The permafrost under the East Siberian Arctic Shelf, long thought to be an impermeable barrier sealing in the methane, is perforated and is starting to leak seven to eight million tonnes of methane each year into the atmosphere.
The scientists said release of even a fraction of the methane stored in the shelf could trigger abrupt climate warming.
"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," said University of Alaska Fairbanks scientist Natalia Shakhova, a researcher at the university's International Arctic Research Center. "Subsea permafrost is losing its ability to be an impermeable cap."
Current average methane concentrations in the Arctic average about 1.85 parts per million, the highest in 400,000 years, she said.
In her office at the University of Alaska, Natalie Shakova demonstrates how methane escapes from permafrost. (Photo courtesy U. Alaska, Fairbanks)
Shakhova and colleague Igor Semiletov published their research results in the March 5 edition of the journal "Science."
Methane is a greenhouse gas more than 30 times more potent than carbon dioxide. It is released from previously frozen soils in two ways. When the organic material stored in permafrost thaws, it decomposes and gradually releases methane.
Methane also can be stored in the seabed as methane gas or methane hydrates and released as subsea permafrost thaws. These releases can be larger and more abrupt.
The East Siberian Arctic Shelf is a methane-rich area that encompasses more than two million square kilometers of seafloor in the Arctic Ocean. The shelf is shallow, 50 meters (164 feet) or less in depth, which means it has been alternately submerged or terrestrial, depending on sea levels.
During the Earth's coldest periods, it is a frozen arctic coastal plain, and does not release methane. As the Earth warms and sea level rises, it is inundated with seawater, which is 12-15 degrees warmer than the average air temperature.
"Our concern is that the subsea permafrost has been showing signs of destabilization already," she said. "If it further destabilizes, the methane emissions may not be teragrams, it would be significantly larger."
Shakhova notes that the Earth's geological record indicates that atmospheric methane concentrations have varied between about .3 to .4 parts per million during cold periods to .6 to .7 parts per million during warm periods.
The International Arctic Research Center measured concentrations above the East Siberian Arctic Shelf that are even higher.
From 2003 through 2008, Shakhova, Semiletov and their colleagues took annual research cruises throughout the shelf and sampled seawater at various depths and the air 10 meters (33 feet) above the ocean.
In September 2006, they flew a helicopter over the same area, taking air samples at up to 2,000 meters (6,562 feet) in the atmosphere. In April 2007, they conducted a winter expedition on the sea ice.
View from the research vessel as it traveled the East Siberian Arctic Shelf, August 2009. (Photo courtesy Igor Semiletov University of Alaska Fairbanks)
They 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 study is a testament to sustained, careful observations and to international cooperation in research," said Henrietta Edmonds of the National Science Foundation, which partially funded the study.
"The Arctic is a difficult place to get to and to work in, but it is important that we do so in order to understand its role in global climate and its response and contribution to ongoing environmental change," Edmonds said. "It is important to understand the size of the reservoir - the amount of trapped methane that potentially could be released - as well as the processes that have kept it 'trapped' and those that control the release. Work like this helps us to understand and document these processes."
"Regardless of the cause, methane is increasing," said Durwood Zaelke, president of the Institute for Governance & Sustainable Development, based in Washington, DC. The institute provides the secretariat to the International Network for Environmental Compliance and Enforcement, a network of over 4,000 environmental professionals in 150 countries.
Zaelke and his group recommend immediate reduction of "short-lived climate forcers" such as black carbon soot, tropospheric ozone, and methane produced from activities such as agriculture, coal mining, and production of oil and natural gas.
Because they are short-lived, implementing aggressive mitigation measures can lead to major near term climate benefits, Zaelke said. He mentioned expanding biochar production to sequester carbon and increasing urban white surfaces to reflect solar radiation as important strategies to reduce the possibility of passing climate tipping points.
Scientists deploy sonar device to take measurements over the East Siberian Arctic Shelf. (Photo courtesy U. Alaska, Fairbanks)
"We can't afford to wait and see what happens," added Zaelke. "Taking fast action on powerful, short-lived pollutants, capturing carbon through biochar, and reducing the absorption of solar radiation by increasing urban albedo - this group of strategies is our critical insurance policy against abrupt climate change that can offset the effects of CO2 by as much as 40 years or more."
A 2008 study on methane funded by the National Science Foundation found that over 600 million years ago, a sudden release of methane from ice sheets set in motion an abrupt change in climate, transforming the Earth from a cold environment into a much warmer one.
While the scientists involved in the East Siberian Arctic Shelf study note that there is no way to determine how much methane it would take to reach that threshold, the current trend of rising emissions is deeply troubling.
"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 three to four times," Shakhova said. "The climatic consequences of this are hard to predict."
Shakhova, Semiletov and collaborators from 12 institutions in five countries plan to continue their studies in the region, tracking the source of the methane emissions and drilling into the seafloor in an effort to estimate how much methane is stored there.
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