Climate change has made heavy rains in Europe more likely

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Scientists have shown that the deadly floods that devastated northern Europe in July would have been less likely in a world without climate change.

Global warming has made heavy summer precipitation between 3% and 19% heavier, and 1.2 to nine times more likely, according to a study released Tuesday by an international group of climatologists from the World Weather Attribution (WWA). It’s a find that could play a critical role in Germany – the fourth-largest polluter in history and the country with the highest number of flood deaths – ahead of the September elections.

“These floods have shown us that even developed countries are not sure,” Friederike Otto, climatologist at the Environmental Change Institute at the University of Oxford and co-author of the study, said in a press release. .

The floods, which killed more than 220 people taking away buildings and livelihoods, were the result of heavy rains between July 12 and 15. They followed three weeks of hot and humid weather that had left the soils of two German states almost saturated.

Unable to absorb such large amounts of water in a short period of time, the ground quickly flooded when a record 93 mm (3.7 inches) of rain fell in a single day around the German rivers Ahr and Erft, and 106 mm fell over two days in the Belgian Meuse. Rivers overflowed, breaking bridges and cutting roads. People asleep in their homes – many of whom were warned late by authorities – had little time to evacuate.

Rain attribution

When weather conditions strike like this, WWA researchers run computer models fueled by historical data to quantify the role climate change plays in a disaster – working against the clock before journalists, policymakers and the public do. ‘forget. Scientists publish their results before submitting their research for peer review so they can draw conclusions in weeks, instead of waiting months or years.

The study found that climate change made rains heavier and more likely. At the lower end of the study range, this made rains at least 20% more likely and 3% heavier. At the high end, that made them nine times more likely and 19% stronger.

The level of rain was so rare and fell over such a small area that it is “extremely difficult” to attribute such a specific event to climate change, said Matthias Mengel, a scientist at the Potsdam Institute for Research on climate impact (PIK), which did not participate in the study. Instead, WWA scientists broadened their analysis to include data from all over Western Europe. “At the most general level, the study is conclusive: global warming has increased the risk and force of such events,” Mengel said.

Beneath the complex climate changes that make precipitation hard to attribute lies a simple physics principle: warmer air holds more moisture. The Earth has already warmed by at least 1.1 degrees Celsius (2 degrees Fahrenheit) since the end of the 19th century – Germany by nearly 2 ° C – and scientists predict that the heavy rains will continue to rise. increase as the planet warms.

“We thought we knew the situation”

One of the areas most affected by the rains was Oberbergischer Kreis, near the city of Cologne, in western Germany, which received 150mm of rain in 72 hours.

“We thought we knew about the situation because we have light flooding every now and then,” said Christian Kettler, managing director of SN Maschinenbau, a packaging machinery exporting company that was hit when the Wupper River overflowed. “What was special this time was the speed, the suddenness and the quantity [of rain] It came. “

Scientists at the government-funded Climate Service Center Germany research group project that if emissions rise so high that they heat the planet by 4 to 5 ° C, the number of days with more than 20 mm of precipitation will increase in the world. ‘Oberbergischer Kreis from nearly 11 a year to 14. This is a larger increase than in any other of Germany’s 401 districts except one.

In the neighboring district of Ahrweiler, prosecutors filed a lawsuit against local authorities for negligent homicide after the drowning of 12 disabled people in a residential house in the town of Sinzig. “They couldn’t get out, they were trapped inside,” said Jean-Marie Dumaine, who runs a nearby restaurant for which the residents of the house had grown herbs. The WWA study highlights that people with disabilities are among the most vulnerable to flooding.

The Ahr River had become dangerous, like a big boa constrictor, Dumaine said. “Like a snake that had eaten and whose stomach contained a bomb of water. A tsunami.”

No more extreme extremes

In recent years, WWA studies have confirmed links between climate change and several extreme events around the world. More recently, they established that the deadly heat wave in Canada and the United States in June was 150 times more likely and about 2 ° C hotter than it would have been in a world without fossil fuels. Before that, they had shown that climate change had exacerbated Australian forest fires in 2019 and 2020, European heatwaves in 2018 and 2019 and Tropical Storm Imelda, which hit the United States in 2019.

Not all attribution studies find a link. Sometimes this is because there is not enough data to detect the influence of climate change amid natural variations. But other times, it’s because warmer temperatures alter the climate in complex ways that simultaneously increase and decrease the likelihood of extremes. The bottom line sometimes defies global trends.

A study published in the journal Bulletin of the American Meteorological Society in 2014, when the science of attribution was just beginning, did not find a climatic link when heavy summer rains resulted in flooding on the Danube and Elbe in Germany the previous year.

But since then, research has come a long way. A landmark report released by the Intergovernmental Panel on Climate Change (IPCC) in August found that while the world is warming 4 degrees Celsius above pre-industrial temperatures, the heavy rains that once hit once in a while. decade will strike almost three times as often and carry 30% more rain. Western and central Europe will be exposed to increasingly extreme rainfall and flooding.

“[The flooding in Europe] demonstrates once again in 2021 that far-breaking extremes, exacerbated by climate change, can strike anywhere, cause enormous damage and cause deaths, “said Frank Kreienkamp of the German Meteorological Service and co-author of the WWA study in a press statement.

World leaders pledged in 2015 to limit global warming to ideally 1.5 ° C by the end of the century in order to prevent the climate from becoming more chaotic and less hospitable to human life. But they’re currently pursuing policies that put him on track for 3C.

As the death toll from extreme weather events has declined for decades – as increasingly accurate computer models have allowed scientists to predict bad weather before it happens – the number of people displaced by such disasters increased. Much of the change has occurred in countries like Bangladesh and China, which have built flood defenses, implemented early warning systems, and developed sophisticated evacuation plans to quickly put people out of danger.

But many evacuees have no homes or livelihoods to return to. This was also found to be true in Germany.

“The events observed this summer in Ahr and Erft and the Meuse region will become stronger and more frequent in the future,” said Mengel, PIK scientist. “We must stabilize the global climate to stop the trends of more severe and more frequent extremes.”


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