From what I understand carbon comes from the soil as well as from the air, and it's supposed to be "stored" back into the soil by various means but our agriculture and more generally human activities tend to accelerate release of carbon from the soil and refrein the storing process. And that provokes a climate disimbalance. There was a good video on the relation between carbon cycle and massive extinctions throuhout history (1h long) : https://www.youtube.com/watch?v=uxTO2w0fbB4