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January 28, 2002

Scientists detect thickening of West Antarctic ice sheet

By Tim Stephens

The stability of the West Antarctic ice sheet has long been a concern because of the potentially catastrophic rise in sea level that would result from its collapse. Researchers at UCSC and NASA now report that, contrary to previous studies, at least one part of the ice sheet is actually growing rather than shrinking.

The Ross Ice Shelf in Antarctica, shown here at the Bay of Whales, will disappear if the ice streams that feed it shut down. Photo: Michael Van Woert, NOAA NESDIS, ORA
Assistant professor of Earth sciences Slawek Tulaczyk and Ian Joughin of NASA's Jet Propulsion Laboratory used satellite radar images to map the flow of ice in the ice sheet and estimate how its mass is changing. They reported their findings in the January 18 issue of the journal Science.

"The West Antarctic ice sheet has been retreating for several thousand years, so to look now and see that it is growing is staggering to me," Tulaczyk said. "Within the past 200 years, the ice sheet seems to have switched fairly rapidly from a negative mass balance to a positive mass balance."

Antarctica's huge ice sheets are fed by snow falling in the interior of the continent. The ice gradually flows out toward the edges. The West Antarctic ice sheet is considered less stable than the larger East Antarctic ice sheet because much of it rests on land that is below sea level, and parts of it, called ice shelves, are floating on the sea.

A positive mass balance means that more ice is accumulating than is leaving the ice sheet. The reason, according to Tulaczyk and Joughin, is that major ice streams have slowed or stopped moving altogether. Ice streams are fast-moving currents of ice within the ice sheet that carry large volumes of ice out onto the floating ice shelves.

Earlier work by Tulaczyk may explain why the ice streams are slowing down. The ice streams slide over a bed of sediment saturated with liquid water, but an ice stream will grind to a halt if its bed becomes cold enough for the water to freeze. Tulaczyk showed that thinning of the ice sheet allows more heat to escape from the bed, eventually leading to freezing conditions (see previous Currents story,

The ice sheet has been retreating and thinning since the end of the last ice age more than 10,000 years ago. The changes now being detected by Tulaczyk and Joughin may signal the end of this process.

"It is either some kind of short-term fluctuation that we don't quite understand, or it's a trend and we just happened to come along at the right time to observe an event that only happens once in 10,000 years," Tulaczyk said. "To think that it just happens to be doing this now when we can observe it leaves me feeling a little queasy," he added.

One reason for that queasiness is that no one is quite sure what the long-term implications of these changes may be. Tulaczyk noted that if the ice streams continue to slow and stop, the ice shelf that covers the Ross Sea is likely to break up. The removal of the lid of ice that currently covers the Ross Sea could have significant effects on global ocean circulation and the global climate, he said.

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