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How white roofs shine bright green

Painting homes a lighter shade does more than save money on A.C.

Eco-vation: If all urban rooftops in temperate and tropical zones were white, like these houses in Bermuda, they could slow global warming by up to 11 years, a new study finds.

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Can you help save the planet by painting your roof white?

Hashem Akbari thinks so.

Global warming’s complexity and momentum have led to a try-everything approach by scientists. In that spirit, Dr. Akbari offers his simple yet profound innovation for slowing that warming way down.

It has long been known that a white roof makes a dwelling cooler. That saves energy and cuts carbon emissions. But until Akbari, a researcher at the Lawrence Berkeley National Laboratory in California, picked up a pencil to do the calculations, few realized the major climate effect that millions of white rooftops could have by reflecting sunlight back into space.

It turns out that a 1,000 square foot area of rooftop painted white has about the same one-time impact on global warming as cutting 10 tons of carbon dioxide emissions, he and his colleagues write in a new study soon to be published in the journal “Climatic Change.”

As sunlight pours down into Earth’s atmosphere, some of the energy is filtered out or bounces off clouds. About half the energy shines through as visible light and some of that hits the tops of houses. If a roof is white, most sunlight reflects back into space and doesn’t heat the earth. But if a roof is a dark color, the sunlight converts to heat rather than bouncing off as light. That thermal energy then radiates off the roof back toward space, where it is trapped by CO2 in the atmosphere, and then absorbed by this greenhouse gas. As a result, the world’s thermometer reads just a little higher than it did before.

If the estimated 360,000 square miles (less than 1 percent of the world’s land surface) covered by urban rooftops and pavement were a white or light color, enough sunlight would be reflected back into space to delay climate change by about 11 years, the study shows.

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