Breakaway Iceberg Should Not Sound Global Warming Alarms

WHEN Antarctica's ice sheets and coastal glaciers break off, sending huge blocks of ice drifting into the ocean, often the only ones to notice are penguins and shipowners.

But when the Larsen Ice Shelf calved recently, it gave birth to a veritable Babe the Blue Ox of icebergs -- roughly the size of Rhode Island and some 600 feet thick. And while explanations vary as to the cause of the split, the event is giving researchers a good look at the mechanics of ''deglaciation,'' one predicted result of long-term global warming theories.

Researchers seem to agree on two things: that temperatures on the Antarctic Peninsula, which shelters the Larsen Ice Shelf, have been rising; and that calving is part of the natural cycle of ice sheets.

''There is no doubt that the climate on the Antarctic Peninsula has warmed significantly over the last few decades,'' says David Vaughan, a glaciologist with the British Antarctic Survey. Researchers note the average annual air temperature there has risen 2.5 degrees Celsius since the 1940s.

''The temperature increase is quite striking,'' agrees Charles Bentley, director of the Geophysical and Polar Research Center at the University of Wisconsin at Madison.

But trying to tie the increase to human-induced climate change is a shaky endeavor. ''Global climate models do not have the geographic resolution, especially in regions with such uncertain boundary conditions,'' he says of the peninsula's geography. It faces open ocean on one side, ice on the other, has mountains running its length, and is among the northernmost reaches of the continent. These factors make the region highly sensitive to climate fluctuations. Moreover, he adds, researchers in other regions of the continent have been reporting suspected cooling trends.

Indeed, three major climate models -- from researchers at Princeton University, the National Aeronautics and Space Administration, and the National Center for Atmospheric Research in Boulder, Colo. -- would not have projected this kind of warming at all, says Jim Titus, an analyst in the climate-change division of the US Environmental Protection Agency's Office of Policy Planning and Evaluation. ''This means either that something else is causing the temperature fluctuations or that the models don't have everything in them they need,'' he says. He adds that under the three major models, temperatures at the poles don't increase for the first 100 years or so.

''Ice shelves, particularly large ones, characteristically move seaward for decades at a time, building a bulge into the ocean,'' Dr. Bentley says. When the sheet no longer can support the bulge, it breaks off.'' He notes that images of the ice sheet's boundary, minus the latest chunk, closely resembles the shelf's boundary in the 1960s -- with one glaring exception. The shelf has disintegrated around James Ross Island, allowing circumnavigation of the island for the first time in recorded history.

The disappearance of ice from the island ''is interesting,'' Bentley says. He speculates that the ice may be disappearing because it is not being replaced by ice sliding off the mainland. Such sliding forms one of the bases for concern about sea-level rises in scenarios about human-accelerated global warming. Antarctic ice would not melt, but slide into the sea as the major ice sheets blocking its paths break up. This would raise the sea level over time, much as adding an ice cube to water raises the water level in a glass. Bentley adds that in other instances where Antarctic shelves have disintegrated, little evidence has been found of glaciers behind them accelerating.

The immediate importance of ice calving along the Larsen shelf may lie less in what it says about global warming theories and more in its value as a laboratory for studying deglaciation, Mr. Titus says. ''We're going to learn something about deglaciation, regardless of global warming.''