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Comparison of equilibrium and transient responses to CO2 increase in eight state-of-the-art climate models


*Corresponding author.


We compared the climate response of doubled CO2 equilibrium experiments (2 × CO2) by atmosphere–slab ocean coupled general circulation models (ASGCMs) and that of 1% per year CO2 increase experiments (1%CO2 by atmosphere–ocean coupled general circulation models (AOGCMs) using eight state-of-the-art climate models. Climate feedback processes in 2 × CO2 are different from those in 1%CO2, and the equilibrium climate sensitivity (T) in 2 × CO2 is different from the effective climate sensitivity (T2×,eff) in 1%CO2. The difference between T and T2×,eff is from −1.3 to 1.6 K, a large part of which can be explained by the difference in the ice-albedo and cloud feedback. The largest contribution is cloud SW feedback, and the difference in cloud SW feedback for 2 × CO2 and 1%CO2 could be determined by the distribution of the SAT anomaly which causes differences in the atmospheric thermal structure. An important factor which determines the difference in ice-albedo feedback is the initial sea ice distribution at the Southern Ocean, which is generally overestimated in 2 × CO2 as compared to 1%CO2 and observation. Through the comparison of climate feedback processes in 2 × CO2 and 1%CO2, the possible behaviour of the time evolution of T2×,eff is discussed.