Dust. Volcanic ash. Belching smokestacks. Wildfire plumes that send particles skyward. These are some of the many phenomena captured in the planetary boundary for atmospheric aerosol loading. The boundary includes aerosols produced by both natural and human sources. Aerosols are defined as the tiny solid and liquid particles suspended in the atmosphere and exhibit significant diversity in their composition, distribution, and in their duration in the atmosphere. All of this diversity creates significant uncertainties about their total impact. Calculating these effects is further complicated by whether aerosols are liquid or solid. These and other factors affect how the planetary boundaries framework tallies the impact of atmospheric aerosol loading on the Earth system. The measurement for aerosol loading uses what is known as aerosol optical depth, or AOD, which is a calculation of how aerosols in the atmosphere interact with clouds and ice crystals to variously absorb or scatter radiation from the Sun. By scattering radiation aerosols can cool the climate, an effect that partially offsets global warming driven by greenhouse gas emissions (Bellouin et al. Reference Bellouin, Quaas and Gryspeerdt2020).

The measure of AOD is an attempt to provide an ‘integrated measure of the overall reduction in sunlight reaching Earth’s surface caused by all absorption and scattering in the vertical air column’ (Richardson et al. Reference Richardson, Steffen and Lucht2023: 7). In other words, a maximum value for AOD would mean that no radiation reaches the Earth’s surface; the radiation, in strict terms, would be extinct. In the 2009 planetary boundaries report the value for aerosol loading remained to be determined. By 2023, it was set in a way that recognized that aerosol loading had an asymmetric impact on the planet, such as by cooling one area but also potentially shifting precipitation patterns elsewhere. Interestingly, accounting for these differences means that the planetary boundary for AOD is a measure of differences between hemispheres and is set at 0.1 and based on annual averages across hemispheres. These interhemispheric impacts, combined with the many kinds of matter that comprise the category of ‘aerosols’, create significant uncertainties about aerosol interactions with the climate and especially with global and regional water cycles. For instance, aerosols are known to affect the water cycle, and especially the monsoon rains upon which large populations of humans and non-humans rely across South and Southeast Asia (Li et al. Reference Li, Lau and Ramanathan2016). Aerosol loading in the atmosphere can affect the timing and intensity of monsoons, and aerosols in one hemisphere can affect precipitations patterns in the other (Krishnamohan & Bala Reference Krishnamohan and Bala2022; Roose et al. Reference Roose, Bala and Krishnamohan2023).

Currently, atmospheric aerosol loading is one of the few planetary boundaries that remain in a ‘safe space’ for humanity. This is despite well-known effects on human health of particulates at the bottom of the atmosphere where humans and other species breathe aerosols in and out. Extreme air pollution in urban centres like Delhi, Beijing, London, or Riyadh has significant effects on human health, local economies, and regional ecosystems. These are recognized in the planetary boundaries framework, but they are not the driving concern when it comes to planetary resilience.

The two chapters in this part consider atmospheric aerosol loading from the perspectives of international law and environmental geography, respectively. The first, by Leslie-Anne Duvic-Paoli, troubles the distinction between the air being breathed daily and atmospheric loading of aerosols. In Duvic-Paoli’s view, the tools of international law are critical yet underequipped to address the global governance challenge aerosols present. Her concerns then turn to the ethical considerations of fairness and equity in response to what international law can and cannot (yet) do. The next chapter, by Jeremy Schmidt, examines the ethics of purposefully loading the atmosphere with aerosols in attempts to cool the climate – often referred to as solar geoengineering or solar radiation management. After explaining some of the polarizing debates over these proposals, he examines the ethics at work in a growing global agenda that does not necessarily seek to manage solar radiation but instead to experiment with technologies that could do so. Together, the two chapters offer insights into how ethical issues on aerosol loading pivot not only on how calculations are made with respect to the Earth system, but also on the instruments and techniques of governing aerosols, and how the impacts of having more or less of them loaded into the atmosphere are known.

24.1 Introduction

Almost the entire world’s population breathes unsafe air that threatens their health and degrades the environment. Many urban areas are regularly found to have illegally high levels of air pollutants (WHO 2021). Since pre-industrial times, the global concentration of aerosol particles in the atmosphere has more than doubled and air pollution has been identified as the single largest environmental health risk by the World Health Organisation (Tsigaridis et al. Reference Tsigaridis, Krol and Dentener2006: 5143; WHO 2021). Their harmful impacts on human health and the environment have justified the inclusion of ‘atmospheric aerosol loading’ as one of the nine planetary boundaries, the scientific framework designed by Rockström and his co-authors to identify the processes that regulate the stability of the Earth system and set safe operating conditions for a healthy planet (Reference Rockström, Steffen and Noone2009: 17).

The essay identifies selected ethical issues posed by the global governance of air pollution, using the planetary boundaries framework as a conceptual tool. An analysis of legal systems and institutions in the context of the planetary boundaries framework is essential because law, as a regulatory institution, could set limits on human behaviour and thus keep us within planetary boundaries (Chapron Reference Chapron, Epstein and Trouwborst2017; Kotzé & French Reference Kotzé, French, French and Kotzé2021). In addition, law has an intimate and complex relationship with justice and can offer insights into how to design safe but also just boundaries (see Gupta et al. Reference Gupta, Liverman and Prodani2023). Notably, the refined framework, drawn by Rockström et al. and renamed ‘Earth system boundaries’, features an aerosol loading boundary that considers the safety of the Earth system and also accounts for human well-being, considering variations in the harmful effects of aerosols on human health across countries, communities, and generations (Reference Rockström, Gupta and Qin2023).

