The fourth task: creating an environment for animal life.

Plant photosynthesis will slowly take atmospheric CO₂ (plus water and sunlight) to produce nutrients and oxygen.

On Earth, the efficiency with which plants produce oxygen from sunlight is only a hundredth of 1 percent. With this efficiency, converting Martian carbon dioxide to oxygen would take more than a million years (recall the same process on Earth took over two billion years). This could be reduced to less than 1000 years with human intervention.

As plants consume the atmospheric carbon dioxide (and as it becomes locked in carbonate rocks), the greenhouse effect would lessen, and Mars would once again become cold (at least until the Sun brightens).

To keep the surface temperatures warm with an atmosphere that contained mostly nitrogen and oxygen and only 1 percent carbon dioxide, the concentrations of super-greenhouse gases would have to be maintained at a few parts per million (harmless to living things).