Long-term air temperature changes in Ljubljana (Slovenia) in comparison to Trieste (Italy) and Zagreb (Croatia)

Abstract

The cities of Ljubljana, Trieste and Zagreb are proximate in terms of distance but differ in terms of geographic and climatic conditions. Continuous meteorological measurements in these cities began in the mid-19th century. The 100-year trends of changes in mean annual and seasonal air temperatures for these cities are presented here, evaluating the differences between them which result from their different geographical and climatic positions. Differences in trends between Ljubljana and Zagreb that result from different measurement histories and the impact of urban climate are also presented: the impact of city growth on air temperatures in Ljubljana after 1950 was not completely eliminated in the process of data homogenization. The lowest air warming trends occur in the maritime climate of Trieste (mean annual air temperature: + 0.8 °C x 100 yr-1), where measurements were continuously performed in the densely built-up section of the city. The strongest trends occur in Ljubljana, mainly due to city growth (mean annual air temperature: + 1.1 °C x 100 yr-1). Comparing the linear trends in Zagreb-Grič and in Ljubljana, the impact of Ljubljana's urban heat island on the 100-year warming trend was assessed at about 0.2 °C, at 0.3-0.4 °C for the trend after 1950, and if non-homogenized data are used, at about 0.5 °C.