Arctic seafloor is "leaking" methane

As noted in prior posts, the evidence for climate change can be found in the many changes occurring across the globe.  One of the most disturbing such events is the recent evidence that the Arctic seafloor is emitting methane into the atmosphere.  As noted in several prior posts, methane is a significant GHG because its warming effects are many times that of CO2.

Very large amounts of carbon are known to be trapped in the former Siberian wetland.  The East Siberian Arctic Shelf - a 2.1-million-square-kilometer patch of Arctic seafloor that was exposed during the most recent ice age, when sea levels were lower - is three times larger than all of today's land-based Siberian wetlands.  When the region was above sea level, tundra vegetation pulled carbon dioxide from the air as plants grew.  That organic material, much of which did not fully decompose in the frigid Arctic, accumulated in the soil and is the source of the methane found in this region.  It was believed that this methane was trapped beneath a layer of permafrost, but such is not the case with the portion that is beneath the sea.

However, recent field studies suggest that this reservoir of carbon has begun to leak.  During six cruises in the region from 2003 to 2008, the researchers gathered data at more than 1,000 spots in the Greenland-sized stretch of shallow ocean.  The researchers also took atmospheric readings of methane concentration during one helicopter survey and a wintertime excursion from shore onto the ice-covered sea.  The researchers found unexpectedly high amounts of methane dissolved in seafloor waters across 80% of the area they studied.  In some spots, methane concentrations during those six years averaged more than 80-times normal.  Because the water over the shelf is relatively shallow - average depth in the region is about 45 meters - much of the methane reaches the ocean surface and then wafts into the atmosphere.

Previously, scientists presumed that the carbon trapped in sediments on the East Siberian Arctic Shelf was sealed by permafrost, as nearby deposits on land are.  However, the researchers note a major difference.  Much of the permafrost on land remains intact because it is exposed to bitter winter cold, whereas the seafloor permafrost is bathed in cold, but not freezing, salt water. The annual average temperature of seafloor permafrost is between 12 and 17 degrees warmer than that of nearby land-based permafrost.

The researchers speculate that the warmth of the seawater, as well as heat flowing up from within the Earth, has thawed the seafloor permafrost, releasing the methane.  Sonar images show plumes of methane bubbling from the seafloor, indicating that the gas originates in sediments there.  Other measurements show that the methane is not generated in the water by microbes or brought to the seas by rivers.  Each year, the researchers estimate, nearly 8 million metric tons of methane make their way to the atmosphere over the East Siberian Arctic Shelf.  That amount is more than previous estimates for all of the world's oceans.

Although the Siberian seafloor sediments are spewing much more methane than previously thought, they are still only providing a small fraction of the estimated 440 million tons of methane gas emitted to the atmosphere each year.  Nevertheless, the release of a sizeable fraction of the carbon trapped in these sediments would lead to warmer atmospheric temperatures, which would in turn cause more methane to be released.

Even though we are far from the disaster that visited the Earth during the Permian extinction, the best scientific evaluation of that event was that the coup d' grace was administered by the sublimation of the methane hydrates in the oceans, which drove the average temperature up to the point where most life of Earth was extinguished.  Thus, many climate scientists are very concerned that if temperature increases to a point where the existing methane hydrates begin to defrost, then the Earth is in for a true disaster of Biblical proportion.  We are far from that point at present, but anytime one sees seafloor methane starting to gasify, there is cause for concern.

The report on the research can be found at http://www.sciencemag.org/cgi/content/abstract/327/5970/1246.