The Equilibrium Climate Sensitivity (ECS) is the amount of surface warming that would occur in response to doubled carbon dioxide. It is a widely-used diagnostic in climate science and is also important for informing policy decisions. Estimates of ECS from 20th century observations predict a lower value than values obtained from climate models. However, studies based on observations typically assume that all forcing agents affect the climate equally. We apply the concept of forcing efficacy, which is the amount of warming per unit global average forcing, divided by the warming per unit forcing from carbon dioxide, to our observation-based estimate. We find an ECS of 2.3 K (5%-95%-confidence range of 1.6-4.1 K), which is near the bottom of the IPCC's likely range of 1.5-4.5 K, but is consistent with other observational studies, under the traditional assumption that forcing efficacy is unity. We show that our calculation of ECS is sensitive to the assumed efficacy of aerosol and ozone forcing and that increasing the efficacy of these two agents to 1.33 yields an ECS of 3.0 K (1.9-6.8 K). This value agrees well with model results, demonstrating a way to reconcile different estimates of ECS.
