GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 24.?Ice in the Climate System

https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-24

Sea Ice [00:01:16]

So let's start with sea ice. And what I've shown you here are some diagrams that illustrate how sea ice changes with season in the two hemispheres. Now remember, this is frozen seawater. So about this time of year, late October, you begin to get cold air blowing over the sea surface in the high northern latitudes.?

And as the cold air blows over the ocean surface, it draws heat out of the ocean, cools it down. When you get the temperature down to about what? Not zero, but about minus two Celsius, because the salt depresses the freezing point by that much, then you start to get sea ice forming. It starts usually in small little patches of ice called pancake ice.?And then as the season progresses, you freeze the water in between and those little pieces of ice stick together.

And a month or two later, you've got a more or less continuous layer of sea ice in these high latitude regions. And then during the winter, it deepens. In other words, you're drawing heat out of the bottom and you're freezing more ocean water onto the bottom. So the thickness of the sea ice increases.

So here we are in September, which is at the end of the warming season. That would be the minimum in the sea ice. And yet there is some sea ice remaining from the previous year. Then we begin to build new sea ice. And about six months later, this is typically--or at least was--we'll talk about the fact that it's changing with time. But this is typically how far sea ice extends at the end of the cooling season.

Then it begins to warm again. And over the summer, you melt away a lot of this. And you end up back towards the minimum. So there's a big seasonality in sea ice. And this is the way we understand it. In March, when you've got maximum sea ice, you have a stream of sea ice coming down on the east side of Greenland. Most of Baffin Bay is filled with sea ice. All of Hudson's Bay and James Bay is filled with sea ice. And really, it even comes out a bit into the North Pacific.

But this area stays generally free of sea ice. And you know why that is, because you remember that the--part of the Gulf Stream splits off and moves up into that region. So even at the maximum of sea ice, you still have open water along the coast of Norway and even up here in an area that's called North Cape.

Now, of course, in the southern hemisphere, the seasons are reversed. And so it's in March that you'll have the minimum of sea ice, because that's when you're at the end of the warming season. And it pretty much drops to zero. In other words, you have a little bit of permanent sea ice remaining, but really very little. And then six months later, at the end of the cooling season, you've grown quite a remarkable area of sea ice, by, again, freezing seawater.

Now I want you to notice something. In the Arctic Ocean, it's ocean at the North Pole. And it's confined by continent surround. And you typically fill that up. And you can't go any further, of course, because you can't make any sea ice over the land.

So if you're going to monitor this sea ice year by year, the way it's typically done in the Northern Hemisphere is to monitor the minimum sea ice, which is in September. Maximum wouldn't make much sense, because you're not going to fill that in with ice. And you've already taken it shore to shore in the Arctic Ocean. So the way you monitor interannual variations, or trends, is to keep track of the minimum.

On the other hand, in the Southern Hemisphere, the minimum is pretty much zero. You have a continent filling the South Pole. And you're not going to form any sea ice there. And the sea ice itself pretty much goes back to that shoreline. So the way we monitor sea ice in the Southern Hemisphere typically is by the maximum sea ice that you grow. Notice, then, that that means September is kind of the month where we monitor sea ice in both hemispheres, even though the seasonality is opposite in the two hemispheres.?

Pancake ice looks like this. So when the winter is beginning, and the cold air is drawing heat out of the ocean, for a period of some days or weeks, you have little chunks of ice that are from half a meter to a couple meters in diameter. They have rounded edges, because with the wave action, they keep bouncing into each other breaking off any sharp corners.

And then a few weeks later you begin to freeze and get a more continuous ice. This happens to be continuous sea ice with compression ridges. In other words, when you look at these features like this or that, that's an area where the wind has blown the sea ice together and it has crumpled along a line.

The ridges will get typically only four or five meters high. But remember, they've got a root which you don't see. But in order to support that extra weight, they also have a root that goes down beneath the ice. So if you measured it from top to bottom, it might be 20 or 30 meters, but the part you have to climb over is just 3 or 4 meters.

