Ancient woolly mammoth bones and grasslands locked in a 400,000 sq mile stretch of Siberian permafrost are starting to thaw, and have the potential to unleash billions of tons of carbon and accelerate global warming, Russian and American scientists have concluded.

‘‘It’s like taking out food out of your freezer. Leave it on your counter for a few days, and it rots,’’ said University of Florida botany professor Ted Schuur, describing the process by which decaying animal and plant matter in the soil is converted by bacteria into carbon dioxide, methane and other harmful greenhouse gases. The study, published in last week’s issue of the journal Science, concluded that while other global warming researchers are including carbon reserves in the ocean, and in current soils and vegetation on the earth in their calculations, they have inadvertently ignored vast amounts of carbon trapped in permafrost in the north plains of Siberia and central Alaska.

If all the permafrost thawed and was released as heat-trapping carbon dioxide, it could nearly double the 730 billion metric tons of carbon now in the atmosphere, the scientists said. They said what was most surprising was the size and depth of the terrain that they found that could be affected—a piece two-thirds the size of Alaska and an average of 80 ft deep containing roughly 500 billion metric tons of carbon.

‘‘It’s like finding a new continent under the earth,’’ said lead author Sergey Zimov. He said the vast, carbon-rich area had been buried over many millenniums by a unique layer of wind-borne ‘‘loess’’ dust that covered bones of mammoth, bison, saber toothed tiger and abundant grasses they fed on, then froze about 10,000 years ago into permafrost.

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