Solar Geoengineering Explained: What Harvard’s SCoPEx Was, Why It Ended, and Where Research Stands in 2026

A decade ago, headlines announced that Harvard researchers, backed in part by Bill Gates, were preparing to “block out the sun.” The project, called the Stratospheric Controlled Perturbation Experiment, or SCoPEx, proposed a small balloon experiment to study how tiny particles released high in the atmosphere might reflect sunlight, a concept known as solar geoengineering. The coverage fueled years of public alarm, protests, and online conspiracy theories.

SCoPEx never released anything into the stratosphere. After repeated delays and opposition, Harvard formally ended the project in March 2024. But the broader debate it symbolized has only grown. The United Kingdom is now funding small outdoor experiments, U.S. states have passed laws banning geoengineering, a startup has sold “cooling credits” for sulfur-filled balloons, and the U.S. Environmental Protection Agency has launched a public information page on the subject. This article explains what solar geoengineering is, what SCoPEx actually planned, why it ended, where research and regulation stand in 2026, and what the science says about risks and claims.

What Solar Geoengineering Is

Solar geoengineering, also called solar radiation modification (SRM) or sunlight reflection methods, covers proposals to cool the planet by reflecting a small percentage of incoming sunlight back to space. It is distinct from carbon dioxide removal, which aims to take greenhouse gases out of the atmosphere.

The idea draws on a natural analogy. When large volcanoes erupt, they can inject sulfur dioxide into the stratosphere, where it forms tiny sulfate particles that reflect sunlight. According to the U.S. Geological Survey, Mount Pinatubo’s 1991 eruptions in the Philippines injected nearly 20 million tons of sulfur dioxide into the stratosphere, and the resulting haze caused global temperatures to drop temporarily, from 1991 through 1993, by about 1°F (0.5°C).

The main proposed approaches include:

Method	How it would work	Scale of effect	Key concerns
Stratospheric aerosol injection (SAI)	Release reflective particles, such as sulfates or calcium carbonate, high in the stratosphere	Potentially global	Ozone effects, regional rainfall changes, termination shock, governance
Marine cloud brightening (MCB)	Spray fine sea-salt droplets to make low marine clouds more reflective	Regional	Uncertain effectiveness, effects on weather downwind
Cirrus cloud thinning	Thin high ice clouds that trap heat	Regional to global	Very uncertain physics
Surface brightening	Reflective roofs, surfaces, or ice-preserving techniques	Local	Limited overall impact; generally low risk

All of these approaches share a fundamental limitation: they would mask warming without removing its cause. Carbon dioxide would keep accumulating unless emissions fell, and ocean acidification would continue. If a large-scale program were stopped suddenly, warming that had been masked could return rapidly, a risk researchers call termination shock.

What SCoPEx Actually Planned

SCoPEx was first proposed in a research paper in 2014 by Harvard scientists including David Keith and Frank Keutsch. As MIT Technology Review described it, the plan was to launch a high-altitude balloon, equipped with propellers and sensors, that could release a small amount of material, a few kilograms of calcium carbonate, sulfuric acid, or other substances, into the stratosphere. The balloon would then fly back through the plume to measure how the particles dispersed, how they interacted with the surrounding air, and how much sunlight they reflected.

The quantities involved were tiny compared with a volcano or with any real-world deployment. The researchers argued that the experiment was about basic atmospheric chemistry and physics that computer models could not resolve, and that understanding the risks of solar geoengineering required careful experiments before anyone considered using it.

The project was funded through Harvard’s Solar Geoengineering Research Program. A briefing by the ETC Group’s Geoengineering Monitor, which opposed the project, listed funders including the William and Flora Hewlett Foundation, the Alfred P. Sloan Foundation, the Open Philanthropy Project, and several individual donors, among them Bill Gates. The “Bill Gates-funded” headline was therefore partly accurate but incomplete: Gates was one of a number of funders of a broader research program, not the sole backer of an effort to dim the sun.

Why SCoPEx Ended

From the start, SCoPEx became a proxy for a larger fight over whether solar geoengineering should be researched at all. Opponents, including environmental groups and Indigenous organizations, argued that even small experiments would normalize the idea, build momentum toward deployment, and distract from cutting emissions. They also questioned who should have the authority to experiment with the shared atmosphere.

