Once, when the Earth was young, its atmosphere was a nasty mix of
CO2 and Methane. No oxygen. Maybe down by a hydrothermal vent in the ocean life was stirring. Some of them got near the surface and needed pigments to protect them from the sun. So happened some of these pigments also helped absorb light and channel
energy into the metabolism. So they got to eat CO2 and pollute the planet with oxygen - which was very detrimental to many other species. But these proto-plants developed into the plant kingdom, and buried all that carbon. So if you think we're a good way through the Earth's stockpile, think again: there's
enough down there to really scupper the climate. It's within the ingenuity of mankind to extract it all. If we're really Homo sapiens, the wise one, we'll quit ASAP.
We have a unique role on this planet: we alone have the capacity to sense large meteors on a collision course, and we're not far off being able to deflect them, and protect all the other surviving life-forms. We may need fossil fuels to do so. And in the long distant future, the Sun will become a red giant, starting to swallow up the innermost planets. Time to go swapping
solar systems - that'll be a long ride, a long time before we get there, during which the Earth would have to be coccooned to retain energy, and burn existing stocks slowly. I don't know if that'll be possible, but if we're still around, that'd be our duty to try.