The mCDR deployments will necessarily modify the ocean,
but intended and unintended ecological impacts are poorly
known. This will require monitoring ecological variables in
at least the upper 1,000 m as well as deeper, including at
the seafloor where benthic systems could be affected. Side
effects would be assessed through modeling studies of the
impacts of biogeochemical changes on marine ecosystems.
Unacceptable ecological side effects, acute or chronic, may
lead to the termination of an mCDR deployment.
MODEL VALIDATION, EXCLUSION,
AND ADDITIONALITY
To achieve these three objectives, model simulations will
be run, with and without mCDR. In order to fulfill this role,
simulations will have to capture many ongoing changes
in the ocean that include those due to climate change,
the hysteresis effects from climate change, the effects
of other mCDR (and terrestrial CDR), and the effects of
emissions reductions on the ocean carbon cycle. It will
require advanced modeling capabilities that could effec-
tively simulate the state of the coupled physical and bio-
geochemical ocean and its changes under the different
mCDR scenarios. Such models pose many scientific and
technological challenges that impede the development of
Digital Twins of the Ocean (DTO). The DTO will combine
next- generation ocean modeling, artificial intelligence, and
high- performance computing to create digital replicas of
the ocean that are regularly informed and improved with
observations. Some of the observations will be used in the
models, and others will be kept for model validation.
Extensive validation of these models and their improve-
ments (e.g., optimization of model parameters) will be
needed well in advance of and during mCDR deploy-
ments. Confirming that the simulations are consistent
with observations will require the initiation of monitoring
a long time before any mCDR deployment. During this pre-
deployment period, no mCDR could be undertaken in the
mCDR- intended region.
However, there may be cases where model validation
could not be undertaken well in advance of the mCDR
deployments, for example, where a deployment would
take place without prior, long- term consultation with the
authority to which monitoring would be reported. In such
a case, the results of model simulations run with and with-
out mCDR could be compared with observations made in
the mCDR- deployment region and one or more control
regions (with long- term time- series observations) where
conditions would be comparable to those in the selected
mCDR region and where no mCDR of any type would
be deployed. Of course, control regions will eventually
become “contaminated” by the spread of DIC—via ocean
circulation—from mCDR deployed elsewhere (e.g., Boyd
and Bressac, 2016), but they could be used for model vali-
dation before this occurs and even after.
Failure to implement either pre- deployment periods
or control regions would make attribution impossible
and therefore compromise the monitoring of all mCDR
deployments in a given ocean region. In addition, an open
registry or metadatabase of the mCDR pilot studies and
deployments would be very useful in this context. It would
provide information, in particular for modelers, on the
location and depth of each activity and key information
on its technical aspects (e.g., for alkalinization, the mineral
type, timing, and amount of alkalinity added).
Furthermore, we assume here the desirable exclusion
principle, whereby the deployment of one type of mCDR
in a given ocean region excludes the possibility of deploy-
ing other types there. This principle stems from the like-
lihood that multiple- type deployments in a given region,
especially as mCDR deployments aim to sequester carbon
at the gigaton scale,1 would make attribution of individ-
ual deployments impossible (Boyd and Bressac, 2016).
Exclusion is also important because multiple-type deploy-
ments could potentially cause interactive side effects that
were not anticipated by single-type mCDR pilot studies.
Pre- deployment periods or control regions—in situ or
in models—are also needed to assess the additionality of
mCDR deployments, defined in IPCC (2022) as: “The prop-
erty of being additional. Mitigation is additional if the
greenhouse gas emission reductions or removals would
not have occurred in the absence of the associated policy
intervention or activity.” Thus, additionality is the require-
ment that the net increase in the air- to- sea CO2 flux due to
an mCDR deployment (i.e., based on detection and attribu-
tion) exceeds the flux in the absence of this mCDR.
Monitoring for additionality will be especially challeng-
ing as the change in net carbon flux into the ocean and the
magnitude of carbon sequestration caused by an mCDR
deployment will be very small compared to natural air-
sea carbon fluxes and the magnitude of the ocean carbon
sink. In addition, the models will need to take into account
the inherent uncertainties of field measurements. This will
pose challenges for both observation and modeling.
1 The intended gigaton-scale magnitude of mCDR deployments would be much larger than existing multiple overlapping uses and perturbations.
For example, global marine capture fisheries and aquaculture harvested 112 Mt of animals and 36 Mt of seaweeds (fresh weight) in marine waters
in 2020 (FAO, 2022), which represented ~0.02 GtC yr–1.