WEBVTT 00:01.267 --> 00:03.997 RONALD SMITH: As you know, because we just got this subject started 00:04.000 --> 00:06.500 last time, I'm going to talk today about ice 00:06.500 --> 00:07.500 in the climate system. 00:07.500 --> 00:11.570 Let me just review the types of ice that 00:11.567 --> 00:14.497 we'll be speaking about. 00:14.500 --> 00:17.130 Sea ice, which is frozen seawater. 00:17.133 --> 00:23.533 Ice sheets, which there's only two of those existing today in 00:23.533 --> 00:24.373 the modern world. 00:24.367 --> 00:26.327 And that's Greenland and Antarctica. 00:26.333 --> 00:27.773 We'll talk about those. 00:27.767 --> 00:31.467 Later on, next week, when we talk about climate change, 00:31.467 --> 00:34.627 I'll describe to you how in earlier days there were some 00:34.633 --> 00:35.873 other ice sheets on Earth. 00:35.867 --> 00:38.167 But at the moment there's just those two. 00:38.167 --> 00:40.227 We'll talk about how they work. 00:40.233 --> 00:42.633 Glaciers, both coming down from ice sheets and from 00:42.633 --> 00:44.373 mountain glaciers. 00:44.367 --> 00:47.467 Ice shelves, icebergs, and permanent and seasonal 00:47.467 --> 00:48.027 snowfields. 00:48.033 --> 00:51.803 So we'll be going through those subjects today. 00:51.800 --> 00:53.900 And I mentioned these items last time, as well. 00:53.900 --> 00:56.770 There are certain important physical properties that 00:56.767 --> 01:01.897 you'll need to understand in order to see--in order to 01:01.900 --> 01:05.630 understand really how ice behaves on our planet. 01:05.633 --> 01:07.773 And I won't go through those again today. 01:07.767 --> 01:10.227 I went through them at the beginning--at the end of the 01:10.233 --> 01:12.173 period last time. 01:12.167 --> 01:13.797 But just review those. 01:13.800 --> 01:16.400 We'll be discussing those as we go. 01:16.400 --> 01:18.570 So let's start with sea ice. 01:23.933 --> 01:26.733 And what I've shown you here are some diagrams that 01:26.733 --> 01:30.973 illustrate how sea ice changes with season in the two 01:30.967 --> 01:31.567 hemispheres. 01:31.567 --> 01:33.567 Now remember, this is frozen seawater. 01:33.567 --> 01:39.727 So about this time of year, late October, you begin to get 01:39.733 --> 01:44.503 cold air blowing over the sea surface in the 01:44.500 --> 01:46.730 high northern latitudes. 01:46.733 --> 01:49.903 So we'll focus on these two diagrams. 01:49.900 --> 01:54.100 And as the cold air blows over the ocean surface, it draws 01:54.100 --> 01:58.000 heat out of the ocean, cools it down. 01:58.000 --> 02:00.830 When you get the temperature down to about what? 02:00.833 --> 02:05.203 Not zero, but about minus two Celsius, because the salt 02:05.200 --> 02:09.230 depresses the freezing point by that much, then you start 02:09.233 --> 02:11.973 to get sea ice forming. 02:11.967 --> 02:15.767 It starts usually in small little patches of ice called 02:15.767 --> 02:16.597 pancake ice. 02:16.600 --> 02:18.200 I'll show you that in a minute. 02:18.200 --> 02:22.900 And then as the season progresses, you freeze the 02:22.900 --> 02:25.970 water in between and those little pieces 02:25.967 --> 02:27.127 of ice stick together. 02:27.133 --> 02:30.103 And a month or two later, you've got a more or less 02:30.100 --> 02:34.570 continuous layer of sea ice in these high latitude regions. 02:34.567 --> 02:37.397 And then during the winter, it deepens. 02:37.400 --> 02:39.470 In other words, you're drawing heat out of the bottom and 02:39.467 --> 02:44.727 you're freezing more ocean water onto the bottom. 02:44.733 --> 02:49.033 So the thickness of the sea ice increases. 02:49.033 --> 02:52.333 So here we are in September, which is at the end of the 02:52.333 --> 02:53.103 warming season. 02:53.100 --> 02:55.330 That would be the minimum in the sea ice. 02:55.333 --> 02:58.233 And yet there is some sea ice remaining from 02:58.233 --> 03:00.733 the previous year. 03:00.733 --> 03:03.533 Then we begin to build new sea ice. 03:03.533 --> 03:08.373 And about six months later, this is typically or at least 03:08.367 --> 03:11.667 was--we'll talk about the fact that it's changing with time. 03:11.667 --> 03:15.767 But this is typically how far sea ice extends at the end of 03:15.767 --> 03:17.867 the cooling season. 03:17.867 --> 03:19.627 Then it begins to warm again. 03:19.633 --> 03:22.103 And over the summer, you melt away a lot of this. 03:22.100 --> 03:24.100 And you end up back towards the minimum. 03:24.100 --> 03:26.970 So there's a big seasonality in sea ice. 03:26.967 --> 03:29.867 And this is the way we understand it. 03:29.867 --> 03:32.497 In March, when you've got maximum sea ice, you have a 03:32.500 --> 03:36.200 stream of sea ice coming down on the east side of Greenland. 03:36.200 --> 03:39.730 Most of Baffin Bay is filled with sea ice. 03:39.733 --> 03:47.003 All of Hudson's Bay and James Bay is filled with sea ice. 03:47.000 --> 03:51.400 And really, it even comes out a bit into the North Pacific. 03:51.400 --> 03:54.670 But this area stays generally free of sea ice. 03:54.667 --> 03:57.067 And you know why that is, because you remember that 03:57.067 --> 04:01.027 the--part of the Gulf Stream splits off and moves up into 04:01.033 --> 04:01.573 that region. 04:01.567 --> 04:06.497 So even at the maximum of sea ice, you still have open water 04:06.500 --> 04:11.470 along the coast of Norway and even up here in an area that's 04:11.467 --> 04:12.697 called North Cape. 04:15.633 --> 04:18.503 Now, of course, in the southern hemisphere, the 04:18.500 --> 04:21.200 seasons are reversed. 04:21.200 --> 04:25.870 And so it's in March that you'll have the minimum of sea 04:25.867 --> 04:27.867 ice, because that's when you're at the end of the 04:27.867 --> 04:29.067 warming season. 04:29.067 --> 04:31.367 And it pretty much drops to zero. 04:31.367 --> 04:34.027 In other words, you have a little bit of permanent sea 04:34.033 --> 04:37.003 ice remaining, but really very little. 04:37.000 --> 04:40.170 And then six months later, at the end of the cooling season, 04:40.167 --> 04:43.767 you've grown quite a remarkable area of sea ice, 04:43.767 --> 04:47.327 by, again, freezing seawater. 04:47.333 --> 04:49.603 Later on I'm going to be showing you what the ice on 04:49.600 --> 04:51.970 Antarctica is doing. 04:51.967 --> 04:53.697 We'll talk about ice shelves. 04:53.700 --> 04:56.470 But keep that distinct from what we're seeing here. 04:56.467 --> 05:00.067 This is the frozen seawater, the sea ice that 05:00.067 --> 05:02.667 I'm referring to. 05:02.667 --> 05:03.797 Any questions on that? 05:03.800 --> 05:04.170 Yes. 05:04.167 --> 05:06.497 STUDENT: Is it because the Gulf Stream waters are warmer, 05:06.500 --> 05:07.900 or they're just moving faster? 05:07.900 --> 05:09.330 PROFESSOR: They're warmer. 05:09.333 --> 05:13.403 So even in wintertime, the temperature of the sea surface 05:13.400 --> 05:19.870 here is about plus three or plus four Celsius. 05:19.867 --> 05:21.997 And remember, the freezing point is minus two. 05:22.000 --> 05:25.470 So you're not going to get freezing of these waters, even 05:25.467 --> 05:28.527 though you get terrific cold winds coming 05:28.533 --> 05:31.873 off of that sea ice. 05:31.867 --> 05:34.397 I was on an expedition up there 20 years ago, right at 05:34.400 --> 05:39.330 the edge of this sea ice, looking at how that cold wind 05:39.333 --> 05:42.303 was coming off and trying to freeze the water. 