How is it possible ? How could torrential rains have caused the death of at least 183 people, in Belgium but mainly in Germany (156 killed)? German Chancellor Angela Merkel is expected Sunday in the village of Schuld in Rhineland-Palatinate, described as «martyr» because almost everything was destroyed there by the floods. In this Land alone, the local police reported 110 deaths in a statement, against 98 in the previous report.
Four days after the devastating floods that hit the center of Europe, scientists, residents and journalists are groping their way in search of plausible explanations. If the impact of climate change on extreme weather events is (almost) consensus, other causes can explain such a heavy toll. Review (not exhaustive) of hypotheses.
Exceptional weather event
“Air masses, loaded with a lot of water, were blocked at altitude by low temperatures, which made them stagnate for four days in the region”, explains Jean Jouzel, climatologist, former vice-president of IPCC, the UN climate expert group. Meteorologists talk about “The cold drop” this presence of a mass of cold air which is fixed above a precise geographical area. It can then reinforce the power of a depression – responsible for the agitated weather – which would already be there.
“This cold air revolves around the pole with westerly winds, forming a kind of ring. But this circulation can be disturbed, it is not completely regular. There are like meanders. We can thus sometimes observe deformations, opening the way to the detachment of small pockets which will then descend towards our latitudes ”, specifies François Gourand, forecaster at Météo France, to West France.
Result: intense precipitation, between July 14 and 15, which reached “100 and 150 millimeters” that’s the equivalent of two months of rain, according to the World Meteorological Organization.
If the west of Germany is customary to heavy rains, they were “Exceptional, both for the quantity of water spilled and for their violence”, comments Kai Schröter, hydrologist at the University of Potsdam. In places, 148 liters of water per square meter were measured in 48 hours in an area of Germany where there are usually 80 liters for the entire month of July. At Cologne Stammheim Station, 154 mm of rain fell in 24 hours, erasing the city’s previous record of 95 mm.
In less than a month, the planet has recorded record temperatures, especially in western Canada but also in Spain and the Maghreb – a phenomenon that France could experience shortly – before this week’s flood in Northern Europe . “This summer is a strong signal, insists Jean Jouzel in La Croix. All these events should invite our political and economic decision-makers to take the warnings of scientists seriously. ”
Since Thursday and the first images of destruction, as if a tsunami had devastated certain villages, several European politicians have clearly made a link between climate change and bad weather. “These floods confirm what science says about global warming ”, thus affirmed Friday in Dublin the President of the European Commission, Ursula von der Leyen, two days after the presentation of an ambitious plan to reduce greenhouse gas emissions at the level of the Union.
In Germany, where the electoral campaign for the legislative elections in September is in full swing, the left and the ecologists also make this link and even the conservative candidate for Angela Merkel’s succession, Armin Laschet, asks – finally – for more resolute action against reheating. That he is president of the German region of North Rhine-Westphalia, the most affected by the disaster, owes nothing to chance. The German far right refutes this explanation and cries out “Instrumentalisation”.
What about the scientific debate? Can we link the deteriorating climate and the unleashed weather? “Even if we have to keep a certain nuance in the diagnosis, I think we can answer yes. We can no longer imagine a meteorological event that is not influenced by global warming ”, explain to Release Jean-Pascal van Ypersele, professor of climatology at the Catholic University of Louvain and former vice-president of the IPCC.
Kai Schröter, from the University of Potsdam, is more careful: “For the moment, we cannot say with certainty that this event is linked to climate change”, but such extreme phenomena become “More frequent and more likely” because of the warming. The rise in the temperature of the planet mechanically increases the evaporation of water from oceans and rivers, which brings “Larger water masses in the atmosphere”.
This phenomenon can increase the risk of intense and violent precipitation. More generally, extreme weather events are made more likely by global warming, according to the IPCC.
“With climate change, we predict that all extreme weather events will become even more extreme. What we have just seen in Germany sticks to this global trend ”, insists on The Guardian Carlo Buontempo, who heads the Global Warming Department of the European Center for Medium-Range Weather Forecasts (ECMWF).
Poorly protected watercourses
Why such a heavy balance sheet? The precipitation suddenly swelled many small rivers and tributaries of rivers, which did not have the capacity to withstand such a shock and whose banks were not sufficiently high. In places, the plains are under water. In Erftstadt, the pressure of precipitation and the Erft river caused a gigantic landslide.
“The Rhine is used to flooding, the biggest problem is the small rivers, the tributaries”, said the president of the German region of North Rhine-Westphalia, Armin Laschet, on Friday. “The rivers are slower and wider, the water rises less quickly and we have more time to prepare, unlike small rivers.», Confirms Kai Schröter.
Some German media and experts question the unpreparedness of the authorities, who would not have issued alerts early enough to the population. “Forecasters […] issued warnings, yet the warnings were not taken seriously and the preparations were insufficient “, says Hannah Cloke, professor of hydrology at the University of Reading in the UK.
In addition, a lack of awareness of these risks, upstream, among the population living in flood-prone areas is singled out. The water sometimes rose in a few minutes, trapping the inhabitants in the floors for the lucky ones.
“Some victims underestimated the danger and did not respect two basic rules during heavy rains. First: Avoid basements where water gets in. Second, turn off the electricity immediately ”, said Armin Schuster, president of the BBK, a public body specializing in natural disasters, on a daily basis Picture.
Some experts point the finger at the increasing concreteization of soils in the highly urbanized west of Germany, in the heart of the “Blue banana”, economic center of Europe. “The significant urbanization of these regions has played a role. Would the toll have been so heavy forty years ago? ”, asks Jean Jouzel.
The artificialisation of the land prevents water from infiltrating the soils, which no longer play a sponge role, which increases the risk of flooding. “If the surfaces affected are not natural and they have been waterproofed (roads, parking lots, etc.) or if we have drainage systems which have been designed for the flows that we knew a hundred years ago and not today’s flows during extreme rains, we have overflows like the ones we see now ”, confirms the former president of the IPCC, Jean-Pascal van Ypersele.