The project’s most concrete step came in 2021, when the team planned a test flight of its balloon hardware, without any particle release, from the Esrange Space Center near Kiruna, in northern Sweden. The Saami Council, representing the Indigenous Saami people, together with Swedish environmental groups, objected. An independent advisory committee Harvard had set up recommended postponing the flight, and it did not proceed.

On March 18, 2024, Harvard announced that Keutsch was “no longer pursuing the experiment.” Scientific American, republishing E&E News reporting, noted that the project failed to conduct field tests because of opposition from environmentalists and Indigenous residents. The balloon platform was to be repurposed for unrelated stratospheric research. Keith, who had moved to the University of Chicago, told MIT Technology Review that the experiment had become a proxy for the whole debate over solar geoengineering research, and that given the weight it had come to carry, it no longer made sense to move forward.

After SCoPEx: Who Is Doing What in 2025–2026

A startup’s balloons, and a national ban

While Harvard’s carefully reviewed experiment stalled, a small startup went ahead on its own. Climate Home News reported that in 2022 the U.S. company Make Sunsets launched balloons carrying sulfur dioxide from two sites in Baja California, Mexico, without monitoring or recovering them, and began selling “cooling credits” for future releases, with each credit corresponding to about one gram of material. The Mexican government said the launches took place without notice or consent and announced plans to prohibit solar geoengineering experiments in the country.

The company later launched balloons in the United States. EPA’s geoengineering FAQ states that, as of July 2025, the agency was aware of only one private actor, Make Sunsets, that had actively deployed stratospheric aerosol injection or marine cloud brightening in the United States, and that the amount of material intentionally released to date was very small relative to natural events such as volcanic eruptions. In 2025, EPA Administrator Lee Zeldin sent the company a formal information request under the Clean Air Act.

A halted cloud-brightening test

In 2024, University of Washington researchers, working with nonprofit partners, began a small marine cloud brightening experiment spraying saltwater aerosols from the deck of the decommissioned aircraft carrier USS Hornet in Alameda, California. The city halted the test after officials said they had not been adequately informed. EPA’s FAQ notes that it later learned the canceled experiment may have been intended as a precursor to a larger-scale experiment that had not been disclosed to the public.

The United Kingdom funds outdoor experiments

The most significant government move has come from the United Kingdom. Its Advanced Research and Invention Agency (ARIA) launched a program to explore climate cooling approaches with about £57 million in funding, making it the largest single funder of solar geoengineering research to date, according to the research platform SRM360. A Carbon Brief fact check reported that around £24.5 million is earmarked for controlled, small-scale outdoor experiments, including attempts to thicken Arctic sea ice, brighten clouds over Australia’s Great Barrier Reef, and fly weather balloons carrying natural mineral particles into the stratosphere that would be retrieved after hours or weeks. An independent oversight committee chaired by climate scientist Piers Forster reviews the outdoor work and has stated that it does not exist to legitimize the program.

U.S. states move to ban geoengineering

In the United States, the political momentum has run the other way. According to EPA’s FAQ, Tennessee passed a law in 2024 banning the intentional release of substances into the atmosphere to affect temperature, weather, or sunlight, and Florida passed a law in June 2025 prohibiting acts intended to affect the temperature, weather, or intensity of sunlight in the state, covering both solar geoengineering and traditional weather modification such as cloud seeding. Bills have been introduced in many other states, including Texas, Pennsylvania, North Carolina, and Vermont. Some of this legislative activity has been driven by long-running conspiracy theories about secret spraying, a point discussed below.

International governance

There is no binding international treaty governing solar geoengineering. Parties to the UN Convention on Biological Diversity agreed in 2010 on a de facto moratorium on geoengineering activities that could affect biodiversity, with exceptions for small-scale scientific research in controlled settings. A panel of scientific advisors to the European Union published policy recommendations in December 2024, as EPA’s FAQ notes. Proposals range from an international non-use agreement to coordinated research programs with strong governance.

How Scientists Study It Without Outdoor Tests

Most solar geoengineering research happens in computers and laboratories. Climate modelers have run coordinated experiments for more than a decade through the Geoengineering Model Intercomparison Project, in which research groups around the world simulate the same scenarios with different climate models, allowing them to compare results and identify robust findings and uncertainties. Laboratory work studies how candidate particles behave and react chemically under stratospheric conditions.

