Change of return periods for low-flow extremes across storylines in a warming Danube River Basin

The Danube River Basin is a vital artery for European energy production, food security, and inland transport, yet it is increasingly emerging as a hotspot for hydroclimatic extremes, particularly droughts. Although global thermodynamic warming signals are robust, regional climate change projections remain uncertain due to the large impact of atmospheric circulation at these scales. In particular, the seasonal response of the North Atlantic jet stream to forcing is not robust across models. Here, we define physical climate storylines based on CMIP6 data to partition the uncertainty associated with diverging jet stream responses in speed and latitude. Subsequently, we use bias-adjusted CMIP6 projections to generate hydrological simulations for the Upper Danube Basin, focusing on the high-emission scenario SSP5-8.5 but finding similar results for SSP2-4.5. We identify an intensification of historically rare low-flow events in several storylines at a +2°C and +3°C global warming level. Notably, return periods in winter are modulated depending on the jet stream response. Consequently, adaptation planning must move beyond historical benchmarks to prepare for a reality of more frequent water scarcity in the future.