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Using the three‐dimensional coarse‐resolution climate model ECBILT‐CLIO, 1000‐year long ensemble simulations with natural and anthropogenic forcings have been performed to study the long‐term variation of the ice cover in the Southern Ocean. Over the last 250 years, the ice area has decreased by about 1 × 106 km2 in its annual mean. A comparison with experiments driven by only natural forcings suggests that this reduction is due to both natural and anthropogenic forcing, the latter playing a larger role than natural forcing over the last 150 years. Despite this contribution from anthropogenic forcing, the simulated ice area at the end of the 20th century is similar to that simulated during the 14th century because of the slow response of the Southern Ocean to radiative forcing. Sensitivity experiments performed with the model show that the model's initial conditions have a large influence on the simulated ice cover and that it is necessary to start simulations at least two centuries before the period of interest in order to remove this influence. Copyright © 2005 Royal Meteorological Society. [ABSTRACT FROM AUTHOR]