Longer, rainier summers are thawing permafrost at an accelerated rate in interior Alaska, according to a new study, begging the question: what does this mean for rainy summers in the Canadian North?
“Thawing is happening even faster than we thought,” said Thomas Douglas, an environmental engineer with the U.S. Army Cold Regions Research and Engineering Laboratory and lead author of the study. “We’ve had these crazy wet summers. It’s gonna be bad for permafrost.”
The study, published in Nature’s Climate and Atmospheric Science journal, found that between 0.6 and 0.8 centimetres of permafrost thawed for every centimetre of above-average rainfall in Alaska between 2013 and 2017.
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Many Yukoners likely noticed, and commented on, the many rainy summer days this year compared to last, but Fabrice Calmels, research chair of permafrost and geoscience at Yukon University Research Centre, said that’s a matter of public perception and doesn’t necessarily mean the territory’s permafrost cover was impacted.
“We’ll have to check the precipitation record at the end [of the summer] to know the quantity of water that came to the ground,” he said. “There’s a difference between several large events and small events spread out through the summer.”
Considering all of the factors at play in permafrost thaw, including winter and summer temperatures, and snow cover, as well as the territory’s broad zone of frozen ground, Calmels said it will take some time to see how Yukon fared this summer.
“If we have a very cold winter with little snow, maybe the impact of this summer raining will be mitigated, even cancelled,” he said. “To get this perspective it takes seasons, or years.”
At the moment, he said the impact of groundwater on permafrost is more of a concern in the sites his team monitors, though the relationship between precipitation and temperature, as well as snow accumulation and other factors, are taken into account in their surveys.
According to a 2015 report by Yukon University, annual precipitation in the territory has increased by six per cent over the past 50 years, with summers seeing the most rainfall compared to other seasons.
“Rain water, especially in the summer, is pretty warm and it can move warm, thermal mass down through the soil a lot faster than just warm air temperatures can,” Douglas said. “If you lose three to four weeks of winter to summer, what used to be falling as snow is now falling as rain.”
Study finds effects of precipitation on permafrost vary depending on vegetation
Ground cover plays a role in insulating the permafrost that underlies 24 per cent of the northern hemisphere from thaw brought on by heavy summer rainfall, according to the Alaska study.
Wetter and warmer conditions that are expected across much of the Arctic as a result of climate change will affect the vegetation. And as that vegetation changes, so too does its ability to protect the frozen ground.
Researchers looked at four permafrost sites near Fairbanks, Alaska, with differing types of vegetation cover such as grassy tundra, wetlands and mixed forests. They took measurements in the same locations every year throughout the study to monitor changes in the permafrost.
Over the five years of the study, the depth of the active layer — the layer of soil above permafrost that freezes in the winter and thaws in the summer — increased to varying degrees across the sites, meaning the permafrost had started to thaw.
The summers of 2014 and 2016 were the first and third wettest seasons on record since meteorological data first started to be tracked in the area 91 years ago. The study shows this led to an increase in active layer depth.
After the extremely wet summer of 2014, permafrost didn’t recover to 2013 levels, even after drier seasons that followed, the study shows. Several areas now have “taliks,” or pockets of earth surrounded by permafrost that no longer freeze up.
The study illustrated that some environments are better equipped to protect permafrost from precipitation. Forests, particularly those with peat moss, are the best, Douglas said, adding that thick canopies and moss-covered grounds do a good job of intercepting rainfall.
“That moss is an amazing sponge and it literally sucks up the water,” he said, adding that there’s no standing water in these areas. There was about 0.3 centimetres of permafrost thaw for every centimetre of increased rainfall in sites with black spruce and peat moss; in mixed forests, there were 0.6 centimetres of thaw, Douglas said.
However, he added, the earth beneath the forests tends to contain more dry sand than moisture-bearing silt and has low ice content to begin with. Soil temperatures, on average, are warmer under mixed forests than other vegetation and are increasing in temperature, Douglas said. The winter freeze, he said, is no longer enough to refreeze the grounds that thawed the previous summer.
But disturbed areas — lands affected by human development like trail crossings or lacking a tree canopy due to, say, forest fires — fared the worst among the test sites. In these areas, about one centimetre of permafrost thawed per centimetre of increased rainfall.
These are relatively low-lying areas with little to no tree or underbrush coverage. “Water can linger in those areas and work its magic on thawing,” Douglas said.
Wetlands are also on the more-vulnerable list, Douglas said, because rainfall causes more standing water and has higher potential to penetrate permafrost. These areas saw 0.9 centimetres of thaw for every centimetre of additional rainfall.
Douglas plans to return to the sites this fall to investigate how another summer of heavy rainfall affected permafrost depths.
“I expect we’ll continue to see this top-down thaw,” he said. “I’m assuming we’re going to continue to see that this year, maybe even at a greater pace because it was so wet.”
But it’s not only the North that is impacted by thawing permafrost. Arctic permafrost stores an estimated 1.4 million megatonnes of carbon in frozen organic matter. As it thaws, microorganisms that were dormant when frozen start to break down that matter, releasing carbon and methane into the atmosphere.
“It has global ramifications,” Douglas said.
Holistic approach needed to understand permafrost thaw
While there’s no doubt that large volumes of water affect permafrost thaw, there are other variables at play that also need to be considered — some of which are missing from the study, said Philip Marsh, a hydrologist at Wilfrid Laurier University who has studied permafrost in the Canadian Arctic for more than four decades.
He had further questions around the amount of rain that fell, how much water from the ground ended up in the atmosphere and how long the snow remained on the ground.
“The depth of thaw and the thaw of the permafrost is really controlled by a whole range of factors,” he continued. “Different types of vegetation trap different amounts of snow, for example, and snow is good at insulating the ground during the winter.”
Understanding the rate of permafrost thaw requires a holistic approach, he said, including the impacts of air temperature, humidity and solar heat, as well as water seeping into permafrost.
While you can’t paint all of the North with the same brush, Marsh said there is value in applying the findings of the study to areas that are receiving less snow and rainfall than they historically did, such as Inuvik, N.W.T., to understand future outcomes.
He said we could see all Arctic precipitation levels change in the coming years as sea ice continues to disappear, leaving more open water and more evaporation that eventually becomes precipitation.
“As the Arctic Ocean becomes more ice-free in the summer, you would expect many of these areas to become eventually wetter,” Marsh said.