I have stressed the enormity of the environmental crisis we face. We see the evidence all around us: in congestion, in the effects of acid rain, in the difficulty many children and old people have in breathing. And it is clear that cities are generating most of the damage.
The ecological problems our cities face are intertwined with social ones. The city is a rich, multi-faceted place, which offers private pleasures, but also the chance for public life. The problem is that for too long individual interests and the search for short-term profit have overridden the needs of the community.
The division of the city into different centres of isolated activity, the proliferation of private, opaque, and ugly buildings, the promotion of the car, have undermined the public realm. In place of the bustling street, we have the supermarket; instead of the square, we get the car park. Eventually the rich flee, and the poor are left stranded in inner- city ghettos.
But our cities are not beyond help. Almost everyone agrees on the supreme value of the natural environment. The most important principle to emerge is sustainability - the principle that economic development today must not compromise the ability of future generations to meet their own needs.
Sustainability needs to be applied to cities, which consume a disproportionate amount of our natural resources. The key lies, as the ecologist Herbert Girardet has stated, in cities aiming at a circular "metabolism" by conserving their resources, using renewable energies, and recycling waste.
Ironically, Third World cities, too poor to squander resources, often come closer to the ideal than their developed counter-parts. Cairo, for instance, although larger than London, recycles most of its solid waste. Some European cities are moving in the right direction. The Germans have passed the Materials Recirculation Law, which comes into effect next year. This creates an obligation to reduce the generation of waste and makes industries fully responsible for its disposal.
We are getting used to thinking about nature as being of ultimate value; we now need to think of the civic realm in the same way. The terms that describe our relationship to nature - the idea we are not owners but trustees - apply just as well to the city's public life.
Perhaps the best reason for promoting public life is simply that it is immensely pleasurable. Public life fosters tolerance and community. It is no coincidence that in racist or fascist societies the city was segregated. Sharing public space forces us to acknowledge what we have in common.
The public life of cities also plays an increasingly vital economic function. Today, competition is more international. Cities compete for investment not just against neighbouring cities, but against global rivals. In this post-industrial age, investment is attracted by factors beyond the normal economic costs of labour or rent. The quality of the cultural and civic life of a city is now critical.
In Europe this principle has become a political commonplace. Indeed, there is a race on, with cities striving to outdo each other in commissioning prestigious architectural and cultural projects, and inaugurating extensive programmes of civic regeneration. What the mayor of Stuttgart does, the mayors of Frankfurt, Lyon and Amsterdam will determine to do better.
Nowhere can this be seen more clearly than in Barcelona. Here autonomy, vision and strong leadership have totally transformed the city. Barcelona was catapulted into reforming its congested, polluted and decaying inner city by hosting the 1992 Olympic Games. The development went much further than the provision of Olympic facilities. It included the realisation of a masterplan, including the refurbishment of Barcelona's streets and squares and the construction of new housing and services. The city council created a network of more than 150 interlinked public parks and places. World attention provoked the city into the wholesale revitalisation of its defunct waterside dock areas. A new mixed waterside district and linear park put the city back in touch with the sea. Barcelona again became a city in which people longed to work and live.
But in Britain we have still not grasped the economic importance of a thriving urban culture.
The city that I describe turns its back on the dominant tradition of 20th-century urban planning. Ebenezer Howard's Garden City, Le Corbusier's Ville Radieuse and Frank-Lloyd Wright's Broad-Acre City rejected, in their different ways, the dense mixed-activity city, in favour of more spread-out, greener urban forms. Their aim was to combine the merits of the city and the countryside.
Yet it has become clear that these modernist cities lack the dense critical mass needed to sustain the vib-rancy of urban life. The city I advocate takes the tradition of dense overlapping activity as its starting point, but reinvents it.
The sustainable city of the future will, I believe, need to be many things. First, it should be a dense and polycentric city - because this form of settlement protects the countryside, focuses communities around neighbourhoods, and minimises dependence on cars.
