In a Summer of Deadly Deluges, New Research Shows How Global Warming Fuels Flooding

New attribution research shows that climate change primed the pump and increased the odds of deadly July floods in northwestern Europe.

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Volunteers and residents start the clean up process at their shops and restaurants following severe flash flooding on July 18, 2021 in Bad Neuenahr-Ahrweiler, Germany. Credit: Thomas Lohnes/Getty Images
Volunteers and residents start the clean up process at their shops and restaurants following severe flash flooding on July 18, 2021 in Bad Neuenahr-Ahrweiler, Germany. Credit: Thomas Lohnes/Getty Images

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While global warming shifts some parts of the world into an age of persistent fires, others have been ravaged by intensifying rainfall and deadly floods, sure signs that Earth’s water cycle is becoming more volatile, with increasingly intense rain and floods punctuating longer dry periods.

Most recently, at least 21 people died in Aug. 21 flooding in Tennessee after 17 inches of rain fell in less than a day, by far the highest rainfall total recorded in the state for a 24-hour period. Right around the same time, New York City reported it’s greatest-ever 24-hour rainfall total.

In June, floods in Chad displaced about 20,000 people after destroying more than 4,000 houses and 30 schools. In July and August, widespread Northern Hemisphere flooding killed more than 300 people in northwestern Europe, at least 81 in Turkey and more than 300 in China’s Henan province.

“It’s been relentless and bad, and we’ve been seeing an onslaught all around the world,” said flood researcher Hannah Cloke, with the University of Reading. “We’re seeing the future now. With global warming, we’re cascading very rapidly into a future of more rainfall and more flooding.” 

Other climate-related flood risks, not directly related to rainfall, are also increasing, including glacial outburst floods in the Andes and Himalaya as icy mountains thaw; run-off floods and debris flows in areas scorched by wildfires in western North America, and destructive coastal river flooding from melting ice in Greenland. According to the World Meteorological Organization, floods have killed more than 58,000 people in the last 50 years.

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The recent climate science report from the Intergovernmental Panel on Climate Change dedicated a full chapter to the intensification of the water cycle, concluding that even more sudden and disruptive changes are ahead if warming remains unchecked, and Cloke said new climate attribution research released this week bolsters the IPCC’s assessment.

The rapid attribution analysis by an international team of 39 scientists analyzed the floods that killed at least 184 people as they washed away centuries-old towns in northeastern Germany and the adjacent Low Countries in mid-July after record extreme rainfall persisted across the region. 

World Weather Attribution, a climate research group that focuses on links between extremes and global warming, led the study and coordinated the work of scientists from universities and meteorological and hydrological agencies in Belgium, France, Germany, Luxembourg, the Netherlands, the U.S. and the U.K. The research has not been peer reviewed yet. 

For a time during the four-day flood that started July 12, the small, regional Ahr River flowed at 1,000 cubic meters per second, a rate equivalent to the powerful Upper Rhine River, said study co-author Enno Nilson, of the German Federal Institute of Hydrology. The floods swept away some of the streamflow gages, he said, so the team is still not sure at what level the flows peaked.

The study analyzed historical weather data and used climate models to determine that global warming made the flooding between 1.2 and 9 times more likely, and made rainstorms in the region 3 percent to 19 percent more intense.

The researchers said the rainfall broke old records by margins so great that it’s difficult to accurately model how frequently such events might happen in a warmer future, because there are no similar past events for comparison, which “challenges the limits” of attribution science. The study also identified similar risks in a larger region extending from the Netherlands to the Alps, and shows that the intensity of one- and two-day extreme rain would increase another 0.8 percent to 6 percent if global warming reaches 2 degrees Celsius above the pre-industrial level.

Seeing the risks ramp up quickly means there is “no time to wait for science to catch up with urgent need for action,” said University of Reading hydrologist Linda Speight. “The message I take away from this is that when floods occur they are getting more extreme, and if the earth continues to warm they will continue to do so.”

