On 12 May 2026, the United Nations Environment Programme (UNEP) released a working paper presented as a synthesis of the current state of scientific knowledge on solar radiation modification (SRM). Its headline finding is unequivocal: SRM is not “a viable climate response” (key finding 1). The paper is clear and candid about the severe limitations of SRM modelling, the idealised assumptions on which projected benefits rest, the concentration of research and funding in a handful of high-income countries, and the profound governance and justice problems that any SRM deployment would raise.
We welcome this candour. These positions also align very well with those taken by more than 600 academics advocating for an international non-use agreement on solar geoengineering.
Yet the paper’s conclusions do not consistently follow from its own evidence. Having documented why SRM cannot presently be considered viable, it nonetheless leaves the door open to future use and frames expanded research — including outdoor experimentation — as the appropriate response to the very risks it identifies. Four features of the paper warrant a response. Each of these is as much a question of international law, as of science or policy.
1. “More research” as the answer to problems research cannot solve
The paper’s central move is a non sequitur. It identifies risks that are, on its own account, profound and in part irreducible — termination shock, geopolitical instability, uneven regional impacts, and committed multi-generational dependency (pp. 8, 9, 11) — and then prescribes more modelling and field research as the response. But it concedes in the same breath that idealised models “assume globally coordinated governance, stable deployment over time and well-controlled interventions” (p. 4), and that field experiments are “inherently limited in scope” and cannot resolve uncertainties in global-scale climate responses (p. 8). Better models and more experiments cannot dissolve what are not, at root, knowledge gaps. The decisive risks of SRM are social, political and intergenerational: they concern who would control a planetary thermostat, on whose behalf, and with what recourse for those harmed. Finer parameterisation of aerosol microphysics does not touch on these political questions.
This is also where the paper’s treatment of equity falls short. It rightly notes the concentration of research in the Global North and calls for “reciprocal capacity sharing, regionally-led research and full recognition of diverse knowledge systems” (key finding 4). But capacity sharing in research is not the same as decision-making power over whether the technology is developed at all, and the working paper does not confront that distinction. The deeper point, which the non-use literature has also emphasized, is that SRM at planetary scale is not governable in a globally inclusive and just manner within the existing international system. More research cannot make an ungovernable technology governable. It risks instead entrenching the research momentum and funding architectures that — as the paper itself warns — narrow future choices and legitimise deployment pathways (p. 17). This is the risk of lock-in that the paper flags but does not allow to discipline its own recommendations.
Furthermore, greater care must be taken when applying the ‘research’ label. It is imperative to safeguard the right to freedom of scientific research and to protect the integrity of institutions dedicated to research and knowledge production against subversion. While legitimate research warrants protection, speculative and risky endeavours should not be legitimised under the guise of ‘research’. Ultimately, such an approach amounts to a strategy of ‘learning by doing’ on a pathway to potential deployment, constituting ‘post-caution’ rather than pre-caution.
2. The precautionary principle is a constraint, not a licence
The paper’s most consequential error is legal. Instead of treating precaution as a constraint, it asserts that there is “a growing consensus that precaution does not mean inaction, but rather demands careful, inclusive and globally coordinated oversight” (p. 16) and that “precaution does not mean paralysis” (p. 18). This inverts the principle. Precaution operates as a constraint on activities that may cause serious or irreversible harm under conditions of scientific uncertainty; it is not a warrant to generate that harm in order to study it.
The confusion has a recognisable structure. The precautionary principle does not itself need to be “balanced” against competing considerations; instead, it structures and disciplines the balancing exercise. Applying it involves, at a minimum, two steps: a scientific assessment of impacts, probabilities and uncertainties, followed by a decision-making phase in which the costs and benefits of action and inaction are weighed against the objective of reducing risk to an acceptable level. Where serious and unacceptable risks remain after that process, the principle justifies prohibition even absent full scientific proof. What it does not authorise is the pitting of one speculative risk against another — the “risk–risk” framing now common in the SRM debate — so as to license experimentation on the grounds that unmitigated warming is itself dangerous. The dangers of warming are real; and are an argument for the mitigation the working paper rightly prioritises, rather than the justification for a second planetary-scale intervention whose fundamental risks are known to be and/or likely to be grave and unevenly distributed.
This reading is now firmly anchored in international jurisprudence. In its 2024 Advisory Opinion (para 257), the International Tribunal for the Law of the Sea held that the due-diligence obligation to protect the marine environment is closely linked to the precautionary approach, such that States breach it if they disregard or fail adequately to account for risks of serious harm. The International Court of Justice, in its Advisory Opinion of 23 July 2025, confirmed the precautionary approach as a guiding principle of the applicable law and located the duty to prevent significant environmental harm in customary international law binding on all States (paras 224, 293–294 and 315). The Inter-American Court of Human Rights, in Advisory Opinion OC-32/25, went furthest: it grounded States’ heightened due-diligence obligations in the precautionary principle and expressly applied a reversal of the burden of proof in environmental matters. That last point is decisive for SRM. Under a precautionary standard with a reversed burden, it falls to those who would develop and deploy the technology to demonstrate that it is safe and governable — not to the exposed to prove, in advance and against idealised models, that it is not. Framing precaution as a permission slip for outdoor experimentation is unsupportable in light of this jurisprudence.
We therefore reject the self-serving framing of precaution that runs through parts of the paper. Insistence on more research and field experimentation is not precautionary. Instead, if applied to a planetary-scale, partly irreversible and ungovernable intervention, the precautionary principle points in one direction: non-use.
