The Iron Hypothesis

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New Research Finds that Adding Iron to the Oceans to Capture More Carbon Isn’t Worth the Risk
Geoengineering has gained popularity in recent years as a potentially effective way to alleviate the Earth’s warming climate. Proponents argue that artificially boosting the planet’s carbon sinks will curb carbon’s effects on rising global temperatures. The Earth’s largest active carbon sink is the ocean, which naturally absorbs more than a quarter of the carbon dioxide that is put into the air. Presently, it’s estimated that one-third of human-generated carbon dioxide is being absorbed by the ocean

There are two natural processes that the ocean uses to sequester carbon dioxide. One process is the solubility pump, which is responsible for the majority of carbon uptake in the ocean. The second natural process is the biological pump, in which dead organisms and algae carry carbon from the ocean’s surface into deeper waters.

Geoengineering research on the ocean has focused mainly on boosting the natural biological pump in the ocean to sequester more carbon dioxide. Re-searchers have investigated the possibility of dispersing large quantities of iron salts in an effort to fertilize more barren parts of the ocean and trigger the growth of algal blooms and other photosynthesizing marine life. When the algae die, they sink to the bottom of the sea taking carbon with them.

This proposal has been met with controversy. While some researchers argue that adding iron to the oceans is both cost effective and an easy and efficient means to reduce the carbon dioxide in the atmosphere, there has been compelling evidence of serious cost and ecological concerns. In 2010, researchers at the University of Western Ontario published evidence that iron enrichment sharply increases the chances of developing toxic diatom blooms.


The diatoms they observed during their research in the Gulf of Alaska was a species known to be responsible for the death and illness of thousands of marine mammals and birds along the west coast of North America. The water samples from the testing sites that were enriched with iron developed into the population of the toxic algae, doubled the level of toxin in each cell and created conditions that give the toxic species an advantage over nontoxic species. On a larger scale, this algae would be devastating for marine life.

More recently, the International Journal of Global Warming published a study that found that a single round of ocean iron fertilization would result in a net sequestration of only 11 tons of carbon per square kilometer, sequestered for a century or more, at a cost of almost $500 per ton of carbon dioxide. “Previous estimates of cost fail to recognize the economic challenge of distributing low concentrations of iron over large areas of the ocean surface and the subsequent loss processes that result in only a small net storage of carbon per square kilometer fertilized,” says Daniel Harrison of the University of Sydney Institute of Marine Science, the research author

Harrison’s calculations accounted for a number of variables that prior scenarios have not shown—not only the carbon dioxide that will be permanently sequestered in the deep ocean, but also taking into account the many losses due to ventilation, nutrient stealing and greenhouse gas production. Harrison’s calculations also subtracted the carbon dioxide emitted by the burning of fossil fuels to produce the iron salts and to power their transportation and distribution at sea. His figures directly contradict assertions that we could sequester around one billion tons of carbon dioxide by adding 33,000 tons of iron to the Southern Ocean, the generally accepted figure.

JENNIFER SANTISI is a freelance science and environmental writer living in Washington, D.C.