The northern section of the Larsen B ice shelf--a thick slab of floating ice on the eastern side of the Antarctic Peninsula--has collapsed and separated from the continent, researchers report. The incident, which was monitored and recorded by satellite images, aerial photography and a research vessel navigating through the resulting icebergs, is the largest single event in a 30-year series of ice shelf retreats in the area.

Temperatures at the Antarctic Peninsula have increased by 2.5 degrees Celsius over the last 50 years, warming that is much faster than the concurrent average global temperature increase. One response to this warming, scientists say, is the retreat of five ice shelves, including Larsen B. "Since [1998], warming on the peninsula has continued and we watched as piece-by-piece Larsen B has retreated," David Vaughan of the British Antarctic Survey says. "We knew what was left would collapse eventually, but the speed of it is staggering." Over the course of 35 days, 3,250 square kilometers of shelf area disintegrated--an area larger than that of Rhode Island.

Ted Scambos of the University of Colorado at Boulder and colleagues analyzed the satellite imagery of the ice shelf's demise and found substantial observational proof for a theory of ice disintegration first proposed more than two decades ago. They posit that ponds of melt water present on the icy surface in late summer (the result of climate warming in the area) enhance fracturing of the shelf by filling small cracks in the ice. The weight of the water then drives the cracks through the ice, causing it to shatter.

Because the ice shelf was already floating, its break-up will not cause global sea levels to rise. The shelves do, however, act as braking systems for glaciers moving on the continent. "Loss of ice shelves surrounding the Antarctic continent could have a major effect on the rate of ice flow off the continent," Scambos notes. "The Ross ice shelf for instance, is the main outlet for the West Antarctic Ice Sheet, which encompasses several large glaciers and contains the equivalent of five meters of sea level in its perched ice."