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Why would ocean fertilization require a long-term effort across much of the ocean?

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Illustration of various geoengineering techniques to address climate change, including carbon capture and storage methods like artificial trees, industrial scrubbers, and deep ocean CO2 storage, as well as solar reflectance strategies such as sulfur aerosols, reflective crops, cloud whitening, and orbital mirrors. The scene shows these interventions applied to land, sea, and atmosphere, with labeled features explaining each process. — Geoengineering Various geoengineering proposals designed to increase solar reflectance or capture and store carbon.

Ocean fertilization is a geoengineering technique that aims to increase the uptake of atmospheric carbon dioxide by stimulating the growth of phytoplankton in regions where primary productivity is low.12

For this process to be effective, a sustained, long-term effort involving a fleet of vessels covering most of the ocean is considered necessary. This requirement stems from the need to achieve a significant and lasting impact on carbon sequestration, as the process relies on phytoplankton blooming, dying, and sinking to the ocean floor to store carbon as sedimentary rock. Because much of the carbon may be recycled through upwelling or consumed by zooplankton near the surface, authorities suggest that the scheme would need to unfold over decades to potentially yield meaningful results.12

Furthermore, scientists warn that such large-scale interventions could disrupt marine food chains and potentially harm ecosystems by promoting harmful algal blooms or depleting oxygen in deep-sea environments.1