The Paris floods, that saw extreme rainfall swell the river Seine to its highest level in decades, were made almost twice as likely because of the manmade emissions driving global warming, scientists have found.
A three-day period of heavy rain at the end of May saw tens of thousands of people evacuated across France, and the capital’s normally busy river closed to traffic because the water levels were so high under bridges. As artworks in the Louvre were moved to safety and Paris’s cobbled walkways were submerged, the French president, François Hollande, blamed the floods on climate change.
Now a preliminary analysis by a group of scientists, including the Dutch weather agency and the University of Oxford, has concluded the risk of the flooding event in Paris was almost doubled – multiplied by a factor of 1.8 – by humanity’s influence on the climate.
“Hollande was right to say climate change is playing a role but at the same time it’s important to say that this event could’ve happened without climate change,” said Prof Myles Allen, a climate scientist at the University of Oxford.
“But it means what was a 1 in 200 year event is more like a 1 in 100-something year event. Is that a big change? In terms of weather events, that’s not nearly as big an increase in risk as we’ve seen in heatwave events, where we often come up with a factor of 10. But for precipitation this is kind of what we’re seeing.”
The climate science community is speeding up its efforts to draw the links – the attribution – between extreme weather events and climate change, while such events are fresh in the public and politician’s minds. Previous quick turnaround research has shown flooding in England and heatwaves in Europe were made more likely because of global warming.
“The crucial thing is decisions… they get made in the aftermath of these events, when minds are focused on the impacts,” said Allen. “Getting this information out while people are still thinking about the event is useful. Also, it guards against the risk of over-attribution [overegging climate change’s role in an extreme weather event].”
Richard Black, director of the Energy and Climate Intelligence Unit, an environmental thinktank, 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.”
The analysis by the World Weather Attribution project published this week used a variety of approaches to look at how much the risk of the rainfall on 29-31 May had been increased by global warming, including statistical analysis of the historical temperature model, trends in climate models and “the results of thousands of simulations of possible weather with a regional climate model”.
All the approaches agreed climate change had made the heavy rains in the Seine and Loire river basins more likely. However, the link between global warming and rainfall in Germany at the same time, which saw intense thunder storms, proved inconclusive.
While the work has not yet been peer-reviewed it is based on peer-reviewed methods, and Allen said it would be submitted in a week to a journal for peer review and publication, as similar past research has been by the group.
Almost all previous floods on the Seine and Loire have occurred during winter, the researchers said, making the summer timing very unusual. The rains saw the Seine peak at 6.1 metres above its normal height on 3 June, a 34-year high. Its record peak was in 1910, when an eight-metre rise caused the catastrophic ‘Great Flood of Paris’.