We are now an urban species. Half the global population lives in cities and this three billion people will double to six billion in only one generation. This historic transition is pregnant with both peril and promise. Cities have emerged at the epicenter of both the planetary and global systems. In this dual context both cities and nation states are evolving in new, often divergent directions.

But this process remains poorly understood and riddled with confusion. This was evident at the United Nations World Urban Forum 5 in Rio de Janeiro, attended by nearly 14,000 people and delegations from 130 nations. For the first time there was a parallel Social Urban Forum organized by social movements and organizations in Brazil.

Overall, both the narrative of the challenge and the choices considered fell far short of a systematic analysis of and response to urbanization. For example, there was discussion of the conference theme, "The Right to the City," but not the Rights of the City required for the right to the city to become operational. The challenges facing cities are growing geometrically in relation to the arithmetic growth of the choices made by cities. While the proportion of the urban population living in slums has decreased from 39 percent to 32 percent in fifteen years, the absolute number of people living in slums grew dramatically, increasing by 58 million annually, or a country the size of Italy.

The challenge is all the more difficult to comprehend because cities are the most complex of all human creations. Few among us have any experience in the multi cultural contexts of the world's largest cities nor do we understand the complex hard and soft infrastructure required for everyday life.

The challenges of the city can be simplified as two inter related problems: equity and ecology. But these two problems are not equal. There will be no successful choices on the ecological front without equity as the road map. The only kind of sustainable environment is a just and equitable social system, one that actually insures the human needs of everyone. This requires that we break the intra and inter-generational transmission of poverty and inequality.

The first challenge is that the current global system inherently breeds inequality, injustice and marginalization. Economically it is privatization and private accumulation that undermines human needs and human rights. Politically it is the absence of consistent democracy, the power required by people to collectively transform themselves and their world.

Therefore access to services, greater participation, better governance, decentralization, transparency, inclusion (in a system designed to produce exclusion), even the movement for the "right to the city," produce improvements, but are not sufficient to redesign the condition of people, place and power. As long as human rights are viewed as individual rights to be safeguarded by the nation state, not enough will change.

The second challenge is that the planetary system, the ecology of the earth, is being systematically poisoned by the global system. Whether excessive production of carbon gases, species extinction, ecosystem collapse or resource depletion, there is abundant evidence that the existing social and economic system is incompatible with a sustainable planet. The ecological footprint of large and small cities must be brought within equitable limits.

In this new crossroads it is often cities that act and nations that talk. That is, most nation states have proven themselves unwilling or incapable of seriously addressing either the challenges of equity or ecology. The challenge is nothing less than redesigning a system of production, distribution and consumption that is both equitable and ecological. In the words of David Harvey, a "humanizing urbanism." A system in which human beings redesign themselves to redesign the social, economic, and cultural systems required to reduce and reverse both inequality and non-sustainable development.

A better world and a better city is not enough. The existing urban planning tools are not enough. The closing window of global warming requires that we find dramatic new responses to re-organize production, distribution and consumption. This will be a far more difficult transition than the industrial revolution. One way or another a change is coming. It is far more likely to be swift and unpredicted than slow and predictable. Whether we change to meet this evolutionary challenge depends on the choices we make in cities in the next few years.

Barry Weisberg is the author of Beyond Repair, The Ecology of Capitalism (1971) and the Global Cities Reporter for Worldview, Chicago Public Radio/National Public Radio, USA. He lives between Shanghai and Chicago. E-Mail: barryweisberg@att.net. Longer version at http://www.citieschallengeshoices.info.