Their probe sought to shed light on a fiercely-debated but poorly-understood concern: the future of organic matter that today is locked up in the frozen soil of Alaska, Canada, northern Europe and Siberia.

The fear is that, as the land thaws, this material will be converted by microbes into carbon dioxide, which will seep into the atmosphere, adding to the greenhouse effect.

This in turn will stoke warming and cause more permafrost to thaw, which in turn pushes up temperatures, and so on.

But how and when this vicious cycle could be unleashed is unclear.

Indeed, some voices have argued that it will not present a significant threat, as plants will start to grow on the soggy, warmer earth and suck in carbon dioxide (CO2) from the atmosphere through photosynthesis, thus blunting the problem.

A team led by University of Florida ecology professor Ted Schuur investigated an area of tundra at Eight Mile Lake in central Alaska, where permafrost thaw has been monitored since 1990 but had begun to start many years before.

Schuur's team used hand-built, automated chambers, which they deployed at three sites that represented minimal, moderate and extensive amounts of thaw.

From 2004 to 2006, the chambers measured how much carbon was escaping from the soil and how much was being absorbed by any vegetation.

In areas that had thawed for the previous 15 years, there was a net uptake of carbon, meaning that the newly-established plants sucked up more CO2 than was lost from the soil.

But in areas that had begun to thaw decades before, the reverse was true.

There was a net loss of CO2, the principal greenhouse gas blamed for global warming, as older stocks of carbon were gradually released to the atmosphere.

"At first, with the plants offsetting the carbon dioxide, it will appear that everything is fine, but this actually conceals the initial destabilisation of permafrost carbon," Schuur said in a press release.

"But it doesn't last, because there is so much carbon in the permafrost that eventually the plants can't keep up."

Most of the 13 million square kilometres (five million square miles) of permafrost remain frozen, but thawing is already under way around the region's southern fringes and is thought likely to expand this century.

In that scenario, the permafrost could release around a billion tonnes a year of carbon, roughly equivalent to the contribution to greenhouse emissions each year by deforestation in the tropics, the paper said.

Even as the Arctic greens, the rising loss of older carbon "could make permafrost a large biospheric carbon source in a warmer world," it said.

Burning fossil fuels adds about 8.5 gigatonnes of emissions each year, but it is a process that can theoretically be controlled.

Permafrost thaw, though, would be self-reinforcing and could be almost impossible to brake.

"It's not an option to be putting insulation on top of the tundra," Schuur said.

"If we address our own emissions either by reducing deforestation or controlling emissions from fossil fuels, that's the key to minimising the changes in the permafrost carbon pool."

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