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

visitor to Swiss Camp is the glaciologist Jay Zwally, one of Hansen's colleagues at NASA. He made the alarming discovery that during warm years the half-mile-thick ice lifts off the bedrock and floats on the water-rising half a yard or more at times. And it floats toward the ocean. Ice sheets are never entirely still, of course. But Swiss Camp is already more than a mile west of where it started. And Zwally found that in summer, when the surface is warmer and more water pours down the crevasses, the velocity of the ice sheet's flow increases. Acceleration starts a few days after the melting begins at the surface. It stops when the melting ceases in the autumn.
    This discovery is a revelation, glaciologists admit. "These flows completely change our understanding of the dynamics of ice-sheet destruction," says Richard Alley, of Penn State. "We used to think that it would take Io,ooo years for melting at the surface to penetrate down to the bottom of the ice sheet. But if you make a lake on the surface and a crack opens and the water goes down the crack, it doesn't take io,ooo years, it takes ten seconds. That huge lag time is completely eliminated."
    As ever, Alley has a good analogy. "The way water gets down to the base of glaciers is rather the way magma gets up to the surface in volcanoesthrough cracks. Cracks change everything. Once a crack is created and filled, the flow enlarges it and the results can be explosive. Like volcanic eruptions. Or the disintegration of ice sheets." The lakes on the surface of Greenland are, he says, the equivalent of the pots of magma beneath volcanoes. "More melting will mean more lakes in more places, more water pouring down crevasses, and more disintegration of the ice." No wonder, in a paper in Science, Zwally called the phenomenon "a mechanism for rapid, large-scale, dynamic responses of ice sheets to climate warming."

    Could such processes be close to triggering a runaway destruction of the Greenland ice sheet? It is hard to be sure, but Greenland does have past form, says David Bromwich, Box's colleague at Ohio State. There is good evidence that the ice sheet lost volume around 120,000 years ago, during the warm era between the last ice age and the previous one. "Temperatures then were very similar to those today," he says. "But the Greenland ice sheet was less than half its present size." He believes that the Greenland ice sheet is a relic of the last ice age whose time may finally have run out. "It looks susceptible, and with the drastic warming we have seen since the 198os, the chances must be that it is going to melt, and that water will go to the bottom of the ice sheet and lubricate ice flows."
    Greenland melting seems to have set in around 1979, and has been accelerating ever since. The interior, above the rising equilibrium line, may still be accumulating snow. But the loss of ice around the edges has more than doubled in the past decade. The NASA team believes that "dynamic thinning" under the influence of the raging flows of meltwater may be responsible for more than half of the ice loss. In early 2006, it reported the results of a detailed satellite radar study of the ice sheet showing that it was losing 18o million acre-feet more of ice every year than it was accumulating through snowfall. That was double the estimated figure for a decade before. And all this gives real substance to the evidence accumulating from Greenland's glaciers, the ice sheet's outlets to the ocean.
    Swiss Camp is in the upper catchment of a glacier known as Jakobshavn Isbrae. It is Greenland's largest, flowing west from the heart of the ice sheet for more than 400 miles into Baffin Bay. It drains 7 percent of Greenland. Jakobshavn has for some decades been the world's most prolific producer of icebergs. From Baffin Bay they journey south down Davis Strait; past Cape Farewell, the southern tip of Greenland; and out into the Atlantic shipping lanes. Jakobshavn was the likely source of the most famous