Analysis of long-term temperature data (1948–2022) shows the growing threat of extreme heat in the city of Ljubljana, Slovenia

Climate change is the greatest threat and risk to urban health and manifests itself in more intense, more frequent, and longer extreme weather events. Although Ljubljana has a relatively small population compared to other European capitals, the city faces problems in summer due to the urban heat island. The aim of this study was to characterize the intensity, frequency and duration of extreme high temperature events and their variability over a 75-year period (1948–2022) for Ljubljana. This study uses 23 indices recommended by the WMO Expert Team on Climate Change Detection and Indices (ETCCDI). The indices are calculated with the Climpact-v2 software. For each index, the type of trend and significance were calculated using the Mann–Kendall test and the magnitude of the trend was calculated using Sen’s slope estimator. The trends in annual mean maximum (TXm), minimum (TNm) and daily mean (TMm) temperatures were all positive and statistically significant with rates of 0.37 °C/decade, 0.41 °C/decade and 0.39 °C/decade, respectively. Both the annual minimum value of daily TN (TNn) and the annual maximum value of daily TN (TNx) have increased significantly in recent times, showing that the coldest and hottest nights of the year in Ljubljana are now warmer, with an increase of almost 7 °C on the coldest nights and a smaller increase of 3.8 °C on the hottest nights. The number of events with a fixed threshold temperature, summer days (TX > 25 °C), hot days (TX ≥ 30 °C) and very hot days (TX ≥ 35 °C) showed statistically significant positive trends. Ljubljana has an average of 68 summer days and 18 hot days per year. Until recently, these days were only typical of summer, but now they occur in May and even continue into September. In the period 1948–2012, we could expect a very hot day only once every three years, and the average of the last 10 years, 2013–2022, is 4 very hot days per year. The number of tropical nights (TN > 20 °C) increased at a rate of 0.3 days/decade, especially after 2000. Thermal heat sum indices, heating degree days (HDDheat) and cooling degree days (CDDcool), indicators of weather-related energy consumption for heating and cooling buildings, showed a significant decrease for HDDheat and an increase for CDDcool. Heat waves (HW) are becoming a growing problem in Ljubljana as all investigated HW indices are increasing, i.e. HW number (trend 0.5 events/decade), frequency (2.0 days/decade), magnitude (0.36 °C/decade) and maximum amplitude (0.73 °C/decade).

According to the research results, it is necessary to accelerate the adoption of a heat action plan that will include observations, forecasts, warnings, and education for the city's residents.