White roofs to fight global warming?

As part of our new “Journal Club” feature series, we’ll be highlighting a scientific journal article and explaining a major concept in Earth science.  This week we start with a recent publication on white roofs by Oleson, et al., from Geophysical Research Letters:

Oleson, K. W., G. B. Bonan, and J. Feddema (2010), Effects of white roofs on urban temperature in a global climate model, Geophys. Res. Lett., 37, L03701, doi:10.1029/2009GL042194.

Introduction:

Increasing the albedo of urban surfaces has received attention as a strategy to mitigate urban heat islands. Here, the effects of globally installing white roofs are assessed using an urban canyon model coupled to a global climate model. Averaged over all urban areas, the annual mean heat island decreased by 33%. Urban daily maximum temperature decreased by 0.6°C and daily minimum temperature by 0.3°C. Spatial variability in the heat island response is caused by changes in absorbed solar radiation and specification of roof thermal admittance. At high latitudes in winter, the increase in roof albedo is less effective at reducing the heat island due to low incoming solar radiation, the high albedo of snow intercepted by roofs, and an increase in space heating that compensates for reduced solar heating. Global space heating increased more than air conditioning decreased, suggesting that end-use energy costs must be considered in evaluating the benefits of white roofs.

Commentary:

While the promise is interesting, the deliverables are not there.  The authors started with a noble idea — to see if white roofs in urban centers would significantly impact global warming by refelcting more sunlight back into space.  Sometimes the smallest change in the land surface can have a global impact, but in this case, the there is just not much of an effect.  The authors conclude that the transition to white roofs in urban centers would not significantly change global climate, and would only slightly change local climate — these benefits may be outweighed by the costs (monetary and environmental) of the work required.

Still, it is a conclusion worthy of publication.  Since white roofs would reflect more sun, and therefore more heat, a building may be able to use less electricity for cooling and air conditioning with the lighter-colored surface.  UPS, for instance, has recently painted the tops of their famous brown delivery trucks white to save energy.

The full article (licensed access required).

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