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[music] Narrator: NASA’s airborne surveys are

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are giving scientists astonishingly accurate views of how

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Greenland’s glaciers are changing.

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Laser altimeters map the very details of glacier surfaces

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and flights spanning two decades reveal the dramatic changes

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that have taken place.

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[atmospheric music]

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On Greenland’s rugged eastern coast, spilling into

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a mountainous fjord, lies the four-mile-wide

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Helheim Glacier, named for the Viking world of the dead.

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It’s had a wild ride over the last 20 years,

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first rapidly retreating and thinning, then partially recovering

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its former extent. NASA science

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missions have flown the glacier’s center line year after year,

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collecting a wealth of valuable data.

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It all begins

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with a single beam of light.

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Firing several thousand pulses per second, laser instruments on board

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research aircraft measure the height of the surface below.

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The lasers spin in a

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250 meter circle. providing a swath of data that can be

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turned into a topographic map of the ice.

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Here we’ve shown higher elevations

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in red and orange, and lower elevations in green and blue,

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all the way down to the glacier’s calving front –

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where Helheim’s mighty icebergs break off into the sea.

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But one snapshot

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only tells part of the story. Here’s a 1998

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swath compared with 2013.

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We’ve changed the color scale to highlight the local

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differences in elevation.

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We’re now moving below the surface of the ice as it was in 1998,

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and over the mélange of icebergs and ocean that were present in the same spot

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in 2013. The calving front of the glacier

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has retreated significantly, by two and half miles.

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The glacier has thinned as well. We couldn't have flown at this elevation

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15 years ago. This all would have been ice.

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NASA’s Operation IceBridge, which has been measuring Greenland

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since 2009, has added to the laser data from previous missions

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with new instruments like ice penetrating radar,

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a magnetometer, and a gravimeter. It’s also used a

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high-resolution camera system, taking overlapping images of the ice

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throughout its eight-hour flights.

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These images can be pieced together

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into a mosaic, and since they overlap, provide us with a

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stereoscopic view of the ice, and an elevation model of their own.

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Here is that model overlaid onto the laser data,

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as we approach Helheim’s 70 meter high calving front.

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As we get

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close to the glacier’s terminus, large cracks in the ice, called crevasses

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get longer and deeper, a sign new icebergs

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will soon join their comrades on their way out to sea.

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Until the launch of a

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new NASA satellite, ICESat-2, Operation IceBridge

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will return to Greenland every spring to continue measurements

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of this large and ever-changing glacier.

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[beep beep]

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[beep beep, beep beep]