Does Rising CO₂ Come from the Oceans?

(c) 2025 by Barton Paul Levenson

The hypothesis

A lot of global warming deniers insist that rising CO₂ is coming from the oceans, not from burning fossil fuels. After all, as the temperature rises, the oceans must hold less carbon dioxide in solution. Therefore the new stuff is bubbling out of the ocean, not coming from fossil fuels. This actually happens in a natural deglaciation! Milankovic cycles (the Earth's cycles of axial tilt, orbital eccentricity, and precession) cause solar illumination at the poles to change slightly, there is a slight warming, and CO₂ bubbles out of the oceans--where it causes a further increase of temperature through the greenhouse effect. Nonetheless, there are problems with using this to explain the present rise of CO₂.

1. It fails to establish what is causing the warming. Milankovic cycles are too slow (tens to hundreds of thousands of years).
2. It fails to explain where all the CO₂ generated by burning fossil fuels is going.
3. It fails to account for the fact that fossil fuel CO₂ and background-biosphere CO₂ have very different radioisotope signatures.
4. It fails to demonstrate, quantitatively, how much CO₂ is coming out of the ocean, and how this relates to how much new CO₂ is appearing in the atmosphere.

Let's establish some baseline facts. CO₂ in the preindustrial era averaged about 280 parts per million by volume (ppmv). It is currently about 425 ppmv. That means 145 ppmv has been added in the last 200 or so years (NOAA 2025).

Analysis of the short-term carbon cycle shows that the oceans are actually a net sink for CO₂, not a source: The oceans give off 90 gigatons of carbon a year, but take in 92, for a net -2 gigatons of carbon per year (IPCC 2022). But let's ignore that. Assume the oceans are only a source. How much carbon dioxide would the increase in temperature since the 1800s have produced?

Henry's Law

There is a relationship called Henry's Law between the partial pressure of a gas and the amount dissolved in a liquid under the gas. It is an equilibrium relation, meaning that everything has to have come to a stop before it applies. And it assumes there is no further chemical reaction in the dissolved gas--for carbon dioxide, changes in seawater are extensive. But it can be a fair approximation even when things are changing, and chemistry is going on, assuming they're changing slowly enough. Henry's Law can be written as:

p_X = [X] / k_X

where p_X is the partial pressure of the gas in the atmosphere, [X] is the molar concentration of the gas in the liquid medium, and k_X is the Henry's Law constant for the gas. Here X stands for whatever gas we're considering.

The Henry's Law constant for carbon dioxide in seawater, at the reference temperature of 298.15 K, is 0.034 moles per liter per atmosphere. It varies with temperature according to the van't Hoff equation, which can be expressed as:

H(T) = H₀ exp (-(Δ_solH / R) (¹/_T - ¹/_T0))

Here H(T) means the Henry's Law constant at the temperature T we want. H₀ is the constant at the reference temperature, T₀. And Δ_solH / R is the temperature dependence of the Henry's Law constant. The latter term has a value of about 2400 in the units we want.

So let's get calculating.

Doing the math

It should be noted that the second equation above is an approximation, valid for only about 20 K of temperature change either way. But we're going to be within the envelope, so it's all right to use it.

Preindustrial mean global annual surface temperature was about 286.8 K. The present temperature is about 1.4 K higher than that, or 288.2 K. That means the Henry's Law constant was about 0.0468 in the preindustrial era and is about 0.0449 now. Notice that the figure is lower for the higher temperature.

Let's calculate the molarity of CO₂ in the preindustrial ocean. To do this we rearrange the Henry's Law equation (the first equation above):

[X] = k_X p_X

And Henry's Law tells us the molarity of CO₂ in the preindustrial ocean was about 13.1 moles per liter.

Now, that's how much carbon dioxide we've got to work with. Let's use the original form of the equation, p_X = [X] / k_X, and plug in [X] = 13.1 and the modern-era k = 0.0449. We then predict a partial pressure of carbon dioxide of 292 ppmv.

What? But it was supposed to be 425 ppmv, wasn't it? Instead of increasing by 145 ppmv, it only increased by 12 ppmv--one twelfth of what's needed!

So even if

• the warming was coming from something other than increased greenhouse gases, and
• the CO₂ from burning fossil fuels was mysteriously disappearing, and
• CO₂ from the ocean had the same radioisotope signature as CO₂ from burning fossil fuels, and
• the ocean were only a source, and not a sink, for CO₂, then...

...the ocean could only have supplied 8.3% of the observed increase.

So the cry of "the new carbon dioxide is coming from the oceans, not from fossil fuels!" is defeated by actually doing the math--something global warming deniers who tout Henry's Law as proof for their thesis tend to be averse to doing.

References

IPCC 2022. Climate Change 2022: Impacts, Adaptation, and Vulnerability. Contribution of Working Group II to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change. Pörtner, H.-O., Roberts, D.C., Tignor, M., Poloczanska, E.S., Mintenbeck, K., Alegría, A., Craig, M., Langsdorf, S., Löschke, S., Möller, V., Okem, A., Rama, B. (eds). Cambridge, UK: Cambridge University Press, 3056 pp.

NOAA 2025. Global Monitoring Laboratory. https://gml.noaa.gov/ccgg/trends/, accessed 05/19/2025.