No Major Changes Seen In Stability Of Antarctic Ice Sheet

That’s the conclusion drawn by an international team of scientists who analyzed five years of satellite radar measurements covering a large part of the southernmost continent. Their report was published in a recent issue of the journal Science.

The study is important in that it offers one of the best investigations so far of possible mass balance changes in the ice covering Antarctica. While global warming has been blamed for possible reductions in the Antarctic ice sheet, scientists have argued over whether there was definitive evidence of such ice loss. It’s also unclear whether the west Antarctic ice sheet would be unstable in a warmer world.

The new study suggests that the answer is no, at least forthe middle of the ice sheet.

“Based on our short, five-year period of observation of the interior of Antarctica, we do not seem to detect that the ice is melting more than one centimeter per year,” explained C.K. Shum, an associate professor of civil and environmental engineering and geodetic science at Ohio State University.

“That would mean that the interior Antarctic ice sheet does not seem to be contributing to sea level rise more than 1 millimeter per year.” Shum said that a one-centimeter (0.4 inch) decrease in Antarctic ice sheet volume roughly converts into a one-millimeter (0.04 inch) rise in global sea level.

Shum, along with other scientists from University College in London and the Delft University of Technology in the Netherlands, analyzed radar altimetry data retrieved from two European Space Agency remote sensing satellites -- ESA-1 and ESA-2 -- used to measure ice altitudes from 1992 through 1996.

The orbits of the satellites reached to 81.5 degrees N, allowing them to regularly monitor at least 60 percent of the continent’s grounded ice.

The majority of the West Antarctic ice sheet sits atop dry land while the East Antarctic ice sheet is grounded below sea level. Changes in the East Antarctic sheet would have little effect on sea level since the ice displaces sea water. But a complete melt of West Antarctic ice would pour new water into the oceans, raising sea levels.

“We assume that global warming is underway now,” Shum says, “and it may be enhanced by human activities but, until now, its effect on ice loss in Greenland and the Antarctic has been mostly speculation. We wanted to look at ice sheets to see how they contribute to sea level rise.”

Researchers understand some causes for sea level rise, Shum said, but the role ice sheets may play “remains an uncertainty.” During the last ice age -- 18,000 years ago -- sea levels were at least 100 meters lower than they are today.

Shum and his colleagues turned to the ESA satellites to look for ice sheet growth. The two, circling the globe in 800-kilometer (497-mile) orbits, were able to measure the height of the ocean surface to an accuracy of about 5 centimeters (two inches).

“But over ice, the readings are less accurate,” Shum said. “The satellites also have problems accurately measuring coastal ice regions.”

The researchers had to devise new algorithms to decipher the raw, ice sheet altimetry data and correct for several variables such as radar penetration below the ice surface and snow accumulation. Even with these limitations, the study represents the longest time series for which data is available.

Shum said the National Aeronautics and Space Administration is planning a new mission for the year 2001 called ICESAT. It would position a new satellite in a near-polar orbit, increasing the amount of ice sheet coverage, and use a more accurate laser altimeter to take measurements.

These, combined with the radar altimetry data, would give a much better assessment of mass balance changes, if any, in the Antarctic ice sheets.

This project was supported in part by the NASA Physical Oceanography Program and the United Kingdom Natural Environment Research Council.