If average temps go up 5°C (the current fear is that 2°C is the deadline, and at 3°C we're experiencing CATASTROPHIC DAMAGE including major loss of breadbasket regions, mass famine leading to mass revolt (think Arab Spring x 100)) then we are long, long past the current idea of modern civilization, billions of humans etc. At 5°C I bet 75% of world population has died to either lack of resources or wars over scarce resources.

https://www.ipcc.ch/sr15/chapter/chapter-3/

I'd link more to IPCC but their website appears down, so sorry.

I will say that the 5 degree scenario is called the "RCP 8.5" and is basically the worst disaster scenario published here https://link.springer.com/article/10.1007/s10584-011-0149-y