A Triple Threat
Nasa revealed new data that shows there has been a dramatic rise in the intensity of weather events including droughts and floods in the past five years. Urban centres are exposed to extremes: flooding overwhelms drainage systems, while drought leaves reservoirs and aquifers depleted. Rising heat compounds both problems, intensifying evaporation, straining energy systems, and worsening health risks for city residents. In many cities, when the rain comes, it rains hard. Wide expanses of concrete prevent water from seeping into soil, sending torrents of runoff into drains that overflow. In dry periods, those same impermeable surfaces prevent water from being absorbed and stored in the ground.
Mexico City is an example of this imbalance. In 2024, the city endured a severe drought that drove key reservoirs to critically low levels, prompting concerns about “Day Zero” scenarios and significant water cuts. At its worst, parts of the city faced water rationing, and the Cutzamala reservoir system dropped to around 34.7 % of its capacity. One year later, in 2025, the city is facing a different stress: torrential rains. June 2025 was the city’s rainiest June in 21 years, as reported by national and international media. The paradox is that both drought and flooding stem from a disrupted urban water cycle. Without space for rainfall to be absorbed, stored, and gradually released, cities swing between scarcity and excess.
Sponge as a Solution
The sponge city concept emerged in China in the early 2010s, after catastrophic flooding spurred a search for better urban water management. A sponge city uses the landscape to retain water at its source, slow down the flow and clean it during the process. It is designed to behave more like a natural system: it soaks up rainfall where it falls and slowly releases it when needed. A city’s ability to absorb water, or its ‘sponginess’ is actually a key measure of climate resilience.
Hydrologists such as Dr Heiko Sieker (Honorary Professor of Urban Hydrology at the Technical University of Berlin) often describe the sponge city not as a single technology, but as a “toolbox” combining low-tech interventions like green roofs, permeable pavements, and swales (vegetated trenches that capture runoff). Together, these elements transform urban surfaces into absorptive systems.
By enabling rain to infiltrate soil, sponge cities can recharge groundwater, reduce flood peaks, and buffer drought stress. The same green areas that absorb rainfall also help filter pollutants and support biodiversity.
How a Sponge Approach Offers Cooling
The cooling power of sponge cities derives from evapotranspiration: water stored in soil and vegetation evaporates, pulling heat out of the surface and lowering air and surface temperatures. Vegetation also shades buildings and pavements, reducing the amount of solar radiation absorbed and further curbing heat buildup in dense urban areas. Modelling and empirical studies show that sponge-inspired designs can mitigate the urban heat island effect by altering surface moisture and vegetation cover. Other literature notes that sponge city interventions help moderate radiant and energy balance dynamics in dense urban settings.
Global Proof
China has a national sponge city programme that moved the concept from theory into practice. In 2013, the Chinese government launched the initiative which included running a pilot project in Wuhan, a city known for its many lakes. Wuhan was chosen due to its susceptibility to flooding but also the fact it was undergoing rapid urbanisation.
Wuhan undertook a comprehensive approach, integrating nature-based solutions such as rain gardens, green roofs, permeable pavements, and artificial wetlands. The city implemented 380 sponge projects designed to absorb and divert rain water to artificial lakes. Since their construction, local air quality and biodiversity have improved, and lower temperatures have been recorded at the Yangtze River Beach Park.
Between 2015 and 2016, the government of China invested US$2billion for 16 pilot sponge cities stating that 20% of the city’s land should be constructed to sponge city standards by 2020 and 80% by 2030.
After extreme heat and floods caused problems across Germany, the country has turned to nature-based solutions to strengthen its resilience. A major focus has been expanding cities’ capacity to absorb water and cool buildings through green roofs and façades, which imitate the natural water cycle by soaking up rainfall where it lands. These measures not only lower surface and indoor temperatures but also ease pressure on sewer systems, support biodiversity, and improve air quality.
By 2019, around two-thirds of German cities had made green roofs mandatory in local development plans which is double the proportion in 2010. That same year, Germany’s total green roof area was estimated at 120 million m², more than double the area a decade earlier, with an additional 7.2 million m² added in 2019 alone.
