Importance of vegetation feedbacks in doubled‐CO2 climate experiments

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Abstract
The rising atmospheric concentration of carbon dioxide resulting from the burning of fossil fuels and deforestation is likely to provoke significant climate perturbations, while having far‐reaching consequences for the terrestrial biosphere. Some plants could maintain the same intake of CO2 for photosynthesis by reducing their stomatal openings, thus limiting the transpiration and providing a positive feedback to the projected surface warming. Other plants could benefit from the higher CO2 level and the warmer climate to increase their productivity, which would on the contrary promote the transpiration. The relevance of these feedbacks has been investigated with the Météo‐France atmospheric general circulation model. The model has been run at the T31 spectral truncation with 19 vertical levels and is forced with sea surface temperature and sea ice anomalies provided by a transient simulation performed with the Hadley Centre coupled ocean‐atmosphere model. Besides a reference doubled‐CO2 experiment with no modification of the vegetation properties, two other experiments have been performed to explore the impact of changes in the physiology (stomatal resistance) and structure (leaf area index) of plants. Globally and annually averaged, the radiative impact of the CO2 doubling leads to a 2°C surface warming and a 6% precipitation increase, in keeping with previous similar experiments. The vegetation 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 and 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 case of the European summertime climate and the Asian summer monsoon. The strong sensitivity of the climate in these areas emphasizes the large uncertainties of climate change predictions for some of the most populated regions of the world and argues for the need to include more interactive land surface processes in current generation climate models.