Shaping the mid-Miocene warmth: a sensitivity study on paleogeography, CO₂ and model physics

Abstract. The mid-Miocene (15.98 to 13.82 Ma) was a period characterized by substantially warmer temperatures than today and atmospheric CO₂ concentrations comparable to near-future projections. Climate models have generally struggled to reproduce proxy-based reconstructions from this interval, particularly at high latitudes where model temperatures are consistently too low. Here, we present new mid-Miocene simulations using an unpublished geography and evaluate the climate's sensitivity to several key components: paleogeography (including land-sea distribution, topography and ice sheets), atmospheric CO₂ concentration, atmospheric model choice, and solar forcing. Our baseline mid-Miocene climate yields a global mean surface temperature (GMST) of 19.8 °C. GMST varies by up to 3.2 °C between simulations with CO₂ concentrations of two and four times pre-industrial values, which is consistent with estimates for the mid-Miocene. Removal of the Antarctic ice-sheet leads to expected local warming, but nevertheless records an overall global cooling of 1.3 C. Solar forcing and subtle changes of land-sea mask each impact GMST by around 0.2 °C. The choice of atmospheric model substantially affects the simulated mid-Miocene climate through modified feedback mechanisms. We estimate an equilibrium climate sensitivity (ECS) of 2.9 °C for the mid-Miocene, similar to modern-based estimates from our model, indicating the potential for the Miocene to contribute to constraining equilibrium ECS. Global precipitation is tightly coupled to GMST across all our simulation. As with previous studies, all our simulations, regardless of specific configuration, underestimate high-latitude proxy-reconstructed temperatures. This highlights the need to improve our understanding on polar amplification and the need to use high concentrations of CO₂ to compensate for a cold modeled Miocene climate.