probably 50 years ago, that by the 21st century, with the deforestation of the Amazon rain forest, most of the world's oxygen would come from North America.

Both ecosystems consume a lot of their own oxygen output. Oceans consume a lot of oxygen--then again, so do rainforests (sh*t rots).

As for Jared Diamond, he has some good points and if you get past his main point there's a lot of info to be learned. But many of his examples were strained and iffy at the time--and more than one completely refuted by facts on the ground. It's a nice narrative, but even as I read it counterexamples kept coming to mind. Counterexamples that his explanation encountered and then went "splat" against because once you step back temporally, his always-present-conditions can't handle large civilizational changes. Thing is, the "nice narrative" seemed to handle all kinds of things and meant that humans--esp. Europe--really deserved no credit, while some areas--notably those underdeveloped--just deserved understanding. So it was comforting for many. Still, ultimately he's positing what I'd label structural enviro-racism. Nature itself was arrayed against some groups. So if it means some groups are disadvantaged by the way the Earth is put together, can't blame those who are advantaged. (Nobody likes that elaboration, but I think it's pretty self-evidence and did when I read it.)

Yes, when I point out that oceans contribute more oxygen than land, some students harrumph. So do some when I say the universe originated 13.6 bya and yet others when I point out that the 'hot' Big Bang doesn't actually account for the universe's origin, just it's development fairly soon after that. (And by fairly soon we're talking 1 x 10^-45 s or so, just a bit more than the time allotted for a public school teacher's lunch).