(introduction...)

The existence of plausible threats to biospheric stability, even survival (questions 2.3 and 2.4 above), is by no means obvious. The question about whether or not pollution constitutes a possible limiting factor for economic growth (question 2.3) is perhaps the one most debated at present. It is highly controversial. If there is any consensus on this issue it is merely that "toxification" - in the sense of "toxic wastes anywhere near my neighbourhood"- is unacceptable (i.e. must be prohibited regardless of cost). The next section discusses this further. But the extent to which pollution constitutes a limitation on growth itself, or on the welfare generated by economic activity, remains an open question.

The problem of climate warming has been extensively studied and debated (as mentioned above), though there are still significant areas of disagreement among experts with regard to economic damage and the appropriate response strategies. On the other hand, the issue of environmental acidification and/or toxification has never been considered seriously as a global threat to human survival. However, concerns are beginning to arise, especially in regard to cancer and human reproductive capacity. The link between various chemical agents and biological impacts none the less remains largely speculative, and is likely to remain so for many years. Damage mechanisms and thresholds are known in some cases, but not in others. However, it is fairly easy to construct a simple catalogue of measures of materials flux and consequent waste generation that self-evidently cannot continue to increase indefinitely.

The issue of whether or not there is a threat to biospheric stability itself (question 2.4) is rather deep. There are two aspects: the first has to do with phenomenology; the second has to do with the essential indeterminacy of the risk. Not only is there no consensus on either of these points, there has been almost no discussion up to now. 1 return to this question at greater length below.

As regards the third main question, concerning the least-cost (or "least-pain") transition to a sustainable trajectory, the problem of slowing population growth (question 3.1) has already been mentioned. The existence of plausible technological "fixes" (question 3.2) and the possible existence of large numbers of "win-win" opportunities or "free lunches" (question 3.3) are much more controversial. There is also no consensus as yet. These questions are discussed after the two digressions - on toxicity and on biospheric stability.