One way in which geoengineering increases societal fragility is if we pump particles into the atmosphere and then find ourselves obliged to keep pumping particles into the atmosphere in order to maintain the effects, and then suffer a significant collapse of infrastructure that makes us not capable of this any longer. This could result in extremely sudden warming and a rapid, unpredictable change in weather patterns. Something would have to go very wrong first, of course, but it could compound an existing catastrophe and take it from recoverable to irrecoverable.
See the recent paper by Parker and Irvine on termination shock. The catastrophe required to terminate solar geoengineering efforts would be extraordinarily specific, making the use of planes or hot air balloons impossible for months or making the production of aerosols such as sulphates impossible for months. While this is possible, it doesn’t seem like a big enough risk to make solar geoengineering a significant concern—how exactly could this happen? Other parts of our infrastructure, such as the production of fertiliser, could also be interrupted by some incredibly specific catastrophe, but these aren’t usually thought to be among the top risks we should consider.
As I argue in a recent paper on solar geoengineering, many of the alleged risks of solar geoengineering are overblown. The main problem is getting governance of it over an extended period, which looks extremely difficult, but would presumably also be a disincentive to use it in the first place.