The heat transported northwards by the North Atlantic thermohaline circulation warms the climate of western Europe(1-3) previous model studies(4-6) have suggested that the circulation is sensitive to increases in atmospheric greenhouse-gas concentrations, but such models have been criticised for the use of unphysical ＇flux adjustments＇ (7-9) (artificial corrections that keep the model from drifting to unrealistic states), and for their inability to simulate deep-water formation both north and south of the Greenland-Iceland-Scotland ridge, as seen in observations(10,11), Here we present simulations of today＇s thermohaline circulation using a coupled ocean-atmosphere general circulation model without flux adjustments, These simulations compare well with the observed thermohaline circulation, including the formation of deep water on each side of the Greenland-Iceland-Scotland ridge. The model responds to forcing with increasing atmospheric greenhouse-gas concentrations by a collapse of the circulation and convection in the Labrador Sea, while the deep-water formation north of the ridge remains stable. These changes are similar in two simulations with different rates of increase of CO2 concentrations. The effects of increasing atmospheric greenhouse-gas concentrations that we simulate are potentially observable, suggesting that it is possible to set up an oceanic monitoring system for the detection of anthropogenic influence on ocean circulation.