"White roofs" are, as I've mentioned before, one of the easiest ways to cool down the planet. As Hashem Akbari at the Lawrence Berkeley National Laboratory estimated last year, if we took all the roofs and pavement in the world's large urban areas, and either painted them white or replaced the black asphalt with brighter material, thereby increasing their solar reflectivity, the global cooling effect alone would be enough to offset 44 metric gigatons of carbon-dioxide—roughly equivalent to taking all the world's cars off the road for eleven years. By itself, that wouldn't prevent many of the other nasty effects of CO2 emissions, like ocean acidification—an area where "geo-engineering" schemes will always prove a poor substitute for curbing emissions. But it would buy us some time to implement other longer-lasting climate mitigation strategies.
Anyway, Joe Romm has a more in-depth look at white roofs over at his blog. I'd add a bit more: Yes, it sounds like a daunting (and costly) task to rip up all that old asphalt, but consider that roughly three-quarters of all buildings that will exist in the United States in 2035 either haven't even been built yet or will soon be renovated—this is something we can easily do as we're replenishing our housing stock. California already requires white material for all new flat roofs, though white sloped roofs seem to be less popular, and aren't required. Another side benefit, meanwhile, is that white roofs—or even "green" roofs packed with vegetation—tend to keep buildings cooler in the summer, cutting down on A/C use, and hence, emissions. On the list of no-brainer environmental moves, this has to rank very high up there.