A detailed mapping of the legal instruments available to protect the atmospheric aerosol loading boundary has shown that they are inadequate to respond to the scale of the challenge (Duvic-Paoli & Webster Reference Duvic-Paoli, Webster, French and Kotzé2021), thus unable to mitigate increasing risks of planetary destabilization. On this basis, the essay aims to shed light on the ethical implications of the legal governance of air pollution facing decision-makers in search of fair, equitable, and effective solutions to keep atmospheric aerosol loading within safe limits.

As a result of the failure of environmental law to respond to the challenges of the Anthropocene era, legal scholarship is becoming more vocal about the need for a fundamental paradigm shift in how law operates (see Adelman Reference Adelman, French and Kotzé2021: 75). A line of criticism has been that environmental law operates in an ethical vacuum that has prevented it from addressing fundamental questions of vulnerability, equity, and fairness. The emphasis of legal positivism on the law as it stands, rather than as it should be, has been criticized for separating law from ethics and discouraging an ethical examination of existing norms (Voigt Reference Voigt2005: 123–124). An evaluation of the law regulating air pollution can therefore offer insightful lessons for the field: it raises multiple ethical questions, ranging from how legal systems can be designed to protect particularly vulnerable communities from the health impacts of air pollution, to whether it is legitimate to rely on global norms, often representative of the interests of powerful countries, to govern localized impacts.

In order to explore the ethical implications of global air governance, the essay uses the planetary boundaries framework as a conceptual tool. The framework proposes quantitative boundaries within which humankind can operate in a stable Earth system, and beyond which the risk of large-scale and irreversible environmental damage is likely. It offers scientifically based thresholds for decision-makers to design sustainable systems and has become a useful concept for legal scholars to imagine new solutions to overcome the ongoing inability of legal systems to mitigate environmental degradation (Kotzé & French Reference Kotzé, French, French and Kotzé2021: 7–8).

The essay proceeds as follows: it sets the scene with a brief overview of how law regulates the problem of atmospheric aerosol loading (Section 24.2). It then moves to identify three central ethical questions that decision-makers must confront when regulating air pollution – related to why govern, how to govern, and where to govern. It uses the planetary boundaries framework as a conceptual lens to explore the ethical implications behind the motivations driving air quality governance (Section 24.3), the framing of a multifaceted environmental problem (Section 24.4), and the global governance of multi-scale impacts (Section 24.5). In doing so, the essay reflects on how the design of more ethically oriented legal systems can enhance environmental law’s response to planetary problems.

24.2 Legal Protection of the Atmospheric Aerosol Loading Planetary Boundary

Before evaluating the ethical questions carried by the planetary boundaries framework, the essay starts with a brief description of legal responses to the problem of atmospheric aerosol loading.

Global cooperation on air pollution has increased in recent years, as evidenced by explicit references to air quality in UN Environment Assembly resolutions (2016, 2018), and in the 2015 Sustainable Development Goals (UNGA 2015: 11.6 and 3.9). However, responses to the problem have not been unified: no global legally binding instrument exists on air pollution, contrary to the international treaties governing climate change (UNFCCC 1992) or biodiversity (CBD 1992).

Instead, global governance on air pollution is fragmented; it is regulated by a multiplicity of legal instruments, each limited materially and/or geographically. This reality is ill-suited to the logic of the planetary boundary that portrays each environmental problem as a coherent and self-contained ensemble. However, a comprehensive mapping of the instruments available to ensure that safe thresholds are not exceeded reveals substantial gaps in the governance of the planetary boundary (Duvic-Paoli & Webster Reference Duvic-Paoli, Webster, French and Kotzé2021). Indeed, instruments relevant to the planetary boundary have restrictive mandates to regulate specific air pollutants or polluting industries (such as aviation or shipping). They are also spatially fragmented, as air pollution is governed through a patchwork of regional frameworks, each differing widely in terms of their maturity and effectiveness. Most regional air pollution frameworks lack legally binding mandates; instead, they operate either as scientific projects (such as the Acid Deposition Monitoring Network in East Asia) or as political networks (such as the Southern African Development Community Regional Policy Framework on Air Pollution). This is explained by the fact that, in relation to air pollution, states generally prefer instruments that facilitate multilateral cooperation while preserving national discretion and accommodating regional differences. However, the general lack of regulatory ambition means that existing multilateral instruments and institutions for the governance of air quality are insufficient to be transformational.

An important exception is the Convention on Long-Range Transboundary Air Pollution (CLRTAP), a sophisticated regional treaty adopted in 1979 under the auspices of the UN Economic Commission for Europe (UNECE 1979). Through an advanced web of eight protocols, the CLRTAP regime regulates different sources of pollution (fixed, mobile, new, existing), different types of pollutants (sulphur dioxide, nitrogen oxides, volatile organic compounds, heavy metals, persistent organic pollutants), and mitigates various environmental problems (acidification, eutrophication, tropospheric ozone). The Convention has been remarkably successful at reducing emissions of air pollutants by 40–80 per cent since 1990 in Europe (UNECE 2019: 3). The decrease in sulphur emissions by at least 80 per cent is noteworthy, although nitrogen emissions remain high (UNECE 2019: 3). The Convention illustrates the positive role that international law can play in improving air quality; however, the scope of the treaty is geographically restricted to Eurasia and North America, which means it cannot adequately accommodate the global scale of the problem as outlined by the planetary boundaries framework.