Scientists also learn from natural and accidental experiments. Large volcanic eruptions such as Pinatubo provide data on how stratospheric particles spread, how long they last, and how they affect temperature, rainfall, and ozone. Ship tracks, bright lines of cloud that form where ship exhaust particles seed marine clouds, have been used to study cloud brightening. When the International Maritime Organization sharply limited the sulfur content of shipping fuel starting in 2020, reducing sulfur pollution over the oceans, researchers began studying the change as an unplanned experiment in how fewer particles affect clouds and regional warming.

These methods have limits, which is why some scientists argue that small outdoor experiments are needed. Models may not capture small-scale processes, and volcanoes and ships are imperfect analogs for deliberate, controlled releases.

How It Compares With Other Climate Responses

Approach	What it does	Speed	Addresses root cause?
Cutting emissions	Stops adding greenhouse gases	Slow effect on temperature	Yes
Carbon dioxide removal	Takes CO₂ out of the air (forests, soils, engineered capture)	Slow; limited scale today	Yes
Adaptation	Reduces harm from climate impacts (sea walls, heat plans, drought-tolerant crops)	Local, ongoing	No
Solar geoengineering	Reflects sunlight to mask warming	Potentially fast	No

Scientific assessments consistently treat emissions reductions as essential. Carbon removal is seen as a complement for hard-to-eliminate emissions. Solar geoengineering, if ever used, would at most be a temporary measure to reduce peak warming while emissions fall, not an alternative.

The Governance Problem

One reason solar geoengineering worries policy experts is that it could be relatively cheap compared with the costs of climate change and with cutting emissions. Studies have estimated that a stratospheric program could cost billions of dollars per year, within reach of a single wealthy country or even a very wealthy private actor. Economists have described this as a “free driver” problem, the opposite of the free-rider problem that plagues emissions cuts: instead of everyone waiting for others to act, one actor could act alone, with consequences for everyone.

That possibility raises hard questions. Who would decide how much cooling is appropriate? How would countries harmed by changes in rainfall be compensated? What would prevent a program from being started or stopped for political reasons? No international institution currently has the authority to answer these questions, which is why many experts argue that governance discussions should come before, not after, any move toward larger experiments.

Public Engagement and Indigenous Rights

The SCoPEx experience showed that public consent is not a side issue. The Saami Council argued that an experiment associated with changing the global climate should not take place on Saami land without their consent, and many Indigenous and civil society groups have called for their voices to be central in decisions about solar geoengineering. The ARIA program’s independent oversight committee, public disclosure of experiment plans, and environmental reviews represent attempts to address these concerns, though critics argue they fall short without international agreement.

Key Terms

Solar radiation modification (SRM): Deliberate efforts to reflect sunlight to cool the planet.

Stratospheric aerosol injection (SAI): Adding reflective particles to the stratosphere, roughly 10 to 50 kilometers above Earth.

Marine cloud brightening (MCB): Spraying sea-salt particles to make low ocean clouds more reflective.

Termination shock: Rapid warming that could follow an abrupt end to a large solar geoengineering program.

Carbon dioxide removal (CDR): Methods that remove CO₂ from the atmosphere and store it.

Contrail: A condensation trail of ice crystals formed from aircraft engine exhaust.

What the National Academies Recommended

In 2021, the U.S. National Academies of Sciences, Engineering, and Medicine published Reflecting Sunlight, a major report on solar geoengineering research and governance. The committee concluded that solar geoengineering is not a substitute for reducing greenhouse gas emissions, but that the stakes of climate change justify a carefully governed research program to understand its feasibility, risks, and impacts. It recommended that the United States establish a transdisciplinary research program, coordinated with other countries and with strong public engagement and governance, including transparency and independent oversight for any outdoor experiments.

That recommendation remains controversial. Some scientists and advocacy groups argue that any research program risks creating momentum toward deployment and should be avoided in favor of an international non-use agreement. Others argue that the world needs to understand these options before a crisis tempts some country or actor to try them without adequate knowledge.

The Main Risks Scientists Worry About

Regional climate disruption. Reflecting sunlight cools the planet unevenly and can alter rainfall patterns, including monsoons that billions of people depend on. Models suggest some regions could face drier conditions even as global average temperatures fall.

Ozone damage. Sulfate particles in the stratosphere can accelerate chemical reactions that deplete the ozone layer, as happened temporarily after Pinatubo. Alternative materials such as calcium carbonate have been proposed partly for this reason, but their behavior is less understood.

