Painting roofs white or covering them with a reflective coating is a more effective way to cool buildings in cities like London compared to vegetation-covered ‘green roofs’, University College London (UCL) researchers have said.
As the climate changes, residents within cities are particularly vulnerable to warming temperatures. Cities generally trap heat, inducing the urban heat island effect that can lead to greater discomfort and mortality for residents during hot spells.
The study used a three-dimensional urban climate model of Greater London to test the thermal effects of different passive and active urban heat management systems, including painted ‘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.
Previous research using satellite data has found that rooftop gardens and greenery can help to alleviate severe heat in cities.
But research from the UCL team showed that if adopted widely throughout London, cool roofs would be more effective, reducing outdoor temperatures across the city by an average of 1.2°C, and up to 2°C in some locations. Conversely, extensive use of air conditioning would warm the outside environment by as much as 1°C in London’s dense city centre.
Other systems, such as extensive street-level vegetation or solar panels, would provide a smaller net cooling effect – only about 0.3°C on average across London. However, they do offer other environmental benefits. A report in January suggested that London is “underprepared” for the impact of climate change , which could include severe flooding, extreme heat and wildfires.
Similarly, while green roofs offer benefits like water drainage and wildlife habitats, their net cooling effect on the city was found to be negligible on average.
To gauge the potential full effect of each method, the team modelled each one as though they had been widely adopted throughout Greater London.
Dr Oscar Brousse, lead author of the study, 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.”
In comparison to previous research, the UCL team found that the effect of green roofs was negligible and that their effect on temperature varied significantly throughout the day.
During the warmest times of day, the wide adoption of green roofs could lower urban temperatures by an average of 0.5°C. However, this would be offset overnight as the thermal mass from the roofs would retain daytime heat, releasing when the sun was down and increasing night-time temperatures by about the same amount.
Source: E&T
