Ocean CO2 Outgassing With Temperature – Watts Up With That?

Guest Post by Willis Eschenbach (yes, I’m still banned from X/Twitter, see here).

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:

• [1] https://andthentheresphysics.wordpress.com/2013/10/28/henrys-law/
• [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC7474059/
• [3] https://phys.org/news/2024-03-sea-surface-temperature-evidence-human.html
• [4] https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2008gb003449
• [5] https://worldoceanreview.com/en/wor-8/the-role-of-the-ocean-in-the-global-carbon-cyclee/how-the-ocean-absorbs-carbon-dioxide/
• [6] https://www.pnas.org/doi/10.1073/pnas.2318779121
• [7] https://oxfordre.com/climatescience/display/10.1093/acrefore/9780190228620.001.0001/acrefore-9780190228620-e-885?p=emailAMn6v3nlVKpFs&d=%2F10.1093%2Facrefore%2F9780190228620.001.0001%2Facrefore-9780190228620-e-885
• [8] https://www.annualreviews.org/doi/pdf/10.1146/annurev-environ-120920-111307?download=true
• [9] https://cp.copernicus.org/articles/20/1177/2024/
• [10] https://cp.copernicus.org/articles/11/991/2015/cp-11-991-2015.pdf

This was interesting for a couple of reasons. First, my estimate of the long-term change (~18 ppmv/°C) is the same as theirs. However, they point to something that I hadn’t thought of—the short-term increase is due to the warming in the entire mixed layer (depending on the location, a depth of from 15m to 200m depending on location, average ~60 m), not just warming of the sea surface temperature.

They estimate the short-term increase using the mixed layer as being on the order of 6 ppmv/°C. Given the very slow rate of mixing across the “thermocline” (the area of rapid temperature drop at the bottom of the mixed layer), I would think that would be more relevant to our current situation of explaining a short-term rise in CO2.

So for purposes of this analysis, let me take a value of ~ 10±4 ppmv for each 1°C warming of the top 100 meters of the ocean.

And how much has the top 100 meters of the ocean warmed?

About 0.33°C from 1969 to 2020, per NASA.

During that time, CO2 went up by about 100 ppmv.

How much of that was from the ocean? Well, given the data above, it is something on the order of ~2 ppmv to ~ 5 ppmv, which is also about 2% to 5%.

Conclusion? While I agree with the good Dr. M. that outgassing due to ocean temperature rise is a “missing part of the climate puzzle”, at 2% to 5% of the rise in CO2, it’s a small part of the climate puzzle, and as such it is generally ignored in the overall analysis.

My thanks to Dr. Marohasy for all of her excellent contributions to the climate debate.

Here, I’m back on the hill overlooking the ocean after a week in Australia, two weeks in Fiji, and a week in the Solomon Islands. Here’s dawn from where I stayed with friends in the Solos …

It was wonderful to visit old friends, I got to scuba dive the reef wall around Kennedy Island, complete with a moray eel, two turtles, giant clams, and the fish with the longest Hawaiian name, the “Humuhumunukunukuāpuaʻa” … and hey, the waves aren’t going to surf themselves …

My very best regards to everyone,

w.

PS—As always, I ask that when you comment you quote the exact words you are referring to. It avoids all kinds of misunderstandings.