White roofs could cut city temperatures

Posted by Emma Woollacott

Painting roofs white really could cool cities significantly and help mitigate global warming, a new study suggests.

"Our research demonstrates that white roofs, at least in theory, can be an effective method for reducing urban heat," says NCAR scientist Keith Oleson, the lead author of the study.

"It remains to be seen if it's actually feasible for cities to paint their roofs white, but the idea certainly warrants further investigation."

The National Center for Atmospheric Research (NCAR) used a new computer model to reach its conclusions.

Its simulations provide an idealized view of different types of city around the world and indicate that, if every roof were entirely painted white, the urban heat island effect could be reduced by 33 percent.

This would cool the world's cities by an average of about 0.7 degrees F.

In the real world, however, the cooling impact might be somewhat less because dust and weathering would cause the paint to darken over time and parts of roofs would remain unpainted because of openings such as heating and cooling vents.

In addition, white roofs would have the effect of cooling temperatures within buildings. As a result, the amount of energy used for space heating and air conditioning could change - potentially affecting the consumption of fossil fuels.

"It's not as simple as just painting roofs white and cooling off a city," Oleson says.

The research indicated that some cities would benefit more than others from white roofs, depending on such factors as roof density, construction and location.

While the model lacked enough detail to capture individual cities, it did show the change in temperatures in larger metropolitan regions. The New York area, for example, would cool in summer afternoons by almost 2 degrees Fahrenheit.