We have covered green roofs extensively (see the many links at the bottom of this post), but we haven't given quite as much coverage to white roofs, which is unfortunate because it's one of these simple things that could make a big difference. New Scientist has a little piece on the subject: "Would it be possible to reduce the impact of the greenhouse effect by painting roofs of buildings white to reflect sunlight in the same way the polar icecaps do?" To answer that question, the the Earth Institute at Columbia University (New York) has undertaken the Global Rural Urban Mapping Project (GRUMP). Its results show that roughly 3 per cent of the Earth's land surface is covered with buildings.
The Earth has an albedo of 0.29, meaning that it reflects 29 per cent of the sunlight that falls upon it. With an albedo of 0.1, towns absorb more sunlight than the global average. Painting all roofs white could nudge the Earth's albedo from 0.29 towards 0.30. According to a very simple "zero-dimensional" model of the Earth, this would lead to a drop in global temperature of up to 1 °C, almost exactly cancelling out the global warming that has taken place since the start of the industrial revolution. A zero-dimensional model, however, excludes the atmosphere and, crucially, the role of clouds. [But!] It would be interesting to see if more sophisticated models predict a similar magnitude of cooling.
Of course, doing that would only be a way to gain a bit of time. On its own, it is no cure for our pollution and greenhouse gas problems.
A better use of roofs would be to use them as mini power stations by installing photovoltaic tiles. This would displace a significant proportion of the fossil carbon that we emit without relying on perturbing the Earth's delicate and complex climate system. Sure prevention is much better than uncertain cure.
So next time you replace the tiles on your roof or buy a new house, you know what to do (if you can't/won't get solar panels, white tiles or a green roof, at least try to get a light color).
More global warming prevention tips & green roof info
Ways to Prevent Global Warming On Your Vacation
Green Roofs: A primer
Inhabitat on Green Roofs
First-ever analysis of green roofs' ability to keep buildings warm in winter
Yet more on Green Roofs
Green Roofs by Earth Pledge Foundation