Abstract
Abstract. The Pliocene epoch has great potential to improve our understanding of the long-term climatic and environmental 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 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 the pre-industrial era with a multi-model mean value of 3.2 ∘C. Annual mean total precipitation rates increase by 7 % (range: 2 %–13 %). On average, surface air temperature (SAT) increases by 4.3 ∘C over 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 system response to a 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 an ECS range of 2.6–4.8 ∘C. This result is in general accord with the ECS range presented by previous Intergovernmental Panel on Climate Change (IPCC) Assessment Reports.
Highlights
1.1 Pliocene climate modelling and overview of the Pliocene Model Intercomparison ProjectEfforts to understand climate dynamics during the midPiacenzian warm period (MP; 3.264 to 3.025 million years ago), previously referred to as the mid-Pliocene warm period, have been ongoing for more than 25 years
The Pliocene Model Intercomparison Project Phase 2 represents one of the largest ensembles of climate models of different complexities and spatial resolution ever assembled to study a specific interval in Earth history
PlioMIP2 builds on the findings of PlioMIP1 and incorporates state-of-the-art reconstructions of Pliocene boundary conditions and new temporally consistent sea surface temperature proxy data which underpin the new data–model comparison
Summary
Efforts to understand climate dynamics during the midPiacenzian warm period (MP; 3.264 to 3.025 million years ago), previously referred to as the mid-Pliocene warm period, have been ongoing for more than 25 years. This is because the study of the MP enables us to address important scientific questions. The inclusion of a Pliocene experiment within the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental protocols underlines the general potential of the Pliocene to address questions regarding the long-term sensitivity of climate and environments to forcing, as well as the determination of climate sensitivity . PlioMIP Phase 1 (PlioMIP1) proposed a single set of model boundary conditions based on the U.S Geological Survey PRISM3D dataset (Dowsett et al, 2010) and a unified experimental design for atmosphere-only and fully coupled atmosphere–ocean climate models (Haywood et al, 2010, 2011)
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