3.1. Current International Law and Climate Engineering

Present international law imposes virtually no control on any state’s conduct of most forms of CE, whether conducted for the purposes of research or operational climate modification. Multiple regimes are relevant but none meaningfully constrain CE, with the result that any state may legally conduct CE, on or over its own territory, or that of other consenting states, or over the high seas.^{Footnote 18}

The reasons for this lack of legal control are unique to each treaty and institution, but generally lie in the narrowness and specificity of obligations imposed by environmental treaties.^{Footnote 19} The regimes of greatest relevance are those on stratospheric ozone depletion, climate change, and long-range air pollution. Yet the concrete obligations of the Montreal Protocol on the ozone layer are limited to controls on the production and consumption of listed chemicals, and do not include comprehensive controls on other activities that affect ozone.^{Footnote 20} Similarly, the Kyoto Protocol on climate change only limits national emissions of six listed GHGs, and only for Parties listed in Annex B.^{Footnote 21} None of the sulphur-based species now considered promising candidates for stratospheric aerosol injection appear on the list of controlled substances in either of these treaties. National emissions of sulphur dioxide (SO₂) are controlled under the 1999 Gothenburg Protocol^{Footnote 22} to the Convention on Long-Range Transboundary Air Pollution.^{Footnote 23} But this Convention is a regional treaty the membership of which includes only European nations plus the US and Canada, and the way in which the 1999 Protocol specifies national emissions limits appears likely to seriously constrain participation in a CE programme for only the smaller European states.^{Footnote 24} Another treaty of apparent relevance, the 1977 Environmental Modification Convention (ENMOD), prohibits large-scale environmental modification, but only if undertaken for military or other hostile purposes, and includes an explicit exemption for activities carried out for peaceful purposes.^{Footnote 25} The result is that none of these treaties impose concrete obligations that would be violated by proposed CE interventions.

Pushed by vigorous advocacy by a few non-governmental organizations (NGOs), two treaties have taken explicit steps to limit or discourage CE activities: (i) the London Convention and Protocol under the International Maritime Organization,^{Footnote 26} and (ii) the Convention on Biological Diversity (CBD).^{Footnote 27} Even these initiatives, however, currently impose no binding legal restrictions on CE. The initiatives in the CBD, despite claims by their NGO proponents that they comprise a moratorium, impose no binding controls.^{Footnote 28} At most, they express a generalized disapproval for CE, in language that is remarkable for its weakness, opacity, and multiple escape clauses.^{Footnote 29} Action within the London Convention and Protocol has been more focused, but is limited to ocean fertilization, a CE method that appears increasingly unlikely to be effective.^{Footnote 30} London Parties have asserted that ocean fertilization falls within the scope of these treaties and have expressed concern about its potential adverse impacts,^{Footnote 31} but have constructively decided that it does not comprise ‘dumping’ and so falls outside the Protocol’s general prohibition. Further, Parties have drawn an even stronger distinction between dumping and ‘legitimate scientific research’ into ocean fertilization, and have developed an ‘assessment framework’ to which such research should be subject – a rather generic set of procedures for environmental impact and risk assessment – and are now developing legal measures to implement this framework.^{Footnote 32} The upshot is that ocean fertilization is at present subject only to generalized normative statements of concern urging caution, not yet to any legally binding control, while other forms of CE, including stratospheric aerosol injection, are under even less international legal control. In the specific case of controlling US conduct, the legal situation is even weaker because the US is not a party to either the CBD or the London Protocol. Consequently, even if binding controls were adopted under one of these treaties, the US as a non-party would not be bound by them.^{Footnote 33}

In the absence of specific treaty provisions that would constrain national CE activities, the points of existing international law of potential relevance to CE fall into two classes: (i) general obligations to protect and preserve the environment that appear in many treaties, such as the Vienna Convention for the Protection of the Ozone Layer^{Footnote 34} and the United Nations (UN) Convention on the Law of the Sea (UNCLOS);^{Footnote 35} and (ii) relevant principles of customary international law, such as the duty to avoid transboundary harm.^{Footnote 36} Although any of these could be elaborated or interpreted to apply to CE, they lack the specificity to provide operational guidance on what CE interventions, and under what conditions, would be permissible or impermissible – particularly in view of the tension between the potential of CE both to reduce climate change risks and to introduce new risks. Other provisions of customary international law, such as the duty to undertake environmental impact assessment, would not limit CE itself but may create procedural obligations related to how it is conducted.^{Footnote 37}

