Phoenix tries to rise from the flames
The Sunbelt city is one of the nation's most sweltering urban heat islands. But simple solutions to help cool it are at hand.
The parking lot at the corner of First and Polk streets, in the heart of downtown Phoenix, doesn't look unusual until you see it from a nearby high-rise. Instead of the usual black or dingy gray, the pavement reflects a dull green – more like an algae-encrusted desert watering hole for thirsty cattle than a rest stop for commuters' cars.
Most drivers don't give the pavement a second thought. But some of the neighbors do. And so does Sheri Roese, who sold the city on the innovative green asphalt coating. Roese is the founder of Emerald Cool Pavements, whose mission is to cool the world's overheated cities, one parking lot at a time.
"People in the neighborhood didn't want a 90,000-square-foot patch of black asphalt in the middle of downtown, where it's already hot," Roese says. "With this, we're changing the surface from a heat-absorbing surface to a heat-reflecting one. And green is a psychologically cool color, too. You get the feeling of nature, even though it's just a parking lot."
In the furnace heat of a summer day, Roese might seem a utopian: After all, downtown Phoenix is hot no matter what. But studies show that asphalt finished with her green and blue coatings stays about 20 degrees cooler than conventional black lots. Spread that across the thousands of lots that freckle the valley's sprawl, and you could begin to combat a problem largely of the city's own making: the hellish heat that now dominates its summers.
Phoenix is the poster child for what scientists call the "urban heat island" effect, or the strong tendency cities have to retain more heat than their unpaved surroundings. In the Sonoran Desert, it's not just a nuisance: It's an actual killer as well as an economic drain and a contributor to global warming. But Roese and a handful of like-minded technophiles believe that solutions are at hand – and might even turn a profit.
Many cities are warmer than their surroundings because their abundant building materials trap heat. But few have suffered from the problem as much, or documented it as thoroughly, as Phoenix. Set in an expansive valley whose bowl shape traps the sun's heat, the city is naturally hot. As early as 1921, a meteorologist noticed that downtown Phoenix, then a small town, cooled off more slowly after a scorching day than the surrounding desert, which cools quickly after sunset.
"At night, natural materials like desert soils and gravels lose their heat really fast," says Arizona state climatologist Nancy Selover, a professor of geography at Arizona State University. "But if you're standing over an asphalt parking lot or a block wall, you'll feel that heat coming off all night long. The nights aren't long enough in summer to dissipate all that heat."
Phoenicians once dealt with the sun's assault using low-tech means –– building covered porches on houses and commercial buildings, planting shade trees, soaking in irrigation ditches and sleeping outside. But with the widespread adoption of air conditioning after World War II, those simple solutions gave way to an orgy of paving and building.
The result has been misery. Nighttime temperatures here have risen 10 to 12 degrees Fahrenheit over the last 40 years. In 2013, Phoenix set a new record when the temperature one night dropped to a low of only 96 degrees – one of 15 nights that year that sweltered above 90.
Most Phoenicians react by blasting the air conditioner. The local utility estimates that a 1-degree rise in summertime temperatures results in a 2 to 3 percent increase in electricity use, adding millions to monthly utility bills. Since more than two-thirds of Arizona's electricity is generated by fossil fuels, that increased usage releases more greenhouse gases.
There's a more immediate human cost, too. Since 2006, Maricopa County has reported an average of about 80 heat-related deaths each summer. Sharon Harlan, an ASU sociologist who is wrapping up a six-year study on the Phoenix heat island's health impacts, notes their uneven distribution.
"There are more heat-related deaths and emergency room visits in the low-income areas where high temperatures are concentrated," she says. "People who live in hotter neighborhoods tend to be minority and lower-income, because they tend to cluster together more in the inner city. There's less vegetation there. And in this environment, that's connected to water, which costs money."
Most of the dead are homeless, or poor residents who can't afford air conditioning. The city operates cooling centers where the needy can get water and experience cooler temperatures during severe heat waves. But those are Band-Aids on a problem that, left untreated, will get worse. A 2012 study estimates that climate change could increase average temperatures in Arizona by almost 2 degrees Fahrenheit by 2050. If development continues unchecked, however, the Phoenix area could warm by up to 7 degrees by then.
