# Future changes in the carbon cycle from deliberate ocean-based climate interventions are

Philip Williamson, Nina Bednaršek, Jean-Pierre Gattuso, Nianzhi Jiao, Robert C. Steenkamp, Erik van Doorn, Philip W. Boyd

► To cite this version:

Philip Williamson, Nina Bednaršek, Jean-Pierre Gattuso, Nianzhi Jiao, Robert C. Steenkamp, et al. Future changes in the carbon cycle from deliberate ocean-based climate interventions are. Robin-son, C; Sabine, C; Lemcke, S; Isensee, K. Integrated Ocean Carbon Research: a vision primed for implementation, IOC of UNESCO, pp.39-43, 2026, 10.71245/fulk2623 . hal-05520991

HAL Id: hal-05520991

https://hal.sorbonne-universite.fr/hal-05520991v1

Submitted on 26 Feb 2026

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>
<figure><img src="image_2.png" /></figure>

Integrated
Ocean
Carbon
Research

A vision
primed for
implementation

<figure><img src="image_3.png" /></figure>

unesco
Intergovernmental
Oceanographic
Commission
Published in 2026 by the United Nations Educational,
Scientific and Cultural Organization,
7, place de Fontenoy, 75352 Paris 07 SP, France

<figure><img src="image_4.png" /></figure>

This document is available in Open Access under the
Attribution-ShareAlike 3.0 IGO (CC-BY-SA 3.0 IGO)
license (http://creativecommons.org/licenses/by-sa/3.0/igo/).

By using the content of this publication, the users accept to be bound
by the terms of use of the UNESCO Open Access Repository
(http://www.unesco.org/open-access/terms-use-ccbysa-en).

This document should be cited as follows: IOC of UNESCO.
2026. *Integrated Ocean Carbon Research: a vision primed for
implementation*. Paris, UNESCO. (IOC Technical Series, 214.)
https://doi.org/10.71245/FULK2623

The work of section 3.c Carbon exchanges across the land-ocean-
ice continuum is under UNESCO and Canadian Crown Copyright
(© Fisheries and Oceans Canada).

The designations employed and the presentation of material
throughout this document do not imply the expression of any
opinion whatsoever on the part of UNESCO concerning the legal
status of any country, territory, city or area or of its authorities, or
concerning the delimitation of its frontiers or boundaries.

The ideas and opinions expressed in this document are those of
the authors; they are not necessarily those of UNESCO and do not
commit the Organization.

Cover photo: Kogia - Karim Iliya

Design: UNESCO

Design and typesetting: UNESCO

Printed by UNESCO

Printed in France

(IOC/2026/TS/214)
7. **Future trajectories of plastics production**, waste generation and the proportion that accumulates in the surface ocean, throughout the water column, and at the seafloor.

8. **The current and future impacts of plastics** upon critical ocean functions including carbon cycling, particle sinking rates, microbial ecology and fisheries productivity.

## Actions to be taken

I. **Generate information on the key processes within the biological pump** (such as grazing, particle sinking, remineralization and plankton mortality), that are likely to be impacted by industrial extraction and the addition and degradation of plastics. Multiple approaches to address this can be envisaged ranging from in vitro incubations (Geisen et al., 2022), mesocosm studies (e.g. Lagaria et al., 2017) and *in situ* studies potentially around mine tailing discharge (e.g. Ramirez-Llodra et al., 2022) as an analogue for deep sea mining discharges.

II. **Quantify the lability and bioavailability of material released from the seabed during industrial activities and that produced through photodegradation of plastics.** Studies to address this should take place at a range of scales, from bottle incubations, through to *in situ* studies along trophic gradients (e.g. Pusceddu et al., 2005) and large scale meta analyses / modelling (e.g. Zhang et al., 2024).

III. **Improve knowledge on the resilience of carbon sequestration** to distortions / reorganisations of food webs by fisheries.

IV. **Undertake coherent and targeted research on the effects of fishing, industrial processes (including wind farms) and plastic pollution** on plankton ecology and biogeochemistry in a range of settings, including the continental shelves.

V. **Improve numerical models to receive, use and generate new information:** many of the models that we currently use to predict the future evolution of climate contain simplified representations of the ecosystem that do not consider or parameterise explicitly the key processes (such as photochemistry and competition between bacteria and phytoplankton for nutrients), taxa and functional groups that will be impacted by human activity in the ocean.

