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NEW YORK — On a mild April afternoon, the sun baked a Bronx rooftop at more than 104 degrees. Just yards away, another rooftop measured about 70 degrees.
The cooler roof was a green roof, 5,600 square feet of shrubby plants called sedum. And the temperature difference between the two roofs is just one type of data that climatologist Stuart Gaffin is gathering to convince politicians and developers that green roofs are crucial to the future of cities.
Gaffin, an atmospheric researcher at Columbia University's Center for Climate Systems Research, has studied how green roofs affect the environment since January, when he began gathering data from his rooftop laboratory on the recently constructed buildings of the campus of the Ethical Culture Fieldston School, a private school in the Bronx. Gaffin is one of a growing number of climatologists who think green roofs are a powerful technology to help cities confront the threats of global climate change.
”Global warming has been my issue for my whole career,“ Gaffin said. The data he's collecting is bolstering the case for green roofs. Compared to a conventional roof, a green roof stays cooler in the sun, drinks up rainwater and reflects sunlight that would otherwise heat the roof like a blowtorch on a steel girder.
Cities are hot spots of worry for climate scientists because expansive urban areas, coated in asphalt and belching the summertime exhaust of countless air conditioners, already pose problems of air quality, heat and excess rainfall, which can foul water systems.
In coming years, global warming is expected to make those problems worse. ”Two of the biggest predictions for cities is more heat and more rain,“ Gaffin said. ”Global warming is going to actually amplify both of those things.“
Global warming will increase the intensity, duration and number of heat waves in the United States, said Radley Horton, another climatologist at Columbia. That's in addition to an existing problem: the urban heat island, a stifling pocket of air saturated with pollutants that surrounds a city on hot summer days. Horton won't link global warming to the urban-heat-island effect, but he thinks the combination poses a dramatic threat.
A single green roof benefits just one building, but advocates said a wide area of green-roofed buildings can affect the climate of a whole community.
”Is one roof (going to) make the entire neighborhood better air quality? A little,“ said Mark Thomann, a landscape architect at Balmori Associates, a design firm in New York. ”Would a whole bunch of green roofs do it? Sure.“
Experts said the sedum plants are easy to maintain, requiring weeding for only the first few seasons until they fill the roof with a dense, verdant carpet. The sedum plant can survive long periods of drought, heavy rainstorms and severe winds.
In the Midwest, interest in green roofs is growing, and entrepreneurs are taking notice. From 2004 to 2005, green roof square footage grew 80 percent in the United States, led by Chicago and Washington, D.C., according to Green Roofs for Healthy Cities, a green-roof industry organization.
Green roof technology is off to a strong start in some cities, but it struggles to grow in others.
”We've had a lot of requests from architects,“ said Thomann of the Balmori firm, which created a green roof for the Sopranos studio at Silvercup Studios in Queens. ”But they just want a deck terrace with a couple of planters.“
