Past warm climates and their implications for future climate sensitivity

The early Eocene and mid-Cretaceous were periods of extremely warm climate with no evidence of polar ice. The extreme warmth of both periods has been attributed to higher greenhouse gas concentrations, making them analogues for our future high-CO2 world. Climate models with prescribed estimates of past high carbon dioxide levels have been unable to reproduce the warm conditions and reduced meridional thermal gradient indicated by proxy data. This failure has led both to suggestions that current generation climate models are deficient in critical ways and to proposals for exotic climate forcings in the past. Here we report on new simulations of the early Eocene and mid-Cretaceous using NCAR CESM 1.2 that show improved agreement with climate proxy data. In our paleoclimate simulations, CESM 1.2 exhibits an equilibrium climate sensitivity that is double that of the present day. We discuss the reasons for the improved model performance and the feedbacks that lead to a higher climate sensitivity. Our simulations suggest a risk of extreme warming in the future.