05:42.300 --> 05:45.130 But there was so much heat convecting out of the water 05:45.133 --> 05:47.703 that you weren't able to cause any new sea ice 05:47.700 --> 05:49.170 forming in that area. 05:49.167 --> 05:52.267 So it's the warmth of that Gulf Stream extension that's 05:52.267 --> 05:56.727 keeping that free of ice. 05:56.733 --> 05:57.503 Yes? 05:57.500 --> 05:58.800 STUDENT: How deep is the ice? 05:58.800 --> 05:59.830 PROFESSOR: I'll talk about that. 05:59.833 --> 06:04.033 But typically, it's from--at the end of the freezing 06:04.033 --> 06:08.903 season, you can freeze ice about one meter deep. 06:08.900 --> 06:12.630 But if you mechanically stack it up, which happens in some 06:12.633 --> 06:15.703 parts, you can get it to be about the height of this room, 06:15.700 --> 06:17.730 three or four meters high. 06:17.733 --> 06:19.973 So compared to the ocean depth, it's tiny, just the 06:19.967 --> 06:21.727 thinnest skin. 06:21.733 --> 06:25.673 But in terms of real physical dimensions, I would say maybe 06:25.667 --> 06:28.597 one meter at the end of the cooling season, in some cases. 06:28.600 --> 06:30.930 Up to four where the ice has been 06:30.933 --> 06:32.573 mechanically crushed and stacked. 06:35.900 --> 06:38.270 Now I want you to notice something. 06:38.267 --> 06:43.527 In the Arctic Ocean, it's ocean at the North Pole. 06:43.533 --> 06:45.733 And it's confined by continent surround. 06:45.733 --> 06:48.033 And you typically fill that up. 06:48.033 --> 06:50.673 And you can't go any further, of course, because you can't 06:50.667 --> 06:52.997 make any sea ice over the land. 06:53.000 --> 06:58.370 So if you're going to monitor this sea ice year by year, the 06:58.367 --> 07:02.297 way it's typically done in the Northern Hemisphere is to 07:02.300 --> 07:07.930 monitor the minimum sea ice, which is in September. 07:07.933 --> 07:10.533 Maximum wouldn't make much sense, because you're not 07:10.533 --> 07:11.833 going to fill that in with ice. 07:11.833 --> 07:14.433 And you've already taken it shore to shore 07:14.433 --> 07:15.973 in the Arctic Ocean. 07:15.967 --> 07:19.297 So the way you monitor interannual variations, or 07:19.300 --> 07:22.870 trends, is to keep track of the minimum. 07:22.867 --> 07:26.727 On the other hand, in the Southern Hemisphere, the 07:26.733 --> 07:29.603 minimum is pretty much zero. 07:29.600 --> 07:34.070 It goes--you have a continent filling the South Pole. 07:34.067 --> 07:36.827 And you're not going to form any sea ice there. 07:36.833 --> 07:38.603 And the sea ice itself pretty much goes 07:38.600 --> 07:39.970 back to that shoreline. 07:39.967 --> 07:45.297 So the way we monitor sea ice in the Southern Hemisphere 07:45.300 --> 07:50.930 typically is by the maximum sea ice that you grow. 07:50.933 --> 07:54.503 Notice, then, that that means September is kind of the month 07:54.500 --> 07:58.500 where we monitor sea ice in both hemispheres, even though 07:58.500 --> 08:02.470 the seasonality is opposite in the two hemispheres. 08:02.467 --> 08:03.727 So be aware of that. 08:03.733 --> 08:06.333 It's a curious way that we decide to 08:06.333 --> 08:07.203 monitor these things. 08:07.200 --> 08:09.330 So pancake ice looks like this. 08:09.333 --> 08:12.633 So when the winter is beginning, and the cold air is 08:12.633 --> 08:18.403 drawing heat out of the ocean, for a period of some days or 08:18.400 --> 08:22.530 weeks, you have little chunks of ice that are from half a 08:22.533 --> 08:25.073 meter to a couple meters in diameter. 08:25.067 --> 08:27.597 They have rounded edges, because with the wave action, 08:27.600 --> 08:29.970 they keep bouncing into each other breaking 08:29.967 --> 08:33.467 off any sharp corners. 08:33.467 --> 08:36.067 And then a few weeks later you begin to freeze and get a more 08:36.067 --> 08:37.467 continuous ice. 08:37.467 --> 08:39.667 This happens to be continuous sea ice 08:39.667 --> 08:41.267 with compression ridges. 08:41.267 --> 08:44.067 In other words, when you look at these features like this or 08:44.067 --> 08:48.167 that, that's an area where the wind has blown the sea ice 08:48.167 --> 08:53.527 together and it has crumpled along a line. 08:53.533 --> 08:59.533 And so if you're traveling over sea ice, as the early 08:59.533 --> 09:02.803 explorers were trying to do, travel over the sea ice to 09:02.800 --> 09:06.800 reach the North Pole, it's not like some skating rink where 09:06.800 --> 09:10.630 you can just ski or skate across the sea ice. 09:10.633 --> 09:14.673 In fact, every few hundred meters, you end up climbing 09:14.667 --> 09:19.167 over one of these compression ridges, which makes traveling 09:19.167 --> 09:21.497 there extremely difficult. 09:21.500 --> 09:24.630 You have to pull your sledges or whatever kind of equipment 09:24.633 --> 09:27.303 you have up over these ridges and then down. 09:27.300 --> 09:29.170 You get maybe 50 or 100 yards, and then 09:29.167 --> 09:30.027 you're doing it again. 09:30.033 --> 09:32.733 So it's very rough going when you're trying to 09:32.733 --> 09:33.533 cross the sea ice. 09:33.533 --> 09:33.873 Yes? 09:33.867 --> 09:35.697 STUDENT: How tall do the ridges get? 09:35.700 --> 09:40.370 PROFESSOR: So the ridges will get typically only 09:40.367 --> 09:42.927 four or five meters high. 09:42.933 --> 09:46.303 But remember, they've got a root which you don't see. 09:46.300 --> 09:50.000 But in order to support that extra weight, they also have a 09:50.000 --> 09:52.530 root that goes down beneath the ice. 09:52.533 --> 09:56.303 So if you measured it from top to bottom, it might be 20 or 09:56.300 --> 10:00.070 30 meters, but the part you have to climb over is just 3 10:00.067 --> 10:01.667 or 4 meters, something like that. 10:01.667 --> 10:02.097 Yes? 10:02.100 --> 10:03.430 STUDENT: How are they formed? 10:03.433 --> 10:05.303 PROFESSOR: By compression. 10:05.300 --> 10:08.930 So the wind, for example, blowing over the ice will try 10:08.933 --> 10:10.273 to push it. 10:10.267 --> 10:13.027 But if it comes up against a shoreline, it can't be pushed 10:13.033 --> 10:14.173 any further. 10:14.167 --> 10:17.197 And so you'll end up just crumpling that ice and 10:17.200 --> 10:20.900 stacking it up along these ridges. 10:20.900 --> 10:21.230 Yeah. 10:21.233 --> 10:23.333 Good questions. 10:23.333 --> 10:27.573 So here's another picture of sea ice. 10:27.567 --> 10:30.267 And I wanted to ask you, first of all, was this picture taken 10:30.267 --> 10:34.427 in winter or summer would you guess? 10:34.433 --> 10:35.203 STUDENT: Summer? 10:35.200 --> 10:36.430 PROFESSOR: Why? 10:40.633 --> 10:42.103 It was taken in summer. 10:42.100 --> 10:43.800 How would you know that? 10:43.800 --> 10:45.570 STUDENT: The ice isn't continuous? 10:45.567 --> 10:46.027 PROFESSOR: Sorry? 10:46.033 --> 10:47.803 STUDENT: So the ice isn't continuous? 10:47.800 --> 10:47.870 PROFESSOR: Yes. 10:47.867 --> 10:49.867 It's got some melt water in between here. 10:49.867 --> 10:52.267 But there's even a more obvious answer than that. 10:52.267 --> 10:54.527 STUDENT: Do they hibernate in the winter? 