Solar radiation modification

• 1. Introduction

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  Solar radiation modification sits at one of the most contested frontiers in climate science. The basic premise is arresting in its simplicity: if Earth reflects just a little more sunlight back into space, the planet cools. But the questions that follow from that premise are anything but simple. Who decides? Who gets to run the experiments? And what happens if the intervention suddenly stops?

  The idea is not new. In 1965, the President's Science Advisory Committee delivered a report to Lyndon B. Johnson's administration warning of the dangers of carbon dioxide from fossil fuels. Even then, the document raised the possibility of deliberately raising Earth's reflectivity. Nine years later, in 1974, Russian climatologist Mikhail Budyko proposed that aircraft burning sulfur could generate reflective aerosols in the stratosphere, cooling the planet if warming ever became a serious threat.

  For decades the concept stayed on the margins. Then, in 2006, Nobel Laureate Paul Crutzen published an influential paper arguing that, given how little progress had been made on emissions reductions, research into SRM's feasibility and environmental consequences deserved serious attention. That paper pulled the topic into mainstream scientific debate, and it has not left since.

  Today, SRM research is funded by governments and billionaires alike, opposed by Indigenous groups and Republican state legislatures, governed by almost no formal law, and studied under a thickening cloud of urgency as warming continues. The story of how a physics concept became a geopolitical flashpoint is what this documentary explores.

• 2. The Physics of Reflection

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  A 1% increase in Earth's planetary albedo, its overall reflectivity, could reduce radiative forcing by 2.35 W/m2. A 2% increase could counteract the warming effect of a full doubling of atmospheric carbon dioxide. Those numbers explain why SRM attracts attention: the lever is physically powerful.

  The most studied method is stratospheric aerosol injection, or SAI. Small reflective particles would be lofted into the upper atmosphere, mimicking the cooling that follows large volcanic eruptions. Sulfates are the most commonly proposed aerosol because they appear naturally in volcanic events. Calcium carbonate and titanium dioxide have also been discussed as alternatives. Delivery by custom-designed aircraft is generally considered the most feasible approach, though artillery and balloons have occasionally been suggested. At its theoretical maximum, SAI could produce up to 8 W/m2 of negative radiative forcing.

  The IPCC concluded in 2021 that SAI is the most-researched SRM method, with high agreement in the literature that it could limit warming to below 1.5 degrees Celsius. But the comparison to volcanoes only goes so far. A volcanic eruption releases material in a single pulse; SAI would require sustained injection over years or decades. That distinction matters when projecting long-term atmospheric effects.

  Other proposed methods work on different principles. Marine cloud brightening would increase the reflectivity of clouds over the oceans. Cirrus cloud thinning would seed high-altitude cirrus clouds to reduce their optical thickness, allowing more heat to escape into space rather than bouncing back. The catch with cirrus cloud thinning is that some studies suggest it could cause net warming, not cooling, due to complex cloud-aerosol interactions.

  Space-based proposals, including mirrors at the L1 Lagrange point between Earth and the Sun, appear in historical assessments. The Royal Society in 2009 evaluated them and concluded that while such methods might be viable far in the future, costs and deployment challenges make them infeasible for near-term use. The most recent IPCC report, published in 2021, did not assess them at all.

• 3. What the Models Show

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  Climate models have consistently concluded that moderate SRM use would significantly reduce many impacts of global warming, including changes to extreme temperatures, extreme precipitation, Arctic ice, cyclone intensity and frequency, and the Atlantic Meridional Overturning Circulation. Of all known methods, SRM is the only one capable of lowering global temperatures within months of deployment.

  The IPCC Sixth Assessment Report states that SRM could offset some effects of increasing greenhouse gases on global and regional climate, including the carbon and water cycles. However, the same report notes substantial residual or overcompensating climate change at regional scales and seasonal timescales, and acknowledges that large uncertainties associated with aerosol-cloud-radiation interactions persist.

  The regional mismatch is a recurring theme. Greenhouse gases warm throughout the globe and year, whereas SRM reflects light more effectively at low latitudes and in the hemispheric summer due to the angle at which sunlight strikes the atmosphere, and only during daytime. The result is that no deployment regime can perfectly reverse the climate change already baked in by elevated carbon dioxide.

