In the context of climate change, methane (CH4) is a powerful greenhouse gas (GHG) that accumulates in the troposphere, where it traps heat escaping from the Earth’s surface. Over 20 years, it traps 84 times more heat than carbon dioxide (CO2). 1
Along with other man-made GHGs like CO2, nitrous oxide and the fluorocarbons, methane drives the greenhouse effect which is at the root of the present climate crisis. Since preindustrial times, global methane concentrations have increased two and a half times, from 722 ppb (parts per billion) to 1863 ppb, by August 2019. 2 This is the highest level of atmospheric methane for 800,000 years. 3
Man-made methane emissions account for about 16 percent of global greenhouse gas emissions, or 8.8 billion tons of CO2 equivalent. An estimated 62 percent of atmospheric methane comes from sources which scientists consider to be man-made. The main sources include: livestock farming and industrial seepage from natural gas and petroleum industry installations. 4 The remainder comes from natural sources that existed before humans destabilized the climate system by burning so many fossil fuels. The most important natural sources include emissions from: wetlands, thawing permafrost, termites and marine methane deposits (clathrates) on the sea bed.
Methane’s active life in the atmosphere (about 12 years) is relatively short-lived compared to other GHGs like CO2. This is because methane is mostly scrubbed out of the air by chemical reactions initiated by “hydroxyl radicals”, within about ten years, while a significant percentage of carbon dioxide can survive in the atmosphere for millennia.
Curiously, between about 1999 and 2007, for reasons that are still not properly understood, atmospheric methane levels remained static at around 1780 ppb, although since 2007 they have been on the rise again – reaching a peak of 1900 ppb in November 2018. This recent increase has raised fears that a climate tipping point has been reached, whereby warming causes the emission of CH4 which causes more warming, and so on. Professor Euan Nisbet of Royal Holloway University of London says researchers are very concerned about the latest rise. “I’m not sure but it looks as if the warming is feeding the warming,” says Nisbet.
Looking ahead, other potential climate problems regarding methane include the exploitation of natural gas from shale formations around the world. These could result in major new emissions of methane if gas industry leakage rates are not improved. Secondly, and in the longer term, the continuing thaw of permafrost could lead to a significant release of CH4 emissions, with potentially catastrophic effects on global warming. In the short term, the problem is that methane levels are rising faster than anyone expected. Atmospheric levels of CH4 jumped by 10.77 parts per billion in 2018, the second highest annual increase in the past two decades, according to U.S. agency NOAA. (For more, see: Why Are Methane Levels Rising?)
- How Does Methane Greenhouse Gas Affect Global Warming?
- Difficulty Measuring Methane Greenhouse Gas Emissions
- What Are The Latest Statistics For Global Methane Emissions?
- What Are The Main Sources Of Methane Greenhouse Gas?
- What Are The Main Methane Sinks?
- Methane-Oxidizing Bacteria In The Soil
- How Can Methane Emissions Be Reduced?
How Does Methane Greenhouse Gas Affect Global Warming?
The greenhouse effect is the name given to a mechanism in the lower atmosphere, whereby certain gases (such as water vapor, carbon dioxide and methane) absorb some of the heat given off by the planet and radiate it back down to the surface. This process, which has been in operation for millions of years, keeps the planet at a nice cosy 15°C (59°F) instead of the minus 18°C (0 °F) it would be without it. Over the last 250 years, however, this natural “warming mechanism” has been completely destabilized by the burning of fossil fuels, which have resulted in the emission of huge quantities of greenhouse gas.
