[hyperman1]

Citing a post in this same discussion: warming by 5 degrees will cause all phytoplankton to die off and as a result we'll run out of oxygen. So no, that's not enough.

Even if it would be, how would you feed these people? How would their underground cave get power for light etc in a CO2-negative world?

AFAIK, the 'biosphere 2' experiment and the experiences with the space stations demonstrated humanity is not capable of surviving long term without mother earth. There are plenty of unknown unknowns.

[adrianN]

Breeder reactors are really nice in that they produce lots of energy that you can use to produce food and oxygen. Biosphere 2 was a hippy project that tried to reproduce a complex ecosystem that nobody really understood. The Russians had much more pragmatic approaches. For this project https://en.wikipedia.org/wiki/BIOS-3 it's much easier to see how it could scale up to true self-containment.