tion is recognized and encouraged through the "polluter pays" principle. The United States and other countries in the Organization for Economic Cooperation and Development (OECD) already accept this principle, in theory at least, although implementation through. law and regulation at national and international levels is sporadic, weak, and generally inefficient.2 Thus, environmental improvements in industrial ecosystems could be facilitated globally through wider acceptance, and improved implementation, of the polluter-pays principle at the international level.
One method for internalizing environmental costs that is now being considered in the context of efforts to halt global warming is to sell limited numbers of marketable pollution rights, or permits, on open markets. This method is particularly appealing internationally with regard to resources generally thought of as part of the global commons (e.g., the oceans and the atmosphere). While pollution rights raise interesting questions of environmental ethics and equity, such rights do force companies to include at least some artificial measure of their environmental costs as real costs, and thus provide an incentive for companies to reduce them.
An alternative method of encouraging international internalization of environmental costs is to allow national governments to place countervailing duties on imported products equal to the production subsidy the goods receive in the country of origin as a result of less stringent environmental regulations.3 There are, however, a number of practical difficulties with implementing such a system.
The precautionary principle requires that if a particular action can be shown to pose a threshold risk of harm, then the proponents of the action must prevent or terminate that activity unless they can prove by a preponderance of the evidence that the activity will not degrade the environment.4 The precautionary principle is particularly effective at guiding decision making where little or no evidence exists as to the potential environmental risks of an action.
The precautionary principle is closely related to the principle of pollution prevention; each seeks to avoid environmental harms before they occur. The precautionary principle is also closely related to the principle of sustainable development; each maintains that humankind will confine its actions to those activities that do not cause irreparable damage to the environment. Thus, the precautionary principle provides important common ground between international environmental law and efforts to prevent environmental damage and is a natural starting point for the wider incorporation of such preventive measures into international environmental law.
It is therefore encouraging that the precautionary principle is increasingly being developed as a central tenet of international environmental law, as in Principle 15 of the Rio Declaration, recently adopted at the United Nations Conference