How climate change can produce extreme rainfall

Germany’s National Meteorological Service said the two most impacted states, Rhineland-Palatinate and North Rhine-Westphalia, recorded between 4 and 6 inches of rain in the 24 hours between July 14 and 15. According to CNN meteorologist Brandon Miller, that amounts to almost as much as the region usually sees in a month.

There are two main links between climate change and extreme rainfall events like the one in northwestern Europe. First, as Hayley Fowler, professor of climate change impacts in the School of Engineering at Newcastle University, told me, a warmer atmosphere can hold more moisture. “According to the Clausius-Clapeyron equation, a one-degree rise in temperature has the potential to give you a 7 percent increase in the intensity of rainfall,” Fowler said.

“The second point is that the Earth’s poles are increasing in temperature at two to three times the rate of the equator,” Fowler said. That, she said, “weakens the jet stream of the mid-latitudes, which is basically over Europe. In summer and autumn, the weakening of the jet stream has a knock-on effect causing slower-moving storms. So there’s a double whammy of increasing intensity, but the storm lingers longer too.”

And that kind of double whammy can have devastating impacts on the land and infrastructure.

“All this happened very fast, and I’ve never experienced a situation which developed that fast,” Tanja Krok, head of volunteering service in the German Red Cross in North Rhine-Westphalia, told me. She’s been working in the region for nearly 30 years. “In 2002, we had flooding in the east of Germany but it impacted one region and developed slowly,” Krok said.

The powerful flow of water has also caused landslides, leaving some roads unusable if not completely washed away. “We’ve never had landslides before. We feel like our houses here are stable and fixed. It’s not often that you see houses collapse,” Krok said.

Europe’s flood-warning system is also to blame

In addition to climate change, experts have also pointed to communication failures in the European Flood Awareness System.

The German weather service issued warnings for the event on Monday, three days before it actually happened. The hydro-logical services in Germany also issued a warning. Given the number of warnings in place, experts have said that the problem is not as much forecasting as communicating the severe impacts of flooding events to the greater population.

Core Concept: How does climate change influence extreme weather? Impact  attribution research seeks answers | PNAS

“The issue is not that there wasn’t a warning in place. There was. We’ve got really good forecasting models now. So, both these events, and also the floods that we saw in New York and London earlier in the week, there were flood warnings in place for those. We knew that heavy rainfall was coming,”Linda Spejght, a flood forecasting specialist at the University of Reading in England, told me.

“Over 100 people should not have died in a flood in Germany. That shouldn’t happen in Western Europe in 2021,” she said.

Speight, who works at the nexus of hydrology and meteorology to understand how the weather will cause flooding, thinks the high loss of life could be because people did not understand the seriousness of the warnings.

“If you issue a weather warning which says there’s going to be 200 milli-meters of rain tomorrow, that doesn’t mean anything. It doesn’t mean a lot to me — and that’s my area of specialism, so I doubt it means very much to the general public,” Speight said. “We need to change how we communicate warnings. For example, instead of saying, ‘There will be 200 milli-meters of rain,’ we need to say, ‘There will be rapidly rising water levels, damage to properties, a risk to life.’”

And as extreme weather events like these become more and more common, learning how to communicate the danger effectively will be even more critical. “Across the world, we need to get better prepared for these kinds of events,” Speight said. “Everybody can learn lessons from the flood in Germany and see how they can apply them to improve to be more prepared in their own countries.”

But while early-warning systems can help reduce the loss of life, the ultimate answer is for humans to stop emitting carbon dioxide and other planet-warming greenhouse gases.

“The climate is warming, and it will keep on warming as long as we emit CO2. Last time I checked, we’re still emitting huge amounts of CO2,” Geert Jan van Oldenborgh, a visiting professor at Oxford University who studies the impact of climate change on extreme weather events, said.