Response of a Coupled Ocean–Atmosphere Model to Increasing Atmospheric Carbon Dioxide: Sensitivity to the Rate of Increase

The influence of differing rates of increase of the atmospheric CO2 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 CO2 increase ranging from 4% yr21 to 0.25% yr21. By the time of CO2 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 CO2 concentration. The thermohaline circulation (THC) weakens in response to increasing atmospheric CO2 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 CO2 doubling with the CO2 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 CO2 increase, indicating that the coupled system has more time to adjust to the forcing when the rate of CO2 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 CO2 integrations. Considering only the sea level changes due to the thermal expansion of seawater, the integration with the slowest rate of increase in CO2 concentration (i.e., 0.25% yr21) has the largest globally averaged sea level rise by the time of CO2 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.