Increasing Earth System Sensitivity in mid-Pliocene simulations from CCSM4 to CESM2

Three new equilibrium Mid-Pliocene (MP) simulations are implemented with the Community Climate System Model version 4 (CCSM4), Community Earth System Model version 1.2 (CESM1.2), and 2 (CESM2). All simulations are carried out with the same boundary and forcing conditions following the protocol of Pliocene Model Intercomparison Project Phase 2. These simulations reveal amplified MP climate change relative to preindustrial going from CCSM4 to CESM2, seen in global mean and polar amplification of surface warming, sea ice reduction in both Arctic and Antarctic, and weakened Hadley circulation. The enhanced global mean warming arises from both enhanced Earth System Sensitivity (ESS) and Equilibrium Climate Sensitivity (ECS) to CO forcing. ESS is amplified by up to 70% in CCSM4, and up to 100% in CESM1.2 and CESM2 relative to ECSs of respective models. Simulations also agree on the strengthened Atlantic Meridional Overturning Circulation, but disagree on several other climate metrics. Compared to preindustrial,...