How does Earth maintain a constant level of oxygen?

It doesn’t! The oxygen level of the planet has varied quite dramatically in the last 500 million years. It was 35 per cent during the Carboniferous period, around 300 million years ago; as the climate, cooled and land plants died off, oxygen fell to as low as 12 per cent by the beginning of the Triassic. Back then, the air at sea level would have felt thinner than at the top of the Alps today.

Burning fossil fuels has reduced oxygen levels very slightly – about 0.057 per cent over the last 30 years. Deforestation only has a small effect because when rainforest is cut down, other plants are usually grown in its place. But it’s marine phytoplankton (plant plankton), rather than trees, that produces about 75 per cent of atmospheric oxygen. Global warming will have a significant impact on phytoplankton, which is a much more serious threat to oxygen levels.