GLOBAL GEODETIC SIGNATURES OF THE ANTARCTIC ICE-SHEET

Four scenarios of present day Antarctic ice sheet mass change are deve loped from comprehensive reviews of the available glaciological and oc eanographic evidence. The gridded scenarios predict widely varying con tributions to secular sea level change xi over dot ranging from -1.1 t o 0.45 mm/yr, and predict polar motion m over dot and time-varying low -degree gravitational coefficients J(l) that differ significantly from earlier estimates. A reasonably linear relationship between the rate of sea level change from Antarctica xi over dot(A) and the predicted A ntarctic J(l) is found for the four scenarios. This linearity permits a series of forward models to be constructed that incorporate the effe cts of ice mass changes in Antarctica, Greenland, and distributed smal ler glaciers, as well as postglacial rebound (assuming the ICE-3G degl aciation history), with the goal of obtaining optimum reconciliation b etween observed constraints on J over dot(l) and sea level rise xi. Nu merous viable combinations of lower mantle viscosity and hydrologic so urces are found that satisfy ''observed'' xi over dot in the range of 1 to 2-2.5 mm/yr and observed J over dot (l) for degrees 2, 3, and 4. In contrast, rates of global sea level rise above 2.5 mm/yr are incons istent with available J over dot (l) observations. The successful comp osite models feature a pair of lower mantle viscosity solutions arisin g from the sensitivity of J over dot (l) to glacial rebound. The paire d values are well separated at xi over dot = 1 mm/yr, but move closer together as xi over dot is increased, and, in fact, merge around xi ov er dot = 2 - 2.5 mm/yr, revealing an intimate relation between xi over dot and preferred lower mantle viscosity. This general pattern is qui te robust and persists for different J over dot (l) solutions, for var iations in source assumptions, and for different styles of lower mantl e viscosity stratification. Tighter J over dot (l) constraints for l > 2 may allow some viscosity stratification schemes and source assumpti ons to be excluded in the future. For a given total observed xi over d ot, the sea level rise from Antarctica xi over dot (A) is tightly cons trained and ranges from 0 to + 1 mm/yr (corresponding to an ablating i ce sheet) as estimates of xi over dot are raised from 1 to 2.5 mm/yr. However, when the degree 3 zonal harmonic constraint is: removed, the solutions show little sensitivity to Antarctic mass balance, emphasizi ng the need for a well determined odd-degree secular zonal harmonic fo r determining polar ice mass balance.