Well, if we’re thinking apocalyptic scenarios, where we have massive run-away global warming and end up somewhat like Venus, it’s probably be a competition between lots of different effects. The thing is, humans are pretty quick at adapting, so some of us will be able to move to higher ground to avoid flooding, and build some kind of shelter from the heat, build heavy homes that aren’t damaged by extreme weather.
The problem is not so much what we humans would do, but the damage that will occur to the biosphere as we go along.
It wouldn’t take much destabilisation of the weather patterns for us to have serious problems growing enough food to feed everyone. It’s not like we can easily build sci-fi domes over the countryside! With populations starving I suspect we’d quickly slide towards a situation of warfare everywhere as countries tried to gain control of drinking water and farmland.
In all honesty I think the prospect is terrifying. This is why I believe we should try our absolute utmost to try and understanding what’s going on with climate change (work like John is doing!), and to move as quickly as possible to de-carbonise our energy infrastructure and reduce our environmental impact. It’s going to be expensive to do this, and it will require massive multi-national cooperation, but I think it’s a better future to aim for than one of famine and war.