THE idea of sustainable development has been around for nearly two decades, but its vagueness has kept it from being a useable guide for policy.  It received its most prominent international acknowledgment at the recent U.N. Summit on Sustainable Development in Johannesburg, South Africa. Although some found the September 2002 summit to be one in a long line of U.N.  environmental conferences thick with sentiment and unspecific goals, the dynamic at Johannesburg was in fact markedly different, and may represent a turning point in the global environmental debate. The outcome of Johannesburg suggests that the sustainable development project may be moving away from its Malthusian roots toward a broader, more productive view of the relationship between economic development and environmental quality.