Humans require a pretty particular ratio of CO2 in their atmosphere.

Plants use oxygen to live, and only become net producers of oxygen when they're photosynthesizing.

If the sunlight were cut off to, say Earth for example, would humans, animals (and plants, I don't have good numbers for their sensitivity to CO2 - but they are sensitive to some degree) choke on the raising CO2 levels before freezing to death?

How long would it take to raise the CO2 levels to various thresholds? Obviously once you start killing off life you'll end up with less CO2 pollution, so at some point you'll level off the CO2 ratio.

Any idea of what a choked planet's atmosphere would look like?

  • $\begingroup$ How long it takes to raise the CO2 ratio depends on how fast the atmosphere is being changed. I'm not sure what you're asking. $\endgroup$
    – Frostfyre
    Jun 29, 2015 at 3:10
  • $\begingroup$ The current estimated atmospheric CO2 concentration is 387ppm which already exceeded 1 of the planetary boundaries for climate change claims CO2 tipping point at 350ppm. In a way Earth is already suffocated in a state of coma. Perhaps you can clarify how much concentration of CO2 either gaseous state or dissolved in water to qualify as choked. $\endgroup$
    – user6760
    Jun 29, 2015 at 4:25
  • 1
    $\begingroup$ I think user3082 is asking how long it would take, if plants stopped producing oxygen due to lack of light, for the air to become unbreathable to oxygen-breathing creatures like ourselves. $\endgroup$ Jun 29, 2015 at 5:30

1 Answer 1


No, we'd freeze to death faster.


Within a week, temperatures would drop to 0 degree F, or -17 C. Within a year we'll be at -100 F or -73 C. Quick reminder: Carbon dioxide freezes at -55 C.

Now, in a year, CO2 emission from all sources is about 800 gigatonnes per year.

https://www.skepticalscience.com/human-co2-smaller-than-natural-emissions-intermediate.htm https://www.skepticalscience.com/print.php?r=45

Assuming that continues for a year (very likely an overestimate because everything will be dying) that translates to 102 ppm increase. So CO2 levels in the atmosphere will increase from 0.04% to 0.05%. Workplace safety regulations say long term levels below 0.5% are perfectly acceptable.

  • $\begingroup$ I think that, because the cold, one can assume that CO2 generation will at first increase due to burning of wood et.al. to generate heat. Yet i doubt that would have any significant influence on the numbers you provided. $\endgroup$
    – Burki
    Jun 29, 2015 at 12:04
  • 1
    $\begingroup$ Within a one-week timeframe, there is simply no way to greatly increase CO2 production. The amount of oil which is available for burning is fixed by refinery capacity, which cannot change rapidly. Likewise, coal use cannot change quickly - the mining equipment is already working at peak capacity, and distribution networks are fixed. Burning wood will not increase noticeably - you can't burn green wood. By the end of the first week most of the planet will be snowbound, except for coastal areas where the oceans will moderate temperatures, so most forests will be impassable. $\endgroup$ Jun 29, 2015 at 15:08

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