MELTING OF ICE SHELVES AND THE MASS BALANCE OF ANTARCTICA

We calculate the present ice budget for Antarctica from measurements of accumulation minus iceberg calving, run-off and in situ melting beneath the floating ice shelves. The resulting negative mass balance of 469 Gt year-1 differs substantially from other recent estimates but some components are subject to high temporal variability and budget uncertainties of 20-50%. Annual accumulation from an earlier review is adjusted to include the Antarctic Peninsula for a total of 2144 Gt year-1. An iceberg production rate of 2016 Gt year-1 is obtained from the volume of large icebergs calculated from satellite images since 1978, and from the results of an international iceberg census project. Ice-shelf melting of 544 Gt year-1 is derived from physical and geochemical observations of meltwater outflow, glaciological field studies and modeling of the sub-ice ocean circulation. The highest melt rates occur near ice fronts and deep within sub-ice cavities. Run-off from the ice-sheet surface and from beneath the grounded ice is taken to be 53 Gt year-1. Less than half of the negative mass balance need come from the grounded ice to account for the unattributed 0.45 mm year-1 in the IPCC "best estimate" of the recent global sea-level rise.