Environmental decisions frequently distribute benefits to the present while transferring ecological risks to persons who cannot participate in today’s political process. The problem becomes acute when the threatened loss is irreversible: extinction, aquifer collapse, permanent contamination, or climate-system change cannot simply be compensated by a later legislature. Contemporary international law increasingly responds through prevention, precaution, long-term assessment, and attention to future generations. Islamic environmental scholarship, meanwhile, has developed rich accounts of stewardship, balance, harm, public interest, and the objectives of the Sharīʿa. Yet a public-law question remains underdeveloped: when do these normative materials generate a legal duty of the state, rather than an ethical appeal to individuals, and what does that duty require before irreversible harm occurs? Using doctrinal analysis of Islamic legal sources, conceptual analysis of intergenerational justice, and functional comparison with recent climate and environmental jurisprudence, this article argues for an irreversibility-sensitive duty of prevention in Islamic public law. The duty is triggered by a credible risk of serious, non-substitutable ecological loss within public authority’s sphere of control and requires science-based assessment, explicit consideration of future interests, precaution under uncertainty, preference for reasonably available reversible alternatives, and reasoned periodic review. Irreversibility does not create an absolute veto on development. It changes the burden and quality of public justification because the state may otherwise foreclose ecological options that later generations cannot recover.