The manuscript by Oschlies et al. is the introductory chapter in a Best Practices Guide to OAE Research. The full guide will contain seven chapters, which compare and synthesise previously published methods, and offer guidance for future research. Given that Oschlies et al. have only written the introduction chapter it does not present new results or new research. The manuscript is very well written and presents a comprehensive overview of the need for climate dioxide removal (CDR), and the role the ocean can, or should, play. The description of ocean alkalinity enhancement (OAE) is detailed and comprehensive, yet the nuances are easy to follow and understand. The referencing is comprehensive, and I have not identified key publications that have not been, but should be, referenced here. I commend the authors on their overview.  

The only issue, and the reason I suggest minor revisions, is about the timeline of our climate goals and how this affects the realism of the suggested CDR methods. The authors correctly state that the goals are to reach net-zero emissions by mid-century. We are now in 2023 so mid-century is quite close in time. Yet the technology necessary for CDR, and marine CDR in particular is in its infancy or non-existent (as noted by the authors of this manuscript).

I have just read the introductory chapter so this topic may be covered elsewhere in the Best Practices Guide. But a (brief) section should be added to discuss the necessary timeline, and how realistic/unrealistic it is to achieve operational technology and methods to successfully implement marine CDR. Even if covered elsewhere this aspect deserves mention in the introductory chapter.

General comments

To introduce the OAE best practices guide, this chapter provides the background, motivation and aims of this document, whilst impressing the relevance and timeliness of advancing CDR science, and the role OAE can play. The chapter is well-written and includes references that appropriately situate this text within the conversation of relevant scientific literature.

Section 1 does a good job of providing the salient information for the reader to understand the importance and need of CDR in climate action portfolios to achieve the Paris Agreement goals. However, at times I felt the flow of the arguments and the structure of the paragraphs could be improved for increased clarity, and have made a few suggestions in the “Technical comments” section of my review.

Section 2 establishes the need for ‘novel’ approaches outside of land-based CDR (i.e. the role the ocean can play) in order to reach net-zero. Importantly, the authors highlight the pressing need to address knowledge gaps in efficacy, risks and benefits, as well as intersecting societal factors. Crucially, the authors make the case for the urgent need for research now, in order to make well-informed decisions in future. At the end of this section, the concept of MRV is introduced. Since this text forms an introduction, and considering the nature of this guide, a short paragraph explaining MRV, its role and general current challenges could be useful here.

Section 3 provides a clear summary of OAE – how it works, where it can be applied and where research on OAE currently sits. Addressing these knowledge gaps provides the motivation for the guide presented in Section 4, which provides an overview of the contents. Section 5 situates the this guide within the context of similar initiatives as well as outlines the project development, protocol for transparency and stakeholder involvement.

Overall, I congratulate the authors on this chapter. Minor specific and technical comments can be found in the attached pdf file.

This MS provides the introduction to a new Best Practices Guide for research on ocean alkalinity enhancement (OAE), giving the background to the development of that document.  It gives the climate policy context for ocean-based methods for carbon dioxide removal, with specific focus on OAE.  It is well-structured and clearly written, covering the main science issues and knowledge gaps that are to be later discussed in greater detail.

The only significant gap would seem to be (brief) consideration of the current status of OAE governance, primarily from a regulatory perspective at international level – since this was considered to be a very high constraint on the OAE feasibility by IPCC (Bindoff et al., 2019; Fig 5.23).  The authors may consider such issues to be out of scope for the guide; however, that would not seem to be the case, since substantive subsequent content is indicated (“The guide also discusses the legal context in which research occurs”; line 179). 

The nature of decision-making at UN bodies is relevant here. Since formal decisions are based on consensus and agreement, a cautionary approach is the most usual outcome – as is evident by decisions to date by the Convention on Biological Diversity (CBD) and the London Convention/London Protocol (LC/LP) on ‘marine geoengineering’ in general and ocean fertilization in particular.  A major concern by CBD and LC/LP parties is the risk of adverse transboundary effects, with the actions of one nation state negatively affecting another; such effects may be unlikely for OAE, nevertheless, they could occur even if OAE deployments are limited to territorial waters.  For climate-scale OAE, there would also need to be international agreement on carbon accounting within the UNFCCC framework, a topic that is likely to be highly contentious (and therefore taking a very long time to resolve). .

It is relevant that the LC/LP has recently identified OAE as an approach requiring further attention: https://www.imo.org/en/MediaCentre/PressBriefings/pages/Marine-geoengineering.aspx.  GESAMP (2019) could also usefully be cited in the context of governance issues.

