by W. Eschenbach, Apr 13, 2025 in WUWT

Over at Dr. Jennifer Marohasy’s always interesting blog, she makes an interesting claim about ocean outgassing of CO2.

Picture this: it’s a hot day, and you grab a soda can that’s been in the sun. You crack it open—psssht—and CO₂ fizzes out, tickling your nose, maybe spraying your shirt if you’re slow. It’s a tiny chaos, a burst you can’t control. Now imagine that fizz across the ocean’s sun-warmed surface, covering 71% of Earth, bubbling CO₂ into the air we breathe. Wild, right? A bit mad. I reckon it’s a missing piece of the climate puzzle.

Now, it’s undeniable that warmer ocean water contains less CO2 than colder water. As the temperature rises, the CO2 outgasses from the ocean. But that wasn’t my question. After reading her post, my question was, “How much?”.

How much increase in CO2 do we get from a 1° increase in ocean temperature?

My first step was to look at the Vostok ice core data. It shows that as the globe rose by ~ 5°C, the CO2 level rose by ~ 100 ppmv. This gives us a CO2 outgassing trend of ~20 ppmv per 1°C temperature rise.

However, it’s not quite that simple. More of the ocean was covered by sea ice during the glaciation, reducing the outgassing ocean area. How much? Estimates are on the order of 30 million km^2, which is about 8.2% of the current ice-free ocean. So that would drop the value to ~ 18 ppmv/°C.

Next, I went to my go-to AI, perplexity, which gave me the following:

To estimate the increase in atmospheric CO₂ from a 1°C rise in ocean temperatures, Henry’s law and equilibrium dynamics between the ocean and atmosphere are key. Here’s a synthesis of insights from relevant studies:

• Henry’s Law and Temperature Dependence

Henry’s law states that the concentration of dissolved CO₂ in seawater (C) is proportional to its partial pressure in the atmosphere (p):  C = k_H * p, where(k_H) is Henry’s constant, which decreases with rising temperature (reducing CO₂ solubility). For example, a 1°C increase reduces(k_H) by ~4%[1][6]. This shifts the equilibrium, releasing CO₂ from the ocean to the atmosphere.

• Model Estimates

1. Simplified Ocean Layer Model:  

 Using a 100m-deep surface layer exchanging CO₂ with the atmosphere, a 1°C rise increases atmospheric CO₂ by ~6 ppm[1]. However, this assumes limited vertical mixing and may underestimate the full ocean response.  

   – For deeper layers (e.g., 3800m), the same temperature rise could yield ~20 ppm[1], aligning with glacial-interglacial transitions where a 5°C warming corresponded to ~90 ppm CO₂ rise (~18 ppm/°C) [1][7].

2. Modern Context:  

Recent studies highlight complexities like the ocean’s thermal skin layer and vertical mixing. Correcting for temperature gradients between the ocean surface and sampling depth increases estimated CO₂ uptake by 0.8–0.9 PgC/yr[2], but this focuses on current anthropogenic fluxes rather than equilibrium responses.  

• Key Variables

– Mixed-Layer Depth: Shallow layers (e.g., 100m) show smaller CO₂ releases than deeper layers[1].

– Buffering by Carbonate Chemistry: CO₂ dissolution forms carbonic acid, which dissociates into ions, adding complexity beyond Henry’s law[5][6]. This buffers pH changes but does not prevent long-term CO₂ release as temperatures rise.  

• Synthesis

The estimated increase ranges from 6–20 ppm/°C, depending on assumptions about ocean mixing depth and timescales. The higher end (~18 ppm/°C) aligns with paleoclimate evidence[1][7], while modern models suggest lower values due to ongoing oceanic uptake[2][3]. These estimates exclude feedbacks like biological responses or changes in ocean circulation.

• Citations

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