Why the Air Stays Breathable
Biology · · 5 min read · By No Design Editorial
Oxygen sits at 21 percent. Below 15 fires cannot burn, above 25 a lightning strike could take a continent. Something holds it there, and it is fire itself.
Start with the air you are breathing. Roughly a fifth of the atmosphere is oxygen, and that figure is not casual. Combustion chemists have worked out that if oxygen fell much below about 15 percent, fires could not sustain themselves, and large, active, warm-bodied animals that burn fuel quickly would struggle to power themselves at all.
The opposite failure is worse. If oxygen rose much above 25 percent, fires would ignite so readily that even damp vegetation would burn, and a single lightning strike could take out a continent's forests. Life needs oxygen in a narrow band, high enough to run an animal and low enough not to torch the biosphere. There it sits, at 21 percent.
What holds it there is the part worth knowing, because it is not luck being maintained by more luck. It is a loop that corrects itself, and fire is the regulator.
Suppose oxygen begins to climb. Fires start more easily and spread further, so more forest burns, so less vegetation is left photosynthesising, so less oxygen is produced and the level falls back. Suppose it drops instead. Fires become harder to start, vegetation accumulates rather than burning, photosynthesis increases across a greener planet, and oxygen rises again. The very thing that makes high oxygen dangerous is what stops oxygen getting high.
That is a thermostat. The atmosphere is not a lucky mixture we happen to tolerate. It is a regulated quantity, held where life needs it by a mechanism that pushes back in both directions.
From Chapter 2: Signs in the Green.
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