River floods are among the costliest natural disasters worldwide, causing annual damage of more than $100 billion. They affect millions of people each year because many towns and cities are built along rivers.
Examining flood data across 50 years, researchers found significant shifts in timing along the Atlantic coast of Western Europe from 1960 to 2010.
According to a paper published in the journal Science, half of the measurement stations from England to Portugal showed floods were occurring on average at least 15 days earlier by 2010 compared with a half century earlier.
In northeastern Europe, earlier snowmelts also brought river floods forward by at least eight days over the 50 years, while areas around the North Sea are now seeing floods happen more than a week later than in 1960.
'If the trends in flood timing continue, considerable economic and environmental consequences may arise, because societies and ecosystems have adapted' to the average timing of floods, the authors concluded.
Farming, water provision
They cited possible harm to farming around the North Sea from later winter floods that leave the ground softer going into spring. Water utility companies in northeastern Europe may need to begin filling reservoirs with the earlier water surges rather than waiting for later flooding to ensure sufficient supply for hydropower plants and irrigation, they said.
The study's authors, led by Guenter Bloeschl of Vienna's Technical University, cautioned that the precise mechanism by which flood patterns change is complex and still needs to be fully understood.
While data on the timing of floods showed a clearer link with climate change than past studies that looked into flood severity, the researchers noted that several factors affect the timing of floods - including the amount of rainfall, the nature of the soil, upriver snowmelt and land use. Not all the shifts are necessarily caused by man-made global warming, they said.
Bloeschl said the researchers would try to use their findings to predict future changes in the timing of seasonal floods.
'We really expect these trends to increase,' he said.