New York City has nearly one million buildings—many of them woefully energy inefficient. Insulation is spotty at best, single-paned windows leak heat in winter and cool air in summer, and the untold millions of electric appliances they hold suck energy from the grid—many even when turned off. As a result, buildings contribute 79 percent of the Big Apple's 60 million metric tons of greenhouse gas (GHG) emissions, according to the Mayor's Long-Term Planning and Sustainability Office; the remaining 21 percent stems from cars, trucks and mass transit. In fact, New York City emits roughly the same amount of GHGs as the entire country of Ireland and contributes 1 percent of total U.S. emissions. Worldwide, buildings—both commercial and residential—contribute roughly one third of all GHG emissions despite covering only 0.2 percent of land worldwide. And experts say that reining in pollution from them will be key in the fight to contain climate change.

The good news? "By 2030, about 30 percent of the projected GHG emissions in the building sector can be avoided with net economic benefit," scientists write in the Intergovernmental Panel on Climate Change (IPCC) report on ways to stave off the effects of global warming.

Five international economic institutions around the world—ABN AMRO, Citi, Deutsche Bank, JPMorgan Chase and UBS—as well as four multinational energy services companies—Honeywell, Johnson Controls, Siemens and Trane—cut a $5 billion deal to work together to retrofit existing buildings in 16 of some of the world's biggest cities, including New York, London, Johannesburg, Karachi, Mexico City, Mumbai and Tokyo.

The effort calls for the banks to loan cash to the cities and building owners to make needed changes, and for the power companies to provide the retrofits and guarantees of energy savings. The cities and building owners will repay the loans from monies saved by reduced energy costs. It is hoped that the plan will stimulate the sluggish U.S. and world market for such retrofits: The U.S., which has had an energy efficiency retrofit program for 25 years, is the world leader in this area, yet fewer than 1 percent of the country's buildings have been redone.

"We will all benefit from this, whether small or big, rich or poor," says Berhanu Deresa, mayor of Addis Ababa in Ethiopia, whose city is not yet part of the initiative. For example, he notes, the many international buildings that hold United Nations and African Union offices in this capital city could be retrofitted with solar panels to generate electricity. "We have 13 months of sunshine," Deresa adds. "Even if [the U.N.] only equipped their own buildings, that would be a great help."

But retrofitting alone is not enough, says Mark Levine, a senior staff scientist at Lawrence Berkeley National Laboratory in California, a federal institution that studies a wide range of engineering and scientific issues. He says that builders constructing new commercial and residential buildings in the developing world must incorporate more energy-efficient—and more costly—materials and designs to control greenhouse emissions. "China built an awful lot of buildings very quickly," Levine says. "Until fairly recently [its] buildings were not very good from an energy point of view."

New and existing homes hold the greatest and cheapest hope for gains. According to the Clinton Climate Initiative (an effort to curb climate change launched by former President Bill Clinton), installing thick insulation and double-paned windows as well as using energy-efficient electrical appliances can cut emissions by as much as 50 percent. Simply using efficient ovens, dishwashers, refrigerators, washers, dryers and other appliances as well as compact fluorescent light bulbs instead of incandescent ones could negate the need for the energy output equivalent of 110 coal-fired 600-megawatt power plants that might otherwise be needed in the U.S. by 2020, according to a report by the consultancy think tank McKinsey Global Institute.

Adding features like solar generators or solar water heaters can pay for themselves over a longer term as well. "I think residential is a slam dunk," Levine says. "It's just a matter of applying technology that we already have."

Another area where cities can play a major role in warding off warming is waste management, enhancing efficiency by taking relatively cheap and simple steps such as capturing the latent power in landfills or recycling. "Things like waste recycling or waste minimization actually avoid greenhouse gas generation," says waste management expert Jean Bogner, a lead author of the IPCC report. For instance, making a beverage can from recycled aluminum uses only 5 percent of the energy it would take to create one from scratch. "If you recycle in the case of aluminum, you are talking about a 95 percent savings of energy," says IPCC author Lenny Bernstein, an environmental consultant.

The bottom line: buildings contribute a tremendous amount of GHG emissions whereas garbage produces relatively little (roughly 5 percent, according to the IPCC), but both offer immediate cost-effective remedies for climate change. A key to limiting global warming will be making sure that cities from New York to Addis Ababa implement those changes. "Now is the time to start doing real work," says Ken Livingstone, mayor of London. "Retrofitting our buildings and bringing down our carbon emissions."