CPD Climate of the Past Discussions CPD 1814-9359 Copernicus GmbH Göttingen, Germany 10.5194/cpd-7-1-2011 Impact of CO<sub>2</sub> and climate on the Last Glacial Maximum vegetation Woillez M.-N. 1 Kageyama M. 1 Krinner G. 2 de Noblet-Ducoudré N. 1 Viovy N. 1 Mancip M. 1 LSCE/IPSL INSU, UMR 8212, CE Saclay, l'Orme des Merisiers, 91191 Gif-sur-Yvette Cedex, France LGGE, UMR CNRS 5183, 54 rue Molière, 38402 St. Martin d'Hères Cedex, France 03 01 2011 7 1 1 46 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.clim-past-discuss.net/7/1/2011/cpd-7-1-2011.html The full text article is available as a PDF file from http://www.clim-past-discuss.net/7/1/2011/cpd-7-1-2011.pdf

Vegetation reconstructions from pollen data for the Last Glacial Maximum (LGM), 21 kyr ago, reveal lanscapes 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 account to explain these changes in comparison to today's potential vegetation: a generally cooler and drier climate and a lower level of atmospheric CO<sub>2</sub>. In order to assess the relative impact of climate and atmospheric CO<sub>2</sub> changes on the global vegetation, we simulate the potential modern vegetation and the glacial vegetation with the dynamical global vegetation model ORCHIDEE, driven by outputs from the IPSL_CM4_v1 atmosphere-ocean general circulation model, under modern or glacial CO<sub>2</sub> levels for photosynthesis. ORCHIDEE correctly reproduces the broad features of the glacial vegetation. Our modelling results support the view that the physiological effect of glacial CO<sub>2</sub> is a key factor to explain vegetation changes during glacial times. In our simulations, the low atmospheric CO<sub>2</sub> is the only driver of the tropical forests regression, and explains half of the response of temperate and boreal forests to glacial conditions. Our study shows that the sensitivity to CO<sub>2</sub> changes depends on the background climate over a region, and also depends on the vegetation type, needleleaf trees being much more sensitive than broadleaf trees in our model. This difference of sensitivity leads to a dominance of broadleaf types in the remaining simulated forests, which is not supported by pollen data, but nonetheless suggests a potential impact of CO<sub>2</sub> on the glacial vegetation assemblages. It also modifies the competitivity between the trees and makes the amplitude of the response to CO<sub>2</sub> dependent on the initial vegetation state.

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