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On the mechanisms for warming the mid-Pliocene and the inference of a hierarchy of climate sensitivities with relevance to the understanding of climate futures

We present results from our investigation into the physical mechanisms through which the mid-Pliocene with pCO2 only ~400 ppmv could have supported the same magnitude of global warmth as that which has been inferred for the end of the century climate when pCO2 is expected to be three times larger. These mechanisms explore changes to the radiative properties of the surface, the clouds, greenhouse gases and changes to the meridional heat transport. Furthermore, we provide a mid-Pliocene perspective towards ongoing efforts to understand the climate system’s sensitivity at various timescales and using multiple lines of evidence. The similarities in the boundary conditions between the mid-Pliocene and the present day, together with the globally elevated temperatures, make the mid-Pliocene an ideal palaeo time period from which to derive inferences of climate sensitivity and assess the impacts of various timescale dependent feedback processes. We assess a hierarchy of climate sensitivities of increasing complexity to explore the response of the climate over a very large range of timescales. The various sensitivities that we calculate provide insight on not only how the climate responds to a given forcing over a short timescale, but also on intermediate and very-long timescales. The latter category includes the impact of oceanic heat uptake and the feedback from the glacial isostatic adjustment of the Earth’s surface in response to the melting of the polar ice sheets.