Climate change shifts timing of European floods

News   •   Aug 11, 2017 14:26 BST

When a major flood event occurs it is often attributed to climate change, however, a single event is not proof, and so far it has been unclear whether climate change has a direct influence on large scale river flooding across Europe. A study conducted by TU Wien along with 30 European partners has now shown that the timing of the floods has shifted across much of Europe.

The study, led by Prof. Guenter Bloeschl from the Institute of Hydraulic Engineering and Water Resources Management at TU Wien, showed that climate change has had a real impact on flood events in some regions, and this has been seen by a shift in the timing of floods over the years, dramatically in some areas. Depending on the cause of the flood events, they occur earlier in some regions, and later in others.

"In flood research, we are often concerned with the annual probability of the occurrence of floods," says Prof Guenter Bloeschl from TU Wien. "By observing their magnitudes one can estimate a one hundred-year flood as a high-water event that occurs with a probability of 1% in any one year. If one only examines the magnitude of flood events, the role of the climate can be masked by other effects. Land use change by urbanisation, intensifying agriculture and deforestations are other factors affecting flood events."

In order to understand the connection between climate and floods, Bloeschl and his team looked closely at the timing of the flood events in different regions of Europe. "The timing of a flood provides information about its likely cause," says Bloeschl. For example, in much of north-west Europe and the Mediterranean, floods occur more frequently in the winter, when evaporation is low and precipitation is intense. In Austria, on the other hand, the highest magnitude floods are associated with summer downpours. In north-eastern Europe, the risk of flooding is at its highest in spring because of snow melt. The timing at which floods occur is thus much more directly related to the climate, in contrast with the absolute magnitude of the flood event.

Flood data from all over Europe have been meticulously compiled, screened and statistically analysed. These show that the floods in Europe have indeed shifted considerably over the last 50 years: "In the north-east of Europe, Sweden, Finland and the Baltic States, floods now tend to occur one month earlier than in the 1960s and 1970s. At that time, they typically occurred in April, today in March," says Guenter Bloeschl. "This is because the snow melts earlier in the year than before, as a result of a warming climate."

In parts of northern Britain, western Ireland, coastal Scandinavia and northern Germany, on the other hand, floods now tend to occur about two weeks later than they did a couple of decades ago. 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. The study sheds light on the complexity of flood processes in north-western Europe; on the Atlantic coasts of western Europe, 'winter' floods in fact typically occur earlier, in the autumn, as maximum soil moisture levels are now reached earlier in the year. In parts of the Mediterranean coast, flood events occurring later in the season are aligned with the warming of the Mediterranean.

"The timing of the floods throughout Europe over many years gives us a very sensitive tool for deciphering the causes of floods," says Guenter Bloeschl. "We are thus able to identify connections that previously were purely speculative."

Adverse weather, which can lead to the conditions that can cause flooding, featured fifth in the list of concerns that business continuity professionals have, as identified in the Business Continuity Institute's latest Horizon Scan Report. Climate change is not yet considered an issue however, as only 23% of respondents to a global survey considered it necessary to evaluate climate change for its business continuity implications.