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

When evaluating the effect of carbon dioxide (CO) changes on Earth's climate, it is widely assumed that instantaneous radiative forcing from a doubling of a given CO concentration (IRF) 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 is not constant, but rather depends on the climatological base state, increasing by about 25% for every doubling of CO, 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, a problem that has persisted among climate models for nearly three decades.