New WWA analysis: strong climate link to French deluge but data on Germany inconclusive

Human-caused climate change played an important role in the heavy rains that pounded parts of France late last month, scientists with World Weather Attribution (WWA) have concluded in an analysis just released.

“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 (IPSL).

“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.”

Vautard worked with IPSL colleague Florence Habets and WWA scientists from the University of Oxford, the Royal Netherlands Meteorological Institute (KNMI), the Climate Centre, and Climate Central using observations and climate models to conduct near real-time analyses of the likelihood that climate change contributed to the recent heavy rain and flooding in both France and Germany.

The latest European floods are reported to have killed at least 18 people in Germany, France, Romania and Belgium.

By reviewing the on-the-ground impacts, the team say, they realized they needed to define the meteorological event differently in France and Germany.

In France, it was characterized 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 48° to 51° N and 7° to 13° E – since January.

The probability of three-day extreme rainfall in this season has increased by at least 40 per cent in France, the scientists found, with a best estimate of about 80 per cent for the Seine and about 90 per cent for the Loire.

‘Very consistent numbers’

All four climate models that simulated the statistical properties of the extremes were “in good overall agreement”.

Results of the analyses for Germany, however, were inconclusive.

“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.”

Karsten Haustein from Oxford University said: “The same overall weather pattern – a stalled system of low pressure – triggered the heavy rains in both France and Germany, but affected each country very differently.

“In France, problems were caused primarily by rivers bursting their banks, requiring mass evacuations.

“In Germany, thunderstorms dropped large amounts of rain in a very short period in mountainous terrain, causing devastating flash floods.”

In one location in southern Germany, walls of water crushed cars and houses, forcing residents onto rooftops, news reports said.

In parts of central and north-east France, historic river floods caused widespread power cuts and threatened Paris.

‘Full reservoirs’

Climate Centre Director Maarten van Aalst said: “These latest lethal floods in Europe illustrate the rising impact of extreme-weather events, including on developed and well-prepared countries like Germany and France.

“Sadly we saw that even advanced infrastructure and water management cannot prevent some areas and neighbourhoods being overwhelmed and people sometimes dying.

“And hardware goes hand-in-hand with awareness: Well-briefed citizens know what to do when intense rainfall and thunderstorms are forecast, and they can get out of harm’s way.

“In the Paris area managing flood risk is also particularly challenging at this time of year because many reservoirs would be nearly full in preparation for the typically dry summer.”

Heidi Cullen of Climate Central said: “People affected by extreme events need an objective assessment of the causes.

“There is often speculation about the possible role of climate change when extreme weather events take place, generally lacking sound scientific evidence.

“Our goal is to provide quantitative answers using multiple peer-reviewed methodologies so we can let the science speak for itself.”

A scientific paper detailing the analyses is being submitted to the journal Hydrology and Earth System Sciences and a non-technical summary is also now online at Climate Central site.

The French Red Cross said last week it had mobilized hundreds of volunteers to assist people displaced by heavy rains. A new scientific analysis has shown that human-induced climate change played an important role in the floods and destruction they triggered. (Photo: French Red Cross via Twitter)