Timing of floods shifts and reveals climate change impact

Timing of floods shifts and reveals climate change impact (photo: GFZ).

10.08.2017: Climate change is abstract, however, weather events like extreme rainfalls that cause floods are very real. How are they connected? An international multi-author study led by the University of Vienna and with the participation of the GFZ now demonstrates for the first time that an observable link between climate change and floods exists on continental scale. The results are now published in the scientific journal Science.

When a major flood event occurs the question comes up if it is caused by climate change. Climate is, however, a statistical parameter that describes trends in the occurrence of weather events over a time period of at least 30 years. Thus, single events cannot be explained by climate change. Only the observation of longer time spans can provide information on the connection between climate change and weather events.

A large international project led by main author Günter Blöschl, Vienna University, has now collected and analysed 50 years of data from over 4,000 hydrometric stations from 38 European countries. Bruno Merz, head of the GFZ section Hydrology, interpreted the results concerning the causes of the flood events for Germany and Europe.

According to the study, the occurrence of flood events has been shifted over the years. Depending on the cause of the flood events in some regions they now occur earlier in the year, in other regions later in the year. This is the link to climate change: the cause of changes in the timing of the annual flood season can be linked to changes in climate. The magnitude of an event, however, for example the high water level or runoff, are not necessarily sensitive characteristics for detecting the impact of climate change, as they do not only depend on the climate. Effects like soil sealing by increasing urbanisation, an intensified agriculture, and deforestation can mask climate change impacts.

Many causes, one trigger: climate change

For every region the scientists determined the timing of floods and the causes behind. A significant change in timing was identified over the past decades: In much of north-west Europe floods occur most frequently in March and therewith about one month earlier than back in the 1960th and 70th. This is caused by an earlier snow-melt. At the Atlantic Coast of western Europe climate change causes the maximum soil moisture levels being reached earlier in the year. The ground becomes saturated resulting in floods.

In Great Britain, and northern Germany, on the other hand, floods now tend to occur about two weeks later. The scientists assume that later winter storms are likely to be associated with a modified air pressure gradient between the equator and the pole, which may also reflect climate warming. A warming of the Atlantic results in higher water vapour in the atmosphere that is transported towards the Mediterranean where flood events then occur later in the season.

The change in timing of the floods throughout Europe is caused by several different processes. All these processes, however, can be traced back to climate change. (ak)

Original study: Blöschl, G. et al., 2017. Changing climate shifts timing of European floods. Science Vol. 357, Issue 6351, pp. 588-59011 Aug 2017. DOI: xx

>>Press release of the University of Vienna