Emissions of Greenhouse Gases in the United States 2003
Methane
U.S. emissions of methane in 2003 were 0.5 percent higher than in 2002, at 26.2 million metric tons of methane or 601.9 million metric tons carbon dioxide equivalent (8.7 percent of total U.S. greenhouse gas emissions). Total U.S. methane emissions in 2002 were 26.1 million metric tons of methane. The 2003 increase resulted primarily from small increases in methane emissions from landfills, coal mines, animal waste, and natural gas systems. Despite the 0.5-percent increase from 2002, methane emissions in 2003 were 4.6 million metric tons (15 percent) below the 1990 level.
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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 42.5, 30.4, and 26.6 percent, respectively, of total 2003 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) increased from 254.7 million metric tons carbon dioxide equivalent in 2002 to 256.1 million metric tons carbon dioxide equivalent in 2003, representing a 0.5-percent increase in emissions from energy sources. Methane emissions from energy sources have fallen by 6.9 percent since 1990. The rise in 2003 was the result of increases in emissions from coal mines and natural gas systems.
Methane emissions from agricultural sources (182.8 million metric tons carbon dioxide equivalent) decreased by 0.3 percent in 2003. 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 2003 were 0.3 percent lower than in 2002, whereas methane emissions from animal waste were 0.6 percent higher than in 2002. The overall decline in agricultural methane emissions resulted mainly from one of the smallest sources, emissions from rice cultivation, which fell by 0.7 million metric tons carbon dioxide equivalent or 6.8 percent. Agricultural emissions have increased by 4.7 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 rose by 1.2 percent, from 158.5 million metric tons carbon dioxide equivalent in 2002 to 160.4 million metric tons carbon dioxide equivalent in 2003. Contributing to the increase was a 1.3-percent increase in emissions from landfills, which would have been larger but for a 0.3 million metric ton increase in methane recovery for energy use. Methane emissions from wastewater increased by 0.8 percent to 15.5 million metric tons carbon dioxide equivalent. Emissions of methane from waste management have declined by 37.2 percent since 1990 as a result of an increase in the amount of methane recovered from landfills (5.0 million metric tons more in 2003 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 volumes of municipal waste generated and recycled, as well as the amount of methane recaptured at landfills (see discussion on "The Methane to Markets Partnership").