Scientists Wonder Whether Painting the World's Roofs White May Be One Way to Slow Global Warming - washingtonpost.com

Making roofs white "changes the reflectivity . . . of the Earth, so the sunlight comes in, it's reflected back into space," Chu said. "This is something very simple that we can do immediately," he said later.
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But if anybody is seriously considering a global whitewash, "simple" and "immediate" are probably not words that come to mind.

"I don't think that it could ever be done at a sufficient scale," said Ken Caldeira, a climate scientist at the Carnegie Institution in Stanford, Calif. He added: "It's hard enough, in many of the cities of the world, to keep the streets swept, much less to keep the city reflective."
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"We may have to figure out a way to artificially cool the planet while the atmosphere is still super-saturated with greenhouse gases," said Mike Tidwell of the Chesapeake Climate Action Network. This could be it, he said, "because the planet, it's a closed system, it's an absolutely closed system, except for one thing: sunlight."