北极海底甲烷的排放量超估算值两倍

The seafloor off the coast of Northern Siberia is releasing more than twice the amount of methane（甲烷） as previously estimated, according to new research results published in the Nov. 24 edition of the journal Nature Geoscience. The East Siberian Arctic Shelf is venting at least 17 teragrams of the methane into the atmosphere each year. A teragram is equal to 1 million tons.

"It is now on par with the methane being released from the arctic tundra, which is considered to be one of the major sources of methane in the Northern Hemisphere," said Natalia Shakhova, one of the paper's lead authors and a scientist at the University of Alaska Fairbanks. "Increased methane releases in this area are a possible new climate-change-driven factor that will strengthen over time."

Methane is a greenhouse gas more than 30 times more potent than carbon dioxide. On land, methane is released when previously frozen organic material decomposes. In the seabed, methane can be stored as a pre-formed gas or asmethane hydrates. As long as the subsea permafrost（永久冻土） remains frozen, it forms a cap, effectively trapping the methane beneath. However, as the permafrost thaws, it develops holes, which allow the methane to escape. These releases can be larger and more abrupt than those that result from decomposition.

The findings are the latest in an ongoing international research project led by Shakhova and Igor Semiletov, both researchers at the UAF International Arctic Research Center. Their twice-yearly arctic expeditions have revealed that the subsea permafrost in the area has thawed（融解） much more extensively than previously thought, in part due to warming water near the bottom of the ocean. The warming has created conditions that allow the subsea methane to escape in much greater amounts than their earlier models estimated. Frequent storms in the area hasten its release into the atmosphere, much in the same way stirring a soda releases the carbonation more quickly.

"Results of this study represent a big step forward toward improving our understanding of methane emissions from the East Siberian Arctic Shelf," said Shakhova. She noted that while the ESAS is unusual in its expansive and shallow nature, the team's findings there speak to the need for further exploration of the subsea Arctic. "I believe that all other arctic shelf areas are significantly underestimated and should be paid very careful attention to."

The East Siberian Arctic Shelf is a methane-rich area that encompasses more 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 of atmospheric methane. Previous estimates performed for the ESAS suggested that the area was releasing 8 teragrams of methane into the atmosphere yearly.

During field expeditions, the research team used a variety of techniques -- including sonar and visual images of methane bubbles in the water, air and water sampling, seafloor drilling and temperature readings -- to determine the conditions of the water and permafrost, as well as the amount of methane being released.

Methane is an important factor in global climate change, because it so effectively traps heat. As conditions warm, global research has indicated that more methane is released, which then stands to further warm the planet. Scientists call this phenomenon a positive feedback loop.

"We believe that the release of methane from the Arctic, and in particular this part of the Arctic, could impact the entire globe," Shakhova said. "We are trying to understand the actual contribution of the ESAS to the global methane budget and how that will change over time."