Response of the North Atlantic Thermohaline Circulation to Atmospheric CO2 Concentration Octupling

Recent studies on climate change arising from increases in atmospheric CO₂ concentrations have demonstrated not only the possibility of a weakened North Atlantic thermohaline circulation (THC), but also a total recovery in its intensity. The present study considers the case of 8 times the present concentration by carrying out a long-term integration of a GFDL low-resolution coupled model. Results are compared with previous quadrupling experiments. It is found that the THC eventually recovers, but the time of recovery is prolonged by about 5000 years in comparison to the 4xCO₂ case. During the 3000 years prior to complete recovery, little change is seen in the THC whose intensity varies around 8Sv, but there is a distinctive centennial variability, also seen in the time series of the globally averaged sea-surface temperature. The final recovery stage occurs rapidly about 6200 years after the start of integration and is thereafter marked by a large decadal variability. The average value of the final THC intensity is approximately 16Sv, slightly smaller than those of the control and 4xCO₂ cases. The initial forcing due to changes in the hydrological cycle are mainly amplifications of those in the 4xCO₂ case. In other words, there are larger increases in net precipitation over the polar regions, the Himalayas and the central tropical Pacific, and larger decreases in the rest of the low latitudes. The globally averaged sea-surface temperature increases from about 17.6^oC to 26.3^oC (23.7^oC for the 4xCO₂ case), mostly within the first 1500 years. In the deeper ocean (3km), the average temperature increases from 1.1^oC and gradually reaches 8.9^oC (6.6^oC for the 4xCO₂ case) at the time of complete THC recovery.