Azoteas Blancas

Research Highlights
LBNL Heat Island Group
September 7, 2008

White Roofs Cool the World, Offset CO2, and Delay Global Warming
As the threat of global warming becomes widely recognized,scientists have proposed geoengineering techniques (“the artificial manipulation of the environments of the Earth, especially as a means of counteracting global warming”1) for quick response to counter thethreat. Most proposed geo-engineering techniques are novel but unproven. Simple ideas, such as cool roofs and cool pavements, that save energy, improve comfort, and have been practiced for thousands ofyears should be the first adopted geo-engineering project. Increasing the solar reflectance of urban surfaces results in reflecting more of the incoming global solar radiation, hence, counters globalwarming. In a recent study to be published in journal Climatic Change, Akbari, Menon, and Rosenfeld have quantified the effect of increasing the solar reflectance of urban surfaces in terms of CO2offset2. Most existing flat roofs are dark and have a typical solar reflectance of 10%-20% that can be increased by using white materials with initial reflectance of 70%-80%, aging to over 60%, anincrease of over 40% in the solar reflectance. They estimate that for a house with 1000 ft2 of roof area (or a non-residential building), the CO2 emission offset is 10 tonnes. Currently, in Europe emittedCO2 is traded at ~$25/tonne; this total CO2 offset of 10 tonnes is worth $250. Rosenfeld notes that “the idea of a white roof to cool a glass greenhouse is well known. A greenhouse traps heatefficiently in winter, but then it overheats in warm weather. The manager then whitewashes the panes so as to reflect solar radiation back out through the transparent atmosphere instead of permitting thatradiation to enter the greenhouse where it is converted to heat and trapped.” It is fairly easy to persuade (or to require) owners of buildings with flat roofs to select white for their roof color, and...