IN 1978 Mercer(1) discussed the probable effects of climate warming on the Antarctic Ice Sheet, predicting that one sign of a warming trend in this region would be the retreat of ice shelves on the Antarctic Peninsula. Analyses of 50-year meteorological records have since revealed atmospheric warming on the Antarctic Peninsula(2,3), and a number of ice shelves have retreated(4-8). Here we present time-series of observations of the areal extent of nine ice shelves on the Antarctic Peninsula, showing that five northerly ones have retreated dramatically in the past fifty years, while those further south show no clear trend. Comparison with air-temperature data shows that the pattern and magnitude of ice-shelf retreat is consistent with the existence of an abrupt thermal limit on ice-shelf viability, the isotherm associated with this limit having been driven south by the atmospheric warming. Ice shelves therefore appear to be sensitive indicators of climate change.