Energy Secretary Steven Chu stunned the audience at a London scientific conference Tuesday with a radical but simple proposal to combat global warming: Paint all the roofs of all the buildings in the world white.

If we did so, the Nobel Prize-winning physicist said, and if we also made sure the world's roads and sidewalks were light-colored, it would have the same effect on global warming as taking all the cars in the world off the world's roads for 11 years.

The idea is to harness the "albedo effect" -- the theory that a reflective planet warms up less as heat from the sun is bounced back into space.

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Chu, speaking at the St. James' Palace Nobel Laureate Symposium, said the calculations are based on work done at Lawrence Berkeley National Laboratory, where he used to work and where three researchers concluded last year that changing surface colors in the world's 100 largest cities would offset 44 billion tons of carbon dioxide emissions.

But at least one science expert thinks Chu is nuts.

"It's past simplistic -- it's ridiculous," says Steven Milloy, publisher of junkscience.com and an avowed climate-change skeptic. "Imagine the glare on roads, in urban areas, imagine the UV radiation bouncing around. Snow blindness would be replaced by road blindness."

But Dr. Gordon Bonan, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., says there's a kernel of truth in the science behind Chu's idea.

"That's been a pretty standard idea many for many years now," says Bonan. "It's related to the idea of an urban heat island -- that a big city will generate a large amount of heat. In urban planning and urban design, the idea is that painting roofs white will absorb less solar radiation and keep the city cooler."

"In terms of roads, that does work," he continued. "You can test it yourself by walking barefoot on a hot summer day. The asphalt is going to be much hotter than the concrete and the white lines painted on top of the asphalt."

Still, it's not clear how well the practice would work on the world as a whole.

"You could try to extend this idea to the entire planet, but I've never seen any numbers on how much impact this would have on the Earth's surface temperature," says Bonan. "The urban [surface] area of the world is pretty small."

Milloy says he's certain that it would be a huge waste of time and money.

"How would this accomplish anything? What's the expense?" he asks. "This shows you how even Nobel winners get lost when they step outside their fields of expertise."

Bonan says that there wouldn't be any "unintended consequences," but Milloy disagrees.

"What if we do this, and solar activity decreases?" he wonders. "We need sunlight to make vitamin D. Plants need it to make photosynthesis."

It's well known that people in hot regions, such as Greece or the American Southwest, make sure their houses are light-colored to keep out the heat. But what about residents of colder climes, who might appreciate the extra solar boost from a dark roof during winter months?

"For a house, the heat loss would be pretty small," said Bonan. "It would depend on how well the attic was insulated."

As for the assertion that painting everything white would offset the greenhouse-gas emissions of hundreds of millions of cars for more than a decade, Bonan demurred that he wasn't qualified to judge.

Milloy was more critical: "It sounds like some dubious global-warming calculation that someone made on the back of an envelope."