This article written by Peter Hongler and Georgia-Cristiana Cozac examines the international legal duty to prevent significant harm to the environment and explores whether this obligation can extend to state tax policy decisions. The inquiry is conducted in four parts. First, the article studies the evolution of the no-harm principle from its foundation in international jurisprudence, to its elucidated content – which includes its application to risks affecting the climate system, its due diligence standard and its erga omnes character – as clarified by the International Court of Justice in its advisory opinion on obligations of states in respect to climate change rendered in mid-2025. Second, it analyses the conditions for establishing state responsibility in the event of a breach of this duty, focusing on issues of attribution, causation and respective legal consequences. Third, against this doctrinal background, the article assesses whether and how tax policy measures – with a particular focus on fossil fuel subsidies – may constitute conduct capable of triggering international state responsibility in contexts where they contribute to significant transboundary harm. Finally, the article addresses the enforcement of the duty through the lens of available and effective remedies. By situating taxation within the broader framework of international climate and environmental obligations, the authors argue that tax policy can no longer be viewed as normatively neutral in the climate change era. Fiscal measures can therefore play an important role both in compliance with and enforcement of states duties under international climate law.
