The certain activities case: what implications for the no-harm rule?

The customary law duty to prevent significant transboundary harm and harm to the global commons (‘no-harm’ rule) has developed considerably since it was first enunciated in the 1938/1941 Trail Smelter arbitration. This article reflects on this development and analyses what implications the 2015 Certain Activities case has for existing understandings of the no-harm rule. The International Court of Justice (ICJ)'s judgment provides greater clarity concerning procedural obligations flowing from the no-harm rule by establishing a positive obligation to ascertain risk and a sequence in which procedural obligations arise. However, it raises questions concerning the nature of the substantive obligation under the no-harm rule. Specifically, whether breach of the substantive obligation is subject to establishing that an activity has resulted in significant transboundary harm. The ambiguity in the Certain Activities case highlights the need to further clarify and develop the content of the no-harm rule to better enable it to contribute to the governance of contemporary transboundary and global environmental problems.