Differentiating Solar Radiation Modification Field Experiments: Scale, Technical Characteristics, and Governance Implications

Proposed solar radiation modification (SRM) field experiments are receiving growing scientific and policy attention, with several localized experiments having occurred or under development. While they may be critical for improving technical understanding and reducing uncertainties, SRM field experiments remain controversial and inadequately captured under current legal, ethical, and political frameworks, which often rely on binary “small-scale” or “large-scale” distinctions. In practice, SRM field experiments may vary widely in scale, geography, materials, scientific purpose, and environmental impacts, making uniform governance approaches impractical. Here, we develop a typology of plausible, scientifically motivated SRM field experiments across three leading approaches: stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus and mixed-phase cloud thinning (CCT/MCT). We identify six SAI, five MCB, and three CCT/MCT experiments across a range of scales, assessing their interaction with existing environmental and legal frameworks, particularly within the EU. The study also addresses governance challenges such as scale perception and stakeholder legitimacy, and highlights procedural tools, including exit ramps and transparency requirements. We introduce a phase-based typology of plausible SRM field experiments: near-phase (technically and regulatorily feasible), intermediate-phase (technically feasible but likely to cross regulatory thresholds), and distant-phase (plausible but requiring comprehensive review and new governance mechanisms). The intermediate-phase highlights that SRM research cannot be reduced to a small-/large-scale binary: many plausible experiments occupy a gray zone where scientific value is high, but governance remains underdeveloped. Recognizing this distinction may help prioritize governance, guide proportionate regulation, and ensure field experiments are evaluated by intent and their regulatory implications.