New York Times: "Starting just a few feet below the surface and extending tens or even hundreds of feet down, it contains vast amounts of carbon in organic matter — plants that took carbon dioxide from the atmosphere centuries ago, died and froze before they could decompose. Worldwide, permafrost is thought to contain about twice as much carbon as is currently in the atmosphere."
"Once this ancient organic material thaws, microbes convert some of it to carbon dioxide and methane, which can flow into the atmosphere and cause even more warming. Scientists have estimated that the process of permafrost thawing could contribute as much as 1.7 degrees Fahrenheit to global warming over the next several centuries, independent of what society does to reduce emissions from burning fossil fuels and other activities."