The planet’s oceans are an ally for fighting climate change. They absorb about a third of all human-generated carbon dioxide emissions. Many researchers are looking at ways to speed up the ocean’s ability to remove carbon dioxide from the atmosphere.

One proposed solution is to dump iron into ocean waters. This would trigger a bloom of phytoplankton—microscopic plant-like organisms that use sunlight to produce food—which would pull large amounts of carbon dioxide from the atmosphere.

But research published in the journal Nature shows that not all places are equal when it comes to ocean iron fertilization (OIF). The carbon dioxide removal strategy could have unintended side-effects, and its ecological impact and effectiveness depends on where it is put into action.

The team from the University of California, Irvine has found that implementing OIF at higher latitudes rather than near the equator could reduce environmental impacts while still achieving removal of carbon dioxide

“The Central Pacific should be avoided for OIF due to the severe ecological risks, and the Southern Ocean may offer the best balance between carbon sequestration and ecological risk over decadal timescales,” says Jun Yu of UCI’s department of earth systems science.

 

 

Artificial OIF is more than a theoretical concept, Yu says. Researchers have conducted field tests in the past. But those short-term OIF experiments were designed to see whether iron “is the limiting nutrient for phytoplankton growth in ocean regions, not to serve as a climate solution.”

To work as a climate solution, OIF would need to be done at much larger scales over longer periods. That could have unintended environmental consequences such as biodiversity loss, expansion of oxygen minimum zones, and disturbance of fish habitats. That’s because the added iron can deplete nutrients from water. That results in less zooplankton, tiny organisms that serve as food for fish and larger marine animals.

Yu, J. Keith Moore, Adam Martiny and colleagues set out to see which regions can achieve efficient, durable carbon dioxide removal, and what the ecological impact would be. They used an ocean iron fertilization model to analyze the fertilization potential and ecosystem impact across ten ocean biomes over 60 years.

The team found that OIF removed up to 5.3 parts per million of carbon dioxide, or 0.70 gigatons of carbon dioxide per year. They also found that the areas with the largest plankton blooms, and hence the most effective at carbon dioxide removal, were the Southern Ocean and the equatorial Pacific.

However, the strategy depleted nutrients near the equator. Fertilizing 0.35% of the ocean in the equatorial Pacific leads to at least 10% decline in zooplankton. Meanwhile, fertilizing 0.2% of the Southern Ocean led to a 10% increase in zooplankton.

“The next step would be to explore the fishery impact of OIF, with additional model experiments at more locations in the Southern Ocean to test the benefits and risks,” Yu says.

Source: Jun Yu et al. Climate benefit and ecological cost trade-offs for ocean iron fertilization. Nature, 2026.

Image credit: NASA Earth Observatory