Keywords: Keywords: thermal regime, climate change, representative concentration pathways, climate characteristic projections, regional climate model (RCM), date of persistent air temperature transition

The first article in a series of analyses on changes in thermal periods in Ukraine until the end of the 21st century is presented here. It focuses on analyzing the characteristics of the warm period, defined by a persistent transition of the average daily temperature above 0°C, separating it from the winter period. With the improvement in the spatial resolution of regional climate models (RCMs) and the updating of scenarios, there is a need to refine the characteristics of the warm period, crucial especially for agriculture and forestry. Based on data from the E-Obs database, the start dates, end dates, and duration of the warm period in Ukraine during the standard climatic period of 1961-1990 were analyzed, along with changes in these characteristics further in 1991-2010. An assessment of changes in values for future periods 2021-2040, 2041-2060, and 2081-2100 was also conducted using scenarios of representative concentration pathways (RCP 4.5 and RCP 8.5) with an ensemble of 34 RCMs from the Euro-CORDEX project with a spatial grid resolution of approximately 12×12 km. It's worth noting that previous studies by other authors have already assessed changes in the characteristics of the warm period, but using previous A1B scenarios and a much smaller number of RCMs characterized by a coarser grid of 25×25 km. According to updated climate scenarios, the duration of the warm season in the 21st century will increase across the entire territory of Ukraine, with more pronounced changes expected in the north and east. Under RCP 4.5 and RCP 8.5 scenarios, an increase in the duration of the warm period is projected in 2021-2040 relative to 1991-2010: from 7-14 days in the south to 20-23 days in the north and east of Ukraine. In the period 2041-2060, under RCP 4.5, changes will range from 7 days in the south, Transcarpathia, and the Carpathians to 20-25 days in the northeast, while under RCP 8.5, changes will range from 7-14 to 30-35 days, respectively. By the end of the century, these changes will continue, and in Crimea, there is a high likelihood that temperatures will not drop below 0°C, meaning there will be no winter season. Analysis of other thermal periods will be presented in subsequent parts of the study.

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