Résumé
The atmospheric CO2 content is expected to continue to increase and probably induce warming at the global scale during the next century. The impact of such an increase will affect the composition and distribution of ecosystems on the same scale. To predict the integrated whole-ecosystem response to the CO2 increase in the Mediterranean region we used a vegetation biogeochemical model. This model (BIOME3) integrates monthly temperature and precipitation, some soil characteristics, cloudiness and CO2 concentration as inputs to simulate the vegetation in terms of biomes. First we demonstrate the ability of the model to simulate past vegetation when tested versus pollen data. Second we use the vegetation model for different climate scenarios and report results of future changes in the Mediterranean vegetation. These simulations indicate that an increase of the atmospheric CO2 to 500 ppmv, jointly with an increase of about 2 degreesC of the mean annual temperature, as simulated by several atmospheric general circulation models, should be accompanied by a severe reduction (more than 30%) of the present annual precipitation to change significantly the present vegetation surrounding the Mediterranean. When precipitation is maintained at its present-day level, an evergreen forest spreads in the eastern Mediterranean and a conifer forest in Turkey. In NW Africa, a woody xerophytic vegetation occupies a more extensive territory than today and replaces part of the present steppe area.