Evaluating Physical Climate Model Emulators for Global Warming Projections

Abstract Climate model emulators were extensively used in the IPCC's Sixth Assessment Report for their computational efficiency and consistency with key climate metrics. The emulators were calibrated to historical observations and used for future climate projections without systematic evaluation of their robustness. Here, we develop a framework to evaluate the performance of physical climate model emulators against global climate model (GCM) large ensembles. This is demonstrated by constraining a two‐layer energy balance model (EBM) to historical simulations of four GCMs and comparing their 21st century warming projections. The EBM matches projected warming in three of the four GCMs but exhibits substantial ensemble spread. It fails to reproduce the time‐evolving global climate feedback in GCMs, with compensating biases between feedbacks and ocean heat uptake efficiency allowing seemingly accurate projections for incorrect reasons. Our results underscore the importance of evaluating the accuracy and physical realism of emulators before using them for warming projections.