Here's some examples of thickness.?The mean, from 1982 to 2000--now it's difficult to measure ice thickness. There are various ways to do it.?The earliest way was to take a submarine. In the late '60s they finally got nuclear submarines that could pass under the ice sheet. And then with an up-looking sonar you could measure the bottom and the top of the ice and get a thickness measurement that way.

You can't grow ice that thick thermodynamically?by freezing at the bottom. Because after it gets to a certain thickness, remember you have to take the heat out the top. That's where the atmosphere is cold. And yet you've got to freeze the water at the bottom. And after a while, it becomes too hard to get that heat to conduct out through that layer of sea ice. So you can only grow sea ice thermodynamically to this kind of thickness. This is a mechanical stacking process.

And you can understand that if you remember that across the Arctic Ocean there is a current called the Transpolar Drift. And also the Beaufort Gyre. But that's going to be carrying ice and pushing it up against the north coast of Greenland and along the Canadian Archipelago, and then causing that compression ridge to form, those compression ridges. And that deepens the average ice thickness to as great as four meters, in other words, typically a little higher than the ceiling in this room.?

Here's what's been going on the last couple of decades. So this is the Northern Hemisphere extent anomaly. What they do is take a measurement and then subtract off a mean value to just emphasize the changes that have occurred. And this is expressed in percent.?They've taken 1979 to the year 2000 as their reference time, subtracted off the mean, and then here's what's left as the residual or as the anomaly.

Basically, in the earlier part of the time it was--had a positive anomaly by definition, as it's decreasing. And then near the end you get a negative anomaly. So really the change has been remarkable. Overall that's about a 40%--if you count this, maybe even a 50% decrease in the area covered by ice in September. That's in the minimum period, after the warming season.

So the Southern Hemisphere,?again September. This is the maximum, because the minimum doesn't make any sense. It pulls right back pretty much to the shores of Antarctica. That's pretty flat. Or, if you looked at it very closely, it's even increasing ever so slightly. And so this trend in sea ice that we see in the Northern Hemisphere is not found in the Southern Hemisphere. When we're talking about things like global warming, we'll come back and refer to measurements such as this one.

But remember, this has a somewhat different meaning, because this is the maximum extent of sea ice. Whereas the other one, for the Northern Arctic Ocean was the minimum extent of sea ice.

And of course what's also in the news is that now, as in the Arctic Ocean, you get a retreat of that minimum sea ice in September. You've opened up these two long-sought navigational routes. For years and years in the 17th and 18th and 19th centuries, sailors were seeking the Northwest Passage, trying to find a way to get between the Atlantic and the Pacific without going all the way under Cape Horn. And this is what they were trying to do. They never achieved it, but now today you can. There is a month or so in the summer when you can take a ship through the Canadian Archipelago free of ice and transport back and forth.

So navigation is now becoming possible in the Arctic Ocean. And so is things like oil drilling. So there's been a lot of discussion about who's going to lead the charge to now developing oil resources in the Arctic Ocean. It's quite a change going on up there.

Ice Sheets [00:20:34]

So now we turn to ice sheets, which is generally defined as a large plateau of ice. First of all, on the high altitude parts of the ice sheet--this happens to be Greenland, but it would be the same for Antarctica--you have an excess of accumulation every year. So more snow falls during the winter than would melt off if indeed there's any melt at all in the summertime. And so year after year, you just accumulate.

You have to balance that. And what happens is that, under its own weight, it begins to sag and to flow. So the snow gets compressed into more or less solid ice.?And then that ice itself, under high pressure conditions with a lot of weight above it, will begin to flow out toward the edges of the continent.?

Then near the edge, there could be some loss by melting or direct sublimation. Sublimation is when you go directly from ice to vapor. But if that's not sufficient, then the ice will actually be squeezed out and will reach the ocean. And then you'll break off those chunks as icebergs.

But in any case, you've got to balance the mass by having this flow moving from the center towards the edge. And that's the way ice sheets work. That speed is not very great. It may be only a meter per day or even a meter per week. Those flows can be quite slow. But nevertheless, they balance this annual accumulation of snow on the high parts of the ice sheet.

It's called an ice divide, so that the snowflake that falls here will end up being squeezed out towards the west coast of Greenland. A snowflake that falls there will be squeezed out towards the east coast and the north and so on.?Antarctica has similar divides. Each snowflake then will be squeezed out ultimately towards one part of the coast or the other depending where they fall relative to these ice divides.?

