Climate Change Wreaking Havoc on Europe’s Flood Season

Flooding in August 2005 in Kappl-Nederle (Paznaun Valley, Tyrol), Austria.
(Photo: TU Wien/ASI/Land Tirol/BH Landeck)

(CN) – New large-scale research shows that global warming has shifted the timing of floods across Europe, highlighting the current and ongoing effects of climate change.

An international team of scientists collected and analyzed 50 years of data from more than 4,000 hydrometric stations across 38 European nations. The data revealed climate change has a significant impact on flood events in several regions.

The findings, published Thursday in the journal Science, show that the changing climate has caused flood events to occur earlier in some regions, and later in others.

“The timing of a flood provides information about its likely cause,” said lead author Guenter Bloeschl, a professor at the Vienna University of Technology.

For instance, floods occur more frequently in the winter in northwest Europe and the Mediterranean, when evaporation is low and precipitation is elevated. In contrast, the highest magnitude floods in Austria result from summer rains.

Northeastern Europe faces the greatest risk of floods in spring due to snow melt – the exact timing of which is more closely associated with the climate, rather than the absolute magnitude of the event.

The volume of the data reviewed, along with a focus on timing, allowed the team to determine the impact of climate change on flood events. Measuring the effects of global warming on these events can be challenging when only using essential aspects of flood risk management – probabilities and magnitudes.

“If one only examines the magnitude of flood events, the role of the climate can be masked by other effects,” Bloeschl said. Those effects include land-use changes stemming from urbanization, increased agricultural production and deforestation.

Average time of floodings over the year: winter floodings – blue arrows (up), summer floodings – red arrows (down). Credit: TU Wien

The data show that the timing of floods in Europe has shifted drastically over the past 50 years.

“In the northeast of Europe, Sweden, Finland and the Baltic states, floods now tend to occur one month earlier than in the 1960s and 1970s. At that time, they typically occurred in April, today in March,” Bloeschl said. “This is because the snow melts earlier in the year than before, as a result of a warming climate.”

On the other hand, some regions of Europe now experience flooding later than they did a couple of decades ago, such as in parts of northern Britain, western Ireland, northern Germany and coastal Scandinavia. Later winter storms are likely connected to a modified air pressure gradient between the pole and the equator, which may also stem from a warming climate, according to the team.

The research also points out the complexity of flood processes in northwestern Europe. For example, the Atlantic coasts of Western Europe experience “winter” floods in autumn as maximum soil moisture levels are now reached earlier in the year.

“The timing of the floods throughout Europe over many years gives us a very sensitive tool for deciphering the causes of floods,” Bloeschl said. “We are thus able to identify connections that previously were purely speculative.”

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