This science package is EMBARGOED until 14:00, U.S. Eastern Time (18:00GMT), Thursday July 18, 2002 

July 18, 2002. Copyright 2002. Graphic News. All rights reserved.

EarthÕs ice on the melt

LONDON, July 18, Graphic News: Glaciers in the Alaskan mountains are thinning by more than 20 inches (520mm) a year -- possibly because of global warming -- making a larger contribution to rising sea level than any other glacial region. 

ThatÕs the conclusion of a study using laser altimetry to measure the volume changes of 67 Alaskan glaciers from the mid-1950s to the mid-1990s. In addition, repeat measurements of 28 glaciers made in 2000-2001 showed average rates of thinning increasing to 71 inches (1.8 metres) a year. The data is reported in this weekÕs edition of the journal Science.

Mountain glaciers tend to be remote and inhospitable, so much of what we know about them is based on studies of a few of the smaller glaciers, according to Anthony Alan Arendt of the Geophysical Institute, University of Alaska. 

Glaciers in Alaska and neighbouring Canada cover 34.7 thousand square miles (90 thousand square kilometres), or approximately 13 percent of the mountain glacier area on Earth. The authors surveyed volume and area changes of part of this region and found that the glaciers were thinning enough to produce a sea level rise of approximately 0.01 inches (0.14 millimetres) per year. The recent losses are nearly double the amount of water lost from the entire Greenland Ice Sheet, according to the authors. The melt rate also seems to have increased in recent years, they say.

Arendt and his colleagues used a laser altimeter flown on a light aircraft to monitor the thickness of the ice. The equipment transmits laser beams to the surface of the glacier and picks them up again when they are bounced back. The time span between transmission and return indicates the distance covered and thus the relative elevation of the measured point on the ground relative to the aircraft. To determine the absolute elevation, the position and location of the aircraft is determined using a minimum of four GPS (Global Positioning System) satellites.

The researchers concluded that if their data were extrapolated to all of AlaskaÕs glaciers then the total annual loss of ice would amount to around 13.7 trillion U.S. gallons (52 cubic kilometres, 52 trillion litres, 11.4  trillion Imperial gallons).

ÒSince 1999, we have reprofiled 28 glaciers, covering about 13 percent of the glacerized area of Alaska,Ó explained Arendt. The repeat measurements involved flying the aircraft laterally to within 50-80 feet (15-25 metres) of an earlier profile. 

ÒWe try to fly the new profile at the same time of the year, usually within the same week, as the older profile,Ó said Arendt. ÒComparisons are then made at the crossing points between the old and new profiles, providing measurements of glacier change during the intervening five to seven years.Ó

If the reprofiled data were extrapolated the wastage of all Alaskan glaciers would increase to 25.9 trillion U.S. gallons (96 cubic kilometres, 96 trillion litres, 21 trillion Imperial gallons) a year -- a far higher figure than previously thought.

One-tenth of the worldÕs land area is covered with glaciers, storing about 75 percent of the worldÕs freshwater.
/ENDS

Source: Science