Atmospheric methane is themethane present in Earth'satmosphere.[2] The concentration of atmospheric methane is increasing due tomethane emissions, and is causingclimate change.[3][4] Methane is one of the most potentgreenhouse gases.[5]: 82 Methane's radiative forcing (RF) of climate is direct,[6]: 2 and it is the second largest contributor to human-caused climate forcing in the historical period.[6]: 2 Methane is a major source of water vapour in the stratosphere through oxidation;[7] and water vapour adds about 15% to methane's radiative forcing effect.[8] Theglobal warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe, and 28 in terms of its impact over a 100-year timeframe.[9][10]
Since the beginning of theIndustrial Revolution (around 1750), the methane concentration in the atmosphere has increased by about 160%, and human activities almost entirely caused this increase.[11] Since 1750 methane has contributed 3% ofgreenhouse gas (GHG) emissions in terms of mass[12] but is responsible for approximately 23% ofradiative or climate forcing.[13][14][15] By 2019, global methane concentrations had risen from 722 parts per billion (ppb) in pre-industrial times to 1866 ppb.[16] This is an increase by a factor of 2.6 and the highest value in at least 800,000 years.[17]: 4 [18][19]
Methane increases the amount ofozone (O3) in the troposphere (4 miles (6 km) to 12 miles (19 km) from the Earth's surface) and also in the stratosphere (from the troposphere to 31 miles (50 km) above the Earth's surface).[20] Both water vapour and ozone are GHGs, which in turn add to climate warming.[6]: 2
Methane (CH4) in the Earth's atmosphere is a powerfulgreenhouse gas with aglobal warming potential (GWP) 84 times greater than CO2 over a 20-year time frame.[22][23] Methane is not as persistent as CO2, and tails off to about 28 times greater than CO2 over a 100-year time frame.[10]
Radiative or climate forcing is the scientific concept used to measure thehuman impact on the environment inwatts per square meter (W/m2).[24] It refers to the "difference betweensolar irradiance absorbed by the Earth and energy radiated back to space"[25] The direct radiative greenhouse gas forcing effect of methane was estimated to be an increase of 0.5 W/m2 relative to the year 1750 (estimate in 2007).[26]: 38 (Figure 2.3)
In their 2021 "Global Methane Assessment" report, the UNEP and CCAC said that their "understanding of methane's effect on radiative forcing" improved with research by teams led by M. Etminan in 2016,[13] and William Collins in 2018.[6] This resulted in an "upward revision" since the 2014IPCC Fifth Assessment Report (AR5). The "improved understanding" says that prior estimates of the "overall societal impact of methane emissions" were likely underestimated.[27]: 18
Etminan et al. published their new calculations for methane's radiative forcing (RF) in a 2016Geophysical Research Letters journal article which incorporated the shortwave bands of CH4 in measuring forcing, not used in previous, simpler IPCC methods. Their new RF calculations which significantly revised those cited in earlier, successive IPCC reports forwell mixed greenhouse gases (WMGHG) forcings by including the shortwave forcing component due to CH4, resulted in estimates that were approximately 20–25% higher.[13] Collins et al. said that CH4 mitigation that reduces atmospheric methane by the end of the century, could "make a substantial difference to the feasibility of achieving the Paris climate targets", and would provide us with more "allowable carbon emissions to 2100".[6]
In addition to the direct heating effect and the normal feedbacks, the methane breaks down to carbon dioxide and water. This water is often above the tropopause, where little water usually reaches. Ramanathan (1998)[28] notes that both water and ice clouds, when formed at cold lower stratospheric temperatures, are extremely efficient in enhancing the atmospheric greenhouse effect. He also notes that there is a distinct possibility that large increases in methane in future may lead to a surface warming that increases nonlinearly with the methane concentration.
Mitigation efforts to reduce short-lived climate pollutants like methane andblack carbon would help combat "near-term climate change" and would supportSustainable Development Goals.[29]
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Any process that results in the production of methane and its release into the atmosphere can be considered a "source". The known sources of methane are predominantly located near the Earth's surface.[12] Two main processes that are responsible for methane production includemicroorganismsanaerobically converting organic compounds into methane (methanogenesis), which are widespread inaquatic ecosystems, andruminant animals.
