Solar geoengineering without the hype: questions and answers

In 2024, solar geoengineering moved out of seminar rooms and into places where ordinary politics happens. A university abandoned its flagship experiment. A city council in California voted to stop a test on the deck of a museum ship. A UN resolution collapsed in Nairobi. A start-up kept selling "cooling credits" from balloon launches. In January 2025, the European climate service Copernicus reported that 2024 was the first calendar year in its records to average more than 1.5°C above pre-industrial levels, which will not lower the temperature of the argument.

What follows is an attempt to set out the basics without the evangelism or the panic.

What is solar geoengineering?

It is the deliberate reflection of a little more sunlight back to space to cool the planet. You will also see it called solar radiation modification or solar radiation management, both shortened to SRM. The Royal Society's 2009 report on geoengineering drew the distinction that most people still use: carbon dioxide removal takes greenhouse gases out of the air, while solar techniques leave them there and offset some of their warming effect.

That difference is the heart of the matter. Solar geoengineering treats a symptom. Carbon dioxide would keep accumulating, the oceans would keep acidifying, and the cooling would last only as long as someone kept doing it.

How would stratospheric aerosol injection work?

The model is the volcano. When Mount Pinatubo in the Philippines erupted in 1991, it put millions of tonnes of sulphur dioxide into the stratosphere. The gas formed a haze of sulphate particles that reflected sunlight, and average global temperatures fell by roughly half a degree Celsius over the following year or so before recovering as the particles settled out.

The proposal is to do something similar on purpose, continuously, with specially designed high-flying aircraft releasing sulphur dioxide or another reflective material such as calcium carbonate. The particles fall out within a year or two, so injection would have to continue for as long as cooling was wanted. Atmospheric chemist Paul Crutzen revived serious discussion of the idea with an essay in 2006, arguing that research was justified because emissions cuts were too slow.

The known risks are substantial. Models suggest changes in rainfall patterns, including monsoons that billions of people depend on. Sulphate particles can damage the ozone layer, a concern serious enough that the Montreal Protocol's scientific assessment in 2022 included a chapter on it. Skies would become slightly whiter. And there is "termination shock": if injection stopped abruptly, decades of masked warming could return within a few years, faster than ecosystems or farmers could adjust.

And marine cloud brightening?

This targets low clouds over the oceans. Spraying fine sea-salt particles into them gives water vapour more surfaces to condense on, producing more and smaller droplets, which make a cloud brighter and possibly longer-lived. The physicist John Latham proposed the idea in 1990.

Its effects would be more local and short-lived, measured in days rather than years, which supporters see as an advantage. Australian researchers have trialled sprayers over the Great Barrier Reef since 2020, hoping to shade and cool parts of the reef during marine heatwaves. Ships have already been doing a version of this by accident: the sulphur in their exhaust seeded "ship tracks" in clouds. When international rules sharply cut the sulphur content of shipping fuel in 2020, those tracks thinned, and several studies have argued this added to recent warming, though how much is disputed.

The catch is that clouds are among the least understood parts of the climate system. Brightening clouds in one region could shift rainfall far away, and models do not agree on how much.

Has anyone actually tested this outdoors?

A few have tried, and 2024 was mostly a story of stoppages.

SCoPEx. Harvard's Stratospheric Controlled Perturbation Experiment planned to release a tiny amount of material from a steerable balloon to study particle chemistry in the stratosphere. A preliminary flight, with no release, was planned from northern Sweden in June 2021. It was called off after the Saami Council, which represents the Indigenous Saami people, and Swedish environmental groups objected, and the project's own advisory committee recommended waiting. In March 2024 Harvard announced the project was over.

Alameda. In April 2024, a University of Washington team began testing a salt-spray machine on the deck of the USS Hornet, a decommissioned aircraft carrier turned museum in Alameda, California. The point was to check how the spray behaved, not to change any clouds. City officials said they had not been properly informed and ordered a pause. In June the city council voted unanimously to end the tests, even though a review it had commissioned found no significant health or environmental risk. The objection was less about the salt than about who had been asked.

Make Sunsets. Not everyone asks. In 2022 a US start-up launched weather balloons carrying sulphur dioxide from Mexico's Baja California peninsula without seeking permission, then sold "cooling credits" to cover them. Mexico's government responded in January 2023 by announcing it would prohibit solar geoengineering experiments. The company has continued launching from the United States.

The pattern is uncomfortable. The projects run by universities, with advisory boards and public notices, were the ones that got stopped. The one that told nobody in advance carried on.

What do "cooling credits" buy?

As sold, a credit claims to offset the warming of a tonne of carbon dioxide for a year by putting a small amount of sulphur dioxide into the upper atmosphere. Even accepting the physics, the comparison is lopsided. A tonne of CO2 warms the planet for centuries; a puff of sulphate cools it for months. Nobody independently checks where the particles go or how much reflecting they do, and no standard or registry exists for this kind of credit. It is a product looking for a market, and for now the market is mostly buyers who want to be part of something new.

