Importance of vegetation feedbacks in doubled-CO2 climate experiments

The rising atmospheric concentration of carbon dioxide resulting from the b urning of fossil fuels and deforestation is likely to provoke significant c limate perturbations, while having far-reaching consequences for the terres trial biosphere. Some plants could maintain the same intake of CO2 for phot osynthesis by reducing their stomatal openings, thus limiting the transpira tion and providing a positive feedback to the projected surface warming. Ot her plants could benefit from the higher CO2 level and the warmer climate t o increase their productivity, which would on the contrary promote the tran spiration. The relevance of these feedbacks has been investigated with the Meteo-France atmospheric general circulation model. The model has been run at the T31 spectral truncation with 19 vertical levels and is forced with s ea surface temperature and sea ice anomalies provided by a transient simula tion performed with the Hadley Centre coupled ocean-atmosphere model. Besid es a reference doubled-CO2 experiment with no modification of the vegetatio n properties, two other experiments have been performed to explore the impa ct of changes in the physiology (stomatal resistance) and structure (leaf a rea index) of plants. Globally and annually averaged, the radiative impact of the CO2 doubling leads to a 2 degrees C surface warming and a 6% precipi tation increase, in keeping with previous similar experiments. The vegetati on Feedbacks do not greatly modify the model response on the global scale. The increase in stomatal resistance does not systematically lead to higher near-surface temperatures due to changes in the soil wetness annual cycle a nd the atmospheric circulation. However, both physiological and structural vegetation feedbacks are evident on the regional scale. They are liable to modify the CO2 impact on the hydrological cycle, as illustrated for the cas e of the European summertime climate and the Asian summer monsoon. The stro ng sensitivity of the climate in these areas emphasizes the large uncertain ties of climate change predictions for some of the most populated regions o f the world and argues for the need to include more interactive land surfac e processes in current generation climate models.