I wasn't able to put my finger on it. Avatar wiki says:

Pandora's toxic atmosphere is unbreathable by humans due to the presence of high concentrations of carbon dioxide (>18%) and hydrogen sulfide (>1%).

However, entire planet seemed to be covered in plants, including water plants. Earth isn't half as green as Pandora is, but it still has pretty low levels of carbon dioxide (0.035%) even with all the emissions that come from human society.

I understand about the air toxicity due to other chemicals found in the atmosphere, but if the plants are performing photosynthesis, shouldn't it be enough to keep the atmosphere relatively free of carbon dioxide?

  • 3
    It could be that the plants aren't even performing the equivalent of photosynthesis, what with the entire eco-system being linked via an information network.
    – Monty129
    Mar 4 '13 at 12:02
  • 6
    That's a reducing atmosphere, like early earth en.wikipedia.org/wiki/Reducing_atmosphere eesc.columbia.edu/courses/ees/climate/lectures/earth.html Must be a lot of iron or similar around to maintain microaerophillic conditions up thru the advent of multicellular life. Mar 4 '13 at 15:15
  • Carbon dioxide is an excellent source of food for plants, it's well known that they grow better in high CO2 environments. Hence CO2 being pumped into greenhouses, this is NOT to "cause a greenhouse effect", the glass roof does that, but to enable the plants to grow faster and more larger.
    – jwenting
    Mar 19 '13 at 8:11
  • I think that thing about more than 1% hydrogen sulfide is inaccurate--Avatar: An Activist's Survival Guide says "Another gas, hydrogen sulfide, is spewed out by pervasive vulcanism, and is toxic in concentrations far less than 1 percent." They also mention that Xenon makes up about 5.5% of the atmosphere, not sure if that would have any ill effects on humans.
    – Hypnosifl
    Nov 27 '14 at 17:09

Alien planet, alien plants. Just because they appear to be similar to Earth plants doesn't mean they necessarily behave identically to Earth plants. It would be entirely possible for an alien plant to do the opposite of our plants: take in oxygen and expel carbon dioxide. The plants on Pandora are specially suited to that planet's ecosystem, so while they may perform something like photosynthesis they likely absorb and expel different types of chemicals and in different amounts than Earth plants.

  • 11
    @Monty129, if you're talking about isotopes of Elements, then yes, however, i have never heard about possibility of "new" element, excluding those that are extremely heavy and radioactive and only synthesized in laboratory conditions and remain stable for a few seconds. PseudoPsyche, somehow i saw so much similarities between earth's plants and Pandora's (including color, bio-luminescence, etc, I've somehow assumed that biosynthesis should somehow be similar to ours).
    – ertx
    Mar 4 '13 at 12:15
  • 7
    You can't just reverse the direction and say that the plants take up O2 and produce CO2, that would have some major consequences. Photosynthesis allows plants to build up biomass, if they don't do photosynthesis they would need another way to get the building blocks they need.
    – user496
    Mar 4 '13 at 13:49
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    @Monty129 : saying that unknown elements can be found on an alien planet is the same as saying "wow, I discovered a new integer in this account book I've read."
    – vsz
    Mar 4 '13 at 15:08
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    @ertx ...Ahem...Unobtainium
    – Monty129
    Mar 4 '13 at 17:23
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    -1 The idea that an alien "plant" would have alien biology is not objectionable and the sort-of default answer for a question like this. We could speculate endlessly what that biology looks like, however even alien biology would be limited by the physical laws of the universe and that would require any autotrophic carbon-based organism to use an external source of energy to reduce low energy inorganic C (i.e., CO_2) into high energy organic matter (i.e., CH_2_O). So no, an alien plant cannot only take in O_2 and release CO_2 and still be a "plant" by any functional definition of the word.
    – DQdlM
    Mar 5 '13 at 1:17

I understand about the air toxicity due to other chemicals found in the atmosphere, but if the plants are performing photosynthesis, shouldn't it be enough to keep the atmosphere relatively free of carbon dioxide?

What about the effect of non-plant life? Pandora seems to be a rich ecosystem with many hexapodal lifeforms. Couldn't the different carbon dioxide ratio just be due to a different equilibrium reached between the plant and animal life?

The Avatar wiki also mentions the atmosphere as a high concentration of Xenon. It has this to say about the increased atmospheric density:

The increased mass of the denser air means that more force is required to accelerate it as it is moved out of the path of a moving object.