  Precipitation adds another layer of complexity. Models indicate SRM would reverse warming-induced precipitation changes more effectively than it reverses temperature changes. Using SRM to fully return global mean temperature to a preindustrial level would therefore overcorrect for precipitation. But soil moisture, which matters more directly to plant life than average annual rainfall, responds differently. Because SRM reduces evaporation, it more precisely compensates for soil moisture changes than for raw precipitation figures.

  Some studies suggest SRM could prevent coral bleaching events by reducing sea surface temperatures. Agricultural effects are mixed: the CO2 fertilization effect that enhances plant growth under high carbon dioxide would continue, but reduced overall sunlight could slightly decrease productivity for some crops. The ratio between direct and diffuse solar radiation would also shift, with implications for photosynthesis and solar energy generation.

• 4. The Risks Left Open

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  The 2023 UNEP report concluded that while an operational SRM deployment could reduce some climate hazards, it would also introduce new risks to ecosystems and human societies. Ecosystem impacts, the report found, are not yet well understood.

  SAI poses a specific threat to stratospheric ozone. Sulfates, the most commonly proposed aerosol, would delay the current recovery of stratospheric ozone, which protects organisms from harmful ultraviolet radiation. The Montreal Protocol and the Vienna Convention for the Protection of the Ozone Layer, which obligate countries to cooperate to protect human health against activities that modify the ozone layer, are therefore directly relevant to how SAI would be governed.

  A 2022 assessment by the World Meteorological Organization confirmed that SAI has the potential to limit the rise in global surface temperatures but comes with significant risks and can cause unintended consequences. Tropical monsoon intensity, which is increased by climate change, would generally be decreased by SAI. The net effect on agriculture and ecosystems in monsoon-dependent regions would depend heavily on how any deployment was designed and at what scale.

  SRM would not reduce atmospheric carbon dioxide concentrations. This means ocean acidification, driven by oceans absorbing excess CO2, would continue regardless of how much sunlight was reflected away. The IPCC is explicit on this point, stating that SRM cannot be the main policy response to climate change and is, at best, a supplement to achieving net zero or net negative emissions.

  The termination risk concentrates minds most sharply. If SRM were masking significant warming and then abruptly ceased, the climate would rapidly warm toward the level it would have reached without any intervention. A sudden halt in a world of high greenhouse-gas concentrations would trigger rapid global temperature rise, intensified precipitation changes, sea level rise, land drying, weakened carbon sinks, and accelerated CO2 accumulation. The IPCC notes that a gradual phase-out combined with mitigation would reduce these impacts, but any large-scale deployment would likely require a commitment lasting from multiple decades to a century.

• 5. Governing the Ungovernable

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  No comprehensive global framework exists to regulate SRM research or deployment. That gap is not for lack of relevant law; it is because no existing treaty was written with SRM in mind.

  The Environmental Modification Convention, known as ENMOD, is the only international treaty that directly regulates deliberate manipulation of natural processes with widespread, long-lasting, or severe transboundary effects. SRM falls within ENMOD's definitions and is therefore subject to its prohibition on military or hostile use. At the same time, the treaty explicitly states that it shall not hinder the use of environmental modification techniques for peaceful purposes.

  The UN Convention on the Law of the Sea could support SRM research through its provisions permitting legitimate scientific activities, but UNCLOS could also constrain large-scale outdoor activities that risk harming marine ecosystems. Because SRM does not address ocean acidification, its alignment with UNCLOS environmental protection objectives is uncertain.

  The Convention on Biological Diversity has made several decisions on climate-related geoengineering. A 2010 decision established a non-binding normative framework requiring that such activities be scientifically justified, environmentally assessed, and subject to regulatory oversight. A 2016 decision called for more transdisciplinary research and sharing of knowledge.

  The Oxford Principles, developed by academics rather than governments, remain the most prominent non-governmental framework. They call for SRM to be regulated as a public good, with public participation in decision making, open publication of results, independent impact assessments, and governance before deployment.

  The governance vacuum has practical consequences. SAI's relatively low direct costs and technical feasibility raise the real possibility of unilateral deployment: one country or even a well-funded non-state actor acting alone. The startup Make Sunsets released balloons containing helium and sulfur dioxide in Mexico, prompting the Mexican government to announce in 2023 that it would prohibit SRM experiments within its borders. In April 2025, the U.S. Environmental Protection Agency demanded information from the firm about its sulfur dioxide releases.

