Running Out Of Oxygen (III). Prof. Denning’s Answer

Scott Denning

Professor of Atmospheric Science, Colorado State University

In reply to Patrice Ayme

Patrice Ayme,

Thank you for this thoughtful comment. I agree with almost all of what you’ve written, and you seem to agree with almost everything I wrote in the article.

First and foremost, I completely agree that burning forests is a terrible thing for many reasons. You are quite correct that CO2 produces carbonic acid in the oceans in addition to heating the climate. Also, as you write, without strong policy to eliminate CO2 emissions it is likely that carbon in permafrost will thaw and decompose over a period of some centuries.

For all of these reasons, it is urgent that we decarbonize the entire world economy as soon as possible. The nations of the world have agreed to limit warming to no more than 2 C above preindustrial temperatures, and have also indicated that they prefer a target of 1.5 C. Achieving this means driving CO2 emissions to near zero in the next 20 years.

Indeed forest clearing is a part of CO2 emissions, perhaps 10%, and the recent fires in thee Amazon are a fraction of that 10%. But let’s not forget the other 90% too!

Regardless of our 100% agreement on these facts, calculating the effect of fossil fuels and warming on atmospheric oxygen is a matter of arithmetic.

Every atom of carbon that is burned or decomposed consumes one molecule of oxygen. There are about 35 x 1018 molecules of oxygen in the world, but only about 1.7 x 1017 moles of organic carbon on land – yes, this estimate includes all of the carbon in permafrost.

If (God forbid!) all of the terrestrial organic matter on Earth were to burn – every plant, microbe, leaf, and grain of frozen muck – the loss of oxygen would be (1.7 x 1017) / (35 x 1018) = about 0.5%.

It would certainly be possible to measure the change from 100% of our current O2 to 99.5%. But of course there would be nobody to make such a measurement since everyone would already have been oxidized!

Regarding the oceans, carbonate chemistry has no effect on dissolved oxygen. Only the organic parts of marine organisms participate in the CO2 and oxygen dance of photosynthesis and respiration.

If all life on Earth were to perish, in some kind of planetary disaster the likes of which have never happened in all the long story of our blue gem, then indeed the remaining air would eventually become anoxic. This would happen over geologic time as the minerals themselves slowly oxidized.

I have dedicated my professional life to explaining the climate crisis, to researching the problem and possible solutions, and to writing and speaking about the urgency of a just transition to a carbon-free civilization. A fair reading of the article will show I am no sycophant of catastrophe.

But it’s simply not true that there is now or any time in the coming thousands of centuries any reason to expect atmospheric oxygen to become scarce.

Best regards, Scott Denning


Here is a picture of (melting) permafrost on Herschel island:


Dear Professor Denning:

Thank you for your kind and considerate answer. This is how communications about the fate of Earth’s biosphere should be done:frank and thorough. I thank you for your time. 

When I wrote my comment, I was actually aware of the alleged “official in 2019”, quantity of carbon in the permafrost which is the core of your logic…

Here is what the International Panel On Climate Change says on this, Sept 24, 2019:

 Arctic and boreal permafrost contain 1460–1600 Gt organic carbon, almost twice the carbon in the atmosphere (medium confidence). There is medium evidence with low agreement whether northern permafrost regions are currently releasing additional net methane and CO2 due to thaw. Permafrost thaw and glacier retreat have decreased the stability of high-mountain slopes (high confidence).

I am very skeptical of the permafrost carbon number. Moreover, there is more “carbon” which can rot away in regions which are cold now, although not frozen.

Well, I disagree strongly with professor Denning’s position, and I have a strong philosophical argument or two, going that way, and several half-baked scientific points too, which I can add to the mix and which boost my philosophical positions (all of this in coming essay(s)).  I do not believe professor Denning and his colleagues have any counters to all these points.

Yes, I believe we can run out of oxygen on the scale of at most a few centuries, the way things are going, and I can prove it. Sort of. Let’s say it’s definitively in the realm of possibilities.

First a reminder, six years old:

I don’t think we know for sure how much carbon is in permafrost, by an order of magnitude.. Melting permafrost, Herschel island, Canada, 2013…

Patrice Ayme


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