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A current estimate of the cost of reducing the concentration of carbon dioxide in the atmosphere by Ocean Nourishment is provided. A scenario of fertilisation of the ocean in regions of excess phosphorous, carried out using a ship to distribute ammonium hydroxide, is examined.

Ocean fertilisation could be deployed to draw down the carbon dioxide already in the atmosphere and store it for millennia in the deep ocean.

The costs of fertilising the ocean with macronutrient depends mostly on the cost of producing the nutrient and the cost of its delivery. Macronutrient fertilisation has been calculated, for a particular scenario, to cost US$20 per tonne of carbon dioxide emission avoided for 100 years.

There is a collateral benefit of increased fish stocks, which is not considered here. The ocean, plausibly, has the capacity to sequester more than one Gigatonne per year of carbon (∼3.7 Gt CO2/yr) via macronutrient fertilisation.

This modest cost of reducing climate change justifies further research and development of ocean macronutrient fertilisation.

The modest cost allows climate change to be addressed without serious economic disruption.

The study reported is a contribution to mitigation of climate change.