Economically efficient injection of aerosols into the stratosphere for the purpose of deflecting incoming sunlight and managing the Earth's energy budget would require high-altitude deployment platforms. Studies suggest that high-altitude injection at 25 km would substantially enhance the forcing efficacy of the aerosols compared to injections at 20 km. While platforms capable of lofting and releasing aerosols up to an altitude of 20 km have been explored in other studies, similar studies assessing the feasibility of deployment platforms at an altitude of 25 km seem to be lacking. No existing aircraft is suitable for this purpose. In this paper, we review five possible concepts for deployment at 25 km and conclude that all of them would multiply costs, complexity, and operational risk substantially relative to deployment at 20 km.
