| Summary: | To achieve a sustainable or steady-state economy a society must be able to hold negative environmental impacts, whether defined as resource depletion or as pollution, to desired maxima. That is, it must control throughput defined as resource inputs plus low-value outputs. This can be done directly through capping input substances or approached indirectly through various sustainability paths, fruitfully classified into three groups: control of the number of people, control of the quantity of goods and services per person, and increasing the ratio of goods and services per unit of either resource consumption or pollution. This taxonomy coincides with the formula I = PAT. It is found that each indirect path suffers from rebounds: whatever inputs are left temporarily fallow after reducing population, and/or the affluence of some people, and/or the technological ratio of inputs to economic output, are merely consumed to support more people, raise the affluence of others, and/or increase the amount of economic output. In order however to decrease throughput to sustainable levels – solving the primary problem of human ecology, that of the scale of the human economy in relation to biophysical limits – we needn’t measure exact levels of rebound: directly imposing caps achieves this goal. Once overall caps are in place, population, affluence and efficiency will find their levels decentrally and democratically. Social problems to be addressed in a lower-throughput economy concern the just distribution of rights to consume or pollute common property, unemployment that can result from economic shrinkage, and in general just distribution of the pie when the pie is getting smaller. It is suggested that environmental research should focus on winning democratic majorities for sustainability and identifying the ethical, aesthetic and quality-of-life benefits of a smaller economy. Only worldwide political action – not that of individuals, firms or even countries – is likely to achieve sustainability.
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