Reply #30: Could happen

[cosmicaug]

The OP hypothesizes that there is enough CO2 to make the air viscous and buoyant.

That amount of CO2 would displace virtually all the O2 available.

As long as you sufficiently increase the pressure, it does not need to displace all the oxygen. As long as there is about one fifth of an atmosphere worth of pressure due to oxygen it doesn't matter if the pressure due to CO2 is over 300 atmospheres because there will be enough oxygen (actually, you'd have a major fuckup with the bicarbonate buffering system in air breathing organisms but that's not really related to oxygen being displaced). Of course, I just looked at the CO2 phase diagram and things get a little weird (supercritical fluid) at well below those pressures.

Of course, it's still retarded.