All cracked up from the heat? Major hunk of an Antarctic ice shelf shatters and drifts away. (This Week).

A Rhode Island-size section of an Antarctic ice shelf splintered into thousands of icebergs in a mere 5-week period during the area's warmest summer on record.

The Larsen B ice shelf--or rather, the 40 percent of it that's left--rims a small portion of the eastern coastline of the Antarctic Peninsula. Satellite images (see series at right) show that about 3,250 square kilometers of Larsen B disintegrated between January 31 and March 5. Over the past 5 years, the 220-meter-thick ice shelf--one of the continent's northernmost--has lost about 5,700 square kilometers, says Ted A. Scambos, a glaciologist at the National Snow and Ice Data Center in Boulder, Colo.

Over the long term, ice shelves normally stay within a stable size range. That's because the ice that calves as icebergs from the front of the shelf is replenished by flow from onshore glaciers and ice sheets. But Scambos contends that ice-sheet stability along the Antarctic peninsula has been disrupted by a long-term warming trend.

The annual average temperature in the region has risen about 2.5 [degrees] C since the late 1940s. Because of high summertime temperatures in recent years, there's a 50-to-80-day period when snow and ice melt atop the shelf. The large ponds that resulted, some of them more than 1 km wide, spelled doom for the ice shelf, says Scambos. The water spilled into crevasses crisscrossing the surface of the ice shelf, and the pressure it exerted at the tapered bottoms of those fissures forced the walls apart and extended the cracks even deeper. Scambos and his colleagues calculated that once a water-filled crack reaches a depth of 6 to 15 m, the pressure would drive the fissure through the ice shelf.

British scientists propose that cracks from the bottom of the ice shelf are the bigger worry. They recently measured the crack resistance of ice cores drilled from the Ronne Ice Shelf, an ice mass south of Larsen B along the peninsula. Those experiments show that the top 50 m of material, composed of unconsolidated snow and ice, isn't as tough as the layers below it, which have been packed by the weight of the overlying material The bottommost layers, which formed when seawater froze to the underside of the ice shelf, are just as crack-resistant as the midlevel ice formed from dense-packed snow, says Martin A. Rist, a glaciologist at the University College London. He and his colleagues reported their findings in the Jan. 10 Journal of Geophysical Research (Solid Earth). Cracks growing into the lower surface are filled with water under high pressure. At a depth of 220 m, water pressure would be about 21 times that of the atmosphere at sea level.

Sediment cores have been collected from the ocean bottom in an area, just north of Larsen B, exposed by ice-shelf disintegrations in the early 1990s. The cores indicate that the shelf there was only about 2,000 years old (SN: 9/8/01, p. 150). However, a preliminary analysis of sediment layers in cores taken in December 2001 from seafloor near Larsen B suggests that this shelf has been in place for more than 12,000 years, says Eugene W. Domack, a marine geologist at Hamilton College in Clinton, N.Y. He'll present those results at an international workshop on Antarctic climate variability hosted by his college next week.