Earth after most disasters short of getting hit by a full-on planetoid that liquefies the entire crust is still much easier to live on than Mars, and hardened Earth-based bunkers are a lot cheaper and easier to make than a Martian colony.
Really, for most planet-wide disasters the thing they do is force us to live a little bit like we'd have to on non-disaster-stricken Mars.
[EDIT] for the record I'm not against Martian exploration or even a Martian colony—I'm susceptible both to the "because it was there" and the "we'll come up with some cool tech in the process" arguments—I'm just very unconvinced Martian colonization is a good path to better survivability for the human species.
Really, for most planet-wide disasters the thing they do is force us to live a little bit like we'd have to on non-disaster-stricken Mars.
[EDIT] for the record I'm not against Martian exploration or even a Martian colony—I'm susceptible both to the "because it was there" and the "we'll come up with some cool tech in the process" arguments—I'm just very unconvinced Martian colonization is a good path to better survivability for the human species.