Ice Sheet Mass Balance From Satellite Altimetry

A major uncertainty in understanding the current sea level rise of about 2mm/yr has been the uncertainty in the present mass balance of the Greenland and Antarctica ice sheets. For predicting changes in ice balance over the next century, in response to predictions of climate change, a principal uncertainty has been in the prediction of polar precipitation and the consequent rates of ice accumulation. Predictions of surface temperature change and consequent surface melting and runoff are more reliable. However, the non-linear dynamical effects of increased melting that cause accelerated flow of grounded ice are only beginning to be taken into account. Now, decadal-length measurements of changes in the surface elevations of the ice sheets by satellite and airborne altimeters are providing good estimates of the recent ice-sheet mass balance and the current contributions of the ice sheets to the rise of sea level. Time-series of ice-sheet surface elevations constructed from about 10 years of ERS-1 and 2 radar altimeter data show patterns of ice elevation increases and decreases that are significant in terms of regional-scale mass balances. Generally, the Greenland ice sheet has been growing inland and shrinking at the margins, which is the expected response of the ice sheet to climate warming. Much of the West Antarctic ice sheet has been losing mass while the East Antarctica ice sheet and grounded ice of the Antarctic Peninsula are gaining mass. The net contribution to sea-level rise is only a few tenths of a mm/yr. The floating ice shelves show corresponding regional mass losses and gains and a significant net loss, which may lead to an increased loss of grounded ice. Overall, the measurements are beginning to show patterns of growth and shrinkage characteristic of ongoing climatic change. For the future, the race between increasing precipitation and increasing melting is underway. A program of continuous monitoring of ice sheet elevation, similar to the continuous monitoring of sea-level rise, is clearly needed.