Not much of an effect, though, compared to breeding out intelligence, which is a secondary effect of welfare states, and general material abundance. Low infant mortality is allowing us to collapse from that much faster than Rome did. Alleles related to higher intelligence are reducing in concentration the world over, and CO2 can't make that happen. Women that don't have to fear a lack of planning ability and complex problem solving skills from their baby daddies, however, do make that happen.

Natural intelligence in recorded history peaked around the middle of the Industrial Revolution, in the West. It would have begun lowering later anyway, but removing natural infant mortality gave it a head start. IQs rose for a bit yet, based on environmental factors helping to maximize the, "book learming," aspects of IQ tests, before the general negative trend could be seen. FI, Jeb, a rural farmer of the past, would do poorly on the more scholastic parts, but would not actually have been dumber than his long lost rich city boy twin, that would have scored several points higher, thanks to years of going to school.

If we need to be genetically more intelligent to compensate for a bit more CO2, natural and sexual selection will do that. It will also increase lung capacity over time. Brains are energy hogs, so they don't get bigger or more complex than they need to be, to keep the population reproducing.