As Europe swelters through increasingly brutal summers, a simple resource may offer relief: the rain that falls on our rooftops. New research from the University of Manchester suggests that harvesting this water and using it strategically to cool buildings could significantly reduce both urban temperatures and the energy consumed by air conditioning.
The study, published in the journal Earth's Future, modelled a system where rainwater collected from roofs is stored and automatically sprayed onto building surfaces during heat spikes. The findings indicate that cooler roofs transfer less heat into buildings, cutting the need for air conditioning and the associated energy use. Less air con also means less waste heat is pumped back into the streets, creating a virtuous cycle that could lower peak temperatures and reduce the number of heatwave days.
While the case study focused on Tokyo, the implications for European cities from Madrid to Warsaw are clear. With heatwaves intensifying across the continent, urban planners are searching for passive cooling solutions that don't rely on fossil fuels or strain the grid.
Timing matters more than tank size
The research, led by Dr Zhonghua Zheng, found that the trigger for activating the sprinklers—such as a specific roof temperature—had a greater impact than either the size of the storage tank or the volume of water applied. Surprisingly, very large tanks delivered only "modest additional reductions" in energy use and extreme heat. Applying extra water didn't always lead to more cooling because some of it remained on the roof instead of evaporating.
"Cities around the world are facing growing challenges from extreme heat," says Dr Zheng. "Air conditioning can help keep people safe and comfortable but it also consumes large amounts of energy and releases additional heat into the urban environment. Our study shows that harvesting rainwater from roofs and using it strategically for cooling could provide a practical way to reduce both energy demand and urban temperatures."
The benefits become "even greater during hotter years," he adds, suggesting the approach could become increasingly vital as the climate warms. The system also offers a co-benefit: reducing extreme urban runoff during heavy downpours, a growing concern as storms become more intense.
"The rainwater tank also provides an additional co-benefit on reducing the extreme urban runoff," says Junjie Yu, a PhD researcher at Manchester. "This approach exemplifies a 'natural solution to natural challenges', in which rainwater serves as a natural resource to mitigate both thermal stress and hydrological extremes."
Why air con isn't the answer
The International Energy Agency (IEA) estimates that space cooling—mostly air conditioning, but also fans—consumed around 2,100 terawatt-hours (TWh) of power in 2022, roughly seven per cent of global electricity. With installations projected to triple by 2050, the strain on Europe's energy grids could become critical, especially as hydropower output has hit record lows during droughts.
Beyond energy, air conditioning relies on refrigerants such as hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs), which are potent greenhouse gases. The EU and UK are phasing out these fluorinated gases in favour of natural alternatives like propane and CO2, but propane's flammability complicates installation. Even with cleaner refrigerants, the very act of dumping heat from inside buildings to the outside exacerbates the urban heat island effect—where heat is trapped between buildings and absorbed by asphalt and concrete, making cities hotter than surrounding rural areas.
Cool roofs: a complementary strategy
The Manchester study adds to a growing body of evidence that passive measures can make a real difference. A 2024 study by UCL and the University of Exeter analysed the impact of "cool roofs"—roofs painted white or reflective colours—on London's air temperature during the summer of 2018, one of the hottest on record. The researchers found that if cool roofs had been widely installed, the city could have cooled by about 0.8°C on average.
"If widely adopted, cool roofs can significantly reduce the ground-level air temperature of a city," says lead author Dr Charles Simpson from the UCL Bartlett School Environment, Energy & Resources.
For European policymakers, the message is clear: a combination of rainwater harvesting, cool roofs, and green infrastructure could help cities withstand the growing threat of extreme heat without relying solely on energy-intensive air conditioning. As schools and businesses rethink their routines, these nature-based solutions offer a practical, low-cost path to a cooler future.
