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nytimes – What we know about the link between climate and flooding in Europe

The images of Germany are startling and horrifying: houses, shops and streets in picturesque towns and villages along the Ahr and other rivers violently washed away by rapid flood waters.

The flooding was caused by a storm that slowed to crawl across parts of Europe on Wednesday, dumping up to six inches of rain over the area near Cologne and Bonn before finally starting to slow on Friday. There was flooding in Belgium, the Netherlands and Switzerland as well, but the worst impacts were in Germany, where the official death toll topped 125 on Friday and was sure to climb.

The storm was a frightening example of an extreme weather event, with some places receiving a month of rain per day. But in the age of climate change, extreme weather events are more and more common.

The question is to what extent has climate change affected this specific storm and the resulting flooding?

A full response will have to wait for analyzes, almost certain to be undertaken given the scale of the disaster, which will investigate whether climate change has made this storm more likely, and if so, by how much.

But for many scientists, the trend is clear. “The answer is yes – all major weather conditions these days are affected by climate change,” said Donald J. Wuebbles, professor of atmospheric sciences at the University of Illinois.

Studies have already shown an increase in extreme downpours as the world warms, and the Intergovernmental Panel on Climate Change, the United Nations-backed group that reports on the science and impacts of global warming climate, said the frequency of these events will increase as temperatures continue to rise.

Geert Jan van Oldenborgh, researcher at the Royal Netherlands Meteorological Institute, said that in studies of extreme rainfall events in the Netherlands, “the observed increase is greater than expected”.

Dr van Oldenborgh is a leading scientist in the Global Weather Allocation, a loose group that quickly analyzes specific extreme weather events for any impact on climate change. He said the group, which had just completed a rapid analysis of the heat wave that hit the Pacific Northwest in late June, were wondering if they would study German flooding.

One of the reasons for heavier downpours has to do with basic physics: warmer air holds more moisture, which increases the likelihood that a specific storm will produce more precipitation. The world has warmed by just over one degree Celsius (roughly 2 degrees Fahrenheit) since the 19th century, when societies began pumping huge amounts of heat-trapping gases into the atmosphere.

For every degree Celsius of warming, the air can contain 7% more humidity. As a result, said Hayley Fowler, professor of climate change impacts at the University of Newcastle in England, “These types of storms are going to increase in intensity.”

And while it’s still a matter of debate, some studies suggest that the rapid warming of the Arctic is affecting the jet stream, reducing the temperature difference between the northern and southern parts of the northern hemisphere. One of the effects in summer and fall, Dr Fowler said, is that the high-altitude air current that circles the globe weakens and slows down.

“This means the storms have to move more slowly,” Dr Fowler said. The storm that caused the recent flooding was virtually stationary, she noted. The combination of higher humidity and a blocked thunderstorm system can result in very heavy rains in any given area.

Kai Kornhuber, a climatologist at the Earth Institute at Columbia University, said his research and that of his colleagues, as well as papers from other scientists, have drawn similar conclusions about slowing weather systems. “They all point in the same direction – that the mid-latitude summer circulation, the jet stream, slows down and is a more persistent weather pattern,” meaning that extreme events like heat waves and heavy rains are likely to continue indefinitely.

Michael E. Mann, climatologist at Penn State University, studied the effects of another summer jet phenomenon known as “wave resonance” in locking weather systems in place.

Climate change, he said, is making stagnant weather events more frequent. But he said it was premature to say that the European catastrophe was caused by resonance of the waves.

Jennifer Francis, senior scientist at the Woodwell Climate Research Center in Massachusetts, said that while lagging weather systems can have many causes, they usually don’t happen in a vacuum.

The European storm is “part of this larger picture of extremes that we have seen throughout the northern hemisphere this summer,” she said, which includes heat in the US West and Northwest. the Pacific, heavy rains and cooler temperatures in the Midwest and heat waves in Scandinavia and Siberia.

“It’s never isolated when it comes to a strange configuration of the jet stream,” said Dr Francis. “An extreme in the same place is always accompanied by extremes of different types. “

“Everything is connected, and it is really the same story,” she added.

When it comes to flooding, however, other factors can come into play and complicate any analysis of the influence of climate change.

On the one hand, local topography must be taken into account, as this can affect precipitation patterns and the amount of runoff in which rivers.

Human impacts can further complicate an analysis. Developments near rivers, for example, often replace open land, which can absorb rain, with buildings, streets and parking lots that increase the amount of water flowing into rivers. Infrastructure built to cope with heavy runoff and flooding of rivers can be under-designed and inadequate.

And weather conditions can sometimes lead to different conclusions.

A 2016 study by World Weather Attribution on flooding in France and Germany in May of the same year found that climate change had affected flooding in France, caused by three days of rain. But the situation in Germany was different; the flooding was caused by a one-day storm. Computer simulations did not reveal that the likelihood of shorter storms in this region increased in a changing climate.

While some developments can make flooding worse, other projects can reduce flooding. This appears to have been the case in the Netherlands, which was not so badly affected by the storm.

After several major floods on the Meuse in the 1990s, the Dutch government launched a program called Room for the River to reduce flooding, said Nathalie Asselman, who advises the government and other clients on flood risks.

The works included lowering and widening the riverbeds, lowering floodplains and excavating side channels. “The purpose of these measures is to bring down the flood level,” she said.

While a dike near the Meuse in the south of the Netherlands suffered a breach that caused flooding until it was repaired on Friday, the measures appear to have worked.

Flood levels on the Meuse were about a foot lower than they would have been without them, Ms Asselman said. This meant that small tributaries receded less where they met the Meuse, producing less flooding.

“If we had not implemented these measures, the situation would have been worse,” she said. “Both on the main river and the tributaries. “

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