Longer, hotter and generated heat waves especially in Lugano and Geneva
Zurich – Hot days followed by hot nights with no cooling in between could become the new norm towards the end of the 21st century. Researchers from the University of Zurich analyzed the frequency, intensity and duration of extreme events for five Swiss cities. Lugano and Geneva would be most affected.
Climate change is causing extreme weather events such as heat waves, floods, and droughts to occur and become more intense. The hot summer of recent years has already had a serious impact on human health, especially in the cities, due to the accumulation of heat during the day and the lack of cooling at night. Such simultaneously occurring periods of extreme heat during the day (with temperatures of 28 to 35 degrees) and at night (with temperatures of 15 to 20 degrees) are more health-conscious than individual hot days or hot nights. In addition, these will last longer and occur again in Swiss cities in the future, as a study by the UZH shows.
Heat waves could be recorded up to 8 times
Researchers from the Department of Geography at UZH, the University of Friborg and the Swiss Federal Research Institute WSL have investigated how different future scenarios will affect the frequency and intensity of hot daytime and night-time storms occurring simultaneously in the five Swiss cities of Basel, Bern, Geneva, Lugano and Zurich, for example. They assume that by the end of the century, extreme heat waves that exceed past daytime and nighttime temperatures will increase by up to 7.8 times in all Swiss cities and the corresponding events by up to 5.3 times. could be times longer. “Our results show the need for Swiss cities to adapt to extreme heat waves, thereby reducing daytime warming, improving night-time cooling and strengthening the population’s resilience to more severe heat waves,” says lead author Saeid Ashraf Vaghefi.
Three scenarios analyzed for three periods
In their study, the researchers defined three parameters for analyzing heat waves:
- a) the annual number of concurrent hot days and nights exceeding the threshold for both day and night,
- b) the regular frequency of those events exceeding the probability for at least two consecutive days, and
- c) the actual duration of the relevant events. These metrics were combined with three emission scenarios—low, moderate, and high—and fitted to three time periods of thirty years each: the past (1980–2020), the near future (2020–2050), and the far future (2070–2100).
Lugano and Geneva are affected
The results show that the number of hot days and nights is increasing significantly in all cities, but is more pronounced in cities at lower latitudes: the highest increase – calculated per decade – is in Lugano with 65.8 days in the past, 110 days in the near future and 371 days in the distant future, then from Geneva with 48 days in the past, 108 days in the near future and 362 days in the distant future. Still clear, but less pronounced are the largest in Basel (48 / 74 / 217 days), Bern (15 / 44 / 213 days) and Zurich (14 / 50 / 217 days).
Adaptations to heat waves are necessary
“In our study, we predict that the composite hot day and night tremes will become more likely and intense in all cities and under all emission scenarios, but significantly under high emission scenarios and after the 2050s,” summarizes Vaghefi. “A hot day followed by a hot night without humans being able to recover could become the new norm by the end of the century. Therefore, we recommend that policymakers and stakeholders conduct a systematic analysis of options before implementing adjustments to such extreme events.”
In the ongoing project, UZH researchers are investigating how various adaptation measures such as increasing vegetation density, making better use of existing bodies of water, reducing anthropogenic heat through the use of green and renewable energies and changing outdoor activity times are the various possible drivers of extreme events. (UZH/mc/pg)