3.2. Distribution of State Capabilities

The present lack of any controlling international law, however, does not necessarily imply a serious threat of unilateral action to develop or deploy CE technologies. The severity of this risk will depend additionally, indeed primarily, on the distribution of relevant state capabilities and interests. Focusing on these, one common way to express the strategic novelty and challenge of CE has been to contrast its basic structure to that of cutting emissions. Cutting emissions is generally understood as a problem of collective action in which the basic strategic challenge is to motivate and enforce costly contributions to a shared goal, while for CE the basic problem is to bring a widely distributed capability under competent and legitimate collective control. One recent discussion used the vivid ‘free-rider versus free-driver’ image to illustrate this distinction: for effective global policy, the basic problem of emissions control is to overcome free-rider incentives, while the basic problem of CE is to corral multiple potential drivers, each able to act alone, into a collective decision process.^{Footnote 38}

Taken to an extreme, this logic would suggest that virtually anyone can do CE – as has been proposed in various colourful scenarios of CE conducted by terrorist groups, apocalyptic cults, or wealthy individuals.^{Footnote 39} But these scenarios overstate the distribution of capabilities and thus the risk of unilateral action, because they focus too narrowly on financial cost as the determinant of capability and neglect other, non-financial, requirements and constraints. To assess these other constraints, it is crucial to note that achieving a non-trivial, sustained alteration of global climate requires continued large-scale material inputs. These, in turn, depend upon delivery equipment and supporting infrastructure – for example, balloons, tethered pipes, aircraft or ships, backed up by airports, bases and ports – that is visible, hard to conceal, and vulnerable to military attack. This is not to claim that even powerful states would take such military action lightly, in view of the substantial associated costs and risks; yet such action will clearly be a feasible response for some states under some conditions, if they judge another state’s CE actions to threaten their vital interests and have been unable to stop it through other means.

In view of the possibility of such military interdiction, unilaterally achieving a climate alteration that matters would require not just the money, technological capability, and delivery assets, but also the command of territory, global stature, and ability to deploy and project force necessary to protect a continuing operation against opposition from other states, including deterring their threats of stopping it through military action. These requirements exclude the nightmare scenarios of climate alteration by megalomaniac billionaires, terrorist groups, or apocalyptic cults, and also exclude the prospect of unilateral action by most states. Rather, the capability is likely to be limited to a few major world powers, probably numbering fewer than a dozen. Precisely which states could act unilaterally is indeterminate, not just because the feasibility, the precise requirements, and the effects of particular CE interventions are uncertain, but also because who can do it (or, more precisely, who can do what) will also depend on the intensity of other states’ interests in who does what. The more strongly others care – in particular, the more other powerful states are intensely opposed – the fewer states will be able to conduct unilateral CE in the face of that opposition.

However, while the distribution of unilateral capability is narrower than the most apocalyptic commentaries have suggested, it is still broad enough to be significantly destabilizing. The potential for destabilization and conflict arises, on the one hand, from the virtual certainty that any intervention by one state strong enough to alter its own climate would also exert similarly large effects on other states and, on the other hand, from the presumption that no state would undertake a major CE programme lightly. That is, in the event that any state seriously pursues a CE programme, its leaders and polity must perceive the intervention as a matter of high-order national interest because of the realized and impending harm from climate change they see and their expectation that CE can reduce these. Given the likely inability to limit the effects of a CE intervention to one state, if the state making the intervention perceives such acute interest then so also will other states.

3.3. Distribution of State Interests

Characterizing the severity of the resultant risks of international conflict requires a closer examination of states’ likely interests, in particular the degree to which the interests of major states over the available set of CE choices are aligned or opposed. State interests in CE will depend on (i) what specific capabilities are available, including how controllable they are; (ii) the specific projected regional effects of available capabilities; and (iii) how these effects are expected to interact with ongoing greenhouse heating and natural climate variability. Although these factors are uncertain and likely to change with further research, the range of possibilities can instructively be clustered into three alternative degrees of interest alignment.

At one extreme, the interests of states over available CE options might be closely aligned. This situation would be most likely to arise under three conditions:

• • Experienced and anticipated climate change harms are widely distributed worldwide, so states perceive a broadly shared peril.

• • The projected effect of CE is to limit these harms in a manner that is also roughly consistent across world regions. This would require that even the imperfect joint correction of temperature and precipitation noted above is roughly consistent across regions, facilitating agreement among states on some preferred compromise between restoring prior precipitation and prior temperature.

