WEBVTT FILE 1 00:00:00.000 --> 00:00:02.961 [helicopter sound] 2 00:00:02.961 --> 00:00:05.922 Narrator: Five years ago, a NASA-funded science 3 00:00:05.922 --> 00:00:08.883 team ventured onto an ever-changing region 4 00:00:08.883 --> 00:00:11.845 of the Greenland Ice Sheet in the peak of summer melt season, 5 00:00:11.845 --> 00:00:14.806 when the ice was literally melting 6 00:00:14.806 --> 00:00:17.767 out from under their feet. 7 00:00:17.767 --> 00:00:20.729 [music builds, sound of crunching ice] 8 00:00:20.729 --> 00:00:23.690 What they learned is changing the way we think about the movement of ice sheets, 9 00:00:23.690 --> 00:00:26.651 and possibly changing our computer models 10 00:00:26.651 --> 00:00:29.612 models that predict how fast ice will melt, 11 00:00:29.612 --> 00:00:33.700 a question which matters to every coastline on the planet. 12 00:00:34.617 --> 00:00:41.041 Smith: So the number one reason we are here is all about global sea level rise. 13 00:00:42.751 --> 00:00:46.296 Greenland is the single largest melting chunk of ice in the world. 14 00:00:46.838 --> 00:00:51.843 What really matters to the world is how much of that water melted on the ice sheet gets out to the ocean. 15 00:00:52.260 --> 00:00:55.221 Narrator: In order to collect this data, the team had to first transport 16 00:00:55.221 --> 00:01:00.060 scientific equipment and survival gear to Greenland 17 00:01:00.060 --> 00:01:03.688 and then travel via helicopter to set up camp 18 00:01:03.688 --> 00:01:07.567 in the ablation zone, a region of melting ice. 19 00:01:08.109 --> 00:01:10.820 Chu: Camping out here logistically is very difficult. 20 00:01:11.613 --> 00:01:14.574 We’re camping in the ablation zone. It’s very wet, as you can see. 21 00:01:15.033 --> 00:01:17.994 The ablation zone is where it is melting over the summer. 22 00:01:18.244 --> 00:01:21.498 Even talking to the logistics coordinators, they’re very interested in our camp 23 00:01:21.498 --> 00:01:25.210 because they’re trying to learn things about, how do you camp in the ablation zone? 24 00:01:26.211 --> 00:01:31.716 Narrator: One lesson is to be quick and nimble – the team had to evacuate from the first spot they scouted, 25 00:01:32.133 --> 00:01:35.095 because the surface started melting right under their camp. 26 00:01:36.262 --> 00:01:40.767 So what big science questions are at the heart of this bold undertaking? 27 00:01:41.351 --> 00:01:47.315 Smith: In 2015, when we started this study, there was surprisingly little attention paid to 28 00:01:47.816 --> 00:01:54.030 the hydrology of streams and rivers on the ice sheet, especially inland away from the ice edge. 29 00:01:55.365 --> 00:01:59.160 And we felt that this was a critical scientific gap. 30 00:01:59.452 --> 00:02:04.165 Just from looking at satellite images of the ice sheet, it was very apparent that 31 00:02:04.165 --> 00:02:08.002 very large volumes of meltwater were moving through these systems. 32 00:02:08.002 --> 00:02:11.381 And one of the things we learned is that the 33 00:02:11.381 --> 00:02:19.889 total volume of water passing through these river systems far exceeds the volume of water contained by lakes. 34 00:02:19.889 --> 00:02:26.479 Much like the terrestrial land surface, you know, lakes catch your eye because they're so big, but the real action the real flux is through the rivers. 35 00:02:26.479 --> 00:02:29.440 [sound of rushing water] 36 00:02:29.440 --> 00:02:34.362 All of these rivers terminate in a stunning 37 00:02:34.362 --> 00:02:39.367 and dangerous feature called a moulin, which is essentially a sinkhole 38 00:02:39.367 --> 00:02:46.082 in the glacier surface that develops when these large rivers 39 00:02:46.916 --> 00:02:52.630 melt down into the ice to a point where they encounter a crack of some type. 40 00:02:52.630 --> 00:02:58.845 At that point, the river is captured, and it ceases to flow over the surface of the ice sheet 41 00:02:58.845 --> 00:03:02.432 and instead plummets down into the interior. 42 00:03:02.432 --> 00:03:08.730 And this year we mapped 538 of these very large blue rivers 43 00:03:08.730 --> 00:03:13.193 and showed that every single one them terminates in one of these moulins. 44 00:03:13.193 --> 00:03:19.407 So water that's melted on top of the ice sheet is quickly and effectively gathered 45 00:03:19.407 --> 00:03:24.370 and transferred through these branching stream and river network systems. 46 00:03:24.370 --> 00:03:31.169 They are swept off the surface of the sheet within a matter of a few hours or even less 47 00:03:31.169 --> 00:03:35.340 and ultimately emerge 80 kilometers from here at the ice edge. 48 00:03:36.549 --> 00:03:40.470 Narrator: The team used a couple innovative techniques to measure the river. 