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
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.
"The radar images showed a large area of completely shattered ice, indicating an ice front 35 kilometers back from its previous extent," says Scambos. "The sudden appearance of thousands of small icebergs suggests that the shelves are essentially broken up in place and then flushed out by storms or currents afterward."
Using computer models, the BAS scientists had predicted that the Larsen B was nearing its stability limit--the shelf has retreated too far to be supported by adjacent islands and shorelines. While both the U.S. and U.K. scientists expected the two shelves to fail soon, the current rate of decay is more rapid than they anticipated.
"We have evidence that the shelves in this area have been in retreat for 50 years, but those losses amounted to only about 7,000 square kilometers," says David Vaughan, a researcher with the Ice and Climate Division of BAS. "To have retreats totaling 3,000 square kilometers in a single year is clearly an escalation. Within a few years, much of the Wilkins ice shelf will likely be gone."
Even so, the effect on the sea level of all this melting remains enigmatic. Sea levels are indeed rising--but at the stately pace of about two millimeters a year. That amount could be accounted for by the increase in volume of seawater as it expands from warmer temperatures. Moreover, sea level changes are difficult to monitor--the land on some coasts is rising, and it is sinking on others. Some researchers believe that the melting of midlatitude glaciers may have a more significant impact on sea levels than the ice sheets because their water is added directly to the oceans, whereas much of the polar ice is floating and has displaced its mass. Others predict that the system will remain in balance--warming will increase moisture and cause more snow to fall in polar regions, replenishing the supply of ice.
For now, the Intergovernmental Panel on Climate Change, a body established in 1988 by the World Meteorological Organization and the United Nations Development Program, has taken the position that both the polar ice caps are most likely to remain constant in size through the next century. The panel's forecasts for sea-level rise range from 20 centimeters to almost one meter--a tolerable rate of change that humanity has dealt with successfully in the past.And yet the new findings are still a bit unsettling. What if Mercer wasn't all wet?