Termination shock. If a large-scale program masked substantial warming and then stopped abruptly, temperatures could rise rapidly, giving ecosystems and societies little time to adapt.

Moral hazard. Critics fear that the prospect of a technological fix would weaken political will to cut emissions.

Governance and conflict. Because stratospheric aerosols affect the whole planet, a decision by one country or a small group to deploy them could provoke international conflict, especially if blamed for droughts or floods elsewhere.

Unequal consent. Communities most exposed to climate impacts, and Indigenous peoples such as the Saami who objected to the Swedish test, argue they should have a decisive voice in decisions that affect their environment.

At the scale of small experiments like SCoPEx or ARIA’s projects, the direct physical risks are minimal; the concerns are mainly about precedent, governance, and trust.

Contrails, “Chemtrails,” and the Evidence

Discussion of solar geoengineering often becomes entangled with the long-running “chemtrails” theory, which holds that the lines of cloud left behind aircraft are secret chemical spraying programs. Those lines are contrails, short for condensation trails, which form when hot, humid engine exhaust mixes with cold air at altitude and the water vapor freezes into ice crystals. Whether contrails dissipate quickly or spread into cirrus-like clouds depends on humidity and temperature at flight altitude.

In a 2016 study in Environmental Research Letters, researchers surveyed 77 leading atmospheric chemists and geochemists about evidence commonly presented for secret spraying. Seventy-six of the 77 said they had found no evidence of a secret large-scale atmospheric spraying program, and they attributed the data offered as evidence to other factors, such as normal contrail formation and poor sampling methods. EPA’s own geoengineering FAQ, cited above, reports that intentional releases to date have been very small.

Distinguishing the real debate from conspiracy theories matters. Solar geoengineering is a genuine, openly discussed research and policy issue with serious questions about risk and governance. Claims that it is already being secretly deployed on a massive scale through airline flights are not supported by evidence.

Arguments For and Against Research

The case for careful research: Climate change is already causing severe harm, and emissions cuts may not happen fast enough to avoid dangerous warming. Supporters argue that decision-makers need reliable information about solar geoengineering’s effects before a crisis, and that transparent, publicly funded research with independent oversight is safer than leaving the field to private actors or unilateral governments.

The case against: Opponents argue that research creates institutional momentum toward deployment, that the technology is inherently ungovernable at a global scale, that it could distract from emissions cuts, and that the risks to vulnerable regions are too great. Many call for an international agreement not to use solar geoengineering.

Between these positions, many researchers support indoor and modeling research while opposing outdoor experiments without international governance, and many governments have yet to take formal positions.

What Solar Geoengineering Would Not Fix

Even in the most optimistic scenarios, reflecting sunlight would leave major climate problems unaddressed. Carbon dioxide would continue to dissolve into the oceans, making them more acidic and harming corals, shellfish, and marine ecosystems. Rising carbon dioxide would continue to affect plant growth and ecosystems in ways unrelated to temperature. Reflecting sunlight also does not perfectly cancel greenhouse warming, which traps heat day and night and across seasons, so some patterns of change would remain. Sulfate-based approaches would add particles that eventually fall to the surface, though in quantities small relative to current air pollution.

These limits are why nearly every scientific assessment describes solar geoengineering as, at most, a possible supplement to emissions reductions and carbon removal, not a substitute.

Why the Headlines Got It Wrong

Coverage of SCoPEx frequently described it as a plan to “block out the sun” or “dim the sun,” phrases that suggested a single experiment could darken the sky. In reality, the planned release of a few kilograms of material would have had no measurable effect on sunlight at the surface, and the project’s stated aim was to study risks. Framing a small science experiment as a global intervention helped fuel fears and conspiracy theories, while also making it harder for the public to engage with the real policy questions.

At the same time, critics of SCoPEx made a fair point that headlines often missed: the concern was less about that specific balloon than about what it represented, the first step toward normalizing outdoor experimentation with technologies that could affect everyone. Understanding both points is necessary for an informed opinion.

How to Follow the Debate

For readers who want reliable information, government and scientific sources such as the EPA’s geoengineering pages, the National Academies’ reports, and summaries from established science journalism outlets provide grounded coverage. Public consultations on research programs and proposed state laws offer opportunities to comment. Watching for a few warning signs helps separate evidence from speculation: claims of secret global programs without verifiable data, confusion between contrails and deliberate spraying, and headlines that describe small experiments as planetary interventions.

