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