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.