• • Crucially, the available CE capabilities remain rather crude, with little ability to control effects beyond choosing the aggregate intensity of intervention. In particular, there is no ability to tune interventions to achieve differential control of climate effects in different regions.

At the opposite extreme from this first possibility, states’ interests in CE might be strongly opposed. The conditions favouring this situation would be the opposite of those above – that is, the severity of climate disruptions varies strongly among regions, so some states perceive a climate change crisis requiring urgent response while others do not. The anticipated effects of CE in limiting harmful climate impacts also show large regional differences, in their effects or how they are valued, so states disagree over whether and, if so, how to use CE even if they agree that a climate crisis is occurring. Finally, scientific and technological advances have brought a strong ability to control regional effects of CE interventions. In the extreme, plausible advances in information technology and nanotechnology over several decades might lead to individually controllable, optimally asymmetric stratospheric particles, allowing some degree of real-time, regional controllability of climate and weather.^{Footnote 40} Like CE itself, such extreme advances in regional control would present sharp double-edged possibilities. They would greatly advance the ability to reduce harm from climate change and manage climate and weather for global benefit. However, precisely the same advances would enable control of CE to distribute large regional benefits and harm on command, even on a time-scale of days or hours, starkly raising the stakes in how the capability is used and who controls it.^{Footnote 41} If states even suspected others of pursuing such capabilities, or of withholding information about capabilities and effects, opportunities for international mistrust and tension would be substantial.

Perhaps the most likely is the intermediate possibility in which states’ interests are mixed and variable, aligned for some choices and opposed for others. Early model studies have already cast doubt on the strongest commonality of interests from shared benefits of CE. Even if the only available dimension of CE control is the aggregate intensity of an intervention, it appears that different regions most closely approach their prior climate at different levels of intervention.^{Footnote 42} As interventions come to vary in respect of more dimensions – they have more dials to turn – the possibility of trade-offs among different regional harm-reduction objectives increases.^{Footnote 43} Current early study of these questions is highly tentative, of course: it has only identified the possibility of trade-offs, not characterized the intensity or degree of opposition of the resultant interests, which will depend on currently unknown future advances in capabilities. Advancing knowledge may perhaps reveal large-scale regional patterns in joint effects of climate change and CE that create foreseeable common or opposed interests, for example, by latitude band (low-latitude versus mid-latitude countries) or position relative to major mountain ranges (China versus India, for example, over how precipitation and storms are distributed over the Himalayas). Although the details of these interests are unknown, it appears generally likely that increased CE controllability, including more dimensions for control of interventions, will increase the potential for opposing interests.^{Footnote 44}

Moreover, the discussion thus far may understate the prospects for opposition, because it assumes some rational process of forming nationally aggregated interests, based on realized or projected climate effects, with each region viewing its recent climate as ideal; but any of these assumptions might not hold. State interests could be driven by smaller-scale patchiness of climate effects within countries and resultant domestic political conflict. Alternatively, climate preferences might shift in response to realized climate changes or to recognition of the possibility of intentional climate control, such that a region’s present climate is no longer judged to be ideal. State interests in CE might also be dominated by non-consequential or non-rational processes, such as religious or symbolic commitments, general technological optimism or pessimism, or generalized suspicion about other states’ intentions. To the extent that these other processes show strong regional variation, they could further increase the possibility of interstate conflict over CE.

From this sketch of potential state capabilities and interests in CE, two large-scale implications can be drawn about unilateralism in CE. On the one hand, major powers such as the US are likely to face significant temptations to unilateralism – that is, to develop CE capabilities unilaterally; to conceal information about plans, research results and capabilities; and to act diplomatically to preserve a unilateral right of action. On the other hand, such unilateral actions are likely to be dangerous and disruptive to international stability.

