Flooding: A Complex Challenge in the Era of Climate Change
Günter Blöschl
Vienna University of Technology, Austria
Winning article: Megafloods in Europe can be anticipated from observations in hydrologically similar catchments (Nature Geoscience, 2023)
“It is essential to move beyond national flood risk assessment and share information on megafloods across countries and continents.”
As global temperatures rise, weather patterns are becoming increasingly unpredictable, and flooding is emerging as an urgent and escalating threat. Our key question is: Is climate change increasing the frequency and severity of flooding, and, if so, how? We found some answers to be straightforward, while others require more nuanced analysis. This piece explores the complexities of flooding, its causes, and its evolving relationship with climate change.
Flooding has many triggers, and to better understand its causes and patterns, we must utilize historical case studies. Coastal floods often result from powerful storms, as seen in the North Sea storm surge of February 1953. Historically, ice jams caused flooding along the Danube, such as in 1830, but rising temperatures and hydropower plants have since reduced this risk. River floods, in contrast, are more commonly fueled by prolonged rainfall, especially when Mediterranean air masses bring heavy moisture (e.g., Vb weather patterns), key drivers of the Danube floods in 2013 and 2024. Soil moisture also plays a crucial role. When the ground is already saturated, additional rainfall has nowhere to go, increasing flood risks. Snow cover can further contribute by slowly releasing water into the soil, amplifying the effect. Meanwhile, flash floods are often triggered by intense thunderstorms, sometimes bringing landslides in their wake. Given the wide range of flooding causes, each of which interacts with climate change differently, it is essential to take a comprehensive and nuanced approach to assessing flood risk.
Another facet of understanding flood risk is the size of the catchment area. Streams with small catchment areas respond quickly to intense rainfall. These areas are therefore more prone to flash floods, particularly during convective events with high rainfall intensity, short duration, and small spatial extent. The Clausius-Clapeyron relation suggests that for every degree of temperature rise, the atmosphere’s water-holding capacity increases by 7%, leading to more rainfall and flooding. This relationship is confirmed by existing literature on rainfall and flooding, showing that increased energy in the atmosphere enhances convective rainfall, leading to higher flooding risk in smaller catchment areas in many regions.
In contrast, larger catchment areas, ranging from a few to thousands of square kilometers, react more slowly. Floods in these areas are typically caused by widespread rainfall events of longer duration and lower intensity, rather than convective events. Long-term flow measurements show that flood peaks in Northwestern Europe have increased by more than 5% per decade (Fig. 1) while flood peaks in large catchments in Eastern and Southern Europe have decreased over the same period.
So, why are floods changing? We must consider whether shifting flood patterns are linked to an increase in low-pressure systems, such as the Vb (five-b) weather pattern. Analysis of atmospheric reanalysis data reveals that, while Vb weather patterns have decreased in frequency since the 1960s, their intensity has increased.
Other shifts in rainfall patterns are tied to changes in atmospheric circulation. The expansion of the Hadley cell, a global wind pattern, has shifted storm tracks towards the north contributing to decreased rainfall in Southern Europe and increased rainfall in the north, matching the observed flood trends. The role of extreme rainfall in flooding is clear, but soil moisture and snow processes are equally important. Floods in Northwestern Europe, for instance, occur in winter when the soil is wetter after heavy rainfall in the autumn. In Northern Europe, spring floods are often the result of snowmelt, despite the heaviest rainfall occurring in the summer.
Figure 1: Observed flood trends from 1960-2010 based on streamflow measurements at stations with significant trends in medium and large catchment areas in Europe. Blue colors indicate an increasing trend, while red colors indicate a decreasing trend. Redrawn from Blöschl, G. et al. (2019) Changing climate both increases and decreases European river floods. Nature, 573 (7772)
In regions like Austria, a combination of rainfall, soil moisture, and snowmelt throughout the year dictates flooding patterns. While heavy rainfall is the main cause of flooding in Northwestern Europe, large catchments in Southern Europe (but not the small ones, where the floods seem to increase) have seen reduced runoff due to drier soils, and Eastern Europe has experienced fewer snowmelt floods due to warmer temperatures.
Floods tend to occur in cycles, with periods of frequent and severe floods followed by drier periods. Over the past 500 years, Europe has experienced several flood-rich periods, with the most significant between 1760–1800, 1840–1870, and 1990–2016. Interestingly, most of these periods were colder than the surrounding intervals, despite the fact that the warmer atmosphere today holds more water. This suggests that the increased frequency of floods in recent decades cannot be attributed solely to higher atmospheric moisture content. Instead, atmospheric circulation, combined with soil moisture and snow dynamics, plays a more significant role in flooding trends, in particular in large catchments.
It is also essential to understand that flooding isn’t solely driven by climate change - land use and water infrastructure also contribute. For example, agricultural practices, such as soil compaction from heavy machinery, increase surface runoff, but this is only significant in small areas. Large river basins are influenced by saturation areas, where soil properties have little effect on runoff. Land use generally has a smaller impact on large-scale flooding. Urbanization and land development, such as ski slopes or settlements, can intensify local flooding, but their impact on entire river basins is limited due to their small land area.
River regulations, designed to speed up water drainage and flood embankments, can reduce flooding locally but may increase flood risk downstream by preventing water retention at the source. The net impact of land use and infrastructure changes varies by region and catchment size, but it is clear that climate change influences flood risk more than land use alone. In northern Austria, for example, trends show a 5–10% increase in flood runoff per decade, with evidence that this trend may continue, though possibly weakened by shifts in seasonal precipitation patterns.
In order to protect themselves from larger floods, many countries have adopted an integrated approach to flood risk management. This approach combines water management, spatial planning, engineering measures, and community awareness. In line with the principle of "after the flood is before the flood," the flood risk cycle encompasses prevention, protection, awareness, preparedness, and recovery. Prevention includes measures such as spatial planning to avoid construction in flood-prone areas and addressing the residual risk of extreme events. Efforts are focused on retaining as much water in the area as possible to reduce flooding. In small catchment areas, flood retention basins are particularly effective for this purpose.
While reforestation does not significantly reduce the risk of large floods, it is valuable for other reasons, such as protection against avalanches and landslides. The same is true for river restoration, as the resulting retention capacity is typically small, so also the effect on flooding is usually small. For larger rivers, such as the Danube, the floods of 2013 and 2024 underscored the importance of functional dams, and reliable early warning systems are essential for disaster preparedness, including coordination with emergency services. The right combination of these measures, tailored to specific regional needs, is crucial in mitigating flood risks and adapting to the challenges posed by climate change.
Flooding is a complex and multifaceted issue that requires a comprehensive and adaptive approach to manage. While climate change is influencing flood patterns, it’s not the only factor at play. By understanding the various causes of flooding and the changing dynamics of rainfall, soil moisture, and snowmelt, we can better prepare for and mitigate the risks associated with floods. Climate change is certainly increasing the risk in many regions, but proactive measures such as integrated flood risk management and the effective use of infrastructure can help us cope with the challenges ahead.
The time to prepare for future floods is now.