21st century scenario forcing increases more for CMIP6 than CMIP5 models

We present new estimates of the forcing for models participating in Coupled Model Intercomparison Project 6 (CMIP6) by applying the method developed in Fredriksen et al. (2021). Validating our approach, these estimates are overall consistent with the fixed-SST estimates available for a small subset of the models. We estimate forcing for experiments with abrupt changes of CO 2 , 1% increase of CO2, historical forcings, and future scenarios. Furthermore, we compare our new estimates to CMIP5 forcing, and demonstrate that CMIP6 forcing is lower than CMIP5 forcing at the end of the historical period, but grows faster than CMIP5 in the future scenarios, ending up at higher levels than CMIP5 at the end of the 21st century. The radiative efficiency of CO 2 has not changed, suggesting that the stronger future increase in CO 2 concentrations in CMIP6 compared to CMIP5 explains the forcing difference.