Now turning to the potential aggregate effect of domestic legislation on the protection of the planetary boundary, it appears to be very limited. Indeed, domestic legislation on air pollution across the world is largely immature. A UN Environment Programme report explains:

Not only is air pollution poorly regulated but even when regulation is in place, it does not guarantee good air quality – in the European Union, for example, member states regularly exceed their legally binding air quality targets (European Environment Agency 2022).

This brief overview of global responses to atmospheric aerosol loading shows that existing legal instruments are inadequate to respond to the scale of the planetary challenge. The question of how to design more effective legal systems is becoming increasingly pressing. In this endeavour, paying attention to the ethical implications of different regulatory interventions is essential to ensure that the measures do not only prevent environmental degradation but do so in a manner that is fair for the communities involved.

24.3 Why Regulate Air Pollution?

The essay now turns to the reasons underpinning the governance of the planetary boundary. Understanding the motivations driving the regulation of an environmental problem is important because they dictate different policy approaches with distinct ethical implications. The planetary boundaries framework justifies the adoption of a boundary on atmospheric aerosol loading on two grounds: the effects of aerosols on (1) health, as a major cause of disease and death globally; and (2) the environment, including on ecosystems, cloud formation, weather patterns, and the climate (Rockström et al. Reference Rockström, Steffen and Noone2009: 17).

The justification aligns with the largely anthropocentric reasons used to substantiate political action on air pollution internationally. The harm done by air pollution to human health, leading the World Health Organisation to declare air pollution a ‘silent public health emergency’ (Ghebreyesus Reference Ghebreyesus2018), is the primary reason for regulatory interventions on air quality. Combined ambient and household air pollution contributes to around eight million premature deaths each year globally from increased mortality from stroke, heart disease, chronic obstructive pulmonary disease, lung cancer, and acute respiratory infections. This burden of disease is deemed to exceed the burdens of malaria, tuberculosis, and AIDS combined (WHO n.d.).

The motivations driving decision-makers for acting upon a policy problem have fundamental consequences on legal design. Notably, when the main objective is to tackle air pollution to protect human health, then ambient air quality standards are used to set limit values below which aerosol concentrations are considered safe and acceptable in terms of their effects on human health. The level of concern regarding the human health impacts of aerosols determines the extent to which regulatory interventions protect communities most sensitive to air pollution, such as children or the elderly. Alternatively, it can be considered acceptable to compromise the health of vulnerable communities to pursue other public policy objectives related to the development of air polluting activities, such as transportation or agriculture (see Lovett Reference Lovett2012). Similarly, taking action in the context of scientific complexity and uncertainty – the atmospheric aerosol loading was one of only two boundaries that could not initially be quantified (Rockström et al. Reference Rockström, Steffen and Noone2009: 18) – raises difficult ethical choices about whether, and if so how, to protect human health from uncertain risks without compromising human development and well-being associated with aerosol-producing activities. In other words, how should agricultural activities be regulated when the health consequences of resulting atmospheric aerosols are difficult to quantify, and restrictions might impact access to healthy and affordable food?

Another difficulty relates to the fact that the raison d’être of the planetary boundary is dual, to protect human well-being while maintaining planetary stability. And yet, the two are not necessarily aligned: indeed, air quality standards aimed at protecting human health, especially when set low, do not necessarily avoid environmental harm and stricter air quality targets can, at times, be needed to prevent the environmental consequences of atmospheric aerosol loading. To do so, one commonly used tool is that of ‘critical load’ found, for instance, in the CLRTAP Protocol to Abate Acidification, Eutrophication and Ground-level Ozone (UNECE 1999). It assesses the effects of air pollutants on ecosystems, working out their tolerance to air pollution before damage becomes irreversible. However, as mentioned earlier, higher levels of protection can negatively impact human well-being in other ways and environmentally based motivations tend to have lower traction as the benefits of regulation are less directly visible and are more difficult to be quantified. In addition, Rockström et al.’s framework for safe and just boundaries suggests that considerations over societal fairness might require higher levels of regulatory interventions, even in cases when we remain in a safe operating space from an ecological, as well as potentially a human health, perspective (Reference Rockström, Gupta and Qin2023: 7).

24.4 How to Frame the Air Pollution Problem?

The essay moves to explore the framing of the air pollution problem, showing how varying understandings of the origins and nature of the problem carry different ethical implications in the context of law-making.

The planetary boundaries framework is generally praised for offering a simple, yet powerful conceptualization of environmental thresholds able to reach beyond the scientific community and capture the imagination of decision-makers (Biermann & Kim Reference Biermann and Kim2020: 513). The inclusion of a stand-alone boundary on aerosols has the advantage of putting air pollution on the same level as other environmental problems, such as climate change or biodiversity loss, for which political awareness is higher. The clarity of the message is, however, weakened by the reference to the technical term of ‘aerosol loading’ to describe what is more commonly known as air pollution (that includes, but is not limited to, aerosol loading).

