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Atmospheric methane is the methane present in Earth's atmosphere.[1] The concentration of atmospheric methane—one of the most potent greenhouse gases[2]: 82 [nb 1]—is increasing due to methane emissions, and is causing climate change.[3][4]
Methane's radiative forcing (RF) of climate is direct,[5]: 2 and it is the second largest contributor to human-caused climate forcing in the historical period.[5]: 2 Methane is a major source of water vapour in the stratosphere through oxidation;[6] and water vapour adds about 15% to methane's radiative forcing effect.[7] Methane increases the amount of ozone O3 in the troposphere—4 miles (6.4 km) to 12 miles (19 km) from the Earth's surface— and stratosphere—from the troposphere to 31 miles (50 km) above the Earth's surface.[8] Both water vapour and ozone are GHGs, which in turn adds to climate warming.[5]: 2
Since the beginning of the Industrial Revolution (around 1750) the atmospheric methane concentration has increased by about 260%, with the overwhelming percentage caused by human activity.[9] Since 1750—in terms of mass—methane has contributed 3% of GHG emissions[10] but is responsible for approximately 23% of radiative or climate forcing.[11][12][13] In 2019, global methane concentrations rose from 722 parts per billion (ppb) in pre-industrial times to 1866 ppb,[14] an increase by a factor of 2.6 and the highest value in at least 800,000 years.[15]: 4 [16][nb 2][17]
The IPCC reports that the global warming potential (GWP) for methane is about 84 in terms of its impact over a 20-year timeframe—[18][19]—that means it traps 84 times more heat per mass unit than carbon dioxide (CO2) and 105 times the effect when accounting for aerosol interactions.[20]
Methane is a short-lived climate pollutant (SLCP) with a lifetime in the atmosphere of twelve years.[21][22] The Coalition for Climate Action says that mitigation efforts to reduce short-lived climate pollutants, like methane and black carbon would help combat "near-term climate change" and would support Sustainable Development Goals.[21]
Atmospheric methane and climate change
Methane in the Earth's atmosphere is a powerful greenhouse gas with a global warming potential (GWP) 84 times greater than CO2 in a 20-year time frame.[24][25][26][27]
Radiative or climate forcing is the scientific concept used to measure the human impact on the environment in watts / meter².[28] It refers to the "difference between solar irradiance absorbed by the Earth and energy radiated back to space"[29] The direct radiative greenhouse gas forcing effect of methane relative to 1750 has been estimated at 0.5 W/m2 (watts per meter²) in the 2007 IPCC "Climate Change Synthesis Report 2007".[30]: 38
In their May 21, 2021 173-page "Global Methane Assessment", the UNEP and CCAP said that their "understanding of methane's effect on radiative forcing" improved with research by teams led by M. Etminan in 2016,[11] and William Collins in 2018,[5] which resulted in an "upward revision" since the 2014 IPCC Fifth Assessment Report (AR5). The "improved understanding" says that prior estimates of the "overall societal impact of methane emissions" were likely underestimated.[31]: 18 Etminan et al. published their new calculations for methane's radiative forcing (RF) in a 2016 Geophysical 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 for well mixed greenhouse gases (WMGHG) forcings by including the shortwave forcing component due to CH4, resulted in estimates that were approximately 20-25% higher.[11] 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".[5]
About 40% of methane emissions from the fossil fuel industry could be "eliminated at no net cost for firms", according to the International Energy Agency (IEA) by using existing technologies.[32] Forty percent represents 9% of all human methane emissions.[32] The Economist recommended setting methane emissions targets as a reduction in methane emissions would allow for more time to tackle the more challenging carbon emissions".[32][33]
Methane is a strong GHG with a global warming potential 84 times greater than CO2 in a 20-year time frame. Methane is not as persistent a gas and tails off to about 28 times greater than CO2 for a 100-year time frame.[19]
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 (1988)[34] 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 future methane may lead to a surface warming that increases nonlinearly with the methane concentration.