Methane is also released in the Arctic for example fromthawing permafrost.
Increasingmethane emissions are a major contributor to the rising concentration ofgreenhouse gases inEarth's atmosphere, and are responsible for up to one-third of near-termglobal heating.[31][32] During 2019, about 60% (360 million tons) ofmethane released globally was from human activities, while natural sources contributed about 40% (230 million tons).[33][34] Reducing methane emissions by capturing and utilizing the gas can produce simultaneous environmental and economic benefits.[31][35]
Since the Industrial Revolution, concentrations of methane in the atmosphere have more than doubled, and about 20 percent of the warming the planet has experienced can be attributed to the gas.[36] About one-third (33%) ofanthropogenic emissions are from gas release during theextraction and delivery offossil fuels; mostly due togas venting andgas leaks from both active fossil fuel infrastructure andorphan wells.[37] Russia is the world's top methane emitter from oil and gas.[38][39] The International Energy Agency (IEA) highlights that abandoned coal mines and oil and gas wells have become significant sources of methane emissions. If considered a country, these emissions would rank as the fourth-largest globally, surpassing those of Iran. The IEA estimates that addressing over 8 million abandoned onshore oil and gas sites would cost about $100 billion.[40]
Animal agriculture is a similarly large source (30%); primarily because ofenteric fermentation byruminant livestock such as cattle and sheep. According to the Global Methane Assessment published in 2021, methane emissions from livestock (including cattle) are the largest sources ofagricultural emissions worldwide[41] A single cow can make up to 99 kg of methane gas per year.[42] Ruminant livestock can produce 250 to 500 L of methane per day.[43]Methane was typically measured usinggas chromatography. Gas chromatography is a type ofchromatography used for separating or analyzing chemical compounds. It is less expensive in general, compared to more advanced methods, but it is more time and labor-intensive.[citation needed]
Spectroscopic methods were the preferred method for atmospheric gas measurements due to its sensitivity and precision. Also, spectroscopic methods are the only way of remotely sensing the atmospheric gases.Infrared spectroscopy covers a large spectrum of techniques, one of which detects gases based onabsorption spectroscopy. There are various methods for spectroscopic methods, includingDifferential optical absorption spectroscopy,Laser-induced fluorescence, andFourier Transform Infrared.[citation needed]
In 2011,cavity ring-down spectroscopy was the most widely used IR absorption technique of detecting methane. It is a form oflaser absorption spectroscopy which determines the mole fraction to the order of parts per trillion.
CH4 has been measured directly in the environment since the 1970s.[46][11] The Earth's atmospheric methane concentration has increased 160% since preindustrial levels in the mid-18th century.[11]
Long term atmospheric measurements of methane byNOAA show that the build up of methane nearly tripled since pre-industrial times since 1750.[47] In 1991 and 1998 there was a sudden growth rate of methane representing a doubling of growth rates in previous years.[47] TheJune 15, 1991 eruption of Mount Pinatubo, measuringVEI-6—was the second-largest terrestrial eruption of the 20th century.[48] In 2007 it was reported that unprecedented warm temperatures in 1998—the warmest year since surface records were recorded—could have induced elevated methane emissions, along with an increase in wetland and rice field emissions and the amount of biomass burning.[49]
