Effects at https://en.wikipedia.org/wiki/Hypercapnia
Anyway drowned infrastructure will directly or indirectly kill a lot of people and that could mitigate the growth of all greenhouse gases except maybe H2O, because there is more and more of that in the air as temperature increases. It's a reinforcing feedback loop.
That means that CO2 levels would have to increase 25 times their current level in order for your fear of hypercapnia to become a problem.
> A study of humans exposed in 2.5 hour sessions demonstrated significant negative effects on cognitive abilities at concentrations as low as 0.1% (1000 ppm) CO2 likely due to CO2 induced increases in cerebral blood flow. Another study observed a decline in basic activity level and information usage at 1000 ppm, when compared to 500 ppm.
With some caveats about the quality of those studies.
considering how many people are allowed to die of air pollution as a matter of standard policy, even in "nice" western countries and cities, I don't know that this argument holds
https://keelingcurve.ucsd.edu/
And yes, it is as dire as it sounds. My children are breathing 1/3 more CO2 than my parents were at the same age. But it's not the breathing that is a concern (not until we get above 1000 ppm at least), but rather the warming. There is significantly more sunlight energy being trapped in the biosphere today than there was two generations ago.