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There's about 1.2E21 grams of oxygen in the atmosphere. Photosynthesis produces about 3E17 grams of oxygen per year. Assuming that the amount produced is about equal to the amount used up (after all, the amount of oxygen isn't going up or down significantly), we have around 1.2E21 / 3E17 = 4000 years.
Of course, that assumes that the biosphere continues to use oxygen at the same rate over that entire time, which wouldn't happen. As the oxygen concentration falls, larger animals such as humans will not be able to get enough oxygen into their tissues and so they'll die off, which will drop consumption.
But I'm confident that animals would run out of food before they run out of oxygen.
Maybe if we take your "on the planet" literally, we could keep a very small population of humans alive using plants grown in space, but it would be tough to get enough production going before the last of us dies of starvation.
On average, plants consume (at night) about half of what they produced during the day. So the 3e17g O2/4000 year estimate might be off. It's hard to pin down precisely because different plants respire differently.
I mean, sure. But that's because photosynthetic organisms are a net producer of oxygen and so therefore so is the biosphere. If they stop photosynthesizing then they'll be a net consumer. Combustion uses about 5-6E16 grams annually. That's definitely less than the biosphere.
Animals don't actually consume all that much oxygen. Much More comes from microbes (bacteria, fungi, archaea) decomposing plant materials. And microbes are far more tolerant of hypoxic and anoxic conditions. If photosynthesis stopped, this would cause a runaway greenhouse effect as microbes would produce CO2 without any uptake by plants to counterbalance it, which would increase Earth's temperature, which would then feed back to speed the metabolism of those microbes (which is very temperature sensitive), causing more CO2 emissions, ad infinitum. So even if all the animals die off O2 consumption could be accelerated.
over the last decades we've increased hte co2 content from like 0.02% to 0.04% with catastrophic effects on thermal radiation
the oxygen content depending on the fuel is gonna drop by a bit more so presumably some 0.03%
so from 20% to 19.97%
the natural cycle already can'T keep up compared ot our industrial use and even that would take some 5000 years to get to a point where direct co2 poisoning would become dangerous, 20000 til we literally run out of oxygen
and our breathing alone ia s atiny fraction of that
given that most living things live much less than 100 years and most dead things rot away in way under 100 years we'd DEFINITELY run out of food first
My dude, the answer is food runs out and it is not even close. I’m sorry to this sub, I’m not “doing the math”. It doesn’t need to be done. At any point in time there’s a couple months worth of edible plant material for humans to eat. In the meantime there’s hundreds of years of breathable oxygen in the atmosphere.
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There's about 1.2E21 grams of oxygen in the atmosphere. Photosynthesis produces about 3E17 grams of oxygen per year. Assuming that the amount produced is about equal to the amount used up (after all, the amount of oxygen isn't going up or down significantly), we have around 1.2E21 / 3E17 = 4000 years.
Of course, that assumes that the biosphere continues to use oxygen at the same rate over that entire time, which wouldn't happen. As the oxygen concentration falls, larger animals such as humans will not be able to get enough oxygen into their tissues and so they'll die off, which will drop consumption.
But I'm confident that animals would run out of food before they run out of oxygen.
Maybe if we take your "on the planet" literally, we could keep a very small population of humans alive using plants grown in space, but it would be tough to get enough production going before the last of us dies of starvation.
Does that include the oxygen that is then promptly used by the same organisms producing it, or is it net production?
The biosphere isn't the main net consumer of atmospheric oxygen. Combustion is.
On average, plants consume (at night) about half of what they produced during the day. So the 3e17g O2/4000 year estimate might be off. It's hard to pin down precisely because different plants respire differently.
I mean, sure. But that's because photosynthetic organisms are a net producer of oxygen and so therefore so is the biosphere. If they stop photosynthesizing then they'll be a net consumer. Combustion uses about 5-6E16 grams annually. That's definitely less than the biosphere.
How are the plants surviving without photosynthesis?
The same way anything else is surviving. They're using up their energy reserves. They're going to die off quickly, but not instantaneously.
I was curious if these numbers are based on a static state of the biosphere.
I figured that as stuff started dying off, there'd be more decomposition, and the CO2/O2 curve would change, but have no idea how.
Does decomposition use or rely on oxygen being present? Would it speed up or slow down as the planet dies?
[deleted]
Your image doesn't mention animals or wildfires at all. And I said net.
https://www.sciencedirect.com/science/article/pii/S209592731830375X
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The imbalance between sources and sinks can be found by comparing net production to net consumption.
Animals don't actually consume all that much oxygen. Much More comes from microbes (bacteria, fungi, archaea) decomposing plant materials. And microbes are far more tolerant of hypoxic and anoxic conditions. If photosynthesis stopped, this would cause a runaway greenhouse effect as microbes would produce CO2 without any uptake by plants to counterbalance it, which would increase Earth's temperature, which would then feed back to speed the metabolism of those microbes (which is very temperature sensitive), causing more CO2 emissions, ad infinitum. So even if all the animals die off O2 consumption could be accelerated.
Thanks to everyone who replied to this question, I teach biology and I look forward to sharing this with them.
over the last decades we've increased hte co2 content from like 0.02% to 0.04% with catastrophic effects on thermal radiation
the oxygen content depending on the fuel is gonna drop by a bit more so presumably some 0.03%
so from 20% to 19.97%
the natural cycle already can'T keep up compared ot our industrial use and even that would take some 5000 years to get to a point where direct co2 poisoning would become dangerous, 20000 til we literally run out of oxygen
and our breathing alone ia s atiny fraction of that
given that most living things live much less than 100 years and most dead things rot away in way under 100 years we'd DEFINITELY run out of food first
My dude, the answer is food runs out and it is not even close. I’m sorry to this sub, I’m not “doing the math”. It doesn’t need to be done. At any point in time there’s a couple months worth of edible plant material for humans to eat. In the meantime there’s hundreds of years of breathable oxygen in the atmosphere.