Changing spatial structure of the thermohaline circulation in response to atmospheric CO2 forcing in a climate model

The heat transported northwards by the North Atlantic thermohaline circulation warms the climate of western Europe1,2,3. Previous model studies4,5,6 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’7,8,9 (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 observations10,11. 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 intwo 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.