The Earth has a natural CO2 cycle that moves massive amounts of CO2 into and out of the atmosphere.

The oceans and land vegetation release and absorb over 200 billion metric tons of carbon into and out of the atmosphere each year. When the cycle is balanced, atmospheric levels of CO2 remain relatively stable.

Human activities are now adding about 7 billion metric tons (Gt) of carbon into the atmosphere every year. Since about 740 Gt already are in the air, that's an addition of about 1 percent each year, though recent growth rates are higher.
But that's enough to knock the system out of balance, surpassing nature's ability to take CO2 emissions out of the atmosphere.

Of the total 6-7 billion tons (Gt) of carbon we put in the air, only about half remains there, for a net increase of about 3 Gt a year.
This is what is causing the rapid buildup of CO2 (remaining airborne for 100 years or longer) a buildup that dwarfs natural fluctuations.

The other half is taken up by ocean water or marine organism, stored in plant tissues during photosynthesis, buried in sediments or peat bogs or sequestered in one of several other natural carbon-holding systems known as reservoirs, or 'sinks'.

It's not clear how much more these sinks can hold because their various mechanisms to store and release carbon are complex and incompletely understood.