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Markkula Center for Applied Ethics

An Ethical Evaluation of Stratospheric Aerosol Injection

The SPICE project will investigate the feasibility of one so-called geoengineering technique: the idea of simulating natural processes that release small particles into the stratosphere, which then reflect a few percent of incoming solar radiation, with the effect of cooling the Earth with relative speed. Hughhunt, CC BY-SA 3.0

The SPICE project will investigate the feasibility of one so-called geoengineering technique: the idea of simulating natural processes that release small particles into the stratosphere, which then reflect a few percent of incoming solar radiation, with the effect of cooling the Earth with relative speed. Hughhunt, CC BY-SA 3.0

Brad Zukeran, ’24

Photo by: Hughhunt, CC BY-SA 3.0

Brad Zukeran ’24 is pursuing a major in environmental science and minors in political science and history at Santa Clara University. Zukeran was a 2022-23 environmental ethics fellow at the Markkula Center for Applied Ethics. Views are his own.


By the end of the century, the global mean temperature is on track to increase by 2.7-3.1ºC compared to pre-industrial times, which would far surpass the widely recognized lower-risk threshold of an increase of 1.5ºC of warming [1]. Our trajectory would result in irreversible ecosystem damage, severe heat waves, prolonged droughts, mass extinction, and some areas even becoming uninhabitable. In fact, in many parts of the world today, effects like those are already being experienced to a certain extent. In 2018, after a historic three-year drought, Cape Town, South Africa’s municipal water supply fell to alarmingly low levels which led to city-wide reductions of water consumption to avoid “Day Zero” (the day municipal water would shut off and extreme rationing would be implemented) [2]. Decreased rainfall and increased wind in the past decade created optimal conditions for California to experience the worst wildfire season (2020) in the modern era of record keeping with 8,648 wildfires contributing to a total of 4,304,379 acres burned [3, 4]. These are just a few examples and reasons why climate change is a constant topic of discussion and a key issue on the ballot. Despite important, necessary efforts by international organizations to steer the global community in the right direction, it is unlikely that current pledges made in the 2015 Paris Agreement will keep global temperatures from increasing by less than 2ºC [1]. If the rate of cutting emissions is not quick enough, then what else can be done to prevent the worst effects of climate change?       

The geoengineering technique of solar geoengineering or solar radiation management (SRM) might be able to help (for more on geoengineering techniques, see here [5]). Stratospheric aerosol injection (SAI, also called stratospheric particle injection, SPI) is one of the bolder and more controversial methods of solar radiation management. It involves planes, balloons, or giant pipes from the surface releasing sulfur dioxide into the upper atmosphere to increase Earth’s atmospheric albedo (reflectivity). This method of limiting solar rays is based on naturally observed phenomena, like major volcanic eruptions that spew millions of tons of sulfur dioxide into the atmosphere [6]. The best example of this is Mount Pinatubo’s major eruption in 1991 which lowered global temperatures by around 0.6ºC for over 15 months after the eruption [7]. Likewise, the controlled dispersion of sulfur aerosols into the stratosphere could, in theory, hold or reduce global temperatures to below +1.5ºC warming, which would buy time for energy infrastructure to transition from fossil fuels to renewable sources. SAI remains a contentious proposal, as some researchers remain hopeful for its prospects while others remain skeptical.       

Considered a taboo subject in the 1970s, solar geoengineering has emerged into mainstream conversations of climate change adaptation within the past decade [8]. As of 2023, there are over ten major research groups pursuing stratospheric aerosol injection. Most of these groups propose limited use of stratospheric aerosol injection to supplement an overall reduction in carbon emissions: “the technologies are complementary in that carbon removal can allow temporary SRM to limit the rate and absolute magnitude of climate change” [9]. Despite significant funding for solar radiation management research, there have been no field tests due to pushback by various environmental groups and indigenous peoples. The most recent and well-known example of this occurred in April 2021 when Harvard’s ​​Stratospheric Controlled Perturbation Experiment (SCoPEx) was postponed indefinitely by the Swedish Space Corporation after pressure from the Saami Council, a non-governmental organization representing the indigenous Sámi people from Finland, Norway, Russia, and Sweden. The Saami Council had written an open letter criticizing SCoPEx for planning on testing “a technology that entails risks of catastrophic consequences, including the impact of uncontrolled termination, and irreversible sociopolitical effects that could compromise the world’s necessary efforts to achieve zero-carbon societies” [10]. Heads of the Swedish Society for Nature Conservation, Friends of the Earth Sweden, and Greenpeace Sweden cosigned the letter rejecting the field testing of SAI technologies in Sweden or anywhere else.

