INTERPRETATION OF SHORT-TERM ICE-SHEET ELEVATION CHANGES INFERRED FROM SATELLITE ALTIMETRY

Satellite altimetry offers means of directly measuring changes in surface elevation over the polar ice sheets of Greenland and Antarctica. By relating these changes to variations in ice mass, it becomes possible to detect short-term changes in the Earth's ice sheets. However, it is not immediately obvious that short-term changes in surface elevation are indicative of any (long-term) trend in ice mass. An increase in ice thickness may very well reflect the response of the glacier to random fluctuations in precipitation. The spectrum of this response is dominated by low frequencies, with the majority of the variance contained in the longer time scales. As a result, the ice-thickness record may exhibit trends that have no climatic significance, but are due to a low-frequency response to random forcing. A simple model for the interpretation of observed elevation changes is developed and applied to measurements made over the Greenland Ice Sheet. It appears to be unlikely that the difference between the rate of thickening derived by Zwally and others (1989) using repeat satellite altimetry, and significantly smaller previous estimates, can be explained as being the response of the ice sheet to random climatic forcing or that this difference can be attributed to a recent increase in accumulation rate.