Elevation change of the Greenland Ice Sheet and its measurement with satellite radar altimetry

Satellite radar altimetry is presently the only method that has provided the spatial coverage and density of observations needed to reduce the present uncertainty in the mass balance of the Greenland Ice Sheet and its contribution to change in eustatic sea level. The only such measurement reported, however, estimated that southern Greenland was thickening at 23±6 cm a -1 which is larger than was thought hitherto. This value is reconsidered given more recent information concerning the errors in the measurement. A survey of measurements of specific mass balance of the Greenland Ice Sheet is given, together with estimates of its sensitivity to temperature change. The expected behaviour is described of errors in the satellite position and errors in the range measurement to the ice sheet surface. The treatment of biases and the number of independent observations of random errors is described. It is found in particular that a higher degree of independence was given to the random errors than should have been the case. The total error is recalculated with this accounted for, and is found to remain dominated by the bias estimate and therefore largely unaffected by this change; the estimate is 23 ± 7 cm a -1 . It is concluded that the observation does support a recent thickening of the southern Greenland Ice Sheet.