Streamflow changes in the European Alps indicate increasing drought risk from past to future

Relatively short instrumental streamflow records in the European Alps limit our understanding of the long-term variability of streamflow, which could greatly impact freshwater resources management. To improve the accuracy of future scenarios for rare extreme events that recently impacted severely important water resource systems and communities at the global level, a better understanding of past climatological and hydrological information is essential. Tree-ring-based proxy data have proven to be a viable opportunity for reconstructing streamflow in different regions. Here, the station-based streamflow of six rivers originating from the European Alps are reconstructed dating back to the year 1100 by using a climate-informed framework. Additionally, we use paleo simulations and future projections from state-of-the-art CMIP6 and PMIP4 climate models to study past and future streamflow changes, including the characteristics of rare extreme events like multi-year droughts and floods. By integrating proxy-based reconstructions, climate model simulations and projections, and observations, the changes in streamflow and rare extreme events in the European Alps are put into a millennial perspective covering the past nine centuries and one century into the future. Our findings reveal that the observed worst annual streamflow deficit events are among the most severe in the past 900 years. In addition, climate models project a coherent decrease in streamflow for all the basins, which indicates an unprecedented drought condition in the future. Our framework offers a unique opportunity to assess the risk of extreme events for rivers in the European Alps, thus playing a crucial role in developing robust water management strategies for climate change adaptation.