‘‘These are the three giant stomachs of Lille.’’ Amid the hum of machinery and the warmodor of putrefying autumn leaves, Pierre Hirtzberger is explaining how three giant fermenters can convert household food waste, trimmings from parks and gardens and the slops from school and hospital cafeterias into enough methane gas to power about a third of the buses in this French city.

‘‘The process is exactly the same as in the stomach of a cow,’’ he said, gesturing toward three biodigesters that each hold 20,000 cubic meters, or 215,000 square feet, of rotting liquefied waste.

The objective is to fuel 100 of Lille’s buses on the biogas, out of a total fleet of 350, said Mr. Hirtzberger, head of urban waste research and development for the city.

Cities around the globe are preparing for a new imperative: accommodating the mass of world population growth, and thriving, without further accelerating the release of carbon dioxide.

Half of the world’s population already lives in cities, and the urban population is projected to reach almost five billion by 2030. But it is not just growth that puts cities in the front line of climate change, even when the potential for migrants fleeing drought is factored in.

The fact that 60 percent of the world’s 39 largest metropolises are located in coastal areas puts the cities themselves at risk, in future centuries, from rising seas.

But cities’ solutions will vary. Abundant sunlight (and abundant oil money) allows Abu Dhabi, for instance, to envision Masdar City, a planned sustainable eco-city in which solar panels would power pilotless taxis and trams and residents would drink desalinated water.

For older city centers in gloomier climates, the approach might be more like Lille’s — distilling energy from human waste, discarded restaurant food and the mountains of unsold sandwiches left in supermarket refrigerators at the end of each week.

Cities in France, Sweden, Australia and the United States are looking at an exotic mix of energy sources, and their choices demonstrate that what looks good in architects’ promotional literature is not necessarily what works on the ground.

The approaches are different even within Australia. The government there is planning seven pioneering ‘‘solar cities’’ and is investing 1.5 billion Australian dollars, or $1.4 billion, in four large solar power stations. But a temperate city likeMelbourne will have a very different approach from that of sun-bathed Brisbane.

‘‘If you’re in Brisbane, you’ll probably have solar hot water and solar airconditioning and a bit of electric power as your mix,’’ said Jim Smitham, a renewable energy expert at the Australian state research body CSIRO. ‘‘But if you’re in Melbourne, you’ll be much more interested in heating and power and a little bit of air-conditioning for the summer.’’ Even within cities, the density of solar generation will vary according to the value of land, Mr. Smitham said. In pricey central business districts, solar panels will be stacked on rooftops, but in the suburbs, small-scale solar plants will help supplement households’ own generation. Outside the cities, where land is cheapest, solar power stations will find a niche, feeding power into the metropolis.

Lille, with moist air sweeping in from the North Atlantic, gets about half of Brisbane’s 2,790 annual hours of sunlight.

So the French city is focusing on biogas.

The fuel is actually an ancient energy source. It captured the attention of Marco Polo in China, where he noted that sewage was stored in covered pots to generate energy, and it earned a mention by the writer Daniel Defoe in the 17th century. Lille is looking at the same option. ‘‘Potentially, one could run the entire bus system with biogas from sewage and rubbish,’’ Mr. Hirtzberger said.

Other cities, like Malmo, Sweden, use waste to heat and power buildings. In Malmo, 50 percent of heat is produced from its 550,000 tons of waste a year—a level that could be replicated in most North European cities, said Richard Bengtsson, project manager of E.ON Nordic, which developed Malmo’s heat and power system.

‘‘Waste is an interesting fuel due to the fact that you don’t have to pay for it,’’ said Mr. Bengtsson. ‘‘You get paid to take care of it.’’ Malmo owes much of its success to an existing network of pipes that carries heated water directly to homes and businesses — a highly efficient system known as ‘‘district heating.’’ A similar system is employed in the city of Monsteras, 300 kilometers, or 185 miles, to the northeast, usingwaste heat fromthe local pulp mill. In an added benefit, the heat from the subterranean pipelines keeps ice off of cycle paths during the winter.

In the Finnish capital Helsinki, a power company is working with an information technology firm on an underground data center that would channel excess heat from computers into a district heating network, to warm homes.

Biogas could reap comparable benefits for capitals like London and Paris.

‘‘Biomethane for the grid has such great opportunities, because it uses the existing infrastructure,’’ said Martin Orrill, head of energy technology and innovation at British Gas.

Biomethane is already being injected into gas grids in Germany, France and Austria. And in New York, gas is taken from the Staten Island landfill and injected into the grid.

Most cities are likely to employ a mix of solutions. San Francisco plans to use solar to generate about 5 percent of its power by 2012. Residents can enter their addresses into a Web site for an instant estimate on how much money and carbon they could save with solar panels.

Even new bus stops have solar cells in their red plastic roofs.

The city has just finished a study of small-scale wind turbines that rev up about the time the sun sets, said Adam Browning, executive director of the Vote Solar Initiative that promotes sun power. Next, it plans a study of wave power. In November, the city announced a small-scale hydro plant fed by the mountains to the east, the first trial of a system they estimate could meet about a tenth of the city’s needs.

San Francisco’s carbon emissions are already 5 percent below1990 levels, said Mayor Gavin Newsom, who said the city’s eco-friendly citizens were more tolerant of new ideas like mandatory composting.

‘‘It was easy,’’ he said of the carbon cuts so far. ‘‘It’s just not difficult. We need to disenthrall ourselves about how difficult this stuff is.’’