The major sources of U.S. methane emissions are energy production, distribution, and use; agriculture; and waste management (Figure 15 on right).

U.S. methane emissions in 2008 totaled 737.4 MMTCO₂e, 2.0 percent higher than the 2007 total of 722.7 MMTCO₂e (Table 16 below).

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

From 2002 to 2008, methane emissions rose as a result of moderate increases in emissions related to energy, agriculture, and waste management that more than offset a decline in industrial emissions of methane over the same period.

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, accounting for 295.7 MMTCO₂e in 2008.

Agricultural emissions (primarily from livestock management) and emissions from waste management (primarily landfills) also are large sources of U.S. methane emissions, contributing 225.0 and 212.1 MMTCO₂e, respectively, in 2008.

Natural gas systems and coal mines are the major sources of methane emissions in the energy sector (Figure 16 on right and Table 17 below).

U.S. methane emissions from natural gas systems grew from 1990 to 2008, largely because of increases in natural gas consumption.

Emissions from coal mines declined from 1990 to 2002 and remained nearly steady through 2007. In 2008, emissions from ventilation of underground mines jumped by 24.6 percent, leading to a 15.3-percent increase in total mining emissions over their 2007 level. Much of the 2008 increase can be attributed to the larger number of gassy mines in operation throughout the year and to the fact that, as mining proceeds into deeper seams, more methane emissions tend to be produced per ton of coal mined.

With domestic oil production dropping by 30 percent from 1990 to 2008, methane emissions from petroleum systems also declined.

Residential wood consumption accounted  for nearly 45 percent of U.S. methane emissions from stationary combustion in 2008.

Methane emissions from passenger cars fell by 51.3 percent from 1990 to 2008, as the use of catalytic converters increased. A 9.2-percent drop in annual miles traveled by passenger cars from 2002 to 2008 also contributed to the decrease in emissions.

Livestock management—including emissions from enteric fermentation (66 percent) and management of animal wastes (29 percent)—accounts for most of the U.S. methane emissions from agricultural activities (Table 18 below).

Since 1990, there has been a shift in livestock management to larger facilities that manage waste in liquid systems, increasing the amount of methane generated from livestock waste. Increases in the U.S. swine population since 1990 have also contributed to the rise in methane emissions. Emissions of methane from animal waste rose by 2.0 percent from 2007 to 2008. Swine accounted for 45.9 percent (29.7 MMTCO₂e) and dairy cattle 36.2 percent (23.3 MMTCO₂e) of the 2008 total.

Enteric fermentation (food digestion) in ruminant animals also produces methane emissions, and digestion by cattle accounts for 95 percent of U.S. methane emissions from this source. With little change in the cattle population since 1990, the level of emissions from enteric fermentation has been relatively stable. Small declines in cattle and goat populations in 2008 were offset by increases in horse, goat and swine poulations, which caused methane emissions from enteric fermentation to increase slightly (by 0.5 MMTCO₂e, or 0.3 percent) from their 2007 level.

Despite lower crop yields, U.S. rice production rose by 7 percent in 2008 as a result of increases in area harvested in Missouri, Texas, Arkansas, Louisiana, and Mississippi. Consequently, emissions from this source increased by 7.1 percent from 2007 to 2008.

Although emissions from crop residue burning grew by 2.0 percent in 2008, residue burning remains the smallest contributor to methane emissions from agriculture, representing less than 1 percent of total U.S. methane emissions from agriculture (Figure 17 on right).

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

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 American Recovery and Reinvestment Act of 2009, signed into law on February 17, 2009, included a 2-year extension (through December 31, 2012) of the production tax credit (PTC) for renewable energy, including waste-to-energy and landfill gas combustion.

Wastewater treatment, including both domestic wastewater (two-thirds) and industrial wastewater (one-third), is responsible for 13.1 percent (27.8 MMTCO₂e) of methane emissions from waste management.

Emissions from the treatment of wastewater from ethanol production and from petroleum refineries are included for the first time in the 2008 inventory, increasing the estimates of methane emissions from industrial wastewater in previous years by 8.0 to 11.4 percent. In 2008, emissions from wastewater at petroleum refineries accounted for 7.3 percent (0.7 MMTCO₂e) of total emissions from industrial wastewater, and emissions from ethanol production accounted for 2.9 percent (0.3 MMTCO₂e).

Methane emissions are generated by industrial processes in the production of iron and steel and in chemical production (Figure 19 on right and Table 20 below).

Methane emissions from industrial processes declined by a net 0.6 MMTCO₂e (10.7 percent) from 2007 to 2008, as a result of declines in both chemical production and iron and steel production.

Since 1990, methane emissions from industrial processes have increased by 0.1 MMTCO₂e (2.4 percent). A 32.4-percent decline (0.4 MMTCO₂e) in emissions from iron and steel production since 1990 has been offset by an increase of 0.5 MMTCO₂e (13.9 percent) in emissions from chemical production.

Estimates of industrial emissions of methane in the 2008 inventory are approximately 50 to 65 percent higher for all years, as a result of applying the IPCC’s revised emissions factors for methane from ethylene, ethylene dichloride, and methanol production in the chemical industry and for methane from sinter and coke in iron and steel production.