The seriousness of SAI technology raises many relevant questions. Without even considering the geopolitical aspects of solar radiation management, there is a major question of if humans should try to “engineer” ourselves out of the climate crisis that we brought upon ourselves. Are currently projected greenhouse gas emission cuts enough to prevent devastating climatic events? Who will be most affected by climate change? What are the technical and ethical issues with researching and deploying this technology? Who would be impacted by the deployment of stratospheric aerosol injection?       

When evaluating the validity of research and deployment of stratospheric aerosol injection, it can be useful to assess its research and possible deployment through two sometimes opposing ethical lenses: justice and utilitarianism [11]. A justice perspective judges the ethical sensibility of a decision based on its potential to provide equal treatment to all involved unless there is a relevant reason for differing treatment. In many cases, justice issues emphasize decision-making that supports the needs of those in a disadvantaged position. Utilitarian ethics, in contrast, weigh decisions based on their ability to provide the greatest good for as many actors as possible, despite the possibility that others may suffer as a result. Since stratospheric aerosol injection would have global implications, possibly with many winners and losers, it is appropriate to view the topic from these two perspectives.

The case for geoengineering

Fully pursuing stratospheric aerosol injection methods would be a conscious endeavor to alter the chemical balances of the atmosphere in a way that has never been attempted before. The deployment of SAI technology would require international cooperation and coordination, a feat nothing short of a miracle. Even the research and funding for geoengineering could be considered a watershed moment for how we approach the climate crisis and other issues. This all begs the question: why is SAI needed?      

With the current trend of global warming, the IPCC asserts with high confidence that the regions most vulnerable to climatic hazards are found in West-, Central- and East Africa; South Asia; Central and South America; small island developing states; and the Arctic: “Vulnerability is higher in locations with poverty, governance challenges and limited access to basic services and resources, violent conflict and high levels of climate-sensitive livelihoods” [12]. The injustice of developing and small island nations being the most impacted by the effects of climate change despite contributing the least to it (Western nations having historically added the majority of greenhouse gasses into the atmosphere over the past two centuries), warrants the consideration of solar radiation management with the protection of the global poor and developing nations in mind [13]. Dr. David Keith and Dr. Joshua Horton, both Harvard researchers working on SRM, argue that there is a prima facie (first impression) moral obligation to research solar radiation management techniques because of its plausible ability to minimize the disproportionate effects the global poor will experience. Furthermore, the pair recognize that “SRM technology is subject to a range of significant uncertainties, and if research into SRM were to demonstrate a potential for harmful side effects or unresolvable uncertainties with serious risks for people of the natural environment, SRM should at least be critically reassessed and may ultimately warrant abandonment” [14]. The need for solar geoengineering research is that it accomplishes what other climate change solutions (like emission cuts, adaptation, and carbon removal) cannot: it has the potential to rapidly reduce global temperature [14]. As the only feasible solution to minimize the worst effects of climate change on a global scale and in the near term, stratospheric aerosol injection may effectively address injustices caused by the climate crisis.

An estimated 3.3-3.6 billion people live in locations that are highly vulnerable to the array of effects of climate change [12]. Extreme heat waves, perennial droughts, sea level rise, and other climatic changes will make current locations inhabitable and create up to 1 billion climate refugees (most climate refugees would be within countries rather than transnational refugees). The lives of plant and animal species also must be considered in the calculations of the negative effects of climate change; the past several centuries may have marked the slow beginning of the sixth mass extinction of Earth’s history. While factors such as a change in land and sea usage and direct exploitation of organisms may have a larger relative impact on the loss of species than climate change, there is no denying that the contributions of warming temperatures globally have resulted in the alarmingly high rate of extinction [15]; the current rate of species extinction is up to 100 times the normal natural rate [16]. From a purely utilitarian view, deploying SRM strategies could alleviate much suffering for humans and other organisms. While there would be likely negative regional side effects of geoengineering, the prevailing global average would be positive.

