Down south, shattering ice uncorks the Antarctic
Over three days in March 2002, there occurred one of the most dramatic alterations to the map of Antarctica since the end of the last ice age. It happened on the shoreline of the Antarctic Peninsula—a tail of mountains 1,200 miles long and more than a mile high pointing from the southern part of the continent toward the tip of South America. A shelf of floating ice larger than Luxembourg and some 650 feet thick, which had been attached to the peninsula for thousands of years, shattered like a huge pane of glass. It broke into hundreds of pieces, each of them a huge iceberg that floated away into the South Atlantic.
There were no casualties, except the self-esteem of Antarctic scientists who believed that after a century of studying the continent's ice, they knew how it behaved. Their subsequent papers revealed their shock. "The catastrophic break-up of the Larsen B ice shelf is remarkable because it reveals an iceberg production mechanism far different from those previously thought to determine the extent of Antarctic ice shelves," wrote Christina Hulbe, a peace activist and glaciologist from Portland State University, in Oregon. Rather than the normal "infrequent shedding of icebergs at the seaward ice front," this time "innumerable icebergs were created simultaneously through the entire breadth of the shelf."
The demise of the Larsen B ice shelf was not in itself a surprise. Both the air and the water around the Antarctic Peninsula had been warming since the 1960s. It had become one of the hot spots of global warming. Warm currents had been gradually eating away at the underside of the floating shelf, while warmer air produced pools of melting water on the surface. It was obvious that the sheet was under strain. Some cracks formed across the surface in 1994; a chunk around the edge of the shelf broke off in 1998. But nothing had prepared glaciologists for what was about to happen. During January 2002, the height of the southern summer, temperatures hit a new high and the heavy winter snow on the shelf's surface began to melt. By the end of the month, satellite pictures showed dark streaks across the shelf. Some were ponds, but others were crevasses that had filled with water.
Water is denser than ice. So, once inside the crevasses, it created pressure that levered them ever wider. There were, in effect, thousands of mechanical wedges pushing ever deeper into the ice shelf. Then, in three climactic days at the start of March, the entire structure gave way. Some 500 billion tons of ice burst into the ocean. In many ways, says Richard Alley, what happened at Larsen B mirrored the processes under way in Greenland. "Water-filled cracks more than a few tens of yards deep can be opened easily by the pressure of water. Ponding of water at the ice surface increases the water pressure wedging cracks open." In their enthusiasm to study ice, glaciologists had forgotten about water.
Larsen B was one of a series of floating shelves formed by ice draining from the mountains of the Antarctic Peninsula. The shelves are the floating front edges of glaciers, and where they meet the ocean, icebergs regularly break off.
In recent years, Larsen B had been moving forward by about a yard a day. Despite this constant movement, the ice shelf itself, at more than 650 feet thick, was a surprisingly permanent structure. After its collapse, study of the diatoms in the sediment beneath the former shelf suggested that Larsen B had been there for the entire 12,000 years since the end of the last ice age, when a single ice sheet covered the whole region.
Larsen B wasn't alone; nor has it been alone in disappearing. In all, more than 500 square miles of ice shelves have been lost from around the Antarctic Peninsula in the past half century. The Larsen A ice shelf, the other side of an ice-covered headland called Seal Nunatak, broke up in a storm in 1995. And before that, the Wordie shelf, on the west side of the peninsula, disappeared between 1974 and 1996, triggering a dramatic thinning of the glaciers that fed it. But both were much smaller than Larsen B, and neither disappeared in the catastrophic manner of Larsen B.
"Really we don't think there is much doubt that the collapse of the Larsen B shelf was caused by man-made climate change," says John King, chief climatologist at the British Antarctic Survey (BAS), the inheritor of the great tradition of explorers such as Robert Scott and Ernest Shackleton. From their base at Rothera, on Adelaide Island, BAS researchers have mapped in detail how a pulse of warmer air temperatures has pushed south across the peninsula over the past fifty years, lengthening the summer melt season, sending glaciers into retreat, and destabilizing ice shelves as it goes.
Armed with the evidence of Larsen B, glaciologists are reassessing the stability of dozens of peninsula ice shelves—starting with Larsen C, immediately to the south, which is thinning and widely expected to be the next to go. Eventually, they say, the warming will reach the Ronne ice shelf, a slab of ice the size of Spain at the south of the peninsula. And on the other side of the continent is the Ross ice shelf, the continent's largest. It, too, now seems to be vulnerable, says Hulbe.
Disappearing ice shelves do not contribute to sea level rise because their ice is already floating. Their loss no more raises sea levels than an ice cube melting in a drink causes the glass to overflow. But their disappearance does change what happens inland. Ice shelves buttress the glaciers that feed them. After Larsen B disappeared, it was "as if the cork had been removed from a bottle of champagne," says the French glaciologist Eric Rignot, who works at NASA's Jet Propulsion Laboratory, in California. The glaciers that once discharged their ice onto the Larsen B shelf are now flowing into the sea eight times faster than they did before the shelf collapsed. Similar acceleration has happened after other ice sheet collapses. And that faster discharge of ice from land into the ocean is raising sea levels. With the Ross Sea being the main outlet for several of the largest glaciers on the West Antarctic ice sheet, which contains enough ice to raise sea levels by six yards, the stakes are rising.
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