Next week, world leaders gather for a momentous occasion — the United Nations Conference on Sustainable Development in Rio de Janeiro. Will it be a success? In my opinion, yes. To be sure, the negotiations have been lengthy. Even now there is more disagreement than agreement on the details of the so-called “outcome document” that will emerge. Yet that will not be the defining measure. Far more important is what the Rio conference has already accomplished. And that is to build a global movement for change.