By Curt Suplee
Washington Post Staff Writer
Sunday, April 19, 1998; Page A17

A section of an Antarctic ice shelf as big as the District of Columbia has broken away from the continental ice mass, perhaps in response to decades of gradual warming in the South Polar region.

The break occurred three weeks ago in a segment of the Antarctic Peninsula ice cover known as Larsen B, which is about the size of Connecticut. The area, near the southern tip of South America, is the northernmost of the many floating ice shelves that are usually frozen tight to the continent.

The break may indicate a shaky prognosis for the entire Larsen B formation. "This is the biggest ice shelf yet to be threatened," said Ted Scambos, whose group at the National Snow and Ice Data Center (NSIDC) in Boulder, Colo., detected the fracture in satellite images.

"This may be the beginning of the end for the Larsen Ice Shelf," Scambos said, and "there may only be a few years left" before the whole sheet disintegrates, "or even as little as one more warm season" -- that is, next December, January and February, when temperatures in the region are highest.

Bits and pieces of various Antarctic ice shelves have been cracking off for decades as regional average temperatures have risen approximately 4.5 degrees Fahrenheit since the 1940s. The current rupture, which dislodged an area of about 75 square miles, is less than one-tenth the size of an iceberg that snapped off the Larsen shelf in January 1995, noted Claire L. Parkinson, a climatologist and ice expert at NASA's Goddard Space Flight Center in Greenbelt.

At 24 miles long by 3 miles wide, "the new one is indeed large," she said, "but by no means a record setter." Records show that since 1974, approximately 100 ice shelf sections exceeding 15 miles in length have detached themselves from the continental ice mass, which is anchored to rock. The largest known was a piece from the Ross Ice Shelf due south of New Zealand, which measured 96 by 22 miles, Parkinson said.

In all, Scambos said, the amount of Antarctic ice shelf lost to break-off is less than one-tenth of 1 percent of the total ice cover in the area. The fragments do not contribute directly to sea-level rise, he added, because the shelves by definition already are floating on the surface. The more menacing question, he said, is "what happens to the glaciers behind the ice sheets."

Scientists at the NSIDC, part of a joint project of the University of Colorado and the National Oceanic and Atmospheric Administration, routinely monitor the ice shelves using time-sequence images from polar-orbiting NOAA satellites.

It is unknown whether global warming caused the new break or its predecessors. It is possible, Parkinson said, "that internal ice dynamics -- that is, the flow of the ice caused over thousands of years -- could be a more important factor than [air temperature increases] over the past few decades."

Whatever the cause, she said, "the one thing that's kind of a relief is that [the Larsen break] is not as closely tied to the rest of the western Antarctic ice sheet" as other adjacent formations. If it were, she said, "that would raise a huge concern that all of the west Antarctic ice might collapse."

Scambos said there is no "smoking gun" evidence that this break or the previous Larsen sheet breaks were caused by rising temperatures. But "the warming trend appears to be related to a reduction in sea ice," he said in a statement Friday. In addition, he said, "ice shelves appear to be good bellwethers for climate change since they respond to change within decades, rather than the . . . centuries sometimes typical of other climate systems."