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Urban Heat: Can White Roofs Help Cool World’s Warming Cities?

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  • It has long been known that installing white roofs helps reduce heat buildup in cities. But new research indicates that making surfaces more light-reflecting can have a significant impact on lowering extreme temperatures – not just in cities, but in rural areas as well.
  • There can be unintended consequences, both on temperature and other aspects of climate, like rainfall. Even local geoengineering needs to be handled with care

….Keith Oleson of the National Center for Atmospheric Research in Boulder, Colorado looked at what might happen if every roof in large cities around the world were painted white, raising their reflectivity — known to climate scientists as albedo — from a typical 32 percent today to 90 percent. He found that it would decrease the urban heat island effect by a third — enough to reduce the maximum daytime temperatures by an average of 0.6 degrees C, and more in hot sunny regions such as the Arabian Peninsula and Brazil.

Other studies suggest even greater benefits in the U.S. In a 2014 paper, Matei Georgescu of Arizona State University found that “cool roofs” could cut temperatures by up to 1.5 degrees C in California and 1.8 degrees in cities such as Washington, D.C….

…But things might not always be quite so simple. Reducing local temperatures would, for instance, limit evaporation, and so potentially could reduce rainfall downwind. A modeling study by Irvine found that messing with the reflectivity of larger areas such as deserts could cause a “large reduction in the intensity of the Indian and African monsoons in particular.” But the same study concluded that changing albedo in cities or on farmland would be unlikely to have significant wider effects. ..

…Solar panels “cool daytime temperatures in a way similar to increasing albedo via white roofs,” according to a study by scientists at the University of New South Wales. The research, published in the journal Scientific Reports last year, found that in a city like Sydney, Australia, a city-wide array of solar panels could reduce summer maximum temperatures by up to 1 degree C. …

…The urban heat island can be a killer. Counter-intuitively, the biggest effects are often at night. Vulnerable people such as the old who are stressed by heat during the day badly need the chance to cool down at night. Without that chance, they can succumb to heat stroke and dehydration. New research published this week underlines that temperature peaks can cause a spike in heart attacks. …

…A combination of rising temperatures and high humidity is already predicted to make parts of the Persian Gulf region the first in the world to become uninhabitable due to climate change. And a study published in February predicted temperatures as much as 10 degrees C hotter in most European cities by century’s end. No wonder the calls to cool cities are growing….

…Whitewashed walls, arrays of photovoltaic cells, and stubble-filled fields can all provide local relief during the sweltering decades ahead. But policymakers beware. It doesn’t always work like that. There can be unintended consequences, both on temperature and other aspects of climate, like rainfall. Even local geoengineering needs to be handled with care.

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