What percentage of the world’s oxygen does the Amazon produce?

How to get close to this number

Reason from area. The forest covers a few percent of the planet's surface, and oceans cover most of it. A single land biome cannot plausibly produce a fifth of global oxygen when phytoplankton have seventy percent of the surface to work with — the geometry alone rules out the famous figure.

The net figure is the correction that matters, and the explanation states it: the forest consumes almost all the oxygen it produces, through decomposition of its own dead material. A mature forest is roughly in balance, so its net contribution to atmospheric oxygen is near zero regardless of how much it produces gross.

That does not make the forest unimportant — it matters enormously for carbon storage, biodiversity and regional rainfall. But the oxygen claim is the wrong argument for the right conclusion, and repeating a figure that is off by a factor of three makes the genuine case easier to dismiss.