Published by City Farmer, Canada's Office of Urban Agriculture


Green Roofs Cool City Rooftop Gardens in Chicago to Fight Smog, Heat

by Tammy Webber
The Associated Press
C h i c a g o, April 5, 1999

City officials want to add a new architectural element to Chicago buildings -- green rooftops. That's green as in grass, flowers and other plants. The city environment department plans to plant gardens atop several city buildings this summer as part of a U.S. Environmental Protection Agency program studying ways to help cool cities and reduce smog.

Dark-roofed buildings -- along with miles of pavement -- absorb the sun's rays, making Chicago a huge heat island and adding as much as 4 to 6 degrees to city thermometers, said William Abolt, acting commissioner of the city environment department.

Out of the Darkness

"The city's wearing dark clothes and it's making the city hotter," Abolt said. "We want to dress the city in lightercolors." Hotter temperatures increase smog by forcing electric plants to work harder and emit more pollution, city and EPA officials said. Pollution reacts with heat and sunlight to create smog. That's a big issue for Chicago, which consistently violates federal air quality standards. The rooftop gardens could begin with City Hall and other public buildings, such as schools, said Alexandra Holt, deputy director of the environment department. The first gardens could be planted this year.

Trees and Lighter Pavements

The city also will plant trees and other vegetation in medians to help cool pavement, and will consider light-colored pavingsurfaces, she said. Nobody knows how many roofs would have to be resurfaced or trees planted to make a measurable difference in heat and smog, said Virginia Gorsevski, program analyst with the EPA's office of airand radiation.

Computer modeling predicts that widespread heat-reduction measures could easily lower a city's temperature 5 degrees, said Hashem Akbari, a researcher at Lawrence Berkeley National Laboratory in California, which performs the modeling. But no city has ever implemented wholesale changes to test the models'accuracy. Chicago is one of five cities participating in the EPA program and the only one promoting rooftop gardens, Gorsevski said.

Go Away, Sunlight!

The others -- Baton Rouge, La.; Houston; Sacramento, Calif.; and Salt Lake City -- are concentrating on roofing materials that reflect sunlight, Gorsevski said. Holt said Chicago officials focused on gardens because reflective roofing might make buildings more difficult to heat during the winter. Akbari said gardens would offer benefits similar to those of reflective roofing, but could be more expensive. Still, researchers are eager to see Chicago's results, Gorsevski said, adding that she knows of no U.S. city where such gardenshave been tested.

Abolt said reducing emissions from vehicles and small, unregulated businesses -- the biggest contributors to smog -- or cracking down on industrial smokestack emissions isn't enough. Large industry contributes only about 20 percent of thepollution, he said. "We don't think traditional approaches to dealing with smog will solve the problem," Abolt said.

Cooling City Is the Best Smog-Fighter

"He's right," Akbari said. "Based on modeling and results of small-scale testing, cooling a city could reduce smog more than almost every other pollution-fighting measure," Akbari said. "The chemicals need an oven to cook in to produce smog," he said. City officials will encourage private corporations to plant rooftop gardens. But the city must first monitor the benefits of its own project to prove it works, he said.

The city is working with engineers and designers to plan the first gardens, which could be simple prairie grass or more elaborate plantings, Abolt said. He said the gardens must require little maintenance and the roofs must be able to bear the weight of the gardens, which will not be designed for people to use. "The point is to make it green and efficient," Abolt said. Akbari said the potential benefits make the project worthwhile. "It is a win, win, win case," Akbari said. "You have a better environment, lower temperature and you save energy. "This is not rocket-science type of business."

David Foley Holland and Foley Building Design
232 Beech Hill Road Northport, Maine USA 04849
p: (207) 338-9869 f: (207) 338-9859

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Revised Friday, April 23, 1999

Published by City Farmer
Canada's Office of Urban Agriculture