Atmospheric methane

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 (RF) 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 (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.

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-6—was the second-largest terrestrial eruption of the 20th century. 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.

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 [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." 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 (ppb). 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. 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.

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 (NOAA) 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 (e.g., fossil fuels).

However, this conclusion is not universally accepted. Changes in isotopes and sinks, such as OH (hydroxyl radicals), 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 (for example, landfills, manure management and wastewater treatment) can each account for roughly half of the atmospheric increase, with anthropogenic sources (agriculture + fossil fuels) 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.

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.

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. Since methane oxidation consumes OH (hydroxyl radicals), higher methane concentrations can reduce global mean OH levels and slightly increase methane's atmospheric lifetime (originally approximately 9 years). This creates a positive feedback between methane sources and sinks that is explicitly considered in contemporary atmospheric chemistry–climate studies.

Carbon isotope analysis allows scientists to identify the source of the carbon in a methane sample by carefully measuring the ratio of the most common form of carbon, carbon-12, and its stable isotope, carbon-13 (13C), which has one extra neutron and is thus heavier. That means 13C is ever so slightly heavier than 12C. By analyzing the isotopic composition of methane in the atmosphere, it is possible to distinguish between microbial, thermal, and combustion sources of methane in atmospheric observations and inversion models.

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.

This atmospheric chemical process is the most effective methane sink, as it removes 90% of atmospheric methane. This global destruction of atmospheric methane mainly occurs in the troposphere.

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". 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 in 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.

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. 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.

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. 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.

Atmospheric methane could last about 120 years in the stratosphere until it is eventually destroyed through the hydroxyl radicals oxidation process.

Mean lifespan

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. 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.

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.

CH₄ + Cl → CH₃ + HCl

The HCl produced in this reaction leads to catalytic ozone destruction in the stratosphere.

Methanotrophs in soils and sediments

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. 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

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. 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 also occur in the underwater sediments. Their presence can often efficiently limit emissions from sources such as the underwater permafrost in areas like the Laptev Sea.

Removal technologies

Atmospheric methane removal is a category of potential approaches being researched to accelerate the breakdown of methane that is in the atmosphere, for mitigating some of the impacts of climate change. Atmospheric methane has increased since pre-industrial times from 0.7 ppm to 1.9 ppm, more than doubling in concentration. The increase in methane from industrialization is linked to natural methane sinks failure in accommodating for the increase in methane due to anthropogenic activities. From 2010 to 2019, methane emissions caused 0.5 °C (about 30%) of observed global warming. Global methane emissions approached a record 600 Tg CH₄ per year in 2017. 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 a global warming potential 34 times higher over and 86 times higher over 20 years. 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. 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. 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. 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.

An abundance of atmospheric methane removal methods have been studied, including photocatalysts and metal catalysts associated with zeolites and porous polymer networks. 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. Many of the methane removal methods involve oxidation chemistry, such as thermal-catalytic, photocatalytic, and biological oxidation. The targeted locations for methane destruction include ambient air, methane-enriched air, high-methane air, very high-methane air, and near-explosive air. Descriptions of these targets and their respective atmospheric mixing ratios can be seen in table 1 with information as provided in Nisbet-Jones.

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.

In order to remove methane from the atmosphere, energy which most likely involves the emission of greenhouse gases is directly involved. This makes some more anthropogenic-based forms of methane removal potentially less productive and more damaging than leaving the methane in the atmosphere. 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.

Atmospheric methane removal is generally considered a supplementary climate mitigation strategy rather than a primary solution, as emissions reduction at the source is typically more effective, less-energy intensive, and lower cost. Emissions from sectors such as gas systems, agriculture, and waste management can be reduced by existing technologies such as: leak detection and repair, improved manure management, and landfill gas capture. These methods have been identified as cost effective.The low atmospheric methane concentration (approx 1.9 ppm) means that large volumes of air need to be processed to achieve measurable removal, which can be correlated with higher energy and infrastructure requirements. Marginal abatement cost analyses generally show that methane mitigation measures lower-cost climate actions, whereas atmospheric methane removal technologies are in early stages of development. This can be correlated with higher projected costs and uncertainties. Current research often focuses on emissions reduction, with methane removal considered as a supplementary application.

Methane removal does play a significant role in addressing residual emissions that are difficult to eliminate and to reduce legacy methane already present in the atmosphere. Methane has an atmospheric lifetime of approximately 10-12 years, reducing its concentration can yield relatively rapid climate benefits compared to carbon dioxide mitigation. Large-scale deployment of methane removal technologies face many significant challenges related to scalability, monitoring, and environmental impacts. Engineered systems are constrained by many factors such as the volume of air that can be processed, catalyst efficiency, and overall energy requirements. Additionally, atmospheric approaches, including oxidation, may introduce unintended chemical effects such as changing ozone levels and air quality. Accurately measuring and verifying methane removal at atmospheric scales remains difficult due to natural variability and diffuse nature of methane sources and sinks. These factors contribute to ongoing research regarding long-term feasibility and optimal role of atmospheric methane removal within broader climate mitigation strategies.

Methane concentrations in the geologic past

From 1996 to 2004, researchers in the European 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". 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. There were low values of GHG in ice ages, and high values during the warm periods.

This 2016 EPA illustration above is a compilation of paleoclimatology showing methane concentrations over time based on analysis of gas bubbles from EPICA Dome C, Antarctica—approximately 797,446 BCE to 1937 CE, Law Dome, Antarctica—approximately 1008 CE to 1980 CE Cape Grim, Australia—1985 CE to 2015 CE Mauna Loa, Hawaii—1984 CE to 2015 CE and Shetland Islands, Scotland: 1993 CE to 2001 CE.

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 rapid global warming events in the Earth's distant past, such as the Paleocene–Eocene Thermal Maximum, and the Great Dying.

In 2001, NASA's Goddard Institute for Space Studies and Columbia University's Center 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 the American Geophysical Union (AGU). They offered a theory on the 100,000-year long Paleocene–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 in Science, confirmed NASA research that the contribution of methane to global warming had previously been underestimated.

Early in the Earth's history carbon dioxide and methane likely produced a greenhouse effect. The carbon dioxide would have been produced by volcanoes and the methane by early microbes. During this time, Earth's earliest life appeared. According to a 2003 article in the journal Geology, 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.