More than 100 billion tonnes of carbon locked in permafrost could be released as the atmosphere warms over the course of the century, say scientists who have written a sweeping new assessment.
But the escape of that carbon will likely be gradual, they say, not an abrupt time bomb that could overwhelm the global climate system in one fell swoop.
The assessment, published on Wednesday in the journal Nature, offers the most comprehensive look to date at the connection between climate change and permafrost, a layer of permanently frozen Arctic and sub-Arctic soil that covers about one quarter of the northern hemisphere's land mass.
The assessment shows that thawing permafrost will play a role in accelerating climate change as inert organic material lying frozen in the ground becomes available to microbes that break it down, dumping carbon-bearing gases into the air.
Researchers estimate that, worldwide, permafrost contains about 1.2 to 1.4 trillion tons of carbon, about twice the total amount in the atmosphere. The release of so much carbon over a decade or less would be catastrophic.
"What we can safely say is that is not going to happen," said Merritt Turetsky, an ecologist at the University of Guelph who was part of the international team that produced the assessment.
"There will be an effect," she added, "but it's going to be more like a dial on the current system."
By combining data from many previous studies conducted across the Arctic, Dr. Turetsky and her colleagues found that 5 to 15 per cent of the carbon in permafrost is likely to make its way back into the atmosphere over many decades rather than in a rapid burst.
The rate of the release will depend on how fast the climate is warming, which is primarily driven by the burning of fossil fuels. Carbon that originates from permafrost could add an extra 0.13 to 0.27 degrees Celsius of warming by 2100, the study says. In comparison, humans are on course to warm the planet by at least a few degrees above pre-industrial levels. As nations negotiate how to reduce those emissions, the permafrost effect means they will have even less leeway before the planet crosses a two-degree limit on warming established by international agreement.
"This highlights the urgent need to think about human emissions and controlling those so that we don't have additional emissions from places like the Arctic," said Ted Schuur, a biologist at Northern Arizona University who led the assessment.
Another variable in the equation is the form the release carbon will take. In dry and well-aerated conditions, it should mainly emerge as carbon dioxide. Where melting permafrost creates a wet environment, the breakdown of organic matter is more likely to produce methane, a more potent greenhouse gas.
Much of the work behind the assessment was supported by the U.S. National Science Foundation, which funds an extensive research program on permafrost based in Alaska.
In Canada, the situation is somewhat different, said Christopher Burn, a professor of environmental studies at Carleton University, who was not involved in the assessment. While federal dollars have been directed toward research into the physical impact of melting permafrost on Arctic infrastructure, less support is generally available for studies on the consequences of permafrost loss for climate change. About half of Canada's total land area is permafrost.
"I think it's one of the two biggest issues in Arctic science at the moment," Prof. Burn said. "The other one being the reduction in sea ice extent."