I recently watched a TED talk by a journalist, Stephen Petranek, in which he talks about colonization of Mars. Reaching the end of the presentation he talks about terraforming the planet, making it "more like Earth".
According to him, Mars is cold because of its thin atmosphere. One way to solve this though, would be to heat the vast amounts of carbon dioxide found in the planet's poles, which would then sublime, and this CO2 would warm the planet, allowing plants to grow.
Some people pointed out in the comment section that because there is less gravity and no magnetic field, like on Earth, the new atmosphere wouldn't last, so it wouldn't be feasible. I was quite fascinated by the idea of changing the whole planet to accomodate life, like in Earth, and I got very curious: would that really be a hidrance to forming the desired atmosphere, or is it actually possible to accomplish?