Painting roofs white could be more effective at cooling cities than vegetation-covered ‘green roofs’, street-level vegetation, or solar panels, a new study has found.
Researchers at UCL used a three-dimensional urban climate model of Greater London to test the thermal effects of different passive and active urban heat management systems. These included covering roofs with a reflective coating or white paint to turn them into ‘cool roofs’, rooftop solar panels, green roofs, ground-level tree vegetation, and air conditioning during the two hottest days of the summer of 2018, the warmest on record.
Global climate change means that by 2050 average temperatures in London are predicted to increase 1.2 deg C in winter and 2.6 deg C in summer, compared with the period 1981-2010. These changes are made worse by the urban heat island effect, which causes cities to become hotter than surrounding rural areas.
The model showed that, if adopted widely throughout London, cool roofs would reduce outdoor temperatures across the city on average by 1.2 deg C, and up to 2 deg C in some locations.
By reflecting rather than absorbing heat, cool roofs have the dual benefit of cooling the outside urban environment as well as the inside of buildings.
Conversely, systems such as extensive street-level vegetation or solar panels would provide a smaller net cooling effect of only 0.3 deg C on average across the UK capital. In the case of green roofs, the model showed that their net cooling effect on the city was negligible on average.
However, these solutions offer other environmental benefits, including water drainage and wildlife habitats, in the case of green roofs.
Mitigating extreme heat
Published in Geophysical Research Letters, the research looked into urban heat management by comprehensively comparing several common passive and active heat mitigation actions.
During 2022, there were an estimated 387 all-cause excess deaths in London associated with heat. In July 2022, a national Level 4 heatwave alert was triggered for the first time in England, with an extreme heat warning put in place for all of England and Wales, and parts of Scotland, for at least two days.
The modelling showed that air conditioning, which transfers heat from within buildings to the outside, would warm the outdoor urban environment by about 0.15 deg C for the city overall, but by as much as 1 deg C in denser central London.
The researchers also found that the increase in the distribution of air conditioning units in their model could be entirely powered by photovoltaic solar panels if they were similarly installed to their fullest extent.
Lead research author Dr Oscar Brousse, from UCL Bartlett School of Environment, Energy and Resources, said: “We comprehensively tested multiple methods that cities like London could use to adapt to and mitigate warming temperatures, and found that cool roofs were the best way to keep temperatures down during extremely hot summer days. Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”
