Earthward: Solar Geoengineering

This week, executives of the European Union released a Joint Communication to the European Parliament, offering a new outlook on the connection between climate change and European security. Although the plan is very broad, covering a wide range of implications arising from global warming, the attention grabber is the EU’s decision to join international discussions on solar geoengineering. The Joint Communication states that “…the EU will support international efforts to assess comprehensively the risks and uncertainties of climate interventions, including solar radiation modification, and promote discussions on a potential international framework for its governance, including research related aspects.” In taking this decision the EU follows the lead of the US and the United Nations Environment Program, both already committed to taking a hard look at controversial planetary-scale approaches to cooling Earth by artificially altering its energy balance with Space.

Solar geoengineering is fundamentally different from other technologies (like atmospheric carbon removal) that are often grouped under the geoengineering banner, because it is really just a way of adapting to global warming without addressing the root cause of the problem. In the most widely considered approach, fleets of custom designed aircraft carrying sulfur aerosol payloads would be continuously flown through the lower stratosphere at 5-12 miles altitude, releasing thousands of tons of sulfur particles that reflect sunlight. The Earth’s surface and atmosphere presently reflect about 30% of the solar radiation that reaches the planet, and this project, if implemented at scale, might increase that reflectivity by 1% or so. The sulfur particles settle out after a year or two; hence the need for ongoing release. Researchers investigating this approach by computer modeling (there have been no sanctioned experimental releases yet) point to the 1991 eruption of Mount Pinatubo in the Philippines as a model for this concept. That natural event generated worldwide sulfur dioxide emissions that, by the same mechanism, produced global planetary cooling of about 1 degree Fahrenheit lasting several years – a bit smaller but roughly on the same scale as the target temperature decrease envisioned from artificial sulfur injection (anthropogenic warming has so far raised Earth’s average surface temperature by 2°F compared to the preindustrial era).

Reading the literature on this subject, I think it a fair inference that many climate scientists investigating sulfur aerosol injection feel that the data provided by Pinatubo and other eruptions offers some reasonable assurance that the risks of the approach are manageable with ongoing monitoring and an effective governance regime. These researchers point to the evident benefits associated with curtailing heat waves, storm intensities and sea level rise. However, such confidence is far from unanimous and is certainly not shared in the academic community at large. About 60 climate science and governance scholars are circulating a non-use agreement that calls for a halt to development and potential use of these technologies, including prohibitions on public funding, patents and (of course) any deployment. The group points to many possible hazards that are poorly understood, including altered rainfall patterns, potential effects on plant photosynthesis and crop yields, and depletion of stratospheric ozone. And despite decades of research, few climate scientists would disagree that the effects of clouds and aerosols remain among the largest unknowns in climate models.

The debate about solar geoengineering is not confined to rarified academic settings, nor is it limited to science. Our international treaties seeking to limit global warming still leave us at high risk of climate chaos, a consequence of the large credibility gap between the “net zero” pledges of many countries under the UN Paris Agreement and the actual policies so far enacted to phase out fossil fuels. Given this, many are highly dubious that international governance of solar geoengineering could possibly be effective. Concern is heightened because any sudden halt to sulfur injection arising from (say) international conflict would create a severe termination shock consisting of extremely rapid planetary heating as the artificial aerosol shield dissipates.

Sulfur aerosol injection is scarily cheap, with estimated price tags in the range of billions to tens of billions of dollars per year, and no fundamentally new technical barriers are expected in its deployment. The relatively low costs are within the means of medium sized countries that may well decide, under domestic pressure, to enact their own programs irrespective of what others do. And many progressive climate activists point to the moral hazard associated with the deployment of a technology that may reduce the urgency for fundamental transformation of the energy system without addressing the root cause of warming. Progressives are also concerned that deployment will be controlled by countries of the global North, with further marginalization of the world’s most vulnerable peoples. For some, the prospect of solar geoengineering even belongs in the same morally impermissible category as research on biological warfare agents, human cloning and nuclear weapons development – utterly beyond the pale.

It is early days for solar geoengineering, and the shape of any international governance regime that may emerge is undefined. It seems likely that the contours of the debate around sulfur aerosol injection and similar technologies will be significantly influenced by global progress towards a clean energy economy. If decarbonization accelerates in the next decade and the 2°C target begins to look more plausible, it is easier to imagine a consensus forming around reasonable use of sulfur injection to alleviate the worst impacts of global heating during the transition. This would also buy time for eventual deployment of more expensive and energy-intensive industrial-scale carbon dioxide removal methods to rebalance the climate system. But more limited progress toward overall climate goals and fracturing of countries into high and low emitters would likely produce a much more volatile situation in which guidance emerging from international negotiations may carry little force.

The EU’s action adds to an ever clearer sense that, for better or worse, the solar geoengineering train is leaving the station. In the US, prestigious universities have begun awarding doctorates in the field, the Biden administration has released a governance framework to provide guidance for publicly funded work, and Senators of both political parties are making favorable noises about directed research funding. At minimum, the recent US and international actions clearly signal an elite consensus in favor of thoroughly exploring the risks and benefits of solar geoengineering technologies.


Solar Engineering Non-Use Agreement Initiative, Solar Engineering Myths Debunked. Briefing Note #1, January 2023.

United Nations Environment Program (2023). One Atmosphere: An independent expert review on solar radiation modification research and deployment. Kenya, Nairobi.

National Academies of Science, Engineering and Medicine 2021. Reflecting Sunlight: Recommendations for Solar Engineering Research and Research Governance, Washington DC. The National Academies Press.

Joeri Rogelj et al., Credibility gap in net-zero climate targets leaves world at high risk. Science, 380, 1014 (9 June 2023),

Kim Stanley Robinson, The Ministry for the Future. Orbit, 2020.

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Earthward is written by Dr. John Perona and is an outgrowth of the climate education work begun with From Knowledge to Power: The Comprehensive Handbook for Climate Science and Advocacy (K2P).