According to Robert Zubrin, the founder of the Mars Society, colonization will take place in three steps, and we've already started with the first: exploration. Multiple human landings are necessary to uncover mineral materials, ice deposits and suitable habitats. Next, bases will be built, and finally, a self-sufficient colony must be established.
Now it's time to start terraforming. For this article, let's say the year is 2150.
Terraforming begins with the warming of the planet. Zubrin describes it as "trying to drive a runaway greenhouse effect." He proposes three methods to get the planetary hot tub started. Critics argue that the first two will be ineffective; the third is sketchy but feasible.