Pete Harrison and Peter Henderson
REUTERS/ULLE, FRANCE

""These are the three giant stomachs of Lille.""

Amid the hum of machinery and warm odor of putrefying autumn leaves, official 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 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 towards three biodigesters which each hold 20,000 cubic meters of rotting liquefied waste.""The objective is to fuel 100 of Lilles buses on this biogas, Out of a total fleet of 350,"" Hirtzberger, head of the citys urban waste research and development, told Reuters.

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

With half the worlds 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 centers, the fact that 60 percent of the worlds 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 is allowing Abu Dhabi to showcase a futuristic eco-city Masdar City is a vision of solar panels powering pilotless 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 a record USS200 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 centers 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 fridges at the end of each week.

Much of it will just 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 myriad different approaches to whittle down societys impact on the planet.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.

In Australia, the government plans seven pioneering ""Solar Cities"" and is putting ASl.S billion into four large power stations driven by the sun.Even within cities, the density of solar generation will vary according the value of land, 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.Most cities will find the answer in a mix.San Francisco plans to use solar to generate about 5 percent of its power by 2012, mostly from small solar arrays which it is helping to underwrite.

Residents can enter their address into a website 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.Next it plans a study of wave power, and this month announced a small-scale hydro plant fed by thf mountains to the east, the first in a system that potentially could meet about a tenth of city needs.Carbon emissions are already 5 percent below 1990 levels and headed towards net zero, said Mayor Gavin Newsom, adding the citys eco-friendly citizens are more tolerant of trying new things such as mandatory composting.""It was easy,"" he said of the carbon cuts so far. ""Its just not difficult. We need to disenthrall ourselves about how difficult this stuff is.""