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.
