Under the Lens

Large-scale SRM has never been deployed.

• Large-scale SRM deployment would be a major operation with observable infrastructure and detectable changes to the planet.
• There have been a few small-scale outdoor experiments, but these have been far smaller than what would be needed to noticeably affect the global climate.

The chemtrails conspiracy theory falsely points to aircraft contrails as evidence of ongoing SRM deployment.¹ Contrails, or condensation trails, form when jet exhausts interact with certain atmospheric conditions, leaving cloud-like streaks across the sky.²

Weather modification also gets confused for SRM.¹ Many countries pursue weather modification efforts like cloud seeding, which aim to influence local weather – usually enhancing rain or snowfall.³ SRM is distinct from weather modification, focused on changing long-term climate outcomes at a much larger scale.³

While some small-scale SRM outdoor experiments have taken place, no large-scale SRM deployment has begun, and it would be easy to detect if it did.

The most researched and feasible SRM approach is stratospheric aerosol injection (SAI), which would use high-flying jets to create a global layer of tiny reflective particles in the upper atmosphere.⁴ Cooling the planet by 1°C would require hundreds of jets each flying hundreds of times per year.⁵

It would not be possible to keep such a large-scale SAI programme secret. The flight activity would be detected,⁶ and existing satellites could detect aerosol injections in the stratosphere before they could produce a significant climate impact.⁷

Large-scale deployment of other SRM approaches like marine cloud brightening would also be detectable through the large fleets required for deployment and the resulting changes to clouds.⁸

While there have been a limited number of small-scale outdoor SRM experiments, these have all had little to no environmental impact and are far smaller than what would be needed to noticeably affect the global climate.⁹

Endnotes:

1. Buck HJ, Shah P, Yang JZ, Arpan L. (2025). Public concerns about solar geoengineering research in the United States. Communications Earth & Environment. 6(1):609. https://doi.org/10.1038/s43247-025-02595-5
2. Singh DK, Sanyal S, Wuebbles DJ. (2024). Understanding the role of contrails and contrail cirrus in climate change: a global perspective. Atmospheric Chemistry and Physics. 24(16):9219-62. https://doi.org/10.5194/acp-24-9219-2024
3. World Meteorological Organization. (2025). WMO Statement on Weather Modification. https://wmo.int/content/wmo-statement-weather-modification
4. Parson EA, Keith DW. (2024). Solar geoengineering: History, methods, governance, prospects. Annual Review of Environment and Resources. https://doi.org/10.1146/annurev-environ-112321-081911
5. Smith W. (2020). The Cost of Stratospheric Aerosol Injection Through 2100. Environmental Research Letters 15: 114004. https://doi.org/10.1088/1748-9326/aba7e7
6. Smith W, Wagner G. (2018). Stratospheric Aerosol Injection Tactics and Costs in the First 15 Years of Deployment, Environmental Research Letters 13: 124001. https://doi.org/10.1088/1748-9326/aae98d
7. Lange A, Niemeier U, Rozanov A, von Savigny C. (2025). Investigating the ability of satellite occultation instruments to monitor possible geoengineering experiments. Atmospheric Chemistry and Physics. 25(19):11673-88. https://doi.org/10.5194/acp-25-11673-2025
8. Latham J, Bower K, Choularton T, et al. (2012). Marine cloud brightening. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences. 370(1974):4217-62. https://doi.org/10.1098/rsta.2012.0086
9. SRM360. (2025). Outdoor SRM Experiments. SRM360.org. https://srm360.org/article/outdoor-srm-experiments/

SRM has the potential to undermine emissions cuts.

Experts agree that emissions cuts would remain essential if SRM were deployed. Nevertheless, powerful actors could see SRM as a reason to slow down efforts to reduce emissions.

SRM may be able to reduce many risks from climate change, but it would be a poor substitute for emissions cuts as it would bring new risks and side effects. A common concern is that advancing SRM could undermine efforts to cut emissions, known as “mitigation displacement” or “moral hazard”.

