Asteroid/​comet impact, super volcanic eruptions, and even nuclear war risks are quantifiable within an order of magnitude or two: link. There are additional uncertainties in the cost and efficacy of interventions such as storing food or alternate foods. However, if you value future generations, one-three orders of magnitude uncertainty is not a significant barrier to making a quantified case.
Asteroid/​comet impact, super volcanic eruptions, and even nuclear war risks are quantifiable within an order of magnitude or two: link. There are additional uncertainties in the cost and efficacy of interventions such as storing food or alternate foods. However, if you value future generations, one-three orders of magnitude uncertainty is not a significant barrier to making a quantified case.