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The influence of differing rates of increase of the atmospheric CO 2 concentration on the climatic response is investigated using a coupled ocean-atmosphere model. Five transient integrations are performed each using a different constant exponential rate of CO 2 increase ranging from 4% yr 1 to 0.25% yr 1 . By the time of CO 2 doubling, the surface air temperature response in all the transient integrations is locally more than 50% and globally more than 35% of the equilibrium response. The land-sea contrast in the warming, which is evident in the equilibrium results, is larger in all the transient experiments. The land-sea difference in the response increases with the rate of increase in atmospheric CO 2 concentration. The thermohaline circulation (THC) weakens in response to increasing atmospheric CO 2 concentration in all the transient integrations, confirming earlier work. The results also indicate that the slower the rate of increase, the larger the weakening of the THC by the time of doubling. Two of the transient experiments are continued beyond the time of CO 2 doubling with the CO 2 concentration maintained at that level. The amount of weakening of the THC after the CO 2 stops increasing is smaller in the experiment with the slower rate of CO 2 increase, indicating that the coupled system has more time to adjust to the forcing when the rate of CO 2 increase is slower. After a period of slow overturning, the THC gradually recovers and eventually regains the intensity found in the control integration, so that the equilibrium THC is very similar in the control and doubled CO 2 integrations. Considering only the sea level changes due to the thermal expansion of seawater, the integration with the slowest rate of increase in CO 2 concentration (i.e., 0.25% yr 1 ) has the largest globally averaged sea level rise by the time of CO 2 doubling (about 42 cm). However, only a relatively small fraction of the equilibrium sea level rise of 1.9 m is realized by the time of doubling in all the transient integrations. This implies that sea level continues to rise long after the CO 2 concentration stops increasing, as the warm anomaly penetrates deeper into the ocean.
Stouffer et al. (Sun,) studied this question.