The planetary boundaries framework presents its environmental thresholds as scientifically determined and, hence, objective. The role of decision-makers is framed as being responsible for a transgression of the boundaries, but no guidance is offered on how this can be achieved (Biermann & Kim Reference Biermann and Kim2020: 503–504). Lawmakers are faced with the complex task of designing measures regulating the multiple activities and actors responsible for air pollution. Many of the air polluting activities, such as transportation and industrial activities, are routine activities that are seen to be indispensable to modern life, and, thus, would be difficult to prohibit out right. Standard setting tools that set legal limits on the emission of certain substances are not enough to encourage the transformational changes needed to reduce air pollution at source, that is, to regulate fossil fuel combustion and land-use changes. As such, despite what the planetary boundaries framework might imply, the protection of the atmospheric aerosol loading boundary does not merely rest on technical choices but rather requires complex economic and societal transformations.

Moreover, identifying the origins of air pollution is not a neutral exercise and rather carries major ethical implications due to the multiple sources of atmospheric aerosol loading. Jasanoff revealed that ‘the finger of blame often points downward – toward the deficient practices of poor, local communities’ (Reference Jasanoff2018: 11). She explained that the pollution in Delhi is due to the annual burning of crop stubble by farmers who are too poor to clear their fields for replanting by less polluting means; but instead of helping farmers buy the equipment they need, states prohibit polluting activities that continue nevertheless due to a lack of affordable alternatives. As a result, legal interventions, often adopting a command-and-control approach that sets specific emission reduction targets, generally fail to incentivize and support transitions to less polluting methods. They also tend to be ineffective because compliance with rules perceived to be unjust and unpractical is generally low.

A final comment related to the framing of the air pollution problem relates to its interactions with other environmental problems. The planetary boundaries framework presents aerosol loading as a self-standing problem operating in isolation from other environmental problems, and interactions between the different boundaries are scientifically difficult to quantify (Richardson et al. Reference Richardson, Steffen and Lucht2023: 2–3). And yet, air pollution is not a self-contained problem; it is closely related to pesticide use, oceanic pollution, waste management, and climate change. As a result, regulatory interventions on air quality can have a positive impact on other environmental problems. But the opposite is also true, and, in this case, complex trade-offs need to be considered. Notably, a reduction in emissions of sulphur dioxide carries destabilizing risks for the climate because they tend to cool the climate (IPCC 2022: C.2.3). Under these circumstances, a major intergenerational equity dilemma emerges as lawmakers must decide whether to prioritize the health of the current generation from the impacts of air pollution over that of future generations that will suffer the climate consequences.

To summarize, the aerosol loading problem has multiple sources, involves various contributing stakeholders, and interacts in complex ways with other environmental problems. But the apparent neutral and narrow framing offered by the planetary boundary logic gives little guidance on how decision-makers should respond to this multifaceted problem and sidelines the ethical implications of regulatory interventions.

24.5 Where to Govern a Multi-Scalar Problem?

A final set of ethical difficulties in the governance of the planetary boundary concerns scale (Lewis Reference Lewis2012). The planetary boundaries framework views the air pollution problem as global, reflecting the intercontinental and regional impacts of atmospheric aerosol loading, but impacts are primarily local. The justifications proposed for the adoption of the atmospheric aerosol loading boundary are paradoxical. On the one hand, the framework approaches aerosol loading as a global challenge; on the other hand, the choice of the boundary is justified in spatially restricted terms, based on regional tipping points (Steffen et al. Reference Steffen, Richardson and Rockström2015: 7; Rockström et al. Reference Rockström, Gupta and Qin2023: 7). The inclusion of atmospheric aerosol loading in the planetary boundaries framework was initially justified by the effects of aerosols on monsoon patterns in Southeast Asia (Rockström et al. Reference Rockström, Steffen and Noone2009: 17), then used as a case study to quantify a regional safe boundary (Steffen et al. Reference Steffen, Richardson and Rockström2015: 5). While scientific advancements have resulted in a preliminary proposal to identify a global threshold beyond which humanity would not be operating within a safe zone (Richardson et al. Reference Richardson, Steffen and Lucht2023: 7–8), it remains to be refined. Indeed, such quantification is complex due to the ‘multiple natural and human-caused sources [of aerosols], differences in chemical composition, seasonality and atmospheric lifetimes, and the consequently very large spatial and temporal heterogeneity in distribution and climatic and ecological impacts of aerosols’ (Richardson et al. Reference Richardson, Steffen and Lucht2023: 7).

The global framing of the planetary boundary, together with its shortcomings, sheds light on the important question of whether air quality can be adequately, and ethically, protected by international law. A first reason for caution relates to the fact that the planetary boundaries framework implicitly reinforces the commonly held idea that air pollution is an outdoor problem and fails to highlight the importance of indoor air quality. Indeed, the planetary boundary logic operates far away from the legal interventions into the intimacy of the home that would be required to, for instance, reduce pollution from the use of open fires and simple stoves. And yet, indoor air quality impacts the health of individuals, and disproportionately vulnerable communities in low- and middle-income countries but is rarely included in air quality legislation globally – only 7 per cent of countries have indoor air quality standards (UNEP 2021a: 9). By framing air pollution as a global and environmental problem, the planetary boundary rationale renders invisible the local and developmental nature of aerosol loading. In turn, this raises the question of whether international law has a role in regulating domestic activities beyond classic collective action problems. If so, does it have sufficient legitimacy to address issues such as indoor pollution, which lie close to individuals’ everyday lives?

