the Creative Commons Attribution-NonCommercial 4.0 International License.
the Creative Commons Attribution-NonCommercial 4.0 International License.
Ideas and perspectives: A framework for understanding efficiency losses of Ocean Alkalinity Enhancement
Abstract. Ocean alkalinity enhancement (OAE) is a proposed carbon dioxide removal (CDR) approach in which seawater alkalinity is increased to facilitate both the enhanced uptake of atmospheric CO2 and long-term storage in the ocean’s dissolved inorganic carbon pool. Carbonate system thermodynamics provide a stoichiometric upper bound on the amount of CO2 that can be taken up per unit of alkalinity added, but realized uptake can be lower due to dynamic physical and (bio)geochemical processes that evolve over time. This Perspective presents a structured framework for understanding these potential efficiency losses. We identify loss pathways across four categories: incomplete mineral dissolution, loss of alkalinity to solid phases, incomplete air–sea exchange, and indirect impacts on ocean biogeochemical cycles. The potential significance of each loss varies across OAE approaches and across spatial and temporal scales. We highlight that substantial knowledge gaps persist across all of these processes, and we suggest targeted research priorities to improve fundamental understanding and reduce uncertainty. By providing a common structure and definitions, this framework aims to support coordination across oceanographic disciplines and to improve assessments of OAE efficiency at both the project and global scale.
Status: open (until 21 Oct 2026)
- RC1: 'Comment on egusphere-2026-5148', Anonymous Referee #1, 11 Sep 2026 reply
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RC2: 'Comment on egusphere-2026-5148', Anonymous Referee #2, 01 Oct 2026
reply
The authors have produced a review paper going through and categorizing various processes and uncertainties that can affect additionality of OAE. They present their results as a framework for understanding efficiency losses for OAE.
While the paper has some nice elements, I nevertheless think it should be returned to the authors for major revisions or rejected at the editor's discretion (as this appears to already be a 3rd draft):
When the contribution to the field is that complicated material is being presented in a way that makes it easier to understand (rather than adding new analysis), then the bar is quite high for the accuracy and clarity of the writing. This manuscript, while solid in some places, does not yet meet that high bar in my opinion. Regarding whether the framework is adding something that is highly original, I’d argue it is not, as I’ve seen closely related frameworks used in other papers (e.g., Ho et al., 2023), MRV protocols (https://registry.isometric.com/protocol/ocean-alkalinity-enhancement/1.0), and websites (https://carbonplan.org/research/cdr-verification/ocean-alkalinity-enhancement-mineral). It is perhaps more clearly phrased than some, but it does not rise to the level of helping me think about any of the issues more clearly, and the other examples were presented as components of broader efforts rather than the main product. The paper does provide a useful list of literature related to some topics, but it is far from exhaustive, has inaccuracies in framing and phrasing, and has some extraneous and redundant text.
My opinion is strongly undercut if other reviewers do find it helpful, so I’m happy to be over-ridden.
Specific comments: (I’ll add a note here that the lack of line or page numbers is a challenge during the review process, though I’m not sure if this was an omission by the authors or a quirk of how the article is displayed)
Title: at least one of these processes considered can lead to a gain in additionality
The introduction acknowledges that the thermodynamic limit varies from place to place, but doesn’t sufficiently consider that it can vary from time to time, and that it is the limit at the time and place of air-sea exchange that matters more than the limit at the time of addition.
Re: “We identify nine loss pathways, grouped into four categories: incomplete mineral dissolution, alkalinity loss to solids, incomplete air–sea equilibration, and indirect impacts on ocean biogeochemical cycles that affect CO₂ uptake (Figure 1).” It is not immediately clear to me which category the feedback related to the loss of atmospheric CO2 and consummate loss of anthropogenic carbon to the atmosphere through air-sea exchange in otherwise unperturbed regions would fit in this scheme.
I think “(and circum-neutral pH),” is gesturing at the idea that the pH is unchanged from ambient, but that is not what those words mean… nor would pH be expected to be unchanged. I suggest focusing on pCO2 instead of pH.
“unequilibrated alkalinity” is a flawed phrase that is used throughout the paper… consider the instance in which alkalinity is quickly added to high pCO2 water until it reaches air-sea equilibrium. This water has not resulted in any mCDR yet, and yet it is now fully equilibrated. The key is rather whether net air-sea exchange has been modified to the extent expected based on thermodynamics.
