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 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 lower than observations. Here, we present new mid-Miocene simulations using a previously unpublished geography and evaluate the climate's sensitivity to several key components: paleogeography (including land-sea distribution, topography and ice sheets),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. In mid-Miocene sensitivity experiments of two and four times pre-industrial CO2 concentrations, consistent with estimates for the mid-Miocene, GMST varies by up to 3.2 °C between simulations. Removal of the Antarctic ice-sheet leads to expected local warming of around 25 °C at the maximum height of the ice sheet, 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 (2.5–3.3 °C, 95 % prediction interval) for the mid-Miocene, similar to modern-based estimates from our model (2.8 °C, 2.2–3.4 °C, 95 % prediction interval), indicating the potential for the Miocene to contribute to constraining ECS. Global precipitation is tightly coupled to GMST across all our simulations. 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 on the limitations affecting the proxy record.