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Atmospheric methane is methane (CH₄) in the atmosphere. Its global mean mole fraction was about 1,900 parts per billion (ppb) in the early 2020s, about 2.6 times the preindustrial value of roughly 720 to 730 ppb. It is the second most important anthropogenic greenhouse gas after carbon dioxide. This article concerns the behavior of methane once it is in the air: its chemical removal, its lifetime, its oxidation products, its isotopic composition, and its recent growth. The sources of methane, and its role in climate policy, are treated in the article on methane.

Methane is removed from the atmosphere mainly by chemical oxidation, and not by uptake into the ocean or the land biosphere as for CO₂. Its lifetime in the atmosphere is therefore about a decade. This is short compared with the centuries to millennia over which a pulse of CO₂ persists, so that changes in methane emissions are reflected in the atmospheric concentration within years to decades.

The hydroxyl radical sink

The main sink of methane is its reaction with the hydroxyl radical (OH) in the troposphere:

This reaction accounts for about 85 to 90 percent of the global loss. OH is formed when ultraviolet radiation photolyzes ozone, which produces an excited oxygen atom, O(¹D), that reacts with water vapor. OH is also recycled in reactions of HO₂ with nitric oxide or ozone. It reacts with most trace gases, which is why it is often called the "detergent" or the main oxidant of the atmosphere. Its concentration is very low, a global mean of about 10⁶ molecules per cubic centimeter, and its lifetime is about a second, so it cannot be measured directly with global coverage. Estimates come from the budget of methyl chloroform, from the budget of ¹⁴CO, from direct measurements in campaigns, and from chemistry models, and they carry uncertainties of about 10 to 20 percent.

The rate of the reaction depends strongly on temperature. The rate coefficient is about 6.3 × 10⁻¹⁵ cm³·molecule⁻¹·s⁻¹ at 298 K, and it is about half this value at 270 K, so that most of the loss occurs in the warm, humid lower troposphere of the tropics. The first-order loss rate and lifetime are:

For an effective temperature-weighted k of about 3.6 × 10⁻¹⁵ cm³·molecule⁻¹·s⁻¹ and [OH] = 10⁶ cm⁻³, τ_OH = 1 / (3.6 × 10⁻⁹ s⁻¹) ≈ 2.8 × 10⁸ s, which is about 8.8 years. The total lifetime against all sinks, which includes soils and the stratosphere, is somewhat shorter.

Oxidation chain and ozone

The methyl radical reacts rapidly with oxygen to form the methylperoxy radical, CH₃O₂. The subsequent steps depend on the amount of nitrogen oxides (NO_x):

• High NO_x. CH₃O₂ reacts with nitric oxide to form methoxy (CH₃O) and NO₂. The photolysis of NO₂ produces ozone, so methane oxidation is a source of tropospheric ozone. CH₃O reacts with oxygen to form formaldehyde (HCHO) and HO₂. Formaldehyde is photolyzed or reacts with OH, and it forms carbon monoxide (CO) and, finally, CO₂.
• Low NO_x. CH₃O₂ reacts with HO₂ to form methyl hydroperoxide (CH₃OOH), and the chain consumes HOx radicals, so that it removes ozone.

The final products are CO₂ and water vapor, with a yield of CO₂ that is close to 1 per methane molecule oxidized. The ozone produced contributes to the indirect radiative forcing of methane, and the CO₂ formed from fossil methane contributes to the global warming potential of fossil methane.

Other sinks

Smaller sinks are:

• Soil uptake by aerobic methanotrophic bacteria, about 30 to 40 Tg per year (about 5 percent), which is decreasing in some places because of the use of land and nitrogen fertilizer.
• Reactions with chlorine atoms in the marine boundary layer and in the stratosphere. The size of the tropospheric chlorine sink is uncertain, of the order of a few percent, and it is a subject of research.
• Loss in the stratosphere by reaction with OH, atomic oxygen, and chlorine, about 5 percent of the total.

