Understandably, environmentalists generally reject the concept of weak sustainability and argue that the environment should not be degraded for future generations. Even those who view the environment as merely an economic asset may argue that each generation should inherit at least a similar natural environment. David Pearce and his colleagues (1989, chapter 2) give the following reasons:

Non-substitutability
There are many types of environmental assets for which there are no substitutes: for example, the ozone layer, the climate-regulating functions of ocean phytoplankton, the watershed protection functions of tropical forests, the pollution-cleaning and nutrient-trap functions of wetlands. For those people who believe that animals and plants have an intrinsic value, there can be no substitute.

Uncertainty
We cannot be certain whether or not we will be able to substitute for other environmental assets in the future. Scientists do not know enough about the functions of natural ecosystems and the possible consequences of depleting and degrading natural capital. And 'if we do not know an outcome it is hardly consistent with rational behaviour to act as if the outcome will be a good one'.

Irreversibility
The depletion of natural capital can lead to irreversible losses such as species and habitats, which once lost cannot be recreated through man-made capital. Other losses are not irreversible but repair may take centuries; for example, the ozone layer and soil degradation.

Equity
Poor people are often affected by unhealthy environments more than wealthier people. A substitution of wealth for natural resources does not mean that those who suffer are the same people as those who will benefit from the additional wealth. This will be discussed further on in this part.

Resilience
Human-made capital often lacks an important feature of natural capital; diversity. Diverse ecological and economic systems are more resilient to shocks and stress.

Additionally, the relative value of natural capital to human-created capital can change over time; this is especially true in Australia, where the demand for environmental resources is likely to increase as they become more scarce, as incomes increase and people have more leisure time, and as the tourism and recreation industries are developed.

A United States professor of international and environmental law, Edith Brown Weiss (1990, p. 8), argues that not only can resource consumption increase the real prices of those resources for future generations, but that resources may be depleted before they are identified as useful or before their best use is discovered. She gives the example of helium-bearing natural gas. Developing substitutes may well be more expensive than conserving existing supplies. When resources are depleted and species extinct, the options available to future generations are narrowed.

Weiss points out that 'conservation of options' is a principal criterion for intergenerational equity. Current generations should not try to second-guess what future generations will need, but rather should let future generations choose their own goals by allowing them the flexibility through keeping options open and maintaining diversity.

Moreover, there is an equity issue involved in replacing natural resources and environmental assets that are currently freely available to everyon with human-made resources that have to be bought and may only be accessible to some people in the future. Weiss points out that the principle of 'conservation of access' implies that not only should current generations ensure equitable access to that which they have inherited from previous generations, but they should also ensure that future generations can also enjoy this access.

Vili Fuavao, representing the South Pacific Regional Environmental Programme, argues:

Good living and foreign life-style are perhaps one's right when one can afford them. But it would be selfish and therefore irresponsible if such luxuries were to be attained at the expense of our future generations who have as much right as we have to a clean environment and an adequate resource base to support their own needs. (1992, p. 14)

Weiss argues that our responsibilities to future generations should be institutionalised. She suggests a number of measures to this end, including representation of future generations in decision-making (ombudsmen), intergenerational conservation assessments, sustainable use of renewable resources, intergenerational trust funds, and education to foster a new planetary ethos.

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