This is the first time that changes to flooding have been linked to climate change on such a large geographic scale. The findings ranged from a shift of 13 days per decade earlier, to 9 days per decade later. That corresponds to total shifts forward in time by 65 days over the 50 year period or later by 45 days.
"Previous studies gave somewhat confusing results because floods are not only driven by rainfall, but also by soil moisture and snow melt," said Günter Blöschl of the Technische Universität Wien, Austria. "Because floods are also affected by land-use change, such as deforestation, and by river training, it is therefore difficult to isolate climate change effects...if one looks at the magnitude of the floods, which is what researchers have done in the past."
Climate change is projected to change the magnitude, frequency and timing of floods by intensifying the water cycle.
As Blöschl explained, several earlier studies that looked at the magnitude of floods did not find trends. By examining shifts in flood timing, he and colleagues discovered that northeastern Europe is now experiencing earlier spring snowmelt floods as a result of warmer temperatures. A region around the North Sea, meanwhile, is seeing later winter floods, which may be linked to polar warming delaying winter storms.
"In northwestern Europe and in the Mediterranean, floods usually occur in winter when evaporation is low and precipitation is intense," said Blöschl. "In central Europe the biggest floods occur in the summer due to summer storms. And in northeastern Europe, floods occur in spring because of snow melt."
Floods in Western Europe now tend to be earlier in winter because peak soil moisture levels are occurring earlier, the analysis also indicated. Here, soil stores autumn rainfall until it reaches saturation; then sustained winter rainfall – rather than extreme precipitation – may lead to flooding.
Blöschl and colleagues examined more than 200,000 records of river discharge or water level from more than 4000 measurement stations in 38 European countries between 1960 and 2010.
The largest changes were in western Europe along the North Atlantic coast from Portugal to England, where half of stations showed a shift towards earlier floods of at least 15 days over the 50 years. And the most consistent changes were in northeastern Europe, with 81% of stations showing a shift towards earlier floods.
The link between climate change and shifts in flooding seasonality was clearest in northeastern Europe, where warming temperatures are typically melting snow earlier in the year. In western Europe, the picture is more complex and natural variability in climate may also be playing a role.
"If the trends in flood timing continue, considerable economic and environmental consequences may arise, because societies and ecosystems have adapted to the average within-year timing of floods," wrote the researchers in Science. "Later winter floods in catchments around the North Sea, for example, would lead to softer ground for spring farming operations, higher soil compaction, enhanced erosion, and direct crop damage, thereby reducing agricultural productivity." And earlier spring floods in the northeast could limit the replenishment of reservoirs, reducing water supply, irrigation and hydropower, if managers expect later floods that don't arrive.
One of the next steps, according to Blöschl, is to model these changes in flooding seasonality. "In order to predict or project floods into the next decades, we need mathematical models," he said. "In the past we have tested these mathematical models only on the basis of the floods' magnitudes."
Next up is looking at changes in flood magnitude shown by the data. "But this is more complicated because we need to account also for the changes of land use and for the changes of river training and of hydraulic structures in the river network," Blöschl explained.
The team reported the findings in Science.