Another challenge for global governance is to remain legitimate in the context of a multi-scalar problem. It has been noted that the planetary boundaries framework offers little guidance on ‘how to determine a fair share of the safe operating space and concomitant responsibilities among various actors at multiple levels of governance to stay within this space and to avoid breaching the boundaries’ (Kim & Kotzé Reference Kim and Kotzé2021: 7). Beyond the need to ensure that different scales of governance all work together towards an efficient and just protection of the planetary boundary, the sharing of responsibilities between states at the global level is also an ethical issue. Indeed, imposing identical legal obligations on states when major variations occur depending on the geographic level of industrialization and economic and technological capabilities can appear unfair (Okowa Reference Okowa2000: 6). This is particularly important given that, while the global aerosol loading boundary has not yet been crossed, regional transgressions have been identified (Richardson et al. Reference Richardson, Steffen and Lucht2023: 9).

Past experience in the development of air pollution law suggests that flexible international obligations, tailored to national circumstances, are perceived as fairer. By way of example, the 1985 Helsinki Protocol on the reduction of sulphur emissions adopted a single target, with a common timetable, for all Parties (UNECE 1985); as a result, countries like the United Kingdom and the United States did not join the protocol because it did not account for the particularities of each country. Rather, participation increased, and environmental outcomes were better when states were given the choice of several options about how to reduce their emissions (such as in the 1991 Geneva Protocol on volatile organic compounds (UNECE 1991)), or by setting national emission standards tailored to the circumstances of each state (as in the 1988 Sofia Protocol on nitrogen oxides (UNECE 1988)). Hence, the global lens of the planetary boundaries framework can help increase collective awareness but should not be understood to encourage a uniform approach to the aerosol loading problem; rather, global instruments that accommodate local specificities tend to offer more flexibility and share the burdens of environmental action more fairly.

24.6 Conclusion

The planetary boundaries framework alerted to the scientific limits of the Earth but remained silent on how this should translate in terms of human behaviour. The revised framework, which proposes both safe and just boundaries, emphasizes that safe limits cannot be considered sustainable where resources are unevenly distributed and inequalities high (Rockström et al. Reference Rockström, Gupta and Qin2023). Law, as a regulatory tool designed for social organization, has a critically important role to play to build just societal systems operating within their environmental limits. While a complex web of international treaties, texts, and institutions seeks to mitigate air pollution, it has, so far, been unable to provide an adequate response to protect the planetary boundary. In our search for more efficient legal systems, significant ethical issues ought to be considered.

This essay explored three sets of ethical questions that decision-makers must confront when regulating air pollution. It questioned the motivations underpinning legal responses to air pollution, showing how legal design varies depending on what is deemed worth protecting. It also emphasized the complexity of governing air pollution problem when its sources are multiple, and it reflected on how to reconcile a global focus with the localized impacts of air pollution.

The planetary boundaries framework used as a conceptual tool for legal analysis makes visible essential dilemmas about how to design more efficient and more just legal systems; even its own shortcomings alert us to the complex choices facing decision-makers. Although the planetary boundaries framework initially offered little in terms of guidance on how to prompt radical change, it opens important avenues to craft a viable role for law and governance and build more ethically oriented legal systems to better protect the environment.

In May 2021, after a planned experiment in solar radiation management in Sweden was cancelled, the journal Nature published an editorial with a plea to ‘give research into geoengineering a chance’ (Editorial Board 2021). Solar radiation management involves several potential technologies that are designed to reduce the amount of the sun’s energy reaching Earth’s surface. These include, among other options, injecting aerosols into the stratosphere and brightening clouds over the ocean, both of which are meant to reflect a portion of solar radiation back out to space. The editorial reasoning was straightforward: (1) the best option for addressing climate change is mitigation, but the failure to arrest greenhouse gas emissions makes it increasingly unlikely that sufficient steps will be taken prior to the Earth system crossing critical climate thresholds. So, other options must be considered; (2) geoengineering has risks, including the moral hazard of misplaced confidence in technology, but without conducting experimental tests there is no evidentiary basis upon which to assess these risks against those of unabated climate change. The one-two punch of this line of reasoning supported an increasingly familiar position that does not advocate for geoengineering per se, but instead makes the case for geoengineering research. A few months later, in October 2021, an op-ed in the New York Times by David Keith (Reference Keith2021) made the same case from the opposite direction, arguing that geoengineering is the ‘least bad’ way to cool the planet. The shared premise of these arguments is that it is prudent to pursue experimental research into geoengineering. Indeed, it may even be the ethical thing to do.

Set in the context of the planetary boundaries, solar radiation management complicates concerns about aerosols. In the planetary boundaries framework, concerns over aerosols like dust, sulphates, and other particulates pivot on how they reduce the amount of sunlight reaching Earth’s surface and in so doing affect planetary functions. Reducing radiation affects monsoon rains across Asia and affects biosphere integrity. It can affect regional climates and hemispheric patterns of precipitation, while too many aerosols can also have systematic effects on air quality for ‘land and marine ecosystems’ (Richardson et al. Reference Richardson, Steffen and Lucht2023: 8). But in the case of solar radiation management, where injecting aerosols is done intentionally, the concern is not over limiting aerosols but instead reckoning with whether some amount of additional aerosol loading may be needed for a last-ditch effort to stop runaway planetary heating. Here, then, is a case where it is not only a matter of two different planetary boundaries affecting one another, as so many of them do. Rather, at least one boundary is potentially being sacrificed for another: aerosols for climate. Other boundaries will be affected, too; aerosol loading affects the ozone layer, has knock-on effects on the biosphere and water cycle, and could forge unpredictable alliances with novel entities.

