Over the past two centuries, remarkable gains in living standards encouraged some of us to believe that natural limits to human well-being had been conquered. But now the sheer number of human beings, the natural desire of all of them to share the prosperity so far enjoyed only by a few, and the unprecedented rate at which we are using energy and other resources, have taken us into uncharted territory.

We should no longer imagine either that a fifth of humanity can indefinitely enjoy prosperity while much larger numbers live lives of deprivation and squalor, or that patterns of production and consumption that destroy the environment can bring us lasting prosperity.

The issue is not environment versus development, or ecology versus economy. It is wrong, for example, to depict all economic growth and development as leading inexorably to the apocalypse. But it is also wrong to downplay the real ecological problems we face, or to assure ourselves that some spontaneous technological breakthrough will come to our rescue. Rather, the issue is how to integrate environment with development.

We thought we had found a way out of this predicament 10 years ago, with the agreements reached at the Earth Summit in Rio de Janeiro. But progress since then has been slower than we had hoped. Developed countries, in particular, have not lived up to the promises they made either to protect the environment or to help the developing world. Discussions on finance and the economy, from the local to the global, still treat the environment like an unwelcome guest.

Now we have another chance to get this right: the World Summit on Sustainable Development, to be held in Johannesburg in three months' time. Of course, one summit meeting by itself will not change history. But I believe this summit will be seen to have marked a turning-point, if we win clear commitments to change, and new initiatives to make it happen, in five specific areas.

The first of these is water. In order to save the more than 3m people who die each year from water-related diseases, we must improve water and sanitation services, and access to them, by finding new money for water development and management. And in order to save two-thirds of the world's population from facing serious water shortages in the decades ahead, we must reduce leakage and waste, particularly in agriculture ("more crop per drop"), and provide for regional management of watersheds vital to more than one country.

The second area is energy. In order to give poor people a chance to escape from poverty, we must provide clean energy for the 2bn people who now lack it. And in order to make sure this advance is not accompanied by disastrous climate change, we must improve energy efficiency, use more renewable energy, implement the Kyoto protocol, put an end to perverse subsidies and tax incentives and fund research on new types of clean energy and carbon sequestration.

Third, we must focus on health. In order to save the lives of millions who die each year from an unsafe environment - dirty water, indoor air pollution, toxic wastes, insects that transmit deadly diseases - we must redouble our efforts to create a safe environment, make immunisation and treatment available to all and increase our research on the tropical diseases that impose huge human and economic burdens on the world's poorest countries.

The fourth priority should be agriculture. In order to ensure that food production keeps pace with the number of mouths to feed, we must find ways to halt land degradation and reverse the sharp decline in agricultural productivity, particularly in Africa.

That means planning and managing land use more responsibly, implementing the United Nations Convention to Combat Desertification and funding research on new drought-resistant crops.

The fifth area of focus is biodiversity. To halt the galloping extinction of other species, which has devastating implications for human life, we must clamp down on illegal and unsustainable fishing and logging practices; we must help people who currently depend on such activities to find other, more sustainable ways of earning their living; and we must fund new research into ecosystems and biodiversity.

In all these areas there are things we can do now, with the technologies already at our disposal, provided we give the right incentives. But science will bring us many more solutions if we make the right investment in research. Knowledge has always been the key to human development. It will also be the key to sustainability.

This agenda will sound impossibly ambitious to some, disappointingly narrow to others. But I believe it represents the essential, achievable start we must make if we are to preserve the hope of a decent life for our children and grandchildren. And that is what Johannesburg is all about.

The writer is secretary-general of the United Nations