The idea of dumping tons of iron into the ocean to feed CO₂-absorbing phytoplankton is making a comeback. Yet the effectiveness of this method remains unproven, and it may be disastrous for ecosystems.
This is a demiurgic project that scientists had long shelved, but which has resurged in recent months: fertilizing the ocean. On paper, the idea may appear enticing: dump tons of nutrients—mainly iron—into the ocean to stimulate the growth of phytoplankton, which in turn absorbs CO₂ from the atmosphere as it grows. This would reduce the greenhouse effect and help mitigate climate change.
Except, of course, the real world is far more complex. In the 1990s and 2000s, numerous studies, models, and no fewer than 13 small-scale field experiments produced mixed results. The very uncertain climate efficiency of the process, combined with multiple biodiversity risks, eventually dampened enthusiasm for ocean fertilization.
Until now. Boosted by the expansion of carbon markets, an increasing number of economic players are turning their attention to ocean fertilization—an approach that risks slipping entirely out of scientific control.
In January 2025, the Israeli startup Gigablue announced it had secured the largest contract ever in this field: the sale of 200,000 carbon credits to SkiesFifty, a company promoting “sustainable aviation,” in exchange for the promise to sequester 200,000 tons of CO₂ in the ocean through fertilization over four years.
A bloom of startups
Behind Gigablue, many other candidates are lining up.
“I’m completely stunned by how fast this is emerging,” says marine biogeochemist Marion Fourquez, a specialist in ocean fertilization processes. “Over the past year, I’ve been approached by numerous startups that barely understand the subject. Not to mention the countless conferences and articles that have been circulating.”
Hélène Planquette, researcher at the Laboratory of Marine Environmental Science, also sees an explosion of dubious initiatives, especially from the United States and Asia.
“At the UN Ocean Conference [June 2025, in Nice], we saw some unbelievable things—presentations claiming these geoengineering methods work, with little to no impact studies,” she says.
Indeed, the surge of companies falsely marketing “scientifically proven” technologies is proving difficult to regulate.
The 1996 London Protocol, signed by more than 50 states, regulates ocean dumping. A 2013 amendment prohibits “all ocean fertilization activities” except for “legitimate scientific research.” However, this amendment has not entered into force because many countries—including France—have not ratified it.
Startups are not the only ones eyeing fertilization. All technologies for removing atmospheric CO₂, particularly ocean-based solutions, are attracting growing institutional interest.
In the background: the realization that overshooting 1.5°C of global warming is now inevitable due to the failure of states to act. In all IPCC scenarios, the only hope of returning below this threshold is to combine deep emission cuts with carbon-removal technologies.
A counterproductive technique?
But can ocean fertilization truly be a viable candidate given the risks and uncertainties surrounding it?
Even among scientists, opinions diverge. Some—especially in the Anglo-Saxon world—are taking a renewed interest in these technologies, and scientific institution reports have multiplied in recent years. Their core argument: uncertainties must be resolved, potential benefits and dangers assessed, and expertise should not be left solely to industrial lobbies.
Others believe we already know enough, and that further research would open Pandora’s box, inevitably leading to the deployment of dangerous projects.
In a report published in October, the French Academy of Sciences warned against these false “miracle technologies,” which risk becoming “climate illusions”—a pretext for inaction that diverts attention from real emission-reduction efforts.
Reasons for skepticism abound.
A 2023 NOAA report estimates the CO₂ sequestration potential of fertilization at 0.1 to 1 gigaton of CO₂ per year.
The U.S. National Academy of Sciences suggests a theoretical maximum of 3.72 Gt CO₂ per year—marginal compared to the 40+ Gt emitted annually worldwide.
“If you’re iron-deficient, licking a rusty nail won’t help you much”
The wide range of estimates reflects extremely variable results between experiments, revealing the enormous complexity of biogeochemical processes involved. Fertilization must occur in iron-poor but nutrient-rich waters, with sufficient light penetration, among other criteria.
And not all types of iron are equal.
“If you’re deficient in iron, licking a rusty nail won’t do you much good—your body can only absorb specific forms of iron made bioavailable in capsules,” explains Fourquez. “It’s the same for phytoplankton.”
“Dumping iron sulfate, for example, will never replace the natural organic ligands that stabilize iron and keep it at the surface in forms that are more easily assimilated,” she adds.
Multiple threats to ecosystems
Even if iron fertilization allows phytoplankton to grow and absorb CO₂, the carbon must sink into the deep ocean for thousands of years to count as long-term sequestration. Yet a significant portion may resurface within weeks or months, releasing CO₂ back into the atmosphere.
Worse, fertilization could produce more greenhouse gases than it captures. Stimulating phytoplankton could increase emissions of nitrous oxide and methane, both extremely potent greenhouse gases. Dumped iron may also feed bacteria that release CO₂ instead of capturing it.
Even if these risks were ignored, the overall carbon and ecological footprint of such a massive logistical operation would need to be assessed.
An October 2025 report by Friends of the Earth and the Center for International Environmental Law estimated that large-scale fertilization would require thousands of vessels treating 10–20% of the ocean surface, 15 times per year, for decades—plus pollution from mining and transporting iron.
Biodiversity could also suffer. Increasing phytoplankton in one region may disrupt nutrient flows and affect ecosystems elsewhere. Added iron could favor certain species over others, creating potential imbalances across the food web.
Iron could promote some phytoplankton while reducing the krill populations essential to penguins, seals, and whales—or trigger toxic algal blooms harmful to seabirds.
Industry waiting in the wings
Proponents of further research are not deterred. To them, uncertain harmful effects are “exactly why we need pilot studies” in the ocean—large enough to yield meaningful results but not at commercial scale—so society can make informed decisions, argues Ken Buesseler, marine chemist at the Woods Hole Oceanographic Institution and executive director of Exploring Ocean Iron Solutions (ExOIS).
ExOIS plans to dump iron over 30 km² of ocean in coming years—an area they consider necessary for answers.
The operation may take place in the North Pacific, causing concern among Alaska’s coastal communities, who warned on 6 November about potential ecosystem threats.
ExOIS researchers responded that ethical science and governance with local communities is essential.
“Commercial demand for ocean carbon capture is skyrocketing, with credit sales already reaching 795,000 tons by mid-2025. (…) If global market demand outpaces sound governance, it will be too late to restore trust,” they wrote.
Meanwhile, in 2025, global CO₂ emissions continued to rise, surpassing 42 Gt.
The more this climate debt grows, the more such dilemmas will multiply—off the coast of Alaska and elsewhere.
Source: Reporterre
