Satellite gravity and the mass balance of the antarctic ice sheet

Changes in the Earth's gravity field with time have important applications to a broad range of disciplines. Any process that involves a large enough horizontal redistribution of mass, either within the Earth or on or above its surface, is potentially detectable. In particular, when ice sheets grow or shrink, gravity changes as mass is redistributed in the solid earth and between the oceans and the ice sheets. The sources of global sea-level rise (about 2 mm a over the last century) and in particular the contribution of the Antarctic ice sheet thereto are not well understood. Gravity measurements can help to diminish this uncertainty. The technology currently exists to measure gravity with higl accuracy by a dual-satellite mission in which the distance between the satellites is precisely monitored. We estimate from recent studies that temporal changes in the gravity field as determined by a satellite gravity mission lasting 5 years at an orbital height of 400 km would be sensitive e to changes in the overall mass of the Antarctic ice shect to a precision corresponding to better than 0.01 mm a of sea-level change. However, the effects of three other phenomena that could cach produce a temporally varying gravity signal with characteristics comparable to that caused by a change in Antarctic ice - postglacial rebound, inter-annual variability in snow fall, and atmospheric pressure trends - also need to be evaluated. Postglacial rebound could be partly separated from ice-mass changes with the aid of global positioning system campaigns and numerical models of rebound that use improved determinations of mantle viscosity also provided by the gravity mission. Determination of inter-annual ice-mass changes will be aided by measurements of moisture-flux divergence around the perimeters of the ice sheets and direct observations of inter-annual changes by the gravity satellite itself. The removal of pressure effects over Antarctica will become more effective as the number of automatic weather stations in the interior of the continent increases. Even after corrections are made for these factors the uncertainties they cause limit the accuracy in the determination of the contribution of the Antarctic ice sheet to sea-level change to about 0.5 mm a -1 . However, there is a strong complementarity between gravity measurements and the surface-height measurements that will be produced by NASA's laser altimeter mission early next century. Together, they should be able to determine that contribution to an accuracy of about 0.1 mm a -1 .