A wet or dry European and Northern African climate during the Miocene Climatic Optimum

End-of 21st Century hydroclimate projections suggest an expansion of the subtropical dry zone, with Europe and Northern Africa becoming drier. However, paleoclimate evidence primarily from paleobotanical assemblages from a past warm climate period, the Miocene Climatic Optimum (MCO) ~14-17 Ma, suggests both regions were instead wet and humid environments. Here, we simulate the MCO with the Community Earth System Model (CESM 1.2) forced by compiled sea surface temperature (SST) proxy data that are 5-6°C warmer than Preindustrial in the North Atlantic (NA). Given these boundary conditions, the climate model better matches paleobotanical proxy evidence for wetter continents relative to coupled simulations. The prescribed SST simulations show enhanced ocean evaporation and integrated water vapor flux that overrides any drying effects associated with warming, increasing evaporation on land. The vegetation model (BIOME4) forced by the climatologies from our simulations predicts a mixed forested landscape dominated Europe and Northern Africa during the MCO, with largely consistent paleobotanical evidence. This proxy-model study of MCO climate reveals the potential for wetter Mediterranean climates associated with warming and presents an alternative scenario from future drying projections. The critical difference identified in our MCO simulations is localized SST warming governing regional climate.