How Safe is Ocean Fertilization?

A scientific strawman is blocking one of our best pathways to climate restoration

Photo by Zia Ur Rehman on Unsplash

If you Google “risks of ocean iron fertilization,” you’ll find hundreds of papers warning about toxic algal blooms, oxygen-depleted dead zones, and disrupted ecosystems. Reading them, you’d think scientists are planning to dump iron across the entire Pacific and pray.

Here’s the strange part: nobody has ever seriously proposed doing that.

“Full-basin ocean iron fertilization”—spreading iron across whole ocean regions—would cost more than the Pentagon’s annual budget, slow CO₂ rise by less than 10%, and genuinely risk ecological chaos. It’s a terrible idea. There’s widespread agreement.

So why do hundreds of papers discuss its dangers?

The Accidental Strawman

The oceanographic community seems to have spent decades thoroughly documenting why full-basin OIF would be disastrous. It appears to be an effort to align with environmentalists in setting up a bad (“false”) solution as a strawman, to pull for research funding.

Years of models and piles of papers later, though, AI syntheses confidently warn about “ocean fertilization risks” by citing articles about something nobody intends to do. It’s like writing extensive policy tomes about the dangers of damming the entire Mississippi River when someone only proposed building a levee in Louisiana.

Meanwhile, a new approach—Localized Ocean Fertilization (LOF)—risks getting tarred with the same brush. Yet LOF is expressly targeted, scaled, and designed to optimize photosynthesis and CO₂ removal while avoiding the common concerns (well-founded or not) about OIF.

What “LOF” Actually Refers To

After the 1991 Mount Pinatubo eruption, iron-rich volcanic ash fell into Pacific Ocean regions containing “downwelling eddies”—rotating currents that pull surface water deep. Our hypothesis is that phytoplankton bloomed, rapidly pulling down large amounts of CO₂. Then nitrogen-fixing bacteria provided more nutrients, sustaining the bloom. And the resulting biocarbon was efficiently pulled into the deep ocean before fish could return it to the atmosphere as CO₂, sequestering it permanently.

Following Pinatubo, atmospheric CO₂ levels leveled off for over a year, even as smokestacks and cars continued to send billions of tons of CO₂ into the sky. Put another way, roughly 20 billion tons of CO2 “disappeared” from the atmosphere, naturally. The most plausible explanation is enhanced photosynthesis in the ocean. The world achieved net-zero for about a year. No ecological disasters occurred.

Nature had demonstrated a proof of concept for natural CO₂ removal on a large scale.²

LOF is based on this blueprint. The process would deposit minute amounts of iron dust into carefully targeted downwelling eddies to enable health phytoplankton blooms. We project the LOF approach to be so efficient that implementing it at full scale would require only about 1 percent of ocean surface. That’s a far cry from the “whole-basin” approach.

No one’s proposing to turn the ocean green. LOF supporters are working with natural systems that have safely removed atmospheric carbon for eons. If we just replicate nature’s 1992 photosynthesis, LOF practiced intermittently in small areas of the ocean could collectively offset all CO₂ emissions within a few years. That’s an exciting possibility.

Addressing the Concerns

“Isn’t this banned as dangerous geoengineering?”

Although a ban was proposed, small-scale research projects are excepted, and that’s how LOF must start. The London Convention reflects governance concerns—who decides and who bears risks. It seeks to avoid having commercial interests damage the whole ocean with full-basin fertilization in search of lucrative carbon offsets. The regulatory caution is appropriate and manageable.³

“Won’t this damage ecosystems?”

OIF—basically replenishment of iron in anemic areas of the ocean—is a natural process to which ecosystems are well adapted. More than a dozen intentional OIF experiments since 1994 have produced no significant ecosystem harm.⁴ Rather than distorting ecosystems, conditions remained within natural year-to-year variation. No fisheries collapsed—just the opposite in some cases.

LOF is designed as an extremely strategic process, with monitoring and safeguards. The eddies it would use (200-300km diameter) allow mobile species to relocate easily should an unexpected problem arise.

Toxic algal blooms occur in polluted coastal areas, related to excess nutrients from agricultural runoff — exactly where OIF/LOF would never be conducted.

“Doesn’t the carbon just get released back to the air?”

Studies show that in traditional OIF, only 1-5 percent of surface carbon falls deep enough in the ocean to be stored long term, with the remainder returning to the air within months. That’s a major limitation. And it’s why LOF targets eddies to sink a lot more of the biocarbon.⁶ As a precedent, the 2004 EIFEX experiment achieved ~50% deep export under optimal eddy conditions.⁵

The Safety Record

After decades of observations and experiments, the evidence is actually reassuring:

• Natural iron fertilization (dust storms, volcanic eruptions) occurs regularly without any documented damage or catastrophes

• Intentional experiments and volcanic eruptions since 1994 consistently show ecosystem responses within natural ecosystem variability

• Small-scale, eddy-confined LOF mimics natural processes, keeping impacts local and temporary.

