Climate change worsened Europe’s devastating July floods, according to a study released Tuesday, but it was just one factor in the catastrophe.
Two days of record-shattering rain and subsequent river floods brought devastation to parts of Belgium and Germany, killing more than 200 people.
Even before the water had drained from the towns where it had swept away cars, homes and lives, many politicians — including European Commission President Ursula von der Leyen — said Europe had suffered a climate disaster.
On Tuesday, 39 scientists from the World Weather Attribution (WWA) service backed up those political assertions. They said the additional 1.2 degrees by which humans had warmed the planet since the Industrial Revolution had made events such as this summer’s floods between 20 percent and 900 percent more likely. The climate-driven increase in rainfall intensity was also stretched across a large range: between 3 percent and 19 percent.
With or without the effect of climate change, it was a massive and very rare event, the scientists said. This amount of rain would fall on any single location in northwestern Europe on average once every four centuries, the study found. The Ahr River in Germany last reached a similar level in 1804.
“The extreme precipitation overtopped almost everything we knew in that region before,” said study author Enno Nilson from the German Federal Institute of Hydrology. But factors like the rain also falling on already soaked soil, in a region where natural flood defenses have been removed to make way for farming or urban development, made the floods worse.
The study reflects recent findings from the Intergovernmental Panel on Climate Change, which said that climate change was the major driver in the increase of extreme weather, including heavy rainfall.
Other factors at play
But while climate change made the rains worse, the extra water wasn’t what turned something more manageable into a catastrophe, said Linda Speight, a hydrologist at the University of Reading, who was not involved in the study. Even without the maximum 19 percent extra precipitation attributed to a warmer climate, “if this amount of rainfall was to fall then I expect major flood damage to occur regardless.”
While the WWA study focused on the climate’s effect on rainfall, said Hannah Cloke, a natural hazards researcher at the University of Reading, deadly floods “are caused by a combination of weather, the conditions of rivers, soils and vegetation, and where people are and how they respond.”
While politicians highlighted the need to do more on climate change in the immediate aftermath of the floods, Cloke and others were asking why early warning systems had been ignored.
“It is crucial to highlight how climate change makes floods and other impacts on people more likely,” she said. “But it is important that we do not use climate change as an excuse for inaction. Individuals, local authorities and governments still have the power to make specific, local changes that can save lives and protect property from the worst impacts of floods. Blaming others far away, or long ago, for these problems just shirks responsibility.”
The large range of uncertainty highlights limitations in analyzing single, rare events that take place over relatively small areas, Speight added. “It is very challenging to do this kind of attribution study for heavy localized rainfall events like we saw in Germany.” Most climate models are on a scale of larger than 12 square kilometers, she said, too large to pick up rainfall over a single river catchment.
“Similarly the floods in Germany were very extreme and rare,” she said. “The available observed data records and climate model runs were not long enough to be confident in the results, hence the wide confidence bands.”
These challenges pushed the scientists into new territory as they tried to assess the summer cloudbursts. That meant using models that could account for convection — or rising heat and moisture — for the first time, said Sarah Kew, a climate researcher at the Royal Dutch Meteorological Institute. “Such analyses will become more routine as more and longer convection-permitting model simulations become available.”
If the world warms further to 2 degrees above preindustrial levels, very heavy rainfall events like this summer’s would return to the region between 1.2 and 1.4 times more often than without such warming, the authors of the study said.
The message from the study, said author Maarten van Aalst, the lead climate specialist for the Red Cross/Red Crescent Climate Center, is that “natural variability certainly played a big role, it is a very rare event … but a rare event we should increasingly be prepared for.”