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Start with a planet just like Earth as of today (whatever today means when you are reading this). For simplicity's sake, disregard mankind's continuous spewing of greenhouse gases into the atmosphere; if you like, think of this new planet as not having humans, and maybe not even any fossil-carbon fuels such as coal or oil.

Now, suppose instead of a CO2 level around 0.04% (400 ppm), which is where we're at, this planet's atmosphere has a stable concentration of gaseous CO2 of around five percent by volume (50,000 ppmv), along with a significant amount of gaseous oxygen in the atmosphere.

That will of course have huge ramifications throughout the biosphere, but my question here is simple:

What is the smallest change to the planet which will enable it to support such a level of atmospheric CO2 as a stable level, while still supporting oxygen-breathing life broadly similar to what we might be used to?

Note that "smallest" does not necessarily mean of least magnitude (something like changing the geology of a planet could be far-reaching) but rather more like requires the least amount of handwavium in order to explain in another planet. There does not necessarily need to exist a simple way to get from where Earth is today to where this planet is (or the other way around), but there needs to exist a plausible explanation of how a planet (some planet) otherwise similar to Earth could end up in the situation this planet is in.

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  • $\begingroup$ Well, this very Earth had about 0.15% CO2 in the atmosphere during the Carboniferous period, about 360 million years ago. (And maybe more than 0.5% in the Permian.) As you may guess from the name of the period, all that carbon was eventually converted into vegetable biomass and then deposited as coal. By burning the coal we are doing our best to recreate that atmosphere... Note that CO2 has detrimental physiological effects above a concentration of 1% and is actually toxic above 2.5% or so. $\endgroup$ – AlexP Aug 19 '17 at 16:30
  • $\begingroup$ @AlexP It's a big difference between 1,500 ppmv, or even 5,000 ppmv, and 50,000 ppmv. And high levels of CO2 certainly has detrimental effects on life adapted to a relatively high oxygen, low carbon dioxide, atmosphere; but that doesn't imply that life cannot exist at a much higher CO2 level, only that it would need to be different. $\endgroup$ – a CVn Aug 19 '17 at 18:48
  • $\begingroup$ Obviously; that's why I made a comment and not an answer... $\endgroup$ – AlexP Aug 19 '17 at 18:58
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Move the planet further from its sun. Then if CO2 levels drop, it will get too cold for much plant life, so CO2 levels will rise. The same mechanism kept the amount of CO2 in the Earth's atmosphere considerably higher than the present day, hundreds of millions of years ago when our Sun was cooler.

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  • $\begingroup$ Killing plants would decrease CO2 sequestration by plants. But where does additional CO2 come from to cause the rise? You need a lot of it and much of the original CO2 in the atmosphere is locked away as biomass, carbonates and petrochemicals? $\endgroup$ – Willk Aug 19 '17 at 23:56
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    $\begingroup$ @Will Geological processes release new CO2 all the time. It's then taken up and sequestered by plants, but not if there aren't any. That's how the "Snowball Earth" period ended, around 650 million years ago -- it's estimated that Earth's CO2 levels would have to have hit 13% for that to happen. en.wikipedia.org/wiki/… $\endgroup$ – Mike Scott Aug 20 '17 at 5:15
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Limestone and chalk rocks exist because life has been hauling CO2 out of the atmosphere and making it into rocks for billions of years. I'm pretty sure limestone is responsible for more CO2 sequestration than fossil fuels are. So to have more CO2 then, over geological time, you should:

  1. Restrict the evolution or extent of reef-building organisms. These include various prehistoric and extinct things (rudist bivalves, calcite secreting sponges), as well as modern corals.
  2. Alter the proportion of planktonic creatures with calcareous shells down, and the proportion with siliceous shells up. So down with the coccoliths and foraminifera! Boost the importance and diversity of diatoms and radiolarians.

Meanwhile, try to guarantee your planet has permanent ice caps at the poles. So over geological time it varies from interglacial (modern Earth) to glacial (ice age earth) but never - or rarely - slips into greenhouse Earth without ice caps (e.g. Cretaceous Earth, Devonian Earth). Ice caps are the pumping mechanism to get oxygen down to the deepest depths of the ocean basins.

If there is oxygen way down there, organic carbon (dead sea beasties and plankton poo) get quickly recycled back into the ocean by scavengers and aerobic bacteria. The ocean is in equilibrium with the atmosphere, so the C ends up back there as CO2.

