In the debate over how to confront climate change, carbon dioxide gets most of the attention. But at the city level, new research suggests, we ought to be looking just as critically at how urban growth is raising temperatures.
A group of researchers found that as urban areas in the United States expand, so too will the "heat island effect," in which pavement, rooftops, parking lots and other hard surfaces absorb heat and slowly release it, boosting temperatures higher than rural surroundings.
By century's end, the built environment will contribute as much to rising temperatures in cities across the United States as climbing greenhouse gas levels in Earth's atmosphere, adding up to 3.6 degrees of additional warming, according to a new scientific paper.
“If you live in an urban area, you are in an environment that is warming because of greenhouse gas-induced climate change plus the added effect of an urban heat island,” said lead author Matei Georgescu, a climate scientist at the School of Geographical Sciences and Urban Planning at Arizona State University.
The upside? Urban areas can offset that warming by slowing their expansion, painting roofs white to reflect heat or planting rooftop vegetation to cool the air, according to the study.
Georgescu and three researchers at the U.S. Environmental Protection Agency examined six "megapolitan" regions in California, Arizona, Texas, Florida, the Midwest and the Mid-Atlantic to predict how much they would warm under different growth and development scenarios.
Slowing urban expansion would cut warming from the heat island effect by one-third to one-half, the study found. Equipping all buildings with "green" roofs planted with vegetation would cut warming even more, and painting roofs white would do the most to roll back rising temperatures, the study found.
The payoff of white-colored "cool roofs" would be greater in drier regions such as California, cooling down by 2 degrees compared with "green" ones. Florida would see a cool-down of only one-third of a degree by painting roofs white.
In the worst-case scenario, in which cities would grow without changing their rooftops, parts of Chicago and Detroit could see temperature increases of more than 5.4 degrees by 2100, the study predicts. Florida and other humid regions would see less warming than low-lying Mediterranean climates such as California's Central Valley.
Large-scale adoption of "cool" or "green" roofs could, however, bring unintended consequences.
Painting rooftops white would cause a pronounced drop in the amount of rainfall in Florida and the Southeast, the study says, while planting "green" roofs would boost precipitation in the Mid-Atlantic, Chicago and Detroit.
Such regional disparities mean that local governments seeking to keep their residents cool will have to tailor their planning and design efforts to consider their effects on the climate, Georgescu said.
"Local decisions matter," he said. "That's really one of the take-home messages here."
The study, published in the Proceedings of the National Academy of Sciences of the United States of America, expanded on earlier research on how light-reflecting roofs could lessen the heat island effect for residents of the Arizona cities of Phoenix, Tucson, Prescott and Nogales.