Anthropogenic climate change amplified flood peaks during the September 2024 Storm Boris floods more strongly than it intensified rainfall. This is the conclusion of an international research team led by the Helmholtz Centre for Environmental Research (UFZ), to which the Czech University of Life Sciences Prague (CZU) also contributed. Without climate change, event precipitation would have been around 6–9% lower, depending on the climate model, while flood peaks would have been 14–22% lower on average. The study was published in Science Advances.
Storm Boris brought extreme precipitation and widespread flooding to parts of Central and Eastern Europe in September 2024. Czechia, particularly Moravia and Silesia, as well as regions in Poland and Austria, were severely affected. In places, several hundred millimetres of rain fell within a few days and rivers burst their banks.
Previous attribution studies of Storm Boris mainly examined how anthropogenic warming intensified the precipitation itself. The new study goes a step further: using hydrological models, it directly estimates how this intensification propagated into flood peaks, i.e. the highest river discharge reached during a flood. The response of catchments is nonlinear.
Flood peaks respond more strongly than precipitation
“The soil can initially absorb some of the rain during a heavy rainfall event. But eventually the soil becomes saturated, and every additional millimetre of rain flows directly into the river,” says lead author Peter Miersch of UFZ. Relative to the change in precipitation, the hydrological response during Storm Boris was more than twice as large.
“Anthropogenic climate change substantially amplified the September 2024 flooding. Our results show that without this influence, precipitation across the affected region of Central Europe would have been roughly 6 to 9 percent lower, but flood peaks would have been 14 to 22 percent lower on average. The key finding is that a relatively small percentage increase in extreme precipitation can lead to a considerably larger amplification of flooding,” explains co-author Oldřich Rakovec, a hydrologist at the Faculty of Environmental Sciences, CZU.
The researchers compared the observed event with a counterfactual scenario in which human influence on the climate is largely removed. “We then propagated these differences through two structurally different hydrological models that simulate how rivers respond. This allowed us to quantify not only the change in precipitation but, above all, its effect on flood magnitude,” says Rakovec, who contributed to the study through hydrological modelling with the mesoscale Hydrologic Model (mHM).
Implications for flood protection planning
These findings are particularly relevant for flood protection, which is often designed for a specific flood magnitude and water level. If a flood is amplified more than expected from changes in precipitation alone, existing design assumptions may prove inadequate.
“If climate change increases extreme precipitation by 10%, it does not mean flooding will increase by only 10%. It could also be 20%. If flood levels exceed the thresholds for which flood protection measures are designed, extensive damage can occur very quickly,” says co-author Prof. Jakob Zscheischler of UFZ and TUD Dresden University of Technology.
To assess the robustness of the results, the researchers combined two climate models with two hydrological models, one of which uses machine learning. Despite the different methodological approaches, all combinations pointed to the same conclusion: flood peaks increased considerably more than precipitation as a result of climate change. The study focuses on how climate change altered this specific event while keeping its large-scale atmospheric circulation fixed. It therefore does not establish whether climate change has made storms like Boris more or less likely.