Peatlands are the largest terrestrial store of carbon, locking away twice as much heat-trapping CO2 from the atmosphere in their waterlogged soils as all the world's forests.
These carbon-rich reservoirs, composed of partially decayed organic matter, only cover three percent of Earth's surface, and generally fade out in the far north, where harsh weather limits plant growth.
But warmer temperatures caused by climate change have improved growing conditions for plants in the Arctic, and satellite data has shown a general "greening" of this frosty region.
Using drones, satellite imagery and on-the-ground observations, an international team of scientists assessed peatlands in the European and Canadian Arctic to see if they had benefited from warmer climes.
They found strong evidence that peatlands "have likely undergone lateral expansion over the last 40 years" in the Arctic, which is the fastest-warming region on Earth.
"The permafrost thaws a little, provides a water source for vegetation, and surface vegetation recovers. In this study, we specifically see a lateral expansion," Michelle Garneau, a professor at the University of Quebec in Montreal, and co-author of the study, told AFP.
The most marked change was observed where summer temperatures have risen the most, such as in the Norwegian archipelago of Svalbard.
"All these new vegetated surfaces that didn't exist three decades ago are currently actively absorbing carbon," Garneau added.
How Arctic peatlands respond to climate change in future is "still highly uncertain", said the study, published in the peer-reviewed journal Communications Earth & Environment.
Recent modelling suggests these northern peatlands "may become a carbon source from mid-century" as they dry out and permafrost thaws, the study said.
They are also at risk from wildfires, which release masses of stored up carbon at once.
"This means that in the short term, these expanding peatlands are a growing carbon sink in the Arctic," said study co-author Karen Anderson, a professor from the University of Exeter, which led the research.
"But in the future, this could switch back, and this will happen because of higher temperatures and potentially things like fires," she told AFP.
As peatlands expand, they also release methane, a potent greenhouse gas with far more heat-trapping capacity than CO2, though these emissions decline over time.
"So while our study gives us some positive news, it does not detract from the urgent need to reduce greenhouse gas emissions and stabilise our climate," said Anderson.
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