Temptations to unilateralism may arise from several factors. The scientific and technical challenges of doing CE well – developing high-benefit, low-risk interventions – are sufficiently large that rich, scientifically advanced nations are likely to have substantial advantages in developing them. Scientific and government elites in such nations may be confident of these advantages, and may also be confident – perhaps over-confident – of their ability to persuade others to their view of CE. Temptations to unilateralism may be exacerbated by anticipation of economic benefits if CE research produces private intellectual property. They may also be exacerbated by the polarization of early debates on CE governance, in which widespread hostility to CE and calls for bans may lead those who favour developing CE capability to judge that doing it unilaterally may be the best way to ensure that it gets done. These temptations are already evident in US policy debate, both in a few explicit calls to preserve US freedom of action and in more widespread scepticism about international consultation over early-stage research.^{Footnote 45}

Yet unilateral pursuit of CE is likely to carry serious risks, which also follow from the same observations about the likely distribution of state capabilities and interests. The ability to develop CE capability, and even to deploy it, will not be limited to the US or to any single state. Other world powers can do it, possibly just as well; and even if some leading state achieves a technological breakthrough – for example, with an approach that is cheaper, safer, or more controllable – less advanced approaches can make similarly large climate perturbations, albeit more crudely. Other states can also assert the same legal arguments for a unilateral right of action. Indeed, states with programmes of regional weather modification may be favoured in advancing these arguments because of the blurry line between these activities, which clearly lie within their sovereign authority, and early CE development. With both capabilities and potential justifications broadly distributed, at least among major powers, unilateral pursuit of CE by any world power, including the US, would risk others deciding to do the same. Once any major power decides to pursue this course, attempting to stop them would be difficult and risky.

Moreover, states are likely to perceive strong interests in whether and how other states pursue CE, not just at the deployment stage but also from early unilateral steps to develop capabilities that might make future deployment more likely. As discussed above, the severity of these risks will depend on how states’ future interests in CE are aligned or opposed. But, given the current uncertainties about CE capabilities and effects, these interests might be subject to some degree of influence. In particular, states’ perceived interests may in part form reactively in response to early acts by other states that signal either anticipated rivalry or cooperation over CE. Thus, early unilateral acts by a major state – including development of capabilities, secrecy about intentions, or aggressive declaration of rights of action – may induce others to perceive CE as predominantly rivalrous and to pursue similar acts, either because they interpret these acts as indicating hostile or rivalrous intent or because they infer from these acts that it is valuable to have an independent CE capability. Conversely, early signals of cooperation and openness may have the opposite effect, steering others’ perceptions and choices towards cooperation. Given the uncertain and labile nature of future CE capabilities, such cooperative early moves may even influence the direction in which future capabilities are developed towards those that pose less risk of conflict.

In sum, following a unilateral approach to CE – including not just eventual deployment but also early steps to pursue research and development alone, maintain secrecy about capabilities and results, and reserve unilateral legal rights – is a superficially tempting but dangerous course of action, for the US and other major powers. States should anticipate and resist these temptations and instead pursue a cooperative approach to CE. Such an approach could start immediately, with informal consultations on research programmes, agreement on common standards for transparency, and joint development of assessment frameworks.^{Footnote 46} A cooperative approach need not involve universal participation, but could start with only the dozen or so nations likely to be most interested in developing CE and most able to pursue it unilaterally. It also need not await a comprehensive climate regime. By building cooperation and transparency on CE while the stakes are relatively low, such early cooperation may help to build norms for cooperative management of CE, which would then be available to help in resolving the more challenging governance problems raised by future proposals for operational interventions.

4.1. Linking Climate Engineering with Mitigation: Four Possible Forms of Linkage

The first scenario, Plan B Linkage, represents the simplest, most minimal, form of linkage between CE and mitigation. Effectively, it makes explicit the relationship often presumed to exist between the two responses even in the absence of intentional linkage. In this scenario, states keep trying to pursue serious emissions cuts. At the same time, they conduct research to develop CE capability for future interventions and agree that these will be used if and as needed to limit future severe climate changes and impacts.

For this minimal form of linkage to promote an effective climate response that includes increased near-term mitigation efforts, it must present future CE deployment as a threat – an outcome so abhorrent that it motivates increased mitigation efforts to avoid it. In effect, the scenario presumes that the normal ‘moral hazard’ concern about CE can be reversed, even with no specific measures to change incentives. This presumption seems odd if we assume rationality in future decision-making, since any future decision to deploy CE would presumably be made only if CE promised to reduce climate harms otherwise anticipated – when the prospect of these worse climate harms at present is failing to provide adequate motivation for mitigation.