For example, humans have set about creating a global livestock industry, as well as a huge natural gas and petroleum industry, that every day pumps out huge amounts of methane into the atmosphere. Methane may be only the third most common greenhouse gas after water vapor and CO2, and may be present in the atmosphere in relatively small amounts (CO2 is roughly 200 times more abundant), but its 20-year global warming potential is 84 – which means, over a 20 year period it traps 84 times more heat escaping from the planet’s surface, than carbon dioxide. 1
In addition to warming the planet directly via the greenhouse effect, methane is also part of an important climate feedback loop, which works like this. The methane in wetlands comes from naturally occurring bacteria and other microbial decomposers. But when temperatures rise, the bacteria are more active and produce more methane, according to a study published in Science magazine. “The higher the temperature, the more efficient they are at producing methane,” says scientist Paul Palmer co-author of the study. In other words, global warming causes wetlands to emit methane. The methane then creates more warming which creates more methane, and so on. 5
This methane feedback loop applies especially to the vast areas of permafrost in the Northern Hemisphere, where melting ice is thawing the methane-rich soils even in winter, due to unprecedented heatwaves across Alaska, Canada and Russia. Scientists worry that this may lead to runaway thawing and the release of massive quantities of methane, with incalculable effects on climate.
WANT TO REDUCE YOUR METHANE EMISSIONS?
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Difficulty Measuring Methane Greenhouse Gas Emissions
Creating and maintaining accurate databases of methane emissions is a Herculean task, made even more difficult by methane’s shorter atmospheric lifetime, stronger warming potential, and its variable atmospheric levels (compared to CO2), the causes of which are still debated.
The two main problems are: the variety of diverse and overlapping sources of CH4 and the destruction of CH4 in the atmosphere by short-lived hydroxyl radicals (OH), of which more later. In addition, the two principal methods of estimating emissions – “bottom-up” and “top-down” – typically produce differing sets of statistics.
Meantime, methane monitoring remains problematic. In some important areas, such as the tropics and the Arctic, ground-based sensor networks are lacking and cloud cover hinders satellites observation.
Occasionally, scientists get lucky. In 2011, NASA satellite expert Christian Frankenberg published imagery from SCIAMACHY, the Scanning Imaging Absorption Spectrometer for Atmospheric Cartography on the European Space Agency’s Envisat satellite. It revealed that the Sichuan Basin – a fertile, low-lying area in southwestern China – was releasing more methane into the atmosphere than anywhere else in the world, due to its natural gas installations, and a high density of rice paddies and animal farms. The Indo-Gangetic plain and eastern China – both major farming areas – also had particularly high emissions, as did the swampy Sudd wetlands of southern Sudan. 6
In the Four Corners region of the southwestern United States, SCIAMACHY observed a methane cloud about the size of the state of Delaware. Hovering over the San Juan Basin, it was the largest methane hot spot ever detected by satellite in North America. The area was well known for its gas, coal, and oil industries, but until the SCIAMACHY observations, regulators had no idea that such pollution existed. 7
For these and other reasons, most methane emissions statistics – from whatever source – should be regarded as approximate only. This applies both to “bottom-up” and “top-down” measurements. See also: Greenhouse Gas Statistics Lack Consistency.
What Are The Latest Statistics For Global Methane Emissions?
In addition to the IPCC, Global emissions of methane are now monitored by a consortium of scientists under the auspices of the Global Carbon Project. The latter is a Global Research Project of Future Earth and a research partner of the World Climate Research Programme. The aim of the new consortium is to synthesize and stimulate new research aimed at regularly updating the global methane budget. The most important source of uncertainty in the methane budget for 2008-2017 is the contribution of natural emissions, especially those from wetlands.
At present, bottom-up methods suggest larger CH4 emissions (737 million tonnes/year) than top-down methods, mostly because of larger estimates of CH4 coming from natural wetlands, other inland water systems, and geological sources. However, scientists at the Global Carbon Project believe these bottom-up figures are overestimated. As a result, the above chart is based on top-down estimates only.
Global Methane Greenhouse Gas Concentrations
What Is Biogenic, Thermogenic And Pyrogenic Methane?
As well as being classified man-made or natural, methane sources can be separated by process (biogenic, thermogenic, or pyrogenic). Each of these three process categories has human and natural components.
