Disasters include catastrophic events that would severely impede scientific progress, such as supervolcano eruption, asteroid impact, cosmic rays, climate change, pandemic, nuclear war, biological warfare, an explosion of nanotechnology, and so on. The risk of such disasters and others are assessed in Bostrom & Cirkovic (2008).

A bit as a side note, but I don't think going into the details here adds much to your argument; Bostrom's discussion of global catastrophic risks is interesting but it's a distraction here.