The scientific rationale and logistical plans described in this proposal have been developed by a team of University of Colorado and NOAA scientists (Waleed Abdalati, David Bahr, Roger Bilham, Kristine Larson, Anne Nolin, Doug Robertson, Glenn Sasagawa, Konrad Steffen, Tonie vanDam, and John Wahr)
Our objective is to use GPS and absolute gravity measurements to help estimate temporal variations in ice thickness 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 displacement 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 a paper that should appear in the March 1, 1995 issue of Geophysical Research Letters.
We will make absolute gravity and GPS position measurements at locations on the bedrock along the edge of the Greenland ice sheet. We will install year-round, continuous GPS receivers at each location, to monitor seasonal signals and long-term trends in the vertical displacement. We will re-visit each site with an absolute gravimeter once per year, so that we can remove the effects of past changes in ice from the GPS-determined trends. It could conceivably require nearly a decade of yearly measurements to obtain useful accuracies for the secular trends. Useful results for the seasonal variations should be obtained every year. The continuous GPS receivers will be kept in place during the entire project lifetime however, because seasonal signals are apt to be variable from one year to the next. Not only is this variability of interest in its own right, but it could alias into the secular trend estimates if not determined separately.
Since measurements at a single point mostly constrain changes in ice within a few hundred kilometers of that point, we plan on eventually visiting 4 sites: Sondre Stromfjord (W Greenland), Jakobshavn (W), Narsarsuaq (S), and Ammassalik (SE), all of which have airports. These sites are located around the bottom third of Greenland, the region of the ice cap with by far the largest accumulation and discharge rates.
We plan on beginning an abbreviated measurement campaign this coming summer (1995), with visits to Sondre Stromfjord. Continuous GPS receivers will be installed and absolute gravity measurements will be made.
The number of yearly gravimeter visits needed to obtain the secular trend at each site depends on the size of the secular signal and on the measurement accuracies. Our best guess is that we may be able to obtain useful secular uplift rates from yearly visits over 6-7 years - possibly shorter if there are large secular changes in mass near the station or improvements in instrument accuracies over the next few years, though possibly longer if there are significant signals from such things as local fluctuations in ground water or inter-annual variations in the snow/ice load. Useful seasonal signals should be obtained every year with the GPS measurements alone.
NOAA's Geophysical Laboratory will provide an absolute gravimeter for this project, along with personnel to operate the meter and to analyze the gravity data. The operation of the meter would probably require only a single NOAA employee, although an additional person would be needed at each airport to help with the loading and unloading. The continuous GPS receivers would be installed by a different individual (possibly a NOAA employee) during the first year a site is occupied. Any later replacement of GPS receivers would be done by the person making the gravity measurements. The analysis of the GPS and gravity data would be done by the scientists named at the top of this proposal.