Equilibrium climate sensitivity (ECS), the global surface temperature increase to the radiative forcing caused by a doubling of atmospheric CO2 concentration, is the single number that describes the severity of long-term climate change. Global climate models have been an important tool for estimating ECS and understanding the relevant physics. Recent Coupled Model Intercomparison Project phase 6 (CMIP6) models simulated an ECS range of 1.8–5.6 ℃, with an upper end substantially higher than previ