Blue and black containers filled with air from Algeria, Alaska, China and Samoa line up ready to be tested. “We collect these bottle samples and 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 lab measures the levels of different gases in the samples, from carbon dioxide to nitrous oxide and sulfur hexafluoride, compiling a meticulous record that forms the basis for major climate models. About 15 years ago, his researchers noticed an increase in atmospheric methane, a powerful greenhouse gas with a warming effect 80 times greater than CO2. Many researchers initially assumed that the increase was linked to fossil fuel production. Methane is the main component of natural gas, but it is also produced by other human activities such as landfills, rice paddies and cattle ranching. In recent years, however, this rise has accelerated exponentially. The implications for global warming are huge: of the 1.1C global warming since pre-industrial times, about a third can be attributed to methane. Atmospheric methane had its highest rate of growth ever recorded by modern instruments in 2020, and then that record was broken again in 2021. No one knows exactly why. “It’s shocking,” says Lindsay Xin Lan, a researcher at the Boulder lab who is analyzing the data. “A lot of research, a lot of scientists, are trying to explain it.”
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One thing they are beginning to identify is the type of methane responsible for the increase. Methane from fossil sources contains more of the isotope carbon-13 than atmospheric methane, while that produced from 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 to carbon-13 isotopes in the atmosphere. But around 2007, when atmospheric methane began to rise again, this isotopic ratio reversed. The recent increase in methane is not primarily from fossil fuels, but from other sources. This suggests that the planet itself could be emitting more methane and is not slowing down. “We’re seeing a very substantial change,” says Dlugokencky. “After 200 years of increase . . . suddenly we start to see a decrease in delta carbon-13. That means something important has happened.” Determining what that “something important” is requires careful 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 “cesspools” 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 track,” says Dlugokencky. “You have to look at it like you’re a detective trying to solve a crime mystery, that’s how I think about it.”
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Unraveling the mystery will reveal whether or not the world can face the worst-case scenario of a ‘methane bomb’ – a feedback loop where a warmer planet emits more of the natural gas, raising temperatures further. It’s a scary prospect, one that scientists who study this topic tend to tiptoe around, particularly in interviews. “We can feel that climate feedback may be happening,” says Lan. “But it can be difficult to separate the signals from the noise.” Others are more direct. “If you think fossil fuel emissions are putting the world on a slow boil, methane is a torch cooking us today,” says Durwood Zaelke, president of the Institute for Governance and Sustainable Development and an advocate for stricter policies. reducing methane emissions. “The fear is that this is a self-reinforcing feedback loop. . . If we let the earth get warm enough to start warming, we’re going to lose that battle.”
Hunt for clues
For years methane was somewhat ignored by the scientific community and policy makers, who tended to focus more on CO₂ emissions. Part of the reason for this is because atmospheric methane seemed to level off between 2000 and 2007. Now, researchers are using both isotopic measurements and satellite data to determine the origin of the methane increase. They know the increase comes from microbial sources due to the change in the ratio of carbon-13 — but exactly which microbial sources? Wetlands, cattle and landfills produce “microbial” methane, in which microbes break down carbon and produce the greenhouse gas. To determine how much each of these sources contributes, scientists scour the world 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. “Satellite data will give you the location of the murder,” says Palmer. “And the isotopes will give you the weapon – the source type.”
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Wetlands and cattle appear to be the biggest culprits, says Euan Nisbet, professor of earth sciences at Royal Holloway, University of London. “Biological sources are growing faster,” he says. “The strongest growth appears to be coming from the tropics.” The global increase in livestock and landfills is also fueling the increase in microbial emissions. In a forthcoming paper, Lan and Dlugokencky reach a similar conclusion: 85 percent of the increase in atmospheric methane since 2007 is due to microbial sources. And about half of that comes from the tropics. The sources of methane may be natural, but a climate warmed by human activity is fueling these emissions. Climate change is expected to lead to more intense rainfall in eastern 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 work on the ground, physically traveling around the world to capture methane samples in containers to be sent to laboratories. 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 laboratory is filled with boxes of samples sent from all over the planet. “These are exciting,” says Rebecca Fisher, a lecturer in atmospheric sciences at the university, pointing to a box that has just arrived. “They’re air bottles from Halley Research Station in Antarctica.” Because Antarctica has no vegetation, the air there contains very little locally produced methane, making it ideal for providing background measurements. Fisher is preparing for a trip to Finland, near the other side of the planet from Antarctica, to collect samples that will measure what she calls the isotopic “fingerprint” of Arctic wetland emissions. By measuring not only the carbon-13 isotope, but also the hydrogen isotope deuterium, known as heavy hydrogen, her team and others are working to create a library of these fingerprints. “We have really different signatures in the Arctic versus the tropics,” says Fisher. “By taking these isotopic measurements, we can see if that matches what’s in the atmosphere.” In addition to helping scientists piece together the current rise in methane emissions, the Arctic also gives an idea of what future emissions might be like: the region is warming three times faster than the rest of the planet. “Permafrost itself contains about 1,500 billion tons of carbon,” says Katey Walter Anthony, professor of ecology and biogeochemistry at the University of Alaska, Fairbanks. As the permafrost thaws, this carbon can be converted to methane by microorganisms known as methanogens. She’s flown all over Alaska to measure methane coming out of lakes, and what she saw recently surprised her. “In the last five to six years, I’ve just seen incredible changes,” he says. “It seems like we’ve crossed a threshold and we’re seeing crazy things happen.” One of those crazy things is that lakes form – lots of them – as permafrost melts. These pools, known as thermokarst lakes, are spreading rapidly. And methane-producing microbes thrive on all the freshly thawed organic material at the bottom of these new lakes. “In interior Alaska we’ve seen almost 40 percent…
