The Battle Over Blocking the Sun

The global community’s efforts to reduce greenhouse gas emissions are faltering and the world is getting hotter. On its current trajectory, the world is unlikely to meet the limits it set for itself in the 2015 Paris Agreement to halt global warming. According to the UN Intergovernmental Panel on Climate Change, average global temperatures have already increased by 1.3 degrees Celsius (2.3 degrees Fahrenheit) relative to preindustrial levels, are likely to exceed the 1.5 degrees Celsius (2.7 degrees Fahrenheit) Paris goal by as soon as 2040, and could reach an increase of between 3 and 4 degrees Celsius by 2100. That level of warming would be catastrophic.

As this reality sets in, once fringe ideas about how to artificially cool the planet are gaining traction. One such idea is lowering global temperatures by effectively shading the planet, a process known as solar geoengineering. Recent reports from major players in the policy world, including the White House Office of Science and Technology Policy, the U.S. director of national intelligence, and the UN Intergovernmental Panel on Climate Change (IPCC), have all considered this controversial technique to combat global warming, at least as an interim measure until greenhouse gas concentrations in the atmosphere can be reduced. As usual, however, policy trails practice: in 2022, a California-based startup called Make Sunsets began launching balloons filled with sulphate particles high into the atmosphere where, in theory, they would cool the planet by reflecting incoming sunlight.

This approach, known as stratospheric aerosol injection, is behind much of the recent surge in attention paid to solar geoengineering because it is widely believed to be among the cheapest and least complex approaches. But SAI is still intensely controversial: public opposition has forced the cancellation of several planned experiments, and the UN Environment Assembly rejected a modest proposal to form a solar geoengineering study group at its meeting earlier this year.

It is not hard to understand the source of the controversy. Although SAI might help counteract some of the temperature increase from climate change, it could also disrupt global weather and climate patterns, creating risky, unintended consequences for the entire planet. That makes it especially worrying that there is little in the way of international law or global governance currently in place to prevent unilateral SAI efforts on the part of individual countries or even nonstate actors, perhaps including radical environmental groups or ambitious tech startups. Unilateral SAI efforts could even become a trigger for armed conflict, as countries resort to military force to prevent what they see as dangerous tinkering with the world’s climate.

To prevent such developments, the world urgently needs to establish stronger rules to govern SAI. Regulating these technologies does not necessarily mean entirely prohibiting research into solar geoengineering or even deploying it. There may be future scenarios in which some form of it, including SAI, may be deemed necessary as a temporary emergency measure in conjunction with other efforts to mitigate the causes of climate change. But any solar geoengineering initiative should be collectively governed rather than unilaterally imposed on the planet.

NO QUICK FIXES

The basic idea of SAI draws inspiration from volcanic eruptions that cause temporary global cooling by blowing sulfate particles high into the atmosphere. These particles then act as nuclei for cloud formation, in turn reflecting heat back into space. The 1991 eruption of Mount Pinatubo in the Philippines, for instance, lowered global temperatures by about 0.5 degrees Celsius for over a year by injecting 20 million tons of sulfur dioxide into the stratosphere. Proponents of SAI argue that it is possible to replicate this cooling effect through the deliberate injection of reflective aerosols that partially block incoming solar radiation, preventing it from contributing to the warming of the earth’s surface and lower atmosphere.

With the world already experiencing devastating heat waves, floods, and other extreme weather events, and with far worse consequences looming, SAI promises a cheap and relatively straightforward way to blunt global warming by quickly turning down the thermostat while the difficult work of transitioning from fossil fuels to clean energy continues. Some estimates have suggested that injecting just one million tons of sulfur dioxide annually could offset one degree Celsius of warming at a cost of tens of billions of dollars per year, a fraction of the trillions of dollars required for rapid decarbonization. Moreover, SAI would not require radical new technology. Most SAI proposals envision using high-altitude aircraft to seed the upper atmosphere with particulate matter, usually a sulfate. Purpose-designed aircraft would likely be within the technical and financial capabilities of many large economies, and alternative delivery mechanisms such as modified existing aircraft or high-altitude balloons could likely be deployed by companies and other nonstate entities, and even some very wealthy individuals.

