Free With Speed and Violence: Why Scientists Fear Tipping Points in Climate Change by Fred Pearce Page A

iceberg of all-the one that sank the Titanic in 1912. But it has been in overdrive since 1997, after suddenly doubling the speed of its flow to the sea. It is now also the world's fastest moving glacier, at better than 7 miles a year.
    Jason Box has installed a camera overlooking the glacier to keep track. It takes stereo images every four hours throughout the year. As well as flow ing ever faster toward the sea, he says, the glacier is becoming thinner, and in 2003 a tongue of ice 9 miles long that used to extend from its snout into the ocean broke off. "What is most surprising is how quickly this massive volume of ice can respond to warming," says Box. There seems to be a direct correlation between air temperatures in any one year and the discharge of water from glaciers into the ocean. Long time lags, once thought to be a near-universal attribute of ice movement, are vanishing. Jakobshavn, he estimates, could be shedding more than 40 million acre-feet a year, an amount of water close to the flow of the world's longest river, the Nile. Half of that volume is water flowing out to sea from beneath the glacier, and half is calving glaciers.

    Other Greenland glaciers are getting up speed, too. The Kangerdlugssuaq glacier, in eastern Greenland, which drains 4 percent of the ice sheet, was flowing into the sea three times faster in the summer of 2005 than when last measured in 1988. At an inch a minute, its movement was visible to the naked eye. Meanwhile, its snout has retreated by three miles in four years. This familiar pattern of faster flow, thinning ice, and rapid retreat of the ice front has also shown at the nearby Helheim glacier, where Ian Howat, of the University of California in Santa Cruz, concludes that "thinning has reached a critical point and begun drastically changing the glacier's dynamics."
    Most of these great streams of ice are exiting into the ocean beneath the waterline, in submarine valleys, via giant shelves of floating ice that buttress them. But as the oceans warm, these ice shelves are themselves thinning. It is, says Hansen, a recipe for rapid acceleration of ice loss across Greenland.
    The picture, then, is of great flows of ice draining out of Greenland, lubricated by growing volumes of meltwater draining from the surface to the base of the ice sheet and uncorked by melting ice shelves at the coast. All this is new and frightening. "The whole Greenland hydrological system has become more vigorous, more hyperactive," says Box. "It is a very nonlinear response to global warming, with exponential increases in the loss of ice. I've seen it with my own eyes. Even five years ago we didn't know about this." Alley agrees: "Greenland is a different animal from what we thought it was just a few years ago. We are still thinking it might take centuries to go, but if things go wrong, it could just be decades. Everything points in one direction, and it's not a good direction."

    "Building an ice sheet takes a long time-many thousands of years," says Hansen. "It is a slow, dry process inherently limited by the snowfall rate. But destroying it, we now realize, is a wet process, spurred by positive feedbacks, and once under way it can be explosively rapid."

     

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    THE SHELF
    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