Siberia isn't the only place on Earth with massive lodes of permafrost -- parts of Alaska, Canada and northern Europe have them too. The Siberian area is possibly the world's largest, covering nearly 400,000 square miles (1.036 million sq km), with an average depth of 82 feet (25 metres), and probably holds about 500 billion metric tons of carbon.

By any measure, this is a lot, and it is in fact twice what scientists previously believed was there, ecologist Ted Schuur of the University of Florida said in a telephone interview.

"There's a huge pool of carbon, even more than people thought before, perhaps double the amount of carbon that we thought," said Schuur, one of the article's co-authors. "If you have twice as much carbon there, essentially in the future twice as much could be released into the atmosphere."

Cars, power plants and other fossil fuel burners release at least 6 billion metric tons of carbon dioxide into the atmosphere annually, contributing to global warming, the scientists said.

As the Siberian permafrost thaws, it will release the carbon contained in old grass roots and buried animal bones into the atmosphere, in what could be an unstoppable contributor to global climate change, according to the researchers.

Earlier climate models may have failed to account for this possible component of global warming, he said.

Schuur said this source of atmospheric carbon could create a vicious global warming cycle.

"You have anthropogenic (human-generated) carbon that's making things a little bit warmer, and that causes the permafrost to warm up and carbon is then released from the permafrost," he said. "It goes into the atmosphere and makes things warmer yet again, so then more permafrost thaws."

If all Siberian permafrost thawed and released its carbon in the form of heat-trapping carbon dioxide, it could nearly double the 730 billion metric tons of carbon now in the atmosphere, the scientists said.