I haven't read any novels about terra forming Mars, but have watched a few shows on Discovery about it. One of the ways discussed, just to start the atmosphere building process, was to being with Algae. Algae has proven to be able to survive and often prosper in some of the worst environments on Earth. So if Algae can get started on Mars, even if only on a small level initially, it will gradually help to change the planet. Then over time as things change other plant life can be introduced. It's expected that it could take a very long time for that process to work, none of us would still be around - but a backup plan for Earth might still be a good idea, even if it takes that long to accomplish.