What We Know Concerning the Local weather Connection to the European Floods

The images from Germany are frightening and terrifying: houses, shops and streets in the picturesque towns and villages along the Ahr and other rivers that are forcibly washed away by fast-flowing floods.

The flooding was caused by a storm that slowed to a creep over parts of Europe on Wednesday, pouring up to six inches of rain over the region near Cologne and Bonn before finally subsiding on Friday. There was also flooding in Belgium, the Netherlands and Switzerland, but the worst impact was in Germany, where the official death toll on Friday would surpass 125 and is sure to rise.

The storm was a terrifying example of an extreme weather event, with a month of rain falling in some locations in one day. But in times of climate change, extreme weather events are becoming more and more common.

The question is, how much did climate change affect this particular storm and the resulting floods?

A full answer will have to wait for analysis, which, given the scale of the disaster, will almost certainly be done to find out if climate change 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 events today are affected by climate change,” said Donald J. Wuebbles, professor of atmospheric science at the University of Illinois.

Studies have previously shown that as the world warms, extreme rainfall increases, and the Intergovernmental Panel on Climate Change, the United Nations-backed group that reports on the science and effects of global warming, has said the frequency of these events is increasing will increase as temperatures continue to rise.

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

Dr. van Oldenborgh is one of the key scientists at World Weather Attribution, an informal group that quickly analyzes certain extreme weather events in terms of the effects of climate change. He said the group, which had just completed a quick analysis of the heat wave that hit the Pacific Northwest in late June, was discussing whether they would investigate the German floods.

One reason for heavier downpours has to do with physical physics: warmer air stores more moisture, making it more likely that a given storm will produce more precipitation. The world has warmed itself a little more than 1 degree Celsius (about 2 degrees Fahrenheit) since the 19th century when societies began pumping huge amounts of heat-storing gases into the atmosphere.

For every 1 degree Celsius warming, the air can absorb 7 percent more moisture. As a result, Hayley Fowler, Professor of Climate Change Impacts at Newcastle University in England, said, “These types of storms will intensify.”

And while it’s still controversial, there are studies to suggest that rapid warming in the Arctic affects the jet stream by reducing the temperature difference between northern and southern parts of the northern hemisphere. An effect in summer and fall, said Dr. Fowler, is that the high altitude airflow orbiting the earth is weakening and slowing down.

“That means the storms have to move more slowly,” said Dr. Fowler. The storm that caused the recent flood was virtually stationary, she noted. The combination of more moisture and a deadlocked storm system can lead to particularly heavy rainfall in a given area.

Kai Kornhuber, a climate scientist at Columbia University’s Earth Institute, said his and his colleagues’ research, as well as the work of other scientists, draw similar conclusions about the slowing down of weather systems. “They all point in the same direction – that the summer circulation in mid-latitudes, the jet stream, is pointing down and is a more persistent weather pattern,” which means that extreme events like heat waves and heavy rains are likely to continue.

Michael E. Mann, a climate scientist at Penn State University, has studied the effects of another summer jet stream phenomenon known as “wave resonance” on the latching of weather systems.

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

Jennifer Francis, a senior scientist at Woodwell Climate Research Center in Massachusetts, said that dawdling weather systems can have many causes, but generally don’t occur in a vacuum.

The European storm is “part of this bigger picture of extremes we saw across the northern hemisphere this summer,” she said, including the heat in the American West and the Pacific Northwest, heavy rainfall and cooler temperatures in the Midwest, and heat waves in Scandinavia and Siberia.

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

“It’s all connected and it’s really the same story,” she added.

In the case of floods, however, other factors can come into play and make it difficult to analyze the impact of climate change.

On the one hand, the local topography must be taken into account, as this can influence the precipitation pattern and how much runoff enters which rivers.

Human influences can make an analysis even more difficult. For example, building near rivers often replaces open land that can absorb rain with buildings, roads, and parking lots that increase the amount of water that drains into rivers. Infrastructure built to handle heavy drains and rising rivers may be undersized and inadequate.

And meteorological conditions can sometimes lead to different conclusions.

A 2016 study by World Weather Attribution of floods in France and Germany in May this year found that climate change affected the French floods caused by three days of rain. But the situation in Germany was different; the flood was caused by a day-long storm. The computer simulations did not indicate that the likelihood of shorter storms in this area had increased in a changing climate.

While some developments can make flooding worse, other projects can reduce flooding. That seems to have been the case in the Netherlands, which were not as 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 risk.

The work included the lowering and widening of river beds, the lowering of floodplains and the digging of side channels. “The aim of these measures is to lower the flood levels,” she said.

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

The floods on the Meuse were about a foot lower than they would have been without them, said Ms. Asselman. This meant that smaller tributaries would back up less when they met the Meuse, resulting in less flooding.

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

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