Legitimacy and justifiability of non-state geoengineering

Solar Radiation Modification (SRM) geoengineering could moderate anthropogenic global warming. SRM could be privatised, e.g. using voluntary carbon offsets; alternatively, philanthropic geoengineering is possible. Analogues to private SRM can be drawn from situations where citizens are empowered to act unilaterally in crises such as stopping trains, deploying fire hoses, and using lethal force for self-defence. The question arises: could citizens ethically and responsibly conduct SRM? To explore philanthropic geoengineering, issues of justifiability and legitimacy are examined in various scenarios: Greenfinger, Billionaires' Club, Crowdfunding, Social Movement, and Technocrats. The governance, support, rules & laws, and informed & effective decision-making in these scenarios are evaluated through discussions of legitimacy types: normative, descriptive, rational-legal, and output-input lrespectively. In conclusion, legitimacy is neither guaranteed nor impossible - but particular accountability and stability problems arise in the lone-actor Greenfinger model and loosely-structured crowdfunded movements. However, there is some merit to Technocrats, particularly when overseen by external funders. Blending various approaches appears to offer significant benefits. Finally, a set of criteria for legitimacy is proposed.