Yeah, the Jet and Gulf streams are serious causes for concerns, and would absolutely play havoc with the world that we know.

I'm just of the opinion that our technology will insulate us against the worst of it. I could easily be wrong, in fact history says I likely will be.

We've experienced collapses in civilization and 'dark ages' because of climate change at least once before, the Bronze Age.

I think our current civilization is both more resilient than ancient cultures, and at the same time much more fragile. Our technology should make it considerably more difficult to experience a significant collapse, but once it starts I think we we would be much worse off than previous civilizations.

Billions would die from stuff we take for granted on a daily basis.