Map shows alarming increase in hot days in Austria

According to the report, only three districts recorded a "very high" heat stress index in 2023, while in 2024 there were already 25. In 49 districts there were at least 34 hot days, significantly more than in the peak year of 2019 (15 districts).

The team led by Hannah Schuster from the Complexity Science Hub (CSH) Vienna and the Vienna University of Economics and Business (WU) has now updated the heat stress map, first presented last year, with data for 2024. The interactive map presents a heat index for all districts and municipalities based on the number of hot days (with temperatures above 30 degrees Celsius). In addition, a five-level heat stress index (from "very low" to "very high") can be read off, which also takes into account the proportion of older people (over 65 years of age).

For 2023, the map shows the districts of Hollabrunn, Rust, and Eisenstadt-Umgebung in deep red—meaning they have a "very high" heat stress index. Last year, an arc encompassing 25 districts and statutory cities stretched from the eastern Waldviertel region through the northern Weinviertel region toward Burgenland, the Industrieviertel region, and the Mürztal valley to southern Styria and large parts of Carinthia, where all administrative units except for Klagenfurt and Villach, as well as the Hermagor district, are highlighted in dark red. This color represents the highest heat stress index.

According to the study's definition, the heat stress index is "very high" if the district in question experiences more than 27.6 hot days per year and more than 22.8 percent of the population is older than 65. In contrast, the risk is "very low" if there are fewer than 14.9 hot days per year and the proportion of people over 65 is below 18 percent.

In terms of the number of hot days, all districts in Burgenland had a "very high" heat index (more than 34 hot days) in 2024; in Lower Austria, this applied to 20 of the 24 districts or statutory cities. Due to the high number of hot days and the higher proportion of older people, the heat stress index was "very high" in seven of the ten Carinthian districts.

There are clear differences between western and eastern Austria: Examples of the significantly higher number of hot days and tropical nights (over 20 degrees Celsius) in the east are Rust and Neusiedl with 52 hot days each, or Wiener Neustadt (50) in contrast to Reutte (2).

Vienna experienced a sweaty year in 2024, with 49 hot days and 41 tropical nights, with some districts such as Wieden, Margareten, and Mariahilf experiencing up to 53 tropical nights. However, the heat stress index was not as pronounced due to the comparatively young population (only 16.4 percent over 65 years of age).

Besides Vienna, the most tropical nights were recorded in Eisenstadt (33) and Rust (27). In contrast, residents of the districts of Hermagor, Spittal an der Drau, Salzburg, Hallein, Sankt Johann im Pongau, Tamsweg, Zell am See, Leoben, Murtal, and all districts of Tyrol enjoyed restful sleep because temperatures there never climbed above 20 degrees Celsius at night. The only district in Austria with a "very low" heat stress index was Dornbirn, with 13 hot days and a share of 17.7 percent of people over 65.

The scientists at the CSH assume that this year's summer "will most likely bring another severe heat wave – and more widespread than in previous years." Furthermore, demographic change is progressing, and in a few years, significantly more regions will have a high proportion of older – and thus vulnerable – people: While only around ten percent of districts had a senior citizen share of over 25 percent in 2024, forecasts predict that this will rise to 95 percent by 2050.

This is problematic because, according to the research team, each additional hot day increases mortality by 2.4 percent per 1,000 inhabitants. Heat waves, i.e., a week with more than three hot days, triple the risk of death compared to single hot days.