Major floods increased in temperate climates but decreased elsewhere: Oxford study

The first global examination of recent changes in the size, frequency, and probability of extreme river floods using historical river records, the paper shows that dangers of extreme river flooding demand close monitoring of rivers for decades to come, to understand and account for the potential impact of such changes.

Flood maps need to be regularly updated to keep up with these changes and protect people and their livelihoods...these changes cannot be ignored

Dr Louise Slater

Dr Louise Slater, the main author and Oxford expert in flood risks, warns, ‘Our work shows the magnitude and frequency of floods are changing substantially in different climates of the globe. Flood maps need to be regularly updated to keep up with these changes and protect people and their livelihoods.’

She maintains, ‘These changes cannot be ignored. We need to keep monitoring our rivers because long-term records are necessary to understand and communicate how major floods are changing relative to the past.’’

Using historic recorded magnitudes of river flow, the team found that overall, floods have mostly decreased at the global scale. In arid, tropical, polar, and cold climate zones, 20-year floods decreased between −33% and −12% on average since the 1970s. This was most visible in regions such as north eastern Brazil, eastern Europe, parts of western US and parts of northern China.

The team suggests these decreases may be caused by a range of factors such as flood control measures, climate changes and decreasing soil moisture, where drier antecedent conditions offset flood magnitudes (due to increasing temperatures, decreasing rainfall, or groundwater depletion).

In temperate countries, it is not just the size of major river floods that is increasing. The paper shows that major floods have also been occurring more frequently

By contrast, the size of 20-year river floods increased by 22% on average since the 1970s in temperate climate zones such as Atlantic-facing Europe, south eastern Brazil, and south eastern China. According to the team, it is likely some of these increases result from climate change and variability, but no direct link was drawn within the research paper.

In temperate countries, it is not just the size of major river floods that is increasing. The paper shows that major floods have also been occurring more frequently, which is why it is essential to maintain up-to-date flood risk mapping.

Authors of the study are Dr Louise Slater (University of Oxford), Professor Gabriele Villarini (University of Iowa, US), Dr Stacey Archfield (U.S. Geological Survey, US), Dr Duncan Faulkner (JBA Consulting, UK), Professor Rob Lamb (JBA Trust and Lancaster University, UK), Dr Abdou Khouakhi (Cranfield University, UK) and Dr Jiabo Yin (Wuhan University, China).