Other cities around the world\u2014from Hong Kong to New York\u2014have adopted similar green-gray approaches to curb urban inundation. But as the aptly named \u201cSponge City\u201d 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\u2019s absorbent abilities are being pushed to the brink.\u00a0<\/p>\n
Sponge Cities\u00a0<\/p>\n
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\u2014such as concrete and asphalt\u2014often wick water into other streets or storm drains.\u00a0<\/p>\n
\u201cWe superimposed what we wanted onto the landscape \u2026 and then by doing that, we essentially sealed the surface of the landscape,\u201d Franco Montalto, a civil engineer at Drexel University, told me.\u00a0<\/p>\n
While these drainage systems may have held up to storms when they were first constructed, many aren\u2019t equipped to withstand the increasingly severe rainfall brought by climate change, he added.\u00a0<\/p>\n
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<\/a>. As an NYC resident, I\u2019ve seen (and smelled) this firsthand.\u00a0<\/p>\n But in recent years, the Big Apple and many other American cities<\/a> 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<\/a>.\u00a0<\/p>\n The problem? At the moment, these green-gray efforts are more of a patchwork than a network in the U.S., according to Montalto.<\/p>\n \u201cI think the problem with the way green infrastructure has happened in the United States is that it\u2019s been this sort of opportunistic approach where we do it where we can, where it\u2019s easy, where it\u2019s not too expensive, and that hasn\u2019t really turned out to be enough,\u201d he said. \u201cYes, 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.\u201d\u00a0<\/p>\n That\u2019s partially because it\u2019s 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 \u201cmore radical transformation of landscapes,\u201d and that some areas in China\u2014where the sponge city movement took off after Chinese President Xi Jinping endorsed it about a decade ago\u2014are seeing more success because officials are able to integrate this effort earlier in the process of urbanization.\u00a0<\/p>\n On top of that, extreme storms supercharged by climate change may hinder nature\u2019s ability to help us handle them, experts say.\u00a0<\/p>\n Nature\u2019s Sponge<\/p>\n A study published in May<\/a> 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.\u00a0<\/p>\n 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.\u00a0<\/p>\n Too much water can also be bad<\/a>, as any plant owner has likely learned the hard way. In 2021, China\u2019s Zhengzhou\u2014which has invested billions to construct sponge city elements\u2014was overwhelmed by the heaviest rainfall in the city\u2019s recorded history. Experts told Reuters<\/a> that it\u2019s doubtful any level of green infrastructure in the developed area would have been able to handle this storm, which brought more than a year\u2019s worth of rain<\/a> in a few days.\u00a0<\/p>\n \u201cThere\u2019s kind of a sweet spot, like you want your soils to be a little bit wet,\u201d climate scientist Justin Mankin told me. He\u2019s an associate professor of geography at Dartmouth College, and he co-authored the May study.\u00a0<\/p>\n But Boise State University climate scientist Jen Pierce stressed that increasing tree cover and vegetated areas in cities has a number of benefits for people and the environment, including improving mental health<\/a>, cleaning up waterways<\/a> and absorbing climate-warming carbon. And even in the most severe storms, green spaces are always going to be better at absorbing water than impermeable cement or asphalt, she told me.\u00a0<\/p>\n \u201cIf you\u2019ve already paved paradise and put in a parking lot,\u201d Pierce said, \u201cthen you really don\u2019t have many options.\u201d<\/p>\n More Top Climate News<\/p>\n Last week, a federal judge temporarily blocked the Trump administration\u2019s effort to remove or edit signs and materials at national parks that \u201cinappropriately disparage Americans\u201d or cast the United States \u201cin a negative light,\u201d Maxine Joselow reports for The New York Times<\/a>. After President Donald Trump issued the directive, the National Park Service took down signs all over the country related to such issues as slavery, racial inequality and climate change. In this latest ruling, the judge wrote: \u201cNot only does this undermine the integrity of the national parks; it sets a dangerous precedent of censorship and sanitization.\u201d\u00a0<\/p>\n An Ebola virus is spreading across Africa, with hundreds infected in the hardest-hit Democratic Republic of the Congo. Humans aren\u2019t the only ones at risk: Experts are concerned the outbreak could hurt vulnerable western lowland gorillas in the country, Kayleigh Long reports for Mongabay<\/a>. The DRC has had at least 17 Ebola outbreaks over the past 50 years, and many of them have jumped into critically endangered gorilla populations, killing thousands of the animals since the early 2000s. The gorillas\u2019 highly social nature makes them more at risk of spreading the disease if they catch it, according to virologists.\u00a0<\/p>\n
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