10:54.533 --> 10:55.433 PROFESSOR: That's true also. 10:55.433 --> 10:59.003 But there's even a more obvious answer than that. 10:59.000 --> 10:59.600 STUDENT: The sunlight. 10:59.600 --> 11:00.700 PROFESSOR: It's sunlit. 11:00.700 --> 11:01.300 Exactly. 11:01.300 --> 11:03.570 So you could not take a picture like that in the 11:03.567 --> 11:04.127 winter time. 11:04.133 --> 11:05.733 It would be pitch dark. 11:05.733 --> 11:07.333 So it is true it is melt ice. 11:07.333 --> 11:09.673 It is true the polar bears walking around. 11:09.667 --> 11:13.967 But even more obvious is it's sunlit, so that means in those 11:13.967 --> 11:17.397 latitudes that's got to be a summertime picture. 11:17.400 --> 11:19.700 Just a trick question for you this morning. 11:19.700 --> 11:21.070 Seeing if you're awake. 11:21.067 --> 11:24.867 So let's take a look at what happens. 11:24.867 --> 11:27.397 This is a short movie. 11:27.400 --> 11:31.330 And it'll just show you remember the date is up here. 11:31.333 --> 11:33.573 And you'll watch that change. 11:33.567 --> 11:37.227 And so we'll just watch how one season's 11:37.233 --> 11:39.473 sea ice grows here. 11:39.467 --> 11:46.467 [VIDEO PLAYBACK] 11:46.467 --> 11:49.397 PROFESSOR: So it's growing now, 11:49.400 --> 11:50.500 filling out the basin. 11:50.500 --> 11:53.230 Look what's happening in Hudson's Bay. 11:53.233 --> 11:55.033 Filling in James Bay too. 11:55.033 --> 11:56.873 Still growing a bit up here. 11:56.867 --> 11:59.297 But it's now about as far as it's going to get. 11:59.300 --> 12:02.030 A little bit of extra growth there in Baffin Bay. 12:02.033 --> 12:04.403 A little bit more growth, though, in the current 12:04.400 --> 12:05.170 bringing it down. 12:05.167 --> 12:05.297 [END VIDEO PLAYBACK] 12:05.300 --> 12:06.570 PROFESSOR: And then there we are in March. 12:06.567 --> 12:08.367 So that's kind of at the maximum. 12:08.367 --> 12:12.697 The movie just takes you from minimum to maximum last year, 12:12.700 --> 12:16.070 in the year 2010. 12:16.067 --> 12:18.397 Now what will happen after that, not shown, is that this 12:18.400 --> 12:19.630 will all began to melt back. 12:19.633 --> 12:24.003 And we'll end up back towards that minimum September 12:24.000 --> 12:25.270 distribution. 12:28.633 --> 12:31.073 Let me play that again, just so you can-- in case you 12:31.067 --> 12:32.497 missed something. 12:32.500 --> 12:34.370 Every place you look at you see a little 12:34.367 --> 12:35.967 different story there. 12:35.967 --> 12:37.297 [VIDEO PLAYBACK] 12:37.300 --> 12:39.300 PROFESSOR: Also, the zooming in is distracting a 12:39.300 --> 12:40.530 little bit. 12:52.133 --> 12:54.633 The shade of white gives you something about the thickness 12:54.633 --> 12:56.703 or the continuity of the sea ice. 12:56.700 --> 13:00.530 If there's gaps, that will show up as a slightly darker 13:00.533 --> 13:01.933 shade of gray. 13:01.933 --> 13:04.803 [END VIDEO PLAYBACK] 13:04.800 --> 13:07.300 PROFESSOR: So Rachel asked about thickness. 13:10.867 --> 13:12.897 Here's some examples. 13:12.900 --> 13:16.700 The mean, from 1982 to 2000 now it's difficult to measure 13:16.700 --> 13:17.430 ice thickness. 13:17.433 --> 13:21.003 There are various ways to do it. 13:21.000 --> 13:23.270 The earliest way was to take a submarine. 13:23.267 --> 13:26.667 In the late '60s they finally got nuclear submarines that 13:26.667 --> 13:28.997 could pass under the ice sheet. 13:29.000 --> 13:33.200 And then with an up-looking sonar you could measure the 13:33.200 --> 13:35.700 bottom and the top of the ice and get a thickness 13:35.700 --> 13:37.170 measurement that way. 13:37.167 --> 13:40.097 Of course, you can go there in situ, but then how do you--you 13:40.100 --> 13:42.900 want to get some measurement that's kind of representative 13:42.900 --> 13:44.270 of a large area. 13:44.267 --> 13:46.867 It's hard to do that if you're just walking around on the ice 13:46.867 --> 13:48.667 with some kind of way of drilling through. 13:48.667 --> 13:54.927 So now there's a way to fly over it, and use down-scanning 13:54.933 --> 13:58.803 atmospheric acoustic waves or electromagnetic waves to 13:58.800 --> 14:00.600 measure sea ice thickness as well. 14:00.600 --> 14:02.300 So it's not a perfectly solved problem. 14:02.300 --> 14:04.470 But this will give you some idea. 14:04.467 --> 14:05.967 The scale is down on the bottom. 14:05.967 --> 14:09.067 It's in meters, from zero to four meters. 14:09.067 --> 14:13.767 And it's all over on the north slope of Canada--north slope 14:13.767 --> 14:17.097 of Alaska and then along the Canadian Archipelago where you 14:17.100 --> 14:19.670 get this deeper ice. 14:19.667 --> 14:24.697 And that, I've made the case for you, is not you can't grow 14:24.700 --> 14:28.470 ice that thick thermodynamically, by freezing 14:28.467 --> 14:28.927 at the bottom. 14:28.933 --> 14:32.333 Because after it gets to a certain thickness, remember 14:32.333 --> 14:34.703 you have to take the heat out the top. 14:34.700 --> 14:36.330 That's where the atmosphere is cold. 14:36.333 --> 14:39.373 And yet you've got to freeze the water at the bottom. 14:39.367 --> 14:42.027 And after a while, it becomes too hard to get that heat to 14:42.033 --> 14:44.133 conduct out through that layer of sea ice. 14:44.133 --> 14:48.403 So you can only grow sea ice thermodynamically to this kind 14:48.400 --> 14:49.130 of thickness. 14:49.133 --> 14:51.733 This is a mechanical stacking process. 14:51.733 --> 14:54.203 And you can understand that if you remember that across the 14:54.200 --> 14:56.430 Arctic Ocean there is a current called 14:56.433 --> 14:58.933 the Transpolar Drift. 14:58.933 --> 15:00.403 And also the Beaufort Gyre. 15:00.400 --> 15:03.830 But that's going to be carrying ice and pushing it up 15:03.833 --> 15:07.003 against the north coast of Greenland and along the 15:07.000 --> 15:11.030 Canadian Archipelago, and then causing that compression ridge 15:11.033 --> 15:13.173 to form, those compression ridges. 15:13.167 --> 15:18.567 And that deepens the average ice thickness to as great as 15:18.567 --> 15:21.667 four meters, in other words, typically a little higher than 15:21.667 --> 15:24.597 the ceiling in this room. 15:24.600 --> 15:27.870 Questions on that? 15:27.867 --> 15:33.197 So here's what's been going on the last couple of decades. 15:33.200 --> 15:38.370 So this is the Northern Hemisphere extent anomaly. 15:38.367 --> 15:42.667 Remember we're learning to understand this word anomaly, 15:42.667 --> 15:45.827 because it's often used in climate analysis. 15:45.833 --> 15:48.203 What they do is take a measurement and then subtract 15:48.200 --> 15:52.370 off a mean value to just emphasize the changes that 15:52.367 --> 15:53.397 have occurred. 15:53.400 --> 15:55.570 And this is expressed in percent. 15:55.567 --> 15:56.567 And zero is here. 15:56.567 --> 15:58.297 So they've taken a year. 15:58.300 --> 16:04.700 They've taken 1979 to the year 2000 as their reference time, 16:04.700 --> 16:08.870 subtracted off the mean, and then here's what's left as the 16:08.867 --> 16:11.097 residual or as the anomaly. 16:11.100 --> 16:14.800 So basically, in the earlier part of the time it was--had a 16:14.800 --> 16:18.430 positive anomaly by definition, as it's 16:18.433 --> 16:19.073 decreasing. 