| 29 Jun 2026
The role of temperature and salinity on ocean alkalinity enhancement performance
Abstract. Ocean alkalinity enhancement (OAE) is a promising carbon dioxide removal approach but its effectiveness is constrained by uncertainties in dissolution kinetics and carbonate precipitation under varying ocean conditions. Here, we systematically quantified the dissolution and net alkalinity delivery of three common OAE feedstocks, i.e., NaHCO3, Ca(OH)2, Mg(OH)2, across 16 temperature–salinity combinations (T = 4, 12, 20, 28 °C and S = 24, 29, 34, 38). Each treatment targeted an alkalinity increase of 500 µmol kg-1 and was monitored over 11 days to track changes in total alkalinity (TA) and dissolved inorganic carbon (DIC). Results showed that NaHCO3 dissolved rapidly and nearly completely under all conditions, reliably delivering the intended alkalinity. Ca(OH)2 delivered high net alkalinity in cold waters, but its effectiveness declined with increasing temperature and salinity. Net TA losses occurred at 28 °C and S ≥ 29, likely driven by secondary CaCO3 precipitation and particle passivation. Mg(OH)2 dissolved more slowly and exhibited strong salinity dependence: near–complete dissolution occurred at S ≤ 34 and lower temperatures, while higher salinity (S = 38) significantly inhibited dissolution, causing net TA losses at higher temperatures. The observed dissolution kinetics, including likely effects of secondary CaCO3 precipitation, were well described by a modified Noyes–Whitney function. A first application suggests that NaHCO3 is the most predictable feedstock across tested global conditions; Ca(OH)2 has more potential in cold environments but loses efficiency when temperature increases; and Mg(OH)2 is kinetically slower and susceptible to high–salinity inhibition. Experiment and global extrapolation imply that the carbon dioxide removal potential and efficiency are highly dependent on the combination of feedstock and environmental control.
Competing interests: JH is consulting for the Planeteers GmbH. All authors declare that they have no competing interests.
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Review of “The role of temperature and salinity on ocean alkalinity enhancement performance”
Summary
The study investigates the dissolution and subsequent alkalisation potential of three different alkaline feedstocks (NaHCO3, Ca(OH)2 and Mg(OH)2) under different T–S combinations. Their major outcome is that NaHCO3 is the most reliable material as the target ∆TA of 500 µmol/l was achieved under all conditions tested. The other two materials both showed reduced alkalisation potential under higher temperatures and salinities, culminating in actual TA loss. The authors derive an expression in the form of a Noyes–Whitney equation that expresses the ratio of realised TA to target TA as a function of T and S. Based on that, they extrapolate globally the precipitation risk associated with the addition of each of the materials to the surface ocean.
Overall impression
This study is well executed and certainly tackles an interesting question. Still it leaves the reader with a certain frustration. In other words: The study lacks a story that really catches the reader’s attention. The discussion is very detailed on small-scale processes, but it completely fails to incorporate these findings into the broader context of OAE. Upon reading I felt like I was always asking: In what direction do the results point when it comes to where OAE research should look now? What material has which potential to serve as a feedstock for actual application? Is this entire process efficient at all? Right now, this study presents itself as a very tame, carefully executed lab exercise.
Recommendation
This said, in theory I would endorse publication with moderate revisions, as there are no major flaws in the study, except for one: No Triplicates. That is an absolute red flag and must be addressed. Hence, I recommend major revisions, not because the science is flawed, but because the methodology is too weak to support a global extrapolation exercise.
Major Comments
The experiments were not conducted in peplicates.The implication is quite severe as each result could be a coincidence. Not that I believe that, but as scientists we have to ensure that the results we produce are robust, especially when we aim to build a global extrapolation on them. I therefore ask the authors to either (a) run triplicates for four selected T–S combinations (two additional bottles per treatment would suffice), or (b) repeat eight of the single experiments and show that the duplicates agree within measurement uncertainty.
The bottles were capped without gas exchange, and this design choice compromises the transferability of the results considerably: the experiments by Suitner et al. and Flipkens et al. have shown that equilibration with air has a strong impact on secondary mineral formation. I ask the authors to (i) justify this design choice explicitly, (ii) discuss in which direction and roughly by how much the absence of air equilibration is expected to bias their precipitation thresholds — and therefore the derived global precipitation-risk maps — and (iii) temper the global extrapolation accordingly.
The entire discussion is on the alkalisation potential of the different materials rather than on the actual “performance”. In hindsight this term appears more and more ambiguous, as I expected it to encompass questions like sequestration efficiency or cost efficiency. This discrepancy between my expectation and what how you understand it indicates that the terminology might not be appropriate. Try to rephrase this in the title and the entire manuscript, or clarify at a very early point in the manuscript how you define the term.
Line-by-line comments
Line 23: “Climate change is one of humanity’s most urgent challenges”. While I personally agree with this statement, it is a very subjective statement. Whether it is urgent or not basically depends on the individual interest. If you want to keep it, use the next sentence to underpin it. This requires combining the sentences to something like: “…urgent challenges, since according to the IPCC…”. The resulting sentence is a tad bit long, but otherwise the first sentence stands there quite alone and sounds more like cheap clickbait than the opener for a scientific publication.
Line 32: What do you mean by technical viability? That is too generic.
Line 32b: Precipitation risk is a term that needs explanation. Up to this point, secondary mineral formation has not been mentioned.
Line 35: You want to cite Rigopoulos 2018 and Fuhr et al. 2022 for secondary mineral precipitation.
Line 36: First: explain runaway precipitation. Second: the “even” in the sentence is misplaced — it should go before “a runaway”.
Lines 73–74: I know that 48 bottles are already quite a bit, but no replicates in a lab-based experiment are nowadays a very red flag. Especially if the sample volume is only one litre. And I must correct the authors: replicates are not run to estimate variability between them. They are run to ensure reproducibility. And capturing process dynamics requires that the result is not a coincidence. I kindly ask the authors to choose either 4 combinations of salinity and temperature to perform triplicates (an additional two bottles of each kind would suffice) or otherwise repeat 8 single experiments to show the duplicates fall within error of measurements. I will continue to review this paper but want to make clear that from my side this repetition is a non-negotiable requirement for publication.
Line 86/87: The results of the measurements should theoretically be presented in the results part. Alternatively, show them as a supplement along with the entire grain-size distribution. Why did the authors not perform a BET measurement?
Line 89: The fact that the bottles were capped without gas exchange compromises the results drastically. Experiments by Suitner et al. and Flipkens et al. have shown that the equilibration with air has a strong impact on secondary mineral formation.
Line 100: Was the pH probe additionally calibrated in AMPY and BIS or at least in a TRIS buffer for verifying the salinity offset of the reference electrode?
Lines 254–261: You are labelling this section (if we include header 4.1) “Overall performance of NaHCO3”, but ultimately all you report is that the material did dissolve completely. That is not performance. That’s physics. The interesting question is: By how much has the pCO2 been lowered? How much CO2 uptake can be expected (and down the road: How efficient is the process?).
Line 333: Delete “in a study”.
Lines 385/386: The technical replication of analyses shows that the analytical part is ok. It does not say anything about the question whether the observation is statistically significant.
Line 387: I strongly disagree. The results must be understood as a potential coincidence. And even if the authors were right, their interpretation would not allow for any sort of extrapolation. If the results are to be meaningful, they must reflect universal material performance.