Both Speight and Cloke emphasized there are protective actions that can begin now to avoid deaths during the next floods, and that discussions over what role global warming played in the floods shouldn’t be used as an excuse for inaction.

“I don’t like the way many people are blaming climate change and then shirking responsibility,” Cloke said.

“The potential for extreme rain and flooding was forecast several days in advance,” she added. “It crushes my spirit to see so many people not taking action and ignoring warnings.”

Slow Storms Bringing Extremes Fasters

The results of the study show that “it’s all going faster, and that we have less time left to stay on track” to keep global warming under a safe limit, said co-author Maarten van Aalst, director of the Red Cross Red Crescent Climate Centre

“The extremes are coming a lot faster than what I had anticipated,” said van Aalst, who is also a coordinating lead author on one of the next major IPCC reports, which is due in 2022. “These were scenarios for farther away, things we wanted to avoid, but now we are already dealing with them today.”

A warmer atmosphere holds more moisture, but “that is not going to just gradually drizzle out,” he said. “No, it’s going to be more extreme precipitation and more dry periods, at the same time.” 

While the attribution study found a global warming fingerprint on this one extreme event, there are also larger shifts in weather patterns linked to climate change, said Stefan Rahmstorf, head of Earth Systems Analysis at the Potsdam Institute for Climate Impact Research, who was not involved in the new research.

“I think the event was instead made worse by the general slowing of the summer circulation,” he said. “Indeed this flooding event was characterised by a lingering of the extreme rain over the same region.”

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A 2015 study suggests global warming has weakened westerly summer winds that shunt weather systems around the mid-latitudes of the Northern Hemisphere as measured by several key indicators, including average zonal winds and the size of atmospheric waves. Those measurements show a decline in the intensity of the prevailing westerly winds, which can cause rainy patterns to stall in one area, as did the July rainstorm over Germany.

And a study published this June shows that slow-moving storms could be 14 times more frequent over land by 2100, increasing rainfall and “enhancing the risk of flash floods across Europe beyond what was previously expected,” the authors wrote. The main reason for the slowdown seems to be a reduced temperature difference between the poles and tropics, they added.

Rahmstorf said the attribution study’s projections of rainfall frequency and intensity from observations were much greater than those based on models, suggesting that the simulations don’t include all the mechanisms that could contribute to the increase in extremes.

Those triggers could include surges of icy Arctic freshwater into the North Atlantic, which can intensify rainstorms, according to a 2020 study. Other studies show how global warming is driving large-scale shifts in the patterns of sea surface salinity and temperatures, as well as changes to major ocean currents running parallel to continental coasts. All those changes affect 

how much moisture developing storms carry, which direction they go and how fast they move. 

Double Whammy of Climate and Human Stupidity’

One of the lessons from this summer’s deadly floods is that, “Sadly, people tend to prepare for the last disaster,” van Aalst said. “We thought the climate was something we could count on to help us plan,” using the frequency and severity of past events to guide preparations for the future.

But the attribution research suggests that, with global warming, there is a growing chance that flooding and other extremes will become more frequent very rapidly—faster than emergency planning can adapt. The results show a great and growing uncertainty, and the need “to be prepared for the unprecedented.”

And with marginalized, poor communities around the world at the greatest risk, “we don’t have the luxury of doing these attribution studies everywhere,” he said. Rapid adaptation to extraordinary and unexpected weather extremes is critical to preventing more flooding deaths. 

A forthcoming climate mortality atlas from the World Meteorological Organization shows that water-related hazards, including storms, droughts and floods, are the deadliest and costliest climate and weather catastrophes. The atlas’s grim tally includes 650,000 deaths caused by drought, 577,232 deaths caused by storms and 58,700 deaths caused by floods between 1970 and 2019. In terms of economic losses, the WMO calculates a $521 billion cost for storms and $115 billion for floods. The atlas will be published in September.

“It’s going to happen more frequently,” added Cloke. “It’s a double whammy of climate and human stupidity of not taking these risks seriously. We really could do something about it,” she said. “We’re not going to survive as the human race if we throw our hands up and just say, ‘oh well.’”

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