Biogenic methane comes from the decomposition of organic matter by methanogenic bacteria in anaerobic environments, such as wetlands, bogs, rice paddies, marine sediments, landfills, wastewater treatment facilities, manure stores, or inside animal stomachs. Thermogenic methane is formed over millions of years by the decomposition of buried organic matter due to geological heat and pressure within the Earth’s crust. Thermogenic methane escapes into the atmosphere via oceanic and terrestrial gas seeps. These particular emissions are considerably boosted by human processes, for instance the production, refining and distribution of fossil fuels. Pyrogenic methane derives from the incomplete combustion of biomass, such as peat, wood, crop residues, and other organic material. 4
Methane clathrates (methane deposits trapped inside structures of frozen water) typically found on continental shelves and below land permafrost, can be either biogenic or thermogenic.
What Are The Main Sources Of Methane Greenhouse Gas?
The main sources of atmospheric methane include fossil fuel industries, livestock agriculture, biomass burning, wetlands, permafrost thawing and other natural sources.
Oil And Gas Supply Chains
The fossil fuel industry still seems to have difficulties preventing serious discharges of greenhouse gases, as illustrated by the 22,500 square mile methane cloud tracked by satellite above the southwestern USA. The investigation that followed, concluded: “the source is likely to be from established gas, coal, and coalbed methane mining and processing.” Globally speaking, the coal, oil and natural gas industries account for roughly one fifth of all methane emissions. 4
Natural gas is 85 to 95 percent methane, so any leaks are predominantly methane. During the production, storage and transmission of natural gas, a significant amount of the gas seeps into the atmosphere, through normal procedures, routine maintenance and venting, as well as fugitive leaks and system failures. In the United States, for example, according to the EPA, methane seepage from natural gas and petroleum installations and infrastructure totals a massive 8.1 million tonnes: 6.5 million tonnes from the natural gas system, 1.6 million tonnes from petroleum supply chains. 8
However, a 2017-18 review of emissions studies revealed that the figure of 8.1 million tonnes cited by the EPA was actually a significant underestimate. The true figure, the review said, was 13 million tonnes – roughly 60 percent more than the figure in the original EPA report. 9
During the 2008-2017 decade, emissions from upstream and downstream oil and gas sectors represented about 63 percent of all methane emissions from fossil fuel sources.
When coal is mined, methane is often released from the coal seam and also the disturbed rock strata in the immediate vicinity. In its annual report on the outlook for global energy, the International Energy Agency (IEA) estimates that methane emissions from the world’s coal mining operations reached just under 40 million tonnes in 2018. 10 This latest figure, a significant increase over 2017, represents about one third of 2018 methane emissions from fossil fuels. In the same report, the IEA stated that carbon emissions from the global energy industry had reached a new record in 2018 despite the much-touted boom in renewable energy in recent years.
Livestock manure is a significant source of global methane emissions. Manure from a single American cow, for instance, produces an average of 74 kg of methane per year. [Source: “A Review of Livestock Methane Emission Factors.” Donal O’Brien and Laurence Shalloo. EPA Research Program 2014-2020.]
In a digestive process known as enteric fermentation, significant amounts of methane are produced by tiny bacteria known as methanogens who live inside the multi-chambered stomachs of cows, sheep and other ruminants, where they break down plant cellulose into simple molecules that are nutritious to the host animal. Roughly 13 percent of the methane is released via the flatus, while the other 87 percent is discharged through eructation. Without these methanogens to break down plant cellulose, animals such as cattle would not be able to consume grasses. Although the exact amount varies with diet, a sheep can produce about 30 litres of methane each day and a dairy cow up to about 200. 11
According to a report by the Food and Agriculture Organization (FAO), livestock generate more greenhouse gas emissions than the entire global transportation sector. Henning Steinfeld, co-author of the study, states: “Livestock are one of the most significant contributors to today’s most serious environmental problems.” 12 Currently, livestock are responsible for 31 percent of all anthropogenic methane emissions.
