Geoengineering and the Use of Force

(Published in Opinio Juris, Jan. 20, 2021).

It is now widely accepted that the climate change crisis is going to contribute to increasing levels of armed conflict among and within states in the coming decades. Less widely considered is the effect the crisis may have on the jus ad bellum regime. In a two-part essay in Opinio Juris (and in a much longer law review article), I have suggested that there will be growing pressure to relax the jus ad bellum regime when the more dire consequences of the climate change crisis begin to manifest themselves. That is, there will be mounting claims that the threat or use of force may be justified against those “climate rogue states” perceived to be recklessly and unlawfully contributing to the growing threat to international peace and security.

This argument may seem rather radical and unlikely from today’s perspective. But in this essay, I will examine how the case of geoengineering may help to illustrate just how some of the threats posed by climate change will create real tension for the jus ad bellum regime. The essay explores the hypothetical situation in which one country moves to unilaterally engage in a geoengineering scheme that many other states think will cause catastrophic harm to the climate and the ecosystem. How would the international community likely respond, and with what implications for international law?


As most readers will know, the term geoengineering refers to large-scale intervention and manipulation of the environmental systems for purposes of either reducing the pace or countering the effects of climate change. There are many different avenues being explored, ranging from different methods of carbon dioxide removal (CDR), to various forms of solar management regulation (SRM). The latter is a broad category of methods that aim to lower or maintain the temperature of the Earth’s atmosphere by reducing the exposure of the Earth’s surface to the full brunt of energy from the sun.

One method of SRM that has received considerable attention contemplates salting the stratosphere with sulphate particles or aerosols, which would reflect sunlight and have a cooling effect, in much the same way that tiny volcanic particles have done after major volcanic eruptions. The delivery of such sulphate aerosols into the stratosphere could be implemented using specialized aircraft over a period of months, and so is one of the more technically and financially feasible methods of SRM.

It should be noted that while this method could be undertaken by one state, it cannot be implemented on a national or regional level. It would have to be applied to the entire planet – one cannot salt only the Northern (or Southern) hemisphere without causing a massive climate imbalance. Also, the sulphates would only remain in the Stratosphere for a couple of years, so once started the process has to be continued to maintain the moderating effect, or else it would be followed by abrupt temperature rises, particularly if greenhouse gas emissions have continued to increase unabated in the meantime. Among the many risks, one is the possibility that the sulphates falling out of the atmosphere would further acidify the oceans (as will the every increasing levels of carbon dioxide) and create a biodiversity disaster, and the impact on local and regional weather patterns, including monsoons, would be quite unpredictable.

Research on this method of SRM geoengineering is more advanced, and the potential for implementation closer at hand, than most people realize. The Harvard University geoengineering project, among others, will be soon conducting preliminary SRM field tests. While it is certainly not imminent, developments have led to calls for greater control over research and development of such methods, and even a complete ban on SRM. There is a growing recognition that SRM efforts could have huge geopolitical and national security implications.

A Hypothetical – Unilateral Geoengineering Effort

When the consequences of the climate change crisis begin to cause increasing havoc and human suffering in the more vulnerable countries, the pressure on the governments of those states to take protective action will become intense. This is already foreshadowed in the activism of the low-lying island states. In the face of insufficient international coordination and institutional paralysis, those states that are both among the most vulnerable and have sufficient technological and financial capacity, will begin to press for geoengineering solutions. And in the face of perceived dithering on the part of the international community, it is not difficult to image one, or a small group of, such countries deciding to act unilaterally. Scholars, such as my colleague Frederic Sourgens, have even begun to develop some of the legal rationales that might help justify or legitimate such unilateral action.

Let us suppose that a country like India, which clearly has the requisite technical capability, announces that it is going to unilaterally commence a sulphate SRM program. Suppose further that the scientific community voices increasing alarm over the possible risks of the proposed program, with some models predicting catastrophic consequences for both regional climates and various marine ecosystems, resulting in significant loss of life and wellbeing in various states. Governments around the world express strong objections in institutional fora such as the UN, through direct diplomatic demarches, and in the public media. The Indian government is unmoved, citing the need to take immediate action to protect its 1.35 billion citizens. It announces that the program will commence in two months, and that the initial phase is projected to continue for one year.

How would the international community likely respond? On the one hand, it could be argued that the unilateral action would itself be unlawful, a violation of the no-harm principle and any number of other obligations. Indeed, it could be viewed as a violation of the Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques, to which India is a party. India of course could respond that its effort is entirely peaceful and consistent with the commitment to develop methods of ameliorating the consequences of climate change (e.g. UNFCCC, Art. 4(1)(c)), and with the precautionary principle. And when the argument is deadlocked, all diplomatic and other avenues have been exhausted, and India is poised to launch, time will be of the essence—there will be, in Webster’s famous words, “no choice of means, no moment for deliberation,” many will then argue that a surgical strike on the Indian SRM launch facilities, designed to prevent the program while minimizing other damage or loss of life (and ideally not provoking a retaliation), would constitute a necessary and proportionate response to the existential threat posed. But how is such a strike to be legally justified?

