Fertilizing the ocean does two things, both of them good

Restoring ocean ecosystems and removing carbon dioxide are two sides of the same coin.

Say you’re a microscopic marine plant —a phytoplankton— and you’ve just come into existence.

Congratulations! You’re among the septillions of beings that sustain all ocean life. Alas, your time will be short. You’ll float around turning sunlight and carbon dioxide into your own body for a few days, a week max. Then, something will eat you, or you’ll die and sink slowly down to the ocean depths.

By far your more likely fate is the former: something will eat you. Probably, it’ll be a marine animal that’s dozens of times bigger than you, yet still so tiny humans can’t see it without a microscope: a zooplankton. Maybe it’ll be a jellyfish or a krill or a fish. Maybe it’ll be a whale. One way or another, you —and all the carbon dioxide you built your body with— will become part of another being. Then something else will eat that being, and later something bigger still will eat that other one too, and so on: food webs are like that.

The CO₂ you absorbed through photosynthesis is going to end up getting passed around from one organism to another for weeks, years or maybe even decades. Ultimately, one of two things will happen: the CO₂ you took up will either go up into the air, or down to the deep ocean. The same particle can’t be in two places at once.

There are lots of ways the carbon dioxide you absorbed could end up back in the air. It could end up in some organism’s pee, dissolving in the water and eventually reentering the atmosphere during a storm. Or it could end up inside a fish that a human being catches and filets and serves fried with a bit of lemon; it’ll return to the atmosphere through the diner’s breath. Either way, the carbon dioxide you mixed with sunlight to create your body will recycle into the atmosphere, playing its miniscule part in the much bigger drama of a warming planet.

Alternatively, the little CO₂ packet may sink, and avoid all the predators on its way down—unlikely, but not impossible. Your body could end up inside a whale that eventually dies and sinks, and it’s so big it can’t be eaten by the ocean’s bottom dwellers. Or you could end up in fish poo that sinks down through the water column. The farther down the CO₂ goes, the longer it will stay out of the air.

The climate repair technique of ocean fertilization is about leveraging the awesome power of photosynthesis to absorb CO₂ across the 70% of the surface of the earth that’s wet.

The twist is that the absorbing organisms moonlight as the base of the ocean food chain. Tiny marine plants turn solar energy into the chemical energy every other living thing in the ocean depends on. (And yes I know they’re technically “protists”, but they photosynthesize: to normal people, that’s a plant!)

When you increase phytoplankton populations, you do two things at once: increase the rate at which the ocean sucks CO₂ out of the atmosphere and boost the total amount of ocean life, aka biomass.

This dual effect gives rise to contrasting approaches to ocean fertilization.

One set of researchers sees phytoplankton mostly as little wet CO₂ scrubbers: nanomachines for pulling carbon from the air. From this perspective, the ecological role phytoplankton play as the base of marine food webs is almost an annoyance: the more your single phytoplankton gets eaten by other things, the more likely it is that the CO₂ comes back out into the air soon. That diminishes the technique’s efficiency as a CO₂ sink. As a result, these researchers focus on ways of preventing this: thinking up ocean fertilization strategies meant to help phytoplankton biomass make its way down the water column undisturbed, taking as much carbon out of the fast cycle as possible.

But not everybody working on ocean fertilization sees it this way. A second group, often led by marine ecologists, views feeding the ocean food web as a feature, not a bug. From this point of view, removing carbon dioxide is a nice co-benefit of stimulating phytoplankton growth, not the point of it. The point is to slow the collapse of marine ecosystems; the fast decline in fish stocks and whale populations and the web of life that sustains them. As these folks see it, ocean biodiversity is important for its own sake, as well as for livelihoods. They’ll point out that people have a direct economic interest in thriving ocean ecosystems, whether we’re talking coastal fishing communities or places reliant on tourist dollars from whale watchers, divers, or sports fishermen.

My sense is that there’ll always be some tension between these two groups. But it’s easy to overstate it. The beauty of ocean fertilization is that whichever side of the divide you’re on, the biggest side-effects of your project doubles as the main benefit the other group is focused on. Even if you’re mostly concerned about scrubbing carbon dioxide from the air, you can’t be that upset that you also stimulated the ocean food web in the process. And even if your concern is mostly for restoring degraded marine ecosystems, why on earth would you begrudge a bit of carbon dioxide removal on the side? The cowboy and the farmer should be friends!

Ocean fertilization is the ultimate twofer: climate repair and marine ecosystem restoration, all in one go. This one technique offers the chance of affordable gigaton-scale CO₂ removal and fisheries restoration, whale conservation and (in some versions) coastal ecosystem clean-up too. You can do it in ways that stress more one aspect or the other, but however you do it, you’ll be doing both.

I can’t help but notice that groups that are ideologically opposed to ocean fertilization struggle to articulate a reasonable objection to it. They end up hand-waving about unknown-unknowns or scaremongering about geoengineering; sure signs that they can’t come up with a specific negative effect without resorting to science fiction.

Lost in the tidal wave of doomerism, this simple message seems to have gotten lost: with a bit of sustained experimentation, we could reverse the buildup of carbon dioxide in the atmosphere with an affordable technique that also regenerates our marine ecosystem. Why on earth aren’t we focusing more on this?