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White roofs could be most effective at cooling

UK: Painting roofs white would be more effective at cooling cities like London than vegetation-covered “green roofs”, street-level vegetation or solar panels, a new study claims.

The study, led by University College London (UCL) researchers and published in Geophysical Research Letters, 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 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.

It found that if adopted widely throughout London, cool roofs could reduce outdoor temperatures across the city on average about 1.2ºC, and up to 2ºC in some locations. 

Other systems, such as extensive street-level vegetation or solar panels would provide a smaller net cooling effect, only about 0.3ºC on average across London, though they offer other environmental benefits. 

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.

Air conditioning would warm the outdoor urban environment by about 0.15ºC for the city overall, but by as much as 1ºC in dense central London, the study suggests. 

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.

To gauge the potential full effect of each method, the team modelled each one as though they had been as widely adopted as theoretically feasible across housing, commercial and industrial buildings throughout Greater London.

“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,” commented lead author Dr Oscar Brousse of UCL Bartlett School Environment, Energy & Resources. “Other methods had various important side benefits, but none were able to reduce outdoor urban heat to nearly the same level.”

Though on average the effect of green roofs was negligible, the researchers found 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 0.5ºC. 

However, researchers say that 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.

Converting city green spaces from grass to deciduous tree cover would cool temperatures overnight but at best would have mixed net effects during the day, the report states. In addition, it would likely increase the amount of water vapour in the air, which would effectively increase the air humidity and could affect residents’ thermal comfort.

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