Beneath Antarctic ice, a potent greenhouse gas?

Large volumes of the greenhouse gas methane could have been produced under the Antarctic Ice Sheet over millions of years, which could add to global warming if released into the atmosphere by a thaw, a study said on Wednesday.

Scientists from the universities of Bristol, Utrecht, California and Alberta simulated the accumulation of methane in Antarctic sedimentary basins using models and calculations.

They found it was likely there were micro-organisms there that would have been able to convert the ice sheet's large deposits of organic carbon into the potent gas.

If present, methane would most likely be trapped under the ice.

But it could be released into the atmosphere as rising global temperatures melt the ice sheet, fuelling even more global warming, the scientists said in the paper published in the journal Nature.

"The Antarctic Ice Sheet could constitute a previously neglected component of the global methane hydrate inventory although significant uncertainty exists," the scientists said.

Methane stays in the atmosphere for up to 15 years. Levels have been on the rise over the past few years, following a period of stability since 1998.

The gas is normally trapped as "methane hydrate" in sediments under a seabed. Methane hydrate is a form of water ice containing a large amount of methane which is usually stable.

As temperatures rise, the hydrate breaks down and methane is released from the sea bed, mostly dissolving into the seawater.

But if trapped methane broke sea surfaces and escaped into the atmosphere, it would intensify global warming.

Scientists have already identified thousands of sites in the Arctic where methane is bubbling into the atmosphere but the potential for methane formation under the Antarctic Ice Sheet has been less well studied.

Conditions under the Antarctic Ice Sheet seem capable of producing methane as the water is oxygen-depleted, hosts micro-organisms and contains significant reservoirs of organic carbon, Wednesday's study said.

"We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost."

In 2008, American and Russian experts estimated that 0.5 megatonnes of methane are released per year and at least 1,400 gigatonnes of carbon is trapped as methane and methane hydrates under the Arctic permafrost.

But up to 50 gigatonnes of hydrate storage could be released at any time, which would increase the methane content of the Earth's atmosphere by a factor of 12, they warned.

(Editing by Andrew Heavens)