Credit: University of Alaska Fairbanks
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Part of the Arctic Ocean seafloor that holds vast stores of frozen methane is showing signs of instability and widespread venting of the powerful greenhouse gas, according to new research findings.
The study, published this month in the journal Science, shows that the frozen seabed under the East Siberian Arctic Shelf, long thought to be an impermeable barrier sealing in methane, is perforated and leaking large amounts of the gas into the atmosphere.
The Shelf is a very methane-rich area encompassing more than two million square kilometres of seafloor in the Arctic - more than three times as large as the nearby Siberian wetlands, which have long been considered the primary source of atmospheric methane in the Northern Hemisphere.
With methane a potent greenhouse gas - with 25 times the warming potential of carbon dioxide - release of even a fraction of the Shelf's stores could trigger abrupt climate warming, according to lead researchers Natalia Shakhova and Igor Semiletov, from the University of Alaska in Fairbanks.
"The amount of methane currently coming out of the East Siberian Arctic Shelf is comparable to the amount coming out of the entire world's oceans," Shakhova said. "Subsea permafrost is losing its ability to be an impermeable cap."
Her team's research shows that the Shelf is already releasing around seven million tonnes of methane each year, about equal to the amount of methane coming from the rest of the ocean.
"Our concern is that the subsea permafrost has been showing signs of destabilisation already," she said. "If it further destabilises, the methane emissions may not be in [millions of tonnes], it would be significantly larger."
Also of concern, she said, is that in addition to holding large stores of frozen methane, the Shelf area is also very shallow. In deep water, methane gas can be converted into the less potent carbon dioxide before it reaches the surface and enters the atmosphere. In the shallows, there isn't enough time for this conversion to occur. That, combined with the sheer amount of methane in the region, could add a previously uncalculated variable to climate models.
"The release to the atmosphere of only one per cent of the methane assumed to be stored in shallow...deposits might alter the current atmospheric burden of methane up to three to four times," Shakhova said. "The climatic consequences of this are hard to predict."
Shakhova also noted that the Earth's geological record indicates that atmospheric methane concentrations have varied between about 0.3 to 0.4 parts per million during cold periods to 0.6 to 0.7 parts per million during warm periods. Current average methane concentrations in the Arctic average about 1.85 parts per million, the highest in 400,000 years, she said. Concentrations above the East Siberian Arctic Shelf are even higher.