Data from the last 500 years shows that river flooding has become more extreme and follows a completely different pattern in recent decades.
The findings are the result of a major international study involving 34 research groups led by Austrian flood expert Professor Günter Blöschl from the Vienna University of Technology.
The research provides clear evidence that river flooding in Europe over the last three decades has intensified, and global warming is to blame.
Compared to the past five centuries, floods are now larger in many places, the timing has shifted, and the relationship between flood occurrence and air temperatures has reversed.
“From our previous research, we already knew how climate change has affected Europe’s floods in the past 50 years,” said study co-author Alberto Viglione.
“For forecasts of the next decades, however, it is also important to understand whether this is a completely new situation or whether this is just a repetition of something that has already occurred.”
“So far, the available data had not been sufficient to ascertain whether this is the case or not. We have examined this question in great detail and can now say with confidence: Yes, flooding characteristics in recent decades are unlike those of the previous centuries.”
For the current study, researchers and historians collaborated to analyze tens of thousands of documents containing river flood reports from 1500 to 2016.
“The particular challenge of this study consisted in making the very different texts of the different centuries and different cultural regions comparable,” said study co-author Andrea Kiss.
“We managed to achieve this comparability by putting all the texts in their respective historical contexts with painstaking attention to detail.”
The team pinpointed nine flood-rich periods and found that historically, these events were triggered by cooler weather.
“This finding seems to contradict the observation that, in some areas such as in the northwest of Europe, the recent warmer climate is aligned with larger floods,” said Professor Blöschl. “Our study shows for the first time that the underlying mechanisms have changed: While, in the past, floods have occurred more frequently under colder conditions, the opposite is the case now. The hydrological conditions of the present are very different from those in the past.”
The seasonal timing of the floods has also changed. Central European floods have become much more frequent in the summer due to shifts in precipitation, evaporation, and snowmelt.
The study is the first of its kind to evaluate historical flood periods for an entire continent in such elaborate detail. The findings will help to distinguish the role of climate change from the contribution of other factors such as deforestation and river management.
Professor Blöschl recommends the use of flood-risk assessment tools that capture the physical processes involved, and management strategies that can incorporate the recent changes that were identified in the study.
“Regardless of the necessary efforts of climate change mitigation, we will see the effects of these changes in the coming decades,” said Professor Blöschl. “Flood management must adapt to these new realities.”
The research is published in the journal Nature.