Vast Reservoirs of Methane in Arctic Seafloor Are Leaking Large Amounts of Powerful Greenhouse Gas to Atmosphere

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Researchers report in tomorrow's journal Science (5 March 2010) that the East Siberian Arctic Shelf (ESAS) is leaking methane to the atmosphere, in volumes "on par with previous estimates of methane venting from the entire World Ocean."  They warn that "remobilization to the atmosphere of only a small fraction of the methane" held in the ESAS sediments "could trigger abrupt climate warming."

In Extensive Methane Venting to the Atmosphere from Sediments of the East Siberian Arctic Shelf, researchers led by University of Alaska Fairbanks (UAF) scientists Natalia Shakhova and Igor Semiletov say that the 2 million square kilometer ESAS now is releasing an estimated 7 teragrams of methane (1 teragram = 1.1 million tons) annually.  Methane is a much more powerful greenhouse gas than carbon dioxide, but is shorter lived.

The permafrost of the East Siberian Arctic Shelf (an area of about 2 million kilometers squared) is more porous than previously thought. The ocean on top of it and the heat from the mantle below it warm it and make it perforated like Swiss cheese. This allows methane gas stored under it under pressure to burst into the atmosphere. The amount leaking from this locale is comparable to all the methane from the rest of the world's oceans put together. Methane is a greenhouse gas more than 30 times more potent than carbon dioxide. Credit: Zina Deretsky, National Science Foundation
Above: Methane is leaking from the East Siberian Arctic Shelf.  Credit: Credit: Zina Deretsky, National Science Foundation


The leakage is helping to drive average atmospheric methane concentrations in the Arctic up to 1.85 parts per million, the highest in 400,000 years -- with even higher concentrations in the atmosphere directly above the ESAS. 

The permafrost of the ESAS has in the past served as a barrier impeding the release of methane held there. However, Shakhova says in an press release from the National Science Foundation (NSF) that "subsea permafrost is losing its ability to be an impermeable cap" and is "showing signs of destabilization." 

According to the NSF press release:

They [the researchers] found that more than 80 percent of the deep water and more than 50 percent of surface water had methane levels more than eight times that of normal seawater. In some areas, the saturation levels reached more than 250 times that of background levels in the summer and 1,400 times higher in the winter. They found corresponding results in the air directly above the ocean surface. Methane levels were elevated overall and the seascape was dotted with more than 100 hotspots. This, combined with winter expedition results that found methane gas trapped under and in the sea ice, showed the team that the methane was not only being dissolved in the water, it was bubbling out into the atmosphere.

These findings were further confirmed when Shakhova and her colleagues sampled methane levels at higher elevations. Methane levels throughout the Arctic are usually 8 to 10 percent higher than the global baseline. When they flew over the shelf, they found methane at levels another 5 to 10 percent higher than the already elevated Arctic levels.

"The release to the atmosphere of only one percent of the methane assumed to be stored in shallow hydrate deposits might alter the current atmospheric burden of methane up to 3 to 4 times," Shakhova says in the NSF press release. "The climatic consequences of this are hard to predict."

A report released by the U.S. Global Change Research Program, Abrupt Climate Change, said in December 2008 (during the Bush Administration) that warming in the Arctic could cause sea levels to rise substantially beyond scientists’ previous predictions and could result in massive releases of methane.  The report said that the "rapid release to the atmosphere of methane trapped in permafrost and on continental margins" was among "four types of abrupt change in the paleoclimatic record that stand out as being so rapid and large in their impact that if they were to recur, they would pose clear risks to society in terms of our ability to adapt." 

Scientists deploy an apparatus that will allow the research team to take sonar measurements from the seafloor on the East Siberian Arctic Shelf during a research cruise in August 2009. The researchers used sonar to record clouds of bubbles rising from the seafloor.  Credit: Photo courtesy of Igor Semiletov, University of Alaska Fairbanks.

Above: Scientists deploy an apparatus that will allow the research team to take sonar measurements from the seafloor on the East Siberian Arctic Shelf during a research cruise in August 2009. The researchers used sonar to record clouds of bubbles rising from the seafloor.  Credit: Photo courtesy of Igor Semiletov, University of Alaska Fairbanks


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