Climate of the Past Impact of CO2 and climate on the Last Glacial Maximum

Abstract. Vegetation reconstructions from pollen data for the Last Glacial Maximum (LGM), 21 ky ago, reveal lan-scapes radically different from the modern ones, with, in particular, a massive regression of forested areas in both hemispheres. Two main factors have to be taken into ac-count to explain these changes in comparison to today’s po-tential vegetation: a generally cooler and drier climate and a lower level of atmospheric CO2. In order to assess the relative impact of climate and atmospheric CO2 changes on the global vegetation, we simulate the potential mod-ern vegetation and the glacial vegetation with the dynami-cal global vegetation model ORCHIDEE, driven by outputs from the IPSL CM4 v1 atmosphere-ocean general circula-tion model, under modern or glacial CO2 levels for photo-synthesis. ORCHIDEE correctly reproduces the broad fea