Solar geoengineering is no longer a thought experiment. It is a technically feasible option that has been seriously discussed in climate research circles for years, with growing urgency and growing unease in equal measure.
The best-known method is stratospheric aerosol injection, or SAI. Tiny reflective particles are released into the stratosphere, part of the sunlight is reflected back into space, and the Earth's surface cools. Volcanoes have demonstrated the principle for millions of years. The 1991 eruption of Mount Pinatubo cooled the Earth by about 0.4 degrees Celsius for roughly two years. What researchers like Claudia Wieners at Utrecht University or Shuchi Talati of the Degrees Initiative are examining is whether this effect can be reproduced in a controlled way: deliberately, dosably, and under what conditions responsibly.
The physics is by now well understood. Models consistently show that SAI can dampen temperature peaks. The costs would be comparatively low: a few billion dollars annually for a globally perceptible effect, while unchecked climate change produces costs in the hundreds of trillions. A handful of specially equipped aircraft could, by some estimates, achieve measurable effects within months.
That sounds tempting. And that is exactly where the problem begins.
There is a line in the research debate that stays with you. Wieners says it almost in passing: „We are already doing geoengineering. We are doing geoengineering with CO₂.“ This is not a rhetorical device. The climate system is already in the state of an uncontrolled, accumulating experiment, without governance, without mandate. The position of non-intervention no longer exists. Inaction is a path decision. The only question is which intervention produces which consequences.
SAI does not act uniformly. Atmospheric circulation systems are not homogeneous surfaces. Monsoon regimes respond differently from Arctic systems. Some models show that a hemispherically asymmetric injection could shift rain bands. Heavy injection in the northern hemisphere makes it relatively cooler, the tropical rain band shifts southward, and the Sahel, which depends on exactly that rain band, gets less rain. The Sahel region, sub-Saharan Africa, the Indian subcontinent: areas already under climate stress could benefit from a SAI regime or suffer significantly from it. What would happen if droughts and crop failures suddenly became events for which someone could be held responsible?
The first serious problem is structural rather than physical. Andy Parker, founder of the Degrees Initiative, makes it plain: the countries with the highest vulnerability are not at the table. Not in the research, not in the governance discussion. It is a pattern familiar from climate politics. With SAI it takes on particular force because what is being negotiated are interventions into the system on which literally everything else depends, not merely emissions targets.
The governance vacuum is difficult to miss. There is no international institution that could decide on SAI deployment, control it, or sanction violations. Stefan Schäfer of the Helmholtz Centre Potsdam describes the drift from coordinated to uncoordinated scenarios as likely: if reality does not look like the model calculations, one can very quickly drift into a scenario in which deployment becomes uncoordinated or is abandoned altogether. While institutions debate frameworks, the startup Make Sunsets is already sending SO₂ weather balloons into the stratosphere without a mandate or governance, arguing that the situation is too urgent for institutional process. Stardust Solutions argues along similar lines. In 2012, Russ George released iron particles into the sea off the Canadian coast without regulatory approval to stimulate phytoplankton growth, with unclear ecological consequences.
This is the point at which the issue stops being only a climate question and becomes a question of power structures, dependencies, and responsibility.
Path Ethics is not another ethical framework with a list of values. It is an evaluative architecture for systems: structural, substrate-independent, explicitly anti-anthropocentric. What makes it particularly suited to the SAI problem is an analytical category that emerged from this very case analysis: the infrastructure path.
The climate system is not an ordinary object of evaluation. It is a system with such high connectivity that interventions in its structure cannot be treated as local events. Agriculture depends on climatic conditions. Water supply depends on precipitation patterns. Cultural forms of life that have grown over generations in specific climatic rhythms depend on their reliability. When the climate system is reconfigured, all of these couplings are reconfigured at the same time, whether those affected were consulted or not.
