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THE IDEA of sustainability is simple: an arrangement is sustainable if it can go on indefinitely; it is unsustainable if it cannot. Living on the interest from an inheritance is sustainable; drawing down the principal is not. An unsustainable ecology is one that, for mathematical or biophysical reasons, undermines the conditions necessary for its own continuance, while a sustainable ecology is one that is self-supporting. Thus sustainability is not a moral evaluation (although it has large moral implications); it has to do with whether or not a certain thing can work in a certain way.
We are in the crucial phase of an ecological experiment that is unprecedented in the history of earthly life. It is to determine whether the planet's ecosystem can support, for more than a few years, nearly seven billion talking primates together with their energy-intensive infrastructure. The evidence is not encouraging: resources such as topsoil, forests, fisheries, freshwater and petroleum face imminent degradation or depletion, while accelerating climate change (caused primarily by our profligate use of our hydrocarbon heritage) threatens to become catastrophic. It is not surprising that knowledgeable observers have expressed despair about the future of the human species.
Despite the acute danger, there is room for guarded...