This study investigates the transient response of a coupled ocean-atmosphere model to gradual changes in atmospheric CO₂ concentrations. The model, a general circulation model of the coupled atmosphere-ocean-land surface system with global geography and seasonal insolation variation, is adjusted to offset biases in its equilibrium state. Three numerical integrations are performed: one with a normal CO₂ concentration, and two with CO₂ concentrations increasing and decreasing at a rate of 1% per year, respectively. The responses are compared to the equilibrium response of an atmosphere-mixed layer ocean model to CO₂ doubling or halving. Key findings include: - The circulation of deep sea surface temperature is slow in the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere due to deep vertical mixing. - The distribution of surface air temperature changes resembles the equilibrium response of the atmosphere-mixed layer ocean model, with increases in the Northern Hemisphere and over continents. - The penetration depth of thermal anomalies is deeper in the CO₂ reduction experiment compared to the CO₂ growth experiment, due to differences in static stability and salinity distribution. - Despite the difference in penetration depth, the magnitude of the time-dependent response of global mean surface air temperature in the CO₂ reduction experiment is similar to that in the CO₂ growth experiment, influenced by the larger area covered by snow and sea ice in the colder climate. - The ratio of the transient to equilibrium response of surface air temperature is significantly different between the CO₂ growth and reduction experiments, with the reduction experiment showing a smaller ratio in high-latitude oceans.This study investigates the transient response of a coupled ocean-atmosphere model to gradual changes in atmospheric CO₂ concentrations. The model, a general circulation model of the coupled atmosphere-ocean-land surface system with global geography and seasonal insolation variation, is adjusted to offset biases in its equilibrium state. Three numerical integrations are performed: one with a normal CO₂ concentration, and two with CO₂ concentrations increasing and decreasing at a rate of 1% per year, respectively. The responses are compared to the equilibrium response of an atmosphere-mixed layer ocean model to CO₂ doubling or halving. Key findings include: - The circulation of deep sea surface temperature is slow in the northern North Atlantic and the Circumpolar Ocean of the Southern Hemisphere due to deep vertical mixing. - The distribution of surface air temperature changes resembles the equilibrium response of the atmosphere-mixed layer ocean model, with increases in the Northern Hemisphere and over continents. - The penetration depth of thermal anomalies is deeper in the CO₂ reduction experiment compared to the CO₂ growth experiment, due to differences in static stability and salinity distribution. - Despite the difference in penetration depth, the magnitude of the time-dependent response of global mean surface air temperature in the CO₂ reduction experiment is similar to that in the CO₂ growth experiment, influenced by the larger area covered by snow and sea ice in the colder climate. - The ratio of the transient to equilibrium response of surface air temperature is significantly different between the CO₂ growth and reduction experiments, with the reduction experiment showing a smaller ratio in high-latitude oceans.