Every year, 6,000 flasks arrive at a laboratory in Boulder, Colorado. Inside each is a sample of air, taken from one of a chain of 50 monitoring stations that spans the globe. Together, these samples could help answer one of the most important questions facing the planet: why is there so much methane in the atmosphere?

Blue and black canisters filled with air from Algeria, Alaska, China and Samoa are lined up ready for testing. “We collect these flask samples, then they come back here,” says Ed Dlugokencky, a chemist at the Global Monitoring Laboratory, run by the US National Oceanic and Atmospheric Administration.

The laboratory measures the levels of different gases inside the samples, from carbon dioxide to nitrous oxide and sulphur hexafluoride, compiling a meticulous record that forms the basis for major climate models. About 15 years ago, its researchers observed an uptick in atmospheric methane, a potent greenhouse gas with a warming impact 80 times greater than CO₂.

Many researchers initially assumed the increase was linked to fossil fuel production. Methane is the primary ingredient in natural gas but is also produced by other human activities such as landfills, rice paddies and raising cattle.

In the past few years, however, that uptick has accelerated into a surge. The implications for global warming are immense: of the 1.1C increase in global temperatures since pre-industrial times, about a third can be attributed to methane. Atmospheric methane had its highest growth rate ever recorded by modern instruments in 2020, and then that record was broken again in 2021. Nobody knows exactly why.

“It is shocking,” says Lindsay Xin Lan, a researcher based in the Boulder laboratory who is analysing the data. “A lot of research, a lot of scientists, are trying to explain it.”

One thing they have begun to identify is what kind of methane is culpable for the increase. Methane derived from fossil sources contains more of the carbon-13 isotope than atmospheric methane, while that produced by microbial sources — such as wetlands, cattle and landfills — contains less.

Since the beginning of the industrial revolution, fossil fuel emissions have tilted the ratio of methane isotopes in the atmosphere towards carbon-13. But around 2007, when atmospheric methane started to climb again, that isotopic ratio went into reverse. The recent increase in methane is not coming primarily from fossil fuels, but from other sources. That suggests the planet itself could be emitting more methane, and it is not slowing down.

“We are seeing a very substantial change,” says Dlugokencky. “After 200 years of increasing . . . all of a sudden we start to see a decrease in delta carbon-13. That means something significant has happened.”

Determining what that “something significant” is requires close study of methane emitted from a variety of sources — from wetlands and shallow lakes in the tropics to melting permafrost in the Arctic; from landfills and agriculture to the fossil fuel industry — as well as the chemical “sinks” that remove it from the atmosphere.

“Methane is a very interesting type of greenhouse gas because it has so many kinds of sources and sinks that you have to keep track of,” says Dlugokencky. “You have to look at it like you are a detective trying to solve a criminal mystery, that is how I think of it.”

Unravelling the mystery will reveal whether or not the world might face the worst-case scenario of a “methane bomb” — a feedback loop where a warmer planet emits more of the gas naturally, driving temperatures up further. It’s a terrifying prospect, one that scientists studying this topic tend to tiptoe around, particularly in interviews.

“We can have a gut feeling that the climate feedback might be happening,” says Lan. “But it can be difficult to separate the signals from the noise.”

Others are more direct. “If you think of fossil fuel emissions as putting the world on a slow boil, methane is a blow torch that is cooking us today,” says Durwood Zaelke, president of the Institute for Governance & Sustainable Development, and an advocate of stricter policies to reduce methane emissions. “The fear is that this is a self-reinforcing feedback loop . . . If we let the earth warm enough to start warming itself, we are going to lose this battle.”

Hunting for clues

For years methane was somewhat overlooked by the scientific community and by policymakers, who tended to focus more on CO₂ emissions. Part of the reason for that is because atmospheric methane appeared to be levelling off between 2000 and 2007.

Now, researchers are using both isotopic measurements and satellite data to determine the origins of the surge in methane. They know that the increase is coming from microbial sources because of the shift in the carbon-13 ratio — but which microbial sources exactly? Wetlands, cattle and landfills all produce “microbial” methane, in which microbes break down carbon and generate the greenhouse gas. To determine how much each of these sources is contributing, scientists are scouring the globe for data points.

Paul Palmer, an atmospheric chemist at the University of Edinburgh, compares it to a game of Cluedo, the children’s detective board game. “The satellite data will give you the location of the murder,” say Palmer. “And the isotopes will give you the weapon — the type of source.”

Wetlands and cattle appear to be the biggest culprits, says Euan Nisbet, professor of earth sciences at Royal Holloway, University of London. “The biological sources are increasing faster,” he says. “The most intense growth seems to be coming from the tropics.” A global increase in cattle-raising, and in landfills, is also fuelling the growth in microbial emissions.

In an upcoming paper, Lan and Dlugokencky reach a similar conclusion: 85 per cent of the increase in atmospheric methane since 2007 is due to microbial sources. And about half of that is from the tropics.

Using satellite data, Palmer has zeroed in on east Africa as a source of increased emissions, such as the Sudd wetland in South Sudan. “We are seeing a huge methane enhancement due to wetland emissions. We’ve only known that since 2019,” he adds. Other tropical areas with wetlands, such as south-east Asia and the Amazon, are showing an increase as well. When wetlands get wetter, it leads to more methane emissions because the microbes that produce methane have more organic matter on which to feed.

The sources of the methane may be natural, but a climate warmed by human activity is fuelling these emissions. Climate change is expected to lead to more intense rainfall in east Africa; and these wetter, warmer wetlands will produce more methane. Other natural sources of methane — melting permafrost, and wildfires — are also linked to climate change.

