Oxygen in the atmosphere

Often, I hear passionate environmentalists claim that we must not cut down the Amazon rainforest because it supplies the oxygen that we breathe.  Many refer to the rainforest as the lungs of the planet.  As in many such arguments, there is a kernel of truth in that statement, but much is misleading. Now, don’t get me wrong.  I am all in favor of saving the rainforests of the world, but I am not at all worried about the world running out of oxygen anytime soon.

The Earth’s atmosphere contains about 1.2 x 1021 grams of oxygen (O2).  Every year about 2.66 x 1017 g of oxygen is added to the atmosphere by the photosynthesis of land plants and marine phytoplankton.  Nearly all the O2 added to the atmosphere each year is consumed by organisms that decompose dead organic materials. Thus, the net amount that is added is determined by the amount of dead organic material that is buried in soils and marine sediments, which is normally only a small fraction (<1%) of what is produced by photosynthesis.

If we were to cut down and burn all the world’s forests, the world’s oxygen content would drop about 0.02%–from 21% to 20.98%.  The current removal of rainforest could account for a drop of 0.0002% in oxygen in the atmosphere each year. Thus, the current storage of carbon in plant biomass and soil organic matter in the tropics cannot account for the oxygen that is in Earth’s atmosphere.

The oxygen in our atmosphere has accumulated from millions of years of burial of organic matter, largely in ocean sediments, and not from current production in the rainforest.  If we were to cut off all sources of O2 to the atmosphere, the amount in the atmosphere would begin decline at a rate that would leave half of it in the atmosphere after about 4000 years. At that point, humans would have difficulty breathing, but the presence or absence of the rainforest appears to have only a modest impact on the O2 available for life in the near-term.

Likewise, the extraction and combustion of fossil fuels also has only a trivial impact on the Earth’s atmospheric content of O2.  Each year, the combustion of fossil fuel drops the Earth’s oxygen content about 0.001%–equivalent to a decline from 21% to 20.9998%.

For Earth’s future environment, we have much to worry about, but running out of oxygen can be placed far down the list.



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