Remember white roofs? They were supposed to be a cheap, easy way to cool down the planet. If the cities of the world simply slathered their roofs with lighter shades of white and gray, the theory went, then they’d trap less heat and reflect more sunlight back into space. This scheme couldn’t halt global warming, because it’d only provide a one-time benefit, but it might buy us some time. By one estimate, refitting the 30 billion square feet of commercial roof space in the United States would be equivalent to taking 75 million cars off the road for a year.

Except, as it now transpires, white roofs might actually make the planet hotter, not cooler. That’s the conclusion that Stanford’s Mark Jacobson reached in a new studypublished in the Journal of Climate. The crux of the issue, he told me by phone, is that most of the early white-roof studies simply looked at the change in radiative forcing and figured that less absorbed sunlight equals lower temperatures. “None of these studies had ever looked at the climate feedback effects, how clouds and pollutants would be affected,” he says. And he found that white roofs tend to decrease cloud cover — which, in turn, makes the world warmer.

How does this work? Jacobson’s climate modeling found that white roofs would, in fact, cool urban surfaces. But by cooling that surface air, the roofs increase overall air stability, which prevents moisture from traveling upward to form clouds. Fewer clouds means that a greater share of sunlight ends up hitting the Earth’s surface, which means hotter overall temperatures. This cloud effect outweighs the roof effect. After all, says Jacobson, clouds cover a far greater surface area than roofs do, and clouds are more reflective than white roofs, which can get grimy after a while. (Jacobson also found that the increased reflectivity seems to increase the potency of heat-trapping particles such as black carbon.)

Granted, Jacobson’s study isn’t the last word on the subject. I e-mailed Hashem Akbari of Concordia University, who’s done a lot of work on reflective surfaces (his 2008 study found that making most urban surfaces worldwide more reflective — including paved sidewalks and roads — could have a dramatic effect). He said that he was still reviewing the paper but that his more recent work had suggested the benefits of white roofs on cooling have actually been understated so far. We still need to see how this all shakes out. (I’ll update with Akbari’s response when it comes in.)

What’s more, white roofs can influence energy use — though this too is subject to debate. One theory holds that white roofs would reduce the amount of heat that buildings absorb in the summer, thereby curbing the need for air conditioning. But another recent study by the National Center for Atmospheric Research found that this could potentially get offset by the need for greater heating in the winter. Jacobson’s paper didn’t weigh in on this debate, although he notes that installing solar photovoltaic panels on roofs seems like the optimal way to go, since they convert the absorbed sunlight into clean electricity. “Still,” he says, “I’m wary of predicting all the effects without studying it further.”

And that’s the rub. White roofs have always been considered a “soft” geoengineering scheme — an easy, painless way to cool the planet slightly in order to buy us time to reduce emissions and halt the global rise in temperatures. But even this simple solution can create unforeseen side effects. If white roofs are more complicated than expected, then what aboutall those other proposed schemes — like pumping sulfur into the air or lacing the ocean with iron — to artificially cool the planet?

It’s no wonder that most scientists who at least want to study geoengineering tend to say, like the Carnegie Institution for Science’s Ken Caldeira told Joe Romm, that “99% of our effort to avoid climate change should be put on emissions reduction, and 1% of our effort should be looking into these options.”

original article at the Washington Post