16:19.067 --> 16:21.727 And then near the end you get a negative anomaly. 16:21.733 --> 16:25.833 So really the change has been remarkable. 16:25.833 --> 16:30.573 Overall that's about a 40% if you count this, maybe even a 16:30.567 --> 16:36.267 50% decrease in the area covered by ice in September. 16:36.267 --> 16:41.097 That's in the minimum period, after the warming season. 16:41.100 --> 16:43.200 There was quite a shock, and you read about this in the 16:43.200 --> 16:46.770 newspaper perhaps, back in 2007. 16:46.767 --> 16:50.527 We had this remarkable drop in sea ice extent 16:50.533 --> 16:52.003 in the Arctic Ocean. 16:52.000 --> 16:53.700 Then it rebounded. 16:53.700 --> 16:55.000 And people were saying, well, maybe 16:55.000 --> 16:56.300 that was just an anomaly. 16:56.300 --> 16:57.670 Well, it was an anomaly. 16:57.667 --> 16:59.097 But now it's come down again. 16:59.100 --> 17:02.330 So the point is it's been noisy all along. 17:02.333 --> 17:05.003 But nevertheless the trend, the downward 17:05.000 --> 17:06.700 trend is very clear. 17:06.700 --> 17:08.930 And what I would expect to happen in the future, it would 17:08.933 --> 17:13.173 decline further, but be very noisy from year to year. 17:13.167 --> 17:16.197 And you can extrapolate this for yourself. 17:16.200 --> 17:19.870 In another 20 or 30 years, it seems like we'll have an 17:19.867 --> 17:26.597 ice-free Arctic Ocean at the end of the warming season. 17:26.600 --> 17:27.830 Questions on that? 17:30.067 --> 17:35.267 So the Southern Hemisphere, again September. 17:35.267 --> 17:37.197 This is the maximum, because the minimum 17:37.200 --> 17:38.130 doesn't make any sense. 17:38.133 --> 17:40.733 It pulls right back pretty much to the shores of 17:40.733 --> 17:41.733 Antarctica. 17:41.733 --> 17:42.903 That's pretty flat. 17:42.900 --> 17:46.070 Or, if you looked at it very closely, it's even increasing 17:46.067 --> 17:48.297 ever so slightly. 17:48.300 --> 17:51.200 And so this trend in sea ice that we see in the Northern 17:51.200 --> 17:55.500 Hemisphere is not found in the Southern Hemisphere. 17:55.500 --> 17:57.670 When we're talking about things like global warming, 17:57.667 --> 18:03.497 we'll come back and refer to measurements such as this one. 18:03.500 --> 18:05.800 But remember, this has a somewhat different meaning, 18:05.800 --> 18:09.870 because this is the maximum extent of sea ice. 18:09.867 --> 18:13.867 Whereas the other one, for the Northern Arctic Ocean was the 18:13.867 --> 18:15.827 minimum extent of sea ice. 18:15.833 --> 18:17.073 Any questions on that? 18:19.467 --> 18:24.427 And of course what's also in the news is that now, as in 18:24.433 --> 18:29.433 the Arctic Ocean, you get a retreat of that minimum sea 18:29.433 --> 18:31.333 ice in September. 18:31.333 --> 18:36.333 You've opened up these two long-sought 18:36.333 --> 18:38.333 navigational routes. 18:38.333 --> 18:42.533 For years and years in the 17th and 18th and 19th 18:42.533 --> 18:46.473 centuries, sailors were seeking the Northwest Passage, 18:46.467 --> 18:48.697 trying to find a way to get between the Atlantic and the 18:48.700 --> 18:52.030 Pacific without going all the way under Cape Horn. 18:52.033 --> 18:53.873 And this is what they were trying to do. 18:53.867 --> 18:56.497 They never achieved it, but now today you can. 18:56.500 --> 18:59.570 There is a month or so in the summer when you can take a 18:59.567 --> 19:05.227 ship through the Canadian Archipelago free of ice and 19:05.233 --> 19:06.703 transport back and forth. 19:06.700 --> 19:14.130 And in the last well, at least in 2007, and in this year, 19:14.133 --> 19:17.233 there's actually a northern sea route along the north 19:17.233 --> 19:21.273 coast of Russia that you could use also to get back and forth 19:21.267 --> 19:23.327 between the two oceans. 19:23.333 --> 19:26.903 So navigation is now becoming possible in the Arctic Ocean. 19:26.900 --> 19:29.570 And so is things like oil drilling. 19:29.567 --> 19:31.927 So there's been a lot of discussion about who's going 19:31.933 --> 19:36.833 to lead the charge to now developing oil resources in 19:36.833 --> 19:37.903 the Arctic Ocean. 19:37.900 --> 19:41.970 It's quite a change going on up there. 19:41.967 --> 19:43.627 So any questions on sea ice? 19:43.633 --> 19:46.173 This was all about frozen seawater. 19:46.167 --> 19:47.967 Any questions on that? 19:47.967 --> 19:48.867 Yes, Julia 19:48.867 --> 19:49.297 STUDENT: Can you quickly review how the wind--is it 19:49.300 --> 19:50.570 convection? 19:53.267 --> 19:54.327 PROFESSOR: Yes, so the cold wind 19:54.333 --> 19:55.973 blows over the sea. 19:55.967 --> 20:00.067 It draws heat out of the ocean, drops the temperature 20:00.067 --> 20:03.327 down to zero degrees Celsius, then below, down 20:03.333 --> 20:04.833 to minus two Celsius. 20:04.833 --> 20:07.903 And at that point then, you start to freeze. 20:07.900 --> 20:12.230 And of course, I showed you on the first slide, you've got to 20:12.233 --> 20:15.273 draw certain amount of heat out for every kilogram of ice 20:15.267 --> 20:16.567 that you make. 20:16.567 --> 20:18.327 That's called the latent heat of freezing. 20:18.333 --> 20:21.633 So as you continue to draw heat out, the temperature 20:21.633 --> 20:25.073 doesn't drop anymore, but you continue to build deeper and 20:25.067 --> 20:26.467 deeper ice. 20:26.467 --> 20:29.597 So that continues through the month of March in the Northern 20:29.600 --> 20:30.870 Hemisphere. 20:34.400 --> 20:39.370 So now we turn to ice sheets, which is generally defined as 20:39.367 --> 20:42.827 a large plateau of ice. 20:42.833 --> 20:46.473 And there's two things you have to know 20:46.467 --> 20:47.327 to understand this. 20:47.333 --> 20:50.273 First of all, on the high altitude parts of the ice 20:50.267 --> 20:52.527 sheet this happens to be Greenland, but it would be the 20:52.533 --> 20:56.703 same for Antarctica you have an excess of accumulation 20:56.700 --> 20:58.230 every year. 20:58.233 --> 21:03.633 So more snow falls during the winter than would melt off if 21:03.633 --> 21:06.403 indeed there's any melt at all in the summertime. 21:06.400 --> 21:10.300 And so year after year, you just accumulate. 21:10.300 --> 21:11.870 Well, that can't go on forever. 21:11.867 --> 21:17.527 I mean, if you accumulated a meter of snow or 10 meters of 21:17.533 --> 21:22.003 snow every year, by the time you got to 10 years or 100 21:22.000 --> 21:25.070 years or 1,000 years or 10,000 years, you would have an 21:25.067 --> 21:26.967 enormous pile of snow. 21:26.967 --> 21:28.597 So you have to balance that. 21:28.600 --> 21:31.130 And what happens is that, under its own weight, it 21:31.133 --> 21:34.673 begins to sag and to flow. 21:34.667 --> 21:38.897 So the snow gets compressed into more or less solid ice. 21:38.900 --> 21:42.730 And then that ice itself, under high pressure conditions 21:42.733 --> 21:46.833 with a lot of weight above it, will begin to flow out toward 21:46.833 --> 21:49.503 the edges of the continent. 21:49.500 --> 21:52.200 You can simulate this next time you've got pancakes for 21:52.200 --> 21:55.630 breakfast. Just keep pouring that syrup right in the middle 21:55.633 --> 21:58.233 the pancake, and it only get so deep. 21:58.233 --> 22:01.933 After a while it begins to flow off the edges. 