Flooding in Europe Is a ‘Clear Reminder’ of the Threat of Extreme Weather

Climate change is influencing extreme weather events like the flooding across six countries that has led to mass evacuations of cities as the waters continue to rise.

Floodwaters surged across Central Europe this week, inundating entire towns and transit systems with a rippling layer of murky brown water.

More than 20 people have been killed by flooding across Romania, Poland, the Czech Republic and Austria, with many others missing in the days since heavy rainfall began last Thursday. Thousands more, including people in Hungary and Slovakia, have been displaced. On Monday night, the mayor of a Polish town asked 42,000 residents to evacuate ahead of continued rainfall.

“These floods are a clear reminder of the growing threat of climate-induced extreme weather events,” said Sissi Knispel de Acosta, the general secretary of the European Climate Research Alliance, which is made up of research groups that study global warming.

The record rains are part of a slow-moving, low-pressure system called Storm Boris that has dumped five times September’s average rainfall over four days.

The weather system was fueled by a blast of Arctic air that moved in from the north, causing temperatures to plummet within 24 hours. While it’s not unprecedented for a polar blast to hit Europe in late summer, it could become more likely to happen in the future under a changing climate, said Richard Rood, a climatologist at the University of Michigan.

That cold air collided with warmer air from the south that was dense with water vapor. The overloaded moisture came from an unusually warm Mediterranean Sea that hit the highest temperature ever recorded last month.

“The climate is so warm that every storm or weather event is influenced by a warming climate,” Dr. Rood said. “It’s impossible to have an event, especially an extreme event, that doesn’t have some relation to climate change.”

While flooding has always been a natural occurrence, heavier rains are arriving more often as greenhouse gas emissions, largely caused by the burning of fossil fuels, continue to rise. Higher temperatures on both land and sea mean more moisture is held in the atmosphere. And a hotter planet creates more energy that can cause storms to more efficiently rain out that moisture, potentially leading to a more violent storm.

“To prevent such catastrophic outcomes in the future, we as Europe have to accelerate our flood adaptation,” Dr. Knispel de Acosta said. Those adaptations could include improved storm water management systems, better urban planning, more accessible early warning systems and growing investments in green infrastructure, like replacing concrete surfaces with more permeable materials or planting more trees.

“Our infrastructure was built for a climate that no longer exists,” said Diana Urge-Vorsatz, a professor at Central European University and vice chair of the Intergovernmental Panel on Climate Change. But while these storms become more common, even experts can forget how pervasive they have become.

Before Storm Boris hit Central Europe, Dr. Urge-Vorsatz was warned by another climate scientist to stock up on enough food and emergency supplies to last at least three days. It’s going to be really bad, she said he had told her. She didn’t listen. Then floodwaters and strong winds threatened her home just outside Vienna.

“We know that with climate change, the rain is getting more intense and frequent, but no one really believes it when it’s coming,” Dr. Urge-Vorsatz said. “We always think it’s happening to others, and that it can’t happen to me.”

Winds pummeled her neighborhood, knocking down big trees. Schools closed, roadways and transit lines were submerged, power outages flickered across the region, affecting tens of thousands of homes.

The waters are still rising. Forecasters expect flooding to reach eastern Germany by Wednesday.