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Harry Turtledove worked out what the temperature range would be for an alternate universe story he wrote where the planet in Mars' orbit was called Minerva (because it radically changed brightness through it's year, due to advancing and retreating ice sheets) and was almost identical to Earth in all but it's location. It was mostly covered with ice, I personally think an earthlike planet would be TOTALLY covered in ice (once the albedo reached a certain point because of the growing ice sheets the cooling would become a vicious circle), but I'm not the famous science fiction author. 8^)
It seems that you are overlooking the fact that it would take a lot more than increased pressure to make Mars livable, and it's hard to think of a way of increasing the pressure anyway. If there was liquid water and a little higher pressure plants might do OK since the atmosphere is almost completely CO2 and that's all they really need...but how much of it would they convert to oxygen? And even if Mars' atmosphere was 100% O2, it would still be too thin for humans to breathe. We need a neutral gas to fill the space - any combination of just CO2 and O2, even at our pressures, would be unbreathable, for the long run at least. But I don't think Mars has much nitrogen. Could land plant life alone be able to convert the atmosphere? Mars is also lacking in oceans, which were very important in turning our reducing atmosphere to the breathable one we have now.
Terraforming would require a LOT more time and energy than it is worth, and even if you did make Mars somewhat livable somehow, the atmosphere would probably not stick around very long since the reason it has such a thin atmosphere is it is so small (has a lot less than half our gravity to hold it down).
Colonization to decrease the population is impractical at best. How many spacecraft would have to leave the Earth every day just to keep the population where it is at? Figuring one child born per second (I know that's probably off, but probably close) you would have to move 86,400 people each day. And once you can move people en masse from the surface of the planet (unless we come up with new propulsion systems, you would probably require something like a Beanstalk to move thousands per hour economically), there is no real point to returning them to a gravity well. Anything you need can be found in space - minerals, water, oxygen. Then you have to worry about spending all that fuel to get them back out of the hole if they want to move. Just build orbital colonies (take a fairly large nickle-iron asteroid, dig a cavity in the middle, fill it up with water ice, seal it, spin it, and focus a lot of big mirrors on it), scatter them in stable orbits throughout the solar system. We probably will never find much use for any of the planets in the solar system, and I think eventually, once we start to colonize space, we will adapt ourselves so we don't even need much protection out there.
I think we may have colonies on the moon - it is conveniently close to Earth and has a shallow gravity well, so it might make a good spaceport if you needed one in a gravity well for some reasons. I doubt people will ever live on Mars, except maybe like people live in Antarctica now - not for long at a time, and only to do research in small communities.
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