Our objective is to use GPS and absolute gravity meas- urements to help estimate temporal variations in ice thick- ness on the Greenland ice cap. We will be looking both at multi-year secular trends and at seasonal periods.
If the ice thickens over a region, then the increasing load on the underlying earth will cause the crust around that region to subside. GPS measurements of the subsidence can provide an observational constraint on the changing ice. The subsidence at any given point depends mostly on the change in ice thickness averaged over the surrounding few hundred kilometers. Depending on the ice model we adopt, our estimates for the long-term vertical crustal displace- ment rates along the edge of the Greenland ice sheet range anywhere from 0 mm/yr to possibly about +- 10 mm/yr. Seasonally-varying displacements could well be as large as 10-20 mm at places.
The interpretation of any observed long-term trend is complicated by the fact that the earth is not a perfectly elastic body, but tends to flow slowly under the application of surface loads. Thus, the crust under Greenland is apt to be gradually subsiding or uplifting not only because of present-day changes in ice, but also because of any changes in ice thickness which might have occurred over the last several hundred to several thousand years. If we used only GPS receivers to measure secular uplift rates, we would have no way of separating the part of the subsidence caused by the present-day change in ice from that caused by past thickness changes. (This should not be an issue for our estimates of seasonal variability.) We have discovered, though, that if we also measure changes in gravity, we can combine the gravity and GPS observations so as to separate the secular effects of present-day changes in ice from those of past variability. These findings are described in Wahr et al., Geophysical Resarch Letters, 22, 977-980, 1995.
We are currently operating receivers at Kellyville and Kulusuk.