So how much carbon can we stash in the ocean, really?

Why ocean photosynthesis is like baking a cake

Say you want to bake a cake, and the recipe calls for:

• 120 grams of flour

• 100 grams of sugar

• 100 grams of eggs

• 5g of baking powder

You go to the kitchen and you realize you have plenty of flour, a big bag of sugar, two dozen eggs and…no baking powder at all.

How many cakes can you bake?

Zero.

Looking at the ingredients list, the amount is baking powder is out of proportion to everything else. Five grams is just not that much mass! But, as anyone who’s tried to bake a cake and forgotten to put in the baking powder can tell you, you really do need it.

Substitute “iron” for “baking powder” and “marine photosynthesis” for “cake”, and you have a rough idea of what ocean iron fertilization is about. Across much of the ocean, tiny marine plants (=phytoplankton) can’t photosynthesize because they’re missing just one ingredient: iron.

The carbon-to-iron ratio in phytoplankton varies, but it’s generally huge: often north of 50,000:1. Provided all the other ingredients are there, a single atom of iron enables enough phytoplankton to lock up 50,000 atoms of carbon.

That’s the kind of eye-popping ratio that gets biogeochemists to sit up and take notice.

Though, of course, it’s not quite that simple. You can’t just drive a dump truck of baking powder up to somebody’s house and say “there, now you can bake a billion cakes.”

Once you clear the baking powder bottleneck, other ingredients start to run out. Maybe you’re going to run out of flour first, maybe sugar, or maybe eggs. Whichever you have least of at home will be your new limiting factor. Very much the same thing holds in the ocean.

Marine photosynthesizers need miniscule quantities of iron to grow, but they need much bigger quantities of other nutrients. They need quite a bit of phosphorous, and substantial amounts of bioavailable nitrogen too. Some crucial micro plants need substantial amounts of silica, too.

Experiments in the 1990s and 2000s conclusively proved that if you add trace quantities of iron to bits of the ocean where all nutrients except iron are plentiful, then you very quickly get big phytoplankton blooms.

You run into this situation mostly in colder waters: the North Pacific, the North Atlantic, and especially the cold ocean around Antarctica, the Southern Ocean.

Those areas tend to be “well-mixed” — a term of art that means deep and shallow waters churn enough that nutrients get brought up from the depths regularly. That’s why the surface is rich in nutrients: it has everything, except the iron.

This sounds like a good thing, but it comes with drawbacks. Well mixed oceans, churning oceans, aren’t very well suited to keeping carbon dioxide locked away for a long time. The same churn that brings nutrients up from the deep also eventually brings the carbon dioxide up again. In cold waters, the CO₂ that photosynthesis captures tends to get re-emitted into the atmosphere within a few years.

For long-term carbon sequestration, you’d prefer waters that aren’t so well mixed: stratified waters, where whatever falls down through the water column stays down for a long time. Those are conditions you usually find in warmer oceans, particularly in the giant circulation patterns just north and south of the equator known as subtropical gyres (though not along the equator itself, for reasons I’ll get into another day.)

Those subtropical gyres are excellent at locking away carbon for a long time, precisely because they’re not well-mixed. But because they don’t churn, they’re nutrient poor. All the macronutrients end up at depths, where marine plants can’t use them.

It’s as though in the places where nature had put a nice hot oven, it had neglected to stock not just baking powder, but the flour and the eggs too.

Flour, in this analogy, is bioavailable nitrogen, which quickly becomes a limiting factor in subtropical gyres, right behind iron. That’s why a lot of the new types of Ocean Fertilization trials now being proposed are aimed not so much at fertilizing phytoplankton in general, but at fertilizing nitrogen-fixing ocean bacteria in particular.

It’s a bit like trying to get oceans to produce the flour they will need to bake a cake. Which may be straining my analogy, granted, but is more or less what some groups are now proposing.

In practice, you’d still be sprinkling iron minerals into ocean waters, though you’d be doing it to stimulate the kinds of nitrogen-fixing bacteria it would take to lift nitrogen as a limiting factor at the same time you clear the iron bottleneck. Whether this can really work is a subject of intense debate in the field, with very eminent scientists disagreeing.

And it gets even more involved, because some scientists think even if you did manage to get enough bioavailable nitrogen into subtropical gyres, you’d then only run into the next bottleneck: no sugar, that is, not enough phosphorus.

Here, the technical arguments get a little bit arcane, with top experts just not aligned on how much phosphorus it takes to sustain how much phytoplankton growth.

And sitting just behind that bottleneck there’s the question of eggs —silica— which isn’t very important for phytoplankton in general but is crucial to diatoms, the specific type of phytoplankton that does the best job at locking carbon away in the ocean depths.

Depending on where exactly an expert comes down on these questions, the additional amount of carbon dioxide the world’s oceans could lock away via fertilization might be as little as one billion tons per year (2% of global human emissions) or as much as 50 billion tons per year (which would offset all human emissions.)

Today, we have no way to know if the real number will come out towards the bottom of that range or the top. The only way to find out is to head out to the oceans and start experimenting, carefully but boldly, to see precisely what happens when you fertilize which types of water how.

It may be, as some groups argue, that you need to fertilize with more than just iron. It may be, as others suspect, that you’re best off sticking to coastal waters with plenty of silica already in them, and shying away from subtropical gyres miles from land. It may be, as still others think, that ocean fertilization need not lock carbon away permanently to be a useful tool against climate change: that it can stash away enough carbon inside the bodies of plankton and krill and fish and whales for decades (if not milenia) to buy us a desperately needed few decades while we transition away from fossil fuels. It could be that focusing on fertilizing seaweed —kelp and sargassum and such— is a much more efficient route than the alternatives. And it might, I’m afraid, turn out that ocean fertilization isn’t actually that useful at all.

However it all turns out, figuring out exactly what minerals to use to fertilize which bit of ocean and how, precisely, is going to be a major scientific challenge. We have a lot to learn about the nitty-gritty of fertilization before we really understand its potential, its ecological impact, its limits, and its drawbacks.

What I know for sure is that there is only one way to answer those questions. And that’s to get baking.