to avoid the specter of Malthusian scarcity through resource substitution and technical ingenuity.

Others, however, notably the ecologists Paul and Anne Ehrlich (1990) and the economist Herman Daly (see Daly and Cobb, 1989), believe that the scale of human pressure on natural systems already is well past a sustainable level. They point out that the world’s human population most likely will at least double before stabilizing, and that to achieve any semblance of a decent living standard for the majority of people, the current level of world economic activity must grow, perhaps fivefold to tenfold. They cannot conceive of already stressed ecological systems tolerating the intense flows of materials use and waste discharge that presumably would be required to accomplish this growth, no matter what level of investment in built capital and technology occurs. On the contrary, an implication of the Ehrlich/Daly argument is that even the present global population and economy are unsustainable.

KEY CONCEPTUAL ISSUES

As noted above, intergenerational fairness is a key component of sustainability. The standard approach to intergenerational trade-offs in economics involves assigning benefits and costs according to some representative set of individual preferences, and discounting costs and benefits accruing to future generations just as future receipts and burdens experienced by members of the current generation are discounted. The justifications for discounting over time are first, that people prefer current benefits over future benefits (and weigh current costs more heavily than future costs); and second, that receipts in the future are less valuable than current receipts from the standpoint of the current decision maker, because current receipts can be invested to increase capital and future income.

Critics of the standard approach take issue with both rationales for unfettered application of discounting in an intergenerational context. They maintain that invoking impatience to justify discounting entails the exercise of the current generation’s influence over future generations in ways that are ethically questionable. The capital growth argument for intergenerational discounting also is suspect, critics argue, because in many cases the environmental resources at issue—for example, the capacity of the atmosphere to safely absorb greenhouse gases or the extent of biological diversity—are seen to be inherently limited in supply. These criticisms do not imply that discounting should be abolished (especially since this could increase current exploitation of natural and environmental capital), but they do suggest that discounting might best be applied in tandem with safeguards on the integrity of key resources such as ecological life-support systems.

Critics also question whether the preferences of an “average” member of the current generation should be the sole or even primary guide to intergenerational resource trade-offs, particularly if some current resource uses threaten the future well-being of the entire human species. (Adherents of “deep ecology” even taken



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