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.