The Biggest Carbon Sink of All - International Science Council

This article was first published in Anthropocene Magazine.

One of the brightest hopes for sequestering carbon lies in the darkest place on earth: the abyssal depths of the deep ocean. For millennia, dead plants and animals have sunk to the bottom of the sea, where they form sediment that eventually turns into rock (and sometimes fossil fuels).

Some climate researchers think we can now accelerate this natural process and clean up our modern carbon mess by deliberately sinking millions of tons of seaweed and fish.

But is carbon sinking the climate equivalent of sweeping dirt under the rug? Our knowledge of deep-sea ecosystems is still sketchy; and even if the carbon math works out, the logistics of sinking carbon could disrupt the marine systems we rely on for food, transport, and recreation.

A Natural Solution As Vast As The Problem

1. Sinking seaweed. A 2016 paper in Nature Geoscience estimated that marine macroalgae—aka seaweed or kelp—could store around 175 million tons of carbon each year, either by burying it in coastal sediments or exporting it to the deep sea. In 2020, the Energy Futures Initiative, a climate tech non-profit, published a report that found marine carbon dioxide removal could one day sequester CO2 at a billion ton scale, thanks to the sheer amount of available space in the ocean and the absence of land use complications.

Pathways for sequestration of macroalgae carbon into the deep sea.
Figure was adapted from Krause-Jensen and Duarte, 2016.

2. Just add water (and money). There seem to be no major technical barriers to farming seaweed, which can grow at almost 3 centimeters an hour. Several start-ups are already experimenting with growing or moving coastal kelp far out to sea, where it would settle to the ocean floor instead of washing up on shore. Running Tide is using carbon buoys to suspend kelp “microforests” above deep water: when the buoys are deflated, the kelp naturally sinks. Pull to Refresh wants to use semi-autonomous solar-powered vessels to grow and sink its carbon-sucking seaweed.

3. Scaling back on industrial fishing. Gaël Mariani, a marine ecologist at the University of Montpelier has calculated that big fish like tuna also sink to the seafloor when they die—and that global fishing has interrupted that natural process to the tune of 730 million tons of CO2 since 1950. Letting the big ones get away can be a win for carbon.

The Numbers Are Still Murky

1. The science ebbs and flows. The 2016 Nature paper estimated that a square kilometer of seaweed absorbs around 50 tons of carbon. But when University of Tasmania scientist John Barry Gallagher ran the numbers, he was surprised to find that kelp forests are actually a net source of carbon. He calculates that tiny sea creatures feeding on seaweed breathe out their own CO2, meaning a square kilometer of kelp actually emits an average of 20 tons. His helpful article from March suggests more research is needed before we scale up seaweed farms.

2. The detail is in the weeds. In this excellent MIT Technology Review article from last year, James Temple attempts to untangle the prospects for kelp carbon removal. He notes that scholars worry about a billion-ton blue carbon effort blocking the paths of marine mammals, disrupting local ecosystems, interfering with shipping, and encroaching on protected areas and indigenous territories.

3. Don’t sink it, eat it. Any discussion of kelp is complicated by its growing popularity as a climate-friendly agricultural foodstuff, biofuel and replacement for plastics. These reduce the world’s carbon footprint but are only carbon-neutral solutions: recycling carbon dioxide already in the atmosphere rather than permanently removing it. To sink and sequester the kelp instead, a generous carbon price (or other incentive) would be needed to make it a more attractive option.

What To Keep An Eye On

1. Early experiments. Running Tide has already deployed about 1600 kelp buoys, and you can now buy its “carbon-negative” oysters online.

2. Kelp Coins. Whether you consider “a unique, serialized digital security token comprising a forward contract on a ton of seaweed” to be a great way of building natural capital based on marine permaculture, or another example of cryptocurrency gone mad, it will be interesting to see what happens to the Kelp Coin’s $200 face value when it matures (at a so-far unspecified point in the future).

3. Public and political pushback. In 1990, protestors managed to prevent a seaweed farm intended to produce nori for the then-hottest food trend of sushi from going ahead in Washington State. Permitting is still a big issue for kelp cultivation—although Washington recently advanced a bill to streamline regulation.

Mark Harris is an investigative science and technology reporter originally from the UK but now based in Seattle, with a particular interest in robotics, transportation, green technologies, and medical devices. He is a contributing editor at IEEE Spectrum and writes for a wide range of outlets including The Economist, The Guardian, and Wired.

Image: by SIMON LEE on Unsplash