Not one shoe fits all: Applicability of hydrodynamic models for the simulation of ocean alkalinity enhancement in the Baltic Sea

[Geoengineering News]

https://www.sciencedirect.com/science/article/abs/pii/S0924796326000485

Authors: Anna-Adriana Anschütz, Thomas Neumann, Peter Holtermann, Hagen Radtke

14 May 2026

https://doi.org/10.1016/j.jmarsys.2026.104226

Highlights

•A passive tracer in the central Baltic Sea was run in two models to assess transport.

•Transport estimates can be highly uncertain even in calibrated models.

•Validating transport beyond salt and temperature is key for reliable estimates.

Abstract

Ocean alkalinity enhancement (OAE) has become the focus of intensive research as a potential method for future carbon dioxide removal (CDR). In the Baltic Sea, the deposition of alkaline minerals on the seafloor is one of the considered options, as their dissolution raises local alkalinity. Once the additional alkalinity reaches the sea surface, it increases the ocean’s potential for uptake of atmospheric CO2. A reliable estimation of the vertical transport of dissolved constituents is therefore essential for a model’s applicability to ocean alkalinisation research. While concentrations of seawater ingredients are often straightforward to measure, the corresponding transports of these ingredients by advection and turbulent diffusion are not. Based on an example from the study of ocean alkalinity enhancement, we demonstrate that these transport estimates can exhibit a considerable range of uncertainty even when well-calibrated models are employed. To evaluate the reliability of such estimates by current physical ocean models, we recreated a passive tracer release experiment in the Gotland Deep (Baltic Sea) in three model setups: two hydrodynamic models with differing vertical coordinate systems, including two resolutions for one of the models. The resulting simulations produced substantially different estimates of vertical tracer transport. In the case of OAE, this could lead to considerable variability in the predicted time lags between mineral deposition and sea surface contact and thereby in the desired CO2 drawdown. We conclude that, in addition to typical comparisons with salt and temperature observations, a specific validation of transport processes as a prestudy is necessary to assess whether a hydrodynamic model setup is suitable for research questions that require reliable physical transport estimates. These research questions encompass a range of typical applications of hydrodynamic modelling, including simulating the transport of nutrients, toxic substances, or fish larvae.

Source: ScienceDirect 

[Greg Rau]

There seems to be a misunderstanding here about how carbonate minerals on the seafloor might effect OAE mCDR. 

In the abstract: "In the Baltic Sea, the deposition of alkaline minerals on the seafloor is one of the considered options, as their dissolution raises local alkalinity. Once the additional alkalinity reaches the sea surface, it increases the ocean’s potential for uptake of atmospheric CO2."

and in the intro:

"for OAE to effectively remove atmospheric CO2, the added alkalinity must be transported from the deep basin to the surface."

Similar statements are in a related, previous paper by some of the same authors.

If carbonate minerals are placed in contact with actively respiring ocean sediments (as advocated in the paper), the CO2 being generated can, indeed, react with the added carbonate minerals to produce alkalinity. eg: CaCO3s + aCO2 + bH2O ---> Ca++ + cHCO3- + dCO3-- ....   However, as the preceding shows, such alkalinity will be fully carbonated and no further CDR will occur once/if that alkalinity reaches the ocean surface and contacts air. In fact the likely increase pH  between the seafloor and the ocean surface means that there will be some loss of the originally captured CO2 due less efficient C storage of alkaline C at elevated pH. 

Anyway, modeling ocean physics (the thrust of the paper) is still important in determining the effectiveness of such CDR because it is necessary to estimate the counterfactual of how much CO2 would otherwise escape to the atmosphere in a given time period in the absence of this form of OAE.  By analogy adding uncarbonated alkalinity to CO2-supersaturated surface ocean water (eg, in upwelling areas) can effect CDR solely by reducing marine CO2 emissions to the atmosphere rather than "removing atmospheric CO2". Again, it is necessary to realistically model ocean physics in order to quantify how much marine CO2 emissions are avoided (and thus gross CDR achieved). Then all of the emissions associated with installing, maintaining, monitoring and verifying the OAE activity has to be subtracted to arrive at a net CDR. Good luck, considering that in the Baltic Sea-carbonate case at best only about 0.7 mols of gross CDR can be achieved per mol of CaCO3 added (0.3 t CDR/t CaCO3).  Or what am I missing?

🌊

Greg

--
You received this message because you are subscribed to the Google Groups "Carbon Dioxide Removal" group.
To unsubscribe from this group and stop receiving emails from it, send an email to CarbonDioxideRem...@googlegroups.com.
To view this discussion visit https://groups.google.com/d/msgid/CarbonDioxideRemoval/CAHJsh9-A8AgXbD0qPcL58d%3D5TcSzD4aNUUH1YtYw8oT-X%3DKgJA%40mail.gmail.com.

--

Greg H. Rau, Ph.D.

Senior Research Scientist
Institute of Marine Sciences
Univer. California, Santa Cruz
https://www.researchgate.net/profile/Greg_Rau
Co-founder and manager, the Carbon Dioxide Removal Google group
Co-founder and Senior Scientist, Planetary Technologies, Inc.
510 582 5578

510 363 1519 c