Cities around the world are trying to drink the rain.
After Copenhagen's catastrophic 2011 cloudburst dumped more than 5 inches of water on the Danish capital in a single day and caused over $1 billion in damages, officials there spent the next decade rebuilding the urban surface to absorb storms rather than repel them. The approach — green roofs, rain gardens, constructed wetlands, permeable basins — has since spread from Hong Kong to New York under the banner of the "sponge city" movement. The question now is whether sponges can keep up with the storms climate change is producing.
The conventional wisdom treats green infrastructure as a clear win. It mostly is. But recent reporting complicates the optimism with two harder truths: U.S. cities are installing this stuff in scattershot fashion, and the climate that made the projects necessary is also degrading the soil and vegetation those projects rely on.
American cities have often built rain gardens and green roofs wherever it's cheap and politically easy, rather than designing connected systems sized for extreme events. The result is real progress that still falls short of what climate-driven storms now demand.
The numbers help frame the scale. In New York City, roughly 60 percent of the sewers are part of a centuries-old combined system where stormwater and sewage share the same pipes, meaning heavy rain routinely pushes raw sewage into local waterways. In Los Angeles, by contrast, green spaces and shallow porous basins helped absorb 8.6 billion gallons of water when an atmospheric river hit in 2024 — a concrete demonstration of what well-placed green infrastructure can do.
China has gone further than most. After President Xi Jinping endorsed the sponge city concept about a decade ago, billions of dollars flowed into retrofits and new construction. Then came Zhengzhou. In 2021, the city was hit with the heaviest rainfall in its recorded history, with more than a year's worth of rain falling in a few days. Experts told Reuters it is doubtful any amount of green infrastructure in a built-up area could have absorbed that volume.
Climate change is also changing how soil behaves. A study published in May forecasts that annual rainfall in much of the world will increasingly arrive in short, heavy bursts that fall faster than land can absorb. Pooled surface water then evaporates more readily, drying out soil over time. Dried-out soil, in turn, becomes hydrophobic — less able to soak up the next storm.
None of this argues against building sponge cities. Trees and vegetated areas still outperform concrete on almost every measure that matters — mental health, water quality, carbon absorption, and yes, flood control. Even an overwhelmed rain garden absorbs more than asphalt does.
What this suggests is a recalibration of expectations. Green infrastructure works best when it is networked rather than scattered, integrated early in urban planning rather than retrofitted into existing grids, and paired with honest acknowledgment that some storms will exceed any system's capacity. The co-benefits — cooler streets, cleaner waterways, more habitat, healthier residents — justify the investment even when the flood math gets harder.
For travelers and city dwellers paying attention to how places adapt, the sponge city story is a useful frame. Climate adaptation is rarely a clean victory. It is a slow, expensive, partially successful rebuild of systems that were never designed for the weather they now face. The cities doing it imperfectly are still doing better than the ones not doing it at all.
