The importance of plant-water stress for predictions of ground-level ozone in a warm world

Evapotranspiration is important for Earth’s water and energy cycles as it strongly affects air temperature, cloudcover and precipitation. Leaf stomata are the conduit of transpiration and thus their opening is sensitive to weather and climateconditions. This feedback can exacerbate heat waves and droughts and can play a role in their spatio-temporal propagation.Therefore, the plant response to available water is a key element mediating vegetation-atmosphere interactions. Sustained hightemperatures strongly favor high ozone levels with significant negative effects on air quality and thus human health. Our studyassesses the process representation of evapotranspiration in the atmospheric chemistry model ECHAM/MESSy. Diverse waterstress parametrizations are implemented in a stomatal model based on CO2 assimilation. The stress factors depend on eithersoil moisture or leaf water potential and act directly on photosynthetic activity, mesophyll and stomatal conductance. Overall,the new functionalities reduce the initial overestimation of evapotranspiration in the model globally by more than one orderof magnitude which is most important in the Southern Hemisphere. The intensity of simulated warm spells over continentsis significantly enhanced. With respect to ozone, we find that a realistic model representation of plant-water stress depressesuptake by vegetation and enhances its photochemical production in the troposphere. These effects lead to a general increasesin simulated ground-level ozone which is most pronounced in the Southern Hemisphere over the continents. The uncertaintiesfor plant dynamics representation due to too shallow roots can be addressed by more sophisticated land surface models withmulti-layer soil schemes. In regions with low evaporative loss, however, the representation of precipitation remains the largestuncertainty