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19 Jul 2002
A report in today's Science shows that Alaska's glaciers are melting faster than expected.
Using airborne laser altimetry, scientists have shown that glaciers in Alaska and Canada are retreating much faster than previous estimates. The team from the University of Alaska says that the thinning North-American ice sheets make the largest glaciological contribution to rising sea level ever measured. (Science 297 382).
The altimetry system consists of a nadir-pointing laser range finder mounted in a small aircraft and a gyroscope to measure the orientation of the ranger. A global positioning system continuously feeds back the exact position of the aircraft as it flies over each glacier.
Keith Echelmeyer and colleagues used this laser-based technique to measure both the changes in area and volume of 67 glaciers in Alaska and Canada. "Most glaciers have thinned several hundred feet at low elevations in the last 40 years and about 60 feet at higher elevations," he said.
The team's results show that rates of glacier thinning have more than doubled over the last five to seven years. From the mid-1950s to the mid-1990s the estimated thinning rate was 0.7 m/year but this has risen to 1.8 m/year during the past five to seven years.
This rate of thinning is equivalent to a rise in sea level of 0.14 ± 0.04 mm/year over the 40 year period and 0.27 ± 0.10 mm/year over the past seven years.
According to Echelmeyer, this means that Alaska's glaciers have contributed about 9% (1.5 ±0.5 mm/year) of the observed rate of sea level rise over the past 50 years. He says that, compared with the estimated inputs from the Greenland ice sheet and other sources, Alaska's glaciers have made the largest, single glaciological contribution to rising sea level ever measured.
Echelmeyer stresses though that factors other than climate warming over the past decades may be responsible for the rise.
Author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.
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