Climate change is modifying the magnitude of floods in Europe

An international study in which the Polytechnic University of Madrid (UPM) has participated has shown, for the first time, that climate change is causing floods to reach greater magnitude in Europe. It is estimated that worldwide the annual flood damage is more than 90 billion euros. The forecasts are that this figure will continue to increase due to economic growth, urbanization processes and climate change.

Until now, the influence of climate change on floods has not been clear as no coherent trends had been found at the global level. However, an international study led by the Technical University of Vienna, in which 35 European research groups have participated, including the Polytechnic University of Madrid (UPM), has found that the changes in flood magnitudes observed in recent decades can be clearly attributed to climate change.

The effect on flooding is not the same in all regions of Europe: floods are becoming more severe in northwestern Europe, while they are milder in southern and eastern Europe. The results have been published in the prestigious journal Nature.

Previous research by the same group, published in the journal Science in 2017, found that climate change is changing the timing of occurrence of floods— that is, at what time of year they occur. Now, researchers have wondered if climate change also affects the magnitude of flooding.

According to Luis Mediero, member of the research team and professor at the ETSI of Caminos Canales y Puertos of the UPM, “previously, it had not been possible to answer this question on a continental scale in Europe. However, we can now answer with confidence that yes, indeed the impact of climate change on the magnitude of floods is clearly visible at European level.”

A thorough search

To carry out the study, data recorded in 3,738 flow measurement stations in Europe, from 1960 to 2010, collected in the European Flood Database, were used. The study of these data revealed different results depending on the region of Europe considered. In central and northwestern Europe, from Iceland to Austria to the Czech Republic, flooding has increased in recent decades as a result of an increase in the magnitude of rainfall and the fact that soils are wetter at the beginning of storms.

However, in southern Europe, from the Iberian Peninsula to Turkey, flooding tends to decrease, as climate change has led to a reduction in the magnitude of rainfall, along with drier soils due to rising temperatures, which cause greater evaporation of water into the soil. In Eastern Europe, from Poland to western Russia, with a more continental climate, floods are also decreasing, due to the reduction in the depth of snow layers in winter as a result of rising temperatures.

“There are consistent flood change patterns across Europe and they are in line with large-scale climate change forecasts. This indicates that we are already in the midst of climate change,” says Günter Blöschl, head of the research team.

Changes in Spain

In most of Spain, a decrease in the magnitude of flooding has been observed in recent decades. Climate change is causing the expansion of the Hadley cell (a closed circulation cell of the Earth’s atmosphere that dominates the global atmospheric circulation in equatorial and tropical latitudes) and now the upper edge of this cell rises in latitude. As a consequence, the subtropical jet stream of the northern hemisphere is also ascending in latitude, modifying the trajectories of the storms that normally cause flooding in the Iberian Peninsula.

In a previous nationwide study, conducted by the Polytechnic University of Madrid, the reduction in the magnitude of floods in Spain was also attributed to the increase in evapo-transpiration rates due to the increase in temperatures, causing soils to be drier at the beginning of storms.

“These changes occur in rivers with medium and large slope basins, since the database used does not include the smallest basins of a few square kilometers, which are not normally monitored,” explains Luis Mediero. “In these smaller basins,” he continues, “the floods are caused by convective storms of short duration and great intensity, such as those that have caused the floods in Madrid this week. With climate change these storms are expected to be more severe, as as temperatures rise, the air is able to store a greater amount of water vapor. Therefore, an increase in the magnitude of pluvial floods in urban and river areas in ravines and wadis is expected.”

Help for better management

The results of the work show that the size of the observed changes in the magnitude of floods is remarkable: they range from a decrease of 23% per decade to an increase of 11% per decade, compared to their long-term average value. If these trends continue in the future, significant effects on flood risks in many regions of Europe are to be expected.

However, such trends cannot be easily extrapolated into the immediate future, for the coming years. “Atmospheric conditions are not stable in time, that is, every year it does not rain the same, nor on the same date. Therefore, the variability of climate and extreme weather events can cause oscillations of wetter periods with floods of greater magnitude, followed by drier periods with milder floods, “says Luis Mediero.

In any case, in the opinion of the research team, these new findings should be included in flood management strategies. “Regardless of efforts to mitigate climate change, the effects of these changes will be seen in the coming decades, therefore flood management must adapt to these new realities,” they say.

Source: Renewable Energies