Thank you for your reasoned analysis, I suspect that when the ice ages started it reduced global O2 due to reducing the amount of surface area for all plants and the seasonal variation being so much colder globally. Grassland produces less O2 than old growth forest. The amount of combustion after the meteor impact may have had some influence. A minor point but T, rex doesn’t appear in the fossil record until about 68 million years, but isn’t really relevant.
I assume you would agree that O2 is a result of photosynthesis and is a biological process, and that respiration is also one. So to discount biology as a significant factor in the atmospheric make up seems to leave out the significance of life producing that O2. True that there is also the geophysical aspect involved as well in the atmospheric O2 content. As in the case of oxidation and combustion and ice ages. However, humans are responsible for a significate amount of combustion and that is the result of biological activity.
There really isn’t any sound debate that humans don’t have a global atmospheric impact. The current increase in CO2 as measured from satellites shows vividly where it is originating over cities. Which are a product of the biological human organism.
Checkout this time lapse of CO2 concentrations, and note the point sources at the locations of US cities.
Watch Carbon Dioxide Move Through Earth’s Atmosphere - NASA Science
The medical students primary concern I believe, was about the human physiological response being impacted at a 40% reduction to O2 over an evolutionally short period of time. I am not good enough to judge the math but do feel there are assumptions that can and should be challenged there. I also agree that 20 years isn’t enough data to make an accurate 3,600 year projection of O2 reaching a 40 % reduction in concentration in the atmosphere. What can be said with some degree of certainty from the graph is over the past 20 years it has been decreasing by an average of 4ppm annually.
This paper shows that O2 concentration isn’t the constant it was thought to be.
That O2 concentrations vary on a diurnal, seasonal, and interannual time frames and can be used as a way to determine the biological production over short time spans and quantified for the surface waters of the Strait of Georgia. I think this demonstrates the biological component of O2 production on a detailed level.
