University of Toronto researchers predict that in the event of the collapse of the West Antarctic Ice Sheet (WAIS), the coasts of North America and countries bordering the southern Indian Ocean will be hardest hit by rising sea levels.
Jerry Mitrovica and Natalya Gomez from U of T’s Department of Physics, along with Peter Clark from the University of Oregon’s Department of Geosciences, used computer models to predict what would happen following the complete disintegration of the WAIS.
The results of their study, published in the February 6 edition of Science, indicate that sea level rise is likely to vary significantly around the globe.
The coasts of North America and countries in the southern Indian Ocean would see the most drastic change, up to 25 per cent greater than predicted. This could mean an increase of six to seven meters in some locations, rather than the typical estimate of five meters, the figure used by the Intergovernmental Panel on Climate Change.
In the past, predictions of sea level rise were calculated by converting the amount of water released from melting ice and spreading it uniformly—or eustatically—around the world’s oceans. The problem with this method, Mitrovica says, is that it fails to take three critical points into account.
The first is the elastic nature of Earth’s surface. As the ice sheet melts, the ground beneath it, compacted under the weight of the ice, bounces back, expelling water away from where the glacier once was.
The second factor is the gravitational attraction exerted by the ice sheet on the surrounding water. If the WAIS collapses, ocean levels within 2,000 km of its borders will fall as water moves away, rising incrementally at greater distances.
The final factor is the ice sheet’s effect on Earth’s rotational axis, which will experience a 500 metre tilt if the WAIS disappears completely. This shift would cause water to migrate northwards out of the southern Pacific and Atlantic oceans.
Canada’s coastal regions, including major centers like Halifax and Vancouver, would see a rise “in excess of six metres if all of the WAIS disappears,” says Mitovica. Some of the United States’ most populous cities, including New York, Los Angeles, and Washington D.C. would see similar increases.
Mitrovica is keen to avoid apocalyptic predictions, pointing out that the ice sheet will likely collapse over a period of “several centuries or more.” However, he continues, “this work shows that the sea-level rise that would occur at many coastal sites would be much larger than currently estimated.”
There is evidence that the WAIS is already becoming unstable. This is in part due to its structure: the WAIS is a grounded, marine-based ice sheet, sitting on the earth’s surface below sea level. Its edges are surrounded by a protective barrier of floating ice shelves, which are particularly vulnerable to rising air and water temperatures. Losing them, Mitrovica says, means the ice sheet “will have a lot less impediment to collapse.”
He adds that the WAIS “is only part of the story.” Other sources of meltwater, including mountain glaciers and ice sheets in Greenland and the eastern Antarctic, also need to be considered to fully understand the degree and effect of eventual sea level rise.
This study marks a significant improvement in the ability of researchers to make accurate predictions about the world’s oceans. “We are providing the ‘ingredients’ if you will,” Mitrovica says. “The final recipe—that is, the final combination of the different sources—remains to be seen.”
Natalya Gomez is currently embarking on a study of the unique “fingerprint” of Greenland’s ice sheet—another vulnerable ice reservoir—on sea level rise.
Mitrovica hopes this study will relate that “scientists as well as policy makers should focus on projections that avoid simplistic assumptions.” Instead, they should understand that “sea-level change will vary significantly from place to place, [and if they] want to prepare for our warmer future, they need to take this into account.”
More information about what rising sea levels might look like is available at the website for the Center for Remote Sensing of Ice Sheets: www.cresis.ku.edu/research/data/sealevelrise
