Too much work. Just blow up the planet and build a new one with proper CO2 levels.
But airships are fine too. I am not quite sure why aren't we making floating
dwellings on Earth. Sure, living on the surface is many times more efficient, but if we're to adapt to living on another world, preferably self-sustained, we ought to try living and surviving in the different environments here on Earth first. Either that, or where's the eccentric megalomaniac billionaire who thinks living there makes them awesome? I love the Japanese project(s) for ocean cities that were posted here some time ago.
Habitable poles and inhabitable equator would be kind of awesome too, but not for the possibility to settle there. There would be more independence in the evolution between the hemispheres, which might (or might not) lead to stranger results. I've been exploring the idea for some time. I like it. The societal aspect of it is also worth looking at. Societies living on the two halves would be more disconnected and travel between them would be much difficult. At best ships would have to cross the tropical regions during the night (one night!), at worst the crossing would be more deadly then our poles are. The ability to travel easily between them would have been seen as much more miraculous than our ability to presently live on the poles.