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Sources and Emissions

Photo collage of selected methane emission sources (natural gas and petroleum systems and agricultural operations) and utilzation options (electricity generation and direct gas use)
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Where Does Methane Come From?

Methane (CH4) is emitted from a variety of both human-related (anthropogenic) and natural sources. Human-related activities include fossil fuel production, animal husbandry (enteric fermentation in livestock and manure management), rice cultivation, biomass burning, and waste management. These activities release significant quantities of methane to the atmosphere. It is estimated that more than 50 percent of global methane emissions are related to human-related activities (U.S. EPA). Natural sources of methane include wetlands, gas hydrates, permafrost, termites, oceans, freshwater bodies, non-wetland soils, and other sources such as wildfires.

Methane emission levels from a source can vary significantly from one country or region to another, depending on many factors such as climate, industrial and agricultural production characteristics, energy types and usage, and waste management practices. For example, temperature and moisture have a significant effect on the anaerobic digestion process, which is one of the key biological processes that cause methane emissions in both human-related and natural sources. Also, the implementation of technologies to capture and utilize methane from sources such as landfills, coal mines, and manure management systems affects the emission levels from these sources.

Emission inventories are prepared to determine the contribution from different sources. The following sections present information from inventories of U.S. man-made sources and natural sources of methane globally. For information on international methane emissions from man-made sources, visit the International Analyses Web site.

Human-Related Sources in the United States

In the United States, the largest methane emissions come from the decomposition of wastes in landfills, ruminant digestion and manure management associated with domestic livestock, natural gas and oil systems, and coal mining.

Table 1 shows the level of emissions from individual sources for the years 1990, 2000 and 2005 to 2009.

Table 1 U.S. Methane Emissions by Source (TgCO2 Equivalents)

Source Category 1990 2000 2005 2006 2007 2008 2009
Natural Gas Systems 189.8 209.3 190.4 217.7 205.2 211.8 221.2
Enteric Fermentation 132.1 136.5 136.5 138.8 141 140.6 139.8
Landfills 147.4 111.7 112.5 111.7 111.3 115.9 117.5
Coal Mining 84.1 60.4 56.9 58.2 57.9 67.1 71
Manure Management 31.7 42.4 46.6 46.7 50.7 49.4 49.5
Petroleum Systems 35.4 31.5 29.4 29.4 30 30.2 30.9
Wastewater Treatment 23.5 25.2 24.3 24.5 24.4 24.5 24.5
Forest Land Remaining Forest Land 3.2 14.3 9.8 21.6 20 11.9 7.8
Rice Cultivation 7.1 7.5 6.8 5.9 6.2 7.2 7.3
Stationary Combustion 7.4 6.6 6.6 6.2 6.5 6.5 6.2
Abandoned Underground Coal Mines 6 7.4 5.5 5.5 5.6 5.9 5.5
Mobile Combustion 4.7 3.4 2.5 2.3 2.2 2 2
Composting 0.3 1.3 1.6 1.6 1.7 1.7 1.7
Petrochemical Production 0.9 1.2 1.1 1 1 0.9 0.8
Iron and Steel Production &
Metallurgical Coke Production
1 0.9 0.7 0.7 0.7 0.6 0.4
Field Burning of Agricultural Residue 0.3 0.3 0.2 0.2 0.2 0.3 0.2
Ferroalloy Production + + + + + + +
Silicon Carbide Production and
+ + + + + + +
Incineration of Waste + + + + + + +
International Bunker Fuels 0.2 0.1 0.1 0.2 0.2 0.2 0.1
Total for U.S. 674.9 659.9 631.4 672.1 664.6 676.7 686.3

Source: Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009 (459 pp., 14.4MB, About PDF)

The principal human-related sources of methane are described below. For each source, a link is provided to the report entitled "U.S. Emissions Inventory 2011: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009," prepared by EPA, which provides detailed information on the characterization and quantity of national emissions from each source. This report, hereafter referred to as the "U.S. inventory report", provides the latest descriptions and emissions associated with each source category and is part of the United States' official submittal to the United Nations Framework Convention on Climate Change. The U.S. inventory report also describes the procedures used to quantify national emissions, as well as a description of trends in emissions since 1990.

Also, for those sources where EPA has established voluntary programs for reducing methane emissions, a link to those program sites is provided.

