White roofs, cool cities

Light-colored (high-albedo) surfaces reflect more sunlight than dark surfaces and therefore have a lower surface temperature and are surrounded by cooler air. The proposal that painting a building’s roof white can save energy for the occupant has been around for more than a decade. In recent years, region-wide modeling of so-called urban heat islands has included albedo effects. Keith Oleson (National Center for Atmospheric Research, Boulder, Colorado) and his colleagues have now gone global. They started with a dataset of urban extent and urban properties in 33 regions of the world, and a sophisticated model that includes factors like building heights, street widths, and thermal and radiative properties of roofs, walls, and streets. Next, they imposed interior building temperature ranges consistent with climate and socioeconomic conditions. Finally, they coupled the model to a global climate model and varied the roofs’ albedos. All grid cells in the final model contained rural regions and some also had urban areas. The figure shows the average annual difference in the heat island due to white roofs. (White cells on the map included no urban areas.) The heat-island effect of cities is noticeably reduced. During the summer months, the use of air conditioning would also be reduced. Interestingly a closer look at data for the winter months showed a reversal at high latitudes, where the extra albedo effect prompts additional internal heating of buildings. (K. W. Oleson, G. B. Bonan, J. Feddema, Geophys. Res. Lett. 37 , L03701, 2010 http://dx.doi.org/10.1029/2009GL042194 .)