The city is a growing paradox. Humanity needs its very efficiency: People living more densely and taking up less land – with easy access to decarbonizing public transport – collaborating and innovating as urbanites have always done. But as the climate warms, city dwellers suffer worse heat than their rural counterparts due to the “urban heat island effect”. All that concrete, asphalt and brick absorbs the sun’s energy and accelerates urban temperatures far above those in the surrounding countryside.
In the United States, heat already kill more people than any other form of extreme weather, and nowhere is it more dangerous than in cities. So scientists and urban designers are now scrambling to research and introduce countermeasures, especially in the Southwest – no more energy-hungry air conditioning, but more passive, simple cooling techniques. “Cool roofs,” for example, reflect the sun’s energy back into space with the help of special coatings or reflective shingles. And creating urban green spaces full of plants that cool the surrounding air.
“In the same way that the urban environment we’ve built around us can exacerbate heat, it can also be adapted to reduce that heat,” says Edith de Guzman, a researcher at UCLA and director of the Los Angeles Urban Cooling Collaborative . “If we also invest in increasing the reflectivity of existing materials in the built environment, we can significantly reduce the number of ER visits and the number of deaths, in some cases more than 50 percent.”
While scientists have long known about the heat island effect, they are getting more of the granular data they need to determine which interventions cities should invest in and where. Realizing the many benefits of greening cities with more vegetation at ground level, local governments have already handed out incentives to plant more trees. But they could do much more to encourage the spread of cool roofs, which would make heat waves less dangerous.
New research suggests that cities are ignoring the power of cool roofs at their peril. A study in the journal Geophysical Research Letters earlier this month modeled how much cooler London would have been on the two hottest days in the extra hot summer of 2018 if the city had widely adopted cool roofs compared to other interventions, such as green roofs, rooftop solar panels and ground-level vegetation. Although simple from an engineering point of view, cool roofs have proven to be the most effective in reducing temperatures.
“We saw it as practical everywhere,” said Oscar Brousse, a geographer specializing in urban climatology at University College London and the study’s lead author. “Because in theory there’s no reason – other than heritage or protection by UNESCO or something like that – that would prevent you from doing that.”
Cool roofs have the luxury of scale: You can basically swap out any dark, heat-absorbing roof for one made of reflective materials, or simply paint the structure white. (Think how much hotter you’ll get on a 95-degree day wearing a black shirt than a white one.) Even clay tiles can be painted with light-colored coatings.
It’s a little more difficult to put them on top of single-family homes, given the spread of dark wood shingles. “It’s both about the industry being locked into a specific type of roof shingles and municipal building codes not pushing for something better, despite a growing awareness of the importance of cool roofs,” says Vivek Shandas, who works on the urban heat -island effect at Portland State University, but was not involved in the new study.
However, with the right policies and incentives, cities can increase the adoption of more reflective shingles. In 2015, Los Angeles became the first major city to require all new residential construction comes with cool roofs by default. While a cool roof can cost the same or slightly more than a traditional one, the Los Angeles Department of Water and Power offers rebates for homeowners to make the switch. But until more municipal codes push the industry to switch to cool roofs, “wide adoption will remain woefully inadequate for the scale of the challenge we face,” Shandas said.
One tricky thing about the heat island effect is that no two neighborhoods heat up in the same way. Differences in geography, such as proximity to lakes that provide cooling and hills that block winds, help determine how hot a given neighborhood is already getting and how effective different interventions might be. Wealthier neighborhoods tend to be greener to begin with, while lower-income neighborhoods been often intentionally zoned to host more industrial activity – lots of big buildings and heat-absorbing concrete.
“Each neighborhood has its own unique characteristic of heat,” Shandas said. “We have to start with what’s on the ground and build from there, instead of taking the whole city and throwing a bunch of different interventions at it.”
While the new study found that widely deployed cool roofs could reduce temperatures across London by an average of about 2 degrees Fahrenheit, in some places by up to 3.6 degrees F. Both ground-level vegetation and rooftop solar panels will not have the same kind of success. : They lowered temperatures in London by about half a degree F on average. Green roofs will lower temperatures during the day, but then raise them again at night by releasing stored heat, so the effects cancel each other out on average.
To be clear, this study only looked at temperatures, not the many other benefits of efforts to cool cities. A green roof, for example, acts as a haven for native plant and animal species. Green spaces on the ground can also prevent flooding if consciously designed to be absorbent. And green is simply beautiful, promoting the mental health of residents.
While solar panels wouldn’t cool London as much as cool roofs, they can still provide a building with a host of climate-friendly benefits. Electricity from those panels can power highly efficient heat pumps, which provide warmth in the winter and then reverse in the summer to act like air conditioners. “So even if you don’t lower the temperature, you’ll have the means to lower it indoors and provide cool shelters,” Brousse said.
However, deploying more air conditioners would raise temperatures across London by an average of 0.27 degrees F, but up to 1.8 degrees F in the dense city centre. This is because air conditioners cool a space by pumping indoor heat outside, essentially recycling heat across a metropolis.
The research suggests that the more passive cooling techniques cities use, the less dependent they will be on air conditioning to provide indoor shelter for the vulnerable. And the better scientists and urban designers can characterize heat in a given neighborhood, the better they will be able to work with that community on solutions. “We have to resist the urge to find just one way to do it,” said de Guzman of the Los Angeles Urban Cooling Collaborative. “From a scientific and heat mitigation point of view, we need to have a combined approach.”
