The Pliocene Model Intercomparison Project Phase 2: large-scale climate features and climate sensitivity

• Main Authors: Haywood, A, Tindall, J, Dowsett, HJ, Dolan, AM, Foley, KM, Hunter, SJ, Hill, DJ, Chan, W-L, Abe-Ouchi, A, Stepanek, C, Lohmann, G, Chandan, D, Peltier, WR, Tan, N, Contoux, C, Ramstein, G, Li, X, Zhang, Z, Guo, C, Nisancioglu, KH, Zhang, Q, Li, Q, Kamae, Y, Chandler, MA, Sohl, LE, Otto-Bliesner, BL, Feng, R, Brady, EC, von der Heydt, AS, Baatsen, MLJ, Lunt, DJ
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2020
• Subjects: Pacific; Ice Sheet
• Online Access: https://eprints.whiterose.ac.uk/167658/; https://eprints.whiterose.ac.uk/167658/1/cp-16-2095-2020.pdf

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.