Scientists officially report in a paper in today’s issue of Science (subscription required) what has already, er, leaked a while ago: Methane gas is released at quite substantial rates from beneath the frozen Arctic Ocean floor.

The international research team observed and recorded the leaks from the methane-rich shallow East Siberian Arctic Shelf during a series of eight expeditions from 2003 to 2008 on board Russian ice breakers.

The permafrost (frozen soil) on the sea floor – a relic of the last ice age – was previously thought to act as a tight lid for the methane – a highly potent greenhouse gas – stored underneath. But the team, led by Natalia Shakhova and Igor Semiletov of the University of Alaska in Fairbanks – found that the cap has become permeable.

They discovered that 80% of the bottom water and 50 % of surface water over the shelf are already supersaturated with methane. As a result, large amounts of the gas are bubbling out into the atmosphere.

Methane released from the region – around eight million tons per year (compared to around 600 million tonnes globally) – is no immediate threat for global climate stability. But the scientists say that future permafrost thawing in response to rising sea and air temperatures does raise concerns for the future.

Arctic permafrost — on land and underwater — could hold trillions of tons of methane, most of which is stored in the form of frozen gas hydrates. The scientists now plan to drill into the seafloor off Siberia to estimate how much of the stuff is stored there.

Image: Fluxes of methane venting to the atmosphere over the ESAS / Science/AAAS