Even for an era of more extreme weather, this autumn has seemingly shifted into yet another 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 flooding that sent cars piling up like flotsam on the beach, while an ocean away much of the United States bakes through a nearly rain-free October that has created a flash drought.
Scientists trying to explain what's happening, especially with a spate of deadly European downpours, see two likely connections to human-caused climate change. One is that warmer air holds and then dumps more rain. The other is possible changes in the jet stream — the river of air above land that moves weather systems across the globe — that spawn extreme weather.
Several climate scientists and meteorologists said the immediate cause of the flooding is called 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 poured rain. This happens often enough that in Spain they call them DANAs, for the Spanish acronym for the system, meteorologists said.
In America, it was a sunny, high-pressure system with no moisture that plunked down like a dome and kept storms away.
“If we're getting all the dryness, somebody else is getting all the rain,” said Yale Climate Connection meteorologist Jeff Masters, co-founder of Weather Underground.
“The same extremely wavy jet stream that is causing the U.S. drought is also responsible for the horrific flooding in eastern Spain,” said climate scientist Jennifer Francis at the Woodwell Climate Research Center in Cape Cod. She’s a pioneer in a theory that attributes a wavier and slower-moving jet stream to climate change because the Arctic is warming so much it’s no longer a lot colder than the rest of the planet. That theory is gaining more acceptance, but it's not fully embraced by the climate science community.
“Attributions are always tricky. Generally speaking, the jet stream, because of the changes we are seeing due to climate change, is having more pronounced undulations," said Maria Jose Sanz, scientific director of the BC3 Basque Center for Climate Change. These DANAs happen when there are more undulations, often in the winter, she said.
ETH Zurich climate scientist Erich Fischer isn't fully sold about the wavy jet stream theory, but then he ticks off the cut-off low storm systems that have doused and flooded Europe this fall: last week in France, twice in Italy in September and October, flooding in Austria and Czech Republic in September. And then there are the October floods in the Balkans, but Fischer isn't sure they are quite similar enough. Parts of Poland, Germany and the Czech Republic got three months of rain in just five days in September, according to the European climate service Copernicus.
“I’ve only been talking about the ones in autumn. We had a whole series in the Alps causing flash floods during summer,” Fischer said. “Starting with Bavaria, southern Germany in June, and then it was something like six events in Austria and Switzerland in the mountains, extreme thunderstorms, and now this autumn. So in terms of heavy rainfall, it was an extremely unusual stretch.”
He said the systems, especially in Spain, France and Austria, got stuck and “the rain did not move” from the same valleys for hours.
“It's incredible,” he said.
Even without the potential changes to the jet stream, several scientists said they are sure that basic physics are making storms like this wetter.
It's a core equation in physics called the Clausius-Clapeyron relation. It says 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 because of greenhouse gases, so it's about 9% to 10% heavier rain, at the least, said Imperial College climate scientist Friederike Otto. She helps run World Weather Attribution that checks for human fingerprints in extreme weather, sometimes finding them, sometimes not.
“It is very clear that climate change did play a role," especially in short bursts like what happened in Valencia, Otto said.
That air holding more moisture may be “just for starters,” meteorologist Masters said. When the moisture condenses it releases heat energy, which goes into the storm, invigorates it, increases its updrafts and allows it to pull even more moisture from a larger area, which could boost rainfall as much as 20%, he said.
“It just kind of feeds and you get a vicious cycle," he said.
Fischer found a similar storm in the same place in 1957. But this year's storm, with warmer air stoking it, was much wetter. The 1957 storm dumped around 250 millimeters of rain (10 inches), but this week's had reports of more than 490 millimeters (19 inches) in just eight hours, Fischer said. There could be rain gauge issues involved, but part of this is the atmosphere holding and dumping more water.
And then you add a toasty Mediterranean sea.
It had its warmest surface temperature on record in mid-August, with a mean temperature of 28.47 Celsius, said Carola Koenig of the Centre for Flood Risk and Resilience at Brunel University of London.
“This facilitates a greater uptake of moisture in the air, resulting in more rain when the atmosphere starts to cool in the autumn," she said. "As things stand, Spain needs to embrace itself for continued heavy rain for the next few days.”
There may be different ways of counting and attributing climate change and the havoc it wreaks, Otto said, but one thing is for certain: “Burning fossil fuels causes climate change and climate change causes death and destruction.”
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