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.

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.