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

We compared the climate response of doubled CO 2 equilibrium experiments (2 × CO 2 ) by atmosphere–slab ocean coupled general circulation models (ASGCMs) and that of 1% per year CO 2 increase experiments (1%CO 2 by atmosphere–ocean coupled general circulation models (AOGCMs) using eight state-of-the-art climate models. Climate feedback processes in 2 × CO 2 are different from those in 1%CO 2 , and the equilibrium climate sensitivity (T 2× ) in 2 × CO 2 is different from the effective climate sensitivity (T 2×,eff ) in 1%CO 2 . The difference between T 2× and T 2×,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 × CO 2 and 1%CO 2 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 × CO 2 as compared to 1%CO 2 and observation. Through the comparison of climate feedback processes in 2 ×CO 2 and 1%CO 2 , the possible behaviour of the time evolution of T 2×,eff is discussed.