The Middle-to-late Eocene Climate: Simulated using the CESM 1.0.5

The middle-to-late Eocene period ( ~ 42 to 34 million years ago) experienced an extreme greenhouse climate, preceding one of the major climatic transitions in the known record: the Eocene-Oligocene transition (EOT). Throughout the Eocene, polar regions were mostly free of ice while the land surface was mainly covered with either tropical or sub-tropical vegetation. The EOT is believed to concur with the formation of a continental ice sheet on Antarctica marking the shift from a greenhouse towards an icehouse state. In order to better understand this particular transition, it is primordial to have an accurate reconstruction of the prevailing climate prior to the event. This is achieved using a set of new geographical boundary conditions and higher resolution climate model simulations. The results of those simulations allow to get a better understanding of the Eocene climate as well as the subsequent changes leading to Antarctic glaciation at the EOT. The Eocene greenhouse climate is significantly warmer and more equable compared to that of the present-day mainly owing to a combination of changes in geography, land cover and atmospheric composition. Mild high latitude conditions are sustained by regionally altered radiative fluxes rather than meridional heat fluxes. Climate sensitivity and an enhanced sensitivity of polar regions to an external radiative forcing appear to be universal, but the processes behind them are dependent of the climatic background state. The presence of multiple equilibria in the ocean circulation can increase and complicate the responses to different external forcings. Despite being small globally, regional impacts from late Eocene geography changes can be significant due to changes in the circulation pattern. Finally, a surprisingly warm and wet climate is seen on Antarctica that exhibits a subtropical-like summer monsoon. This helps explain the presence of abundant vegetation, but greatly limits the potential for ice growth. As a consequence, the conventional idea of a gradual global cooling eventually triggering the glaciation of the Antarctic continent at the EOT is challenged