Abstract

Stratospheric aerosol injection (SAI) is a solar radiation modification technique, proposed as an interim measure to offset warming while greenhouse gas (GHG) emissions are reduced. This paper discusses a possible SAI implementation route - an alternative to sulfate aerosols formed in situ - based on engineered solid particles having dedicated properties such as size, composition, surface chemistry, and traceable origin, supporting safety, controllability, and functionality needed for SAI systems. These engineered properties also open up options for any future multi-state coordination of SAI through two technical building blocks: (1) the SAI-induced radiative forcing (SRF) - the magnitude of the cooling effect attributable specifically to the SAI layer - as an operator-independent quantity, derivable from direct aerosol-layer measurements; and (2) particle traceability through identifying signatures embedded at production. Both could feed into a shared, publicly accessible monitoring database open to independent interrogation, addressing several governance challenges by anchoring compliance assessments in measurable parameters. Drawing on precedents from the Montreal Protocol, IAEA safeguards, and other regimes, we show that shared technical metrics have historically enabled multi-state cooperation, and we argue the same could apply to SAI. We describe a phased pathway in which the technical capabilities and coordination practices that would use them are developed and tested together, at scales orders of magnitude below operational deployment. To be clear - we regard SAI deployment as premature; the conditions under which it might be considered have not been met. The paper does not propose a governance framework; rather, it identifies technical infrastructure that could support a wide range of such frameworks.

Plain-language summary

Stratospheric aerosol injection (SAI) - adding reflective particles into the upper atmosphere - has been proposed as a way to temporarily cool the planet while greenhouse gas emissions are reduced. This paper does not argue that SAI should be deployed or that it is ready for that. It identifies two features of the engineered particle layer - how much sunlight the layer reflects back to space, and the exact identity of each particle - that would give cooperating states a shared, observable basis for coordinating any such climate stabilization program. Particle identity enables tracing every contribution to the layer, whether from participating states, outside actors, or gaps in compliance. Both features depend on using purpose-built solid particles, not sulfates. Those same measurements could ground questions of responsibility, verification, and dispute resolution in observed facts rather than disputed estimates of climate impacts. This kind of measurement-first approach has supported international cooperation on hard problems before, from nuclear arms control to ozone protection. The paper also outlines a step-by-step pathway in which the technical tools and the governance practices around them are built and tested together, well before SAI itself could possibly be deployed.