the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Evaluating ocean alkalinity enhancement as a carbon dioxide removal strategy in the North Sea
Abstract. Ocean Alkalinity Enhancement (OAE) is a climate mitigation strategy aimed at increasing the ocean’s capacity to absorb and store atmospheric CO2. The effect of OAE depends significantly on local physical conditions, underscoring the importance of selecting optimal locations for alkalinity addition. Using a regional coupled physical-biogeochemical-carbon model, we examine OAE responses in the North Sea, including CO2 uptake potential, enhanced carbon storage and cross-shelf export, and the associated changes in the carbonate chemistry. Alkalinity is continuously added as a surface flux in three distinct regions of the North Sea. Our simulations show that the Norwegian Trench and the Skagerrak serve as sinks for added alkalinity, reducing its interaction with the atmosphere. Alkalinity addition along shallow eastern coasts results in a higher CO2 uptake efficiency (~0.79 mol CO2 uptake per mol alkalinity addition) than offshore addition in ship-accessible areas (~0.66 mol CO2 uptake per mol alkalinity addition), as offshore alkalinity is more susceptible to deep-ocean loss. Long-term carbon storage, measured by excess carbon accumulation in deep ocean layers and cross-shelf export below permanent pycnoclines, is similar across the three scenarios, accounting for less than 10 % of total excess CO2 uptake. The smallest changes in pH occur when alkalinity is added offshore, with effects nearly an order of magnitude lower than alkalinity addition in the shallow German Exclusive Economic Zone, where pH increases from 8.1 to 8.4. The model's resolution (~4.5 km in coastal areas) limits its ability to capture rapid, localized carbonate responses, leading to a nearly tenfold underestimation of chemical perturbations. Thus, finer-scale models are needed to accurately assess near-source alkalinity impacts.
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Status: open (until 12 Mar 2025)
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RC1: 'Comment on egusphere-2025-81', Lester Kwiatkowski, 11 Feb 2025
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Liu et al present a study of ocean alkalinity addition (OAE) in the North Sea using a regional ocean model. They vary the location of OAE deployment and explore the impacts on alkalinity transport, ocean carbon uptake and long-term carbon storage. This was an enjoyable read and is a really nice addition to the growing OAE literature that provides a number of interesting and often counterintuitive findings which are robustly explored. These findings include:
- The North Sea is a relatively efficient OAE deployment region despite its shallow depth and short flushing time.
- Small differences in the location of North Sea OAE deployment can lead to considerable differences in alkalinity dispersion, ocean carbon uptake enhancement and long-term carbon storage.
- Loss of alkalinity to the deep ocean does not necessarily equate to inefficient CDR as although the total carbon flux enhancement is reduced, a greater proportion of this carbon may be transported below the permanent pycnocline.
The manuscript is clear and well-written and I recommend publication subject to a few minor corrections. My main suggestion is that the authors provide a little additional detail on the validation of simulated carbonate chemistry (even though this is published in greater detail elsewhere). Alongside this, it would be good to provide some detail on the maximum attainable CDR efficiency possible in North Sea waters based on mean present-day carbonate chemistry conditions.
Minor comments
L90 “efficiently transported” I recommend phrasing this differently.
L101 Deployment areas appear stippled not “hatched”.
L161. I see that carbonate chemistry evaluation is published elsewhere. Nonetheless I think some comment on the ability of the model to simulate DIC/TA/pCO2 in the North Sea is required here.
L176. Can you clarify here whether there are sediment fluxes of alk and DIC and whether these fluxes are influenced by benthic carbonate chemistry and OAE?
L301. Typo “of the” tracer
L318-340 Some context on the maximum attainable uptake efficiency in the North Sea would be useful here. Is this around 0.85 given the mean simulated surface ocean DIC and alkalinity fields?
Figure 9. It’s difficult to distinguish between the thick and thin lines in panel a. I suggest using another line type (e.g. dotted or a different dash type).
L386. Recommend not using “captures”
L389-390. Are these values still increasing at the end of the simulations? Does this explain why the deep DIC inventory plateaus within the model domain?
L454-456. Can the authors explain this finding here as they do in the supplementary material? How do they know full equilibration has occurred? I recommend phasing this as CO2 system equilibration not alkalinity equilibration.
Citation: https://doi.org/10.5194/egusphere-2025-81-RC1
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