BOULDER—Global warming may decimate the top 10 feet (3 meters) or more of perennially frozen soil across the Northern Hemisphere, altering ecosystems as well as damaging buildings and roads across Canada, Alaska, and Russia. New simulations from the National Center for Atmospheric Research (NCAR) show that over half of the area covered by this topmost layer of permafrost could thaw by 2050 and as much as 90 percent by 2100. Scientists expect the thawing to increase runoff to the Arctic Ocean and release vast amounts of carbon into the atmosphere.

The study, using the NCAR-based Community Climate System Model (CCSM), is the first to examine the state of permafrost in a global model that includes interactions among the atmosphere, ocean, land, and sea ice as well as a soil model that depicts freezing and thawing. Results appear online in the December 17 issue of Geophysical Research Letters.

"People have used models to study permafrost before, but not within a fully interactive climate system model," says NCAR's David Lawrence, the lead author. The coauthor is Andrew Slater of the University of Colorado's National Snow and Ice Data Center.

About a quarter of the Northern Hemisphere's land contains permafrost, defined as soil that remains below 32 degrees F (0 degrees C) for at least two years. Permafrost is typically characterized by an active surface layer, extending anywhere from a few centimeters to several meters deep, which thaws during the summer and refreezes during the winter. The deeper permafrost layer remains frozen. The active layer responds to changes in climate, expanding downward as surface air temperatures rise. Deeper permafrost has not thawed since the last ice age, over 10,000 years ago, and will be largely unaffected by global warming in the coming century, says Lawrence.