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Methane (CH₄) is the simplest alkane, a colorless, odorless gas that is the main component of natural gas. In the atmosphere it is a trace gas, present at a mole fraction of about 1.9 parts per million (ppm), or about 1,900 parts per billion (ppb), which is about 2.6 times the preindustrial value of about 720 to 730 ppb. It is the second most important anthropogenic greenhouse gas after carbon dioxide, and it accounts for roughly 0.5 W·m⁻² of the direct radiative forcing since 1750, and about 0.6 W·m⁻² or more when indirect effects on ozone and stratospheric water vapor are included. Methane is studied in atmospheric chemistry, climatology, biogeochemistry, energy policy, and Earth system science.

Methane is far less abundant than CO₂, but each molecule absorbs much more infrared radiation, so that per unit mass it has a much larger effect on the Earth's energy budget over short timescales. Its atmospheric lifetime is only about a decade, in contrast to the centuries to millennia over which a pulse of CO₂ persists, so that cutting methane emissions changes the atmospheric concentration, and the rate of warming, relatively quickly.

Absorption of infrared radiation

Methane is a tetrahedral molecule with four C–H bonds. It absorbs infrared radiation mainly in the bending band near 7.7 μm and in the stretching bands near 3.3 μm. The 7.7 μm band lies on the shoulder of the Earth's emission spectrum, in a region where water vapor absorption is partly weaker, although the band overlaps with water vapor and nitrous oxide absorption, and the statement that the methane band is "relatively unblocked" is an oversimplification. The effect of an added molecule depends on the existing concentration, because the absorption by a gas saturates as its concentration rises. CO₂ is more strongly saturated in the center of its 15 μm band, and the forcing of CO₂ depends logarithmically on its concentration. The forcing of methane depends approximately on the square root of its concentration:

where M is the methane concentration in ppb, M₀ is the reference concentration, and f is a correction for the overlap with N₂O. For a simplified estimate that neglects the overlap, the increase from 730 to 1,900 ppb gives 0.036 × (√1,900 − √730) ≈ 0.036 × (43.6 − 27.0) ≈ 0.6 W·m⁻², which is close to the assessed values. Because of the square root dependence, a molecule added at a low concentration is more effective than one added at a high concentration. On a per-molecule basis at present concentrations, methane is about 25 to 30 times more effective than CO₂, with a larger value for the mass basis (about 80 to 90 times at 20 years, as noted below).

Atmospheric lifetime and sinks

Methane is removed from the atmosphere mainly by reaction with the hydroxyl radical (OH) in the troposphere:

This accounts for about 85 to 90 percent of the sink. The methyl radical is oxidized further, through formaldehyde and carbon monoxide, mainly to CO₂ and water vapor. Smaller sinks are the uptake by oxidation in aerobic soils by methanotrophic bacteria (about 5 percent), reactions with chlorine atoms (a few percent, uncertain), and loss in the stratosphere. The oxidation in the stratosphere is also a source of stratospheric water vapor, as each methane molecule that is oxidized yields about two water molecules. The mean lifetime τ against the removal is the burden divided by the loss rate:

where B is the burden and S is the global sink. For a burden of about 4.9 petagrams (5 × 10³ teragrams) and a sink of about 550 teragrams per year, τ ≈ 9 years. The "perturbation lifetime", which describes how long it takes a pulse of added methane to decay, is longer, about 12 years, because methane reduces its own sink by consuming OH, a positive feedback in the chemistry. The OH concentration is also affected by emissions of nitrogen oxides, carbon monoxide, and by the concentration of water vapor, so that methane is part of a chemically coupled system. The OH radical is the main oxidant in the atmosphere, and its global mean concentration is about 10⁶ molecules per cubic centimeter and it is difficult to measure directly. Estimates are made with methyl chloroform and with models.

If the sources are constant, the concentration approaches a steady state C = E τ / (conversion factor), and it relaxes after a change with a time constant of about 10 years. For example, a sudden cut of emissions to zero would reduce the excess methane by about 63 percent in 12 years and by about 90 percent in 28 years (e^−_t_/_τ_ with τ = 12 years).

Global warming potential

The global warming potential (GWP) compares the integrated radiative forcing of a pulse of a gas with that of the same mass of CO₂ over a time horizon. For methane of fossil origin, the Sixth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR6) gives a GWP of about 82.5 for 20 years and about 29.8 for 100 years (about 80 and 27 for biogenic methane, because the CO₂ formed from fossil methane adds to the atmospheric CO₂). Earlier reports gave 28 to 36 for 100 years and 84 to 87 for 20 years. The difference between the two horizons arises because methane is short-lived. The GWP depends on the choice of time horizon, which is a value judgment, and on the treatment of indirect effects. An alternative, GWP*, treats short-lived gases as a flux and not as a pulse, so that a sustained constant emission of methane has an effect that is closer to a one-time change in warming, which is relevant for the design of targets. The choice of metric is debated.

The global methane budget is estimated by the Global Carbon Project, with bottom-up methods (inventories and process models) and top-down methods (atmospheric measurements and inversions). Total emissions are about 580 to 600 teragrams per year in the 2010s and 2020s, of which roughly 60 percent are anthropogenic (about 350 to 380 teragrams per year) and about 40 percent are natural. The bottom-up and top-down estimates differ, and the sum of the individual sources in the bottom-up approach exceeds the top-down total.