SRM would be inexpensive to deploy compared to the investments needed for large-scale emissions cuts or carbon removal.¹ This could incentivise policymakers to weaken efforts to cut emissions, and those with a vested interest in fossil fuels might exaggerate the potential of SRM to rationalise continued emissions.²

However, several studies using surveys and games have found that people’s support for emissions cuts tends not to be significantly affected by SRM.^3,4 Some studies even found that SRM increases rather than reduces support for emissions cuts,^5,6 though others show that results depend strongly on how SRM is presented and framed.⁷

These studies may not provide much insight into the long-term politics of SRM, though. Public opinion can shift dramatically over time as policy debates evolve, and political decisions are often driven by factors beyond public opinion.⁸

SRM’s potential effect on emissions is not only difficult to predict – it would also be difficult to determine in retrospect.² This is because of the lack of a “counterfactual” – an alternative reality without SRM to compare to. Similar concerns were raised around adaptation, but we can’t know whether the growing attention to adaptation has reduced efforts to cut emissions or not.²

Endnotes:

1. Moreno-Cruz J, McEvoy DM, McGinty M, Cherry TL. (2025). The economics and governance of solar geoengineering. Review of Environmental Economics and Policy. 19(1):1-24. https://doi.org/10.1086/733652
2. McLaren D. (2016). Mitigation deterrence and the “moral hazard” of solar radiation management. Earth’s Future. 4(12):596-602. https://doi.org/10.1002/2016EF000445
3. Andrews TM, Delton AW, Kline R. (2022). Anticipating moral hazard undermines climate mitigation in an experimental geoengineering game. Ecological Economics. 196:107421. https://doi.org/10.1016/j.ecolecon.2022.107421
4. Schoenegger P, Mintz-Woo K. (2024). Moral hazards and solar radiation management: Evidence from a large-scale online experiment. Journal of Environmental Psychology. 95:102288. https://doi.org/10.1016/j.jenvp.2024.102288
5. Cherry TL, Kallbekken S, Kroll S, McEvoy DM. (2021). Does solar geoengineering crowd out climate change mitigation efforts? Evidence from a stated preference referendum on a carbon tax. Climatic Change. 165(1):6. https://doi.org/10.1007/s10584-021-03009-z
6. Cherry TL, Kroll S, McEvoy DM, et al. (2023). Climate cooperation in the shadow of solar geoengineering: an experimental investigation of the moral hazard conjecture. Environmental politics. 32(2):362-70. https://doi.org/10.1080/09644016.2022.2066285
7. Merk C, Wagner G. (2024). Presenting balanced geoengineering information has little effect on mitigation engagement. Climatic Change. 177(1):11. https://doi.org/10.1007/s10584-023-03671-5
8. Parson EA, Keith DW. (2024). Solar geoengineering: History, methods, governance, prospects. Annual Review of Environment and Resources. 49. https://doi.org/10.1146/annurev-environ-112321-081911

SRM would not make emissions cuts unnecessary.

• SRM could not fully counteract the effects of greenhouse gases and would have its own risks and side effects.
• SRM might play a useful role in limiting climate risks, but without emissions cuts there is no sustainable solution to the climate crisis.

Climate change is caused by greenhouse gas build-up – mainly CO₂ – trapping heat and warming the planet. SRM may be able to help by reflecting a small portion of sunlight back to space, cooling the planet. This might help to reduce many impacts of climate change,¹ but SRM has shortfalls and risks that mean it cannot replace emissions cuts.

SRM would offer only a short-lived cooling effect, while CO₂ persists in the environment for centuries.² And it wouldn’t stop ocean acidification, which threatens marine life.³

SRM also couldn’t fully counteract the effects of greenhouse gases on the climate. Rainfall patterns would still change significantly, and some regions may see greater changes under SRM than under climate change alone.⁴

And SRM approaches would also have environmental side effects of their own. For example, stratospheric aerosol injection (SAI) using sulphates would add to acid rain⁵ and air pollution, while slowing the recovery of the ozone hole – though these harms may be outweighed by the benefits of reduced temperatures.⁶

Using SRM to maintain temperatures without cutting emissions would require deploying more and more SRM – amplifying side effects and risks. And if deployment suddenly stopped, temperatures would spike rapidly.⁷

At best, SRM could temporarily reduce some climate impacts while emissions cuts scale up.⁸

Endnotes:

