Compound drought and heatwave events, known in the literature as CDHEs, occur when drought conditions and heatwave episodes overlap in the same place at the same time. Scientists regard these combined extremes as far more damaging than either hazard alone, because dry soils amplify surface heating while heat accelerates moisture loss from vegetation and waterways. The twin stresses have been linked to crop failures, forest dieback, elevated mortality among people with respiratory conditions, strains on water supplies, and increased wildfire risk. In a rapidly warming world, understanding where and how often these overlapping extremes will strike has become a central question for climate adaptation planning.

To answer that question for the eastern Baltic, researchers Laurynas Klimavičius and Egidijus Rimkus of Vilnius University turned to downscaled projections from five climate models participating in the Coupled Model Intercomparison Project Phase 6, or CMIP6. The team drew on the NASA NEX-GDDP-CMIP6 database, which provides statistically downscaled daily climate data at a fine enough resolution to capture regional and local patterns. They evaluated two Shared Socioeconomic Pathways: the moderate SSP2-4.5 scenario, in which emissions decline gradually over the coming decades, and the high-end SSP5-8.5 scenario, which assumes continued heavy reliance on fossil fuels throughout the century.

The study focused on the warm season from April through September, when both droughts and heatwaves exert their greatest impact on agriculture, forestry, and human health. Droughts were identified using the Standardised Precipitation Index, a widely adopted drought metric originally developed in the 1990s that compares precipitation totals against long-term climatological norms. Heatwaves were defined using a percentile-based threshold, with a heatwave day counted whenever daily maximum air temperature exceeded the 90th percentile of the historical distribution. This dual-index approach allowed the researchers to pinpoint the days on which both a drought, according to the SPI, and a heatwave, according to the temperature threshold, were recorded simultaneously across the study area.

The results reveal a striking asymmetry between the two components of the compound hazard. Although most of the models foresee an increase in the number of drought days across the region during the final decades of the century, running from 2081 to 2100, those changes are mostly not statistically significant. Precipitation patterns over the eastern Baltic are inherently variable, and projected shifts in rainfall are smaller relative to natural variability than projected shifts in temperature. Heatwave days, by contrast, are expected to increase significantly across the entire study area by the end of the 21st century under both scenarios, a consequence of the relentless rise in mean and extreme temperatures that virtually all climate models agree upon.

Because heatwaves are the more robustly projected element, it is the warming of the atmosphere that primarily drives the increase in compound events. As air temperatures rise and heatwaves become more frequent and more likely to coincide with existing dry spells, the recurrence of compound drought and heatwave events increases almost mechanically. By the end of the century, the study projects an additional 1 to 5 CDHE days per year under the moderate SSP2-4.5 pathway, but a far more alarming 6 to 18 additional days per year under the high-emissions SSP5-8.5 scenario. In practical terms, that means that in the worst case, more than two additional weeks of simultaneous drought and extreme heat each warm season compared with the recent past.

Perhaps even more consequential than the frequency increase is the finding that both the duration and the maximum spatial extent of compound events will also grow. Longer, more expansive CDHEs are particularly threatening because they strain emergency response systems, dry out entire landscapes rather than isolated pockets, and allow compounding damage to accumulate across successive weeks. The magnitude of these changes, the authors note, is ultimately determined by precipitation patterns, meaning that regions within the eastern Baltic that receive the most pronounced declines or reorganisations of summer rainfall will face the steepest increases in compound hazard.

The findings place the eastern Baltic within a broader global picture that has emerged over the past decade. Research across Europe, China, Australia, and North America has consistently shown that compound hot-dry extremes are intensifying faster than individual hazards, and that high-end warming pathways carry a disproportionately amplified risk. Some studies have warned that climate change will accelerate the high-end risk of these events globally, while European-scale analyses suggest that hot and dry summers will become more frequent and expand northward, bringing Mediterranean-style compound extremes into the mid-latitudes. The new study extends this evidence into a region where adaptation infrastructure for such hazards remains comparatively limited.

The authors are careful to acknowledge the uncertainties inherent in their projections. Results depend on the choice of climate models, the scenarios selected, the study period and index definitions, and other methodological factors. Only five models were used, and the statistical downscaled dataset, while high-resolution, carries its own biases. The weak statistical significance of drought trends in particular underscores how difficult it remains to pin down the future of regional precipitation, even as temperature projections grow increasingly confident. Nonetheless, the direction of change in compound events appears clear across the model ensemble, because the temperature-driven component dominates the overlap.

For policymakers and communities across Lithuania, Latvia, and neighboring territories, the study delivers a sobering planning signal. Agriculture, forestry, water management, and public health systems in the region will need to prepare for a warm season in which combined drought and heat stress becomes a routine rather than exceptional feature of the late-summer landscape. The gap between the two emissions scenarios is the study’s clearest message: the difference between a manageable rise of a few compound days per year and an 18-day escalation lies almost entirely in the emissions choices made over the coming decades.

Subject of Research: Future projections of compound drought and heatwave events in the eastern Baltic Sea region using CMIP6 climate models

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

Article References: Future projections of compound drought and heatwave events in the eastern part of the Baltic Sea region. (n.d.). https://doi.org/10.1007/s00704-026-06563-7

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06563-7

Keywords: compound drought and heatwave events, Baltic Sea region, CMIP6, SSP2-4.5, SSP5-8.5, Standardised Precipitation Index, heatwaves, drought, climate change, NEX-GDDP, extreme weather, climate projections