Data from 2007 suggested methane concentrations were beginning to rise again.[50] This was confirmed in 2010 when a study showed methane levels were on the rise for the 3 years 2007 to 2009. After a decade of near-zero growth in methane levels, "globally averaged atmospheric methane increased by [approximately] 7 nmol/mol per year during 2007 and 2008. During the first half of 2009, globally averaged atmospheric CH4 was [approximately] 7 nmol/mol greater than it was in 2008, suggesting that the increase will continue in 2009."[51] From 2015 to 2019 sharp rises in levels of atmospheric methane have been recorded.[52]
In 2010, methane levels in the Arctic were measured at 1850 nmol/mol which is over twice as high as at any time in the last 400,000 years.[citation needed] According to the IPCC AR5, since 2011 concentrations continued to increase. After 2014, the increase accelerated and by 2017, it reached 1,850 (parts per billion) ppb.[53] The annual average for methane (CH4) was 1866 ppb in 2019 and scientists reported with "very high confidence" that concentrations of CH4 were higher than at any time in at least 800,000 years.[14] The largest annual increase occurred in 2021 with current concentrations reaching a record 260% of pre-industrial—with the overwhelming percentage caused by human activity.[11]
In 2013, IPCC scientists said with "very high confidence", that concentrations of atmospheric methane CH4 "exceeded the pre-industrial levels by about 150% which represented "levels unprecedented in at least the last 800,000 years."[14][54] The globally averaged concentration of methane in Earth's atmosphere increased by about 150% from 722 ± 25 ppb in 1750 to 1803.1 ± 0.6 ppb in 2011.[55][56] As of 2016, methane contributedradiative forcing of 0.62 ± 14% Wm−2,[13] or about 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases.[10] The atmospheric methane concentration has continued to increase since 2011 to an average global concentration of 1911.8 ± 0.6 ppb as of 2022.[16] The May 2021 peak was 1891.6 ppb, while the April 2022 peak was 1909.4 ppb, a 0.9% increase.[56]
The Global Carbon Project consortium produces the Global Methane Budget. Working with over fifty international research institutions and 100 stations globally, it updates the methane budget every few years.[57]
In 2013, the balance betweensources and sinks of methane was not yet fully understood. Scientists were unable to explain why the atmospheric concentration of methane had temporarily ceased to increase.[58]
The focus on the role of methane in anthropogenic climate change has become more relevant since the mid-2010s.[59]
The amount of methane in the atmosphere is the result of a balance between the production of methane on the Earth's surface—its source—and the destruction or removal of methane, mainly in the atmosphere—its sink— in anatmospheric chemical process.[60]
Another major natural sink is through oxidation bymethanotrophic or methane-consuming bacteria in Earth's soils.
These 2005 NASA computer model simulations—calculated based on data available at that time—illustrate how methane is destroyed as it rises.
As air rises in the tropics, methane is carried upwards through the troposphere—the lowest portion of Earth's atmosphere which is 4 miles (6.4 km) to 12 miles (19 km) from the Earth's surface, into the lower stratosphere—theozone layer—and then the upper portion of the stratosphere.[60]
This atmospheric chemical process is the most effective methane sink, as it removes 90% of atmospheric methane.[58] This global destruction of atmospheric methane mainly occurs in the troposphere.[58]
Methane molecules react withhydroxyl radicals (OH)—the "major chemical scavenger in the troposphere" that "controls the atmospheric lifetime of most gases in the troposphere".[61] Through this CH4 oxidation process, atmospheric methane is destroyed and water vapor and carbon dioxide are produced.
While this decreases the concentration of methane in the atmosphere, it is unclear if this leads to a net positive increase inradiative forcing because both water vapor and carbon dioxide are more powerful GHGs factors in terms of affecting the warming of Earth.
