The French scientists employed a standard biogeochemical model (BIOME3) - which uses monthly temperature and precipitation data, certain soil characteristics, cloudiness and atmospheric CO2 concentration as inputs - to simulate the responses of various biomes in the region surrounding the Mediterranean Sea to changes in both climate (temperature and precipitation) and the air's CO2 content.  Their first step was to validate the model for two test periods: the present and 6000 years before present (BP).  Recent instrumental records provided actual atmospheric CO2, temperature and precipitation data for the present period; while pollen data were used to reconstruct monthly temperature and precipitation values for 6000 years BP and ice core records were used to determine the atmospheric CO2 concentration of that earlier epoch.  These efforts suggested that winter temperatures 6000 years ago were about 2°C cooler than they are now, that annual rainfall was approximately 200 mm less than today, and that the air's CO2 concentration averaged 280 ppm, which is considerably less than the value of 345 ppm the authors used to represent the present, i.e., the mid-point of the period used for calculating 30-year climate normals at the time they wrote their paper.  Applying the model to these two sets of conditions, they demonstrated that "BIOME3 can be used to simulate ... the vegetation distribution under ... different climate and [CO2] conditions than today," where [CO2] is the abbreviation they use to represent "atmospheric CO2 concentration."

Cheddadi et al.'s next step was to use their validated model to explore the vegetative consequences of an increase in anthropogenic CO2 emissions that pushes the air's CO2 concentration to a value of 500 ppm and its mean annual temperature to a value 2°C higher than today's mean value.  The basic response of the vegetation to this change in environmental conditions was "a substantial southward shift of Mediterranean vegetation and a spread of evergreen and conifer forests in the northern Mediterranean."

More specifically, in the words of the authors, "when precipitation is maintained at its present-day level, an evergreen forest spreads in the eastern Mediterranean and a conifer forest in Turkey."  Current xerophytic woodlands in this scenario become "restricted to southern Spain and southern Italy and they no longer occur in southern France."  In northwest Africa, on the other hand, "Mediterranean xerophytic vegetation occupies a more extensive territory than today and the arid steppe/desert boundary shifts southward," as each vegetation zone becomes significantly more verdant than it is currently.

What is the basis for these positive developments?  The authors say "the replacement of xerophytic woodlands by evergreen and conifer forests could be explained by the enhancement of photosynthesis due to the increase of [CO2]."  Likewise, they note that "under a high [CO2] stomata will be much less open which will lead to a reduced evapotranspiration and lower water loss, both for C3 and C4 plants," adding that "such mechanisms may help plants to resist long-lasting drought periods that characterize the Mediterranean climate."

Contrary to what is often predicted for much of the world's moisture-challenged lands, therefore, the authors were able to report that "an increase of [CO2], jointly with an increase of ca. 2°C in annual temperature would not lead to desertification on any part of the Mediterranean unless annual precipitation decreased drastically," where they define a drastic decrease as a decline of 30% or more.  Equally important in this context is the fact that Hennessy et al. (1997) have indicated that a doubling of the air's CO2 content would in all likelihood lead to a 5 to 10% increase in annual precipitation at Mediterranean latitudes, which is also what is predicted for most of the rest of the world.  Hence, the results of the present study - where precipitation was held constant - may validly be considered to be a worst-case scenario, with the true vegetative response being even better than the good-news results reported by Cheddadi et al., even when utilizing what we believe to be erroneously-inflated global warming predictions.

So bring on that dreaded global warming.  The biosphere can handle it ... just as long as the air's CO2 content continues to climb.

PS: See also our Editorial of 15 October 1999: The Fortunate Coupling of Atmospheric CO2 and Temperature.

References
Cheddadi, R., Guiot, J. and Jolly, D.  2001.  The Mediterranean vegetation: what if the atmospheric CO2 increased?  Landscape Ecology 16: 667-675.

Hennessy, K.J., Gregory, J.M. and Mitchell, J.F.B.  1997.  Changes in daily precipitation under enhanced greenhouse conditions.  Climate Dynamics 13: 667-680.