Half the world’s population currently lives in cities, and that number is spiraling upward, as urban settlements gobble up most of the world’s natural resources and emit the most pollutants. No wonder that these panelists perceive the challenge (and opportunity) of sustainability as much bigger than getting people to switch from incandescent light bulbs to fluorescents.

The “latest craze in city governance,” says Judith Layzer is making your city as sustainable as possible. New York for instance, has vowed to plant one million trees, and convert its entire taxi fleet to hybrids. Chicago is covering its rooftops in green; Toronto composts. Layzer believes there are “good reasons to worry we’ll see symbolic commitments with not much done.”

Cities struggle to undertake systemic change, partly because they don’t control the supply and demand mechanism for energy resources such as oil, which helps drive commuting and mass transit behaviors. Cities have also historically supported unfettered growth to keep their tax base high, and when confronted with a sensible, pollution saving plan such as switching traffic lights to LED lightbulbs, cringe at the high upfront costs. Layzer thinks successful urban sustainability initiatives will depend on national governments pricing natural resources appropriately (e.g., eliminating subsidies on fossil fuels); effective local leadership that makes the case for often unpopular schemes like parking fees and congestion pricing; and major coalition building.

No amount of green construction will help with reducing greenhouse gases to desirable levels if today’s buildings aren’t altered to reduce their CO2 emissions, says Milton Bevington. His brief with the Clinton Climate Initiative (CCI) in 40 cities worldwide is to provide market-based solutions, not handouts or tax rebates, to get efficient heat and power into millions of residential and commercial buildings. A large part of Bevington’s job is educating landlords and others about new financing approaches for retrofitting old buildings. One example: a Chicago bank designed a loan enabling the owners of the city’s 550 thousand multifamily housing units to use an “energy performance guarantee” as collateral. Borrowed funds go into reducing water and energy use, and “every single dollar required to pay back the bank” comes from a reduction in energy use. Bevington would like to see more investor-driven financing for energy efficient projects, which he believes could spread swiftly in both rich and poor countries “to change a large sector of the built environment.”

There’s a dilemma brewing for most of the world’s big businesses, says Bill Sisson, who is United Technologies’ point man in a business consortium effort on energy efficient buildings. While firms recognize the importance of energy efficiency, only 13% are rising to the challenge. Sisson’s group seeks to create a roadmap for zero net energy use in buildings, involving technology, improved financial mechanisms, and behavior change. Says Sisson, this is “really about managing risk and directing the future of business in the right way; we see this aspect of buildings as critical for our growth and presence in the market.