Some heat-scorched cities in the U.S. have turned to cool pavement projects, which use a bit of photochemical alchemy to make greyer roads that better reflect back the sun’s rays rather than absorbing all of its heat.Handout
Late this spring, city crews in San Antonio applied five different products to thousand-foot sections of road across the city, each designed to dress the black asphalt with substances that are lighter in colour – and, perhaps, less hot.
The technology has been likened to sunscreen for streets. Its formal name is cool pavement. Its idea is simple: Use a bit of photochemical alchemy to make greyer roads that better reflect back the sun’s rays rather than absorbing all of its heat.
Paved surfaces cover 40 per cent of urban areas in many U.S. cities. The industrial makers of pavement coatings promise their cool products will reduce surface temperature by five and up to 14 degrees Celsius.
Better yet, by keeping asphalt cooler those coatings – which are expensive – can extend a road’s life, making it a solution that can be effectively free in the long run.
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“We’re hoping there’s a cost-neutral point,” said Murray Myers, senior municipal sustainability manager in San Antonio.
It sounds like a Cinderella solution to cities baking in evermore intense temperatures. June was the hottest month in global recordkeeping. Cities across the U.S. shattered records. Extreme heat is already the top weather-related cause of death in the country. Now it’s raising economic fears, too.
In San Antonio, “we’re hearing from our convention centre that no one wants to come in July and August,” Mr. Myers said, referring to visitors. As temperatures rise, “they’re worried people may not want to come in May, June and September, either.”
Enter cool pavement, an idea gaining new adherents in the scorched south. Phoenix expects to complete cool coating of nearly 200 kilometres of road by the end of this year. Los Angeles intends to have nearly 300 kilometres of cool pavement by 2028. (Canadian cities are less enthusiastic: The City of Toronto says it is not considering the idea.)
But the performance of cool pavement is uneven, at best – in some ways, it may actually raise temperatures – and the complications around its use underscore the difficulties for cities grappling with how they can remain habitable in the heat.
Some of the hottest places on the continent, including Phoenix, Los Angeles and Miami-Dade County, have created chief heat officers, tasked in part with shrinking their cities’ heat islands: Buildings and roads absorb the sun’s energy during the day, before releasing it into what become abnormally warm nights.
San Antonio is evaluating a road paint that makes pavement more reflective. That means roads absorb less heat during the day keeping them cooler under the baking sun and radiating less heat into the air at night.
Some solutions are obvious enough that their use is ancient, such as the iconic white buildings of Santorini, the Greek island. White roof tile is common in Florida, too. California has since 2005 mandated white surfaces for commercial flat roofs.
Cool roofs and cool walls – which use lighter, more reflective colours and surfaces – offer a double virtue. They diminish the need for cooling individual buildings while also reflecting solar heat out of a city.
“You can lower daytime temperatures by maybe two to three degrees Celsius, depending on how much of your surface you’re making more reflective and how much more reflective it is,” said Ronnen Levinson, who leads heat island studies at the Lawrence Berkeley National Laboratory.
“You can actually make an appreciable dent in the urban heat island effect.”
A 2021 study done for the city of Phoenix suggested spending US$1.5-billion to install cool roofs across the city and a further $4-billion to double the city’s tree cover, to 25 per cent. Those investments together could yield $23-billion in savings by 2050, the study found.
But this summer has made the search for solutions particularly acute. In Phoenix, which broke its all-time high temperature record this week at 47 degrees Celsius, darker surfaces can reach 70 degrees in the sun. Fresh, pitch-black asphalt absorbs 95 per cent of the sun’s energy.
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When Mac McCullough was recently a health economist at the Maricopa County Department of Public Health in Phoenix, he kept a set of gloves in his car. “The steering wheel was too hot if you parked into the sun,” he said. A fall on blistering pavement can burn skin. “There are costs to this,” he said.
But modern cities have tended to pay little notice to heat in urban design. “A lot of it had to do with the advent of air conditioning and inexpensive electricity,” said David Sailor, a scholar at the Arizona State University who is former director of its Urban Climate Research Center. “We just air-conditioned our way out of challenges for so long.”
Cool pavement creates some of its own problems. Research has shown the solar energy it reflects can make it hotter for nearby pedestrians and actually increase the need to cool nearby buildings – by up to 11 per cent, one 2012 study found.
Other experiments have given more cause for optimism. In the Los Angeles neighbourhood of Pacoima, crews applied DuraShield Solar Gray asphalt coating, an epoxy-fortified acrylic, to 10 square city blocks – not just on streets but also on adjacent parking lots, playgrounds and athletic areas. Air temperatures declined by nearly 1 degree Celsius on average days, and up to two degrees on the hottest days, when the sun was most intense.
That suggests neighbourhoods could benefit in ways not appreciated by studies that haven’t installed cool pavement across a contiguous area.
“When we think about the adoption of this, it’s going to be important to be at scale, because that’s where we’re seeing the greatest cooling effect,” said Eliot Wall, a general manager at GAF Materials LLC, which makes DuraShield.
Even a more modest effect can matter. In Phoenix, cool pavement streets lower air temperatures by about 0.3 degrees Celsius. That’s enough to trim electricity and water use by 1 per cent, said Prof. Sailor, which could cut air-conditioning costs by tens of millions of dollars.
Cool pavement can be rolled out at scale by a single municipal public-works department, rather than relying on individual building owners to retrofit buildings. It can be brought to lower-income neighbourhoods, too, making it a more equitable response. And it can be applied without the difficult work of remaking the hard contours of modern city life.
It’s “one of the more underappreciated” technologies, said Randolph Kirchain, co-director of the Massachusetts Institute of Technology Concrete Sustainability Hub. “You can’t convert every urban area into a greened area.”
Nathan VanderKlippe