I stumbled upond this "paper" which appears to be published in a reputable journal, but oh boy is it naive: The human physiological impact of global deoxygenation

Basically, they look at just over 20 years of atmoshperic oxygen concentration measurements, find that oxygen content is currently declining at about 4ppm per year and accelerating, and without bothering to try and understand the drivers of that decline (mostly the burning of fossil fuels), fit a parabolic curve to it and project it millennia into the future. That "methodology", to use the term loosely, leads them to conclude that global oxygen concentrations will reach half their current value in just roughly 3600 years. The rest of the "paper" discusses the effects of such a decline, apparently taken as a given, on human physiology, the processes by which humans adapt to lowered oxygen, and the estimated point in time by which the planet becomes uninhabitable, all in the millennia timescale.

Here's another paper that actually discusses the drivers of said oxygen decrease, and they conclude that the burning of fossil fuels is responsible for about 90% of the change rate and model (assuming the worst case, i.e. essentially unmitigated burning of fossil fuels well into the 22nd century) a new equilibrium of atmospheric oxygen at 20.825% (down from 20.95): https://www.sciencedirect.com/scienc...9592731830375X In terms of the air becoming unbreathable, that translates to a sea level partial oxygen pressure identical to what we now have at an altitude of 330 meters. Yes, 330, not 3300. That doesn't sound very threatening to me, and the least of our problems if we do burn up all currently known reserves of fossil fuels - at which point we'd have burned about 3 times the limit suggested to avoid devastating climate change. Even if we do burn not only currently explored reserves but the entirety of the crust's fossil fuel resources, chances are that, other than altitude sickness setting in at (at most) a couple thousand meters lower altitudes than it used to, little will change in terms of the air being fit for breathing. This might be a problem for Tibet and parts of Bolivia, but globally it would be peanuts compared to the effect of rising sea levels on some of our largest population centres.

Maybe I'm missing something, but is the "human physiological impact" paper really any better than predicting the oceans will boil by November by looking at the temperature trends between January and March in your own (northern hemisphere) backyard?