I need a way to make life possible in the troposphere and lower stratosphere for my scenario set in a distant future. My excuse to move life to these altitudes is to increase the oxygen levels in the atmosphere, so that living at sea level is toxic, and the reason for the increased oxygen levels is some kind of great oxidation event using a new type of cyanobacterium.

As it is, the temperature of the troposphere goes from -60° C by the tropopause to around 0° when we reach the upper stratosphere and ozone layer. Would this change in any significant way if the oxygen levels doubled, tripled or quadrupled? Could life otherwise unable to inhibit these zones suddenly have access to them?

  • $\begingroup$ "Plant life would surge": Nope, it won't. Oxygen is detrimental to photosynthesis. The more oxygen, the more dificult would be for plants to fulfill their purpose in the world. $\endgroup$
    – AlexP
    May 22, 2021 at 16:07
  • $\begingroup$ Ah you're right! I misunderstood, I read it would change and that there would be more ferns, moss and mushrooms. I'm nor sure why though? I'll edit the question 🙏 $\endgroup$
    – Glowworm
    May 22, 2021 at 21:29
  • $\begingroup$ Is this before the rampaging fires turn a lot of this oxygen into carbon dioxide? $\endgroup$
    – Mary
    May 22, 2021 at 23:00
  • $\begingroup$ @Mary, that's a good point and would make an excellent frame challenge. However, it's important to give the OP the benefit of the doubt and answer the question in its context. How all that oxygen came to pass and continues to be isn't what Glowworm asked. 😉 $\endgroup$
    – JBH
    May 23, 2021 at 4:04
  • $\begingroup$ @Mary I'm thinking something similar to the great oxidation event with cyanobacteria. I realize I should have written this into my question too (this is my first question here, so I'm learning - I'll edit it in). Wildfires may even feed the cyanobacteria and potentially lead to even more oxygen I think? The exact "why" isn't all that important as long as long as I can create enough suspension of disbelief to make it believable that humans could live in the upper troposphere and lower stratosphere at least for a few decades. $\endgroup$
    – Glowworm
    May 23, 2021 at 10:25

2 Answers 2


Yes... and No

If we were talking about a greenhouse gas like CO2 or Methane the answer might be yes (kinda), but there's the thing.

The atmosphere, while very thin compared to the diameter of the planet, is quite thick. How thick depends on how you define the atmosphere. NASA says 60 miles, Space.com says 300 miles, etc.

But you're talking about the troposphere. That's the nice, comfy layer right next to the surface. Temperature calculation in the troposphere is very complicated because it has as much to do with what's under the atmosphere (sand, rocks, plants, water...) as it does what's in the atmosphere (air, greenhouse gasses, clouds...).

The basic problem with greenhouse gasses (I know, O2, I'm getting to that) is that photons hit the Earth, reflect back into the atmosphere, and rather than passing to the higher levels of atmosphere or right back out into space, they're trapped by the greenhouse gasses. And that really affects us humans because, compared to most gasses in the atmosphere (O2, N, etc.), they're heavy.

When you increase O2, it's going to float (ignoring wind and simplifying the situation to make angels weep) above the heavier elements, like greenhouse gasses.

Conclusion #1: O2 won't have nearly the effect on temperature that greenhouse gasses would (and do).

But, what if we, for example, quadrupled the O2 in the atmosphere?

Earth’s atmosphere is composed of about 78 percent nitrogen, 21 percent oxygen, 0.9 percent argon, and 0.1 percent other gases. (Source)

So, from one point of view you're moving O2 from 21% to 84% by pushing something else, obviously nitrogen, out of the mix.

Conclusion #2: You can't actually do what you're suggesting without seriously changing the chemical makeup of the Earth — especially its crust. All that nitrogen doesn't just come from plants (where'd they get it in the first place, right?). Without nitrogen a lot of things on Earth stop working. Like life.

