The dolts that think colonizing Mars would be easy have absolutely no understanding of it.
Problem one - atmosphere. You can't keep a thick atmosphere on Mars. It's got no magnetic field, so the solar wind rips the atmosphere from the planet. It's also got less gravity to begin with.
You'd have to keep outgassing, which, of course, would keep lowering the mass of the planet, making it harder and harder to keep an atmosphere.
Since Earth is denser, and has a larger core, it's not even clear how strong a magnetic field you could create, if you could somehow find a way to liquefy the core of Mars. Of course, finding enough water for life would be difficult, and of course, it would keep getting ripped by the solar wind.
In short, it's not going to happen any time in the time frame where human overpopulation problems need to be met. At least at the rate we're going.
Carbon dioxide based global warming isn't a bad thing. It's actually the answer for overpopulation, since plants grow much faster with it, and growing seasons are longer. They also keep sucking it out of the air, faster as more gets in there, so it's not a bottomless pit, unless we keep killing plants.
Humans are parasites. Pretty much all animals are. Life begins and ends with plants. That's where most energy comes from, from fueling our cells, to making our cars go (Uranium, solar power, wind power, hydroelectric are exceptions, but make up a small portion of energy harnessed today, and you can't eat them). So, what do we do? We keep cutting down forests and killing plants. As long as plants are thriving, the planet can heal itself. When we destroy this, we've gone to being a parasite to being a cancer - and we die with the organism we killed.
It's easy to say colonize other planets, etc..., but the answer is to save Earth. It starts and ends with plants.