From the elevated train rumbling through downtown Chicago, the view is a blur of black rooftops, wide expanses of parking lots, and a paucity of greenery. It is standard urban scenery to passengers, but to environmental experts, rooftops and pavement are the next frontier for combating smog and higher temperatures in cities.
So this spring, Chicago and four other US cities are participating in an experiment to transform the rooftops of America. Here along Lake Michigan, officials plan to plant gardens on the roofs of a handful of public buildings, which could mean a lovelier view for passengers on public trains. But more important, some environmental officials say covering dark, heat-absorbing roofs with prairie grasses, purple coneflowers, and other plants, will also will reduce air pollution and cool the city - cutting energy costs.
Rooftop gardens are good for urban environments because they absorb industrial emissions and reflect heat. "Basically, anything you can do to cover areas with vegetation, whether it's urban forestry, tree planting - you'll have a natural cooling effect," says Dennis Church, president of EcoIQ, an environmental consulting firm in Cupertino, Calif.
For their part, the other cities involved in the program - Houston, Salt Lake City, Baton Rouge, La., and Sacramento, Calif. - are focusing on using lighter colored, energy-efficient roofing materials instead of roof gardens. Researchers at Lawrence Berkeley National Laboratory in California found in a study of buildings in Sacramento that those with lightly colored roofs used 40 percent less energy from air conditioning than those with darker roofs.
With technical help from the US Environmental Protection Agency (EPA), environmental officials hope such programs will lessen the detrimental effects of "urban heat islands," a phenomenon in which city temperatures can rise six degrees higher than those in surrounding rural areas.
Scientific research has shown that over the past 30 to 80 years, July's maximum temperatures in major cities worldwide, from Baltimore to Tokyo, have risen by as much as 1 degree per decade. The warmer temperatures create higher demand for air conditioning, which results in more air pollution.
Since 1990, the EPA has encouraged communities to save energy through voluntary tree-planting programs in residential areas. The increased shade over homes has helped residents save energy and electricity costs, although the evidence is mostly anecdotal, says Jack Barnette, chief of the EPA's climate-change program in Chicago.
The current program aimed at urban rooftops is "a more formalized approach" because NASA plans to measure the effects on the urban heat islands using satellite photographs, he says.
"There's some very good evidence to show this will save money, save energy, and reduce pollution," Mr. Barnette adds.
Chicago officials are still ironing out the technical kinks of planting rooftop gardens. There are structural issues such as putting heavy soil on tops of buildings, as well as concerns over how to avoid roof damage from garden watering.
But an expert in urban heat islands says Chicago's program raises even larger concerns. Unless city officials plant gardens on a majority of the metropolitan area's rooftops, the experiment will have only a "minimal" effect on smog, says Hashem Akbari, a lead researcher at Lawrence Berkeley National Laboratory.
"The only benefit you'll get is the direct effect on [lowering] the air-conditioning use of that building," Dr. Akbari says.
He stresses that he is not opposed to Chicago's program, but he believes city officials should focus on touting the aesthetic benefits rather than energy savings.
Switching from dark to light-colored roofs when they need to be repaired is a much cheaper and more effective approach, Akbari adds.
Still, Chicago environmental officials are confident that the program will provide a host of perks. "One is the citywide benefit of mitigating urban heat islands," says Jessica Rio, a spokeswoman for Chicago's Department of Environment. "Another is energy-efficient benefits, particularly lowering energy needs and costs. Third is the aesthetic benefit, so people can enjoy the greenery."