In 2011, a brief but devastating downpour hit Copenhagen, inundating parts of the Danish city with more than 5 inches of rain in one day.
This storm caused more than $1 billion in damage. It also spurred city-wide transformation. Officials spent the next decade implementing a matrix of green space and designing stormwater infrastructure to absorb future flooding.
Other cities around the world, from Hong Kong to New York, have adopted similar green-gray approaches to controlling urban flooding. But as the aptly named “sponge city” movement grows around the world, experts say major challenges are preventing urban sponges from realizing their full potential. And as global warming causes more rain and more severe droughts, research shows that nature’s absorption capacity is being pushed to the brink.
sponge city
From expansive skyscrapers to busy highways, many of the features that make large cities iconic also place them at serious risk of flooding. During storm events, water often seeps from the mostly impermeable materials used to construct roads and sidewalks, such as concrete and asphalt, into other roads and storm drains.
“We superimposed what we wanted onto the landscape…and in doing so, we essentially sealed off the surface of the landscape,” Franco Montalto, a civil engineer at Drexel University, told me.
While these drainage systems may have been able to withstand storms when they were first built, many are not equipped to withstand the increasingly intense rainfall caused by climate change, he added.
For example, in New York City, approximately 60 percent of the sewer system is part of an integrated system that has been in use for centuries, with stormwater and sewage flowing through the same pipes to wastewater treatment plants. This means, as my colleague Lauren Dalban reported in 2024, that extreme rains often result in sewage flooding into major waterways. As a New York City resident, I’ve seen (and smelled) this firsthand.
But in recent years, the Big Apple and many other American cities facing similar problems have spent billions of dollars installing mosaic rain gardens, green roofs, constructed wetlands and other stormwater mitigation measures. In Los Angeles, as I reported that year, recently created green spaces and shallow basins of porous soil helped absorb 8.6 billion gallons of water when the atmospheric river flooded in 2024.
problem? For now, these green-gray efforts are more of a patchwork than a network in the U.S., Montalto said.
“I think the problem with the way green infrastructure works in the United States is this kind of opportunistic approach of doing it where you can, where it’s easy, where it’s not very expensive, and that’s not really 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 reduces the risk of flooding from extreme events.”
Part of the reason is that it is difficult and expensive to retrofit existing urban infrastructure to accommodate the amount of green space and artificial stormwater structures needed to cope with climate change-induced flooding. Montalto said the sponge city initiative is a “more fundamental transformation of the landscape,” noting that in some parts of China, where the sponge city movement began after Chinese President Xi Jinping endorsed it about a decade ago, it has been more successful because authorities can incorporate the initiative earlier in the urbanization process.
On top of that, experts say extreme storms intensified by climate change could hamper nature’s ability to help us cope with them.
natural sponge
A study published in May predicted that annual rainfall would become more concentrated in many parts of the world. This means that during intense storms, rain falls faster than the land can absorb it, making it easier for water that collects on the ground to evaporate. Overall, the study found, the phenomenon is actually drying out the land.
Additionally, prolonged droughts can kill organic matter and make certain soil types dry and relatively hydrophobic, allowing rainwater to repel rather than soak in.
As any plant owner has learned the hard way, too much water is also bad. In 2021, the Chinese city of Zhengzhou, which has invested billions of dollars in building sponge cities, experienced the heaviest rainfall in the city’s history. Experts told Reuters it was questionable whether any level of green infrastructure in the developed region could have coped with the storm, which dumped more than a year’s worth of rain in just a few days.
“There’s kind of a sweet spot where you want to keep the soil a little bit moist,” climate scientist Justin Mankin told me. He is an associate professor of geography at Dartmouth College and a co-author of the May study.
But Jen Pearce, a climate scientist at Boise State University, emphasized that increasing tree cover and vegetation in urban areas has many benefits for people and the environment, including improving mental health, cleaning waterways and absorbing climate-warming carbon. And even in the heaviest storms, green spaces are always better at absorbing water than impermeable cement or asphalt, she said.
“If you’ve already paved your paradise and installed a parking lot, you don’t have many options,” Pearce said.
More top climate news
last week, A federal judge has temporarily blocked the Trump administration’s efforts to remove or edit national park signs and materials. It “inappropriately belittles Americans” and casts America “in a negative light,” Maxine Joserow reports in the New York Times. After President Donald Trump issued the order, the National Park Service removed billboards across the country related to issues such as slavery, racial inequality and climate change. In this latest ruling, the judge said: “This not only undermines the integrity of our national parks, but also sets a dangerous precedent for censorship and sanitization.”
Ebola has spread across Africa, infecting hundreds of people in the Democratic Republic of the Congo, the worst-hit country. Humans aren’t the only ones at risk. Experts are concerned that the outbreak could harm the country’s vulnerable western lowland gorillas.Kayleigh Long is a reporter in Mongabay. The DRC has seen at least 17 Ebola outbreaks in the past 50 years, many since the early 2000s, jumping into endangered gorilla populations and killing thousands of animals. According to virologists, gorillas are highly social in nature, which puts them at a higher risk of spreading infection if they become infected.
According to a new United Nations report, Approximately half of the world’s children are regularly exposed to at least three types of climate change.Mason W.C. Bunting reported in the Guardian. From heatwaves to severe flooding, these threats are destroying children’s health, education and quality of life. The report says children in high-income countries are not immune, but developing countries are most affected.
“The lives of children continue to be transformed by the effects of heatwaves, wildfires, droughts and floods,” Catherine Russell, executive director of the United Nations Children’s Fund, said in a statement.
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Kylie Price is a reporter for Inside Climate News with a particular interest in wildlife, ocean health, food systems and climate change. She writes ICN’s newsletter, Today’s Climate, which features the most pressing environmental news each week.
She earned a master’s degree in science journalism from New York University and a bachelor’s degree in biology from Wake Forest University. Her work has appeared in National Geographic, Time, Scientific American, and more. She is a former Pulitzer Press Fellow who spent a month in Thailand covering the intersection of Buddhism and the country’s environmental movement.
