is explaining how three giant fermenters can convert household food waste, trimmings from parks and gardens and the slops from school and hospital canteens into enough methane gas to power about a third of the buses in the French city.

"The process is exactly the same as in the stomach of a cow," he said, gesturing toward three biodigesters which each hold 20,000 cubic meters of liquefied waste. "The objective is to fuel 100 of Lille's buses on this biogas, out of a total fleet of 350," Hirtzberger, head of the city's urban waste research and development, said.

From San Francisco to Malmo, Sweden, cities around the globe are preparing for a new imperative: to accommodate the mass of population growth and thrive, without further accelerating the release of carbon dioxide that threatens their existence.

With half the world's population already living in cities and the urban population projected to reach almost five billion by 2030, it is not just growth that puts them in the front line of climate change.

Even if populations escaping drought migrate to urban centres, the fact that 60 per cent of the world's 39 largest metropolises are located in coastal areas puts the cities themselves at risk in future centuries , from rising seas.

Sunshine, tech creativity and a clued-in population help widen the range of options for places like San Francisco - the first city to make it a crime not to compost food and waste in city bins, in a bid to cut landfill use to zero. Plenty of money on top of abundant sun are allowing Abu Dhabi to showcase a futuristic eco-city : Masdar City is a vision of solar panels powering pilot less taxis and trams and feeding desalinated water to citizens and its verdant palms.

Such visions make dazzling prospectuses for those eyeing a market which analysts expect to be worth $200 billion next year, and sunshine will be a major source of clean power as the cost comes down to make it competitive with fossil fuels.

But for many cities, particularly older centres in gloomier climates, the reality will be more like Lille - distilling energy from the excrement of citizens, the waste from restaurants and the mountains of unsold sandwiches left in supermarket refrigerators at the end of each week.

Much of it will be plain boring - pumping insulation foam into loft spaces and wall cavities, fitting double or triple glazing - the stuff that can keep small builders busy even if economic slowdown stalls grand construction projects. In all, it will require different approaches to whittle down society's impact on the planet.

HERE COMES THE SUN

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

Even within cities, the density of solar generation will vary, he added. 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.

As solar power costs have fallen due to economies of scale, an initially subsidised power source is becoming viable in some places. "In countries like Spain, southern Italy and Greece, the cost of energy from solar is already, or will soon be, at parity with the cost of electricity from the grid," said Winfried Hoffmann, president of the European Photovoltaic Industry Association . "Germany is less sunny so it will take longer, but it will reach parity by 2016 at the latest," he added.

EXCREMENTAL GAINS

Lille is focusing on waste. Biogas - the fuel that will power some buses is actually an ancient energy source. It captured the attention of 13th-century adventurer Marco Polo in China, where he noted covered pots of sewage stored to generate energy.

"We're studying the possibility of getting biogas from sewage sludge at one of the city's two sewage treatment plants, and that has the potential to do at least 150 more buses," said Hirtzberger. "Potentially, one could run the entire bus system with biogas from sewage and rubbish. This would be typical of most cities in Europe."

Other cities, such as Malmo, Sweden, use waste to heat buildings. In Malmo, 50 per cent of heat is produced from its 5,50,000 metric tons of waste a year - a level that could be replicated in most north European cities.

Malmo owes its success to an existing network of pipes to carry heated water from the Sysav plant direct to homes and businesses - a system popular in eastern Europe known as "district heating."

The system is also used in the city of Monsteras, 300 km to the northeast, using waste heat from the local pulp mill, Sodra Cell. As an added benefit, the heat from the subterranean pipelines keeps ice from cycle-paths during the winter.

MODEL TOWN

Basics of a sustainable city

Bulk of energy would be generated from renewable sources rather than relying on fossil fuels. Buildings would be properly insulated. Wasteful industries would be made more efficient or eliminated altogether A sustainable city would have massively improved public transport systems with reduced reliance on cars and better provision for cyclists and pedestrians. Over time, we would redesign our towns and cities to ensure that the era of the long commute was over People will have longer holidays and, thus, choose greener ways to travel long distances. We could develop fast, efficient and cheap railways for long distance transport. Collective social institutions, such as creches and launderettes, would be much more common