VI. **Adapt fishery ecosystem models to provide explicit knowledge about the consequences of removing large numbers of fish**, e.g. reduced deadfall and defecation flux. This problem is much more difficult for the shelf than for the off-shelf seas, and effort should be focussed there.

VII. **Develop regionally focussed ocean system pathways to drive numerical models**, informed by representative stakeholder consultations encompassing both the groups that regulate human activities in the ocean and those that seek to undertake these actions for profit.

## 3.e Future changes in the carbon cycle from deliberate ocean-based climate interventions

Phillip Williamson, Nina Bednaršek, Jean-Pierre Gattuso, Nianzhi Jiao, Robert C. Steenkamp, Erik van Doorn, Philip W. Boyd

There are two main groups of ocean-based climate interventions: those aiming to reduce atmospheric CO2 (mCDR; **Figure 8**); and those intended to reflect sunlight (solar radiation management, SRM). Whilst ocean-based SRM methods (e.g. marine cloud brightening and sea-surface foams) are likely to impact ocean ecosystems, and therefore the carbon cycle, such effects would be indirect, through changes in light availability, temperature, and ocean mixing. SRM approaches are therefore not considered further here.

Climatically-meaningful mCDR (i.e. at Gt scale) would necessarily change the carbon cycle: that is its purpose. However, many marine processes can be used for initially capturing carbon, with many different pathways for the subsequent fate of captured carbon. As a result, there is high method-specificity for carbon-related consequences, and for other environmental impacts, scalability, and storage durability. These factors affect acceptability and cost, hence the likelihood of implementation.

**Biologically-based mCDR approaches include:**

* **Coastal blue carbon:** the restoration of mangrove, saltmarsh and seagrass ecosystems that accumulate organic carbon in their sediments.

* **Large-scale seaweed cultivation** (also known as ocean afforestation), with either deep-sea sinking of macro-
algal biomass (with impacts on benthic ecosystems and seafloor pH and $O_2$) or its removal to land for processing into biofuel (with carbon capture and storage) or biochar.

* Ocean fertilization to increase phytoplankton productivity and subsequent deep-water carbon sequestration through the biological pump. Fertilization can be achieved by directly adding limiting micronutrients (ocean iron fertilization) or macronutrients (nitrogen or phosphorus), or by physical manipulation, increasing nutrient supply through enhanced upwelling. The latter has the counterproductive effect of bringing $CO_2$-rich water to the surface.

Chemically-based mCDR involves the manipulation of carbonate chemistry, for example:

* Adding alkaline minerals, to convert dissolved $CO_2$ to carbonate and bicarbonate ions, thereby increasing the ocean capacity to take up anthropogenic $CO_2$ from the atmosphere, and potentially mitigating ocean acidification at the local level. This is referred to as ocean alkalinity enhancement (OAE).

* Electrochemical $CO_2$ removal from seawater, with its subsequent collection and geological storage (direct ocean removal, DOR). After further processing, climate benefits are obtained when $CO_2$-depleted seawater is released and re-equilibrates with the atmosphere.

<figure><img src="image_5.png" /><figcaption>**Figure 8.** Overview of carbon dioxide removal approaches using ocean processes and ocean-land linkages.</figcaption></figure>

Source: "Carbon dioxide removal: overview of options II" by Rita Erven, GEOMAR, licensed under CC BY 4.0, www.cdrmare.de/en/spp-1689-materialien/

In addition, a range of land-ocean hybrid methods would involve ocean storage for carbon initially captured on land (e.g. Raven et al., 2024; Zhu et al., 2024).

For further details of specific methods, and discussion of their effectiveness, risks, costs and governance, see Gattuso et al. (2018, 2021), GESAMP (2019), Hoegh-Guldberg et al. (2023), Cai and Jiao (2022), NASEM (2022), Williamson et al. (2022), Zhang et al. (2022),

Cross et al. (2023), Jiao et al. (2023), Doney et al. (2024), Roberts et al. (2024), and Oschlies et al. (2025).

