Secular Gravity and Topography Variations of ice Sheets - the Case for Global Inverse Approach and Data Combination

One outstanding issue in solid Earth geophysics is the possibly large uncertainty associated with the mantle viscosity profile, particularly, the viscosity value in the lower mantle. The difficulty in resolving such issue is understandable because of its complexity and lack of data. First, the solid Earth deforms in response to both historical and present-day ice mass changes. Secondly, the spatial pattern as well as magnitude of the rebound deformation depend not only on mantle viscosity profile but also on the load geometry and time history. Unambiguous resolution of either parameter of the great load deformation phenomenon demands multiple data sets with sufficient spatial coverage and simultaneous consideration of all relevant factors. The upcoming global measurements of secular gravity variations by the GRACE mission will undoubtedly contribute significantly to this quest for understanding and information. The information content of GRACE's secular gravity variation measurements has been investigated in a global inverse platform allowing global sources of change in gravity and combination of other data sets. While the combination of gravity and ICESAT altimeter data can potentially separate present-day ice mass change signature from that of rebound and constrain the Last Glacial Maximum (LGM) heights to an uncertainty level of 100 meter, a number of difficulties and ambiguities have to be resolved, or they will significantly contaminate or even corrupt the solution. For example, unmodeled long term hydrological mass trend over the rebound area may increase the LGM ice height uncertainty by a factor of 3. Over the current continental ice sheets, the contemporary gravity/altimetry data combination will not be able to separate late-Pleistocene deglaciation from more recent changes in the last few thousand years. Also, such data sets still contain an ambiguity between global LGM ice mass and lower mantle viscosity value. The great scientific value and significant limitations of these data sets make a compelling case for further combination with relative sea level records and dynamic ice models.