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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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.