The latest Intergovernmental Panel on Climate Change (IPCC) assessment described a rising sea level as one of the most serious potential consequences of global warming. "If the oceans rise a few inches, in the low-lying flat coastal regions that can translate to the encroachment of water inward by tens of feet, if not more," says Waleed Abdalati, program manager at NASA's Cryospheric Sciences Branch, which studies ice cover. "Add to that the fact that the water erodes beaches," Abdalati says. "The more it moves inland, the more it erodes the beaches, and the problem becomes greater. Add to that still that some areas are sinking -- the surface is lowering as the Earth's crust moves -- and the relative sea level problem becomes even greater. Finally, with the rising seas, the barrier islands diminish and with them, their capacity to mitigate damage from storm surges, etc. So in the big picture, the effects are much greater than the small number might lead you to believe."

For these reasons and others, he adds, ice sheet research is crucial. "In order to understand and minimize the potential impacts of rising seas, it is important to understand how fast they are rising and the mechanisms that cause them to rise. Curt's work is an important element of those efforts."

Davis's IDS study involved using data from two NASA satellites to chart surface elevation changes on East Antarctica. The first satellite orbited from 1992 to 1996 and the second from 1995 to 2003. Every day, from a height upwards of 800 kilometers, they circled the planet more than a dozen times. All the while an on-board altimeter, an instrument that uses radar signals to measure range, kept track of the distance from the satellites to the surface of the Earth. "You've got something 800 kilometers [above the planet], in orbit, making measurements that can be on the order of, on a yearly rate of change, centimeters per year," Davis says. Compare that to airplane altimeters which only have to be accurate within a couple of hundred feet, and from just thousands of feet in the air.

The satellites' altimeters provided such precise measurements because they sent out "very, very, very short radar pulses," Davis says. And the shorter the pulse, the more accurate the reading. "A pulse [from the satellites' altimeters] lasts for a few nanoseconds, or one billionth of a second."

Even with that precision, Davis and his fellow researchers can't use altimeter range measurements alone to track elevation change. That's because there are tiny variations in the satellites' positions from orbit to orbit, variations that affect the range measurements. For example, the range measurement might decrease a couple of centimeters because the satellite had moved that much closer to Earth, not just because the ice sheet had added that much to its surface.

To correct for such problems, scientists first use ground tracking stations and global positioning systems to record the satellites' positions with respect to the center of the Earth. Next they subtract the altimeters' range measurements from distances recorded between the satellites and the Earth's center. This provides a measure of surface elevations. Researchers then compare the elevation measurements from orbit to orbit and, eventually, develop a picture of how ice elevations change over time.

During the period examined for Davis' study, researchers gathered tens of thousands of surface elevation measurements for myriad points in East Antarctica. All totaled, the data added up to some 347 million usable elevation change measurements. "We combined these to get a picture over 11 years, which is one of the real strengths of this study," Davis says. "It represents the longest period of time of continuous observations of the Antarctic ice sheet."

Published by the Office of Research.

©2006 Curators of the University of Missouri. Click here to contact the editor.