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