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Last year, global carbon emissions from burning fossil fuels reached an all-time high. As the world heats up, many influential bodies—such as the United Nations Intergovernmental Panel on Climate Change, the governments of China and the United States, and especially fossil fuel companies—are calling for the development of carbon removal technologies. These techniques pull carbon dioxide, a potent greenhouse gas, out of the air or water and lock it away in an inaccessible form. At a big enough scale, these technologies can theoretically counterbalance emissions and help cool things down—or at least slow the rate of warming.

That’s why, in November 2021, Edwina Tanner, a marine scientist at the Australia-based biotechnology company Ocean Nourishment Corporation, dumped a mix of nutrients from a boat into the water in Botany Bay, on the south side of Sydney, Australia. As waves rocked the craft, currents pulled the red-dyed slurry in every direction, permeating one tiny patch of the world’s largest carbon sink: the ocean.

The limiting factor for the abundance of life at the ocean’s surface is often the availability of essential nutrients like iron, nitrogen, and phosphorus. So when a glut of nutrients arrives in the form of volcanic dust, wildfire ash, water upwelled from the deep, or a lab-made mixture, the sudden bounty allows tiny photosynthesizing phytoplankton to flourish. Like plants, these single-celled organisms use sunlight and carbon dioxide as fuel. The important thing for those concerned with climate change is that when these phytoplankton die, some of them sink, dragging the carbon in their bodies to the seafloor where it becomes trapped.

Oceanographer John Martin first proposed the idea of manipulating the ocean’s nutrients to store carbon in the late 1980s. There have been a few experiments since, but in general, says Tanner, getting real-world data on how well nutrient fertilization works is incredibly challenging. The public doesn’t have a big appetite for large-scale climate experiments at sea, she says.

The last large-scale attempt was a decade ago and, to Tanner’s point, it was spectacularly controversial. So in recent years, scientists have instead turned to laboratory work, computational models, and smaller field trials to better understand ocean nutrient fertilization. Modeling published in 2017, for instance, suggests that adding nitrogen and phosphorus to the ocean could lock away up to 1.5 gigatonnes of carbon per year from the atmosphere.

Tanner and her team at Ocean Nourishment Corporation are among the many scientists striving to learn more. Although she hopes to run larger field experiments, it’s difficult to get permission from the Australian government for trials exceeding 2,000 liters of the nutrient mixture. In the Botany Bay experiment, the researchers added only 300 liters of their nutrient mix. Working with such small quantities makes calculating the consequences very challenging. To circumvent the restrictions, they’re building a bioreactor to test how different mixes of nutrients stimulate phytoplankton growth and affect the rate of carbon storage.

Other researchers, too, are digging into nutrient fertilization. In 2023, for example, Joo-Eun Yoon, an applied mathematician at the University of Cambridge in England, conducted experiments with a team in the Arabian Sea off Goa, India, to find out how to best deliver nutrients to the ocean. Maximizing carbon storage, it turns out, is not as simple as just dumping nutrients overboard.

Yoon says nutrient fertilization could potentially be made more effective. The key is whether scientists can stimulate the growth of bigger—that is, physically larger—phytoplankton species. Bigger phytoplankton “are very heavy,” she says. “[They] sink quickly onto the seafloor, and so they can reduce carbon dioxide more efficiently.”

Yoon is hoping to learn more through her work with the international Exploring Ocean Iron Solutions research consortium, which is aiming to run its own iron fertilization field experiments by 2025.

Yet even if nutrient fertilization can be made more efficient, Alessandro Tagliabue, an ocean biogeochemist at the University of Liverpool in England, is skeptical of its value. He says that even at its peak performance, the technique just can’t store that much carbon.

Modeling work published by Tagliabue that looks into ocean iron fertilization—a scenario where just iron is added to the ocean—shows that by the year 2100, the amount of carbon we could trap and store through this technique would amount to about 78 gigatonnes. For context, over just the past four years, the world has emitted about 75 gigatonnes of carbon.

In practice, inefficiencies and unforeseen complications mean iron fertilization would likely lead to even less carbon storage.

For example, setting up a large-scale nutrient fertilization project would require mining the minerals and building infrastructure to get them into the ocean. These activities would emit carbon, lowering the overall carbon sequestration potential by the time the nutrients hit the water. Even at its most efficient, says Tagliabue, “it buys us a handful of years.”

Worse still is the potential for negative side effects. Scientists already expect that nutrient stocks in the upper ocean will decrease as ocean temperatures rise. Tagliabue’s research suggests that the flurry of phytoplankton growth triggered by iron fertilization could also use up the available nitrogen or phosphorus, ultimately leading to a drop in animal biomass in the upper ocean.

Tagliabue didn’t study what would happen if a geoengineer added nitrogen and phosphorus to the mix, too. Doing so could presumably avoid throwing the ocean’s nutrient balance as far out of whack as only adding iron, he says. But increasing the complexity of this marine multivitamin would mean more mining and more infrastructure, complicating the process and likely further reducing the carbon that’s captured and stored.

Other modeling suggests that adding nitrogen and phosphorus to the ocean could reduce oxygen levels and increase the global volume of low-oxygen dead zones by 17.5 percent.

Like Tagliabue, Tanner doesn’t shy away from sharing the fact that ocean fertilization will only be able to counteract a couple of years’ worth of current carbon emissions. She says the technique is only one in a broader suite of potential carbon sequestration technologies being looked at, like storing carbon in seaweed.

There are going to be a mix of approaches that will transition us along the way to net zero, she says. Ocean Nourishment Corporation will not solve the climate crisis, she adds, “but we will provide part of the answer.”

Tagliabue is less enthusiastic. If iron fertilization can only capture a few years’ worth of emissions, he says, that’s “not useful in terms of global climate change.”