Urban thermal data analysis over the period 1948–2022: a case study of Ljubljana, Slovenia

The aim of the study was to characterize the intensity, frequency and duration of extreme high temperature events and their variability over a period of 75 years (1948-2022) for Ljubljana, Slovenia. This study uses 23 thermal indices recommended by the WMO (ETCCDI) based on daily maximum and minimum air temperatures, retrieved from the Slovenian Environment Agency. The study conducted showed an increase in heat stress risk during the summer months over the last 75 years, with particularly pronounced changes since the 1990s. The observed increase in air temperature was greater for extreme than for average temperatures. The trends in annual average maximum, minimum and daily temperatures were all positive and significant with rates of 0.37 °C/decade, 0.41 °C/decade and 0.39 °C/decade respectively. As a result of these changes, the number of hot days, tropical nights, intensity, frequency and duration of heatwaves (HW) have also increased. HW are becoming a growing problem in Ljubljana, as all HW indices examined are increasing: number of HW (trend 0.5 events/decade), frequency (2.0 days/decade), magnitude (0.36 °C/decade) and maximum amplitude (0.73 °C/decade). Until recently, these events were only typical of summer, but now they occur in May and even last into September. The 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 clear change, namely a decrease in HDDheat and an increase in CDDcool. The city has experienced pronounced urban growth, which is accompanied by significant changes in the area surrounding the measurement site, which, together with climate change, exacerbate the risk of heat exposure. Despite numerous measures already taken to reduce heat stress in the city, it remains a problem in the summer months, especially given the prediction that conditions will worsen in the future. It is therefore necessary to continue monitoring temperature conditions and local and temporal changes, which is the responsibility of the National Meteorological Service. Further studies on urban characteristics and human thermal comfort parameters are also needed to assess local vulnerability. In addition, some complementary measurements could be carried out to collect data on spatial variations, which is an important step in developing a plan to combat heat stress.