A range of actors – from scientists, to innovators, to self-styled mavericks – have long been pushing for research into solar geoengineering to leave the laboratory.

As currently envisioned, such field experiments are small-scale and early-stage. Intended to test deployment mechanics or atmospheric processes at a much-reduced scale, planned tests have involved a room-sized balloon attached to aerosol-dispensing and measurement equipment, with negligible, time-limited physical impacts.

Yet, these tests never went ahead. Two tests organized by scientists have been cancelled in the face of fierce civic opposition and expert dispute – one in the UK by an alliance of NGOs (Stilgoe et al., 2013), and a second in Sweden by the Saami Council (Osaka, 2021). A third demonstration, organized on a shoestring budget by a pair of Silicon Valley enthusiasts in Baja California, and involving no motives for technical testing, caused an uproar that prompted the Mexican government to ban all such activity (Temple, 2022).

Our research asks: Who opposes these small-scale tests? How have they done so? And most importantly, why? At much larger scales these approaches would certainly constitute deliberate manipulation or management of the global climate system. But even now, as experiments and concept-proofs, they exercise an outsized influence on the debate over future climate governance – and they are contested in equal measure.

To answer this, we examine a wider and longer arc of early-stage experiments in radical climate interventions for both carbon removal or sunlight reflection. We draw upon multiple historic cases of ocean iron fertilization experiments (e.g. Strong et al., 2009) – still labelled as an archetypical case of ‘marine geoengineering’ – that shows how they were shaped by the same scientific and commercial motives that now underpin solar geongineering. We draw further connections to antecedent and incoming kinds of carbon removal (three different kinds of enhanced weathering) and sunlight reflection (marine cloud brightening and ice protection. In all, we undertake a qualitative comparative analysis (QCA) that examines the substance and drivers of social opposition to radical climate intervention experiments: spanning 45 cases across 5 sociotechnical approaches, and treating 22 in-depth.

With this bird’s eye view, we show that the key scientific, commercial, and civic narratives – and in some cases, even the actors – are common across all climate intervention approaches and cases. We outline the key tactics through which scientific or entrepreneurial proponents have sought to defend field experiments – and conversely, how civic opponents have sought to prevent to delegitimize them. We show how governance – from informal research norms, to public engagement, to international law at multiple regimes – have come to be changed by these contestations.Nature

Finally, we show that planetary-scale ‘geoengineering’ remains a key motif of contestation in early-stage experiments. Experiment planners are eager to describe (or camouflage) their work under different terms, while opponents treat ‘geoengineering’ less as a technical description, and more as a totem for concerns about inequities in the carbon economy and Anthropocentric hubris.

Our work targets scientists, technologists, and practitioners in the field, that all might better understand the issues and dynamics that surround the risks of exploring climate interventions.

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