Unlike land-based CDR, mCDR has yet to be included in IPCC mitigation scenarios. That is due to scientific uncertainties regarding effectiveness and potential environmental risks, in addition to legal constraints. In particular, open-ocean mCDR methods are currently strongly discouraged as geoengineering under the UN Convention on Biological Diversity (CBD), and the
deployment of ocean fertilisation is restricted under
the London Protocol (IMO, 2023). Furthermore, the
International Tribunal for the Law of the Sea (ITLOS)
has recently designated anthropogenic CO₂ as a marine
pollutant, with hazards that should not be transferred
from one area to another (ITLOS, 2024; Klerk, 2025).

In contrast, coastal blue carbon has relatively high public
and political acceptability. This 'nature-based' mCDR
approach was identified as a climate mitigation policy
in 23 initial Nationally Determined Contributions to the
Paris Agreement (Gallo et al., 2017) and was considered
a no-regrets action by Gattuso et al. (2021) on account
of its important co-benefits. However, IPCC assessed
the mitigation potential of coastal blue carbon as 'very
modest' or 'small', at 0.05 – 0.3 Gt CO₂ per year (Bindoff
et al., 2019; Canadell et al., 2021), and there are concerns
that such removal rates may be over-estimated
(Williamson and Gattuso, 2022).

Major knowledge gaps

1. Demonstrating safety and long-term effectiveness.

The potential climatic benefits of other mCDR
methods (with much greater theoretical CO2
removals) could arguably be achieved if their main
knowledge gaps could be resolved, demonstrating
their safety and long-term effectiveness. NASEM
(2022) recommended a ~$1.5 billion mCDR research
program to address such issues, in the context of
increasing need for CDR to meet the internationally-
agreed goal of net zero (IPCC, 2023), together with
realisation of the limitations of land-based CDR
approaches (Boysen et al., 2017; Deprez et al. 2024).
There are also market opportunities for UNFCCC-
approved carbon trading under Article 6.4 of the Paris
Agreement.

2. Demonstrating additionality.

To qualify for emission credits, mCDR methods need
to demonstrate additionality: i.e. that their climate
benefits are unequivocal and can be reliably measured
relative to a well-established baseline (Michaelowa
et al., 2019). Uncertainties relating to additionality
are crucial for mCDR research and development
(Boyd and Bressac, 2016; Bach 2024; Bach et al.,
2024), requiring information on the energy used (as

CO₂e) to carry out the mCDR action as well as an
assessment of all its climate-related biogeochemical
consequences; i.e. a fully comprehensive life
cycle analysis (Terlouw et al., 2021; Delval et
al., 2025), relative to a counterfactual baseline.

Many of those consequences (shown conceptually in
**Figure 9**) will likely occur over large spatial and temporal
scales, influenced by ocean conditions over thousands
of km² and decadal-to-century time periods. Their
robust quantification is extremely challenging, with
future conditions depending on emission scenarios.

Although models can greatly assist in determining
mCDR additionality for open-ocean mCDR (Zhou et al.,
2024), direct measurements will still be needed, not
only of carbon-related parameters (Doney et al., 2024)
at much greater accuracy than currently possible
(McKinley et al., 2024) but also of other environmental
impacts (that may be adverse or beneficial) within a
regulatory framework of Monitoring, Reporting and
Verification (MRV; Smith et al., 2024). Such monitoring
is itself likely to involve significant CO₂e emissions,
affecting the cost-effectiveness of the intervention.

The relative importance of factors determining mCDR
additionality is technique-specific. However, for most
mCDR actions, the following are relevant (in addition
to CO₂e costs for implementation and MRV):

**2.1 Reduction in natural carbon sinks,** shown as a dashed rectangle in **Figure 9**. Increasing local productivity through ocean fertilization can decrease productivity 'downstream' through nutrient depletion (Gnanadesikan et al., 2003; Aumont and Bopp, 2006; Tagliabue et al., 2023), with similar effects occurring for large-scale seaweed cultivation (Berger et al., 2023). Abiotically-induced regional changes in the ocean carbon sink may also affect terrestrial sink efficiency (Keller et al., 2018; Boyd et al., 2025), as also likely for ocean storage of land-derived biomass. These feedback effects need to be taken into account when comparing mCDR effectiveness with any other CDR approaches (Yamamoto et al., 2024; Jeltch-Thömmes et al., 2024; Oschlies et al., 2025).
<figure><img src="image_6.png" /><figcaption>**Figure 9.** Conceptual representation of additionality (excluding albedo effects) for a generic mCDR action, adapted from Bach et al. (2024). See text for details.</figcaption></figure>

