Readers of this blog know of my advocacy for nuclear power as an essential tool for reducing greenhouse gas emissions. While emissions reductions are necessary, mitigation alone is no longer sufficient. Since the dawn of the industrial age, humanity has added hundreds of gigatons (Gt) of carbon dioxide (CO₂) to the atmosphere. Even under aggressive decarbonization pathways, this accumulated stock of CO₂ will continue to drive warming for decades.
Recognizing this reality, the National Academies of Sciences, Engineering, and Medicine conclude that achieving net-zero—and ultimately net-negative—emissions will require large-scale carbon removal technologies (NASEM, 2021). Carbon drawdown—actively removing CO₂ from the atmosphere—must therefore complement emissions reductions, not replace them.
Among the various approaches for removing CO2 from the atmosphere, marine carbon dioxide removal (mCDR) stands out for its scale, physical feasibility, and potential co-benefits. The ocean already plays a central role in Earth’s carbon cycle, absorbing roughly 25–30% of anthropogenic CO₂ emissions each year through physical and biological processes (Friedlingstein et al., 2023). At the core of this system is the biological carbon pump, in which phytoplankton convert dissolved CO₂ into organic matter, a portion of which sinks into the deep ocean and is sequestered for centuries or longer while the rest of the biomass becomes the base of the food web for all ocean creatures.
A defining advantage of mCDR is its potential to generate tangible co-benefits. By strengthening the base of the marine food web, enhanced primary productivity can translate into greater fish biomass, more resilient ecosystems, and stronger fisheries—particularly in nutrient-limited regions. This matters politically as much as scientifically. Climate policies that impose visible costs while delivering diffuse or delayed benefits often struggle for public support. In contrast, mCDR offers a pathway that could support fishing industries, strengthen coastal economies, and improve food security, while simultaneously contributing to climate stabilization.
This natural system, however, is constrained. In vast regions of the open ocean, phytoplankton growth is limited not by sunlight, but by the availability of trace nutrients such as iron or silica. This limitation presents an opportunity to amplify the ocean’s natural carbon uptake by selectively adding the missing nutrients in those regions. Could this approach work?
Natural events provide important clues about what is possible. Following the 1991 eruption of Mount Pinatubo, scientists observed a temporary slowdown in the rise of atmospheric CO₂ concentrations recorded in the Keeling Curve (Keeling et al., 1995; Sarmiento, 1993). While part of this effect reflected cooling-induced changes on land, volcanic ash also delivered iron and other micronutrients to surface waters.
Subsequent satellite and oceanographic observations support the conclusion that such nutrient inputs stimulated large phytoplankton blooms, increasing oceanic carbon uptake. Estimates of the resulting enhancement to the global carbon sink reach tens of gigatons of CO₂, demonstrating that the oceans system is capable of large, rapid increases in carbon sequestration under favorable conditions.
Controlled ocean iron fertilization experiments over the past three decades reinforce this lesson. Small additions of iron to high-nutrient, low-chlorophyll (HNLC) regions reliably stimulate phytoplankton blooms (Boyd et al., 2007). These experiments validated the underlying mechanism of nutrient-based mCDR, while also revealing important complexities. Not all carbon fixed during blooms is exported to the deep ocean; much is recycled at the surface through grazing and microbial activity. As a result, early experiments—limited in scale and duration—produced modest sequestration efficiencies relative to initial expectations (Buesseler et al., 2008).
Crucially, these studies were never designed to test large-scale or long-term deployment. More recent research emphasizes precision approaches: combining multiple nutrients, using mineral carriers such as clay, and targeting regions where ocean circulation naturally favors long-term carbon storage (NASEM, 2022). mCDR, in other words, is not about indiscriminate fertilization of the seas. It is about targeted interventions informed by ecology and geography that work with ocean physics rather than against it.
Before industrial whaling, large whale populations played a significant role in ocean nutrient cycling through the so-called “whale pump”, redistributing iron and other nutrients via fecal plumes (Roman & McCarthy, 2010). The near elimination of great whales likely reduced this natural fertilization process across large ocean regions. While whale recovery remains essential, it is necessarily slow. This raises an uncomfortable but important question: should human systems attempt to partially replicate this lost ecological service?
Global shipping fleets offer a compelling answer. Commercial and passenger vessels already traverse most of the world’s oceans on predictable routes. With appropriate safeguards, ships could be equipped to disperse carefully measured nutrients or mineral carriers, mimicking key aspects of the whale pump. This would transform global shipping from being a polluter to an agent for climate and ecosystem restoration.
None of this should be pursued casually. mCDR carries real risks that must be rigorously managed. Among the most prominent concerns is the potential for harmful algal blooms (HABs), which can disrupt marine ecosystems and produce toxins. These blooms occur most often in coastal regions already overloaded with nitrogen and phosphorus from agricultural runoff, underscoring the need for precise calibration and careful site selection.
In conclusion, carbon drawdown is no longer optional—it is indispensable to climate stabilization. The ocean offers one of the few mechanisms capable of operating at the necessary scale. Guided by natural analogues such as volcanic eruptions, informed by decades of ocean research, and implemented with care, mCDR has the potential to deliver both climate stabilization and ecological restoration. At a moment when political feasibility is as important as technical validity, it stands out as a rare solution that aligns environmental urgency with economic and ecological opportunity—helping to restore not only the stability of the climate system, but the abundance of the oceans themselves.
References
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Boyd, P. W., et al. (2007). Mesoscale iron enrichment experiments 1993–2005. Science.
https://www.science.org/doi/10.1126/science.1131669 - Buesseler, K. O., et al. (2008). Ocean iron fertilization—moving forward in a sea of uncertainty. Science. https://www.science.org/doi/10.1126/science.1154305
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Friedlingstein, P., et al. (2023). Global Carbon Budget 2023.
https://essd.copernicus.org/articles/15/5301/2023/ -
Keeling, C. D., et al. (1995). Interannual extremes in the rate of rise of atmospheric CO₂. Nature.
https://www.nature.com/articles/375666a0 -
National Academies of Sciences, Engineering, and Medicine (2021).
Negative Emissions Technologies and Reliable Sequestration.
https://nap.nationalacademies.org/catalog/25259 -
National Academies of Sciences, Engineering, and Medicine (2022).
A Research Strategy for Ocean-based Carbon Dioxide Removal.
https://nap.nationalacademies.org/catalog/26278 -
Roman, J., & McCarthy, J. J. (2010). The whale pump. PLoS ONE.
https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0013255 -
Sarmiento, J. L. (1993). Atmospheric CO₂ stalled. Nature.
https://www.nature.com/articles/365697a0