A few minor comments:

Line 59: “several Gt CO2 per year globally”.  It would be helpful to be more specific regarding the amount of residual emissions that are included in IPCC scenarios (presumably 2-3 Gt pa, on the basis of “close to 20%”).

Line 65:  Change “current global CDR deployment” to “current CO2 removal” (since this is ‘unintentional’ rather than purposeful CDR).

Line 108: After “macrophytes”, insert “(e.g. seaweed)”; that is more understandable.

Line 114: “with high (> Gt CO2 yr-1 scale) theoretical sequestration”.  It would be more informative if actual estimates of maximum CO2 removal can be given here, by several authors; e.g.: “between 3 -30 Gt CO2 yr-1 theoretical sequestration (Kohler et al. 2013; Renforth & Henderson, 2017; Feng et al 2017”).

Line 140:  What is meant by “at the expense of imperfect CDR”?  Explain – or delete.

Line 185-6:  “We have to widen the space of options… climate targets” seems rather wordy - and too prescriptive.  Simplification and linkage with previous sentence is suggested: “… is urgently needed, to enable society to define and design appropriate actions to reach agreed climate goals”.

EXTRA REFERENCES CITED ABOVE

Bindoff, N.L. et al (2019) Changing Ocean, Marine Ecosystems, and Dependent Communities. Chapter 5 in: IPCC Special Report on the Ocean and Cryosphere in a Changing Climate [eds H.-O. Pörtner et al]. Cambridge University Press.

Feng EY et al (2017). Model‐based assessment of the CO2 sequestration potential of coastal ocean alkalinization. Earth's Future, 5(12): 1252-1266.

Köhler P et al (2013) impact of open ocean dissolution of olivine on atmospheric CO2, surface ocean pH and marine biology. Environmental Research Letters, 8(1): 014009.

GESAMP (2019) High level review of a wide range of proposed marine geoengineering techniques”. (Boyd, P.W. and Vivian, C.M.G., eds.). (IMO/FAO/UNESCO-IOC/UNIDO/WMO/IAEA/UN/UN Environment/ UNDP/ISA Joint Group of Experts on the Scientific Aspects of Marine Environmental Protection). Rep. Stud. GESAMP WG 41, Reports & Studies Series.

I commend the authors for writing a concise and informative introduction to their Best Practices Guide For Ocean Alkalinity Enhancement Research, and for organizing the production of a guide that will undoubtedly prove useful for researchers, as well as practitioners, in this nascent field. My comments and suggestions on this introductory chapter are below.  

Line 29: ‘net-zero requirement for avoiding further temperature rise’ – should clarify that this is ‘further temperature rise beyond the 1.5 – 2 deg IPCC target’ (not beyond the present-day mean global temperature)

Line 34: The discussion and corresponding figure showing how to achieve IPCC’s target of net zero CO2 emissions by 2050 is useful for illustrating the need for CDR in addition to emissions reductions, but it would be helpful (and potentially compelling for skeptics) to see the corresponding mean atmospheric pCO2 and mean global temperatures that correspond to that target emissions trajectory, since those relationships are the basis of the authors’ rationale for pursuing CDR in the first place. Otherwise, the non-specialist may not grasp what achieving ‘net zero by 2050’ means in terms of global climate change. Perhaps include another panel above the net zero figure showing the corresponding changes in mean global pCO2 and temperature over the same interval.

Line 77: should clarify here and elsewhere whether the 2/1.5 deg C warming target is relative to pre-Industrial mean global temp or present-day mean global temp.

Line 100: The authors should differentiate between particulate inorganic carbon and dissolved inorganic carbon when referencing ‘inorganic carbon’ in this sentence (presumably they are referring only to DIC): ‘The ocean holds more than 50 times as much inorganic carbon (in the form of dissolved inorganic carbon) as the pre-industrial atmosphere’

Line 104: should ‘marine CDR’ instead be ‘marine CDR by OAE’ here?

Line 125: should differentiate between the time needed for pCO2 of air and seawater to fully vs. partially equilibrate. It is true that air/sea can take years to fully equilibrate, but the equilibration will be an inverse exponential function of time, meaning a disproportionate share of the equilibration will occur in the beginning of the equilibration interval. This is an important but often overlooked distinction that has important implications regarding the perceived challenges of quantifying CDR by OAE.

