Future intensification of compound and cascading drought and heatwave risks in Europe

Abstract. The risks of extreme weather events, such as droughts and heatwaves, are expected to rise across Europe due to global warming, leading to more severe and worsening impacts. These impacts become even more pronounced when compound and cascading (CnC) drought and heatwave hazards occur. Yet, most studies on drought and heatwave have focused on single hazard rather than their impacts. This study aims to identify the future characteristics of both single and compound drought and heatwave hazards across Europe. More specifically, we analyzed changes in the total number of events, average duration, total duration, and frequency. Droughts were identified using the Standardized Soil Moisture Index (SMI) and heatwaves were detected using the Variable Threshold Method (VTM). Both hazards were assessed using bias corrected simulations from the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) models from 1953 to 2014 for historical period and from 2039 to 2100 for future climate scenarios under SSP1-2.6 and SSP5-8.5. Furthermore, we employ a machine learning (ML) approach to project the impacts of droughts and heatwaves, using Germany as a case study. The ML models were developed using hazard characteristics as predictors and drought and heatwave impact data as response variables. Results indicate that the number, duration, and frequency of both drought and heatwave events are projected to increase under SSP1-2.6, with even higher increase for SSP5-8.5, not only when analyzed independently but also as CnC hazards. This applies not only in the south but also across multiple other European regions. Drought hotspots were identified in the West Europe, with projections showing an expansion toward the South and East under SSP1-2.6, and across nearly all of Europe under SSP5-8.5 except for the northern regions. Heatwave hotspots were primarily located in eastern and southern Europe, particularly in Russia, Italy, and Portugal. Future scenarios suggest that southern Europe will remain a key hotspot for heatwaves. The occurrence of compound drought and heatwave events was projected to increase sixfold compared to the reference period, while cascading drought and heatwave events might rise by up to 3.5 times under SSP5-8.5. Additionally, results also reveal that drought impacts on economic, non-economic, and ecosystem are projected to double in Germany, while heatwave impacts on human health and mortality may increase ninefold by 2100. Our findings highlight the need to consider CnC hazards and show once more the urgency of climate mitigation in limiting impacts across multiple sectors.