White roofs reflect far more of the sun’s heat than do darker ones, so buildings with white roofs will need less air conditioning in the summer months. Moreover, the heat absorbed by a black roof both heats the space below it, and is also carried into the air — raising the ambient temperature of cities in what is known as the urban heat island effect.
Consequently, U.S. Dept. of Energy is pushing to more broadly implement cool roof technologies on DOE facilities and buildings. DOE offices will install cool roofs, whenever economical over the lifetime of the roof, when constructing new roofs or replacing old ones.
The white roof idea is confirmed by a Berkeley Lab study that used a detailed global land surface model from NASA which contained regional information on surface variables, such as topography, evaporation, radiation and temperature, as well as on cloud cover. For the northern hemisphere summer, Berkeley researchers found that increasing the reflectivity of roof and pavement materials in cities with a population greater than 1 million would, among other things, give a one-time offset of 57 gigatons (1gigaton equals 1 billion metric tons) of CO2 emissions (31 Gt from roofs and 26 Gt from pavements). That’s double the worldwide CO2 emissions in 2006 of 28 gigatons. Their results were published online in the journal Environmental Research Letters.
Researchers used a conservative assumption of increasing the average albedo (solar reflectance) of all roofs by 0.25 and of pavements by 0.15. That means a black roof (which has an albedo of 0) would not have to be replaced by a pure white roof (which has an albedo of 1), but just by a roof of a cooler color -- hot pink, say.
Roofs and pavements cover 50 to 65% of urban areas. More than half of the world’s population now lives in cities; by 2040 the proportion of urbanites is expected to reach 70%.
The Berkeley Lab study found that global land surface temperature decreased by a modest amount—an average of roughly 0.01degrees C, based on an albedo increase of 0.003 averaged over all global land surfaces.
The press release describing the work can be found here:
http://newscenter.lbl.gov/news-releases/2010/07/19/cool-roofs-offset-carbon-dioxide-emissions/
