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Using GPS and Gravity to Infer Ice Mass Changes in Greenland

There are strong indications that the global mean sea level rise is accelerating. The IPCC reports a mean sea level rise of ~0.17m (or 1.7 mm/year) during the previous century [1]. Recent satellite altimeter data indicate almost a doubling in this rate since 1993 (3.2 mm/year over 1993-2005 [2]) Roughly half the sea level rise is attributed to thermal expansion of ocean water. The rest of the sea-level rise is due to the addition of water to the oceans (from melting of mountain glaciers and polar ice). Model projections for the 21st century show a wide range of estimates varying from 0.22 m to 0.50 m. However some scientists believe these are conservative estimates. They predict a sea-level rise of 1 m for this century. A sea level rise of this magnitude will have significant consequences for coastal communities around the globe.
The mass balance and equilibrium state of an ice sheet is a complex function of external climate forcing and internal dynamical processes. During the last two decades scientists have witnessed large and rapid changes on the Greenland ice sheet. Large outlet glaciers thinned more rapidly than could be explained by increased summer melting alone. This observation therefore supports the theory that dynamical changes in ice flow have occurred [3]. Southeast Greenland, in particular, seemed to be affected by these dynamical changes. Indeed glacier flow there increased significantly and some glaciers doubled or nearly tripled in velocity: Kangerdlugssuaq glacier, a large outlet glacier in southeast Greenland, accelerated from 5 km yr-1 to 14 km yr¬-1 in only a few years [4], becoming perhaps the fastest glacier in the world. More recently, the sudden speed-up of glacier flow appears to have ceased [5]. Nonetheless, the sudden and near-synchronous dynamic changes of Greenland’s largest outlet glaciers suggest that the ice sheet is able to respond rapidly to environmental changes. Monitoring these changes and fluctuations has therefore become a key focus for climate scientists.
The most efficient way to assess contemporary ice mass changes on a large and remote ice sheet such as Greenland is to use remote sensing. Modern geodetic techniques that measure the mass balance of the Greenland Ice Sheet, such as time variable gravity (e.g. GRACE), must be corrected for postglacial rebound (PGR). PGR is the visco-elastic response of the earth to changes in ice loading caused by melting of the great ice sheets after the last glacial maximum.
The Geophysics Laboratory from the University of Luxembourg collaborates with researchers from the USA and Denmark in a project called GNET. The main goal of GNET is to determine PGR adjustments necessary to correct satellite measurements. As part of GNET more then 30 continuously operating GPS receivers have been placed at the edge of the Greenland Ice Sheet. The GPS antennas are placed on metal rods that are anchored firmly in the bedrock. The instruments can measure the vertical uplift rate of the earth’s crust very precisely. We analyze the GPS data and combine this with in-situ gravity measurements to separate changes in crustal uplift resulting from PGR and present day ice mass changes. The results of this study will contribute to better estimates of the current mass imbalance of the Greenland Ice Sheet and its contribution to global sea level rise.


GPS and Gravity in Luxembourg

In order to study trends and patterns of crustal motion in Luxembourg, detailed geodetic GPS measurements are carried out by the Administration du Cadastre et de la Topographie in collaboration with the University of Luxembourg. A network of continuous GPS stations is installed at 7 geologically stable sites across the country (see figure). The GPS stations record data continuously and data is processed at the Geophysics Laboratory. Using a wide network of reference stations in Europe the vertical movement of the Luxembourg sites is determined very accurate. We analyze the GPS data and combine this with absolute gravity measurements and rain gauge data to evaluate seasonal changes in ground height resulting from rainfall and groundwater variations.

Locations of permanent GPS stations in Luxembourg.


International Intercomparison of Absolute Gravimeters

 In 2000, a laboratory dedicated to the intercomparison of absolute gravimeters was built within the Walferdange Underground Laboratory of Geodynamics with the power and space requirements that it is able to accommodate up 16 instruments operating simultaneously.

 The WULG has the additional advantage of environmental stability (i.e. constant temperature and humidity within the lab), as well as being isolated from anthropogenic noise. International comparison of absolute gravimeter are organized every 4 years on alternate with the comparison organized at the Bureau International des Poids et Mesures in Sèvres, France. These comparisons are the only way to check that the gravimeters work properly.