WWA is an international group established to accelerate the scientific community’s ability to analyse and communicate the possible influence of climate change on extreme weather events such as storms, floods, heat waves and droughts.
“We know that global warming leads to more downpours in general,” said Robert Vautard, a senior scientist with France’s Laboratory for Climate and Environment Sciences and Institut Pierre-Simon Laplace. “But with this attribution analysis, we found we could tie global warming directly to the recent rainstorms in France that triggered so much flooding and destruction.”
Overall, the probability of three-day extreme rainfall in this season has increased by at least 40% in France, with the best estimate about 80% on the Seine and about 90% on the Loire. All four climate models that simulated the statistical properties of the extremes are in good overall agreement.
WWA scientists also ran analyses for Germany, but the results there were inconclusive.
Vautard worked with a colleague from the Institut Pierre-Simon Laplace and fellow WWA scientists from the University of Oxford, the Royal Netherlands Meteorological Institute (KNMI), the Red Cross Red Crescent Climate Centre and Climate Central, using observational data and climate models to conduct near real-time analyses of the likelihood that climate change contributed to the recent heavy rains in France and Germany.
By reviewing the on-the-ground impacts, the team realised it needed to define the meteorological event differently in France and Germany. In France, the event was best characterised by looking at three-day rainfall extremes from April to June over the Seine and Loire River basins. In Germany, the team analysed trends in one-day maximum precipitation over the hardest hit area from January to 5 June.
“Using an ensemble of different climate models and different methods we got very consistent numbers for the impact of climate change on the rainfall in France, giving us confidence in the results,” said Geert Jan van Oldenborgh, a researcher with KNMI who led the analysis. “For such late-spring thunderstorms in Germany this was not the case, so we’re holding off on drawing conclusions for those events at this stage.”
In parts of central and northeastern France, flooding of rivers led to widespread power outages and the closing of Paris landmarks like the Louvre museum. The deluge is reported to have killed at least 18 people in Germany, France, Romania and Belgium.
“These latest lethal floods in Europe illustrate the rising impact of extreme-weather events, including developed and well-prepared countries like Germany and France,” commented Maarten van Aalst, director of the Red Cross Red Crescent Climate Centre.
“Sadly we saw that even advanced infrastructure and water management cannot prevent some areas and neighbourhoods being overwhelmed and people sometimes dying.”
Energy and Climate Intelligence Unit (ECIU) director Richard Black said: “The ability to analyse scientifically whether man-made climate change has played a role in specific extreme weather events is advancing at a startling pace. Until very recently, scientists weren’t able to make this sort of judgement, but that’s changing fast.
“Within the last year or so, we’ve learned that climate change made both last year’s European heat wave and last December’s extreme rainfall in parts of the UK more likely; and now scientists conclude man-made climate change probably almost doubled the likelihood of the recent French floods.
“This kind of information is really useful. It helps civil authorities plan for increasing extreme weather events of the future, and shows policymakers the increased risks that lie ahead if they choose not to curtail greenhouse gas emissions.”