Equilibrium climate sensitivity (ECS) is the global temperature change expected after doubling atmospheric CO2 concentration. This Commentary reviews how Sherwood et al. (2020) used Bayesian statistics and evidence from climate-process physics, historical observations, and earlier proxies to reduce the likely range of ECS from 1.5-4.5 K to 2.6-4.1 K. They may have overestimated ECS by adding non-equilibrium short-term adjustments to the radiative forcing of greenhouse gases and by underestimating the effect of solar irradiance and aerosols. Two alternative periods during the Holocene show that forcing by agents other than CO2 was significant and requires further research.