Alaska Wildfire Study Tracks Permafrost Thaw and Climate Feedbacks

In early July, graduate student Andrew Haverdink drilled a cylindrical core of dark permafrost from soil at a research plot marked by surveyor’s tape. The team was gathering samples and data from the site. During their first survey in 2016, the researchers found that in places where the fire burned hottest and deepest, the thawed soil above the permafrost, known as the active layer, extended between a foot and nearly two feet deeper than in plots outside the burn scar.

The fire also crossed two survey grids that the National Park Service established in 2001, including the one where the researchers worked that morning. Xanthe Walker, one of the principal investigators on the project, said that pre-fire snapshot offered a rare opportunity to study how the blaze altered vegetation, carbon and permafrost over time. “There’s a lot of uncertainty,” Walker said. “These are huge biomes with massive fires that no one’s ever been to or ever looked at.”

The project is part of a growing body of research into what happens after fires in the Arctic-boreal region as blazes occur more often and burn hotter, exposing landscapes and frozen permafrost to a warming climate. The work is providing clues about cascading climate feedback effects: as warming temperatures fuel more frequent and severe fires, more permafrost thaws, releasing greenhouse-gas emissions that threaten to drive more warming, fires and permafrost loss.

Permafrost makes up 15% of the land surface of the Northern Hemisphere and holds about 1.5 trillion tons of carbon, roughly twice the amount in the atmosphere. Most of it formed during the last ice age, but it has been thawing at an accelerating rate during the last half-century amid rising temperatures and worsening fires. As it melts, microbes begin to break down organic matter, releasing carbon dioxide and methane, both powerful greenhouse gases.

Scientists have long debated what would have the strongest effect on greenhouse-gas exchange over the next few centuries: the loss of carbon in soil and permafrost or the carbon uptake from “Arctic greening,” the spread of carbon-absorbing plants as temperatures rise. Thawing permafrost can contribute to greening by deepening the unfrozen soil layer and making more nitrogen, phosphorus and other nutrients accessible to plants. Past modeling suggested that carbon dioxide uptake from expanding trees and plants may largely offset the release of carbon from warming soils and melting permafrost, at least over long periods and under moderate levels of warming. But improving observations and analyses have revealed worrisome signs about the impacts of rising Arctic-boreal emissions in the immediate decades ahead, as the world struggles to keep global warming within relatively safe boundaries.

Editor's Summary The 2013 Denali fire burned nearly 35,000 acres and crossed National Park Service survey grids established in 2001, giving researchers pre-fire baseline data. Initial surveys found deeper thaw in severely burned areas, and the project is examining whether carbon release from thawing permafrost could outpace plant uptake. The research adds observational evidence to projections of Arctic-boreal climate feedbacks.