In the absence of international statutes prohibiting nations from causing each other environmental damage, scholars of international environmental law typically focus on trying to identify and clarify norms of "customary international law": that body of commonly accepted—but not formalized—legal principles that is manifest in the behavior of nations toward one another. Two environmental principles are often held to be established norms of customary international law. One of these principles proscribes transboundary harm: nations should prohibit activities within their borders that cause significant environmental harm beyond those borders. The other is the closely allied precautionary principle, which requires nations to exercise due care to avoid putting other nations at significant risk of environmental harm.

In debating whether a given principle should be classified as a norm of customary international law for the purposes of deciding international cases, scholars of international environmental law generally accept an established criterion: principles are norms only if nations customarily abide by the principles in actual practice rather than merely affirming them as desirable standards. In fact, however, many purported "norms" of customary international environmental law, including the duty to prevent transboundary harm and the precautionary principle, do not reflect the actual behavior of many nations. Although systematic empirical studies are lacking, experience shows that harmful pollutants constantly cross most international borders, and that nations have only rarely attempted to remedy this situation.

Even though nations only rarely abide by these environmental "norms," they nevertheless routinely profess to accept them. Similarly, while scholars discussing customary international law claim to focus on what nations do, their debates are almost invariably based on what nations profess. In reality, international environmental "norms" primarily reflect the evaluative standards that nations use to justify their own actions and criticize those of other nations. Hence these environmental "norms" are not properly classifiable as norms of customary international law. They might more accurately be characterized as an ideological system, since they merely represent some collective ideals of the international community.

In light of this fact, those scholars who seek in customary international law a firm grounding for decisions in international environmental cases are misdirecting their efforts. This is especially true given that international treaties and direct negotiations, rather than international court decisions, are now the principal means of resolving international environmental disputes. It would be more productive for scholars to study such disputes from perspectives more consonant with this trend. Specifically, attention should be directed toward how the affirmation of core environmental principles can contribute to negotiations and treaty formation. This approach would promote progress toward agreements that could effectively hold nations to appropriate standards of environmental conduct.