Natural sources of methane include wetlands, gas hydrates, permafrost, termites, oceans, freshwater bodies, non-wetland soils, and other sources such as wildfires.
Natural wetlands are responsible for approximately 80% of global methane emissions from natural sources. Wetlands provide a habitat conducive to methane-producing 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.
Global emissions of termites are estimated to be about 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 are estimated to be responsible for about 8% of the global methane emissions from natural sources.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.
Global emissions from methane hydrates is estimated to account 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.
Human-related activities that produce methane include fossil fuel production, livestock industry, rice cultivation, biomass burning, and waste management. This is broken down into smaller categories above. 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.
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 conditions (with no oxygen). The amount of methane created depends on the quantity and moisture content of the waste and the design and management practices at the site.
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.
Methane trapped in coal deposits is released during normal mining operations in both underground and surface mines. In addition, handling of the coal after mining results in methane emissions.
Among domesticated livestock such as cattle, buffalo, sheep, goats, and camels, these animals 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 (cow farts if you will). (Methane is also produced in smaller quantities by the digestive processes of other animals, including humans, but emissions from these sources are insignificant.)
Wastewater from sewage 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 (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.
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. There have been cultivation practices that have shown promise for reducing methane emissions from rice cultivation.