Climatic Response to a Gradual Increase of Atmospheric Carbon Dioxide

The transient response of a coupled ocean-atmosphere model to an increase of carbon dioxide has been the subject of several studies (Bryan et al., 1982; Spelman and Manabe, 1984; Bryan and Spelman, 1985; Schlesinger and Jiang, 1988; Schlesinger et al., 1985; Bryan et al., 1988; Manabe et al., 1990; Washington and Meehl, 1989). The models used in these studies explicitly incorporate the effect of heat transport by ocean currents and are different from the model used by Hansen et al. (1988). Here we evaluate the climatic influence of increasing atmospheric carbon dioxide using a coupled model recently developed at the NOAA Geophysical Fluid Dynamics Laboratory. The model response exhibits a marked and unexpected interhemispheric asymmetry. In the circumpolar ocean of the Southern Hemisphere, a region of deep vertical mixing, the increase of surface air temperature is very slow. In the Northern Hemisphere of the model, the rise of surface air temperature is faster and increases with latitude, with the exception of the northern North Atlantic, where it is relatively slow because of the weakening of the thermohaline circulation.