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 × 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_2×) in 2 × CO₂ is different from the effective climate sensitivity (T_2×,eff) in 1%CO₂. 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 × 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 T_2×,eff is discussed.