Evaluation of the CMIP7 historical stratospheric aerosol forcing dataset

Stratospheric aerosol forcing, which primarily represents aerosols from explosive volcanic sulfur emissions, is a key natural forcing dataset for Phase 7 of the Coupled Model Intercomparison Project (CMIP7) climate modelling experiments. The CMIP7 stratospheric aerosol forcing datasets for the historical period (1750-2023) include (1) stratospheric sulfate aerosol optical properties, and (2) upper tropospheric-stratospheric volcanic sulfur dioxide emissions. Understanding how historical volcanic forcing has changed from CMIP6 to CMIP7 is essential for interpreting differences in simulation results across CMIP phases and assessing model performance. A key methodological advance in CMIP7 is the emission-driven approach for pre-satellite era stratospheric aerosol optical properties, which incorporates additional ice-core-based volcanic sulfur emission data compared to CMIP6. In this study, we present a systematic evaluation comparing the CMIP6 and CMIP7 stratospheric aerosol forcing datasets against observations, including aerosol optical depth estimates from lunar eclipses, stellar extinction, and satellite retrievals. The comparison provides an in-depth analysis of spatial and temporal patterns in stratospheric aerosol optical depth across CMIP6 and CMIP7, examining both background climatology and selected large-magnitude eruptions in the pre-satellite and satellite eras. This evaluation highlights the key differences between CMIP6 and CMIP7 datasets, improvements achieved through updated methodologies, their potential implications for climate simulations, and directions for future forcing dataset development.