Even in an era of more extreme weather, this fall appears to have shifted into a new gear, especially in a rain-weary Europe where massive and deadly flooding in Spain’s Valencia region is the latest incarnation.

At least 95 people have been killed in floods that left cars piled up like wreckage on the beach, while an ocean away, much of the United States is experiencing a virtually rain-free October that caused a sudden drought.

Scientists trying to explain what’s happening, especially as Europe experiences a spate of deadly downpours, see two likely links: man-made climate change. One of these is that warmer air holds more rain and then transports it away. The other is possible changes in the jet stream – the river of air over land that moves weather systems around the world – that are producing extreme weather.

Several climate scientists and meteorologists said the direct cause of the flooding is a cut-off lower-pressure storm system that migrated from an unusually wavy and stalled jet stream. That system simply parked over the region and dropped rain. This happens often enough that in Spain they call them DANAs, the Spanish abbreviation for the system, meteorologists say.

In America it was a sunny high-pressure system without moisture that came down like a dome and kept storms at bay.

“If we get all the drought, someone else gets all the rain,” says Yale Climate Connection meteorologist Jeff Masters, co-founder of Weather Underground.

“The same extremely wavy jet stream that is causing the drought in the US is also responsible for the horrific flooding in eastern Spain,” said climate scientist Jennifer Francis of the Woodwell Climate Research Center in Cape Cod. She has pioneered a theory that attributes a wavy and slower-moving jet stream to climate change, because the Arctic is warming so much that it is no longer much colder than the rest of the planet. That theory is gaining acceptance, but not yet fully embraced by the climate science community.

“Attributions are always difficult. In general, because of the changes we see due to climate change, the jet stream shows more pronounced waves,” said Maria Jose Sanz, scientific director of the BC3 Basque Center for Climate Change. These DANAs happen when there are more undulations, often in winter, she said.

ETH Zurich climate scientist Erich Fischer isn’t quite sold on the wavy jet stream theory yet, but then he ticks off the low-level storm systems that have swamped and flooded Europe this fall: last week in France, twice in Italy September And October, flow in Austria and the Czech Republic in September. And then there are the October floods in the Balkans, but Fischer isn’t sure if these are similar enough. Parts of Poland, Germany and the Czech Republic three months of rain in just five days in September, according to the European climate agency Copernicus.

“I only talked about the one in the fall. We had a whole series in the Alps that caused flash flooding over the summer,” Fischer said. “Starting with Bavaria, southern Germany in June, and then it was about six events in Austria and Switzerland in the mountains, extreme thunderstorms, and now this fall. So in terms of heavy rainfall it was an extremely unusual stretch.”

He said systems, especially in Spain, France and Austria, were stalling and “the rain was not moving from the same valleys for hours.”

“It’s unbelievable,” he said.

Even without the potential changes in the jet stream, several scientists said they are confident that fundamental physics makes storms like this wetter.

It’s a core equation in physics called the Clausius-Clapeyron relationship. It says that for every degree Celsius the air warms, it can hold 7% more moisture (4% more for every degree Fahrenheit). The world has warmed 1.3 degrees Celsius due to greenhouse gases, so it’s at least about 9% to 10% heavier rain, said Imperial College climate scientist Friederike Otto. She helps run World Weather Attribution, which checks for human fingerprints in extreme weather, sometimes finding them and sometimes not.

“It’s very clear that climate change has played a role,” especially in short bursts like what happened in Valencia, Otto said.

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