In the 1990s and early 2000s, a series of experiments tested iron fertilization in the open ocean. These tests consistently found that adding iron led to phytoplankton blooms, However, the extent to which that carbon sank to the depths wasn’t always measured, and the phytoplankton were not able to use all of the iron for growth before the mineral sank.
Researchers did document changes in phytoplankton communities, with diatoms becoming more abundant than many other types of phytoplankton. These algae can be up to 1,000 times larger than cyanobacteria, allowing them to take up more carbon dioxide via photosynthesis. Diatoms create silica-based glass-like shells that add weight, increasing the likelihood that they will sink faster than other, smaller phytoplankton when they die. Their fast growth rates and loss to the deep sea bodes well for potential removal of carbon from the atmosphere and its sequestration deeper in the ocean.
Some diatoms, however, release toxins that can contribute to harmful algal blooms, although these did not occur following any of the field experiments. In addition, iron fertilization has the potential to alter where and how nutrients are allocated in the marine ecosystem. Until experiments are done to test these potential outcomes and determine how much carbon can be sequestered in the ocean depths, iron fertilization should not be put to use as a method of slowing climate change.
Early iron fertilization experiments faced resistance, due to the many unknowns. Despite this, scientists are returning to the idea as one CDR tool that should be on the table in the fight against climate change. They are currently working to create codes of conduct, so that research can be conducted in a transparent manner to better understand both intended and unintended consequences of adding iron to the ocean’s surface.
New technologies using autonomous platforms and sensors now exist that allow scientists to fully investigate the potential for iron to remove atmospheric carbon and track the subsequent movement of that carbon through the ocean. Because iron fertilization would be relatively inexpensive, it could be an important part of a suite of CDR activities aimed at removing excessive amounts of carbon dioxide from our atmosphere. But it’s important to remember that such approaches do not replace the need for immediate and major reductions in the use of fossil fuels that produce carbon dioxide in the first place.