Until we started burning fuels, it balanced itself. What we call fossil fuels is literally the buried carbon of life that came before. The stuff the natural process already dealt with once before. We brought it back, and now it all has to be reprocessed and put back in the ground or deep ocean where we got it or there's no balance in the system and it will take very long times for natural processes to bring it back to normal levels, maybe never.

Imagine a full bath tub with the drain open and the water running., Water is flowing in and out at an even rate, meaning the tub is draining but stays pretty much full. Now imagine somebody took the drain pipe and routed it back into the tub. Now the drain has to deal with that water again, and the new water coming through the tap.