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gwern comments on What is up with carbon dioxide and cognition? An offer - Less Wrong Discussion
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The CO2 intervention is doubling the density of CO2, and decreasing the density of oxygen by < 1%.
Small proportional changes seem unlikely to drive big effects, unless there is some feedback mechanism that is keeping the level precisely balanced. But 1% changes in oxygen should be happening all over the place. It seems much more plausible for doubling the density of CO2 to have a direct effect.
Similarly, the nitrogen intervention is a significant proportional change.
Such as in the body, dealing with tightly regulated and critical aspects of metabolism like oxygen consumption.
Perhaps they are. You don't know the effect because the existing experiments do not vary or hold constant oxygen levels. All you see is the net average effect, without any sort of partitioning among causes.
Existing experiments do vary oxygen levels systematically, albeit usually unintentionally, by geography. Going up 100 meters from sea level gives you a 1% drop in oxygen pressure and density. If that was enough for a detectable effect on IQ, then even the 16% lower oxygen levels around Denver should leave Coloradans obviously handicapped. IIRC altitude sickness does show a strong effect on mental performance, but only at significantly lower air pressures still.
And they also vary CO2 levels systematically by geography as well; if that was enough for a detectable effect on IQ, then the lower CO2 levels around Denver should make the rest of us at lower altitudes, such as sea level, look obviously handicapped. If you believe the altitude point refutes effects of oxygen, then it must refute effects of carbon dioxide and nitrogen as well...
Which is part of my original point about implausible effect sizes: the causal effect is underidentified, but whether it's oxygen or CO2 or nitrogen, it is so large that we should be able to see its repercussions all over in things like the weather (or altitude, yes).
The magnitude of the variation isn't nearly the same in the O2 vs CO2 cases. "16% O2 reduction is lost in the noise" is devastating evidence against the theory "0.2% O2 reduction has significant cognitive effects", but "16% CO2 reduction is lost in the noise" is weaker evidence against the theory "66% and 300% CO2 increases have significant cognitive effects".
I'm not arguing with you about implausible effect sizes, though. We should especially see significant seasonal effects in every climate where people typically seal up buildings against the cold or the heat for months at a time.
It seems possible (I know of no evidence for or against) that human bodies adapt slowly to differences in O2 and CO2 level. In that case, newcomers to Denver might be smarter or stupider for a while, but after (say) a few months they might be back to baseline, but short-term fluctuations (e.g., sitting for a few hours in an office with slightly depleted O2 and slightly raised O2) could still have detectable cognitive effects.
I meant, changing a level by 1% probably won't have a huge effect (e.g. 1/2 of a standard deviation) unless that level is itself controlled by a homeostatic process (or else has almost no variation).