A chunk of ice the size of Connecticut that had hovered along the coast of Antarctica for hundreds of years has disintegrated in less than a decade, and scientists believe they have finally figured out why.
If they are right, it took only a slight increase in summertime temperatures to spell the doom for the Larsen Ice Shelf.
Much attention has been focused on the ice shelf since it began dramatically breaking up in 1995, and it appears that the driving force that ripped it apart was simply water.
A team of scientists led by Ted Scambos of the University of Colorado at Boulder used satellite images, a NASA computer and historical records to conclude that many other areas of Antarctica could face a similar future. It is a matter of great concern because massive melting of the glaciers at the bottom of the world could cause the seas to rise globally, inundating low-lying areas.
Small Warming Means Big Melting
What's troubling about the scientists' report, published in the January issue of the Journal of Glaciology, is it didn't take much to trigger the process that virtually destroyed the Larsen Ice Shelf.
"What we saw was an indication that climate doesn't have to change a whole lot to have a much more rapid effect than was previously thought," says Scambos.
He insists, however, that he's no prophet of doom. It would take decades, if not centuries, to melt Antarctica's glaciers, he says. But the team's research suggests that the beginning of the process could come sooner than most scientists had thought.
One reason many scientists have doubted the doomsday scenario for Antarctica is the mean annual temperature there has remained fairly constant over the years. Some areas have experienced a rise of a few degrees over the last 50 years, but many experts believed that the temperature would probably have to rise about 20 degrees to make much difference.
But Scambos and his team found that the average annual temperature is not nearly as important as seasonal changes. A warmer summer can cause a dramatic change, even if the overall temperature for the year remains fairly constant.
That, apparently, is what happened on the Antarctic Peninsula, a long finger of land that points toward South America. The summertime temperature there has been two or three degrees warmer in recent years, playing havoc with the Larsen Ice Shelf along its shoreline.
Water in the Cracks
The warmer temperature caused some surface melting during the summer, the researchers say, and that caused cracks in the ice to fill with water.
Since water is denser than ice, it caused pressure to build within the crack, forcing the ice to break further, Scambos says.
Team member Christine Hulbe of NASA's Goddard Space Flight Center in Greenbelt, Md., used a NASA computer to measure the impact of water in a surface crack. The computer model revealed an astonishing fact. If the crack was only 15 feet deep, Hulbe found, the water would force its way down, propagating the crack all the way through a sheet of ice more than 600 feet thick.
That process apparently left the Larsen in tatters, held together here and there by narrow bands of ice that bridged across two sections of the shelf. When the bridges broke, like during a storm, a giant iceberg was released into the sea.
Mark Fahnestock of the University of Maryland studied two decades of satellite images of the area and found a key correlation between "ponds" of melted ice and the breakup of the Larsen Ice Shelf. The ice broke up the most during years with a lot of melting.
In 1995, when a 775-square-mile chunk of the ice shelf broke off during a violent storm, the "melt season" lasted 80 days, which was 20 days longer than normal, Fahnestock found.
So what appears to be happening is that rising summertime temperatures cause surface ice to melt, flooding cracks and wedging apart the ice. All it takes is a storm, or a nudge from a drifting chunk of ice, to break off a piece.
Hemming in the Continent
The loss of the Larsen shelf in itself is not all that important, unless you happen to be aboard a ship that has to dodge icebergs in the area, but the finding holds profound implications for the rest of Antarctica.
Researchers at Pennsylvania State University, for example, have shown that the huge shelves of floating ice along Antarctica's coast serve a very useful purpose. In effect, they hold back the glaciers on the mainland, providing a "braking system" that keeps the glaciers from sliding into the sea.
That hasn't been much of a problem along the peninsula, because the glaciers there are wedged in mountain valleys and even with the loss of the braking system, it's hard for them to move very fast. And they are smaller glaciers, at least for Antarctica, so their contribution to sea level rise would be inconsequential, Scambos says.
Not too far away, however, is the Ross Ice Shelf, a humongous chunk of ice that holds back giant glaciers that rest on mud. If the Ross were to disintegrate, those glaciers could slide into the sea, although that could take several decades.
Scientists estimate that if the landlocked ice in just the Western Hemisphere of Antarctica were to be released into the ocean, the seas would rise about 15 feet around the world.
What are the chances of that happening?
Drip by Drip
If the Ross experiences a summertime warming trend over the next 50 years equal to the warming of the Larsen over the last half century, it could also disintegrate. And, Scambos says, we're talking a couple of degrees, not a heat wave.
Of course, that might not happen. No one knows yet exactly how much warmer Antarctica is likely to get over the next few decades. But one thing is clear. It's changing.
"There's something fundamentally different about climate now than over the last several centuries," Scambos says. "The Larsen Ice Shelf had been there for several centuries," and it broke up almost overnight, geologically speaking.
And other areas of Antarctica "are closer than we thought to undergoing the same thing," he adds.
Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.