Final Report Summary - PLIO-ESS (Pliocene Constraints on Earth System Sensitivity) The magnitude of long-term global temperature rise due to an increasing concentration of carbon dioxide (CO2) in the atmosphere is a question of relevance to policy makers and society. Previous studies have addressed this issue on the basis of the equilibrium response of the climate system due to fast feedbacks such as clouds and sea ice-albedo, often referred to as Climate Sensitivity. Plio-ESS explored the new concept of Earth System Sensitivity that additionally includes slow feedbacks such as those derived from changes in the major ice sheets and vegetation. The project produced robust estimates of the Earth System Sensitivity using the last interval in Earth history when CO2 was at modern or near future levels – the mid-Pliocene Warm Period. Plio-ESS integrated records of mid-Pliocene vegetation and ice sheets into climate and Earth system models, and pushed the frontier of palaeoclimatology by using state-of-the-art models which enable the importance of resolution, improved model physics and the inclusion of additional Earth System components on model estimates of Earth System Sensitivity to be identified. Ensembles of experiments exploring the plausible range in model boundary conditions and physics quantified the uncertainty on our estimates of Earth System Sensitivity. The project investigated the causes of Pliocene warmth and in particular the drivers of high latitude warming. Higher concentrations of CO2 in the atmosphere have been identified as the primary driver for Pliocene warmth. The amplification of warming in the higher latitudes is driven by changes in surface albedo, as well as CO2, and different ways to understand how ice sheets on Greenland and Antarctica changed in the warm Pliocene were explored. Differences between models that have been set up in an identical way for the Pliocene have been assessed, enabling a determination of what are robust model features of Pliocene climate. We used this modelling framework to identify the similar to modern nature of ocean circulation, the changing nature of Arctic sea-ice coverage and to explore changes in the intensity of monsoon systems. Our work supports previous estimates that Earth System Sensitivity is greater than Climate Sensitivity, but the ESS/CS ratio has been demonstrated to be larger than previous studies. However, to constrain Earth System Sensitivity further it is necessary to further understand and quantify uncertainties in Pliocene and environmental reconstruction and climate modelling and to better constrain the nature of Pliocene climate variability driven by orbital cycles.
