from Climate Alert Volume 8, No. 3 May-June 1995

Greenhouse Warming Could Pop the Cork On a
Bottled-up East Antarctic Ice Sheet

The following is a transcript of an interview with Professor Terence J. Hughes,
Institute for Quaternary Studies, University of Marine, Orono, Maine on July 23, 1995

The West Antarctic Ice Sheet, now only about 10 percent of the size of the East Antarctic Ice Sheet, was one-third of Antarctica roughly between 14 - 20,000 years ago. Two-thirds of that has collapsed. But there's still enough ice in the West Antarctic Ice Sheet to raise sea level by 6 meters if it disappeared; in the East Antarctic there's enough to raise it by 60 meters.

The question arises why hasn't the East Antarctic Ice Sheet undergone a collapse similar to that of the West Antarctic Ice Sheet? We cannot dismiss the possibility that greenhouse warming could cause something similar in the decades ahead, because we just don't know enough about the East Antarctic Ice Sheet; the knowledge is not available.

The West Antarctic Ice Sheet probably collapsed in two areas: one in the Ross Sea producing the Ross Sea embayment (a shallow sea that floods part of a continent, like Hudson Bay in Canada) where the Ross ice shelf is floating; and the other in the Weddell Sea embayment where the Ronne-Filchner ice shelves are floating. In between these two embayments is the Antarctic Peninsula where ice shelves have been disintegrating recently.

The East and the West Antarctic Sheets are separated (and were even when the West Antarctic Ice Sheet was three times bigger than it is now) by the Transantarctic Mountains. Because of the collapse that created the Ross and Weddell Sea embayments, ice from East Antarctica that had been abutting the West Antarctic ice sheet now discharges directly into those two embayments through gaps in the Transantarctic Mountains

The West Antarctic Ice Sheet is like a cork in a bottle. The bottle is the East Antarctic Ice Sheet and the bottleneck is the narrowest part of the West Antarctic Ice Sheet that today is buttressing the East Antarctic Ice Sheet between the Ross and Weddell Sea embayments. It happens to be at the part of the Transantarctic Mountains that is not a continuous mountain range. There are large gaps in the bedrock of the bottleneck that go clear down to sea level and even below .

The collapse of the West Antarctic Ice Sheet would leave the East Antarctic Ice Sheet - in an area just 3 or 4 degrees from the South Pole, 3000 meters high - with no support, no buttressing. The East Antarctic Ice Sheet would be destabilized in quite a dramatic way. If the remaining one-third of the West Antarctic Ice Sheet collapsed, there would be very wide gaps in the Transantarctic Mountains where the East Antarctic ice would come flooding into the sea. So collapse of the West Antarctic Ice Sheet is not important just for the 6 meters that it would raise global sea level. It would also free a large area of the East Antarctic ice that would come into the sea too. I have no idea what fraction of the 60 meters of sea level trapped in East Antarctica could be released through those gaps in the mountains.

The East Antarctic Ice Sheet ends in the sea in the Indian Ocean, in the westernmost Pacific Ocean and in the easternmost Atlantic Ocean. Like the West Antarctic Ice Sheet, it is drained by ice streams which are fast currents of ice near the edge of the ice sheet that discharge about 90 percent of the ice. They are very much like the rivers of North America that drain the precipitation over the continent back into the sea: the Mississippi, the Rio Grande, the Colorado, the Columbia, the Fraser, the Yukon, the Mackenzie and the St. Lawrence Rivers. These rivers and smaller ones flowing into Hudson Bay would probably return something like 90 percent of the water falling over North America back into the sea. The ice sheets have the same role when they discharge the snow precipitation over Antarctica back into the sea. If both the East and West Antarctic Ice Sheets were in stable equilibrium, they would be discharging ice back into the sea as icebergs at a rate that would equal the whole precipitation all over the continent in terms of the volume of ice.

Today we have something called the West Antarctic Ice Sheet Program. [See West Antarctic Ice Sheet Program (WAIS) box.] One of the things that gives this West Antarctic Ice Sheet Program some sense of urgency is greenhouse warming. The warming is moving down the Antarctic Peninsula and is probably responsible for some of these ice shelves on both sides of the Peninsula breaking up in recent years. (See accompanying story on the calving of icebergs on the Antarctic Peninsula.)

