The top 11 feet of soil in the Arctic continues to thaw. Sinkholes are opening, highways are buckling, houses and forests are tilting, all of which is wreaking havoc on landscapes, wildlife and cities from Murmansk to Juneau. This permafrost layer--defined as soil that remains icy cold for more than two years--covers nearly a quarter of the land in the Northern Hemisphere. But that total is shrinking and new models show that it may nearly disappear by the end of this century.

"Our model predicts that the area with permafrost in the top [11 feet] will fall sharply over the next century," explains climate change researcher David Lawrence of the National Center for Atmospheric Research in Colorado. "Deeper permafrost, which can extend down [more than one mile] and which our model does not represent, will be largely unaffected by global warming." Lawrence and his colleague Andrew Slater of the University of Colorado developed a model that accounts for the effects of the atmosphere, land, ocean, sea ice and the soil itself. "People have used models to study permafrost before, but not within a fully interactive climate system model," Lawrence notes.

The researchers set the model loose on several scenarios to determine how permafrost would react under different concentrations of greenhouse gases in the atmosphere: a scenario in which concentrations skyrocket from their current value of 350 parts per million (ppm) to 810 ppm by the year 2100; a scenario in which such concentration levels off at around 550 ppm due to deployment of emission-free technologies; and a scenario, dubbed "business as usual," in which concentrations climb to 700 ppm.

Under the skyrocketing scenario, permafrost declines from more than four million square miles today to just 400,000 by 2100. Even in the best case scenario, permafrost shrinks to cover just 1.5 million square miles. "Permafrost degradation of this magnitude is likely to have significant adverse ecological and societal impacts," the researchers write in the current issue of Geophysical Research Letters. (In the image above, light blue denotes current permafrost coverage; dark blue represents future coverage under the "business as usual" scenario.)

Arctic rivers already carry 7 percent more fresh water to the ocean today than they did in the 1930s. The model correctly predicts this rise and points to as much as a 28 percent increase in water flow by century's end, as water drains without being blocked by frozen soil. This could impact everything from the formation of sea ice to the circulation of currents in the Arctic Ocean.

Even more troubling, this permafrost layer contains anywhere from 20 to 60 percent of the carbon trapped in soils in the world. Thawing could lead to a massive "positive feedback loop" that pushes global greenhouse gas concentrations ever higher. "The major feedback is the possible accelerated rate of decomposition of organic matter that is currently 'locked up' in the frozen soil, leading to increased atmospheric concentrations of greenhouse gases such as [carbon dioxide] and methane," Lawrence says. "If this emission occurs on a large scale, it could speed up or enhance global warming."

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