Areas of permanently frozen ground in northern regions are now emitting more carbon into the atmosphere than they absorb, causing the planet to heat even further, according to the first Arctic-wide estimate of all three major greenhouse gases.
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| A crater formed by thawing permafrost in Russia Padi Prints/Troy TV Stock/Alamy |
Frozen ground, or permafrost, which underlies 15 per cent of the northern hemisphere and contains twice as much carbon as the atmosphere, has shrunk in area by an estimated 7 per cent in 50 years as it thaws. Recent research suggests the thaw will slow but not stop if we successfully limit global warming to 1.5°C above pre-industrial levels.
Scientists, however, haven’t been sure whether the permafrost region has become a net emitter of greenhouse gases. Even as the thaw releases more carbon compounds from the once-frozen biological matter in the ground, increased summertime plant growth is absorbing more CO2 from the atmosphere.
A 2019 study found that the Arctic was emitting more CO2 than it was absorbing, but research in 2021 and 2023 suggested that it was still a net sink for CO2.
Now, Justine Ramage at the Nordregio research institute in Stockholm and her colleagues have found the permafrost region has tipped from sink to source, emitting 144 million tonnes of carbon per year between 2000 and 2020. That is largely because methane emissions were included in the measurement in addition to CO2. Permafrost also emitted 3 million tonnes of nitrogen per year, partly in the form of nitrous oxide, an even more powerful greenhouse gas.
“You put some frozen food in a freezer, it’s OK. As soon as you take it out, it starts rotting very fast,” says Ramage. “The [microbial] activity starts increasing, and when it’s not positive for climate, it will have a strong impact.”
While previous research often relied on satellite data or machine learning, Ramage and her colleagues compiled ground-level emissions observations from 200 sites across Scandinavia, Russia, Alaska and Canada and extrapolated them to areas with similar plants and moisture.
Vegetated areas were mostly carbon sinks, but these were offset by emissions from rapidly expanding lakes, as well as wildfires, which weren’t taken into account by earlier studies.
The estimates have large uncertainty ranges because greenhouse gas monitoring, while improving, is still patchy in remote Arctic areas. “Abrupt thaw”, which includes collapses and landslides, is especially difficult to quantify, says Eugénie Euskirchen at the University of Alaska Fairbanks, who published observations in January showing that emissions from permafrost bogs and forests are increasing.
“Abrupt thaw is kind of a wild card,” she says. “It’s really hard to constrain and really hard to make measurements because you have to be right on top of it.”
Ramage’s study found that abrupt thaw released 31 million tonnes of CO2 and 31 million tonnes of methane per year, but didn’t include these figures in the total carbon estimate for fear of double counting. If anything, the total probably underestimates abrupt thaw, as well as wintertime emissions, says Euskirchen.
Susan Natali at the Woodwell Climate Research Center in Massachusetts, who led the 2019 permafrost emissions study and was a co-author of the “more complete story” in Ramage’s paper, says permafrost’s shift from sink to source will make climate change even worse than expected.
“This is a new source of greenhouse gases to the atmosphere that are not fully accounted for in global climate models,” she says.
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