I'm never certain whether I'm just imagining this, but you look out the window of an airplane at 30,000+ feet, and the sky isn't as blue as it is at ground level. And even standing on top of the kind of mountain an average person can climb - I made it to the crater plateau, but not the summit of Kilimanjaro (altitude issues) - and realizing that every breath you take has only half as much air in it as you're used to - makes you realize just how thin this atmosphere is.
I think scientists really do a terrible job of explaining that. When you look out the window, it appears the atmosphere goes on forever. Why not burn coal, let cars exhaust, and generally treat the atmosphere like a cheap sewer?
In reality, the more I've thought about this, the more appalled I become.
Many people have issues at 10k feet elevation. The FAA starts having different o2 requirements at 10k feet. 12k elevation gets very challenging to climb.
Yet I'll drive 12k feet to go to the grocery store and never thing twice about it.
