> It doesn't need to be in space, the earth will look like Mars soon enough
You're disregarding how well humans are adapted to earth-conditions. An earth ravaged by draughts, flooding, wildfires, volcano eruptions and superstorms is still more hospitable to humans than Mars - by several orders of magnitude.
I think the whole earth could look a lot like the Sahara or Gobi desert in a few generations. If the biology of our fertile soils change too much, and nature can't evolve fast enough to adapt to the changes, we could lose a lot of what we rely on.
I think its more likely that a few large famines will wipe out most humans before a total biological collapse, which will end the carbon emission problem.
Mars is colder than Antarctica, drier than the Sahara, and has lower air pressure than the top of Everest.
What little atmosphere Mars does have is 95% CO2, but it's so cold in the Martian winter that the sky literally falls each winter as 25% of it by mass condenses into solid CO2 "dry ice" on the poles. Mars has no ozone layer (not that you'd survive outside without a space suit on), and the entire thickness of the atmosphere is so tenuous that a large coronal mass ejections that happens to hit the planet will kill basically all humans walking or driving around outside on the surface.
The Martian soil has about a million times the concentration of calcium perchlorate (toxic to both humans and plants) than the perchlorate concentration in water found in literal superfund cleanup sites.
Even then, we'd be able to breathe without helmets. Mars is already like the Gobi, without oceans, precipitation or the breathable air that Earth has. It's not close to being the same thing
You're disregarding how well humans are adapted to earth-conditions. An earth ravaged by draughts, flooding, wildfires, volcano eruptions and superstorms is still more hospitable to humans than Mars - by several orders of magnitude.