Global warming may cause permanently frozen ground to thaw over a large area of Siberia, threatening release of carbon from soils, British researchers say. A global rise of 2.7 degrees Fahrenheit could thaw Siberia's permafrost -- ground frozen throughout the year -- and release more than 1,000 gigatons of the greenhouse gases carbon dioxide and methane into the atmosphere, potentially accelerating warming, an international team led by Oxford University scientists said. The researchers studied stalactites and stalagmites from caves located along the "permafrost frontier," where ground begins to be permanently frozen in a layer tens to hundreds of yards thick. Because stalactites and stalagmites only grow when liquid rainwater and snow melt drips into the caves, they record 500,000 years of changing permafrost conditions, including warmer periods similar to the climate of today, an Oxford release said. Evidence of a warming period 400,000 years ago suggests global warming of 2.7 degrees F would enough to cause substantial thawing of permafrost far north from its present-day southern limit, the researchers said. "As permafrost covers 24 percent of the land surface of the Northern hemisphere significant thawing could affect vast areas and release gigatons of carbon," Oxford earth sciences Professor Anton Vaks said. "This has huge implications for ecosystems in the region, and for aspects of the human environment."