No, it wouldn’t be comparable to what it was like before life. If nothing else, the oxygen in Earth’s atmosphere was made by, and is maintained by, living things. In fact, life is responsible for major changes in Earth’s geology; more than half the minerals found on Earth require the oxygen in our atmosphere to form.
But to a lesser extent: yes, by burning fossil fuels we’re returning carbon that has been buried and kept out of the atmosphere for eons, and returning the Earth to a state of higher carbon dioxide in the atmosphere. Before the formation of those deposits, Earth was much hotter – there were palm trees at the poles – and in the most extreme climate change scenarios that’s basically what we go back to. (Realistically, not all fossil fuels are economical, and that shift will take a very long time. So we won’t see that in our lifetimes even in the worst cases.)
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