The carbon cycle contained the CO2 levels to a range that did not exceed around 350 ppm for at least a million years until the industrial age.
Since the industrial age we have overwhelmed the ability of the carbon cycle to hold back CO2 levels and the CO2 that can't be assimilated into carbon sinks is building up in the atmosphere.
Forests have been overwhelmed and with warming oceans their ability to take in CO2 is lessening.
Once positive feedback mechanisms kick in its game over and humans will need some incredible innovations to once reign in CO2 levels.
