Cooler Roofs, Drier Desert?

The benefits of painting roofs white to combat the urban heat-island effect aren’t so glaringly obvious. In last winter’s Journal Watch section, David Malakoff reported on research suggesting that additional reflected sunlight could actually amplify heating when absorbed by atmospheric pollutants such as black carbon (“White Out,” Winter 2012). A study published this September in Environmental Research Letters warns of another side effect of widespread whitewashing: a regional decrease in rainfall.

Researchers at Arizona State University looked at the Sun Corridor, the sprawling “megapolitan” region surrounding Phoenix, Arizona, in an attempt to understand how urban growth could affect climate in the area. Their models showed that highly reflective roofs did reduce temperatures, along with building energy costs. However, they also found that the additional reflectivity reduced evaporation from soil and transpiration from plants, resulting in a four percent decline in precipitation. With the population of the Sun Corridor expected to top 9 million by 2040, their results emphasize the importance of tradeoffs in sustainable urban development.

Georgescu, M., A. Mahalov and M. Moustaoui. 2012. Environmental Research Letters doi:10.1088/1748-9326/7/3/034026.