What if humanity burns all fossil fuels it can find? Putting the negative effects of climate change aside, would the enormous amount of $\small\mathsf{CO_2}$ in the atmosphere eventually make it unbreathable?

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    $\begingroup$ in what time span, if they burned it all in a day it would have a strong immediate impact. $\endgroup$
    – John
    Commented Oct 20, 2019 at 0:54

4 Answers 4


Not directly. What possibly could happen is that rising temperatures turn the parts of the ocean anoxic, which leads to large releases of hydrogen sulfide, which is toxic: https://www.chemistryworld.com/news/ocean-chemistry-changes-triggered-earths-greatest-extinction-event/2500368.article


tl,dr: The direct effect of burning most of the fossil fuels (6 to 10°C temperature rise) may be survivable for some. But that does not take into account secondary effects triggered by the warming, like heat waves, ocean deoxygenation, flooding, melting of ice caps, etc.

The study below is already a bit dated, latest research has shown that earlier projections were too optimistic, but here you go (paywalled):

The climate response to five trillion tonnes of carbon

The study assumes a linear response from the temperature rise to carbon emissions, until the end of the next century.

This would heat the earth's atmopshere by 6.5 to 9.5°C. So, this is a theoretical play with numbers (for now), the effects on complex life can only be imagined (anoxic ocean ?).

Pop science discussions of it can be found here: phys.org, vice.com, NatGeo

It is not possible to say with confidence how much oxygen levels will fall. They did fall in the Pleistocene and today fall faster than CO2 rises. But that still does not account for the loss of ecosystems like forests and parts of the ocean becoming an oxygen sink. It is safe to say that this is not sustainable, it alone will not render the atmopshere unbreathable in the next 200 years, but in the long run it will endanger our species.

Of course, this should all be taken with the proverbial grain of salt (local product :-)).

  • $\begingroup$ I'm fairly sure this question is not about climate change at all. It's very specifically asking whether the atmosphere would become unbreathable. $\endgroup$ Commented Jan 7, 2020 at 17:09
  • $\begingroup$ @SemidiurnalSimon Edited. Hope it fits better now. $\endgroup$
    – user18607
    Commented Jan 7, 2020 at 17:30
  • $\begingroup$ thanks :-) 12345 $\endgroup$ Commented Jan 7, 2020 at 17:34

Although conventional oil, gas and coal reserves total between 829 and 1,501 GtC, estimates of the global recoverable fossil fuel resource including oil shales range upwards from about 4,000 GtC. If exotic resources, especially methane hydrates, are included the total may be 15,000-25,000.
Millenial timescale carbon cycle and climate change in an efficient Earth system model

There are 2.12 GTC for 1 PPM of atmospheric CO2 so the high estimate would be ~10,000 ppm, or 1% of the atmosphere, consuming 5% the O2. If the FF are mostly CH4, the hydrogen would consume another 5% of the O2.

Also Elevated CO2 Levels Directly Affect Human Cognition (1400 ppm).

  • $\begingroup$ err ... 10,000ppm are 1% ... ? $\endgroup$
    – user18607
    Commented Jan 5, 2020 at 22:20
  • $\begingroup$ @ebv Thanks. Not nearly as doomy, then. $\endgroup$ Commented Jan 5, 2020 at 22:56
  • $\begingroup$ Problem is the heating, but that wasn't the question. I recently read that if we continue to burn fossil fuels as we do for the next 100 years, climate might end up 10°C higher than today. We can speculate that most complex life has given up in heatwaves by then ... extremeophiles maybe ... $\endgroup$
    – user18607
    Commented Jan 5, 2020 at 23:17
  • $\begingroup$ @ebv All the fossil fuels came from periods where complex life existed. I really, really doubt that it’d kill everything, even in the worst case. $\endgroup$
    – nick012000
    Commented Jan 6, 2020 at 1:44

No. For a start, humanity will not burn all the fossil fuels it can find. But let's suppose for the sake of argument that humanity recognises no restrictions and fulfils all its energy needs by burning fossil fuels. Only a limited amount of fossil fuels are recoverable; the vast majority will have to stay in the ground.

Oil, for example, won't suddenly run out. What would happen is that it would gradually become scarcer, and increasingly expensive to recover. Eventually it would become too expensive to burn, so humans would have to turn to other fossil fuels, coal being the obvious one. Some would be used to make synthetic oil, and some burned in its natural state. The world has very large recoverable reserves of coal There could be enough to last humanity for a thousand years, but eventually recoverable reserves would become so scarce that it too would become too expensive to burn and, like oil, would be used as a source of chemicals and plastics.

All these fuels together, oil, gas and coal, might conceivably be burned for 2,000 years before becoming too scarce and expensive to be used as fuels. During that time,plants,especially in the oceans, would not be idle. Much of the CO2 generated would be sequestered by natural means, and although there would be a big rise in atmospheric CO2 with consequent climate warming, it wouldn't be nearly enough to make the atmosphere unbreathable.

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    $\begingroup$ Once again, Michael, citation needed. The scientific literature oftentimes runs contrary to your claims. $\endgroup$ Commented Oct 19, 2019 at 7:50
  • $\begingroup$ I have explained that it is impossible to extract all fossil fuels from the ground, and even if mankind said to hell with chemicals and plastics, lets burn it all,it wouldn't make very much difference. CO2 is not very toxic,your longs always contain plenty, and air would still be breathable with1 percent CO2 as opposed to the tiny fraction of one percent we now have. If my tweets were often contradictory to scientific thinking, people would soon let me know about it, but they don't. $\endgroup$ Commented Oct 19, 2019 at 9:17
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    $\begingroup$ @Michael Walsby: CO2 is not very toxic? The people around Lake Nyos might disagree with you, if you had a good medium: en.wikipedia.org/wiki/Lake_Nyos_disaster Also the approximately 90 people killed (in the US) by CO2 intoxication: ncbi.nlm.nih.gov/pmc/articles/PMC5380556 $\endgroup$
    – jamesqf
    Commented Oct 19, 2019 at 18:36
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    $\begingroup$ Anything is toxic if you take too much of it. Dozens of culinary substances can be fatal if you take too much: water, salt, sugar, vinegar etc. Arsenic can be fatal if you take either too much or too little. In some circumstances oxygen and nitrogen can be toxic, Your lungs have a lot of CO2 in them at this very moment. $\endgroup$ Commented Oct 19, 2019 at 18:49
  • $\begingroup$ @MichaelWalsby: A pointless remark. Just hold your breath if you don't fear the CO2 in your lungs. $\endgroup$
    – user18607
    Commented Jan 6, 2020 at 10:24

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