Rebecca Zisser / Axios

A massive survey of European waters has found that across the continent, climate change has caused flood times to shift — sometimes by several weeks.

Why it matters: Major shifts in flooding could have big impacts for local water management. "It opens up a lot of questions about how these changes in flood timing could interact with other kinds of processes that impact people," Anna Michalak, an earth scientist at the Carnegie Institution for Science in Stanford who was not involved in the study tells Axios. "Societies and ecosystems have adapted to the average within-year timing of floods. We're used to them coming at a certain time of year, and our land management practices have evolved to respond to that."

Farmers wait until after flood seasons to fertilize their fields, if they can, and reservoirs are drained in anticipation of heavy rainfall. Flooding can also make the soil too soft for farmers to till, and delay or change planting seasons.

What they did: The team created a massive database of European floods over a period of 50 years, including data from 4262 stations in 38 countries, and took the date of the highest peak flood of each year. They looked for potential flood causes by cross-referencing that data with three different measures: the 7-day maximum precipitation (to control for storms), the middle day of the month with the highest soil water saturation, and the middle day of the first week of the year above freezing (as an indicator of snowmelt). The research was conducted across a consortium of European institutes and universities and published Thursday in Science.

"There hasn't really been a definitive study of this magnitude that's shown a connection between floods and climate before," Donald Burn of the University of Waterloo in Canada tells Axios.

The findings:

  • In the North Sea and the Mediterranean, changes to storm patterns mean winter floods are coming earlier than before.
  • In Western Europe, increasing autumn rains are saturating soil, so even though peak rainfall is happening later, underground aquifers are overflowing earlier. Because these results are varied and flood magnitude is influenced by land use, it's hard to tell what the impacts of these changes will be, say the authors.
  • But in Northeastern Europe, warmer winters mean earlier snowmelt-caused floods and less snowpack. The arctic and subarctic ecosystems of the region have evolved to rely on these floods, and scientists are very concerned about what this could mean for the environment, says Berit Arheimer, an author on the study from the Swedish Meteorological and Hydrological Institute.
"This study shows climate change is not some abstract thing happening in the future, it's having an impact now," says Michalak.

Take note: This study does not show if the magnitude of floods is increasing or decreasing — that's very difficult to do, in part because flooding is so strongly influenced by things like land use. The authors hope future research will parse out what changes in magnitude were due to climate change instead of other factors, to get an even better idea of how human-caused changes are impacting Europe's watersheds.

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