The Big Apple wants its skyscrapers to turn greener.

That’s greener in terms of the environment, not in terms of money.

On a grassy rooftop in midtown Manhattan, Mayor Michael Bloomberg announced Wednesday – Earth Day – that the city will try to pass legislation mandating environmental changes that could reduce the city’s total carbon output by 5 percent. The proposed legislation is aimed at getting significant-sized buildings to meet tougher energy requirements anytime they renovate, to conduct an energy audit every 10 years, and to make energy improvements that will pay for themselves within five years. According to the city, the energy improvements could save landlords $750 million a year – assuming they upgrade everything from their lighting systems to boilers.

New York’s effort comes at a time when Congress is considering legislation to cap carbon emissions and then allow emitters to trade in carbon credits or allowances. The proposal, which had its first day of hearings on Wednesday, is considered to be President Obama’s second greatest legislative priority after healthcare reform.

On Wednesday, Mr. Obama, visiting a wind-power plant in Newton, Iowa, called on America to view a green emphasis as a way to lead the global economy in the 21st century.

For its part, New York is looked to by other cities as an environmental leader. Its latest effort is being watched by other cities, and if they adopt such plans, too, the impact could be big: The New York model requires older buildings, not just new ones, to become energy-efficient.

"This is a game-changer, because it's the first large-scale citywide program to cut emissions from existing buildings. It includes the private sector, and it focuses on efficiency upgrades that have a financial cost savings," writes Fred Krupp, president of Environmental Defense Fund in New York, in an e-mail. "And it does all that at the scale of New York City – harnessing billions of square feet of real estate to help solve global warming."

According to the US Green Building Council, buildings in the US are responsible for 39 percent of the nation’s carbon emissions and 32 percent of the energy usage. In New York, which has a low transportation carbon footprint because of mass transit, buildings represent 80 percent of carbon emissions.

“Buildings must be a part of the solution to the climate crisis and the economic crisis,” says Richard Fedrizzi, CEO of the council, which is known for bestowing green status on buildings.

Despite the endorsement of some of America’s most well-known environmental groups and some City Council leaders, it’s not a given that the council will adopt the legislation.

Two years ago, the City Council considered requiring buildings to use biodiesel to cut down on emissions. But that effort ran into opposition because there was concern that the biofuel would cause food prices to rise.

In addition, landlords in the city are starting to feel the effects of the economic downturn. Office vacancy rates are rising. Some prominent building owners, who purchased properties at the wrong time, are defaulting on payments.

Mayor Bloomberg’s latest initiative is part of his plaNYC, an effort now 2 years old to substantially reduce the city’s carbon footprint. New York has already planted 174,189 trees (with a goal of 1 million), added 80.9 miles of bike lanes, and launched 224 energy-efficiency projects for city buildings.

Among the major points of the new proposal:

• A new energy code that must be met by existing buildings of 50,000 square feet (approximately equivalent to 50 apartment units) whenever they make reservations. This would go into effect in 2010.

• The requirement for an energy audit every 10 years, which would go into effect in 2012.

• A requirement for buildings to upgrade their lighting by 2010.

To try to increase support for these moves, Bloomberg is characterizing them as creating 19,000 construction jobs. At the announcement on Wednesday, a host of union leaders endorsed the plan.

The jobs would come as building owners replaced light fixtures, added energy-efficient faucets and showers, and cleaned and tuned their boilers. Some $16 million in federal funds would go toward training such workers.