Line 131: It seems that the definition of marine CDR should be expanded from ‘OAE qualifies as marine CDR if CO2 is transferred directly from the atmosphere into seawater’ to ‘OAE qualifies as marine CDR if CO2 is transferred from the atmosphere or seawater into stable carbonate or bicarbonate ions in seawater’, as both processes will result in the eventual drawdown of CO2 from the atmosphere. Otherwise, we will miss an important and efficient pathway in CO2 removal. Likewise, CO2 removal from the atmosphere alone is not sufficient for CDR, as increasing the pCO2 of the atmosphere through increased CO2 emissions would increase the flux of CO2 from the atmosphere to the ocean, but surely this should not constitute marine CDR (as the alleviation of the atmospheric CO2 pressure would cause off-gassing of the dissolved CO2 back to the atmosphere unless balanced by alkalinity addition). This point is also illustrated by the author in an earlier paragraph, where they state: ‘Alkalinity enhancement results in the consumption of protons, a corresponding increase in the pH, which results in a decrease of the partial pressure of CO2 in seawater. If applied to the surface ocean, and depending on the initial air-sea CO2 gradient, it would promote CO2 uptake from - or lessen CO2 release to - the atmosphere, in both cases leading to a net reduction in atmospheric CO2 at the expense of an increase in the oceanic carbon pool.’ In the case that the flux of CO2 from the ocean to the atmosphere is lessened by OAE, this would not satisfy the authors’ current requirement that ‘CO2 is transferred directly from the atmosphere into seawater’, but would instead reduce the rate that seawater CO2 is released to the atmosphere, thereby resulting in a theoretical ‘net reduction in atmospheric CO2’ – which should qualify the activity as successful marine CDR. Perhaps a more useful framing for CDR is the transfer of C from shorter residence time reservoirs (atmospheric CO2, seawater CO2, terrestrial biomass, marine biomass in mixed layer etc.) to longer residence time reservoirs (bicarbonate ion reservoir, carbonate ion reservoir, terrestrial and marine biomass transported to deep ocean below mixed layer, etc.) (c.f., Prentice, I. C., 2001, The carbon cycle and atmospheric carbon dioxide. Climate change 2001: the scientific basis, Intergovernmental panel on climate change. hal-03333974) or, more colloquially, transferring C from the ‘fast C cycle’ to the ‘slow C cycle’, as this encompasses the ultimate goal of marine CDR – i.e., net reduction of atmospheric CO2, regardless of the strict and not necessarily relevant ‘transfer of CO2 between ocean and atmosphere’.

Line 143: ‘leakage’ by OAE-induced precipitation (or reduced dissolution) of CaCO3 at the seafloor (or really anywhere in the water column below the mixed layer) is probably not relevant over climate-relevant timescales because it will take 100-1000s of years for those waters to return to the surface and re-equilibrate with (i.e., offgas CO2 to) the atmosphere, just as shoaling of the carbonate compensation depth in response to CO2-induced OA will not sequester anthropogenic CO2 over fast enough timescales to prevent warming (hence, the bind we are in).

Line 150: should probably add ‘so long as any CO2 emitted in their production (e.g., Ca(OH)2 or Mg(OH)2 produced through calcination of CaCO3 or MgCO3, respectively) is accounted for’

Line 161: Was Albright et al (2016) the first ocean acidification field experiment (see Hall-Spencer et al. 2008 field experiments using volcanic vents, etc.)? Or just the first field experiment to modify seawater pH through alkalinity addition rather than direct pCO2 manipulation? May also be worth mentioning that insight into impact of OAE on marine organisms can be gained from past research by the shellfish industry investigating the utility of so-called ‘sweetening’ the water through addition of mainly soda ash (Na2CO3), a practice utilized in shellfish hatcheries for decades, and also in the academic and industrial fields of ‘river liming’, which dissolved primarily CaCO3 and dolomite in higher latitude watersheds to offset the effects of acid rain (due to NOx and SOx emissions) in the 1960s and 1970, but is still practiced today in Canada and some Scandinavian countries, among other places.

Line 173: need more concise phrasing than ‘enhancing technological readiness’; perhaps ‘developing (or implementing) scalable methodologies’

Line 184: perhaps change ‘in a situation where’ to ‘at a time when’

Misc:

Confirm whether ‘Ocean Alkalinity Enhancement’ and ‘Carbon Dioxide Removal’ should be capitalized in title and throughout manuscript.

Use of term ‘monitoring, reporting and verification’ in abstract without defining the term may be confusing to readers, as the phrasing really only has meaning when the three terms are defined and understood in aggregate.

Respectfully submitted,

J. Ries