Cool roofs under scrutiny

For anyone who has spent nine straight days in heat above 110 degrees, as the residents of Phoenix did this August, it’s easy to recognize the appeal of “cool roofs.” Like dressing a building in a white T-shirt, these reflective roofs offer relief for city-dwellers baking in a habitat of concrete and asphalt. But a new study indicates that in certain regions, cool roofs may have the unintended consequence of reducing rainfall.

“We need to start looking at the impact of cool roofs on the entire climate system, not just temperature,” said climatologist Matei Georgescu, whose paper was recently published in Environmental Research Letters.

Cool roofs are painted white or covered with reflective materials, instead of dark asphalt. They absorb less solar radiation and are quicker to emit the heat they do absorb, thereby keeping buildings cooler and lowering energy costs. U.S. Energy Secretary Steven Chu has advocated cool roofs as a way to reduce greenhouse gas emissions, and Arizona regulations mandate cool roofs on state-funded buildings.

Georgescu and his colleagues at Arizona State University in Tempe used computer simulations to investigate how 40 more years of rapid urban growth would change the climate in Arizona’s Sun Corridor, the sprawling megapolitan region that includes Phoenix and Tucson. They concluded that if the urban landscape expands as much as high-end projections indicate, summer temperatures could jump up to 7.2 degrees Fahrenheit in some places and annual rainfall could decrease 12 percent. That’s without including the effects of global climate change, which are expected to aggravate extreme heat and drought in the region.

In the simulation, installing cool roofs on every building halved the warming from the urban heat island effect — the phenomenon of cities creating their own pockets of heat as they absorb solar rays. But worryingly, the cool roofs also reduced rainfall an additional four percent. The authors reason that with less hot air rising from the cooler roofs, condensation decreases and rain clouds are less likely to form.  This could reduce water storage in the Salt River watershed, which overlaps the northern end of the Sun Corridor and supplies water to the metro Phoenix area.

Hashem Akbari, an engineering professor at Concordia University in Montreal who researches and promotes cool roofs, said these modeling efforts are welcome. But he finds them “inconclusive” as “there is a very, very high degree of uncertainty among scientists” about how to capture cloud formation in computer models. By contrast, he said, the benefits of cool roofs on temperature, energy costs and water use are well understood.

When it comes to disrupting rainfall, the larger culprit in the ASU study is urbanization itself. Urban growth, which covers the natural landscape with roads, parking lots, buildings and sidewalks, blocks the ground from storing and recycling moisture, a prerequisite for rain.

Georgescu said cities should consider using permeable paving materials like porous asphalt, which allows water to pass through to the soil. “I don’t think it’s something that’s been implemented at any scale,” he said. As his paper and a wealth of previous studies have made clear, however, once humans change the climate, there is really no changing it back.