erm Just in case it has escaped anyone's notice, if you put garbage (stuff that is not normally there in large quantities) into the water, you get polluted water. If you put garbage into the ground, you get polluted ground. So it kind of follows that if you put garbage into the air .....
So yes, I think humans have have an impact on the atmosphere, just not as much of an impact as the Chicken Littles rant about. It is not as if nature does not have air filters and other ways of dealing with H²S (it makes a very nice acid rain and components of the ground like calcium react with it) and other compounds.
It is just my theory. =)