Climate change increases flooding in some European regions and decreases it in others
Human-driven climate change is causing significant changes to the pattern and extent of river floods in Europe, according to a major new study.
A large, multi-national team of researchers analysed river flow data from 3,738 of catchments taken over a 50 year period between 1960 and 2010. Writing in the journal Nature, the team – led by Prof Günter Blöschl and Dr Julia Hall – outline clear regional patterns in the data.
Regional flood discharge trends in Europe range from an increase of around 11% per decade to a decrease of 23%. The study authors identify three European regions where flooding has either increased or decreased over the study period.
Regional patterns of flooding change
Increasing autumn and winter rainfall – and the resulting wetter soils – over the last 50 years in northwestern Europe has caused an increase in flooding. Around 69% of river flow stations in this region show an increasing flood trend, with an average local increase of flow of 2.3% per decade.
In medium and large catchments in southern Europe, floods are decreasing because of lower rainfall and increasing evaporation from the soil. However, in some Mediterranean areas, small rivers can experience more floods due to frequent thunderstorms and alterations to their catchments, such as deforestation. In this region, around 74% of stations show a decreasing flood trend, with a regional average decrease in flow of 5% per decade.
In Eastern Europe, flood levels are decreasing due to less extensive spring snow cover, a shift to rainfall (rather than snow), and earlier snowmelt as a result of higher air temperatures. However, extreme precipitation in the summer has increased in the summer in this region. Here, around 78% of stations show a decreasing flood trend, with an average decrease in flow of 6% per decade.
Joint lead author of the study, Professor Günter Blöschl of the Vienna University of Technology, says: “We already knew that climate change is shifting the timing of floods in a year, but the key question had been, ‘Does climate change also control the magnitude of flood events?’. Our study did in fact find there are consistent patterns of flood change across Europe and these are in line with predicted climate change impacts, such as a contrast between increasing severity of flooding in the north and decreases in the south.”
Jamie Hannaford of the UK‘s Centre for Ecology & Hydrology, one of the scientists who was involved in the research, says: “This timely study adds to a growing body of evidence that shows that flood magnitude has increased in the UK over the last five decades, particularly in parts of northern and western Britain.
“We show this is part of a continent-wide pattern of changes in flooding which is in line with what we may expect in a warming world. This highlights the importance of long-term hydrological monitoring and the benefits of data sharing and collaboration at a European scale in order to better understand the mechanisms behind observed changes in flooding.”
Flood risk management
The research team’s results have implications for flood risk management in European river catchments. River managers often use the ‘return period’ concept to model how often floods of a certain size (and impact) will recur in their catchments.
The study authors highlight that in regions with increasing flood discharges, the 100-year flood discharge of 50 years ago now has a return period shorter than 100 years. In other words, large floods now be expected more regularly. As a result, the flood defences built to mitigate the risk of extreme flooding may no longer be sufficient.
In contrast, in regions where flooding is decreasing (such as Eastern Europe), the return period of large floods is increasing. The authors estimate that large floods which happened only once every 100 years, will now have a 125 to 250 year return period.
The study authors suggest that changing patterns of flood risk driven by climate change across Europe must be taken seriously by policy makers in order to mitigate the potential devastating impact on affected communities.
Dr Neil Macdonald of the University of Liverpool, a co-author of the study, says: “Flood management must adapt to the realities of our changing climate and associated flood risk over the coming decades.”
Dr Thomas Kjeldsen of the University of Bath, another co-author, adds: “Incorporating the evidence of increasing flood risk into engineering design and general flood management would ensure we are better prepared for future changes – a point also raised in the UK Government’s National Flood Resilience Review.”
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