This additional water vapor in the stratosphere caused by CH4 oxidation, adds approximately 15% to methane's radiative forcing effect.[62][7]
By the 1980s, the global warming problem had been transformed by the inclusion of methane and other non-CO2 trace-gases—CFCs, N2O, and O3— on global warming, instead of focusing primarily on carbon dioxide.[63][64] Both water and ice clouds, when formed at cold lower stratospheric temperatures, have a significant impact by increasing the atmospheric greenhouse effect. Large increases in future methane could lead to a surface warming that increases nonlinearly with the methane concentration.[63][64]
Methane also affects the degradation of the ozone layer—the lowest layer of the stratosphere from about 15 to 35 kilometers (9 to 22 mi) above Earth, just above the troposphere.[65] NASA researchers in 2001, had said that this process was enhanced by global warming, because warmer air holds more water vapor than colder air, so the amount of water vapor in the atmosphere increases as it is warmed by the greenhouse effect. Their climate models based on data available at that time, had indicated that carbon dioxide and methane enhanced the transport of water into the stratosphere.[66]
Atmospheric methane could last about 120 years in the stratosphere until it is eventually destroyed through the hydroxyl radicals oxidation process.[67]
There are different ways to quantify the period of time that methane impacts the atmosphere. The average time that a physical methane molecule is in the atmosphere is estimated to be around 9.6 years.[69][70][68] However, the average time that the atmosphere will be affected by the emission of that molecule before reaching equilibrium – known as its 'perturbation lifetime' – is approximately twelve years.[29][71]
The reaction of methane and chlorine atoms acts as a primary sink of Cl atoms and is a primary source ofhydrochloric acid (HCl) in the stratosphere.[72]
CH4 + Cl → CH3 + HCl
The HCl produced in this reaction leads to catalyticozone destruction in the stratosphere.[67]
Soils act as a major sink for atmospheric methane through the methanotrophic bacteria that reside within them. This occurs with two different types of bacteria. "High capacity-low affinity" methanotrophic bacteria grow in areas of high methane concentration, such as waterlogged soils in wetlands and other moist environments. And in areas of low methane concentration, "low capacity-high affinity" methanotrophic bacteria make use of the methane in the atmosphere to grow, rather than relying on methane in their immediate environment.[74] Methane oxidation allows methanotrophic bacteria to use methane as a source of energy, reacting methane with oxygen and as a result producing carbon dioxide and water.
Forest soils act as good sinks for atmospheric methane because soils are optimally moist for methanotroph activity, and the movement of gases between soil and atmosphere (soil diffusivity) is high.[74] With a lower water table, any methane in the soil has to make it past the methanotrophic bacteria before it can reach the atmosphere. Wetland soils, however, are often sources of atmospheric methane rather than sinks because the water table is much higher, and the methane can be diffused fairly easily into the air without having to compete with the soil's methanotrophs.[74]
Methanotrophic bacteria also occur in the underwatersediments. Their presence can often efficiently limit emissions from sources such as the underwaterpermafrost in areas like the Laptev Sea.[73]
Atmospheric methane removal is a category of potential approaches being researched to accelerate the breakdown ofmethane that is in the atmosphere, for mitigating some of the impacts ofclimate change.[75] Atmospheric methane has increased since pre-industrial times from 0.7 ppm to 1.9 ppm, more than doubling in concentration.[76] The increase in methane from industrialization is linked to natural methane sinks failure in accommodating for the increase in methane due to anthropogenic activities.[76] From 2010 to 2019,methane emissions caused 0.5 °C (about 30%) of observedglobal warming. Global methane emissions approached a record 600 Tg CH4 per year in 2017.[77] Methane is a potent greenhouse gas with a lifetime of up to 12 years. With the ability to undergo reactions converting methane to carbon dioxide which has a lifetime in the hundreds of thousands of years, methane is an impactful greenhouse gas. Atmospheric methane is a more powerful greenhouse gas than carbon dioxide with aglobal warming potential 34 times higher over and 86 times higher over 20 years.[78] Since methane is a more powerful greenhouse gas, removing smaller amounts of atmospheric methane compared to carbon dioxide from the atmosphere would result in a similar climate impact.[78] With background levels of atmospheric carbon dioxide being approximately 420 ppm and background levels of atmospheric methane being approximately 1.92 ppm, lower concentrations of methane result in an increase in difficulty in methane capture compared to carbon dioxide.[76][78][75] Alongside the scarcity of methane in the atmosphere, methane's stability and atmospheric conditions in terms of thermodynamics, kinetics, and mass transfer are large contributing factors in the difficulty of removal.[76] The removal of carbon dioxide from the atmosphere has proven to be impactful in terms of greenhouse gas mitigation efforts, with diverse research efforts and a technological foundation.[78]
