THE NEW CLIMAT
Sea levels would rise unevenly as ice sheet melts, study says
Globe and Mail, Canada -
Coasts of Argentina would see no change, but Vancouver would be inundated
ENVIRONMENT REPORTER
The collapse of the massive ice sheet covering West Antarctica has always been one of the nightmare scenarios of global warming. So much water is locked away in the ice that if it were distributed evenly in the world's oceans, it would raise sea levels by an average of five metres.
But a Canadian-led research team has made an unusual discovery about what will happen if the ice melts: Not all coastlines of the world will be affected equally.
Some lucky areas, notably in Southern Chile and Argentina, will experience no sea-level change on their coasts. But other places along the populous Atlantic and Pacific coasts of Canada and the United States would have seas rise by six to seven metres, inundating cities such as Vancouver and New York with a much worse-than-expected increase.
Although no one is predicting the imminent demise of the West Antarctic ice sheet, some scientists worry that it is at risk from global warming. It would likely take centuries or even millenniums to melt, so it is a modest near-term threat to humanity - but a possible major problem over the longer term.
A research study issued last month indicated that temperatures on the ice sheet are beginning to increase, suggesting that additional melting is more than a remote possibility. Among the world's ice sheets, the one on West Antarctica causes an unusual amount of fretting among climate scientists because it rests on bedrock that is beneath sea level, making it more vulnerable to washing into the ocean.
It might, at first glance, seem intuitive that if an ice sheet melts, the world's oceans would be affected equally, just like adding more water to a bathtub causes the same rise everywhere. Most previous reports on the Antarctic ice sheet have assumed just that.
But in a paper in today's issue of the journal Science, Dr. Mitrovica and a pair of other researchers say this seemingly common-sense, bathtub type outcome won't occur.
One reason is that there is so much ice on West Antarctica that it exerts a huge gravitational pull on the surrounding ocean water, much like the moon causes tides on Earth.
If the ice were to melt, this attractive force would vanish and cause sea levels around the ice sheet in Antarctica to plunge, perhaps by as much as 30 metres, according to Dr. Mitrovica. But the water now attracted by gravity around the coast of the southern continent will flow away if there is melting, causing additional sea-level rise elsewhere.
Another odd effect identified by the researchers is that the Antarctic bedrock on which the ice sheet rests will rise if there is melting. The ice is so thick it has caused the bedrock to be depressed by an estimated 500 metres to one kilometre. That means some land will rise, Atlantis-like, out of the ocean, if the ice melts.
The water displaced by this rebounding would flow into the world's oceans, adding to the sea level elsewhere, although Dr. Mitrovica said this would take thousands of years.
The researchers also believe the melting would cause the Earth's rotation axis to shift by about 500 metres, causing the equator to move northward by that amount and water to shift from the Southern to the Northern Hemisphere.
According to Dr. Mitrovica, these physical properties would cause sea level to rise 25 per cent more than is currently expected along some coastlines. The worst effects would be in places such as New York City, Washington, D.C., and the coast of California. Southern Florida would disappear if sea levels were to rise more than six metres.
"We aren't suggesting that a collapse of the West Antarctic ice sheet is imminent," said Peter Clark, a professor of geosciences at Oregon State University, who also worked on the study.
The third researcher was U of T graduate student Natalya Gomez.
The study was funded by a number of organizations, including the Canadian Institute for Advanced Research.
Oceans rising
A new study concludes that, if global warming causes the West Antarctic ice sheet to collapse, sea levels would rise much higher in some parts of the world than current models predict.
WEST ANTARCTIC ICE SHEET
Covers the portion of Antarctica west of the Transantarctic Mountains.
Onshore ice flows continuously out toward the coast.
Ice continues to flow outward onto the water, creating a floating ice shelf attached to the continent.
Melt water flows under the ice sheet and weakens it close to the coast.
Floating ice shelf has largely broken away in places.
Weight of ice has pushed bedrock below sea level.
UNEQUAL DISTRIBUTION
The new model predicts that, if the West Antarctic ice sheet were to melt, sea levels would rise most in the North Atlantic and North Pacific Oceans. Much of Asia, though, would still see levels rise by five metres or more.
Gulf of Mexico
The new model predicts sea levels could rise as much as 6.4 metres above current levels.
Western Europe
Sea levels would rise about 5.5 metres, devastating the Netherlands and Belgium.
The Indian subcontinent
Sea levels could rise by five metres - the global average - overrunning the Ganges delta.
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North America at greater risk from Antarctic ice sheet melt: study
