September 2025 | Oceanography
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As mCDR becomes increasingly investigated as
a potential tool to help mitigate climate change,
understanding how GOA-ON’s goals intersect with
and support mCDR initiatives is crucial. GOA-ON’s
three high-level goals are: (1) improving under-
standing of global OA conditions; (2) improving
understanding of ecosystem responses to OA, and
(3) acquiring and exchanging data necessary to opti-
mize modeling for OA and its impacts (Figure 1).
Here, we explore the GOA-ON community per-
spective for informing and supporting mCDR activ-
ities in relation to the GOA-ON goals. The aims of
this paper are to consider how mCDR could inter-
act with OA, either by mitigating or exacerbating it;
to identify what knowledge gaps exist; to consider
lessons learned from OA research when approach-
ing mCDR; and finally to make recommendations
about how OA knowledge and the GOA-ON com-
munity can support mCDR research.
POTENTIAL FOR mCDR TO
MITIGATE OR EXACERBATE OA
In order to reverse the OA that has already occurred
without further manipulating other components
of the carbonate system, CO2 needs to be removed
from the ocean while maintaining alkalinity (reverse
of “Continued CO2 emissions” line in Figure 2). As
the end goal of mCDR is to remove carbon from
the atmosphere and lock it away in long-term stor-
age, all mechanisms have the overall potential of pre-
venting further OA in the future. However, depend-
ing on where the carbon is stored in the ocean and
the manipulation involved, ongoing OA and/or
its impacts could either be ameliorated or exacer-
bated at different scales, and we discuss these here
(see Figures 2 and 3).
Given the chemical equilibrium underpinning
the carbonate chemistry in seawater during OAE,
the reduction of H+ following alkalinity injection
or mineral dissolution would lead to an increase in
FIGURE 2. Dissolved inorganic carbon (DIC)–Total alkalinity (TA) plots showing (a) pCO2, (b) pH, and (c) aragonite saturation state. [Overlaid trajectories indi-
cate when different mCDR approaches are applied in their unequilibrated phase] [Overlaid trajectories indicate effects of applying different mCDR approaches
in their unequilibrated phases?]. The relative concentrations of these variables apply only to a specific temperature and salinity (here T = 15°C, S = 34); the
absolute values will change at other combinations of temperature (T), salinity (S), dissolved inorganic carbon (DIC), and TA. In this example, present day is
represented with a pCO2 of 420 μatm and shown as the yellow dot in each panel. At some point along the “continued CO2 emissions” trajectory, the mCDR
approaches are applied, noting that the closer to present day CO2 levels they are applied, the less mCDR will be needed to return to present day or historic
levels, and that without continued emissions reductions no mCDR approach will work. The mCDR approaches include: “DIC removal, no TA addition” (purple),
which represents direct ocean carbon capture and storage (DOCCS); three ocean alkalinity enhancement (OAE) options as represented in Schulz et al. (2023):
“NaOH addition” (dark blue), which adds TA and not DIC; “Na2CO3 addition” (mid blue), which adds TA and half as much DIC; “NaHCO3 addition” (light blue),
which adds equal amounts of TA and DIC; “Photosynthesis” (green), which is all photosynthetically driven mCDR (e.g., macroalgae growth, nutrient fertiliza-
tion, artificial upwelling; these may have additional impacts on alkalinity depending on the form of nutrient additions and the type of photosynthetic organism).
Also shown is “Respiration” (brown), representing eventual remineralization of organic matter. Here, photosynthesis and respiration are assumed to occur as
a result of nitrate or nitrite being the N source and product, respectively (as opposed to ammonia). See Wolf-Gladrow and Klaas (2024).