This warming is moving down the Peninsula toward the heart of the West Antarctic Ice Sheet. But the margin of the East Antarctic Ice Sheet that is on the Indian Ocean and parts of the Pacific and Atlantic Oceans is closer to the equator than the northernmost coastline of the West Antarctic Ice Sheet. Therefore the East Antarctic Ice Sheet is more vulnerable to greenhouse warming even earlier than the West Antarctic Ice Sheet, simply because it ends further north. And if we already see the effects of warming on the Peninsula of the West Antarctic Ice Sheet with the breaking up of these ice shelves, then those ice shelves at the northern margins of the East Antarctic Ice Sheet, extending for 8000 km over an enormous front, are a much greater concern.

One part of the East Antarctic Ice Sheet partially collapsed many years ago. It is south of India, the Amery Ice Shelf, another one of these floating ice shelves that is fed primarily by the largest ice stream in East Antarctica, the Lambert Glacier. A lot of East Antarctic ice has been discharged by the Lambert Glacier to produce an embayment now occupied by the Amery Ice Shelf. We don't know when the collapse occurred but we do know from the glacial geology of the exposed rock that it occurred quite a long time ago. It followed a subglacial trough under the ice sheet that is below sea level and extends right into the central part of the East Antarctic Ice Sheet where today ice is 4000 meters above sea level.

Ice streams drain into the sea in a number of other places along the coast of East Antarctica. They are underlain by deep bedrock troughs that are below sea level and that extend into broad subglacial basins, also below sea level. When two-thirds of the West Antarctic Ice Sheet collapsed, most of it was grounded on land that was below sea level. It started to collapse probably 20,000 years ago, and collapse really became rapid after 14,000 years ago when sea level was rising rapidly as the Northern Hemisphere ice sheets disintegrated. That destabilized the ice streams draining the West Antarctic Ice Sheet because as sea level rose the ice streams became increasingly afloat instead of being grounded; they lost their traction with the bed. It was as though the West Antarctic Ice Sheet slipped on a banana peel. It lost its support from the bed and just spread out into the sea. That could still be going on with present day ice streams if they become thinner because of surface melting from the greenhouse warming. The surface meltwater could get down through crevasses and lubricate the bed. That could reduce the traction under the ice streams and cause the same kind of instability that rising sea level caused when the Northern Hemisphere ice sheets were disintegrating.

Speed of Ice Sheets The fastest known ice stream is in Greenland, called Jakobshavn Isbrae. It's on the west coast of Greenland just above the Arctic Circle, and it drains about 6 or 7 percent of the Greenland ice sheet. Its speed averages about 7 kms a year. It's very heavily crevassed; very little of the water from the summer melting drains off the ice streams. Most of it drains into the crevasses, which probably explains why this ice stream is moving so rapidly. Some other ice streams in Greenland are moving almost as fast but they are much smaller and are contained in fjords that have high bedrock sills or steps at the head of the fjord. When the grounding line (the inland boundary of ice shelves) retreats along an ice stream, it gets anchored against these sills or steps and can't retreat further.

This situation can exist not only in the West Antarctic Ice Sheet but also in the East Antarctic Ice Sheet. I have no idea why it didn't affect the East Antarctic Ice Sheet when sea level was rising except to say that the rock on which that ice sheet was grounded was closer to sea level. But the measurements haven't been made with radar and seismic soundings to determine the thickness of the ice under these ice streams in East Antarctica where they become afloat. Those studies just haven't been conducted. Until they are we won't know how vulnerable the East Antarctic Ice Sheet is to greenhouse warming. If there were no ice stream crevasses, the meltwater would run off the ice surface and into the sea. But there are crevasses, so greenhouse warming would cause a lot of surface melting, and that meltwater would not drain off into the sea. It would drain into the crevasses - which are always present because these ice streams are moving so fast - and find its way down to the bed and lubricate it. That would allow these ice streams to move much faster than they do today.

For the East Antarctic Ice Sheet to begin collapsing we'd have to have enough greenhouse warming to cause meltwater to get down through the crevasses to lubricate the bed, and there couldn't be any high bedrock sills or steps that would come up close to sea level under these ice streams. If those conditions are satisfied then I see no reason whatever why ice streams that drain the East Antarctic Ice Sheet couldn't acquire the velocity we see today in Jakobshavn Isbrae in Greenland.

