The major sources of U.S. methane emissions are energy production, distribution, and use; agriculture; and waste management (see Figure 12 on right).

U.S. methane emissions in 2006 totaled 605 million metric tons carbon dioxide equivalent (MMTCO2e), down slightly from the 2005 total (see Table 14 below).

Methane emissions declined steadily from 1990 to 2001, as emissions from energy sources and waste management fell.

Emissions rose from 2001 to 2005, as emissions from waste management increased in every year except 2004.

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.

This is the first annual EIA emissions inventory that directly incorporates estimates of methane emissions from industrial wastewater treatment in the waste management category.

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 from 2000 to 2006 (see Figure 13 on right and Table 15 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, which declined from 1990 to 2003 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 16 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.

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.

Decreases in U.S. rice production—particularly in Louisiana, Mississippi, and Arkansas—have reduced the estimated emissions from rice cultivation.

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 14 on right).

Decomposition of solid waste in municipal and industrial landfills is the largest source of U.S. methane emissions from waste management (see Table 17 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, annual increases in the total amount of waste deposited in landfills have resulted in increasing methane emissions (see Figure 15 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.

Methane recovery may also have been increased by the U.S. EPA’s New Source Performance Standards and Emission Guidelines, which require large landfills to collect and burn landfill gas.

As part of the American Jobs Creation Act of 2004, a tax credit for electricity generation from landfill gas was added to Section 45 of the Internal Revenue Code. The credit was augmented under the Energy Policy Act of 2005, providing a 10-year tax credit valued at 0.9 cents per kilowatthour for landfill gas-to-energy projects placed in service between October 22, 2004, and December 31, 2007.

About 14 percent of methane emissions from waste management are attributable to wastewater treatment, including both domestic wastewater (two-thirds) and industrial wastewater (one-third).

Methane emissions are generated by industrial processes in the production of iron and steel and in chemical production (see Figure 16 on right and Table 18 below).

The slight growth in methane emissions from U.S. chemical production from 1990 to 2006 (0.2 MMTCO2e) has been more than offset by declines in emissions from iron and steel production (0.5 MMTCO2e) over the same period, leading to a net decline of 0.3 MMTCO2e in methane emissions from industrial processes.