Solar geoengineering, or solar radiation management (SRM), brings together techniques aimed at reflecting a fraction of the Sun's energy back into space to cool the planet, without touching the concentration of greenhouse gases. Its promise is real and its natural precedent documented; but its principal danger is neither chemical nor secret. It lies in irreversibility and in the absence of governance. Here, with real sources to support it, is what science actually establishes.
Definition. SRM denotes a set of methods, still largely under study, intended to reflect a small part of solar radiation in order to lower the surface temperature. The most studied avenue is stratospheric aerosol injection (SAI): dispersing reflective particles, such as sulphate compounds, in the upper atmosphere. The IPCC, in its Sixth Assessment Report (AR6, 2021), examines three main aerosol-based methods: stratospheric aerosol injection, marine cloud brightening, and cirrus cloud thinning.
The idea is not science fiction: it rests on a measured natural precedent. In 1991, the eruption of Mount Pinatubo in the Philippines injected roughly 20 million tonnes of sulphur dioxide into the stratosphere. The resulting aerosol veil cooled the Earth's surface by about 0.5 °C for more than a year — an effect extensively studied, notably by Soden, Wetherald, Stenchikov and Robock (Science, 2002). Aerosol injection proposes, in short, to deliberately mimic this volcanic mechanism.
This is precisely what makes SRM attractive to some: it would act quickly, at a relatively low cost, and could mask part of the warming while emission cuts take effect. But that same speed, that same low cost, are also the source of its most serious dangers. The Reflecting Sunlight report of the U.S. National Academies (2021) underlines, moreover, that this apparent ease says nothing about the poorly understood consequences for regional climate.
To date, SRM is overwhelmingly a matter of research, modelling, and laboratory experimentation, not of real deployment. In 2021, the U.S. National Academies of Sciences, Engineering, and Medicine (NASEM) published the report Reflecting Sunlight, recommending a public, transparent, and supervised research programme funded at 100 to 200 million dollars over five years — while insisting on one essential point: research must not amount to a commitment to deployment.
Public institutions confirm this. In 2023, the United Nations Environment Programme (UNEP), in its One Atmosphere report, concludes that SRM is not ready for large-scale deployment and cannot replace a rapid reduction in emissions. On the U.S. side, the Government Accountability Office (GAO) recalls, in its science overview devoted to solar geoengineering, that potentially harmful effects — air pollution, damage to the ozone layer, changes in precipitation patterns — have been identified but are still poorly understood.
In other words, research on solar geoengineering does exist, and it is partly public. This is enough to set aside two symmetrical errors: believing that nothing exists (false: serious work is conducted and published) and believing that a secret deployment is already under way (nothing attests to it). What is documented is research in the open — proposed and debated — not a hidden programme nor an operational device.
The risk most cited by researchers has a name: the termination shock. The climatologist Alan Robock formalised it among his "20 reasons why geoengineering may be a bad idea" (Bulletin of the Atomic Scientists, 2008), and the IPCC takes it up in its AR6 report. The reasoning is as follows:
SAI therefore does not address the cause: it masks a symptom, creating a potentially irreversible dependency. This is why nearly all specialists insist: solar geoengineering could in no case replace the reduction of emissions at the source. The IPCC notes, however, that a gradual phase-out combined with genuine mitigation would reduce the magnitude of this termination shock. At best a bandage, never a cure.
The deepest danger of SRM is not technical but political. A relatively inexpensive technology, which a single actor — a state, even a billionaire — could theoretically trigger, would have planetary consequences on rainfall regimes, monsoons, and the agriculture of others. The vertigo is not secrecy: it is the absence of a democratic mandate to touch the common thermostat.
The SCoPEx case illustrates this concretely. This Harvard University project planned a modest experiment: a stratospheric balloon releasing a very small quantity of material to study its behaviour. A preparatory flight was envisaged in Sweden, near Kiruna, in 2021. It never took place: faced with the opposition of environmental groups and, decisively, of the Saami Council — which had not been consulted and refused tests on its territory — the project's advisory committee recommended cancellation. In March 2024, Harvard announced the definitive abandonment of SCoPEx. Real research, funded and technically ready, stopped not by a scientific failure but by an objection over consent.
The governance vacuum is, likewise, documented. In 2010, the United Nations Convention on Biological Diversity (CBD) adopted a decision (X/33) inviting states to refrain from geoengineering activities liable to affect biodiversity — a de facto "moratorium," though non-binding. The Reflecting Sunlight report (2021) and the UNEP One Atmosphere review (2023) both call for transparent, participatory, and independently assessed governance frameworks. To date, no global institution is empowered to gather the consent of those a deployment would affect.
Is solar geoengineering already deployed? No. SRM today is a matter of research, modelling, and a few laboratory experiments, as indicated by the Reflecting Sunlight report of the U.S. Academies (2021) and the UNEP One Atmosphere review (2023). No large-scale deployment is under way, and nothing attests to a secret programme. These techniques are proposed and studied, not operational.
What is the termination shock? It is the danger that an aerosol veil, deployed to mask the warming and then abruptly halted, would cause a very rapid catch-up warming, more dangerous than a gradual warming. Formalised by Alan Robock (2008) and taken up by the IPCC (AR6, 2021), it follows from the fact that SRM masks the symptom without treating the cause.
Can SRM replace the reduction of emissions? No. It does not reduce the concentration of CO₂; it merely masks part of the warming. The institutions — U.S. Academies, UNEP, IPCC — present it at best as a temporary complement to be studied, never as a substitute for the reduction of emissions at the source.
To understand how solar geoengineering differs both from the (debunked) "chemtrails" myth and from CO₂ removal (CDR), our full investigation lays out each piece, marked out level of certainty by level of certainty: the ebook Geoengineering Without Consent gathers all the primary references. For the overall picture, see also our pillar article on geoengineering: science versus chemtrails.
No. SRM today is a matter of research, modelling, and a few laboratory experiments, as indicated by the Reflecting Sunlight report of the U.S. Academies (2021) and the UNEP One Atmosphere review (2023). No large-scale deployment is under way, and nothing attests to a secret programme. These techniques are proposed and studied, not operational.
It is the danger that an aerosol veil, deployed to mask the warming and then abruptly halted, would cause a very rapid catch-up warming, more dangerous than a gradual warming. Formalised by Alan Robock (2008) and taken up by the IPCC (AR6, 2021), it follows from the fact that SRM masks the symptom without treating the cause.
No. It does not reduce the concentration of CO₂; it merely masks part of the warming. The institutions — U.S. Academies, UNEP, IPCC — present it at best as a temporary complement to be studied, never as a substitute for the reduction of emissions at the source.
Dossier : Géo-ingénierie & modification du climat
MINI12: La Géo-Ingénierie Sans Consentement
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