The climate response to a large increase in atmospheric CO2 was investigated in a numerical experiment with a coupled ocean-atmosphere model. The study is focused on one aspect of the experiment, the predicted response of the ocean to the warming episode. A fourfold increase in atmospheric CO2 causes a warming sufficiently intense to produce a partial collapse of the thermohaline circulation of the ocean. Surprisingly, the wind-driven circulation of the ocean is maintained without appreciable change. The global hydrological cycle intensifies without a major shift of the pattern of net precipitation over the model ocean. In the warming episode the downward pathways for heat, which include diffusion and Ekman pumping, remain open. The partial collapse of the thermohaline circulation closes the normal upward pathways associated with abyssal upwelling and high-latitude convection. As a result the thermocline is able to sequester almost twice as much heat than would be predicted from the behavior of a neutrally buoyant tracer introduced at the surface under normal climatic conditions. An enhanced sequestering of heat would produce a negative feedback for greenhouse warming. However, the partial collapse of the thermohaline circulation found in the numerical experiment would also affect the global carbon cycle, possibly producing a climatic feedback as strong as that caused by an enhanced uptake of heat from the atmosphere.