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.