If you’re at #OSM26 in Glasgow, I’ve just signed lots of copies of Blue Machine that are now on sale in the shop at @glasgowscience.bsky.social 🌊🌊 Share the ocean love!
21.02.2026 14:46
👍 170
🔁 19
💬 5
📌 1
If you’re at #OSM26 in Glasgow, I’ve just signed lots of copies of Blue Machine that are now on sale in the shop at @glasgowscience.bsky.social 🌊🌊 Share the ocean love!
Extremely insightful and complex study on ocean alkalinity enhancement under different future emission scenario. Congratulations to @jhauck.bsky.social and team. This is a great primer for many of the things we plan to elaborate further in #OAEMIP.
🙏 Thanks to Hanna van de Mortel and Nina Bednarsek for leading the effort, and to Nicolas Gruber, @banjogreg.bsky.social , and Richard Feely for great contributions.
⚠️ Caveat
We focus on the direct effects of alkalinity enhancement. The broader benefit that CO₂ removal — irrespective of the method — reduces ocean acidification is not considered here.
📝 Implication
The more efficient OAE is at removing CO₂, the less effective it becomes at mitigating biological impacts of ocean acidification. Nevertheless, OAE can provide localized acidification relief around the deployment site, motivating deployments with explicit biological objectives.
Time series for the restoration of pHT for four levels of TA additions from 1985 to 2022 compared to the unperturbed conditions. Computations are done assuming ηmax = 0.832 (coastal region) and 80% CDR efficiency.
Taking a “time-machine” perspective inspired by @davidho.bsky.social , we asked: How far back in time does a coastal alkalinity addition of 50 µmol kg⁻¹ take calcification conditions?→ About 20 years, for an assumed 80% CDR efficiency and depending on location.
Fully restoring preindustrial calcification conditions under equilibrated CO₂ would require an average alkalinity increase of ~300 µmol kg⁻¹ — far exceeding the ~60 µmol kg⁻¹ surface-ocean DIC increase since the industrial era.
Calcification rate decline due to OA, calcification rate increase upon 50 μmol/kg NaOH addition, and NaOH addition required to restore preindustrial conditions. (a) Decline of species-specific calcification rates from preindustrial to current conditions, (b) the relative increase in calcification rate compared to the current conditions baseline upon 50 μmol kg−1 NaOH addition, and (c) the concentration of NaOH addition required to restore calcification to the preindustrial conditions assuming unequilibrated conditions.
📉 What this means in numbers
Increasing total alkalinity by 50 µmol kg⁻¹ enhances calcification by ~14% on average across species — substantial when compared to an average 17% decline since preindustrial times. However, once CO₂ uptake is realized, the recovery drops to <5% on average.
![Changes in seawater−carbonate chemistry due to OAE and subsequent uptake of atmospheric CO2 returning pCO2 to the pretreatment level. Conceptual diagram illustrating the effect of OAE on pH and [CO32−], considering a two-step procedure in which first TA is enhanced under otherwise isochemical conditions (DIC = const., Step 1) followed by a re-equilibration of the seawater sample with current atmospheric pCO2 (Step 2).](https://cdn.bsky.app/img/feed_thumbnail/plain/did:plc:vnatj6ys5y2fjs6suxxokbh4/bafkreigsjjqb6kqlg765kwprjldtbtnbngxzfwwstyiqeiiaykwskm4ese)
Changes in seawater−carbonate chemistry due to OAE and subsequent uptake of atmospheric CO2 returning pCO2 to the pretreatment level. Conceptual diagram illustrating the effect of OAE on pH and [CO32−], considering a two-step procedure in which first TA is enhanced under otherwise isochemical conditions (DIC = const., Step 1) followed by a re-equilibration of the seawater sample with current atmospheric pCO2 (Step 2).
🧪 Why?
This outcome follows directly from marine CO₂ chemistry: under constant atmospheric pCO₂, even substantial increases in alkalinity lead to only small changes in pH and carbonate ion concentration.
