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 concent ration on the climatic response is investigated using a coupled ocean-atmos phere model. Five transient integrations are performed each using a differe nt constant exponential rate of CO2 increase ranging from 4% yr(-1) to 0.25 % yr(-1). By the time of CO2 doubling, the surface air temperature response in all the transient integrations is locally more than 50% and globally mo re than 35% of the equilibrium response. The land-sea contrast in the warmi ng, which is evident in the equilibrium results, is larger in all the trans ient experiments. The land-sea difference in the response increases with th e rate of increase in atmospheric CO2 concentration. The thermohaline circu lation (THC) weakens in response to increasing atmospheric CO2 concentratio n in all the transient integrations, confirming earlier work. The results a lso 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 c ontinued beyond the time of CO2 doubling with the CO2 concentration maintai ned at that level. The amount of weakening of the THC after the CO2 stops i ncreasing 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 chang es due to the thermal expansion of seawater, the integration with the slowe st rate of increase in CO2 concentration (i.e., 0.25% yr(-1)) has the large st globally averaged sea level rise by the time of CO2 doubling (about 42 c m). 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 inte grations. This implies that sea level continues to rise long after the CO2 concentration stops increasing, as the warm anomaly penetrates deeper into the ocean.