Congratulations on becoming confused by your own analogy, that's impressive. You were making an analogy between the relative size of all humanity's vapour emissions and natural vapour sources, and the relative size of one person to all of the people on the planet.

So saying there is a kind of slippery slope because as soon as one person decides to pollute then the next person decides to pollute etc. is nonsense because it corresponds to another entire population of Earth deciding to emit water vapour.

And er, no, not poisonous. Do you know why Earth isn't as barren as the surface of the moon with its extreme temperature variations? Guess what, it's greenhouse gases. Insulation from space is poison? Okay. You realise your sentence reads like something on trollscience.com ?