Budget equation

The mean lifetime against all sinks is the burden divided by the loss rate. One part per billion of methane in the whole atmosphere corresponds to about 2.8 teragrams (Tg) of CH₄, so at 1,900 ppb the burden is about 5,300 Tg. For a total sink of about 560 Tg per year, the lifetime is:

The change of the concentration C (in ppb) over time is a balance of sources E and first-order loss:

At steady state, C = E τ / 2.8. For E = 580 Tg per year and τ = 9.1 years, C ≈ 580 × 9.1 / 2.8 ≈ 1,885 ppb, close to the observed value. A change of the concentration relaxes with the time constant τ when the sink is constant.

Perturbation lifetime and the OH feedback

Methane consumes OH, so that an increase of methane reduces the OH concentration, which lengthens the lifetime. This is a positive feedback of the chemistry. If the lifetime changes with the burden as s = −∂ ln τ / ∂ ln B, which is about 0.28, then the perturbation lifetime τ_pert is related to the global mean lifetime by a feedback factor:

For s = 0.28, f ≈ 1.39, so that a mean lifetime of about 9 years gives a perturbation lifetime of about 12 years (9.1 × 1.39 ≈ 12.6). The perturbation lifetime is the time that governs how long an added pulse of methane persists. Emissions of CO, NO_x, and volatile organic compounds also affect OH. CO and organic compounds consume OH and increase the methane lifetime, whereas NO_x increases OH recycling and shortens it. The OH response to the loss of OH by carbon monoxide is the reason for the statements that carbon monoxide emissions can increase the methane concentration, but a spike of CO from a large fire is short-lived (CO has a lifetime of about 1 to 2 months), and the effect on the methane lifetime is generally small and transient. The idea of "sink saturation" is not an accurate description of this feedback, because OH is continuously regenerated and the sink does not saturate, and the feedback acts on the rate of loss in a way that is modest, of about 30 to 40 percent in the lifetime.

A small fraction of methane reaches the stratosphere, mainly through the tropical tropopause. There it is oxidized by OH, by O(¹D), and by chlorine. Each oxidized methane molecule yields about two molecules of water, as well as hydrogen. The total hydrogen is conserved, so in the middle stratosphere the relation 2[CH₄] + [H₂O] ≈ 7 ppm by volume holds approximately. Water vapor entering the stratosphere is about 3.5 to 4 ppm, and oxidation raises it to about 5 to 6 ppm at upper levels. Methane is therefore a source of stratospheric water vapor, though not the only one, because water also enters through the tropical tropopause and from volcanic or pyroconvective injections.

Stratospheric water vapor is a greenhouse gas, and the increase of this water from the additional methane is assessed as a radiative forcing of about +0.02 to +0.07 W·m⁻², which is included in the effective radiative forcing of methane, and is not an additional "double" forcing. The statement that stratospheric water vapor accumulates because there is no precipitation is partly correct. Stratospheric water vapor is removed on timescales of a few years by transport back to the troposphere (the Brewer-Dobson circulation), and by condensation in the polar vortices.

Carbon isotopes

The stable isotope ratio ¹³C/¹²C of methane is expressed as δ¹³C, in per mil relative to a standard. The atmospheric value is about −47 to −48 per mil. Sources differ in their signature:

• Biogenic (microbial) methane, from wetlands, ruminants, rice, and landfills, is depleted in ¹³C, with δ¹³C of about −55 to −70 per mil, because methanogenic archaea discriminate against the heavier isotope.
• Thermogenic methane, from natural gas and coal, is less depleted, with about −30 to −50 per mil.
• Pyrogenic methane, from the burning of biomass, is enriched, with about −15 to −25 per mil in C₄ plant fires and in most burning, and lower for fires of C₃ plants.

The δ¹³C of the atmosphere is the result of the source mix and of the fractionation by sinks. The reaction with OH enriches the remaining methane in ¹³C (the kinetic isotope effect is of the order of 4 to 6 per mil), and chlorine has a much larger effect, so the fraction of chlorine in the sink influences the interpretation. A simple mixing model gives the signature of a source mix with a fraction f of biogenic methane:

For δ_bio = −62 and δ_fossil = −42, an equal mix gives −52 per mil, and a mix of 70 percent biogenic gives −56.

