Abstract. The Middle Miocene was one of the last warm periods of the Neogene, culminating with the Middle Miocene Climatic Optimum (MMCO, approximatively 17-15 Ma). Several proxy-based reconstructions support warmer and more humid climate during the MMCO. The mechanisms responsible for the warmer climate at the MMCO and particularly the role of the atmospheric carbon dioxide are still highly debated. Here we carried out a series of sensitivity experiments with the model of intermediate complexity Planet Simulator, investigating the contributions of the absence of ice on the continents, the opening of the Central American and Eastern Tethys Seaways, the lowering of the topography on land, the effect of various atmospheric CO 2 concentrations and the vegetation feedback.Our results show that a higher than present-day CO 2 concentration is necessary to generate a warmer climate at all latitudes at the Middle Miocene, in agreement with the terrestrial proxy reconstructions which suggest high atmospheric CO 2 concentrations at the MMCO. Nevertheless, the changes in sea-surface conditions, the lowering of the topography on land and the vegetation feedback also produce significant local warming that may, locally, even be stronger than the CO 2 induced temperature increases. The lowering of the topography leads to a more zonal atmospheric circulation and allows the westerly flow to continue over the lowered Plateaus at mid-latitudes. The reduced height of the Tibetan Plateau notably prevents the development of a monsoon-like circulation, whereas the reduction of elevations of the North American and European reliefs strongly increases precipitation from northwestern to eastern Europe.Correspondence to: A.-J. Henrot (alexandra.henrot@ulg.ac.be)The changes in vegetation cover contribute to maintain and even to intensify the warm and humid conditions produced by the other factors, suggesting that the vegetationclimate interactions could help to improve the model-data comparison.

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In Europe and the Mediterranean region, the vegetation and climate of the Neogene is well understood, due to the abundance of pollen data, allowing the climate evolution at a time of global cooling to be described. This paper presents a climatic reconstruction of four key timeslices of the Neogene: the Mid-Miocene (c. 14 Ma), the Late Miocene (c. 10 Ma), the Early Pliocene (c. 5-5.3 Ma) and the Mid-Pliocene (c. 3.6 Ma). The results show that Neogene climate was warmer than today and that the transition from a weak latitudinal thermic gradient (around 0.48 8C/degree in latitude) to a gradient similar to that of today (0.6 8C/degree in latitude) took place at the end of the Miocene. The latitudinal precipitation gradient was more accentuated than today from the Mid-Miocene to the Mid-Pliocene, with higher precipitation than today in northwestern Europe and the northwestern Mediterranean but with conditions that were drier than or equivalent to today in the southwestern Mediterranean region.