This is honestly one of the scenarios that scares the shit out of me. For all of the problems we see coming down the pike w.r.t. climate change, a major volcano would yield a lot of ecosystem collapse in novel/unexpected ways. All of this would force the existing political system to actively make choices about geoengineering that are very likely to be badly informed, poorly executed, and insufficiently tracked. Once we're geoengineering on the fly I suspect we'll lose the whole planet.