Chris Mooney in Mother Jones:

Scientists have known for some time about the risk of large-scale carbon emissions from thawing permafrost. But in recent years, they’ve become increasingly attuned to an additional—and very worrisome—aspect of this threat. As climate change proceeds, larger and more intense wildfires are increasingly scorching and charring the forests of the north. While these fires have always been a natural and recurring aspect of forest ecosystems, they now appear to be undergoing a major amplification. And that, in turn, may further increase the threat of permafrost thawing and carbon releases—releases that would, in turn, greatly amplify global warming itself (and potentially spur still more fire activity).

“You have this climate and fire interaction, and all of a sudden permafrost can thaw really rapidly,” explains Jon O’Donnell, an ecologist with the National Parks Service’s Arctic Network. Scientists call it a “positive feedback,” and it’s one of the scariest aspects of global warming because, in essence, it means a bad situation is making itself worse.

When it comes to understanding the wildfire-permafrost feedback and just how bad it could be, one factor is clear: Wildfires are definitely getting worse. “The area burned by wildfires has been increased quite a bit over the last couple of decades,” says Terry Chapin, a biologist at the University of Alaska-Fairbanks. Indeed, a new study just out in theProceedings of the National Academy of Sciences suggests that recent fire activity in these “boreal” regions of the globe is higher than anything seen in the last 10,000 years.

Fires are also becoming more severe, says O’Donnell. Finally, the seasonality of fires appears to be changing, with burns extending later into the summer, when permafrost has thawed more completely—once again, amplifying the overall impact of burning on frozen soils and the carbon they contain.

And here’s where the feedback kicks in: Large northern fires don’t just burn huge swaths of forest. They can also burn off the upper layer of lichen and mosses on the forest floor. When intact, this forest surface layer insulates the underlying permafrost and protects it from thawing—but getting rid of it takes away that protection, even as it also exposes the area to the heating of direct sunlight.

Plus, there’s an added effect: After a fire burns through a region, O’Donnell notes, it leaves behind an area of the earth’s surface that is blackened in color. And these dark areas absorb more heat from the sun, thus further upping temperatures and thawing permafrost. As the soil thaws, meanwhile, microbes have a much easier time decomposing its organic matter. “The microbes can start to crank on that carbon,” says O’Donnell, adding that the process results in the release of carbon dioxide to the atmosphere.