Weather clearly affects people in a lot of ways, do disentangling the CO2 effects will be hard.
Any idea how high will CO2 go in a room in a normal building, say during winter in a well-sealed residential house? Offices and apartments buildings typically have HVAC systems which have standards for air exchange and such, but a single-family house can do whatever it wants to, including turning itself into an airtight box in the name of energy efficiency...
Weather clearly affects people in a lot of ways, do disentangling the CO2 effects will be hard.
Weather affects people, but as I said, the effects generally seem to be small and not as large as the experiment claims. If there were large weather effects, they could be due to CO2 or other aspects of weather; but since there are not, that implies that all aspects of weather including CO2 are not that important. (Modus ponens, modus tollens, etc.)
One or two research groups have published work on carbon dioxide and cognition. The state of the published literature is confusing.
Here is one paper on the topic. The authors investigate a proprietary cognitive benchmark, and experimentally manipulate carbon dioxide levels (without affecting other measures of air quality). They find implausibly large effects from increased carbon dioxide concentrations.
If the reported effects are real and the suggested interpretation is correct, I think it would be a big deal. To put this in perspective, carbon dioxide concentrations in my room vary between 500 and 1500 ppm depending on whether I open the windows. The experiment reports on cognitive effects for moving from 600 and 1000 ppm, and finds significant effects compared to interindividual differences.
I haven't spent much time looking into this (maybe 30 minutes, and another 30 minutes to write this post). I expect that if we spent some time looking into indoor CO2 we could have a much better sense of what was going on, by some combination of better literature review, discussion with experts, looking into the benchmark they used, and just generally thinking about it.
So, here's a proposal:
Some clarifications:
(Thanks to Andrew Critch for mentioning these results to me and Jessica Taylor for lending me a CO2 monitor so that I could see variability in indoor CO2 levels. I apologize for deliberately not doing my homework on this post.)