Icy indicators of global warming.

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 cautious U.N.-sponsored Inter-governmental Panel on Climate Change concluded that receding mountain glaciers arc "among the clearest and best evidence" for climate change over the last century. The panel also concurred that glaciers in the Alps have been melting 10 times faster than they did at the end of the last ice age, 10,000 to 20,000 years ago.

"There's no doubt in my mind that mountain glaciers are thinning and retreating due to climate change - both warmer temperatures and less snowfall," says Mark Meier, director of the Institute for Arctic and Alpine Research in Boulder, Colorado.

In the last century, there has been a general trend of warming, punctuated by brief cooler intervals. The global average surface temperature on land rose 1 degree Fahrenheit. While that may sound trivial, climatologists know that a 1-degree rise in temperature over a century exceeds the normal rate.

The consequences may be most pronounced among glaciers in the tropics and subtropics. In these regions, mountain glaciers are located where the temperature is often at or just below freezing. A slight rise in temperature can quickly accelerate the normal rate of melting. "Many of the glaciers in the tropics and subtropics have disappeared in the last century," says Lonnie Thompson, a glaciologist with the Byrd Polar Research Center at Ohio State University. "Of the ones that remain, virtually all are rapidly retreating."

Along with the melting glaciers disappear histories of past environmental and climactic conditions, frozen in time and waiting to be studied. With each snowfall, specks of dust, volcanic ash, meteoric particles, and pollution carried by atmospheric currents are deposited on glaciers and eventually compressed into ice. By analyzing cores of glacial ice, scientists can recreate records of temperature and precipitation dating back thousands of years.

Thompson has extracted cores from a tropical glacier in the Peruvian Andes and subtropical glaciers in the Tibetan Plateau. At each of these high-altitude sites, he found that over the last 50 years, temperatures have been warmer and glaciers melting faster than at any other time in the 1,500- to 12,000-year ice record. While several computer models of climate change predict that global warming will be greatest near the North and South Poles little evidence of comparable warming has been found so far in ice cores drilled in these regions. Instead, Thompson says that "recent warming may be expressed more strongly and earlier at higher elevation, lower latitude sites."

If these natural climactic records - the best among a very few that exist - disappear, climatologists will lose critical data against which they can test their complex models of climate change.

The impacts of full-scale glacial retreat would be widely felt. Glaciers act essentially as giant sponges that trap water from the air and periodically release it. They are found at almost every latitude and altitude, ranging from the equator in Ecuador to the far shores of Canada's Ellesmere Island, and from sea level on the Gulf of Alaska to heights of 22,000 feet in Tibet. For the amount of land they cover - a total area of just 216,500 square miles (slightly larger than France) - mountain glaciers store a surprising amount of precipitation, equal to 260 years of all the rain and snow that falls on Earth.

Meltwaters from glaciers irrigate farmland, drive hydroelectric turbines, and quench the thirst of millions of people around the world. Mauri Pelto estimates that glaciers in the North Cascades provide up to 30 percent of Washington's summer water supply. Already, shrinking glaciers in the Alps, Andes, and Cascades have caused late-summer water shortages for countless people, plants, and animals living downstream.

But the effects of increased glacial melt are even more widespread. According to Mark Meier, melting alpine glaciers are responsible for almost half of the 5-inch rise in sea levels measured since 1940. The other half of the sea-level rise comes from the depletion of groundwater and the thermal expansion of oceans (warm water occupies more space than cold water). If global temperatures rise by 3 degrees by the year 2050, as most climate scientists predict, then Meier expects that meltwater from mountain glaciers alone win raise sea levels by another eight inches.

If temperatures rise that high so quickly, then the prospects for mountain glaciers are bleak. Already, the loss of mountain glaciers has accelerated in the 1980s, the warmest decade since record-keeping began 130 years ago. Could the day be far off when most of these moving masses of ice exist only on old survey maps?