Icy indicators of global warming.

• Author: Denniston, Derek
• Position: Melting of glaciers

When the Danish Geodetic Survey mapped the Ok ice cap in western Iceland in 1910, it registered the glacier's area at six square miles. Were members of the survey team to return to the same crags today, they'd probably double check their maps: Ok has shrunk to just one square mile. A crater is now exposed between patches of snow where ice once stood 100 feet thick. "It's unlikely the glacier will survive this century," says Richard Williams, a glaciologist with the U.S. Geological Survey in Woods Hole, Massachusetts.

Far from being a unique phenomenon, the melting of Ok is only one example among hundreds of glaciers known to have retreated this century, prompting scientists like Williams to scour historical records and the glacier regions for evidence of a global trend. What they have found is that mountain glaciers and small ice caps around the planet are melting at an unusually fast rate. They estimate that 15 percent of the total volume of glaciers has vanished during the last 100 years. Glaciologists estimate that glaciers in the Alps have fared even worse: they've lost up to half their mass since 1850.

"No matter where you look around the world today, you see most glaciers thinning and retreating," says Williams.

Other studies seem to confirm this. Researchers with the Soviet Geophysical Committee studied 408 glaciers in Asia's Tien Shan, Altai, and Caucasus mountain ranges. They found that over the last 40 years, more than 85 percent of the glaciers retreated.

Mauri Pelto, geology professor at Nichols College in Dudley, Massachusetts, and director of the North Cascade Glacier Climate Project, has chronicled the same distressing trend in Washington State. Of the 114 glaciers he monitored over the last decade, 91 have retreated. Similar glacier retreat has been documented in Kazakhstan, Kenya, New Guinea, New Zealand, Scandinavia, the Canadian Rockies, and around the Gulf of Alaska.

While many weather factors - notably winter snowfall and cloud cover - influence the dynamics of a glacier and vary widely among mountain ranges, glaciologists note that one climactic pattern - rising temperatures - has consistently preceded the shrinking of mountain glaciers in this century. This cause-and-effect relationship, they say, provides tangible evidence that the planet has been warming.

A few well-known climate scientists dispute claims that melting glaciers are evidence of global warming, but most climatologists are convinced. Last year, the...