Seaweed Cultivation Models Show Limited Potential for Large-Scale Carbon Removal

Modeling studies indicate that large-scale seaweed cultivation for carbon dioxide removal would face severe nutrient constraints and could reduce phytoplankton growth by up to 8 percent. The research examined cultivation across waters up to 200 nautical miles from coastlines and found growth would decline 95 percent after 25 years due to depletion of nitrogen, phosphorus and iron.

@NewScientist reported that tens of millions of dollars have been invested in the approach.

Running Tide raised $70 million to grow seaweed on wood pucks intended to sink to the deep sea but closed last year after running out of financing. Kelp Blue raised at least $2 million to expand cultivation in Namibia for agricultural fertilizer and claims its operations could eventually sequester up to 500 million tonnes of CO2 per year.

Manon Berger at the University of Bern led one study, published in Nature Communications, that modeled annual production of 20 billion tonnes of seaweed.

Only patches off Senegal and southern Australia, covering about 0.05 percent of the ocean, allowed growth without decreasing phytoplankton, the model showed. In many scenarios the approach could increase atmospheric CO2 by half a tonne for every tonne of seaweed carbon grown.

A second study, published in Biogeosciences and led by Andrew Yool at the UK National Oceanography Centre, examined iron-fertilized cultivation.

It found potential removal of up to 40 billion tonnes of CO2 annually but projected that plankton in the ocean would be halved. The infrastructure required would cover 14 percent of the ocean surface, mainly in the Southern Ocean and northern Pacific and Atlantic.