Carbon is a greenhouse gas. Just look up greenhouse gas theory. 3% is significant. If we were generating some insane number like 50%, the earth wouldn't be inhabitable.
Actually, carbon is an element on the periodic table. I assume you were referring to carbon dioxide. If we were generating 50% of the earth's carbon dioxide, we would still only be responsible for 0.019% of the atmosphere. Carbon dioxide is food for the plant life all over the globe, not to mention all the plant life under the oceans.
I believe you were trying to say that if man generated 50% of the total amount of CO2, the earth wouldn't be HABITABLE.
Oh, and my ciphering was a bit off. man is not responsible for 0.114% - it's actually at most 0.00114%.