Well, I can tell you I have seen the effects of humans on the atmosphere, every time I descend into LAX or SAN, there is a huge brown layer of smog you come into at about 5000', give or take a few, depending on temps and surface winds. It has gotten better in the past 20 years, due to the emission controls put on the cars, and better fuel blends, etc. but it's still there.

And then there's China and India, you should see some of the smog they generate! It makes the worst day in LA look like a spring morning in the alps!

Now, compare that to what Mother Nature throws up into the atmosphere all by herself, in the way of volcanic ash and forest fire smoke, and the big red dust storms coming off the African coast, well, I'd call it a draw.

BUT...one thing is certain, we humans can at least try to do something about it, in the way of conservation and/or emissions reductions. Will it change anything, long term?

Who knows, but at least we could try to clean up after ourselves. It couldn't hurt.