POSTED 17 JAN 2002
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bobsled poised above a perilous run, the Antarctic ice sheets are in position
for a quick downhill slide into the Southern Ocean. And while their velocity
is measured in meters per year rather than per minute, the stakes in this
plunge vastly exceed the contest for Olympic gold in bobsledding.
By plunging into the ocean, the East and West Antarctic Ice Sheets could raise sea level by 70 meters -- drowning coastal regions that are home to billions. Aside from a major asteroid impact, it's perhaps the nightmare scenario of global change.
The West Antarctica Ice Sheet is a massive accumulation of ice -- more than a kilometer thick -- that flows into the Ross Sea. Like a bobsled in extreme slow motion, the sheet crunches downward.
Also like a bobsled, gravity's downward pull causes movement; resistance comes from friction against the land below.
It's easy to measure a bobsled's speed, but that's a monumental task for a vast ice sheet. Previously, researchers had to visit the inhospitable ice desert to make measurements.
Got it on my fuzz-buster
New measurements by Ian Joughin of the Jet Propulsion Laboratory show that parts of the West Antarctic sheet are grinding to a halt. The Whillans Stream, one component of the sheet, slowed 23 percent between 1973-4 and 1997, when Joughin's radar data were collected. It's now moving about 400 meters per year. At the current rate, Whillans could grind to a halt in 80 years or so.
Another major component, poetically named Ice Stream C, stopped about 150 years ago.
Previously, scientists estimated that the West Antarctic sheet was losing 20 billion tons of ice per year. Based on the new radar velocity data, Joughin and co-author Slawek Tulaczyk estimate that it is actually gaining about 27 billion tons.
Scientists have focused on the West Antarctic ice sheet because it disappeared in past warm periods between the ice ages. "There is evidence that the Ross Sea was open, had no ice," says Joughin. "At some time in past interglacials, much of the West Antarctic ice sheet went away."
Were it to happen again, bye-bye Tokyo and Rome....
Earth is always bleeding out some internal heat which combines with heat from friction to melt ice beneath the glaciers, "greasing the skids" so the giant ice sheets can slide toward the Ross Sea.
Theoretically, the ice sheet's movement tends to limit itself. If the ice thins, the cold surface gets closer to the base, freezing those water bearings and causing the ice stream to pile up. When the ice gets thick enough, the blanket of ice insulates the base, and earth heat thaws the bearings so the downhill flow can resume.
While the Antarctic glaciers have been melting and thinning since the last ice age ended about 15,000 years ago, Joughin says this process may be almost over. "The ice sheet seems to be thickening instead of thinning," he says. "We think the thinning may have come to an end, but we will probably not live long enough to see [this hypothesis] tested."
Joughin's technique is a major advance in studying ice movement, and a positive sign for coastal communities. "A sea-level rise is not imminent, at least from this sector of ice sheet," he says. However, the larger Pine island Ice Stream, also in West Antarctica, seems to be speeding up, so the net effect is unclear.
The stakes in gaining a better grasp of ice flow are considerable. Ice, for example, reflects more heat back to space than water or soil, so melting ice could change the global energy balance. And a major infusion of cold, fresh water from melted glaciers could alter the ocean currents which redistribute heat around the globe.. But in the short term, at least, people in Bangladesh and New York may breathe a bit easier about the giant ice blanket at the south end of the world.
-- David Tenenbaum