What can be learned about the polar ice sheets by combining ICESAT/GLAS and GRACE satellite data?

I. Velicogna (CIRES and Dept. of Physics, Univ of Colorado, Boulder, CO 80309, USA; phone: 1-303-4925141; fax: 1-303-4927935; e-mail: isabella at colorado.edu)

J. Wahr (CIRES and Dept. of Physics, Univ of Colorado, Boulder, CO 80309, USA; fax: 1-303-4927935; e-mail: wahr@colorado.edu)

D. Wingham (Department of Space and Climate Physics, University College London, London, England WC1H OAH)

C. Bentley (Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706)

Measurements from the GLAS laser altimeter aboard NASA's ICESAT satellite and from the NASA-DLR dedicated gravity satellite mission GRACE, both scheduled for launch in 2001 and both with expected lifetimes on the order of 5 years, can be combined to learn about ongoing changes in polar ice mass and viscoelastic rebound of the lithosphere under the ice sheet. GRACE will map the Earth's gravity field orders of magnitude more accurately and with considerably higher resolution than any existing satellite, and will do this at 30-day intervals. This will permit resolution of the time-varying gravity field with sufficient accuracy to constrain surface hydrology and glacial ice variations to $\leq$1 cm of water-equivalent mass at scales of a few hundred km and greater. GLAS will monitor changes in ice-sheet topography, and can provide estimates of secular changes in the total polar ice sheet mass over the lifetime of the mission. The largest error sources in GLAS measurements of ice sheet mass will be uncertainties in the effects of post glacial rebound (PGR) and complications caused by decadal and inter-annual variability in accumulation rate. The total PGR signal for the polar ice sheets depends on the time history and spatial distribution of the ice sheet over the last many hundreds to thousands of years and on the earth's viscosity profile. The effects of the PGR signal can be reduced by combining the GLAS data with GRACE gravity measurements, using the approximation that changes in the geoid caused by PGR are mostly due to mass anomalies associated with vertical motion of the solid Earth surface. The improvement obtained by adding the gravity data would be substantially greater for multiple, successive gravity missions.