Biogenic sources

Methane is made biologically by methanogenic archaea (the order Methanobacteriales and others) in anoxic environments in which other electron acceptors (oxygen, nitrate, sulfate, iron) are depleted. The main routes are the reduction of CO₂ by hydrogen (hydrogenotrophic methanogenesis), the cleavage of acetate (acetoclastic methanogenesis), and the use of methylated compounds. The natural wetlands of the tropics and the high latitudes, the rumen of cattle, rice paddies, termite guts, freshwater lakes and reservoirs, and landfills are the main habitats. The process is a type of anaerobic respiration, and it is not "the largest natural source" in all estimates, since the estimates of inland waters are uncertain. Some methane is also made in aerobic conditions, for example by plants and by marine phytoplankton, although the global contribution is small and uncertain.

Thermogenic and pyrogenic sources

Thermogenic methane forms deep in the crust by the thermal breakdown of organic matter, and it is the main component of natural gas (about 70 to 95 percent). It reaches the atmosphere in leaks from the production, processing, transport, and use of natural gas, from coal mines, and from oil wells and from natural seeps. Satellites have identified many "super-emitter" events, in which a small number of point sources, such as oil and gas facilities, landfills, and coal mines, contribute a large part of the emissions. The use of hydraulic fracturing is only one of several oil and gas activities. The emissions depend on the rate of leakage along the gas supply chain, and not on the technique alone. Fires burn biomass and release methane from incomplete combustion.

Isotopes and the recent increase

The sources can be separated partly by the stable isotope ratio δ¹³C, because biogenic methane is more depleted in ¹³C (about −55 to −70 per mil) than thermogenic (about −30 to −50 per mil) and pyrogenic (about −15 to −25 per mil) methane. The atmospheric concentration was nearly constant for a thousand years before about 1750. It rose rapidly to about 1,780 ppb by the 1990s, grew slowly or leveled off around 1999 to 2006, and then resumed growth from 2007. The growth rate reached a record in 2020 to 2021 (about 15 to 18 ppb per year), and it has remained high. The shift of the δ¹³C toward lower values since 2007 indicates that the increase is dominated by biogenic sources (wetlands, agriculture, and waste), with a possible contribution from changes in the sink (OH). The cause of the increases in 2020 and 2021 is debated. A reduction of OH from the decline in nitrogen oxide emissions during the COVID-19 lockdowns and an increase in wetland emissions in warm and wet years are both proposed. Wetland emissions respond to temperature and rainfall, which are changing, so there is a risk of a climate feedback.

Wetlands and permafrost

Warmer and wetter conditions increase methanogenesis, so that the emissions from natural wetlands are expected to increase with warming, as a positive feedback. Arctic permafrost holds about 1,400 to 1,600 petagrams of carbon in soils. As it thaws, decomposition releases CO₂ and methane. The balance depends on the oxygen content of the soil. Aerobic decomposition, in drier soils, mostly gives CO₂, and anaerobic decomposition, in waterlogged soils, gives methane in part, so that the methane fraction of the carbon released is typically a few percent by mass, but it has a larger radiative effect than its mass suggests. Thermokarst lakes, which form when ice-rich permafrost collapses, are strong sources of methane. Assessments conclude that the permafrost carbon feedback will add to the warming in this century, but that it is unlikely to dominate it, and the size of the methane part is uncertain. The Arctic is warming about two to four times faster than the global average (a range that depends on the period and the data), which is called Arctic amplification, and not strictly "four times".

Methane hydrates

Methane clathrates (hydrates) are ice-like solids in which methane is trapped in cages of water molecules. They occur in marine sediments on continental margins, usually at water depths of more than about 300 to 500 m, and in permafrost. The global inventory is uncertain, with estimates from several hundred to several thousand petagrams of carbon. The stability of the hydrate depends on temperature and pressure. Warming of bottom waters and of sediments can dissociate hydrates at the shallow edge of the stability zone. Most of the methane released from the seafloor is oxidized in the sediment by anaerobic methanotrophic archaea (the anaerobic oxidation of methane), or is oxidized in the water column, and only a small fraction reaches the atmosphere, because of the depth. Observations do not at present show a significant contribution from hydrates to the atmospheric methane growth. The "clathrate gun hypothesis", in which warming triggers a rapid and large release that causes further warming, is considered unlikely on timescales of centuries by most assessments, although the long-term response is uncertain. The statement that hydrates risk "rapid, massive venting" is therefore not supported at present.

Because of its short lifetime and large radiative effect, methane has a large potential for near-term mitigation. A 2021 assessment by the United Nations Environment Programme and the Climate and Clean Air Coalition estimated that human-caused methane emissions could be cut by about 45 percent by 2030, avoiding about 0.3 °C of warming by the 2040s, with about 60 percent of the options being in the fossil fuel, waste, and agriculture sectors that are cheapest. The Global Methane Pledge, launched in 2021, aims to cut anthropogenic methane emissions by 30 percent from the 2020 levels by 2030, and it has been joined by over 150 countries. Options include the detection and repair of leaks from oil and gas systems, the capture of methane from coal mines and landfills, the management of manure, dietary additives and breeding for ruminants, and changes in the management of water in rice paddies, as well as reductions of the demand for fossil fuels and of food waste. The monitoring of methane is improved with satellites (such as TROPOMI, GHGSat, and MethaneSAT) that can detect large emission plumes, with aircraft and tower networks, and with global networks such as that of the National Oceanic and Atmospheric Administration.

• Greenhouse gas
• Global warming potential
• Methane emissions
• Methane clathrate
• Permafrost
• Atmospheric methane
• Hydroxyl radical