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Ground water and tundra fires may work together to thaw permafrost, study says

Montreal and Victoria – Ground water may play an unrecognized role in thawing Arctic permafrost following wildfires, according to new Canadian-led research.

A new study indicates that, after wildfire burns off a portion of organic rich soil that normally insulates permafrost, summer warmth penetrates deeper into the frozen soils, allowing ground water to flow downgradient and potentially contributing to greater release of greenhouse gases, the Geological Society of America said in a news release.

The study’s lead author, ecohydrologist Samuel Zipper from McGill University and the University of Victoria, shared his findings on Oct. 22 at the Geological Society of America Annual Meeting in Seattle, Wash.

Conventionally, Zipper said in the release, researchers tend to estimate permafrost thaw by vertically measuring thawed soil depth at single locations, offering a one-dimensional perspective of the aftereffects of fire on the Arctic landscape.

Instead, he suspected that ground water might play an unrecognized role in the process, where it thaws permafrost in other areas after flowing through once-frozen soils. Once wildfire burns through the organic-rich soil that usually insulates permafrost, summertime temperatures can reach deeper into the frozen ground, contributing to even greater thaw.

“What’s interesting to me,” Zipper said, “is if you burn one location, how do other parts of the landscape respond to that fire? Can ground water flow transmit the impacts of burning from one spot to another by moving water and heat through the subsurface?”

To explore the question, Zipper and his colleagues ran over 20,000 simulations of the largest Tundra fire in recorded history: Alaska’s Anaktuvuk River Fire, which scorched over 400 square miles. They ran simulations both with and without ground water flow to tease out its contribution to permafrost thaw following fires. Zipper suspected that, after summertime temperatures penetrate the permafrost, ground water would flow more rapidly, delivering a greater amount of water into nearby streams.

They found that, indeed, wildfire does lead to deeper permafrost thaw, which is enhanced by ground water flow. But, surprisingly, more water evaporated following fire, so there was less overall ground water flow reaching the stream.

“Even though the water was able to flow more rapidly,” Zipper said, “there was actually less water being supplied to the subsurface because of this change in evaporation. We found this kind of interesting dynamic where there was more room for water to flow, but less water available to flow.”

Additionally, the results suggest that wildfire may trigger a positive feedback loop of permafrost thaw. When permafrost thaws simply from the Sun’s warmth, ground water flows from the thawed soil into more permafrost, melting it, which releases more ground water to thaw more permafrost. Zipper suspects that fire could prematurely set this loop into motion and intensify its effects.

To follow up on the results of the study, Zipper plans to work with data from more Alaskan field sites, which will help him determine the pervasiveness of the phenomenon. “We want to know, how real and widespread is this process?”

Permafrost contains many organic compounds left by long-dead but not degraded frozen plants. Thawing permafrost releases greenhouse gases from those compounds into the atmosphere, which further intensifies warming. The Arctic is among the most rapidly warming regions on Earth, and hotter summers tend to bring more fire-prone vegetation, according to Zipper.

To properly manage and protect these landscapes, Zipper said it’s important to form a complete picture of their response to fire, and incorporating ground water into that picture is essential.

“Our findings show that you can’t really understand how permafrost is going to respond to disturbances like fire without understanding what’s happening in the ground water system,” Zipper said. “Ground water is a component of the Arctic’s water cycle that’s been under appreciated just for lack of data. It’s kind of the next frontier for figuring out what’s going to happen in the Arctic’s future.”