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

Abstract

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 maximum spatial extent 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 Pathway scenarios (SSP2–4.5 and SSP5–8.5) were used. 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, 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) and 6–18 days (SSP5–8.5) per year are projected. The duration and maximum spatial extent of CDHEs will also increase with the magnitude of these changes, which is determined by precipitation patterns. Nevertheless, it should be noted that the results are subject to inherent uncertainties associated with the choice of climate models, scenarios, the study period, and other factors.