Abstract
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 CO2, 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
CO2 has not changed, suggesting that the stronger future
increase in CO2 concentrations in CMIP6 compared to
CMIP5 explains the forcing difference.