CMIP6 Doubling vs. Quadrupling Data and Analysis

Abrupt CO2 quadrupling experiments are frequently used to characterize general circulation models; how applicable is this large forcing to smaller amplitude climate change? We compare abrupt quadrupling (abrupt-4xCO2) and doubling (abrupt-2xCO2) CO2 experiments across eight CMIP6 models. Two models, CESM2 and MRI-ESM2-0 warm at substantially different rates between abrupt-4xCO2 and abrupt-2xCO2, unexplained by non-logarithmic scaling of CO2 forcing. Using an energy balance model, we evaluate the impact of two important factors on the warming rate: radiative feedbacks and effective heat capacity. Fractionally larger declines in the Atlantic Meridional Overturning Circulation in abrupt-2xCO2 simulations modulate the transient climate through low cloud cover (radiative feedbacks) and heat storage depth (heat capacity). We conclude that abrupt-2xCO2 experiments have significant differences in the Earth system compared to abrupt-4xCO2 and may be more relevant to near future warming.