(This is about O₂, not CO₂)

How did the concentration of oxygen change?

Over geologic time frames since plants began to produce it, and during the time since the carbon dioxide increase caused by humans began is most interesting.

It would be most interesting to have an abstract description, even if simplified.

Plots of the concentration over time are useful too, but not central.


2 Answers 2


Refer to the stratigraphic chart for timeline and stratigraphic units. And i tried to be as up to date as possible in the limited time. Which is also the reason why i omitted some details in favour of an overview.

It is assumed that during the early and middle Archaen only very sporadic oxygen production took place. This changes towards the end of the Archean, when a flow of oxygen from shallow ocean parts to the atmosphere began.

This article suggests that the following "great oxygenation event" (goe), at the early Proterozoic 2.33Gy just took up to 10 million years. During the goe, atmopsheric oxygen increased to 1 or 2 percent, the exact percentage seems unclear. But it seems that in some places, a fully oxygenated ocean existed slightly before the goe at the end of the Archean, at ~2.5Gy. These waters would have been able to produce excess oxygen for the atmosphere in a small or regional scale slightly earlier.

Atmospheric O2 stays low during the Proterozoic, probably because it is used up in weathering processes.

At the beginning of the Cambrian, O2 was still low (~5%). This article describes the development thorugh the Phanerozoic by combining data from modelling and proxies. See Figure 3 (there's your plot;-). It shows that not before the end of the Ordovician atmospheric O2 rises to above 10%, topping off at the end of the Permian at ~32%.

These days, oceanic as well as atmopsheric O2 levels are falling and will continue to do so because of fossil fuel combustion.


There aere too many unknowns to give reliable figures for the varying levels of oxygen in the atmosphere in Precambrian times. What can be said with a fair degree of certainty is that photosynthetic cyanobacteria evolved about 3.5 billion years ago. At first the oxygen they produced was absorbed by the ocean, but by about 3 billion years ago it began to build up in the atmosphere. This led to the Great Oxygenation Event of almost 2.5 billion years ago which lasted some 250 million years and oxidised iron in the ocean, causing it to be deposited as rusty sediments and thus leaving a geological record of the event. About 1.5 billion years ago more advanced photosynthetic organisms evolved, but they were still microscopic.

We know that at the start of the Cambrian there was plenty of oxygen in the atmosphere and ocean, because primitive animal lifeforms which couldn't exist without it became abundant, but there was no one to take measurements so we can't give precise figures. It is said that by the late Carboniferous oxygen levels in the atmosphere may have been as high as 35 percent. One of the reasons given for this guestimate is the presence of giant-size dragonflies with a wingspan of about 2.5 feet, which it is theorised would have needed such high levels of O2 in order to remain airborne. The atmosphere was probably denser at that time, which also would have made flight easier. Forests on land were making a large contribution to oxygen levels and burying carbon in the ground.

Anthropogenic carbon dioxide in vast quantity happened only yesterday, and is a controversial topic, but it is still the photosynthetic life in the ocean which plays the major part in disposing of it and in the process generates oxygen.

  • $\begingroup$ How was the atmosphere more dense? N₂ is lighter than O₂, but not so much that I expect a significant difference in flight physics. $\endgroup$ Commented Dec 24, 2019 at 1:10
  • $\begingroup$ It's another guestimate. We know that in earlier times, Earth, like Mars, had a much denser atmosphere, but on neither planet were there people making measurements. So did enough leak away by the carboniferous to reduce pressure to 14.7lb per square inch? Probably not, because as any pilot will tell you, aerofoils lose lift in rarified air, We have to explain how massive insects the size of crows were able to take flight when their modern counterparts have to be small to remain aloft. $\endgroup$ Commented Dec 24, 2019 at 8:23
  • $\begingroup$ @Michael Walsby: Might not be the pressure that enabled giant flying insects, since birds the size of crows and larger have no problems flying today, nor do we see non-flying insects that size. Could easily be that the percentage of oxygen was higher, so that insects' respiration was more efficient. Or both, of course. Maybe the giant pterosaurs like Quetzalcoatlus northropi would be a better example. $\endgroup$
    – jamesqf
    Commented Dec 25, 2019 at 5:20
  • $\begingroup$ Insects have a primitive respiratory system which becomes less efficient as they grow larger,and therefore differ from birds,which have a vey efficient respiratory system. We can't dissect a pterosaur, so we can only speculate about their respiratory systems. Quetzalcoatlus relied on thermals,much as today's vultures do,so didn't require a very efficient respiratory system.The flightless NZ giant weta,a kind of cricket, is the world's heaviest insect, heavier than a mouse. We don't know when atmospheric pressure came down to its present level, but it was probably well after the carboniferous. $\endgroup$ Commented Dec 25, 2019 at 23:42

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