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et al., 2023), as well as incomplete understanding of the carbonate
pump in Earth system models (Planchat et al., 2023).
Some model approaches go beyond biogeochemistry and
attempt to model impacts on species and ecosystems. However,
the complexity and uncertainty of responses to OA, coupled
with lack of representation of biodiversity in numerical models,
make them generally less certain, but still informative. For
instance, pelagic calcification is included in several models as a
back-calculation from particulate organic carbon production
(e.g., Aumont et al., 2015; Yool et al., 2015). Few models explicitly
represent calcifying plankton functional types (e.g., Krumhardt
et al., 2019), and the benthos is usually represented in a very
simple manner focused on the recycling of organic matter into
nutrients and CO2. Feedbacks on pelagic carbonate systems are
represented in some models (e.g., DIC and TA fluxes associated
with remineralization of organic matter), but benthic calcifica-
tion is not usually represented.
The implementation of these coupled physical-biogeochemical-
ecosystem models has allowed identification of further areas of
uncertainty that require both deeper understanding of processes
and further model development to improve the representation of
the carbonate chemistry system. For example, improving repre-
sentation of freshwater input of DIC and TA and providing bet-
ter constraint of the continuous representation of the carbonate
system along the salinity gradient are particularly important for
simulating the spatial and temporal variability in coastal envi-
ronments. Including the impact of sediment processes on car-
bonate chemistry dynamics is similarly important for reducing
uncertainty in the coastal environment.
HOW GOAL 3 SUPPORTS mCDR
Data and knowledge synthesis for use in products and models
is essential for mCDR. Before deployment (i.e., in the research
stage), they are needed to predict feasibility, scalability, efficiency,
ecosystem response, and impact, and thereby to support decision-
making to optimize mCDR approaches. If the mCDR activities
are taken forward beyond the research and development stage to
deployment (i.e., field trials and commercialization), these tools
are needed to evaluate MRV, carbon accounting, and environmen-
tal monitoring, therefore supporting the regulation and imple-
mentation of safe and sustainable practices.
The spatial and temporal scales over which mCDR may ulti-
mately be deployed may be very large, making observational
monitoring potentially (even prohibitively) expensive. A joint
model-observation approach to MRV is therefore recommended,
especially to assess alterations to ecosystems that may be too
small to measure observationally. Uncertainties associated with
both models and observations will still limit accurate prediction
(Bach et al., 2023) and provide challenges for governance and
social license to operate, thus requiring interdisciplinary, collab-
orative approaches.
Models that already incorporate carbonate chemistry and bio-
logical response provide the foundation for the mCDR commu-
nity to build upon. However, some of the mCDR methods could
push the chemical composition of seawater outside of the nor-
mal range and therefore could require a more detailed approach
like the one developed in the SCOR working group Modelling
Chemical Speciation in Seawater to Meet 21st Century Needs
(MarChemSpec; Clegg et al., 2023). In relation to OA, models can
help to predict how different mCDR strategies will interact with
OA processes and ultimately whether they will result in long-term
benefits or result in disruptions to marine ecosystems.
Models provide a reliable basis for assessing the long-term
effectiveness of upscaled CDR as governed by macroscale hydro-
dynamics and the biological pump, which operate on decadal
to millennial timescales. While large-scale approaches provide
the climate context for mCDR impacts and benefits, the efficacy
and impacts of mCDR at local to regional scales can only be con-
veyed through higher-resolution and local-scale modeling. Such
models often have sub-kilometer resolution and can be adapted
to address individual or clusters of CDR deployments, assessing
environmental impact, dispersion of chemical and biological par-
ticles and plumes, local sequestration, and export. These models
will require validation studies to demonstrate that they are fit-for-
purpose, and local model approaches will need to be site-specific
(Khangaonkar et al., 2024).
Modeling is already offering a holistic picture of different
mCDR deployment scenarios; for example, regional simulations
(Wang et al., 2023) and century-long simulations (González and
Ilyina, 2016) show artificial OAE can effectively remove atmo-
spheric CO2 and alleviate OA. However, emissions-driven Earth
system modeling demonstrates that an abrupt ending of OAE
might act to accelerate OA and atmospheric warming, thus
threatening vulnerable ecosystems that are struggling to adapt to
existing environmental pressures (González et al., 2018).
The community has already made critical investments in
ocean biogeochemical and ecosystem models. Such models
are crucial for simulating present-day and future predictions
of mCDR impacts and ecosystem consequences. Better under-
standing of the implications of greenhouse gas emissions and
CDR for the coupled carbon climate system is essential for pro-
viding reliable guidance to policymakers and other stakeholders.
While such global-scale approaches provide the climate context
for CDR impacts, answering questions about the effectiveness
and ecosystem impacts of local to regional-scale CDR approaches
may require both higher-resolution and regional-scale model-
ing as well as incorporation of additional modeling strategies.
These tools will allow the combination of CDR scenario assess-
ment, detection, and attribution; observation system simulation;
and process studies to increase understanding and inform sound
management decision-making.