The carbon in fossil fuels came out of the atmosphere. Burning fossil fuels will put the carbon back in the atmosphere. The concentration of CO2 in the atmosphere today is a bit less than 400 parts per million. Back at the beginning of the Carboniferous period 360 million years ago it was 1500ppm (parts per million) and declined to close to present day levels by the end of that era. CO2 increased thereafter, peaking at 2500ppm during the Jurassic period 120 million years ago. It has been declining ever since. There has been a slight uptick in the modern era but we are are going to have to burn orders of magnitude more fossil fuel to get CO2 levels back to where they were. We’re talking tens of thousands of years, assuming we can find it.

Funny thing is, though, the planet survived those high CO2 concentrations without turning into a fireball. In fact, during the Ordovician period 450 million years ago, when CO2 concentrations were above 4000 ppm, the Earth was a snowball. This link provides heaps of useful information about CO2 concentations and global temperatures over the long term. The author notes:

The Carboniferous Period and the Ordovician Period were the only geological periods during the Paleozoic Era when global temperatures were as low as they are today. To the consternation of global warming proponents, the Late Ordovician Period was also an Ice Age while at the same time CO2 concentrations then were nearly 12 times higher than today– 4400 ppm. According to greenhouse theory, Earth should have been exceedingly hot. Instead, global temperatures were no warmer than today. Clearly, other factors besides atmospheric carbon influence earth temperatures and global warming.

The current low CO2 concentrations are virtually unprecedented in the history of the planet over the last 500 million years. It is our solemn duty to pump CO2 back into the atomophere. Plants will thank us and reward us with more O2, food and biomass for conversion to fuel.