Errors in Simple Climate Model Emulations of Past and Future Global Temperature Change

Climate model emulators are widely used to generate temperature projections for climate scenarios, including in the recent Intergovernmental Panel on Change Sixth Assessment Report. Here we evaluate performance of a two-layer energy balance emulating historical and future from Coupled Model Intercomparison Project Phase 6 models. We find that emulation errors can be large (>0.5°C SSP2-4.5) differ markedly between models, forcing scenarios time periods. Errors arise near-surface response both greenhouse gas aerosol forcing; some periods due these forcings oppose one another, giving spurious impression better emulator performance. feedbacks assumed constant emulator, introducing time-varying or state dependent may reduce prediction errors. Close emulations produced given period but, crucially, this does not guarantee reliable other Therefore, rigorous out-of-sample evaluation is necessary characterize