Tuesday, May 13, 2014

NASA: Antarctic Glacial Melting Past Point of No Return

PASADENA, California -- Glacial melting on a section of the West Antarctic Ice Sheet has passed the point of no return, according to NASA scientists.

Researchers say that a new study presents multiple lines of evidence, incorporating 40 years of observations, that indicate the glaciers in the Amundsen Sea sector of West Antarctica are in an irrevisable decline.

According to NASA, that these glaciers already contribute significantly to sea level rise, releasing almost as much ice into the ocean annually as the entire Greenland Ice Sheet.  They contain enough ice to raise global sea level by 4 feet (1.2 meters) and are melting faster than most scientists had expected. 

Eric Rignot, of UC Irvine and NASA's Jet Propulsion Laboratory said these findings will require an upward revision to current predictions of sea level rise.  "This sector will be a major contributor to sea level rise in the decades and centuries to come," Rignot said. "A conservative estimate is it could take several centuries for all of the ice to flow into the sea."

The glaciers flow out from land to the ocean, with their leading edges afloat on the seawater. The point on a glacier where it first loses contact with land is called the grounding line. Nearly all glacier melt occurs on the underside of the glacier beyond the grounding line, on the section floating on seawater.

Just as a grounded boat can float again on shallow water if it is made lighter, a glacier can float over an area where it used to be grounded if it becomes lighter, which it does by melting or by the thinning effects of the glacier stretching out.  The Antarctic glaciers studied by Rignot's group have thinned so much they are now floating above places where they used to sit solidly on land, which means their grounding lines are retreating inland.

The accelerating flow speeds and retreating grounding lines reinforce each other.  As glaciers flow faster, they stretch out and thin, which reduces their weight and lifts them farther off the bedrock.  As the grounding line retreats and more of the glacier becomes waterborne, there's less resistance underneath, so the flow accelerates.

"The collapse of this sector of West Antarctica appears to be unstoppable," Rignot said. "The fact that the retreat is happening simultaneously over a large sector suggests it was triggered by a common cause, such as an increase in the amount of ocean heat beneath the floating sections of the glaciers. At this point, the end of this sector appears to be inevitable."

Article Source: NASA / Carol Rasmussen