from International Institutions and Global Governance Program

The Anticipatory Governance of Solar Radiation Management

Sun setting over the ocean in North Devon, United Kingdom, on February 7, 2011. Tim Graham/Getty Images

Emission reductions alone are unlikely to prevent severe climate change effects. Geoengineering proposals are a way forward, but they need legitimate and effective governance.

July 2, 2019

Sun setting over the ocean in North Devon, United Kingdom, on February 7, 2011. Tim Graham/Getty Images

This Global Governance Working Paper is a feature of the Council of Councils (CoC), an initiative of the Council on Foreign Relations. Targeting critical global problems where new, creative thinking is needed, the working papers identify new principles, rules, or institutional arrangements that can improve international cooperation by addressing long-standing or emerging global problems. The views and recommendations are the opinion of the authors only. They do not necessarily represent a consensus of the CoC members, and they are not the positions of the supporting institutions. The Council on Foreign Relations takes no institutional positions on policy issues and has no affiliation with the U.S. government.

The Challenge

Oliver Geden

Head of the European Union Division, German Institute for International and Security Affairs

Susanne Dröge

Senior Fellow, Global Issues Division, German Institute for International and Security Affairs

More than twenty-five years after the adoption of the UN Framework Convention on Climate Change (UNFCCC), global emissions of greenhouse gases are still rising. The Paris Agreement aims to keep the average temperature increase to “well below” 2 degrees Celsius (preferably 1.5 degrees Celsius), but projections show 3 to 3.5 degrees Celsius of warming by 2100, compared to the roughly 1 degree Celsius above pre-industrial levels experienced today. A recent report by the Intergovernmental Panel on Climate Change (IPCC) concludes that in order to mitigate the most severe effects of climate change, the world would need to reduce carbon dioxide emissions by 50 percent between 2018 and 2030 and reach net-zero emissions by mid-century—a mitigation pathway that seems politically and economically implausible.

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Emission reductions alone are unlikely to prevent severe climate change impacts. Researchers and policymakers are already considering additional measures, often clustered under the term geoengineering (or climate engineering), which encompasses deliberate large-scale interventions into the climate system. Carbon dioxide removal methods would draw down previously emitted carbon dioxide from the atmosphere and enhance the effects of conventional mitigation over time. Solar radiation management (SRM) would temporarily suppress the temperature increase and some of the associated effects, instead of addressing emissions, the root cause of global warming (see figure 1). Such proposals require legitimate and effective governance.

Figure 1. Potential Effects of Solar Radiation Management in Long-Term Climate Policy

Figure 1. Potential Effects of Solar Radiation Management in Long-Term Climate Policy
Source: MacMartin, Ricke, and Keith, “Solar Geoengineering as Part of an Overall Strategy for Meeting the 1.5 Degrees Celsius Paris Target,” Philosophical Transactions of the Royal Society A: Mathematical Physical and Engineering Sciences, 2018.

The most prominent SRM method would quickly reduce the amount of incoming sunlight by continuously injecting reflective particles into the lower stratosphere (see figure 2). A small coalition of powerful countries could potentially perform stratospheric aerosol injection and alter the global mean temperature much faster than any other climate policy measure. SRM thus poses significant risks and governance challenges, both geophysical and geopolitical. Depending on the chosen deployment scheme, there could be adverse effects on regional precipitation patterns and crop growth or a risk of ozone depletion, among other risks. Given the natural variability of the climate system and an incomplete understanding of the earth’s climate system, attributing harms geophysically would be quite difficult. For example, SRM methods could regionally reduce precipitation from one year to another, but precipitation could also be reduced without SRM. Therefore, geopolitically, perceptions will matter much more than scientific accuracy. The belief that countries deploying SRM causes extreme weather events would create fertile ground for international conflict. Furthermore, it is unclear what the exact target global mean temperature should be, and who could legitimately make that decision, given the regionally uneven distribution of risks and benefits.

Figure 2. Stratospheric Aerosol Injection and Other Sunlight Reflection Methods

Figure 2. Stratospheric Aerosol Injection and Other Sunlight Reflection Methods
Source: Council on Foreign Relations.

No targeted governance for SRM exists, and discussions address its potential rather than a ready-to-deploy technology. Some multilateral agreements—such as the Convention on Biological Diversity (CBD), the Vienna Convention for the Protection of the Ozone Layer, and the Environmental Modification Convention (ENMOD)—cover parts, but none is comprehensive. SRM is still at an early stage of research and development, and the potential risks and benefits of techniques such as stratospheric aerosol injection, marine cloud brightening, and other sunlight reflection methods differ substantially and have varying governance requirements. For SRM, the dominant approach by national governments and in multilateral forums can be characterized as “wait and see.” Yet the first outdoor experiment releasing particles in the stratosphere—currently planned for 2020—will likely shift the perception of SRM worldwide and create a sense of urgency around governance, probably highlighting fears of unregulated research and “emergency-mode” deployment by a small group of countries and companies.