No ice caps and the deep oceans become anoxic and uninhabitable. Anaerobic bacteria aren't as efficient at recycling, so the organic carbon mostly gets incorporated into sediment, and that sediment eventually turns into rocks. That's the reason why Wales is full of grey rocks like shales and siltstones - the grey colour is specks of organic carbon which never got digested or decayed.

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Considering humans as exemplary of oxygen breathing life, the issue for us is the gradient of CO2. We generate CO2 and to expel it, the concentration in exhaled air must be greater than the concentration in ambient air. We cannot expel CO2 against a gradient. This author estimates the toxic level of atmospheric CO2 to be 6%.

From http://principia-scientific.org/at-what-concentration-does-co2-becomes-toxic-to-humans/

Now we need to make connection between the partial pressure of CO2 in the air and in the blood, before breathing in and after breathing out. For the gas molecule to cross from the lungs to the blood it needs to have the higher partial pressure in the lungs than in the blood, and obviously, the opposite is true – if the partial pressure of the gas molecule is higher in the blood than in the lungs, it will cross from the blood to the lungs. When it comes to CO2, its concentration in the blood, after the blood has collected all the CO2 generated by the bio-chemical process keeping the cells alive reaches the pressure of 45 mm Hg, while the pressure inside the lungs after we breathe in the air is 0.3 mm Hg. Therefore, as long as the partial pressure of CO2 in the air that we breathe in is less than 45 mmHg, the human body will be able to clear out the cell-produced CO2. By the way, when people suffer serious brain damage which affects breathing, the function of those machines that maintain the life is to bring in the oxygen and make sure that all the CO2 is cleared from the blood stream. The number to watch for is 45 mm Hg of CO2 in the air, or 6% or 60,000 PPM – that is the concentration of CO2 that needs to be reached for the humankind to become extinct. If my math is serving me right, if we divide 60,000 PPM with 400 PPM we get the ‘kill factor’ for CO2: 150.

Humans tolerate elevated levels of CO2 OK. People with sleep apnea or emphysema compensate for elevated levels of CO2 via various metabolic means.

This piece of the answer is meant to address the oxygen breathing life component of your question - the answer: we will do ok.

As regards why the planet might have a stably high level of CO2 that turns on issues of input and output. Where does the CO2 come from and where does it go. It is the same for any budget: income vs expenses. The atmosphere used to have loads of CO2. Then photosynthetic organisms chipped away at it for a billion years or so. If you want more CO2 you could

  • Increase CO2 addition to the atmosphere - from combustion, from increased degradation of carbonate rocks, from volcanic outgassing, from extraterrestrial inputs.

  • Decrease CO2 sequestration. The main consumer of CO2 is photosynthetic organisms. You could cripple photosynthetic CO2 fixers somehow; perhaps low light? Nutrient scarcity? Increased ambient radiation, UV or ionizing? Ocean got too hot or too acidic? Any methods such that additions exceed more than the photosynthesizers can fix.

  • Another consumer of CO2 is water - it is soluble. The oceans act as a buffer. You can do an entire PhD on this subject and so will not provide links. The ability of water to solvate CO2 (or any gas) increases as water temperature decreases. Hot water dissolves less gas. If the oceans warmed up they would hold less CO2. A warmer and more acidic ocean could theoretically cripple oceanic photosynthesizers in the short to intermediate term which might help with that aspect of your world.

You could have increased volcanic activity, increased outgassing of internal CO2, increased breakdown of carbonate rocks and also heat up the oceans nicely from below if you had the earths core start to heat up. I was wondering about this in the context of a previous question and so asked on the physics stack:

https://physics.stackexchange.com/questions/351327/is-decay-heat-proportional-to-half-life

The answer is a little heady for me but I take away yes - if there is a decay product with a shorter half-life, it is possible for total decay heat to temporarily increase. Your warming earth would warm from the inside out and could lead to the above described changes affecting CO2 homeostasis.

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Carbon dioxide is generated by living organisms on Earth and then converted into Oxygen via photosynthesis, if we are disregarding man made emissions and forest fires and tectonic activity etc. Thus any mechanic which reduces the amount of photosynthetic organisms relative to oxidisers will increase CO2 levels. Alternatively a geological mechanic such as a shutting down or slowing of tectonic activity would increase CO2 levels as less gas is absorbed by the oceans and recycled into the mantle by tectonic subduction.

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