The case against geoengineering

In the face of the catastrophic global effects of climate change, calls to ban the research and deployment of solar geoengineering have never been higher. Books like Half-Earth Socialism by Drew Pendergrass and Troy Vettese paint a dystopian future with mass extinction and widespread environmental damage due to the reckless, overuse of SAI. An open letter advocating for an international solar geoengineering non-use agreement has been published and signed by over 430 academics from 61 countries and 34 organizations [17]. Along with the Saami Council, the National Congress of American Indians and other regional indigenous peoples’ organizations around the globe have announced their opposition to field testing and funding of SAI technology [18]. 

Jennie Stephens and Kevin Surprise, authors of “The hidden injustices of advanced solar geoengineering research,” recognize that the main issue with SAI discourse is the attention it draws away from the fundamental goal of cutting greenhouse gas emissions: “By investing in extreme technical solutions to climate change, those advocating for solar geoengineering research are avoiding extreme (and necessary) social changes that are rapidly gaining political traction,” [19]. Referred to as the “moral hazard” by experts, the risk with researching and developing stratospheric aerosol injection is that if the technology is successful in halting global warming, countries and corporations will be disincentivized to reduce their emissions. Even the prospects of the technology during the research would disincentivize current action to reduce emissions. If humankind were to fall into the “moral hazard” trap, we would be actively choosing to add sulfur dioxide into the stratosphere for potentially a century without ever planning to quit deployment. This scenario would result in an unnecessarily high concentration of sulfates in the stratosphere that could lead to negative impacts down the line. A United Kingdom survey study, conducted in 2013 of people nationally representative of ethnic, gender, socio-economic, age, political status, and geographical region, suggests that “for people who are sceptical [sic] about climate change, wealthier and more self-oriented, the prospect of geoengineering may reduce their own motivation to engage in sustainable behaviour” thus falling into the “moral hazard” trap [20]. Similarly, in a nationally-representative 2016 British survey, respondents expected to vote for the Conservative Party were more likely to trust climate science when told that scientists were developing methods to reduce global temperatures to combat rising emissions [21]. Both of these suggest that while not all people would fall into the “moral hazard” trap, the concern of the “moral hazard” is valid when considering geoengineering technologies. If humankind were to shift our attention away from reductions in greenhouse gas emissions and towards SAI, and SAI technology did not pan out to be a success, climate change would continue to unequally harm the global poor and likely at a higher rate due to the reduction in mitigation efforts. Even if the “bandage” approach of SAI were a success it would allow the dominating economic systems to cause irreparable damage to the planet and kick the can (the transformation of our energy and food systems) down the road.

The opposition to deploying SAI approaches additionally stems from the unpredictable nature of the technology that could cause varying regional effects and its potential misuse having cataclysmic impacts. Across the globe, different climatic changes would likely occur with the introduction of millions of tons of sulfur dioxide into the atmosphere per year: higher precipitation rates in Argentina and South Africa, increased likelihood of drought in Benin, destabilizing the Asian monsoon, and many other region-dependent effects are possible [22]. The disparity in the beneficial and negative regional effects of geoengineering is already a logical environmental justice reason for being against the deployment of SAI strategies, but connecting the justice lens with a utilitarian lens is the potential of “termination shock.” If millions of tons of sulfur dioxide were to be ejected into the upper atmosphere on an annual basis for decades and then suddenly, permanently halted, global temperatures would rapidly rise [23]. This phenomenon would result in rapid warming that would surpass current warming trends, thus leading to more extreme climatic events. Cessation of SAI triggering “termination shock” could be due to voluntary abandonment (e.g., due to a strong rise in opposition to SAI) or forced desertion of the technology (e.g., the destruction of infrastructure and/or economic and political means). If a utilitarian ethical lens weighs the usage of SAI against a “business-as-usual” scenario with the intent of achieving the most “good” for the highest amount of people,the uncertainty of this scenario and the potential of a “termination shock” could lead to the conclusion that the technology may not result in an overall positive outcome. The riskiness of SAI and its potential to worsen expected outcomes is an argument against the deployment without extensive research and field testing. 

