon dioxide emissions in the commercial sector fell by 0.5 percent, from 1,018.3 million metric tons in 2001 to 1,012.9 million metric tons in 2002. Emissions attributable to purchased electricity decreased by 1.2 percent, from 791.1 million metric tons in 2001 to 781.8 million metric tons in 2002. Carbon dioxide emissions from the direct combustion of fossil fuels in the commercial sector, primarily natural gas, rose from 227.1 million metric tons in 2001 to 231.1 million metric tons in 2002, a 1.7-percent increase. Despite the slight decrease in 2002, commercial sector emissions of carbon dioxide have increased at an average annual rate of 2.2 percent since 1990. Energy-related carbon dioxide emissions in the industrial sector in 2002 are estimated at 1,673.7 million metric tons, which is approximately equal to the level of emissions in 1990. After peaking in 1997, industrial emissions have generally fallen with the exception of a slight upturn in 2000. Historically, industrial energy consumption and carbon dioxide emissions have been more sensitive to economic growth than to the weather. The most recent decline, in 2002, is a case in point: industrial emissions fell by 0.7 percent in 2002, coinciding with a 1.1-percent decrease in manufacturing output. Industrial energy consumption and emissions are concentrated in a few energy-intensive industries, and their performance is more closely correlated with carbon dioxide emissions than is the performance of the industrial sector as a whole. Among the six energy-intensive industry groups, which traditionally account for about 65 to 70 percent of total industrial carbon dioxide emissions and 80 percent of carbon dioxide emissions from manufacturing, changes in output were mixed in 2002 (see text box in Chapter 2). Declines in output relative to 2001 were seen in 2002 for the primary metals and pulp and paper industries (-3.0 percent and -2.3 percent, respectively), while increases in output were seen for the chemicals industry (3.9 percent) and stone, clay and glass products (1.0 percent). Smaller increases in output were seen for the other energy-intensive industries: food (0.5 percent) and petroleum refining (0.5 percent). By fuel type, industrial sector carbon dioxide emissions from purchased electricity rose by 0.9 percent, while emissions from natural gas and coal fell by 2.0 percent and 6.9 percent, respectively. Carbon dioxide emissions from petroleum use in the industrial sector increased by 0.7 percent in 2002. Carbon dioxide emissions in the transportation sector, at 1,849.7 million metric tons, were 1.2 percent higher in 2002 than in 2001. Emissions of carbon dioxide from gasoline consumption (61.6 percent of transportation sector emissions) grew by 2.5 percent, while emissions from jet fuel use for air travel fell by 2.5 percent. Transportation sector carbon dioxide emissions have grown by an average of 1.4 percent annually since 1990. Methane U.S. emissions of methane in 2002 were 2.7 percent lower than in 2001, at 26.6 million metric tons of methane or 612.8 million metric tons carbon dioxide equivalent (8.9 percent of total U.S. greenhouse gas emissions). Total U.S. methane emissions in 2001 were 27.4 million metric tons of methane or 630.2 million metric tons carbon dioxide equivalent (9.2 percent of total U.S. greenhouse gas emissions). The 2002 decline resulted primarily from decreases in methane emissions from landfills and, to a smaller extent, from reductions in methane emissions from coal mining.
Methane emissions come from four categories of sources, three major and one minor. The major sources are energy, agriculture, and waste management, and the minor source is industrial processes. The three major sources accounted for 41.1, 29.9, and 28.6 percent, respectively, of total 2002 U.S. emissions of methane. Trends in the major sources of anthropogenic methane emissions since 1990 are illustrated in Figure ES4. Methane emissions from energy sources (coal mining, natural gas systems, petroleum systems, stationary combustion, and mobile source combustion) declined from 11.0 million metric tons of methane in 2001 to 10.9 million metric tons of methane in 2002, representing a 0.7-percent reduction in emissions from energy sources. Methane emissions from energy sources have fallen by 8.5 percent since 1990. The drop in 2002 was the result of reductions in emissions associated with natural gas production, lower emissions from underground coal mines, a drop in residential wood combustion, and lower emissions from petroleum systems and the transportation sector. Methane emissions from agricultural sources (8.0 million metric tons) decreased by 0.5 percent in 2002. Agricultural methane emissions have several sources but are dominated by emissions from domestic livestock, including the animals themselves (enteric fermentation) and the anaerobic decomposition of their waste. Methane emissions from enteric fermentation in 2002 were slightly (0.03 percent) lower than in 2001, and methane emissions from animal waste were 0.9 percent lower. Agricultural emissions have increased by 5.0 percent since 1990. Methane emissions from waste management sources include two subcategories: emissions from the anaerobic decomposition of municipal solid waste in landfills and emissions from wastewater treatment facilities. Methane emissions from waste management decreased by 7.7 percent, from 8.2 million metric tons in 2001 to 7.6 million metric tons in 2002. Contributing to the decrease was an 8.5-percent decrease in emissions from landfills that resulted from a 0.6 million metric ton increase in methane recovery for energy use. Emissions of methane from waste management have declined by 34.3 percent since 1990 as a result of an increase in the amount of methane recovered from landfills (4.7 million metric tons more in 2002 than in 1990) that would otherwise have been emitted to the atmosphere. The estimates for methane emissions are more uncertain than those for carbon dioxide. U.S. methane emissions do not necessarily increase with growth in energy consumption or the economy. Energy-related methane emissions are strongly influenced by coal production from a relatively small number of mines; agricultural emissions are influenced in part by the public’s consumption of milk and beef and in part by animal husbandry practices; and waste management emissions are influenced by the volume of municipal waste generated and recycled, as well as the amount of methane recaptured at landfills. Nitrous Oxide
