And according to one commentary on the research, current models of climate change have not taken this extra source of greenhouse gas into account.

Scientists have long known that organic carbon trapped inside a blanket of frozen permafrost covering one fifth of the world's land mass would, if thawed, release greenhouse gases into the atmosphere.

But until now they simply did not have a good idea of how much carbon is actually locked inside this Arctic freezer.

To find out, a team of American researchers led by Chien-Lu Ping of the University of Alaska Fairbanks examined a wide range of landscapes across North America.

They took soil samples from 117 sites, each to a depth of at least one metre, in order to provide a full assessment of the region's so-called "carbon pool."

Previous estimates of the Arctic carbon pool relied heavily on a relative handful of measurements conducted outside of the Arctic, and only to a depth of 40 centimetres (15.5 inches).

The study, published in the British journal Nature Geoscience, found that the stock of organic carbon "is considerably higher than previously thought" -- 60 percent more than the previously estimated.

This is roughly equivalent of one sixth of the entire carbon content in the atmosphere.

And that is just for North America. The size and mix of landscapes in the northern reaches of Europe and Russia are about the same, and probably contain a comparable amount of carbon-dioxide producing matter currently held in check only by the cold, the study said.

And the danger of a thaw is real, note climate scientists.

The Nobel Prize-winning UN panel of climate change scientists project temperature increases by century's end of up to six degrees Celsius (10.8 degrees Fahrenheit) in the Arctic region, which is more sensitive to global warming than any other part of the planet.

Commenting on the research, Christian Beer of the Max Planck Institute in Jena, Germany, pointed out that the climate change models upon which future projections are based, do not include the potential impact of the gases trapped frozen Arctic soils.

"Releasing even a portion of this carbon into the atmosphere, in the form of methane or carbon dioxide, would have an significant impact on Earth's climate," he noted in his commentary, also published in Nature Geoscience.

Methane, another greenhouse gas, is less abundant than carbon dioxide but several times more potent as a driver of global warming.

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