22:01.933 --> 22:03.203 And that's exactly what's happening. 22:03.200 --> 22:09.930 It's just gravity forcing that viscous fluid toward the edges 22:09.933 --> 22:11.673 of the pancake. 22:11.667 --> 22:15.997 Then near the edge, there could be some loss by melting 22:16.000 --> 22:18.800 or direct sublimation. 22:18.800 --> 22:23.970 Sublimation is when you go directly from ice to vapor. 22:23.967 --> 22:26.527 But if that's not sufficient, then the ice will actually be 22:26.533 --> 22:29.273 squeezed out and will reach the ocean. 22:29.267 --> 22:34.927 And then you'll break off those chunks as icebergs. 22:34.933 --> 22:38.073 But in any case, you've got to balance the mass by having 22:38.067 --> 22:42.227 this flow moving from the center towards the edge. 22:42.233 --> 22:45.673 And that's the way ice sheets work. 22:45.667 --> 22:47.227 That speed is not very great. 22:47.233 --> 22:53.133 It may be only a meter per day or even a meter per week. 22:53.133 --> 22:55.473 Those flows can be quite slow. 22:55.467 --> 23:00.127 But nevertheless, they balance this annual accumulation of 23:00.133 --> 23:04.473 snow on the high parts of the ice sheet. 23:04.467 --> 23:07.897 You can imagine then, if you have this spreading going on, 23:07.900 --> 23:12.030 there has to be some kind of a divide, just like when we did 23:12.033 --> 23:16.133 the river trip, we understood that there is a divide between 23:16.133 --> 23:17.733 water sheds. 23:17.733 --> 23:20.773 Rain that falls on one side goes into one river. 23:20.767 --> 23:23.297 Rain that falls on the other side goes into another river. 23:23.300 --> 23:24.770 The same is true with ice. 23:24.767 --> 23:27.967 It's called an ice divide, so that the snowflake that falls 23:27.967 --> 23:30.967 here will end up being squeezed out towards the west 23:30.967 --> 23:31.867 coast of Greenland. 23:31.867 --> 23:35.327 A snowflake that falls there will be squeezed out towards 23:35.333 --> 23:37.473 the east coast and the north and so on. 23:37.467 --> 23:39.527 And Antarctica has similar divides. 23:39.533 --> 23:41.433 They're drawn here. 23:41.433 --> 23:43.503 There's quite a number of them, actually. 23:43.500 --> 23:47.330 But each snowflake then will be squeezed out ultimately 23:47.333 --> 23:51.403 towards one part of the coast or the other depending where 23:51.400 --> 23:56.500 they fall relative to these ice divides. 23:56.500 --> 23:57.730 Any questions on that? 24:01.767 --> 24:03.997 So when you're up on the Greenland ice sheet, 24:04.000 --> 24:06.470 it looks like this. 24:06.467 --> 24:09.497 More or less continuous sheets of deep ice. 24:09.500 --> 24:13.670 There may be a kilometer or more of ice beneath you. 24:13.667 --> 24:15.927 Very deep ice. 24:15.933 --> 24:19.833 Occasionally you may get a rocky outcrop, a so-called 24:19.833 --> 24:22.933 nunatak, where a mountain ridge sticks 24:22.933 --> 24:24.673 up through the ice. 24:24.667 --> 24:27.497 But for the most part, there's very deep ice, maybe a 24:27.500 --> 24:31.770 kilometer or two of ice beneath you. 24:31.767 --> 24:35.667 And you don't have any sense of that slow creeping motion, 24:35.667 --> 24:37.667 at least not here where it's smooth. 24:41.267 --> 24:47.027 When the ice squeezes out to the edge, in some places when 24:47.033 --> 24:51.503 it reaches the sea it'll maintain its continuity as an 24:51.500 --> 24:55.870 ice sheet, but spread out and begin to float over the sea. 24:55.867 --> 24:57.967 Those are called ice shelves. 24:57.967 --> 25:00.467 And I've shown you the dominant ice shelves here for 25:00.467 --> 25:01.827 Antarctica. 25:01.833 --> 25:07.133 There's the Ross ice shelf, the Ronne, and the Larsen. 25:07.133 --> 25:10.903 And here's a zoom of the Antarctic Peninsula showing 25:10.900 --> 25:14.370 the Larsen ice sheet there. 25:14.367 --> 25:19.897 So while that's floating out over the ocean surface, do not 25:19.900 --> 25:21.800 confuse it with sea ice. 25:21.800 --> 25:24.500 It is still compacted snow. 25:24.500 --> 25:28.670 It's fresh water in compacted snow that's flowing out and 25:28.667 --> 25:31.497 floating over the ocean, being squeezed off the 25:31.500 --> 25:34.900 continent in that way. 25:34.900 --> 25:38.770 So it looks like this. 25:38.767 --> 25:40.867 At least the part you can see looks like this. 25:40.867 --> 25:45.027 But remember, there's another 90% or so. 25:45.033 --> 25:47.673 Most of it is beneath the water's surface because of 25:47.667 --> 25:49.727 buoyancy considerations. 25:49.733 --> 25:52.403 But that's an example of an ice shelf, a 25:52.400 --> 25:53.200 thick layer of ice. 25:53.200 --> 25:55.970 Here's a ship for scale. 25:55.967 --> 26:00.397 And basically, the ice is attached to the land. 26:00.400 --> 26:04.670 But eventually as it flows off it gets water underneath. 26:04.667 --> 26:05.997 It floats up. 26:06.000 --> 26:10.070 Remember, ice is less dense than water, so it floats on 26:10.067 --> 26:14.197 the water and flows on out as an ice shelf. 26:14.200 --> 26:18.000 And then eventually it breaks off to form icebergs. 26:18.000 --> 26:20.800 Questions there? 26:20.800 --> 26:25.100 Now, back in 19--in 2002, it was in the news that the 26:25.100 --> 26:27.800 Larsen ice shelf, or part of it, had collapsed. 26:27.800 --> 26:31.170 So here's a sequence of satellite images. 26:31.167 --> 26:35.767 January, 2002, February, late February, and then March. 26:35.767 --> 26:38.027 So here is the Larsen ice shelf. 26:38.033 --> 26:41.173 Remember, that's on the coast of the Antarctic Peninsula. 26:41.167 --> 26:43.127 I just showed that to you. 26:43.133 --> 26:46.673 It looks like there's already some weakening going on here. 26:46.667 --> 26:51.167 But then over the next month or so, most of that broke away 26:51.167 --> 26:54.827 and floated on off as icebergs. 26:54.833 --> 27:00.203 There's some argument in the scientific literature about 27:00.200 --> 27:03.870 the significance of the sudden loss of an 27:03.867 --> 27:05.227 ice shelf like this. 27:05.233 --> 27:07.573 Some would say, well, it doesn't really mean anything, 27:07.567 --> 27:10.427 because that ice has already left the continent. 27:10.433 --> 27:13.533 It's already floating over the sea. 27:13.533 --> 27:15.973 And it'll be replaced. 27:15.967 --> 27:18.267 In other words, in the natural flow of things, even in a 27:18.267 --> 27:22.767 steady state climate, this ice has to be replaced by new ice 27:22.767 --> 27:23.997 coming off the land. 27:24.000 --> 27:28.800 Others would say, no, a catastrophic loss like this 27:28.800 --> 27:31.770 will actually have an effect back over the land. 27:31.767 --> 27:35.567 And will accelerate the rate at which glaciers are flowing 27:35.567 --> 27:37.667 off the land onto the sea. 27:37.667 --> 27:41.367 So that argument's a little bit unresolved, but there 27:41.367 --> 27:44.967 probably is some climatic significance when you see a 27:44.967 --> 27:47.997 sudden collapse of an ice shelf like that. 27:48.000 --> 27:52.800 But it can be argued one way or the other. 27:52.800 --> 27:54.030 Questions there? 27:56.200 --> 28:02.030 Greenland doesn't have much in the way of ice shelves. 28:02.033 --> 28:03.733 There's glaciers being produced 28:03.733 --> 28:06.133 all along both coasts. 