Characteristics of future warmer base states in CESM2

Simulations of 21st century climate with Community Earth System Model version 2 (CESM2) using the standard atmosphere (CAM6), denoted CESM2(CAM6), and the latest generation of the Whole Atmosphere Community Climate Model (WACCM6), denoted CESM2(WACCM6), are presented, and a survey of general results is described. The equilibrium climate sensitivity (ECS) of CESM2(CAM6) is 5.3 degrees C, and CESM2(WACCM6) is 4.8 degrees C, while the transient climate response (TCR) is 2.1 degrees C in CESM2(CAM6) and 2.0 degrees C in CESM2(WACCM6). Thus, these two CESM2 model versions have higher values of ECS than the previous generation of model, the CESM (CAM5) (hereafter CESM1), that had an ECS of 4.1 degrees C, though the CESM2 versions have lower values of TCR compared to the CESM1 with a somewhat higher value of 2.3 degrees C. All model versions produce credible simulations of the time evolution of historical global surface temperature. The higher ECS values for the CESM2 versions are reflected in higher values of global surface temperature increase by 2,100 in CESM2(CAM6) and CESM2(WACCM6) compared to CESM1 between comparable emission scenarios for the high forcing scenario. Future warming among CESM2 model versions and scenarios diverges around 2050. The larger values of TCR and ECS in CESM2(CAM6) compared to CESM1 are manifested by greater warming in the tropics. Associated with a higher climate sensitivity, for CESM2(CAM6) the first instance of an ice-free Arctic in September occurs for all scenarios and ensemble members in the 2030-2050 time frame, but about a decade later in CESM2(WACCM6), occurring around 2040-2060.