Osborne, C.P., Mitchell, P.L., Sheehy, J.E. and Woodward, F.I. 2000. Modellng the recent historical impacts of atmospheric CO2 and climate change on Mediterranean vegetation. Global Change Biology 6: 445-458.
What was done
The authors used a mechanistic model of Mediterranean shrub vegetation to address two questions: (1) Has recent climate change, especially increased drought, impacted Mediterranean shrublands, and (2) Has the concurrent increase in atmospheric CO2 concentration modified this impact? These model predictions were compared with observations of a range of indigenous species located at various sites scattered throughout the Mediterranean region.
What was learned
The warming and reduced precipitation experienced in the Mediterranean area over the past century were predicted to have had negative impacts on net primary production and leaf area index. When the measured increase in atmospheric CO2 concentration experienced over this period was factored into the calculation, however, these negative influences were overpowered, with the net effect that both measures of vegetative prowess increased, net primary productivity by 25% and leaf area index by 7%.
What it means
The authors' results, in their own words, "indicate that the recent rise in atmospheric CO2 may already have had significant impacts on productivity, structure and water relations of sclerophyllous shrub vegetation, which tended to offset the detrimental effects of climate change in the region."
How can we relate this observation to climate change predictions for the earth as a whole? For a nominal doubling of the air's CO2 concentration from 300 to 600 ppm, earth's mean surface air temperature is predicted by current climate models to rise by approximately 3°C, which equates to a temperature rise of 0.01°C per ppm CO2. In the case of the Mediterranean region here described, the temperature rise over the past century was quoted by the authors as being 0.75°C, over which period of time the air's CO2 concentration rose by approximately 75 ppm, for an analogous climate response of exactly the same value: 0.01°C per ppm CO2.
With respect to model-predicted changes in earth's precipitation regime, a doubling of the air's CO2 content is projected to lead to a modest intensification of the planet's hydrologic cycle. In the specific case of the Mediterranean region over the last century, however, there has been a recent tendency toward drier conditions. Hence, the specific case investigated by the authors represents a much-worse-case scenario than what is predicted by current climate models for the earth as a whole.
So how does the region's plant life respond to this dreaded environmental change? It does even better than it did before the change, thanks to the over-powering beneficial effects of the concurrent rise in the air's CO2 content. By analogy, therefore, we would expect the entire biosphere to do likewise, as all indications are that it indeed is doing.
Reviewed 23 August 2000