The Pliocene Model Intercomparison Project Phase 2: Large scale climate features and constraining 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 1.7 and 5.2 °C relative to pre-industrial with a multi-model mean value of 3.2°C. Annual mean total precipitation rates increase by 7% (range: 2%-13%). On average, 40 surface air temperature (SAT) increases by 4.3°C over the land and 2.8°C over the oceans. 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 2.3. In the Atlantic and Pacific Oceans, meridional temperature gradients are reduced, while tropical zonal gradients remain largely unchanged. 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 Pliocene surface temperature response. The mean ensemble Earth 45 system response to doubling of CO2 (including ice sheet feedbacks) is 67% greater than ECS, this is larger than the increase of 47% obtained from the PlioMIP1 ensemble. Proxy-derived estimates of Pliocene sea-surface temperatures are used to assess model estimates of ECS and give a range of ECS between 2.6 and 4.8°C. This result is in general accord with the range in ECS presented by previous IPCC Assessment Reports.

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