"Horrible catastrophe" works, but it doesn't stop there. How are humans and our political and military structures going to react?
It is debatable how close we have been, during various past crisis, to an actual large-scale nuclear conflict. I don't think we can deny our raw capability to cause a near-extinction-level with our existing technology (nuclear, biological or otherwise). Our political and military ecosystems, flawed as they may be, have kept that capability in check.
But would such a "horrible catastrophe" stress that ecosystem way past the point where it will remain confidently under control? Absolutely yes, I believe that beyond any reasonable doubt.
With our global structure out of control, is there is a significant probability for that destructive capability to be put into action? Absolutely yes, I believe that probability to be significant, beyond any reasonable doubt. I don't see how it can't be otherwise.