Is it ethically “right” to pursue stratospheric aerosol injection research and deployment?

Justice and utilitarian lenses were selected to evaluate stratospheric aerosol injection because of their contrasting priorities regarding the focus on potential benefits and harms. The justice lens emphasizes the need for equal conditions for all people from the most economically developed countries to the least economically developed countries. Climate change disproportionately harms those with the least resources to adapt to the harsher conditions. This could strengthen the argument to invest in SAI research and deployment to alleviate the climatic stresses. The “moral hazard” of solar geoengineering does caution against the temptation of committing to solar geoengineering research and deployment due to it harming the overall movement to mitigate climate change through emission reduction. A utilitarian perspective on climate change and SAI focuses on the need to take the course of action that would benefit the most people possible. If SAI solutions were to be successfully deployed, they could reduce global warming and the severity of climate change for most of the people on Earth. However, the chance that it could do much more harm than good worldwide, through “termination shock,” compared to a scenario of not pursuing it at all, pushes back on the promises of solar radiation management.      

However contentious the debate surrounding stratospheric aerosol injection may be, the commitment we make to it does not necessarily have to be all or nothing. Due to the unlikeliness of current international pledges to reduce greenhouse gas emissions, global temperatures will likely rise at least 2ºC above pre-industrial levels which will cause major harmful climatic events. In this case, temporary, moderate usage of SAI in tandem with the permanent transition to renewable energy could significantly reduce injustices and suffering for the most vulnerable people on our planet. It cannot be overstated how crucial it is for emission reduction pledges to continue to be made and strengthened, but the reality of our inadequate progress would seem to justify alternative research. The National Academy of Science recommends that the U.S. Global Change Research Program receive a $100-200 million research budget over five years to explore “the range of climate and other environmental effects” and “unintended impacts” of SAI. This seems like a logical step to take in case the technology is needed. By funding SAI research, including SAI field testing, we are laying a foundation for us to make rational and informed decisions if the day comes that SAI is truly needed, for example, if a major climate disaster strikes which absolutely must be prevented from recurring. Record-topping heat waves in India in March and April of 2022 contributed to the country’s heatwave-related mortality of 24,000 deaths since 1992 [24]. In 2017, Hurricanes Irma and Maria devastated Puerto Rico through infrastructure damage, flooding, and nearly 3,000 deaths [24]. As the climate increasingly creates extreme conditions like these, it is better to have researched and tested potential technologies ahead of time, rather than the alternative situation in which SAI technology is needed, but either impossible to deploy in time, or is deployed without necessary innovations, understanding, and environmental impacts having been accounted for. David Keith and Joshua Horton from Harvard put it best: “An obligation to investigate is not the same as an obligation to use” [14].

Works Cited

[1] “Global Update: Paris Agreement Turning Point.” Climate Action Tracker, December 1, 2020.  

[2] Pascale, Salvatore, Sarah B. Kapnick, Thomas L. Delworth, and William F. Cooke. “Increasing Risk of Another Cape Town ‘Day Zero’ Drought in the 21st Century.Proceedings of the National Academy of Sciences 117, no. 47 (November 9, 2020): 29495–503. 

[3] “2020 Incident Archive.” Cal FIRE. Accessed June 2, 2023. 

[4] Goss, Michael, Daniel L Swain, John T Abatzoglou, Ali Sarhadi, Crystal A Kolden, A Park Williams, and Noah S Diffenbaugh. “Climate Change Is Increasing the Likelihood of Extreme Autumn Wildfire Conditions across California.Environmental Research Letters 15, no. 9 (August 20, 2020). 

