ECS, Climate Feedback, and TCRE: A Comparison of CMIP6 Values to Estimates Inferred from the Century-Long Climate Record

We will present an analysis of Equilibrium Climate Sensitivity (ECS), climate feedback, and Transient Climate Response to Cumulative Emissions (TCRE) inferred from archived output of near surface air temperature (SAT) provided by the General Circulation Models (GCMs) that have participated in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). The CMIP6 values of ECS and climate feedback will be based on analysis of the global mean surface temperature anomaly (GMST) computed from SAT, for GCMs that participated in the abrupt-4×CO₂ experiment. The CMIP6 values of TCRE are based on analysis of GMST from GCM runs that use prescribed future abundances of greenhouse gases from the Shared Socioeconomic Pathway (SSP) 3 - 7.0 scenario, for which about 6/7 of the radiative forcing of climate in 2100 is due to the rise in CO₂. We will also present values of ECS, climate feedback, and TCRE found using our Empirical Model of Global Climate (EM-GC), a multiple linear regression energy balance model that quantitatively accounts for natural influences on GMST such as major volcanic eruptions, the 11-year solar cycle, ENSO, and variations in the strength of the Atlantic Meridional Overturning Circulation as well as anthropogenic influences due to rising greenhouse gases, tropospheric aerosols, and land use change (Hope et al., doi:10.1007/978-3-319-46939-3_2, 2017). The EM-GC simulations are constrained by observations of GMST since the late 1800s and oceanic heat content (OHC) since the early 1960s. Most importantly, the EM-GC provides a framework for quantifying the impact on ECS, climate feedback, and TCRE of incomplete knowledge of the radiative forcing due to tropospheric aerosols as well as uncertainty in OHC. Empirically-based estimates of ECS, climate feedback, and TCRE, all with uncertainties, will be compared to the CMIP6-based values of these three quantities. We'll conclude by offering some thoughts on the reason for the differences apparent in our preliminary analysis: i.e., most of the CMIP6 GCMs appear to be warming more rapidly than the actual climate system.