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News :: Miscellaneous |
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Alaskan glaciers raise sea level |
Current rating: 0 |
by PHILIP BALL
(No verified email address) |
21 Jul 2002
Modified: 10:26:43 AM |
Mountain melt-water is rising fast.
The Alaskan Columbia Glacier: on the verge of collapse?
© S. McGuire |
Alaskan glaciers have thinned twice as fast over the past 5 years as during the preceding four decades, say US researchers. Their melting is making the largest contribution to rising sea levels of any ice-bound region on Earth1.
Understanding and predicting sea level rise is vital. Some have forecast flooded coasts and submerged islands within the next hundred years.
Alaska's mountain glaciers cover barely 1% of the surface area of the great Greenland ice sheet. Yet the new findings suggest that their melt-water may account for up to 9% of the total rise in sea level over the past 50 years.
Global average sea level has risen by around 3 millimetres each year over the past decade. Melting of the Alaskan glaciers has previously been estimated to account for only a very small proportion of this - perhaps just 0.02 millimetres per year. The new reckoning ups that number by more than a factor of ten.
Ice work
Anthony Arendt and co-workers at the University of Alaska reached these conclusions by analysing measurements of glacier thickness made since the mid-1950s. Since 1999, the researchers have been re-measuring the ice profiles of 28 glaciers last studied between 1993 and 1996.
Until the 1970s, glacier ice volumes were estimated simply from aerial photographs. Now more detailed and accurate measurements are made by laser altimetry. An aircraft flies over the glacier surface and bounces a laser beam off the ice to measure its elevation. Ice thickness and extent are calculated from this, and changes in ice volume are deduced.
On average, the Alaskan glaciers are now losing nearly 2 metres of ice thickness every year. But a few have got thicker. Glaciers aren't simply like ice cubes, steadily melting in response to global warming. They grow or retreat according to a complex mixture of factors.
On average, the Alaskan glaciers are now losing nearly 2 metres of ice thickness every year
It depends, for example, on where they are going: some end in lakes, some at the coast, some on land. These tongues of ice advance sluggishly downhill under the pull of gravity, but sometimes they surge forward more rapidly, depending on the lubrication at their base. Increased snowfall, which is a possible consequence of global warming at high latitudes, can actually thicken the ice.
All the same, some Alaskan glaciers thinning dramatically. The Columbia Glacier, which creaks into the Gulf of Alaska, is 150 metres lower at its seaward end than it was five years ago. This glacier has been predicted to be on the verge of collapse for the past couple of years.
Arendt and colleagues doubt that the changes are as marked in other mountain glaciers, except possibly those in West Antarctica and Patagonia.
Also this week, Stan Jacobs and co-workers at the Lamont-Doherty Earth Observatory in New York say that the water in the Ross Sea off Antarctica has been getting less salty over the past four decades because of heavier snowfall and melting of the West Antarctic Ice Sheet2. This change will alter the circulation of water in the Southern Ocean |
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