The CO2 graph over decades is painfully clear.[1] From 321ppm in 1970 to 428ppm in mid-2005, measured in Hawaii atop Mauna Loa, far from any major CO2 sources. Everything else is noisy and statistical, but the CO2 measurement increases very steadily.
Yes, and the scary thing is that soon the atmospheric carbon PPM will be high enough to start affecting how we think, act, and feel on a day to day basis.
Surprisingly, no. Humans adapt to higher CO2 concentrations over a period of days to weeks. Submarines run as high as 5000ppm, which is way above normal atmospheric concentration.[1] Many indoor environments are above 1000ppm.
This seems to be like high altitude adaptation. It's going back and forth between concentrations that causes problems at moderate concentrations. The adaptation doesn't happen.
[1] https://www.nationalacademies.org/read/11170/chapter/5#51