Analysis of Climate Change in Croatia Based on Calculation of Temperature Thresholds

The equivalent of the phenological data for the beginning of active vegetation and the provision of plant growth stage development with heat is the date when the average daily air temperature exceeds a certain threshold or biological minimum, and is determined from the average annual temperature pattern. The aim of the paper was to calculate occurences of temperature thresholds of 5, 10, 15, 20 and 25 °C for the reference period 1961-1990 and the recent period from 1991 to 2018, at three locations - Osijek, Gospić and Zadar, covering three agricultural regions of Croatia - Pannonian, Mountain and Adriatic region, respectively, and to indicate potential climate change in selected areas through analysis of time series and calculation of trends in the beginning, end and duration period of a certain temperature. Based on the results of the calculation of temperature thresholds in the observed period from 1991 to 2018, warming occurred in all three agricultural regions and the last four years - 2015, 2016, 2017 and 2018 have been the warmest years so far. Furthermore, temperature thresholds on average appear earlier than in the period from 1961 to 1990 and often have later endings. The most significant extension of the number of days is calculated for the temperature thresholds of 5 °C and 10 °C in the area of the Pannonian and Mountain agricultural regions, and in the Adriatic region for the temperature threshold of 10 °C. Results indicate pronounced winter/ spring warming which can induce advanced start of active vegetation and extension of warm season in mountainous areas