Multiple-Century Response of a Coupled Ocean-Atmosphere Model to an Increase of Atmospheric Carbon Dioxide

To speculate on the future change of climate over several centuries, three 500-year integrations of a coupled ocean-atmosphere model were performed. In addition to the standard integration in which the atmospheric concentration of carbon dioxide remains unchanged, two integrations are conducted. In one integration, the C02 concentration increases by 1% yr−1 (compounded) until it reaches four times the initial value at the 140th year and remains unchanged thereafter. In another integration, the C02 concentration also increases at the rate of 1% yr−1 until it reaches twice the initial value at the 70th year and remains unchanged thereafter. One of the most notable features of the C02-quadrupling integration is the gradual disappearance of thermohaline circulations in most of the model oceans during the first 250-year period, leaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the North Atlantic during the fist 200 years of the integration. In the Weddell and Ross seas, thermohaline circulation becomes weaker and shallower, thereby reducing the rate of bottom water formation and weakening the northward flow of bottom water in the Pacific and Atlantic oceans. The weakening or near disappearance of thermohaline circulation described above is attributable mainly to the capping of the model oceans by relatively fresh water in high latitudes where the excess of precipitation over evaporation increases markedly due to the enhanced poleward moisture transport in the warmer model troposphere. In the C02-doubling integration, the thermohaline circulation weakens by a factor of more than 2 in the North Atlantic during the first 150 years but almost recovers its original intensity by the 500th year. The increase and downward penetration of positive heat and temperature anomaly in low and middle latitudes of the North Atlantic helps to increase the density contrast between the sinking and rising regions, contributing to this slow recovery. The recovery is aided by the gradual increase in surface salinity that accompanies the intensification of the thermohaline circulation. During the 500-year period of the doubling and quadrupling experiments, the global mean surface air temperature increases by about 3.5°C and 7°C, respectively. The rise of sea level due to the thermal expansion of sea water is about 1 and 1.8 m, respectively, and could be much larger if the contribution of meltwater from continental ice sheets were included. It is speculated that the two experiments described above provide a probable range of future climate change.