CESM2 Equilibrium Climate Sensitivity

The Community Earth System Model Version 2 (CESM2) has an equilibrium climate sensitivity (ECS) of 5.3 K, when subject to a doubling in the present-day value of carbon dioxide (CO2). This is elevated significantly from the value of 4.1 K seen in the previous model version (CESM1), and is primarily the result of changes in cloud feedbacks. Although there are contributions from other feedbacks associated with the land model, much of the change in ECS are a consequence of efforts to reproduce the long-term trend of global and regional surface temperatures over the 20th century, in response to climate forcings. Changes made to reduce sensitivity to aerosols also impacted cloud feedbacks, which significantly influence ECS. CESM2 simulations compare very well to observations of present climate, so it is critical to understand whether the high ECS, which lies outside the current PICC best estimate of 1.5-4 K, is plausible.

Reference

  • Gettelman, A., Hannay, C., Bacmeister, J. T., Neale, R. B., Pendergrass, A. G., Danabasoglu, G., et al., 2019. High climate sensitivity in the Community Earth System Model Version 2 (CESM2). Geophysical Research Letters, 46, 8329– 8337. https://doi.org/10.1029/2019GL083978