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Climate fix? ‘Fertilizing’ oceans with iron unlikely to sequester more carbon

Phytoplankton. Image courtesy of NOAA MESA Project via Flickr (CC BY 2.0).

  • Since the 1980s, scientists have studied whether adding iron to the oceans might represent a relatively simple and inexpensive solution to climate change.
  • The idea is that adding iron would encourage the growth of carbon-munching marine phytoplankton that would pull carbon out of the atmosphere on a global scale.
  • But a new study by researchers at the Massachusetts Institute of Technology suggests that iron fertilization, as the process is called, is unlikely to work.

It’s a beguiling proposition: pulling carbon dioxide out of the atmosphere by supercharging populations of microscopic, carbon-munching marine phytoplankton through the relatively simple and inexpensive process of adding iron to the ocean.

But a new study by researchers at the Massachusetts Institute of Technology (MIT) suggests that iron fertilization, as the process is called, is unlikely to help mitigate climate change.

“According to our framework, iron fertilization cannot have a significant overall effect on the amount of carbon in the ocean because the total amount of iron that microbes need is already just right,” Jonathan Lauderdale, an oceanographer and the report’s lead author, said in a press release.

The world’s oceans play a key role in keeping atmospheric carbon levels in check, largely through the work of phytoplankton. These organisms consume carbon dioxide from the atmosphere as they photosynthesize; those that aren’t eaten, as well as their waste, sink to the seafloor and take some of that carbon with them. And there it can lie for hundreds or thousands of years.

Phytoplankton need iron to grow, as well as macronutrients like nitrate and phosphate. Iron is the second most abundant mineral in the Earth’s crust, but it only enters the sea via dust from the continents and quickly sinks to the seafloor, so some parts of the ocean have lower concentrations of the mineral than others. The Southern Ocean, for example, has relatively low iron levels, and a correspondingly low phytoplankton population, despite being rich in other macronutrients.

In the late 1980s, marine biogeochemist John Martin carried out bottle-based experiments that showed that adding iron to water from these low-iron, high-nutrient areas rapidly boosted phytoplankton populations. He was quick to make a connection between iron fertilization and climate, claiming half-seriously to colleagues in 1988, “Give me half a tanker of iron, and I’ll give you the next ice age.”

Since then, a number of scientists have pursued the possibility of iron fertilization as a relatively simple way to draw down carbon dioxide from the atmosphere. “The perceived wisdom is that those nutrients represent a missed opportunity to fix carbon,” Lauderdale told Mongabay, referring to nutrients left unconsumed by phytoplankton in low-iron areas.

The allure of carbon-credit cash has motivated a number of individuals, community groups and private-sector companies to pursue the cause, too. In the mid-2000s, several companies were established with the aim of raising carbon credits through iron fertilization, though none managed to bring their projects to fruition. In 2012, U.S. entrepreneur Russ George worked with a Haida Nation community in coastal British Colombia, Canada, to carry out a controversial iron-fertilization project that George claimed would not only generate carbon credits to sell but also boost salmon stocks. The project organizers dumped 100 tons of iron sulfate into the ocean, creating a large algal bloom that lasted for months; there’s no evidence, though, that the process actually sequestered carbon or boosted fish populations in the longer term.

A scanning electron micrograph of Emiliania huxleyi, a species of single-celled marine phytoplankton. Image by Alison R. Taylor (University of North Carolina Wilmington Microscopy Facility) via Wikimedia Commons (CC BY 2.5).

From the bottle to the big blue sea

According to Lauderdale, the impacts of iron fertilization in a dynamic ocean ecosystem are much less straightforward than those Martin saw in his bottles. “Over the last 20 or 30 years, we’ve gotten a much better idea of how interconnected the ocean is,” he said. For instance, the excess nutrients that are not consumed by phytoplankton in the Southern Ocean “get folded into the ocean circulation, and they outcrop in more dusty regions where there’s lots of iron,” he said; the process provides around 75% of the nutrients that feed phytoplankton growth in the northern oceans.

