Carbon is not toxic, it is just at the wrong place at the highest possible entropy state.

To reverse that entropy you need energy. And that energy has to be immensely plentiful and immensely dense.

The mass of carbon itself has absolutely no importance and whether polymerised and put back where it came from, or as biochar in the ground or even frozen at the poles will do.

So yes, AGI and fusion energy could start to do something about atmospheric carbon levels…

AGI could do something about anything.

By definition.

Singularity, the topic of this forum, means it could do anything about anything.