[jyl]

It seems to me that the hundreds of trillions of dollars and hundreds of years it will take to make Mars a home for more than a few hundred people, if that is even possible, could be used to solve pretty much every existential environmental problem we face on earth.

Climate change: 100% renewable energy or fossil with carbon capture, orbiting sun shades, relocation and flood defense, etc. It's not a technological problem, it's a political one.

Food supply: large scale irrigation and agriculture/aquaculture programs, shift to more grain/veg and less meat.

Population: This is only a problem in the least developed regions, and naturally solves itself with as economies become developed and urbanized, also contraception programs help.

Species extinction: Case by case, but most species are not necessary for human survival.