‘Sponge Cities’ Are Catching On. But Can They Handle Supercharged Storms?

In 2011, a short but catastrophic cloudburst hammered Copenhagen, flooding parts of the Danish city with more than 5 inches of rain in a single day.

The storm caused more than $1 billion in damages. It also catalyzed a transformation across the city . Officials spent the next decade implementing a matrix of green spaces and engineered stormwater infrastructure to sop up future flooding.

Other cities around the world—from Hong Kong to New York—have adopted similar green-gray approaches to curb urban inundation. But as the aptly named “Sponge City” movement grows worldwide, experts say major challenges are keeping cities from reaching their full spongy potential. And with global warming giving rise to wetter storms and more severe droughts, research shows nature’s absorbent abilities are being pushed to the brink.

From sprawling skyscrapers to busy highways, many of the characteristics that make major cities so iconic also put them at risk of severe flooding. When a rainstorm hits, the mostly impermeable materials used to construct roads and sidewalks—such as concrete and asphalt—often wick water into other streets or storm drains.

“We superimposed what we wanted onto the landscape … and then by doing that, we essentially sealed the surface of the landscape,” Franco Montalto, a civil engineer at Drexel University, told me.

While these drainage systems may have held up to storms when they were first constructed, many aren’t equipped to withstand the increasingly severe rainfall brought by climate change, he added.

In New York City, for example, roughly 60 percent of the sewers are part of a centuries-old combined system where stormwater and sewage run through the same pipes to wastewater treatment plants. That means extreme rain events often trigger sewage overflows into key waterways, as my colleague Lauren Dalban reported in 2024 . As an NYC resident, I’ve seen (and smelled) this firsthand.

But in recent years, the Big Apple and many other American cities experiencing similar problems have spent billions of dollars installing a mosaic of rain gardens, green roofs, constructed wetlands and other stormwater-control measures. In Los Angeles, green spaces and shallow basins with porous soil implemented in recent years helped soak up 8.6 billion gallons of water when an atmospheric river hit in 2024, as I covered that year .

The problem? At the moment, these green-gray efforts are more of a patchwork than a network in the U.S., according to Montalto.

“I think the problem with the way green infrastructure has happened in the United States is that it’s been this sort of opportunistic approach where we do it where we can, where it’s easy, where it’s not too expensive, and that hasn’t really turned out to be enough,” he said. “Yes, we have a lot of green infrastructure, but that green infrastructure is not designed, cited, scaled [and] implemented in a way that helps us to reduce flood risks from the extreme events.”

That’s partially because it’s difficult and costly to retrofit existing city infrastructure to accommodate the amount of green space and engineered stormwater structures necessary to take on climate-fueled inundation. Montalto noted the sponge city concept is a “more radical transformation of landscapes,” and that some areas in China—where the sponge city movement took off after Chinese President Xi Jinping endorsed it about a decade ago—are seeing more success because officials are able to integrate this effort earlier in the process of urbanization.

On top of that, extreme storms supercharged by climate change may hinder nature’s ability to help us handle them, experts say.

Nature’s Sponge

A study published in May forecasts that the annual rainfall in much of the world will condense: more rain falling in heavy storms faster than the land can absorb it, which means that the water pooled on the surface is more readily evaporated. Overall, this phenomenon actually dries out the land, the research found.

Additionally, prolonged droughts can also kill organic matter and dry out certain types of soils enough to make them relatively hydrophobic, repelling rainwater rather than soaking it in.

Too much water can also be bad , as any plant owner has likely learned the hard way. In 2021, China’s Zhengzhou—which has invested billions to construct sponge city elements—was overwhelmed by the heaviest rainfall in the city’s recorded history. Experts told Reuters that it’s doubtful any level of green infrastructure in the developed area would have been able to handle this storm, which brought more than a year’s worth of rain in a few days.

“There’s kind of a sweet spot, like you want your soils to be a little bit wet,” climate scientist Justin Mankin told me. He’s an associate professor of geography at Dartmouth College, and he co-authored the May study.

But Boise State University climate scientist Jen Pierce stressed that increasing tree cover and vegetated areas in cities has a number…