Popular Science has a brief article laying out the three steps to terraform Mars.
The recipe for creating a habitable planet turns out to be surprisingly simple: Just add water—and atmospheric gases. Mars has both, relics from four billion years ago when the planet was warm and wet. “When it comes to Mars, and only Mars, the notion of terraforming is no longer in the realm of science fiction,” says NASA astrobiologist Chris McKay. Humans could warm the planet and restore a thick atmosphere in a matter of decades, but producing breathable levels of oxygen would take 100,000 years with today’s best technology: plants. New inventions could, in theory, speed that along too. “Living off the land is going to be essential for long-term human explorers beyond Earth,” says Laurie Leshin, a geochemist on the Mars Curiosity team. “We have to figure out how to do this stuff.”
The article talks about three broad requirements: raising the temperature, building the atmosphere, and releasing water. But it omits discussion of the easiest thing we could do to live there: modify ourselves. Altering humans to live in the Martian environment would be a lot faster and require a lot less energy than modifying the entire planet.
Of course, we don’t know how to do that yet, but as the excerpt above relays, we also don’t currently know how to terraform Mars in less than 100,000 years. We might figure out how to transform Mars more rapidly before we figure out how to transform ourselves, but I suspect we’ll still know how to do the self transformation long before we can complete the actual planet transformation.
This assumes we don’t completely cede Mars to increasingly intelligent machines, machines we might someday be uploaded into. Long term, distinguishing sophisticated machines from engineered life (or re-engineered life) might eventually be semantic.