2 00:00:06,420 --> 00:00:15,970 The Greenland ice sheet, in contrast, is not grounded below sea level, 4 00:00:15,970 --> 00:00:21,850 but it is extra vulnerable because it's warmer at the surface. 5 00:00:21,850 --> 00:00:27,520 In Antarctica the surface of the ice is always pretty much below freezing. 6 00:00:27,520 --> 00:00:30,430 And so the only way to melt the Antarctic ice sheet is 7 00:00:30,430 --> 00:00:33,390 to flow it off into the water and melt it in the water. 8 00:00:33,390 --> 00:00:36,330 Maybe let it drift away, up into warmer climates. 9 00:00:38,660 --> 00:00:40,100 But Greenland, it gets warm enough to 10 00:00:40,100 --> 00:00:43,020 melt at the surface, especially around the edges. 11 00:00:43,020 --> 00:00:47,560 And, what seems to be happening, is that meltwater from the surface is able 12 00:00:47,560 --> 00:00:52,670 to penetrate through the ice sheet and run down, and lubricate the bed. 13 00:00:52,670 --> 00:00:58,940 And so, there are fluctuations in the flow velocity of 14 00:00:58,940 --> 00:01:03,930 the ice according to the seasonal cycle. 15 00:01:03,930 --> 00:01:05,850 They flow faster in the summer when there's 16 00:01:05,850 --> 00:01:08,570 more of this meltwater than there is in the winter. 17 00:01:08,570 --> 00:01:11,930 Without this mechanism, if the only way to get heat from the 18 00:01:11,930 --> 00:01:16,930 surface to the bed is by diffusion, 19 00:01:16,930 --> 00:01:20,510 by conduction of heat, it would take a very long time for a warming 20 00:01:20,510 --> 00:01:24,980 at the surface to translate into faster flow of the ice. 21 00:01:24,980 --> 00:01:29,110 But with this meltwater mechanism the ice is 22 00:01:29,110 --> 00:01:35,100 able to respond and flow much more quickly than it would otherwise. 23 00:01:35,100 --> 00:01:37,120 It's still a mystery how this meltwater 24 00:01:37,120 --> 00:01:39,500 is able to get through all of this ice. 25 00:01:39,500 --> 00:01:44,530 This is supposed to be subfreezing through here, but it's observed happening. 26 00:01:44,530 --> 00:01:48,410 Here you can see a photograph of what's called a moulin, where the 27 00:01:48,410 --> 00:01:53,660 water is running and just going down the drain to the bed. 28 00:01:53,660 --> 00:01:59,680 They can trace the water as it goes in here and comes out there, but 30 00:01:59,680 --> 00:02:05,120 mechanistically it's difficult to understand how this works. 31 00:02:05,120 --> 00:02:06,670 Ice sheets are very complicated. 32 00:02:06,670 --> 00:02:09,600 The Greenland ice sheet in particular is very complicated. 33 00:02:09,600 --> 00:02:17,650 You have these regions of very fast flow, called ice streams. 34 00:02:17,650 --> 00:02:19,840 There seems to be some a positive 35 00:02:19,840 --> 00:02:24,380 feedback mechanism that if the ice starts to flow quickly enough 36 00:02:24,380 --> 00:02:29,180 the deformation crumbles the ice or melts it to lubricate the bed, 37 00:02:29,180 --> 00:02:31,560 and allows it to keep going quickly, 38 00:02:31,560 --> 00:02:36,300 so there are places that ice streams are 39 00:02:36,300 --> 00:02:39,010 going very quickly, and then suddenly stop, 40 00:02:39,010 --> 00:02:42,080 and then start again, it's very unpredictable and quirky. 41 00:02:43,340 --> 00:02:45,380 The ice sheet can 42 00:02:45,380 --> 00:02:50,570 be pinned by local islands and rocks and things like that. 43 00:02:50,570 --> 00:02:56,730 Lateral stress can hold the ice from moving. 44 00:02:56,730 --> 00:02:59,150 it'd be much easier if we could just model it 45 00:02:59,150 --> 00:03:01,450 in a one-dimensional sense, so the models that you'll be 46 00:03:01,450 --> 00:03:05,100 playing with in the lab are just one-dimensional models, but 47 00:03:05,100 --> 00:03:08,860 those aren't really good enough to capture everything that's going on. 48 00:03:08,860 --> 00:03:10,890 Then you have this ice shelf here 49 00:03:10,890 --> 00:03:16,160 which can also act to buttress the ice 50 00:03:16,160 --> 00:03:20,520 slowing down its rate of flow into the ocean. 51 00:03:20,520 --> 00:03:25,980 After the ice shelf on the Jakobshavn 52 00:03:27,760 --> 00:03:32,310 glacier in in Greenland collapsed. 53 00:03:32,310 --> 00:03:36,190 It was observed that the flow coming down 54 00:03:36,190 --> 00:03:39,630 the mountain into the sea about doubled. 55 00:03:39,630 --> 00:03:44,763 All these different pieces have to be put together 56 00:03:44,763 --> 00:03:49,941 to predict how an ice sheet will respond to changes in climate.