Other cities are adopting similar sponge principles to build drought resilience. Los Angeles is also scaling up sponge-style systems, aiming to capture enough stormwater by 2040 to meet the needs of over half a million residents each year. Whereas in El Salvador is in a programme called CityAdapt which includes a forest and coffee farm restoration project. The idea is that when vegetation is replaced with concrete, the ground loses its permeability. Trees and other vegetation act as sponges, drawing water into the earth, preventing erosion, limiting floods and recharging groundwater supplies for times of drought. When cities treat water as a circular resource, sponge infrastructure can mitigate both flooding and drought simultaneously.
These varied experiences show that large-scale urban resilience depends on innovation and policy commitment. Part of their success lies in codifying sponge and green infrastructure principles into national or municipal regulations, turning what were once pilot projects into standard practice.
Turning Ambition into Action
Despite the promise, sponge city initiatives face some hurdles. Retrofitting dense urban districts is costly, spatially complex, and disruptive. Beyond technical challenges, barriers often lie in governance. Responsibility for stormwater, urban planning, parks, and infrastructure typically resides in separate agencies, making coordination difficult.
As with many sustainability initiatives, success for sponge cities is only possible with political commitment and stakeholder buy-in from architects, city planners, and engineers. Collaboration is essential. As we have seen with the examples above, cities that succeed with sponge programmes do so because sponge principles are codified, not optional.
Moreover, maintenance is an important aspect of planning. Without proper upkeep, infiltration systems clog, vegetation fails, and performance declines. Studies on the success of sponge cities in practice emphasise that clear maintenance regimes are critical for long-term success.
Looking Ahead
As the climate crisis deepens, sponge cities offer a compelling path forward. They demonstrate that low-tech, nature-based solutions can restore urban water balance and buffer against floods, heat, and drought simultaneously.
By restoring the ability to soak rather than repel water, sponge design helps cities transition from oscillating between deluge and drought to a more stable, sustainable hydrological cycle. But sponge cities are about more than just water management – they reshape how urban life interacts with nature, offering a blueprint for healthier, more resilient cities that prioritize both human and environmental well-being.
However, the success of sponge cities depends on more than just innovative design. It requires commitment from governments, collaboration across sectors, and most importantly, long-term maintenance. Sustainable change is driven not only by the technologies implemented but also by the policies and partnerships that support them.
As a member of the World Economic Forum’s Global Commission on Nature-Positive Cities, Impact One played an active role in shaping the ‘Nature-Positive: Guidelines for the Transition in Cities’ report. This comprehensive guide supports local governments and businesses by providing clear steps on how to reverse biodiversity loss by 2030. It outlines practical solutions for incorporating nature into urban planning, regulations, incentives, and economic decisions – all aimed at building resilient, nature-positive cities.
But the challenge extends beyond technical solutions. Collaboration across sectors is crucial. Public and private entities must work together to implement solutions that benefit both people and nature. This means integrating nature-based solutions into urban design, creating policies that incentivize sustainable development, and ensuring that every aspect of urban life is aligned with the goal of a more sustainable future. By bringing together the best minds from governments, businesses, and local communities, we can build cities that not only address current climate challenges but also set a new global standard for sustainability and resilience.
Sponge Cities: Managing Floods, Heat, and Drought
Cities worldwide are increasingly vulnerable to extreme weather events. Droughts are lasting longer, heatwaves are becoming more intense, and storms are bringing rainfall that existing infrastructure can no longer handle. Sponge cities offer a nature-based solution to these challenges by restoring natural water cycles. Through innovative urban design, sponge cities incorporate green infrastructure like green roofs, permeable pavements, and swales to absorb and manage rainfall, mitigate the urban heat island effect, and provide resilience against droughts. Originally developed in China, the sponge city concept has been adopted globally. As climate change continues to intensify, sponge cities provide an affordable, scalable solution for creating sustainable, climate-resilient urban environments.