updated 4/27/2012 7:19:09 PM ET 2012-04-27T23:19:09

Earth day, April 22, comes on a Sunday this year, making a weekend paint project a perfect way to celebrate. Just painting the surface of a black roof white can lower its temperature on a hot afternoon by as much as 50 to 65 degrees.

And cooling down the house is good for the wallet and the environment.

Research published in Environmental Research Letters online finds that if all of the world’s urban areas were outfitted with white roofs and light-colored pavement, it would cool the planet by as much as 0.13 degrees Fahrenheit, saving up to 150 billion tons of carbon dioxide emissions--equal to 50 years’ worth of emissions from all the world’s cars.

WIDE ANGLE: Green and White Roofs: Low-Tech Ways to Save Tons of Energy

The new work, by Hashem Akbari and colleagues of Concordia University in Montreal, used estimates for the amount of urban roof and pavement surfaces globally combined with climate models to determine the effect on global temperatures of changing the reflectivity of a surface, known as its albedo.

Albedo values range from zero, for a completely black surface, to one, for a perfectly white surface. Akbari’s team said resurfacing roofs could easily increase their albedo by 0.25 on average, while a 0.15 average increase is easy to achieve for pavement.  This would translate to an overall increase in albedo for an entire city of around 0.1.

Lightening a roof can be as simple as painting it white or choosing a lighter colored roof material. Some higher-tech shingles exist that are dark yet still reflect infrared light. For pavement, the necessary increase in albedo can usually be achieved by incorporating light-colored aggregate into the asphalt, Akbari said.

 “The technology is a thousand-year-old technology,” he said. “Look all around the Middle East.” Places like Greece and the Bahamas also have traditionally incorporated white roofs, he said.

ANALYSIS: White Roofs Help Cities Chill Out

Yet, how realistic is it to change the albedo of cities on such a large scale? “Every roof is going to be changed every now and then,” Akbari said, creating an opportunity for the roof to be replaced with a lighter material. The same goes for pavement.

Every bit counts: each square meter of roof that’s made 0.01 albedo units lighter is the equivalent of taking over 15 pounds of carbon dioxide out of the atmosphere.

It saves on air conditioning bills in the summer--perhaps 10 to 15 percent--since less heat is absorbed by the building, and cool roofs may last longer, since they expand and contract less with heat and cold.

ANALYSIS: Big Apple Goes Green with White Roofs

For flat roofs, it makes no aesthetic difference to make them white since they can’t be seen from the ground. Many states have building codes that require commercial roofs to be made white when they are redone, and cool roofs are a part of the federal Energy Star program.

Several cities have taken steps to incorporate white roofs. Through its NYC °CoolRoofs program, which provides no-cost labor to coat roofs, and with homeowner’s own efforts, New York has whitened 2.5 million square feet of roof, said Danielle Grillo, Executive Director of Community Partnerships at the New York City Department of Buildings.  That’s the equivalent of 750 New Yorkers not driving for a year.

HOW STUFF WORKS: Earth Day Coverage

© 2012 Discovery Channel


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