Keywords: thermal regime, climate change, RCP scenarios, climate characteristic projections, date of persistent air temperature transition, Euro CORDEX

In the context of ongoing climate change and the rapid development of regional climate models, there is an increasing demand for detailed assessments of the duration, onset, and end dates of thermal periods, which are crucial for multiple sectors of the economy. Previous publications have examined changes in the warm period (mean daily air temperature t > 0°C), the vegetation period (t > 5°C), and the period of active vegetation (t > 10°C). This article concludes a series of studies on thermal periods in Ukraine and focuses on the characteristics of the climatic summer (t > 15°C), a critical indicator for human health, recreation, tourism, energy supply, and agricultural activities. The main goal of the presented research was to analyze the spatial and temporal patterns of the summer season and evaluate how these characteristics may change under future climate scenarios. Using E-OBS observational data, the dates of onset, termination, and duration of the summer period were calculated for the standard climatological period 1961–1990, along with observed changes during 1991–2010. Projections for future changes were conducted for three periods—2021–2040, 2041–2060, and 2081–2100—under moderate (RCP 4.5) and high (RCP 8.5) greenhouse gas representative concentration pathways, based on an ensemble of 34 Euro-CORDEX regional climate models with a high spatial resolution of 12?12 km, covering over 7,300 grid points across Ukraine. The analysis revealed that during 1961–1990, the summer season typically began between May 10 and 20 and ended between September 17 and 27. In 1991–2010, the season lengthened by 5–15 days, with the onset occurring 2–5 days earlier. Future projections suggest further extension of the summer season by 7–60 days depending on the region and scenario. Under RCP 8.5, the maximum summer duration could reach 180–200 days in Crimea and southern regions by the end of the century. In contrast, the Carpathians may experience a climatic summer lasting 80–120 days, similar to the Pre-Carpathian region at the end of the twentieth century, while in the Polissya region, the summer may extend to 140–160 days, resembling current conditions in Crimea. The results presented in this and previous parts of the study have substantial practical significance. They can support agricultural planning, risk assessment for food security, energy demand forecasting, and the design of climate change adaptation strategies. In addition, these findings are essential for evaluating the impacts of climate change on human health, planning recreational activities, promoting sustainable tourism, and managing territorial resources effectively.

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