Below are commonly raised concerns about ocean fertilization, along with what has been actually observed.

Analyzing the Theoretical Risks

1. Deep-ocean oxygen depletion

   Oxygen levels vary naturally in the deep ocean. Whenever surface water becomes productive, oxygen levels in the depths fall. Habitats then “compress,” or shrink temporarily, like a forest during a flood. No evidence has been found of permanent damage or extinctions in this constantly occurring process. Fish and plankton below a fertilized eddy can easily swim away if oxygen actually gets low.

2. Harmful Algal Blooms (HABs)

   Harmful algal blooms nearly always occur in coastal areas with a surfeit of nutrients. With high nutrient levels already, these are not areas that would be fertilized.

3. Emitting of nitrous oxide (N₂O) or methane

   As with HABs, these emissions occur mostly in high-nutrient coastal areas, where intentional ocean fertilization would not take place. No one has reported N₂O or methane production related to iron distribution in the deep ocean.

4. Nutrient robbing (removal of nutrients from nearby areas). This typically occurs from overfishing, which removes fish that normally recycle nutrients within the ecosystem through waste and decomposition. As these nutrient ‘recyclers’ disappear, less nitrogen and phosphorus return to the marine food web. Nutrient robbing has never been reported from natural or intentional ocean fertilization. In theory, large-scale sinking of phytoplankton to the deep seafloor could remove nutrients from upper waters. Yet no significant biocarbon showed up on the seafloor after the Pinatubo eruption. In addition, EIFEX and other experiments indicate that most additional carbon was remineralized above the deep seafloor, with no reports of thick new organic blankets.

   
   As to nitrogen, LOF is designed to nourish naturally occurring nitrogen-fixing bacteria to replenish supplies. Phosphorus and other minerals would be recycled into the water after organisms die, fall with the downwelling current and dissolve.

5. Seafloor (benthic) ecology changes. When large amounts of biomass such as seaweed or diatoms sink, they can add biological sediment to the seafloor and change the chemistry or ecology. Yet, as noted above, neither natural nor intentional ocean fertilization appears to contribute significant seafloor sediment. There is no record of such seafloor changes.

The Real Risk? Inaction

We face a choice between two benefit/risk profiles. Which would you choose?

Exploring LOF

• Localized ecosystem perturbations appear to fall within natural variability.

• “Unknown unknowns.” The ocean is hugely complicated, and something unforeseen and negative could happen. In this case, the intervention would be adjusted or, in a worst case, halted.

• Governance challenges will need to be worked out, as both the public and regulators are understandably wary of human intervention in the ocean.

• Initial hostility has come from the climate community since LOF does not reduce the use of fossil fuels—the longstanding, key goal of climate action. We must help shift their focus to a safe climate, rather than just clean energy.

• On the upside, benefits include potential sequestration of an enormous amount of carbon: enough to quickly attain net-zero emissions, then pull atmospheric CO₂ down toward preindustrial levels. This climate restoration could stop sea-level rise, death of coral reefs, deadly heat waves, floods, storms, and ecosystem collapses.

Climate Inaction

While great strides have been made in the transition to clean energy, emissions reduction alone does not touch the huge store of legacy CO₂ already in the air from 200 years of industry. In the absence of efforts to pull significant amounts of CO₂ out of the sky, we are nearly certain to see:

• Widespread ecosystem collapse and ongoing mass extinctions

• Irreversible ice sheet loss and fast-rising seas that sink coastal cities

• Increasingly devastating and costly climate-related disasters

• Agricultural disruption threatening the food security of billions

• Cascading geopolitical crises, and

• A grim future for humanity.

From Speculation to Strategy

Ocean iron fertilization has suffered mistaken identity. The fearsome “full-basin OIF” dominating the literature is something nobody proposed. Localized Ocean Fertilization is fundamentally different: biomimicry of natural processes, strategically targeted, designed for efficiency.

Questions remain, but they’re research questions, not deal-breakers. Questions about optimal sites, exact storage rates, and governance frameworks are what staged pilots with robust monitoring are designed to answer.

The real issue isn’t whether LOF is perfectly safe. (Can you claim that for any medication you take?) The issue is whether the potential risks and benefits of LOF offer a better bet for humanity and life on Earth than the certainty of climate catastrophe.

References

1. Boyd, P.W., et al. (2018). “Ocean iron fertilization experiments,” Biogeosciences 15: 5847–5889.

2. Sarmiento (1993).

3. IMO London Convention/Protocol Resolution LP.4(8) (2013).

4. Boyd et al. (2018), Biogeosciences.

5. Ward, B.A., et al. (2025). “Techno-economic analysis of ocean iron fertilization,” Frontiers in Climate 7: 1509367.

6. Paleoclimate evidence from ice core records and ocean sediment analyses.