It seems unlikely to me that the results of climate change will be so dramatic as to kill 80% of all humans, unless climate change sparks some sort of global war involving nuclear weapons. That doesn't sound very good for the planet.
https://www.thoughtco.com/bacterial-growth-curve-phases-4172...
Yes we can go ahead blindly, and destroy our living space and therefore our viable numbers. Or, we can control our growth, and bring it back to sustainable levels.
If we destroy our environment (pollution, climate change, overfishing, etc), it will hold less people sustainably than if we don't destroy our environment and reduce our numbers voluntarily to those current sustainable levels.
But, as I indicated above, humans are incapable of acting in their own best interests and it will happen forcibly by Nature and not to our liking.
https://www.smithsonianmag.com/history/climate-wars-conflict...