Model-data synthesis of benthic isotopes suggests a warmer Miocene Climatic Optimum

The Miocene Climatic Optimum (MCO, ~ 15 Ma) offers insights into warm-climate dynamics and future climate change. However, its global warmth magnitude remains uncertain due to limitations in surface and benthic proxy records. Here, we develop long-run Miocene simulations featuring deep-ocean equilibration and water isotope capability, and present a probabilistic inference framework integrating them with a global compilation of benthic foraminiferal δ18O to better constrain the MCO warmth. Our approach yields a maximum likelihood estimate of MCO global mean surface temperature of 7.5∘C above preindustrial, significantly warmer than previous benthic δ18O-based reconstructions, implying a higher MCO-derived estimate of Earth system sensitivity. The corresponding surface temperature field shows among the best agreements with independent surface temperature proxies. This study highlights the importance of deep-ocean equilibration and proxy-model integration for accurately estimating both deep-ocean and surface temperatures, and offers a method applicable to improving global climate reconstructions across other time intervals. Combining long climate-isotope simulations with deep-sea oxygen isotope records, the Miocene Climatic Optimum (15 Ma) is estimated to have been 7.5∘C warmer than preindustrial -- 30% above earlier estimates.