Responses to solar radiation management proposals are often anchored in ethical concerns. The ethics of sacrificing some degree of biosphere integrity, regional climate shifts, and hemispheric precipitation patterns all make it a significant moral wager. This is the case regardless of whether one appraises those risks in terms of utilitarian welfare functions or in terms of meeting (or foregoing) different obligations or duties. Additionally, many argue there are moral hazards to deploying solar geoengineering technologies (see Preston Reference Preston2011, Reference Preston2013; Tuana et al. Reference Tuana, Sriver and Svoboda2012). For instance: solar geoengineering could kick-start an uncertain and complex chain of relationships and consequences that create unanticipated empirical and ethical challenges. Further, without a plan for how to stop using the technology there could be a kind of ‘lock-in’ where the climate would be hit with a warming shock should geoengineering be suspended. Worse, still, for many critics, is that geoengineering could become a license for further pollution since it avoids confronting the main driver of climate change: excessive greenhouse gas emissions. These types of concerns underpinned a response to the 2021 Nature editorial, in which Biermann (Reference Biermann2021: 30) represented 17 signatories who argued that geoengineering was an unwelcome distraction from decarbonization efforts and that ‘the current world order is unfit to devise and implement’ the types of agreements required to manage the planet. The riposte from these critics drew on arguments that connect geoengineering to issues of governance, transparency, and democracy across different scales and actors, from states to Indigenous peoples (see also Gannon & Hulme Reference Gannon and Hulme2018; Whyte Reference Whyte2018; Reynolds Reference Reynolds2019). The critical response was followed a short time later with a call for a global ‘non-use’ agreement on geoengineering, which argues these technologies present intractable governance challenges that prevent their use in a just, globally inclusive manner (Biermann et al. Reference Biermann, Oomen and Gupta2022).

Despite the power of these responses, there is an ethical issue that they miss. And if we return to the context of the 2021 Nature editorial, this additional issue comes into view. That context includes, especially, a major report from the US National Academies of Sciences, Engineering, and Medicine (2021), which went to special lengths to make an ethical and empirical case for further research into geoengineering, particularly solar radiation management (SRM). The report represents one of the clearest expressions of a central normative assertion in support of geoengineering research: that experimentation is the best route through which to reach ethically justified decisions regarding solar radiation management. When reading the report, it is hard not to notice the pairing of ethics and experimentation. References to ethics occur on 77 pages and experimentation on 104; issues of justice and equity are not far behind, on 36 and 30 pages respectively. But it is not just the co-occurrence of these concerns that warrants attention. Rather, what is significant is that experimentation is deployed as the vehicle through which to reach decisions on ethical issues. As the National Academies of Sciences, Engineering, and Medicine (2021: 18) report states in the context of achieving an effective solar geoengineering (SG) research programme, a programme of experimentation should examine ‘not only the atmospheric effects of SG but also the ecosystem, and economic, political, and ethical implications’.

The notion that experimentation is an appropriate venue for assessing empirical and ethical elements of geoengineering is the fundamental concern of this chapter. Major reports, editorials in major science journals, and op-eds in influential newspapers all suggest experimentation is a reasonable path forward – not ideal, perhaps, but prudent under the circumstances. For the rest of the chapter, I am going to argue that experimentation cannot be the venue for determining both the empirical and ethical considerations in geoengineering research. This is because experimentation is itself ethical, not neutral ground for adjudicating moral concerns. The upshot of my argument is that there may be a sound ethical basis for geoengineering research, but the view that experimentation provides that basis cannot simply be asserted. Nor is it sufficient to pepper reports with ethical concerns yet fail to appreciate how experimentation is not a value-free vehicle for addressing them. Rather, an argument is needed; one that addresses at least the two concerns that this chapter examines. The main argument, and where the bulk of the chapter focuses, explains the case for why appeals to experimentation are ethically inadequate. The second and much shorter argument is practical. It responds to arguments that claim those who quickly declared against geoengineering research – such as in the global non-use agreement – haven’t yet considered what people in the Global South may have to contribute to debates and decisions regarding solar radiation management. I conclude by returning to considerations of aerosols and planetary boundaries.

25.1 Experimentation as Ethical Disposition

The idea that experimentation is an appropriate arena in which to convene empirical and ethical discussions regarding solar radiation management is not new. David Keith (Reference Keith2013) deployed this argument in his ‘case for geoengineering’ as he laid further groundwork for pursuing research on planet shaping technologies (see also Morton Reference Morton2016). Yet the idea of experimentation has witnessed a resurgence in view of increases to greenhouse gas emissions that continue to climb higher. In 2023, UNESCO’s World Commission on Scientific Knowledge and Technology published its assessment of ethics and geoengineering. That report, like the US National Academies assessment, emphasizes that any experiments with SRM should be at small scales (ideally indoors) and constrained by a series of ethical considerations regarding transparency and conflicts of interest, and that they should also account for different cultural and disciplinary concerns (UNESCO 2023). That same year, the United Nations Environment Programme (UNEP) came to a similar conclusion: there was not enough known about SRM technologies to appraise their risks on matters ranging from impacts on the ozone layer to implications for Indigenous peoples; so, its first recommendation was that a more robust scientific review process by a global body of experts in natural and social sciences be put in place (UNEP 2023: 25). The history of solar radiation management and its current momentum, then, both point towards experimentation as needed to answer empirical questions and to thereby provide the evidence through which to settle ethical concerns.