Second, it must be a city of overlapping activity, because it maximises contact and diversity. Third, it should be an equitable city - a self- governing, participatory city where wealth and justice are fairly distributed. Fourth, it must be an ecological city, which gives as much to the environment as it takes out. Fifth, it should be an open city, which embraces new ideas with architectural form. And, finally, it needs to be a beautiful city, where art, architecture and landscape move the spirit.
Encouragingly, current trends are underpinning this approach. The industrial age is giving way, at least in the developed countries, to the post-industrial: telecommunications, cheap computer power, the information superhighway, clean robotic and microscopic manufacturing. All this is transforming our cities.
The raw material of this new economy is citizens and their knowledge, creativity and initiative. Art and science will be the lifeblood of these knowledge-based cities, and the key to further wealth. Networks of small- scale companies are emerging as the driving force of the future. Multimedia technologies and industries could end the division of the city into zones of housing, offices and factories. Indeed, the distinctions between office and home, work and play, education and entertainment are themselves set to dissolve. This will give the city a more diverse texture of overlapping activities, which will facilitate the emergence of a more dynamic, greener, more community-based city.
Cities such as Barcelona, Glasgow and Lyon are already striking out on their own, doing everything to establish themselves as centres of the newcommunications age. That is why it is so important that we invest in London and other British cities.
It is not just patterns of work that are being affected by technological developments. They are also revolutionising architecture. As homes, schools and workplaces all come to centre on a common communication network, buildings themselves will become less and less defined by a single function: the same fundamental structure will serve as a school, an office and a "factory".
In the future, buildings will become dematerialised. It will be an age not of solids but of transparency and veils - of indeterminate, adaptable and floating structures, which respond to the needs of users. The buildings of the future - already foreshadowed in Britain by the work of Will Alsop, Future Systems and Zaha Hadid, among others - will be less like the fixed Classical temples and more like moving, thinking robots.
As structures become lighter, buildings will become more permeable. There is no reason why pedestrians should not walk through, rather than around them. The street and the park may be part of the building or the building might hover above or below them.
We can finally expect a similar revolution in transport. The car is an outdated piece of polluting technology. If its cost reflected the social and ecological damage it caused, it would rapidly be replaced by a new generation of clean, quiet vehicles. The technology for this exists.
High-speed train links, allowing fast, comfortable movement from city centre to city centre, are also set to reinforce the importance of cities as hubs of communication. The newest generation of trains already travel at nearly 200mph, but magnet-levitation trains, which are now being tested, will double that speed. In the not-too-distant future, a vacuum-tunnel transport system allowing speeds of 1,000 mph will make trains compete with planes on long-haul journeys. We could soon travel underground from London to New York or Moscow.
But how do we implement change?
Present-day market economics is based on producing goods, including urban development, at minimum short- term cost. The result of this short-termism on health, global warming and general quality of life is catastrophic.
Take petrol. Today it is cheaper than mineral water, despite the pollution that its consumption causes. This is because goods in the free market are priced entirely on the basis of manufacturing cost and exclude all external damage. We are essentially consuming cheap petrol now at the expense of long-term environmental damage.
Environmental economists such as David Pearce argue that government can manage the market so that it avoids creating short-term efficiency at the cost of long-term ecological damage. This requires government to apply levies on activities that reflect their external costs. This would have the effect of steering researchtowards greener solutions.
Governments can encourage sustainable city development by applying levies to deter the market from constructions - such as out-of-town shopping centres and business parks - which undermine communities and generate pollution and congestion. These types of levies would encourage developers and architects to create sustainable developments, without governments having to be directly prescriptive and risk choking innovation.
It is important that governments make the purpose of "sustainability" levies transparent by earmarking the revenue they raise for specific sustainable projects. For example, the revenue from an increased tax on petrol could be used to improve or subsidise public transport.
