One thing that bugs me is they measure CO2 levels in ppm instead of moles/m^3 (or whatever would be appropriate). Why is the denominator assumed constant? Why can't nitrogen, oxygen, argon, water vapor, etc be changing too?

In fact people have been extracting N2 from the atmosphere at scale for ~120 years: https://en.wikipedia.org/wiki/Haber_process

And apparently O2 has been depleting as well: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5138252/

We are at the point where gradeschoolers are questioning why the tables of atmospheric constituents don't add up to 100%:

>Two recent reliable sources cited here have total atmospheric compositions, including trace molecules, that exceed 100%. They are Allen's Astrophysical Quantities[5] (2000, 100.001241343%) and CRC Handbook of Chemistry and Physics[4] (2016–2017, 100.004667%), which cites Allen's Astrophysical Quantities. Both are used as references in this article. Both exceed 100% because their CO2 values were increased to 345 ppmv, without changing their other constituents to compensate. This is made worse by the April 2019 CO 2 value, which is 413.32 ppmv. https://en.wikipedia.org/wiki/Atmosphere_of_Earth#cite_note-...