A vast outcrop of the Arctic Siberian coast that had been frozen for tens of thousands of years is releasing huge carbon deposits as rising temperatures thaw parts of its coastline, a study warned Wednesday.

The carbon, a potential source of Earth-warming CO2, has lain frozen along the 7,000-kilometre (4,400-mile) northeast Siberian coastline since the last Ice Age.

But atmospheric warming and coastal erosion are gnawing at the icy seal, releasing about 40 million tonnes of carbon a year -- 10 times more than previously thought, said a study in the journal Nature.

About two-thirds of the carbon escapes into the atmosphere as carbon dioxide (CO2) and the rest becomes trapped in higher layers of ocean sediment.

About half the carbon pool in soil globally is held in permafrost in the Arctic, a region that is experiencing twice the global average of climate warming, said the study led by researchers at Stockholm University.

Earlier this week, US scientists said the sea ice in the Arctic Ocean had melted to its smallest point ever.

The region covered by the Nature study, called Yedoma, is twice the size of Sweden but has been poorly researched because it is so remote.

The finding touches on a vicious circle, or positive feedback in climate parlance.

Under this, man-made warming caused by the burning of fossil fuels releases naturally-occurring stocks of CO2 that have been stored in permafrost since the last Ice Age, called the Pleistocene.

The released gases in turn add to global warming, which frees even more locked-up carbon, and so on.

"Thermal collapse and erosion of these carbon-rich Pleistocene coastline and seafloor deposits may accelerate the Arctic amplification of climate warming," the paper warned.

The atmospheric leakage from Yedoma is equivalent to the annual emissions of around five million passenger cars, on the basis of average carbon output (five tonnes per year) of vehicles in the United States.

In a separate study also in Nature, researchers in Britain, the Netherlands and the United States used computer models to estimate there could be as much as four billion tonnes of methane under Antarctica's icesheet.

Methane is 25 times more efficient at trapping solar heat than carbon dioxide.

Before it froze over, the region teemed with life whose organic remains became trapped in sediment later covered by ice sheets.

"Our modelling shows that over millions of years, microbes may have turned this old organic carbon into methane," which could boost climate warming if released by icesheet collapse, the researchers said in a statement.

The collapse of the Antarctic icesheet is considered an extremely remote scenario by most climatologists, and some studies have suggested that parts of it could be thickening, due to localised increases in snowfall.