White and reflective roofs are the best way to keep cities cool.

In cities like London, painting roofs white or adding reflective coatings is the best way to cool them down during the heatwaves, and these “cool roofs” could work better than solar panels, green roofs or adding more trees to the ground.

Oskar Bruss Researchers at University College London ran a climate simulation to look at how temperatures in London would have changed on the hottest two days of the summer of 2018 if the city had implemented widespread cooling measures, from cooled roofs to air conditioners to cooling solar panels. The highest temperature was 35.6℃. (96.1°F).

The researchers found that the cool roofs outperformed all other countermeasures, reducing the city’s average outdoor temperature by 1.2°C over two days, and by as much as 2°C in some places. By comparison, adding more trees reduced temperatures by just 0.3°C, and solar panels reduced temperatures by 0.5°C.

The study found that while widespread air conditioning may keep indoor temperatures cooler, it could increase outdoor temperatures by up to 1°C in parts of central London. “In London, cool roofs were the most effective at reducing outdoor temperatures at pedestrian level,” Bruce says.

While the study only used two days of data for its simulations, Bruce said: Similar published studies.

Applying reflective coatings or brightening roof surfaces across cities would be a relatively simple, low-cost climate adaptation measure, Bruce says. “In fact, I think it’s probably the easiest thing to implement,” he says. “I see very little reason not to do this more widely.”

Beyond cooling, he says there are other reasons to install technologies: trees and green roofs can boost biodiversity and resident well-being, for example, and solar panels can provide clean electricity.

2023 Report A report by the Greater London Authority suggests that cool roofs could become a policy focus for the city government as summer temperatures rise due to climate change. “As temperatures in London are expected to rise and heatwaves become more frequent, reflective roofs are likely to become a key element of climate adaptation strategies,” the report said.