0:00:01.010,0:00:03.010 [ music ] 0:00:03.030,0:00:04.080 No Turning Back: 0:00:04.100,0:00:06.770 West Antarctic Glaciers in Irreversible Decline -- 0:00:06.790,0:00:09.140 Presented by Science@NASA 0:00:09.160,0:00:10.980 Over the years, 0:00:11.000,0:00:13.510 as temperatures around the world have ratcheted upward, 0:00:13.530,0:00:16.760 climate change researchers have kept a wary eye on one place 0:00:16.780,0:00:18.520 perhaps more than any other: 0:00:18.540,0:00:20.190 The West Antarctic Ice Sheet, 0:00:20.210,0:00:23.260 and particularly the fastest melting part of it, 0:00:23.280,0:00:25.670 the glaciers that flow into the Amundsen Sea. 0:00:25.690,0:00:27.360 In that region, 0:00:27.380,0:00:29.640 six glaciers hang in a precarious balance, 0:00:29.660,0:00:31.900 partially supported by land, 0:00:31.920,0:00:34.680 and partially floating in waters just offshore. 0:00:34.700,0:00:36.950 There's enough water frozen in the ice sheet 0:00:36.970,0:00:38.470 that feeds these icy giants 0:00:38.490,0:00:40.950 to raise global sea levels by 4 feet- 0:00:40.970,0:00:42.430 if they were to melt. 0:00:42.450,0:00:45.320 That's troubling because the glaciers are melting. 0:00:45.340,0:00:47.470 Moreover, a new study finds that 0:00:47.490,0:00:49.890 their decline appears to be irreversible. 0:00:49.910,0:00:51.850 'We've passed the point of no return,' 0:00:51.870,0:00:53.360 says Eric Rignot, 0:00:53.380,0:00:56.560 a glaciologist working jointly at NASA's Jet Propulsion Laboratory 0:00:56.580,0:00:59.370 and the University of California, Irvine. 0:00:59.390,0:01:01.190 Rignot and colleagues 0:01:01.210,0:01:03.790 have used 19 years of satellite radar data 0:01:03.810,0:01:05.400 to map the fast-melting glaciers. 0:01:05.420,0:01:07.050 In their paper, 0:01:07.070,0:01:08.770 which has been accepted for publication 0:01:08.790,0:01:10.410 in Geophysical Research Letters, 0:01:10.430,0:01:11.800 they conclude that 0:01:11.820,0:01:13.350 'this sector of West Antarctica 0:01:13.370,0:01:15.920 is undergoing a marine ice sheet instability 0:01:15.940,0:01:18.560 that will significantly contribute to sea level rise' 0:01:18.580,0:01:20.590 in the centuries ahead. 0:01:20.610,0:01:23.120 A key concept in the Rignot study is the 0:01:23.140,0:01:24.570 'grounding line'- 0:01:24.590,0:01:26.860 the dividing line between land and water underneath a glacier. 0:01:26.880,0:01:29.600 Because virtually all melting occurs 0:01:29.620,0:01:31.870 where the glaciers' undersides touch the ocean, 0:01:31.890,0:01:34.470 pinpointing the grounding line is crucial 0:01:34.490,0:01:36.340 for estimating melt rates. 0:01:36.360,0:01:37.330 The problem is, 0:01:37.350,0:01:40.930 grounding lines are buried under thousands of feet of glacial ice. 0:01:40.950,0:01:43.340 'It's challenging for a human observer 0:01:43.360,0:01:45.420 to figure out where they are,' Rignot explains. 0:01:45.440,0:01:48.650 'There's nothing obvious that sticks out on the surface to say, 0:01:48.670,0:01:50.670 'This is where the glacier goes afloat.'' 0:01:50.690,0:01:52.990 To find the hidden grounding lines, 0:01:53.010,0:01:55.280 they examined radar images of the glaciers 0:01:55.300,0:01:58.850 made by the European Space Agency's Earth Remote Sensing satellites 0:01:58.870,0:02:01.210 from 1992 to 2011. 0:02:01.230,0:02:04.060 Glaciers flex in response to tides. 0:02:04.080,0:02:06.150 By analyzing the flexing motions, 0:02:06.170,0:02:08.160 they were able to trace the grounding lines. 0:02:08.180,0:02:10.289 This led to a key discovery. 0:02:10.310,0:02:12.800 In all the glaciers they studied, 0:02:12.820,0:02:15.830 grounding lines were rapidly retreating away from the sea. 0:02:15.850,0:02:17.420 'In this sector, 0:02:17.440,0:02:18.830 we are seeing retreat rates 0:02:18.850,0:02:21.310 that we don't see anywhere else on Earth,' Rignot says. 0:02:21.330,0:02:23.780 Smith Glacier's line moved the fastest, 0:02:23.800,0:02:25.920 retreating 22 miles upstream. 0:02:25.940,0:02:29.190 The other lines retreated from 6 to 19 miles. 0:02:29.210,0:02:31.980 As the glaciers melt and lose weight, 0:02:32.000,0:02:34.130 they float off the land where they used to sit. 0:02:34.150,0:02:36.420 Water gets underneath the glacier 0:02:36.440,0:02:38.360 and pushes the grounding line inland. 0:02:38.380,0:02:39.760 This, in turn, 0:02:39.780,0:02:42.100 reduces friction between the glacier and its bed. 0:02:42.120,0:02:43.560 The glacier speeds up, 0:02:43.580,0:02:45.640 stretches out and thins, 0:02:45.660,0:02:48.210 which drives the grounding line to retreat farther inland. 0:02:48.230,0:02:50.630 This is a positive feedback loop 0:02:50.650,0:02:52.580 that leads to out of control melting. 0:02:52.600,0:02:56.550 The only natural factor that can slow or stop this process 0:02:56.570,0:02:58.470 is a 'pinning point' in the bedrock -- 0:02:58.490,0:03:01.690 a bump or projection that snags the glacier from underneath 0:03:01.710,0:03:03.720 and keeps it from sliding toward the sea. 0:03:03.740,0:03:06.020 To investigate this possibility, 0:03:06.040,0:03:09.100 the researchers made a novel map of the bed beneath the glaciers 0:03:09.120,0:03:11.560 using radar and other data from satellites 0:03:11.580,0:03:13.440 and NASA's airborne IceBridge mission. 0:03:13.460,0:03:17.070 The map revealed that the glaciers had already floated off 0:03:17.090,0:03:19.400 many of their small pinning points. 0:03:19.420,0:03:21.910 In short, there seems to be no turning back. 0:03:21.930,0:03:24.240 'At current melt rates,' concludes Rignot, 0:03:24.260,0:03:27.670 'these glaciers will be 'history' within a few hundred years.' 0:03:27.690,0:03:30.990 For more news about climate change and other hot topics, 0:03:31.010,0:03:34.414 stay tuned to Science.nasa.gov.