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.
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