...Earth is a closed system for the most part.

True, but it is also a dynamic system and continually adjusts itself through various feedback mechanisms.

An increase in carbon dioxide causes an increase in plant growth which, in turn, cause on increase in atmospheric oxygen which is returned to carbon dioxide by animals. Around and around it goes. An ever-changing system.

The system also corrects problems like pollution as best it can. About 35% of atmosphereic pollution is removed each year by natural causes such as rain. If we reduce our pollution, things would return to "normal" in a few years. Some folks try to scare people into thinkng that air pollution is permanent and that what went up since the start of the Industrial Revolution is still there. Probably not much.