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

We compared the climate response of doubled CO2 equilibrium experiments ( 2 x 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 x CO2 are different from those in 1% CO2, and the equilibrium climate sensitivity (T-2x) in 2 x CO2 is different from the effective climate sensitivity (T-2x,(eff)) in 1% CO2. The difference between T-2x and T-2x,T-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 x 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 x CO2 as compared to 1%CO2 and observation. Through the comparison of climate feedback processes in 2 x CO2 and 1%CO2, the possible behaviour of the time evolution of T-2x,T-eff is discussed.