I still think it's many, many orders of magnitude easier to survive on a messed up earth than on a fresh Mars or Venus.

There is pretty much nothing we can do to this planet to make it less habitable then Mars short of filling it with self sustaining death robot armies set to kill all humans or something. Incredibly severe global warming? Still a better temperature then mars. Nuclear Winter? Still can breathe the atmosphere.

"Self sustaining death robot armies" gets me thinking. What if the biggest threat is bad ideologies? Humans make pretty good self sustaining death robot soldiers if you can shape their brain accordingly.

> What if the biggest threat is bad ideologies?

Then changing planet doesn't help unless the colony has people who are protected from them compared to those left behind.

Depends on how messed up it is. Earth ravaged by global warming and pollution? Yeah, still a lot easier than Mars. Earth ravaged by a strike from a gigantic asteroid that sterilizes the crust to a depth of three meters? Mars is probably easier.

The chances of the second one are pretty low, but some people think that any chance is too high.

Earth is still more hospitable in the case of your giant asteroid. Those in nuclear submarines and bunkers will survive unless they're very close to the epicenter. When they emerge, they'll find temperatures and atmosphere which are much more hospitable than on Mars. The survivors will also be able to scavenge abundant refined metals from the dead for rebuilding.

We can also start improving these facilities today, if we care about the survival of homo sapiens. Even out the gender ratio at Cheyenne mountain, give it a small nuclear reactor, a hydroponic farm, and a seed bank and we can be reasonably confident in it weathering anything but a very near miss.

As far as we're aware of, the crust of Mars is even more sterile.

I'm pretty sure humanity currently possesses the knowledge and resources to build multiple underground shelters around the Earth (as seen in Hollywood movies) that can sustain a population of at least one million indefinitely. Food can be grown with artificial lighting. Air and water can be brought in from outside, purified and recycled. If you need anything else, send robots (or people in protective suits) to scavenge the surface. The only thing we lack is political will.

Yeah in a lot of circumstances that is true, but the degree to which it is messed up is important. Eventually it will be too messed up right? Eventually the Solar System will be too messed up as well, but we need to start somewhere.

Just comes down to how well prepared you want the human race to be for a planetary or solar system wide disaster.

Then we should probably prefer levels of organization that have a track record of pulling off these types of feats, instead of private sector wanking.