Increasing risks of extreme salt intrusion events across European estuaries in a warming climate

Estuaries are the semi-enclosed transition zones between river and open sea, where saline and fresh water mix. Over the last decade, many estuaries worldwide have faced increased salt intrusion as a result of human activities and a changing climate, leading to lower food production and reduced accessibility to drinking water. Despite its socio-economic importance, our current projections on the statistics of future salt intrusion are limited to case studies in certain regions. Here, we show that, compared to present-day conditions, river discharge in the summer months is projected to be reduced by 10 - 60 % in 22 major European river basins at the end of the 21st century under a high CO2 emission scenario. We find that the reduced future river discharge in the summer months, in turn, increases salt intrusion lengths by 10 - 30 % in 8 representative European estuaries at low and mid latitudes. Our analysis further indicates that the representative European estuaries are projected to experience more than eight times more frequent extreme salt intrusion events at the end of the century as compared to present day. Hence, urgent and drastic mitigation measures are required to guarantee sustainable future freshwater supplies in low-lying coastal regions.