Natural emissions are balanced by natural sinks, so that the balance is close to zero, while human emissions are not balanced by anything. The actual variation of CO2 levels in the atmosphere is almost exclusively due to human emissions tipping the equilibrium.

The effect of CO2 on the climate is old science, it goes back to the 19th century, it seems a little bit insincere to ask this kind of question, if you had actual interest you would have found the answer very easily already.