Green Roofs: Reflecting Light and Radiating Cool

A new study indicates that painting building roofs white could significantly cool cities and mitigate some of the effects of global warming.

The effects of urban heat on human health can be significant and are likely to worsen as a result of climate change. If supported by further studies, the research results have clear policy implications, highlighting an adaptive mechanism via which humans can ameliorate the impact of climate change on urban populations.

Cities are particularly vulnerable to climate change because they are warmer than rural areas. Asphalt roads, tar roofs and other artificial surfaces absorb heat from the sun, creating a heat-island effect that can raise temperatures by an average 2-5 degrees Fahrenheit (about 1-3 degrees Celsius). White roofs reflect some of that heat back into space and thus cool temperatures--much as wearing a white shirt on a sunny day causes one to feel cooler than wearing a dark shirt.

Researchers used a new computer model, designed to assess the impacts of a changing climate on urban populations and to explore options for countering rising temperatures. While the model did not capture individual cities, it did show the white roof effect in large metropolitan regions. The New York area, for example, would cool by almost 2 degrees Fahrenheit on summer afternoons.

The model simulates the amount of solar radiation that urban surfaces absorb or reflect, capturing such factors as the influence of roofs, walls, streets and green spaces on local temperatures. The researchers then analyzed the interactions of global climate change and urban areas.

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