Due Diligence, Obligations to Cooperate and to Regulate Private Actors: Insights from Three Climate Change Advisory Opinions

This article analyses three noteworthy aspects of the 2024–2025 Advisory Opinions (AOs) on climate change from the International Court of Justice (ICJ), the International Tribunal for the Law of the Sea (ITLOS), and the Inter-American Court of Human Rights (IACtHR). Primarily, the article considers what the three AOs suggest about the variable, fact-dependent content of the various obligations to exercise due diligence falling upon States. It is argued that while the content of due diligence obligations varies depending upon various context-specific factors, including the capabilities of the State concerned, this does not lead to unmanageable indeterminacy nor provide excuses for climate inaction based on national circumstances. Rather, these international courts and tribunals (ICTs) have provided significant guidance on objectively reasonable approaches to implementing the relevant obligations and have emphasised that applying due diligence standards is a matter for objective determination. Secondly, the article considers what the AOs suggest about the continuing nature of States’ obligations to cooperate, which are not exhausted by specific climate-related treaties or processes. Third, the article demonstrates that all three AOs recognise and elaborate upon a duty of States to regulate effectively the activities of private, non-State actors that cause climate change.