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From what I understand, Dissolved Oxygen is nothing but Molecular Oxygen (O2) loosely bonded with water -- and thus the bond is not chemical, and breaking it would not result to any changes in the chemical composition.

The maximum amount (%) of oxygen water may dissolve under constant temperature and pressure is almost fixed -- proportional to the solubility constant of Oxygen in water.

Usually, the solubility should decrease as the temperature increases, as the molecules will have more energy to break loose bonds like hydrogen bonds (governed by Arrhenius equation I think).

This extra oxygen would thus be freed and go back to the atmosphere.

Onward to the question,

We know that oceans contain a good amount of dissolved oxygen. If we start gradually increasing the temperature (upto the boiling point of salty water), this would liberate more and more of the dissolved oxygen back to the atmosphere. Question is, what will be the % increase in Earth's atmospheric oxygen level if we were to start boiling oceans. Will it scale linearly (or following some monotonically increasing curve) with the temperature of the oceans, or will it hit a maximum and degrade, and what could be that maximum?

We further have two cases- (1) Assume that the aquatic plants/ creatures still keep functioning like normal even in boiling water ; (2) Assume what would happen in reality -- they would die.

Thank you!

Addendum: Let's assume we have high alien-quality nuclear fusion tech to cater for our energy needs. Also we can, for now, forget about the side effects, even if the world is damned.

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  • $\begingroup$ Is it even physically possible to boil all the oceans or is the best you can hope for is to merge the oceans and the lower parts of the atmosphere into a layer of supercritical fluid? $\endgroup$
    – biziclop
    Commented Jun 16, 2023 at 15:38
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    $\begingroup$ The amount of oxygen would increase but so would the amount of nitrogen and CO2 because they are dissolved in the ocean as well. However, the amount of steam would go up a lot more, so in terms of percentages everything else would go down. $\endgroup$
    – N. Virgo
    Commented Jun 16, 2023 at 23:22

2 Answers 2

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  • Mass of the atmosphere: 5.5E18 kg. One fifth is oxygen, about 1E18 kg.

  • Mass of the ocean: 1.35E21 kg. Which looks good, about 250 times larger than the mass of the atmosphere.

  • But oxygen is very poorly soluble in water, with a maximum concentration of 40 mg/L. Even if the entire mass of the ocean was maximally oxygenated, which is isn't, there would be only 1.35E21 ÷ 25,000 = 5.4E16 kg of oxygen in the ocean.

    Gross handwaving here, because on the one hand solubility increases with pressure, but on the other hand not even the upper layer of the ocean is maximally oxygenated. In reality, the top layer of the ocean has about 8 mg of oxygen per liter of water. For comparison, air at sea level contains about 210 mg of oxygen per liter of air.

  • Overall, the oxygen dissolved in the ocean is less than 6% of the oxygen in the atmosphere.

    Actually much less; a better value would be maybe 1%.

  • So that if by some extraordinary means we could extract all the oxygen dissolved in the ocean, the proportion of oxygen in the air would increase from about 21% to about 22%.

    In reality, much less; a better number would be 21.3%.

Note:

While the amount oxygen in the ocean is a small fraction of the amount of oxygen in the atmosphere, the opposite is true with respect to carbon dioxide. Carbon dioxide is readily soluble in water, with the effect that the amount of carbon dioxide in the ocean is about 16 times as large as the amount of carbon dioxide in the air. Releasing all that carbon dioxide into the atmosphere would increase the concentration of carbon dioxide in the air from about 0.04% to about 0.64%, which is huge.

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  • $\begingroup$ Does that take into account the carbon dioxide, nitrogen and other gases that are also currently dissolved in the oceans and that would also be released along with the oxygen if the ocean is heated? $\endgroup$ Commented Jun 16, 2023 at 14:16
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    $\begingroup$ @KerrAvon2055: Nitrogen is very more poorly soluble in water than oxygen, so it doesn't count. And there is basically no carbon dioxide in the air or in the ocean, surely nowhere near enough to register in such a brutal approximation. Carbon dioxide is a trace gas. $\endgroup$
    – AlexP
    Commented Jun 16, 2023 at 14:19
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    $\begingroup$ Good answer. In addition, it would be worth noting that boiling the water would also drive out all the dissolved carbon dioxide in the ocean. CO2 is more soluble in water than O2. $\endgroup$ Commented Jun 16, 2023 at 15:43
  • $\begingroup$ @WalterMitty: Good observation. Edited. $\endgroup$
    – AlexP
    Commented Jun 16, 2023 at 17:25
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    $\begingroup$ The CO2 increase might be temporary if there are still photosynthesizing plants around. They would drive the CO2 levels back down through photosynthesis (making more O2). Curious about what effect that might have on vegetation mass increase. $\endgroup$ Commented Jun 16, 2023 at 21:40
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If we boiled the oceans, the Earth’s temperature would be so high that the permafrost would entirely melt, releasing CH4, and then the carbon in the permafrost would burn. The resulting recombination with oxygen could spectacularly reduce the O2 in the atmosphere. The usual number advanced for permafrost mass seems to evaluate only the first two meters. Computed that way, there’s a huge amount of carbon stored in permafrost — an estimated 1,500 gigatons, or twice as much as the atmosphere contains carbon. Recombining it with oxygen, this would produce around 5,000 gigatons of CO2, one hundred times the present yearly emissions of CO2. Mass of O2 in the atmosphere is roughly one million gigatons So recombining that O2 with the disappeared permafrost, we get that roughly .3% of O2 would disappear. However in some places, in the northern Lena and Yana River basins in Siberia, drilling has shown the permafrost to be 1,493 meters thick. Permafrost stops in depth due to geothermal heating. I have not seen an evaluation for the total permafrost mass. If the permafrost is 200 meters deep in the average, then, under full recombination with O2, an order of magnitude around 30% of the present atmospheric O2 would disappear. (This says that there is a danger of global hypoxia under catastrophic anthropogenic warming…)
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