Once fringe ideas about how to artificially cool the planet are gaining traction.

If a quick fix to climate change sounds too good to be true, that’s because it probably is. Large-scale SAI would likely disrupt climate and weather patterns in unpredictable ways, including shifting monsoons and potentially causing severe droughts in some regions while flooding others, imperiling food security and habitability across entire regions in the process. These negative consequences have been observed before: global rainfall in the year following the 1991 Pinatubo eruption was roughly half the normal amount. SAI efforts could also damage the ozone layer, accelerate ocean acidification, and destroy entire ecosystems. Worse, once started, SAI efforts would have to be maintained indefinitely, or only gradually ended, as suddenly discontinuing the injections would lead to rapid warming, particularly if greenhouse gas emissions have continued unabated. This risk, known evocatively as “termination shock,” would effectively unleash the collective effect of years of accumulated emissions all at once, making it even more difficult for humans and ecosystems to adapt. There are also serious concerns that relying on SAI could undermine the global push to address the actual causes of climate change, either by diverting attention and resources from the efforts to reduce greenhouse gas emissions or by giving political cover to climate action laggards to slow roll or stall decarbonization.

But the biggest risk posed by SAI is that it could easily be undertaken unilaterally, with little or no consultation with affected parties. The relatively low cost and technical feasibility of SAI, which could make it appealing to anyone seeking a quick, albeit deeply flawed, fix to climate change, combined with the fact that its risks and harms are widely distributed, could create a scenario in which there are strong incentives for individual actors to take unilateral action that affects everyone (what economists call a “free-driver problem”). It is easy to envision how this might happen. A brutal sequence of climate disasters might induce a government to respond to public pressure by turning to the quick climate fix promised by SAI. Alternatively, a major foundation, an environmental group, or even a wealthy individual could be tempted to try to save the planet by undertaking SAI on its own. Even limited SAI interventions, which would be well within the means of such actors, and which could be undertaken on the bet that other countries would then be forced to join the effort, could produce a range of negative consequences, such as reduced rainfall and food insecurity at planetary scales. Despite these risks, policymakers have given little attention to the potential dangers of unilateral SAI. And what is more, none of the treaties and customary international law that comprise the international climate change and environmental law regimes provide for any meaningful constraints or any specific regulation of SAI or solar geoengineering more generally.

CLIMATE WARS

It is high time for serious and open discussion of the risks posed by SAI and action to develop new rules and institutions to govern it—before it leads to geopolitical strife, or even warfare. If done without coordination, SAI has the potential to cause serious climatic effects in some regions, affecting food and water security and even human habitability. The risk of such catastrophic consequences may be perceived by a state as such a threat to its national security that it justifies using military force to prevent another state from engaging in the actions that would cause such harm. States have viewed foreign interference with water and food security as an act that justifies war from earliest history, and the 1977 Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques reflects the extent to which states have more recently viewed climate modification as a potential threat to national security. The defense and intelligence establishments of the United States and many of its allies already characterize the similar consequences of climate change as threats to national security, and a likely cause of increased incidence of armed conflict. Moreover, a range of national and international institutions, including the White House and the IPCC, have identified unilateral SAI as creating the risk of geopolitical instability, including the risk of war.

The international law regime governing the use of force would provide only a modest constraint on states that perceived their vital interests to be at stake, as events in Ukraine and elsewhere over the last few decades have tragically illustrated. International law prohibits countries from using force against other states, with only two exceptions: when authorized by the UN Security Council or when used as an act of self-defense in response to an armed attack. Given deep divisions among the Security Council’s permanent members, it would be highly unlikely that anyone could obtain a consensus to authorize a use of force to stop unilateral SAI. Nor would a use of force in response to the threat of unilateral SAI efforts come within the traditional doctrine of self-defense. But we may predict that states would either try to expand the doctrine or create new exceptions to the prohibition on the use of force.

