Future projections of compound drought and heatwave events in the eastern part of the Baltic Sea region

Human-induced climate change is contributing to an increase in the frequency of weather extremes. Among these are compound drought and heatwave events (CDHEs), which occur when both phenomena are recorded simultaneously. This study aimed to assess projected changes in the recurrence, duration, and intensity of CDHEs during the warm season (April–September) by the end of the 21st century in the eastern part of the Baltic Sea region. Downscaled projections from five CMIP6 climate models representing the two Shared Socioeconomic Pathways (SSP2–4.5 and SSP5–8.5) scenarios were used for this analysis. Projections of CDHEs were generated for 2081–2100, with 1995–2014 selected as the baseline period. Droughts were identified using the Standardised Precipitation Index (SPI), while heatwaves were defined based on the 90th percentile of maximum air temperature. Although most models foresee an increase in the number of drought days in 2081–2100, the projected changes are mostly not statistically significant. In contrast, the number of heatwave days is expected to increase significantly across the entire study area by the end of the 21st century. As air temperatures rise and heatwaves become more frequent, the recurrence of CDHEs will increase. By the end of the 21st century, an additional 1–5 CDHE days (SSP2–4.5 scenario) and 6–18 days (SSP5–8.5 scenario) per year are projected. The duration and spatial extent of CDHEs will also rise with the magnitude of these changes determined by precipitation patterns.