Aug 23, 2021 - Energy & Environment

Climate change made deadly European floods more severe: study

Damage from flooding on a narrow street in Germany.

The town of Euskirchen, Germany, after severe rain and flash floods hit on July 18. Photo: Abdulhamid Hosbas/Anadolu Agency via Getty Images

Human-caused climate change increased the likelihood and intensity of the heavy rainfall that helped cause the devastating floods in Europe in July, an international team of 39 climate scientists stated Monday.

Why it matters: The study, released Monday, demonstrates how global warming is already influencing extreme weather events in ways that ramp up disaster risks.

The big picture: A slow-moving low pressure system that tapped into a plume of water vapor-laden air over the Mediterranean helped dump record amounts of rain in Germany, Belgium, the Netherlands and Luxembourg from July 12 to 15 of this year.

  • The resulting flash floods and river flooding tore apart communities, and killed at least 220 people in Belgium and Germany alone.

What they found: The study found that human-caused climate change to date made the heavy rainfall amounts, which caused the floods, between 1.2 and 9 times more likely.

  • Even the low end of that range would still be an increase of 20% that's attributable to global warming, according to Sarah Kew, a study co-author and climate scientist with the Royal Netherlands Meteorological Institute. The upper end would be an increase of 800%, Kew told Axios.
  • On a call with reporters Monday, some of the report's authors said the paucity of long-term river flow records made it hard to narrow down that wide range of results, but added that it is clear that this was an unprecedented rainfall event for the regions most affected.
  • For example, more than 3.67 inches of rain fell during a single day in the vicinity of the Ahr and Erft rivers in Germany, which broke all-time rainfall records.
  • While the new research did find clear ties between the human-driven increase in greenhouse gases and the floods, it also illustrated some of the limitations of what's known as extreme event attribution, particularly when it comes to data that has sparse periods of record or is influenced by multiple factors.
  • Challenges for this analysis included the localized nature of some of the heaviest rains, and the lack of reliable long-term river flow data, in part because the floods themselves destroyed so many river measurement stations.
  • Many factors other than climate events can affect flooding, too, including the built environment and geography in and around waterways.

How they did it: To determine the role that climate change played in influencing the rainfall rates and totals, researchers examined historical weather data as well as different types of computer models run with and without the effects of human-caused global warming.

  • The simulations with climate change most closely matched the event that occurred.
  • The study looked closely at two key areas affected: the Ahr and Erft river regions of Germany and the Meuse river region in Belgium. The latter area saw 4.2 inches of rainfall over a two-day period, and this water fell on already saturated soils, which meant it ran off directly into rivers and streams.
  • In looking at this smaller region, the scientists found too much variability in the data to reach firm conclusions, which prompted them to zoom out slightly to look at summer rainfall across a broader area to find the likelihood of such extreme rainfall events in much of Germany, Belgium and the Netherlands.
  • The data pulled from the larger region revealed the upward trend in odds as well as intensity. The study concludes that human-caused climate change boosted the amount of single-day rainfall in this event by between 3% and 19%.

Context: The new study is the product of a global network of scientists who use peer-reviewed research methods to rapidly assess climate change's role, or lack thereof, in extreme weather events. The effort is known as the World Weather Attribution project.

What's next: The study finds that for a climate that is 2°C (3.6°F) warmer than the preindustrial era, the intensity of a similar 1-day rainfall event would jump by another 0.6% to 6%, and the likelihood by a factor of 1.2 to 1.4.

What they're saying: "Climate change is hitting us everywhere now," said Maarten van Aalst, a study coauthor and director of the Red Cross Red Crescent Climate Center.

"The increase in risk that we found in this study is something we need to manage by thinking about our flood risk management, about our preparedness, about early warning systems, being ready to tell people what's coming their way and people knowing what to do at that point," he said.

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