Actually someone has worked out a way of making a profit from reducing CO2 by causing algal blooms in the deep ocean via phosphate fertilization [0]. The basic concept is you use the blooms to support fisheries that provide the profit - fish are expensive these days.

0.https://en.wikipedia.org/wiki/Ocean_fertilization#Phosphorus

I don't think it's economically feasible. The amount of CO2 reduction needed will drop the cost of fish enormously, such that there isn't the profit to make it be sustainable.

The world produces about 5 metric tons of CO2 per capita ("only fossil fuels and cement manufacture", says Wikipedia). 27% of that is carbon.

That's 1 ton of carbon per person.

Let's assume fish are pure carbon.

http://www.greenfacts.org/en/fisheries/l-2/06-fish-consumpti... says "Preliminary estimates for 2006 indicate a slight increase in global per capita fish supply to about 16.7 kg."

That's rather smaller than 1 ton.

https://www.popsci.com/were-catching-way-more-fish-than-we-t... says we currently take about "130 million tons of fish out of the oceans".

Fish aren't pure carbon. In humans, carbon is 18% of the body. I'll assume it's about the same for fish. That's 7.5 metric tons of fish, per capita, per year.

Another way to think of the sizes, and realize how small the fish population is, comes from https://what-if.xkcd.com/33/ :

> Marine fish biomass dropped by 80% over the last century, which—taking into consideration the growth rate of the world’s shipping fleet—leads to an odd conclusion: Sometime in the last few years, we reached a point where there are, by weight, more ships in the ocean than fish.

It is a lot more complex than this (more than a HC post), but the simple version is most of the carbon captured is due to carbon rich waste products falling below 200m (about 400t of carbon for every t of phosphate), while the money is made from selling the fish.

I did a rather detailed investigation into the scheme a few years ago. The end result is provided you have a cheap source of phosphate (phosphate-rich rock is fine) and a good slow release system (think something like pumice), then the whole thing is both a massive money spinner and will capture enough carbon to reverse the current CO2 rise.

I agree it's complex. I see I also made a mistake - my calculations were for "fish and fish wastes", not "fish".

I still don't see how the numbers work out. You need about 7 tons of fish+waste per year to keep CO2 levels as they are. Only a few tens of kilos of those will be extracted via fishing.

The numbers I see are something like 1.5 kg feed for 1kg of salmon, so there's not that much waste.

You're going to have to cover huge amounts of ocean, as otherwise you're limited by the diffusion rate.

And what's the supply curve for fish? If twice as many fish are harvested, would the prices be the same? What about 10x as many fish?

How much fish are needed to reverse the current CO2 rise?

Then there are other questions like, will it induce algal blooms? if there's a surplus of phosphate then what becomes the limiting nutrient?

I think where you might be going astray is confusing the biomass of the fish with the carbon fixed. The fish sit at the top of the food chain and account for a tiny percentage of the carbon captured - they are just the profit center.

Most of the carbon is fixed by phytoplankton and the next stage in the food chain. As these die the carbon in the water column increases and eventually the excess carbon settles below 200m where it is trapped for thousands/millions of years.

The limiting factor to primary productivity in most oceans is the phosphate concentration. As long as you don't release too much phosphate at once then you limit the algal growth. You get around 400 tons of carbon fixed for every ton of phosphate added.

Yes we are going to have to use a large amount of the deep ocean, but there is more than enough ocean to do the job.

Thank you. Yes, that was what I was missing.

"400 tons of carbon for every ton of phosphate added" ... how long does that take? It's diffusion limited, yes?

Ahh, I followed the Wikipedia link to https://www.atmos-chem-phys.net/9/5539/2009/acp-9-5539-2009.... .

> Ocean fertilisation options are only worthwhile if sustained on a millennial timescale and phosphorus addition may have greater long-term potential than iron or nitrogen fertilisation.

How long would it take a company to be in the black with phosphorous mining? First the phytoplankton, then eventually the fish, so that's a few years of waiting, yes?

Since fish move, isn't this the sort of project where one company pays to put the phosphorus in but other companies can profit off the increased fish? So it requires a change to the international fishing laws for the fertilisation company to make a profit?

You actually have to provide homes for the fish (floating structures) that act like a surface wreck. The fish then hang around the floating material and and breed and grow.

The response to phosphate addition is surprisingly fast - depending the fish you wanted to harvest you are looking at a couple of years at most before you had a harvestable amount of fish to sell.

The law in this area is uncertain. Under the various international treaties it is not legal to dump garbage in the oceans, but no one is exactly sure where phosphate fertilisation sits. Even if turns out to be illegal all you need to do is find a friendly pacific island nation and do all the phosphate dumping in their waters and let the ocean currents do the work for you :)

The nice thing about this idea is that it turns what appears to be an impossible dream and makes it possible to make a profit solving it.

Thank you for your patience with me. Your comments were quite informative. I'm now well beyond the point where I can make even somewhat relevant questions, much less comments. :)

EDIT: Oh, wait, I do have one more question. In order for this to be an effective form of carbon reduction, how much will the fish population increase? As the supply goes up, the prices will go down, and I'm still not convinced that the amount of carbon capture needed is economically justified by the increase in fish supply.

This is a rather hard question to answer as it would depend on how elastic demand is for fish and how efficiently the phosphate is converted to fish.

You could choose to run the whole operation semi-inefficiently where you chose to only catch and sell a portion of the fish, or use the fish as feed for other higher value fish like salmon and bluefin tuna.

My minor contribution to the idea was to workout that you don’t need purified phosphate to fertilise the oceans. We could just use phosphate rich rock to make a slow release pumice-like fertiliser that floats on the surface (using a photo-degradable polymer). This gets the phosphate cost down from around US$1600/t to something under $50/t. I wrote a paper on this many years ago, but I never got around to publishing it. I might try and turn it into a blog post if I get a chance.