In introducing Chen et al.'s new study, Science magazine (where it was published) says "satellite measurements of gravity variations show that the Greenland Ice Sheet now is disappearing at the rate of about 240 cubic kilometers per year," stating as factual what is really only suggested by the authors' analysis, and that only tenuously. But why take our word for it? Simply consider what the three researchers themselves have to say about the subject.
Forming the basis for the ice sheet mass balance assessment of Chen et al. were data returned from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, which was launched in March of 2002 and has since been providing measurements of earth's gravity field at roughly monthly intervals. These data, however, which covered the period April 2002 to November 2005 in the Chen et al. study, had to be significantly "massaged" before they were usable.
First came what the scientists call the "GRACE dealiasing process," which was designed to remove atmospheric and oceanic contributions from the gravity signals and that requires an entire User Handbook to describe its particulars. Then came corrections for post-glacial rebound or PGR, which is the rebounding of earth's crust and mantle from the depressing effects of the crushing weights of past glacial ice loads. With respect to the complexities of this procedure, Chen et al. state that "because PGR effects are present within the same geographical regions as current deglaciation, a PGR model [our italics] is required to separate the effects."
Expanding on this complication and its potential resolution, the three University of Texas at Austin scientists note that "different PGR models may show large discrepancies in modeling the Greenland surface deformation effect," but because "average PGR effects over all of Greenland are estimated [again by models] to be small," they neglect them altogether, not only over the bulk of Greenland, but also over vast regions of the ocean surrounding Greenland.
A third problem is the limited spatial resolution of GRACE gravity fields, which shortcoming the researchers attempt to overcome by using "numerical simulations [our italics] to assign [our italics] mass changes to regions suggested by remote sensing or other observations." But in spite of this dubious attempt at improvement, they still find that the "limited spatial resolution of GRACE estimates causes a large portion of variance to be spread into the surrounding oceans, even though the actual source location is likely on the continent."
A fourth nagging problem, in the words of the researchers, is that "PGR effects from nearby regions such as Hudson Bay may contribute to variations over Greenland." In fact, they admit that "many [our italics] error sources may affect our GRACE estimates, which include the remaining GRACE measurement error (after spatial smoothing), uncertainty in the background geophysical models used in GRACE, and unquantified [our italics] other leakage effects." And for even more errors associated with GRACE assessments of ice sheet mass balance, see our review of the study of Velicogna and Wahr (2006).
So just how questionable might the GRACE results possibly be? In a comparative study of the ice sheet mass balance over Svalbard, Chen et al. note that the technique they employ yields a mass wastage of about 75 km3/year, but that absolute gravity measurements indicate a melting rate of about 50 km3/year, and that surface deformation data suggest an ice loss rate of about 25 km3/year. If one or the other of these results is correct, the GRACE approach employed by Chen et al. could be anywhere from 50% to 200% too high.
Even more telling is Chen et al.'s open acknowledgement that "another study (Zwally et al., 2005) based on 10 years of radar altimetry data during the period 1992 to 2002 suggests a small mass gain [our italics] for Greenland," which is even opposite in sign to their estimate. In light of these many observations, all of which come from Chen et al.'s own paper, we find no need to say anything more. Most rational people should be able to draw their own conclusions about the matter.
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