L. S. Jackson1, A. C. Maycock1, T. Andrews2, C. J. Smith1, and P. M. Forster3
1 School of Earth and Environment, University of Leeds, Leeds, UK. 2 Met Office Hadley Centre, Exeter, UK. 3 Priestley International Centre for Climate, University of Leeds, Leeds, UK.
Corresponding author: L. S. Jackson (l.s.jackson@leeds.ac.uk)
Key Points:
Abstract
Climate model emulators are widely used to generate temperature projections for climate scenarios, including in the recent IPCC Sixth Assessment Report. Here we evaluate the performance of a two-layer energy balance model in emulating historical and future temperature projections from CMIP6 models. We find that prediction errors can be large (greater than 0.5oC in a given year) and differ markedly between climate models, forcing scenarios and time periods. Errors arise in emulating the near-surface temperature response to both greenhouse gas and aerosol forcing; in some periods the errors due to these forcings oppose one another, giving the spurious impression of better emulator performance. Time-varying and state-dependent feedbacks may contribute to prediction errors. Close emulations can be produced for a given period but, crucially, this does not guarantee reliable emulations of other scenarios and periods. Therefore, rigorous out-of-sample evaluation is necessary to characterize emulator performance.
Plain Language Summary
Complex climate models are state-of-the-art tools used to produce projections of future climate but they are expensive and take a long time to run. Climate model emulators are simple statistical or physically based models which can reproduce projections from complex climate models at lower cost and more quickly. In this study, we use a climate model emulator to reproduce projections of twentieth and twenty-first century temperatures for eight complex climate models. We show that close emulations can be produced for pre-defined climate scenarios and time periods. Close emulations are not guaranteed, however, when the emulator is used for other climate scenarios or other periods. This is important because climate model emulators are frequently used to produce projections that are not available from complex climate models. Evaluation of climate model emulators and characterization of their uncertainties, therefore, should use data not used in the calibration of the emulator.