1. The Royal Society. (2025). Solar Radiation Modification Policy Briefing. https://royalsociety.org/news-resources/projects/solar-radiation-modification/
2. Archer D, Eby M, Brovkin V, et al. (2009). Atmospheric lifetime of fossil fuel carbon dioxide. Annual review of earth and planetary sciences. 37(1):117-34. https://doi.org/10.1146/annurev.earth.031208.100206
3. Roberts KE, Rohr T, Raven MR, et al. (2026). Potential impacts of climate interventions on marine ecosystems. Reviews of Geophysics, 64, e2024RG000876. https://doi.org/10.1029/2024RG000876
4. Ricke K, Wan JS, Saenger M, Lutsko NJ. (2023). Hydrological Consequences of Solar Geoengineering. Annual Review of Earth and Planetary Sciences. 51(Volume 51, 2023):447–70. https://doi.org/10.1146/annurev-earth-031920-083456
5. Visioni D, Slessarev E, Macmartin DG, et al. (2020). What goes up must come down: Impacts of deposition in a sulfate geoengineering scenario. Environmental Research Letters; 15. https://doi.org/10.1088/1748-9326/ab94eb
6. Harding A, Vecchi GA, Yang W, Keith DW. (2024). Impact of solar geoengineering on temperature-attributable mortality. Proceedings of the National Academy of Sciences. 121(52):e2401801121. https://doi.org/10.1073/pnas.2401801121
7. Parker A, Irvine PJ. (2018). The risk of termination shock from solar geoengineering. Earth’s Future. 6(3):456-67. https://doi.org/10.1002/2017EF000735
8. Parson EA, Keith DW. (2024). Solar geoengineering: History, methods, governance, prospects. Annual Review of Environment and Resources. 49. https://doi.org/10.1146/annurev-environ-112321-081911

There is no evidence of fossil fuel interests – individuals or organisations heavily involved in fossil fuels – funding SRM research.

Our SRM Funding Tracker is the most comprehensive analysis available of who’s funding solar geoengineering work. Across all the funding we analysed, we found no fossil fuel money. Many non-profits working on SRM explicitly state that they wouldn’t accept such funding.

*That said, we couldn’t confidently identify the source of about $1 million (less than 1% of total funding analysed). And it’s possible some funding remained undetected. So, while we found no evidence of fossil fuel involvement, we can’t rule it out completely.

SRM is not banned under international law.

• While there are international laws relevant to SRM, none amount to an outright ban.
• SRM deployment would still likely face legal challenges, as countries might argue it conflicts with certain legal principles.

International law comes in two main forms: treaties between agreeing countries, and customary international law (consistent practices countries accept as being legally required).

Several treaties have some relevance to SRM, but none amount to a ban. The main ones are:

• The Environmental Modification Convention, which would prohibit the hostile use of SRM but not peaceful uses;¹
• The Convention on Biological Diversity, which invites governments to consider prohibiting SRM activities that may affect biodiversity, but does not itself prohibit SRM;²
• The London Protocol, which prohibits specific “marine geoengineering” activities but does not currently apply to SRM.³

While no treaties ban SRM at any scale, countries may argue that large-scale tests or deployment would conflict with certain principles of customary international law – particularly the “no-harm rule”. This requires countries to limit environmental harms to each other and could apply to SRM, since it could pose significant risks to countries other than the deployer(s).⁴

However, customary international law can lack detail and be open to interpretation.⁵ The UN International Law Commission, whose work to some degree reflects this body of law, did not recommend a prohibition on SRM in its Draft Guidelines on the Protection of the Atmosphere.⁶ Instead, it states that such activities “should only be conducted with prudence and caution, and subject to any applicable rules of international law”.⁶

While customary international law could present legal obstacles to SRM, it doesn’t amount to an outright ban. That said, some countries may support one. The Mexican government has said it would rally countries to ban SRM,⁷ and African ministers have called for a global “non-use agreement”.⁸

Endnotes:

1. Convention on the prohibition of military or any other hostile use of environmental modification techniques. (1976). https://treaties.un.org/pages/ViewDetails.aspx?src=IND&mtdsg_no=XXVI-1&chapter=26&clang=_en
2. Convention on Biological Diversity. (2010). Decision X/33. https://www.cbd.int/decision/cop?id=12299
3. London Protocol. (2013). Resolution LP.4(8). https://wwwcdn.imo.org/localresources/en/KnowledgeCentre/IndexofIMOResolutions/LCLPDocuments/LP.4(8).pdf
4. Brent K. (2021). Solar geoengineering is prohibited under international law. Debating climate law. Cambridge University Press, Cambridge. 274-84. https://doi.org/10.1017/9781108879064.021 See also Reynolds JL. (2021). Solar Geoengineering Could Be Consistent with International Law. Debating climate law. Cambridge University Press, Cambridge. 257-73. https://doi.org/10.1017/9781108879064.020
5. Hakimi M. (2020). Making Sense of Customary International Law. 118. Michigan Law Review. 1487. https://doi.org/10.36644/mlr.118.8.making
6. International Law Commission. (2021). Guideline 7 in Draft guidelines on the protection of the atmosphere, with commentaries. https://legal.un.org/ilc/texts/instruments/english/commentaries/8_8_2021.pdf
7. Garrison C. (2023). Insight: How two weather balloons led Mexico to ban solar geoengineering. Reuters. https://www.reuters.com/business/environment/how-two-weather-balloons-led-mexico-ban-solar-geoengineering-2023-03-27/
8. AMCEN. (2025). Tripoli Declaration. https://wedocs.unep.org/handle/20.500.11822/48291

SAI has the potential to disproportionately benefit poorer regions.

Climate change has disproportionate impacts on poorer regions – they’re generally more vulnerable, less able to adapt, and more dependent on climate-sensitive livelihoods.¹ Many poorer countries are also closer to the equator, with already hot climates, and are expected to suffer greater harms as the world warms.¹

By reducing temperatures globally, stratospheric aerosol injection (SAI) could produce disproportionate benefits for hotter, poorer regions, including reductions in temperature-related deaths² and other climate impacts like agricultural losses.³

However, SAI would produce uneven changes in regional rainfall and water availability.⁴ For example, it could decrease the risk of droughts in some poorer regions, while increasing it in others.⁵

SAI could also have side effects, such as delaying ozone recovery and adding to air pollution, though these harms may be small compared to the benefits of reduced temperatures.²

Irresponsible SAI deployment – like in just one hemisphere or with abrupt abandonment – could cause severe harms,⁶ likely felt more strongly by poorer regions. But careful, moderate deployment could help reduce changes to most climate hazards in most regions.⁷

Endnotes:

1. IPCC. (2022). Climate Change 2022: Impacts, Adaptation and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press. http://doi.org/10.1017/9781009325844
2. Harding A, Vecchi GA, Yang W, et al. (2024). Impact of solar geoengineering on temperature-attributable mortality. Proceedings of the National Academy of Sciences, 121(52). https://doi.org/10.1073/pnas.2401801121
3. Fan Y, Tjiputra J, Muri H, et al. (2021). Solar geoengineering can alleviate climate change pressures on crop yields. Nature Food. 2(5):373-81. https://doi.org/10.1038/s43016-021-00278-w
4. Ricke K, Wan JS, Saenger M, et al. (2023). Hydrological consequences of solar geoengineering. Annual review of earth and planetary sciences. 51(1):447-70. https://doi.org/10.1146/annurev-earth-031920-083456
5. Fu W, Yue X, Tian C, et al. (2025). Unequal socioeconomic exposure to drought extremes induced by stratospheric aerosol injection. Atmospheric Chemistry and Physics. 25(20):13103-21. https://doi.org/10.5194/acp-25-13103-2025
6. Tang A, Kemp L. (2021). A fate worse than warming? Stratospheric aerosol injection and global catastrophic risk. Frontiers in Climate. 3:720312. https://doi.org/10.3389/fclim.2021.720312
7. Irvine PJ, Keith DW. (2020). Halving warming with stratospheric aerosol geoengineering moderates policy-relevant climate hazards. Environmental Research Letters. 15(4):044011. http://doi.org/10.1088/1748-9326/ab76de

Public opposition to SRM is not stronger in the Global South than in the Global North.