Hydrogen isotopes (_δ_D) and radiocarbon (¹⁴C, which is absent from fossil methane) are also used. Because several source categories overlap in their signatures, and because the sinks and the OH concentration are uncertain, the isotopic data constrain the source mix but do not give a unique answer.

Recent changes

The atmospheric δ¹³C changed little, or increased slightly, in the 1980s and 1990s. It then began to decline from about 2007, as the growth of the concentration resumed after a period in which the concentration was almost constant (about 1999 to 2006). The decline of δ¹³C means that the additional methane is isotopically light, and the most common interpretation is that the increase since 2007 is dominated by biogenic sources, such as wetlands, agriculture (ruminants and rice), and waste. Other explanations include a role for fossil emissions combined with a large reduction of the isotopically heavy sources, and changes in OH, which affect the mass balance without a change of the sources, and the relative weight of these explanations is debated. The earlier trend toward heavier values has been attributed to changes in the balance between fossil, biomass burning, and microbial sources, and not only to the "expansion of the fossil fuel industry".

Long-term record

Ice cores show that the preindustrial concentration varied between about 350 ppb in glacial periods and about 700 ppb in interglacial periods, and it was about 720 to 730 ppb in the late Holocene. It has increased since about 1750 as a result of agriculture, fossil fuel use, and waste. Direct measurements by global networks, such as that of the National Oceanic and Atmospheric Administration and AGAGE, show a rise of about 1,630 ppb in 1984 to about 1,780 ppb in the early 1990s, a slowing to near zero growth from about 1999 to 2006, and then a renewed increase. The growth rate reached about 15 to 18 ppb per year in 2020 to 2021, the largest since systematic measurements began in the 1980s, and it has remained high in later years. Growth rates of this size are related to the budget by:

so that, for example, an increase in net emissions of about 40 Tg per year would raise the concentration by about 14 ppb per year before the sink responds.

Causes of recent growth

The growth since 2007 and the peak of 2020 to 2021 are attributed to a combination of increased emissions, mainly microbial, and of changes in OH. Estimates of the contributions differ. Wetland emissions respond to temperature and rainfall, and anomalously warm and wet years, such as the La Niña conditions of 2020 to 2022, increased emissions in the tropics. A decrease of OH, which lengthens the methane lifetime, was proposed for 2020, because the decline of NO_x emissions during the lockdowns of the COVID-19 pandemic reduced OH production, but this explanation accounts for only part of the change, and other studies attribute most of the growth to emissions. Concern exists that warming could increase emissions from wetlands and permafrost, which is a positive feedback, but the size of the feedback so far is not established.

Methane is measured in air samples that are analyzed by gas chromatography with flame ionization detection, by cavity ring-down and other laser spectroscopy at towers and surface stations, by aircraft profiles, and by satellites. Satellite instruments such as GOSAT and TROPOMI measure the column-averaged mole fraction from reflected sunlight, and instruments such as GHGSat and MethaneSAT detect large point-source plumes. Global emissions are estimated by inversions, which combine measurements and a transport model, and these results are compared with the bottom-up inventories in the Global Methane Budget of the Global Carbon Project. The measurements of OH are indirect, and the uncertainty of OH is one of the largest sources of uncertainty in the inversions.

The original version of this article contained statements about a "2026 Super El Niño", record-breaking wildfires, and a record methane growth rate in 2026, attributed to an "atmospheric chemistry breakdown". I could not verify these statements and I did not search for them, so they were removed. The mechanism that was proposed, in which smoke carbon monoxide depletes OH enough to cause a record growth, would also need support from measurements, because the reduction of OH by a fire plume is short-lived and small on the global scale. If a record growth rate of methane in 2026 has been reported by the National Oceanic and Atmospheric Administration or the World Meteorological Organization, it should be added with a citation and with the stated attribution of the causes.

• Methane
• Hydroxyl radical
• Greenhouse gas
• Global warming potential
• Atmospheric chemistry
• Methane emissions
• Tropospheric ozone