A plan to clean this city's notoriously filthy air by planting millions of trees and curtailing driving could help counter the greenhouse effect if copied by other urban areas, an environmentalist said.

"If we're going to address this problem, it will be a grass-roots effort by activists throughout the cities of the world," said Jeremy Rifkin, director of the Foundation on Economic Trends and a member of the Global Greenhouse Network, a coalition of environmental activists in 35 countries.In response to federal pressure to reduce smog and local anxiety about growth and traffic congestion, Mayor Tom Bradley recently proposed planting 2 million to 5 million trees throughout the city during the next five years, encouraging the use of public transport and ride-sharing, and weaning industries away from fossil fuels that generate high levels of carbon dioxide.

The 63-point proposal represents the first urban effort to curb the worldwide greenhouse effect, said Rifkin, who joined the mayor at a City Hall news conference Wednesday. Bradley said he would ask mayors of cities throughout the world to emulate the plan.

"This landmark project, initiated in a major American metropolis, will focus on the urban areas throughout the world," Rifkin said.

"It is vital for the future of the planet that we begin to sharply reduce emissions of carbon dioxide and other gases that contribute to the greenhouse effect."

The greenhouse effect is a term used by scientists to describe a potential warming of the Earth's temperature by pollution and carbon dioxide that trap solar radiation in the atmosphere. Many scientists fear it could cause the planet's temperature to rise several degrees, melting polar ice, raising sea levels and flooding coastal cities and possibly disrupting agriculture.

Rifkin said the most dramatic element of the plan is the proposed planting of trees, which absorb carbon dioxide and produce life-giving oxygen.

"Up to 5 million trees planted in the right places in Southern California will conserve the energy equivalent of one to two new power plants," said Andy Lipkis, executive director of TreePeople, a conservation and community-action group that is organizing the planting campaign.

The city has donated $25,000 of an estimated $100,000 needed to organize the planting, which probably will cost several million dollars, Lipkis said.

Among the specific recommendations of the plan, which Bradley unveiled Jan. 4, are mandatory parking fees at shopping malls, replacing the city's cars and trucks with newer models that get better mileage and establishing tax incentives for home energy conservation.