Satellite images reveal that a large chunk of the Wilkins ice shelf fell into the ocean sometime around the end of February, suggesting that climate change could be causing it to disintegrate faster than scientists had predicted.
The 160-square-mile piece, about seven times the size of Manhattan, had been attached to Antarctica for hundreds, or maybe even 1,500 years.
Scientists fear that the entire Wilkins shelf, more than 6,000 square miles, could be next.
British Antarctic Survey researcher David Vaughan blamed global warming for the smaller piece's disintegration.
Satellite images of the initial breakaway on Feb. 28 were noticed within hours, and scientists diverted satellite cameras and even flew an airplane over the ongoing collapse for rare pictures and video.
"It's an event we don't get to see very often," said Ted Scambos, lead scientist at the National Snow and Ice Data Center in Boulder, Colo. "The cracks fill with water and slice off and topple... That gets to be a runaway situation."
While icebergs naturally break away from the mainland, collapses like this are unusual but are happening more frequently in recent decades, Vaughan said. The collapse is similar to what happens to hardened glass when it is smashed with a hammer, he said.
The rest of the Wilkins ice shelf is holding on by a narrow beam of thin ice. Scientists worry that it too may collapse.
Larger, more dramatic ice collapses occurred in 2002 and 1995.
Vaughan had predicted the Wilkins shelf would collapse about 15 years from now. The part that recently gave way makes up about 4 percent of the overall shelf, but it's an important part that can trigger further collapse.
There's still a chance the rest of the ice shelf will survive until next year because this is the end of the Antarctic summer and colder weather is setting in, Vaughan said.
Scientists said they are not concerned about a rise in sea level from the latest event, but say it's a sign of worsening global warming.
Such occurrences are "more indicative of a tipping point or trigger in the climate system," said Sarah Das, a scientist at the Woods Hole Oceanographic Institute.
"These are things that are not re-forming," Das said. "So once they're gone, they're gone."
Climate in Antarctica is complicated and more isolated from the rest of the world.
Much of the continent is not warming and some parts are even cooling, Vaughan said.
However, the western peninsula, which includes the Wilkins ice shelf, juts out into the ocean and is warming. This is the part of the continent where scientists are most concern about ice-melt triggering sea level rise.