Global monitoring of atmospheric methane concentrations
CH4 has been measured directly in the environment since the 1970s.[36][9] The Earth's atmospheric methane concentration has increased 260% since preindustrial levels in the mid-18th century, according to the 2022 United Nations Environment Programme's (UNEP) "Global Methane Assessment".[9]
Long term atmospheric measurements of methane by NOAA show that the build up of methane nearly tripled since pre-industrial times since 1750.[37] In 1991 and 1998 there was a sudden growth rate of methane representing a doubling of growth rates in previous years.[37] The June 15, 1991 eruption of Mount Pinatubo, measuring VEI-6—was the second-largest terrestrial eruption of the 20th century.[38] According to the 2007 IPCC AR7, unprecedented warm temperatures in 1998—the warmest year since surface records were recorded—could have could have induced elevated methane emissions, along with an increase in wetland and rice field emissions and the amount of biomass burning.[39]
Data from 2007 suggested methane concentrations were beginning to rise again.[40] 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 7 nmol/mol per year during 2007 and 2008. During the first half of 2009, globally averaged atmospheric CH4 was 7 nmol/mol greater than it was in 2008, suggesting that the increase will continue in 2009."[41] From 2015 to 2019 sharp rises in levels of atmospheric methane have been recorded.[42]
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. 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.[43] 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.[12] 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.[9]
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."[12][44] 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.[45][46] As of 2016, methane contributed radiative forcing of 0.62 ± 14% Wm−2,[11] or about 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases.[19] According to NOAA, the atmospheric methane concentration has continued to increase since 2011 to an average global concentration of 1895.3 ± 0.6 ppb as of 2021.[14] The May 2021 peak was 1891.6 ppb, while the April 2022 peak was 1909.6 ppb, a 0.9% increase.[46]
The Global Carbon Project consortium funded by the French BNB Paribas Fondation, produces the Global Methane Budget. Working with over fifty international research institutions and 100 stations globally and update the methane budget every few years.[48]
A 2013 Nature Geoscience article based on data collected between 1980 and 2010 said that at that time the balance between sources and sinks of methane was not fully understood. Scientists were unable to explain why the atmospheric concentration of methane had temporarily ceased to increase.[49]
The focus on the role of methane in anthropogenic climate change has become more relevant since the mid-2010s.[50]
Methane cycle
This graphic depicts the flow of methane from natural and anthropogenic sources into the atmosphere as well as the sinks that capture, convert, or store methane.[51][52][53][54][55][56][57][58]
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.[10] Two main processes that are responsible for methane production include microorganisms anaerobically converting organic compounds into methane (methanogenesis), which are widespread in aquatic ecosystems, and ruminant animals. Other natural sources include melting permafrost, wetlands, plants, and methane clathrates.[citation needed]
Natural sinks or removal of atmospheric methane
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 an atmospheric chemical process.[59]
Another major natural sink is through oxidation by methanotrophic 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—the ozone layer—and then the upper portion of the stratosphere.[59]
This atmospheric chemical process is the most effective methane sink, as it removes 90% of atmospheric methane.[49] This global destruction of atmospheric methane mainly occurs in the troposphere.[49]
Methane molecules react with hydroxyl radicals (OH)—the "major chemical scavenger in the troposphere" that "controls the atmospheric lifetime of most gases in the troposphere".[60] 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 also increases radiative 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.[61][6]
By the 1980s, the global warming problem had been transformed by the inclusion of methane and other non-CO2 trace-gases—CFCs, N20, and 03— on global warming, instead of focusing primarily on carbon dioxide, according to V. Ramanathan in his 1997 Volvo Environmental Prize Lecture.[62][63] Ramanathan summarized research benchmarks that deepened understanding of the role of the stratosphere, the troposphere, methane and other gases, including the feedback loop that increased the amount of water vapor in the stratosphere where little water usually reaches.[63] He said that both water and ice clouds, when formed at cold lower stratospheric temperatures, have a significant impact by increasing the atmospheric greenhouse effect. He cautioned that large increases in future methane could lead to a surface warming that increases nonlinearly with the methane concentration.[62][63]
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.[64] 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, according to Drew Shindell.[65]
According to a 1978–2003 study of the impact of methane and hydrogen in the stratosphere on the "abundance of stratospheric water vapor" published in 2006 in the Journal of Geophysical Research: Atmospheres, atmospheric methane could last about 120 years in the stratosphere until it was eventually destroyed through the hydroxyl radicals oxidation process.[66]
As of 2001, the mean lifespan of methane in the atmosphere was estimated at 9.6 years. However, increasing emissions of methane over time reduced the concentration of the hydroxyl radical in the atmosphere.[67] With less OH˚ to react with, the lifespan of methane could also increase, resulting in greater concentrations of atmospheric methane according to Holmes et al. in 2013.[68]
By 2013, methane's mean lifetime in the atmosphere was twelve years.[21][22]
The reaction of methane and chlorine atoms acts as a primary sink of Cl atoms and is a primary source of hydrochloric acid (HCl) in the stratosphere, according to a 2000 publication.[69]
CH4 + Cl → CH3 + HCl
According to a 2006 Journal of Geophysical Research article, the HCl produced in this reaction leads to catalytic ozone destruction in the stratosphere.[66]
Methanotrophs in soils
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.[70]
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.[70] 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.
