 December 27, 2000 -- New evidence suggests that the West Antarctic Ice Sheet is retreating more slowly and contributing less to rising global sea levels than scientists once thought. In fact, said researchers at a recent meeting, the sheet was still growing as recently as 8,000 years ago -- thousands of years after the most recent Ice Age.
"Our previous best estimates that the ice sheet was adding 1 millimeter per year to global sea level are almost certainly too high," says Robert Bindschadler, a glaciologist at NASAs Goddard Space Flight Center in Greenbelt, Maryland. Right: Antarctica is divided by the Transantarctic Mountains into East Antarctica, or "Greater Antarctica," and West Antarctica, or "Lesser Antarctica." Most of Antarctica is covered by ice, with an average thickness of nearly a mile -- constituting roughly 90 percent of the Earths total amount of ice. The Antarctic ice sheet is the largest body of fresh water on our planet, amounting to 70 percent of the total. This revised assessment is based on a synthesis of new data including past sea-level rise estimates presented at a workshop this fall on the West Antarctic Ice Sheet. Bindschadler, who organized the fall workshop, discussed the latest research results and changing views of the West Antarctic Ice Sheets history at the American Geophysical Unions annual meeting in San Francisco on December 16. Calculations of how much and how fast the ice sheet has thinned and retreated since the peak of the Earths last major ice age 20,000 years ago are based in large part on a recent reconstruction of how big the ice sheet was during that last glacial maximum. That reconstruction included a West Antarctic Ice Sheet three times as large as it is now. Currently, the ice sheet averages 2000 meters thick, covers an area the size of Mexico, and contains enough water to raise global sea level 5 meters.
But analysis of a 30-by-50-mile rise in the ice sheet near the Ross Ice Shelf called Siple Dome suggests that this feature was not overrun by a massive ice sheet in the past, which is what the reconstruction suggests. A team of glaciologists from the University of Washington led by Charles Raymond used an ice-penetrating radar to study the subsurface layering of Siple Dome. Another line of evidence that throws the ice sheets ancient bulk into question is the discovery that the ice sheet was still growing as recently as 8,000 years ago. The reconstruction assumed that the ice sheet reached its maximum growth 20,000 years ago and has only been in retreat since then. 
Above: These two figures show former temperatures with major periods of glaciation labeled. The dashed lines are the present global average temperature of about 15?C (59?F). The solid curves show small changes from this average. Note that the temperature drops only about 5?C during a glaciation. This has occurred about every 100,000 years, with smaller wiggles in between. The most recent glaciation, 20,000 years ago, is called the Laurentide, and Earth is still recovering from it. According to a new reconstruction of historic sea level around the world by W. R. Peltier of the University of Toronto, a major jump in sea level occurred before the West Antarctic Ice Sheet began its current retreat, but there is no sign of a subsequent rise large enough to account for melting of so much West Antarctic ice.  The question of how fast the ice sheet retreated still challenges scientists. Recent work, however, leads Bindschadler to conclude that the ice sheet experienced a rapid retreat phase some 7,000 years ago that was preceded and followed by a slower retreat that continues today. Bindschadler points to the geologic record of dated stages in the retreat of the ice sheets continental base as evidence that it has shrunk in fits and starts. Such episodic retreats may be controlled more by the varying depth of the underlying surface and water than by the changing climate.
Above: Sea level rise is one of the greatest threats of climate change. In New York, the Battery tide gauge measured an increase in sea level of roughly .25 meters since 1920. (Graph based on data from the Goddard Institute for Space Studies) "The portion of the West Antarctic Ice Sheet we have focused on for the past ten years appears to be in a stage of near-zero retreat now," says Bindschadler, "but what it will do in the future is still uncertain." "If you extend the new evidence and the new line of reasoning into the future, the behavior of the ice sheet is more difficult to predict," he says. "It suggests, however, that if the ice sheet loses its hold on the present shallow bed it is resting on, the final retreat could be very rapid." |