Equilibrium and Effective Climate Sensitivity Inferred from the Historical Climate Record Using an Empirical Model of Global Climate Compared to CMIP6

Effective climate sensitivity is an estimate of the response of global mean surface temperature (GMST) to 2×pre-industrial CO2 for non-equilibrium conditions, while equilibrium climate sensitivity (ECS) represents the warming of GMST at equilibrium due to 2×pre-industrial CO2. We will use the Empirical Model of Global Climate (EM-GC) (McBride et al., ESD, 2021) to estimate values of both effective climate sensitivity and ECS. The EM-GC accounts for the natural influences of ENSO, the 11-year solar cycle, major volcanic eruptions, as well as the anthropogenic influence of greenhouse gases and aerosols, and the efficiency of ocean heat uptake on GMST. To estimate ECS, we will train the EM-GC to the HadCRUT5 GMST record from 1850-2019 for the model spin-up period, and then simulate the Abrupt 4×CO2 experiment utilized by CMIP6 applying values of climate feedback and aerosol radiative forcing determined by the model spin-up period until the EM-GC reaches equilibrium. We will compare the estimates of effective climate sensitivity and ECS based on the EM-GC to values derived from the CMIP6 Global Climate Models. Preliminary analysis shows that the best estimate of effective climate sensitivity based on the observed temperature record is 2.33°C with a range of 1.40-3.57°C (5th and 95th percent confidence interval) compared to a value for effective climate sensitivity of 3.74°C with a range of 2.19-5.65°C (5th and 95th percent confidence interval) from the current CMIP6 multi-model ensemble. We will conclude by discussing possible reasons for this discrepancy by comparing satellite derived estimates of cloud properties to those inferred from the CMIP6 multi-model ensemble.