Ice Shelves [00:24:41]

When the ice squeezes out to the edge, in some places when it reaches the sea it'll maintain its continuity as an ice sheet, but spread out and begin to float over the sea. Those are called ice shelves.?And I've shown you the dominant ice shelves here for Antarctica. There's the Ross ice shelf, the Ronne, and the Larsen. And here's a zoom of the Antarctic Peninsula showing the Larsen ice sheet there.?While that's floating out over the ocean surface, do not confuse it with sea ice. It is still compacted snow. It's fresh water in compacted snow that's flowing out and floating over the ocean, being squeezed off the continent in that way.

There's another 90% or so. Most of it is beneath the water's surface because of buoyancy considerations. But that's an example of an ice shelf, a thick layer of ice. Here's a ship for scale.?And basically, the ice is attached to the land. But eventually as it flows off it gets water underneath. It floats up. Remember, ice is less dense than water, so it floats on the water and flows on out as an ice shelf. And then eventually it breaks off to form icebergs.?

In 2002, it was in the news that the Larsen ice shelf, or part of it, had collapsed. So here's a sequence of satellite images. January, 2002, February, late February, and then March.?So here is the Larsen ice shelf. Remember, that's on the coast of the Antarctic Peninsula.?

Greenland doesn't have much in the way of ice shelves. There's glaciers being produced all along both coasts. But there's an area up here, called the Petermann Glacier that it might be argued has the nature of an ice shelf. We're up in the northwest corner of Greenland. There's some barren land with no ice on it. But here comes a glacial stream, coming down from the ice sheet. And at some point, it does float out over water.

And while it is confined to a fjord, it is floating as a more or less continuous ice sheet. Even though it's confined, it could be called an ice shelf. And then it will have a point where it breaks off and becomes icebergs at that point.?And back just last year, in 2010, this was in the news, that a big chunk--between July and August of last year, a big chunk of the Petermann Glacier broke off. And then this big flat, huge, tabular iceberg drifted northwest into the Strait of Nares and ended up blocking that channel for a number of months.

Icebergs [00:29:36]

We're going to right on to icebergs then. The?verb for what happens when you take a floating chunk of ice and break it off to form an iceberg is called calve. So we say that icebergs are formed from calving. That can be either from mountain glaciers that have come down to the sea, or I've just given you several examples of where ice sheet glaciers come down to the sea. And then pieces can break off to form icebergs.

Once they're floating, they drift under the influence of wind and ocean currents. And don't forget to indicate that the Coriolis force can also play a big role in this too. For example, if the wind pushes on an iceberg in that direction, it'll move off at right angles. So that's because of the Coriolis force.

They float with about 90% of their volume under water. That means there's a lot of ice underneath that you don't see. And if you're in a shallow part of the ocean, they can hit the bottom and get stuck. So very often you'll find icebergs that are not moving, even though the wind is pushing on them and the current is pushing on them. But they're just stuck in place. That would have to be because their bottom is touching.?

They tend to melt the fastest right around their water line. Right where the waves splash up against them you tend to get this rapid melting right around the water line. What that will do is change the shape of these icebergs by carving away these grooves around the water line.

And at some point, their shape changes so much that their current orientation is no longer stable. And then, without warning, these things will suddenly begin to tip. And they'll roll to a new orientation and then stay that way for several weeks while melting continues around their new water line. And that's rather remarkable.

Archimedes law. It basically says that the buoyancy force pushing up on a submerged obstacle—submerged object is equal to the weight of the fluid displaced.

So when you have an object?like an iceberg, floating in a stable configuration, there's going to be a balance of its own weight with the buoyancy force pushing it up. So I've left the acceleration of gravity off of both terms, but that would be the same in both cases. So the weight would be the total volume of the iceberg times the density of ice. That product will give you the mass in kilograms of the total iceberg.?And then that's going to be balanced by a buoyancy force pushing up, which is the volume submerged--it's only this part--multiplied by the density of the fluid displaced. Well, it's displacing seawater. So we use the density of seawater. So that's the statement of equilibrium for this floating object.

