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Exploring the Potential of Ocean Carbon Capture through Seaweed Farming

Here is a plan for ocean carbon capture: Step one, grow seaweed. Step two, sink the seaweed to anoxic ocean depth, where it will not decompose into CO2. Step three, cash in your carbon credits..

A recent journal article, "Economic and biophysical limits to seaweed farming for climate change mitigation," attempts to analyze the feasibility of this plan. This is good, because seaweed has attracted a lot of interest and some venture investment, and because that venture investment so far has not turned out great. The failures so far might be endemic to the particular company that went first, or might represent flaws in the idea. A solid techno-economic analysis is the only way to tease this apart.

According to the article's findings, the cost of farming seaweed for carbon capture can be as low as $190 per ton. The article presents pretty big error bars, however, because of lack of knowledge about how many nutrients are really available in different ocean regions, whether the seaweed will outcompete native phytoplankton, and the like. Land-based carbon capture is easy to model - you know all of your inputs, and can characterize all of your outputs. Aquaculturists have to try a pilot site, and then hope like heck that they can scale any successes to the next geographic location. Farming is hard to do consistently from place to place and year to year, as any farmer will tell you.

And location matters a lot. The $190/ton number is low enough to garner interest, but it's only viable in a few places on the globe, with an area for farming that represents a fraction of a fraction of a percent. Farming only in locations offering capture at <$200/ton would deliver negligible climate impact. If seaweed farmers were asked to sequester 1 gigaton of carbon per year, the cost would increase as less productive ocean areas are utilized. The article estimates a cost of $480 per ton, which is uncompetitive compared to air capture technologies.

Despite these drawbacks, seaweed farming for climate change mitigation shouldn't be entirely dismissed. Technological advancements may improve seeding methods and nutrient delivery, potentially making seaweed aquaculture more economically viable. Additionally, harvesting some of the seaweed for animal or human consumption could free up land for reforestation, indirectly offsetting carbon emissions. Although this approach complicates accounting, it shouldn't be overlooked. While the current economic feasibility of ocean aquaculture for climate change mitigation seems out of reach, we should not abandon the potential of seaweed farming just yet. But a skeptical eye is needed, as well as a lot of cost modeling before investments are made.

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