This is all very true. No one "knows" that global warming is fact. There is a general consensus among scientists all over the world that the climate is changing. Proving that this is caused by human pollution is difficult if not impossible.

Keep in mind that the tobacco companies had dozens of scientists stating that there is no, nor has ever been evidence that nicotine is addictive and that smoking doesn't cause cancer.

Most people dismissed the threat of acid rain, until huge swaths of forest started dying.

My argument is not that it is or is not happening, I'll try to keep my beliefs to myself in this regard. Just that NO ONE can dismiss it off hand, particularly when there is (at least) some evidence supporting that theory.