Unless you're a believer in the theological "young Earth" claptrap, it's obvious that biological and geological forces have been sequestering enormous amounts of carbon in the earth's crust for hundreds of millions of years. It's also obvious that CO2 is a greenhouse gas.
The notion that putting a large amount of that sequestered carbon back into the atmosphere in a few hundred years will have little to no effect is wishful thinking and denial. Might as well take the POV of Reagan's secretary of the Interior, James Watt- none of it will matter when the Rapture comes.
Quite what will happen is, as you suggest, difficult to predict, particularly given that any changes will lag increased CO2 concentrations for decades, if not longer, and that there are other factors very much in play. None of that denies the obvious fact of high correlation between Man's use of fossil fuels and increased CO2 levels in the atmosphere. Current levels are well above anything recorded for the last 400K years, and climbing fast.
A thoughtful and non-hysterical article on the subject-
http://www.daviesand.com/Choices/Precautionary_Planning/New_Data/