Scientists may have solved the mystery behind a major climate threat

Shannon Osaka
| The Washington Post

Nearly two decades ago, levels of methane in the atmosphere – a dangerous greenhouse gas that is more than 80 times more potent than carbon dioxide in the short term – began to rise. And climb.

Methane concentrations, which had been stable for years, increased by 5 to 6 parts per billion every year from 2007. Subsequently, the growth rate almost doubled in 2020.

Scientists were baffled and concerned. Methane is the big question mark in world climate estimates; Although it breaks down in the atmosphere much faster than fossil fuels, it is so powerful that higher than expected levels of methane could shift the world to much higher temperatures.

But now a study is shedding light on the cause of the record methane emissions. The culprits, according to scientists, are microbes: the small organisms that live in the stomachs of cows, agricultural fields and swamps. And that could mean a dangerous feedback loop – in which these emissions cause warming that releases even more greenhouse gases – is already underway.

“The changes we have seen in recent years – and even since 2007 – are microbial,” said Sylvia Michel, lead author of the paper published last month in the Proceedings of the National Academy of Science. “As wetlands become warmer and wetter, they may produce more methane than before.”

It is difficult for scientists to identify all sources of methane in the world. It comes from leaking oil and gas operations, from burping cows, from landfills and swamps, and from thawing permafrost in the Arctic. When methane emissions increase, finding the cause is like solving a complicated algebra problem with too many unknowns.

And it is a problem that will determine the fate of the climate.

For a while, scientists thought that rising methane emissions came from the growth in the use of natural gas, which is largely methane. Leaks from drilling or pipelines can leak the greenhouse gas into the atmosphere.

But the new paper points to microbes as the biggest source of the methane spike. Michel and her co-authors analyzed samples of methane, or CH4, from 22 locations around the world in a laboratory in Colorado. They then measured the ‘heaviness’ of that methane – specifically how many of the molecules had a heavier isotope of carbon in them, known as C13.

Different sources of methane give off different carbon signatures. Methane produced by microbes – usually small, single-celled organisms known as archaea, which live in cow stomachs, swamps and agricultural fields – tends to be ‘lighter’ or has fewer C13 atoms. Methane from fossil fuels, on the other hand, is heavier, with more C13 atoms.

As the amount of methane in the atmosphere has increased over the past fifteen years, it has also become lighter. The scientists used a model to analyze those changes and found that only a large increase in microbial emissions could explain both the rising methane and its changing weight.

“The most important thing about their conclusion is that it was not fossil or geological,” says Professor Rob Jackson of Stanford University, who was not involved in the study and is part of the Global Methane Budget, a project that examines the sources and emissions of methane in keeps track of all of Europe. the planet.

However, the research does not show how many of those emissions are natural or man-made. Although microbes in wetlands are largely natural, the tiny creatures can also pump methane from reservoirs, farmlands and landfills.

Another recent study found that two-thirds of current methane emissions are caused by humans – from fossil fuels, rice farming, reservoirs and other sources.

“Methane is produced biologically in warm, wet, low-oxygen environments,” says Jackson. “The wetlands of a rice field and the intestines of a cow all look alike.”

But evidence is also emerging that natural wetlands may be responding to global warming by pumping out more methane. Satellite data in recent years has revealed global methane hotspots in the tropical wetlands of the Amazon and Congo.

“Wetlands will emit more methane as temperatures rise,” Jackson said. “This could be the start of a reinforcing feedback that more methane is released from natural ecosystems at higher temperatures.”

According to Michel, it is still too early to say whether this is the start of a vicious circle. “Are these from human-induced changes in freshwater systems, or are they some kind of scary climate feedback?” she said. “I want to be careful about what we can and cannot say with this data.”

Researchers say this doesn’t mean the world can just keep burning natural gas. If wetlands are releasing methane faster than ever, they argue, there should be even greater pressure to reduce methane from the sources that humans can control, such as cows, agriculture and fossil fuels.

More than 100 countries have pledged to reduce their methane emissions by 30 percent by 2030, compared to 2020 levels, but so far that pledge has not yielded results. Instead, satellite measurements show concentrations increasing at a rate consistent with worst-case climate scenarios.

“You can turn a wrench in an oil and gas field to reduce methane emissions,” Jackson said. “There is no wrench for Congo or the Amazon.”