Antarctic ice shelf loses large piece.

The icy apron surrounding parts of Antarctica got a little smaller in February, when a 200-square-kilometer chunk suddenly broke off. The loss of this patch may have destabilized surrounding areas enough to trigger the collapse of an entire ice shelf, says Ted Scambos of the National Snow and Ice Data Center in Boulder, Colo.

Scambos and his colleagues discovered that a section of the coastline was missing when they compared sequential satellite images of the Antarctic Peninsula. The Larsen B ice shelf--one of the many floating sections of ice that rim Antarctica--appears intact in an image from Feb. 15. In a shot taken 11 days later, part of the shelf had started breaking away. By March 23, a section of ice measuring 5 km by 40 km had disappeared.

The recent event comes 4 years after the 1,000-square-km Larsen A ice shelf disintegrated over a few days. Other ice shelves have disappeared since the 1950s, as temperatures along the Antarctic Peninsula have climbed. "The significance of these breaking up is that things that had been stable for several centuries are no longer stable," says Scambos.

The recent loss may cause the entire Larsen B shelf, which covers more than 10,000 square km, to crumble in the next year or two, says Scambos. In the Feb. 19 Nature, an international team of researchers used a computer model to assess the health of the shelf. Although it appeared stable at the time of their writing, they suggest that "if the [Larsen B] ice front were to retreat by a further few kilometers, it too is likely to enter an irreversible retreat phase."