• 6. Who Is Paying

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  Through 2024, roughly 200 million US dollars had been spent on SRM research globally, with the annual rate rising to more than 30 million dollars in recent years. As of May 2025, an additional 164 million dollars had been committed for the 2025-2029 period.

  Governments account for 42% of research funding. Countries that have contributed include the United States, the United Kingdom, Australia, Argentina, Germany, China, Finland, Norway, and Japan, as well as the European Union. The US National Oceanic and Atmospheric Administration spent 22 million dollars from 2019 to 2022 and, as of 2024, provides about 11 million dollars per year through its solar geoengineering research program.

  The UK has moved most aggressively among governments. In 2025, the UK government invested more than 60 million pounds in SRM research, including outdoor geoengineering experiments. The Advanced Research and Invention Agency announced in late 2024 that 57 million pounds, roughly 75 million US dollars, would be made available for projects exploring climate cooling, including outdoor experiments. Combined with a separate 10 million pound program from the Natural Environment Research Council, the UK has positioned itself as one of the largest funders of geoengineering research in the world.

  Philanthropy accounts for 48% of research funding. The Simons Foundation, Quadrature Climate Foundation, and Open Philanthropy are the largest donors. The Quadrature Climate Foundation plans to provide 40 million dollars for work in this field over roughly three years as of 2024. Among individual donors, Bloomberg News reported that Mike Schroepfer, Sam Altman, Matt Cohler, Rachel Pritzker, Bill Gates, and Dustin Moskovitz have all been identified as notable supporters of geoengineering research.

  Opposition has its own organized structure. The ETC Group, the Heinrich Böll Foundation, the Center for International Environmental Law, and Climate Action Network all oppose outdoor experiments or SRM deployment. A coalition proposing an international non-use agreement had gathered endorsements from nearly 540 academics and 60 advocacy organizations as of December 2024. The campaign's launch article was later flagged by publisher Wiley after it emerged that the journal's editor-in-chief, Mike Hulme, had co-authored an earlier version of the same article, a conflict of interest that led to Hulme's resignation during the publisher's investigation.

• 7. The Political Divide

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  In 2021, researchers at Harvard University paused plans for a small-scale SRM field experiment in Sweden after the Saami Council, an Indigenous advocacy group, objected to a planned test flight over their ancestral land. The flight itself would not have released any material, but the Saami Council criticized the lack of consultation and expressed broader concerns about the ethics and risks of SRM. The pause illustrated how a governance vacuum at the international level plays out in practice: even small, benign experiments can become contested ground.

  Since 2024, lawmakers in at least 28 US states have introduced or supported bills to prohibit SRM or related practices. These efforts are heavily shaped by the chemtrails conspiracy theory, which has nothing to do with stratospheric aerosol injection but has been politically conflated with it. In 2024, Tennessee enacted such a bill, approved along party lines and signed by Governor Bill Lee. Florida followed with a similar law signed by Governor Ron DeSantis. Congresswoman Marjorie Taylor Greene announced plans to introduce a federal bill that would make outdoor SRM or weather modification activities a felony.

  Robert F. Kennedy Jr., serving as Secretary of Health and Human Services in the Trump administration, posted on the platform X in support of the state-level bans, framing them as opposition to dousing citizens with toxins. EPA Administrator Lee Zeldin, when the agency took action against Make Sunsets, was quoted describing its activities as climate extremism that had overtaken common sense.

  In the meantime, some developing-world populations show greater openness to SRM than European counterparts. Surveys consistently find that southern European and Global South populations are more accepting, particularly where climate impacts are perceived as more immediate. The Degrees Initiative, a UK-registered charity, works specifically to build capacity in developing countries to evaluate SRM, though a researcher from Geoengineering Monitor has argued that the initiative imposes its research agenda onto the Global South and is predominantly funded by foundations run by technology and finance billionaires based in the Global North.

  In 2025, James Hansen and co-authors published a call for research on purposeful global cooling, citing recommendations from the US National Academy of Sciences. The World Climate Research Programme launched a Lighthouse Activity on Research on Climate Intervention in 2024, covering both large-scale carbon dioxide removal and SRM. As of 2025, the federal US government has no formal policy on SRM, even as researchers inside and outside its borders push the science forward.