Across Europe, rivers aren’t flooding when they used to.
Long-term changes in temperature and precipitation are making some rivers flood days, weeks or even months earlier than they did 50 years ago, and pushing flooding in other areas much later, researchers report August 11 in Science. Those changes could impact people, wildlife and farms near rivers.
Previous studies have shown that climate change is likely to increase the severity and frequency of coastal floods, but it can be tricky to concretely link river flooding to climate change, says Günter Blöschl, a hydrologist at the Vienna University of Technology who led the study.
Coastal flooding is worsened largely by one overriding variable that can be tracked: sea level rise. But river flooding is affected by a complex set of factors, says Rob Moore, a policy analyst at the Natural Resources Defense Council in Chicago who specializes in water issues. Both the timing and quantity of precipitation matter, as does the type of soil and whether it’s dry or already waterlogged when rain hits. What’s more, changes in land use around a river or engineering projects such as dams that change river flow can also affect flood risk — but aren’t necessarily related to the climate.
Many rivers across Europe now typically flood at different times in the year than they did 50 years ago. The shift is as small as a few days in some areas and as big as several months in others. This map shows those changes, which vary widely across Europe. In red regions, floods are happening earlier; blue regions are flooding later.
So instead of tracking the size or frequency of river floods, the researchers examined the seasonal timing of those floods. That measurement is less impacted by factors that have nothing to do with climate. Blöschl worked with researchers from 38 countries to analyze hydrological data collected at 4,262 sites across Europe from 1960 to 2010.
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Flood season shifted as much as 13 days earlier or nine days later per decade, the researchers found. Over the entire study period, that shift added up to floods in some regions occurring, in the most extreme cases, as much as 65 days earlier or 45 days later. The biggest changes were in Western Europe, where a quarter of the monitoring sites recorded flood timing shifts of more than 36 days over the 50-year period. Elsewhere, effects were more moderate, though still measurable: In northeastern Europe and the area around the North Sea, for instance, more than half of the stations showed shifts of more than 8 days.
The effect varies substantially by region because not all parts of Europe experience the same sorts of floods, says Blöschl. In southern Sweden and the Baltics, floods are mostly driven by snowmelt. Warmer local temperatures make the snow melt earlier in the spring, shifting flood season up, too. In southern England, on the other hand, heavy autumn rains saturate the soil, and subsequent winter deluges can cause flooding. Flood season there is driven by when the soil gets too waterlogged to take in more moisture.
The study shows that flood timing has changed, but does not address specific consequences. It’s clear, though, that off-season flooding could have far-reaching effects, especially if these trends continue. Animals that rely on river conditions at a certain time of year in order to breed or find food could be affected by surprise floods. Out-of-season floods or unexpected dry spells could damage crops.
Plus, people are less prepared when big floods happen off-season, says Moore. While a comprehensive study like this one hasn’t been done in the United States, floods are occurring at unusual times here, too, he notes. Moore cites devastating floods that swelled the upper Mississippi River to a record size in December 2015 — not the time of year when the river is expected to overflow its banks. That flooding, combined with tornadoes spurred by the same storm system, killed more than 50 people and caused almost $2 billion in damage.