Atmospheric CO₂ concentrations continue to rise, yet uncertainty in equilibrium climate sensitivity (ECS) remains large, with implications for future warming projections. In this talk, I use simulations of past climate states to assess Earth system model performance and to diagnose sources of model bias. By comparing multiple generations of the Community Earth System Model (CESM) against paleoclimate proxy constraints, I show that very high ECS values (as in CESM2) overestimate both Eocene warmth and glacial cooling, whereas a moderate, state-dependent ECS—one that increases with warming—better reconciles models and data. These results demonstrate how paleoclimate benchmarks can identify physical and parameterization issues (especially in cloud microphysics and turbulence schemes), refine model sensitivity, provide insights into the longstanding “equable climate problem”, and ultimately improve confidence in projections of future climate change.

Bio
Dr. Chris Poulsen is the Tykeson Dean of Arts and Sciences and a professor in the Department of Earth Sciences at the University of Oregon. A climate scientist with 146 peer-reviewed publications, his research focuses on past and future climate change, climate dynamics, and earth system modeling. Prior to joining UO in 2022, he held leadership roles at the University of Michigan, including associate dean for natural sciences and chair of Earth and Environmental Sciences. He is a fellow of AAAS, the Geological Society of America, and the Humboldt Foundation, and a recipient of the UM John Dewey Award for teaching. He holds a PhD from Penn State.