Evidence that end-of-century emulation errors under overshoot remain largely within internal variability in an Earth system model

Abstract: Overshoot emission scenarios, in which a warming threshold is temporarily exceeded before returning to lower temperatures, are becoming increasingly relevant in the face of insufficient reductions of greenhouse gas emissions. Climate model emulators offer an efficient framework to study these pathways and are seeing growing uptake for this purpose. Yet most existing approaches for spatial emulation are driven solely by global mean surface temperature (GMST). This prevents them from capturing non-reversible local climate responses that persist as GMST declines, raising questions regarding their performance under overshoot trajectories. 

Here, we consider an aggressive overshoot scenario and evaluate end-of-century biases in mean temperature, maximum daily temperature, and precipitation from a GMST-driven emulator against a 10-member ensemble of simulations from an Earth system model. The results show that while the emulator displays biases, these biases are largely dominated by internal variability for 99% of the land regions. Land regions where biases approach or exceed variability are primarily associated with rapid reductions in regional aerosol emissions. We demonstrate that extending the emulator to include aerosol optical depth as an additional predictor helps diagnose and reduce these biases. 

These results provide evidence that GMST-driven emulators provide useful information for end-of-century projections under overshoot, and that a key priority for future emulator development is to improve the representation of aerosol forcing.