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Assessing Antarctica's Ice Mass Balance Via Measurements of Time-Variable Gravity from Satellites ... Again
Reference
Ramillien, G., Lombard, A., Cazenave, A., Ivins, E.R., Llubes, M., Remy, F. and Biancale, R. 2006. Interannual variations of the mass balance of the Antarctica and Greenland ice sheets from GRACE. Global and Planetary Change 53: 198-208.

Background
A few months ago we reviewed the study of Velicogna and Wahr (2006), wherein the two researchers used measurements of time-variable gravity obtained from the Gravity Recovery and Climate Experiment (GRACE) satellites to estimate mass variations of the Antarctic ice sheet for the 34-month period April 2002 to August 2005. From this work they 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 was done
Hard on the heels of Velicogna and Wahr's calculations, Ramillien et al. derived new estimates of the mass balances of the East and West Antarctic ice sheets that were also based on GRACE data, but that pertained to the somewhat shorter period of July 2002 to March 2005.

What was learned
The most-recently-reporting group of seven researchers obtained some significantly different ice sheet mass balances than those obtained by Velicogna and Wahr: a loss of 107 23 km3/year for West Antarctica and a gain of 67 28 km3/year for East Antarctica, which results yielded a net ice loss for the entire continent of only 40 km3/year (which translates to a mean sea level rise of 0.11 mm/year), as opposed to the 152 km3/year ice loss calculated by Velicogna and Wahr (which translates to a nearly four times larger mean sea level rise of 0.40 mm/year).

What it means
Clearly, the Ramillien et al. mean sea level rise of 0.11 mm/year is much less ominous than the much larger value calculated by Velicogna and Wahr; but it is of the same order of magnitude as the 0.08 mm/year Antarctic-induced mean sea level rise calculated by Zwally et al. (2005), which was derived from elevation changes based on nine years of satellite radar altimetry data obtained from the European Remote-sensing Satellites ERS-1 and -2.

Even at that, the GRACE approach is still laden with a host of potential errors, as we report in our review of the Velicogna and Wahr paper and as both they and Ramillien et al. readily admit. In addition, as the latter scientists note in their closing paragraph, "the GRACE data time series is still very short and these results must be considered as preliminary since we cannot exclude that the apparent trends discussed in this study only reflect interannual fluctuations."

In light of these several observations, it would behoove everyone to "keep their cool" about Antarctica's possible contribution to global sea level change over the last few decades and not jump to any irreversible conclusions about the matter until longer and more definitive data series of more than one type of measurement have been obtained and correctly analyzed.

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

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.

Reviewed 27 December 2006