NEW YORK (NYTIMES) - For the first time since satellites began observing Antarctica nearly a half-century ago, an ice shelf has collapsed on the eastern part of the continent, scientists said.

The collapse of the 1165 sq km Conger ice shelf in a part of the continent called Wilkes Land occurred in mid-March. It was first spotted by scientists with the Australian Bureau of Meteorology and appeared in satellite images taken last Thursday (March 17), according to the National Ice Centre in the United States.

Ice shelves are floating tongues of ice at the end of glaciers that in Antarctica serve as outlets for the continent's massive ice sheets. Stresses cause cracks in the floating ice, and meltwater and other factors can cause the fissures to erode and grow to a point where the shelf disintegrates rapidly.

According to the National Ice Centre, the largest fragment of the Conger shelf after the collapse was an iceberg, named C-38, that was about 518 sq km in size.

The loss of a shelf can allow faster movement of the glaciers behind it, which can lead to more rapid ice sheet loss and thus greater sea level rise. Ice shelf loss is a major concern in West Antarctica, where warming related to climate change is having a greater effect than in the east.

Several very large glaciers in West Antarctica are already flowing faster, and if their ice shelves were to collapse completely, sea levels could rise on the order of 3m over centuries.

But the two glaciers behind the Conger sheet are small, and even if they were to accelerate, they would have minimal effect on sea level, on the order of fractions of about 2.5cm over a century or two, said Ted Scambos, a senior researcher at the Earth Science and Observation Centre at the University of Colorado Boulder.

While some ice shelves have collapsed in West Antarctica - notably, the much larger Larsen B, in 2002 - the Conger collapse is the first observed in East Antarctica since the era of satellite imagery began in 1979, said Catherine Walker, a glaciologist at Woods Hole Oceanographic Institution in Massachusetts.

Walker, who had been monitoring the ice shelf for a few months, said it had been retreating for several years. "It was an unhealthy little ice shelf to begin with," she said. But it had appeared to become stabilised, she said, between the mainland and a small island.

So while the collapse was not a complete surprise, it occurred sooner than expected, she said. She and Scambos agreed that recent weather in that part of Antarctica may have played a role.

In mid-March, an atmospheric river, a plume of air heavy with water vapour, swept into East Antarctica from the ocean to the north. It resulted in record-setting warmth in some locations, with temperatures as much as 21 deg C higher than normal for this time of year.

The warmth could have led to more surface melting of the Conger ice shelf, helping to further erode its fissures and hastening its collapse. But Scambos said it was likely that the windy conditions resulting from the atmospheric river, combined with record-low sea ice around Antarctica this season, played a larger role.

Sea ice serves as a buffer, damping the swells that roll in to the coast from the Southern Ocean. With little ice, and with the wind stirring the ocean even more, the floating shelf flexed more than it normally would. "The flexing probably weakened the more fixed parts of the ice that held the shelf together," Scambos said.

"The warm pulse probably didn't do a lot," he said, "but wind events and warm temperatures in the air and in the ocean certainly don't help with ice shelf stability."

East Antarctica has been considered to be the more stable region of Antarctica, with less warming and even ice gains in some areas. The collapse of the Conger ice shelf does not really change that view, Walker said. "We don't see any indication that this is going to happen in the rest of East Antarctica anytime soon," she said.

Scambos, who studies the more at-risk ice shelves and glaciers in West Antarctica, said it will be interesting to see what happens with the glaciers behind Conger. "Every time one of these things happens," he said, "it tells us a little bit more about how bigger parts of Antarctica are going to respond when bigger events occur."