Some cataclysmic event has transpired on planet Earth in the not-too-distant-future, and the outside air is now thick with CO2, causing all of the problems that come with it. Human beings can no longer breathe the air directly, global temperatures have spiked, and the flora and fauna of the world have been affected drastically. A few pockets of civilization have survived in highly advanced compounds, and they want to start working to undo the CO2. I'd like this process to have a decently long time period before it is sufficient for humans to survive on the surface again, at least a century.
Assuming that all scientific knowledge has survived, and assuming that my survivors can get creative with how they acquire resources, what are some methods to begin to slowly change the planet's atmosphere back?