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Recent Trends in Southern Greenland Ice Sheet Thickness
Reference
Davis, C.H., Kluever, C.A. and Haines, B.J. 1998. Elevation Change of the Southern Greenland Ice Sheet. Science 279: 2086-2088.

What was done
Elevation changes of the Greenland ice sheet were calculated over the period 1978 to 1988 using radar altimeter data from the Seasat and Geosat satellites. Similar elevational studies had been performed previously, but they were deemed deficient in their treatment of orbit errors, retracking errors, and systematic biases in the data. The present study used various algorithms and calibration techniques to reduce these biases to approximately 0.5 centimeters per year. Spatial coverage of both satellites provided elevation data from 62 to 72N latitude, with over 95% of the data coming from elevations greater than 2000 meters near the north-south orientated ice divide.

What was learned
Vertical ice growth rates varied spatially across the ice sheet from -15 to +18 centimeters per year. Overall, the 1978-1988 spatially-averaged change in Greenland ice sheet elevation was reported to be + 2.0 0.5 centimeter per year. Near the west of the ice sheet divide - between 65 and 69N - the elevation increased by 10 to 15 centimeters per year, agreeing with ice sheet growth rates determined from ground survey and airborne laser altimeter data from 1980 to 1994. Seasonal and interannual variations in ice-sheet surface elevation were reported to be 15 centimeters and 8 centimeters, respectively.

What it means
Studies of the growth or decay of polar ice sheets are of considerable importance in global climate change research because of the potential of changes in ice sheet mass to influence the rate and magnitude of sea level change. Many of these studies have focused on the mass balance of the Greenland ice sheet, because it is much warmer there than it is over the Antarctic ice sheet. In fact, many general circulation models have predicted that an initial warming of the Greenland ice sheet may set in motion a positive feedback cycle leading to further melting, as more and more of the surface becomes ice-free and able to absorb an increasingly greater amount of solar radiation. This increased absorption of solar radiation, the models suggest, will raise temperatures over the ice sheet, causing it to lose mass at its surface. The empirical results presented in this article, however, do not substantiate these predictions, as the radar altimeter data indicate a mean elevation rise of 10 to 15 centimeters over the period 1978 to 1988 for the major measured portion of the Greenland ice sheet.

Update
In a technical comment on this article reported in Science (Vol. 281, p. 251), H. J. Zwalley et al. present results from an analysis of additional data that show an even greater ice thickening (by 3-fold, in fact) than that reported by Davis et al.

Reviewed 15 October 1998