3. Non-use is the outcome of expanded deliberation, not its suppression
A recurring rhetorical move in the paper is to distinguish those “calling for strengthened governance” from those who “wish to silence discourse” (p. 16), and to characterise advocates of restrictive governance as implicitly silencing debate and discourse (p. 16). This is not a neutral empirical observation; it mischaracterises and delegitimises a considered legal and political position calling for restricting a dangerous activity. Academics advocating for non-use of SRM, for example, have not silenced discourse, to the contrary, they have enlivened it.
Opposition to SRM does not seek to foreclose inquiry. Instead, it actively engages in and broadens discourse and debates on SRM. While expanding the space for debate and discourse, it calls for restricting risky and unjust practices and systems. This non-use position builds on two decades of research and deliberation across the natural and social sciences, law and global governance — recently including the peer-reviewed case for a non-use agreement, the comparative legal analysis of how such a regime could be designed, and the assessment of its political feasibility. This position is also reflected in the decisions of States: the African Ministerial Conference on the Environment has called for a non-use mechanism and reaffirmed its unequivocal rejection of stratospheric aerosol injection and other forms of solar geoengineering; Mexico announced a ban on all SRM-related experiments, and Pacific Island States prevented a permissive SRM resolution at UNEA-6 and secured reaffirmation of the de facto moratorium on geoengineering under the Convention on Biological Diversity.
To recast this body of considered judgement as censorship is both factually wrong and rhetorically corrosive. It is also in tension with the working paper’s own, more careful acknowledgement elsewhere that effective governance must rest on legal, ethical and scientific grounds (p. 16) —standards that restrictive-governance positions meet. A non-use agreement also does not call for bans on atmospheric or climate science; instead it targets the specific development pathway — funding, patents, outdoor experiments, deployment and institutionalisation — that turns research into a deployed technology. These distinctions, which the working paper’s own evidence supports, are precisely what its “silencing discourse” framing of opposition to SRM obscures.
4. Participation is not consent, and consultation is not governance
The paper repeatedly invokes “inclusive governance”, “inclusive participation” and “inclusive deliberation” as the answer to SRM’s equity problems (e.g. pp. 5, 8, 9, 11, 13, 15–18). Inclusion matters. But the paper avoids acknowledging the significant difference between participation and the power to decide — and this avoidance carries a precise legal cost.
It matters in law — and the normative and ethical considerations underpinning it — that the communities most exposed to outdoor experimentation and to any future SRM deployment are overwhelmingly those least represented in decision-making arenas. The principle of free, prior and informed consent — codified in the UN Declaration on the Rights of Indigenous Peoples and affirmed by the Inter-American Court in the climate context — cannot be approximated by “transparent, internationally supervised research” (p. 14). Consultation that informs but does not bind is not consent; and consent over a discrete project is not collective governance of a planetary intervention.
The UN Human Rights Council Advisory Committee has already concluded that new technologies intended for climate protection can interfere with the enjoyment of human rights and pose physical, political and social risks to frontline communities, including Indigenous Peoples (paras 57 and 66). Against that background, a regime that offers the exposed a seat at a consultative table while reserving decision-making to a few capable States and their funders is not “inclusive governance”; it is incompatible with States’ positive obligations under international human rights law. The Inter-American Court has now recognised a peremptory (jus cogens) prohibition on massive and irreversible environmental harm — a standard that a technology designed to alter the entire climate system, with acknowledged potential for irreversible and inequitably distributed effects, cannot credibly satisfy.
Operationalising UNEP’s own conclusion
The working paper’s own finding is that SRM is not a viable climate response. The task now is to give that finding effect. Calling for broad consultation and improved modelling is not a substitute for taking a clear decision or stance; and the decision on which the evidence, the precautionary principle and the human rights framework converge is restriction, not facilitation. What is needed is not a global research programme but a global agreement restricting the development of solar geoengineering technologies.
Such an agreement is neither unprecedented nor utopian. International law already offers a rich repertoire of prohibitory and restrictive regimes from which States could draw — among them the Environmental Modification Convention, the Anti-Personnel Land Mine Ban Convention, the Montreal Protocol’s control of ozone-depleting substances, the Basel Convention’s restrictions on hazardous-waste movements, the London Protocol’s reverse-list approach and its moratorium on ocean fertilisation, and the moratorium on mining in Antarctica. Furthermore, restrictions on development and use of biological and chemical weapons in the Biological and Chemical Weapons Conventions neither silenced discourse nor impeded decades of progress in the biological and chemical sciences or technologies. More broadly, the legal principle of systemic integration means that any future SRM regime would in any event need to account for existing legal obligations and the customary no-harm rule — a body of law that, as our prior research shows, is far better suited to constraining this technology than to enabling it. Building on that foundation, an International Non-Use Agreement on Solar Geoengineering could commit States to five core measures: no public funding for development, no outdoor experiments, no patents, no deployment of technologies developed by third parties, and no support for the institutionalisation of SRM as a policy option in international fora.
Governments in Africa and the Pacific have already shown the way. It is within UNEP’s mandate, and consistent with its own conclusion, to support rather than dilute that leadership. A UNEP working paper should not be used to justify research budgets and activities that the same report’s analysis does not warrant. We urge UNEP to operationalise key finding 1 — and to help move the international community from a posture of open-ended exploration to one of principled non-use.
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This blog post is based on discussions within the academic initiative calling for an International Non-Use Agreement on Solar Geoengineering. It has been led by Margaretha Wewerinke-Singh, with contributions from Heleen Bruggink, Dana Ruddigkeit, Aarti Gupta, Varun Mohan, Jennie C. Stephens, Stacy D. VanDeveer, Sean Low, and Wolfgang Cramer. The views expressed here do not necessarily represent all signatories of the open letter calling for an international non-use agreement on solar geoengineering.