The East Antarctic ice sheet is the largest mass of ice on our planet and so its stability and potential contribution to global sea level change is of great interest. It was thought to be relatively stable but new research suggests it may have begun to loose mass, although the true significance of the change is unclear.

Measuring the Antarctic ice sheets is difficult. Over the years estimates of its mass balance has swung between positive and negative values. Radar altimetry observations covering the entire continent between 1992 - 2003 suggested that the ice mass balance was between -5 and + 85 GT/yr, although as usual with ice such estimates have large errors. Another satellite-borne technique, Synthetic Aperture Radar, using observations taken over the past decade, suggested that ice loss exceeded model predictions of snow accumulation. The data also indicated a recent increase in mass loss with 196 +/- 92 GT in 2006.

The latest data uses observations taken monthly between 2002 and 2009 from a pair of satellites flying in formation measuring gravity perturbations. Analysing data from the GRACE satellites is tricky and relating them to ice sheet mass estimates involves several steps and assumptions.The new study - published in Nature Geoscience by J Chen and researchers at the University of Texas at Austin in Austin, Texas - say the mass loss for the entire continent is - 190 +/- 77 GT/yr averaged over 2002 - 2009. This is much larger than previous estimates (although consistent with the most recent Synthetic Aperture Radar estimates of 196 +/- 92 GT/yr.)

Loss from the West Antarctic ice sheet is 132 +/- 26 GT/yr. East Antarctica, the researchers say, is losing mass at about 57 +/-  52 GT/yr (note large uncertainty - it could be consistent with zero ice loss). Most of the loss appears to be from coastal regions and to stem from increased ice loss after 2006. The finding is significant because the east of the continent has been regarded as the more stable half. It should be stressed however that although these seem large quantities of ice they are tiny in terms of their significance for global sea level changes.

The importance of this research is that it highlights the close scrutiny needed of the region, especially for the East Antarctic ice cap, to see if its rate of loss increases. At present it is not known if the loss is due to climate change or if such losses are in any way exceptional or cyclical. If the changes persist, and accelerate, then, “Antarctica may soon be contributing significantly more to global sea-level rise”, the researchers write.

Others have expressed surprise at the results. A study in 2005 suggested that the East Antarctic ice sheet was actually gaining mass. What this latest work indicates is the need for more observations, an appreciation of the large uncertainties in individual measurements and the problem of relating them to each other. Overall it would be unwise to draw strong conclusions from this research.

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