The equilibrium climate sensitivity (ECS, in K) to $$\hbox {CO}_{2}$$ CO 2 doubling is a large source of uncertainty in projections of future anthropogenic climate change. Estimates of ECS made from non-equilibrium states or in response to radiative forcings other than $$\hbox {2}\times \hbox {CO}_{2}$$ 2 × CO 2 are called “effective climate sensitivity” (EffCS, in K). Taking a “perfect-model” approach, using coupled atmospher...