At what scale could Ocean Iron Fertilization make sense?

Photo by henrique setim on Unsplash

Ocean Iron Fertilization (OIF) is a process that aims to reduce carbon dioxide (CO2) levels in the atmosphere by encouraging phytoplankton growth in certain ocean areas where iron is scarce. It's generally considered a form of geoengineering, even though it does not change the planet's surface or atmosphere. It simply tries to remove the bottlenecks to nature's own processes for turning CO2 into biomass.

As with other forms of nature-based solutions, modeling its actual cost/benefit is difficult. Nevertheless, we persist, most recently in the recently published preprint Identifying the most (cost-)efficient regions for CO2 removal with Iron Fertilization in the Southern Ocean. To determine whether OIF is effective at removing CO2 from the atmosphere, the paper considered five important questions that must be answered in any assessment:

1. (Does nutrient use by phytoplankton in iron-seeded regions deny nutrients to other phytoplankton downstream?

2. Is iron actually a limiting mineral all year, and if not, when is the appropriate seeding time?

3. Does the availability of light during the summer months limit phytoplankton more than iron does?

4. Will the phytoplankton actually sink to anoxic depth as desired, or will ocean currents drag the phytoplankton to regions where they will be recycled back to the atmosphere?

5. Will a phytoplankton bloom consume so much local CO2 that carbonate will become the limiting reagent, limiting the effects of iron

Needless to say, this is all hard to model, and it would be a real technical challenge to construct an effective carbon credits program for ocean iron fertilization. The study takes its best shot, finding that cost-efficient CO2 removal can be less than $100 per ton of CO2 in limited areas on the Antarctic Shelf. In contrast, costs are typically greater than $1,000 per ton in offshore regions where past OIF experiments have taken place. (Note that the work only considered the Southern Hemisphere.)

The conclusion is that OIF on the Antarctic shelves appears to be a cost-efficient option, but of course the approach must contend with serious legal concerns due to overlapping international laws in the area. All models are tentative, and this one of course could be undone in the future. But the researchers data suggests it's going to be difficult to focus the world's politicians on crafting rules to allow ocean iron fertilization, given how limited the impact appears to be even in the best-case scenarios. There is a large ante for any bet on geoengineering-like interventions, the payoff here may not be large enough to make it worth even coming to the table.