There is also evidence water levels used to be much higher.

I theorize that lots of the carbon based fuels we extract from ground and burn would have eventually worked its way to the surface (depressurized) via volcanoes or tar pits if man were not intervening and depressurizing the reservoirs via extraction. A single volcano eruption is equivalent to one year of 400 million motor vehicles chugging gasoline. So I've heard. Meaning that nature can pollute its own atmosphere with volcanic ash. It seems the ring of fire is near where lots of natural oil reservoirs are. My theory could be wrong though.