State-dependence of CO2 Forcing and its Implications for Climate Sensitivity

Abstract

When evaluating the effect of CO2 changes on the earth's climate, it is widely assumed that instantaneous radiative forcing from a doubling of a given CO2 concentration (IRF2xCO2) 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 IRF2xCO2 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 primarily due to stratospheric cooling, implying a proportionate increase in climate sensitivity. This base-state dependence also explains about half of the inter-model spread in IRF2xCO2, a problem that has persisted among climate models for nearly three decades.