An exceptional wildfire in northern Alaska in 2007 put as much carbon into the air as the entire Arctic tundra absorbs in a year, scientists say.
The Anaktuvuk River fire burned across more than 1,000 sq km (400 sq miles), doubling the extent of Alaskan tundra visited by fire since 1950.
With the Arctic warming fast, the team suggests in the journal Nature that fires could become more common.
If that happens, it could create a new climate feedback, they say.
Temperatures are low even in summer, and the ground can also remain wet after the ice has melted.
But 2007 saw unusually warm and dry conditions across much of the Arctic - resulting, among other things, in spectacularly fast melting of Arctic sea ice.
This created conditions more conducive to fire, and when lightning struck the tundra in July, the Anaktuvuk River fire ignited.
"Most tundra fires have been very small - this was an order of magnitude larger than the historical size," said Michelle Mack from the University of Florida in Gainesville, who led the research team on the Nature paper and is currently conducting further field studies in Alaska.
"In 2007, we had a hot, dry summer, there was no rain for a long period of time.
"So the tundra must have been highly flammable, with just the right conditions for fire to spread until the snow in October finally stopped it." (more)