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SRM governance could draw on existing international regimes or could emerge from principles and procedures for research experiments. A prerequisite, however, is for state and private actors to agree on the potential role of SRM in the overall climate change regime and to recognize its limitations. To achieve legitimate and effective SRM governance, state and private actors should pursue the following steps.

Acknowledge the trade-off between targets and instruments. A Swiss-led proposal that suggested an expert assessment on geoengineering and its governance, made to the Fourth Session of the UN Environment Assembly (UNEA) in March 2019, failed, unsurprisingly. Until now, most governments, civil society organizations, and climate researchers have avoided an in-depth conversation on SRM. The reasons for reluctance differ substantially. Most climate policy advocates and scientists fear that debating governance and normalizing SRM as part of the policy mix could obstruct mitigation efforts by creating the misleading perception that injecting aerosols could be a substitute for reducing emissions. Governments fundamentally opposed to massive emissions reductions either do not want to debate SRM because it would mean acknowledging that climate change is a serious threat, or they avoid a governance conversation because they do not want to bind their hands prematurely.

In light of a trajectory to warming of more than 3 degrees Celsius and already visible climate change effects, governments will have to deal with the trade-off between ambitious temperature targets and SRM. Its potential as a concept is not going to disappear. This does not mean that SRM would have to be deployed in the likely case of an overshoot of the 1.5 to 2 degrees Celsius threshold and accelerating harms. But it does mean that governments have to assess different types of risks, address the underlying norm conflict between precaution and harm minimization, and be held accountable for their decisions, even if they opt for an SRM moratorium or ban.

Strive for flexible and anticipatory governance. Today, deploying SRM is still a speculative concept, built on natural analogues (for example, temperature effects of large-scale volcano eruptions) and globally coordinated computer modeling. SRM methods could be unable to deliver on their promises, and geophysical and geopolitical side effects could arise. Such emerging and potentially disruptive technology cannot be governed through comprehensive institutional architectures designed in advance, since those would likely prove either too restrictive or too permissive in light of subsequent developments.

Many technological, environmental, and political assumptions held today could change in the future. For example, one popular notion is that SRM will be in the interest of industrialized countries but not the Global South. In the 2030s, however, governments from the most vulnerable regions might demand SRM deployment. Likewise, the widely held assumption that deployment of stratospheric aerosol injections would be comparatively inexpensive could turn out to be false, because of either unexpected engineering challenges or the costs associated with accompanying liability schemes. Existing legal schemes and the role of insurance regimes for testing or deploying SRM do not live up to the anticipated challenges.

Governance arrangements should coevolve with respective SRM technologies.

Accordingly, governance arrangements should coevolve with respective SRM technologies, aiming to be at least one step ahead of research, development, demonstration, and—if the world ever reaches this stage—deployment. Such an approach will necessarily be patchy, using existing forums and regulations and also creating new ones better fit for this purpose.

Treat SRM as part of the global climate policy regime. In order to establish a regular exchange on governance, the UNFCCC should make stratospheric aerosol injection and other geoengineering methods a legitimate object of global climate governance. In this global setting, states and private actors have confronted SRM with distrust and demands for an outright ban (sometimes even on research). But events like the launch of the Stratospheric Controlled Perturbation Experiment (SCoPEx), currently planned as a U.S.-based research activity for 2020, could change the public perception and swiftly open up the conversation on SRM. Critical parties in the global climate regime, like China and the European Union, would have to start talking about the need for SRM governance, even if only in response to other parties’ actions.

Two issues can be addressed by integrating SRM methods under the UNFCCC regime: acceptance and control. Regarding acceptability, the parties to the UNFCCC should aim for a comprehensive consensus that SRM cannot be a substitute for mitigating emissions. Mitigation is part of the genetic code of the UNFCCC and a fundamental environmental protection claim. The more the world achieves with conventional mitigation and carbon dioxide removal, the smaller the scope of potential SRM deployment schemes (see figure 1). Moreover, parties support the climate regime on the basis of multilateralism and the balancing of interests on a variety of contested issues (mitigation, adaptation, finance, and loss and damage). Thus if some parties seriously consider SRM—for example, as a possible emergency approach—the extension of the negotiations agenda could raise the acceptability of talking about SRM as part of international climate policymaking.

The UNFCCC might worry about a shift of the control over climate system parameters into the hands of only a few countries. This concern could be addressed by a mix of trust-building and surveillance, politically as well as technically, because it touches upon many geopolitical tensions that exist beyond the climate regime. Embedding SRM in the climate regime would entangle the most potent parties (large countries with the means and will to deploy) in climate-bargaining.

