The impact of increasing atmosphere CO2 concentration on hydrological trends in France over the 21st century

The elevated CO₂ concentration in the atmosphere has warmed the planet and modified the global precipitation pattern. Typical impact studies that investigate the regional hydrological response to climate change is based on the hydrological models forced by climate model projections. However, the physiological CO₂ effects of plants that manifests as reduced transpiration via partially closed leaf stomata and enhanced photosynthesis is often overlooked in these impact studies. 

 

Here, the potential impact of the physiological CO₂ effects on hydrological trends in France over the 21st century is assesed using the validated high-resolution Organising Carbon and Hydrology in Dynamic Ecosystems (ORCHIDEE) land surface model (Huang et al., 2023). The ORCHIDEE land surface model is forced with 4 regionalized climate projections narrowed from the CMIP5 ensemble projection under the RCP 8.5 scenario, with which we test the effect of two atmospheric CO₂ conditions: a constant CO₂ level of year 2005 and an increasing CO₂ concentration of the RCP 8.5. 

 

We find that the physiological CO₂ effects result in a decrease of evapotranspiration and an increase of total runoff over France for the 4 projections. Therefore, the physiological CO₂ effects enhance the increasing trend of river discharges in wet projections and alleviate the decreasing trend of river discharges in dry projections over the 21st century. Despite the model uncertainties, our study confirms the important physiological CO₂ effects on French water availaibility in the future, and this result likely holds at a broader scale.