A team of researchers from Vienna has calculated a heat stress index for each district and municipality for the years 2018 to 2023, taking into account the number of hot days and the proportion of the population over 65 years of age. The number of heat risk districts is already increasing during this period; without greenhouse gas reduction, almost every district in Austria will be at high risk by 2050.
The increasing number of hot days represents a significant health burden, especially for older people, scientists from the Complexity Science Hub (CSH) Vienna and the Vienna University of Economics and Business (WU) stressed in a press release on Wednesday. Between 2015 and 2022, each additional hot day – that is, days with temperatures over 30 degrees Celsius – increased mortality in Austrian districts by 2.4 percent per 1,000 inhabitants, they write in their work, which has not yet been peer-reviewed.
If more than 25 percent of the population in a district is over 65, this effect doubles. During heat waves (defined as a week with more than three hot days), the increase in death rates is almost three times as high as the increase on a single hot day.
Given the ageing population, researchers say heat-related health problems will increase: while in 2023 only a quarter of the people in 15 percent of districts were older than 65, by 2050 this figure will have risen to 80 percent.
The risk increases with age
This increase in heat risk is already evident in the years 2018 to 2023, which the research team led by Hannah Schuster from the CSH and WU Vienna used for their interactive map: During this period, there are more and more districts with a medium and very high heat risk. The east of Austria in particular is exposed to high heat stress.
In 2023, the districts of Hollabrunn, Eisenstadt Umgebung and Rust had a “very high” health risk. In the same year, Graz, Rohrbach, Tamsweg, Reutte, Imst, Dornbirn and Feldkirch only recorded a “low” heat risk index.
According to the definition in the study, the heat stress index is “very high” if there are more than 27.6 hot days per year in the district in question 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 less than 18 percent.
The risk index in Vienna is relatively low overall, which scientists attribute to the low average age of the population. The fact that a higher average age increases the risk is evident in the Vienna districts of Innere Stadt and Hietzing, for example, with a “very high” burden.
Individual cooler years
The map shows that despite rising temperatures, there can also be individual cooler years, such as 2020. However, the scientists prove that the heat trend is steadily increasing over time by looking ahead to the future, calculated using various emission scenarios. If greenhouse gas emissions remained as they are now, almost every district in Austria would be classified as very high risk by 2050. Even with a strict climate policy, the health risk would remain in the medium to high range, which is due, among other things, to the steadily aging population.
In the study, the research team also shows that green spaces in cities might significantly reduce the impact of heat on mortality. Local politicians are unlikely to be able to reverse warming or demographic trends, the scientists write in the paper, “but they have the opportunity to create more green spaces in their cities or communities. Such investments are effective because cities can be greened bit by bit without high initial costs,” explained Schuster.
