Cool roofs could curb urban warming

Painting roofs white could help lower urban temperatures by up to 2 degrees Celsius compared to ‘green’ roofs planted with vegetation, but they could also have a negative effect on rainfall in some regions.

Urban growth is expected to increase surface temperatures in major metropolitan areas by up to 3 C. This is because pavement, rooftops, parking lots and other hard surfaces absorb heat and slowly release it, creating the so-called ‘heat island effect’, reports the Los Angeles Times. Add greenhouse-gas induced climate change to the equation, and some megapolitans could experience warming of more than 5.4 C by 2100.

A team of researchers led by Matei Georgescu from Arizona State University (ASU) set out to examine which adaptation strategies can best offset urban warming. In a recently published study, they claim that slowing urban expansion would cut warming by one-third to one-half. Equipping buildings with ‘green’ roofs planted with vegetation would cut warming even more, while painting roofs white – so-called ‘cool’ roofs – are even more effective at countering climate change.

But they also found that cool roofs perform differently depending on the region and even the season. For instance, cool roofs would be effective at curbing warming in drier regions with a Mediterranean climate such as California, but could reduce rainfall by 2 to 4 millimetres per day in humid regions such as Florida. Green roofs would be a better option in northerly locations where they could provide insulation during the winter. The conclusion: There is no one-size-fits-all solution when it comes to fighting urban-induced climate change.

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