While the jury might be out on the benefits of white roads, most urbanists agree that planting trees is one of the best ways to cool cities down – without the danger of reflecting sunlight where it isn’t wanted. Initial measurements showed the coating could reduce temperatures by up to 5C, although others have suggested that, while the road surface itself may be cooler, the reflected sunlight actually makes nearby pedestrians feel much hotter. In response, the city has trailed painting roads with a white reflective coating – at a cost of $40,000 a mile. Meanwhile, the road-riddled city of Los Angeles faces a different dilemma: more than 10% of the urban land area is black asphalt, which absorbs up to 95% of the sun’s energy. Paris's buried river Bièvre was described as 'oily and black, streaked with acids'. The city’s Cool Roofs campaign, which was launched in 2009, has already seen over 900,000 sq metres of roof space covered in a white reflective coating, saving almost 4,000 tonnes of CO 2 a year from cooling emissions. Research by Nasa has shown that a white roof in New York City can be 23C cooler than a typical black asphalt roof on the hottest day of the summer. In the US, the roof-lightening crusade is already under way. Modifying only half of the industrial and commercial buildings, with their expansive rooftops, had the same impact on lowering temperatures as adapting all of the residential buildings in the city – suggesting that real change could be easily implemented with a tweak to planning policy. The more reflective the rooftop, the lower the surrounding air temperature will be.” The modelling study, which focused on Birmingham and the West Midlands, found that the type of building made a difference, too.
“Think of Mediterranean countries, where the houses are painted white. “Cool roofs can be really simple,” said co-author of the study Dr Clare Heaviside, who is now associate professor at University College London’s Institute for Environmental Design and Engineering. Photograph: Anthony Wallace/AFP/Getty Images
Powerful cooling effect … the revived Cheonggyecheon River in Seoul the project cost $900m. That might not sound like a huge difference, but the scientists concluded that such a drop could reduce the number of heat-related deaths by up to a quarter – a massive life-saving measure, given there were over 2,500 excess deaths from heat during the 2020 heatwave. Researchers at the University of Oxford found that making the rooftops of buildings a lighter, more reflective colour could reduce daytime temperatures by up to 3C during a heatwave. It sounds too simple a solution, but some argue that one of the most effective measures to cool cities down is to make their surfaces reflect light, rather than absorb it – particularly where you might not think to look: up on the roof. The biggest cause of the urban heat island effect – which can make cities up to 10C warmer than neighbouring rural areas – is the stuff they are made of: hard, dark, dense materials like concrete, brick, tarmac and asphalt, which absorb the sun’s heat during the day, and re-radiate it at night.
So how can we adapt our buildings, streets and public spaces to cope, without resorting to pumping out energy-hungry, mechanically chilled air and further heating our planet in the process? As hot, developing nations become more prosperous, and prosperous nations become hotter, the International Energy Agency estimates that the energy spent on air conditioning will triple by 2050 – a growth equivalent to the current electricity demand in the US and Germany combined. The US expends as much energy on it each year as the UK uses in total, while during a recent heatwave in Beijing, half of the city’s power capacity was going on AC. Photograph: Tom Dulat/Getty ImagesĪir conditioning is almost uniquely power-hungry, and its use is only set to grow.
Colossal emissions … outdoor air conditioning in Dubai’s Citywalk.
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