Ocean iron fertilization removes CO2—but what does nature pay?

Phytoplankton may be microscopic, but their influence reaches from the surface of the ocean to the global climate system. These drifting, plant-like organisms absorb atmospheric carbon dioxide through photosynthesis, converting it into organic matter. When plankton die or are consumed, a portion of that carbon sinks into deeper water, where it can remain isolated from the atmosphere for years, decades or, in some cases, much longer. This natural process, known as the biological carbon pump, has made the ocean one of Earth’s most important carbon reservoirs. Now, a new study suggests that deliberately adding iron to selected ocean regions could increase carbon removal—but the potential climate benefit comes with ecological costs that may spread far beyond the original intervention site.

In vast areas of the open ocean, phytoplankton growth is constrained not by a lack of sunlight or carbon dioxide, but by the scarcity of iron. This trace element is required for photosynthetic machinery, electron transport and nitrogen metabolism, meaning that even tiny concentrations can determine how much biological production an ecosystem supports. The idea behind ocean iron fertilization is straightforward: add dissolved iron to iron-deficient waters, stimulate a plankton bloom and encourage more carbon to sink into the deep ocean. For decades, scientists have tested this concept through field experiments and computer models. The central question, however, has never been simply whether iron can stimulate growth. It is whether the additional carbon removal is large, durable and environmentally acceptable.

A study led by Professor Adam Martiny of the Technical University of Denmark addresses that question using an advanced ocean model that links carbon, nutrients, oxygen and plankton communities. The researchers simulated 60 years of iron addition in ten different ocean regions, allowing them to examine both the immediate response and the consequences that emerge as currents transport water across the planet. Their analysis, published in Nature, reveals a pronounced geographical trade-off. Some regions deliver greater carbon dioxide removal but trigger extensive ecological disruption, while others produce a more moderate climate benefit with fewer and shorter-lived effects on marine life. The results provide one of the broadest comparisons yet of where iron fertilization might be most effective—and where it could create serious unintended consequences.

The Southern Ocean, encircling Antarctica, emerged as the region offering the most favorable balance between climate benefit and ecological risk. Its powerful circulation connects distant parts of the ocean and can carry iron and unused nutrients into areas where they support additional biological production. In the model, this circulation helped extend carbon storage beyond the initial fertilization zone rather than concentrating all the effects in one location. The Southern Ocean ecosystem also appeared comparatively resilient. When iron addition stopped, biological conditions moved back toward their original state relatively quickly, suggesting that the intervention’s ecological footprint was less persistent than in several other regions. That resilience does not make the approach risk-free, but it distinguishes the Southern Ocean from areas where nutrient disturbances can remain embedded in the wider circulation system.

The equatorial Pacific presented a very different picture. Iron addition there also produced substantial increases in carbon dioxide uptake, but the resulting plankton blooms consumed large quantities of other nutrients, including nitrogen and phosphorus. Once these nutrients were depleted, ocean currents transported nutrient-poor water into distant regions, potentially suppressing plankton growth far from the fertilization site. This effect illustrates why ocean iron fertilization cannot be assessed only by measuring the size of a local bloom. The ocean is a connected, moving system in which a chemical change in one region can alter the resources available to organisms thousands of kilometers away. By changing nutrient ratios, iron fertilization could therefore reshape food webs and productivity patterns on a basin-wide scale.

The model indicated that the ecological consequences were not limited to microscopic organisms. In the equatorial Pacific simulations, the biomass of larger zooplankton declined. These animals are an important link between phytoplankton and fish, so a reduction could affect organisms higher in the food chain. The study also found an expansion of low-oxygen areas, a result of increased biological activity and the decomposition of organic matter. As microbes break down sinking material, they consume oxygen in deeper waters. Persistent or widespread oxygen loss can place stress on fish, invertebrates and other marine organisms, while creating conditions that favor some species over others. In this scenario, the area affected by the intervention became many times larger than the region where iron was originally introduced.

The researchers estimate that, depending on the location and scale of fertilization, 60 years of iron addition could remove between 0.14 and 0.70 billion metric tons of carbon dioxide from the atmosphere per year. That is a significant quantity in absolute terms, but it remains small compared with current global emissions of roughly 40 billion metric tons annually. Ocean iron fertilization could therefore function only as a supplement to deep emissions cuts, not as a substitute for them. More importantly, the carbon removal would not necessarily be permanent. The simulations show that more than half of the carbon dioxide removed during fertilization could return to the atmosphere within the following decades if the program were stopped. This limited durability complicates claims that iron fertilization could provide a long-term solution to climate change.

The study’s authors compared their simulations with previous field experiments in which scientists added iron to real ocean waters and tracked plankton growth, carbon uptake and ecological change over several weeks. According to Martiny, the model reproduced the broad patterns observed in those experiments, strengthening confidence that it captures key biological processes. Nevertheless, predicting the climate effect of a large-scale intervention remains difficult. Carbon may be absorbed by plankton at the surface but later return to the atmosphere if it is respired before reaching the deep ocean. Even when carbon sinks, researchers must determine how long it remains isolated and whether the process changes oxygen levels or nutrient availability elsewhere. These uncertainties make precise measurement, verification and the creation of reliable carbon credits particularly challenging.

The findings also raise questions that extend beyond biology and climate modeling. Ocean currents ignore national borders, so an action authorized in one country could influence marine ecosystems in international waters or affect biodiversity in distant coastal regions. Monitoring such consequences would require sustained observations across enormous areas, including measurements of plankton communities, nutrient concentrations, carbon export and oxygen levels. Regulation is equally complex because existing international frameworks were not designed to govern interventions whose effects can travel across entire ocean basins. As the United States invests more heavily in technologies intended to remove carbon dioxide from the atmosphere, ocean-based approaches are receiving increasing attention. Martiny argues that Europe should follow these developments closely and participate actively in research, oversight and international rule-making.

Ocean iron fertilization remains a controversial tool precisely because it combines a potentially measurable climate benefit with ecological consequences that are difficult to predict and even harder to contain. The new study does not present iron addition as a cure for global warming. Instead, it shows that location determines nearly everything: the amount of carbon removed, the distance over which effects spread, the pressure placed on food webs and the speed at which ecosystems recover after fertilization ends. The Southern Ocean may offer the least damaging balance among the regions examined, but even there, uncertainties remain. Any future consideration of the technology will require transparent experiments, long-term monitoring and international governance. The ocean may be able to absorb more carbon with human assistance, but the study makes clear that it cannot do so without changing—and potentially disrupting—the living systems that make that carbon sink possible.

Web References: https://www.nature.com/articles/s41586-026-10795-y

References: Nature, DOI: 10.1038/s41586-026-10795-y