Deforestation leads to less O2 because trees and other photosynthetic plants make O2 for us to use. Less O2 leads to less people. Too much CO2 leads to a hotter world, and melting ice caps, which means that one of our most important clean water (or non-salt) is gone. Less drinkable water leads to less people, since people, like just about every other living thing, need water to survive. Air pollution leads to unclean air (no, really?) which we need to breathe and maintain homeostasis. Air pollution also leads to acid rain, which is bad for fish and other organisms in the oceans and lakes which we eat and that make O2 for us (the photosynthetic ones, at least...) to respire. And of course, acid water also leads to a decline in drinkable water. Everything is like a fishing net. If one string breaks, the fish can escape... I guess that doesn't really relate to the question, but the 'jist' of it is that if one bad thing occurs, it is very likely that the others will follow too, leading to the destruction of the world, or human population.... And now tat I think of it, a decline in human population may be GOOD for the Earth, since we are the ones doing all of this stuff to the world... What a thought!