## 2.2 Emission of non-CO₂ greenhouse gases.

Anaerobic conditions in coastal sediments and mid-water oxygen minimum zones favour methane (CH₄) and nitrous oxide (N₂O) production, with the potential to significantly offset the CO₂ removal benefits of coastal blue carbon (Rosentreter et al., 2021, 2023) and ocean fertilization (Law and Ling, 2001). Measurement of the efflux of these gases therefore needs to be included in MRV protocols. For seaweed cultivation, the production of dimethyl sulphide (DMS) and halomethanes (e.g. bromoform, CHBr₃) may also have climatic impacts (Carpenter et al., 2009).

**2.3 Leakage.** The non-permanence of carbon storage for most biotic mCDR approaches is a cause for concern, since the durability of CO₂ removal is critical for effective climate mitigation (Brunner et al., 2024). For ocean fertilization, leakage results in declining effectiveness over time for global-scale deployment (Aumont and Bopp, 2006; Oschlies et al., 2025); similar effects are likely for seaweed cultivation with deep-sea biomass sinking. For coastal blue carbon, rising sea level, warmer temperatures, and human disturbance all threaten the long-term integrity of carbon stores (Williamson and Gattuso, 2022).

## Actions to be taken

### I. Model improvement

Roberts et al. (2024) reviewed the inclusion of CDR-relevant parameters in marine ecosystem models and ESMs. They identified 14 deficiencies with medium/high relevance to specific mCDR methods: most related to seaweed cultivation, including the effects of changes to surface ocean ecosystems, benthic smothering, and the attraction of seafloor biomass to deep sea fauna.

Modelling priorities identified by Buesseler et al. (2024) for ocean iron fertilization included observing system simulation experiments, storage durability, and the biological carbon pump. Despite the research attention given to the biological carbon pump, many aspects remain poorly understood (Henson et al., 2022; Baker et al., 2022; **Sections 3.a, 3.b, 3.d** and **4.a**).

### II. Field experiments and operational deployment

Buesseler et al. (2024) also proposed a new generation of iron fertilization field experiments, of larger size and longer duration than previously,
to improve understanding of carbon fluxes and
environmental impacts. MRV protocols should also be
developed and tested, using autonomous platforms
and novel biogeochemical sensors (including for
total alkalinity and dissolved inorganic carbon).
Relatively few field experiments have been carried
out on ocean alkalinity enhancement (Cyronak et al.,
2023). Although controversial (Cornwall, 2024), these
are needed to demonstrate safety (or otherwise) and
should be closely linked to experimental studies of
potential biological impacts (Bednaršek et al., 2025;
Ferderer et al., 2022), as well as high-resolution
modelling of physico-chemical processes and
exposure risk (Laurent et al., 2025; Wang et al., 2025)

The only mCDR method currently in operational
deployment is coastal blue carbon, regulated through
the voluntary carbon market. It is unclear whether
the MRV protocols developed to date (e.g. VERRA,
2023) meet UNFCCC certification requirements,
since major methodological uncertainties remain
(Kristensen et al., 2025; Dahl et al., 2025; Williamson
et al., 2025).

### III. Updated comprehensive assessment of proposed marine-based climate interventions

An updated version of the GESAMP (2019) report
is needed to provide a systematic, policy-relevant
review of the effectiveness and feasibility of mCDR
methods. In addition to potential for climate
mitigation, assessments of wider implications
should include environmental effects on biodiversity,
ecosystem functioning, and ecosystem services –
issues not always well-covered in other reviews. The
new Scientific Summary for Policymakers released
in 2025 provides a first overview in this respect
(GESAMP, 2025).

#### IV. Improving mCDR credibility

The scientific literature shows that a wide range of
interventions to increase the ocean uptake of CO₂ are
theoretically possible. Further research on the ocean
carbon cycle and related ecological processes should
help resolve key knowledge gaps; such research will
also be necessary to establish a wider consensus on
whether or not mCDR can contribute to a credible
and scalable climate mitigation strategy, that is also
both societally and politically acceptable.