Presumably, then, the animal life requires more energy to move in that atmosphere, and therefore requires a higher metabolism. We see Pandora as being not only rich in plant life, but also full of high-metabolism animal life, which I assume would expel even greater quantities of carbon dioxide. Obviously the animal life is adapted to the high carbon dioxide concentration and have particularly efficient respiratory systems.

And who knows what types of microbal life exist in the oceans, etc. Volcanism isn't the only explanation for high hydrogen sulfide concentrations.

  • 1
    Denser air actually makes flight easier. When you are in the air, gravity is pulling down, and the air is what is keeping you up. This denser air helps. Consider that water is far, far denser than air, yet fish expend the less energy (require less oxygen) than birds. Of course there may be other reasons for higher metabolism. Mar 5 '13 at 3:31
  • @StevenBurnap fair point about flight, I was thinking more of those huge lumbering herds of rhino-like things wading through the denser air. :)
    – ghoppe
    Mar 5 '13 at 15:25
  • I am struggling to see why this wasn't marked as the answer: it is different than the earth because the equilibrium between C02 sources and sinks is different. One obvious further point: there is limited surface area for photosynthesis, limiting the potential volume of CO2 consuming plant life.
    – horatio
    Mar 5 '13 at 22:38
  • @horatio Another good angle! Consider also that Pandora orbits a Gas Giant, and according to the wiki it has a significant axial tilt. Not only is there a day/night cycle, but a cycle where the sun is eclipsed by the Gas Giant.
    – ghoppe
    Mar 5 '13 at 22:51

Oceans on Earth are deadly to humans yet conducive to life.

Which means unprotected humans are only really viable in less than 10% of Earth's biosphere. And we evolved here, adapted over millennia. What hope have we when dropped into an alien biosphere?

The idea that xenoclimates will be pleasant, let alone survivable, is an SF trope that Avatar has not adopted.

  • 1
    indeed. And remember that oxygen is a deadly toxin, extremely corrosive, and could well be seen by other lifeforms as making earth impossible to live on (which is according to one theory the reason we've not yet experienced an alien invasion :) ).
    – jwenting
    Mar 19 '13 at 12:09

Since it was not addressed yet - hydrogen sulfide has similar toxicity as cyanide (for us, similar in the sense of same order of magnitude). Wikipedia lists the lethal dose (50% death within an hour) for mice around 700ppm which is 0.07% (cyanide has 270ppm). It is usually is not dangerous as much lower concentrations stink horribly. But Pandora concentrations (need to be corrected for different pressure) should lead to death within minutes or even seconds.

But as other answers mention it is easy to imagine plants and wildlife that develop tolerance to such concentrations (we have some on Earth).

I am surprised how close these numbers fit what we see in the movie - short exposure isn't a death sentence - but a few minutes will kill. Looks like they had someone calculate it exactly.

  • This is the best answer so far.
    – Marvel Boy
    May 1 '19 at 3:13

One possible explanation is that the CO2 levels are dependent on the greenhouse effect. The more CO2 there is in the air, the warmer the planet is. If we assume that high CO2 levels drive plant growth, which then reduces CO2, then we have a situation where the greenhouse effect loses steam and the planet gets colder. The cold will reduce plant life, which will then allow CO2 levels to rise again.

If this is true, then the CO2 levels in the air may be dependent on how much solar radiation there is. If we assume that Earth gets relatively high amounts of radiation, then plant life can expand until CO2 levels are relatively low before cold starts reducing plant growth. Pandora then is further away, and gets less solar radiation and so can only sustain heavy plant growth with a high CO2 driven greenhouse effect.

That would explain why CO2 isn't being reduced. As to how such lush plant life can exist, one would have to assume that there's also just more carbon in general available to be captured. Part of the explanation might also be that we're just seeing the equatorial jungle portions and that the rest of it isn't so lush.


Maybe the plants are not completely/at all photosynthetic. Given the composition of Pandoran athmosphere, they could even be chemosynthetic, as shown in Wikipedia :

Hydrogen sulfide chemosynthesis:

12H2S + 6CO2 → C6H12O6 (=carbohydrate) + 6H2O + 12S


Our own flora gives off carbondioxide at night and oxygen in the light of day. It is established that pandora is a low light planet, hence the bioluminesence. It could be that the gas exchange is such that oxygen levels can' t get above a certain level. Odd because animals are very large whoch usually indicates oxygen levels of 25% or higher, at least here on earth. It is an alien planet where earth checks and balances don't exist. Ill stay with the balance of gas theory for now.

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