Image: USGS GOODBYE, GRINNELL. Three views of the famous Grinnell glacier in Glacier National Park, Montana, taken in 1914, 1938 and 1997, show that what remains today is but a remnant of the glacier at the beginning of the century.

Within the next 50 years or so, the last of the rivers of ice that gave Glacier National Park in Montana its name will run dry. In 1850 the area was home to more than 150 glaciers; today there are fewer than 50. More inland glaciers are melting away in Europe and around the world. Last year, researchers from the National Snow and Ice Data Center (NSIDC) at the University of Colorado in Boulder reported that the largest glacier on Africa's Mount Kenya had lost 92 percent of its mass and that the glaciers in the Russian Caucasus Mountains had shriveled to half their former size the past century.

Glaciologists monitor these midlatitude glaciers, such as those in Glacier National Park, as a barometer of global climate change. And there is no question that the world--whether as part of natural cycle, from the result of human activity, or both--is becoming a warmer place. The impact of this global meltdown is more than aesthetic--the water released ends up in the oceans, raising sea levels and having potentially devastating economic and social impacts.

But the mountain glaciers contain only 6 percent of the world's ice. More worrisome to researchers are the vast ice sheets of Antarctica and Greenland, which hold about 90 percent of the world's fresh water. If just 10 percent of the water locked in these frozen reservoirs is added to the oceans, geologists predict that sea levels will rise by more than 20 feet, drowning lowlying islands and inundating continental coasts.

So far, these ice sheets have seemed to be stable and firmly anchored to islands and the nearby coasts, but new data indicate that they, too, are crumbling in the face of the thermal onslaught. Over the past 50 years, the Antarctic has warmed by about 4.5 degrees Celsius. In a 1978 article J. H. Mercer of Ohio State University predicted that if the south polar regions were warmed five degrees Celsius, the floating ice shelves surrounding the West Antarctic ice sheet would quickly disintegrate, flooding coastlines around the world in the process.

Image: NSIDC, University of Colorado 'FULL RETREAT.' Satellite images document that the Larsen B ice shelf in Antarctica has caved off more than 1,714 square kilometers of area since November 1998. Dark areas are warmer, indicating open water.

Last October, Mercer's once controversial idea gained credibility when scientists at the National Oceanic and Atmospheric Administration's National Ice Center revealed satellite data showing a massive iceberg 92 miles long by 30 miles wide, with an area of 2,751 square miles, breaking free from the Ronne Ice Shelf in Antarctica.

In early April, investigators from the Colorado group and the British Antarctic Survey (BAS) reported that radar images of two ice shelves in Antarctica known as the Larsen B and Wilkins showed they are in "full retreat" and have lost nearly 3,000 square kilometers of their total area in the past year.

Surface features on the Larsen B indicate that it has existed for at least 400 years. But as climate inches toward an average summertime temperature just above the melting point of water, the Larsen and Wilkins ice shelves have begun to disintegrate. Images recorded by the Advanced Very High Resolution Radiometers on NOAA weather satellites show that the Larsen B ice shelf has begun to crumble rapidly after an initial retreat in the spring of 1998. In a series of events that began in last November, an additional 1,714 square kilometers of shelf area caved away, says Ted Scambos of NSIDC. On the opposite side of the Antarctic peninsula, the Wilkins Ice Shelf, which is nearly twice the size of Delaware, has retreated by nearly 1,100 square kilometers since early March 1998.

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