Researchers have surveyed public opinion on SRM and compared findings across regions. While most people globally are unfamiliar with SRM, studies consistently find that the public in the Global South tend to be modestly more open to it than those in the Global North.^1–3

Key factors that could explain this difference: the Global South is generally younger and more exposed to climate harms.^1,4 Younger people will likely suffer more harms from climate change and tend to be more open to new technologies.⁵

The surveys also found differences in concerns about SRM. Respondents in the Global South were more concerned with power imbalances and SRM’s potential to undermine emissions cuts, while those in the Global North emphasised unintended consequences and harms to humans and nature.¹

Public opinion on SRM is likely to shift as the field develops and emerges more clearly into public consciousness, and will depend heavily on how it’s framed in media coverage.⁶

Endnotes:

1. Baum CM, Fritz L, Low S, Sovacool BK. (2024). Public perceptions and support of climate intervention technologies across the Global North and Global South. Nature Communications. 15(1):2060. https://doi.org/10.1038/s41467-024-46341-5
2. Contzen N, Perlaviciute G, Steg L, et al. (2024). Public opinion about solar radiation management: A cross-cultural study in 20 countries around the world. Climatic Change. 177(4):65. https://doi.org/10.1007/s10584-024-03708-3
3. Sugiyama M, Asayama S, Kosugi T. (2020). The north–south divide on public perceptions of stratospheric aerosol geoengineering?: a survey in six Asia-Pacific countries. Environmental Communication. 14(5):641-56. https://doi.org/10.1080/17524032.2019.1699137
4. Fritz L, Baum CM, Brutschin E, et al. (2024). Climate beliefs, climate technologies and transformation pathways: Contextualizing public perceptions in 22 countries. Global Environmental Change. 87:102880. https://doi.org/10.1016/j.gloenvcha.2024.102880
5. Sovacool BK, Evensen D, Baum CM, et al. (2024). Demographics shape public preferences for carbon dioxide removal and solar geoengineering interventions across 30 countries. Communications Earth & Environment. 5(1):642. https://doi.org/10.1038/s43247-024-01800-1
6. Bolsen T, Palm R, Luke RE. (2023). Public response to solar geoengineering: how media frames about stratospheric aerosol injection affect opinions. Climatic Change. 176(8):112. https://doi.org/10.1007/s10584-023-03575-4

SRM is poorly suited to being used as a weapon.

It can’t be precisely targeted to cause predictable harm in a certain region. However, it could shift climate conditions in ways that affect many countries, potentially raising geopolitical tensions.

A weapon is something designed or used for inflicting damage – usually physical. While SRM could cause harm, its effects would be so uncertain and imprecise that it would be a poor choice for inflicting harm on a specific target.

SRM could potentially help reduce many of the impacts of climate change, but it might also be used in more selfish or disruptive ways.¹ It wouldn’t allow for influence over specific weather events – instead, it would change long-term climate outcomes in ways that would affect many or all countries.²

Stratospheric aerosol injection, the most researched SRM proposal, is inherently global – its effects can’t be confined to a certain country.³ Regional SRM approaches, like marine cloud brightening, could offer more influence over local impacts, but they would still have indirect, remote effects across the planet and are currently poorly understood.⁴

Compared to existing weaponry, SRM lacks many of the crucial qualities that would make it a useful weapon. Nevertheless, it could potentially be deployed to advance the interests of some over others or in a deliberately disruptive way, which could raise geopolitical tensions.¹

Endnotes:

1. Corry O, McLaren D, Kornbech N. (2024). Scientific models versus power politics: How security expertise reframes solar geoengineering. Review of International Studies. 1-20. https://doi.org/10.1017/S0260210524000482
2. The Royal Society. (2025). Solar Radiation Modification Policy Briefing. https://royalsociety.org/news-resources/projects/solar-radiation-modification/
3. Lee WR, MacMartin DG, Visioni D, et al. (2021). High‐latitude stratospheric aerosol injection to preserve the Arctic. Earth’s Future.11(1):e2022EF003052. https://doi.org/10.1029/2022EF003052
4. Wan JS, Chen CC, Tilmes S, et al. (2024). Diminished efficacy of regional marine cloud brightening in a warmer world. Nature Climate Change. 14(8):808-14. https://doi.org/10.1038/s41558-024-02046-7