I have a few interesting questions ... to be fair, I've done no science or math to make a conclusion but there would certainly be a sense of irony involved.
1. Could the green-house gases produced to launch a civilization to Mars be the final straw that breaks our eco-system's back?
2. What are the moral implications of terra-forming Mars and moving a small population there, knowing that the population remaining on Earth is doomed?
3. If you're willing to allow an epic depopulation of the Earth, why not stay here and let it happen?
4. If any of the above are true, why would you expect the (current) larger population of the Earth to fund the colonization of Mars?
I realize this is a very dystopian view ... and that I read/watch too much science fiction, but if this is a novel (or a Netflix original), the Mars population moves back to Earth after it's "recovered"
EDIT: I'm not against the StarShip, SpaceX or Musk at all - in fact I'm a big fan as I was in elementary school during the Apollo missions and believe that SpaceX in particular has reinvigorated the passion for engineering and tackling big problems. This is only a Gedankenerfahrung.