Title: Interactions between Vegetation and Water Cycle In the Context of Rising Atmospheric Carbon Dioxide Concentration
Description: Predicting how increasing atmospheric carbon dioxide concentration will affect the hydrologic cycle is of utmost importance for water resource management, ecological systems and for human life and activities. A typical perspective is that the water cycle will mostly be altered by atmospheric effects of climate change, precipitation and radiation, and that the land surface will adjust accordingly. Terrestrial processes can however feedback significantly on the hydrologic changes themselves. Vegetation is indeed at the center of the carbon, water and energy nexus. This work investigates the processes, the timing and the geography of these feedbacks. Using Earth System Models simulations from the Coupled Model Intercomparison Project, Phase 5 (CMIP5), with decoupled surface (vegetation physiology) and atmospheric (radiative) responses to increased atmospheric carbon dioxide concentration, we first evaluate the individual contribution of precipitation, radiation and physiological forcings for several key hydrological variables.

Over the largest fraction of the globe the physiological response indeed not only impacts, but also dominates the change in the continental hydrologic cycle compared to either radiative or precipitation changes due to increased atmospheric carbon dioxide concentration. It is however complicated to draw any conclusion for the soil moisture as it exhibits a particularly nonlinear response. The physiological feedbacks are especially important for extreme temperature events. The 2003 European heat wave is an interesting and crucial case study, as extreme heat waves are anticipated to become more frequent and more severe with increasing atmospheric carbon dioxide concentration. The soil moisture and land-atmosphere feedbacks were responsible for the severity of this episode unique for this region. Instead of focusing on statistical change, we use the framework of Regional Climate Modeling to simulate this specific event under higher levels of surface atmospheric carbon dioxide concentration and to assess how this heat wave could be altered by land-atmosphere interactions in the future.

Increased atmospheric carbon dioxide concentration modifies the seasonality of the water cycle through stomatal regulation and increased leaf area. As a result, the water saved during the growing season through higher water use efficiency mitigates summer dryness and the heat wave impact. Land-atmosphere interactions and carbon dioxide fertilization together synergistically contribute to increased summer transpiration if rainfall does not change. This, in turn, alters the surface energy budget and decreases sensible heat flux, mitigating air temperature rise during extreme heat periods. This soil moisture feedback, which is mediated and enabled by the vegetation on a seasonal scale is a European example of the impacts the vegetation could have in an atmosphere enriched in carbon dioxide. We again use Earth System Models to systematically and statistically investigate the influence of the vegetation feedbacks on the global and regional changes of extreme temperatures. Physiological effects typically contribute to the increase of the annual daily maximum temperature with increasing atmospheric carbon dioxide concentration, accounting for around 15% of the full trend by the end of the XXIth Century.

Except in Northern latitudes, the annual daily maximum temperature increases at a faster pace than the mean temperature, which is reinforced by vegetation feedbacks in Europe but reduced in North America. This work highlights the key role of vegetation in influencing future terrestrial hydrologic responses. Accurate representation of the response to higher atmospheric carbon dioxide concentration levels, and of the coupling between the carbon and water cycles are therefore critical to forecasting seasonal climate, water cycle dynamics and to enhance the accuracy of extreme event prediction under future climates in various regions of the globe.