The devastating weather that hit Germany and Belgium at the weekend left at least 191 dead. If scientists and meteorologists knew this could happen, the scale of the phenomenon surprised everyone.

How is it possible ? How could torrential rains have caused the deaths of at least 191 people, Belgium, but mainly in Germany (160 killed)? German Chancellor Angela Merkel was expected Sunday in the village of Schuld in Rhineland-Palatinate, described as a “martyr”, because almost everything was destroyed there by the floods. In this Land alone, the local police reported 112 deaths in a press release, against 98 in the previous report.

Four days later these devastating floods that have struck 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 the IPCC, the UN climate expert group. Meteorologists speak of “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, with from West France.

Result: intense precipitation, between July 14 and July 15, which reached “100 and 150 millimeters” or the equivalent of two months of rain, according to the World Meteorological Organization.

If the west of Germany is used to heavy rains, they were “exceptional, both in terms of the quantity of water discharged and 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, 154mm of rain fell in 24 hours, erasing the city’s previous record of 95mm.

In less than a month, the planet has recorded record temperatures, especially in western Canada, but also in Spain and the Maghreba phenomenon that France could experience shortly – before the flood this week in Northern Europe. “This summer is a strong signal, insists Jean Jouzel in The cross. All these events should invite our political and economic decision-makers to take the warnings of scientists seriously. “

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Global warming

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”, said the press in Dublin on Fridaydente from the European Commission, Ursula von der Leyen, two days after the presentation of an ambitious plan to reduce greenhouse gas emissions at EU level.

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 candconservative idat to the succession d’Angela Merkel, Armin Laschet, calls – finally – for more resolute action against global warming. That he was 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 for “instrumentalization”.

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, of the University of Potsdam, is more cautious: “At the moment, it cannot be said with certainty that this event is linked to climate change”, but such extreme events are becoming “more frequent and more likely” in cause of warming. The rise in the temperature of the planet mechanically increases the evaporation of water from oceans and rivers, which brings “larger masses of water into 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 fits in with this global trend ”,underline in The Guardian Carlo Good time, who heads the Global Warming Department of the European Center for Medium-Range Weather Forecasts (ECMWF).

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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 river Erft 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, Friday. “Rivers are slower and wider, the water rises less quickly and we have more time to prepare, unlike small rivers,” confirms Kai Schröter.

Unpreparedness?

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 preparations were insufficient, ”said 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 pointed 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, cut off the electricity immediately, ”said Armin Schuster, president of the BBK, a public body specializing in natural disasters, on a daily basis. picture.

Town planning

Some experts point to the increasing concreteization of soils in the highly urbanized west of Germany, at the heart of the “blue banana”, the 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 artificialization of land prevents water from seeping into the soil, which no longer acts as a sponge, 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 those we see now, ”confirms Jean-Pascal van Ypersele, former president of the IPCC.

With Agence France-Presse

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