The UNFCCC—supported by closely affiliated institutions like the IPCC, the UN Environment Program (UNEP), and the World Meteorological Organization (WMO)—should become the major political venue and information hub on SRM and its governance. This would ensure that both political debate and research and development are embedded in the multilateral climate regime.

Embedding SRM in the climate regime would entangle the most potent parties climate-bargaining.

Hedge the risk of uncoordinated action. The parties to the UNFCCC could be unwilling or unable to deal with SRM constructively. Since decision-making under the UNFCCC is consensus based, a minority of countries could block any meaningful debate, or agreed-upon outcomes could deliver overly restrictive or weak guidance. A restriction from the UNFCCC would probably not hinder interested parties such as nation-states, private enterprises, or scientists from moving forward with SRM research, development, demonstration, or even deployment. Their motivations may not align with the global climate regime. International law does not prohibit SRM, although some provisions of existing environmental agreements can be applied—such as the Montreal Protocol on Substances That Deplete the Ozone Layer, CBD, and ENMOD.

Potentially disruptive technology options like stratospheric aerosol injections—which could affect international security or agricultural yields—could also be considered by a different set of political and commercial actors, thus going beyond the realm of climate or environmental policymaking under the United Nations. Decisions on deployment would ultimately lie in the hands of heads of state and governments, bringing broader considerations into play, including the need to show political or technological leadership and the capability to act in times of perceived crisis.

If governments and multilateral and civil society organizations want to hedge the risk that a single state or a small coalition could move ahead with emergency SRM deployment, they should establish institutions that not only allow for thorough dialogue on governance but also can coordinate the next practical steps in areas beyond climate governance. Given the early stage of research on SRM, major powers such as the United States and China will be reluctant to restrict their options by consenting to a tight set of rules. At the same time, none of them would want to see a competing power acting unconstrained and unmonitored. As long as high uncertainty exists about the benefits of unilateral action, countries will have little interest in striving for global governance.

A high-level representative body could be another entry point for global governance, for example a high-level commission on SRM or geoengineering, following the example of the UN-mandated World Commission on Environment and Development under Gro Harlem Brundtland (1983–87), which elaborated the basic interconnections that enable sustainable development. Practical outcomes of such a process—which would require broad participation beyond national governments—could be programs to strengthen existing international agreements related to SRM, improve capacities for regional cooperation and conflict resolution, or develop monitoring systems. Supported by a transparent stocktaking, assessment, and knowledge hub, national interests could evolve in the context of a comprehensive international debate. Such a process could make the benefits of a global governance approach on SRM much more tangible.

Enhance and use research governance. Research on SRM, notably field experiments, requires principles and procedures that over time could—in a bottom-up manner—lead to mutual learning and feed into norms and regulations for global SRM governance.

A decade of coordinated modeling studies assessing the climate system effects of different SRM interventions and a vast body of accompanying social science research, including on public perceptions and governance issues, explains the strong demand for field experiments. They help sharpen understanding of physical and chemical processes and test technical equipment that could be used in SRM deployment. Outdoor experiments could also deliver evidence that SRM does not live up to its promises. Experiments could therefore help avoid risky bets on future technological options to address global warming at the expense of ongoing emission reductions.

Norms could be established by setting up enforceable codes of conduct for responsible SRM research.

Past SRM experiments, like the controversial Stratospheric Particle Injection for Climate Engineering (SPICE) project in the United Kingdom that aimed to test an aerosol delivery mechanism, show that transparency and accountability of research is critical, including openness about research objectives, methods, and results. Norms could be established by setting up enforceable codes of conduct for responsible SRM research, adopted by project funders and national research organizations. Some of those organizations—for instance, the U.S. National Academies of Sciences, Engineering, and Medicine—have already started to chart out SRM research agendas. Codes of conduct could develop in parallel in different countries and add to mutual learning about appropriate research procedures that could facilitate global norm-building in SRM governance. If high standards for responsible SRM research evolve in countries where such research takes place, it would be reasonable for the UNFCCC or the WMO to host an international clearinghouse for SRM research. This would allow policymakers to coordinate research priorities and enable effective public participation.


Governance of SRM is at an early stage, despite a decade of intensive theoretical research and first experiments. Governance issues should be advanced in a step-by-step manner and involve governments, researchers, companies, and other nonstate actors. The uncertainties and risks around SRM—including physical effects, political tensions, liability, and costs—demand anticipatory and flexible approaches and a dose of trial and error regarding rules. Parallel points of departure for governance could also help establish global agreement on what SRM could add to tackle the overall challenge of global warming.


The authors would like to thank the participants of the Council of Councils 2019 annual meeting for valuable comments on this paper. In particular we thank Luca Bergamaschi (Institute of International Affairs), Romy Chevallier (South African Institute of International Affairs), Maria Panezi (Centre for International Governance Innovation), and Samir Saran and Aparajit Pandey (Observer Research Foundation) for their detailed review and helpful suggestions on an earlier version.

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