As research practitioners working on climate and disasters in the humanitarian sector, we are at once fascinated and terrified by the prospect of geoengineering. Altering the Earth's climate, whether inadvertently through anthropogenic greenhouse gas emissions or deliberately through geoengineering, is an experiment in which every person on our planet is potentially a test subject. Vulnerable populations will be differentially and disproportionately impacted by deployments. Geoengineering deployments that shift the burden of impacts constitute humanitarian externalities. This chapter proposes framing geoengineering research and policy agendas in ways that explicitly integrate the role of the most vulnerable through Learning, Preparing, and Preventing as a way to internalise humanitarian externalities. Additionally, we should support research and policy efforts to innovate through corrective mechanisms and financial instruments designed to internalise the negative externalities incurred by the most vulnerable. The growing geoengineering community is actively exploring options. They submit that there is a moral imperative to facilitate involving the most vulnerable in decision-making about geoengineering.