Greenland isn't just a huge ice-covered island. It is also a crucial factor in how global warming may reshape the world. If all the ice and snow on Greenland were to melt, the oceans would rise 20 feet. Say goodbye to Miami and big chunks of Florida and Louisiana. New York's LaGuardia Airport and parts of Brooklyn and Manhattan would be submerged. Hundreds of millions of people would be displaced in Bangladesh.
So how fast is Greenland melting? To figure that out scientists have been clocking the speed at which ice sheets are sliding to the sea, and have shown that the speed has increased. New research published Apr. 17 in the online journal Science Express explains how this actually occurs. Here's what happens: Big lakes of meltwater form on the surface of the ice in the summer. The pressure from the water then creates cracks in the ice sheet that go all the way down to bedrock, more than half a mile below. The water then gushes down through the ice in a cataclysmic flow rivaling Niagara Falls.
Down at the bedrock, the water actually lifts up the massive ice sheet and acts like grease, doubling the speed of the glaciers' journey over the bedrock to the sea. "It matters," says Richard Alley, professor of geosciences at Pennsylvania State University, and an ice-sheet expert: "It is not 'run for the hills, we are doomed,' but this tells us that loss of the Greenland ice could happen in centuries, not millennia."
The Mystery of the Disappearing Lakes
The new research proves a theory proposed by Alley and others. Scientists had been puzzled by the fact that the glaciers on Greenland seemed to be moving at an increasing speed (though at less than a tenth of a mile per year, the pace is glacial). The researchers already knew, from observations and calculations, that Greenland's ice is slowly shrinking simply from melting. That process is slow, however. "It would take a lot of centuries to melt the whole thing," says Alley. Yet if the glaciers also started sliding faster to the sea, the loss of ice could be more rapid.
In poring over satellite images, researchers noticed that large lakes form on the surface of the glaciers during the summer. Those lakes then suddenly disappear. "We see these things come and go," says Sarah Das, glaciologist at the Woods Hole Oceanographic Institution in Woods Hole, Mass. One possibility was the lakes simply drain into rivers on the surface. Or, scientists theorized, the water might generate enough pressure to crack the 3,000-foot-thick ice all the way to bedrock, pouring down through the ice.
A team led by Das and Ian Joughin of the University of Washington set out to prove which idea was correct. They flew to an area of glaciers with lakes, and set up camp on the ice. Then they installed an array of instruments. They put sensors in a lake 2½ miles wide to measure the changes in the amount of water it held. They deployed seismometers to detect rumbles in the ice, and put global-positioning units on the ice to chart its movement. They left the instruments in place when they left the study area, and waited.
Ice Disappearing Faster Than We Thought
It wasn't a long wait. "The lake drained about 10 days after we were there," Das says. When they went back, gathered up the instruments, and began to look at the data, it was clear that the crack theory was correct. The water had indeed rushed down to the bedrock. It had spread out under the ice, and raised the huge ice sheet by more than three feet. But there was a also a surprise: It happened in a relative flash. "The entire lake drained in about two hours," says Das. "It was a much more catastrophic drainage than we expected." The volume of water flowing down to bedrock matched the torrent over Niagara Falls. That realization led the team to ditch plans to explore other, still-intact lakes aboard a rubber boat. "We decided we'd leave the boat in its crate," laughs Das.
The data also showed that as the water flowed underneath it, the glacier did indeed speed up, doubling its velocity. That doesn't mean Greenland's ice is about to fall into the sea, since the glaciers move pretty slowly to begin with. But it does mean that the ice will disappear faster than just from melting alone, cutting the time from thousands of years to hundreds.
Ice Sheets' Tenuous Survival
And there's still a big unknown. How widespread is this phenomenon? The lake studied by Das and Joughin was 12 miles from the sea. Further inland, Greenland is still so cold that the ice is frozen to the bedrock, "so it doesn't slide at all," explains Penn State's Alley. Warming will probably create more lakes in these regions, which then could open up their own cracks to the bedrock.
"If you dump a flow of water like Niagara down, it would be hard for the ice sheet to stay frozen to the bedrock, so we could see more speed-up than Ian and Sarah observed," says Alley. "That still gives us centuries [before the ice disappears], but it makes the survival of the ice sheet more tenuous." So go ahead and buy that beachfront property in Florida now, but don't expect it to still be above water for too many generations to come.