Satellite altimetry offers means of directly measuring changes in surf
ace elevation over the polar ice sheets of Greenland and Antarctica. B
y relating these changes to variations in ice mass, it becomes possibl
e to detect short-term changes in the Earth's ice sheets. However, it
is not immediately obvious that short-term changes in surface elevatio
n 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 ran
dom fluctuations in precipitation. The spectrum of this response is do
minated by low frequencies, with the majority of the variance containe
d 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 l
ow-frequency response to random forcing. A simple model for the interp
retation of observed elevation changes is developed and applied to mea
surements 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 b
e attributed to a recent increase in accumulation rate.