The insatiable hunger of cities was brought home powerfully to me in the Eighties when I was working as a documentary film-maker in Latin America. In the Amazon, I saw huge areas of tropical forest incinerated and converted into cattle ranches. I encountered armies of displaced people digging up bauxite, tin and gold in mines deep in the forest. I saw huge stacks of mahogany cut from virgin rainforests. Piled up in ports like Belem, where were all these 'goods' going? Crates were stamped with the legends 'London', 'Paris' and those of other leading European cities.
Cities consume the bulk of the world's output. Look at Paris, a great and beautiful city, but, like its siblings and rivals, a vast transformer of resources gleaned from across the globe to be processed into consumer products and services, forever finding new ways of stimulating appetites, whether in terms of stylish restaurants or modish art galleries.
In the process, Paris and London, driven by cars and an insatiable desire to move onwards and upwards, pile up mountains of rubbish and spew billions of tons of waste gases into the atmosphere. In the summer months they are wrapped in a veil of toxic haze, from power station and factory chimneys, and from the exhausts of cars and lorries on their streets, and on the motorways that link them to a network of lesser and rival cities.
With nearly half the world's population now urban dwellers, the fate of the environment depends on the future development of the city - the way it functions, the way it is planned, the way it relates to the rest of the world, its architecture - and not a retreat from it, as so many ecologically minded people used to believe. The city can be seen as a monster - and it often behaved like one - but it is here to stay.
Cities, whatever their faults, are stimulating: places of rich cultural mix, and political and social freedoms. When I first arrived in London in the early Sixties I was astounded to meet people from all parts of the world in one place. Yes, there were tensions, even riots, yet in Notting Hill, where I settled and helped to set up the first carnivals, a new global culture seemed to be in the making.
Unlike many of my friends at that time, I did not travel to the corners of the earth for new experiences. I did not need to. I was intrigued just listening to the stories of people newly settled in London: tales from Nigeria, Sri Lanka and India. The city appeared to be giving something back to some of the peoples it had drawn its resources from.
We need to assess our cities not only by their cultural output, but also by the implications they have for the environment. We need a clearer understanding of how to reduce their dependence on global resources.
Cities will always be centres of trade, but how can that trade be conducted within the concept of sustainability? The onus on us now is to make the city work responsibly, so that what it consumes in terms of energy and resources, it gives back: in ecological terms, we need to audit the city.
Urban people - citizens - with their command of the world's resources, are particularly well placed to assess their impact on the biosphere. Will they take responsibility for the global impact of their culturally rich lifestyles? Can they initiate an urban ecological culture as well as culture itself?
It is here that planners, architects and urban authorities have a crucial new role. There is now plenty of evidence that cities can dramatically improve their environmental performance. The starting point is an analysis of the urban metabolism, with its input of raw materials and output of wastes. Can we account for the full environmental impact of cities? If we can, then we begin to know the true cost of cities, how much we should expect them to pay for the damage they cause and if we feel the damage is acceptable.
Economists have recently developed new tools for environmental accountancy. One of these is quantifying the role of cities in the context of 'GND' - annual 'gross national damage'. In Germany, GND is assessed at some 6 per cent of GNP, or around DM100bn ( pounds 42bn) a year. This includes damage to national forests caused by air pollution, ground water contamination, soil degradation and corrosion damage to buildings. Much of this is due to the waste discharges of cities. Using such analytical tools, substantial efforts are now being made to move towards greater urban sustainability.
Take energy. In Holland, Germany, Austria, Switzerland and Italy, 200 cities and towns - including Vienna, Berlin, Hamburg, Frankfurt, The Hague, Utrecht and Genoa - have formed a 'Climate Alliance', committed to reducing their fossil fuel consumption by 50 per cent by the year 2010. This exceeds by far the commitments towards greater energy efficiency made by their governments. Such a dramatic commitment to urban energy conservation requires a major rethink of our approaches to both energy supply and energy consumption, involving engineers, architects, transport planners, as well as householders.
London, with its 7 million people, needs the energy equivalent of a supertanker full of oil every day, year-in, year-out. At the other end of the equation, it blows some 180,000 tons of carbon dioxide into the atmosphere every day. Only a dramatic increase in energy efficiency of its buildings and transport systems, and the supply of energy from renewable resources such as the widespread use of solar energy, could help to make London's energy supply sustainable in the medium term. A study in Berlin, with a similar climate, has shown that 30 per cent of its current electricity consumption could be supplied from the solar-panels installed in roofs.
Beyond the issue of energy, it is the broader understanding of the functioning of cities that is of key importance. Modern cities are the home of the 'amplified man', in whom biology and technology merge almost indistinguishably. We are both man and machine, apparently unable to function without wheels. Our biological requirements - food, water, air - and technological ones - cars, washing machines, hi-fis - all have their own environmental implications. Food wastes, aluminium cans, glass bottles and plastic bags are all mixed in one unmanageable, sticky mess and to disentangle this is a massive undertaking.
Other European capitals, such as Rome and Madrid, have developed much clearer perspectives of the alternatives. Rome has now achieved virtually total recycling of its wastes, greatly helped by the fact that it actually has a city government, as London once had and needs again. The British government's current obsession with deregulating and breaking down the infrastructure of our cities is ecologically damaging, because it undermines concerted city-wide efforts to tackle a problem that spreads from Piccadilly Circus to the heart of the Amazon. Only concerted action, democratically organised, can make sense.
Architecture, too, has a major role to play in reducing the environmental impact of cities. Architects may have to re-examine their role in the urban scene. Their cherished concept of building monuments of wilful inventiveness may have to give way to design in the service of ecological sustainability.
Ecological design means many things to many people, but for me, the most important criterion is low environmental impact during use. Buildings can be made to run on a minimum of energy, requiring a minimum of heating or cooling, whatever the prevailing climate. Low-conductivity building materials, super insulation, the use of solar energy, hi-tech metal-coated glass and high-efficiency lighting systems are available for all buildings, new and existing. Who will make these the standard requirements for contemporary architecture?
None of these requirements means that we need turn the clock back in terms of design. An ecologically sound building does not have to look tweedy, or borrow its form from past styles. Architects like the Richard Rogers Partnership are working at the cutting edge of ecologically responsible design as they research and plan sophisticated new buildings for the turn of the century.
The city will not have to be populated with neo-Georgian or Victorian buildings in order to become ecologically sound. In fact, many of the most interesting experiments in ecologically responsible buildings in recent years have come from architects and engineers working at the leading-edge of engineering and technology. Many of London's old buildings are very unsound from an ecological point of view; poorly insulated, they waste energy.
However, to make our cities sustainable in the long term requires much more than improvements to the environmental performance of buildings. Culturally we need to develop a new concept of the city. The urban mind map can no longer stop at the city limits. The tentacles of our cities stretch around the globe.
Understanding this may change the way we view our cities, as places where culture is as much about the ecological implications of city living as about art and architecture, theatre and music, food and recreation. Cities are here to stay, but we should want them only if they can be made to function efficiently without further damaging the earth: Paris, London and Frankfurt in balance with the Amazon.
Herbert Girardet is an ecologist, film-maker and author. His book 'The Gaia Atlas of Cities; New Directions for Sustainable Urban Living' has just been published by Gaia Books, pounds 9.99.