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

[Geoengineering News]

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025EF007303

Authors: B. H. Redmond Roche, I. Hernandez-Galindo, A. Määttänen, J. C. Moore, O. Boucher, B. M. Dollner, A. Duffey, B. Gasparini, J. Henneberger, A. N. Koyun, D. McGrath, I. Steinke, J. Vinders, D. Visioni, P. J. Irvine

23 May 2026

https://doi.org/10.1029/2025EF007303

Abstract

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.

Plain Language Summary

Solar radiation modification (SRM) refers to a proposed set of methods for cooling the planet by adding small particles to the atmosphere. This study examines the three main approaches: stratospheric aerosol injection (SAI), marine cloud brightening (MCB), and cirrus and mixed-phase cloud thinning (CCT/MCT). SAI and MCB work by reflecting some sunlight back into space, while CCT/MCT aim to reduce the heat-trapping effect of clouds. SRM field experiments are important for improving scientific understanding of how these methods work in practice. To explore what such experiments could look like, we convened an expert workshop and identified six SAI, five MCB, and three CCT/MCT experiments that would be scientifically useful and could plausibly be undertaken. We also considered the challenges that previous field experiments had experienced. From these defined experiments, we developed a phase-based classification: near-term activities that are technically and legally feasible, intermediate activities that may cross regulatory thresholds and raise societal concerns, and distant activities that would require new international governance. Our analysis shows that existing laws could already be applied to some experiments and highlights areas where additional rules are required. This helps create a common language for researchers and policymakers to discuss SRM responsibly.

Source: AGU