Where does methane come from?

Methane 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 60% of global methane emissions are related to human-related activities (IPCC, 2001c). 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 emissionf from man-made sources, visit the International Analysis section of this web site.

Human-related Sources

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. Figure 1 shows the contribution of the major human-related sources to methane emissions in the United States in 2001. Table 1 shows the level of emissions from individual sources for the years 1990 and 1998 to 2001.

Figure 1

Source: US Emissions Inventory 2003: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001

Table 1 U.S. Methane Emissions by Source (Gigagram (Gg) of Methane)

The principal human-related sources of methane are described below. For each source, a link is provided to the report entitled "US Emissions Inventory 2003: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2001," 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.

Landfills. Landfills are the largest human-related source of methane in the U.S., accounting for 34% of all methane emissions. 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.

Natural 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").

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.

Coal 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.

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.

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.

Wastewater 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").

Rice 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

Emissions from natural sources are largely determined by environmental variables such as temperature and precipitation. Although much uncertainty remains as to the actual contributions of these natural sources, available information indicates that global methane emissions from natural sources are around 190 Tg per year. The figure below shows the relative contribution of different natural sources to global atmospheric methane emissions.

Source: Prepared from data contained in IPCC, 2001c .

Wetlands. Natural wetlands are responsible for approximately 76% of global methane emissions from natural sources, accounting for about 145 Tg of methane 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 termites are estimated to be about 20 Tg per year, and account for approximately 11% of the global methane emissions from natural sources. 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. Oceans are estimated to be responsible for about 8% of the global methane emissions from natural sources, accounting for approximately 15 Tg of methane. 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 is estimated to be around 10 Tg of methane per year, accounting for approximately 5% of the global methane emissions from natural sources. 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.