Antarctica's Wilkins Ice Shelf eroding at an unforeseen pace

A crumbling ice shelf along the West Antarctic Peninsula has become the latest polar poster child for global warming.

This week, researchers in the United States, Britain, and Taiwan released images of long stretches of ice shearing away from the shelf. What started with the loss of a relatively thin, 26-mile-long iceberg at the end of February cascaded into the loss of 160 square miles of ice by the end of last week.

Its erosion won't affect sea levels. Like an ice cube in a filled cup, it's already in the water. And the handful of glaciers that feed into the shelf, called the Wilkins Ice Shelf, are small. Still, researchers say, the event represents a marker. The region has seen unprecedented rates of warming during the past 50 years. Two of the 10 shelves along the peninsula have vanished within the past 30 years. Another five have lost between 60 percent and 92 percent of their original extent. Of the 10, Wilkins is the southernmost shelf in the area to start buckling under global warming's effects.

"Wilkins is a stepping stone in a larger process," says Ted Scambos, a glaciologist at the National Snow and Ice Data Center in Boulder, Colo., who discovered the breakup in satellite images. "It's really a story of what's yet to come if the mainland of Antarctica begins to warm."

So far, the shelf has lost about 3 percent of its total extent, which covers an area more than twice the size of Rhode Island and is up to 820 feet thick. But all that sits between the shelf's new seaward edge and a vast expanse of much weaker shelf ice is what researchers dub a "thread" of strong ice. And Wilkins's erosion is happening faster than researchers projected.

"In 1993, we predicted that this was going to be a vulnerable ice shelf," says David Vaughan of the British Antarctic Survey. "But we got the time scales completely wrong. We were saying 30 years at that time, and now it's happened within 15."

Glaciologists are concerned about Antarctica's ice shelves because most of them represent brakes of solid ice that slow the glaciers' flow to the sea. Without those brakes, the glaciers would surge, calve into icebergs, and significantly raise the sea level.

The region of greatest concern is West Antarctica, which includes the peninsula. Using satellites, scientists have been tracking snowfall, ice loss, and changes in the region's gravity field to gauge the amount of mass the continent's two large ice sheets are gaining or losing. The West Antarctic Ice Sheet is separated from its eastern sibling by a long chain of mountains, so gains or no change in mass for the continent as a whole may still mask significant changes on the West Antarctic Ice Sheet.

Recent studies have added to a growing body of evidence that key glaciers flowing from the West Antarctic Ice Sheet are thinning at rates not seen since the last ice age. For instance, for the past 4,700 years, the Pine Island Glacier has thinned at a rate of about 1-1/2 inches a year, according to a team of scientists from Britain and Germany. That rate is similar to those of other major glaciers in the region. But between 1992 and 1996, Pine Island Glacier thinned at an average rate of 63 inches a year. Their results appear in the March edition of the journal Geology.

Meanwhile, a team led by the Jet Propulsion Laboratory's Eric Rignot published satellite radar data showing that while East Antarctica's ice sheet lost virtually no mass between 1992 and 2006, the West Antarctic Ice Sheet was losing 132 billion tons of ice a year by the end of that period. Scientists attribute the losses to warmer air and ocean temperatures, which melt the glacier's shelves from above and from below.

But the Wilkins Ice Shelf is a different breed in the way it forms, explains Dr. Vaughan.

Although it's been stable for as long as scientists have been able to reach the continent and study it, the shelf scientists see crumbling today appears to have formed either between the Roman era and the Medieval Warm Period or with the onset of the Little Ice Age. Indeed, he adds, "it's kind of a come-and-go ice shelf" compared with the other vanishing shelves, which have been stable far longer.

Wilkins appears to have started as seasonal sea ice that gradually thickened, Dr. Vaughn adds. Shelves built of glacial ice are stout, because the weight of each succeeding winter's snow has compressed the layers beneath until the glacial ice becomes solid.

By contrast, Wilkins and a handful of ice shelves like it are more porous. Over centuries they too thickened with each winter's snowfall. The weight of the new snow, however, drives a preceding year's layer under water before the upper layers build enough weight to squeeze the tiny nooks and crannies out of it.

The task now is to tease out the precise mechanisms triggering the recent collapse, researchers say. By figuring out the breakup mechanism in detail, scientists should be able to improve the models they use to anticipate the behavior of other ice shelves as climate and ocean conditions continue to change, Dr. Scambos says.