How tightly does oxygen limit biological mCDR ?

[Brad Warren]

How much has the road to climate solutions just narrowed?

For years, Oschlies et al have been helping the rest of us see the enormous lungs of the deep. Back in 2021, Oschlies and co-authors showed that legacy carbon emissions (those already gushed into the atmosphere) will ultimately drive another 4x more oxygen depletion in the ocean. In a June 2025 paper, Oschlies and co-authors just showed that efforts to accelerate the ocean's biological powers for carbon cleanup could exact a steep price, paid in oxygen. How much does this narrow our options for mopping up the carbon mess?

Excerpt:
Global warming is a main cause for current ocean deoxygenation. A deployment of marine carbon dioxide removal (CDR) for mitigating global warming could therefore also be viewed as a measure for mitigating ocean deoxygenation if, and only if, the respective CDR measure itself does not lead to a larger oxygen loss than the reduction in atmospheric CO2 would prevent. We here review the current state of knowledge regarding the potential impacts of various marine CDR (mCDR) options onto ocean oxygen, a key ocean state variable and an essential element for all higher forms of marine life. Using results from global model simulations, we show that biotic approaches, such as ocean fertilization, macroalgae cultivation and sinking, and placement of organic matter that is prone to remineralization, can lead to a loss in seawater dissolved oxygen that is 4–40 times larger than the oxygen gain that would result from the CDR-induced reduction in global warming only.


Source, Potential impacts of marine carbon dioxide removal on ocean oxygen
Andreas Oschlies et al 2025 Environ. Res. Lett. 20 073002
DOI 10.1088/1748-9326/ade0d4
https://lnkd.in/gv-qbsJp
hashtag#climate hashtag#climatesolutions hashtag#carbonremoval hashtag#mCDR 

Sharing this post, which I first put on LInkedin and Global Ocean Health website.
hashtag

[Greg Rau]

Thanks Brad. Shouldn’t the bio CDR game be to find ways of preventing oxidation of present or enhanced marine primary production, thus minimizing O2 consumption and maximizing C storage? Depositing in existing anaerobic zones is one approach. But the problem with increasing storage of C is that you also increase the storage of N and P accompanying that C, and thus short circuit nutrient recycling and reduce the primary production/C drawdown based on that. Selectively storing C rich, nutrient poor biomass (in anaerobic zones or underground), which could involve marine or land biomass sources, is required? Then there is the marine C extraction and long-lived use scenario for CDR, eg, long-lived, marine C-based plastics? Plus brownie points for avoiding fossil fuel based plastic use?

Greg  

Sent from my iPhone

On Jul 28, 2025, at 8:08 AM, Brad Warren <br...@globaloceanhealth.org> wrote:



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[Brad Warren]

Thank you Greg. You are exploring pathways to mitigate the trade-offs. This leads to two main questions: 

1) Got any clarity on which biomass forms (processed or otherwise) might meet that description: C-rich, nutrient-poor? 

2) Suggestions regarding scientific approaches that could inform decisions on how to manage the tradeoffs here?

One such trade-off is this: If we haul away such carbon-rich biomass for sequestration, will that carbon be missed in ecosystems that produced it? 

Some forest conservation experts have raised this concern. In fact, some authorities in that field oppose commercialization of biochar made from forest “slash” that is removed to reduce fire risk associated with “ladder fuels” left in the forest. This usually consists of material like tree tips, branches, stumps, undergrowth, small trees that are judged to be crowding too densely to thrive. Critics fear incentivizing activities that rob needed carbon from forest floors.

Choosing a path through such trade-offs is a familiar challenge for land managers. Their conundrum is growing sharper as accelerating wildfires consume more forests (and sometimes towns). In response, forest managers, including many Tribes, are undertaking extensive “fuels management” work to reduce the heat and destructive power of fires when they break out. 

That work isn’t cheap. Paying for it might be easier if they can find practical and sustainable ways to sell some of the forest biomass they remove (instead of, say, piling it up and burning it in place to avoid feeding the next wildfire). In many cases, the cheapest practical option is to burn those slash piles. That practice may undercut at some of the purpose of leaving the material in place: the fire lofts much (often most) of the C into the atmosphere. 

In some cases, companies using forest wastes for carbon sequestration may be able to offer useful alternatives. Yet here too, the trade-offs continue: is carbon removal a higher priority than conserving carbon stocks in soil? Or vice versa? What modeling and measurement approaches can help to resolve this quandary and support sound decisions?

Global Ocean Health is an ocean organization, but we have worked closely with Tribes for many years, and our friends in the Tribes have drawn us ashore to join them in efforts to intercept trouble before it reaches the sea. We and several collaborators have just launched a learning network for Tribal forest managers around forest biomass and its potential uses in sequestration. 

We would be glad to hear from folks in this group whose research and companies may open new options. In a field where trade-offs are difficult to avoid, we are interested in exploring ways of easing them —as you suggest, Greg— and ways of understanding the tradeoffs well enough to manage them intelligently.