The Pliocene Model Intercomparison Project Phase 2: A return to large-scale features of Pliocene climate sensitivity

The Pliocene epoch has great potential to improve our understanding of the long-term climatic and environmental 35 consequences of an atmospheric CO2 concentration near ~400 parts per million by volume. Here we present the large-scale features of Pliocene climate as simulated by a new ensemble of climate models of varying complexity and spatial resolution and based on new reconstructions of boundary conditions (the Pliocene Model Intercomparison Project Phase 2; PlioMIP2). As a global annual average, modelled surface air temperatures increase by between 1.4 and 4.7°C relative to pre-industrial with a multi-model mean value of 2.8°C. Annual mean total precipitation rates increase by 6% (range: 2%-13%). On 40 average, surface air temperature (SAT) increases are 1.3°C greater over the land than over the oceans, and there is a clear pattern of polar amplification with warming polewards of 60°N and 60°S exceeding the global mean warming by a factor of 2.4. In the Atlantic and Pacific Oceans, meridional temperature gradients are reduced, while tropical zonal gradients remain largely unchanged. Although there are some modelling constraints, there is a statistically significant relationship between a model's climate response associated with a doubling in CO2 (Equilibrium Climate Sensitivity; ECS) and its simulated 45 Pliocene surface temperature response. The mean ensemble earth system response to doubling of CO2 (including ice sheet feedbacks) is approximately 50% greater than ECS, consistent with results from the PlioMIP1 ensemble. Proxy-derived estimates of Pliocene sea-surface temperatures are used to assess model estimates of ECS and indicate a range in ECS from 2.5 to 4.3°C. This result is in general accord with the range in ECS presented by previous IPCC Assessment Reports. 50