Painting roofs of buildings white can cool off cities and help mitigate global warming, new research finds, but it would work better in cities with densely packed roofs and in warm climates where sunlight is strong year-round.

In a study accepted for publication in Geophysical Research Letters, scientists say computer modeling suggests that Energy Secretary Steven Chu and others may be onto something in advocating white roofs as a tool to help mitigate the effects of climate change.

"Our research demonstrates that white roofs, at least in theory, can be an effective method for reducing urban heat," Keith Oleson, lead author, says in a news release from the American Geophysical Union. Oleson, a scientist at the National Center for Atmospheric Research, cautions that further study is needed to determine if it's feasible.

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Cities tend to be warmer because asphalt roads, tar roofs and other urban surfaces absorb heat from the sun. Modeling found that if every roof were painted completely white, that "urban heat island effect" could be reduced by a third. The average temperature reduction for all the world's cities would be less than a degree Fahrenheit, but the cooling effect would be felt the most on summer days.

The modeling isn't good enough yet to look at how well white roofs work in specific cities, researchers say. So it's too early to say how valuable it is in someplace like Baltimore, where the Civic Works service corps and private contractors have been painting rowhouse roofs white for several years now.

The merits of doing it, researchers say, depend on several factors, including the density of roofs and their construction.  Roofs covered in metal and with little insulation would get less benefit from being painted white, they say, because that would let more of the sun's heat penetrate into the building.  Cities in warm climates also are likely to get the most out of white roofs, they point out.

(2003 Baltimore Sun photo by David Hobby)

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