What is the objection to experimentation? It seems like a good antidote to what is currently happening. For instance, in 2022 the start-up company Make Sunset lofted balloons filled with sulphur dioxide high into the atmosphere and boasted they could release more aerosols if people wanted to buy the ‘cooling credits’ they claimed this achieved. Shortly after, another article in Nature pounced on the idea, arguing that the scary fact that anybody can pump aerosols skyward is all the more reason to research geoengineering (see Ricke Reference Ricke2023). This wasn’t the first time this kind of thing happened. In 2012 the Haida people hired a fly-by-night scientist to dump 120 tons of iron oxides and iron sulphates off the coast of Haida Gwaii (in the Pacific Ocean off Canada’s west coast). This geoengineering project by an Indigenous nation was meant to stimulate the ocean life that ultimately draws carbon down to the ocean floor (Gannon & Hulme Reference Gannon and Hulme2018). There is, of course, an important difference between start-up publicity stunts, ill-defined research, and careful experimentation. Shouldn’t we favour systematic programmes rather than just accept ad hoc interventions? Before digging in on this question, it is important to also note that geoengineering research does not always entail real-world experimentation. For instance, atmospheric modelling doesn’t necessarily entail actual release of aerosols. Here, however, I focus on experimental arguments involving actual releases because these are among the most salient sources of knowledge once the theories and models recommend a particular technological path for intervening in Earth’s solar radiation budget.

A key aspect of my argument against experimentation as a vehicle for ethical decision-making was put succinctly by the late philosopher Bernard Williams. In his work Philosophy and the Limits of Ethics, Williams (Reference Williams1985: 172) argued that a society given over to ‘experiments in living’ does not necessarily increase ‘the chances of living in the best way. It is one sort of society rather than another, and there are various forms of living that it rules out; indeed, those ruled out could include those most worth living’. When Williams made this argument he had a specific target in mind – liberal forms of experimental rule proffered by the utilitarian John Stuart Mill. My argument is not beholden to a critique of either liberalism or utilitarianism, but it equally points out the fact that experimentation orients ethical questions about geoengineering in one way and not in others. In other words, experimentation operates as a kind of ethical disposition – a particular way of arranging concerns and the relations among them. In so doing, experimentation rules out alternate, non-experimental modes of ethics by fiat. This ring-fencing is an important part of what makes experimentation inappropriate as a venue for convening ethical decisions.

One objection that a proponent of experimental research into geoengineering might make is to point out the difference between what Williams termed experimental ‘ways of living’ versus the more constrained aims of a scientific experiment. This sort of objection emphasizes that experimentation in one area is different than saying everything is experimental. If that is true, the proponent could then argue, we can hive off the experimental aspect of geoengineering in a way that does not commit us to making ethics experimental too. There are two challenges this objection faces. The first is that scientific experiments are not conducted in social vacuums. Historians and philosophers of science have routinely made this point plainly and powerfully enough over recent decades that the scientific knowledge once declared to be value-free is now increasingly recognized for the many judgments through which theories, technologies, norms, and institutions make science reliable (Daston & Galison Reference Daston and Galison2010; Oreskes Reference Oreskes2019; Cartwright et al. Reference Cartwright, Hardie and Montuschi2022). Risks, too, are assessed not with respect to the universe of total possibility but through culturally situated practices and bounded rationalities that make satisfying multiple constituencies not only a matter of presenting experimental results (cf. Rayner & Cantor Reference Rayner and Cantor1987). As result of this tangle, the idea that experimentation retains some kind of special status amidst value-loaded sciences is not tenable. This is part of why, I would suggest, major reports by scientific and environmental agencies are trying to show that geoengineering research can be ethical in the first place – because it needs normative legitimacy even at the experimental phase.

The second problem with trying to parse geoengineering research from the argument that it commits decisions to one ethical disposition by fiat is that, if it does not, then proponents face a different challenge: exactly how will experimentation inform ethical decisions? Philosophically, there is an important logical difference between what Nancy Cartwright (Reference Cartwright2007) termed ‘hunting causes and using them’. Suppose, for instance, that an experiment suggested with strong evidence that x would be the result of applying solar geoengineering at scale. This still would not provide us with the full range of morally relevant information about how the causal explanations of x, as determined by the experiment, ought to bear on different uses of the technology – for how long, at what scale, and with what weight assigned to other trade-offs. These kinds of ethical decisions are a species of what Marcus Düwell (Reference Düwell2013: 9) terms ‘mixed judgments’ in which different disciplines and practices offer partial perspectives on a shared moral concern. These broader considerations are among the most salient questions that opponents of solar geoengineering raise. In other words, if proponents of solar radiation management seek to keep experimentation cloistered from its social context (as my first point earlier suggests is hard to do), then they are left no better off in terms of being able to settle ethical questions through answers to empirical questions. The cloistering has the opposite effect: it cuts off the presumptive connection of ethics and inquiry that the case for geoengineering research via experimentation rests on.

The ball is back in the court of those advancing experimentation as the vehicle for making decisions across the empirical and ethical considerations entailed by using aerosols to manage solar radiation. What can be noted in this, the first of my arguments, is that I have not closed the door to experimentation being ethically appropriate. Rather, I have pointed out that this requires a defensible argument that convincingly shows why this is so. To set up experimentation as the site of decision-making about the prospects for planetary geoengineering is to invoke an entire apparatus of inquiry that goes far beyond the boundaries of any single experiment or set of experiments. Additionally, and because experimentation is entangled with the values, theories, and technologies implicated in establishing scientific claims, the argument for experimentation will, in fact, need to confront the central insight provided by Bernard Williams: giving decisions over to experimentation rules out other ways that decisions might be reached. Experimentation entails a particular way of arranging ethical concerns – a disposition – in relation to scientific practices and this is not a morally neutral endeavour.

