Suppression of orbital-induced polar amplification by lower GHG concentrations improves Mid-Holocene simulations in PMIP4

Abstract. The mid-Holocene (MH) provides a crucial window for investigating the impact of orbital forcing on climate. However, there is no consensus on MH temperature changes relative to the preindustrial. Substantial differences exist between the simulation results of the third and fourth phases of the Paleoclimate Model Intercomparison Project (PMIP3 and PMIP4), thereby increasing the uncertainty in MH temperature changes. To investigate these inter-generational discrepancies, we apply a climate feedback-response analysis method (CFRAM) to quantify the contributions of external forcings and internal feedbacks to MH temperature variations. Results suggest that the discrepancy is primarily driven by external forcings, but is substantially modulated by greenhouse gas (GHG)-conditioned nonlinear feedbacks. In PMIP3, orbital-forcing-induced warming more readily triggers strong polar amplification. In PMIP4, however, lower GHG concentrations lead to a colder background state and nonlinearly weaken key positive feedbacks including sea-ice albedo and surface heat storage processes, thereby suppressing polar warming. This GHG-modulated feedback suppression brings PMIP4 simulations into closer agreement with multiple proxy reconstructions, supporting a relatively cooler MH. Our findings highlight the combined influence of orbital parameters and GHG forcing on MH temperature change and emphasize the nonlinear role of GHG changes in damping polar amplification. These results offer a process-based perspective for understanding the intergenerational differences among climate models.