Mar 9, 2010
Arctic shelf belches methane
The East Siberian Arctic Shelf is releasing around 8 teragrams of methane from subsea sediments each year, about the same amount as the whole of the rest of the ocean. That is according to researchers who have measured methane concentrations in the region’s ocean and atmosphere from ships, a helicopter and the ice.
“Seabed deposits on the Arctic Shelf hold an enormous amount of methane,” said Natalia Shakhova of the University of Alaska Fairbanks, US, and Russian Academy of Sciences. “Its involvement in the modern biogeochemical cycle might give a very significant implication for climate models.”
Until recently, scientists believed that subsea permafrost was acting as a giant seal and preventing methane from sediments from entering the oceans. A study in the East Siberian Arctic Shelf region in 2005 revealed that this was not the case; now Shakhova and colleagues from the University of Alaska Fairbanks, US, the Russian Academy of Sciences and Stockholm University, Sweden, have used data from across the area to estimate how much methane the East Siberian Arctic Shelf is releasing each year. Their measurements, taken between 2003 and 2008, included readings at different ocean depths and atmospheric data.
“The Arctic Siberian Shelf is about 2 million square kilometres and holds huge carbon deposits,” said Shakhova. “It is very shallow, with a mean depth less than 50 m. If it was deeper it would allow methane to oxidize in the water column [before reaching the surface].”
Methane is around 30 times more powerful a greenhouse gas than carbon dioxide. Natural emissions of the gas make up about 40% of the 440 Teragrammes of carbon emitted in the form of methane each year; the bulk of these natural emissions come from wetlands.
Compared to this figure, the 8 Teragrammes of carbon in the form of methane released in the East Siberian Arctic Shelf is relatively small.
“Our concern is that the subsea permafrost has been showing signs of destabilization already,” said Shakhova. “If it further destabilizes, the methane emissions may not be teragrams, it would be significantly larger.”
Shakhova says that the team’s initial calculations of annual emissions did not incorporate all factors, for example missing out seasonal components such as sudden releases determined by seasonal storm events, the ice formation period and ice break up.
The seabed of the East Siberian Arctic Shelf consists of permafrost left behind by the last glaciation. The area has been alternately underwater and on dry land as climate has fluctuated over the last 8,000–10,000 years. This has caused the temperature of the permafrost to swing between –17 °C when aboveground and just below freezing when covered by water.
“We should have it in mind that the long-lasting process of natural permafrost degradation by seawater is accelerated by the contribution of global warming,” said Shakhova.
The team believes that global climate change has caused additional warming to the subsea permafrost by raising the temperature of river run-off entering the ocean. The permafrost is also experiencing geothermal heating from the rift zone below.
The atmospheric concentration of methane above the Arctic is the highest measured for the last 400,000 years, said the researchers. In cold periods, the methane concentration was typically between 0.3 and 0.4 parts per million, whereas in previous warm periods it was 0.6–0.7 ppm. In this current warm epoch the atmospheric methane concentration is 1.7 ppm on average across the globe, 1.85 ppm above the Arctic and roughly 2.6 above the East Siberian Arctic Shelf, with spikes in some areas up to 8.2, according to Shakhova.
“To discern whether this extensive methane venting over the East Siberian Arctic Shelf is a steadily ongoing phenomenon or signals the start of a more massive methane-release period, there is an urgent need for expanded multifaceted investigations into these inaccesible but climate-sensitive shelf seas north of Siberia,” write the researchers in Science.
About the author
Liz Kalaugher is editor of environmentalresearchweb.