Due diligence and climate change: Insights from the International Court of Justice's advisory opinion

Abstract Due diligence plays a central role in the International Court of Justice's advisory opinion on States' obligations in respect of climate change, which places particular emphasis on this substantive standard of conduct. The authors examine how the Court articulates a generally ‘stringent’ due diligence standard applicable in the context of climate change. While the Court did not provide much guidance on what is implied by characterising the due diligence standard as ‘stringent’, this article proposes that stringency requires a greater depth and breadth of policies, as well as balancing competing policy interests in favour of preventing climate change. In particular, the article addresses two issues relevant to an in concreto determination of the due diligence standard that remain insufficiently clarified following the advisory opinion. The first concerns the temporal application of the customary law obligation to exercise due diligence in preventing significant transboundary harm in the context of climate change. The second concerns the methodology for differentiating the applicable standard of due diligence, following the Court's reliance on the principle of common but differentiated responsibilities and respective capabilities as a factor in determining a varying standard of due diligence. With respect to the first issue, the article argues that, since knowledge of the risks and the causal link is a prerequisite for due diligence, the customary due diligence obligations to prevent significant harm to the environment became applicable to climate change in the late 1980s or 1990, coinciding with the emergence of sufficiently reliable scientific evidence. With respect to the second issue, the authors argue that, although the Court refers to historical responsibility, the level of diligence required of a State should be determined primarily by its present‐day capacity, and current and future emissions, as the duty to prevent significant environmental harm is a forward‐looking obligation.