Too much coal. The earth could only have supported so much biomass,
including tree bark, tree trunks, and every other kind of plant/animal life
you want to include.
I looked up some US Almanac data to test this: Here are some numbers:
900,000,000 tons per year (US) are mined
2,000 pounds/ton
1,800,000,000,000 pounds/year (US) are mined
150 years of mining
270,000,000,000,000 pounds in 150 years have been mined
3,787,425 Area of the US (sq mi)
x 5,280
x 5,280
105,587,349,120,000 Area of the US (sq ft)
3 pounds/coal per sq ft
If it takes 10 pounds of biomass to form 1 pound of coal, this means 30
pounds of biomass
per square foot must have existed in Noah's time, and ALL of it converted
to coal. And all that just to create the coal that has already been mined.
I am told there is still quite a bit left).
All calculations above are very rough; refinements welcomed.
Burgy