Wednesday, July 23, 2008

Glaciers in the balance

An informative, detailed article by Graham Cogley, from Environmental Research Web, summarizing quite a bit of research: That glaciers are shrinking is a commonplace observation in the media. The journalists are right, but defining "shrinkage", pinning down its rate, and deciding how unsure we ought to be about that rate, are not as straightforward as you’d think. There are four main ways of describing glacier change. Here I will leave changes of length to one side and focus on changes of area, volume and mass. I will also focus on the global scale and on "small glaciers". The others, the two ice sheets of Antarctica and Greenland, are so big that they call for completely different approaches to measurement.

A measurement of shrinkage, defined as reduction of the glacier’s area, requires two measurements of the area at known dates. In the dataset described here, nine-tenths of the measurements record shrinkage and the rest show expansion. In fact, most glaciers have been shrinking for the last 100-200 years, since the culmination of the so-called Little Ice Age when many deposited prominent terminal moraines. These enable the estimation of maximum extents with fair accuracy. Accuracy improves as we enter the era of good maps and of photography, especially from the air or space.

Both the quantity and quality of such information have increased greatly in recent decades. Glaciers are remote from most inhabited regions, and visiting them is expensive. But satellites with cameras fly over regularly, and we only have to wait for a cloudless overflight to get valuable observations. It is a little surprising that nobody has yet tried to put the accumulating evidence of shrinkage together. Recently I began an attempt to do just this; it has meant trawling the scientific literature, including some of its darkest depths.

… Glaciers are often described as "sensitive indicators of climatic change". This cliché misses an important point: glaciers are also independent indicators of climatic change. Mass-balance measurements assume nothing about the temperature at weather stations, so glaciers give us a better-rounded and more secure picture of how the natural world is coping with the changes being forced upon it.

Piz Buin from Ochsentaler Glacier, German Wikipedia, shot by "Saperaud," Wikimedia Commons, under the terms of the GNU Free Documentation license, Version 1.2

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