[5] Brad Zukeran and Shivani Dharanipragada, “A Brief Introduction to Climate Engineering,Markkula Center website, August 2023. 

[6] Burns, Lizzie, David Keith, Peter Irvine, and Joshua Horton. “Technology Factsheet Series: Solar Geoengineering.” Belfer Center, 2019. 

[7] “Global Effects of Mount Pinatubo.” NASA Earth Observatory. Accessed June 2, 2023.  

[8] Grossman, Daniel. “Geoengineering: A Worst-Case Plan B? Or a Fuse Not to Be Lit? " Yale Climate Connections.” Yale Climate Connections, June 8, 2021.  

[9] Keith, David W., and Douglas G. MacMartin. “A Temporary, Moderate and Responsive Scenario for Solar Geoengineering.Nature Climate Change 5, no. 3 (February 16, 2015): 201–6.  

[10] Kahn, Natalie L, and Simon J Levien. “SEAS Researchers Postpone Test Flight for Controversial Geoengineering Project to Block Sun.” The Crimson, April 5, 2021.  

[11] Velasquez, Manuel, Dennis Moberg, Michael J Meyer, Thomas Shanks, Margaret R McLean, David DeCosse, Claire André, Kirk O Hansen, Irina Raicu, and Jonathan Kwan. “A Framework for Ethical Decision Making.” Santa Clara University, November 8, 2021. 

[12] Pörtner, Hans-O et al. “Summary for Policymakers - Climate Change 2022: Impacts, Adaptation and Vulnerability.Intergovernmental Panel on Climate Change, 3–33. Accessed June 2, 2023.  

[13] Popovich, Nadja, and Brad Plumer. “Who Has the Most Historical Responsibility for Climate Change?” The New York Times, November 12, 2021.  

[14] Horton, Joshua, and David Keith. “Solar Geoengineering and Obligations to the Global Poor.” Harvard John A. Paulson School of Engineering and Applied Sciences. Accessed June 3, 2023.  

[15] “UN Report: Nature’s Dangerous Decline ‘Unprecedented’; Species Extinction Rates ‘Accelerating.’” UN Sustainable Development Goals, May 6, 2019. 

[16] Pearce, Fred. “Global Extinction Rates: Why Do Estimates Vary so Wildly?” Yale E360, August 17, 2015.  

[17] Biermann, Frank, et al. “Solar Geoengineering: The Case for an International Non-Use Agreement.Wires Climate Change, November 16, 2021. 

[18] “The National Congress of American Indians Resolution #AK-21-025.” The National Congress of American Indians, June 24, 2021. 

[19] Stephens, Jennie C., and Kevin Surprise. “The Hidden Injustices of Advancing Solar Geoengineering Research.” Global Sustainability 3 (2020): e2. doi:10.1017/sus.2019.28 

[20] Corner, Adam, and Nick Pidgeon. “Geoengineering, Climate Change Scepticism and the ‘Moral Hazard’ Argument: An Experimental Study of UK Public Perceptions.Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 372, no. 2031 (December 28, 2014). 

[21] Fairbrother, Malcolm. “Geoengineering, Moral Hazard, and Trust in Climate Science: Evidence from a Survey Experiment in Britain.Climatic Change 139, no. 3–4 (October 3, 2016): 477–89.  

[22] McKibben, Bill. “Dimming the Sun to Cool the Planet Is a Desperate Idea, yet We’re Inching toward It.” The New Yorker, November 22, 2022.  

[23] Parker, Andy, and Peter J. Irvine. “The Risk of Termination Shock from Solar Geoengineering.Earth’s Future 6, no. 3 (March 11, 2018): 456–67.  

[24] Debnath, Ramit, Ronita Bardhan, and Michelle L. Bell. “Lethal Heatwaves Are Challenging India’s Sustainable Development.PLOS Climate 2, no. 4 (April 19, 2023).  

[25] “Mitigation Assessment Team Report: Hurricanes Irma and Maria in Puerto Rico - Building Performance, Observations, Recommendations, and Technical Guidance.” FEMA, October 2018. 


Aug 17, 2023