I don’t think this is a relevant point. It is after all used for photosynthesis despite the low concentration. The low concentration is an advantage in one way,

I don’t think this is a relevant point. It is after all used for photosynthesis despite the low concentration. The low concentration is an advantage in one way, because you have to remove a relatively small amount. The low concentration means CCS will be slow. However, we can’t drop CO2 to pre-industrial levels in a week, that would probably cause significant climate instability. The logical thing to do would be to drop the levels at around the same rate as we have increased them. That may be too slow to prevent significant climate change in the meantime. Perhaps the rate could be adapted to how bad things seem.