Atmospheric methane

Since the beginning of the Industrial Revolution, the methane concentration in the atmosphere has increased by about 160%, and human activities almost entirely caused this increase. Since 1750 methane has contributed 3% of greenhouse gas emissions in terms of mass but is responsible for approximately 23% of radiative or climate forcing. By 2019, global methane concentrations had risen from 722 parts per billion in pre-industrial times to 1866 ppb. This is an increase by a factor of 2.6 and the highest value in at least 800,000 years.
Methane increases the amount of ozone in the troposphere and also in the stratosphere. Both water vapour and ozone are GHGs, which in turn add to climate warming.

Role in climate change

Methane in the Earth's atmosphere is a powerful greenhouse gas with a global warming potential 84 times greater than CO₂ over a 20-year time frame. Methane is not as persistent as CO₂, and tails off to about 28 times greater than CO₂ over a 100-year time frame.
Radiative or climate forcing is the scientific concept used to measure the human impact on the environment in watts per square meter. It refers to the "difference between solar irradiance absorbed by the Earth and energy radiated back to space" 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.
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, and William Collins in 2018. This resulted in an "upward revision" since the 2014 IPCC Fifth Assessment Report. The "improved understanding" says that prior estimates of the "overall societal impact of methane emissions" were likely underestimated.
Etminan et al. published their new calculations for methane's radiative forcing 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 forcings by including the shortwave forcing component due to CH₄, resulted in estimates that were approximately 20–25% higher. 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".
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 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 and black carbon would help combat "near-term climate change" and would support Sustainable Development Goals.

Measurement techniques

Methane was typically measured using gas chromatography. Gas chromatography is a type of chromatography 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.
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 on absorption spectroscopy. There are various methods for spectroscopic methods, including Differential optical absorption spectroscopy, laser-induced fluorescence, and Fourier-transform infrared.
In 2011, cavity ring-down spectroscopy was the most widely used IR absorption technique of detecting methane. It is a form of laser absorption spectroscopy which determines the mole fraction to the order of parts per trillion.

Global monitoring

Atmospheric methane is the methane present in Earth's atmosphere. The concentration of atmospheric methane is increasing due to methane emissions, and is causing climate change. Methane is one of the most potent greenhouse gases. Methane's radiative forcing of climate is direct, and it is the second largest contributor to human-caused climate forcing in the historical period. Methane is a major source of water vapour in the stratosphere through oxidation; and water vapour adds about 15% to methane's radiative forcing effect. The global warming potential 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.
image:Mlo ch4 ts obs 03437.png |thumb|upright=1.35|Methane concentration at NOAA's Mauna Loa observatory through July 2021: A record-high of 1912 ppb was reached in December 2020.CH₄ has been measured directly in the environment since the 1970s. The Earth's atmospheric methane concentration has increased 160% since preindustrial levels in the mid-18th century.
Long term atmospheric measurements of methane by NOAA show that the build up of methane nearly tripled since pre-industrial times since 1750. In 1991 and 1998 there was a sudden growth rate of methane representing a doubling of growth rates in previous years. The June 15, 1991 eruption of Mount Pinatubo, measuring VEI-6was the second-largest terrestrial eruption of the 20th century. In 2007 it was reported that unprecedented warm temperatures in 1998the warmest year since surface records were recordedcould have induced elevated methane emissions, along with an increase in wetland and rice field emissions and the amount of biomass burning.
Data from 2007 suggested methane concentrations were beginning to rise again. 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." From 2015 to 2019 sharp rises in levels of atmospheric methane have been recorded.
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. The annual average for methane 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. The largest annual increase occurred in 2021 with current concentrations reaching a record 260% of pre-industrialwith the overwhelming percentage caused by human activity.
Since 2006, atmospheric methane concentrations have risen, while the global average δ¹³C–CH₄ value has shown a gradual decline. Based on this, the National Oceanic and Atmospheric Administration concluded that the recent methane growth primarily originated from microbial emissions with lighter isotopes such as wetland and agricultural emissions, rather than increased thermal or combustion sources.
However, this conclusion is not universally accepted. Changes in isotopes and sinks, such as OH, indicate that the recent atmospheric methane growth is not dominantly driven by fossil fuels. However, isotopic changes reflect the combined effects of sources and sinks of methane. Thus, the atmospheric δ¹³C–CH₄ evolution can be influenced by multiple factors. Lighter δ¹³C values only indicate relatively greater microbial sources; therefore, they cannot directly rule out concurrent increases in fossil fuel emissions. Several recent inversion models show that emissions from fossil fuels, agriculture, and waste can each account for roughly half of the atmospheric increase, with anthropogenic sources dominating the overall increase. In addition, substantial uncertainties exist in both the δ¹³C–CH₄ isotopic fingerprint of sources and the emission estimates themselves, rendering source apportionment results sensitive to prior assumptions. Overall, this is a subject that remains open to scientific debate.
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." 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. As of 2016, methane contributed radiative forcing of 0.62 ± 14% Wm⁻², or about 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases. The atmospheric methane concentration has continued to increase since 2011 to an average global concentration of 1911.8 ± 0.6 ppb as of 2022. The May 2021 peak was 1891.6 ppb, while the April 2022 peak was 1909.4 ppb, a 0.9% increase.
Image:Ch4 gr gl.png |thumb|upright=1.35|Annual atmospheric methane concentrations from 1984 to 2021
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.
In 2013, the balance between sources 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.
The focus on the role of methane in anthropogenic climate change has become more relevant since the mid-2010s.