Methanogenesis (also called biomethanation) is the production of methane by microorganisms known as methanogens. These methane-makers are single-celled microbes that belong to the domain of archaea. Like bacteria, only simpler, they operate in low- or no-oxygen environments where they break down organic matter into simple molecules, producing methane as a metabolic byproduct. Oxygen actually inhibits the growth of methanogens. Methanogenesis takes place in two basic situations: (a) inside the stomachs of ruminant animals like cattle; (b) in warm wet environments containing abundant organic matter, like wetlands. Globally, methanogenesis is one of the largest sources of methane emissions responsible for a significant amount of global warming.
Rice paddies are ideal environments for methane-producing decomposers, such as fermentative bacteria and methanogenic archaea. The water prevents oxygen from penetrating the soil, thus creating ideal conditions for their metabolic activities which involve the emission of methane as a by-product. Due to the ever-increasing world population, rice cultivation has become a growing source of food and thus a growing source of atmospheric methane. Rice farming now accounts for roughly 15 percent of anthropogenic methane emissions. 13
Accounting for 11 percent of all global CH4 emissions, municipal solid waste is considered the third largest anthropogenic source of methane emissions. 14 Landfills are another ideal environment for anaerobic methane-producing decomposers. Depleted oxygen levels combined with large amounts of decaying organic matter allow these microorganisms to dominate the decomposition process, which continues (along with the emission of CH4) for years after the landfill is closed. 15 In America, landfill waste is the third largest source of atmospheric methane, accounting for about 18 percent of global methane emissions in 2014. 16
Biomass Burning (Forest, Peat, Crop & Domestic Biofuel Burning)
This category includes methane emissions from biomass burning involving forests, grasslands, peat soils, agricultural residues, as well as biofuels in the domestic sector (boilers, stoves, fireplaces). In general, the amount of methane released into the atmosphere from these categories of biomass burning has decreased by about 12 percent, between the early 2000s and the more recent period of 2007 to 2014.
This finding is based on satellite measurements of methane and carbon monoxide actually in the atmosphere. These “top-down” recordings came from the Measurements of Pollutants in the Troposphere (MOPITT) sensor installed on the Terra satellite, and the Tropospheric Emission Spectrometer (TES) sensor on Aura (EOS CH-1), the multi-national NASA scientific research satellite.
Of course, data relating to the extensive wildfires in the Arctic during the summer of 2019 may alter the situation.
Methane Emissions From Wood Burning
The burning of fuelwood is being touted as a form of green biofuel, on the basis that its CO2 emissions are offset by the amount of CO2 the wood (and its successors) sequesters from the atmosphere.
Of course, under the proper conditions, all the hydrocarbons present in wood are oxidized to CO2 and water when burned. In fact, this level of combustion happens every day in analytical chemistry laboratories. But most cooking fires, for instance, don’t use wood that’s been thoroughly dried, mixed with a catalyst and subjected to a heat source of 950°C in an oxygenated atmosphere, to eliminate the possibility of partial combustion. And partial combustion is the key to the emission of the unburned hydrocarbons, including methane, carbon monoxide and carbon dioxide. 17
So usually, whenever wood is burned methane is also produced – roughly 70g of CO2 equivalent per kilogram of wood. And remember, methane traps 84 times more heat than CO2. 18 For more, see: What is the Effect of Wood Burning on Climate Change?
The burning of agricultural waste in particular generates significant amounts of methane due to its typically high water content. On a global scale, methane emissions from biomass burning may be decreasing, but they still account for 30 million tonnes of methane per year.
One major exception to the wood problem, is the burning of wood chips as an alternative to industrial fossil fuels. For example, an electrical power plant fuelled by (say) renewable and fully combusted pine wood chips obtained from ‘waste wood’, instead of coal, would have a much-reduced net greenhouse gas impact compared to traditional coal or gas fired power stations.
