roadly interpreted, the precautionary principle requires that decisions by governments, institutions and individuals need to allow for and recognise conditions of uncertainty, particularly with respect to the possible environmental consequences of those decisions, and to act to prevent or avoid effects which may be damaging even if this cannot be proven. As a principle, it remains full of ambiguity: as with the polluter pays principle, the precautionary principle has been widely adopted not just because of its alliterative resonance, but because the emergent field of environmental policy needs principles:

• environmental policy is still in the early stages of its development, compared with some other policy areas;
• as environmental issues are no respecters of national boundaries, action needs to be a greed at an international level, and so different cultures and priorities have to be reconciled;
• the substantive policy area includes considerable scientific complexity and real uncertainties, such as climate change, requiring a much longer term commitment than most political horizons, and hence the need for agreed principles.

The adoption of broad principles as such therefore provides useful foundations for a new, long term, and possibly costly area of international and global action. The precautionary principle in particular has been stimulated by

• the need for collective action, to protect critical life-support processes such as the assimilative capacity of natural systems;
• the need to share the burden of environmental responsibility amongst nations and groups;
• the notion of global citizenship, extending a duty of care across both space and time to existing and future generations.

In these circumstances of global obligations requiring changes in national economies ahead of conventional scientific proof, and with uncertain, and possibly unpredictable, global environmental change, the precautionary principle offers some form of comforting authority. The principle has accordingly been adopted at both national and international level. The Rio Declaration from the UNCED Summit of 1992, for instance, proclaimed that `in order to protect the environment, the precautionary approach shall be widely applied by states according to their capabilities. Where there are threats of serious or irreversible damage, lack of full scientific certainty shall not be used as a reason for postponing cost-effective measures to prevent environmental degradation' (UNCED, 1993, Article 15). The European Union's Fifth Environmental Action Programme Towards Sustainability also explicitly states that one of `the guiding principles for policy decisions under this Programme' derives from the precautionary approach (CEC, 1992, chapter 2).

From its origins in the German concept of Vorsorge, adopted in the 1970s as an element of domestic policy, the principle has come to have a variety of meanings. It now has seven distinct elements:

• proaction: the readiness to take action in advance of scientific proof where inaction may be socially or environmentally costly;
• cost-effectiveness of action: to include in conventional CBA an examination of possible environmental costs and a presumption in favour of high environmental quality;
• safeguarding ecological space: leaving wide margins of tolerance in environmental capacities;
• awarding the environment intrinsic value: the grant of natural rights which may well challenge conventional views of the humans/nature relationship;
• shifting the onus of proof: imposing a duty of care on those who intend to develop the environment;
• futurity: a recognition that the future is uncertain, but that it needs to be given due weight;
• paying for ecological debt: an implication that past ecological exploitation should be compensated.

These elements potentially pose fundamental challenges to the conventional legal and political weight given to environmental concerns, and also to the extent to which we have come to rely on science to provide us with answers.