A doubled CO2 coupled ocean-atmosphere experiment has been run for over 800 years. The 'effective' equilibrium climate sensitivity to a doubling of CO2 (the equilibrium response of the model assuming the feedbacks remained constant at the value found at any given point of the transient response) is calculated throughout the run and found to increase by around 40%. The time-dependence is associated with differences in cloud feedback arising from inter-hemispheric temperature differences due to the slower warming rate of the Southern Ocean. The time-dependence of the climate response has implications for the use of simpler models in scaling GCM results to different scenarios of forcing. ¿ 2000 American Geophysical Union