1 00:00:00,030 --> 00:00:04,040 VO: Of all the freshwater on Earth, 99 percent is stored 2 00:00:04,060 --> 00:00:08,070 in ice sheets, the large frozen masses that form over land. 3 00:00:08,090 --> 00:00:12,100 As climate changes, melting ice sheets can contribute to rising 4 00:00:12,120 --> 00:00:16,130 sea levels, which can place vulnerable cities around the world in 5 00:00:16,150 --> 00:00:20,170 jeopardy. For decades 6 00:00:20,190 --> 00:00:24,210 NASA has studied the polar regions, and a new mission, the Ice, Cloud 7 00:00:24,230 --> 00:00:28,240 and Land Elevation Satellite, or ICESat-2, will elevate our 8 00:00:28,260 --> 00:00:32,290 understanding of these complex ice sheets. Brunt: ICESat-2 is a NASA mission 9 00:00:32,310 --> 00:00:36,320 whose goals include precisely measuring changes in our ice sheets 10 00:00:36,340 --> 00:00:40,350 and how that's actually contributing to mean sea level rise. 11 00:00:40,370 --> 00:00:44,390 Ice sheets form in our polar regions. We have two major ice sheets. 12 00:00:44,410 --> 00:00:48,450 We have Greenland, and we have Antarctica. Antarctica is the larger one, 13 00:00:48,470 --> 00:00:52,530 it's approximately the size of the continental United States, and it's really 14 00:00:52,550 --> 00:00:56,600 thick. In places it gets to be about 4500 meters, 15 00:00:56,620 --> 00:01:00,710 or just under 15,000 feet thick. So it's really thick. 16 00:01:00,730 --> 00:01:04,740 Ice sheets are actually really dynamic and they flow under their own weight from 17 00:01:04,760 --> 00:01:08,800 the center of the ice sheet out to the perimeter of the continent. 18 00:01:08,820 --> 00:01:12,840 In the really cold regions and way high up on our ice sheets, 19 00:01:12,860 --> 00:01:16,950 we get a lot of snow accumulation and over time that accumulation 20 00:01:16,970 --> 00:01:21,120 can build up. If it stays cold enough and that snow persists, 21 00:01:21,140 --> 00:01:25,160 and then you get another year of snow and another year of snow, you can imagine the 22 00:01:25,180 --> 00:01:29,190 weight of the snow on top of itself forces some of the lower layers to compact. We call that the 23 00:01:29,210 --> 00:01:33,230 firn densification of the top layer of the ice sheet. 24 00:01:33,250 --> 00:01:37,350 When we talk about the health of our ice sheets, we talk about 25 00:01:37,370 --> 00:01:41,390 the mass balance of the ice sheet. Basically that means snow coming 26 00:01:41,410 --> 00:01:45,460 in is in balance with all the terms of water or ice 27 00:01:45,480 --> 00:01:49,510 going out. VO: The health of the ice sheets depends on a balance 28 00:01:49,530 --> 00:01:53,540 of these terms of input and output, but the interaction of the atmosphere, ocean 29 00:01:53,560 --> 00:01:57,590 currents and temperatures can force the ice sheets out of this balance. 30 00:01:57,610 --> 00:02:01,610 Brunt: At a big scale, the winds in Antarctica are kind of 31 00:02:01,630 --> 00:02:05,770 spinning in a big clockwise direction around the continent. 32 00:02:05,790 --> 00:02:09,800 But you can imagine a big dome of ice has very little obstruction, 33 00:02:09,820 --> 00:02:13,850 like trees or mountains kind of steering the winds. Consequently, 34 00:02:13,870 --> 00:02:17,990 winds that are sort of gravity-driven come down the continent can build up speed 35 00:02:18,010 --> 00:02:22,010 really quickly, and again, uninterrupted by any sort of 36 00:02:22,030 --> 00:02:26,030 disturbance. And we call those katabatic winds. And they have a major 37 00:02:26,050 --> 00:02:30,050 influence on what happens at the edge of the continent. Around Antarctica 38 00:02:30,070 --> 00:02:34,080 there's a massive current that we call the Antarctic Circumpolar 39 00:02:34,100 --> 00:02:38,130 Current, and it flows clockwise around the continent. 40 00:02:38,150 --> 00:02:42,170 Close to the continent, we also have the Antarctic Coastal Current, stays really 41 00:02:42,190 --> 00:02:46,190 close to the coastline and flows counterclockwise around the 42 00:02:46,210 --> 00:02:50,210 continent. In addition to these continent-scale currents, we 43 00:02:50,230 --> 00:02:54,250 also have regional scale currents, such as gyres. Gyres are these 44 00:02:54,270 --> 00:02:58,310 parts of the oceans that are sort of isolated because of topography, 45 00:02:58,330 --> 00:03:02,390 or ocean bottom topography. They're usually closed currents that 46 00:03:02,410 --> 00:03:06,420 often circulate. The gyres have a big role in 47 00:03:06,440 --> 00:03:10,470 sea ice formation and also in the currents that actually 48 00:03:10,490 --> 00:03:14,480 flow underneath our ice shelves. You can imagine that 49 00:03:14,500 --> 00:03:18,540 around the edge of the continent, near those ice shelves, 50 00:03:18,560 --> 00:03:22,570 warm water from the ocean can intrude into that cavity and contribute to 51 00:03:22,590 --> 00:03:26,600 basal melting, the melting from warm ocean waters 52 00:03:26,620 --> 00:03:30,700 of the bottoms of our ice shelves. Calving in Antarctica 53 00:03:30,720 --> 00:03:34,770 is a little bit sporadic and it's hard to actually model, but some of the 54 00:03:34,790 --> 00:03:38,790 contributing factors associated with calving include those strong katabatic 55 00:03:38,810 --> 00:03:42,830 winds pushing on the on the edge of the ice sheet, pushing on 56 00:03:42,850 --> 00:03:47,000 the edge of the ice shelf and calving large icebergs. 57 00:03:47,020 --> 00:03:51,030 So we're measuring surface elevation, and we can take that 58 00:03:51,050 --> 00:03:55,050 vertical measurement and kind of integrate it over a whole ice sheet and get a volume 59 00:03:55,070 --> 00:03:59,090 change. And then the real science of ICESat-2 is taking that volume change 60 00:03:59,110 --> 00:04:03,120 and turning it into a mass change. And from that we can determine how much 61 00:04:03,140 --> 00:04:07,150 ice is actually turning into water in our oceans and raising sea levels. 62 00:04:07,170 --> 00:04:11,160 [music] 63 00:04:11,180 --> 00:04:13,591 [music]