A Cool Idea for Fighting Urban Warming

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The benefits of roof gardens are no longer limited to the aesthetic — recent research shows that green roofs can play a significant role in reducing temperatures in urban centers.

Green roofs — which can be easily installed on many urban structures and are composed of layers of thermal insulation and waterproofing topped by growing medium and vegetation — have long been touted for their private benefits, which include sound insulation, improved drainage, and savings on private heating and cooling costs. Yet research conducted at Cardiff University in the U.K. suggests that green roofs could have a much wider reaching impact, possibly even as a solution to the urban heat island.

Taking into account a city's climate (humid or dry) and urban geometry (shaded or exposed), researchers Eleftheria Alexandri and Phil Jones used urban canyon models to calculate the mitigating influence of green roofs on high summer temperatures in Athens, Riyadh and Mumbai. Their findings were startling: a 30° C (54° F) difference in the surface temperature of the roof itself, and a lowering of the entire urban temperature by as much as 4° C (about 7° F).

While the eco-construction market is well established in Europe and green roofs have enjoyed recent popularity in U.S. cities like Chicago and San Francisco, the phenomenon has yet to gain real traction in most of the world. As it is now clear that the potential is not restricted to personal penny pinching, it would be in many cities' best interest to adopt a proactive stance in regard to recent global temperature increases and promote green roofs as a simple, relatively inexpensive solution to uncomfortable and often dangerous levels of summer warming.
And for those green enthusiasts who have already traded in their roofing for rosemary? The next frontier is green walls, which, though more difficult to install, have a greater cooling effect at the street level than their horizontal cousins.