At a ceremony last week on the Magnificent Mile, Mayor Richard M. Daley cut the ribbon on Chicago's newest green roof.

"There are huge benefits," Daley said, citing the environmental effects of green roofs.

Daley's green roof program started with installation of trees and shrubs on top of City Hall in 2001, and has grown to include 300 additional projects.

But assessing the environmental impact of green roofs is a difficult task. Advocates say the roofs reduce temperatures in urban areas, control storm run-off and cut energy bills.  Five scientists interviewed by the Daily News, though, said there are no studies proving those benefits on a citywide level.

"I don't think Green Roofs alone will have an immediate effect," said Stephen Bell, director of the Chicago Center for Green Technology. "But more green initiatives will help."

The benefits of green roofs for individual buildings are clear. They can be up to 60 degrees cooler than traditional roofs during the summer, cutting air-conditioning bills and energy usage.

"A one-degree drop can really benefit energy consumption," said Brad Bass, a professor at the University of Toronto's Centre for Environment.

The roofs also absorb rainwater that runs off conventional buildings and into the sewer system, where it can carry pollutants into waterways.

But researchers have been unable to document that green roofs can reduce overall storm run-off in a large city, or cut energy consumption on a wide scale, according to several scientists.

It's also uncertain that green roofs can help the urban heat island effect, which causes higher temperatures in cities because roofs and other building materials trap heat.

Because green roofs are not widely used, no city has installed enough of them to produce a measurable benefit, said Tom Liptan, who is an enviromental specialist for the Bureau of Enviromental Services in Portland, Ore.

"No city in the U.S. or Canada really has enough [green roofs] to do any monitoring," said Liptan.

"We know that it works at an individual level," said Merritt.

And benefits from an individual level will translate into a city wide benefit, Merritt said.

At the Notebaert Nature Museum, a green roof has helped cut runoff during rain storms, said technical operations manager Chris Dunn.

"We have kind of turned an unnatural area into a natural area," said Dunn.