The major sources of U.S. methane emissions are energy production, distribution, and use; agriculture; and waste management (Figure 13 on right). 

U.S. methane emissions in 2007 totaled 699.9 million metric tons carbon dioxide equivalent (MMTCO₂e), up from the 2006 total of 686.9 MMTCO₂e (Table 16 below).

Methane emissions declined steadily from 1990 to 2001, as emissions from coal mining and landfills fell.

Emissions have risen from 2001 to 2007, with moderate increases in each of the major emission sources.

The energy sector—including coal mining, natural gas systems, petroleum systems, and stationary and mobile combustion—is the largest source of U.S. methane emissions.

Agriculture (primarily livestock management) and waste management (primarily landfills) also are large contributors to U.S. methane emissions.

Natural gas systems and coal mines are the major sources of methane emissions in the energy sector. 

Methane emissions from natural gas systems grew between 1990 and 2000, in parallel with increases in U.S. natural gas consumption, then leveled off between 2000 and 2005 before resuming growth (see Figure 14 on right and Table 17 below).

Emissions from coal mines declined from 1990 to 2002 and have remained low since then, because production increases have been largely from surface mines that produce relatively little methane.

Methane emissions from petroleum systems have declined as domestic oil production has dropped by more than 30 percent since 1990.

Residential wood consumption accounts for almost 90 percent of methane emissions from stationary combustion.

Methane emissions from passenger cars (a major component of mobile sources), which declined from 1990 to 2004 as more efficient catalytic converters were added on newer models, have rebounded with increases in total vehicle miles traveled.

Livestock management—including emissions from enteric fermentation (two-thirds) and management of animal wastes (one-third)—accounts for most of the U.S. methane emissions from agricultural activities (see Table 18 below). 

Since 1990, there has been a shift in livestock management to larger facilities that are more likely to manage waste in liquid systems, which increase the amounts of methane generated from livestock waste. In addition, increases in the U.S. swine population have contributed to the rise in methane emissions.

Because 95 percent of all methane emissions from enteric fermentation (digestion in ruminant animals) are attributable to cattle, trends in emissions are correlated with trends in the size of the U.S. cattle population. With little change in the cattle population size since 1990, the level of methane emissions from enteric fermentation has been relatively stable.

Decreases in U.S. rice production—particularly in California, Louisiana, Oklahoma, and Texas—have reduced the estimated emissions from rice cultivation. In 2007, a rebound in Louisiana’s rice cultivation was more than offset by declines in cultivation in Arkansas and Missouri.

Crop residue burning remains the smallest contributor to methane emissions from agriculture, representing less than 1 percent of total U.S. methane emissions (see Figure 15 on right).

Methane emissions from waste management are dominated by the decomposition of solid waste in municipal and industrial landfills (see Table 19 below). 

Emissions from landfills declined substantially from 1990 to 2001 as a result of increases in recycling and in the recovery of landfill methane for energy; since 2001, increases in the total amount of waste deposited in landfills have resulted in increasing methane emissions (see Figure 16 on right).

Rapid growth in methane recovery during the 1990s can be traced in part to the Federal Section 29 tax credit for alternative energy sources, which provided a subsidy of approximately 1 cent per kilowatthour for electricity generated from landfill gas before June 1998.

The U.S. EPA’s New Source Performance Standards and Emission Guidelines, which require large landfills to collect and burn landfill gas, have also played an important role in the growth of methane recovery.

The Emergency Economic Stabilization Act of 2008 included a 2-year extension (through December 31, 2010) of the production tax credit for waste-to-energy and landfill gas, as well as other renewable energy sources.

Wastewater treatment, including both domestic wastewater (two-thirds) and industrial wastewater (one-third), is responsible for about 14 percent of methane emissions from waste management.

Methane emissions are generated by industrial processes in the production of iron and steel and in chemical production (see Figure 17on right and Table 20 below). 

Methane emissions from industrial processes declined by a net 0.3 MMTCO₂e from 1990 to 2007, as small increases in emissions from chemical production (0.2 MMTCO₂e) were more than offset by declines in emissions from iron and steel production (0.5 MMTCO₂e).