The largest natural source of methane, wetlands (swamps, marshes, bogs, fens, peatlands) produce around 150 million tonnes of biogenic methane a year, accounting for up to a third of global methane emissions. 19
Like rice paddies, water-logged soils are ideal environments for methanogens as well as other microbes, like those that specialize in fermentation to break down essential nutrients. Microorganisms from the archaea domain, for example, produce methane by fermenting acetate and H2-CO2 into methane and carbon dioxide, in a process called acetoclastic methanogenesis.
As always, for maximum methanogenesis, the essential components are an environment low in oxygen (in wetlands, water blocks the flow of oxygen), containing plenty of organic matter, preferably already semi-decayed.
A common feature of wetlands is the presence of microorganisms known as methanotrophs, tiny creatures that depend on methane for energy. These methane-eating microbes are drawn to water-saturated areas precisely because of the methane produced there, although they prefer areas with a lower water table. In these locations where water doesn’t block as much oxygen, methanotrophs dominate the decomposition process since methanotrophic bacteria do not function well in aerobic conditions. As a result, the melanotrophs eat most of the methane, preventing it from reaching the atmosphere.
Permafrost: A Huge Store of Carbon
The Global Methane Budget 2000-2017 does not have a separate category for methane emissions from thawing permafrost. Instead they are hidden away in the Wetlands category. This seems almost certain to change in future, given the recent series of heatwaves and wildfires that spread throughout the Arctic Circle in 2019, as well as the number of eyewitnesses reporting widespread areas of exposed permafrost and smouldering peatlands.
The temperature rise is critical. Right now, the Arctic is warming twice as fast as the rest of the world. 20 Since the 1970s, the average temperature of the Arctic has risen by 2.3°C. 21 Because of this, sea ice is at its lowest extent, and the Greenland ice sheet is melting fast. The high temperature also accounts for the record-breaking wildfires in 2019, as well as the concern shared by many scientists that the thawing of the Arctic permafrost. 22
As rising temperatures cause the permafrost to melt, the methane that was frozen in the permafrost before it had a chance to reach the atmosphere, is slowly released. And when the ice melts, lakes form, and the soil thaws and becomes food for decomposers that generate greenhouse gases, like CH4. This methane adds to the warming, which melts more permafrost, which adds to the warming, and so on. It’s a classic example of a positive climate feedback. The thawing of the ground also leads to land subsidence and collapse which only accelerates the sub-surface soil melt. Already, an ancient type of Siberian permafrost known as yedoma, has become a significant source of atmospheric methane (about 4 million tons of CH4 per year). 23
By 2100, it is estimated that 5-15 percent of the terrestrial permafrost carbon pool will be vulnerable to release in the form of methane and carbon dioxide, corresponding to 130-160 billion tonnes of carbon. 24 The progressive discharge into the atmosphere of this level of carbon as CO2 and CH4 may have a significant impact on climate change trajectory. 25
Methane clathrate (also called methane hydrate) is a solid clathrate compound with a significant amount of methane trapped within its ice-like structure. Large deposits of methane hydrate have been discovered beneath sediments on the ocean floor and on the bottom of deep lakes, like Lake Baikal. If these deposits were released into the atmosphere, it might lead to irreversible global warming – similar to the Permian–Triassic extinction event – the most extreme mass extinction event on earth.
A recent scientific expedition detected concentrations of methane in the atmosphere of the Siberian Arctic – thought to have originated from methane clathrates in the area between the Laptev Sea and East Siberian Sea – that were 100 times above normal. 26 27
However, Carolyn Ruppel, chief of the U.S. Geological Survey’s gas hydrate program, is emphatic that none of this is a problem.
