Twenty years ago, there was the Earth Summit. Gathering in Rio de Janiero, world leaders agreed on an ambitious blueprint for a more secure future. They sought to balance the imperatives of robust economic growth and the needs of a growing population against the ecological necessity to conserve our planet's most precious resources - land, air and water. And they agreed that the only way to do this was to break with the old economic model and invent a new one. They called it sustainable development.

Two decades later, we are back to the future. The challenges facing humanity today are much the same as then, only larger. Slowly, we have come to realize that we have entered a new era. Some even call it a new geological epoch, where human activity is fundamentally altering the Earth's dynamics.

Global economic growth per capita has combined with a world population (passing 7 billion last year) to put unprecedented stress on fragile ecosystems. We recognize that we can not continue to burn and consume our way to prosperity. Yet we have not embraced the obvious solution - the only possible solution, now as it was 20 years ago: sustainable development.

Fortunately, we have a second chance to act. In less than a month, world leaders will gather again in Rio - this time for the U.N. Conference on Sustainable Development, or Rio+20. And once again, Rio offers a generational opportunity to hit the reset button: to set a new course toward a future that balances the economic, social and environmental dimensions of prosperity and human well-being.

More than 130 heads of state and government will be there, joined by an estimated 50,000 business leaders, mayors, activists and investors - a global coalition for change. But success is not guaranteed. To secure our world for future generations - and these are indeed the stakes - we need the partnership and full engagement of global leaders, from rich nations and poor, small countries and large. Their overarching challenge: to galvanize global support for a transformative agenda for change - to set in motion a conceptual revolution in how we think about creating dynamic yet sustainable growth for the 21st century and beyond.

This agenda is for national leaders to decide, in line with the aspirations of their people. If I were to offer advice as U.N. secretary general, it would be to focus on three "clusters" of outcomes that will mark Rio+20 as the watershed that it should be.

First, Rio+20 should inspire new thinking - and action. Clearly, the old economic model is breaking down. In too many places, growth has stalled. Jobs are lagging. Gaps are growing between rich and poor, and we see alarming scarcities of food, fuel and the natural resources on which civilization depends.

At Rio, negotiators will seek to build on the success of the Millennium Development Goals, which have helped lift millions out of poverty. A new emphasis on sustainability can offer what economists call a "triple bottom line" - job-rich economic growth coupled with environmental protection and social inclusion.

Second, Rio+20 should be about people - a people's summit that offers concrete hope for real improvements in daily lives. Options before the negotiators include declaring a "zero hunger" future - zero stunting of children for lack of adequate nutrition, zero waste of food and agricultural inputs in societies where people do not get enough to eat.

Rio+20 should also give voice to those we hear from least often: women and young people. Women hold up half the sky; they deserve equal standing in society. We should empower them, as engines of economic dynamism and social development. And young people - the very face of our future: are we creating opportunities for them, nearly 80 million of whom will be entering the workforce every year?

Third, Rio+20 should issue a clarion call to action: waste not. Mother Earth has been kind to us. Let humanity reciprocate by respecting her natural boundaries. At Rio, governments should call for smarter use of resources. Our oceans must be protected. So must our water, air and forests. Our cities must be made more liveable - places we inhabit in greater harmony with nature.

At Rio+20, I will call on governments, business and other coalitions to advance on my own Sustainable Energy for All initiative. The goal: universal access to sustainable energy, a doubling of energy efficiency and a doubling of the use of renewable sources of energy by 2030.

Because so many of today's challenges are global, they demand a global response - collective power exercised in powerful partnership. Now is not the moment for narrow squabbling. This is a moment for world leaders and their people to unite in common purpose around a shared vision of our common future - the future we want.