This article examines the role of customary international law in regulating SRM by analysing competing interpretations of the customary law principle of prevention and their implications for SRM governance. Existing customary law limitations are largely overlooked in current policy and expert discussions around the future of SRM, which seemingly proceed on the basis that there are no universally applicable limitations for states to develop SRM technologies should they decide so. The paper contrasts this view and argues, from a positivist point of view, that the customary principle of prevention does pose certain limits for states even before SRM-caused transboundary environmental harm occurs. It distinguishes between a retrospective and future-oriented dimension of the prevention principle, and it depicts three scenarios for how the prevention obligation may limit the development and deployment of SRM technology depending on how States (and international legal advisors) conceptualise the temporal scope and normative content of the prevention principle. The article also examines the implications of the different configurations of the customary law obligation for an eventual SRM treaty.

States’ current emission policies are far from being aligned with what is needed to meet the Paris Agreement’s temperature goal. Against this backdrop, an increasing number of lawsuits have been filed around the world. As of March 2024, most courts have exercised restraint in imposing substantive limits on the legislator’s discretion in determining emission levels. Judicial restraint commonly rests on two premises: Climate models yield wide uncertainty ranges and choosing emission reduction levels is a normative decision belonging to the political domain. By engaging best available science on climate tipping points, this article examines the reasoning in favour of political discretion through a due diligence and equity lens. The analysis concludes that all factual requirements are met for states to be under an obligation to align their mitigation policies with a global carbon budget which is expected to limit global warming to 1.5°C at a likelihood as high as state capacities allow for.