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Ocean Fertilisation (OF)

Promises

OF aims to increase the rate at which the ocean draws down atmospheric CO2 and sequesters it in the deep oceans through the growth of phytoplankton.
This can be achieved by adding nutrients such as nitrogen or phosphorus, or trace minerals such as iron. These components help stimulate photosynthesis in phytoplankton allowing some of the transformed carbon dioxide to sink into the deeper ocean.
OF is governed by an existing international treaty, the London Protocol.

Opportunities

The Sixth Assessment Report (AR6) of the IPCC estimates the CDR potential of OF at 1-3 gtCO2 per year, with costs ranging from $US 50-500 per ton of carbon removed.

Concerns

The added nutrients could restructure marine ecosystems and lower biological diversity.
OF may decrease to a small extent pH in deeper zones and cause the deep ocean to become more acidic.
OF could change phytoplankton species, leading to the development of undesired algal blooms.

Boundaries

It is not necessarily the case that the phytoplankton in fact sinks to the bottom of the ocean.
The extra carbon could be returned to the atmosphere on a timescale anywhere from decades to millennia.

Individual technologies

Stratospheric aerosol

Ocean fertilization

Bio energy with carbon

Ground based Albedo

Afforestation & reforestation