The carbon that is found in fossil fuels does not cycle through the earth, rather it is stored locked in these fossil fuels.
When human beings burn fossil fuels, the stored carbon is released into the atmosphere as carbon dioxide. Once this happens, the carbon in the CO2 is then free to cycle through the earth and can thus upset the natural balance of carbon available, disrupting the way the natural process of carbon cycling occurs.
The carbon that is in storage should remain in storage unless released by purely natural means and not by unnatural actions of human begins.