An image of a landfillLandfills. Landfills are the third–largest human–related source of methane in the U.S., accounting for 17 percent of all methane emissions in 2009. Methane is generated in landfills and open dumps as waste decomposes under anaerobic (without oxygen) conditions. The amount of methane created depends on the quantity and moisture content of the waste and the design and management practices at the site. The U.S. inventory report provides a detailed description on methane emissions from landfills and how they are estimated (see the Chapter entitled "Waste").

EPA has also established a voluntary program to reduce methane emissions from landfills. This program, known as the Landfill Methane Outreach Program (LMOP), works with companies, utilities, and communities to encourage the use of landfill gas for energy.

An image of an oil fieldNatural gas and petroleum systems. Methane is the primary component of natural gas. Methane losses occur during the production, processing, storage, transmission, and distribution of natural gas. Because gas is often found in conjunction with oil, the production, refinement, transportation, and storage of crude oil is also a source of methane emissions. The U.S. inventory report provides a detailed description on methane emissions from natural gas and petroleum systems and how they are estimated (see the Chapter entitled "Energy"). An additional technical note has been provided on the 1990-2009 emissions estimates for the natural gas industry.

EPA has also established a voluntary program to reduce methane emissions in the natural gas industry. This program, known as the Natural Gas STAR Program (Gas STAR) is a voluntary partnership between EPA and the natural gas and oil industries to reduce emissions of methane from the production, transmission, and distribution of natural gas.

An image of a coal mineCoal mining. Methane trapped in coal deposits and in the surrounding strata is released during normal mining operations in both underground and surface mines. In addition, handling of the coal after mining results in methane emissions. The U.S. inventory report provides a detailed description on methane emissions from coal mining and how they are estimated (see the Chapter entitled "Energy").

EPA has also established a voluntary program to reduce methane emissions in the coal mining industry. This program, known as the Coalbed Methane Outreach Program (CMOP) helps the coal industry identify the technologies, markets, and finance sources to profitably use or sell the methane that coal mines would otherwise vent to the atmosphere.

A picture of cows on the range.Livestock enteric fermentation. Among domesticated livestock, ruminant animals (cattle, buffalo, sheep, goats, and camels) produce significant amounts of methane as part of their normal digestive processes. In the rumen, or large fore-stomach, of these animals, microbial fermentation converts feed into products that can be digested and utilized by the animal. This microbial fermentation process, referred to as enteric fermentation, produces methane as a by-product, which can be exhaled by the animal. Methane is also produced in smaller quantities by the digestive processes of other animals, including humans, but emissions from these sources are insignificant. The U.S. inventory report provides a detailed description on methane emissions from livestock enteric fermentation and how they are estimated (see the Chapter entitled "Agriculture").

EPA has studied options for reducing methane emissions from enteric fermentation and has developed resources and tools to assist in estimating emissions and evaluating mitigation options. For more information, please visit the Ruminant Livestock site.

A picture of biogas recovery system.Livestock manure management. Methane is produced during the anaerobic (i.e., without oxygen) decomposition of organic material in livestock manure management systems. Liquid manure management systems, such as lagoons and holding tanks, can cause significant methane production and these systems are commonly used at larger swine and dairy operations. Manure deposited on fields and pastures, or otherwise handled in a dry form, produces insignificant amounts of methane. The U.S. inventory report provides a detailed description on methane emissions from livestock manure management and how they are estimated (see the Chapter entitled "Agriculture").

EPA has also established a voluntary program to reduce methane emissions in the livestock industry. This program, known as the AgSTAR Program, encourages adoption of anaerobic digestion technologies that recover and combust biogas (methane) for odor control or as an on-farm energy resource.

An image of a wastewater treatment facilityWastewater treatment. Wastewater from domestic (municipal sewage) and industrial sources is treated to remove soluble organic matter, suspended solids, pathogenic organisms, and chemical contaminants. These treatment processes can produce methane emissions if organic constituents in the wastewater are treated anaerobically (i.e., without oxygen) and if the methane produced is released to the atmosphere. In addition, the sludge produced from some treatment processes may be further biodegraded under anaerobic conditions, resulting in methane emissions. These emissions can be avoided, however, by treating the wastewater and the associated sludge under aerobic conditions or by capturing methane released under anaerobic conditions. The U.S. inventory report provides a detailed description on methane emissions from wastewater treatment and how they are estimated (see the Chapter entitled "Waste").

