Has the Meltdown Begun?

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.”

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.”