Re: Table 1… where does feedstock burial fit in? Category 2 appears to be strictly but not explicitly abiotic? Re: “Early timepoints have lower apparent efficiency than later.” This is frequently untrue when using the eta efficiency definition presented in this paper. Eta typically peaks on the 5-15 year time horizon because the atmosphere has responded to the surface perturbation but the deep ocean has not yet responded to the atmospheric change by degassing anthropogenic carbon (Yamamoto et al. : https://iopscience.iop.org/article/10.1088/1748-9326/ad7477/meta). This is a common problem in the literature because people are confusing eta with an air-sea gas exchange efficiency (Yamamoto’s “relative efficiency”). The subduction row seems to focus on loss of excess TA to the ocean interior, but the return of excess TA from the ocean sediments/interior to the ocean surface is also a key pathway for alkalinity that is only added below the surface. Is seawater Omega really likely to be more impactful than surface pCO2 for organic matter cycling?
Re: “Slower than expected or incomplete dissolution results in discrepancies between expected and actual alkalinity enhancement, lowering efficiency” Should the possibility of faster than expected dissolution not be acknowledged?
“However, environmental conditions at the mineral surface may differ from bulk seawater,
precluding direct calculation of dissolution rates.” Semantics, but calculation is not precluded per se… I’d argue that this is just a challenge to trusting the result to high precision.Re: “For faster-dissolving feedstocks, high Ω around particles can also result in a dynamic interplay
between dissolution and secondary mineral precipitation (described in the next section). If
dissolution is sufficiently rapid but transport of resulting alkalinity away from the grain surface is
limited, supersaturation with respect to carbonate minerals may occur, leading to mineral
precipitation (aragonite in seawater and calcite in freshwater) (Cloud Jr et al., 1962; Hartmann et
al., 2023; Moras et al., 2022; Pytkowicz, 1965). Further, precipitation on the feedstock surface
potentially slows dissolution rates. Therefore, the role of dissolution kinetics on OAE efficiency
must be considered in tandem with loss of alkalinity to carbonate formation, especially for
applications with fast-dissolving feedstock.” Omega here is being used somewhat loosely for unspecified minerals and seemingly perhaps for multiple minerals in a single sentence.Re: “In open ocean applications, faster-dissolving minerals can be used for OAE if they dissolve sufficiently rapidly before sinking below the mixed layer (Renforth et al., 2022).” With patience, dissolution below the mixed layer is also effective. The authors frame their discussion frequently around timescales that matter for mCDR without sufficient consideration of what those timescales are or what depths equate to what timescales. The current treatment seems oriented around the idea that anything below the mixed layer can be neglected.
Re: “Following mineral dissolution, alkalinity can potentially be lost via subsequent precipitation to, or reaction with, solid minerals. This concept is sometimes referred to as alkalinity stability (Hartmann et al., 2023).” As noted earlier, this seems to be limited in consideration to abiotic losses, so why not use the more common term of secondary or abiotic precipitation? The authors should probably not self cite when making a case that a term is in common use.
R: “CaCO3 precipitation can be conservatively assumed to be a long-term loss of alkalinity, given that the ocean is oversaturated with respect to CaCO3 (Berner, 1995; Renforth and Henderson, 2017).” This statement is unneeded and more incorrect than correct… Much of the ocean is undersaturated with respect to CaCO3 and there is a great deal of literature suggesting that carbonate minerals dissolve throughout the ocean, including in supersaturated parts of the ocean (admittedly some effectively contend that this is the result of undersaturated microenvironments).
Re: “The stability of precipitated Mg(OH)2 is not well understood, though some research suggests it may dissolve over longer timescales, i.e., months to years (Moras et al., 2026).” Why only point out the longer time horizon studies? Shaw et al., 2025 and Ringham et al., 2024 (same research group) found much shorter time horizons for Mg(OH)2 precipitated from seawater.
“Precipitation thresholds may be lower in sediments compared to the water column, due to buffering by in situ minerals, alkalinity production by anaerobic respiration,” These first two would affect the omega, not the threshold, I think.
“Alkalinity losses to sediments would logically be most significant for shallow coastal deployments, where alkalinity is elevated in water masses with prolonged sediment contact, and/or for mineral deployments on the seafloor.” This could be paraphrased as “Alkalinity losses to sediments would logically be most significant where sediments are present” which doesn’t add much insight.
“full equilibration is reached over years to decades” for a beaker of seawater on the counter, this is much too long. For surface ocean mixed layers the usual reference is Jones et al., which states months to years. Decades become relevant primarily due to exchange with the ocean interior, which you have dealt with as a separate mechanism, so this time horizon is much too long for this part of your paper.
“may result in effectively permanent efficiency loss” it is not helpful to only give the upper limit. Seasonal mixed layers are often re-entrained months after they are detrained.
