How accurately can the climate sensitivity to [Formula: see text] be estimated from historical climate change?

The equilibrium climate sensitivity (ECS, in K) to [Formula: see text] 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 [Formula: see text] are called “effective climate sensitivity” (EffCS, in K). Taking a “perfect-model” approach, using coupled atmosphere–ocean general circulation model (AOGCM)...