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 made the heavy summer rainfall between 3% and 19% stronger, and 1.2 to nine times more likely, according to a study published Tuesday by an international group of climate scientists from the World Weather Attribution (WWA). It is a finding that could play a critical role in Germany — the fourth-biggest historical polluter and the country with the highest death toll from the flooding — ahead of elections in September.
"These floods have shown us that even developed countries are not safe," said Friederike Otto, a climate scientist at Oxford University's Environmental Change Institute and co-author of the study, in a press statement.
The floods, which killed more than 220 people as they swept away buildings and livelihoods, were the result of heavy rainfall between July 12 and 15. They followed three weeks of warm and wet weather that had left soils in two German states nearly saturated.
Unable to soak up such large quantities of water in a short space of time, the ground quickly flooded when a record-breaking 93 mm (3.7 inches) of rain fell over a single day around Germany's Ahr and Erft rivers, and 106 mm fell over two days in Belgium's Meuse region. Rivers burst their banks, breaking bridges and cutting off roads. People asleep in their homes — many warned late by authorities — were left with little time to evacuate.
When such weather strikes, WWA researchers run computer models fed by historical data to quantify the role climate change played in a disaster — working against the clock before journalists, policymakers and the public have forgotten about it. The scientists release their results before submitting their research for peer review so they can draw conclusions in a matter of weeks, instead of waiting months or years.
The study found that climate change made the rains stronger and more likely. At the lower end of the study's range, it made the rains at least 20% more likely and 3% stronger. At the higher end, it 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 hard" to attribute such a specific event to climate change, said Matthias Mengel, a scientist at the Potsdam Institute for Climate Impact Research (PIK), who was not involved with the study. The WWA scientists instead broadened their analysis to include data from across western Europe. "At the most general level the study is conclusive: Climate warming increased the chance and the strength of such events," Mengel said.
Beneath the complex climate shifts that make rainfall difficult to attribute is a simple principle of physics: Hotter air holds more moisture. The Earth has already warmed at least 1.1 degrees Celsius (2 degrees Fahrenheit) since the late 19th century — Germany by almost 2 C — and scientists project that heavy rains will continue to grow heavier still as the planet heats up.
'We thought we knew the situation'
One of the districts hit by the most rain was the Oberbergischer Kreis, near the western German city of Cologne, which received 150 mm of rain in 72 hours.
"We thought we knew the situation because we have light floods every now and then," said Christian Kettler, a managing director at SN Maschinenbau, a company exporting packing machines that was hit when the Wupper River overflowed its banks. "What was special this time was the speed and suddenness and quantity [of rain] that came."
Scientists from the government-funded research group Climate Service Center Germany project that, if emissions rise so high as to heat the planet by 4-5 C, the number of days with more than 20 mm of rainfall will rise in the Oberbergischer Kreis from nearly 11 per year to 14. That's a greater increase than in any other of Germany's 401 districts bar one.
In the nearby Ahrweiler district, public prosecutors have brought a lawsuit against local authorities for negligent homicide after 12 people with disabilities drowned in a residential home 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 home had grown herbs. The WWA study highlights that people with disabilities are among the most vulnerable to floods.
The Ahr River had grown dangerous, like a big boa constrictor, said Dumaine. "Like a snake that had eaten and whose stomach contained a bomb of water. A tsunami."
More extreme extremes
In recent years WWA studies have confirmed links between climate change and several extreme events across the world. Most recently, they established that the deadly heat wave in Canada and the US in June was made 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 exacerbated the Australian wildfires in 2019 and 2020, the European heat waves in 2018 and 2019 and Tropical Storm Imelda, which hit the US in 2019.
Not every attribution study finds a link. Sometimes that's because there isn't enough data to detect the influence of climate change amid natural variation. But other times, it's because hotter temperatures alter the climate in complex ways that simultaneously increase and decrease the likelihood of extremes. The net result sometimes defies global trends.
A study published in the journal Bulletin of the American Meteorological Society in 2014, when attribution science was just kicking off, did not find a climate link when heavy summer rains led to flooding on the Danube and Elbe rivers in Germany the previous year.
But since then, the research has come a long way. A landmark report published by the Intergovernmental Panel on Climate Change (IPCC) in August found that, if the world warms 4 degrees Celsius above pre-industrial temperatures, heavy rains that used to hit once a decade will hit nearly 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 extremes breaking observed records by far, exacerbated by climate change, can strike anywhere, induce huge damages and cause fatalities," said Frank Kreienkamp from the German Meteorological Service and a co-author of the WWA study in a press statement.
World leaders in 2015 pledged to limit global warming to ideally 1.5 C by the end of the century in a bid to prevent the climate from growing more chaotic and less hospitable to human life. But they are currently pursuing policies that put it on track for 3 C.
While the death toll from weather extremes has fallen 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 has risen. Much of the shift has happened in countries like Bangladesh and China, which have built flood defenses, set up early-warning systems and developed sophisticated evacuation plans to swiftly move people out of harm's way.
But many evacuees do not have homes or livelihoods to go back to. That has also proven true in Germany.
"Events seen this summer in the Ahr and Erft and the Meuse region will become stronger and more frequent in the future," said PIK scientist Mengel. "We need to stabilize the global climate to stop the trends of more severe and frequent extremes."