West Antarctic Glaciers in Irreversible Decline   
       
   May 12, 2014: Over the years, as temperatures around the world have ratcheted   
   upward, climate change researchers have kept a wary eye on one place perhaps   
   more than any other:  The West Antarctic Ice Sheet, and particularly the   
   fastest melting part of it, the glaciers that flow into the Amundsen Sea.   
       
   In that region, six glaciers hang in a precarious balance, partially supported   
   by land, and partially floating in waters just offshore.  There's enough water   
   frozen in the ice sheet that feeds these icy giants to raise global sea levels   
   by 4 feet-if they were to melt. That's troubling because the glaciers are   
   melting. Moreover, a new study finds that their decline appears to be   
   unstoppable.   
       
   https://www.youtube.com/watch?v=W2pYHMx5bN8   
       
   A new ScienceCast video lays out the evidence for irreversible decline of the   
   West Antarctic glaciers.  Play it   
       
   "We've passed the point of no return," says Eric Rignot, a glaciologist   
   working jointly at NASA's Jet Propulsion Laboratory and the University of   
   California, Irvine.  Rignot and colleagues have used 19 years of satellite   
   radar data to map the fast-melting glaciers. In their paper, which has been   
   accepted for publication in Geophysical Research Letters, they conclude that   
   "this sector of West Antarctica is undergoing a marine ice sheet instability   
   that will significantly contribute to sea level rise" in the centuries ahead.   
       
   A key concept in the Rignot study is the "grounding line"-the dividing line   
   between land and water underneath a glacier.  Because virtually all melting   
   occurs where the glaciers' undersides touch the ocean, pinpointing the   
   grounding line is crucial for estimating melt rates.   
       
   The problem is, grounding lines are buried under thousands of feet of glacial   
   ice. "It's challenging for a human observer to figure out where they are,"   
   Rignot explains. "There's nothing obvious that sticks out on the surface to   
   say, `This is where the glacier goes afloat.'"   
       
   To find the hidden grounding lines, they examined radar images of the glaciers   
   made by the European Space Agency's Earth Remote Sensing satellites from 1992   
   to 2011. Glaciers flex in response to tides.  By analyzing the flexing   
   motions, they were able to trace the grounding lines.   
       
   This led to a key discovery.  In all the glaciers they studied, grounding   
   lines were rapidly retreating away from the sea.   
       
   http://tinyurl.com/lx4yhxn   
       
   Click on the image to view more animations related to this story. More   
       
   "In this sector, we are seeing retreat rates that we don't see anywhere else   
   on Earth," Rignot says. Smith Glacier's line moved the fastest, retreating 22   
   miles upstream. The other lines retreated from 6 to 19 miles.   
       
   As the glaciers melt and lose weight, they float off the land where they used   
   to sit.  Water gets underneath the glacier and pushes the grounding line   
   inland. This, in turn, reduces friction between the glacier and its bed.  The   
   glacier speeds up, stretches out and thins, which drives the grounding line to   
   retreat farther inland.   
       
   This is a "positive feedback loop" that leads to out of control melting.   
       
   The only natural factor that can slow or stop this process is a "pinning   
   point" in the bedrock -- a bump or projection that snags the glacier from   
   underneath and keeps it from sliding toward the sea. To investigate this   
   possibility, the researchers made a novel map of the bed beneath the glaciers   
   using radar and other data from satellites and NASA's airborne IceBridge   
   mission. The map revealed that the glaciers had already floated off many of   
   their small pinning points.   
       
   In short, there seems to be no turning back.   
       
   "At current melt rates," concludes Rignot, "these glaciers will be 'history'   
   within a few hundred years."   
       
   Credits:   
   Author: Dr. Tony Phillips | Production editor: Dr. Tony Phillips | Credit:   
   Science@NASA   
       
   Web Links:  The Unstable West Antarctic Ice Sheet -- a primer  Loss of   
   Glaciers Appears Unstoppable -- press release   
       
       
   Regards,   
       
   Roger   
      
   --- D'Bridge 3.99   
    * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)