Nitrogen and Oxygen have nearly identical atomic construction (being next-door neighbors on the periodic table), which suggests the troposphere would heat up a little bit from the increased density, but not that much.

But from another point of view you could be making the atmosphere more dense. In this regard, you're not increasing O2 from 21% to 84% (requiring you to replace something), you're increasing atmospheric pressure. Some fast math off the top of my head suggests the new percentage is closer to 51%, but you've increased average tropospheric air pressure by more than 50%.

Conclusion #3: Increasing air pressure would definitely increase temperature. Probably a lot — but you're changing the planet again to get it. Increasing air pressure on average requires increasing gravity.

Next we need a chart, courtesy Wikimedia Commons:

enter image description here

Oxygen has displaced various gasses over time, but the operative word is displaced. You can change the composition of the ground (more or less water, more or less flora, even the nature of the soil), but it's a whomping big deal to change the density or diameter of a world.

Conclusion 4: You want to add O2 by displacing another gas, this will basically not change the temperature (other issues would/will have a much greater effect on T), but you do have some practical limits on how much you can displace. 2X feels like the most you could get without violating suspension of disbelief and having to explain how you got it.

  • $\begingroup$ Thank you soo much! I definitely did not think about the replacement-factor. Maybe increasing air pressure could solve my problem instead, as you said... what I'm after is strictly making life possible in the stratosphere and upper troposphere. I just need an excuse for gravity to suddenly increase then. Or use CO2 (but I liked O2 as an excuse to make the earth's surface uninhabitable, like a new Great Oxidation Event). $\endgroup$
    – Glowworm
    May 22, 2021 at 21:58
  • $\begingroup$ Could I use CO2 to increase air pressure perhaps? I read that pre-historic animals apparently needed higher air pressure to fly, like 3.7–5.0 bar instead of the 1 bar we have now. If I increase CO2 drastically instead of O2 to increase temperatures and then have the O2 float above the heavier gasses and slowly increase again over a long period of time, could I achieve both a higher temperature and a higher air pressure in the troposphere? If the stratosphere violates the suspension of disbelief, I'll just abandon it... $\endgroup$
    – Glowworm
    May 23, 2021 at 11:40
  • $\begingroup$ Also, if you significantly alter the O2 levels, pretty much every large air-breathing organism could die $\endgroup$
    – Codosaur
    May 23, 2021 at 11:55
  • $\begingroup$ @Codosaur In my scenario they did all die in the end - they just tried to survive by "climbing" so to speak. $\endgroup$
    – Glowworm
    May 23, 2021 at 13:17
  • $\begingroup$ @Glowworm You certainly can and the historical chart supports a higher percentage of CO2 having been the case. Keep in mind, it's your world, so you can do what you want. What you're searching for isn't necessarily "scientific purity" in your "reality." You're looking for enough science to rationalize someone's suspension of disbelief - and most people are willing to suspend that disbelief at far lower levels of scientific purity than you might imagine (e.g., the entire Marvel universe). $\endgroup$
    – JBH
    May 23, 2021 at 17:57

Why not have a light potent greenhouse gas. It will float to upper atmosphere capturing sunlight and heating up. This will heat up earth a bit too, but if it sticks to higher altitudes, you will be heating atmosphere more that the earth itself. You may fast forward time (a billion years or so) to have less radioactive decay so that temperature on the ground will remain as today.

I am not a chemist, someone might come up with a compound for this scenario. But I guess a natural or man made system could produce enough of the compound to have that effect.

  • $\begingroup$ I could use geo-engineering as an excuse to have a light greenhouse gas in the upper atmosphere, and I can definitely fast-forward time in my scenario! But why would there be radioactive decay? $\endgroup$
    – Glowworm
    May 23, 2021 at 11:48
  • $\begingroup$ Some of the heat of earth is coming from radioactive decay. Some is from friction through gravity. Getting rid of one will cool the ground. $\endgroup$ May 23, 2021 at 18:36

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