It’s not a one-way relationship either, the researchers found by simulating the mineral concentrations and circulation patterns found in different parts of the ocean to investigate the interplay of microbes, iron and other nutrients. Phytoplankton have evolved the ability to produce organic molecules called ligands, which make iron more bioavailable. The researchers found that ligands secreted by phytoplankton in the North Atlantic carry iron as they ride currents back into the Southern Ocean to feed the phytoplankton there. “The microbes have this ability to tune the marine chemistry for their own purposes,” Lauderdale said, “and so they’ve engineered this system to be optimal for themselves.

“But eventually, they’re going to run out of something else,” he said — other nutrients or sufficient sunlight to support a bigger population, for instance. “And so that feedback mechanism, in our models and in our hypothesis, matches the availability of iron to the other resources that the phytoplankton need to grow.”

That means adding iron to the Southern Ocean to stimulate plankton growth will reduce the amount of macronutrients being delivered to the North Atlantic, which will affect the productivity of phytoplankton there — and may actually reduce the amount of bioavailable iron in the Southern Ocean, too, in the longer term. “So the net effect of that is zero,” Lauderdale said.

What’s more, phytoplankton sit at the base of the marine food chain, so interfering with their populations would significantly impact marine ecosystems — particularly in the North Atlantic, where millions of people depend on fisheries for their livelihoods. Some experiments have also shown that fertilization favors certain kinds of phytoplankton over others, so the process could cause toxic algal blooms and deoxygenate water, depleting marine life. Concerns over this possibility led the United Nations Convention on Biological Diversity to establish a moratorium on all ocean-fertilization projects in 2008, apart from small-scale, coastal experiments.

Satellite image shows a phytoplankton bloom off Newfoundland, Canada, on Sept. 19, 2019. The bloom occurred unusually late for the region, possibly because of higher temperatures and more sunlight than is typical for that time of year. Image courtesy of NASA.

Impossible dream or important possibility?

Given the dearth of research showing conclusive long-term carbon capture from iron fertilization, and the potential large-scale ecosystem impacts of doing so, is it time to lay the idea to rest? Lauderdale said he believes so: “I think we should tackle the source of the problem — reducing our carbon emissions — rather than trying to come up with band-aids,” he said.

But some researchers, governments and companies think the concept is still worth exploring. The South Korea’s Ministry of Oceans and Fisheries and the Korea Polar Research Institute led a 2016-2020 project to explore the possibility of experimenting with small-scale iron fertilization in the Southern Ocean. Joo-Eun Yoon, a Ph.D. candidate at Incheon National University who led a 2018 study to establish design guidelines for the project, told Mongabay in an email that iron fertilization “could still be an effective method for stimulating oceanic carbon sequestration.” To overcome the circulation dynamics outlined in the recent study, he recommended conducting experiments in specific regions where iron and nutrients are more likely to stay put, “for example, in an eddy formed in the polar front of the Southern Ocean.”

David Emerson, a geomicrobiologist at the Bigelow Laboratory for Ocean Sciences in Maine, told Mongabay in an email that when it comes to iron fertilization there are still “critical questions worthy of research,” such as whether alternative forms of iron would interact differently with phytoplankton and ocean currents. However, he also emphasized the “unknown cost” of ecosystem impacts from large-scale fertilization.

“We shouldn’t do it, unless there are concomitant major reductions in emissions,” he said. “We shouldn’t do it until we know significantly more about how effective it will be. We should only do it if the alternative is major ecosystem/human civilization collapse.”

Phytoplankton. Image courtesy of NOAA MESA Project via Flickr (CC BY 2.0).


Lauderdale, J. M., Braakman, R., Forget, G., Dutkiewicz, S., & Follows, M. J. (2020). Microbial feedbacks optimize ocean iron availability. Proceedings of the National Academy of Sciences, 201917277. doi:10.1073/pnas.1917277117

Yoon, J. E., Yoo, K. C., Macdonald, A. M., Yoon, H. I., Park, K. T., Yang, E. J., … Kim, I. N. (2018). Reviews and syntheses: Ocean iron fertilization experiments – past, present, and future looking to a future Korean Iron Fertilization Experiment in the Southern Ocean (KIFES) project. Biogeosciences15(19), 5847-5889. doi:10.5194/bg-15-5847-2018

Monica Evans is a freelance writer based in Aotearoa/New Zealand, who loves exploring the relationships between social and environmental issues. She has a master’s degree in development studies from Victoria University of Wellington. Find her at

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