The carbon gets trapped in rock and then subducted. This explains Venus, there is no subduction on Venus so all the carbon stays on the surface. In the far future(1 billion years) the ever increasing luminosity of the sun will boil the oceans away causing subduction (subduction needs water) to end on Earth. Thus carbon will have nowhere to go and we will become Venus.
Volcanoes constantly replace carbon in the atmosphere but subduction regulates the amount of it, take the regulator way and massive buildup will happen.
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