In the West Antarctic Ice Sheet, in the Pine Island Bay region of the Amundsen Sea, there are two big ice streams. One of them, the Thwaites Glacier, is moving a little over 3 km a year and the Pine Island Glacier a little over 2 km a year. They are not moving as fast as Jakobshavn Isbrae, only about a half to a third as fast. That's without any surface melting. It seems from our present knowledge that surface melting has the capability of doubling or tripling the discharge of these ice streams. The East Antarctic ice streams are moving about one km a year, the biggest ones, but in principle they could move 7 km a year like Jakobshavn Isbrae, yielding 7 times as much discharge .

There are a number of ice streams that have subglacial trenches that go deep into the interior of the ice sheet. There's no reason that we know of why the kind of collapse along the trench under Lambert Glacier couldn't happen in multiple sites around the perimeter of the East Antarctic Ice Sheet where other ice streams occupy subglacial trenches.

Conditions have existed in the earth's climate history to evacuate enormous parts of the East Antarctic Ice Sheet. Perhaps the greenhouse warming that we've already begun to experience is the sort of thing that could evacuate other parts of East Antarctica where the big ice streams already exist.

Compared to the West Antarctic Ice Sheet there's been almost no work done on the East Sheet by glaciologists. The American glaciological interest has always been concentrated in West Antarctica and therefore our knowledge about the ice sheet is much greater in West Antarctica than East Antarctica. We've put 10 times as much money into our studies as any other country and yet we still cannot answer the question, is the West Antarctic Ice Sheet disintegrating or not? We know that two-thirds of it has disintegrated as a result of the sea level rising when the Northern Hemisphere ice sheets melted. But whether it's still going on today. ... In some sectors it does seem to be collapsing, but in others it's not. So we can't reach a firm conclusion, despite all the effort that's been put into this very question.

A 10-fold Sea Level Rise In looking at the effect of greenhouse warming, a lot of attention is paid to the West Antarctic Ice Sheet and the 6 meter rise in sea level that in principle could result from that. But very little attention is paid to the 60 meters rise in sea level that could come from East Antarctica or some fraction, some substantial fraction of it. Think of the 60 meters sea level locked up in the East Antarctic Ice Sheet. As far as I can see in some respects it’s even more vulnerable than the remainder of the West Antarctic Ice Sheet because so much of its perimeter extends up into these warmer latitudes. This perimeter, some 8000 km long, will be the first to be affected by greenhouse warming.

There's an enormous dearth of basic fundamental information, such as: ice thickness, especially in the ice streams, mapping the bedrock topography underneath the ice streams, accurately measuring the surface melting rate along the ice streams, and changing sea ice conditions that should accompany greenhouse warming.

Are the many small ice sheets that fringe the Antarctic Ice Sheets going to disintegrate as the ones along the Antarctic Peninsula seem to be doing now? This is a very important question. If they do disintegrate, then they might unbuttress some very important ice streams that today aren't discharging much. Those ice shelves (when they are anchored to islands) are like little corks in the bottles of these East Antarctic ice streams. If they disintegrated those corks would be popped out of the bottles and maybe the East Antarctic Ice Sheet could start discharging 10 times as much ice as its ice streams do now. We just don't have the data to more than just ask these questions. There isn't a lot of money available ... We continue to pour almost all of ours into West Antarctica. But nonetheless we're aware of the East Antarctic Ice Sheet and the possibilities of its collapse.

At some time in the past the warming did extend along the Transantarctic Mountains almost all the way to the South Pole. If it was that warm then, the northern ice margins would have been extra warm. It would be hard to imagine that the East Antarctic Ice Sheet would not have undergone massive collapse on those northernmost perimeters. That's looking back 12-15 million years ago. But I think that potential exists.

If the East Antarctic Ice Sheet were to collapse, Memphis TN would be a deep sea port; all of Florida would be underwater; New England and Maritime Canada would be an island; San Francisco Bay would be 200 miles long and 100 miles wide. ... Something like 90 percent of the US population would have to be relocated. You can say similar things about Europe and India and China and all the heavily populated parts of the world. Even though it may be a remote possibility, the consequences if it happened would be an order of magnitude greater than if just the West Antarctic Ice Sheet collapsed. The reason why studying the West Antarctic Ice Sheet is important is not because two-thirds of the ice sheet has already collapsed; it is because collapse of the remaining one-third will pull the cork on the bottle of the East Antarctic Ice Sheet, pull the cork near the South Pole, where East Antarctic ice is ten thousand feet above shorelines all around the world.