"Painting the Town White—and Green" by Arthur H. Rosenfeld, Joseph J. Romm, Hashem Akbari, and Alan C. Lloyd, in Technology Review (Feb.–Mar. 1997), Bldg. W59, MIT, Cambridge, Mass. 02139.

Los Angeles could be a cool place. But right now, it isn’t. On a typical summer day, the temperature in central L.A. is a full five degrees F. higher than in the surrounding suburbs and rural areas. Many other big cities are also overheated. Is this, as many assume, due mainly to heat generated by cars, office buildings, and factories in the city? Guess again, say Rosenfeld, Romm, and Akbari, who are with the U.S. Department of Energy, and Lloyd, who works at the Desert Research Institute in Reno, Nevada. That heat accounts for only one percent of the temperature difference. The chief culprit, they say, is dark surfaces, such as roofs and asphalt pavements, which absorb heat that lighter surfaces would reflect away.

"With white roofs, concrete-colored pavements, and about 10 million new shade trees," the authors point out, "Los Angeles could be cooler than the semidesert that surrounds it, instead of hotter." Besides providing cooling shade, the trees would soak up groundwater, which then would "evapotranspire" from the leaves, indirectly cooling the surrounding air.

Reducing the average summer afternoon temperature in Los Angeles by five degrees, the authors calculate, would cut the need for air conditioning by 18 percent and lower the smog level. The energy savings, not to mention the reduction in medical costs, would be substantial. But it would take about 15 years to achieve this effect. "Los Angeles, or any other large city," the authors note, "cannot be cooled in a day."