Everything we're using is already part of the earth anyway. Seems like we're just evaporating it into the atmosphere and going through a water cycle with different materials. Digging it up is condensation, burning it as fuel is evaporation, and I would think it would eventually have to reform. It will then come back to the ground even and everything will still be the same. I would think it would really be more of a danger if we were using foreign substances that the earth isn't used to.