This scenario is not completely implausible, however, and could come about under various assumptions, related to uncertain CE effects or non-rational decision-making. For example, future CE use could be perceived as a gamble carrying the risk of outcomes worse than uncontrolled climate change. If future decision-makers regard CE as likely to improve matters on average, but have not learned enough to be fully confident that it will not increase harms, they might still favour deploying it as a desperate measure in the face of severe climate change. Looking ahead to this possibility, current decision-makers might be motivated to greater mitigation efforts to avoid this awful future choice. Alternatively, the prospect of deploying CE might somehow gain more saliency or mobilize more horror about the severity of human disruption of the global environment than severe climate change alone. At first glance, these eventualities appear barely plausible – suggesting that this scenario is unlikely to motivate much strengthening of near-term mitigation – but cannot be completely dismissed.

The second scenario, Reverse Linkage, would reverse the contingency relationship between mitigation and future CE use from that in the Plan B scenario. In this scenario, states would jointly agree to withhold CE, no matter how severe the climate impacts occurring or anticipated, unless states had achieved some agreed level of acceptable performance on cutting emissions. This scenario admittedly requires some suspension of disbelief, yet is still instructive to explore.

The linkage in this scenario would aim to motivate states to cut emissions early, to avoid the prospect of facing severe future climate change without access to CE to moderate the impacts. The most obvious difficulty with the scenario is credibility: how could a threat to refuse CE in response to some future climate emergency be credible? As preposterous as this may first appear, such refusal could be plausible in a political setting marked by intense domestic opposition to CE in the states otherwise able to deploy it. In such a setting, it would be difficult to achieve agreement to use CE under any conditions; but achieving this agreement would be easier if states had made serious contributions to reducing emissions, in part because moral hazard concerns would then be less severe.

In this regard, it is important to recall that future climate change risks come from two distinct routes – either continued failure to cut emissions, or unfavourable resolution of major uncertainties. In this scenario CE would be available to respond to a climate crisis arising from the second cause – unlucky resolution of uncertainties – because under this condition it would be easier to overcome general opposition to CE. CE would not be available, however, to respond to climate harms caused or confounded by failure to make agreed emissions cuts. Given sufficiently strong opposition to CE, the threat to withhold it under these conditions might be credible – or at least, credible enough to provide stronger motivation for near-term mitigation than the first scenario. And, of course, if it does motivate stronger mitigation, the credibility of the threat to refuse future CE in the event of inadequate mitigation would not be tested.^{Footnote 52}

The scenario poses two further difficulties, however. Firstly, while one can construct plausible accounts in each case of how the required future threats could be made credible, both the first and second scenarios involve long time lags between the present mitigation decisions to be influenced, and the future use or non-use of CE that provide the incentive. Both thus suffer from the special credibility problems that afflict any attempt to link acts widely separated in time, as changes in conditions, actors, preferences, or capabilities can easily intervene to prevent the promised (or threatened) future actions. Second, both scenarios make future CE decisions depend on aggregate, collective mitigation performance in the interim. They thus suffer from the same free-rider, collective action problems that hinder current attempts at mitigation. Even if the inter-temporal linkage can be made credible in aggregate, each state will still have incentives to weaken its mitigation efforts in the hope that others do enough to trigger the desired future condition, as long as mitigation remains costly or difficult.

The third scenario, Real-time Linkage, aims to address the first of these problems, the inter-temporal disconnect, by linking actions on mitigation and CE concurrently rather than through future commitments. This scenario thus diverges from the most widely proposed way to use CE, as a response held in reserve for use in some future climate crisis. Instead, in this scenario CE would be used in one or both of the two proposed near-term modes: incremental use to shave the peak off near-term heating coupled with a phased programme of steep emissions cuts, with CE gradually phased out thereafter; or use targeting regional processes (such as Arctic summer sea ice loss or tropical hurricane formation) that are strongly linked to global climate risks.

In this scenario, participating states would simultaneously pursue agreed programmes of steep emissions cuts and limited CE deployment in one or both of these modes. Such real-time linkage of mitigation and CE could ease several strategic and political problems, making both responses more politically feasible and effective by coupling them. Linkage would make mitigation easier by addressing the distinct inter-temporal disconnect that has obstructed efforts thus far, which may be an even more severe obstacle than the collective action character of mitigation.^{Footnote 53} Whereas emissions cuts carry immediate costs to reduce climate risks decades in the future, introducing some small, modulated level of CE concurrently with mitigation would reduce climate risks in the near-term when they are politically salient. In addition to this clear political benefit, mitigation linked to concurrent CE could be less costly, because reductions could build gradually to match capital turnover cycles and allow the orderly development and roll-out of new technologies.

