Painting roofs white or covering them with plants could help fight global warming, but they don’t offer the same bang for the buck everywhere, says a study of six U.S. “megapolitan” regions.
Cool roofs – painted white or other light colors to reflect sunlight – as well as green roofs have surged in popularity as cities such as New York promote their use and research shows they lower the need for air conditioning. Now comes another federally funded study that reinforces that idea – with caveats.
“Each can completely offset the warming due to urban expansion and can even offset the warming due to greenhouse gas emissions,” said lead author Matei Georgescu, a sustainability scientist at Arizona State University.
He said cool and green roofs can become an important way to deal with rising temperatures, especially as more Americans live in urban areas that are warmed by asphalt roads and tar roofs.
The study, co-authored by three scientists at the U.S. Environmental Protection Agency, used climate models to look at the potential benefits of cool and green roofs in metropolitan areas that are sprawling into “megapolitan” regions.
Georgescu said cool roofs, which lower a building’s temperature year-round, offer lesser benefits in Northern than Southern areas. The reason’s simple: In northern cities, the energy savings from less air conditioning use in the summer are offset by more heating requirements in the winter.
“There’s always some (energy) savings,” he said. But they’re smaller in the Chicago/Detroit region and the Mid-Atlantic than in Texas, Arizona, California or Florida.
His study finds green roofs don’t seem to cool buildings in the winter the way white roofs do, because the addition of water vapor promotes warming – a potential plus for Northern areas. Yet roofs with plants offer less overall cooling than those painted white – a potential downside for Southern areas.
“There are trade-offs that need to be considered,” Georgescu said, adding: “We don’t yet know enough about the various trade-offs.”
His study, funded by the National Science Foundation, was published in the Proceedings of the National Academy of Sciences journal.
“The study is a step in the right direction and is consistent with other studies that suggest that cool roofs can offset localized urban heat island impacts,” said Mark Jacobson, an environmental engineering professor at Stanford University. He said, however, it does not show the global impact.
In a 2011 study, Jacobson’s computer modeling found that white roofs do cool urban surfaces, but they also exacerbate global warming by reducing cloudiness and heating up particles of soot and dark pollutants that hover in the atmosphere. His study did not include the potential benefit of reduced air conditioning use.
Most studies tout the benefits of cool or white roofs. The U.S. Department of Energy says reflective surfaces can keep a roof 50 degrees Fahrenheit cooler than traditional dark tar.
© 2014. USA TODAY. All rights reserved.
© 2014. USA TODAY. All rights reserved.
