Climate of the Past www.clim-past.net 1814-9324 1814-9332 5 4 2009 10.5194/cp-5-785-2009 http://www.clim-past.net/5/785/2009/ http://www.clim-past.net/5/785/2009/cp-5-785-2009.html http://www.clim-past.net/5/785/2009/cp-5-785-2009.pdf 785 802 2009-12-15 Warm Paleocene/Eocene climate as simulated in ECHAM5/MPI-OM M. Heinemann malte.heinemann@zmaw.de J. H. Jungclaus J. Marotzke Max Planck Institute for Meteorology (MPI-M), Bundesstraße 53, 20146 Hamburg, Germany International Max Planck Research School on Earth System Modelling (IMPRS-ESM), Bundesstraße 53, 20146 Hamburg, Germany We investigate the late Paleocene/early Eocene (PE) climate using the coupled atmosphere-ocean-sea ice model ECHAM5/MPI-OM. The surface in our PE control simulation is on average 297 K warm and ice-free, despite a moderate atmospheric CO₂ concentration of 560 ppm. Compared to a pre-industrial reference simulation (PR), low latitudes are 5 to 8 K warmer, while high latitudes are up to 40 K warmer. This high-latitude amplification is in line with proxy data, yet a comparison to sea surface temperature proxy data suggests that the Arctic surface temperatures are still too low in our PE simulation. <br><br> To identify the mechanisms that cause the PE-PR surface temperature differences, we fit two simple energy balance models to the ECHAM5/MPI-OM results. We find that about 2/3 of the PE-PR global mean surface temperature difference are caused by a smaller clear sky emissivity due to higher atmospheric CO₂ and water vapour concentrations in PE compared to PR; 1/3 is due to a smaller planetary albedo. The reduction of the pole-to-equator temperature gradient in PE compared to PR is due to (1) the large high-latitude effect of the higher CO₂ and water vapour concentrations in PE compared to PR, (2) the lower Antarctic orography, (3) the smaller surface albedo at high latitudes, and (4) longwave cloud radiative effects. 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