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.