28:06.133 --> 28:08.533 But there's an area up here, called the Petermann Glacier 28:08.533 --> 28:15.433 that it might be argued has the nature of an ice shelf. 28:15.433 --> 28:16.633 There's the Petermann glacier. 28:16.633 --> 28:18.033 It certainly looks like that. 28:18.033 --> 28:20.873 It's got a broad, flat appearance. 28:20.867 --> 28:24.327 It's floating at this point. 28:24.333 --> 28:27.473 I'll show you the geometry of that here. 28:27.467 --> 28:28.067 So here we are. 28:28.067 --> 28:31.697 We're up in the northwest corner of Greenland. 28:31.700 --> 28:34.100 There's some barren land with no ice on it. 28:34.100 --> 28:38.630 But here comes a glacial stream, coming down 28:38.633 --> 28:40.933 from the ice sheet. 28:40.933 --> 28:43.573 And at some point, it does float out over water. 28:43.567 --> 28:50.127 And while it is confined to a fjord, it is floating as a 28:50.133 --> 28:52.273 more or less continuous ice sheet. 28:52.267 --> 28:54.567 And so I guess even though it's confined, it could be 28:54.567 --> 28:58.167 called an ice shelf. 28:58.167 --> 29:01.497 And then it will have a point where it breaks off and 29:01.500 --> 29:06.200 becomes icebergs at that point. 29:06.200 --> 29:11.600 And back just last year, in 2010, this was in the news, 29:11.600 --> 29:15.970 that a big chunk--between July and August of last year, a big 29:15.967 --> 29:18.367 chunk of the Petermann Glacier broke off. 29:18.367 --> 29:22.867 And then this big flat, huge, tabular iceberg drifted 29:22.867 --> 29:29.097 northwest into the Strait of Nares and ended up blocking 29:29.100 --> 29:31.770 that channel for a number of months. 29:36.267 --> 29:40.397 We're going to right on to icebergs then. 29:40.400 --> 29:41.170 OK. 29:41.167 --> 29:43.367 Here's what you need to know. 29:43.367 --> 29:48.327 The word--the verb for what happens when you take a 29:48.333 --> 29:50.833 floating chunk of ice and break it off to form an 29:50.833 --> 29:52.703 iceberg is called calve. 29:52.700 --> 29:57.800 So we say that icebergs are formed from calving. 29:57.800 --> 29:59.970 That can be either from mountain glaciers that have 29:59.967 --> 30:02.827 come down to the sea, or I've just given you several 30:02.833 --> 30:07.273 examples of where ice sheet glaciers come down to the sea. 30:07.267 --> 30:10.097 And then pieces can break off to form icebergs. 30:10.100 --> 30:13.570 Once they're floating, they drift under the influence of 30:13.567 --> 30:16.667 wind and ocean currents. 30:16.667 --> 30:20.427 And don't forget to indicate that the Coriolis force can 30:20.433 --> 30:22.103 also play a big role in this too. 30:22.100 --> 30:25.600 For example, if the wind pushes on an iceberg in that 30:25.600 --> 30:29.600 direction, it'll move off at right angles. 30:29.600 --> 30:33.230 So that's because of the Coriolis force. 30:33.233 --> 30:37.533 They float with about 90% of their volume under water. 30:37.533 --> 30:41.803 We'll work out the math for that in just a moment. 30:41.800 --> 30:43.900 That means there's a lot of ice underneath 30:43.900 --> 30:45.170 that you don't see. 30:45.167 --> 30:49.197 And if you're in a shallow part of the ocean, they can 30:49.200 --> 30:52.830 hit the bottom and get stuck. 30:52.833 --> 30:55.403 So very often you'll find icebergs that are not moving, 30:55.400 --> 30:57.700 even though the wind is pushing on them and the 30:57.700 --> 30:58.970 current is pushing on them. 30:58.967 --> 31:01.167 But they're just stuck in place. 31:01.167 --> 31:04.697 That would have to be because their bottom is touching. 31:04.700 --> 31:05.930 So they get stuck there. 31:08.500 --> 31:12.800 They tend to melt the fastest right around their water line. 31:12.800 --> 31:17.000 Right where the waves splash up against them you tend to 31:17.000 --> 31:20.530 get this rapid melting right around the water line. 31:20.533 --> 31:24.333 What that will do is change the shape of these icebergs by 31:24.333 --> 31:28.473 carving away these grooves around the water line. 31:28.467 --> 31:32.127 And at some point, their shape changes so much that their 31:32.133 --> 31:35.473 current orientation is no longer stable. 31:35.467 --> 31:39.427 And then, without warning, these things will suddenly 31:39.433 --> 31:41.103 begin to tip. 31:41.100 --> 31:45.270 And they'll roll to a new orientation and then stay that 31:45.267 --> 31:48.927 way for several weeks while melting continues around their 31:48.933 --> 31:51.803 new water line. 31:51.800 --> 31:53.030 And that's rather remarkable. 31:53.033 --> 31:56.803 If you get a chance to visit these latitudes and if you see 31:56.800 --> 32:00.730 a broad field of icebergs, and if you're patient and wait a 32:00.733 --> 32:03.333 few minutes, because there's so many of them, you'll 32:03.333 --> 32:06.903 probably see one or two of them do that roll. 32:06.900 --> 32:08.530 And that's the reason why you don't want 32:08.533 --> 32:09.373 to be close to them. 32:09.367 --> 32:11.867 Because when they roll, they'll put off a wave. And 32:11.867 --> 32:14.467 they would swamp you if you're in a small boat. 32:14.467 --> 32:20.197 That'll happen suddenly, without any warning. 32:20.200 --> 32:22.530 So I want to show this little calving movie. 32:22.533 --> 32:24.903 I found it on the web. 32:24.900 --> 32:28.530 And I want to be sure the sound is here. 32:32.800 --> 32:34.430 Let's see if I can get this to work. 32:38.267 --> 32:51.127 [VIDEO PLAYBACK] 32:51.133 --> 32:54.603 -Oh, look at it go. 32:54.600 --> 32:55.600 Oh my gosh. 32:55.600 --> 32:58.070 Cindy, you have to get a picture of that. 32:58.067 --> 32:59.327 [INAUDIBLE] 33:01.633 --> 33:02.873 -Wow. 33:07.600 --> 33:10.070 Woo-hoo. 33:10.067 --> 33:11.567 Look at all that. 33:17.533 --> 33:18.033 Fabulous. 33:18.033 --> 33:20.003 I'm glad we're not next to that. 33:20.000 --> 33:23.000 Holy cow. 33:23.000 --> 33:24.500 Oh my gosh. 33:28.967 --> 33:30.467 -I can feel that. 33:30.467 --> 33:32.427 -Oh my gosh. 33:35.433 --> 33:36.673 [INAUDIBLE] 33:47.333 --> 33:49.273 Oh my gosh. 33:49.267 --> 33:51.867 [END VIDEO PLAYBACK] 33:51.867 --> 33:53.327 PROFESSOR: I don't know what happened after that. 33:56.733 --> 33:59.203 So that's the calving process. 33:59.200 --> 34:01.070 So that ice has come out. 34:01.067 --> 34:03.897 And then eventually it has to break off pieces. 34:03.900 --> 34:06.970 And it happens kind of in a dramatic 34:06.967 --> 34:10.927 fashion, as you saw there. 34:10.933 --> 34:15.173 So from that bit of fun to the mathematics of the situation. 34:15.167 --> 34:17.997 So remember, Archimedes law. 34:18.000 --> 34:19.570 We've talked about it before in this course. 34:19.567 --> 34:23.197 It basically says that the buoyancy force pushing up on a 34:23.200 --> 34:28.830 submerged obstacle--submerged object is equal to the weight 34:28.833 --> 34:32.473 of the fluid displaced. 34:32.467 --> 34:37.367 So when you have an object like this, like an iceberg, 34:37.367 --> 34:40.267 floating in a stable configuration, there's going 34:40.267 --> 34:44.497 to be a balance of its own weight with the buoyancy force 34:44.500 --> 34:46.030 pushing it up. 34:46.033 --> 34:48.973 So I've left the acceleration of gravity off of both terms, 34:48.967 --> 34:50.827 but that would be the same in both cases. 