Analysis of trends and projected changes in summer and very hot days in Bosnia and Herzegovina

Bosnia and Herzegovina is highly vulnerable to climate change impacts, particularly to rising air temperatures, prolonged heat waves, and altered precipitation regimes that increase the frequency and intensity of floods and droughts. This study investigates long-term trends of summer days (SU25) and very hot days (SU35) based on homogenized observations from five representative meteorological stations (Banja Luka, Sarajevo, Sanski Most, Sokolac, and Mostar) for the period 1961–2023. The above stations have a homogeneous data set and are located in different climate types. Results indicate a statistically significant increase in the number of summer days, ranging from 4 days per decade in Mostar to 8 days per decade in Banja Luka. Also, the increase in very hot days is pronounced, varying between 1.5 days per decade (Sarajevo and Sanski Most) and 5.5 days per decade (Mostar). Climate projections under the RCP4.5 and RCP8.5 scenarios suggest a substantial intensification of these trends by the end of the 21st century. The projected increase in summer days ranges from 5.8 to 66.6 days, depending on the scenario and location, while very hot days could increase by up to 58 days annually. These findings demonstrate that thermal extremes in Bosnia and Herzegovina are intensifying at a rapid rate, posing significant risks to human health, agriculture, and water resources. The results underscore the need for urgent, evidence-based adaptation strategies tailored to regional climate extremes.