News release

From: Springer Nature

Climate: Reducing the ecological footprint of ocean iron fertilization (N&V)

Deploying ocean iron fertilization — a strategy for carbon dioxide capture — in higher latitudes rather than near the equator could reduce environmental impacts while still achieving removal of carbon dioxide from the atmosphere, according to a modelling study in Nature. The findings offer insight into best practices for maximizing carbon dioxide removal whilst reducing potential ecological trade-offs typically associated with iron fertilization.

Current carbon emissions are projected to surpass the Paris Climate Agreement’s limit of 2 °C above pre-industrial levels. Ocean iron fertilization — 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 fertilization model to assess the climate benefits and ecological trade-offs of the approach. By analysing over six decades of fertilization 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 fertilization 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 fertilization was deployed in higher latitudes. Specifically, they note that fertilizing 0.35% of the ocean in the equatorial Pacific leads to a minimum 10% decline in macrozooplankton biomass, while fertilizing 0.2% of the Southern Ocean led to a 10% increase in macrozooplankton.

The authors suggest that fertilizing the Southern Ocean offers the best balance between ecological disturbance and carbon dioxide removal from the atmosphere. However, they caution that ocean iron fertilization has downstream effects and that any localized interventions will inherently have non-local effects.