Keywords: thermal regime, climate change, RCP scenarios, representative concentration pathways, climate characteristic projections, date of persistent air temperature transition

With the ongoing climate change, updated climate scenarios, and higher-resolution Regional Climate Models (RCMs), there is a growing need for a detailed assessment of the duration and changes in start and end dates of thermal periods, which are critical for various economic sectors. Previous studies have analyzed changes in the warm period (when temperatures stay above 0°C) and the growing season (above 5°C). This article is the next part in the series examining shifts in thermal periods across Ukraine till the end of the 21st century, focusing specifically on the Period of Active Vegetation (PAV) which is defined by the persistent transition of daily average temperatures above 10°C and is a key factor in determining suitable conditions for heat-loving crops and plays a crucial role in planning agricultural practices. The goal of this study was to analyze the spatial and temporal characteristics of the PAV and how they are changing over time. Using data from the E-Obs database, we calculated duration, the start and end dates of the PAV across Ukraine for the baseline climate period of 1961-1990 and assessed changes in 1991-2010. Additionally, we estimated future shifts for three periods: 2021-2040, 2041-2060, and 2081-2100, using Representative Concentration Pathways of both moderate (RCP 4.5) and high (RCP 8.5) concentrations. The analysis is based on an ensemble of 34 RCMs from the Euro-CORDEX project, with a spatial resolution of approximately 12×12 km that corresponds to over 7300 grid nodes in Ukraine. In period 1991-2010, the duration of the PAV in Ukraine increased compared to 1961-1990, particularly in the northwest, where it extended by at least 10 days. By 2021-2040, under RCP 4.5, the PAV is expected to lengthen by about 7 days across most of Ukraine and by 7-14 days in the steppe regions of Crimea, southern Odesa Oblast, and the Carpathians. Under RCP 8.5, the increase is projected to be mostly 7-14 days, with the Carpathians seeing an increase of 14-21 days, and up to 7 days in Dnipropetrovsk Oblast. By 2041-2060, under RCP 8.5, the PAV is expected to increase significantly—by an average of 14-21 days, and up to 30 days in western Ukraine, Crimea, and southern Odesa Oblast. In 2081-2100, under RCP 4.5, the duration of the PAV in Ukraine is projected to resemble that of the RCP 8.5 scenario for 2041-2060. Under RCP 8.5, the extension of this period relative to 1991-2010 could range from 31 to 70 days, with the most pronounced increases occurring from east to west. Increasing the duration of the growing season and PAV can strengthen the agroclimatic potential of Ukraine in the future and will contribute to obtaining higher crop yields, if plants will be provided with moisture, that is, the timely implementation of modern methods of adaptation to changes in hydrothermal conditions and other agrotechnologies for plant protection.

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