Fertilising the ocean involves adding iron to stimulate phytoplankton to grow, which in turn removes carbon dioxide from the atmosphere. The team compared the likely amount of carbon dioxide that could be removed by fertilising different parts of the oceans around the world, and also looked at the environmental impact the fertilisation would have. 

The findings offer insight into best practices for maximising carbon dioxide removal whilst reducing potential ecological trade-offs typically associated with iron fertilisation.

Current carbon emissions are projected to surpass the Paris Climate Agreement’s limit of 2 °C above pre-industrial levels. Ocean iron fertilisation — in which iron is added to surface water to stimulate phytoplankton growth for carbon dioxide removal — is a potential solution.

However, the ecological impacts of this strategy, such as creating runaway algal blooms and depleting nutrients in the water, have been a limitation.

Jun Yu, Adam Martiny, and colleagues used an ocean iron fertilisation model to assess the climate benefits and ecological trade-offs of the approach.

By analysing over six decades of fertilisation data, they found that net carbon dioxide removal was up to 5.3 parts per million, which corresponds to a removal rate of 0.70 gigatons of carbon dioxide per year.

Furthermore, Yu and colleagues found that the most productive areas — that is, the areas with the largest plankton blooms — after iron fertilisation were the Southern Ocean and the equatorial Pacific.

A key ecological issue with the strategy involves the depletion of macronutrients from the water column, particularly in areas near the tropical Pacific, which can proliferate through the food chain and result in declines in macrozooplankton biomass.

However, the authors determined that these disturbances were a lower risk when fertilisation was deployed in higher latitudes.

Specifically, they note that fertilising 0.35% of the ocean in the equatorial Pacific leads to a minimum 10% decline in macrozooplankton biomass, while fertilising 0.2% of the Southern Ocean led to a 10% increase in macrozooplankton.

The authors suggest that fertilising the Southern Ocean offers the best balance between ecological disturbance and carbon dioxide removal from the atmosphere.

However, they caution that ocean iron fertilisation has downstream effects and that any localised interventions will inherently have non-local effects.