It's a novel and intriguing idea, and in the excerpt of his talk embedded below, he claims that a worldwide whitening of roofs and roads is capable of removing as much carbon dioxide as removing every car for 11 years. A study conducted by Akbari et al. (2012) estimated that light-coloured roofs, pavements and roads could increase urban albedo by about 10%, and if undertaken globally could offset 130 - 150 billion tonnes of CO2, equivalent to removing every car for 50 years.
It shocked me that such a seemingly easy procedure like painting a roof could have such a significant impact, so I was eager to investigate the potential of this method and how suitable it was for urban areas in colder environments.
How can painting a roof white help to combat global warming?
Figure 1: Approximate albedos of various urban surfaces. (Source: https://weather.msfc.nasa.gov/urban/urban_heat_island.html).
As Figure 1 illustrates, the reflectivity of roofs varies immensely. Roofs and roads together account for roughly half of urban areas and contribute to the Urban Heat Island (UHI) effect by preventing evaporation and absorbing sunlight. The idea is simple: increase the albedo (solar reflectance) of these roofs and roads, which in turn will reflect more incoming solar radiation and hence tackle global warming (Akbari et al., 2008).
White roofs are also capable of reducing energy use through air conditioning by up to 40% in some climates and through reducing the impact of the UHI effect, could also provide better air quality and comfort as well as mitigating the UHI contribution to global warming (Akbari et al., 2008).
How effective is it?
There is currently much debate within the scientific community about the effectiveness of white roofs. Most notably, a modelling study by Jacobson and Ten Hoeve (2011) estimated that the UHI effect might contribute to 2-4% of global warming, in comparison to 79% from greenhouse gases and 18% from dark particulates. The study suggested that a worldwide conversion to white roofs could cause a net warming of the Earth due to less hot air rising resulting in fewer clouds being formed, as well as the increased surface reflectance creating an increase in sunlight absorbed by dark pollutants like black carbon. Jacobson and Ten Hoeve (2011) suggested that attaching photovoltaic panels to roofs would be a much better alternative, but their study on white roofs did not account for any reduction in electricity use for cooling.
However, later research such as the previously mentioned Akbari et al. (2012) study disagree with Jacobson, and a more recent study by Sproul et al. (2014) comparing roofs in the United States suggested that white roofs are the most economic to install and are three times more effective than green roofs (vegetated) at achieving global cooling.
What about in cold climates?
Within the research, there is a problem that the studies are often undertaken in hot climates (the bulk of research coming from the Lawrence Berkeley Laboratories) and do not assess their full environmental consequences. More recent research suggests that net negative environmental impacts can occur in colder climates due to a large heating penalty occurring from white roofs; that is, the resulting increase in heating required from high solar reflectance (Cubi et al., 2015).
The website of Steven Chu's former department, the U.S. Department of Energy, now explicitly states that cool roofs can increase energy costs in colder climates and acknowledge other potential problems such as increased susceptibility to the accumulation of moisture.
Regardless, the idea is limited in its potential impacts by the fact that less than 1% of the Earth's surface is urban. It appears to be an easy, inexpensive, effective strategy when applied to particular hot and dry climates but lacks the potential to be scaled globally as a significant geoengineering technique.
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