for National Geographic News
Like a tiny seed carried by a late summer breeze, the idea of cultivating plants on rooftops has spread from Europe to North America and around the world.
Toronto is the most recent city to hop on the green roof bandwagon. Last month it joined Canada's federal government and Green Roofs for Healthy Cities, a public-private lobby and research network, in announcing a $1 million (U.S. $640,000) partnership program aimed at monitoring the performance of two local green roofs over a two-year period.
Although several cities in the United States have also undertaken green roof programs, Europe is in the vanguard of the green roof experiment, with many cities having put incentive systems in place.
In Germany, for instance, more than 140 million square feet (13 million square meters) of rooftops have green roof systems thanks to incentives provided by more than 75 local governments.
Green roofs have also sparked interest in Japan, where in Tokyo the municipal government last year mandated that structures with a roof area greater than or equal to 10,765 square feet (1,000 square meters) must cultivate at least 20 percent of that area.
Combating Urban Heat Effects
The impetus behind these incentives is the hope that green roofs will reduce the "urban heat island effect"a phenomenon created primarily by urban surfaces such as pavement and conventional roofs that absorb solar energy and re-radiate it as heat. This raises the temperature of a city several degrees higher than that of surrounding rural areas.
The urban heat island effect is further exacerbated by added heat generated by running air conditioners and machinery, and from vehicle exhaust.
The higher temperatures mean city buildings need more air conditioning, and therefore use more energy than they otherwise would. Cranking up the air conditioning provides temporary relief for those indoors, but makes the heat island effect even worse as the warm exhaust of a city's air conditioning units actually makes it hotter outside.
In addition, the increased use of fossil fuels to provide extra electricity for air conditioners also increases emissions of greenhouse gases and other pollutants. This, combined with higher summer temperatures, increases the incidence of severe smog.
Growing natural cover on a building's roof can help combat all these effects and more, scientists say.
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