25.2 Experimentation in Planetary Perspective

In February 2023, it was widely reported that Earth’s temperature averaged over 1.5°C above the pre-industrial average for the first calendar year ever. That year, then the hottest on record, saw significant rises in ocean surface temperatures in the north Atlantic and elsewhere. Explaining the rapid rise in temperatures, which are a decade ahead of what was predicted in many climate models, has often turned to aerosols. It has been understood that aerosols have long been having a cooling effect on Earth’s temperature, even though the sensitivity of climate to them was not fully understood (see Andreae et al. Reference Andreae, Jones and Cox2005). One of the first efforts to quantify these effects was published in 2018 and estimated that aerosols were already cooling Earth by 0.5°C (possibly up to 1.1°C) and that reductions in aerosol loading would increase precipitation 2–4.6 per cent and make extreme weather even more likely (Samset et al. Reference Samset, Sand and Smith2018). The sixth (and most recent, as of this writing) Intergovernmental Panel on Climate Change (2021) report considered the upper limit of aerosol cooling to be as high 0.8°C. The counterintuitive concern then arose – what are the ethics of reducing polluting aerosols when they are already a kind of de facto geoengineering? Would rapid reduction lead to the kind of climate shock that worries those who want a clear and agreed plan for how to phase down any future, purposeful form of solar geoengineering?

In this context, the case for experimental research into solar radiation management takes on a different tenor. There is already unintentional cooling of the earth through historical aerosol emissions. Could this be levered into a need to understand how to avoid rapid temperature increases that could arise from effective reductions in aerosol emissions? If so, that might be a further basis for legitimating geoengineering research. This is because temperature rises that are much faster than anticipated pose the potential for a climate shock for which species, cities, economies, and social stability might not have adequate adaptive equipment or capacity. In this case there is a kind of practical urgency. Reducing emissions is critical to reducing planetary heating, but it is also critical to avoid moral harms caused by rapid climate heating from aerosol emissions reductions. This is especially the case for those most vulnerable to climate change, such as those in the majority world (for example: the Global South). This argument is not simply about ‘giving geoengineering research a chance’ as the editorial in Nature put it. Rather, it is about supporting the best way to learn about interactions of aerosols and climate.

In this context, Táíwò and Talati (Reference Táíwò and Talati2022) recently argued that one of the problems with current debates about geoengineering research is that many critics base their arguments on standards their own arguments don’t meet. This is especially the case, they argue, when the argument is made that harms may accrue disproportionately to people and environments in the Global South through decisions made by actors in the Global North. Such arguments, Táíwò and Talati (Reference Táíwò and Talati2022) reason, often succumb to the inconsistency of never having asked people in the Global South their perspective. So, in this way, the argument that geoengineering cannot be done owing to lack of transparent or democratic global governance fails to meet its own standard of confronting undemocratic or non-transparent forms because it simply asserts its decision without consulting those in the Global South. They think it too presumptive to declare no geoengineering research should take place without consulting those most vulnerable to the risks and trade-offs between climate heating versus managing radiation.

My argument is not of the sort Táíwò and Talati (Reference Táíwò and Talati2022) target. I have not claimed no research could ever be justified, only that the argument for experimentation cannot also double for ethical warrant. My argument also holds for practical questions around urgency. Responses to urgent moral questions are not appropriately confined to experimental dispositions either. They require a suite of tactics. If experimentation is to be principal or even prominent among them, it will require an argument for why this is so, not a default response to crisis. Through deliberations of the kind that Táíwò and Talati (Reference Táíwò and Talati2022) don’t wish to prematurely exclude, a more transparent and democratic appraisal could include experimentation as part of a governance package. Here, however, experimentation is already situated differently. Experimentation would be one among many aspects of ethical decision-making – transparency, democracy, and the values these rely on – regarding solar radiation management, not its primary disposition.

25.3 Conclusion

The argument for research into solar radiation management needs a justification that does not collapse ethical decision-making to an experimental disposition. This is an issue that is also likely to become increasingly important for the planetary boundaries framework. That framework, at present, recognizes existing aerosol loading is already reducing monsoon rains and that there are large uncertainties and asymmetric effects across hemispheres in the interactions among different types and sources of aerosols, clouds, and atmospheric climate forcing (Richardson et al. Reference Richardson, Steffen and Lucht2023; cf. Intergovernmental Panel on Climate Change 2021). It also places aerosol loading among the remaining boundaries that have not yet been crossed – implying, potentially, that adding aerosols could be permissible from the perspective on Earth system stability and resilience that the framework commits itself to. The most recent assessment, however, also states that ‘a much better systemic and quantitative understanding of the hydroclimatic, ecological, and biogeochemical effects of asymmetric aerosol forcing is needed to refine the aerosol loading boundary’ (Richardson et al. Reference Richardson, Steffen and Lucht2023: 8). How will this more systematic, quantitative understanding be achieved? If it is to be through experiments that involve purposeful aerosol loading, a much more robust argument will be needed than just giving over ethical decisions to experimentation.