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The grim origins of an ominous surge in methane

The likely culprit is actually sneakier and more threatening than the scenario where scientists leak a massive pipeline leak somewhere. Write today in the diary Naturean international team of researchers has found that humanity’s methane emissions actually decreased in 2020, but of nature no: wetlands spewed significantly more gas than in 2019. In fact, it was the fastest rate of methane growth since atmospheric measurements began in the early 1980s. This could be a hint of a potential climate feedback loop, which could release even more methane as the world warms. And ironically, due to chemistry quirks, even civilization’s reduced emissions during the first year of the pandemic ended up exacerbating the atmospheric methane problem.

Methane is a perfectly natural thing to have in the sky, as many environmental processes produce the gas. As the the climate warms up quicklyfrozen ground in the far north known as permafrost can thaw, allowing microbes buried within it to munch on organic material and release methane as a byproduct. Wetlands absorb carbon from the atmosphere as plants grow, then release methane as those plants die and decompose. Forest fires they also burp methane while chewing on vegetation.

In the human realm, the fossil fuel industry is a major source of methane. Rotting food waste also releases the gas, as a wetland would. And let’s not forget that cow burps: The bovine stomach acts as a fermentation vat, where microbes break down plant cellulose and excrete methane.

The authors of this new paper calculated humanity’s methane emissions in 2020 by collating data such as agricultural productivity and fossil fuel production. They found that anthropogenic methane emissions fell by 1.2 trillion grams (one teragram, in scientific terms) between 2019 and 2020, as the world locked down and the economy reeled.

The researchers also knew that Siberia suffered unprecedented heat in 2020, potentially permafrost thaw, and that the northern wetlands had been exceptionally warm and humid. “If you have a warmer temperature in the northern hemisphere, you’ll get more methane produced by microbes in the wetlands,” says Shushi Peng, an atmospheric scientist at Peking University and the paper’s lead author. “If you get a wetter climate, the wetlands expand.” Essentially, you grow a natural methane factory.

Using models, the team estimated the amount of gas coming from those landscapes: as humanity’s methane emissions decrease, emissions from wetlands increased of 6 teragrams, mainly from Siberia, boreal North America and the northern tropics. This accounts for about half of the increase in atmospheric methane in 2020.

The story of the other half is more ironic. When we burn fossil fuels, they produce CO2, but also nitric oxide, or NOx. How As NOx enters the atmosphere, it produces a molecule known as a hydroxyl radical (OH), which breaks down methane. All told, OH removes about 85 percent of annual methane emissions. “During the lockdown, NOx emission was decreasing,” Peng says. “So the OH of the atmosphere, the methane sink, could be slowed down.”

That is, because we’ve polluted less—heavy industry has gone bankrupt, flights have been cancelled, people have stopped commuting—we’ve also produced less of the pollutant that normally breaks down methane. It’s a second unfortunate and surprising consequence of reducing pollution: burning fossil fuels also produces aerosols that bounce some of the sun’s energy back into space, somehow refresh the climate. While it is imperative to decarbonise as quickly as possible, eliminating the beneficial effects of NOx and aerosols has some unintended and convoluted side effects.

“Burning fewer fossil fuels will cause there to be fewer OH radicals in the atmosphere, which will cause methane concentrations to rise,” says Earth scientist George Allen of Virginia Polytechnic Institute and State University, who wrote an accompanying commentary on the ‘article but was not involved in the research. “So this will reduce the effectiveness of measures to combat global warming.”

This makes it all the more urgent for humanity to take drastic measures to reduce methane and CO2 emissions, especially given the alarming degradation of northern lands as the planet warms. The growth of emissions from nature makes the fight against it even more urgent keep those lands. For example, people are draining the soggy bogs and setting them on fire to convert them into agricultural land, which transforms them from carbon sinks into carbon sources. And why the Arctic is warming more than four times faster than the rest of the planet, human development may encroach further north, churning up carbon sequestered in the soil as people build roads and homes. All of this only exacerbates the problem.

This type of degradation is blurring the line between human and natural sources of methane. “While some sectors are clearly anthropogenic – industry, transport, landfills and waste – other ‘natural’ sectors such as polluted waterways and wetlands may be little, moderately or heavily influenced by humans, which in turn can increase ‘natural’ methane emissions,” says Judith Rosentreter, a senior research scientist at Southern Cross University who studies methane emissions but was not involved in the new research.

Meanwhile, the Arctic region is greening, thanks to the new vegetation, which darkens the landscape and further warms the soil. Permafrost, which covers 25 percent of the Northern Hemisphere’s land surface, is thawing so rapidly that it is dig holes in the earthknown as thermokarst, which fills with water and provides ideal conditions for methane-spewing microbes.

“There’s a lot of organic carbon stuck in there, it’s like a frozen compost pile in your garden,” says Torsten Sachs of the German Research Center for Geosciences GFZ, who was not involved in the new research. “There is a lot of talk, a lot of speculation, and a lot of modeling of how many greenhouse gases will come out of these thawing and warming areas of permafrost. But until you have real data from the field, you can’t really prove it.

Sachs did exactly that, venturing into the Siberian tundra for months on end to collect data. In a paper recently published in Nature Climate change, found that methane production each June and July has increased by 2% annually since 2004. Interestingly, while this corresponds to significantly higher atmospheric temperatures in the region, it does not appear to correspond to permafrost thaw. Instead, the extra methane could come from wetlands that sit above the permafrost.

This is the extreme complexity that scientists are trying to understand better. While the new paper’s modeling may make fun of the methane emitted by humans and nature, field data is also needed to fully understand the dynamics. The main concern is that runaway carbon emissions could initiate climate feedback loops: We burn fossil fuels, which warm the planet, which thaws permafrost and forms larger wetlands that emit methane. This will have serious consequences for the rest of the planet.

Scientists can’t yet tell, however, whether we’re already seeing a feedback loop. This new study focused on 2020, so researchers will need to continue collecting methane data for consecutive years and pinpoint the source of those emissions. But methane emissions were even higher in 2021. “The idea that warming is fueling warming is definitely something to worry about,” says James France, senior international methane scientist at the Environmental Defense Fund. “This is very difficult to mitigate. So it really reinforces the idea that we need to double down and really focus on mitigation in the areas that we do Power check.”