Solar Cycle Temperature Response as an Independent Constraint of Transient Climate Sensitivity

As a quasi-periodic and well-constrained process, solar cycle radiative forcing holds promise as a means to empirically constrain global climate sensitivity on decadal timescales. Due to methodological disagreements, however, past estimates of global temperature response to solar cycle forcing have been too widely dispersed to yield a meaningful constraint on observed climate sensitivity. Using Bayesian likelihood-maximization of surface temperature response estimates to solar cycle forcing and accounting for relevant scalings to convert variation in total solar irradiance to equivalent CO2 forcing, we find a 95% confidence interval of climate sensitivity inferred from solar cycle forcing of 2.1 to 6.4 K/(W m2). Through comparison of this empirical constraint to temperature responses produced in CMIP6 solar radiative forcing experiments, we explore the degree to which variations in solar cycle sensitivity are indicative of transient and equilibrium responses in CMIP6. This analysis offers a means of testing the validity of a GCM-derived climate sensitivity estimate by comparing its solar forcing sensitivity to the empirical solar cycle constraint.