Scared passengers in flooded subway cars in Zhengzhou, China. Water tumbles down stairs on the London Underground. A woman wades through murky, waist-high water to reach a subway platform in New York City.

Metro systems around the world are struggling to adapt to an era of extreme weather conditions caused by climate change. Their designs, many based on expectations from another era, will be overwhelmed and investment in upgrades could be weighed down by a drop in passenger numbers caused by the pandemic.

“It’s scary,” said Sarah Kaufman, assistant director of the Rudin Center for Transportation at New York University. “The challenge is how can we prepare for the next storm, which should be 100 years away,” she said, “but could it happen tomorrow?”

Local public transport plays a crucial role in reducing car traffic in large cities and thereby reducing emissions from cars that contribute to global warming. If commuters are frightened by images of flooded train stations and avoid subways for private cars, traffic experts could have a significant impact on urban air pollution and greenhouse gas emissions.

Some networks, like those of London or New York, were designed and built more than a century ago. While a few, like Tokyo, have succeeded in strengthening their flood defenses, the crisis in China this week shows that even some of the world’s newest systems (Zhengzhou’s system is less than a decade old) can be overwhelmed.

Retrofitting subways to prevent flooding is “a huge undertaking,” said Robert Puentes, executive director of the Eno Center for Transportation, a nonprofit think tank focused on improving transport policy. “But when you compare it to the cost of doing nothing, it makes a lot more sense,” he said. “Doing nothing costs a lot more”

Adie Tomer, a senior fellow in the Brookings Institution’s Metropolitan Policy Program, said subways and rail systems are helping fight sprawl and reduce people’s energy use. “Underground trains and fixed railways are part of our climate solution,” he said.

The recent floods are another example of extreme weather conditions compatible with climate change around the world.

Just a few days before the underground nightmare in China, around 160 people were killed in floods in Germany. Great heat waves have plunged Scandinavia, Siberia, and the Pacific Northwest of the United States into misery. Forest fires in the American West and Canada last week sent smoke across the continent, setting off health warnings that gave the sun an eerie reddish hue in cities like Toronto, Philadelphia and New York City.

Flash floods have also inundated streets and highways in recent weeks. The collapse of part of California’s Highway 1 into the Pacific Ocean after heavy rains that year was a reminder of the fragility of the country’s roads.

However, more intense flooding poses a particular challenge to the aging metro systems in some of the world’s largest cities.

In New York, the Metropolitan Transportation Authority has invested $ 2.6 billion in resilience projects since Hurricane Sandy flooded the city’s subway system in 2012, including the paving of 3,500 subway shafts, stairwells and elevator shafts Floods. Even on a dry day, a network of pumps pulls about 14 million gallons, mostly groundwater, out of the system. Still, this month flash floods showed that the system is still fragile.

“It’s challenging to work within the confines of a city with aging infrastructure and a pandemic-recovering economy,” said Vincent Lee, assistant director and technical director for water at Arup, an engineering firm working on upgrading eight U. – Was involved in rail stations and other facilities in New York after the 2012 storm.

London’s sprawling Underground is facing similar challenges.

“Much of London’s drainage system dates back to the Victorian era,” said Bob Ward, policy director at the Grantham Research Institute on Climate Change and the Environment in London. And that has a direct impact on the city’s subway system. “At the moment it is simply not able to cope with the increase in heavy rain that we are experiencing due to climate change.”

Meanwhile, the crisis in China this week shows that even some of the world’s newest systems can be overwhelmed. Robert E. Paaswell, professor of civil engineering at the City College of New York, said: “Subway trains are flooding. They are flooded because they are underground. “

To understand how underground flooding works, Taisuke Ishigaki, a researcher in the Department of Civil Engineering at Kansai University in Osaka, Japan, built a diorama of a city with a busy subway system and then set off a deluge that lasted around Nov. Inches of rain equals a single day.

In a matter of minutes, floods broke through several subway entrances and began flowing down the stairs. Only 15 minutes later, the platform of the diorama was under eight feet of water – a sequence of events that Dr. Ishigaki this week when it happened in real life in Zhengzhou. There, floods quickly overwhelmed the passengers who were still standing in the subway cars. At least 25 people died in and around the city, 12 of them in the subway.

The research of Dr. Ishigaki is now flowing into a flood monitoring system used by Osaka’s extensive underground network, which uses special cameras to monitor surface flooding during heavy rainfall. Water above a certain security level activates emergency protocols, where the most vulnerable entrances are cordoned off (some can be closed in less than a minute) while passengers are promptly evacuated from the subway through other exits.

Japan has made other investments in its flood infrastructure, such as cavernous underground cisterns and sluice gates at subway entrances. Last year, the private rail operator Tokyu, with the support of the Japanese government, completed a huge cistern to collect and divert up to 4,000 tons of flooding at Shibuya Station in Tokyo, an important hub.

However, should the many rivers that flow through Japanese cities break, “even these defenses will not be enough,” said Dr. Ishigaki.

Public transportation advocates in the United States are calling for pandemic relief supplies to be provided for public transportation. “The scale of the problems has grown larger than our cities and states can handle,” said Betsy Plum, executive director of the Riders Alliance, an advocacy group for subway and bus drivers.

Some experts suggest a different approach. With more extreme flooding on the line, it will be impossible to keep subways safe all the time, they say.

Instead, investments are required in buses and cycle paths that can serve as alternative means of public transport in the event of subway flooding. Natural defenses could also provide relief. Rotterdam in the Netherlands has grown crops along its trams that make it possible to soak up rainwater from the ground and reduce heat.

“During the pandemic, you saw people get around on their bicycles, the most resilient, least disruptive, inexpensive, and low-carbon mode of transport,” said Anjali Mahendra, director of research at the World Resources Institute’s Ross Center for Sustainable Cities. a Washington-based think tank. “We really have to do a lot more to connect parts of cities and neighborhoods with these bike corridors that can be used to get around.”

Some experts wonder why public transport has to be underground in the first place, saying that local public transport should recapture the road. Street-level trams, bus systems, and bike lanes are not only less prone to flooding, they’re cheaper to build and more accessible, said Bernardo Baranda Sepúlveda, a Mexico City-based researcher at the Institute of Transportation Development, a transportation nonprofit.

“We have this indolence from the last century to give cars so much space above the ground,” he said. “But one bus lane carries more people than three car lanes.”