Everyone calls the Amazon the lungs of the planet, and the stat attached to it is that it pumps out 20 percent of Earth's oxygen. When the rainforest was burning in 2019, this number was everywhere, in headlines, in a French president's tweet, on placards. The logic feels airtight: huge forest, lots of trees, trees make oxygen, therefore the air you are breathing is largely thanks to the Amazon.
The Amazon's net contribution to the oxygen in the atmosphere is close to zero. Trees do produce oxygen by day through photosynthesis, but they also consume it at night through respiration, and the real oxygen sink is the army of microbes and insects breaking down dead leaves and wood on the forest floor, which burns through almost everything the trees make. Mature forests sit near balance. Scientists who study this, including the ecologists quoted by National Geographic during the fires, put the Amazon's net oxygen output at roughly nothing. Most of the oxygen we actually breathe was banked over hundreds of millions of years and comes today largely from ocean phytoplankton. The Amazon is staggeringly important for biodiversity, carbon storage, and rainfall. Just not for the oxygen in your next breath.