What do you suppose will happen when hundreds of millions of people find that their local geography no longer supports human habitation? They will be welcomed into neighboring regions with open arms?
Unfortunately, killing hundreds of millions of starving people can be done very easily -- much more easily than solving the underlying problem. The areas that become uninhabitable will then not be able to produce more starving people, so such an atrocity need only be carried out once.
The worst part is, once the killing starts, people will rationalise it to themselves, saying "We didn't have a choice".
Of course they will be. That's just what happened during the Syrian civil war; the refugees were welcomed with open arms in Europe, Germany didn't experience any political turmoil that reverberates to this day at all.
I guess I'm in a sarcastic mood today. Yes, of course that will cause mayhem. It's one of the reasons I see climate change as such a threat to our all civilization. We're fearsome tribal creatures and the coming waves of desperate peoples will almost certainly play into the hands of nationalist fearmongers and firebrands.
I wonder if we will see some kind of -literally - ecofashism rise from that.
The worst part is, once the killing starts, people will rationalise it to themselves, saying "We didn't have a choice".