The atmospheric concentration of oxygen is 21%, while the minimum necessary for humans is around 17%, therefore only roughly 1/4 of the atmosphere would have to be respired before the concentration could no longer support people. Global warming caused by higher CO2 levels would be a complicated issue. The amount of carbon dioxide that can be stored in the ocean is very limited, so there would be less space for other gases such as O2 or N2 for example. However, since CO2 is a gas that allows plants to grow more efficiently, removing it from the atmosphere may not be as disastrous for humanity as one might think.