Mankind's activities have upset the CO2 balance
Humanity's extraction and burning of long-buried fossil fuels for energy releases CO2 into the atmosphere. This has significantly disturbed the natural carbon cycle.
The natural processes that could restore the balance are too slow compared to the rates at which human activities - also including deforestation - are adding CO2 to the atmosphere so the balance of the carbon cycle remains disturbed.
As a result, a substantial fraction of the CO2 emitted from human activities accumulates in the atmosphere, where some of it will remain not just for decades or centuries, but for thousands of years.
How do we know which CO2 is natural and which is due to human activities?
We can identify which CO2 is from burning fossil fuels as it has low amounts of the carbon isotope 13C and none of the isotope 14C.
Measurements show a decrease in 13C and 14C together with a small decrease in atmospheric oxygen concentration. This shows that increases in CO2 are primarily caused by burning of fossil fuels, and from deforestation and land use changes that also release carbon.
(An amount equivalent to about 1% of the CO2 due to burning fossil fuels can be attributed to volcanic activity.)
This paper explains the analysis based on isotopes in more detail.
It also includes this important fact "The increase (in CO2) between the year 1800 and today is 70% larger than the increase that occurred when Earth climbed out of the last ice age between 17,500 and 11,500 years ago, and it occurred 100-200 times faster."