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 will use 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 distribution. This has the potential to revolutionise the scientific debate on anthropogenic emissions of greenhouse gases and climate stabilisation targets. The aim of the project is to produce a robust estimate 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. Using a combined modelling and geological data approach, Plio-ESS will integrate reconstructions of mid-Pliocene vegetation and ice sheets into climate and Earth system models. In this context Plio-ESS will push the frontier of palaeoclimatology by using state-of-the-art models which will 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 will also quantify the uncertainty on estimates of Earth System Sensitivity. The outcome of the project will be a rigorous estimate of Earth System Sensitivity, which can be used by climate scientists and policy makers in defining stabilisation targets for greenhouse gas emissions and global temperatures to avoid dangerous levels of climate change.