White roofs work because of the physics of sunlight. Dark roofs absorb and hold more than 80 percent of solar energy, while white ones can reflect 75 percent of it away. That makes a white-roofed building cooler and cheaper to air-condition.
Because of that energy savings, California has since 2005 required most flat-roofed buildings to have white tops, and Walmart has installed them on about 75 percent of its U.S. stores. In January, the District will require new flat roofs on commercial buildings to be covered in vegetation or a reflective material.Now scientists are wondering whether white roofs might keep the world cooler, too.
In his talk, Chu cited new research from his former laboratory, the Lawrence Berkeley National Laboratory in California, which imagined the result of painting about 63 percent of the roofs white in 100 large cities in tropical and temperate areas worldwide.
It estimated that would provide about the same climate benefits as taking all the world's cars off the road for 10 years.