Global warming, especially in the last 50 years, had almost doubled the likelihood of the kind of three-day downpour that burst the banks of the Seine and Loire rivers, they calculated.
At the very least, the probability of such an extreme rainfall event had increased by more than 40 percent.
"We found that we could tie global warming directly to the recent rainstorms in France that triggered so much flooding and destruction," Robert Vautard, a senior scientist with France's Laboratory for Climate and Environment Sciences, said in a statement.
In southern Germany, heavy rains also caused flash flooding that swept away houses and cars. At least 18 people were killed in four European countries.
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Unlike for France, the evidence was not strong enough to establish a direct link between warming and the destructive rainfall in Germany, the researchers said.
This does not mean that climate change did not play a key role, only that observations failed to line up with the models well enough to draw similarly robust conclusions.
"Until recently, scientists weren't able to make this sort of judgment, but that's changing fast," said Richard Black, director of the Energy and Climate Intelligence Unit, an advocacy group, in London.
"We've learned that climate change made both last year's European heatwave and last December's extreme rainfall in parts of the UK more likely."
Part of the explanation lies in basic physics. A warmer atmosphere can hold -- and discharge -- more water.
On current trends, that temperature is set to rise by another 2 degrees Celsius, even taking into account national pledges made by virtually all the world's nations last year to slash carbon pollution.