“People tend to get very alarmed about gas hydrates because of how much methane is stored in these deposits,” she says. “But climate change – even assuming the worst scenarios – should have little or no impact on the vast majority of it.” Most of the methane hydrate is buried beneath layers of sediment in deep-water environments, where temperatures are cold enough and the pressures are great enough for the deposits to remain stable” she explains. So even if global warming continues for thousands of years, it would probably have little effect on them. 28
Like cattle and sheep, termites also have methanogenic microorganisms living in their complex stomach system. Fortunately, termites affect climate less than livestock for two reasons. First, their methanogens break down food into ethanol, as well as methane. Second, termites can afford to eat less food than ruminants in order to obtain the same amount of energy, and therefore emit proportionally less CH4. Termites will not cause runaway global warming.
What Are The Main Methane Sinks?
Global methane emissions are currently estimated to be around 572 million tonnes a year. But methane levels are increasing at an annual rate of only 16-17 million tonnes. This means that methane sinks must be removing about 556 million tonnes of atmospheric methane every year.
Unfortunately, we don’t know exactly how sources and sinks of methane interact. The IPCC have stated that there remain “large uncertainties in the current bottom-up estimates of components of methane sources”, and that the balance between sources and sinks is not yet well known. 29 This lack of understanding was clearly demonstrated between the year 2000 and 2006 when methane levels stopped rising altogether, for reasons that are still being investigated. 30
Hydroxyl Free Radical (Oh)
We do know that the most successful methane sink is the hydroxyl free radical (OH) which is produced photochemically in the atmosphere. As methane rises into the air, it reacts with the hydroxyl free radical to create water vapor and the less harmful greenhouse gas carbon dioxide. The hydroxyl free radical is often called the “detergent” of the troposphere because it “cleans up” a number of pollutants. It has a particularly important role in neutralizing some greenhouse gases like methane and ozone. 31
In 2001, the active lifetime of atmospheric methane used to be 9.6 years. Since then, however, increasing emissions of methane have reduced the concentration of OH in the atmosphere. 32 Scientists worry that less OH means more (i.e. longer-living) methane. 33
If it is not neutralized in the troposphere, methane may last up to 120 years before it is eventually destroyed in the stratosphere. This occurs in the same way that it does in the troposphere: methane is converted into CO2 and water vapor. During the period between 1978 and 2003, stratospheric methane levels rose by more than 13 percent. 34
In total, the hydroxyl free radical methane sink removes around 518 million tonnes of methane from the atmosphere every year. 4
Note: Hydroxyl radicals are traditionally counted as methane sinks, but this is, strictly speaking, incorrect, since they do not result in methane storage or physical removal from the atmosphere. 35
Methane-Oxidizing Bacteria In The Soil
Methane can be consumed – oxidized to carbon dioxide – in soils by certain methanotrophic bacteria. Woodland soils are very effective sinks for both atmospheric methane, and for methane produced in deeper soil layers. The reason that forest soils tend to be good sinks is because the trees help keep the water table well below the surface, thus allowing the methanotrophs to grow and thrive. However, whenever the soil become waterlogged, the balance of power shifts from methanotrophs to their rival decomposers, the methane-producing methanogens (who thrive when water blocks the flow of oxygen), and the soil becomes a methane source. This applies all year round to wetlands with a high water table, and to forests in winter.
There are two main types of methanotroph in soil: (a) The so-called “high capacity – low affinity” methanotrophs, specially adapted for high methane levels (several 1000 parts per million in air), like those arising from waterlogged soil layers. (b) The so-called “low capacity – high affinity” methanotrophs, who are built to make use of tiny amounts of methane (around 1.8 parts per million in air).
How Can Methane Emissions Be Reduced?
There are three essential pathways to lowering methane emissions.
- The natural gas and petroleum industry need to reduce the amount of methane seepage from their installations and infrastructure. Governments need to incentivize success in this regard and penalize failure. Fossil fuel companies must pay the cost of their pollution.
