The second task: creating a water world.

Water is trapped as ice in the polar caps and deep underground layers of Mars.

Thawing this source of water will take much longer: perhaps 600 years for 10 percent efficiency in greenhouse gas trapping of solar energy.

The third task: introducing plant life.

Plants and bacteria can thrive in a warm, wet, CO₂ atmosphere.

Begin with algae and lichens such as those found a few millimeters below the surface of porous sandstone rocks in Antarctica. Continue to introduce new fauna as temperature increases (think of a hike down from a mountaintop).