Gee! What Do You Scientific American took a break from their |
|
Melting AwayThe shrinking of an immense swath of Antarctic Now new geologic evidence and one-of-a-kind satellite images are shedding light on West Antarctica's disappearing act. The bad news: the ice sheet may continue to shrink whether or not humanity curbs its release of heat-trapping greenhouse gases. The good news: its potential collapse may be slow enough that people will have time to move their cities out of harm's way. Scientists worry about the West Antarctic ice sheet more than its counterparts in East Antarctica and Greenland, which cover bedrock that sits well above sea level. In contrast, West Antarctica's rocky foundation lies up to 2,500 meters below the ocean surface. The danger is that if the ice shelves that extend seaward from the continent start floating higher, they may pull the "grounded" ice away from the bedrock, making it more apt to crack into icebergs and melt. A complete breakup of the ice sheet, which is about the size of Mexico, would raise sea level by five or six meters.
As the planet warmed, the ice that had gripped much of North America melted, the oceans swelled and the grounded ice in West Antarctica pulled away from the bedrock in response. To find out just how fast this separation happened, Hall and her team needed to figure out the age of a beach that had formed along the ice sheet after its first known step inland. The bits of organic matter needed to perform radiocarbon dating are difficult to come by in the barren Antarctic landscape. Trowels and tweezers in hand, Hall and her team often hiked 30 kilometers a day, scouring the rocky soil for the mollusk shells and sealskin that prove that seasonal open water must have existed there in the past. Radiocarbon dates for the shells found at the oldest beach, which today juts out into the Ross Sea near McMurdo Sound, indicate that the region was free of grounded ice by 7,600 years ago. And based on organic beach material and radar images of the subsurface ice at two points farther inland, the ice has been retreating at an average rate of 120 meters per year ever since. While Hall's team plotted how fast the edge of the ice sheet has been shrinking, a different research group has found evidence of when the ice began its retreat--using rocks stranded along the flanks of Mount Waesche volcano, which sits in the middle of the ice sheet and records the highest elevation the ice ever reached. "We use the volcano like a dipstick," says Robert P. Ackert, Jr., of the Woods Hole Oceanographic Institution. Ackert and his colleagues looked at the accumulation of cosmic particles that first struck the rocks when they were left exposed on the volcano, as the ice began to thin. The time that has passed since the rocks' exposure indicates that the ice did not begin its retreat until 10,000 years ago--at least 3,000 years after the oceans began to rise. These findings together suggest that the ice of West Antarctica is slow to react and can continue to change even long after an external trigger--in this case, rising sea level--has stopped. What's more, the ice sheet shows no signs of halting its inland march, Hall says. At its current pace, it will disappear in 7,000 years regardless of global warming. But that prediction is extrapolated from only four past positions of the ice sheet. "We don't have enough data to know whether it has retreated in jumps and spurts," Hall notes. Jumps and spurts are especially hard to predict because of the way the continent sheds its icy load. Antarctica may be shrinking, but oddly enough, it is not melting, at least not directly. Meltwater pours off Greenland's icy veneer, but in much colder West Antarctica "streams" of swift-moving ice do the shedding. Snow falls in the interior, and the streams carry ice to the sea, where it breaks into icebergs. Until now, no one knew what was happening at the streams' source, but researchers are a giant leap closer to understanding just how these ice streams work, thanks to new images made by a Canadian satellite called Radarsat. On two occasions during the fall of 1997, the satellite measured reflections of cloud-penetrating radar over much of the ice sheet's thick interior upstream from the Ross Ice Shelf. Using a technique called interferometry, Ian R. Joughin of the Jet Propulsion Laboratory in Pasadena, Calif., and his co-workers mathematically compared the two sets of reflections to determine the speed and direction of the ice at each point [see illustration above]. "You don't see the picture until you connect all the dots," Joughin says. "That's what our image does." In times past, a single velocity measurement required that someone go to the spot and plant a stake with a Global Positioning System receiver in the ice, leave for a certain amount of time, then go back and see how far it had moved. In the barren chill of the Antarctic, that's no easy task. "The step forward is just remarkable," says glaciologist Richard B. Alley of Pennsylvania State University. "In the past we were really unclear about what the ice sheet looked like and how it changes." Before Radarsat, some specialists had suspected a stable "lake" of accumulating snow might feed these swift streams, but it turns out that long tributaries nourish the streams from snowy regions deeper in the ice sheet's interior. "There's always more chance for instability when the ice flow extends so far inland," Joughin says. These tributaries flow at about 100 meters per year--roughly 10 times as fast as the ice sheet itself. At that pace there could be enough friction that the ice is actually melting along the bottom, he adds. Alley points out that lubricated streambeds are probably not new. Ice has been sliding quickly out of the interior for a long time, he notes: "Otherwise the ice would have been much thicker at Mount Waesche." The satellite images also revealed that tributaries are still feeding one stream that previous researchers had given up for dead when it dammed up 140 years ago. That means if global warming melts ice elsewhere, rising sea level could tear up the Ross Ice Shelf and break the dam, which would allow ice from inland to flow faster, Joughin says. Alley cautions that scientists are still far from being able to predict the fate of West Antarctica. "We'd like tell you whether it's going to fall in the ocean, but there's a lot of fundamental science we still just don't know," he says. This summer Alley and his colleagues will begin analyzing a kilometer-long ice core from West Antarctica that could reveal whether the ice sheet vanished in the warm times before the last ice age. If it did, that may give New Yorkers and the rest of the world more reason to be wary of a future meltdown. --Sarah Simpson |
|
Satellites | Oil | Cars Suck! |
