Changes Tracked In Polar Ice Sheet Elevations

"We are using GPS (Global Positioning System) technology to make elevation measurements of the ice sheets in Antarctica and Greenland," says Gordon Hamilton, a research assistant professor. "We want to understand if the ice sheets are currently changing size and contributing to sea level rise. The current estimate is that of the average two millimeters of global sea level rise that occurs annually, we can explain one millimeter by thermal expansion of the oceans and groundwater pumping. The remaining contribution is unknown, but an assumption is melting of the ice sheets in the polar regions."

Scientists do not yet know how the ice sheets are likely to respond if average global temperatures rise in the future. If snowfall increases over Antarctica and Greenland, possibly due to a warmer, moister atmosphere, the ice sheets could grow. However, if the sea continues to rise or increased temperatures warm the ice, they will flow faster and could shrink.

So far, the results of work by Hamilton and others suggest that the ice sheets may be thinning at the edges but getting thicker inland. Nevertheless, some stations show little movement while others are rising or falling, and no clear pattern has emerged. Hamilton has spent the better part of the last decade perfecting field techniques to collect reliable data. His experiments entail measuring the vertical velocity of ice and comparing that with snow accumulation rates derived from ice core analyses. If vertical velocity exceeds snow accumulation, the ice sheet is getting thinner, and vice versa.

In the last eight years, he has placed more than three dozen sites on the West Antarctic Ice Sheet and 12 more in Greenland. To get a picture of whether the ice is growing or shrinking in a region, he looks for areas that are well away from fast moving ice streams. Hamilton and a colleague, Ian Whillans of The Ohio State University, are currently funded by the National Science Foundation to carry out measurements as part of the International Trans-Antarctic Scientific Expedition (ITASE). The purpose of ITASE is to collect data that will help scientists understand the Antarctic environment and serve as a baseline for measurements of future changes.

"Ice sheets do not change much on the scale of a few decades," says Hamilton. "They’re so large that it takes a very long time for them to respond to changes in the environment. It’s currently thought, for example, that the West Antarctic Ice Sheet is still responding to the end of the last Ice Age about ten thousand years ago. The vertical movements of points on the ice sheet are very slow and constant over a 500 to 1000-year time frame. Having said that, we are still unsure how fast future changes might occur if climate enters an unstable phase."

Data collected by Hamilton will assume new importance after the December, 2001, launch of ICESat (Ice, Cloud and Land Elevation Satellite) by NASA. ICESat will use lasers to measure elevations across the world’s ice sheets, and the results of his work will be critical to the process of validating data from the satellite. "We know the elevation of our stations and the rates of ice thickness changes, so we can compare our numbers to those coming from the satellite. If there’s a discrepancy, we’ll know that the satellite has a problem."

In advance of ICESat’s launch, Hamilton and colleagues have been conducting measurements in Greenland as part of the Program for Arctic Regional Climate Assessment (PARCA). This NASA-funded initiative combines ground-based GPS work with overflights by a research aircraft equipped with a laser similar to that onboard ICESat. PARCA is providing valuable experience that will guide future work interpreting data from the satellite mission.

Hamilton has a PhD from the University of Cambridge and came to UMaine from the Byrd Polar Research Center at Ohio State and the Norsk Polarinstitutt in Oslo, Norway. He is a native of Scotland.