Humans can definitely affect the amount of CO2 in the atmosphere; every time we exhale, we emit it.
This. I recently had an inspiration for an editorial cartoon in which the EPA would be putting the screws to a Lamaze class. Unfortunately, editorial cartoons need be punctual and I was a little behind on my inspiration.

It is hiliarious to me that the American government did officially declare CO2 to be a regulated pollutant, and you don't need a cartoon to laugh at that.