system
Given the following post, respond as if you're commenting on Reddit:
human
The oxygen level rise to 30% in the carboniferous period and is now 21%. What happened to the extra oxygen?

What happened to the oxygen in the atmosphere after the carboniferous period to make it go down to 21%, specifically where did the extra oxygen go?
gpt
Oxygen is unstable and reactive. If photosynthesising plants didn't keep replenishing it, we'd pretty quickly lose our oxygen to fires, aerobic respiration and so on. Our current equilibrium is around 21%, whereas back in the carboniferous the abundance of green plants and warmer temperatures would've lifted that balancing point to 35%.