How does sinking seaweed in the ocean remove Carbon Dioxide from the atmosphere?

This is a really interesting topic for discussion, and one which we have found ourselves talking about with investors, colleagues, collaborators and anyone else who’ll listen to us. There are actually a few different ways you can go about answering this question, but all you really need is an understanding of how the global carbon cycle works to really get to grips with it. There’s a lot of great resources online to tell you more about it, this is one of our favourites and it helped us come up with diagrams below.

Seaweed is growing photosynthetically and sourcing its carbon dioxide from the ocean. Simple, right? Er, not quite! It’s a little bit more complex than that. As you know, Planet Earth is covered ~70% by water. This water is continually exchanging gases (including CO₂) with the atmosphere¹. On a massive scale. Take a look at the diagram below. There are about 875 gigatonnes of carbon (GtC) in the atmosphere (or petagrams if you prefer those units…).

There’s slightly more in the surface ocean waters: about 900 GtC. The CO₂ in the surface waters is constantly moving into the atmosphere, and the CO₂ in the atmosphere is constantly moving into the surface waters. The scale of this movement is absolutely massive: about 80 GtC per year. To put that into perspective, our industrial emissions of CO₂ are about 4 GtC per year.

With that rate of exchange, removing CO₂ from the surface ocean is just as good as removing it from the atmosphere. They are two sides of the same coin. Atmospheric or Surface Ocean… It’s all the same CO₂ pool, the CO₂ is moving freely in between the two. We actually think it’s better to remove it from the ocean. And here’s why.

At the local scale, when the surface of the ocean has more CO₂ in it than the atmosphere, there will be a net movement of CO₂ out of the sea. Like a fizzy drink going flat when you open the bottle or can. But, when the atmosphere has more CO₂ than the water it’s in contact with, there will be a net movement into the ocean. Different parts of the world at different times will have a net losses or net gains of variable sizes, it’s very dynamic process with an almost infinite number of moving parts². We are forever reading stories about the latest work proving seaweed X in location Y produces netflux Z under very precise conditions ABCDEFGHIJKLMNOP has a net gain or a net loss^345678910. It’s pretty confusing to the layman. It’s all rather contentious and the debates get quite heated.

What you need to remember is this: overall, globally, the net direction of travel is CO₂ goes INTO the oceans. That’s exactly the situation we’re making worse with our CO₂ emissions.

The oceans have become the major sink for atmospheric carbon dioxide, indeed, they have been doing a very successful job of hiding/mitigating against the massive amounts we have been producing - absorbing a third of human emissions already¹¹. Before the industrial revolution (at 280 ppm) the ocean was a sink for CO₂ at around 0.5 GtC per year. Now it’s up to 2.5 GtC per year (as I write this on the 12th August 2022 it stands at 417 ppm).

Because of the lag in reaching an equilibrium (you’ve already seen the scales and complexity involved here), it doesn’t matter what we do about removing from the atmosphere via processes like direct air capture, the oceans will still be a net receiver of CO₂ and will continue to absorb CO₂. This is a terrible situation for oceanic life: ocean acidification in the surface oceans is causing massive problems. So, we must get it out of there.

The difference between ocean based and non-ocean based CDR is that removing it from the atmosphere won’t have much impact on the oceans.  But removing it from the oceans will have an impact on both the atmospheric AND the oceanic CO₂ levels.

The overall impact of removing the seaweed at the surface of the ocean to the bottom of the deep ocean is relatively simple to understand: the seaweed has utilised lots of CO₂ in order to grow by photosynthesis, so by sinking it to the bottom of the ocean and removing it from the surface carbon cycle¹², we are effectively reducing what’s known as the partial pressure (pCO₂) of the water, which means more atmospheric CO₂ will have to enter the oceans to balance it. So, we remove it from the oceans, and the atmospheric transfer responds. It’s very dynamic and of course there’s a time delay, nothing on a planetary scale is instantaneous, but our conservative modelling suggests most (>90%) of the CO₂ will enter from the atmosphere to replace the oceanic CO₂ we have removed and it will do this over a period of 8-9 months.