Methane

U.S. anthropogenic methane emissions totaled 639.5 MMTCO₂e⁸ (27.8 million metric tons of methane) in 2004, representing 9.0 percent of total U.S. greenhouse gas emissions. Methane emissions in 2004 were 0.9 percent (5.6 MMTCO₂e) higher than the 2003 level of 633.9 MMTCO₂e (Table ES4). The increase is attributable primarily to increases in methane emissions from landfills and, to a lesser extent, emissions associated with animal waste, coal mines, and rice cultivation. Despite the 0.9-percent increase in 2004, methane emissions still were 81.9 MMTCO₂e (11.4 percent) below their 1990 level of 721.4 MMTCO₂e.

Figure Data

Methane emissions come from four source categories, 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 40, 29, and 31 percent, respectively, of total 2004 U.S. emissions of methane. Trends in the major sources of anthropogenic methane emissions since 1990 are illustrated in Figure ES4.

Energy Sources

Methane emissions from energy sources (coal mining, natural gas systems, petroleum systems, stationary combustion, and mobile source combustion) were nearly unchanged at 256.3 MMTCO₂e in 2004 from the 2003 level of 256.4 MMTCO₂e. Methane emissions from energy sources have fallen by 6.8 percent since 1990; however, after bottoming out in 2001, methane emissions from energy sources rose in each of the next 2 years.

Agricultural Sources

Methane emissions from agricultural sources, at 182.3 MMTCO₂e, represented 29 percent of total U.S. anthropogenic methane emissions in 2004. Emissions increased by less than 1 percent (0.9 MMTCO₂e) in 2004 relative to 2003, as a small decrease in emissions from enteric fermentation in domesticated animals was offset by increases in emissions from animal waste management, rice cultivation, and burning of crop residues. Of the total 2004 methane emissions from agricultural sources, 93 percent (170.0 MMTCO₂e) resulted from livestock management, of which 68 percent (115.2 MMTCO₂e) was from enteric fermentation and the remainder (54.7 MMTCO₂e) was from anaerobic decomposition of livestock wastes. Methane emissions from rice cultivation (11 MMTCO₂e) and crop residue burning (1.3 MMTCO₂e) together represented about 6.7 percent of total methane emissions from agricultural sources in 2004, which have increased by 5.1 percent since 1990.

Waste Management

Methane emissions from waste management, which at 198.2 MMTCO₂e accounted for 31 percent of U.S. anthropogenic methane emissions in 2004, were 2.4 percent above the 2003 level of 193.6 MMTCO₂e. Landfills—the largest single source of U.S. anthropogenic methane emissions—represented 92 percent (182.6 MMTCO₂e) of total U.S. methane emissions from waste management in 2004. The remainder (15.6 MMTCO₂e) of 2004 methane emissions from waste management was associated with domestic wastewater treatment. Methane emissions from waste management have fallen by 27 percent (72.0 MMTCO₂e) from their 1990 level of 270.2 MMTCO₂e, due largely to increased methane recovery at landfills, which increased to 122.1 MMTCO₂e in 2004 from 21.7 MMTCO₂e in 1990. Even at these higher methane recovery levels, however, waste management emissions, after bottoming out in 2002, increased in both 2003 and 2004.

Industrial Processes

Methane emissions from industrial processes totaled 2.7 MMTCO₂e in 2004, including 1.6 MMTCO₂e from chemical manufacturing and 1.1 MMTCO₂e from iron and steel production. Since 1990, growth in methane emissions from chemical production has been balanced by reductions in emissions from iron and steel production, leaving the 2004 total for industrial processes approximately equal to their 1990 level.

Notes and Sources