OCEAN-CLIMATE NEXUS
Observations for
Marine Carbon Dioxide
Removal
Operational Monitoring of Open-Ocean Carbon Dioxide Removal Deployments:
Detection, Attribution, and Determination of Side Effects
By Philip W. Boyd*, Hervé Claustre*, Louis Legendre*, Jean-Pierre Gattuso, and Pierre-Yves Le Traon (*equal first authors)
Human activities are causing a sustained increase in the
concentration of carbon dioxide (CO2) and other green-
house gases in the atmosphere. The resulting harmful
effects on Earth’s climate require decarbonizing the econ-
omy and, given the slow pace and inherent limitations of
decarbonization of some industries such as aviation, also
the active removal and safe sequestration of CO2 away
from the atmosphere (i.e., carbon dioxide removal or CDR;
NASEM, 2022). Limiting global warming to 1.5°C—a target
that may already have been exceeded—would require
CDR on the order of 100–1,000 Gt CO2 over the twenty- first
century (IPCC, 2018).
Natural terrestrial and ocean processes already remove
about half of human CO2 emissions from the atmosphere,
with half of this amount (i.e., a quarter of the total) ending
up in the ocean. These natural processes slow down global
warming; without the continuous removal of atmospheric
CO2 since the beginning of the Industrial Era (1750), the
present (2022) level of 420 ppm would have been reached
in the 1980s. In the ocean, CO2 combines with water (H2O)
to form dissolved inorganic carbon (DIC: CO2 gas, H2CO3,
and HCO3
– and CO3
2– ions), and photosynthetic organisms
use some of the DIC to synthesize the organic matter that
is the basis of pelagic marine food webs. Marine organic
carbon exists in both particulate and dissolved forms (POC
and DOC, respectively). A number of physical, chemical,
and biological processes, collectively called ocean carbon
pumps, transfer carbon from surface waters downward
and store it in the ocean as DIC and refractory (i.e., long-
lived) DOC and POC in the ocean. Some of this storage takes
place on climatically significant timescales and is called car-
bon sequestration. Sequestration of DIC can occur at any
depth, but its potential is higher at greater depths.
There is increasing discussion of implementing marine
CDR (mCDR) approaches, which range from methods
based on natural processes to more industrial tech-
niques (NASEM, 2022). Here, we focus on open- ocean
mCDR approaches, including alkalinization (i.e., adding
alkaline substances, such as olivine or lime, to seawater
to enhance the ocean’s chemical uptake of CO2 from the
atmosphere) and nutrient fertilization (i.e., adding a nutri-
ent that limits phytoplankton photosynthesis, such as iron,
to surface waters to enhance the photosynthetic uptake
of DIC), which aim to enhance DIC sequestration resulting
from increased CO2 influx from the atmosphere.
There is a growing body of literature on various aspects
of mCDR approaches. Published mCDR studies have
addressed the appropriateness of implementation, testing
the efficiency of sequestering CO2 and/or assessing det-
rimental ecological effects (laboratory/mesocosm studies,
field trials), and identifying potential deployment sites.
Such pilot studies are precursors to possible future mCDR
deployments (NASEM, 2022), which should only occur in
cases where the pilot studies indicate that mCDR would not
unduly disrupt marine ecosystems. In contrast, this paper
addresses the situation where mCDRs are to be deployed
at scales commensurate with the target of removing giga-
tons of atmospheric carbon.
Here, considering information from satellites and auton-
omous platforms combined with artificial intelligence (AI)
and models (Figure 1), we describe a future operational
monitoring system for the detection, attribution, and deter-
mination of side effects of open- ocean mCDR deployments.
We mainly address the monitoring challenge described
in NASEM (2022), based upon the current and expected
readiness of observational platforms and sensors. This
approach ensures that the proposed monitoring system
would be tractable and deployable. The assessment of
future mCDR deployments will include three components,
together referred to as MRV: measurement or monitoring