I do believe, however, that it will be at least a couple hundred years before a colony on Mars will not only be self-sufficient, but capable of flourishing, if some extinction event were to befall Earth. That doesn't mean that we should give up or stop working on it right now, but I think the reality check is important.
Isn't the Terra-forming technology that we will have to develop to make Mars habitable, basically aligned with what we will have to learn to repair/manage our relationship with the Earth's environment?
The issue isn't "messing up life", life will be fine. The issue is that we're knocking the supports in the forms of ecosystems out from under ourselves. Not to mention the human cost, e.g. potentially hundreds of millions of refugees. The potential for war, and the possible destructiveness of war, is immense.
Besides, starting "afresh" where there are no such supports to begin with, without Earth to be supported by initially is still facing same issues, but worse. Depending on that of all things saving humankind is like hoping a child "might" save their parents from their troubles 5 minutes after being born.
> Isn't the Terra-forming technology that we will have to develop to make Mars habitable, basically aligned with what we will have to learn to repair/manage our relationship with the Earth's environment?
Here too I see it the other way around: our civilizations not collapsing in massive wars over scraps is what is needed for civil space missions to go on, and the sustainability -- materially, socially individually -- which we would need to avert that from happening is also what we would need for space colonies to both come into grasp and to not turn into the wrong end of Star Trek episodes. You know, where we're not the visiting crew, but said crew shakes their heads in horror at.
No, Mars or Venus are as hostile, more hostile probably than what would result if we screwed up the earth utterly. Radiation, lack of atmosphere, lack of sufficient gravity, lack of the resources that millions of years of life give. These aren't small problems.
Humans are extraordinarily dependent on the conditions of life on earth. That might be changeable in the long term but sending rockets into space isn't really related to such change. Extreme revolutions in biology and materials are whats needed.
But I agree that it's important to make planetary colonization a priority now rather than waiting. We can -- and should -- develop the technology and expertise needed to build a settlement on Mars while simultaneously trying to arrest and reverse some of the damage we've done to Earth. The latter is unfortunately largely a political process, not as much a technological and scientific one, so it will take a lot more time and effort than it should.