“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 river floods at large scales in Europe. Thanks to the new database we can show that climate has changed the floods and there is a clear pattern across Europe. The timing of high flows has changed over the years. Depending on the cause of the flood events, they occur earlier in some regions and later in others”, explains Berit Arheimer, researcher at SMHI who participated in the study.
SMHI contributed with 30 other hydrological institutes to the large international project, which collected and analysed 50 years of data from over 4,000 hydrometric stations from 38 European countries. . This is an unprecedented dataset in terms of coverage across Europe and the sheer number and diversity of river systems that have been included. The project was led by Prof. Günter Blöschl from the Vienna University of Technology and the results have now been published in the “Science” journal.
The magnitude does not tell you everything
"In flood research, we are often concerned with the annual probability of the occurrence of floods," says Prof. Günter Blöschl from the Vienna University of Technology in a Press Release. "By observing their magnitudes one can estimate a one hundred-year flood as a high-water event that occurs with a probability of one percent in any one year."
However, while probabilities and magnitudes are an essential aspect of flood risk management, they are not necessarily the most sensitive characteristics for detecting the impact of climate change, as they do not only depend on the climate: "If one only examines the magnitude of flood events, the role of the climate can be masked by other effects," explains Günter Blöschl. "Land use change by urbanisation, intensifying agriculture and deforestations are other factors affecting flood events."
The timing provides information on the influence of the climate
In order to understand the connection between climate and floods, Blöschl 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 Blöschl.
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 central Europe, 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. Dr Berit Arheimer, head of Hydrological Research at SMHI, participated in the working group and explains that the results confirm the findings from previous studies in Sweden.
"We know from previous analysis of Swedish observations that the flood in spring now tends to occur one month earlier than in the 1960s and 1970s. This is because the snow melts earlier in a year than before, as a result of a warming climate. With this new European study we can see that this tendency is valid also for large parts of north-east of Europe, including Finland and the Baltic States”, says Berit Arheimer, scientist at SMHI.
Useful monitoring programmes
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 of long-term monitoring 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.
"Data with timing of the floods throughout Europe over many years gives us a very sensitive tool for deciphering the causes of floods," says Günter Blöschl. "We are thus able to identify connections that previously were purely speculative."
The advances in flood research have been made possibly by persistent and long-term monitoring by the national institutes, and an ERC Advanced Grant awarded to Günter Blöschl in 2012, which allowed him to establish the database through numerous international cooperations across Europe and then closely examine the connection between climate and floods.