Filming at the Brazilian port of Belem a few years ago - while producing a television documentary on the deforestation of the Amazon basin and the resulting loss of biodiversity - I saw a huge stack of mahogany timber with 'London' stamped on it being loaded into a freighter. As a result, I started to take an interest in the connection between urban consumption patterns and human impact on the biosphere. It seemed that the logging of virgin forests, or their conversion into fields of soya beans for cattle fodder (in Brazil's Mato Grosso region) or of manioc for pig feed (in the former rainforest regions of Thailand), was perhaps not the most suitable way to supply urban markets. I began wondering how the vast appetites of cities for resources, and their huge discharges of wastes, could be curtailed - and how cities, the main habitat of one species, could come to live in peace with the global habitat of millions of others.

The world's major environmental problems can only be solved as part of the way we run our cities. In just one century, urban populations have increased ten-fold, to some 2.5 billion people, and they now take centre stage in the global dramas of pollution, land degradation and loss of species diversity. Cities occupy only 2 per cent of the world's land surface, but use some 75 per cent of the world's resources, and release a similar percentage of wastes. Their concentration of intense economic processes and high levels of consumption both increase and stimulate their demands on resources. They profoundly affect rural economies - and their cultural diversity - far beyond their boundaries. As better roads are built and access to urban products is assured, rural people acquire urban standards of living and the mindset to go with them.

Recently the Canadian economist William Rees, started a debate about the footprint of cities - the area of land required to supply them with food and timber products, and to absorb their carbon dioxide output through growing vegetation. I thought it might be useful to examine the impact of London, the 'mother of megacities', the first - 190 years ago - to exceed 1 million people. Today its footprint, following Rees's definition, is 125 times its surface area; nearly 20 million hectares compared to 159,000. This means that - although it contains only 12 per cent of Britain's population - London requires the equivalent of all the country's productive land, though, of course, this extends to the wheat prairies of Kansas, the tea gardens of Assam, the copper mines of Zambia and other far-flung places.

The critical question, as humanity moves to even greater urbanization, is whether living standards in our cities can be maintained while their environmental impacts are curbed. It helps, in answering this question, to draw up balance sheets quantifying urban resource flows. Similar sized cities supply their needs with a greatly varying throughput of resources - but most large cities have been studied in considerable detail and it is often not difficult to compare them.

Demand for energy defines modern cities more than any other single factor. All their key activities - transport, electricity supply, heating, manufacturing and the provision of services - depend on a ready supply of fossil fuels. London, for instance, currently requires 20 million tonnes of oil equivalent per year - the equivalent of two supertankers a week - and discharges some 60 million tonnes of carbon dioxide. Its per capita energy consumption is amongst the highest in Europe, yet the know-how exists to bring these figures down by 30 to 50 per cent without affecting living standards - and creating large numbers of jobs in the process.

Cities need a whole range of new resource-efficient technologies to make them more sustainable and reduce their impact on the biosphere. These include combined heat-and-power systems, heat pumps, photovoltaic modules and low energy road transport. Major steps can also be taken towards urban sustainability in waste management. Every day London disposes of 6,600 tonnes of household waste - of which only some 4 per cent is recycled. Meanwhile most cities in Western Europe are developing and adopting ambitious technologies for recycling and composting waste.

From linear to circular

The metabolism of most 'modern' cities is essentially linear, with resources flowing through the urban system without much concern either about their origin, or about the destination of their wastes: inputs and outputs are considered to be largely unrelated. Nutrients are taken from the land as food is grown and not returned to it. Trees are felled for timber or pulp but forests are not replenished. Raw materials are extracted, combined and processed into consumer goods which end up as rubbish that cannot beneficially be reabsorbed into living nature. Fossil fuels are extracted from rock strata, refined and burned; their fumes are discharged into the atmosphere.

This linear system is profoundly different from nature's own circular metabolism where every output is also an input which renews, and thus sustains, life. Cities which take responsibility for their global environmental impact, and the potential benefits for their own populations, will adopt circular metabolic systems, assuring a sustainable relationship with the natural world.

Some cities have made resource efficiency a top priority, installing sophisticated equipment for resource recovery. Austrian, Swiss and French cities have taken the lead in installing waste recycling and composting systems. Twenty seven composting plants, with a combined annual capacity of 600,000 tonnes, are currently under construction in German towns and cities. Throughout the developing world, cities have also made it their business to encourage recycling and composting of wastes. Brazil's Curitiba is often cited for its energetic efforts towards urban sustainability, not only in waste management but also in creating a system of fast, convenient bus routes and persuading drivers to leave their cars at home.

Cities worthy of a new millennium will be energy and resource efficient, and culturally rich, with active democracies assuring the best uses of human energies. In Northern megacities, such as London and New York, prudent inward investment will contribute significantly to achieving higher levels of employment. In cities in the South, significant investment in infrastructure will make a vast difference to health and living conditions.

Some writers have argued that cities can actually be better for the global environment than the adjacent rural areas. They emphasize that they often contain an impressive range of plant and animal species. They suggest that the very density of human life in cities makes for energy efficiency in both home heating and transport. Systems for recycling wastes are more easily organized in densely inhabited areas. And urban agriculture too, if well developed, could make a significant contribution to feeding cities.

Growing food in urban areas is certainly common enough - a new book published by the United Nations Development Programme (UNDP) proves the point. Singapore, it shows, is fully self-reliant in meat and produces a quarter of the vegetables its people need. Bamako, Mali, is self-sufficient in vegetables and produces half or more of the chickens it consumes. Dar-es-Salaam, one of the world's fastest growing large cities, now has 67 per cent of families engaged in farming compared with 18 per cent in 1967. Two-thirds of Moscow's families are now involved in food production compared with a fifth in 1970. There are 80,000 community gardeners on municipal land in Berlin with a waiting list of 16,000. The 1980 United States census found that urban metropolitan areas produced 30 per cent of the value of United States agricultural production: by 1990, this figure had increased to 40 per cent, according to UNDP's Urban agriculture: Food, jobs and sustainable cities.

Cities, particularly those in the North, have yet to prove that they can be compatible with a healthy biosphere. This is an unaccustomed challenge for business people, planners, architects, politicians and citizens alike. Yet central and local governments are increasingly aware that efforts to improve the living environment must focus on cities. Eco-friendly urban development could well become the greatest challenge of the 21st century, not only for human self-interest, but also for the sake of a sustainable relationship between cities and the biosphere, on which humanity ultimately depends.