The fundamental reason why carbon dioxide in the atmosphere is critically important to biology is that there is so little of it. A field of corn growing in full sunlight in the middle of the day uses up all the carbon dioxide within a meter of the ground in about five minutes. If the air were not constantly stirred by convection currents and winds, the corn would stop growing.
Vegetation is really controlling what happens...whereas the emphasis in the climate models has always been on the atmosphere.
We simply don't know yet what's going to happen to the carbon in the atmosphere.