We need to reform our taxation system to sponsor sustainable development. We need to discourage short-termism by offering tax incentives for long- term investment. Land taxes for retail, business and residential developments on out-of-town sites should reflect the real contribution of publicly financed infrastructure. This will make locating locally more competitive, and in the long run encourage developers and retailers to consolidate the sustainability of the city rather than making it sprawl.
Environmental policies will never be effective unless we can co-ordinate them internationally. In the first place, we need to establish a common international standard of measurement, enabling us to assess the performance of cities. Governments that have committed themselves to ecological targets must also suffer penalties if those targets are not, in fact, reached. So unless eco-taxes are levied and enforced across borders, then any polluting industries will simply relocate from one country to another.
As our relationship with the environment becomes ever more complex and precarious, so city planning becomes one of the most important disciplines.
Our planning legislation must be specifically revised to promote sustainability and architectural quality. One particularly invidious problem is that a planning permission, which is secured on the basis of a high-quality scheme, can be passed on to build a low-quality building in its place.
Taxation and regulation can be used to advance the cause of sustainable cities. But governments have direct control over enormous purchasing power, which, if used properly, could have a hugely beneficial effect on the environment.
Consider the following paradox. In 1992, the US government purchased approximately 135,000 passenger cars. At about the same time General Motors exhibited a prototype electric car, which it announced would not be put into production until annual sales of 100,000 could be guaranteed. The US still has no electric car on the production line. Governments, such large consumers, are in a position to directly influence the market. Encouraging innovation in these areas would produce a huge growth in technological development and employment.
For the past 25 years, France has used its public buildings budgets to improve the quality of its buildings. Competition and innovation are at the heart of this policy. Although we in Britain are perhaps aware only of the grands projets in France, there is one competition for each and every government building, be it a public housing project, a school, a post office, a local square or a new town. A local competition of any significance will be decided by a jury comprised of the mayor, representatives of the users and the local community, technical experts and architects. The major international competitions normally involve the president himself. This system is now being extended to encourage ecological design. It alsosucceeds in giving young talent real experience, and in attracting international architects to work in France.
This long-term policy has led to countless good contemporary buildings throughout the country. It has also attracted enormous international attention and boosted tourism.
Contrast this with Britain. Here taxpayers spend £4bn annually on their public buildings and yet the Government has no architectural policy. In 1992, Britain held 10 public design competitions to France's 2,000. Britons complain about their architecture and yet we have a generation of talented young architects who - almost without exception - have received no public work in this country.
The Government is failing in its responsibilities as a patron. We should be going out of our way to involve the citizen and to raise the quality of the architectural profession. Public buildings have a crucial role to play in our everyday lives, from teaching our children to embellishing our town centres. One good public building can revitalise a whole neighbourhood.
But a new opportunity is being presented by the Lottery and by the Millennium Fund. Much of this money will be spent on public buildings - museums, theatres and sports centres. The Government should step in to establish a competition structure for these and all other public commissions.
Education is fundamental to a far-reaching change in our perception of the built environment. Cities are a great tool for education, yet they are hardly present in the curriculum. Children should be introduced at school to the issues raised by the buildings and cities in which they live. Sustainability should be at the heart of many of the core subjects, a linking theme between biology, geography, history, art and technology.
We need to involve citizens in the problems of their environment. The Victorians built public libraries, we should build architecture centres.
The architecture centre is where the planning committee would meet in public. It would become the focus for public debate on strategic plans, planning applications and competitions. It would also hold lectures, exhibitions, courses and debates. And at the very heart of each centre should be adaptable working models of the city and the neighbourhood.
The policies that I have been advocating are easy to implement. Equitable cities that are beautiful, safe and exciting are quite within our grasp.
Our aim must be to search for a dynamic equilibrium between society, cities and nature. I believe that with vigilance and popular determination the concept of sustainability will grow in importance until it becomes the dominant philosophy of our age.