Painting roofs white can help reflect sunlight and reduce heat in urban areas. Maxppp via ZUMA Press
This article was originally published by Grist and is reprinted here as part of the Climate Desk collaboration.
Cities are increasingly fraught with contradictions. Humanity needs many of the efficiencies of cities: People pack more densely together to occupy less land, have easy access to decarbonized public transportation, and collaborate and innovate as city dwellers have always done. But as the climate warms, city dwellers suffer more extreme heat than their rural counterparts as a result of a phenomenon called the urban heat island effect. Concrete, asphalt, and brick all absorb the sun’s energy, causing cities to heat up much more dramatically than the surrounding countryside.
Heatwaves already kill more people in the United States than any other extreme weather event, and nowhere is it more dangerous than in cities. So scientists and urban planners, particularly in the Southwest, are rushing to replace energy-hungry air conditioners with more passive, simpler cooling techniques. “Cool roofs,” for example, use special coatings or reflective materials to bounce the sun’s energy back into space, and urban green spaces full of plants cool the surrounding air.
“Just as the urban environment we’ve built around us makes it worse, we can also improve it to make it less hot,” says Edith de Guzman, a researcher at the University of California, Los Angeles and director of the Los Angeles Urban Cooling Collaborative. “If we also invested in making the existing materials in our built environment more reflective, we could significantly reduce emergency room visits and deaths, in some cases by more than 50 percent.”
Scientists have known about the urban heat island effect for some time, but they now have more detailed data to help cities decide what measures to invest in and where. Recognizing the many benefits of greening cities with more vegetation on the ground, local governments are already offering incentives to plant more trees. But more could be done to promote the uptake of cool roofs, which help reduce the dangers of heat waves.
New research suggests that cities are ignoring the power of cool roofs at their own peril. A study published earlier this month in Geophysical Research Letters modeled how much cooler London would have been if they were widely deployed, compared with other measures like green roofs, rooftop solar panels, and ground-level vegetation, during the city’s hottest two days during the scorching summer of 2018. Though simple from an engineering perspective, cool roofs turned out to be the most effective at lowering temperatures.
“We thought it could be done anywhere,” said Oskar Bruce, a geographer specializing in urban climatology at University College London and lead author of the study, “because in theory there’s nothing stopping you from doing it, apart from heritage sites, UNESCO protection, and so on.”
Cool roofs offer the benefit of scale: Essentially, you can replace a dark, heat-absorbing roof with one made of reflective materials, or paint the entire building white. (Think about how much hotter you’ll be wearing a black shirt on a 95-degree day than a white shirt.) Even clay roof tiles can be painted with a light-colored paint.
“Each district has its own unique heat characteristics… We have to start with where we are and build from there.”
Installing cool roofs on single-family home roofs is a bit of a challenge, given the prevalence of dark wood roofing materials. “Despite increasing awareness of the importance of cool roofs, the industry is both locked into certain types of roofing materials and local building codes don’t promote better ones,” said Vivek Shandas, who studies urban heat islands at Portland State University but was not involved in the study.
But with the right policies and incentives, cities can encourage the adoption of more reflective roofing materials. In 2015, Los Angeles became the first major city to mandate that cool roofs be standard on all new homes. Although cool roofs cost the same or slightly more than traditional ones, the Los Angeles Department of Water and Power offers homeowners discounts if they switch to cool roofs. But unless more local ordinances encourage the industry to switch to cool roofs, “widespread adoption will remain totally inadequate for the scale of the challenge we face,” Shandas said.
The tricky thing about the urban heat island effect is that not every area warms equally. Differences in geography, such as proximity to a cooling lake or wind-blocking hills, determine how much an area heats up and how effective various measures are. Wealthy areas tend to have more natural greenery, while lower-income neighborhoods are often purposely zoned for larger, heat-absorbing buildings, more concrete, and more industrial activity.
“Each neighborhood has its own unique heat characteristics,” Shandas said. “Rather than just taking the whole city blindly and applying a bunch of interventions, we need to start with where we are and build from there.”
A new study finds that widespread cool roofs could reduce temperatures across London by about 2 degrees Fahrenheit on average, but by as much as 3.6 degrees Fahrenheit in some places. Ground-based vegetation and rooftop solar panels would not have the same effect; London’s temperatures would drop by about half a degree Fahrenheit on average. Green roofs reduce temperatures during the day, but warm them again at night by releasing the heat they’ve stored, so on average the effect is offset.
To be clear, this study only looked at temperature, not the many other benefits of urban cooling efforts: green roofs, for example, provide shelter for native plants and animals; above-ground green space can also prevent flooding if deliberately made absorbent; and greenery is simply good: it improves residents’ mental health.
While solar panels can’t cool London as much as cool roofs can, they can still bring a range of climate-friendly benefits to buildings. The power from these panels can run super-efficient heat pumps that provide warmth in the winter and act like air conditioners in reverse in the summer. “So even if you don’t turn the temperature down, you have a means of lowering the temperature indoors and providing a cooler shelter,” Bruce said.
But installing more air conditioners would increase temperatures across London by an average of 0.27 degrees Fahrenheit, but by as much as 1.8 degrees Fahrenheit in the densely populated centres. This is because air conditioners cool spaces by dumping heat outdoors, essentially circulating heat throughout the metropolis.
Research suggests that the more passive cooling technologies a city deploys, the less it will need to rely on air conditioning to provide shelter for vulnerable people indoors. And the more precisely scientists and urban planners can understand how hot a particular area is, the more likely they are to work with communities to find solutions. “We should resist the urge to find just one solution,” says de Guzman of the Los Angeles Urban Cooling Collaborative. “From a science and heat mitigation perspective, we need a multi-disciplinary approach.”