📊 Key result
OAE leads to only moderate improvements in calcification, especially once the additional CO₂ uptake from the atmosphere is realized.
🔬 Approach
Using experimental data from 27 marine calcifiers negatively affected by acidification, we quantified how calcification would recover under different levels of alkalinity enhancement. Crucially, we distinguish between scenarios with and without the intended uptake of atmospheric CO₂.
🌊 New paper out assessing the potential of Ocean Alkalinity Enhancement (OAE) to mitigate the impacts of ocean acidification on marine calcifiers
doi.org/10.1021/acs....
Editorial note: I just learned that additional credits have been issued in the meantime, for the continued CO2 uptake simulated to have occurred after the first crediting period.
🌊
Lots of things to be done, and I look forward to tackling some of them by coordinating the upcoming #OAEMIP. Stay tuned.
⏳ The largest cut to the gross CDR estimate (>30%) comes from limiting the crediting period to three months beyond deployment. This is required for ex-post crediting, but I’m wondering how we will handle this in the future — especially for larger-scale interventions.
📉 There is no established routine to propagate the individual uncertainty components in the simulated CO₂ uptake into a transparent uncertainty budget. Developing these routines would strengthen verification and reveal where model improvements are most urgently needed.
⚖️ Model uncertainties are subtracted from the gross CDR estimate to obtain the carbon removal that can be credited. I like this conservative approach, but based on my experience with quantifying the “natural” ocean carbon sink, I’m wondering if the reported ~10% uncertainty is high enough.
If you agree, consider submitting a proposal to our joined learning opportunity accompanying the next major OAE experiment: www.carbontosea.org/2025/11/10/h...
🚢 It is not required to demonstrate through measurements at sea that the added feedstock dissolved and increased alkalinity as expected. Quantifying OAE in the far field is certainly tough — but shouldn’t it be possible in the near field?
🎉 But first of all: huge congratulations to everyone involved! Quantifying the carbon removal required combining four different types of models with measurements from the field and in the lab. It’s a massive effort and truly at the forefront of what science can deliver today.
As a newbie to the field, moving over from fundamental ocean carbon research, there are a couple of aspects that surprised me.
Want to understand how the world’s first carbon credits for ocean alkalinity enhancement were issued? Anna Madlener — one of the new colleagues I’m extremely excited to work with at the Carbon to Sea Initiative — has you covered:
www.carbontosea.org/2025/11/25/a...
The assessment includes ocean alkalinity enhancement (OAE) as a mCDR approach, but the field trials that are reported involved different chemical compositions of the feedstock to achieve the alkalinity addition.
Yes, to my understanding these numbers represent reported activities with the direct purpose to achieve CDR. But I'll confirm this with the colleagues from the @stateofcdr.bsky.social , and ask how liming / wastewater treatment are considered.
Great progress! Maybe resolve more decimals places to make the orders of magnitude between conventional and novel CDR comparable? Could be done at least in the data file.
Many thanks to @pfriedling.bsky.social for leading the effort, and @jhauck.bsky.social and Peter Landschützer for onboarding my to the GCB ocean team. It was a great experience to be part of this community.
🔬 Strengthening and expanding our global carbon flux observing systems across the ocean, atmosphere, and land seems more important than ever. @wmo-global.bsky.social
⚖️ Despite advances in understanding the global carbon cycle, the 2024 budget imbalance is one of the largest in six decades, due to an overestimate of atm. CO₂ growth or of land and/or ocean sinks.
This underscores major challenges in tracking carbon-cycle changes and detecting emerging shifts.
🔧 We also included ocean-based carbon dioxide removal (CDR) estimates for the first time. These fluxes are still tiny compared to CO₂ emissions, but we wanted to start documenting their growth. Thanks to Kirsty Harrington from the @stateofcdr.bsky.social report for sharing updated 2024 estimates.