Garden in Chicago Sky Is a Cool Idea
For the crankiest connoisseurs of grit in a proudly gritty town, the new City Hall rooftop may be the capper in what one critic called Mayor Richard M. Daley’s “Martha-Stewartization of Chicago”: 20,000 square feet of downy phlox, mother of thyme, purple prairie clover, silky rye.
But for most residents, the rooftop prairie will represent a milestone in Daley’s largely lauded greening of a once graying city--and, more important, a promising experiment in the fight against urban heat and pollution.
Chicago’s will be the prettiest project in a five-city effort by the Environmental Protection Agency to study the problems of rising urban temperatures and smog. Workers began last month reinforcing the 11th-story rooftop of the downtown building. By March, nearly 500 cubic yards of soil, 21,000 plants and shrubs, and two oak trees should be in place to help cool the structure and fight the production of ozone.
“Cities are getting hotter and more polluted, but there are things we can do about it--some of them even fun,” said William F. Abolt, commissioner of Chicago’s Department of the Environment.
As greenery has given way to heat-absorbing tar paper and asphalt in urban America, “heat islands” have appeared--geographic swatches of unusually high temperatures.
Shade Trees to the Rescue
In Chicago, where the sweltering summers are only slightly less infamous than the icy winters, the temperature on top of a tar-roofed building like City Hall can reach upward of 190 degrees. The same is true for Los Angeles, Phoenix and dozens of other cities.
A single shade tree with a 15-foot canopy, however, will lower the temperature beneath its leaves by 3 or 4 degrees. At the same time, the tree cools the surrounding atmosphere by releasing water through pores in its leaves.
The garden atop City Hall, to be sown mostly with hearty plants that will withstand high winds and thrive on rainwater alone, will save an estimated $3,000 to $4,000 annually in air-conditioning costs. A similar county building nearby will retain its blacktopped roof as a scientific benchmark.
With about 50% of many urban areas paved or tarred over, summer temperatures in the heat islands rise as much as 8 degrees above those in surrounding areas, according to computer models created at California’s Lawrence Berkeley National Laboratory.
The higher temperatures mean increased energy consumption in the form of air-conditioning--energy that in Chicago comes mostly from high-polluting coal-fired plants--and an increase in ozone.
A key ingredient of smog, lung-searing ozone is created during a photochemical reaction among airborne pollutants--a reaction that intensifies with temperature. In Los Angeles, for every degree the mercury rises above 70, smog increases about 3%.
While workers in Chicago prepare to spread the soil, the other cities in the EPA’s Heat Island Reduction Initiative--Sacramento, Salt Lake City, Houston and Baton Rouge, La.--are experimenting mostly with more reflective roofing and paving materials.
A light-colored roof on a building in sunny Houston can lower rooftop temperatures by 30 degrees in the summer and reduce energy costs by as much as 15 cents per square foot, studies show. In cooler cities, such as Denver and Washington D.C., savings run closer to 3 or 4 cents a square foot.
The price of most lighter-colored roofing materials is only slightly higher than for dark colors. So scientists propose that cities and their residents use lighter materials when it’s time for a new roof.
“In 20 years, you have done all the roofs,” said Hashem Akbari of the heat island group at the Berkeley lab.
A massive greening and lightening of the Los Angeles Basin, according to a pilot study by the lab, would save $170 million a year in energy costs and an additional $360 million from improved air quality.
Decreasing the overall temperature in the Los Angeles area by about 4 degrees would result in a 12% reduction in smog, according to the study. Achieving the same 12% goal by cutting vehicle emissions would require every single car in Los Angeles County to be electric.
Rooftop Gardens Far From Cheap
If lighter roofing and paving materials are relatively cheap, rooftop gardens are not.
Chicago is sinking $1 million from a $1.1-billion settlement with its primary energy provider, Commonwealth Edison, into the City Hall garden. The city is plowing another half a million dollars into a handful of smaller gardens and similar projects in other parts of the city--knowing in advance they will never recoup its money from energy savings alone.
Then again, the 135,000 trees Daley has had planted in recent years--one every 15 feet when a new sidewalk is poured--and the hundreds of flower boxes installed throughout Chicago don’t pay for themselves either. But cultural critics agree that the nation’s third-largest city, which also goes to great lengths to preserve classic building facades and beautify its newspaper boxes, is enjoying an extraordinary renaissance.
Said Virginia Gorsevski of the EPA’s Heat Island Reduction Initiative: “There are things like aesthetics that you can’t quantify.”
