In a comment to a recent question, it was pointed out that Biosphere II failed in part because too much CO2 was absorbed by the concrete, leaving insufficient oxygen in available CO2 to be released by plants in photosynthesis.

We know that one of the ways scientists and engineers are hypothesizing to reduce CO2 levels, in order to reduce the effects of global warming, is to investigate carbon capture systems, wherein CO2 is captured in, say, calcium carbonate and then buried.

This evoked in me an interesting story line and plot device.

It's about the Law of Unintended Consequences.

Is it feasible to capture so much CO2 on earth and make it permanently unavailable to photosynthesis, that the levels of oxygen in our atmosphere fall to levels that can no longer sustain life on earth?

This is NOT about HOW this capture is done, or even if it is doable. It's about, theoretically, given the current amount of CO2 that needs to be captured to reduce global warming, and the future ongoing rate of production and release of CO2 through the burning of fossil fuels, respiration, farting, and so on, if humans captured enough CO2 (and unintentionally made it unavailable to photosynthesis) to reverse global warming, would so much oxygen be captured that there is not enough left to sustain life?

In other words, is it feasible that we could unintentionally substitute one extinction event while trying to prevent another?


Some useful data that I have researched is:

Oxygen levels today 20.9%

Minimum oxygen levels for proper human functioning without adverse effects 19.5%

Oxygen levels have been dropping as CO2 levels are increasing, hypothesized to be in part from oxygen used in combustion

Any oxygen removed by 'sinking' CO2 is no longer available to replenish this depleted oxygen through photosynthesis - a double whammy - oxygen used up in combustion and not replaced by photosynthesis.

Closest figure I have for projected oxygen depletion WITHOUT CO2 sinking is a drop to 20.8 by the end of the century.

I have not found any research that gives a figure that says how LOW CO2 levels have to fall to REVERSE global warming (i.e., is it below the C3 plant compensation point?).

But I have not found any research that ties it all together in a future projection. The closest is this

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    $\begingroup$ OK, I'm changing my comments. Is it true that you are only asking, "if we magically remove CO2 from the atmosphere, is there a point the planet cannot compensate?" correct? (I love the idea for a story, BTW.) $\endgroup$ – JBH Feb 19 at 16:49
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    $\begingroup$ @JBH That is correct. It is not about HOW, that is left to 'future discoveries' and 'hand waving'. it is about the numbers game of removing enough CO2 and how much oxygen is left. $\endgroup$ – Justin Thyme Feb 19 at 16:51
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    $\begingroup$ Mandatory comic reference: dilbert.com/strip/2019-02-11 and dilbert.com/strip/2019-02-12 $\endgroup$ – Cyn Feb 19 at 17:34
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    $\begingroup$ @Cyn Very timely. It pretty much addressees my question. pointy-haired bosses solving the wrong problem. $\endgroup$ – Justin Thyme Feb 19 at 18:31
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    $\begingroup$ @AlexP Do you have a reference to that 200 ppm number? that would make a great answer. $\endgroup$ – Justin Thyme Feb 19 at 20:43

The photosynthesis would cease only as a secondary consequence.

CO2 plays an important part in the modern climate. link

75% of the greenhouse effect is caused by water vapor and clouds, which rain out of the atmosphere if it cools. This makes water vapor a strong positive feedback to any change in non-condensing greenhouse gases. CO2 constitutes 80% of the non-condensing greenhouse gas forcing. Removing CO2 would remove most of the water, cancelling most of the greenhouse effect and cooling the Earth by 30 C.

This temperature drop would kill most of the flora and fauna first. Mostly because they are not used to this temperature, but also, because photosynthesis efficiency drops:

At low temperatures, between 32 and 50 degrees Fahrenheit – 0 and 10 degrees Celsius – the enzymes that carry out photosynthesis do not work efficiently, and this decreases the photosynthetic rate.

Some would likely survive, as there are plants living on the poles, but i doubt they would be able to maintain the O2 level. But the O2 level would be highly unlikely to fall, as most of the life was killed by the temperature drop.

Also see this xkcd graphic, and this NASA article, showing that the CO2 concentration in the atmosphere never went under c.a. 180ppm.

Edit: I just found this on biology stackexchange:

That being said, the answer depends on other concept: CO2 compensation point. For a C3 plant, carbon dioxide compensation point is around 50ppm (Tolbert, Benker and Beck, 1995).

Thus, we can say that, as a rough estimate, the value you want lies between 50ppm and 170ppm (probably closer to 170ppm than to 50ppm).

(Extra: the plant cells have a respiration)

This would mean, if you instantly lowered the CO2 concentration under this point (and kept the temperature normal), the plants would slowly suffocate during the day. Probably they would produce some CO2 until they die, but probably this rate would not be enough to raise the CO2 level to the minimum for most of the plants.

Yes, this would lead to an extinction event (the species which have higher compensation points would be effected the most), also in the case if you do not consider the temperature drop.

  • $\begingroup$ This is not about global warming per say, nor does it have anything to do with reducing temperatures except as an artifact. In essence, it is about how much CO2 is produced now, how much of it needs to be removed to lower CO2 levels to levels that produce a sustainable temperature, and how much oxygen would be removed in doing so if all of this CO2 were captured. $\endgroup$ – Justin Thyme Feb 19 at 17:22
  • $\begingroup$ Ok, I think I understand your question. I updated the answer with some new info. $\endgroup$ – G. B. Feb 19 at 17:49
  • $\begingroup$ I never would have thought that reducing CO2 would cause a deadly consequence. $\endgroup$ – Mr.J Feb 20 at 2:18
  • $\begingroup$ @ Mr.J That's how the Law of Unintended Consequences works. $\endgroup$ – Justin Thyme Feb 20 at 14:14

This is a very good point to consider. The concrete absorbed CO2, which prevented the plants from using it in order to produce sugars and release oxygen back into the atmosphere. So, if CO2 is captured by some industrial process, and never released again, plant growth is stunted, and the animals will have to compete over a smaller share of plant food. First, the normal biomass will diminish as more carbont is sucked-out the system and less oxygen will find its way back into the atmosphere.

Humans response to suffocation is triggered by high CO2 and not low oxygen. This has caused the participants in the experiment to behave strangely, to our standards.

Even if the concrete was replaced with a more inert material, there is another problem: Volume of air. Our atmospheric mass is huge, so that biogenic factors, like breathing, raises CO2 very slowly. This allows plants to grow, absorb more CO2 and eventually catch-up with the pace of rising CO2. A few miles-high column of air above an earth-sized planet, is not the same as an aquarium loaded with plants and animals.


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