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What jump-started the glaciers' outflow isn't precisely clear, but scientists point to two likely triggers. The first, says Julian Dowdeswell of the Scott Polar Research Institute at the University of Cambridge, is the breakup of ice "tongues" that reach out into the sea at the glaciers' leading edges. It's likely, he says, that removing that barrier allowed the glaciers to flow more freely. The second is that ice on the glaciers' surfaces has melted at a record rate in two of the past four years. "Some of that water," says Dowdeswell, "presumably percolates down through crevasses," lubricating the soft sediments at the base of the glaciers and allowing the huge ice floes to slip more quickly to the sea.
What's even more ominous than the speedup is the fact that it's spreading northward. Between 1996 and 2000, says Rignot, glaciers started accelerating, but only up to the 66th parallel. Over the next five years, the speedup moved north to the 70th. "If it spreads even further north," says Dowdeswell, "the implications are that much greater."
It isn't just the rise in sea level that makes the surprising news out of Greenland so disturbing. That is only one more hint that climate change may hinge on tipping points, where relatively small changes in temperature can suddenly cause disproportionately large effects. In Greenland, it's meltwater greasing the way for massive outflows of ice. In Antarctica, which has one ice sheet the size of Greenland's and another nearly 10 times as large, the same sort of leverage could eventually come into play, with even greater consequences. Yet another tipping point could come as ice sheets shrink and the polar caps start absorbing rather than reflecting energy from the sun.
In the North Atlantic, meanwhile, scientists have been warning for more than two decades about an influx of freshwater--not just from Greenland but also from melting icebergs and increasing mainland runoff. The resulting drop in salinity could change the density of surface water enough to prevent it from sinking as it cools and returning south to the tropics where it can replenish ocean currents like the Gulf Stream. And because the Gulf Stream is the only reason much of Western Europe has so mild and temperate a climate, such a shutdown of that conveyor belt of heat could be nothing short of catastrophic. Oceanographers reported late last year the ominous news that one element of that family of currents has slowed 30% since 1992. It's not clear yet that this is the beginning of the feared shutdown, but if so, it represents yet another tipping point.
And there could be many more that scientists haven't yet uncovered. "I worry every day about other surprises," says Oppenheimer. "It would be the height of arrogance to assume that there won't be--as these results prove." Sure, he says, some of the surprises may cut in our favor, but adds, "I'd hate to count on that. We have only one world to play with."