Extreme events have become an integral aspect of the unusually intensified climate change characterizing this century. This study examines extreme heat waves and tropical nights—phenomena historically uncommon in the mid-latitude Southeastern Baltic Sea region. Extreme heat and heat waves are defined as any period during which the daily maximum air temperature exceeds 30 °C, and a tropical night is one in which the daily minimum air temperature does not fall below 20 °C. Both in situ observations and model output from the Copernicus Climate Change Service were employed in the 1982–2024 analysis. The results reveal that the frequency of extreme heat waves is increasing. Since 2018, the southeastern Baltic Sea coast has experienced at least one extreme heat wave and one tropical night each year. The observed rise in mean air and sea-surface temperatures has driven an uptick in tropical night occurrence. Forecasts of tropical-night formation could be substantially improved by integrating sea-surface temperature assessments for the southeastern Baltic coast. Moreover, timely adaptation to evolving weather conditions—through enhanced forecasting techniques and the incorporation of high-resolution reanalysis datasets—is essential for optimizing early-warning systems capable of safeguarding human health and lives. Climate change increases the frequency and intensity of heat waves, posing significant challenges to public health, the economy, the environment, and infrastructure. Therefore, advancing the understanding of extreme heat events through the use of cutting-edge technologies, remote sensing, and Copernicus reanalysis data represents a key sustainability task. Such approaches enable more accurate assessments and forecasts of extremes, thereby supporting a safer, healthier, and more resilient future.

Climate warming is increasing heat-related risks and the need for sustainable adaptation across Europe, including Lithuania. This study assessed thermal stress in Lithuania during May–September over a 44-year period (1982–2025), using Copernicus ERA5 and ERA5-HEAT data alongside national meteorological station observations. Long-term changes in maximum air temperature (Tmax), the Universal Thermal Climate Index (UTCI), Humidex, and the frequency of hot days and tropical nights were examined. The comparison focused on Klaipėda, influenced by a maritime climate, and Vilnius, located approximately 300 km inland. The annual number of heat-stress days with moderate or greater heat stress (UTCI ≥ 26 °C) increased by 0.30 days per year in Klaipėda and 0.61 days per year in Vilnius, equivalent to approximately 3 and 6 additional days per decade, respectively. Increases were most pronounced in the moderate heat-stress category (26 °C ≤ UTCI < 32 °C). Strong heat-stress days (32 °C ≤ UTCI < 38 °C) were less frequent, with a slight increase observed only in Vilnius and rare occurrences in Klaipėda. Humidex also indicated increasing thermal discomfort at both locations. Although warm-season temperature percentiles were lower at the coast, tropical nights (minimum air temperature ≥ 20 °C) increased significantly in Klaipėda by 0.76 nights per decade (p = 0.002). A supplementary exploratory pilot spectral analysis identified indications of recurring Tmax variability at 2–3-year timescales, more pronounced in Klaipėda. These findings highlight the importance of assessing complementary daytime and nighttime heat indicators in maritime and continental settings.