1 00:00:00,020 --> 00:00:04,040 The lakes forming on the surface of Greenland are storing a large amount 2 00:00:04,060 --> 00:00:07,290 of meltwater throughout the season. 3 00:00:07,310 --> 00:00:12,480 Sometimes you see really shallow, kind of lagoon, almost snow swamp sort of thing, 4 00:00:12,500 --> 00:00:17,160 all the way up to lakes that are almost 30 feet in depth. 5 00:00:17,180 --> 00:00:20,200 As a glaciologist, I am looking at this and 6 00:00:20,220 --> 00:00:24,240 I see bright ice, some darker ice, 7 00:00:24,260 --> 00:00:28,280 that happens to have more dust in it. I see really bright snow 8 00:00:28,300 --> 00:00:31,350 I see the lakes. You can see kind of a network structure 9 00:00:31,370 --> 00:00:34,600 where they are draining together. 10 00:00:34,620 --> 00:00:41,030 So intuitively, you see this beautiful blue color, and you know the darker the blue, the deeper the lake. 11 00:00:41,050 --> 00:00:44,120 Which is nice, but we want to be able to quantify that. 12 00:00:44,140 --> 00:00:48,220 So imagine that you're a little beam of light. You get emitted by the sun, 13 00:00:48,240 --> 00:00:53,730 you hit the surface of the lake. As the light goes through the lake, some of it is absorbed by the water 14 00:00:53,750 --> 00:00:56,340 and a little bit of it gets scattered back. 15 00:00:56,360 --> 00:01:00,830 Then the light hits the bottom of the lake and a bunch gets return reflected, 16 00:01:00,850 --> 00:01:04,680 goes back through the column of water in the lake, 17 00:01:04,700 --> 00:01:07,260 and that is what Landsat can see. 18 00:01:07,280 --> 00:01:13,500 So we need to be able to understand how much energy is being absorbed by the water 19 00:01:13,520 --> 00:01:17,630 what the reflectance of the bottom of the lake is, 20 00:01:17,650 --> 00:01:22,350 and what proportion of light is being scattered back by the water. 21 00:01:22,370 --> 00:01:25,090 Once we have those three pieces of information 22 00:01:25,110 --> 00:01:28,640 and what the brightness that particular pixel is, 23 00:01:28,660 --> 00:01:33,290 then we can say how deep the water is in the lake there. 24 00:01:33,310 --> 00:01:36,920 These lakes are darker than the ice around them so they are going to start absorbing 25 00:01:36,940 --> 00:01:39,340 more energy from the sun and melt more. 26 00:01:39,360 --> 00:01:41,590 A little bit of melt concentrates in one place, 27 00:01:41,610 --> 00:01:46,440 and then melts more, which melts more, and it's a feedback mechanism. 28 00:01:46,460 --> 00:01:50,490 And when the lakes get big enough they can force open fractures 29 00:01:50,510 --> 00:01:54,050 that then drill all the way down to the bed of the glacier, 30 00:01:54,070 --> 00:01:59,320 transporting this water to the base where it can temporarily speed up the flow of the ice. 31 00:01:59,340 --> 00:02:02,180 And so we're interested in the lakes 32 00:02:02,200 --> 00:02:06,760 because they might be important for speeding up the ice sheet. 33 00:02:06,780 --> 00:02:10,810 34 00:02:10,830 --> 00:02:16,446