On IQs, I remember seeing higher CO2 levels can lower IQs a little bit, higher CO2, IQs get lower. Maybe that could be part of the reason at least for that.
And this would creep up on us very slowly. Not only would we not notice, but we’d lose our ability to notice. It’d be like the whole species slowly descending into dementia.
Most of what I’ve seen suggest that the effect starts really kicking in around 1000ppm. But as outdoor levels rise, indoor levels, which are usually higher, rise too. They can already hit those levels.
If we keep burning more and more carbon outdoor levels could hit that by the 22nd century.
If we start getting here we need to put away the green moralistic finger wagging and start looking at ways to suck CO2 out of the air like it’s an emergency. Things like ocean seeding or huge scale industrial scrubbers powered by solar or nuclear power, or all of the above.
This is a far scarier possibility than just climate change. The only thing scarier than this is the clathrate gun, which luckily seems to be considered unlikely by most climate scientists (at least unless we get to really insane CO2 levels like thousands of ppm).
Good news on that front. Because you constantly produce waste CO₂, you have detection mechanisms letting you know when the amount in your lungs is excessive.
You picked the only gas where accidental suffocation isn't a possibility as the one you wanted to worry about.
CO2 outdoors has gone from about 280ppm to 420ppm in the last hundred years. If we kept going for another century it could approach 1000ppm, which is where you start seeing these effects.
The scary question is whether there is any effect at 420ppm. We don't think so, but very marginal effects are hard to detect. Yet a global loss of an average of 5 IQ points would have significant large scale effects on society. It's also possible that it doesn't affect all people equally, since most things don't.
There have been times in Earth's history where CO2 concentrations were in the thousands of ppm, but there were no humans then.