- Global eating habits need to change. Dietary patterns need to move away from meat-every-day, to meat-occasionally. Again, this needs government intervention to manipulate subsidies to encourage less meat eating and lower meat production.
- We must reduce our emissions of CO2! Carbon dioxide is the most prevalent and long-lasting greenhouse gas, and it’s the one doing the most damage to our polar regions and our ecosphere. In order to help achieve this, governments must reduce reliance on fossil fuels by developing same-price renewable fuels and a supportive infrastructure. For example, simply imposing a carbon tax on ordinary citizens, or advising them to buy electric cars, without at the same time providing same-price green options and car-charging facilities, is completely counter-productive.
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- “Four corners: The largest US methane anomaly viewed from space.” Kort, E.A. et al. (2014, October 9). Geophysical Research Letters, 41 (19), 6898-6903.
- EPA Inventory of U.S Greenhouse Gas Emissions and Sinks: 1990–2015 report.
- “Assessment of methane emissions from the U.S. oil and gas supply chain”. Alvarez, Ramon A. et al; (2018-07-13). Science. 361 (6398): 186–188.
- “World Energy Outlook 2019.” International Energy Agency (IEA)
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- “Quantitative analysis of the methane gas emissions from municipal solid waste in India.” Chander Kumar Singh, Anand Kumar & Soumendu Shekhar Roy. Scientific Reports volume 8, Article number: 2913 (2018)
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- “Methane Feedbacks to the Global Climate System in a Warmer World.” Joshua F. Dean, Jack J. Middelburg, Thomas Rockmann, Rien Aerts, Luke G. Blauw, Matthias Egger, Mike S. M. Jetten, Anniek E. E. de Jong, Ove H. Meisel, Olivia Rasigraf. Reviews of Geophysics. Volume 56, Issue 1, Pages 207-250. March 2018.
- “Arctic Warming Twice as Fast as Rest of World.” Sid Perkins. American Association for the Advancement of Science (AAAS) Aug. 6, 2013.
- “Arctic Climate Change. World Wildlife Fund for Nature. WWF.com
- “Climate change and the permafrost carbon feedback”. Schuur, E.A.G.; et al. (2015). Nature. 520 (7546): 171–179.
- “Methane bubbling from Siberian thaw lakes as a positive feedback to climate warming”. Walter, K. M. et al. (September 2006). Nature. 443 (7107): 71–75.
- “Permafrost carbon-climate feedback is sensitive to deep soil carbon decomposability but not deep soil nitrogen dynamics.” Charles D. Koven, David M. Lawrence, and William J. Riley. PNAS March 24, 2015 112 (12) 3752-3757; March 9, 2015.
- “Climate change and the permafrost carbon feedback.” E. A. G. Schuur, A. D. McGuire, C. Schadel, G. Grosse, J. W. Harden, D. J. Hayes, G. Hugelius, C. D. Koven, P. Kuhry, D. M. Lawrence, S. M. Natali, D. Olefeldt, V. E. Romanovsky, K. Schaefer, M. R. Turetsky, C. C. Treat & J. E. Vonk. Nature volume 520, pages 171–179. (2015)
- Yr.no – the Joint online weather service from the Norwegian Meteorological Institute (met.no) and the Norwegian Broadcasting Corporation (NRK)
- “Massive Siberian forest fire could melt permafrost, freeing massive methane stores.” Nicole Karlis. Salon.com August 27, 2019.
- “Methane Matters.” Adam Voiland. Earthobservatory.nasa.gov/ March 8, 2016.
- IPCC. Fourth Assessment Report (2007) Chapter 2. IPCC Working Group I.
- “Three decades of global methane sources and sinks”. Kirschke, Stefanie; et al. (September 22, 2013). Nature Geoscience. 6 (10): 813–823.
- “Trends in the Hydroxyl Free Radical” (PDF) (IPCC AR4 WG1). IPCC.
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