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The Easy Fix That Isn’t: White Roofs May Increase Global Warming

Installing white roofs (or painting them white) has been  promoted as a way to help slow global warming. New research shows that white roofs may actually add to global warming. Credit: NNSA/flickr.

[Update, Nov. 8, 2011: WhiteRoofProject.com has posted a new blog in response to some of the coverage of Jacobson’s recent study on white roofs, including mine below. They offer some important points that I didn’t cover in my piece, so their blog is worth reading. In particular, previous research has shown that in some specific regions, like those near the equator, white roofs probably offer more benefits than drawbacks, from an emissions perspective – something Jacobson’s computer models haven’t yet accounted for.

I should stress that Jacobson’s study is just one piece of research in a broad field, but it’s an important study nonetheless. More than anything, it points out that the science isn’t settled on white and cool roofs, and that more research can help figure out where and how they can be put to good use.]

If you’re interested in staving off climate change without trying too hard, painting your roof white seems like a complete no-brainer. It’s far cheaper than trading in your SUV for a Prius, and it turns the laws of physics to best advantage. Dark roofs absorb sunlight that heats up your house, office tower, or apartment building. That means you’re bound to crank up the energy-intensive air conditioner to keep pace in the summer months — and since electricity in the U.S. comes largely from fossil fuels, the net result is more heat-trapping greenhouse gas emissions, and more global warming.

But a white roof does just the opposite. It bounces sunlight right back into the sky, just as light clothing helps you stay cool in the summer. Cooler buildings need less air conditioning, which translates to fewer emissions of heat-trapping gases. That’s why Energy Secretary (and Physics Nobel prizewinner) Steven Chu endorsed the idea back in 2009 and why cities like New York and Philadelphia have launched white-roof projects.

Unfortunately, what seems obvious is not always true, and a new study available online and soon coming out in the Journal of Climate reveals some potentially bad news for white roofs. When Stanford University engineer Mark Jacobson, and his grad student John Hoeve modeled the total climate response to white roofs and other urban surfaces, they found the lightening may actually cause more global warming.  

Here’s why: the sunlight that bounces off white roofs doesn’t all fly out into space. A lot of it is absorbed by particles of soot and other dark-colored pollutants that float around in the atmosphere (those same particles are already responsible for a good portion of global warming). The particles heat up, just like your house would have, and the net result is a warmer atmosphere. You house might be cooler, but it would be at the expense of heating the planet.

In short, says Jacobson in a press release: "There does not seem to be a benefit from investing in white roofs. The most important thing is to reduce emissions of the pollutants that contribute to global warming." So much for trying to take the easy way out.

On the other hand, says Jacobson, there is another way to use your roof in the fight against climate change: cover it with solar panels. The panels intercept sunlight before it hits the roof, so your house doesn’t heat up so much. They don’t bounce the light back into the atmosphere where it can heat up soot particles. And they generate at least some electricity without emitting greenhouse gases. It’s not quite as cheap as painting your roof. But unlike that feel-good solution, it’s actually likely to be effective. 

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