U.S. nitrous oxide emissions decreased by 1.1 percent from 2001 to 2002, to 1.1 million metric tons of nitrous oxide or 333.1 million metric tons carbon dioxide equivalent (4.9 percent of total U.S. greenhouse gas emissions). The 2002 decline in nitrous oxide emissions resulted from decreases in emissions from agricultural sources. Since 1990, U.S. nitrous oxide emissions have fallen by 0.2 percent. Emissions estimates for nitrous oxide are more uncertain than those for either carbon dioxide or methane. Nitrous oxide is not systematically measured, and for many sources of nitrous oxide emissions, including nitrogen fertilization of soils and motor vehicles, a significant number of assumptions are required for the derivation of emissions estimates. U.S. nitrous oxide emissions include two large categories of sources, agriculture and energy use, and two smaller categories, industrial processes and waste management (Figure ES5). Agricultural sources, at 791 thousand metric tons of nitrous oxide or 234.2 million metric tons carbon dioxide equivalent, account for 70.3 percent of total U.S. nitrous oxide emissions. Emissions associated with nitrogen fertilization of soils, at 583 thousand metric tons or 172.5 million metric tons carbon dioxide equivalent, account for 73.7 percent of nitrous oxide emissions from agriculture. Emissions from the solid waste of animals, at 207 thousand metric tons or 61.2 million metric tons carbon dioxide equivalent, make up 26.1 percent of agricultural nitrous oxide emissions. Nitrous oxide emissions from agriculture have decreased by 2.8 percent since 1990. U.S. nitrous oxide emissions associated with fossil fuel combustion in 2002 were 0.3 million metric tons of nitrous oxide, or 23.4 percent of total nitrous oxide emissions. Of these energy-related emissions, 81.4 percent comes from mobile sources, principally motor vehicles equipped with catalytic converters. The remainder comes from stationary source combustion of fossil fuels. Nitrous oxide emissions from energy sources have increased by 31.5 percent since 1990. Industrial processes and wastewater treatment facilities are responsible for 6.2 percent of total nitrous oxide emissions. Industrial process emissions increased from 47 thousand metric tons (14.0 million metric tons carbon dioxide equivalent) in 2001 to 50 thousand metric tons (14.9 million metric tons carbon dioxide equivalent) in 2002. This represents the first increase in industrial nitrous oxide emissions since 1996. The increase can be attributed to the leveling off of emissions reductions from adipic acid production, as well as a slight increase in emissions from nitric acid production. Emissions from wastewater treatment facilities were 20 thousand metric tons of nitrous oxide (6.0 million metric tons carbon dioxide equivalent), an increase of 0.3 thousand metric tons (0.1 million metric tons carbon dioxide equivalent) or 1.4 percent from 2001. Other Gases: Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride HFCs, PFCs, and SF6 are three classes of engineered gases that account for 1.8 percent of U.S. GWP-weighted emissions of greenhouse gases. At 120.6 million metric tons carbon dioxide equivalent in 2002, their emissions were 6.2 percent higher than in 2001. The increase in emissions of the engineered gases from 2001 to 2002 resulted largely from increases in HFC emissions (8.4 percent) that counteracted decreases in emissions of PFCs (-2.2 percent) and SF6 (-4.7 percent). At 83.1 million metric tons carbon dioxide equivalent, emissions of HFCs make up the majority of this category, followed by SF6 at 16.7 million metric tons and PFCs at 7.6 million metric tons. Another group of engineered gases, consisting of other HFCs, other PFCs, and perfluoropolyethers (PFPEs), includes HFC-152a, HFC-227ea, HFC-4310mee, and a variety of PFCs and PFPEs. They are grouped together in this report to protect confidential data. In 2002, their combined emissions totaled 13.2 million metric tons carbon dioxide equivalent. Emissions in this “other” group in 2002 were 13.5 percent higher than in 2001 and orders of magnitude higher than in 1990, when emissions were less than 180,000 metric tons carbon dioxide equivalent. Since 1990, HFC emissions from U.S. sources have increased by 126.0 percent, PFC emissions have decreased by 60.8 percent, and SF6 emissions have decreased by 58.7 percent. Emissions of the high-GWP gases specified in the Kyoto Protocol are very small (at most a few thousand metric tons). On the other hand, some of the gases (including PFCs and SF6) have atmospheric lifetimes measured in the thousands of years, and consequently they are potent greenhouse gases with GWPs thousands of times higher than that of carbon dioxide per unit of molecular weight. Some of the commercially produced HFCs (134a, 152a, 4310mee, 227ea), which are used as replacements for chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), have shorter atmospheric lifetimes, ranging from 1 to 33 years. Land Use and Forestry Forest lands in the United States are net absorbers of carbon dioxide from the atmosphere. Absorption is enabled by the reversal of the extensive deforestation of the United States that occurred in the late 19th and early 20th centuries. Since then, millions of acres of formerly cultivated land have been abandoned and have returned to forest, with the regrowth of forests sequestering carbon on a large scale. The process is steadily diminishing, however, because the rate at which forests absorb carbon slows as the trees mature, and because the rate of reforestation has slowed. The U.S. Environmental Protection Agency (EPA) estimates annual U.S. carbon sequestration for the year 2001 at 838.1 million metric tons carbon dioxide equivalent. In 1990, land use change and forestry practices represented an offset of more than 17 percent of total U.S. anthropogenic carbon dioxide emissions. By 2001, the offset had declined to 12 percent.
Released: October 2003 |
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