While Palmer works with satellites, other scientists are working on the ground, physically travelling around the globe to capture methane samples in canisters to be sent to laboratories.

A satellite image of the Upper Nile River Province in South Sudan
Scientists are studying increased emissions in wetlands, such as those in South Sudan, seen here in a satellite image © NASA Library/Alamy

The Royal Holloway lab is filled with boxes of samples shipped in from around the planet. “These are exciting,” says Rebecca Fisher, a lecturer in atmospheric sciences at the university, gesturing to a box that has just arrived. “They are flasks of air from the Halley Research Station in Antarctica.” Because Antarctica has no vegetation, the air there contains very little locally produced methane, making it ideal to provide background measurements.

Fisher is preparing for a trip to Finland, near the other end of the globe from Antarctica, to collect samples that will measure what she calls the isotopic “fingerprint” of wetland emissions in the Arctic. By measuring not only the carbon-13 isotope, but also the hydrogen isotope deuterium, known as heavy hydrogen, her group and others are working to build a library of these fingerprints.

“We get really different signatures in the Arctic versus the tropics,” Fisher says. “By taking these isotopic measurements, we can see if that fits with what is in the atmosphere.”

As well as helping scientists piece together the current surge in methane emissions, the Arctic also gives an idea of what future emissions might look like: the region is warming three times faster than the rest of the planet. “Permafrost itself contains around 1,500bn tonnes of carbon,” says Katey Walter Anthony, Professor of ecology and biogeochemistry at the University of Alaska, Fairbanks. As permafrost thaws, that carbon can be turned into methane by micro-organisms known as methanogens.

She has flown all over Alaska to measure the methane coming out of lakes, and what she has seen recently has surprised her. “In the last five to six years, I have just seen incredible change,” she says. “It seems like we crossed a threshold and we are seeing crazy things happening.”

Maps showing methane emissions by source

One of those crazy things is that lakes are forming — a lot of them — as permafrost melts. These pools, known as thermokarst lakes, are spreading rapidly. And the microbes that produce methane thrive on all the newly thawed organic material at the bottom of these new lakes.

“In interior Alaska we’ve seen a nearly 40 per cent increase in lake area since the 1980s, of new thermokarst lakes forming,” says Anthony. “Those lakes emit methane at least 10 times higher than a normal lake, they are hotspots.”

Anthony says current climate models greatly underestimate the methane released by these lakes. In a 2018 research paper in Nature Communications, she calculated that methane could become the dominant source of atmospheric warming from permafrost gases this century, between 2050 and 2070, if our greenhouse gas emissions continue on current trends.

For now, the methane emissions from those lakes are much lower than from wetlands in the tropics. “We are watching the Arctic, but at the moment the Arctic does not appear to be leading the problem,” says Nisbet. “There are a lot of potential feedbacks here. In the Arctic, we need to keep watching that.”

Cutting the gas

While there is little that governments can do about the methane bubbling out of wetlands and thermokarst lakes, they have already promised to reduce the amount produced from human-caused sources. Around half of methane emissions come from anthropogenic sources, and half from natural sources.

More than 100 countries signed up to the Global Methane Pledge launched at the COP26 climate summit last year, pledging to collectively cut methane emissions 30 per cent by the end of this decade.

Cutting methane emissions would have a more immediate impact on temperatures — within a human lifetime — than cutting CO₂. That’s because methane lasts only a decade in the atmosphere, depending on conditions, compared to the century or more CO₂ remains there.

Salvagers collect materials for recycling at a landfill site in India. A global increase in landfills is fuelling the growth in microbial methane emissions © Anindito Mukherjee/Bloomberg

A concerted global effort to reduce methane emissions using existing technologies could slash anthropogenic emissions by 45 per cent by 2030, according to a May 2021 report from the UN Environment Programme, avoiding 0.3C of warming by the 2040s.

The quickest methane fixes are in the fossil fuel sector, which accounts for about one-third of anthropogenic emissions. Special venting installed in coal mines; early detection of gas leaks; reducing methane venting during oil and gas production and other “readily available” measures could cut methane emissions by more than 40mn tonnes a year, according to the report. Capturing natural gas from landfills would even pay for itself because of its resale value.

Still, it’s not clear this will be enough. The world’s biggest methane emitters — China and Russia — have not signed the COP26 pledge. And even if they did, it’s not clear that reductions in human-caused methane will be enough to compensate for the increase from natural sources.

If the warming Earth is already starting to release more methane, then this vicious cycle — in which warming triggers more warming — could become self-perpetuating. Although that moment could still be decades in the future, once that tipping point is reached, it will be very hard to reverse.

Anticipating this, some groups are starting to study methane removal — whether there might be ways to pull methane directly from the air. While mostly hypothetical at this point, the ideas include increasing the amount of chemical sinks in the atmosphere, for example by adding tiny iron-oxide particles to the air. Other approaches include using methane-eating bacteria to act as a “filter” for methane, such as in dairy farms.

Yet even as methane rises in priority, funding to monitor it has not kept pace. “There really hasn’t been a big expansion of our network for the last 15 or 20 years,” says Dlugokencky. Federal funding cuts a decade ago mean the network today is smaller than it used to be.

Still, the methane hunters keep going. “Right now we are trying to work out just the source signatures; we need to get a global database,” says Nisbet. He has recently returned from a trip to Canada where he was collecting swamp samples.

Although Nisbet is nearing retirement, he keeps going out in the field, searching for the answer to the riddle. “Our level of primary knowledge is still very, very low,” he says. “Methane keeps kicking up these surprises.”

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