I doubt that increased atmospheric CO2 concentration is going to be a direct threat to human health. Humans have for a fair amount of time at this point, spent

I doubt that increased atmospheric CO2 concentration is going to be a direct threat to human health. Humans have for a fair amount of time at this point, spent most of our time (and decision making time) indoors, where CO2 concentration grows far higher than atmospheric. In fact, reaching the 1000s ppm is already fairly common, and does not seem to have lead to major obvious health problems (although it does, as you identify, lead to short term cognitive decline). The bigger problem is that in our quest for energy efficiency, we would further reduce ventilation, increasing the indoor CO2 concentration; but this can be countered with better ventilation (using a thermal exchange system so it is more energy efficient than opening a window). The reduction of indoor CO2 possible from this is probably greater in most places than the ~100ppm increase in background concentration.