“During that time, biological processes within the subducted water mass may consume some of the unequilibrated alkalinity, however this will have no impact on atmospheric CO2 (Wolf-Gladrow et al., 2007).” Probably this belongs in the next section.
“Air-sea equilibration and CO2 uptake (and/or decreased CO2 outgassing) may proceed when the water mass returns to the surface, if any excess alkalinity remains.” Is the implication here that biological feedbacks might entirely negate the OAE perturbation? Given that interior OA has not even been remotely close to compensated by biological processes or carbonate compensation, I don’t see why it is worth using text to imply that OAE might be entirely compensated by these same processes.
“Indirect impacts are sometimes referred to as “additionality” considerations (Bach, 2024).” Aren’t all of these considerations additionality considerations? This aside is beating up on a strawman argument that additionality only refers to indirect impacts is not a great use of text.
Check the math… unless I am misinterpreting this somehow, 1 Gt of inorganic C is orders of magnitude different from 10 Mmol of TA
“Given the early stages of research on this topic, much of what is known is extrapolated from ocean acidification studies, which do not show clear directional impacts between carbonate export and seawater pH (Planchat et al., 2024).” see also Barrett et al. 2025: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GB008358
Citation: https://doi.org/10.5194/egusphere-2026-5148-RC2
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The authors present a broad synthesis describing the processes that affect the efficiency losses in ocean alkalinity enhancement. The identified processes include incomplete mineral dissolution, alkalinity loss to solids, incomplete air–sea equilibration, and indirect impacts on ocean biogeochemical cycles. The manuscript is largely well written although the discussion feels somewhat superficial (with some errors), as most of the potential deficiencies pointed out here are relatively well known, and this manuscript does not offer practical solutions, and perhaps many of what they point out are difficult to solve, either via modeling or field trials. Nevertheless, it is worth documenting these processes in a perspective article.
Because the manuscript doesn’t have page and line numbers, I’ll simply list section -> paragraph/line in the list of minor comments below:
Mineral dissolution kinetics -> 3rd paragraph, 3rd line, slow “down” over time…
Four lines below, dissolution rate increase or decrease relative to water column, this statement is vague, what conditions can cause increase and what cause decrease?
Particle sinking -> 9th line, don’t use unit in sentence w/o a number, “micrometer” instead.
2. Alkalinity loss to minerals -> 6th line, what’s “porewater cycling”?
Same paragraph, second to last line, “mechanistically” reads like a filler word, check entire manuscript to make it less verbose.
Loss to minerals in the water column -> 2nd paragraph, 3rd line, “surface” ocean is oversaturated, not the entire ocean.
Loss to minerals in sediments -> last sentence, stochiometrically precipitation of CaCO3 sequesters CO2 as in the last page, the authors still want to emphasize that this is sequestration to a much smaller extent.
Incomplete air-sea CO2 equilibration -> 3rd line, the text is written as if equilibrated high alkalinity water mixing with seawater has no further air-sea exchange, different temperature and alkalinity levels in feedstock do result in disequilibrium.
Kinetics of air-sea CO2 exchange -> current open ocean formulation “underestimated” air-sea exchange rate? I think the authors misread this reference. In fact, this paper clearly states the widely used formulation overestimates compared to tracer measured k.
Next paragraph -> 3rd line, elaborate local and regional conditions. While it’s fine to cite other’s work, leaving this ambiguous statement makes the text read sloppy.
Decreased sediment alkalinity flux -> while carbonate dissolution represents a significant benthic alkalinity source, another source that involves anoxic alkalinity production through sulfate reduction etc was not mentioned. Adding solid materials to surface sediment can potentially smother oxic respiration in surface sediments, it may be worth discussing what implications this might have.
Same section -> 3rd paragraph, 3rd line from bottom, John Morse and Andreas Andersson also have papers on this exact topic a decade before these citations.
Increased carbonate counterpump -> Dissolution of biogenic CaCO3 has also been shown to “be slower” at higher Ω.
Last paragraph ->
“To date, most assessments of OAE efficiency have focused on the third of these -- air-sea equilibration -- whereas losses associated with the other processes are less well constrained and often not considered.”
I don’t completely agree with this statement and suggest the authors tone down the wording a bit. Air-sea equilibration has received the most quantitative treatment in OAE efficiency discussions, but the remaining pathways regarding the loss in efficiencies are not simply ignored but rather constrained at different scales. For example, secondary alkalinity loss to solids has been extensively characterized under controlled lab and mesocosm conditions, and incomplete mineral dissolution has also been quantified for silicate feedstocks. That leaves field trial, both in modeling and field trials, as well as impact on ocean biogeochemistry much less well examined.