In recent decades, some countries, notably the United States, have pushed for an expanded interpretation of the doctrine of self-defense in response to perceived new threats to national security, such as transnational terrorism, the development of weapons of mass destruction, and potential cyberattacks. Unilateral SAI could become the latest justification added to this list. Similarly, there have been efforts to create new exceptions, such as “humanitarian intervention” for purposes of dealing with other threats to international peace and security. Regardless of whether such arguments might succeed in expanding the legal exceptions to accommodate the threat of SAI, today’s international law regime is not sufficient to deter states from resorting to force if they feel sufficiently imperiled by such a threat. Thus, a much more robust set of international rules and governance structures is needed to regulate and constrain the development of SAI efforts.

BEFORE IT’S TOO LATE

Ultimately, stopping states from launching SAI efforts on their own will require a new multilateral treaty with the primary purpose of prohibiting unauthorized deployment and establishing a collective decision-making process for approving and governing any potential future use. Given the planetary implications of SAI, such an agreement must be truly multilateral, with fair representation for countries most vulnerable to climate change effects. Such a treaty would also oblige states to prevent nonstate actors from engaging in SAI efforts from within their territory or under their jurisdiction, just as the Montreal Protocol, for instance, requires states to prevent corporations from producing or using ozone-depleting chemicals within their territory. The United Nations may not be able to stop Elon Musk or Make Sunsets from undertaking SAI—but the U.S. government certainly could.

Such an agreement would need to address specific aspects of SAI, including requirements for broad information sharing on SAI-related research and development, international decision-making procedures for authorizing any SAI deployment, monitoring and verification mechanisms, and measures for dispute resolution and enforcement in the case of noncompliance. An agreement should also include certain exceptions and safeguards for SAI research in support of possible multilateral deployment.

A new agreement is, admittedly, a tall order for an international system groaning under the weight of multiple ongoing crises. Fortunately, there are existing international institutions that could potentially serve as forums for negotiating such an agreement relatively quickly. The Montreal Protocol on ozone-depleting substances provides one promising model. It has near-universal membership, an established institutional secretariat, and a strong track record of success, and it has already been expanded to address certain climate change issues through the Kigali Amendment on hydrofluorocarbons. An SAI governance treaty could potentially be negotiated as a new protocol or amendment under this framework.

The biggest risk posed by SAI is that it could easily be undertaken unilaterally.

Alternatively, the UN Framework Convention on Climate Change and its Conference of the Parties could provide an institutional home for SAI governance negotiations. Although addressing SAI under the UNFCCC is controversial, SAI clearly implicates the convention’s core purpose of preventing “dangerous anthropogenic interference with the climate system” and arguably is already within the mandate of the treaty. A new agreement on SAI could be developed as a new protocol to this existing treaty. Negotiating such a protocol, whether under the Montreal Protocol or the UNFCCC, would be far easier and faster than trying to establish an entirely new treaty and develop a new set of governance mechanisms and institutions.

To be sure, no matter where discussions of SAI governance take place, they are likely to remain intensely controversial. But major powers are becoming more open to the idea of talks. Although the U.S. government has historically taken a dim view of solar geoengineering in international climate talks, a high-level U.S. State Department advisory panel included a recommendation to initiate multilateral dialogues on geoengineering governance in its March 2024 report. In private, Chinese negotiators have also signaled that they may be receptive to broad-based efforts to prevent unilateral SAI. At the same time, major players such as the IPCC, the UN Environmental Program, and the U.S. intelligence community now acknowledge that unilateral SAI poses considerable geopolitical risk. If nothing else, these shifts suggest a growing openness to discussion on the issue of geoengineering governance.

What is certain is that, as the climate crisis intensifies, the temptation to deploy technologies to artificially shade the planet will only grow stronger. Guardrails must be put in place before a solar geoengineering experiment or unilateral deployment destabilizes the world’s climate patterns. A robust governance regime is needed not only to mitigate the direct risks of unilateral SAI but also to reduce the chances of armed conflict in response to such interventions. By establishing clear rules, decision-making processes, and enforcement mechanisms regarding SAI, states can help ensure that any future solar geoengineering efforts happen cooperatively under international oversight rather than chaotically and perilously through unilateral action. The stability of the global climate—and potentially international peace and security—may depend on it.