So far, research on the effects of climate change on floods has mostly investigated how climate change affects water runoff and the extent of floods. A new study addresses the temporal component for the first time in a Europe-wide study.

Globally, more people are threatened by floods than by any other natural hazard. In Austria, too, we are aware of the devastating damage that a flood disaster can cause. These events require homeowners to hire water damage restoration services like the Temecula water damage restoration to get back to their homes. However, rivers do not stop at national borders and can sometimes flood large catchment areas across borders. Since floods can endanger many people, there are many efforts at the national and EU levels to protect the population from flooding. This is the primary objective of the EU Floods Directive 2007/60/EC. It stipulates that each EU member state must draw up and regularly update flood hazard and flood risk maps as well as flood management plans with protection and precautionary measures. This ensures that the negative impacts of floods on people, the environment, and the economy are minimized.

Many studies examine the increase in floods due to climate change

Since many major flood disasters have occurred in recent years and decades, the cause is often sought. Climate change is often used in this context. Many are wondering: Does climate change increase the likelihood of flood disasters?

Reliable statements are lacking, as there are hardly any long-term records of floods. In addition, many aspects influence whether and when floods occur: precipitation, snow, and soil moisture are decisive factors. These influencing variables overlap with human activities in watersheds, such as land use or river regulation. These additionally influence the occurrence of floods. Conclusions about more frequent or stronger flood events due to climate change are difficult to derive due to the complex relationships and therefore cannot be proven or disproved with certainty.

New approach: taking into account the timing of floods

An international research team led by Günter Blöschl, Professor of Engineering Hydrology, and Julia Hall from the Vienna University of Technology pursued a different approach. They did not investigate whether climate change affects the frequency and extent of floods, but how closely floods are linked to weather-related processes. “Analyzing the timing of the floods provides information about the direct influence of the climate,” says study author Hall.

Major international project

Previous studies examined the temporal shift of floods at the regional level. For the first time, Blöschl and his team evaluated the timing of flood events for the whole of Europe. The research team collected and analyzed over 4,000 data sets from measuring stations on rivers on soil moisture and the amount of snow. Data from 38 European countries between 1960 and 2010 were included in the study.

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Result: Surprisingly clear climate signal

Previous studies have found a temporal shift at the regional level due to earlier snowmelt. The result of the new study shows a clear pattern of a temporal shift of floods for the whole of Europe. The authors expected that, similar to the small-scale studies, there would be temporal shifts due to global warming in Europe. “What is surprising is that the large-scale relationships are visible at the European level,” says Hall.

Where do which changes occur?

The observed temporal shifts are clearly related to changes in the climate system: For example, earlier snowmelt due to higher temperatures in Scandinavia and the Baltic states in northeastern Europe leads to earlier floods in spring.

In the countries around the North Sea, on the other hand, floods occur later in winter due to later winter storms with heavy rainfall, which is probably influenced by changes in air pressure conditions over the Atlantic and the warming of the polar region.

In the western regions influenced by the Atlantic, from England to Portugal, the so-called maximum soil moisture is of greater relevance for flooding than extreme precipitation. In these regions, floods occur mainly in winter, when the soil is saturated and can no longer absorb water after long periods of rainfall. The examined time series showed a shift of the winter floods by about four to eight days forward.

On the northern Adriatic coast, floods occur later due to climate change. The reason for this is the large-scale influence of the Atlantic on the cyclones in the Mediterranean, which bring heavy rainfall with them. Changes in the Atlantic cause altered trajectories of the storms. As a result, the storms and thus also the floods in the Mediterranean occur later in winter.

Effects of the time shift

Over centuries, regions have adapted to the regular occurrence of floods. A further time lag on the environment and the economy will have certain effects. For example, if winter floods occur later, the drainage of the soil is delayed, making agricultural cultivation more difficult due to increased erosion and soil compaction. Furthermore, the water supply and the possibility of irrigation are affected if the spring flood occurs earlier than usual and water reservoirs can no longer be sufficiently replenished later.