Is there a quick and handy way to cool down the planet? Maybe—and it might just take a few buckets of white paint and a little extra concrete. Fine, more than a few buckets, but still: The Los Angeles Times reports that Hashem Akbari, a physicist with the Lawrence Berkeley National Laboratory, has crunched the numbers and estimated that if the 100 biggest cities in the world simply turned all of their roofs white and used more reflective material for the pavement in their sidwalks, parking lots, and roads—concrete instead of asphalt, say—the cooling effects would be tremendous:

Globally, roofs account for 25% of the surface of most cities, and pavement accounts for about 35%. If all were switched to reflective material in 100 major urban areas, it would offset 44 metric gigatons of greenhouse gases, which have been trapping heat in the atmosphere and altering the climate on a potentially dangerous scale.

That is more than all the countries on Earth emit in a single year. And, with global climate negotiators focused on limiting a rapid increase in emissions, installing cool roofs and pavements would offset more than 10 years of emissions growth, even without slashing industrial pollution.

Keith Johnson of the Wall Street Journal pours a healthy bit of cold water on the idea, noting that "[t]he scale and cost of any program that would re-top all the roofs and paved surfaces in cities the size of Los Angeles, Mexico City, New Delhi, and Tokyo simultaneously" might well dwarf anything Al Gore's proposing. True. On the other hand, roughly three-quarters of all the buildings that will exist in the United States in 2035 either haven't even been built yet or will soon be renovated, so that's one easy place to start. (Presumably the figures are even higher for China and India.)

One side-benefit of painting roofs a blinding white—or installing vegetation-heavy green roofs—is that it can cut down on the need for air-conditioning in the summer: Buildings with green roofs are typically 30 percent cooler than their blacktopped counterparts. California, for one, has already revamped its building codes to require white roofs on all commercial buildings. Using more reflective material on roads and highways sounds more contentious, though the concrete industry is certainly keen on the idea.

--Bradford Plumer