A decline in Antarctic sea-ice extent is a commonly predicted effect of a warming climate. Direct global estimates of the Antarctic sea-ice cover from satellite observations, only possible since the 1970s(1-4) have shown no dear trends. Comparisons(1) between satellite observations and ice-edge charts obtained from early ship records(5) suggest that sea-ice extent in the 1970s was less than during the 1930s, an indication supported by limited regional observations(6). But these observations have been regarded as inconclusive, owing to the limited spatial and temporal scope of the early records(2). A significant data source has, however, been overlooked. The southern limit of whaling was constrained by sea ice, and since 1931 whaling records have been collected for every whale caught(7), giving a circumpolar coverage from spring to autumn until 1987. Here, an analysis of these catch records indicates that, averaged over October to April, the Antarctic summer sea-ice edge has moved southwards by 2.8 degrees of latitude between the mid 1950s and early 1970s. This suggests a decline in the area covered by sea ice of some 25%. This abrupt change poses a challenge to model simulations of recent climate change, and could imply changes in Antarctic deep-water formation and in biological productivity, both important processes affecting atmospheric CO2 concentrations.