An image of a rice fieldRice cultivation. Methane is produced during flooded rice cultivation by the anaerobic (without oxygen) decomposition of organic matter in the soil. Flooded soils are ideal environments for methane production because of their high levels of organic substrates, oxygen-depleted conditions, and moisture. The level of emissions varies with soil conditions and production practices as well as climate. Several cultivation practices have shown promise for reducing methane emissions from rice cultivation. The U.S. inventory report provides a detailed description on methane emissions from rice cultivation and how they are estimated (see the Chapter entitled "Agriculture").

Natural Sources - Global Emissions

Methane and Nitrous Oxide Emissions From Natural Sources cover
See EPA's Methane and Nitrous Oxide Emissions From Natural Sources (PDF) (194 pp, 1.9MB, About PDF), published April 2010.

Emissions of methane from natural sources are largely determined by environmental variables such as temperature and precipitation. The best available information indicates that global methane emissions from natural sources are around 208 Tg per year. However, the value is highly uncertain. (Source: Prepared from data contained in EPA Methane and Nitrous Oxide Emissions From Natural Sources, April 2010 (PDF) (194 pp, 1.9MB, About PDF).)

Wetlands. Natural wetlands are responsible for the majority of global methane emissions from natural sources, accounting for about 170 Tg of methane per year (range 105-278 Tg per year). Wetlands provide a habitat conducive to methane-producing (methanogenic) bacteria that produce methane during the decomposition of organic material. These bacteria require environments with no oxygen and abundant organic matter, both of which are present in wetland conditions.

Termites. Global emissions of methane due to termites are estimated to be between 2 and 22 Tg per year, making them the second largest natural source of methane emissions. Methane is produced in termites as part of their normal digestive process, and the amount generated varies among different species. Ultimately, emissions from termites depend largely on the population of these insects, which can also vary significantly among different regions of the world.

Oceans, Rivers, and Estuaries. Oceans, rivers, and estuaries are estimated to be responsible for approximately 9 Tg of methane per year (range 2-16 Tg per year). The source of methane from oceans is not entirely clear, but two identified sources include the anaerobic digestion in marine zooplankton and fish, and also from methanogenisis in sediments and drainage areas along coastal regions.

Hydrates. Global emissions from methane hydrates are estimated to be around 2 to 9 Tg of methane per year. Methane hydrates are solid deposits composed of cages of water molecules that contain molecules of methane. The solids can be found deep underground in polar regions and in ocean sediments of the outer continental margin throughout the world. Methane can be released from the hydrates with changes in temperature, pressure, salt concentrations, and other factors. Overall, the amount of methane stored in these hydrates globally is estimated to be very large with the potential for large releases of methane if there are significant breakdowns in the stability of the deposits. Because of this large potential for emissions, there is much ongoing scientific research related to analyzing and predicting how changes in the ocean environment affect the stability of hydrates.

Geologic. Geologic emissions are estimated to be between 42 and 64 Tg of methane per year. Geologic emissions are difficult to quantify because there are many small point sources all over the Earth. One of the dominant sources of geologic methane is mud volcanoes. These structures can be up to 10 kilometers in diameter, though most are much smaller, and often form on tectonic plate boundaries or near fossil fuel deposits. Over 1,000 such structures have been located on land or in shallow water. Mud volcanoes release methane gas from within the Earth, as well as smaller amounts of carbon dioxide, nitrogen and helium. Other structures which emit methane that would qualify as geologic sources include gryphons, steam vents and bubbling pools.

Wildfires. Wild fires are estimated to release between 2 and 5 Tg of methane per year. Methane is released during fires due to incomplete combustion of organic material. A large fraction of these emissions come from deforestation in tropical areas, however only the emissions from natural forest fires are considered here. In addition to emissions from direct combustion, fires can lead to the release of large amounts of methane from soil, especially in high latitude regions. Here, fires melt permafrost which traps methane in the soil. In addition, warmer soil temperatures after fire events lead to greater microbial activity. Greater microbial activity increases the diffusion of methane from soils to the atmosphere.

Wild animals. Another highly uncertain source of methane emissions is wild animals. Bison and buffaloes are examples of animals that release methane. Estimates of how much methane is emitted from wild animals can be derived based on estimates of the population of these species. Estimates have suggested the source strength of methane emissions from wild animals could be up to 15 Tg per year.


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