The moral is clear. GNP does not measure environmental degradation, nor is it necessarily correlated directly with it. So it cannot be used as an environmental indicator. It might be possible to adjust GNP for environmental impact. Publishing a figure for "Environmentally Adjusted National Income" at the same time as GNP would certainly promote debate about the nature of economic welfare and the objectives of government policy. But it is notoriously difficult to make the adjustments. Neither the items to subtract nor their value are easy to identify.

Since no rate of economic growth, whether positive, zero or negative, can tell us what is happening to the environment, none is a useful target for environmental policy.

This does not necessarily mean that environmentalists are wrong when they argue that growth should be reduced, if they are using the term to mean growth of resource consumption, measured in physical, not monetary, terms. While GNP growth may not be a useful indicator, the consumption of physical resources is surely important. In a finite world with growing population, there are clearly limits to the quantities of resources which each person can consume, both because of scarcity and, probably more importantly, because of the inability of the biosphere to absorb the consequent wastes.

But what does zero growth in this sense actually mean? Must consumption of all resources remain at current levels? This would be absurd. The environmental impact of consuming different resources varies enormously. While certain types of resource consumption have probably already reached their limits, and may actually need to be contracted, others clearly have not and expansion remains possible.

It might be argued that it is not the consumption of specific resources which must be maintained at current levels, but overall consumption. But this is precisely the problem. What does "overall" mean ? In what units are different resources measured, such that an increase in the consumption of one can be weighed against a fall in another ? Consumption clearly cannot be limited to a certain total weight or volume; this would be no guarantee of environmental impact. Nor can resources be valued in money terms for this purpose, since prices do not reflect environmental damage either. But without a common way of measuring the consumption of different resources, the concept of "overall zero growth" is meaningless.

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