An abundance of atmospheric methane removal methods have been studied, including photocatalysts and metal catalysts associated with zeolites and porous polymer networks.[78][79] Biological-based methane removal methods that have been studied include iron-salt aerosol formation, industrial approaches, and approaches managing soils in a variety of ecosystems.[78][79] Many of the methane removal methods involve oxidation chemistry, such as thermal-catalytic, photocatalytic, and biological oxidation.[75] The targeted locations for methane destruction include ambient air, methane-enriched air, high-methane air, very high-methane air, and near-explosive air.[75] Descriptions of these targets and their respective atmospheric mixing ratios can be seen in table 1 with information as provided in Nisbet-Jones.[75]
| Target | Mixing ratio (ppm) | Example location |
| Ambient air | 1.9 ppm | Anywhere on Earth (0.1 ppm higher in the Northern Hemisphere than the Southern Hemisphere) |
| Methane-enriched air | 10 ppm | In wider vicinities of cattle, directly above large area sources such as wetlands, rice paddies, landfills, and blast zones of an open-cast coal mine. |
| High-methane air | 100 ppm | Above a tank of manure or above feeding troughs in a barn holding cattle. |
| Very high-methane air | 1000 ppm | Near a gas-field dewatering installation or a leaky compressor. |
| Near-explosive air | 1% methane | Near a deliberately venting oil-field installation, leaking gas distribution or landfill gas extraction pipe, etc. |
Table 1: This table directly restates information from Nisbet-Jones providing information on different targets for methane detection and the respective mixing ratios and examples listed in the opinion piece.[75]
In order to remove methane from the atmosphere, energy which most likely involves the emission of greenhouse gases is directly involved.[75] This makes some more anthropogenic-based forms of methane removal potentially less productive and more damaging than leaving the methane in the atmosphere.[75] Regardless of potential environmental impacts, costs versus benefits, and other notable factors, all methods of atmospheric methane removal will be thoroughly discussed in this article including related information regarding atmospheric methane removal..png)
From 1996 to 2004, researchers in theEuropean Project for Ice Coring in Antarctica (EPICA) project were able to drill and analyze gases trapped in the ice cores in Antarctica to reconstruct GHG concentrations in the atmosphere over the past 800,000 years".[80] They found that prior to approximately 900,000 years ago, the cycle of ice ages followed by relatively short warm periods lasted about 40,000 years, but by 800,000 years ago the time interval changed dramatically to cycles that lasted 100,000 years.[80] There were low values of GHG in ice ages, and high values during the warm periods.[80]
This 2016 EPA illustration above is a compilation ofpaleoclimatology showing methane concentrations over time based on analysis of gas bubbles from[81] EPICADome C, Antarctica—approximately 797,446 BCE to 1937 CE,[82]Law Dome, Antarctica—approximately 1008 CE to 1980 CE[83]Cape Grim, Australia—1985 CE to 2015 CE[84]Mauna Loa, Hawaii—1984 CE to 2015 CE[85] andShetland Islands, Scotland: 1993 CE to 2001 CE[86]
The massive and rapid release of large volumes of methane gas from such sediments into the atmosphere has been suggested as a possible cause for rapidglobal warming events in the Earth's distant past, such as thePaleocene–Eocene Thermal Maximum,[88] and theGreat Dying.[89]
In 2001, NASA'sGoddard Institute for Space Studies andColumbia University'sCenter for Climate Systems Research scientists confirmed that other greenhouse gases apart from carbon dioxide were important factors in climate change in research presented at the annual meeting of theAmerican Geophysical Union (AGU).[90] They offered a theory on the 100,000-year longPaleocene–Eocene Thermal Maximum that occurred approximately 55 million years ago. They posited that there was a vast release of methane that had previously been kept stable through "cold temperatures and high pressure...beneath the ocean floor". This methane release into the atmosphere resulted in the warming of the earth. A 2009 journal article inScience, confirmed NASA research that the contribution of methane to global warming had previously been underestimated.[91][92]
Early in theEarth's history carbon dioxide and methane likely produced agreenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. During this time, Earth's earliest life appeared.[93] According to a 2003 article in the journalGeology, these first, ancient bacteria added to the methane concentration by converting hydrogen and carbon dioxide into methane and water. Oxygen did not become a major part of the atmosphere until photosynthetic organisms evolved later in Earth's history. With no oxygen, methane stayed in the atmosphere longer and at higher concentrations than it does today.[94]
{{cite book}}:|work= ignored (help) See Table 8.7.{{cite web}}: CS1 maint: multiple names: authors list (link){{cite web}}: CS1 maint: numeric names: authors list (link)Russia is the world's top source of methane emissions from the oil-and-gas industry
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