Assessing Antarctica's Mass Balance Via Measurements of Time-Variable Gravity from Satellites

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Reference

Velicogna, I. and Wahr, J. 2006. Measurements of time-variable gravity show mass loss in Antarctica. Sciencexpress: 10.1126science.1123785.

What was done

Using measurements of time-variable gravity from the Gravity Recovery and Climate Experiment (GRACE) satellites, the authors determined mass variations of the Antarctic ice sheet for the 34 months between April 2002 and August 2005.

What was learned

Velicogna and Wahr concluded that "the ice sheet mass decreased significantly, at a rate of 152 ± 80 km3/year of ice, equivalent to 0.4 ± 0.2 mm/year of global sea level rise," all of which mass loss came from the West Antarctic Ice Sheet, since they calculated that the East Antarctic Ice Sheet mass balance was 0 ± 56 km3/year.

What it means

What these results imply about the real world is highly dependent upon their ability to truly represent what they presume to describe; and in this regard Velicogna and Wahr say there is "geophysical contamination ... caused by signals outside Antarctica," including "continental hydrology ... and ocean mass variability." The first of these confounding factors, according to them, "is estimated our italics using monthly, global water storage fields from the Global Land Data Assimilation system," while "the ocean contamination is estimated our italics using a JPL version of the Estimating Circulation and Climate of the Ocean (ECCO) general circulation model our italics."

In addition to these problems, the two researchers note that the GRACE mass solutions "do not reveal whether a gravity variation over Antarctica is caused by a change in snow and ice on the surface, a change in atmospheric mass above Antarctica, or post-glacial rebound (PGR: the viscoelastic response of the solid Earth to glacial unloading over the last several thousand years)."

To adjust for the confounding effect of the variable atmospheric mass above Antarctica, Velicogna and Wahr utilize European Centre for Medium-Range Weather Forecasts (ECMWF) meteorological fields, but they acknowledge that "there are errors in those fields," so they "estimate our italics the secular component of those errors by finding monthly differences between meteorological fields from ECMWF and from the National Centers for Environmental Prediction."

With respect to post-glacial rebound, Velicogna and Wahr say "there are two important sources of error in PGR estimates: the ice history and Earth's viscosity profile." To deal with this problem, they "estimate our italics the PGR contribution and its uncertainties using two ice history models our italics."

All of these estimates and adjustments are convoluted and complex, as well as highly dependent upon various models. In addition, the estimates and adjustments do not deal with miniscule effects, as Velicogna and Wahr acknowledge that "the PGR contribution is much larger than the uncorrected GRACE trend." In fact, their calculations indicate that the PGR contribution exceeds that of the signal being sought by nearly a factor of five!!! And they are forced to admit that "a significant ice mass trend does not appear until the PGR contribution is removed."

In light of the latter humungous confounding problem, Velicogna and Wahr rightly state in their concluding paragraph that "the main disadvantage of GRACE is that it is more sensitive than other techniques to PGR." In fact, considering the many other adjustments they had to make, based upon estimations utilizing multiple models and databases with errors that had to be further estimated, we are led to totally discount the significance of their final result, particularly in light of the additional fact that it did not even cover a full three-year period. Much more likely to be much more representative of the truth with respect to Antarctica's mass balance are the findings of Zwally et al. (2005), who determined Antarctica's contribution to mean global sea level over a recent nine-year period to be only 0.08 mm/year compared to the five-times-greater value of 0.4 mm/year calculated by Velcogna and Wahr.

Reference

Zwally, H.J., Giovinetto, M.B., Li, J., Cornejo, H.G., Beckley, M.A., Brenner, A.C., Saba, J.L. and Yi, D. 2005. Mass changes of the Greenland and Antarctic ice sheets and shelves and contributions to sea-level rise: 1992-2002. Journal of Glaciology 51: 509-527.