Colloquium: Dr. Cristian Proistosescu, Univ. of Washington

The latest Intergovernmental Panel on Climate Change Assessment Report widened the equilibrium climate sensitivity (ECS) range from 2° to 4.5°C for a doubling of atmospheric CO2 to an updated range of 1.5° to 4.5°C. The widened uncertainty range accounts for the lack of consensus between estimates based on numerical models and recently available historical observations. These historical estimates of ECS are derived either by examining slow changes in the earth's energy budget since pre-industrial times, or by a fluctuation-dissipation approach, where the sensitivity is inferred from stochastic internal variability. Both estimates are derived under the assumption of a linear radiative response to surface warming.

We document curvature in the radiative response to warming from an evolving contribution of interannual to centennial modes of radiative response. Centennial modes display stronger amplifying feedbacks and ultimately contribute 28 to 68% of equilibrium warming, yet they comprise only 1 to 7% of current warming. A similar time-scale dependence is documented for stochastic modes of variability.  Accounting for these unresolved centennial contributions brings historical records into agreement with model-derived ECS estimates.