Extreme heat during the day and a lack of cooling at night put strain on the body and impair health, especially in older people and young children or people with poor health. Heat during the day makes physical and mental work more difficult and can exacerbate existing conditions such as heart, circulatory and respiratory diseases. In the worst case, heatwaves can even lead to death. Heat events also have an impact on infrastructure systems, the energy sector, agriculture and forestry.

The website of the National Centre for Climate Services (NCCS) provides further information on the impacts of climate change and measures in various sectors.

Observed increase in heat

The sharp rise in temperatures in Switzerland has been accompanied by an increase in extreme heat and the setting of new heat records. Temperatures that used to be very rare and extreme are now occurring much more frequently.

Observations at the four measuring stations in Zurich, Geneva, Basel and Bern show that the highest annual maximum daily temperature rose by 3.4 degrees Celsius between 1901 and 2024. The highest 7-day average of maximum daily temperatures even increased by 3.7 degrees Celsius during the same period. Many heat indicators, such as the number of hot days, also show a significant increase in Switzerland. This is illustrated by the example of the Lucerne measuring station: until the early 1980s, there were a maximum of 10 hot days per year, which is now roughly the average. Since 1981, there has not been a single year without a hot day in Lucerne.

The results of Climate CH2025 show that even higher maximum temperatures would be possible in today's climate. In the event of a highly improbable, but physically possible extreme event, the current temperature records (as of 2025) could be exceeded by 5 to 7 degrees Celsius.

The observed increase in the warmest nights is even more pronounced, than that of the warmest days. The five highest daily minimum temperatures since measurements began, have all occurred in the last ten years. At the four measuring stations in Zurich, Geneva, Basel and Bern, the highest daily minimum temperature rose by an average of 4.5 degrees Celsius between 1901 and 2024. For the highest 7-day average of daily minimum temperatures, the increase is as much as 4.6 degrees Celsius. The number of tropical nights is also increasing, as can be seen here in the example of Neuchâtel.

With heat waves becoming more frequent and increasingly intense, the demand for cooling has risen in recent decades, particularly in low-lying areas. In Geneva/Cointrin, for example, the number of cooling days and cooling degree days has risen significantly since the 1960s.

Heat stress is particularly pronounced in urban areas, as the heat island effect further reduces night-time cooling. Heat stress in urban areas is explained in detail here.

Heat waves to become even more frequent and intense in future

In future, Switzerland must expect significantly more frequent and intense heat events. The increase in extreme heat in Switzerland is significantly greater than the average temperature increase in summer. Annual maximum temperatures will rise significantly as global warming increases. With global warming of 1.5 degrees Celsius – in a so-called 1.5-degree world (GWL1.5) – the warmest day of the year in Switzerland will be 1.5 degrees Celsius (0.6 to 2.7 degrees Celsius) warmer than in the reference period 1991-2020. A 1.5-degree world is imminent. In a 3-degree world (GWL3.0), the warmest day will be 4.4 degrees Celsius (2.4 to 6.8 degrees Celsius) warmer. For example, in a 3-degree world, this means a daily maximum temperature at the Basel (Binningen) monitoring station of 38.8 degrees Celsius instead of 34.4 degrees Celsius measured in the reference period 1991—2020. The highest daily minimum temperature in the year will be 1,1 degrees Celsius (0,6 to 2,3 degrees Celsius) warmer in a 1.5-degree world and 3.8 degrees Celsius (2.3 to 5.1 degrees Celsius) warmer in a 3-degree world.

The frequency and intensity of extreme temperatures will increase significantly. Extremely hot days, which currently only occur every 50 years, will become approximately 2.6 times more frequent in a 1.5-degree world and around 17 times more frequent in a 3-degree world. The intensity of such extremely hot days shows a clear increase as well. For heat events that occur once every 50 years, the temperature in a 1.5-degree world will be 1,6 degrees Celsius higher and in a 3-degree world 5.6 degrees Celsius higher than in the reference period 1991-2020

More frequent and more intense heatwaves will lead to higher energy requirements for cooling buildings in the future. In Geneva/Cointrin, the average number of cooling days per year in the reference period 1991–2020 is around 72, and the cooling degree days are 216 degrees Celsius. In a 3-degree world, there would be an average of 115 cooling days per year, and the cooling degree days would more than double to 543 degrees Celsius.