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.