I heard a cool idea this week (seriously, I didn't intend to pun) on Living On Earth, the weekly environmental public radio show. California's energy commissioner, Arthur Rosenfeld, is a big advocate of white or light-reflecting roofs, which are now required for new roofs and re-roofs in California. He'll soon travel to China and India to urge those nations to require white roofs, too.

"If white roofs took over the world or the urban world over a twenty year program, we would save twenty five billion tons of CO2, which is the same as turning off the whole world's emissions of CO2 for one year," said Rosenfeld.

You can reduce the energy you consume to cool your house by 10 to 20 percent if your roof is white instead of dark. In addition, the white roof, because it reflects heat back into space, cools your neighborhood and the world directly. A thousand square feet of white roof cools the world enough to offset the heating effects of ten tons of carbon dioxide. That's about two and a half years of emissions from your family car or one year's emissions from your house, according to Rosenfeld.

What happens in winter? The amount you save on cooling is a lot more than you lose on heat, at least in most places. In Boston, you lose 15% of the summer savings on increased winter energy use, but in Birmingham, Alabama, you only lose 5%. The winter loss is low because of our tilted planet: In winter, the sun is low and shines mostly on the south wall of your house, and in summer, it's high and shines mostly on the roof. So we should paint our roofs white and our southern walls dark. Read Bruce Gellerman's interview of Rosenfeld here. And send me your tips! Have a cool week.

Bonney