Global warming is driving big changes in floods across Europe by fueling the atmosphere with more moisture and changing the path and speed of rain storms, new research shows.

In some areas, that means more rainfall and surging rivers that could overwhelm levees if communities don't plan for increasing flooding. Other regions have seen a decline in rain and snow, which sets up a different challenge: as flood risk there decreases, it could discourage investments in defensive measures, leaving communities vulnerable to less frequent but still damaging extreme storms.

The study shows "clear flood risk patterns across Europe that match the projected impacts of climate change," said Günther Blöschl, lead author of the study, published Wednesday in the journal Nature, and director of the Centre for Water Resource Systems at the Vienna University of Technology.

Sign up for InsideClimate News Weekly Our stories. Your inbox. Every weekend.

To assess the continent's changing flood risks, a team of scientists from across Europe tracked the highest annual river flows at more than 3,700 stations over 50 years, from 1960 to 2010. In some of the local hotspots in northwestern Europe, they found the flows had increased by nearly 18 percent every decade. In other parts of Europe, flows had declined up to 23 percent per decade.

"For each year, we picked the maximum discharge and looked at how these annual peaks change over time," Blöschl said. He said the study is the first to clearly show regional patterns of flood magnitude across Europe driven by global warming.

How Is Global Warming Involved?

To understand how global warming affects floods differently in different areas, it's necessary to think about climate on a continental scale.

For Europe, the areas seeing the biggest increases in the magnitude of flooding are in the north and northwest. Scotland, coastal France and parts of Norway are hotspots, Blöschl said.

Floods are increasing in northwestern Europe because global warming is increasing moisture in the atmosphere, making storms wetter, and shifting the track of incoming storms northward, bringing more rainfall to the region. The storms are also moving slower, so they drop more rain over river catchments, he said.

From Iceland to the Alps, the study found that river flooding had increased regionally by 11.4 percent per decade, with increases of up to 17.8 percent in some areas.

The northward shift of the storm tracks also reflects an expansion of semi-arid subtropical zones in the south, the study found. As a result, precipitation has declined in southern and eastern Europe, and warming temperatures have also increased the evaporation of water from the soil there. But while flooding overall is down in those regions, they are still at risk from flash floods in smaller river basins as extreme rainfall events increase.

"We know the mechanism. A warmer atmosphere holds more moisture. Rainfall is increasing, so there's more water for the floods," Blöschl said.

"Also, the soils are wetter so they can't take up water, but this is not the most relevant factor," he said. "More relevant is that the storm tracks are farther north than they were in the past, which means the Mediterranean gets less, the northwestern regions get more rain."

Regional Details Can Help Communities Prepare

Globally, river flooding averages about $100 billion in damages a year, and understanding how global warming affects the threat is critical for at-risk communities.

Earlier research had tracked seasonal changes in Europe's flood risk, finding that spring flooding was starting earlier, but there had been little agreement as to how global warming affected the magnitude of floods on a regional scale.

Blöschl said that's because most studies looked at data from just a few hundred sites, not enough to show clear regional trends. The new study looked at about 3,700 flood-measuring stations.

Flooding from heavy rain in 2018 caused a bridge in Villegailhenc, France, to collapse into the Trapel River. Credit: Eric Cabanis/AFP/Getty Images

There's plenty of scientific evidence showing more frequent flooding, but less research has been done on flood magnitude, particularly at the regional level, so the new research will help identify growing flood risks driven by global warming, said Kristy Dahl, a climate researcher with the Union of Concerned Scientists USA. One earlier study had looked at the frequency and magnitude of the largest flood events on a broader scale and found they had increased by about 9 percent in Europe overall and about 8 percent in the United States starting in 1980.

Local officials could use local and regional data, like that in the new study, to make better decisions for their communities, said Sven Willner, who studies floods at the Potsdam Institute for Climate Impact Research.

"This new study makes the point that flood risks are already changing and have already changed, due to climate change," Willner said. "Municipalities have to find their own way to become more resilient to the increased risks they face."

Regional cooperation is also important, in part because flood control measures in one area can create new flood risks downstream.

Frequency of Severe Flooding Is Also Changing

The study's results show how both the extent and timing of severe floods in Europe is changing.

"While past studies have focused on a few catchments or were clustered around western Europe, this study provides a continental perspective, which allows for an analysis of climate processes that manifest themselves at larger scales. Isolated local or national scale studies, however, are broadly, consistent with our findings," the authors wrote.

The data show that, in northwestern Europe, 100-year floods—those with about a 1 percent chance of happening in any given year and often used as a benchmark in flood risk management—are now occurring more frequently, putting them closer to 50- to 80-year floods. But in much of eastern Europe, 100-year floods are happening less frequently, the study found.

The study doesn't make specific projections for the future, but "it implies that what we have seen in the past may continue into the future, and what we've seen is well accounted for in climate models," Blöschl said. He hopes the findings will encourage better flood preparation and planning.

"There is a saying: 'After a flood is before a flood,'" he said. "It's wise to build levees before a flood, but it never happens. Public spending only starts if there is visible damage."

Published Aug. 28, 2019