This is not inevitable, however. The simplest means of cooling a city, or neighborhood, is low-tech and obvious: shade. A return to more traditional hot-weather building styles, such as shaded porticos and narrower streets, could help make neighborhoods more comfortable, says Harvey Bryan, an architecture professor at ASU. But current zoning regulations often make that difficult. For example, developers of commercial buildings are typically required to provide open sidewalks instead of shaded walkways.
More widespread tree planting would also help. In 2010, Phoenix completed a master plan that calls for approximately doubling the shade provided by trees by 2030. But planting trees conflicts with water-conservation goals. Before the 1970s, many neighborhoods were cooled by trees growing along open irrigation ditches; when those were replaced by more efficient underground canals, the trees vanished. Today, residents can receive a rebate for replacing thirsty trees and turf with desert vegetation.
"If everybody on the block did that," Bryan says, "it would dramatically change the neighborhood. You'll eventually see a 10-degree difference in temperature. I understand the water-conservation issue. But I think there are ways to compromise."
Bryan says that selective planting of shade trees would be wise. So would the use of innovative cooling technologies for outdoor spaces, such as water-wicking Saltillo tiles and artificial fabric canopies. But for a more widespread impact, he says, Phoenix and other hot cities will have to address their biggest heat sinks: their pavement.
Researchers from the Lawrence Berkeley National Laboratory's Heat Island Group have estimated that impermeable surfaces – mainly roofs and pavements – cover about 60 percent of many cities, suggesting a tremendous, unmet cooling potential. Former Energy Secretary Stephen Chu believes that simply lightening surface colors in the world's 100 largest cities would offset the projected growth in global carbon emissions for the next decade.
Sunbelt cities have made some progress in implementing white roofing, which significantly cools commercial buildings and lowers utility bills. But asphalt still tends to be very black – and very hot.
The Arizona Department of Transportation, which oversees a lot of pavement, has helped ameliorate Phoenix's heat island almost by accident. It's paved highways with permeable asphalt studded with crumb rubber from recycled tires. The primary goal is to reduce noise levels, but the side effect is cooler pavement. According to Kamil Kaloush, an engineer at ASU, rubberized asphalt retains less heat, and because it transmits less heat to the underlying concrete base, overall road maintenance costs are lower.
Even cooler, though, is pavement coated with reflective sealants like those in the parking lot at First and Polk. When city officials proposed a temporary square-block parking lot there in 2010, neighbors objected, claiming that a swath of black asphalt would roast an already hot neighborhood.
Enter Roese, who has developed sealant in a variety of cool tones – turquoise, teal, beige – that reflect the sun's heat, and persuaded the city to finish the parking lot with it. At $1 a square foot, the sealant is expensive and has been used on only a couple of local sites. But Roese sees potential in the metropolitan area's many big-box stores. Last summer, she monitored parking lot temperatures at a Walmart near Phoenix. Covering a 200,000-square-foot lot with a reflective surface would pay for itself within two to three years, she claims, thanks to savings in air conditioning.
"Chain stores are really where this belongs," she says, "because they are so large. Whoever that first store is that adopts this will change the world. Think of the economic benefits, and not just in cooling costs. Right now, a lot of people in Phoenix say, 'I don't go shopping here in the summer. I'm going to stay home and shop online.' "
So far, though, those who control the most asphalt haven't agreed to pay the tab, and the mixed solutions to the heat issue haven't coalesced into a single broad strategy – even though almost everyone agrees on the magnitude of the problem. More affluent neighborhoods, including ASU's campus in Tempe, boast shade trees, water features and permeable pavements, while poorer neighborhoods swelter.
But a more cohesive regional strategy is emerging. Mick Dalrymple, a green building expert at ASU's Global Institute of Sustainability, is drafting a project aimed at reducing Phoenix' average summer temperatures by 1 degree over the next five years. The plan will call for zoning changes, retrofitting of existing buildings and roads, and energy-efficiency initiatives. It's ambitious, but – compared to the steps that need to be taken to address the much bigger issue of planetary warming – it's easy.
"If we were to succeed in lowering Phoenix's temperature by just 1 degree in five years," Dalrymple says, "think what a message that would send about what we can do about climate change."
Peter Friederici is an associate professor of journalism at Northern Arizona University who writes frequently about science and the environment. He has authored several nonfiction books, and curated What Has Passed and What Remains: Oral Histories of Northern Arizona's Changing Landscapes.