Lower satellite-gravimetry estimates of Antarctic sea-level contribution

Models of Antarctic ice loss have been extremely difficult to construct because of the shortcomings of satellite measurements. Previously, scientists have used a satellite called Gravity Recovery and Climate Experiment (GRACE), which is unable to estimate ice loss within reconcilable errors because of mass changes due to glacial isostatic adjustment (King et al. 2012). However, new imaging techniques, called W12a GIA modeling, have allowed for more accurate predictions by accounting for changes in the ocean’s basins. Consequently, new data predict that the ice-mass change in Antarctica occurs at a rate of about one third or one half the recent GRACE estimates, or about –69 ± 9 Gt yr–1. Furthermore, this new modeling suggests that ice loss in Antarctica is concentrated around the Amundsen Sea coast in West Antarctica while the ice-mass in East Antarctica is gaining substantial mass. Ultimately, the Antarctic ice sheet is likely not contributing to sea level changes as rapidly as previously predicted. —Olivia Jacobs
King, M.A., Bingham, R.J., Moore, P., Whitehouse, P.L., Bentley, M.J., Milne, G.A., 2012. Lower satellite-gravimetry estimates of Antarctic sea-level contribution. Nature 491, 586589.[GSS king milne satellite]

            King et al. (2012) used a new system of measuring ice-mass loss, which allowed for more accurate predictions and inferences than the previously used GRACE model. This is because it accounts for the changes in the glacial isostatic adjustment. Since the Last Glacial Maximum, the surface loads on the ocean floor have changed substantially, and this change in mass causes the earth’s basins to change levels. Particularly, North America is no longer covered with several kilometers of ice, so the mantle is now slowly uplifting. With new modeling techniques (W12a), the authors were able analyze 26 independent Antarctic drainage basins from August 2002 to December 2010 and account for this uplift data. 
            Across all basins, the new mass change was estimated to be –69 ± 9 Gt yr–1, indicating a significant shrinkage pattern. East Antarctica is partially compensating for the great losses in West Antarctica as it is gaining ice-mass at a rate of about +60 ±13 Gt yr­–1 while West Antarctica is losing mass at about –118 ­­± 9 Gt yr­–1. However, this rate of loss is only 36–48% the recent GRACE estimates. The authors note that the Antarctic ice sheet’s rate of loss has also been increasing over time, but not as much as previous estimates allowed. King et al. found that the rate of mass loss for the continent as a whole has been increasing over the analysis period (about nine years) by about –4 ­­± 16 Gt yr–2. This is about 15% of previous estimates, and is not statistically significant from zero.
While the earth’s sea levels have been rising at an average rate of 3.2 mm yr–1, the GRACE measurements have been too varied to give insight into which ice-masses contribute the most to this change. The W12a estimates that the Antarctic ice sheet is contributing about +0.19 ± 18 Gt yr–1 to this increase in sea levels, which is less than 10% of the total observed increase during this study’s time period. Furthermore, King et al. used similar models to predict total glacial contribution to sea levels, and estimated that they contribute about 38% of total sea level rise, while the rest is attributed to ocean warming and continental hydrology. Ultimately, future predictions suggest that the basin that contains Pine Island Glacier, a glacier along the Admundsen Sea coast, could increase sea levels by a maximum of 7.6 cm this century. Additional melt from the Antarctic ice sheet may mean that the total glacial melt could exceed this value by 2100. 

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