Well, one point I didn't make here but that made me pick that example is that, with the exception of a gamma ray burst, there is no tragedy that could realistically affect the Earth in such a way as to make it even CLOSE to how uninhabitable Mars is.

Global warming, nuclear winter, asteroid hit, massive solar flare, supervolcano, plate tectonics - anything we care to imagine will leave the Earth much closer to being habitable than Mars.