The results offer fresh hope to would-be geoengineers hoping to draw down ever-increasing concentrations of industrial CO2 in the atmosphere, such as the ill-fated company Planktos and its failed bid to fertilize the ocean off Ecuador with iron. This new experiment induced carbon to fall 34 times as fast as natural rates for nearly two weeks—the highest such rate ever observed outside the laboratory. As the deceased oceanographer John Martin of Moss Landing Marine Observatories in California famously said in 1988: "Give me half a tanker of iron, and I'll give you the next ice age."
But such fallen carbon only resides in the deep for a few centuries at best. Eventually, it makes its way back to the surface as the ocean's bottom water circulates and rises anew near the equator (although carbon buried in sediment might stay buried longer). And such techniques might be capable, at best, of sequestering one billion metric tons of carbon dioxide per year (based on the extent of iron-deficient waters around the globe), compared with annual human emissions of more than eight billion metric tons and rising. "There is massive uncertainty in this figure, and until much more research is done no serious scientist should express any confidence in such estimates," of iron fertilization's geoengineering potential, cautions oceanographer Richard Lampitt of the National Oceanography Center in England, who also argues that more research into such potential geoengineering techniques is needed due to the failure of global efforts to curb greenhouse gas emissions.
One key to the whole experiment's success turns out to be the specific diatoms involved, which use silicon to make their shells and tend to form long strands of cellular slime after their demise that falls quickly to the seafloor. A similar cruise and experiment in 2009 failed despite dumping even more iron fertilizer over an even larger area of the Southern Ocean. The eddy chosen for that experiment lacked enough silicon to prompt these particular diatoms to grow. Instead, the experiment yielded bloom of algae, which was readily and rapidly eaten by microscopic grazers. As a result, the CO2 in the algal bloom returned to the atmosphere.
In fact, these iron-seeding experiments could backfire by producing toxic algal blooms or oxygen-depleted "dead zones," such as the one created in the over-fertilized waters at the mouth of the Mississippi River. At present, scientists have no way to ensure that the desired species of silica-shelled diatoms bloom. In short, Smetacek says, the type of bloom—and therefore the ability to sequester CO2—"cannot be controlled at this stage."