High Equilibrium and Transient Climate Sensitivity in the Community Earth System Model: Evolution from CESM1 to CESM2

Equilibrium climate sensitivity (ECS) in the Community Earth System Model version2 (CESM2) has increased to around 5.3K from a value of around 4.1K in CESM1. Transient climate sensitivity derived from fully-coupled abrupt 4xCO2 runs has also increased in CESM2. The increase in TCS in CESM2 has been accompanied by significant qualitative changes in the time evolution of surface temperature and top-of-atmosphere (TOA) radiative balance in 4xCO2 experiments. Increased climate sensitivity has been reported by several major modeling centers in their CMIP6 generation models, when compared with previous CMIP5 estimates.

In this presentation we show a systematic investigation of how key development steps impacted the climate sensitivity of CESM2. CESM2 incorporated substantial changes to parameterizations compared to CESM1. This includes changes in PBL turbulence, shallow convection, cloud microphysics and cloud "macrophysics". In addition, during CESM2 development atmosphere model physics were adjusted at several points to achieve a TOA radiation balance within 0.1 Wm -2 . CESM2 also incorporated changes to ocean and land-surface components.

We find a relatively large range of ECS values in our development versions of CESM2, with some versions registering ECS as low as 4.5K depending on details in land and atmospheric physics parameterizations. However, qualitative changes in the transient behavior of fully-coupled 4xCO2 simulations between CESM1 and CESM2 are more pronounced. Qualitative differences in the time-evolution of regional as well as global mean surface temperatures are evident, which are suggestive of significantly altered thermohaline circulation sensitivity between model versions. We will describe these changes, and attempt to better define the relationship between different estimates of climate sensitivity in CESM.