Large amounts of a powerful greenhouse gas are bubbling up from a long-frozen seabed north of Siberia, raising fears of far bigger leaks that could stoke global warming, scientists said.

It was unclear, however, if the Arctic emissions of methane gas were new or had been going on unnoticed for centuries -- since before the Industrial Revolution of the 18th century led to wide use of fossil fuels that are blamed for climate change.

The study said about 8 million tonnes of methane a year, equivalent to the annual total previously estimated from all of the world's oceans, were seeping from vast stores long trapped under permafrost below the seabed north of Russia.

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