Now, a very interesting place on the west coast of Greenland is Jacobshaven. So here is the ice sheet, the Greenland Ice Sheet out to the east. There's Baffin Bay to the west. And the glacier comes down here. But here is indicated where that calving front was in 1851, 1875, and back to 2003. So it's been retreating rather dramatically.

Even when this retreats all the way back here--this is where the calving is now taking place--still, this channel is choked with ice. It's not open water. It's got lots of icebergs sitting in it, some of them making their way out. Some of them grounded and kind of stuck there. But the calving front is back there.

If the channel with the fjord is still choked with ice, does it make any difference whether the calving is here or the calving is--I would think it would make a difference. If you're calving back here, that's going to allow a faster flow down from the ice sheet. You're going to be moving more ice from the Greenland Ice Sheet if the calving is back here than if it has to go all the way up here before it can calve off.

The other reason I wanted to show you this particular location, Jacobshaven, is that there's good reason to think that it was a Jacobshaven iceberg that sunk the?Titanic.?

Mountain Glaciers [00:41:54]

Now we'll turn to mountain glaciers. And there again, a bit like the ice sheets, you have an accumulation zone and an ablation zone. So every year, full 12 months that goes by, you've added a bit of ice here. And that cannot continue. You've got to balance that mass in some way. And that is eventually balanced by a flow, a liquid-like flow down the mountain slope by the glacier itself.

Some of it is accomplished by distorting the ice. And some of it is accomplished by just sliding the ice along the bottom. But in any case, it is moving under the influence of gravity. And then in some cases, it makes it all the way down to the ocean surface.?

So in this case, for these two glaciers in Alaska, they've actually made it all the way down to sea level. This one easily. This one just barely. But any glacier that makes it down to sea level is called a tidewater glacier. It makes it down, and then partly because of the tides--but it would happen anyway--you'll get calving, and you produce icebergs. So a tidewater glacier will produce icebergs that will then float out over the sea.

You're likely to have these in high latitudes. You can have mountain glaciers in low latitudes too. Mount Kilimanjaro in Africa, almost exactly on the equator, has a glacier at the top. But it certainly doesn't come down very far. And it certainly does not reach any ocean where you would produce icebergs. But these tidewater glaciers do produce icebergs.

Here's an example in Alaska of one that does not, Stevens Glacier. So you can see glaciers from various parts of the mountain chain combining and merging. Here is the last merger right here. But then there's the terminus of the glacier. There is some melt water and the river will carry that water away to the sea. The water will get to the sea, but not the ice itself. So this is happening more and more. As glaciers--as the climate warms and glaciers retreat, their terminus moves further up. But eventually that water, of course, always returns to the sea.

Very often you can see the seam. Where two glaciers come together you have a little bit of rock rubble. And in some glaciers you can see way down at their snout you can see five or six of these little black lines. And if you trace them back, each one of them originates at a point like this. So you can actually keep track of all the tributaries by the little line of rock rubble that gets permanently put into the glacier.?

So again, the speed of these things are quite slow. They might be only a few centimeters per day or a few centimeters per week, in some cases. But what they can do is drive a stake here, go up on the bedrock, triangulate, get the location. And then come back a week or two later and see how much that stake has moved. And that will at least give you the speed of the ice at the top of the glacier.?

Here's a couple of glaciers from Norway. The word, well, "breen" simply means ice or glacier, and so all the glaciers in Norway end with the suffix "-breen." But there's the one in Briksdal, and there's the one in Jenn--and of course, "dal" just means "valley" in Norwegian, so Jennsdals glacier and Briks valley glacier. They're all named like that.

But these are not tidewater glaciers. This is not the sea. This is a little lake at the bottom of the glacier, a freshwater lake. And it has that milky appearance to it because underneath the glacier, where there's a grinding process going on, you're developing a kind of powder from the grinding of rock. And that ends up in the water.

And then if you see that river, even miles away, it'll have that characteristic milky appearance. And you can say, that one came from a glacier. That river water came from a glacier, because it has that so-called glacial milk in it that’s characteristic of water that flows out from a mountain glacier like that.?