State dependence of CO 2 forcing and its implications for climate sensitivity

When evaluating the effect of carbon dioxide (CO₂) changes on Earth's climate, it is widely assumed that instantaneous radiative forcing 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 dependence of these feedbacks on the climatological base state. Here, we show that the IRF_2×CO2 is not constant, but rather depends on the climatological base state, increasing by about 25% for every doubling of CO₂, and has increased by about 10% since the preindustrial era primarily due to the cooling within the upper stratosphere, 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.