Using a coupled climate model of intermediate complexity including oxygen 18, CLIMBER-2, we investigate the evolution of the distribution of surface water 18O composition under warm climate conditions. We then determine the impact of changes of the surface water 18O distribution on ocean surface temperatures inferred from calcite oxygen 18. Our results show that published temperature reconstructions based on oxygen 18 from calcite are systematically biased by 2¿ to 4¿C in the absence of major oceanic circulation changes and up to 7¿C in the presence of major oceanic circulation changes. As the bias introduced is shown to vary with latitude, our work has major implications on past latitudinal temperature gradient reconstructions based on oxygen 18 measurements.