34:50.833 --> 35:00.333 So the weight would be the total volume of the iceberg 35:00.333 --> 35:02.473 times the density of ice. 35:02.467 --> 35:05.967 That product will give you the mass in kilograms 35:05.967 --> 35:08.727 of the total iceberg. 35:08.733 --> 35:12.073 And then that's going to be balanced by a buoyancy force 35:12.067 --> 35:16.427 pushing up, which is the volume submerged it's only 35:16.433 --> 35:21.473 this part multiplied by the density 35:21.467 --> 35:23.427 of the fluid displaced. 35:23.433 --> 35:26.433 Well, it's displacing seawater. 35:26.433 --> 35:28.203 So we use the density of seawater. 35:28.200 --> 35:32.370 So that's the statement of equilibrium for 35:32.367 --> 35:35.697 this floating object. 35:35.700 --> 35:41.070 If I simply divide through, I can get this expression. 35:41.067 --> 35:46.267 The ratio of the submerged part of the iceberg to the 35:46.267 --> 35:50.827 total is simply going to be dividing through the ratio of 35:50.833 --> 35:55.403 the two densities, rho ice over rho of seawater. 35:55.400 --> 35:57.970 And we've already used these values before. 35:57.967 --> 36:02.167 The density of ice is about 917 kilograms per cubic meter. 36:02.167 --> 36:05.297 Seawater, with the salt, is about 1,025. 36:05.300 --> 36:06.830 Divide that out, you get 0.9. 36:06.833 --> 36:12.473 So the point is the ratio of this part to the total is 36:12.467 --> 36:15.027 about 0.9, or 90%. 36:15.033 --> 36:24.073 So that is a solid derivation of why it is that most of the 36:24.067 --> 36:27.167 iceberg is below sea level there. 36:27.167 --> 36:28.397 Any questions on that? 36:33.100 --> 36:37.070 Now, a very interesting place on the west coast of Greenland 36:37.067 --> 36:38.297 is Jacobshaven. 36:40.467 --> 36:43.497 So here is the ice sheet, the Greenland Ice Sheet out to the 36:43.500 --> 36:48.800 east. There's Baffin Bay to the west. And the glacier 36:48.800 --> 36:49.800 comes down here. 36:49.800 --> 36:54.330 But here is indicated where that calving front was in 36:54.333 --> 36:59.003 1851, 1875, and back to 2003. 36:59.000 --> 37:03.070 So it's been retreating rather dramatically. 37:03.067 --> 37:05.197 But I want to make something clear to you, that even when 37:05.200 --> 37:08.800 this retreats all the way back here this is where the calving 37:08.800 --> 37:11.830 is now taking place still, this channel 37:11.833 --> 37:13.103 is choked with ice. 37:13.100 --> 37:15.070 It's not open water. 37:15.067 --> 37:17.867 It's got lots of icebergs sitting in it, some of them 37:17.867 --> 37:19.167 making their way out. 37:19.167 --> 37:23.227 Some of them grounded and kind of stuck there. 37:23.233 --> 37:25.873 But the calving front is back there. 37:25.867 --> 37:29.297 So again the question is, is this a significant change? 37:29.300 --> 37:32.870 If the channel with the fjord is still choked with ice, does 37:32.867 --> 37:36.697 it make any difference whether the calving is here or the 37:36.700 --> 37:39.000 calving is I would think it would make a difference. 37:39.000 --> 37:41.870 Because again, if you're calving back here, that's 37:41.867 --> 37:46.327 going to allow a faster flow down from the ice sheet. 37:46.333 --> 37:49.173 You're going to be moving more ice from the Greenland Ice 37:49.167 --> 37:52.427 Sheet if the calving is back here than if it has to go all 37:52.433 --> 37:56.033 the way up here before it can calve off. 37:56.033 --> 37:59.873 I was there last year and took some pictures. 37:59.867 --> 38:02.597 There's nothing for scale here, but that's about 40 38:02.600 --> 38:04.800 meters high. 38:04.800 --> 38:08.070 And then when you think what lies below that oh, and I'm 38:08.067 --> 38:09.497 looking across the channels. 38:09.500 --> 38:12.000 Through the little gap there you can see the land on the 38:12.000 --> 38:15.770 other side of that fjord. 38:15.767 --> 38:16.997 But those are pretty impressive 38:17.000 --> 38:19.600 glaciers, just the part-- 38:19.600 --> 38:22.200 icebergs, just the part that are above water. 38:22.200 --> 38:24.300 Here's a small ship for scale. 38:24.300 --> 38:26.230 You can get some idea. 38:26.233 --> 38:28.873 Has anybody seen something like this, traveled to high 38:28.867 --> 38:31.097 latitudes and seen something like this? 38:31.100 --> 38:31.830 Where were you? 38:31.833 --> 38:32.633 STUDENT: Antarctica. 38:32.633 --> 38:33.373 PROFESSOR: Antarctica. 38:33.367 --> 38:33.867 Nice. 38:33.867 --> 38:35.627 Yes. 38:35.633 --> 38:40.433 So they look a bit different in Antarctica, but the physics 38:40.433 --> 38:41.403 is the same. 38:41.400 --> 38:44.300 It depends on the shape of the ice, how much it's crumpled as 38:44.300 --> 38:46.370 it comes down before it breaks off. 38:46.367 --> 38:49.227 But basically, you've got the same process happening in both 38:49.233 --> 38:50.473 hemispheres. 38:53.500 --> 38:56.030 The other reason I wanted to show you this particular 38:56.033 --> 39:02.833 location, Jacobshaven, is that there's good reason to think 39:02.833 --> 39:08.973 that it was a Jacobshaven iceberg that sunk the Titanic. 39:08.967 --> 39:12.767 And I want to show you the geometry of that. 39:12.767 --> 39:20.527 So here is Jacobshaven, on the west coast of Greenland. 39:20.533 --> 39:25.073 And if an iceberg breaks off there, it'll be carried first 39:25.067 --> 39:27.127 northward by the West Greenland Current. 39:27.133 --> 39:29.473 Then at some point it'll get drawn into the Labrador 39:29.467 --> 39:32.667 current, come down past if it's large enough. 39:32.667 --> 39:34.527 It will be melting as it comes down through here. 39:34.533 --> 39:37.933 But if it's large enough, it will persist and possibly get 39:37.933 --> 39:44.333 down into this area south of 50 north, and maybe even south 39:44.333 --> 39:47.433 of 45 degrees north, down in this area. 39:47.433 --> 39:51.233 And at that point now, it becomes into the great circle 39:51.233 --> 39:57.603 route for ships between say New York and England. 39:57.600 --> 40:04.970 And that's what the Titanic was doing in April of 1912. 40:04.967 --> 40:07.467 It was making its way from well, it started in 40:07.467 --> 40:12.227 Southampton, England, and was making its way to New York. 40:12.233 --> 40:14.073 And that's where it hit the iceberg and sunk. 40:14.067 --> 40:16.967 So almost certainly it came from the 40:16.967 --> 40:18.097 west coast of Greenland. 40:18.100 --> 40:22.970 And because Jacobshaven is known to be the most prolific 40:22.967 --> 40:27.927 generator of icebergs, I guess it's a fair guess that it was 40:27.933 --> 40:31.933 one of those Jacobshaven icebergs that was the one that 40:31.933 --> 40:36.503 did the deed for the Titanic. 40:36.500 --> 40:37.730 Any questions on that? 40:41.700 --> 40:44.800 Now we'll turn to mountain glaciers. 40:44.800 --> 40:49.700 And there again, a bit like the ice sheets, you have an 40:49.700 --> 40:53.700 accumulation zone and an ablation zone. 40:53.700 --> 40:57.670 So every year, full 12 months that goes by, you've added a 40:57.667 --> 41:00.227 bit of ice here. 41:00.233 --> 41:02.573 And that cannot continue. 41:02.567 --> 41:04.927 You've got to balance that mass in some way. 41:04.933 --> 41:08.973 And that is eventually balanced by a flow, a 41:08.967 --> 41:12.967 liquid-like flow down the mountain slope 41:12.967 --> 41:14.367 by the glacier itself. 