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“I used to think of the trees as mine, to use as I saw fit, but now I see things differently,” said Marques, a father of five and grandfather of five. “The trees that grow here are mine, but lots of other people depend on them, too, so by saving even just one single tree, I’m performing a service for all of humanity.”

Since he joined the project three years ago, the dried-up stream has been resuscitated. At first it was a mere trickle, he said, but now it’s grown into a thick rope of water.

With real, measurable gains for 9 million consumers in Rio de Janeiro, for the forest and for the locals who call it their home, the Guandu Water Fund embodies the win-win situation for people and the environment that sustainable development aspires to be.

Such initiatives are gaining traction among policymakers as a way to slow the kind of wholesale environmental destruction that has been blamed for recent years’ rise in devastating droughts, floods and other natural disasters.

The notion of sustainable development was born well ahead of the current conference’s precursor, the U.N.’s 1992 Earth Summit, which helped put climate change on the world agenda. Still, it remains an amorphous, and divisive, concept.

“Definitions of just what is sustainable development vary, society by society,” said Jeffrey Sachs, the economist and who heads Columbia University’s Earth Institute. “But while there are big debates about the specifics and how to balance … the economy, the environmental and the social concern, I think that the basic idea that we have three bottom lines, not one, is the most important idea.”

Still, it has been hard to agree on how to implement it.

Weeks of bickering between rich and poor countries delayed agreement on the final summit conclusions and the result has disappointed environmental groups, who have lambasted it as toothless and inadequate.

“What most people at the summit are talking about when they talk about sustainable development is nothing but business as usual under a different name, something that will deliver misery to many and profit to a few,” said Daniel Mittler, a political director at Greenpeace who is heading the environmental group’s delegation at Rio+20.

“But it doesn’t have to be: Sustainability is an agenda that can deliver for people and the planet at the same time,” Mittler said, adding that political will and direction are needed to make it work on a global scale. “The tragedy of Rio+20 is that governments are failing to grasp that opportunity.”

While decision makers squabble over language, farmer Marques in Rio Claro says he’s sold on sustainable development.

“I need money to live, but I also need clean air and clean water,” he said. “This project gives me all three at the same time.”