Climate Change Is Altering Flood Timing in Europe

Climate-related shifts in European river flooding from 1960 to 2010. | Carla Schaffer/ AAAS

Climate change is altering the timing of river flooding across Europe, according to a new study in the August 11 issue of Science. The data, collected over a 50-year period, reveal that rivers are flooding earlier in the spring in the western and northeastern regions of the continent and later in the season around the North Sea and some areas of the Mediterranean coast.

River flooding affects more people worldwide than any other natural hazard, with an estimated global annual average loss of $104 billion. Günter Blöschl of Vienna University of Technology, an author of the study, notes that flooding involves more than just monetary costs. "River floods also pose a hazard to the lives of people. We do have hundreds, if not thousands, of fatalities around the world due to river floods every year. The number of fatalities strongly depends on how well prepared a region is."

Knowledge of local flooding patterns offers communities some degree of preparedness. Climate change has great potential to alter flooding patterns, but the possible extent of these changes has not been quantified until now.

"There are different processes through which climate change may change floods," explained Blöschl. "Rainfall may become heavier, the soils may become wetter, due to more frequent storms, and snowmelt may become more intense."

To better understand changing patterns in river flooding, Blöschl and colleagues analyzed data collected from more than 4,200 hydrometric stations in 38 European countries between 1960 and 2010. The large-scale project, called FloodChange, was funded by the European Research Council.

The researchers looked at changes in flood timing in particular because it is directly linked to climate change. Other features of flooding tend to be less ideal to study for understanding the impacts of climate change because additional factors can influence the outcome. For example, flood magnitude can be affected by factors beyond climate change, such as changes in land use from urbanization, intensifying agriculture and deforestations.

The data reveal that the most substantial changes occurred in western Europe along the North Atlantic coast from Portugal to England. Here, about half of the stations recorded a shift toward earlier floods by at least 15 days over the 50-year period. The authors attribute this shift largely to the nature of soil (in particular, its ability to retain moisture) in this region.

Nearly all stations in northeastern Europe recorded a shift to earlier flooding as well, with about half recording a shift that exceeds eight days; these changes are largely due to increases in snowmelt that occur in a warmer climate. In speculating how this might impact the region, Blöschl said, "Spring floods shifting to earlier in the season in northeastern Europe may leave reservoirs half empty, if managers expect later floods that never arrive, with adverse consequences for hydropower production."

Around the North Sea, about half of the stations recorded a shift toward later flooding by more than eight days, which the authors attribute to extreme precipitation occurring during the winter and changes in the North Atlantic Oscillation, a major weather pattern affecting winds and storm tracks in the North Atlantic region. These changes could impact ecosystems and agriculture, Blöschl suggested.

"Later winter floods in Scotland and Ireland may delay the migration of salmon and hence jeopardize their sustainability. Later winter or spring floods in catchments around the North Sea may reduce agricultural productivity due to softer soils and enhanced erosion in spring," he said.

Based on these results, Blöschl emphasized the need for management strategies to be reconsidered, in order to optimize resources and minimize potential damage from "off-season" events. His team plans to take advantage of the unique flood data set they have compiled and analyze it further in terms of changes in flood frequency and magnitudes.

[Credit for related image: ASI / Land Tirol / BH Landeck]