What can happen to our planet that would make it less hospitable than Mars or deep space? Most of the threats (global warming, asteroid strike, etc) still leave Earth with a usable atmospheric pressure and breathable oxygen, they just affect food supply (there is no food on Mars), or other things that still leave you better off than facing the vacuum of space.
I remember reading a story when I was a kid, about a future society that moved underground to get away from unfavorable planet conditions. The description was remarkably like what I'd picture as a large scale off-planet settlement -- everyone living in a rather confined space, and kids only knowing about what Earth was like from reading books and watching old movies.
If you believe the James Lovelock and the Gaia hypothesis, the environment is stabilised by multiple negative feedback loops. However, the feedback is limited and will only compensate for a certain range of inputs - if these are exceeded, the entire system could "flip" into another stable configuration. Lovelock suggests that this has already happened once on Earth; the change from an anaerobic atmosphere to the present oxygen/nitrogen atmosphere, caused by the "pollution" excreted by anaerobic lifeforms (the bacteria now mostly relegated to your intestines).
It's an interesting hypothesis with a fair bit of research ("daisyworld" simulation of global temperature stabilisation via albedo effects, the methyl iodide cycle, etc.)
I believe his current position is that we're all doomed and the "flip" is underway.
I remember reading a story when I was a kid, about a future society that moved underground to get away from unfavorable planet conditions. The description was remarkably like what I'd picture as a large scale off-planet settlement -- everyone living in a rather confined space, and kids only knowing about what Earth was like from reading books and watching old movies.