What Could Happen Next

Over the next several years, the most likely developments are incremental: results from the UK’s small outdoor experiments and the oversight committee’s public reviews; further state-level legislation in the United States, both bans and narrower rules; continued modeling studies of regional effects; and international discussions about whether to pursue a non-use agreement or a governance framework for research. Large-scale deployment by any government remains unlikely in the near term, but the growing severity of climate impacts will keep the question on policy agendas.

Myths and Facts

Myth: Solar geoengineering is already cooling the planet. Fact: No large-scale program exists. Documented deliberate releases have been tiny.

Myth: SCoPEx sprayed chemicals over Sweden. Fact: The planned 2021 flight would have tested equipment only and was called off; no release ever took place.

Myth: Contrails that linger are proof of spraying. Fact: Whether contrails persist depends on humidity and temperature at flight altitude.

Myth: Scientists want to replace emissions cuts with geoengineering. Fact: Major scientific assessments stress that it cannot substitute for cutting emissions.

Myth: It would be simple to stop once started. Fact: A large program stopped abruptly could cause rapid rebound warming.

Frequently Asked Questions

Did Harvard and Bill Gates try to block out the sun?

No. Harvard’s SCoPEx proposed releasing a few kilograms of material from a research balloon to study particle behavior, which would have had no measurable effect on sunlight reaching Earth. Bill Gates was one of several funders of Harvard’s broader research program. The experiment was canceled in 2024 without any release.

Is solar geoengineering happening now?

Not at any meaningful scale. EPA reports that the only private actor it knows of deploying such methods in the U.S. has released very small amounts. The UK is funding small, controlled outdoor research experiments.

Is solar geoengineering legal?

It depends on where. Tennessee and Florida have banned it, Mexico has moved to prohibit experiments, and there is an international de facto moratorium under the Convention on Biological Diversity with exceptions for small-scale research. There is no comprehensive global treaty.

Could it stop climate change?

It could temporarily reduce warming but would not remove greenhouse gases or stop ocean acidification, and it carries significant risks. Scientific bodies describe it as, at most, a potential supplement to emissions cuts, not a substitute.

Are chemtrails real?

The lines behind aircraft are contrails formed by water vapor from engine exhaust. A survey of atmospheric scientists found no evidence of secret large-scale spraying programs.

Who funded Harvard’s geoengineering research?

Harvard’s Solar Geoengineering Research Program was supported by several philanthropic foundations and individual donors, including Bill Gates, according to published funder lists.

What is the UK doing on solar geoengineering?

The UK’s Advanced Research and Invention Agency is funding a roughly £57 million research program, including about £24.5 million for small, controlled outdoor experiments overseen by an independent committee.

What did the EPA say about geoengineering?

EPA’s public FAQ says that, as of July 2025, it knew of only one private company actively deploying stratospheric aerosol injection or marine cloud brightening in the U.S., and that the amounts released were very small compared with natural events like volcanic eruptions.

References

1. U.S. Geological Survey. The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines. Fact Sheet 113-97. pubs.usgs.gov
2. MIT Technology Review. Harvard has halted its long-planned atmospheric geoengineering experiment. March 18, 2024. technologyreview.com
3. MIT Technology Review. The hard lessons of Harvard’s failed geoengineering experiment. April 4, 2024. technologyreview.com
4. Scientific American / E&E News. High-Profile Geoengineering Experiment Shuts Down. March 2024. scientificamerican.com
5. Geoengineering Monitor (ETC Group). Current Geoengineering Attempts Briefing: SCoPEx. geoengineeringmonitor.org
6. U.S. Environmental Protection Agency. Geoengineering: Frequent Questions. epa.gov
7. Carbon Brief. Factcheck: How the UK is and is not studying solar geoengineering. 2025. carbonbrief.org
8. Climate Home News. Mexico plans to ban solar geoengineering after rogue experiment. January 18, 2023. climatechangenews.com
9. National Academies of Sciences, Engineering, and Medicine. Reflecting Sunlight: Recommendations for Solar Geoengineering Research and Research Governance. 2021. nap.nationalacademies.org
10. Shearer C, West M, Caldeira K, Davis SJ. Quantifying expert consensus against the existence of a secret, large-scale atmospheric spraying program. Environmental Research Letters. 2016;11(8):084011. iopscience.iop.org

Last updated: September 26, 2026