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White Roofs Could Raise Global Temperatures

A worldwide conversion to white roofs may actually warm the earth by about 0.07 C (0.13 F), according to Stanford University researchers.

In Effects of Urban Surfaces and White Roofs on Global and Regional Climate, in the Journal of Climate, Mark Z. Jacobson and John E. Ten Hoeve combined land use, vegetation, albedo (reflecting power of the earth’s surface) and soil-type data to model temperatures 20 years into the future, estimating the effects of white roofs on climate.

They found that a worldwide conversion to white roofs, accounting for their albedo effect only, would cool population-weighted temperatures by about 0.02 C but raise overall global temperatures by much more. The researchers said that the extra sunlight reflected by white roofs can increase how much light is absorbed by dark pollutants such as black carbon, according to the Guardian.

They noted that white-roof local cooling may also affect energy use, and therefore emissions – a factor not accounted for by the study. But, Jacobson said, “Cooling your house with white roofs at the expense of warming the planet is not a very desirable trade-off. There are more effective methods of reducing global warming.”

White roofs have been frequently mentioned in media outlets as an easy fix for reducing energy consumption and costs. In the Atlantic in July, president Bill Clinton described them as the single best idea to jumpstart job creation. Clinton, founder of green building funder the Clinton Climate Initiative, pointed to a New York City program that hires young people to paint the city’s roofs white.

In 2008 California scientists said white roofs can cut a building’s energy use by 20 percent, and said they have a formula to calculate how much CO2 can be offset by increasing the reflectivity of rooftops.

Last year, U.S. Department of Energy Secretary Steven Chu announced a series of initiatives to more broadly implement cool roof technologies on DOE facilities and buildings across the federal government.

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11 thoughts on “White Roofs Could Raise Global Temperatures

  1. This study is rather useless without its noted lack of comparison to how much energy is saved by the buildings with white roofs- 20%! The offset by white roofs using less energy therefore less pollution negates this study.

  2. It seems that white roofs in warm or hot climates is beneficial. Without disregarding the presented work,if dark pollutants were not absorbed on the way down, will they be absorbed on the way up to space?
    However, also important is to allow small spaces between tiles, i.e., no mortar between them, as to avoid hot air bubble trapped in the roof, increasing air conditioning thermal loads.

  3. White roofs would be used in cooling-dominated climates only, not world-wide. This study is terribly incomplete without accounting for the reduced electric generation caused by reduced cooling loads.

  4. Sounds like an big oil based study…white roofs do not use the traditional roofing materials such as tar and shingles. And consider the basis of a “worldwide conversion”…how can this be credible.

  5. The first sentence of this article is not supported by the journal article. The authors of the journal article clearly state that they are not accounting for the effect of white or cool roofs on energy use, which could have a greater impact than the change in the energy absorption by the atmosphere. The paper only loks at a part of the picture, not the whole picture.

  6. Now they need to look into what are the tradeoffs for reducing energy using by 20% and warming the earth by .13F. Would the reduced energy usage have such a benefit that it would offset and actually more greatly benefit the earth’s usage of electricity? I understand the Earth would be warmer but we also would need to know how the Earth’s temperature would compound(ly) increase over the next 100+ years. If this would adversely affect the benefits of decreased energy usage by such amount that the energy savings are thus zero or negative then we have a problem.

  7. The aim of white roofs is to reflect the sun’s high frequency light back to the sky, rather than absorb it on earth and then radiate it out as lower frequency heat energy. The reason being that high frequency (higher energy) light penetrates our greenhouse gases (primarily CO2 and Water vapour) more efficiently than does lower frequency heat energy being radiated back out from the warmed surface of the earth.

    It seems to me, that for the research to be correct (ignoring the fact they didn’t look at the energy and GHG savings of white roofs) then black pollution in the sky would need to absorb high frequency reflected light selectively over low frequency heat energy being radiated up from the warmed surface of the earth. Given that black pollution is made of physical particles of soot, I can’t see how it would absorb high frequency light waves but allow low frequency energy waves through it.

    It will be interesting to see what scientists make of the study once it gets some public peer reviewing.

  8. Congrats to the readership for so quickly spotting obvious flaws in this study. Let me point out two more flaws:

    1) The study “… combined land use, vegetation, albedo (reflecting power of the earth’s surface) and soil-type data to model temperatures …”. So this study pulled in widely disparate factors to arrive at their conclusion. Why then does the conclusion exclusively blame the white roofs for the entire result? Sounds like ‘greenbashing’ to me.

    2) Leaving out those other disparate factors, consider the only true difference between white and black roofs – black ones instantly absorb alot of heat, while white ones reflect much of the long-wave heat radiation. In both cases, the long-wave radiation has already traversed the entire atmosphere to arrive at the rooftop. So now we are to believe that this reflected radiation will somehow lead to an increase in temperature, over and above what already would have ocurred when that heat radiation warmed the black rooftop upon the instant of its arrival? Yeah – right. To accomplish that, the reflected radiation would have to be absorbed by atmospheric constituents on its way back out to space – by an amount greater than 100%! It would have to be greater than 100%, since we are only talking about radiation that was not absorbed in the case of a black roof. This of course is complete scientific nonsense.

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