At the same time, real-time linkage could make CE less politically explosive, both because its deployment would be limited in intensity or spatial extent (albeit also performed earlier, when it is arguably not ‘needed’ to manage an imminent climate crisis); and, crucially, because the parallel enactment of mitigation and CE would address the strongest concern about harmful effects of CE, that it may undermine mitigation incentives. Moreover, concurrent linkage would enhance the credibility of nations’ mitigation commitments, because ongoing agreement and authorization to undertake CE – which states would presumably want to continue because of their real-time risk-reduction benefits – would depend on continuing mitigation efforts, with performance verifiable year by year. In sum, this scenario would link the two responses in a ‘both or neither’ political bargain, under which opponents of both mitigation and CE each tolerate the response they oppose because its scale, cost, and risks are limited by parallel pursuit of the response they favour.

Like the first two scenarios, however, this one does not specify by what process or subject to what inducements these collective decisions are made. By implicitly treating all action and consequences as collective, all three scenarios assume a return to some form of full cooperation – if not globally, then at least among the participating major powers. They thus fail to address many strategic and operational difficulties with making real deals work. Most importantly, they do not specify the linkage between the actions of individual states and the consequences that shape their incentives. Since the benefits of both mitigation and CE are likely to be distributed globally, these scenarios thus all remain vulnerable to the collective action challenges that have hindered cooperation on mitigation thus far. In any scenario that links collective CE decisions to aggregate mitigation performance, states will still have incentives to underperform on mitigation, so long as they expect their free-riding will not unravel the global agreement.

The fourth scenario, Pay to Play Linkage, aims to address this problem. This scenario follows the third in proposing real-time coupling of a strong, agreed trajectory of mitigation with a parallel programme of incremental or targeted CE. However, this scenario also provides individual incentives to deter free-riding, by making each state’s mitigation performance a condition for its participation in decision-making on CE. Under the dual assumptions that (i) all participating states strongly desire a voice in CE decisions, and (ii) the threat to exclude them from such participation is credible, this approach would address the problem of providing effective incentives for states to accept and meet strong mitigation commitments. To explore whether this scenario could be plausible, we must thus examine these two assumptions.

Firstly, is the threatened consequence of non-performance – exclusion from participation in CE – sufficiently painful that states will be motivated to pursue strong emissions cuts in order to avoid it? This will turn, in part, on what it means to be excluded. A narrow interpretation of exclusion might be that non-performing states and their citizens and enterprises may not participate in implementing the agreed CE programme. Such narrow exclusion would mainly target commercial interests – for example, firms that want CE contracts – which are unlikely to be large components of overall national interest, although they could still be effective political motivators if sufficiently concentrated. More broadly, exclusion might mean that non-performing governments are also barred from participating in decisions on the design and implementation of the CE programme – what is done, where, and how. Oddly, the extent to which such exclusion is disagreeable to states is likely to vary with the alignment of their interests in CE, but in a manner opposite to the discussion of interest alignment above. If states’ interests in CE are strongly aligned – at least over the range of choices being considered – then the cost to any state of being excluded from decision-making is likely to be low. In this case, other states are likely to implement essentially the same programme as they would if the excluded state was present. As states’ interests diverge – as discussed above, a function of regional variation in climate impacts, how CE limits these, CE capabilities and their controllability, and states’ beliefs and suspicions about these – the cost of being excluded from decisions, and thus the motivating power of the threat of exclusion, would grow.

If we stipulate that exclusion is sufficiently disagreeable to states to be motivating, is the threat of exclusion credible? This will depend strongly on which states are in the participating group, and which state is the presumed target of the threat. Clearly the threat to exclude grows more credible if the group of states collectively binding themselves is large and strong, and the state considering free-riding is small and weak. The credibility of the threat will also depend on the alignment of states’ interests in CE, now once again in the direction that opposed interests pose a greater challenge. If states’ interests in specific uses of CE are strongly opposed, then the threat to exclude is constructively a threat to design a CE programme that takes no account of the excluded state’s interests. Under some conditions, this might inflict significant harm on the excluded state and thus risk serious conflict, raising the possibility that the threatened act may be too severe for the threat to be credible. This problem might be eased by the incremental and real-time nature of the CE interventions being made in this scenario, which are less intense or less global in scale than those that would be deployed in response to some future climate emergency. Over this more limited decision space, regional disparity of interests may be attenuated, allowing a balance between the disagreeability of exclusion and the credibility of threatening it. Such intermediate degrees of interest alignment may thus be close to a politically optimal configuration of interests for CE to be able to motivate contribution to mitigation.