"There will come a day when the temperature won't fall below 100 degrees in Phoenix during the nighttime," Dr. Andrew Ross, a professor of social and cultural analysis at New York University who wrote "Bird on Fire: Lessons from the World's Least Sustainable City," told Salon. "That will be a threshold of some kind."
The American Southwest has long been a refuge for those seeking the health benefits of warm, dry air and sunny days. But too much of a good thing is not a good thing — for human health or for the natural ecosystem. Now, the Southwest is facing a reckoning: decades of human development, coupled with rising global temperatures as a result of carbon emissions, means that many major cities in the Southwest may become uninhabitable for humans this century.
The reason has to do with something called the Heat Island Effect, a concept that describes the effect in which the densely-populated, central parts of a city with lots of concrete and asphalt will have higher temperatures compared to the less populous areas, as Dr. Juan Declet-Barreto, senior social scientist at the Union of Concerned Scientists, explained to Salon. The term "island" is not a metaphor here, Declet-Barreto said, because when you look at a thermal map of many cities, "the temperatures inside the central parts of a city resemble an island, surrounded by a cooler ocean in the surrounding more rural areas." Obviously, the effect is apt to be more dire in desert cities like Phoenix.
Sarah Mincey, associate professor at Indiana University's O'Neill School of Public and Environmental Affairs, added that the Heat Island Effect is caused by urban centers gradually losing their tree canopies, meaning that sunlight is absorbed and held in by materials like roads and rooftops, which are typically darker in color. When they finally do release that heat back into the air, it increases the temperature experienced by the people in those urban environments.
"Tree canopies mitigate this as they can shade these surfaces, avoiding the absorption of heat in the first place and through the cooling effects of transpiration – releasing of moisture into their surrounding environments," Mincey explained. "In general, western US cities have less urban tree canopy cover than eastern US cities, so mitigation of UHI [Urban Heat Islands] there is likely more difficult."