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Update Apr. 7, 2022, 11:10 AM ET: The National Oceanic and Atmospheric Administration (NOAA) announced that in 2021 atmospheric levels of methane had increased by a record amount for the second straight year. The original methane story from April 2021 continues below.

The potent greenhouse gas methane continues to increase in Earth's atmosphere, and it shows no sign of slowing down.

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The NOAA graph below paints a clear picture. Following a temporary flattening or stabilization of methane levels in the aughts, the odorless, invisible gas has continually risen since around 2008, and more recently has accelerated.

Methane is a problematic greenhouse gas because it traps heat on the planet 28 times better than carbon dioxide. (Methane lives in the atmosphere for about a decade before breaking down into the major greenhouse gas carbon dioxide.) Over the last two centuries, as fossil fuel use and cattle ranching expanded around the globe (both make methane emissions), atmospheric methane levels have over doubled, causing a whopping quarter of the human-caused warming on Earth. It's why methane is considered the "second most important greenhouse gas" created in large part by human activity (behind CO2).

But explaining the recent, stark methane rise, at a time when climate scientists emphasize greenhouse gas emissions must rapidly fall, isn't simple (sorry!). A number of human-caused and natural factors are at play. Yet scientists are investigating, and closely watching the potential culprits.

"Methane levels are going up but our community does not have a clear answer about why," said Manvendra Dubey, an atmospheric chemist at Los Alamos National Laboratory. "Many natural and anthropogenic [human-caused] sources are contributing."

Rising methane levels in the atmosphere. Methane concentrations reached a record high in 2021, averaging 1,895.7 ppb (parts per billion). Credit: NOAA

Why solving the methane mystery is a challenge

Methane is a tough nut to crack. Comparatively, it's easier to account for the sources of carbon dioxide. That's because when carbon-rich fossil fuels like gasoline, coal, or methane (aka "natural gas") are burned, they create CO2. So knowing how much fuel civilization burns (we do) gives a relatively clear idea of how much CO2 humanity is loading into the atmosphere (a colossal amount).

"CO2 is much more clear-cut from a scientific point of view," explained Steven Smith, an earth scientist at the Department of Energy's Pacific Northwest National Laboratory.

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In contrast, methane can come from some disparate, indirect, awfully hard-to-monitor sources. "Methane is amuch more complicated beast," said Dubey. To track and estimate these emissions, scientists collect emission data from world nations, observe emissions from space, take readings from aircraft, towers, and cars, and more.

Though the major methane contributors are detailed in the section below, some elusive methane sources include "fugitive gases" (like leaking methane from oil drilling sites) and methane from remote biological sources (like bacteria decomposing plants in wetlands). Atmospheric scientists can actually identify when methane comes from biological sources, as opposed to fossil fuels. But,scientists can't easily distinguish between the types of biological sources (such as methane from wetlands versus methane from cows). This leaves a somewhat murky methane picture, for now.

What's more, it's possible that natural events in the atmosphere have also impacted methane numbers. Atmospheric scientists sometimes call the atmosphere a great big laboratory teeming with chemical reactions between different gases and particles. And in this atmospheric laboratory, researchers in 2017 found evidence of a decline in a molecule (called OH) that naturally breaks down methane. This would make methane more abundant. But, this effect remains uncertain: Other research has found the declines of OH to be quite small.

Methane is the second most important greenhouse gas. Credit: NOAA

What are the sources of methane?

There's a diversity of methane sources. But the factors or processes contributing to the current surge is the big question. "The [methane] budget is complex, so many combinations of processes could be responsible for the increase since 2007," explained Ed Dlugokencky, a research chemist at NOAA's Global Monitoring Laboratory.

Here are some major methane sources:

• Wetlands: Wetlands — places like marshes, bogs, and swampy areas where microbes release methane — are naturally responsible for some 30 percent of global methane emissions. This makes wetlands, particularly in the warm tropics, the single largest source of methane. They'll always emit lots of methane. But there's evidence that methane emissions from wetlands may have increased, explained Scot Miller, a professor of environmental health and engineering at Johns Hopkins University who researches greenhouse gas emissions. And in the future, research suggests that as the globe warms and rainfall increases in many areas, wetlands may grow wetter or expand, resulting in more methane emissions.

• Fossil fuels:"Oil and natural gas emissions on a global scale are a really important source of methane," noted Miller. Methane often leaks from pipelines, or is released when oil or coal is extracted from the earth. China, for example, emits more methane than any other nation (largely from coal mining), and these emissions rose each year between 2010 and 2015. In the U.S., methane leaks prodigiously from some oil and gas sites, and emissions are significantly higher than government estimates. Like wetlands, fossil fuels are a major methane contributor and could also be a player in the recent methane surge.

• Agriculture: Cows burp methane, and cows dominate the world's sprawling pasturelands, which take up an area almost the size of North America. "By far and away the largest agricultural source of methane is cows," said Miller. (Other types of agriculture, like rice paddies, contribute methane too.) Overall, humanity produces three times more meat than it did half a century ago, in part resulting in some 1 billion million cattle alive, and burping, today. Cattle and pasturelands, a potent methane source, might certainly contribute to the recent methane surge.

• Landfills: Ourdecomposing waste contributes around 12 percent of methane into the atmosphere each year. This is a sizable problem, and environmental experts propose capturing this gas, so less of it enters the atmosphere.

What's not *currently* a major methane source

The Arctic's permafrost — a layer of soil that stays frozen for years at a time — stores bounties of carbon, in the form of dead plants. There's more than twice as much carbon stored in these frozen soils than in the entire atmosphere. And the Arctic is rapidly heating, meaning some permafrost is thawing.

But, as of now, researchers haven't found evidence that methane is leaching in significant amounts from the warming Arctic — not yet, anyway. (But this is a worrisome possibility that could unleash a vicious cycle of more warming, known as a "feedback loop," as the release of more greenhouse gases drives more thawing soil.)

"There's no direct evidence of permafrost contributing to the global methane rise," said Dubey. "I'm not saying it won't happen in the future," he added.

What about that methane "pause"?

Methane levels in the atmosphere stayed relatively steady from around 2000 to 2007. This temporary pause in the methane rise remains an open area of investigation, too.

"That pause is still a real mystery," said Miller.

It's possible the collapse of the Soviet Union in the nineties resulted in a major fall in agricultural emissions, leading to the temporary methane plateau. Some research suggests that nations like the U.S. clamped down on "fugitive emissions," meaning fewer methane releases from the fossil fuel industry at the time. Or the pause could be part of a natural atmospheric cycle, as the atmosphere naturally broke down methane during that period.

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Atmospheric scientists don't have a timetable of when we might expect a more certain picture of why methane started rising, and accelerating, again. Yet the bigger climate picture is already clear: Methane levels are headed in the wrong direction.

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The more heat-trapping greenhouse gases in the atmosphere, the more Earth will warm. Already, the warming trend is stark. The last time Earth had a month of normal temperatures— compared to the 20th century — was in February 1985.