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

When evaluating the effect of carbon dioxide (CO 2 ) changes on Earth's climate, it is widely assumed that instantaneous radiative forcing from a doubling of a given CO 2 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 CO 2 , 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.