Understanding long-term multi-year drought characteristics in Europe under climate change

Characterizing the evolution of drought frequency and severity under anthropogenic global warming remains a key challenge because of the complexity of the underlying physical processes and the mismatch between the length of instrumental records and the long-term variability of drought features. Multiyear droughts, given their rare occurrence and the limited availability of historical data, make it more difficult to determine their occurrence patterns and develop reliable future predictions. This thesis aims to enhance the understanding of long-term multiyear meteorological and hydrological drought risk in European regions from the past, present, to future by integrating paleo-hydroclimatic, observational, and climate-model-based data. The study first assesses the ability of Global Climate Models (GCMs) to reproduce multiyear meteorological drought statistics in Northern Italy using a 209-year rainfall record. Results indicate that while GCMs effectively simulate rainfall seasonality and drought frequency, they tend to underestimate drought duration and severity. A modeling framework is then developed to integrate streamflow observations, paleo-hydrological reconstructions, and climate model simulations, addressing the limitations of short instrumental records. Applied to the Po River, the result indicates a projected 10% decline in mean annual flow during the 21st century, with drought duration and severity increasing by approximately 11% and 12%, respectively. Future drought conditions are likely to match or exceed the driest period of the Medieval Climate Anomaly under different emissions scenarios. Expanding this framework to six additional Alpine basins, the study demonstrates that tree-ring-based streamflow reconstructions effectively capture long-term variability and multiyear droughts. Large-scale climate oscillations may play a critical role in influencing multiyear droughts, with future projections indicating an increased likelihood of extreme events under similar climate patterns. In summary, this thesis underscores the value of a multidisciplinary framework to assess hydroclimatic extremes and improve drought risk projections, providing crucial insights for climate adaptation and regional water resource management.