Multimodel Assessment of Nitrogen Pollution in European River Systems under Changing Climate and Shared Socioeconomic Pathways

Nitrogen pollution in European landscapes poses persistent challenges to aquatic ecosystems, human health, and water quality. The European Union has set a goal to achieve zero pollution by 2050, including the reduction of air, water, and soil pollution to levels that no longer harm health or natural ecosystems. However, the feasibility of achieving this goal for legacy contaminants like nitrogen (N) under changing climate and land-use management is not well understood. This study employs a multimodel approach to provide a comprehensive assessment of nitrogen pollution across European river systems under varying climate emission and land-use management scenarios. We used a suite of hydrological and biogeochemical models (mHM-mQM, SWAT, and IMAGE-GNM) driven by an ensemble of climate projection datasets (CMIP) operating under diverse emission scenarios (RCPs; 2.6, 4.5, and 8.5) and shared socioeconomic pathways (SSPs; 1-5). These climate-driven runs were complemented with nitrogen input scenarios adhering to different SSPs, accounting for strategies managing agricultural land and technological innovations while considering future factors such as food production, economic growth, and environmental requirements. Ensemble hydrologic and nitrogen export simulations are constructed for the period spanning 1971 to 2070. Our analysis highlights notable progress in reducing nitrogen loads across European river systems by the 2050s compared to the 2010s. Regionally, our ensemble simulations identify Central Europe as a persistent area of concern, with relatively higher nitrogen exports projected under both conservative (SSP1-RCP2.6) and conventional development (SSP5-RCP8.5) scenarios. Despite overall improvements, many European river systems are projected to exceed critical nitrogen concentration thresholds (e.g., 2–3 mg N/L) by the 2050s. The majority of ensemble simulations consistently reveal similar hotspot regions in countries like Germany, France, Poland, Italy, and Spain. This may be linked to ongoing nitrogen exports that gradually deplete legacy reservoirs (e.g., soil and groundwater). By integrating multimodel insights, our study aims to provide a robust framework and assessment for anticipating and addressing the challenges of nitrogen pollution in pursuit of realizing EU zero-pollution goals.