http://en.wikipedia.org/wiki/Paleoclimatology#Earliest_atmos...
Wikipedia says that the earth's earliest atmosphere was a stunning 10% C02.
Total coal reserves are 930 billion (non-metric?) tons: http://www.eia.doe.gov/oiaf/ieo/coal.html
But the earth's atmosphere is 5 quadrillion metric tons: http://en.wikipedia.org/wiki/Atmosphere_of_Earth#Density_and...
So getting to 10% is impossible. Adding in oil, bitumen, and natural gas probably won't get you past 1% (which is still pretty bad: http://en.wikipedia.org/wiki/Carbon_dioxide#Toxicity .)