Thinking at the megaton scale: Community reflections across three different marine carbon-dioxide removal strategies

Investigations of marine carbon dioxide removal are unfolding rapidly as countries seek net zero greenhouse gas emissions by 2050. A key priority for interdisciplinary research is public engagement in system design, social conditions, and acceptance. Social scientists warn that ‘large-scale’ proliferation—at megaton or gigaton levels—raises distinct challenges. Yet most work examining CDR has relied on simplified descriptions with little attention to scale. This study examines public reflections on three depictions of marine CDR deployed at ‘scale’ in British Columbia, Canada. Scaled descriptions included material, transportation, and energy needs, along with spatial requirements for generating 2MT of annual removals for each of three technologies: (1) ocean alkalinity enhancement; (2) direct air capture of CO₂, with sub-seabed mineralization for storage; and (3) macroalgae cultivation and sinking. Results of two daylong deliberative workshops with First Nations and members of environmental groups found several theory-relevant results when thinking ‘at scale’. First, there was an evident willingness to grapple with each CDR component and process; participants struggled but remained engaged despite the cognitive difficulty of thinking at scale. Second, amongst participants who initially preferred macroalgae sinking, there was evidence of a backlash response about the ‘unnaturalness’ of this approach when the full scale of activities was discussed. Third, full component descriptions encouraged contemplation of some trade-offs across material and energy demands, system lifespan, decommissioning, financing, and governance. Overall, these results demonstrate the need for researchers to develop engagement designs that ensure consideration of scale, siting, components and the many considerations involved in mCDR. • Perceptions research on carbon removal has yet to address deployment scale. • Thinking at a 2 MT scale produces a willingness to grapple with full details and cognitive burdens. • At MT scale, the heuristic sheen of naturalness converts to more negative views. • Future public engagement needs to address scale and fuller system component designs.