For years, people have held out hope that the wide surface of the oceans could be used in some sort of way to suck carbon dioxide out of the atmosphere and pull it away permanently down into the depths of the oceans. One proposal was to fertilize the oceans with iron, increasing the growth of diatoms, a kind of phytoplankton that has shells that contain significant amounts of carbon. The diatoms would then die, and sink down with their relatively heavy shells forever, taking the carbon with them.
On the face of it, there were a few big problems with this plan:
1. It requires massive manipulation of ocean ecosystems, treating them like farmers’ fields. The oceans are already in a state of ecological crisis, so the iron fertilization plan could contribute to the oceans’ ruin.
2. A consequence of increased levels of atmospheric carbon dioxide levels is increased ocean acidification. Ocean acidification makes it more difficult for marine life such as diatoms to make carbon-based shells.
3. Only a small amount of carbon would be sequestered, which would probably be offset by the carbon emitted in the iron mining and shipping process.
Even so, scientists at the Alfred-Wegener Institute figured it was worth a try, so they went to the Southern Ocean and conducted an iron fertilization experiment. It failed. There was a plankton bloom, but with kinds of phytoplankton that don’t have any shells. The plankton was eaten right up, and the animals that ate the plankton breathed out more carbon.
Some scientists explained that the Southern Oceans may not have the right chemistry for diatoms growth to be stimulated. They pointed out that previous iron fertilization experiments had shown some carbon sequestration, but that was only at a rate of one to two percent of the carbon involved in the process. At that rate, the massive dumping of iron into ocean waters could reduce atmospheric carbon dioxide in perhaps centuries of constant activity.
Post script: Here’s another little problem with the iron fertilization scheme: Some types of diatom create a neurotoxin called domoic acid which can accumulate in marine sediments long after the diatoms who created the poison has gone.