Fairbanks, Alaska—A section of the Arctic Ocean seafloor that holdsvast stores of frozen methane is showing signs of instability andwidespread venting of the powerful greenhouse gas, according to thefindings of an international research team led by University of AlaskaFairbanks scientists Natalia Shakhova and Igor Semiletov.
The research results, published in the March 5 edition of the journalScience, show that the permafrost under the East Siberian ArcticShelf, long thought to be an impermeable barrier sealing in methane,is perforated and is leaking large amounts of methane into theatmosphere. Release of even a fraction of the methane stored in theshelf could trigger abrupt climate warming.
“The amount of methane currently coming out of the East SiberianArctic Shelf is comparable to the amount coming out of the entireworld’s oceans,” said Shakhova, a researcher at UAF’s InternationalArctic Research Center. “Subsea permafrost is losing its ability to bean impermeable cap.”
Methane is a greenhouse gas more than 30 times more potent than carbondioxide. It is released from previously frozen soils in two ways. Whenthe organic material—which contains carbon—stored in permafrost thaws,it begins to decompose and, under oxygen-free conditions, graduallyrelease methane. Methane can also be stored in the seabed as methanegas or methane hydrates and then released as subsea permafrost thaws.These releases can be larger and more abrupt than those that resultfrom decomposition.
The East Siberian Arctic Shelf is a methane-rich area that encompassesmore than 2 million square kilometers of seafloor in the Arctic Ocean.It is more than three times as large as the nearby Siberian wetlands,which have been considered the primary Northern Hemisphere source ofatmospheric methane. Shakhova’s research results show that the EastSiberian Arctic Shelf is already a significant methane source: 7teragrams yearly, which is equal to the amount of methane emitted fromthe rest of the ocean. A teragram is equal to about 1.1 million tons.
“Our concern is that the subsea permafrost has been showing signs ofdestabilization already,” she said. “If it further destabilizes, themethane emissions may not be teragrams, it would be significantlylarger.”
Shakhova notes that Earth’s geological record indicates thatatmospheric methane concentrations have varied between about .3 to .4parts per million during cold periods to .6 to .7 parts per millionduring warm periods. Current average methane concentrations in theArctic average about 1.85 parts per million, the highest in 400,000years, she said. Concentrations above the East Siberian Arctic Shelfare even higher.
The East Siberian Arctic Shelf is a relative frontier in methanestudies. The shelf is shallow, 50 meters or less in depth, which meansit has been alternately submerged or terrestrial, depending on sealevels throughout Earth’s history. During Earth’s coldest periods, itis a frozen arctic coastal plain, and does not release methane. As theplanet warms and sea levels rise, it is inundated with seawater, whichis 12-15 degrees warmer than the average air temperature.
“It was thought that seawater kept the East Siberian Arctic Shelfpermafrost frozen,” Shakhova said. “Nobody considered this huge area.”
Earlier studies in Siberia focused on methane escaping from thawingterrestrial permafrost. Semiletov’s work during the 1990s showed,among other things, that the amount of methane being emitted fromterrestrial sources decreased at higher latitudes. But those studiesstopped at the coast. Starting in the fall of 2003, Shakhova,Semiletov and the rest of their team took the studies offshore. From2003 through 2008, they took annual research cruises throughout theshelf and sampled seawater at various depths and the air 10 metersabove the ocean. In September 2006, they flew a helicopter over thesame area, taking air samples at up to 2,000 meters in the atmosphere.In April 2007, they conducted a winter expedition on the sea ice.
They found that more than 80 percent of the deep water and greaterthan half of surface water had methane levels more than eight timesthat of normal seawater. In some areas, the saturation levels reachedat least 250 times that of background levels in the summer and 1,400times higher in the winter.
They found corresponding results in the air directly above the oceansurface. Methane levels were elevated overall and the seascape wasdotted with more than 100 hotspots. This, combined with winterexpedition results that found methane gas trapped under and in the seaice, showed the team that the methane was not only being dissolved inthe water, it was bubbling out into the atmosphere.
These findings were further confirmed when Shakhova and her colleaguessampled methane levels at higher elevations. Methane levels throughoutthe Arctic are usually 8 to 10 percent higher than the globalbaseline. When they flew over the shelf, they found methane at levelsanother 5 to 10 percent higher than the already elevated arctic levels.
The East Siberian Arctic Shelf, in addition to holding large stores offrozen methane, is more of a concern because it is so shallow. In deepwater, methane gas oxidizes into carbon dioxide before it reaches thesurface. In the shallows of the East Siberian Arctic Shelf, methanesimply doesn’t have enough time to oxidize, which means more of itescapes into the atmosphere. That, combined with the sheer amount ofmethane in the region, could add a previously uncalculated variable toclimate models.
“The release to the atmosphere of only one percent of the methaneassumed to be stored in shallow hydrate deposits might alter thecurrent atmospheric burden of methane up to 3 to 4 times,” Shakhovasaid. “The climatic consequences of this are hard to predict.”
Shakhova, Semiletov and collaborators from 12 institutions in fivecountries plan to continue their studies in the region, tracking thesource of the methane emissions and drilling into the seafloor in aneffort to estimate how much methane is stored there.
Source: University of Alaska Fairbanks