41:14.367 --> 41:19.327 Some of it is accomplished by distorting the ice. 41:19.333 --> 41:22.273 And some of it is accomplished by just sliding the ice along 41:22.267 --> 41:23.667 the bottom. 41:23.667 --> 41:26.467 But in any case, it is moving under the 41:26.467 --> 41:29.167 influence of gravity. 41:29.167 --> 41:32.727 And then in some cases, it makes it all the way down to 41:32.733 --> 41:34.173 the ocean surface. 41:34.167 --> 41:35.527 Question, yes? 41:35.533 --> 41:36.903 STUDENT: Can you explain what you mean by 41:36.900 --> 41:38.270 distorting the ice? 41:38.267 --> 41:39.567 PROFESSOR: So if you do a viscous 41:39.567 --> 41:41.027 experiment, if you-- 41:41.033 --> 41:44.733 let me get the lights on for a second. 41:44.733 --> 41:54.973 If you tilt your pancake and pour that syrup on it, you'll 41:54.967 --> 42:00.397 find that the syrup is not slipping along the bottom. 42:00.400 --> 42:04.100 It actually has a shear within it. 42:07.633 --> 42:09.533 Icebergs do--I'm sorry, glaciers do some of this, but 42:09.533 --> 42:12.433 they also slip a little bit along the base. 42:12.433 --> 42:14.673 So it would be more accurate to draw it like this. 42:14.667 --> 42:17.067 There's some internal deformation, but there's also 42:17.067 --> 42:18.897 some slip right at the bottom. 42:18.900 --> 42:23.070 So it's a combination of the two that allows it to slide 42:23.067 --> 42:26.197 down the slope like that. 42:28.833 --> 42:29.333 Thanks for that. 42:29.333 --> 42:37.733 So in this case, for these two glaciers in Alaska, they've 42:37.733 --> 42:40.403 actually made it all the way down to sea level. 42:40.400 --> 42:41.700 This one easily. 42:41.700 --> 42:43.870 This one just barely. 42:43.867 --> 42:47.767 But any glacier that makes it down to sea level is called a 42:47.767 --> 42:49.567 tidewater glacier. 42:49.567 --> 42:55.027 It makes it down, and then partly because of the tides 42:55.033 --> 42:57.673 but it would happen anyway you'll get calving, and you 42:57.667 --> 42:58.727 produce icebergs. 42:58.733 --> 43:02.873 So a tidewater glacier will produce icebergs that will 43:02.867 --> 43:06.267 then float out over the sea. 43:06.267 --> 43:08.527 You're likely to have these in high latitudes. 43:08.533 --> 43:10.373 Remember, you can have mountain glaciers in low 43:10.367 --> 43:11.597 latitudes too. 43:14.333 --> 43:19.603 Mount Kilimanjaro in Africa, almost exactly on the equator, 43:19.600 --> 43:21.570 has a glacier at the top. 43:21.567 --> 43:23.297 But it certainly doesn't come down very far. 43:23.300 --> 43:25.870 And it certainly does not reach any ocean where you 43:25.867 --> 43:28.527 would produce icebergs. 43:28.533 --> 43:33.573 But these tidewater glaciers do produce icebergs. 43:33.567 --> 43:35.997 Here's an example in Alaska of one that does 43:36.000 --> 43:37.430 not, Stevens Glacier. 43:37.433 --> 43:42.273 So you can see glaciers from various parts of the mountain 43:42.267 --> 43:44.697 chain combining and merging. 43:44.700 --> 43:46.800 Here is the last merger right here. 43:46.800 --> 43:49.700 But then there's the terminus of the glacier. 43:49.700 --> 43:52.800 There is some melt water and the river will carry that 43:52.800 --> 43:54.030 water away to the sea. 43:54.033 --> 43:57.873 The water will get to the sea, but not the ice itself. 43:57.867 --> 44:01.397 So this is happening more and more. 44:01.400 --> 44:04.500 As glaciers--as the climate warms and glaciers retreat, 44:04.500 --> 44:06.400 their terminus moves further up. 44:06.400 --> 44:09.300 But eventually that water, of course, always 44:09.300 --> 44:12.070 returns to the sea. 44:15.067 --> 44:16.767 Very often you can see the seam. 44:16.767 --> 44:19.497 Where two glaciers come together you have a little bit 44:19.500 --> 44:21.630 of rock rubble. 44:21.633 --> 44:25.803 And in some glaciers you can see way down at their snout 44:25.800 --> 44:29.630 you can see five or six of these little black lines. 44:29.633 --> 44:33.033 And if you trace them back, each one of them originates at 44:33.033 --> 44:33.973 a point like this. 44:33.967 --> 44:38.227 So you can actually keep track of all the tributaries by the 44:38.233 --> 44:45.103 little line of rock rubble that gets permanently put into 44:45.100 --> 44:47.330 the glacier. 44:47.333 --> 44:48.503 Questions there? 44:50.500 --> 44:53.630 So again, the speed of these things are quite slow. 44:53.633 --> 44:59.073 They might be only a few centimeters per day or a few 44:59.067 --> 45:01.467 centimeters per week, in some cases. 45:01.467 --> 45:05.767 But what they can do is drive a stake here, go up on the 45:05.767 --> 45:08.427 bedrock, triangulate, get the location. 45:08.433 --> 45:11.403 And then come back a week or two later and see how much 45:11.400 --> 45:12.630 that stake has moved. 45:12.633 --> 45:15.473 And that will at least give you the speed of the ice at 45:15.467 --> 45:16.927 the top of the glacier. 45:16.933 --> 45:19.503 It won't give you everything you need to know about the 45:19.500 --> 45:21.730 depth all the way through, the speed 45:21.733 --> 45:26.233 down through the glacier. 45:26.233 --> 45:31.033 Here's a couple of glaciers from Norway. 45:33.667 --> 45:41.127 The word, well, "breen" simply means ice or glacier, and so 45:41.133 --> 45:46.103 all the glaciers in Norway end with the suffix "-breen." But 45:46.100 --> 45:49.800 there's the one in Briksdal, and there's the one in Jenn 45:49.800 --> 45:51.670 and of course, "dal" just means "valley" in Norwegian, 45:51.667 --> 45:56.097 so Jennsdals glacier and Briks valley glacier. 45:56.100 --> 45:57.070 They're all named like that. 45:57.067 --> 46:03.197 But these are not tidewater glaciers. 46:03.200 --> 46:04.300 This is not the sea. 46:04.300 --> 46:07.470 This is a little lake at the bottom of the glacier, a 46:07.467 --> 46:08.727 freshwater lake. 46:08.733 --> 46:11.973 And it has that milky appearance to it because 46:11.967 --> 46:15.497 underneath the glacier, where there's a grinding process 46:15.500 --> 46:20.270 going on, you're developing a kind of powder from the 46:20.267 --> 46:21.367 grinding of rock. 46:21.367 --> 46:22.897 And that ends up in the water. 46:22.900 --> 46:26.100 And then if you see that river, even miles away, it'll 46:26.100 --> 46:30.200 have that characteristic milky appearance. 46:30.200 --> 46:32.430 And you can say, that one came from a glacier. 46:32.433 --> 46:36.533 That river water came from a glacier, because it has that 46:36.533 --> 46:40.603 so-called glacial milk and it's characteristic of water 46:40.600 --> 46:43.730 that flows out from a mountain glacier like that. 46:46.367 --> 46:49.597 Questions there? 46:49.600 --> 46:51.230 Here's another one. 46:51.233 --> 46:54.503 You can see the terminus of that and the water rushing out 46:54.500 --> 46:58.330 from underneath in that stream. 47:03.167 --> 47:06.997 We are, let's see, yeah... I guess we'll quit here 47:07.000 --> 47:09.570 and just finish up. 47:09.567 --> 47:12.467 There's just a little bit left on ice, but I want to do a 47:12.467 --> 47:13.127 good job on that. 47:13.133 --> 47:15.403 So we'll wait until Monday and finish that up.