Methanotrophic bacteria in soils – Methanotrophic bacteria that reside within soil use methane as a source of carbon in methane oxidation.[70] 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.
- CH4 + 2O2 → CO2 + 2H2O
Quantifying the global methane CH4 budget
In order to mitigate climate change, scientists have been focusing on quantifying the global methane CH4 budget as the concentration of methane continues to increase—it is now second after carbon dioxide in terms of climate forcing.[50] Further understanding of atmospheric methane is necessary in "assessing realistic pathways" towards climate change mitigation.[50] Various research groups give the following values for methane emissions:
| Reference: | Fung et al. (1991) | Hein et al. (1997) | Lelieveld et al. (1998) | Houweling et al. (1999) | Bousquet et al. (2006)[72] | Saunois et al. (2016)[10] | Saunois et al. (2020)[47] |
|---|---|---|---|---|---|---|---|
| Base year: | 1980s | – | 1992 | – | – | 2003–2012 | 2008-2017 |
| Natural emission sources | |||||||
| Wetlands | 115 | 237 | 225[nb 3] | 145 | 147±15 | 167 (127–202) | 181 (159-200) |
| Termites | 20 | – | 20 | 20 | 23±4 | 64 (21–132) | 37 (21–50) |
| Ocean | 10 | – | 15 | 15 | 19±6 | ||
| Hydrates | 5 | – | 10 | – | – | ||
| Anthropogenic emission sources | |||||||
| Energy | 75 | 97 | 110 | 89 | 110±13 | 105 (77–133) | 111 (81-131) |
| Landfills | 40 | 35 | 40 | 73 | 55±11[nb 4] | 188 (115-243) | 217 (207-240) |
| Ruminants (livestock) | 80 | 90[nb 5] | 115 | 93 | |||
| Waste treatment | – | [nb 5] | 25 | – | [nb 4] | ||
| Rice agriculture | 100 | 88 | [nb 3] | – | 31±5 | ||
| Biomass burning | 55 | 40 | 40 | – | 50±8 | 34 (15–53) | 30 (22-36) |
| Other | – | – | – | 20 | 90±14[nb 6] | ||
| Sinks | |||||||
| Soils | 10 | 30 | 40 | 21±3 | 33 (28–38) | 38 (27-45) | |
| Tropospheric OH | 450 | 489 | 510 | 448±1 | 515 | 518 (474–532) | |
| Stratospheric loss | 46 | 40 | 37±1 | ||||
| Source versus sink imbalance | |||||||
| Total source | 500 | 587 | 600 | 525±8 | 558 (540–568) | 576 (550-594) | |
| Total sink | 460 | 535 | 580 | 506 | 548 | 556 (501–574) | |
Human-caused methane emissions
| Part of a series on the |
| Carbon cycle |
|---|
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The AR6 of the IPCC said, "It is unequivocal that the increases in atmospheric carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) since the pre-industrial period are overwhelmingly caused by human activities."[36][9][12] Atmospheric methane accounted for 20% of the total radiative forcing (RF) from all of the long-lived and globally mixed greenhouse gases.
According to the 2021 assessment by the Climate and Clean Air Coalition (CCAC) and the United Nations Environment Programme (UNEP) over 50% of global methane emissions are caused by human activities in fossil fuels (35%), waste (20%), and agriculture (40%). The oil and gas industry accounts for 23%, and coal mining for 12%. Twenty percent of global anthropogenic emissions stem from landfills and wastewater. Manure and enteric fermentation represent 32%, and rice cultivation represents 8%.[31]
The most clearly identified rise in atmospheric methane as a result of human activity occurred in the 1700s during the industrial revolution. During the 20th century—mainly because of the use of fossil fuels—concentration of methane in the atmosphere increased, then stabilized briefly in the 1990s,[37] only to begin to increase again in 2007. After 2014, the increase accelerated and by 2017, reached 1,850 (parts per billion) ppb.[43][73]
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