Effective Radiative Forcing and Rapid Adjustments in CMIP6

The Radiative Forcing Model Intercomparison Project (RFMIP) experiments evaluate the present-day effective radiative forcing (ERF) from greenhouse gases, aerosols, land use, and all anthropogenic agents, as well as 4×CO₂. It will provide model-based estimates of ERF in the IPCC's Sixth Assessment Report and is an important line of evidence in the overall assessment. Here we show and discuss the differences in ERF for the models that have submitted results to the CMIP6 archive. Using a radiative kernel decomposition we can separate the ERF into its direct (instantaneous) component and its rapid adjustments. These separations allow insight into the driving mechanisms behind certain forcing agents. Many models now provide ISCCP simulator diagnostics allowing a direct estimate of rapid adjustments due to changes in cloud top pressure and cloud optical depth. We see a large spread in cloud adjustments in the 4×CO₂ experiment between models, and we can also decompose the forcing from the aerosol experiment into contributions from aerosol-radiation and aerosol-cloud interactions. The effective radiative forcing has consequences for equilibrium climate sensitivity (ECS) given its relationship between planetary energy imbalance, climate feedback parameter and temperature, and understanding the variance in ERF in models may enable us to narrow the uncertainty in ECS.