University of Toronto geophysicists have shown that should the West Antarctic Ice Sheet collapse and melt in a warming world – as many scientists are concerned it will – it is the coastlines of North America and of nations in the southern Indian Ocean that will face the greatest threats from rising sea levels.

"There is widespread concern that the West Antarctic Ice Sheet may be prone to collapse, resulting in a rise in global sea levels," says geophysicist Jerry X. Mitrovica, who, along with physics graduate student Natalya Gomez and Oregon State University geoscientist Peter Clark, are the authors of a new study to be published in the February 6 issue of Science magazine. "We've been able to calculate that not only will the rise in sea levels at most coastal sites be significantly higher than previously expected, but that the sea-level change will be highly variable around the globe," adds Gomez.

Digital image of what Antarctica would look like if it consisted only of land actually above sea level. Credit: OSU College of Science

"Scientists are particularly worried about the ice sheet because it is largely marine-based, which means that the bedrock underneath most of the ice sits under sea level," says Mitrovica, director of the Earth Systems Evolution Program at the Canadian Institute for Advanced Research. "The West Antarctic is fringed by ice shelves which act to stabilize the ice sheet – these shelves are sensitive to global warming, and if they break up, the ice sheet will have a lot less impediment to collapse." This concern was reinforced further in a recent study led by Eric Steig of the University of Washington that showed that the entire region is indeed warming.

"The typical estimate of the sea-level change is five metres, a value arrived at by taking the total volume of the West Antarctic Ice Sheet, converting it to water and spreading it evenly across the oceans, says Mitrovica. "However, this estimate is far too simplified because it ignores three significant effects:

1. when an ice sheet melts, its gravitational pull on the ocean is reduced and water moves away from it. The net effect is that the sea level actually falls within 2,000 km of a melting ice sheet, and rises progressively further away from it. If the West Antarctic Ice Sheet collapses, sea level will fall close to the Antarctic and will rise much more than the expected estimate in the northern hemisphere because of this gravitational effect;

2. the depression in the Antarctic bedrock that currently sits under the weight of the ice sheet will become filled with water if the ice sheet collapses. However, the size of this hole will shrink as the region rebounds after the ice disappears, pushing some of the water out into the ocean, and this effect will further contribute to the sea-level rise;

3. the melting of the West Antarctic Ice Sheet will actually cause the Earth's rotation axis to shift rather dramatically – approximately 500 metres from its present position if the entire ice sheet melts. This shift will move water from the southern Atlantic and Pacific oceans northward toward North America and into the southern Indian Ocean.

"The net effect of all of these processes is that if the West Antarctic Ice Sheet collapses, the rise in sea levels around many coastal regions will be as much as 25 per cent more than expected, for a total of between six and seven metres if the whole ice sheet melts," says Mitrovica. "That's a lot of additional water, particularly around such highly populated areas as Washington, D.C., New York City, and the California coastline." Digital animation of what various sea-level rise scenarios might look like for up to six metres is at www.cresis.ku.edu/research/data/sea_level_rise.

"There is still some important debate as to how much ice would actually disappear if the West Antarctic Ice sheet collapses – some fraction of the ice sheet may remain quite stable," he says. "But, whatever happens, our work shows that the sea-level rise that would occur at many populated coastal sites would be much larger than one would estimate by simply distributing the meltwater evenly. Any careful assessment of the sea-level hazard associated with the loss of major ice reservoirs must, of course, account for the sea-level fingerprint of other sources of meltwater, namely Greenland, the East Antarctic and mountain glaciers. The most important lesson is that scientists and policy makers should focus on projections that avoid simplistic assumptions."

Both digital images and video of the impact around the world of sea level increases up six meters can be obtained at Cresis web site.