21st-century storms are overwhelming 20th-century cities

In just a few hours on Wednesday night, between 6 and 10 inches of rain fell on New York City—more than has fallen on San Jose, California, in the past year. Water rose in basement apartments and leaked through roofs. Rain streamed into subway stations and pooled on the tracks. The remains of Hurricane Ida, which had thrashed the Gulf Coast earlier in the week, brought floods to the Northeast. Across the region, the death toll reached 40 by Thursday evening. Subway delays and suspensions continue.

The city’s infrastructure, you see, was built in the late 19th and early 20th centuries to withstand the sort of storm that comes every five to 10 years. Now brutal, record-breaking storms are an annual occurrence. What was left of Ida transformed the scene of everyday commutes into a disturbing reminder that climate change comes for us all. Wildfire thunderclouds in the West, blackouts in Texas, hurricanes in the South, torrential downpours in the East: “It’s all the stuff we said would happen 20 years ago,” says Zeke Hausfather, a climate scientist and the director of climate and energy at the Breakthrough Institute. “It’s just a little crazy to see it all happening at once.”

The storm flooded roadways. But it also inundated the alternatives aimed at getting people out of their cars: bike lanes, sidewalks, and public transit systems. For a time in New York on Thursday, all that was underwater. The images of water spilling into subway stations brought the crisis home. “You don’t have to be a person with a great understanding of infrastructure to know that that is a problem,” says Michael Horodniceanu, former president of the Metropolitan Transportation Authority’s Capital Construction Company and now the chair of the Institute of Construction Innovations at NYU. “We’re starting to see the results of what is, in my view, a certain amount of lax attention to what our infrastructure is doing.”

New York had its first climate-related wake-up call nine years ago, when Hurricane Sandy brought a storm surge that flooded low-lying areas and, yes, subway stations. Since then, the city has spent almost $20 million on climate-proofing the city, according to the Mayor’s Office of Resiliency. But some of that funding went to solving a different problem than the one presented by Ida: water coming from the rivers. This week, all the wet stuff fell from the sky, threatening even areas above sea level.

Ida’s remnants dumped all that water on the Northeast because of a climatic quirk. You might expect less rainfall on a warming planet, but some parts of the world, including the US’ Northeast and Midwest, are seeing an increase in heavy precipitation. Temperature directly affects how much moisture the atmosphere can “hold” before it starts raining, says Hausfather. Cooler air holds less moisture—and hotter air holds more moisture that then falls as rain.

A hurricane feeds on heat: Ida intensified so quickly because abnormally warm waters in the Gulf of Mexico boosted it just before landfall, resulting in 150-mile-per-hour winds. As a swirling mass of warm air, Ida held on to a whole lot of moisture. So even though the winds abated as it pushed inland, the storm carried an incredible amount of moisture north, drenching states along the way.

Climate change did not create Hurricane Ida, but scientists know how climate change is making hurricanes like Ida worse. “It’s one of the most basic physical relationships we have in the climate: For every one degree [Celsius] you warm the atmosphere, you get about 7 percent more moisture in the air, and that means that you can have much heavier rainfall events,” says Hausfather. “Hurricanes have gotten wetter in the last few decades, and that’s projected to continue into the future.” Scientists have also shown that hurricanes have been intensifying more rapidly in recent years, as Ida did, due to warming waters in the gulf.

No one could foresee this when the bones of New York City were pieced together more than 100 years ago. When engineers dream up a sewer system, they imagine the worst storm the system could drain, a storm that may only come once in 10 or 20 years. New York’s is designed for a once-in-five-years storm. Scientists still need to tabulate the monster that just inundated the city, but it sure as hell wasn’t a one-in-five. The metric would be more like centuries.

The nature of Wednesday’s storm posed another problem. Intense rainfall is often caused by small cells moving over a city, says Carnegie Institution for Science environmental engineer David Farnham, who’s studied New York’s sewer system. “So it may be raining everywhere, but it’s really intense in a smaller area.”

So in Maplewood, New Jersey, 8.39 inches of rain fell between Wednesday night and Thursday morning. But Millburn, just 3 miles away, got about half as much—4.4 inches. Even that left its downtown muddy and full of puddles come morning.

Now, after years of updates, 60 percent of New York City has a combined sewer system, which uses a single pipe to carry both wastewater and stormwater to treatment plants. During heavy rainstorms, the system can get quickly overwhelmed. The detritus of city living—trash, plants, general gunk—clogs drains, further gumming up the works. “So if you get a really big kahuna like this, I don’t think it really has a shot at draining that out fast enough to avoid flooding,” says Farnham.

The city has worked to separate those combined sewer systems and to clear clogged drains, especially when storms threaten. It has raised and in some cases eliminated subway grates, which were built to allow fresh air to flow down to dank underground spaces but which now look like holes to let more water in. In some places, the MTA constructed flood-proof doors, which can close when the water gets too close.

More generally, cities like New York can create more green infrastructure to help with their water problems—basically, less pavement and more dirt. You might, for instance, create roadside green spaces where water can percolate before moving into stormwater drains, removing trash and pollution in the process. Los Angeles has been doing this to catch rainwater. “This is a long-term thing,” says Horodniceanu. Retrofitting cities to deal with what’s coming, and what’s already come, will take gobs of one of the scarcest resources of all: much more funding.

This story originally appeared on wired.com.