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I have done some rough calculations of how long it might take humanity: approx 80,000 years (that's taking Earth's population as 7.5 billion, 11,000 litres a day of breathing per person, the weight of 1 litre of air at sea level at 1.225 grams, and the total weight of the atmosphere at 5140 trillion tonnes)

Please let me know if my calculations are massively out!

My question now is - how long would it take all animal species combined to breathe the equivalent sum total weight of the atmosphere.... and how long would it take all life combined??

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    $\begingroup$ When you exhale, has your body taken up all of the oxygen you breathed in? $\endgroup$ – Wolfgang Bangerth Jan 13 at 4:47
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    $\begingroup$ No - we take in about 5% of oxygen from the air.we breathe in, which represents about 24% of the available oxygen. The question is about how much total air enters and leaves the human body via the lungs, and in comparison the total air that enters and leaves the stomata on plants. Understandably these are not easy comparisons to make due the different nature of aerobic respiration in different organisms, so the question becomes very complex to answer $\endgroup$ – Amphibio Jan 13 at 7:31
  • $\begingroup$ I wonder if the folks in Biology SE would be better suited to formulate an answer to this? They might be easier for them to field questions about total size of animal population and it's respiration rate than Earth scientists. I think this can be on-topic here too of course, but in terms of getting an answer, if nothing happens here, you might consider closing this and asking for migration at some point. (not the same, bu cf. How much energy does a small spider expend per day just waiting for its web to vibrate?) $\endgroup$ – uhoh Jan 17 at 4:22
  • $\begingroup$ All life combined breathing is meaningless, since some life reverses the "breathing" of other life. $\endgroup$ – Oscar Lanzi Feb 7 at 2:30
  • $\begingroup$ Oscar, I wouldn't say its meaningless. That's like saying measuring rainfall is meaningless, just because it is being evaporated somewhere else. There is still a net movement of water, just like there is a net movement of air which is filtered through living organisms $\endgroup$ – Amphibio Feb 8 at 9:27

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