Persistently rising atmospheric greenhouse gas concentrations challenge dominant Liberal hopes that science and multilateralism might deliver rational, global climate outcomes. Emerging Realist climate approaches that take geopolitics and national interests more seriously have yet to explore Morgenthau’s concern that ‘scientism’ – exaggerated faith in scientific rationality to solve political problems – would lead to disastrous underestimations of power and irrationality. Recently, Realists have mooted ‘solar geoengineering’ designs as a ‘lesser evil’ option to deliberately cool the Earth independently of emissions reductions. However, assessments of solar geoengineering prospects barely factor in Realist concerns, focusing instead on idealised scientific modelling of bio-physical effects and Liberal governance scenarios. To explore how geoengineering techno-science would be ‘translated’ into security assessments, geopolitical logics were elicited through interviews and group discussions with (mainly Arctic-oriented) national security professionals. Security experts reframe solar geoengineering in three significant ways: (a) from a climate ‘global public good’ to a source of geopolitical leverage and disruption; (b) from a risk-reduction tool to a potential source of distrust and escalation; and (c) from a knowledge-deficit problem solvable by more research, to a potential disinformation vector. This expands Realist scholarship on climate change and identifies serious risks to ongoing scientific and commercial pursuit of such technologies.