Solar Radiation Modification (SRM) geoengineering could moderate Anthropogenic Global Warming (AGW). SRM could be privatised, e.g. using Voluntary Carbon Offsets (VCOs); alternatively, philanthropic geoengineering is possible. Analogues for private SRM exist, as citizens are empowered to act unilaterally in crises: stopping trains; evacuating buildings; deploying fire hoses; and using lethal force for self-defence. The question arises: could citizens ethically and responsibly conduct SRM? Such philanthropic geoengineering’s ethics and regulation are considered, specifically issues of justifiability and legitimacy. Normative, descriptive, rational-legal, and output-input legitimacy address governance, support, rules & laws, and informed & effective decision-making respectively. In conclusion, legitimacy is neither guaranteed nor impossible–but particular accountability and stability problems occur in the lone-actor Greenfinger model, and with 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.