Flash floods in the Alps: How climate change is supercharging summer storms

In June 2018, the city of Lausanne in Switzerland experienced an extreme and short-lived rainfall episode, with 41 millimeters of precipitation falling in just 10 minutes. Large parts of the city were flooded, resulting in estimated damage of 32 million Swiss Francs. These short, extreme events, often causing severe damage to property and posing risks to lives, are still very rare in Switzerland today. However, with the rise in temperatures caused by global warming, they are likely to become more frequent in the future, particularly over the Alpine mountains and their surroundings. Warm air retains more moisture (around 7% more per degree) and intensifies thunderstorm activity. As the Alpine region is warming faster than the global average, it is particularly hard hit. It is therefore urgent to assess the impact of global warming in these regions. In a new study published in npj Climate and Atmospheric Science (a Nature portfolio journal), scientists from UNIL’s Faculty of Geosciences and Environment, in collaboration with the University of Padua (UNIPD) have demonstrated that an average temperature rise of 2°C could double the frequency of short-lived summer rainstorms in the Alpine region. With such warming, an intense storm currently expected every 50 years could occur every 25 years in the future.

To obtain these results, the researchers examined data from almost 300 weather stations in the European Alps, spread across Switzerland, Germany, Austria, France, and Italy. They focused on record-breaking rainfall events (lasting from 10 minutes to an hour) between 1991 and 2020, as well as temperatures associated with these storms.

Based on these observations, a statistical model incorporating physics principles has been developed to establish a link between temperature and rainfall frequency, and then to simulate the future frequency of extreme precipitation using regional climate projections. “Our results show that an average temperature rise of 1°C would already be highly problematic,” warns Nadav Peleg, researcher at UNIL and first author of the study. “The sudden and massive arrival of large volumes of water prevents the soil from absorbing the excess. This can trigger flash floods and debris flows, leading to infrastructure damage and, in some cases, casualties,” he adds. “It is therefore crucial to understand how these events may evolve with climate change in order to plan appropriate adaptation strategies, such as improving urban drainage infrastructure where necessary.” Francesco Marra, researcher at UNIPD and one of the main authors of the study adds: “An increase of 1°C is not hypothetical, it is likely to occur in the coming decades. We are already witnessing a tendency for summer storms to intensify, and this trend is only expected to worsen in the years ahead.”