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> recent photosynthesis AFAIK our oxygen was not made recently. It accumulated over billions of years. https://en.wikipedia.org/wiki/Great_Oxidation_Event htt

by ralfd 6y ago

> recent photosynthesis AFAIK our oxygen was not made recently. It accumulated over billions of years. https://en.wikipedia.org/wiki/Great_Oxidation_Event https://en.wikipedia.org/wiki/Great_Oxidation_Event This here tries to project oxygen levels: > It is found that anthropogenic fossil fuel combustion is the largest contributor to the current O2 deficit, which consumed 2.0 Gt/a in 1900 and has increased to 38.2 Gt/a by 2015. Under the Representative Concentration Pathways (RCPs) RCP8.5 scenario, approximately 100Gt (gigatonnes) of O2 would be removed from the atmosphere per year until 2100, and the O2 concentration will decrease from its current level of 20.946% to 20.825%. https://www.sciencedirect.com/science/article/pii/S209592731830375X https://www.sciencedirect.com/science/article/pii/S209592731...

twic 6y ago
> AFAIK our oxygen was not made recently. It accumulated over billions of years. According to wikipedia [1], the atmosphere has 34e18 mol of oxygen, and photosynthesis produces 8800e12 mol/yr of oxygen, meaning the atmosphere turns over in a little under four thousand years. There's a separate estimate of atmospheric oxygen's "residence time" in the text, of 4500 years. So although oxygen levels have been high for billions of years, there is a sense in which the oxygen in the atmosphere right now has only been there for thousands. [1] https://en.wikipedia.org/wiki/Oxygen_cycle https://en.wikipedia.org/wiki/Oxygen_cycle
- the8472 6y ago
  That calculation is simplistic as it is based on the current steady-state. The oxygen catastrophe was a massive change away from the previous equilibrium state which first had to deplete all the buffers. That's how we got iron ore veins, by oxidizing most of the iron out of the oceans. And there are other sinks besides iron.
  - samatman 6y ago
    All true, this all depends on what question you're trying to answer. If it's "how long is it likely that the oldest free oxygen molecule I'm inhaling right now has been in the atmosphere", that's about 4k years. If it's "how long has the atmosphere had a double-digit partial pressure of O2", very different question with a different answer.
    - mncharity 6y ago
      > oldest [...] [is] about [residence time] Only if the reservoir is a fifo queue. :) Otherwise there's a distribution, with a tail. But oldest in atmosphere still looks (envelope scribbling...) order Myr? Rather than Gyr. Similar for breath?

by ralfd 6y ago

twic 6y ago

> AFAIK our oxygen was not made recently. It accumulated over billions of years. According to wikipedia [1], the atmosphere has 34e18 mol of oxygen, and photosynthesis produces 8800e12 mol/yr of oxygen, meaning the atmosphere turns over in a little under four thousand years. There's a separate estimate of atmospheric oxygen's "residence time" in the text, of 4500 years. So although oxygen levels have been high for billions of years, there is a sense in which the oxygen in the atmosphere right now has only been there for thousands. [1] https://en.wikipedia.org/wiki/Oxygen_cycle https://en.wikipedia.org/wiki/Oxygen_cycle

the8472 6y ago
That calculation is simplistic as it is based on the current steady-state. The oxygen catastrophe was a massive change away from the previous equilibrium state which first had to deplete all the buffers. That's how we got iron ore veins, by oxidizing most of the iron out of the oceans. And there are other sinks besides iron.
- samatman 6y ago
  All true, this all depends on what question you're trying to answer. If it's "how long is it likely that the oldest free oxygen molecule I'm inhaling right now has been in the atmosphere", that's about 4k years. If it's "how long has the atmosphere had a double-digit partial pressure of O2", very different question with a different answer.
  - mncharity 6y ago
    > oldest [...] [is] about [residence time] Only if the reservoir is a fifo queue. :) Otherwise there's a distribution, with a tail. But oldest in atmosphere still looks (envelope scribbling...) order Myr? Rather than Gyr. Similar for breath?