Our ape ancestors evolved in an environment where temperatures were far higher than today. In fact, we're in a cool phase, as far as the global climate is concerned.
In longer scale, Ocean gets warmer, plankton dies, we start running out of oxygen. Not immediately - but slowly the percentage of it in the atmosphere will start dropping. I wouldn't be surprised if in a 100 years it will be common to have oxygen supply stations on the streets and if you can afford them - personal supplies of oxygen just so you can operate at 100% mental capacity. The rest will have to make do with feeling crappy all day long.
And of course we're already starting to see signs that we might have triggered a self reinforcing loop - melting clathrates releasing methane, methane heating up the atmosphere, releasing more methane.
No, humans won't die, absolutely not. But I'm 100% certain that the civilization as we know it is fucked.
[0]https://link.springer.com/article/10.1007/s12576-016-0501-0
Climate change, at any large enough timescale, is completely unavoidable. It's also (to me) inconceivable that humanity will agree to limit its standard of living now just so that climate change can be somewhat mitigated.
> No, humans won't die, absolutely not. But I'm 100% certain that the civilization as we know it is fucked.
In that regard, I'd be much more worried about the prospects of a nuclear war than about climate change.
Climate change seems like just the sort of thing that might start wars.
Again, some of our ape ancestors evolved in a time with far lower oxygen levels. After that, oxygen levels rose higher, but they have been drifting downwards for millions of years since then.
If fossil fuels become too expensive to pump out of the ground, some other process of creating them will replace that. Perhaps it'll come out of the atmosphere: