McClymont, Erin L.; Ford, Heather L.; Ho, Sze Ling; Tindall, Julia C.; Haywood, Alan M.; Alonso-Garcia, Montserrat; Bailey, Ian; Berke, Melissa A.; Littler, Kate; Patterson, Molly O.; Petrick, Benjamin; Peterse, Francien; Ravelo, A. Christina; Risebrobakken, Bjørg; De Schepper, Stijn; Swann, George E. A.; Thirumalai, Kaustubh; Tierney, Jessica E.; van der Weijst, Carolien; White, Sarah; Abe-Ouchi, Ayako; Baatsen, Michiel L. J.; Brady, Esther C.; Chan, Wing-Le; Chandan, Deepak; Feng, Ran; Guo, Chuncheng; von der Heydt, Anna S.; Hunter, Stephen; Li, Xiangyi; Lohmann, Gerrit; Nisancioglu, Kerim H.; Otto-Bliesner, Bette L.; Peltier, W. Richard; Stepanek, Christian; Zhang, Zhongshi

A range of future climate scenarios are projected for high atmospheric inline-formulaCO2 concentrations, given uncertainties over future human actions as well as potential environmental and climatic feedbacks. The geological record offers an opportunity to understand climate system response to a range of forcings and feedbacks which operate over multiple temporal and spatial scales. Here, we examine a single interglacial during the late Pliocene (KM5c, ca. inline-formula3.205±0.01 Ma) when atmospheric inline-formulaCO2 exceeded pre-industrial concentrations, but were similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were almost identical to today, we are able to focus on equilibrium climate system response to modern and near-future inline-formulaCO2. Using proxy data from 32 sites, we demonstrate that global mean sea-surface temperatures were warmer than pre-industrial values, by inline-formula∼2.3inline-formulaC for the combined proxy data (foraminifera inline-formulaMg∕Ca and alkenones), or by inline-formula∼3.2–3.4 inline-formulaC (alkenones only). Compared to the pre-industrial period, reduced meridional gradients and enhanced warming in the North Atlantic are consistently reconstructed. There is broad agreement between data and models at the global scale, with regional differences reflecting ocean circulation and/or proxy signals. An uneven distribution of proxy data in time and space does, however, add uncertainty to our anomaly calculations. The reconstructed global mean sea-surface temperature anomaly for KM5c is warmer than all but three of the PlioMIP2 model outputs, and the reconstructed North Atlantic data tend to align with the warmest KM5c model values. Our results demonstrate that even under low-inline-formulaCO2 emission scenarios, surface ocean warming may be expected to exceed model projections and will be accentuated in the higher latitudes.

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McClymont, Erin L. / Ford, Heather L. / Ho, Sze Ling / et al: Lessons from a high-CO2 world: an ocean view from  ∼ 3 million years ago. 2020. Copernicus Publications.

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