49 00:03:41.054 --> 00:03:46.601 First, working in shifts they measured stream flow for 72 straight hours 50 00:03:46.601 --> 00:03:49.812 using an instrument mounted on a boogie board 51 00:03:49.812 --> 00:03:55.068 that uses sonic beams to measure the depth of the water and the speed of the current. 52 00:03:55.068 --> 00:03:58.863 To do so they had to climb out to the very edge of the water. 53 00:04:00.490 --> 00:04:04.953 Rennermalm: Basically, the most important here is that we all come back home. 54 00:04:05.620 --> 00:04:10.375 The reason why this is a dangerous place is because only a couple of 55 00:04:10.375 --> 00:04:14.837 100 meters or maybe 200 meters downstream to where these guys are working right now, 56 00:04:14.837 --> 00:04:21.427 is a moulin. This is a vertical passageway – a hole – where melt water 57 00:04:21.427 --> 00:04:28.101 goes straight into the ice. You see this river behind us? 58 00:04:28.101 --> 00:04:33.898 This blue river, flowing very fast, very powerful, very cold. 59 00:04:33.898 --> 00:04:40.238 If one of us would fall into this river without being secured to something, 60 00:04:40.238 --> 00:04:47.245 we would just flow like a little leaf into that big hole and that’s it. 61 00:04:48.997 --> 00:04:54.502 Smith: By far the best solution to ever having to deal with someone taking a spill 62 00:04:54.502 --> 00:04:59.299 is to make it impossible for them to fall in in the first place, and so the way we do that 63 00:04:59.299 --> 00:05:03.469 is to put them on a leash where the leash is exactly long enough 64 00:05:03.469 --> 00:05:07.724 to get close to the water's edge but not one inch more. 65 00:05:07.724 --> 00:05:09.559 [helicopter noise] 66 00:05:09.559 --> 00:05:13.187 Narrator: In addition to measuring stream flow with instruments on boogie boards, 67 00:05:13.187 --> 00:05:19.485 they also flew several kilometers upstream, to three different tributaries of their study river, 68 00:05:19.485 --> 00:05:27.744 and deployed the last three autonomous drifters built by the late scientist and engineering wizard Alberto Behar. 69 00:05:28.161 --> 00:05:32.999 [crunches boot into ice, drifter gently splashes in water] 70 00:05:39.505 --> 00:05:42.467 [boots crunch away over the ice] 71 00:05:45.261 --> 00:05:49.349 Chu: These are GPS autonomous drifters which 72 00:05:49.349 --> 00:05:54.103 will send the GPS coordinates of the location as they flow down our river. 73 00:05:54.103 --> 00:06:00.735 What that tells us is its velocity. And that's very helpful because when we set up these cross sections, we’re in one point location. 74 00:06:00.735 --> 00:06:06.866 With the drifters we get a longitudinal long profile and then we lose the signal when they go down into a moulin. 75 00:06:06.866 --> 00:06:11.037 And as it gets closer to a moulin, the rivers actually don’t get that much deeper. 76 00:06:11.037 --> 00:06:15.833 They just get kinda faster and then they incise into the ice, so these big canyons 77 00:06:16.626 --> 00:06:22.673 So the point of the drifters is to map the hydraulics of the big fast rivers that we can’t get close to. 78 00:06:24.842 --> 00:06:31.432 Narrator: An hour later, all three drifters, which had been placed in three separate streams at different times, 79 00:06:31.432 --> 00:06:37.522 came floating into view at once, sending a chill of excitement through everyone on the team. 80 00:06:40.191 --> 00:06:43.152 [sounds of helicopter and rushing stream] 81 00:06:48.366 --> 00:06:54.122 After analyzing their hard-won 2015 data, the team was a bit puzzled by one thing. 82 00:06:54.122 --> 00:07:02.130 The heat budget calculated by satellite observations, computer models, and the scientists’ mobile weather stations 83 00:07:02.130 --> 00:07:07.135 predicted that current temperatures should be warm enough to melt more ice 84 00:07:07.135 --> 00:07:11.139 and create more runoff than the scientists were actually measuring 85 00:07:11.931 --> 00:07:14.100 So what was missing from the models? 86 00:07:14.976 --> 00:07:18.312 The team returned to the field site the following year, 87 00:07:18.312 --> 00:07:21.983 this time collecting an entire week of flow data, 88 00:07:21.983 --> 00:07:27.071 and also decided to look more closely at the surface of the ice itself. 89 00:07:27.864 --> 00:07:33.536 Smith: And when we drilled into it, we found up to a meter of soaking wet 90 00:07:33.536 --> 00:07:38.666 rotten, fragmented ice. You could break it apart with your hands. 91 00:07:38.666 --> 00:07:43.337 And it stores a non-trivial amount of water. And it also creates the opportunity 92 00:07:43.337 --> 00:07:47.133 for water that is melted during the day to refreeze at night. 