The Amazon produces 20 percent of the world's oxygen

What actually happened

The arithmetic is the giveaway. A mature rainforest is roughly in balance: photosynthesis by day, respiration by the trees themselves at night, and decomposition by the enormous microbial and fungal community consuming the rest. A forest only nets oxygen while it is accumulating biomass. The Amazon is old and mostly at steady state, and Yadvinder Malhi and Scott Denning, among others, made this point loudly during the 2019 fires when politicians and celebrities were repeating the 20 percent line.

The reservoir argument is even more decisive. The atmosphere is about 21 percent oxygen, built up over roughly two billion years, and it is enormous relative to any annual flux. If every plant on Earth stopped photosynthesizing tomorrow, and we burned every scrap of fossil fuel we have, oxygen would still be around 20 percent. We would die of many things. Suffocation is not one of them.

None of which makes the Amazon less important, and the myth is worth killing precisely because it distracts from the real case. The Amazon is critical for carbon storage, holding roughly 150 to 200 billion tonnes, for regional rainfall through flying rivers that irrigate South American agriculture, and for biodiversity that exists nowhere else. Parts of it have already flipped from carbon sink to carbon source. Those are the reasons, and they do not need a fake one.