Structural similarities and differences in climate responses to CO2 increase between two perturbed physics ensembles

The equilibrium climate sensitivity (ECS) of the two perturbed physics ensembles (PPE) generated using structurally different GCMs, MIROC3.2 and HadSM3, is investigated. We develop a method to quantify the SW cloud feedback by clouds with different cloud top pressure. We find that the difference in the ensemble means of the ECS between the two ensembles is mainly caused by differences in the SW low level cloud feedback. The ensemble mean SW cloud feedback and ECS of the MIROC3.2 ensemble is larger than the HadSM3 ensemble and this is likely related to the 1XCO2 low level cloud albedo of the former being larger than that of the latter. We also find that the largest contribution to the within-ensemble variation of ECS comes from the SW low level cloud feedback in both ensembles. The mechanism which causes the within-ensemble variation is different between the two ensembles. In the HadSM3 ensemble, members with large 1XCO2 low level cloud albedo have large SW cloud feedback and large ECS; ensemble members with large 1XCO2 cloud cover have large negative SW cloud feedback and relatively low ECS. In the MIROC3.2 ensemble, the 1xCO2 low level cloud albedo is much more tightly constrained, and no relationship is found between it and the cloud feedback. These results indicate that both the parametric uncertainties sampled in PPEs and the structural uncertainties of GCMs are important and worth further investigation.