Other factors could account for the discrepancy, acknowledges Clark Wilson, one of the University of Texas team. For instance, scientists do not fully understand the ocean tides in the Arctic Ocean, and there are not a lot of weather stations to monitor air pressure there. GRACE only measures changes in gravity due to changing mass - it cannot tell if that results from changes in air, water, rock or ice.Over time GRACE and related satellite measurements will become very useful, as the results form a set of data over time which can be calibrated with surface observations. In the meantime they tell us something, but it is hard to be sure about what.
So to find changes due to ice loss alone, the researchers have to subtract the estimated contribution of water and air. If that is not well known, it results in higher uncertainties in the interpretation.
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posted by mischief at 10:11 PM on August 10, 2006