The Antarctic peninsula is a long, skinny curve of rock, and it reaches north toward the tip of Chile, like a gnarled finger beckoning you toward the pole. In October 2001, still spring on the peninsula, an Argentine glaciologist named Pedro Skvarca was on top of an ice shelf known as Larsen B doing fieldwork. Research in Antarctica imposes a special form of isolation; even in summer, scientists must surround their tents with snow banks to keep out the wind. The Argentines have a permanent base on the peninsula, and Skvarca had spent more time on Larsen B, and knew it more intimately, than nearly any other scientist.
Over the years, as he visited the ice shelf, Skvarca had watched the entire landscape change. Huge crevasses had opened up, rifts in the ice, the biggest among them visible from orbiting satellites. Skvarca found himself surrounded by meltwater, ponds of shimmering blue water up to 100 feet across. He could barely get any work done safely: Each time he pitched a tent it filled with water. This year, the melting was far more extensive. The ponds were most numerous in the north, where the weather was warmest, but they had spread out across the entire ice shelf. If Larsen B was a cork, then it looked like it was about to come unstuck. When he returned to the Argentine base, Skvarca e-mailed several of his colleagues in the United States and Europe. "I think this is it," he wrote.
Scientists had been worried about this portion of the peninsula for years. Over the past half-century, temperatures in this part of Antarctica have leapt by five degrees, and wind speeds have increased by 15 percent. Climatologists believe that the amount of carbon in the atmosphere and the size of the ozone hole control the winds like a dial: The more we've warped the climate, the faster the winds blow. At Larsen B, the combination operated like a convection oven, baking the ice each summer and melting it from above and below. "The stronger winds push more warm air over the peninsula," says John Turner, project leader for climate variability and modeling with the British Antarctic Survey. "It's been the nail in the coffin."
During the first week of March 2002, a few months after Skvarca sent his e-mail warning, Larsen B was obscured by clouds for several days; it was so overcast that orbiting satellites couldn't get a good image of the area. When the clouds parted, on March 5th, and the satellites could see through again, the scientists stared at the images in disbelief. Nearly two-thirds of Larsen B, an ice shelf the size of Delaware, had disappeared into the sea. The glaciers of the peninsula had come uncorked, altering the shape of Antarctica's map in only a few days. "How rapidly and completely Larsen B broke was beyond our imagination," says Ted Scambos, lead scientist at the National Snow and Ice Data Center in Boulder, Colorado.
Since then, scientists have pieced together an extremely detailed model of how Larsen B shattered, bit by painstaking bit. The ice shelf, they believe, was so profoundly weakened from years of melting that it would have taken only a small disturbance at the water line — likely a wave of precisely the right amplitude — to rock the shelf back and calve off a long, narrow iceberg, sending it toppling into the water. The splash of that first berg rebounded against the edge of the shelf, in waves as high as a tsunami, breaking off a second iceberg, along the cracks opened up by the water, and the second berg begat a third. "It's like what happens in a mosh pit, where you have a chain-reaction feedback of energy," says Doug MacAyeal, a geophysicist at the University of Chicago who has studied Larsen B. Before long, a huge semicircle of water was rushing into the opening left behind by the ice shelf — moving inland at a rate of more than a dozen miles a day. "It looked like a giant disintegration machine had started eating into the ice sheet," Scambos says. "Here was unequivocal evidence of something happening because of climate change — and I think it really scared a lot of people."
A few days later, at the end of March, a British research ship sailed to the edge of the harbor, still too choked with bergs to actually enter. From the deck, a group of oceanographers took in the scene: The collapse of Larsen B had left behind exposed ice cliffs hundreds of feet high, so blue and so precisely angled that they looked almost unnatural, as if they'd been cut by a giant buzz saw. The physics had been so intense that the ice was shattered into pieces as tiny as gravel. Humpback whales occasionally drifted into these waters, but the scientists, looking around, saw that the sea was packed with them, drawn by the reverberating energy of the collapse, breaching everywhere they looked. With Larsen B gone, it seemed that one of the most enduring questions in glaciology might now be solved: What happens when you remove an ice shelf? Would the large glaciers that had once snaked down to Larsen B from the continent rush into the sea, like uncorked champagne? Or would they stay put, held in place by the rocks below?
Satellites over Antarctica don't work well in the lightless winter, so it took until the next spring — late October, early November — for photos to appear. Soon scientists were rushing to get papers into print about the glaciers behind Larsen B. Satellites examining one of the main tributaries of the ice shelf, Crane Glacier, showed that not only had the edge of the glacier begun to race rapidly toward the ocean, but that the speedup was taking place much farther inland than expected. Even more remarkably, the Hektoria Glacier, the largest river feeding ice into Larsen B, the largest had dropped in height by more than 80 feet in just six months. The leading edge of the glacier had slid out into the sea, its front decomposing from smooth ice to crunched, crevassed fragments, like stretching toffee. In more normal times, a drop in elevation of just a few feet had been considered big news.
"What we were able to see at Hektoria and at Crane was that the ice shelves do, in fact, have a huge impact on the glaciers behind them," says Scambos, who led one of the two scientific teams analyzing the data. "They are the Achilles' heels of the ice sheet." Nature, as glaciologists say, had provided the perfect experiment at Larsen B and resolved the debate: Remove the ice shelves, and the glaciers behind them would go racing for the sea.
Larsen B was, by geological standards, a vast formation of ice, but it is dwarfed by much larger ice shelves farther south, on the main part of the Antarctic continent. Unlike Greenland, which holds a small society, Antarctica is a planetary lockbox, a third the area of the moon and nearly as remote; it only holds ice. The ice sheets on both sides of the continent contain huge amounts of ice that lay below sea level — in West Antarctica alone, enough to raise global seas by more than 10 feet. A single shelf — the Ross Ice Shelf, the world's largest — is the size of France. "The lesson of Larsen B," Scambos says, "is that if you remove an ice shelf, then you will quickly tap deep into the center of the ice sheet."
Still, scientists weren't too worried that the glaciers behind the biggest ice shelves would rush into the ocean. Larsen B had been removed by sustained melting from a warmed atmosphere, a process almost impossible to imagine further south, where air temperatures never warm past the freezing point. So Antarctica was safe. Or it was so long as nature, evolving, didn't find some other way to remove the remaining ice shelves.
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