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A simplified climate model of the coupled ocean-atmosphere system is used to perform extensive sensitivity studies concerning possible future climate change induced by anthropogenic greenhouse gas emissions. Sup- plemented with an active atmospheric hydrological cycle, experiments with different rates of CO 2 increase and different climate sensitivities are performed. The model exhibits a threshold value of atmospheric CO 2 concen- tration beyond which the North Atlantic Deep Water formation stops and never recovers. For a climate sensitivity that leads to an equilibrium warming of 3.68C for a doubling of CO2 and a rate of CO2 increase of 1% yr21, the threshold lies between 650 and 700 ppmv. Moreover, it is shown that the stability of the thermohaline circulation depends on the rate of increase of greenhouse gases. For a slower increase of atmospheric pCO2 the final amount that can be reached without a shutdown of the circulation is considerably higher. This rate-sensitive response is due to the uptake of heat and excess freshwater from the uppermost layers to the deep ocean. The increased equator-to-pole freshwater transport in a warmer atmosphere is mainly responsible for the cessation of deep water formation in the North Atlantic. Another consequence of the enhanced latent heat transport is a stronger warming at high latitudes. A model version with fixed water vapor transport exhibits uniform warming at all latitudes. The inclusion of a simple parameterization of the ice-albedo feedback increases the model sensitivity and further decreases the pole-to-equator temperature difference in a greenhouse climate. The possible range of CO2 threshold concentrations and its dependency on the rate of CO 2 increase, on the climate sensitivity, and on other model parameters are discussed.

The Stability of the Thermohaline Circulation in Global Warming Experiments