Polski
Science, Issue 13/2025
Flash floods in the Alps may be twice as frequent
Jul
As global warming continues, the landscape of the Alps could undergo dramatic changes. A new study by researchers at the University of Lausanne suggests that a 2°C increase in average temperature relative to the 1991-2020 period could double the frequency of extreme, short-lived rainfall events in the region. The consequences? More frequent flash floods, debris flows, landslides and more costly damage to infrastructure.
Nadav Peleg, Marika Koukoula and Francesco Marra, a trio of researchers from the Universities of Lausanne and Padova, conducted the study, the results of which were published June 16 in the scientific journal Climate and Atmospheric Science.
Extreme precipitation in the Alps – a real threat
Violent summer storms in the Alps are a phenomenon that residents and emergency services have not been familiar with for a long time. They can bring intense rainfall in just a few minutes, for example, on June 11, 2018, 41 mm of rain fell in Lausanne in 10 minutes. That’s the equivalent of more than 240 mm/h, and the damage was estimated at 32 million euros. In July 2009, in the Eastern Alps (Upper Adige), an hour’s rain brought 98 mm of water, triggering mudslides and changing the course of the river. The losses amounted to 700,000 euros.
Switzerland recorded €77.5 million worth of damage caused by natural phenomena in 2023 alone, and 67 percent of that amount was due to heavy rainfall.
More heat means more water in the air
Rising temperatures cause the atmosphere to hold more water vapor – by about 7 percent for every 1°C of warming, according to the Clausius-Clapeyron relationship. Combined with stronger convection, this leads to more intense storms. Although the overall amount of summer precipitation in the Alps may be decreasing, extreme episodes, such as rains exceeding the 99th percentile in intensity, will become more frequent, according to the researchers.
TENAX model: new tool, accurate forecasts
The international team of researchers used the TENAX statistical model, which, based on data from 299 high-altitude measurement stations from Austria, France, Germany, Switzerland and Italy, allows them to assess how precipitation of very short duration (10 minutes and 1 hour) will change under different climate scenarios (1°C, 2°C and 3°C warming).
The analysis showed an average increase in precipitation intensity of 9 percent for every additional 1°C. Interestingly, the higher up you go, the greater the increase will be – stations above 1,500 meters above sea level recorded average rates as high as 12 percent/1°C, compared to 6.2 percent/1°C for locations below 500 meters above sea level.
A 2°C rise in temperature? Flash floods will occur twice as often
Simulations for the +2°C warming scenario show that today’s 50-year extreme precipitation (i.e., precipitation that statistically occurs once every 50 years) could occur up to twice as often – once every 25 years, and in places even once every 16 years. For 10-minute precipitation, this change is even more drastic than for hourly precipitation.
With a 3°C increase in temperature, the frequency of some precipitation events can increase up to fourfold – events considered “once every 100 years” today can occur every 25.
Uncertainty increases with altitude – but the trend is clear
The study’s authors emphasize that forecast uncertainty increases with altitude – especially above 1,500 meters above sea level, where it is more difficult to model weather due to complex orography. Still, the direction of change is clear: the intensity and frequency of extreme precipitation events will increase. Flash floods may become more frequent.
Comparisons with other models (GEV and SMEV) confirm the effectiveness of TENAX, which, while slightly overestimating rainfall intensity, provides realistic and consistent forecasts.
Time to update engineering standards
Most infrastructures in Europe are designed to accommodate so-called extreme precipitation return periods. In cities, sewage systems are adapted to 10-30 year recurrence intervals of precipitation, while roads and bridges are adapted to 50-100 year intervals. A 2°C rise in temperature, however, could change these assumptions.
Current standards based on historical data do not match the realities of a changing climate. The study’s authors call for an urgent revision of engineering guidelines, especially in mountainous regions that are most vulnerable to the effects of severe weather events such as flash floods.
Reduce warming – reduce risk
Limiting global warming to 1.5°C – the target set by the Paris Agreement – could significantly reduce the risk of extreme precipitation. With the current level of temperature increase of about 1.2°C relative to the pre-industrial era, implementing climate commitments (such as those of Glasgow) could stop the increase at 1.6-1.8°C.
However, the Alps are warming faster than other regions of the world. This means that even if the global increase is limited to 2°C, the phenomenon will not stop at this level in the mountains. Its effects will be more intense, such as more frequent, violent precipitation. Action to reduce greenhouse gas emissions is therefore crucial for the future of infrastructure, the safety of residents and the protection of Alpine ecosystems.
