1) The shift in the chemistry of the atmosphere, coupled with the removal of forestation, certainly can change the environment, which will change the climate, and the weather. 2) Overpopulation of the world is the root cause of all of that. 3) I do not think we really have enough historical perspective to say - for certain - that the changes observed were 100% the result of the two items above. From a scientific and logical perspective, it make sense that there is some impact, but I don't think there's enough scientific evidence to prove, beyond a doubt, that is is causing all of the observed "changes". Every system has some natural variability in it. That GISS graph shows that clearly. Are the ups and downs man-made? Not 100%, no, but denying that there is some impact is ridiculous, as well. The thing that concerns me is that when you try to compensate for the natural variation of a system, it is called "tampering", and usually results in even more variation, long term. That makes me wonder how much "correcting a man-made problem", and how much "tampering with natural variation" is going on, and going back to point 3, I don't think we know.