As we burn things (or even breathe in and out) we terrapods release carbon dioxide into the atmosphere. The trees and other green beings with roots in soil appreciate this carbon dioxide, breathe it in and exhale oxygen into the atmosphere.
We like that oxygen. Go to an old forest and simply breathe in and out to appreciate your connection with the plant kingdom. (image by David Tarsi)
By the way, most of what we breathe in is nitrogen. The atmosphere is primarily nitrogen, Plants love nitrogen and the atmosphere is full of it. Terrapods don’t do much with nitrogen except pass it through lungs on a regular basis.
This oxygen - carbon dioxide exchange with the plant kingdom is a pretty good cycle and it has tended to balance itself over geologic time by means that are pretty complex and not completely understood. Homo Industrialus, the 6 billion human beings on the planet today, are clearly sending this system out of balance right now. There really is no question that the number of human beings with their current living habits are loading the atmosphere with carbon dioxide.
One of the ways that our biosphere responds to higher level of carbon dioxide in the atmosphere is for the oceans to simply absorb a higher level of carbon dioxide. The oceans are really big and it is simply stunning that carbon dioxide absorption by the oceans could change the oceans, but that is the case.
As we “firemen” have burned everything imaginable for power, warmth, and nutrition we have loaded the atmosphere with so much carbon dioxide that we are changing the acidity of the planet’s oceans. The ocean absorption of carbon dioxide has raised the acidity of the oceans and that has profound consequences for all the sea life that works with shells. The “shell” life in the ocean is dissolved by more acidic oceans. We are talking about knocking out a significant portion of sea life with consequences that we cannot foresee.