State Dependence of CO2 Forcing and Its Implications for Climate Sensitivity

Instantaneous radiative forcing (IRF) is a fundamental metric for measuring the extent to which anthropogenic activities and natural events perturb the Earth's energy balance. This perturbation initiates all other forced climate responses. Among all the anthropogenic forcing agents, CO₂ is the dominant driver of warming over the past century and the defining forcing variable for quantifying climate sensitivity. When evaluating the effect of CO₂ changes on the earth’s climate, it is universally assumed that the IRF from a doubling of a given CO₂ concentration (IRF_2×CO2) is constant and that variances in climate sensitivity arise from differences in radiative feedbacks, or a dependence of these feedbacks on the climatological base-state. In this paper, we show that the IRF_2×CO2 is not constant, but also depends on the climatological base-state, increasing by ~25% for every doubling of CO₂, and has increased by ~10% since the pre-industrial era, implying a proportionate increase in climate sensitivity. This base-state dependence also explains about half of the intermodel spread in IRF_2×CO2, a problem that has persisted among climate models for nearly three decades. It may also have important implications for elucidating the causes and consequences of deep-time paleoclimates, where changes in the climatological base-state can strongly modulate the magnitude of the CO₂ IRF.