Implications for International Law

Security Council Authorized Intervention?  The most obvious recourse for legal authority for such action is through the UN Security Council. It is not difficult to imagine the UN Security Council determining that the Indian SRM program constitutes a “threat to international peace and security” pursuant to Article 39 of the Charter. There has been increasing attention on the role of the Security Council in combatting climate change (e.g. Shirley Scott and Charlotte Ku), and Mark Nevitt among others have argued that the UN Security Council could declare climate change itself a threat to international peace and security. The Security Council has already expanded the scope of Article 39 determinations to include such non-military threats as the Ebola pandemic (U.N. Sec. Res. 2177, 2014). Such a determination of course opens the door to authorizing collective action to respond to the threat, whether in the form of non-violent measures such economic sanctions pursuant to Article 41, or the use of force in accordance with Article 42 of the UN Charter.

There cannot be much doubt that the UN Security Council could authorize a use of force against a state deemed to constitute a threat in this fashion, just as it has authorized uses of force for purposes of humanitarian interventions. And the logic for such a use of force would be more compelling than the justification for humanitarian intervention. If a state is said to lose or abdicate some of its sovereign rights against intervention when it causes sufficient suffering to populations within its own territory, as is articulated in the principles of R2P, it should surely also abdicate some of its sovereign rights if its actions are likely to cause (or disproportionately contribute to) significant human suffering to populations all over the world. But the security council is of course notoriously dysfunctional and prone to gridlock. Supposing one of the permanent members opposes such an “atmospheric intervention” – what then?

Self-Defense and Necessity?  As readers know all too well, the only other currently accepted exception to the prohibition on the use of force is the right of self-defense, as provided for in both Article 51 of the UN Charter and customary international law. The exercise of that right is only permissible in response to an armed attack. This remains pretty well settled, despite the many debates over precisely what level of force constitutes an armed attack, and whether a response to an armed attack may be pre-emptive. It may be difficult to imagine, even within the context of recent debates, that the concept of “armed attack” could be stretched to include India’s proposed SRM program, though one can surely anticipate creative arguments invoking such authority as the Chemical Weapons Convention.

Then again, 20 years ago it would have been difficult to understand a cyber-attack as being sufficient to trigger the right to use force in self-defense. Now, however, the Tallin Manual 2.0 (see Rule 71), relying on the ICJ Advisory Opinion on the Legality of the Threat or Use of Nuclear Weapons, and state practice in relation to other non-kinetic weapons, suggests that cyber operations that cause damage and injury of sufficient scale and effect (that is, equivalent to the effects of a kinetic armed attack) would constitute an armed attack triggering the right of self-defense. States have since adopted as policy the position advanced by the Tallin Manual.

What is more, cyber is only one example of how pressure has been brought to bear for a relaxation of the jus ad bellum—claims in favor of preventative self-defense and the unwilling or unable doctrine similarly tried to undermine the standards relating to the scale, gravity, attribution, and imminence of armed attacks as the triggering condition for self-defense. All of these efforts have in one way or another sought to alter the nature or timing of the acts that successfully trigger the right of self-defense. The imminent launch of a geoengineering effort that is thought likely to risk catastrophic climate consequences may not fit within the current understanding of armed attack justifying self-defense, but as that threat begins to emerge more clearly, one can certainly anticipate that arguments will be made that it is close enough.

To the extent that there is strong resistance to such efforts to relax the “armed attack” standard, scenarios such as explored here will tend to blur the lines between self-defense and such justifications as necessity and countermeasures. It is widely accepted that necessity cannot be invoked as a justification or defense for violating the prohibition on the use of force (see, e.g., Olivier Corten, chapter 4, and ILC commentary on ARSIWA). Similarly, states may not use force as a means of countermeasure. But when certain actions of states, not military in nature and certainly not armed attacks as currently understood, can nonetheless pose an existential or grave threat to the population of other states, and the use of force is seen as the only remaining option for preventing the threat from materializing, the rationale for these exclusions is likely to be challenged and come under pressure for change.


I am not here suggesting that either the jus ad bellum regime or the law of state responsibility be relaxed to permit the use of force against unilateral geoengineering efforts—just as in my larger project I was not advocating for a change in the law to permit the use of force against climate rogue states. Quite the contrary, I am in favor of defending the jus ad bellum. But I am arguing that we will be confronted with these issues as the climate change crisis worsens, and that we need to begin thinking about them now.

On geoengineering, it means first and foremost getting involved in the development of multilateral understandings on the governance and control of such efforts, perhaps most obviously within the UNFCCC COP framework. More importantly, it means greater discussion of how to ensure that the Security Council considers all of the ways in which both contributions to climate change and responses to its consequences fall within the scope of international peace and security. In short, it means that policy makers, jurists, and legal scholars should be discussing how to address the many ways that the climate change crisis will exert pressures on the relevant legal regimes.