"It's quite remarkable that an ice stream would reverse its flow direction," said Howard Conway, a University of Washington glaciologist who has extensively studied the West Antarctic Ice Sheet, where the stream is located.

Conway and colleagues from the UW earth and space sciences department, the University of Colorado and the California Institute of Technology found evidence that what they call Ice Stream C-zero has completely reversed its flow in the last 250 years.

It used to be a tributary of the larger Ice Stream C, but the configuration of the ice stream system has changed. The nearby Whillans Ice Stream has thinned substantially and Ice Stream C has slowed dramatically and thickened because other tributaries continue to flow into it. As a result, it has risen higher, acting as a dam to Ice Stream C-zero, which now moves ice away from Ice Stream C and into the adjacent Whillans stream.

The research, paid for by a grant from the National Science Foundation, appears in the Oct. 3 edition of the journal Nature.

The ice streams studied are part of a complex network of ice flowing in the West Antarctic Ice Sheet, near the Ross Sea, where the scientists spent several months gathering data in 2000-01 and again in 2001-02. They collected ice core data, images from ice-penetrating radar pulled behind snowmobiles, and information from global positioning sensors placed on poles set in the ice.

Using that data along with satellite images, they pieced together evidence showing that, about 250 years ago, Ice Stream C-zero quit flowing at what is regarded as a fast rate, at least 100 meters (325 feet) a year, a pace fast enough to open crevasses in the ice. It gradually halted its flow toward Ice Stream C and now flows into the Whillans Ice Stream at 20 meters a year. That is because Ice Stream C thickened and deflected the flow from Ice Stream C-zero.

"We think it's in response to long-term climate change, but also to the internal dynamics of the ice sheet," Conway said. "It responds like a river system might. We just have to be careful using shorter-term observations to make conclusions about long-term changes of the ice sheet."

Fast-flowing streams that rapidly move ice from the interior of the ice sheet to the ocean are one of several methods by which Antarctica is losing its mass.

The West Antarctic Ice Sheet � as large as Texas and Colorado combined � has shrunk substantially in the last 7,500 years. In 1999, Conway and colleagues published research indicating the West Antarctic sheet could completely disintegrate within 7,000 years, raising global sea levels 15 to 20 feet. It remains unclear, however, what role human causes have played in the ice sheet's shrinkage.

Overall, Antarctic ice is shrinking, though recent research shows that the Ross Sea ice streams might have been growing for the last 150 years. But the new work by Conway and his colleagues demonstrates that there is a great deal of variability in the Ross Sea stream system during geologically short periods of 1,000 years or less.

If the section of the ice sheet now flowing into the Whillans stream continues to gain speed, the Ross Sea sector of the ice sheet could start to thin again, Conway said. That would leave scientists to puzzle whether the recent thickening has simply been part of short-term variability or if, in fact, it marks the end of the shrinking that's gone on for more than seven millennia, he said.

Besides Conway, the authors are Ginny Catania, a UW doctoral student; Charles Raymond, a UW earth and space sciences professor; Anthony Gades, a former UW research associate; Theodore Scambos, a research associate in the National Snow and Ice Data Center at the University of Colorado; and Hermann Engelhardt, a senior research associate in geophysics at CalTech.

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