93 00:07:47.133 --> 00:07:53.097 And when it refreezes at night, it needs to be refrozen the following day in order to turn back into meltwater again. 94 00:07:54.015 --> 00:07:57.727 Narrator: Melting again the next day requires more energy. 95 00:07:57.727 --> 00:08:04.734 This was the energy that models assume was only melting ice once, rather than having to do it twice. 96 00:08:05.943 --> 00:08:10.573 Smith: And this is great because we're working with modelers and we're going to get that process now into the models 97 00:08:10.573 --> 00:08:14.869 and the models will get even better. And field teams and modelers 98 00:08:14.869 --> 00:08:19.832 have been working this way, hand-in-hand since the 1960s. And that’s why the models keep getting better and better. 99 00:08:20.958 --> 00:08:25.546 Narrator: In addition to measuring stream flow and solving the mystery of the missing meltwater, 100 00:08:25.546 --> 00:08:30.885 the team learned something about the reflectivity of the ice, known as the ice albedo. 101 00:08:31.677 --> 00:08:37.475 Basic physics tells us that the darker something is, the faster it absorbs the sun's heat. 102 00:08:38.226 --> 00:08:42.522 Smith: And you can see this when you fly around Greenland and you look at it when that snowline pulls back, 103 00:08:42.522 --> 00:08:46.192 you can see the darker blue ice revealed 104 00:08:46.192 --> 00:08:52.281 and that bare exposed, darker ice absorbs more sunlight than it would if it was snow covered. 105 00:08:53.199 --> 00:08:57.745 Narrator: And likewise, other things covering the ice, like algae, or dust, 106 00:08:57.745 --> 00:09:01.916 or soot from engines and factories, or volcanic ash, 107 00:09:01.916 --> 00:09:07.088 can also have a darkening affect. But their study showed that the snowline itself 108 00:09:07.088 --> 00:09:12.093 was five times more important to melt rates than these other processes. 109 00:09:12.885 --> 00:09:18.808 Smith: One factor that's a little worrisome is that owing to the topographic profile of the ice sheet, 110 00:09:18.808 --> 00:09:22.436 as it gets flatter as you go to higher elevation. 111 00:09:22.436 --> 00:09:28.776 So what that means is, as the snowline elevation goes higher under a warming climate, 112 00:09:28.776 --> 00:09:35.366 the area of ice exposed will increase as we approach the flatter parts of the ice sheet. 113 00:09:36.617 --> 00:09:42.873 Narrator: Finally, the team observed that when surges of water enter a moulin in a particular location, 114 00:09:42.873 --> 00:09:48.045 it’s often followed within a couple hours by a surge of ice movement above. 115 00:09:48.921 --> 00:09:56.053 That meltwater can act like a layer of lubrication and allow regions of the ice sheet to slide more rapidly 116 00:09:56.887 --> 00:10:01.392 Increased ice motion can result in an increase in iceberg calving 117 00:10:01.392 --> 00:10:04.895 and other positive feedbacks which affect sea-level rise. 118 00:10:06.314 --> 00:10:12.069 A new study including members of the team and NASA Goddard glaciologist Lauren Andrews, 119 00:10:12.069 --> 00:10:18.868 concluded that the most important factor influencing daily changes in glacier speed in southwest Greenland 120 00:10:18.868 --> 00:10:27.043 was not necessarily the volume of the water, but how quickly the volume of water entering the subglacial system changes. 121 00:10:27.043 --> 00:10:32.840 The faster water enters the subglacial system, the higher the subglacial water pressure, 122 00:10:32.840 --> 00:10:37.219 essentially creating an effect like when the tread on tires of a car 123 00:10:37.219 --> 00:10:42.183 are overwhelmed by water on a wet road, causing the car to hydroplane. 124 00:10:44.810 --> 00:10:51.817 The intimate workings of the Greenland Icesheet may seem like a distant concern to those of us thousands of miles away, 125 00:10:51.817 --> 00:10:54.487 but their effects will be widespread. 126 00:10:55.780 --> 00:11:00.326 Smith: Sea level rise presents an existential threat to 127 00:11:00.326 --> 00:11:04.330 core cities and populations all around the world. 128 00:11:05.247 --> 00:11:11.128 A majority of our cities are on coastal deltas. So it's very important. 129 00:11:11.879 --> 00:11:17.718 And our ice sheets are a biggest contributor to that. And Greenland is one of them, but 130 00:11:17.718 --> 00:11:23.516 of course, Antarctica and the stability of the West Antarctic ice sheet is the other elephant in the room. 131 00:11:27.436 --> 00:11:33.275 I think five years from now, our sea level rise projection models from Greenland, 132 00:11:33.275 --> 00:11:37.863 which are already excellent, will be even better and more dependable. 133 00:11:38.155 --> 00:11:41.117 We know quite a lot now, but not enough.