The bigger question is what happens if this turns into a real business with real investors. A company that profits from injections has an interest in continuing them, which is the opposite of what any careful governance system would want.

Is there any law on this?

Not much, and none of it was written with solar geoengineering in mind.

• The Convention on Biological Diversity. At its 2010 meeting in Nagoya, the convention's parties adopted decision X/33, which invites governments to ensure that no climate-related geoengineering activities that may affect biodiversity take place until there is an adequate scientific basis and appropriate consideration of the risks, with an exception for small-scale scientific research in controlled settings. Later meetings reaffirmed it. It is not legally binding, and the United States is not a party to the convention.
• The London Protocol. A 2013 amendment created a permit system for marine geoengineering, but it covers only ocean fertilisation so far and is not yet in force.
• The ENMOD Convention of 1976 bans hostile military use of environmental modification. Peaceful use is outside its scope.
• The Montreal Protocol could become relevant because of the ozone effects, but nobody has proposed using it that way.

Beyond treaties, governance is improvised. Mexico's 2023 announcement was a national response. In the United States, Tennessee passed a law in 2024 banning intentional releases into the atmosphere meant to affect temperature, weather or sunlight, a measure pushed partly by people who believe aircraft are already secretly spraying the population. Policy on this subject is being written by an odd coalition of cautious scientists, Indigenous organisations and conspiracy theorists.

What happened at the UN?

The UN Environment Assembly, the body created when UNEP's governing council was opened to all countries after the Rio+20 summit in 2012, has tried twice. In 2019 a Swiss-led resolution asking for an assessment of geoengineering was withdrawn after the United States and Saudi Arabia, among others, resisted it. In February 2024 Switzerland tried again, proposing an expert group to look at solar radiation modification. This time the split ran the other way as well. African governments, which had called in 2023 for an international non-use agreement, wanted language rejecting the technology, while others wanted a neutral assessment, and some were reluctant to start any new UN process. The sponsors withdrew the text.

What is the case for research?

It goes like this. Emissions are not falling fast enough, and the world is likely to overshoot 1.5°C for some period. If governments one day face a heatwave catastrophe and reach for this tool, it would be better if they knew what it would do. Ignorance does not prevent deployment; it only makes deployment worse.

That argument has institutional support. A 2021 report from the US National Academies of Sciences, Engineering, and Medicine recommended a US research programme of $100 million to $200 million over five years, under strong governance. In 2023 the White House published a congressionally requested report outlining what such a programme might cover, while noting that no comprehensive federal research programme was being planned. The Climate Overshoot Commission, an independent group of former leaders, recommended in 2023 a moratorium on deployment and large-scale outdoor experiments, alongside more research and governance talks. Groups such as the Degrees Initiative fund scientists in the global South to model regional effects, on the reasoning that the countries most exposed should not depend on Northern models.

And the case for a non-use agreement?

In 2022 a group of academics published a call for an International Non-Use Agreement on Solar Geoengineering, which hundreds of scholars have since signed. It asks governments to commit to no public funding for developing the technology, no outdoor experiments, no patents, no deployment, and no endorsement in international bodies.

The reasoning is that solar geoengineering cannot be governed fairly at planetary scale. Some institution would have to decide how much cooling, where, and for how long, and there is no legitimate body that could. Research builds constituencies and momentum that make deployment more likely. The prospect of a cheap fix weakens the pressure to cut emissions, a problem often called mitigation deterrence. And the regions most at risk from shifted monsoons have the least say over the controls. Supporters of this view see the long argument over governing synthetic biology as a warning about what happens when governance is left to the people building the technology.

Who decides, and who carries the risk?

This is where the technical debate turns political. Estimates of the direct cost of a stratospheric programme run to a few billion dollars a year in its early stages: cheap enough for a single mid-sized government, or a coalition of the willing, to try without asking anyone else. The economist's term is "free driver", the reverse of the free rider who benefits from cuts made by others.

The consequences would not fall where the decision was made. A monsoon shift would be felt by farmers in South Asia or the Sahel, not by the ministry that ordered the flights. That is why the most insistent voices for a ban come from Africa, while the most generous research funding comes from wealthy universities and philanthropies. It echoes older climate fights, including the long dispute over who controls climate technologies, in which the countries that would live with the outcome had the least say over the design.

The next chance for governments to take this up formally is the UN Environment Assembly's next session, due in December 2025. Between now and then, the useful question to ask about any proposal, whether for a new experiment, a research budget or a ban, is the same one the Alameda council ended up asking: who was consulted before it started, and who would have the power to stop it?