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 paint, the theory went, then they’d absorb 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.

(Mark Gail/The Washington Post)

How does this work? Jacobson’s climate modeling found that white roofs would, in fact, cool urban surfaces. But by cooling that air near the ground, the roofs increase local air stability, which prevents moisture and energy 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 could well offset 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. In many locations, white roofs can 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 these savings are sometimes 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 about all 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.”