Summer rainfall accelerates permafrost thaw in Alaska

In a phenomenon known as Arctic amplification, regions like Alaska and Siberia are warming at about twice the rate of the rest of the planet. The accelerated climate change is producing record-breaking heat waves and wildfires in Siberia, while Alaska is experiencing excessive rainfall.

Arctic amplification is expected to intensify, and scientists are urgently trying to predict what this extreme warming means for the Arctic permafrost that covers about 25 percent of the Northern Hemisphere.

Permafrost not only supports infrastructure and buildings, but also stores about twice the amount of carbon that is currently in the atmosphere. This means that thawing permafrost has enormous potential to accelerate global warming even further.

In a new study from UC Boulder, researchers have investigated how extreme rainfall is impacting Alaska’s permafrost, which underlies about 85 percent of the state. The research is the first to compare how rainfall is affecting permafrost across time, space, and a variety of ecosystems. 

Study lead author Thomas A. Douglas is a scientist with the U.S. Army Cold Regions Research and Engineering Laboratory, and a resident of Fairbanks, Alaska.

“In our research area the winter has lost almost three weeks to summer,” said Douglas.”This, along with more rainstorms, means far more wet precipitation is falling every summer.”

Over five years, the experts took 2,750 measurements to document how far below the land’s surface permafrost had thawed by the end of summer across a wide range of environments near Fairbanks. 

The five-year study period consisted of two summers with average precipitation, one that was unusually dry, and two summers that broke records for high rainfall. The differences in annual rainfall were found to have a major influence on the amount of permafrost thaw, with more rainfall leading to deeper thaw across all sites. 

“I was just at one of our field sites and you need hip waders to get to areas that used to be dry or only ankle deep with water,” said Douglas. “It is extremely wet out there. So far this year we have almost double the precipitation of a typical year.”

As a result of a wet summer in 2014, the researchers found that permafrost did not freeze back to previous levels, even after two dry summers that followed. The most thawing was observed in wetlands and disturbed sites, like trail crossings and clearings.

Forest ecosystems were found to be the most resistant to permafrost thaw, especially spruce forests with thick sphagnum moss layers.

Charlie Koven, an Earth system modeler with the Lawrence Berkeley National Laboratory, used the field measurements to build a model that showed how rain was driving heat down into the permafrost ground.

The study highlights the major influence that land cover types have on the relationships between summer rainfall and permafrost thaw. 

Alaska’s vegetation is expected to be altered by climate change, and wildfires will disturb larger areas of the landscape. These conditions could lead to a reinforcing feedback loop between more permafrost thaw and wetter summers. As climate conditions change rapidly, the researchers will continue to investigate.

Study co-author Merritt Turetsky is the director of UC Boulder’s Institute of Arctic and Alpine Research (INSTAAR).

“This study adds to the growing body of knowledge about how extreme weather, ranging from heat spells to intense summer rains, can disrupt foundational aspects of Arctic ecosystems,” said Turetsky. “These changes are not occurring gradually over decades or lifetimes; we are watching them occur over mere months to years.”

The study is published in the journal Climate and Atmospheric Science.

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By Chrissy Sexton, Earth.com Staff Writer