This makes the climate system what Path Ethics calls an infrastructure path: a system that meets three criteria simultaneously. First, high connectivity across different substrate types. Second, universal obligatoriness: there are no systems of relevant scale that can opt out of the coupling. Third, asymmetric damage potential that cannot be compensated for by other systems. The negative list matters: individual platforms such as Google, individual states, even the Amazon rainforest do not fully meet these criteria. The concept is not meant to be inflated. Infrastructure paths are rare, and that is the point.
For systems of this kind, Path Ethics applies stricter protection criteria. Interventions are permissible only under maximum inclusion of all structurally dependent systems, proportionate to their vulnerability rather than their power resources. External support of the system takes precedence over external replacement. And no intervention is permissible if its termination would create irreversible consequences for highly relevant dependent systems.
This allows a more precise description of what SAI does structurally. The climate system regulates itself through internal feedback loops: ice-albedo dynamics, ocean currents, carbon cycles, monsoon systems. These internal tensions constitute the system. SAI that stabilises temperature externally while the CO₂ burden remains in place overlays this self-regulation. The system persists along the observed temperature axis while the continuing CO₂ forcing is externally counteracted. The apparent stability does not remove the underlying change. Path Ethics describes this as the difference between transductive persistence, in which the system sustains itself through internal transformation, and rigid persistence, in which the system is stabilised from outside. Rigid persistence looks like stability. It is not the same thing.
Wieners calls the consequence termination shock: if SAI ends because war breaks out, an alliance collapses, or funding disappears, the radiative effect that had previously been masked can become effective as rapid warming over a short period. The problem is not that the climate system has „forgotten“ an ability, but that external stabilisation does not remove the underlying forcing. In Path-Ethics terms, SAI without emissions reduction can therefore be read as an attempt to fix a central state variable externally while the underlying tensions continue.
This is not an argument against SAI as such. Wieners herself sketches a scenario she calls peak shaving: SAI as a temporary bridging instrument that buffers the most dangerous temperature peak while emissions fall seriously in parallel. That has a Path-Ethics logic. The external stabilisation would be time-limited and tied to an exit path; the problematic path dependency would therefore be lower than under an open-ended SAI regime. Whether that condition could ever be met politically is another question. Schäfer considers the coordinated scenario the less likely one.
No simple yes-or-no judgments follow from this analysis. Unilateral SAI by private actors is structurally impermissible because of destructive dominance: forced coupling without reciprocity and irreversible loss of possibility space for all systems that did not participate in the decision. Radical emissions reduction is required from a Path-Ethics perspective. Basic research that actively includes the Global South remains ambivalent and is permissible only under the strict condition that research logic and implementation logic remain separate.
As an evaluative architecture, Path Ethics asks three questions of every SAI proposal. First: does the project replace the climate system's internal feedback architecture or support it? Second: which regional systems lose stable coupling conditions through the intervention, and were they included in the decision? Third: is there an exit path that does not pull the structural stability of dependent systems into a collapse sequence?
If the third question has no answer, the proposal is structurally incomplete. The diagnosis is structurally incomplete. This language connects to UNEP processes, IPCC syntheses, and the Degrees Initiative. It avoids moral overloading and gains structural sharpness in return.
Path Ethics also has limits that it states openly. The first is this: what if warming reaches tipping points at which coordinated SAI is the only remaining option for preventing mass losses? Requirements of reversibility and democratic legitimacy could then themselves become part of the problem. The theory still lacks an explicit emergency mechanism.
The second limit is subtler. The climate system is not the only infrastructure path. The global financial system also meets the criteria. Radical emissions reduction that is beneficial for the climate system can destabilise the financial system: fossil assets lose value, energy costs rise, and economic systems come under pressure. When two infrastructure paths conflict, Path Ethics has no standard procedure. It has only a heuristic: the path with greater temporal depth in its own self-regulation takes precedence. The climate system has regulated itself for billions of years. The financial system for a few centuries. The heuristic gives a direction without becoming an algorithm.
But frameworks that state their limits are more useful than frameworks that pretend they have none.