White roofs and unintended consequences

Obama’s Energy Secretary Steven Chu once pitched painting roofs white as a solution to global warming. Maybe, but new research finds that there are unintended consequences: reduction of rainfall in the Southwest U.S.

Researchers from Arizona State University have found that “warming resulting from megapolitan expansion is seasonally dependent, with greatest warming occurring during summer and least during winter. Among the most practical ways to combat urbanization-induced warming – the painting of buildings’ roofs white – was found to disrupt regional hydroclimate, highlighting the need for evaluation of tradeoffs associated with combating urban heat islands (UHI).” (See press release.)

In a paper published in Environmental Research Letters (see full paper here) the researchers studied possible effects of white roofs on Arizona’s sun corridor (Phoenix, Tucson, Prescott, and Nogales). Note that this is a computer simulation study.

Urbanization, with its tendency to cover natural surface with various artificial surfaces such as asphalt, concrete, and buildings, which reduce natural evapotranspiration causes a reduction in rainfall. Their maximum urbanization scenario leads to a 12% reduction in rainfall. Painting roofs white leads to an additional 4% reduction in rainfall.

The researchers do note: “Integration of highly reflective cool roofs within the built environment offsets about half of urban-induced warming, leading to small regional-scale cooling during the winter season.” But, “the combined effects of UHI-induced warming and evapotranspiration changes act in the same direction and reinforce one another” thereby making the already arid region dryer.

The researchers provide a caveat in the usual scientific jargon: “While we consider our results robust, we recognize the value of a model intercomparison and differing parameterization selection (e.g., incorporation of various convective schemes) that could improve simulated diagnosis of uncertainty quantification.” Translation: we think we are right, but with computer models it’s garbage in, garbage out.

Update: More unintended consequences of white roofs.

See also:

Urban heat island effect on temperatures, a tale of two cities

Warmer nights no proof of global warming