I agree with the various critiques of the Aluminum Sequin proposal.
It's doomed from all points of view - practicality, effectiveness, and
political impossibility.

Then again, maybe it's not so politically impossible. Perhaps the
administrations that gave us Star Wars would find something like
this very much to their taste

But all that aside, I wonder if anyone has an answer to AP's
specific question?

Anyone know what the mechanism as to why plants would stop
growing with increasing Global Warming and increasing CO2??

Clearly, increasing CO2 will only increase photosynthetic rates to the
extent that they are limited by CO2 rather than other factors. Does anyone
know if currently available quantities of CO2 are a limiting factor? I
wouldn't think they pose a limit currently.

Temperature is also a factor, but global warming may have both positive
and negative effects on total plant growth. Warming may increase
photosynthetic activity in areas of the globe that are currently too cold
for part or all of the year, but may desertify other areas, reducing total
plant life in those areas.

But, as another poster pointed out, it's probably the effects in the
ocean that will be most telling.

Alan