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Global Oxygen Cycle sources and sinks. Illustration of oxygen being exchanged between land, sea, air and space, plus mankind's industry..
An illustration of the Global Oxygen Cycle sources and sinks. I'm not sure all these numbers are right, but it does show how complicated this process is. Credit: Wikipedia, but updated.

Global Oxygen Cycle sources and sinks. Illustration of oxygen being exchanged between land, sea, air and space, plus mankind's industry..
An illustration of the Global Oxygen Cycle sources and sinks. I'm not sure all these numbers are right, but it does show how complicated this process is. Credit: Wikipedia, but updated.

How much oxygen is consumed by the Earth per year by natural causes?

This article is part of a series

There are a lot of ways oxygen is burned/consumed/oxidized. It is a very reactive element; it even reacts with itself — the oxygen we breathe is a combination of two oxygen molecules. In a previous article, we calculated how much oxygen is burned through unnatural (manmade) causes, like fossil fuels, chemicals and plastics. Here, I discuss ways the Earth consumes oxygen in natural processes. 

Measuring all these sources would be difficult. And I haven’t found a lot of reliable sources. However, we don’t really need to know all these numbers since I have already roughly calculated how much excess oxygen photosynthesis produces. As we saw, that number is very small. So, unlike my other articles where I go into detailed explanations and show all the math, this will be a summary of some sources I found online. 

I hope to show that the Earth itself is consuming most of the excess oxygen produced by photosynthesis. And I think it suggests that mankind is tapping into a resource that is essentially non-renewable in a human timescale.   

Respiration

Everything — even plants — needs to breathe oxygen to convert food into energy. Here are a few examples from one of our primary resources, Science Bulletin:

Livestock (farm animals) breathe 2.24 gigatonnes of oxygen/year, and 
Humans breathe 3.09 gigatonnes of oxygen/year.

For some reason, all other animals, like fish and birds, aren’t calculated. But the number must be huge. The biomass of earthworms alone is 11 times bigger than humans. Yes, worms “breathe” oxygen through their moist skin.  

And a residual term of 2.69 gigatonnes of oxygen/year. This means there is an extra 2.69 Gt observed to be missing that doesn’t match their theoretical model.

Decomposition (Microbial oxidation)

When plants and animals die, they decompose, and this consumes oxygen a little by weathering, more so by being eaten by microbes. 

Microbial oxidation consumes 51 gigatonnes of oxygen/year.*

Livestock methane emissions

Perhaps you’ve heard the discussion about cattle flatulence and how this contributes to global warming?  It sounds ridiculous, but it is significant. Methane is a much more powerful greenhouse gas than carbon dioxide, and it reacts in the atmosphere with oxygen as if being burned. (By the way, livestock is considered a manmade problem instead of deer or elephants.)

Per the Food and Agricultural Organization, livestock emits 0.119 gigatonnes of methane.**

Methane is natural gas, so using my previous formula, we get =
0.24 gigatonnes of oxygen burned by livestock methane emissions per year.

I admit that seems like a small number when it comes to oxygen. Keep in mind this number does not include wild animals. 

Forrest fires and deforestation

Under normal circumstances, deforestation, rotting wood and forest fires would all be part of the natural decomposition process. When a tree dies, it releases its nutrients, like carbon dioxide, back into the environment, and a new tree grows, reabsorbing them. Again, this is usually a break-even process, but humans are deforesting the planet at a huge rate. We could call this artificial decomposition. I don’t have an easy number for this, so let’s use forest fires as a proxy measure. Science Bulletin states:

5.87 gigatonnes of oxygen are consumed by forest fires a year.

Land oxidation (rust)

Oxygen is highly reactive. If you have seen rust or newspapers turn yellow, this is an example of oxidation. It’s the same reason Mars is red. It’s also called chemical weathering. This process can gradually erode mountains. The number given on Wikipedia is 0.5 gigatonnes of oxygen consumed by chemical weathering. But all the numbers here look dated. And considering the exposed land due to deforestation and mining, and the surface area of cities, roads, buildings and vehicles, I’m going to guesstimate this number is much higher: 

2.0 gigatonnes of oxygen consumed by oxidation, like rust.

Ocean saturation

This is a tricky one. As I already mentioned, most of our breathable air comes from phytoplankton in the ocean. But the ocean also absorbs oxygen. When photosynthesis first originated on Earth and began producing oxygen, it took 1.5 billion years before the oceans became saturated and started outgassing the excess into the atmosphere. It took another billion years for the oxygen to build up in the atmosphere to sustain life on land. So, if the oxygen levels in the ocean are low in some places, it reabsorbs oxygen from the air. The massive amount of dead zones are certainly reabsorbing oxygen. This is an understudied area. 

Oxygen being reabsorbed by the ocean. Unknown.

Global oxygen depletion logo. An illustration of the Earth  as a big O2 molecule.

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Miscellaneous causes of oxygen depletion

We’ve covered the main ways breathable oxygen disappears, but there are a lot of little ways:

  • Lost in space. 200 tonnes per day. (A negligible amount.)***
  • Nitrogen fixation by lightning or industry (fertilizer). 0.12 and 0.1 Gt/a *
  • Oxidation of volcanic gases. 0.05 Gt/a.*
  • Inefficiency of enzyme rubisco (part of photosynthesis). Possibly 25% of the total oxygen production of photosynthesis. Yikes! A gigantic number, but a loss that photosynthesis compensates for by just making more. So, we aren’t counting it here. 
  • Seashells, coral and other sediments which eventually become limestone. Unknown amount. 
  • And many more little ones. 

Summary

These are the major sources of natural oxygen depletion. Again, this is all offset by photosynthesis. If it weren’t, there would be no oxygen in the atmosphere. However, it becomes apparent that the Earth itself is consuming most of the excess. Please see our other articles in this Global Oxygen Depletion series as we examine if and when the Earth may run out of air to breathe. 

Footnotes

Science Bulletin: The global oxygen budget and its future projection.

* Oxygen cycle. Capacities and fluxes. Knoll AH, Canfield DE, Konhauser K (2012). “7”. Fundamentals of geobiology. Chichester, West Sussex: John Wiley & Sons. pp. 93–104. ISBN 978-1-118-28087-4. 

** The article reads, “Livestock supply chains emitted an estimated total of 8.1 gigatonnes CO2-eq in 2010 (using 298 and 34 as global warming potential for N2O and CH4 respectively). Methane (CH4) accounts for about 50 percent of the total.” I converted the number back to methane. 

*** Earth Loses Hundreds of Tons of Atmosphere to Space Every Day

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