Has the Meltdown Begun?
The discovery that Greenland's glaciers are melting faster than anyone expected has experts worried anew about how high the seas will rise
By MICHAEL D. LEMONICK
The usual argument put forth by global-warming skeptics for why we shouldn't rush to do anything yet is that the science behind climate change is uncertain--and in fact it is. While there's little doubt that humans are helping heat up the planet, the questions of how much, how quickly and leading to what consequences are fiendishly difficult to pin down. That's because the actual climate is still far more complicated than any existing computer model can accurately reflect, making predictions iffy at best. Some natural processes nobody has yet thought of could end up blunting the severest impact of global warming.
Or, conversely, they could make the impact even worse than expected. And according to a study that sent tremors through the scientific community last week, that is exactly what seems to be happening in Greenland. Glaciers that flow toward the ocean in the southern half of that enormous frozen island are among the world's fastest moving, and their massive outpouring of ice now contributes fully a sixth of the annual rise in sea level. According to a study in the current issue of Science, they have nearly doubled their rate of flow over the past five years, to about 8 miles a year, dumping icebergs and meltwater into the already rising ocean faster than anyone expected. "In 1996 Greenland was losing about 100 cu km of ice per year," says Eric Rignot of NASA's Jet Propulsion Laboratory, lead author of the study, which he presented at last week's meeting of the American Association for the Advancement of Science in St. Louis, Mo. "This year it will lose more than twice as much." By comparison, he says, in 1996 Greenland dumped 90 times as much water into the sea as Los Angeles consumed; last year it was up to 225 times. "In the next 10 years," says Rignot, "it wouldn't surprise me if the rate doubled again."
No computer climate model anticipated that increase, which means that all current predictions about how much sea level could rise--the latest U.N. report estimated it at a half-meter (about 1.5 ft.) by the end of the century--are too low and will have to be revised upward. Greenland's ice cap covers more than 650,000 sq. mi. and in places stands nearly 2 miles thick. "If it all melted or otherwise slid into the ocean, sea level would rise by 20 ft. or so," says Michael Oppenheimer, a professor of geosciences and international affairs at Princeton. Under conventional global-warming scenarios, that will eventually happen--but over a period of several thousand years. The new study suggests that it could happen in a few hundred years. "That's a few feet per century," says Oppenheimer, "which may not sound like a lot, but it's more than society can handle. In places like the Eastern seaboard of the U.S., a 1-ft. vertical rise in sea level means a 100-ft. retreat of shoreline." In low-lying countries like Bangladesh, the resulting flooding could dwarf the 2004 tsunami.
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."
http://www.time.com/time/magazine/printout/0,8816,1161231,00.html
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