Central Europe’s worst flooding in many decades has left the region in shock, with corridors of destruction carved across the landscape of western Germany and eastern Belgium. More than 200 people have died – at least 170 in Germany and 36 in Belgium – making it Europe’s deadliest flood since 1985, and Europe’s 10th-deadliest flood in the past 100 years. Over 200 people remain missing, and the death toll may grow higher.
The last flood to take so many lives in Germany was the massive North Sea coastal flood of 1962, which killed 347 people. A multi-billion-dollar damage toll is assured, with huge amounts of infrastructure in ruins. Insurance broker Aon is estimating damages will exceed $10 billion, making it the costliest European flood disaster since the $13.2 billion 2013 Central European floods.
The worst flooding played out in the western German states of Rhineland-Palatinate and North Rhine-Westphalia on the evening of Wednesday, July 14. Rivers and streams in the Eifel Mountain range were especially hard hit. As an area of disturbed weather rotated around an unusually strong, slow-moving upper low, a large, persistent cluster of heavy thunderstorms dumped more than four inches of rain within a few hours over a broad area where soils were already at or near saturation.
The Cologne-Stammheim weather station maintained by Deutsche Wetterdienst (DWD), the German meteorological service, measured a total rainfall on July 14 of 153.5 millimeters (6.04 inches), and as noted by DWD and the World Meteorological Organization, a station at Rheinback-Todenfeld reported 158 mm (6.22”). These amounts are roughly twice Cologne’s average rainfall for the entire month of July. In eastern Belgium, Jalhay reported 271.5 mm (10.69”) over 48 hours, a national record for two-day rainfall.
While such amounts are far less than the one- and two-day totals one can see in lower-latitude areas, such as the U.S. Gulf Coast or southern China, most of Europe is cut off from the rich atmospheric moisture that leads to such heavy rainfall closer to the equator. The totals in Europe were actually quite extreme for a human and natural landscape accustomed to rains that are much less intense.
“It is simply surreal,” German meteorologist Michael Theusner (mtwetter.de) told us. “We have seen flash flood events in the past in Germany. However, those occurred in relation to heavy thunderstorms in a limited area, with a few villages affected. Now, a massive area was hit, with hundreds of towns. This also explains the high death toll. People did not die in one place, but a few in many places.”
Theusner pointed out that the heavy rains themselves were amply predicted by computer models and well forecast by DWD. However, problems may have emerged in how these warnings were responded to by cities and districts.
Climate change and the European floods
Intensified short-term rainfall events are one of the hallmark manifestations of a human-warmed climate. Higher temperatures allow more storm-fueling water vapor to evaporate from a warmer ocean even as they also pull moisture from the parched landscapes where drought is in play.
In a 2020 paper, Joel Zeder and Erich Fischer (ETH Zurich) showed that maximum precipitation totals across Central Europe increased from 1901 to 2013 on a variety of time scales, from single-day to monthly. “The presented findings highlight considerable seasonal differences, but still support the expected tendency of increasing extreme precipitation intensity with continuing global warming,” the authors wrote (see also Zeder’s recent Twitter thread).
Along with extreme precipitation rates, the slow movement of the upper low parked over central Europe for more than a week is consistent with the hypothesis that quasi-resonant behavior within the jet stream may be allowing summertime weather features to stall out more easily.
Improvements in climate modeling of quasi-resonant behavior, as studied by Michael Mann (Pennsylvania State University) and others, might be able to better capture such effects. “The models are not yet run at a resolution where they resolve planetary wave resonance … and this is a CRUCIAL factor behind persistent weather extremes,” Mann tweeted.
A new study using high-resolution climate simulations, published in June and led by Abdullah Kahraman (Newcastle University), found that in a high-end emissions scenario (RCP 8.5), virtually stationary heavy-rain-producing environments across Europe – settings in which rainstorms are both more intense and more prolonged – could become up to 14 times more frequent by century’s end. This scenario is unrealistically too extreme, but illustrates the type of changes that might occur in a warming climate.
The World Weather Attribution project, which recently found that the Pacific Northwest heat wave of June 2021 would have been “virtually impossible” without climate change, will be conducting a rapid-attribution analysis of the Central Europe flooding over the next few weeks.
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