According to the WSJ (27 May 2010):
President Obama’s energy and climate officials have a habit of touting geoengineering as a solution to climate change.
The latest? Energy secretary Steven Chu, who while in London pitched an idea from his former colleagues at the Lawrence Berkeley National Laboratory: painting roofs white to reflect sunlight can make a huge difference in global warming.
For the best results, the roofs need to be flat. However, most roofs are sloped. Using cool-colors on sloped roof is still useful but the global-warming benefits are reduced.
Proponents of the idea tout other advantages–reflective surfaces also reduce tempertures in urgan “hear islands,” and reduce the need for air-conditioning, which in turn reduces the demand for electricity, which generates its own greenhouse-gas emissions.