It is easy to measure for any gas mix with O2 the only oxidizer. Just measure the pressure is a closed container before and after passing electric current thru wire in the container, is how this physicist would do it. I.e. as red hot wire grows thinner the resistance will increase. When that stops, turn the current off an let internal temperature return to the original temperature. The pressure will then be lower as all the O2 is now in the metal oxide. - I bet chemists have even better ways. Your assumption that extinction will be by "suffocating" (lack of O2 or excess of CO2) is almost surely wrong. With AGW, both the peak temperature and humidity are increasing. When the wet bulb temp is 35C (95F) for an hour of exposure to that, you die. - internally over heat as can't dump to the environment your metabolic heat, even if just sitting in a chair.* Dogs can only perspire via their tongues, so they will all die before you who can perspire with most of you skin doing so. Humans, not in "couch potato" mode but in their ancient state are really good running machines. Could run all day long after many animals that could not perspire via their skin. They eventually died of internal over heating. - I believe this is a main factor in why man became the dominate creature, not his bigger brain, which had not yet become "bigger." Then he learned to cook roots etc. so didn't need to spend most of his awake hours munching on leaves etc. as apes do between his infrequent animal kills. With "free time" the discovery of cooking gave, his small brain became larger and useful and improved with use plus evolution selection, but it all started with being the world's best running machine. * the rich will have air conditioning but that soon fails when linemen can't remove tree branches from wires etc. - no food delivered etc.